US3609063A - Sublimation pump and method of regulating it - Google Patents

Sublimation pump and method of regulating it Download PDF

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Publication number
US3609063A
US3609063A US17158A US3609063DA US3609063A US 3609063 A US3609063 A US 3609063A US 17158 A US17158 A US 17158A US 3609063D A US3609063D A US 3609063DA US 3609063 A US3609063 A US 3609063A
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Prior art keywords
electric
metal
container
filament
heating
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US17158A
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English (en)
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Thomas A Jennings
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Hull Corp
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Hull Corp
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    • G—PHYSICS
    • G05—CONTROLLING; REGULATING
    • G05D—SYSTEMS FOR CONTROLLING OR REGULATING NON-ELECTRIC VARIABLES
    • G05D5/00—Control of dimensions of material
    • G05D5/02—Control of dimensions of material of thickness, e.g. of rolled material
    • G05D5/03—Control of dimensions of material of thickness, e.g. of rolled material characterised by the use of electric means
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J7/00—Details not provided for in the preceding groups and common to two or more basic types of discharge tubes or lamps
    • H01J7/14—Means for obtaining or maintaining the desired pressure within the vessel
    • H01J7/18—Means for absorbing or adsorbing gas, e.g. by gettering

Definitions

  • Sublimation pumps provided heretofore are controlled as to on" and off" time by observed variations in pump chamber pressure.
  • This type of control depends for its accuracy and consistency upon proper attention by operating personnel. The requirement of continuous supervision by operating personnel burdens this type of control with excessive labor costs, and the dependence upon human estimations and attention renders the accuracy and consistency of such control susceptible of significant variation.
  • the off time i.e. the time interval during which gas adsorption on the cooled metal surface is taking place
  • the off time i.e. the time interval during which gas adsorption on the cooled metal surface is taking place
  • the on time i.e. the time required to deposit a clean metal surface over the adsorbed gas layer
  • the on time is allowed to continue for too long a period of time, excessive vaporizable metal is wasted, whereas if the on time is made too short, insufficient trapping of adsorbed gas results, with consequent reduction in effective pumping area.
  • this invention provides a sublimation pump and method of regulating it, in which changes in current-voltage characteristics of a space-charged limited current diode, resulting from changes in the extent of adsorption of gas and of deposition of vaporizable metal, are utilized to maintain automatically a preadjusted pumping speed.
  • Another important object of this invention is the provision of a sublimation pump of the class described which is of simplified construction for economical manufacture, which is precise in its operation and requires a minimum of maintenance and supervision.
  • FIGURE of the drawing illustrates a sublimation pump embodying the features of this invention and includes a cross-sectional view of the pump and a schematic electrical diagram of control circuitry associated therewith.
  • the pump includes a cylindrical container having a closed bottom end 12 and removably sealed at its top end by a cover 14.
  • a laterally extending conduit 16 is adapted for connection, as by cooperating flanges, to a conduit 18 leading from a vessel (not shown) to be evacuated.
  • the container thus defines a pumping chamber which communicates with the vessel to be evacuated.
  • At least the inner surface of the container, and preferably the entire container, is made of stainless steel, or any other electrically conductive metal suitable for use in high vacuum.
  • Means for cooling the inner surface of the container so that the latter will adsorb such gases as oxygen, nitrogen, hydrogen, ammonia, carbon monoxide, carbon dioxide, sulfur dioxide, tetracyanoethylene, water vapor and many others.
  • such means is provided by a vessel 20 made in the form of a Dewar flask. The top of the vessel is open to receive the cylinder which thus may be immersed in a liquid coolant 22 contained in the vessel.
  • the coolant may be water, liquid nitrogen or other suitable form of liquid. It will be understood that various other well-known means may be utilized to effect cooling of the inner surface of the container.
  • a source 24 of active metal i.e. metal capable of adsorbing the desired gas particles
  • the metal also is capable of being vaporized for deposition in film form on the inner surface of the container.
  • titanium is most commonly used for this purpose, molybdenum, tungsten, tantalum, zirconium, chromium, barium and various other metals also may be employed, as will be understood. Evaporation of the metal may be effected by various means, such as an electron beam source.
  • the vaporizable metal source 24 is provided in the form of a filament rod which is supported by electrodes 26 and 28. These extend from the cover 14 and are connected to terminals 26' and 28'. It will be understood that the electrodes and terminals are insulated electrically from the cover, if the latter is made of electrically conductive material. The terminals are connected through electrical conductors 30 and 32 to a power supply circuit 34.
  • the conductors 30 and 32 are connected, preferably through an ammeter 36, across the secondary winding of a stepdown transformer 38.
  • One end of the primary winding of the transformer is grounded, while the opposite end is connected to the variable contact of a Variac '40.
  • One end of the Variac coil is grounded, while the opposite end is connected to one end of the secondary winding of the transformer 42.
  • the opposite end of this secondary winding is connected to ground selectively by means of a pair of switches which, in the embodiment illustrated, are in the form of oppositely arranged silicon controlled rectifiers 44 and 46. Each silicon controlled rectifier conducts during opposite half cycles of an alternating current signal supplied from a signal source described in detail hereinafter.
  • the silicon controlled rectifiers may be bypassed by a switch 48 for manual operation of the power supply.
  • the primary winding of transformer 42 is connected through the control switch 50 to the terminals 52 of a source of electric potential.
  • a cathode filament 54 of tungsten or other suitable metal Surrounding the vaporizable metal rod 24 is a cathode filament 54 of tungsten or other suitable metal.
  • the cathode filament is in the form of a coil, although it will be apparent that other configurations may be selected as desired.
  • the cathode filament is connected at its opposite ends to electrodes 56 and 58 which are supported by the cover 14. The electrodes are connected through terminals 56' and 58 to electrical conductors 60 and 62.
  • Battery 64 provides an isolated power supply for heating the cathode filament to electron emission temperature, and it is connected across the conductors through a control switch 66 and variable resistance 68. The latter serves to adjust the cathode temperature such that the diode current is space-charged limited, and also to prevent deposition of metal vapors on the filament.
  • the inner surface of the container 10 and the cathode filament 54 are made the anode and cathode, respectively, of a space-charged limited current diode, and means is provided for detecting variations in current-voltage characteristics thereof.
  • the electrically conductive container is connected through the electrical conductor 70 to the grounded junction 72 of a bridge circuit.
  • One end of the cathode filament 54 is connected through the electrode 58 and conductor 62 to a junction 74 of the bridge circuit which forms with the grounded junction 72 one branch of the bridge circuit.
  • the other branches of the bridge circuit include variable reference resistance 76 and fixed resistances 78 and 80.
  • the junctions 74 and 82 of the bridge circuit are interconnected through the series arrangement of control switch 84, direct current constant voltage bridge power supply 86, connected in the polarity indicated, and ammeter 88.
  • the output junction 90 of the bridge circuit is connected to the input of a direct current differential amplifier 92 through control switch 94 and diode 96, and preferably also through ganged selector switches 98 and 100 which accommodate operation with either electronegative or electropositive gases.
  • control switch 94 and diode 96 and preferably also through ganged selector switches 98 and 100 which accommodate operation with either electronegative or electropositive gases.
  • electronegative gases is afforded with the selector switches arranged in the positions illustrated. when these switches are transferred to their alternate positions, the signal from the bridge is fed to the inverting input of the operational amplifier 102.
  • the noninventing input is connected to ground. With resistors 104 and 106 being equal, the bridge input is amplified by the factor i, and therefore the input and output voltages are equal and only the polarity is reversed.
  • the output of the amplifier 92 is connected through conductor 108 to the silicon controlled rectifiers 44 and 46. It will be apparent that various vacuum tube of solid state type differential amplifiers may be substituted for the one which is illustrated in the drawing merely for purposes of explanation.
  • This invention utilizes the current-voltage characteristics of a space-charge limited current diode to select and maintain a desired sublimation pumping speed.
  • the potential difference V can be expressed as the sum of the applied cathode to anodg voltage V, and the average anode electron work function 0, in volts.
  • the electron work function is defined as the energy required to remove an electron in the Fermi energy state to a point outside the metal and beyond the influence of its electric fields.
  • Adsorbed gas layers then can either increase or decrease the apparent electron work function, depending upon whether the adsorbed gas is electronegative or electropositive. To increase the apparent electron work function the adsorbed gas layer must be electronegative. Examples of such gases are oxygen and nitrogen.
  • the very high electric fields existing between the metal anode surface and the negatively charged adsorbed layer retard electrons from leaving the surface, thereby increasing the work function of the surface.
  • the average anode electron work function may be expressed by the equation wherein 0,, is the electron work function of the clean metal anode surface, fl, is fraction of the unit surface area free of an adsorbed gas layer, and jiis the fraction of the anode surface that has a work function Q,,- Thus, if Brepresents the fraction of the unit surface area which is free of an adsorbed gas layer and 1-8) is set equal to Efl-Q then 9 min QB wherein Q is the electron work function for the adsorbed gas layer, in volts, and
  • the potential difference across the bridge circuit is related to the extent of gas adsorption on the anode and the extent of deposition of vaporized metal film on the anode.
  • the amplified bridge output voltage can no longer fire the appropriate silicon controlled rectifier. Accordingly the source of current for the vaporizable metal rod 24 is cut off and metal vapor deposition is stopped. Adsorption of a next subsequent gas layer on the clean anode surface then proceeds.
  • the pump also may be adjusted to increase or decrease the pumping speed. It has been shown hereinbefore that the pumping speed is proportional to the fraction of the area of the anode not covered by an adsorbed gas layer and that the latter is a function of the diode current density.
  • the bridge voltage across the junctions 90 and 72 is not sufficient, when amplified, to fire the silicon control rectifier and that the pumping speed is S
  • the diode current density 1. must be increased to a valve 1, This is accomplished by decreasing the resistance of the reference resistor 76 to increase the potential difference across the junctions 90 and 72 to a magnitude which, when amplified, effects firing of the silicon controlled rectifiers.
  • the resulting deposition of metal vapors results in an increase in the fraction of the anode area not covered by the adsorbed gas layer and a corresponding increase in the pumping speed.
  • the amplified bridge output voltage can no longer fire the silicon controlled rectifier and metal vapor deposition is stopped.
  • the resistance of the reference resistor 76 is increased. This increases the magnitude of the bridge output voltage, but the polarity at junction 90 is reversed from negative to positive with respect to the potential at junction 72.
  • the diode 96 functions to prevent large grid currents when the grid is positive with respect to the cathode of the input tube of the amplifier 92. The amplified output from the amplifier is insufficient to effect firing of the silicon controlled rectifiers.
  • the pumping speed will be reduced from the valve of S I to S, and the current through the reference resistor 76 will be equal to the total space-charged limited current diode. Further adsorption of gas causes a negative output voltage from the bridge sufficient to effect firing of the silicon controlled rectifiers and consequent vapor deposition of metal on the anode.
  • the surface area of the anode is 0.081 square meter
  • the cathode to anode distance is 0.038 meter
  • the electron work functions for clean titanium is 5.8 volts
  • the electron work function for an adsorbed oxygen layer is 6.8 volts
  • the maximum diode current density at a maximum sublimation pumping speed of 1,215 liters per second is 0.015 amperes per square meter, providing cathode to anode voltage of 10.24 volts.
  • the total voltage applied across the bridge is 20.48 volts and each of the resistors is 8,450 ohms.
  • Power supply 86 supplies 36 volts and the differential amplifier provides an amplification of 1,000.
  • the operation of the sublimation pump is as follows: First pressure in the vessel to be evacuated, and hence in container 10, is reduced, either by a mechanical pump or an adsorption pump, to about i XlOtorr. Liquid nitrogen then is filled into the vessel 20 to cool the anode surface.
  • the differential amplifier 92 now is adjusted by first closing the switches 110, 112 and 114.
  • the potential difference across the resistor 116 is fixed at 0.005 volts and thus by means of the variable resistors 118 and 120 the output voltage of the amplifier is adjusted to five volts, as measured on the voltmeter 122.
  • the switches then are opened.
  • the bridge circuit now is adjusted first by setting the reference resistor 76 to a low pumping speed. With switches 98 and 100 in the positions illustrated, for operation with electronegative oxygen gas, switches 66, 84 and 94 then are closed and the resistance of the variable resistor 68 is decreased until further reduction in resistance does not increase the current measured by the meter 88.
  • the switch 110 in the amplifier circuit now is closed.
  • the voltmeter 122 will indicate the amplifier output to be about 9 to 10 volts.
  • the titanium filament power supply circuit 34 now is activated by closing e switch 50 to provide filament current of, for example, not more than about 50 amperes.
  • the variac 40 now is adjusted slowly to increase the filament current until it suddenly decreases to zero. This occurs deposition of vaporized titanium film on the cooled anode surface.
  • the reference resistor 76 of the bridge circuit now is adjusted slowly toward the desired pumping speed, care being taken to limit the output voltage of the amplifier to not more than about 11 volts.
  • the rate at which the output voltage of the amplifier decreases is controlled by the variac 40.
  • the pumping speed may be adjusted over a range by varying the cathode to anode voltage, by adjusting the reference resistor 76, as previously explained. Although the diode current density may be varied, care should be taken to limit its maximum value to prevent desorption of gas from the cooled surface due to electron impact.
  • the sublimation pump is turned 05 by first opening the switch 50 in the filament power supply 34 and then opening the switches 110, 84 and 66 in that order.
  • a sublimation pump comprising a. a container having an inner surface of electrically conductive metal and defining a chamber arranged for communication with a vessel to be evacuated,
  • cooling means in contact with the container for cooling the inner surface thereof
  • electric circuit means connected to the inner surfaces of the container and to the filament means and forming said inner surface and filament means as the anode and cathode, respectively, of a space-charged limited current diode
  • electric signal producing means having an input connected to said electric circuit means and operable upon changes in the current-voltage characteristics of the diode to produce an electric output signal
  • j. means connecting the output of the signal producing means to the electrical control means for activating the vaporizable metal heating means when the electrical output signal reaches a predetermined value.
  • the signal producing means includes an electrical bridge circuit in which the diode forms one branch, and an electric signal amplifier has its input connected to the output of the bridge circuit and its output connected to the electrical control means.
  • control means comprises electric switch means operable by the electric output signal of the amplifier.
  • the heating means includes an electric power supply connected to the rod,
  • the filament means is a coil surrounding the rod
  • the signal producing means includes an electric bridge circuit in which the diode forms one branch, and an electric signal amplifier has its input connected to the output of the bridge circuit and its output connected to the electrical control means, and
  • control means comprises a silicon controlled rectifier in the circuit of the power supply operable upon activation by the electric output signal of the amplifier to activate the power supply.
  • the sublimation pump of claim 1 wherein the vaporizable metal is an elongated rod, the heating means includes an electric power supply connected to the rod, and the filament means is a coil surrounding the rod.
  • control means comprises electric switch means in the circuit of the power supply operable by the electric output signal of the signal producing means.
  • a sublimation pump including a container having an inner surface of electrically conductive metal and defining a chamber arranged for communication with a vessel to be evacuated, cooling means in contact with the container for cooling the inner surface thereof, vaporizable metal in the chamber, means for heating the vaporizable metal to vaporize the latter for deposition on the inner surface of the container, and electrical control means for said heating means, the method of controlling automatically the adsorption of gas and deposition of vaporizable metal on said cooled surface, comprising a. placing a cathode filament in the chamber,
  • the method of claim 8 including the steps of utilizing the changes in current-voltage characteristics of the diode to produce an electric signal, amplifying said electric signal, and utilizing said amplified electric signal to operate the electrical control means for said heating means.

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  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Engineering & Computer Science (AREA)
  • Automation & Control Theory (AREA)
  • Compressors, Vaccum Pumps And Other Relevant Systems (AREA)
  • Electron Tubes For Measurement (AREA)
  • Particle Accelerators (AREA)
US17158A 1970-03-06 1970-03-06 Sublimation pump and method of regulating it Expired - Lifetime US3609063A (en)

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US1715870A 1970-03-06 1970-03-06

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JP (1) JPS5027883B1 (member.php)
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Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3371853A (en) * 1966-06-17 1968-03-05 Wisconsin Alumni Res Found Orbitron vacuum pump with getter vaporization by resistance heating
US3429501A (en) * 1965-08-30 1969-02-25 Bendix Corp Ion pump

Patent Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3429501A (en) * 1965-08-30 1969-02-25 Bendix Corp Ion pump
US3371853A (en) * 1966-06-17 1968-03-05 Wisconsin Alumni Res Found Orbitron vacuum pump with getter vaporization by resistance heating

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CA922798A (en) 1973-03-13
JPS5027883B1 (member.php) 1975-09-11
GB1254659A (en) 1971-11-24

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