US3532615A - Rf cathode sputtering method - Google Patents

Description

Oct. 6, 1970 p, DAVIDSE ETAL 3,532,615

RF CATHODE sPiJTTERING METHOD Original Filed Jan. 28. 1965 2 ShetsfSheet 1 FIG. 1

v I "Hill in! KL M n LEW n Film boom/am RETURN VACUUM PUMP a 1 1 l l l I I I INVENTORS PiETER DI DAVIDSE LEON I. MAISSEL I 1 BY I I AT ORN United States Patent 3,532,615 RF CATHODE SPUTTERING METHOD Pieter D. Davidse and Leon I. Maissel, Poughkeepsie,

N .Y., assignors to International Business Machines Corporation, Armonlr, N.Y., a corporation of New York Original application Jan. 28, 1965, Ser. No. 428,733, new Patent No. 3,369,991, dated Feb. 20, 1968. Divided and this application Dec. 22, 1967, Ser. No. 692,855

Int. Cl. C23c 15/00 US. Cl. 204192 5 Claims ABSTRACT OF THE DISCLOSURE A method of applying an insulating layer to the surface of a substrate wherein the substrate is mounted on an anode electrode and a target of the material to be applied is mounted on a cathode electrode spaced from the anode electrode in a chamber containing a gas at a pressure less than 30 microns of mercury which is adapted to support a glow discharge. A high frequency alternating voltage is applied across the anode electrode and cathode electrode to thereby generate a glow discharge which results in a bombardment of the target by gas ions during periods when the target is at a negative potential with respect to the ions.

CROSS-REFERENCE TO RELATED APPLICATION This application is a divisional application of Ser. No. 428,733, filed Jan. 28, 1965 now Pat. No. 3,369,991.

BACKGROUND OF THE INVENTION This invention relates to the sputtering of solid dielectric materials.

The process of sputtering involves exposing a material to be sputtered (called the target") to bombardment by ions in a low-pressure gaseous glow discharge, thereby causing atomic particles of the bombarded material to be dislodged and deposited upon an object which is to be coated. The sputtering of metallic substances is well understood and has been accomplished with relative case, but prior attempts to sputter dielectric materials have been of questionable utility. A dielectric target, by its very nature, tends to acquire a repellant charge when bombarded by ions, and if this charge reaches a suflicient level, it will inhibit the glow discharge and prevent sputtering action. To sputter a dielectric material, therefore, the charge that tends to gather on the target must be dissipated or prevented from building up in order that the necessary ion sheath, with its concentrated bombarding action, may be formed around the target. The unwanted ion-repelling charge may be eliminated by using high-frequency alternating-current excitation instead of the usual direct-current excitation, thereby causing a discharging action to occur during alternate half-cycles at a rate suificient to prevent the build-up of a substantial repellent charge on the target. In the past, however, the use of high-frequency excitation for this purpose has been found unsatisfactory for the following reasons:

In a dielectric sputtering apparatus the excitation cannot be applied directly to the dielectric target, so it is applied to a, metal electrode that adjoins the target. If this electrode is positioned within the ionization enclosure, then it must be adequately shielded from bombardment by the gas ions; otherwise the metal in the electrode will sputter off and contaminate the dielectric material which is being sputtered. When high-frequency excitation is employed, the presence of an anti-sputtering shield near the electrode creates an undesirable capacitive coupling between the electrode and the grounded shield, and if this coupling is too great, it will inhibit the necessary 3,532,615 Patented Oct. 6, 1970 ionization and sputtering actions. If the distance separating the shield from the electrode is increased in order to reduce the capacitive coupling between them, this may defeat the purpose of the shield by permitting the electrode to be bombarded by ions which will cause the metal to sputter 011? and contaminate the sputtered dielectric material. Alternatively, it has been proposed to construct the dielectric target as the envelope enclosing the ionization chamber, with the metal electrode being positioned on the outer surface of this envelope where it has no adverse effect upon the sputtering process. The choice of dielectric target materials is very limited if one must use the envelope of the chamber as a target. Materials which are suitable for use in making an enclosure do not provide satisfactory sputtered coatings of the type contemplated by the present invention. As a. practical matter, therefore, it is preferable not to use the envelope of the ionization chamber as the target.

SUMMARY OF THE INVENTION An object of the present invention is to provide an improved dielectric sputtering apparatus which enables the target to be made of any desired solid dielectric material without requiring that such material also be used as the enclosing envelope for the ionization chamber.

A further object is to provide an effective anti-sputtering shield for the gas-contacting surface of a target electrode which is contained within the vacuum chamber of a high-frequency dielectric sputtering apparatus, said shield having no adverse effect upon the ionization process notwithstanding the inherent capacitive impedance that may exist between the shield and the electrode.

Another object is to provide a novel dielectric sputtering apparatus having a commercially acceptable yield greatly exceeding that obtainable by any prior means.

Still another object is to provide a radio-frequency dielectric sputtering apparatus characterized by ease of tuning and high stability.

An important feature of the invention is the spacing that is provided between the exposed surface of the target electrode and its shield. It has been found that when this spacing lies within a certain critical range, hereinafter specified, an optimum condition is achieved whereby the radio-frequency capacitive coupling between the electrode and its shield is low enough to be disregarded; yet there is no perceptible tendency to sputter metal or other contaminants from the electrode. The invention also involves an optimum configuration of the shield in relation to the target for obtaining uniform deposition of the sputtered material. Still further improvement is obtained by applying to the gas in the ionization chamber a steady magnetic field which enhances the ionizing action, this magnetic field also having the advantage of improving (in a hitherto unrealized manner) the stability of the glow discharge and the ease with which the radio-frequency power supply can be tuned and matched to the load.

BRIEF DESCRIPTION OF THE INVENTION The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of a preferred embodiment of the invention, as illustrated in the accompanying drawings.

In the drawings:

FIG. 1 is a vertical view, partly in section, of a dielectric sputtering apparatus which embodies the invention.

FIG. 2 is a vertical sectional view showing in greater detail the shielded electrode structure on which the dielectric target is mounted.

FIG. 3 is a horizontal view taken on the line 3-3 of FIG. 1, showing the articles to be coated as they may be positioned in relation to the other electrode of the sputtering apparatus.

DESCRIPTION OF THE PREFERRED EMBODIMENT The embodiment of the invention disclosed herein is adapted to coat an article such as a silicon wafer with insulating material sputtered from a dielectric target. It is contemplated that the invention also will find a variety of other uses, wherever it is feasible to deposit dielectric materials by this sputtering method.

In any sputtering operation, the active agent is a glow discharge maintained between spaced electrodes in a suitable gaseous medium. In a direct-current sputtering operation the target is the negative electrode or cathode. In an alternating-current sputtering operation, as will be explained presently, the target may be referred to as a cathode, even though it is not always negative. Under the influence of the electric field established between the electrode, ionization of the gas is produced by the collision of free electrons with the gas molecules, producing positively charged gas ions. These ions are attracted toward the cathode, thereby creating what is known as an ion sheath around the cathode. Where a glow discharge exists, the region around the cathode which contains this concentration of ions also is known as the Crookes dark space. Within this region the ions are subjected to a high potential which accelerates them toward the cathode so that they bombard the target with sufficient impact to eject atomic particles therefrom. These ejected or sputtered particles of target material will be deposited upon nearby objects. The apparatus may be so designed, for example, that the sputtered material will deposit upon articles that are mounted on the opposite electrode or anode of the apparatus.

As explained hereinabove, direct-current excitation cannot be successfully employed in the sputtering of an insulating or dielectric material, because when such a material is bombarded by positive ions, it will build up a positive charge that repels the ions so that they cannot bombard the target with sufficient impact to cause sputtering. Hence, it is necessary to employ an alternating current excitation when sputtering a dielectric material, in order to discharge the target periodically. Sputtering action takes place during those periods when the target is at a sufficiently negative potential with respect to the glow discharge. During the intervening periods, when the polarities of the electrodes are reversed, electrons are attracted to the target for removing the positive ion-repelling charge therefrom. Due to the fact that the electrons have greater mobility than the ions, there will be a tendency for many more electrons than ions to flow toward the target, but inasmuch as there cannot be any net direct current flowing through a dielectric, the target will bias itself negatively in a suflicient amount to prevent any net direct current from flowing (assuming that the target is the only path through which current can flow between the electrodes). In this negatively biased condition the target is analogous to the cathode in a direct-current glow discharge apparatus; hence the term cathode is applied to it.

In order to maintain a glow discharge with a dielectric target or cathode, the frequency of the applied voltage must be high enough so that the number of ions reaching the target during the negative half-cycle is not sufficient to neutralize the desired negative charge on the surface of the target. If the target were to acquire a substantial positive potential, this would cause reverse sputtering of the object being coated, as well as undesirable sputtering of metal parts associated with the electrode which normally functions as the anode. It has been found that a radio-frequency excitation in the low megacycle range gives the best results. With the properly selected frequency and magnitude of applied voltage, the sputtering action will be confined to the dielectric target, and the anode will not reach a sufficient high negative potential at any time to produce reverse sputtering or other undesired sputtering effects. Under these conditions the target, being negatively charged most of the time, performs a function analogous to that of a cathode in direct-current sputtering, and for that reason it may be referred to herein as the RF cathode, while the opposite electrode is called the RF anode. The glow discharge maintained by the applied radio-frequency excitation has the familiar characteristics of a direct-current glow discharge, including the existence of a Crookes dark space adjoining the RF cathode. The thickness of this dark space is inversely proportional to the gas pressure. At a pressure of around 20 microns, for example, the dark space thickness is about inch (assuming the absence of an applied magnetic field, the effect of which will be explained hereinafter).

In any sputtering operation it is desirable, of course, that only the target material be consumed. Therefore, it is necessary to protect other parts of the structure associated with the cathode from the bombarding action of the ions. This customarily is done by placing a grounded shield on one side of the cathode opposite the side thereof on which the target is mounted. This prevents the ion sheath from extending back of the cathode and thereby confines the bombarding action of the ions to the target itself. However, in sputtering a dielectric material using radio-frequency excitation as contemplated by the present invention, the shielding of the cathode becomes a difficult problem. In order that the dielectric target may serve as an RF cathode, it must be mounted on or positioned next to a metal electrode which is connected to the ungrounded side of the radio-frequency power source. If a grounded shield is placed near this electrode in order to prevent the electrode metal from being sputtered, an undesirable capacitive coupling may exist between the electrode and the shield at the high frequency of the applied excitation. This capacitive coupling will tend to bypass the applied excitation to ground and prevent the gas from being ionized sufficiently to give reasonable sputtering rates or even to establish a glow discharge. On the other hand, if the spacing between the shield and the target electrode is made too great in an effort to minimize the capacitive coupling between them, this may defeat the purpose of the shield by allowing ions to bombard the metal electrode, causing deterioration of the latter and contamination of the sputtered dielectric. This is one of the problems to which the present invention is addressed. The invention also is addressed to other problems which are associated with the radio-frequency sputtering of dielectrics, such as the difficulty of maintaining a stable glow discharge under radio-frequency excitation, as well as the difficulty of tuning the radio-frequency power source and matching it to this type of load.

DETAILED DESCRIPTION Referring now to FIGS. 1 and 2, which show an exemplary form of sputtering apparatus incorporating the principles of the present invention, a low-pressure gas ionization chamber is enclosed by an envelope 10 in the form of a bell jar made of suitable material, such as Pyrex glass, which is removably mounted on a base plate 12. A gasket 11 is disposed between the jar 10 and plate 12 to provide a vacuum seal. A suitable gas such as argon, supplied by a source 13, is maintained at a desired low pressure in the enclosure by means of a vacuum pump 14. Within the gas-filled enclosure are positioned a cathode structure, generally designated 16, and an anode structure, generally designated 18, FIG. 1. The terms cathode and anode are employed merely for convenience herein. Inasmuch as the sputtering apparatus is excited by a radio-frequency power source 20, FIG. 1, the portions of the structure respectively designated the cathode and anode will actually function as cathode and anode, respectively, during the negative half-cycles of the applied radio-frequency excitation. During the intervening positive half-cycles the polarities of the electrodes are reversed, but in the present apparatus this does not effect a reversal of the sputtering operation.

Considering now the detailed construction of the cathode assembly which is generally designated 16, FIGS. 1 and 2, a target T, consisting of the dielectric material which is to be sputtered, is mounted on or positioned adjacent to a metal electrode 22. This electrode 22 is indirectly supported by, while being insulated from, a hollow supporting column or post 24, the bottom flanged portion of which is secured to the base plate 12. The post 24 is electrically conductive, and being in direct electrical contact with the base plate 12 (which is grounded as indicated in the drawings), the post 24 is maintained at ground potential. Supported on the upper flanged end of the cylindrical post 24 is a metallic shield 26 having an upwardly-extending annular lip portion 28, FIG. 2, that partially encloses the electrode 22 adjoining the target T. A cylindrical metal sleeve 30, FIG. 2, is secured to and depends from the lower face of the shield 26 in concentric relation to the cylindrical post 24, which encloses it. Within this sleeve 30 is disposed a narrow sleeve 32 of suitable insulating material, such as Teflon, which extends upwardly into a central aperture in the shield member 26. A metal tube or pipe 34 extends vertically through the insulating sleeve 32 and is frictionally held in this vertical position by the sleeve 32. A ferrule or bushing 36 engaged with a projecting annular portion of the sleeve 32 is screw-threaded onto the outer surface of the sleeve 30, and with the ferrule 36 tightened, a firm frictional engagement is maintained among the parts 30, 32, and 34, whereby the tube 34 is effectively supported along the vertical axis of the post 24 while being electrically insulated therefrom. The lower portion of the tube 34 extends down through an opening 38 in the base plate 12 aligned with the interior space of the hollow post 24. The upper and lower flanges of the post 24 have airtight seals with the shield 26 and the base plate 12, respectively, and the insulating sleeve or gasket 32 maintains an airtight seal between the tube 34 and the shield 26. Hence, the interior of the post 24 is sealed from the space surrounding the post 24, which is part of the lowpressure gas chamber. The interior of the post 24 is at normal air pressure.

The electrode 22 is supported on the upper end of the vertical tube 34 as shown in FIG. 2. The electrode 22 is generally disc-shaped and has an annular, downwardly projecting portion 40 that seats upon a metal disc 42 secured to the upper end of tube 34. The disc 42 and annular lip 40 are secured to each other for enclosing a central space 44, FIG. 2, within which water or other cooling fluid may be circulated to keep the temperature of the electrode 22 from rising too high while the apparatus is operating. To insure a uniform cooling action, a disc-shaped baflle member 46, FIG. 2, is disposed within the space 22, this baffle 46 being positioned therein by bosses 48 which engage the interior faces of the electrode 22 and the enclosing disc 42. The baflle 46 has a central opening that communicates with the upper end of a vertical tube 50 of small diameter that extends through the interior of the tube 34 in coaxial relation therewith. The lower end of the tube 34 extends into a metal bushing or sleeve 52, with which it has a tight fit. An inlet pipe 54, through which water or other cooling fluid may flow, communicates with the interior of the bushing 52 and with the tube 34. A fluid-tight seal between the bushing 52 and the tube 34 is provided by means of a gasket 56 and a ferrule 58 threaded onto the bushing 52. The tube 50 extends entirely through the bushing 52 and serves as a return conduit for the cooling fluid which leaves the interior space 44 of the electrode 22. A gasket 60 and ferrule 62 threaded onto the loWer end of the bushing 52 afford a fluid-tight seal between the tube 50 and the interior of the bushing 52. In operation, water or other cooling fluid enters the outer tube 34 through the inlet pipe 54, is circulated around the baflle 46 within the space 44 inside the electrode 22, and then leaves through the exit tube 50, thereby cooling the electrode 22 and the target T mounted thereon. This helps to prevent excessive deterioration and sagging of the target. If water or any other electrically conductive fluid is used, inlet pipe 54 and exit tube 50 are connected respectively to the faucet and drain by means of long plastic or rubber tubing. This creates a high resistance path to ground. With fifteen feet of A I.D. tubing, a resistance to ground of about 10 megohms is obtained. With this arrangement, substantially no power is lost to ground.

Provisions also are made for cooling the shield 26. As shown in FIG. 2, an annular space 64 is provided within the shield 26, this space being closed by a disc 66 fitted into the shield 26. An inlet pipe 68 and outlet pipe 70 communicate with the space 64 for circulating a cooling fluid through this space and thereby cooling the shield 26. These inlet and outlet pipes 68 and 70 extend vertically through the opening 38 in the base plate 12 and are coupled at their upper ends to the shield 26, as shown in FIG. 2.

Voltage is applied to the electrode 22 from the radiofrequency power source 20, FIG. 1. One side of the source 20 is grounded, and the other side thereof is connected to a lug or terminal 72 on the bushing 52. The electrical connection is continued throughthe bushing 52 and the tube 34 to the electrode 22. As explained hereinabove, the tube 34 is electrically insulated from the shield 26. Ground potential is maintained on the shield 26 by virtue of the fact that this shield is electrically connected to the supporting post 24 which is mounted on the grounded base plate 12. The grounded shield 26 serves to suppress any glow discharge that otherwise might take place behind the target T in the vicinity of the target electrode 22.

The shape of the shield 26 and its spacing from the electrode 22 are important factors. As shown in FIG. 2, the lip 28 of the shield 26 does not project upwardly past the electrode 22, nor does it project laterally beyond the outer edge of the target T. Furthermore, the space D, FIG. 2, between the shield 26 and the electrode 22 is chosen to fall within certain limits. It has been experimentally determined that for effective radio-frequency sputtering of dielectric materials, this distance D should have a lower limit of about one-quarter inch and an upper limit not greater than the thickness of the Crookes dark space in the glow discharge. This means that radiofrequency sputtering of dielectrics, in accordance with the principles of this invention, must be accomplished with gas pressure no greater than about 30 microns, and preferably much less than that, whereas in conventional direct-current sputtering it is customary to use much higher gas pressures, on the order of 50 microns or higher. Where these higher pressures are used, the thickness of the Crookes dark space becomes very small, less than one-quarter inch. If the shield 26 were spaced by that small amount from the electrode 22, an excessive capacitive coupling would exist between the shield 26 and the electrode 22 at radio frequency. As explained hereinabove, the sputtering of dielectric materials in accordance with this invention requires radio frequencies in the megacycle range, and at this frequency the spacing D, FIG. 2, between the shield 26 and the electrode 22 should not be less than the critical distance (about one-quarter inch) just mentioned. The maximum spacing is approximately the thickness of the Crookes dark space, which is affected by the gas pressure and also by the pressure of a magnetic field, as will be explained.

Referring now to FIG. 1, the RF anode 18 is secured to the underside of a plate 76 supported by posts 78, through which the anode 18 is electrically connected to the grounded base plate 12. The objects to be coated, such as the silicon wafers W shown in FIG. 3, are mounted in suitable holders 80 secured to the underside of the anode 18 in spaced parallel relationship to the target T.

(It will be understood, of course, that substrates other than silicon wafers may be mounted in holders such as 80 to be coated with dielectric materials sputtered from the target T.) The distance separating the substrates W from the target is approximately one inch.

If the substrates which are to be coated are of a nature such that they would be damaged by excessive heat, means can be provided to keep the temperature of the substrates as W from rising too high during the sputtering process. To this end, a cooling coil 82 is positioned in close association with the metal plate 76 on which the RF anode 18 is mounted. Inlet and outlet pipes 84 and 86 extending through gas-tight bushings in the base plate 12 conduct a suitable coolant through coil 82 for removing excessive heat from the anode 18 and the substrates mounted thereon. On the other hand, if the nature of the substrates is such that the deposition of sputtered material thereon is improved by heat, a suitable heating means can be mounted near the anode to achieve the desired substrate temperature.

With radio-frequency voltage applied to the electrode 22, the target T functions as an RF cathode during those half-cycles When the potential of the electrode 22 is negative with respect to ground. During the intervening positive half-cycles the potential of the electrode 22 rises above ground level, thereby attracting electrons to the target T for removing the positive charge previously placed on the target T by the bombarding ions. As mentioned hereinabove, electrons are attracted to the target T in far greater numbers than the heavier ions, but since the target T is a dielectric, and its electrode 22 is well shielded, no net direct current can flow through the RF cathode structure 16. Hence, as a result of the aforesaid interaction of the ions and electrons, the target T maintains itself at a generally negative potential with respect to ground, and if it does momentarily acquire a positive potential, this is not sufiicient to reverse the sputtering process or to cause undesired sputtering of any metal parts associated with the RF anode structure 18.

Establishment of a glow discharge at radio frequency between the target T and the RF anode 18 causes a positive ion sheath to form around the negative target T. As the target is bombarded by the ions in this sheath, atomic particles of the target material are sputtered off and are deposited upon the substrates carried by the holders 80 on the counter-electrode or anode 18. The arrangement is such that very little of this sputtered di electric material is deposited elsewhere.

It is well known that the application of a magnetic field to a glow discharge will enhance the ionizing action, and this effect has been used to increase the sputtering rate of prior direct-current sputtering devices. According to generally accepted theory, the magnetic field imparts a spiral motion to the electrons traveling between the electrodes, thereby lengthening the path traveled by each electron and increasing the opportunity for collisions between the electrons and the gas molecules. In the present radio-frequency sputtering apparatus it has been found that the application of a magnetic field aids the ionizing action as expected, but in addition to this there are certain other advantages, hitherto unrealized, which seem to result from using a magnetic field in this particular environment. Thus, it has been observed that the radio-frequency glow discharge is rendered much more stable when a steady magnetic field is applied thereto, and it is much easier to tune the radio-frequency power source and match it to the load under these circumstances. The reason for this is not precisely known at the present time. Pursuant to this discovery, means have been incorporated in the present apparatus for applying a magnetic field to the inter-electrode space where the glow discharge is maintained. Referring to FIG. 1, a set of toroidal permanent magnets 90 is stacked above the RF anode 18 to provide a steady magnetic field along Cir the vertical axis 92 of the toroids, normal to the surface of the target T. Experimentation indicates that the downward or upward polarity of this magnetic field is immaterial. The number of permanent magnets 90 which may be employed is selected according to the operating conditions involved. Once the desired intensity of the magnetic field is established by experiment, a single permanent magnet or equivalent magnetizing means (such as a solenoid) may be utilized to provide such a field. Ionization and sputtering actions are greatly expedited by this magnetic field, as will be explained presently with reference to a specific example.

The presence of a magnetic field furthermore appears to have the same effect upon the gas discharge as increasing the gas pressure. In the presence of an applied magnetic field, a glow discharge can be maintained at a gas pressure that otherwise would be too low for that purpose, and the thickness of the Crookes dark space also is reduced. Good results have been obtained by using a gas pressure of only 5 microns and a magnetic field of from 70 to 110 gausses in the present apparatus, which gives a glow discharge having a Crookes dark space no thinner than about one-quarter inch, the critical dimension discussed hereinabove.

Although the RF cathode 16 is represented in FIG. 1 as being positioned beneath the RF anode 18, this physical relationship may be inverted so that material is sputtered from the target T in a downward direction rather than upward. This inverted arrangement has the advantage that the substrates which are to be coated with dielectric material need not be anchored to the anode but are merely held by gravity thereon, thereby eliminating the holder 80 and enabling more substrates to be posititoned on an anode of given size. The back of the target T can be metallized and bonded to the electrode 22 for holding the target in place.

Applied Applied voltage magnetic field (peak to peak), volts Deposition rate (approx.) in

Target material (gausses) angstroms/min.

None 3, 200 27 The rate at which quartz, for example, can be deposited using this invention is on the order of sixty times as great as the rate which can be achieved by the use of any other dielectric sputtering apparatus known to be available at the present time. Furthermore, because of the fact that the target electrode in the present apparatus is adequately shielded while supporting the target inside of the ionization chamber, it is now feasible to sputter a wide variety of dielectric materials, most of which would be unsuitable in an apparatus which has to use the enclosure envelope as the target. This enables one to choose a target material having physical properties (such as coefficient of expansion and annealing temperature) compatible with those of the articles being coated. It also increases the variety of objects that can be coated by dielectric sputtering.

The above table brings out the important part played by a magnetic field in improving the deposition rate of a radio-frequency sputtering apparatus. Even more important, however, is the great improvement effected by this field in stabilizing the glow discharge and making the radio-frequency power source easier to tune and match to the load.

While the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and detail may 'be made therein without departing from the spirit and scope of the invention.

What is claimed is:

1. A method of applying an insulating layer of a material to the surface of a substrate comprising;

positioning in the chamber adapted to support a glow discharge, the substrate to be coated supported on an anode, a target of the insulating material to be applied on a cathode spaced from said anode,

a grounded anti-sputtering shield arranged with the distance separating said shield from the exposed surface of the cathode not substantially less than one-quarter inch and not substantially greater than the thickness of the Crookes dark space associated with the particular gas pressure between said shield and cathode, said particular gas pressure being less than 30 microns of mercury,

applying a high-frequency alternating voltage directly to said anode and said cathode to thereby create a glow discharge in the region between the anode and cathode which results in a bombardment of the target by gas ions during the period when the target is at a negative potential with respect to said glow, and

continuing the application of high-frequency alternating voltage across said anode and said cathode until the desired insulating layer thickness is achieved.

2. The method of claim 1 wherein the gas in said chamber is argon at a pressure in the range of 5 to microns of mercury.

3. The method of claim 1 wherein a magnetic field is applied to the ionized gas in said chamber for stabilizing and enhancing the radio frequency glow discharge.

4. The method of claim 1 wherein said substrate is made of a semiconductor material.

5. The method of claim 4 wherein said insulating layer is an inorganic material.

References Cited UNITED STATES PATENTS 3,347,772 10/1967 Laegreid et al. 204-298 3,242,006 3/1966 Gerstenherg 204192 3,170,810 2/1965 Kagan 204-192 1,926,336 9/1933 Hunter 204-492,

JOHN H. MACK, Primary Examiner S. KANTER, Assistant Examiner

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