Vacuum chamber for ultra high vacuum processing at high temperatures

Abstract: A vacuum processing chamber (10), such as a sputter deposition chamber, includes at least a cryogenic pump (22) and a getter pump (26) to provide a vacuum in the chamber (10). The cryogenic pump (22) provides pumping from a pressure obtainable by a roughing pump (20) to a first vacuum pressure, and the getter pump (26) allows pumping of the chamber down to a lower, second, vacuum pressure by gettering reactive species outgassed from the chamber walls, The chamber (10) may be operated at elevated temperatures, such as that needed for the reflowing of contacts, and the combination of the getter (26) and cryogenic pumps (22) permits the maintenance of high or ultra high vacuum conditions where the elevated temperature increases the rate of outgassing from the chamber walls.

Description: The present invention relates to vacuum processing equipment. More particularly, the present invention relates to the creation and maintenance of ultra high vacuum environments in which substrates, including semiconductor substrates, may be processed by sputtering or other processes. Thin film deposition apparatus and techniques are used, among other techniques, to provide film layers on semiconductor substrates. One well known prior art deposition process is sputtering. A sputter deposition apparatus generally comprises a high vacuum chamber which encloses the substrate and a target composed of the material desired to be deposited on the substrate. A negative voltage applied to the target excites into a plasma state an inert gas (typically argon) supplied to the chamber. Ions from the plasma bombard the target and eject particles of target material from the target toward the substrate. These particles deposit on the substrate to form the desired film. In some sputtering process applications the substrate may he heated to temperatures on the order of 450°C to 510°C, to reflow the film layer deposited on the substrate by sputtering. A combination of a roughing pump and a cryogenic pump is typically used to provide the vacuum in the sputtering chamber. Where the chamber is maintained at a temperature of below approximately 300°C, the roughing pump can reduce the pressure within the chamber to about 10 -3 torr, and the cryogenic pump can then reduce the chamber pressure to a pressure on the order of 10 -8 torr. To minimize the inclusion of impurities or contaminants in the film deposited on the substrate, it is generally considered desirable to maintain the sputtering chamber at the highest possible vacuum, that is, the lowest possible pressure. However, the greater the vacuum in the chamber and the higher the temperature in the chamber, the greater the influx of undesirable hydrogen molecules into the chamber due to outgassing of hydrogen molecules from the chamber components. Since hydrogen is an undesirable impurity in the deposited film, the highest vacuum and highest temperature at which the sputtering chamber can be operated is limited by the ability of the cryogenic pump to remove the hydrogen. Because of the low condensation temperature of hydrogen, cryogenic pumps are relatively ineffective at removing hydrogen. The difficulty of maintaining a sufficiently low partial pressure of hydrogen has limited the ability of existing sputtering chambers to operate at high temperatures and ultra-high vacuums. Sputter deposition of aluminum films on semiconductor substrates is an application in which ultra-high vacuum at high temperatures is especially desirable. The ultra high vacuum is used to obtain a clean environment in the chamber. The chamber pressure for sputtering, commonly as high as the milli-torr range, is commonly achieved by first evacuating the chamber to an ultra high vacuum pressure as low as the 10 -8 torr range to remove contaminants from the chamber, and then re-filling ("back filling") the chamber with a clean process gas at a total chamber pressure regulated at about 10 -3 . torr. High substrate temperatures are used to enhance the step coverage of the deposited film, that is, to improve the ability of the film to completely fill holes and trenches in the surface of the semiconductor substrate. Higher temperatures promote the diffusion of deposited atoms, called "reflow," to fill any gaps in, and equalize the thickness of, the film as it is being deposited. Lower chamber pressures promote reflow of the deposition material on the substrate by reducing the occurrence of reflow inhibiting contaminants on the substrate surface. Reflow may occur simultaneously with the deposition of the film layer, at the end of the deposition step, or in a separate chamber dedicated to reflow. For aluminum reflow, the substrate generally is heated by heating the pedestal which holds the substrate during processing. This heating increases the outgassing of hydrogen from the pedestal. The hot pedestal also radiantly heats the chamber walls and other internal components, thereby increasing their hydrogen outgassing rate. Consequently, existing sputter deposition chambers have achieved a chamber pressure of about 1.5 x 10 -8 torr at a substrate temperature of 510°C. At this temperature and pressure, the major gas constituent in the chamber is hydrogen. The limited ability of the cryogenic pump to pump hydrogen, in combination with the outgassing of hydrogen from the chamber components, creates an equilibrium point at this temperature and pressure when a cryogenic pump is used as the high vacuum pump. However, reflow can be achieved at temperatures as high as 650°C, and higher temperatures are beneficial to the reflow rate and quality of the reflow. Therefore, there is a need to provide higher vacuum levels and/or higher temperatures within the vacuum chamber in order to improve step coverage of the deposited film, reduce the time required for reflow, and/or reduce the impurity concentration in the film. Alternatively, achieving higher vacuum, i.e., lower pressure levels, would allow the use of lower reflow temperatures to achieve a desired reflow of the deposition layer, because fewer reflow inhibiting contaminants will form on the substrate at the lower pressure. By reducing the reflow temperature, the risk of damage to devices formed on the substrate is reduced. In response to the above need the present invention provides the vacuum processing apparatus according to independent claim 1, and the method according to independent claim 14. Further advantageous features, aspects and details of the invention are evident from the dependent claims, the description and the drawings. The claims are intended to be understood as a first non-limiting approach of defining the invention in general terms. The present invention employs a cryogenic pump and a getter pump in combination to enable a higher vacuum to be achieved at elevated temperatures within an enclosure, for example within a high vacuum chamber such as a sputtering chamber. The getter pump removes outgassed hydrogen from the vacuum chamber, thereby enabling the cryogenic pump to reduce the chamber pressure to a lower level (higher vacuum) than the cryogenic pump could achieve alone. The invention will be more fully understood and further advantages will become apparent when reference is made to the following detailed description of the invention and the following drawing in which Figure 1 is a perspective view, partially in section, of a representative vacuum chamber of the present invention; Figure 2 is a partial sectional view of the representative vacuum chamber of the present invention, showing the getter pump in section; and Figure 3 is a perspective view of an alternative gettering pump plate configuration useful with the present invention. Figures 1 and 2 depict an enclosure such as vacuum chamber 10 for processing a semiconductor substrate 8. The chamber 10 includes a substrate support member or pedestal 12, a sputtering target 14, a gas inlet 16 into which a gas, preferably an inert gas such as argon, is supplied, and an exhaust port 18. The target 14 preferably is a conventional magnetron sputtering target such as that described in United States Patent No. 5,242,566, Parker , and the pedestal 12 preferably is internally heated. The pedestal 12 includes a conventional internal heater (not shown) such as that described in United States Patent No. 5,228,501, Tepman . To pump the chamber 10 down to the ultra high vacuum, the chamber 10 includes a roughing pump 20 ported to the chamber exhaust port 18, a cryogenic pump 22 ported to the exhaust port 18 through a valve 24, and a getter pump 26 disposed within the chamber 10 adjacent, and preferably below, the pedestal 12. The purpose of the roughing pump 20 is to initially reduce the chamber pressure to a level at which a cryogenic pump can operate effectively, typically about 1 x 10 -3 torr. The cryogenic pump 22 is then engaged to reduce the chamber pressure down to a pressure as low as approximately 7 x 10 -9 torr. The getter pump 26 functions to remove hydrogen and possibly other reactive gases which enter the chamber 10 primarily due to the outgassing of chamber components. The cryogenic pump 22 functions to exhaust the argon process gas from the chamber 10. In a preferred process for depositing aluminum film on a semiconductor substrate 8 having holes or trenches in its top surface, the pedestal heater is operated to heat the pedestal 12 and the substrate 8 to temperatures as high as 650°C. In prior art sputtering systems, such high temperature could not be achieved together with a chamber pressure as low as 5 to 7 x 10 -9 torr, because at that temperature and pressure the pedestal 12 and perhaps other chamber components would outgas hydrogen at a rate faster than a conventional cryogenic pump 22 could remove it. In contrast, using our invention, the getter pump 26 removes sufficient hydrogen by chemisorbtion to allow the combination of the cryogenic pump 22 and getter pump 26 to maintain the chamber pressure at about 5 to 7 x 10 -9 torr. The present invention allows the substrate 8 to be maintained at a higher temperature, thereby improving step coverage of the deposited film and/or reducing the time which must be allotted for the film to reflow. Such high temperature can be achieved simultaneously with ultra-high vacuum so as to minimize impurities in the deposited film. The roughing pump 20 and cryogenic pump 22 are of conventional construction. The roughing pump 20 can be a conventional mechanical pump, although other pumps capable of reducing the chamber pressure into the range of 1 x 10 -3 torr also would be suitable. The cryogenic pump 22 preferably includes a conventional condenser array 23 having multiple chevrons through which liquid helium is flown. Gases impinging on the chevrons will condense and flow downwardly on the condenser array to be collected in an exhaust chamber/valve. One such configuration of multiple chevrons in a cryogenic pump is shown in detail in United States Patent Number 4,873,833, Pfeiffer, et al . which is fully incorporated hereby reference. Although the invention could be implemented with a conventional getter pump, Figures 1 and 2 show the details of construction of the preferred getter pump 26. The getter pump 26 preferably is constructed as a pair of rectangular plates 30 of getter material, although the plates 30 may be formed of a non-gettering base material having the gettering material plated or otherwise affixed thereto. Each of the plates 30 is preferably folded into a pleated profile as shown in Figure 1. Although each of the plates 30 is shown as including one pleated member, multiple pleated members 31 may be stacked to form each of the gettering plates 30 as shown in Figure 3. Each of the plates 30 terminates, at either end thereof, in a support flange 34, and a rod 36 extends from the chamber base and connects to the support flange 34 to support the plate 30 above the chamber base. An electrical feedthrough 32 extends through the chamber base to provide power to an internal electric resistance heater 13 (shown in Figure 2) maintained within each of the plates 30. When a reactive gas species such as hydrogen contacts either of the plates 30, it is sorbed (i.e., gettered) preferably chemisorbed into the plate 30. Thus, as reactive gas species diffuse or migrate within the chamber 10 at the ultra-low chamber pressure, for example at a pressure of less than 1.5 x 10 -8 torr, they are chemisorbed upon contact with the plates. The surface area of the gettering material of the plates 30 is sized large enough to provide chemisorption of the outgassing hydrogen sufficient to maintain the lower chamber pressure of 5 to 7 x 10 -9 torr. However, non-reactive species, such as argon which is commonly used as the process gas, is not absorbed into the getter material. The getter plates 30 are preferably mounted directly underneath the substrate pedestal 12 as shown in Figures 1 and 2 so that the pedestal 12 acts as a shield between the substrate 8 and the gettering plates 30. Thus, if any particles should be dislodged from the gettering plates 30, such particles will be highly unlikely to lodge on, and thereby contaminate, the substrate 8. The gettering plates may be placed elsewhere in the chamber so long as the uppermost surface of the plates 30 is located below the upper surface of the substrate 8, where the substrate is positioned on the support member as shown in Figures 1 and 2. Additionally, electrically grounded shields 40 may be placed surrounding the region between the target 14 and the substrate support pedestal 12 to confine the plasma to a region above the substrate 8 so as to prevent sputtering of the gettering plates 30 which are positioned below the substrate. However, the plates 30 are also positioned as high as possible underneath the substrate 8 so as to minimize the distance between the plates 30 and the substrate 8, and thereby maximize chemisorbtion of reactive species near the surface of the substrate 8. The region near the substrate surface 8 is where it is most crucial to minimize the concentration of impurities such as hydrogen to prevent these impurities from being incorporated in the film deposited on the substrate. The getter pump 26 operates by sorbing preferably chemisorbing chemically active species such as hydrogen, but it has virtually no effect on inert gases such as the argon used as the principal process gas for sputtering. Therefore, the gettering pump 26 may be used to retrofit existing chambers 10, without the need to reconfigure or recalibrate the gas flows in the chamber 10. The preferred plate 30 material is a non-evaporable gettering material that may be reactivated by exposure to high temperatures after multiple process cycles in the chamber. One preferred gettering material is a Zirconium Aluminum material, sold by Saes Getters S.p.A., of Milan, Italy under the designation SORB-AC st101. An additional gettering material that may he used in the chamber is also sold by Saes Getters S.p.A., under the designation st707, which is a Zirconium Vanadium Iron alloy. The st101 material is preferred because it has negligible iron content. Over time, the chemisorption rate of the getter pump 26 may decline as a result of the cumulative quantity of reactive species that are chemisorbed into the plates 30. At this point, the gettering material forming, or located over, the plates 30 must be reactivated by heating the plates 30 to a temperature on the order of 600 to 750°C, for time periods ranging from 10 to 1000 minutes, to desorb, or drive out, the hydrogen and other gases absorbed by the plates 30. In the preferred embodiment, this heat is supplied by the electric resistance heater 13 disposed within each plate 30. The time required for desorption is inversely proportional to the temperature of desorption and may be optimized for the particular chamber 10 and getter plate 32 configuration. It is believed that the gettering plates may be desorbed up to five times, before they must be replaced. The high vacuum enabled by the present invention provides benefit to both sputtering and reflow processes. Where sputtering is performed in the chamber, the lower base pressure obtainable in the chamber before backfilling with the process gas yields a cleaner chamber environment. Where the chamber is configured for reflow, the lower pressures enhance reflow by reducing the incidence of particle or chemical contamination of the film layer. Therefore, it is specifically contemplated that the gettering pump 30 may be used for sputtering chambers, sputtering chambers wherein reflow occurs during and/or after sputtering, or dedicated reflow chambers. Although the present invention has been described herein primarily with respect to a high temperature sputtering chamber, the improved hydrogen pumping and the higher temperature ultra high vacuum conditions made possible by the present invention may be used with other equipment or processes where ultra high vacuums and/or high temperatures are desired.

Claims: A vacuum processing apparatus, comprising an enclosure (10); a support member (12) for holding a semiconductor substrate (8) within the enclosure (10); a heater for raising the temperature of the substrate (8) to a temperature high enough to cause reflow of a layer of material on the substrate (8); a cryogenic pump (22) being capable of evacuating said enclosure (10) to a first vacuum pressure; and a getter pump (26) arranged within said enclosure (10) and capable of reducing the pressure in said enclosure from said first pressure to a second lower pressure by sorbing, preferably chemisorbing reactive gas species in the enclosure (10). The vacuum processing apparatus according to claim 1, wherein said cryogenic pump (22) is ported to said enclosure (10). The apparatus of claim 1 or 2, wherein the apparatus is a reflow chamber. The apparatus of claim 1 or 2, wherein the apparatus is a sputtering chamber. The apparatus of one of the preceding claims, wherein said second pressure is in the range of 5 to 7 x 10 -9 torr. The apparatus of any of the preceding claims, wherein said layer of material contains aluminum, and the reflow temperature exceeds 510°C. The apparatus of one of the preceding claims, wherein said getter pump (26) includes a plurality of plates (30) formed of a gettering material. The apparatus of one of claims 4 to 7, wherein said sputtering chamber further includes a sputtering target (14) and a substrate support member (12), said substrate support member (12) including a substrate support surface disposed adjacent said target (14) to receive a substrate (8) thereon; and said substrate support member (12) is disposed intermediate said gettering pump (26) and said substrate support surface. The apparatus of claim 7 or 8, wherein said gettering material is non-evaporable. The apparatus of one of the preceding claims, further including a roughing pump (20) to produce a pressure in the enclosure (10) higher than the first pressure. The apparatus of one of the preceding claims, wherein the cryogenic pump (22) is disposed in communication with said enclosure (10) and capable of evacuating said enclosure (10) to a chamber pressure of at least 1.5 x 10 -8 torr at a substrate reflow temperature. The apparatus of any of the preceding claims, wherein said heater heats said substrate (8) to a temperature in excess of 510°C and said film layer is aluminum. The apparatus of any of the preceding claims, wherein said enclosure (10) is maintained at pressures below 7 x 10 -9 torr and the substrate (8) is maintained at temperatures above 510°C. A method of providing a vacuum in a processing environment comprising the steps of: providing a sealable enclosure; providing a substrate support member in the enclosure; pumping the enclosure to a first vacuum pressure with a cryogenic pump; raising the temperature of the support member to a contact reflow temperature; and pumping the enclosure to a second, lower, pressure with a getter pump located within the enclosure so as to reduce the pressure to the second pressure by sorbing, preferably chemisorbing reactive gas species. The method of claim 14, wherein the second pressure is in the range of 5 to 7 x 10 -9 torr. The method of one of claims 14 to 15, wherein the reflow temperature of the substrate exceeds 510°C. The method of one of claims 14 to 16, further including the step of locating the substrate support member intermediate the substrate and gettering pump. The method of one of claims 14 to 17, wherein the pressure in the enclosure is maintained at a pressure no greater than 7 x 10 -9 torr as the temperature of the substrate support member changes from an ambient temperature to a contact reflow temperature. The method of one of claims 14 to 18, including the additional step of first pumping the chamber to a pressure, higher than the first pressure, with a roughing pump.

Inventors: DAVENPORT ROBERT, NULMAN JAIM, RAMASWAMI SESHADRI, DAVENPORT, ROBERT, NULMAN, JAIM, RAMASWAMI, SESHADRI
Applicants: APPLIED MATERIALS INC, APPLIED MATERIALS, INC.
Application year: 1995
Main classification: F04B37
IPC codes: H01L21, C23C14, B01J3, F04B37