GB2141680A - Automated cassette rail transport system operating under clean air conditions - Google Patents

Abstract

The system includes a series of tunnels, transfer stations and interconnections with semiconductor fabrication equipment. The cassettes of wafers 175 are carried on trolleys 176 which are driven by motors 181 on track means 177, 178 provided within the tunnels. Within the tunnels and transfer stations a continuous flow of clean room quality air is maintained through filter 173 in order to keep contaminants and particulates from settling on the surfaces of the wafers. Preferably, a component of laminar flow is maintained across the surfaces of the wafers. Filtration of the clean room quality air is provided adjacent the trolleys to achieve this laminar flow. Transfer stations such as elevators and turntables are linked with linear and curved tunnel sections to provide hands-off station-to-station transport of cassettes of semiconductor wafers. At substantially all points in the system access to the wafers is obtainable through the base of the cassettes by means of the longitudinal opening which allows the flow of air to be discharged into the external environment. <IMAGE>

Description

SPECIFICATION Automated cassette transport system incorporating HEPA filtration This invention relates to a system for transporting cassettes of semiconductor wafers, and more particularly, relates to a system for the automated transfer of cassettes carrying semiconductor wafers where the cassettes move on trolleys within modular units having positive air flow provided through high quality filters.

In the semiconductor industry, it has always been found necessary to fabricate transistors and integrated circuits in semiconductor wafers in a clean environment. Thus, for example, deionized filtered water has typically been available for use in semiconductor fab facilities to ensure particularly that electronic contaminants would not come in contact with semiconductor wafers as they were watertreated. Similarly, rooms free from particulates and other contaminants have been established and utilized from the earliest years. These socalled "clean rooms" use positive air flow over the 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. As clean room specifications become progressively more stringent, it may become necessary to wear space suits. See, e.g., M. S. Dahlstrom, "Clean Room Garments: Where From Here?", Semiconductor International, April, 1983, p.

111. Within these clean rooms wafers have been handled individually by gripping the edges with tweezers and have typically been transported between processing stations in cassettes holding from about 13 to about 25 wafers.

The earliest perceived need for cleanliness was to avoid affecting the electronic properties of the devices being fabricated. The presence of electrically active contaminants would directly affect properties such as carrier concentration, sheet resistance, contact resistance and the like. Similarly, the presence of particles could interfere with the successful execution of such processing steps as diffusion and oxidation. See, for example, J. Amick, "Cleanliness and the Cleaning of Silicon Waf ers ', Solid State Technology, November 1976, p. 47.

In the relentless drive to reduce the widths of conductive lines (line widths) and the dimensions of other geometric features of semiconductor devices, process specifications have become progressively more rigorous. Newer technologies such as electron beam lithography, ion implantation and dry plasma etching have been displacing older technologies such as furnace diffusion and wet chemical processing. Along with this trend, the presence of particles whose mean diameter approaches the diameter of circuit features has become anathema. Such particles could well produce a break in a conductive line, obliterate some other circuit feature, or otherwise alter the electrical properties of a transistor. To avoid the presence of such particles the use of high quality clean rooms is mandatory. See, e.g., J. A.Lange, "Sources of Semiconductor Wafer Contamination", Semiconductor International, April 1983, p. 124. Thus, for VLSI (very large scale integrated) circuit production it is becoming increasingly necessary to ensure that all processing occurs in a quality clean room environment. 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 is designated as Class 100 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 flow rate of approximately 100 ft./min. Such rooms are costly with costs running many hundreds of dollars per square foot of floor space.

Particles are generated by abrasion of silicon wafers against hard surfaces, as the body byproducts of human agents, and from clothing, cosmetics, and bacterial media. And in any normal ambient there will be a collection of particles from various sources which can simply be called dust. In theory, if the sources of particles were eliminated, then it would be possible to use less powerful cleaning techniques. In any event the reduction in the source of particles is a desideratum. However, as much as one tries to eliminate the sources of particles there will always be a background count which has to be taken into account.

In most semiconductor fabrication facilities, semiconductor wafers are placed in cassettes which hold a number of individual wafers.

These cassettes are transported by hand or on rolling carts from process station to process station. See, for example, P. P. Castrucci, ''Pneumatic Distribution and Control System", IBM Technical Disclosure Bulletin, v. 15, No.

6, November 1972, p. 1763. With each step there is the involvement of a human operator at least in the handling of the cassette and at times in the removal and reinsertion of wafers into the cassettes. The use of such cassettes is an alternative to individual wafer transport from station to station by means such as air track, vibration track or moving belts. With such large scale handling and transportation by cassettes there is no alternative to the traditional clean room environment. This approach in a sense is self defeating since the manual handling of semiconductor wafers in a clean room both produces particles and stirs up particles which then must necessarily be removed from the presence of the wafers by the costly clean room techniques described above.

One approach to the transport of semiconductor wafers in a cleaner-than-ambient environment without resorting to a clean room is the wafer transport system sold by 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. The magnetic forces attract and pull the magnetized cassette carriage. 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 to stir up the ever present particulates and distribute them on the wafers.

According to the invention there is provided an automated cassette transport system as set out in claim 1 or claim 24 or a system for transporting wafers as set out in claim 33 of the claims of this specification.

An example of the invention will now be described with reference to the accompanying drawings in which: FIG. 1 is a schematic diagram of an automated cassette transport system installed in a semiconductor fabrication facility; FIG. 2 is a block diagram of the electrical controls of the system of the present invention; FIG. 3 is a perspective view of an automated transport cassette system; FIG. 4 is a cross sectional view of a cassette trolley in place on a turntable within the automated cassette transport system; FIG. 5 is a cross sectional view of a cassette trolley in place in a length of tunnel with an illustration of the direction of air flow; FIGS. 6a-6d are plan schematic diagrams of the logical operations of trolley car movement along a linear track and through a turntable;; FIGS. 7a-7b are plan and side schematic diagrams of the logical operations through and up an elevator; FIG. 8 is a frontal view of an elevator; FIG. 9 is a side view of the elevator of FIG.

8; and FIG. 10 is a schematic diagram of the collision avoidance and trolley halt circuit.

As discussed in detail previously, it is both desirable to eliminate the sources of particulate contaminants and to keep such contaminants away from the working surfaces of semiconductor wafers. The maxim that the elimination of people eliminates particles is applicable. Also, the less individual wafers are handled, the less likelihood there is that silicon particulates will be generated from the abrasion of silicon wafers. The automated cassette transport system described below applies these principles in providing a hands-off cassette transport system. The entire transport and manipulation of cassettes is accomplished in a clean room environment even though the system itself may be installed in an ordinary factory environment. This is accomplished by utilizing a source of clean air or by using high quality filters along the length of the tunnels within which the cassettes travel.The preferred filters are so-called high efficiency particulate air (HEPA) filters. Throughout this specification the term HEPA will be used as representation of high quality filters having a specified maximum pore size of 0.5 microns or less. At all times and at all points the cassettes are bathed in HEPA filtered gas.

typically air, so that even though the volume of the tunnel is open to the ambient of the factory environment, gas flow is in the direction from the HEPA filter toward the factory environment. Class 100 clean rooms as defined by Federal Standard 209B are readily maintainable. By using advanced quality filters as they become available, Class 10 clean rooms or better are achievable and will be useful for VLSI processes with submicron design rules. In a preferred embodiment of the system particulate counters are located at intervals within the system. A major benefit of this modular tunnel approach to the transport of cassettes is the reduction of work in progress and therefore a net reduction in inventory. And the work that is in process is arbitrarily alterable. Also, a significant increase in wafer yield results from the hands-off transport of both wafers and cassettes.

A simple system is shown in FIG. 3. No processing equipment is shown, but one can visualize such equipment as being located at virtually any point within the system. For example, a linear tunnel section could terminate at a piece of production equipment and a second linear tunnel section could lead away from the equipment, thereby providing automatic delivery and automatic removal of cassettes from the equipment. Or side, storage tracks could be provided. As shown in FIG. 3 a conventional HEPA-type clean hood 142 is provided for the introduction of cassettes of wafers and the removal of wafers from the system. In a factory system, the cassettecarrying trolleys which ride on rails 146 would be carried to a succession of processing chambers, discussed subsequently in reference to FIG. 1, but the simple system of FIG.

3 serves to show the components of the modular tunnel system and the logical transfer steps that may be carried out. The rails 146 as discussed subsequently, are electrified so that the wafer-carrying trolleys are self-propelled along the rails in accordance with instructions programmed into on-board electronics or in response to control signals provided to each trolley as it passes intermittent communication points. For example, a cassettecarrying trolley placed on rail 146 would logically travel through linear tunnel section 135 until it approaches turntable 139. Upon approaching turntable 139, a communication link, seen for example as LED 106, 108. .110 in FIG. 2, will communicate with the trolley and direct it to halt should turntable 139 not be available to accept the trolley or, alternately, will direct it to proceed if turntable 139 is oriented to accept the trolley.

Thereupon, the cassette-carrying trolley will travel onto the turntable 139 and stop in an appropriate central location. Next, turntable 139 will execute a 90 turn so that the forward end of the cassette-carrying trolley will be directed toward linear section 130.

When turntable 139 is in place so that its rails mate with the rails of linear section 130.

As the trolley moves onto the rails of linear tunnel section 130. it will pass a further communication point which will provide a central computer. shown as computer 70 in FIG. 2, with information that the turntable may be returned to its receiving position in line with linear section 135. The trolley then moves along within section 130 until it approaches turntable 138. The turntable sequence is again executed as described above.

Once the turn is made, then a decision point is reached between sending the trolley up elevator 141 for delivery to upper linear section 132 or passing through to lower linear track 133. This decision will be made in accordance with information directed from the central computer to the elevator 141. Alternately, this information may be transmitted to the trolley via an appropriately placed communication link. If the process sequence designated for the wafers on the cassette requires transport through the upper track 132 then the trolley will pass from the turntable 138 onto the elevator platform 141. Once the trolley is stopped centrally on the platform, as described in detail subsequently, it will be raised up to a level where the rails of the platform are mated to the rails on the upper track 132.Subsequently, the trolley will propel itself onto the rails of linear section 132 so that trolley will proceed to the waiting platform of elevator 140. Since all working components in the system are tied to the central computer, the platform of elevator 140 may be directed to be ready and waiting to receive the trolley. When the trolley is received and in place, the elevator 140 is lowered and the trolley is discharged into turntable 137, providing as always that the turntable is oriented appropriately to receive the trolley. Alternately to taking the overhead route of linear section 132, the trolley will have moved along lower section 133 and be delivered to turntable 137. In a semiconductor fabrication layout such as shown, for example, in FIG. 1 and discussed subsequently, processing will have been carried out at numerous locations throughout the system.

Finally, the cassette-carrying trolley will turn through turntable 136 onto linear tunnel section 134 and will be returned to clean hood 142. Throughout its transport in the automated cassette transport system, as described in detail subsequently, the trolley and cassette will have continuously been bathed in quality clean room air, typically Class 100 or better.

The air is provided by one or more blowers 143 through ducts 144 into a series of plenums which are located on the upper side of each of the tunnels, as shown specifically in FIGS. 4 and 5.

A semiconductor fabrication facility system is shown in FIG. 1. The room in which the modular units are located need not be a clean room. Instead, each of the modular tunnels, transfer stations (turntables, elevators), and work stations, must provide a clean environment for the cassettes of semiconductor wafers. In FIG. 1 as shown in the legend, T signifies turntable, E signifies elevator, C signifies communication station, and W signifies work station or clean hood. The logic of the system of FIG. 1 is that the cassettes are transported in one direction with an opportunity being provided for processing at successive work stations. With sophisticated computer control and on-board trolley electronics, the trolleys may travel in either direction.

Before each transfer station, e.g., a turntable or elevator, which would deliver a trolley to a particular processing loop, a communication point C is provided. At this point, the central computer 70, shown in FIG. 2 can identify the trolley and consult its own data base to determine whether the wafers in the cassette on that particular trolley should be processed in the upcoming loop. If the answer is affirmative, then transfer through the transfer station into the processing loop is effected. For example, a given cassette may contain wafers which have process specification requiring processing at work stations in loops 3, 5 and 7.Thus, as the trolley exits turntable 30, it passes communication point 70 whereupon the central computer is informed of the identity of the trolley, the run sheet for the wafers in the cassette on the trolley is called up and the instruction is given to turntable 28 to accept the trolley, stop it and turn it onto loop 7. Then, the trolley will proceed to turntable 50, turntable 48, will pass through elevator 41 and be processed in work station 19. The trolley then passes through elevator 39 to turntable 26. When the trolley is released from turntable 26 it passes communication point 72. The identification of the trolley is perceived by the central computer, the run sheet is called up and an instruction is given to turntable 24 to pass the trolley straight through since the processing step of loop 6 is not required.In succession then, the trolley is diverted to loop 5 by means of turntable 20, passes by loop 4 by passing through turntables 16 and 14 and is diverted into loop 3 by means of turntable 12. Alternately, should there be any reason in terms of queing or sequence for cassettes to be processed strictly serially, the trolleys can be directed to travel through each loop. Where a given process is not required, the cassette-carrying trolley may be diverted around a particular work station, for example, by means of elevators 41 and 39 in loop 7, elevators 37 and 35 in loop 6, and the like. The layout of the automated cassette transport system requires that all cassettes pass by work station 9. This could be a point of introduction and removal from the system or could be a preliminary process step experienced by all the semiconductor wafers.The turntables T and the elevators E function in accordance with the principles discussed subsequently with respect to FIGS. 4 and 8-9.

The control provided by the central computer 70 for the processing of wafers in the system of FIG. 1 is so precise that the run sheets for individual wafers may be called up.

For example, if a cassette is presented at a particular work station, e.g., work station 13 in loop 4, the central computer can identify the cassette and either have stored in its memory or read from the on-board electronics or have read from the on-board electronics at communication point 76 the identities of individual wafers on the cassette. Thus, at work station 13 certain ones of the wafers can be designated to receive the treatment. Also, each wafer can receive a particularized treatment, e.g., the dosage level in an ion implanter would be adjusted on a wafer specific basis. This feature is particularly aided by the ready access to individual wafers through the base of the cassette-carrying trolley as described in copending application (Agents Ref. 230PS1220), filed on even date herewith.

Central computer 70 shown in FIG. 2 is the heart of the preferred embodiment of the automated cassette transport system of the present invention. In simpler systems and in straightforward fabrication facilities not involving large numbers of work stations or a desire to individually distinguish between wafers on cassettes or to make significant distinctions between cassettes, it may not be necessary to have a central computer. Thus, a simple system could consist of a linear tunnel section connecting two work stations or a linear tunnel section having a succession of work stations at selected locations along the tunnel.

However. for very large scale integration in highly automated fabrication facilities, it is likely that wafers will be required to be treated individually, and that large distinctions will be made between wafers and cassettes. For the preferred embodiment then, a central computer is in communication with each active component of the system including the trolleys. As shown, central computer 70 may receive status information from each of elevators 87, 89...91 and may send actuation signals in return to each elevator. Similarly, central computer 70 may receive status information from each of turntables 81, 83...85 and may send actuation signals to each of the turntables.Most importantly, central computer 70 may actively interact with each of work stations 86, 88...90 to specify process parameters and to direct wafer handling apparatus to select specific wafers out of identified cassettes to provide particularized processing.

The key to such particularized processing is the provision of identification means on each of trolleys 80, 82...84. Each trolley has onboard electronics including memory such as ROMs or RAMs which will provide, for example, the process history of each wafer, and perhaps the process requirements for each wafer. As shown, LEDs 106, 108 and 110 provide actuation information from central computer 70 to respective associated sensor elements 100, 102...104. Information is transmitted to the central computer 70 by means of light emitting diodes 101, 103...105 which are associated with sensors 107, 109...111 respectively. Alternatively, the communication means may be magnetic, such as disclosed, for example, in P. P. Castrucci, "Pneumatic Distribution and Control System", IBM Technical Disclosure Bulletin, v. 15, no. 6, November 1972, p. 1763, or the communication means may provide oneway identification information to the central computer such as would be provided by an optical bar reader which would read a bar code on each cassette car. Optimally, the trolley electronics will include on-board memory so that detailed information could be carried by the trolley and two-way communication would be possible.

The detailed structure of a linear section of an embodiment of the tunnel used in the automated cassette transport system is shown in FIG. 5. The three-sided structure in this embodiment comprises walls 171, 172 and roof 170. In the preferred mode of operation of this embodiment, the tunnel is oriented as shown in the Figure so that roof 170 is parallel to the ground. Thus, the air flow shown by the arrows is coincident with the effects of gravity and is parallel to the plane of the wafers 175 which are held in the semiconductor cassette 174. Underneath and adjacent roof 170 a plenum 169 communicates with an external source of gas, typically air.

The over pressure from the external air supply forces the air through high efficiency particulate filter (HEPA) 1 73. Such filters are commercially available, for example, from HEPA Air Filtration Corporation, Anaheim, California 92806. They are specified to not allow passage of any particle larger than a certain size, for example, no larger than 0.5 microns. In a preferred embodiment the filter 173 contains a large number of conduits whose axes are aligned with the direction of flow of the air into the tunnel. These conduits serve to evenly distribute the air in the tunnel and contribute to the desired laminar flow over the wafers. The arrows show the flow of air into the bottom portion of the tunnel containing trolley 176. The flow of air proceeds downwardly around cassette-carrying trolley 176 and out the opening in the bottom portion 168 of the tunnel.Such flow is essentially laminar rather than turbulent so that particulates will not be allowed to settle or adhere to the surfaces of the wafers. Wafers 175 held in standard semiconductor cassette 174 are thus continuously bathed in a clean room quality air. The trolley 176 will move along rails 177, 178 as the cassette is being transported through the tunnel. As shown in FIGS.

4 and 5 the tunnel is open along the bottom side to accommodate the discharge of the flow of air. In another embodiment the bottom side may be partially enclosed, for example, by baffles or louvers. Here, laminar flow is preserved over the cassette and wafers, thereby keeping contaminants off the surface, even though some turbulent flow may be introduced adjacent the baffles or louvers. A feature of the present invention that effectively reduces contamination is that there is always a component of the continuous flow which is parallel to the surface of the wafer. Settling and adhesion of particulates on the semiconductor wafer is continuously thwarted by this parallel component of flow.

Cassette-carrying trolley 176 may be of the type disclosed in the copending application mentioned above, filed on even date herewith.

Briefly, the body 167 of the trolley has an opening through its base to permit continuous access to wafers 175 which are held in cassette 174. Such access may be by a knifeedge blade with VEE slot as shown by phantom blade 166. Semi-cylindrical linear bearing 180 allows trolley 176 to ride smoothly on rail 178. Such a bearing is available from Thomsen Industries, Manhasset, New York 11030. Electric motor 181 has its shaft in contact with rail 177 to propel the trolley along the tracks 177, 178. In a preferred embodiment the tracks are cylindrical in cross section to particularly accommodate the semicylindrical linear bearings. The mated relationship provides a particularly smooth ride.In a most preferred embodiment, an exceptionally stable trolley movement is obtained by triangulating the linear bearings, i.e., by having two bearings on one side and one on the other with the three locations forming the corners of a triangle, preferably an equilateral triangle. To further stabilize the trolley car the solitary bearing on the one side may have a lateral degree of freedom to adjust for variations in distance between the rails. Continuous traction is provided by means of rubber wheels 182, 184 which are held against rail 177 by means of spring 183. Electric motor 181 is energized by tapping the electricity in the rails by contact means (not shown).

Within an automated cassette transfer system there will be a number of logical switching points such as are provided by turntables and elevators. These will interconnect linear tunnel sections or might interface a linear section with a piece of semiconductor fabrication equipment. As shown in FIG. 6b a trolley 200 is approaching a turntable 201. The turntable is not oriented to receive the trolley.

Stationary optical means 209 communicates with sensor 207 mounted on that trolley to instruct the on-board electronics on trolley 200 to stop the motor until the turntable is in place. When the turntable is properly oriented, e.g., on command from a central computer, mated with the end of the rails 210, 211 then stationary optical means 209 informs on-board sensor 207 that the trolley may proceed. Thereupon, trolley 200 moves onto the rails 202, 203 of turntable 201. As shown in FIG. 6c the trolley will approach an optical means 204 which will instruct the trolley electronics through sensor 207 to stop the trolley in the middle of the turntable.

Thereafter, the turntable moves toward the direction of travel for the trolley. Optical means 204 instructs sensor 207 that the trolley should now proceed. Thereupon, trolley 200 proceeds onto the track 211, 212 in the next section of linear tunnel. When trolley 200 passes sensor 206 optical source 208 provides a signal that it is now off the turntable so that the next logical operation of the turntable may be performed when necessary.

Another feature of the trolleys used in the automated cassette transport system is shown in FIG. 6a. Cassette-carrying trolley 190 would potentially collide with stationary trolley 191. However, optical source 193 directs a light signal forwardly which then reflects off reflective surface 192 on the rear of stationary trolley 191. This reflected light is sensed by sensor 194. A warning signal is thereby provided to the on-board electronics on trolley 190 so that it stops within a distance which is determined by the sensitivity of the on-board electronics on trolley 190.

The on-board electronics shown as blocks 80, 82...84 in FIG. 2 whose function is described in part in the preceding paragraph is fully shown in the circuit diagram of FIG.

10. Two main functions are performed. First, the light emitted from optical source 193, such as an LED, must be detected by detector 199 to stop a trolley as it is reflected off a reflective surface 192 of the trolley immediately in front of it (for cross reference see FIG.

6a). Second, phototransistor 208 must serve as a means of receiving information from light source 209 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 collision avoidance function is performed by pulse generator 250 which drives LED 193 in a pulsed mode. Whenever a collision is imminent light from LED 193 will be reflected rearwardly off reflective surface 192 of the next-up trolley. This reflected light will be received by photodetector 194 which will provide an input to monostable multivibrator 251 which provides a continuous signal to Schmidt trigger 253. The changed input to Schmidt trigger 253 will result in a changed output which is held and passed through optoisolator 260 to the base of the first transistor in Darlington pair 261. The Darlington pair 261 shuts off thereby removing the short in four phase loop 262 and turning off motor 263. The motor remains off so long as pulsed light is being received by photodetector 194 and the output of Schmidt trigger 253 remains the same.When a light signal above the threshold of sensitivity is no longer being received by detector 194 then the Schmidt trigger 253 returns to its original state, Darlington pair 261 switches back, shorting loop 262 is decoupled and motor 263 returns to operation, its normal mode.

The halt function is performed by monostable multivibrator 251 which receives pulses and converts them to a continuous signal.

When a light signal is sensed by phototransistor 207 from light source 209 then Schmidt trigger 254 changes state and through optoisolator 260 a signal is sent to the input transistor of Darlington pair 261. As a result four phase shorting loop 262 inhibits the operation of motor 263. Motor 263 remains off so long as a light signal is received from optical source 209. When it is safe to proceed the light signal from source 209 disappears and Schmidt trigger 254 sends a reverse signal to return Darlington pair 261 to its original state thereby decoupling shorting loop 262 and turning motor 263 back on.

The detailed structure of a turntable is shown in FIG. 4. A stepping motor 156 is connected by means of shaft 157 to turntable 158. One rail is held on turntable 158 by means of vertical supports 161, the other by vertical supports 162. These rails are mounted in line with but separated from the rails 160 of the adjacent linear section. It is preferred to contour the spaced apart rail ends so there is a narrow gap in between but so that the rail ends clear each other as the turntable turns. In FIG. 4 the trolley 176 is shown in a stationary, control position on the turntable 158. The turntable has rotated as far as allowed by stop 1 59. The position of the turntable is sensed by magnetic reed switches 157. 158 which verify the extent of travel of the turntable.The turntable is housed in a continuation of the HEPA-filtered environment of the linear sections. See. e.g.. FIG. 5.

Thus, the wafers are continuously bathed in HEPA-filtered air which is at least a component of laminar flow and preferably is parallel to the surface of all the wafers.

The logical function of an elevator is shown in FIGS. 7a and 7b. Elevator 223 is shown in ground position in FIG. 7a in between pair of rails 225, 226 and pair of rails 227, 228. It serves to translate trolley 222 between different sections of horizontal track. It would be used to take a trolley over a doorway, to another floor or over a piece of production equipment. Any given trolley may travel through the elevator and stay on the same level, as shown in FIG. 7a where trolley 222 is moving onto rails 227, 228 after leaving rails 225, 226. In the lift mode, shown in FIG. 7b, trolley 222 has stopped on elevator 223, for example, by means of a halt signal from an appropriately positioned LED as described elsewhere. Elevator 223 rides on vertical rods 220, 221 by means of cylindrical linear bearings such as are available from Thomsen Industries, Manhasset, New York.

Vertical drive is supplied by differential pneumatic air cylinder 228 such as supplied by Origa Corporation, Elmhurst, Illinois 60126.

When elevator 223 rises to the level of an upper track (not shown) the trolley 223 will proceed onto that track providing no halt or collision avoidance signals are received.

The detailed structure of an elevator is shown in FIGS. 8 and 9. L-bracket 229 is attached to a float member within pneumatic cylinder 228 which moves upwardly or downwardly as determined by the differential air pressure as between the upper and lower portions of the cylinder. Alternatively, a ball and screw lift means may be used. Vibration free, level ascent and descent is accomplished by cylindrical, linear bearings 230, 231, 232 which ride on vertical rods 222, 221, 233. Communication links are provided by optical means 234, 235 in order, for example, to provide a halt signal to a given trolley so it will remain on elevator 223 rather than passing through to the continuation of the linear track.

The tunnel and trolley structures described heretofore are exemplary. The trolley may have different motive means and a different configuration which yet allows access to semiconductor wafers through the base of the cassette. For example, the cassette may be carried below the trolley rather than above it akin to the lumber carriers which grab their load and hold it within the vehicular frame. Or the cassette may be carried on the side of a trolley which attaches to drive means located along the inside of the tunnel. In both instances the wafers in the cassette may be accessed through the base. Also, the track sections may be curved or sloped as the requirements of the semiconductor fabrication facility dictate.

Claims (39)

1. An automated cassette transport system, comprising: at least one tunnel section for transport of said cassettes therethrough in a clean room environment, said tunnel section having a gas plenum disposed on one side thereof along its length and having an opening for discharging from said tunnel, gas introduced through said plenum; track means positioned longitudinally within said tunnel intermediate said gas plenum and said opening; and trolley means having a drive linkage with said track, said trolley means adapted to receive a semiconductor wafer-holding cassette.

2. An automated cassette transport system in accordance with claim 1 in combination with a source of clean room quality gas interconnected with said gas plenum.

3. An automated cassette transport system in accordance with claim 1 in combination with means associated with said plenum for maintaining a positive gas pressure in said tunnel section.

4. An automated cassette transport system in accordance with claim 3 wherein trolley receives said cassette in detachably attahable relationship.

5. An automated cassette transport system in accordance with claim 1 in combination with a HEPA filter positioned longitudinally within said tunnel section inbetween said gas plenum and said track means whereby gas introduced to said plenum passes through said HEPA filter to provide clean room quality gas in laminar flow fashion over said trolley and over said wafers in said cassette.

6. An automated cassette transport system in accordance with claim 5 wherein said cassettes carry said wafers in upright parallel orientation.

7. An automated cassette transport system in accordance with claim 6 wherein said HEPA filter contains numerous conduits whose axes are substantially aligned with the planes of said wafers.

8. An automated cassette transport system in accordance with claim 5 wherein said trolley means provides access to said wafers through the bottom of said cassette at least along a portion of the length of said track.

9. An automated cassette transport system in accordance with claim 8 wherein said opening is located in opposition to said HEPA filter.

10. An automated cassette transport system in accordance with claim 9 wherein said cassette is detachably attachable to the top of said trolley means, thereby allowing said wafers to be oriented parallel to the direction of said positive flow of gas.

11. An automated cassette transport system in accordance with claim 8 wherein said cassette is detachably attachable to the underside of said trolley means.

12. An automated cassette transport system in accordance with claim 8 wherein said cassette is detachably attachable to the side of said trolley.

13. An automated cassette transport system in accordance with claim 10 wherein said trolley means includes on-board electronic circuitry including collision avoidance and halt functions.

14. An automated cassette transport system in accordance with claim 13 wherein said onboard circuitry includes memory means to record process specifications and process history.

15. An automated cassette transport system in accordance with claim 8 in combination with a central computer for controlling transfer stations within said system, and for storing process information for work stations within said system.

16. An automated cassette transport system in accordance with claim 15 further including communication links for effecting communication between said central computer and individual ones of said trolleys.

17. An automated cassette transport system in accordance with claim 15 wherein said transfer stations include turntable means contiguous with one end of said at least one tunnel section, said turntable having a track which abuts said track means in said at least one linear section in mated fashion when said turntable is turned to receive a trolley from said at least one linear section, said turntable means forming a continuation of said tunnel so that a trolley on said turntable means is continuously bathed in clean room quality air as it passes from said linear section through said turntable.

18. An automated cassette transport system in accordance with claim 15 wherein said transfer stations include elevator means contiguous with one end of said at least one tunnel section, said elevator means forming a continuation of said tunnel section so that a trolley within said elevator means is continuously bathed in clean room quality air as it is transferred from said at least one linear tunnel section.

19. An automated cassette transport system in accordance with claim 13 wherein said track means comprises electrified rail means and said trolley includes an electric motor for self-propulsion of said trolley along said rails in response to actuation signals from said onboard electronic circuitry.

20. An automated cassette transport system in accordance with claim 19 wherein said rail means comprises a pair of rails and wherein said trolley rides on said rails on semi-cylindrical linear bearings which permit the ready removal of said trolley from said rails.

21. An automated cassette transport system in accordance with claim 16 in combination with a piece of semiconductor fabrication equipment interconnected with said at least one tunnel section or one of said transfer stations.

22. An automated cassette transport system in accordance with claim 9 in combination with a modular clean room chamber placed adjacent the end of said at least one tunnel section to permit the introduction and removal of cassettes from said cassette transport system.

23. An automated cassette transport system in accordance with claim 9 in combination with particulate measurement equipment located in said at least one linear tunnel section.

24. A system for transporting wafer-holding cassette means under filtered air conditions comprising: means defining a trackway for movement of said cassette means, with wafers oriented generally upright relative to said trackway; a passageway section at least partially enclosing said track means; means cooperating with said passageway for establishing a generally laminar gas flow into the volume about said trackway, said gas flow having a component parallel to the surfaces of said wafers; means cooperating with said laminar air flow means for filtering the air flowing into said volume about said trackway; whereby said wafers may be transported through passageway while bathed only in filtered air having an air flow component parallel to said wafers.

25. A system as in claim 24 in which said passageway section is provided with trackway entrance and trackway exit openings, and in addition is provided with exhaust openings to promote said laminar gas flow.

26. A system as in claim 24 in which said passageway section is provided with trackway entrance, trackway exit and gas exhaust openings, but otherwise fully encloses said trackway.

27. A system as in claim 24 in which said passageway section is elongated in the direction of said trackway.

28. A system as in claim 25 or claim 26, in which said trackway entrance and trackway exit openings terminate in workstations for processing said wafers.

29. A system as in claim 24 in which said means for establishing air flow establishes a positive air pressure within said volume about said trackway.

30. A system as in claim 24 in which said passageway section is open on its lower side so as to provide access to said trackway from below, but otherwise encloses said trackway on all remaining sides.

31. A system as in claim 24 in which said wafer-holding cassette means includes a trolley and detachable cassette, said trolley supporting said cassette and riding upon said rail means, said trolley being apertured, whereby access to said wafers may be had from below said track.

32. A system as in claim 30 which further includes blade means positioned below said trackway and operable transversely thereto to engage individual wafers from below and move them to and from said cassette means for processing.

33. A system for transporting under clean conditions wafers carried side by side within cassettes allowing access to said wafers from above and below, comprising: at least one axially elongated enclosure enabling passage of cassettes in the axial direction, said enclosure having at least one opening on a side thereof; means for establishing a positive gas pressure and flow throughout at least a substantial volume of said enclosure, said air flow having at least a component transverse to the axis of said enclosure; means for filtering gas flowing from outside said volume into said means for establishing said positive gas pressure; trackway means axially extending through said enclosure; ; trolley means engaging said trackway and adapted for movement along said trackway within said volume, and further adapted to support at least one of said cassettes, said trolley means being apertured to permit access to said wafers from below; and blade means aligned with said passage- way side opening and moveable up and down in the planes of said wafers for engaging said wafers through said trolley aperture to successively raise and lower individual ones of said wafers to permit individual processing of each wafer; whereby a plurality of wafers may be transported, and individual ones thereof accessed and processed, all under clean conditions regardless of the ambient atmosphere.

34. A system in accordance with claim 29 wherein said means for filtering comprises a high efficiency particulate filter.

35. A system in accordance with claim 30 wherein said trackway means comprises a pair of spaced-apart rails adapted to receive said trolley.

36. A system in accordance with claim 31 in combination with a semiconductor fabrication machine positioned so as to receive wafers raised by said blade means.

37. A system in accordance with claim 32 in which said means for establishing a positive gas pressure and flow includes means for establishing a laminar air flow through said volume generally parallel to said cassette and wafers.

38. A system in accordance with claim 36 wherein said means for establishing laminar flow comprises a high effifiency filter having conduits whose axes are substantially aligned with the planes of said wafers.

39. A transport system substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.