Abstract:
An apparatus and method is provided for capturing, heating and degassing a wafer without using moving parts and without exposing the wafer to external stress. A degassing chamber is backfilled with a dry gas that improves wafer heating ramp rates and wafer heating uniformity. The backfilled gas efficiently conducts heat at relatively low pressures. Thus the degassing chamber may be evacuated via a cryo-pump without the need for an intermediate rough pumping step. Further, because the wafer is heated primarily by conduction, wafer temperatures are easily and precisely controlled independent of layers previously deposited on the wafer. Frontside heating elements such as heat generators and/or heat reflectors are provided that further improve wafer heating ramp rates and wafer heating uniformity by directing heat toward the front surface of the wafer. Preferably as heat radiates from the wafer it is reflected back to the wafer by a frontside reflector. The improved wafer heat uniformity provides more uniform desorption of contaminants which are then entrained by the dry gas and pumped from the degassing chamber. An isolation valve such as a slit valve provides a highly reliable and inexpensive means of isolating the cryo-pump from the degassing chamber.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is related to commonly owned U.S. Pat. application Ser. No. 08/889,990 filed Jul. 10, 1997. 
    
    
     FIELD 
     The present invention relates generally to the field of semiconductor device fabrication, and specifically to degassing semiconductor wafers. 
     BACKGROUND 
     Semiconductor substrates and the layers deposited thereon (collectively referred to herein as a “wafers”) absorb water vapor and other gases and impurities (e.g., hydrocarbons) when exposed to the same (e.g., when a wafer is removed from a vacuum chamber). These gases and impurities degrade film properties and therefore must be desorbed and driven off the wafer (i.e., the wafer must be degassed) before further films are deposited thereon. 
     One conventional degas module uses infrared radiation to heat the wafer to a desired temperature. The infrared radiation originates from an array of lamps positioned above the wafer. The wafer&#39;s temperature is measured using infrared pyrometry, a measure of the infrared radiation emitted from the wafer and, if the emissivity of the wafer is known, a measure of the wafer&#39;s absolute temperature. A major disadvantage of heating a wafer by infrared radiation and measuring its temperature by infrared pyrometry is the substantial transparency of most common substrate materials (e.g., silicon) to infrared wavelength radiation at the temperature range of interest for degassing (150-500° C.). Because most substrates are transparent to infrared wavelengths, the rate at which a wafer heats is dependent on the presence of other non-transparent (i.e., energy absorbing) layers, and device patterning placed on the wafer before it enters the degas chamber. Furthermore, any layers or device patterns which may be present from previous process steps affect the wafer&#39;s emmissivity making it difficult to obtain an accurate wafer temperature measurement by infrared pyrometry. 
     In order to achieve heating rates independent of wafer patterning some conventional degas methods employ a heated substrate support. However, due to surface roughness of both the wafer and the substrate support, small interstitial spaces may exist between the substrate support and the wafer which effectively decreases the contact area and heat transfer rate therebetween. Particularly in vacuum environments, this decreased contact area causes heat transfer to be dominated by radiation, and thereby to be slow, (as little radiation is produced at typical degassing temperatures). These spaces interfere with and cause non-uniform heat transfer from the substrate support to the wafer. To promote more uniform heat transfer, a heat transfer gas such as argon, helium or nitrogen is often used to fill the interstitial spaces between the wafer and the substrate support. This gas has better heat transfer characteristics than the vacuum it replaces and therefore acts as a thermal medium for heat transfer from the substrate support to the wafer. Accordingly the heat transfer coefficient of such a system is dependent on the spaces between the wafer and the substrate support and on the pressure, the atomic mass and the accommodation coefficient of the heat transfer medium. Small spaces and high pressures generate the best heat transfer. 
     In an effort to achieve smaller spaces, more efficient heating and more uniform wafer temperatures, conventional degassing apparatuses mechanically clamp the wafer to the substrate support using a clamp ring which contacts the outer edge of the wafer&#39;s frontside (i.e., a side that faces into the chamber). The clamp ring holds the wafer against the substrate support to maintain the necessary gas pressure between the substrate support and the wafer&#39;s backside (i.e., a side that faces the substrate support); a lower pressure is therefore maintained along the wafer&#39;s frontside than the pressure along the wafer&#39;s backside. However, the opposing forces applied to the wafer by the clamp ring and by the backside gas pressure may cause the wafer to bow. For example, a 10 Torr backside pressure causes an 8 inch wafer to bow about 1 mm at the wafer&#39;s center, and causes a 12 inch wafer to bow about 5 mm at the wafer&#39;s center. This bow increases the space between the substrate support and the wafer&#39;s backside, thereby decreasing the backside pressure and reducing heat transfer. Moreover, a 10 Torr backside pressure can cause the stress in the substrate to exceed the substrate&#39;s yield strength and break the wafer. 
     In addition to the disadvantages described above, mechanical clamping of the wafer is undesirable because the clamp ring consumes otherwise patternable surface area and because the surface contact between the wafer and the clamp ring promotes particle generation particularly as the wafer heats and expands. Accordingly, a need exists for a degassing apparatus and method that heats wafers independent of individual wafer patterning, that reduces wafer bowing, that reduces particles generated by contact between moving parts, and that increases patternable surface area. 
     SUMMARY OF THE INVENTION 
     The present invention provides a degassing apparatus and method for flowing gas into a degassing chamber until the degassing chamber reaches a pressure at which wafer heating occurs primarily via gas conduction rather than radiation. Thus, wafer heating and degassing occur uniformly regardless of pre-existing wafer patterning. Gas is preferably gradually flowed into the degassing chamber via a needle valve or flow controller so as to prevent the wafer from being unseated from the heated substrate support  15 . Thus, the wafer need not be clamped. Because pressures as low as a few Torr provide adequate heat conduction (the heat conductivity of Argon, for example, gas varying from near zero at high vacuum levels to full conductivity at 4 Torr.), after a degassing process the degassing chamber may be evacuated via a cryo-pump, without the need for an additional rough pumping step. Thus the configuration of the present invention provides faster evacuation times and increased throughput as compared to conventional systems that require both rough and high vacuum pumping. A roughing pump may nonetheless be employed with the present system to increase the time between cryo-pump regenerations. Alternatively the cryo-pump can be replaced with both a roughing pump and a turbo molecular pump. 
     An isolation valve such as a slit valve provides reliable cost effective isolation between the degassing chamber and a cryo-pump. A plurality of pins may be placed under the wafer to facilitate gas flow to the wafer&#39;s backside. To further enhance wafer temperature heat-up rate and wafer temperature uniformity during the degassing process and to reduce wafer temperature drop during the chamber evacuation for wafer transfer, a frontside heating element (e.g., a heat source and/or heat reflector) is placed preferably parallel to the wafer&#39;s frontside and in sufficiently close proximity to reflect heat that radiates from the wafer back to the wafer. Thus wafer heating and degassing continues to occur even as the chamber is being pumped out for wafer transfer. 
     By reducing the number of particles (via elimination of the clamp ring), by reducing reabsorbed moisture (via frontside heating elements contained within the vacuum chamber), and by reducing wafer stress (via a uniform frontside/backside pressure) the present invention greatly improves the wafer degassing process. Accordingly with use of the present invention less contamination and stress induced failures occur, and product yields increase. Also, since heat-up rates and degassing rates are higher with the present invention than with conventional radiatively heated degassing systems, the throughput of the overall semiconductor fabrication system is increased. These and other objects, features and advantages of the present invention will become more fully apparent from the following detailed description of the preferred embodiments, the appended claims and the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is a side elevational view of a degassing apparatus made in accordance with the invention; 
     FIG. 1B is a partial side elevational view providing a closer view of the wafer lift hoop of the degassing apparatus of FIG. 1A; 
     FIG. 2 is a top elevational view of the substrate support of the degassing apparatus of FIG. 1A; and 
     FIG. 3 is a top plan view of a fabrication tool that employs the inventive degassing apparatus of FIG.  1 A. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     For convenience the following table lists the reference numerals used in FIGS. 1-3 and the items identified thereby. 
     
       
         
               
               
             
           
               
                   
               
               
                 REFERENCE NUMERAL 
                 DESCRIPTION 
               
               
                   
               
             
             
               
                 11 
                 degassing apparatus 
               
               
                 13 
                 vacuum chamber 
               
               
                 15 
                 substrate support 
               
               
                 17 
                 gas inlet 
               
               
                 19 
                 dry gas source 
               
               
                 21 
                 gas outlet 
               
               
                 23 
                 gas pump 
               
               
                 25 
                 wafer 
               
               
                 27a-c 
                 pins 
               
               
                 29 
                 wafer lift hoop 
               
               
                 29a-c 
                 fingers 
               
               
                 30 
                 wafer shelf portion 
               
               
                 31 
                 side portion 
               
               
                 33 
                 sloped lower portion 
               
               
                 35 
                 needle valve/flow controller 
               
               
                 37 
                 isolation valve 
               
               
                 39 
                 reflector 
               
               
                 41 
                 buffer chamber slit valve 
               
               
                 43 
                 buffer chamber 
               
               
                 45 
                 fabrication system 
               
               
                 47 
                 first load lock 
               
               
                 49 
                 process chamber 
               
               
                 51 
                 wafer handler 
               
               
                   
               
             
          
         
       
     
     FIG. 1A is a side elevational view of a degassing apparatus  11  configured in accordance with the invention. In order to conveniently describe the inventive apparatus  11  its components will be described with reference to the object to be degassed. However, it will be understood that the object itself is not a part of the apparatus. 
     As shown in FIG. 1A the degassing apparatus  11  comprises a vacuum chamber  13  containing a heated substrate support  15 . A gas inlet  17  couples a dry gas source  19  (such as a noble gas or nitrogen with preferably less than  10  parts per billion of general contaminants, such as water, hydrogen, hydrocarbons, etc.) in fluid communication with the vacuum chamber  13 . The gas emitted from the dry gas source  19  may be further “dried” via a getter or cold trap (not shown) within the gas inlet  17 . A gas outlet  21  couples the vacuum chamber  13  in fluid communication with a gas pump  23 . 
     A wafer  25  is shown mounted on the heated substrate support  15 . Optionally, a plurality of pins  27  (preferably, three pins  27   a-c  as shown in FIG. 1A) may be positioned beneath the wafer  25  so as to facilitate gas flow along the backside of the wafer  25  and so as to reduce contact between the wafer  25  and the substrate support  15  (thereby reduce particles generated by such contact). Short pin heights facilitate heat transfer from the substrate support  15  to the wafer  25 ; preferrably the pins  27   a-c  are between 0.005-0.02 inches in height. The positioning of the plurality of pins  27  can be seen with reference to FIG. 2 which shows the heated substrate support  15  from a top plan view. 
     In order to easily place and extract a wafer from the heated substrate support  15 , a conventional wafer lift hoop  29  (the operation of which is well known in the art) or the like is employed. The wafer lift hoop  29  preferably is of the type having three fingers  29   a-c  that extend under the wafer. Thus wafer contact is limited to the area above the three fingers  29   a-c , and fewer particles are generated. More specifically the fingers  29   a-c  extend upwardly from the wafer lift hoop  29  and have a wafer shelf portion  30  preferably extending inwardly a horizontal distance of between 0.030-0.050 inches. Not only do the fingers  29   a-c  extend beneath the wafer  25  they also comprise a side portion  31   a-c , respectively, (shown in FIG. 1B) which extends along the edge of the wafer  25 . The side portion is preferably sloped sufficiently to avoid contact with the edge of the wafer  25  (avoiding particle generation thereby) as the wafer  25  is placed on or removed from the wafer handler (not shown). Similarly, to reduce contact between the wafer shelf portion  30  and the backside of the wafer  25 , he fingers  29   a-c  have a sloped lower portion  33  which lopes away from the wafer shelf portion  30  at an angle greater than or equal to 10°. Thus, if the wafer  25  should slide off of the wafer shelf portion  30 , the wafer  25  will be supported by the sloped lower portion  33 , and thus will avoid a potentially catastrophic fall. Thus, even after the wafer lift hoop  29  has lowered (and the horizontal portion of the fingers  29   a-c  are housed in appropriately located recesses in the surface of the substrate support  15 , the side portions  30   a-c  of the fingers  29   a-c  capture the wafer, preventing the wafer from moving out of center, or becoming unseated from the substrate support  15 . 
     The rate of the gas flowing into the vacuum chamber  13  is preferably controlled via a needle valve or flow controller  35  operatively coupled along the gas inlet  17 . Preferably, the gas pump  23  comprises a cryo-pump and the gas outlet  21  comprises an isolation valve  37 , such as slit valve or a gate valve, operatively coupled to the gas pump  23  to control the gas flow rate from the vacuum chamber  13 . A reflector  39  is positioned in close proximity above the wafer&#39;s  25  frontside such that heat radiating from the wafer  25  will reach the reflector  39  and be reflected back to the wafer  25 . As an alternative to the reflector  39 , a heater may be placed in close proximity above the wafer&#39;s  25  frontside such that heat radiating from the heater will each the wafer  25 . Preferably such a heater would comprise reflective metal and would be positioned in close roximity to the wafer  25  such that heat radiating from the wafer  25  will reach the heater and be reflected back to the wafer  25 . In this manner the wafer heat-up rate and the wafer temperature uniformity are greatly enhanced. Also, the cooling rate of the wafer  25  is significantly reduced, allowing the wafer to maintain a higher temperature as the vacuum chamber  13  is pumped out and as a buffer chamber wafer handler (not shown) reaches into the vacuum chamber  13  to extract the wafer  25  therefrom. The higher the temperature of the wafer  25  the less contaminants will reabsorb thereon. 
     As shown in FIG. 1A, the gas inlet  17  is positioned adjacent the heated substrate support  15 . However, the gas inlet  17  could alternatively be coupled opposite the heated substrate support  15  and could comprise a manifold having a plurality of openings which diffuse gas emitted from the gas inlet  17  into the vacuum chamber  13  and cause a substantially uniform flow of dry gas over the wafer&#39;s  25  frontside. The design of such a manifold is well known to those of ordinary skill in the art of CVD reactor design. U.S. Pat. No. 4,854,263 entitled “Inlet Manifold and Method for Increasing Gas Dissociation and for PECVD of Dielectric Films” is incorporated herein for it teaching of a specific inlet manifold. When such a manifold is employed, the reflector  39  can be coupled to the manifold. 
     In operation, prior to placing a wafer  25  within the vacuum chamber  13 , an isolation valve  37  is opened and an ion gauge (not shown) is turned on to monitor the pressure within the vacuum chamber  13 . Thereafter, a buffer chamber slit valve  41  that operatively couples the vacuum chamber  13  to a buffer chamber  43  opens and a buffer chamber wafer handler (not shown) extends therethrough, carrying the wafer  25  into position above the heated substrate support  15 . The wafer lift hoop  29  (via the three fingers  29   a-c ) lifts the wafer  25  from the buffer chamber wafer handler and lowers it onto the heated substrate support  15  after the buffer chamber wafer handler has sufficiently retracted. The isolation valve  37  is then shut and the ion gauge is turned off. 
     Thereafter the needle valve or flow controller  35  is turned on and a dry gas is flowed from the dry gas source  19  into the vacuum chamber  13  via the gas inlet  17 . The dry gas is flowed into the vacuum chamber  13  until the pressure within the vacuum chamber  13  reaches a set point (e.g., 3-10 Torr). Any conventional pressure measurement device may be employed to monitor the pressure within the vacuum chamber  13 . (It takes approximately 5 seconds to create a 10 Torr pressure within an evacuated 15 liter vacuum chamber with a dry gas (e.g., argon) flow rate of 2 s.l.m.) 
     After the vacuum chamber  13  reaches the pressure set point the needle valve or flow controller  35  is closed, shutting off the flow of dry gas to the vacuum chamber  13 . The gas pressure within the vacuum chamber  13  aids the transfer of heat from the heated substrate support  13  to the wafer  25 . As the wafer  25  heats, moisture and other contaminants are desorbed therefrom and mixed with the dry gas. Heat radiates from the wafer  25  to the reflector  39  and is reflected back to the wafer  25 ; in this manner the wafer  25  heats more efficiently. After only approximately 5-30 seconds the wafer  25  will reach the desired degassing temperature. Thereafter the isolation valve  37  is opened to begin evacuation of the vacuum chamber  13 , and the ion gauge (not shown) is turned on to monitor the pressure within the vacuum chamber  13 . As the vacuum chamber  13  evacuates, the dry gas and desorbed contaminants are carried away. Meanwhile the heat that radiates from the wafer  25  is reflected back to the wafer  25  by the reflector  39  so the wafer  25  maintains a high temperature and continues to desorb contaminants. 
     After the pressure within the vacuum chamber  13  is reduced to 1×10 −5  Torr or less, the wafer lift hoop  29  elevates, and the three fingers  29   a-c  thereof raise the wafer  25  above the heated substrate support  15 , the buffer chamber slit valve  41  opens, the wafer handler (not shown) reaches into the vacuum chamber  13 , extends under the wafer  25 , the wafer lift hoop  29  lowers (transferring the wafer  25  to the wafer handler) and the wafer handler retracts carrying the wafer  25  into the buffer chamber  43 . The wafer  25  continues to absorb heat reflected back from the reflector  39  until the wafer handler begins to retract. Even as the wafer handler retracts the wafer  25  maintains a sufficient temperature to degas contaminants and to prevent contaminant reabsorption. After the wafer handler retracts from the vacuum chamber  13  the isolation valve  37  closes. The wafer  25  is then transferred to a process chamber (not shown) for further processing. 
     FIG. 3 is a top plan view of a fabrication system  45  that employs the inventive degassing apparatus of FIGS. 1A and 1B. The fabrication system  45  comprises at least a first load lock  47 , at least one process chamber  49 , at least one wafer handler  51  and the inventive degassing apparatus  11 . 
     In operation, a wafer carrier containing at least one wafer is loaded into the first load lock  47 , and the first load lock  47  is pumped to the desired vacuum level. The wafer handler  51  extracts a first wafer and transports it to the inventive degassing apparatus  11 . A sealable port such as the slit valve  37  (FIG. 1A) on the vacuum chamber  13  opens allowing the wafer handler  51  to reach into the vacuum chamber  13  and deposit the first wafer on the heated substrate support  15 . The wafer handler  51  retracts and the slit valve  37  closes. The wafer is then degassed in accordance with the invention as described with reference to FIGS. 1A,  1 B and  2 . After degassing within the inventive degassing apparatus  11  is complete and the vacuum chamber  13  is evacuated as previously described, the slit valve  37  opens and the wafer handler  51  extracts the first wafer and carries it to the process chamber  49  for further processing. The process chamber  49  preferably performs chemical or physical vapor deposition or argon ion etching, as these processes will be more successful on a thoroughly degassed wafer. After the first wafer is processed within the process chamber  49  it is returned to the first load lock  47 . The sequence repeats until each wafer within the wafer carrier has been processed and returned to the first load lock  47 . 
     The foregoing description discloses only the preferred embodiments of the invention, modifications of the above disclosed apparatus and method which fall within the scope of the invention will be readily apparent to those of ordinary skill in the art. For instance, although the components of the inventive degassing apparatus and the configuration described herein are presently preferred, numerous variations may occur and yet remain within the scope of the invention. For instance, physical or chemical stimulation (e.g., radiation sources within the manifold and/or reactive gases such as  0   3  or CO within the gas flux) may be used to further enhance wafer degassing. The needle valve or flow controller and the isolation valve can be manually adjusted but are preferably computer controlled. The inventive degassing apparatus is not limited to processes which mount the object to be degassed on the lower portion of the chamber; the invention applies equally to top or side wall wafer mounting. Appropriate alteration of the process so as to prevent the wafer from falling from the heated substrate support  15  will be readily apparent to those of ordinary skill in the art. Further, numerous objects other than wafers (for example liquid crystal display panels and glass plates) may benefit from the inventive process. 
     Accordingly, while the present invention has been disclosed in connection with the preferred embodiments thereof, it should be understood that other embodiments may fall within the spirit and scope of the invention, as defined by the following claims.