Single level masking process with two positive photoresist layers

This single level masking process includes the use of two layers of a positive photoresist. A pattern is formed in the first layer of photoresist. This photoresist pattern is heated and polymerized to a degree which permits it to be resistant to attack when covered with a second layer of the same positive photoresist, that is, the first photoresist pattern will maintain its integrity. After the heat treatment, the first layer pattern is substantially insensitive to actinic radiation and is easily stripped with conventional solvents. A pattern is formed in a second layer of photoresist that is different from the pattern formed in the first layer. After a first metal is deposited on portions of the substrates exposed in the second layer pattern, the second layer pattern is removed. A second metal is deposited on the portions of the substrate exposed in the first layer pattern and then that pattern is removed.

FIELD OF THE INVENTION 
This invention relates to fabrication of microelectronic devices, and more 
particularly the fabrication of magnetic bubble domain chips having 
multiple metallic films thereon which are provided by a process having 
only a single critical masking step. 
DESCRIPTION OF THE PRIOR ART 
In the fabrication of microelectronic devices, such as semiconductor 
devices and bubble domain devices, it is frequently necessary to form 
multiple layers of material which must be in accurate alignment with one 
another. For instance, in the fabrication of magnetic bubble domain 
devices, it is often necessary that the conductor layers be precisely 
aligned with propagation layers used to move magnetic bubble domains in 
the magnetic material. This is described fully in the IBM Technical 
Disclosure Bulletin article, appearing in Vol. 15, No. 6, November 1972, 
at page 1826. In that article, a plurality of masking steps and alignments 
are used to provide the final device structure. 
In the bubble domain art, attempts have been made to provide improved 
fabrication processes which will not require extensive use of high 
resolution masks and which will not require multiple masking steps where 
critical alignments must be maintained. For instance, A. H. Bobeck et al 
describes such a process in IEEE Transactions on Magnetics, Volume MAG-9, 
No. 3, September 1973, at page 474. In the process of Bobeck et al., 
single level metallurgy for producing bubble domain devices is described. 
In particular, a shadow mask is used to protect the sensor area of the 
bubble domain chip during deposition of the conductor layers which are 
used for various device functions and for providing current to the sensor. 
In a copending patent application, Ser. No. 555,645 filed Mar. 5, 1975, now 
U.S. Pat. No. 3,957,552, and assigned to the assignee of this application, 
describes a method for making multilayer devices using only a single 
critical masking step. A two metal layer -1 pattern structure is formed 
with the use of a positive photoresist layer and a negative photoresist 
layer. The patent to Horst, U.S. Pat. No. 3,873,313 describes a process 
for forming a resist pattern using a positive photoresist layer and a 
negative photoresist layer to form a two metal layer -1 or 2 pattern 
structure. 
The use of a double coating of a positive photoresist is described in an 
IBM Technical Disclosure Bulletin, Vol. 10, No. 12, May 1968 at p. 1865. 
In that article the double coating of the positive photoresist was used to 
form a single pattern which was pinhole free. This method is suitable for 
a two metal layer -1 pattern structure. 
In the single level metallurgy process the resolution required exceeds that 
normally obtained in conventional photolithography. As a result expensive 
equipment and materials such as E-beam, X-ray and deep UV exposure units 
and polymethylmethacrylate photoresist which are not suitable for 
widespread use are required. 
SUMMARY OF THE INVENTION 
It is a primary object of this invention to provide a new method for 
selectively metallizing a microelectronic device. 
It is another object of this invention to provide a method for metallizing 
having high resolution. 
It is still another object of this invention to provide a photolithography 
method having a high degree of resolution. 
These and other objects are accomplished by a method that utilizes two 
layers of a positive photoresist. The first layer of photoresist is 
exposed to actinic radiation through a first mask. After developing, the 
exposed portion of the first layer is removed to form a first pattern 
remaining in the photoresist layer. This pattern is baked at a 
temperature, for example, 105.degree. C, for a time period, for example, 
16 hours, in order to sufficiently insolubilize the first pattern in 
positive photoresist and to cause the first pattern to be substantially 
insensitive to further exposure of actinic radiation. A second layer of a 
positive photoresist is applied on top of the first pattern. The second 
layer of photoresist is exposed to actinic radiation through a second mask 
which is different from the first mask. The second layer is developed and 
the exposed portions of the second layer are removed to form a second 
pattern. A first metal is then deposited on the substrate in those exposed 
areas common to the first and second layer patterns. A suitable metal for 
this first layer is gold. The second pattern is removed by exposing to 
actinic radiation and dissolving with developer without affecting the 
integrity of the first pattern. A second metal is deposited on the 
portions of the substrate exposed in the first pattern. The first pattern 
is then removed. 
Other objects of this invention will be apparent from the detailed 
description reference being made to the accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION 
This metallization process involves the use of two layers of a positive 
photoresist in which the first layer of photoresist is subjected to a 
treatment which permits it to retain its integrity during the processing 
of the second layer of photoresist. 
As shown in FIG. 1 a microelectronic device 10 has a substrate 12. The 
substrate 12 in a bubble domain device consists of, typically, a layer of 
gadolinium gallium garnet having an LPE (liquid phase epitaxial) film 
thereon of a material suitable for supporting bubble domains therein. 
Examples of LPE films for bubble use are Y.sub.1.95 Sm.sub..09 Lu.sub..09 
Ca.sub..87 Ge.sub..87 Fe.sub.4.13 O.sub.12 and Y.sub.1.8 Sm.sub..1 
Tm.sub..2 Ca.sub..9 Ge.sub..9 Fe.sub.4.1 O.sub.12. It is understood, of 
course, that in semiconductors that the substrate would be silicon or 
silicon dioxide or other suitable material. 
On top of the substrate 12 is a layer of positive photoresist 14. A 
positive photoresist becomes more soluble upon exposure to actinic 
radiation. Positioned between the photoresist layer 14 and the substrate 
12 is a layer of metal (not shown) for plating such as Permalloy metal 
which is about 225 angstroms thick. This conductive layer is generally 
positioned on top of a spacer layer (not shown) which is on top of the 
substrate 12. The positive photoresist 14 may be any of the positive 
photoresist well known in the art. An example is AZ-1350J, a positive 
photoresist sold by Shipley which contains n-cresol formaldehyde novolak 
resin, napthoquinone diazide, cellosolve acetate, butyl acetate and 
xylene. 
As is the practice in the art, the layer of positive photoresist 14 is 
subjected to a prebake step to remove excess solvent. Typically, the 
prebake step is done at a temperature of the order of 85.degree. C for a 
time period of about 20-45 minutes. 
As shown in FIG. 2 a mask 16 is placed on top of photoresist layer 14. The 
mask 16 is a high resolution mask and no alignment is required. Portions 
of the photoresist layer 14 are exposed to actinic radiation. 
After the exposure to actinic radiation the positive photoresist layer 14 
is developed. The photoresist portions that have been exposed to actinic 
radiation are dissolved in a solution of a conventional developer to form 
a first layer pattern 18 as shown in FIG. 3. 
In accordance with this invention the pattern 18 is subjected to a heat 
treatment in order to sufficiently insolubilize pattern 18 in the original 
positive photoresist material as well as to cause pattern 18 to be 
substantially insensitive to any subsequent exposure to actinic radiation. 
With respect to the solubility characteristics, it is necessary that the 
first pattern be sufficiently insoluble in the photoresist so that the 
pattern does not dissolve in the photoresist and thereby lose its 
integrity and its resolution. On the other hand the pattern must not be 
too insoluble since it must be removed by typical solvents. If the pattern 
is too insoluble and not soluble in typical photoresist solvents, it would 
be necessary to use acids which etch away the photoresist. The use of acid 
etches is to be avoided since acid etches are deleterious to the 
metallization. Since pattern 18 is a positive photoresist material, it is 
necessary to desensitize this material so that the portions of pattern 18 
that are exposed to actinic radiation in forming a second pattern will not 
become more soluble. 
It has been determined that a heat treatment involving the baking of 
pattern 18 at a temperature of higher than 95.degree. C and less than 
120.degree. C will satisfy the requirements set forth above. The preferred 
temperature is of the order of 105.degree. C. Typically, the baking step 
requires a time of 6 hours or more, preferably of the order of 16 to 18 
hours. Longer times may be used if more convenient. Temperatures below 
105.degree. C, that is between 100.degree. C and 105.degree. C may require 
a somewhat longer time than the 6 hours. Temperatures above 105.degree. C, 
that is from 105.degree. C to 120.degree. C may require less time than 6 
hours. At temperatures between 110.degree. C and 120.degree. C the 
photoresist material starts to reflow. As a result, heat treatment at 
temperatures in this range should not be continued for a time period in 
which reflow occurs. In otherwords the heat treatment at elevated 
temperatures is for a time preferably less than that required to produce 
reflow. 
As shown in FIG. 4 the pattern 18 is covered with a layer of positive 
photoresist layer 20. The positive photoresist in layer 20 is similar to 
the positive photoresist in layer 14 described above. 
A mask 22 is positioned on top of photoresist layer 20 as shown in FIG. 5. 
The photoresist layer 20 is then exposed to actinic radiation through the 
openings through the mask 22. The mask 22 used on the photoresist layer 20 
is different than the mask 16 used on the photoresist layer 14. Because of 
this mask difference, there are areas or portions of pattern 18 which are 
exposed to actinic radiation in the same fashion as the photoresist layer 
20. During this exposure step the portions of layer 20 not covered by mask 
22 is made more soluble, as is the practice with positive photoresist. 
However, the portions of the pattern 18 that are exposed to the same 
actinic radiation are not made more soluble because they have been 
previously made insensitive to the actinic radiation by the heat 
treatment. 
The exposed portions of layer 20 are dissolved in developer and removed to 
leave pattern 24 in the second layer as shown in FIG. 6. A metal is then 
plated through the openings in pattern 24 which extends through the 
openings in pattern 18 to the substrate 12 as shown in FIG. 7. The plating 
is made possible by the thin film of Permalloy metal or other metal not 
shown in the figure but described in the description on FIG. 1. While any 
metal may be deposited, gold is an example of a metal commonly used for 
bubble domain devices. The metal 26 is deposited in the openings 
previously mentioned. 
As shown in FIG. 8 the pattern 24 is then removed by exposing to actinic 
radiation and dissolving in developer. The pattern 18 is unaffected since 
it has previously been made insensitive to actinic radiation by heat 
treatment and is substantially insoluble in the developer used on the 
pattern 24. A metal 28, such as Permalloy metal, is then deposited on 
substrate 12 in all openings of the pattern 18. The metal 28 is also 
deposited on top of the metal layers 26 to form a double metal layer in 
those places. The double metal layers 26 and 28 are useful for particular 
functions in bubble domain devices. It is understood, of course, that 
other metals other than the Permalloy type may be deposited in this 
metallizing step as well as the previous metallizing step. 
As shown in FIG. 9 the pattern 18 is removed by the use of a suitable 
solvent, for example, Acetone. The Permalloy or metal layer that was 
described in FIG. 1 (although not shown) is then removed by sputter 
etching to form the structure as shown in FIG. 9. 
EXAMPLE NO. 1 
A first layer of AZ-1350J positive photoresist 1.2 micron thick was spun 
onto a garnet substrate and subjected to a prebake treatment at 85.degree. 
C for 45 minutes to remove excess solvent. The first layer of photoresist 
was subjected to actinic radiation for 5 seconds through a high resolution 
first mask. The soluble photoresist was removed with an AZ developer and 
the resulting first pattern dried in nitrogen. The first pattern was then 
baked for 18 hours at a temperature of 105.degree. C. The pattern 
maintained its integrity during this heat treatment and no reflow of the 
photoresist was observed. A second layer of AZ-1350J photoresist 1.2 
micron thick was spun onto the top of the first pattern. The second layer 
was subjected to a prebake treatment of 85.degree. C for 20 minutes. The 
first pattern was not dissolved by the second layer of photoresist. The 
second layer of photoresist was exposed for 10 seconds to actinic 
radiation through a second mask which was different from the first mask. 
The soluble photoresist in the second layer was removed with an AZ 
developer to form a second pattern. Gold was then plated on the portions 
of the substrate common to the first and second patterns. The surface was 
then blanket exposed to actinic radiation for 10 seconds. The second 
pattern was then developed and removed. The first pattern retained its 
integrity and had not been affected by the second layer processing. 
Permalloy metal was then plated onto the areas exposed by the first 
pattern. The first pattern was then stripped with a solvent, acetone. 
EXAMPLES 2-14 
The same procedure as Example 1 was used on Examples 2-14. The results are 
tabulated in the following table. 
______________________________________ 
First Pattern 
Dis- Affected 
solved 
by 2nd 
Ex- Sub- Temp. Time Resist 
by 2nd 
layer 
ample strate 0.degree. C 
Hours Flow layer processing 
______________________________________ 
2 glass 90 48 No Yes -- 
3 glass 95 18 No Yes -- 
4 glass 100 48 No No No 
5 garnet 100 24 No No No 
6 glass 100 18 No No slightly 
7 glass 100 18 No No No 
8 glass 105 48 No No No 
9 glass 105 24 No No No 
1 garnet 105 18 No No No 
10 glass 105 6 No No No 
11 glass 110 18 No No No 
12 glass 115 18 very No No 
slight 
13 glass 120 1/2 slight 
Yes -- 
14 glass 130 1/2 Yes -- -- 
______________________________________ 
Examples 2 and 3 show that at temperatures of 90.degree. and 95.degree. C 
the first pattern is dissolved by the second layer of photoresist. 
Examples 4-7 show that a temperature of 100.degree. C a time of 18 hours 
or longer is necessary. 
Examples 1, 8, 9 and 10 show that at a temperature of 105.degree. C the 
time can be as short as 6 hours or as long as 48 hours. In Example 12 at a 
temperature of 115.degree. C for 18 hours there was an indication of very 
slight reflow of the photoresist. The time should not exceed 18 hours at 
115.degree. C. In Examples 13 and 14 at temperatures of 120.degree. C and 
130.degree. C for 30 minutes there were problems with either resist reflow 
or dissolving in the second layer of photoresist. 
Although a preferred embodiment of this invention has been described, it is 
understood that numerous variations may be made in accordance with the 
principles of this invention.