A laser sustained plasma stable control system and method for wafer defect detection

Abstract

The application belongs to the technical field of semiconductor wafer defect detection, and provides a laser sustained plasma stable control system and method for wafer defect detection, which comprises a multi-wavelength laser driving module, a dynamic light spot modulation module, a plasma cavity module, a closed-loop feedback control module, a gas thermophysical property adjustment module and an optical collection and output module; the plasma thermal equilibrium is maintained through 1064nm laser, the 808nm modulation laser disturbs the shear laminar flow field, and the 10-500Hz dynamic light spot scanning is combined to destroy the vortex periodic structure; the high-speed imaging and radiation intensity real-time feedback are combined, the laser parameters, the light spot trajectory and the Xe / Ar / He mixed gas ratio are adjusted through the PID+MPC control algorithm, so that the plasma oscillation frequency is reduced from 20-50Hz to below 5Hz; the application realizes the closed-loop stable regulation and control of the plasma, outputs a wide-spectrum stable light source, and meets the wafer defect detection requirement.

Description

Technical Field

[0001] This invention belongs to the field of semiconductor wafer defect detection technology, specifically relating to a laser-maintained plasma stabilization control system and method for wafer defect detection, which is particularly suitable for optical detection equipment for wafer surface defects that require high brightness and wide-spectrum stable light sources. Background Technology

[0002] In advanced semiconductor manufacturing, wafer defect detection places stringent requirements on light sources: high radiance, broad spectral coverage (ultraviolet-visible-infrared), and high temporal stability (radiation fluctuation <1%). Laser-sustaining plasma (LSP) sources are considered important candidates for high-end detection light sources due to their blackbody-like broad spectral radiation characteristics.

[0003] However, existing laser-maintained plasma light sources have the following technical problems when applied to wafer bright-field inspection:

[0004] 1. The plasma generates periodic oscillations in the 20-50Hz range, accompanied by plasma position drift and radiation intensity fluctuations, resulting in a decrease in image signal-to-noise ratio and an increase in defect misjudgment rate;

[0005] 2. Buoyancy-driven vortex shedding causes periodic fracturing of the shear layer, resulting in an imbalance between heat diffusion and energy input, leading to localized overheating and asymmetric plasma morphology;

[0006] 3. Fluctuations in the collection efficiency of the ellipsoidal mirror and a decrease in fiber coupling efficiency further exacerbate the instability of the output light intensity;

[0007] 4. Traditional single-wavelength continuous laser sustaining schemes (such as single 1064nm heating) cannot effectively suppress the periodic generation of shear layer vortices; fixed-focus heating methods are prone to forming steady-state thermal plumes, which exacerbate the periodic vortex shedding; lack of high-speed real-time feedback on plasma state and multi-parameter coordinated control capability; single gas medium (such as pure xenon) with unadjustable thermal properties makes it difficult to suppress oscillations from the root of fluid dynamics.

[0008] Therefore, there is an urgent need for a novel laser-maintained plasma source system and method that can actively suppress low-frequency plasma oscillations, stabilize output radiation, and improve the signal-to-noise ratio of wafer inspection. Summary of the Invention

[0009] To address the shortcomings of existing technologies, this invention provides a laser-maintained plasma stabilization control system and method for wafer defect detection, overcoming the deficiencies of existing technologies.

[0010] To achieve the above objectives, the present invention provides the following technical solution:

[0011] This invention provides a laser-maintained plasma stabilization control system for wafer defect detection, comprising:

[0012] Multi-wavelength laser drive module;

[0013] The dynamic beam modulation module has its optical input end connected to the optical output end of the multi-wavelength laser driving module via optical fiber.

[0014] The plasma cavity module has its optical input end coupled to the optical output end of the dynamic spot modulation module through a quartz window;

[0015] The closed-loop feedback control module has its signal acquisition terminal electrically connected to the detection output terminal of the plasma cavity module via signal lines, and its control output terminal electrically connected to the control input terminals of the multi-wavelength laser driving module, the dynamic spot modulation module, and the gas thermophysical property adjustment module via signal lines.

[0016] The gas thermophysical property adjustment module has its gas outlet connected to the gas inlet of the plasma cavity module via a sealed pipeline;

[0017] An optical collection and output module, with its optical input end aligned with the plasma radiation output end of the plasma cavity module;

[0018] The system utilizes the synergistic effect of the multi-wavelength laser driving module and the dynamic spot modulation module to perform dynamic spot scanning.

[0019] Preferably, the multi-wavelength laser driving module includes:

[0020] First laser source;

[0021] Second laser source;

[0022] Low-power shortwave laser source;

[0023] The laser source, the second laser source, and the low-power short-wavelength laser source are respectively coupled to the input end of the laser combiner via optical fibers, and the output end of the laser combiner serves as the optical output end of the multi-wavelength laser driving module.

[0024] Preferably, the dynamic spot modulation module:

[0025] MEMS adjustable reflectors;

[0026] A spatial light modulator, the input of which is optically coupled to the output of the MEMS adjustable mirror via a reflected light path;

[0027] The time modulation scanning unit has its control terminal electrically connected to the control output terminal of the closed-loop feedback control module via a signal line, and its output terminal is electrically connected to the control input terminals of the MEMS adjustable reflector and the spatial light modulator via a drive circuit.

[0028] The dynamic spot modulation module performs periodic or random scanning of the laser focus, and the dynamic spot modulation frequency is 10-500Hz.

[0029] And satisfy the following formula: ,

[0030] In the formula, For a moment Location Heat flux density at that point The average heat flux density, For modulation amplitude, To modulate the angular frequency, This is the initial phase.

[0031] Preferably, the plasma flow field feedback control module includes:

[0032] A high-pressure xenon gas chamber, with a plasma generation zone inside;

[0033] The gas inlet is located on the high-pressure xenon gas chamber and is connected to the gas outlet of the gas thermophysical property adjustment module through a sealing flange.

[0034] Quartz window, fixed to the cavity wall.

[0035] Preferably, the closed-loop feedback control module includes:

[0036] A high-speed imaging device with a frame rate of not less than 10kfps, and its acquisition end is aligned with the plasma region through an optical window set on the plasma cavity module;

[0037] A radiation intensity detector, the detection end of which is aligned with the radiation output window of the plasma cavity module via an optical fiber;

[0038] The oscillation identification module has its input terminal electrically connected to the data output terminal of the high-speed imaging device and the signal output terminal of the radiation intensity detector via signal lines;

[0039] The control algorithm unit has its input terminal electrically connected to the output terminal of the oscillation identification module via an internal bus, and its control output terminal is connected to the power control terminal of the multi-wavelength laser driving module, the modulation parameter control terminal of the dynamic spot modulation module, and the gas distribution and flow rate control terminal of the gas thermophysical property adjustment module via signal lines, respectively.

[0040] The control algorithm unit aims to achieve the following control objectives:

[0041] , ,

[0042] in, For the plasma temperature gradient, Radiation intensity;

[0043] Plasma closed-loop stable control is achieved through the following PID control law:

[0044] ,

[0045] in, The error between the set value and the actual value of radiation intensity. , , These are the proportional, integral, and derivative coefficients, respectively; and they simultaneously satisfy the following fluid-optical coupling control equations:

[0046] ,

[0047] in, The intensity of plasma radiation. For laser power, For temperature, For density, Flow rate.

[0048] Preferably, the gas thermophysical property adjustment module includes:

[0049] Xe gas source, Ar gas source, He gas source; each gas source outlet is equipped with an independent mass flow controller;

[0050] The gas distribution unit has its inlet connected to the Xe gas source, Ar gas source, and He gas source respectively via pipelines;

[0051] The gas outlet of the gas distribution unit is connected to the gas inlet of the plasma cavity module via a pipeline.

[0052] The control input terminal of the gas distribution unit and the control output terminal of the control algorithm unit are electrically connected via signal lines.

[0053] Preferably, the optical collection and output module includes:

[0054] A high numerical aperture mirror, the reflecting surface of which faces the radiation output window of the plasma cavity module;

[0055] A broadband collection unit, the input end of which is aligned with the output optical path of the high numerical aperture mirror through a focusing lens group, the collection band of the broadband collection unit covers ultraviolet, visible and infrared light;

[0056] The input end of the fiber optic output module is coupled to the output end of the broadband collection unit via a fiber optic connector, or a free-space output module can be used to replace the fiber optic output module.

[0057] The present invention also provides a laser-suppressed plasma stabilization control method for wafer defect detection, for implementing the laser-suppressed plasma stabilization control system described in any of the above claims, comprising the following steps:

[0058] Step 1: The control algorithm unit of the closed-loop feedback control module sends a proportional command to the gas distribution unit of the gas thermophysical property adjustment module via a signal line; the gas distribution unit draws gas from the Xe gas source, Ar gas source, and He gas source according to a set ratio, and fills the high-pressure xenon gas chamber through a sealed pipeline and the gas inlet of the plasma chamber module.

[0059] Step 2: The closed-loop feedback control module sends an ignition command to the multi-wavelength laser drive module to start the low-power short-wavelength laser source or the second laser source. After passing through a beam combiner, the laser enters the dynamic spot modulation module through an optical fiber, and is focused by a MEMS adjustable reflector and a spatial light modulator before being shot into the plasma cavity module through the quartz window to ignite the gas. Simultaneously, the high-speed imaging device of the closed-loop feedback control module begins to acquire shadow images of the plasma region through the radiation detection port, and the detection end of the radiation intensity detector begins to monitor the real-time radiation intensity through the quartz window or optical fiber. The acquired data is transmitted to the oscillation recognition module via a signal line.

[0060] Step 3: After successful ignition, switch to the first laser source in the multi-wavelength laser drive module to output 1064nm continuous laser, enter plasma maintenance mode, and turn off or reduce the power of the low-power short-wavelength laser source; the 1064nm laser is injected into the cavity in the form of an initial fixed spot through the dynamic spot modulation module to maintain plasma thermal balance.

[0061] Step 4: The high-speed imaging device continuously generates image data, and the radiation intensity detector synchronously generates radiation intensity data; both send the data to the oscillation identification module through a signal line; the oscillation identification module performs FFT or time series analysis on the image data, extracts the characteristic frequency and amplitude of plasma oscillation, and transmits the analysis results to the control algorithm unit through the internal bus to determine whether there is a low-frequency oscillation in the range of 20-50Hz;

[0062] Step 5: When low-frequency oscillations are detected, the control algorithm unit determines the appropriate response based on the fluid-optical coupling equation. The required modulation parameters are calculated, and control commands are sent to the time modulation scanning unit of the dynamic spot modulation module via signal lines. The time modulation scanning unit drives the MEMS adjustable mirror and spatial light modulator through a driving circuit, so that the laser focus performs periodic or random scanning at 10-500Hz. At the same time, the power and phase of the second laser source are adjusted via signal lines to ensure that the heat flux density distribution meets the requirements. ;

[0063] Step 6: The control algorithm unit is based on PID control law. ,by To control the target, the following actuators are adjusted in real time via signal lines:

[0064] The power of the first laser source;

[0065] Modulation parameters of the second laser source;

[0066] The scanning trajectory and frequency of the dynamic spot modulation module;

[0067] The output gas flow rate and pressure gradient of the gas distribution unit in the gas thermophysical property regulation module;

[0068] The iteration period is on the order of milliseconds, until the oscillation frequency reported by the oscillation identification module drops below 5Hz or is completely eliminated;

[0069] Step 7: When the control algorithm unit determines that the plasma meets the requirements... After the oscillation frequency is less than 5Hz, the high numerical aperture mirror of the optical collection and output module collects plasma radiation. After passing through the broadband collection unit, it is coupled to the fiber output module or free space output module with a fiber core diameter of 200-600μm through the fiber connector, and the stable broadband radiation light is guided to the wafer defect detection equipment. At the same time, the closed-loop feedback control module continues to monitor at an update frequency of not less than 10 times per second.

[0070] This invention provides a laser-maintained plasma stabilization control system and method for wafer defect detection. It offers the following advantages:

[0071] By using multi-wavelength laser synergy (1064nm continuous laser to maintain thermal equilibrium, 808nm modulated laser to disturb the shear layer flow field structure) and dynamic spot scanning (10-500Hz periodic or random scanning), the periodic generation conditions of plasma shear layer vortices are disrupted, reducing plasma oscillation amplitude by >70% and controlling radiation intensity fluctuations to <1%, thus meeting the stability requirements of light source time for high-end wafer inspection.

[0072] Stable broadband radiation output (UV-VIS-IR) combined with closed-loop feedback control (high-speed imaging + radiation intensity detection + MPC + PID regulation) improves image contrast by 30-50% and significantly reduces the defect misjudgment rate.

[0073] By optimizing the thermal diffusivity and buoyancy driving strength through a gas thermophysical property adjustment module (Xe / Ar / He mixed gas ratio adjustable), the deposition rate of thermally induced contaminant particles on the quartz window surface is reduced, thereby extending the window maintenance cycle.

[0074] Based on high-speed imaging (≥10kfps) and real-time feedback of radiation intensity, combined with PID+model predictive control algorithm, the laser power, spot scanning trajectory, gas flow rate and pressure gradient are adjusted in real time to make the plasma temperature gradient tend to be constant and the radiation intensity derivative is zero, so as to achieve fluid-optical coupling stable control.

[0075] The optical collection and output module provides a high-NA reflector, a wide-spectrum collection unit, and an optical fiber (200-600μm) / free space output interface, which can flexibly match the optical path requirements of different wafer defect detection equipment. Attached Figure Description

[0076] Figure 1 This is a hardware module topology diagram of the system of the present invention;

[0077] Figure 2 This is the closed-loop feedback control signal flow topology diagram of the present invention;

[0078] Figure 3 This is a flowchart of the method steps of the present invention. Detailed Implementation

[0079] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of 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, 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.

[0080] like Figure 1 As shown, in one embodiment, a laser-maintained plasma stabilization control system for wafer defect detection includes:

[0081] A multi-wavelength laser drive module is used to provide laser energy of different wavelengths to achieve plasma ignition, heat maintenance, and flow field disturbance, respectively.

[0082] The dynamic beam modulation module has its optical input end connected to the optical output end of the multi-wavelength laser driving module via optical fiber to achieve spatial transmission and mode matching of the laser beam.

[0083] The plasma cavity module has its optical input end coupled to the optical output end of the dynamic spot modulation module through a quartz window, so that the modulated laser is focused into the cavity to excite and maintain the plasma.

[0084] The closed-loop feedback control module has its signal acquisition terminal electrically connected to the detection output terminal of the plasma cavity module via signal lines, and is used to acquire plasma images and radiation intensity signals in real time. Its control output terminal is electrically connected to the control input terminals of the multi-wavelength laser driving module, the dynamic spot modulation module and the gas thermophysical property adjustment module via signal lines, and is used to dynamically adjust the laser power, spot scanning parameters and gas composition and flow rate according to the feedback signal.

[0085] A gas thermophysical property adjustment module, whose gas outlet is connected to the gas inlet of the plasma cavity module through a sealed pipeline, is used to fill the cavity with a controllable proportion of mixed gas to change the thermodynamic properties of the plasma.

[0086] An optical collection and output module, with its optical input end aligned with the plasma radiation output end of the plasma cavity module, is used to collect broadband light emitted by the plasma and couple it to subsequent detection equipment.

[0087] The multi-wavelength laser driving module and the dynamic spot modulation module work together to scan the multi-wavelength laser, thereby suppressing the plasma shear layer vortex generation process and reducing low-frequency oscillations. The shear layer refers to the velocity gradient region between the plasma and the surrounding gas. Vortex generation is the main fluid instability that causes oscillations.

[0088] Specifically, the multi-wavelength laser driving module includes:

[0089] The first laser source (e.g., fiber laser or diode-pumped solid-state laser) is used to output 1064nm continuous laser to maintain plasma thermal equilibrium.

[0090] The second laser source is used to output an 808nm modulated laser to disturb the shear layer flow field structure (this wavelength can be absorbed by the plasma edge region and generate periodic thermal disturbances through modulation).

[0091] Low-power short-wavelength laser sources (e.g., 405nm or 532nm lasers, power <1W) are used to assist in ignition stabilization. Short-wavelength photons have high energy and can easily break down gases to generate initial seed electrons.

[0092] The laser source, the second laser source, and the low-power short-wavelength laser source are coupled to the input of the laser combiner via optical fibers. The output of the laser combiner serves as the optical output of the multi-wavelength laser driving module, thereby achieving multi-channel coaxial laser output.

[0093] Specifically, the dynamic spot modulation module:

[0094] MEMS Adjustable Reflector (Micro-Electro-Mechanical Systems Two-Dimensional Scanning Reflector, Angle Adjustment Range ±10°);

[0095] The spatial light modulator has its input end optically coupled to the output end of the MEMS tunable mirror through a reflected light path. The SLM can modulate the phase or amplitude of the laser pixel by pixel to generate arbitrary spot shapes.

[0096] The time modulation scanning unit (a waveform generator based on FPGA or DSP) has its control terminal electrically connected to the control output terminal of the closed-loop feedback control module via a signal line (receiving commands such as modulation frequency, amplitude, and waveform), and its output terminal is electrically connected to the control input terminals of the MEMS adjustable reflector and the spatial light modulator via a drive circuit (providing scanning voltage and modulation signal).

[0097] The dynamic spot modulation module performs periodic or random scanning of the laser focus, and the dynamic spot modulation frequency is 10–500 Hz. (This frequency range covers the dominant frequency and its harmonics of plasma shear layer vortex shearing, which can effectively disrupt periodic structures and is used to disrupt the formation of periodic plasma structures.)

[0098] And it satisfies the following equation (quantitatively describing the spatiotemporal distribution of the scanned heat source): ,

[0099] In the formula, For a moment Location Heat flux density at a given location (unit: W / m²). The average heat flux density (contributed by the 1064nm laser). The modulation amplitude (determined by the intensity of the 808nm modulated laser). For modulation angular frequency ( , (Take 10-500Hz) This is the initial phase (adjustable, used to optimize phase matching).

[0100] Specifically, the plasma flow field feedback control module includes:

[0101] The high-pressure xenon chamber (made of stainless steel or aluminum alloy, designed to withstand pressure of 1-10 atm) has a plasma generation zone inside (a region with a volume of about 1-10 mm³ near the laser focal point).

[0102] The gas inlet is located on the high-pressure xenon gas chamber and is connected to the gas outlet of the gas thermophysical property adjustment module through a sealing flange. Metal gaskets or O-rings are installed between the flanges to ensure airtightness.

[0103] Quartz windows (2-5mm thick, coated with antireflective film, transmittance >95%) are fixed to the cavity wall and used for laser injection and plasma radiation output; the window material must be heat resistant and sputter resistant.

[0104] like Figure 2 As shown, specifically, the closed-loop feedback control module includes:

[0105] A high-speed imaging device (e.g., a CMOS high-speed camera equipped with a long working distance microscope lens) has a frame rate of not less than 10kfps, and its acquisition end is aligned with the plasma region through an optical window set on the plasma cavity module to capture the dynamic evolution of plasma morphology and vortex structure.

[0106] A radiation intensity detector (e.g., a photodiode or photomultiplier tube, equipped with a narrowband filter or a broadband response) has its detection end aligned with the radiation output window of the plasma cavity module via an optical fiber to monitor the radiation power of the plasma in a specific or full-band wavelength range in real time.

[0107] The oscillation identification module (implemented by a digital signal processor or FPGA) has its input terminals electrically connected to the data output terminal of the high-speed imaging device and the signal output terminal of the radiation intensity detector via signal lines. It receives image sequences and intensity voltage signals and is used to perform FFT or time series analysis on the image data (fast Fourier transform to extract the oscillation frequency, and time series analysis to identify the periodicity of vortex shedding).

[0108] The control algorithm unit (adopting a parallel structure of model predictive control (MPC) and PID) has its input terminal electrically connected to the output terminal of the oscillation identification module via an internal bus (receiving oscillation frequency, amplitude, and error signals). Its control output terminal is connected via signal lines to the power control terminal of the multi-wavelength laser drive module, the modulation parameter control terminal of the dynamic spot modulation module, and the gas distribution and flow rate control terminal of the gas thermophysical property adjustment module (outputting 0-10V analog quantity or PWM signal).

[0109] The control algorithm unit aims to achieve the following control objectives (objective: to make the plasma temperature gradient approach uniform and the rate of change of radiation intensity approach zero):

[0110] , ,

[0111] in, The plasma temperature gradient is expressed in K / m. Radiation intensity (unit: W / sr);

[0112] And (the plasma closed-loop stable control is achieved through the following PID control law, which relates the rate of change of radiation intensity to the error and its integral and derivative, equivalent to a first-order system):

[0113] ,

[0114] in, This is the error between the set value and the actual value of the radiation intensity (the set value is pre-calibrated based on the illuminance required for wafer inspection). , , These are the proportional, integral, and differential coefficients (tuned using the Ziegler-Nichols method or a genetic algorithm); simultaneously (satisfying the following fluid-optical coupling control equation, which shows that the rate of change of radiation intensity is a nonlinear function of laser power, temperature, density, and flow velocity):

[0115] ,

[0116] in, Plasma radiation intensity (unit: W / sr). Laser power (unit: W). Temperature (unit: K). Density (unit: kg / m³). Flow velocity (unit: m / s).

[0117] Specifically, the gas thermophysical property adjustment module includes:

[0118] Xe gas source, Ar gas source, He gas source (high pressure gas cylinder, pressure reducing valve output 0.2-0.5MPa); each gas source outlet is equipped with an independent mass flow controller (MFC, accuracy ±1%FS).

[0119] The gas distribution unit (static mixer or dynamic proportional valve group) has its inlet connected to the Xe gas source, Ar gas source, and He gas source respectively through pipelines (pipeline inner diameter 3-6mm, pressure resistance 1MPa) (used to adjust the ratio of Xe, Ar, and He in the mixed gas (Xe: 0-100%, Ar: 0-100%, He: 0-100%, total flow rate adjustable), thereby changing the gas's thermal diffusivity, specific heat capacity, density, and buoyancy driving strength (for example, adding He increases thermal diffusivity, adding Xe increases radiation efficiency).

[0120] The gas outlet of the gas distribution unit is connected to the gas inlet of the plasma cavity module via a pipeline (a buffer tank can be installed in between to stabilize the airflow).

[0121] The control input terminal of the gas distribution unit and the control output terminal of the control algorithm unit are electrically connected via a signal line (to receive the flow rate setting values ​​of each component).

[0122] Specifically, the optical collection and output module includes:

[0123] A high numerical aperture mirror (NA≥0.5, parabolic or ellipsoidal mirror, aluminum or gold plated) with its reflecting surface facing the radiation output window of the plasma cavity module (to collect plasma radiation at the maximum solid angle).

[0124] A broadband collection unit (lens group or compound parabolic concentrator) has its input end aligned with the output optical path of the high numerical aperture mirror through a focusing lens group. The collection band of the broadband collection unit covers ultraviolet, visible and infrared (200-1100nm, meeting various illumination requirements in wafer defect detection).

[0125] The fiber optic output module (multimode silica fiber, numerical aperture 0.22) has a core diameter of 200-600 μm. Its input end is coupled to the output end of the broadband collection unit via a fiber optic connector (SMA905 or FC / PC standard interface). Alternatively, a free-space output module (beam collimator or beam expander) can be used to replace the fiber optic output module to output stable broadband radiation light to the wafer defect detection equipment. Free-space output is suitable for applications requiring high power density or specific spot shapes.

[0126] The present invention provides a laser-sustaining plasma stabilization control method for wafer defect detection. The laser-sustaining plasma stabilization control method for wafer defect detection described below can be referred to in correspondence with the laser-sustaining plasma stabilization control system for wafer defect detection described above.

[0127] like Figure 3As shown, in one embodiment, a laser-maintained plasma stabilization control method for wafer defect detection includes the following steps:

[0128] Step 1: Gas filling and thermophysical property preset

[0129] The control algorithm unit of the closed-loop feedback control module sends a proportional command (e.g., Xe:Ar:He = 60%:30%:10%) to the gas distribution unit of the gas thermophysical property adjustment module via a signal line. The gas distribution unit draws gas from the Xe gas source, Ar gas source, and He gas source according to the set ratio (total flow rate 0.5-5L / min), and fills the high-pressure xenon gas chamber through the gas inlet of the plasma chamber module via a sealed pipeline (to make the chamber pressure reach 0.5-2atm, and wait 10-30 seconds to stabilize the airflow) to adjust the thermal diffusivity and buoyancy driving strength.

[0130] Step 2: Laser Ignition

[0131] The closed-loop feedback control module sends an ignition command (lasting 5-20ms) to the multi-wavelength laser drive module, activating a low-power short-wavelength laser source (e.g., 405nm, power 50-200mW) or a second laser source (808nm, power 0.5-2W pulse mode). This laser, after passing through a beam combiner, enters the dynamic spot modulation module via an optical fiber. After being focused sequentially by a MEMS adjustable reflector and a spatial light modulator (beam waist diameter approximately 50-200μm), it is shot into the plasma cavity module through the quartz window, igniting the gas (generating an initial plasma fire nucleus). Simultaneously, the high-speed imaging device (exposure time 1-100μs) of the closed-loop feedback control module begins acquiring shadow images of the plasma region (magnification 10-50×) through the radiation detection port. The radiation intensity detector's detection end begins monitoring real-time radiation intensity (sampling rate ≥100kHz) through the quartz window or optical fiber. The acquired data is transmitted to the oscillation recognition module via a signal line.

[0132] Step 3: Switch to sustain mode

[0133] After successful ignition (judgment condition: radiation intensity exceeds the threshold and remains stable), the system switches to the first laser source in the multi-wavelength laser drive module, outputting a 1064nm continuous laser (power gradually increases from 10W to 200W, rise rate 10-50W / s), entering plasma sustaining mode, and shutting down or reducing the power of the low-power short-wavelength laser source (reducing it to 0 or maintaining it below 10mW); the 1064nm laser passes through the dynamic spot modulation module (at this time, the scanning unit outputs an initial fixed bias, i.e.) The plasma is injected into the cavity in the form of an initial fixed spot (the spot diameter is about 1-2 mm) to maintain plasma thermal balance (so that the plasma temperature rises to 5000-10000K).

[0134] Step 4: Real-time monitoring and oscillation identification

[0135] The high-speed imaging device (≥10kfps) continuously generates image data (each frame resolution ≥256×256 pixels), and the radiation intensity detector synchronously generates radiation intensity data; both send the data to the oscillation identification module via a signal line; the oscillation identification module performs FFT (window length 1024-4096 points) or time series analysis (autocorrelation function or phase space reconstruction) on the image data, extracts the characteristic frequency and amplitude of plasma oscillation (the dominant frequency is usually 20-50Hz, and the amplitude exceeding 10% of the average intensity is considered a significant oscillation), and transmits the analysis results (including error signals) to the relevant data. =Set radiation intensity - current radiation intensity) is transmitted to the control algorithm unit via the internal bus to determine whether there is a low-frequency oscillation in the range of 20-50Hz (if it exists, proceed to step 5; otherwise, remain in step 3).

[0136] Step 5: Dynamic spot modulation intervention

[0137] When low-frequency oscillations are detected, the control algorithm unit applies the fluid-optical coupling equation. The required modulation parameters are calculated (identified online using a backpropagation neural network or linear model), and control commands (including modulation frequency) are sent to the time modulation scanning unit of the dynamic spot modulation module via a signal line. Select 10-500Hz, amplitude / Take a value of 0.1-0.5 for the initial phase. (Optimization to minimize error); the time-modulated scanning unit drives the MEMS adjustable reflector (scanning angle ±5° corresponds to focal point movement ±2mm) and the spatial light modulator (generating a ring or checkerboard phase map) through a driving circuit, so that the laser focus performs periodic or random scanning at 10-500Hz (the scanning trajectory can be Lissajous, spiral, or pseudo-random point sequence), and at the same time adjusts the power and phase of the second laser source through the signal line (808nm laser power accounts for 5%-20% of 1064nm power) so that the heat flux density distribution meets the requirements. (This disrupts the periodic structure of the vortex, and by destroying spatial coherence, it broadens or eliminates the frequency of vortex shedding.)

[0138] Step 6: Closed-loop stability control

[0139] The control algorithm unit is based on PID control law. ,by To control the target, the following actuators are adjusted in real time via signal lines:

[0140] The power of the first laser source (adjustable) (Range 10-300W, resolution 0.1W).

[0141] Modulation parameters of the second laser source ( , , (Update rate 1kHz);

[0142] The scanning trajectory and frequency of the dynamic spot modulation module (updated by the time-modulated scanning unit, which updates the trajectory every 10ms).

[0143] The output gas flow rate and pressure gradient of the gas distribution unit in the gas thermophysical property adjustment module are adjusted by a mass flow controller, with a response time of <100ms.

[0144] The iteration period is in the millisecond range (control period 1-5ms), until the oscillation frequency fed back by the oscillation identification module drops below 5Hz or is completely eliminated (i.e. (The value is less than 1% of the set threshold for 100 ms consecutively).

[0145] Step 7: Stabilize the light source output

[0146] When the control algorithm unit determines that the plasma meets the requirements After the oscillation frequency is <5Hz (stabilization duration is at least 1 second), the high numerical aperture mirror (collection efficiency ≥80%) of the optical collection and output module collects plasma radiation. After passing through the broadband collection unit (transmittance >70%), it is coupled to the fiber output module or free space output module with a fiber core diameter of 200-600μm through the fiber connector, and the stable broadband radiation light is guided to the wafer defect detection equipment (optical power fluctuation <±1%, spectral stability <±2% over 1 hour). At the same time, the closed-loop feedback control module continues to monitor at an update frequency of not less than 10 times per second (10-50Hz) to maintain a stable state (if oscillation recurrence is detected, it automatically jumps back to step 5).

[0147] In Example 1, the system operates under typical conditions in a conventional wafer defect detection scenario, with the following specific parameters and configurations:

[0148] Multi-wavelength laser drive module: First laser source (1064nm continuous laser): Output power is 300W (CW), used to maintain plasma thermal balance.

[0149] The second laser source (808nm modulated laser) has an output power of 50-80W, a modulation waveform of sine wave, and an initial modulation frequency of 20Hz (when dynamic spot scanning is not performed). It is used to initially disturb the shear layer flow field structure.

[0150] Low-power short-wavelength laser source: Used only during the ignition phase, and turned off after ignition is complete;

[0151] Gas thermophysical property adjustment module: The mixed gas ratio is: Xe + 5%Ar (volume fraction), without adding He.

[0152] The gas distribution unit fills the plasma cavity module with gas through a sealed pipeline, and the absolute pressure inside the cavity is 2 atm.

[0153] Gas thermophysical parameters: Xe is the main luminescent gas, and Ar is used to moderately adjust the thermal conductivity without intentionally reducing the buoyancy driving strength.

[0154] Dynamic spot modulation module: In this embodiment, dynamic spot scanning is not enabled (the time modulation scanning unit outputs a fixed bias), and the laser focus remains at a fixed position.

[0155] The light spot size is approximately 1-2 mm, and it is statically aligned using a MEMS adjustable reflector.

[0156] Closed-loop feedback control module: The frame rate of the high-speed imaging device is set to 10kfps, and the sampling rate of the radiation intensity detector is 1kHz.

[0157] The oscillation identification module performs FFT analysis on the image to monitor the plasma oscillation frequency (typical value 20-30Hz).

[0158] The control algorithm unit only uses PID control (MPC is not enabled), and the adjustment target is to control the oscillation amplitude within ±5%, without requiring complete elimination of oscillation.

[0159] Control output: Only adjust the 1064nm laser power (±10%), do not adjust the 808nm modulation parameters and spot position.

[0160] Optical collection and output module: The fiber output module is adopted with a fiber core diameter of 200μm and a numerical aperture NA=0.22.

[0161] Output spectral range: 200-1000nm, used for bright or dark field illumination of wafer surface defects.

[0162] This embodiment can achieve stable plasma maintenance, with the low-frequency oscillation frequency suppressed to below 20Hz and radiation intensity fluctuation <5%, meeting the requirements of conventional wafer inspection.

[0163] In Example 2, dynamic spot scanning and model predictive control (MPC) are introduced based on Example 1 to achieve highly stable plasma output and completely eliminate periodic oscillations.

[0164] Multi-wavelength laser drive module: First laser source (1064nm): 300WCW, power adjustable range ±20%.

[0165] Second laser source (808nm): 80W modulation, programmable modulation waveform (sine wave, square wave or custom), initial modulation frequency 50Hz.

[0166] Dynamic spot modulation module: Enable time modulation scanning unit, and set the spot scanning frequency to 50-300Hz (adaptively adjusted according to real-time feedback).

[0167] Scanning mode: Two-dimensional Lissajous trajectory or circular scan, with the scan amplitude covering the plasma core region (approximately 5 mm in diameter).

[0168] The spatiotemporal distribution of heat flux density satisfies: ,

[0169] The modulation angular frequency It is calculated in real time by the closed-loop feedback control module.

[0170] Closed-loop feedback control module: The control algorithm unit adopts MPC+PID hybrid control, and the MPC control cycle is 1kHz (i.e., the control refresh frequency is 1000Hz).

[0171] Feedback input: Real-time image sequence from a high-speed imaging device (frame rate ≥ 10kfps) + signal from a radiation intensity detector (sampling rate 10kHz).

[0172] The oscillation identification module extracts the oscillation frequency and amplitude in real time using FFT. If periodic oscillations (such as 20-50Hz) are detected, the MPC calculates the optimal control sequence and outputs instructions to:

[0173] Dynamic beam modulation module: Adjusts scanning frequency, trajectory, and modulation depth. ;

[0174] Multi-wavelength laser drive module: Synchronously adjusts the modulation phase of 808nm laser To achieve destructive interference.

[0175] Control objective: Completely eliminate periodic oscillations (oscillation amplitude < 0.1%).

[0176] Gas thermophysical property adjustment module: Gas ratio: Xe + 5%Ar + 1%He (volume fraction). The addition of He enhances the thermal diffusivity and suppresses low-frequency vortices driven by buoyancy.

[0177] The pressure is maintained at 2 atm, but a pressure disturbance suppressor (passive buffer chamber) is added.

[0178] Optical collection and output module: Same as in Example 1, 200μm optical fiber, NA=0.22.

[0179] This embodiment can reduce the plasma oscillation frequency to below 5Hz and completely eliminate the periodicity, with radiation stability better than 0.5%, making it suitable for high-precision wafer defect detection (such as nanometer-level linewidth measurement).

[0180] In Example 3, based on the previous examples, random phase modulation (RPSM) and gas flow field micro-perturbation control are further introduced to suppress the plasma oscillation amplitude to <0.5% and achieve an extremely low noise light source.

[0181] Multi-wavelength laser drive module: First laser source (1064nm): 300WCW, with additional low-frequency random amplitude perturbation (±2%, bandwidth 0.1-10Hz) to disrupt the periodic establishment of thermal equilibrium.

[0182] Second laser source (808nm): 50-80W modulation, modulation phase Random function generation, i.e., random phase modulation (RPSM), is used, with a random phase update frequency of 500Hz, so that the shear layer disturbance has no periodicity, thereby completely eliminating the regular shedding of vortices.

[0183] The heat flux density distribution is corrected to:

[0184] in Let be a random variable that follows a uniform distribution. Small random fluctuations can also be added.

[0185] Dynamic spot modulation module: The spot scanning frequency randomly changes within the range of 50-300Hz (controlled by a random number generator to modulate the scanning unit), and the scanning trajectory is a chaotic mapping (such as a Lorentz trajectory) to avoid any repeated paths.

[0186] Simultaneously, a gas flow field micro-disturbance control signal is superimposed: the micro piezoelectric valve in the gas thermophysical property adjustment module is controlled by the closed-loop feedback control module to apply a small pressure disturbance (amplitude <1% of cavity pressure) to the inlet airflow at a random frequency (20-200Hz), thereby disrupting the large-scale vortex generation conditions of the shear layer.

[0187] Closed-loop feedback control module: The control algorithm unit adopts adaptive stochastic control + traditional PID. The MPC period remains 1kHz, but a random component is added to the reference trajectory.

[0188] The oscillation identification module not only analyzes the FFT peak value, but also calculates the Lyapunov exponent of the time series to ensure that the system enters a non-periodic, low-amplitude chaotic state.

[0189] Feedback control objectives:

[0190] And the oscillation amplitude is <0.5% (peak-to-peak value).

[0191] Control output:

[0192] Send random scanning parameters to the dynamic spot modulation module;

[0193] Send a random phase modulation sequence to the multi-wavelength laser drive module;

[0194] Send a micro-disturbance valve drive signal to the gas thermophysical property regulation module.

[0195] Gas thermophysical property adjustment module: Gas ratio: Xe + 3%Ar + 5%He, further increasing the He ratio, accelerating thermal diffusion, and shortening the vortex survival time.

[0196] A micro-flow disturbance device (piezoelectric ceramic driven diaphragm) is connected in series at the outlet of the gas distribution unit. It is directly controlled by the closed-loop feedback control module to realize active micro-disturbance of the gas flow field (pressure fluctuation range ±0.02atm, frequency 20-200Hz random).

[0197] The chamber pressure remains at 2 atm, but an active pressure stabilizer is used to eliminate macroscopic drift.

[0198] Optical collection and output module: A free-space output module is used (to avoid inter-mode noise introduced by optical fiber). The output light is directly irradiated on the wafer after beam expansion and collimation.

[0199] Simultaneously, a monitoring light is output to the reference detector to calculate the real-time radiation intensity fluctuation rate.

[0200] This embodiment can achieve a plasma radiation intensity fluctuation of <0.5% and an oscillation spectrum with a broad spectrum without spikes, which fully meets the application scenarios with extremely high requirements for light source stability, such as extreme ultraviolet lithography mask inspection and defect-free wafer full-surface scanning.

[0201] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A laser-maintained plasma stabilization control system for wafer defect detection, characterized in that, include: Multi-wavelength laser drive module; The dynamic beam modulation module has its optical input end connected to the optical output end of the multi-wavelength laser driving module via optical fiber. The plasma cavity module has its optical input end coupled to the optical output end of the dynamic spot modulation module through a quartz window; The closed-loop feedback control module has its signal acquisition terminal electrically connected to the detection output terminal of the plasma cavity module via signal lines, and its control output terminal electrically connected to the control input terminals of the multi-wavelength laser driving module, the dynamic spot modulation module, and the gas thermophysical property adjustment module via signal lines. The gas thermophysical property adjustment module has its gas outlet connected to the gas inlet of the plasma cavity module via a sealed pipeline; An optical collection and output module, with its optical input end aligned with the plasma radiation output end of the plasma cavity module; The system utilizes the synergistic effect of the multi-wavelength laser driving module and the dynamic spot modulation module to perform dynamic spot scanning.

2. The system according to claim 1, characterized in that: The multi-wavelength laser driving module includes: First laser source; Second laser source; Low-power shortwave laser source; The laser source, the second laser source, and the low-power short-wavelength laser source are respectively coupled to the input end of the laser combiner via optical fibers, and the output end of the laser combiner serves as the optical output end of the multi-wavelength laser driving module.

3. The system according to claim 1, characterized in that: The dynamic spot modulation module: MEMS adjustable reflectors; A spatial light modulator, the input of which is optically coupled to the output of the MEMS adjustable mirror via a reflected light path; The time modulation scanning unit has its control terminal electrically connected to the control output terminal of the closed-loop feedback control module via a signal line, and its output terminal is electrically connected to the control input terminals of the MEMS adjustable reflector and the spatial light modulator via a drive circuit. The dynamic spot modulation module performs periodic or random scanning of the laser focus, and the dynamic spot modulation frequency is 10-500Hz. And satisfy the following formula: , In the formula, For a moment Location Heat flux density at that point The average heat flux density, For modulation amplitude, To modulate the angular frequency, This is the initial phase.

4. The system according to claim 1, characterized in that: The plasma flow field feedback control module includes: A high-pressure xenon gas chamber, with a plasma generation zone inside; The gas inlet is located on the high-pressure xenon gas chamber and is connected to the gas outlet of the gas thermophysical property adjustment module through a sealing flange. Quartz window, fixed to the cavity wall.

5. The system according to claim 1, characterized in that: The closed-loop feedback control module includes: A high-speed imaging device, wherein its acquisition end is aligned with the plasma region through an optical window disposed on the plasma cavity module; A radiation intensity detector, the detection end of which is aligned with the radiation output window of the plasma cavity module via an optical fiber; The oscillation identification module has its input terminal electrically connected to the data output terminal of the high-speed imaging device and the signal output terminal of the radiation intensity detector via signal lines; The control algorithm unit has its input terminal electrically connected to the output terminal of the oscillation identification module via an internal bus, and its control output terminal is connected to the power control terminal of the multi-wavelength laser driving module, the modulation parameter control terminal of the dynamic spot modulation module, and the gas distribution and flow rate control terminal of the gas thermophysical property adjustment module via signal lines, respectively. The control algorithm unit aims to achieve the following control objectives: ， ， in, For the plasma temperature gradient, Radiation intensity; Plasma closed-loop stable control is achieved through the following PID control law: ， in, The error between the set value and the actual value of radiation intensity. , , These are the proportional, integral, and derivative coefficients, respectively; and they simultaneously satisfy the following fluid-optical coupling control equations: ， in, The intensity of plasma radiation. For laser power, For temperature, For density, Flow rate.

6. The system according to claim 1, characterized in that: The gas thermophysical property adjustment module includes: Xe gas source, Ar gas source, He gas source; each gas source outlet is equipped with an independent mass flow controller; The gas distribution unit has its inlet connected to the Xe gas source, Ar gas source, and He gas source respectively via pipelines; The gas outlet of the gas distribution unit is connected to the gas inlet of the plasma cavity module via a pipeline. The control input terminal of the gas distribution unit and the control output terminal of the control algorithm unit are electrically connected via signal lines.

7. The system according to claim 1, characterized in that: The optical collection and output module includes: A high numerical aperture mirror, the reflecting surface of which faces the radiation output window of the plasma cavity module; A broadband collection unit, the input end of which is aligned with the output optical path of the high numerical aperture mirror through a focusing lens group, the collection band of the broadband collection unit covers ultraviolet, visible and infrared light; The input end of the fiber optic output module is coupled to the output end of the broadband collection unit via a fiber optic connector, or a free-space output module can be used to replace the fiber optic output module.

8. A laser-sustaining plasma stabilization control method for wafer defect detection, used to implement the system according to any one of claims 1 to 7, characterized in that, Includes the following steps: Step 1: The control algorithm unit of the closed-loop feedback control module sends a proportional command to the gas distribution unit of the gas thermophysical property adjustment module via a signal line; the gas distribution unit draws gas from the Xe gas source, Ar gas source, and He gas source according to a set ratio, and fills the high-pressure xenon gas chamber through a sealed pipeline and the gas inlet of the plasma chamber module. Step 2: The closed-loop feedback control module sends an ignition command to the multi-wavelength laser drive module to start the low-power short-wavelength laser source or the second laser source. After passing through a beam combiner, the laser enters the dynamic spot modulation module through an optical fiber, and is focused by a MEMS adjustable reflector and a spatial light modulator before being shot into the plasma cavity module through the quartz window to ignite the gas. Simultaneously, the high-speed imaging device of the closed-loop feedback control module begins to acquire shadow images of the plasma region through the radiation detection port, and the detection end of the radiation intensity detector begins to monitor the real-time radiation intensity through the quartz window or optical fiber. The acquired data is transmitted to the oscillation recognition module via a signal line. Step 3: After successful ignition, switch to the first laser source in the multi-wavelength laser drive module to output 1064nm continuous laser, enter plasma maintenance mode, and turn off or reduce the power of the low-power short-wavelength laser source; the 1064nm laser is injected into the cavity in the form of an initial fixed spot through the dynamic spot modulation module to maintain plasma thermal balance. Step 4: The high-speed imaging device continuously generates image data, and the radiation intensity detector synchronously generates radiation intensity data; both send the data to the oscillation recognition module through a signal line. The oscillation identification module performs FFT or time series analysis on the image data, extracts the characteristic frequency and amplitude of plasma oscillation, and transmits the analysis results to the control algorithm unit through the internal bus to determine whether there is low-frequency oscillation in the range of 20-50Hz. Step 5: When low-frequency oscillations are detected, the control algorithm unit determines the appropriate response based on the fluid-optical coupling equation. The required modulation parameters are calculated, and control commands are sent to the time modulation scanning unit of the dynamic spot modulation module via signal lines. The time modulation scanning unit drives the MEMS adjustable mirror and spatial light modulator through a driving circuit, so that the laser focus performs periodic or random scanning at 10-500Hz. At the same time, the power and phase of the second laser source are adjusted via signal lines to ensure that the heat flux density distribution meets the requirements. ; Step 6: The control algorithm unit is based on PID control law. ,by To control the target, the following actuators are adjusted in real time via signal lines: The power of the first laser source; Modulation parameters of the second laser source; The scanning trajectory and frequency of the dynamic spot modulation module; The output gas flow rate and pressure gradient of the gas distribution unit in the gas thermophysical property regulation module; The iteration period is on the order of milliseconds, until the oscillation frequency reported by the oscillation identification module drops below 5Hz or is completely eliminated; Step 7: When the control algorithm unit determines that the plasma meets the requirements... After the oscillation frequency is less than 5Hz, the high numerical aperture mirror of the optical collection and output module collects plasma radiation. After passing through the broadband collection unit, it is coupled to the fiber output module or free space output module with a fiber core diameter of 200-600μm through the fiber connector, and the stable broadband radiation light is guided to the wafer defect detection equipment. At the same time, the closed-loop feedback control module continues to monitor at an update frequency of not less than 10 times per second.

Images