Single-wavelength ellipsometry with improved spot size capability

Abstract

Metrology system (100), comprising: a narrowband illumination source (110) configured to generate an illumination light beam (114); an illumination optics subsystem (112) configured to direct the illumination light beam (114) from the illumination source (110) to a measurement object arranged on a sample (115) to be measured, the illumination optics subsystem (112) comprising: a linear polarizer (111) configured to receive the illumination light beam (114) generated by the narrowband illumination source (110) and to impart a linear polarization to the illumination light beam (114); and an elliptical polarizer (113) configured to receive the illumination light beam (114) from the linear polarizer (111) and to impart an elliptical polarization to the illumination light beam (114); a detector (123) having a planar two-dimensional surface sensitive to incident light, the detector (123) configured to generate a plurality of output signals (125) indicative of a response of the sample (115) to the illuminating light beam (114); and a collection optics subsystem (119) configured to collect a collection light beam (117) from the sample (115) and direct the collection light beam (117) onto the surface of the detector (123), the collection optics subsystem (119) comprising: an analyzer (122) configured to receive the collected light beam (117) and to impart a linear polarization to the collected light beam (117); and a field stop (121) arranged in the collecting light beam (117) close to or at an image plane of the subsystem of a collecting optics (119), and / or a pupil stop (120) arranged in the collecting light beam (117) at or close to a pupil plane of the subsystem of a collecting optics (119).

Description

Reference to related applications

[0001] This patent application claims priority to U.S. Provisional Patent Application 62 / 286,279, entitled “Methods of Improved Spot Size Capability in Single Wavelength Ellipsometry,” filed January 22, 2016. Technical field

[0002] The described embodiments relate to metrology systems and methods, and in particular to methods and systems for improved measurement of semiconductor structures. Background of the invention

[0003] Semiconductor devices such as logic and memory devices are typically fabricated through a sequence of processing steps applied to a sample. The various structures (features) and multiple structural levels of the semiconductor devices are formed through these processing steps. For example, lithography, among others, is a semiconductor manufacturing process that involves creating a structure (pattern) on a semiconductor wafer. Additional examples of semiconductor manufacturing processes include, but are not limited to, chemical mechanical polishing, etching, deposition, and ion implantation. Multiple semiconductor devices can be fabricated on a single semiconductor wafer and then separated into individual semiconductor devices.Metrology processes are used at various steps during a semiconductor manufacturing process to detect defects on wafers to promote higher yield. Optical metrology techniques offer the potential for high throughput without the risk of sample destruction. A range of optical metrology-based techniques, including scatterometry and reflectometry implementations and associated analysis algorithms, are commonly used to characterize critical dimensions, film thicknesses, composition, overlay, and other parameters of structures at the nanoscale.

[0004] Progressive reductions in feature size, increasing geometric complexity, and more diverse material compositions of semiconductor devices place challenging demands on optical metrology systems that support process development and process monitoring. To meet device performance requirements, the thickness and composition of the thin films (e.g., oxide, nitride, metal layers, etc.) formed on a silicon wafer must be precisely controlled during the semiconductor device manufacturing process.

[0005] Single-wavelength ellipsometry (SWE)-based metrology techniques and systems are widely used to measure thin film properties. SWE systems use model-based metrology techniques to determine physical properties of film structures based on polarization properties of light reflected from the structure being measured. Exemplary metrology systems and techniques are described in detail in U.S. Patent No. 6,734,968 B1, issued May 11, 2004; U.S. Patent No. 7,006,222 B2, issued February 28, 2006; and U.S. Patent No. 7,253,901 B2, issued August 7, 2007, all assigned to KLA-Tencor Corporation. US 2013 / 0 114 085 A1 describes a metrology system in which an illumination pupil, a collecting pupil and field stops in the illumination and collecting beam paths can be dynamically adjusted by being controlled by a control system.

[0006] In many advanced film measurement applications, SWE systems are preferred due to their excellent measurement repeatability and optical stability. The single-wavelength polarized light source delivers consistent light output and excellent wavelength stability. SWE systems provide excellent matching performance between one tool and another among multiple tools in the same or different manufacturing facilities. This enables the sharing of ellipsometry models, measurement recipes, and optical constants across multiple SWE systems.

[0007] However, the light sources used by SWE systems have large coherence lengths, for example, on the order of tens of meters. This leads to significant coherence artifacts in measurement signals, which can degrade system performance. Coherence-based artifacts occur under many different circumstances. In one example, light diffracted from the edges of a measurement object (metrology target) leads to interference along the propagation path between the light reflected within the boundary of the measurement object and the light reflected from outside the measurement object. In another example, ghost images arise due to interference between even-numbered reflections from surfaces of optical elements.In another example, measurement data are contaminated by scattering effects from optical surface roughness and coatings, particulate contaminants, black surface treatments, and other light interactions with opto-mechanical structures.

[0008] Contaminated light does not exclusively carry information about the target box. Any amount of contaminated light detected by the SWE system contributes to measurement errors. The minimum object size that can be measured within a given tolerance variation of the thickness measurement error is often referred to as the "spot size" (also called "spot size" or "point size"). The measurement spot size is a function of the detected contaminated light. The greater the amount of contaminated light detected, the larger the measured "spot size" for a given thickness error criterion. In some examples, the levels of contaminated light must be less than 10 -5of the detected light to meet the measurement error specification for a reasonable spot size. As spot size and measurement error requirements become increasingly stringent, further reductions in contaminated light are required.

[0009] Future metrology applications pose challenges due to small feature size and multiparameter correlation. Improvements to SWE systems are desired. Brief description of the invention

[0010] Methods and systems for performing single wavelength ellipsometry (SWE) measurements with reduced spot size are presented herein.

[0011] In one aspect, a pupil stop is disposed at or near a pupil plane in the optical collection path to reduce sensitivity to diffraction effects at the edge of the object. In some embodiments, a circular aperture in the beam of collected light (collecting light beam) is disposed at or near the pupil plane of the collection optics of a SWE system. In a preferred embodiment, the pupil stop 120 is located in the pupil plane of the collection optics at a location where the spatial separation of the desired measurement signals from contaminated signals is maximized.

[0012] In another aspect, a field stop is disposed at or near an image plane conjugate to the wafer plane in the optical collection path to reduce sensitivity to unwanted optical-structural interactions. In some embodiments, a rectangular-shaped field stop is included in the optical collection path. In a preferred embodiment, the field stop is disposed in the image plane in front of the analyzer.

[0013] According to another aspect, a linear polarizer acting on the input beam of the SWE system comprises a thin, nanoparticle-based polarizer element. The nanoparticle-based polarizer element improves the quality of the illumination beam and reduces astigmatism at the wafer level. The thin, nanoparticle-based polarizer element is positioned downstream of the illumination source in the illumination path, but upstream of the elliptical polarizer of the SWE system.

[0014] According to another aspect, the dimensions of the apertures of the field stop, the pupil stop, or both are adjusted to increase the measurement sensitivity for structure-specific features, such as film thickness, by limiting light transmission. By limiting light transmission, light originating external to the measurement object at the wafer level, light associated with interference from the interaction between light generated within the measurement object and light generated external to the measurement object, and stray light reflected into the collected beam are absorbed or otherwise diverted away from the detector. As a result, the measurement spot size is reduced and the matching between one tool and another for small measurement objects is significantly improved.

[0015] The foregoing is a summary and thus necessarily contains simplifications, generalizations, and omissions of detail; consequently, those skilled in the art will recognize that the summary is only illustrative and not in any way limiting. Other aspects, inventive features, and advantages of the devices and / or methods described herein will become apparent in the detailed description, which is not limiting herein. Short description of the drawings Fig. 1 shows an exemplary SWE system 100 with reduced measurement spot size. Fig. Figure 2 shows a diagram illustrating a simulation of the irradiance at the wafer plane in and around an illuminated measurement object. Fig. 3 shows a diagram 160 representing a simulation of the irradiance distribution of the collected beam at a collecting pupil plane due to light originating from the interior of the measurement object. Fig. 4 shows a diagram 165 representing a simulation of the irradiance distribution of the collected beam at a collecting pupil plane due to light originating from outside the measurement object. Fig. 5 shows a diagram 170 representing a simulation of the irradiance distribution of the collected beam at a collection image plane due to light originating from the interior of the measurement object. Fig. 6 shows a diagram 175 representing a simulation of the irradiance distribution of the collected beam at a collection image plane due to light originating from outside the measurement object. Fig. 7 shows a diagram 180 representing a simulation of the irradiance distribution of the collected beam at a collection image plane due to interference between light originating from inside the measurement object and light originating from outside the measurement object. Fig. Figure 8 shows a diagram 185 representing a measurement of the irradiance distribution at a conjugate plane with respect to the collecting lens before filtering. Fig. 9 shows a diagram 190 representing a measurement of the irradiance distribution at a conjugate plane with respect to the collecting lens after filtering. Fig. Figure 10 shows a diagram 191 illustrating measurement spot sizes obtained with different focal plane offset distances for a conventional SWE system. Fig. Figure 11 shows a diagram 194 illustrating measurement spot sizes obtained with different focal plane offset distances for a SWE system including a nanoparticle-based input beam polarizer. Fig. 12 shows a diagram 197 representing the SWE measurement results of a test object with a rectangular, well-formed structure. Fig. Figure 13 shows an embodiment of a field stop comprising four movable absorbing plates configured to move relative to each other to obtain a rectangular shaped aperture of a desired dimension and position. Fig. 14 illustrates a method 200 for performing SWE measurements according to at least one new aspect. Detailed description

[0016] Reference will now be made in detail to background examples and some embodiments of the invention, examples of which are illustrated in the accompanying drawings.

[0017] Methods and systems for performing single-wavelength ellipsometry (SWE) measurements with reduced spot size are presented herein. Reducing the spot size is achieved by 1) providing a pupil stop in the optical collection path at a location near a pupil plane of the SWE system; 2) providing a field stop located in the optical collection path at a location near an image plane conjugate to the wafer plane of the SWE system; 3) providing a linear polarizer comprising a thin, high-extinction-rate, nanoparticle-based polarizing element disposed in the illumination path between the illumination source and the wafer of the SWE system; or any combination thereof. Reducing the spot size enables more accurate thin-film measurements and improved tool-to-tool matching.In some examples, a match specification between one tool and another of 0.02 angstroms is achieved for a thin film measurement with spot sizes ranging from 400 microns to 40 microns.

[0018] A single-wavelength ellipsometer performs thin-film measurements with improved beam quality and reduced sensitivity to diffraction effects at object edges and unwanted optical-structural interactions. The SWE system is configured to include an aperture near the converging pupil plane, an aperture near the image plane conjugate to the wafer plane, and / or a linear polarizer with a thin nanoparticle-based polarizer element. Each element is selected to optimize measurement sensitivity for the parameters of interest, measurement accuracy, and system tuning. The nanoparticle-based polarizer element improves the quality of the illumination beam and reduces astigmatism at the wafer plane.The pupil and field stops effectively block out and absorb unwanted collected light that is diffracted or reflected from outside the target before it reaches the detector. As a result, thin-film measurement accuracy is greatly improved for small targets (e.g., 40 µm × 40 µm targets).

[0019] Fig. 1 shows an exemplary SWE system 100 with a reduced measurement spot size. The SWE system 100 includes an illumination source 110 that generates an illumination light beam 114 incident on a wafer 115. The illumination light beam 114 passes through a linear polarizer comprising a thin, nanoparticle-based input beam polarizer element 111 with a high extinction rate, illumination optics 112, and an elliptical polarizer element 113 (e.g., a quarter-waveplate compensator, phase retarder, etc.) as the beam propagates from the illumination source 110 toward the wafer 115. The elements between the illumination source 110 and the wafer 115 are part of an illumination optics subsystem. The beam 114 illuminates a portion of the wafer 115 on which a measurement spot 116 is located. A beam of collected light 117 is collected by the measuring spot 116.The collected light 117 passes through a compensator 118 (e.g., a quarter-wave plate compensator, phase retarder, etc.), a collection optics 119 with a collection pupil stop 120, a collection field stop 121, and an analyzer element 122. The beam of collected light 117 impinges on the surface of a detector 123. The elements between the wafer 115 and the detector 123 are part of a collection optics subsystem.

[0020] In one example, detector 123 is a photovoltaic detector. However, in general, other detector technologies may also be considered (e.g., a position-sensitive detector (PSD), an infrared detector, a charge-coupled device (CCD), etc.). Detector 123 converts the collected light into electrical signals 125 that indicate the intensity of the collected light. Light collected by measurement spot 116 is projected onto detector 123 via captured spot 124. In this sense, measurement spot 116 includes locations on the wafer that generate the portion of the collected light that is ultimately projected onto the surface of detector 123.

[0021] A computer system 130 receives measured signals 125 and performs model-based measurements of parameters of interest of the measurement object based on the measured signals 125.

[0022] As in Fig. 1, the illumination light beam 114 impinges on the surface of the wafer 115 at an oblique angle. Generally, illumination light can be provided to the surface of the wafer 115 at any oblique angle or any number of oblique angles.

[0023] The illumination light beam 114 is a narrowband illumination light. In one example, the narrowband illumination light has a central wavelength of approximately 632.8 nanometers with an extremely narrow band (e.g., in the sub-nanometer wavelength range). In some examples, the emission spectrum of the narrowband light source is generated by a laser (e.g., a helium-neon laser). In general, however, any suitable narrowband illumination source within the scope of this patent document may be considered.

[0024] As in Fig. 1, the Z-axis is oriented perpendicular to the surface of wafer 115. The X- and Y-axes are coplanar with the surface of wafer 115 and thus perpendicular to the Z-axis. The principal ray 126 of the illumination light beam 114 and the principal ray 127 of the collected light beam 117 define an incidence plane perpendicular to the XY plane. The illumination light beam 114 impinges on the surface of wafer 115 at an angle of incidence AOI with respect to the Z-axis and lies within the incidence plane X'Z. The incidence plane is oriented with respect to a coordinate frame XY that is fixedly oriented to wafer 116 at an azimuth angle Az.

[0025] The geometric projection of an illuminating light beam onto the surface of a sample at an oblique angle results in an elongation of the illuminating beam cross-section in the direction aligned with the plane of incidence. As a non-limiting example, a circular beam of illuminating light (illuminating light beam) projected onto the wafer surface results in an illumination area that has an elliptical shape. In general, oblique illumination of a surface thus results in a projected illumination area that is elongated relative to the illuminating cross-section, and the direction of the elongation is aligned with the plane of incidence. Furthermore, the magnitude of the elongation increases with increasing angle of incidence. More specifically, the beam shape is inversely proportional to the cosine of the angle of incidence in the direction of the plane of incidence in the absence of diffraction and aberration effects.

[0026] Fig. Figure 2 shows a diagram illustrating a simulation of the irradiance at the wafer plane centered around the center of a measurement object 128 arranged on the wafer 115. The incoming beam (e.g., a beam with a Gaussian beam profile) illuminates both the measurement object and the areas surrounding the measurement object. The boundaries of the measurement object 128 (e.g., a thin-film pad) are shown. The irradiance existing within the boundaries of the measurement object 128 is denoted by I in The irradiance that exists outside the boundaries of the measurement object 128 is denoted by I out As in Fig. As shown in Figure 2, the measuring object has a square shape with dimensions of 40 µm by 40 µm.

[0027] Similar to coronagraphic measurements in astronomical applications, the illuminating light is diffracted around the edges of the object. In some examples, the diffraction of illuminating light at the object edge creates an undesirable halo in the projected image. The exact physical realization of the halo effect depends on the projected shape and size of the object. However, this diffracted light interferes with light originating from within the object area, thus causing errors in the thickness measurement.

[0028] According to one aspect, a circular aperture is arranged in the collection light beam near the pupil plane of the collection optics of the SWE system 100. In the Fig. In the embodiment illustrated in Figure 1, the collection optics 119 includes a pupil stop 120 integrated into the collection lens. In the illustrated embodiment, the pupil stop 120 includes a non-transparent structure with a round aperture through which light can freely pass.

[0029] In a preferred embodiment, the pupil diaphragm 120 is located in the pupil plane of the collection optics. At this location, there is maximum separation between light originating from inside the measurement object and light originating from outside the measurement object. At this location, the pupil diaphragm 120 blocks the most unwanted light. Furthermore, the collected light beam has a maximum extension at this location.

[0030] The total irradiance I(r) at each plane along the optical axis of the collected light (i.e., optical z-axis) can be expressed by equation (1): I(r)=Iin+Iout+Iint where I in is the irradiance resulting from light within the object being measured, I out is the irradiance resulting from light outside the object being measured, and I int is the irradiance resulting from the interference between light inside the measurement object and light outside the measurement object, as expressed by equation (2): Iint=2Iin Iout|a^in.a^out| cos(kΔ.r+φΔ+φp) where â = a x x + a y x + a z x̂ is the polarization vector of each wave with ||â|| = 1, and φ Δ is the phase difference between the components and φ p = arg(â in . â out ). Assuming the worst-case scenario, the polarization vectors associated with light inside the measurement object and the polarization vectors associated with light outside the measurement object are set equal: â in = â out ·

[0031] Equation (3) expresses a signal-to-contamination metric (SCR), which defines a ratio of desired signals (i.e., signals originating from inside the target region) to contamination signals (e.g., signals originating from outside the target region and interference signals) as a function of the location along the optical axis of the collected light (i.e., the optical z-axis). SCR(z)=IinIout+Iint

[0032] A search is performed to identify the location of the highest SCR value along the optical path. In some embodiments, the location of the highest SCR value is located where there is maximum separation between desired signals and contamination signals, where a physical obscuring device, such as an aperture, most effectively blocks contamination signals with minimal blockage of desired signals. In one example, only interference is included as a source of unwanted signal contamination. According to Babinet's principle, the diffracted field from the aperture is segmented and propagated separately for modeling purposes.

[0033] Fig. 3 shows a diagram 160 representing a simulation of the irradiance distribution of the collected beam at the collecting pupil plane due to light originating from the interior of the measuring object.

[0034] Fig. Figure 4 shows a diagram 165 representing a simulation of the irradiance distribution of the collected beam at the collecting pupil plane due to light originating from outside the measurement object.

[0035] The Fig. 3 are desirable and should be recorded, whereas the signals shown in Fig. 4 signals are undesirable and should be removed before detection.

[0036] The light diffracted by the measurement object 128 as imaged in the pupil plane is redistributed and concentrated near the edges of the pupil, as in Fig. 3. Thus, a high SCR is achieved at this point. The unwanted signals concentrated at the periphery of the beam are removed by a pupil stop with a relatively large aperture (e.g., on the order of millimeters). The irradiance distribution of the collected light source is most spatially concentrated in the wafer plane. Therefore, diffraction effects at edges or borders are most noticeable for relatively small targets. Therefore, the use of a pupil stop to mitigate these effects is most effective for relatively small targets.

[0037] Fig. 5 shows a diagram 170 representing a simulation of the irradiance distribution of the collected beam at the collection image plane due to light originating from the interior of the measurement object.

[0038] Fig. 6 shows a diagram 175 illustrating a simulation of the irradiance distribution of the collected beam at the collection image plane due to light originating from outside the measurement object.

[0039] Fig. 7 shows a diagram 180 representing a simulation of the irradiance distribution of the collected beam at the collected image plane due to interference between light coming from inside the measuring object and light coming from outside the measuring object.

[0040] Fig. 5 shows an image of the desired light that should be detected. As in Fig. 6 and Fig. However, as shown in Figure 7, unwanted light due to edge diffraction and interference is simply not spatially separated from the desired light at the image plane of the collection optics. Thus, a low SCR is obtained at this location. For this reason, an aperture located at the image plane is not effective in blocking unwanted light signals induced by diffraction at the edges of a target.

[0041] According to another aspect, a rectangular-shaped field stop is included in the optical collection path near an image plane of the collection optics of a SWE system. Fig. 1, the field stop 121 is located in the image plane in front of the analyzer 122. In the illustrated embodiment, the field stop 121 comprises a non-transparent plate with a rectangular aperture through which light can freely pass.

[0042] A field stop, located in the optical collection path at an intermediate image, prevents light from outside the field of view from reaching the detector. In one example, the field stop removes stray light resulting from interactions between the collection light beam and optomechanical elements in the optical path. These interactions typically manifest as a ring of relatively large radius around the collection light beam. This particular manifestation is due to the symmetrical cylindrical housings of the optical elements in the optical path.

[0043] Stray light resulting from interactions between the collecting light beam and optomechanical elements in the beam path is particularly prevalent when measuring larger objects. In one example, a projection of the object's image onto an image plane of the collecting lens is determined based on the size of the measurement object. The size of the field stop aperture is calculated based on the magnification of the collecting lens.

[0044] Fig. Figure 8 shows a diagram 185 representing a measurement of an illuminating beam at a conjugate plane with respect to the collecting lens without filtering. As in Fig. As shown in Figure 8, stray light appears in the image as a ring 128 around the light beam.

[0045] Fig. 9 shows a diagram 190 illustrating a measurement of an illumination beam at a conjugate plane with respect to the collection lens for the case where a field stop of appropriate size is arranged in the collection optical path as described herein. As in Fig. As shown in Figure 9, the field stop filters out unwanted light reflected from outside the object being measured.

[0046] According to another aspect, a linear polarizer disposed in the illumination path of a SWE system between the illumination source and any elliptical polarizer element comprises a thin, high-extinction-rate, nanoparticle-based polarizer element. Fig. In the embodiment shown in Figure 1, a thin, nanoparticle-based polarizer element 111 is arranged in the illumination path after the illumination source 110, but before the elliptical polarizer 113. Therefore, the polarizer element 111 is sometimes referred to as an input beam polarizer for conditioning the input beam, whereas the elliptical polarizer is used to impart a reference polarization to the illumination beam for measurement purposes. In the embodiment shown in Fig. In the embodiment shown in Figure 1, the thin, nanoparticle-based polarizer element 111 absorbs light polarized perpendicular to the transmission axis. Polarization is created in the thin, nanoparticle-based polarizer element 111 by spherical ellipsoidal nanoparticles embedded in sodium silicate glass. Exemplary thin, nanoparticle-based polarizers are available from Thorlabs, Inc., Newton, New Jersey (USA).

[0047] Thin nanoparticle-based polarizers provide a similar extinction rate to a conventional Glan-Thompson (GT) polarizer and a significantly higher damage threshold compared to conventional polymer-based components. In some embodiments, the thin nanoparticle-based polarizer element 111 is approximately 200 micrometers thick, while a comparable GT polarizer is approximately 10 millimeters thick. The thin polarizer provides better aberration control, and the refractive index of the thin nanoparticle-based polarizer is independent of the angle of incidence. This results in a significant reduction in astigmatism in the optical system and thus a reduced spot size capability. In contrast, a conventional GT polarizer configuration separates beam polarizations by refraction, but the extraordinary refractive index of the beam is angle-dependent, while the ordinary refractive index is constant.This leads to aberrations and a reduction in beam quality.

[0048] Fig. Figure 10 shows a graph 191 illustrating measurement spot sizes obtained with different focal plane offset distances for a conventional SWE system. Curve 192 illustrates the measurement spot size at the wafer plane in the X' direction, which is Fig. 1 for different focal plane offset distances. Curve 193 illustrates the measurement spot size at the wafer plane in the Y'-direction, which is Fig. 1 for different focal plane offset distances. Diagram 191 is generated based on experimental data collected from a SWE system using a conventional GT polarizer. Due to astigmatism effects, the minimum spot sizes measured in the X' and Y' directions reach their minimum sizes at different focal plane offset distances. Therefore, it is not possible to select a focal plane offset distance that minimizes the measurement spot size in both directions.

[0049] Fig. 11 shows a graph 194 illustrating measurement spot sizes obtained with different focal plane offset distances for a SWE system including a nanoparticle-based input beam polarizer, such as the SWE system 100. Curve 195 illustrates the measurement spot size at the wafer plane in the X' direction, which in Fig. 1 for different focal plane offset distances. Curve 196 shows the measurement spot size at the wafer plane in the Y'-direction, which is Fig. 1 for different focal plane offset distances. In this scenario, the minimum spot sizes measured in the X'-direction and the Y'-direction reach their minimum sizes at almost the same focal plane offset distance (i.e., focal plane offset distance of approximately 15 micrometers, as in Fig. 11). Thus, it is possible to select a focal plane offset distance that minimizes the size of the measurement spot in both directions.

[0050] Illustrated by example Fig. 12 Improvements (i.e., a reduction) in the effective measurement spot size by applying field and pupil stops as described herein. Fig. Figure 12 shows a diagram 197 depicting the SWE measurement results of a measurement object with a rectangular, well-formed structure. The illumination spot was scanned over the object. In the given example, a film thickness characterizing the object was measured by identifying a film thickness parameter that provided the best fit to measured SWE signals. The film thickness variation is expected to remain within a prescribed range while the illumination beam remains within the well-formed structure. Therefore, it is assumed that unwanted interaction of the illumination beam with the edges of the object area occurs when the measurement results move outside this range.

[0051] Curve 198 shows measurements performed with a conventional SWE system that does not include a pupil stop, a field stop, or a thin nanoparticle-based input beam polarizer as described herein. In the example shown, the measurement is stable over a linear scan of approximately 20 micrometers (that is, when the center of the measurement beam is scanned between 30 micrometers and 50 micrometers). Beyond this limited range, the beam begins to interact with the edges of the well-formed object, and the measured thickness begins to increase.

[0052] Curve 199 shows measurements taken with an SWE system incorporating the pupil stop, field stop, and thin nanoparticle-based input beam polarizer as described herein. In the illustrated example, the measurement is stable over a linear scan of approximately 40 micrometers (that is, when the center of the measurement beam is scanned between 20 micrometers and 60 micrometers). Beyond this range, the beam begins to interact with the edges of the well-formed object, and the measured thickness begins to increase. Note that the effective measurement spot size is smaller for the SWE system with pupil stop, field stop, and thin nanoparticle-based input beam polarizer because the linear scan range is significantly larger before edge / fringe effects affect the measurement.

[0053] Measurements by a SWE system incorporating the pupil stop, field stop, and thin nanoparticle-based input beam polarizer as described herein demonstrate tool-to-tool matching of film thickness measurements down to 0.02 angstroms for a 40 µm × 40 µm target. Similar measurements performed on a SWE system not incorporating the pupil stop, field stop, and thin nanoparticle-based input beam polarizer as described herein demonstrate tool-to-tool matching of film thickness measurements greater than 0.2 for the same 40 µm × 40 µm target.

[0054] In some embodiments, the SWE system 100 is configured as a discrete measurement polarizer and a rotating compensator system. In these embodiments, measurements at discrete polarizer angles are performed using a continuously rotating compensator (e.g., the one shown in Fig. 1) for one or more azimuth angles and angles of incidence. In some embodiments, the polarizer 113 comprises a rotatable polarizer element, and the computer system 130 transmits a command signal 141 indicating a desired polarization state to the polarizer 113. In response, the polarizer 113 rotates and stops at the desired polarization state. In this way, the SWE system 100 is configured to stop at a fixed polarization angle.

[0055] In some embodiments, SWE system 100 is configured as a rotating polarizer system. In these embodiments, measurements are taken while the polarization state continuously changes for one or more azimuth angles and angles of incidence. In some embodiments, polarizer 113 includes a rotatable polarizer element, and computer system 130 transmits a command signal 141 indicating a desired rate of change of the polarization state to polarizer 113. In response, polarizer 113 rotates at a desired angular velocity.

[0056] In some embodiments, the SWE system 100 is configured as a rotating polarizer and a rotating compensator system. In these embodiments, measurements are taken with a continuously rotating polarizer (for example, the one shown in Fig. 1) and a continuously rotating compensator (for example the one shown in Fig. 1) for one or more azimuth angles and angles of incidence. In some embodiments, compensator 118 includes a rotatable polarizer element, and computer system 130 transmits a command signal 145 indicating a desired rate of change of the polarization state to compensator 118. In response, compensator 118 rotates at the desired angular velocity.

[0057] In another aspect, the SWE system 100 also includes a selective analyzer angle. Similarly, the computer system 130 transmits a command signal 142 indicating a desired analyzer angle to the selective analyzer 122.

[0058] Regardless of the polarization state, in some embodiments, the SWE system 100 is also configured to select regions of the AOI and Az for measurement.

[0059] In some embodiments, the field stop 121 is adjusted to achieve the desired signal rejection. As in Fig. 1, the computer system 130 communicates a command signal 143 indicating a desired aperture size to the field stop 121. In response, the field stop 121 adjusts to the desired aperture size. In some embodiments, the field stop 121 includes active elements such as movable slits, knife edges, MEMS-based mirror elements, etc., configured to adjust the aperture size. Fig. Figure 13 shows an embodiment of a field stop 121 having four movable absorbing plates 151-154 configured to move relative to each other to obtain a rectangular shaped aperture having a desired dimension and position.

[0060] In some embodiments, the pupil diaphragm 120 is adjusted to achieve the desired signal blocking. As in Fig. 1, the computer system 130 transmits a command signal 144 indicating a desired aperture size of the pupil stop 120 to the pupil stop 120. In response, the pupil stop 120 adjusts to the desired aperture size. In these embodiments, the pupil stop 120 includes active elements such as movable shutter elements, knife edges, MEMS-based mirror elements, etc., configured to adjust the aperture of the pupil stop 120.

[0061] In some embodiments, the azimuth angle is selected by rotating the measurement object with respect to the plane of incidence of the metrology (measuring) system. For example, the SWE system 100 may include a turntable for supporting the sample 115. In these embodiments, the computer system 130 transmits a command signal to the turntable to rotate the sample 115 with respect to the optics system (e.g., rotation about the z-axis in Fig. 1) to achieve the desired azimuth angle.

[0062] According to another aspect, the dimensions of the apertures of the field stop, the pupil stop, or both are adjusted to increase measurement sensitivity to pattern specific features, such as film thickness, by limiting light transmission. By limiting light transmission, light associated with light originating from outside the measurement object at the wafer level, interference light generated by the interaction between light generated inside the measurement object and light generated outside the measurement object, and stray light reflected into the collected beam are absorbed or otherwise redirected away from the detector. In this way, light rays most sensitive to changes in the parameter of interest are detected.

[0063] Fig. 14 illustrates a method 200 for performing SWE measurements according to at least one novel aspect. The method 200 is adapted for implementation by a metrology system such as that described in Fig. 1, according to the present invention. In one aspect, it is recognized that data processing blocks of method 200 may be executed via a preprogrammed algorithm, wherein the preprogrammed algorithm is executed by one or more processors of computer system 130 or any other general-purpose computer system. It is recognized herein that the specific structural aspects of SWE system 100 are not limiting and should be interpreted as illustrative only.

[0064] In block 201, a narrowband illumination light beam is generated by an illumination source.

[0065] In block 202, the narrowband illumination light beam is directed by an illumination optics subsystem onto a measurement object disposed on a sample (e.g., a semiconductor wafer) that is currently being measured.

[0066] In block 203, the narrowband illumination light beam is linearly polarized by a linear polarizer. In some embodiments, the linear polarizer is a thin nanoparticle-based polarizer.

[0067] After linear polarization, the narrowband illumination light beam is elliptically polarized in block 204.

[0068] In block 205, a collection light beam is collected from the measurement object illuminated by the narrowband illumination light beam.

[0069] In block 206, a first portion of the collected light beam is removed from the remainder of the collected light beam at or near a pupil plane of a collection optics subsystem configured to collect the collected light beam from the measurement object. In some embodiments, the first portion of the collected light beam is removed by an aperture configured to absorb or redirect the removed light.

[0070] In block 207, a second portion of the collected light beam is removed from the remainder of the collected light beam at or near an image plane of the collection optics subsystem. In some embodiments, the second portion of the collected light beam is removed by an aperture configured to absorb or redirect the removed light.

[0071] In block 208, a third portion of the beam of collected light is detected on a surface of a detector that is sensitive to incident light.

[0072] In block 209, a plurality of output signals indicative of the detected portion of the beam of collected light are generated by the detector.

[0073] In general, the signal contamination control methods and systems described herein are not limited to application in single-wavelength ellipsometer systems and can be implemented in any semi-coherent metrology system, including spectroscopic ellipsometry systems, etc.

[0074] In another embodiment, system 100 includes one or more computer systems 130 used to perform measurements of actual device structures based on measurement data collected according to the methods described herein. The one or more computer systems 130 may be communicatively coupled to a detector (e.g., detector 123). In one aspect, the one or more computer systems 130 are configured to receive measurement data 125 associated with measurements of the structure of sample 115.

[0075] It should be appreciated that one or more steps described in the present disclosure may be performed by a single computer system 130 or, alternatively, a multiple computer system 130. Furthermore, various subsystems of system 100, such as detector 123, may include a computer system suitable for performing at least a portion of the steps described herein. Therefore, the foregoing description should not be interpreted as a limitation of the present invention, but merely as an illustration.

[0076] Additionally, computer system 130 may be communicatively coupled to detector 123 in any manner known in the art. For example, the one or more computer systems 130 may be connected to computer systems associated with detector 123. In another example, detector 123 may be directly controlled by a single computer system coupled to computer system 130.

[0077] The computer system 130 of the SWE system 100 may be configured to receive and / or acquire data or information from the system's subsystems (e.g., detector 123 and the like) through a transmission medium, where the transmission medium may include wired and / or wireless portions. In this way, the transmission medium may serve as a data connection between the computer system 130 and other subsystems of the system 100.

[0078] The computer system 130 of the SWE system 100 may be configured to receive and / or acquire data or information (e.g., measurement results, modeling inputs, modeling results, reference measurement results, etc.) from other systems through a transmission medium, where the transmission medium may include wired and / or wireless portions. In this way, the transmission medium may serve as a data connection between the computer system 130 and other systems (e.g., a memory-on-board SWE system 100, an external storage, or other external systems). For example, the computer system 130 may be configured to receive measurement data from a storage medium (e.g., memory 132 or an external storage) via a data connection.For example, the intensity measurement results obtained using detector 123 may be stored in a permanent or semi-permanent storage device (e.g., memory 132 or external memory). In this regard, the measurement results may be imported from the on-board memory or from an external storage system. Furthermore, computer system 130 may send data to other systems via a transmission medium. For example, a measurement model or an actual device parameter value determined by computer system 130 may be communicated and stored in external memory. In this regard, measurement results may be exported to another system.

[0079] Computer system 130 may include, but is not limited to, a personal computer system, mainframe computer system, workstation, image computer, parallel processor, or any other device known in the art. In general, the term "computer system" can be broadly defined to include any device having one or more processors that execute instructions from a storage medium.

[0080] Program instructions 134 implementing methods such as those described herein may be transmitted over a transmission medium such as a wire, cable, or wireless transmission connection. For example, as described in Fig.1, program instructions 134 stored in memory 132 are transferred to processor 131 via bus 133. Program instructions 134 are stored in a computer-readable medium (e.g., memory 132). Example computer-readable media include read-only memory (ROM), random access memory (RAM), a magnetic or optical disk, or magnetic tape.

[0081] In some examples, the measurement models are implemented as an element of a SpectraShape® critical dimension optical measurement system, available from KLA-Tencor Corporation, Milpitas, California, USA. In this way, the model is created and ready for use immediately after the measurement signals are collected by the system.

[0082] In some other examples, the measurement models are implemented offline, for example, by a computer system implementing AcuShape® software available from KLA-Tencor Corporation, Milpitas, California, USA. The resulting learned model may be incorporated as an element of an AcuShape® library accessible by a measurement system performing measurements. According to yet another aspect, the measurement model results described herein may be used to provide active feedback to a process tool (e.g., lithography tool, etch tool, deposition tool, etc.). For example, values of measured parameters determined based on measurement methods described herein may be communicated to a lithography tool to adjust the lithography system to achieve a desired output. In a similar manner, etch parameters (e.g., etch time, diffusivity, etc.) may be adjusted.) or deposition parameters (e.g., time, concentration, etc.) may be included in a measurement model to provide active feedback to etch tools and deposition tools, respectively. In some examples, corrections to process parameters determined based on measured device parameter values and a learned measurement model may be communicated to a lithography tool, an etch tool, or a deposition tool. As described herein, the term "critical dimension" includes any critical dimension of a structure (e.g., bottom critical dimension, middle critical dimension, top critical dimension, sidewall angle, grating height, etc.), a critical dimension between any two or more structures (e.g., distance between two structures), and a displacement between two or more structures (e.g., overlay displacement between overlying grating structures, etc.).Structures may include three-dimensional structures, patterned structures, overlay structures, etc.

[0083] As described herein, the term “critical dimension application” or “critical dimension application” includes any measurement of critical dimensions.

[0084] As described herein, the term "metrology system" encompasses any system used, at least in part, to characterize a sample according to any aspect, including metrology applications such as critical dimension metrology, overlay metrology, focus / dose metrology, and composition metrology. However, such terms of art do not limit the scope of the term "metrology system" described herein. Additionally, the SWE system 100 may be configured to measure structured (patterned) wafers and / or unstructured wafers.The measurement system (metrology system) can be configured as an LED inspection tool, edge inspection tool, backside inspection tool, macro inspection tool, or multimode inspection tool (incorporating data from one or more platforms simultaneously), and any other metrology or inspection tool that benefits from the techniques described here.

[0085] Various embodiments are described herein for a semiconductor processing system (e.g., an inspection system or a lithography system) that can be used to process a sample. The term "sample" is used herein to refer to a wafer, a reticle, or any other sample that can be processed (e.g., printed or inspected for defects) by means known in the art.

[0086] As used herein, the term "wafer" generally refers to substrates formed from a semiconductor or non-semiconductor material. Examples include, but are not limited to, monocrystalline silicon, gallium arsenide, and indium phosphide. Such substrates may be commonly found and / or processed in semiconductor manufacturing facilities. In some cases, a wafer may include only the substrate (i.e., the bare wafer). Alternatively, a wafer may include one or more layers of different materials formed on a substrate. One or more layers formed on a wafer may be "structured" ("patterned") or "unstructured." For example, a wafer may include a plurality of dies with repeatable structural features.

[0087] A "reticle" can be a reticle at any stage of a reticle manufacturing process or a fully fabricated reticle that may or may not be released for use in a semiconductor manufacturing facility. A reticle, or "mask," is generally defined as a substantially transparent substrate having substantially opaque regions formed thereon and configured into a structure (pattern). The substrate may, for example, comprise a glass material such as amorphous SiO2. A reticle may be disposed over a resist-covered wafer during an exposure step of a lithography process so that the structure on the reticle can be transferred to the resist.

[0088] One or more layers formed on a wafer may be patterned or unpatterned. For example, a wafer may comprise a plurality of dies, each having repeatable structural / pattern features. The formation and processing of such material layers may ultimately result in finished devices. Many different types of devices may be formed on a wafer, and the term wafer, as used herein, is intended to encompass a wafer on which any type of device known in the art is fabricated.

[0089] In one or more example embodiments, the described functions may be implemented in hardware, software, firmware, or any combination thereof. When implemented in software, the functions may be stored or transmitted as one or more instructions or as code on a computer-readable medium. Computer-readable media includes both computer storage media and communications media, including any medium that facilitates the transfer of a computer program from one location to another. A storage medium may be any available medium that can be accessed by a general-purpose or special-purpose computer.By way of example and not limitation, such computer-readable media may include RAM, ROM, EEPROM, CD-ROM, or other optical disk storage, magnetic disk storage or other magnetic storage devices, or any other medium usable for carrying or storing desired program code means in the form of instructions or data structures and accessible by a general-purpose or special-purpose computer or processor. Each combination is properly referred to as a computer-readable medium.For example, if the software is transmitted from a website, server, or other remote source using coaxial cable, fiber optic cable, twisted pair cable, digital subscriber line (DSL), or wireless technologies such as infrared, radio, and microwave, then the coaxial cable, fiber optic cable, twisted pair cable, DSL, or wireless technologies such as infrared, radio, and microwave are included in the definition of medium. Disk and disc, as used herein, include compact disc (CD), laser disc, optical disc, digital versatile disc (DVD), floppy disk, and Blu-ray Disc, with disks typically reproducing data magnetically, while discs reproducing data optically using lasers. Combinations of these should also be considered computer-readable media.

[0090] Although certain specific embodiments have been described above for instructional purposes, the teachings of this patent document have general applicability and are not limited to the specific embodiments described above. Accordingly, various modifications, adaptations, and combinations of various features of the described embodiments may be made without departing from the scope of the invention as set forth in the claims. List of reference symbols 100 SWE system 110 Lighting source 111 Input beam polarizer element 112 Lighting optics 113 elliptical polarizer element 114 Beam of illuminating light (illuminating light beam) 115 Sample, wafer 116 measuring spot 117 Beam of collected light (collected light beam, collected light beam) 118 Compensator 119 Collectible Optics 120 Collecting pupil diaphragm 121 Collector panel 122 Analyzer element 123 Detector 124 spots 125 electrical signal, measurement data 126 Main beam 127 Main beam 128 Measurement object (metrology target) 130 computer system, computer system 131 processor 132 memory 133 buses 134 program command 141 Command signal 142 Command signal 143 Command signal 144 Command signal 145 Command signal 151 plate 152 plate 153 plate 154 plate 160 Diagram 165 Diagram 170 Diagram 175 Diagram 180 diagram 185 Diagram 190 Diagram 191 Diagram 192 curve 193 Curve 194 Diagram 195 Curve 196 curve 197 Diagram 198 Curve 199 Curve 200 procedures

Claims

[1] Metrology system (100), comprising: a narrowband illumination source (110) configured to generate an illumination light beam (114); an illumination optics subsystem (112) configured to direct the illumination light beam (114) from the illumination source (110) to a measurement object arranged on a sample (115) to be measured, the illumination optics subsystem (112) comprising: a linear polarizer (111) configured to receive the illumination light beam (114) generated by the narrowband illumination source (110) and to impart a linear polarization to the illumination light beam (114); and an elliptical polarizer (113) configured to receive the illumination light beam (114) from the linear polarizer (111) and to impart an elliptical polarization to the illumination light beam (114); a detector (123) having a planar two-dimensional surface sensitive to incident light, the detector (123) configured to generate a plurality of output signals (125) indicative of a response of the sample (115) to the illuminating light beam (114); and a collection optics subsystem (119) configured to collect a collection light beam (117) from the sample (115) and direct the collection light beam (117) onto the surface of the detector (123), the collection optics subsystem (119) comprising: an analyzer (122) configured to receive the collected light beam (117) and to impart a linear polarization to the collected light beam (117); and a field stop (121) arranged in the collecting light beam (117) close to or at an image plane of the subsystem of a collecting optics (119), and / or a pupil stop (120) arranged in the collecting light beam (117) at or close to a pupil plane of the subsystem of a collecting optics (119). [2] The metrology system (100) of claim 1, wherein the linear polarizer (111) comprises a nanoparticle-based polarizer element. [3] The metrology system (100) of claim 1, wherein the field stop (121) is a rectangular shaped opening. [4] The metrology system (100) of claim 1, wherein the pupil diaphragm (120) is a circular opening. [5] Metrology system (100) according to claim 1, wherein a region of an aperture of the pupil stop (120), a region of an aperture of the field stop (121) or both regions are adjustable. [6] The metrology system (100) of claim 1, wherein the pupil stop (120), the field stop (121), or both are arranged in an optical collection path of the metrology system (100) at one or more locations where a signal to the contamination metric is maximized. [7] Metrology system (100) according to claim 1, further comprising: a computer system (130) configured to transmit a first command signal (144) to the pupil stop (120), causing the pupil stop (120) to adjust the area of the aperture of the pupil stop (120) to block light rays associated with light diffracted from edges of the measurement object, to transmit a second command signal (143) to the field stop (121), causing the field stop (121) to adjust the area of the aperture of the field stop (121) to block light rays associated with unwanted light interactions with optomechanical elements of the collection optics subsystem (119), or to transmit both the first (144) and the second (143) command signals. [8] The metrology system (100) of claim 1, wherein the elliptical polarizer (113) rotates at a predetermined angular velocity. [9] The metrology system (100) of claim 1, wherein the metrology system (100) is configured as a single wavelength ellipsometer. [10] The metrology system (100) of claim 1, wherein the illumination light beam (114) is narrowband light having a wavelength range of less than one nanometer. [11] Method comprising: generating a narrowband illumination light beam (114); Directing the narrow-band illuminating light beam (114) onto a measurement object arranged on a sample (115) to be measured; linearly polarizing the narrowband illumination light beam (114); elliptically polarizing the narrowband illumination light beam (114) after the linear polarization; Collecting a collecting light beam (117) from the measurement object illuminated by the narrowband illuminating light beam (114); Removing a first portion of the collection light beam (117) near or at a pupil plane of a collection optics subsystem (119) configured to collect the collection light beam (117) from the measurement object; Removing a second portion of the collecting light beam (117) near or at an image plane of the subsystem of a collecting optics (119); Detecting a third portion of the collected light beam (117) on a surface sensitive to incident light; and Generating a plurality of output signals (125) indicative of the detected portion of the collected light beam (117). [12] The method of claim 11, wherein linearly polarizing the narrowband illumination light beam (114) involves a nanoparticle-based polarizer element. [13] The method of claim 11, wherein removing the first portion of the collection light beam (117) near or at the pupil plane of the collection optics subsystem (119) involves a circular aperture. [14] The method of claim 11, wherein removing the second portion of the collection light beam (117) near or at the image plane of the collection optics subsystem (119) involves a rectangular shaped aperture. [15] The method of claim 11, further comprising: Transmitting a first command signal (144) to a pupil diaphragm (120), causing the pupil diaphragm (120) to adjust a portion of an aperture of the pupil diaphragm (120) to block light rays associated with light diffracted by edges of the measurement object; and Transmitting a second command signal (143) to a field stop (121) causing the field stop (121) to adjust a portion of an aperture of the field stop (121) to block light rays associated with unwanted light interactions with optomechanical elements of a collection optics subsystem (119). [16] Metrology system (100), comprising: a narrowband illumination source (110) configured to generate an illumination light beam (114); an illumination optics subsystem (112) configured to direct the illumination light beam (114) from the illumination source (110) to a measurement object arranged on a sample (115) to be measured, the illumination optics subsystem (112) comprising: a linear polarizer (111) configured to receive the illumination light beam (114) generated by the narrowband illumination source (110) and to impart a linear polarization to the illumination light beam (114); and an elliptical polarizer (113) configured to receive the illumination light beam (114) from the linear polarizer (111) and to impart an elliptical polarization to the illumination light beam (114); a detector (123) having a planar two-dimensional surface sensitive to incident light, the detector (123) configured to generate a plurality of output signals (125) indicative of a response of the sample (115) to the illuminating light beam (114); a collection optics subsystem (119) configured to collect a collection light beam (117) from the sample (115) and direct the collection light beam (117) onto the surface of the detector (123), the collection optics subsystem (119) comprising: an analyzer (122) configured to receive the collected light beam (117) and to impart a linear polarization to the collected light beam (117); and a field stop (121) arranged in the collecting light beam (117) near or at an image plane of the subsystem of a collecting optics (119), and / or a pupil stop (120) arranged in the collecting light beam (117) at or near a pupil plane of the subsystem of a collecting optics (119); and a non-transitory, computer-readable medium comprising: Code to cause a computer system (130) to transmit a first command signal (144) to the pupil diaphragm (120) causing the pupil diaphragm (120) to adjust a portion of an aperture of the pupil diaphragm (120); and Code to cause the computer system (130) to transmit a second command signal (143) to the field stop (121) causing the field stop (121) to adjust a portion of an opening of the field stop (121). [17] The metrology system (100) of claim 16, wherein the linear polarizer (111) comprises a nanoparticle-based polarizer element. [18] The metrology system (100) of claim 16, wherein the field stop (121) is a rectangular shaped opening and wherein the pupil stop (120) is a circular opening. [19] The metrology system (100) of claim 16, wherein the metrology system (100) is configured as a single wavelength ellipsometer. [20] Metrology system (100) according to claim 16, wherein the illumination light beam (114) is directed at the measurement object at a plurality of angles of incidence.

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