Abstract:
A radial turbomolecular vacuum pump that includes a rotor made from a silicon rotor surface comprising monolithically fabricated micro blades, and a stator made from a silicon stator surface comprising corresponding monolithically fabricated grooves. The micro blades and grooves are arranged in multiple rings, and the rotor and stator disks are placed in proximity, creating interdigitated stator and rotor blade rings. The interdigitated stator and rotor blade rings form a multi-stage compression in the radial direction.

Description:
FIELD 
       [0001]    The present invention relates generally to the field of vacuum pumps. More particularly, the present invention relates to the design and manufacture of miniature turbomolecular vacuum pumps. 
       BACKGROUND 
       [0002]    Turbomolecular pumps are a type of vacuum pump used to obtain and maintain high vacuum. A conventional turbomolecular pump is capable of high vacuum level and pump speed. However, the conventional turbomolecular pump is too bulky for most portable applications, such as use with micro-instrumentation. For example, the rotor diameter is typically sized to greater than 75 mm, which results in the turbomolecular pump size being inconveniently large for portable applications. 
         [0003]    In addition, a conventional turbomolecular pump is only able to operate against a fairly low exhaust pressure, typically less than 10 millitorr, and a first-stage or foreline pump must be used between the turbomolecular pump exhaust and the atmosphere. It is difficult to find a miniature pump capable of reaching values of less than 10 millitorr pressure to be used as the first stage pump. For example, most commercially available miniature pumps at present typically are only able to achieve pressures on the order of 100 torr. Therefore, it is desirable for a miniature turbomolecular pump to be able to work against exhaust pressures larger than 10 torr. 
         [0004]    Attempts to produce portable vacuum pumps have had limited success due to difficulty in achieving required pumping capacity with a compact pump design. 
       SUMMARY 
       [0005]    The present invention relates to a turbomolecular pump that can produce a high vacuum against a high exhaust pressure, such as pressures larger than 10 torr, yet may be used in instrumentation where the application may be portable, hand held or space limited. Examples of these types of instruments are mass spectrometers, electron microscopes, small gas detectors, and gas analyzers. 
         [0006]    In one embodiment, the present invention features a radial turbomolecular vacuum pump that includes a gas inlet, a gas outlet, a motor, a rotor, a stator, and a casing. The rotor includes a silicon rotor surface comprising monolithically fabricated micro blades. The stator includes a silicon stator surface comprising monolithically fabricated blades and grooves. The micro blades on the rotor and stator, respectively, are arranged in multiple rings, and the rotor and stator disks are placed in proximity, forming interdigitated stator and rotor blade rings. The interdigitated stator and rotor blade rings form a multi-stage compression in the radial direction. 
         [0007]    The microfabrication allows for the creation of blades on the rotor and stator in dimensions smaller than the mean free path of gas molecules, even at the high limit of the exhaust pressure of 10 torr. With the silicon etching method used for monolithic fabrication, multiple blade rings can be made easily and the number of stages of compression in the radial direction (e.g., 100 stages) can be achieved. One advantage of increasing the number of stages within each rotor and stator pairing is that the same compression ratio can be achieved at a lower rotor speed, leading to lower power consumption and less stringent bearing requirements. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  illustrates an exemplary view of a radial flow turbomolecular pump; 
           [0009]      FIG. 2A  illustrates an exemplary cutaway view of a stator and rotor; 
           [0010]      FIG. 2B  is an enlarged partial view of the stator; 
           [0011]      FIG. 2C  is an enlarged partial view of the rotor; 
           [0012]      FIG. 3  is an exemplary cutaway view of the rotor and stator in a position in which the rotor and stator operate; 
           [0013]      FIG. 4  illustrates an exemplary fabrication process for a turbomolecular pump with an external motor; and 
           [0014]      FIG. 5  illustrates an exemplary fabrication process for a turbomolecular pump with an on-chip electromagnetic motor and bearing. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]      FIG. 1  illustrates an exemplary view of a radial flow turbomolecular pump  10 . The pump  10  includes an inlet  12 , a rotor  14 , a stator  16 , a motor  18 , a casing  20 , and a plurality of outlets  22 . The rotor  14  includes a plurality of rotor blades  24 . The stator  16  includes a plurality of stator grooves  26 . 
         [0016]    Casing  20  encloses rotor  14  stator  16 . Motor mount  19  holds motor  18  in place and aligns motor  18  with rotor  14  and stator  16 . Inlet  12  serves to allow an amount of gas  13  to enter casing  20  and flow into rotor  14 . 
         [0017]    Each groove of the plurality of stator grooves  26  is positioned to receive each blade of the plurality of rotor blades  24 . In the assembled state, stator  16  is fixed and rotor  14  is free to spin. 
         [0018]    A rotor circumferential speed can range from 10 to 300 m/s, depending on a needed compression ratio and gas flow. The rotation of rotor  14  relative to stator  16  causes gas to be pumped radially outward, away from an axial centerline  28  or radially inward toward axial centerline  28 . 
         [0019]    In operation, motor  18  moves rotor  14  so that the rotor rotates relative to stator  16 . When the rotor rotates, the plurality of rotor blades  24  passes through a respective one of the plurality of stator grooves  26 . As gas molecules enter via inlet  12 , the plurality of rotor blades  24  and stator grooves  26  impact the molecules, causing the molecules to gain momentum in the radial direction. This process is continued until the gas molecules are lead through an outlet of the plurality of outlets  22  and outside casing  20 . The dimension of the gas flow path parallel to a radial centerline  30  is greater than the dimension of the gas flow path parallel to axial centerline  28 , resulting in a radial flow of the gas molecules. 
         [0020]      FIG. 2A  illustrates a cutaway view of an exemplary stator  32  and rotor  34 .  FIG. 2B  illustrates a partial augmented view of rotor  34 . Rotor  34  and stator  32  are each formed from a silicon substrate. Each blade of the plurality of rotor blades  36  extends axially from a first rotor surface  40 , that is, in a substantially parallel direction to an axial centerline  44 . The plurality of rotor blades  36  are arranged in rotor concentric rings  46  on rotor surface  40 . A plurality of stator blades  37  and a plurality of stator grooves  38  are arranged on a stator surface  42  in stator concentric rings  48 . The plurality of stator grooves  38  are arranged to fit between concentric rings  46  of plurality of rotor blades  36 . 
         [0021]    The plurality of rotor blades  36  may be monolithically fabricated on first rotor surface  40  of rotor  34  using an etching process. The plurality of stator blades  37  may be monolithically fabricated on first stator surface  42  of stator  32  using an etching process. 
         [0022]    An exemplary etching process may be deep reactive ion etching. In this process, silicon substrates are places inside a vacuum chamber and are usually grounded and electrically isolated from the rest of the chamber. An etch mask is placed on the silicon substrate to selectively protect areas of the substrate. Gas then enters the chamber and etches the unprotected portions of the silicon substrate; thus the substrate takes on a form that is dictated by the mask. An example of a gas that may be used for this process is sulfur hexafluoride. However, the fabrication process is not limited to using sulfur hexafluoride, and a number of other gases may be used. As an alternative to gas, plasma may be used to etch the silicon substrate. Another etching process may be photo assisted wet chemical etching. Both sides of rotor  34  or stator  32  may be processed using an etching process. The etching process allows for the blades to be monolithically fabricated on the rotor. With monolithic fabrication, multiple rings of blades and grooves can be easily made, forming a large number of stages, for example, 100 stages. When the number of stages is increased, the same compression ratio can be achieved at a lower rotor speed. When turbomolecular pump  10  operates at a lower rotor speed, less power is consumed. Additionally, the ability to operate at a lower rotor speed will result in less stringent bearing requirements and less wear on the bearings of the turbomolecular pump. A plating/LIGA process may be used as well. In this process, a layer of photosensitive polymer is coated on the silicon substrate, followed by x-ray radiation using an x-ray mask. 
         [0023]    Each blade of the plurality of rotor blades  36  is thick enough to be stable under high speed rotation, yet thin for efficient compression. In one embodiment, the size of each blade of the plurality of rotor blades  36  is approximately 10 micron, which allows the pump to work against 10 torr exhaust pressure. However, the blades are not limited to this size, and a number of other sizes may be used for other operating conditions. 
         [0024]    The plurality of rotor blades  36  are shaped and positioned to achieve a certain pumping speed, compression, and efficiency. The pitch of each blade of the plurality of rotor blades  36  generally determines the pumping speed and compression. As an example, tilting the plurality of rotor blades  36  towards a radial direction  50  will generally result in a higher pumping speed. Tilting the plurality of rotor blades  36  towards a circumferential direction  52  will result in higher compression, yet lower pumping speed. The blades of the plurality of rotor blades  36  near the center of rotor  34  may be larger than the blades at the edge of rotor  34 , because the pressure near the center of rotor  34  is lower. 
         [0025]    The silicon monolithic fabrication allows for stator  32  and rotor  34  to be manufactured very small in size. In an exemplary embodiment, stator  32  and rotor  34  are each 10 mm in diameter. However, stator  32  and rotor  34  are not limited to this size. 
         [0026]      FIG. 3  illustrates a cutaway view of the rotor and stator pair from  FIG. 2  in the operating position. As is shown in  FIG. 3 , first rotor surface  40  is positioned in a substantially parallel direction to radial centerline  50 . First stator surface  42  is positioned proximate to first rotor surface  40 , and is also substantially parallel to a radial centerline  54 . First stator surface  42  is positioned so that it does not touch first rotor surface  40 . The plurality of stator grooves  38  effectively form grooves for receiving the corresponding plurality of rotor blades  36 . 
         [0027]    The silicon monolithic fabrication allows for the plurality of rotor blades  36  to be made with high precision, so that each blade of the plurality of rotor blades  36  fits within the corresponding stator groove  38  within a specified tolerance. In an exemplary embodiment, a lateral clearance between at least one blade of the plurality of rotor blades  36  and the first and second plurality of stator grooves  38  is approximately 5 micron. However, the distance is not limited to 5 micron, and may comprise other values as well. The rotor blades may be as close to the grooves as possible. 
         [0028]    Assembling the rotor  34  and stator  32  generally requires a tolerance of about 5 to 10 micrometers. This can be accomplished by assembling rotor  34  and stator  32  in a precision bore tubing after aligning and bonding rotor  34  and stator  32  to carrier disks, which have precision matching outer diameters. 
         [0029]      FIG. 4  illustrates an assembly for the turbopump with an external motor. In this configuration, rotor  14  and stator  16  are concentrically mounted in the direction of arrow  56  on motor  18  so that rotor  14  is placed on motor  18 , as shown in the assembled state in  FIG. 1 . Motor  18  may be concentrically mounted via a precision mount base. 
         [0030]      FIG. 5  illustrates an alternative configuration for the turbopump with on-chip magnetic bearing. In this embodiment, rotor  14  is operated using permanent magnets  60 . Permanent magnets  60  may be embedded in rotor  14  and driven by planar coils  62  positioned in a facing surface  64 . A levitation external circuit may drive planar coils  62 . Position sensors  66  determine the position of rotor  14 .