Semiconductor device manufacturing: process – With measuring or testing – Optical characteristic sensed
Reexamination Certificate
2002-07-01
2004-08-24
Thompson, Craig A. (Department: 2813)
Semiconductor device manufacturing: process
With measuring or testing
Optical characteristic sensed
Reexamination Certificate
active
06780659
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a stencil mask wherein the shape of a stencil hole is corrected, a method of producing the stencil mask, a semiconductor device produced using the stencil mask and a method of producing the semiconductor device.
Semiconductor devices continue to be refined more and more such that the patterning thereof with light is getting more and more difficult. Thus, lithography techniques based on an X-ray, an electron beam, an ion beam and so forth have been proposed, investigated and developed.
One of the novel techniques uses a stencil-like mask formed from a plate which does not allow a used beam to transmit therethrough and having a hole of a desired pattern shape formed therein for allowing the beam to pass through, which is different from a mask structure conventionally used in light lithography and formed from a hard plate which allows a used beam to transmit therethrough and having a pattern provided thereon by a substance which intercepts, absorbs or scatters the beam.
The stencil mask is in most cases formed as a thin film in order that the pattern accuracy may not be deteriorated by a beam reflected by a side wall of a hole in the stencil mask when the beam passes through the hole.
In a process of the formation of a stencil mask, a thin film before a pattern is formed, that is, before a stencil-like hole is perforated, is in an equilibrium state in a condition wherein the initial stress which appeared upon crystal growth and the internal stress, such as stress caused by a mask structure, remains; and, when a stencil-like hole is formed, the internal stress at the pattern portion of the hole is released. Consequently, the pattern itself is distorted and distorts some other pattern. Particularly, in the case of a stencil mask of a thin film, depending upon the material, the distortion is so great that it cannot be ignored.
Further, generally in the case of a stencil mask, since the shape of the mask material itself is a pattern to be transferred, there is a restriction that a pattern having a shape which itself cannot be kept from a condition of a pattern shape, a material, a thickness or the like, such as a doughnut-shaped pattern or a long cantilever structure, cannot be formed. Therefore, a system called complementary mask is used. According to the complementary mask system, transfer from a mask to a wafer is not performed by irradiation with a single mask, but by successive irradiation with a plurality of masks produced in advance and representative of different divisional parts of an object pattern to transfer the object pattern to a wafer. Since the distortion called in question here appears in a different fashion depending upon the pattern shape, linkage of the pattern may not be performed accurately between different irradiation cycles.
Similarly, because the distortion depends upon the pattern, degradation of the overlapping accuracy between different mask layers of the pattern occurs.
Various contrivances for a process of production of a mask and contrivances for a mask structure for reducing the internal stress which causes the distortion or the degree of influence of the stress upon the distortion have been proposed. Separately from the contrivances, a method of predicting distortion caused by internal stress by a stress analysis or the like and producing a mask with a pattern corrected in advance using a result of the prediction is disclosed in Japanese Patent Laid-Open No. Hei 9-326349 or No. Hei 9-218032. The method disclosed therein relates to lithography wherein an X-ray is used as a light source and is directed not to a stencil mask but to the elimination of distortion by a pattern of a material which absorbs an X-ray because a substrate material which transmits an X-ray therethrough is thin. The method is characterized in that, in order to save the processing time for a stress analysis in the process of elimination of distortion, the shape of the absorber for realizing the pattern to be transferred is not directly stress-analyzed but the area density of the absorber is used as a film thickness upon the stress analysis.
From the point of view of elimination of the distortion, it is considered that, in principle the techniques described above can also be applied to a stencil mask. However, in order for the area density to represent a pattern to be handled, the following prerequisites are required: each pattern to be handled must be small, a variation in shape thereof can be ignored, and the distortion must appear only at the position of the pattern and not in the shape itself. Among patterns of an actual LSI, a very great pattern when compared with the size of contacts which form scribe lines and so forth is present at an outer peripheral portion of a chip even with a mask for a contact layer, which only includes patterns of almost the same shape within a chip area. If the patterns including such very great patterns are handled with the area density, then the distortion calculation in each of the areas which include the very great patterns involves great errors.
Thus, it is demanded that stress correction of a stencil mask be performed by calculation with a high accuracy and at a high speed to correct patterns, even if a large-scale pattern is involved.
Internal stress acting in a stencil mask is known if the history of the material and the process of production of the stencil mask are known. The distortion can be readily calculated in accordance with a technique of the strength of materials which uses stress information and information of pattern shapes and properties of the mask material. Since the stencil mask is in the form of a flat plate, the plane stress analysis can be applied satisfactorily as the analysis means, and usually the finite-element method is used as the particular calculation method.
In order to perform the plain stress analysis in accordance with the finite-element method, the shape of the object of the analysis is divided into simple elements. If the object of the analysis is, for example, a stencil mask
11
shown in
FIG. 3A
, which is shaped so as to have a single large stencil hole
12
and four small stencil holes
13
, then the surface of the stencil mask
11
, except for the stencil holes
12
and
13
, is divided into an aggregate of simple triangular elements, as seen in FIG.
3
B.
Although the elements may have various forms such as quadrangles or complicated elements having nodes for analysis on the sides thereof, a triangle is the simplest form and is utilized frequently.
In the case of a triangle element, it is determined that the displacement amounts at the nodes of each element, that is, at the vertices i, j, k of a triangle, in the direction of the X-axis caused by stress are represented by U
i
, u
j
, U
k
and the displacement amounts in the direction of the Y-axis are represented by v
i
, v
j
, V
k
, as seen in FIG.
4
. These displacement amounts can be determined by a stress analysis.
In the related art described above, the original pattern is corrected so that a desired pattern shape may be reached as a result of the displacement of the nodes. Strictly, therefore, mathematical processing is applied to determine an inverse function or the like to determine the amounts to be corrected. This, however, requires complicated processing and involves a more exact calculation than is necessary.
In order to eliminate such a drawback, a process illustrated in the flow chart of
FIG. 5
is performed. Referring to
FIG. 5
, in the process illustrated, desired mask pattern data are prepared as an original pattern first at step S
1
, and are then copied to produce a first corrected pattern at step S
2
. A stress analysis is then performed for the first corrected pattern at step S
3
, and displacement amounts obtained by the stress analysis are regarded as correction amounts of negative values and subtracted from coordinate values of the nodes of the original pattern. A result of the subtraction is regarded as a second corrected pattern and a stress analysis of the second correcte
Kananen Ronald P.
Rader & Fishman & Grauer, PLLC
Thompson Craig A.
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