Reticle Comprising Phase Shifter With a Tapered Edge

1. Field of the Invention

The present invention relates to a reticle used as a mask for forming a device pattern on a semiconductor wafer, at the photolithographic step in the production of a semiconductor. More particularly, the present invention relates to a reticle having a phase shifter capable of providing a fine pattern.

2. Description of the Related Art

According to the conventional phase shifting method, in the boundary area between a glass substrate of a reticle and a 180.degree. phase shifter the light intensity is reduced and an unnecessary pattern is formed. As a reticle overcoming this disadvantage, there is known a reticle having a multiple-stage phase shifter, whereby the phase shift quantity is stepwise changed, arranged between the glass substrate portion of the reticle and the 180.degree. phase shifter, as disclosed in 27P-ZG-6 of Collection of Preliminary Lectures at the '90 autumn meeting of Association of Applied Physics. FIG. 3a is a diagram illustrating the sectional structure of the reticle having this structure. In FIG. 3a, reference numeral 2 represents a reticle, reference numeral 7 represents a 180.degree. phase shifter, and reference numeral 8 represents a 90.degree. phase shifter. A reticle having a similar structure is disclosed in Japanese Unexamined Patent Publication No. 57-62052.

In the reticle having the above-mentioned structure, as shown in FIG. 3b, a reduction of the light intensity at the wafer during the light exposure can be controlled in the boundary area between the glass substrate and the 180.degree. phase shifter.

Nevertheless, the conventional multiple-stage phase shifter forming process is disadvantageous in that, since it is necessary to form a plurality of shifter films having a precisely controlled thickness, the reticle-preparing process becomes very complicated.

Under this background, an object of the present invention is to provide a reticle in which an effect equivalent to the effect attained by the multiple stage phase shifter forming process can be attained by a single layer process.

According to the present invention, this object is attained by a reticle in which a 180.degree. phase shifter is uniformly formed on a glass substrate of the reticle, and the edge of the phase shifter is tapered by etching at the step of forming a shifter pattern.

FIG. 1a is a diagram illustrating the section of a reticle according to one embodiment of the present invention, and FIG. 1b is a diagram illustrating the intensity of the light exposed on a wafer;

FIGS. 2a-2c are diagrams showing an embodiment of the method of tapering the shifter edge;

FIG. 3a is a diagram illustrating the section of the reticle in the conventional multiple stage phase shifter process, and FIG. 3b is a diagram illustrating the intensity of the light exposed on the wafer in the conventional reticle.

According to the present invention, by adopting the above-mentioned structure, a shifter pattern in which the formation of an unnecessary pattern due to a reduction of the intensity of the transmitted light in the boundary area between the glass substrate of the reticle and the 180.degree. phase shifter does not occur can be prepared by the single layer process.

An embodiment of the present invention will now be described with reference to the accompanying drawings. FIG. 1a is a diagram illustrating the section of one embodiment of the reticle of the present invention.

The edge of a 180.degree. phase shifter 1 on a glass substrate 2 of the reticle has a tapered structure, and thus a reduction of the intensity of the transmitted light by an interference in the boundary area between the phase shifter 1 and the glass substrate 2 is controlled as shown in FIG. 1b.

FIGS. 2a through 2c are diagrams illustrating the process of forming the shifter edge, wherein SOG as the phase shifter is formed at a baking temperature of 400.degree. C. on the glass substrate 2 (synthetic quartz) of the reticle by coating. The refractive index of the formed SOG film 3 is 1.42, and if the exposure light is an i-ray (wavelength: 365 nm), the thickness of the SOG film acting as the 180.degree. shifter is 435 nm (see FIG. 2a).

Next, the surface of the SOG film is subjected to an O.sub.2 plasma treatment to form a plasma treatment layer 4 and reduce the adhesion to a resist, and then a resist 5 is coated and a pattern is transferred by photolithography. Thereafter, a wet etching is carried out with a solution of hydrofluoric acid (see FIG. 2b).

Accordingly, a turn-around of the etchant occurs between the shifter film and the resist, and a tapered structure is formed on the edge of the shifter as shown in FIG. 2c.

The SOG film formed by the coating method (the baking temperature is 400.degree. C.) has an etching rate with hydrofluoric acid about 10 times as high as synthetic quartz, as the glass substrate of the reticle, and therefore, substantially no influence is imposed on the glass substrate 2 of the reticle.

The thickness D of the 180.degree. phase shifter can be determined from the wavelength .lambda. of the exposure light and the refractive index n of the shifter material according to the following relationship formula:

Examples of the thickness of the 180.degree. phase shifter obtained from various combinations of the shifter material and the exposure light are shown in the following table.

In the phase shifter constructed in the above-mentioned manner, the taper angle .theta. (see FIG. 1a) of the tapered structure of the pattern edge of the phase shifter is preferably 50.degree. to 80.degree..

If the taper angle .theta. exceeds 80.degree., an effect of controlling the attenuation of the light is not obtained, and thus such a large taper angle is not preferred. If the taper angle .theta. is smaller than 50.degree., good results are not attained with respect to the effect of making the device pattern fine.

As is apparent from the foregoing description, the present invention is advantageous in that a shifter pattern can be formed by the single layer process, compared with the multiple stage shifter process in which a plurality of shifter films, the thickness of which is controlled at a very high precision, must be formed.