Abstract:
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.

Description:
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: &#34;Growth of Beta-SiC Thin Films and Semiconductor Devices Fabricated Thereon,&#34; Ser. No. 113,921, filed Oct. 26, 1987; &#34;Homoepitaxial Growth of Alpha-SiC Thin Films and Semiconductor Devices Fabricated Thereon,&#34; Ser. No. 113,573, filed Oct. 26, 1987; &#34;Sublimation of Silicon Carbide to Produce Large, Device Quality Single Crystals of Silicon Carbide,&#34; Ser No. 113,565, filed Oct. 26, 1987; and &#34;Implantation and Electrical Activation of Dopants into Monocrystalline Silicon Carbide,&#34; 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° and 1300° 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 4 ) and oxygen, as well as nitrogen trifluoride (NF 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 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. 
    
    
     The foregoing objects and advantages and other features of the invention will be set forth in the accompanying detailed description, in which preferred and exemplary embodiments are set forth, taken in conjunction with the accompanying drawings in which: 
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows two Auger electron spectra obtained for an unetched silicon carbide surface and a silicon carbide surface after reactive ion etching in carbon tetrafluoride using a stainless steel cathode; 
     FIG. 2 is an electron micrograph of a silicon carbide surface after reactive ion etching in carbon tetrafluoride using a stainless steel cathode; 
     FIG. 3 is an electron micrograph of an etched surface of silicon carbide following a reactive ion etching in carbon tetrafluoride using anodized aluminum cathode; 
     FIG. 4 illustrates two Auger electron spectra of an etched surface of silicon carbide following (a) reactive ion etching in carbon tetrafluoride, and (b) reactive ion etching in NF 3 , both using an anodized aluminum cathode; 
     FIG. 5 is an electron micrograph of an etched surface of silicon carbide after reactive ion etching in NF 3  using an anodized aluminum cathode; 
     FIG. 6 illustrates two Auger electron spectra for a sample of silicon carbide etched with (a) CF 4 , and (b) NF 3 , both using a carbon cathode; 
     FIG. 7 is an electron micrograph of an etched surface of silicon carbide following reactive ion etching in carbon tetrafluoride, using a carbon cathode; 
     FIG. 8 is an electron micrograph of an etched surface of silicon carbide following reactive ion etching in NF 3  using a carbon cathode; and 
     FIG. 9 is another electron micrograph of an etched surface of silicon carbide following reactive ion etching in NF 3  using a carbon cathode. 
    
    
     SUMMARY OF THE INVENTION 
     The invention comprises a method of etching monocrystalline thin films and single crystals of silicon carbide using nitrogen trifluoride (NF 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 3  and a carbon cathode. 
     DETAILED DESCRIPTION 
     The present invention comprises a method of dry etching of silicon carbide using NF 3 , as the etchant. In this technique, the NF 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 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 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 3  tend to be very clean. Where desired, the NF 3  can be mixed with other gases, particularly oxygen (O 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 2 ). The chamber was evacuated to a pressure of 5×10 -5  Torr and the NF 3  was introduced at 25 standard cubic centimeters per minute (sccm) while the pressure was maintained at 40 millitorr (4×10 -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 5×10 -3  and 60×10 -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 2 ), with densities of between about 0.4 and about 0.9 W/cm 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 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 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 2  O 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 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 2  O 3  which has sputtered from the cathode and masked small portions of the surface, a process referred to as &#34;micromasking.&#34; 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 2  O 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 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 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: