GB2141681A

GB2141681A - Cassette trolley with access through the base

Info

Publication number: GB2141681A
Application number: GB08412730A
Authority: GB
Inventors: Martin Albert Hutchinson
Original assignee: Varian Associates Inc
Current assignee: Varian Medical Systems Inc
Priority date: 1983-05-20
Filing date: 1984-05-18
Publication date: 1985-01-03
Also published as: JPS59220445A; GB8412730D0; DE3418244A1; FR2548449A1

Abstract

The cassette 13 is held by the trolley 10 so as to permit access through the bottom of the cassette to semi-conductor wafers held in the cassette. Drive linkage means 23, 25, 26 are provided on the body 9 of the trolley so the trolley may move smoothly and continuously on the track of an automated cassette transport system. In one embodiment, a precision motor 23 such as a DC stepping motor located on the body of the trolley is energized by bushing means (33 Fig. 2, not shown) which contact electrified rails 11, 12. Traction rollers 25, 26 connected to the motor grip at least one rail to provide the motive forces for the movement of the trolley. in a preferred embodiment the drive linkage means 23, 25, 26 are semi-cylindrical linear bearings 19 positioned in a triangular pattern on a pair of rails. <IMAGE>

Description

SPECIFICATION Cassette trolley with access through the base This application relates to apparatus for transporting a semiconductor wafer cassette and, more particularly, relates to a trolley for transporting a semiconductor wafer cassette in an automated cassette transport system.

In most semiconductor fabrication facilities, semiconductor wafers are stored and transported in cassettes which hold from about thirteen to about twenty-five individual wafers.

These cassettes offer an alternative to the approach of individual wafer transport from station to station by means such as air track, vibration track or moving belts. The cassettes are transported by hand or on rolling carts from process station to process station. In one scheme, sealed cassettes are transported pneumatically from process station to process station. See P. P. Castrucci, "Pneumatic Distribution and Control System", IBM Technical Disclosure Bulletin, v. 15, No. 6 November 1972, p. 1763. With most cassette arrangements, a human operator is involved, at least in the handling of the cassette and at times in the removal and reinsertion of wafers into cassettes.With such large scale handling and transportation of semiconductor wafers in cassettes, especially in light of the semiconductor industry maxim-people cause particula tes *here is no alternative to the traditional clean room environment in semiconductor fabrication facilities. See, e.g., J. A. Lange, "Sources of Semiconductor Wafer Contamination", Semiconductor International, April 1983, p. 124.

Clean rooms free from particulates and other contaminants have long been used in the semiconductor industry. Such clean rooms use positive air flow over working areas, filter incoming air through high efficiency particulate air (HEPA) filters and are operated under the strictest conditions. For example, protective caps, gowns, shoe coverings, and pants must be worn over street clothing, no foreign objects are allowed and entry must often be gained through decontamination chambers.

See, M. S. Dahlstrom, "Clean Room Garments: Where From Here?", Semiconductor International, April, 1983, p. 111. The purpose of such clean rooms has been to avoid the presence of electrically active impurities or particles on the working surface of semiconductor wafers. Electrically active impurities could affect device operation. Particles could well produce a break in a conductive line, obliterate some other circuit feature, or otherwise alter the electrical properties of a transistor. The standard practice in the semiconductor industry has been to use clean room facilities which meet the criteria of Federal Standard 209B.The typical clean room meets the specification of a Class 100 facility in which by definition there are fewer than 100 particles of mean diameter greater than or equal to 0.5 microns in a cubic foot of volume at a typical flow rate of approximately 100ft./minute (30 m/minute).

One prior art approach to the transport of semiconductor wafers in a cleaner-than-ambient environment without resorting to a clean room is the wafer transport system available from Nacom Industries, Inc., 2852A Walnut Avenue, Tustin, Ca 92680. An enclosed tunnel, purged along its length with nitrogen, is provided for the rolling transport of a cassette carriage. The carriage rolls along the bottom of the tunnel as it is pulled by the magnetic forces from a permanent magnet located in a second, lower tunnel positioned adjacent the bottom of the upper tunnel. Straight or curved sections may be provided to transport a cassette from one work station to another. With this system there is access to the cassettes and the wafers only at the end of each tunnel section, not intermittently along the track.

Also, provision must be made for returning the permanent magnet, located in the lower, second tunnel, to the entrance zone of each track section. Although the tunnel which houses the cassette is purged with dry nitrogen, there is no provision for removing particulate contamination which may settle on the semiconductor wafers. In fact the motion of the carriage and the purge gas is such that it would tend to both generate particulates and stir up the ever present particulates and distribute them on the wafers.

In order to reduce the cost of semiconductor fabrication facilities and to provide an efficient and clean interconnection between production equipment within such facilities, it has been proposed in the copending patent application (agents reference 230PS1219) to transport cassettes of semiconductor wafers within tunnels having a laminar flow of clean room quality air passing over the wafers.

Briefly, in this automated system, cassettecarrying trolleys ride on electrified rails through partially enclosed tunnels. HEPA-fil- tered air passes downwardly over the cassettes and out the open side of the tunnel into the fabrication facility. The downward flow includes at least a component of laminar flow parallel to the surface of the semiconductor wafers held in the cassettes. A central computer monitors the status of transfer points such as elevators and turntables. It communicates with each trolley at various locations along the tunnel by means such as optical links, including optical bar code scanners, magnetic readers or radio frequency transmission. The central computer can thereby control the process sequence experienced by cassettes of wafers and even the sequence experienced by individual wafers within a cassette.In order to facilitate the hands-off auto mated handling of cassettes and wafers in accordance with this proposed system, it is desirable that the cassette-carrying trolleys provide access to the cassettes and to individual wafers in the cassettes at substantially all points within the system.

It is therefore an object of the present invention to provide a trolley for carrying cassettes of semiconductor wafers which provides access to the wafers through the base.

According to the invention there is provided a trolley for carrying cassettes of semiconductor wafers as set out in claim 1 of the claims of this specification.

An example of the invention will now be described with reference to the accompanying drawings, in which: Figure 1 is an end view of the cassettecarrying trolley with a cassette in place and a knife blade lifter of narrow width in contact with a particular wafer; Figure 2 is a plan view of the cassette trolley (without cassette on lifter) taken through line 2-2 of Fig. 1; Figure 3 is a bottom view of the cassettecarrying trolley taken through line 3-3 of Fig.

1; Figure 4 is a side view of the cassettecarrying trolley taken through line 4-4 of Fig.

2; Figure 5 is a partial inside view taken through line 5-5 of Fig. 2; and Figure 6 is a schematic diagram of the circuitry to stop the trolley in case of a collision signal or a halt command.

Cassette-carrying trolley 10 is shown in end view in FIG. 1. A cassette 13 of semiconductor wafers straddles the center of trolley 10 as lower tabs 14 form a press fit over U-extension 15 attached to body 9. This cassette may be metal or plastic and is of the type, for example, of Flouroware OPA182-50M which holds 25 5-inch wafers. A wafer 16 is shown in place in a slot within standard cassette 13.

Typically, the wafer has some ability to move in the slot since the wafer must move in and out of the slot without abrading. The open central region of the body 9 a trolley 10 permits a knife blade lifter 17 to reach upwardly through the open bottom of trolley 10 and into the open bottom of cassette 13 to push the wafer 16 upwardly and out of the cassette 13 either directly into processing equipment or for presentation to some type of pick-up arm such as a vacuum chuck which takes the wafer and transports it to a desired destination. Access to the wafers through the base of the cassette is permitted by the open central region of trolley 10. Such access permits wafers to be transported, manipulated into and removed from processing equipment without a need for human operators.In operation, the trolley may take a cassette directly up to the load lock chamber of a production machine where in a simple, single step, knifeblade lift the wafer is introduced into the production machine. See for example, G. L.

Coad, et al., "Wafer Transfer System", U.S.

Patent No. 4,311,427 (British Patent Specification No.

The manner by which cassette-carrying trolley 10 rides on rails 11, 12 may be seen with reference to FIGS. 1-3. Commercially available semi-cylindrical linear bearing 19 (available from Thompson Industries, Manhasset, New York 11030) rides on rail 12. As trolley 10 rides on rails 11, 12 the individual bearings 20 progress in a linear track within the housing of the bearing to provide a smooth motion for the trolley along the rail. This is shown particularly in the broken away side view of FIG. 5 which shows the individual bearings 20 within the grooved track. On the opposing rail 11, semi-cylindrical linear bearings 21 and 22 similarly provide a smooth movement for the trolley. The use of semicylindrical linear bearings also produces the result that in case of equipment failure the whole trolley may readily be removed from the rails of the automated cassette transport system.To accommodate variations in the spacing between the rails, linear bearing 19 is free to slide laterally on pins 22 to lessen or increase the net lateral distance between the linear bearing 19 and the linear bearings 21, 22. In a preferred embodiment the bearings are located at the corners of a triangle to provide three-point, stable support.

D.C. motor 23 is mounted on one corner of the body 9 of trolley 10. In a preferred embodiment, by using a stepping motor and digitally counting the steps taken by the motor the precise location of a cassette can be known and a particular wafer can be accessed. Even though there is some variation in the position of wafers within a slot the knife blade lifter can be designed to be oversized with a vee groove to scoop up the wafer; alternately a cassette with off-vertical orientation can be used. See E. W. Lee, "Wafer Boat," U.S. Patent No. 4,355,974. The motor is energized from electrified rails 11, 12 by means of conductive bushings 33. The shaft 24 of DC motor 23 extends down through rubber wheel 25 which tangentially contacts rail 11. The other side of rail 11 is contacted by rubber wheel 26. The two rubber wheels are kept against rail 11 by spring 27 which draws wheels 25 and 26 together in friction fit against rail 11.

As discussed in the above-referenced copending application, the trolley 10 will be controlled in its movement by a central computer whose data base reflects the status of upcoming transfer points and knows the correct processing sequence for a given cassette.

Since multiple trolleys will be moving within the system at any time control over the movement of a particular trolley cannot optimally be accomplished by switching the electrical power to the rails. Rather, through communication links the central computer communicates with on-board electronics located on printed circuit board 30. In the simpler trolleys a photodiode 31 can receive go/stop instructions from an LED connected to the central computer. The on-board electronics 30 in this case constitutes a simple switch for DC motor 23 and is discussed in detail with reference to FIG. 6. Typically, the rails will be energized by direct current of a value, for example, of 24 volts. In more complex trolleys, on-board electronics 30 may include ROM memory which can store information on the process specifications and process history of the whole cassette or of individual wafers.

Communication with the central computer may be two-way so that remote control may be exercised over all cassettes and wafers.

Such communication can also by by magnetic means or by radio frequency transmission.

Since multiple trolleys will be in use in any automated system, it is desirable to prevent the collision of trolleys. Thus, each trolley may be equipped with a reflective sticker 32 on its rear end and a light source such as LED 34 and light sensor such as photodiode 33 on its front end. If one trolley approaches another too closely, the light from LED 34 will reflect from reflective sticker 32 and be detected by photodiode 33. This in turn will operate the circuitry of FIG. 6 to cut the electricity to the drive motor 23, as described subsequently.

The sensitivity of the detection circuitry associated with photodiode 33 may be adjusted so that the minimum distance between trolleys may be controlled.

The circuitry incorporated in on-board electronics 30 in one embodiment of the cassette trolley of the present invention is shown in FIG. 6. Two functions are performed. First, the light emitted from optical source 34, such as an LED, must be detected when it is reflected off of a reflective surface 32 of a trolley located ahead of it. Second, phototransistor 31 must serve as a means of receiving information from light source 42 to stop the movement of the trolley if it is unsafe to proceed due to the status of upcoming transfer stations, e.g., of turntables or elevators.

The collison avoidance function is performed by pulse generator 40 which drives LED 34 in a pulsed mode. Whenever a collision is imminent, light from LED 34 will be reflected rearwardly off reflective surface 32 of the next-up trolley. This reflected light will be received by photodetector 33 which will provide an input to monostable multivibrator 41 which provides a continuous signal to Schmidt trigger 45. The changed input to Schmidt trigger 45 will result in a changed output which is held and passed through optoisolator 49 to the base of the first transistor in Darlington pair 50. The Darlington pair 50 shuts off thereby removing the short in four phase loop 51 and turning off motor 52.

The motor remains off so long as pulsed light is being received by photodetector 33 and the output of Schmidt trigger 45 is held in the position as described. When a light signal above the threshold of sensitivity of detector 33 is no longer being received then the Schmidt trigger 45 returns to its original state, Darlington pair 50 switches on, shorting loop 51 is decoupled and motor 52 is turned back on, its normal state.

The halt function is performed by monostable multivibrator 41 which receives pulses and converts them to a continuous signal.

When a light signal is sensed by phototransistor 31 from light source 42 then Schmidt trigger 44 changes state and through optoisolator 49 a triggering signal is sent to the input transistor of Darlington pair 50. The Darlington pair shuts off thereby allowing four phase loop 51 to turn off motor 52. Motor 52 remains off so long as a light signal is received by phototransistor 31 from optical source 42. When it is safe to proceed, source 42 turns off as directed, for example, by a central computer, Schmidt trigger 44 sends a reverse signal to return Darlington pair 50 to its original state thereby decoupling four phase loop 51 Hand turning motor 52 back on.

While the cassette trolley with access through the base is described for use in a dual rail system, an essential feature is access to wafers through the base of the cassettes.

Alternative embodiments of this invention include a trolley providing access through the base yet being adapted to ride on a monorail, a trolley which carries a cassette within its body yet allows access through the base much like lumber carrying vehicles or a trolley which rides in a slot on the side of a tunnel in an automated cassette transport system and holds the cassette in lateral spaced apart relationship yet allows access through the base.

Claims

1. A trolley for carrying cassettes of semiconductor wafers on track means through tunnels in an automated cassette transport system, comprising: a trolley body adapted to receive a semiconductor-wafer cassette whereby said cassette is held so as to permit access to said wafers through the bottom of said cassette; and drive linkage means mounted on said body and linked with said track means whereby said body may be propelled on said track means within said tunnels in said automated cassette transport system.

2. A trolley for carrying cassettes in accordance with claim 1 wherein said trolley body holds said cassette in detachably attachable relationship.

3. A trolley for carrying cassettes in accordance with claim 1 wherein said body holds said cassette on at least one side thereby permitting access to said cassette from underneath.

4. A trolley for carrying cassettes in accordance with claim 3 wherein said drive linkage means comprises an arm connected to drive means positioned longitudinally within said tunnel.

5. A trolley for carrying cassettes in accordance with claim 4 wherein said drive means comprises a differential pneumatic cylinder.

6. A trolley for carrying cassettes in accordance with claim 1 wherein said body holds said cassette within its interior while permitting access to said cassette from underneath.

7. A trolley for carrying cassettes in accordance with claim 1 wherein said body holds said cassette on its bottom side yet permitting access to said cassette from underneath.

8. A trolley for carrying cassettes in accordance with claim 7 wherein said body comprises a frame having an opening in its central region so as to permit a semiconductor wafer cassette to rest on the top of said frame yet permitting access through said opening from the underside of said body to wafers in said cassette.

9. A trolley for carrying cassettes in accordance with claim 8 wherein said drive linkage means comprises a motor mounted on said frame, and said track means comprises a rail, said motor having a shaft which contacts said rail means to propel said trolley through said tunnel.

10. A trolley for carrying cassettes in accordance with claim 9 wherein said drive linkage means further comprises semi-cylindrical linear bearings which ride on said rails.

11. A trolley for carrying cassettes in accordance with claim 10 wherein said rails comprise a pair of parallel rails.

12. A trolley for carrying cassettes in accordance with claim 11 in combination with means to electrify said rails.

13. A trolley for carrying cassettes of semiconductor wafers in accordance with claim 12 wherein said linear bearing means comprise multiple linear bearing means so that said body rides on each one of said rails by means of linear bearings and wherein at least one of said linear bearings means has a lateral degree of freedom to permit adjustment for variations in the spacing between said rails.

14. A trolley for carrying cassettes of semiconductor wafers in accordance with claim 12 wherein said body rides on each of said rails by means of said linear bearing means and wherein said linear bearing means include at least three separate linear bearings, two on one rail, and one on the other, the three bearings being placed at the corners of a triangle to provide three-point, stable support for said trolley.

15. A trolley for carrying cassettes of semiconductor wafers in accordance with claim 9 in combination with on-board electronic cicuitry responsive to external communications signals to be able to control the operation of said motor.

16. A trolley for carrying cassettes of semiconductor wafers in accordance with claim 15 in combination with optical source means and photodetector means mounted on one end of said frame whereby light from said optical source is deflected off a reflective surface on a nearby trolley and is received by said photodetector means to provide a collision avoidance signal to said on-board circuitry to turn off said motor.

17. A trolley for carrying casettes of semiconductor wafers substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.