Synchronous multiplexed near zero overhead architecture for vacuum processes

Workpieces, such as, semiconductor wafers, are continuously manufactured by repetitively alternately switching a common radio frequency power source between a plurality of downstream or in-chamber processing reactors and actively processing one workpiece in a vacuum in an operating one of the processing chambers while simultaneously executing with a robot at atmospheric pressure the overhead tasks relative to next processing another workpiece in the other processing chamber. The active processing of the workpieces in alternate chambers does not overlap, and the robot starts and completes all of its preparatory tasks during the active processing step during the time when a chamber's door is closed thereby providing virtual zero overhead. System architecture allows eliminating all redundant components other than the dual chambers which operate in parallel. For a modest cost increase for the second chamber throughput is trebled and overall costs significantly reduced. Preferred modes include switching a common microwave power source between the pair of processing chambers, pumping down with a common vacuum pump, and stabilizing the chamber pressure with a common throttle valve.

BACKGROUND OF THE INVENTION
 1. Fields of the Invention
 The present invention relates generally to an apparatus and method for
 reducing workpiece handling overhead relative to active workpiece
 processing time in a manufacturing process, and, more particularly, to
 reducing the relative time spent pumping a vacuum in semiconductor wafer
 processing chambers, venting such chambers to atmosphere, and transferring
 wafers to and from such chambers while increasing the relative time spent
 actively processing the wafers in the chambers, such as, by actively
 etching, stripping, and depositing the semiconductive layers of the
 wafers, and, even more particularly, to the process of switching common RF
 or microwave power supply sources alternately between dual downstream or
 in-chamber plasma reactors and alternately actively processing in one
 reactor while performing the aforesaid overhead tasks on the other reactor
 thereby significantly increasing the throughput of the overall machine at
 overall reduction in equipment cost compared to conventional dual or
 multiple reactor systems.
 2. Discussion Of Background And Prior Art
 a. Generating A Plasma
 The primary reason to generate a plasma is to generate an intense amount of
 heat in a localized area to excite atoms or molecules of gas to an
 elevated state. The energy can be so intense that some common compounds
 dissociate.
 For example, CF.sub.4, a stable compound gas, can have some of the fluorine
 atoms stripped off to form monatomic F which is extremely reactive. In the
 case of oxygen, it exists in nature as O.sub.2 and not O.sub.1. When
 oxygen is passed through an intense RF electric field, often produced by
 microwave radiation, a significant percentage actually separates into
 monatomic oxygen, O.sub.1, and is far more reactive than O.sub.2. The
 monatomic oxygen is also heated to temperatures as high or higher than
 1000 degrees C when in the plasma field. Photoresist is a carbon based or
 organic compound. When the wafer is heated to typically 220 to 270 degrees
 C and subjected to the hot monatomic oxygen, the photoresist is combusted
 often within 10 to 15 seconds depending on thickness and other variables.
 The new compounds formed are generally CO.sub.2, CO, and H.sub.2 O which
 are gaseous at ambient conditions. The photoresist effluent is then pumped
 off in the gaseous state through a vacuum pump. In etch applications,
 different gas volatiles are formed.
 Although plasmas can be created at atmospheric pressure, they are very
 difficult to ignite, difficult to control, consume enormous power,
 generate intense UV light, and are awkward to confine. Nearly all
 semiconductor process applications operate at greatly reduced pressures
 relative to atmospheric conditions. As a general class, plasma processing
 is typically conducted from a low range of 1 milli-Torr to a high range of
 roughly 10 Torr. For example, photoresist ashing or surface cleaning and
 preparation pressure ranges operate from about 0.1 Torr to 2 Torr.
 Examples of methods to generate a plasma at rarified pressures are
 capacitively coupled electrodes, parallel plate reactive ion etchers
 (RIE), inductively coupled plasma (ICP) methods involving a resonate RF
 coil, microwave cavities, and electron cyclotron resonance (ECR). Some
 embodiments also include magnetic fields for shaping the plasma.
 b. Vacuum Processing
 Semiconductor silicon wafer processes, such as, actively etching,
 stripping, and depositing the semiconductor layers of the wafers, are
 generally performed under greatly reduced atmospheric pressure conditions
 ranging generally from well under the 10.sup.-3 Torr regime to a few Torr
 in order to readily establish plasma conditions exciting the process gas
 with RF or microwave energy. Plasma excited process gas species have
 greatly elevated reaction levels to speed combination with the intended
 semiconductor layer to be processed. This vacuum processing contributes
 substantially to the high cost of semiconductor processing equipment
 itself (vacuum pumps alone cost about $40,000.00) and the relatively
 excessive amount of time (typically 65-80% of a complete cycle) spent in
 the overhead tasks of pumping a vacuum, venting to atmosphere,
 transferring wafers to and from, opening and closing doors, metering gases
 in and out, and the like. Moreover, each of these subsystems is typically
 contained in a multi-chamber machine, providing 100% redundancy at great
 expense in terms of equipment cost and space. Nakane U.S. Pat. No.
 4,483,651 (2:10-13).
 c. Cost of Semiconductor Equipment
 On one hand, when examining the cost of semiconductor wafer processing
 equipment, one quickly realizes that relatively few system components
 comprise the vast majority of the total equipment costs. The process gas
 mass flow controllers, the RF power sources, such as, microwave and lower
 frequency RF power supplies, and the vacuum pumps are among the largest
 high cost items followed by the actual process reactor. Significant
 savings can be immediately achieved simply by developing a process which
 increases throughput by using dual, side-by-side replicas of selected
 critical costly items, but one of everything else.
 Accordingly, it is an object of the present invention to significantly
 reduce the overall costs of semiconductor processing equipment, and
 especially atmosphere to vacuum equipment, by eliminating all redundant
 components of a plural setup other than the dual, side-by-side, wafer
 processing chambers.
 d. Overhead Time
 On the other hand, when examining the typical overhead tasks performed by
 the semiconductor wafer processing equipment, including (1) wafer transfer
 from the cassette to the chamber entrance and later back to the cassette;
 (2) chamber door opening and closing; (3) wafer exchange to and from the
 process chamber; (4) wafer heating or cooling preparation; and (5) pumping
 or venting the process reactor chamber to the desired vacuum level, or
 conversely, to an elevated pressure, one quickly recognizes that a
 significant savings can be immediately achieved by coordinating and
 sharing , that is, synchronously multiplexing, common material support
 sub-assemblies which perform overhead tasks.
 As one example of the relative speed of one type of semiconductor
 processing machine, such as a photoresist ashing machine, the fastest
 photoresist, strip, dual process chamber module unit in the marketplace
 today runs at typically 110-130 wafers per hour.
 Accordingly, it is an object of the present invention, by adding redundancy
 of one critical stage of a single process machine at only a modest cost
 increase while synchronously multiplexing the remaining components of the
 processing system, virtually 100% of the wafer exchange overhead, the
 pump/vent overhead, and the wafer temperature conditioning overhead
 commonly associated with preparing an environment for wafer chemical
 processing to commence is eliminated or masked.
 e. Current Semiconductor Atmosphere to Vacuum Continuous, Synchronously
 Multiplexed, Wafer Transfer Systems
 (1). Solo Wafers, Early Attempts
 Early attempts to solve the problem of high expense, high overhead wafer
 processing are seen in Uehara U.S. Pat. No. 4,149,923 in which single
 wafer, single processing chambers, each separately pumped, were replaced
 by multi-chamber, sequential processing of single wafers all in one common
 vacuum, and in the system by Nakane '651 referenced above in which a
 computer controlled a common main wafer transfer mechanism operating in
 parallel with plural plasma reactors each fed by its own branch shuttle
 wafer transfer mechanism and each having its own vacuum pump. While
 achieving some economies through a common vacuum and by an automatic
 parallel scheme, Uehara and Nakane left too much redundancy in their
 entire systems.
 (2). Group Batch Process
 Another method commonly in use reduces the pump and vent overhead time by
 pumping and venting an entire group of wafers in a load lock all at one
 time, typically a cassette of 25 or more wafers. In an early system by
 Blake in U.S. Pat. No. 5,019,233 plural, separately pumped, single
 process, chambers are serviced by a central interior substrate handling
 robot which services dual load lock chambers alternately loaded with
 25-piece substrate batches. While one batch is being subjected to load
 lock evacuation providing thermal desorption, the other batch, having been
 previously evacuated is transferred one at a time to selected process
 chambers by the central interior robot. Therefore, load lock dwell times
 are ordered parallel to the active (coating) process, and extended load
 lock dwell times do not impair apparatus productivity until the batch lock
 dwell time exceeds the time needed for the serial processing of a full
 batch of individual cassettes.
 There are good reasons for transferring wafers within a vacuum environment
 including toxic gas control, sequential processes where exposure to air
 affects the process, and moisture control for cold process where moisture
 can freeze to surfaces. Moreover, while wafer transfer time within a
 vacuum equalized environment is faster, nonetheless, it continues to be an
 appreciable percentage of the total productive wafer process time.
 Thus, while Blake's system improves overall system throughput, it is far
 from optimal because the cost of a cassette vacuum load lock is quite
 expensive especially when coupled with an expensive vacuum wafer transfer
 robot, and it can never provide true zero overhead because of the overhead
 inherently required in sequential processing of the substrate stack
 through plural process chambers.
 In a more recent attempt by Begin in U.S. Pat. No. 5,310,410, plural,
 individually pumped, processing chambers each for a different function
 serviced by an interior central robot in a sequential processing process
 were provided with atmospheric level processing chambers interspersed
 therebetween. While this system no doubt provided the claimed increase in
 flexibility, it did so at a great increase in equipment cost and overhead.
 Finally, in an even more recent example, by Higashi in U.S. Pat. No.
 5,611,861, after describing the failure of early solo wafer processing
 systems to achieve sufficient throughput, after describing the failure of
 early batch wafer processing systems to achieve sufficient uniformity and
 throughput, after describing the failure of early multi-chamber wafer
 processing systems to achieve sufficient increase in throughput because
 the transfer overhead time exceeded the processing time of the processing
 chambers and finished wafers had to wait for transfer, and, therefore, in
 a further effort to increase throughput in even multi-chamber processing
 systems, Higashi proposed putting the plural processing chambers on a
 carousel supported by two fixed, spaced apart, transfer stations adjacent
 the carousel, one for transferring unprocessed wafers into a processing
 chamber in hermetically sealed fashion and the other for transferring
 processed wafers to the outside as the carousel rotates the processing
 chambers sequentially into alignment with one or the other transfer
 station while wafer processing is taking place therebetween. Each of
 Higashi's processing chambers has its own high frequency power supply for
 its plasma reactors. Again, we see the prior art achieve increased
 throughput at the expense of significantly increased equipment cost with
 complete 100% redundancy of critical costly items in each processing
 chamber without significantly improving machine component efficiency or
 utilization.
 f. Current Semiconductor Dual Chamber Alternating Power Supply System
 In a semiconductor alternating dual processing chamber system by Oramir
 located in Israel, it is known to alternate the power source between the
 dual chambers in an application which uses a raster scan laser to ablate
 photoresist on a semiconductor wafer in an ozone ambient at atmospheric
 pressure. However, this feat is accomplished by merely deflecting the
 laser beam with a mirror alternately from one process chamber to the
 other. Obviously, Oramir's system is extremely expensive and slow and is
 not price or throughput competitive even with conventional machines on the
 market. Moreover, the entire process is performed at atmospheric or near
 atmospheric conditions, not at typical low pressure vacuum levels.
 Furthermore, due to the special complexities associated with deep vacuum
 processing and RF power switching, there is no suggestion by Oramir to
 provide, and, accordingly, it is an object of the present invention to
 provide, a switchable radio frequency power supply alternately,
 synchronously multiplexed with atmosphere to vacuum dual processing
 chambers.
 It has been amply demonstrated above that there is still a long felt need
 for, and it is an object of the present invention to provide, a process of
 providing radio frequency power alternately between dual, adjacent,
 downstream or in-chamber or in chamber plasma reactors and actively
 processing in one without overlapping the processing in the other while
 performing the aforesaid overhead tasks in the other thereby significantly
 increasing the throughput of the overall machine at a substantial overall
 reduction in equipment cost.
 SUMMARY OF THE INVENTION
 Set forth below is a brief summary of the invention which achieves the
 foregoing and other objects and provides the foregoing and hereafter
 stated benefits and advantages in accordance with the structure, function
 and results of the present invention as embodied and broadly described
 herein. Applicant's invention includes independently both the apparatus
 and the methods described herein which achieve the objects and benefits of
 the present invention. Both formats of the invention are described below,
 and it is applicant's intention to claim both formats even though from
 time to time below for purposes of clarity and brevity applicant will use
 either one or the other format to describe various aspects and features of
 the invention.
 A first aspect of the invention is a method of continuously processing a
 plurality of workpieces which includes the steps of supplying radio
 frequency power to one of a pair of processing chambers, actively
 processing a workpiece in a deep vacuum solely in the one processing
 chamber while simultaneously executing at substantially atmospheric
 pressure substantially all post-processing and pre-processing overhead
 tasks relative to processing another workpiece in the other processing
 chamber, and then reversing the aforesaid steps by power supplying the
 other processing chamber and actively processing the other workpiece in a
 deep vacuum solely therein while simultaneously executing at substantially
 atmospheric pressure substantially all overhead tasks relative to
 processing a third workpiece in the one processing chamber, and
 continuously repeating all of the aforesaid steps in sequence.
 Further features of this aspect of the invention include placing the
 chambers adjacent, supplying microwave power, synchronously multiplexing a
 common radio frequency power source between the pair of processing
 chambers, actively processing the workpieces in a downstream or in-chamber
 plasma reactor, actively processing one workpiece in one processing
 chamber without overlapping the processing of another workpiece in the
 other chamber, beginning the actively processing step with closing the
 door of the chamber with the workpiece to be processed next inside and
 ending that step with the opening of the door of that chamber at the
 completion of the processing of that workpiece, and robot starting and
 completing all of the overhead tasks between the beginning and ending of
 the actively processing step.
 A second aspect of the invention is a method of continuously manufacturing
 a plurality of workpieces which includes the steps of alternately
 switching a common radio frequency power source between plural processing
 chambers, and actively processing a workpiece in a deep vacuum processing
 chamber while simultaneously executing at substantially atmospheric
 pressure the post-processing and pre-processing overhead tasks relative to
 next processing another workpiece in a non-operating other one of the
 processing chambers, and continuously repeating all of the aforesaid steps
 in sequence.
 Further features of this aspect of the invention include the steps of
 processing in dual chambers, placing the processing chambers adjacent,
 synchronously multiplexing a microwave power source between the pair of
 processing chambers, actively processing the workpieces in a downstream or
 in-chamber plasma reactor, actively processing one workpiece in one
 processing chamber without overlapping the processing of another workpiece
 in the other processing chamber, beginning the actively processing step
 with closing the door of the chamber with the workpiece to be processed
 next inside and ending that step with the opening of the door of that
 chamber at the completion of the processing of that wokpiece, and a robot
 starting and completing all of said tasks between the beginning and ending
 of the actively processing step.
 A third aspect of the invention is a continuous method of processing
 workpieces through dual processing chambers which includes the steps of
 (a) switching a common power source to the first chamber ON and to the
 second chamber OFF,(b) processing the workpiece in the first chamber to
 completion while simultaneously removing the processed workpiece from the
 second chamber and reloading the second chamber with another workpiece to
 be processed, (c)switching the common power source to the first chamber
 OFF and to the second chamber ON, (d) processing the workpiece in the
 second chamber to completion while simultaneously removing the processed
 workpiece from the first chamber and reloading the first chamber with yet
 another workpiece to be processed , and (e) continuously repeating steps
 (a)-(d).
 A fourth aspect of the invention is a continuous method of processing
 workpieces through a single first stage processor and dual chambers of a
 second stage processor comprising the steps of (a) switching the output of
 a first stage processor to a first path while blocking the output of the
 first stage processor to a second path and closing the second path, (b)
 processing the workpiece in a first chamber of a second stage to
 completion while simultaneously removing a processed workpiece from a
 second chamber of the second stage and reloading the second chamber of the
 second stage with another workpiece to be processed , (c). switching the
 output of the first stage processor to the second path, while blocking the
 output of the first stage processor to the first path and closing the
 second path, (d) processing the workpiece in the second chamber of the
 second stage of completion using the output of the first stage while
 simultaneously removing the processed workpiece from the first chamber of
 the second stage and reloading the first chamber of the second stage with
 another workpiece to be processed, and (e)continuously repeating steps
 (a)-(d).
 Some of the many advantages of the present invention are:
 1. The overall system architecture eliminates redundant parts and, through
 sharing of components allows, the overwhelming majority of high cost
 subsystems to be amortized between the two process chambers, including,
 for examnple, the common switchable RF power supply, throttle valve, gas
 box, pressure transducers, and vacuum pumps.
 2. This invention is applicable to both deposition and etch semiconductor
 processes and their variations, including, for example, photoresist
 removal, wafer cleaning, and the like.
 3. The system architecture utilizing two process chamber reactors with
 power supply switching and alternate synchronous multiplexed processing is
 novel.
 4. At least for those wafer processes that do not require sequential wafer
 transfer within a vacuum, the system achieves results which exceed those
 of a vacuum load-locked system without the added cost of the load-locks
 and vacuum wafer transfer robot.
 5. The system achieves virtual continuous non-stop wafer processing
 essentially negating all of the normal overhead associated with wafer
 transfer between atmospheric and vacuum conditions.
 6. The system greatly improves overall machine throughput and utilization
 without materially increasing system cost for atmosphere to vacuum
 processes.
 7. Recognizing that the alternate use of two process reactors is in actual
 practice a single process module, the projected system throughput can more
 than double to beyond 200 wafers per hour with generally only a 10 to 20%
 increase in system cost.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
 1. Overview
 As best seen in FIGS. 15, 17, 18 the synchronous multiplexed near zero
 overhead system architecture 10 of the present invention has the typical
 set of one group of mass flow controllers controlling process gases 20,
 microwave and RF power sources 22, and optionally a single source of
 vacuum 34. The key difference is the addition of the second of two process
 reactor chamber 30,32. Wafer processing alternates between the two process
 chambers 30,32. While one reactor chamber 30 is processing a wafer, the
 other chamber 32 is being vented 43 (FIG. 15) back to atmosphere, the
 completed wafer is exchanged with a new unprocessed wafer, and the other
 chamber 32 is pumped back to the desired vacuum level. Optional wafer
 temperature conditioning 31,33 is performed just prior to pump down or
 during vacuum pump down. Process gases and vacuum sources are alternated
 20 to the appropriate process chamber 30,32 along with the associated
 microwave or RF power sources 22 via a relay and/or microwave switch 24.
 To reach peak optimization, the robotic transfer overhead plus pump and
 vent overhead time should be less than the total active wafer processing
 time. (FIG. 17). In this mode, a near 100% equipment utilization condition
 for active process is achieved versus the typical situation of 30%
 utilization, or even less for short process times. While the additional
 process chamber 30,32 adds to system cost, even so, the added amount
 represents only 10 to 20% of the total system cost. For advanced systems
 in the marketplace today, the present invention increases overall
 throughput substantially by roughly double the normal throughput of the
 prior art.
 More particularly, as best seen in FIGS. 1-13, the overall assembly 10 of
 the computer controlled, synchronous, multiplexed, near zero overhead,
 architecture for vacuum processes of the present invention is shown in a
 preferred embodiment that includes in its most general form a computer
 shown in FIG. 7, a front panel 12 (FIGS. 1,3), at least one cassette 14,16
 (FIGS. 1,3,6), a robot 15 (FIGS. 4,6), a back panel 18 (FIGS. 2,7), a
 process gas distribution box 20 (FIGS. 2,7,8), a microwave generator 22
 (FIGS. 1,2,7,9), a microwave switch 24 ((FIGS. 2,5,7,10), at least one
 plasma source 26, 28 (FIGS. 2,5,7,11) and at least two processing chambers
 30,32 (FIGS. 1,7,12), and at least one vacuum pump 34 (FIGS. 7,13).
 In a typical general process of the present invention as seen in schematic
 form in FIGS. 14, 17, 18 the process chambers 30,32 are identical single
 process chambers set up to run the same or similar process in a vacuum.
 The process cycle can be best explained by examining the movements of the
 robot 15. We can start by looking at the robot 15 in mid-cycle removing a
 previously processed wafer from one previously vented 20 chamber 30 with
 one gripper and replacing it with an unprocessed wafer previously removed
 from a cassette 14,16 (one or more cassettes may be used) with its other
 gripper. A vacuum 34 is drawn in that chamber 30 and the active wafer
 processing begins there by igniting the plasma 26 for that chamber by
 switching 24 on the common microwave energy generator source 22 to it
 While the first wafer is being processed in the one chamber 30, the robot
 15 moves to the cassette 14,16 while the still hot processed wafer (in the
 case of photoresist ashing) it is carrying is cooled and loads that wafer
 into the cassette 14,16 while removing a second unprocessed wafer and
 turns to the now vented 20 other chamber 32 which has just finished
 processing a wafer also previously deposited in it. The robot 15 removes
 the processed wafer from the other chamber 32 replacing it with the second
 unprocessed wafer in its other gripper. While the second wafer is being
 processed in the other chamber 32, the robot again moves to the cassette
 14,16 while the still hot processed wafer it is carrying is cooled and
 loads that wafer into the cassette 14,16 while removing a third
 unprocessed wafer and again turns back to the now vented 20 one chamber 30
 which has just finished processing the first wafer, and the cycle repeats.
 The overhead is near zero if all chamber 30,32 overhead processes and robot
 15 overhead processes (that is, all pre- and post-active processing
 preparatory steps) are begun and completed during the time that each
 chamber 30,32 alternately and synchronously begins and completes active
 processing of its wafer (between the time the power supply is switched on
 and the time the power supply is switched off).
 As described in greater detail below, the power supply, vacuum pump and
 throttle valve can all be shared by the dual chambers and used in
 alternation synchronously with the chambers 30,32 operations.
 Additionally, however, just as the aforesaid components may be shared, so
 too, can the other components of the system be shared, including , but not
 limited to, the end-point detector, the pressure transducers (manometers
 measuring pressures in the chambers), the gas box, the backfill pressure
 tank and lines, the reservoir and pressure equipment and lines and the
 like. Additionally, other configurations and sharing than those shown
 herein are possible.
 2. The Power Supply
 Plasma power sources come in many forms. In the preferred embodiment of the
 present invention microwave energy is used to excite the process gases at
 rarified pressures. However additional power supplies may be used in the
 present invention to produce a plasma condition. As a matter of practice,
 the FCC only allows certain frequencies to be used for high power
 non-communications commercial applications while the vast majority of the
 radio frequency spectrum is reserved for communications. Typically, the RF
 frequencies used in commercial semiconductor applications are 100 KHz, 400
 KHz, 13.56 MHZ, 915 MHZ, and 2.45 GHZ. The bands above 900 MHZ are
 generally called microwave frequencies because of their very short
 wavelengths. But, in fact, they are all radio frequencies and it is the
 intent of the present invention to cover all applicable radio frequencies.
 3. Robotic Interfaces
 The robotic interface to the process reactors can also take on many forms.
 In one embodiment described in this patent application in working example
 2 in FIGS. 19-20 a single arm, dual gripper is used (one arm linkage, but
 ability to hold two wafers at a time). The preferred robotic form is a
 dual arm robot, such as is described below in working examples 3 (FIGS.
 21-31), each with a single gripper to hold one wafer on each arm for an
 overall total of two wafers. A single arm linkage with one gripper to hold
 only one wafer ordinarily would be too slow for many applications, but may
 be acceptable for long processes.
 The particular configuration used in this application has the cassettes and
 chambers on 90 degree X-Y coordinates which means that the robot moves in
 a straight line path perpendicular to a line joining a cassette on one
 side and a processing chamber on the other side, that is, in an aisle
 between the two cassettes on one gripper and the two chambers on the other
 gripper.
 If a circular arrangement is used, then the traverser is not needed. In the
 circular arrangement, the robot is at the center and the cassettes and
 process reactors are on a fixed radius around it. Newer robots have
 additional degrees of movement freedom to operate with cassettes and
 process reactors on 90 degree X-Y configurations, yet are located at a
 central fixed position. It is the intent of this application to cover all
 robot types.
 4. The Process Gas Flow
 As best seen in the schematic diagram of FIG. 15, the process gases of the
 present invention are selected and then flow through a plurality of lines
 according to the position of a plurality of valves mounted on the process
 gas distribution box 20 and which are under the control of a computer
 program (not shown). While the gases and the distribution hardware 20 used
 in the present invention are all conventional, the system architecture and
 the software processes which control the flow of gases therethrough are
 proprietary.
 Process gases originate from a common source for both processing chambers
 30,32. However, the gas flow is alternately enabled through separate lines
 to each plasma applicator 26,28 and chamber 30,32 combination in
 synchronism with the loading of an unprocessed wafer therein by the robot
 15, the application of a vacuum by the common pump 34 in the chamber
 30,32, the heating of the wafer on the chuck 31,33, the switching on by
 the common microwave switch 24 of the common microwave power supply 22,
 the stabilization of the pressure in the chambers 30,32 by the throttle
 valves 36,37, the venting 41,43 of the chambers 30,32 and the removal of
 the processed wafers from the chambers 30,32 by the robot 15. As is well
 known in the trade, provision is made for brief slow pump bypassing of the
 throttle valves 36,37 by closing isolation valves 38,39, opening bypass
 valves 45, 47 and applying the vacuum 34 to the chambers 30,32 through the
 orifices 49,51. This step reduces the speed of adiabatic expansion and,
 consequently, the temperature drop in the chamber 70, 72. Initial fast
 pumping is not used because it creates condensation and particles.
 5. The Pumping System
 As best seen in FIG. 16, in one embodiment of the present invention the
 pumping system subsystem of the pump/vent system 40 includes two pumps
 96,98 and one throttle valve 94 for the two process chambers 70,72. This
 architecture is based on the condition that only one of chambers 70,72 is
 running process at any one time. To pump down a chamber 70,72 the
 appropriate one of stop valves 86,92 is opened and wet pump 98 is
 operated. Manometer 64 indicates the pressure in the chamber 70,72 which
 is processing by opening the appropriate one of stop valves 66,68. To
 stabilize the operating chamber pressure the throttle valve will close
 correspondingly if the chamber pressure is too low, and, will open
 appropriately if the chamber pressure is too high.
 The pump subsystem 96,98 provides the following additional significant
 operating features and advantages when compared to one vacuum pump for one
 processing chamber setup:
 a. Dry/Wet Pump Setup Reduces Cost Without Sacrificing Performance
 In the embodiment of FIG. 16, one of the two pumps 96,98 is used solely for
 chamber pump-down while the other pump is used only during the processing
 of the wafer.
 For pumping down a chamber 70, 72, wet pump 98 is operated and the
 appropriate one of the stop valves 86,92 is opened while its associated
 stop valve 88,90 is closed. Wet pump 98 requires oil for lubricating the
 pumping mechanism, and, therefore, it is highly possible that the chamber
 70,72 may be contaminated by the lubrication oil due to back-streaming.
 However, oil back-streaming occurs in higher vacuum conditions where fewer
 gas molecules exist in the pump line, and the oil molecules require some
 time to travel back-stream to the chamber 70,72. During pump-down, the
 chamber pressure is reduced from 760 Torr to 1 Torr and a pump-down cycle
 is completed in 3 to 5 seconds. The higher pressure translates into a lot
 of gas molecules and creates a sweeping effect in a short period of time,
 virtually eliminating the back-streaming effect.
 For the processing of the wafer, the dry pump 96 is operated with throttle
 valve 94 and the appropriate one of the stop valves 88,90 open and its
 associated pump down stop valve 86,92 closed. A dry pump 96 is preferable
 for the processing pump because it does not require lubrication oil for
 the pumping mechanism, and, therefore, eliminates the possibility of
 process contamination by oil back-streaming to the processing chamber
 70,72 through the pump line. The dry pump 96 costs about twice as much as
 the wet pump 98. The dry/wet pumping system setup reduces the overall cost
 of the system and efficiently utilizes the function of the pumps 96,98.
 b. Single Throttle Valve Further Reduces Cost
 In the embodiment of FIG. 16, only one throttle valve 94 is necessary since
 only one of the chambers 70,72 is running a process at any one time. This
 setup configuration provides the capability of switching the throttle
 valve from servicing one chamber to the other by opening one of the stop
 valves 88,90, as desired, while closing the other one and also closing
 stop valves 86,92.
 c. Less Process Variations Between Chambers
 Because both chambers 70,72 share the same throttle valve 94 and processing
 pump 96, process variations from chamber to chamber are significantly
 reduced.
 d. Bypassing the Throttle Valve For Pump-Down Reduces Overall Process Time
 If pump-down is conducted through the throttle valve 94, the throttle valve
 94 has to be wide open for a faster pump-down. When the chamber 70, 72
 reaches the base pressure, the throttle valve 94 starts to move to the
 throttling position while process gas starts to flow. It usually takes the
 throttle valve 94 about 5 seconds to control the chamber to reach the
 desired process pressure. Nearly this entire 5 seconds can be saved by
 bypassing the throttle valve 94 for pump-down, since the throttle valve
 can be preset at the desired position for a faster process pressure
 stabilization.
 6. The Venting System
 As best seen in FIG. 16, in one embodiment of the present invention the
 venting subsystem of the pump/vent system 40 includes a source of N.sub.2
 gas which enters the system through a conventional gas box 42 having a
 pressure gauge 44, a stop valve 46, a pressure regulator 48 which is
 adjustable in a range, for example between 0-100 psig, a filter 50, and a
 stop valve 52. The gas box 42 distributes the N2 gas to a pressurized
 back-fill tank 53 and then to chamber 70 through a pair of parallel valves
 56,60 and to chamber 72 through a pair of parallel valves 58,62. The
 miniconvectorns 78,80 measure the pressure in the chambers 70,72 through
 valves 82,84 in a broad range from the base vacuum of a few mTorr to
 atmospheric level of 760 Torr. The vent subsystem provides the following
 additional operating features and advantages:
 a. Initial Slow Venting Particle Contamination
 Upon completion of the processing in a chamber 70,72, initial slow venting
 is achieved by opening the appropriate one of the small orifice (.25
 inches) valves 60,62.
 b. Fast Venting by the Pressurized Back-Fill Tank
 The short initial slow venting step described in the preceding section is
 immediately followed by a fast venting which is achieved by shutting the
 small orifice valves 60,62 and opening the appropriate one of the large
 orifice valves 56,58 backfilling the chamber with N2 from the pressurized
 back-fill tank 53 which is kept at a preferred pressure of 30 psig on
 pressure gauge 54.
 c. The Pressurized Back-Fill Tank Reduces The Pressure Drop In The N2 Line
 Venting requires a lot of N2. Without a pressurized tank 53, if chamber 1
 were venting while chamber 2 was running a process, the N2 pressure in the
 gas line to chamber 2 might drop, and, if it did, the process gas flow of
 chamber 2 might be disrupted. Pressurized back-fill tank 53 is equivalent
 to the capacitor of an electrical circuit where energy can be stored and
 released in a short period of time thereby minimizing process disruption.
 d. Continuous Purge Prevents Moisture From Entering the Chamber
 Moisture is the root cause of corrosion. When a chamber 70,72 has been
 vented to atmospheric pressure by opening the gate valve 41,43 (FIG. 15)
 and the robot 15 is transferring wafers in and out of the chamber 70,72,
 the venting subsystem of the pump/vent system 40 can provide a gentle
 trickle purge of N2 gas through the bleed valve 60,62 (FIG. 16) to keep
 air and moisture from entering the chamber 70,72.
 7. The Vacuum Resevoir Equivalent
 One need not be concerned about connecting both chambers to a single vacuum
 pump out of fear of interaction when one chamber is processing a wafer and
 the other chamber begins to pump down from atmosphere, expecting that the
 burst of air could potentially travel down the vacuum line to the pump and
 back up to the chamber processing the wafer. The most negative pressure is
 going to be at the pump head. If the vacuum lines are long enough and big
 enough in diameter, the pressure will equalize and expand to fill the
 space. By the time that the side that is pumping down reaches the pump,
 the pressure will be very low. Meanwhile on the side where the wafer is
 being processed, process gas is being delivered through the mass flow
 controllers. In the case of ashing, the total process gas flow is on the
 order of 5 liters per minute. Therefore, the gas going through the chamber
 being processed should be at a higher pressure than what is in the line.
 The key to this working is sufficiently long vacuum lines to provide
 isolation between the two process chambers. To assist in isolation the
 vacuum lines should be fairly large in diameter to provide more volume for
 the air to expand from the chamber being pumped down. Furthermore, a
 bypass valve is provided with a 1/4 inch line to slow the initial burst of
 air from the chamber being pumped down. A second or two later, the main
 ISO 80 valve is opened providing a higher conductance to rapidly pump the
 remaining air from the chamber.
 8. The End-Point Detector
 The fiber optic cable transmits UV light from the chamber being processed.
 Again, since only one chamber at a time is processing a wafer, the two
 fiber optic cables go into an optical summing junction where the signals
 from the two chambers are added together. Obviously, only one produces UV
 light at a time, so an optical switch is unnecessary. The end point
 detector is a monochromator which selects only one spectrum emission line,
 typically an OH line. End-point detectors are relatively expensive.
 Normally, one end-point detector is required per chamber.
 10. Working Example 1
 As seen in FIGS. 17-18, the line graph displays a task function along the Y
 coordinate and time along the X coordinate of a complete cycle of a first
 working example of one embodiment of the present invention in which two
 conventional downstream reactors each running the same or similar process
 are serviced by two exterior (relative to the vacuum) 25-wafer cassettes
 and a single exterior robot with one front arm and one rear arm. The same
 sequence would be followed using a single 25-wafer cassette.
 Actually, the process reactors can be different and still alternate
 overhead /process tasks. Wafers would be mapped cassette #1 to process
 chamber #1 and cassette #2 to process chamber #2.
 As seen in FIG. 17, and as more generally disclosed above, the process
 cycle can be best explained by examining the movements of the robot 15
 which are traced out schematically in FIG. 18. Starting at the top
 function of FIG. 17 at chamber 1 at the point in the cycle where chamber 1
 has been backfilled to atmospheric pressure, its pins raised and the
 chamber 1 door opened, the robot 15 removes a previously processed wafer 1
 from chamber 1 with his back gripper and rotates 180 degrees placing an
 unprocessed wafer 3 previously removed from cassette 1 (one or two
 cassettes may be used) with its front gripper. The chamber 1 door is then
 closed its pins are lowered, a vacuum 34 is applied to the chuck 31, the
 chamber 1 is pumped down, the process gas is turned on, the process gas is
 stabilized by adjusting the throttle valve 36 (or 94 in a single pump
 process (FIG. 16)), and the active wafer 3 processing begins by igniting
 the plasma 26 for chamber 1 by switching 24 on the common microwave energy
 generator source 22 to it. Meanwhile from the point that the robot places
 the new wafer 3 into the chamber 1 for processing and thereafter while
 wafer 3 is being processed in chamber 1, the robot traverses to cassette 2
 and holds the still hot processed wafer 1 at the cooling station which is
 actually a heatsink area on the robot's body combined with cool air from
 blower 35 (FIG. 15) directed at the robot. The robot then loads wafer 1
 into the cassette 2 with its rear gripper, retracts and indexes to a new
 slot and removes unprocessed wafer 4 with the rear arm, and waits for the
 door to open of the now vented chamber 2 which has just finished
 processing a wafer 2 also previously deposited in it. The robot 15 removes
 the processed wafer 2 from chamber 2 with its front gripper, rotates 180
 degrees, places the unprocessed wafer 4 into chamber 2 with its rear
 gripper, traverses to cassette 1 while the still hot processed wafer 2 is
 cooled and loads wafer 2 into the cassette 1, retracts and indexes and
 removes unprocessed wafer 5 and waits for the door to open to the now
 vented chamber 1 which has just finished processing the first wafer 3. The
 cycle repeats.
 The overhead is near zero if each chamber 1,2 alternately and synchronously
 finishes processing its wafer just as the robot 15, finishes removing a
 fresh wafer from the cassette adjacent that chamber 1,2 with its one
 gripper ready for an exchange with its other gripper and the chamber
 overhead is shorter than the process time. In other words if the wait time
 is zero, then the overhead is near zero and 100% utilization of the
 processing capacity of the machine is being realized. Achieving near zero
 wait time is simply a function of doing what is necessary to shorten and
 equalize the processing times in the adjacent chambers while speeding up
 the robot to finish his tasks in the same or similar amount of time.
 In working example 1 both cassettes are processing simultaneously, and all
 of the wafers are removed from one cassette before processing, but, are
 returned to the other cassette after processing.
 11. Working Example 2
 As seen schematically in FIGS. 19-20 the robot movements are those of a
 modified form of the working example of FIG. 17 but still similar in many
 respects. In FIGS. 19-20 the robot may be viewed as always standing facing
 toward chamber 2 and cassette 2 as seen in FIG. 19 step #1 with his left
 arm adjacent chamber 1 and the right arm adjacent cassette 1 and not
 turning 180 degrees to move a wafer from a cassette to a chamber, and,
 vice versa. In all other respects working examples 1 and 2 are identical.
 Thus, as seen sequentially in FIGS. 19,20, wafer 1 is removed from cassette
 1 (#1) and placed in chamber 1 for processing (#2); wafer 2 is removed
 from cassette 1 (#3) and placed in chamber 2 for processing (#4); wafer 3
 is removed from cassette 1 (#5) and, as chamber 1 finishes processing,
 wafer 1 is removed from chamber 1 (#6) and replaced by wafer 3 (#7), and
 wafer 1 is stored in its original slot in cassette 1 (#8); wafer 4 is
 removed from cassette 1 (#9) and, as chamber 2 finishes processing, wafer
 2 is removed from chamber 2 (#10) and replaced by wafer 4 (#11), and wafer
 2 is stored in its original slot in cassette 1 (#12). The cycle repeats.
 Note that in this example, even though two cassettes are used, all of the
 wafers are taken from only one cassette and all wafers are returned to
 their original slot in the same cassette before wafers in the other
 cassette begin processing. This procedure is typical because human or
 robot operators in the plant typically only come by every 10 to 15 minutes
 or so to change cassettes which allows continuous use of the machine,
 whereas in working example 1 both cassettes were processing simultaneously
 and all of the wafers are removed from one cassette before processing but
 are returned to the other cassette after processing.
 12. Working Example 3
 As seen in FIG. 30 the movements are those of a robot of a third working
 example of an embodiment of the present invention in which dual downstream
 or in-chamber reactors are each running the same single process using a
 single exterior (relative to the vacuum) 25-wafer cassette and a single
 exterior robot with two front arms (as was the robot of working example 2
 in FIGS. 19-20), eight separate portions of which are shown enlarged in
 FIGS. 22-29 for ease in reading the function steps. In this working
 example, all of the odd numbered wafers are processed in chamber 1 and all
 of the even numbered wafers are processed in chamber 2, but all wafers are
 returned to their original slots in the single cassette. Otherwise, the
 process steps are the same as in the prior working examples. Microwave
 energy is used in this example. Additionally, while the sequence is shown
 for a single cassette, actually the sequence can repeat relative to a
 cassette #2, #3, and #4, or even for 6 or more cassettes if desired.
 10. Working Example Parameters
 A set of specifications of the typical process results and typical
 operating parameters of the present invention are as follows:
 Microwave frequency: 2,450 MHZ
 Microwave power: 1,000 watts
 Operating Pressure: 1.2 Torr
 Total flow rate: 4,400 sccm
 O.sub.2 4,000 sccm
 N.sub.2 400 sccm
 11. Additional Embodiments
 The present invention may still take further forms. For example, as seen in
 FIG. 32 further economies and reductions in overall cost may be achieved
 by using a single common plasma applicator 26 the output flow of plasma
 excited process gases from which are selectively directed alternately 100
 to one of a pair of separate processing chambers 14, 16. In this
 embodiment, not shown, while the plasma exhaust will be extremely hot,
 nonetheless, materials exist which could withstand this intense heat blast
 and not contaminate the downstream plasma excited process gases flow. In
 this embodiment there is a contemporaneous requirement for sealing off
 102, 104 the path to the non-processing process chamber to preserve vacuum
 integrity. From such heat resistant materials referenced above, a common
 diverter plate is constructed and is mounted adjacent the plasma exhaust
 and is mechanically electromechanically operated to alternately deflect
 the plasma exhaust in one path through an open very large orifice stop
 valve 104 to and in synchronism with the processing one of the two
 processing chambers while a very large orifice stop valve 102 is
 synchronously closed in the other path to the non-processing chamber, and
 vice versa. The complete elimination of one entire plasma applicator
 further eliminates a costly redundancy and is in keeping with the objects
 of the present invention.
 The foregoing description of a preferred embodiment and best mode of the
 invention known to applicant at the time of filing the application has
 been presented for the purposes of illustration and description. It is not
 intended to be exhaustive or to limit the invention to the precise form
 disclosed, and obviously many modifications and variations are possible in
 the light of the above teaching. The embodiment was chosen and described
 in order to best explain the principles of the invention and its practical
 application to thereby enable others skilled in the art to best utilize
 the invention in various embodiments and with various modifications as are
 suited to the particular use contemplated. It is intended that the scope
 of the invention be defined by the claims appended hereto.