Classifications

• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F16—ENGINEERING ELEMENTS AND UNITS; GENERAL MEASURES FOR PRODUCING AND MAINTAINING EFFECTIVE FUNCTIONING OF MACHINES OR INSTALLATIONS; THERMAL INSULATION IN GENERAL
  • F16F—SPRINGS; SHOCK-ABSORBERS; MEANS FOR DAMPING VIBRATION
  • F16F3/00—Spring units consisting of several springs, e.g. for obtaining a desired spring characteristic
  • F16F3/08—Spring units consisting of several springs, e.g. for obtaining a desired spring characteristic with springs made of a material having high internal friction, e.g. rubber
  • F16F3/10—Spring units consisting of several springs, e.g. for obtaining a desired spring characteristic with springs made of a material having high internal friction, e.g. rubber combined with springs made of steel or other material having low internal friction
  • F16F3/12—Spring units consisting of several springs, e.g. for obtaining a desired spring characteristic with springs made of a material having high internal friction, e.g. rubber combined with springs made of steel or other material having low internal friction the steel spring being in contact with the rubber spring
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F04—POSITIVE - DISPLACEMENT MACHINES FOR LIQUIDS; PUMPS FOR LIQUIDS OR ELASTIC FLUIDS
  • F04B—POSITIVE-DISPLACEMENT MACHINES FOR LIQUIDS; PUMPS
  • F04B37/00—Pumps having pertinent characteristics not provided for in, or of interest apart from, groups F04B25/00 - F04B35/00
  • F04B37/06—Pumps having pertinent characteristics not provided for in, or of interest apart from, groups F04B25/00 - F04B35/00 for evacuating by thermal means
  • F04B37/08—Pumps having pertinent characteristics not provided for in, or of interest apart from, groups F04B25/00 - F04B35/00 for evacuating by thermal means by condensing or freezing, e.g. cryogenic pumps

Definitions

• a cryopump typically includes a vacuum vessel, enclosing a cryopump chamber, which can be mounted to a work chamber which is to be evacuated.
• a cryogenic refrigerator is mounted to the vacuum vessel and a cold finger of the refrigerator extends into the vessel.
• the refrigerator typically operates on the Gifford-MacMahon cycle.
• High pressure refrigerant gas such as helium, is introduced into the cold finger and there displaced and expanded by movement of a reciprocating displacer/regenerator.
• Cryopanels mounted to the cold finger within the cryopump chamber are cooled to cryogenic temperatures. Gases from the work chamber condense on those cryopanels and, with the gases thus removed from the work chamber, its pressure is reduced to a high vaccu in the order of 10 torr.
• cryopump is a capture system in which no moving parts are exposed to the vacuum environment, it is extremely clean. Cryopumps are also fast and efficient. Cryopumps cooled by closed cycle refrigerators are particularly convenient and economical because there is no need for an expendible cryogen such as liquid nitrogen. The refrigeration is developed in the cold finger which protrudes into the vacuum vessel. Cryopumps driven by closed cycle refrigerators have suffered one disadvantage which has made such cryopumps unsuitable for certain applications. In such applications as electron microscopes and electron beam lithography, vibrations generated in a closed cycle refrigerator can be detrimental.
• the closed cycle refrigerator is mounted directly to the cryopump vacuum vessel.
• the refrigerator is vibration isolated from the work chamber.
• the vibration isolator may be posi ⁇ tioned between the cryopump chamber and the work chamber or between the closed cycle refrigerator and the cryopump chamber.
• a bellows is utilized. The bellows may surround the flow path between the cryopump chamber and the. work chamber or it may surround the cold finger of the cryogenic refrigerator.
• the spring action of the bellows isolates the cryopump chamber from refrigerator vibration frequencies above a resonant frequency. Below the resonant frequency there is no isolation, and at the resonant frequency vibrations are actually amplified.
• the isolator system can be designed to have a resonant frequency below the most significant vibra ⁇ tion frequencies and thus effectively isolate the refrigerator from the work chamber.
• a primary- deficiency of the Venuti system is that it requires the additional cumbersome mass. Further, that mass acts to stretch the bellows out when there is no vacuum in the vacuum chamber, so the refrigerator must have added support.
• the system is only suited to cryopumps mounted below a work chamber.
• the bellows is provided for vacuum sealing, and elastomer vibration isolators are positioned about the bellows between the refrigerator and the work chamber.
• Such a system relies on isolation and damping of vibration amplitudes by the elastomer isolators which also prevent collapse of the bellows.
• An example- can be found in U.S. Patent No. 4,539,822 to Sundquist.
• an elastomeric ring surrounds the bellows. A metal band around the elastomeric isolator ring prevents the collapse of the bellows and the cryopump vessel under vacuum.
• the elastomeric isolator ring provides both vibration isolation and damping of vibration amplitudes.
• the present invention relates to a vibration isolator placed between a cryopump and a work chamber for eliminating vibration transferred to the chamber.
• two isolators are connected in. series and are separated by an intermediate mass.
• the two isolators provide for a two degree of freedom system which increases vibration attenuation.
• the first of the two isolators forms a vacuum housing about cryopanels and that the other isolator be mounted between to the first isolator and a refrigerator which cools the cryopanels.
• the intermediate mass preferably comprises weighted flanges used to mount the two isolators together.
• the vibration isolator comprises a cylindrical bellows having circumferential corrugations.
• An elastic vibration damping material fills the spaces between the corrugations of the bellows to prevent collapse of the bellows and to isolate and damp vibrations transmitted through the bellows.
• the dynamic shear modulus and the dynamic Young's modulus of the damping material are substantially less than that of the bellows in order to maintain isolation as the bellows is collapsed against the material.
• the bellows has high dynamic moduli and provides rigidity against transverse bowing of the assembly but-its corrugated shape provides good isolation.
• the dynamic shear modulus of the material of the bellows should be at least in the order of 10 dynes/square centimeter and that of the damping material should be less than 10 7 dynes/cm2.
• the damping material can provide high or low damping depending on the frequen- cies and amplitude of the primary vibrations of, concern.
• a metal bellows provides the preferred surface for exposure to the vacuum environment. " It is impermeable to gases and minimizes outgasing.
• the metal bellows can be of welded diaphragms or it can be a formed convoluted bellows.
• Figure 1 is a longitudinal cross sectional view of a cryopump provided with a welded bellows isolator between the refrigerator and the cryopump chamber.
• Figure 2 is an enlarged longitudinal cross sectional view of an alternative convoluted bellows isolator.
• Figure 3 is a longitudinal sectional view of another convoluted bellows isolator.
• Figure 4 is a side view of an isolator positioned between the cryopump chamber and the work chamber.
• Figure 5 is a longitudinal sectional view of the cryopump chamber employing an isolator as a vacuum chamber.
• Figure 6 is a longitudinal sectional view of a pair of isolators connected in series.
• Figure 7 is a longitudinal sectional view of an alternative pair of isolators connected in series.
• Figure 8 is a longitudinal sectional view of an ' alternative embodiment of the invention having one damped isolator and one undamped isolator forming a cryopump housing.
• Figure 9 is a longitudinal sectional view of a second alternative embodiment of the invention having a collar placed between one of the corrugations of the bellows.
• the cryopump of each of the figures comprises a vacuum vessel 12 which is to be mounted to the wall of a work chamber along a flange 14.
• a front opening 16 in the vessel 12 communicates with a circular opening in a work chamber.
• the cryopump assembly may protrude into the chamber and a vacuum seal be made at a rear flange.
• a two stage cold finger 18 of a refrigerator protrudes into the vessel 12 through an opening 20.
• the refrigerator is a Gifford-MacMahon refrigerator such as disclosed in U.S. Patent No. 3,218,8i5 to Chellis et al., but others may be used.
• a two stage displacer in the cold finger 18 is driven by a motor 22.
• a first stage heat sink, or heat station, 28 is mounted at the cold end of the first stage 29 of the refrigerator.
• a heat sink 30 is mounted to the cold end of the second stage 32.
• the primary pumping surface is, in this system, an inverted cup 34 mounted to the heat sink 30.
• a low temperature adsorbent 36 such as charcoal particles is epoxied to the inside surface area of the cu .
• a cup shaped radiation shield 44 is mounted to the first stage, high temperature sink 28.
• the second stage of the cold finger extends through an opening 45 in that radiation shield.
• This radiation shield 44 surrounds the second stage array to the rear and sides to minimize heating of the array by radiation.
• the temperature of this radiation shield is less than about 120 K.
• a frontal cryopanel array 46 serves as both a radiation shield for the primary cryopanel and as a cryopumping surface for higher boiling temperature gases such as water vapor.
• This array comprises louvers 48 joined by spokes 50.
• the frontal array 46 ⁇ is mounted to the radiation shield 44, and the shield both supports the frontal array and serves as the .thermal path from the heat sink 28 to that array.
• the refrigerator In a conventional cryopump, the refrigerator is connected directly to the vacuum vessel 12. In the system of Figure 1, however, the refrigerator is mounted to the vessel through a vibration isolator 51.
• the vibration isolator 51 includes a bellows 52 which in Figure 1 is a welded diaphragm bellows.
• Vibration damping material 54 is integrally formed with the bellows to fill the outer spaces between the corrugations of the bellows and to cover the bellows.
• the bellows is welded at each end to flanges 56 and 58 which allow it to be coupled into the system.
• the bellows should be stainless steel of thickness between .008 inch and
• Silicon rubbers such as methyl phenyl are particularly suited as the damping material because they can be poure ' about the bellows in liquid orm and then cured.
• the stainless steel of which the bellows is formed has a dynamic-shear modulus of about 1 x 10 12
• the corrugated shape of the thin metal results in a relatively low spring constant of the bellows as ⁇ sembly.
• the bellows configuration does permit significant changing length of the bellows assembly along its axis, but the circumferential length through each cross section of the bellows is main ⁇ tained constant due to the high dynamic modulus of the metal.
• the bellows is free to change in axial length but does not bow inwardly or outwardly with ' changes in pressure acting radially on the bellows and with collapsing of the bellows.
• the damping material 54 is of a synthetic rubber which has a high damping factor, preferably around one, and a relatively low dynamic shear modulus, preferably less than 5 x 10
• the isolator does not include multiple isolators which must be bolted onto flanges as in the Sundquist system or a bulging elastomer held in place by a metal ring.
• Another significant advantage of the present isolator is that it provides damping of all frequen ⁇ cies transmitted into the isolator.
• some vibrations could be transmitted through the bellows with minimal damping.
• the damping material integral with the bellows those vibrations are likely to be absorbed by the damping material.
• some vibrations trans- mitted along the bellows follow a path through alternating layers of metal and rubber.
• the isolator' provides the improved damping of constrained layer damping.
• the typical vibrations produced by a cryopump have both a low frequency component and a high frequency component.
• the piston stroke of a typical cryopump provides low frequency vibrations, while the high frequency components are due to the vibrations of the stepper motor of the refrigerator and due to the vibrations of the vacuum chamber.
• the present isolator as in the case of many typical isolators, can be modeled as a spring-mass system having a spring "constant" and a damping factor dependent on frequency and temperature. In such vibration isolat ⁇ ing models the resonant frequency is determined by the relationship
• Any vibration isolating spring scheme has a vibration transmissibility of greater than one for a given resonarit frequency f .
• Such a system has a lower transmissibility below and above that resonant frequency.
• vibration frequencies above v2 f are attenuated (transmissi ⁇ bility is less than 1) while below v2 f frequency vibration levels are amplified (transmissibility is greater than one) .
• the transmissibility amplitude is determined by the damping factor.
• High damping factors lower the amplitude at frequencies below v2 f ; but the higher damping factor increases the transmissibility above v2 f relative to a lighter damped system, even though the transmissibility is still less than one.
• the low frequency components of cryopump vibrations do not need be greatly reduced, so they can be attenuated easily via high-damping rubberlike material, and they are much lower in frequency than the resonant frequency of the vibra ⁇ tion isolator so as not to be samplified by the isolator.
• the high frequency vibration components are well above the resonant frequency and are easily attenuated by the isolator.
• the damping material can be of low damping or high damping rubberlike material.
• the low damping rubberlike materials at room temperatures exhibit low damping factors and provide better isolation where high frequency vibrations are of primary concern. Higher damping materials typically provide better results where lower frequencies near the resonant frequency are a primary concern.
• neoprene rubber, natural rubber, SBR rubber (75/25 butadiene-styrene) are filled natural rubber (50 parts by weight of HAF carbon black) exhibit shear moduli and damping factor that vary by a factor of* two in the frequency range of 1 Hz to 10,000 Hz. at 35°C.
• the high-damping rubberlike materials like plasticized polyvinyl butyral resin, thiokol Rd, plasticized polyvinyl acetate and filled
• the bellows is required to provide the gas flow path from the work chamber to the cryopump chamber.
• Such positioning of the vibration isolator is suitable for isolating the work
• one isolator 69 serves as the vacuum vessel 12 while the second isolator 67 is positioned between the cryopump vacuum vessel and the work chamber.
• the refrigerator may be mounted to the first vibration isolator forming the vacuum vessel 12, as described above, through a second vibration isolator 51. This embodiment is shown in figure 7.
• the two vibration isolators shown in figures 6 and 7 are joined at their respective flanges.
• the joined flanges serve as a weighted intermediate mass which provides for further vibrational attenuation by creating a two degree of freedom system having two resonant frequencies.
• the weight of the intermediate mass may vary, however, depending on the operating frequency of the system in order to avoid one of the resonant frequencies from equaling the operating frequency.
• An alternative method for controlling the resonant frequencies is accomplished by removing the damping material surrounding one of the isolators as shown in figure 8.
• weight may be added to the refrigerator.
• the combined weight of the refrigerator and the added weight is 30 to 35 pounds for a refrigerator which operates at 53 to 60 Hz.
• thermoster 70 may be coupled to the intermediate mass thereby eliminating vibration that would otherwise be transmitted to the system from the external environment. Similary, a valve 72 connecting the vacuum chamber to a roughing pump is incorporated into the intermediate mass.
• a collor 73 is placed between one of the corrugations of the bellows.
• This collor serves as an intermediate weighted mass and may be placed in any one of the corrugations along the bellows.
• the purpose of the collor is to separate the bellows into two bellows for creating a two degree of freedom system discussed above. By splitting the bellows into two bellows having different lenghts, different frequencies can be attenuated. Damping material filling the outer spaces between- the corrugations of the bellows and the collor further attenuates vibration.

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• Engineering & Computer Science (AREA)
• General Engineering & Computer Science (AREA)
• Mechanical Engineering (AREA)
• Compressors, Vaccum Pumps And Other Relevant Systems (AREA)

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Citations (5)

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• 1986
  • 1986-09-23 WO PCT/US1986/001994 patent/WO1987001768A1/en active Application Filing
  • 1986-09-23 DE DE19863690477 patent/DE3690477T1/de not_active Ceased
  • 1986-09-23 GB GB8710617A patent/GB2190460B/en not_active Expired

Patent Citations (5)

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