Wet etching method for forming metal film pattern having tapered edges

A wet etching method for forming a metal film pattern having tapered edges includes preparing a metal film deposited on a substrate, performing a treatment for making the surface of the metal film hydrophilic, forming a photoresist layer pattern with phenol novolac resin as a main component on the surface of the metal film, post-baking the photoresist layer pattern at a predetermined temperature for a predetermined time and etching the metal film with etchant including nitric acid, thereby obtaining a metal film pattern having edges with a uniform taper angle desirably controlled with precision.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to a wet etching for forming a 
metal film pattern, and more particularly, to an improvement in etching 
for forming a metal film pattern (hereinafter referred to as a 
taper-etching) having tapered edges. 
2. Description of the Background Art 
FIG. 1 is a flow chart illustrating a conventional wet etching for forming 
a metal film pattern. As can be seen from the diagram, a metal film is 
deposited on a substrate of silicon or glass by, for example, sputtering 
in step S1. In step S2, the metal film is covered by a photoresist layer 
including phenol novolac as a main component. In step S3, the photoresist 
is pre-baked. In step S4, predetermined regions of the pre-baked 
photoresist layer are exposed to light by known photolithography. In step 
S5, the exposed photoresist layer is developed to have a predetermined 
pattern. The photoresist pattern is dried in step S6 and fully cured by 
post-baking at a fixed temperature for a fixed time in step S7. Then in 
step S8, the metal film covered by the resist pattern is etched to have a 
predetermined pattern and by a known wet etching method such as a shower 
method, a spray method and a paddle method with appropriate etchant. 
FIG. 2 schematically shows a section of a thin film transistor (TFT). The 
TFT is commonly used as a drive transistor in a liquid crystal display 
apparatus. In the TFT, a gate electrode 2a is formed on an insulator 
substrate 1 of glass or the like by etching a metal film of chromium or 
tantalum. The gate electrode 2a is covered with a first insulator layer 3 
of SiO.sub.2 or Si.sub.3 N.sub.4. An intrinsic type amorphous silicon 
layer 4 is deposited on the first insulator layer 3. The intrinsic type 
amorphous silicon layer 4 is covered with a second insulator layer 5 of 
Si.sub.3 N.sub.4. An n.sup.+ type amorphous silicon layer 6 is deposited 
on the second insulator layer 5. Source/drain regions of the n type 
amorphous silicon layer 6 are in contact with the intrinsic type amorphous 
silicon layer 4 through holes formed in the second insulator layer 5. 
Source/drain electrodes 7 of aluminum or the like are formed on the 
source/drain region of the n type amorphous silicon layer 6. The 
source/drain electrodes 7 are covered with a protective insulator film of 
SiO.sub.2 or the like. 
In such TFT as shown in FIG. 2, edges of the second gate electrode 2a are 
desirably tapered in order to obtain an excellent coverage of upper 
partial layers in the vicinity of the edge and avoid undesired electric 
field concentration in the vicinity thereof. The reason is that an 
improved coverage attributes to prevent disconnection of a lead line, 
thereby improving a yield rate and that the attenuation of the electric 
field concentration increases a breakdown voltage of the TFT. 
FIGS. 3A, 3B and 3C are sectional views showing the taper etching disclosed 
in Japanese Patent Laying-Open No. 64-86524. In FIG. 3A, a chromium film 2 
is formed on a substrate of silicon or the like by plating or vacuum 
evaporation, for example. A pattern 10 of photoresist OFPR-77E (product of 
Tokyo Ohka Corporation) with phenol novolac as a main component is formed 
on the chromium film 2 by known photolithography. 
Thereafter, the chromium film 2 is etched with etchant including ammonium 
cerium (IV) nitrate of 19 g, nitric acid of 13 cc and water of 87 cc. At 
this time, the nitric acid included in the etchant starts peeling the 
edges of the resist layer 10 off from the metal film 2 and the etchant 
dissolves the chromium film 2 at the same time, as shown in FIG. 3B. As a 
result, the edges of the patterned chromium film 2a are tapered to have an 
inclination angle .theta. as shown in FIG. 3C. 
The taper angle .theta. can be controlled by adjusting the temperature and 
the concentration of the nitric acid of the etchant as shown in FIG. 4. In 
the graph of FIG. 4, the abscissa represents the concentration of nitric 
acid (mol/l) and the ordinate represents a taper angle .theta. (deg). The 
curves A, B, C and D respectively show the etching with the etchant 
temperatures of 22.degree. C., 32.degree. C., 42.degree. C. and 52.degree. 
C. 
Such taper-etching according to the prior art as described above requires 
the concentration of nitric acid and the temperature of the etchant to be 
controlled when the thickness of the metal film 2 and the taper angle 
.theta. are changed. In addition, it is difficult to control the taper 
angle of 20.degree. or less with precision, as can be seen from the graph 
of FIG. 4. Furthermore, the surface of the metal film 2 is liable to 
become nonhomogeneous with the passage of time after the deposition 
thereof, because the surface is partially oxidized or adsorbs moisture. 
The nonhomogeneous surface of the metal film 2 results in variations of 
the taper angle .theta. depending on the location on the substrate. 
SUMMARY OF THE INVENTION 
In view of the above described prior art, an object of the present 
invention is to provide a wet taper-etching enabling a metal film pattern 
having a desired uniform taper angle to be formed with ease and the taper 
angle of 20.degree. or less to be controlled with precision. 
The taper-etching for forming a metal film pattern having tapered edges 
according to the present invention includes the steps of preparing a metal 
film deposited on a substrate, performing a treatment for making the 
surface of the metal film hydrophilic, forming a photoresist layer pattern 
with phenol novolac resin as a main component on the surface of the metal 
film, post-baking the photoresist layer pattern at a predetermined 
temperature for a predetermined time period and etching the metal film 
with etchant including nitric acid, thereby obtaining a metal film pattern 
having edges with a desired uniform taper angle controlled with precision. 
The foregoing nd other objects, features, aspects and advantages of the 
present invention will become more apparent from the following detailed 
description of the present invention when taken in conjunction with the 
accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Wet taper-etching for forming a metal film pattern according to one 
embodiment of the present invention will be described with reference to 
FIGS. 5 and 6A to 6F. 
In step S1 of FIG. 5, a metal film 2 of chromium, tantalum, aluminum, 
copper or titanium is deposited to have a thickness of about 500-5000 
.ANG. on a substrate 1 of silicon or glass by sputtering, for example, as 
shown in FIG. 6A. 
In step S1a, a surface 2s of the metal layer 2 is irradiated by far 
ultraviolet light having main wave lengths of 185 nm and 254 nm to become 
hydrophilic as shown in FIG. 6B. The surface 2s of the metal layer 2 can 
be made hydrophilic also by plasma irradiation and anodic oxidation. The 
surface 2s of the metal layer 2 becomes homogeneous all over by such 
treatment for making the surface 2s hydrophilic. 
Thereafter, steps S2 to S6 similar to those of the prior art shown in FIG. 
1 form a photoresist pattern 10 on the hydrophilic surface 2s of the metal 
layer 2 as shown in FIG. 6c. The resist layer 10 can be made of resin with 
phenol novolac as a main component to have a film thickness of about 0.5-3 
.mu.m. 
In step S7a, the resist layer 10 is post-baked at a predetermined 
temperature for a predetermined time period in order to control the 
adhesiveness between the resist layer 10 and the metal film 2, resulting 
in a cured resist pattern 10a as shown in FIG. 6D. The post-baking such as 
convection type or hot plate type heating, is generally performed at about 
120.degree. C. within 30 minutes. 
In step S8, as shown in FIG. 6E, the metal film 2 is etched with such 
etchant including nitric acid of 2 mol/l or more as described in Japanese 
Patent Laying-Open No. 64-86524. At this time, the liability of the edge 
portion of the resist film 10a to peel off the metal film 2 under the 
presence of nitric acid in the etchant depends on the temperature and the 
time of the above-described post-baking. 
The taper angle .theta. of the edges of the metal film pattern 2a formed by 
etching shown in FIG. 6F can be therefore controlled by modifying the 
post-baking conditions. 
FIG. 7 shows a relationship between the post-baking conditions and the 
taper angle. In the graph of FIG. 7, the abscissa represents a post-baking 
time (min) and the ordinate represents a taper angle .theta. (deg), 
wherein the post-baking temperature is fixed to be 120.degree. C. It can 
be seen from FIG. 7 that the present invention enables a taper angle of 
20.degree. or less to be controlled with precision. The post-baking 
temperature can be changed within the range from 100.degree. C. to 
160.degree. C., and the curve shown in FIG. 7 tends to have a steep 
inclination with an increase of the temperature. 
As described in the foregoing, the treatment for making the surface of the 
metal film 2 hydrophilic serves to make the surface uniform. It is 
therefore possible to reduce the variation in the taper angle .theta. of 
edges of the formed metal pattern depending on the location, that is, to 
obtain a uniform taper angle .theta.. 
In addition, controlling the temperature and the time of the post-baking 
enables a desired taper angle .theta. of 20.degree. or less to be 
precisely controlled. 
The etching method according to the present invention can be applied to a 
formation of a gate electrode of a TFT and can provide a TFT having a high 
breakdown voltage at a high yield rate accordingly. 
Although the present invention has been described and illustrated in 
detail, it is clearly understood that the same is by way of illustration 
and example only and is not to be taken by way of limitation, the spirit 
and scope of the present invention being limited only by the terms of the 
appended claims.