Wide bandgap semiconductor defect characterization by UV four-wave-mixing imaging

Abstract

An optical microscope includes a light source emitting coherent laser light having a wavelength less than 550 nm; an optical system; and an electronic module. The microscope generates two copies of the laser light, modulates a phase or amplitude of the first copy with respect to the second copy, and recombines the first copy with the second copy to form an excitation beam. The microscope focuses the excitation beam to a position on a material and measures a response beam from material with an optical detector. The response beam includes transmitted and reflected light from the material. The microscope extracts, from the response beam a response signal representing a nonlinear optical response of the material indicating a characteristic of the material. The optical detector is a semiconductor photodiode. A type of the semiconductor photodiode is selected based on a photon energy of the laser light beam.

Description

WIDE BANDGAP SEMICONDUCTOR DEFECT CHARACTERIZATION BY UV FOUR-WAVE-MIXING IMAGINGCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present patent application claims priority to U.S. provisional patent application 63,537,370 filed on September 8, 2023, which is incorporated herein by reference in its entirety.GOVERNMENT LICENSE RIGHTS

[0002] This invention was made with Government support under grant number 2208201 awarded by the National Science Foundation. The Government has certain rights to this invention.BACKGROUND

[0003] Wide bandgap, compound semiconductor materials are preferred over silicon in applications requiring higher breakdown voltages and currents, higher charge densities, and higher charge mobilities, including high-power electronics and electric vehicles. In-demand materials include silicon carbide (SiC) and gallium nitride (GaN). One of the challenges in the current production is that the metrology tools for measuring defects in SiC and GaN do not offer sufficient sensitivity or specificity toward all killer defects and suffer from low throughput.

[0004] Defects in SiC wafers can be found in both the substrate and in the few microns thick epitaxial (epi) layer deposited on top of the substrate. Substrate defects are inherited from the seed and the bulk from which the substrate is cut or from processing. Defects in the substrate subsequently lead to defects in the epi layer, where the growth of the epi layer is affected by defects present in the substrate. Defect characterization of wafers has two goals: to determine 1) the location and 2) the type of defect to distinguish between non-killer and killer defects that detrimentally affect device performance.

[0005] The current state of the art to optically detect defects in SiC wafers is a combination of surface white light scattering, differential interference contrast (DIC), visible photoluminescence (PL), and ultraviolet (UV) PL. Surface white light scattering uses a white light or laser source and a detector that is intentionally not illuminated by the laser light reflection unless it is scattered at a defect, in which case some of the scattered light reaches the detector. DIC microscopy uses the interference of light that is offset by a small amount in the shear direction to provide surfaceshear direction to provide surface contrast. Both of these imaging methods detect morphological inhomogeneity in the surface. The techniques have very high throughput but are only sensitive to some of the defects at the surface of the epi layer.

[0006] In contrast, PL has a lower throughput but higher sensitivity to some defects. PL is a light-matter interaction in which light is first absorbed by the material, and, following relaxation processes, low er energy light is emitted by the material. Because defects can alter those relaxation processes through changes in the band structure, defects can either glow or darken in PL. Unfortunately, PL is a relatively inefficient process as the emitted photons are emitted in every direction. Due to the omnidirectional emission, PL either requires high numerical aperture optics, or a longer integration time, either of which limits the throughput. PL throughput can be further delayed by the fundamental decay time of the optical emission. For instance, if the decay time is much longer than the acquisition time at a single pixel, the collection efficiency will be reduced proportionally. Due to its sensitivity to band structure changes, PL is good at detecting decorated basal plane defects and stacking faults, but scratches and threading dislocations are not as easily accessible.SUMMARY

[0007] Four-wave-mi.xmg systems utilizing ultrafast laser imaging to detect defects in semiconductors are known. For example, international patent application PCT / US2022 / 025503 filed by Applicant on April 20, 2022, the subject matter of which is incorporated by reference to the full extent allowable by law in each region in which this patent application is pursued, discloses a system for detecting defects in semiconductors based on exciting and measuring a nonlinear response of the material with modulated laser beams. This system overcomes many of the w eaknesses of previous systems, enabling fast scanning of a material of interest, while enabling detection and characterization of defects. Four- w ave-mixing systems have, however, proven difficult to apply for detecting and characterizing defects in wide bandgap semiconductors, which requires the use of near ultraviolet range laser light.

[0008] According to the disclosure, an optical microscope detects defects in wade bandgap semiconductor materials by using four-wave mixing and laser light with a w avelength in the near ultraviolet range. The wavelength of the laser light is selected based on physical characteristics of the material of interest. The system includes a semiconductor photodiode with a bandgap energy greater than 1.12eV as the photodetector for the optical system. Other system elements, such as the acousto-optic modulators (AOMs) and optical lenses are adaptedto process the shorter wavelengths and related high energy levels of laser pulses in the near ultraviolet range.

[0009] In an example, the techniques described herein relate to an optical microscope including: a light source emitting coherent laser light beam having a wavelength less than 550 nm: an optical system; and an electronic module. The optical microscope is configured to generate two copies of the coherent laser light beam, the two copies including a first copy and a second copy; modulate a phase or amplitude of the first copy of the coherent laser light beam with respect to the second copy of the coherent laser light beam; recombine the modulated first copy of the coherent laser light beam with the second copy of the coherent laser light beam to form an excitation beam; focus the excitation beam to a laser spot position on a material of interest; and measure a response light beam received from the material of interest based on the excitation beam with an optical detector, wherein the response light beam includes transmitted light from the material of interest and can further include reflected light from the material of interest. The optical microscope is further configured to extract from the response light beam by the electronic module, a response signal representing a nonlinear optical response of the material of interest, wherein the nonlinear optical response indicates a characteristic of the material of interest. The optical detector is a semiconductor photodiode; and a type of the semiconductor photodiode is selected based on a photon energy of the coherent laser light beam.

[0010] In some aspects, the techniques described herein relate to an optical microscope, wherein the optical detector is a photodiode including a semiconductor with a bandgap greater than 1.12eV.

[0011] In some aspects, the techniques described herein relate to an optical microscope, wherein the optical detector is a photodiode including a semiconductor with a bandgap greater than one half times the photon energy of the coherent laser light beam.

[0012] In some aspects, the techniques described herein relate to an optical microscope, wherein the optical detector is a photodiode including a compound semiconductor.

[0013] In some aspects, the techniques described herein relate to an optical microscope, wherein the optical detector is a photodiode including gallium phosphide.

[0014] In some aspects, the techniques described herein relate to an optical microscope, wherein the wavelength of the coherent laser light beam is selected based on a physical characteristic of the material of interest.

[0015] In some aspects, the techniques described herein relate to an optical microscope, wherein the physical characteristic is a spectral feature of interest.

[0016] In some aspects, the techniques described herein relate to an optical microscope, wherein the physical characteristic of the material of interest is an energy level of a bandgap of the material of interest.

[0017] In some aspects, the techniques described herein relate to an optical microscope, wherein the physical characteristic of the material is an ultraviolet absorption resonance.

[0018] In some aspects, the techniques described herein relate to an optical microscope, wherein the light source includes a femtosecond laser.

[0019] In some aspects, the techniques described herein relate to an optical microscope, wherein the femtosecond laser is a titanium sapphire femtosecond laser.

[0020] In some aspects, the techniques described herein relate to an optical microscope, wherein the light source includes a frequency multiplying element.

[0021] In some aspects, the techniques described herein relate to an optical microscope, wherein the frequency multiplying element is a Beta Barium Borate nonlinear optical cry stal.

[0022] In some aspects, the techniques described herein relate to an optical microscope, wherein the light source emits pulses with a duration less than 1 picosecond.

[0023] In some aspects, the techniques described herein relate to an optical microscope, wherein the light source emits pulses with a duration less than 100 femtoseconds.

[0024] In some aspects, the techniques described herein relate to an optical microscope, wherein the optical system includes an acousto-optic modulator to modulate the phase or the amplitude of the first copy of the coherent laser light beam with respect to the second copy of the coherent laser light beam.

[0025] In some aspects, the techniques described herein relate to an optical microscope, wherein the acousto-optic modulator includes quartz.

[0026] In some aspects, the techniques described herein relate to an optical microscope, wherein the optical system includes one or more UV-enhanced aluminum mirrors.

[0027] In some aspects, the techniques described herein relate to an optical microscope, wherein the optical system includes one or more dielectric ultrafast mirrors.

[0028] In some aspects, the techniques described herein relate to an optical microscope, wherein the optical microscope is configured to modulate a phase or frequency of the second copy of the coherent laser light source.

[0029] In some aspects, the techniques described herein relate to an optical microscope, wherein the optical microscope is configured to: modulate the first copy of the coherent laser light beam with a first frequency fO and a second frequency fl; and modulate the second copy of the coherent laser light beam with a third frequency f2 and a fourth frequency 1'3.

[0030] In some aspects, the techniques described herein relate to an optical microscope, wherein a repetition rate of the light source is fiep, the first frequency fO is 0, the second frequency fl is frep / 8, the third frequency f2 is 5 / 12 frepand the fourth frequency 13 is frep / 2.

[0031] In some aspects, the techniques described herein relate to an optical microscope, wherein the nonlinear optical response is a third-order nonlinear optical response.

[0032] In some aspects, the techniques described herein relate to an optical microscope, wherein the nonlinear optical response is a resonant four-wave-mixing signal.

[0033] In some aspects, the techniques described herein relate to an optical microscope, wherein extracting the response signal includes application of a lock-in detection.

[0034] In some aspects, the techniques described herein relate to an optical microscope, further configured, after completing the measurement at the laser spot position, to move the focus to a subsequent laser spot position.

[0035] In some aspects, the techniques described herein relate to an optical microscope, further including a scan mechanism, wherein moving the laser spot position to the subsequent laser spot position is performed by the scan mechanism and the optical system.

[0036] In some aspects, the techniques described herein relate to an optical microscope, wherein moving the laser spot position relative to the material includes moving a stage onto which the material is mounted rapidly along a first (X) axis while a galvanometer mirror included in the optical system scans along a perpendicular (Y) axis.

[0037] In some aspects, the techniques described herein relate to an optical microscope, configured to: measure multiple laser spot positions on the material via the scan mechanism and the optical system; and generate a two-dimensional image based on the measurements performed at each of the laser spot positions.

[0038] In some aspects, the techniques described herein relate to an optical microscope, wherein the two-dimensional image is used to identify a location of a defect in the material of interest.

[0039] In some aspects, the techniques described herein relate to an optical microscope, wherein the two-dimensional image is used to identify' a type of a defect in the material of interest.

[0040] In some aspects, the techniques described herein relate to an optical microscope, wherein a linear resonant optical response is measured simultaneously with the nonlinear optical response.

[0041] In an example, the techniques described herein relate to a method including: emitting, by a laser light source, a coherent laser light beam having a wavelength less than 550 nm; generating two copies of the coherent laser light beam, the two copies including a first copy and a second copy; modulating a phase or amplitude modulation of the first copy of the coherent laser light beam with respect to the second copy of the coherent laser light beam; recombining the modulated first copy of the coherent laser light beam with the second copy of the coherent laser light beam to form an excitation beam; focusing the excitation beam on a laser spot position on a material of interest; and measuring a response light beam received from the material of interest based on the excitation beam with an optical detector, wherein the response light beam includes transmitted light from the material of interest and can further include reflected light from the material of interest. The method further includes extracting a nonlinear optical response signal indicating a characteristic of the material of interest from the response light beam. The optical detector is a semiconductor photodiode, and a type of the semiconductor photodiode is selected based on a photon energy of the coherent laser light beam.

[0042] In some aspects, the techniques described herein relate to a method, wherein extracting the nonlinear optical response signal is performed by a photodiode including a semiconductor having a bandgap greater than 1. 12eV.

[0043] In some aspects, the techniques described herein relate to a method, wherein extracting the nonlinear optical response signal is performed by a photodiode having a bandgap greater than one half times a photon energy of the coherent laser light beam.

[0044] In some aspects, the techniques described herein relate to a method, wherein extracting the nonlinear response signal is performed by a photodiode detector including gallium phosphide.

[0045] In some aspects, the techniques described herein relate to a method, wherein emitting the coherent laser light beam having the wavelength less than 550 nm includes: generating laser light with a laser; and doubling a frequency of the laser light with a nonlinear optical crystal.

[0046] In some aspects, the techniques described herein relate to a method, wherein a wavelength of the coherent laser light beam is selected based on a physical characteristic of the material of interest.

[0047] In some aspects, the techniques described herein relate to a method, wherein the physical characteristic is a spectral feature of interest.

[0048] In some aspects, the techniques described herein relate to a method, wherein the physical characteristic of the material of interest is an energy’ level of a bandgap of the material of interest.

[0049] In some aspects, the techniques described herein relate to a method, wherein the physical characteristic of the material is an ultraviolet absorption resonance.

[0050] In some aspects, the techniques described herein relate to a method, wherein the coherent laser light beam includes pulses less than 1 picosecond in duration.

[0051] In some aspects, the techniques described herein relate to a method, wherein the coherent laser light beam includes pulses less than 100 femtoseconds in duration.

[0052] In some aspects, the techniques described herein relate to a method, wherein modulating the phase or the amplitude of the first copy of the coherent laser light with respect to the second copy of the coherent laser light beam is performed by a quartz acousto-optic modulator.

[0053] In some aspects, the techniques described herein relate to a method further including: scanning the material of interest to measure a plurality' of laser spot positions; and generating a two-dimensional image based on the measurements at the respective laser spot positions.

[0054] In some aspects, the techniques described herein relate to a method, wherein scanning the material of interest includes moving a stage onto which the material is mounted rapidly along a first (X) axis while a galvanometer mirror scans along a perpendicular (Y) axis.

[0055] In some aspects, the techniques described herein relate to a method, further including: identifying a location of a defect in the material of interest based on the two- dimensional image.

[0056] In some aspects, the techniques described herein relate to a method, further including: identifying a type of a defect in the material of interest based on the two-dimensional image.

[0057] In some aspects, the techniques described herein relate to a method, wherein measuring the response light beam includes measuring a linear resonant response of the material of interest.BRIEF DESCRIPTION OF THE DRAWINGS

[0058] Figure 1 is a diagram of an example optical microscope for detecting characteristics of a material.

[0059] Figure 2 is a diagram of a light source including a frequency multiplier for the optical microscope of figure 1.

[0060] Figure 3 is a schematic diagram of an example signal processing subcircuit for processing a response signal in the optical microscope of figure 1.

[0061] Figure 4 is a schematic diagram of an example imaging setup for the optical microscope of figure 1.

[0062] Figure 5A shows plots in the frequency domain of example optical signals in the optical microscope of figure 1.

[0063] Figure 5B shows plots in the frequency domain of example radio frequency signals it the optical microscope of figure 1.

[0064] Figures 6A - 6C show example images of a defect under different conditions.

[0065] Figures 7A - 7B show example images of a defect and the effect of delaying a probe signal.DETAILED DESCRIPTION

[0066] A femtosecond laser is used to induce a nonlinear response from a material, typically near its bandgap energy or other spectral feature. The nonlinear response described here is a four- wave mixing (FWM) signal, caused by the absorption of photons near the spectral feature. Because FWM requires absorption of photons by the material, the femtosecond laser photon energy is ty pically tuned within 100 meV of the semiconductor bandgap, when applied to semiconductors. Because defects alter band structure in semiconductors, changing transition dipole moments and resonance energies, the FWM signal is strongly sensitive to those defects.

[0067] When compared to linear absorption and reflection, FWM reveals certain defective features with a higher signal-to-noise ratio (SNR). Like PL, FWM processes are sensitive to band-structure changes. However, FWM has a couple of advantages over PL: 1) the FWMsignal is coherent and can hence be collected efficiently by lower numerical aperture optics, and 2) the FWM response is instantaneous.

[0068] The device used in this example can also be used to simultaneously measure resonant linear reflection, which provides complementary information to the FWM signal. The resonant linear reflection signal is just the magnitude of the laser light reflected off the sample, which is typically six or more orders of magnitude greater than the FWM signal. This method is similar to conventional microscopy, except that the laser light is specifically tuned to be resonant with a spectral feature of interest in the sample. Therefore, the resonant linear reflection has two advantages compared to conventional microscopy: 1) sitting at the band edge makes the resonant linear reflection more sensitive to changes in the band structure, and 2) resonant linear reflection can detect features below the wafer surface if the focus of the imaging setup is set accordingly. The linear signal, when measured in conjunction with the FWM signal, is useful to distinguish defects that are otherwise indistinguishable in FWM.

[0069] Figure 1 illustrates an example system 100 for measuring a material of interest 140, wherein the material of interest 140 is a wide bandgap semiconductor. Examples of wide bandgap semiconductor materials include silicon carbide (SiC) and gallium nitride (GaN).

[0070] The example system 100 can include: a computing device 170; a light source 104 producing a coherent laser light beam SI 12; an interferometer 105 including a pump optical path 106 and a probe optical path 107; first and second modulators (MOD) 114, 116 included respectively in the pump optical path 106 and the probe optical path 107 for modulating the coherent laser light beam S 112 to generate a pump beam S 126 and a probe beam S 128; a delay element 118 included in the probe optical path 107 for potentially adjusting a timing of the probe beam S128 relative to the pump beam S126; and an electronic circuit 160.

[0071] The example system 100 further includes abeam splitter 130 to colinearly combine the pump beam S126 and probe beam SI 28 and generate a modulated light beam SI 32, a scan mechanism 154 for directing the modulated light beam SI 32 to a location on a material of interest 140, a beam splitter 134 to direct a response beam S142+ S133 from the material of interest to a photodetector 144. The photodetector 144 converts the response beams S142 and SI 33 from the material 140 to an electrical signal SI 46.

[0072] The electronic circuit 160 includes control electronics 166 to control some operations of system 100, an RF generation electronics 162 that generates electrical signals SI 22 and SI 24 used to modulate the coherent laser light beam S 112 to generate the pump beamS126 and probe beam S128, and signal processing electronics 164 to extract a signal of interest from the electrical signal SI 46 received from the material of interest 140.

[0073] To generate the FWM response to a material 140 having a wide bandgap. a light source 104 with a high peak intensity and a wavelength in the near ultraviolet range is needed. With reference to figure 2, the light source 104 includes a laser source 201 and a frequency multiplier 202. The frequency multiplier can be, e.g., a crystal that doubles or quadruples the energy of the photons emitted by the laser source 201. In an example, the laser can be a titanium sapphire femtosecond laser (Chameleon Vision S by Coherent) that is tunable from 680 to 1050 nm and the frequency can be doubled using a Beta Barium Borate nonlinear optical crystal (for example, HarmoniXX SHG by Angewandte Physik und Elektronik GmbH (APE)). A wavelength of the coherent laser light beam SI 12 can be selected based on a physical characteristic of the material of interest 140. For example, the coherent laser light beam SI 12 can be optimized by tuning the doubled light to a wavelength having a photon energy near the bandgap energy level or other spectral feature of the material of interest 140. As an example, for defect characterization of SiC, where the bandgap energy of SiC is 3.26eV (379 nm wavelength), the wavelength of the coherent laser light beam SI 12 can be tuned to roughly 390 nm.

[0074] The coherent laser light beam S 112 may typically have a repetition frequency (frep) in a range from 100 kilohertz (kHz) to 1 gigahertz (GHz). In an example, the repetition rate is 80 megahertz (MHz) such that a time between pulses is 12.5 nanoseconds (ns). The pulses generated by the ultrafast laser typically have a duration of less than 1 picosecond (ps), for example, 75 femtoseconds (fs). For FWM scanning with near UV light, a wavelength of the coherent laser light beam SI 12 is typically in a range of 350 to 530 nanometers (nm).

[0075] Returning to figure 1, the system 100 generates two copies of the coherent laser light beam 112, a pump beam S126 and a probe beam S128. The coherent laser light beam SI 12 is input into the interferometer 105 comprising two optical paths, the pump optical path106 to generate the pump beam SI 26 and the probe optical path 107 to generate the probe beam S128. The pump optical path 106 includes the modulator (MOD) 114. The probe optical path107 includes the delay element 118 and the second MOD 116. The pump beam SI 26 and probe beam SI 28 are colinearly combined by the beam splitter 130.

[0076] The first MOD 1 14 modulates the coherent laser light beam S 112 based on a signal S122 received from the RF generation electronics 162. In an example, the signal S122 is an electrical signal including two frequency components, fo and fi. By applying the two drivingfrequencies to first MOD 114, a portion of the coherent laser light beam SI 12 experiences diffraction by one frequency, fo, and another portion experiences diffraction by the other frequency, fi. The resulting pump beam S 126 is the sum of these two frequency -shifted beams, the amplitude of which oscillates at a difference frequency fi-fo. The pump beam SI 26 can therefore be understood to be an amplitude modulation of the coherent laser light beam SI 12.

[0077] In an example, the probe beam path 107 of the interferometer 105 includes a delay element 118 followed by the second MOD 116.

[0078] The delay element 118 can be used to adjust to a delay between the pump beam S 126 and the probe beam SI 28 to be equal or to create a temporal delay between the two pulses. The delay element 118 receives the coherent laser light beam S 112 and outputs a version of the coherent laser light beam S112D that can be delayed by an amount of time relative to the coherent laser light beam SI 12, depending on a setting of the delay element 118.

[0079] In an example, the delay element 118 can include a retroreflector mounted onto a mechanical delay stage with an adjustable position that allows for adjusting a length of an optical path and therefore a transit time of the coherent laser light beam SI 12 being reflected by the retroreflector from the light beam source 104 to the light beam destination, second MOD 116. The adjustable delay element 118 makes it possible to adjust a timing of the probe beam S 128 reaching the beam splitter 130 relative to the timing of the pump beam S126 reaching the beam splitter 130.

[0080] MOD 116 receives the coherent laser light beam S112D from the delay element 118 and modulates the coherent laser light beam S 112D based on the electrical signal S 124 to generate the probe beam S128. In an example, the electrical signal S124 includes two frequency components, fz and fi. By applying the two driving frequencies to second MOD 116, a portion of the coherent laser light beam SI 12D experiences diffraction by the frequency fz and another portion experiences diffraction by the other frequency fi. The resulting probe beam S 126 is the sum of these two frequency-shifted beams, the amplitude of which oscillate at a difference frequency of fi-fi.

[0081] The pump beam SI 26 and probe beam SI 28 are subsequently recombined collinearly by beam splitter 130. meaning they are spatially overlapped and propagating in the same direction to generate an excitation beam SI 32. The excitation beam SI 32 is directed to the material 140 via a lens 138. The material 140 can also be referred to as the material of interest 140 herein.

[0082] With continued reference to figure 1, the first and second MOD 114, 116 are devices that can modulate a received optical signal based on a frequency, phase, and amplitude of a received radio frequency (RF) signal. For operation with near ultraviolet laser light, example types of AOMs may be TeO2 or Quartz (fused silica). TeO2 is very efficient, but typically shows high dispersion. Quartz is less efficient than TeO2, but dispersion of Quartz is lower than TeO2. An example of a commercially available acousto-optic modulator is CQM- 120-8-BC manufactured by Brimrose Corporation of America at address 19 Loveton Circle, Hunt Valley Loveton Center, Baltimore. Maryland 21152-9201, USA.

[0083] Alternatively, the first and second MOD 114, 116 may be electro-optic modulators or mechanical intensity modulators such as choppers.

[0084] The lens 138 receives the excitation beam S132, after it passes, through the beam splitter 134, and directs the excitation beam S132 to a location on the material 140. In a nonlinear interaction between the excitation beam SI 32 and the material 140, the material 140 generates a nonlinear response beam SI 42. All spectral components in the nonlinear response beam S142 are within 1 THz of spectral components of the excitation beam SI 32. The nonlinear optical processes create light with new wavelengths, but the nonlinear signals that are nearly degenerate, i.e.. having nearly the same wavelength as the excitation beam S132. cannot be isolated using an optical spectral filter. Because an optical spectral filter cannot be used to isolate the nonlinear signal, electrical filtering such as the technique described herein is used. The material 140 additionally emits and / or reflects a linear response beam S133 in response to the excitation beam S 132. The linear response beam SI 33 includes the same spectral components as the excitation beam SI 32.

[0085] The photodetector 144, which can be a photodiode, detects the nonlinear response beam SI 42 along with the linear response beam SI 33 and generates an electrical signal SI 46 based thereon. The photodetector output signal SI 46 is input to the signal processing electronics 164.

[0086] For use with light having wavelengths in the near ultraviolet range, the photodetector 144 is made using a semiconductor with a wide bandgap compared to silicon, which is traditionally used. As an example, the photodetector 144 can be a compound semiconductor such as a gallium phosphide detector with a bandgap of 2.24 eV. As further discussed below, using a photodiode with a bandgap greater than half of the photon energy of the laser light source 104 suppresses a nonlinear response from the photodiode, thereby reducing artifacts in the signal S 146 and allowing the extraction of a signal of interest from thesignal SI 46. The signal of interest can be a signal having characteristics of the nonlinear response beam S142 received by the photodetector 144 from the material of interest 140.

[0087] As noted above, the electronic circuit 160 can include an RF generation electronics 162, a signal processing electronics 164 and a control electronics 166. The electronic circuit 160 includes electronic components such as resistors, capacitors, inductors, and transistors. It further includes logic elements such as logic gates, flip-flops, and registers. Still further, the electronic circuit can include one or more processors and one or more data storage elements. In an example, the electronic circuit 160 can include a storage element that stores instructions for a processor for setting up, adjusting, or performing operations with other elements in the system 100 and can also store collected data from the material 140. The electronic circuit can be or include one or more FPGAs, integrated circuits and / or circuits on printed circuit boards or substrates such as ceramic substrates.

[0088] The RF generation electronics 162 generates the signal SI 22 to control the modulation of the pump beam S126, and the signal S124 to control the modulation of the probe beam S128. As described above, the signal S122 can include two frequency components fo and fi. The RF generation electronics 162 may, for example, generate the two signals at two frequencies fo and fi. and combine the two signals using an adder, to generate the signal SI 22. Similarly, the RF generation electronics may generate two signals at two frequencies fz and ft. and combine the tw o signals using an adder, to generate the signal S124.

[0089] The RF generation electronics 162 receives as an input signal Si l l, hich is a signal including the repetition rate frepof the pulses of the coherent laser light beam SI 12 generated by the UV light source 104. The RF generation electronics 162 can use the signal Si l l to synchronize the clock of the RF generation electronics 162 with the pulses generated by the UV light source 104. Synchronizing the clock of the RF generation electronics 162, and accordingly, the signals S122 and S124, enables the system to operate at the high data acquisition rates necessary for processing and extracting a FWM signal of interest from the response signal S146 received from the photodetector 144. As discussed below, in an example, the modulation frequencies of the pump beam SI 26 and the probe beam SI 28 can be set to be integer multiples of the data acquisition rate, and to integer fractions of the repetition rate of the laser pulses frep. allowing for effective extraction of the signal of interest (signal with values of characteristics of nonlinear response beam 142) from the electrical signal S146 received from the photodetector 144.

[0090] With reference to figure 3, the signal processing electronics 164 receives and processes the output signal S146 from the photodetector 144 to determine values of characteristics of the nonlinear response beam S142 included in the output signal S 146. The signal processing electronics 164 can discriminate the photodiode output signal S146 to determine values of characteristics of the nonlinear response beam S 142. The signal processing electronics 164 can further output the values of the characteristics of the nonlinear response beam SI 42 as digital values to a computing device such as the computing device 170 or a display or output the values as an analog output signal.

[0091] As shown in figure 3, a difference generator 302 in the signal processing electronics 164 receives the modulation signal SI 22 and modulation signal SI 24 as inputs and generates a difference signal S304. In an example, the difference generator 302 can receive the modulation signal S122 and modulation signal S124 as analog signals. In another example, the difference generator 302 can receive digital tuning words representing the frequencies used to generate the modulation signal SI 22 and modulation signal SI 24. The difference signal S304 can be referred to herein as the demodulation signal S304.

[0092] As a next step, a lock-in-type detector 306 in the signal processing electronics 164 receives as an input the photodetector output signal S146 and the demodulation signal S304. In this example the lock-in-type detector 306 includes a mixer 308 and a low-pass filter 312. The mixer 308 mixes the photodetector output signal S146 with the demodulation signal S304. The output of the mixer 308 is a signal S310. The signal S310 includes signals at multiple frequencies based on the frequencies in the photodetector output signal SI 46 and the demodulator signal S304, including a signal at 0 Hz (zero Hertz). The signals at the multiple frequencies (excluding the signal at 0 Hz) include demodulations of the modulation frequencies of the pump signal SI 26 and the probe signal SI 28. The signal at 0 Hz includes values of characteristics of the nonlinear response beam S142. In an example, an amplitude of the signal at 0 Hz is proportional to the amplitude of the nonlinear response beam SI 42.

[0093] The low-pass filter 312 filters the signal S310 to discriminate a portion of the electrical signal SI 46 including the value or values proportional to the characteristics of the nonlinear response beam SI 42 from other components of the electrical signal S146 and generates a signal S314. The resulting signal S314 includes at least one characteristic of the nonlinear response beam S142. In an example, the resulting signal S314 includes an amplitude of the nonlinear response beam SI 42.

[0094] In an example, the low-pass filter 312 is a constant- weighted moving average filter, in which case the lock-in-type detector 306 is referred to as a box lock-in detector 306. The moving average filter is characterized in that it performs a moving average of a fixed number of samples in the time domain and having one or more notches in a transfer function of the filter 312 in the frequency domain. The moving average filter is further characterized by a wait time, which is equal to the number of samples included in the moving average multiplied by the period of a sample.

[0095] FWM is a nonlinear response and has a unique modulation frequency that depends on both the pump and probe modulation frequencies. Using lock-in detection, the signal processing electronics 164 isolates the FWM signal from all other signals impinging the photodetector 144. Specifically, lock-in detection uses a finite impulse response (FIR) filter to isolate the FWM signal at a high data acquisition rate. Notches in the response curve of the FIR align with and suppress signals other than the FWM signal.

[0096] A control electronics 166 can control operation of the system 100. The control electronics 166 can, for example, select the frequencies fo and fi used to generate modulation signal SI 22, and the frequencies fz and h used to generate modulation signal SI 24 by the RF generation electronics 162. The control electronics 166 can further select parameters for the delay element 118. The control electronics 166 can further select parameters for signal processing performed in the signal processing electronics 164 such as the ty pe of filter to be used, the coefficients of the filter, and the time constants of the filter, which determine the pixel wait time. The control electronics 166 can further control parameters such as the step vector (length and direction) and step timing of scanning performed by the scan mechanism 154. Still further, the control electronics 166 can control outputting of data collected by the signal processing electronics 164 during measurement of the material 140.

[0097] As described above, frequency shifts applied as modulations to the pump beam S126 and probe beam S128 are imparted onto the response beam S33 by the material 140. In an example, the control electronics 166 can select the frequencies generated by the RF generation electronics 162 and parameters of filtering performed in the signal processing electronics 164 such that these frequency shifts of the pump beam S126 and probe beam SI 28 are suppressed by notches in the transfer function of the filtering applied by the signal processing electronics 164. The signal processing electronics 164 can further include a port 165 for outputting data from the signal processing electronics 164 to devices such as the computing device 170, a display, or a measurement device such as an oscilloscope. The port165 can include one or more electrical connections to support different digital bus configurations or the outputting of one or more analog signals.

[0098] The system 100 may further include a computing device 170. The computing device 170 may be laptop computer, desktop computer, mobile telephone, etc. The computing device 170 may include one or more processors and one or more memories and may be programmed to receive digital communications, for example according to a wireless standard such as IEEE 802. 11, Bluetooth, or via wired communications such as a Universal Serial Bus (USB). Ethernet, or a Peripheral Component Interconnect (PCI).

[0099] The computing device 170 may be programmed to receive an output SI 65 from the signal processing electronics 164, wherein the output can be digital data indicating a value proportional to a characteristic of the nonlinear response beam SI 42 for locations on the material 140. The computing device 170 may store the value together with its respective location on the material 140. The computing device 170 may further be programmed to present the data graphically, for example on a display, such that characteristics of the material at locations, for example defects, can be identified.

[0100] Figure 4 is a schematic diagram of an imaging setup 400 including a galvo mirror 410, followed by a scan lens 420. a tube lens 430, and a microscope objective 440. An excitation beam S412 impinges on the galvo mirror 410, is passed through the scan lens 420, the tube lens 430 and the microscope objective 440. The microscope objective 440 focuses the beam down to a 0.2-10 micron-sized diameter spot on a wafer 444. The wafer 444 generates a response beam S446. which can include both linear and nonlinear components. The microscope objective 440 collects the response beam S446 and passes it back through the imaging setup 400 which directs the response beam S446 to a photodetector (not shown). An aperture, or spatial mode filter, can be used in front of the photodetector to reduce contributions from depths of the material outside the laser spot, including reflections from the back surface of the wafer.

[0101] To generate two-dimensional images of the response signal from the wafer 444, a wafer stage 450 onto which the wafer 444 is mounted rapidly moves along a direction of an (X) axis while the galvanometer mirror scans a perpendicular direction of a (Y) axis. After going through one motion, such as a full line along the (X) axis, the wafer stage 450 is offset along the direction of the perpendicular (Y) axis by a width of the galvo field of view and moves back along its original direction of the (X) axis. In this way the scanner measures the sample in rows where the row height is the microscope objective field-of-view (width of the galvo field) scanned out by the 1-D galvanometer mirror.

[0102] In an example, the imaging setup 400 can be used together with the example system 100 as shown in figure 1. In this case, the excitation beam S412 of the imaging setup can be the excitation beam SI 32 of figure 1, the wafer 444 of the imaging setup corresponds to the material of interest 140 of figure 1, the microscope objective corresponds to the lens 138 of figure 1, the Galvo mirror 410, scan lens 420, tube lens 430 and wafer stage 450 correspond to the scan mechanism 154, and the response beam S452 corresponds to the combination of signals SI 33 and S142 as shown in figure 1.

[0103] FWM is a third-order nonlinear optical response, and it is the primary nonlinear response measured using a pump-probe technique such as the one described above. The response can be understood by recognizing the signal as the differential material response to the probe beam S128 resulting from whether the pump beam S126 is on or off. Because all materials have some third-order nonlinearity, the optical properties of all materials will depend on whether they are illuminated by a bright pump beam S126. We can measure this, for instance, by taking the difference between a measurement of how much probe light from the probe beam SI 28 reflects from a material with no pump beam SI 26 exciting the material and with a pump beam SI 26 exciting the material. Amplitude modulating the pump beam SI 26 is equivalent to rapidly turning the pump beam SI 26 on and off. In microscopy, when the pump beam S 126 and probe beam S 128 are collinear with one another, the pump beam S 126 impinges the photodetector 144. If only the pump beam S126 is modulated, the modulation of the nonlinear response is indistinguishable from the pump beam modulation S126. To distinguish the nonlinear signal from the modulated pump beam SI 26, we also amplitude modulate the probe beam SI 28. One last issue remains, which is that the pump beam SI 26 and probe beam S128 electric fields interfere when they overlap. To mitigate issues due to a phase term on the nonlinear signal, it is also necessary to phase cycle the pump pulses in the pump beam S126 and probe pulses in the probe beam S128 with respect to each other.

[0104] To amplitude modulate, phase cycle, and use a high sampling rate that is synchronous with the repetition rate of the laser, frep, it is necessary7to carefully select 4 distinct modulation frequencies with which to drive the MOD 114 and MOD 116. In an example, the frequency shifts can be 0. frep / 8, 5 / 12 frep, and fiep / 2. That is, according to the example above, fo = 0, fi = fiep / 8. f2 = 5 / 12 frep. and fs = fiep / 2. Each of these frequencies are radio-frequency shifts applied to the coherent laser light beam SI 12, resulting in the pump beam SI 26 and probe beam SI 28. The optical frequencies are roughly 800 THz, and the radio-frequency shifts are on the order of 10 MHz where frepof the laser used here is 80 MHz. The radio-frequency spectrum of aperiodically pulsed laser measured with a photodiode has signals at every harmonic of the repetition frequency frep. Therefore, the radio-frequency spectrum measured at the photodetector 144 will contain all harmonics of the repetition frequency, every beat note between each frequency pair, and the RF spectrum between the Oth and 1st repetition frequency harmonic is repeated between every other harmonic. The linear and 3rd-order nonlinear signals for this set of applied frequency shifts are plotted in figures 5A and 5B.

[0105] The optical frequencies plot in figure 5 A shows all the frequency-shifted laser lines as solid lines and all FWM signals induced in the material as dashed lines. The frequency -shifted laser lines include the peaks and corresponding frequencies as shown in the table 1, below.Table 1

[0106] Each FWM signal is the combination of three frequencies where two are positive and one is negative. This is because the measured signals are all at the same wavelength, meaning in the order of 800 THz. Summing three frequency-shifted beams would therefore be at 2400 THz, which is significantly outside the bandwidth of the measurement. The FWM peaks and related frequencies appear in figure 2, below.Table 2

[0107] The radio frequencies plot in figure 5B shows all the beat notes between each of the optical frequency lines. The radio frequency peaks and associated frequencies appear in table 3, below.|Table 3

[0108] The left and right sides radio frequencies plot in figure 5B are mirror images of each other. This is because FWM signal intensities are typically 5-9 orders of magnitude smaller than the linear signals, and FWM signals at the same frequency as a linear signal are not show n. The radio-frequency signal on which the signal processing electronics focus is (fi-fo) - (fs-f2). In the example, this is the frep / 24 signal, which does not overlap any linear signals or any other FWM signals.

[0109] Because all four frequencies are unique, the phase of each beam is constantly modulated with respect to each other beam. Therefore, there is not a delay-dependent optical phase that needs to be considered for optimizing the signal as in an interferometer with pure amplitude modulation. This dynamic phase cycling method ensures that every beam pair is modulated, and the acquisition rate conditions required for lock-in detection using a finite impulse response filter are met.

[0110] A challenge in attaining a measurable signal using the near UV light necessary to measure materials having a wide bandgap is the spurious FWM of silicon photodetectors used for these types of measurements. The result has been that the spurious FWM signal due to the photodetector 144 is much greater than the FWM emitted by the material of interest 140, rendering silicon photodiodes ineffective for UV optical systems.[OHl] The spurious FWM issue can be overcome by using a wide bandgap photodetector 144 with a bandgap that is more than half the energy of the excitation photon energy from thecoherent laser light beam SI 12. For example, for excitation using 390 nm light (3.2eV), half the photon energy7is 1.6eV. The silicon bandgap is 1.12eV, which is less than half the photon energy. This means that one blue photon (from the UV light) can excite 2 electrons in a silicon detector with some probability. In contrast to more common silicon detectors, a gallium phosphide (GaP) photodiode having a bandgap of 2.24eV does not suffer from the spurious FWM at 390 nm excitation. Using a photodetector 144 having a bandgap that is more than half the energy of the excitation photon energy of the laser light overcomes this issue. Measuring a FWM response using a wavelength of 550 nm requires using a photodetector containing a semiconductor with a bandgap that is greater than 1.12eV and less than 2.24 eV at which point even the fundamental light will not be absorbed by the semiconductor. Shorter FWM wavelengths will require photodetector semiconductors having bandgaps at higher energies.

[0112] The most readily available photodetectors / diodes having a bandgap greater than the bandgap of silicon (1.12ev) are GaP, SiC, and GaN. The bandgaps of SiC and GaN are potentially too wide for laser light with a wavelength in a range of 390 nm, so GaP is a practical choice. To the extent that InGaN is or becomes available, it may also be a good candidate because the bandgap can be tuned by tuning the In:Ga concentration ratio.

[0113] Because FWM is a nonlinear process, high peak intensities are required. Several engineering steps are required in order to achieve a high peak intensity in the material 140. Initially, a high-power seed laser is used. In an example, a titanium sapphire laser can be used that outputs 75 fs pulses at 80 MHz with an average power of 3.2W. In the example, this laser output is focused into a frequency-doubling crystal (e.g., Beta Barium Borate) which is used to generate 1W of near-UV light. Throughout the setup, it is important to use optics designed for near-UV light, such as UV-enhanced aluminum mirrors or dielectric ultrafast mirrors.

[0114] Further, to maintain a high peak intensity of light at the sample it is also necessary7to maintain short pulses. Optical components will stretch the pulse in time, thereby reducing the peak intensity of light in the pulse. Many common optical components, particularly those used in acousto-optic modulators, are extremely dispersive at near-UV and UV wavelengths. In an example, AOMs made of quartz can be used. Quartz has a low er acousto-optic efficiency than more common materials such as tellurium dioxide or lead molybdate but has much lower dispersion. Reducing dispersion can alternatively be achieved by using dispersion compensation. However, dispersion compensation methods typically lead to more than 70% loss of intensity, w hich also reduces the peak intensity of the sample.

[0115] The FWM signal is also complementary to linear resonant reflection, which has two advantages compared to the linear process of white light scattering mentioned above: 1) sitting at the band edge makes the linear resonant reflection more sensitive to changes in the band structure, and 2) it can detect features below the wafer surface if the focus of the imaging setup is set accordingly.

[0116] Figures 6A - 6C show the appearance of a common surface triangle defect in FWM under different conditions. Figure 6A shows an image of a defect generated using FWM and near UV light. The triangle, measured using light having a wavelength of 390 nm, glows brightly because of extraction of the FWM signal with a photo diode having a bandgap with a bandgap energy greater than 1.6 eV. This glow can only be observed with a wide bandgap detector where signal artifacts due to detector supralinearity are eliminated.

[0117] Figure 6B, for comparison to figure 6A, shows a detected FWM image for a silicon photodiode. The image is obscured due to the supralinearity of the silicon photodiode.

[0118] Figure 6C shows a linear resonant reflection of the defect captured with a silicon photodiode. The silicon-detected FWM image correlates strongly with the linear resonant reflection because it is generated by the reflected beams in the detector photodiode. The silicon FWM signal, dominated by this artifact, exceeds the wide bandgap detector signal by an order of magnitude, rendering any real FWM from the sample negligible.

[0119] Because some defect states have a slower decay time (are longer lived) than others, adjusting the delay generated by the delay element 118 can result in improved detection of different types of defects. For example, a zero delay (no delay) is good for detecting an absence of absorption from a bulk semiconductor. As an example, a zero delay can be used to detect a defect called a micropipe, which is a small hole in the semiconductor.

[0120] As another example, a 2 ps delay has been shown to generate higher contrast measurements compared to zero delay for long-lived defect states, such as a polyt pe inclusion. Figures 7A and 7B show the appearance of a common surface triangle defect using FWM with two different delay settings for the pump and probe signals. Figure 7A shows an image of the defect generated with zero (0) delay between the pump and probe signals. Figure 7B shows the improved contrast of an image of the defect generated with a 2-picoscond delay between the pump and probe signals, relative to figure 7A with zero delay.

[0121] All terms used in the claims are intended to be given their plain and ordinary meanings as understood by those skilled in the art unless an explicit indication to the contrary is made herein. In particular, use of the singular articles such as "a," "the," "said," etc. shouldbe read to recite one or more of the indicated elements unless a claim recites an explicit limitation to the contrary.

[0122] The term "based on” herein means based on in whole or in part.

[0123] The term “exemplary” is used herein in the sense of signifying an example, e.g., a reference to an “exemplary widget” should be read as simply referring to an example of a widget.

[0124] In the drawings, the same reference numbers indicate the same elements. Further, some or all of these elements could be changed.

[0125] In general, the computing systems and / or devices described may employ any of a number of computer operating systems, including, but by no means limited to, versions and / or varieties of the Microsoft Windows® operating system, the Unix operating system (e.g., the Solaris® operating system distributed by Oracle Corporation of Redwood Shores, California), the AIX UNIX operating system distributed by International Business Machines of Armonk, New York, the Linux operating system, the Mac OSX and iOS operating systems distributed by Apple Inc. of Cupertino, California, the BlackBerry OS distributed by Blackberry7, Ltd. of Waterloo, Canada, and the Android operating system developed by Google, Inc. and the Open Handset Alliance. Examples of computing devices include, without limitation, network devices such as a gateway or terminal, a computer workstation, a server, a desktop, notebook, laptop, or handheld computer, or some other computing system and / or device.

[0126] Computing devices generally include computer-executable instructions, where the instructions may be executable by one or more computing devices such as those listed above. Computer-executable instructions may be compiled or interpreted from computer programs created using a variety of programming languages and / or technologies, including, without limitation, and either alone or in combination, Java™, C, C++, Visual Basic, Java Script, Perl, Verilog, VHDL, etc. Some of these applications may be compiled and executed on a virtual machine, such as the Java Virtual Machine, the Dalvik virtual machine, or the like. In general, a processor (e.g., a microprocessor) receives instructions, e.g., from a memory, a computer- readable medium, etc., and executes these instructions, thereby performing one or more processes, including one or more of the processes described herein. Such instructions and other data may be stored and transmitted using a variety of computer-readable media.

[0127] A computer-readable medium (also referred to as a processor-readable medium) includes any non-transitory (e.g., tangible) medium that participates in providing data (e.g., instructions) that may be read by a computer (e.g., by a processor of a computer). Such amedium may take many forms, including, but not limited to, non-volatile media and volatile media. Instructions may be transmitted by one or more transmission media, including fiber optics, wires, wireless communication, including the internals that comprise a system bus coupled to a processor of a computer. Common forms of computer-readable media include, for example, RAM, a PROM, an EPROM, a FLASH-EEPROM, any other memory chip or cartridge, or any other medium from which a computer can read.

[0128] Databases, data repositories or other data stores described herein may include various kinds of mechanisms for storing, accessing, and retrieving various kinds of data, including a hierarchical database, a set of files in a file system, an application database in a proprietary format, a relational database management system (RDBMS), etc. Each such data store is generally included within a computing device employing a computer operating system such as one of those mentioned above, and is accessed via a network in any one or more of a variety of manners. A file system may be accessible from a computer operating system, and may include files stored in various formats. An RDBMS generally employs the Structured Query Language (SQL) in addition to a language for creating, storing, editing, and executing stored procedures, such as the PL / SQL language mentioned above.

[0129] In some examples, system elements may be implemented as computer-readable instructions (e.g., software) on one or more computing devices (e.g., servers, personal computers, etc.), stored on computer readable media associated therewith (e.g., disks, memories, etc.). A computer program product may comprise such instructions stored on computer readable media for carrying out the functions described herein.

[0130] With regard to the processes, systems, methods, heuristics, etc. described herein, it should be understood that, although the steps of such processes, etc. have been described as occurring according to a certain ordered sequence, such processes could be practiced with the described steps performed in an order other than the order described herein. It should be further understood that certain steps could be performed simultaneously, that other steps could be added, or that certain steps described herein could be omitted. In other words, the descriptions of processes herein are provided for the purpose of illustrating certain embodiments, and should in no way be construed so as to limit the claims.

[0131] Accordingly, it is to be understood that the above description is intended to be illustrative and not restrictive. Many embodiments and applications other than the examples provided would be apparent upon reading the above description. The scope should be determined, not with reference to the above description, but should instead be determined withreference to the appended claims, along with the full scope of equivalents to which such claims are entitled. It is anticipated and intended that future developments will occur in the technologies discussed herein, and that the disclosed systems and methods will be incorporated into such future embodiments. In sum, it should be understood that the application is capable of modification and variation.

[0132] The Abstract is provided to allow the reader to quickly ascertain the nature of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. In addition, in the foregoing Detailed Description, it can be seen that various features are grouped together in various embodiments for the purpose of streamlining the disclosure. This method of disclosure is not to be interpreted as reflecting an intention that the claimed embodiments require more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive subject matter lies in less than all features of a single disclosed embodiment. Thus, the following claims are hereby incorporated into the Detailed Description, with each claim standing on its own as a separately claimed subject matter.

Claims

CLAIMSWhat is claimed is:

1. An optical microscope comprising: a light source emitting a coherent laser light beam having a wavelength less than 550 nm; an optical system; and an electronic module; wherein: the optical microscope is configured to: generate two copies of the coherent laser light beam, the two copies including a first copy and a second copy; modulate a phase or amplitude of the first copy of the coherent laser light beam with respect to the second copy of the coherent laser light beam; recombine the modulated first copy of the coherent laser light beam with the second copy of the coherent laser light beam to form an excitation beam; focus the excitation beam to a laser spot position on a material of interest; measure a response light beam received from the material of interest based on the excitation beam with an optical detector, wherein the response light beam includes transmitted light from the material of interest and can further include reflected light from the material of interest; extract, from the response light beam by the electronic module, a response signal representing a nonlinear optical response of the material of interest, wherein the nonlinear optical response indicates a characteristic of the material of interest; the optical detector is a semiconductor photodiode; and a type of the semiconductor photodiode is selected based on a photon energy of the coherent laser light beam.

2. The optical microscope according to claim 1, wherein the optical detector is a photodiode comprising a semiconductor with a bandgap greater than 1. 12eV.

3. The optical microscope according to claim 1, wherein the optical detector is a photodiode comprising a semiconductor with a bandgap greater than one half times the photon energy of the coherent laser light beam.

4. The optical microscope according to claim 1, wherein the optical detector is a photodiode comprising a compound semiconductor.

5. The optical microscope according to claim 1, wherein the optical detector is a photodiode comprising gallium phosphide.

6. The optical microscope according to claim 1, wherein the wavelength of the coherent laser light beam is selected based on a physical characteristic of the material of interest.

7. The optical microscope according to claim 6, wherein the physical characteristic is a spectral feature of interest.

8. The optical microscope according to claim 6, wherein the physical characteristic of the material of interest is an energy7level of a bandgap of the material of interest.

9. The optical microscope according to claim 6, wherein the physical characteristic of the material of interest is an ultraviolet absorption resonance.

10. The optical microscope according to any one of claims 1-9, wherein the light source includes a femtosecond laser.

11. The optical microscope according to claim 10, wherein the femtosecond laser is a titanium sapphire femtosecond laser.

12. The optical microscope according to any one of claims 1-9. wherein the light source includes a frequency multiplying element.

13. The optical microscope according to claim 12. wherein the frequency multiplyingelement is a Beta Barium Borate nonlinear optical cry stal.

14. The optical microscope according to any one of claims 1-9, wherein the light source emits pulses with a duration less than 1 picosecond.

15. The optical microscope according to any one of claims 1-9, wherein the light source emits pulses with a duration less than 100 femtoseconds.

16. The optical microscope according to any one of claims 1-9, wherein the optical system includes an acousto-optic modulator to modulate the phase or the amplitude of the first copy of the coherent laser light beam with respect to the second copy of the coherent laser light beam.

17. The optical microscope according to claim 16, wherein the acousto-optic modulator comprises quartz.

18. The optical microscope according to any one of claims 1-9. wherein the optical system includes one or more UV-enhanced aluminum mirrors.

19. The optical microscope according to any one of claims 1-9, wherein the optical system includes one or more dielectric ultrafast minors.

20. The optical microscope according to claim 1, wherein the optical microscope is configured to modulate a phase or frequency of the second copy of the coherent laser light beam.

21. The optical microscope according to claim 20, wherein the optical microscope is configured to: modulate the first copy of the coherent laser light beam with a first frequency fo and a second frequency fit and modulate the second copy of the coherent laser light beam with a third frequency fz and a fourth frequency fs.

22. The optical microscope according to claim 21, wherein a repetition rate of the light source is rep, the first frequency fo is 0, the second frequency fi is fiep / 8, the third frequency fz is 5 / 12 fiep and the fourth frequency fi is frep / 2.

23. The optical microscope according to claim 21, wherein the nonlinear optical response is a third-order nonlinear optical response.

24. The optical microscope according to any one of claims 21-23. wherein the nonlinear optical response is a resonant four-wave-mixing signal.

25. The optical microscope according to any one of claims 1-9, wherein extracting the response signal includes application of a lock-in detection.

26. The optical microscope according to claim 1, further configured, after completing the measurement at the laser spot position, to move the focus to a subsequent laser spot position.

27. The optical microscope according to claim 26, further comprising a scan mechanism, wherein moving the laser spot position to the subsequent laser spot position is performed by the scan mechanism and the optical system.

28. The optical microscope according to claim 27, wherein moving the laser spot position relative to the material of interest includes moving a stage onto which the material of interest is mounted rapidly along a first (X) axis while a galvanometer mirror included in the optical system scans along a perpendicular (Y) axis.

29. The optical microscope according to claims 27 or 28, configured to: measure multiple laser spot positions on the material of interest via the scan mechanism and the optical system; and generate a two-dimensional image based on the measurements performed at each of the multiple laser spot positions.

30. The optical microscope according to claim 29. wherein the two-dimensional image isused to identify a location of a defect in the material of interest.

31. The optical microscope according to claim 29. wherein the two-dimensional image is used to identify a type of a defect in the material of interest.

32. The optical microscope according to any one of claims 1-9, wherein a linear resonant optical response is measured simultaneously with the nonlinear optical response.

33. A method comprising: emitting, by a laser light source, a coherent laser light beam having a wavelength less than 550 nm; generating two copies of the coherent laser light beam, the two copies of the coherent laser light beam including a first copy and a second copy; modulating a phase or amplitude modulation of the first copy of the coherent laser light beam with respect to the second copy of the coherent laser light beam; recombining the modulated first copy of the coherent laser light beam with the second copy of the coherent laser light beam to form an excitation beam; focusing the excitation beam on a laser spot position on a material of interest; measuring a response light beam received from the material of interest based on the excitation beam with an optical detector, wherein the response light beam includes transmitted light from the material of interest and can further include reflected light from the material of interest; and extracting a nonlinear optical response signal indicating a characteristic of the material of interest from the response light beam; wherein: the optical detector is a semiconductor photodiode, and a type of the semiconductor photodiode is selected based on a photon energy of the coherent laser light beam.

34. The method according to claim 33, wherein extracting the nonlinear optical response signal is performed by a photodiode comprising a semiconductor having a bandgap greater than 1.12eV.

35. The method according to claim 33, wherein extracting the nonlinear optical response signal is performed by a photodiode having a bandgap greater than one half times the photon energy of the coherent laser light beam.

36. The method according to claim 33, wherein extracting the nonlinear optical response signal is performed by a photodiode detector comprising gallium phosphide.

37. The method according to claim 33, wherein emitting the coherent laser light beam having the wavelength less than 550 nm includes: generating laser light with a laser; and doubling a frequency of the laser light with a nonlinear optical crystal.

38. The method according to claim 33, wherein the wavelength of the coherent laser light beam is selected based on a physical characteristic of the material of interest.

39. The method according to claim 38, wherein the physical characteristic is a spectral feature of interest.

40. The method according to claim 38, wherein the physical characteristic of the material of interest is an energy level of a bandgap of the material of interest.

41. The method according to claim 38, wherein the physical characteristic of the material of interest is an ultraviolet absorption resonance.

42. The method according to any one of claims 33-41, wherein the coherent laser light beam comprises pulses less than 1 picosecond in duration.

43. The method according to any one of claims 33-41, wherein the coherent laser light beam comprises pulses less than 100 femtoseconds in duration.

44. The method according to any one of claims 33-41, wherein modulating the phase or the amplitude of the first copy of the coherent laser light beam with respect to the second copy of the coherent laser light beam is performed by a quartz acousto-optic modulator.

45. The method according to any one of claims 33-41 further comprising: scanning the material of interest to measure a plurality of laser spot positions; and generating a two-dimensional image based on the measurements at the respective laser spot positions.

46. The method according to claim 45, wherein scanning the material of interest includes moving a stage onto which the material of interest is mounted rapidly along a first (X) axis while a galvanometer mirror scans along a perpendicular (Y) axis.

47. The method according to claim 45, further comprising: identifying a location of a defect in the material of interest based on the two- dimensional image.

48. The method according to claim 45, further comprising: identifying a type of a defect in the material of interest based on the two- dimensional image.

49. The method according to any one of claims 33-41, wherein measuring the response light beam includes measuring a linear resonant response of the material of interest.

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