Mask defect detection system and method based on high-sensitivity detection module

Abstract

The present invention discloses a multi-beam confocal mask defect detection system and method based on a high-sensitivity detection module. The present invention realizes the segmentation of the detected Airy disk based on a microlens array, and uses optical fibers to transmit the corresponding signals to the photomultiplier tubes for detection. The microlens array, optical fibers and photomultiplier tubes together form a high-sensitivity detection module to realize accurate detection of mask defects. Compared with the traditional detection method that only uses photomultiplier tubes, the present invention gets rid of the dependence of the traditional detection method on the single-side illumination method and the wedge-shaped spectrometer, and has the advantages of higher detection resolution, more accurate detection results and more accurate defect morphology judgment.

Description

Technical Field

[0001] The present invention relates to the field of photolithography technology, and in particular to a mask defect detection system and method based on a high-sensitivity detection module. Background Art

[0002] In the semiconductor manufacturing process, the detection of mask defects is essential. Tiny depressions and protrusions on the mask will cause changes in the amplitude and phase of the light field, resulting in defects in the lithography elements, which restricts the further development of the lithography process. In order to minimize the impact of mask defects on the lithography yield, it is necessary to detect the mask defects in advance and distinguish their types. Although the existing point scanning mask defect detection system has a good positioning effect on mask defects, it often needs to introduce shading treatment or wedge-shaped spectrometers to achieve single-sided lighting when distinguishing the defect type. The accuracy of distinguishing the defect type is poor, and it is difficult to make a more accurate judgment on parameters such as the size and shape of the detected defects. Therefore, it is a technical problem that needs to be solved urgently to obtain as much defect morphology information as possible while ensuring the defect detection accuracy and detection speed. Summary of the invention

[0003] The purpose of the present invention is to address the deficiencies of the prior art and provide a mask defect detection system and method based on a high-sensitivity detection module.

[0004] The present invention uses a microlens array to segment the Airy disk obtained by detection, and uses four optical fibers to input the signals of each direction of the segmented Airy disk into a photomultiplier tube for detection. The obtained signals can be processed according to actual usage requirements, and the multi-directional defect morphology analysis of the mask can be realized through the above-mentioned high-sensitivity detection module. A diffraction optical device is used to obtain a multi-point scanning light field, and a multi-point confocal rapid scanning of the mask is achieved under the joint action of the illumination system and the confocal scanning system, thereby improving the imaging flux and detection speed of the system. The mask focusing system based on differential confocal technology, in combination with the actuator in the system, realizes real-time and accurate focusing of the system, thereby improving the detection accuracy of the system.

[0005] The present invention is achieved through the following technical solutions: The present invention provides a multi-beam confocal mask defect detection system based on a high-sensitivity detection module, which is mainly composed of a light source, an illumination system, a confocal scanning system, a microscope objective, a mask to be tested, a workbench system, a high-sensitivity detection module, a clock synchronization system, a signal processing system and a focus system. The high-sensitivity detection module includes a focusing lens, a microlens array, an optical fiber and a photomultiplier tube detector.

[0006] The system uses a 532nm laser as the light source. After collimation and beam expansion, the laser enters the illumination system, and obtains a multi-point illumination light field through the internal diffraction optical device. The multi-point illumination light field enters the confocal scanning system, and the light field is scanned in the X direction after being deflected by the high-speed rotating mirror. The confocal scanning system has a built-in beam splitter. After the multi-point scanning beam is split, it enters the microscope objective and the clock synchronization system respectively. The linearly polarized light entering the microscope objective is focused on the surface of the mask to be tested, and after being reflected by the mask, it enters the microscope objective again. After passing through the confocal scanning system, it is received by the high-sensitivity detection module to obtain the corresponding reflected light intensity information in different directions. The linearly polarized light entering the clock synchronization system is used as the synchronization of the clock signal to realize the corresponding relationship between the detection signal and the detection position. The data obtained by the clock synchronization system and the high-sensitivity detection module are synchronized to the signal processing system, and the defect morphology information is obtained after data processing.

[0007] Furthermore, when the reflected light carrying information related to the mask surface is incident on the high-sensitivity detection module, the converging lens in the detection module converges the reflected light into multiple Airy disks. A microlens array is placed at the position of each Airy disk, forming a conjugate relationship with the multi-point scanning light field. The Airy disk of the reflected light beam is focused to the center of the microlens array and is split into four parts under the control of the microlens array. The parts are received by the optical fibers at the corresponding positions respectively, and the corresponding light intensity signals are converted into electrical signals under the action of the photomultiplier tube detector and input into the signal processing system for processing. Combined with the established intensity signal change curve and the corresponding relationship between the defect morphology, the defect position and the defect morphology are located and analyzed.

[0008] Furthermore, the system's lighting system obtains a multi-point illumination light field through an optical diffraction device. In the process of designing the optical diffraction device, the actual detection needs of the system should be considered first, and the number of diffracted light beams and their arrangement spacing should be determined in combination with the field of view of the microscope objective used, so that the system's detection speed and the required detection accuracy match each other. In particular, in the actual system, multiple different optical diffraction devices can be designed to generate 4, 16 and 64 diffracted light beams respectively, and based on the arrangement method perpendicular to the point column direction, the mutual overlapping of high-order diffraction can be avoided to achieve switching between different detection modes.

[0009] Furthermore, the system realizes linear scanning in the X direction through the high-speed rotating mirror in the confocal scanning system, and cooperates with the movement of the workbench system in the Y direction to realize multi-point confocal imaging of the whole field of view. In particular, the X-direction scanning distance realized by the high-speed rotating mirror should be greater than the spacing between adjacent illumination beams. At the same time, the movement speed of the workbench system needs to match the scanning speed of the high-speed rotating mirror to ensure that the system scanning area can completely cover every part of the field of view, so as to realize high-precision detection of mask defects.

[0010] Furthermore, the device uses a real-time focus tracking system to achieve a real-time fixed focus effect on the microscope objective lens. The focus tracking system is based on a differential confocal imaging optical path, uses a detection laser as a detection light source, detects two signals before and after the system is focused, respectively, to obtain the differential response curve of the system, and adjusts the actuator in the system in real time through a differential confocal processing algorithm to achieve a real-time focus tracking effect of the microscope objective lens.

[0011] Compared with the prior art, the present invention has the following beneficial technical effects: the present invention uses a microlens array to segment the detected Airy disk, and uses optical fibers to input the segmented Airy disk signals into the photomultiplier tube detector for detection, which is equivalent to making the detection light incident on the mask surface along four different directions, getting rid of the dependence on unilateral illumination in the traditional detection method. Compared with directly using multiple optical fibers to segment the Airy disk, the present invention has a higher detection light utilization efficiency, and compared with using a wedge-shaped spectrometer to split the detection light, the present invention has a simpler optical structure, and can combine the detection results of each detector to achieve more accurate defect positioning and more precise defect morphology discrimination, which greatly improves the detection effect of the system without affecting the basic structure of the detection optical path. The present invention combines the above advantages to achieve a high-precision mask defect detection system with more accurate morphology detection effect.

[0012] Another aspect of the present invention provides a multi-beam confocal mask defect detection method based on a high-sensitivity detection module, the method comprising: Design of optical diffraction device: In the lighting system, an optical diffraction device is used to obtain a multi-point illumination light field. There are multiple two-dimensionally distributed diffraction units on the surface of the optical diffraction device, which can finely control the laser phase. The incident light diffracts after passing through each diffraction unit and interferes at a certain distance to generate a specific light field distribution. By designing the distribution of the diffraction units, any light field distribution can be obtained in theory. The weighted GS algorithm is used in the system to calculate the required diffraction hologram, which has faster iteration speed and better uniformity than the traditional GS algorithm.

[0013] In particular, the specific process of the weighted GS algorithm is as follows Figure 1 As shown in the figure, its basic process is consistent with the traditional GS algorithm, but during the iteration process, the amplitude of the desired target light field can be modified according to the actual iteration results, thereby accelerating the iteration process and improving the uniformity. The changes are as follows:

[0014] in is the amplitude distribution of the initial target light field, and They represent the light field distribution at the target plane during the iteration process, It is an adaptive parameter used to improve the convergence of the weighted GS algorithm. Its iteration process is determined by the following formula:

[0015] Take the initial value of the adaptive parameter in the experiment for , as the iteration process increases, the adaptive parameters The value of gradually increases, and the algorithm iteration process gradually transitions from the traditional GS algorithm to the weighted GS algorithm, which effectively improves the convergence and stability of the weighted GS algorithm. At the same time, according to the energy conservation relationship, the energy in the target plane is equal to the energy in the holographic plane, that is:

[0016] Under the constraints of the weighted GS algorithm, we have:

[0017] That is, by controlling the energy in the holographic plane, the energy in the target plane can be constrained indirectly. The noise in the non-signal area can be effectively suppressed through iteration, which greatly improves the uniformity of the results obtained by the algorithm. Figure 2 As shown in the figure, the results show that the multi-beam uniformity obtained by simulation can be close to the ideal 100%. Based on the calculated diffraction hologram, the distribution of diffraction units on the surface of the optical diffraction device is determined, thereby completing the design of the required optical diffraction device.

[0018] Airy disk segmentation: In the high-sensitivity detection module, a microlens array is used to segment the Airy disk. The microlens array consists of four microlenses with the same focal length, each of which has an independent focus, which can evenly divide the Airy disk focused on the center of the array into four parts, which are then incident on the corresponding optical fiber and received by the detector. In particular, in the high-sensitivity detection module, each Airy disk corresponds to a microlens array, and the arrangement of each microlens array is determined according to the spacing of the multi-point light field obtained by the optical diffraction device, so as to achieve accurate alignment between the center of the Airy disk and the center of the microlens array.

[0019] Differential confocal focus tracking technology: Differential confocal technology is used in the focus tracking system to achieve accurate focus tracking of the microscope objective lens. In the focus tracking system, a separate laser is used as the detection light source, and a front focus optical path and a back focus optical path are built to collect the corresponding light intensity signals, which are recorded as and , if the additional noise brought by the detector in the optical path is , then its differential confocal response is:

[0020] in is the axial optical normalized coordinate, whose value is determined by the effective numerical aperture of the microscope objective. , the wavelength of the detection light source , the actual axial coordinate Joint decision, i.e. , In the vertical coordinate The value is the defocus amount The normalized value is obtained when the noise is added. When the differential confocal response curve has good linearity, the change value of the differential response signal can be and the slope value of the linear region Focus Amount Calculate, where In particular, when adjusting the microscope objective actuator according to the calculated fixed focus amount, the numerical aperture and other parameters of the system should be considered to limit the fixed focus distance to achieve real-time focus tracking of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 It is a flowchart of the weighted GS algorithm iteration used in the present invention; Figure 2 This is the intensity distribution diagram of the multi-point light field obtained using the weighted GS algorithm; Figure 3 A schematic diagram of a multi-beam confocal mask defect detection system based on a high-sensitivity detection module provided as an example of the present invention; Figure 4 This is a schematic diagram of the system's high-sensitivity detection module structure. DETAILED DESCRIPTION

[0022] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the specific embodiments of the present invention are described in detail below with reference to the accompanying drawings.

[0023] In the following description, many specific details are set forth to facilitate a full understanding of the present invention, but the present invention may also be implemented in other ways different from those described herein, and those skilled in the art may make similar generalizations without violating the connotation of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below.

[0024] The present invention provides a mask defect detection system based on a high-sensitivity detection module, comprising: The system uses a 532nm laser as the light source, and obtains a multi-point uniform light field after modulation by the optical diffraction device inside the illumination system. After the multi-point uniform light field enters the confocal scanning system, a multi-point scanning light field is obtained under the action of the multi-faceted rotating mirror. After passing through the field lens and the collimating lens, it is divided into two beams by a beam splitter. One part enters the microscope objective and focuses on the surface of the mask to be tested. The reflected light on the mask surface passes through the microscope objective again and is received by the high-sensitivity detection module to obtain the defect information on the mask surface; the other part enters the clock synchronization system, and uses the grating device and photomultiplier tube detector that have been calibrated in advance to obtain the corresponding relationship between the scanning position and time, and inputs it into the signal processing system to complete the synchronization of the time zero point coordinates of different detectors, so as to realize the accurate positioning of the real-time detection area. The focus tracking system in the device uses an independent laser as the detection light source, and uses the differential confocal optical path to obtain the front focus and back focus signals of the microscope objective respectively, and uses the actuator to realize the real-time and accurate focus tracking of the microscope objective according to the differential response curve.

[0025] Furthermore, the principle of using a high-sensitivity detection module to detect mask defects is as follows: when the reflected light from the mask surface carrying relevant defect information is incident on the high-sensitivity detection module, the converging lens in the module converges the reflected light into multiple equally spaced Airy disks, whose positions satisfy a conjugate relationship with the multi-point scanning light field used. The detection module is designed in advance to completely align the center of each microlens array with the center of each Airy disk. Therefore, when the Airy disk is projected onto the surface of the microlens array, it is evenly divided into four parts, each of which is received by the photomultiplier tube detector through an optical fiber to obtain the corresponding light intensity information. From the detection results, the spectral effect of the microlens array on the reflected light is equivalent to the detection result of making the detection light incident at a specific angle. Under oblique lighting conditions, if there are defects on the mask surface, the scattering effect of the defects will cause the intensity of the reflected light received by the detector to change. Based on this, the specific type of defect in the mask can be judged. At the same time, the reflected light intensity information collected by different detectors can also be used to judge the types of mask defects in multiple dimensions, and the obtained defect model can be used to achieve more accurate detection of mask defects. The system uses the multi-faceted rotating mirror in the lighting system and the workbench system to achieve two-dimensional scanning of the mask, and uses the time synchronization module to calibrate the intensity signal obtained by detection in time. The data obtained by each system is processed in the signal processing system to achieve accurate positioning of defects and accurate identification of defect types.

[0026] Furthermore, the workflow of detecting mask defects based on the high-sensitivity detection module is as follows: the light source generates a multi-point uniform light field through the illumination system, and realizes a two-dimensional scan of the mask surface under the joint action of the confocal scanning system and the workbench system. The high-sensitivity detection module, the workbench system and the clock synchronization system respectively record the intensity-time relationship, the longitudinal coordinate-time relationship and the transverse coordinate-time relationship in the defect detection process, and input them into the signal processing system to obtain the accurate mapping relationship between the scanning position and the light intensity information. The signal processing system analyzes the light intensity information in each direction obtained by the high-sensitivity detection module, and combines the mask defect neural network detection model built inside the system to realize the rapid positioning of the defect position and the accurate identification of the defect type. The focus tracking system in the system will fine-tune the actuator of the microscope objective lens at a specific time period, record the front focus signal and the back focus signal value before and after the actuator is fine-tuned, and calculate the corresponding differential signal response value change, and obtain the fixed focus amount parameter according to the above differential confocal adjustment formula, and adjust the actuator based on this to ensure that the microscope objective lens is always positioned to the ideal position.

[0027] The following is a specific implementation example of the device of the present invention, but is not limited thereto. The multi-beam confocal mask defect detection system based on the high-sensitivity detection module of this example is as follows: Figure 3 As shown, it includes a light source 1, an illumination system 2, a confocal scanning system 3, a microscope objective 4, a mask to be tested 5, a workbench system 6, a high-sensitivity detection module 7, a clock synchronization system 8, a signal processing system 9 and a focus tracking system 10.

[0028] Light source 1 uses 532nm laser as the detection light source, which is split and modulated after passing through the optical diffraction device inside the illumination system 2 to obtain a multi-point uniform light field. The multi-point uniform light field is deflected after passing through the multifaceted rotating mirror inside the confocal scanning system 3, and is modulated by the system's field mirror, tube mirror and other optical devices before passing through a splitter. Linearly polarized light in the transmission direction is incident on the microscope objective 4, generating a multi-point detection light field on the surface of the mask plate 5 to be tested. The mask plate 5 to be tested is fixed on the workbench system 6, and the longitudinal scanning of the detection light on the mask plate to be tested is achieved by the movement of the workbench system. After the detection light field is modulated by the mask plate surface, its reflected light carries the relevant information of the mask plate surface, and is received by the high-sensitivity detection module 7 after passing through the microscope objective 4 and the confocal scanning system 3 again. The multiple microlens arrays inside the high-sensitivity detection module respectively complete the segmentation of the corresponding Airy disk, and the light intensity information sequence of each part of the Airy disk is obtained through the optical fiber and the detector.

[0029] After passing through the spectroscopic device, the linearly polarized light in the reflection direction enters the clock synchronization system 8. Under the action of the grating device inside the system, only part of the light incident at a specific angle can be detected by the detector inside the system. The angle information obtained by this pre-calibration can be used to locate the horizontal coordinates of the current scanning position. The clock synchronization system 8 and the information sequence collected by the detector in the high-sensitivity detection module 7 are input into the signal processing system 9, which synchronizes and processes the obtained light intensity information, and completes the location of defects and the identification of defect types in combination with the neural network model obtained by the previous training.

[0030] During the operation of the device of the present invention, the focus tracking system 10 fine-tunes the microscope objective lens at a certain period, and uses the front focus light path and back focus light path detectors therein to respectively obtain the front focus signal and the back focus signal of the system, calculates and obtains the change of the corresponding differential confocal response signal, and determines the fixed focus amount of the system based on the differential confocal technology, and adjusts the actuator inside the focus tracking system to achieve real-time and accurate focus tracking of the microscope objective lens.

[0031] The detectors used in the system can be avalanche photodiodes and photomultiplier tubes.

[0032] The above is only a preferred embodiment of the present invention. Although the present invention has been disclosed as a preferred embodiment, it is not intended to limit the present invention. Any technician familiar with the art can make many possible changes and modifications to the technical solution of the present invention by using the above disclosed methods and technical contents without departing from the scope of the technical solution of the present invention, or modify it into an equivalent embodiment of equivalent changes. Therefore, any simple modification, equivalent change and modification made to the above embodiments according to the technical essence of the present invention without departing from the content of the technical solution of the present invention still falls within the scope of protection of the technical solution of the present invention.

Claims

1. A mask defect detection system based on a high-sensitivity detection module, characterized in that: include: It includes light source, lighting system, confocal scanning system, microscope objective, mask to be tested, workbench system, high-sensitivity detection module, clock synchronization system, signal processing system and focus tracking system. The light source is split and modulated after passing through the optical diffraction device inside the illumination system to obtain a multi-point uniform light field; the multi-point uniform light field is deflected after passing through the multi-faceted rotating mirror inside the confocal scanning system, and is modulated by the system's field mirror, tube mirror and other optical devices, and then passes through a splitter device. The linearly polarized light in the transmission direction is incident on the microscope objective lens, and a multi-point detection light field is generated on the surface of the mask to be tested. The mask to be tested is fixed on the workbench system, and the longitudinal scanning of the detection light on the mask to be tested is achieved by the movement of the workbench system. After the detection light field is modulated by the mask surface, its reflected light carries the relevant information of the mask surface, and is received by the high-sensitivity detection module after passing through the microscope objective lens and the confocal scanning system again. The multiple microlens arrays inside the high-sensitivity detection module respectively complete the segmentation of the corresponding Airy disk, and the light intensity information sequence of each part of the Airy disk is obtained through the optical fiber and the detector; After passing through the spectrometer, the linearly polarized light in the reflection direction enters the clock synchronization system. Under the action of the grating device inside the system, only a part of the light incident at a specific angle can be detected by the detector inside the system. The angle information obtained by pre-calibration can be used to locate the lateral coordinates of the current scanning position. The information sequences collected by the clock synchronization system and the detector in the high-sensitivity detection module are input into the signal processing system, which synchronizes and processes the obtained light intensity information to complete the location of the defect and the determination of the defect type.

2. A mask defect detection system based on a high-sensitivity detection module according to claim 1, characterized in that: When the reflected light carrying information related to the mask surface is incident on the high-sensitivity detection module, the converging lens in the detection module converges the reflected light into multiple Airy disks. A microlens array is placed at the position of each Airy disk, forming a conjugate relationship with the multi-point scanning light field. The Airy disk of the reflected light beam is focused to the center of the microlens array and is split into four parts under the control of the microlens array. The four parts are received by the optical fibers at the corresponding positions respectively. The corresponding light intensity signal is converted into an electrical signal under the action of the photomultiplier tube detector and input into the signal processing system for processing. Combined with the established intensity signal change curve and the corresponding relationship between the defect morphology, the defect position and the defect morphology are located and analyzed.

3. A mask defect detection method based on a high-sensitivity detection module, characterized in that: include: A fiber laser array is used to obtain a multi-point light beam, which is continuously deflected by a collimating lens, a reflector, a first beam splitter, and a multifaceted mirror. After deflection, the light beam is expanded by an imaging lens and incident on a second beam splitter. At this time, the light beam is divided into linear polarized light in a transmission direction and a reflection direction. The linear polarized light in the transmission direction is modulated into circular polarized light after continuously passing through a third beam splitter and a quarter-wave plate. The light beam is focused onto a mask sample to be tested fixed on a moving stage through a microscope objective lens placed on an actuator. The reflected light on the surface of the mask sample to be tested is returned along the original optical path to the first beam splitter through which the incident light passes. The split reflected light is focused by the first lens onto a photomultiplier light detection array to obtain a sequence of reflected light intensity information at each position. The linearly polarized light in the reflection direction split by the second beam splitter is incident on the clock synchronization module to obtain the time sequence of the impulse signal. Combined with the reflected light intensity information sequence, the corresponding two-dimensional mask detection icon is obtained to achieve rapid positioning and type identification of defects. The reflected light from the surface of the mask sample to be tested is split along the original optical path through the third beam splitter through which the incident light passes, and is then split into the differential confocal module. The beam splitter in the differential confocal module splits the light beam into the back focus optical path and the front focus optical path respectively, and the light beams are collected by the corresponding detectors to obtain the corresponding back focus signals and front focus signals. After scanning by the actuator, the corresponding axial differential confocal response curve is obtained, and the accurate focusing information of the microscope objective is obtained after fitting the data, so that the microscope objective is always in the ideal fixed focus position.

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