This invention relates to sputtering apparatus capable of forming thin films having uniform thickness and crystal structure on cylinders or columns.
The term sputtering means as technique wherein a material mounted on a cathode electrode electrode is sputtered by the bombardment of ions formed by electric discharge and then deposited on an anode electrode or a substrate located close to the anode electrode to form a thin film of the material.
There have been proposed many types of the sputtering apparatus including diode or triode structure apparatus. More particularly, in one type electrodes themselves act as a target and a member to be coated with sputtered material respectively. In the other type, a third electrode is provided to form a plasma. In still another type a magnet is used to focus a plasma. However, direct current (DC) sputtering apparatus including two electrodes for creating glow discharge is used most widely.
FIG. 1 schematically illustrates such a DC sputtering apparatus. A cathode electrode 2 and an anode electrode 3, each in the form of a flat plate, are disposed in parallel opposed relationship in a vacuum chamber 1 connected to an evacuating system, not shown. The target material is mounted on the cathode electrode, or the cathode electrode itself is comprised by the target material, and a substrate 4 on which the sputtered material is to be deposited is secured on the surface of the anode electrode facing the target. The anode electrode is grounded while a high potential is applied to the cathode electrode. While maintaining a steady flow of argon gas, for example, through the vacuum chamber a high potential is applied across the anode and cathode electrodes to create glow discharge between them so as to bombard cathode electrode 2 with positively charged argon ions thus depositing the material of the cathode electrode on the substrate 4.
With the sputtering apparatus referred to above, however, as anode and cathode electrodes are made of parallel flat plates, the distribution of electric field established between these electrodes is not uniform at the edges thereof. Thus, at the central portion of electrodes, lines of electric force are perpendicular to the surface of electrodes but are bowed at the edges. This means that the speed of deposition of the sputtered material on the substrate secured to the surface of the anode electrode is different at the central portion and at the edge thus resulting in the nonuniformity of thickness and crystal structure of the deposited film.
Moreover, because both the substrate and target are flat, the resulted film is also limited to flat one. Where a cylindrical substrate is used, it is necessary to provide a mechanism for rotating the substrate.
An object of this invention is to provide a sputtering apparatus capable of establishing a uniform field distribution in a discharge space.
Another object of this invention is to provide a new and improved sputtering apparatus which can form sputtered films having crystals of uniform thickness and structure.
Yet another object of this invention is to provide a sputtering apparatus capable of forming thin films on the surface of cylindrical or columnar substrate.
According to this invention, these and other objects can be accomplished by providing a sputtering apparatus comprising a vacuum chamber in which a gas is introduced a first cylindrical electrode disposed in said chamber, a second electrode disposed in said first electrode, said second electrode having an outer peripheral surface of substantially the same configuration as the inner peripheral surface of said first electrode, and means to establish a plasma between said first and second electrodes.
The first electrode acts as the cathode while the second electrode as the anode. The cathode electrode is supplied with a negative high potential whereas the anode electrode is grounded. For example, the cathode electrode is made of copper and the anode electrode is made of tungsten. After introducing a suitable gas in vacuum chamber, when discharge is created by impressing a high voltage across the anode and cathode electrodes, the copper comprising the cathode electrode is bombarded by the ions of the gas to deposit copper on the surface of the anode electrode. With the above described arrangement the cathode electrode serves as of target material and the anode electrode acts also as a substrate on which the sputtered material is to be deposited.
Because cathode and anode electrodes are cylinders disposed concentrically, the field distribution between them is quite uniform thus increasing the effective discharge space region which is particularly effective to obtain uniform thickness and crystal structure of the deposited film. Further, in accordance with this invention it is very easy to form thin films on the surface of cylinders without the necessity of rotating the substrate by a rotating mechanism of special design as heretofore been the practice.
While in the above described arrangement a thin film is formed directly on the surface of the anode electrode, such film may be formed on an independent cylinder which is concentrically disposed around the anode electrode.
By reversing the relative position of cathode and anode electrodes, a thin film may be formed on the inner surface of a cylindrical body.
In accordance with a modified embodiment of this invention the sputtering apparatus comprises a vacuum chamber an electroconductive rod disposed in the chamber, a cylindrical target coaxially disposed about the rod, and a cathode electrode and an anode electrode disposed adjacent opposite openings, respectively of the target. This embodiment is a sputtering apparatus of the three electrode type. In operation, a suitable gas is introduced in the vacuum chamber, a low voltage is impressed across the anode and cathode electrodes to create electric discharge and a negative potential is applied to the target. Positive ions of the gas formed by the discharge are caused to bombard the target so that sputtered target material is deposited on the surface of the rod through the plasma. With this embodiment the rate of deposition of the film is increased, contamination by the residual gas is decreased and a film of uniform thickness and crystal structure can be formed on the surface of the cylindrical or columnar rod.
The invention is now described in conjunction with a preferred embodiment with reference to the accompanying drawing, in which:
FIG. 1 is a longitudinal sectional view schematically illustrating an electrode arrangement of a prior art sputtering apparatus;
FIG. 2 shows a side elevation, partly in section, of one embodiment of the sputtering apparatus according to the present invention;
FIG. 3 is a perspective view of electrodes of a modified sputtering apparatus according to the present invention; and
FIG. 4 shows a longitudinal side elevation, partly in section, of another modification of this invention.
The sputtering apparatus of this invention will be described in detail by referring to FIGS. 2 to 4 of the accompanying drawing.
As shown in FIG. 2, within a vacuum chamber 11 of glass or metal, tungsten, for example and connected on an evacuating system, not shown, is disposed a cylindrical cathode electrode 12 or a first electrode of tantalum and having dimensions of 1.0 mm. thick, 30 mm. inside diameter and 50 mm. height. Within the cathode electrode is concentrically disposed a cylindrical or columnar nickel anode 13, of 1.8 mm. in diameter. The anode electrode 13 is secured to a support 14 which is grounded. A negative high voltage is supplied to cathode electrode 12 through a conductor 15 connected to a power source of high voltage, not shown, the conductor 15 being insulated from the support 16 by means of an insulator 17.
If desired a cooling device for the cathode electrode 12 and a cylindrical shutter may be provided between anode and cathode electrodes.
In operation, the interior of the vacuum chamber 11 is evacuated by the evacuating system. When a pressure of the order of 1×10-6 torr. is reached, argon gas is introduced into the chamber through a variable leak valve 18 to a pressure of 10-1 -10-2 torr. and a voltage of from 3 to 5 kv. is impressed across the cathode and anode electrodes. It is advantageous to utilize a high reactance transformer to prevent an abnormal discharge.
Generally the current capacity is determined dependent upon the area of the cathode electrode. In this case the current density amounts to several milliamperes/cm.2 on the average. Under these conditions a maximum rate of deposition of tantalum of 10 A./min. can be obtained, and the deposited film has substantial adhesion and hardness. With this sputtering apparatus a film comprised by sputtered particles can be formed on the external surface of anode electrode 13. Of course cathode electrode 12 acts as a target during sputtering.
Positively charged ions of gas created by the electric discharge bombard cathode electrode 12 with high energy to sputter atoms of the material comprising the target. Under this condition substantially equal number of positively charged ions and negatively charged particles or electrons exist to form a plasma. In front of the cathode cylinder 12 facing to the plasma there is formed a region termed as the cathode drop in which the density of the charged particles is low and the intensity of the electric field is high. Positively charged ions are accelerated in this cathode drop region to bombard cathode electrode 12 thus causing sputtering as well as secondary electron emission (γ function). Electrons emitted by the γ function are accelerated to further ionize the discharge gas to sustain discharge.
While in FIG. 2, cathode and anode electrodes 12 and 13 are shown concentric, it is not always necessary. Thus for example, even when the axes of anode and cathode electrodes are not in exact alignment the thickness of the film deposited on the surface of anode electrode 13 becomes slightly thinner at portions remote from the inner surface of the cathode electrode. Where it is desired to locally vary the thickness of the deposited film, cathode and anode electrodes may be arranged eccentrically. Should both electrodes become eccentric by some reason, where it is desired to obtain a film of uniform thickness, the anode electrode may be rotated.
Although in the above described example films are formed on the surface of anode electrode 13, such films can be formed on the inner surface of a cylinder in the following manner. This can be accomplished by mere change of the electrode material and the relative polarity of the potential. More particularly, in the arrangement shown in FIG. 2, anode electrode 13 is made of a target material, cathode electrode 12 is grounded and a high negative potential is applied to anode electrode 13. Then the target material comprising the anode will be sputtered and deposited on the inner surface of cathode electrode 12.
While in the sputtering apparatus shown in FIG. 2, films are formed on the surface of anode electrode 13, in the modification shown in FIG. 3, the films are deposited on the surface of a cylinder independent of the anode electrode.
More particularly, as shown in FIG. 3, an additional cylinder 19 of glass, for example is mounted on support 14 to coaxially surround anode electrode 13. Upon application of high voltage across cathode and anode electrodes 12 and 13 to create glow discharge therebetween, the material, tantalum for example, of the cathode will be sputtered to deposit on the outer surface of glass cylinder 19. By vertically reciprocating the glass cylinder the width of the deposited film can be increased. Where it is desired to deposit a film on the inner surface of glass cylinder 19, as above described, the relative potential of anode and cathode electrodes is reversed and the anode electrode is made of a target material such as tantalum. As diagrammatically shown in FIG. 3, the lower end of anode electrode 13 is received in an opening (not shown) in support 14 and secured thereto by means of a set screw 20. The glass cylinder 19 may be merely placed on support 14.
To prepare a thin film of an insulator a cylinder of the insulator such as SiO2, Si3 N4, metal oxides of high melting point such as Ta, Nb, Zr, Ti and the like is fit in the cathode electrode 12 shown in FIG. 2 and a high-frequency voltage of about 10 MHz. is applied to the cathode electrode. Then positively charged particles collected on the surface of the cylindrical insulator will be periodically neutralized by negatively charged particles having a mobility several thousand times larger than the former so that the surface of the insulator will become negative with respect to the grounded anode electrode at each half cycle of the high-frequency voltage, during which sputtering is effected to deposit a film of the insulator on the surface of anode electrode 13.
This example illustrates a modified sputtering apparatus employing three electrodes. Thus, as shown in FIG. 4, a cylindrical target or a first electrode 32 of tantalum and having dimensions of 1.5 mm. thick, 70 mm. inside diameter and 50 mm. height is disposed in a vacuum chamber 31 connected to an evacuating system, not shown, and a glass or quartz tube to be coated 33, 10 mm. inside diameter, 80 mm. height and 1.0 mm. wall thickness is mounted on a support 34 of stainless steel for example. Within tube 33 is disposed an electroconductive metal rod 35, or a second electrode made of stainless steel and having a diameter of 5 mm., which is also supported by support 34. An anode electrode 36 comprising a metal circular disk is disposed close to and in parallel with the upper surface of target 32. A cathode electrode 37 is disposed spaced from the lower end of target 32. Target 32 is maintained at a high negative potential with respect to metal rod 35 while the metal rod 35 and the cathode electrode 37 are grounded. Anode electrode 36 is maintained at a low positive potential. As shown, a focusing magnet 38 is disposed on the outside of vacuum chamber 31 in parallel with target 34. The purpose of the focusing magnet is to combine the plasma created between anode electrode 36 and cathode electrode 37 within a predetermined region.
The operation of this modification is as follows:
When the interior of the vacuum chamber 31 is evacuated to a vacuum of the order of 1× 10-6 torr. argon gas is introduced into the chamber through a variable leak valve 39 to a pressure of 1× 10-3 torr.
Then a potential of 50 V is impressed across anode electrode 36 and cathode electrode 37 to establish a plasma while at the same time a DC power of 1.0 kv. and 60 milliamperes is applied to target 32. The plasma is formed inside target cylinder 32 and the positively charged ions in the plasma bombard the target 32 to deposit a tantalum film on the outer surface of tube 33.
In the foregoing examples 1 and 2 although sputtered films are deposited on the outer surface of a cylindrical member, no film is formed on the inner surface thereof. Where it is desired to form films on the inner surface of tube 33, target 32 and electroconductive metal rod 35 may be interchanged with consequent reversal of the polarity of the potential as has been discussed with reference to Example 1.
As above described, since in accordance with this invention, parallel plate electrodes are not employed as in the prior art but instead a target cylinder and an electroconductive metal rod respectively acting as a cathode electrode and an anode electrode are disposed concentrically, it is possible to increase the volume of uniform discharge, thus resulting in uniform thickness and crystal structure of the deposited film. Further, it is possible to deposit films on both inside and outside surfaces of a cylinder.