Semiconductor carrier lifetime imaging method based on chirp photoluminescence
By using chirped photofluorescence and CCD to acquire image data in photofluorescence imaging technology, the problems of slow measurement speed and unoptimized signal-to-noise ratio in traditional photofluorescence imaging technology are solved, and fast, quantitative, online carrier life imaging of large-size semiconductor wafers is achieved.
Patent Information
- Application Number
- CN202411921823.8
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2024-12-25
- Publication Date
- 2025-03-28
AI Technical Summary
The existing CCD-based photofluorescence imaging technology is slow in measuring semiconductor carrier life and the signal-to-noise ratio is not optimized, making it impossible to achieve full coverage and fast imaging of large-size wafers.
Using an imaging method based on chirped photofluorescence, a chirped signal is generated by a function generator to modulate the light intensity output by the laser, forming a laser with linear frequency modulation over time, excitating the semiconductor sample and generating a fluorescent signal. The CCD is used to collect dynamic image data and perform signal processing through a computer, including cross-correlation and convolution operations, to obtain quantitative spatial distribution images of carrier life.
Non-contact, high signal-to-noise ratio, quantitative, fast, online carrier life imaging of large-size semiconductor wafers is achieved, and the problems of slow measurement speed and unoptimized signal-to-noise ratio in traditional methods are overcome.
Smart Images
Figure CN119715485A_ABST
Abstract
Description
Technical Field
[0001] The present invention relates to the field of semiconductor material property detection, and in particular to a non-contact, non-destructive, quantitative imaging optical characterization method for semiconductor carrier lifetime spatial distribution based on modulated photoluminescence technology. Background Art
[0002] The carrier lifetime measurement technology based on optical detection methods has the advantages of non-contact, non-destructive, pollution-free, and fast. It has become a new generation of key detection technology in the field of semiconductor carrier recombination and transport characteristics characterization. Among the many optical detection technologies used in the field of semiconductor material characteristics detection, photoluminescence technology is famous for its ability to measure non-equilibrium carrier recombination and transport performance, including non-equilibrium carrier transport parameters such as bulk recombination lifetime, mobility, diffusion coefficient, and surface recombination rate. In the process of realizing the measurement of non-equilibrium carrier recombination and transport parameters, photoluminescence technology usually adopts the mode of excitation light intensity modulation. The fluorescence signal (generally in the near-infrared band) generated by the radiation recombination of the excited non-equilibrium carriers in the sample carries the same frequency component. The phase-locked amplifier is used to demodulate the amplitude and phase at this frequency, and the amplitude and phase change data with frequency are obtained by scanning the modulation frequency from low frequency to high frequency, from which the non-equilibrium carrier recombination and transport parameters are extracted. However, with the current trend of increasing semiconductor substrate size (16-inch wafers have gradually become mainstream), traditional photoluminescence technology based on single-point detection can only perform local sampling of non-equilibrium carrier lifetimes, and cannot achieve full coverage scanning measurement of the entire large-size wafer. If a point-by-point scanning mode is used to achieve imaging, it will take too long and cannot achieve online rapid inspection.
[0003] The rapid development of near-infrared CCD performance today makes it possible to perform full coverage rapid imaging of large-size wafers based on photoluminescence technology. Traditional CCD-based photoluminescence imaging technology mainly uses single-frequency excitation and phase-locked demodulation mode for signal modulation and demodulation. In this mode, measuring the semiconductor carrier lifetime requires scanning from low frequency to high frequency, performing phase-locked measurement at each single frequency point to obtain amplitude and phase information, and then using theoretical models to fit the experimentally measured amplitude frequency and phase frequency data to extract the carrier lifetime value. This method is very time-consuming. It generally takes ten minutes to complete a measurement, and the signal-to-noise ratio is not optimized. Therefore, the development of non-contact, high signal-to-noise ratio, quantitative, fast, and online carrier lifetime imaging characterization methods is an urgent need in the field of semiconductor detection. Summary of the invention
[0004] The present invention proposes a semiconductor carrier lifetime imaging method based on chirp photoluminescence, wherein the system comprises a function generator 1, an excitation laser 2, a collimation and beam expansion system 3, a wafer sample to be characterized 4, a fluorescence imaging CCD 5, a long-pass filter 6, and a computer 7, and the method is characterized in that:
[0005] The chirp signal generated by the function generator 1 is used to modulate the laser 2 so that it emits a laser whose light intensity is linearly modulated with time. After passing through the beam collimation and expansion system 3, a large light spot with uniform light intensity is formed to excite the semiconductor sample 4 and generate non-equilibrium carriers whose concentration varies with time. The photoluminescence dynamic image signal emitted by the non-equilibrium carriers through radiation recombination is sampled and photographed by CCD5. The long-pass filter 6 placed in front of the CCD5 lens filters out the excitation light emitted by the laser 2 and allows the fluorescence to enter the CCD5. The captured dynamic image data is transmitted to a computer 7 with a frame grabber. The discrete time domain signals of each pixel point of the CCD5 in different frames are cross-correlated with the reference signal to obtain a quantitative spatial distribution image of the carrier lifetime of the sample.
[0006] The starting and ending frequencies of the chirp signal generated by the function generator 1 are both low frequencies, that is, they satisfy the quasi-steady-state approximation ωτ<<1, where ω is the light intensity modulation angular frequency, and τ is the equivalent carrier lifetime of the semiconductor wafer to be measured.
[0007] The laser 2 should be a continuous laser that can realize analog modulation of light intensity, and the output light intensity should have good linearity with the modulated electrical signal; the photon energy of the laser emitted by the laser 2 should be greater than the bandgap width of the semiconductor material to be measured, so as to excite non-equilibrium carriers; the optical power of the laser 2 should be high enough, and the excitation light intensity of the entire sample after the spot is expanded meets the requirements of the carrier injection dose, which is determined by the specific characterization requirements.
[0008] The laser beam emitted from the laser 2 forms a large-size light spot with uniform light intensity distribution after passing through the collimation and beam expansion system 3, thereby achieving uniform excitation of the semiconductor wafer sample, that is, the incident light intensity at all locations on the wafer is consistent.
[0009] The photosensitivity band of the CCD5 is selected as the fluorescence band of the semiconductor to be tested, generally a near-infrared CCD, and the CCD5 is placed on the back of the wafer, that is, the other side where the excitation light is located. Therefore, the reflected light of the excitation light on the front surface of the wafer cannot reach the CCD5, and the long-pass filter 6 placed in front of the CCD5 can further filter out the stray excitation light to ensure that the image captured by the CCD5 is a pure fluorescence image.
[0010] The CCD5 has an external trigger function, its frame rate is adjustable, and should satisfy the Nyquist sampling theorem, that is, the frame rate is greater than twice the chirp start and cutoff frequencies. The shooting trigger sequence signal of CCD5 is generated by computer 7, and the synchronization control of the entire system is realized by computer 7.
[0011] The overall duration of the image sequence obtained by the CCD5 should be an integer multiple of the chirp period. Generally, this condition is naturally met when the laser modulation frequency is set to an integer multiple of 1 Hz and the overall duration of the image sequence is selected to be an integer multiple of 1 s.
[0012] The computer 7 convolves the laser modulation signal with the response function corresponding to different carrier lifetimes to obtain a series of reference signals. The specific algorithm is to first perform Fourier transform on the chirp modulation signal, and then multiply it with the photoluminescence transfer function C / (1+iωτ) corresponding to different carrier lifetimes, where C is the normalization coefficient and i is the imaginary unit. Then, the complex vector is normalized so that its two-norm is 1.
[0013] The series of reference signals are cross-correlated with the discrete time domain signals of each pixel of the CCD in different frames, wherein the carrier lifetime corresponding to the reference signal with the highest correlation peak is the measured value of the pixel, and the overall image is the quantitative spatial distribution of the carrier lifetime of the sample.
[0014] The beneficial effects of the present invention are: overcoming the shortcomings of the existing CCD photoluminescence imaging method such as slow measurement speed and unoptimized signal-to-noise ratio, completing signal processing and filtering and quantitative measurement of target parameters in the same step, and providing a non-contact, high signal-to-noise ratio, quantitative, fast, online carrier lifetime imaging characterization method for large-size semiconductor wafer production lines. BRIEF DESCRIPTION OF THE DRAWINGS
[0015] Figure 1 It is a schematic diagram of the experimental system of the present invention, wherein 1 is a function generator, 2 is an excitation laser, 3 is a collimation and beam expansion system, 4 is a wafer sample to be characterized, 5 is a fluorescence imaging CCD, 6 is a long-pass filter, and 7 is a computer.
[0016] Figure 2 Schematic diagram of system synchronization trigger sequence signal.
[0017] Figure 3 The block diagram of the algorithm for cross-correlation calculation of chirp photoluminescence signals and quantitative measurement of carrier lifetime. DETAILED DESCRIPTION
[0018] Combine the following Figure 1-3The present invention specifically describes a semiconductor carrier lifetime imaging method based on chirp photoluminescence. However, it should be understood that the accompanying drawings are provided only for a better understanding of the present invention and should not be construed as limiting the present invention. The specific implementation steps are as follows:
[0019] (1) Build the experimental system. Figure 1 The fast quantitative imaging characterization experimental system for the spatial distribution of non-equilibrium carrier lifetime in silicon wafer shown includes a function generator 1, an excitation laser 2, a collimation and beam expansion system 3, a wafer sample to be characterized 4, an InGaAs CCD 5, a long-pass filter 6, and a computer 7.
[0020] a. The excitation laser 2 is selected as a semiconductor laser, which can realize analog modulation of light intensity, and has good linearity between its output light intensity and the modulated electrical signal.
[0021] b. Connect function generator 1 to laser 2 to ensure that the function generator can emit a chirp electrical signal. Based on the drive signal data provided in the laser manual, set the safe range of the function generator output signal amplitude.
[0022] c. The excitation laser is selected as a continuous laser with a wavelength of 808nm and a peak power of 30W. Its photon energy is greater than the silicon bandgap width, so it can excite non-equilibrium carriers. Its optical power can meet the injection dose requirements for carrier characterization of most silicon wafers.
[0023] d. Adjust the beam collimator and expander 3 to form a large-size light spot with uniform light intensity distribution to enter the wafer surface, so as to achieve uniform excitation of the sample.
[0024] e. Place a long-pass filter in front of the InGaAs CCD to completely filter out stray excitation light and only pass the near-infrared fluorescence signal generated by the non-equilibrium carrier radiation recombination in the silicon wafer.
[0025] f. Adjust the CCD lens so that its focal plane is at the wafer sample plane position.
[0026] (2) Wafer photoluminescence dynamic imaging. Based on the above experimental system, photoluminescence dynamic imaging is carried out. Through four steps, namely, modulation signal selection and setting, absolute light intensity measurement and calibration, synchronization control software writing, and dynamic image data acquisition, photoluminescence phase-locked imaging data is finally obtained. As an example, the silicon wafer sample to be tested is p-type FZ single crystal silicon with a resistivity greater than 10 3 Ω . cm, the surface is hydrogen passivated after HF etching.
[0027] a. According to the basic information of the sample to be tested, its equivalent carrier lifetime is in the order of hundreds of microseconds. Therefore, the chirp modulation frequency range generated by the function generator is set to 20-120Hz, satisfying the quasi-steady-state approximation ωτ<<1, where ω is the light intensity modulation angular frequency and τ is the equivalent carrier lifetime of the silicon wafer to be tested.
[0028] b. Measure the incident light power and spot area at the sample position, and calculate the absolute light intensity as the basis for the carrier injection dose of this characterization.
[0029] c. Use LABVIEW to write the system's synchronous control software, and the synchronous trigger sequence signal is as follows: Figure 2 As shown, after the computer sends out the main trigger signal, the function generator starts to modulate the laser light, and at the same time uses 300 Hz as the trigger signal of the CCD, satisfying the Nyquist sampling theorem.
[0030] d. According to the aforementioned system control scheme, the CCD began to continuously capture the image sequence. To ensure an excellent signal-to-noise ratio, the image signal of 100 modulation cycles was continuously recorded.
[0031] (3) Image data processing and carrier lifetime calculation. By performing cross-correlation operation on the discrete time domain signals of each pixel of the CCD in different frames and the reference signal, a quantitative spatial distribution image of the carrier lifetime of the sample can be obtained. The specific algorithm is as follows: Figure 3 shown.
[0032] a. First, perform fast Fourier transform on the measured excitation light intensity time domain signal f(t) to obtain its spectrum F(ω).
[0033] b. Multiply F(ω) by the frequency domain photoacoustic transfer function T corresponding to different thermal diffusivities point by point. The transfer function T(ω,τ) = C / (1+iωτ), where C is the normalization coefficient and i is the imaginary unit.
[0034] c. A series of complex vectors obtained by multiplying F(ω) with T(ω,τ) corresponding to different carrier lifetimes are normalized so that their bi-norms are all 1, thereby obtaining a series of normalized frequency domain reference signals.
[0035] d. Perform fast Fourier transform on the chirped photoluminescence signal s(t), then take the complex conjugate of its spectrum and multiply it with the above series of reference signals, and perform inverse Fourier transform, which is essentially a correlation operation between s(t) and these reference signals.
[0036] f. Compare the time domain correlation peaks of s(t) after correlation operation with these reference signals. The reference signal with the largest peak value is the matched filter, and the corresponding carrier lifetime is the measured value of the sample. The overall image is the quantitative spatial distribution of the carrier lifetime of the sample.
[0037] The present invention overcomes the shortcomings of the existing CCD photoluminescence imaging method, such as slow measurement speed and unoptimized signal-to-noise ratio, and completes signal processing and filtering and quantitative measurement of target parameters in the same step, and can provide a non-contact, high signal-to-noise ratio, quantitative, fast, and online carrier lifetime imaging characterization method for large-size semiconductor wafer production lines.
Claims
1. A semiconductor carrier lifetime imaging method based on chirp photoluminescence, characterized in that: The chirp signal generated by the function generator 1 is used to modulate the laser 2 so that it emits a laser whose light intensity is linearly modulated with time. After passing through the beam collimation and expansion system 3, a large light spot with uniform light intensity is formed to excite the semiconductor sample 4 and generate non-equilibrium carriers whose concentration varies with time. The photoluminescence dynamic image signal emitted by the non-equilibrium carriers through radiation recombination is sampled and photographed by CCD5. The long-pass filter 6 placed in front of the CCD5 lens filters out the excitation light emitted by the laser 2 and allows the fluorescence to enter the CCD5. The captured dynamic image data is transmitted to a computer 7 with a frame grabber. The discrete time domain signals of each pixel point of the CCD5 in different frames are cross-correlated with the reference signal to obtain a quantitative spatial distribution image of the carrier lifetime of the sample.
2. The semiconductor carrier lifetime imaging method based on chirp photoluminescence according to claim 1, characterized in that: The starting and ending frequencies of the chirp signal generated by the function generator 1 are both low frequencies, that is, the quasi-steady-state approximation ωτ<<1 is satisfied, where ω is the light intensity modulation angular frequency, and τ is the equivalent carrier lifetime of the semiconductor wafer to be measured.
3. The semiconductor carrier lifetime imaging method based on chirp photoluminescence according to claim 1, characterized in that: Laser 2 should be a continuous laser that can realize analog modulation of light intensity, and the output light intensity should have good linearity with the modulated electrical signal; the photon energy of the laser emitted by Laser 2 should be greater than the bandgap width of the semiconductor material to be tested, so as to excite non-equilibrium carriers; the optical power of Laser 2 should be high enough, and the excitation light intensity of the entire sample after beam expansion meets the requirements of carrier injection dose, which is determined by the specific characterization requirements.
4. The semiconductor carrier lifetime imaging method based on chirp photoluminescence according to claim 1, characterized in that: The laser beam emitted from the laser 2 passes through the collimation and beam expansion system 3 to form a large-sized light spot with uniform light intensity distribution, thereby achieving uniform excitation of the semiconductor wafer sample, that is, the incident light intensity at all locations on the wafer is consistent.
5. The semiconductor carrier lifetime imaging method based on chirp photoluminescence according to claim 1, characterized in that: The photosensitive band of CCD5 is selected as the fluorescence band of the semiconductor to be tested, generally a near-infrared CCD, and CCD5 is placed on the back of the wafer, that is, the other side where the excitation light is located. Therefore, the reflected light of the excitation light on the front surface of the wafer cannot reach CCD5. The long-pass filter 6 placed in front of CCD5 can further filter out stray excitation light to ensure that the image captured by CCD5 is a pure fluorescence image.
6. The semiconductor carrier lifetime imaging method based on chirp photoluminescence according to claim 1, characterized in that: CCD5 has an external trigger function, its frame rate is adjustable, and should satisfy the Nyquist sampling theorem, that is, the frame rate is greater than twice the chirp start and cutoff frequencies. The shooting trigger sequence signal of CCD5 is generated by computer 7, and the synchronization control of the entire system is realized by computer 7.
7. The semiconductor carrier lifetime imaging method based on chirp photoluminescence according to claim 1, characterized in that: The overall duration of the image sequence obtained by CCD5 should be an integer multiple of the chirp period. Generally, this condition is naturally met when the laser modulation frequency is set to an integer multiple of 1 Hz and the overall duration of the image sequence is selected to be an integer multiple of 1 s.
8. The semiconductor carrier lifetime imaging method based on chirp photoluminescence according to claim 1, characterized in that: Computer 7 convolves the laser modulation signal with the response function corresponding to different carrier lifetimes to obtain a series of reference signals. The specific algorithm is to first perform Fourier transform on the chirp modulation signal, and then multiply it with the photoluminescence transfer function C / (1+iωτ) corresponding to different carrier lifetimes, where C is the normalization coefficient and i is the imaginary unit. Then, the complex vector is normalized so that its two-norm is 1.
9. The semiconductor carrier lifetime imaging method based on chirp photoluminescence according to claim 1, characterized in that: A series of reference signals are cross-correlated with the discrete time domain signals of each pixel of the CCD in different frames. The carrier lifetime corresponding to the reference signal with the highest correlation peak is the measured value of the pixel, and the overall image is the quantitative spatial distribution of the carrier lifetime of the sample.