Process for providing clean lift-off of sputtered thin film layers

A unique photoresist process is provided which achieves clean and complete lift-off of a thin film layer such as a sputtered thin film formed on a photoresist which is formed above a semiconductor substrate. The process of the present invention relies on a reentrant photoresist profile which breaks the continuity of the thin film layer. Accordingly, the process of the present invention ensures a clean lift-off. The desired photoresist profile which breaks the continuity of the thin film layer can be obtained by a typical photoresist process preceded by an oxidation process that takes place on the surface of the semiconductor substrate. The oxidation process provides a thin native oxide layer with thickness ranging from about 30 to 50 .ANG.. No extra processing steps involving dielectric film deposition and etch are required to achieve clean lift-off. Nevertheless, the process of the present invention ensures the clean lift-off of the thin film layer. Accordingly, the process of the present invention provides good visual and electrical yields.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to GaAs microwave monolithic 
integrated circuit (MMIC) fabrication and, more particularly, to the 
lift-off of sputtered metal film which is used to define metal patterns in 
GaAs MMICs. 
2. Description of Related Art 
Sputtered thin films are commonly used in GaAs MMIC fabrication. In 
particular, sputtered thin film resistors have been extensively used in 
GaAs MMIC chips for many military and communication systems. Lift-off 
techniques are commonly employed in GaAs MMIC manufacturing as well as in 
semiconductor device fabrication, in general. Lift-off techniques are 
employed to define metal patterns from sputtered metal films. Obtaining a 
clean lift-off of a thin sputtered film layer such as a sputtered metal 
film is very important in GaAs MMIC fabrication. By clean lift-off is 
meant that the edges of the sputtered metal film are not ragged and that 
the intended metal pattern is formed substantially complete in the 
sputtered metal film. Incomplete lift-off of the sputtered metal film 
causes electrical shorts and visual defects. Sputtering techniques used to 
form sputtered metal films, and other thin sputtered film layers, provide 
superior conformal deposition characteristics. As such, the attainment of 
a clean lift-off of sputtered thin film layers has been a long existing 
challenge in GaAs processing. 
At present, there are two approaches employed in obtaining lift-off of 
sputtered thin film layers: the simple photoresist process and the 
dielectric assisted lift-off. With the simple photoresist process, a 
photoresist is deposited on a semiconductor substrate such as GaAs. The 
photoresist is exposed and developed to define patterns therein. Where the 
photoresist is exposed and developed, the photoresist is removed down to 
the underlying semiconductor substrate. After development of the 
photoresist, photoresist remains only on some regions of the semiconductor 
substrate. Other regions of the semiconductor substrate thus become 
exposed. A sputtered metal film is formed on the patterned photoresist and 
on the regions of the semiconductor substrate that are exposed. The 
sputtered metal film conforms to the surface of the patterned photoresist. 
The patterned photoresist is then lifted off. Portions of the sputtered 
metal film are lifted off as well. Ideally, a patterned metal layer should 
remain intact on the semiconductor substrate. The patterned metal layer 
comprises the portions of the sputtered metal film that were formed on the 
regions of the semiconductor substrate that were exposed. 
The simple photoresist process is simple relative to the alternative 
approach, dielectric assisted lift-off, however, clean lift-off cannot be 
obtained with the simple photoresist process. Typically, the edges of the 
patterned metal layer are ragged. 
The alternative approach, dielectric assisted lift-off, involves additional 
steps comprising dielectric deposition and etch. As in the simple 
photoresist process, photoresist is employed to achieve lift-off. Prior to 
the deposition of the photoresist, however, a dielectric layer is formed 
over the semiconductor substrate. The dielectric layer may comprise a 1000 
.ANG. layer of oxide or nitride. The photoresist is then deposited on the 
dielectric layer. As in the simple photoresist process, the photoresist is 
exposed and developed to define patterns therein. Where the photoresist is 
exposed and developed, the photoresist is removed down to the underlying 
dielectric layer. Openings in the photoresist are thereby formed. These 
openings in the photoresist are defined by the edges of the patterned 
photoresist. As such, photoresist remains only on some regions of the 
dielectric layer. Other regions of the dielectric layer are exposed. 
The dielectric layer is then etched using an additional etching process to 
extend the patterns into the dielectric layer. Where the dielectric layer 
is exposed, the dielectric layer is etched down to the semiconductor 
substrate. Openings in the dielectric layer are thereby formed. These 
openings in the dielectric layer have sidewalls. The additional etching 
process, i.e., the dielectric etch, also attacks the sidewalls in the 
openings in the dielectric layer. Thus, dielectric from beneath the 
photoresist, i.e., the unexposed photoresist, is removed. The dielectric 
layer at the sidewalls is etched to form a reentrant profile in the 
dielectric layer that is formed beneath the edge of the patterned 
photoresist. The term "reentrant" is used herein in its accepted meaning 
of "directed inward". 
Upon completion of the additional etching process, i.e., the dielectric 
etch, photoresist and dielectric remain on only some regions of the 
semiconductor substrate. Other regions of the semiconductor substrate are 
exposed. As above, a sputtered metal film is formed on the patterned 
photoresist and on the regions of the semiconductor substrate that are 
exposed. Again, the sputtered metal film conforms to the surface of the 
photoresist. The photoresist is lifted off. Portions of the sputtered 
metal film are lifted off as well. After the lift-off step, the patterned 
metal layer should remain intact on the semiconductor substrate. The 
patterned metal layer comprises the portions of the sputtered metal film 
that were formed on the regions of the semiconductor substrate that were 
exposed. The dielectric layer is not lifted off along with the 
photoresist. As such, the dielectric layer also should remain intact on 
the semiconductor substrate. The patterned metal layer is surrounded by 
this dielectric layer. 
The dielectric assisted lift-off is effective in obtaining clean lift-off; 
however, as described above, an additional dielectric deposition and etch 
is required. For additional details regarding the dielectric assisted 
lift-off process see, e.g., R. Williams, "Modern GaAs Processing Methods", 
Artech House, 1990, pp. 278-279. 
Thus, there remains a need for a simple photoresist process for obtaining 
clean lift-off of thin films deposited using sputtering deposition 
techniques. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, a process for achieving clean and 
substantially complete lift-off of a thin film layer is provided. The thin 
film layer is formed over a photoresist which is formed above a 
semiconductor substrate having a surface. The process comprises the steps 
of: 
(a) forming a thin native oxide layer on the surface of the semiconductor 
substrate; 
(b) coating the semiconductor substrate having the thin native oxide layer 
formed thereon with the photoresist; 
(c) baking the photoresist; 
(d) exposing the photoresist to provide a latent pattern in the 
photoresist; 
(e) developing the photoresist to form a pattern therein using a developer 
which etches both the photoresist which has been exposed and the thin 
native oxide layer thereby forming at least one opening in the photoresist 
and the thin native oxide layer having sides and thereby creating at least 
one exposed portion of the semiconductor substrate, the developer also 
removing the thin native oxide layer from beneath the photoresist on the 
sides of the opening that has not been exposed and undercutting the 
photoresist on the sides of the opening that has not been exposed thereby 
forming a reentrant angle in the profile of the photoresist and providing 
a patterned photoresist; 
(f) depositing a thin film layer on the patterned photoresist and on the 
exposed portions of the semiconductor substrate; and 
(g) lifting-off the photoresist and the thin film layer, thereby forming a 
patterned thin film layer. 
The process of the present invention enables very clean lift-off of thin 
film layers deposited using sputtering deposition techniques to be 
achieved. The major advantage of the present invention is that the process 
for attaining clean lift-off is simplified. No extra processing steps for 
dielectric deposition and etching are required. A very thin native oxide 
layer (about 40 .ANG.) is provided on the surface of the GaAs substrate 
and a reentrant photoresist profile is created in the photoresist. This 
reentrant photoresist profile breaks the continuity of the thin film 
layer. This unique photoresist profile of the present invention ensures 
the clean lift-off of the thin film layer. Accordingly, good visual and 
electrical yields are provided. In particular, the yields of MMICs are 
improved.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Reference is now made in detail to a specific embodiment of the present 
invention, which illustrates the best mode presently contemplated by the 
inventors for practicing the invention. Alternative embodiments are also 
briefly described as applicable. 
The present invention is directed to a process involving the formation of a 
unique photoresist profile to ensure the clean lift-off of a sputtered 
thin film formed thereon. 
The process procedure of the invention now follows: 
Referring now to FIG. 1, wherein like reference numerals designate like 
elements throughout, a GaAs substrate 10, (semiconductor substrate) is 
depicted having a thin native oxide layer 12 formed thereon. Other 
suitable semiconductor substrates 10 may also be employed in the practice 
of the present invention. The semiconductor substrate 10 may comprise, for 
example, other III-V semiconductor materials. Examples of III-V 
semiconductor materials suitably employed as the semiconductor substrate 
included InP, InSb, and AlAs. 
The thin native oxide layer 12 comprises thin native oxide which is easily 
grown on the surface 14 of the GaAs substrate 10. The typical thickness of 
the thin native oxide is about 40 .ANG.. The thin native oxide layer 12 is 
formed on the GaAs substrate 10 by exposing the surface 14 of the GaAs 
substrate to an oxygen plasma. Alternatively, baking the GaAs substrate 10 
in air at a temperature ranging between about 100.degree. to 150.degree. 
C. will also result in the formation of the thin native oxide layer 12. 
The native oxide is formed by an oxidation process which takes place on 
the surface 14 of the GaAs substrate 10. Either form of surface 
preparation, i.e., exposing the surface 14 of the GaAs substrate 10 to 
oxygen plasma or baking the GaAs substrate, can be employed. For either 
form of surface preparation, the growth of the thin native oxide layer 12 
is self-limiting. The resulting thickness of the thin native oxide layer 
12 is in the range of about 30 to 50 .ANG.. 
After completing the surface preparation described above, the GaAs 
substrate 10, with the thin native oxide layer 12 formed thereon, is 
coated with a photoresist 16. The photoresist 16 is formed using 
conventional photoresist coating techniques and comprises any of the known 
photoresists. The photoresist 16 is baked and exposed to provide a latent 
pattern therein. Conventional bake and expose processes are employed to 
bake and expose the photoresist 16. The photoresist 16 is subsequently 
developed to form a pattern therein, as shown in FIG. 2. 
The development of the photoresist 16 is an important feature of the 
present invention. For a positive photoresist, the exposed photoresist is 
developed out while the unexposed photoresist is not developed out as 
fast; the ratio of the two rates being about 15:1. The developer, which 
comprises any of the known developers for the particular photoresist 
employed, attacks and etches both the photoresist 16, i.e., the exposed 
photoresist, and the thin native oxide layer 12 that lies underneath the 
exposed photoresist. During development of the photoresist 16, the 
developer first removes the exposed photoresist. Where the photoresist 16 
is exposed and developed, the photoresist is removed down to the 
underlying thin native oxide layer 12. An opening 18 is thereby formed in 
the photoresist 16. This opening 18 in the photoresist 16 has sidewalls or 
sides. While one such opening 18 is shown formed in the photoresist 16, it 
will be readily apparent to those skilled in this art that, in fact, other 
openings may be formed in the photoresist as well. 
The developer also attacks and etches the thin native oxide layer 12. The 
thin native oxide layer 12 is removed down to the surface 14 of the GaAs 
substrate 10. Accordingly, the opening 18 is extended down into the thin 
native oxide layer 12 as shown in FIG. 2. The opening 18 in the thin 
native oxide layer 12 also has sidewalls or sides. The thin native oxide 
layer 12 and the photoresist 16 remain on the regions of the GaAs 
substrate 10 indicated by arrows 20. In contrast, the region of the GaAs 
substrate 10 indicated by arrow 22 is exposed. 
The developer also attacks the sidewalls or sides of the opening 18 in the 
photoresist 16 and the thin native oxide layer 12. In particular, the 
developer attacks the sidewalls or sides of the opening 18 in the 
photoresist 16 and the thin native oxide layer 12 at positions indicated 
by arrows 24 in FIG. 2. The developer causes the thin native oxide layer 
12 to be removed from beneath the photoresist 16 at positions indicated by 
arrows 24. Accordingly, the thin native oxide layer 12 is removed from 
beneath the photoresist 16 that has not been exposed, i.e., unexposed 
photoresist. Once the thin native oxide layer 12 is removed from beneath 
the unexposed photoresist 16, the developer undercuts the unexposed 
photoresist at the positions indicated by arrows 24. The developer attacks 
the unexposed photoresist 16 from two directions, namely, from underneath 
and from the side. In this manner, a reentrant angle in the photoresist 
profile is created at the positions indicated by arrows 24. The height and 
width of the reentrant photoresist profile is a function of developing 
time. As described above, the term "reentrant" is used herein in its 
accepted meaning of "directed inward". 
The photoresist profile after the developing process is shown in FIG. 2. 
While the photoresist profile of one such region above the GaAs substrate 
10 is shown, it will be readily apparent to those skilled in this art 
that, in fact, patterns in the photoresist 16 having similar photoresist 
profiles are formed over other portions of the GaAs substrate as well. 
Next, a thin film layer 26, as shown in FIG. 3, is deposited by employing 
conventional sputtering techniques. It will be readily apparent to those 
skilled in this art that, the thin film layer 26 may alternatively be 
deposited by employing conventional evaporation techniques. The thin film 
layer 26 is formed on the photoresist 16 and on the exposed portion of the 
GaAs substrate 10, i.e., above the region indicated by arrow 22 in FIG. 3. 
The thin film layer 26 may comprise metal such as TaN (tantalum nitride). 
The thin film layer 26 may comprise other metals and non-metals as well. 
Examples of other metals suitably employed as the thin film layer 26 
include NiCr (nickel chromium), TiW (titanium tungsten), and Ta 
(tantalum). The thickness of the thin film layer 26 ranges from about 500 
to 1000 .ANG.. The thin film layer 26 has a discontinuity caused by the 
reentrant photoresist profile which is depicted in FIG. 3. The reentrant 
photoresist profile interrupts the continuity of the thin film layer 26 
during deposition thereof. Accordingly, a discontinuity in the thin film 
layer 26 is created. 
The GaAs wafer is then soaked in acetone. Both the thin film layer 26 and 
the photoresist 16 are cleanly lifted off. The presence of the 
discontinuity in the thin film layer 26 ensures that the thin film layer 
26 is cleanly lifted off. FIG. 4 depicts the thin film layer 26 patterned 
and intact on the semiconductor substrate 10. The thin native oxide layer 
12 also remains on portions of the surface 14 of the GaAs substrate 10. 
The thin native oxide layer 12 is not lifted off during the thin film 
layer lift-off process. Also shown in FIG. 4 are the remaining portions of 
the native oxide layer 12 which surround the patterned thin film layer 26. 
The process of the present invention relies on the reentrant photoresist 
profile which breaks the continuity of the thin film layer 26. The desired 
reentrant photoresist profile which breaks the continuity of the thin film 
layer 26 can be obtained by typical photoresist processes preceded by the 
oxidation process that takes place on the surface 14 of the GaAs substrate 
10. The desired reentrant photoresist profile of the present invention 
ensures the clean lift-off of the thin film layer 26 such that the edges 
or sides of the openings in the thin film layer 26 are smooth. 
Accordingly, good visual and electrical yields are provided. In 
particular, the yields of MMICs are improved. 
The major advantage of the present invention is that no extra dielectric 
film deposition and etch processing steps are required to achieve clean 
lift-off. Nevertheless, a unique photoresist profile is provided. 
EXAMPLES 
GaAs MMIC chips were fabricated using the process of the invention and were 
examined. 
Specifically, several GaAs wafers were fabricated in accordance with the 
process of the invention. A thin native oxide layer 12 was formed on the 
surface 14 of several of the GaAs substrates 10 or GaAs wafers. The GaAs 
substrates 10 having the thin native oxide layer 12 formed thereon were 
coated with a photoresist 16. The photoresist 16 was BPRS-100, a positive 
photoresist available from OGC Microelectronics Material Inc. (West 
Paterson, N.J.). The photoresist 16 was baked, exposed and developed to 
form a pattern therein. The developer employed was PLSI, a positive resist 
developer available from OGC Microelectronics Material Inc. A patterned 
photoresist 16 as well as posed portions on the GaAs substrates 10 were 
thereby provided. A reentrant angle in the profile was thus formed in the 
photoresist 16. A thin film layer 26 comprising a TaN film was deposited 
on the patterned photoresist 16 and on the exposed portions of the GaAs 
substrates 10. The TaN film, having a thickness of about 500 .ANG., was 
sputtered on these GaAs wafers 10. The photoresist 16 and the TaN film 
were lifted-off. A patterned TaN film was thereby formed. 
Several of the GaAs wafers 10 were fabricated without a thin native oxide 
layer 12 to serve as a control group. Also evaluated were several GaAs 
wafers 10 using four different photoresist developing times. A summary of 
these results is listed in Table 1. 
TABLE I 
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SUMMARY OF RESULTS FOR GaAs WAFERS FABRICATED 
USING DIFFERENT PHOTORESIST DEVELOPING TIMES 
Developing Time 
Lift-off 
______________________________________ 
120 seconds 10% residue 
150 seconds 5% reside 
180 seconds completely clean 
240 seconds completely clean 
______________________________________ 
The results of the examination confirm the functions and advantages 
expected of the present invention, namely, that a clean lift-off is 
provided. As described above, by clean lift-off is meant that the edges of 
the thin film layer 26 are not ragged and that the intended pattern is 
formed substantially complete in the thin film layer. Without the thin 
native oxide layer 12, the lift-off was not clean and complete. With the 
thin native oxide layer 12, if the developing time was not long enough, 
the lift-off was not completely clean. The lift-off was not completely 
clean when the developing time was not long enough due to the fact that 
the reentrant angle was not wide enough to break the continuity of the 
thin film layer 26 of TaN sputtered on the GaAs substrate 10. 
Thus, there has been disclosed a unique photoresist process which ensures 
the clean lift-off of sputtered thin films. The process of the invention 
for clean lift-off of sputtered thin films can be applied to GaAs MMIC 
fabrication and, in particular, to all MMIC chips utilizing sputtered thin 
film resistors. It will be readily apparent to those skilled in this art 
that various changes and modifications of an obvious nature may be made, 
and all such changes and modifications are considered to fall within the 
scope of the invention, as defined by the appended claims.