Coating apparatus – Gas or vapor deposition – Work support
Reexamination Certificate
2000-01-04
2003-10-07
Bueker, Richard (Department: 1762)
Coating apparatus
Gas or vapor deposition
Work support
C118S715000, C118S729000, C118S733000
Reexamination Certificate
active
06630030
ABSTRACT:
The present invention relates to a method according to the preamble of claim
1
for producing thin films.
In the present method, a substrate located in a reaction space is subjected to alternately repeated surface reactions of at least two different reactants used for producing a thin film. The vapor-phase reactants are admitted repetitively and alternately each reactant from its own source into the reaction space where they are allowed to react with the substrate surface for the purpose of forming a solid-state thin film product on the substrate. Reaction products which have not adhered onto the substrate and any possible excess reactant are removed from the reaction space in vapor phase.
The invention also concerns an apparatus according to the preamble of claim
8
.
Conventionally, thin-films are grown using vacuum evaporation deposition, the Molecular Beam Epitaxy (MBE) and other corresponding vacuum deposition methods, different variants of the Chemical Vapor Deposition (CVD) method (including low-pressure and organometallic CVD and plasma-enhanced CVD), or alternatively, the above-described deposition method of alternately repeated surface reactions called the Atomic Layer Epitaxy (ALE) method. In the MBE and CVD methods, besides other process variables, the thin-film growth rate is also affected by the concentrations of the starting material inflows. To achieve a uniform thickness of the layers deposited by the first category of conventional methods, the concentrations and reactivities of the starting materials must be kept equal all over the substrate area. If the different starting materials are allowed to mix with each other prior to reaching the substrate surface as is the case in the CVD method, for instance, the risk of their mutual reaction arises. Then, the risk of microparticle formation already in the inflow channels for the gaseous reactants is imminent.
Such microparticles usually have a deteriorating effect on the quality of the thin film. Therefore, the possibility of premature reactions in MBE and CVD reactors is avoided by heating the starting materials no earlier than at the substrate surfaces. In addition to heating, the desired reaction can be initiated using, e.g., a plasma or some other similar activator.
In the MBE and CVD processes, the growth of thin films is primarily adjusted by controlling the inflow rates of starting materials impinging on the substrate. By contrast, the ALE process is based on allowing the substrate surface qualities rather than the starting material concentrations or flow properties to control the deposition rate. The only prerequisite in the ALE process is that the starting material is available in sufficient concentration for thin-film formation all over the substrate.
The ALE method is described in, e.g., FI Patent Specifications Nos. 52359, 97730 and 57975, in WO Publication No. 96/17107 and in U.S. Pat. Nos. 4,058,430 and 4,389,973, in which also some apparatus embodiments suited for implementing this method are disclosed. Apparatus constructions for growing thin films can also be found in the following publications: Material Science Report 4(7) (1989), p. 261, and Tyhjiötekniikka (Finnish publication for vacuum techniques), ISBN 951-794-422-5, pp. 253-261.
In the ALE deposition method, atoms or molecules are arranged to 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. 57975, the saturation step is followed by an inert gas pulse forming a diffusion barrier which sweeps away the excess starting material and the gaseous reaction products from the substrate. The successive pulses of different starting materials and of diffusion barriers of carrier gas, the latter separating the former, accomplish the growth of the thin film at a rate controlled by the surface chemistry properties of the different materials. Such reactor is called the “traveling-wave” reactor. To the function of the process it is irrelevant whether it is the gases or the substrates which are moved, but rather, it is imperative that the different starting materials of the successive reaction steps are separated from each other and arranged to impinge on the substrate alternately.
Most vacuum evaporators operate on the so-called “single-shot” principle. Hereby, a vaporized atom or molecule can impinge on the substrate only once. If no reaction with the substrate surface occurs, it is rebound or re-vaporized so as to hit the apparatus walls or the vacuum pump undergoing condensation therein. In hot-walled reactors, an atom or molecule impinging on the reactor wall or the substrate may become re-vaporized and thus repeatedly impinge on the substrate. When applied to ALE reactors, this “multi-shot” principle can provide, i.a., improved efficiency of material consumption.
If the starting materials in ALE deposition, due to flow dynamic or other reasons, are unevenly distributed over different parts of the substrates, it is necessary to pulse each starting material over the substrates in an amount which will guarantee that even at the thinnest flow, a sufficient amount flows at each pulse in order to ensure an even growth of the film. Knowing, on the other hand, that flow geometry can lead to concentration differences of even several decades, it may in the case of a disadvantageous flow geometry be necessary to pulse greater amounts of starting materials than the growth of the film in its entirety would presuppose. This is termed overdosage and other reasons may also exert an influence here, such as the chemistry of the starting materials.
In order for a sufficient amount of starting material to be provided over different parts of the substrate without any significant overdosage, two solutions are applied to achieve an even distribution of the gases:
1. The apparatus is constructed such that the pressure on the substrates is so low that the average mutual collision distance of the gas molecules is greater than the distances between the substrates. Hereby most of the collisions of the gas molecules will hit the substrates and few gases are evenly distributed with regard to the substrates. When the average collision distance is equal to the distance d between walls in a typical system or at least one hundredth part thereof, the gas is called transitional. At a pressure of one millibar and at room temperature the collision distance of one nitrogen molecule is 64 micrometers and at 0.01 mbar, 6.4 mm. In ALE reactors the distances between the substrates are typically in the range of a few millimeters. Thus, when the discussed solution is sought, the pressure must be about 1 mbar or preferably even lower.
2. At a greater pressure the average collision distance of the gas molecules is diminished and the gas is no longer transitional but is instead in a viscous state (collision interval<d/100). In viscous state the flow of the gas is collective movement of its molecules, linked together via the collisions, towards the decreasing pressure. The intermixing of the molecules caused by the thermal movement is expressed as diffusion. In these solutions the aim is to distribute gas evenly over the substrates by means of different gas flow generators and nozzles because the diffusion velocities in the cross direction of the gas flow are small compared to the speed of propagation of the gas.
The problem with the first alternative is the low pressure required in the case of real dimensions. When the pressure decreases by a decade, the pump capacity must also be increased by a decade and the gas velocities are also increased by a decade if the gas flow is kept constant.
The gas velocities cannot surpass acoustic velocity and the pump prices are drastically increased. In addition, with the decreasing pressure the dimensions of the reactor must be enlarged in order to achieve improved transmittance such that pressure losses increasing the pressure are prevented. This would again presuppose that the pressure be reduced.
Bondestam Niklas
Soininen Pekka
Suntola Tuomo
ASM Microchemistry Ltd.
Bueker Richard
Knobbe Martens & Olson Bear LLP.
LandOfFree
Method and apparatus for growing thin films does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method and apparatus for growing thin films, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and apparatus for growing thin films will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3127783