US 3233137 A
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Description (OCR text may contain errors)
1966 G. s. ANDERSON ETAL 3,233,137
METHOD AND APPARATUS FOR CLEANSING BY IONIC BOMBARDMENT 2 Sheets-Sheet 1 Filed Aug. 28, 1961 FIG. I
,H MIHL l INVENTORS GERALD S. ANDERSON ROGER M. MOSEZON ATTORNEY United States Patent Office 3,233,137 Patented Feb. 1, 1966 3,233,137 METHOD AND APPARATUS FOR CLEANSING BY IONIC BOMBARDMENT Gerald S. Anderson, St. Paul, and Roger M. Moseson, Minneapolis, Minn., assignors, by mesne assignments, to Litton Systems Inc., Beverly Hills, Califl, a corporation of Maryland Filed Aug. 28, 1961, Ser. No. 134,457 4 Claims. (Cl. 313-401) This invention relates to a method and apparatus for cleansing by ionic bombardment, and more particularly to such a method and apparatus for cleansing nonconductive surfaces.
The technique of sputtering, which has been known for many years, involves the removal of atoms from a material as a result of bombardment of the material by particles such as ions or atoms. It may be used for atomically cleaning the surface of a material, for etching the surface, or the sputtered material may be deposited as a coating on a nearby surface if desired. Sputtering may also be used for other purposes such as to provide samples of material for spectroscopic examination.
Sputtering has been accomplished in the past by directing a beam of energetic particles, either neutral or charged, at the target material. However, it is difficult to obtain beams with sufiicient intensity to remove atoms from the target material at a rate which is satisfactory for the deposition of the material on another surface to form a film or the like.
Another method of sputtering, which is considerably faster than that using a beam of particles, involves immersing the target in a gas discharge plasma, which may be produced by various methods such as a glow discharge, a low-pressure discharge with an electron source (such as a mercury pool or a thermionic cathode) or by a highfrequency discharge. The plasma in which the target is immersed is composed of ions, electrons and neutral atoms. When a potential that is negative with respect to the plasma potential is placed in the target, the potential difference appears across the Langmuir sheath which forms around the target, and the target is bombarded by ions of a kinetic energy equal to this potential difference. Utilizing this technique, a metallic target can be sputtered very readily. However, it has not been possible to sputter nonconductors by this method because a positive charge builds up on the target surface in a very short time and reduces the potential difference between the target surface and the plasma to such a degree that it is too small for any appreciable sputtering. However, by connecting a source of alternating potential between the electrode that holds the target and one of the other electrodes of the apparatus, a nonconductive material may be sputtered. This technique is described in detail in copending application Serial No. 134,458 filed concurrently.
In utilizing either the direct or alternating potential technique, there is a tendency for the sputtered material to be deposited on the surface of the envelope within which the process takes place. Such envelopes are usually nonconductors and are often transparent so that the process may be observed. Therefore, any deposition on the envelope which decreases its transparency is undesirable. Heretofore, it has been necessary to dismantle the apparatus in order to remove the deposit.
Such deposits tend to form not only in sputtering apparatus, but in virtually any gas discharge apparatus where sputtering occurs as a secondary or unwanted effect. For example, in the ordinary fluorescent lamp, dark deposits tend to form at each end of the glass envelope which consist of material sputtered from the lamps electrodes. Thus far, there has been no practical way of removing such illumination-reducing deposits.
Accordingly, a primary object of the present invention is to provide a method and apparatus for preventing, or, if already formed, removing deposits that tend to form on the enclosing envelope of gas discharge apparatus.
Briefly speaking, the present invention utilizes ions existing in the gas discharge plasma of an apparatus or device to sputter off an undesired deposit from the envelope of the device. This is accomplished by placing an electrode, preferably transparent, adjacent the envelope where the deposit is located or tends to form, and impressing an alternating potential between that electrode and another electrode of the device. Thus, the deposit and the envelope are not only bombarded by positive ions but are also bombarded by electrons for part of each cycle, which prevents a residual or permanent positive charge from building up on the surface of a nonconducting envelope. The deposit is quickly sputtered away.
Other objects and features of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, in which:
FIGURE 1 is a perspective view of a low-pressure mercury plasma tube to which the present invention may be applied;
FIG. 2 is a vertical sectional view of the tube shown in FIG. 1;
FIG. 3 is a horizontal section view taken on the line 33 of FIG. 2;
FIGS. 4 and 5 are circuit diagrams for operating tubes according to the invention; and
FIG. 6 is a fragmentary perspective view of a fluorescent lamp to which the invention is applied.
Although the present invention is illustrated and will be described in conjunction with a low-pressure mercury plasma tube, it is to be understood that it is not limited thereto. The teachings of the invention may be applied with corresponding advantages and benefits to any apparatus or device which utilizes a gas discharge plasma in its operation. Furthermore, the invention is not limited to the use of either a direct current or an alternating current discharge device.
FIG. 1 illustrates a low-pressure mercury plasma tube 10 in conjunction with which the invention will be described. Such a tube is described in detail in a co-pending application of Gottfried K. Wehner, Serial No. 103,- 056, filed April 14, 1961, now Patent No. 3,100,272, and entitled low Pressure Mercury Plasma Discharge Tube. Therefore, it will be described here in general terms only. The tube 10 has an upper chamber or envelope section 11 and a lower section or tank assembly 12 and is provided with a large-diameter conduit 13 which connects the interior of the tank assembly 12 to a mercury diffusion pump (not shown). The tank assembly 12 is also provided with water inlet and outlet means 14 and 15, respectively and with gas inlet means 16.
As best seen in FIG. 2, the tank assembly 12, which may be made of a conducting material such as stainless steel, includes three cylindrical walls 17, 18, and 19, with the walls being spaced apart to form reservoirs 21 and 22 therebetween. The water inlet and outlet pipes 14 and 15 extend through the outer cylindrical wall 17 and provide means for circulating water in the reservoir 21 to regulate the internal temperature of the tube.
The reservoir 22 formed between the walls 18 and 19 contains a mercury pool 23, in the present embodiment of the invention. An igniter electrode 24, making coutaet with the mercury pool 23 is connected to an electrical lead 25.
In order to provide suitable means for mounting the envelope section 11, the Wall 18 is provided at its upper end with a shoulder portion 26. The shoulder portion 26 has an annular groove 27 which contains an O-ring 3 seal 23, and the envelope 11 is firmly but releasably positioned on the O-ring in the recess. Thus, a gas tight connection is provided between the tank assembly 12 and the envelope 11.
The envelope 11 may be of conventional construction, and is customarily of transparent construction such as a Pyrex glass bell jar. Also, it may be provided with a reentrant cold trap cavity 30 extending downwardly into the envelope and so constructed that it can receive and retain a cooling material such as liquid nitrogen.
The interior of the envelope 11 is separated from the interior of the tank assembly 12 by a circular plate 31 which is located in a seat formed in the shoulder portion 26 of the cylindrical wall 18. The plate 31 is insulated from the shoulder by conventional insulating material, and has a mesh graphite grid electrode 32 positioned in a central aperture formed therein. The grid is electrically connected to a lead 33.
A plurality of electrodes are supported by the tank assembly 12, some of which are located within the envelope portion 11 and some of which are located within the tank assembly 12. In effect, those electrodes located within the envelope 11 form the anode assembly, and those located within the tank 12 form the cathode assem- Located within the cathode space in the base structure 12, along with the igniter electrode 24 previously mentioned, is an auxiliary anode 34 which is electrically connected to a lead 35.
Located within the anode space in the envelope portion 11 of the tube are a main anode 36 which is electrically connected to a lead 37, and a target electrode 38, which is adapted to hold a target material 40 to be sputtered. The target electrode 38' is electrically connected to a lead 41. The anode space may also contain an insulating support 42 for a plate or substrate on which material sputtered from the target 41) is to be deposited.
The electrical leads from all of the electrodes thus far described extend out of the base portion 12 of the tube through suitable conventional insulators. In addition, the leads 25, 35, 37 and 41 to the igniter 24, auxiliary anode 34, main anode 36 and target electrode 38 may be of sufiicient strength to support the electrodes physically.
In the course of operation of the tube as thus far set forth, which operation will be later described, material sputtered from the target virtually fills the interior of the envelope 11 and tends to be deposited on the envelope as well as on the substrate. This deposition occurs at such a rate that after several hours of operation the envelope becomes virtually opaque and the apparatus must be dismantled in order to remove the deposit. In accordance with one of the teachings of the invention, such deposition may be removed or a deposit prevented from forming by placing an electrode 50 (having a lead 51) adjacent the outer surface of the envelope where it is desired to keep the envelope transparent.
The structure of the electrode 50 is not critical, and it may take the form of a fine wire screen taped or otherwise secured to the envelope. Alternatively, it may be a transparent conductive paint of conventional type, or it could take the form of a conductive plate provided with a suitable handle so that it could be periodically held adjacent the envelope.
Electrical connections to the apparatus for operation as a direct current discharge device are shown in FIG. 4, which also shows a grounded lead 53 that is connected to the base structure 12 of the tube. The lead 33 from the graphite grid electrode 32 is connected through a variable resistor 54 to the positive side of a conventional 30-volt direct current (DC) power supply, the negative side of which is grounded. The lead 25 from the igniter electrode 24 is connected to one terminal of a two-way switch 55, and another terminal of the switch is connected to the positive side of a conventional ZOO-volt DC. power supply, the negative side of which is grounded.
4 The lever of the switch 55 is connected to ground through a capacitor 56. The lead 35 from the auxiliary anode 34 is connected to the positive side of the ZOO-volt power supply through a resistor 57, and the lead 37 of the main anode 36 is similarly connected through a variable resistor 58. The target electrode 38 is connected by means of its lead 41 through a resistor 59 to the negative side of a conventional 20-volt DC. power supply, the other side of which is connected to the main anode.
In accordance with one important aspect of the inven tion, an alternating electric field is applied between the external electrode 50 and the discharge plasma existing within the tube during operation. Thus, as shown in FIG. 4, the electrode 50 is cdnneeted by means of its lead 51 through an inductance 60 to one side of a conventional alternating current (A.C.) source, preferably of radio frequencies, the other side of which is connected through a DC. blocking capacitor 61 to the main anode lead 37.
Although the alternating current source is shown in FIG. 4 as being connected between the external electrode and the main anode, it has been found in practice that it may be connected between the external electrode and any of the electrodes in the tube or between the external electrode and ground. Also, it may be connected between two electrodes arranged on opposite sides of the envelope. There is a small capacitance existing between the electrode 50 and'the gas discharge plasma, which is shown in broken lines at 62 in FIG. 4, and the purpose of the inductance 60 is to form a series resonance circuit with that capacitance. Without the inductance 60 in the circuit, considerably more power is required for cleansing the envelope at a satisfactory rate. The invention is not limited to the use of a series inductance, as various known methods of impedance matching may be used to increase the power efficiency.
Although the exact range of usable frequencies for the AC. source has not been determined, the preferred range is in radio frequencies from about 25,000 cycles per second upwardly, and successful operation has been attained at a frequency of 50 megacycles. Neither the upper nor lower frequency limits are known, although it is known that as the frequency becomes lower, the rate of cleansing is reduced. A potential of as little as 30-40 volts peak-to-peak is adequate for the purpose, in the particular embodiment described.
In operation, after the envelope 11 is placed in position on the base structure 12 as shown in FIG. 2, the mercury diffusion pump is started to lower the pressure within the tube to about 13 microns of mercury. The gas discharge is started between the mercury pool 23 and the auxiliary anode 34 by means of a voltage pulse supplied to the igniter 24 by momentarily moving the switch 55 from its solid line position to its dotted line position. Thus the capacitor 56 discharges through the igniter 24 and mercury pool 23, and a direct current discharge of approximately 3 amps. at 15 volts potential drop is established between the mercury pool 23 and the auxiliary anode 34. The main anode 36 is connected through its lead 37 and variable resistor 58 to the ZOO-volt power supply, and the main discharge of approximately 5 amps. at 30 volts potential drop is established between the mercury pool 23 and the main anode. This causes the anode section of the tube, that is, the space inside the envelope 11 above the grid 32, to become filled with a low-pressure mercury plasma with a density of the order of 10 -10 ions per cubic centimeter. The target 40 attached to the target electrode 38 becomes surrounded by this highdensity plasma, and, because the target is slightly negatlve with respect to the main anode, it is bombarded by mercury ions. Part of the material sputtered from the target by the ionic bombardment is deposited on the substrate, while part tends to be deposited on the inside of the envelope 11. However, because of the electrode 50 and the potential impressed thereon, the envelope itself adjacent the electrode is subjected to ionic bombardment and any material from the target deposited on the envelope in that area is sputtered off.
Sputtering, and hence cleansing of the envelope occurs, if the material of the envelope is an insulator, because of the alternating potential impressed between the electrode 50 and one of the electrodes in the tube, which, in the present case, is the main anode 36. If a direct potential is used instead of an alternating potential, a positive charge builds up on the envelope surface, which reduces the potential drop between the surface and the plasma to such an extent that there is no appreciable bombardment. With the alternating potential, however, the envelope is not only bombarded by positive ions, but, during a positive portion of each cycle, it is bombarded by electrons, which prevents a positive charge from building up on the material. Thus, the envelope is sputter-cleansed for a major portion of each cycle of the alternating potential. The alternating potential may be approximately the same as the voltage drop between the anode and cathode of the tube or, in this case, about 30 volts. It is pointed out that it may not be necessary to apply the alternating potential continuously, and intermittent cleansing of the envelope may be sufiicient.
Although particular values of voltage and current have been mentioned, it is to be understood that these are illustrative only and the invention is in no way limited to any specific operating conditions.
FIG. 5 shows a modified circuit diagram wherein an alternating potential is placed on the target electrode as well as on the external electrode. Otherwise, the operation of the apparatus is the same as that previously described. It will be apparent, however, that by using an alternating potential on the target electrode, a nonconducting target may be sputtered in the same fashion as the inner surface of the glass envelope may be sputtercleansed.
FIG. 6 illustrates the teachings of the invention applied to removing a deposit from a fluorescent lamp. A conventional fluorescent lamp 70, having an electrode 71 and electrical connector 72, also has a deposit 73 inside its envelope caused by material sputtered from the electrode 71 during operation. To remove the deposit, an electrode 74 is placed adjacent the deposit, outside the envelope, and a source of alternating potential (not shown) is connected between the two electrodes 71 and 74. If the lamp is in operation to provide the plasma for the ionic bombardment of the deposit, only a small alternating field is required to remove the deposit. If the lamp is not in operation, the plasma may be produced by the applied alternating field itself, in which case a higher potential is required to produce the necessary plasma. It has been found that such a deposit may be removed in a matter of seconds. It is also pointed out that the alternating electric field may be applied between two electrodes on opposite sides of the lamp with the deposit therebeween.
Although several embodiments and modifications have been described, it is apparent that many more modifications may be made by one skilled in the art without departing from the scope and spirit of the invention.
1. In apparatus having a transparent envelope and electrode means for providing an electric gaseous discharge within the envelope whereby deposits tend to coat a given area of the inside surface of the envelope, a transparent electrode adjacent the outer surface of the envelope and approximately opposite to said given area coated with a deposit to be removed, and a source of radio frequency alternating potential connected between said transparent electrode and said electrode means to provide an alternating radio frequency electric field therebetween.
2. In an apparatus having a transparent envelope and electrode means for providing an electric gaseous discharge within said envelope, said gaseous discharge forming deposits covering an area of the inside surface of said envelope, the improvement for cleaning said deposits off said area of said inner surface which comprises:
a transparent electrode adjacent the outer surface of said envelope and opposite to said area of said inner surface covered with a deposit to be removed, and
a source of radio frequency alternating potential connected across said transparent electrode and said electrode means for establishing a radio frequency field in said envelope adjacent to said covered area to remove said deposit and clean said area.
3. The method of cleaning an apparatus including a nonconducting envelope containing an ionizable gas, said apparatus also including electrode means for establishing a gas discharge, said gas discharge including ions of said gas within said envelope, said gas discharge being effective to coat a selected area of the inside surface of said envelope with a deposit, which method comprises:
energizing said electrode means to provide said gas discharge in said envelope, and
establishing a radio frequency alternating field across said selected area to develop a radio frequency potential on said selected area and bombard said selected area with said ions so that said deposit is removed from said selected area.
4. A method of cleaning a deposit from a given area of the inside surface of an enclosure provided with electrode means for establishing an electric gaseous discharge therein, which comprises:
energizing said electrode means to provide said electric gaseous discharge in said enclosure, said discharge including ions;
positioning an electrode on the outside surface of said enclosure opposite to said given area of said inside surface of said enclosure, and
applying a radio frequency alternating potential across said electrode means and said electrode for bombarding said given area of said inside surface with said ions to clean said deposit from said inside surface.
References Cited by the Examiner UNITED STATES PATENTS 2,774,013 12/1956 Willoughby 313-20l X 2,877,338 3/1959 Berge 204-192 X 2,947,913 8/1960 Trostler 313-201 X JOHN W. HUCKERT, Primary Examiner.
BENNETT G. MILLER, DAVID J. GALVIN, JOHN W. HUCKERT, Examiners.