A mask defect detection system

Abstract

The present disclosure relates to a mask defect detection system, comprising: a vacuum cavity and a vibration isolation platform, wherein the vacuum cavity is located above the vibration isolation platform, and further comprising: a laser used as a system light source to generate laser light; a beam shaping system used to perform beam shaping on the laser light; a decoherence mirror used to achieve decoherence of the laser light; a pupil conversion wheel used for pupil selection; a zone plate used to image defects on a mask, a flange used to achieve a sealed connection between cavities; a four-claw slit used to fine-tune the size of an illumination spot; the decoherence mirror, the pupil conversion wheel and the zone plate are located in the vacuum cavity; the laser light generated by the laser passes through the beam shaping system, the flange and the four-claw slit, the decoherence mirror and the pupil conversion wheel in sequence, and is irradiated onto a mask for mask defect detection.

Description

Technical Field

[0001] The present disclosure relates to the technical field of mask defect detection, and more specifically, to a mask defect detection system. Background Art

[0002] EUV lithography is the most promising technical means to meet the 7nm and below technology nodes. In the semiconductor manufacturing process, extreme ultraviolet mask technology is considered to be one of the most critical technologies for the successful implementation and development of EUV lithography technology. Since extreme ultraviolet light is strongly absorbed by most materials, reflective optical elements must be used in the optical path, including masks. Therefore, unlike traditional photolithography masks, EUV masks use a multi-layer film structure. The EUV mask preparation process is very complicated, and each process step will inevitably introduce defects. The defects on the EUV mask directly affect the yield of the photolithography process and must be strictly controlled. Today, EUV mask defects are one of the most important challenges related to EUV mask preparation. Therefore, EUV mask defect detection technology has become a core key technology for defect-free EUV mask preparation and EUV lithography mass production.

[0003] With the development of EUV lithography technology, EUV mask defect detection technology has also received more and more attention from scientific research institutions at home and abroad. EUV mask defect detection must be carried out by wavelength detection. At present, there are four main wavelength EUV mask defect detection technologies in the world: Schwarzschild two-mirror imaging technology and projection objective imaging technology for mass production; coherent diffraction imaging technology and zone plate imaging technology for scientific research. Among them, the imaging optical path of the zone plate imaging technology is simple and low-cost. It can achieve high-resolution EUV mask defect detection, and it is the only one that can reproduce all existing and future EUV lithography machine lighting conditions. It can be used to study the problems faced in the EUV mask development process, such as mask architecture and material research, optical proximity correction and auxiliary feature pattern research, the influence of multilayer film roughness on lithography pattern roughness, the influence of high NA, large-angle multilayer film research, and Source-Mask collaborative optimization, etc. It is precisely because of these incomparable advantages that the mask defect detection technology based on zone plate imaging has played an important role in the EUV mask development process since its inception.

[0004] At present, the main research institution for EUV mask defect detection technology based on zone plate imaging is the Lawrence Berkeley National Laboratory in the United States. The illumination light source is a synchrotron radiation light source, and the illumination system adopts Fourier synthesis illumination technology to achieve free control of the illumination pupil, thereby reproducing the illumination pupil conditions of the lithography machine and improving the resolution of the imaging system. However, Fourier synthesis illumination technology requires the use of MEMS mirrors MA for two-dimensional scanning, which has very high scanning rate and accuracy requirements. It combines plane mirrors MB and ellipsoidal focusing mirrors MC to achieve control of the illumination light. All three optical components need to be coated in the extreme ultraviolet band and the roughness requirements must reach the sub-nanometer level, so they are very difficult to manufacture and are expensive. In addition, this illumination method is extremely difficult to assemble and adjust. It can meet the needs of dipole mode illumination pupils, but it is difficult to meet the needs of quadra or even more complex illumination pupils, thus limiting the imaging resolution of the entire system. Summary of the invention

[0005] To solve the technical problems that the existing mask defect detection system is expensive, difficult to assemble and adjust, and cannot meet the user's needs for more complex illumination pupils.

[0006] To achieve the above technical objectives, the present disclosure provides a mask defect detection system, comprising: a vacuum chamber and a vibration isolation platform, wherein the vacuum chamber is located above the vibration isolation platform, and further comprising:

[0007] A laser, used to generate laser light as a system light source;

[0008] A beam shaping system, used for performing beam shaping on the laser;

[0009] A decoherence mirror, used to achieve decoherence of the laser;

[0010] Pupil conversion wheel, used for pupil selection;

[0011] zone plates, used to image defects on the mask;

[0012] Flange, used to achieve sealing connection between cavities;

[0013] The decoherence mirror, the pupil conversion wheel and the zone plate are located in the vacuum chamber;

[0014] The laser light generated by the laser passes through the beam shaping system, the flange, the decoherence mirror and the pupil conversion wheel in sequence, and is irradiated onto the mask for mask defect detection.

[0015] Furthermore, the beam shaping system adopts a KB mirror, and the size of the illumination spot after shaping by the KB mirror is 100-300um.

[0016] Furthermore, it also includes:

[0017] A four-claw slit, located between the flange and the decoherence reflector, for fine-tuning the spot size of the light after beam shaping;

[0018] The laser generated by the laser passes through the flange and then passes through the four-claw slit to be emitted into the decoherence reflector.

[0019] The blade of the four-claw slit can move in two dimensions to fine-tune the size of the shaped illumination spot, thereby selecting the appropriate spot size for different scenes.

[0020] Furthermore, the reflective surface of the decoherence reflector has a sub-reflective surface in a periodic step or groove shape.

[0021] Its main purpose is to divide the incident light spot into several sub-spots so that the optical path difference between adjacent spots is greater than the coherence length, thereby achieving the purpose of decoherence.

[0022] Furthermore, the decoherence reflection mirror is set with a decoherence reflection angle so that the laser irradiates the mask to be inspected at an illumination incident angle of not less than 6°.

[0023] Furthermore, the decoherence reflector is a plane mirror or a spherical mirror.

[0024] The spatial coherence of the laser is very high. Highly coherent illumination will produce speckle on the imaging surface, affecting the imaging resolution of the system. Fourier synthesis illumination is a decoherence technical solution that can scan different illumination pupils at the same time, but the MEMS mirrors it uses are extremely difficult to process and assemble. This device uses a decoherence mirror to decoherent the output light of the laser. The decoherence mirror can be a plane mirror, which only changes the direction of the light path without changing the spot size, or it can be a spherical mirror, which can change the direction of the light path and also play a focusing role. Different mirror surface types can be selected according to actual needs.

[0025] Further, it also includes: a swing table;

[0026] The decoherence mirror is arranged on the swing stage.

[0027] The swing table can swing at a very small angle and high speed, which can effectively improve the uniformity of the reflected light spot, thereby avoiding the impact of uneven illumination spot on the system detection performance.

[0028] Furthermore, the pupil conversion wheel is circular or square.

[0029] Furthermore, the zone plate is a bright field zone plate or a dark field zone plate.

[0030] Furthermore, the pupil conversion wheel has conventional illumination, annular illumination, vertical dipole illumination, horizontal dipole illumination, quadrupole illumination, and asymmetric quadrupole illumination pupil conversion holes.

[0031] The beneficial effects of the present disclosure are:

[0032] Compared with the prior art, the mask detection system disclosed in the present invention has the following advantages:

[0033] The illumination light source of the mask detection system disclosed in the present invention is preferably extreme ultraviolet light output by a free electron laser, which is characterized by very high brightness, which is incomparable to synchrotron radiation light sources and other DPP and LPP light sources. The high brightness can improve the signal-to-noise ratio of the system, and can greatly improve the defect detection rate of the system while obtaining a higher resolution.

[0034] Free electron lasers can achieve very high monochromaticity while ensuring high brightness, which is incomparable to synchrotron radiation sources and other DPP and LPP light sources. High monochromaticity can effectively avoid the influence of chromatic aberration of zone plates on imaging resolution. At the same time, zone plates with a number of rings corresponding to the monochromaticity can be used to obtain a larger focal length of the zone plate, thereby greatly reducing the difficulty of installing and adjusting the zone plate.

[0035] (1) Low device cost, (2) Low manufacturing difficulty, (3) Simple optical path adjustment, (4) High imaging resolution, and (5) High defect detection rate. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 A schematic diagram of a mask detection system according to Embodiment 1 of the present disclosure is shown;

[0037] Figure 2 Schematic diagram a of the reflection surface of the decoherence mirror of the first embodiment of the present disclosure is shown;

[0038] Figure 3 FIG. 2 shows a schematic diagram b of a reflection surface of a decoherence reflection mirror according to the first embodiment of the present disclosure;

[0039] Figure 4 A schematic diagram of a pupil conversion wheel according to a first embodiment of the present disclosure is shown;

[0040] Figure 5 A schematic diagram of a pupil conversion wheel according to a first embodiment of the present disclosure is shown. DETAILED DESCRIPTION

[0041] Hereinafter, embodiments of the present disclosure will be described with reference to the accompanying drawings. However, it should be understood that these descriptions are exemplary only and are not intended to limit the scope of the present disclosure. In addition, in the following description, descriptions of well-known structures and technologies are omitted to avoid unnecessary confusion of the concepts of the present disclosure.

[0042] Various structural schematic diagrams according to embodiments of the present disclosure are shown in the accompanying drawings. These figures are not drawn to scale, and some details are magnified and some details may be omitted for the purpose of clear expression. The shapes of various regions and layers shown in the figures and the relative sizes and positional relationships therebetween are only exemplary, and may deviate in practice due to manufacturing tolerances or technical limitations, and those skilled in the art may further design regions / layers with different shapes, sizes, and relative positions according to actual needs.

[0043] Embodiment 1:

[0044] like Figure 1 As shown:

[0045] The present disclosure provides a mask defect detection system, comprising: a vacuum chamber and a vibration isolation platform, wherein the vacuum chamber is located above the vibration isolation platform, and further comprising:

[0046] Also includes:

[0047] a laser for generating laser light for input into the system;

[0048] A beam shaping system 1 for beam shaping a laser input into the system;

[0049] A decoherence mirror 2 for decohering the laser light generated by the laser;

[0050] Pupil conversion wheel 3 for pupil selection;

[0051] A zone plate 5 for imaging defects on the mask;

[0052] A flange 9 for achieving a sealed connection between the cavities;

[0053] A four-claw slit 10 for fine-tuning the size of the illumination spot;

[0054] The decoherence mirror 2, the pupil conversion wheel 3 and the zone plate 5 are located in the vacuum chamber;

[0055] The laser generated by the laser passes through the beam shaping system 1, the flange 9, the four-claw slit 10, the decoherence mirror 2 and the pupil conversion wheel 3 in sequence, and is irradiated onto the mask 4 for mask defect detection.

[0056] The laser is preferably a free electron laser.

[0057] The illumination light source of the mask detection system disclosed in the present invention is preferably extreme ultraviolet light output by a free electron laser, which is characterized by very high brightness, which is incomparable to synchrotron radiation light sources and other DPP and LPP light sources. The high brightness can improve the signal-to-noise ratio of the system, and can greatly improve the defect detection rate of the system while obtaining a higher resolution.

[0058] Free electron lasers can achieve very high monochromaticity while ensuring high brightness, which is incomparable to synchrotron radiation sources and other DPP and LPP light sources. High monochromaticity can effectively avoid the influence of chromatic aberration of zone plates on imaging resolution. At the same time, zone plates with a number of rings corresponding to the monochromaticity can be used to obtain a larger focal length of the zone plate, thereby greatly reducing the difficulty of installing and adjusting the zone plate.

[0059] Furthermore, the beam shaping system adopts a KB mirror, and the size of the illumination spot after shaping by the KB mirror is 100-300um.

[0060] Furthermore, it also includes:

[0061] A four-claw slit for fine-tuning the spot size of the beam-shaped light;

[0062] The four-claw slit is located between the flange and the decoherence reflector;

[0063] The illumination light generated by the laser passes through the flange and the four-claw slits and then enters the decoherence reflector.

[0064] The blade of the four-claw slit can move in two dimensions to fine-tune the size of the shaped illumination spot, thereby selecting the appropriate spot size for different scenes.

[0065] Furthermore, the reflective surface of the decoherence reflector has a sub-reflective surface in a periodic step or groove shape.

[0066] Its main purpose is to divide the incident light spot into several sub-spots so that the optical path difference between adjacent spots is greater than the coherence length, thereby achieving the purpose of decoherence.

[0067] Furthermore, the decoherence reflection mirror is set with a decoherence reflection angle so that the laser irradiates the mask to be inspected at an illumination incident angle of not less than 6°.

[0068] Furthermore, the decoherence reflector is a plane mirror or a spherical mirror.

[0069] The spatial coherence of the laser is very high. Highly coherent illumination will produce speckle on the imaging surface, affecting the imaging resolution of the system. Fourier synthesis illumination is a decoherence technical solution that can scan different illumination pupils at the same time, but the MEMS mirrors it uses are extremely difficult to process and assemble. This device uses a decoherence mirror to decoherent the output light of the laser. The decoherence mirror can be a plane mirror, which only changes the direction of the light path without changing the spot size, or it can be a spherical mirror, which can change the direction of the light path and also play a focusing role. Different mirror surface types can be selected according to actual needs.

[0070] Further, it also includes: a swing table;

[0071] The decoherence mirror is arranged on the swing stage.

[0072] The swing table can swing at a very small angle and high speed, which can effectively improve the uniformity of the reflected light spot, thereby avoiding the impact of uneven illumination spot on the system detection performance.

[0073] Furthermore, the pupil conversion wheel is circular or square.

[0074] Furthermore, the zone plate is a bright field zone plate or a dark field zone plate.

[0075] The conventional solution only uses a zone plate to image the reflected light, which belongs to bright field detection. The zone plate in the mask defect detection system disclosed in the present invention can be a zone plate for bright field detection, or a zone plate for imaging scattered light, that is, a zone plate for dark field detection.

[0076] Furthermore, the pupil conversion wheel has conventional illumination, annular illumination, vertical dipole illumination, horizontal dipole illumination, quadrupole illumination, and asymmetric quadrupole illumination pupil conversion holes.

[0077] Combine the following Figure 1 to Figure 5 Detailed explanation of the mask defect detection system disclosed in this disclosure:

[0078] An EUV mask defect detection system based on free electron laser and zone plate imaging is used to detect EUV mask defects and study key technologies in the EUV mask development process. The optical path diagram of this device is shown in Figure 1As shown. The working principle of this device is as follows: The illumination light source is extreme ultraviolet light output by a free electron laser, which is characterized by very high brightness, which is incomparable to synchrotron radiation sources and other DPP and LPP light sources. The high brightness can improve the signal-to-noise ratio of the system, and can greatly improve the defect detection rate of the system while obtaining higher resolution. The free electron laser can achieve very high monochromaticity while ensuring high brightness, which is also incomparable to synchrotron radiation sources and other DPP and LPP light sources. High monochromaticity can effectively avoid the influence of chromatic aberration of the zone plate on the imaging resolution. At the same time, a zone plate with a number of rings corresponding to the monochromaticity can be used to obtain a larger focal length of the zone plate, thereby greatly reducing the difficulty of assembling and adjusting the zone plate.

[0079] The extreme ultraviolet light output by the free electron laser forms an illumination spot of the required size after passing through the beam shaping system 1. The shaping system can adopt a KB mirror or other forms. It is recommended that the size of the illumination spot after shaping is 100-300um. The shaped illumination light enters the vacuum cavity 7 of the device through the flange 9 and the four-claw slit 10. The four-claw slit 10 is placed between the flange 9 and the vacuum cavity 7. The blade of the four-claw slit can move in two dimensions to fine-tune the size of the shaped illumination spot, so as to select the appropriate spot size for different scenes. After passing through the four-claw slit, the illumination spot is irradiated onto the decoherence reflector 2 with the required spot size. It should be noted that the spatial coherence of the free electron laser is very high, and the high coherence illumination will produce speckle on the imaging surface, affecting the imaging resolution of the system. Fourier synthesis illumination is a decoherence technical solution, which can scan different illumination pupils at the same time, but the processing and adjustment of the MEMS reflector it uses are extremely difficult. This device uses a decoherence mirror to decoherent the output light of the free electron laser. The decoherence mirror can be a plane mirror, which only changes the direction of the light path without changing the spot size, or a spherical mirror, which changes the direction of the light path and also plays a focusing role. Different reflector surface types can be selected according to actual needs.

[0080] On the reflective surface of the decoherence reflector, some periodic step or groove-shaped sub-reflective surfaces need to be prepared, such as Figure 2 and Figure 3 As shown, or a plurality of irregularly shaped and periodic sub-reflecting surfaces, the main purpose is to divide the incident light spot into a plurality of sub-light spots, so that the optical path difference of adjacent light spots is greater than the coherence length, thereby achieving the purpose of decoherence. The illumination light spot is reflected after being irradiated to the reflector 2, and the reflected light does not have the original high coherence, so that speckle will not be caused on the imaging surface to affect the imaging resolution of the system.

[0081] The reflector 2 needs to be set at a specific angle to ensure that the reflected light can be irradiated onto the mask 4 at an incident angle of 6°, which is the illumination angle currently used by EUV lithography machines. The illumination angle of future EUV lithography machines will increase, so the reflector 2 needs to be able to adjust the mask illumination angle to more than 6°.

[0082] The reflector 2 can also be mounted on a swinging table, which can swing at a very small angle and at high speed, so that the uniformity of the reflected light spot can be effectively improved, thereby avoiding the influence of the uneven illumination light spot on the detection performance of the system. The reflected light passing through the reflector 2 needs to pass through the pupil conversion wheel 3 before irradiating the mask 4 to perform pupil selection on the illumination light. Different illumination pupils determine different diffraction resolution limits. For example, the diffraction limit resolution of dipole illumination is doubled compared to coherent illumination. Therefore, in order to improve the resolution of the system, a complex illumination pupil needs to be adopted. In the existing technology, the illumination system adopts Fourier synthesis illumination to realize the free control of the illumination pupil, thereby reproducing the illumination pupil conditions of the lithography machine and improving the imaging resolution. However, Fourier synthesis illumination technology requires the use of MEMS reflectors for two-dimensional scanning, which has high scanning rate and precision requirements, is expensive and difficult to process, and is difficult to assemble and adjust, so it is difficult to apply to complex illumination pupils.

[0083] The present application adopts a very simple and flexible pupil conversion wheel to control the pupil. The pupil conversion wheel can be understood as a traditional slit or pinhole diaphragm, except that its light-transmitting area is designed to be a shape corresponding to various illumination pupils, such as Figure 4 and Figure 5 shown.

[0084] The pupil conversion wheel can be round or square. Various illumination pupil shapes can be prepared on the pupil conversion wheel by electron beam lithography or laser direct writing. The light-transmitting area is Figure 4 The white color in the image corresponds to various shapes of illumination pupils. By rotating the circular conversion wheel or moving the square conversion wheel horizontally, the pupil shape can be switched on the optical path. The conversion wheel of this solution has low processing cost and much longer life than the MEMS reflector, which can greatly reduce the cost waste caused by damage to the optical reflector. It is also simple to install and adjust. Switching different pupil shapes does not require additional optical path installation and adjustment, which can greatly reduce the difficulty of optical path installation and adjustment and save time.

[0085] Commonly used pupil shapes include conventional illumination a1, annular illumination a2, vertical dipole illumination a3, horizontal dipole illumination a4, quadrupole illumination a5, asymmetric quadrupole illumination a6, etc., among which the conventional illumination a1 referred to in the present disclosure refers to the circular through hole through which the laser directly passes through the pupil. The current micro-nano processing technology can meet the preparation of any pupil shape. Therefore, even the mask illumination collaborative optimization technology, which is currently a hot topic in the field of lithography, can be prepared on the pupil conversion wheel through micro-nano processing technology for the very complex illumination intensity distribution it requires.

[0086] Another advantage of using a pupil conversion wheel is that the energy of the illumination spot can be attenuated. The energy of the illumination light of the free electron laser is very high. After passing through the imaging system and reaching the CCD, it is easy to cause the saturation effect of the CCD, and an effective image cannot be obtained. The energy of the illumination light can be appropriately attenuated by the pupil conversion wheel, which is beneficial to reducing the saturation effect of the CCD. After the illumination spot passes through the pupil conversion wheel 3, it will be irradiated onto the mask 4 with the required spot size, the required incident angle, and the required illumination pupil. After being reflected by the mask 4, the reflected light will be received by the zone plate 5. The zone plate 5 can be a bright field zone plate for imaging the reflected light, or a dark field zone plate for imaging the scattered light. Both the reflected light and the scattered light will carry the defect information on the mask, and will be received by the CCD 6 after being imaged by the zone plate.

[0087] In the existing technical solutions, because the energy of the illumination light source is relatively weak, the signal-to-noise ratio of the imaging is relatively low, and only EUV CCD can be used. The present disclosure uses a free electron laser as the illumination light source, and its energy is much greater than other forms of light sources, so a visible light CCD based on a scintillator can be used. This can greatly reduce the cost of the device. The image obtained by the CCD can accurately obtain the defect information on the EUV mask after being processed by the defect extraction algorithm. EUV mask defect detection at a wavelength requires the use of extreme ultraviolet light. Light of this wavelength must be used in a vacuum environment, otherwise it will be quickly absorbed by the air. Therefore, this device requires a vacuum cavity to provide a vacuum environment, and the vacuum degree generally required is better than 10-6hPa. The EUV mask defect detection device has very strict requirements on the vibration of the system. The relative vibration displacement of the mask and the wave zone plate cannot exceed 5nm. Therefore, the entire device needs to be vibration-isolated. The device needs to be placed on a vibration isolation platform 8, and the vibration isolation platform needs to provide 90% vibration isolation at 2Hz.

[0088] The embodiments of the present disclosure are described above. However, these embodiments are for illustrative purposes only and are not intended to limit the scope of the present disclosure. The scope of the present disclosure is defined by the appended claims and their equivalents. Without departing from the scope of the present disclosure, a person skilled in the art may make a variety of substitutions and modifications, which should all fall within the scope of the present disclosure.

Claims

1. A mask defect detection system, comprising: A vacuum chamber and a vibration isolation platform, wherein the vacuum chamber is located above the vibration isolation platform, and is characterized in that it also includes: A laser, used to generate laser light as a system light source; A beam shaping system, used for performing beam shaping on the laser; A decoherence mirror, used to achieve decoherence of the laser; Pupil conversion wheel, used for pupil selection; zone plates, used to image defects on the mask; Flange, used to achieve sealing connection between cavities; A four-claw slit, located between the flange and the decoherence reflector, for fine-tuning the spot size of the light after beam shaping; The decoherence reflector, the pupil conversion wheel and the zone plate are located in the vacuum chamber; the reflective surface of the decoherence reflector has a sub-reflective surface in a periodic step or groove shape; The laser generated by the laser passes through the beam shaping system, the flange, the four-claw slit, the decoherence mirror and the pupil conversion wheel in sequence, and is irradiated onto the mask for mask defect detection.

2. The mask defect detection system according to claim 1, characterized in that: The beam shaping system adopts a KB mirror, and the size of the illumination spot after shaping by the KB mirror is 100-300um.

3. The mask defect detection system according to claim 1, characterized in that: The decoherence reflection mirror is set at a decoherence reflection angle so that the laser irradiates the mask to be inspected at an illumination incident angle of not less than 6°.

4. The mask defect detection system according to claim 3, characterized in that: The decoherence reflection mirror is a plane mirror or a spherical mirror.

5. The mask defect detection system according to any one of claims 1 to 4, characterized in that: Also includes: Swing table; The decoherence mirror is arranged on the swing stage.

6. The mask defect detection system according to any one of claims 1 to 4, characterized in that: The pupil conversion wheel is circular or square.

7. The mask defect detection system according to any one of claims 1 to 4, characterized in that: The zone plate is a bright field zone plate or a dark field zone plate.

8. The mask defect detection system according to claim 6, characterized in that: The pupil conversion wheel has normal illumination, annular illumination, vertical dipole illumination, horizontal dipole illumination, quadrupole illumination, and asymmetric quadrupole illumination pupil conversion holes.

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