Method of Depositing Uniform Films of Si.sub.x N.sub.y or Si.sub.x O.sub.y in a Plasma Reactor

Broadly speaking, this invention relates to plasma deposition. More particularly, in a preferred embodiment, this invention relates to a method of depositing a two-component film of material from successive, one-component gas plasmas.

Plasma deposition is a technique that is widely used in industry to deposit thin films of material on a variety of workpieces. For example, in the manufacture of integrated circuits, an RF-excited plasma comprising an inert gas, such as argon, and the reactive species silane, (SiH.sub.4,) and ammonia, (NH.sub.3,) is used to deposit a thin film of SiN, the designation given to compositions approaching Si.sub.3 N.sub.4 but not necessarily identical with Si.sub.3 N.sub.4, on a silicon wafer approximately 3 inches in diameter. The SiN film can then be used as mechanical protection during subsequent processing of the wafer, as a dielectric insulating layer between 2 layers of metallization, or (if improved) as a hermetic seal.

It will be appreciated that among the desirable properties that must be possessed by such a silicon nitride film are uniformity of thickness, uniformity of composition and freedom from defects such as impurities and pin-holes. Unfortunately, the prior art plasma deposition processes are unpredictable and such undesirable defects are quite common, occurring randomly, for no apparent reason.

One mechanism that has been proposed to explain at least some of the observed defects is a reaction of the form:

where

4 4 and m.ltoreq.3,

where * indicates some metastable excited configuration and where the component Si.sub.x N.sub.y H.sub.z can be in an excited state or in a ground state, the said reaction taking place in the plasma rather than on the surface of the substrate.

As might be expected, such a reaction would produce particulate matter that would deposit onto the surface of the substrate. One reason that has been suggested for the occurrence of this reaction is that, once a molecule of Si.sub.x N.sub.y H.sub.z has formed in the plama, it provides all of the incentives for plasma deposition that the substrate itself provides.

As a solution to this problem, I propose a method wherein a thin, uniform film of porous a-Si.sub.x H.sub.y is deposited on a silicon substrate by means of a one-component plasma comprising a silicon-containing active radical, such as silane or silicon tetrachloride. After the porous film has been deposited, this one-component plasma is withdrawn and the film oxidized or nitrided in situ by flowing activated oxygen or nitrogen over the substrate.

The invention and its mode of operation will be more fully understood from the following detailed description.

The instant invention provides for the plasma deposition of denser, and more uniform films of Si.sub.x N.sub.y, Si.sub.x N.sub.y O.sub.z or Si.sub.x O.sub.y in a two-step process utilizing successive one-component plasmas.

As an example, consider the deposition of Si.sub.x N.sub.y. First, using an entirely conventional Reinberg-type plasma reaction chamber, or equivalent, a thin, uniform film of Si.sub.x H.sub.y is deposited on a silicon substrate using an RF-excited SiH.sub.y +Ar gas plasma. The Reinberg reaction chamber is described in U.S. Pat. No. 3,757,733 which issued on Sept. 11, 1973 to A. R. Reinberg, which patent is hereby incorporated by reference as if more fully set forth herein. The ratio of y/x in the deposited Si.sub.x H.sub.y film is adjusted by controlling the temperature of the substrate, which will typically fall between 50.degree. C. and 300.degree. C.

The Si.sub.x H.sub.y film that is deposited is actually a porous, amorphous a-Si.sub.x H.sub.y film with its porosity and total hydrogen content being primarily determined by deposition temperature, deposition rate and the RF power which is applied to the reaction chamber. The properties of thin films of a Si.sub.x H.sub.y are described in "Infrared and Raman Spectra of the Silicon-Hydrogen Bonds in Amorphous Silicon Prepared by Glow Discharge and Sputtering" by M. H. Brodsky, M. Cardona and J. J. Cuomo, Phy. Rev. B., Vol. 16, No. 8, 15 October 1977 at Pg. 3556; "Properties of Amorphous Silicon and a-Si Solar Cells" by D. E. Carlson, C. R. Wronski, J. I. Pankove, P. J. Zanzucchi and D. L. Staebler, RCA REview, Vol. 38, June 1977, pp. 211-225 (Table I at Pg. 217 in particular); and "Controlling the Type of Bonded Hydrogen Sites in Glow-Discharge Amorphous Silicon Films" by M. H. Brodsky, IBM Technical Disclosure Bulletin, Vol. 19, No. 11, April 1977, pp. 4447-4450, all of which references are hereby incorporated by reference as if more fully set forth herein.

After an appropriate time interval, the flow of SiH.sub.4 +Ar is replaced by a flow of N.sub.2, or other suitable nitrogen-containing mixture.

Because the flow of SiH.sub.4 has been terminated, its premature decomposition is no longer a problem; thus, the pressure, the frequency of the RF-excitation and/or the input power to the reaction chamber may be increased to a point where activated nitrogen is produced in the chamber. Next, the temperature of the substrate is raised to more than 360.degree. C. so that hydrogen starts to diffuse out of the Si.sub.x H.sub.y. Since a-Si is porous, and since the departing hydrogen leaves behind dangling Si bonds, the diffusion of activated N into Si and the formation of Si-N bonds is accelerated, thus forming a uniform, impurity-free, dielectric film of Si.sub.x N.sub.y.

If extremely uniform layers of Si.sub.x N.sub.y are required, the process may be modified by laying down a very thin layer of a-Si.sub.x H.sub.y and repeating the procedure several times until the desired overall thickness is obtained. As an alternative, one could start the process by depositing a thin layer of Si.sub.x N.sub.y H.sub.z which is porous and N-deficient and complete the nitridation in a N* rich plasma by adding some nitrogen-containing compound to the initial gas plasma. However, the possibility of non-uniformity remains with this alternate embodiment.

The above-described process has the following advantages:

1. In a simple plasma reactor, the uniformity of a one-component film, such as Si, is more easy to obtain than the uniformity of a film which is deposited from a mixture of two gases which differ in activation energy, for example, SiH.sub.4 and N.sub.2 or SiH.sub.4 and NH.sub.3 ;

2. More complex compositions and layer structures can be attempted sequentially;

3. If the diffusion of the active radical (N*) into Si layer dominates, the film will be denser than ordinary SiN and it is possible that the tensile stress will decrease; and

4. Homogenous nucleation of Si.sub.x N.sub.y H.sub.z and the resulting particle contamination are eliminated.

One skilled in the art will appreciate that, by substituting a flow of activated oxygen for the flow of activated nitrogen in the above-described process, the film that would ultimately form on the substrate would be Si.sub.x O.sub.y rather than Si.sub.x N.sub.y. Similiarly, by flowing simultaneously both nitrogen and oxygen, the film would comprise Si.sub.x N.sub.y O.sub.z. It will also be appreciated that while RF-excitation of the plasma is the preferred embodiment, other forms of excitation are possible. Likewise, while silane is the preferred embodiment for the first plasma, other volatile Si compounds can be used.

One skilled in the art may make various changes to the above-described processes without departing from the spirit and scope of the invention.