TWI900915B - Capacitively coupled plasma processor - Google Patents

Abstract

The present invention provides a capacitively coupled plasma processor comprising a reaction chamber containing a base and an upper electrode disposed opposite the base. The base is used to support a substrate to be processed. A source radio frequency (RF) power supply and a bias RF power supply are connected to the base. An RF signal processing device is connected to a unidirectional coupler at the output of the source RF power and is used to detect the reflected power of the fundamental frequency reflected to the high-frequency RF power supply. The RF signal processing device includes an RF power detector, a fast Fourier transform (FFT) module, and a processor. The processor receives the first reflected power value output by the radio frequency power detection module and the power ratio parameter of each frequency obtained from the fast Fourier transform module, and calculates a second reflected power value obtained by removing the intermodulation reflected power according to a preset algorithm.

Description

Capacitively coupled plasma processor

The present invention relates to the field of semiconductor device technology, and in particular to a plasma processor and a radio frequency matching method thereof.

In the field of semiconductor manufacturing, plasma etching is a key process for wafer processing. With the evolving requirements of semiconductor devices, multiple RF power supplies are often required to power plasma processors. As shown in Figure 1, a capacitively coupled plasma processor comprises a reaction chamber 10 containing an upper electrode 24 and a susceptor 20 serving as a lower electrode. Above the susceptor 20 lies an electrostatic chuck 22 for securing a wafer W. A high-frequency RF power source HF supplies high-frequency (fH) RF power to the susceptor 20 through a matching circuit 11. This power enters the plasma processor as source RF power, igniting and maintaining the plasma. A low-frequency RF power source LF supplies low-frequency (fL) RF power to the susceptor 20 through a matching circuit 13 to control the energy of ions incident on the wafer W. The matching circuits 13 and 11 ensure that the RF outputs from the two RF power sources are supplied to the reaction chamber 10 as much as possible to minimize reflected power. For applications requiring rapid changes in output power, such as pulsed RF power output, the RF power supply's output frequency also needs to be adjusted. By rapidly adjusting the output RF frequency, impedance matching can be adjusted within microseconds, achieving rapid impedance matching for the pulsed RF power.

Impedance matching through frequency modulation of the RF power supply output requires accurate detection of the RF power reflected from the reactor to the RF power supply. However, existing technologies are unable to quickly and accurately detect reflected RF power. Existing technologies typically filter the desired RF signal and noise signals through a filter before detecting the reflected power signal. However, for RF power supplies with a large frequency conversion range, the filter must reject noise frequency signals while allowing fH, which has a wide frequency range, to pass with low impedance. Conventional methods in existing technologies, which rely on selecting a set of optimized inductor and capacitor combinations to form a special parameter filter, cannot solve this problem. Therefore, a new detection device or method is needed to quickly and accurately detect the reflected power within the plasma reaction cavity. This detection method can be applied to rapidly changing the output frequency within a certain range while still effectively filtering out RF signals in other noise bands besides the incident frequency.

The present invention provides a capacitively coupled plasma processor, comprising a reaction chamber, wherein the reaction chamber comprises a base and an upper electrode arranged opposite to the base, wherein the base is used to support a substrate to be processed. A first RF power source outputs a first incident power with a first frequency to the base for igniting and maintaining the plasma in the reaction chamber; a second RF power source outputs a second incident power with a second frequency to the base, wherein the first frequency is greater than 13 MHz and the second frequency is greater than 10 kHz and less than or equal to 400 kHz. The invention is characterized in that: an RF signal processing device is connected between the output end of the first RF power source and the base, and the RF signal processing device includes a sampling device, an RF power detection module, a fast Fourier transform module, and a processor; wherein the RF power detection module is used to detect the reflected power signal obtained on the sampling device, and the reflected power The signal includes reflected power having a first frequency and intermodulation reflected power having the first frequency ± a second frequency. The radio frequency power detection module obtains a first reflected power value based on the reflected power signal. The fast Fourier transform module decomposes the reflected power signal to obtain power ratio parameters of each frequency. The processor receives the first reflected power value output by the radio frequency power detection module and the power ratio parameters of each frequency obtained from the fast Fourier transform module, and calculates a second reflected power value by removing the intermodulation reflected power according to a preset algorithm. The first radio frequency power supply adjusts the output first frequency based on the second reflected power value to achieve impedance matching.

The preset algorithm includes calculating the sum of power ratios within the range of the first frequency ± the spread frequency based on a power ratio parameter provided by a fast Fourier transform module to obtain a second reflected power value, where the spread frequency is less than half the second frequency. Alternatively, the preset algorithm includes determining the split frequencies fd1 and fd2 corresponding to the power minimum points between the first frequency signal and two intermodulation reflection signals, and calculating the sum of power ratios within the range fd1-fd2 to obtain the second reflected power value. This algorithm enables accurate detection of reflected power at the base frequency, thereby achieving effective matching based on accurate reflected power data.

To more clearly illustrate the technical solution of the present invention, the following briefly introduces the figures required for the description. Obviously, the figures described below are an embodiment of the present invention. For those with ordinary knowledge in this field, other figures can be obtained based on these figures without inventive effort:

The following will provide a clear and complete description of the technical solutions in the embodiments of the present invention, combined with the drawings in the embodiments. Obviously, the described embodiments are only a portion of the embodiments of the present invention, and not all of them. All other embodiments obtained by persons of ordinary skill in the art based on the embodiments of the present invention without inventive effort are also within the scope of protection of the present invention.

The inventors discovered that the primary cause of distortion in the detection of the high-frequency RF signal (fH) output by the RF power supply is that when the high-frequency RF power source HF and the low-frequency RF power source LF are simultaneously applied to the plasma processor, plasma is generated. The nonlinear impedance of the plasma causes intermodulation between the two RF signals. As a result, the high-frequency signal reflected back from the RF power source HF not only has the reflected power of the basic incident frequency fH, but also produces two intermodulation reflection frequency signals at frequencies fH ± fL. Calculations show that the power of these two intermodulation reflection frequency signals can reach approximately 5-20% of the incident frequency (fH), making them non-negligible. If these two intermodulation frequency signals cannot be effectively eliminated, the subsequent RF matching algorithm, which adjusts the HF power supply's HF output fundamental frequency (fH), will have significant errors, preventing fast and effective matching. Existing technologies typically use a low-frequency RF power (fL) of 2 MHz. When the HF power supply's HF output fundamental frequency fH is relatively high, such as 60 MHz, the 58 MHz and 62 MHz signals generated by the intermodulation of the two output frequencies in the plasma can be filtered out of the reflected power signal. However, when the LF output frequency is ultra-low, that is, when fL is only 10kHz-400kHz, for example, when fL is 400kHz, the intermodulation frequencies of 59.6MHz and 60.4MHz are very close to the fundamental incident frequency of 60MHz, and cannot be separated by filtering.

Based on the above findings, the present invention proposes a new high-frequency RF signal matching control circuit as shown in Figure 2a. The RF power output by the high-frequency RF power source HF with variable frequency output is output to the matching circuit 11 through the unidirectional coupler 14, and finally fed into the reaction chamber 10 to form plasma for plasma processing. An RF signal processing device 30 is connected to the unidirectional coupler 14 to detect the fundamental frequency reflected power reflected to the high-frequency RF power source HF. The RF signal processing device 30 includes an RF power detector 32, a fast Fourier transform module 34 and a processor 36. The RF power detector 32 is connected to the unidirectional coupler 14 to detect the RF power reflected from the plasma processor to the unidirectional coupler 14. A filter is installed in the matching circuit 11 or between the HF power supply HF and the base 20. The filter's conduction frequency is selected to ensure that the RF power output by the variable-frequency HF power supply HF passes within its frequency range. For example, if the fH output frequency needs to be adjustable within a 3 MHz range around the center frequency of 60 MHz, meaning the HF power supply's output range varies between 57-63 MHz, the filter must be able to pass signals in this frequency band with low impedance. fH can also be other frequencies, such as 27 MHz. By configuring the filter parameters, signals within a frequency range of approximately 5% surrounding the center frequency can pass with low impedance while blocking signals at other frequencies. The filter settings can concentrate the power detected by the RF power detector into a frequency band centered around the fundamental frequency, preventing other frequency signals, such as harmonics, from interfering with power detection. Therefore, the RF power detector 32 detects the filtered first reflected power signal within the 57-63 MHz frequency band, and the power value of the first reflected power signal can be obtained through detection. The first reflected power signal is mixed with a fundamental frequency fH signal and corresponding intermodulation frequency signals fH±fL.

A fast Fourier transform module 34 is also connected to the output of the one-way coupler 14. Through sampling and rapid calculation, it can obtain the spectral power distribution data of the filtered signal. Figure 3 shows an exemplary spectral power distribution diagram when fL is 400 kHz.

The power ratio of the intermodulation reflection frequencies is related to the LF power output. Increasing the LF power output will also lead to a corresponding increase in the signal strength of the two intermodulation reflection frequencies (58.1 MHz and 58.9 MHz) in Figure 3.

A processor 36 receives the output signals from both the RF power detector 32 and the fast Fourier transform module 34. Based on the proportion of the fundamental frequency (fH) in the fast Fourier transform data (that is, the ratio of the area of the 58.5 MHz peak to the area of all frequency peaks in the figure), it can calculate the proportion of the reflected power detected by the RF power detector 32 that corresponds to the fundamental frequency of 58.5 MHz in the first reflected power signal. When calculating the power fraction corresponding to the fundamental frequency, a spread frequency fE can be set, as shown in Figure 3. The power in the frequency range fH ± fE is counted as the reflected power corresponding to the fundamental frequency fH. The spread frequency fE can be less than half of fL to ensure that intermodulation reflected frequency signals are excluded. For example, in Figure 3, the spread frequency fE is 0.2 MHz, and the sum of the power fractions in the range 58.3-58.7 MHz is 85%. In this case, the reflected power corresponding to the fundamental frequency of 58.5 MHz is 85% of the first reflected power value mentioned above. Alternatively, based on the power distribution curve in Figure 3, the two lowest power points between the fundamental frequency fH and the two intermodulation reflection frequency peaks fH±fL are calculated. The frequencies fD1 and fD2 corresponding to these two lowest power points serve as the two split frequencies. The sum of the power ratios corresponding to the frequency bands between the two split frequencies is multiplied by the detected first reflected power signal to obtain the optimized second reflected power. For example, if frequencies fD1 and fD2 are 58.25 MHz and 58.68 MHz, respectively, and the sum of the ratios within the two split frequencies is 85.5%, then the reflected power corresponding to the fundamental frequency of 58.5 MHz is 85.5% of the first reflected power value.

The above-mentioned frequency band segmentation or calculation methods are merely examples. Other algorithms can be selected based on specific needs and application scenarios. As long as they can remove the signal representing the intermodulation reflection frequency, other calculation methods are also embodiments of the present invention.

Processor 36 multiplies the received first reflected power value by the power ratio corresponding to the fundamental frequency to obtain a second reflected power value corresponding to the fundamental frequency, after removing the intermodulation frequency from the first reflected power signal. This second reflected power value is then fed back to the high-frequency RF power supply HF. Based on the precise reflected power signal received, the HF power supply HF performs multiple frequency adjustments within a preset frequency range, gradually optimizing the output frequency of the HF power supply HF and ultimately achieving optimal matching. As the output frequency fH of the high-frequency RF power source HF changes, the fundamental frequency fH component signal detected by the corresponding RF power detector 32 and the fast Fourier transform module 34 also changes accordingly. The processor 36 can determine the current fundamental frequency based on the output fundamental frequency signal provided by the high-frequency RF power source HF or the fH frequency signal detected by an additional detector. It then calculates the proportion of the corresponding fundamental frequency in the reflected power based on the current fundamental frequency. The corresponding second reflected power value in the current first reflected power signal is then calculated and provided to the high-frequency RF power supply HF, allowing the high-frequency RF power supply HF to perform the next matching operation and adjust the output frequency fH in the next step. This cycle repeats until a matching state is achieved.

The reflected power signal sampling in the present invention is achieved through a unidirectional coupler, enabling separation of the incident and reflected RF power. The RF signal at the RF power supply output can also be directly digitally sampled, achieving A/D conversion and then processed by a calculator. During signal processing, the voltage and current phase differences between the incident and reflected RF signals are separated, and the incident power and the reflected power value, which is a mixture of the fundamental frequency fH and the intermodulation reflection frequencies fH±fL, are calculated. The power ratio corresponding to the fundamental frequency in the reflected RF signal is then calculated using the fast Fourier transform module proposed in the present invention, ultimately obtaining the second reflected power value after eliminating the intermodulation reflection frequencies, which is required for RF matching.

The output frequency of the low-frequency radio frequency power source LF in the present invention can be 10Khz-300Khz. This low-frequency signal is more suitable for the reflected power detection device and detection method proposed in the present invention.

The RF signal processing device proposed in this invention enables faster and more accurate detection of the reflected power of the fundamental frequency reflected from the plasma processor, eliminating the need for designing filters with precise parameters. Furthermore, the sampling and calculation speed of the Fast Fourier Transform (FFT) can reach micrometers or even nanoseconds, faster than the corresponding frequency modulation step of the HF power supply itself. This ensures that the HF power supply achieves frequency modulation matching at the fastest speed.

The present invention also provides another embodiment as shown in Figure 2b, which is characterized in that the RF signal processing device 30' includes a second RF power detector 31 connected to one of the output terminals of the unidirectional coupler 14, for detecting the power value Pf of the RF signal having a fundamental frequency fH output by the high-frequency RF power source HF. Combined with the RF power detector 32, fast Fourier transform module 34, and processor 36' shown in Figure 2a, the RF signal processing device 30' can fully detect the fundamental frequency incident power Pf and the second reflected power. Through the built-in matching frequency algorithm of the processor 36', which can be customized and optimized by the plasma processor manufacturer, the matching algorithm is directly assigned to the high-frequency RF power supply HF, so that the high-frequency RF power supply HF directly outputs the frequency specified by the processor 36'. By utilizing the RF signal processing device 30', the internal algorithm can be unaffected by differences in RF power supply suppliers and models, improving the consistency of the performance of the same algorithm across different matching circuits and reducing the development complexity of new plasma processors. The processor 36' of the present invention also stores the FM matching program. This integration of the matching program within the processor 36 significantly reduces the design complexity of the high-frequency RF power supply and increases the flexibility of matching program design.

The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any person skilled in the art will readily conceive of various equivalent modifications or substitutions within the technical scope disclosed herein, and such modifications or substitutions should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention shall be subject to the scope of protection of the patent application.

10: Reaction chamber 11: Matching circuit 13: Matching circuit 14: Unidirectional coupler 20: Base 22: Electrostatic chuck 24: Upper electrode 30, 30': RF signal processing device 31: Second RF power detector 32: RF power detector 34: Fast Fourier transform module 36, 36': Processor W: Wafer LF: Low-frequency RF power supply HF: High-frequency RF power supply fE: Spreading frequency fD1, fD2: Frequency

Figure 1 is a schematic diagram of a prior art plasma processing device; Figures 2a and 2b are schematic diagrams of the first and second embodiments of the processing device of the present invention; Figure 3 is a schematic diagram of the reflected power spectrum obtained through fast Fourier transform (FFT) of the present invention, where the HF frequency is 58.5 MHz and the LF frequency is 400 kHz.

10: Reaction chamber

11: Matching circuit

13: Matching circuit

14: One-way coupler

20: Base

22: Electrostatic chuck

24: Upper electrode

30: RF signal processing device

32: RF power detector

34: Fast Fourier Transform Module

36: Processor

W: Wafer

LF: Low frequency radio frequency power supply

HF: High frequency radio frequency power supply

Claims (11)

A capacitively coupled plasma processor includes a reaction chamber, wherein the reaction chamber includes a base and an upper electrode disposed opposite the base, wherein the base is used to support a substrate to be processed, a first radio frequency power source outputs a first incident power having a first frequency to the base to ignite and maintain plasma in the reaction chamber, and a second radio frequency power source outputs a second incident power having a second frequency to the base, wherein the first frequency is greater than 13 MHz. , the second frequency is greater than 10KHz and less than or equal to 400KHz, wherein: a sampling device is connected between the first RF power output terminal and the base, the output terminal of the sampling device is connected to a RF signal processing device, the RF signal processing device includes a RF power detection module, a fast Fourier module, and a processor; wherein the RF power detection module is used to detect the reflected power signal obtained on the sampling device, and the reflected power The radio frequency signal includes the reflected power of the first frequency and the power of two intermodulation reflected frequency signals with frequencies being positive and negative (±) of the first frequency and the second frequency, the radio frequency power detection module obtains a first reflected power value according to the reflected power signal; the fast Fourier transform module is used to obtain the spectrum power ratio distribution data of the reflected power signal, the processor receives the first reflected power value output by the radio frequency power detection module and the first reflected power value output by the fast Fourier transform module. The fast Fourier transform module obtains the spectral power distribution data and calculates the proportion of the first frequency in the reflected power signal based on the spectral power distribution data and a preset algorithm. The first reflected power value is multiplied by the first reflected power value to obtain a second reflected power value obtained by removing the intermodulation reflected power. The first radio frequency power supply adjusts the output of the first frequency based on the second reflected power value to achieve impedance matching. The capacitively coupled plasma processor of claim 1, wherein the preset algorithm includes calculating the sum of power ratios within a positive and negative (±) spread frequency range of the first frequency based on the spectral power ratio distribution data provided by the fast Fourier transform module to obtain the proportion of the first frequency in the first reflected power signal, wherein the spread frequency is less than half of the second frequency. The capacitively coupled plasma processor of claim 1, wherein the preset algorithm includes determining a split frequency fd1 based on the frequency of the lowest point of a power curve between the peak of the reflected power signal at the first frequency and the peak of the intermodulation reflected frequency power signal between the first frequency and the second frequency, determining a split frequency fd2 based on the frequency of the reflected power signal at the first frequency and the peak of the intermodulation reflected frequency power signal between the first frequency and the second frequency, and calculating the sum of the power proportions within the range from the split frequency fd1 to the split frequency fd2 to obtain the proportion of the first frequency in the first reflected power signal. The capacitively coupled plasma processor of claim 1, wherein the sampling device is at least one unidirectional coupler. The capacitively coupled plasma processor of claim 1, wherein the processor receives the first frequency value and the first incident power value signal output by the first radio frequency power source via a conductive line. The capacitively coupled plasma processor of claim 1, wherein the radio frequency power detection module is further configured to detect the first incident power and output the first incident power value to the processor. The capacitively coupled plasma processor of claim 5 or 6, wherein the processor further stores a matching frequency calculation program, and the processor calculates a modified first frequency value corresponding to the next adjustment step based on the second reflected power value and the first incident power value, and the first RF power supply modifies the output first frequency to the modified first frequency value. The capacitively coupled plasma processor of claim 1, wherein the first frequency has a center frequency and an amplitude variation range of plus or minus (±) 5% of the center frequency, and further includes a filter circuit disposed between the first RF power source and the base, the filter circuit allowing RF signals within the center frequency and the amplitude variation range to pass with low impedance while blocking signals of other frequencies. The capacitively coupled plasma processor of claim 1, wherein a matching circuit is further connected in series between the first RF power source and the base. The capacitively coupled plasma processor of claim 1, wherein the second frequency is greater than 10 kHz and less than 300 kHz. A capacitively coupled plasma processor includes a reaction chamber, wherein the reaction chamber includes a base and an upper electrode disposed opposite to the base, wherein the base is used to support a substrate to be processed, a first radio frequency power source outputs a first incident power having a first frequency to the base, for igniting and maintaining plasma in the reaction chamber, wherein the first frequency is adjustable within a variable frequency range; a second radio frequency power source outputs a second incident power having a second frequency to the base, and the reaction chamber Reflecting a reflected power signal to the first RF power source; wherein the first frequency is greater than 13 MHz, and the second frequency is greater than 10 kHz and less than or equal to 400 kHz, wherein: a sampling device is connected between the output end of the first RF power source and the base, and the output end of the sampling device is connected to a RF signal processing device, and the RF signal processing device includes a RF power detection module, a fast Fourier module, and a processor; wherein the RF power detection module is used to detect The sampling device measures the reflected power signal obtained from the sampling device, wherein the reflected power signal includes the reflected power of the first frequency and the power of two intermodulation reflected frequency signals having frequencies positive and negative (±) of the first frequency and the second frequency, and the radio frequency power detection module obtains a first reflected power value according to the reflected power signal; the fast Fourier transform module is used to obtain the spectrum power ratio distribution data of the reflected power signal; the processor receives the output of the radio frequency power detection module. The first reflected power value and the spectral power distribution data obtained from the fast Fourier transform module are used to determine the proportion of the first frequency in the reflected power signal based on the spectral power distribution data and a preset algorithm. The first reflected power value is then multiplied by the first reflected power value to obtain a second reflected power value in which the intermodulation reflected power is removed. The first RF power source adjusts the output of the first frequency based on the second reflected power value to achieve impedance matching.