Double rim phase shifter mask

This invention describes the fabrication and use of a double rim phase shifter mask comprised of patterned layers of phase shifting material, partially transmitting material, and opaque material formed on a transparent substrate. Since the partially transmitting material is used as a rim at the pattern edge the light intensity transmitted to the wafer is not limited by the transmitting percentage of the partially transmitting material. The mask is self aligned and is readily manufacturable. Image resolution and depth of focus is improved over other photomasks.

BACKGROUND OF THE INVENTION 
1. FIELD OF THE INVENTION 
The invention relates to the use of a phase shifting mask to improve 
resolution in the fabrication of sub-micron integrated circuits and more 
particularly to the fabrication and use of a self aligning double rim 
phase shifter mask with phase shifting regions, half-tone or partially 
transmitting regions, opaque regions, and non phase shifting transparent 
regions. 
2. DESCRIPTION OF THE RELATED ART 
As optical lithography advances to 0.5 to 0.35 microns and below new 
technologies are needed to improve the resolution of the imaging lens. 
Phase-shifting photomasks have been used to improve resolution as well as 
depth of focus. Phase shifting photomasks are described in a paper 
"LITHOGRAPHY'S LEADING EDGE, T 1: PHASE-SHIFT TECHNOLOGY," published in 
Semiconductor International, February 1992, pages 42-47. The description 
includes rim type phase shifter in the Table on page 45. This rim type 
phase shifter uses an opaque material in conjunction with a phase shifting 
material. Phase shifting photomasks are described in U.S. Pat. No. 
5,045,417 to Okamoto. 
While improvements have been made in image resolution and depth of field, 
the linear dimensions used in integrated circuit technology call for even 
greater improvements in resolution and increased depth of field. This 
invention uses a double rim type phase shifting mask and uses a partially 
transmitting material in the mask. 
SUMMARY OF THE INVENTION 
A principle object of the invention is to provide a phase shift photomask 
which combines the advantages of rim and half-tone, or partially 
transmitting, phase shift photomasks to achieve improved image resolution 
and depth of focus tolerance. 
Another object of the invention is to provide a very manufacturable method 
of fabricating a phase shift photomask combining the advantages of rim and 
partially transmitting photomasks using a single alignment step with all 
other alignment steps being self aligning. 
These objectives are achieved by means of a double rim phase shifter mask 
wherein a patterned layer of partially transmitting material is formed on 
a transparent substrate. A patterned layer of phase shifting material is 
formed on the patterned layer of partially transmitting material with the 
edges of the patterned layer of phase shifting material extending beyond 
the edges of the patterned layer of half tone material. A patterned layer 
of opaque material is formed on the patterned layer of phase shifting 
material so that the edges of the patterned layer of half tone material 
extend beyond the edges of the patterned layer of opaque material. 
The double rim phase shifter mask combines a partially transmitting rim 
with a phase shifting rim at the edge of the opaque pattern of the mask. 
This combination provides a very sharp image at the pattern edge which 
achieves improved image resolution and depth of focus tolerance. Since 
light is transmitted through the partially transmitting material only at 
the pattern edge, the light power reaching the mask is not significantly 
reduced by the transmitting percentage of the partially transmitting 
material. The fabrication of the double rim phase shifter mask uses a 
number of self aligning steps which improves the manufacturability of the 
mask. There is only one alignment step in fabrication of the mask, with 
all other alignment steps self aligning. 
The double rim phase shifter mask is used in fabrication of integrated 
circuit wafers where improved image resolution and depth of focus are 
needed. Light is projected through the mask and focused on the surface of 
the integrated circuit wafer. Interference in the light passing through 
the phase shifting, partially transmitting, opaque, and transparent areas 
of the mask cause nulls in the intensity of the light at the pattern edges 
and thereby provides improved image resolution at the surface of the 
wafer. The partially transmitting material is present as a rim at the 
pattern edges of the opaque material in the mask and does not 
significantly reduce the light intensity transmitted through the mask.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Refer now more particularly to FIG. 1 through FIG. 4, there is shown an 
embodiment of the double rim phase shifter mask. FIG. 1 shows the 
transparent quartz substrate 25 with a thickness of between about 1 and 5 
millimeters. A patterned layer of thin chromium between about 100 and 500 
Angstroms thick is formed on the quartz substrate 25. A patterned layer of 
spin-on-glass 29 with a thickness between about 3800 and 4200 Angstroms is 
formed on the patterned layer of thin chromium 27 so that the pattern 
edges of the spin-on-glass 29 extend beyond the pattern edges of the thin 
chromium 27 by between about 0.25 and 0.5 microns. A patterned layer of 
chromium 31 with a thickness between about 1000 and 3000 Angstroms is 
formed on the patterned layer of spin-on-glass 29 so that the pattern 
edges of the patterned layer of spin-on-glass 29 extend beyond the pattern 
edges of the patterned layer of chromium 31 by between about 0.5 and 1.0 
microns. 
Light 37 passes through the double rim phase shifter mask onto the surface 
of the integrated circuit wafer. As the light passes through the double 
rim phase shifter mask it is shifted in phase by some areas of the mask 
and partially attenuated by some areas of the mask. FIG. 2 shows the 
electric field 41 of the light as it exits the double rim phase shifter 
mask. FIG. 3 shows the electric field 43 of the light at the surface of 
the integrated circuit wafer. FIG. 4 shows the intensity 45 of the light 
at the surface of the integrated circuit wafer. The image resolution and 
depth of focus have been improved by 50% over that achieved using other 
photomasks. The partially transmitting material is used as a rim at the 
pattern edge so that the light intensity reaching the wafer is not limited 
by the transmitting percentage of the partially transmitting material. 
Refer now more particularly to FIG. 5 through FIG. 10, there is shown an 
embodiment of the method of forming the double rim phase shifter mask. 
FIG. 5 shows the mask prior the formation of the photoresist layer with a 
layer of chromium 31 having a thickness of between about 1000 and 3000 
Angstroms formed on a layer of spin-on-glass 29 having a thickness of 
between about 3800 and 4200 Angstroms formed on a layer of thin chromium 
27 having a thickness of between about 100 and 500 Angstroms formed on a 
quartz substrate having a thickness of between about 1 and 5 millimeters. 
As shown in FIG. 6 the photoresist layer 33 is then formed on the layer of 
chromium 31 and exposed and developed. As shown in FIG. 7 the layer of 
chromium 31 is then over etched isotropically by means of a ceric ammonium 
and perchloric acid solution through the pattern in the photoresist layer 
33 so that the edge of the pattern in the photoresist layer 33 extends 
beyond the edge of the pattern in the chromium layer 31 by between about 
0.25 to 0.5 microns. Next, as shown in FIG. 8, the layer of spin-on-glass 
29 is vertically anisotropically etched by means of reactive-ion-etching 
through the pattern in the photoresist layer 33 so that the edge of the 
pattern in the photoresist layer 33 is directly over the edge of the 
pattern in the spin-on-glass layer 29. Next, as shown in FIG. 9, the layer 
of thin chromium 27 is over etched isotropically by means of a ceric 
ammonium and perchloric acid solution through the pattern in the 
photoresist layer 33 so that the edge of the pattern in the photoresist 
layer 33 extends beyond the edge of the pattern in the layer of thin 
chromium 27 by between about 0.25 and 0.5 microns. During this etching 
step the chromium layer 31 on the layer of spin-on-glass is again 
over-etched so that the edge of the photoresist pattern extends beyond the 
edge of the pattern in the layer of chromium on the spin-on-glass by 
between about 0.5 and 1.0 microns. The photoresist layer is then stripped 
and the double rim phase shifter mask is completed as shown in FIG. 10. 
The fabrication of the mask requires only one aligning step, that of 
exposing the pattern in the photoresist layer. All other alignment steps 
in the mask formation are self aligning. 
While the invention has been particularly shown and described with 
reference to the preferred embodiments thereof, it will be understood by 
those skilled in the art that various changes in form and details may be 
made without departing from the spirit and scope of the invention.