Interferometer configured to measure small structures

Abstract

An interferometer is configured to characterize a plurality of structures in a sample including High Aspect Ratio structures. The interferometer illuminates the sample with illumination optics having a numerical aperture that is smaller than the numerical aperture of the collection optics that collect the reflected light in order to increase the amount of light that penetrates the depth of the structures and improves the signal to noise ratio. During measurement, the focal position of the collection optics with respect to the sample is fixed, while an optical path difference between the sample light and the reference light is altered to scan the coherence fringes through the depth of the structures. Images of a region of the sample are collected at different optical path differences between the reference light and the sample light and are analyzed to characterize the structures in the sample.

Description

INTERFEROMETER CONFIGURED TO MEASURE SMALL STRUCTURESTECHNICAL FIELD

[0001] Subject matter described herein is related to interferometry and more particularly to methods and apparatus for measurement of deep and narrow structures.BACKGROUND

[0002] Interferometry is an established method for precision measurement of a variety of physical parameters including the surface shape or profile of solid objects. Multiple data frames are acquired, each at a different relative phase-shift between a reference beam and a test beam and data is processed by computer to determine the relative path-difference between the reference and test with high accuracy.

[0003] Semiconductor and other similar industries are producing increasingly smaller and more complex structures that require non-contact evaluation during fabrication. For example, one type of structure being produced is Through Silicon Vias (TSV), which are used for connecting devices and chips in integrated circuit’s advanced packaging. TSVs, for example, may include both trenches and holes. Structures, such as TSVs, are sometimes referred to as High Aspect Ratio (HAR) due to their large depth and narrow width. As the depth of complex structures, such as TSVs, continue to increase while the width narrows, it is becoming increasingly difficult if not impossible to measure a profile of these devices using conventional optical metrology methods. For example, with conventional interferometers, an insufficient amount of light is able to penetrate the depth of the structures and return to the detector to provide an adequate signal.SUMMARY

[0004] An interferometer is configured to characterize a plurality of structures in a sample, including structures having a High Aspect Ratio. The interferometer illuminates the sample with illumination optics having a numerical aperture that is smaller than the numerical aperture of the collection optics used to collect reflected light from the sample, thereby increasing the amount of light that penetrates the depth of the structures to improve the signal to noise ratio. Additionally, the collection optics are focused at a fixed focal position with respect to the sample that is not changed, while the optical path difference between the sample light and the reference light is altered or the periodicity of the spectrumof light is modulated to scan the coherence fringes through the depth of the structures during measurements. Images of a region of the sample are collected at different optical path differences between the reference light and the sample light and are analyzed to characterize the structures in the sample.

[0005] In one implementation, a method of characterizing with an interferometer a region of a sample including a plurality of structures includes splitting illumination light into sample light to be incident on and reflected by the sample and reference light to be incident on and reflected by a reference mirror. The method includes illuminating, with illumination optics having a first numerical aperture, the region of the sample with the sample light and focusing collection optics at a focal position with respect to the sample, the collection optics having a second numerical aperture that is greater than the first numerical aperture. One or more coherence envelopes produced by reflected sample light from the sample interfering with reflected reference light from the reference mirror are scanned through a depth of the sample while not changing the focal position of the collection optics with respect to the sample. The method further includes imaging, with the collection optics, the reflected sample light from the sample interfering with the reflected reference light from the reference mirror at a plurality of depths of the one or more coherence envelopes to produce multiple images. The multiple images of the region of the sample including the plurality of structures are analyzed to characterize the region of the sample.

[0006] In one implementation, an interferometer configured for characterizing a region of a sample including a plurality of structures includes means for splitting illumination light into sample light to be incident on and reflected by the sample and reference light to be incident on and reflected by a reference mirror. The interferometer further includes means for illuminating, with illumination optics having a first numerical aperture, the region of the sample with the sample light and means for focusing collection optics at a focal position with respect to the sample, the collection optics having a second numerical aperture that is greater than the first numerical aperture. The interferometer further includes a means for scanning one or more coherence envelopes produced by reflected sample light from the sample interfering with reflected reference light from the reference mirror through a depth of the sample while not changing the focal position of the collection optics with respect to the sample. The interferometer further includes means forimaging, with the collection optics, the reflected sample light from the sample interfering with the reflected reference light from the reference mirror at a plurality of depths of the one or more coherence envelopes to produce multiple images. The interferometer further includes means for analyzing the multiple images of the region of the sample including the plurality of structures to characterize the region of the sample.

[0007] Tn one implementation, an interferometer configured for characterizing a region of a sample including a plurality of structures includes a beam splitter configured split illumination light into sample light to be incident on and reflected by the sample and reference light to be incident on and reflected by a reference mirror. The interferometer further includes illumination optics having a first numerical aperture configured to illuminate the region of the sample with the sample light and collection optics configured to be focused at a focal position with respect to the sample, the collection optics having a second numerical aperture that is greater than the first numerical aperture. An actuator is configured to scan one or more coherence envelopes produced by reflected sample light from the sample interfering with reflected reference light from the reference mirror through a depth of the sample while not changing the focal position of the collection optics with respect to the sample. The interferometer further includes a detector configured to image, with the collection optics, the reflected sample light from the sample interfering with the reflected reference light from the reference mirror at a plurality of depths of the one or more coherence envelopes to produce multiple images. The interferometer further includes at least one processor configured to analyze the multiple images of the region of the sample including the plurality of structures to characterize the region of the sample.

[0008] In one implementation, a method of characterizing with an interferometer a region of a sample including a plurality of structures includes splitting illumination light into sample light to be incident on and reflected by the sample and reference light to be incident on and reflected by a reference mirror. The method includes illuminating, with illumination optics having a first numerical aperture, the region of the sample with the sample light and focusing collection optics at a focal position with respect to the sample, the collection optics having a second numerical aperture that is greater than the first numerical aperture. An optical path difference between the sample light and the reference light is altered while not changing the focal position of the collection optics with respect to thesample. The method further includes imaging, with the collection optics, reflected sample light from the sample interfering with the reflected reference light from the reference mirror at a plurality of optical path differences to produce multiple images. The multiple images of the region of the sample including the plurality of structures are analyzed to characterize the region of the sample.

[0009] Tn one implementation, a method of characterizing with an interferometer a region of a sample including a plurality of structures includes splitting illumination light into sample light to be incident on and reflected by the sample and reference light to be incident on and reflected by a reference mirror. The method includes illuminating, with illumination optics having a first numerical aperture, the region of the sample with the sample light and focusing collection optics at a focal position with respect to the sample, the collection optics having a second numerical aperture that is greater than the first numerical aperture. The method includes modulating a periodicity of the spectrum of the illumination light or interfering light produced by reflected sample light from the sample interfering with reflected reference light from the reference mirror while not changing the focal position of the collection optics with respect to the sample. The method further includes imaging, with the collection optics, reflected sample light from the sample interfering with the reflected reference light from the reference mirror at a plurality of changes in modulation of the periodicity of the spectrum of light to produce multiple images. The multiple images of the region of the sample including the plurality of structures are analyzed to characterize the region of the sample.BRIEF DESCRIPTION OF THE DRAWINGS

[0010] FIG. 1 illustrates a schematic view of an interferometer with a conventional interference objective.

[0011] FIG. 2A illustrates operation of a conventional interference objective scanning a focal position.

[0012] FIG. 2B illustrates a sample with superimposed interference signals produced by scanning the focal position.

[0013] FIGs. 3A and 3B illustrate a Mirau interference objective at different focal positions with respect to a sample.

[0014] FIG. 4A illustrates a scanning electron microscope (SEM) cross-sectional profile, a corresponding low coherence interferometer (LC1) cross-sectional profile of the sample, and a corresponding correlogram illustrating LCI fringes.

[0015] FIGs. 4B and 4C illustrate LCI fringes produced from samples with high aspect ratio vias have widths of 5pm and 1pm, respectively.

[0016] FIG. 5A illustrates a schematic view of a modulation interferometer that modulates the spectrum of the light provided to the metrology interferometer.

[0017] FIG. 5B illustrates a schematic view of a modulation interferometer that modulates the spectrum of the light received from the metrology interferometer.

[0018] FIG. 5C illustrates fringes produced by modulating the spectrum of the light.

[0019] FIG. 5D illustrates input light before modulation of the spectrum of light by a modulation interferometer.

[0020] FIG. 5E illustrates output light showing the periodic spectrum after modulation of the spectrum of light by a modulation interferometer.

[0021] FIG. 6 illustrates examples of the beam collimation spread in systems with different source size and lens focal length.

[0022] FIG. 7A illustrates a schematic view of an interferometer with low NA illumination optics and a high NA collection optics that characterizes structures on a sample by altering the optical path difference between the reference light and the sample light while maintaining the focal position of the collection optics.

[0023] FIG. 7B illustrates a schematic view of an interferometer with low NA illumination optics and a high NA collection optics that characterizes structures on a sample by modulating the periodicity of the spectrum of light while maintaining the focal position of the collection optics.

[0024] FIG. 8 illustrates a schematic view of an interferometer that provides illumination light to the interference objective below the objective lens.

[0025] FIG. 9 illustrates an implementation of an interference objective that may be used with the interferometer shown in FIG. 8.

[0026] FIG. 10 illustrates a schematic view of an interferometer that provides illumination light to the interference objective through the objective lens.

[0027] FIGs. 11 A-l 1C illustrate various implementations of interference objectives that may be used with the interferometer shown in FIG. 10.

[0028] FIG. 12 illustrates a comparison of scanning a single reference beam path and scanning multiple reference beam paths.

[0029] FIGs. 13A, 13B, and 13C illustrate different implementations of producing multiple reference beam paths that may be used with the interferometers shown in FIGs. 7 A, 7B, 8, and 10.

[0030] FIGs. 14A, 14B, and 14C illustrate examples of correlograms for differently sized vias as seen by a single pixel scan by an interferometer as discussed herein.

[0031] FIG. 15 illustrates a full field of view of a sample in multiple images produced by an interferometer as discussed herein.

[0032] FIGs. 16-18 are flow charts illustrating methods of characterizing with an interferometer a region of a sample including a plurality of structures, as discussed herein.DETAILED DESCRIPTION

[0033] Interferometers, such as low coherence interferometers (LCI), generate sample light that is reflected from the sample and reference light that is reflected from a reference surface. The reflected sample light and reference light interferes producing interference patterns. When the optical path difference between the sample light and reference light is less than the coherence length, the strength of the wave interference increases. By scanning the focal position of the interference objective over the sample, the optical path difference between the sample light and the reference light is altered and may be recorded with respect to the changing focal position. Using the recorded interference data with respect to focal position, the sample may be characterized, e.g., the depth profile of the sample may be determined.

[0034] Interferometry has been relied upon to accurately measure the profiles of structures on samples. Structures on samples, however, continue to shrink, which poses difficulties for conventional interferometry techniques. For example, LCI optical profilers may accurately measure depths in vias, e.g., trenches or holes, that are over 10pm wide. Vias with smaller widths, however, become increasingly difficult to measure using LCI techniques. For example, the depth of vias that are 5pm wide may be measurable with some difficulties, but with vias less than 5pm wide, and particularly 3pm wide or less, the depth measurements are not repeatable and may not be possible to measure particularly for deeper vias, e.g., depths of 30pm or more.

[0035] As discussed herein, an interferometer is configured to characterize small structures, such as High Aspect Ratio (HAR) vias that may have widths of 5pm or less and depths of 30pm or greater, using illumination optics that have a numerical aperture (NA) that is smaller than the NA of the collection optics. In some implementations, for example, the illumination light may be provided to the interference objective below the objective lens, so that the NA of the illumination optics excludes the objective lens, while the collection optics include the objective lens. In some implementations, the illumination light may be provided through the objective lens, but the NA of the illumination light is reduced, so that the illumination optics (including the objective lens) has an NA smaller than the NA of the objective lens operating as the collection optics.

[0036] Additionally, instead of scanning the focal position of the interference objective to alter the optical path difference between the reference light and the sample light, the interferometer alters the optical path difference while holding the focal position at a fixed position with respect to the sample. For example, the length of the reference beam path may be altered by moving the reference mirror or a beam splitter or a combination thereof, without changing objectives focal position. In another implementation, instead of scanning the focal position of the interference objective or the reference mirror, the interferometer may modulate the periodicity of the spectrum of light used by the interferometer while holding the focal position at a fixed position with respect to the sample.

[0037] In some implementations, multiple reference light paths may be used, with the length of each reference light path configured based on the nominal positions of the surfacesof the sample. Each of the multiple reference light paths may be scanned simultaneously or sequentially to reduce throughput time of the measurements.

[0038] FIG. 1 shows a schematic view of an interferometer 100 that uses an interference objective that is scanned vertically with respect to the sample to alter the optical path difference with the reference beam. The interferometer 100 may be a low coherence interferometer and includes a broadband light source 110 and a beam splitter 1 12 that directs light towards an interference objective 120. The interference objective 120 is illustrated as a Mirau objective lens, which includes an objective lens 122, a beam splitter 124 and a reference mirror 126. The objective lens 122 directs the light towards the sample 102 and beam splitter 124 splits the light into a reflected reference beam 123, which is incident on and reflects from the reference mirror 126, and a sample beam 125 that is incident on and reflects from the sample 102. The reflected reference beam 123 and reflected sample beam 125r are recombined at the objective lens 122. The resulting interference beam is directed by the beam splitter 112 to a lens 142 that provides an image of interfering beams at the detector 140, which is coupled to a processing system 150. During measurement, the reflected reference beam 123 interferes with the reflected sample beam 125r when the light differs by less than the coherence length.

[0039] The interference objective 120 is coupled to an actuator 128 that is controlled by the processing system 150 to adjust the vertical position of the interference objective 120. In operation, the interferometer 100 vertically scans the interference objective 120, as indicated by the arrow 129, with respect to the sample, to alter the optical path difference with the reference beam 123. In some implementations, the sample 102 may be moved vertically (via a stage) to adjust the relative vertical position of the interference objective 120 and the sample 102. The resulting interference patterns are received by the detector 140 at the different vertical heights and used by the processing system 150 to determine characteristics of the sample 102, e.g., such as the surface profile of the sample 102.

[0040] FIG. 2A, by way of example, illustrates the operation of an interference objective 210 that scans a focal position over the sample to alter the optical path difference. The interference objective 210 produces a sample beam 212 that is directed towards and is reflected by sample 202. The focal position 213 of the sample beam 212 is illustrated as a dotted line. During measurement of the sample 202, the focal position 213 of the samplebeam 212 is scanned (moved) vertically over the sample 202, as illustrated by arrow 218, by moving the interference objective 210 towards and away from the sample (or moving the relative position of the sample 202).

[0041] FIG. 2B illustrates sample 202 with superimposed interference signals 220A, 220B, and 220C produced by scanning the focal position of the interference objective 210 over the sample 202 at different scan points. When the optical path difference between the reference beam and the sample beam is less than the coherence length, the strength of the wave interference increases. By vertically scanning the focal position 213 over the sample 202 at multiple horizontal scan points and recording the resulting interference signal with respect to the focal position, the depth profile of the sample 202 may be obtained.

[0042] FIGs. 3A and 3B illustrate a Mirau interference objective 310 scanning the focal position of the sample beam over a sample 302, as illustrated by arrows 312. The sample 302, for example, may include complex structures and is illustrated with a structure 304 that may be a hole or trench. FIG. 3A, for example, illustrates the interference objective 310 with the focal position of the sample beam at the top surface of the sample 302, i.e., the top of the structures 304. FIG. 3B illustrates the interference objective 310 with the focal position of the sample beam after scanning to the bottom of the structures 304.

[0043] FIG. 4A illustrates a scanning electron microscope (SEM) cross-sectional profile 410 of a sample 402, a corresponding low coherence interferometer (LCI) cross- sectional profile 420 of the sample 402, and a corresponding correlogram 430 illustrating LCI fringes. The LCI cross-sectional profile 420 and corresponding correlogram 430, for example, may be produced by interferometer 100 shown in FIG. 1 by scanning the focal position of the sample beam over the sample 402 vertically. As illustrated, the sample 402 has a top surface 406 illustrated with dotted line, and a plurality of TSVs 404 with bottom surface 408 illustrated with dotted line. The TSVs 404 in FIG. 4 A are 5pm wide with a depth of 10pm.

[0044] The LCI fringes show the LCI cross-sectional profile 420 and in the correlogram 430 illustrate that the strength of the interference signal increases at the top surface 406 of the sample 402 and at the bottom surface 408. The LCI fringes illustrated inthe correlogram 430 are produced at two different positions, i.e., a first position that is located outside a TSV 404 to measure top surface 406 of the sample 402, and at a second position inside the TSV 404 to measure the bottom surface 408 of the TSV 404. By scanning the interference objective vertically so that the focal position of the sample beam scans both the top surface 406 of the sample 402 and the bottom surface 408, the depth of the TSV may be determined based on the distance between the top and bottom fringes that correspond to the top surface 406 and at the bottom surface 408. Diffraction from vias may produce additional fringes, sometimes referred to as ghost fringes, which may further complicate measurements. With relatively large TSVs, e.g., TSVs that are 5pm wide as illustrated in FIG. 4A, ghost fringes are barely visible in the opening of the via or extension outside of via bottom as shown in the LCI cross-sectional profile 420.

[0045] As devices continue to shrink, however, TSVs and other high aspect ratio structures are becoming more difficult, if not impossible, to measure using a conventional LCI interferometer due to narrowing widths and large depths of the structures. For example, with narrow widths of the TSVs, a conventional LCI interferometer that scans the focal position of the sample beam over the depth of the TSV cannot provide sufficient light to the bottom of the TSV to produce an adequate signal, and the presence of ghost fringes may further complicate measurements.

[0046] FIGs. 4B and 4C illustrate LCI fringes 450 and 460 produced from samples with high aspect ratio vias. FIG. 4B illustrates the LCI fringes 450 from a sample having vias with a width of 5pm and a depth of 10pm. As can be seen in FIG. 4B, the fringes 452 from the top surface are clearly visible as are the fringes 454 from the bottoms of the vias, and accordingly, the depth of the vias in the structure illustrated in FIG. 4B may be accurately measured. As the vias narrow and / or deepen, however, the amplitude of the fringes from the bottom of the vias significantly decrease. FIG. 4C, for example, illustrates the LCI fringes 460 from a sample having vias, such as trenches, with a width of 1.5pm and a depth of 8pm. As can be seen in FIG. 4C, the fringes 462 from the top surface are clearly visible, however, the amplitude of the fringes 464 from the bottoms of the vias is greatly reduced and may be inadequate for accurate characterization of the vias. In particular, it should be noted that the via illustrated in FIG. 4C are relatively shallow, e.g., with an aspect ratio (width:depth) of about 1 :5, and the amplitude of the fringes 464 at the bottoms of thevias are very weak. It is desirable to measure high aspect ratio (HAR) vias, which may have an aspect ratio of 1:20 smaller, e.g., 1:50. For conventional LC1 measurement of vias, the typical aspect ratio limit for vias less than between 5pm and 3pm in diameter is about 1:12, and for narrower structures, the limit is worse, e.g., 1 :10 and less. Moreover, as the structures narrow to below, e.g., 4pm, additional diffraction from the vias incudes ghost fringes 466, which are strong at the via opening at the top surface level, which may further complicate measurements.

[0047] In order to adequately characterize structures, such as HAR structures, it has been found that the strength of the fringes at the bottom of the vias depends on factors including the numerical aperture of the illuminating light, whether the via is a hole or a trench, the diameter / width, the depth, as well as polarization of the illuminating light, relative reflectivities of sample with respect to the reference mirror, the spectral bandwidth of the illuminating light (e.g., shorter wavelengths will travel farther in narrow structures), and position of the reference mirror. In particular, it has been found that maintaining the focal position of the objective lens at a single height, while altering the optical path difference between the reference beam and the sample beam generates fringes with increased amplitude of the fringes from the bottom of the structures. The optical path difference may be altered and low coherence fringes scanned through object depth, while maintaining the focal position of the objective lens (i.e., while holding the focal position of the object lens fixed at one location), for example, by scanning the reference mirror, scanning a beam splitter that produces the reference light, or a combination thereof. In another implementation, low coherence fringes may be produced and scanned through object depth by modulating the periodicity of the spectrum of light while maintaining the focal position of the objective lens at a single height. In some implementations, both the optical path difference may be altered and the periodicity of the spectrum of light may be modulated to produce coherence fringes that are scanned through object depth without moving the focal position of the objective lens. By scanning the coherence fringes through object depth without moving the focal position of the objective lens, e.g., by scanning the reference mirror or by modulating the periodicity of the spectrum of the light, it is possible to avoid imaging the diffraction influence from objective defocus, e.g., to avoid varying the background intensity and or affects the fringe visibility.

[0048] Scanning low coherence fringes through the objects depth by modulating the periodicity of the spectrum of the light source has a similar effect as altering the optical path difference while maintaining the objective focal position at the same height with respect to object. Modulation of the periodicity of the spectrum of the light, while maintaining the focal position of objective lens may be performed, e.g., by modifying a broadband spectrum to be periodic and then varying the spectrum in time periodicity. The periodicity of the spectrum of light may be modulated, for example, using a modulation interferometer that is combined in one of a number of ways with the primary metrology interferometer, e.g., in a tandem interferometer configuration.

[0049] FIG. 5A, by way of example, illustrates a schematic view of a modulation interferometer 500 that may be used to modulate the periodicity of the spectrum of the light and that may be combined with the metrology interferometer, e.g., in a tandem interferometer configuration. For example, the modulation interferometer 500 may receive light from the light source and modulate the spectrum of the light, which is provided as input light to the metrology interferometer, or the modulation interferometer 500 may receive light from the metrology interferometer and modulate the spectrum of the light, which is provided to the detector of the metrology interferometer. As illustrated, the modulation interferometer 500 may have a Michelson configuration, but other configurations may be used. The input light (e.g., from a light source or from the metrology interferometer) is received and provided to a beam splitter 504. The beam splitter 504 directs a portion of the light to a first return mirror 506 in a first arm and another portion of the light to a light to a second return mirror 508 in a second arm. The light reflected from first return mirror 506 and second return mirror 508 is recombined at beam splitter 504 and is provided as output light (e.g., as input light to the metrology interferometer or to the detector of the metrology interferometer). As illustrated, the first return mirror 506 is coupled to an actuator 507 that is controlled by a processing system to vary the position of the first return mirror 506, thereby altering the optical path difference between the reference arms of the modulation interferometer 500. If desired, both the first return mirror 506 and the second return mirror 508 may be moved.

[0050] In another implementation, rather than moving first return mirror 506, the periodicity of the spectrum of light may be modulated using an unbalanced optic fiberinterferometer, e.g., which stretches one optic fiber with respect to the other, to change the balance of the interferometer thereby changing the position of the internal reference plane, and thus modulating the periodicity of the spectrum of the light.

[0051] The periodicity of spectrum of the light may be modulated using other techniques. FIG. 5B, for example, illustrates a spectrum modulator 520 that may be used to modulate the periodicity of the spectrum of the light and that may be combined with the metrology interferometer. For example, the spectrum modulator 520 may receive light from the light source and modulate the spectrum of the light, which is provided as input light to the metrology interferometer, or the spectrum modulator 520 may receive light from the metrology interferometer and modulate the spectrum of the light, which is provided to the detector of the metrology interferometer. The spectrum modulator 520 receives input light and spectrally decomposes the spectrum of the light with a dispersive optic 522, such as a prism or diffraction grating. The spectrum of the light is modulated using a spatial light modulator 524, thereby creating a varying periodicity of the spectrum of the light, which is recombined using recombination optic 526, which may be a prism or diffraction grating, to produce the output light.

[0052] FIG. 5C illustrates low coherence fringes 550 for creating a periodic spectrum, as discussed above. FIG. 5D, by way of example, illustrates an example of the input light 562 from the source before modulation, and FIG. 5E illustrates an example of the output light 564 showing the periodic spectrum after modulation. As illustrated in FIG. 5C, three sets of low coherence fringes, 552, 554, and 556 are shown, each bounded by coherence envelopes. The outside fringes 554 and 556 have a distance D from the middle fringe 552, and the distance D changes to change the periodicity of the spectrum, e.g., by altering the optical path difference of the reference arms in the modulation interferometer 500 or by modulating the spectrum with the spatial light modulator 524 in the spectrum modulator 520. By modulating the periodicity of the spectrum of the light, as discussed above, or using other methods, the coherence envelope of the outside fringes 554 and / or 556 may be moved through the depth of object, e.g., via, while maintaining the focal position of the objective lens, which advantageously avoids mechanical scanning.

[0053] Additionally, the interferometer may use a design that separates the illumination and imaging optics. For example, the optics design may use a low numericalaperture (NA) optics for illumination of the sample while using high NA imaging optics. The imaging optics may be designed for best imaging in terms of resolution, e.g., high NA, distortion, chromatic aberration etc. The illumination optics, which may be separate optics under the objective lens, may be designed for best wavefront quality, better collimation e.g., lower NA. Defocus of collimating optics may additionally control the wavefront shape and shape the illuminating beam. Wavelength bandwidth filters and polarization elements may also influence light propagation in structures such as HAR TSVs. The design of the illuminating optics separately from the imaging optics, allows for a control or enhancement of the shape of fringe modulation envelope allowing for characterization of deeper vias.

[0054] FIG. 6, by way of example, illustrates an example of beam collimation spread in systems with different source sizes and lens focal lengths. FIG. 6 illustrates three systems 610, 620, and 630 including a light source 602 that is regulated by the size of light source aperture 614 (in systems 610 and 630) and the light source aperture 624 (in system 620), and the numerical aperture NA (illustrated with focal length) of the optics 612 (having focal length fl 1 in systems 610 and 620) and 632 (having focal length ±12 in system 630). As illustrated, the light source aperture 624 is larger than the light source aperture 614 thereby providing a larger light source 602, and the focal length fl2 of optics 632 is shorter than the focal length fll of optics 612, e.g., the NA of optics 632 is larger than the NA of optics 612.

[0055] As illustrated by systems 610 and 620, which use light source 602 and optics 612, the size of the light source aperture for the source 602 affects the beam collimation spread. System 610, for example, includes a light source aperture 614 that is relatively smaller than the light source aperture 624 of system 620, which results in a relatively smaller range of angle of series (spread) of the collimated beam in system 610, illustrated by angle a, than in system 620, illustrated by angle [31, e.g., [31> a. The smaller size of the light source aperture 614 in system 610 results in less collimation beam spread than found in system 620 with the larger size of the light source aperture 624. Accordingly, the system 610 has better beam collimation than system 620 but provides less light for an equivalent sized optic 612.

[0056] Additionally, as illustrated by systems 610 and 630, which use the same light source 602 and light source aperture 614, the NA (focal length) of the optics affects thebeam collimation spread. System 610, for example, includes optics 612 with a relatively longer focal length ill (smaller NA) than the focal length fl2 (higher NA) of the optics 632 of system 630, which results in a relatively smaller range of angle of series (spread) of the collimated beam in system 610, illustrated by angle a, than in system 630, illustrated by angle [32, e.g., P2> a. The longer focal length ill (smaller NA) of the optics 612 in system 610 results in less collimation beam spread than found in system 630 with the shorter focal length fl2 (larger NA). Accordingly, system 610 has better beam collimation than system 630 but provides less light for an equivalent sized light source 602 and light source aperture 614.

[0057] Thus, separating the illumination optics design and imaging optics design allows for control over the light propagation to the structures, e.g., vias, without affecting the imaging optics. By reducing the NA of the illumination optics, e.g., to nominal collimated light, more light may be provided to the bottom of the structure under test, while using a high NA for the imaging optics allows for better imaging of small lateral features on object.

[0058] FIGs. 7A and 7B, by way of example, illustrates schematic views of interferometers 700 and 700’, respectively, that characterize structures on a sample by scanning the coherence fringes through object depth while maintaining the focal position of the collection optics at a single height, and using illumination optics and collection optics with different effective NAs so that the illumination light is more collimated than the collected sample light, as discussed herein. Interferometer 700’ is similar to interferometer 700, with like designated elements being the same, and are sometimes collectively referred to herein as interferometers 700.

[0059] Interferometer 700 illustrated in FIG. 7A is configured to alter the optical path difference to scan the coherence fringes through object depth between the reference light and the sample light while maintaining the focal position of the collection optics, and interferometer 700’ illustrated in FIG. 7B is configured to modulate the periodicity of the spectrum of light to scan the coherence fringes through object depth while maintaining the focal position of the collection optics. In some implementations, one or more aspects of interferometer 700’ may be combined with interferometer 700. Interferometers 700 may use low NA illumination optics for illumination optics and high NA collection optics toproduce illumination light that is more collimated than the collected sample light. In some implementations, interferometer 700 may be an LCI interferometer, coherence scanning interferometry or the like.

[0060] As illustrated in FIGs. 7A and 7B, interferometer 700 and interferometer 700’ include optical heads 710 and 710’, respectively, (sometimes collectively referred to as optical head 710), which include a light source 712 and a detector 720, and an interference objective 713 that includes a beam splitter 714, a reference mirror 716, and an objective lens 718. In some implementations, interferometers 700 may be an LCI interferometer, and the light source 712 may have a spectral bandwidth and angular bandwidth that is sufficient to produce low coherence fringes. Additionally, the light source 712 may be configurable based on the type of structures on the sample being measured. For example, the light source 712 may adjust at least one of the spectral bandwidth and the angular bandwidth, at least one of cylindrical optics and spatially linear source, and polarization control, or any combination thereof, based on the configuration of the structures being measured. For example, the spectral and / or angular bandwidth and type of optics may be selected based on the shape, width, and depth of the vias. For example, shorter wavelengths may be used to better propagate in narrow vias compared to longer wavelengths, while use of a wide spectral bandwidth may be used to better determine the coherence envelope, e.g., the coherence envelope is narrower as spectral bandwidth increases. Angular bandwidth may be used to control the spread of the collimated beam, e.g., as illustrated in FIG. 6.Moreover, the use of cylindrical optics and or linear light source may be used to control the spread of collimation in the x and y directions, which may be beneficial to control for trench measurements. Further, the polarization of the light may be controlled in response to the shape, width, and depth of the vias. For example, circular polarization may be used for a variety of vias, including holes and trenches with various orientation. In specific instances, such as trenches having one particular orientation, linear polarization with the same orientation as the trench may be used for improved propagation in the trench. Additionally, relative reflectivities of sample with respect to the reference mirror may be controlled, for example, by selecting mirrors of different reflectivity or by varying the ratio of beam splitting to the different arms of the interferometer. In some implementations, additional components may be included in the optical head 710, such as beam splitters, polarizers, lenses, etc. Additionally, the relative location of the light source 712 in the optical head 710may be altered, as discussed herein. For example, as illustrated, illumination light from the light source 712 is provided to beam splitter 714 below the objective lens 718, but in some implementations, the illumination light from the light source 712 may be provided above the objective lens 718 (as illustrated by the objective lens 718 with dotted lines), e.g., via a second beam splitter. The beam splitter 714 splits the illumination light and produces reference light 715 that is directed to and reflects from the reference mirror 716 and sample light 717 that is directed to the sample 702.

[0061] In some implementations, the optical head 710 may include additional components such as an aperture 719 to effectively reduce the NA of the illumination light. While the interference objective 713, illustrated in FIGs. 7 A and 7B, has a Michelson configuration, it should be understood that other types of interference objective configurations may be used in the optical head 710, including a Mirau objective configuration and a Linnik objective configuration.

[0062] The illumination light from the light source 712 is received by beam splitter 714, which splits the illumination light and produces reference light 715 that is directed to and reflects from the reference mirror 716 and sample light 717 that is incident on and reflected by the sample 702. The reflected light from the reference mirror 716 and the sample 702 is recombined and interferes producing interference light 722 that is received by the detector 720, from which one or more characteristics of a structure 704 on the sample 702 may be measured, such as the depth, width, or other profile parameters. The sample light 717 is focused by the objective lens 718 in optical head 710, e.g., at or near the top surface of the sample 702.

[0063] Interferometers 700 maintain the focal position of objective lens 718 with respect to the sample 702, i.e., the focal position is fixed, while coherence fringes are scanned through the object depth during measurements of the sample 702. As illustrated in FIG. 7 A, for example, the optical path difference between the reference light 715 and the sample light 717 is altered to scan the coherence fringes through the object depth. The optical path difference between the reference light 715 and the sample light 717 may be altered by increasing and / or decreasing the length of the optical path of the reference light 715 (as illustrated by arrow 721 ). For example, the length of the optical path of the reference light 715 may be increased or decreased by moving the reference mirror 716 or bymoving the beam splitter 714 (e.g., if the interference objective 713 has a Mirau type objective configuration).

[0064] As illustrated in FIG. 7B, the periodicity of the spectrum of the light may be modulated to scan the coherence fringes through the object depth, e.g., using modulation optics 772 or 782, which may be similar to the modulation interferometer 500 shown in FIG. 5 A or the spectrum modulator 520 shown in FIG. 5B. For example, the modulation optics 772 may modulate the periodicity of the spectrum of light produced by the light source 712 that is provided to the beam splitter 714. In another example, the modulation optics 782 may receive light from the beam splitter 714, e.g., via mirrors 784 and 786, and may modulate the periodicity of the spectrum of light, which is provided to the detector 720, e.g., via mirror 788. In some implementations, a combination of techniques illustrated in FIGs. 7A and 7B may be used, including any combination of moving the reference mirror 716, moving the beam splitter 714, and modulating the periodicity of the spectrum of light.

[0065] Additionally, the interferometers 700 may use illumination optics having with an effective NA for the illumination light incident on the sample 702 (e.g., illustrated by shaded sample light 724) that is lower than the effective NA of the collection optics for the collected light from the sample 702 (e.g., illustrated by collected light 726). The illumination optics for the illumination light, for example, may have a relatively low, effective NA that is less than 0.05, and may produce collimated or near collimated light, while the collection optics for the collected light may have a relatively high, effective NA of at least 0.2. By way of example, in some implementations, the low NA illumination optics may be one or more lenses (not shown) that provide the illumination light to the interference objective 713 after the collection optics in the beam path, i.e., between the objective lens 718 and the sample 702. The beam splitter 714 directs a portion of the illumination light to illuminate the sample 702 and directs another portion of the illumination light to the reference mirror 716. In another implementation, the effective NA of the illumination optics may be reduced by limiting the spatial extent of the light source 712, e.g., with aperture 719 (e.g., as discussed in FIG. 6), and thus, for example, the low NA illumination optics may include the objective lens 718, and an aperture 719 to reduce the spatial extent of the light source 712. There are different ways of reducing the NA of the illumination optics, such as through the use of Kohler illumination, critical illumination, aswell as other afocal or non- focused or critical illuminations may be used to control the NA of the illumination optics.

[0066] The interferometers 700 are further configured to image a region of the sample 702 that includes a plurality of structures 704 so that multiple structures 704 may be characterized simultaneously. For example, the detector 720 may include an imaging tube lens that images a region of sample that is received by the detector 720, and multiple images of the region may be collected as the optical path difference is varied.

[0067] The detector 720 in the optical head 710, as well as other components of the interferometers 700, such as the light source 712, stage 730, chuck 732, actuators to move the reference mirror 716 and / or beam splitter 714, reference mirrors in the modulation optics 772 or 782, etc. may be coupled to a processing system 750, such as a workstation, a personal computer, central processing unit or other adequate computer system, or multiple systems. It should be understood that one processor, multiple separate processors or multiple linked processors may be used, all of which may interchangeably be referred to herein as processing system 750. The processing system 750 may be included in, or connected to, or otherwise associated with interferometers 700. The processing system 750, for example, may control the positioning of the sample 702, e.g., by controlling movement of the stage 730 on which the sample 702 is held. The stage 730, for example, may be capable of horizontal motion in either Cartesian (i.e., X and Y) coordinates, or Polar (i.e., R and 0) coordinates or some combination of the two. The stage 730 may also be capable of vertical motion along the Z coordinate, e.g., for focusing the objective lens 718 on or near the surface of the sample 702. In some implementations, the stage 730 may be held stationary while the optical head 710 or portions of the optical head 710 moves relative to the sample 702 or both the stage 730 and the optical head 710 (or portions thereof) may move relative to the other. The processing system 750 may further control the operation of a chuck 732 on the stage 730 used to hold or release the sample 702.

[0068] The processing system 750 may also collect and analyze the data obtained from the optical head 710. The processing system 750 may analyze the interferometer data to determine characteristics of the structure 704 on the sample 702, such as the depth, width, or other profile parameters, as discussed herein. The processing system 750, which includes at least one processor 752 with memory 754, as well as a user interface including e.g., adisplay 756 and input devices 758. A non-transitory computer-usable storage medium 759 having computer-readable program code embodied may be used by the processing system 750 for causing the processing system 750 to control the interferometers 700 and to perform the functions including the analysis described herein. The data structures, software code, etc., for automatically implementing one or more acts described in this detailed description can be implemented by one of ordinary skill in the art in light of the present disclosure and stored, e.g., on a non-transitory computer-usable storage medium 759, which may be any device or medium that can store code and / or data for use by a computer system such as the at least one processor 752. The non-transitory computer-usable storage medium 759 may be, but is not limited to, flash drive, magnetic and optical storage devices such as disk drives, magnetic tape, compact discs, and DVDs (digital versatile discs or digital video discs). A communication port 757 may also be used to receive instructions that are used to program the processing system 750 to perform any one or more of the functions described herein and may represent any type of communication connection, such as to the internet or any other computer network. The communication port 757 may further export signals, e.g., with measurement or inspection results and / or instructions, to another system, such as external process tools, in a feed forward or feedback process in order to adjust a process parameter associated with a fabrication process step of the samples based on the measurement results. Additionally, the functions described herein may be embodied in whole or in part within the circuitry of an application specific integrated circuit (ASIC) or a programmable logic device (PLD), and the functions may be embodied in a computer understandable descriptor language which may be used to create an ASIC or PLD that operates as herein described. The results from the analysis of the data may be stored, e.g., in memory 754 associated with the sample and / or provided to a user, e.g., via display 756, an alarm or other output device. Moreover, the results from the analysis may be fed back to the process equipment to adjust the appropriate patterning step to compensate for any errors detected in the measurements or inspection.

[0069] FIG. 8 shows a schematic view of an interferometer 800, which may be a low coherence interferometer, and may be an implementation of the interferometer 700 shown in FIG. 7A. The interferometer 800 includes an interference objective that changes the reference light path length to alter the optical path difference between the reference light and sample light to scan coherence fringes through the object depth. Additionally, theinterferometer 800 provides nominally collimated illumination light to provide a low NA for the illumination light that is incident on the sample and a high NA for the collected light from the sample 802. If desired, interferometer 800 may include a modulation optic, as illustrated in FIG. 7B and FIGs. 5 A and 5B, in addition or alternatively to changing the optical path difference between the reference light and the sample light to scan coherence fringes through the object depth.

[0070] As illustrated, the interferometer 800 includes a light source 810 and one or more lenses 811 to produce collimated or near collimated light that is received by the interference objective 820. The interference objective 820 is illustrated in FIG. 8 as a Michelson objective lens, which includes an objective lens 822, a beam splitter 812 and a reference mirror 826, but other types of objective lenses may be used. The collimated light is received by the interference objective 820 below the objective lens 822, i.e., between the objective lens 822 and the sample 802. A beam splitter 812 splits the collimated light into reference light 823, which is incident on and reflects from the reference mirror 826, and sample light 825 that is incident on and reflects from the sample 802. The reference mirror 826 is coupled to an actuator 828 that is controlled by the at least one processor 850 to adjust the position of the reference mirror 826. In operation, the interferometer 800 moves the reference mirror 826, as indicated by the arrow 827, to alter the optical path difference between the reference light 823 and the sample light 825. In some implementations, an assembly 830 including, e.g., the reference mirror 826, beam splitter 812, and light source 810 with one or more lens 811, may be coupled to an actuator 831 and moved, as indicated by arrow 832, to alter the optical path difference between the reference light 823 and the sample light 825. The objective lens 822 operates as collection optics and is focused on or near the top surface of the sample 802 as illustrated by collected light 829. The focal position of the collection optics (i.e., objective lens 822) does not change, i.e., the focal position is fixed relative to the sample 802, while the optical path difference is altered by moving the reference mirror 826. With collimated or nearly collimated illumination light provided below the objective lens 822, the interferometer 800 may use low NA illumination optics (e.g., lens(es) 811) for the sample light 825 (e.g., nominally collimated), while using high NA collection optics (e.g., objective lens 822) for the collected light 829.

[0071] The reference light 823 and sample light 825 reflected from the sample 802 are recombined at the beam splitter 812. The resulting interference light is directed by the beam splitter 812, the objective lens 822, and lens 842, e.g., an imaging tube lens, to a detector 840, which is coupled to at least one processor 850, which may be part of processing system 750. The resulting interference patterns are received by the detector 840 at the different optical path differences and used by the at least one processor 850 to determine characteristics of the sample 802, e.g., such as the surface profile of the sample 802.

[0072] The interferometer 800 uses the broadband light source 810 with a scanning reference mirror 826 to achieve depth resolution that is determined by the positioning of the coherence envelope that is scanned through the depth of the object. The coherence envelope is the propagation distance over which a coherent wave (e.g., an electromagnetic wave) maintains a specified degree of coherence. The wave interference is strong when the optical path difference between the reference light 823 and sample light 825 differs by less than a coherence length. As discussed in FIGs. 5 A and 5B and FIG. 7B, in some implementations, the periodicity of the spectra of the light, e.g., provided by the broadband light source 810 or the interference light provided by the beam splitter 812, may be modulated to scan a coherence envelope through the depth of the object in addition to or instead of scanning the reference mirror 826. The at least one processor 850 receives the reflected optical data from each single scan point, which is interpreted as an interference pattern along the scan, and records it as a depth profile. The interferometer 800 may simultaneously collect data over a large field of view, e.g., by imaging on a detector array, e.g., a CCD array or polarized pixelated camera if polarized states are introduced in the beams, a region of the sample 802 that includes a number of structures to be measured. Each pixel in the image corresponds to a location in the region of the sample 802 and each image may correspond to a different optical path difference, which corresponds to a depth.

[0073] FIG. 9 illustrates an implementation of the interference objective, which may be used as the interference objective 820 in interferometer 800 shown in FIG. 8. FIG. 9 illustrates an interference objective 900 (can be a Michelson type interference objective) that includes an objective lens 902, beam splitter 904, and a reference mirror 906 that moves to alter the optical path difference, as indicated by the arrow. Collimated, or nearlycollimated, illumination light, is provided by a light source (not shown) via one or more lenses 908 and is received by the interference objective 900 below the objective lens 902, i.e., between the objective lens 902 and the sample 901. The beam splitter 904 splits the illumination light into reference light 905, which is incident on and reflects from the reference mirror 906, and sample light 907, which is incident on and reflects from the sample 901. The collection optics, e.g., objective lens 902, is focused on or near the top surface of the sample 901, as illustrated by collected light 909, and maintains the focal position, while the optical path difference is altered by moving the reference mirror 906. In some implementations, the reference mirror 906, beam splitter 904, and source of the illumination light may be coupled to an actuator and moved vertically, as indicated by the dashed arrow, to alter the optical path difference between the reference light 905 and the sample light 907. As illustrated, because the sample light 907 is provided with a low NA illumination optics (lenses 908), e.g., nominally collimated, more light is provided to the bottom of the structures of the sample 901 than would be provided via objective lens 902.

[0074] FIG. 10 shows a schematic view of an interferometer 1000, which may be a low coherence interferometer, and may be an implementation of the interferometer 700 shown in FIG. 7A. The interferometer 1000 includes an interference objective that changes the reference light path length to alter the optical path difference between the reference light and sample light to scan coherence fringes through the object depth. Additionally, the interferometer 1000 uses low NA illumination optics to provide illumination light that is incident on the sample and a high NA imaging optics for the collected light from the sample 1002. If desired, interferometer 1000 may include a modulation optic, as illustrated in FIG. 7B and FIGs. 5 A and 5B, in addition or alternatively to changing the optical path difference between the reference light and the sample light to scan coherence fringes through the object depth.

[0075] As illustrated, the interferometer 1000 includes a light source 1010 (can be a broadband light source) and one or more lenses 1011 and an aperture 1012 to reduce the aperture size of the light source 1010. The interferometer 1000, for example, may use Kohler illumination. The illumination light is directed towards the interference objective 1020 by a beam splitter 1014. The interference objective 1020 is illustrated in FIG. 10 as a Mirau objective lens, which includes an objective lens 1022, a beam splitter 1024, and areference mirror 1026, but other types of objective lenses may be used. The illumination light is received by the objective lens 1022 and focused on or near the top surface of the sample 1002 as illustrated by the cone of the sample light 1025. Due to the reduced aperture size of the light source 1010, the illumination light incident on the sample 1002 is provided with the objective lens 1022 with an effectively low NA. The objective lens 1022, however, collects, e.g., images, reflected light from the sample 1002 with a higher NA, as illustrated with collected light 1029. Accordingly, as illustrated, the interferometer 1000 uses a low NA optics for sample light 1025 but uses a high NA optics for the collected light 1029.

[0076] A beam splitter 1024 splits the illumination light into reference light 1023, which is incident on and reflects from the reference mirror 1026, and sample light 1025 that is incident on and reflects from the sample 1002. The length of the reference light path may be altered in the interference objective 1020 by moving the reference mirror 1026 and / or the beam splitter 1024. For example, the reference mirror 1026 may be coupled to an actuator 1028 that is controlled by the at least one processor 1050 to adjust the position of the reference mirror 1026. Additionally, or alternatively, the beam splitter 1024 may be coupled to the actuator 1028 that is controlled by the at least one processor 1050 to adjust the position of the beam splitter 1024. In operation, the interferometer 1000 maintains, i.e., fixes relative to the sample 1002, the focal position of the objective lens 1022, while the optical path difference is altered by, e.g., moving the reference mirror 1026 and / or the beam splitter 1024. The reference mirror 1026 (and / or beam splitter 1024) may be scanned, as indicated by the arrows, to alter the optical path difference between the reference light 1023 and the sample light 1025.

[0077] The reference light 1023 and sample light 1025 reflected from the sample 1002 are recombined at the beam splitter 1024. The resulting interference light is directed by the objective lens 1022, beam splitter 1014, and lens 1042 to a detector 1040, which is coupled to at least one processor 1050. The resulting interference patterns are received by the detector 1040 at the different optical path differences and used by the at least one processor 1050 to determine characteristics of the sample 1002, e.g., such as the surface profile of the sample 1002. The at least one processor 1050, for example, may be part of processing system 750 shown in FIGs. 7A and 7B.

[0078] The interferometer 1000 uses the light source 1010 with a reference mirror 1026 (can be a scanning reference mirror) and / or beam splitter 1024 to achieve depth resolution that is determined by the positioning of the coherence envelope that is scanned through the depth of the object. The coherence envelope is the propagation distance over which a coherent wave (e.g., an electromagnetic wave) maintains a specified degree of coherence. The wave interference is strong when the optical path difference between the reference light 1023 and sample light 1025 differs by less than a coherence length. As discussed in FIGs. 5A and 5B and FIG. 7B, in some implementations, the periodicity of the spectra of the light, e.g., provided by the broadband light source 1010 or the interference light provided by the beam splitter 1024, may be changed to scan a coherence envelope through the depth of the object in addition to or instead of scanning the reference mirror 1026. At least one processor 1050 receives the reflected optical data from each single scan point, which is interpreted as an interference pattern, and records it as a depth profile. The interferometer 1000 may simultaneously collect data over a large field of view, e.g., by imaging on a detector array, e.g., a CCD array or polarized pixelated camera if orthogonal polarized states are introduced in the beams, a region of the sample 1002 that includes a number of structures to be measured. Each pixel in the image corresponds to a location in the region of the sample 1002 and each image may correspond to a different optical path different, which corresponds to a depth.

[0079] FIGs. 11 A- 11C illustrate various implementations of interference objectives, which may be used as the interference objective 1020 in interferometer 1000 shown in FIG. 10.

[0080] FIG. 11 A illustrates an interference objective 1100 (can be a Mirau type interference objective) that includes an objective lens 1102, beam splitter 1104, and a reference mirror 1106, where the reference mirror 1106 and / or beam splitter 1104 is moved to alter the optical path difference, as indicated by the arrows. The illumination light is provided by a light source with a reduced aperture size (not shown) and is received by the interference objective 1100 and focused by the objective lens 1102 as illustrated by sample light 1107. The beam splitter 1104 splits the illumination light into reference light 1105, which is incident on and reflects from the reference mirror 1106, and the sample light 1107, which is incident on and reflects from the sample 1101. The objective lens 1102 is focusedon or near the top surface of the sample 1101 to image the sample 1101 with relatively high NA optics, as illustrated by collected light 1109, and maintains the focal position, while the optical path difference is altered by moving the reference mirror 1106, the beam splitter 1104, or both the reference mirror 1106 and the beam splitter 1104.

[0081] FIG. 1 IB illustrates an interference objective 1120 (can be a Michelson type interference objective) that includes an objective lens 1 122, a beam splitter 1 124, and reference mirror 1126 that moves to alter the optical path difference, as indicated by the associated arrow. Similar to interference objective 1100 discussed in FIG. 11A, illumination light is provided by a light source with a reduced aperture size (not shown) and is received by the interference objective 1 120 and focused by the objective lens 1122 as illustrated by sample light 1127. The beam splitter 1124 splits the illumination light into reference light 1125, which is incident on and reflects from the reference mirror 1126, and the sample light 1127, which is incident on and reflects from the sample 1121. The objective lens 1122 is focused on or near the top surface of the sample 1121 to image the sample 1121 with relatively high NA optics, as illustrated by collected light 1129, and maintains the focal position, while the optical path difference is altered by moving the reference mirror 1126, or the beam splitter 1124 and the reference mirror 1126 are moved vertically together, as illustrated by the dashed arrow.

[0082] FIG. 11C illustrates a Linnik type interference objective 1150 that includes matching objective lenses 1152 and 1153, beam splitter 1154, and a reference mirror 1156 that moves to alter the optical path difference, as indicated by the arrow. Illumination light is provided by a light source with a reduced aperture size (not shown) and is received by the beam splitter 1154 that splits the illumination light into reference light 1155, which is focused on the reference mirror 1156 with objective lens 1153, and sample light 1157, which focused by the objective lens 1152 on and reflects from the sample 1151. The objective lens 1153 and reference mirror 1156 may move together to alter the optical path difference, while in some implementations, only the reference mirror 1156 is moved. In some implementations, the beam splitter 1154, objective lens 1153, and reference mirror 1156 are all moved vertically, as illustrated by the dashed arrow. The objective lens 1152 is focused on or near the top surface of the sample 1151 as illustrated by collected light 1159, to image the sample 1 151 with relatively high NA optics, and maintains the focal position,while the optical path difference is altered by moving the reference mirror 1156 and objective lens 1153.

[0083] In some implementations, the multiple reference paths may be used. For example, if a single reference path is used for measuring structures having multiple levels, e.g., a top surface and a bottom surface, the scan to alter the reference beam path to produce fringe patterns corresponding to each level may be long, which will decrease throughput. By using multiple reference paths, with each reference path configured for a different level, the scan length may be greatly reduced.

[0084] FIG. 12, by way of example, illustrates a structure 1200 with a top surface 1202 and a bottom surface 1204 with corresponding correlograms 1210 and 1230 generated by an interferometer, as discussed herein. The correlograms 1210, 1230 include top fringes 1212, 1232 corresponding to the top surface 1202 of the structure 1200 and bottom fringes 1214, 1234 corresponding to the bottom surface 1204 of the structure 1200.

[0085] With a single reference beam path, the scan to alter the reference beam path will have a length at least as great as the full depth of the structure 1200. For example, a single reference mirror 1220 may scan a distance 1222 of the entire depth of the structure 1200 to produce the correlogram 1210 with top fringes 1212 corresponding to a first location at the top surface 1202 and bottom fringes 1214 corresponding to a second location at the bottom surface 1204, but are illustrated along the same line. The depth of the structure 1200 may be determined based on the distance between the top fringes 1212 corresponding to the top surface 1202 and bottom fringes 1214. If a single reference beam path is used, it may be desirable to adjust the range, i.e., distance 1222, scanned by the reference mirror 1220, e.g., to correspond to the depth of the structure 1200.

[0086] By using multiple reference paths, however, separate scans over the levels of interest may be used and it is unnecessary to scan over the entire depth of the structure 1200. For example, as illustrated with a first reference mirror 1240 scanning a distance 1242, which is only a portion of the full distance 1222, and a second reference mirror 1244 separately scanning a distance 1246, which is only a portion of the full distance 1222, the correlogram 1230 may be generated with top fringes 1232 corresponding to the first position at the top surface 1202 (produced by the scan of first reference mirror 1240) andbottom fringes 1234 corresponding to the second position at the bottom surface 1204 (produced by the scan of second reference mirror 1244). It should be understood that because the interferometer images an entire region the scans of the first reference mirror 1240 and the second reference mirror 1244 may be performed without relative lateral movement between the optics and the sample, and the scans may be performed simultaneously (or sequentially) as discussed in FIGs. 13A-13C. The depth of the structure 1200 may be determined based on a distance between the top fringes 1232 and bottom fringes 1234, which may be calibrated based on a known difference between the lengths of the reference beam paths. The multiple reference beam paths may be scanned simultaneously or sequentially. Because each reference beam path is scanned over a much shorter distance than if a single reference beam path is used, the acquisition time may be greatly reduced. Additionally (or alternatively), the periodicity of the spectra of the light may be modulated, e.g., as discussed in FIGs. 5A-5C and FIG. 7B, to further reduce (or eliminate) the distance that reference mirrors 1240 and 1244 are physically scanned.

[0087] FIGs. 13A, 13B, and 13C illustrate different implementations of producing multiple reference beam paths, which may be used with the interferometers 700, 800, 100, discussed herein using Michelson or Linnik type interference objectives, as illustrated in FIGs. 9, 1 IB, and 11C, discussed herein.

[0088] As illustrated in FIG. 13 A, the interference objective 1300 includes multiple reference beam paths with multiple reference mirrors. The interference objective 1300 includes an objective lens (not shown), a first beam splitter 1304, a second beam splitter 1306, and two reference mirrors 1308 and 1310 that move separately to alter the optical path difference in each reference beam path, as indicated by the arrows. The illumination light 1303, which is illustrated as collimated, is provided by a light source (not shown) and is received by the interference objective 1300. The first beam splitter 1304 splits the illumination light 1303 into reference light 1305 and sample light 1307, which is incident on and reflects from the sample 1301. The second beam splitter 1306 directs a first portion of the reference light 1305 to the first reference mirror 1308 and a second portion of the reference light 1305 to the second reference mirror 1310. The length of the reference beam paths associated with the first reference mirror 1308 and the second reference mirror 1310 may correspond to be near the nominal locations of the different levels in the structure, e.g.,top surface 1202 and bottom surface 1204 shown in FIG. 12. The reference mirrors 1308 and 1310 may independently move to alter the optical path difference for each reference beam path. In some implementations, the second beam splitter 1306 may be a dichroic beam splitter that directs different spectral ranges of the reference light 1305 to the first reference mirror 1308 and the second reference mirror 1310. The detector, e.g., detector 720, 840, or 1040 in FIGs. 7A, 7B, 8, or 10, may include another dichroic beam splitter and separate sensors to simultaneously detect the fringes produced in response to each reference beam path. Alternatively, the first reference mirror 1308 and the second reference mirror 1310 may scan sequentially, so that the fringes produced by one reference beam path may be collected before the fringes produced by the other reference beam path are collected. In some implementations, a flip mirror may be used to prevent reference light from the second reference mirror 1310 from being collected while the first reference mirror 1308 is scanned and to prevent reference light from the first reference mirror 1308 from being collected while the second reference mirror 1310 is scanned.

[0089] As illustrated in FIG. 13B, the interference objective 1320 includes multiple reference beam paths with multiple reference surfaces. The interference objective 1320 includes an objective lens (not shown), a first beam splitter 1324, a first reference mirror 1326 that is movable and functions as a beam splitter that directs a portion of the incident light towards the first beam splitter 1324 and directs another portion of the light towards a second reference mirrors 1330 that is movable separately from the first reference mirror1326. The illumination light 1323, which is illustrated as collimated, is provided by a light source (not shown) and is received by the interference objective 1320. The first beam splitter 1324 splits the illumination light 1323 into reference light 1325 and sample light1327, which is incident on and reflects from the sample 1321. The first reference mirror 1326, which may be a partially transparent pellicle, reflects a first portion of the reference light 1305 back to the first beam splitter 1324, and transmits a second portion of the reference light 1305 to the second reference mirror 1330, which is reflected and returned via the first reference mirror 1326. The first reference mirror 1326 is movable, as indicated by the associated arrow, and the second reference mirror 1330 is movable as indicated by the associated arrow. The length of the reference beam paths associated with the first reference mirror 1326 and the second reference mirror 1330 may correspond to the nominal locations of the different levels in the structure, e.g., top surface 1202 and bottom surface 1204 shownin FIG. 12. The reference mirrors 1326 and 1330 may independently move to alter the optical path difference for each reference beam path. In some implementations, the second reference mirror 1 30 may move with the first reference mirror 1326 while the first reference mirror 1326 is scanned, and then may move separately to scan the second reference mirror 1330. In some implementations, the first reference mirror 1326 may be a dichroic beam splitter that reflects one spectral range of the reference light 1325 back to the first beam splitter 1324 and transmits a different spectral range of the reference light 1325 to the second reference mirror 1330. As discussed in FIG. 13 A, the detector, e.g., detector 720, 840, or 1040 in FIGs. 7A, 7B, 8, or 10, may include a dichroic beam splitter and separate sensors to simultaneously detect the fringes produced in response to each reference beam path. Alternatively, the first reference mirror 1326 and the second reference mirror 1330 may scan sequentially, so that the fringes produced by one reference beam path may be collected before the fringes produced by the other reference beam path are collected.

[0090] As illustrated in FIG. 13C, the interference objective 1350 includes multiple reference beam paths with multiple reference surfaces produced by a single optical element. The interference objective 1350 includes an objective lens (not shown), and a glass plate 1356 with a partially reflective front surface that operates as a first reference mirror 1358, and a reflective back surface that operates as the second reference mirror 1360. The illumination light 1353, which is illustrated as collimated, is provided by a light source (not shown) and is received by the interference objective 1350. The first beam splitter 1354 splits the illumination light 1353 into reference light 1355 and sample light 1357, which is incident on and reflects from the sample 1351. The front surface of the glass plate 1356 operates as the first reference mirror 1358 and reflects a first portion of the reference light 1305 back to the first beam splitter 1354, and transmits a second portion of the reference light 1305 to the back surface of the glass plate 1356, which operates as the second reference mirror 1360 and reflects the reference light 1305. The glass plate 1356 is movable as indicated by the associated arrow, to move the first reference mirror 1358 and second reference mirror 1360. The length of the reference beam paths associated with the first reference mirror 1358 and the second reference mirror 1360, i.e., the distance between the front surface and back surface of the glass plate 1356, may correspond to the nominal locations of the different levels in the structure, e.g., top surface 1202 and bottom surface 1204 shown in FIG. 12. In some implementations, the glass plate 1356 may be a dichroicbeam splitter that reflects one spectral range of the reference light 1355 back to the first beam splitter 1354 and transmits a different spectral range of the reference light 1355 to the second reference mirror 1360. As discussed in FIG. 13A, the detector, e.g., detector 720, 840, or 1040 in FIGs. 7A, 7B, 8, or 10, may include a dichroic beam splitter and separate sensors to simultaneously detect the fringes produced in response to each reference beam path. Alternatively, the first reference mirror 1358 and the second reference mirror 1360 may scan sequentially, so that the fringes produced by one reference beam path may be collected before the fringes produced by the other reference beam path are collected.

[0091] FIGs. 14A, 14B, and 14C illustrate examples of correlograms for differently sized vias as seen by a single pixel scan. In the correlograms illustrated in FIGs. 14A-14C, the top fringes are ghost fringes, and may be used, along with the bottom fringes, to produce an approximate via depth, but an accurate depth measurement would be made using fringes produced by the top surface of the sample, i.e., outside the via, and the bottom fringes illustrated in FIGs. 14A, 14B, and 14C, and the ghost fringes, i.e., top fringes illustrated in FIGs. 14A, 14B, and 14C, would be ignored.

[0092] FIG. 14A illustrates correlograms for 5pm wide by 50 pm deep vias. Correlogram 1402, for example, is from a sample scan obtained by an interferometer with a conventional scanning interference objective, i.e., scanning the focal position of the objective lens, while correlogram 1404 is from a sample scan obtained by an interferometer with a scanning reference mirror as discussed herein. As can be seen, both correlograms 1402 and 1404 have visible fringes 1402t and 1404t, respectively, that correspond to the ghost fringes produced at the top surface of the sample. Correlogram 1402, produced with a conventional scanning interference objective, however, has significant background intensity variation illustrated by the skew to the right and has low contrast of the bottom fringes 1402b, corresponding to the bottom of the via, which may be the results of diffraction and multiple reflections within the structure. The background intensity is the sum of intensities of reference, object and any other beam at the detected position. Correlogram 1404, produced with a scanning reference mirror, on the other hand, has a constant background intensity value and has good contrast of the bottom fringes 1404b, corresponding to the bottom of the via.

[0093] FIG. 14B illustrates correlograms for 3|im wide by 30 |im deep vias.Correlogram 1412, for example, is from a sample scan obtained by an interferometer with a conventional scanning interference objective, i.e., scanning the focal position of the objective lens, while correlogram 1414 is from a sample scan obtained by an interferometer with a scanning reference mirror as discussed herein. As can be seen, both correlograms 1412 and 1414 have visible fringes 1412t and 1414t, respectively, that correspond to the ghost fringes produced at the top surface of the sample. Correlogram 1412, produced with a conventional scanning interference objective, however, has significant background intensity variation illustrated by the skew to the right and the bottom fringes corresponding to the bottom of the via are not visible, which may be the results of diffraction and multiple reflections within the structure. Correlogram 1414, produced with a scanning reference mirror, on the other hand, has a constant background intensity value and has good contrast of the bottom fringes 1414b, corresponding to the bottom of the via.

[0094] FIG. 14C illustrates a correlogram for 3pm wide by 50 pm deep vias. Correlogram 1424, for example, is from a sample scan obtained by an interferometer with a scanning reference mirror as discussed herein. A corresponding correlogram obtained by an interferometer with a conventional scanning interference objective is not shown as no useful fringes are produced that correspond to the bottom of the vias. As can be seen, however, correlogram 1424, produced with a scanning reference mirror, shows visible fringes 1424t that correspond to the ghost fringes produced at the top surface of the sample, has a constant background intensity value, and has good contrast of the bottom fringes 1424b, corresponding to the bottom of the via.

[0095] FIG. 15 illustrates, by way of example, a collection 1500 of multiple images (illustrated as images 1500A, 1500B, and 1500C), produced by an interferometer as discussed herein, such as interferometers 700. Each image in collection 1500 is a frame of data including a full field of view of a sample with a plurality of structures 1502, e.g., vias, as collected during the scan. The image 1500 A, for example, is produced when the reference mirror is at a position at which interference fringes are not visible at this given moment / frame of scan. The full field of view, for example, may be 0.45mmx0.45mm, and includes a region of the sample that includes a plurality of structures 1502, e.g., vias. Multiple images of the region of the sample are produced in the collection 1500, each with adifferent optical path difference produced by scanning the reference mirror, beam splitter, or altering the spectrum provided by the light source, or any combination thereof. It should be understood that while FIG. 15 illustrates three images in collection 1500, the collection of images from a scan may include a significantly greater number of images.

[0096] FIG. 16 is a flow chart illustrating a method of characterizing with an interferometer a region of a sample including a plurality of structures, which may be performed by any of interferometers 700, 800, and 1000 shown in FIGs. 7 A, 7B, 8, and 10, respectively, e.g., using any of the interference objectives discussed in relation to FIGs. 9 and 11 A-l 1C and / or modulation of the periodic spectrum discussed in relation to FIGs. 5A- 5E. In some implementations, the plurality of structures may be at least one of trenches, holes, and high aspect ratio (HAR) structures. The HAR structures, for example, may have a width of 5pm or less and a depth of 30pm or greater.

[0097] As illustrated by block 1602, the method includes splitting illumination light into sample light to be incident on and reflected by the sample and reference light to be incident on and reflected by a reference mirror, e.g., as illustrated by beam splitter 714 in FIGs. 7A and 7B. A means for splitting the illumination light into sample light to be incident on and reflected by the sample and reference light to be incident on and reflected by a reference mirror, by way of example, may include a beam splitter positioned below the objective lens (i.e., the beam splitter is positioned between the objective lens and the sample), such as beam splitters 812, 904, 1024, 1104, and 1124 in FIGs. 8, 9, 10, 11A, and 1 IB, or a beam splitter positioned above the objective lens (i.e., the objective lens is positioned between the beam splitter and the sample), such as beam splitter 1154 in FIG. 11C.

[0098] As illustrated by block 1604, the method includes illuminating, with illumination optics having a first numerical aperture, the region of the sample with the sample light, e.g., as illustrated by sample light 724 in FIGs. 7A and 7B. A means for illuminating, with illumination optics having a first numerical aperture, the region of the sample with the sample light may include, e.g., optical elements positioned to provide illumination light below a collection lens, such as illustrated by lens(es) / beam splitters / sample light 81 1 / 812 / 825 and 908 / 904 / 907 shown in FIGs. 8 and 9, respectively, or may include, e.g., optical elements positioned to provide illumination light before thecollection lens, such as illustrated by aperture / objective lens / sample light 1012 / 1022 / 1023 shown in FIG. 10 and objective lens / sample light 1102 / 1107, 1152 / 1157 shown in FIGs. 11A, 11B, and 11C.

[0099] At block 1606, the method includes focusing collection optics at a focal position with respect to the sample, the collection optics having a second numerical aperture that is greater than the first numerical aperture, e.g., as illustrated by collected light 726 in FIGs. 7A and 7B. In some implementations, the collection optics may be one of a Michelson objective lens, a Mirau objective lens, or a Linnik objective lens. A means for focusing collection optics at a focal position with respect to the sample, the collection optics having a second numerical aperture that is greater than the first numerical aperture may include, e.g., an objective lens / collected light 822 / 829, 902 / 909, 1022 / 1029, 1102 / 1109, 1122 / 1129, and 1152 / 1159 shown in FIGs. 8, 9, 10, 11A, 11B, and 11C, respectively.

[0100] At block 1608, the method includes scanning one or more coherence envelopes produced by reflected sample light from the sample interfering with reflected reference light from the reference mirror through a depth of the sample while not changing the focal position of the collection optics with respect to the sample, e.g., as illustrated by arrow 721 in FIGs. 7A and 7B. A means for scanning one or more coherence envelopes produced by reflected sample light from the sample interfering with reflected reference light from the reference mirror through a depth of the sample while not changing the focal position of the collection optics with respect to the sample may include, e.g., a means for altering an optical path difference between the sample light and the reference light, such as one or more moving reference mirrors 716, 826, 906, 1026, 1106, 1126, and 1156 shown in FIGs. 8, 9, 10, 11A, 1 IB, and 11C, and / or moving beam splitters 1024 and 1104 shown in FIGs. 10 and 11 A, and / or the light source 810 or 1010 in FIGs. 8 and 10 altering the spectra of the illumination light, or a means for modulating a periodicity of the spectrum of the illumination light or interfering light produced by the reflected sample light from the sample interfering with the reflected reference light, such as modulation interferometer 500 or an unbalanced optic fiber interferometer, as discussed in reference to FIGs. 5A-5E.

[0101] As illustrated by block 1610, the method includes imaging, with the collection optics, the reflected sample light from the sample interfering with the reflected reference light from the reference mirror at a plurality of depths of the one or more coherenceenvelopes to produce multiple images, e.g., as illustrated by interference light 722 and detector 720 shown in FIGs. 7 A and 7B and in FIG. 15. A means for imaging, with the collection optics, the reflected sample light from the sample interfering with the reflected reference light from the reference mirror at a plurality of depths of the one or more coherence envelopes to produce multiple images, may include, e.g., objective lens 822, lens 842, and detector 840 in FIG. 8 or objective lens 1022, lens 1042, and detector 1040 in FIG. 10.

[0102] As illustrated by block 1612, the method includes analyzing the multiple images of the region of the sample including the plurality of structures to characterize the region of the sample, e.g., as discussed in reference to processing system 750 shown in FIGs. 7A and 7B. A means for analyzing the multiple images of the region of the sample including the plurality of structures to characterize the region of the sample may include, e.g., at least one processor 752, 850, and 1050 in FIGs. 7A, 7B, 8, and 10, that may be configured with computer-readable program code as discussed herein.

[0103] In some implementations, the method may scan the one or more coherence envelopes produced by the reflected sample light from the sample interfering with the reflected reference light from the reference mirror through the depth of the sample by altering an optical path difference between the sample light and the reference light, e.g., using one or more moving reference mirrors 716, 826, 906, 1026, 1106, 1126, and 1156 shown in FIGs. 8, 9, 10, 11 A, 1 IB, and 11C, and / or moving beam splitters 1024 and 1104 shown in FIGs. 10 and 11 A, and / or the light source 810 or 1010 in FIGs. 8 and 10 altering the spectra of the illumination light.

[0104] In some implementations, the method may further include providing the illumination light into an optical path of the interferometer between the collection optics and the sample, e.g., as discussed in reference to FIGs. 8, and 9. A means for providing the illumination light into an optical path of the interferometer between the collection optics and the sample, for example, may include, e.g., the light source 810, lens 811 and beam splitter 812 shown in FIG. 8, or the lens / beam splitter 908 / 904 shown in FIG. 9. In one example implementation, the optical path difference between the sample light and the reference light may be altered while not changing the focal position of the collection optics with respect to the sample by moving the illumination optics and the reference mirror, e.g.,as discussed in reference to assembly 830 in FIG. 8. A means for moving the illumination optics and the reference mirror may include, e.g., the actuator 831 shown in FIG. 8. In one example implementation, the illumination optics may include collimating optics and a beam splitter that splits the illumination light into the sample light and the reference light, e.g., as illustrated by lens / beam splitters 811 / 812 and 908 / 904 shown in FIGs. 8 and 9, respectively.

[0105] In some implementations, the method may alter the optical path difference between the sample light and the reference light while not changing the focal position of the collection optics with respect to the sample by at least one of moving the reference mirror, moving a beam splitter that splits the illumination light into the sample light and the reference light, and modulating a spectrum of the illumination light, e.g., as discussed in reference to FIGs. 7A, 7B, 8, and 10.

[0106] hi some implementations, the method may further include splitting the reference light to produce second reference light to be incident on and reflected by a second reference mirror; altering a second optical path difference between the sample light and the second reference light while not changing the focal position of the collection optics with respect to the sample, wherein the second optical path difference is different than the optical path difference; imaging, with the collection optics, interfering reflected sample light from the sample and reflected second reference light from the second reference mirror at a plurality of second optical path differences to produce a second set of multiple images; and analyzing the second set of multiple images of the region of the sample including the plurality of structures to characterize the region of the sample, e.g., as discussed in reference to FIG. 12 and FIGs. 13A-13C. For example, a means for splitting the reference light to produce second reference light to be incident on and reflected by a second reference mirror may include, e.g., the second beam splitter 1306, first reference mirror 1326, and glass plate 1356 shown in FIGs. 13A, 13B, and 13C, respectively. A means for altering a second optical path difference between the sample light and the second reference light while not changing the focal position of the collection optics with respect to the sample, wherein the second optical path difference is different than the optical path difference may include, e.g., the second reference mirror 1310, 1330, 1360 shown in FIGs. 13A, 13B, and 13C, respectively. A means for imaging, with the collection optics, interfering reflected sample light from the sample and reflected second reference light from the second reference mirrorat a plurality of second optical path differences to produce a second set of multiple images may include, e.g., objective lens 822, lens 842, and detector 840 in FIG. 8 or objective lens 1022, lens 1042, and detector 1040 in FIG. 10. A means for analyzing the second set of multiple images of the region of the sample including the plurality of structures to characterize the region of the sample may include, e.g., at least one processor 752, 850, and 1050 in FIGs. 7A, 7B, 8, and 10, that may be configured with computer-readable program code as discussed herein.

[0107] By way of example, the interfering reflected sample light from the sample and reflected reference light from the reference mirror at optical path differences in the multiple images produces top interference fringes that correspond to a top surface of the sample, e.g., as illustrated by top fringes 1232 in correlogram 1230 in FIG. 12, and the interfering reflected sample light from the sample and reflected second reference light from the second reference mirror at second optical path difference in the second set of multiple images produces bottom interference fringes that correspond to bottoms of the plurality of structures, e.g., as illustrated by bottom fringes 1234 in correlogram 1230 in FIG. 12.

[0108] By way of example, the illumination light may have a spectral bandwidth and splitting the reference light directs a first spectral range to be incident on the reference mirror and directs a second spectral range to be incident on the second reference mirror, e.g., as discussed in reference to FIG. 12 and FIGs. 13A-13C. Additionally, imaging the interfering sample light and the reference light may use a first sensor and imaging the interfering sample light and the second reference light may use a second sensor, e.g., as discussed in reference to FIG. 12 and FIGs. 13A-13C.

[0109] In some implementations, the method may further include reducing a numerical aperture of a light source that produces the illumination light and providing the illumination light into an optical path of the interferometer with a beam splitter that directs the illumination light to an objective lens; wherein the illumination optics having the first numerical aperture comprise the numerical aperture of the light source and the objective lens, wherein the collection optics having the second numerical aperture that is greater than the first numerical aperture comprise the objective lens, e.g., as discussed in reference to FIGs. 10, 11 A, 1 IB, and 1 1C. A means for reducing a numerical aperture of a light source that produces the illumination light may include, e.g., aperture / objective lens 1012 / 1022shown in FIG. 10, or the aperture may be adjusted at the lens, or further may be adjusted in an intermediate image of the source, or anywhere along the beam. A means for providing the illumination light into an optical path of the interferometer with a beam splitter that directs the illumination light to an objective lens may include, e.g., beam splitter 1014, 1154 shown in FIGs. 10 and 11C.

[0110] In some implementations, the illumination light may have at least one of spectral bandwidth and angular bandwidth to produce low coherence fringes in the multiple images, e.g., as discussed in reference to light source 712 in FIGs. 7A and 7B.

[0111] In some implementations, an illumination source that produces the illumination light is configurable to alter one or more of the following in the illumination light: at least one of spectral bandwidth and angular bandwidth; at least one of cylindrical optics and spatially linear source; and polarization control, e.g., as discussed in reference to light source 712 in FIGs. 7A and 7B.

[0112] In some implementations, the one or more coherence envelopes may be scanned over a range that is at least as great as a depth of the plurality of structures, e.g., as discussed in reference to FIG. 12. For example, in some implementations, the range may be adjustable, e.g., as discussed in reference to FIG. 12. In some implementations, the method may further include detecting, in the multiple images, top interference fringes that correspond to a top surface of the sample, detecting, in the multiple images, bottom interference fringes that correspond to bottoms of the plurality of structures, and determining depths of the plurality of structures based on a distance between the top interference fringes and the bottom interference fringes, e.g., as discussed in reference to FIG. 12 and 15. A means for detecting, in the multiple images, top interference fringes that correspond to a top surface of the sample, means for detecting, in the multiple images, bottom interference fringes that correspond to bottoms of the plurality of structures, and means for determining depths of the plurality of structures based on a distance between the top interference fringes and the bottom interference fringes may include, e.g., at least one processor 752, 850, and 1050 in FIGs. 7A, 7B, 8, and 10, that may be configured with computer-readable program code as discussed herein.

[0113] hi some implementations, the method may scan the one or more coherence envelopes produced by the reflected sample light from the sample interfering with the reflected reference light from the reference mirror through the depth of the sample by modulating a periodicity of the spectrum of the illumination light or interfering light produced by the reflected sample light from the sample interfering with the reflected reference light, e.g., using as modulation interferometer 500 or an unbalanced optic fiber interferometer, as discussed in reference to FIGs. 5A-5E.

[0114] FIG. 17 is a flow chart illustrating a method of characterizing with an interferometer a region of a sample including a plurality of structures, which may be performed by any of interferometers 700, 800, and 1000 shown in FIGs. 7A, 8, and 10, respectively, e.g., using any of the interference objectives discussed in relation to FIGs. 9 and 11 A-l 1C. In some implementations, the plurality of structures may be at least one of trenches, holes, and high aspect ratio (HAR) structures. The HAR structures, for example, may have a width of 5pm or less and a depth of 30pm or greater.

[0115] As illustrated by block 1702, the method includes splitting illumination light into sample light to be incident on and reflected by the sample and reference light to be incident on and reflected by a reference mirror, e.g., as illustrated by beam splitter 714 in FIG. 7A. A means for splitting the illumination light into sample light to be incident on and reflected by the sample and reference light to be incident on and reflected by a reference mirror, by way of example, may include a beam splitter positioned below the objective lens (i.e., the beam splitter is positioned between the objective lens and the sample), such as beam splitters 812, 904, 1024, 1104, and 1124 in FIGs. 8, 9, 10, 11 A, and 1 IB, or a beam splitter positioned above the objective lens (i.e., the objective lens is positioned between the beam splitter and the sample), such as beam splitter 1154 in FIG. 11C.

[0116] As illustrated by block 1704, the method includes illuminating, with illumination optics having a first numerical aperture, the region of the sample with the sample light, e.g., as illustrated by sample light 724 in FIG. 7A. A means for illuminating, with illumination optics having a first numerical aperture, the region of the sample with the sample light may include, e.g., optical elements positioned to provide illumination light below a collection lens, such as illustrated by lens(es) / beam splitters / sample light811 / 812 / 825 and 908 / 904 / 907 shown in FIGs. 8 and 9, respectively, or may include, e.g.,optical elements positioned to provide illumination light before the collection lens, such as illustrated by aperture / objective lens / sample light 1012 / 1022 / 1023 shown in FIG. 10 and objective lens / sample light 1102 / 1107, 1152 / 1157 shown in FIGs. 11A, 11B, and 11C.

[0117] At block 1706, the method includes focusing collection optics at a focal position with respect to the sample, the collection optics having a second numerical aperture that is greater than the first numerical aperture, e.g., as illustrated by collected light 726 in FIG. 7 A. In some implementations, the collection optics may be one of a Michelson objective lens, a Mirau objective lens, or a Linnik objective lens. A means for focusing collection optics at a focal position with respect to the sample, the collection optics having a second numerical aperture that is greater than the first numerical aperture may include, e.g., an objective lens / collected light 822 / 829, 902 / 909, 1022 / 1029, 1102 / 1109, 1122 / 1129, and 1152 / 1159 shown in FIGs. 8, 9, 10, 11A, 11B, and 11C, respectively.

[0118] At block 1708, the method includes altering an optical path difference between the sample light and the reference light while not changing the focal position of the collection optics with respect to the sample, e.g., as illustrated by arrow 721 in FIG. 7A. A means for altering an optical path difference between the sample light and the reference light while not changing the focal position of the collection optics with respect to the sample may include, e.g., a moving reference mirrors 716, 826, 906, 1026, 1 106, 1126, and 1156 shown in FIGs. 8, 9, 10, 11 A, 11B, and 11C, and / or moving beam splitters 1024 and 1104 shown in FIGs. 10 and 11A, and / or the light source 810 or 1010 in FIGs. 8 and 10 altering the spectra of the illumination light.

[0119] As illustrated by block 1710, the method includes imaging, with the collection optics, the reflected sample light from the sample interfering with the reflected reference light from the reference mirror at a plurality of optical path differences to produce multiple images, e.g., as illustrated by interference light 722 and detector 720 shown in FIG. 7A and in FIG. 15. A means for imaging, with the collection optics, the reflected sample light from the sample interfering with the reflected reference light from the reference mirror at a plurality of optical path differences to produce multiple images, may include, e.g., objective lens 822, lens 842, and detector 840 in FIG. 8 or objective lens 1022, lens 1042, and detector 1040 in FIG. 10.

[0120] As illustrated by block 1712, the method includes analyzing the multiple images of the region of the sample including the plurality of structures to characterize the region of the sample, e.g., as discussed in reference to processing system 750 shown in FIG. 7A. A means for analyzing the multiple images of the region of the sample including the plurality of structures to characterize the region of the sample may include, e.g., at least one processor 752, 850, and 1050 in FIGs. 7A, 8, and 10, that may be configured with computer-readable program code as discussed herein.

[0121] FIG. 18 is a flow chart illustrating a method of characterizing with an interferometer a region of a sample including a plurality of structures, which may be performed by interferometer 700’, shown in FIG. 7B, e.g., using the modulation of the periodic spectrum discussed in relation to FIGs. 5A-5E. In some implementations, the plurality of structures may be at least one of trenches, holes, and high aspect ratio (HAR) structures. The HAR structures, for example, may have a width of 5pm or less and a depth of 30 m or greater.

[0122] As illustrated by block 1802, the method includes splitting illumination light into sample light to be incident on and reflected by the sample and reference light to be incident on and reflected by a reference mirror, e.g., as illustrated by beam splitter 714 in FIG. 7B. A means for splitting the illumination light into sample light to be incident on and reflected by the sample and reference light to be incident on and reflected by a reference mirror, by way of example, may include a beam splitter positioned below the objective lens (i.e., the beam splitter is positioned between the objective lens and the sample), such as beam splitters 812, 904, 1024, 1104, and 1124 in FIGs. 8, 9, 10, 11 A, and 1 IB, or a beam splitter positioned above the objective lens (i.e., the objective lens is positioned between the beam splitter and the sample), such as beam splitter 1154 in FIG. 11C.

[0123] As illustrated by block 1804, the method includes illuminating, with illumination optics having a first numerical aperture, the region of the sample with the sample light, e.g., as illustrated by sample light 724 in FIG. 7B. A means for illuminating, with illumination optics having a first numerical aperture, the region of the sample with the sample light may include, e.g., optical elements positioned to provide illumination light below a collection lens, such as illustrated by lens(es) / beam splitters / sample light811 / 812 / 825 and 908 / 904 / 907 shown in FIGs. 8 and 9, respectively, or may include, e.g.,optical elements positioned to provide illumination light before the collection lens, such as illustrated by aperture / objective lens / sample light 1012 / 1022 / 1023 shown in FIG. 10 and objective lens / sample light 1102 / 1107, 1152 / 1157 shown in FIGs. 11A, 11B, and 11C.

[0124] At block 1806, the method includes focusing collection optics at a focal position with respect to the sample, the collection optics having a second numerical aperture that is greater than the first numerical aperture, e.g., as illustrated by collected light 726 in FIG. 7B. In some implementations, the collection optics may be one of a Michelson objective lens, a Mirau objective lens, or a Linnik objective lens. A means for focusing collection optics at a focal position with respect to the sample, the collection optics having a second numerical aperture that is greater than the first numerical aperture may include, e.g., an objective lens / collected light 822 / 829, 902 / 909, 1022 / 1029, 1102 / 1109, 1122 / 1129, and 1152 / 1159 shown in FIGs. 8, 9, 10, 11A, 11B, and 11C, respectively.

[0125] At block 1808, the method includes modulating a periodicity of the spectrum of the illumination light or interfering light produced by reflected sample light from the sample interfering with reflected reference light from the reference mirror while not changing the focal position of the collection optics with respect to the sample, e.g., as illustrated by arrow 721 in FIG. 7B. A means for modulating a periodicity of the spectrum of the illumination light or interfering light produced by reflected sample light from the sample interfering with reflected reference light from the reference mirror while not changing the focal position of the collection optics with respect to the sample may include, e.g., a modulation interferometer 500 or an unbalanced optic fiber interferometer, as discussed in reference to FIGs. 5A-5E.

[0126] As illustrated by block 1810, the method includes imaging, with the collection optics, the reflected sample light from the sample interfering with the reflected reference light from the reference mirror at a plurality of changes in modulation of the periodicity of the spectrum of light to produce multiple images, e.g., as illustrated by interference light 722 and detector 720 shown in FIG. 7B and in FIG. 15. A means for imaging, with the collection optics, the reflected sample light from the sample interfering with the reflected reference light from the reference mirror at a plurality of changes in modulation of the periodicity of the spectrum of light to produce multiple images, may include, e.g., objectivelens 822, lens 842, and detector 840 in FIG. 8 or objective lens 1022, lens 1042, and detector 1040 in FIG. 10.

[0127] As illustrated by block 1812, the method includes analyzing the multiple images of the region of the sample including the plurality of structures to characterize the region of the sample, e.g., as discussed in reference to processing system 750 shown in FIG. 7B. A means for analyzing the multiple images of the region of the sample including the plurality of structures to characterize the region of the sample may include, e.g., at least one processor 752 in FIG. 7B, that may be configured with computer-readable program code as discussed herein.

[0128] Those skilled in the art will understand that the preceding implementations of the present disclosure provide the foundation for numerous alternatives and modifications that are also deemed within the scope of the present disclosure. For example, the present disclosure has been described throughout without polarization of the light, but orthogonally polarized light in interfering beams may be used and images analyzed using polarized pixelated camera, if desired. The above description is intended to be illustrative, and not restrictive. For example, the above-described examples (or one or more aspects thereof) may be used in combination with each other. Other implementations can be used, such as by one of ordinary skill in the art upon reviewing the above description. Also, various features may be grouped together and less than all features of a particular disclosed implementation may be used. Thus, the following aspects are hereby incorporated into the above description as examples or implementations, with each aspect standing on its own as a separate implementation, and it is contemplated that such implementations can be combined with each other in various combinations or permutations. Therefore, the spirit and scope of the appended claims should not be limited to the foregoing description.

Claims

CLAIMSWhat is claimed is:

1. A method of characterizing with an interferometer a region of a sample comprising a plurality of structures, the method comprising: splitting illumination light into sample light to be incident on and reflected by the sample and reference light to be incident on and reflected by a reference mirror; illuminating, with illumination optics having a first numerical aperture, the region of the sample with the sample light; focusing collection optics at a focal position with respect to the sample, the collection optics having a second numerical aperture that is greater than the first numerical aperture; altering an optical path difference between the sample light and the reference light while not changing the focal position of the collection optics with respect to the sample; imaging, with the collection optics, reflected sample light from the sample interfering with reflected reference light from the reference mirror at a plurality of optical path differences to produce multiple images; and analyzing the multiple images of the region of the sample comprising the plurality of structures to characterize the region of the sample.

2. The method of claim 1, further comprising providing the illumination light into an optical path of the interferometer between the collection optics and the sample.

3. The method of claim 1, wherein altering the optical path difference between the sample light and the reference light while not changing the focal position of the collection optics with respect to the sample comprises at least one of moving the reference mirror, and moving a beam splitter that splits the illumination light into the sample light and the reference light.

4. The method of claim 1, further comprising: splitting the reference light to produce second reference light to be incident on and reflected by a second reference mirror; altering a second optical path difference between the sample light and the second reference light while not changing the focal position of the collection optics with respect to the sample; imaging, with the collection optics, interfering reflected sample light from the sample and reflected second reference light from the second reference mirror at a plurality of second optical path differences to produce a second set of multiple images; and analyzing the second set of multiple images of the region of the sample comprising the plurality of structures to characterize the region of the sample.

5. The method of claim 4, wherein the second optical path difference is different than the optical path difference, and wherein the interfering reflected sample light from the sample and reflected reference light from the reference mirror at optical path differences in the multiple images produces top interference fringes that correspond to a top surface of the sample, and the interfering reflected sample light from the sample and reflected second reference light from the second reference mirror at second optical path difference in the second set of multiple images produces bottom interference fringes that correspond to bottoms of the plurality of structures.

6. The method of claim 1, further comprising: reducing a numerical aperture of a light source that produces the illumination light; providing the illumination light into an optical path of the interferometer with a beam splitter that directs the illumination light to an objective lens; andwherein the illumination optics having the first numerical aperture comprise the numerical aperture of the light source and the objective lens; wherein the collection optics having the second numerical aperture that is greater than the first numerical aperture comprise the objective lens.

7. The method of claim 1, wherein the plurality of structures comprises at least one of trenches, holes, and high aspect ratio (HAR) structures, wherein the HAR structures have a width of 5pm or less and a depth of 30 m or greater.

8. The method of claim 1, further comprising: detecting, in the multiple images, top interference fringes that correspond to a top surface of the sample; detecting, in the multiple images, bottom interference fringes that correspond to bottoms of the plurality of structures; and determining depths of the plurality of structures based on a distance between the top interference fringes and the bottom interference fringes.

9. An interferometer configured for characterizing a region of a sample comprising a plurality of structures, the interferometer comprising: means for splitting illumination light into sample light to be incident on and reflected by the sample and reference light to be incident on and reflected by a reference mirror; means for illuminating, with illumination optics having a first numerical aperture, the region of the sample with the sample light;means for focusing collection optics at a focal position with respect to the sample, the collection optics having a second numerical aperture that is greater than the first numerical aperture; means for altering an optical path difference between the sample light and the reference light while not changing the focal position of the collection optics with respect to the sample; means for imaging, with the collection optics, reflected sample light from the sample interfering with reflected reference light from the reference mirror at a plurality of optical path differences to produce multiple images; and means for analyzing the multiple images of the region of the sample comprising the plurality of structures to characterize the region of the sample.

10. The interferometer of claim 9, wherein the means for splitting the illumination light into sample light and reference light is positioned between the collection optics and the sample.

11. The interferometer of claim 9, wherein the means for altering the optical path difference between the sample light and the reference light while not changing the focal position of the collection optics with respect to the sample performs at least one of moving the reference mirror, and moving a beam splitter that splits the illumination light into the sample light and the reference light.

12. The interferometer of claim 9, further comprising: means for splitting the reference light to produce second reference light to be incident on and reflected by a second reference mirror;means for altering a second optical path difference between the sample light and the second reference light while not changing the focal position of the collection optics with respect to the sample; means for imaging, with the collection optics, interfering reflected sample light from the sample and reflected second reference light from the second reference mirror at a plurality of second optical path differences to produce a second set of multiple images; and means for analyzing the second set of multiple images of the region of the sample comprising the plurality of structures to characterize the region of the sample.

13. The interferometer of claim 12, wherein the second optical path difference is different than the optical path difference, and wherein the interfering reflected sample light from the sample and reflected reference light from the reference mirror at optical path differences in the multiple images produces top interference fringes that correspond to a top surface of the sample, and the interfering reflected sample light from the sample and reflected second reference light from the second reference mirror at second optical path difference in the second set of multiple images produces bottom interference fringes that correspond to bottoms of the plurality of structures.

14. The interferometer of claim 9, further comprising: means for reducing a numerical aperture of a light source that produces the illumination light; means for providing the illumination light into an optical path of the interferometer that directs the illumination light to an objective lens; and wherein the illumination optics having the first numerical aperture comprise the numerical aperture of the light source and the objective lens;wherein the collection optics having the second numerical aperture that is greater than the first numerical aperture comprise the objective lens.

15. The interferometer of claim 9, wherein the plurality of structures comprises at least one of trenches, holes, and high aspect ratio (HAR) structures, wherein the HAR structures have a width of 5pm or less and a depth of 30pm or greater.

16. The interferometer of claim 9, further comprising: means for detecting, in the multiple images, top interference fringes that correspond to a top surface of the sample; means for detecting, in the multiple images, bottom interference fringes that correspond to bottoms of the plurality of structures; and means for determining depths of the plurality of structures based on a distance between the top interference fringes and the bottom interference fringes.

17. An interferometer configured for characterizing a region of a sample comprising a plurality of structures, the interferometer comprising: a beam splitter configured to split illumination light into sample light to be incident on and reflected by the sample and reference light to be incident on and reflected by a reference mirror; illumination optics having a first numerical aperture configured to illuminate the region of the sample with the sample light; collection optics configured to be focused at a focal position with respect to the sample, the collection optics having a second numerical aperture that is greater than the first numerical aperture;an actuator configured to alter an optical path difference between the sample light and the reference light while not changing the focal position of the collection optics with respect to the sample; a detector configured to image, with the collection optics, reflected sample light from the sample interfering with reflected reference light from the reference mirror at a plurality of optical path differences to produce multiple images; and at least one processor configured to analyze the multiple images of the region of the sample comprising the plurality of structures to characterize the region of the sample.

Images