Microwave reflected light conductance measurement system for wafer film carrier lifetime

Abstract

The invention relates to the technical field of semiconductors, and discloses a microwave reflected light conductance measurement system for the service life of a wafer film carrier, and the system comprises a reconfigurable differential metasurface probe which interacts with a to-be-measured wafer film in a near-field coupling manner; the optical excitation module is used for carrying out pulse illumination on the wafer film below the measurement area so as to generate non-equilibrium carriers; the dual-channel microwave receiving and transmitting module feeds microwave signals into the measurement area and the reference area; the closed-loop feedback control module is used for receiving the differential signal as input before the optical excitation module works; and the data acquisition and processing module is used for acquiring the transient change of the differential signal after the optical excitation module works. The microwave response of the reference area of the reconfigurable probe is actively adjusted through the closed-loop feedback control module, and a differential signal baseline is locked to a zero point before measurement, so that common-mode noise is suppressed, and the signal-to-noise ratio and the measurement accuracy of a transient attenuation curve are improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of semiconductor, in particular to a microwave reflection photoconductance measurement system for wafer thin film carrier lifetime. BACKGROUND

[0002] The minority carrier lifetime of semiconductor material is a core physical parameter determining the key device performances such as the conversion efficiency of photovoltaic cell, the switching speed of high-speed transistor and the quantum efficiency of light-emitting diode. In the research and production process of semiconductor devices, the fast, accurate and non-destructive measurement of carrier lifetime of wafer thin film has a direct and important role in the optimization of material growth process, the prediction of device performance and the control of production yield.

[0003] To realize the non-contact measurement of carrier lifetime, the existing technology has developed a microwave reflection photoconductance measurement scheme based on the differential probe structure. This scheme uses a probe containing a measurement area and a reference area to feed the microwave signal into the two areas simultaneously. The design principle is that when the distance between the probe and the sample changes as a whole due to external disturbance, the reflection signals of the measurement area and the reference area will produce similar fluctuations. By differentiating the two signals, most of the common-mode noise introduced by mechanical vibration and other factors can be effectively eliminated.

[0004] However, the effectiveness of the above static differential measurement technology depends on the ideal symmetry of the measurement area and the reference area in physical structure and electromagnetic characteristics, but this premise is difficult to achieve in practice. The inherent micron-level tolerance in the probe manufacturing process, as well as the unevenness of the electrical characteristics of the wafer thin film to be measured in different areas, will destroy this symmetry, resulting in an inherent signal imbalance between the two channels in the initial state. Therefore, when the system is subjected to external disturbances such as vibration, this inherent asymmetry will make the response amplitudes of the two channels to the same disturbance not completely consistent, resulting in residual noise after subtraction. This static differential scheme lacks active adjustment capability and cannot compensate for the inherent asymmetry of the system, so there is an upper limit to its common-mode noise suppression capability, which constitutes a fundamental technical bottleneck that limits the measurement signal-to-noise ratio and accuracy of the existing technology. SUMMARY

[0005] In view of the shortcomings of the prior art, the present application provides a microwave reflection photoconductance measurement system for wafer thin film carrier lifetime, which solves the problem that in the existing static differential microwave reflection photoconductance measurement technology, there is an inherent asymmetry between the measurement area and the reference area of the probe, and there is a lack of active adjustment mechanism to compensate for the asymmetry, resulting in incomplete common-mode noise suppression and limited measurement signal-to-noise ratio.

[0006] To achieve the above purpose, the present application realizes the following technical solutions: The first aspect of the present application provides a microwave reflection photoconductance measurement system for wafer thin film carrier lifetime, comprising: A reconfigurable differential metasurface probe interacts with the wafer thin film to be measured in a near-field coupling manner, the reconfigurable differential metasurface probe is divided into a measurement area and a reference area, and a tunable element is integrated in the reference area; An optical excitation module impinges pulsed light on the wafer thin film below the measurement area to generate non-equilibrium carriers; A dual-channel microwave transceiver module feeds microwave signals to the measurement area and the reference area, and receives respective reflected signals to generate a differential signal representing the difference between the two; A closed-loop feedback control module receives the differential signal as input before the optical excitation module works, and generates a control signal based on the differential signal to be applied to the tunable element to actively suppress common-mode noise; A data acquisition and processing module acquires the transient change of the differential signal after the optical excitation module works, and calculates the carrier lifetime of the wafer thin film based on the transient change.

[0007] In the technical solution of the present application, by setting a differential probe structure comprising a measurement area and a reference area, and combining a closed-loop feedback control module running before light measurement, the tunability of the reference area is used to actively compensate for common-mode noise introduced by factors such as change in lift-off distance until the baseline of the differential signal tends to be stable. On this basis, optical excitation and transient signal acquisition are performed again, thereby providing original signals for accurate calculation of carrier lifetime without interference from common-mode noise.

[0008] Preferably, the reconfigurable differential metasurface probe specifically comprises a dielectric substrate and a metasurface formed on the dielectric substrate. The metasurface is composed of subwavelength periodic metal microstructures, which are divided into different physical regions to form the measurement area and the reference area. The measurement area and the reference area are distributed in concentric ring shapes on the dielectric substrate. The structural design of the probe generates an evanescent wave field on the surface of the probe at the working frequency, which is used to realize near-field coupling with the wafer thin film to be measured.

[0009] In one specific embodiment, the tunable element is a varactor diode. The varactor diode is integrated in the subwavelength periodic metal microstructure in the reference area. The capacitance value of the varactor diode changes in response to the control signal output by the closed-loop feedback control module, thereby changing the resonance characteristics of the subwavelength periodic metal microstructure.

[0010] In one specific embodiment, the optical excitation module specifically comprises: A pulsed laser for generating a laser beam with a predetermined wavelength and pulse width; An optical focusing unit is arranged in the optical path of the pulsed laser, for focusing the laser beam into a spot and ensuring that the spot only irradiates the wafer film under the measurement area; A synchronous triggering unit is electrically connected to the pulsed laser and the data acquisition and processing module, for ensuring that the data acquisition and processing module starts to acquire data when the pulsed laser emits a laser beam.

[0011] In one embodiment, the dual-channel microwave transceiver module specifically comprises: A microwave signal source for generating a raw microwave signal with a stable frequency; A power distribution unit, having an input end connected to the microwave signal source and output ends connected to the measurement area and the reference area respectively, for dividing the raw microwave signal into two paths; A differential processing unit, which is a 180° hybrid ring, receives reflected signals from the measurement area and the reference area respectively, and performs a differential operation on the two received reflected signals to output the differential signal.

[0012] In one embodiment, the closed-loop feedback control module specifically comprises: A low-pass filter for receiving the differential signal, filtering out high-frequency transient components in the differential signal and extracting its direct current or slowly varying components; A controller connected to the output end of the low-pass filter, and having an output end for outputting the control signal.

[0013] Further, the controller adopts a proportional-integral-derivative (PID) control algorithm, taking the output of the low-pass filter as error input, to generate the control signal.

[0014] The proportional-integral-derivative control algorithm is used to generate the control signal , which is linearly superimposed by the following three parts: ; In the formula, is the output control signal at time ; is the input error signal at time , which is the output of the low-pass filter; is the gain coefficient of the proportional term, which is used to proportionally generate a control action according to the current error, to respond to the deviation of the system; is the gain coefficient of the integral term, which is used to accumulate historical errors, to eliminate the steady-state error of the system and ensure that the error signal tends to zero after feedback locking; Kd is the gain coefficient of the differential term, which acts to produce a control action based on the rate of change of the error signal, to anticipate error changes and provide damping to suppress oscillations and speed up the settling process; t is the variable representing time; is the function symbol representing the error signal value at the integration time point ; is the infinitesimal increment of the integral variable ; is the differential symbol representing an infinitesimal increment or differential; is the infinitesimal increment of the time variable .

[0015] In one specific embodiment, the data acquisition and processing module specifically comprises: a high-speed data acquisition unit configured to receive the differential signal and perform high-speed analog-to-digital conversion on the transient change of the differential signal after the optical excitation module is operated; a lifetime calculation unit connected to the high-speed data acquisition unit and configured to process the converted digital signal to calculate the carrier lifetime.

[0016] Further, the lifetime calculation unit performs an exponential decay fitting algorithm on the transient change of the differential signal collected by the high-speed data acquisition unit to determine the carrier lifetime of the wafer film . The exponential decay fitting algorithm is based on the following model: ; wherein, is the theoretical value of the transient alternating component of the differential signal over time ; is the initial amplitude of the signal at the end of the laser pulse; is the effective carrier lifetime to be solved; t is the variable representing time; is the mathematical function symbol representing the natural exponential function, i.e., the exponential function with the natural constant as the base.

[0017] The present application provides a microwave reflection photoconductance measurement system for carrier lifetime of wafer film. It has the following advantages: 1. The present application sets a reconfigurable differential metasurface probe, and combines a closed-loop feedback control module running before optical excitation, which can actively adjust the microwave response characteristics of the reference area by using the tunable elements therein. This makes the system able to compensate for common-mode noise introduced by factors such as changes in the distance between the probe and the wafer film in real time, thereby establishing a stable zero-point reference before the formal measurement begins.

[0018] 2. Because common-mode noise is effectively suppressed by the closed-loop feedback control module before optical excitation occurs, the transient changes in the differential signal acquired by the data acquisition and processing module mainly contain only the effective signal caused by non-equilibrium carrier recombination. This avoids the effective signal being overwhelmed by a large noise baseline, improves the signal-to-noise ratio of the transient decay curve, and provides high-quality raw data for subsequent accurate carrier lifetime calculations.

[0019] 3. This invention, through its active noise suppression mechanism, reduces the stringent requirements on the physical stability of the measurement environment and the flatness of the wafer film under test. The system has a higher tolerance for measurement errors caused by external vibrations or inherent sample warpage, thereby enhancing the system's applicability under different operating conditions and the repeatability of measurement results. Attached Figure Description

[0020] Figure 1 This is a system architecture diagram of the present invention; Figure 2 This is a schematic diagram of the optical excitation module of the present invention.

[0021] Among them, 10 is a reconfigurable differential metasurface probe; 20 is an optical excitation module; 30 is a dual-channel microwave transceiver module; 40 is a closed-loop feedback control module; and 50 is a data acquisition and processing module. Detailed Implementation

[0022] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0023] Reference Figure 1 and Figure 2 The present invention provides a microwave reflective photoconductivity measurement system for wafer thin film carrier lifetime. The system includes: a reconfigurable differential metasurface probe 10, an optical excitation module 20, a dual-channel microwave transceiver module 30, a closed-loop feedback control module 40, and a data acquisition and processing module 50.

[0024] The reconfigurable differential metasurface probe 10 is positioned with its surface parallel to the surface of the wafer thin film under test during operation, maintaining a predetermined distance between them. The optical excitation module 20 has its optical path output aligned with the wafer thin film region corresponding to the measurement area of ​​the reconfigurable differential metasurface probe 10.

[0025] The two microwave signal output ports of the dual-channel microwave transceiver module 30 are electrically connected to the measurement region and the reference region of the reconfigurable differential metasurface probe 10 respectively; and the two microwave signal input ports thereof are also electrically connected to the measurement region and the reference region respectively for receiving reflected signals. The differential signal output port of the dual-channel microwave transceiver module 30 is simultaneously electrically connected to the signal input end of the closed-loop feedback control module 40 and the signal input end of the data acquisition and processing module 50.

[0026] The control signal output end of the closed-loop feedback control module 40 is electrically connected to the tunable element in the reference region of the reconfigurable differential metasurface probe 10. A synchronous trigger connection is provided between the optical excitation module 20 and the data acquisition and processing module 50 to ensure the consistency of the two in timing.

[0027] The working principle of the system of the present application is based on a two-stage timing process. The first stage is a feedback locking stage, and the second stage is an excitation measurement stage.

[0028] In the feedback locking stage, the optical excitation module 20 does not work. The dual-channel microwave transceiver module 30 continuously feeds in equal-amplitude in-phase microwave signals to the measurement region and the reference region of the reconfigurable differential metasurface probe 10. Due to the initial non-parallelism or slight mechanical vibration between the probe 10 and the wafer film, there will be a difference between the reflected signals of the measurement region and the reference region, thereby generating a non-zero differential signal at the output end of the dual-channel microwave transceiver module 30.

[0029] The non-zero differential signal is transmitted to the closed-loop feedback control module 40. After processing this signal, the closed-loop feedback control module 40 generates a control signal and applies it to the tunable element in the reference region of the reconfigurable differential metasurface probe 10. The control signal changes the electrical parameters of the tunable element, thereby adjusting the microwave reflection characteristics of the reference region. This feedback process continues until the difference between the two reflected signals is compensated, so that the direct current baseline of the differential signal is driven to a stable and minimum value close to zero. At this time, the system reaches a locked state.

[0030] In the excitation measurement stage, the closed-loop feedback control module 40 continues to maintain the locked state of the system. At this time, the optical excitation module 20 emits a short pulse of laser light via a synchronous trigger signal. The laser pulse only irradiates the wafer film under the measurement region, instantaneously generating non-equilibrium carriers, resulting in a sharp change in the electrical conductivity of the region.

[0031] The change in electrical conductivity only changes the microwave reflection signal of the measurement region, while the reflection signal of the reference region remains unchanged, thereby breaking the established balance. The dual-channel microwave transceiver module 30 thus outputs a transiently changing differential signal, and the decay process of the differential signal directly reflects the recombination process of the non-equilibrium carriers.

[0032] The data acquisition and processing module 50 starts to collect the transient differential signal waveform at the same time when the synchronization trigger signal is received. After the collection is completed, the data acquisition and processing module 50 processes the collected data to calculate the carrier lifetime of the wafer film.

[0033] Referring to Figure 1 and Figure 2 , the reconfigurable differential metasurface probe 10 in the present application is composed of a dielectric substrate. The dielectric substrate is selected from materials with low dielectric loss at the target working frequency, such as quartz, sapphire or high-frequency circuit board material. On the surface of the dielectric substrate, a metasurface composed of metal thin film is made.

[0034] The metasurface is composed of an array of sub-wavelength periodic metal microstructures, i.e. the geometric size of each metal microstructure constituting the array is much smaller than the free-space microwave wavelength at which the system works. The metasurface is physically divided into two isolated regions: a central region as a measurement zone, and an outer annular region surrounding the measurement zone as a reference zone. There is a physical gap between the measurement zone and the reference zone to achieve electromagnetic isolation between the two regions.

[0035] The structural design of the reconfigurable differential metasurface probe 10 makes it form an evanescent wave field on the surface of the reconfigurable differential metasurface probe 10 when it is excited by a microwave signal. The electromagnetic energy of the evanescent wave field is highly concentrated near the surface of the reconfigurable differential metasurface probe 10, and its field strength decreases exponentially with the increase of the vertical distance from the surface of the reconfigurable differential metasurface probe 10. When the reconfigurable differential metasurface probe 10 is close to the wafer film to be measured, the evanescent wave field interacts with the carriers in the wafer film through near-field coupling, and the local conductivity of the wafer film is equivalent to a load, affecting the microwave reflection characteristics of the corresponding region of the reconfigurable differential metasurface probe 10.

[0036] In the reference zone, some or all of the sub-wavelength periodic metal microstructures in the array have tunable elements integrated therein. In a specific embodiment, the sub-wavelength periodic metal microstructure is an open resonant ring structure, and the tunable element is a varactor diode. The varactor diode is welded or integrated at the opening gap of the open resonant ring.

[0037] The capacitance value of the varactor diode is a function of the reverse bias voltage applied across it, i.e. the control signal output by the closed-loop feedback control module 40. When the control signal changes, the junction capacitance of the varactor diode changes accordingly. This change in capacitance directly changes the overall equivalent capacitance of the metal microstructure in which it is located. ​​, and further modulate the electromagnetic resonance characteristics of the microstructure.

[0038] In particular, the resonance frequency of a single metal microstructure is determined by its equivalent inductance and equivalent capacitance . When integrated with a varactor diode, the resonance frequency of the microstructure in the reference region becomes a function of the control signal : ; wherein is the resonance frequency of a single metal microstructure; is the control signal; is the equivalent inductance; is the equivalent total capacitance of the microstructure, which is determined by the geometric capacitance of the microstructure itself and the capacitance of the varactor diode . By applying the control signal , the resonance frequency of the reference region can be directly adjusted. Since the number of microwave reflections of the probe is most sensitive near the resonance frequency, adjusting the resonance frequency can effectively change the amplitude and phase of the reflected signal of the reference region at the time-determining operating frequency, thereby achieving active adjustment of the microwave reflection characteristics of the reference region.

[0039] Referring to Figure 1 and Figure 2 , the optical excitation module 20 is configured to pulse the wafer film under test to instantaneously generate non-equilibrium carriers in a local region during the excitation measurement phase. In one specific embodiment, the optical excitation module 20 includes a pulsed laser, an optical focusing unit, and a synchronous triggering unit.

[0040] The pulsed laser is configured to generate a laser beam with a specific wavelength and pulse width. The wavelength of the pulsed laser is selected to satisfy the condition that the photon energy is greater than the band gap of the semiconductor material of the wafer film under test, so as to ensure that the laser photons can be effectively absorbed and excite the generation of electron-hole pairs. This condition can be expressed as: ; wherein is the Planck constant; is the speed of light in a vacuum; is the wavelength of the laser output by the pulsed laser; and is the band gap of the semiconductor material of the wafer film under test. In addition, the pulse width of the laser output by the pulsed laser should be much smaller than the carrier lifetime of the wafer film under test, so as to ensure instantaneous excitation and distortionless recording of the carrier recombination decay process.

[0041] An optical focusing unit is arranged in the light path of the pulsed laser. The optical focusing unit is composed of one or more sets of lenses, mirrors and diaphragms, and its function is to shape and focus the laser beam emitted from the pulsed laser, and finally form a light spot of a predetermined size and shape on the surface of the wafer film to be measured. The position and optical path of the optical focusing unit are precisely adjusted to ensure that the irradiation area of the light spot is strictly limited to the wafer film position corresponding to the measurement area of the reconfigurable differential metasurface probe 10, and the wafer film under the reference area is not illuminated.

[0042] A synchronization trigger unit is a digital delay pulse generator or similar timing control circuit. Its output is electrically connected to the trigger input of the pulsed laser and the external trigger input of the data acquisition and processing module 50. In the excitation measurement phase, the synchronization trigger unit generates a synchronization trigger signal which is sent to the pulsed laser and the data acquisition and processing module 50 simultaneously. This design ensures that the time of the pulsed laser emitting a laser pulse and the starting time of the data acquisition and processing module 50 starting to collect the transient differential signal are strictly synchronized in time, thereby ensuring the accuracy of the time axis zero point of the collected decay curve.

[0043] Referring to Figure 1 and Figure 2 , the dual-channel microwave transceiver module 30 is used to generate and feed a microwave signal for detection to the reconfigurable differential metasurface probe 10, receive the reflected signals returned by the reconfigurable differential metasurface probe 10, and perform differential processing on the two reflected signals to generate a differential signal. In a specific embodiment, the dual-channel microwave transceiver module 30 includes a microwave signal source, a power distribution unit, and a differential processing unit.

[0044] The microwave signal source is used to generate a continuous wave microwave signal with stable frequency and power, and the frequency of the signal is the fixed operating frequency of the system.

[0045] The input end of the power distribution unit is connected to the output end of the microwave signal source. The power distribution unit equally divides the input original microwave signal into two signals, and the two output signals have equal amplitudes and the same phase. The two output ports of the power distribution unit are respectively electrically connected to the feed ports of the measurement area and the reference area of the reconfigurable differential metasurface probe 10 through phase-matched transmission cables.

[0046] The differential processing unit has two input ports respectively receiving reflected signals from the measurement area and the reference area. In a specific embodiment, the differential processing unit is a 180° hybrid ring. The 180° hybrid ring performs vector operations on the two input reflected signals and outputs a voltage signal from its differential port, which is the differential signal and its value is related to the vector difference of the two input signals.

[0047] If the microwave reflection coefficients of the measurement area and the reference area are respectively denoted as... and The differential signal output by the differential processing unit This is proportional to the vector difference between the two reflection coefficients. This relationship can be expressed as: ; In the formula, It is the complex voltage of the differential signal output by the differential processing unit; and These are the complex reflection coefficients of the measurement area and the reference area, respectively, and their amplitude and phase depend on the coupling state between the probe and the thin film of the wafer under test. It is a proportionality constant whose value is related to the output power of the microwave signal source and the gain of the system.

[0048] The differential signal The output terminal is connected to both the input terminal of the closed-loop feedback control module 40 and the input terminal of the data acquisition and processing module 50.

[0049] Reference Figure 1 and Figure 2 The input terminal of the closed-loop feedback control module 40 receives the differential signal output from the dual-channel microwave transceiver module 30. Its output is electrically connected to a tunable element within the reference region of the reconfigurable differential metasurface probe 10. The closed-loop feedback control module 40 functions to form a negative feedback loop before the optical excitation module 20 operates, thereby stabilizing the differential signal. The DC baseline. In one specific embodiment, the closed-loop feedback control module 40 includes a low-pass filter and a controller.

[0050] The low-pass filter receives the differential signal at its input. . Differential signaling This signal contains DC or slowly varying components caused by factors such as probe-sample distance drift, as well as inherent high-frequency noise from the system. The function of a low-pass filter is to filter out the high-frequency noise components, extracting only the DC or slowly varying components characterizing common-mode noise. The cutoff frequency of this low-pass filter... It is set to a specific value that is lower than the system's high-frequency noise frequency but higher than the characteristic variation frequency of common-mode noise drift, thereby ensuring effective tracking of common-mode noise.

[0051] The output signal of the low-pass filter is defined as the error signal. The signal is transmitted to the input of the controller. The controller can be implemented using an analog circuit composed of operational amplifiers, or a digital circuit such as a microcontroller or digital signal processor. The function of the controller is to adjust the input error signal based on the input error signal. , a control signal is calculated by a preset control algorithm .

[0052] In a specific embodiment, the controller employs a proportional-integral-derivative (PID) control algorithm. This control algorithm calculates a control signal according to the current value, historical cumulative value and future trend of the error signal , which is linearly superimposed by three parts of proportion, integral and derivative, and the mathematical relationship is: ; wherein, is the control signal output at time ; is the error signal input at time , i.e. the output of the low-pass filter; is the gain coefficient of the proportional term, which is to proportionally generate a control action according to the current error to respond to the deviation of the system; is the gain coefficient of the integral term, which is to accumulate the historical error to eliminate the steady-state error of the system and ensure that the error signal can approach zero after feedback locking; is the gain coefficient of the derivative term, which is to generate a control action according to the change rate of the error signal to predict the error change and provide damping to suppress the oscillation of the system and speed up the stabilization process; represents the variable of time; is a function symbol, which represents the error signal value at the integral time point ; is the integral part of the integral operation, which represents an infinitesimal increment of the integral variable ; is a differential symbol, which represents an infinitesimal increment or differential; is the differential part of the differential operation, which represents an infinitesimal increment of the time variable .

[0053] The control signal output by the controller is applied to the tunable elements in the reference area of the reconfigurable differential metasurface probe 10. The control signal changes the electrical parameters of the tunable elements, thereby adjusting the microwave reflection coefficient of the reference area . The adjustment direction is opposite to the error signal , thereby forming a negative feedback loop to continuously drive the error signal to decrease. When the system reaches stability, approaches zero, at which time the microwave reflection characteristics of the reference area match the microwave reflection characteristics of the measurement area, and the differential signal The DC baseline of the differential signal is locked at a stable minimum value.

[0054] Referring to Figure 1 and Figure 2 , the data acquisition and processing module 50, the input end of which is electrically connected with the differential signal output end of the dual-channel microwave transceiver module 30, and the trigger input end of which is electrically connected with the synchronous trigger unit in the optical excitation module 20. The function of this module is to collect and digitize the transient changes of the differential signal after receiving the synchronous trigger signal, and calculate the carrier lifetime of the wafer thin-film based on the collected data. In a specific embodiment, the data acquisition and processing module 50 includes a high-speed data acquisition unit and a lifetime calculation unit.

[0055] The high-speed data acquisition unit can be a high-speed digital oscilloscope or a data acquisition card (DAQ). When the high-speed data acquisition unit receives the synchronous trigger signal from the synchronous trigger unit, it immediately starts to perform analog-to-digital conversion on the input differential signal at a preset high sampling rate. The sampling process converts the continuous analog voltage signal into a discrete time series digital signal . The sampling rate and vertical resolution of this unit are key parameters to ensure measurement accuracy, and the sampling rate needs to meet the Nyquist sampling theorem, which is much higher than the highest frequency component of the measured transient signal.

[0056] The lifetime calculation unit can be composed of a general-purpose computer, a microprocessor or a field programmable gate array (FPGA) and the software algorithm running on it. The lifetime calculation unit receives the digital signal sequence output by the high-speed data acquisition unit as input. Since the baseline of the differential signal has been locked near zero by the closed-loop feedback control module 40 before optical excitation, the transient signal collected directly represents the photogenerated carrier recombination decay process.

[0057] The lifetime calculation unit determines the carrier lifetime by performing an exponential decay fitting algorithm on the collected transient decay signal. This decay fitting algorithm is based on the assumption that the carrier recombination process follows a first-order kinetic model, i.e. the signal decay pattern is a single exponential decay. Its mathematical model is: ; In the formula, is the theoretical value of the transient alternating current component of the differential signal as a function of time; is the initial amplitude of the signal at the end of the laser pulse; is the effective carrier lifetime to be solved; represents the variable of time; is the mathematical function symbol, representing the natural exponential function, i.e. Exponential function with base e.

[0058] In the specific calculation, the lifetime calculation unit adopts the least square method to fit the collected data point sequence . By adjusting the values of the model parameters and , the residual square sum between the model calculation value and the actual collected value is minimized. The expression of the residual square sum is as follows: ; In the formula, is the residual square; is the initial amplitude of the signal at the end of the laser pulse; is the effective carrier lifetime to be solved; is the mathematical function symbol, representing the natural exponential function, that is, the exponential function with base ; is the summation index variable; is the collected data point sequence; is the total number of data points involved in the fitting; is the sampling period of the high-speed data acquisition unit. By solving the value of that minimizes by using a numerical optimization algorithm, the wafer film carrier lifetime result of this measurement is obtained.

[0059] The above-described embodiments are merely preferred embodiments of the present application, and do not limit the scope of the present application. Without departing from the design spirit of the present application, various modifications and improvements to the technical solutions of the present application made by those of ordinary skill in the art shall fall within the protection scope of the present application as defined by the claims.​

Claims

1. A system for measuring wafer thin-film carrier lifetime by microwave reflection photoconductance, characterized in that, The application relates to a reconfigurable differential metasurface probe, which is used for near-field coupling with a wafer film to be measured, and comprises a measurement area and a reference area, and a tunable element is integrated in the reference area. An optical excitation module is used for pulsed light irradiation on the wafer film under the measurement area to generate non-equilibrium carriers. A dual-channel microwave transceiver module is used for feeding microwave signals to the measurement area and the reference area, receiving the reflected signals of the measurement area and the reference area respectively, and generating a differential signal representing the difference between the measurement area and the reference area. A closed-loop feedback control module is used for receiving the differential signal as an input before the optical excitation module works, and generating a control signal according to the differential signal and applying the control signal to the tunable element to actively suppress common-mode noise. A data acquisition and processing module is used for collecting transient changes of the differential signal after the optical excitation module works, and calculating the carrier lifetime of the wafer film based on the transient changes. The reconfigurable differential metasurface probe comprises a dielectric substrate, a metasurface formed on the dielectric substrate and composed of subwavelength periodic metal microstructures, and the subwavelength periodic metal microstructures are divided into different physical regions to form the measurement area and the reference area.

2. The system for measuring wafer thin-film carrier lifetime by microwave reflection photoconductance according to claim 1, wherein, The measurement area and the reference area are distributed in a concentric ring shape on the dielectric substrate, and the structure design of the reconfigurable differential metasurface probe generates an evanescent wave field on the probe surface at a working frequency, which is used for realizing near-field coupling with the wafer film to be measured. The tunable element comprises a varactor diode. The varactor diode is integrated in the subwavelength periodic metal microstructure in the reference area. The capacitance value of the varactor diode changes in response to the control signal of the reference area, thereby changing the resonance characteristics of the subwavelength periodic metal microstructure.

3. The system for measuring the microwave reflectance photoconductance of a wafer thin-film carrier lifetime according to claim 1, wherein, The optical excitation module comprises a pulsed laser for generating a laser beam with a predetermined wavelength and pulse width, an optical focusing unit located on the light path of the pulsed laser, and a synchronous trigger unit electrically connected to the pulsed laser and the data acquisition and processing module. The dual-channel microwave transceiver module comprises a microwave signal source for generating an original microwave signal with a stable frequency, a power distribution unit connected to the original microwave signal, a differential processing unit for receiving the reflected signals from the measurement area and the reference area respectively, and the differential processing unit is a 180-degree hybrid ring used for differential operation on the two received reflected signals to output the differential signal. The closed-loop feedback control module comprises a low-pass filter for receiving the differential signal, filtering out high-frequency transient components in the differential signal and extracting direct current or slowly varying components. ​ 4. The system for measuring the microwave reflectance photoconductance of a wafer thin-film carrier lifetime according to claim 1, wherein, ​ ​ ​ ​ 5. The system for measuring the microwave reflectance photoconductance of a wafer thin-film carrier lifetime according to claim 1, wherein, ​ ​ ​ ​ 6. The system for measuring the microwave reflectance photoconductance of a wafer thin-film carrier lifetime according to claim 1, wherein, ​ ​ A controller is connected to the output of the low-pass filter, and an output of the controller is used to output the control signal.

7. The system for measuring the microwave reflectance photoconductance of a wafer thin-film carrier lifetime according to claim 6, wherein, The controller uses a proportional-integral-derivative control algorithm to generate the control signal, taking the output of the low-pass filter as an error input.

8. The system for measuring the microwave reflectance photoconductance of a wafer thin-film carrier lifetime according to claim 7, wherein, The proportional-integral-derivative control algorithm is used to generate the control signal, which is linearly superimposed by the following three parts: A proportional term, the value of which is proportional to the output of the low-pass filter; An integral term, the value of which is proportional to the integral of the output of the low-pass filter with respect to time; A derivative term, the value of which is proportional to the derivative of the output of the low-pass filter with respect to time.

9. The system for measuring the microwave reflectance photoconductance of a wafer thin-film carrier lifetime according to claim 1, wherein, The data acquisition and processing module specifically includes; A high-speed data acquisition unit for receiving the differential signal and performing high-speed analog-to-digital conversion on the transient change of the differential signal after the optical excitation module is working; A lifetime calculation unit connected to the high-speed data acquisition unit for processing the converted digital signal to calculate the carrier lifetime.

10. The system for measuring the microwave reflectance photoconductance of a wafer thin-film carrier lifetime according to claim 9, wherein, The lifetime calculation unit determines the carrier lifetime of the wafer film by performing an exponential decay fitting algorithm on the transient change of the differential signal collected by the high-speed data acquisition unit.

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