Radio Frequency Sputter Apparatus And Method

Abstract

A radio frequency sputter apparatus is disclosed. The apparatus includes first and second spaced electrode structures with a grounded shield structure disposed around the outside of one of the electrode structures to define a sputter cathode or target electrode. Radio frequency energy is applied to the space between the electrode structures to produce a radio frequency plasma discharge in the region between the electrode structures. A radio frequency inductor is placed in a direct current circuit connection between the non-target electrode (substrate table) and ground potential. The target electrode is permitted to self bias to a negative d.c. potential. Heating of the non-target electrode and objects being plated supported therefrom is substantially reduced due to the provision of the inductor. A switch is provided for selectively connecting the non-target electrode directly to ground with or without the inductor in the connection to ground.

Parent Case Text

This application is a continuation of Ser. No. 832,246 filed June 11, 1969 and now abandoned.

Description

DESCRIPTION OF THE PRIOR ART

Heretofore, radio frequency sputter apparatuses have been built wherein one of the electrode structures was permitted to float at a self biased negative d.c. potential relative to the opposed substrate electrode which was grounded. Objects to be sputter plated were disposed on the grounded substrate electrode And positive ions generated within the plasma discharge bombarded target material disposed adjacent the self biased electrode to produce sputtering of target material onto the objects disposed on the substrate table electrode. Such an apparatus, when it was operated at substantial power levels, as of 2.5 kilowatts of energy being dissipated in the r.f. discharge, caused substantial heating of the objects on the substrate table. More particularly it was found that when plating certain substrate members, such as silicon, with certain material, such as gold or copper, that excessive heating Of the objects to be plated was obtained, such heating exceeding 356.degree. C which, when depositing gold, resulted in producing an alloy of the gold with the substrate material. Such prior art sputter apparatuses are described and claimed in copending U.S. applications Ser. No. 662,637 filed Aug. 23, 1967; Ser. No. 625,733 filed Mar. 24, 1967, and Ser. No. 674,539 filed Oct. 11, 1967 all assigned to the same assignee as the present invention.

While such prior art apparatuses are particularly useful for sputter plating many devices where substantial temperatures of the surface being plated can be tolerated, there exists a need for r.f. sputter apparatus wherein the temperature of the substrate can be maintained below certain temperatures at which alloying occurs between the sputter layer and the surface material being plated. The removal of heat from the substrates is difficult and has not been successful at practical substantial power levels.

SUMMARY OF THE PRESENT INVENTION

The principal object of the present invention is the provision of an improved radio frequency sputter apparatus.

One feature of the present invention is the provision of a radio frequency inductor connected between the substrate table electrode and ground while permitting the opposed target electrode structure to assume a self bias d.c. potential, whereby the inductor serves to reduce the heating of the objects being plated as carried upon the substrate electrode structure.

Other features and advantages of the present invention will become apparent upon a perusal of the following specification taken in connection with the accompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic line diagram of a radio frequency sputter apparatus incorporating features of the present invention,

FIG. 1A is an alternative circuit for that portion of FIG. 1 delineated by line 1A--1A, and

FIG. 2 is a schematic circuit diagram of another sputter apparatus incorporating features of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 1, there is shown, in schematic line diagram form, a radio frequency, r.f., sputter apparatus 1 incorporating features of the present invention. Briefly, the r.f. sputter apparatus 1 includes a bell jar 2 forming an evacuable chamber in gas communication with a vacuum system 3 utilized for evacuating the bell jar to a suitable operating pressure such as 2 .times. 10.sup.-.sup.5 Torr. A suitable ionizable gas, such as argon, is utilized as the fill, at a pressure of 5 .times. 10.sup.-.sup.3 Torr, for the bell jar 2 such that an intense plasma discharge may be obtained therein. A pair of r.f. electrodes 4 and 5, respectively, are supported within the bell jar 2 and are preferably flat plate structures, as of copper. A grounded shield electrode structure 6 is disposed around the outside of one of the electrodes 4. A grounded radio frequency screen shield 7 is disposed outside the surface of the bell jar 2 to prevent escape of radio frequency energy from the bell jar 2.

A radio frequency generator 8 at a suitable radio frequency, as of 13.56 megahertz and having a relatively high power output as of 2.5 kilowatts, is coupled to electrode 4 via an impedance matching transformer 9 and a variable coupling capacitor 11, as of 10 to 300 picofarads. A second variable capacitor 12, as of 10 to 300 picofarads, is connected between the electrode 4 and ground for impedance matching the radio frequency generator 8 to the impedance of the radio frequency discharge produced in the region between electrodes 4 and 5. Capacitors 11 and 12 are adjustable for impedance matching and they serve to isolate electrode 4 from ground potential for d.c. potentials, such that electrode 4, in operation, may assume a d.c. potential independent of ground potential.

Electrode 5 forms a substrate table onto which objects 13 which are to be plated are disposed. Objects 13 to be plated may be conductive or nonconductive. A target material 14 is placed adjacent target electrode 4 to be bombarded by ions generated within the plasma discharge region between electrodes 4 and 5 to produce sputtering of the target material 14 onto the objects 13 to be plated or coated. The target material 14 may be conductive or nonconductive. Typical target materials include gold, copper, stainless steel, alumina, glass, and many other materials.

The apparatus 1 of FIG. 1 may be operated in two modes depending upon the circuit connections made to the electrodes 4 and 5. More specifically, a pair of at least two position switches 15 and 16, respectively, are ganged together by means of a mechanical interconnection 17. In a first position of the switches 15 and 16, indicated by I, the upper or target electrode 4 is connected via switch 15 to a floating open circuit terminal 18, whereas the sbustrate table electrode 5 is connected to ground. This is the conventional prior art r.f. sputter plating connection. In operation, the radio frequency energy applied across the r.f. electrodes 4 and 5 produces a radio frequency plasma discharge in the region between electrodes 4 and 5 resulting in a plasma discharge zone 19. In the plasma discharge zone 19, there are approximately as many positive ions as negative electrons such that the plasma zone is electrically neutral and it assumes a relatively low potential with respect to ground, such as plus 100 volts, depending upon the gas utilized in the discharge and the pressure of the discharge. The plasma discharge zone 19 does not extend to the surfaces of the electrodes 4 and 5. More specifically, it is found that a dark space sheath is formed immediately adjacent the interior surfaces of the electrodes 4 and 5 with the thickness of the dark sheath being substantially greater adjacent the target electrode 4.

In the presence of the r.f. plasma discharge, the target electrode structure 4, which also includes the target material 14 when it is conductive, assumes a negative d.c. potential which is approximately equal to the magnitude of the average of the peak-to-peak amplitude of the r.f. potential applied across electrodes 4 and 5. This relatively large d.c. self biased potential which develops on electrode 4 has been attributed to the high mobility of the electrons relative to that of the ions. Thus, during one cycle of operation, more electrons are attracted to the electrode 4 during the positive half cycle than there are ions attracted during the negative half cycle, such that the negative charge builds up on the electrode, including the target electrode if it is conductive. The result is that electrode 4 and the target 14, when it is conductive, acquires a large negative d.c. potential with respect to the plasma, such potential typically falling within the range of -1 KV to -3 KV and being a function of the gas utilized to form the plasma discharge and the pressure of the discharge.

Positive ions within the discharge zone 19 are accelerated across the dark sheath and bombard the target 14 with an energy corresponding to the d.c. bias, namely, typically about 2.3 KV. This results in sputtering of the target material. The sputtered material passes through the discharge zone 19 and impinges upon the devices 13 to be plated, as disposed upon the opposed substrate table electrode 5.

When the positive ions bombard the target material 14, in addition to dislodging target material, they also knock out secondary electrons in accordance with the secondary emission ratio of the target material. These secondary electrons "see" the r.f. voltage as superimposed on the self bias potential and under certain phase angles of the r.f. voltage they "see" very substantial accelerating voltages which propel the secondary electrons through the neutral plasma discharge 19 to the substrate table electrode 5. Since the discharge zone is operating at a relatively low pressure, for example, 1 .times. 10.sup.-.sup.3 Torr to 3 .times. 10.sup.-.sup.3 Torr the mean free path for the electrons is on the same order of magnitude as the typical dimensions of the evacuated system and, therefore, a large fraction of the propelled secondary electrons will pass through the discharge and strike the plating which is being deposited upon the objects 13, thus, producing unwanted heating of the plating by the secondary electron bombardment.

In addition, positive ions are present in the plasma zone 19. Since the plasma zone is operating at a potential slightly positive with respect to ground potential, i.e., 100 volts, a certain small fraction of the positive ions within the discharge will be drawn out of the discharge and propelled against the grounded substrate electrode 5 and objects 13 thereon.

Thus, both positive ion and electron bombardment of the objects 13 being plated is obtained for the r.f. sputter plating mode I. It is found that some bombardment of the surface being plated is desired during the initial phases of the plating process. It is believed that this bombardment of the surface serves to increase the adhesion between the plating and the surface being plated by removing surface contaminants or by the formation of surface defects serving as nucleation sites for the plating. However, in some cases, during the major portion of the plating time, and particularly during the final portion of the plating cycle, it is desired not to have substantial bombardment of the plating by either the electrons or the ions as this bombardment can cause substantial overheating of the film being plated and it can also result in destruction of the substrate or surface on which the plating is being deposited. The result is that low reflectivity, film cracks and crazing of the plating are obtained and even flaking is obtained in some cases.

In addition to the electron and ion bombardment of the objects 13 being plated, these objects are also heated by r.f. heating, by infrared heating obtained from the glow discharge, and by bombardment of the surface being plated with the sputter material which impinges with approximately 300 electron volts of energy.

It has been found that the unwanted heating of the surface being plated can be substantially reduced by switching switches 15 and 16 to the second mode of operation indicated by II. In this mode of operation electrode 4 is connected to free floating terminal 22 such that the electrode 4 operates in substantially the same manner as indicated above with regard to mode I and has a relatively high negative d.c. self bias potential. However, the substrate table electrode 5 is connected to terminal 23 which serves to connect the substrate table 5 through switch 16 and an inductor 24 to ground. The radio frequency inductor 24, as of 84 microhenries, serves as a radio frequency choke. Thus, in mode II the substrate electrode 5 is connected for d.c. to ground through the radio frequency inductor 24. It is found that the inductor 24 serves to substantially reduce the temperature of the surface of the objects 13 being plated. Measurements of the power density impinging on the objects to be plated when 1 kw r.f. power is applied across the electrodes in mode I is 0.33 watts per square centimeter and in mode II is 0.16 watts per square centimeter. A consistent 20 percent reduction in temperature of the substrate objects is observed. A fully satisfactory explanation of the mechanism whereby the inductor 24 reduces the temperature of the surface being plated has not as yet been found. It is surmised that it reduces the temperature of the surface being plated by reducing one or more of the aforementioned heating factors, namely, electron bombardment, ion bombardment, r.f. heating, infrared heating or by reducing the energy with which the sputtered material bombards the surface being plated.

It has been found that use of the inductor 24 permits plating of gold onto silicon semiconductive wafers 13 with the temperature of the surface of the wafer which is being plated remaining below 350.degree. C such that the gold is not alloyed with the silicon semiconductor wafer material. More particularly, it was found that alloying of gold with the silicon did not occur when approximately 400 watts of radio frequency energy was applied to the discharge and the gold was being deposited at the rate of approximately 1,000 A per minute per kilowatt of energy applied to the discharge.

Referring now to FIG. 1A, there is shown an alternative embodiment of the structure of FIG. 1 wherein the switches 15 and 16 are replaced by a floating terminal 28 and a switch 31 which shunts the inductor 24 to ground. Typical r.f. sputter plating (mode I operation) is obtained by closing switch 31 and shunting the substrate table 5 to ground. Low temperature r.f. sputter plating (mode II operation) is obtained by opening switch 31 causing the current flowing between the substrate table and ground to flow through the inductor 24.

Use of the switches 15 and 16, as shown in the embodiment of FIG. 1, or alternatively employing the arrangement of FIG. 1A readily permits switching the mode of operation of the sputter apparatus from one mode to another either in preparation for or during the plating process.

Referring now to FIG. 2 there is shown an alternative sputter apparatus incorporating features of the present invention. The apparatus of FIG. 2 is substantially the same as that shown in FIG. 1. The only exception is that the r.f. energy from generator 8 is also applied to a radio frequency coil 35 disposed in the space between the electrodes 4 and 5, respectively. The apparatus of FIG. 2 is operable in the aforedescribed modes I or II, by setting of switches 15 and 16 to the positions I or II, as aforedescribed with regard to the embodiment of FIG. 1, or by employing the circuit of FIG. 1A, as aforedescribed. An apparatus incorporating an RF coil encircling the space between the target and the substrate holder is disclosed in Varian Data Sheet VAC 2311 of April, 1967. The RF-induced-plasma sputtering process is superior to conventional diode-type and triode-type processes for the following reasons:

1. At operating pressures as low as 2 .times. 10.sup.-.sup.4 Torr, two difficulties inherent in higher-pressure diode-type sputtering are avoided: impurity inclusion and non-reproducibility of film properties.

2. No heated filament electron source is needed to sustain the plasma as in triode-type sputtering. Therefore, a major cause of film contamination is eliminated and the problem of filament burn-out is avoided.

3. No magnetic focusing coil is required to confine the ionic plasma between the electrode plates. Thus density gradients are not induced in the plasma and high uniformity of film deposition can be achieved.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

Claims

We claim:

1. In a radio frequency sputter apparatus, means forming a target electrode structure, means forming a second electrode structure spaced from said target electrode, said second electrode structure being adapted to support a substrate to be sputter coated, a source of radio frequency energy connected between said target electrode and ground to produce a radio frequency plasma discharge in the region between said target and second electrode structures, said target electrode structure being substantially isolated from ground potential for direct current potentials such that said target electrode structure is self-biased in the presence of the plasma discharge to a negative direct current potential relative to ground potential, THE IMPROVEMENT COMPRISING means forming a direct current circuit connection between said second electrode structure and ground potential, a radio frequency choke connected in series with said direct current circuit connection, and switch means selectively operable to bypass said radio frequency choke in said direct current connection to ground potential.

2. The apparatus of claim 1 further comprising a radio frequency coil disposed around the space between said target and second electrodes and connected to a radio frequency source.

3. The apparatus of claim 1 further comprising a grounded shield structure disposed around the outside of said target electrode structure.

4. The method of operating sputter apparatus which comprises a target electrode structure, a substrate holder for supporting at least one substrate to be coated with material sputtered from said target electrode structure, a source of radio frequency energy connected between said target electrode structure and ground to produce a radio frequency plasma discharge in the region between said target electrode structure and said substrate holder, said target electrode structure being substantially isolated from ground potential for direct current potentials such that said target electrode structure is self biased in the presence of the plasma discharge to a negative direct current potential, first mode circuit means forming a direct current and radio frequency current conducting circuit connection between said substrate holder and ground potential, and second mode circuit means forming a direct current circuit connection including a radio frequency choke in series therewith connecting said substrate holder to ground potential, SAID METHOD comprising the steps of operating part time in one of said modes and part time in the other of said modes during coating of the same substrate.

5. The method of claim 4 in which said apparatus is operated first in said first mode and then in said second mode during coating of the same susbtrate.

6. In a radio frequency sputter apparatus, means forming a target electrode structure, means forming a second electrode structure spaced from said target electrode, said second electrode structure being adapted to support a substrate to be sputter coated, a source of radio frequency energy connected between said target electrode structure and ground to produce a radio frequency plasma discharge in the region between said target and second electrode structures, said target electrode structure being substantially isolated from ground potential for direct current potentials such that said target electrode structure is self-biased in the presence of the plasma discharge to a negative direct current potential relative to ground potential, THE IMPROVEMENT COMPRISING an electrical circuit connected between said second electrode structure and ground, said circuit forming an open circuit to ground for direct current, a radio frequency choke connected in series in said direct current grounding circuit; and except for stray reactance, said second electrode structure being isolated from ground other than through said circuit.