JPH04133061A - Method for inspecting photomask with pellicle - Google Patents

Abstract

PURPOSE:To measure the flatness of a mask pattern surface by irradiating the mask pattern surface with measuring light through a pellicle, reflecting this light on the mask pattern and interfering the reflected light past the pellicle with reference light. CONSTITUTION:The collimated beams of light from a laser L are passed through a convex lens L1 and are changed to diverging light. This light is passed through a collimator lens L2 via a half mirror M and is converted to the magnified collimated beams of light. The magnified collimated beam of light are made incident on a reference translucent plane mirror MR and are partly reflected in the opposite direction via a half mirror HM on the rear surface thereof. On the other hand, the light transmitted through the reference translucent plane mirror MR is reflected on the photomask pattern 5 and return in the opposite direction. In succession, these reflected light are interfered with each other via the half mirror M and arrive at a screen S. The interference fringes are observed on the screen S surface and the measurement is made concerning the flatness and position accuracy of the mask pattern 5 surface. The sharp pattern exposing is thus possible even with an exposing device of a shallow focal depth.

Description

Detailed Description of the Invention [Field of Industrial Application] The present invention relates to a method for inspecting the flatness of a photomask with a pellicle and the positional accuracy of a mask pattern used in a lithography process in the manufacture of semiconductor integrated circuits such as VLSIs. It is something.

[Conventional technology 2] As the degree of integration of semiconductor integrated circuits increases, the projection exposure apparatus used in the lithography process in their manufacture is required to have higher resolution, and it is necessary to increase the numerical aperture of the projection lens system and increase the exposure wavelength. The trend is to make it shorter. As a result, according to Rayleigh's theory, the depth of focus becomes significantly shallower, and the flatness of the photomask is required to be more strict.

According to Rayleigh's theory, the resolution and depth of focus are as follows.

R=k・λ/NAO1 δ=λ/(2(NA.)2) Where, R: Resolution (μm), δ: Depth of focus (μm),
λ: Wavelength (μm), NAo: Numerical aperture of lens, k
: A constant determined by the state of the device, etc.

In addition, as semiconductor integrated circuits have become more highly integrated in recent years, it has become necessary to increase the overlay accuracy of each layer, which requires stricter positional and dimensional accuracy of the mask pattern on the photomask. ing.

By the way, if dust adheres to the mask pattern surface of a photomask, this dust will be projected onto the surface of the semiconductor substrate and be baked into the product, causing defects in the product. Therefore, a pellicle (transparent thin film) is used as a mask to prevent dust from forming on the photomask. A photomask with a pellicle is installed at a distance from the pattern surface so that dust adheres to this pellicle, so that even if the image is focused on the mask pattern surface and the image is projected, the attached dust will not form an image and will be blurred. Although it has already been proposed (Japanese Patent Publication No. 54-28716), this photomask with a pellicle is increasingly being used as semiconductor integrated circuits have become more highly integrated in recent years. FIG. 4 shows a cross-sectional view of an example of a photomask with a pellicle. In the figure,
1 is a pellicle frame made of metal such as aluminum;
A pellicle 2 is uniformly stretched and bonded to the upper surface of the pellicle frame 1. 4 is a photomask substrate,
A photomask pattern 5 is provided on the surface of the frame 1, and a pellicle 2 covers the photomask pattern 5.
is bonded to the surface of the photomask substrate 40 with MM3.

[Issues that Hathu tries to solve]

By the way, the pellicle frame 1 is a photomask base Fi,
4, considerable pressure is applied to the photomask substrate 4. Furthermore, since the pellicle frame 1 itself has some distortion, the flatness of the photomask and the pattern position WM may be affected before and after bonding the pellicle frame.
The degree may vary. There is a relationship between this change in flatness and the change in pattern position accuracy. For example, when a flat plate with a side width of 100 mm is bent by 1 μm at the center, the total of the pattern drawn on the flat plate is The pitch changes by about 0.09 μm. Ultra LSIs such as 4M bits and 16M bits have become a problem in recent years.
In memory, such a large positional error of the mask pattern is unacceptable. Therefore, changes in the flatness of the photomask before and after attaching the pellicle should be minimized. In addition, as mentioned above, when projecting such a highly integrated photomask with high resolution, the depth of focus becomes very shallow, so the flatness of the photomask after attaching the pellicle is very high. required.

However, conventionally, it has been assumed that the change in the flatness of the photomask after the pellicle is attached is negligible, and that the flatness before the pellicle is attached is maintained. Furthermore, regarding the pattern position accuracy, since there is no device to measure the pattern position accuracy of photomasks with pellicles,
It was assumed that the value before the pellicle was attached would be maintained.

The present invention has been made in view of the problems of the prior art, and its purpose is to provide a method for inspecting the flatness of a photomask with a pellicle and its pattern position accuracy.

[Means to solve the problem]

A first method for inspecting a photomask with a pellicle according to the present invention that achieves the above object is to attach a pellicle spaced apart on the mask pattern surface of a photomask having a mask pattern on one side, and then pass through the pellicle to inspect the mask pattern surface. This method is characterized in that the flatness of the mask pattern surface is measured by irradiating measurement light onto the mask pattern and causing the reference light to interfere with the reflected light that has been reflected from the mask pattern and transmitted through the pellicle.

In addition, the second pellicle-equipped photomask inspection method of the present invention irradiates measurement light from the mask pattern side of a photomask having a mask pattern on one side, and combines reflected light reflected from the mask pattern and transmitted through the pellicle with reference light. Measure the flatness of the mask pattern surface before attaching the pellicle by interfering with the pellicle. Next, after attaching the pellicle to the mask pattern surface at a distance, measurement light is transmitted through the pellicle and irradiated onto the mask pattern surface. The flatness of the mask pattern surface after attaching the pellicle is measured by interfering the reflected light reflected from the mask pattern and transmitted through the pellicle with the reference light, and the change in the flatness of the mask pattern surface before and after attaching the pellicle is measured. This method is characterized by inspecting the positional accuracy of the mask pattern by determining the positional accuracy of the mask pattern.

In either method, the reference target may be the light reflected by a half mirror placed perpendicular to the optical axis in the optical path of the irradiated measurement light, or the light reflected in the optical path of the irradiated measurement light. The light may also be reflected by a half mirror disposed obliquely to the optical axis and reflected again in the opposite direction to the direction of travel.

[Function 2] According to the first method of the present invention, after mounting the pellicle, measurement light is transmitted through the pellicle and irradiated onto the mask pattern surface, and the reference light is caused to interfere with the reflected light that is reflected from the mask pattern and transmitted through the pellicle. Since the flatness of the mask pattern surface is measured by the method, the flatness of the mask pattern surface of a photomask with a pellicle can be determined under actual conditions of use, and even when printing using an exposure device with a shallow depth of focus, the entire surface will be in focus. We can guarantee the supply of photomasks with pellicles that can expose sharp patterns without blur.

Further, according to the second method of the present invention, the flatness of the mask pattern surface is measured by interfering the reflected light reflected from the mask pattern with the reference light before and after attaching the pellicle,
Since the positional accuracy of the mask pattern is inspected by determining the changes during that time, the positional accuracy of the mask pattern of a photomask with a pellicle can be accurately determined under actual usage conditions, making it suitable for use in highly integrated semiconductor integrated circuits. We can guarantee the supply of photomasks with pellicles.

〔Example〕

As explained with reference to FIG. 4, the pellicle frame 1 with a height of several mm is attached to the pattern 5 surface of the photomask with pellicle, and the pellicle 2 is stretched on the surface above it. It is impossible to directly measure the flatness on the pattern surface side using a contact method, and it is also impossible to measure it close to the pattern surface using a microscope or the like. Although it is possible to measure the flatness of the back side of the photomask substrate opposite to the mask side and replace it with the flatness of the pattern side,
This is undesirable as it may cause stains and scratches. Therefore, it is desirable to measure the flatness of the mask pattern surface of a photomask with a pellicle in a non-contact manner through the pellicle. In the present invention, this will be realized by optical interferometry.

An example of an arrangement for this purpose is shown in FIG. In the figure, L is a laser, Ll is a convex lens, and M is a half mirror 1.
L2 is a collimator lens, RM is a reference semi-transparent plane mirror, and a half mirror HM is provided on the back surface of the mirror.
F is a photomask with a pellicle as shown in Figure WJ4 to be measured, which includes a pellicle 2, a photomask substrate 4, a photomask pattern 5, and the like. S is a screen.

In such power distribution, the parallel light emitted from the laser L is converted into diverging light by the convex lens Ll, and /"t-7-
The light passes through the mirror M and is converted into parallel light that is expanded by the collimator lens L2. The convex lens L1 and the collimator lens L2 constitute a beam expansion system so that the entire surface of the photomask F is irradiated with laser light. The expanded parallel light is incident on the reference semi-transparent plane mirror RM, and a part of it is reflected in the opposite direction by the half mirror HM on the back surface thereof.

On the other hand, the light transmitted through the reference semi-transmissive plane mirror RM illuminates the entire surface of the photomask F, and most of the light is reflected by the photomask pattern 5 and returns in the opposite direction. The light reflected by the half-mirror HM and the photomask pattern 5 is a half-mirror.
They are reflected by M toward the screen S and interfere with each other.

These two lights have an optical path difference of 2d, where d is the distance between the half mirror HM and the photomask pattern 5. When these two lights interfere with each other, the optical path difference is an odd number multiple of 1/2 of the incident light wavelength λ/2X (2n-1)
In this case, they cancel each other out and become dark; on the other hand, when the optical path difference is an integral number n times n·λ of the wavelength λ, they strengthen each other and become bright. In other words, equal straight lines of the unevenness of the mask pattern 5 of the photomask F are formed every 1/2 wavelength. In this case, the incident light wavelength λ needs to be a wavelength that is not significantly attenuated by the pellicle 2. Note that since the intensity of the reflected light from the pellicle 2 is usually much smaller than the intensity of the reflected light from the mask pattern 5, the interference fringes representing the equilinear lines of the pellicle 2 are not visible. In addition, the observed interference fringes representing equilinear lines of the mask pattern 5 are superimposed with fine interference fringes based on the fine pattern structure, forming complex interference fringes on a microscopic scale, but on a macroscopic scale. When viewed (with a resolution that does not resolve the pattern structure), it represents equilinear lines of unevenness on the surface of the photomask substrate 4 on which the mask pattern 5 is placed.

In this way, by observing the interference fringes on the screen S surface, the flatness of the mask pattern surface 5 of the photomask F with a pellicle can be inspected, and the photomask F with a pellicle as it is can be used for semiconductor printing. It is possible to determine whether or not the photomask can be projected with sufficient resolution without blurring the focus when projected using a projection exposure device, that is, whether the photomask is usable. The flatness of a photomask with a pellicle manufactured using the same method can be guaranteed. As described above, one important point in the method of the present invention is to discover and utilize the fact that even if the reflected light from the mask pattern 5 having a fine structure interferes with the reference light, its macro flatness can be measured. That is what we are doing.

In addition to the interferometer arranged as shown in FIG. 1, various known interferometers can also be used to obtain interference fringes representing equilinear lines of the unevenness of the mask pattern 5 of the photomask F. An example is shown in FIG.

In this case, a Michelson interferometer is used, and the same components as in FIG. 1 are used. However, in this case, the reference plane mirror RM that reflects the reference light is not a semi-transparent mirror but a total reflection mirror. A detailed explanation may not be necessary.

The above is a method for inspecting whether the mask pattern surface of a photomask with a pellicle has the necessary flatness, but it is also possible to measure the positional accuracy of the mask pattern of a photomask with a pellicle in the same way. can. That is, for example, in FIG. 4, the mask pattern 5 of the photomask with a pellicle is formed on the substrate 4 before the pellicle 2 is attached.
It is created with sufficient positional and dimensional accuracy. Therefore, it can be assumed that the mask pattern 5 before the pellicle is attached has accurate position and dimensions. However, when the pellicle 2 is attached as described above, the mask pattern 5
The flatness of the surface changes, and an error occurs in the position of the mask pattern 5. Changes in the positional accuracy of the mask pattern based on such changes in flatness can be indirectly measured by measuring the change in the flatness of the mask pattern surface after the pellicle is attached compared to before the pellicle is attached. Then, by checking whether the change in flatness is larger than a predetermined value, it is possible to inspect the positional accuracy of the mask pattern of this photomask.

For this purpose, for example, by using the interferometry shown in FIG. 1 or 2, the mask pattern 5 before and after mounting the pellicle 2 is
The interference fringes representing isolines are obtained, and the amount of change in flatness is calculated from the difference between the two.

Since such calculations are too complex to be performed manually, it is preferable to use a device such as that shown in FIG. 3, for example. That is, the interference fringes before and after the pellicle 2 are attached, which are obtained as shown in FIG. The information is input to the computer 11, and the computer 11 calculates the change in flatness before and after attaching the pellicle. If this change is larger than a predetermined value, it is determined that the positional error of the mask pattern exceeds the allowable value. Note that instead of such indirect determination, the position error can also be determined more directly.

That is, before attaching the pellicle, the position data of the pattern on the photomask is measured using another device, and in addition, the flatness of the mask pattern surface before attaching the pellicle is measured using the device shown in Fig. 3. , calculates and stores three-dimensional positional information of the pattern on the photomask.

After that, after attaching a pellicle to the photomask, its flatness is measured in the same manner, and the pattern position accuracy of the photomask after attaching the pellicle is calculated from the stored three-dimensional position information and the measured flatness. . In this way,
The positional error of the mask pattern can be determined. By the method described above, it is possible to calculate the positional accuracy of the pattern of the pellicle-equipped photomask, which cannot be directly measured, and to guarantee the accuracy of the pellicle-equipped photomask manufactured to the specification value.

Although several embodiments of the pellicle-equipped photomask inspection method of the present invention have been described above, the present invention is not limited to these embodiments, and various modifications are possible. For example, as a method of obtaining interference fringes, Figs.
It is clear that the arrangement is not limited to the one shown in the figure, and that various known interferometers may be used.

〔Effect of the invention〕

In the pellicle-equipped photomask inspection method of the present invention as described above, after the pellicle is attached, measurement light is transmitted through the pellicle and irradiated onto the mask pattern surface, and the reference light and the reflected light that has been reflected from the mask pattern and transmitted through the pellicle are combined. Since the flatness of the mask pattern surface is measured by interference, it is possible to determine the flatness of the mask pattern surface of a photomask with a pellicle under actual use, and even when printing using an exposure device with a shallow depth of focus. , we can guarantee the supply of a photomask with a pellicle that enables sharp pattern exposure without blurring of focus over the entire surface.

In addition, the flatness of the mask pattern surface is measured by interfering the light reflected from the mask pattern with the reference light before and after attaching the pellicle, and the positional accuracy of the mask pattern is inspected by determining the change between them. The positional accuracy of the mask pattern of a photomask with a pellicle under actual use conditions can be accurately determined, and a photomask with a pellicle suitable for highly integrated semiconductor integrated circuits can be guaranteed and supplied.

[BRIEF DESCRIPTION OF THE DRAWINGS] FIG. 1 is a diagram showing the arrangement of an apparatus for implementing a photomask inspection method with a pellicle according to an embodiment of the present invention, FIG.
FIG. 3 is a diagram showing the arrangement of an apparatus for implementing a photomask inspection method with a pellicle according to another embodiment, and FIG. 4 is a sectional view of an example of a photomask with a pellicle. 1... Pellicle frame, 2... Pellicle, 3...
・Adhesive, 4... Photomask substrate, 5... Photomask pattern, 10... Image human power device, 11...
Computer, L...Laser, Ll...Convex lens, M...Half mirror 1L2...Collimator lens, RM...Reference plane mirror, HM...Half mirror
1F...Photomask with pellicle, S...Screen Applicant Dai Nippon Printing Co., Ltd.

Claims (4)

[Claims] (1) After mounting a pellicle spaced apart on the mask pattern surface of a photomask having a mask pattern on one side, irradiating measurement light onto the mask pattern surface through the pellicle,
1. A method for inspecting a photomask with a pellicle, the method comprising measuring the flatness of a mask pattern surface by interfering a reference light with reflected light that has been reflected from a mask pattern and transmitted through a pellicle. (2) Measurement light is irradiated from the mask pattern side of a photomask that has a mask pattern on one side, and the reflected light that is reflected from the mask pattern and transmitted through the pellicle interferes with the reference light to measure the mask pattern surface before the pellicle is attached. After measuring the flatness, a pellicle is mounted at a distance on the mask pattern surface, and measurement light is transmitted through the pellicle and irradiated onto the mask pattern surface, and the reflected light that is reflected from the mask pattern and transmitted through the pellicle is measured. The flatness of the mask pattern surface after attaching the pellicle is measured by interfering with the reference light, and the positional accuracy of the mask pattern is inspected by determining the change in the flatness of the mask pattern surface before and after attaching the pellicle. Characteristic photomask inspection method with pellicle. (3) The method for inspecting a photomask with a pellicle according to claim 1 or 2, wherein the reference light is light reflected by a half mirror disposed perpendicular to the optical axis in the optical path of the irradiated measurement light. . (4) A claim characterized in that the reference light is light that is reflected by a half mirror disposed obliquely to the optical axis in the optical path of the irradiated measurement light and reflected again in a direction opposite to the traveling direction of the reference light. 2. The method for inspecting a photomask with a pellicle according to 1 or 2.