Abstract:
A valve system having high maximum gas flow rate and fine control of gas flow rate, includes a valve housing for blocking gas flow through a gas flow path, a large area opening through said housing having a first arcuate side wall and a small area opening through said housing having a second arcuate side wall, and respective large area and small area rotatable valve flaps in said large area and small area openings, respectively, and having arcuate edges congruent with said first and second arcuate side walls, respectively and defining therebetween respective first and second valve gaps. The first and second valve gaps are sufficiently small to block flow of a gas on one side of said valve housing up to a predetermined pressure limit, thereby obviating any need for O-rings.

Description:
BACKGROUND OF THE INVENTION 
   Most plasma processes for semiconductor circuit fabrication require the plasma reactor chamber to be maintained at a sub-atmospheric pressure using a vacuum pump coupled to the chamber. Typically, the vacuum pump is operated at a nominal constant rate, while the chamber pressure is adjusted by a butterfly valve coupled between the chamber and the vacuum pump. The butterfly valve has a rotatable disk-shaped flap whose rotational position determines the flow rate to the vacuum pump and therefore controls the chamber pressure. The valve flap typically has an O-ring around its perimeter that seats on the edge of the valve housing whenever the valve is in the closed position. The O-ring is necessary in order to ensure a seal when the valve flap is in the closed position. The O-ring suffers wear when it is in a slightly opened position at which the desired chamber pressure is achieved. Plasma and gases flowing past the O-ring react with the O-ring material and degrade it or remove it. As a result, the valve must be serviced periodically to replace the O-ring, which entails significant maintenance costs and down-time of the reactor. 
   Another problem is that there is a trade-off between the maximum flow capacity of the valve and its ability to regulate chamber pressure accurately. The resolution with which pressure can be controlled is roughly inversely proportional to the valve diameter. This is because control of the rotational angle of the valve flap is limited to a minimum angular excursion, depending upon the motor or servo employed to rotate the flap. The minimum angular excursion or resolution may be less than 1 degree. For a very small diameter valve flap and opening, this resolution can afford highly accurate or fine control of the chamber pressure. However, for a larger diameter valve flap or opening, movement of the flap through the minimum angular excursion causes a relatively large change in chamber pressure, so that fine control of chamber pressure is not possible. This problem can be overcome by employing a smaller diameter valve flap and opening. However, such an approach limits the rate at which the chamber can be evacuated or cleaned. For example, cleaning the chamber with NF3 gases with a fast “dump” of the cleaning gases and by-products is not possible with a small diameter valve. 
   What is desired is a pressure-control valve that has a very high maximum flow rate (maximum opening size) but which, despite the large maximum opening size, can control chamber pressure as accurately as a very small valve, and requires no periodic replacement of an O-ring. 
   SUMMARY OF THE INVENTION 
   A valve system having high maximum gas flow rate and fine control of gas flow rate, includes a valve housing for blocking gas flow through a gas flow path, a large area opening through said housing having a first arcuate side wall and a small area opening through said housing having a second arcuate side wall, and respective large area and small area rotatable valve flaps in said large area and small area openings, respectively, and having arcuate edges congruent with said first and second arcuate side walls, respectively and defining therebetween respective first and second valve gaps. The first and second valve gaps are sufficiently small to limit conductance of a gas through said valve housing up to a predetermined pressure limit for a predetermined minimum gas flow limit, thereby obviating any need for O-rings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1A  illustrates a plasma reactor including a valve assembly of the invention. 
       FIG. 1B  is an enlarged cross-sectional side view of a valve in the valve assembly of  FIG. 1A . 
       FIG. 2  is a perspective view of the top side of a valve flap in the valve of  FIG. 1B . 
       FIG. 3  is a perspective view of the bottom side of the valve flap of  FIG. 2 . 
       FIG. 4  is a cross-sectional side view of the valve assembly. 
       FIG. 5  is a perspective view of a valve flap and housing in the valve assembly of  FIG. 4 . 
       FIG. 6  is a cross-sectional end view of the valve assembly. 
       FIG. 7  is a perspective view of the valve system in the reactor of  FIG. 1  including the drive motors and feedback control system. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to  FIGS. 1A and 1B , a plasma reactor  10  has a ceiling  12  and sidewall  14  enclosing a vacuum chamber  16  with a wafer support pedestal  18  inside the chamber  16  for holding a silicon wafer to be processed. A process gas supply  11  furnishes process gas or a process gas mixture into the chamber  16  through gas injection apparatus  13 . Plasma bias power is applied from a RF bias power generator  15  through an impedance match circuit  17  to the wafer pedestal  18 . Plasma source power may be applied from an RF source power generator  19  and through an impedance match circuit  21  to a source power applicator  23  (which may be an antenna or an electrode, for example, and may be located at the ceiling  12  or at the pedestal  18 ). A pumping annulus  20  is defined between the sidewall  14  and pedestal  18 . A pumping conduit  22  is coupled between the pumping annulus  20  and an external vacuum pump  24 . 
   The internal surface of the pumping conduit  22  has a shoulder  26  supporting a valve housing  28  extending across the diameter of the pumping conduit  22 . The valve housing  28  supports a pair of tandem butterfly valves, namely a large high volume valve  30  and a small fine control valve  32 . The high volume valve  30  consists of a circular opening  34  through the valve housing  28  and a rotatable valve flap  36 . The valve flap  36  is a flat section of a sphere, and therefore has an arcuate edge  38 . The edge  40  of the opening  34  through the valve housing  28  has a similar arcuate shape matching that of the valve flap arcuate edge  38 . When the flap  36  is in the closed position (i.e., parallel with the plane of the valve housing  28 ), the arcuate edges  38 ,  40  define an arcuate gap having a thickness T. The radius of the arcuate gap is sufficiently small so that there is no straight-line path through the arcuate gap from the internal side of the valve to the external side (i.e., from the chamber  16  to the pump  24 ). This helps limit the rate of gas escaping through the arcuate gap. Preferably, the arcuate gap thickness T is less than the mean free collision path of the gases or plasma in the chamber  16  within a given pressure range. This pressure range may be 2 mT to 200 mT, in one embodiment. In other words, the gap thickness in that embodiment is sufficiently small to be less than the mean free collision path for chamber pressures up to 200 mT. The gap thickness T may be between about 0.010 and 0.030 inch, or lower, for example, depending upon the intended chamber pressure operating range. Coarse control over chamber pressure is obtained by controlling the rotational position of the high volume valve flap  36 , which determines the valve opening size d ( FIG. 1B ). With such a small gap and a circular path through the gap, the rate at which process gases or plasma products escape through the gap is low. This feature obviates the need for any O-ring to seal the gap, a significant advantage. 
   Conductance of the high volume valve  30  is determined by the valve opening size d, which is monotonically related to the angular or rotational position of the valve flap  36  (for angular position range of 0 degree (closed) up to 90 degree (wide open)). The conductance and gas flow rate determines the chamber pressure, and therefore chamber is regulated by controlling the valve opening size d through rotation of the valve flap  36 . 
   The fine control valve  32  consists of a circular opening  44  through the valve housing  28  and a rotatable valve flap  46 . The valve flap  46  is a flat section of a sphere, and therefore has an arcuate edge  48 . The edge  50  of the opening  44  through the valve housing  28  has a similar arcuate shape matching that of the valve flap arcuate edge  48 . When the flap  46  is in the closed position (i.e., parallel with the plane of the valve housing  28 , the arcuate edges  48 ,  50  define an arcuate gap  51  having a thickness t. The radius of the arcuate gap  51  is sufficiently small so that there is no straight-line path through the arcuate gap  51  from the internal side of the valve to the external side (i.e., from the chamber  16  to the pump  24 ). This helps limit the rate of gas escaping through the arcuate gap  51 . 
   In accordance with another feature that helps prevent gas escaping through the gap  51 , the gap thickness t is less than the mean free collision path of the gases or plasma in the chamber  16  within a given pressure range. This pressure range may be 2 mT to 200 mT, in one embodiment. In other words, the gap thickness t in that embodiment is sufficiently small to be less than the mean free collision path for chamber pressures up to 200 mT. The gap thickness may be about 0.010 to 0.030 inch, for example. Fine control over chamber pressure is obtained by controlling the rotational position of the flap  46 , which determines the valve opening size. With such a small gap and a circular path through the gap, the rate at which process gases or plasma products escape through the gap is low. This feature obviates the need for any O-ring to seal the gap, a significant advantage. 
   Conductance through the valve  32  is determined by the valve opening size, which is monotonically related to the angular or rotational position of the valve flap  46  (for angular position range of 0 degree (closed) up to 90 degree (wide open). The parallel sum of the individual conductances of the respective tandem valves  30 ,  32  and the gas flow rate determine the chamber pressure, and therefore chamber pressure is regulated by controlling the valve opening size of each valve  30 ,  32  through rotation of the valve flaps  36  and  46 . An advantage of the high volume valve  30  is that extremely large chamber evacuation rates can be attained by rotating the flap  36  of the high volume valve  30  to its fully opened angular position (i.e., perpendicular relative to the plane of the valve housing  28 ). The high volume valve&#39;s diameter may be very large (e.g., 9 inches), to accommodate a large chamber evacuation rate required during chamber cleaning operations using a cleaning gas such as NF3 or using depositing gases during film deposition, for example. However, the high volume valve does not provide the most accurate regulation of chamber pressure to a desired pressure value because small angular rotations of its flap  36  produce large changes in the chamber pressure. Accurate control is provided by the fine control valve  32 , whose opening diameter may be as small as one inch (for example). In the case of the fine control valve  32 , a small angular rotation of its flap  46  produces a relatively small change in chamber pressure, facilitating small, accurate adjustments in chamber pressure. Moreover, the fine control valve flap  46 , due to its small diameter, has a relatively small moment of inertia, which permits a motor of modest torque capability to effect very rapid corrections or changes in the chamber pressure, enhancing the fine control capability of the valve  32 . 
   In operation, the high volume valve  30  is set to a rotational position (or opening size d) that establishes a chamber pressure above the desired chamber pressure by a difference that is sufficiently small to be within the ability of the fine control valve  32  to compensate. The chamber pressure is then accurately adjusted to the exact value of the desired pressure by opening the fine control valve  32  until the chamber pressure has decreased to the desired value. Because the fine control valve  32  has such a small opening, the rotational movement of the fine control valve flap  46  effects very small changes in chamber pressure, thereby facilitating accurate adjustment of the chamber pressure. 
     FIG. 2  shows that one surface of the valve flap  36  facing the chamber  16  (the “plasma” side) is a smooth continuous surface. The perspective view of  FIG. 3  shows that the opposite side of the valve flap  36  partially encloses a hollow volume surrounded by a circumferential skirt  60  defining the arcuate edge  38  and extending axially from the planar top surface  62 . Radial struts  64  extending through the flap&#39;s center between opposite sides of the skirt  60  provide rigidity. An axle or shaft  66  extends partially across the diameter and through one side of the skirt  60 . The shaft  66  is overlies and is fastened to one of the radial struts  64 . A pin  61  aligned with the shaft  66  extends radially outwardly from the opposite side of the skirt  60 . Referring to the cross-sectional view of  FIG. 4 , a top ring  70  nests in a hollow annulus  72  in the top half of the housing  28 . The top ring  70  forms a top half of the arcuate edge  40  while the housing  28  forms the remaining half of the arcuate edge  40 . A half-cylindrical shaft hole  28   a  formed in the valve housing  28  and a matching half-cylindrical shaft hole  28   b  in the top ring  70  form a cylindrical shaft hole  27  that encloses the shaft  66  when the top ring  70  is bolted in place. A half-cylindrical shaft sleeve  71  overlies the portion of the shaft  66  that extends beyond the top ring  70 . In similar manner, a half-cylindrical pin hole  29   a  formed in the valve housing  28  and a matching half-cylindrical pin hole  29   b  in the top ring  70  form a cylindrical pin hole  29  that encloses the shaft  66  when the top ring  70  is bolted in place. A pair of teflon spacers  74   a ,  74   b , surrounding the shaft  66  and pin  61 , respectively, maintain the axial position of the flap  36  within the valve opening  34 . For a gap thickness t on the order of about 0.010 to 0.030 inch, each teflon spacer  74   a ,  74   b  has a thickness of about 0.010 inch, for example. 
   The small fine control valve  32  and flap  46  is a smaller but identical version of the larger coarse control valve  30  and flap  36 , and therefore has the same structure illustrated in  FIGS. 2 ,  3  and  4 , but of reduced size. For example, the fine control valve  32  is about 1/10 the diameter of the coarse control valve  30 , in one embodiment. 
   One optional feature is to increase the conductance in the high volume valve  30  by forming flow-enhancing slots  90  in the arcuate surface  40  of the valve opening  34 , as illustrated in  FIGS. 5 and 6 . The slots  90  may be arcuate with a radial slot depth s which is maximum depth at the surface of the valve housing  28  and is zero at some depth p below the surface of the housing  28 , the slot depth s decreasing with depth below the surface of the housing. The slot depth s reaches zero (so that the slot  90  disappears) at a selected distance p below the surface of the housing  28 . A similar slot  91  may extend from the opposite face of the housing  28 , tapering in the opposite direction in the same manner as the slot  90 . The upper and lower slots  90 ,  91  are in alignment and may have the same depth p. Their common slot depth p is less than half the thickness of the valve housing  28 , so that a surface region  40   a  of the curved opening surface  40  lies between the two slots  90 ,  91 . The result, as illustrated in  FIG. 6 , is that when the valve flap  36  is in the closed (parallel) position, there is a gap of the desired thickness T between the periphery of the valve flap  36  and the surface region  40   a . The gap thickness T is sufficiently small to limit the rate of leakage of gas or plasma through the valve without requiring an O-ring, as discussed previously in this specification. 
   The purpose of the slots  90 ,  91  is to increase the rate at which conductance through the valve grows as the flap is rotated from its closed position. This increase is augmented by increasing the number of slots  90  in the upper housing face and  91  in the bottom housing face. 
     FIG. 7  illustrates how a pair of fast speed positive coupler motors  93 ,  95  can control the rotation of the respective valve flaps  36 ,  46 . The motors  93 ,  95  are coupled to the shafts of the valves  30 ,  32 , respectively. A feedback control system  97  compares actual chamber pressure measurements received from a pressure sensor  98  within the chamber with a desired chamber pressure, and controls the positions of the two valves  30 ,  32  through the motors  93 ,  95 . The control system  97  may be programmed to achieve an approximate match of the actual chamber pressure with the desired value by operating the motor  93 , and then achieve exact match between actual and desired chamber pressure by operating the motor  95 . 
   The surfaces of the valve housing and valve flaps facing the plasma reactor are preferably of a material compatible with plasma processing. For some processes, this material may be aluminum. The opposite side of the valve housing and valve flaps facing the vacuum pump, as well as the shaft, may be formed of other materials (e.g., steel or stainless steel). No O-ring is necessary to seal the valve assembly, a significant advantage. 
   While the invention has been described in detail by reference to preferred embodiments, it is understood that variations and modifications thereof may be made without departing from the true spirit and scope of the invention.