BACKGROUND OF THE INVENTION 1. Technical Field This invention relates generally to a method and apparatus for supporting semiconductor wafers, and more specifically to methods and apparatuses for supporting a semiconductor wafer during deposition and heating in a minibatch furnace.

2. Description of Related Art Furnaces are commonly used in a wide variety of industries, including in the manufacture of integrated circuits or semiconductor devices from semiconductor substrates or wafers. Thermal processing of semiconductor wafers include, for example, heat treating, annealing, diffusion or driving of dopant material, deposition

or growth of layers of material, and etching or removal of material from the substrate. These processes often call for the wafer to be heated to a temperature as high as 250 to 1200 degrees Celsius before and during the process. Moreover, these processes typically require that the wafer be maintained at a uniform temperature throughout the process, despite fluctuations in the temperature of the process gas or the rate at which it is introduced into the process chamber.

A conventional furnace typically consists of a voluminous process chamber positioned in or surrounded by a furnace. Substrates to be thermally processed are sealed in the process chamber, which is then heated by the furnace to a desired temperature at which the processing is performed. For many processes, such as chemical vapor deposition, the sealed process chamber is first evacuated, after which reactive or process gases are introduced to form or deposit reactant species on the substrates.

There are several design challenges to meeting the support requirements of wafers inside a semiconductor heat treatment apparatus. For instance, semiconductor wafers subjected to the extreme temperatures inside the process chamber generally undergo thermal expansion. Similarly, as the process chamber is cooled, the wafers contract. During the deposition process, the wafer must be adequately supported, but must not be so firmly supported that thermal expansion or contraction causes the wafer to slip or bind. An example of a wafer boat is shown in U. S. Patent No. 4,770, 590, issued September 13,1988 to Hugues et al.

Further, the contact area between a support and a wafer (colloquially, the "footprint"of the support) may limit or block vapor deposition in the contact area, creating a shadow or mark on the wafer surface. These shadowed areas are useless when forming semiconductors, and effectively constitute wasted space on the wafer

surface. Generally, prior art solutions marked off approximately a 3mm exclusion zone along the wafer edge Accordingly, there is a need for an apparatus and method to overcome the aforementioned problems.

BRIEF SUMMARY OF THE INVENTION Generally, an embodiment of the present invention takes the form of a T- shaped support. The T-shaped support ("T-rail"), when viewed in top-down cross- section, has a relatively large, semicircular portion ("head") at one end, tapering to a series of relatively thin ledges. In side-view cross-section, the ledges preferably are uniformly spaced along the length of the T-rail. The top of each ledge contacts the semicircular portion at an angle slightly greater than ninety degrees. For example, in one embodiment, the angle between the top surface of the ledge and the rail head is ninety-one degrees.

The T-rail, including the head and series of ledges preferably, is uniformly created from a single quartz rod. In one embodiment, clear fused quartz is used.

The T-rail is generally created by grinding the rod to shape through a two-step cutting process. The first step defines the head and a protrusion by making a pair of radiused cuts running the length of the rod. The second step shapes each of the ledge by cutting multiple times into the width of the rod along the protrusion. Each cut into the rod's width forms a ledge from the previously-defined protrusion.

One or more T-rails may be used in a wafer carrier. The wafer carrier typically accepts and supports one or more semiconductor wafers for thermal processing inside the process chamber. In one embodiment of the wafer carrier, three T-rails are attached to a cylindrical base and a semicircular top. The T-rails

are spaced along the base in such a manner that two side T-rails are approximately equidistant from the back T-rail, with each set of three ledges (one on each T-rail) forming a level plane. A wafer is placed in each plane defined by a set of ledges, and is balanced on the ledges. Unlike some prior art systems, the wafer is supported only from the bottom, rather than both the top and bottom. This minimizes boat markings on the wafer surface.

Further, because each ledge is slightly back-cut, the contact surface between a ledge and a supported wafer is decreased. Thus, markings are again reduced, as is thermal transfer between the ledge and the wafer. Finally, since each wafer is supported at only three points, the wafer is free to thermally expand and contract without binding or slipping in the wafer carrier, thus helping the wafer to remain centered during processing.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 displays an exemplary operating environment for one embodiment of the present invention.

Fig. 2a displays a side plan view of a T shaped support ("T-rail") in accordance with one embodiment of the present invention.

Fig. 2b displays a front plan view of a T-rail in accordance with one embodiment of the present invention.

Fig. 3 displays a top-down view of a T-rail in accordance with one embodiment of the present invention, taken along line B-B of Fig. 2b.

Fig. 4 displays a detail view of the base of a T-rail in accordance with one embodiment of the present invention.

Fig. 5a displays a back view of a wafer carrier in accordance with one embodiment of the present invention.

Fig. 5b displays a side view of a wafer carrier in accordance with one embodiment of the present invention.

Fig. 6 displays a top-down view of a wafer carrier in accordance with one embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION 1. Operating Environment Fig. 1 displays an exemplary operating environment for an embodiment of the present invention, namely a semiconductor minibatch furnace. The furnace 140 generally includes a process chamber 102 having a support 104 adapted for receiving a carrier 106 with a batch of wafers 108 held therein by a series of T-rails 100, and heat source or furnace 140 having a number of heating elements 112 for raising a temperature of the wafers to the desired temperature for thermal processing. The furnace 140 further includes one or more optical or electrical temperature sensing elements 114, such as a resistance temperature device (RTD) or thermocouple, for monitoring the temperature within the process chamber 102 and/or controlling operation of the heating elements 112. Such control, for example, might be accomplished via a feedback loop.

In the embodiment shown, the temperature sensing element is a profile thermocouple 114 that has multiple independent temperature sensing nodes or points for detecting the temperature at multiple locations within the process chamber 102.

Alternately, the temperature sensing element may be a series of spike thermocouples unrelated to one another. The furnace 140 can also include one or more injectors 116 for introducing a fluid, gas, or vapor, into the process chamber 102 for

processing and/or cooling the wafers 108, and one or more vents or purge ports 118 (only one of which is shown) for introducing a purge element into the process chamber. A chamber liner 120 may increase the concentration of processing gas or vapor near the wafers 108, and reduces contamination of the wafers from flaking or peeling of deposits that can form on interior surfaces of the process chamber 102.

Generally, the process chamber 102 is sealed by a seal, such as an o-ring 122, to a platform or baseplate 124 to completely enclose the wafers 108 during thermal processing. Openings for the injectors 116, thermocouples 114 and purge ports 118 are sealed using seals such as O-rings, VCR@, or CF fittings. Gases or vapor released or introduced during processing are evacuated through an exhaust port 126 formed in a wall of the process chamber 102 or via a plenum 127 of the baseplate 124, as shown in Fig. 1. The process chamber 102 can be maintained at atmospheric pressure during thermal processing or evacuated to near-vacuum via a pumping system (not shown) including one or more roughing pumps, blowers, hi- vacuum pumps, and roughing, throttle and/or foreline valves.

The process chamber 102 and liner 120 can be made of any metal, ceramic, crystalline or glass material that is capable of withstanding the thermal and mechanical stresses of high temperature and high vacuum operation, and which is resistant to erosion from gases and vapors used or released during processing.

Preferably, the process chamber 102 is made from an opaque, translucent or transparent quartz glass having a sufficient thickness to withstand the mechanical stresses and that resists deposition of process byproducts, thereby reducing potential contamination of the processing environment. More preferably, the process chamber 102 and liner 120 are made from an opaque quartz that reduces or eliminates the

conduction of heat away from the region or processing zone 128 in which the wafers 108 are processed to the seal 122.

In accordance with one embodiment, semiconductor wafers are loaded into a wafer carrier 106 consisting of three T-shaped rails 100 arrayed in a triangle, each of which has a plurality of ledges projecting therefrom. The T-rail 100 and associated ledges are typically machined from a single rod of quartz, and so are integral. One wafer is placed in each plane defined by a set of three ledges, one on each T-rail 100, equidistant from the base of the carrier 106. The wafer carrier 106 is then placed into the process chamber 102, which is in turn sealed. Thermal processing of the wafers takes place within the chamber while the wafers are supported inside the carrier. After processing, the chamber 102 is opened and the wafer carrier 106 removed. In the present embodiment, placement and removal of the carrier 106 is accomplished mechanically, with the carrier being raised into and lowered from the process chamber 102 by an elevator platform. In alternate embodiments, the carrier 106 may enter the chamber 102 through a side or top port, or may be fixed inside the chamber.

2. The T-Rail Structure Generally, one embodiment comprises a machined quartz rod 200, as shown in side plan view in Fig. 2a and front plan view in Fig. 2b. The quartz rod 200 includes a generally semicircular portion (the"head") 210 running along one side from top to bottom. As shown in the top-down view of Fig. 3 (taken along line B-B of Fig. 2b), two radius cuts 300, 310 angle inwards from the widest part of the head 210 and define a first and second sidewall for the head. With reference to both Figs.

2a and 3, the radius cuts generally terminate in a flat face 220.

The quartz rod 200 further includes a number of ledges 230 formed by cutting or planing into the rod surface. The sidewalls 320,330 of each ledge are also generally radiused, and may be formed in the same cutting process used to form the head sidewalls. The top surface of each ledge is generally planar, while the bottom surface may be planar along a length of the surface, then transition into a radiused surface. These ledges are generally evenly spaced along the length of the rod, with the exception of the bottom portion of the rod. A detail view of the bottom of one rod 200 is shown in Fig. 4.

As most clearly seen in Fig. 4, each ledge 230 is an integral element of the quartz rod 200, rather than being formed of a separate material and attached to the rod surface. By forming the entire t-rail 200 from a contiguous piece of quartz, heat is more evenly distributed throughout the t-rail 200, while the possibility of a ledge 230 snapping or otherwise disconnecting from the main rail 200 is minimized.

Still with reference to Fig. 4, in the present embodiment 200 the ledges 230 are slightly downwardly angled with respect to the rod head 210. That is, the ledge slopes as it extends outwardly from the rod 200 body. In the present embodiment, the angle formed at the top of the juncture of the ledge 230 and rod 200 is approximately ninety-one degrees. Alternate embodiments may employ different angles, so long as the angle is greater than ninety degrees. By slightly back-cutting each ledge, surface contact between the ledge and a supported wafer is minimized.

In fact, in the current embodiment 200, the wafer effectively balances on the ledge in such a manner that only a single point of contact is formed between each ledge and the supported wafer. By minimizing contact area, thermal transfer between the wafer and ledge is similarly minimized, thus leading to a more uniform heat

distribution across the wafer during processing. This, in turn, permits more uniform deposition and curing inside the process chamber 102.

Referring now to Figs. 2a, 3, and 4, various measurements of the present embodiment 200 will be given. It should be remembered that these measurements are illustrative of a single embodiment only; alternate embodiments may use different dimensions.

Generally, the instant T-rail 200 measures approximately 23.255 inches from <BR> top to bottom. Each of the ledges 230 is roughly. 120 inches high, . 120 inches wide, and. 385 inches deep. The spaces 240 between ledges 230 are approximately. 75 inches high at the widest point, the head 210 is. 786 inches wide. The radius for the top angle formed by the ledge 230 and rod 200 is roughly. 015 inches, while the bottom angle has a radius of roughly. 120 inches. Finally, the base of the rod extends about 1.5 inches before the first ledge 230 is formed.

3. The T-Rail in Operation Figs. 5a and 5b display a set of three T-rails 500,502, 504 mounted in an exemplary wafer carrier 510 in a back and side view, respectively. The wafer carrier 510 consists of the three T-rails 500,502, 504, a bottom cylindrical support 520, a top semicircular support 530, and a bottom plate 540 upon which the bottom support rests. Each of these elements is made of fused quartz in the present embodiment in order to minimize thermal expansion or contraction, as well as promote uniformity of heat transfer between any supported wafers and the process chamber 102. The various elements may be affixed to one another by any means known to those skilled in the art. As best seen in Fig. 5b, the top support 530 is semicircular, having a

radius approximately equal to that of the bottom support 520, but extending only slightly forward of the T-rail juncture point 550.

Generally, the wafer carrier 510 is mounted within the process chamber 102 and receives a series of wafers, each of which rests in a unique plane defined by coplanar ledges 230 on each of the three T-rails 500,502, 504. Thus, each wafer is supported at three points, effectively balancing on each of the back-cut ledges and minimizing contact therebetween.

Because each wafer is supported at only three points in three-dimensional space, rather than the six typical of prior-art systems, the wafers are not confined within the carrier 510. Accordingly, the wafers have freedom to thermally expand or contract without slipping or binding as the process chamber 102 temperature changes, which leads to twisted or slipped wafers. This same freedom of movement, however, makes properly centering the wafer within the carrier more difficult than if the wafer were clamped or otherwise held in place. The back-cut angles on the various ledges 230, as well as the circular edge along one side of the wafer, assist with centering.

The wafer is inserted into the wafer carrier 510 with a point low on the semicircular edge resting on a ledge 230 of the backmost T-rail 502. The radius of the wafer along its outer edge contacts only a small portion of the sloped ledge 230.

The weight of the wafer is concentrated in this contact area. The combination of the radiused edge, along with the slope of the ledge, leads the wafer to self-align within the carrier 510.

That is, when the wafer is placed into the carrier, it is inserted as far into the carrier interior as possible. The sides of the wafer are supported by the ledges 230 of the side T-rails 500,504, which act as guides. Because the side T-rails 500,504

prevent the wafer from shifting dramatically from side to side, the point along the curved edge farthest from the rear wafer wall (i. e. , the edge of the wafer directly opposite the semicircular edge) rests on the ledge 230 of the backmost T-rail 502.

Still with reference to Figs. 5a and 5b, general dimensions for the wafer carrier 510 will be given. It should be understood that these measurements are illustrative of the present embodiment only; alternate embodiments may use different dimensions.

Generally, the carrier 510 is approximately 12.99 inches in diameter, measured along the bottom plate 540. The first tier of ledges 230 occurs approximately 11.467 inches from the base of the carrier 510, plus or minus approximately. 02 inches. The overall height of the carrier is roughly 33.256 inches.

Fig. 6 displays a top down view of the carrier 510, indicating the contact points between the T-rails 500,502, 504 and the top support 530. The distance from the center of the carrier 500 to the center of any T-rail is approximately 5.98 inches.

The angle formed between the back T-rail 502 and either side T-rail 500,504 is 100 degrees, when measured with respect to the center of the wafer carrier 500.

Accordingly, the angle between the two side T-rails is 160 degrees. Finally, the distance between the centers of the side T-rails 500,504 is no more than approximately 11.959 inches in the present embodiment. Again, different embodiments may have different measurements, dimensions, tolerances, relationships between elements, and so forth. Accordingly, the above numbers should be considered exemplary only of the present carrier 500, rather than defining all wafer carriers embraced by the present invention.

4. T-Rail Manufacture Returning to Fig. 2, one embodiment of a T-rail 200 is manufactured through the use of a pair of grinding wheels joined together by a central support. The support is attached to the middle of both grinding wheels, which are of approximately the same diameter. The wheels'grinding surfaces are generally pointed inwards, towards each other. The grinding wheels pass along the sides of the quartz rail from top to bottom, creating the radiused cuts best seen in the top- down view of Fig. 3. Alternately, two non-connected grinding wheels may be used to create these cuts, or a single wheel may make two passes.

Next, a second set of grinding wheels, connected in the same manner as the first, is used to create the ledges 230. The second (bottom) wheel is a standard grinding wheel, while the first (top) is angled to create approximately a one-degree back-cut. These wheels make a series of cuts into the narrow face of the T-rail 200, defined by the first cuts, in order to produce the various ledges 230. The spacing and angling of the bottom grinding wheel may be varied during operation to produce the smoothly curved incision typical of the ledge base, as shown to best effect in Fig.

4.

The grinding wheels are made of any suitable material capable of cutting quartz in the fashion described above, such as a diamond-impregnated resin.

5. Conclusion As will be recognized by those skilled in the art from the foregoing description of example embodiments of the invention, numerous variations on the described embodiments may be made without departing from the spirit and scope of the invention. For example, a T-rail may have different physical measurements, or may be manufactured from different materials. Further, while the present invention has been described in the context of specific embodiments and processes, such descriptions are by way of example and not limitation. Accordingly, the proper scope of the present invention is specified by the following claims and not by the preceding examples.