Abstract:
A physical vapor deposition coating system to produce orthogonal lift-off coatings. The system incorporates multiple domes that rotate about the source centerline and about another axis of rotation in order to assure an even coating and to utilize a large percentage of a material evaporated from the source.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is related to U.S. Pat. No. 6,342,103 to Ramsay, entitled “Multiple Pocket Electron Beam Source,” which is hereby incorporated by this reference in its entirety. This application is also related to U.S. Pat. No. 6,287,385 to Kroneberger, entitled “Spring clip for sensitive substrates,” which is hereby incorporated by this reference in its entirety. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates generally to semiconductor processing and optical coatings, and more specifically to physical vapor deposition onto substrates.  
           [0004]    2. Related Art  
           [0005]    Electron beam evaporation is commonly used to coat wafers with a thin metallic layer in a process known as metalization. Generally, in typical silicon wafer fabrication, the metallic layer deposited is then etched to form circuit traces of an integrated circuit. However, gallium arsenide (GaAs), indium phosphide (InP) and numerous alloys between the two and similar electro-optical materials are now typically utilized as a substrate for high frequency integrated circuits of cellular devices, and etching of gold to form circuit traces on a GaAs substrate does not work well.  
           [0006]    Gold is often used as the conductor of integrated circuits because in addition to being highly conductive, as a passive metal, gold will not form a superficial oxide. Therefore, a high frequency current applied to a circuit trace made of gold can easily flow through the skin of the circuit trace because it is not a resistive oxide layer. This is the well known skin effect in the conduction of high frequency power. This is critical in order to reduce power consumption in high powered GaAs IC&#39;s common in cellular devices. There are two problems with depositing a gold layer directly upon a GaAs substrate. First, the gold will leach into the substrate. Second, the gold will not adequately adhere directly to the substrate. Therefore, in order to prevent the gold from leaching into the substrate, a diffusion barrier of palladium or platinum separates the gold from the GaAs. Additionally, an adhesion layer of titanium or chromium is deposited upon the GaAs substrate between the substrate and the diffusion barrier in order to make the gold, and the diffusion barrier adhere to the substrate. These barrier and adhesion layers must typically be very thin yet very uniform.  
           [0007]    [0007]FIG. 1 illustrates a cross section of a circuit trace showing adhesion layer  104  upon GaAs substrate  102 . Upon adhesion layer  104  is diffusion barrier  106  upon which is the gold  108  that forms the circuit traces. The gold circuit traces cannot be etched away from the GaAs substrate in a typical etching process as they can on a silicon substrate because the etchant would remove the adhesion layer and diffusion barrier thus freeing the circuit trace from the substrate, a clearly undesirable consequence.  
           [0008]    Therefore, the gold circuit traces are typically made according to a “lift-off” process. In the lift-off process a photoresist pattern having a trench  110  is formed upon the GaAs substrate, as can be seen in FIG. 2. First an adhesion layer, followed by a diffusion barrier are deposited sequentially, and finally gold is then deposited upon the photoresist pattern such that a portion  108   a  is deposited upon diffusion barrier  106  above photoresist layer  112  and a portion  108   b  is deposited upon diffusion barrier  106  within trench  110  . The gold  108   b  that is directly deposited upon the diffusion barrier will form the circuit traces. Any gold  108   a  and adhesion layer  104  plus diffusion barrier  106  that are deposited upon the photoresist will “lift-off’ the substrate when the photoresist layer is dissolved, as long as it is not connected in any way to the circuit formed from gold portion  108   b  deposited within the trench  110 . Therefore, it is of the utmost importance that the sidewalls of the trench not be coated so that gold portions  108   a  and  108   b  are not connected to each other. Any gold connecting the two portions, even fine metal filaments, would result in an improper lift-off and defective circuit formation.  
           [0009]    Thus, a source  120  of metal to be deposited must achieve a trajectory as close to 90 degrees with respect to the substrate surface as possible in order not to coat the sidewalls of trench  110 . This is referred to as orthogonal deposition and the optimal resultant coating is referred to as a “lift-off” coating or as zero step coverage. A commonly used method of physical vapor deposition in lift-off processes is electron beam evaporation. In practical applications where multiple wafers must be precisely coated by a single source, this requires complex machinery with specific setups for specific power levels and materials.  
           [0010]    One prior setup is illustrated in FIG. 3. Source  120  is located at the center of a sphere  130  having a radius R. The sphere is shown to illustrate that the lift-off dome has a dome spherical radius R such that all points on lift-off dome  124  are equidistant from source  120 . At the top portion of the sphere is lift-off dome  124  that has multiple holes for holding wafers or other substrates to be coated. Lift-off dome  124  rotates around the source center line  132 . One wafer  122  is illustrated on lift-off dome  124 . Although all points on the lift-off dome are equidistant from the source, the source to substrate distance is not constant because each wafer is not arced, but flat. However, the difference is substantially negligible for the purposes of this application, and thus the source to substrate distance can be said to equal the dome spherical radius R.  
           [0011]    In order to coat the wafers on lift-off dome  124 , for example wafer  122 , source  120  is heated by an electron beam (not shown) and the coating material is evaporated in a straight line towards wafers held within openings of lift-off dome  124 . The vapor is not uniform and the distribution of the vapor varies with the power supplied to the electron beam, and also with the material to be evaporated. The vapor vector field is often described as a vapor cloud. If the lift-off dome and the wafers on the dome were stationary, the variations in the cloud would result in a very unevenly distributed coating upon the surfaces of each wafer. Rotation of the dome about source center line  132  substantially reduces the unevenness by averaging the variation in a circular path around the center line  132 . However, the magnitude of the vapor vector field is much greater at the center line and tapers off as the distance on the dome from the center line increases as can be seen in FIG. 4. The thickness of the coating deposited is directly proportional to the magnitude of the vector field, and the thickness is also proportional to the distance on the dome from the center line or axis of rotation. FIG. 4 is a graph of the thickness distribution over different distances r from the centerline  132 . The thickness of the coating deposited can be mathematically modeled according to the following relation:  
           T   r        α            cos   N          (   θ   )         R   2         ,                         
 
           [0012]    where T r  is the thickness at point r, R is the dome spherical radius, N is a characteristic number for each power and material being vaporized, and θ is the angle from source centerline  132  to point r.  
           [0013]    In order to reduce the thickness of the coating deposited nearest to the source center line  132 , a stationary uniformity mask  126  is fixed between the source and the dome and has a width that tapers off with increasing radius. Thus, as lift off dome  124  rotates uniformity mask  126  blocks a larger portion of the vapor near the centerline (as θ approaches zero) than far from the centerline (as θ approaches ninety degrees). Because the vapor vector field varies for each different coating material and power level of the electron beam, a unique uniformity mask must be custom tailored not only for each coating material but for each given power level for each material. Changing the uniformity masks requires stopping the coating process and thus results in downtime of the coater. In order to deposit the multiple different metallic layers needed to make the circuit traces upon a GaAs wafer, at least 3 different uniformity masks would be needed to deposit the gold layer  108 , adhesion layer  104  and diffusion barrier  108  seen in FIGS. 1 and 2. Furthermore, much of the evaporated metal is wasted because some is collected on the uniformity mask rather than on the wafer.  
         SUMMARY OF THE INVENTION  
         [0014]    Thus, there is a need for an electron beam coater capable of the orthogonal deposition necessary for lift-off applications that does not require a uniformity mask to evenly deposit a number of different coatings on multiple wafers simultaneously. Additionally, one that is less sensitive to process variations such as evaporant material, power level, beam position, and the like.  
           [0015]    The planetary lift-off deposition system and method of the present invention deposits a uniform “lift-off” coating on a large amount of wafers in a short period of time. In comparison with prior vapor deposition devices and methods, the system and method of the present invention utilize a greater percentage of the vaporized material, do not require that any components be changed when vaporizing different materials, and reliably and consistently deposits a more uniform and precise coating.  
           [0016]    In the preferred embodiment, the vapor deposition device utilizes dome shaped wafer holders positioned such that each point of a first face of the dome is equidistant from a source material to be evaporated. A rotating structure holding the dome shaped wafer holders rotates about a central axis passing through the source. The dome shaped wafer holders rotate about axes that passes through the center of the dome and the source thus eliminating the need for the uniformity mask. Although in the preferred embodiment illustrated, a dome shaped wafer holder is described, in other embodiments a support structure can locate the wafers such that the center of each wafer is equidistant from the source without the dome shaped carrier. 
       
    
    
     BRIEF DESCRIPTION OF THE FIGURES.  
       [0017]    [0017]FIG. 1 is a cross section of a prior art circuit trace on a GaAs substrate.  
         [0018]    [0018]FIG. 2 is a cross section of a prior art lift-off deposition circuit trace.  
         [0019]    [0019]FIG. 3 is an illustration of a prior art lift-off deposition system.  
         [0020]    [0020]FIG. 4 is a graph of coating thickness as a function of horizontal distance from the source centerline.  
         [0021]    [0021]FIG. 5 is a top view of planetary lift-off system  200 .  
         [0022]    [0022]FIG. 6 is a cross section of planetary lift-off system  200 .  
         [0023]    [0023]FIG. 7 is an enlarged section of planetary lift-off system  200 .  
         [0024]    [0024]FIG. 8 is a graph of coating distribution achieved with planetary lift-off system  200  in comparison to a coating made with a prior art system.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0025]    The planetary lift-off deposition system and method of the present invention deposits a uniform “lift-off” coating on a large amount of wafers in a short period of time. In comparison with prior vapor deposition devices and methods, the system and method of the present invention utilizes a greater percentage of the vaporized material, does not require any uniformity mask, does not require that any components be changed when vaporizing different materials, and reliably and consistently deposits a more uniform and precise coating. Finally, it is less sensitive to process variations than prior designs.  
         [0026]    FIGS.  5 - 7  illustrate a planetary lift-off deposition system  200 . FIG. 5 is a top view, FIG. 6 is a cross section, and FIG. 7 is an enlarged cross section of planetary lift-off deposition system (“PLDS”)  200 . The invention will now be described with reference to the figures.  
         [0027]    Electron beam vapor deposition generally occurs in a vacuum and thus, as seen in FIGS. 5 and 6, lift-off domes  212  are within a sealed vacuum chamber  240 . PLDS  200  ranges in size and has a radius R anywhere from about 17.5 to 54 inches or more. The number and configuration of the domes  212  also varies depending on the application and wafer size, and can be anywhere from one to seven. Preferably three to six lift-off domes are arranged around centerline axis  220  positioned at the centerline of source  222 . In the preferred embodiment used to illustrate the invention, five domes  212 , each carrying six six-inch wafers, rotate about centerline axis  220  so that 30 wafers can be coated simultaneously. Each lift-off dome also rotates about dome axes  230 —each dome has its own axis  230   a, b, c, d, e  respectively. If smaller wafers are to be coated, the number that can be simultaneously coated increases, and vice versa. The lift-off dome is the preferred way of holding the wafers in position, but it is only one way. Other ways are within the scope of this invention. For example, a support frame or structure with one or more arms may hold the wafers in similar positions without having a dome shape. Many different configurations of structures can be constructed by one skilled in the art to individually position, interconnect, orient, and rotate the wafers according to the invention.  
         [0028]    Two wafers  214  and  216  are shown within one of the lift-off domes  212 . PLDS  200  can be thought of as a planetary system wherein each lift-off dome  212  is like a planet rotating about its own axis  230  as well as rotating about a centerline axis  220  (the sun). For clarity, the support frame  210  that locates the lift-off domes  212  in the proper position and controls the rotation of the domes about both centerline axis  220  and dome axes  230  is not shown in FIGS. 5 and 6 but can be seen in FIG. 7. One of the lift-off domes  212  seen in FIG. 5 is shown within chamber  240  in FIG. 6.  
         [0029]    Lift-off domes  212  are constant radius domes such that any point on the surface of the domes is equidistant from source  222 . Lift-off domes  212  are part of a sphere  204 , at the center of which is located source  222 . A portion of sphere  204 , can be seen in FIGS. 6 and 7. Sphere  204  is theoretical and is only shown to make it clear that the lift-off domes have a constant radius R, the radius of sphere  204 , and that source  222  is approximately equidistant from all points on the surface of domes  212  facing source  222 . The source  222  contains a material to be evaporated by an electron beam. Material evaporated from source  222  will, generally speaking, travel outward from the source in a straight line along a radius R towards the domes  212 , and thus the material will coat the domes  212  orthogonally, i.e. the trajectory of the material is normal to the surface of the domes  212  because the source  222  is located at the center of the sphere  204  of which domes  212  are a part. The source contains multiple pockets. Each pocket can hold a different material to be evaporated, and in order to evaporate and deposit multiple coatings, each pocket is rotated into the proper position to be evaporated by the electron beam. For more information please refer to U.S. Pat. No. 6,342,103 to Ramsay, entitled “Multiple Pocket Electron Beam Source” which is hereby incorporated by this reference in its entirety.  
         [0030]    Referring to FIG. 7, one of the lift-off domes  212  is shown. For simplicity and clarity of illustration, only one lift-off dome  212  is shown, although lift-off deposition system  200  incorporates multiple domes  212 , each of which holds multiple wafers. Lift-off domes  212  rotate about dome axes  230  that are aligned with source  222 . Dome axes  230  are radii of the theoretical sphere  204  of which dome  212  is a part, and θ, the angle from axis  220  to axes  230  is equal for all axes  230   a - e  (or however many dome axes there may be).  
         [0031]    Domes  212  rotate about dome axes  230 . Space frame  210  positions lift-off domes  210  along sphere  204 . Only a portion of space frame  210  can be seen in the cross section of FIG. 7. Space frame  210  rotates about axis  220  and also provides for the rotation of lift-off domes  212  about axes  230 . Space frame  210  can be made of many materials well known to those skilled in the art, but is preferably made of a material such as stainless steel that is corrosion resistant and will give off a minimum amount of outgassing that may contaminate the vapor deposited coating. Any number of mechanisms including motors, gears, shafts, pulleys and other well known drive mechanisms can be used to provide for the rotation about the multiple axis. One such mechanism uses a stainless steel spring used as a pulley between spindle  206  and pulley  208 . Alternatively, individual motors, flexible shafts, or a system of interconnected gears and motors can provide the planetary rotation. Each one or two rows of the wafers within domes  212  are equidistant from the axis of rotation  230 . At the axes of rotation  230 , domes  212  are normal to the axes  230 . Wafers  214  and  216  are shown in this cross section of deposition system  200 . As the distance along any of the wafers, for instance  214  or  216  increases from the centerline, a negligible deviation from orthogonal deposition occurs because the wafers are not arced, but flat. However, this deviation is quite minimal and does not significantly affect the lift-off properties of the coating, when R is chosen correctly for the wafer diameter of interest. For more information please refer to a paper hereby incorporated by reference entitled, “Improved Evaporation Deposition for Lift-Off Patterning,” by R. J. Hill, Society of Vacuum Coaters, 32 nd  Annual Technical Conference Proceedings p.278, 1989.  
         [0032]    As discussed in the background with regard to FIG. 4, the thickness of the coating is greatest just above the source and diminishes as the distance r from axis of rotation  220  (the centerline) increases. The diminution of the coating with increasing distance r is averaged out by the rotation of the wafer holders about axes  230 . Because the wafers rotate about the axes  230 , a given point on a wafer is exposed to a higher deposition rate resulting in a thick coating when nearest to centerline axis  220  (source center line) and to an increasingly thinner coating as the distance from centerline axis  220  (source center line) increases. In one planetary revolution or cycle any point on any wafer is thus coated with the same thickness as any other point on any of the wafers, and therefore each of the wafers are evenly coated. This evenly deposited coating is achieved without the use of a uniformity mask that is typical in prior art lift-off deposition systems.  
         [0033]    In prior deposition systems, the coating was evenly deposited by blocking the thicker portion of the coating with the uniformity mask. As can be seen in FIG. 4 the thickness utilized near the edge of the curve can be as little as 20% as that deposited near the center, but is typically 60-90%. A minimal thickness point on the curve governed the final thickness to be deposited upon the wafer, as the thicker portion of the curve was selectively blocked from arriving at the wafer surface by the uniformity mask. Thus, a large portion of the material to be deposited was wasted because it ended up coating the mask rather than the wafers. This was thus an inefficient and costly system. With the present invention, a much higher percentage of the material that is evaporated is actually deposited on the wafer, because no material must be blocked in order to achieve a uniform coating.  
         [0034]    Typically, gold is used to form the circuit traces in GaAs applications as discussed earlier. What follows is an example to illustrate the amount of money saved during a year of usage of the planetary lift-off deposition system (“PLDS”) of the present invention vs. a prior existing design (“Prior Art”) incorporating a uniformity mask.  
                                                   TABLE 1                           Amount of Gold Consumed to Deposit 7000 Å Coating.                Characteristics for 1 run   PRIOR ART   PLDS                            Coating thickness sought, Å   7000   7000           Minimum thickness, Å   7000   7000           Maximum thickness, Å   10387   7383           Average thickness, Å   9095   7251           Coating uniformity, %   18.62   2.64           Total evaporated, grams   36.21   32.63                      
 
         [0035]    As can be seen in Table 1, the PLDS exemplifying the present invention uses about 3.58 less grams per run and is approximately 9.9% more efficient than the prior art systems. Assuming a typical shift is 1880 hours per year (47 weeks @ 40 hrs per week) and there are three shifts per week, a deposition system (PLDS or the prior art system) can be used 5640 hours per year. In each hour there are two runs. Factoring in a downtime of 10% for each machine, and a yield of 98%, the present invention saves roughly 35,500 grams of gold per year, as can be seen in Table 2. Assuming the cost of gold is $280 per Troy ounce, the PLDS system will save an operator over $320,000 per year.  
                                                           TABLE 2                           Cost of Operation per Year.                    Grams evaporated   Grams per               Runs   per run to   year (10%   Cost per year           per   7000 Å   downtime and   ($280 per           year   coating   98% yield)   Troy Oz.)                        PLDS   11,280   32.63   324,634   $2,922,750       PRIOR ART   11,280   36.21   360,252   $3,243,426                  
 
         [0036]    [0036]FIG. 8 illustrates the uniformity of the coating deposited using PLDS vs. that deposited with a prior art system. Note that in depositing a 9000 Å coating with the PLDS, the coating deposited is within about 1.2% of the target across the surface of the wafer whereas the coating deposited with the prior art is thinner at the edge of the wafer by about 6.8%. Therefore the present invention is not only more efficient but also deposits a more uniform and precise coating than prior art systems.  
         [0037]    While particular embodiments of the present invention and their advantages have been shown and described, it should be understood that various changes, substitutions, and alterations can be made therein without departing from the spirit and scope of the invention as defined by the appended claims. For example, any device or method of positioning and rotating the domes in the proper position is within the scope of the invention as defined by the appended claims.