Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Making electrical device
Reexamination Certificate
2000-01-20
2002-08-20
Chea, Thorl (Department: 1752)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Making electrical device
C430S005000, C430S302000
Reexamination Certificate
active
06436608
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
This invention relates to lithography, such as that used for fabricating semiconductor devices. More particularly, the invention relates to patterning with phase-shifting lithographic masks.
BACKGROUND OF THE INVENTION
Lithography is utilized in semiconductor device manufacturing to pattern features on semiconductor workpiece layers for integrated circuit fabrication.
FIG. 1
shows a lithographic fabrication system
100
for defining features in a workpiece
120
, in accordance with prior art. Typically, workpiece
120
comprises a semiconductor substrate, together with one or more layers of substances (not shown) such as silicon dioxide and a resist layer
101
, affixed to a surface of workpiece
120
.
Typically, radiation of wavelength &lgr; is emitted by an optical source
106
, such as a mercury lamp or a laser. The radiation propagates through an optical collimating lens or lens system
104
, a patterned lithographic mask
103
having a pattern of opaque and transparent features, and an optical projection lens or lens system
102
. The radiation transmitted through mask
103
is imaged by lens
102
onto resist layer
101
, thereby exposing a patterned area corresponding to the mask pattern. If resist layer
101
is positive, exposed areas will be subject to removal after development and if it is negative, exposed areas will remain intact. Thus, the pattern of mask
103
is transferred to (“printed on”) resist layer
101
. “Mask” as used herein means “mask” and/or “reticle”.
As known in the prior art, the indicated distances L
1
and L
2
satisfy, in cases of a simple lens
102
, 1/L
1
+1/L
2
=1/F, where F is the focal length of lens
102
. A pattern produced by mask
103
on resist layer
101
will be substantially in focus if resist layer
101
is a distance L
2
from projection lens
102
. This conclusion is based on a geometrical optics analysis which assumes light travels in straight lines. However, when the feature size is comparable to &lgr;/NA, where &lgr; is the illumination wavelength, and NA is the numerical aperture of the projection lens, a physical optics analysis should be considered which includes the wave nature of light. Under this analysis diffraction effects are likely to be produced, decreasing the image resolution even at distance L
2
, thereby reducing resolution of component features. For semiconductor devices it is desirable to maximize the number of circuit components per unit area by minimizing component size. As component size decreases, diffraction effects become more significant, thereby limiting reduction in component size. Decreased sharpness of mask images caused by diffraction effects may reduce product yield and increase device failure rate.
Diffraction effects may be severe for conventional and binary masks.
FIG. 2A
depicts a cross-sectional view of a prior art binary mask
10
. Binary mask
10
typically comprises a glass or quartz layer
12
with a patterned chromium layer
40
affixed thereto. The patterned chromium layer comprises a plurality of substantially transparent areas
14
,
15
and
16
and a plurality of attenuating areas
18
,
19
,
20
and
21
. Electromagnetic radiation propagating through areas
14
,
15
and
16
have electric fields associated therewith. Amplitudes of the electric fields at the mask level are represented with respect to a cross-section of the mask in
FIG. 2B
, wherein steps
36
,
37
and
38
correspond to electric fields from radiation propagating through apertures
14
,
15
and
16
, respectively. Because of the wave-nature of the radiation it spreads as it propagates. Therefore, even though the electric fields are separated from one another at mask level they may interfere with one another a distance away from the mask, such as at a workpiece surface. This is shown in FIG.
2
C. Due to the diffraction effect, it is clear that the electric field at the workpiece surface spreads wider relative to that at the mask level. The smaller the feature sizes, as represented by transparent areas
14
,
15
, and
16
, the wider the spread.
Solid lines
22
and
24
in
FIG. 2C
represent electric fields from apertures
14
and
16
, respectively, and broken line
26
represents an electric field from aperture
15
. The amplitudes of the electric fields from adjacent openings (
14
and
15
, for example) overlap in cross-hatched regions
30
and
32
. As shown in
FIG. 2D
, this interference or constructive addition of electric field amplitudes results in an electric field
34
which has a higher intensity at the workpiece surface in regions
30
and
32
, relative to the surrounding areas than at mask level. Therefore, there is less contrast in the light intensity distribution at the workpiece surface than at mask level, thereby reducing the resolution capability of the tool.
Undesirable diffraction effects become more significant with small dimension pattern features. “Small dimension” as used herein means small size and small spacing between transparent regions relative to &lgr;/NA, where &lgr; is the wavelength of the optical source and NA is the numerical aperture of the projection system.
It is known in the art to improve the system resolution by employing phase-shifting masks. The mask imparts a phase-shift to the incident radiation, typically by &pgr; radians. Phase-shifting masks generally comprise transparent areas having an optical intensity transmission coefficient T, near 1.0 at the incident radiation wavelength &lgr;, attenuating areas or partially transparent areas having T at &lgr; in the range of about 0.05 to about 0.15, and, optionally, opaque areas, having T less than or equal to about 0.01.
FIG. 3A
depicts a cross-sectional view of a prior art &pgr; radian-phase-shifting mask
300
. Mask
300
is substantially similar to binary mask
10
but includes a phase-shifter layer
310
over transparent regions
14
and
16
. Phase-shifter layer
310
reverses the direction of the electric field vectors at apertures
14
and
16
relative to aperture
15
as shown in
FIG. 3B
at
320
,
322
and
330
. The &pgr; radian phase-shift is created by employing a phase-shifter layer
310
with a thickness of d=&lgr;/2 (n−1) where &lgr;is the wavelength of the optical source and n is the refractive index of layer
310
at &lgr;. The phase-shifter layer modifies the optical distance traveled by incident radiation, thereby producing a phase-shift. As is shown in
FIG. 3C
, by peaks
340
,
345
and
350
, the overlapping regions of adjacent electric fields have opposite amplitudes, and therefore, a destructive interference occurs. The cancellation of the electric field at those locations improves the contrast of the intensity field as shown in FIG.
3
D.
FIG. 3E
depicts a vector diagram of the electric field at a workpiece level produced by radiation propagating through a n radian-phase-shifting mask. Vector
380
represents an electric field from unshifted radiation such as passes through aperture
15
. Vector
390
corresponds to phase-shifted radiation such as that which propagates through aperture
14
and phase-shifter
310
. The amplitude of vector
390
equals the negative of the amplitude of vector
380
, thereby canceling it out upon interference.
Phase-shifting masks producing &pgr; radian shifts are an improvement over binary masks. However, they do not fully resolve all resolution problems, for example a phase conflict may arise for feature configurations in which a phase transmission is generally unavoidable. Whenever a phase transition occurs a dark line will result.
Electric field interference has been addressed by using a mask having a &pgr;/2 radian shift and a 3/2 &pgr; radian shift. Liebmann et al, “Alternating Phase Shifted Mask for Logic Gate Levels, Design and Mask Manufacturing”, SPIE vol. 3679 p. 27 (1999). It is also known in the art to use &pgr;:2/3 &pgr;:1/3 &pgr;:0 radian shifting masks.
It is therefore desirable to reduce phase conflict thereby substantially eliminating undesirable lines, and thus facilitating f
Agere Systems Guardian Corp.
Chea Thorl
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