JPH08124827A - Method of forming pattern - Google Patents

Abstract

PURPOSE: To enable a substrate large in level difference to be subjected to a light exposure process large enough in depth of a focus so as to stably form a resist pattern on the substrate. CONSTITUTION: A photoresist film 3 formed on a silicon substrate 1 possessed of a difference 2 in surface level is exposed to light using a mask possessed of a first light transmitting region 4A used for forming an optically positive image on the lower surface of the silicon substrate 1 and a second light transmitting region 4B used for forming an optically reversed image on the upper surface of the silicon substrate 1 under such a condition that the distance between the focal points of the positive image and the negative image is set coincident with the difference 2 in surface level.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for forming a pattern, and more particularly, to a method for forming a fine pattern on a substrate having a step using ultraviolet exposure.

[0002]

2. Description of the Related Art In a process of manufacturing a semiconductor device or a superconducting device, a fine pattern is often formed on a substrate having a step. For example, in a dynamic random access memory (DRAM) or the like, a memory element portion and a peripheral circuit portion are clearly separated in a chip. However, since a capacitor and an element isolation layer are present in the memory element portion, the peripheral circuit portion is not provided. On the other hand, it has a step of 1-2 μm.
FIG. 7 shows a case where a line pattern is formed using a positive photoresist on a semiconductor substrate having steps.
Will be explained.

[0003] First, as shown in FIG.
A positive photoresist film 3 is formed on a silicon substrate 1 having a step 2 of. Next, exposure is performed with KrF excimer light 5 using a mask 4 for forming four dense line patterns of 0.3 μm on the lower step surface and the upper cross section of the step.

Since the light passing through the mask 4 is diffracted at the opening of the mask 4, an optically normal image when reaching the photoresist film 3 (an image in which the light intensity is maximized below the opening of the mask:
As shown in FIG. 8, the light intensity 8 of a normal image (hereinafter simply referred to as a normal image) is lower than the light intensity 7 after passing through the mask shown by the broken line as shown by the solid line. Light intensity ratio is given by the following equation from a maximum value I _max and the minimum value I _min of the light intensity at this time.

Light intensity ratio = (I _max −I _min ) / (I _max + I _min ) Positions 11A and 12A of the lower surface and the upper surface in the optical axis direction.
FIG. 9 shows the relationship between the light intensity ratios 11 and 12 of the lower surface and the upper surface with respect to. In the light intensity distributions of the lower surface and the upper surface, the center of the range in which the light intensity ratio of the normal image is equal to or higher than a certain value is set as the focus setting position 10A during exposure, but as shown in FIG. It will be narrow.

That is, under the conditions described with reference to FIG.
When a stepper having a numerical aperture NA = 0.45 of the projection lens is used, the depth of focus of a line pattern that provides a light intensity ratio of 0.4 is about 2.0 μm as shown in FIG. Is about 0.5 μm.
The focus setting position 10A is set at the center of the range where the common depth of focus can be obtained.

In this way, the focus position 10A is determined, exposed, and developed, whereby four dense 0.3 .mu.m line patterns 6 made of photoresist are stepped, as shown in FIG. 7B. Is formed on the lower and upper surfaces.

As described above, by forming four dense line patterns of 0.3 μm, the lens numerical aperture NA = 0.4.
5. When a KrF excimer stepper with a coherence factor of illumination σ = 0.7 is used, the depth of focus when forming a line pattern on both the upper and lower surfaces of the step is 0. It becomes as narrow as 5 μm. 0.
At a focal depth of 5 μm, it is extremely difficult to actually form a stable pattern in consideration of defocus factors such as wafer warp, stepper lens aberration, and focus setting error.

As a countermeasure, a multilayer resist method, a multi-focus exposure method, and the like have been proposed.

The multilayer resist method is disclosed in, for example, Japanese Patent Publication No. 4-66.
As described in No. 097, a thick lower layer resist film for planarization, an intermediate layer made of SiO ₂ and a thin upper layer resist film are sequentially formed on a semiconductor substrate having a step, and a flat upper layer resist film is finely divided. After forming a pattern, this pattern is transferred to an intermediate layer, and then the lower layer resist is etched by using this intermediate layer as a mask.

A multifocal exposure method is disclosed in, for example,
As described in JP-A-42122, after a photoresist film is formed on a substrate having a step, exposure is performed at a plurality of positions on an optical axis to increase the depth of focus.

[0012]

However, among the above-mentioned conventional pattern forming methods, the multilayer resist method involves:
In order to form a resist pattern, three layers of films must be formed and each film must be etched, which causes a problem that the process becomes complicated.

Further, in the multi-focus exposure method, when forming one pattern, it is necessary to move the substrate in the optical axis direction a plurality of times to perform the exposure, which complicates the operation in the manufacturing apparatus, and the processing is performed. There are problems such as reduced ability.

An object of the present invention is to form a fine pattern with high accuracy by exposing a photoresist film on a lower step and an upper step having a high step with a sufficient depth of focus without requiring a complicated process. Another object of the present invention is to provide a simple pattern forming method.

[0015]

A pattern forming method of the present invention comprises a step of forming a photoresist film on a main surface of a substrate having a step, and an optical normal image (or a reverse image) on a lower surface of the step. And a second light-transmitting region for forming an optically inverted image (or a normal image) on the upper surface of the step.
The photoresist film is exposed using a mask having a light-transmitting region and then developed to simultaneously form a pattern of the photoresist film on the lower surface and the upper surface.

[0016]

When performing exposure using a mask having a repeating pattern, a normal image of a pattern as shown in FIG. 10A is usually used. At this time, the light intensity of the normal image is at the maximum point.

Next, when the focus position of the pattern is shifted, the light intensity distribution changes, the maximum point decreases as shown in FIG. 10B, and the optically inverted image (the light intensity becomes maximum under the light shielding portion of the mask). (Hereinafter, simply referred to as an inverted image) appears, and finally, as shown in FIG. 10C, the inverted image has the maximum light intensity. The present invention utilizes this phenomenon to form a resist pattern on the lower and upper surfaces of a stepped substrate by the same exposure operation.

The focal position having the highest light intensity ratio between the normal image and the inverted image is several μm apart, and the distance (inter-focal distance) varies depending on the optical illumination conditions of the stepper. Therefore, by selecting an optical illumination condition such that the distance between the focal points coincides with the step of the substrate, a normal image (or inverted image) is formed on the lower surface of the substrate, and a reversed image (or normal image) is formed on the upper surface. Can be simultaneously formed.

The optimal optical illumination conditions for forming the optically normal image and the inverted image are determined using a simulator.

For example, the pattern size (line ant space) is 0.3 μm, and the illumination coherence factor σ
Is set to 0.3, the wavelength λ of the exposure light (KrF excimer stepper) is set to 248 nm, the numerical aperture NA of the lens is set to a certain value (for example, 0.5), and a normal image and an inverted image of the pattern are formed. The optimum light intensity ratio of each image and the inter-focal distance at this time are obtained by shifting the position (distance) of.
FIG. 3 shows an example of the inter-focal distance obtained by changing the numerical aperture NA.
Shown in According to FIG. 3, when the substrate has a step of 1.3 to 2.0 μm, the normal image and the inverted image are easily formed on the same substrate by adjusting the numerical aperture NA to 0.4 to 0.6. See what you can do.

When the light intensity ratio of the normal image and the reverse image used for exposure is significantly different, the accuracy of the formed resist pattern may be reduced, but the mask size and the exposure amount are locally adjusted. By performing the above, it is possible to accurately form a fine resist pattern having the same width.

[0022]

Embodiments of the present invention will now be described with reference to the drawings.

FIGS. 1A and 1B are cross-sectional views of a semiconductor chip for explaining a first embodiment of the present invention.

First, as shown in FIG. 1A, a positive photoresist film 3 is formed on a silicon substrate 1 on which a step 2 of 1.7 μm is formed by forming elements and the like. Next, a mask is used to form four dense line patterns having a width of 0.3 μm simultaneously on the lower step surface 2A and the upper step surface 2B using a mask.
Exposure is performed with excimer light 5. The mask used at this time is a first light-transmitting region 4A having a lattice-like opening for forming a normal image on the lower surface 2A, and a reversal image on the upper surface. And a second light-transmitting region 4B having a lattice-shaped opening.

When exposure is performed using this mask, the normal image 8A shown in FIG. 2A is obtained on the lower surface 2A, and the inverted image 9A shown in FIG. 2B is obtained on the upper surface 2B. In addition, the optical illumination condition is used so that the focal length of each image is 1.7 μm. In the case of the first embodiment, the lens numerical aperture NA
By using a KrF excimer stepper with a coherence factor of illumination = 0.45 and a coherence factor of illumination = 0.3, the distance between the focal points and the step (1.7 μm) can be matched.
FIG. 4 shows the relationship between the focus position and the light intensity ratio at this time. When a photoresist film that resolves when the light intensity ratio is 0.4 or more is used, the depth of focus of each of the normal image and the inverted image is 1
8, 19 are 1.3 μm and 1.0 μm, and the depth of focus at which a pattern can be formed on both the lower surface and the upper surface at one time is also 1.0 μm. At this time, the focal position 10 of the normal rotation image is adjusted to the lower step surface.

By developing the photoresist film 3 exposed in this manner, as shown in FIG. 1B, four dense line patterns 6A made of a resist film are formed on the lower surface 2A and the upper surface 2B. , 6B.

FIGS. 5A and 5B are cross-sectional views of a semiconductor chip for explaining a second embodiment of the present invention, in which the present invention is used for forming four dense space patterns. Show.

First, as shown in FIG.
After a positive photoresist film 3 is formed on a silicon substrate 1A having a step, a first light-transmitting region 14A having four lattice-shaped openings for forming a normal rotation image on a lower step surface;
Using a mask having a second light-transmitting region 4B having five lattice-shaped openings for forming an inverted image on the upper step surface 2B, K
Exposure is performed using rF excimer light 5.

When exposure is performed using this mask, a normal image 8B shown in FIG. 6A is obtained on the lower surface 2A, and a reverse image 9B shown in FIG. 6B is obtained on the upper surface 2B. Optical illumination conditions are used so that the focal length of each image is 1.5 μm. In the case of the second embodiment, the lens numerical aperture NA
By using a KrF excimer stepper having a coherence factor of σ = 0.5 and an illumination coherence factor of σ = 0.3, the distance between the focal points of each image and the step (1.5 μm) can be made to coincide. When a photoresist film that resolves when the light intensity ratio is 0.4 or more is used, the respective focal depths of the normal image and the inverted image are substantially the same as those of the first embodiment shown in FIG. The depth of focus at which a pattern could be formed on both the lower surface and the upper surface at one time was also 1.0 μm.

By developing the photoresist film 3 exposed in this manner, as shown in FIG. 5B, four dense space patterns 16A made of a resist film are formed on the lower surface 2A and the upper surface 2B. , 16B.

In the above embodiment, a case was described in which four dense patterns were formed using a positive type photoresist film. However, the negative type photoresist film may be used.
One or more resist patterns can be formed. Further, the present invention is applicable not only to a semiconductor substrate but also to a substrate having a high step when forming a superconducting element or the like.

[0032]

As described above, according to the present invention, the first transparent film for forming the optical normal rotation image (or reverse image) on the lower surface of the step of the photoresist film on the substrate having the step is provided. Using a mask having a region and a second light-transmitting region for forming an optically inverted image (or a normal rotation image) on the upper surface of the step, the inter-focal distance of the normal rotation image and the reverse image and the step are matched. By performing the exposure under the conditions, the exposure can be performed with a sufficient depth of focus without requiring a complicated process, and thus a fine pattern can be accurately formed.

[Brief description of drawings]

FIG. 1 is a cross-sectional view of a semiconductor chip for explaining a first embodiment of the present invention.

FIG. 2 is a diagram showing a normal image and a reverse image during exposure in the first embodiment.

FIG. 3 is a diagram showing a relationship between a numerical aperture of a lens and a focal length.

FIG. 4 is a diagram showing a relationship between a focus position of a normal image and a reverse image and a light intensity ratio in the first embodiment.

FIG. 5 is a sectional view of a semiconductor chip for explaining a second embodiment of the present invention.

FIG. 6 is a diagram showing a normal image and a reverse image during exposure in a second embodiment.

FIG. 7 is a sectional view of a semiconductor chip for explaining a conventional pattern forming method.

FIG. 8 is a view showing a light intensity distribution for explaining a light intensity ratio.

FIG. 9 is a diagram showing a relationship between a focus position of an image and a light intensity ratio in the related art.

FIG. 10 is a diagram showing a light intensity distribution for explaining a change in an image when a focus is shifted.

[Explanation of symbols]

 1, 1A Silicon substrate 2 Step 2A Lower surface 2B Upper surface 3 Positive photoresist film 4 Mask 4A, 14A First translucent area 4B, 14B Second translucent area 5 KrF excimer light 6A, 6B Line pattern 7 Mask Light intensity after passing 8, 8A, 8B Forward image 9A, 9B Inverted image 10, 10A Focus position 11 Light intensity ratio of lower stage 12 Light intensity ratio of upper stage 16A, 16B Space pattern

Claims (2)

[Claims] 1. A step of forming a photoresist film on a main surface of a substrate having a step, and a first light-transmitting region for forming an optical normal image (or an inverted image) on a lower surface of the step. The photoresist film is exposed and developed using a mask having a second light-transmitting region for forming an optically inverted image (or a normal image) on the upper surface of the step, and then the lower surface and the A pattern forming method, wherein a pattern made of the photoresist film is simultaneously formed on the upper surface. 2. The exposure is performed under the condition that the distance between the focal points of the optical normal image and the optically inverted image is equal to the step of the substrate.
The pattern forming method described in the above.