Abstract:
An oven for processing semiconductor wafers comprised of a first cylindrical canister, a second cylindrical canister that surrounds the first canister, and a third cylindrical canister that surrounds the second canister. The first canister is comprised of thin stainless steel so that it can be heated and cooled rapidly by band heaters positioned around its exterior. The second canister is comprised of stainless that it thermally insulates the first canister. The third canister is comprised of stainless steel that is thicker than the second canister so that the third canister can hold a sufficient vacuum for processing the wafers.

Description:
This appln. claims the benefit of Provisional No. 60/109,936 filed Nov. 25, 1998. 
    
    
     TECHNICAL FIELD 
     The present invention relates to an oven for processing semiconductor wafers and more particularly to an oven having a lightweight processing canister that can be heated and cooled rapidly, surrounded by a thermal insulating canister and a heavy vacuum canister. 
     BACKGROUND OF THE INVENTION 
     A trend in semiconductor device manufacturing is to replace the silicon dioxide dielectric layer with a thinner dielectric layer. Materials suitable for thinner dielectric layers include polyimides, BCB and perylene. When polyimides are used, the semiconductor wafer is coated with the polyimide and the wafer is baked at 450° C. for about one hour in order to solidify the polyimide and remove contaminants from the polyimide layer. Since oxygen reacts with the polyimide at high temperatures, the baking must be completed in an oxygen free environment, such as in a vacuum oven or in a diffusion furnace. 
     Prior art vacuum ovens comprise heavy stainless steel chambers and thus take a very long time to heat up to 450° C. and cool back down to room temperature. Therefore, what is needed by semiconductor manufacturers is a vacuum oven that can be heated and cooled rapidly. 
     SUMMARY OF THE PRESENT INVENTION 
     Briefly, the present invention is a rapid heating and cooling oven comprised of a first canister, a second canister and a third canister. The first canister is adapted for holding one or more items to be heated to a temperature of approximately 450° C. A plurality of band heaters positioned around the outside circumference of the first canister provide the heating. A first chamber is formed in the first canister for allowing an inert gas to be introduced uniformly into the first canister and a second chamber is formed in the first canister through which the inert gas is removed. 
     The second canister encircles the first canister and has an inner surface and an outer surface, with a first cavity being formed between the first canister and the inner surface of the second canister. The heating elements for heating the first canister are positioned in the first cavity in thermal contact with the outside of the first canister. The inner surface of the second canister has a reflective finish so that heat is reflected back toward the first canister. 
     The third canister encircles the second canister and is adapted to hold a vacuum of at least approximately one torr around and inside of the first and second canisters. An inert gas inlet is positioned so as to direct an inert gas into the first chamber and a vacuum outlet is positioned so as to allow a vacuum to be drawn on the second chamber. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an isometric view of an oven according to the present invention; 
     FIG. 2 is a cross-sectional view taken along the line  2 — 2  shown in FIG. 1; 
     FIG. 3 is a cross-sectional view taken along the line  3 — 3  shown in FIG. 1; 
     FIG. 4 is a cross-sectional view of another embodiment taken along the line  2 — 2  shown in FIG. 1; and 
     FIG. 5 is a schematic diagram of a gas pre-heater. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 illustrates an oven  10  positioned in a cabinet  14 . A door  18  seals one end of the oven  10 . The door  18  opens and closes so as to provides access to the inside of the oven  10  and allows items to be inserted or removed from the oven  10 . The cabinet  14  surrounds the oven  10  so that the oven  10  is enclosed within the cabinet  14 . However, this is mainly for cosmetic purposes, so the cabinet  14  is not an essential part of the oven  10 . In the preferred embodiment, the cabinet  14  is comprised of painted mild steel. However, the cabinet  14  could be comprised of many other materials. The cabinet  14  has a length “L”. In the preferred embodiment, L equals approximately twenty inches. 
     FIG. 2 illustrates that the oven  10  is comprised of a first canister  30 , a second canister  34  and a third canister  38 . The first canister  30  is a hollow cylinder that extends from the door  18  to a rear wall  42 . A plurality of items  46  are placed inside the first canister  30  for heating. The items  46  rest on a shelf  50 . A first porous stainless steel layer  54  is positioned underneath the shelf  50 . A second porous stainless steel layer  58  is positioned a distance “c” above the shelf  50 , so that a chamber  62  is formed between the shelf  50  and the layer  58 . 
     A chamber  66  is formed between the stainless steel layer  54  and the underlying wall of the first canister  30 . A valve  70  extends through the rear wall  42  and into the chamber  66 . The valve  70  is an on/off valve adapted for attachment to a vacuum pump. The valve  70  allows the chamber  66  to be evacuated by the vacuum pump attached to the valve  70  so that a vacuum in the range of approximately 0.01 millimeters of mercury (10 −2  torr) to one torr is established inside of the first canister  30 . In other applications, pressures as low as 10 −7  torr are utilized. 
     A chamber  74  is formed between the stainless steel layer  58  and the overlying wall of the first canister  30 . A valve  78  extends through the rear wall  42  and into the chamber  74 . The valve  78  is an on/off valve adapted for attachment to a source of inert gas, such as nitrogen. The valve  78  allows the chamber  74  to be filled with the inert gas, which also fills the rest of the inside of the first canister  30 . 
     The second canister  34  is a hollow cylinder that extends from the door  18  to the rear wall  42 . The first canister  30  is completely surrounded by the second canister  34 . A cavity  82  (also called space  82 ) is defined by the space that exists between outside surface of the first canister  30  and the inside surface of the second canister  34 . A continuous hollow tube  86  is coiled around the first canister  30  from the end of the first canister  30  adjacent to the wall  42  to the end of the first canister  30  adjacent to the door  18 . An on/off valve  90  allows a gas, preferably an inert gas, to be introduced into the tube  86 . An on/off valve  94  allows the gas to exit the tube  86 . In the preferred embodiment, the gas is nitrogen gas and a heater  98  accepts the nitrogen gas after it exits the valve  94 . The heater  98  heats the nitrogen gas and redirects it to the chamber  74  through the valve  78 . 
     A plurality of heating elements  102  are positioned around the first canister  30 . In the preferred embodiment, there are five of the heating elements  102 , and each heating elements  102  comprises a circumferential band heater. Each band heater is connected to a controller which allows the temperature of the band heater to be controlled. Each of the heating elements  102  has a thermocouple  106  associated with it to monitor the temperature of the canister  30  in the region around the heating element. Each thermocouple is connected to one of the controllers to provide temperature information to the controller about the output from the band heater associated with the thermocouple. 
     Each band heater is a resistive heater that is capable of heating a region of the first canister  30  to a temperature of approximately 450° C. and is controllable to approximately ±1° C. The band heaters are powered by a 240 V alternating current source. The band heaters and thermocouples  106  are available from commercial sources such as Tempco Electric Heater Corporation of Wood Dale, Ill., USA. 
     The heating elements  102  are positioned in the cavity  82  in contact with the canister  30  and encircle the canister  30  (see FIG.  3 ). The hollow tubes  86  are positioned in the cavity  82  with a gap  60  existing between the heating elements  102  and the hollow tubes  86  (see FIG.  3 ). 
     The third canister  38  is a hollow cylinder that extends from the door  18  to the rear wall  42  and that completely encloses the first canister  30  and the second canister  34 . In the preferred embodiment, the third canister  38  comprises a heavy stainless steel cylinder capable of holding a vacuum down to 10 −7  torr. 
     FIG. 3 illustrates that in the preferred embodiment, the first canister  30  has a circular cross section with a diameter measurement “d” of approximately thirty centimeters (cm). The wall of the first canister  30  has a thickness of approximately 0.06 inches and is comprised of stainless steel. The stainless steel is a commercially available alloy designated as SS 316L. The first canister  30  can be removed from the oven  10  so that maintenance tasks such as cleaning the first canister  30  and servicing the heating element  102  can be completed. The ends of the first canister  30  are open and do not tightly abut the wall  42  and the door  18 . This loose fit creates a passageway between the ends of the canister  30  and the wall  42  and the door  18  when the door  18  is closed. This allows the atmosphere inside of the canister  30  to equilibrate with the atmosphere within the rest of the oven  10  (i.e. if a vacuum is pulled inside the canister  30 , the entire volume of the oven  10  is evacuated). 
     The second canister  34  has a circular cross section with a diameter measurement “e” of approximately 40 cm. The wall of the second canister  34  has a thickness of approximately 0.06 inches and is comprised of stainless steel. The stainless steel is a commercially available alloy designated as SS 316L. As with the first canister  30 , the ends of the second canister  34  are open and do not tightly abut the wall  42  and the door  18 . 
     The third canister  38  has a circular cross section with a diameter measurement “f” of approximately 50 cm. The wall of the third canister  38  has a thickness of approximately 0.25 inches and is comprised of stainless steel. The stainless steel is a commercially available alloy designated as SS 316L. 
     Examination of FIG. 3 illustrates that the third canister  38  completely surrounds the second canister  34  in the circumferential direction and that the second canister  34  completely surrounds the first canister  30  in the circumferential direction. The heating elements  102  encircle the first canister  30  as do the hollow tubes  86 . A gap  60  exists between the heating elements  102  and the hollow tubes  86 . A chamber  62  is defined by the space between the stainless steel layer  58  and the shelf  50 , and the item  46  is positioned in the chamber  62 . The ends of the third canister  38  tightly abut the wall  42  and the door  18  so that a gas tight seal is formed when the door  18  is closed. This seal enables a vacuum of at least approximately 10 −2  torr to be maintained within the canister  38 , and preferably approximately 10 −7  torr. 
     The stainless steel layers  54  and  58  are comprised of a stainless steel material referred to as “Mott porous metal media” (also called Mott plate). The Mott plate is a rectangular slab of the porous stainless steel having a pore size in the range of 0.5 to 100 micrometers (μm). Preferably, the pore size is in the range of 40 to 100 μm. The pores in the Mott plate allow gas to pass through the stainless steel so that the nitrogen gas entering the chamber  74  can pass through the stainless steel layer  58  into the chamber  62 . Similarly, the nitrogen gas in the chamber  74  can pass through the stainless steel layer  54  and be evacuated by the vacuum in the chamber  70 . The stainless steel layers  54  and  58  thus form a plenum that allows nitrogen to be evenly distributed within the chamber  62 . The Mott plate is commercially available from Mott Industrial (a division of Mott Company) of Farmington, Conn., USA. Mott porous metal media is frequently used in nitrogen filters to screen out particulate matter. 
     FIG. 4 illustrates another embodiment of the oven  10 . FIG. 4 is analogous to FIG.  2  and the components in FIG. 4 that are identical to components shown in FIG. 2 are identified with the same numerals. In FIG. 4, the tube  86  (shown in FIG. 2) has been removed. Instead, the inert gas (which is preferably nitrogen) is introduced into the chamber  74  (also called plenum  74 ) through a gas tube  120  that is connected to the heater  98 . Inside of the chamber  74 , the tube  120  branches into an H-shaped tube  121  perforated with holes, so that the gas is evenly distributed within the chamber  74 . The chamber  74  has an approximately 0.10 inch thick layer  58  of Mott plate extending along its length as was described previously with respect to FIG.  1 . Similarly, the chamber  66  has an approximately 0.10 inch thick layer  54  of Mott plate extending along its length as was described previously with respect to FIG.  1 . 
     The second canister  34  has an inner surface  124  that faces the heating elements  102 , and an outer surface  128  that faces the third canister  38 . The canister  34  is comprised of stainless steel and the inner surface  124  has a number eight (No. 8) mirror finish. The mirror finish on the inner surface  124  causes the canister  34  to reflect heat back toward the first canister  30 . 
     Inert gas (preferably nitrogen) is introduced into the chamber  74  through the gas tube  120 . The gas is heated by the heater  98  before it is introduced into the chamber  74 . The ends of the chamber  74  are sealed by a front wall  132  and a back wall  136  so that the gas cannot escape through the ends of the chamber. A vacuum is pulled on the chamber  66  through a vacuum tube  140 . In the preferred embodiment, the vacuum tube  140  is a hollow elongated stainless steel rod which is connected to a vacuum pump. The rod has holes in it along the portion of the rod that is inside the chamber  66  so that the effect of vacuum is distributed uniformly along the length of the chamber  66 . The ends of the chamber  66  are sealed by a front wall  144  and a back wall  146 . A lower vacuum tube  148  is positioned in a chamber  152  so that a vacuum can be pulled on the chamber  152  formed between the second canister  34  and the third canister  38 . 
     Each heating element  102  has an individual thermocouple  106  associated with it to control the temperature of the particular heating element  102 . The thermocouples  106  are in turn connected to a controller  150  that controls the thermocouples and keeps them within ±1° C. of the target temperature. A suitable controller  150  is commercially available from Oakleaf Engineering, Inc., of Redwood City, Calif., under the trademark BeyondPID™ controller. The controller  150  is positioned outside of the cabinet  14  to allow the user to select specific temperatures, lengths of heating time and other parameters for the heating elements  102 . The power for the heating elements  102  is preferably 208 volt three phase power controlled by solid state relays. 
     The ends of the oven  10  are sealed in a symmetrical fashion so that the heating along the entire length of the of the first canister  30  is as uniform as possible. Specifically, at the end adjacent to the door  18 , a stainless steel plate  158  is attached to the door  18 . A stainless steel plate  162  is attached to the plate  158 . Preferably, the plate  162  is separated from the plate  158  by a two inch space. A first baffle  166  is attached to the plate  162  separated by a one inch space, and a second baffle  170  is attached to the first baffle  166  separated by a one inch space. The baffles  166  and  170  are both comprised of stainless steel and extend inside the chamber  62  to reflect heat back into the chamber  62 . In the preferred embodiment, these components have the following approximate thicknesses: plate  158 —0.75 inches; plate  162 —0.187 inches; baffles  166  and 170-0.030 inches. 
     A gasket is positioned in the plate  162  so that an airtight seal with the third canister  38  is formed. The canisters  30  and  34  abut the plate  162 , but do not form an airtight seal with it. This allows the inert gas from the chamber  74  to fill the chambers  82  and  152 , and allows the vacuum from the chamber  66  to reach the chambers  82  and  152 . The door  18  is mounted on hinges so that it can be opened and closed as desired. 
     Similarly, to maintain the thermal symmetry of the system, the end of the oven  10  away from the door  18  is sealed by a plate  174  (analogous to plate  158 ), a plate  178  (analogous to plate  162 ), and baffles  182  and  186  (analogous to baffles  166  and  170  respectively). A gasket is positioned in the plate  178  so that an airtight seal with the third canister  38  is formed. The canisters  30  and  34  abut the plate  178 , but do not form an airtight seal with it. 
     In the preferred embodiment, an aluminum housing  190  is positioned around the oven  10 , inside of the housing  14 , so that air can be pumped around the oven  10  to assist in cooling down the oven  10 . 
     In the embodiment of the oven  10  illustrated in FIG. 4, the length “L” (shown in FIG. 1) is approximately forty-one inches. The length of the canister  30  is approximately twenty-four inches. The diameters “d”, “e” and “f” (shown in FIG. 3) are approximately 15.0 inches, 21.25 inches and 22.75 inches, respectively. 
     FIG. 5 illustrates that the heater  98  is comprised of a cartridge heater  152  and a hollow stainless steel tube  156 . A segment  160  of the tube  156  extends outside of the heater  98  and is connected to a gas source (preferably nitrogen). The tube  156  coils around part of the cartridge heater  152  so that gas flowing inside the tube  156  is heated. The gas tube  120  is a segment of the tube  156  that directs heated gas into the chamber  74 . A thermocouple  194  controls the temperature of the cartridge heater  152 . The thermocouple is in turn controlled by the controller  150 . Preferably, the controller  150  will hold the cartridge heater  152  at the same temperature as the heaters  102 , so that the inert gas is heated to the same temperature as the inside of the chamber  62 . A housing  198  surrounds the cartridge heater  152 . 
     The oven  10  illustrated in FIG. 4 functions as follows: In the preferred embodiment, the items  46  are a plurality of semiconductor wafers (e.g. silicon wafers) on which a uniform layer of polyimide has been coated to act as a dielectric layer. The oven  10  is used to bake the wafers so that the polyimide (or other dielectric layer) will degas and solidify. 
     The heating elements  102  are used to heat the oven  10  to approximately 150° C. The door  18  is then opened, the items  46  are positioned in the chamber  62  and the door  18  is closed. The canisters  30 ,  34  and  38  are evacuated to about 10 torr using the vacuum pump attached to the vacuum tubes  140  and  148 . The tubes  140  and  148  are then closed and the chamber  62  is filled with gas using the gas tube  120  to fill the chamber  62  with gaseous nitrogen (preheated to approximately 150° C.). The nitrogen gas enters the chamber  74  and moves through the porous stainless steel layer  58  to fill the chamber  62 . The gas flow is then stopped and the canister  30  is evacuated again to about 10 torr, and then refilled with nitrogen. This cycle is repeated several times (preferably a total of at least three vacuum/gas fill cycles) until there is essentially no oxygen in the chamber  62 . 
     A steady stream of hot nitrogen gas is then allowed to flow through the chamber  62  with the gas flow and vacuum tube  140  adjusted to maintain a reduced pressure of about 200 torr in the chamber  62 . The vacuum tube  148  remains closed during this steady state process. This creates a vertical laminar flow of nitrogen across the vertical axis of the items  46  placed in the chamber  62 . The heating elements  102  continue to heat the first canister  30  up to a temperature of approximately 450° C. The heater  98  heats the nitrogen gas up to approximately 450° C. at the same rate because the thermocouple  194  is synchronized with the thermocouples  106  by the controller  150 . 
     With the present invention, this heating occurs at a rate in the range of 10 to 30° C./min. This rapid rate of heating is achievable because of the minimal thickness of the first canister  30 . However, because of the minimal thickness of the first canister  30 , heat tends to radiate out from the canister  30 . To minimize this problem the inside surface  124  of the second canister  34  is finished with a number eight (No. 8) mirror finish to reflect the radiated heat back toward the canister  30 . The baffles  166 ,  170 ,  182  and  186  also radiate heat back into the canister  30 , but they are not finished with a No. 8 mirror finish. 
     For polyimide baking, once the temperature reaches 450° C., it is held at that level for approximately sixty minutes. After sixty minutes, the heating elements are turned off and the first canister  30  is allowed to cool to about 150° C. The cooling occurs at a rate of about 10 to 30° C./min. The cooling process is assisted by pumping air through the housing  190  around the outside of the canister 38 . By keeping the third canister from heating all the way up to 450° C., the cool down cycle occurs more quickly than in ovens of the prior art. 
     It should be noted that some or all of the parameters given above for processing polyimide wafers can be changed to fit the requirements of a particular process. For example, in some cases it may be desirable to place the wafers in the oven  10  when the oven is at room temperature, and then begin the heating. The controller  150  for the heating elements  102  provides the flexibility to change the temperature of the heating elements  102  to any value that is desired. Additionally, the time that the heating elements  102  remain at a temperature can also be set by the controller  150 . 
     Although the present invention has been described in terms of the presently preferred embodiment, it is to be understood that such disclosure is not to be interpreted as limiting. Various alterations and modifications will no doubt become apparent to those skilled in the art after having read the above disclosure. Accordingly, it is intended that the appended claims be interpreted as covering all alterations and modifications as fall within the true spirit and scope of the invention.