Wafer defect detection multi-reference arm optical coherence tomography apparatus based on dual-wavelength light source

Abstract

The application provides a wafer defect detection multi-reference arm optical coherence tomography device based on a dual-wavelength light source, and belongs to the field of optical imaging; solves the problem that the existing single-wavelength optical coherence tomography technology cannot simultaneously consider high resolution and deep penetration detection when applied to wafer defect detection; the device controls the 840nm light source and the 1310nm light source to work at the same time, and when scanning each position, the optical switch is rapidly switched among multiple different optical path reference arms, and multiple groups of mixed interference spectra containing dual-band information are synchronously collected; a computer selects, from the multiple groups of spectral data, a data subset with the optimal signal-to-noise ratio in high-resolution shallow layer imaging for the 840nm band, and a data subset with the strongest signal in deep penetration imaging for the 1310nm band, and then independently reconstructs two tomographic images which are respectively optimized, to realize high-resolution and large-depth-range detection of wafer surface and internal multi-layer structure defects.

Description

Technical Field

[0001] This application relates to the field of optical imaging technology, and in particular to a multi-reference arm optical coherence tomography device for wafer defect detection based on a dual-wavelength light source. Background Technology

[0002] As the fundamental material for manufacturing semiconductor integrated circuit chips, wafers are circular thin films made of semiconductor materials such as silicon. They are widely used in key fields such as electronic communications, new energy, aerospace, and artificial intelligence. Their production capacity and yield rate have a decisive impact on the development quality of the entire semiconductor industry chain. With the continuous increase in chip design complexity, more and more precision circuits are integrated onto the wafer surface. This not only makes chips more powerful and has larger storage capacity, but also makes wafer manufacturing processes extremely complex—from photolithography and etching to deposition, even minor deviations in each step can lead to defects, and the probability and types of defects also increase significantly. Therefore, wafer defect detection has become a crucial link in the semiconductor manufacturing process to ensure product quality.

[0003] Optical coherence tomography (OCT) achieves three-dimensional imaging based on the principle of low-coherence interference. Belonging to the category of tomographic imaging technology, it possesses characteristics such as non-invasiveness and high resolution, and has been used for wafer surface inspection. When traditional single-reference-arm structures are applied to the detection of defects in multi-layered wafer structures, a large imaging depth is required to capture interface defects tens to hundreds of micrometers deep. However, the single-shot imaging depth of traditional spectral domain OCT (SD-OCT) is limited by the spectrometer resolution, making it difficult to cover the complete range from the surface to the deep layers without reducing lateral resolution or scanning speed. Secondly, the significant differences in refractive index between wafer layers result in very strong reflected signals from surface metal circuitry, while scattering signals from defects between deep dielectric layers are very weak. A reference arm with a fixed optical path cannot simultaneously optimize the signal-to-noise ratio of such a large dynamic range signal, leading to a situation of "overexposure of shallow layers and loss of deep layers."

[0004] However, even OCT systems employing multi-reference-arm optimization techniques still have inherent limitations when based on a single broadband light source. Firstly, resolution and penetration depth are mutually exclusive: high axial resolution requires a short-wavelength light source, but its penetration capability in silicon is limited; deep-penetration capability requires a long-wavelength light source, but its resolution decreases. Current technologies cannot simultaneously acquire optimal surface high-resolution images and deep high signal-to-noise ratio images without distortion in a single scan. Therefore, a novel OCT detection device is needed that can overcome the performance limitations of a single wavelength while integrating depth optimization and multi-parameter measurement capabilities. Summary of the Invention

[0005] To address the limitations of existing single-wavelength optical coherence tomography (OCT) technology in wafer defect detection, which cannot simultaneously achieve high resolution and deep penetration, and to resolve the signal-to-noise ratio imbalance across different depth layers through active switching of optical path lengths in multiple reference arms, this application proposes a multi-reference-arm OCT device for wafer defect detection based on a dual-wavelength light source. This device enables high-resolution, wide-depth detection of multi-layer structural defects on the wafer surface and within the wafer.

[0006] The technical solution adopted in this application is as follows: a multi-reference-arm optical coherence tomography device for wafer defect detection based on dual-wavelength light source, comprising a dual-wavelength light source module, a wavelength combiner, an optical isolator, a broadband fiber coupler, a reference arm switching device, multiple reference arms, a sample arm, and a probe arm. The dual-wavelength light source output from the dual-wavelength light source module is coupled by the wavelength combiner and then enters the broadband fiber coupler through the optical isolator to form two optical signals. One of them is guided by the sample arm to different depth positions of the wafer under test for detection, and the reflected signal returns to the broadband fiber coupler along the original optical path. The other one enters the multiple reference arms through the reference arm switching device, and the reflected signals of each reference arm also return to the broadband fiber coupler. After the two optical signals interfere in the broadband fiber coupler, they are finally guided into the probe arm to complete signal acquisition.

[0007] Furthermore, the dual-wavelength light source module includes a first broadband light source and a second broadband light source. The output ends of both the first and second broadband light sources are connected to the input end of a wavelength combiner, and the output end of the wavelength combiner is connected to the input end of an optical isolator. The output end of the optical isolator is connected to port a of a broadband fiber coupler.

[0008] Furthermore, the first broadband light source is an SLD light source with a center wavelength of 840nm, used to provide high axial resolution and optimize surface and shallow layer imaging; the second broadband light source is an SLD light source with a center wavelength of 1310nm, used to optimize deep interface imaging.

[0009] Furthermore, the reference arm switching device consists of a polarization controller and an optical switch. The b port of the broadband fiber coupler is connected to the input of the polarization controller; the output of the polarization controller is connected to the input of the optical switch; and the output of the optical switch is connected to each of the multiple reference arms, thereby alternately realizing the optical path length of different reference arms through optical switching.

[0010] Furthermore, the multi-reference arm includes at least three reference arms, each of which includes a collimating lens, a slit, a focusing lens, and a reflecting mirror. The collimating lens is connected to the focusing lens through the slit to illuminate the reflecting mirror.

[0011] Furthermore, the sample arm includes a first collimator, a scanning galvanometer, a first focusing lens, and a wafer under test. The C port of the broadband fiber coupler is connected to the input of the first collimator; the output of the first collimator is connected to the input of the scanning galvanometer; the output of the scanning galvanometer is connected to the input of the first focusing lens; and the output of the first focusing lens illuminates the wafer under test.

[0012] Furthermore, the detection arm includes a second collimator, a transmission grating, a second focusing lens, a CCD detector, a data acquisition card, and a computer. The D-port of the broadband fiber optic coupler is connected to the input of the second collimator; the output of the second collimator is connected to the input of the transmission grating, which disperses light to different angles according to its wavelength; the output of the transmission grating is connected to the input of the second focusing lens; the output of the second focusing lens is connected to the input of the CCD detector; the output of the CCD detector is connected to the input of the data acquisition card; and the output of the data acquisition card is connected to the input of the computer for data processing.

[0013] Furthermore, the CCD detector receives the spectra of different pixel regions in parallel at the 840nm and 1310nm bands.

[0014] Furthermore, the computer simultaneously reconstructs a high-resolution surface image corresponding to the 840nm band and a deep penetration image corresponding to the 1310nm band using a fast Fourier transform.

[0015] Furthermore, the optical path of each reference arm is pre-set to match the characteristic reflective surfaces of the wafer under test at different depths from the surface to the interior.

[0016] The advantages of this application over the prior art are as follows: I. This application achieves a leapfrog improvement in imaging performance by simultaneously obtaining a "high-resolution shallow image" and a "high signal-to-noise ratio deep image" in a single measurement through dual-wavelength collaborative operation and active switching of the optical path of the reference arm. This is something that traditional single-wavelength systems would require two separate measurements to acquire.

[0017] Second, the main modifications in this application are concentrated on the reference arm and the light source. There is no need to make complex modifications to the optical design of the sample arm, which makes it easy to upgrade and implement on existing equipment and lowers the application threshold.

[0018] Third, this application uses a spectral domain optical coherence tomography (SD-OCT) system, which features fast imaging speed, high resolution, and high sensitivity. Attached Figure Description

[0019] The following description, in conjunction with the accompanying drawings, further illustrates this application: Figure 1This is a schematic diagram of the structure of a multi-reference arm optical coherence tomography device for wafer defect detection based on a dual-wavelength light source, provided in an embodiment of this application.

[0020] In the diagram: 1 is the first broadband light source, 2 is the second broadband light source, 3 is the wavelength combiner, 4 is the optical isolator, 5 is the broadband fiber coupler, 6 is the reference arm switching device, 7 is the polarization controller, 8 is the optical switch, 9 is the multiple reference arms, 10 is the collimating lens of the reference arm, 11 is the slit of the reference arm, 12 is the focusing lens of the reference arm, 13 is the reflecting mirror of the reference arm, 14 is the collimating lens of the second reference arm, 15 is the slit of the second reference arm, and 16 is the focusing lens of the second reference arm. 17 is the second reflecting mirror of the reference arm, 18 is the third collimating mirror of the reference arm, 19 is the third slit of the reference arm, 20 is the third focusing lens of the reference arm, 21 is the third reflecting mirror of the reference arm, 22 is the sample arm, 23 is the first collimator, 24 is the scanning galvanometer, 25 is the first focusing lens, 26 is the wafer under test, 27 is the probe arm, 28 is the second collimator, 29 is the transmission grating, 30 is the second focusing lens, 31 is the CCD detector, 32 is the data acquisition card, and 33 is the computer. Detailed Implementation

[0021] like Figure 1 As shown, this application provides a multi-reference-arm optical coherence tomography device for wafer defect detection based on dual-wavelength light sources. The device mainly consists of core components such as a 1310nm center wavelength light source, an 840nm center wavelength light source, a wavelength combiner 3, an optical isolator 4, a broadband fiber coupler 5, a reference arm switching device 6, multiple reference arms 9, a sample arm 22, and a probe arm 27. During operation, the 1310nm and 840nm center wavelength light sources are coupled by the wavelength combiner 3 and then enter the broadband fiber coupler 5, where they are split into two optical signals. One signal is guided by the sample arm 22 to different depth positions on the wafer 26 under test for detection, and the reflected signal returns to the broadband fiber coupler 5 along the original optical path. The other signal enters the multiple reference arms 9 through the reference arm switching device 6, and the reflected signals from each reference arm also return to the broadband fiber coupler 5. After interference within the broadband fiber coupler 5, the two optical signals are finally guided to the probe arm 27 for signal acquisition.

[0022] The core of this application lies in the dual-wavelength detection and multi-reference-arm optimized depth coupling. By controlling the simultaneous operation of the 840nm and 1310nm light sources, and rapidly switching the optical switch 8 between multiple reference arms with different optical paths during scanning at each position, multiple sets of mixed interference spectra containing dual-band information are simultaneously acquired. In subsequent processing, the computer 33 selects the data subsets with the best signal-to-noise ratio for the 840nm band in high-resolution shallow imaging and the data subsets with the strongest signal for the 1310nm band in deep-penetration imaging from the multiple sets of spectral data, thereby independently reconstructing two tomographic images that have been specifically optimized.

[0023] like Figure 1 As shown in the embodiment of this application, a specific structure of a multi-reference-arm optical coherence tomography device for wafer defect detection based on a dual-wavelength light source is provided. The reference arm switching device 6 consists of a polarization controller 7 and an optical switch 8. The multi-reference arm 9 can be configured with at least three reference arms, each of which is equipped with a collimating lens, a slit, a focusing lens, and a reflecting mirror. The sample arm 22 consists of a first collimator 23, a scanning galvanometer 24, a first focusing lens 25, and a wafer under test 26. The detector arm 27 consists of a second collimator 28, a transmission grating 29, a second focusing lens 30, a CCD detector 31, a data acquisition card 32, and a computer 33.

[0024] The outputs of the first broadband light source 1 and the second broadband light source 2 are connected to the input of the wavelength combiner 3; the output of the wavelength combiner 3 is connected to the input of the optical isolator 4; the output of the optical isolator 4 is connected to port a of the broadband fiber coupler 5; port b of the broadband fiber coupler 5 is connected to the input of the polarization controller 7; the output of the polarization controller 7 is connected to the input of the optical switch 8; the output of the optical switch 8 is connected to the collimating lens of each reference arm in the multi-reference arm 9; port d of the broadband fiber coupler 5 is connected to the input of the second collimator 28; the output of the second collimator 28 is connected to the input of the transmission grating 29; the output of the transmission grating 29 is connected to the input of the second focusing lens 30; the output of the second focusing lens 30 is connected to the input of the CCD detector 31; the output of the CCD detector 31 is connected to the input of the data acquisition card 32; the output of the data acquisition card 32 is connected to the input of the computer 33 for data processing.

[0025] The output terminals of the first broadband light source 1 and the second broadband light source 2 are connected to the input terminal of the wavelength combiner 3. The first broadband light source 1 is an SLD (Superluminescent Diode) light source with a center wavelength of approximately 840nm. The first light source is a diode (DID) used to provide high axial resolution and optimize surface and shallow imaging. The second broadband light source 2 is an SLD light source with a center wavelength of approximately 1310 nm, which has better penetration in silicon and is used to optimize deep interface imaging. The wavelength combiner 3 is a wavelength division multiplexer used to efficiently couple light from two wavelength bands into the same single-mode fiber. The output of the wavelength combiner 3 is connected to the input of the optical isolator 4. The optical isolator 4 can prevent light reflected back from the broadband fiber coupler 5 from re-entering the light source, ensuring the working stability of the light source and reducing system noise. The output of the optical isolator 4 is connected to port a of the broadband fiber coupler 5. Port b of the broadband fiber coupler 5 is connected to the input of the polarization controller 7. The output of the polarization controller 7 is connected to the input of the optical switch 8 to adjust the polarization state of the light entering each reference arm channel 9, ensuring that it is optimally matched with the polarization state of the light returned from the sample arm 22. The output of the optical switch 8 is connected to the collimating mirror of each reference arm in the multi-reference arm 9, and different reference arm optical paths are alternately realized through optical switching. The optical path structure is simple and can realize three or more reference arm optical path selections. The optical path length of each optical path is preset according to the thin film thickness and refractive index of the target wafer 26 to match its surface, intermediate layer and deep layer interfaces respectively.

[0026] In this embodiment, the multiple reference arms 9 are divided into three groups. One group consists of a collimating lens 10 of reference arm 1 connected to a focusing lens 12 of reference arm 1 through a slit 11 of reference arm 1, which illuminates a reflecting mirror 13 of reference arm 1. Another group consists of a collimating lens 14 of reference arm 2 connected to a focusing lens 16 of reference arm 2 through a slit 15 of reference arm 2, which illuminates a reflecting mirror 17 of reference arm 2. The third group consists of a collimating lens 18 of reference arm 3 connected to a focusing lens 20 of reference arm 3 through a slit 19 of reference arm 3, which illuminates a reflecting mirror 21 of reference arm 3. After the reference arms reflect the light beam, it returns to the broadband fiber coupler 5 along the original optical path.

[0027] The C port of the broadband fiber coupler 5 is connected to the input of the first collimator 23; the output of the first collimator 23 is connected to the input of the scanning galvanometer 24; the output of the scanning galvanometer 24 is connected to the input of the first focusing lens 25; the output of the first focusing lens 25 illuminates the wafer 26 under test; the multi-reference arm 9 receives light information reflected at different depths through the reference arm switching device 6, and the optical path of each reference arm is pre-set to match the characteristic reflective surfaces of the wafer 26 under test at different depths from the surface to the interior. The D port of the broadband fiber coupler 5 is connected to the input of the second collimator 28; the output of the second collimator 28 is connected to the input of the transmission grating 29, which disperses the light to different angles according to the wavelength; the output of the transmission grating 29 is connected to the input of the second focusing lens 30.

[0028] On the CCD detector 31, the spectral frequencies of the 840nm band and the 1310nm band fall into two different sets of pixel regions. The data acquisition system can easily extract these two spectral signals separately and perform fast Fourier transforms on them to obtain depth images at the two wavelengths of 840nm and 1310nm independently. The output of the second focusing lens 30 is connected to the input of the CCD detector 31. The output of the CCD detector 31 is connected to the input of the acquisition card 32. The output of the acquisition card 32 is connected to the input of the computer 33 for data processing, and finally, two tomographic images are output synchronously: one is a high-resolution image based on the 840nm signal and shallow optimized reference arm data, and the other is a high-transmittance image based on the 1310nm signal and deep optimized reference arm data.

[0029] The device proposed in this application utilizes dual broadband light sources with center wavelengths of 840nm and 1310nm in synergy when inspecting wafers. On one hand, by rapidly switching the optical path of the reference arm, it optimizes the acquisition of interference signals from different depths of the wafer, solving the problem of unbalanced signal-to-noise ratio caused by large differences in the dynamic range of signals from different layers. On the other hand, it uses spectral dispersion technology to perform real-time separation and parallel processing of the returned mixed interference signals. Finally, after processing by computer, a clear tomographic image of the wafer's interior from the surface to the bottom can be reconstructed for automatic defect identification and location.

[0030] This application employs two broadband light sources with center wavelengths of 840nm and 1310nm, respectively. The probe light emitted from the broadband light source is coupled by a wavelength combiner 3, and then passes through an optical isolator 4 to a broadband fiber coupler 5. Part of the light enters the multi-reference arm 9 through a reference arm switching device 6, and the optical path of different reference arms is alternately achieved through optical switching. The other part of the light enters the sample arm 22. The light entering the sample arm 22 passes through the first collimator 23, the scanning galvanometer 24, and the first focusing lens 25 in sequence before illuminating the wafer under test 26. The backscattered light from the wafer under test 26 returns to the broadband fiber coupler 5 along the original path. The light entering the multi-reference arm enters multiple sets of reference arms and returns to the broadband fiber coupler 5 along the original path, thus interfering with the light returning from the sample arm 22. In the probe arm 27, the emitted light is collimated by the second collimator 28, split by the transmission grating 29, and focused by the second focusing lens 30. The 840nm and 1310nm wavelength spectrum contained therein is received in parallel by different pixel regions of the CCD detector 31. The signal is transmitted to the computer 33 via the acquisition card 32 for processing. Through fast Fourier transform, a high-resolution surface image corresponding to the 840nm band and a deep penetration image corresponding to the 1310nm band can be reconstructed simultaneously, thereby realizing the integrated detection and positioning of wafer cross-scale defects.

[0031] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of this application.

Claims

1. A multi-reference arm optical coherence tomography device for wafer defect detection based on a dual-wavelength light source, characterized in that: The system includes a dual-wavelength light source module, a wavelength combiner (3), an optical isolator (4), a broadband fiber coupler (5), a reference arm switching device (6), multiple reference arms (9), a sample arm (22), and a probe arm (27). The dual-wavelength light source output from the dual-wavelength light source module is coupled by the wavelength combiner (3) and then enters the broadband fiber coupler (5) through the optical isolator (4) to form two optical signals after beam splitting. One of them is guided to different depth positions of the wafer under test (26) through the sample arm (22) for detection, and the reflected signal returns to the broadband fiber coupler (5) along the original optical path. The other one enters the multiple reference arms (9) through the reference arm switching device (6), and the reflected signals of each reference arm also return to the broadband fiber coupler (5). After the two optical signals interfere in the broadband fiber coupler (5), they are finally introduced into the probe arm (27) to complete signal acquisition.

2. The multi-reference arm optical coherence tomography device for wafer defect detection based on a dual-wavelength light source according to claim 1, characterized in that: The dual-wavelength light source module includes a first broadband light source (1) and a second broadband light source (2). The output ends of the first broadband light source (1) and the second broadband light source (2) are both connected to the input end of the wavelength combiner (3). The output end of the wavelength combiner (3) is connected to the input end of the optical isolator (4). The output end of the optical isolator (4) is connected to port a of the broadband fiber coupler (5).

3. The multi-reference arm optical coherence tomography device for wafer defect detection based on a dual-wavelength light source according to claim 2, characterized in that: The first broadband light source (1) is an SLD light source with a center wavelength of 840nm, used to provide high axial resolution and optimize surface and shallow layer imaging; the second broadband light source (2) is an SLD light source with a center wavelength of 1310nm, used to optimize deep interface imaging.

4. The multi-reference arm optical coherence tomography device for wafer defect detection based on a dual-wavelength light source according to claim 3, characterized in that: The reference arm switching device (6) consists of a polarization controller (7) and an optical switch (8). The b port of the broadband fiber coupler (5) is connected to the input end of the polarization controller (7); the output end of the polarization controller (7) is connected to the input end of the optical switch (8); the output end of the optical switch (8) is connected to each reference arm in the multi-reference arm (9), and the optical path of different reference arms is alternately realized through optical switching.

5. A multi-reference arm optical coherence tomography device for wafer defect detection based on a dual-wavelength light source according to claim 3 or 4, characterized in that: The multi-reference arm (9) includes at least three reference arms, each of which includes a collimating lens, a slit, a focusing lens and a reflecting mirror. The collimating lens is connected to the focusing lens through the slit to illuminate the reflecting mirror.

6. The multi-reference arm optical coherence tomography device for wafer defect detection based on a dual-wavelength light source according to claim 5, characterized in that: The sample arm (22) includes a first collimator (23), a scanning galvanometer (24), a first focusing lens (25), and a wafer under test (26). The C port of the broadband fiber coupler (5) is connected to the input end of the first collimator (23); the output end of the first collimator (23) is connected to the input end of the scanning galvanometer (24); the output end of the scanning galvanometer (24) is connected to the input end of the first focusing lens (25); and the output end of the first focusing lens (25) illuminates the wafer under test (26).

7. The multi-reference arm optical coherence tomography device for wafer defect detection based on a dual-wavelength light source according to claim 5, characterized in that: The probe arm (27) includes a second collimator (28), a transmission grating (29), a second focusing lens (30), a CCD detector (31), a data acquisition card (32), and a computer (33). The D port of the broadband fiber coupler (5) is connected to the input end of the second collimator (28). The output end of the second collimator (28) is connected to the input end of the transmission grating (29). The transmission grating (29) disperses light to different angles according to different wavelengths. The output end of the transmission grating (29) is connected to the input end of the second focusing lens (30). The output end of the second focusing lens (30) is connected to the input end of the CCD detector (31). The output end of the CCD detector (31) is connected to the input end of the data acquisition card (32). The output end of the data acquisition card (32) is connected to the input end of the computer (33) for data processing.

8. The multi-reference arm optical coherence tomography device for wafer defect detection based on a dual-wavelength light source according to claim 6, characterized in that: The CCD detector (31) receives the spectra of the 840nm and 1310nm bands of different pixel regions in parallel.

9. The multi-reference arm optical coherence tomography device for wafer defect detection based on a dual-wavelength light source according to claim 6, characterized in that: The computer (33) synchronously reconstructs a high-resolution surface image corresponding to the 840nm band and a deep penetration image corresponding to the 1310nm band through a fast Fourier transform.

10. The multi-reference arm optical coherence tomography device for wafer defect detection based on a dual-wavelength light source according to claim 5, characterized in that: The optical path of each reference arm is pre-set to match the characteristic reflective surfaces of the wafer under test (26) at different depths from the surface to the interior.

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