Technique for the fabrication of an iron oxide mask

Iron oxide films for "see-through" photomasks are deposited upon heated substrates by chemical vapor deposition from iron pentacarbonyl in the presence of oxygen. The solubility of the iron oxide film is found to be higher, the lower the temperature of the substrate during deposition. At temperatures below 160 degrees C., films are obtained which can be patterned at reasonable rates by photolithographic procedures using conventional mild etchants.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a technique for the fabrication of a mask 
destined for use in resist and lithographic processes. More particularly, 
the present invention relates to technique for the fabrication of iron 
oxide photomasks utilizing chemical vapor deposition techniques. 
2. Description of the Prior Art 
The need for processing localized areas in microcircuit technology has 
generated a technology directed to the efficacious preparation and 
utilization of masks to define diffusion, evaporation and related 
operations. The use of well-known photomask processes for attaining this 
end has been universally applied in the microcircuit processing industry 
with varying degrees of success. Typically, such techniques involve the 
preparation of a suitable photomask defining the pattern of interest and 
the use of this mask to transfer an image to a photoresist pattern. 
Until recently, it had been conventional to form the mask pattern in a 
photographic emulsion. In numerous applications, the masks so formed are 
used repetitively and due to the inherent softness of photographic 
emulsions deteriorate rapidly due to abrasion. Accordingly, workers in the 
art turned their attention toward the development of masks manifesting 
greater durability. 
This end was attained by the use of hard inorganic opaque materials, 
typically metal on glass. A popular mask falling within the scope of this 
class is prepared by evaporating chromium upon a glass substrate and 
thereafter forming the desired pattern in photoresist on this surface. 
Thereafter, the pattern is etched into the chromium. These masks have been 
found to be very durable and manifest a potentially higher resolution 
capability than photographic emulsion masks, such being attributed both to 
the thinness of the deposited metal and the lack of grain and thinness in 
the image defining photoresist. Although satisfactory from many 
standpoints, such photomasks are opaque and reflect a high percentage of 
incident radiation including the light normally used during alignment of 
the photomask with respect to pattern previously imposed on the substrate. 
Both the opacity and reflectivity contribute to the difficulty of carrying 
out this alignment, especially on equally reflective metallized 
substrates. These masks also reflect the light normally used to expose the 
photoresist after alignment, so creating a problem of fringing with the 
concomitant loss of resolution at the edges of the pattern due to multiple 
reflections between the substrate and the photomask. 
Reactively sputtering a hard inorganic compound upon a glass substrate and 
etching the deposited layer to form the desired pattern represented a 
suggested approach for improved masks. Some transition metal oxides were 
found promising in being transparent to the light used by the operator to 
align the photomask with the substrate and highly absorbing at the 
wavelength used to expose the photoresist on the substrate to be 
processed. But iron oxide films which were produced in this manner and 
which had these desirable optical properties were, to the date of the 
present invention, considered unsuitable for patterning into masks by the 
usual photoresist processing because of their relative insolubility in the 
usual etchants compatible with the photoresists. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, the prior art limitations have 
been effectively obviated by a novel procedure wherein a layer of iron 
oxide is deposited upon a heated substrate member at a temperature not 
exceeding 160 degrees C. by chemical vapor deposition techniques in an 
oxidizing ambient. Films so deposited are readily etched in solvents 
compatible with conventional photoresist processing materials.

DETAILED DESCRIPTION 
With reference now more particularly to the FIGURE, there is shown a 
schematic representation of an apparatus suitable for use in the practice 
of the present invention. Shown in the FIGURE is a glove box 11 within 
which there is disposed a deposition chamber 12, positioned upon a hot 
plate 13, and connected by means of T conduit 14 to a chamber 15 capable 
of holding a source solution 16. Conduit 14 additionally includes an inlet 
17 which may be used for the introduction of an oxidizing medium thereto 
during the operation of the process. Chamber 15 also includes an inlet 18 
for the introduction thereto of a carrier gas. 
In the operation of the process, a suitable substrate, which may be 
ordinary glass or any well-known material which is transparent, over the 
range of 3000 to 6000 Angstroms, is inserted within chamber 12 and iron 
pentacarbonyl introduced into chamber 15. Thereafter, a carrier gas is 
bubbled through the source solution, so resulting in the transport of the 
iron pentacarbonyl in vapor form to deposition chamber 12. The carrier gas 
employed may be selected from among any of the well-known inert gases such 
as argon, krypton, xenon, helium, carbon dioxide, nitrogen, et cetera. The 
substrate member of interest is next heated to a temperature within the 
range of 100 to 160 degrees C. and a quantity of oxygen sufficient to form 
iron oxide is admitted to the chamber (either as an impurity in the inert 
gas or from another source), so resulting in the decomposition of the iron 
pentacarbonyl and deposition upon the surface of the substrate of a layer 
of iron oxide. Studies have revealed that the use of temperatures less 
than the noted minimum of 100 degrees C. fail to result in satisfactory 
decomposition of the iron compound and hence result in inadequate 
deposition upon the surface of the substrate. Temperatures appreciably in 
excess of 160 degrees C. have been found to result in the formation of the 
highly crystalline film typically obtained by sputtering in accordance 
with the prior art procedure alluded to hereinabove, such crystalline film 
being capable of being etched only with great difficulty. 
As indicated, it is essential in the practice of the present invention that 
oxygen be present. This may be accomplished by using an impure inert gas 
or by mixing the iron pentacarbonyl-carrier gas mixture with oxygen or a 
mixture of oxygen and water vapor. It has been theorized that iron 
pentacarbonyl reacts with oxygen upon a reactive surface. Water and other 
molecular species which can be strongly absorbed on the reactive surface 
suppress the decomposition and consequent film formation. Nonetheless, 
increasing temperatures free the surface from absorbed water and lead to 
higher deposition rates. Accordingly, it is evident that the primary 
purpose of utilizing oxidizing media is to permit greater control of the 
deposition rate and to assure the formation of iron oxide. 
The volume of carrier gas or carrier gas-oxidizing gas mixture is dictated 
by considerations relating to the desired rate of deposition. It has been 
found convenient to operate at flow rates of carrier gas within the range 
of 20 to 80 cubic centimeters per minute. However, it will be understood 
that such range is not absolute and variations may be made by one skilled 
in the art without departing from the spirit and scope of the invention. 
Similarly, flow rates of comparable range may be used with respect to the 
oxidizing medium when employed. Deposition is continued for a time period 
calculated to produce the desired thickness for photomask purposes, 
typically within the range of 1000 to 2000 Angstroms. The resultant iron 
oxide film is found to be selectively absorbing, typically exhibits less 
than one percent transmission at a wavelength of 4000 Angstroms and 
exhibits more than 50 percent transmission at 5890 Angstroms, the 
wavelength of a sodium lamp which is commonly used to facilitate 
alignment. Examples of the present invention are set forth below: 
EXAMPLE I 
A film of iron oxide approximately 1500 Angstroms in thickness was 
deposited upon a 2.times.2 inch glass substrate member approximately 60 
mils in thickness by the thermal decomposition of iron pentacarbonyl. The 
procedure involved adding 50 cubic centimeters of iron pentacarbonyl to 
source chamber 15 in an apparatus similar to that shown in the FIGURE. 
Thereafter, argon was introduced into the source chamber at a rate of 80 
cubic centimeters per minute and bubbled through the iron pentacarbonyl, 
thereby resulting in its transport to deposition chamber 12 within which 
was contained the substrate member. The substrate member was heated by 
means of a hot plate to a temperature of 125 degrees C. for a time period 
of three minutes, so resulting in the decomposition of the iron 
pentacarbonyl and the deposition upon the substrate of a layer of iron 
oxide. 
Subsequent to the deposition of the iron oxide film, it was coated with a 
commercially available photoresist, exposed to a light pattern and 
developed by conventional commercial techniques. Next, an aqueous solution 
of hydrochloric acid (typically 5.8 molar at 50 degrees C.) was employed 
to etch the pattern in the iron oxide film. Etching was completed within 
20 seconds. 
EXAMPLE II 
The procedure of Example I was repeated with the exception that deposition 
was effected at 105, 115, 130, 140 and 150 degrees C. The time required to 
effect etching was an inverse function of deposition temperature and 
ranged from about 15 seconds at 105 degrees C. to 45 seconds at 150 
degrees C. It will be appreciated by those skilled in the art that 
variations in the temperature of the etchant will result in modification 
of etching rate. 
For comparative purposes, an iron oxide photomask of comparable thickness 
to that described in the examples above obtained by reactive sputtering 
techniques was subjected to etching in the described manner and the rate 
of etching was found to be unreasonably long. 
Studies of the resultant films reveal that, from the standpoint of scratch 
resistance, they are as good or better than the chromium masks. The 
described films were also found to be superior to chromium films in that 
they are essentially pinhole free.