Sputtering Apparatus

Abstract

Sputtering apparatus of the glow discharge type operable at relatively low gas pressures and utilizing a combination of magnetic and electric fields and at least one cathode disposed within the magnetic field and constituting a source of atoms which are emitted through a path defined by the cathode for deposition of thin films on a suitable supporting surface.

Description

This invention relates to sputtering apparatus for production of thin films of metal and dielectric material and more specifically to a novel and improved sputtering device wherein cathode disintegration is effected by an electric glow discharge and cooperating magnetic field in an improved manner that permits operation at very low gas pressures and provides films having adherence adherence and improved uniformity in mechanical and electrical properties.

The production of thin films utilizing cathode disintegration is well known and is effected by gas ions impinging upon the cathode and physically ejecting atoms from the cathode surface. This phenomenon is generally known as sputtering and is particularly useful for coating bodies with thin films. In coating a body, it is generally positioned in the path of the atoms ejected from the cathode surface so that the atoms will be deposited on the surface of the body. In addition to the utilization of an electric field to produce the glow discharge, magnetic fields have been employed in an effort to obtain some improved operation. However, known devices whether using an electric field alone or in combination with a magnetic field have not been found to be entirely satisfactory for a number of reasons. For instance, known sputtering devices including those using magnetic fields require relatively high gas pressures with the result that the gas atoms within the device tend to deflect many of the sputtered atoms away from the body to be coated causing relatively low deposition efficiency. Furthermore, the relatively high number of gas atoms present in the apparatus causes many of them to become entrapped in the metal film being deposited producing nonuniform films wherein the density varies throughout the area of the film. Known devices also require that the surface to be plated either form part of the anode or cathode or be positioned in the apparatus in such a manner that operation of the apparatus must be terminated upon completion of the coating operation in order to insert a new workpiece or pieces to be coated, and substantial difficulty has been experienced in attaining satisfactory adherence of the film to the substrate or workpiece.

With this invention which embodies a novel and improved arrangement of elements including electric and magnetic fields, many difficulties heretofore encountered with known devices have been overcome and an improved device is provided having a relatively high deposition or disintegration efficiency, producing a film of substantially uniform density throughout its area, and greatly improved adherence of the film to the work surface whether the work surface is conductive or nonconductive. By the improved coordination of the electric and magnetic fields, the magnetic field in accordance with the invention functions to deflect electrons from the paths they would normally follow in the absence of such magnetic field, and furthermore directs the electrons in such a manner that the lengths of the paths are greatly increased. This action greatly increases the number of ionizing collisions between the electrons and the gas molecules. As a result, the gas pressure in apparatus in accordance with this invention can be materially reduced while at the same time maintaining a relatively high discharge current. Furthermore, since the invention facilitates the use of relatively low gas pressures the deposition efficiency is greatly improved and a greatly increased quantity of the liberated atoms will be deposited on the surface to be coated. In addition, the material reduction in the number of gas atoms present within the device results in a corresponding reduction in contamination of the film by the inclusion of gas atoms therein. Through an improved arrangement and organization of elements the substrate or workpiece being coated assumes an automatic high-bias voltage which provides a vastly improved film having excellent adherence. This is particularly important in the case of nonconductive substrates.

Another object of the invention is to produce a novel and improved relatively high-efficiency sputtering device for coating surfaces with thin films wherein the surfaces to be coated need not form part of either the anode or the cathode.

Still another object of the invention resides in the provision of a novel and improved sputtering apparatus utilizing a relatively low gas pressure and at the same time producing a highly stable and intense glow discharge between the cathode and the anode which greatly facilitates operation of the apparatus and produces high quality, high-density films.

A still further object of the invention resides in the provision of a highly efficient and stable sputtering device that is adaptable for use in normal production operations in that a plurality of individual surfaces may be successively coated without interrupting the operation of the device.

The above and other objects of the invention will become more apparent from the following description and accompanying drawings forming part of this application.

In the drawings:

FIG. 1 is a cross-sectional view of one embodiment of the apparatus in accordance with the invention and which embodies a glow discharge device;

FIG. 2 is an enlarged cross-sectional view of the glow discharge device illustrated in FIG. 1; and

FIG. 3 is a modified form of glow discharge device in accordance with the invention.

Referring now to FIG. 1, the sputtering apparatus in accordance with the invention includes a baseplate 10 disposed in a substantially horizontal position and having a bell jar 11 removably sealed to the top surface of the plate 10 by suitable sealing means 12. The sputtering apparatus is generally denoted by the numeral 13, and as will be described, is sealed to the bottom side of the baseplate 10 and opens into the interior of the bell jar 11. A circular work-supporting plate 14 is disposed within the bell jar and is pivotally mounted at 15 to the upper surface of the plate 10. The plate 14 can be rotated by any suitable means as for instance a motor 16 carrying a drive wheel 17 of resilient material which contacts the periphery of the plate 14. It is evident that other forms of work or substrate supporting surfaces may also be used.

A conduit 18 extends through the baseplate and is connected on its outer end to a two-way valve 19. A second conduit 20 connects the valve to a vacuum source for exhausting air from within the bell jar 11 and the sputtering device 13. A third conduit 21 has one end connected to the valve and the other end connected to a source of inert gas such as argon or the like so that the gas pressure can be adjusted to the desired magnitude. Reactive gases such as oxygen and nitrogen can also be used when metal and dielectric films such as tantalum nitride or aluminum oxide are to be formed. To remove the bell jar 11 when the plating process has been completed, air can be admitted by disconnecting the vacuum source and opening conduit 20 to the atmosphere.

In the instant embodiment of the invention the circular plate 14 has a plurality of openings 22 spaced uniformly from the center of the plate and the work pieces or substrates on which thin films are to be deposited overlie these openings. During the operation one of the pieces 23 is aligned with the sputtering device 13 and is maintained in that position until the desired thickness of film is deposited on the underside thereof. Then without interrupting the operation of the sputtering device 13, the plate 14 is rotated to bring the next successive work piece into alighment with the sputtering device, and it remains in that position until the desired film is deposited thereon. The operation is then continued in this manner until all of the work pieces have been coated. If desired, the plate 14 can be continuously rotated so that each workpiece or substrate will receive some sputtered material during each revolution. Operation of the sputtering device 13 is then terminated and air is admitted into the bell jar 11 by the valve 19 which permits the bell jar to be raised for removal of the coated workpieces 23 and placement of the next set of work pieces to be coated. The glow discharge for operating the sputtering device 13 is obtained by applying substantially zero voltage through lead 24 to the housing or outer casing thereof and a high negative voltage to leads 25 and 26 which connect with the cathodes as will be described. A fourth lead 27 may be connected with the plate 14 to apply an additional bias voltage to the plate and thus to the workpieces when the latter are of a conductive material.

The sputtering device 13 is shown in detail in FIG. 2 and corresponding elements of FIGS. 1 and 2 have been denoted by like numerals. In the instant embodiment of the invention, the anode 28 is an essentially cylindrical tube and has an annular flange 29 which is secured to the underside of the base 10 by screws or other fastening means 30. The anode 28 is aligned with an opening 31 in the baseplate 10 and the flange 29 includes an annular seal 32 to insure an airtight attachment of the anode to the baseplate.

A first cathode 33 preferably of a hollow cylindrical configuration extends downwardly through the baseplate and into a portion of the anode 28. The top edge of the cathode 33 includes an annular flange 34 having a recess 35 in the underside thereof for receiving a ring 36 of insulating material. The ring 36 may be cemented in the recess 35 and to the top surface of the base plate 10 to maintain the cathode in concentric relationship with the anode. With this arrangement, the cathode 33 is insulated from the anode to permit the application of a high negative voltage thereto by the lead 25. While it is desirable to have the configuration of the cathode 33 conform with the internal configuration of the anode 28, the spacing between the anode and the cathode should be as small as possible in order to prevent a glow discharge from occuring therebetween. Furthermore, the cathode 33 though illustrated as being of tubular configuration may of course take other suitable forms. It is generally desirable however that the cathode have a substantially uniform cross-sectional configuration in order to produce more uniform films, but such a configuration may be modified depending on the particular film requirements. In any event, ion scrubbing can be achieved which enables the attainment of improved adherence of the film to the substrate and film contamination is materially reduced.

The bottom of the anode 28 is closed by an annular member 37 which is hermetically sealed in position on the anode by an annular weld 38 or other suitable sealing means. The member 37 carries a second cylindrical cathode 39 which is centrally disposed within the anode 28. The supporting means for the cathode 39 consists of a metallic sleeve 40 which extends through the member 37 and has an annular flange 41 which engages the top side of the member 37 and is sealed thereto by a gasket 42. The lower end of the metallic sleeve 40 has threads 43 for engaging a nut 44 to firmly hold the sleeve 40 in position within the opening 37' of the member 37. An insulating sleeve 45 extends through a central opening in the metallic sleeve 40 and a rod 46 of conductive material extends through the sleeve 45. A spacer 47 of insulating material surrounds the upper portion of the insulating sleeve 45 and the cathode 39 threadably engages the upper end of the rod 46. The insulating sleeve 45 is hermetically sealed to the rod 46 and to the metallic sleeve 40 and the lower end of the rod 46 terminates in a contact 48 to which the lead 26 is attached. An annular magnet 49 preferably of permanently magnetized material surrounds the anode 28 and the upper end of the magnet preferably extends beyond the lower edge of the upper cathode 33.

With the invention as described above, the anode 28 is connected to ground or substantially zero potential by the lead 24 while a voltage of the order of 1,000 volts negative is applied to both the leads 25 and 26. Under these conditions the glow discharge can be produced with an internal pressure of the order of 2.times.10.sup..sup.-3 torr. The magnet 49 produces a magnetic field in the direction of the arrow A at a point centrally of the magnet and the cathode 33 is either formed of or carries one or more materials to be sputtered. Under these conditions atoms will be liberated from the cathode and move upwardly striking the underside of the workpiece 23 and forming a film thereon. In actual practice it has been found that when the cathode 33 is about 2 inches in diameter, it is preferable to maintain a distance between the underside of the workpiece 23 and the lower edge of the cathode 33 of about 6 centimeters. If this distance is exceeded, it has been found that the coating rate may be reduced and the film deposited tends to be nonuniform. Furthermore, it is also preferable that the lower edge of the cathode extends below the plane of the upper edge of the permanent magnet 49 a distance of approximately one-quarter of an inch for maximum sputtering efficiency.

In the operation of the device in accordance with the invention, the substrate or workpiece whether of a conductive or nonconductive material attains almost immediately a bias voltage of about 50 volts negative when floating electrically. This self-biasing feature is most advantageous since the surface of the workpiece is automatically "scrubbed" by ions attracted by the surface, and this increases the adherence of the film to the workpiece. Ion-scrubbing effects removal of many contaminants such as water vapor and hydrocarbons which not only interfere with adhesion but also contaminate the film. In prior known devices self-biasing voltages of the order of 6 to 8 volts are generally attained but such voltages are below the threshold for ion- bombardment cleaning and thus have no effect. Moreover, the use of probes for biasing is not helpful since a conductive base film of at least 50 angstroms in thickness is required for affixing the probe and there is no affect on either the base film or its adhesion to the workpiece. If desired, a bias can be applied to the workpiece 23 by means of lead 27 which of course would fix the bias to any desired value. It is of course evident that the anode of the sputtering device is formed of a nonmagnetic material so that the desired magnetic field will be produced within the device.

The form of the invention described in connection with FIG. 2 is particularly useful when a DC voltage is applied between the anode and the cathodes and the reentrant cathode 33 extends below the top surface of the magnet 49. In this way a high-intensity glow discharge is obtained with a high degree of stability notwithstanding the relatively low gas pressure within the device. It is believed that the high-intensity glow discharge results from the fact that the specific relationship of the magnetic field to the cathodes deflects electrons from the paths which they would normally follow in the absence of such a field with the result that the lengths of such paths are increased. This action increases the number of ionizing collisions between the electrons and the gas molecules and such collisions are necessary in order to obtain a high deposition rate of atoms from the cathode 33 onto the work piece 23.

A modified embodiment of the invention is illustrated in FIG. 3 and differs from the structure shown in FIG. 2 in that the cathode 39 is omitted. Since both forms of the invention are identical with the exception of the cathode 39 corresponding elements are denoted by like numerals. With the elimination of the cathode 39, the bottom of the anode 28 is merely closed by a plate 28' hermetically sealed thereto. This form of the invention operates in the same manner as the form of the invention described in connection with FIG. 2 and produces an automatic bias on the substrate being plated.

With both forms of the invention described above, operation can be effected with exceedingly low gas pressures as low as 2.times. 10.sup..sup.-3 torr., which results not only in purer films but also in increased adherence of the film to the substrate or work piece. Furthermore, since the workpiece need not be secured to the cathode or anode, the sputtering apparatus is particularly adaptable for use in continuous production processes as there are well-known means for moving the substrates in position for plating and removing finished workpieces without interruption of operation. While the forms of the invention have been illustrated with the axis of the anode in a vertical position, it is of course possible to operate the sputtering device in any desired position.

While only certain embodiments of the invention have been illustrated and described, it is apparent that alterations, modifications and changes may be made without departing from the true scope and spirit thereof as defined by the appended claims.

Claims

I claim

1. Sputtering apparatus for coating a workpiece comprising an anode shell of conductive material having an opening on one end and a conductive closure hermetically sealing the other end, an open-ended tubular cathode at said one end of said anode shell and having at least a portion disposed within an in close proximity to said anode shell to provide uninterrupted access into said anode shell, means for supporting work to be coated at said one end of said anode shell to intercept atoms emerging from the cathode, means hermetically enclosing said one end of said anode shell, said cathode and said work-supporting means and means producing magnetic and electric fields in said anode shell with the direction of said magnetic field being disposed centrally of said anode shell and substantially axially aligned therewith, said magnetic field having a nonlinear portion intercepted by said tubular cathode.

2. Sputtering apparatus according to claim 1 wherein said magnetic field- generating means surrounds said anode shell.

3. Sputtering apparatus according to claim 1 wherein said cathode extends from a point spaced outwardly from the other end of said anode shell into said anode shell and intercepts said nonlinear portion of said magnetic field.

4. Sputtering apparatus according to claim 1 wherein said magnetic field-generating means comprises a magnet surrounding said shell and wherein said cathode extends inwardly from the other end of said shell to a point beyond the plane defined by the adjoining edge of said magnet.

5. Sputtering apparatus according to claim 4 wherein said cathode, has an average internal diameter of about 2 inches and extends into said anode a distance at least one fourth inch beyond the plane defined by said adjoining edge of said magnet.

6. Sputtering apparatus according to claim 1 including a second cathode within and insulated from said anode shell, said second cathode being positioned in spaced relationship to the first said cathode and electrically interconnected therewith.

7. Sputtering apparatus comprising an open-ended tubular anode of nonmagnetic conductive material, nonmagnetic conductive means closing one end of said anode and electrically connected to said anode, magnetic field means producing magnetic flux within and in a direction generally in line with the central portion of the axis of the anode and having nonlinear end portions, an open-ended tubular cathode projecting into the other end of said anode and intercepting at least a part of a nonlinear portion of said magnetic field, a work support facing the opening in said tubular cathode, means enclosing and sealing the other end of said anode, said cathode and work support whereby said anode can be at least partially evacuated and means for applying a voltage between said anode and cathode to produce a glow discharge causing gas ions to strike said cathode and liberate atoms from said cathode for deposition on said work surface.

8. Sputtering apparatus according to claim 7 wherein said magnetic-field-producing means includes a permanent magnet surrounding said tubular anode and said tubular cathode extends into said anode to a point beyond the adjoining end of said magnet.

9. Sputtering apparatus according to claim 8 wherein said cathode and magnet overlap at least one fourth inch.

10. Sputtering apparatus according to claim 7 including a second cathode disposed within and insulated from said anode.

11. Sputtering apparatus comprising an anode shell of conductive material and having an opening on one end and a conductive closure hermetically sealing the other end, an open-ended tubular cathode adjoining said open end and at least partially disposed within said anode shell, means supporting a substrate to be plated in proximity to said cathode, means hermetically enclosing said one end of said anode shell, cathode and work-supporting means to permit evacuation of said anode shell and produce a gaseous atmosphere therein, and means producing electric and magnetic fields within said anode and cathode shell with at least a portion of the magnetic field being nonlinear and intercepted by said cathode, said fields producing a glow discharge therein and causing atoms released from said cathode to be deposited on said substrate and form a film thereon, said electric and magnetic fields producing a relatively high negative bias on said substrate to effect cleaning of said substrate by ion bombardment and minimize contamination of the film deposited thereon.

12. Sputtering apparatus according to claim 11 including a second cathode disposed within said anode and in spaced relationship to the first said cathode.

13. Sputtering apparatus for coating a workpiece comprising a single open ended hollow shell, magnetic means surrounding said shell to produce a magnetic field within said shell having a substantially linear central portion and nonlinear end portions, a cathode having an opening therein adjoining the open end of said shell, the surface of said opening carrying material to be sputtered, said cathode opening intercepting at least a portion of the nonlinear magnetic field, means including said cathode and an anode for producing an electric field within said shell and work-supporting means spaced from said cathode opening and on the side of said cathode opposite said shell.