Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
1999-07-09
2001-07-10
Huff, Mark F. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
Reexamination Certificate
active
06258490
ABSTRACT:
DESCRIPTION
1. Field of the Invention
The present invention generally relates to semiconductor fabrication technology and more particularly to a photomask used in the production of semiconductor devices.
2. Background of the Invention
The photolithography process generally involves exposing light through a photomask onto a photoresist coating on a wafer. The photomask typically includes a transparent portion and an opaque portion, thereby forming a pattern and selectively exposing the photoresist coating to the light according to the pattern. Photoresist coatings are produced from organic solutions which, when exposed to light of the proper wavelength, are chemically changed in their solubility to certain solvents (developer). For example negative-acting resist is initially a mixture which is soluble but after light exposure becomes polymerized and insoluble. Thus, when the light is exposed through a photomask having a pattern formed by its transparent and opaque portions onto the photoresist, the unexposed resist selectively dissolves, softens, or washes away, leaving the desired pattern on the underlying wafer. Positive-acting resists work in the opposite fashion, i.e., exposure to light makes the polymer mixture soluble in the developer. Thus, in positive-acting resists, the transparent portions of the mask correspond to the desired pattern or image to be provided in the resist coating. The remaining pattern then is treated through the rest of the fabrication process for forming a desired semiconductor device.
As the number of circuits printed on a semiconductor chip increases, the pattern features must become smaller, mandating a high degree of accuracy in transferring the correct pattern into the photoresist. However, as the linewidth of the features which are printed on semiconductor wafers decreases, a phenomenon referred to as “foreshortening” (line end shortening) occurs. Foreshortening is the behavior typically observed in near or subresolution lithography whereby a photoresist line prints shorter than the corresponding line design on the photomask when forcing the width of the printed line to be correctly reproduced.
FIG. 1
shows a photomask
102
with a chrome or opaque region
106
and a clear region
104
, whose mask image is printed on positive resist
108
. In
FIG. 1
, the cross hatching
106
represents chrome or opaque regions, and
112
represents photoresist. The exposure, baking, and resist dissolution process steps are adjusted in conformance to a known technique to ensure that the width of the line prints according to the desired width. As illustrated by the image printed on the resist
108
in
FIG. 1
, “foreshortening” typically occurs, so that the printed length of the line is shorter than the design dimension, even though the printed width dimension matches the width design dimension.
One known method employed to recover from such shortening of the printed length is to place serif shapes called “anchors” at the ends of the rectangular lines on a photomask. This technique of eliminating foreshortening is shown in FIG.
2
A. The anchors
204
a
are the serifs projecting from the sides of the mask rectangular shape, turning the long rectangular line into a rectangular line with serifs, making the line into a capital “I” shape. Anchors
204
a
are an effective technique for eliminating or reducing foreshortening as can be seen in the pattern
210
formed on the resist. However, there are several drawbacks associated with this technique. First, this technique increases the data volume in the mask image data sets. Second, and perhaps more importantly, as the space between shapes decreases, it is difficult to add the anchors. For example, as shown in
FIG. 2B
, due to the limited space between the shapes, the anchors
204
a
need to be spaced closely to each other, worsening another known problem in photolithography, i.e., the proximity effect. Briefly, the proximity effect is due to diffraction of light incident on the photomask, producing an undesirable distortion of the image pattern. Therefore, it is highly desirable to have a mask which reduces the “foreshortening” effect on the image patterns without increasing the undesired proximity effect.
Further, with a typical conventional mask with clear image patterns of lines and holes, the lines get foreshortened and the holes do not print at all when the process is adjusted to correctly print the width of the lines. Such an example is shown in FIG.
4
. As illustrated, the clear image region
404
on the mask
402
is foreshortened when printed as an image
410
on a positive resist
408
. Moreover, a clear hole
414
pattern on the mask
402
is not printed on the positive resist
408
. Therefore, it is also highly desirable to enable pattern formations of lines and holes present on the same mask.
SUMMARY OF THE INVENTION
The present invention is directed to a transmission controlled mask (TCM) for providing effective and accurate printing of images. Moreover, the transmission controlled mask provides the ability to accurately print a line and a hole from the same mask. The transmission controlled mask (TCM) of the present invention includes opaque regions, clear regions, and transmission controlled (TC) regions, each region have different transmittance. The opaque regions block nearly 100% of light incident on it. The clear regions generally allow nearly all light to pass through: that is, the clear regions have nearly 100% transmittance. The transmission controlled (TC) regions of the TCM according to the present invention have a transmittance between those of the clear and opaque regions, for example, generally between 5% and 95%, with 85% transmittance preferred for clear images exposed onto positive resists.
The patterns on the mask include clear regions, transmission control regions, and opaque regions. For example, a line may be formed by having the transmission controlled region as the center and the clear regions as the two opposite ends. Modeling may be performed to modulate and determine different sizes and placements of the transmission control regions in an image pattern of the mask.
Moreover, in the present invention, small holes may also be printed along with the lines from the same mask by including a line comprising transmission control regions and clear contact holes. The intensity of light which a conventional rectangular region would receive is then decreased and the exposure time increased. Such a method results in the printing of both the line and the hole.
Further features and advantages of the present invention as well as the structure and operation of various embodiments of the present invention are described in detail below with reference to the accompanying drawings. In the drawings, like reference numbers indicate identical or functionally similar elements.
REFERENCES:
patent: 4704342 (1987-11-01), Lehrer et al.
patent: 4902899 (1990-02-01), Lin et al.
patent: 5126006 (1992-06-01), Cronin et al.
patent: 5324600 (1994-06-01), Jimbo et al.
patent: 5478678 (1995-12-01), Yang et al.
patent: 5636002 (1997-06-01), Garofalo
patent: 5716738 (1998-02-01), Garza
patent: 5725972 (1998-03-01), Takeshita
patent: 5725973 (1998-03-01), Han et al.
patent: 5783336 (1998-07-01), Aoki et al.
patent: 5786114 (1998-07-01), Hashimoto
patent: 5789116 (1998-08-01), Kim
patent: 5827623 (1998-10-01), Ishida et al.
patent: 5863712 (1999-01-01), Von Bunau et al.
patent: 5914202 (1999-06-01), Nguyen et al.
Bula Orest
Cole Daniel C.
Conrad Edward W.
Leipold William C.
Huff Mark F.
International Business Machines - Corporation
Mohamedulla Saleha R.
Sabo, Esq. William D.
Scully Scott Murphy & Presser
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