JP2002164280A - Exposure method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an exposure method whereby no pattern narrowing occurs at a wafer periphery. SOLUTION: The exposure method comprises adjusting an iris of a restriction mechanism, provided in a reducing lens system of a stepper for setting an optimum numerical aperture(NA) of the reducing lens system to, e.g. 0.55 for forming a pattern in an exposure process applied to a wafer, using the stepper, exposing shot sections 26 of effective regions 24 of the wafer in this optical condition to transfer the pattern, adjusting the iris of the restriction mechanism to set the numeral aperture of the reducing lens system to a smaller value of, e.g. 0.45 than 0.55, and in this optical condition, exposing shot sections 30 of a wafer periphery 28 specified as a dummy pattern region located outside the effective regions 24 of the wafer for planarizing the wafer, thereby transferring the same pattern.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exposure method for fabricating a semiconductor device, and more particularly to a method for preventing a thinning of a pattern at a peripheral portion of a wafer and a reduction in a yield due to scattering of the pattern from the peripheral portion of the wafer. And an exposure method.

[0002]

2. Description of the Related Art With the recent miniaturization and high integration of semiconductor devices, wiring widths have become increasingly smaller. As a result, further miniaturization and high-precision line width control technology are regarded as extremely important in the optical lithography technology used when patterning wiring.

Here, an optical lithography technique based on a reduced projection exposure method will be described with reference to FIG. The photolithography technology using the reduced projection exposure method uses a reduced projection exposure device to expose a photoresist film formed on a wafer.
This is a technique of forming an etching mask on which a wiring pattern of a reticle is transferred by developing. FIG. 3 is a schematic sectional view showing the basic configuration of the reduction projection exposure apparatus. As shown in FIG. 3, the reduction projection exposure apparatus 10 includes a light source unit 12, a condenser lens system 14, a reticle holding table 18 for holding a reticle 16 on which a pattern is drawn, and a reduction lens system 20 having an aperture mechanism. And a wafer stage 22 for moving the wafer W in a three-dimensional direction. Although not shown, the reduction lens system 20 is provided with an aperture. When the aperture is reduced, the numerical aperture (NA) decreases, and when the aperture is increased, the numerical aperture increases.

In the reduction projection exposure apparatus 10, the wafer stage 22 is moved to position the wafer W at the exposure position, and then the light emitted from the light source unit 12 is condensed by the condenser lens system 1.
The light is condensed by passing through 4, and the pattern drawn on the reticle 16 is reduced by the reduction lens system 20, and is exposed and imaged on the resist film of the wafer. By developing the exposed resist film, a mask having a pattern having the same shape as the pattern of the reticle 16 can be formed.

In order to further miniaturize a semiconductor device, it is necessary to further reduce the wiring width, and hence the pattern width. For that purpose, it is necessary to further increase the resolution of the reduction projection exposure apparatus. The resolution R of the projection optical system is represented by R = k · λ / NA according to Rayleigh's equation, that is, the exposure wavelength λ at the diffraction limit and the numerical aperture NA of the lens. Here, k is a value that depends on performance variations of the lens itself, light source coherency, and the like.
It is a value between 0.8 and 0.9. For miniaturization, the resolution is increased, and therefore the numerical aperture (N
A) needs to be increased, but by increasing the NA, the focus margin (DOF, depth of focus) Δ
Is smaller because it follows the equation of Δ = ± λ / 2 (NA) ² . Therefore, in order to advance the miniaturization, a technique for overcoming a small DOF is also required.

By the way, when etching is performed using the etching mask formed as described above, if a pattern is not formed on the periphery of the wafer which is out of control, there is a gap between the periphery of the wafer and the effective area inside the periphery. A step occurs. here,
The effective region means a chip formation region, and is a region where a semiconductor device to be a product is formed. On the other hand, the peripheral portion of the wafer is a non-chip forming area, which is a discarding area of a wafer on which no semiconductor device is formed. For example, as shown in FIG. 4A, when only the shot areas 26 in the effective area 24 are exposed, no mask layer is formed on the peripheral portion 28 of the wafer, so that as shown in FIG. The surface of the effective region 24 is higher than the surface of the wafer peripheral portion 28, and a step occurs between the effective region 24 and the wafer peripheral portion 28. FIG.
4A is a plan view of a wafer showing each shot area in an effective area of the wafer, and FIG. 4B is a cross-sectional view taken along line AA ′. If there is a step between the effective area 24 and the peripheral portion 28 of the wafer, in the effective area 24 close to the step, the etching will slightly affect the etching rate in the effective area 24, and the pattern cannot be patterned according to the pattern. There is.

Accordingly, the effective area 24 and the wafer peripheral portion 28
Conventionally, in order to prevent a step from occurring, the wafer peripheral portion 28 is also subjected to exposure processing under the same optical conditions as the effective area, and a dummy pattern is formed thereon. That is,
As shown in FIG. 5A, each shot area 30 of the wafer peripheral portion 28 is also exposed to light under the same optical conditions as the effective region 24 to form a dummy pattern, and a gap between the wafer peripheral portion 28 and the effective region 24 is formed. To prevent a step from occurring. FIG.
5A is a plan view of a wafer showing respective shot areas in an effective area of the wafer and in a peripheral portion of the wafer, and FIG.
It is sectional drawing in A '.

[0008]

However, in the above-mentioned conventional exposure method, as described below, the product yield is reduced due to the pattern narrowing at the peripheral portion of the wafer and the accompanying scattering of the pattern from the peripheral portion of the wafer to the effective area. However, there was a problem that was reduced.

At first glance, the wafer is seemingly uniform and flat, but the periphery of the wafer is slightly warped concavely with respect to the center of the wafer, ie, the effective area. Therefore, the focus position substantially differs in the peripheral portion of the wafer due to the influence of the warpage. As a result, when exposure is performed under the same optical conditions as that of the effective area, the wafer is affected by warpage, defocus occurs, and a normal pattern is not formed. (A)
As shown in FIG. To explain schematically, when exposure is performed under an optical condition of a numerical aperture NA = 0.55, defocus occurs around the wafer, and as the focus fluctuates to ± with respect to the pattern width of the focused position, FIG. As shown in (b), the pattern width is reduced. Then, as a result, the pattern with a small pattern width formed on the peripheral portion of the wafer is separated from the wafer and scatters in the effective area of the wafer as shown in FIG. 6C. as a result,
Subsequent processing causes various problems and lowers product yield.

SUMMARY OF THE INVENTION It is an object of the present invention to provide an exposure method which does not cause pattern thinning in a peripheral portion of a wafer.

[0011]

The inventor of the present invention has considered that the solution of the present invention is to prevent the pattern from being thinned in order to prevent the pattern from scattering around the wafer. The peripheral portion of the wafer is a dummy pattern region for preventing a step from occurring, and it is important that the pattern is not thinned, but the precision of the pattern is relatively low. Therefore, in order to prevent the pattern from being thinned, the resolution at the time of exposure of the peripheral portion of the wafer should be reduced and the depth of focus should be deepened, that is, the numerical aperture should be smaller than that of the effective area where the precision of the pattern is required. With the idea of reducing, the present invention was invented after experiments.

In order to achieve the above object, based on the above findings, an exposure method according to the present invention provides an exposure method for a reduction lens system of a reduction projection exposure apparatus when performing exposure processing on a wafer using the reduction projection exposure apparatus. A first step of exposing an effective area of the wafer under an optical condition in which a numerical aperture (NA) is set to a first numerical aperture and transferring a reticle pattern, and a peripheral portion of the wafer located outside the effective area of the wafer Is defined as a dummy pattern area, and the wafer peripheral portion is exposed under optical conditions in which the numerical aperture of the reduction lens system is set to a second numerical aperture smaller than the first numerical aperture, and the pattern of the reticle in the first step is changed. And a second step of transferring.

In the method of the present invention, the order of the first step and the second step is not limited, and the first step, then the second step, or vice versa, the second step, then the first step. The order of the steps may be used. In the method of the present invention,
Is the most appropriate numerical aperture for pattern formation in consideration of exposure conditions. To set the numerical aperture in the method of the present invention, the numerical aperture is set to a predetermined numerical aperture by adjusting the aperture with an aperture mechanism provided in a reduction lens system of the reduction projection exposure apparatus.

According to the method of the present invention, the resolution is reduced and the depth of focus is increased by reducing the numerical aperture for exposing the peripheral portion of the wafer as compared with the numerical aperture for exposing the effective area. The focus shift caused by the warpage of the peripheral portion is reduced to prevent the pattern from being thinned. In the method of the present invention, since the same pattern as the effective area of the wafer is transferred also to the peripheral part of the wafer as the dummy pattern area, there is no step between the peripheral part of the wafer and the effective area. It is flattened over the effective area and the periphery of the wafer.

[0015]

Embodiments of the present invention will be described below in detail with reference to the accompanying drawings. Embodiment Example This embodiment is an example of an embodiment of an exposure method according to the present invention. FIG. 1 is a plan view of a wafer showing each shot area for explaining the exposure method of this embodiment. In this embodiment, first, when an exposure process is performed on a wafer using a reduction projection exposure apparatus (commonly called a stepper), an aperture mechanism (see FIG. 3) provided in a reduction lens system 20 of the reduction projection exposure apparatus 10 (see FIG. 3). The aperture of the reduction lens system 20 (not shown) is adjusted.
Numerical aperture (NA) is set to the most appropriate numerical aperture (first numerical aperture) for pattern formation, for example, in consideration of exposure conditions.
Set to 55. Under these optical conditions, as shown in FIG.
Each shot area 26 of the active area 24 of the wafer is exposed to transfer the pattern.

Next, the stop of a stop mechanism (not shown) provided in the reduction lens system 20 is adjusted to reduce the numerical aperture of the reduction lens system 20 to a second numerical aperture smaller than the first numerical aperture, for example, 0.2 mm. Set to 45. Under these optical conditions, as shown in FIG. 1, each shot area 30 of the wafer peripheral portion 28 located outside the effective area 24 of the wafer and defined as a dummy pattern area for flattening the wafer is exposed, and Transfer the pattern.

In the present embodiment, when exposing the wafer peripheral portion 28, the resolution is reduced and the depth of focus is increased by reducing the numerical aperture. Therefore, as shown in FIG. Even if it shifts, the pattern width of the transferred pattern hardly changes. As a result, in the wafer peripheral portion 28,
Although the focal position is substantially different due to the influence of the warpage of the wafer, by changing the optical conditions, the pattern thinning is suppressed as shown in FIG. As a result, pattern scattering does not occur, so that a reduction in product yield can be prevented.

Experimental Example In order to evaluate the exposure method of this embodiment, an evaluation test was performed as follows. First, a resist film having a thickness of 0.835 μm was formed on a sample wafer using a coater / developer having a model name, Clean Track, model number ACT-8 manufactured by Tokyo Electron Limited.
As a resist material, product name SEPR manufactured by Shin-Etsu Chemical Co., Ltd.
-3404T was used. Subsequently, the wafer was heated at 120 ° C. for 90 seconds to perform a pre-bake treatment.

Next, as a reduction projection exposure apparatus (stepper), Nikon model number NSR-2205EX14
Using C, exposure was performed at an exposure wavelength of 248 nm according to the following optical conditions, and a 0.25 μm-wide wiring pattern was transferred onto the resist film. At the time of exposure, first, as shown in FIG.
Each shot area 26 within the active area 24 of the wafer was exposed under a first optical condition, and then each shot area 28 at the wafer periphery 28 was exposed under a second optical condition. First optical condition: NA = 0.55, σ = 0.55 Optical condition for effective area Second optical condition: NA = 0.45, σ = 0.75 Optical condition for dummy area

Next, the exposed wafer is heated at 110 ° C. for 90 minutes.
After heating for 2 seconds, a post-exposure bake treatment (PEB) was performed. Subsequently, TMA manufactured by Tokyo Ohka Co., Ltd. was used as a developer.
It developed using NMD-3 whose brand name of H is 2.38%. Next, the wafer was heated at 100 ° C. for 90 seconds to perform post-baking treatment, thereby forming an etching mask having a wiring pattern.

Observation of the etching mask formed in this experimental example shows that the pattern width of the dummy pattern area around the warped wafer is almost the same as the effective area, and the pattern thinning is reduced. Did not scatter into the effective area of the wafer, thereby lowering the product yield.

[0022]

According to the method of the present invention, when an exposure process is performed on a wafer using a reduction projection exposure apparatus, the numerical aperture of the reduction lens system is set in the effective area of the wafer under the optical conditions set to the first numerical aperture. A first step of exposing, and an optical condition in which the periphery of the wafer is defined as a dummy pattern area for flattening the wafer, and the numerical aperture of the reduction lens system is set to a second numerical aperture smaller than the first numerical aperture. Exposure is performed in a second step of transferring the same pattern as the pattern in the first step. In the method of the present invention, since the same pattern as the effective area of the wafer is transferred also to the peripheral portion of the wafer as the dummy pattern area, there is no step between the peripheral portion of the wafer and the effective area. And the wafer surface is flattened over the periphery of the wafer. Thereby, while realizing the flattening of the peripheral portion of the wafer, the pattern thinning at the peripheral portion of the wafer and the accompanying pattern scattering are prevented,
Product yield can be improved.

[Brief description of the drawings]

FIG. 1 is a wafer plan view showing each shot area for explaining an exposure method according to an embodiment.

FIG. 2A is an explanatory diagram for explaining that a pattern width does not greatly change even if the focus is shifted, and FIG.
FIG. 4 is an explanatory diagram for explaining that pattern thinning does not occur in a peripheral portion of a wafer.

FIG. 3 is a schematic sectional view showing a configuration of a reduction projection exposure apparatus.

4A is a plan view of a wafer showing each shot area in an effective area of the wafer, and FIG. 4B is a cross-sectional view taken along line AA '.

5A is a plan view of a wafer showing respective shot areas in an effective area of the wafer and in a peripheral portion of the wafer, FIG.
(B) is a sectional view taken along line AA '.

FIGS. 6A to 6C are schematic diagrams for explaining that a problem of thinning of a pattern and accompanying pattern scattering occur at a peripheral portion of a wafer.

[Explanation of symbols]

10 reduction projection exposure apparatus, 12 light source unit, 14
... Condenser lens system, 16 reticle, 18 reticle holder, 20 reduction lens system, 22 wafer stage, 24 wafer effective area, 26 effective area shot area, 28 ... Wafer peripheral portion, 30... Wafer peripheral portion shot area.

Claims (2)

[Claims] When performing exposure processing on a wafer using a reduction projection exposure apparatus, the effective area of the wafer is set under an optical condition in which the numerical aperture (NA) of a reduction lens system of the reduction projection exposure apparatus is set to a first numerical aperture. A first step of exposing a reticle pattern and exposing a peripheral portion of the wafer located outside the effective area of the wafer as a dummy pattern area, and setting the numerical aperture of the reduction lens system to be smaller than the first numerical aperture. A second step of exposing a peripheral portion of the wafer under an optical condition set to a small second numerical aperture and transferring a reticle pattern in the first step. 2. The exposure method according to claim 1, wherein a numerical aperture is set to a predetermined numerical aperture by adjusting a diaphragm with a diaphragm mechanism provided in a reduction lens system of the reduction projection exposure apparatus.