US 3649512 A
Description (OCR text may contain errors)
-Mamh 14, 1972 J, w. AcKLEY LARGE AREA SPUTTERING TARGET ELECTRODE STRUCTURE Filed June 25, 1970 United States Patent 3,649,512 LARGE AREA SPUTTERING TARGET ELECTRODE STRUCTURE James W. Ackley, Los Altos, Calif., assignor to Varian Associates, Palo Alto, Calif. Filed June 25, 1970, Ser. No. 49,797
Int. Cl. C23c 15/00 US. Cl. 204-298 11 Claims ABSTRACT OF THE DISCLOSURE 'A target electrode structure for sputtering by a gaseous discharge is described which enables the economical construction and use of large sputtering areas. The electrode structure includes as a unit a ground plate which is removably secured within a controlled environment chamber, such as a vacuum coating chamber, and a target electrode plate. The target electrode plate has a face providing the large sputtering area and is insulatively supported from the ground plate a distance assuring that no gaseous discharge can be formed in the space between the target and ground plates. In order to provide adequate cooling of the target electrode plate without causing capacitive losses and the unwanted gaseous discharge between the target electrode and the ground plate, cooling fluid tubing is recessed into a groove which follows a sinuous path over the back side of the plate, i.e., the side of the plate facing the ground plate.
DISCLOSURE Background of the invention This invention relates to apparatus for sputtering material from a face of a target electrode by bombarding the face with ions from a gas discharge and, more particularly, to such an apparatus having a simple and yet effective target electrode structure providing a large surface area from which material can be sputtered.
As the sputtering art advances, the instances in which it is desirable to be able to sputter material from a target having a large sputtering area are increasing. For example, in vacuum coating apparatuses such as that disclosed in copending application Ser. No. 15,169 filed Feb. 27, 1970 and entitled Substrate Object Orienting Mechanism for Vacuum Coating Apparatus it is desirable for efficiency and economic reasons to be able to coat a large number of the desired substrate objects at one time. In the past, the number of such objects which can be coated simultaneously has been limited by the inability to provide a practical sputter target electrode having a sputtering area of any size. In this connection, it will be appreciated that a large size target will require an efficient cooling arrangement in order to dissipate the large amounts of heat which will be generated. Moreover, such a target should be light weight and simply supported within a chamber so that it can be easily removed or replaced.
The most common way of cooling a sputter target is by providing the same with hollow internal passages through which a cooling fluid can be circulated. Such an arrangement is not practical for cooling a large area sputtering surface, though, since the mass the target would have to have in order to assure that it has sufficiently thick walls to withstand the pressure of the cooling fluid without flexing makes the target have too great of a weight for portability and too great of a mass to be cooled. One other cooling method which has been used in the use of a cooling tube or the like secured to the side of the target electrode opposite to that from which the sputtering is to take place. However, such arrangements have normally been used only when the side of the target to which the tube is secured is at atmospheric pressure. In addition, the heat conductivity between such tubes and the target is quite limited in view of the limited contact area and unable to provide the amount of heat conduction necessary to dissipate the large amounts of heat which will be generated by a large area sputtering target.
One other problem which has faced those attempting to design a large area sputter target is that large potential differences must be generated between the target electrode sputtering surface and the gaseous discharge providing the ions for the sputtering. This has created major shielding design problems to assure that gaseous discharges are not formed at unwanted locations about the target electrode while yet minimizing the capacitive losses between the target electrode and the shielding.
Summary of the invention The instant invention provides a relatively simple and inexpensive target electrode structure providing a large area sputtering surface and which overcomes the mass, cooling and shielding problems discussed above. In its basic aspects, the target electrode structure includes a ground plane member from which a target electrode having a face providing the large sputtering area is supported with the side thereof opposite the large area face adjacent to but spaced from the ground plane member by a distance less than the mean free path of ionizing particles such as electrons within the environment of use of the structure. Means are provided defining a tubular passage distributed over the side of the target electrode opposite its sputtering face for the circulation of a cooling medium to cool the face. Such means, desirably in the form of a cooling tube, is recessed into the side of the target electrode over which it is distributed to provide an extended heat dissipating area in close proximity to a surface of the electrode for conduction of heat therefrom.
Most desirably, the depth to which the means defining the tubular passage on the side of the target electrode opposite its sputtering face is recessed is sufllcient to space such means from the ground plane member a great enough distance to prevent appreciable capacitive buildup between the electrode and ground plane with its consequent electrical losses at commonly used high frequencies. Moreover, it is preferred that the target electrode be directly supported by the ground plane member in an insulated manner, and that the ground plane member be removably secured within the controlled environment chamber in which the electrode structure is used. This results in the target electrode structure being simply removable as a unit from the chamber for replacement and the like.
It has been found that the cooling arrangement described above in which the means defining the tubular passage is recessed into the 'side of the target electrode results in a sufiiciently great conduction of heat from the target electrode to enable the structure to be practical as a large area sputtering target. The recessing to a depth providing the distance relationship discussed above, together with the stated spacing between the electrode and the ground plane member, has the added advantage of preventing unwanted gaseous discharge at the back side of the target electrode while yet minimizing capacitive losses between it and the ground plane member.
The target electrode structure of the invention has other advantages which will be discussed or will become apparent from the following description of a preferred embodiment thereof and which make the structure most attractive for large area sputtering.
Brief description of the drawing With reference to the accompanying sheet of drawing: FIG. 1 is a partial side sectional view of a preferred embodiment of the sputtering apparatus of the invention; FIG. 2 is a partial elevational view of the apparatus depicted in FIGS 1 taken on a plane indicated by the lines 22 in FIG. 1;
FIG. 3 is an enlarged sectional view taken on a plane indicated by the line 3-3 of FIG. 1 illustrating the sinuous cooling tube arrangement for conducting heat from the target electrode; and
FIG. 4 is an enlarged partial sectional view taken on a plane indicated by the line 4-4 in FIG. 3 and showing the manner in which a cooling tube is embedded within the target electrode structure.
Detailed description of the preferred embodiment With reference to the accompanying drawing, a vacuum coating apparatus, generally referred to by the reference numeral 11, designed for the simultaneous coating of a plurality of substrate objects 12 is partially depicted. The target electrode structure with which the invention is concerned is especially adaptable for use in large volume vacuum coating apparatuses such as that described and claimed in the previously mentioned copending application Ser. No. 15,169, the disclosure of which is hereby incorporated by reference.
Vacuum coating apparatus 11 includes a chamber for defining a controlled environment for the formation of a gaseous discharge and the vacuum coating, such chamber being partially shown and referred to by the reference numeral 13. The target electrode structure of the invention, generally referred to by the reference numeral 14, is removably secured within the chamber 13. More particularly, the target electrode structure includes a ground plane member in the form of a circular plate or disc 16 and a target electrode 17 which is supported on the ground plate 16 by means of spaced ceramic insulators 18. In order to assure that a gaseous discharge is not formed in the space between the ground plate 16 and the target plate 17, these members should be spaced by a distance less than the mean free path within such space of the ionizing particles such as electrons so that a gaseous discharge and its consequent major power loss is not supportable in the space. In this connection, an annular shield flange 20 secured to the ground plate by means of a groove circumscribes the periphery of the target electrode plate. The distance between the shield 20 and the target electrode should also be less than the mean free path of the ionizing particles in the controlled environment so that it will aid in confining the target sputtering to the face 26.
The ground plate 16 is mounted within chamber 13 by an annular array of four equally spaced bolts 19' which extend inwardly from wall 21 of the chamber. Such bolts are welded or otherwise permanently secured to the inner surface of the wall 21 and register with radially extending slots 22 in the periphery of ground plate 16. Nuts 24 threadably received on the free end of bolts 19 bear against the plate 16 to maintain the same and, hence, the full target electrode structure within the chamber.
Target electrode 17 is also in the form of a disc or circular plate, and the outwardly facing surface 26 thereof provides the desired large area sputtering face. It will be appreciated that although in the embodiment illustrated the target electrode itself is of the material to be sputtered, in some instances it will be desirable to provide a separate material on the face 26 in order to obtain a sputter coat-v ing of that desired material.
As a particularly salient feature of the instant invention, includes means for adequately cooling the large area sputtering surface of target electrode 17 in a quite simple and effective manner. For this purpose, and as is best illustrated in FIG. 3, the back side or face 27 of target electrode 17 includes means defining a tubular passage distributed over such side for the circulation of a cooling medium, which means is recessed, or in. other words, embedded in the face to provide an extended surface area to receive heat from the target electrode for dissipation by the cooling medium. More particularly, the face 27 has a plurality of concentric circular grooves 28 cut therein. Such grooves communicate with one another at spaced radial positions by reentrantly curved grooves 29. As is seen from FIG. 3, the reentrantly curved grooves 29 connect alternate ones of the concentric circles 28 at opposite sides of a radial area which extends between-the periphery and center of the concentric groove arrangemenuand a radial groove 31 extends from the outer concentric groove to the center of the plate through such radial area.
The above described groove pattern enables a continuous piece of cooling medium tubing 32 to be recessed within the side 27 of target plate 17 along a path defined by a plurality of reentrantly connected, concentric circles which enable the passage of one end 33 of the tubing from the outer concentric circular groove to the center of the target adjacent the other end 34 of the tubing with out requiring traversal by the end 33 of any of the circular portions of the tubing. Thus, both ends of the tubing can be brought to one position on the target side 27 to' simplify the structure required for passing the tubing through the remainder of the apparatus whileyet enabling a full distribution of the tubing over the side of 'the'plate for good heat dissipation. I As depicted by FIG. 4, the tubing closely spaced relative thereto. That is, the groove is generally semicircular in cross section and the tubing is circular in cross section with the radius of its outer'sur by placing the coolingtubing in closer proximity to the sputtering surface 26 at which the heat is generated It has been found that the heat dissipation which can be provided by this arrangement is sufiiciently good to enable the use of a low thermal conductivity adhesive;
such as an epoxy resin to secure the tubing within the 3 groove. That is, since the tubing 32 is closely spaced to' the wall of the groove, an epoxy resin adhesive-36 can be used to secure the tubing to the target electrode plate without the low thermal conductivity of such adhesive appreciably reducing the flow of heat from the target electrode to the cooling medium within the tubing 32.
Those skilled in the art will appreciate the importance of this securing arrangement in enabling the elimination of the expensive, time consuming and critical brazing and welding techniques normally employed. Moreover; this ability to use an epoxy resin adhesive allows the joining of materials, such as a'luminurn'and copper, which otherf wise can not be joined by conventional brazing or wield-' ing techniques. As another important feature of the instantinvention,
the tubing 32 is recessed or embedded within the targeltii electrode structure 17 to a depth which assures thatwhe ni' the target electrode is mounted onthe ground plate 16, the tubing will be spaced from the ground plate a'sufiii" cient distance to prevent appreciable capacitive or other}. electrical losses between the ground plate and the target" electrode. By appreciable electrical losses is meantelec-' trical losses of such an extent that the power requirements? for the sputtering operation become too large for pr ac} tical economical use of the apparatus. This is particu larly important when the sputter target is used asa radio" frequency diode and a high frequency such as 13.56
32 conforms tethewall of the groove within which it is embedded and is" megahertz is applied directly to the target electrode. Capacitive losses in this case can represent a large power loss.
This recessing of the tubing within the target electrode enables the close spacing of the target electrode to the ground plate necessary to assure that unwanted gaseous discharge within the space between the same is not supportable while yet preventing appreciable electrical losses between the shielding and the target electrode.
With reference to FIG. 1, it will be seen that the ends 33 and 34 of the tubing 32 are brough through a central aperture 37 in ground plate 16 and pass through the wall 21 of chamber 13 by way of a simple small diameter vacuum feedthrough 38 for connection outside of the chamber to a source of a cooling fluid such as water. Such tubing can also be used to carry the biasing potential to the target and the target current. Thus, in order to remove the target electrode structure 14 from the chamber 13, it is merely necessary to disconnect the exterior connections to the tubing 32, release the vacuum feedthrough 38, and remove nuts 24 on bolts 22.
In accordance with conventional practice, means are provided for initiating and sustaining a gaseous glow discharge within the chamber 13 adjacent the target electrode face 26. For this purpose, an electrically conducting coil 39 is positioned in front of face 26 for energization by radio frequency energy. Means are also included within the chamber 13 for supporting one or more objects to be coated at a location at which they will receive material sputtered from the target electrode. That is, a rod 41 having diametrically opposed catch pins 42 which are fittable within the necks of the bottles 12, the objects to be coated, are provided in accordance with the arrangement described in the previously mentioned copending application Ser. No. 15,169.
The target electrode structure of the invention was successfully used in providing a sputtering target area having a 21.5 inch diameter. The target electrode was .5 inch thick aluminum disc having a .281 inch diameter and depth cut groove 33 for receiving the tubing in its rear side. The tubing 32 was nominal .25 inch diameter copper tubing, and the epoxy resin 35 was that marketed under the trademark ECOBOND 285. The ceramic insulators 18 were .75 inch long and slightly recessed into both the ground plate and the target electrode to provide a .5 inch spacing between the target electrode and the ground plate. It was found that a negative potential of 1500 volts D.C. could be applied to the target to attract ions from the gaseous discharge without undue capacitive losses to the ground plate. Moreover, the cooling arrangement provided a heat dissipation of almost 7 watts per square inch.
Although a preferred embodiment of the invention has been described in some detail, it will be appreciated by those skilled in the art that many variations are possible within the scope of the invention.
What is claimed is:
1. Apparatus for sputtering material from a face of a target electrode by bombarding the face with ions from a gaseous discharge, comprising a chamber defining a controlled environment for the formation of said gaseous discharge; means for initiating and sustaining a gaseous discharge within said chamber adjacent said target electrode face; and a target electrode structure within said chamber providing said face, said target electrode structure including a target electrode having said face at one side thereof and being supported with a side thereof opposite said face spaced from a ground plane by a distance less than the mean free path of ionizing particles within said controlled environment whereby a gaseous discharge is not supportable in the space therebetween, and means defining a tubular passage distributed over the side of said electrode opposite said face for the circulation of a cooling medium therethrough, said means being recessed into said side to provide proximity of an extended surface area of said means to said side of said electrode for conduction of heat from said target electrode to a cooling medium flowing through said means.
2. Apparatus for sputtering material according to claim 1 wherein said ground plane is defined by a conductive plate which is separably mounted within said chamber and said target electrode is insulatively supported on said plate whereby said ground plate and said target electrode are separable as a unit from said chamber.
3. Apparatus for sputtering material according to claim 2 wherein said means defining a tubular passage comprises a fluid tubing which is embedded within a groove in said side of said face, providing, said recessing to a depth spacing same from said conductive ground plate a distance preventing appreciable electrical losses between said ground plate and said target electrode, and wherein said tubing passes through an aperture in said ground plate and through a vacuum feedthrough to the exterior of said chamber.
4. Apparatus for sputtering material according to claim 3 wherein the distribution path of said tubing within said side of said plate follows a plurality of reentrantly connected, concentric circles providing a radial area free of circular portions for the passage within said side of one end of said tube to a position adjacent the other end thereof without requiring traversal of said tubing.
5. Apparatus for sputtering material according to claim 4- wherein said target electrode structure is in the form of a plate mounted on said ground plate, and a shield flange is secured to said ground plate circumscribing the periphery of said target electrode plate.
6. Apparatus for sputtering material according to claim 1 wherein said means defining a tubular passage comprises a fluid tubing which is embedded within a groove in said side of said face providing said recessing.
'7. Apparatus for sputtering material according to claim 6 wherein said tubing conform to the wall of the groove within which it is embedded and is closely spaced relative thereto in order to minimize the heat conduction path therebetween.
8. Apparatus for sputtering material according to claim 7 wherein said tube is secured within said groove by an epoxy adhesive.
9. Apparatus for sputtering material according to claim 6 wherein said tubing is embedded within said electrode side a depth spacing same from said ground plane a distance preventing appreciable electrical losses between said ground plane and said target electrode.
30. Apparatus for sputtering material according to claim 1 wherein said means defining a tubular passage on the side of said electrode opposite said face is distributed over said side in a sinuous path.
Sill. Apparatus for sputtering material according to claim 1 wherein said apparatus is a vacuum coating apparatus and means are provided for supporting an object to be coated at a location at which it will receive material sputtered from said target electrode.
References Qited UNITED STATES PATENTS 3,525,680 8/1970 Davidse et a1. 204-498 FOREIGN PATENTS 1,056,985 2/1967 Great Britain 204298 JOHN H. MACK, Primary Examiner S. S. KANTER, Assistant Examiner