Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
2000-06-19
2002-08-06
Rosasco, S. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
C430S296000, C430S394000
Reexamination Certificate
active
06428938
ABSTRACT:
BACKGROUND OF THE INVENTION
(1) Field on the Invention
The present invention relates to a phase-shift photomask (reticle) and a method of fabrication for printing high-resolution images in photoresist, and more particularly to a phase-shift photomask for making closely spaced active areas, plug contacts, and capacitors for high density Dynamic Random Access Memory (DRAM) cells on semiconductor substrates.
(2) Description of the Prior Art
Photolithographic techniques are used to pattern material layers on semiconductor substrates, such as silicon substrates, for making integrated circuits. In these techniques a photoresist layer is coated on a product substrate. The photoresist is then exposed through a photoresist mask, for example a reticle (hereafter referred to simply as a mask), that is stepped across the product substrate (wafer) and a short wavelength radiation, such as ultraviolet (UV) light, is used to expose the photoresist. Typically the mask is composed of a fused silica (commonly referred to as quartz) plate having a patterned opaque layer, such as a thin chromium (Cr) film. After exposing the photoresist on the product wafer through the mask, the photoresist is then developed to define patterns that are used in subsequent processing steps, for example as plasma etch masks, to fabricate the integrated devices. The photoresist mask and etching are used to pattern semiconductor material layers, such as silicon oxide, polysilicon, suicides, and metals on the semiconductor substrate.
As the circuit density continues to increase, such as for ultra-large-scale integration (ULSI), it is necessary to form photoresist patterns having spacings that are less than the exposure wavelength (lambda) of the UV radiation used to expose the photoresist. As is well known in the physical sciences, because of the wave nature of light, diffraction occurs at the edge of the image of the opaque film (Cr film). This diffraction results in the propagation of the light underneath the edge of the patterned Cr film, which limits the resolution when the photoresist layer on the product wafer is exposed through the mask.
To better appreciate this problem, an enlarged top view of a portion of a conventional photomask
10
, having alternating transparent areas
4
and opaque areas
2
, is shown in FIG.
1
A. The opaque areas
2
are formed by patterning a thin Cr layer on the optically transparent quartz plate, also labeled
10
. The transparent regions
4
are through the quartz plate where the Cr is removed. Typically the Cr is patterned using a photoresist layer that is exposed using an electron beam (e-beam) exposure tool. Because of the short wavelength associated with the electron beam, it provides a high resolution image that is not achieved using a longer wavelength UV. After developing the e-beam photoresist and patterning the Cr film by etching, the mask is used to expose product wafers using UV light.
When UV light is projected through the mask
10
of
FIG. 1A
to expose the photoresist on a product substrate, the UV light intensity I varies across the pattern.
FIG. 1B
shows the variation in the light intensity I across the mask as depicted for
1
A-
1
A′ in FIG.
1
A. As shown in
FIG. 1B
, the light intensity I is a maximum in the transparent regions
4
but does not decrease to zero in the opaque areas
2
because of the constructive interference of the diffracted light at the edge of the opaque areas
2
. Since the diffracted light under the opaque areas
2
arrives in phase from adjacent transparent areas
4
, the intensity I is not zero but has a finite value I
o
. Therefore as the image width decreases, it is difficult to expose and develop a high-resolution pattern in the photoresist on the product substrate.
One method of reducing the diffracted light under the narrow opaque regions is described in the reference entitled “Improved Resolution in Photolithography with a Phase-Shifting Mask” by M. D. Levenson et al. published in the IEEE Trans. on Elec. Devices, Vol. ED-29, No. 12 December, 1982, page 1828. To better understand using phase-shift masks, one approach is depicted in FIG.
2
A. As shown in
FIG. 2A
, the method involves forming alternating transparent areas
4
′ that are 180° out of phase with the adjacent areas
4
. When UV light is projected through the mask
10
to expose the photoresist on the silicon substrate, the UV light intensity I varies across the pattern through the cross section
2
A-
2
A′, as shown in FIG.
2
B. The transmitted UV light intensity I is a maximum in the transparent regions
4
and
4
′ but the UV intensity I
o
is essentially zero under the opaque areas
2
because of the destructive interference of the diffracted light from adjacent transparent areas
4
and
4
′ which have a difference in phase of 180°.
Other methods of making phase-shift photomasks have been reported. In U.S. Pat. No. 5,700,731 to Lin et al. a method is described for making capacitor bottom electrodes using adjacent regions on a phase-shift mask that have a phase shift of 180° to expose a photoresist layer. However, Lin et al. do not address the problem of closely spaced openings on a mask that results in poor depth of focus (DOF) due to the diffracted light under the opaque regions. Oi et al. in U.S. Pat. Nos. 5,541,025 and 5,761,075 teach a method of designing layouts for phase-shifting masks to compact the layout of the circuit elements without violating the design ground rule. Oi et al. do not teach a method for making the phase-shift mask. In U.S. Pat. No. 5,786,114 to Hashimoto, a method is described for making halftone attenuated phase-shift masks in which the phase-shift layer at the boundary regions of the mask is removed to reduce the light scattering between adjacent chip areas on the product substrate without requiring an additional opaque boundary area. However, Hashimoto does not address the problem associated with diffracted light between closely spaced transparent openings at the edges of a patterned opaque layer. In U.S. Pat. No. 5,702,847 to Tarumoto et al., a method is described for forming an attenuated phase-shift mask on an optically transmissive substrate and then they describe a method of using the halftone layer on the perimeter of the mask to minimize the transmittance of the radiation. This improves product reliability and turn-around time (TAT).
However, there is still a strong need in the semiconductor industry to fabricate phase-shift masks using a simplified process that is more manufacturing cost effective with greater DOF for improved photoresist resolution. These phase-shift masks are particularly useful for increasing the memory cell density on future DRAM devices having smaller dimensions.
SUMMARY OF THE INVENTION
A principal object of the present invention is to provide a phase-shift mask and a method for making this phase-shift mask for exposing photoresist with improved resolution for semiconductor devices having sub-micron dimensions, and more specifically for forming photoresist images having an improved Depth Of Focus (DOF) for active areas, plug areas, and capacitor areas on DRAM devices.
Another object of this invention is to make these phase-shift masks having patterned chrome (opaque) areas and alternate recessed areas on a fused silica plate (commonly referred to as a quartz plate) that form part of the phase-shift mask (reticle).
Still another object of this invention is to use a single photoresist masking step to reduce manufacturing cost.
This invention features a novel phase-shift photomask and a method of fabrication for improving the depth of focus (DOF) for exposing closely spaced patterns in photoresist on a product (semiconductor) substrate. This novel mask is particularly useful for making arrays of closely spaced patterns for the active device areas, polysilicon plugs, and capacitors for memory cell areas on DRAM devices. The phase-shift photomask structure minimizes diffraction at the edge of the opaque patterns resulting in greater DOF and higher resolution when the photoresist
Chin Sheng-Chi
Chou Wei-Zen
Lin Chin-Hsiang
Lin Shy-Jay
Ackerman Stephen B.
Rosasco S.
Saile George O.
Taiwan Semiconductor Manufacturing Company
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