Semiconductor device manufacturing: process – With measuring or testing
Reexamination Certificate
1997-12-19
2001-01-16
Bowers, Charles (Department: 2823)
Semiconductor device manufacturing: process
With measuring or testing
C438S011000, C438S016000, C438S018000, C430S005000, C430S030000, C430S312000
Reexamination Certificate
active
06174741
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to semiconductor fabrication and, more particularly, to compensation for proximity effects that result during fabricating integrated circuits.
2. Description of the Related Art
Photolithography is an important part of fabricating integrated circuits. In photolithography, a light opaque pattern imprinted on a mask or reticle interposed between a radiation source and a photosensitive resist (photoresist) layer on a semiconductor wafer. If the photoresist polarity is positive, the exposed portions of the photoresist with respect to the radiation source are easily dissolved or otherwise removed in a subsequent development step. The unexposed portions of the positive photoresist remain polymerized and will not be removed during the development step. After the exposed portions of the photoresist are dissolved and removed, the resulting wafer uses the remaining patterned photoresist layer as a protective layer to, for example, block the deposition of dopants or to prevent etching of one or more layers underlying the remaining photoresist.
One type of projection photolithographic process uses a mask (containing the entire wafer pattern) which is spaced close to the wafer. In this process, no lens system is required to focus the mask image onto the wafer surface. Another type of projection photolithographic process uses a mask spaced away from the wafer, wherein a lens system interposed between the mask and wafer is used to focus the pattern of the mask onto the entire wafer.
An improved type of projection photolithographic process uses a reticle, which contains a pattern for a single die or a relatively small portion of the wafer. This process uses a stepper, wherein the reticle is mounted typically 50 centimeters to 1 meter from the wafer, and a lens system focuses the reticle pattern on a small portion of the wafer to expose the photoresist. The wafer is then slightly shifted relative to the reticle image, and the exposure process is repeated until substantially the entire wafer has been exposed by the same reticle in a repeated pattern.
As is well known, a major limiting factor in image resolution when using any photolithographic process is the diffraction of light, where light bends around the mask or reticle pattern. Due to diffraction, the mask or reticle pattern is slightly distorted when the pattern image is projected onto the wafer surface. This distortion is often referred to as the proximity effect.
With conventional projection photolithographic methods using conventional masks and reticles, line widths of lines within a dense pattern of lines formed on a wafer surface are narrower than line widths of isolated lines, even though all line widths on the mask or reticle are identical. Such is the case where a positive photoresist is used and the opaque portions of the mask or reticle correspond to the lines and other features to be formed on the wafer surface. Where a negative photoresist is used, causing the clear portions of the mask or reticle to correspond to the lines and other features formed on the wafer surface, the effect would be the opposite.
Thus, the resulting wafer contains feature sizes that are dependent upon whether a feature is isolated or within a dense pattern. This results in unpredictable feature sizes. One skilled in the art of integrated circuit design will be aware of the various problems which may result from unpredictable feature sizes, such as differing electrical characteristics. In any case, the change in feature size due to the impact of other nearby features is known as the proximity effect. Although the following discussion assumes the features are lines, the features can have any geometric shape and are not limited to lines.
FIGS. 1A and 1B
are diagrams that illustrate the proximity effect with respect to a simple metalization process. In
FIG. 1A
, a wafer
10
has an unpatterned layer of silicon dioxide
12
formed on its surface. A metal layer
14
, typically aluminum, is then deposited on the surface of wafer
10
over the silicon dioxide
12
using conventional techniques. A layer of positive photoresist is then spun onto the surface of the wafer
10
to completely coat the surface of the wafer. Using well-known techniques, the wafer surface is then selectively exposed to radiation through a reticle in accordance with a reticle pattern. The reticle pattern is represented by light blocking portions
16
and
18
, which block light (radiation) from a lamp that is used to expose the photoresist. Downward arrows represent partially coherent radiation
20
from the lamp. A lens
22
focuses the image of the reticle onto the surface of wafer
10
. The x-axis of the graph in
FIG. 1A
represents the distance along the wafer
10
surface, and the y-axis of the graph represents the resulting light intensity on the wafer
10
surface.
As seen by the intensity of light impinging upon the wafer surface, a certain low level of light intensity exists under light blocking portions
16
and
18
due to the diffraction of light, whereby the light waves traveling in straight paths bend around light blocking portions
16
and
18
. Thus, additional area of the photoresist is exposed to light due to the diffraction of light. As a result of the diffraction, the light waves from radiation
20
have constructively and destructively interfered with one another as a result of the diffraction of light. Hence, where the light intensity is increased due to constructive interference, the photoresist will be even more exposed. The extent of diffraction is a function of light coherency, numerical aperture of the lens used, and other factors, as known in the art.
It is assumed for purposes of illustration that any photoresist exposed to light above a threshold intensity level L
TH
will be dissolved away during development of the photoresist. This threshold light intensity level L
TH
is represented on the y-axis of the graph in FIG.
1
A.
After the wafer
10
is sufficiently exposed to the light pattern and after the exposed photoresist has been removed, the photoresist portions
24
remain. In this example, the width of photoresist portions
24
is 0.74 microns. Next, the exposed metal layer
14
is anisotropically etched, using well-known techniques, and photoresist portions
24
are thereafter removed with a photoresist stripper. The remaining oxide
12
may then be removed as desired. What remains is a metal pattern including parallel metal lines
26
and
28
, whose geometrics are dictated by the geometries of light blocking portions
16
and
18
and by the spaces between the light blocking portions. The width of light blocking portions
16
and
18
corresponds to metal line widths of 0.74 microns. In this example, the pitch or distance between the centers of metal lines
26
and
28
is three microns. Hence, in this example, the pitch of three microns for parallel metal lines
26
and
28
is large enough so that the diffraction from light blocking portion
16
does not influence the shape of metal line
28
and the diffraction from light blocking
18
does not influence the shape of metal line
26
.
FIG. 1B
illustrates a similar example where metal lines
30
,
31
and
32
being patterned have a smaller pitch (i.e., greater density) than did the metal lines
26
and
28
of FIG.
1
A. Here, the pitch between metal lines
30
,
31
and
32
is 1.5 microns. As illustrated, the resulting lengths of metal lines
30
,
31
and
32
are less than 0.74 microns, even though the widths of light blocking portions
40
-
42
of the reticle are identical to the widths of light blocking portions
16
and
18
in FIG.
1
A. This is because light blocking portions
40
-
42
are situated sufficiently close to one another such that the diffraction from light blocking portions
40
and
42
cause a greater amount of photoresist under the center light blocking portion
41
to be exposed above the threshold intensity L
TH
. Also, the diffraction effects from light blocking portion
41
cause a
Faul J{umlaut over (u)}rgen
H{umlaut over (a)}nsch Wilfried
Prein Frank
Bowers Charles
Braden Stanton
Lee Hsien Ming
Siemens Aktiengesellschaft
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