Resolution enhancement for alternating phase shift masks

Details Resolution enhancement for alternating phase shift masks Resolution enhancement for alternating phase shift masks

2001-08-06

2003-08-12

Huff, Mark F. (Department: 1756)

Radiation imagery chemistry: process, composition, or product th

Radiation modifying product or process of making

Radiation mask

C430S030000, C430S311000, C430S313000

Reexamination Certificate

active

06605396

ABSTRACT:

TECHNICAL FIELD
The present invention relates generally to masks used in making semiconductors devices, and more particularly to phase shift masks.
BACKGROUND
In semiconductor device manufacturing, features and geometric patterns are created on semiconductor wafers using conventional optical photolithography. Typically, optical photolithography involves projecting or transmitting light through a pattern made of optically opaque areas and optically clear areas on a mask.
A prior art mask
100
used to pattern a semiconductor wafer is shown in
FIG. 1. A
transparent substrate
102
comprising silicon quartz, for example, is provided. An opaque layer
104
is deposited over the substrate
102
. The opaque layer
104
typically comprises chrome, for example. The opaque layer
104
is patterned with the desired pattern so that light may pass through holes
106
in the opaque layer
104
when the mask is used to pattern a semiconductor wafer. This type of mask
100
is often referred to as a binary chrome-on-glass mask.
The optically opaque areas
104
of the pattern block the light, thereby casting shadows and creating dark areas, while the optically clear areas
106
allow the light to pass, thereby creating light areas. Once the light areas and dark areas are formed, they are projected onto and through a lens and subsequently onto a semiconductor substrate. However, because of increased semiconductor device complexity that results in increased pattern complexity, and increased pattern packing density on the mask, the distance between two of the opaque areas
104
is continually being decreased.
By decreasing the distances between the opaque areas
104
, small apertures are formed which diffract the light that passes through the apertures. The diffracted light results in effects that tend to spread or to bend the light as it passes through the mask
100
so that the space between the two opaque areas is not resolved, therefore making diffraction a severe limiting factor for optical photolithography. More particularly, imaging is degraded because light from clear areas
106
on the mask
100
is diffracted into regions that ideally would be completely dark. The nominally dark region has light diffracted into it from the space on either side.
A conventional method of dealing with diffraction effects in optical photolithography is achieved by using a phase shift mask (PSM), which replaces the previously discussed mask. Generally, with light being thought of as a wave, the term “phase shifting” refers to a change in timing or a shift in waveform of a regular sinusoidal pattern of light waves that propagate through a transparent material. Typically, phase shifting is achieved by passing light through areas of a transparent material of either differing thicknesses or through materials with different refractive indexes, thereby changing the phase or the periodic pattern of the light wave.
Phase shift masks attempt to reduce diffraction effects by combining both diffracted light and phase-shifted light so that constructive and destructive interference takes place. The desired result of using a phase shift mask is that a summation of the constructive and destructive interference results in improved resolution and improved depth of focus.
One particular type of phase shift mask is an alternating phase shift mask. An example of an alternating phase shift mask
200
is shown in
FIG. 2. A
transparent substrate
202
comprising silicon quartz, for example, is provided. An opaque layer
204
is deposited over the substrate
202
. The opaque layer
204
typically comprises chrome, for example. The opaque layer
204
is patterned with a desired patterned so that light may pass through holes
206
in the opaque layer
204
when the mask is used to pattern a semiconductor wafer. A phase shifting material
208
is deposited over the opaque layer
204
. The phase shifting material
208
is patterned, and portions are removed to leave transparent regions
206
and transparent phase shifted regions
209
through which light can pass through to illuminate and pattern a semiconductor wafer.
In an alternating phase shift mask
200
, alternating clear regions
209
cause the light to be phase-shifted 180 degrees, so that light diffracted into the nominally dark area from the clear area
209
L to the left will interfere destructively with light diffracted from the right clear area
209
R. This destructive interference of diffracted light results in improved image contrast, as shown in FIG.
3
.
FIG. 3
illustrates a comparison of the light intensity
110
of a conventional mask
100
such as the one shown in
FIG. 1
with the light intensity
210
of an alternating phase shift mask
200
shown in FIG.
2
. The higher slope of the light intensity
210
curve of the alternating phase shift mask
200
indicates a higher resolution and improved image contrast compared to the light intensity
110
curve of a conventional mask
100
.
The alternating phase shift mask
200
includes clear area
206
(0 degrees) adjacent clear area
209
(shifted by 180 degrees). These phase shifted clear areas
206
/
209
may interfere destructively, resulting in the light intensity distribution profile shown in
FIG. 4
at
212
. This optical image may change the topology of the resist pattern, requiring further process steps or different types of alternating phase shift masks be used to prevent or minimize this effect. For example, excess resist resulting from phase conflicts at line ends on the semiconductor wafer are often trimmed away in a second exposure step. Prior art alternating phase shift masks utilize one phase edge (region where clear area
206
abuts phase shifted clear area
208
) to pattern a feature.
SUMMARY OF THE INVENTION
The present invention achieves technical advantages as an alternating phase-shift mask and method of manufacturing thereof having improved resolution. Assist phase edges are positioned on either side of a phase edge to enhance the ultimate resolution of an alternating phase-shift mask.
A preferred embodiment of a method of manufacturing a phase shift mask includes providing a transparent substrate, patterning the substrate with a geometric pattern, the geometric pattern including a phase edge and at least one assist edge proximate the phase edge, wherein the assist edge is adapted to improve the resolution of the phase edge.
Another preferred embodiment of a phase shift mask includes a substrate that permits light to pass through, the substrate comprising a geometric pattern, the geometric pattern including a phase edge and at least one assist edge proximate the phase edge.
Further disclosed is a preferred embodiment of a method of manufacturing a semiconductor device, comprising providing a semiconductor wafer, depositing a resist layer on the semiconductor wafer, illuminating portions of the resist layer to leave at least a first illuminated resist portion, a first non-illuminated resist portion adjacent the first illuminated resist portion, a second illuminated resist portion adjacent the first non-illuminated resist portion, a second non-illuminated resist portion adjacent the second illuminated resist portion, and a third illuminated resist portion adjacent the second non-illuminated resist portion. The method includes removing at least the first, second and third illuminated resist portions, and removing at least the first non-illuminated resist portion.
Advantages of the embodiments of the present invention include enhancing the resolution of an alternating phase shift mask. The assist edges are positioned at a pre-determined distance from the main phase edge in order to improve the aerial image contrast. Smaller feature sizes may be manufactured on a semiconductor wafer than with prior art phase shift masks in accordance with embodiments of the present invention.


REFERENCES:
patent: 5286581 (1994-02-01), Lee
patent: 5348826 (1994-09-01), Dao et al.
patent: 5362584 (1994-11-01), Brock et al.
patent: 5403682 (1995-04-01), Lin
patent: 5424154 (1995-06-01), Borodovsky
patent: 5563012 (

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