Photomask for Producing Semiconductor Devices

The present invention relates to an improved photomask for producing semiconductor devices.

A series of photoprocesses are used in producing semiconductor devices. Firstly, a mastermask is produced by making reduced copies of reticle patterns, by reducing each pattern to one tenth of its original size. Secondly, a copymask is produced by inversely projecting the pattern of the mastermask. Then, the pattern on the mask is transferred onto a photoresist layer which has been applied on a plain metal layer on a surface of a semiconductor substrate.

A mastermask and a copymask each comprise a transparent substrate, e.g. glass or silica, on which a mask pattern is formed of a metallic film, e.g. chromium, or a metal oxide film, e.g. chromium oxide. The contact exposure technique is commonly used in transferring the pattern from a mask, i.e., light irradiated from the back surface of a mask passes to the plain surface of a photoresist layer to be patterned, where the surface of the photoresist and that of the patterned mask make close contact with each other. However, minute particles of dust of several .mu.m unavoidably adhere to the surface of the transparent substrate of the mask.

During the time of contact exposure, images of the minute particles of dust are transferred together with the mask pattern onto the plain surface to be patterned. Thus, the transferred images of the minute particles of dust result in forming pin holes in the metal conductor when a negative type photoresist is used, and undesired residues of metal are deposited between the pattern designs, when a positive type photoresist is used, leading to disconnection and short circuiting of the metal conductor.

It is a general object of the present invention to provide a photomask, by which exact patterns are formed on a semiconductor device without being disturbed by minute particles of dust.

According to the present invention, there is provided a photomask comprising a transparent substrate, on one surface of which a mask pattern is formed of a photoshielding film, characterized in that the opposite surface to the one surface provided with said mask pattern has been roughened in order that the incident light is refracted diffusedly through this rough surface.

The roughness of the surface is preferably from 1 .mu.m to 50 .mu.m. If the roughness is greater than 50 .mu.m, it becomes difficult to obtain a sharp transferred pattern, and practically the roughness is larger than 1 .mu.m.

These together with other objects and advantages which will become subsequently apparent reside in the details of construction and operation as more fully hereinafter described and claimed, reference being had to the accompanying drawings forming a part hereof, wherein like numerals refer to like parts throughout.

FIG. 1 is a diagrammatic cross-section of the present invention;

FIG. 2 is a pictorial side elevational view of the method of operation of the present invention.

FIG. 1 is a diagrammatic section illustrating a photomask of the present invention with a substrate to be patterned.

A photomask consisting of a glass substrate 1 with a thickness of 2.5 mm is provided, one surface of which has been previously made uneven by sandblasting, the unevenness ranging from 1 to 50 .mu.m. On the other surface of glass substrate 1 patterned chromium 2 has been formed 0.1 .mu.m in thickness and 11.6 .mu.m in width. Minute particles of dust 3 of several .mu.m are show deposited on both surfaces of the substrate 1.

Rays of light incident to the uneven surface of the substrate 1 are refracted diffusedly, as shown in enlarged side elevation in FIG. 2. Because of this diffused refraction the images of the particles of dust 3 almost completely disappeared from the photoresist layer 4 applied on the aluminum layer 5 of the plain silicon substrate 6. However, the image of the patterned chromium 2 was clearly transferred onto the photoresist layer 4 but, having its width slightly increased to 11.8 .mu.m.

In addition, it has been proved that the light energy necessary for exposure increased on the order of 10%, in comparison with the case of the prior art photomask, and was sufficient to obtain substantially the same clear pattern as that obtained through the prior art photomask. The exposure time required at most is only from 1 to 2 seconds more than the known prior art. Therefore, the present invention exhibits practically similar easiness in the photoprocess technique for producing semiconductor devices.

The present invention provides a photomask capable of transferring an exact mask pattern without projecting the images of minute particles of dust and, thus, improves the yield and the reliability of the produced semiconductor devices.

In the above-mentioned example the present invention was applied to a copymask; however, it should be readily understood that the invention can be advantageously applied to a mastermask and, thus, an exact copymask can be produced.