Method of growing a thin film onto a substrate

Details Method of growing a thin film onto a substrate Method of growing a thin film onto a substrate

2001-04-16

2004-08-31

Kunemund, Robert (Department: 1765)

Single-crystal, oriented-crystal, and epitaxy growth processes;

Forming from vapor or gaseous state

With decomposition of a precursor

C117S101000, C117S102000, C117S105000, C117S201000, C117S202000

Reexamination Certificate

active

06783590

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to growing thin films on a surface of a substrate. More particularly, the invention concerns an improved method and apparatus for producing a thin film onto a substrate by subjecting the substrate to alternately repeated surface reactions of vapor-phase reactants.
DESCRIPTION OF THE RELATED ART AND SUMMARY OF THE INVENTION
Conventionally, thin films are grown out using a vacuum evaporation deposition. Molecular Beam Epitaxy (MBE) and other similar vacuum deposition techniques, different variants of Chemical Vapor Deposition (CVD) (including low-pressure and metallo-organic CVD and plasma-enhanced CVD) or, alternatively, the above-mentioned deposition process based on alternate surface reactions, known in the art as the Atomic Layer Deposition, in the following abbreviated ALD, formerly also called Atomic Layer Epitaxy or “ALE”. Equipment for the ALD process is supplied under the name ALCVD™ by ASM Microchemistry Oy, Espoo, Finland.
In the MBE and CVD processes, besides other variables, the thin film growth rate is also affected by the concentrations of the provided starting material inflows. To achieve a uniform surface smoothness of the thin films manufactured using these methods, the concentrations and reactivities of the starting materials must be kept equal over the whole surface area of the substrate. If the different starting materials are allowed to mix which each other prior to reaching the substrate surface, as is the case in the CVD method, the possibility of mutual reactions between the reagents is always imminent. Herein arises a risk of microparticle formation already in the infeed lines of the gaseous reactants. Such microparticles generally have a deteriorating effect on the quality of the deposited thin film. However, the occurrence of premature reactions in MBE and CVD reactors can be avoided, e.g., by heating the reactants not earlier than only at the substrates. In addition to heating, the desired reaction can be initiated with the help of e.g., plasma or other similar activating means.
In MBE and CVD processes, the growth rate of thin films is primarily adjusted by controlling the inflow rates of starting materials impinging on the substrate. By contrast, the thin film growth rate in the ALD process is controlled by the substrate surface properties, rather than by the concentrations or other qualifies of the starting material inflows. In the ALD process, the only prerequisite is that the starting material is provided in a sufficient concentration to saturate the surface of the substrate.
The ALD method is described, e.g., in FI Pat. Nos. 52,359 and 57,975 as well as in U.S. Pat. Nos. 4,058,430 and 4,389,973. Also in FI Pat. Nos. 97,730, 97,731 and 100,409 are disclosed some apparatus constructions suited for implementing the method. Equipment for thin film deposition are further described in publications Material Science Report 4(7), 1989, p. 261, and Tyhjiötekniikka (title in English: Vacuum Techniques), ISBN 951-794-422-5, pp. 253-261. These references are incorporated herein by reference.
In the ALD method, atoms or molecules sweep over the substrates thus continuously impinging on their surface so that a fully saturated molecular layer is formed thereon.
According to the conventional techniques known from FI Patent Specification No. 57,975, the saturation step is followed by a protective gas pulse forming a diffusion barrier that sweeps away the excess starting material and the gaseous reaction products from the substrate. Intermixing of the successive reactant pulses must be avoided. The successive pulses of different starting materials and the protective gas pulses forming diffusion barriers that separate the successive starting materials pulses from each other accomplish the growth of the thin film at a rate controlled by the surface chemistry properties of the different materials.
The pulsing of the gaseous reactants and the purge gas is typically controlled by valves.
An essential feature of the ALD process is that condensation of the reactant should be avoided in the vicinity of the reaction chamber. Condensation of the reactant in particular in the conduit between the reactant source and the reaction chamber and on the substrate in the reaction chamber will seriously impair the quality of the thin film. Particles or droplets condensed or sublimed in the reactant feed lines may disperse into the reactant flow and cause inhomogenity on the thin film. The same applies to condensation of solid particles or liquid droplets on the thin film in the reaction chamber. Therefore, an ALD process is operated in such a manner that the temperature in the equipment interconnecting the reactant source and the outlet of the reaction chamber (the “hot zone”) is not allowed to drop below the condensation temperature of the reactant.
The temperature of the ALD process is determined by the reactants used and by the applied pressure. Generally it lies in the range between the evaporation temperature and the decomposition temperature of the reactant. Usually the temperature is about 25 to 500° C. There is a distinct trend toward the use of less volatile reactants such as solid or high-boiling precursors. Such reactant sources are easier to handle. However, the applicable temperature range is distinctly higher for these than for the gaseous and liquid reactants. Usually solid sources are used at temperatures in the range of 250 to 500° C., typically 300-450° C. The pressure range is typically about 1 to 100 mbar, preferably less than 50 mbar.
When solid reactant sources are used, a carrier gas typically has to be employed for feeding the reactant vapours into the reaction chamber because the vapour pressure of the source is not always sufficient to allow for a sufficiently strong flow of vapour-phase reactant pulses from the source to the reaction chamber. Since many of the solid sources are powders containing extremely finely divided matter (dust), there is a risk for contamination of the vapour-phase reactant pulses with small solid particles when the carrier gas flow is conducted through the reactant material. These particles disturb the growth of the thin film. Similar problems are encountered with liquid reactants having high boiling points in that the flow of the carrier gas may create a mist with finely divided droplet dispersed in the carrier gas flow. Therefore, the vapour-phase reactant pulses may have to be conducted to a purifier, preferably a static purifier, to remove any liquid or droplets or solid particles present in the gas stream, before the pulses are fed into the reaction chamber. Such purifiers may comprise traditional filters in which the gas stream is conducted through a layer of a porous material having macromolecular pores.
Thus, the basic prerequisites of ALD, via operation above the condensation point of the reactant using reactants which are free from particles or droplets which may disturb the homogeneous growth of the thin film, in combination with the trend towards using reactants having high boiling or sublimation points, gives rise to ever more stringent requirement on the ALD equipment. It is necessary to design the apparatus for reliable operation at high temperatures, typically in the range of about 250 to 500° C., at reduced pressure. Not only should the equipment used in the “hot zone” withstand these temperatures as such, but the materials should also be resistant to the action of the reactive vapourised reactants at said temperatures. These conditions are particularly demanding for mechanical valves conventionally employed for, e.g., pulsing of the reactants and purge gas, and for the gaskets and packings of said valves and other fittings. Attrition of the polymer materials used in the gaskets and packings will cause an additional dusting problem resulting in contamination of the vapour-phase reactant pulses. For several reasons it is therefore necessary to incorporate static purifiers into the ALD equipment designed for the use of solid and/or liquid sources.
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