Abstract:
The aim of the invention is to reduce the formation of scratches in a device for the thermal treatment of substrates, in particular, semiconductor substrates, in a chamber in which the substrate is placed upon support elements. According to the invention, said aim is achieved by means of displaceable support elements.

Description:
BACKGROUND OF THE INVENTION 
   The present invention is related to an apparatus for the thermal treatment of substrates, especially semiconductor substrates, in a chamber in which the substrates are placed upon support elements. 
   An apparatus of this type for the treatment of substrates is known, for example, from DE-A-198 21 007 of the same applicant. With this apparatus, the support elements for carrying semiconductor wafers are rigidly connected to a rotary plate that is rotated for the homogenization of the thermal treatment of a semiconductor wafer. 
   An alternative carrier construction for the apparatus of the aforementioned type is illustrated, for example, in FIG.  1 . Here a wafer  1  is placed upon conically shaped support studs  2  which are also called pins. The pins, along with their holding means  4 , are rigidly secured to a carrier frame  3 . The pins have conically shaped support tips in order to keep the contact surfaces between the support elements and the wafer small, and to thus minimize a transfer of heat from the wafer to the support element and to minimize a thereby resulting cooling off of the wafer in the region of the contact surface. Unfortunately, these support tips have the drawback that they break off easily and thereby result in undesired particles. Furthermore, the support tips leave behind mechanical impressions in the wafer that rests upon them, since at high temperatures the wafer material is relatively soft. Especially with wafers having relatively large diameters, such as, for example, 300 mm wafers, which have a weight of 130 g instead of 50 g for wafers having a 200 mm diameter, the pressure forces between support tips and wafer increase and thus the problem of mechanical impressions increases. 
   These mechanical impressions furthermore increase during the thermal treatment of the wafer due to the thermal expansion or distortion of the wafer and a thereby resulting relative movement between wafer and support pin. During the heating up of a wafer from room temperature to 1000° C., the thermally caused increase in diameter is, for example, in the region of 1 mm. Consequently, the tips of the support pins scrape over the surface of the wafer and at those locations leave behind long scratches, as illustrated by way of example in  FIGS. 2   a  and  2   b .  FIGS. 2   a  and  2   b  show defect locations on the back side of a semiconductor wafer, which were produced by the known support pins during the thermal treatment of the wafer. The mechanical impressions and the scratch widths can be examined, for example, by cleavage. With this method, the wafer is broken up into predefined pieces, whereby a line of fracture or a fault line extends through a location that is to be examined. By means of an SIRD process (scanned infrared depolarization), one obtains an indication about the magnitude of the existing stresses at the contact locations. With this method, a double break produced by elastic deformation is measured that results with many transparent and isotropic materials due to deformation. 
   The mechanical impressions and scratches, as well as inhomogeneity of the wafer temperature during the thermal treatment, lead to dislocation errors, so-called slip lines, in the crystalline structure of the semiconductor wafer. Although these occur on the underside, i.e. the support side of the wafer, during the thermal treatment of the wafer they can, if the thermal stress is sufficiently great, propagate to the upper surface and damage or adversely affect the structures that are disposed on the upper surface. Such slip lines are visible, for example, from structural etching. 
   For a good thermal treatment of the semiconductor wafer, a homogeneous temperature distribution over the wafer is necessary. As already mentioned, due to the contact with the support pins, however, there results a localized cooling off of the wafer in the region of the contact, which leads to inhomogeneity of the temperature distribution upon the wafer. This problem was solved in the past in that the contact surface between support pin and wafer was kept small, which, however, worsened the aforementioned problem of scratching. In practice, the support pins were therefore positioned in an edge region of the wafer with a spacing of approximately 1 to 10 mm from the edge of the wafer. This was intended to ensure that the slip lines caused by the support forces did not damage the electronic components or structures disposed upon the surface of the wafer. However, this edge support results in the problem that during the thermal treatment the wafer sags, which again enhances the formation of dislocation errors or slip lines. 
   U.S. Pat. No. 5,817,156 furthermore discloses a substrate treatment apparatus that is provided with holding pins that are movable perpendicular to the plane of the substrate in order to position various regions at varying distances relative to a heating plate. However, with these holding pins the aforementioned problems of scratching occur that result from a relative movement between substrate and pin as a consequence of thermal expansion of the substrate. 
   Proceeding from the aforementioned state of the art, it is an object of the invention to provide an apparatus of the aforementioned type according to which the formation of scratches upon the surface of the wafer is reduced. 
   SUMMARY OF THE INVENTION 
   Pursuant to the invention, this object is realized in that the support elements are movable essentially parallel to the plane of the substrate. Due to the movable configuration of the support elements, they can follow a movement of the wafer during an expansion as a consequence of the thermal treatment. Thus, a scraping of the support elements upon the wafer surface is prevented. Instead of an elongated scratch, the support elements cause only a point-type impression. Furthermore, due to the movement that is directed essentially parallel to the substrate, the height or position of the substrates within the chamber is held essentially constant. 
   In this connection, the support elements are preferably movable radially relative to a central axis of the substrate in order to be able to follow the aforementioned expansion of the wafer, which is directed radially outwardly. 
   Pursuant to one embodiment of the invention, the support elements are suspended in a resilient manner in order to provide the necessary movability of the support elements. In this connection, the support elements are preferably respectively connected with a spring, especially a flat helical spring, via which not only a vertical but also a horizontal springiness can be achieved. The vertical springiness or resilience is expedient especially during the placement of the wafer in order to absorb forces that occur at this point in time. 
   Pursuant to a particularly preferred embodiment of the invention, the support elements are pivotable perpendicular to their longitudinal axis in order in a straightforward manner to provide the necessary movability of the support elements. In this connection, the pivot axis is preferably spaced from the longitudinal axis of the support element, so that in their position of rest the support elements are inclined in a predetermined position. The support elements are preferably inclined toward a central axis of the substrate, as a result of which they can follow an outwardly directed movement of the wafer over a greater distance. 
   Pursuant to another embodiment of the invention, pin-shaped support elements are mounted with great play in a sleeve-like receiving device. Due to the great play, the support elements are guaranteed the necessary freedom of movement in order to be able to follow the thermal expansion of the wafer. Pursuant to a very particular preferred embodiment of this receiving device, the sleeve is disposed so as to be tipped relative to the axis of the wafer so that the support pin is inclined relative to the center of the wafer. 
   Pursuant to a further embodiment of the invention, the support elements are preferably mounted on the free ends of movable carrier arms, whereby the movability of the support elements is provided via the carrier arms. In this connection, the carrier arms are preferably movable parallel to the plane of the substrate in order to follow the radial expansion of the wafer during the thermal treatment and to provide an essentially fixed height or position of the wafer within the chamber. 
   The support elements are preferably received in a guide in order to guide the movement in a prescribed direction. In this connection, the guide means is preferably a slot that preferably extends radially relative to a central axis of the substrate. 
   Pursuant to a further embodiment of the invention, the support elements are provided with a support flange upon which the support elements can, for example, glide in order to thus enable a movement of the support elements. In this connection, the support flange preferably has a curved support surface that provides a pivotable mounting of the support element. With a pivotable mounting of this type, there results the advantage that in contrast to a displaceable support flange, a lower friction occurs and thus a lower risk of damaging abrasion. 
   The support element is preferably light permeable and is embodied as an optical lens. In so doing, a shading effect is prevented with an apparatus with which the substrate is heated via a radiation field. Furthermore, due to the lens effect of the support flange, the radiation can be focused upon the contact location between support element and wafer, thereby compensating for the above described heat loss of the wafer at this location. This leads to a more homogeneous temperature distribution over the wafer. 
   Pursuant to one embodiment of the invention, a movable, especially tiltably mounted, holding element having at least three carrier arms and support elements secured thereon is provided. Due to the tiltable mounting of the hold element, small deviations in height of the support elements and/or of the wafer can be compensated. Furthermore, the support elements are preferably movably, epecially pivotably, mounted on the carrier arms in order to follow a movement of the substrate, especially a radially outwardly directed movement that is caused by the thermal treatment. Preferably, at least three of these holding elements are provided in order to provide a large number of support points and hence to reduce the pressure forces at the support points. 
   Pursuant to a further embodiment of the invention, the support elements are spheres or balls that are preferably respectively guided in a track or groove to enable the balls to roll on the track during a movement of the substrate and thus always contact only one point of the substrate surface. In this connection, the track is preferably inclined relative to a central axis of the substrate so that after the substrate has been raised or removed, the balls always return to a predetermined position of rest. 
   To provide the movability of the support elements, pursuant to one embodiment of the invention, these support elements have a conically tapering base that is pivotably received in a receiving means. 
   To keep a transfer of heat from the wafer to the support elements low, the support elements are preferably provided with substrate support tips, as a result of which the contact surface between support element and substrate is reduced. In this connection, the substrate support tips are preferably formed by a cone that has a larger opening angle than does a subsequent second cone. As a consequence of this double cone, the support tips are less easily damaged, and in particular break off less easily. In this connection, the first cone preferably has an opening angle between 50° and 130° and especially preferably between 80° and 100°. The opening angle of the second cone is preferably between 5° and 45° especially between 5° and 25°. 
   Pursuant to one preferred embodiment of the invention, the support elements are disposed on a circle having a radius of ½ to ⅘ and preferably ⅔ of the substrate radius in order to avoid a sagging of the substrate toward the middle or toward the outside. In the upper region the sagging is the lowest, thereby reducing the formation of dislocation errors or slip lines. 
   To avoid a shading effect, and hence a non uniform thermal treatment, the support elements and/or their holding devices are comprised at least partially of a light-permeable material. In this connection, the surfaces of the support elements and/or their holding devices are at least partially polished, especially fire polished, in order to ensure a good passage of the thermal radiation. 
   Pursuant to one preferred embodiment, the support elements and/or their holding devices are produced from one or a combination of the following materials: quartz, magnesium oxide, zirconium oxide, silicon, silicon nitrite, silicon carbide, aluminum oxide, aluminum nitrite, boron nitrite, sapphire, SAPHAL or ceramic. The present invention is particularly suitable for rapid heating units with which a semiconductor wafer is heated via a radiation field. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be explained in greater detail subsequently with the aid of preferred embodiments with reference to the figures. The figures show: 
       FIG. 1  a schematic, perspective illustration of a substrate carrier pursuant to the state of the art; 
       FIGS. 2   a  and  2   b  defect locations on the back side of a semiconductor wafer that were produced during the thermal treatment of a semiconductor wafer by support pins pursuant to the state of the art; 
       FIGS. 3   a  and  3   b  defect locations on the back of a semiconductor wafer that were produced during the thermal treatment by support pins mounted in a swinging manner pursuant to the present invention; 
       FIG. 4  a schematic, perspective illustration of a resiliently mounted support pin; 
       FIG. 5  a schematic side view of a support pin mounted in a swinging manner pursuant to the present invention; 
       FIG. 6  a schematic side view of the support pin of  FIG. 5  with a viewing angle rotated by 90°; 
       FIG. 7  a plan view of an alternative carrier construction pursuant to the present invention, according to which the support pins are mounted on swinging or resilient carrier arms; 
       FIG. 8  a detail view of a support pin pursuant to the present invention having a conical base end; 
       FIG. 9  a schematic side view of an alternative support pin of the present invention; 
       FIG. 10  a schematic side view of a further embodiment of a support pin pursuant to the present invention; 
       FIG. 11  a guide plate for receiving and guiding a support pin pursuant to  FIG. 9  or  10 . 
       FIG. 12  a schematic cross sectional view through a further embodiment of a support pin pursuant to the present invention; 
       FIG. 13  a further alternative embodiment of a support pin pursuant to the present invention; 
       FIGS. 14   a  and  14   b  a schematic cross-sectional view as well as a schematic plan view of a holding device for receiving support pins pursuant to the present invention; 
       FIG. 15  a perspective view of the holding device of  FIG. 14 ; 
       FIG. 16  alternative embodiments of a holding device for support pins. 
   

   DESCRIPTION OF PREFERRED EMBODIMENTS 
   The invention will be described in greater detail subsequently with the aid of  FIGS. 4  to  16 . The invention relates to an apparatus for the thermal treatment of substrates, especially semiconductor wafers, in a chamber in which the wafers are placed upon support elements. Such an apparatus is known, for example, from DE-A-198 21 007 of the same applicant, and to this extent is made the subject matter of the present invention in order to avoid repetition. The chamber in which the semiconductor wafer is placed upon the support elements is surrounded by a heating device that is comprised of lamps or banks of lamps that are disposed above and below the chamber. Pursuant to the present invention, the support elements are movably or displaceably mounted in order to follow a movement of the wafer, especially during an expansion or other distortion resulting from the thermal treatment. 
     FIG. 4  shows a first embodiment of a carrier construction  6  for a support element  8  in the form of a support pin. The support pin  8  has a first end  10  with a support tip  11  for receiving a semiconductor wafer. An opposite end  13  of the support pin  8  is secured in a suitable manner to a flat helical spring  15 . The helical spring  15  is formed by cutting out a portion  16  from a thin plate  17 , for example a thin quartz plate. Due to the cutout portion  16  in the quartz plate  17 , the flat helical spring  15  is resilient not only in a horizontal direction but also in a vertical direction. 
     FIGS. 5 and 6  show a second embodiment of the present invention for a movable suspension of a support pin  8 . The support pin  8  has a first end  10  on which is formed a support tip  11 . In this connection, the end  10  of the support pin  8  is embodied as a double cone. The support tip  11  is formed by one cone having an opening angle between 50 degrees and 130 degrees, and preferably between 80 degrees and 100 degrees. Following this cone is a second cone having an opening or generating angle between 5 and 45 degrees and preferably between 5 and 25 degrees. The thus-formed support tip is sturdy and reduces the danger that the tip will break off. 
   The support pin  8  has a longitudinal axis A. Mounted on one side of the support pin  8  is a pivot pin  20  having a longitudinal axis B. The longitudinal axis B of the pivot pin  20  extends perpendicular to the longitudinal axis A of the support pin  8 , and is laterally offset relative thereto. 
   The pivot pin  20  is received in a suitable holding device, such as a U-shaped holding device, so that the support pin  8  is mounted in a swinging manner. Due to the fact that the longitudinal axis B of the pivot pin  20  is offset from the central axis A of the support pin  8 , the pivot pin  20  is inclined in a rest position. In this connection, the longitudinal axis B of the pivot pin  20  is disposed in such a way that the support pin  8  is inclined relative to a central axis of the wafer that is to be received. 
     FIGS. 3   a  and  3   b  show defect locations on the back side of a semiconductor wafer that were produced during the thermal treatment by swingingly mounted support pins pursuant to FIG.  5 . As can be clearly recognized in a comparison to the defect locations shown in  FIGS. 2   a  and  2   b , the swingingly mounted support pin produces only a point-type impression rather than an elongated scratch. The damage to the surface of the wafer is thus significantly reduced, which is attributable to the support pin moving along during the thermal expansion of the wafer. 
     FIG. 16  shows a further embodiment for the movable holding of support elements. In this connection, sleeve-like receiving devices are arranged in a tilted manner for receiving support pins. The support pins are seated with great play in the sleeve, while their angle of tilt is in the range of 1°-45°, preferably between 1°-10°. 
     FIG. 7  shows an alternative embodiment for the movable suspension of support pins  8 . With the embodiment of  FIG. 7 , the support pins  8  are mounted on a rigid carrier frame  27  via freely swinging or oscillating carrier arms  25 . A total of three support pins  8  are mounted on the carrier frame  7  and form an equilateral triangle upon which a wafer is placed in a centered manner. During a thermal treatment of a wafer placed upon the support pins  8 , the freely oscillating carrier arms  25  follow a thermally-caused movement, especially an expansion movement, of the wafer. In addition to the freely oscillating carrier arms  25 , the support pins  8  could, of course, also be movably mounted upon the carrier arms. 
   A preferred embodiment for a movable mounting of the support pins  8 , for example in combination with the freely oscillating carrier arms, is illustrated in FIG.  8 . In this connection, the end  13  that is remote from the support tip  11  has a conical configuration. The conical end  13  is received in a receiving means  28 , such as a sleeve, and is pivotably held thereby. Alternatively, the conical end could also simply be received in a hole of the carrier arm  25 . 
     FIG. 9  shows a further alternative embodiment of a movable support pin  8  pursuant to the present invention. The support pin  8  has a first end  10 , which forms a support tip  11 . The end  10  is again provided with a double cone, whereby the cone that forms the support tip  11  has an opening angle of 90 degrees, and the following cone has an opening angle of 15 degrees. 
   Adjoining the second cone is a support flange  30  that provides a considerable widening of the support pin  8  in a middle portion thereof. The support flange  30  forms a support surface  31 , which will be described in greater detail subsequently with reference to FIG.  11 . Connected below the support flange  30  is a lower end  13  of the support pin  8 . In this connection, provided in the transition region between support flange  30  and lower end  13  is an undercut or relief groove  33 . Furthermore, the end portion  13  has a tapered end  34 . 
   The support pin  8  is produced from a light-permeable material, such as quartz, in order to avoid a shading effect due to the support pin  8 . To achieve a smooth surface and a good transparency of the support pin  8 , it is fire polished. The relief groove  33 , as well as the tapered base  34 , are provided due to possible changes in the dimensions during this process. In the region of the support tip  11 , the support pin  8  is not fire polished, so that this region remains relatively opaque. As a result, the tip does not freely allow the heating radiation through, and is thus itself heated up during a thermal treatment. As a result, the temperature gradient between support tip  11  and wafer is reduced, thereby achieving a greater homogeneity of the wafer temperature. 
     FIG. 10  shows a simplified illustration of an alternative embodiment of a support pin  8 . The support pin  8  essentially corresponds to the support pin illustrated in FIG.  9 . It has an upper end  10 , a support flange  30 , and a lower end  13 . Provided on the lower end portion  13  is a widened base  36 , the function of which will be described in greater detail subsequently with reference to FIG.  11 . 
     FIG. 11  shows a receiving plate  40  for receiving support pins  8  pursuant to  FIG. 9  or  10 . The receiving plate  40  has three slots  42  that extend radially relative to a central axis C, and that are provided at their radially outer ends with a widened bore  44 . The slots  42  are angularly uniformly spaced from one another. 
   The dimensions of the slots  42  are such that they receive the lower end portion  13  of the support pins  8  of  FIGS. 9 and 10  and can guide such end portions in a radial direction. The widened bore  44  is dimensioned such that the widened base portion  36  of the support pin of  FIG. 10  passes through the bore  44 . However, the bore  44  is not so large that the support flange  30  of the support pin  8  of  FIGS. 9 and 10  can also pass through. Rather, the support surfaces  31  of the support flanges  30  come to rest upon the upper side of the receiving plate  40 , whereby the support pins  8  are slidingly mounted along the slots  42  and upon the receiving plate  40 . The widened base  36  of the support pin  8  of  FIG. 10  is dimensioned such that it does not fit through the slots. This prevents the support pin  8  from being moved along with the wafer and coming out of the slot  42  when a wafer is raised. In contrast, the support pin of  FIG. 9  has an appropriately lengthened base portion  13  for this purpose. 
   The receiving plate  40  is made of a light permeable material, such as quartz, in order not to adversely affect the thermal treatment of the wafer. 
   Although this is not illustrated, the light permeable support flange  30  of the support pin  8  can have a lens-shaped configuration in order to form an optical lens. In this connection, the lens shape is selected such that light radiation of a heating field is focused on the contact location between support tip  11  and wafer. In this way, thermal losses of the wafer at this location are compensated for, and the temperature distribution is homogenized over the surface of the wafer. 
     FIGS. 12 and 13  show alternative embodiments of support pins  8  having a support flange  30 , according to which the support flange  30  is respectively provided with a curved support surface  31 . 
   With the embodiment of  FIG. 12 , the support flange  30  forms the lowermost portion of the support pin  8 . The support flange  30 , with its curved support surface  31 , is received in a receiving means  50  that is adapted to the curved shape and that is, as is the support pin  8 , made of a transparent material. The support pin  8  is pivotably guided within the receiving means  50 . 
   With the embodiment of  FIG. 13 , an end portion  13  is connected below the support flange  30 . The curved or arc-shaped support surface  31  of the support flange  30  rests upon a receiving ring  52  and thus enables a tilting movement of the support pin  8 . The end portion  13  is received with relatively great play in a receiving sleeve  54  in order to limit the tilting movement of the support pin  8 . 
     FIGS. 14 and 15  show a further alternative embodiment for a movable mounting of support pins  8 . With this embodiment, a holder  60  is provided that has three carrier arms  62  that on their free ends are respectively provided with an opening  64  for receiving a support pin  8 . The carrier arms  62  extend from a central, roof-shaped central portion  66  in which is formed a downwardly directed blind hole  68 . The blind hole  68  serves for receiving a stud  70  having a rounded support end  71 . The other end portion  72  of the stud  70  is received in a closely fitting manner in a receiving means  74  of a rigid carrier arm  75 . 
   The blind hole  68  and the stud  70 , especially the rounded support end  71 , are dimensioned such that the holder  60  is tiltably disposed upon the stud  70 . As a result, when a wafer is placed upon the support pins, a self or automatic correction of the support pins  8  is achieved for small deviations in height. A total of three of these holders  60  are provided, so that a wafer is placed on a total of nine support pins  8 , in contrast to the previous three support pins  8 . As a result, slight pressure forces occur at the respective contact points, thereby reducing damage of the wafer. In addition, a minimal shading of the wafer relative to the heat radiation is achieved, whereby preferably all elements are made of a material, such as quartz, that is transparent for the heat radiation. 
   Although this is not illustrated, the individual support pins  8  can be respectively movably mounted on the free ends of the carrier arms  62  in order to again be able to follow a movement of the wafer during the thermal treatment. In so doing, a tiltable mounting of the support pins  8 , as illustrated by way of example in  FIG. 12  or  13 , is particularly suitable. 
   Pursuant to a further, non-illustrated embodiment of the invention, instead of support pins, spheres or balls can be utilized as support elements. Such balls preferably have a diameter between 0.5 mm and 5 mm. During a thermal expansion of the wafer, the balls roll on a base support in conformity with the magnitude of the expansion, and thus always contact only one point of the wafer surface. 
   In this connection, the movement of the balls can be prescribed by support surfaces, such as grooves. These grooves are preferably inclined relative to a central axis of the substrate in order to ensure that after a wafer has been removed, the balls always roll back into a certain starting position. In this connection, the grooves are preferably disposed relative to one another in the same way as are the slots of FIG.  11 . 
   Pursuant to one preferred embodiment of the invention, in order to receive a wafer three support elements are respectively provided that are disposed at the ends of an equilateral triangle, and hence form a good three-point support. In this connection, the center of the triangle coincides with the central axis of a wafer placed thereupon. Each support element is, in this connection, spaced from the central axis of the wafer by a distance of 0.5 to 0.8 times the wafer radius (R). The spacing is preferably ⅔ of the wafer radius. If the elements are spaced greater than 0.8 R from the central axis, larger wafers, for example having a diameter of 300 mm, sag in the middle during the thermal treatment. If they are disposed too close to the axis, the rim of the wafer sags downwardly during the thermal treatment. If the support elements are disposed in the above mentioned range, and in particular at ⅔ of the wafer radius, the sagging of the wafer is at a minimum, as a result of which the formation of dislocation errors or slip lines is minimized. Instead of individual support elements, it is also possible to respectively provide a holder  60  having respectively three support elements at the corner points of the triangle. 
   Although the invention was described with the aid of preferred embodiments, the present invention is not limited to the specifically illustrated embodiments. In particular, the various ways for mounting or supporting the support elements can be combined with one another. Preferably, all of the described support elements, as well as the support or mounting elements that are correlated therewith, are produced from materials that are transparent for the heat radiation. Such materials are, for example, aluminum nitride, aluminum oxide, zirconium oxide, silicon carbide, boron nitride, sapphire, SAPHAL (trademark of the company Toshiba) or ceramic. Particularly advantageous are quartz, magnesium oxide, silicon and primarily silicon nitride. 
   The specification incorporates by reference the disclosure of German priority document 100 03 639.2 filed 28 Jan. 2000 and International priority document PCT/EP01/00607 filed 19 Jan. 2001. 
   The present invention is, of course, in no way restricted to the specific disclosure of the specification and drawings, but also encompasses any modifications within the scope of the appended claims.