Patent Publication Number: US-2002012749-A1

Title: Method and apparatus for coating and/or treating substrates

Description:
[0001] The present invention relates to a method for coating a surface of a substrate by applying to said surface a gas comprising coat-forming particles necessary for coating, the particles being deposited on and/or reacting with said surface. The invention further relates to an apparatus for coating and/or treating a surface of a substrate by applying a gas to said surface, comprising a plurality of gas-emitting sources as well as gas sinks for gas having reacted with or having been applied to the substrate.  
       [0002] A variety of principles may be utilized for coating substrates or surfaces using a CVD (chemical vapor deposition) process. One approach would be to have a gas flow parallel to the substrate surface, the substrate being either fixed or moving. When the gas is passing along the substrate surface, there is a tendency for the carrier gas to be quickly depleted. With a fixed substrate, non-uniform deposition rates, non-uniform film thicknesses as well as irregular dopings in the direction of the coating thickness and coating surface can also be observed.  
       [0003] While a moving substrate would allow a uniform film thickness to be achieved, it leads to a non-uniform deposition rate. Also, irregular doping call be observed in the direction of the film thickness.  
       [0004] In target-flow reactors, the gas to be applied to a substrate surface flows from above onto the substrate in a vertical direction. A uniform deposition rate can be achieved with substrates of a limited size. A certain uniformity can also be observed in the coating thickness and doping in the direction of the film thickness and in the direction of the film surface. However, there are problems with removing reacted gas when coating large surfaces, so that good results can only be achieved on relatively small substrate surfaces in the case of target-flow reactors.  
       [0005] For achieving uniform depositions on large surfaces, pancake reactors are used that work on a similar principle to the target-flow reactor. This means that the gas flows in a vertical direction onto the surface to be coated. The substrate itself is arranged on a hot substrate pedestal so that the resulting convection in the gas atmosphere allows the gas to be mixed and homogenized this results in uniform deposition rates, uniform film thicknesses as well as uniform dopings in the direction of the film thickness and surface. Uniformity can also be enhanced by rotating the substrate pedestal during the coating process. Although such a coating process is useful for coating large surfaces and results in reproducible, high-grade epitaxial layers, a drawback is that the gas intake is from the center of the system through a disk and its usefulness is therefore limited to the coating of wafers.  
       [0006] Such a method, or reactors used for implementing it, can be found in the cited US publication entitled: “Chemical Vapor Deposition for Microelectronics”, Arthur Sherman, Noyes Publications, USA, pp. 31-39, 150-174.  
       [0007] The problem underlying the present invention is to further develop a method and an apparatus of the type mentioned at the outset in such a way that substrate surfaces are able to be coated or to be treated by reacting them with gases to the desired extent, in particular using CVD processes. Industrial scale treatment or coating of surfaces should also be made possible.  
       [0008] In accordance with the invention, the object is solved using a method of the type mentioned at the outset by separating out the as into partial gas flows whose particle concentration and/or dwell time directly in the region of the surface and/or on the surface of the substrate can be tuned in such a way that equal amounts of constituents can be deposited and/or reacted per surface unit and per time unit. The dwell time can be tuned by a relative movement or speed between the substrate and the sources emitting the partial flows. In particular, the gas partial flows are emitted toward the surface by a plurality of sources, with as sinks arranged between them for gas that has reacted with and/or been applied to, the substrate. The expression “directly in the region of the surface” here means the space between the emission points of the gas partial flows and the surface(s) to be treated.  
       [0009] The partial flows themselves are applied to the surface in a bottom-up direction while the sources and/or the sinks are arranged in a regular pattern across and below such surface. The necessary heating of the substrates and hence of the surface to be coated/treated is done in particular on the surface facing away from the gas.  
       [0010] During the treatment or coating of the surface itself, the substrate should be moved in a direction perpendicular to the gas flow so that continuous processing is possible, which is particularly suitable for substrates having large surfaces, i.e. for large surfaces in general. Arranging the sinks in a regular pattern below the surface to be coated/treated as well as between the sources emitting the partial gas flows ensures that equal amounts of constituents ale deposited from the gas and/or react with the surface per surface unit and per time unit, so that uniform deposition rates, uniform film thicknesses and even, i.e. uniform, doping can be achieved in the direction of the film thickness as well as in the direction of the plane defined by the surface. There is no uncontrolled depletion of the gas, also called carrier or nutrient gas, containing the particles for reacting with or for treating the surface.  
       [0011] An apparatus for coating and/or treating a surface of a substrate, in particular using CVD processes, comprising a plurality of gas-emitting sources as well as gas sinks for said gas having reacted with or having been applied to said substrate is characterized in that the surface of the substrate is arranged above the sources and sinks and in that the sources are distributed over a region in a regular pattern, said region being defined by a vertical projection of the surface of the substrate, with the sinks preferably being arranged between said sources in a regular pattern. In particular, the sources and sinks form a gas distribution system whose surface extension is at least equal to or substantially at least equal to the surface of the substrate itself.  
       [0012] The sources emitting the carrier or nutrient as can comprise or be designed as slots, nozzles or other openings through which the gas can be emitted in a vertical direction or in a substantially vertical direction rising firm the bottom up toward the surface of the substrate.  
       [0013] The sources can be arranged in a first plane parallel to the surface and the sinks can be arranged in a second plane parallel to the surface, where the first and second planes may be at a distance from one another. Preferably, in such an embodiment, the first plane comprising the sources should be closer to the surface than the second plane comprising the sinks.  
       [0014] In a further embodiment of the invention it is envisaged that the sources, such as openings, slots or nozzles, communicate with a space containing uniformly distributed as. The space itself can be a cube or a hollow cylinder, such as a tube. The “space” can also mean a plurality of hollow cylinders or tubes.  
       [0015] When tubes are used such tubes should be arranged parallel to each other below the surface of the substrate with the sources as well as the sinks, in particular slots, being arranged along cach longitudinal axis of each tube. The slot longitudinal axes and the longitudinal axis of the tube would then be parallel.  
       [0016] The gas distribution system itself can be arranged inside the reactor chamber, with the surface of the substrate closing off or defining the chamber. In particular, the substrate is intended to be aligned with the reactor chamber by means of guide rails the reactor chamber being able to be sealed by means of the substrate and the guide rails.  
       [0017] The necessary heating sources, such as radiation heaters and/or microwave radiators, can be arranged above the substrate on the substrate surface facing away from the gas distribution system.  
       [0018] In particular, it is envisaged to house the reaction chambers themselves inside a chamber in order to permit successive treatment or coating of the substrate surface, the reactor chambers being able to contain a variety of nutrient or carrier gases. The chamber arrangement is a continuous processing system, the reactor chambers present within it for successive coating, or treatment of the surface being able to be sealed by the substrate or the surface, The chamber arrangement can be flowed through by an meet gas flowing in a flow direction opposite the direction of movement of the substrate. 
     
    
    
     [0019] Further details, features and advantages of the invention can be seen not only from the claims and the features to be derived from them—singly and/or in combination—but also from the following description of the preferred embodiment taken in conjunction with the accompanying drawing in which:  
     [0020]FIG. 1 shows a first arrangement for coating a substrate.  
     [0021]FIG. 2 shows a sectional view of a second embodiment of an arrangement for coating a substrate.  
     [0022]FIG. 3 shows the principle of a third embodiment of an arrangement for coating a substrate, and  
     [0023]FIG. 4 shows the principle of a continuous processing arrangement. 
    
    
     [0024] For providing substrates such as those of semiconductor components such as thin film solar cells with desired coatings or for forming such coatings on the substrate, CVD processes are usually applied. The substrate is treated with a carrier or nutrient gas containing coat-forming particles deposited on the surface or reacting with the latter. In accordance with the invention, it is provided that to separate out the nutrient or carrier gas are separated out into partial flows in such a way that equal amounts of particles are deposited on and/or react with the surface with respect to the particle concentration in the partial flows and/or the dwell time on the surface per surface unit and per time unit.  
     [0025] With reference to FIG. 1, an apparatus is shown comprising a reactor  10 , in which a gas distribution system  12  is arranged comprising gas sources  14 ,  16 ,  18 ,  20  as well as gas sinks  22 ,  24 ,  26 ,  28 . The gas sources  14 ,  16 ,  18 ,  20  through which the nutrient or carrier gas is separated out into partial flows are designed as flues or hollow cylindrical elements arranged with their openings below substrates  30  to be coated, the substrates themselves being arranged above The reactor  10  being closed by a mask  32  having an opening or a plurality of openings on which, in the present embodiment, a plurality of substrates  30  comprising surfaces  34  to be coated are aligned in a row.  
     [0026] The gas sources  14 ,  16 ,  18 ,  20  are regularly distributed across the surfaces  34  defined by the substrates  30  and arc arranged in a plane which is closer to the surface than the gas sinks  22 ,  24 ,  26 ,  28  with their evacuation openings. The sinks  22 ,  24 ,  26 ,  28  are used to evacuate as that has reacted with the surfaces  34 . The sinks  22 ,  24 ,  26 ,  28  are also regularly distributed across the plane defined by the substrate surfaces  34  An evacuation pipe  38  for evacuating, reacted gas projects from the bottom  36  of the reactor  10  arid has a gas inlet pipe  40  arranged inside it, providing carrier or nutrient as to the gas sources  14 ,  16 ,  18 ,  20  formed as flues or hollow cylinders.  
     [0027] By evenly distributing the gas a cross the plane defined by the surfaces  34  to be coated and also by distributing the sinks  22 ,  24 ,  26 ,  28  evacuating the reacted gas across the plane and between the sources  14 ,  16 ,  18 ,  20 , and by having the gas itself reach the surfaces  34  of the substrates  30  rising vertically from below, a uniform deposition with—at the same time—a uniform film thickness and a uniform doping both in the direction or the film thickness and in the plane defined by the surface  34 .  
     [0028] Insofar as heating of the surfaces  34  is necessary for the coating process, the substrates  30  are provided for example with heat radiation or microwaves an the surface  42  facing away from the gas sources  14 ,  16 ,  18 ,  20 . By evenly distributing the gas sources  14 ,  16 ,  18 ,  20 , i.e. the gas supply, across the plane defined by the surfaces  34 , and by having the gas to be applied to the surfaces  34  rise vertically, while the surface defined by the gas sources  14 ,  16 ,  18 ,  20  and the gas sinks  22 ,  24 ,  26 ,  28  is equal to or greater than the entirety of the surfaces  34  to be coated, a uniform gas distribution is achieved, ensuring a reproducible and desired uniform deposition and hence film thickness and doping. Moreover, the substrates  30  may be moved in a direction perpendicular to the gas flow to further improve uniformity.  
     [0029] With reference to FIG. 2 a further embodiment of the arrangement for coating a surface  34  of a substrate  30  is shown, the substrate being movable along the opening  44  of a reactor (not shown). In the reactor itself, gas distribution tubes  46 ,  48 ,  50  are arranged parallel to one another, between which gas having reacted with the surface  34  is evacuated (arrows  52 ,  54 ). This arrangement also ensures an even distribution of the gas sources and sinks formed by the gas distribution tubes  46 ,  48 ,  50  and the vacuum means  52 ,  54  respectively across the surface  34  of the substrate  30 , ensuring the desired uniformity of the gas stream applied to the surface  34  and hence a uniform deposition rate, film thickness and doping. The gas distribution tubes  46 ,  48 ,  50  comprise openings such as slots or holes arranged along their longitudinal axes, through which the gas partial flows are applied to the surface  34  of the substrate  30 . Preferably, slots arranged along the tubes  46 ,  48 ,  50 , i.e. running parallel to their longitudinal axes, are chosen.  
     [0030] In the embodiment of FIG. 3, a reactor  56  is provided, inside which is a gas distribution system having a gas inlet means  58  and a gas evacuation means  60  circumferentially surrounding the gas inlet means  58  in the form of an annular slot  62  which in turn communicates with a gas outlet tube  64  in the bottom section of the reactor  56 . In the top wall  66  of the reactor  56 , there is an opening  68  having guide rails  70 ,  72  along the edges, along which the substrate  30  to be coated slides, or slides on a gas cushion at a distance from the openings. The distance may be between—for example—1 mm and 20 mm, in particular up to 10 mm, without limiting the invention. Therefore, the substrate  30  can he guided during coating in a movement relative to the emission openings  74 ,  76  forming the gas inlet means  58  or sources and evenly distributed below the surface  32  of the substrate  30 . A kind of gas cushion is formed between the emission openings  74 ,  76  and the surface  32 , the amounts of gas emitted from the openings  74 ,  76  as a function of the position of the opening  74 ,  76  being tuned to one another in such a way that the partial flows in respect of their particle concentration and the dwell time of the partial flows with reference to the surface  32  to be coated are tuned in such a way that equal amounts of particles are deposited on or react with the surface  32  per surface unit aid per time unit.  
     [0031] The pressure differential leads to the desired reproducible uniform deposition rate, film thickness and doping. Uniformity is further improved by a movement of the substrate  30  along the guide rails  70 ,  72  relative to the gas emission openings  74 ,  76 . The reacted gas is evacuated on the side via the annular slot  62  and is passed through the gas outlet  64  out of the reactor  56 .  
     [0032] Above the substrate  30 , i.e. above the surface  42 , a heating means such as radiation or microwave heater may be arranged to heat the substrate or to permit performance of the desired coating processes. The space outside the reactor  56  may additionally be purged with inert gas.  
     [0033] The gas manifold  58  itself is preferably made of quartz.  
     [0034]FIG. 4 is to illustrate that the method in accordance with the invention is also suitable for continuous processing. To achieve this, a sealed continuous processing chamber  78  is provided in which, one after the other, a plurality of reactors  80 ,  82 ,  84 ,  86 ,  88 ,  90 ,  92  are arranged corresponding to one of the structures described above. Each reactor  80 ,  82 ,  84 ,  86 ,  88 ,  90 ,  92  in turn is covered by a substrate  30  to be coated, in order to coat the surface facing the reactor  80 ,  82 ,  84 ,  86 ,  88 ,  90 ,  92  to the desired extent. The interior of the continuous processing chamber  78  itself may be purged by an inert gas, where the direction of flow (arrow  94 ) may be in the opposite direction to the transportation direction (arrow  96 ) of the substrate  30 .  
     [0035] For example, a graphite substrate may be provided with an SiC layer in the first reactor  80 . In the following reactor  82 , a p + -type Si layer highly doped with boron, for example, is deposited on the SiC layer using a CVD process. A capping layer may then be deposited on the p + -type Si nucleation layer in the reactor  84 , followed by a recrystallization process in the reactor  86 . The capping layer is removed in reactor  88 , to be followed by an epitaxy process of a photo sensitive p-type Si layer, and eventually in reactor  92  by the deposition of an n-conducting emitter layer  
     [0036] The embodiment of FIG. 4 is to illustrate that the teachings of the invention are applicable for example for the manufacture of a crystalline Si thin film system, ensuring that the films to be formed have the required uniformity, uniform thickness and doping. At the same time, each reactor  80 ,  82 ,  84 ,  86 ,  88 ,  90 ,  92  is itself sealed during processing by the substrate  30  to be treated.  
     [0037] Preferable process parameters are derived from the following examples:  
     EXAMPLE NO. 1  
     [0038] An apparatus according to FIG. 1 is used to manufacture a large-area silicon film. In a tank able to be evacuated, preferably of stainless steel and having water-cooled walls (and a diameter of about 80 cm), the quartz reactor  36  is arranged having a diameter of about 70 cm. The reactor  36  is closed at its top by a carrier plate  32  having openings for receiving substrates  42  with the dimensions 0.1 m by 0.1 m, or a single opening for a substrate with the dimensions 0.4 m by 0.4 m The substrate  30  or  42  is heated through a transparent covering (e.g. a quartz window) of the tank by means of a lamp array. The space between the rank wall and the quartz reactor is constantly purged by a chemically inert gas such as argon or nitrogen. In addition, for safety reasons, this space is constantly monitored by means of an H 2  sensor to ensure that the lower explosion limit is never even reached. Heating is achieved by means of optical or infrared radiation of a radiator array having a power density of between 400 kW/m 2  and 700 kW/m 2  at temperatures of between 1100° C. and 1300° C. Alternatively, heating is achieved using microwave radiation or by inductive heating. The type of heating depends largely on the properties of the substrate. For high-absorption surfaces (gray to black colors), optical heating is suitable. Conductive substrates are suitable for heating by inductive coupling or by current flowing directly through the substrate. Ceramics having molecular dipole moment are best heated using microwaves. The process gas is a as mixture comprising hydrogen with methyl trichlorosilane (MTCS=CH 3 SiHCl 3 ) for depositing SiC layers or trichlorosilane (TCS=SiHCl 3 ) for depositing Si layers. The doping of SiC is performed by adding small quantities of nitrogen (n-conducting SiC). Silicon can be doped by adding small quantities of BCl 3  to obtain a p-conducting layer.  
     [0039] The chemical reaction can happen at high temperatures according to the following simplified chemical reaction equations:  
     CH 3 SiHCl 3 +H 2 →SiC+3 HCl   (1)  
     SiHCl 3 +H 2 →Si+3HCl   (2)  
     3 BCl 3 +3H 2 →2B+6HCl   (3)  
     [0040] Distributing the gas flow is performed through a gas distribution system, preferably of quartz elements.  
     [0041] An MTCS/H 2  mixture is introduced to deposit SiC layers, a TCS/H 2  mixture to deposit Si layers. Each initial mixture is introduced into the manifold system via the gas pipe  40 . It passes to the space between the two plates  41  and  43  and then flows through the flue-like tubes  147   16 ,  12 ,  20  towards the substrates. The substrates are at a temperature of between 1200° C. and 1550° C. for SiC deposition and of between 900° C. and 1200° C. for Si deposition so that the chemical reaction expressed by the reaction equations (1) through (3) can take place. On the substrate  30  or  42 , SiC or Si is deposited respectively. The gaseous products arc forced via the sinks  22 ,  24 ,  26 ,  28  between the two plates  41  and  43  to the lower part of the reactor chamber, from where they can be extracted through the pipe  38 . The deposition rate is in the range of 0.1 μm to 10 μm per minute. It is an exponential function of the deposition temperature of the substrate and a proportional function of the concentration of MTCS or TCS, respectively, in the process gas.  
     [0042] The deposition rate is critically dependent on the mol ratio of [Si]:[H] or, with MTCS, of [C+Si]:[H]. Typically this mol ratio is between 1:10 and 1:100. The yield is about 10 to 20% depending on the choice of parameters.  
     [0043] For examples an SiC layer having a thickness of 30 μm is deposited on a surface of 0.16 m 2  at a temperature of 1500° C. at a deposition rate of 5 μm/min. The gas flows are 130 slm for hydrogen, 20 slm for MTCS and 1 slm for nitrogen as a dopant gas. The mol ratio MTCS:H 2  is about 1:10.  
     [0044] For example, an Si layer having a thickness of 30 μm is deposited on a surface of 0.16 m 2  at a temperature of 1100° C. at a deposition rate of 5 μm/min. The gas flows are 200 to 2000 slm for hydrogen, 20 slm for TCS and 10 slm for a mixture of BCl 3 :H 2 =1:1000 as a doppant gas. The mol ratio MTCS:H 2  is about 1:100 to 1:10.  
     EXAMPLE NO. 2  
     [0045] In the system of FIG. 2 the initial gas mixture is transported through parallel gas pipelines  46 ,  48 ,  50 , preferably of quartz having bores at their top ends. The gas passes through the bores into the reaction chamber and reaches the heated substrate surface  34 . The substrate  30  is moved in a parallel direction to the plane defined by the gas pipelines. This serves to improve deposition rate uniformity. The substrate is at a temperature of between 1200° C. and 1550° C. in the case of an SiC deposition and at temperatures of between 900° C. and 1200° C. in the case of an Si deposition, so that the chemical reaction expressed by the reaction equations (1) and (2) respectively can take place. SiC or Si is deposited on each respective substrate. The gaseous products  52  and  54  are extracted through spaces between the pipelines.  
     [0046] In a continuous processing system of FIGS.  2  to  4 , the throughput can be considerably enhanced by the drive speed and the choice of the length of the plant. Since deposition rates of 5 μm/min to 10 μm/min can be achieved with the normal-pressure CVD process, only 3 to 6 min are necessary for the deposition of a CVD layer having a 30 μm thickness. By keeping the width of 40 cm and doubling the coating length to 80 cm, a surface of 0.32 m 2  can be manufactured in 3 to 6 minutes in a continuous process using the arrangement of FIG. 2.  
     EXAMPLE NO. 3  
     [0047] To manufacture a large surface layer of silicon dioxide (as a capping layer for the crystallization process in the manufacture of a crystalline Si thin film solar cell) a gas nature of monosilane SiH 4  and oxygen O 2  is used as process gas. The gas is diluted with inert gas, such as nitrogen CO 2 , Argon or other inert gases to avoid spontaneous nucleation, and therefore dust, in the vapor phase. The chemical reaction tales place according to the simplified chemical reaction equation  
     SiCl 4 +2 H 2 O→SiO 2 +4 HCl  
     [0048] at temperatures of about 250° C. to 800° C., preferably at 400° C. to 450° C. The distribution of the gas flow is achieved preferably through the gas distribution system out of quartz elements. The deposition rate is in the order of 0.1 μm to 0.5 μm per minute. The coating is performed at atmospheric pressure.  
     [0049] The coating can also be performed in a vacuum, with the quality of the oxide layer being better, but the deposition rate lower smaller according to the particle concentration. In order to avoid pyroforic SiH 4 , tetraethyl orthosilicate (C 2 H 5 O)4Si (TEOS) can also serve as a source of silicon. The process is carried out at 500 mbar to 1000 mbar and at temperatures of between 600° C. and 800° C., preferably 700° C.  
     EXAMPLE NO. 4  
     [0050] For removing the large surface layer of silicon dioxide (after completion of crystallization) the surface is heated to temperatures of between 1150° C. and 1300° C. and reduced to pure hydrogen. Preferably, a temperature of 1200° C. is suitable. After complete removal of the oxide layer, the crystalline silicon layer is exposed as a nucleation layer, onto which the epitaxial Si semiconductor layer may then be deposited.  
     [0051] As an alternative to oxide removal through reduction with hydrogen, an H 2 O/HF mixture is passed over the oxide-covered sample at temperatures ranging from room temperature to 300° C., and preferably 50° C. to 100° C. (also substrate temperature about 100° C. to 300 ° C.). A chemical reaction takes place as follows:  
     SiO 2 +4 HF+H 2 O→SiF 4 +3H 2 O  
     [0052] The compounds SiF 4  and H 2 O are volatile and evaporate at these temperatures. This is why an etching effect can also be achieved at low temperatures from the vapor phase, so that the oxide layer having a thickness of 2 μm evaporates within about 10 minutes.  
     EXAMPLE NO. 5  
     [0053] The initial mixture is introduced through a pipe into the interior  58  of a gas transportation chamber having a planar wall on the side facing the substrate  30 . The gas is emitted through bores  74 ,  76  and passes to the heated substrate surface  32 . The gaseous products can be extracted to the right and left through the gap  68  between the emission plane and the substrate. The gas is passed around the gas transportation chamber and forced out of the reaction chamber downwards through an outlet means  90  of large cross-section.  
     [0054] A plurality of such reaction chambers may be arranged in series, so that a continuous processing system is developed in which all layers of various composition may be deposited in succession. FIG. 4 schematically shows such a system comprising a plurality of chambers  80 ,  82 ,  84 ,  86 ,  90 ,  92 . The chamber system is in a reaction chamber (reactor) purged with an inert gas. The substrate  30  is pre-heated in the chamber to coating temperature. The individual gas transportation chambers  80  to  100  are the following:  
     [0055] 80 : as transportation chamber for SiC coating,  82 : gas transportation chamber for p + -type Si coating,  84 : gas transportation chamber for oxide capping layer deposition,  86 : crystallization chamber,  88 : gas transportation chamber for oxide removal,  90 : gas transportation chamber for p + -type Si coating (epitaxy). and  100 : gas transportation chamber for n + -type Si coating (diffusion or epitaxy)  
     [0056] The substrate  96  completely coated with the semiconductor layer system can then be taken out of the chamber through the lock for further processing. Using this arrangement, a system may be constructed allowing the deposition of the complete semiconductor system in a continuous process inside a single chamber purged with inert gas.