US3526584A - Method of providing a field free region above a substrate during sputter-depositing thereon - Google Patents

Description

3,526,584 UBSTRATE E. J. SHAW FIELD FREE REGION ABOVE A S DURING SPUTTER-DEPOSITING THEREON Sept. 1, 1910- METHOD OF PROVIDING A 2 Sheets-Sheet 1 Filed Sept. 25, 1964 a v k INVENTOR ZJJaI H/ M ATTORNEY 3,526,584 UBSTRATE E. J. SHAW Sept. 1, 1970 METHOD OF PROVIDING A FIELD FREE REGION ABOVE A S DURING SPUTTER-DEPOSITING THEREON Filed Sept. 25, 1964 v 2 Sheets- Sheet z mama.

United States Patent 3,526,584 METHOD OF PROVIDING A FIELD FREE RE- GION ABOVE A SUBSTRATE DURING SPUT- TER-DEPOSITING THEREON Everett J. Shaw, Hopewell Township, Mercer County,

NJ., assignor to Western Electric Company, Incorporated, New York, N.Y., a corporation of New York Filed Sept. 25, 1964, Ser. No. 399,141 Int. Cl. C23c 15/00 US. Cl. 204-192 9 Claims ABSTRACT OF THE DISCLOSURE A sputtering method includes using a chamber containing a sputtering gas, a cathode of a film-forming material, an anode, and a gride positioned over an article to be coated positioned on the anode or on an article support. During application of a sputtering potential between the anode and the cathode, a voltage is applied to the grid to create an equipotential or field free zone overlying the article. Such zone extends away from the article by a distance greater (preferably, at least three times greater) than the mean free path of negative gas ions within the chamber. Sputtering, both reactive and non-reactive, takes place through this zone to coat the article with a film.

The use of the above method and apparatus precludes damage to the article andor to films previously deposited on the article.

BACKGROUND OF THE INVENTION Field of the invention This invention relates to methods of and apparatus for coating articles, and more particularly to reactive and non-reactive sputtering methods and apparatus for depositing metal and dielectric layers on articles, Without deleteriously affecting the articles.

It is often desirable to provide a substantially pure metal coating on an article. More specifically, in producing a thin film electrical component, one production step may include depositing a metal electrode on a dielectric layer of the partially formed component which may also include another metal layer deposited on a nonconductive substrate and underlying the dielectric layer.

Likewise, it is often desirable to provide a dielectric coating on an article. More particularly when the article is a sensitive thin film electrical component, which may comprise thin metal and thin dielectric films alternately deposited on a substrate, it is often not only expedient, but also necessary to provide a final protective or encapsulative dielectric coating on the component. The dielectric coating may be, for example, a non-conductive metal oxide. The protective dielectric coating both shields the component from adverse environment conditions and electrically insulates the component from other electrical apparatus assembled with the thin film component.

Description of the prior art Controlled cathodic sputtering of a putterable metal onto an article in an inert atmosphere results in the deposition on the article of a substantially pure thin film of the metal. Similarly, controlled cathodic sputtering of a putterable metal onto an article in a reactive atmosphere, which may include a mixture of an inert gas and a reactive gas, results in the deposition on the article of a thin coating comprised of a compound produced by the reaction of the sputterable metal and either the reactive gas or an active species of the gas. The compound may be a metal oxide.

The completed, coated article may comprise alternating metal and dielectric layers produced by alternating such reactive and non-reactive sputtering steps. An ex- 'ice ample of one of the more complex types of coated articles formed by sputtering is a thin film electric capacitor which includes a non-conductive glass substrate or base onto which there may be alternately sputtered a dielectric base layer, a first metal electrode, a capacitor dielectric layer, a second metal electrode or counter electrode, and a final dielectric layer which encapsulates and protects the previously deposited layers (see Berry Pat. 2,993,266). One or more of the sputtered layers may be omitted; for example, deletion of the second electrode and the capacitor dielectric sputtering steps results in a dielectric encapsulated layer which may be patterned to produce a thin film resistor.

It has been established as a result of extensive testing of articles which have been coated by prior art reactive and non-reactive sputtering methods and apparatus that the physical characteristics of the articles and of any coating previously sputtered thereon varied and deteriorated significantly from those characteristics existing prior to sputtering.

More specifically, metal portions of articles, such as metallic electrodes of thin film resistors and capacitors onto which dielectric layers have been reactively sputtered, showed a marked increase in ohmic resistance, indicating that some of the metal had been oxidized during the re active sputtering step. In addition, dielectric articles, such as the dielectric layers of thin-film capacitors broke down in use, indicating the presence of pinholes in the dielectric material which had not previously been present. Such deteriorations of the physical and electrical characteristics of the thin film electrical components were found to be unpredictable, dilficult to control, and rendered many of the thin film components unsuited for use in electrical circuits.

SUMMARY OF THE INVENTION Research conducted in an effort to determine the cause of such variations and deteriorations indicates that the undesirable oxidation of metal articles and the undesirable pinholes formed in dielectric articles by a sputtering step may both be obviated by sputtering through a potential zone adjacent to the surface of the articles which surface is to be coated during the sputtering operation. In addition,

by providing the potential zone, it is found that the reactive and non-reactive sputtering operations could be performed at higher sputtering voltages and in a shorter time than was hitherto possible by prior art methods and apparatus without deleteriously affecting the articles.

An object of this invention is to provide new and improved methods of and apparatus for coating articles.

Another object of this invention is to provide reactive and non-reactive sputtering methods of and apparatus for coating articles with dielectric and metallic coatings without deleteriously affecting the articles.

Still another object of this invention resides in a method of coating thin film electrical components with a metallic or dielectric layer wherein deterioration of the electrical characteristics of the components is avoided.

Yet another object of this invention resides in apparatus for depositing metal or dielectric films on thin film electrical components wherein previously deposited metal and dielectric layers of the component are not damaged.

A further object of this invention resides in the provision of apparatus for producing a potential of a predetermined magnitude and polarity within a zone adjacent to an article during a reactive sputtering operation for coating the article with a dielectric layer.

A still further object of this invention is to provide apparatus for producing a zone having an electrical field with a predetermined gradient directly over a metallic or dielectric article during a non-reactive sputtering step for depositing a metal layer on the article.

With these and other objects in view, the present invention contemplates the provision of grid-like facilities above an article mounted for coating by sputtering apparatus which includes a sputtering cathode and an anode. Concurrent with the application of a negative sputtering voltage to the cathode, a voltage is impressed on the gridlike facilities to establish a potential of a predetermined magnitude and polarity within a zone extending between the grid-like faciliteis and the article. The potential zone is defined by a space of Zero or low potential gradient of either polarity extending between the grid-like facilities and an article mount. To preclude damage to the article during the sputtering operation, the grid-like facilities are spaced from the article by a distance greater than the mean-free paths of ions in the sputtering atmosphere.

Further, with the foregoing objects in view, the present invention contemplates a method of coating articles by reactive or non-reactive cathodic sputtering and simultaneously therewith producing a potential zone of a predetermined magnitude and polarity immediately above the articles. The method includes sputtering electrically neutral atoms of a predetermined material through the potential zone onto the article whereat the atoms alone or in combination with a reactive gas form a coating on the article without deleteriously affecting the article.

BRIEF DESCRIPTION OF THE DRAWINGS A complete understanding of this invention may be had by referring to the following detailed description and the accompanying drawings illustrating specific embodiments thereof, wherein:

FIG. 1 is a cross-sectional, elevational view of a first embodiment of a sputtering apparatus, including a cathode and an anode and showing grid-like facilities positioned above an article mounted on the anode for performing the method of the present invention to coat the article according to the principles of the present invention;

FIG. 2 is a cross-sectional, elevational view of a second embodiment of a sputtering apparatus similar to that shown in FIG. 1 in which the article is mounted on a support other than the anode;

FIG. 3 is an enlarged plan view of the grid-like facilities used in the sputtering apparatus shown in FIGS. 1 and 2 for establishing a potential zone above the article;

FIG. 4 is an enlarged perspective view of a thin film capacitor which is mounted for coating either on the anode shown in FIG. 1 or on the support shown in FIG. 2 prior to being coated; and

FIG. 5 is an enlarged perspective view of the thin film capacitor shown in FIG. 4 illustrating alternating metal and dielectric coatings provided on a substrate according to the principles of the present invention.

DETAILED DESCRIPTION Structure In FIG. 4 of the drawings there is shown a thin film electrical capacitor 10, which may be produced according to the principles of this invention. The capacitor is used throughout this description as an example of an article to be coated only because it is one of the more complex articles which may be produced by the present invention. The thin film capacitor 10 comprises a substrate or base 11 provided with a dielectric base layer 12, a first metal electrode 13, and a dielectric layer 14 overlying the electrode 13. A second metal electrode 15 overlies a portion of the dielectric layer 14. Referring to FIG. 5, the capacitor 10 is shown having upper surfaces 1616 which are exposed during a sputtering operation for receiving a coating, such as a dielectric layer 17 of a metal oxide. All of the layers 12-17 of the capacitor 10 may be deposited on the preceding layer according to the principles of this invention.

Referring now to FIG. 1 a grid-like facility, such as a screen 21 is positioned within sputtering apparatus above an article to be coated, such as the thin film capacitor 10 the capacitor 10 may be mounted in any desired manner to position a surface, such as one or more of the upper surfaces 16, in line with the preferred path of atoms 59 which are sputtered from a cathode sheet 26 of a cathode assembly 22 upon application of a negative voltage to the cathode sheet 26 from a power source 43. A predetermined voltage is applied to the screen 21 to produce a controlled potential zone 23 between the screen 21 and the thin film capacitor 10 both of which may be mounted on an anode 24, or alternatively on a support 25 (FIG. 2), or in any other desired position during the sputtering operation. To preclude damage to the layers 1217 of the capacitor 10, the height X of the zone 23 between the screen 21 and the surfaces to be coated 16 of the thin film capacitor 10 is greater than the mean-free path of ions a, 60b, 63a and 63b of gases which may be used in the sputtering apparatus 20 during the sputtering operation.

The cathode assembly 22 is comprised of the sheet 26 of a sputterable material such as tantalum or silicon and a backing plate 31. A front surface 29 of the sheet 26 faces in the general direction of the capacitor 10, while a back surface 30 faces a backing plate or guard 31, the proximity of which to the back surface 30 of the cathode sheet 26 substantially prevents sputtering of metal atoms' from that surface according to Paschens law. When a negative sputtering voltage is applied to the cathode sheet 26, atoms 59 of the film-forming cathode material are randomly sputtered from the front surface thereof.

The anode 24 may serve as a mount for both the thin film capacitor 10 and the screen 21 during the sputtering operation. The thin film capacitor 10 is supported in such a manner that a portion of one or more of the upper surfaces 16 thereof are selectively exposed by a mask 32 to receive the atoms 59 which are sputtered from the cathode sheet 26. During the sputtering operation the anode 24 is maintained at a potential, such as ground potential, which is positive with respect to the negative sputtering voltage impressed on the cathode sheet 26. The mask 32 is made of a conductive material and may be electrically connected by a wire 27 to the anode 24 or support 25 on which the capacitor 10 rests.

Apparatus 120, similar to that depicted in FIG. 1, is shown in FIG. 2 wherein similar elements are indicated by like reference numerals. In addition to the cathode 22 and the anode 24, there is provided a capacitor support 25 on which the capacitor 10 and the screen 21 may rest during sputtering. The support 25 may be positioned in any desirable manner within the chamber to expose the surfaces 16 through the mask 32 to metal atoms 59 sputtered from the cathode 22. The support 25 may be maintaned at ground potential during sputtering. In this embodiment the support 25 serves as a capacitor rather than the anode 24, and hereinafter, the descriptive word mount will refer to both the anode 24 shown in FIG. 1, and the support 25 shown in FIG. 2.

Referring now to FIGS. 1 and 2, the sputtering apparat us 20 includes a conductive metal base 33 and an upright enclosure 34 mounted on a hermetic seal or gasket 35 provided on the base 33. The enclosure 34 defines a sputtering chamber for containing the sputtering gas or gases and surrounds the cathode assembly 22, the anode 24, the article to be coated, such as the thin film capacitor 10, the screen 21 and the support 25 if one is used. The enclosure 34 may be a conventional upright bell jar 36 or may be of any other desired size and shape, such as a chamber (not shown) of a continuous vacuum sputtering line (such as that discussed in the Western Electric Engineer, April 1963, pages 9-17).

A first conductive mounting post 39 extends upwardly from an electrically insulating, vacuum-tight support 40 through the conductive base 33 and supports the cathode sheet 26 in any desired position, such as the generally horizontal position shown in FIGS. 1 and 2. The backing plate or guard 31 is supported by an electrically conductive tube 41 which extends upwardly from the base 33 and surrounds without touching the mounting post 39.

A first electrical terminal 42 is attached to the first mounting post 39 for connecting the cathode 22 into a sputtering power circuit 43. A second metal mounting post 44 extends upwardly from the base 33 for supporting the anode 24 in any of several desirable positions with respect to the cathode 22. In FIGS. 1 and 2, the anode 24 is shown in a generally horizontal position. If the support 25 (FIG. 2) is used as a mount for the capacitor during sputtering, an additional third metal mounting post 45 extends upwardly from an insulating support 47 through the base 33 to mount the support 25. Second and third electrical terminals 46 and 49 are attached respectively to the second and third mounting posts 44 and 45 for grounding the anode 24 and applying any desired potential to the support 25 during sputtering. The anode 24 and the cathode 22 may be positioned with respect to each other in a variety of ways known in the art to be conducive to efiicient cathodic sputtering, such as in vertical or horizontal alignment, or at right angles to each other. Likewise the cathode 22 and the article to be coated may be relatively positioned in various known manners, the only requirement being that the article be positioned to receive diffused atoms 59 sputtered randomly from the cathode 22. In FIGS. 1 and 2 the cathode 22 is in substantial vertical alignment with both the anode 24 and the capacitor 10 mounted for coating so that the surfaces of the capacitor, such as the upper surfaces 16, are positioned to receive the diffused atoms 59. In addition, the cathode sheet 26 and the anode 24 are spaced apart a predetermined distance Z, the magnitude of which depends on such factors as the sputtering voltage, the composition and pressure of the sputtering gas used within the enclosure 34, and the type of the material comprising the cathode sheet 26.

Non-reactive sputtering takes place within an inert atmosphere which may comprise a noble gas, such as argon. Reactive sputtering takes place within a reactive atmosphere which may comprise a reactive gas mixture including selected portions of an inert gas and a reactive gas. Among the inert gases which are suitable for use are argon and xenon, while examples of the reactive gases which are suitable for use are oxygen and water vapor. A gas supply 50 is provided to introduce the reactive gas or the non-reactive gas mixture into the enclosure 34 to condition the appartus for the sputtering operation. The gases are normally comprised of a majority of electrically neutral gas molecules 61a and 611;, but during the sputtering step are ionized to produce gas ions which include positive ions 60a and 63a, negative ions 60b and 63b and neutral gas molecules and atoms 62a and 62b. Evacuation apparatus 51 is provided to evacuate the enclosure 34 during an inert gas flushing operation, during introduction of the sputtering gas or gases, and throughout the sputtering operation. If an non-reactive, inert gas is used in the enclosure 34, the atoms 59 sputtered from the cathode 22 impinge upon and coat an article such as the thin film capacitor 10 forming a substantially pure metal layer, such as the layers 13 and 15 of the capacitor 10. If a reactive gas mixture, such as one containing oxygen, is used, the atoms 59 sputtered from the cathode 22 combine with reactive gas molecules 61b at the surface of the article to be coated, such as one or more of the upper surfaces 16 of the thin film capacitor 10 to form an dielectric layer, such as a metal oxide 17 thereon.

Referring now to FIGS. 1, 2, and 3, the grid-like facilities or screen 21 are shown including a metal frame 52 which encloses and supports woven strands 53-53 of a conductive metal which define meshes 5454. The individual strands 53 may be comprised of any electrically conductive material, such as tantalum, but are preferably of the same material as the cathode sheet 26. The size of the meshes 5454 is relatively unimportant to proper operation of the present invention, but is preferably quite small to limit field penetration or non-linearity of the potential gradient of the potential zone 23.

Legs are attached to and extend from the frame 52 to position the screen 21 above the thin film capacitor 10. The legs 55 may be electricaly conductive or nonconductive, and are of a length sufiicient to space the screen 21 a predetermined distance X above the thin film capacitor 10. The distance X is greater than the meanfree path of the negative gas ions b and 63b of the sputtering gas, and is preferably a multiple greater than three of the ionic mean-free path.

Proper positioning of the screen 21 over the thin film capacitor 10 defines the potential zone 23 which is bounded above by the screen 21 and below by the capacitor mount such as the anode 24 or the capacitor support 25. The thin film capacitor 10 lies within the potential zone 23 when resting on the mount 24 or 25.

During the sputtreing operation and when the screen 21 is positioned over the thin film capacitor 10, a screen voltage substantially equal to the potential of the mount 24 or 25 is applied to the screen 21. The screen voltage may be applied through the electrically conductive legs 55 which electrically contact the mount 24 or 25. Alternatively, if the legs 55 are non-conductive the screen voltage may be applied through an electrical connection 56 between the frame 52 and the mount 24 or 25.

With the legs 55 made of a non-conductive material, it may also be found suitable to connect an independent screen voltage source (not shown) directly to the screen 21 for selectively adjusting the screen voltage, in which case no direct electrical connection need be provided between the screen 21 and mount 24 or 25.

Referring again to FIGS. 1 and 2, the sputtering power circuit 43 is shown including the variable voltage sources 69 and a series switch 70. The negative side 71 of the voltage source 69 is connected by a line 72 to the first terminal 42 to apply the negative sputtering voltage to the cathode 22 via the post 39. The positive side 73 of the source 69 is connected by a line 74 to ground. The mount voltage which may be ground potential is in turn applied to the screen 21 by the legs 55 or by the connection 56. The potentials applied to both the screen 21 and the mount 24 or 25 being substantially equal, the zone 23 is rendered essentially equipotential and completely overlies the thin film component 10 which rests on the mount 24 and 25. Moreover, if the independent screen voltage source is used, it maybe adjusted to impress a voltage on the screen 21 which is slightly different from that of the mount 21 or 25 to provide a potential zone 23 having a small gradient of either polarity.

Operation In the operation of the apparatus, a component to be encapsulated, the thin film capacitor 10, for example, is placed on the mount 24 or 25. The mask 32 may be provided to expose only selected portions of the upper surfaces 16 of the capacitor 10. The screen 21 is positioned over the component 10 by means of the legs 55 which rest on the mount 24 or 25. If the connection 56 or the independent screen voltage source are to be used, appropriate electrical connections are made. The enclosure 34 is positioned over and sealed to the base 33 by the seal 35. The enclosure 34 is evacuated by the evacuation apparatus 51 flushed with an inert gas, whereafter the sputtering gas mixture is introduced into the enclosure 34 from the supply 50. The switch is closed to apply a sputtering voltage, such as 2,500 volts between the cathode 22 and the anode 24 from the source 64. The mount voltage is applied to the screen 21 through the legs 55 or the connection 56 to produce the potential zone 23. The screen voltage may be of any desired polarity but should be of a magnitude so that the zone 23 is equipotential or nearl so.

During the sputtering operation, electrons 64 are freed from neutral gas molecules which have been ionized, for example, by cosmic radiation. These electrons 64 are accelerated by the field and strike the molecules 61a-61b of the gas, producing additional gas ions such as the positive ions 60a and 63a and the negative ions 60b and 63b, and additional free electrons 64. Under the influence of the negative cathode potential, the positive gas ions 60a and 63a move toward and strike the cathode sheet 26 of the film-forming material to sputter the atoms 59 of the spu-tterable material therefrom. The atoms 59 of the sputterable material are sputtered randomly and many of the atoms 59 pass through the meshes 54 of the screen 21 and through the potential zone 23. The atoms 59 reach the selectively exposed upper surfaces 1616 of the thin film capacitor 10. If a non-reactive gas, such as argon, is used, the metal atoms reaching surfaces 16-46 form metal layers such as the layers 13 and 15 of the capacitor 10 shown in FIG. 4. If the reactive gas mixture, such as one containing oxygen and argon, is used, the metal atoms combine with the reactive gas to form dielectric layers, such as the layers 12 and 14 or the layer 17 which encapsulates the capacitor 10 as shown in FIG. 4.

Components, such as the capacitor 10, onto which the metal or metal oxide layers 12-17 are sputtered while the capacitor 10 is maintained within the zero or low gradient potential zone 23, display little deterioration such as electrode oxidation or dielectric pinholes. In addition, a larger sputtering voltage may be used to deposit the layers 12-17 in a shorter time than has been hitherto possible without deleteriously aifecting the component 10.

EXAMPLES The following examples of thin film components encapsulated with a dielectric layer according to the principles of the present invention are given to illustrate, without limiting, the possible applications of the present invention.

Example I Two resistor slides, A and B each having sixteen thin tantalum film resistors deposited thereon, were coated with a protective layer 17 of tantalum pentoxide (Ta O in a reactive atmosphere comprised of a mixture of argon and oxygen. Slide A was coated without a screen such as the screen 21 in place; whereas a tantalum screen 21 was positioned one-half inch above slide B during the coating thereof in accordance with the principles of this invention. The distance between the screen 21 and slide B was greater than the mean-free path of the negative gas ions 606 and 636 at the pressure and voltage used in the sputtering operation. Slides A and B were sputtered until the Ta O layer 17 thereon was 300-400 angstroms (A.) thick. In both operations, the cathode-to-anode spacing Z was approximately two and one-half inches and the cathode 26 was tantalum. A comparison of the sputtering variables and the resistance deteriorations after sputtering is shown in Table I.

8 as indicated by the relatively small (23%) change of resistance of slide B compared with the high (3.82%) change of ressitance of slide A.

Example II Twenty thin tantalum film capacitors 10 were encap sulated with a layer 17 of Ta O Without a screen such as the screen 21 in place. Initially, the sputtering variables were a sputtering voltage of 1,000 volts, current of 50 ma., and 15 microns of O 185 microns of Ar, for a time of 105 minutes; and subsequently 1,450 volts, ma., 15 microns of O and 185 microns of Ar for 150 minutes. All the capacitors 10 were found to be short-circuited after encapsulation, indicating that the capacitor dielectric had been damaged during sputtering. The sputtering times used were necessary to produce a protective Ta O layer of suflicient thickness to provide protection.

Twenty thin tantalum film capacitors 10 were encapsulated with a layer 17 of Ta O with the tantalum screen 21 in place according to the principles of the invention. The sputtering variables were a sputtering voltage of 2,550 volts, a current of 80 ma., and 10 microns of O partial pressure, and microns of Ar, for 80 minutes. All the capacitors 10 were usable after encapsulation, and showed an average leakage resistance of 52 megohms. The average change in capacitance was 1.54%. The sputtering time of 80 minutes produced a protective layer of Ta O suificient to provide protection.

Conclusion It is apparent that with the screen 21 used during a reactive sputtering operation, in accordance with the principles of the present invention, it is possible to encapsulate capacitors 10 with a layer 17 of Ta O of sufficient thickness without rendering the capacitors unusable. Moreover, use of the screen 21 permits a higher sputtering voltage to be used with the result that the sputtering time decreases.

One possible theory explaining the operation of the screen 21 may be that when the free electrons 64 are repulsed by the cathode 22 to further ionize the sputtering gas, both neutral gas molecules 62a and 62b, positive ions 60a and 63a, and negative gas ions 60b and 63b are formed. The positive gas ions 60a and 63a, as discussed in the preceding paragraphs, are effective to sputter atoms 59 from the cathode 22. It is though that the negative gas ions 60b and 63b are accelerated by the potential gradient between cathode 22 and anode 24 toward the positively biased anode 24 and the article to be coated thereon. With no screen 21 in place, the negative gas ions 60b and 63b attain a velocity suflicient to penetrate the layers 12-17 upon impingement with the capacitor 10. Thus, negative gas ions 63b of the reactive gas internally oxidize the metal layers 13 and 15 of the component, while negative gas ions 6% and 63b of the TABLE I Before After Time Cur- O2 partial Sputtering Resistance Average Resistance Average Percent required Voltage rent pressure gas pressure range resistance range resistance of (minutes) (volts) (ma.) (microns) (microns) (ohms) (ohms) (ohms) (ohms) change Without grid in place Slide A.-- 1, 500 100 5 45 3, 309-3, 127 3, 209 3, 455-3, 227 3, 331 3. 82 With grid in place Slide B 80 2, 550 75 5 18 3, 461-2, 872 3, 009 3, 468-2, 876 3, 016 23 It is apparent that with the screen 21 in place, a 300- 400 angstrom layer 17 of Ta O was produced at a higher voltage and therefore in a shorter time than when the screen 21 was not used. Also less resultant variation or deterioration was produced with the screen 21 in place reactive and non-reactive gases puncture the dielectric layers 12 and 14.

By providing the potential zone 23, spaced above the capacitor 10 by a distance X greater than the mean-free path of the negative gas ions 606 and 636, there is, ac-

cording to Paschens law, great statistical probability that the neagtive ions 60b and 63b will undergo one or more collisions in the zone 23 with other particles. Should a collision occur, the negative ions 60b and 63b lose much, if not most, of their kinetic energy through momentum transfer. The zone 23, being of zero or low potential gradient prevents the now low kinetic energy negative ions 60b and 63b from re-accelerating. That is, the negative ions 60b and 63b after collision are under the influence of an electric field having little or no gradient and undergo little or no accelerative forces. In addition, the field free character of the zone 23 is thought to limit ionization within the zone to a great extent, and provides a buffer zone in which initially accelerated negative ions 60b and 63b undergo a sufiicient number of collisions to reduce ionic velocity to nearly thermal velocity. The supposed absence of any accelerative forces on the ions 6% and 63b after collision leads to the conclusion that should ions 60b and 63b reach a surface of the article to be coated, their kinetic energy would be too low for the ions to penetrate the surface. Thus, the unwanted article deteriorations, specifically metal oxidation and dielectric puncturing, due to ion impact are prevented.

It is to be understood that the above-described arrangements are simply illustrative of the application of the principles of this invention. Numerous other arrangements may be readily devised by those skilled in the art which will embody the principles of the invention and fall within the spirit and scope thereof.

What is claimed is: 1. A method of coating an article with a layer of a predetermined material, the article being mounted on a support in a rarefied, gas atmosphere which supports, be tween an anode and a cathod, a sputtering gas discharge, which method comprises:

sputtering through a zone particles of the predetermined material onto the article to provide the layer on the article, said bone extending away from the article a distance at least as great as the mean-free path of negative gas ions in the atmosphere; and

concurrently with said sputtering, positioning an electrode in the discharge and impressing a potential thereon substantially the same as the potential on the support to render said zone substantially fieldfree whereby deterioration of the article and of the layer is prevented.

2. A method of coating an article with a layer of a predetermined material, said article being mounted on an anode in a rarefied, gas atmosphere supporting between the anode and a cathode a sputtering gas discharge which includes negative ions, which method comprises:

positioning an electrode in the discharge and impressing a potential thereon substantially the same as the potential on the anode to establish a field-free region adjacent to the article and extending away from the article a distance greater than the mean-free path of the negative gas ions; and

sputtering electrically neutral atoms of the predetermined material through said field-free region onto the article, said field-free region precluding re-acceleration of any negative gas ions which experience collisions therewithin.

3. A method of coating an article with a dielectric layer the article being mounted on a support in a rarefied, reactive gas atmosphere which supports, between an anode and a cathode, a supttering gas discharge, which method comprises:

sputtering through a zone particles of a predetermined material toward the article, said zone extending away from the article a distance at least as great as the mean-free path of negative gas ions in the atmosphere;

reactively combining said particles and the reactive 'gas to provide the dielectric layer on the article; and concurrently with said sputtering, positioning an elec- 10 trode in the discharge and impressing a potential thereon substantially the same as the potential on the support to render said zone substantially field-free whereby damage to the article and the layer is prevented.

4. A coating method for providing a thin film of a predetermined, sputterable material on a thin film component, the component including thin dielectric and thin metal films deposited alternately on a non-conductive substrate, the component being mounted on a support in a rarefied, gas atmosphere supporting between an anode and a cathode a sputtering gas discharge which includes negative gas ions, which method comprises:

positioning an electrode in the discharge and impressing a potential thereon substantially the same as the potential on the support to establish a substantially field-free region adjacent to the component and extending away from the component a distance greater than the mean-free path of the negative gas ions, the field-free region having a potential gradient which is substantially zero; and

sputtering particles of the predetermined, material through said fiield-free region onto the component, said field-free region being effective during said sputtering to limit advancement of the negtive gas ions toward the component to prevent the negative gas ions from puncturing the thin dielectric films.

5. A coating method for providing a thin dielectric layer of a predetermined suptterable material on a thin film component, the component including thin dielectric and thin metal films deposited alternately on a non-conductive substrate, the component being mounted on a support in a rarefied, reactive gas atmosphere which both supports, between an anode and a cathode, a sputtering gas discharge and which comprises a mixture of reactive and nonreactive gases, both of which include negative gas ions, which method comprises:

positioning an electrode in the discharge and impressing a potential thereon substantially the same as the potential on the support to establish a substantially field-free region adjacent to the component and extendingaway from the component a distance greater than the mean-free path of the negative gas ions, said field-free region having a potential gradient which is essentially zero; and

reactively sputtering neutral atoms of the predetermined material through said field-free region onto the component, said atoms and the reactive gas combining on the surface of the component to form the dielectric layer, said field-free region being effective during said sputtering to limit advancement of the negative gas ions toward the component to prevent both types of negative gas ions from puncturing the thin dielectric films and to prevent the negative reactive gas ions from oxidizing the thin metal films.

6. A coating method for providing a layer of an oxide of tantalum at least 300 angstroms thick on a thin film component, the component including thin dielectric and thin metal films deposited alternately on a substrate, the component being mounted on a support in a rarefied, reactive gas atmosphere which both supports, between an anode and a cathode, a sputtering gas discharge and which includes a noble gas and oxygen gas, said oxygen gas being at a partial pressure of from 4 to 18 microns, said reactive gas atmosphere including negative oxygen ions, which comprises:

positioning an electrode in the discharge and impressing a potential thereon substantially the same as the potential on the support to establish an essentially field-free region adjacent to the component, the field-free region extending away from the component a distance of one-half inch, said distance being greater than the mean-free path of the negative 11 oxygen ions at a total pressure of from 30 to 100 microns;

reactively sputtering tantalum particles from the cathode held at a potential of negative 2550 volts, said tantalum particles being advanced through said essentially field-free region onto the component, said field-free region being effective during said sputtering to prevent acceleration of the negative oxygen ions toward the component after the negative oxygen ions undergo collisions with ions of the oxygen and argon gases within said region, to prevent the negative oxygen ions from both puncturing the thin dielectric films and from oxidizing the thin metal films, said sputtering being sustained for a least 80 minutes.

7. A method of sputtering thin films from a cathode to an anode, comprising the steps of:

bombarding said cathode with ions to free atoms therefrom said ions being produced by making the potential of said cathode negative with respect to said anode;

forming a field-free region about said anode by making the potential of a grid, which grid is positioned between said anode and said cathode, at least as anodic as that of said anode;

depositing said cathode atoms on said anode, said cathode atoms traveling through said field-free region, so that negative impurity ion contamination of said deposit is reduced.

8. A method of sputtering thin films from a cathode to an anode, wherein negative ions do not impinge upon said anode, comprising the steps of:

bombarding said cathode with ions to free atoms therefrom, said ions being produced by applying a voltage to said cathode so that its potential is negative with respect to that of said anode;

forming a field-free region about said anode by making the potential of a grid positioned between said anode and said cathode, substantially parallel thereto, and

outside the cathode dark space distance, at least as anodic as that of said anode;

depositing said cathode atoms on said anode, said cathode atoms traversing said field-free region to impinge upon said anode.

9. A method of sputtering thin filmsfrom a cathode,

comprising the steps of:

bombarding said cathode with ions to free atoms therefrom, said ions being produced by making the potential of said cathode negative with respect to said anode;

forming a field-free region about said anode by making the potential of a grid, which grid is positioned between said anode and said cathode, at least as anodic as that of said anode; and

depositing said cathode atoms on said anode, said cathode atoms traveling through said field-free 4/1962 France.

OTHER REFERENCES Bertelsen et al., IBM Tech. Disclosure Bulletin, vol. 6, No. 11, April 1964, p. 45.

ROBERT K. MIHALEK, Primary Examiner U.S. Cl. X.R. 204-298

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