JPS5945218B2 - photomask - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a photomask for photolithography used in the manufacture of semiconductor devices.

Semiconductor technology has made remarkable progress in recent years, and LSIs are becoming more and more highly integrated.

As an important process for obtaining highly integrated LSI,
There is a photolithography process, and it is necessary to faithfully reproduce the mask pattern shape on the semiconductor material, but it is quite difficult to faithfully form small-area openings on the semiconductor material substrate with steps. That's a problem. For example, as shown in FIG. 1, openings 4 and 5 are formed in the Si oxide film 3 at points A and B at different height levels of a semiconductor material composed of a Si substrate 1, polycrystalline Si 2, and a Si oxide film 3. In the process of exposing the surfaces of the Si substrate 1 and polycrystalline Si2, the second
As shown in FIG. 3, the surface Si oxide film 3 of the semiconductor substrate 1
For example, after coating a positive type photoresist 6 (described below as an example of a positive type resist), pattern exposure is performed by ultraviolet light irradiation using a photomask 1,
Thereafter, openings are formed in the Si oxide film as shown in FIG. 1 by developing the photoresist and etching the Si oxide film. At this time, if the surface of the substrate to which the photoresist is applied has a shape with uneven steps, the photoresist applied thereon will not faithfully reproduce the uneven steps.
The photoresist applied to the concave portions is thicker than the photoresist film on the convex portions, and the surface of the photoresist becomes almost smooth as shown in FIG. 2, which is a remarkable tendency especially in the case of positive type photoresists. When exposing the pattern of the photomask 1 to the photoresist 6 having different film thicknesses as described above, in order to faithfully reproduce the mask pattern at point A in FIG. However, if the amount of exposure becomes excessive at this time, the S
The photoresist is decomposed to the periphery of the pattern shape due to reflected light from the surface of the i-oxide film, making it impossible to faithfully reproduce the pattern shape.

On the other hand, at point B, if exposure is performed with an appropriate exposure amount corresponding to the photoresist film thickness TA at point A, the photoresist film thickness at point B T
Since B has the relationship TB>TA, the region 6B decomposed by light irradiation does not reach the bottom surface of the photoresist. Therefore, even after the photoresist is developed, undecomposed resist remains at point B, and if this photoresist is used as an etching mask for the Si oxide film,
The Si oxide film at point B will not be opened. On the other hand B
When the exposure amount is determined based on the photoresist film thickness TB at point A, overexposure occurs at point A, where the photoresist film thickness is thin, and the composition of the photoresist is decomposed by light irradiation including the Si substrate and SiO2 reflected light in area 6A. When openings 4 and 5 are formed in the Si oxide film 3 using the photoresist 6 as an etching mask for the Si oxide film 3, the dimensions of the openings in the Si oxide film 3 are as shown in FIG. like,
At point B, the mask pattern dimensions can be roughly reproduced, but at point A
At the points, openings are formed that are larger than the mask pattern dimensions. In particular, the high density of semiconductor devices, which has become increasingly popular recently,
Faithful reproduction of mask patterns is an important factor in achieving high integration, and the above problem is one of the causes that hinder high density. Note that although the above example uses a positive resist, the same problem occurs when a negative resist is used.

An example of this is shown in FIG. 6. Since the negative resist 6 forms a pattern by polymerization and curing by light irradiation, the shadowed area (non-polymerized area) of the mask when overexposure occurs at point A in FIG. The influence of reflected and scattered light on the photoresist is opposite to that of the positive resist, and the opening of the Si oxide film 3 on the Si substrate 1 is
The hole is smaller than the pattern of the photomask 7, and in extreme cases, the hole may not be opened. As mentioned above, whether it is a positive resist or a negative resist, on a substrate with unevenness,
This means that the photomask pattern cannot be faithfully reproduced.

The present invention proposes a photomask that makes it possible to give an appropriate amount of exposure to each area of a photoresist film that is made up of two areas with different photoresist film thicknesses and different appropriate exposure conditions. be. The present invention will be described below based on examples together with drawings.

FIG. 7 is a schematic cross-sectional view showing one embodiment of the photomask of the present invention, and FIG. 8 is a plan view of the same.
It is a local enlarged view of a cross section.

As shown in FIG. 7, the structure is such that a normal mask pattern 9 made of chromium or chromium oxide is formed on the surface of a glass substrate 8, and a mask pattern 9 made of chromium or chromium oxide is formed on the back surface of the glass substrate 8.
3. Alternatively, a transparent pattern electrode 10 made of SnO2+In2O (5%) or the like is positioned on the opposite side of the transmission window 12 of the transmission window 11 of the mask pattern 9 where the amount of transmitted light should be regulated by means such as photoetching. and form it.

Further, as shown in FIG. 8, the electrode 10 has a structure in which the electrodes are connected by a wiring material 13 made of the same material as the electrode material, and a voltage is applied to all the electrodes on the glass substrate 8. On the opposite surface of the transparent pattern electrode 10, SnO2, In2O3, S
A glass plate 15 on which a transparent electrode 14 such as nO2+In2O3 (5%) is formed on the entire surface is arranged with a gap 16 of several microns to several tens of microns, and the gap 16 between the glass substrate 8 and the glass plate 15 is
In this example, a dynamic scattering mode (hereinafter abbreviated as DSM) liquid crystal 17 is injected with the liquid crystal molecules oriented so that the long axis is perpendicular to the glass wall surface. When light is irradiated from the transparent electrode 14 side of a photomask having the above configuration, when no voltage is applied between the transparent electrode 14 and the transparent pattern electrode 10, the incident light 18 is slightly attenuated, but the amount of incident light is almost the same. A transmitted light 19 equal to 18 is obtained from the transmission windows 11,12.

When an AC voltage (DC voltage is also acceptable) is applied between the transparent electrode 14 and the transparent pattern electrode 10, the alignment of the liquid crystal located between the two electrodes is disturbed, and the incident light 18 is scattered and the reflected light 1
8I, the amount of transmitted light 19I is attenuated, and a difference occurs in the amount of transmitted light between the amount of transmitted light 19 that passes through the normally aligned portion to which no electric field is applied.

Figure 9 is a characteristic example showing the relationship between the voltage applied to the liquid crystal and the transmittance. As can be seen from this characteristic, the amount of transmitted light can be controlled over a fairly wide range by adjusting the voltage applied between the electrodes. It becomes possible. If the above photomask is used to form a pattern on photoresist which has different photoresist film thicknesses and different appropriate exposure doses required for decomposing the photoresist as shown in Fig. 4, the photoresist film thickness will be TA in area A. Exposure is performed using the transmission window 12 of the photomask of the present invention, and the exposure of the B area with a film thickness of TB is performed using the transmission window 11, and the exposure amount of the A area is determined based on the exposure amount of the B area.
By controlling the exposure using the transmittance of the liquid crystal, the photoresist in each region is exposed to the appropriate amount of light, and a photoresist mask pattern that is faithful to the pattern mask can be formed even on a sample where the resist film thickness differs due to uneven steps. can.

In addition to the DSM, the amount of transmitted light can be controlled in a low voltage range by using a twisted nematic type liquid crystal with polarizing plates arranged on both sides of the liquid crystal layer.

However, in this case, the change in transmittance with respect to the applied voltage is
It is sharper than M, and also has the disadvantage that the structure of the photomask is slightly more complicated. The structural embodiment of the photomask of the present invention has been described above using a photomask for forming openings using a positive resist as an example, but the basic idea is the same when using a negative resist, and the gap between the transparent pattern electrodes is If the wiring and the like to be connected are designed with consideration, it is possible to obtain a highly useful photomask similar to this embodiment.

As explained above, the photomask of the present invention can control the amount of light at the convex portions of a semiconductor substrate having convex and convex portions by disturbing the alignment of the liquid crystal. can be formed.

[Brief explanation of the drawing]

1 to 6 are cross-sectional views for explaining conventional photomasks, FIGS. 7 and 8 are structural cross-sectional views and plan views showing one embodiment of the present invention, and FIG. 9 is a DSM type liquid crystal. FIG. 3 is a characteristic diagram showing the relationship between applied voltage and transmittance. DESCRIPTION OF SYMBOLS 1...Semiconductor substrate, 3...Si oxide film in which an opening is formed, 6...Photoresist, 6
A, 6B...Soluble area of photoresist, 7...
... Conventional photomask, 8 ... Glass substrate, 9 ... Mask pattern, 10 ... Transparent pattern electrode, 13 ... Transparent pattern electrode connection Wiring, 14...Transparent electrode, 15...
Glass plate, 17...Liquid crystal, 18...Incoming light, 19,19''...Transmitted light.

Claims (1)

[Claims] 1. A first transparent substrate having a mask pattern formed on its front surface and a first transparent electrode formed at a predetermined position on its back surface;
a second transparent substrate installed with a gap from the first transparent substrate and on which a second transparent electrode is formed at a position facing the first transparent electrode; and the second
and a liquid crystal sealed between the transparent substrates, and the first
and applying a voltage between the second transparent electrodes to disturb the orientation of the predetermined portion of the liquid crystal, thereby making the amount of light transmitted between the predetermined portion and other portions of the liquid crystal different. Photomask.