Abstract:
A method of processing a semiconductor wafer includes utilizing a heated gas to heat at least one part of a semiconductor wafer by convection whereupon at least one contaminant is desorbed therefrom. A stream of cooling gas is caused to pass over the one part of the semiconductor wafer in the absence of heated gas to cool the one part of the semiconductor wafer. A metrology tool is then caused to measure at least one part of the semiconductor wafer to determine at least one characteristic thereof.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to semiconductor wafer processing and, more particularly, to desorption of one or more contaminants from a surface of a semiconductor wafer. 
   2. Description of Related Art 
   Semiconductor substrates, especially silicon wafers, can form a layer of surface contamination. This surface contamination is undesirable because it can adversely affect wafer processing and monitoring. This contamination comes from the local untreated atmosphere, which is typically a mixture of numerous gases and gaseous vapors. It is believed that these gases and vapors condensate on the surface of the wafer forming a liquid or semi-liquid film. Although the constituents of this contamination layer are not precisely known, it is believed that water is a primary component. The other contaminants are comprised mostly of various hydrocarbon molecules, which are collectively referred as organics. 
   One approach to removing the contamination layer is heating the wafer. Heretofore, heating a wafer has been accomplished by radiation or conduction. One radiation heating technique includes placing a wafer under a heating element and heating the topside of the wafer primarily through radiation. Practical radiation heating requires holding the heat source at temperatures significantly higher than the desired surface temperature. This heat can create undesirable temperature increases in the surrounding area which would necessitate additional thermal management techniques. 
   Since it is directly over the top surface of a wafer, a radiation heating element must be sealed and non-contaminating even at high temperatures. Sealing the element can reduce radiation effectiveness requiring even a higher temperature heat source. Materials that meet the non-contaminating requirements can also be expensive and hard to manufacture. An array of linear heating elements can create cool zones between the elements resulting in uneven heating of the wafer surface. 
   One conduction heating technique includes placing the wafer onto a heated surface, such as a hotplate. The wafer is then heated primarily through conduction from the backside of the wafer until the topside has reached a desired temperature. One problem with using a heating plate results from the use of robotic arms that transport the wafer by the backside thereof. Setting the wafer down onto the heated surface of the hotplate requires withdrawing of the robotic arm. A mechanical wafer lowering mechanism or a recessed pocket in the surface of the hotplate is usually required to accomplish this. A wafer lowering mechanism adds system complexity while a recessed pocket can create uneven wafer heating. Another problem with conduction heating is a possibility of contaminating the backside of the wafer by the hotplate itself. 
   It would, therefore, be desirable to provide an apparatus and method that avoids the foregoing problems, and others, while enabling the temperature of a semiconductor wafer to be raised sufficiently to facilitate desorption of contaminants thereon for subsequent testing thereof. 
   SUMMARY OF THE INVENTION 
   The invention is a method of desorbing one or more contaminants from a surface of a semiconductor wafer for measurement by a metrology tool. The method includes positioning a wafer in spaced parallel relation with the heating plate and heating the wafer by convection with gas heated by the heating plate for a predetermined time period to remove contaminants from the wafer surface. The wafer is then removed from spaced parallel relation with the heating plate and cooled by blowing a gas thereon. At least one characteristic of the wafer is measured with a metrology tool a predetermined time after wafer heating is complete. 
   The foregoing method can be repeated for a plurality of wafers. Each wafer can be positioned in spaced parallel relation between a pair of spaced parallel heating plates. 
   The invention is also a method of preparing a semiconductor wafer for measurement by a metrology tool. The method includes positioning the wafer in spaced parallel relation with a planar heating element and heating the wafer by convection with gas heated by the heating element for a predetermined heating time to remove contaminants from the wafer surface. The wafer is then removed from its spaced parallel relation with the heating plate and a stream of gas is caused to pass over a surface of the wafer for a predetermined cooling time thereby cooling the wafer. The wafer is then positioned in operative relation to a metrology tool and at least one characteristic of the wafer is measured with the metrology tool. 
   Lastly, the invention is a method of processing a semiconductor wafer. The method includes utilizing a heated gas to heat at least one part of a semiconductor wafer by convection whereupon at least one contaminant is desorbed therefrom. A stream of cooling gas is caused to pass over the one part of the semiconductor wafer in the absence of said heated gas to cool the one part of the semiconductor wafer. A metrology tool is then caused to measure the one part of the semiconductor wafer to determine at least one characteristic thereof. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a side view of a system for desorbing and testing one or more semiconductor wafers; 
       FIG. 2  is a top view of the system shown in  FIG. 1 ; 
       FIG. 3  is the system of  FIG. 1  with the robotic arm assembly in a position for moving a desorbed semiconductor wafer through a stream of cooling gas; and 
       FIG. 4  is an isolated view of the desorption station of  FIGS. 1 and 3  also or alternatively including a heated tube for delivering a heated gas to at least a portion of a semiconductor wafer. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention will be described with reference to the accompanying figures where like reference numbers correspond to like elements. 
   With reference to  FIG. 1 , a system  2  for desorbing one or more contaminants from one or more surfaces of a semiconductor wafer includes a robotic arm assembly  4  for moving a semiconductor wafer  6  from an input/output station  8  to a desorption station  10 . Once semiconductor wafer  6  has been desorbed in response to convection heating of semiconductor wafer  6  by desorption station  10 , and semiconductor wafer  6  has been cooled to a suitable temperature for further testing, robotic arm assembly  4  transfers semiconductor wafer  6  to a metrology tool  12  for testing thereby. An exemplary metrology tool  12  can include a probe tip for contacting a surface of semiconductor wafer  6  and for applying a suitable electrical stimulus thereto. However, this is not to be construed as limiting the invention. 
   Examples of a suitable electrical stimulus include capacitance-voltage (CV), current-voltage (IV), conductance-voltage (GV) or capacitance-time (Ct) type electrical stimulus. Metrology tool  12  measures a response of semiconductor wafer  6  to the electrical stimulus and determines from the response at least one property of semiconductor wafer  6  at the point where the electrical stimulus is applied. Details regarding metrology tool  12  will not be described herein for simplicity of description. However, as will be appreciated by one of ordinary skill in the art, any suitable metrology tool  12  for determining a property of semiconductor wafer  6  can be utilized. 
   With reference to  FIG. 2  and with continuing reference to  FIG. 1 , input/output station  8  includes a base  14  for supporting a carrier  16  that is configured to carry one or more semiconductor wafers  6 . Base  14  can be configured to move carrier  16  upwardly or downwardly, as shown by arrow  18  in  FIG. 1 , under the control of a controller  20 . 
   Robotic arm assembly  4  includes a base  24  coupled to a plurality of arms  26   a – 26   c . Base  24  can be configured to move arms  26   a – 26   c  upwardly or downwardly as shown by arrow  28  in  FIG. 1 . One end of arm  26   a  is rotatably coupled to base  24  while the other end of arm  26   a  is rotatably coupled to one end of arm  26   b . The other end of arm  26   b  is rotatably coupled to one end of arm  26   c . Desirably, arm  26   c  is in the form of a paddle having one or more vacuum holes  30  disposed therein adjacent the distal end thereof. Vacuum holes  30  are coupled to a source of vacuum (not shown) which can deliver a vacuum to vacuum holes  30  under the control of controller  20 . As shown in the figures, semiconductor wafer  6  can be disposed on the distal end of arm  26   c  and held thereon by means of a vacuum supplied to vacuum holes  30 . 
   Desorption station  10  includes a circular heating plate  34  having a diameter that is desirably no less than the diameter of semiconductor wafer  6 . One desirable construction of heating plate  34  includes one or more thick film resistors formed on a circular ceramic substrate. Circular heating plates of this type are available from Watlow Electric Manufacturing Company, #6 Industrial Loop Road, P.O. Box 975, Hannibal, Mo., 63401, USA. However, this particular form of heating plate is not to be construed as limiting the invention. 
   Heating plate  34  is desirably spaced from a base of desorption station  10  by suitable standoffs  36 . Additionally, standoffs  38  are arranged on a top surface of heating plate  34  for maintaining semiconductor wafer  6  in spaced parallel relation to heating plate  34  during desorption of one or more contaminants from semiconductor wafer  6 . Standoffs  36  and  38  are desirably formed from a high-temperature glass-mica ceramic. However, this is not to be construed as limiting the invention since the use of any suitable high-temperature, non-contaminating material to form standoffs  36  and  38  is envisioned. 
   The height of each standoff  38  is selected whereupon the distal end of arm  26   c  can position semiconductor wafer  6  thereon whereafter the source of vacuum to vacuum hole  30  can be terminated, The distal end of arm  26   c  can then be moved from a position in contact with the backside of semiconductor wafer  6  to a position between semiconductor wafer  6  and circular heating plate  34 . Once in this position, the distal end of arm  26   c  can be withdrawn from the space between semiconductor wafer  6  and heating plate  34  thereby facilitating heating of semiconductor wafer  6  by convection with gas heated by heating plate  34 . 
   Electrical current can be supplied to heating plate  34  under the control of controller  20  whereupon the temperature of heating plate  34  can be controlled. Suitable temperature sensing means (not shown) can be positioned on or adjacent heating plate  34  and coupled to controller  20  to enable controller  20  to detect and control the temperature of heating plate  34 . Also or alternatively, desorption station  10  can include a circular heating plate  40 , shown in phantom in  FIG. 1 , disposed above standoffs  38 . Electrical current can be supplied to heating plate  40  under the control of controller  20  for controlling the temperature of heating plate  40 . Desirably, when heating plates  34  and  40 , are provided the spacing therebetween is sufficient to enable semiconductor wafer  6  and the distal end of arm  26   c  to be received therebetween. 
   After semiconductor wafer  6  has been convection heated for a predetermined time period sufficient to desorb one or more contaminants from semiconductor wafer  6 , the distal end of arm  26   c  can be reintroduced into the space between heating plate  34  and semiconductor wafer  6  under the control of controller  20 . Thereafter, robotic arm assembly  4  can be controlled to raise arm  26  into contact with the backside of semiconductor wafer  6  whereupon a vacuum can be applied to vacuum holes  30  to secure semiconductor wafer  6  to the distal end of arm  26   c . Robotic arm assembly  4  can then be controlled to lift semiconductor wafer  6  off standoffs  38  and to withdraw semiconductor wafer  6  from desorption station  10 . 
   With reference to  FIG. 3  and with continuing reference to  FIGS. 1 and 2 , robotic arm assembly  4  is then controlled to move semiconductor wafer  6  through a stream of gas  42 , desirably nitrogen, dispensed by a manifold  44  which receives said gas from a suitable gas source  46 . 
   In the illustrated embodiment, manifold  44  is coupled to a bottom side of desorption station  10 . However, this is not to be construed as limiting the invention since manifold  44  can be positioned at any convenient location accessible to robotic arm assembly  4 . 
   The rate that robotic arm assembly  4  moves semiconductor wafer  6  through stream of gas  42  is selected whereupon semiconductor wafer  6  is cooled sufficiently for testing by metrology tool  12 . To this end, if necessary, robotic arm assembly  4  can move semiconductor wafer  6  through stream of gas  42  a plurality of times. If desired, an on/off gas control valve (not shown) can be fluidly coupled to manifold  44  for on/off controlling stream of gas  42  under the control of controller  20 . 
   Once semiconductor wafer  6  has been cooled sufficiently, robotic arm assembly  4  is controlled to deliver semiconductor wafer  6  to metrology tool  12  for testing. Once semiconductor wafer  6  is in a proper position for testing by metrology tool  12 , controller  20  signals metrology tool  12  that testing of semiconductor wafer  6  can commence. 
   Desirably, metrology tool  12  commences testing semiconductor wafer  6  a predetermined time after convection heating of semiconductor wafer  6  by desorption station  10  is complete. By controlling the time between the end of convection heating of each semiconductor wafer  6  processed by desorption station  10  and the beginning of testing of said semiconductor wafer  6  with metrology tool  12 , variations in testing results between two or more wafers due to exposure to the atmosphere after desorption is avoided. 
   Once testing of semiconductor wafer  6  is complete, metrology tool  12  signals controller  20 . In response to receiving this signal from metrology tool  12 , controller  20  controls robotic arm assembly  4  to remove semiconductor wafer  6  from metrology tool  12  and to return semiconductor wafer  6  to the same location in carrier  16  from which it was initially obtained. Thereafter, the vertical position of base  14  and robotic arm assembly  4  can be controlled to deliver the next semiconductor wafer  6  supported in carrier  16  to desorption station  10 , stream of gas  42  and metrology tool  12  for testing and subsequent return to the same location in carrier  16  in the above-described manner. Desirably, the method of processing each semiconductor wafer  6  in carrier  16  continues until all of the semiconductor wafers  6  in carrier  16  have been processed in the above-described manner. 
   With specific reference to  FIG. 2 , conventional atmospheric gas, i.e., atmospheric air, can be utilized for convection heating of semiconductor wafer  6  in desorption station  10 . However, if desired, semiconductor wafer  6  can be exposed to an atmosphere of a desirable gas, such as nitrogen, during desorption of semiconductor wafer  6  in desorption station  10 . To this end, desorption station  10  can include a manifold  48  for delivering the suitable gas to semiconductor wafer  6  received in desorption station  10  from a suitable source of said gas, e.g., gas source  46 . 
   In the above-described embodiment, heating plate  34  and/or heating plate  40  are described as having a diameter no less than the diameter of semiconductor wafer  6 . However, if desired, the diameter of heating plate  34  and/or heating plate  40  can be smaller than the diameter of semiconductor wafer  6  whereupon only a portion of semiconductor wafer  6  is heated by convection with gas heated by such smaller diameter heating plate(s). 
   With reference to  FIG. 4 , also or alternatively, desorption station  10  can include a tube  50  for directing a stream of gas  52  from a gas source, e.g., gas source  46 , to at least one portion  54  of semiconductor wafer  6 . Tube  50  includes a heating element  56  which is coupled to a suitable source of electric power for heating tube  50  and, in turn, heating the gas flowing therethrough. Stream of gas  52  thus heated can heat portion  54  of semiconductor wafer  6  by convection. Tube  50  and heating element  56  are desirably configured to co-act whereupon the temperature of heated gas  52  is sufficient for desorbing at least one contaminant from portion  54  of semiconductor wafer  6 . 
   Once portion  54  of semiconductor wafer  6  has been desorbed, semiconductor wafer  6  can be exposed to a stream of cooling gas for cooling at least portion  54  thereof. This stream of cooling gas can be supplied to semiconductor wafer  6  via manifold  44  in the manner described above. Alternatively, the stream of cooling gas can be supplied via tube  50  simply by disconnecting heating element  54  from its source of electrical power whereupon stream of gas  52  which is no longer heated by heating element  56  can be utilized to cool portion  54  of semiconductor wafer  6 . 
   Once portion  52  of semiconductor wafer  6  is cooled sufficiently, semiconductor wafer  6  is delivered to metrology tool  12  for testing at least portion  54  thereof. Thereafter, semiconductor wafer  6  is removed from metrology tool  12  and returned to carrier  16 . 
   As can be seen, one or more contaminants on a surface of semiconductor wafer  6  can be desorbed in an efficient and predictable manner whereupon reliable testing of semiconductor wafer  6  by metrology tool  12  can be accomplished. The ability to heat the semiconductor wafer  6  by convection avoids possible contamination of the semiconductor wafer  6  resulting from contact between a heating element and semiconductor wafer  6  during conductive heating thereof and avoids the need for sealed radiation heating elements for radiation heating of semiconductor wafer  6 . Lastly, the ability to convectively heat a portion of semiconductor wafer  6  by blowing a heated gas thereon enables desorption of said portion while avoiding the need to heat the remainder of the semiconductor wafer  6  to a desorption temperature. Thereafter, the desorbed portion of semiconductor wafer  6  can be tested independently of the remainder of the semiconductor wafer  6 . It is envisioned that the ability to selectively heat one or more portions of a semiconductor wafer by convection will enable metrological testing of product semiconductor wafers utilizing conventional metrology tools. 
   The invention has been described with reference to the preferred embodiments. Obvious modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.