Microwave curing of photoresist films

Microwave curing of photoresist films employed in processing semiconductor wafers provides an alternative to conventional drying techniques. The time of curing may be reduced from about 20 to 25 minutes required for conventional air drying to about 5 minutes employing microwave curing. Further, the photoresist film is the only part of the semiconductor assembly that experiences elevated temperatures. The remainder of the wafer remains near ambient conditions, without experiencing possible deleterious effects as a consequence of the high temperature processing.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to processing of semiconductor devices and, in 
particular, to curing of photoresist films employed during such 
processing. 
2. Description of the Prior Art 
As is well-known, photoresist films find a variety of applications in 
semiconductor processing. Typically, photoresist solution is applied to a 
substrate and is prebaked to drive off solvents, thus leaving a thin film 
as a residue. A pattern of some sort, for example, a circuitry pattern, is 
then overlaid over the film, and those portions not covered by the pattern 
are exposed to electromagnetic radiation, typically in the ultraviolet 
region of the spectrum, or to a beam of electrons of appropriate energy. 
During developing of the film employing conventional procedures, the 
exposed portions (positive resist) or the unexposed portions (negative 
resist) are removed. In the areas where the photoresist film has been 
removed, the underlying substrate becomes accessible for further 
processing. Such processing may involve, for example, the selective 
removal of an underlying oxide and the deposition of a metal contact. As a 
final step, the remainder of the photoresist film is removed. 
During the course of the foregoing processing, there are drying or curing 
steps, so-called pre-bake and post-bake steps, which are conventionally 
carried out in air at about 95.degree. to 120.degree. C. for about 20 to 
25 minutes. Such baking consumes a considerable portion of processing time 
and energy. Further, exposing the entire wafer to temperatures on the 
order of 120.degree. C. tends to have a deleterious effect on certain 
finished devices. 
Attempts have been made to speed up the baking process. For example, at the 
1970 International Hybrid Microelectronics Symposium, Beverly Hills, CA, 
November 6-18, 1970, a paper was presented by J. Karp entitled "Curing 
Photoresist Using Microwave Energy". A general purpose microwave oven was 
disclosed for curing photoresist films deposited over metallized layers on 
insulating substrates. Drying time for the resist was given as 
approximately less than 20 seconds for films of about 1 .mu.m thickness. 
The microwave oven used had a power output of 500 watts (nominal). The 
process was limited to a maximum time of about 20 seconds, as noted above, 
since curing cycles ranging between 1 and 5 minutes were found to burn the 
photoresist and/or destroy portions of underlying metallization. Following 
exposure of portions of the film and further photolithographic processing, 
the exposed portions of the cured film were then removed in a chemical 
etchant. 
In contrast to the foregoing publication which discloses use of microwaves 
to cure photoresist films, Japanese Kokai 10-43079 discloses heating 
exposed photoresist films deposited on semiconductor layers by microwave 
radiation to damage the cross-linked structure of the film, which is then 
completely decomposed by ozone. The photoresist film can thus be easily 
removed without the use of chemical etchants. 
SUMMARY OF THE INVENTION 
In accordance with the invention, a process for curing at least a portion 
of a photoresist film comprises: 
(a) applying a photoresist film to a surface of a substrate; 
(b) curing the film by subjecting the photoresist-coated substrate to 
microwave radiation for a period of time sufficient to cure the film for 
subsequent processing; 
(c) subjecting portions of the photoresist film either to electromagnetic 
radiation of sufficient intensity or to a beam of appropriately energetic 
electrons to expose portions of the photoresist film; and 
(d) developing the photoresist film to remove portions thereof. 
At a nominal power level of 400 watts and a frequency of 2.5 GHz, the 
minimum time required for adequate curing is about 3 minutes. While there 
appears to be no maximum time, economic considerations dictate a time 
shorter than about 20 minutes. 
The photoresist film that is obtained is hard-baked and is more resistant 
to chemical attack than films prepared by conventional curing. During 
processing by microwave radiation, only the photoresist film experiences 
elevated temperatures; the remainder of the substrate remains near ambient 
temperature. Finished devices consequently may be expected to evidence 
enhanced reliability. 
DETAILED DESCRIPTION OF THE INVENTION 
The process that follows is given in terms of fabricating gallium arsenide 
devices, more specifically, gallium arsenide diode lasers of the 
heterostructure type. However, it will be appreciated that the disclosed 
method may be advantageously employed in processing semiconductor devices 
employing other materials such as gallium phosphide, silicon, germanium 
and the like. Such devices may optionally have an oxide film, native or 
otherwise, or a metal film covering at least portions of the semiconductor 
surface. 
Gallium arsenide diode lasers typically comprise an n-type GaAs substrate, 
on at least a portion of which are normally grown four successive layers 
of n-(Al,Ga)As, p-GaAs [or p-(Al,Ga)As], p-(Al,Ga)As and p-GaAs as a cap 
layer. The layers of n-(Al,Ga)As and p-GaAs [or p-(Al,Ga)As] form a p-n 
junction, with central areas in the p-GaAs [or p-(Al,Ga)As] layer 
providing light emitting areas. The latter layer is often referred to as 
the active layer. The layers are conveniently formed one over the other in 
one process by liquid phase epitaxy (LPE) or vapor phase epitaxy (VPE). 
The LPE technique is generally carried out in a horizontal sliding boat 
apparatus containing four melts, as is well-known. Metal electrodes in the 
form of stripes parallel to the intended direction of lasing are deposited 
through conventional photolithography techniques onto the p-GaAs cap layer 
and provide means for external contact. A metal layer is deposited on at 
least a portion of the bottom of the substrate. Gold pads, somewhat 
smaller in area than the intended device, are sometimes formed on the 
metal layer and provide means for external contact. Like the metal 
electrodes, the gold pads are also formed by conventional photolithography 
techniques. 
In the fabrication of stripes and pads, a photoresist, for example positive 
photoresist No. AS-1350J (available from Shipley Company, Newton, MA), is 
spun onto the wafer at a fairly high rate of speed for a short period of 
time. Customarily, the solution is spun on at about 6000 rpm for about 40 
seconds. The spun-on photoresist film is then conventionally cured in a 
pre-baking step at about 95.degree. to 120.degree. C. in air for about 20 
to 25 minutes in order to drive off the solvent. Typically, the thickness 
of the photoresist is about 1.4 .mu.m. 
In accordance with the invention, the photoresist film is cured employing 
microwave radiation for a period of time sufficient to cure the film for 
further processing. 
The frequency of microwave radiation employed may be that commonly used in 
commercial microwave ovens. A suitable frequency is 2.5 GHz. The period of 
time of curing and the power experienced by the photoresist film are 
interrelated. However, due to the difficulty in measuring the power 
experienced by the photoresist film, that relationship is not easily 
specified. Rather, employing a particular microwave oven having a 
particular power output, the curing time should be sufficient such that 
further processing does not adversely affect the photoresist film. For 
example, contacting partially cured, exposed portions of a positive 
photoresist film with developer tends to undercut the unexposed portions, 
resulting in less control over subsequent metallization dimensions. 
Accordingly, simple experimentation involving curing times and subsequent 
process conditions will readily establish the minimum desirable curing 
time. For example, for a microwave oven having a nominal power output of 
400 watts, the minimum curing time required is about 3 minutes. At that 
power level, less than 3 minutes results in a photoresist film that is not 
completely cured and is susceptible to undercutting during subsequent 
processing. Five minutes results in a cured film of the requisite 
properties and is accordingly preferred. The maximum time that may be used 
is largely dependent upon economic considerations. For example, curing the 
film for 20 minutes will effect no greater improvement in photoresist 
properties than curing for 5 minutes and is identical in time to prior art 
hot air baking. The process disclosed herein is also applicable to 
negative photoresists, employing the same conditions. 
Any suitable apparatus generating microwave radiation in the range 
disclosed above will suffice in the practice of the invention. 
Advantageously, batch processing of semiconductor wafers may employ 
commercially available microwave ovens, which generally have a frequency 
in the range of about 2.5 GHz. 
Portions of the cured photoresist films are then exposed to a photomask 
pattern employing conventional photolithographic techniques. Typically, an 
ultraviolet (about 365 nm) exposure of 3.4 mW/cm.sup.2 for 30 seconds is 
employed. Alternatively, a beam of electrons having energies on the order 
of about 10 to 100 kV is typically employed. 
The photoresist film is then developed in developer, commonly in an aqueous 
solution in a ratio of 1:1, for about 30 seconds, as is conventional. For 
a positive photoresist, the exposed portions are removed by the developer. 
For a negative photoresist, the unexposed portions are removed. 
In the processing of gallium arsenide diode lasers, an oxide film is 
sometimes formed over the gallium arsenide layer. The oxide film may be 
either a native oxide, grown to a thickness of about 1250 A, or a silicon 
oxide (SiO.sub.2) layer that is grown to a thickness of about 1000 A. 
In the case of a native oxide film, during developing of the photoresist, 
the unwanted portions of photoresist are removed, along with portions of 
the native oxide that are exposed, thus exposing portions of the p-GaAs 
surface for subsequent metallization. A post-cure is then optionally 
performed, depending on the severity of the metallization process with 
respect to the remaining photoresist and native oxide films. The 
metallization process is conventional and forms no part of this invention. 
Electroplating is a comparatively severe process, since the process tends 
to lift portions of the photoresist film and remove any exposed native 
oxide, thereby resulting in wider stripes than desired. A post-curing 
step, performed after the developing step, improves the adhesion of the 
photoresist film to the native oxide film. The post-cure operation may be 
done, as in the prior art, by baking in air at about 95.degree. to 
120.degree. C. for about 20 minutes or, in accordance with the invention, 
employing microwave radiation as described above. After metallization of 
the exposed portions of the p-GaAs surface, the remaining photoresist is 
then removed. 
For a less severe process, such as vacuum deposition, the remaining 
portions of the photoresist film may be removed without removing the 
remaining portions of the native oxide film. The metal may then be 
deposited directly on the native oxide and exposed portions of the p-GaAs 
layer. 
In the case of SiO.sub.2 films (or other non-native oxide films), during 
developing of the photoresist, the developing solution stops at the 
SiO.sub.2 interface. A post-cure is then performed by microwave radiation 
as discussed above, followed by immersing the wafer in a suitable etchant, 
such as buffered HF solution, thus removing the exposed SiO.sub.2 portions 
in preparation for metallization of the exposed portions of the p-GaAs 
layer. Again, the post-cure is employed to improve adhesion of the 
photoresist film to the oxide film in order to avoid formation of 
undesirably wide stripes.

EXAMPLES 
Example 1 
A gallium arsenide substrate (n-type) having layers of GaAs and (Al,Ga)As 
thereon was prepared. The final layer was p-GaAs. A layer of SiO.sub.2 was 
deposited over the surface of the final layer. A film of photoresist 
(Shipley AZ-1350J) was formed on the SiO.sub.2 surface by spinning at 6000 
rpm for 40 sec. The assembly was cured in a microwave oven (400 W nominal 
power; 2.5 GHz frequency) for 2 min. A pattern of stripes was formed on 
the surface of the photoresist film by exposure for 30 sec at 3.4 
mW/cm.sup.2 (362 nm wavelength) through a mask of 10 .mu.m wide stripes. 
The photoresist film was then developed in developer (Shipley AZ-1350J 
Developer), 1:1 dilution in water, for 1 min with agitation, thereby 
removing exposed portion of the photoresist. After rinsing in deionized 
water for 30 sec and blow drying in nitrogen, the assembly was post-cured 
in the microwave oven for 5 min. The underlying exposed portions of 
SiO.sub.2 were removed in buffered HF (10 sec) to expose underlying 
portions of p-GaAs. After rinsing in deionized water and blow drying, the 
remaining resist was removed in acetone. The stripe widths were about 12.6 
.mu.m, or about 26% larger than mask size. 
Example 2 
A second GaAs substrate was processed as in Example 1, except that the 
microwave cure was performed for 3 minutes. The stripe widths were about 
11.25 .mu.m, or about 12.5% larger than mask size. 
Example 3 
A GaAs substrate was processed as in Example 1, except that the microwave 
cure was performed for 5 minutes and the exposure of the photoresist film 
through the mask to UV radiation was performed for 20 sec. The stripe 
widths were about 10.35 .mu.m, or about 3.5% larger than mask size. 
Example 4 
A gallium arsenide substrate was prepared as in Example 3. Metallic stripes 
were then formed over the exposed portions of the p-GaAs layer to a 
thickness of about 1000 A by plating a gold-containing film thereon. The 
gold film was also plated onto the entire n-side, which had previously 
been coated with a silver/tin film. The entire p-side was then protected 
by coating with a film of photoresist as in Example 1, except that the 
film was cured in the microwave oven for 5 min. The n-side (bottom of the 
substrate) was then coated with a photoresist film as in Example 1, 
followed by curing in the microwave oven for 5 min. A pattern of pads was 
formed on the photoresist surface on the n-side by exposure for 40 sec at 
3.4 mW/cm.sup.2 through a mask of 125 .mu.m squares. The photoresist was 
then developed, rinsed and dried as in Example 1. The film was post-cured 
in the microwave oven for 5 min. A film of gold, about 1 to 2 .mu.m thick, 
was electroplated onto the exposed portions of the n-side. After rinsing 
and drying, the remaining portions of the photoresist film on both sides 
were removed in acetone.