1. Field of the Invention
This invention relates to a pattern forming method used in fabricating semiconductor devices. More particularly, it relates to a method of forming a pattern serving as an etching mask, and a method of preparing a photomask for photolithography.
2. Related Background Art
Organic resists have been conventionally used when fine processing is carried out. Using the organic resists as masks, desired patterns are formed by dry etching or the like. Here, in the formation of organic resist fine patterns serving as masks, the processing dimension of organic resists would be limited to of 0.15 .mu.m at best even with an improvement in photomasks used, an improvement in energy ray irradiation apparatus and an improvement in the organic resists.
When the organic resists are developed, the processing size would slightly deviate from the desired size. Accordingly, in photolithography hitherto widely used in VLSI processes, it has been very difficult to control the processing dimension of the organic resists in a good precision of as fine as 0.1 .mu.m or less.
Meanwhile, in photolithographic techniques, photomasks previously provided with patterns of diffusion regions, separating regions, wiring, contact holes, etc. to be formed on a wafer are used and these patterns are successively optically transferred to wafers while keeping their mutual overlap precision. As a system for transferring the patterns on the masks to a wafer, a reduction projection exposure system called a stepper is prevalent at present. In the stepper, patterns on reticles (masks for the stepper are particularly called reticles) are reduced to 1/5 and transferred onto a wafer. At present, it has become possible to transfer a pattern with a width of 0.5 to 0.8 .mu.m onto a wafer at a positional precision of 0.2 .mu.m or less. At this time, the standards of dimensional precision, positional precision, etc. required for light-screening film patterns on the reticle are considered to be values obtained by subtracting errors occurring when patterns are transferred onto a wafer, from the precision actually acceptable on a wafer.
Now, a dimensional error tolerable in a reticle dimension will be estimated, which is intended for an example in which a pattern with a width of 0.5 .mu.m is formed on a wafer at a dimensional precision of .+-.10%. Assume that there are three kinds of dimensional variation factors in a process of forming the pattern onto a wafer, i.e., i) errors in reticle dimension, ii) a resist pattern formation step and iii) an etching step, and their contributions are 1/3 each, the dimensional variation error tolerable as a reticle dimensional error comes to be .+-.0.08 .mu.m around 2.5 .mu.m.
In the meantime, as a method of forming light-screening film patterns on the reticle, electron beam (hereinafter "EB") photolithography is prevalent, this is a process making use of an EB exposure apparatus as an exposure system. Its formation process and main factors of dimensional variations of the light-screening film pattern in each step are shown below.
1. Polishing and washing a light-transmissive substrate (surface flatness, scratches). PA1 2. Depositing a light-screening film on the light-transmissive substrate (layer thickness, adhesion). PA1 3. Coating an EB resist on the light-screening film (layer thickness, sensitivity). PA1 4. Exposure using the EB exposure apparatus (beam precision, amount of exposure). PA1 5. Development of the EB resist exposed (rate of development). PA1 6. Etching of the light-screening film by using the EB resist pattern as an etching mask (difference in pattern conversion).
As shown above, in the conventional EB photolithography, a pattern of highly precise electron beams irradiated from an EB exposure apparatus is first shed on an EB resist to cause a chemical change in the EB resist, and then development is carried out to convert it into an EB resist pattern. Next, using as an etching mask the EB resist pattern thus formed, the light-screening film is etched to form a light-screening film pattern. Hence, it follows that the dimensional variations of the light-screening film pattern finally formed include, in addition to the precision of the pattern of electron beams irradiated from an EB exposure apparatus, an error occurring when the pattern of electron beams is converted into the EB resist pattern, and a difference in pattern conversion produced when the light-screening film is etched using the EB resist pattern as an etching mask.
Since, however, the pattern-generating precision of the EB exposure apparatus is about .+-.0.1 .mu.m at present, it is necessary to keep the pattern-generating precision of the EB exposure apparatus and also make virtually zero the error factors that affect other dimensions and positional precision, in order to achieve the tolerable error .+-.0.08 calculated above. This fact shows that reticles necessary to form pattern dimension of 0.5 .mu.m or less on a wafer can be prepared with great difficulty when a conventional reticle preparation process is used. Hence, in order to establish the formation of fine patterns in a dimension of 0.5 .mu.m or less as a mass production technique in future, it is prerequisite to establish the technique for preparing high-precision reticles.
As the photomasks, those provided with a light-screening film pattern comprising chromium or the like, formed on a transparent substrate, have been hitherto used. However, as LSIs are made to have higher integration and patterns formed on wafers are made finer, it has been attempted to change the construction of photomasks so that the result of transferred pattern can be more improved, and there is a proposal of a technique called a phase-shift mask.
The phase-shift mask is a mask on the surface of which a pattern comprising what is called a phase shifter is formed in addition to a pattern comprising conventional light-screening film regions and light-transmitting regions. The phase shifter has a function to cause changes in the phases of transmitted light, where the positional relationship between the phase of this phase shifter and the light-screening film pattern and light-transmitting regions may be appropriately designed to form a mask. This makes it possible to obtain phase-shift masks with a higher resolution limit than conventional masks even when a projection lens of the same kind is used.
Thus, in the phase-shift mask, different from conventional photomasks, a light-screening film pattern and a phase shifter pattern must be formed on the photomask substrate. In addition, in order to obtain highly precise transfer patterns, both the light-screening film pattern and the phase shifter pattern must be formed faithfully to the designed size. Moreover, the overlap of the light-screening film pattern and the phase shifter pattern must be precise, and hence its preparation process becomes more complicated than that for conventional photomasks and various technical breakthroughs are needed.
The technical breakthroughs leading to a solution include, for one thing, a method of forming a phase shifter pattern. In a conventional photomask preparation process, the electron beam lithography is used to draw a pattern on an electron beam resist film by means of an electron beam exposure apparatus, followed by development to form a resist pattern, and a light-screening film is etched using this resist pattern as an etching mask of light-screening film. A light-screening film pattern is thus formed.
Hence, it is also possible to similarly use the conventional electron beam lithography in the formation of phase shifter patterns. However, an actual attempt to form phase shifter patterns according to the conventional process has revealed that there are the following two main problems.
One of them is the deterioration of mask surfaces that occurs when phase shifter patterns are formed by etching, and the other is a poor dimensional precision of phase shifter patterns that results when phase shifter patterns are formed by etching using an electron beam resist pattern as an etching mask.
It is difficult to avoid these two problems so long as dry etching such as reactive ion etching, capable of achieving anisotropic etching, is used as an etching process. Accordingly, it has been sought to establish a method by which phase shifter patterns can be formed without causing the deterioration of mask surfaces and also with high precision.