Background
Defect-free EUV (extreme ultraviolet) mask fabrication is one of the key issues that limit the mass production of EUV lithography. EUV mask defect detection is therefore a key core technology for implementing EUV lithography. EUV mask defects can be classified into amplitude type defects and phase type defects, wherein phase type defects are the most important defects because their presence cannot be repaired. Phase defects are also the most studied. Because the phase defect is caused by the distortion of the multilayer structure of the EUV mask, the phase defect needs to penetrate through the multilayer structure for detection, and the traditional detection method based on deep ultraviolet or ultraviolet cannot meet the requirement.
For this reason, researchers have utilized the active method, i.e., the EUV band-based light source penetrating the multilayer structure for defect detection, and there are several most typical implementations, among which the most typical is the dark-field detection method based on the Schwarzschild optical system, as shown in fig. 1. The method mainly comprises two curved mirrors and a plane mirror, wherein the plane mirror can shield reflected light besides having a turning effect on incident light, and the two curved mirrors collect scattered light generated by defects to a charge-coupled device (CCD).
The method has the disadvantages that on one hand, the curved surface reflector cannot be too close to the mask plate for defect detection due to the existence of the plane reflector, so that the receiving NA (numerical aperture) of defect scattering light is limited, and meanwhile, if the size of the curved surface reflector needs to be increased in order to increase the receiving NA, the size of the curved surface reflector is increased, so that on the one hand, the processing difficulty of the curved surface reflector is increased, and on the other hand, the processing cost is increased; on the other hand, when the defect is small and the scattered light signal of the defect is comparable to the noise, the system is difficult to detect the defect due to the decrease of the signal-to-noise ratio. In short, a method is urgently needed to enable a receiving system to be infinitely close to a mask plate for detection, the receiving NA is large without increasing the area of a receiving mirror, the signal-to-noise ratio of a detection system can be increased, and the detection sensitivity is further improved.
Disclosure of Invention
In view of the above, it is a primary object of the present invention to provide an EUV mask defect inspection system and method, which are intended to at least partially solve at least one of the above-mentioned technical problems.
As an aspect of the present invention, there is provided an EUV mask defect detecting system including:
the extreme ultraviolet light beam is obliquely emitted to the surface of a mask to be detected;
the reflecting cup is used for collecting and reflecting scattered light caused by the surface defects of the mask to be detected;
the transmission component is used for transmitting the reflected light emitted by the reflecting cup;
and the detector is used for receiving the optical signal transmitted by the transmission component and acquiring the surface defect information of the mask to be detected.
As another aspect of the present invention, there is also provided an inspection method using the EUV mask defect inspection system as described above, the inspection method including the steps of:
step 1: obliquely emitting an extreme ultraviolet light beam onto a mask to be detected, wherein the mask to be detected emits scattered light due to surface defects;
step 2: the light reflecting cup collects and reflects the scattered light;
and step 3: the transmission component transmits the reflected light emitted by the reflecting cup;
and 4, step 4: and the detector receives the optical signal transmitted by the transmission component to acquire the defect information of the surface of the mask to be detected.
Based on the technical scheme, compared with the prior art, the invention has at least one or one part of the following beneficial effects:
according to the EUV mask defect detection system, the extreme ultraviolet light beam is obliquely emitted into the mask to be detected, the reflecting cup is allowed to be used as the collecting component to be closer to the surface of the mask to be detected, the NA of the collecting component is increased, and the scattered light collecting efficiency caused by defects is improved.
In addition, the invention can also utilize two extreme ultraviolet beams to generate interference modulation, when the mask moves, a periodic scattering signal (namely a time series image) caused by a defect is generated, the signal frequency is obtained through Fourier transform, and the defect information equivalent to noise can be extracted.
Detailed Description
In order that the objects, technical solutions and advantages of the present invention will become more apparent, the present invention will be further described in detail with reference to the accompanying drawings in conjunction with the following specific embodiments.
As an aspect of the present invention, there is provided an EUV mask defect detecting system including:
the extreme ultraviolet light beam is obliquely emitted to the surface of a mask 3 to be detected;
the reflecting cup 4 is used for collecting and reflecting scattered light caused by surface defects of the mask 3 to be detected;
a transmission component for transmitting the reflected light emitted by the reflection cup 4;
and the detector is used for receiving the optical signal transmitted by the transmission component and acquiring the surface defect information of the mask 3 to be detected.
It is worth mentioning that the main object of the present invention is to increase the collecting member NA. The invention utilizes the oblique incidence of EUV light, utilizes the reflecting cup 4 to receive scattered light caused by the defects of the mask 3 to be detected in the vertical direction, and finally converges optical signals into the detector through the transmission part at the top of the reflecting cup 4, thereby realizing the increase of receiving NA.
More specifically, NA is the numerical aperture, a dimensionless number that measures the angular range of light that the system can collect.
In the embodiment of the present invention, the extreme ultraviolet light beam includes a single beam, and the single beam of extreme ultraviolet light beam is reflected by the first plane mirror 1 and obliquely enters the mask to be measured 3, in other embodiments of the present invention, the extreme ultraviolet light beam is not limited to a single beam, as shown in fig. 2, the extreme ultraviolet light beam may also include two beams, and the two beams of extreme ultraviolet light beam are reflected by the first plane mirror 1 and the second plane mirror 2 and obliquely enter the mask to be measured, so as to form interference modulation on the surface of the mask to be measured 3.
In the embodiment of the invention, the inner wall surface of the reflecting cup 4 is an even-order paraboloid surface, and the inner wall of the reflecting cup is of a molybdenum-silicon multilayer structure.
The main function of the reflector cup 4 as a collecting member is to increase the collection angle of the scattered light in the illumination area and increase the collecting member NA. The design method of the invention can make the collecting part closer to the mask 3 to be tested, thereby increasing the NA of the collecting part.
In the embodiment of the invention, the transmission component comprises a curved reflector 5, and the curved reflector 5 is arranged at the top of the reflecting cup 4 and is used for reflecting and transmitting the reflected light emitted by the reflecting cup 4;
the curved surface reflector 5 has a double curved surface, and the curved surface reflector 5 has a molybdenum-silicon multilayer structure.
In an embodiment of the invention, the detector is an extreme ultraviolet CCD camera 6.
In the embodiment of the invention, the EUV mask defect detecting system further comprises a scanning table, which is arranged below the mask 3 to be detected and is used for driving the mask 3 to be detected to move in a stepping manner;
the EUV mask defect detection system further comprises a vacuum chamber, wherein the reflecting cup 4, the transmission part, the detector and the scanning platform are arranged in the vacuum chamber.
As another aspect of the present invention, there is also provided an inspection method using the EUV mask defect inspection system as described above, the inspection method including the steps of:
step 1: obliquely irradiating the light beam of the extreme ultraviolet light onto the mask 3 to be detected, wherein the mask 3 to be detected emits scattered light due to surface defects;
step 2: the light reflecting cup 4 collects and reflects scattered light;
and step 3: the transmission component transmits the reflected light emitted by the reflecting cup 4;
and 4, step 4: the detector receives the optical signal transmitted by the transmission component to acquire the defect information of the surface of the mask 3 to be detected.
In the embodiment of the invention, when the extreme ultraviolet light beam is a single beam, the detector obtains a bright spot formed by scattered light caused by a surface defect of the mask 3 to be detected.
In the embodiment of the invention, when the extreme ultraviolet light beam is a double-beam light beam, the detector obtains a time series image, and the surface defect information of the mask 3 to be detected is obtained through Fourier transform.
More specifically, when EUV converging light beams are converged on the surface of a mask 3 to be detected through a first plane reflector 1 (single beam) or as shown in fig. 2, and simultaneously converged on the surface of the mask 3 to be detected through the first plane reflector 1 and a second plane reflector 2 (double beam), the converged EUV light beams are scattered to each direction due to the existence of defects on the surface of the mask 3 to be detected, a large NA reflector 4 collects the reflected light beams and reflects the reflected light beams into a detector EUV light CCD camera 6 through a curved surface reflector 5 positioned on the top of the reflector 4, when single beam illumination is adopted, the detector EUV light CCD camera 6 directly obtains scattered light bright spots caused by the defects, and when double beam illumination is adopted, the detector EUV light CCD camera 6 obtains defect information through fourier transformation on the obtained time series images, and further improves defect detection sensitivity.
The method comprises the steps of generating interference modulation on the surface of a mask 3 to be detected, scanning and translating the mask 3 to be detected at a certain speed, and when the mask 3 to be detected has defects, causing modulated interference fringe scattering, detecting periodic scattered light signals by an extreme ultraviolet CCD camera 6 due to the movement of the mask 3 to be detected, and obtaining the frequency of the scattered light signals through Fourier transformation, wherein the frequency is related to the influence of the defects on an optical field, so that periodic defect scattered signals submerged in noise or equivalent to the noise can be extracted by the method, and the defect detection sensitivity of the system is further enhanced, as shown in FIG. 2. Furthermore, the above definitions of the various elements and methods are not limited to the particular structures, shapes or arrangements of parts mentioned in the examples, which may be easily modified or substituted by one of ordinary skill in the art, for example:
changing the wavelength of the light source;
replacing or altering the shape and size of any optical element;
changing the interference of two beams of light in the figure into single beam of light obliquely incident;
in summary, in the EUV mask defect detecting system of the present invention, the conventional Schwarzschild-based optical system shown in fig. 1 is changed to the beam oblique incidence system shown in fig. 2, and this design can allow the collecting component to be closer to the surface of the mask 3 to be detected, thereby increasing the NA of the collecting component and improving the efficiency of collecting scattered light caused by defects. On the other hand, two beams of light can be used for incidence to generate interference modulation, when the mask 3 to be detected moves, periodic scattering signals caused by defects are generated, signal frequency is obtained through Fourier transform, and defect information equivalent to noise can be extracted.
The above embodiments are provided to further explain the objects, technical solutions and advantages of the present invention in detail, and it should be understood that the above embodiments are only examples of the present invention and are not intended to limit the present invention, and any modifications, equivalents, improvements and the like made within the spirit and principle of the present invention should be included in the protection scope of the present invention.