High-resolution, acoustic-optical metrology system

Abstract

Systems and methods of optical metrology and inspection are provided, including shaping a pump pulse to have a concave wavefront that generates, in a sample, an acoustic wave that converges at a focal location. When a subsurface feature is at the focal location, a reflected acoustic wave is generated, with a convex acoustic wavefront. A probe assembly is configured to shape a probe pulse to have a spherical wavefront and to impinge on the surface simultaneously with the return to the surface of a reflected acoustic wave. A detection assembly is configured to receive a reflected probe pulse, reflected from the surface, and, responsively, to determine whether the reflected probe pulse is indicative of surface changes induced by a reflected acoustic wave from a subsurface feature at the focal location.

Description

HIGH-RESOLUTION, ACOUSTIC-OPTICAL METROLOGY SYSTEMFIELD OF THE INVENTION

[0001] This invention relates to ultrasonic and optical techniques for subsurface measurement and defect detection, particularly for semiconductor metrology.BACKGROUND

[0002] The detection of voids in semiconductor structures is critical for ensuring device performance and long-term reliability. This need is especially acute in bonding processes such as hybrid bonding, where even small voids trapped at the interface can degrade electrical conductivity, thermal conduction, and overall mechanical integrity. Identifying such voids early in manufacturing typically enables higher yield and more reliable advanced semiconductor assemblies.

[0003] A widely used non-destructive approach for assessing subsurface defects is acoustic microscopy. This modality typically employs ultrasound in the approximate range of 10 MHz to 400 MHz to interrogate internal structures in optically opaque materials. A transducer generates high-frequency acoustic waves that propagate into the device under test. As these waves encounter interfaces or inhomogeneities with differing acoustic impedance, a portion of the energy is reflected back to the transducer. By processing these reflected signals, the system may generate images that reveal features such as delamination, cracks, and voids. Acoustic microscopy is typically effective for detecting relatively large defects and provides useful depth information across a range of packaging materials. However, the achievable spatial resolution is governed by diffraction and therefore depends on the acoustic wavelength in the coupling medium, which is usually water. Because practical systems are limited to frequencies below several hundred megahertz, the resulting wavelengths restrict resolution to tens of microns. Voids with dimensions on the order of one micron are therefore extremely difficult to detect using conventional acoustic microscopes.

[0004] Picosecond ultrasonics has emerged as a complementary non-destructive technique that uses ultrashort optical pulses to generate and detect very high frequency acoustic waves. In a typical implementation, an ultrafast pump pulse is absorbed at the surface of an opaque film. The rapid thermal expansion produces a brief strain pulse thatpropagates into the material. A time-delayed probe pulse then measures small changes in reflectivity produced by the returning acoustic wave, which allows the system to determine travel times and thereby infer properties such as film thickness or interface location. Because the acoustic frequencies generated by picosecond excitation can extend into the gigahertz or even terahertz regime, the technique can in principle access nanoscale structural information. In practice, its application to defect and void detection has been limited by weak signal return. A void presents only a very small interface relative to the illuminated region, so the portion of the acoustic energy returned toward the surface is minimal. The resulting back-reflected signal typically exhibits very low amplitude and is difficult to distinguish from noise. These signal -to-noise constraints limit the detectability of buried voids using existing picosecond ultrasonics methods.SUMMARY

[0005] The present invention includes systems and methods for manufacturing process control related to characterizing a semiconductor sample (e.g., a semiconductor wafer), especially with respect to identifying subsurface features, such as voids. Voids may, for example, be created between semiconductor layers during hybrid bonding.

[0006] A sample under inspection is illuminated by a shaped, ultra-short optical pulse. In examples of the present invention, the duration of the pulse may be tens of picoseconds, down to 0.1 ps or even less. The pulse is shaped to have a concave wavefront, such that when it strikes the sample its outer circumference impinges on the sample first, followed by inner concentric rings. The shaped pulse induces an acoustic wave in the sample that converges toward a subsurface region of interest.

[0007] A subsurface feature at the region of interest scatters and reflects the acoustic wave, such that a portion of the acoustic wave is reflected back toward the surface with a diverging, approximately spherical form. At the surface, the reflected acoustic wave causes a change in the surface optical reflectivity. This change in surface reflectivity may be measured by measuring characteristic of an optical probe pulse reflected from the sample surface.

[0008] In examples of the present invention, the probe pulse is first shaped to have a concave (i.e., spherical) optical wavefront, so that impinging concentric portions of the optical wavefront, synchronized to conform to the temporal of the reflected acoustic wave,indicate the surface characteristics affected by the acoustic wave. The reflected optical pulse thus indicates the modified reflectivity of the sample surface, with improved sensitivity due to the affect on each of the multiple concentric rings.

[0009] The present invention thus includes two elements: 1) shaped optical pulse to generate of converging acoustic probe waves, and 2) shaped optical pulse to amplify optical detection of diverging acoustic waves.BRIEF DESCRIPTION OF DRAWINGS

[0010] For a better understanding of various embodiments of the invention and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings. Structural details of the invention are shown to provide a fundamental understanding of the invention, the description, taken with the drawings, making apparent to those skilled in the art how the several forms of the invention may be embodied in practice.

[0011] In the accompanying drawings:

[0012] Figs. 1A, IB, and 2 are schematic diagrams of a sample under acoustic-optical inspection being illuminated first by a shaped pump pulse, to generate an acoustic wave in the sample, and subsequently illuminated by a shaped probe pulse, to detect a reflected acoustic wave, according to embodiments of the present invention;

[0013] Figs. 3A and 3B are schematic diagrams of optical assemblies for generating, respectively, a concave pump pulse and a convex probe pulse, according to embodiments of the present invention; and

[0014] Figs. 4-8 are schematic diagrams of different configurations of the present invention for characterizing subsurface features of a sample by an acoustic-optical method, that is, different configurations of an acoustic-optical microscope, according to embodiments of the present invention. Fig. 4 shows a general configuration for such apparatus. Fig. 5 shows a configuration with a single laser source, producing an optical beam that is subsequently split to pump and probe pulses. Fig. 6 shows a configuration whereby pump and probe pulse paths are separated after reflection from the sample, to shunt the reflected pump prose away from an optical sensor (also referred to herein as the detection assembly). Fig. 7 shows a configuration whereby pump and probe pulses aregenerated at different wavelengths, in order to block the reflected pump pulse from the optical sensor. Fig. 8 shows a configuration whereby pump and probe pulses impinge on the sample at different angles, with the optical sensor oriented according to the angle of the reflected probe pulse.DETAILED DESCRIPTION

[0015] For a better understanding of the system and methods, reference is now made to the figures, which illustrate representative embodiments of the invention. Taken together, these figures demonstrate how the system architecture, optical conditioning, and coordinated process control work in combination to enable stable, accurate, and high- throughput optical metrology.

[0016] Figs. 1 A and IB are schematic diagrams of apparatus for inspecting subsurface features of a sample 20, the subsurface features including elements such as a void 24, which is shown at an interface 26 between semiconductor layers of the sample. In manufacturing semiconductors with hybrid bonding, each layer may have a thickness of a few microns to tens of microns, such that the total sample height is rarely greater than 100 microns or less than 1 micron. As described above, sub-micron features are below the resolution of traditional ultrasound inspection instruments, while the signal -to-noise ratio of higher resolution systems has hindered their effectiveness.

[0017] In examples of the present invention, the sample is illuminated by a shaped, optical pump pulse 30. Absorption of the pump pulse within a region of tens of nanometers results in rapid thermoelastic expansion that radiates into the sample as an acoustic wave 32. The acoustic pulse propagates through the sample at the longitudinal speed of sound, which for silicon is typically on the order of 8430 meters per second (depending on the crystal orientation). Owing to the sub-picosecond pump duration, the induced acoustic pulse may include frequency components extending from tens of gigahertz to several hundred gigahertz. (The pump duration may be, for example, between 100 fs and 1 ps, or even less.)

[0018] As shown in Fig. 1A, the pump pulse, having a radius R and directed toward the sample, is shaped to have a concave wavefront, such that the outer edge of the pumppulse strikes the sample surface before inner portions. In effect, the pump pulse is a series of concentric optical rings, each ring portion having a duration L, each ring progressively delayed toward the center, such that the innermost portion is delayed by a time T relative to the outermost ring. As each ring of the concave pump wavefront impinges on the sample surface, a series of concentric acoustic waves is generated in the sample, which converge to a focal location. The convergence thereby increases the acoustic energy that is reflected from a feature, such as a void, at the focal location.

[0019] For a radius of the optical pump pulse given as R, the edge of the beam impinges on the sample first at a distance R from the point that the beam center impinges upon last. The focal location is at a depth Z, beneath the center of the pulse beam, when the delay time T, plus the time for the acoustic wave to pass from the surface to depth Z, equals the time for the acoustic wave to traverse the diagonal distance, s, from the beam edge point of contact to the focal location, where the speed of acoustic transmission in the sample is v. That is,T ■ v = VR2+ Z2- Z

[0020] The concavity of the wavefront can therefore be set such that the time delay, T, relative to any given distance, r, towards the circumference, is ( (r2+Z2)-Z) / v. It should be noted that the optical delay distance required to create the time delay of T is T- c, where c is the speed of light.

[0021] As described below, the shaped optical pump pulse is typically generated by a laser that emits a beam of one or more frequencies having wavelengths below 400 nm (i.e., violet light, extending into the ultraviolet boundary), to allow for shallow absorption in the sample, such as 100 nanometers or less. The pulse duration is kept short to effectively excite a range of high acoustic frequencies. For example, the pulse duration may be set to 1 picosecond or less, to generate acoustic waves with submicron wavelengths, thereby ensuring reflection from subsurface features of a micron or less, and often as little as tens of nanometers. (As described above, the resolution of typical acoustic microscopes is limited to tens of microns.)

[0022] The shaped pulse may be shaped by optical means, described below, to be focused on the sample surface with a spot size sufficient for acoustic excitation without causing damage to the sample, for example, in the range of microns to tens of microns.

[0023] As indicated in Fig. IB, a subsurface feature (e.g., void 24), generates a reflected acoustic wave 40, which returns toward the sample surface. Given the low energy level of the reflection from a submicron feature, as well as the short wavelength, such a reflected acoustic wave is not readily sensed by an acoustic transducer. However, the acoustic wave may affect the surface reflectivity and may cause surface deformation, phenomena that may be measured by measuring reflection of an optical probe pulse 42. The optical probe pulse is timed to impinge on the surface when the reflected acoustic wave reaches the surface (i.e., at a delay of 2Z / v after the pump pulse). As indicated, the wavefront of the optical pulse probe is shaped, such that concentric rings 44 of the pulse probe (shown in two dimensions in the figure) are synchronized to interact with the surface changes (i.e., reflectivity changes) imparted by the shaped acoustic wave.

[0024] As indicated in Fig. 2, the pulse probe is reflected from the sample surface as a reflected pulse probe 44, whose characteristics (e.g., reflectivity or phase) can be measured by an optical detection assembly (also referred to herein as an optical sensor).

[0025] Figs. 3A and 3B illustrate examples of optical setups for shaping the optical pump pulse 30 and the optical probe pulse 42, respectively.

[0026] Apparatus 50, shown in Fig. 3A, may be applied for shaping the optical pump pulse having a concave wavefront, that is, a wavefront of temporally delayed, concentric rings. As described above, the optical delay between different concentric rings (or “portions”) of the pulse is tailored to the speed of sound in the sample material, such that the concentric optical rings excite a converging acoustic wave to target a desired target location (e.g., a desired axial depth). To produce such a sequence of optical rings (e.g., one or more concentric portions around a central portion), the delays can be achieved by various means, such as using a fiber bundle of different lengths or a free space delay stage. In the example of Fig. 3A, the optical beam is shaped by one or more rings of mirrors 52. The rings redirect an inner portion 54 of an incoming optical beam 56 to follow a delay path 58. As indicated, the delay path is diverting away from the direct beam path and then guided back into the collimated beam.

[0027] Fig. 3B presents an example of optical beam shaping to produce a concave pulse beam. In contrast to the converging acoustic pump wave, the reflected acoustic wave starts at the subsurface reflective feature (e.g., the void) and diverges as the wave moves back to the surface, causing a temporal delay in the signal reaching the surface. To compensate for the delay, and thereby aggregate the generated signal, the probe signal may be shaped as a concave (i.e., spherical) wavefront of concentric rings, with the greatest delay being at the edge, the last position reached by the acoustic wave.

[0028] Such a sequence of concentric optical rings can be achieved by various means, as described above, such as a fiber bundle of different lengths or a free space delay stage. In the example of Fig. 3B, the optical beam is shaped by rings of mirrors 62. The rings redirect an outer portion 64 of an incoming optical beam 66 to follow a delay path 68. As indicated, the delay path is diverted to leave the direct beam path before re-entering the initial columnated beam. It should be noted that while the figure indicates a symmetric arrangement of mirrors, the mechanical arrangement for concentric delay (whether mirrors or other methods, such as fiber bundles) may be arranged asymmetrically about an axis, such as in an oblong arrangement, according to the most appropriate focus for the type of subsurface feature being investigated.

[0029] Fig. 4 illustrates a system 100 configured to generate and detect acoustic waves within a sample, where the acoustic waves are generated by shaped, ultrafast optical pulses. A sample 20 is inspected, as described above, to determine whether there is a subsurface feature 24, such as a void or other anomaly. The sample is typically mounted on a motion stage that provides alignment and scanning capability.

[0030] A laser source 102 produces an ultrashort optical pump pulse having a duration typically below one picosecond. The wavelength of the pulse may be selected below approximately 400 nanometers to ensure shallow optical absorption in materials such as silicon.

[0031] A pump-shaping assembly 104, which may be for example, the mirror assembly 50 described above, receives the output of the laser source 102 and imposes controlled optical path-length differences across concentric radial zones of the pump beam to create a concave wavefront. The shaped pump pulse is then directed to a location of the sample. Subsequently, a delayed probe pulse is directed to the same location. The delay may becaused by the laser, or lasers, 102, generating the two pulses with a timed difference between them, or by generating two pulses simultaneously, where the probe pulse is subsequently delayed by a probe pulse delay assembly 104. The delayed pulse probe is then shaped by convex wavefront shaping apparatus 108, such as by the mirror assembly 60 described above.

[0032] After wavefront shaping, and the delay of the probe pulse, each pulse follows the same propagation path toward the sample, such that both impinge the sample surface at a normal angle of incidence.

[0033] This is achieved by first reflecting the pump pulse at an angle by a mirror 120 such that the pulse is then directed to a common propagation path through a beam splitter 122. The propagation path typically includes a tube lens 124 and an objective lens 126, which focuses the beams to the desired radial dimensions of the beam spots on the sample surface.

[0034] As described above, the temporal, concave pattern produced by the pumpshaping assembly generates within the sample an acoustic wavefront having a converging profile. A subsurface feature at the focal location, such as a void or other anomaly, reflects the acoustic wave. The effect of the acoustic wave on the sample surface then affects characteristics of the reflected probe pulse. The conformance of the probe pulse wavefront to the shape of the diverging, reflected acoustic wave ensures that the wide extent of the reflected acoustic wave is sensed, thereby improving the signal-to-noise ratio. In effect, the timing of the probe pulse compensates for the temporal broadening of the diverging acoustic wave that returns from the subsurface feature.

[0035] The reflected probe pulse is then directed by an optical return path that may include, for example, a second beam splitter 130 that guides the reflected pulse to a detection assembly, such as a sensor 140 (which may include a sensor relay shown in figures described below). The sensor measures parameters such as reflectivity or phase of the reflected probe pulse. A processor 150 processes these parameters to determine the presence as well as characteristics of the subsurface feature, typically by comparing the parameters to modelled parameters or to known parameters from previously acquired measurements.

[0036] It is to be understood that embodiments of the present invention may includeshaping of only the pump pulse or only the probe pulse, such that one or the other has a relatively planar wavefront, and may also include applying different radial dimensions, such that the two wavefronts do not have the same diameters.

[0037] Figs. 5-8 are various exemplary alternative configurations of apparatus for implementing the above acoustic-optical method (i.e., configurations of the acoustic- optical microscope).

[0038] In Fig. 5, a single laser pulse is split by a polarizing beam splitter (PBS) 502, the pulse being separated into pump and probe pulses with orthogonal polarizations. The power ratio between the pulses is determined by first passing the laser pulse through a half wave plate (HWP) 504 having a polarization orientation offset from the orientation of the PBS 502.

[0039] After the pulse paths are split, the pump pulse is directed to the shaping assembly 104, for concave wave shaping, while the probe pulse is directed (for example, by a mirror 520) to the delay assembly 106, followed by the probe shaping assembly 108, for convex shaping.

[0040] After the beams are shaped and the probe pulse is delayed, their paths rejoin at a second PBS 506, which avoids the power loss of non-polarizing beam splitters. Optical elements, such as a mirror 522, have a role in directing the pump pulse to the second PBS. The second PBS is configured according to the polarization of the pump and probe pulses, so as to reflect the probe pulse to be reflected into the common propagation path, while transmitted the orthogonally polarized pump pulse into the path. (The paths can also be switched, with a corresponding switch in the shaping assemblies and in the position of the probe delay assembly.)

[0041] The common propagation path then includes a beam splitter 208 (which has an energy loss, typically of 50%), which subsequently is used to redirect the reflected pulses to the sensor. That is, after being directed toward the sample surface (e.g., by the tube and objective lenses described above) the pump pulse and subsequently the probe pulse are reflected (being normal to the surface) back through the propagation path to the beam splitter 208 (again with a 50% energy loss) to be directed to the sensor. Since both the pump and probe reflections subsequently arrive at the sensor, a sensor relay 142, together with a time-sensitive sensor 144, are used to integrate only the later-arriving probe pulse. (Sensormeasurements are then processed by processor 150 described above with respect to Fig. 4.)

[0042] Fig. 6 shows exemplary apparatus 600 of the acoustic-optical microscope of the present invention, in a configuration that exploits the polarization of pulses described above to reduce the energy losses caused by the beam splitter along the propagation paths of the pulses. After the pump and probe pulses are split by a first PBS 602, the pump pulse is shaped by the shaping assembly 104. The probe pulse is directed by a second PBS 604 to the probe delay 106 and to the probe shaping assembly 108.

[0043] The paths of the pump and probe pulses subsequently are directed (for example, by respective sets of mirrors 620 and 630) to a common propagation path. The pump pulse may also be guided to the common propagation path by transmission through, for example, both the first and second PBSs and through a beam splitter 632 that subsequently directs the reflected probe pulse to the sensor 140.

[0044] The paths of the pump and probe pulses rejoin at a third PBS 634. As with the other PBSs described above, the polarity orientation of PBS 634 is configured to reflect one pulse and to transmit the other. In the configuration shown the probe pulse is reflected into the common propagation path, while the pump pulse is transmitted into the path, due to the orthogonal polarizations of the pulses.

[0045] Subsequently, the common propagation path passes through a quarter wave plate (QWP) 640. Because the pulses traverse the QWP twice (illumination and reflection), the total effect is to impart a half wave rotation, rotating the polarization angle of both pump and probe pulses by 90 degrees. This then causes the two to separate on their return back to the third PBS 634, the pump pulse now being reflected away from the PBS, shunted away from the propagation path, while the probe pulse is transmitted through the PBS, to be guided to the sensor. The beam splitter 632, described above, directs the reflected probe pulse to the sensor, after passing through the sensor relay 142, with a 50% energy loss at the beam splitter.

[0046] Fig. 7 shows exemplary apparatus 700 of the acoustic-optical microscope, with separation of the reflected pump and probe pulse paths before the sensor achieved by a two color scheme, that is, by implementing the pump and probe pulses with different wavelengths (e.g., XI and 2). This could be achieved with externally synchronized laser sources (indicated as lasers 702a and 702b) or by frequency conversion of a commonsource. After shaping and delaying the respective pump and probe pulses as described above, their paths are combined by a dichroic mirror 704 and guided to the sample through a QWP 708, set to rotate the probe wavelength 2, such that the path of the probe pulse reflection can be split from the pump pulse at a PBS 710, to be directed to the sensor 140 (through sensor relay 142). A color filter can block the pump beam from reaching the sensor. In this scheme, there is potentially no power loss.

[0047] Fig. 8 shows exemplary apparatus 800 of the acoustic-optical microscope, whereby the reflected pump and probe pulse paths are separated before reaching the sensor achieved by using different angles of incidence. In configuration shown, the pump is directed toward the sample at a normal angle of incidence (for example, by a mirror 802), whereas the probe path is set to an oblique angle (for example, by a mirror 810). In other configurations, the roles can be swapped (as with the configurations above). As with all configurations, the paths for both the pump and the probe typically are focused toward the sample with a lens configuration such as a tube lens 124 and an objective lens 126. When the pulses do not share a common propagation path, these elements must be provided for the path of each pulse. For the apparatus 800, lenses 820 guide the illumination path of the probe pulse, and lenses 830 guide the reflection (i.e., “collection”) path.

[0048] Examples of the Invention

[0049] An example 1 of the present invention is a system for characterizing a sample that includes a pump assembly that shapes an optical pump pulse to have a temporally concave wavefront and that typically directs the shaped pump pulse to a surface of the sample so that the pulse may generate an acoustic wave that converges at a focal location. When a subsurface feature is at the focal location, a reflected acoustic wave is typically generated. The system includes a probe assembly that shapes an optical probe pulse to have a temporally spherical wavefront and that guides the shaped probe pulse to the sample surface with a synchronized delay so that the probe pulse may impinge on the surface simultaneously with the reflected acoustic wave. The system also includes a detection assembly (including an optical sensor) that receives the reflected probe pulse. The detection system includes a processor to determine whether the reflected probe pulse is indicative of a surface change, such as a change in surface shape and / or optical reflectivity, induced by an acoustic wave. According to the surface change, the processor can then determine the presence of a subsurface feature.

[0050] An example 2 of the present invention includes the features of example 1 and further includes an array of optical path length elements in the pump assembly that delays internal portions of the pump pulse relative to external portions so that the concave wavefront is produced.

[0051] An example 3 of the present invention includes the features of either one of examples 1 or 2 and includes an array of optical path length elements in the probe assembly that delays external portions of the probe pulse relative to internal portions so that the convex wavefront is produced.

[0052] An example 4 of the present invention includes the features of any one of examples 1 to 3 and further includes a configuration in which a single laser pulse is polarized and split at a polarizing beam splitter into oppositely polarized pump and probe pulses, each pulse following a respective path in which shaping occurs. The probe path may add a time delay so that the probe pulse is synchronized to impinge on the surface simultaneously with the return of a reflected acoustic wave.

[0053] An example 5 of the present invention includes the features of any one of examples 1 to 4 and includes a configuration in which the reflected probe pulse is typically directed to a detection assembly and the reflected pump pulse is typically shunted away from the detection assembly, with the routing being determined by the polarization of the pulses.

[0054] An example 6 of the present invention includes the features of any one of examples 1 to 5 and includes a configuration in which the polarized probe pulse, after being shaped and delayed, is redirected to a common optical path toward the sample together with the polarized pump pulse, the two pulses being rejoined at a polarizing beam splitter. The common optical path includes a quarter wave polarizer that typically rotates the polarization of the reflected pulses by a half turn so that, upon returning to the polarizing beam splitter, the reflected probe pulse passes toward a detection assembly and the reflected pump pulse is shunted away.

[0055] An example 7 of the present invention includes the features of any one of examples 1 to 6 and includes a time sensitive detection assembly that typically isolates probe reflections that arrive later in time than pump reflections.

[0056] An example 8 of the present invention includes the features of any one ofexamples 1 to 7 and includes a configuration in which the pump and probe optical beams are generated at different wavelengths and are separated at detection by a wavelength selective filter that typically blocks the pump wavelength while allowing the probe wavelength to be measured.

[0057] An example 9 of the present invention includes the features of any one of examples 1 to 8 and includes an angular detection scheme in which one of the optical beams is directed to the sample at normal incidence and the other is directed at an oblique incidence so that the angular separation may allow probe reflections to be collected independently of pump beam backscatter.

[0058] An example 10 of the present invention includes the features of any one of examples 1 to 9 and includes a pump pulse generated at a wavelength of 400 nanometers or less.

[0059] An example 11 of the present invention includes the features of any one of examples 1 to 10 and the duration of the pump pulse is between 100 fs and 1 ps.

[0060] An example 12 of the present invention includes the features of any one of examples 1 to 11 and includes a scanning stage that typically translates the sample relative to the pump and probe beams for spatial mapping of subsurface features across a semiconductor hybrid bond interface.

[0061] An example 13 of the present invention includes the features of any one of examples 1 to 12 and includes a subsurface feature that is a void.

[0062] An example 14 of the present invention is a method for characterizing a semiconductor sample, including directing an optical pump pulse through a pump assembly configured to optically shape the optical pump pulse to have a temporally concave wavefront, and to direct the shaped pump pulse to a surface of a sample, thereby generating, in the sample, an acoustic wave that converges at a focal location. If a subsurface feature is at the focal location, a reflected acoustic wave is generated. The method further includes directing an optical probe pulse through a probe assembly configured to optically shape the optical probe pulse to have a temporally spherical wavefront, and guiding the shaped probe pulse to the surface of the sample, with a synchronized delay, so as to impinge on the surface simultaneously with the reflected acoustic wave, if one is generated from a subsurface feature. The probe pulse is then reflected from the surface with characteristicindicative of any surface changes caused by a reflected probe pulse. The method further includes receiving, at a detection assembly, the reflected probe pulse, and, responsively, determining whether the reflected probe pulse is indicative of a subsurface feature.

[0063] An example 15 of the present invention is a non-transitory computer readable medium storing instructions for semiconductor metrology that include generating an optical pump pulse that is shaped to have a concave wavefront and guiding the shaped pump pulse to a surface of a sample so that an acoustic wave may converge at a focal location. When a subsurface feature is at the focal location, a reflected acoustic wave having a convex wavefront may be generated. The instructions include splitting the optical pump pulse to generate a probe pulse that is delayed and shaped to have a convex wavefront, guiding the shaped probe pulse to the sample surface by a synchronized delay so that it may impinge simultaneously with the reflected acoustic wave, receiving at a detection assembly a reflected probe pulse, and processing a signal indicative of the reflected probe pulse to determine whether the reflected probe pulse is indicative of surface reflectivity changes induced by a reflected acoustic wave.

[0064] With respect to the schematics described above, it is to be understood that blocks may be implemented in a different order than shown, including concurrent or reversed execution, and may also be realized by special purpose hardware or by a combination of hardware and instructions. It also is to be understood that embodiments of the invention may include a system, a method, and / or a computer program product. A computer program product may include a computer readable storage medium having instructions stored thereon for causing the control unit, or other processing elements, to carry out aspects of the invention. Moreover, any reference to a method may be applied mutatis mutandis to a system capable of executing the method or to a non-transitory computer readable medium storing instructions for implementing the method. Similarly, any reference to a system may be applied mutatis mutandis to a corresponding method executable by the system or to a non-transitory computer readable medium storing instructions executable by the system. Any reference to a non-transitory computer readable medium may likewise be applied mutatis mutandis to a method implemented by executing its stored instructions or to a system capable of executing those instructions.

[0065] Because embodiments of the present invention may be implemented using electronic components and circuits known to those skilled in the art, details have not beenexplained in any greater extent than that considered necessary for an understanding and appreciation of the underlying concepts of the present invention and in order not to obfuscate or distract from the teachings of the present invention.

[0066] It is to be understood that the processing elements described herein may include the control unit as well as other processing elements operating in common. The processing elements may include one or more processors, memory, I / O devices, and a network interface, and may execute instructions stored in non-transitory computer readable memory in order to carry out aspects of the invention.

[0067] The memory of the control unit, or of other processing elements, may include RAM, ROM, fixed memory (e.g., hard drive), removable memory, or other forms of nontransient, computer readable storage medium capable of retaining instructions for execution. Instructions stored in memory of the processing elements, may include assembler code, ISA instructions, machine instructions, microcode, firmware, state-setting data, or source / object code in one or more languages. Execution may occur entirely within a local device, partly local and partly remote, or entirely remote via a LAN, WAN, or the Internet. In some embodiments, programmable logic such as FPGAs or PLAs may execute the instructions by configuring circuitry to perform aspects of the invention.

[0068] In general, terminology used in the above description has been selected to explain the principles, technical improvements, and practical applications, and to enable those skilled in the art to practice the invention. Variations and modifications may be made without departing from the scope of the embodiments.

Claims

CLAIMS1. An acoustic-optical metrology system for characterizing a sample, comprising: a pump assembly configured to optically shape an optical pump pulse to have a temporally concave wavefront, and to direct the shaped pump pulse to a surface of a sample, thereby generating, in the sample, an acoustic wave that converges at a focal location, wherein, if a subsurface feature is at the focal location, a reflected acoustic wave is generated; a probe assembly configured to optically shape an optical probe pulse to have a temporally spherical wavefront, and to guide the shaped probe pulse to the surface of the sample, with a synchronized delay, so as to impinge on the surface simultaneously with the reflected acoustic wave, if one is generated from a subsurface feature, and to be reflected as a reflected probe pulse; and a detection assembly configured to receive the reflected probe pulse, and, responsively, to determine from probe pulse parameters whether the reflected probe pulse is indicative of a subsurface feature.

2. The system of claim 1, wherein the pump assembly comprises an array of optical path- length elements that delay internal portions of the pump pulse relative to external portions to produce the temporally concave wavefront.

3. The system of claim 1, wherein the probe assembly comprises an array of optical path- length elements that delay external portions of the probe pulse relative to internal portions to produce the temporally spherical wavefront.

4. The system of claim 1, wherein a single laser pulse is polarized and split at a polarizing beam splitter into oppositely polarized pump and probe pulses, wherein the split pulses follow separate paths that respectively shape the pump and probe pulses, and wherein the probe path adds a time delay such that the probe pulse is synchronized to impinge on the surface simultaneously with the return to the surface of a reflected acoustic wave.

5. The system of claim 3, configured to direct the reflected probe pulse to the detection assembly and to shunt the reflected pump pulse away from the detection assembly, according to the polarization of the pulses.

6. The system of claim 3, wherein the polarized probe pulse, after being shaped and delayed, is redirected to a common optical path towards the sample as the polarized pump pulse, thetwo pulses being rejoined at a polarizing beam splitter (PBS), and wherein the common optical path includes a quarter-wave polarizer to rotate polarization of the reflected pulses by a half-turn, such that upon returning to the PBS the reflected probe pulse is directed towards the detection assembly and the reflected pump pulse is shunted at the PBS away from the detection assembly.

7. The system of claim 1, wherein the detection assembly is time-sensitive and configured to isolate probe reflections arriving later in time than pump reflections.

8. The system of claim 1, wherein the pump and probe optical beams are delivered at different wavelengths and separated by a wavelength-selective filter at the detection assembly.

9. The system of claim 1, wherein the pump and probe beams follow an angular detection scheme in which one of the beams is directed to the sample at normal incidence and the other at an oblique incidence such that the angular separation allows the probe reflection to be collected independently of pump-beam backscatter.

10. The system of claim 1, wherein the pump pulse is generated at a wavelength of 400 nm or less.

11. The system of claim 1, wherein the pump pulse is generated to have a duration of less than 1 ps.

12. The system of claim 1, further comprising a scanning stage configured to translate the sample relative to the pump and probe beams for spatial mapping of subsurface features across a semiconductor hybrid-bond interface.

13. The system of claim 1, wherein the focal location is set to a hybrid bonding interface between two layers of a semiconductor wafer.

14. The system of claim 13, wherein the subsurface feature at the focal location is a void.

15. The system of claim 14, wherein the void has a diameter of one micron or less.

16. A method for characterizing a semiconductor sample, comprising: directing an optical pump pulse through a pump assembly configured to optically shape the optical pump pulse to have a temporally concave wavefront, and to direct the shaped pump pulse to a surface of a sample, thereby generating, in the sample, an acousticwave that converges at a focal location, wherein, if a subsurface feature is at the focal location, a reflected acoustic wave is generated; directing an optical probe pulse through a probe assembly configured to optically shape the optical probe pulse to have a temporally spherical wavefront, and guiding the shaped probe pulse to the surface of the sample, with a synchronized delay, so as to impinge on the surface simultaneously with the reflected acoustic wave, if one is generated from a subsurface feature, and to be reflected as a reflected probe pulse; and receiving, at a detection assembly, the reflected probe pulse, and, responsively, determining from probe pulse parameters whether the reflected probe pulse is indicative of a subsurface feature.

17. The method of claim 16, further comprising setting the focal location to a hybrid bonding interface between two layers of a semiconductor wafer.

18. The method of claim 17, wherein the subsurface feature at the focal location is a void.

19. The method of claim 16, wherein the pump pulse is generated at a wavelength of 400 nm or less and with a duration of 1 ps or less.

20. A non-transitory computer readable medium that stores instructions for semiconductor metrology, for characterizing a semiconductor sample, the instructions configured to execute steps of: a) generating an optical pump pulse, wherein the optical pump pulse is shaped to have a concave wavefront, wherein the shaped pump pulse is guided to a surface of a sample, to generate, in the sample, an acoustic wave that converges at a focal location, wherein, when a subsurface feature is at the focal location, a reflected acoustic wave is generated, having a spherical acoustic wavefront, wherein the optical pump pulse is beam split to generate a probe pulse that is delayed and shaped to have a spherical wavefront, wherein the shaped probe pulse is guided to the surface of the sample, by a synchronized delay, so as to impinge on the surface simultaneously with the return to the surface of the reflected acoustic wave, when there is a subsurface feature at the focal location, and wherein the reflected probe pulse is received at a detection assembly; andb) processing a signal from the detection assembly indicative of the reflected probe pulse, to determine whether the reflected probe pulse is indicative of surface changes induced by a reflected acoustic wave from a subsurface feature at the focal location.

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