A microwave reflection photoconductance measurement system for wafer thin film carrier lifetime

Abstract

The application relates to the field of semiconductor technology and discloses a microwave reflection photoconductance measurement system for wafer film carrier lifetime, which comprises a reconfigurable differential metasurface probe, which interacts with a wafer film to be measured in a near-field coupling mode; an optical excitation module, which performs pulsed light irradiation on the wafer film below a measurement area to generate non-equilibrium carriers; a dual-channel microwave transceiving module, which feeds microwave signals to the measurement area and a reference area; a closed-loop feedback control module, which receives the differential signal as input before the optical excitation module works; and a data acquisition and processing module, which collects transient changes of the differential signal after the optical excitation module works. The closed-loop feedback control module actively adjusts the microwave response of the reconfigurable probe reference area, the differential signal baseline is locked to zero before measurement, common-mode noise is suppressed, and the signal-to-noise ratio of the transient decay curve and the measurement accuracy are improved.

Description

Technical Field

[0001] This invention relates to the field of semiconductor technology, specifically to a microwave reflective photoconductivity measurement system for wafer thin-film carrier lifetime. Background Technology

[0002] The minority carrier lifetime of semiconductor materials is a core physical parameter that determines the performance of key devices such as photovoltaic cell conversion efficiency, high-speed transistor switching speed, and light-emitting diode quantum efficiency. In the research and development and manufacturing processes of semiconductor devices, rapid, accurate, and non-destructive measurement of the carrier lifetime of wafer thin films plays a direct and crucial role in optimizing material growth processes, predicting device performance, and controlling production yield.

[0003] To achieve non-contact measurement of carrier lifetime, a microwave reflection photoconductivity measurement scheme based on a differential probe structure has been developed in the prior art. This scheme utilizes a probe containing a measurement region and a reference region, simultaneously feeding microwave signals into both regions. Its design principle lies in the fact that when the distance between the probe and the sample changes due to external disturbances, the reflected signals from the measurement region and the reference region will produce similar fluctuations. By performing differential operations on the two signals, most of the common-mode noise introduced by factors such as mechanical vibration can be effectively canceled.

[0004] However, the effectiveness of the aforementioned static differential measurement technique relies on the ideal symmetry between the measurement and reference regions in terms of physical structure and electromagnetic properties, a premise that is difficult to achieve in practice. The inherent micron-level tolerances during probe manufacturing, and the non-uniformity of the electrical properties of the thin film on the wafer under test in different regions, both disrupt 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 causes the response amplitudes of the two channels to the same disturbance to be inconsistent, resulting in residual noise after differential subtraction. This static differential scheme, lacking active adjustment capability, cannot compensate for the inherent asymmetry of the system, and its common-mode noise suppression capability has an upper limit. This constitutes the fundamental technical bottleneck limiting the signal-to-noise ratio and accuracy of existing measurement technologies. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a microwave reflective photoconductivity measurement system for wafer thin-film carrier lifetime. This system solves the problem in existing static differential microwave reflective photoconductivity measurement technologies where the inherent asymmetry between the measurement and reference regions of the probe, coupled with the lack of an active adjustment mechanism to compensate for this asymmetry, leads to incomplete common-mode noise suppression and limited measurement signal-to-noise ratio.

[0006] To achieve the above objectives, the present invention provides the following technical solution:

[0007] The first aspect of this invention provides a microwave reflective photoconductivity measurement system for wafer thin-film carrier lifetime, the system comprising:

[0008] A reconfigurable differential metasurface probe interacts with the thin film on the wafer under test via near-field coupling. 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.

[0009] An optical excitation module applies pulsed light to the wafer thin film below the measurement area to generate non-equilibrium carriers;

[0010] A dual-channel microwave transceiver module feeds microwave signals into the measurement area and the reference area, and receives their respective reflected signals to generate a differential signal characterizing the difference between the two.

[0011] The closed-loop feedback control module receives the differential signal as input before the optical excitation module operates, and generates a control signal accordingly to apply to the tunable element in order to actively suppress common-mode noise.

[0012] The data acquisition and processing module acquires the transient changes of the differential signal after the optical excitation module is activated, and calculates the carrier lifetime of the wafer thin film based on the transient changes.

[0013] In the technical solution of this invention, a differential probe structure comprising a measurement area and a reference area is set up, and combined with a closed-loop feedback control module that runs before illumination measurement, the tunability of the reference area is utilized to actively compensate for common-mode noise introduced by factors such as changes in lift-off distance, until the baseline of the differential signal tends to stabilize. Based on this, optical excitation and transient signal acquisition are then performed, thereby providing a raw signal unaffected by common-mode noise for accurate calculation of carrier lifetime.

[0014] Preferably, the reconfigurable differential metasurface probe specifically includes a dielectric substrate and a metasurface formed on the dielectric substrate. The metasurface is composed of subwavelength periodic metallic microstructures, which are divided into different physical regions to form the measurement region and the reference region. The measurement region and the reference region are concentrically ring-shaped distributed on the dielectric substrate. The probe's structural design enables it to generate an evanescent wave field on its surface at the operating frequency, used to achieve near-field coupling with the thin film of the wafer under test.

[0015] In one specific embodiment, the tunable element is a varactor diode. This varactor diode is integrated within a subwavelength periodic metal microstructure in the reference region. The capacitance value of the varactor diode changes in response to the control signal output by the closed-loop feedback control module, thereby altering the resonant characteristics of the subwavelength periodic metal microstructure.

[0016] In one specific embodiment, the optical excitation module specifically includes:

[0017] A pulsed laser is used to generate a laser beam with a predetermined wavelength and pulse width.

[0018] An optical focusing unit, located in the optical path of a pulsed laser, is used to focus the laser beam into a spot and ensure that the spot only illuminates the wafer thin film below the measurement area.

[0019] The synchronous triggering unit is electrically connected to the pulsed laser and the data acquisition and processing module to ensure that the data acquisition and processing module starts acquiring data synchronously when the pulsed laser emits a laser beam.

[0020] In one specific embodiment, the dual-channel microwave transceiver module specifically includes:

[0021] A microwave signal source is used to generate raw microwave signals with a stable frequency.

[0022] The power distribution unit has its input connected to the microwave signal source and its output connected to the measurement area and the reference area respectively, and is used to split the original microwave signal into two paths.

[0023] The differential processing unit is a 180° hybrid loop that receives reflected signals from the measurement area and the reference area respectively, and performs differential operations on the two received reflected signals to output the differential signal.

[0024] In one specific embodiment, the closed-loop feedback control module specifically includes:

[0025] A low-pass filter is used to receive differential signals, filter out high-frequency transient components in the differential signals, and extract their DC or slowly varying components;

[0026] A controller is connected to the output of a low-pass filter, and its output is used to output the control signal.

[0027] Furthermore, the controller employs a proportional-integral-derivative (PID) control algorithm, using the output of the low-pass filter as the error input to generate the control signal.

[0028] The proportional-integral-derivative (PI-DE) control algorithm is used to generate the control signal. The control signal is composed of the following three linearly superimposed parts:

[0029] ;

[0030] In the formula, It is in time Output control signals; It is in time The input error signal is the output of the low-pass filter; It is the gain coefficient of the proportional term, which generates a control action proportional to the magnitude of the current error in response to the system deviation; It is the gain coefficient of the integral term. The function of this term is to accumulate historical errors, which is used to eliminate the steady-state error of the system and ensure that the error signal can approach zero after feedback locking. It is the gain coefficient of the differential term. The function of this term is to generate a control action based on the rate of change of the error signal, to predict the error change and provide damping, so as to suppress the oscillation of the system and speed up the stabilization process. Variables representing time; The function symbol represents the time point of integration. Error signal value at time; As a component of integration operations, it represents the integration variable. An infinitesimal increment; The symbol for differential represents an infinitesimal increment or derivative; As a component of differential operations, it represents the time variable. An infinitesimal increment.

[0031] In one specific embodiment, the data acquisition and processing module specifically includes:

[0032] The high-speed data acquisition unit is used to receive differential signals and, after the optical excitation module is activated, to perform high-speed analog-to-digital conversion on the transient changes of the differential signals.

[0033] The lifetime calculation unit, connected to the high-speed data acquisition unit, is used to process the converted digital signal to calculate the carrier lifetime.

[0034] Furthermore, the lifetime calculation unit calculates the transient changes of the differential signal acquired by the high-speed data acquisition unit. An exponential decay fitting algorithm is performed to determine the carrier lifetime of the wafer thin film. The exponential decay fitting algorithm is based on the following model:

[0035] ;

[0036] In the formula, It is the transient AC component of the differential signal over time. Theoretical value of the change; It is the initial amplitude of the signal at the end of the laser pulse; It is the effective carrier lifetime to be determined; Variables representing time; The mathematical function symbol represents the natural exponential function, that is, the function expressed using the natural constant. An exponential function with base 0.

[0037] This invention provides a microwave reflective photoconductivity measurement system for wafer thin-film carrier lifetime. It offers the following advantages:

[0038] 1. This invention, by setting up a reconfigurable differential metasurface probe and combining it with a closed-loop feedback control module that operates before optical excitation, can actively adjust its microwave response characteristics using tunable elements within the reference region. This enables the system to compensate in real time for common-mode noise introduced by factors such as changes in the distance between the probe and the wafer thin film, thereby establishing a stable zero-point reference before the formal measurement begins.

[0039] 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.

[0040] 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

[0041] Figure 1 This is a system architecture diagram of the present invention;

[0042] Figure 2 This is a schematic diagram of the optical excitation module of the present invention.

[0043] 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

[0044] 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.

[0045] Reference Figure 1 and Figure 2The 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.

[0046] 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.

[0047] The two microwave signal output ports of the dual-channel microwave transceiver module 30 are electrically connected to the measurement area and reference area of ​​the reconfigurable differential metasurface probe 10, respectively; its two microwave signal input ports are also electrically connected to the measurement area and reference area, 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 terminal of the closed-loop feedback control module 40 and the signal input terminal of the data acquisition and processing module 50.

[0048] The control signal output terminal 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 timing consistency between the two.

[0049] The system working principle of this invention is based on a two-stage timing process. The first stage is the feedback locking stage, and the second stage is the excitation measurement stage.

[0050] During the feedback locking phase, the optical excitation module 20 is inactive. The dual-channel microwave transceiver module 30 continuously feeds equal-amplitude and in-phase microwave signals into the measurement and reference regions of the reconfigurable differential metasurface probe 10. Due to initial non-parallelism or minor mechanical vibrations between the probe 10 and the wafer thin film, a difference in the reflected signals between the measurement and reference regions occurs, resulting in a non-zero differential signal at the output of the dual-channel microwave transceiver module 30.

[0051] The non-zero differential signal is transmitted to the closed-loop feedback control module 40. The closed-loop feedback control module 40 processes this signal, generates a control signal, and applies it to the tunable element within the reference region of the reconfigurable differential metasurface probe 10. This 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, driving the DC baseline of the differential signal to a stable minimum value close to zero. At this point, the system reaches a locked state.

[0052] During the excitation and measurement phase, the closed-loop feedback control module 40 maintains the system in a locked state. At this time, the optical excitation module 20 emits a short-pulse laser beam via a synchronization trigger signal. This laser pulse only illuminates the wafer thin film below the measurement area, instantaneously generating non-equilibrium carriers, causing a sharp change in the conductivity of this region.

[0053] The change in conductivity alters only the microwave reflection signal in the measurement region, while the reflection signal in the reference region remains unchanged, thus disrupting the established equilibrium. The dual-channel microwave transceiver module 30 therefore outputs a transiently changing differential signal, the attenuation of which directly reflects the recombination process of non-equilibrium carriers.

[0054] Upon receiving the synchronization trigger signal, the data acquisition and processing module 50 begins high-speed acquisition of the transient differential signal waveform. After acquisition, the data acquisition and processing module 50 processes the acquired data and calculates the carrier lifetime of the wafer thin film.

[0055] Reference Figure 1 and Figure 2 The reconfigurable differential metasurface probe 10 of this invention is mainly composed of a dielectric substrate. The dielectric substrate is made of a material with low dielectric loss at the target operating frequency, such as quartz, sapphire, or high-frequency circuit board. A metasurface composed of a metal thin film is fabricated on the surface of the dielectric substrate.

[0056] The metasurface is composed of a subwavelength periodic array of metallic microstructures, meaning that the geometric dimensions of each metallic microstructure in the array are much smaller than the free-space microwave wavelength at which the system operates. The metasurface is physically divided into two isolated regions: a central region serving as the measurement region, and an outer annular region surrounding the measurement region serving as the reference region. A physical gap exists between the measurement and reference regions to achieve electromagnetic isolation between the two regions.

[0057] The reconfigurable differential metasurface probe 10 is designed to generate an evanescent wave field on its surface when excited by a microwave signal. The electromagnetic energy of this evanescent wave field is highly concentrated near the surface of the probe 10, and its field strength decreases exponentially with increasing perpendicular distance from the probe surface. When the probe 10 approaches the wafer thin film under test, the evanescent wave field couples with the charge carriers within the wafer thin film in a near-field manner. The local conductivity of the wafer thin film effectively acts as a load, affecting the microwave reflection characteristics of the corresponding region of the probe 10.

[0058] Within the reference region, tunable elements are integrated into some or all of the subwavelength periodic metallic microstructures in its array. In one specific embodiment, the subwavelength periodic metallic microstructure is an open-loop resonant structure, and the tunable element is a varactor diode. The varactor diode is soldered or integrated into the opening gap of the open-loop resonant structure.

[0059] The capacitance value of the varactor diode It is the reverse bias voltage applied across its terminals, i.e., the control signal output by the closed-loop feedback control module 40. The function. When the control signal When the junction capacitance of the varactor diode changes, Consequently, this change in capacitance value directly alters the overall equivalent capacitance of the metal microstructure in which it resides. This modulates the electromagnetic resonance characteristics of the microstructure.

[0060] Specifically, the resonant frequency of a single metal microstructure It can be determined by its equivalent inductance and equivalent capacitance The decision is made based on the resonant frequency of the microstructure within the reference region after the varactor diode is integrated. Become a control signal Functions:

[0061] ;

[0062] In the formula, The resonant frequency of a single metal microstructure; For control signals; Equivalent inductance; The equivalent total capacitance of the microstructure is determined by the geometric capacitance of the microstructure itself. and the capacitance of the varactor diode Joint decision. This is achieved by applying control signals. The resonant frequency of the reference area can be directly adjusted. Since the number of microwave reflection bars of the probe is most sensitive near the resonant frequency, adjusting the resonant frequency can effectively change the amplitude and phase of the reflected signal of the reference area at the time-determined operating frequency, thereby achieving active adjustment of the microwave reflection characteristics of the reference area.

[0063] Reference Figure 1 and Figure 2 The optical excitation module 20 is used to apply pulsed light to the thin film of the wafer under test during the excitation measurement phase, so as to instantaneously generate non-equilibrium carriers in a local area. In a specific embodiment, the optical excitation module 20 includes a pulsed laser, an optical focusing unit, and a synchronous triggering unit.

[0064] The pulsed laser is used to generate a laser beam with a specific wavelength and pulse width. Its wavelength... The selection of the photon energy must satisfy the condition that its photon energy is greater than the bandgap of the semiconductor material of the wafer thin film to be tested. The condition is to ensure that laser photons can be effectively absorbed and excited to generate electron-hole pairs. This condition can be expressed as:

[0065] ;

[0066] In the formula, It is Planck's constant; The speed of light in a vacuum; It is the wavelength of the laser output by the pulsed laser; The band gap is defined by the semiconductor material used in the thin film of the wafer under test. Furthermore, the pulse width of the laser output from the pulsed laser should be much smaller than the lifetime of the carriers under test to ensure instantaneous excitation and distortion-free recording of the carrier recombination and decay process.

[0067] An optical focusing unit is positioned along the light output path of the pulsed laser. This unit comprises one or more lenses, mirrors, and apertures. Its function is to shape and focus the laser beam emitted from the pulsed laser, ultimately forming a spot of predetermined size and shape on the surface of the wafer thin film under test. The position and optical path of the optical focusing unit are precisely adjusted to ensure that the illumination area of ​​the spot is strictly limited to the wafer thin film position corresponding to the measurement area of ​​the reconfigurable differential metasurface probe 10, while the wafer thin film below the reference area remains unilluminated.

[0068] The synchronous trigger unit is a digital delay pulse generator or a 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. During the excitation measurement phase, the synchronous trigger unit generates a synchronous trigger signal, which is simultaneously sent to the pulsed laser and the data acquisition and processing module 50. This design ensures that the timing of the laser pulse emitted by the pulsed laser is strictly synchronized with the start time of the data acquisition and processing module 50 in acquiring the transient differential signal, thereby guaranteeing the accuracy of the zero point of the time axis of the acquired attenuation curve.

[0069] Reference Figure 1 and Figure 2 The dual-channel microwave transceiver module 30 is used to generate and feed microwave signals for detection to the reconfigurable differential metasurface probe 10, receive 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.

[0070] 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.

[0071] The power distribution unit has its input terminal connected to the output terminal of the microwave signal source. The power distribution unit divides the input raw microwave signal into two equal signals, which have equal amplitude and the same phase. The two output ports of the power distribution unit are electrically connected to the feed ports of the measurement area and reference area of ​​the reconfigurable differential metasurface probe 10, respectively, via phase-matched transmission cables.

[0072] The differential processing unit receives reflected signals from the measurement area and the reference area at its two input ports, respectively. In one specific embodiment, the differential processing unit is a 180° mixing loop. This 180° mixing loop performs vector operations on the two input reflected signals and outputs a voltage signal from its differential port. This voltage signal is the differential signal, and its value is related to the vector difference between the two input signals.

[0073] 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:

[0074] ;

[0075] 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.

[0076] 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.

[0077] 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.

[0078] 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.

[0079] 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 generated by calculating using a preset control algorithm. .

[0080] In one specific embodiment, the controller employs a proportional-integral-derivative (PID) control algorithm. This algorithm calculates a control signal based on the current value, historical cumulative value, and future trend of the error signal. The signal is a linear superposition of three parts: proportional, integral, and differential, and its mathematical relationship is as follows:

[0081] ;

[0082] In the formula, It is in time Output control signals; It is in time The input error signal is the output of the low-pass filter; It is the gain coefficient of the proportional term, which generates a control action proportional to the magnitude of the current error in response to the system deviation; It is the gain coefficient of the integral term. The function of this term is to accumulate historical errors, which is used to eliminate the steady-state error of the system and ensure that the error signal can approach zero after feedback locking. It is the gain coefficient of the differential term. The function of this term is to generate a control action based on the rate of change of the error signal, to predict the error change and provide damping, so as to suppress the oscillation of the system and speed up the stabilization process. Variables representing time; The function symbol represents the time point of integration. Error signal value at time; As a component of integration operations, it represents the integration variable. An infinitesimal increment; The symbol for differential represents an infinitesimal increment or derivative; As a component of differential operations, it represents the time variable. An infinitesimal increment.

[0083] Control signals output by the controller A tunable element is applied within the reference region of the reconfigurable differential metasurface probe 10. This control signal alters the electrical parameters of the tunable element, thereby adjusting the microwave reflection coefficient of the reference region. The adjustment direction is related to the error signal. Conversely, this forms a negative feedback loop that continuously drives the error signal. Decrease. When the system reaches stability. Approaching zero, the microwave reflection characteristics of the reference area match those of the measurement area, and the differential signal... The DC baseline is locked at a stable minimum value.

[0084] Reference Figure 1 and Figure 2 The data acquisition and processing module 50 has its input terminal electrically connected to the differential signal output terminal of the dual-channel microwave transceiver module 30, and its trigger input terminal electrically connected to the synchronization trigger unit within the optical excitation module 20. The function of this module is to acquire and digitize the transient changes of the differential signal after receiving the synchronization trigger signal, and calculate the carrier lifetime of the wafer thin-film based on the acquired data. In a specific embodiment, the data acquisition and processing module 50 includes a high-speed data acquisition unit and a lifetime calculation unit.

[0085] The high-speed data acquisition unit can be a high-speed digital oscilloscope or a data acquisition card (DAQ). Upon receiving a synchronization trigger signal from the synchronization trigger unit, the high-speed data acquisition unit immediately begins sampling the input differential signal at a preset high sampling rate. Analog-to-digital conversion is performed. The sampling process converts a 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. Its sampling rate must satisfy the Nyquist sampling theorem and be much higher than the highest frequency component of the transient signal being measured.

[0086] The lifetime calculation unit may consist of a general-purpose computer, a microprocessor, or a field-programmable gate array (FPGA) and software algorithms running on it. This lifetime calculation unit receives a digital signal sequence output from a 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 acquired transient signal This directly represents the recombination and decay process of photogenerated carriers.

[0087] The lifetime calculation unit determines the carrier lifetime by performing an exponential decay fitting algorithm on the acquired 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 single exponential decay. Its mathematical model is as follows:

[0088] ;

[0089] In the formula, It is the transient AC component of the differential signal over time. Theoretical value of the change; It is the initial amplitude of the signal at the end of the laser pulse; It is the effective carrier lifetime to be determined; Variables representing time; The mathematical function symbol represents the natural exponential function, that is, the function expressed using the natural constant. An exponential function with base 0.

[0090] In the specific calculation, the lifetime calculation unit uses the least squares method to process the collected data point sequence. Perform fitting. Adjust the model parameters. and The value of makes the sum of squared residuals between the model calculated values ​​and the actual collected values... Minimize. Sum of squared residuals. The expression is:

[0091] ;

[0092] In the formula, The square of the residual; It is the initial amplitude of the signal at the end of the laser pulse; It is the effective carrier lifetime to be determined; The mathematical function symbol represents the natural exponential function, that is, the function expressed using the natural constant. An exponential function with base 0; For summation index variables; The least squares method is used to process the sequence of collected data points. It is the total number of data points involved in the fitting process; This is the sampling period of the high-speed data acquisition unit. The value is determined through a numerical optimization algorithm. smallest The value is the result of the wafer thin film carrier lifetime measured in this study.

[0093] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A microwave reflective photoconductivity measurement system for wafer thin-film carrier lifetime, characterized in that, include: A reconfigurable differential metasurface probe interacts with the thin film on the wafer under test via near-field coupling. 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 applies pulsed light to the wafer thin film below the measurement area to generate non-equilibrium carriers; A dual-channel microwave transceiver module feeds microwave signals into the measurement area and the reference area, and receives their respective reflected signals to generate a differential signal characterizing the difference between the two. The closed-loop feedback control module receives the differential signal as input before the optical excitation module operates, and generates a control signal accordingly to apply to the tunable element in order to actively suppress common-mode noise. The data acquisition and processing module acquires the transient changes of the differential signal after the optical excitation module is activated, and calculates the carrier lifetime of the wafer thin film based on the transient changes.

2. The microwave reflective photoconductivity measurement system for wafer thin-film carrier lifetime according to claim 1, characterized in that, The reconfigurable differential metasurface probe specifically includes: Dielectric substrate; A metasurface is formed on the dielectric substrate and is composed of subwavelength periodic metal microstructures, which are divided into different physical regions to form the measurement region and the reference region. The measurement area and the reference area are concentrically distributed in a ring on the dielectric substrate, and the structural design of the reconfigurable differential metasurface probe generates an evanescent wave field on the probe surface at the operating frequency to achieve near-field coupling with the thin film of the wafer under test.

3. The microwave reflective photoconductivity measurement system for wafer thin-film carrier lifetime according to claim 1, characterized in that, The tunable element specifically includes: varactor diode; The varactor diode is integrated within a subwavelength periodic metal microstructure in the reference region; The capacitance value of the varactor diode changes in response to the control signal of the reference region, thereby altering the resonant characteristics of the subwavelength periodic metal microstructure.

4. The microwave reflective photoconductivity measurement system for wafer thin-film carrier lifetime according to claim 1, characterized in that, The optical excitation module specifically includes: A pulsed laser is used to generate a laser beam with a predetermined wavelength and pulse width. An optical focusing unit, located in the optical path of the pulsed laser, is used to focus the laser beam into a spot and ensure that the spot only illuminates the wafer thin film below the measurement area; A synchronous triggering unit is electrically connected to the pulsed laser and the data acquisition and processing module to ensure that the data acquisition and processing module starts acquiring data synchronously when the pulsed laser emits a laser beam.

5. The microwave reflective photoconductivity measurement system for wafer thin-film carrier lifetime according to claim 1, characterized in that, The dual-channel microwave transceiver module specifically includes: A microwave signal source is used to generate raw microwave signals with a stable frequency. A power distribution unit is connected to the original microwave signal, and the output of the power distribution unit is connected to the measurement area and the reference area respectively, for splitting the original microwave signal into two paths; The differential processing unit receives reflected signals from the measurement area and the reference area respectively. The differential processing unit is a 180° mixing loop used to perform differential operations on the two received reflected signals to output the differential signal.

6. The microwave reflective photoconductivity measurement system for wafer thin-film carrier lifetime according to claim 1, characterized in that, The closed-loop feedback control module specifically includes: A low-pass filter is used to receive the differential signal and to filter out the high-frequency transient components in the differential signal and extract its DC or slowly varying components. A controller is connected to the output terminal of the low-pass filter, and the output terminal of the controller is used to output the control signal.

7. The microwave reflective photoconductivity measurement system for wafer thin-film carrier lifetime according to claim 6, characterized in that, The controller employs a proportional-integral-derivative control algorithm, using the output of the low-pass filter as the error input to generate the control signal.

8. The microwave reflective photoconductivity measurement system for wafer thin-film carrier lifetime according to claim 7, characterized in that, The proportional-integral-derivative (PI-DI) control algorithm is used to generate the control signal, which is a linear superposition of the following three parts: A scaling 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 low-pass filter's output with respect to time; The differential term, the value of which is proportional to the time derivative of the output of the low-pass filter.

9. The microwave reflective photoconductivity measurement system for wafer thin-film carrier lifetime according to claim 1, characterized in that, The data acquisition and processing module specifically includes: A high-speed data acquisition unit is used to receive the differential signal and, after the optical excitation module is activated, perform high-speed analog-to-digital conversion on the transient changes of the differential signal. The lifetime calculation unit, connected to the high-speed data acquisition unit, is used to process the converted digital signal to calculate the carrier lifetime.

10. A microwave reflective photoconductivity measurement system for wafer thin-film carrier lifetime according to claim 9, characterized in that, The lifetime calculation unit determines the carrier lifetime of the wafer thin film by performing an exponential decay fitting algorithm on the transient changes of the differential signal acquired by the high-speed data acquisition unit.

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