US3442252A

US3442252A - High voltage d.c. converter cathode supply circuit having means for controlling the voltage to the cathode

Info

Publication number: US3442252A
Application number: US473965A
Authority: US
Inventors: James W Ackley
Original assignee: Varian Associates Inc
Current assignee: Varian Medical Systems Inc
Priority date: 1965-07-22
Filing date: 1965-07-22
Publication date: 1969-05-06
Anticipated expiration: 1986-05-06

Description

2 1 v j- R J- UKU Ir-rumor. Qflfinuu uww y 6, 1969 J. W. ACKLEY 3,442,252

HIGH VOLTAGE D.C. CONVERTER CATHODE SUPPLY CIRCUIT HAVING MEANS FOR CONTROLLING THE VOLTAGE TO THE CATHODE Filed July 22, 1965 INVENTOR;

ATTORNEY United States Patent U.S. c1. i1s--s0.1 12 Claims ABSTRACT OF THE DISCLOSURE A first rectifier connected to a high voltage transformer supplies current to a load. A second rectifier in parallel with the first one is connected to a large capacitor which absorbs the energy produced by the transformerreactance following an arc in the load, thus preventing the appearance of a high voltage spike across the load. A resistor across the capacitor dissipates the energy stored therein.

The present invention relates generally to high voltage D.C. converters adapted for higher power uses in a vacuum and more particularly to a converter employing an energy absorbing reactance that is isolatedfrom the high voltage load.

Recently vacuum vapor deposition by electron bombardment heating of an evaporant has been receiving increasingly greater attention because of requirements for extremely pure deposited films and layers. Also, electron beam heating affords the only practical method of'evaporating inany of the refractory material's, e.g., tungsten, because of the extreme temperatures required for vaporization.

To evaporate materials so they can be deposited on a substrate at a high rate by electron bombardment, it is necessary to utilize a higher power electron beam, frequently having 2,000 watts power or more at a potential of 4 kilovolts or more. An electron beam of such power, even in a vacuum less than 4x10" mm. of Hg, sometimes arcs between the electron source and the'evaporant when certain materials, e.g., quartz, are being deposited. The arcs are usually of short duration, between 10 and 1,000 rnicroseconds, and constitute momentary. short circuits Between the high voltage terminal at the electron emitting cathode and the ground potential at which the material being evaporated is maintained. In orderito extinguish these arcs in the fastest and most facile manner, the high voltage power supply for the cathode must be soft, i.e., be capable of delivering only a limited, relatively low short circuit currentfor the time period during which arcing takes place. If the power supply is not soft, but can deliver high short circuit currents, arcing can continue for prolonged time periods, whereby the power supply is possibly destroyed or the evaporant may become contaminated. For these reasons, prior art high voltage powersupplies for electron beam vacuum vapor deposition have generally employed high voltage transformers with considerable series inductance to make them current limiting.

A problem arising with the use of transformers having large series inductance values is in the extremely high voltages that occur in response to extinction of the are. When the arc is extinguished, the transformer current drops very suddenly and its magnetic field collapses to provide valves of Patented. May 6, 1969 "ice this approach is satisfactory for certain high voltage.

vacuum applications, it has not generally proven satisfactory for the high power cathode rays necessary for elec- T tron beam vapor deposition. Connecting a filter capacitor across the rectifier output has been found, through experimentation, to cause repeated additional arcs to occur after the first arc has been extinguished. Apparently, the heavy discharge currents that flow from the capacitor to the high voltage terminal in response to the high voltage spikes cause local heating about the cathode. The cathode local heating is sufiiciently intense to cause gas to be released into the vacuum chamber, whereby a momentary local decrease in the vacuum occurs, enabling an additional arc to be struck between the high voltage terminal and ground.

According to the present invention, a soft, high voltage power supply particularly adapted for electron beam vapor deposition is provided by connecting an auxiliary rectifier in parallel with the rectifier that supplies current to the high voltage output terminal. Connected across the output of the auxiliary rectifier is an energy absorbing capacitor. In response to arc extinction, the capacitor absorbs energy from the current limited power supply to prevent the power supply voltage from becoming excessive. Since the absorbing capacitor is isolated from the high voltage output terminal by the rectifier, its discharge current is not coupled to the high voltage terminal.

It has been found through tests conducted that repeated arcing is substantially eliminated by employing the isolated capacitor of the present invention when evaporating virtually all materials in vacuums of 4 X 10- mm. of Hg.

A further feature of the present invention resides in the use of all solid state components. The rectifiersemploy high voltage semiconductor diodes,that reduce space and input power requirements. The energy absorbing capacitor prevents the large over-voltage, discussed supra, from occurring, whereby presently available semiconductor diodes can be used. If the isolated energy absorbing capacitor is not employed, i.e., the main rectifier operates unfiltered, the peak voltage rating of present-day, relatively inexpensive solid state diodes is exceeded in response to the derivation of high 7 voltage spikes from the transformer.

Another feature of the invention is that the total power supplied by the electron beam to the evaporant is varied by a single control parameter, the current supplied to the filament of the electron gun. The potential delivered to the high voltage output terminal by the supply remains sulficiently constant, at the same value of approximately 4,000 volts, over the 0% ampere range of the beam cur rent, to enable a single knob that varies beam current to be utilized as the only control required to vary deposition rates.

It is, therefore, an object of the present invention to provide a new and improved A.C to high voltage D.C. converter.

Another object of the invention is to provide a new and improved soft. high voltage power supply, particularly adapted for use in producing electron beams necessary for vacuum vapor deposition.

An additional object of the invention is to provide a soft high voltage power supply adapted for deriving relativelyhigh power electron beams used in vacuum vapor deposition and characterized by its arc suppressing qualities- Yet another object of the invention is to provide a set high voltage DC. power supply for use in electron beam vacuum vapor deposition systems, wherein the energy from the high voltage spike that occurs in response to are extinction is absorbed in a manner that prevents recurring arcs from being formed.

Still an additional object of the invention is to provide a new and improved sof high voltage DC. power supply having only solid state elements so that size and power requirements are minimized.

Yet a further object of the invention is to provide a new and improved soft high voltage DC. power supply for use in electron beam vacuum vapor deposition systems, wherein .the'potential from the high voltage. spike that occurs in response to arc extinction is' attenuated sufficiently to enable semiconductor rectifying diodes to be employed.

Another object of the present invention is to provide a new and improved high voltage power supply adapted for use in deriving high power electron beams, wherein the high voltage is regulated suffiiently to enable beam current control to be maintained with a single knob.

The above and still further objects, features and advantages of the present invention will become apparent upon consideration of the following detailed description of one specific embodiment thereof, especially when taken in conjunction with the accompanying drawings, wherein:

The single figure is a schematic diagram of a preferred embodiment of the invention.

Referring now specifically to the single figure, there is illustrated vacuum vapor deposition chamber 11, maintained during operation at a vacuum of no less than 4X l mm. of Hg by a vacuum pump, not shown. Within chamber-11, is electrically grounded metal crucible 12 that serves as a target electrode for an electron beam 14 that is derived from electron gun 15. Within crucible 12, there is contained evaporant material 13 that isvaporized in response to the kinetic energy of electron beam 14. impinging thereon. Electron beam 14 is focused; by conventional means, not shown, to heat material 13- until a puddle is formed. From the puddle, there is derived very pure vapor that drifts upwardly to coat substrate 16, in a a manner well known to those skilled in the art.-

Cathode 15 is maintained at a DC. potential of approximately -4,000 volts by the power supply that comprises the present invention and is connected via a suitable feedthrough into chamber 11. The high voltage DC. potential is derived from a suitable, unregulated AC. low voltage source, such as a 230 volt, single phase, 60-cycle A.C power supply, applied to terminals 21. The AC. voltage across terminals-21 is applied, in parallel, to variable auto-transformer 22 and primary winding 23 of transformer 24. Winding 23 is coupled through core 25 to secondary winding 26, across.which is generated an A.C. voltage of approximately 4.000 volts RMS'. 1

Core 25 is such that transformer 24 is characterized as having a relatively large inductance, whereby the maxi mum short circuit current derived is approximately 2 amperesJHence, transformer 24 is considered as a. soft supply.

The high voltage A.C. across secondary 26 is applied in parallel to full

wave rectifying bridges

27 and 28. Each of

bridges

27 and 28 includes four semiconductor diodes 29 poled so that terminals 31 are maintained ;approxi mately 4,000 volts negative with respect to terminals 32 thereof. Connected in parallel across

output terminals

31 and 32 of bridge 28 is a two microfarad, 10,000-volt energy absorbing capacitor 33 and 700,000 ohm, 20-watt bleeder resistor 34. Terminals 31 and 3-2 of bridge 27 are con= nected unfiltered between ground and cathode 15 through 4 1 electron current monitoring rnilliammeter 3-5, having a full scale deflection of 500 milliamps.

Heating current, of up to 25 amperes at 6 volts AC. is supplied across cathode 15 from the tap on auto-transformer 22 through stepdown transformer 36. One end of secondary winding 37 of transformer 36 is connected to terminal-=31 of bridge 27. As the setting of the tap on auto-transformer 22 is varied, the heating current to cathode 15 is changed, whereby the current in electron beam 14 is altered, as is the rate of vaporization from material 13 in crucible 12. The power supply has sufficient self-regulation to maintain terminal 31 ata relatively constant DC potential of -4,000 volts for all values of beam current from 0 to 0.5 ampere. Therefore, 7

it has been found that the tap on transformer 22 is the only control required to provide the full range of beam current values necessary to vaporize material 13 for many different deposition rates. In normal :operation, electron beam current flows from cathode 15 to its target, evaporant 13. Electron beam current flow varies from virtually 0 ampere to its maximum value, as determined by the setting of the tap for transformer 22 during each half cycle of the AC. source because of the unfiltered nature of the supply.

As described supra, this causes heating of evaporant and deposition on substrate 16. Occasionally, and for many different reasons, arcs are struck' betweent cathode 15 and evaporant 13. These arcs are of short duration, lasting between 10 and 1,000 microseconds, and are limited to two amperes because transformer 24 comprises a soft supply. The voltage between cathode 15 and target 13 drops almost to zero during the occurrence of an arc. The low cathode voltage does not enable the are to be maintained for prolonged time periods, so extinction occurs within the stated interval.

When each arc is extinguished, there is considerable energy stored in, core 25. Because arc extinction is very rapid, the stored energy has a tendency to induce a very large voltage spike across transformer secondary 26. The spike, regardless of polarity, is rectified by bridge 28 and'absorbed by the very low impedance of capacitor 33 to its large amplitude, high frequency components. As the voltage across secondary 26 returns to its normal quiescent value, the charge stored by capacitor 33 leaks through bleeder resistor 34 so that the capacitor can suppress another spike. Because capacitor 33 cannot discharge through bridge 28, due to the polarity of diodes 29 in the bridge, there is no current surge at cathode 15 shorty after arc extinction and a further arc cannot be triggered. Capacitor 33 is sufficiently large to prevent the voltage spike across winding 26 from ever exceeding 6,000 volts so that present state of the art semiconductor diodes, such as type l44 6-C, available from Diodes, Incorporated, can be employed.

While I have described and illustrated one specific embodiment of my invention, it will be clear that variations of the details of construction which are specifically illustrated'and described may be resorted to without departing from the true spirit and scope of the invention as defined in the appended claims.

I claim: I

1. A system for coverting power from an A.C. source to high voltage DC. to be delivered to' a load comprising a transformer having: a primary winding adapted to be connected to said source, secondary winding means across which is derived high voltage AC, and a core; first and second rectifiers connected in parallel across said second- 3. The system of claim 2 wherein said rectifiers are full wave bridge rectifiers.

4. A system for converting power from an AC. source to high voltage to be delivered to a load comprising a transformer having: a primary winding adapted to be connected to said source, secondary winding means across which is derived high voltage A.C., and a core, a rectifier connected across said secondary winding means .for connecting high voltage deriving from said secondary winding means to said load, and an energy absorbing reactance connected across the output of said rectifier, said rectifier being poled to couple energy spikes from the transformer to said reactance and to prevent discharge of energy from said reactance to said load.

5. A system for converting power from an AC. source to high voltage for biasing an electron gun that emits a high powered electron beam directed toward a target within a vacuum chamber comprising a transformer having: a primary winding adapted to be connected to said power source, secondary windings means across which is derived high voltage AC, and a core, said transformer limiting electron beam current in the event of arcing between the cathode and the target and in which energy is stored during said arcing; means for coupling high voltage deriving from said secondary winding means to said cathode so unfiltered electron beam current flows from the cathode to the target during at least every other half cycle of said A.C. supply, rectifying means connected across said secondary winding means, said rectifying means being separately responsive to the voltage across said secondary winding means from the voltage coupled by said secondary winding means to said cathode, a capacitor connected across said rectifying means for ab sorbing energy deriving from said transformer in response to are extinction, said rectifying means being poled to coupled energy from the transformer to said capacitor and to prevent energy stored in said capacitor from being coupled to said cathode.

6. A system for converting power from an A.C. source to high voltage for biasing an electron gun that emits a high powered electron beam directed towarda target within a vacuum chamber comprising a transformer having: a primary winding adapted to be connected to said power source, secondary Winding means across which is derived high voltage AC, and a core, said transformer limiting electron beam current in the event of arcing between the cathode and the target and in which energy is stored during said arcing; means for coupling high voltage deriving from said secondary winding means to said cathode so unfiltered electron beam current flows from the cathode to the target during at least every other half cycle of said A.C. supply, a reactance for absorbing energy deriving from said transformer in response to are extinction, and means responsive to the high voltage A.C. across said secondary winding for coupling energy deriving from the transformer as a result of arc extinction to the reactance and preventing coupling of energy from the reactance to the cathode, said last-named means being separately responsive to the high voltage A.C. across said secondary winding from the voltage coupled to said cathode.

7. A system for converting power from an AC. source to high voltage D.C. for biasing an electron gun that emits a high powered electron beam directed toward a target within a vacuum chamber comprising a transformer having: a primary winding adapted to be connected to said power source, secondary winding means across which is derived high voltage A.C., and a core, said transformer limiting electron beam current in the event of arcing between the cathode and the target and in which energy is stored during said arcing; first rectifying means c0n nected across said secondary means for deriving a high voltage unfiltered rectified replica of the AC. voltage source, means for supplying said replica to the cathode, second rectifying means responsive to the high voltage A.C. across said secondary winding means, said second rectifying means being responsive to the high voltage A.'C. across said secondary winding means separately from the voltage coupled to said first rectifying means, a capacitor connected across said second rectifying means for absorbing energy deriving from said transformer in response to are extinction, said second'rectifying means being poled to couple energy from the transformer to said capacitor and to prevent energy stored in said capacitor from being coupled to said first rectifying means.

8. The system of claim 7 further including stepdown trans-former means responsive to said AC. power source for deriving heating current for said cathode, means responsive to said stepdown transformer means for cou-.

pling said heating current to said cathode, said stepdown transformer means including means for varying the heating current supplied to said cathode as the only control parameter of the system.

9. The system of claim 8 wherein said first rectifying means comprises a full wave bridge rectifier having: a pair of input terminals connected across said secondary winding means and a pair of output terminals connected between said cathode and target; and a current measuring meterfor monitoring the current of the electron beam, said meter being connected in series circuit with said pair of output terminals, said cathode and said target.

10. The system of claim 7 wherein said second rectifying means comprises a full wave rectifying bridge having a pair of input terminals connected across said secondary winding means and a pair of output terminals; said capacitor being connected across said output terminals, and a bleeder resistance for said capacitor connected in parallel with said capacitor.

11. In combination, a transformer having: a low voltage primary winding, a core, and a high voltage secondary winding; first and second full wave rectifier bridges having their inputs connected in parallel across said sec,- ondary winding, the parallel combination of a resistor and capacitor connected across the output terminals of said first bridge, and a pair of high voltage output terminals connected to the output terminals of said second bridgel 12. In combination, a transformer having: a low voltage ,primary winding, a core, and a high voltage secondary winding; first and second full wave rectifier bridges having their inputs connected in parallel across said secondary winding, the parallel combination of a resistor and capacitor connected across the output terminals of said first bridge, a vacuum vapor deposition chamber having a target adapted to carry an evaporant, a cathode for emitting an electron beam to heat said evaporant to vaporization, and means for applying the unfiltered voltage deriving from said second bridge across said cathode and said target.

References Cited UNITED STATES PATENTS 12/1949 Zavales 250 2/1963 Hanks et al. 3l5--107 US. Cl. X.R.

219-12l; 250-495; 31510l, 107, 205; 32l10; 323-8