Abstract:
Device for the floating accommodation of a wafer. This device comprises two mutually opposite parts which delimit a chamber in which the wafer is placed. By gas being supplied from opposite sides, the wafer is held in a floating position. To prevent the wafer from touching the lateral boundaries, it is proposed to provide a discharge of gas at least partially near the circumference of the chamber wherein the wafer is accommodated. This discharge is realised such that if the wafer moves from the intended position to such a gas discharge, by closing of said gas discharge, the pressure is locally increased such that a force is generated acting in opposite direction to return the wafer in the intended position.

Description:
REFERENCE TO RELATED APPLICATIONS 
     This application is the national phase of International Application No. PCT/NL00/00020, filed Jan. 13, 2000, which claims the priority date of Jan. 18, 1999. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to semiconductor processing, and more particularly to an apparatus and method for keeping a floating wafer centered within the apparatus. 
     Dutch Patent 1003538 describes the individual treatment of wafers. A wafer is accommodated in a reactor, with the distance to the walls of the reactor being set to be comparatively small. It is thus possible for a particularly rapid hear transfer to take place. It is possible to heat the wafer to well above 1000° C. within a few seconds. Since the wafer in principle does not have to be supported during this heating operation, but rather is held accurately in position by the gas streams, local temperature differences caused by mechanical contact are avoided. The very uniform heating of the wafer allows said short heating times to be achieved without the wafer becoming warped. Due to the high capacity of the reactor and the relatively low heat capacity of the wafer, in combination with the efficient heat transfer between the walls of the reactor and the wafer, so that the wafer quickly reaches the temperature of the reactor wall, this rapid heating of the wafer can be achieved with a relatively low peak capacity of the heating means in the reactor. The rapid heating of the wafer enables the total treatment time to be reduced, with the result that such a treatment is able to compete with the batchwise treatment of a series of wafers, while the consequences of problems remain limited to only one wafer. 
     In the prior art, it is proposed to provide the bottom part of the chamber with pins which can be displaced in the vertical direction in order to move the wafer into the loading and unloading position. During operation, such pins are inactive, since the wafer is then to be in the floating position. By suitably controlling the supply of gas for the various openings which lie on opposite sides of the wafer within the limits of the chamber in which the wafer is accommodated, the latter can be held floating in a very stable manner. 
     Although in the floating position the wafer is absolutely stable in the axial (vertical) direction, it has been found that the positioning in the radial (horizontal) direction provides problems: the wafer is able to move too easily in the radial direction. This is because of the position of the gas-supply and gas-discharge openings in the reactor, which openings can be regarded as an air bearing or gas bearing. In a radial air bearing which is known in the prior art, the air is supplied at the outside of the disc which is to be supported, and the air is discharged via an opening in the center of the disc which is to be supported; this leads to stable radial positioning. However, this results in considerable pressure differences across the surface of the wafer, while a requirements for a uniform process result is that the range of pressure variation across the wafer is no greater than ±2.5%. In the reactor described in Dutch Patent 1003538, stable positioning is achieved by positioning the gas-discharge openings along the circumferential side of the reactor wall. 
     Therefore, the object of the present invention is to provide a device which allows extremely accurate radial positioning of the wafer during operation, i.e. during treatment. 
     SUMMARY OF THE INVENTION 
     This object is achieved in accordance with one aspect of the invention, with a device for accommodation of a wafer. The device has an accommodation chamber formed by an upper part and a bottom part. The lower part has openings that extend between the accommodation chamber and an underside of the lower part. The openings provide for supply of treatment gas into the accommodation chamber in order to place the wafer in a floating position. Equipment in proximity of an end section of the accommodation chamber is configured to stabilize the wafer in a predetermined floating position. The equipment has a gas discharge and a gas supply that open into the accommodation chamber. The gas discharge and the gas supply are configured so that, upon a displacement of the wafer from the predetermined floating position, a flow resistance between the gas discharge and the gas supply increases at a side of the wafer that faces a direction of displacement such that an increased pressure urges the wafer back in the predetermined floating position. 
     According to a further possible embodiment, the gas is fed near the circumference edge of the wafer. At approaching the feed opening by the edge of the wafer locally the pressure increases and the wafer is moved back to the intended position. It will be understood that also combinations are possible of feed and discharge openings to obtain the centring effect described above. 
     According to a preferred embodiment of the invention, an additional opening is provided for introducing of a positioning-gas near the extremity of the wafer i.e. near the discharge for the gases. This gas moves over a part of the surface of the wafer which is not used to any beneficial effect and does not create any significant pressure differences. As a result, this gas can be supplied in relatively large quantities without producing unacceptable pressure differences, i.e. supplying gas for positioning purposes does not effect the treatment of the wafer, but does stabilise its radial position in the chamber in question. According to one proposal, such pressure differences are limited to ±2.5%. 
     Basically, there are two possible solutions for the positioning of the gas-discharge opening and the positioning-gas supply opening respectively. 
     According to a first embodiment, the center axis of the gas-discharge line is coaxial with the center axis of the central plane of the treatment chamber. In this case, it is assumed that the wafer is situated in the center of the treatment chamber in the position of use. Movement of the wafer towards the gas-discharge opening results in this opening being closed off to a greater or lesser extent, thus producing a force which pushes it back. In this design, the positioning-gas supply openings are preferably arranged substantially perpendicular to the center plane of the wafer to be treated. 
     According to an embodiment, these positions of gas-supply and gas-discharge openings are precisely reversed. 
     Both the gas-discharge opening and the positioning-gas supply opening may be designed in any way which is known in the prior art. The positioning-gas supply opening may comprise a number of openings which are arranged along (part of) a circle. If a complete circle is described, the position of the wafer is controlled in all radial directions. It is also possible for only part of a circle to be described, which is important if the wafer comes out of a supply channel which is connected to the treatment chamber. The positioning-gas supply opening may also be designed as a continuous slot or as a succession of elongate openings. The same applies to the discharge opening, which may be designed as a continuous ring or as a large number of openings or a number of slots. All this depends on the intended design and the desired operating conditions. 
     Since the positioning-gas is in principle not involved in the treatment of the wafer, it is possible to select a much less expensive gas, such as nitrogen, for this positioning-gas. It is, however, desirable for the gas to be supplied at the same temperature as the actual process gas, in order in this way to eliminate the effects caused by temperature differences as far as possible. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is explained in more detail with reference to the drawings, which are meant to illustrate and not to limit the invention, and in which: 
     FIG. 1 shows a perspective, partially cut-away, highly diagrammatic view of a structure according to the prior art; 
     FIG. 2 shows a first variant embodiment according to the invention; 
     FIG. 3 shows an altered version of FIG. 2; 
     FIG. 4 shows a further variant embodiment according to the invention, 
     FIG. 5 an embodiment with anti slip ring in cross-section and opened position; 
     FIG. 6 the structure according to FIG. 5 in closed position of use; and 
     FIG. 7 the structure according to FIG. 6 in a different cross-section. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a highly diagrammatic view of a device for the floating accommodation of a substrate or wafer. This device is denoted overall by  1  and comprises a chamber  9  which is delimited by a top part  2  and bottom part  3 . Treatment-gas supply openings  4  are present in both the bottom part  3  and the top part  2 . Gas is discharged through opening  7 , which is annular, and this opening  7  is connected to an annular channel  6  which is connected to a discharge line  5 . The wafer to be treated is denoted by  10 . FIG. 1 does not show the heating means, which are preferably internal to each of the top part  2  and bottom part  3 , nor does it show the structure for metering the gas through the openings  4 . Accurate metering through each of the openings is of considerable importance in order to ensure that the wafer floats stably. The gas supplied on the one hand keeps the wafer in a floating position and on the other hand treats the wafer (chemically or physically). For a standard 200 mm wafer, the gas consumption is approximately 2-5 slm. With this design, it is readily possible to position the wafer in the axial direction, but radial stability is not achieved. No feed/discharge for the wafer is shown in the drawing. This feed/discharge may be either horizontal or vertical. 
     The preferred embodiments provide stabilisation in the radial direction. 
     A first device which allows such stabilisation is shown in FIG.  2 . In this case, the device is denoted overall by  11 , the top part by  12  and the bottom part by  13 . The supply openings for the process gas are denoted by  14 . The discharge opening is denoted by  17  and is connected to a channel  16  which is connected to a discharge line  15 . Discharge opening  17  is designed as an annular gap. The space between top part  12  and bottom part  13  is denoted as chamber  19 . 
     In addition to the above-described gas-supply openings  14  for treatment gas, there are also two positioning-gas supply openings  18  which lie opposite one another. In this design, these openings are designed as a circular groove which extends along the entire circumference of the top part  12  and bottom part  13 . It is also possible for this groove to extend only over a limited part of the circumference. Gas from a different source (not shown) from the gas which is introduced into the openings  14  is introduced through this positioning-gas supply opening  18 . The volume of gas is considerably greater than the volume of gas which is introduced via the openings  4 . An example which may be mentioned is a ratio of approximately 1.5 or greater. This means that there is an increased flow of gas between openings  18  and discharge  17 . This covers the end part of the wafer. Since this end part is not relevant for subsequent use, the fact that the process conditions are altered in that area compared to the remaining part of the wafer  10  is not a problem. With regard to the remaining part of the wafer  10 , it remains the case that process conditions must be as uniform as possible over the entire surface, and this is achieved by controlling the flow of gas at the various treatment-gas supply openings  14 . 
     This means that a higher flow of gas through the openings  18  is readily permissible, and the result is a strong positioning action at the free end of the wafer. 
     FIG. 3 shows a variant of the design shown in FIG.  2 . The device in accordance with FIG. 3 is denoted overall by  21 . This device comprises a top part  22  and bottom part  23 . The gas-supply openings for the treatment gas are denoted by  24 . The space between top part  22  and bottom part  23  is denoted as chamber  29 . The discharge opening is denoted by  27  and opens out into a channel  26  which is connected to a line  25 . In the example shown in FIG. 3, the discharge opening  27  comprises a number of slots which are distributed along the circumference. This enables further optimisation of the pressure profile between the positioning-gas supply opening  28  and these openings  27  to be provided. In an example for a 200 mm wafer, the distance between the wafer and the top part  22  and bottom part  23  was approximately 0.1-0.15 mm. The distance between the opening  28  and the slots  27  was approximately 4 mm, while the length of the slots  27  was approximately 5 mm, with a height of 0.25-0.50 mm, thirty such slots  27  being present. 
     It should be understood that the described shape of the positioning-gas supply opening and of the gas-discharge opening relates to the boundary face with the chamber. 
     FIG. 4 shows a further variant of the invention, which is denoted overall by  31 . The top part is denoted by  32  and the bottom part by  33 , while the supply openings for treatment gas are denoted by  34 . As in the preceding Figures, the wafer is denoted by  10  and the chamber in which the wafer is situated is denoted by  39 .  37  denotes the gas-discharge opening which is now no longer situated in line with the center plane of the wafer, but rather perpendicular thereto. A channel  36  adjoins this discharge opening in a conventional manner. The positioning-gas is supplied through an opening  38 . This opening may comprise a number of holes  38 , as shown in FIG.  4 . Variants which lie between these options are also possible. 
     Moreover, the end of chamber  39  is adapted to the shape of the end of the wafer (rounded off in an essentially identical way). Such adaptation may also be incorporated in the designs shown in FIGS. 1-3. 
     In FIGS. 5,  6  and  7  a further alternative of the invention is shown. The wafer is transported therein with a so-called anti-slip ring. This ring is referred to by  49  and is supported by a number of support fingers  47  of a support ring  46 . The function of this ring is to realize heating and cooling of the wafer as uniformly as possible. Through the heat capacity of ring  49  it is prevented that the wafer  48  is heated or cooled faster near its periphery that in its center having adverse consequences. The device shown in FIGS. 5 and 6 is generally referred to by  40 . As in the previous embodiments it comprises an upper portion  42  and a lower portion  43 . In the closed position as shown in FIG. 6 a compartment is delimited there between. The closed position of the upper portion  42  and lower portion  43  is determined by abutment cams  50 . Ring  49  extends over the full circumference of wafer  48  and rests in closed position of the reactor on the cams or circumferential edge  51 .  55  shows a duct for introducing of nitrogen. The gas emanating from this channel, functions as shield so that the treatment gas is confined in the reactor.  44  shows a discharge duct. It opens in a groove  53  wherein ring  49  is received. In FIG. 7 the feed of wafer positiong-gas is shown by  45 . Groove  53  is near cams  51  and support fingers  47  higher than on other locations along the circumference of the wafers, where to discharge the gas in outward directly only a narrow gap between the wafer and upper portion, lower portion respectively is provided as shown in the cross-section shown in FIG.  7 . The opening  54  is not shown on full scale and is somewhat enlarged relative to this gap. Because of that in this embodiment in the closed position as shown in FIGS. 6 and 7 this opening  54  will determine the feed of gas through the higher portions of the groove near the cams and support fingers to the enlarged outer portion of the groove  53  and the adjoining ducts  44 . I.e., if for example wafer  48  moves to the left, the opening  54  shown in FIG. 6 is closed and locally a pressure is built up so that the wafer is biased to the right. 
     Through the heat capacity of ring  49  it is prevented that the wafer  48  is heated or cooled faster near its periphery than in its centre having adverse consequences. The device shown in FIGS. 5 and 6 is generally referred to by  40 . As in the previous embodiments it comprises an upper portion  42  and a lower portion  43 . In the closed position as shown in FIG. 6 a compartment is delimited there between. The closed position of the upper portion  42  and lower portion  43  is determined by abutment cams  50 . Ring  49  extends over the full circumference of wafer  48  and rests in closed position of the reactor on the cams or circumferential edge  51 .  55  shows a duct for introducing of nitrogen. The gas emanating from this channel, functions as shield so that the treatment gas is confined in the reactor.  44  shows a discharge duct. It opens in a groove  53  wherein ring  49  is received. In FIG. 7 the feed of wafer positioning-gas is shown by  45 . Groove  53  is near cams  51  and support fingers  47  higher than on other locations along the circumference of the wafers, where to discharge the gas in outward directly only a narrow gap between the wafer and upper portion, lower portion respectively is provided as shown in the cross-section shown in FIG.  7 . The opening  54  is not shown on full scale and is somewhat enlarged relative to this gap. Because of that in this embodiment in the closed position as shown in FIGS. 6 and 7 this opening  54  will determine the feed of gas through the higher portions of the groove near the cams and support fingers to the enlarged outer portion of the groove  53  and the adjoining ducts  44 . I.e. if for example wafer  48  moves to the left, the opening  54  shown in FIG. 6 is closed and locally a pressure is built up so that the wafer is biassed to the right. 
     Except to provide a closure for the sealing gas by admitting nitrogen to channel  43 , through the supply of additional nitrogen a further centring effect of the wafer is realised. 
     It will be understood from the above that there are numerous variants allowing a wafer to be positioned in a chamber. It must be stressed that this positioning is not required along the entire circumference of the wafer. This will depend on the conditions. The variants referred to above lie within the scope of the appended claims.