Patent Publication Number: US-6209221-B1

Title: Wafer rack provided with a gas distribution device

Description:
FIELD OF THE INVENTION 
     The present invention relates to a wafer rack to be placed in a reactor, comprising a carrier frame provided with at least two accommodations for at least two wafers. A wafer rack of this type is generally known in the prior art and is used for batchwise treatment of two or more wafers. The number of wafers that is treated simultaneously can rise to far above 100. 
     BACKGROUND OF THE INVENTION 
     Wafers are subjected to various treatments, including chemical treatments in a wide variety of types of reactor. One treatment comprises etching wafers using HF vapour. Because of a wide variety of circumstances, it can be desirable to etch wafers between different deposition treatments. The problem then arises that such an etching substance also has to penetrate into relatively deep wells present in the wafer in order to be able to etch native silicon dioxide. In particular, problems have been detected if a layer of doped oxides deposited using CVD, such as borophosphosilicate glass (BPSG), has to be etched. The methods known in the prior art for etching batches of wafers do not give a uniform result. 
     This problem increases on increasing the diameter of the wafers. Recently there has been a discernible trend in the market to switch from 200 to 300 mm. If such wafers are etched using conventional techniques, this results in a difference between minimum and maximum etching depth in a range between, for example, 50 Å and 5000 Å. Moreover, it will hardly be possible to remove the native oxides in the mid section of such wafers. 
     In U.S. Pat. No. 4,798,165, drawn up in the name of ASM International N.V. in Bilthoven, it is proposed, for a reactor in which a single wafer is treated, to supply the gas via a distributor plate arranged above the wafer so that uniform distribution of gas over the wafer is produced. Such a process is satisfactory. 
     SUMMARY OF THE INVENTION 
     The aim of the present invention is to achieve the uniform distribution that is obtained with the equipment according to the above mentioned U.S. Pat. No. 4,798,165 for the batchwise treatment of various wafers as well. 
     This aim is achieved with a wafer rack as described above in that a gas distribution device effective for each accommodation is mounted on the carrier frame, said gas distribution device comprising a distributor plate, provided with at least one opening, for each accommodation as well as gas supply means. 
     According to the invention a gas distributor plate is arranged at least above each wafer, which gas distributor plate is provided with a large number of openings through which the gas is supplied to the wafer. That is to say feeding of gas no longer takes place from one side of the reactor in an uncontrolled manner towards the opposing side, producing a non-uniform distribution of gas, but a radial gas flow is now produced over each wafer. The invention has been described above with reference to a reactor in which the wafers are placed above one another in a wafer rack extending in the vertical direction. It must be understood that with the necessary modifications the construction according to the invention can also be used for the batchwise treatment of wafers placed vertically in a wafer rack oriented essentially in the horizontal direction. 
     The gas supply to the gas distribution device can be chosen depending on the design of the construction. If reactors which are still to be produced are taken as the starting point, according to a simple embodiment the wafer rack is provided with feed lines for the gas that is used for treatment or flushing, the inlet thereof being able to engage via a coupling with a corresponding coupling in the reactor. Preferably, coupling is automatically produced by the introduction of the wafer rack. If a wafer rack construction according to the present invention has to be arranged in existing reactors, with which the gas flow takes place from one side to the opposing side, it is preferable so to control the gas flow by fitting gas distributor plates that optimum metering to the distributor plate takes place, whilst provision must be made for directed discharge of the gas. 
     According to an advantageous embodiment of the invention, in a vertical reactor the gas is supplied at the top of the wafers. However, it is also possible to supply the gas at the bottom, on its own or in combination with the gas supply at the top. 
     The present invention also relates to a reactor for accommodating a wafer rack and provided with coupling means to produce a connection with a supply line for gas mounted on the wafer rack as described above. 
     The invention also relates to a method for simultaneous treatment of at least two wafers in a process installation with a gas, comprising placing said wafers in a wafer rack and introducing said filled wafer rack into said process installation, wherein the gas is supplied via a central feed from the reactor separately to, in each case, one chamber above/below the surface of each wafer and is moved over at least said surface of said wafer, uniformly distributed over said wafer. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be described in more detail below with reference to illustrative embodiments shown in the drawings. In the drawings: 
     FIG. 1 shows, highly diagrammatically, a wafer rack according to the invention arranged in a reactor, only part of which is shown; 
     FIG. 2 shows a perspective and partially exposed view of part of the wafer rack according to FIG. 1; and 
     FIG. 3 shows a further embodiment of the wafer rack arranged in a reactor according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     In FIG. 1 the wafer rack according to the invention is indicated in its entirety by  1 . A large number of wafers  2  are accommodated in said wafer rack. The wafer rack  1  consists of two uprights  3 , located opposite one another, which form the carrier frame. Each of the uprights  3  is provided with support ridges  4  for accommodating the wafers  2  between them. A gas distribution chamber  5  is arranged between every two wafers, which gas distribution chamber is connected on opposing sides to, in each case, one channel  6  in each of the uprights  3 . Each gas distribution chamber consists of a top plate  7  and a bottom plate  8 , the bottom plate  8  being provided with a large number of openings. A figure of more than 1000 openings and more particularly approximately 1500 openings with a diameter of approximately 0.3 mm is mentioned here by way of non-limiting example. The channels  6  are in communication with one another and emerge in a connection line  10 , provided at the end with a coupling  11  equipped to interact with a coupling  12  which is arranged in the upper part  13  of a reactor, of which only the walls  14  are also shown. Such a reactor is, for example, designed for etching oxides on wafers using HF and water vapour. To this end a vacuum of, for example, 16 to 30 torr is maintained in such a reactor, the reactor being flushed with nitrogen. This is a dynamic process with continuous flow. The material from which the wafer rack is made is chosen depending on the process. ‘Monel’ is outstandingly suitable for the etching process described here. In FIG. 2 the construction shown in FIG. 1 for a single wafer is shown in detail and partially exposed. 
     With the installation shown in FIGS. 1 and 2, gas originating from coupling  11 ,  12  is fed via a connection line  10  to the various top plates and issues through the openings made therein, uniformly distributed over the surface of the wafers, towards the outside. The gas can then be discharged from the reactor in the conventional manner, for example through discharge openings made over the entire height of the reactor around the periphery thereof, which openings are not shown. 
     FIG. 3 shows a variant of the invention which in particular can be used for subsequent adaptation of existing reactors. The wafer rack according to the invention is indicated in its entirety by  21  and serves to accommodate a number of wafers  22 . Here too the wafer rack consists of two uprights  23  located opposite one another, which form the carrier frame, provided with support ridges  24  for supporting wafers  22  (only one is shown). There is a gas distribution chamber  25 , consisting of a top plate  27  and a bottom plate  28 . Openings  29  have been made in the bottom plate  28 . The vertical furnace walls are indicated by  30  and were originally provided with inlet openings  31  for gas and discharge openings  33  for gas. In the embodiment according to FIG. 3 a number of said existing openings are closed off by plugs  36 . In the embodiment shown here measures have to be taken to collect the gas in the gas distribution chamber  25  and to prevent said gas flowing in an uncontrolled manner from openings  31  to  33 , from left to right in FIG.  3 . After all, in that case non-uniform etching of wafer  22  will take place. End walls  34  and  35  are present to prevent these various aspects. Said walls, together with end plate  32 , form an enclosed chamber, so that the gas entering through openings  31  is forced to move through openings  29 , uniformly distributed over wafer  22 . By means of end plate  32  and top plate  27 , the gas is also forced to emerge again via openings  33 . In this way an individual treatment chamber for each wafer is produced. A number of such treatment chambers are formed by interaction of the walls described above with the reactor walls  30  after the wafer rack  21  according to the invention has been introduced into the reactor. 
     On comparing the embodiment according to FIGS. 1 and 2 and that according to FIG. 3 it will be immediately apparent to a person skilled in the art that numerous variants are possible within the scope of the present application, which variants are obvious and fall within the scope of the appended claims. 
     For instance, it is possible, in particular, to fit further aids for uniform distribution of the gas. This means that blocking and guide plates can be fitted in the gas distribution chamber.