Gas separation: apparatus – Magnetic separating means – Electromagnet
Reexamination Certificate
2000-08-21
2001-11-13
Cain, Edward J. (Department: 1714)
Gas separation: apparatus
Magnetic separating means
Electromagnet
C096S003000, C438S715000
Reexamination Certificate
active
06315819
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a method of dry etching, and more particularly to a method of dry etching a nickel film for making an electrode. The invention also relates to an apparatus of making dry etching exhaust gas non-toxic.
2. Description of the Related Art
There has been used various methods and apparatuses for dry etching of nickel in order to etch a nickel film with high accuracy to thereby form a electrode in a semiconductor device.
For instance, Japanese Unexamined Patent Publication No. 53-20769 has suggested a method of dry etching a nickel film by means of plasma generated from CO or CO
2
gas. The suggested method utilizes a method of refining metal, known as carbonyl process. Iwanami, Scientific and Chemical Encyclopedia, the 3rd edition, pp. 270 defines the word “carbonyl process” as follows: “This is a method of refining metal, utilizing thermal dissociation of metal carbonyl. Mond process for refining nickel is an example of the carbonyl process, and is based on the following equilibrium equation.
Ni(CO)
4
&rlarr2;Ni+4CO
If nickel dioxide (II) were reduced to metal nickel with hydrogen gas, the thus reduced metal nickel were caused to react with CO at 60° C. to thereby produce gaseous nickel carbonyl, Ni(CO)
4
having a boiling point of 42.5° C., and then the thus produced gaseous nickel carbonyl were heated up to 180° C. in a decomposition tower, the above-mentioned equilibrium equation proceeds towards the right. As a result, there is obtained highly purified nickel in the form of powder, including CO in a small content.”
As mentioned above, Ni can be etched merely by causing Ni to directly react with CO gas. The above-mentioned Japanese Unexamined Patent Publication No. 53-20769 further suggests a method of etching Ni by producing plasma of CO gas, but does not mention an advantage of producing plasma of CO gas.
When CO
2
gas is employed, CO
2
gas has to be turned into plasma ill order to etch Ni. This is because CO
2
gas is decomposed by being turned into plasma to thereby produce CO gas necessary for etching Ni.
By etching Ni with either CO or CO
2
gas, there can be obtained an advantage that it is no longer necessary to use chemicals which has a problem that waste solution thereof is quite difficult to properly dispose of.
Japanese Unexamined Patent Publication No. 60-228687 has suggested another method of dry etching nickel by using CO
2
plasma. The Publication mentions that a method of etching nickel at a temperature in the range of 40° C. to 100° C. by using CO gas has a shortcoming of poor etching accuracy relative to a mask pattern due to that etching is isotropic. In addition, the Publication further mentions that if plasma were generated based on CO gas, there would be generated O
2
gas which would oxidize nickel, and hence make it difficult to carry out nickel etching. As a solution to such a problem, the Publication has suggested the use of CO
2
plasma for etching nickel, and addition of H
2
to CO
2
gas for preventing oxidation of nickel.
Japanese Unexamined Patent Publication No. 60-228687 presents a graph showing a relation between a nickel etching rate and input power provided to a discharge electrode, which relation is found when nickel is etched by means of plasma of CO
2
gas. The graph is illustrated in FIG.
1
. According to
FIG. 1
, an input power of about 150 W is required for obtaining an etching rate of 1000 angstroms/min., for instance.
For the purpose of dry etching nickel, there may be used a variety of etching apparatuses. For instance, there may be used a plane parallel plate type reactive ion etching (RIE) apparatus, an electron cyclotron resonant (ECR) etching apparatus, or an inducive coupling type plasma (ICP) etching apparatus.
FIG. 2
illustrates an exhaust system of those etching apparatuses. The exhaust system comprises an exhaust pump system
80
disposed downstream of an etching apparatus
79
, and an apparatus
81
for making an exhaust gas non-toxic, disposed downstream of the exhaust pump system
80
. The exhaust gas
98
made non-toxic by the apparatus
81
is released to atmosphere, or introduced to a gas scrubber (not illustrated) through an exhaust conduit
97
. As illustrated in
FIG. 2
, the exhaust pump system
80
generally includes a booster pump or turbo-molecular pump
95
, and a rotary pump employing oil for rotation or a dry pump
96
employing no oil for rotation, disposed downstream of the pump
95
.
The etching apparatus
79
includes an etching chamber
88
having a gas inlet
85
through which etching gas
84
is introduced into the etching chamber
88
. The etching chamber
88
is provided with a gate
83
which is designed to be open or closed by means of a gate valve
82
. Inside the etching chamber
88
are disposed an anode
89
and a cathode
90
. The anode
89
is supported in the etching chamber
88
with an insulator
86
, and is grounded at
87
. The cathode
90
is supported in the etching chamber
88
with an insulator
99
in the same fashion as the anode
89
, and is electrically connected to RF power source
93
via a matching box
92
. The RF power source
93
is grounded at
94
. A substrate
91
on which a nickel film to be etched is formed is placed on the cathode
90
.
However, the above-mentioned conventional methods of dry etching have problems as follows.
The first problem is that nickel is isotropically etched in accordance with any one of the conventional methods, causing side etching with the result of poor accuracy in nickel etching relative to a mask pattern. This is because that CO gas directly reacts with Ni, which causes isotropic etching, in a conventional method where Ni is etched with CO gas at 40° C. to 100° C.
In a method of dry etching Ni with plasma of CO gas without keeping a substrate at 40° C. or lower, a temperature of a substrate would be over 40° C. by ion radiation to the substrate while nickel is being etched. Thus, as a result, etching is carried out isotropically due to direct reaction of CO gas with Ni.
Similarly, in a method of dry etching Ni with plasma of CO
2
gas without keeping a substrate at 40° C. or lower, a temperature of a substrate would be over 40° C. by ion radiation to the substrate while nickel is being etched. As a result, etching is carried out isotropically due to direct reaction between CO generated by decomposition of CO
2
gas in plasma and Ni.
The second problem is that when nickel is etched with plasma singly of CO gas, excessive deposition is accumulated on a surface of a substrate, which causes reduction in a nickel etching rate, or in some cases, inability of nickel etching. The reason is as follows. In decomposition of CO
2
gas, there are found two stages. Namely, CO
2
gas is first decomposed into CO and O in the first stage, and CO is further decomposed into C and O in the second stage. To the contrary, in decomposition of CO gas, CO gas is decomposed directly into C and O. Hence, non-volatile carbon is generated in a greater amount than CO
2
gas, resulting in that deposition onto a substrate is generated in a greater amount.
The third problem is that plasma singly of CO gas and plasma singly of CO
2
gas could not etch nickel without causing a damage to an underlying substrate. The reason is as follows. If nickel is etched with plasma singly of CO gas or plasma singly of CO
2
gas, nickel is oxidized at a surface thereof by O or O
2
generated by decomposition of CO and CO
2
. Hence, if input power were decreased in order to prevent an underlying substrate from being damaged due to ion radiation, it would be impossible to remove a surface oxidation film of nickel, and as a result nickel could not be etched.
If nickel is etched with plasma of CO
2
and H
2
gases, nickel tends not to be oxidized at a surface thereof, which avoids inability of nickel etching caused by oxides formed on a surface of nickel. However, it would be necessary to increase input power to be provided to a discharge electrode in order to decompose CO
2
gas by plasma to thereby su
Cain Edward J.
Hayes Soloway Hennessey Grossman & Hage PC
NEC Corporation
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