Method of fabricating a phase-shifting photolithographic mask reticle having identical light transmittance in all transparent regions

This invention constitutes a method of fabricating a phase-shifting photolithographic mask reticle having identical light transmittance characteristic in all transparent regions. The method is applicable to those types of phase-shifting reticles that are fabricated by masking alternating transparent regions with photoresist and subjecting the exposed transparent regions to a plasma etch until the thicknesses of the transparent reticle material has been relieved to a degree sufficient to effect diffraction cancellation between neighboring transparent regions. This invention solves the problem of unequal transmittance characteristics of plasma etched transparent regions and unetched transparent regions by subjecting the reticle to a second plasma etch once the photoresist has been removed. Thus, both the transparent regions that were initially etched and the transparent regions that were initially unetched are subjected to an etch. Both types of regions are relieved further, thus maintaining the phase-shifted relationship between the two types of transparent regions, and also equalizing the transmittance characteristics of both types of transparent regions.

FIELD OF THE INVENTION 
This invention relates to semiconductor manufacture and, more particularly, 
to a novel process particularly suited to fabricating phase shifting mask 
reticles that have identical light transmisivity in all transparent 
regions. 
BACKGROUND OF THE INVENTION 
As semiconductor manufacturing advances to ultra-large scale integration 
(ULSI), the devices on semiconductor wafers must be shrunk to sub-micron 
dimensions and the circuit density must be increased to several million 
transistors per die. In order to accomplish this high device packing 
density, smaller and smaller feature sizes are required. This may include 
the width and spaces between device interconnect lines and the surface 
geometry such as contact openings and the like. 
The requirement of small feature sizes with close spacing between adjacent 
features requires high resolution photolithographic processes. In general, 
photolithography utilizes a beam of light, such as ultraviolet (UV) 
radiation, to transfer a pattern from a photolithographic reticle pattern 
onto a photoresist coating through an imaging lens. Reticles are generally 
made of laminar transparent quartz. The reticle pattern includes opaque 
regions (i.e., regions having a thin layer of chromium plating), as well 
as transparent regions. Were it not for the diffraction phenomenon, and 
were all light rays passing through the reticle parallel to one another, 
the pattern on the reticle would be transmitted exactly to the photoresist 
coating. However, when the dimensions of the opaque and transparent 
reticle regions are near the wavelength of the light utilized to project 
the image, diffraction becomes a significant problem. Regions on the 
photoresist that should be dark are illuminated with diffracted light 
rays. 
One technique currently being investigated for mitigating the diffraction 
effect, so as to improve the resolution of the photolithograhic process, 
is known as phase shift lithography. In phase shift lithography, 
diffracted light rays from adjacent transparent regions of the reticle are 
made to cancel one another, thus eliminating the diffraction effect and 
improving the resolution and depth of optical images projected onto a 
target. The cancellation of diffracted rays from adjacent transparent 
regions is effected by adjusting the light path through the various 
transparent reticles such that light passing through any transparent 
region is 180 degrees out of phase with the light passing through any 
adjacent transparent region. Thus, when light rays are diffracted from two 
neighboring transparent regions of the reticle, they cancel one another 
when they coincide at some point below the intervening opaque region. The 
mathematics employed in the construction of a phase-shifting reticle are 
well known in the art, and will not be discussed herein. 
One of the most common and successful techniques for fabricating a 
phase-shifting reticle is to take a conventional reticle consisting of a 
uniformly thick quartz layer on which a chromium layer has been patterned 
to produce a patten of opaque and transparent regions, mask every other 
transparent region with photoresist, and then subject the photoresist 
masked reticle to a plasma etch until the unmasked transparent regions are 
relieved to an extent that, when the photoresist mask is removed, rays of 
light from the coherent source used for the photolithographic exposure 
process will pass through the unetched transparent regions and exit the 
reticle one-half wavelength behind rays of light from the same coherent 
source that pass through neighboring etched transparent regions. Although 
such a process works acceptably in principle, the plasma etch damages the 
optical characteristics of the quartz so that transmittance through the 
etched transparent regions is reduced as compared to the transmittance 
through unetched transparent regions. The result is somewhat less than 
adequate cancellation of the neighboring diffraction patterns. 
SUMMARY OF THE INVENTION 
This invention solves the problem of unequal transmittance characteristics 
of plasma etched transparent regions and unetched transparent regions in a 
phase-shifting reticle fabricated by the aforedescribed process. This is 
accomplished by subjecting the reticle to a second plasma etch once the 
photoresist has been removed. Thus, both the transparent regions that were 
initially etched and the transparent regions that were initially unetched 
are subjected to an etch. Both types of regions are relieved further, thus 
maintaining the phase-shifted relationship between the two types of 
transparent regions, and also equalizing the transmittance characteristics 
of both types of transparent regions.

PREFERRED EMBODIMENT OF THE INVENTION 
Referring now to FIG. 1, a portion of a conventional photolithographic mask 
reticle is depicted in cross-sectional format. The reticle consists of a 
uniformly thick quartz plate 11 and a thin, chromium metal layer 12, which 
has been selectively etched so as to create a pattern of opaque regions 13 
and transparent regions 14. 
Referring now to FIG. 2, the conventional mask reticle of FIG. 1 may 
converted to a phase-shifting reticle by masking every other transparent 
region with photoresist 21, and then subjecting the photoresist masked 
reticle to a highly anisotropic plasma etch (an etch which etches in 
primarily one direction) until the unmasked transparent regions 22 are 
relieved to a depth such that, when the photoresist mask is removed, rays 
of light from the coherent source used for the photolithographic exposure 
process will pass through the unetched transparent regions 23 and exit the 
reticle one-half wavelength behind rays of light from the same coherent 
source that pass through neighboring etched transparent regions. Although 
such a process works acceptably in principle, the plasma etch damages the 
optical characteristics of the quartz so that transmittance through the 
etched transparent regions 22 is reduced as compared to the transmittance 
through unetched transparent regions 23. The result is somewhat less than 
adequate cancellation of the neighboring diffraction patterns on the 
surface which is to be exposed by the reticle. 
Referring now to FIG. 3, the phase-shifting reticle of FIG. 2 has been 
stripped of photoresist, and subjected to a second highly anisotropic 
plasma etch. Thus, both the transparent regions that were initially etched 
22 and the transparent regions that were initially unetched 23 are 
subjected to an etch. Both types of transparent regions are relieved below 
the original upper surface level 31 of the quartz plate 11. However, the 
relative levels of the two types of transparent regions is maintained 
during the second plasma etch, thus preserving the phase-shifting 
relationship of the two types of transparent regions. The second plasma 
etch provides the advantage of equalizing the transmittance 
characteristics of both types of transparent regions. 
Although only a single embodiment of the invention has been disclosed 
herein, it will be obvious to those having ordinary skill in the art of 
photolithographic reticle manufacture, that changes and modifications may 
be made thereto without departing from the spirit and the scope of the 
invention as hereinafter claimed.