Safe Method for Etching Silicon Dioxide

1. Field of the Invention

The present invention is generally related to a method of etching or cleaning off native or other silicon dioxide (SiO.sub.2) from the surface of a silicon (Si) substrate and, more particularly, to a safe method of etching or cleaning off SiO.sub.2 which employs anhydrous ammonium fluoride (NH.sub.4 F) as a source of hydrofluoric acid (HF) which is used for etching or cleaning off the SiO.sub.2.

2. Description of the Prior Art

If the surface of a silicon substrate is exposed to ambient air, a native SiO.sub.2 layer forms on its surface. The native SiO.sub.2 layer presents certain manufacturing difficulties since many semiconductor processes, such as low-temperature epitaxy, polysilicon deposition, and silicidation, require the silicon wafer surface to be free of all native SiO.sub.2. Over the years, many different processes for etching or cleaning off the native SiO.sub.2 from Si surfaces have been developed. One of the most commonly used techniques for etching SiO.sub.2 is to expose the substrate to HF (wet or dry). However, the use of liquid or vapor HF has major drawbacks for the semiconductor manufacturer because it is both highly toxic and highly corrosive. The high toxicity of HF requires the semiconductor manufacturer to implement safety procedures and devices which assure that personnel and the environment are not injured by the HF gas. The high corrosiveness of HF requires the semiconductor manufacturer to utilize non-corrosive storage containers and non-corrosive tubing for delivering the HF gas to a cleaning or etch chamber, and this type of equipment is generally more expensive than containers and tubing that do not have non-corrosive properties.

In U.S. Pat. No. 5,030,319 to Nishino et al. and in the article to Nishino et al., 1989 Dry Process Symposium IV-2, 90-92 (1989), a method of selectively etching native SiO.sub.2 from a silicon substrate is disclosed which employs the fluorine atoms and nitrogen hydrides produced by an NH.sub.3 and NF.sub.3 microwave discharge. In the Nishino et al. etch process, a film of (NH.sub.4).sub.2 SiF.sub.6 is formed by condensation on the surface of the substrate by the reaction of NH.sub.4.sup.+ and F.sup.- with the polarized Si atom of the ionic SiO.sub.2 species. For film formation to occur, the substrate temperature must be maintained below 100.degree. C., and is preferably performed at room temperature. After film formation, the substrate is heated above 100.degree. C. and the (NH.sub.4).sub.2 SiF.sub.6 film vaporizes to leave an oxygen free silicon surface.

The Nishino et al. etch process is not ideal because it requires the transport of corrosive and toxic gases into a reaction chamber; therefore, the cost of non-corrosive tubing and storage chambers as well as the requirement of increased safety procedures are not avoided by the Nishino et al. etch process. Moreover, the Nishino et al. process imposes timing and temperature control parameters on a semiconductor manufacturer since the wafer temperature must first be maintained below 100.degree. C. to allow an etching film to form on the substrate surface, and subsequently, the substrate must be heated above 100.degree. C. to remove the etch products. Nishino et al. acknowledge that if the temperature is kept above 100.degree. C. initially, film deposition does not take place and no SiO.sub.2 etching occurs.

It is therefore an object of this invention to provide a safe and non-corrosive method of removing native or other SiO.sub.2 from silicon surfaces which employs anhydrous NH.sub.4 F as a source of HF.

It is another object of this invention to provide a method of controlling the SiO.sub.2 etch rate with HF by regulating the amount of adsorbed water (H.sub.2 O) present on the SiO.sub.2.

According to the invention, anhydrous NH.sub.4 F is used as a safe source of HF to etch the native SiO.sub.2 from silicon surfaces. A supply of anhydrous NH.sub.4 F is provided either in the etching or cleaning chamber in which the silicon substrates are positioned or is provided in a chamber which is in close proximity to the etching or cleaning chamber and is connected therewith. Upon heating, anhydrous NH.sub.4 F sublimates to form HF gas and NH.sub.3 gas. The HF gas then etches off the native or other SiO.sub.2 on the surface of the silicon substrate.

Controlled amounts of water vapor are used during the etch reaction to ensure complete etching of the thin oxide layers down to within hundredths of a monolayer and to achieve precise etch rate control. Trace quantities of water aid in initiating the SiO.sub.2 etch reaction with HF and anhydrous HF vapor alone cannot be used to achieve complete etching of thin native or chemical oxides on patterned substrates where the rest of the pattern is bulk or thick oxide (see, McIntosh et al., J. of Elect. Mat., 21:57-60 (1992)). Therefore, precise quantities of water vapor are advantageously metered into the etch chamber prior to or during NH.sub.4 F sublimation for SiO.sub.2 etching. Water which is not adsorbed on the SiO.sub.2 surface is either pumped out of the etch chamber or is purged out by flowing dry inert gas over the SiO.sub.2 surface. In addition, water vapor is a by-product of the SiO.sub.2 etch reaction itself. Hence, the etch rate of subsequent oxide etches can vary from the first oxide etch if the quantity of water vapor within the etch chamber is not controlled. Therefore, periodic (e.g., after each SiO.sub.2 etch) water vapor evacuation or purging from the etch chamber assures that the etch rate remains controlled.

Anhydrous NH.sub.4 F sublimates at temperatures above 100.degree. C. and at pressures on the order of one to ten Torr according to the following reaction:

Anhydrous NH.sub.4 F is intrinsically safer than gaseous or aqueous HF because it absorbs water readily and is converted to a less reactive form, aqueous NH.sub.4 F, upon exposure to air. Aqueous NH.sub.4 F is used in making buffered HF solutions and does not form gaseous HF upon heating or evaporation. Anhydrous NH.sub.4 F is commercially available from a variety of sources including Aldrich Chemical, Sigma Chemical, and Alfa Chemical.

In the present invention, anhydrous NH.sub.4 F is provided either in the etching or cleaning chamber in which the silicon substrates having SiO.sub.2 are positioned or is provided in a chamber which is in close proximity to the etching or cleaning chamber and is connected therewith. The anhydrous NH.sub.4 F may be provided in an open or valved ampoule, container or the like. Upon heating the ampoule or container of anhydrous NH.sub.4 F above the sublimating temperature, HF gas is generated and the HF gas is used to etch the SiO.sub.2 from the surface of the silicon substrate. It is possible that the NH.sub.3 gas generated during the sublimation will also assist in the etching by acting as an electron donor in the reaction. Control of the delivery of the HF gas for etching SiO.sub.2 can be accomplished by regulating the temperature to which the ampoule or container of anhydrous NH.sub.4 F is exposed (i.e., modulating the temperature above and below sublimation temperature as HF gas is required, or similar processes). Hence, this invention avoids the problems associated with the handling and transport of HF gas by using anhydrous NH.sub.4 F as a source of HF gas in the same etch chamber as the silicon substrate or in a chamber in close proximity with and connected to the etch chamber.

In a test experiment, a silicon substrate with a thermally grown SiO.sub.2 layer was placed adjacent a container of anhydrous NH.sub.4 F. The substrate was maintained at ambient temperature, but the anhydrous NH.sub.4 F was heated to a temperature of 200.degree. C. such that it sublimated to form HF gas and NH.sub.3 gas. The SiO.sub.2 on the substrate surface was observed to be etched at a rate of approximately 30.ANG./min. Similar results would be achieved when the substrate is maintained at an elevated temperature above the temperature of NH.sub.4 F sublimation.

Water vapor is advantageously added to the etch chamber to assist in the etching of the SiO.sub.2 ; however, care must be taken to prevent the anhydrous NH.sub.4 F from being converted to its aqueous form prior to generation of the HF gas.

Controlled amounts of water vapor are used during the etch reaction to ensure complete etching of the thin oxide layers down to within hundredths of a monolayer (a level suitable for epitaxy) and to achieve precise etch rate control. Trace quantities of water aid in initiating the SiO.sub.2 etch reaction with HF. Moreover, McIntosh et al., J. of Elect. Mat., 21:57-60 (1992), show that anhydrous HF vapor alone cannot be used to achieve complete etching of thin native or chemical oxides on patterned substrates where the rest of the pattern is bulk or thick oxide. Therefore, precise quantities of water vapor are advantageously metered into the etch chamber prior to or during NH.sub.4 F sublimation to HF and NH.sub.3 for SiO.sub.2 etching. Water which is not adsorbed on the SiO.sub.2 surface is either pumped out of the etch chamber or is purged out by flowing dry inert gas over the SiO.sub.2 surface.

Water vapor is a by-product of the SiO.sub.2 etch reaction itself. Hence, the etch rate of subsequent oxide etches can vary from the first oxide etch if the quantity of water vapor within the etch chamber is not controlled. Therefore, periodic (e.g., after each SiO.sub.2 etch) water vapor evacuation or purging from the etch chamber assures that the etch rate remains controlled.

For thick oxide films, a repetitive process which includes the steps of exposure to water vapor, purge or evacuation of excess water vapor, and NH.sub.4 F sublimation for HF etching of SiO.sub.2 allows for fine control over the etch rate as well as ensures complete etching down to hundredths of a monolayer levels of oxide. This degree of etching provides an adequate pre-clean for subsequent processing steps including epitaxy and polysilicon deposition. Preferably, any subsequent processing steps will be performed in the same etch chamber or another chamber vacuum connected to the etch chamber immediately after etching the SiO.sub.2 layer such that regrowth of native SiO.sub.2 is prevented.

While the invention has been described in terms of its preferred embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the appended claims.