Dry etching of silicon carbide

The invention comprises a method of etching silicon carbide targets. In one embodiment, a reactive ion plasma is formed from a gas which is easily dissociated into its elemental species in the plasma, for which all of the dissociated elemental species are volatile in the plasma, and for which at least one of the elemental species is reactive with silicon carbide. The silicon carbide target to be etched is positioned on one of the electrodes which is formed from a material with a low sputter yield and which material reacts with a dissociated species to thereby prevent contamination of the target with either sputtered materials from the electrode or polymerized species from the plasma.

Field of the Invention 
The present invention generally relates to dry etching methods for making 
electronic devices such as semiconductor devices or the like, and 
especially to a dry etching technique for etching silicon carbide (SiC). 
BACKGROUND OF THE INVENTION 
In the manufacture of electronic devices from semiconductor materials, one 
technique of interest is the selective building up, followed by the 
selective removal, of various layers of different materials, so that the 
building up and removal processes result in specific electronic devices 
such as transistors, diodes, capacitors and the like. 
One technique for removing layers of semiconductor or other materials from 
a given substrate is known as etching, which is the removal of a material 
following its interaction with another material generally referred to as 
the etchant. Etching techniques fall into two broad categories: wet 
etching which generally refers to techniques which take place in solutions 
or molten salts or other liquid materials; and dry etching which generally 
refers to the use of gases or plasmas to perform the removal which is 
desired. 
Dry etching techniques are of particular interest in producing electronic 
devices because they generally exhibit better resolution and improved 
dimensional and shape control capabilities than do the various methods of 
wet etching. Accordingly, dry etching is favorably utilized where superior 
pattern control and delineation is required such as the processing of 
semiconductor wafers to form large scale integrated devices and integrated 
circuits. 
Dry etching techniques can be used to micromachine mesas, isolation 
trenches, backside contact via holes, and other forms of pattern 
definition on thin films, substrates, or other materials. 
One perennial candidate material for use in semiconductor devices--and 
which correspondingly requires etching in order to form certain of these 
devices--is silicon carbide (SiC). Silicon carbide has long been 
recognized as having certain favorable characteristics as a semiconductor 
material, including its wide bandgap, high thermal conductivity, high 
saturated electron drift velocity, and high electron mobility. To date, 
however, silicon carbide has not reached the commercial position in the 
manufacture of electronic devices that would be expected on the basis of 
its excellent semiconductor properties. This is a result of the 
difficulties encountered in working with silicon carbide: high process 
temperatures are often required, good starting materials can be difficult 
to obtain, certain doping techniques have heretofore been difficult to 
accomplish, and perhaps most importantly, silicon carbide crystallizes in 
over 150 polytypes, many of which are separated by very small 
thermodynamic differences. Accordingly, controlling the growth of single 
crystals or monocrystalline thin films of silicon carbide which are of a 
sufficient quality to make electronic devices practical and useful, has 
eluded researchers. 
Recently, however, a number of developments have been accomplished which 
offer the ability to grow large single crystals of device quality silicon 
carbide, thin films of device quality silicon carbide, and to introduce 
dopants into silicon carbide, as required in the manufacture of many 
electronic devices. These developments are the subject of co-pending 
patent applications which have been assigned to the common assignee of the 
present invention and which are incorporated herein by reference. These 
include: "Growth of Beta-SiC Thin Films and Semiconductor Devices 
Fabricated Thereon," Ser. No. 113,921, filed Oct. 26, 1987; "Homoepitaxial 
Growth of Alpha-SiC Thin Films and Semiconductor Devices Fabricated 
Thereon," Ser. No. 113,573, filed Oct. 26, 1987; "Sublimation of Silicon 
Carbide to Produce Large, Device Quality Single Crystals of Silicon 
Carbide," Ser No. 113,565, filed Oct. 26, 1987; and "Implantation and 
Electrical Activation of Dopants into Monocrystalline Silicon Carbide," 
Ser. No. 113,561, filed Oct. 26, 1987. 
With the successes offered by these developments, an appropriate technique 
for etching silicon carbide is likewise desirable, for example in the 
production of mesatype structures or any other structures in which etching 
is required. 
A number of investigators have attempted to develop methods for etching 
silicon carbide under circumstances in which silicon carbide is used as a 
mask for a patterning process rather than as the active semiconductor 
portion of an electronic device. An early U.S. Pat. No. 3,398,033, to 
Haga, discusses a method of etching silicon carbide using a mixture of 
oxygen and chloride heated to between 1200.degree. and 1300.degree. C. 
According to Haga this process partially deteriorates the silicon carbide, 
after which the remainder can be removed by a wet reaction in a mixture of 
hydrofluoric and nitric acids. 
Yonezawa, U.S. Pat. No. 4,351,894, also discusses the use of silicon 
carbide as a mask material in manufacturing electronic devices from other 
semiconductor materials. According to Yonezawa, removal of silicon carbide 
is accomplished by either a plasma etching process using carbon 
tetrafluoride and oxygen, or by an electrolytic etching technique in which 
an electrolyte is selected from a mixture of perchloric acid, acetic acid 
and water; or from formic acid; or a mixture of sulfuric acid and water. 
In a later patent, U.S. Pat. No. 4,560,642, Yonezawa discusses a slightly 
different technique for using silicon carbide as a mask material, but 
describes identical etching processes for removing the silicon carbide 
mask. 
Yamazaki, U.S. Pat. No. 4,595,453, discusses a method of forming a 
semiconductor substrate rather than a mask, which may be formed of silicon 
carbide. Yamazaki suggests using hydrogen fluoride gas (HF) as the 
reactive gas plasma for selectively or nonselectively etching the 
semiconductor silicon carbide substrate. 
In the scientific literature, Lu et al., Thermal Oxidation of Sputtered 
Silicon Carbide Thin Films, J. Electrochem. Soc., 131, 1907 (1984), 
discuss masking techniques using amorphous silicon carbide films, and 
using mixtures of tetrafluoramethane (CF.sub.4) and oxygen, as well as 
nitrogen trifluoride (NF.sub.3) as mask-removing reactive ion etching 
plasmas. The thin films described by Lu, however, are sputter deposited 
films of silicon carbide, a technique which results in either amorphous 
layers or partially polycrystalline layers which for practical purposes 
are amorphous. As is known to those familiar with semiconductor materials 
and their properties, such amorphous or polycrystalline materials are 
essentially useless for forming the active portion of semiconductor 
devices. Furthermore, Lu offers only a general discussion of the etching 
he reports carrying out. 
By way of background, reactive ion etching is a procedure in which the 
material to be etched, sometimes called the target, is placed on a cathode 
in an electric field, and in the presence of a selected vaporized 
material. A potential is applied across the anode and cathode which is 
sufficient to ionize atoms and molecules in the vapor, as well as to 
produce some radicals. The potential difference accelerates positively 
charged ions in the vapor towards the target on the cathode. As these ions 
strike the material, they physically etch it away. In reactive ion 
etching, the vaporized material is selected to chemically react with the 
target material, thus enhancing the effects of the physical collisions. 
In producing devices using such dry etching techniques, certain problems 
occur which must be addressed before successful results can be obtained. 
For example, when a gas like tetrafluoromethane (CF.sub.4) is used as the 
reactant gas, polymerization tends to occur among the fluorocarbon 
radicals formed, which in turn cause fluorocarbon compounds to deposit 
onto the surface being etched. These impurities, however, are undesirable 
whenever the etched surface is to be used for silicon carbide-based 
electronic devices. Because high quality ohmic contacts must eventually be 
made on the etched surface in order to produce workable devices, a smooth, 
chemically clean etch is imperative. 
Accordingly, it is an object of the present invention to provide a method 
of dry etching monocrystalline silicon carbide which produces a smooth and 
chemically clean etched surface. 
It is another object of the invention to provide a method for dry etching 
of silicon carbide using plasma etching, reactive ion etching, or reactive 
ion beam etching processes. 
It is a further object of the invention to produce faster etch rates than 
have been possible to date for the dry etching of silicon carbide. 
It is another object of the invention to provide a method of dry etching of 
silicon carbide with nitrogen trifluoride and mixtures of 
nitrogentrifluoride with other gases such as oxygen or argon. 
It is a further object of the invention to provide a method of dry etching 
silicon carbide by controlling the etchant gas used to form the plasma and 
the electrodes used in the ionization process. 
It is another object of this invention to provide a method of dry etching 
of silicon carbide in which the etchant is efficiently broken into free 
radicals and for which all of the by-products of ionization are volatile. 
It is a further object of this invention to provide a method of reactive 
ion etching of silicon carbide using electrode materials which exhibit low 
sputter yield and which will react with the by-products of ionization so 
that they will not affect the etched surface.

SUMMARY OF THE INVENTION 
The invention comprises a method of etching monocrystalline thin films and 
single crystals of silicon carbide using nitrogen trifluoride (NF.sub.3). 
Following etching, the etched surfaces were characterized by Auger 
electron spectroscopy (AES) and scanning electron microscopy (SEM). By 
using a carbon cathode, a very smooth and much cleaner surface resulted 
than was obtained using other cathodes. In comparison to other techniques, 
the optimal conditions for etching silicon carbide are using NF.sub.3 and 
a carbon cathode. 
DETAILED DESCRIPTION 
The present invention comprises a method of dry etching of silicon carbide 
using NF.sub.3, as the etchant. In this technique, the NF.sub.3 gas is 
dissociated in a plasma and the dissociated species (radicals) etch the 
silicon carbide which is immersed in the plasma. As is known to those 
familiar with plasma physics, there is a synergistic effect between the 
radicals and the ions in the plasma that allow very inert materials to be 
etched. The radicals and ions that result from the plasma can be 
introduced to the silicon carbide in one of three different ways; plasma 
etching, reactive ion etching, and reactive ion beam etching. In plasma 
etching, no direct current bias is applied other than the field required 
to generate the plasma. In reactive ion etching (RIE), a direct current 
bias is applied to the system in addition to the plasma-generating field 
and induces directional ion bombardment. Where desired, a magnet can be 
positioned behind the target to enhance the RIE process, a technique known 
as magnetron etching. In reactive ion beam etching, the ions are directed 
from a source chamber to a negatively biased silicon carbide target, which 
is also often maintained in a higher vacuum (lesser pressure) target 
chamber. The NF.sub.3 can be ionized in either a radio frequency (rf), 
microwave, direct current (DC) or electron cyclotron resonance (ECR) 
plasma, depending upon the desired process. 
Silicon carbide is chemically very inert and can only be conventionally 
etched by molten salts, or by chlorine or hydrogen gases at high 
temperatures. The dry etching techniques of the present invention offer a 
more controllable low-temperature method of etching silicon carbide. As 
further stated above, however, previous attempts have shown that 
dry-etching silicon carbide using more conventional gases such as CF.sub.4 
leaves a graphic surface contaminated with materials such as fluorocarbon 
polymers. These surface conditions are undesirable where the etched 
surface is being prepared for making ohmic contacts for an electronic 
device. 
Therefore, a desirable gas for etching silicon carbide should be readily 
dissociable into etching radicals and not into non-etching radicals, ions 
or molecules which merely contaminate the target. Nitrogen trifluoride is 
one such desirable gas because it can be efficiently broken into nitrogen 
and free fluorine radicals, and because all of the possible by-products of 
the ionization process are volatile. As a result, surfaces etched with 
NF.sub.3 tend to be very clean. Where desired, the NF.sub.3 can be mixed 
with other gases, particularly oxygen (O.sub.2) and argon (Ar) in the 
method of the present invention. 
In particular embodiments of the present invention, thin films of beta 
silicon carbide which had been epitaxially grown on silicon (100) 
substrates were etched. As indicated in the co-pending applications 
incorporated by reference, however, it is now possible to successfully 
grow device quality silicon carbide thin films upon silicon carbide 
substrates and the etching techniques of the present invention are equally 
applicable under these circumstances. 
Following the growth of the beta silicon carbide thin films, the samples 
were polished with diamond paste, oxidized, etched in hydrofluoric acid, 
and rinsed in deionized water. The samples that were analyzed using 
scanning electron microscopy (SEM) were masked with evaporated aluminum 
that was patterned using standard photolithographic techniques. The 
samples used for Auger electron spectroscopy (AES) were left unmasked. 
In a particular embodiment of the invention, the reactive ion etching 
techniques were performed in a parallel plate reactor with a 28.0 cm 
diameter aluminum anode, a 17.0 cm diameter anodized aluminum cathode, and 
a plate separation of 5.0 cm. The samples were placed directly on the 
anodized aluminum cathode or, as discussed more fully herein, on a carbon 
cover plate over the anode. The samples were kept at temperatures of 308K. 
A 13.56 megahertz (MHz) rf supply powered the cathode at densities of 
0.440 to 0.548 watts per square centimeter (W/cm.sup.2). The chamber was 
evacuated to a pressure of 5.times.10.sup.-5 Torr and the NF.sub.3 was 
introduced at 25 standard cubic centimeters per minute (sccm) while the 
pressure was maintained at 40 millitorr (4.times.10.sup.-2 torr). In 
general, flow rates of from about 1 to about 500 sccm are appropriate with 
about 20 to about 60 sccm preferred. It should be understood that nitrogen 
trifluoride gas is an extremely toxic material and must be handled 
carefully and under the proper precautions in carrying out the techniques 
of the present invention. 
As is known to those familiar with plasma science, a plasma can be formed 
under a number of gas pressure and electric power conditions. For example, 
natural lightning is a plasma effect which takes place at atmospheric 
pressure under the application of enormous amounts of electric power. For 
scientific and commercial purposes, however, normally available power 
supplies make plasma formation much more feasible at lower gas pressures. 
At moderate power supply levels (typically between about 10 and about 400 
Watts to the powered electrode) a gas pressure of between about 5 and 
about 60 milliTorr (mTorr, i.e. between about 5.times.10.sup.-3 and 
60.times.10.sup.-3 mTorr) are appropriately used, although in some cases, 
pressures high as 10 Torr are still appropriate. Using typical equipment, 
this results in power densities of between about 0.04 and about 2 watts 
per square centimeter (W/cm.sup.2), with densities of between about 0.4 
and about 0.9 W/cm.sup.2 preferred. At generally lower pressures, the 
amount of gas present is insufficient to support the number of collisions 
required to maintain the plasma. At generally higher pressures, so much 
gas is present that the increased number of collisions tends to first 
dampen, and then quench, the plasma. It should be understood, therefore, 
that the selection of temperatures, power levels, and gas pressures used 
in practicing the present invention can vary widely and that those 
specified herein are given by way of example, and not as limitations on 
the scope of the invention. 
Auger electron spectroscopy was used to analyze both the starting materials 
and the etched surfaces. FIG. 1(a) shows a typical AES spectrum for 
unetched beta silicon carbide, and indicates the presence of silicon, 
carbon, and oxygen, present as a native oxide. 
As stated earlier, a polymerization problem is commonly associated when 
halogenated hydrocarbons such as CF.sub.4 are used as the etchant. This is 
illustrated by the AES spectra of FIGS. 4(a) and 4(b) which show a 
significant presence of fluorine (F) on the etched surfaces. It has been 
discovered according to the present invention, however, that fluorine is 
the chief reactant for both silicon and carbon. This reaction scheme makes 
NF.sub.3 a desirable gas for etching SiC because it is more efficiently 
broken into free F radicals than are fluorinated hydrocarbons, and 
secondly because all of the possible by-products of its ionization are 
volatile. The practical advantages of these theoretical advantages were 
observed in the present invention in which etch rates were observed as 
fast as 211 nanometers per minute (nm/min), which are the highest etch 
rates ever reported for the dry etching of silicon carbide using any 
technique. 
In the particular embodiment represented by FIGS. 3, 4 and 5, the reactive 
ion etching was carried out using an anodized aluminum cathode to support 
the SiC target. Under these conditions, however, there existed an apparent 
presence of some aluminum oxide (Al.sub.2 O.sub.3) on the etched surface, 
as demonstrated by the Auger electron spectrum of FIG. 4(b). Additionally, 
the somewhat rough surface shown in the micrograph of FIG. 5 was observed 
following the NF.sub.3 etching using the anodized aluminum cathode. 
Although applicant does not wish to be bound by any particular theory, it 
is believed that this results from Al.sub.2 O.sub.3 which has sputtered 
from the cathode and masked small portions of the surface, a process 
referred to as "micromasking." The presence of iron (Fe) in the AES 
spectrum of FIG. 1(b) and the rough surface seen in the micrograph of FIG. 
2 demonstrate the occurrence of this same effect when a stainless steel 
cathode was used to support the target. 
The presence of fluorine on the etched surface is also demonstrated by the 
AES spectrum in FIG. 4(b). Presently, it is believed that the lack of 
reactivity between the anodized aluminum cathode and the fluorine species 
allows the fluorine species to undesirably accumulate and polymerize on 
the etched surface. 
Therefore, in order to eliminate the surface deposition of both fluorine 
and Al.sub.2 O.sub.3, in a preferred embodiment of the invention a carbon 
cathode cover plate was placed in the RIE chamber. Carbon has several 
advantages in the method of the present invention: it has a very low 
sputter yield, and it is reactive with fluorine. As is known to those 
familiar with these technologies, sputtering occurs when an ion strikes a 
surface knocking an atom, molecule or ion loose from that surface as a 
result of the energy imparted by the impinging ion. This sputtered 
particle can be ejected from the cathode and deposit on the surface being 
etched, leading to the undesired impurity or micromasking referred to 
earlier herein. Of course, under certain circumstances, sputtering is a 
desired technique for depositing a particular material upon another 
material. 
When the carbon cathode cover plate was used for the reactive ion etching 
of silicon carbide in pure NF.sub.3, the greatly improved AES spectrum 
shown in FIG. 6(b) resulted. This indicates that the surface is very 
clean, with no fluorine peak visible and only a minor amount of nitrogen 
present after etching. As stated above, the carbon in the cathode reacts 
with the fluorine, keeping the fluorine from accumulating and polymerizing 
upon the etched silicon carbide surface. 
The differences between the use of the carbon cathode cover and the 
anodized aluminum cathode are demonstrated by FIG. 5 (anodized aluminum 
cathode) and FIGS. 7, 8 and 9 which shows the smooth, etched surface of 
silicon carbide after reactive ion etching in CF.sub.4 (FIG. 7) and in 
nitrogen trifluoride (FIGS. 8 and 9) using the carbon cathode. 
Additionally, in spite of the use of carbon cathode in this embodiment, 
FIG. 6(b) shows no significant increase in the carbon signal of the AES 
spectrum. 
The invention thus demonstrates that in the reactive ion etching of silicon 
carbide, the choice of cathode material plays a major role in the chemical 
and physical characteristics of the etched surface. Because high quality 
ohmic contacts require both a smooth and a chemically clean surface, the 
invention provides an optimal configuration for dry etching of silicon 
carbide by using nitrogen trifluoride and a carbon cathode. Given these 
characteristics, the invention comprises a method of patterning silicon 
carbide in the formation of electronic devices. By forming an appropriate 
masked pattern on the silicon carbide, etching it according to the present 
invention, and then removing the mask, desired patterns and devices can be 
fabricated. 
In the specification, there have been set forth preferred and exemplary 
embodiments, which have been included by way of example and not 
limitation, the scope of the invention being set forth in the following 
claims: