US 3803019 A
Description (OCR text may contain errors)
United States Patent [191 Robison et al.
[451 Apr. 9, 1974 SPUTTERING SYSTEM  Assignee: Hewlett-Packard Company, Palo Alto, Calif.
 Filed: Nov. 16, 1972  Appl. No.: 307,261
Related U.S. Application Data  Division of Ser. No. 187,264, Oct. 7., 1971, Pat. No.
 U.S. Cl. 204/298  Int. Cl. C23c 15/00  Field of Search 204/298  References Cited UNITED STATES PATENTS 3,410,774 ll/l968 Barson et al 204/298 l,644,350 lO/l927 Palmer 204/298 Coolant piral Tubular 20 Calls Coolant Primary E.\'aminerT. M. Tufariello Attorney, Agent, or Firm-R0land I. Griffin 5 7 ABSTRACT The anode and the workpieces of a thin film sputtering system are cooled by coolant that circulates through spiral tubular coils located below the anode and into chambers located within the anode. The anode is rotatable, and the cooling coils expand and contract to permit partial rotation of the anode. The two cathodes of the sputtering system are cooled by coolant that circulates through chambers within the cathodes. The cathodes are designed to minimizecontamination from residue coolant leaking into the gastight enclosure of the sputtering system while the cathodes are being removed. The gastight enclosure is connected to a vacuum pump througha port. A throttle plate, located within the enclosure over the port, aids the vacuum pump to efficiently regulate the gas' pressured in the enclosure during a sputtering run.
3 Claims, 3 Drawing Figures To RF Power pp y Cathode (Target) 12 Rotatable Anode 6 To Vacuum Pumping System mos-L019 PATENTEUAPR 919M SHEET 1 [IF 2 To RF Power pp y Cathode (Torgei) 12 Rororoble Anooe 6 Impedance Matching Network I6 piroi Tubular Coolant 20 V Coolant To Vacuum Pumping System i ur e 2 SPUTTERING SYSTEM This is a division, of application Ser. No. 187,264, filed Oct. 7, 1971 now U.S. Pat. No. 3,718,572.
BACKGROUND OF THE INVENTION In exacting uses of sputtering systems, the presence of contaminants in the environment of the sputtering process is undesirable. During sputtering, the target of highly pure material is bombarded by gas ions that cause atoms of the target material to sputter off the target and deposit on substrate wafers, called workpieces. Contaminants in the process hinder the bonding of the deposits and create impurities in the deposited layer, or film. The contaminants may directly affect the workpieces by adhering to the surfaces of the workpieces, or they may indirectly affect them by contaminating the sputtering apparatus and the target material.
Contamination of the gastight enclosure of the sputtering system may occur in many ways. Leaks in the enclosure seals may allow the entry of water or air particles into the process. These particles may cause oxidation of the deposited material, thereby changing the properties of the film and ruining the process. Similarly, the target may become oxidized, especially if an oxygen-sensitive material such as molybdenum is used. Both air and water often carry contaminants such as oil or dust that may collect on the workpieces or the target if they enter during the sputtering process.
Out-gas from materials during sputtering is also a source of contamination. This may result from the sublimation of the material or from the release of absorbed or adsorbed gases by the material. Elastomer seals are examples of components that exhibit excessive outgassing at low pressures.
An additional problem with vacuum seals arises when a rotating shaft or conduit enters the enclosure. It is more difficult to seal around a rotating element because the rotation eases the entry of contaminants by propelling the particle along the turning shaft through the vacuum seal.
One major source of contamination in a sputtering process is the cooling network. Both the workpieces and the target are cooled by a circulating coolant system to maintain proper operating temperatures. If the anode is rotatable and used as a workpiece holder, the cooling system for the workpieces, which operates by cooling the anode, must also rotate. This necessitates either the entry of a rotating conduit into the vacuum enclosure or the presence of a joint between a rotatable and a stationary conduit inside the enclosure itself. As mentioned above, a seal for a rotating shaft increases contamination problems. The internal joint requires a watertight seal that prevents the escape of coolant in the chamber. The present state of the art has not developed seals for either of the above cases that totally prevent contamination.
Another source of contamination is the cathode assembly. Since it is sometimes desirable to change the target material, most cathodes are composed of a target support and a separable target. If the cathode contains chambers for liquid coolant, the junction between the target and the support is usually sealed by an ring that encircles the cathode. This 0 ring increases the probability of out-gassing and leakage. In addition, whenever the target is detached from the cathode assembly in the gastight enclosure, any residue coolant remaining in the cathode may fall into the enclosure.
The quality of sputtering depends upon other criteria in addition to the absence of contaminants. Constant pressure and temperature are two such factors. During sputtering, the enclosure gas pressure must be maintained at a predetermined pressure. To ease the work load put on the vacuum pump, the cross section of the port leading into the enclosure is reduced in area, usually by the adjustment of a butterfly valve in the pipe connecting the vacuum pump with the enclosure. This is an imprecise technique because the butterfly valve cannot be repeatedly set to the same position. As a result, establishing the proper enclosure pressure is sometimes a lengthy process involving continual readjustment of the butterfly valve and the gas inlet valve that regulates gas flow into the enclosure.
SUMMARY OF THE INVENTION This invention relates to a thin film sputtering system which includes three separate improvements. The cooling system for the rotatable anode includes spiral, tubular coils located inside the gastight enclosure. These coils expand and contract, like a watchspring, as the anode turns. Because the cooling coils do not rotate, a joint between a stationary and a rotary conduit is unnecessary. Thus, there is no need for the water-tight seal that normally encloses such a joint. Moreover, the cooling system no longer needs a seal that can accommodate a rotary shaft because the conduit is stationary at the entry and exit points to the enclosure. With the elimination of these seals, contamination is reduced.
If the target is to be removed, the cathode assembly, consisting of both the target and the target support, can be easily removed as a unit, thereby permitting access to the target outside the vacuum enclosure. The only connections in the cathode cooling system are external to the enclosure. Consequently, disconnection of the cooling system prior to cathode removal confines the remaining coolant within the cathode. When the cathode is removed from the enclosure, the coolant residue is also removed. Thus there is little chance of contamination during this process. The problem of out-gassing from the cathode assembly is considerably reduced because this invention does not require an O ring seal between the target and the target support.
The present invention diminishes the problem of establishing the desired gas pressure inside the gastight enclosure by replacing the butterfly valve with a throttle plate having a central hole. The area of the hole equals the optimum size of the orifice necessary for the vacuum pump to maintain the desired gas pressure during a sputtering run. The plate can be raised and lowered over the port that connects the vacuum pump to the enclosure. Prior to sputtering, the plate is lowered over the port, and the port size is decreased to the size of the central hole. Since this plate is used for every sputtering run, the port is restricted to exactly the same area for every run. Thus, establishing the enclosure gas pressure becomes a quick and easy process when this invention is used. 1
DESCRIPTION OF THE DRAWINGS FIG. 1 shows a cutaway side view of an improved sputtering system according to the preferred embodiment of this invention.
FIG. 2 shows a partially cutaway top view taken along the line 22 of the sputtering system of FIG. 1.
FIG. 3 shows a detailed cutaway side view of the cathode assemblies of the sputtering system of FIG. 1.
DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIGS. 1 and 2, a cylindrical gastight enclosure 2 is connected to a vacuum pumping system through the vacuum port 4. An anode 6 that is capable of rotating on a bearing assembly 8 is driven by a cable drive system 10. The anode acts as a holder for the workpiece wafers 11 that will be coated by the target material during the sputtering process. Each of the two cathodes 12 is composed of a target and a target support. One cathode has a gold target; the other, molybdenum. These materials are used to sputter molybdenum-gold contacts onto semiconductor wafers. Other materials can be used by replacing the targets.
The temperature of the workpieces is controlled by cooling their holder, the anode, by coolant that circulates through chambers within the anode. The coolant, usually water, flows into the gastight enclosure through conduit 13 that passes through a feedthrough 14. Then it flows through the spiral tubular coils 15 and into the anode cooling chambers. It returns through a parallel conduit over the same path.
- The cathodes are cooled .by circulating coolant through chambers within the cathodes. An RF power supply is connected to the cathodes by an impedance matching network, 16. Theimpedance matching network comprises inductors and capacitors interconnected to match the impedance of the power supply with the impedance of the ionized plasma within the enclosure. Impedance matching is performed to minimize high frequency power reflections from the sputtering system back to the power supply. The ground return of the power supply is connected to the anode through the enclosure chassis. Alternatively, a dc. power supply may be used to replace the RF supply and the impedance matching network.
A plasma confining shield 17 that is at the same electrical potential as the anode restricts the plasma and the sputtered particles to the immediate area of the workpieces. Each target in the enclosure is provided with such a shield.
Before the sputtering can begin, the proper atmosphere of inert gas must be established within the gastight enclosure. Before pumpdown, the throttle plate 19 is raised to its upper position above the vacuum port 4 by-turning the knob on the bellows-sealed rotary feedthrough 23, a device well known in the art. The rotary feedthrough is linked to the throttle plate so the plate can be raised or lowered by turning the knob on the rotary feedthrough. With the plate in its upper position, port 4 is unrestricted and the vacuum pumping system can evacuate the enclosure. After the enclosure has been evacuated, the throttle plate is lowered over the vacuum port, closing the port except for the small central hole 18 in the plate. Gas is supplied to the enclosure through the gas inlet 20, and the vacuum pump maintains the proper gas pressure inside the enclosure during the sputtering run.
It is desirable to have maximum gasflow from inlet 20, through the enclosure, and out port 4 during the sputtering run because the gas flow helps sweep any gaseous contaminants that may be present in the enclosure out the port. However, a high gas flow increases the number of gas molecules that the vacuum pump must evacuate. If the gas flow exceeds the capacity of the vacuum pump, the pump may cease to operate and will begin to contaminate the gastight enclosure. Most vacuum pumps include a heavy liquid bath, such as an oil bath, that is used to pull the atmosphere from the enclosure. If the maximum flow rate of the pump is exceeded, molecules of oil will travel from the pump, through the port, into the enclosure. This will contaminate the sputtering apparatus with oil, a highly undesirable contaminant.
The orifice 18 in the throttle plate is designed to provide the highest flow that will not cause oil contamination of the enclosure by the vacuum pump. The flow through the orifice when the throttle plate is lowered over the port is given by the following equation:
Q=5.2APA V TIM where:
Q flow in standard cubic centimeters per second A P= (desired gas pressure of enclosure) (normal pressure of vacuum pump) A area of orifice T= temperature in "K M molecular weight of the gas This equation is derived from the well-known Effusion Law:
F maximum flow rate in liters per second A area of orifice T= temperature in K M molecular weight of the gas 7 The flow, Q, is determined from the specifications of the vacuum pump and from the amount of oil contaminants tolerated in the enclosure during a sputtering run. For. the enclosure used in FIG. 1, it has been experimentally determined that a flow equal to approximately 50 percent of the maximum flow specified by the vacuum pump manufacturer produces optimal conditions for a pure sputtering run. Consequently, for a vacuum pump listed at maximum Q equal to 1 cubic centimeter per second and having a normal pressure of 0.2 micron, the area of orifice 18 according to the above equation is approximately 5.5 square centimeters for an enclosure containing argon gas maintained at a sputtering pressure of 7 microns. This area has been experimentally validated for the sputtering system in FIG. 1.
The areaof central hole 18 may change for different sputtering systems and for the degree of purity required in the sputtered contacts. However, the use of an orifice provides the advantage of precise repeatability of the flow rate for every sputtering run made with the same sputtering system. Once the optimal size of the central hole is determined, the port is always restricted to exactly that size during sputtering by lowering the throttle plate over the port. This exactness cannot be repeatedlyachieved with conventional methods.
Prior to the evacuation of the enclosure, the workpieces are placed on the anode surface in an area under one cathode. To establish uniform sputtering, the wafers must be contained within the boundary established g by the plasma-confining shield. The anode is rotated by the cable drive system until the workpieces are positioned under the cathode that will not be used as the target. Then the enclosure is evacuated and the gas is inserted. The RF supply is connected to the cathode that will be used as the target during this sputtering run. The supply is then energized, causing the target material to be deposited onto the anode within the confines of the plasma shield. This initial sputter is used to remove contamination from the surface of the target. After a sufficient layer has been removed from the target, the anode is rotated l80 so the workpieces are positioned directly under the target. Now the target material is deposited onto the workpieces until the desired layer has been deposited. If it is desired to deposit material from the other target onto the workpieces, the process is repeated with the second target energized.
Throughout the sputtering process, both the anode and the cathode cooling systems are in operation. When the anode rotates from one position to the other, the coolant conduit must rotate with it. Because the conduit is coiled in a spiral (FIG. 2), it can move with the rotating substrate holder. Like a watchspring, the coil expands when the anode moves in one direction and contracts when it moves in the other. As a result, the conduit connected to the underside of the anode can move with the rotating anode while the conduit at the entry feedthrough 14 remains stationary. Thus there is no need for a joint between rotating and stationary conduits, nor for a vacuum seal to accommodate a rotating conduit.
Note that the anode only moves through 1 80 of rotation. If greater rotation is desired, the cooling conduit can be modified to accommodate this. The number of turns in the coil can be increased, or the conduit can be constructed from a more elastic material.
Referring now to FIG. 3, each cathode 12 is composed of a target 21 or 22 and a target support 24. The gold target 21 is brazed to its holder while the molybdenum target 22 is attached by screws 36. Each target support contains cooling chambers 32. Coolant, supplied through the external coolant conduits 34,.cools the targets by circulating through the chambers. The gold target 21 makes direct thermal contact with its target support while the molybdenum target 22 makes thermal contact through copper pads 30. These assemblies eliminate the need for an O ring between the target and the target support.
To change the target material, the cathode must be removed from the vacuum enclosure. First, the joints 40 to the external conduits 34 are disconnected while the cathode remains in position. This allows any coolant remaining in the external conduit to fall outside the enclosure or into the cathode, but not into the enclosure itself. Next, the cathode is disconnected from the RF source by disconnecting the electrical connector 28. Then the clamping nut 26 is unscrewed and removed from the cathode shaft. This releases the cathode from thedisc seal 25 and the ceramic insulator 27. Now the entire cathode assembly, including the target and target support, can be lowered into the enclosure and removed. Any coolant remaining in the cooling chamber can be emptied outside the enclosure. Once the cathode is outside, the target can be removed and a new one installed. As a result of this invention, the target material can be quickly changed with little chance of contamination to the sputtering system.
1. A sputtering system comprising:
an enclosure for maintaining a gas at a selected pressure;
a cathode supported within the enclosure;
an anode supported within the enclosure;
a port in the enclosure to permit evacuation of the enclosure;
a plate positioned within the enclosure above the port, said plate being supported for movement between a raised position defining a first port opening between the port and the enclosure and a lowered position repeatably defining a second port opening between the port and the enclosure of predetermined size smaller than the first port opening; and
means coupled to the plate for raising and lowering the plate.
2. A sputtering system as in claim 1 wherein said plate has a central hole smaller than the first port opening and in the lowered position rests on the surface of the enclosure to restrict the port to the size of the central hole and thereby define the second port opening.
into the enclosure.