Etching of Silicon Nitride

The present invention is concerned with etching silicon nitride and particularly etching silicon nitride at a rate at least as fast as that for silicon dioxide. In addition, the present invention is concerned with certain etching compositions that are suitable for etching silicon nitride at a rate at least as fast as that for silicon dioxide.

In the fabrication of microelectronic components, a number of the steps involved, for instance, in preparing integrated circuit chips and the packaging for the chips (articles to which the chips are attached and protected), are etching processes. Accordingly, over the years, a number of vastly different types of etching processes to remove material, sometimes in selective areas, have been developed and are utilized to varying degrees. Moreover, the steps of etching different layers which constitute, for instance, the finished integrated circuit chip are among the most critical and crucial steps.

One method widely employed for etching is to overlay the surface to be etched with a suitable mask and then immerse the surface and mask in a chemical solution which attacks the surface to be etched, while leaving the mask intact and while only etching other materials of the article to at most, a minimum extent.

In many structures, a silicon nitride layer exists on top or beneath a silicon dioxide layer and it becomes necessary to etch the silicon nitride without overetching the silicon dioxide. The ability to use a fluoride such as hydrogen fluoride in such situation presents a problem since hydrogen fluoride etches silicon dioxide at a much faster rate than it does silicon nitride. It would therefore be desirable to provide an etching procedure that achieves excellent removal of the silicon nitride and especially a procedure that exhibits an etching rate for silicon nitride that is at least as fast as the rate for silicon dioxide.

The present invention is concerned with a method for etching silicon nitride. The process of the present invention comprises contacting an article that contains silicon nitride and silicon dioxide with an etching composition that contains about 0.1 to about 3 molar of a fluoride containing compound, an organic solvent and about 0.1 to about 4 molar of water to thereby etch the silicon nitride at a rate at least as fast as the rate for the silicon dioxide.

In addition, the present invention is concerned with certain preferred etching compositions that are capable of etching silicon nitride at a rate at least as fast as the rate for silicon oxides. The preferred compositions of the present invention contain about 0.1 to about 3 molar of a fluoride containing compound, an organic solvent, and about 0.1 to about 4 molar of water.

The organic solvents employed in the present invention typically have relatively high flash point and provide low viscosity compositions. Suitable solvents include sulfolanes, oxolanes, esters, ketones, aldehydes, lactones, halogenated hydrocarbons, monohydric alcohols, amines and imides.

Other objects and advantages of the present invention will become readily apparent to those skilled in this art from the following detailed description, wherein it is shown and described only the preferred embodiments of the invention, simply by way of illustration of the best mode contemplated of carrying out the invention. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, without departing from the invention. Accordingly, the drawings and description are to be regarded as illustrative in nature and not as restrictive.

The present invention is concerned with etching a silicon nitride. Moreover, the silicon nitride is present in an article that also includes silicon dioxide, wherein both the silicon nitride and silicon dioxide will come in contact with the etching composition. The present invention provides for etching the silicon nitride at a rate at least as fast as the rate for silicon dioxide and preferably at a rate greater than that for silicon dioxide and most preferably at a rate at least about 10 percent greater.

The silicon dioxide is preferably high density silicon dioxide. High density silicon dioxide refers to thermally grown silicon dioxide or silicon dioxide that is chemically vapor deposited (CVD) or physically vapor deposited (PVD) followed by thermal annealing.

The etching compositions employed pursuant to the present invention contain a compound containing fluoride and an organic solvent. The amount of the fluoride containing compound in the composition is about 0.1 to about 3 molar, and preferably about 0.15 to about 1.5 molar, and most preferably about 0.2 to about 1 molar.

Typical compounds providing a fluoride source according to the present invention are hydrofluoric acid, ammonium fluoride, fluoroborates, fluoroboric acid, tin bifluoride, antimony fluoride, tetrabutylammonium tetrafluoroborate, and aluminum hexafluoride. Also, a fluoride salt of an aliphatic primary, secondary or tertiary amine can be used. Such have the following formula:

wherein R.sub.1, R.sub.2 and R.sub.3 each individually represent H or an alkyl group.

Typically, the total number of carbon atoms in the R.sub.1, R.sub.2 and R.sub.3 groups is 12 carbon atoms or less. The preferred fluoride compounds are hydrogen fluoride and ammonium fluoride.

The hydrogen fluoride is typically employed as a 49 weight percent aqueous solution.

Examples of suitable organic compounds include oxolanes, sulfolanes, esters, ketones, aldehydes, lactones, halogenated solvents, amines, imides and monohydric alcohols. Examples of suitable esters are esters of carbonic acids, benzoic acid, phthalic acid, isophthalic acid and terephthalic acid, and especially the C.sub.1 -C.sub.6 alkyl esters. Preferred organic solvents are propylene carbonate, N-methyl pyrrolidone, gamma butyrolactone, methylene chloride, benzyl alcohol, N-formyl morpholine, N-formyl piperidine, cyclohexanone, cyclopentanone, methyl benzoate, diglyme, 2-methyl tetrahydrofuran, and methyl and ethyl esters of phthalic, isophthalic or terephthalic acids. The more preferred solvents employed pursuant to the present invention are propylene carbonate, N-methylpyrrolidone and gamma butyrolactone, with propylene carbonate being the most preferred.

Organic solvents employed in the present invention provide for low viscosity compositions, which in turn render the compositions more workable and providing for better etch uniformity. This is especially so when compared to using polyhydric alcohols such as glycerol and ethylene glycol.

The compositions according to the present invention also include water. Typically, the water is employed in an amount of about 0.1 to about 4 molar, preferably about 0.15 to about 2 molar, and most preferably about 0.2 to about 1.5 molar.

The etching process of the present invention is typically carried out at temperatures of about 20.degree. C. to about 90.degree. C., and preferably about 30.degree. C. to about 70.degree. C. The temperature employed is directly dependent upon the fluoride concentration. In particular, as the fluoride concentration increases, it becomes necessary to also increase the temperature in order to achieve the desired etching rate of the silicon nitride in comparison to that for the silicon dioxide. For instance, at a fluoride concentration of 0.25 molar, to achieve an etch rate for the silicon nitride that is as fast as that for the silicon dioxide, the temperature is about 55.degree. C.; whereas, at a concentration of 1.5 molar, the temperature is about 70.degree. C. The minimum temperature to use for any specific fluoride concentration can be determined without undue experimentation by those skilled in the art once aware of this disclosure.

The ability to use fluoride to etch silicon nitride at a rate equal to or greater than that for silicon dioxide is surprising in view of the prior experience with hydrogen fluoride in etching silicon dioxide at a much higher rate than that for silicon nitride.

The following non-limiting examples are presented to further illustrate the present invention.

An etching composition is prepared by admixing one part by volume of a 49 percent by weight aqueous solution of HF with about 112 parts by volume of propylene carbonate to provide a 0.25 molar HF solution in propylene carbonate. A blanket silicon wafer having a layer thermally grown silicon dioxide beneath a layer of silicon nitride is contacted with the etching composition. The article contains deep trenches thereby exposing the walls of the underlying silicon dioxide layer to the etchant. The etching is carried out at a temperature of about 60.degree. C. for about 20 minutes. The etching solution etches the silicon nitride at a rate of about 0.42 nanometers/minute and the silicon dioxide at a rate of about 0.36 nanometers/minute, resulting in a selectivity of silicon nitride etching relative to the silicon dioxide of about 1.2.

An etching composition is prepared by admixing one part by volume of a 49 percent by weight aqueous solution of HF with about 70 parts by volume of propylene carbonate to provide a 0.4 molar HF solution in propylene carbonate. A blanket silicon wafer having a layer thermally grown silicon dioxide beneath a layer of silicon nitride is contacted with the etching composition. The article contains deep trenches thereby exposing the walls of the underlying silicon dioxide layer to the etchant. The etching is carried out at a temperature of about 65.degree. C. for about 11 minutes. The etching solution etches the silicon nitride at a rate of about 1.2 nanometers/minute and the silicon dioxide at a rate of about 1.0 nanometers/minute, resulting in a selectivity of silicon nitride etching relative to the silicon dioxide of about 1.2.

In this disclosure, there are shown and described only the preferred embodiments of the invention but, as aforementioned, it is to be understood that the invention is capable of use in various other combinations and environments and is capable of changes or modifications within the scope of the inventive concept as expressed herein.