Abstract:
A valve for an effusion cell is disclosed. The valve includes a valve seat having an opening in communication with an interior chamber of a heating crucible. A disk-shaped valve member is axially movable with respect to the valve seat in order to increase or decrease the effective size of the valve seat opening. An outlet nozzle may be provided downstream from the valve seat. The valve provides mechanical adjustment of the deposition rate of the evaporated material replacing thermal changes as the means to control the deposition rate or flux from the effusion cell. The nozzle, valve disk and valve seat may be made of machined graphite coated with a carbon-containing material such as pyrolytic graphite, silicon carbide, or the like. The effusion cell may be used for molecular beam epitaxial growth of various materials incorporating mechanical control of the flux without negatively impacting the state of the art qualities of the deposited films with respect to deposition uniformity, surface morphology and background doping.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims the benefit of U.S. Provisional Patent Application Ser. No. 60/655,124 filed Feb. 22, 2005, which is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The invention relates to vacuum evaporation, and more particularly relates to valves used in effusion-type evaporative cells.  
       BACKGROUND INFORMATION  
       [0003]     Vacuum deposition techniques are used to deposited different types of materials onto substrates to form thin films. Vapor sources that modulate the flux with an integral metal needle valve or a movable metal shutter are known in the art of molecular beam epitaxy (MBE). The majority of solid or molten source materials and the resulting vapors are reactive with metals at elevated temperatures. Thus, the current MBE sources with metal valves or metal shutters may only be used with a few relatively non-reactive source materials. If there is a reaction between the source material and the containing vessel or valve components the thin films become contaminated.  
         [0004]     Valved sources using pyrolytic boron nitride (PBN) have been developed to be used with some of the more corrosive materials. There are limitations on the geometry that is attainable with PBN because of the restrictive fabrication process that is required to produce the components. The use of graphite and silicon nitride as a material for the various components that make up a valved cell including the crucible and valve allow for much greater flexibility in the geometry, function, performance and cost of these cells.  
         [0005]     U.S. Pat. No. 6,162,300, which is incorporated herein by reference, discloses an effusion cell having a ceramic needle valve mechanism.  
       SUMMARY OF THE INVENTION  
       [0006]     The present invention is a thermal evaporation source that includes a ceramic crucible and valve assembly. The reservoir and valve assembly are heated radiatively using resistively heated filaments. The heat shielding metal foils and supporting ceramic pieces are configured to provide for separate thermal zones. The temperature of each thermal zone is typically monitored using a thermocouple or pyrometer that is specific to each thermal zone. A temperature controlling device may be used to both monitor the temperature of the zone and adjust the power supplied to the filament in order to apply any desired thermal gradients to the invention. After the requisite thermal profile is applied to the invention, the flow of vapors evaporating from the source may be modulated by further adjusting the position of a ceramic needle that seats in the ceramic plug at the top of the source. The position of the valve is typically controlled precisely using a micrometer of other micro-positioning device such as a stepper motor or servo motor and position encoder.  
         [0007]     An aspect of the present invention is to provide an effusion cell valve comprising: a crucible having an interior chamber and an open end; a valve seat adjacent the open end of the crucible comprising an aperture in flow communication with the open end of the crucible, and a seating surface surrounding the aperture; and a valve disk comprising a seating surface, wherein the valve disk is axially movable along a length of the effusion cell from a closed position in which the seating surfaces of the valve seat and valve disk contact each other, to an open position in which the seating surfaces of the valve seat and valve disk are disengaged.  
         [0008]     Another aspect of the present invention is to provide an effusion cell valve comprising: a crucible having an interior chamber and an open end; a valve seat adjacent the open end of the crucible comprising a closeable aperture; and an outlet nozzle downstream from the valve seat aperture.  
         [0009]     A further aspect of the present invention is to provide an effusion cell valve comprising: a crucible having an interior chamber, an open end, and an inner diameter C D ; a valve seat adjacent the open end of the crucible comprising a seating surface and an aperture having a diameter A D  in flow communication with the open end of the crucible; and a valve disk having an inlet diameter D ID  and comprising a seating surface, wherein the valve disk is axially movable along a length of the effusion cell from a closed position in which the seating surfaces of the valve seat and valve disk contact each other, to an open position in which the seating surfaces of the valve seat and valve disk are disengaged, and wherein the ratio of A D :C D  and/or the ratio of D ID :C D  is greater than 0.1:1.  
         [0010]     Another aspect of the present invention is to provide an effusion cell valve comprising: a crucible having an interior chamber and an open end; a valve seat adjacent the open end of the crucible comprising an aperture in flow communication with the open end of the crucible; a valve member movable with respect to the valve seat to open and close the aperture; and a outlet nozzle downstream from the valve seat aperture, wherein at least one of the valve seat, valve member and outlet nozzle comprise machined graphite coated with a carbon-containing material.  
         [0011]     These and other aspects of the present invention will be more apparent from the following description. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1  is an isometric view of a portion of an effusion cell including a disk valve in accordance with an embodiment of the present invention.  
         [0013]      FIG. 2  is an end view of the effusion cell of  FIG. 1 .  
         [0014]      FIG. 3  is a longitudinal sectional view of the effusion cell taken through section A-A of  FIG. 2 , with the addition of a cylindrical enclosure and a crucible, showing the disk valve in an open position.  
         [0015]      FIG. 4  is a longitudinal sectional view of the effusion cell taken through section A-A of  FIG. 2 , with the addition of a cylindrical enclosure and a crucible, showing the disk valve in the closed position.  
         [0016]      FIG. 5  is a longitudinal sectional view of an effusion cell valve including a valve seat, valve disk, and outlet nozzle in accordance with an embodiment of the present invention.  
         [0017]      FIG. 6  is a longitudinal sectional view of an effusion cell valve including a valve seat, valve disk, and outlet nozzle in accordance with another embodiment of the present invention.  
         [0018]      FIG. 7  is a longitudinal sectional view of an effusion cell valve including a valve seat, valve disk, and outlet nozzle in accordance with a further embodiment of the present invention.  
         [0019]      FIG. 8  is a graph of beam equivalent pressure (BEP) vs. time for two test runs, demonstrating repeatability of valve position for an effusion cell in accordance with an embodiment of the present invention.  
         [0020]      FIG. 9  is a graph of BEP vs. time for two test runs, demonstrating repeatability of valve position for an effusion cell in accordance with an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0021]      FIGS. 1-4  illustrate an effusion cell  10  in accordance with an embodiment of the present invention. In  FIGS. 1 and 2 , the effusion cell  10  is shown without its outer enclosure, while in  FIGS. 3 and 4  the cylindrical enclosure  11  is shown. The enclosure may be made of any suitable material such as tantalum, e.g., inner and outer tantalum sleeves separated by an insulating layer of knurled tantalum foil. The effusion cell  10  includes a crucible  12 , shown in  FIGS. 3 and 4 , made of a material such as graphite, silicon carbide or the like. The crucible  12  has a crucible chamber  14  with an open end  16 . A valve seat  18  having an opening or aperture  19  is mounted at the open end  16  of the crucible  12  by means such as a press fitting, a threaded connection or fasteners. The crucible  12  has an inner diameter C D , while the valve seat aperture  19  has a diameter A D . In one embodiment, the ratio of A D :C D  may be relatively large, e.g., greater than 0.2:1, typically greater than 0.5:1. However, a smaller A D :C D  ratio may be used for some applications, e.g., as low as 0.1:1 or 0.01:1.  
         [0022]     An outlet nozzle  20  is provided adjacent to the open end  16  of the crucible  12 . In the embodiment shown in the figures, the valve seat  18  and the outlet nozzle  20  are provided as an integral unit. However, the valve seat  18  and outlet nozzle  20  may alternatively be provided as separate pieces. The outlet nozzle  20  includes an inlet end  22  adjacent to the crucible  12 , and an outlet end  24  at the distal end of the effusion cell  10  downstream from the inlet end  22 .  
         [0023]     A valve disk  30  is mounted in the nozzle  20  adjacent the inlet end  22 , and is axially movable with respect to the valve seat  18 . The valve disk  30  includes radially extended ears  32  and  33 . Actuator rods  34  and  35  running along the outside of the crucible  12  are connected to the valve disk ears  32  and  33 , respectively. The actuator rods  34  and  35  are attached to the ears  32  and  33  by fasteners  36  and  37  such as nuts threaded on to the ends of the actuator rods  34  and  35 . A spring-loaded coupling  38  is connected between the actuator rods  34  and  35  and a valve controller (not shown) which controls that axial movement of the actuator rods  34  and  35  in order to selectively open and close the valve disk  30  against the valve seat  18 . The spring-loaded coupling  38  protects the valve seat  18  and valve disk  30  from being damaged when they contact each other. An example of a valve controller that may be used with the present effusion cell valve is disclosed in U.S. Pat. No. 6,162,300. The valve controller allows the valve position to be precisely controlled from outside the vacuum.  
         [0024]     The valve seat  18 , outlet nozzle  20  and valve disk  30  may be made of any suitable materials such as machined graphite coated with a carbon-containing coating, for example, a pyrolytic graphite coating, silicon carbide coating, or the like. Other suitable materials for the valve seat  18 , outlet nozzle  20  and valve disk  30  include ceramics such as pyrolytic boron nitride, silicon carbide and/or metal oxides such as alumina, magnesia, etc.  
         [0025]     The valve disk  30  may have a cylindrical shape, or may have a tapered or conical shape, for example, as shown in  FIGS. 3-5 . The tapered valve disk  30  shown in  FIGS. 3-5  has an inlet diameter D ID , and an outlet diameter D OD . The disk  30  has a thickness D T . The ratio of D ID  or D OD  to D T  may be relatively large, e.g., greater than 2:1, typically greater than 3:1 or 5:1. In one embodiment, the ratio is greater than 10:1. In the embodiment shown in  FIGS. 3-5 , the disk  30  is tapered inwardly toward the outlet end  24  of the nozzle  20  at an angle of about 10°. However, any other suitable taper angle may be used, e.g., from +45° to −45°.  
         [0026]     The inlet end  22  of the nozzle  20  has an inlet diameter N ID  which corresponds to the inside diameter of the nozzle at the inlet end  22 . The outlet end  24  of the nozzle  20  has an outlet diameter N OD  representing the inside diameter at the outlet end  24 . The ratio of N OD :N ID  is typically less than 3:1, e.g., from about 1:1 to 2.5:1. In one embodiment, the ratio of N OD :N ID  is from about 1.4:1 to about 2:1. As shown in  FIG. 5 , the nozzle  20  has an outward taper angle N A , e.g., from about 1 to about 45 degrees, for example, from about 5 to about 20 degrees.  
         [0027]     The nozzle  20  has a length N L  measured from the inlet end  22  to the outlet end  24  of the nozzle  20 . The ratio of N L :N OD  is typically greater than 0.2:1, e.g., from about 0.5:1 to about 10:1. For example, the ratio of N L :N OD  may be from about 0.6:1 to about 3:1, or from about 0.7:1 to about 2:1.  
         [0028]     The ratio of the valve disk inlet diameter D ID  to the nozzle inlet diameter N ID  (D ID :N ID ) is typically greater than 0.3:1, e.g., greater than 0.5:1. The ratio of the valve disk inlet diameter D ID  to the crucible inner diameter C D (D ID :C D ) is typically greater than 0.1:1, e.g., greater than 0.2:1 or 0.5:1.  
         [0029]      FIG. 6  illustrates an effusion cell valve similar to that shown in  FIG. 5 , with a modified valve seat and valve disk. In the embodiment shown in  FIG. 6 , the outlet nozzle  120  has an inlet end  122  and an outlet end  124 . The valve seat  118  has a cylindrical aperture  119  and a tapered conical seating surface. The valve disk  130  has a flat, generally cylindrical shape with a tapered portion forming a conical seating surface which contacts the conical seating surface of the valve seat  118  when the valve is closed. The valve disk  130  has an inlet diameter D ID  measured at the edge between the bottom surface and the conical seating surface of the disk. The valve disk  130  has an outlet diameter D OD  and a thickness D T .  
         [0030]      FIG. 7  illustrates an effusion cell valve similar to that shown in  FIG. 5 , with a modified valve seat and valve disk. In the embodiment shown in  FIG. 7 , the outlet nozzle  220  has an inlet end  222  and an outlet end  224 . The valve seat  218  has a cylindrical aperture  219 . The valve disk  230  has a flat, generally cylindrical shape with a raised ring extending from its bottom surface that is received in an annular recess in the valve seat  218  when the valve is closed. The valve disk  230  has an inlet diameter D ID  which is also equal to its outlet diameter. The valve disk  230  has a thickness D T .  
         [0031]     As shown most clearly in  FIGS. 1-4 , crucible heating element supports  42   a - d  surround the crucible  12  and are used to support conventional heating wires (not shown) which are threaded through holes in the supports  42   a - d  and run axially along the length of the cell. Nozzle heating element supports  44   a - c  surround the nozzle  20  and are used to support conventional heating wires (not shown) which are threaded through holes in the supports  44   a - c  and run axially along the length of the cell. The heating element supports  42   a - d  and  44   a - c  and their respective heating wires provide for independently heated thermal zones. Standard thermocouples (not shown) may be used for control of the temperature of the two independently heated thermal zones.  
         [0032]     The two thermal zones include a lower zone surrounded by the heating element supports  42   a - d  that contains the reservoir portion of the crucible, and an upper zone surrounded by the heating element supports  44   a - c  that contains the valve mechanism. The upper thermal zone may be maintained at a higher temperature preventing condensation of evaporant on the valve mechanism. The lower thermal zone may be maintained at a temperature which provides the desired range of flux exiting the effusion cell.  
         [0033]     In accordance with the present invention, the flux of the vapor material exiting the nozzle  20  of the effusion cell  10  can be controlled, and the flux may be quickly adjusted in comparison with conventional MBE techniques. By selectively controlling the position of the valve disk  30 , the flux is continuously variable to desired levels. This controllable flux may be accomplished in many cases without the necessity of changing the temperature of the crucible and/or nozzle. For example, the flux may be variable over two orders of magnitude, e.g., from 10 −9  to 10 −7  Torr. The ability to maintain a substantially constant temperature while adjusting the material deposition flux represents an improvement over conventional MBE techniques in which temperature adjustments were necessary, e.g., when switching to a different deposition rate.  
         [0034]     Tests were conducted using an effusion cell valve similar to that shown in  FIGS. 1-4  installed on a commercially available MBE system consisting of twelve effusion cell ports, a substrate manipulator capable of rotating and heating a substrate and a beam flux monitor to measure the rate of evaporation from the effusion cells.  
         [0035]     To characterize the cell, the beam flux monitor was used to measure the evaporation rate of material in the cell. In this case, the beam flux monitor used was an ion gauge which measures the total pressure of the volume within the gauge itself. This method of measuring beam flux is generally accepted and the value measured is referred to as the beam equivalent pressure, or BEP, and is commonly given in units of Torr.  
         [0036]     The tests using the beam flux monitor demonstrated the range of the valve, the reproductibility of the BEP vs. valve position, and the stability of BEP for a particular valve position over time. It was found that the BEP of the valve in the off position was nearly two orders of magnitude less than the BEP of the valve in the full on position. The BEP was shown to be reproducible within 1% of the value at each position. Stability also was within 0.5% when the valve was held in one position for a 60 minute period.  
         [0037]      FIG. 8  shows the results when the valve is stepped from 0 to 200 over 40 position increments. Each position equates to 0.001 inch travel. The reproducibility of each position was within 1% and near the noise level of the gauge. This matches the performance of a non-valved effusion cell using temperature changes to change the value of the flux.  
         [0038]      FIG. 9  shows the stability of one valve position over a 60 minute period of time. In this case, once the initial change of opening the valve stabilized, no appreciable change occurred over the duration of the test. This was better than the results seen with a non-valved effusion cell which would see a slight drop in deposition rate due to effects of material depletion.  
         [0039]     Whereas particular embodiments of this invention have been described above for purposes of illustration, it will be evident to those skilled in the art that numerous variations of the details of the present invention may be made without departing from the invention as defined in the appended claims.