WO1995010639A1

WO1995010639A1 - Vacuum seal of heating window to housing in wafer heat processing machine

Info

Publication number: WO1995010639A1
Application number: PCT/US1994/010856
Authority: WO
Inventors: Hiroichi Ishikawa; Lev Chizhik
Original assignee: Materials Research Corporation
Priority date: 1993-10-13
Filing date: 1994-09-26
Publication date: 1995-04-20
Also published as: AU8011394A

Abstract

A wafer processing apparatus (10) is provided with an assembly that effectively seals openings in a housing (11) that encloses a vacuum processing chamber (12) that are subject to heating. An opening cover, such as a quartz window (34) used permit radiant heat to enter the chamber (12) from radiant infrared lamps (32) on the outside, employs a hard metal seal (50). A frame (52) that holds the cover (34) to the housing (11) and presses it against the seal (50) is prevented from expanding under the influence of heat, such as from radiant lamps (32). The expansion is prevented, preferably by cooling the frame (52) with cooling tubes (64) on the surface of the frame (52). Alternatively, a high expansion coefficient spacer ring (68) may be employed that expands an amount equal to the expansion of the gap (62) that holds the cover (34) and seal (50). A further alternative employs a frame (52) of low coefficient of expansion material.

Description

VACUUM SEAL OF HEATING WINDOW TO HOUSING IN WAFER HEAT PROCESSING MACHINE

The present invention relates to wafer processing machines, and more particularly, to the maintenance of vacuum seals in the walls of the chambers of such machines. Background of the Invention;

Semiconductor wafers are subjected to a variety of processing steps in the course of the manufacture of semiconductor devices. The processing steps are usually carried out in sealed vacuum chambers of wafer processing machines. Many of the processes performed on the wafers in such chambers involve the heating either of the wafers or of some component involved in the process. As a result of the heating, thermal expansion, including unequal thermal expansion, of parts of the housing enclosing the chamber is encountered.

The housings enclosing such processing chambers have openings provided in them for various purposes. These openings are closed with covering elements such as panels or support devices, which seal the openings to enable a vacuum to be maintained in the chamber. Where such elements are mounted to seal such openings, seals of various types are used to insure a pressure tight connection between the element and the housing around the opening. Where heating of the housing in the area of the opening occurs during the performance of a process within the chamber, irregular thermal expansion in the area often occurs. When irregular expansion is expected, flexible seals such as O-ring seals are usually employed. Such seals will accommodate dimensional changes without leaking as the housing and covering element expand in different amounts as a result of the heat.

Many semiconductor wafer vacuum processing machines, such as degas, rapid thermal process, anneal, and CVD processing machines or machine modules, use radiant lamp heating systems. Such machines often have lamps positioned outside of a vacuum chamber, adjacent a window in the chamber housing. The window, referred to as a heat window, is a covering panel that separates the atmosphere from the vacuum within the chamber and permits the transfer of heat energy from the lamps to the wafers or to some component within the chamber that requires heating for the process to be carried out. Directing radiant heat through such windows has a side effect of heating the area around the window, such as a frame that holds the window to the housing, causing expansion of the frame and the housing in the vicinity of the opening or general irregular expansion of the structure. Where openings are provided in high vacuum regions of such processing machines, metal seals such as those manufactured under the trademark Helicoflex are used where practical. Metal seals overcome certain disadvantages of O-ring seals, which release gases when subjected to high vacuum. In addition, metal seals withstand higher temperatures without deteriorating or failing due to the heat. Where metal seals are used, however, because they are less elastic than other seals, the thickness of the gap occupied by the seal is critical. A very small increase in the size of a gap in which a metal seal has been compressed to form a seal can cause the seal to leak, and atmospheric gas to enter the vacuum chamber. Because, dimensional changes due to heating are prone to occur in the gaps holding the seals around such windows, metal seals have not been successfully employed in the prior art around heat windows in the housings of such machines.

Accordingly, there is a need for improvement in the sealing of openings in the housings of high vacuum chambers of wafer processing machines where heating that may cause thermal expansion in the vicinity of the openings is expected to be encountered. Summary of the Invention It is a primary objective of the present invention to provide a wafer processing apparatus that employs heat, as for example radiant heat from a source outside of the processing chamber of the machine, with an effective seal around an opening, such as a heat window for example, in the housing of the chamber.

Another objective of the present invention is to provide a wafer processing apparatus with an effective metal seal for use where heating is anticipated that would result in thermal expansion, particularly irregular thermal expansion that might affect the gap containing the seal.

In accordance with the principles of the present invention, a wafer processing apparatus having an opening in the housing thereof, is provided with a heat window or other panel or device mounted to close the opening. The panel is held to the housing in a dimensionally stable gap that is formed between the housing and a frame such that the gap resists dimensional changes from expansion due to heating in the vicinity of the opening. A metal seal is provided in such a gap to seal the panel to the housing and maintain leak free closure of the opening.

In accordance with the preferred embodiments of the invention, there is provided a quartz view window that closes an opening in a housing, and through which radiant heat, from lamps positioned adjacent the window outside of the housing, passes to heat wafers, supported in a vacuum chamber enclosed by the housing, for processing. A frame, which includes a rectangular window clamping rim and bolts that hold the rim to the housing, is mounted to the housing so as to surround the opening and form a gap between the rim and the housing. In the gap is clamped the edge of the window and a metal seal. The bolts are drawn tight enough to compress the seal and window to maintain a pressure tight seal of the window to the housing when the chamber is pumped to a high vacuum for processing of one or more wafers in the chamber. The frame responds to heating in the vicinity of the opening in such a way as to limit dimensional changes in, and particularly an enlarging of, the gap that holds the window edge and the metal seal.

In one embodiment of the invention, the frame surrounding the opening is provided with cooling elements that maintain the frame at approximately the initial temperature at which the compression of the metal seal occurred, thereby limiting the expansion of the frame that would cause an increase in the spacing between the rim and the housing when heating by the lamps and processing of the wafers is taking place.

In a second embodiment of the invention, the frame, and preferably both the rim and the bolts holding the rim to the housing, is formed of a low thermal expansion material, such that, when heating by the lamps is taking place to process wafers within the chamber, substantial thermal expansion of the gap will not occur.

In a third embodiment of the invention, a ring of high thermal expansion material is provided between the rim of the frame and the housing. The thickness of the ring and the coefficient of thermal expansion of the ring are such, in relation to the spacing between the rim and the housing and the coefficients of thermal expansion of the rim and the bolts, that the ring expands when heated an amount approximately equal to the expansion of the rim and bolts. As such, the remaining gap in which the window edge and metal seal are compressed remains generally unchanged. The ring is positioned between the housing and the rim, and is preferably positioned between the window and either the rim or the housing, with a metal seal between the window and either the housing or the rim, respectively. Alternatively, the ring may be placed between the window and the seal. Preferably, the ring is attached to the rim.

With the invention, the gap retains its approximate original dimension as heating occurs, and the use of a metal seal around the opening can be effectively employed without leaking. Thus, the gas tight sealing characteristics of the connection between the window, or other cover, established by the metal seal is maintained.

The above described and other objectives and advantages of the present invention will be more readily apparent from the following detailed description of the drawings in which: Brief Description of the Drawings

Fig. 1 is perspective view, partially cut away, of a portion of a degas module of a silicon wafer processing cluster tool apparatus embodying principles of the present invention.

Fig. 2 is an side elevational diagram, partially cut away, of one embodiment of the apparatus of Fig. 1.

Fig. 3 is side elevational view of a portion of Fig. 2 of an alternative embodiment of the invention. Detailed Description of the Drawings

Referring to Fig. 1, one embodiment of the present invention is illustrated in a semiconductor wafer batch preheating module 10 of a semiconductor wafer processing cluster tool, such as that disclosed in commonly assigned and co-pending U.S. Patent Application Ser. No. 07/701,800, filed May 17, 1991, entitled "Wafer Processing Cluster Tool Batch Preheating and Degassing Method and

Apparatus", hereby expressly incorporated herein by reference.

The module 10 includes a sealed housing 11 enclosing a vacuum chamber 12 in which wafers 14 are processed. In the illustrated embodiment of the module 10, the process performed is one of preheating or preconditioning the wafers 14 for the purpose of removing absorbed gases and vapors prior to the processing of the wafers in other modules of the semiconductor wafer processing apparatus.

In the module 10, the wafers 14 are supported in a multiple wafer support or rack 16 on which they are vertically stacked. The rack 16 is supported in the chamber 12 on a vertically movable and rotatable elevator 20. The rack 16 has a plurality of wafer holders formed by a plurality of slots 22 in four vertical quartz rods 24. The wafers 14 are individually loaded into the rack 16 as the elevator 20 is vertically indexed to bring each of the slots 22 successively into alignment with a wafer loading port (not shown) in the housing 12. The port sealably communicates between the vacuum chamber 12 inside of the housing 11 and the interior vacuum chamber of a wafer transport module. The vacuum in the chamber 12 is maintained by cryogenic vacuum pumps 30 connected to the chamber 12 through the housing 11. In a typical heat treatment process such as the batch preheating process performed with the module 10, wafers 14 are individually loaded through the open gate valve and into the slots or holders 22 of the rack 16 as the elevator 20 is indexed past the port. Then, the gate valve is closed with the vacuum chamber 12 at the same pressure level as that in the chamber of the transport module.

In the preheating or degassing process, the pressure in the chamber 12 is maintained at a vacuum through operation of the pump assembly 30. In the process, the wafers 14 are heated by the energizing of radiant heaters 31 having lamps 32 arranged in sets on the outside of the chamber 12, behind quartz windows 34 that cover openings 35 in opposed walls of the housing 11. The elevated or processing temperature, which may be, for example, 500 °C, is usually maintained for some predetermined processing time of, for example, fifteen minutes. In the process, the housing in the vicinity of the windows 34 are subjected to heat.

Referring to Fig. 2, the mounting of one of the windows 34 over one of the openings 35 in the housing 11 is illustrated in detail. In the illustrated embodiment, the opening 35 is rectangular in shape, though circular and other shaped openings are used. The housing 11 is usually made of metal, for example stainless steel or aluminum. The window 34 is accordingly also rectangular in shape, slightly larger than the opening 11 so that edge 41 of the window 34 overlaps on periphery 42 of the opening 35. The window 34 is mounted on the opening 35 with a metal seal 50 lying between the edge 41 of the window 34 and the periphery 42 of the opening 35. The window 34 is held against the seal 50 by a frame 52, also rectangular, which is secured to the housing 11 and surrounds the opening 35. The frame 52 includes a metal clamping rim 53, of for example aluminum, and screws 54 of, for example, stainless steel, which are threaded into holes 55 in the housing 11 around and near the periphery 42 of the opening 35. The rim 53 has an inwardly projecting lip 57, which overlies the edge 41 of the window 34.

When secured to the housing 11, the frame 52 forms a recess 60 between the lip 57 of the rim 53 and the periphery 42 of the opening 35 in the housing 11. In the embodiment illustrated in Fig. 2, the width of the recess 60 defines the width Wof a gap 62. When the screws 54 are tightened to draw the lip 57 against the window 34, the seal 50 is compressed to a compressed diameter D. In this condition, the width W of the gap 62 is greater than the thickness T of the window 34 by the amount of the compressed diameter D of the seal 50.

The lamps 32 of the heater 31 are positioned outside of the chamber behind the window 34, such that radiant energy from the lamps 32 passes through the window 34. Typically, the lamps are infrared lamps and the window 34 is made of a material such as quartz, has a thermal expansion coefficient which is negligible compared to that of materials such as stainless steel and aluminum. Hence, the temperature-induced dimensional increase in the window thickness is relatively slight.

When the lamps 32 are energized, a portion of the radiant energy radiated from the lamps 32 impinges upon the housing 11 in the vicinity of the opening 35, particularly the periphery 42 thereof, and upon the frame 52. Prior to the present invention, absorption of radiation by the frame 52 caused thermal expansion of the frame 52, partially due to expansion of the rim 53 and partially due to expansion of the screws 54. The thermal expansion of the frame in the prior art greatly exceeded that of the window due in part to the window's relative transparency to the radiation from the lamps and the coefficient of expansion of the quartz of which the window is normally made. This caused the width W of the gap 62 to increase enough relative to the negligible increase in the thickness of the window to require the use of a resilient O-ring seal instead of the metal seal 50.

In the embodiment of the invention of Fig. 2, thermal expansion of the frame 52 is limited by separate cooling tubes 64 provided around the outer side of the rim 53, through which cooling fluid is circulated. The tubes 64 are in thermal contact with the rim 53, which is in thermal contact with the screws 54, thereby maintaining the frame 52 at a sufficiently low temperature to prevent significant and substantial thermal expansion of the frame 52 and a corresponding widening of the gap 62. As a result, the width of the gap is maintained sufficiently constant to allow effective use of the metal seal 50, without the seal 50 leaking when the lamps 32 are in operation. Hence, the gas-tight sealing properties of the metal seal are preserved in the connection between the window and the housing. In the alternative, and suitable in some but not all applications, in lieu of the cooling tubes 64, the frame 52 may be formed of a low expansion coefficient material that expands little when heated to the temperatures experienced in the module 10, thereby maintaining the gas-tight connection formed by the seal between the window and the housing. Such a material, for example, may be that manufactured under the trademark Invar.

Another alternative embodiment of the invention is illustrated in Fig. 3. In Fig. 3, instead of the cooling tubes 64 or the low expansion material, a rectangular ring 68 of high thermal expansion coefficient material is provided. The ring 68 is approximately of the shape and dimensions of the lip 57 and may be positioned in the recess 60 between the lip 57 and the window 34 as illustrated. In this embodiment, the recess 60 is larger than the width Wof the gap 62 by the thickness R of the ring 68.

The material of the ring 68 is selected such that the coefficient of thermal expansion of the ring 68 is greater than the effective coefficient of thermal expansion of the frame 52 by the ratio of W/R. For example, where W = 2 x R, the coefficient of expansion of the ring 68 will equal three times that of the effective coefficient of expansion of the frame 52. By "effective coefficient of expansion of the frame 52" is meant the corresponding dimensional increase of the recess 60. This can be calculated by multiplying the coefficient of expansion of the stainless steel, for example, of which the screw 54 is made, by the length S of shank 71 of the screw 54, adding the product to the product of the coefficient of expansion of the aluminum, for example, of which the rim 53 is made, and the distance L from head 72 of the screw 54, and dividing by the combined dimension S + L.

Without the present invention, it was found that a rise in the temperature from ambient room temperature to 200°C caused an increase of 0.14 mm in the width Wof the gap 62, where L equaled 18.47 mm and S equaled 13.03 mm, with the coefficients of expansion for the aluminum rim 53 equal to 29xlO^"6/°C and for the stainless steel screws 54 equal to 16.4xlO^"*/°C. The expansion of the quartz window 34 is negligible. The 0.14 mm expansion of the gap 62 is too great for a metal seal, which will leak when the chamber is operated at high vacuum, of, for example 5xl0'⁸ Torr. This was confirmed by tests for the presence of water vapor and nitrogen in the chamber after operation of, for example, ten minutes. With the present invention, leaking was dramatically decreased, evidencing that gas-tightness was preserved. From the above, it will be apparent to one skilled in the art that various alternatives to the embodiments described may be employed without departing from the principles of the invention.

Claims

What is claimed is:

1. A wafer processing apparatus comprising: a sealed housing enclosing a thermal processing vacuum chamber therein, the housing having an opening therethrough; a heater including at least one radiant lamp supported outside of the housing adjacent the opening and positioned such that radiant thermal energy from the lamp passes through the opening into the chamber; a wafer support mounted inside the chamber for supporting at least one wafer positioned therein such that radiant energy passing through the opening from the lamp impinges upon and heats the wafer; a frame secured to the housing and forming therewith a window receiving fixed width gap therebetween surrounding the opening, the frame being configured and secured such that the width of the gap increases in direct relation to thermal expansion of the frame; a window formed of a material transparent to the radiant energy and held in the gap between the frame and d e housing and covering the opening, the window having a thickness less than the width of the gap; a non-soft metal seal compressed between the frame and the housing, against and on one side of the window, so as to form a gas tight connection between the housing and the window; and means for limiting the thermal expansion of the gap during the operation of the lamp and thermal processing of the wafer sufficiently to maintain the fixed width of the gap to thereby maintain gas- tightness between the housing and the window.

2. The apparatus of claim 1 wherein the thermal expansion limiting means includes: means in thermal contact with the frame for cooling the frame to limit the expansion of the frame and a widening of the gap during the operation of the lamp and thermal processing of the wafer sufficiently to thereby retain the gas tightness between the housing and the window.

3. The apparatus of claim 2 the thermal expansion limiting means includes: a cooling liquid source; and a cooling tube mounted in thermal contact with the frame; and means connected to the source and to the tube for circulating cooling liquid from d e source through the tube to cool the frame to limit the widening of the gap during die operation of the lamp and thermal processing of the wafer sufficiently to thereby retain the gas-tightness between the housing and the window.

4. The apparatus of claim 2 wherein: the thermal expansion limiting means includes low thermal expansion material; and at least a portion of the frame is formed of the low thermal expansion material that surrounds the gap and connects the frame to the housing.

5. The apparatus of claim 1 wherein the thermal expansion limiting means includes: a spacer positioned between the frame and die housing, thereby reducing the thickness of the gap; and die spacer being formed of a material of high thermal expansion so as to offset the thermal expansion of the frame to limit widening of die gap during the operation of the lamp and thermal processing of the wafer sufficiently to thereby retain the gas tightness between the housing and the window.

6. A wafer processing apparatus comprising: a sealed housing enclosing a thermal processing vacuum chamber therein, the housing having an opening therethrough; a heater including at least one radiant lamp supported outside of the housing adjacent the opening and positioned such that radiant thermal energy from the lamp passes through the opening into the chamber; a wafer support mounted inside the chamber for supporting at least one wafer positioned tiierein such that radiant energy passing through the opening from the lamp impinges upon and heats the wafer; a frame secured to the housing and forming therewith a window receiving gap a fixed width therebetween surrounding die opening, the frame being configured and secured such diat the width of the gap increases in a direct relation to thermal expansion of the frame; a window formed of a material transparent to the radiant energy and held in the gap between the frame and die housing and covering the opening, the window having a thickness less than the widdi of the gap; a non-soft metal seal compressed between the frame and the housing, against and on one side of the window, so as to form a gas tight connection between the housing and the window; and a cooling element in thermal contact with the frame and maintained to cool the frame and limit the expansion of the frame and a widening of the gap during the operation of the lamp and diermal processing of the wafer sufficiently to thereby maintain gas tightness between the housing and the window.

7. The apparatus of claim 6 the cooling element includes: a cooling liquid source; and a cooling tube mounted in thermal contact with die frame and connected to the source such that cooling liquid flows from the source dirough the tube to cool the frame and to limit the widening of the gap during the operation of the lamp and diermal processing of the wafer sufficiently to thereby retain the gas tightness between the housing and the window.

8. A wafer processing apparatus comprising: a sealed housing enclosing a thermal processing vacuum chamber therein, the housing having an opening theredirough; a heater including at least one radiant lamp supported outside of die housing adjacent die opening and positioned such that radiant diermal energy from the lamp passes through the opening into the chamber; a wafer support mounted inside die chamber for supporting at least one wafer positioned tiierein such that radiant energy passing through the opening from the lamp impinges upon and heats the wafer; a frame secured to die housing and forming therewith gap therebetween surrounding die opening, the frame being formed of a material having a tendency to expand when heated and to thereby increase the widdi of die gap; a spacer positioned between the frame and die housing, thereby reducing die diickness of the gap; a window formed of a material transparent to the radiant energy and held in die gap between the frame and the housing and covering the opening, the window having a thickness less than the widdi of the gap; a metal seal compressed in the gap between die frame and die housing so as to form a gas tight connection between the housing and die window; and die spacer being formed of a material of high thermal expansion so as to offset the diermal expansion of the frame to limit widening of the gap during die operation of the lamp and thermal processing of the wafer sufficiently to thereby retain gas tightness between the housing and the window.

9. A wafer processing apparatus comprising: a sealed housing enclosing a thermal processing vacuum chamber therein, the housing having an opening therethrough; a heater positioned near die housing; a wafer support mounted inside die chamber for supporting at least one wafer such that energy from the heater is communicated to die wafer; a frame secured to the housing and forming therewith a cover receiving gap a fixed width therebetween surrounding the opening, the frame being configured and secured such that die width of the gap increases in a direct relation to diermal expansion of the frame; a cover held in the gap between the frame and die housing and covering the opening, die cover having a thickness less than the width of die gap; a non-soft metal seal compressed between the frame and the housing, against and on one side of the window, so as to form a gas tight connection between the housing and the cover; and means for limiting the diermal expansion of the gap during die operation of the heater and diermal processing of die wafer to maintain the fixed widdi of die gap sufficiently to thereby retain the gas tightness between the housing and the cover.

10. The apparatus of claim 9 wherein the thermal expansion limiting means includes: means in thermal contact with the frame for cooling the frame to limit the expansion of the frame and a widening of die gap during die operation of die heater and diermal processing of the wafer sufficiently to diereby retain the gas tightness between the housing and die cover.

11. The apparatus of claim 10 the diermal expansion limiting means includes: a cooling liquid source; a cooling tube mounted in thermal contact with die frame; and means connected to the source and to die tube for circulating cooling liquid from die source through the tube to cool the frame to limit the widening of the gap during the operation of die heater and thermal processing of the wafer sufficiently to thereby retain the gas tightness between die housing and the cover.

12. The apparatus of claim 10 wherein: the thermal expansion limiting means includes low thermal expansion material; and at least a portion of the frame is formed of die low diermal expansion material that surrounds die gap and connects the frame to the housing.

13. The apparatus of claim 9 wherein die thermal expansion limiting means includes: a spacer positioned between die frame and the housing, thereby reducing the diickness of the gap; and die spacer being formed of a material of high thermal expansion so as to offset the thermal expansion of the frame to limit widening of die gap during the operation of the heater and diermal processing of the wafer sufficiently to thereby retain the gas tightness between the housing and the cover.