JPH11160853A - Photomask and photomask blank - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent deterioration of transfer accuracy due to unnecessary light reflected on a wafer surface and a mask surface when projecting and exposing on a wafer using a photomask, and to improve the resolution of a transfer pattern. It is to provide a photomask that can be improved. The A surface light-shielding pattern 2a is formed in the photomask, larger than the refractive index n _S of the transparent substrate against the exposing light, smaller than the refractive index n _C of the light-shielding film forming the light-shielding pattern 2a Thin films 4 and 5 having a refractive index n are formed, and the film thickness d of the thin films 4 and 5 satisfies (Equation 1).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates generally to photomasks used in LSI manufacturing, and more particularly to a Levenson-type phase shift mask and an optical proximity correction photomask.

[0002]

2. Description of the Related Art In a conventional photomask, a light-shielding pattern 12 having a thickness of about 0.1 μm and having a light-shielding property against projection light is formed on one surface of a transparent substrate 11 (see FIG. 5A). Here, the light-shielding pattern 12 may have a multilayer structure in order to reduce the surface reflection.

As the degree of integration of LSIs increases, design rules include fine line widths near the wavelength of the light source, and the projection light passing through the light transmitting portion of the photomask is diffracted on the wafer. The interference causes the light intensity at the pattern boundary to be strengthened, and the resist on the wafer is exposed to light, so that the transferred pattern is not separated and resolved.

Therefore, a phase shift mask technique of improving resolution of a fine pattern by providing a phase difference of 180 degrees between projection lights transmitted through adjacent patterns of a photomask to improve the resolution of a fine pattern has been developed by IBM's Levenson.
(JP-A-58-173744, the principle of which is described in JP-A-62-50811).
That is, by providing a phase shift portion on one side of the adjacent light transmission portion, the transmitted light is diffracted, so that when the interference occurs, both phases are inverted. As a result, the light intensity at the boundary portion is weakened, and the transfer pattern is reduced. Will be separated and resolved. Since this relationship is established before and after the focus, even if the focus is slightly shifted, the resolution is improved as compared with the conventional exposure method, and the focus latitude is improved.

The above-described photomask that provides a phase shift portion on one side of the opening adjacent to the light-shielding pattern and inverts the phase of transmitted light is generally called a Levenson-type phase shift mask or an alternative type phase shift mask ( For example, Literature 1: Mulberry Principle, “The Forefront of Lithography Technology (1) Mask Technology”, Oplus
s E, No. 182, January 1995, p. 9
2). A method of providing a phase shift unit on one side of the opening includes:
There are two methods: a method of forming a transparent thin film pattern called a shifter layer, and a substrate digging type in which a digging portion 13 is formed in a transparent substrate 11. Considering the delicate physical properties of the thin film such as foreign matter defects. Substrate digging type (Figure 5
(See (b)) is becoming mainstream.

Degradation of transfer fidelity due to diffraction and interference of projection light on a wafer is a phenomenon that occurs because a light transmitting portion of a circuit pattern is close to a photomask, and is called an optical proximity effect. An optical proximity effect correction (hereinafter, referred to as OPC) mask (FIG. 5) is used as a photomask that more directly reduces this phenomenon in a manner different from the phase shift mask.
(See (c)) (for example, Reference 2: Haruo Kinoshita, "Reticle Manufacturing Technology in 256M Era", Monthly Semi)
conductor World, June 1996 issue,
p. 96). The OPC mask of FIG. 5C has the same cross-sectional structure as a conventional photomask, but is characterized in that an additional correction pattern is provided around the original circuit pattern in the pattern arrangement.

FIGS. 5A to 5C are schematic sectional views.
All of these are the basic structures of conventional ordinary photomasks, Levenson-type masks, and OPC masks. The light-shielding pattern 12, which is a component of the light-shielding pattern 12, may have a multilayer structure in order to reduce the surface reflection.

[0008]

One of the problems in projecting and exposing a wafer on a wafer using a photomask is a problem of deterioration in transfer accuracy due to unnecessary light reflected on the wafer surface and the mask surface. . In particular, when the base of the resist on the wafer side is a metal layer such as Al, since the reflectance is high,
It is necessary to provide an antireflection film on the metal layer, and selection of an optimal antireflection film has been attempted (for example, Reference 3: UL).
Innovation of SI lithography technology (Science Forum), 1994, p. 60).

However, for the light reflected on the photomask surface, the study of the antireflection film as described above is not sufficient.
It is an issue to be studied to establish an anti-reflection technology capable of coping with an unusual photomask such as a pattern line width miniaturization and a phase shift mask in the future.

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and it is intended to prevent a deterioration in transfer accuracy caused by unnecessary light reflected on a mask surface when a projection exposure is performed on a wafer using a photomask. An object of the present invention is to provide a photomask capable of improving the resolution of a transfer pattern.

[0011]

In order to solve the above-mentioned problems in the present invention, a first aspect of the present invention has a light-shielding pattern formed by patterning a light-shielding film that does not transmit exposure light on a transparent substrate. In the photomask, a thin film having a refractive index n larger than the refractive index n _S of the transparent substrate and smaller than the refractive index n _{C of the} light shielding film for the exposure light is formed on the surface on which the light shielding pattern is formed. The photomask is characterized in that the photomask is formed.

In the present invention, the thickness d of the thin film is d = (2m-1) λ / 4n (1) where m is a natural number, and λ is the exposure light. 2. The photomask according to claim 1, wherein a wavelength n is a refractive index of the thin film with respect to the exposure light.

Further, in claim 3, a photomask blank for manufacturing the photomask according to claim 1 or 2 is provided.

The photomask provided by the present invention basically has a structure in which an antireflection thin film is added to a conventional photomask having a light-shielding pattern. 5 does not depend on the presence or absence of the substrate digging, the case where the present invention is applied to the normal type mask of FIG. The same applies to the case where the present invention is applied to a substrate digging type Levenson type mask.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIGS. 1A to 1D are schematic sectional views showing the structure of a photomask according to the present invention. In the photomask of the present invention, on the surface on which the light-shielding pattern 2a is formed, a light-shielding film that is larger than the refractive index n _S of the transparent substrate with respect to exposure light and that forms the light-shielding pattern 2a , The thin films 4 and 5 having a refractive index n smaller than the refractive index n _C of the film on the side where the thin films 4 and 5 are in contact, and the film thickness d of the thin film satisfies (Equation 1). It was done. The thin film 3 is a film for suppressing the reflection on the surface of the transparent substrate 1 over the entire surface, and when it is formed, it is preferable that the thin film 3 has a refractive index smaller than the refractive index n _S of the transparent substrate 1. .

FIGS. 2A to 2C are schematic sectional views showing the structure of a photomask blank of the present invention for manufacturing a photomask. The photomask of FIG. 1C is converted from the photomask blank of FIG. 2A, and the photomask of FIG. 1B is changed from the photomask blank of FIG.
The photomask of FIG. 1D is obtained from the photomask blank of FIG. The photomask of FIG. 1A can be obtained from a photomask blank having a conventional configuration.
Here, the photomask blank in which the photoresist is formed on the light-shielding film is omitted, but it goes without saying that the photomask blank is included in the photomask blank of the present invention.

Hereinafter, the relationship between the refractive index and the thickness of the thin film formed on the photomask and the photomask blank of the present invention will be described. In general, the light transmittance and reflectance of a thin film formed on a transparent substrate are represented by the optical constants (refractive index, extinction coefficient) and film thickness of these films, and the refractive index of the substrate and the exposure atmosphere (usually air). And is determined as a result of multiple interferences in the film.

The conditions for making the reflectance of a thin film formed on a transparent substrate zero are as follows, assuming normal incidence. Assuming that the refractive indices of the thin film, the transparent substrate and the air are n, n _S and n ₀ respectively

[0019]

(Equation 1)

When the thickness of the thin film is d, d = (2m-1) λ / 4n (1) (where m: natural number, λ: wavelength of exposure light).

The reflectivity becomes zero when (Equation 2) and (Equation 1) are satisfied. However, when quartz is used as the transparent substrate, its refractive index is about n _S = 1.5 at the exposure wavelength. ,

[0022]

(Equation 2)

It is not easy to find a highly transparent thin film having a small refractive index.

Next, when considering the reflectance of the thin film formed on the light-shielding pattern, the light-shielding film forming the light-shielding pattern (in the case where the light-shielding film is a multilayer film) is used instead of n _{S in} (Equation 2). The refractive index n _C of the film on the side where the thin films 4 and 5 are in contact may be used. Since n _C is usually as large as 2.5 or more,

[0025]

(Equation 3)

It is not difficult to find a thin film having a refractive index n such that But usually

[0027]

(Equation 4)

However, if n is larger than n _S , the reflectance on the transparent substrate of the portion where the light-shielding film is removed by patterning becomes large, causing a new problem.

Therefore, in the first aspect, the refractive index n of the thin film is set so that n _S <n <n _C (Equation 4). As a result, the reflectivity on the light-shielding pattern does not become zero, but it is always smaller than when there is no thin film. In addition, since the difference between n and n _S is not so large, the reflectance on the transparent substrate of the portion where the light-shielding film has been removed by patterning can be suppressed to an acceptable level.

Further, in claim 2, a thin film that satisfies (Equation 1) while satisfying (Equation 4) is formed. As a result, the reflectance on the light-shielding pattern always becomes smaller than when there is no thin film, and at the same time, it takes a minimum value with respect to a change in the thickness of the thin film.

[0031]

The present invention will be described in detail with reference to the following examples.

<Embodiment 1> In Embodiment 1, the case of manufacturing the photomask of FIG. 1A will be described. Refractive index n _S =
A chromium oxide film and a chromium film are formed on a transparent substrate 1 made of a quartz substrate having an optical constant of 1.51 and an extinction coefficient k _S = 0 (for a wavelength λ: 248 nm) by a sputtering method. A photomask blank composed of a two-layer light-shielding film having the above structure was manufactured. The chromium oxide film had a predetermined optical constant and thickness, and was made to have low reflection from the transparent substrate side. Here, the optical constants of the chromium film have been confirmed in advance by preliminary experiments, and the refractive index n = 3.0 and the extinction coefficient k =
1.5 (for wavelength λ = 248 nm).

Next, a resist pattern is formed on the photomask blank by a usual electron beam lithography process, and the two-layer light-shielding film is dry-etched with a chlorine-based gas using the resist pattern as a mask. Is removed to form a light-shielding pattern 2a composed of a two-layer film.
Was produced.

Next, a zirconium oxide silicide film was formed by a reactive sputtering method in which an appropriate amount of oxygen gas was added to Ar gas, a thin film 5 serving as an antireflection layer was formed, and a photomask of the present invention was manufactured (FIG. 1 (a)). Here, the optical constant of this zirconium oxide silicide film has been confirmed in advance by a preliminary experiment, and the refractive index n =
1.68, the extinction coefficient k = 0.1 (for the wavelength λ = 248 nm). The film thickness was about 369 Å.

FIGS. 3B and 3C show the results of calculating the reflectance of each part of the photomask of the present invention obtained in Example 1. FIG. In FIGS. 3B and 3C, it is the thickness d of the thin film 5 that actually changes, but is normalized by n × d / λ. Thus, n × d / λ = 1/4, that is, the film thickness is d
= 2480 / (4 × 1.68) = approximately 369 angstroms, so that none of the reflectances exceed approximately 10%. In this manner, a photomask having low reflection on both the light-shielding pattern and the transparent substrate can be obtained.

<Embodiment 2> Here, an example in which the photomask of FIG. 1B is manufactured from the blank of FIG. 2B will be described. First, a zirconium oxide silicide film was formed on a transparent substrate 1 made of a quartz substrate having the same optical constant as in Example 1 by a reactive sputtering method in which an appropriate amount of oxygen gas was added to Ar gas to form a thin film 4. The optical constants of the thin film 4 were confirmed in advance by preliminary experiments, and the refractive index n was 1.68 and the extinction coefficient k was 0.1 (for a wavelength λ of 248 nm).

Next, a light-shielding film 2 composed of a chromium film and a chromium oxide film is formed on the thin film 4 by a sputtering method.
A photomask blank of the present invention was produced (FIG. 2B).
reference). The chromium oxide film has a predetermined optical constant and thickness,
Low reflection from the pattern side. Incidentally, the optical constant of the chromium film at this time has been confirmed in advance by a preliminary experiment, and the refractive index n = 3.0 and the extinction coefficient k = 1.5 (for a wavelength λ = 248 nm).

Next, the light-shielding film 2 was patterned by the same process as in Example 1 to form a light-shielding pattern 2a, thereby producing a photomask of the present invention (see FIG. 1B).

FIGS. 4B and 4C show the results of calculating the reflectance of each part of the photomask of the present invention obtained in Example 2. It should be noted that the abscissas in FIGS. 4B and 4C actually change the thickness d of the thin film 4, but are normalized by n × d / λ. Even if the film thickness changes in this manner, any reflectance is approximately 1
Although it does not exceed 0%, if the film thickness at the point where the light-shielding pattern and the reflectance on the transparent substrate cross each other is set, a photomask with low reflection on both the light-shielding pattern and the transparent substrate can be obtained.

[0040]

Since the photomask of the present invention has the above-described structure, the reflectance of the entire mask is reduced regardless of the light-shielding pattern and the transparent substrate from which the light-shielding film has been removed. Deterioration of transfer accuracy due to unnecessary reflected light can be prevented. Here, since the optical constant and the film thickness of the thin film for lowering the reflectivity of the entire photomask are selected so that the desired effect can be obtained without performing the patterning process of the thin film itself, the conventional photomask is used. It is only necessary to add a film forming process to the manufacturing process. Further, the thin film for anti-reflection used in the photomask of the present invention can also serve as a protective film of the photomask. For example, in the final step of manufacturing a Levenson mask, when the substrate is dug into a chemical solution to form a dug portion and adjust the phase difference, the backside of the substrate can be prevented from being eroded by the chemical solution. In addition, by receiving the contamination generated during repeated use of the photomask during the exposure, the thin film and the transparent substrate exposed as in the conventional photomask are not likely to deteriorate during cleaning.

[Brief description of the drawings]

FIGS. 1A to 1D are schematic cross-sectional views illustrating a configuration of a photomask of the present invention.

FIGS. 2A to 2C are schematic cross-sectional views illustrating a configuration of a photomask blank of the present invention.

FIG. 3A is a schematic cross-sectional view illustrating a configuration of a photomask according to a first embodiment of the present invention. (B)-(c) are graphs showing the reflectance characteristics of the photomask of the present invention manufactured in Example 1.

FIG. 4A is a schematic cross-sectional view illustrating a configuration of a photomask according to a second embodiment of the present invention. (B)-(c) are graphs showing the reflectance characteristics of the photomask of the present invention manufactured in Example 2.

FIG. 5A is a schematic cross-sectional view illustrating a configuration of a conventional normal mask. FIG. 1B is a schematic cross-sectional view illustrating a configuration of a conventional Levenson-type mask. (C) shows the conventional OP
FIG. 3 is a schematic cross-sectional view illustrating a configuration of a C mask.

[Explanation of symbols]

 1, 11 transparent substrate 2 light-shielding film 2a, 12 light-shielding pattern 3, 4, 5 thin film 13 engraved part

Claims (3)

[Claims] 1. A photomask having a light-shielding pattern formed by patterning a light-shielding film that does not transmit exposure light on a transparent substrate, wherein the surface on which the light-shielding pattern is formed is refracted by the transparent substrate with respect to the exposure light. A photomask, wherein a thin film having a refractive index n larger than the refractive index n _{S and} smaller than the refractive index n _{C of the} light shielding film is formed. 2. The film thickness d of the thin film is d = (2m-1) λ / 4n, where m: natural number, λ: wavelength of exposure light, n: refractive index of the thin film with respect to exposure light. The photomask according to claim 1. 3. A photomask blank for producing the photomask according to claim 1.