Plasma Process Monitoring Method and Its Monitoring System

Abstract

The present invention relates to a method for monitoring a plasma process and a monitoring system thereof, comprising: installing an electro-optical aperture device between an incident slit and an optical probe; during etching, collecting spectral signals of a light source in real time, adjusting the size of a light-transmitting aperture to control the pixel response value of the spectral signals within the pixel saturation threshold range; converting the spectral signals into frequency-domain signals through Fourier transform; performing frequency-domain filtering on the frequency-domain spectral signals by combining a peak-finding algorithm with a window function; converting the filtered frequency-domain signals into time-domain signals through inverse Fourier transform; extracting interference signals of reflected light at a specific wavelength from the time-domain signals; calculating the etching rate and the current etching depth; and sending a reminder message indicating that the etching end point has been reached when the current etching depth is equal to a preset etching end point depth. The present invention can avoid overexposure of spectral signals, filter out background light signals to obtain more accurate etching rate and etching depth, and improve the accuracy of plasma process monitoring.

Description

Technical Field

[0001] The present invention relates to the technical field of plasma process treatment, and particularly to a plasma process monitoring method and its monitoring system. Background Art

[0002] Plasma processing technology is widely used in semiconductor manufacturing processes. During the deposition or etching of semiconductor substrates, it is necessary to closely monitor the process. During dry etching, that is, plasma etching, electrons are accelerated by an electric field generated by a radio frequency power supply, collide with gas molecules entering the vacuum chamber through an injection gas pipeline, and ionize them to generate plasma. According to the different forms of radio frequency electric field generation, it can be divided into capacitively coupled plasma CCP, inductively coupled plasma ICP, transformer coupled plasma TCP, etc. Positive ions in the plasma are accelerated by the plasma sheath layer and bombard the surface of the thin film, reacting with it physically or chemically or both, generating gaseous reaction products and being pumped away by a vacuum pump, thereby selectively removing the material on the substrate surface and leaving the target pattern, while the part protected by the photoresist will not be eroded.

[0003] During this processing, it is necessary to accurately know the etching endpoint and timely stop the etching process to prevent the situation of under-etching or etching to the next layer, which may lead to defective wafers. The accuracy of etching endpoint judgment directly affects the yield of chip production.

[0004] Currently, the main methods for etch endpoint detection are Optical Emission Spectroscopy (OES) and Interferometric Endpoint (IEP). A typical Interferometric Endpoint (IEP) method is as follows: The light emitted by the light source is collimated by a collimating mirror and then perpendicularly irradiates the substrate surface. The reflected light also passes through the collimating mirror and is detected by a spectral detection device through an optical fiber. Subsequently, information such as the etch rate can be obtained through data processing methods. Among them, what the spectral detection device detects is the reflected light signal plus the strong emission spectral signal emitted by the plasma during the etching process. Since the signal intensity of the plasma emission spectrum line (background light) is large, it can even exceed the reflected light signal. Therefore, a light source with high intensity is required. One of the choices is a laser. This light source is characterized by strong monochromaticity and high intensity. Therefore, as long as a filter with a specific wavelength is placed in front of the spectral detection device, a reflection spectrum with a relatively high signal-to-noise ratio can be obtained. However, the laser has a single wavelength and a narrow applicable range for the material and thickness of the etched film layer. Another choice for the light source is a broadband light source, such as a pulsed xenon lamp. The advantage of this light source is that it has a high signal intensity and its wavelength covers the ultraviolet to infrared band. Therefore, it has a large applicable range of film layer materials. To eliminate the interference of the strong background light on the reflected light signal, during the etching process, the control module of the spectral detection device can usually be used to control the pulsed xenon lamp. By turning on the pulsed xenon lamp, a background light + reflected light signal spectrum is collected. Subsequently, the pulsed xenon lamp is turned off, and a background light signal is collected separately. During the plasma etching process, the etch rate and other parameters do not change significantly within a very short time. Therefore, the background light signal is relatively stable. Therefore, by combining algorithms to subtract the background light signal, the reflected signal can be detected and analyzed.

[0005] The technical problems existing in the above monitoring method are as follows:

[0006] 1. Currently, the typical solution for the pulsed xenon lamp is to collect two spectra, that is, first collect the background light + reflected light signal spectrum, and then only collect the background light signal, and use subtraction to obtain a pure background light signal. However, due to the very high intensity of the background light, if the reflected light signal is added, the signal response may be overexposed. For example, the saturation value of each pixel point of a general spectral detection device CCD is 65535. Assuming that for the response of the 820nm spectral line, the pure reflected light is 20000 and the pure background light is 50000. Therefore, if the conventional method of subtracting the background light is used, then the background light + reflected light will become 65535 due to overexposure, and thus the pure reflected light response will become 15535 instead of 20000, which will seriously affect the accuracy of the interferometric endpoint detection result.

[0007] 2. In addition, the background light signal during the whole process is not constant. Affected by factors such as gas flow rate change and radio frequency drift, it will have an impact on the plasma background light signal. Moreover, due to environmental factors, the dark noise of the spectral detection device itself will also drift, resulting in a large difference in the background light signal before and after subtraction, ultimately affecting the reflected light signal obtained by subtraction and also affecting the accuracy of the interference end point detection result. Summary of the Invention

[0008] The purpose of the present invention is to overcome the defects of the prior art and provide a plasma process monitoring method and device to solve the problems of low accuracy and non-real-time results in plasma process monitoring in the prior art.

[0009] To achieve the above purpose, the present invention provides a plasma process monitoring method, including the following steps:

[0010] Provide a light source, an optical probe, a spectral detection device, and an electric diaphragm device. Connect the optical probe to the spectral detection device. An adjustable-size light-transmitting hole is formed on the provided electric diaphragm device. Install the electric diaphragm device between the incident slit of the spectral detection device and the optical probe, and align the center of the light-transmitting hole with the center of the incident slit.

[0011] The provided light source is a broadband light source. During the plasma etching process, the light source flashes at a fixed flashing frequency. Use the spectral detection device to collect time-domain spectral signals at a spectral acquisition rate higher than twice the fixed flashing frequency and above, and adjust the light-transmitting hole size, thereby adjusting the light input amount at the incident slit to control the pixel response value of the collected time-domain spectral signals within the pixel saturation threshold range of the detector on the spectral detection device.

[0012] Convert the collected time-domain spectral signals into frequency-domain spectral signals through Fourier transform.

[0013] Perform frequency-domain filtering on the frequency-domain spectral signals through a peak-seeking algorithm combined with a window function.

[0014] Convert the filtered frequency-domain spectral signals into time-domain spectral signals through inverse Fourier transform, and extract the time-domain interference signal of the reflected light at a specific wavelength from the time-domain spectral signals.

[0015] Calculate the corresponding plasma etching rate within the time difference based on the time difference, specific wavelength, and corresponding refractive index between two adjacent wave peaks or adjacent wave valleys on the time-domain interference signal, and calculate the current etching depth in combination with the etching time.

[0016] When the current etching depth is equal to the preset etching end depth, a reminder message indicating that the plasma etching end point has been reached is sent.

[0017] In the present invention, a spectral detection device is used to collect the spectral signal of a light source in real time. The spectral signal includes the reflected light signal and the background light signal of the light source. The pixel response value of the spectral signal is detected by a photodetector on the spectral detection device. The size of the light transmission hole is feedback-adjusted according to the pixel response value of the spectral signal, and then the light input amount of the incident slit is adjusted, so that the pixel response value of the collected spectral signal is controlled within the pixel saturation threshold range, thereby avoiding the influence of overexposure of the detected spectral signal on the accuracy of etching end point judgment. The present invention also filters out the background light signal by means of a Fourier transform filtering method to avoid misleading the judgment of the signal intensity due to the instability of the background light signal, thereby improving the accuracy of the interference end point detection result.

[0018] A further improvement of the plasma process monitoring method of the present invention lies in obtaining the initial size of the light transmission hole and adjusting the size of the light transmission hole by using the principle of the dichotomy method, so that the pixel response value of the collected time-domain spectral signal is controlled within the pixel saturation threshold range.

[0019] A further improvement of the plasma process monitoring method of the present invention lies in that when adjusting the size of the light transmission hole by using the principle of the dichotomy method,

[0020] the pixel response value of the collected time-domain spectral signal is compared and judged with the pixel saturation threshold range;

[0021] If the pixel response value is greater than the upper limit value of the pixel saturation threshold range, the current area value of the light transmission hole is obtained, and the area of the light transmission hole is adjusted to 0.5 times the current area value of the light transmission hole, and then the process returns to the step of pixel response value comparison and judgment for re-judgment;

[0022] If the pixel response value is less than the lower limit value of the pixel saturation threshold range, the current area value of the light transmission hole is obtained, and the area of the light transmission hole is adjusted to 1.5 times the current area value of the light transmission hole, and then the process returns to the step of pixel response value comparison and judgment for re-judgment;

[0023] If the pixel response value is within the pixel saturation threshold range, the size adjustment operation is ended.

[0024] A further improvement of the plasma process monitoring method of the present invention lies in that the provided electric diaphragm device includes a pair of first adjusting plates spaced apart in the vertical direction, a pair of second adjusting plates spaced apart in the horizontal direction, and a driving assembly drivingly connected to the pair of first adjusting plates and the pair of second adjusting plates. The pair of first adjusting plates and the pair of second adjusting plates enclose to form the light-transmitting hole. The driving assembly can respectively drive the pair of first adjusting plates to move towards or away from each other and the pair of second adjusting plates to move towards or away from each other, so as to realize the adjustment of the size of the light-transmitting hole;

[0025] When adjusting the size of the light-transmitting hole, calculate the area value of the light-transmitting hole after adjustment, and combine it with the current area value of the light-transmitting hole, and then calculate the moving amount of each first adjusting plate and each second adjusting plate. Move and adjust the pair of first adjusting plates and the pair of second adjusting plates according to the corresponding moving amount to adjust the area of the light-transmitting hole to the area value after adjustment.

[0026] A further improvement of the plasma process monitoring method of the present invention lies in that when performing frequency-domain filtering on the frequency-domain spectral signal, the peak value and the full width at half maximum of the frequency-domain spectral signal near the fixed flicker frequency are obtained through a peak-seeking algorithm, and the signal waves in the frequency-domain spectral signal whose distance from the fixed flicker frequency is greater than a preset distance value are filtered out by using a window function, so as to obtain the filtered frequency-domain spectral signal;

[0027] The peak-seeking algorithm can find the peak position of the frequency signal near the fixed flicker frequency in the frequency-domain spectral signal and calculate the position and the full width at half maximum of the frequency signal peak position. The peak-seeking algorithm includes the least squares Gaussian fitting, Lorentz fitting, and the first derivative taking extreme value method;

[0028] The window function includes a rectangular window function, a triangular window function, and a Gaussian window function.

[0029] The present invention also provides a plasma process monitoring system, including:

[0030] A light source for illumination, and the provided light source is a broadband light source that flickers at a fixed flicker frequency;

[0031] A spectral detection device for detecting the spectral signal of the light source, and the spectral detection device collects the time-domain spectral signal at a spectral acquisition rate higher than twice the fixed flicker frequency and above;

[0032] An optical probe connected to the spectral detection device, which is used to detect the light emitted by the light source and couple the light into the spectral detection device;

[0033] An electro - optical diaphragm device installed between the incident slit of the spectral detection device and the optical probe. An adjustable - size light - transmitting hole is formed on the electro - optical diaphragm device, and the center of the light - transmitting hole is aligned with the center of the incident slit;

[0034] An adjustment module connected to the spectral detection device and the electro - optical diaphragm device, which is used to adjust the amount of incident light at the incident slit by adjusting the size of the light - transmitting hole during the plasma etching process, so as to control the pixel response value of the collected time - domain spectral signal within the pixel saturation threshold range of the detector on the spectral detection device;

[0035] A Fourier transform module connected to the spectral detection device, which is used to convert the collected time - domain spectral signal into a frequency - domain spectral signal through Fourier transform;

[0036] A filtering module connected to the Fourier transform module, which is used to perform frequency - domain filtering on the frequency - domain spectral signal through a peak - seeking algorithm combined with a window function;

[0037] An inverse Fourier transform module connected to the filtering module, which is used to convert the filtered frequency - domain spectral signal into a time - domain spectral signal through inverse Fourier transform;

[0038] An extraction module connected to the inverse Fourier transform module, which is used to extract the time - domain interference signal of the reflected light at a specific wavelength from the time - domain spectral signal;

[0039] An etching rate calculation module connected to the extraction module, which calculates the corresponding plasma etching rate within the time difference according to the time difference between two adjacent wave peaks or adjacent wave valleys on the time - domain interference signal, the specific wavelength, and the corresponding refractive index;

[0040] An etching depth calculation module connected to the etching rate calculation module, which calculates the current etching depth according to the calculated plasma etching rate and the etching time;

[0041] A reminder module connected to the etching depth calculation module, which is used to send a reminder message indicating the arrival of the plasma etching end point when the current etching depth is equal to the preset etching end - point depth.

[0042] A further improvement of the plasma process monitoring system of the present invention is that the adjustment module is further used to obtain the initial size of the light - transmitting hole and adjust the size of the light - transmitting hole using the principle of the dichotomy method, so as to control the pixel response value of the collected time - domain spectral signal within the pixel saturation threshold range.

[0043] A further improvement of the plasma process monitoring system of the present invention is that the adjustment module is further used to, when adjusting the size of the light - transmitting hole using the principle of the dichotomy method,

[0044] Compare and judge the pixel response value of the collected time-domain spectral signal with the pixel saturation threshold range;

[0045] If the pixel response value is greater than the upper limit value of the pixel saturation threshold range, obtain the current area value of the light-transmitting hole, adjust the area of the light-transmitting hole to 0.5 times the current area value of the light-transmitting hole, and return to the step of comparing and judging the pixel response value for re-judgment;

[0046] If the pixel response value is less than the lower limit value of the pixel saturation threshold range, obtain the current area value of the light-transmitting hole, adjust the area of the light-transmitting hole to 1.5 times the current area value of the light-transmitting hole, and return to the step of comparing and judging the pixel response value for re-judgment;

[0047] If the pixel response value is within the pixel saturation threshold range, end the size adjustment operation.

[0048] A further improvement of the plasma process monitoring system of the present invention is that the provided electric diaphragm device includes a pair of first adjusting plates spaced apart in the vertical direction, a pair of second adjusting plates spaced apart in the horizontal direction, and a driving component drivingly connected to the pair of first adjusting plates and the pair of second adjusting plates. The pair of first adjusting plates and the pair of second adjusting plates enclose to form the light-transmitting hole, and the driving component can respectively drive the pair of first adjusting plates to move towards or away from each other and the pair of second adjusting plates to move towards or away from each other, so as to realize the size adjustment of the light-transmitting hole;

[0049] The adjusting module is further configured to calculate the area value to be adjusted of the light-transmitting hole when adjusting the size of the light-transmitting hole, and combine the current area value of the light-transmitting hole, and then calculate the moving amount of each first adjusting plate and each second adjusting plate, and move and adjust the pair of first adjusting plates and the pair of second adjusting plates according to the corresponding moving amount to adjust the area of the light-transmitting hole to the area value to be adjusted.

[0050] A further improvement of the plasma process monitoring system of the present invention is that the filtering module is further configured to, when performing frequency-domain filtering on the frequency-domain spectral signal, obtain the peak value and the full width at half maximum of the frequency-domain spectral signal near the fixed flicker frequency through a peak-finding algorithm, and use a window function to filter out the signal waves in the frequency-domain spectral signal whose distance from the fixed flicker frequency is greater than a preset distance value, so as to obtain the filtered frequency-domain spectral signal;

[0051] The peak search algorithm can find the peak position of the frequency signal near the fixed flicker frequency in the frequency-domain spectral signal, and calculate the position and full width at half maximum (FWHM) of the frequency signal peak position. The peak search algorithm includes least squares Gaussian fitting, Lorentzian fitting, and the method of taking the extreme value of the first derivative.

[0052] The window function includes a rectangular window function, a triangular window function, and a Gaussian window function. Description of the Drawings

[0053] Figure 1 It is a flowchart of the plasma process monitoring method of the present invention.

[0054] Figure 2 It is a flowchart of adjusting the size of the light transmission hole by using the principle of the bisection method in the plasma process monitoring method of the present invention.

[0055] Figure 3 It is a logic block diagram of the plasma process monitoring system of the present invention.

[0056] Figure 4 It is a spectrogram containing a light source signal, a glow background signal, and a random noise signal at a specific moment in the plasma process monitoring method of the present invention.

[0057] Figure 5 It is a function graph of the intensity of a single wavelength at 820 nm versus frequency in the plasma process monitoring method of the present invention.

[0058] Figure 6 It is a function graph of the intensity of a single wavelength at 820 nm versus frequency after filtering in the plasma process monitoring method of the present invention.

[0059] Figure 7 It is a spectrogram after filtering at a specific moment in the plasma process monitoring method of the present invention.

[0060] Figure 8 It is a spectrogram of a pure light source signal at a specific moment without a glow background signal and a random noise signal.

[0061] Figure 9 It is a spectrogram of the glow background signal. Detailed Embodiments

[0062] In order to make the objectives, technical solutions, and advantages of the present invention clearer and more understandable, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0063] The present invention provides a method and a device for monitoring a plasma process. According to the spectral response value of the spectral signal, the size of the light-transmitting hole on the electro-optical diaphragm device is adjusted to avoid overexposure of the detected spectral signal, thereby improving the accuracy of the interference end-point detection structure. By filtering out the background light signal to obtain more accurate etching rate information, the accuracy of the interference end-point detection result is further improved.

[0064] The method and device for monitoring a plasma process according to the present invention will be described below with reference to the accompanying drawings.

[0065] See Figure 1 , in this embodiment, a method for monitoring a plasma process includes the following steps:

[0066] S101: Provide a light source, an optical probe, a spectral detection device, and an electro-optical diaphragm device. Connect the optical probe to the spectral detection device. An adjustable-size light-transmitting hole is formed on the provided electro-optical diaphragm device. Install the electro-optical diaphragm device between the incident slit of the spectral detection device and the optical probe, and align the center of the light-transmitting hole with the center of the incident slit.

[0067] S102: The provided light source is a broadband light source. During the plasma etching process, the light source flashes at a fixed flashing frequency. Use the spectral detection device to collect the time-domain spectral signal at a spectral acquisition rate higher than twice the fixed flashing frequency and above, and adjust the size of the light-transmitting hole, thereby adjusting the light input amount at the incident slit, so that the pixel response value of the collected time-domain spectral signal is controlled within the pixel saturation threshold range of the detector on the spectral detection device.

[0068] S103: Convert the collected time-domain spectral signal into a frequency-domain spectral signal through Fourier transform.

[0069] S104: Perform frequency-domain filtering on the frequency-domain spectral signal through a peak-seeking algorithm in combination with a window function.

[0070] S105: Convert the filtered frequency-domain spectral signal into a time-domain spectral signal through inverse Fourier transform, and extract the time-domain interference signal of the reflected light at a specific wavelength from the time-domain spectral signal.

[0071] S106: Calculate the corresponding plasma etching rate within the time difference according to the time difference between two adjacent wave peaks or adjacent wave valleys on the time-domain interference signal, the specific wavelength, and the corresponding refractive index, and calculate the current etching depth in combination with the etching time.

[0072] S107: When the current etching depth is equal to the preset etching end-point depth, send a reminder message indicating that the plasma etching end-point has been reached.

[0073] Plasma process monitoring method in this embodiment: First, according to the pixel response value of the detected spectral signal, adjust the size of the light-transmitting hole, and then adjust the amount of incident light at the incident slit, so that the pixel response value of the collected spectral signal is controlled within the pixel saturation threshold range, avoiding the problems that the spectral signal is oversaturated affected by background light or the target signal is too weak after subtracting the background light signal, resulting in unclear interference fringes. Second, in this application, the background light signal is filtered by filtering, so as to obtain a purer reflected light signal, calculate and obtain more accurate etching rate information, and improve the accuracy of the interference endpoint detection result.

[0074] Preferably, the specific wavelength reflected light is selected according to the type of light source and the material of the etched film layer.

[0075] See Figure 2 , further, obtain the initial size of the light-transmitting hole, and use the principle of the dichotomy method to adjust the size of the light-transmitting hole, so that the pixel response value of the collected time-domain spectral signal is controlled within the pixel saturation threshold range.

[0076] See Figure 2 , furthermore, when adjusting the size of the light-transmitting hole using the principle of the dichotomy method,

[0077] Compare and judge the pixel response value of the collected time-domain spectral signal with the pixel saturation threshold range;

[0078] If the pixel response value is greater than the upper limit value of the pixel saturation threshold range, obtain the current area value of the light-transmitting hole, adjust the area of the light-transmitting hole to 0.5 times the current area value of the light-transmitting hole, and return to the step of comparing and judging the pixel response value to re-judge;

[0079] If the pixel response value is less than the lower limit value of the pixel saturation threshold range, obtain the current area value of the light-transmitting hole, adjust the area of the light-transmitting hole to 1.5 times the current area value of the light-transmitting hole, and return to the step of comparing and judging the pixel response value to re-judge;

[0080] If the pixel response value is within the pixel saturation threshold range, end the size adjustment operation.

[0081] Furthermore, the provided electric diaphragm device includes a pair of first adjustment plates spaced apart in the vertical direction, a pair of second adjustment plates spaced apart in the horizontal direction, and a drive assembly drivingly connected to the pair of first adjustment plates and the pair of second adjustment plates. The pair of first adjustment plates and the pair of second adjustment plates enclose to form a light-transmitting hole, and the drive assembly can respectively drive the pair of first adjustment plates to move towards or away from each other and the pair of second adjustment plates to move towards or away from each other, so as to realize the adjustment of the size of the light-transmitting hole;

[0082] When adjusting the size of the light-transmitting hole, calculate the area value of the light-transmitting hole after adjustment, and combine it with the current area value of the light-transmitting hole, and then calculate the movement amount of each first adjusting plate and each second adjusting plate. Move and adjust a pair of first adjusting plates and a pair of second adjusting plates according to the corresponding movement amount to adjust the area of the light-transmitting hole to the area value after adjustment.

[0083] Preferably, the first adjusting plate and the second adjusting plate partially overlap. By moving and adjusting a pair of first adjusting plates and a pair of second adjusting plates, the length of the overlapping part of the first adjusting plate and the second adjusting plate is adjusted, and then the size of the light-transmitting hole formed by enclosing a pair of first adjusting plates and a pair of second adjusting plates can be adjusted.

[0084] Preferably, the pixel saturation threshold range is 50000 - 60000.

[0085] In this embodiment, a slit size feedback adjustment mechanism for the spectral response value of the spectral signal is designed. According to the spectral response value of the spectral signal, a pair of first adjusting plates and a pair of second adjusting plates are moved and adjusted in a feedback manner to adjust the size of the light-transmitting hole, that is, to adjust the range of the pair of first adjusting plates and a pair of second adjusting plates covering the incident slit, so as to realize the adjustment of the light incident amount of the incident slit and achieve automatic feedback adjustment.

[0086] Add a control circuit inside the spectral detection device and add a software algorithm on the computer side. Control the light incident amount at the incident slit by adjusting the size of the light-transmitting hole, so as to prevent the final signal from saturating. However, at the same time, in order to ensure that the signal has a high signal-to-noise ratio, the light incident amount cannot be too small. Specifically, for example, the initial size of the light-transmitting hole is set to 100um. After starting the etching, the spectral detection device collects the reflected light + background light signal of the pulsed xenon lamp. When it is found that the spectral signal saturates, the computer automatically sends a control signal to the electro-optical aperture device to adjust the size of the light-transmitting hole to 50um. If it continues to saturate, the size of the light-transmitting hole is reduced to 25um. If it does not saturate, the size of the light-transmitting hole is expanded to 75um, and then check whether it saturates. Continuously feedback and adjust the size of the light-transmitting hole until the maximum signal of the entire spectrum is finally controlled between 50000 - 60000. After continuously collecting several frames of spectra and adjusting the size of the light-transmitting hole, a reasonable size of the incident slit can be achieved to control the light incident amount of the incident slit.

[0087] In a specific embodiment, when performing frequency-domain filtering on the frequency-domain spectral signal, the peak value and the full-width at half-maximum of the frequency-domain spectral signal near the fixed flicker frequency are obtained through a peak-seeking algorithm, and the signal waves in the frequency-domain spectral signal whose distance from the fixed flicker frequency is greater than the preset distance value are filtered out by using a window function, so as to obtain the filtered frequency-domain spectral signal;

[0088] The peak searching algorithm can find the peak position of the frequency signal near the fixed flicker frequency in the frequency-domain spectral signal, calculate the position and the full width at half maximum (FWHM) of the frequency signal peak position. The peak searching algorithm includes the least squares Gaussian fitting, Lorentz fitting, and the method of taking the extreme value of the first derivative.

[0089] The window function includes the rectangular window function, triangular window function, and Gaussian window function.

[0090] Preferably, the preset distance value is 1.3 times the FWHM.

[0091] Specifically, the distance from the fixed flicker frequency is the difference between the frequency of the signal wave in the frequency-domain spectral signal and the fixed flicker frequency. Filtering out the signal waves in the frequency-domain spectral signal whose distance from the fixed flicker frequency is greater than the preset distance value means filtering out the signal waves outside the range of the preset distance value around the fixed flicker frequency with the fixed flicker frequency as the center line.

[0092] The present invention designs a method for filtering background light signals. The spectral signal is converted into a frequency-domain signal through Fourier transform, and the part of the background light signal in the frequency-domain signal is filtered out, which solves the problem that the instability of the background light signal misleads the judgment of the signal intensity. The filtered frequency-domain signal is converted into a time-domain signal through inverse Fourier transform, and then the interference signal of the reflected light with a specific wavelength is extracted from the time-domain signal, and the parameter information on the interference signal is obtained to calculate and obtain more accurate etching rate information, improving the accuracy of the interference endpoint detection result.

[0093] In this embodiment, the Fourier transform filtering method is used to process the spectrum to remove the influence of the background in the spectrum on the film thickness monitoring. The Fourier transform filtering method converts the continuously acquired spectral signal to the frequency domain through Fourier transform. The optical signal excited by the pulsed xenon lamp has a fixed repetition frequency, which appears as a peak signal with stable intensity and position in the frequency domain; while the background light signal does not have specific frequency characteristics and appears as miscellaneous signals in the frequency domain. Implementing band-pass filtering near the xenon lamp frequency in the frequency domain can filter out the background light, and then performing inverse Fourier transform can reconstruct the time-domain signal after filtering out the background. The interference signal of the reflected light with a specific wavelength is extracted from the time-domain signal, and more accurate etching rate information is calculated and obtained.

[0094] A specific implementation manner of the plasma process monitoring method of the present invention will be described below.

[0095] 1. Refer to Figure 4 、 Figure 8 and Figure 9, when the xenon lamp is triggered, the signal of the sample to be measured reflected by the xenon light source into the spectral detection device is collected. This signal contains the xenon lamp signal, the glow background, and a part of random noise. When it is found that the signal is oversaturated, the size of the light transmission hole is feedback-regulated until the signal intensity is adjusted to an appropriate range.

[0096] 2. Refer to Figure 5 , the spectral signal is Fourier-transformed to convert it into a frequency-domain signal. At this time, there are spikes at the bottom edge of the frequency-domain signal and high peaks at low frequencies. Refer to Figure 6 , the frequency-domain signal is filtered. Refer to Figure 7 , a spectral data graph with the bottom edge and low-frequency signals filtered out can be obtained, so as to filter out the background light signal. Then, the filtered frequency-domain signal is inverse Fourier-transformed to obtain the time-domain signal. In this embodiment, referring to the separate xenon lamp signal, the interference signal at the wavelength of 820 nm is taken from the time-domain signal.

[0097] Figure 7 This is the spectral graph after filtering at a specific moment in the plasma process monitoring method of the present invention. Figure 8 This is the spectral graph of the pure light source signal at a specific moment without the glow background signal and random noise signal. Figure 7 The filtered spectrum in Figure 8 is highly close to the ideal spectrum in

[0098] 3. The processed spectral data is processed in real time. By monitoring the time-sequence diagram of the reflection spectral line at a specific wavelength of 820 nm, information such as the etching rate is calculated, so as to calculate and obtain the current etching depth in real time. When the current etching depth is equal to the preset etching end depth, a reminder message of reaching the etching end is sent.

[0099] Refer to Figure 3 , the present invention also provides a plasma process monitoring system, including:

[0100] A light source for illumination, and the provided light source is a broadband light source that flashes at a fixed flashing frequency;

[0101] A spectral detection device for detecting the spectral signal of the light source, and the spectral detection device collects the time-domain spectral signal at a spectral acquisition rate higher than twice the fixed flashing frequency and above;

[0102] An optical probe connected to the spectral detection device, which is used to detect the light emitted by the light source and couple the light into the spectral detection device;

[0103] An electric diaphragm device installed between the incident slit of the spectral detection device and the optical probe. A light transmission hole with adjustable size is formed on the electric diaphragm device, and the center of the light transmission hole is aligned with the center of the incident slit.

[0104] An adjustment module connected to a spectral detection device and an electric diaphragm device, which is used to adjust the amount of incident light at the incident slit by adjusting the size of the light-transmitting hole during the plasma etching process, so that the pixel response value of the collected time-domain spectral signal is controlled within the pixel saturation threshold range of the detector on the spectral detection device;

[0105] A Fourier transform module connected to the spectral detection device, which is used to convert the collected time-domain spectral signal into a frequency-domain spectral signal through Fourier transform;

[0106] A filtering module connected to the Fourier transform module, which is used to perform frequency-domain filtering on the frequency-domain spectral signal by combining a peak-finding algorithm with a window function;

[0107] An inverse Fourier transform module connected to the filtering module, which is used to convert the filtered frequency-domain spectral signal into a time-domain spectral signal through inverse Fourier transform;

[0108] An extraction module connected to the inverse Fourier transform module, which is used to extract the time-domain interference signal of the reflected light at a specific wavelength from the time-domain spectral signal;

[0109] An etching rate calculation module connected to the extraction module, which calculates the corresponding plasma etching rate within the time difference according to the time difference between two adjacent wave peaks or adjacent wave valleys on the time-domain interference signal, the specific wavelength, and the corresponding refractive index;

[0110] An etching depth calculation module connected to the etching rate calculation module, which calculates the current etching depth according to the calculated plasma etching rate and etching time;

[0111] A reminder module connected to the etching depth calculation module, which is used to send a reminder message indicating that the plasma etching end point has been reached when the current etching depth is equal to the preset etching end point depth.

[0112] See Figure 2 , further, the adjustment module is also used to obtain the initial size of the light-transmitting hole, and adjust the size of the light-transmitting hole using the principle of the dichotomy method, so that the pixel response value of the collected time-domain spectral signal is controlled within the pixel saturation threshold range.

[0113] See Figure 2 , furthermore, the adjustment module is also used to compare and judge the pixel response value of the collected time-domain spectral signal with the pixel saturation threshold range when adjusting the size of the light-transmitting hole using the principle of the dichotomy method;

[0114] Compare and judge the pixel response value of the collected time-domain spectral signal with the pixel saturation threshold range;

[0115] If the pixel response value is greater than the upper limit of the pixel saturation threshold range, obtain the current area value of the light-transmitting aperture, adjust the area of the light-transmitting aperture to 0.5 times the current area value of the light-transmitting aperture, and return to the step of comparing and judging the pixel response value for re-judgment;

[0116] If the pixel response value is less than the lower limit of the pixel saturation threshold range, obtain the current area value of the light-transmitting aperture, adjust the area of the light-transmitting aperture to 1.5 times the current area value of the light-transmitting aperture, and return to the step of comparing and judging the pixel response value for re-judgment;

[0117] If the pixel response value is within the pixel saturation threshold range, end the size adjustment operation.

[0118] Further, the provided electric diaphragm device includes a pair of first adjusting plates spaced apart in the vertical direction, a pair of second adjusting plates spaced apart in the horizontal direction, and a driving assembly drivingly connected to the pair of first adjusting plates and the pair of second adjusting plates. The pair of first adjusting plates and the pair of second adjusting plates enclose a light-transmitting aperture. The driving assembly can respectively drive the pair of first adjusting plates to move towards or away from each other and the pair of second adjusting plates to move towards or away from each other, so as to realize the size adjustment of the light-transmitting aperture;

[0119] The adjusting module is further configured to calculate the area value of the light-transmitting aperture to be adjusted when adjusting the size of the light-transmitting aperture, and in combination with the current area value of the light-transmitting aperture, further calculate the moving amount of each first adjusting plate and each second adjusting plate, and move and adjust the pair of first adjusting plates and the pair of second adjusting plates according to the corresponding moving amount to adjust the area of the light-transmitting aperture to the area value to be adjusted.

[0120] In a specific embodiment, parts of the adjusting module, Fourier transform module, filtering module, inverse Fourier transform module, extraction module, etching rate calculation module, etching depth calculation module, and reminder module in the monitoring system are software programs installed on a mobile terminal device, which can be a computer or a mobile phone, etc. The user manipulates the software program on the mobile terminal device to obtain spectral data, signal wave data before and after filtering, etching rate data, etching depth data, etc. during the etching process. Specifically, the user can set a preset etching end depth according to requirements on the software program. When the current etching depth is equal to the preset etching end depth, a reminder message indicating the arrival of the etching end will be sent on the software program to remind the user, and the reminder message can be a text message or a voice message.

[0121] In this embodiment, the Fourier transform filtering method is used to filter out the background light signal, and the interference signal of the reflected light at a specific wavelength is extracted, and then a more accurate etching rate and the current etching depth are calculated to ensure accurate and timely feedback of the reminder message indicating the arrival of the etching end.

[0122] Preferably, the pixel saturation threshold range is 50,000 - 60,000.

[0123] In a specific embodiment, the filtering module is further configured to, when performing frequency-domain filtering on the frequency-domain spectral signal, obtain the peak value and the full width at half maximum (FWHM) of the frequency-domain spectral signal near the fixed flicker frequency through a peak-seeking algorithm, and filter out the signal waves in the frequency-domain spectral signal whose distance from the fixed flicker frequency is greater than a preset distance value by using a window function, so as to obtain the filtered frequency-domain spectral signal;

[0124] The peak-seeking algorithm can find the peak position of the frequency signal near the fixed flicker frequency in the frequency-domain spectral signal and calculate the position and the full width at half maximum of the frequency signal peak position. The peak-seeking algorithm includes the least squares Gaussian fitting, Lorentz fitting, and the first derivative extreme value method;

[0125] The window function includes a rectangular window function, a triangular window function, and a Gaussian window function.

[0126] Preferably, the preset distance value is 1.3 times the full width at half maximum.

[0127] By adopting the above technical solutions, the present invention has the following beneficial effects:

[0128] 1. Solved the problem that the interference fringes are unclear due to the over-saturation of the detected spectral signal affected by the background light, or the target signal is too weak after deducting the background light signal;

[0129] 2. Solved the problem that the instability of the background light signal misleads the judgment of the signal intensity, thereby affecting the accuracy of the etching rate calculation.

[0130] 3. Has strong robustness and will not have problems such as dislocation due to small frequency errors in the system.

[0131] 4. Enhanced the accuracy of the IEP method for monitoring the etching process.

[0132] It should be noted that the structures, ratios, sizes, etc. shown in the drawings of this specification are only used to cooperate with the content disclosed in the specification for those skilled in this technology to understand and read, and are not used to limit the limited conditions under which the present invention can be implemented. Therefore, they do not have technical substantive meanings. Any modification of the structure, change of the proportional relationship, or adjustment of the size, without affecting the effects that the present invention can produce and the purposes that can be achieved, should still fall within the scope covered by the technical content disclosed in the present invention. At the same time, the terms such as "upper", "lower", "left", "right", "middle", and "one" cited in this specification are only for the convenience of clear narration and are not used to limit the scope under which the present invention can be implemented. The change or adjustment of their relative relationships, without substantial change in the technical content, should also be regarded as the scope within which the present invention can be implemented.

Claims

1. A method for monitoring a plasma process, characterized in that, It includes the following steps: Provide a light source, an optical probe, a spectral detection device, and an electric diaphragm device. Connect the optical probe to the spectral detection device. An adjustable-size light-transmitting hole is formed on the provided electric diaphragm device. Install the electric diaphragm device between the incident slit of the spectral detection device and the optical probe, and align the center of the light-transmitting hole with the center of the incident slit; The provided light source is a broadband light source. During the plasma etching process, the light source flashes at a fixed flashing frequency. Use the spectral detection device to collect the time-domain spectral signal at a spectral acquisition rate that is twice or more than twice the fixed flashing frequency. By adjusting the size of the light-transmitting hole, further adjust the light input amount at the incident slit so that the pixel response value of the collected time-domain spectral signal is controlled within the pixel saturation threshold range of the detector on the spectral detection device; Convert the collected time-domain spectral signal into a frequency-domain spectral signal through Fourier transform; Perform frequency-domain filtering on the frequency-domain spectral signal through a peak-finding algorithm combined with a window function; Convert the filtered frequency-domain spectral signal into a time-domain spectral signal through inverse Fourier transform, and extract the time-domain interference signal of the reflected light at a specific wavelength from the time-domain spectral signal; Calculate the corresponding plasma etching rate within the time difference according to the time difference, specific wavelength, and corresponding refractive index between two adjacent wave peaks or adjacent wave valleys on the time-domain interference signal, and combine the etching time to calculate the current etching depth; When the current etching depth is equal to the preset etching end depth, send a reminder message indicating that the plasma etching end has been reached; When performing frequency-domain filtering on the frequency-domain spectral signal, obtain the peak value and full width at half maximum of the frequency-domain spectral signal near the fixed flashing frequency through a peak-finding algorithm, and use the window function to filter out the signal waves in the frequency-domain spectral signal whose distance from the fixed flashing frequency is greater than the preset distance value, so as to obtain the filtered frequency-domain spectral signal; The peak-finding algorithm can find the peak position of the frequency signal near the fixed flashing frequency in the frequency-domain spectral signal and calculate the position and full width at half maximum of the frequency signal peak position.

2. The plasma process monitoring method according to claim 1, characterized in that, Obtain the initial size of the light-transmitting hole, and use the principle of the dichotomy method to adjust the size of the light-transmitting hole so that the pixel response value of the collected time-domain spectral signal is controlled within the pixel saturation threshold range.

3. The plasma process monitoring method according to claim 2, characterized in that, When using the principle of the dichotomy method to adjust the size of the light-transmitting hole, Compare and judge the pixel response value of the collected time-domain spectral signal with the pixel saturation threshold range; If the pixel response value is greater than the upper limit value of the pixel saturation threshold range, obtain the current area value of the light-transmitting hole, adjust the area of the light-transmitting hole to 0.5 times the current area value of the light-transmitting hole, and return to the step of comparing and judging the pixel response value to re-judge; If the pixel response value is less than the lower limit value of the pixel saturation threshold range, obtain the current area value of the light-transmitting hole, adjust the area of the light-transmitting hole to 1.5 times the current area value of the light-transmitting hole, and return to the step of comparing and judging the pixel response value to re-judge; If the pixel response value is within the pixel saturation threshold range, the size adjustment operation is terminated.

4. The plasma process monitoring method according to claim 3, wherein The provided electric diaphragm device includes a pair of first adjustment plates spaced apart in the vertical direction, a pair of second adjustment plates spaced apart in the horizontal direction, and a drive assembly drivingly connected to the pair of first adjustment plates and the pair of second adjustment plates. The pair of first adjustment plates and the pair of second adjustment plates enclose to form the light transmission hole. The drive assembly can respectively drive the pair of first adjustment plates to move towards or away from each other and the pair of second adjustment plates to move towards or away from each other, so as to realize the size adjustment of the light transmission hole. When adjusting the size of the light transmission hole, calculate the area value of the light transmission hole to be adjusted after adjustment, and combine it with the current area value of the light transmission hole, and then calculate the movement amount of each first adjustment plate and each second adjustment plate. Move and adjust the pair of first adjustment plates and the pair of second adjustment plates according to the corresponding movement amounts to adjust the area of the light transmission hole to the area value to be adjusted after adjustment.

5. The plasma process monitoring method according to claim 1, characterized in that, The peak seeking algorithm includes least squares Gaussian fitting, Lorentz fitting, and the method of taking the extreme value of the first derivative. The window function includes a rectangular window function, a triangular window function, and a Gaussian window function.

6. A plasma process monitoring system, characterized in that, It includes: A light source for illumination, and the provided light source is a broadband light source, and the light source flashes at a fixed flashing frequency. A spectral detection device for detecting the spectral signal of the light source, and the spectral detection device collects the time-domain spectral signal at a spectral acquisition rate higher than twice the fixed flashing frequency and above. An optical probe connected to the spectral detection device, which is used to detect the light emitted by the light source and couple the light into the spectral detection device. An electric diaphragm device installed between the incident slit of the spectral detection device and the optical probe. The electric diaphragm device is formed with a light transmission hole with adjustable size, and the center of the light transmission hole is aligned with the center of the incident slit. An adjustment module connected to the spectral detection device and the electric diaphragm device, which is used to adjust the light input amount at the incident slit by adjusting the size of the light transmission hole during the plasma etching process, so that the pixel response value of the collected time-domain spectral signal is controlled within the pixel saturation threshold range of the detector on the spectral detection device. A Fourier transform module connected to the spectral detection device, which is used to convert the collected time-domain spectral signal into a frequency-domain spectral signal through Fourier transform. A filtering module connected to the Fourier transform module, which is used to perform frequency-domain filtering on the frequency-domain spectral signal through a peak seeking algorithm combined with a window function. An inverse Fourier transform module connected to the filtering module, which is used to convert the filtered frequency-domain spectral signal into a time-domain spectral signal through inverse Fourier transform. An extraction module connected to the inverse Fourier transform module, which is used to extract the time-domain interference signal of the reflected light with a specific wavelength from the time-domain spectral signal. An etching rate calculation module connected to the extraction module calculates the corresponding plasma etching rate within the time difference based on the time difference between two adjacent wave peaks or adjacent wave valleys on the time-domain interference signal, a specific wavelength, and the corresponding refractive index; An etching depth calculation module connected to the etching rate calculation module calculates the current etching depth based on the calculated plasma etching rate and the etching time; A reminder module connected to the etching depth calculation module is configured to send a reminder message indicating the arrival at the plasma etching end point when the current etching depth is equal to a preset etching end point depth; The filtering module is further configured to, when performing frequency-domain filtering on the frequency-domain spectral signal, obtain the peak value and the full width at half maximum of the frequency-domain spectral signal near the fixed flicker frequency through a peak searching algorithm, and filter out the signal waves in the frequency-domain spectral signal whose distance from the fixed flicker frequency is greater than a preset distance value by using a window function, thereby obtaining the filtered frequency-domain spectral signal; The peak searching algorithm can find the frequency signal peak position near the fixed flicker frequency in the frequency-domain spectral signal and calculate the position and the full width at half maximum of the frequency signal peak position.

7. The plasma process monitoring system according to claim 6, wherein The adjustment module is further configured to obtain the initial size of the light transmission hole and adjust the size of the light transmission hole by using the principle of the dichotomy method so that the pixel response value of the collected time-domain spectral signal is controlled within the pixel saturation threshold range.

8. The plasma process monitoring system according to claim 7, characterized in that, The adjustment module is further configured to, when adjusting the size of the light transmission hole by using the principle of the dichotomy method, compare and judge the pixel response value of the collected time-domain spectral signal with the pixel saturation threshold range; If the pixel response value is greater than the upper limit value of the pixel saturation threshold range, obtain the current area value of the light transmission hole, adjust the area of the light transmission hole to 0.5 times of the current area value of the light transmission hole, and return to the step of comparing and judging the pixel response value for re-judgment; If the pixel response value is less than the lower limit value of the pixel saturation threshold range, obtain the current area value of the light transmission hole, adjust the area of the light transmission hole to 1.5 times of the current area value of the light transmission hole, and return to the step of comparing and judging the pixel response value for re-judgment; If the pixel response value is within the pixel saturation threshold range, end the size adjustment operation.

9. The plasma process monitoring system according to claim 8, wherein The provided electric diaphragm device includes a pair of first adjustment plates spaced apart in the vertical direction, a pair of second adjustment plates spaced apart in the horizontal direction, and a driving assembly drivingly connected to the pair of first adjustment plates and the pair of second adjustment plates. The pair of first adjustment plates and the pair of second adjustment plates enclose to form the light transmission hole. The driving assembly can respectively drive the pair of first adjustment plates to move towards or away from each other and the pair of second adjustment plates to move towards or away from each other, thereby realizing the adjustment of the size of the light transmission hole; The adjustment module is further configured to calculate the area value of the light-transmitting hole after adjustment when adjusting the size of the light-transmitting hole, and combine the current area value of the light-transmitting hole, and then calculate the movement amount of each of the first adjustment plates and each of the second adjustment plates, and move and adjust a pair of the first adjustment plates and a pair of the second adjustment plates according to the corresponding movement amount to adjust the area of the light-transmitting hole to the area value after adjustment.

10. The plasma process monitoring system according to claim 6, wherein, The peak-seeking algorithm includes the least squares Gaussian fitting, Lorentz fitting, and the method of taking the extreme value of the first derivative; The window function includes a rectangular window function, a triangular window function, and a Gaussian window function.

Images