Tapered profile etching method

A method of etching an article having a substrate, an etchable film and a mask layer having a pattern formed therein includes the step of exposing the article to an etchant gas mixture which includes a halogen-containing gas and an inert gas. An etching profile is formed which is substantially smooth across an interface between the etchable film and the mask layer. The method is particularly useful in producing components of articles such as flat-panel displays.

FIELD OF THE INVENTION 
The present invention relates generally to an etching method which produces 
a continuous, substantially smooth etching profile, and more particularly 
to a method for etching using an etchant gas mixture of a halogen gas and 
an inert gas. 
BACKGROUND OF THE INVENTION 
Conventional etching methods typically include the steps of applying a 
photoresist to an article having an etchable film, for example, aluminum 
or amorphous silicon, supported on a substrate. The photoresist is 
selectively exposed and developed to form a mask layer over the film. The 
portions of the film not covered by the mask layer are etched to remove 
the exposed portions of the etchable film. Finally, the mask layer is 
removed, for example, by use of a solvent. 
Known etching processes include "wet" etching. In a wet etching process, 
the article to be etched is exposed to an etching solution, such as a 
solution including hydrofluoric acid (HF). Wet etching processes are 
described, for example, in S. Ghandhi, VLSI Fabrication Principles (2d. 
Ed., John Wiley & Sons, Inc., New York, N.Y. 1994), pp. 589-613. 
Wet etching processes, however, suffer from various disadvantages. For 
example, it is usually impracticable to change the etching solution after 
each article is processed; etching solutions typically are changed once 
per week. As a result, particles and residues accumulate in the etching 
solution. These accumulations tend to reduce the quality of the etching 
over time. Another problem is lack of uniformity. Wet etching processes 
are often incapable of uniformly removing an exposed etchable film over 
the entire exposed surface of the article, and in particular can suffer 
from undercutting of the etchable film. Such processes furthermore require 
a substantial area for the required etching tanks, rinsing tanks, drying 
apparatus, etc. Finally, wet etching processes, particularly those using 
strong etchant solutions and/or organic solvents, are environmentally 
hostile and generate significant disposal problems. 
Dry etching methods avoid many of the problems associated with wet etching 
methods in the production of integrated circuits. Dry etching methods are 
described, for example, in the above-noted publication at pp. 613-624. 
Such methods include dry physical etching methods, such as ion beam 
etching and sputter etching, and dry chemical etching. Dry etching methods 
typically utilize fewer chemicals and in smaller quantities, are readily 
automated, and give rise to fewer disposal problems. 
In producing integrated circuits, it is important that the etching process 
employed produce a substantially vertical profile in the etched film. 
FIGS. 1 and 2 illustrate a conventional dry etching process. In FIG. 1, an 
article including a substrate 10, an etchable film 12 (for example, an 
aluminum film or an amorphous silicon film), and a mask layer 14 in which 
a pattern 16 is formed is exposed to an etchant gas. Alternatively, the 
article can be subjected to plasma etching. Mask layer 14 has a mask 
surface 18 which forms a first etch angle .theta..sub.1, with respect to 
etchable film 12. After removal of the exposed portion of etchable film 12 
by the selected etching process, the surface 20 of the etchable film 12 
forms a second etch angle .theta..sub.2 with respect to substrate 10, as 
shown in FIG. 2. Surface 20 preferably forms an angle .theta..sub.2 of 
about 85.degree.-90.degree., preferably approximately 90.degree.. 
In the illustrated process, etchable film 12 etches at rate R.sub.F, and 
mask layer 14 etches at rate R.sub.M. The ratio of the two rates, R.sub.F 
/R.sub.M, preferably is substantially greater than one, and is typically 
about 4 to 10. The high etch rate of film 12 with respect to mask layer 14 
enables formation of a substantially vertical etching profile in film 12, 
as shown in FIG. 2. 
In other applications, however, such as the production of flat panel 
displays (FPDs), it is important to produce a profile which is inclined 
with respect to the vertical, i.e., a tapered profile. As shown in FIG. 3, 
it may be desired to produce an etch angle .theta..sub.2 in etchable layer 
12 which is between about 15.degree. and 60.degree.. To produce such a 
tapered profile, it is desirable that the respective etch rates of film 12 
and mask layer 14 are more nearly equal in value, preferably such that the 
ratio R.sub.F /R.sub.M is less than about 2, very preferably between about 
1 and 2. 
Problems arise in producing tapered profiles. It is difficult, for example, 
to achieve the desired low ratio of R.sub.F to R.sub.M with known dry 
etching techniques. More significantly, it is difficult to completely etch 
the interface 22 at which etchable film 12 and mask layer 14 meet. The 
difficulty arises due the presence of oxidation products, such as alumina 
(Al.sub.2 O.sub.3) in the case of an aluminum film 12 or silicon dioxide 
(SiO.sub.2) in the case of an amorphous silicon film 12, which may be 
present in a surface region 24 of film 12, and/or the presence of mask 
residues. If interface 22 is etched at a lower rate than mask 14 and film 
12, a "ledge" 26 is formed at interface 22, as shown in FIG. 4, and a 
continuous tapered contour cannot be produced. 
In known processes for producing a tapered etch profile, chlorine-based 
etching chemistry is frequently employed. For example, when etchable film 
12 is an aluminum film, boron chloride (BCl.sub.3) and chlorine gas 
(Cl.sub.2) are employed. Initially, BCl.sub.3 removes the Al.sub.2 O.sub.3 
oxidation products from surface region 24 of the exposed aluminum film 12. 
The mixture of BCl.sub.3 and Cl.sub.2 is then used to etch aluminum film 
12 and mask 14. However, the gas mixture does not remove the Al.sub.2 
O.sub.3 from interface 22 as effectively as it removes film 12 and mask 
14. This again results in formation of a ledge 26, preventing a smooth 
etch profile from being formed across film 12 and mask layer 14. 
Processes for etching a film 12 of other known materials also encounter 
problems with ledge formation. When film 12 is an amorphous silicon film, 
BCl.sub.3 typically removes the silicon oxidation products from surface 
region 24 of exposed silicon film 12. Cl.sub.2 is then used to etch 
silicon film 12. Again, Cl.sub.2 does not remove the silicon from 
interface 22 at the same rate as it removes film 12 and mask layer 14, and 
a tapered contour cannot readily be formed. 
A need exists for a dry etching method that enables production of an etched 
article, such as a FPD, having a smooth, continuous tapered etch profile 
which is substantially free of ledges and other irregularities. 
SUMMARY OF THE INVENTION 
In one aspect, the present invention is directed to a method of etching an 
article including a substrate, an etchable film and a mask layer. The 
method includes the steps of forming a pattern in the mask layer and 
exposing the article to an etchant gas mixture which includes a 
halogen-containing gas and an inert gas. By use of the inventive etchant 
gas mixture, an etching profile is formed which is substantially smooth 
across an interface between the etchable film and the mask layer. 
Implementations of the method include the following. The etching profile 
includes a first segment across the etchable film which forms a first 
angle .theta..sub.1 with the substrate. A second segment across the mask 
layer forms a second angle .theta..sub.2 with the etchable layer. The 
first and second angles differ by less than about 15.degree.. The film 
etches at a rate R.sub.F and the mask layer etches at a rate R.sub.M such 
that the ratio R.sub.F /R.sub.M is less than or equal to 2, and in 
particular is between about 1 and 2. 
The etching gas mixture comprises (a) a halogen-containing gas or 
combination of halogen-containing gases, and (b) an inert gas or 
combination of inert gases. 
Halogen-containing gases for use in the inventive etchant gas mixture 
include chlorine-containing gases and fluorine-containing gases. The 
halogen-containing gas may be a chlorine-containing gas such as molecular 
chlorine, boron trichloride, carbon tetrachloride, or mixtures thereof. 
Inert gases for use in the inventive etchant gas mixture may be noble 
gases, more preferably helium, argon, xenon or mixtures thereof. 
The halogen-containing gas and the inert gas are present in the etchant gas 
mixture in a ratio of about 75:25 to 40:60, and more particularly from 
about 70:30 to 40:60. The etching step is carried out at a pressure of 
about 5-50 millitorr and a power of about 0.1-2.0 W/cm.sup.2. 
The inventive method may be particularly useful in etching films such as 
amorphous silicon and aluminum in order to produce etching profiles that 
are substantially smooth. The mask layer may be a polymeric photoresist 
layer. The etchant gas mixture may further include oxygen. 
In another aspect, the present invention is directed to a method of etching 
an article including a substrate, an etchable film and a mask layer, 
including steps of forming a pattern in a polymeric photoresist layer of 
the mask layer, exposing the article to an etchant gas mixture including a 
chlorine-containing gas and a noble gas, such that an etching profile is 
formed which is substantially smooth across an interface between the 
etchable film and the mask layer. 
In accordance with another aspect of the present invention, a method of 
producing an etched article includes the steps of forming a mask layer 
including a polymeric photoresist on an article having a substrate and an 
etchable film, thus producing an article having an interface between the 
etchable film and the mask layer, forming a pattern in the mask layer by 
selectively exposing and developing the mask layer, etching the article by 
exposing the article to an etchant gas mixture including a 
chlorine-containing gas and a noble gas to form the pattern in the 
etchable layer, such that an etching profile is formed which is 
substantially smooth across the interface between the etchable film and 
the mask layer, and removing the mask layer. 
In another aspect, the invention is directed to a method of producing an 
etched article including steps of forming a mask layer having a polymeric 
photoresist on an article including a substrate and an etchable film, 
thereby producing an article having an interface between the etchable film 
and the mask layer. A pattern is formed in the mask layer by selectively 
exposing and developing the mask layer. Other steps include etching the 
article by exposing the article to an etchant gas mixture including a 
chlorine-containing gas and a noble gas to form the pattern in the 
etchable layer, whereby an etching profile is formed which is 
substantially smooth across the interface between the etchable film and 
the mask layer, and removing the mask layer. 
In another aspect, the invention is directed to an article produced by the 
above methods. The article may be a component of an FPD. 
Additional advantages of the invention will be set forth in the description 
which follows, and in part will be obvious from the description, or may be 
learned by practice of the invention. The advantages of the invention may 
be realized and obtained by means of the instrumentalities and 
combinations particularly pointed out in the claims.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Use of an inert gas, such as argon or another noble gas, or combinations 
thereof, as a component of a halogen-containing etchant gas mixture 
enables substantially smooth, continuous etching profiles to be formed. 
The inventive etchant gas mixture is thus particularly suitable for 
producing etched articles, such as FPD's, which require non-vertical 
etching profiles. 
The term "etchable film" as used herein denotes a film that is subject to 
etching by an etchant gas mixture upon exposure thereto, and that remains 
present in unexposed areas of the article after completion of the etching 
process. The etchable film may in particular include an upper portion that 
is at least partially oxidized. Exemplary etchable films include films of 
amorphous silicon and aluminum films. 
The term "mask layer" as used herein denotes a layer in which is defined a 
pattern, typically formed by selective exposure and development of a 
polymerizable photoresist composition, and that is completely removed from 
the article after completion of the etching process. 
The term "etching profile" as used herein denotes a cross-sectional profile 
formed by the underlying etchable film and the overlying mask layer after 
completion of the etching process. The etching profile includes a first 
segment on an edge of the etchable film, which forms a first etch angle 
.theta., with the substrate, and a second segment across the mask layer, 
which forms a second etch angle .theta..sub.1 with the etchable layer (and 
possibly also with the substrate). 
The term "interface" as used herein denotes the plane of contact between 
the etchable film and the mask layer. When used in the context of an 
etching profile, the term "interface" in particular denotes the point on 
the etching profile at which the etchable film contacts the mask layer. 
A "substantially smooth" etching profile as used herein is an etching 
profile which is substantially free of discontinuities, such as ledges, at 
the interface. More specifically, the first etch angle .theta..sub.1, and 
the second etch angle .theta..sub.2 discussed above do not substantially 
differ. For example, the two etch angles do not differ by more than about 
15.degree.. In particular, the etch angles .theta..sub.1 and .theta..sub.2 
may be approximately equal. 
The etchant gas mixture employed according to the present invention 
includes a halogen-containing gas, which may be a chlorine-containing gas 
or a fluorine-containing gas. Particular chlorine-containing gases which 
can be used in the etchant gas mixture include Cl.sub.2, BCl.sub.3, carbon 
tetrachloride (CCl.sub.4) and combinations thereof. Other known 
chlorine-containing gases used in known dry etching processes can also be 
used. Selection of a specific chlorine-containing gas or mixture of 
chlorine-containing gases depends on factors such as the type of etchable 
film to be etched. 
In addition to a halogen-containing gas or combinations thereof, the 
etchant gas mixture employed according to the invention includes at least 
one inert gas. The inert gas may be a noble gas, in particular helium 
(He), argon (Ar) or xenon (Xe). Argon is useful due to its ready 
availability and low cost. Combinations of noble gases, such as Ar/He or 
Ar/Xe, can also be employed. 
The halogen-containing gas or gases, and the inert gas or gases, may be 
present in the inventive etchant gas mixture in a ratio from about 75:25 
to 40:60. Thus, the flowrate of the inert gas or combination of inert 
gases during the etching process of the invention varies from about 30 to 
60% of the total etchant gas flowrate. 
In general, higher inert gas flowrates result in slower etch rates, but 
also afford greater uniformity in the etch profile, that is, a smaller 
difference between the first and second etch angles .theta..sub.1 and 
.theta..sub.2. Conversely, lower inert gas flowrates (and thus, higher 
halogen-containing gas flowrates) generally afford a higher etch rate, but 
may also result in a greater difference between the first and second etch 
angles .theta..sub.1 and .theta..sub.2. Thus, lower inert gas flowrates 
tend to achieve a slightly less smooth etching profile, although still 
significantly smoother than etching profiles produced by known etching gas 
mixtures which do not include an inert gas. The desired etch rate and 
etching profile uniformity can be chosen through selection of the 
halogen-containing gas or gases, the inert gas or gases, and the relative 
proportions of each component in the etching gas mixture. 
In contrast to known dry etching processes in which the etching rate 
R.sub.F of the etchable film is significantly greater than the etching 
rate R.sub.M of the mask layer, such that the ratio R.sub.F /R.sub.M is 
about 4 to 10, the corresponding ratio afforded by the present invention 
can be limited to less than about 2, and preferably to between about 1 and 
2. 
The inventive etching gas mixture can be used in a variety of etching 
processes, including presently known etching processes for producing FPDs. 
One process may use a polymerizable photoresist to form the mask layer. 
The formation of the initial angle .theta..sub.1 in the mask layer, after 
development of the photoresist and prior to exposure of the article to the 
etching gas mixture, can be caused by a reflow step (i.e., by heating the 
mask layer to a high temperature, typically about 120.degree. C.). Other 
known methods, such as UV curing, can also be employed if desired. 
Many known etching devices, including those presently employed in producing 
FPDs, can be utilized to carry out the inventive etching methods. One 
particularly useful apparatus for carrying out the inventive etching 
methods is the Applied Komatsu Technology (AKT) Etcher 1600 or AKT Etcher 
3500, available from AKT of Santa Clara, Calif. Details of such 
apparatuses are included in U.S. patent application Ser. Nos. 08/273,382 
and 08/732,968, co-owned by the assignee of the present invention and 
hereby incorporated by reference. 
Etching methods according to the invention preferably are carried out under 
low pressure and high power. Preferred pressures range from about 5-50 
millitorr, more preferably about 10 to 15 millitorr. Preferred powers 
range from about 0.1 to 2.0 W/cm.sup.2, more preferably about 0.2 to 1.0 
W/cm.sup.2. Etching times will vary depending on the thickness and 
composition of the layer to be etched. Typical etching times range from 
about 0.5 to 5 minutes. For example, etchable layers of amorphous silicon 
or aluminum, having a thickness of about 3000 .ANG., can be etched in 
about 2 minutes at the pressure and power conditions exemplified above. 
The inventive method may operate as follows. By using an etchant gas 
mixture including a halogen-containing gas and an inert gas in a dry 
etching method, etching occurs by two independent mechanisms: halogen 
etching and sputtering. At the preferred low pressures and high powers, 
the sputtering mechanism is enhanced. This sputtering increases the 
effectiveness of the gas etching mixture in removing interfacial 
materials, such as Al.sub.2 O.sub.3 in the case of an aluminum etchable 
layer or SiO.sub.2 in the case of an amorphous silicon etchable layer, as 
well as mask residues. Consequently, the formation of "ledges" and other 
irregularities at the interface between the etchable film and the mask 
layer is reduced, and the resultant etching profile is substantially 
smooth and continuous across the interface. 
FIG. 5 illustrates an etched article produced in accordance with the method 
of the present invention, in which tapered mask surface 18 and tapered 
etchable film surface 20 form etch angles .theta..sub.1 and .theta..sub.2, 
respectively, which are approximately equal. The etched article has the 
desired smooth etching profile across the interface between etchable film 
12 and mask layer 14, with no ledge formation. 
As mentioned, an etchant gas mixture according to the invention can be used 
in presently known etching methods, such as those etching methods used to 
produce FPDs. The inventive etchant gas mixture can also be used in 
combination with oxygen in processes that presently employ oxygen as a 
component of the etchant gas. 
The present invention is further illustrated by the following non-limiting 
examples. 
EXAMPLE 1 
Aluminum film etching 
An article is prepared having layers including a glass substrate, an Al 
film having a thickness of 3000 .ANG., and a polymeric photoresist mask 
layer. The article is selectively exposed and developed to produce a 
pattern in the mask layer. The mask layer after development forms an etch 
angle .theta..sub.2 of 45.degree. with respect to the underlying Al film. 
The article is subsequently etched in an AKT Etcher using an etchant gas 
mixture according to an embodiment of the present invention. The etchant 
gas has a total gas flow rate of 155 standard cubic centimeters per minute 
(sccm), and has the following composition: 70 sccm Ar, 20 sccm BCl.sub.3 
and 65 sccm Cl.sub.2. These values were used in a chamber having a volume 
180 liters, and would scale for larger or smaller chambers accordingly. 
For example, the Ar flow rate could be about 0.4 sccm per liter of chamber 
volume. The flowrate was maintained constant during the etching process. 
The etching process was carried out at a pressure of 10 millitorr and a 
power density of 1 W/cm.sup.2 for 2 minutes. 
After completion of the etching step, the etching profile across the 
mask-Al layer interface is smooth and forms etch angles .theta..sub.1, 
.theta..sub.2 of 45.degree.. 
EXAMPLE 2 
Amorphous silicon film etching 
An article is prepared having layers including a glass substrate, an 
amorphous silicon film having a thickness of 3000 .ANG., and a polymeric 
photoresist mask layer. The article is selectively exposed and developed 
to produce a pattern in the mask layer. The mask layer after development 
forms an etch angle .theta..sub.2 of 45.degree. with respect to the 
underlying amorphous silicon film. 
The article is subsequently etched in an AKT Etcher using an etchant gas 
mixture of the present invention. The etchant gas has a total 145 scam, 
and has the following composition: 70 scam Ar and 75 scam Cl.sub.2. The 
flowrate was maintained constant during the etching process. Flowrates in 
the range of 10 to 1000 scam for Ar and 10 to 500 scam for Cl.sub.2 may 
also be used. The etching process was carried out at a pressure of 10 
millitorr and a power density of 0.4 W/cm.sup.2 for about two minutes. 
Power densities of about 0.1 W/cm.sup.2 to 5 W/cm.sup.2 may generally be 
used. 
After completion of the etching step, the etching profile across the 
mask-silicon layer interface is smooth and forms etch angles 
.theta..sub.1, .theta..sub.2 of 45.degree.. 
EXAMPLE 3 
FPD production 
An article is prepared including layers of a glass substrate, an amorphous 
silicon film having a thickness of 3000 .ANG., and a polymeric photoresist 
mask layer defining a pattern for use in an FPD. The article is etched as 
in EXAMPLE 2, and the mask layer is subsequently removed. The etched 
article is suitable for use in manufacturing an FPD. 
The present invention thus affords an improved method for producing an 
etching profile which is smooth and free from ledges and other interfacial 
irregularities. The method requires only one etchant gas composition for 
use during the entire etching process, and is particularly useful for 
producing articles such as FPDs which require non-vertical etching 
profiles. 
The present invention has been described in terms of a preferred 
embodiment. The invention, however, is not limited to the embodiment 
depicted and described. Rather, the scope of the invention is defined by 
the appended claims.