Patent Application
20090081022
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.
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.
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.
Attempts to produce portable vacuum pumps have had limited success due to difficulty in achieving required pumping capacity with a compact pump design.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
