This application claims the priority benefit under 35 U.S.C. xc2xa7119 to Finnish Patent Application No.  19992797, filed Dec. 28, 1999, the entire content of which is hereby expressly incorporated by reference. 
The present invention relates to an apparatus for growing thin films on a surface of a substrate.  More particularly, the present invention relates to an apparatus for producing thin films on a surface of a substrate by subjecting the substrate to alternately repeated surface reactions of vapor-phase reactants. 
Conventionally, thin-films are grown using vacuum evaporation deposition, the Molecular Beam Epitaxy (MBE) and other similar vacuum deposition methods, different variants of the Chemical Vapor Deposition (CVD) method (including low-pressure and organometallic CVD and plasma-enhanced CVD) or a deposition method of alternately repeated surface reactions called the Atomic Layer Epitaxy (ALE) or Atomic Layer Deposition (ALD). 
In MBE and CVD methods, the thin film growth rate is determined by the concentrations of the provided starting material in addition to other process variables.  To achieve a uniform thickness of the layers deposited by these methods, the concentrations and reactivities of starting materials must be carefully kept constant on different surface areas of the substrate.  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, a chance of their mutual reaction arises.  Then, a risk of microparticle formation already within the infeed channels of the gaseous reactants is imminent.  Such microparticles generally have a deteriorating effect on the quality of the deposited thin film.  Therefore, the possibility of premature reactions in MBE and CVD reactors, for instance, is avoided by heating the starting materials not earlier than at the substrate surfaces.  In addition to heating, the desired reaction can be initiated using, e.g., a plasma discharge or other similar activating means. 
In the MBE and CVD processes, the growth of thin films is primarily adjusted by controlling the infeed rates of starting materials impinging on the substrate.  In contrast, the growth rate in the ALE process is controlled by the substrate surface qualities, rather than the starting material concentrations or flow variables.  The only prerequisite in the ALE process is that the starting material is available in sufficient concentration to saturate the surface of the substrate.  The ALE method is described, for example, in FI patent publications 52,359 and 57,975 and in U.S. Pat.  Nos.  4,058,430 and 4,389,973.  Equipment constructions suited to implement this method are disclosed in patent publications U.S. Pat.  No.  5,855,680 and FI 100,409.  Apparatuses for growing thin films are also described in the following publications: Material Science Report 4(7) (1989), p. 261, and Tyhjixc3x6tekniikka (Finnish publication for vacuum techniques), ISBN 951-794-422-5, pp.  253-261.  These references are incorporated herein by reference. 
In the ALE growth method described in FI Pat.  No.  57,975, the reactant atoms or molecules are arranged to sweep over the substrates, thus impinging on their surface until a fully saturated molecular layer is formed thereon.  Next, the excess reactant and the gaseous reaction products are removed from the substrates with the help of inert gas pulses passed over the substrates or, alternatively, by pumping the reaction space to a vacuum before the next gaseous pulse of a different reactant is admitted.  The succession of the different gaseous reactant pulses and the diffusion barriers formed by the separating inert gas pulses or cycles of vacuum pumping result in a thin film growth controlled by the individual surface-chemical reactions of all these components.  If necessary, the effect of the vacuum pumping cycle may be augmented by the inert gas flow.  For the function of the process, it is typically irrelevant whether the gaseous reactants or the substrates are kept in motion; it only matters to keep the different reactants of the successive reactions separate from each other and to have them sweep successively over the substrate. 
Most vacuum evaporators operate on the so-called xe2x80x9csingle-shotxe2x80x9d principle.  Hereby, a vaporized atom or molecule can impinge on the substrate only once.  If no reaction with the substrate surface occurs, the atom/molecule is rebound or revaporized so as to hit the apparatus walls or the vacuum pump, undergoing condensation therein.  In hot-walled reactors, an atom or molecule that collides with the process chamber wall or the substrate can undergo revaporization and, hence, repeated impingements on the substrate.  When applied to ALE process chambers, this xe2x80x9cmulti-shotxe2x80x9d principle can offer a number of benefits including improved efficiency of material consumption. 
ALE reactions operating on the xe2x80x9cmulti-shotxe2x80x9d principle generally are designed for the use of a cassette unit in which a plurality of substrates can be taken simultaneously into the process chamber or, alternatively, the substrates can be placed unmountedly into the process space formed by a pressure vessel, whereby the process space also serves as the reaction chamber wherein the vapor-phase reactants are reacted with the substrate surface in order to grow thin film structures.  If a cassette unit designed for holding several substrates is employed, the reaction chamber is formed in the interior of the cassette unit.  Use of a cassette unit shortens the growth time per substrate in respect to single-substrate cycling, whereby a higher production throughput is attained.  Furthermore, a cassette unit arranged to be movable into and out from the process chamber can be dismantled and cleaned without interrupting the production flow because one cassette unit can be used in the process chamber while another one is being cleaned. 
Batch processing is preferred in conventional ALE thin film processes because of the relatively slow production pace of the ALE method relative to other thin film growth techniques.  The overall growth time per substrate of a thin film structure can be reduced in a batch process to a more competitive level.  For the same reason, larger substrate sizes are also preferred. 
In one embodiment of the ALE technique, the framework of the cassette unit is formed by a holder box made from titanium, for instance, having a structure with open top and bottom ends, whereby the holder box can support a plurality of substrates inserted therein into a longitudinally parallel position.  The substrates have their ends or perimeters mounted in frames that are further placed into grooves formed at opposite ends of the substrate holder box.  Each substrate frame has two substrates mounted therein with the back sides of the substrates facing each other.  Herein, the back side of a substrate refers to that face of the substrate on which no thin film is to be grown.  The access of reactants into any space that remains between the back sides of the substrates is prevented by covering the longitudinal top and bottom edges of the substrates with protecting continuous seal sections.  The reactants and the inert gas are passed into the holder box via the infeed holes of the parallel infeed channels of a sprayhead manifold located above the holder box.  Of course, the sprayhead construction may be varied as required.  The excess reactants and reaction gases are removed from the holder box via discharge channels of a suction box connected to, at the bottom part of the holder box.  In this manner, the gases are forced to flow through the spaces remaining between the faces of the substrates on which a thin film is to be grown. 
Feeding the gaseous reactants into the holder box from one end only may cause stronger film growth on substrate surface areas located closer to the infeed of reactants.  In order to compensate for this effect, the gases can be fed onto the substrates alternately from opposite directions.  In such an arrangement, the exhaust suction through the outfeed channel are typically arranged to also take place alternately at opposite ends.  Due to the alternating infeed cycles, infeed/discharge nozzles must be located at both the top and bottom ends of the holder box, which complicates the construction substantially. 
It is therefore an object of the present invention to provide a novel type of ALE reaction chamber featuring improved reactant flow conditions over those known in the art, whereby a smooth thin film growth is obtained on the substrate surface. 
Accordingly, one aspect of the invention involves placing the substrates in a substrate holder box of a cassette unit longitudinally side-by-side in an A-shape inclined disposition with their back sides facing each other so that the distance between the opposed surfaces, which are intended to support the thin film growth and between which the gases are arranged to flow, is narrower at the gas infeed end than at the gas outfeed end.  In other words, the A-shape inclined disposition of the opposed surfaces being deposited with a thin film opens in the flow direction of reactant gases, whereby the cross section of the gas flow channel becomes larger in the flow direction and, as a result, the flow speed decreases toward the outfeed end of the substrate holder box.  Furthermore, a preferred embodiment of the invention has the parallel reactant infeed channels of the sprayhead manifold adapted to open at least substantially perpendicular to the longitudinal axis of the substrates, whereby the number of substrates laced in the substrate holder box can be varied without changing the distance between the infeed channels. 
More specifically, the invention relates to an apparatus that comprises a reaction chamber including a reaction space, infeed means connected to the reaction space for feeding into the reaction space the reactants used in the thin film growth process, and outfeed means connected to the reaction space for discharging excess reactants and gaseous reaction products from the reaction space.  At least one substrate is provided within the reaction space and a second surface is opposed to the surface of the substrate on which the thin film is to be grown.  The reactants are forced to flow in relation to the opposed surfaces in the space formed therebetween.  The thin-film growth supporting surface of the substrate and the surface disposed opposing the same are arranged into the reaction chamber so as to subtend an angle opening in the flow direction of the reactants in relation to the opposed surfaces, whereby the distance between the opposed surfaces and the infeed end of reactants is smaller than at the gas outfeed end. 
The invention offers significant benefits.  For example, by placing the substrates in an A-shape inclined disposition opening in the flow direction of the reactants and the reaction gases, the thin film grown on the substrate surfaces becomes more uniform than what is obtained through a constant spacing between the substrates and more uniform than what is obtained by locating the substrate surfaces intended to support the thin film growth into an A-shape opposed disposition so that the gas flow is restricted as it flows toward the closing apex. 
If the sprayhead serving for the infeed of reactants and inert gases is located at the top of the cassette unit, a pair of substrates with their back sides opposing each other will be disposed in a single substrate frame so as to form an inverted letter A. When the substrate frames are then placed in the holder box, they will fit tightly by their own weight against the upward walls in the V-shape aligned grooves made in the ends of the substrate holder box.  The accurate placement of the substrates prevents any gas flow from escaping to the back sides of the substrates. 
To avoid intermixing of reactants, it is preferable to keep the infeed channels of the different reactants separate from each other as long as possible.  In prior-art constructions, the infeed channels of reactants and inert gas have been arranged to open into the cassette unit in the direction of the longitudinal axes of the substrates, whereby any change in the batch of substrates to be processed has also required changes in the distance or number of infeed channels in order to provide gas flow into all the spaces between the substrate surfaces on which a thin film is to be grown.  In the preferred embodiment of the invention, the infeed channels of the sprayheads are located at right angles to the longitudinal axes of the substrates, whereby the number of substrates placed in the cassette unit can be varied without altering the distance between the infeed channels or the number thereof. 
It should be noted that certain objects and advantages of the invention have been described above for the purpose of describing the invention and the advantages achieved over the prior art.  Of course, it is to be understood that not necessarily all such objects or advantages may be achieved in accordance with any particular embodiment of the invention.  Thus, for example, those skilled in the art will recognize that the invention may be embodied or carried out in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other objects or advantages as may be taught or suggested herein. 
It should also be noted that all of these embodiments are intended to be within the scope of the invention herein disclosed.  These and other embodiments of the present invention will become readily apparent to those skilled in the art from the following detailed description of the preferred embodiments having reference to the attached figures, the invention not being limited to any particular preferred embodiment(s) disclosed.