Process of etching with plasma etch gas

A method of etching aluminum in a planar plasma etcher is disclosed wherein a first plasma of carbon tetrachloride is developed followed by a second plasma of carbon tetrachloride and chlorine. By generating the specific claimed plasmas at specific partial pressures in two steps, the selectivity of the etching operation is greatly enhanced while disadvantages encountered in etching aluminum with prior art techniques are greatly reduced.

BACKGROUND OF THE INVENTION 
The gas plasma vapor etching process has recently been utilized for 
performing operations on semiconductor wafers by exposing the plasma to 
remove portions of material carried by the semiconductor structure. The 
extension of reactive plasma etching beyond the patterning of silicon, 
silicon nitride and silicon oxides to include aluminum etching offers 
several potential advantages in the production of small geometry 
integrated circuits. Plasma as compared to chemical etching of metal 
produces better edge definition, less undercutting, considerably less 
photoresist adhesion sensitivity, and the elimination of so-called "knee 
breakdown" due to thinning of the photoresist at sharp edges. This 
thinning where the aluminum goes over and down the side wall of a cut 
leads to premature resist failure during wet etching, thus permitting 
removal of the metal at the near edge. 
Although plasmas are not clearly understood, it is known that a special 
form of chemical materials can be made by exposing the compounds to high 
energy radio frequencies. Under the influence of these radio frequencies, 
compounds break down and rearrange to form transitory species with life 
spans so short that they are difficult to identify. Accordingly, 
unexpected reactions can be effected in a plasma that are difficult or 
impossible to effect using more conventional techniques. 
It has been recognized that the plasma etching of aluminum presents certain 
inherent problems which are not easily overcome. Such a realization was 
presented by Poulsen et al in an article entitled PLASMA ETCHING OF 
ALUMINUM wherein it was stated that aluminum cannot be plasma etched in 
common chlorine-based etch gases (e.g., Cl.sub.2 or HCl) due to the 
protective masking of aluminum by the thin aluminum oxide (Al.sub.2 
O.sub.3) layer that forms on aluminum surfaces upon exposure to air. 
Poulsen accomplished his etching by using boron trichloride (BCl.sub.3) 
and carbon tetrachloride (CCl.sub.4) plasmas. In this way, it was found 
that the aluminum oxide could be removed to allow etching of the exposed 
aluminum via a reaction with chlorine radicals in the plasma. Poulsen used 
a single etching process to remove both the protective aluminum oxide and 
underlying aluminum with one etch gas. 
In a second article entitled PLASMA ETCHING OF ALUMINUM by Herndon et al at 
the M.I.T. Lincoln Laboratory, it was taught that a two-step process could 
be employed by starting with boron trichloride or carbon tetrachloride 
followed by a second chemical etch of chlorine. The first plasma etching 
process removed the aluminum oxide coating while the second chemical etch 
acted to pattern the underlying aluminum layer. 
By practicing the methods of Pousen et al., Herndon et al. and others who 
have attempted to carry out a plasma etching process of aluminum, certain 
serious difficulties were encountered. For example, boron trichloride, 
either taken alone or in an argon, nitrogen or helium carrier gas is 
extremely corrosive. The plasma etching chamber and electrodes deteriorate 
extremely rapidly when boron trichloride is used to remove the aluminum 
oxide coating from the aluminum layer to be etched. Carbon tetrachloride 
is not as corrosive as boron trichloride but has extremely poor 
selectivity over silicon dioxide (Si0.sub.2) and photoresist. Aluminum is 
placed upon a silicon dioxide substrate in order to produce a 
semiconductor device. The aluminum is then protected by the image-wise 
application of a photoresist layer. An ideal plasma etching process would 
remove only the aluminum, and not the photoresist, while leaving the 
silicon dioxide untouched. As stated, carbon tetrachloride as a sole 
etchant gas does a very poor job in accomplishing these goals. 
The use of chlorine to etch aluminum is known and is shown in both articles 
referred to above. Practitioners have used chlorine with boron 
trichloride, but its use with carbon tetrachloride is not universal for it 
was found that chlorine inhibits the etching of aluminum oxide in a carbon 
tetrachloride system. Herndon et al. implies that experiments employing 
chlorine as a second etchant gas have been carried out, but applicant has 
found that the unrestricted use of chlorine as a second stage in the 
etching process causes isantropic etching. This means that chlorine does 
not produce acceptable edge definition for chlorine acts to undercut the 
photoresist and acts to etch aluminum below the photoresist in "protected" 
areas. Furthermore, the use of the combination of carbon tetrachloride and 
chlorine as etchant gases in the plasma etching of aluminum acts to form a 
polymer film, which is not capable of being removed from the substrate 
surface. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to plasma etch aluminum surfaces 
in the fabrication of semiconductor devices without the disadvantages 
outlined above. 
It is a further object of the present invention to carry out plasma etching 
of aluminum without the use of boron trichloride or other highly corrosive 
plasma species. 
It is yet another object of the present invention to carry out the plasma 
etching of aluminum while minimizing isantropic etching. 
It is still another object of the present invention to employ a carbon 
tetrachloride/chlorine etchant gas system without the formation of polymer 
films which are not capable of being removed from the substrate surface. 
It is still another object of the present invention to carry out the plasma 
etching of aluminum while maintaining the characteristic sharp vertical 
edge etching characteristics of carbon tetrachloride while using a carbon 
tetrachloride/chlorine plasma etchant system. 
It is still another object of the present invention to carry out the plasma 
etching of aluminum while decreasing photoresist degradation as much as 60 
percent. 
It is a further object of the present invention to plasma etch aluminum 
under reduced power requirements while producing an improved etching 
operation. 
Plasma etching is carried out in an apparatus similar to that disclosed in 
co-pending application Ser. No. 928,594 entitled "PLASMA ETCHING 
APATUS", filed July 27, 1978. Similar apparatus is also shown in U.S. 
Pat. No. 3,757,733 entitled "RADIAL FLOW REACTOR", issued on Sept. 11, 
1973, the disclosure of which is herein incorporated by reference. 
Generally, the apparatus comprises a holder for aluminum wafers which are 
to be etched. The holder sits on the surface of the lower electrode of a 
planar capacitor structure that forms part of the vacuum chamber. The etch 
gas is bled into the vacuum chamber at a flow rate controlled by a valve 
and the pressure set by throttling a mechanical vacuum pump. Pressure and 
flow rate are accurately monitored with a capacitance manometer and a mass 
flow meter. The upper and lower electrodes are carefully aligned relative 
to one another to achieve radially symmetric gas flow and electric fields 
in the plasma region for best etch rate uniformity. The RF power is 
coupled into the electrodes. Current is monitored for accurate power 
settings. The electrode spacing is adjustable and the temperature of the 
lower electrode is adjustably controlled.

In a typical etching application, the system is opened, the wafers are 
loaded on platens around the lower electrode, the system is evacuated, the 
etch gas is introduced, and etching begins when the plasma is struck 
between the 28" diameter electrodes. 
The objects of the present invention are accomplished by employing a first 
plasma of substantially pure carbon tetrachloride at a partial pressure of 
approximately 300 microns to remove the aluminum oxide coating from the 
pure aluminum layer. The power employed is approximately 3.5 amps and the 
wafer platform temperature is set at 70.degree. C. The protective layer of 
aluminum oxide was removed in approximately three minutes. 
The second stage involves employing a combination of carbon tetrachloride 
and chlorine wherein the partial pressure of the chlorine is approximately 
50 to 100 microns, while the partial pressure of carbon tetrachloride is 
approximately 80 to 150 microns. The total pressure of the system was 
maintained at approximately 250 microns. 
It was found that employing the etchant gases at the partial pressures 
recited above, a 1000 angstroms/min. etch rate could be achieved by 
employing only 2 amps of current, while prior art techniques employing 
pure carbon tetrachloride as the plasma etchant gas required 3 amps to 
maintain a 1000 angstroms/min. etch rate. 
Although the specific etch rates, power requirements and other parameters 
of the system are variable according to the specific design of the plasma 
etching apparatus which is employed, there are certain specific 
characteristics which must be maintained and which form the heart of the 
present invention. More specifically, it was found that if the partial 
pressure of the chlorine exceeded 100 microns, difficult to remove 
polymers built-up upon the aluminum layer. It was also found that if the 
total pressure during the second etchant stage employing chlorine and 
carbon tetrachloride exceeded approximately 250 microns, that a polymer 
film would also develop. It was also found that if the partial pressure of 
chlorine was allowed to exceed that of carbon tetrachloride, that 
isantropic etching occurred. Lastly, it was found that if the partial 
pressure of chlorine was less than one-half the partial pressure of carbon 
tetrachloride, the effectiveness of the chlorine gas as an etchant for 
aluminum was negligent. Thus, the present invention comprises a two-step 
plasma etchant process wherein a first etchant operation removes the 
aluminum oxide from the aluminum surface. The preferred first step 
employed a pure carbon tetrachloride plasma at or below approximately 300 
microns. A second etchant operation was carried out by using carbon 
tetrachloride and chlorine in combination wherein the total pressure of 
the system did not exceed 250 microns, and preferably between 150 to 250 
microns, the partial pressure of chlorine was kept below 100 microns, the 
partial pressure of chlorine was not allowed to exceed that of carbon 
tetrachloride and, lastly, the partial pressure of chlorine was greater 
than one-half the partial pressure of carbon tetrachloride. If was further 
found that the most optimum etchant characteristics were achieved when, 
during the second cycle, the partial pressure of chlorine was kept between 
50 to 100 microns while the partial pressure of carbon tetrachloride was 
kept between 80 to 150 microns. 
By employing the process of the present invention, it was possible to 
maintain an etching selectivity of at least approximately 25 to 1. That 
is, bare aluminum would etch 25 times faster than silicon dioxide. 
It is to be understood that the foregoing description of specific partial 
pressures of the various species is made by way of example and is not to 
be considered as a limitation on the scope of the present invention.