Chemical Dispensing system and method

Described herein is a chemical injection system and method utilizing a chemical storage vessel coupled to a bulk source of chemical and proportioned to contain a volume of chemical useful for multiple applications for which the chemical is to be utilized. A dispense vessel proportioned to contain a volume of chemical useful for a single application is fluidly coupled to the chemical storage vessel. A controller controls operation of a system of valves and sensors, which operate to fill the chemical storage vessel, and to precisely dispense the volume required for a single application from the storage vessel into the dispense vessel and then ultimately into the vessel in which the chemical is needed.

FIELD OF THE INVENTION
 The present invention relates generally to the field of systems and methods
 for dispensing chemicals for industrial applications. More particularly,
 the present invention relates to a system and method for automatically
 dispensing needed volumes of chemicals to a high level of accuracy.
 BACKGROUND OF THE INVENTION
 Accurate dispensing of needed volumes of chemicals is critical to
 manufacturing processes in a variety of industries. For example, in
 semiconductor fabrication processes, wafers are immersed in treatment
 tanks containing process chemicals. Some of the processing steps required
 for semiconductor processing are highly concentration dependent, because
 variations in chemical concentrations can result in variations in surface
 properties between different batches of wafers. It is thus essential to
 precisely regulate the concentration of the chemicals dispensed into the
 treatment tanks so as to insure that the finished wafers have the desired
 surface properties and to further insure consistent processing between
 wafer batches.
 This need is oftentimes at odds with typical operation of the bulk chemical
 supplies utilized within fabrication facilities to supply chemical for
 wafer processing. For example, the bulk supplies have flow pressures that
 vary greatly over time. Time-based dispensing, in which a desired volume
 is dispensed from the bulk supply simply by regulating the time for which
 the chemical is allowed to flow into the process tank, is inherently
 inaccurate in this situation. Without a substantially constant flow rate,
 the dispense "on" time for the needed volume cannot be accurately
 calculated. This leads to inconsistent dispensing between successive
 batches of wafers. As another example, bulk supplies can often shut down
 for periods of time, leaving no chemical available for dispensing into the
 treatment tanks. Each of these difficulties can adversely affect the
 profitability of the fabrication facilities by yielding products having
 inferior or inconsistent surface characteristics or by forcing temporary
 shutdown of processing steps. It is therefore desirable to provide a
 dispensing system and method that will accurately and consistently
 dispense chemicals regardless of the condition of the bulk source
 supplying the chemicals.
 One solution to the problem of bulk flow rate variations is to precisely
 measure a volume of chemical being dispensed from the bulk supply using a
 fluid level sensor positioned in the process tank. The fluid sensor
 provides feedback that triggers termination of chemical flow into the
 vessel once the desired volume has been dispensed.
 While this solution increases accuracy in dispensing, it fails to address
 the problem of bulk supply shut down and is impractical for use with
 process tanks for which the required chemical volume may vary. Multiple
 fluid level sensors would be required to give the system sufficient
 versatility to provide varying amounts of chemical, and even then its use
 would be limited to a finite number of available quantities. Moreover, the
 need for accurate dispensing requires a slow fluid flow rate into the
 process vessel, so as to allow sufficient time for flow to be stopped
 before the dispensed volume significantly exceeds the level detected by
 the fluid sensor. This mandates a lengthy fill time prior to each use of
 the treatment tank and thus detracts from the number of wafer batches that
 can be treated per day.
 It is thus desirable to provide a system and method for accurately
 dispensing chemicals from a bulk supply while minimizing reliance on the
 flow pressures of the bulk supply. It is further desirable to provide a
 system that, in addition to having these attributes, is easily adaptable
 to demands for varying concentrations, which permits quick dispensing of
 chemicals into process vessels so as to avoid delays between process
 steps, and which allows processing to continue through bulk supply down
 times.
 SUMMARY OF THE INVENTION
 Described herein is a chemical injection system and method utilizing a
 chemical storage vessel coupled to a bulk source of chemical and
 proportioned to contain a volume of chemical useful for multiple runs of
 the application for which the chemical is to be utilized. A dispense
 vessel proportioned to contain a volume of chemical useful for a single
 run is fluidly coupled to the chemical storage vessel. A controller
 controls operation of a system of valves and sensors, which operate to
 fill the chemical storage vessel, and to precisely dispense the volume
 required for a single application from the storage vessel into the
 dispense vessel and then ultimately into the vessel in which the chemical
 is needed.

DETAILED DESCRIPTION
 The system and method according to the present invention will be described
 in the context of surface preparation for semiconductor substrates. This
 is done for purposes of illustration only and is not intended in a
 limiting sense. The system and method of the present invention are equally
 suitable for use in other industries in which chemicals must be dispensed
 with a high level of accuracy.
 First and Second Embodiments--Structure
 Referring to FIG. 1, a first embodiment of a chemical dispensing system 10
 in accordance with the present invention generally includes a bulk
 chemical supply 12, a primary dispense vessel 14, a secondary dispense
 vessel 16 and a source 18 of deionized water (DI). The system 10 is used
 to dispense chemical from bulk supply 12 into a stream of DI water as it
 flows into a process tank 20. The resultant chemical solution is then used
 to treat the surfaces of semiconductor wafers immersed in tank 20.
 Primary vessel 14 is preferably a cylindrical vessel having a volume that
 will hold a quantity of chemical sufficient to treat multiple wafer
 batches in the process tank 20. A pressure regulator 22 and a passive vent
 24 are fluidly coupled to primary vessel 14. A three-way valve 23 is
 positioned between pressure regulator 22 and vent 24. Three-way valve 23
 also includes a vent 27.
 Pressure regulator 22 uses N.sub.2 gas to maintain the pressure within the
 primary vessel at a desired level, for example 7-psi. Flow of N.sub.2
 through pressure regulator will continue throughout typical operation of
 the system. Three-way valve 23 has a first position in which N.sub.2 gas
 flow from the regulator is directed into primary vessel 14, and a second
 position in which the N.sub.2 flow is vented from the system via vent 27.
 Passive vent 24 serves as an outlet for fumes released from chemical within
 tank 14, thus preventing the fumes from corroding the metallic components
 of pressure regulator 22. Vents 24 and 27 may be isolated from the
 external environment to prevent air and particles from entering the
 system. This may be carried out, for example, by coupling the vents to an
 N.sub.2 exhaust or by enclosing them within a closed N.sub.2 environment.
 A fill sensor 25 is positioned within primary vessel 14. During use, sensor
 25 detects when the liquid level within the tank has risen to a
 predetermined level corresponding to the volume of chemical that will be
 needed to treat a predetermined number of wafer batches in tank 20.
 At the lower end of the primary vessel is a fluid path 26 that serves as
 both the inlet and outlet path for the vessel. Fluid path 26 is fluidly
 coupled to an inlet line 28 that carries chemical from bulk supply 12 into
 the vessel 14. A valve 30 is used to start and stop the flow of chemical
 into the vessel 14.
 Fluid path 26 is further coupled to an outline line 32 that extends between
 primary vessel 14 and secondary vessel 16. Valve 34 controls the flow into
 secondary dispense vessel 16.
 Secondary dispense vessel 16 is preferably a cylindrical vessel sized to
 contain a volume of chemical sufficient to treat a single batch of wafers
 within the process tank 20. It should be noted with reference to the
 second embodiment shown in FIG. 2 that the system may be provided with
 more than one secondary dispense vessel, such as the pair of vessels 16d,
 16e. The vessels may be arranged such that both can dispense into a single
 tank and/or such that each dispenses into a separate tank.
 An outlet line 36 extends from the lower end of the secondary vessel and is
 fluidly coupled to a venturi 38. A third valve 40 is positioned between
 the secondary vessel and the venturi.
 A conduit 42 extends from DI source 18 to the venturi 38. Exit flow from
 the venturi 38 is directed into the process vessel 20 by conduit 44.
 Process vessel 20 includes a drain 46 that allows the vessel 20 to be
 emptied of process fluids between wafer batches.
 A fill sensor 45 detects when the liquid level in secondary vessel 16
 increases above and falls below a predetermined level. This information is
 used during the process to confirm when a secondary fill has been carried
 out and to further confirm when chemical in the secondary vessel has been
 dispensed.
 Each of the sensors and valves for all of the embodiments described herein
 are preferably electronically coupled to a system controller 48.
 Controller 48 is programmed to govern control and timing of these
 components to automatically open and close the valves and regulate flow of
 fluids etc. to dispense the required volume and concentration of
 chemical/solution required by the process recipe appropriate for the
 treatment process to be carried out. A controller suitable for this
 purpose is a MCS microprocessor controller available from Preco
 Electronics, Inc. Boise, Id. However, any suitable process control
 computer can be used.
 Controller 48 receives feedback from fill sensor 25, controls the opening
 and closing of the valves, and may perform additional functions aside from
 those directly related to the system 10, such as controlling operation of
 wafer transport systems that move wafers into and out of tank 20, closing
 the lid of the tank, etc. The functions of the controller 48 that relate
 most directly to the system 10 will be discussed in greater detail in the
 Operations section below.
 First and Second Embodiments--Operation
 There are three general steps involved during operation of the system 10.
 The first is the bulk fill step, in which primary dispense vessel 14 is
 filled with chemical from the bulk supply 12. The second is the secondary
 fill step, in which the amount of chemical needed for a single process
 step is passed from primary dispense vessel 14 into secondary dispense
 vessel 16. Finally, a dispensing step is carried out in which the chemical
 is delivered from the secondary dispense vessel into a DI stream flowing
 into the process tank.
 The bulk fill step is typically carried out when the primary dispense
 vessel 14 falls below a minimum level. In preparation for the bulk fill
 step, three-way valve 23 is used to halt N.sub.2 flow into the vessel 14
 and to divert the N.sub.2 flow through a vent Valve 30 is then opened,
 causing chemical to flow from bulk supply 12 into primary dispense vessel
 14.
 Fill sensor 25 is configured to provide feedback to the controller
 indicating that the fluid level in the primary vessel has reached a
 predetermined level. This level will preferably be selected to correspond
 to the volume of chemical needed to treat a predetermined number of wafer
 batches in tank 20. For example, if the process to be carried out in tank
 20 requires 200 ml of chemical, primary dispense vessel 14 may be
 initially filled with 2 L of chemical, corresponding to ten runs of the
 process.
 Once fill sensor 25 detects that primary dispense vessel 14 has been filled
 to the desired volume, valve 30 is closed and three-way valve 23 is made
 to divert N.sub.2 flow into primary vessel 14. Pressure regulator 22 then
 uses the N.sub.2 flow to maintain the pressure within the vessel 14 at a
 predetermined level, e.g. 7 psi.
 Valve 34 is opened to begin the secondary fill step. Driving force for the
 secondary fill step is pressure. The pressure source used for this purpose
 can be a pressurized gas source, an interfaced pressurized fluid source, a
 weighted piston, or another solid body resting on the surface of the fluid
 in vessel 14. A preferred configuration utilized the N.sub.2 gas pressure
 described herein. By maintaining a constant pressure through primary
 vessel 14, the system allows an accurate fill of secondary vessel 16 to be
 accomplished by simply monitoring the time for which valve 34 has been
 opened. For example, the flow rate of the system may be such that it takes
 four minutes to dispense 200 ml into the secondary vessel 16. A flow
 restrictor causes fluids to be dispensed slowly into secondary vessel 16
 so as to insure a high level of accuracy during the secondary fill step.
 Once valve 34 has been opened for the required duration, it is closed,
 thereby halting fluid flow into secondary vessel 16.
 If primary vessel 14 is tall, there will be a decrease in head pressure
 within primary vessel 14 each time a quantity of chemical is dispensed
 into secondary vessel 16. This decrease in head pressure translates into a
 slower flow rate from the primary to the secondary vessel. For example,
 assume again that each secondary fill step is intended to fill the
 secondary vessel with 200 ml of chemical. If it takes 4 minutes to
 dispense 200 ml from a full primary vessel, the decreased head pressure
 will cause it to take longer to dispense a second 200 ml into the
 secondary vessel.
 To insure that each secondary fill step will dispense an identical quantity
 of chemical, the system of the present invention is calibrated to take
 into account the decreases in head pressure that occur each time a
 secondary fill step is carried out. For a given chemical, the system
 controller 48 is thus programmed such that, for a first secondary fill
 step, valve 34 must be opened for time T1 in order to dispense 200 ml. For
 a second secondary fill step valve 34 must be opened for time T2 in order
 to dispense 200 ml, etc. Each time primary dispense vessel is refilled in
 a bulk fill step, the secondary fill steps begin again using a fill time
 of T1.
 When it is time to dispense chemical into the process vessel, valve 40 is
 opened to permit chemical to flow from secondary vessel 16 into the
 venturi 38. Deionized water is simultaneously pumped through the venturi
 38 from DI source 18. The DI and chemical mix together and flow into the
 process tank 20. It is important to note that the flow rates of the DI and
 chemical are designed to promote even mixing of the chemical and DI, so
 that the solution flowing into the process tank is at the required
 concentration. This is important because the wafers may be present in tank
 20 as the process fluids are dispensed, and it is necessary to insure that
 only a fully mixed solution at the required concentration comes into
 contact with the wafers.
 After it has been emptied into tank 20, secondary vessel 16 is refilled
 from primary vessel 14. This readies the system for dispensing a fresh
 supply of fluid into tank 20 as soon as a fresh supply is needed. When the
 primary vessel has been fully emptied, the bulk fill step is repeated so
 that it will be ready to replenish the chemical in secondary vessel 16
 after a secondary dispense is carried out.
 Third through Sixth Embodiments
 As described above, care must be taken to insure that each of successive
 secondary fill steps dispenses an identical quantity of chemical
 regardless of changes in the head pressure in primary dispense vessel 14.
 The third through sixth embodiments are directed towards alternative means
 for ensuring the accuracy of the secondary fill process.
 FIG. 3A illustrates a primary vessel system 14a that may be used as an
 alternative for primary dispense vessel 14. Primary vessel system 14a is
 comprised of multiple vessels 15a-15d that are shorter than the vessel 14
 but that are connected to one another by plumbing to effectively form a
 single vessel. Primary vessel system 14a is designed to increase the
 surface area of the chemical within it over the surface area that would be
 possessed by an equivalent volume within vessel 14 of FIG. 1. By
 increasing the surface area, changes in head pressure between successive
 secondary fill steps are effectively eliminated. Thus, each secondary fill
 procedure may be carried out for an identical period of time as the other
 secondary fill procedures and will still dispense an identical volume of
 chemical. If desired, the vessels 15a-15d may be arranged as shown in FIG.
 3B so as to conserve space. It should also be noted that a similar
 increase in surface area may alternatively be accomplished with a single
 yet very wide vessel.
 FIG. 4 illustrates alternative system components that may be used in
 combination with the system of FIG. 1 to ensure accuracy in the secondary
 fill procedure. The FIG. 4 embodiment relies on mass flow control to
 dispense equal volumes of chemicals each time a secondary fill is carried
 out and regardless of the fluid height in primary dispense vessel 14b. It
 includes an N.sub.2 source that directs N.sub.2 gas into a chamber 50 to
 create a calibrated gas volume. A pressure gauge 52 monitors the pressure
 of the chamber 50 and is coupled to the system controller 48. A valve 54
 is disposed between the chamber 50 and vessel 14b. The system is
 calibrated such that the desired amount of chemical will have dispensed
 into secondary vessel 16 when a predetermined drop in pressure in chamber
 50 (e.g. from 20 psi to 5 psi) has occurred.
 To carry out a secondary fill, chamber 50 is filled with gas to increase
 its pressure to a predetermined value (e.g. 20 psi). Valves 54 and 34
 (FIG. 1) are then opened. When the pressure monitor detects that pressure
 has fallen by the predetermined amount, valve 34 is closed. Between each
 secondary fill procedure, the chamber 50 is re-filled to raise the chamber
 pressure back to the predetermined starting pressure. Each secondary fill
 procedure is carried out using the same pressure drop regardless of the
 fluid height in primary vessel 14b.
 Referring to FIG. 5, a fifth embodiment includes another means for
 accurately filling the secondary vessel, which involves measuring the mass
 of chemical dispensed into secondary vessel 16c using a scale 56. Since
 the density of the chemical is known, the system may be configured to
 close valve 34 once the mass of chemical in vessel 16c has reached a
 predetermined level corresponding to the desired volume of chemical. As
 with the embodiment of FIGS. 3A and 4, dispensing according to the
 embodiment of FIG. 5 may be carried out without regard to the fluid height
 in primary vessel 14c.
 Finally, a sixth embodiment, which includes alternative means for
 accurately filling the secondary vessel, may be described again with
 reference to FIG. 1. A liquid level sensor (like fill sensor 45) may be
 provided within the secondary vessel to detect when the secondary vessel
 has been filled to a predetermined volume. Rather than using the
 time-based secondary dispense of the first embodiment, this embodiment
 utilizes the liquid level sensor to detect when the desired volume has
 been dispensed into the secondary vessel. Multiple fill sensors may be
 included, each at a different level in the tank, to provide the user with
 the flexibility to select a volume to be dispensed from a number of
 available volumes.
 Seventh and Eighth Embodiments
 A seventh embodiment of a chemical injection system 300 is shown in FIGS.
 6A and 6B. As with the others, chemical injection system 300 is a
 desirable one in that it permits precise measurement of process chemicals
 despite the variations in pressure that are inherent to the bulk chemical
 supplies typically used at foundries. Moreover, the seventh embodiment is
 further advantageous in that its geometry is such that there is enough
 head pressure from the height of the system such that an additional
 outside pressure source need not be used to drive the secondary fill step.
 Additionally, the seventh embodiment includes a mechanism by which
 dispensed chemical is mixed with a volume of deionized water during the
 final dispensing step.
 As with all of the other embodiments, timing and control of the various
 valves utilized by the chemical injection system 300 are governed by
 process controller 48. The electronic coupling between these components
 and the controller is not represented in the drawings only for reasons of
 clarity.
 Referring to FIG. 6A, chemical injection system 300 includes a chemical
 storage vessel 302 coupled to a bulk chemical supply 304. Chemical storage
 vessel includes a main chamber 306 and a side chamber 308 extending from
 the main chamber. The interiors of the main and side chambers are
 contiguous with one another. In addition, a fluid line 310 extends between
 the main and side chambers and a valve 311 is positioned in fluid line
 310. A liquid level sensor 312 is positioned to monitor the liquid level
 in fluid line 310 and to provide feedback concerning the liquid level to
 system controller 48. A vent 314 extends from a wall of the primary
 vessel.
 A dispense vessel 316 is coupled to chemical storage vessel 302 by line
 318, which includes reduced flow orifice 320. A valve 322 is positioned
 downstream of orifice 320, and a DI line joins line 318 further downstream
 of valve 322. A valve 324 governs flow of DI water from DI source 326 into
 vessel 316.
 An outlet line 328 extends from dispense vessel 316 and includes a valve
 330 and a reduced flow orifice 332. Liquid level sensor 336 is positioned
 in line 328 to detect when fluid is present in line 328 (i.e. once valve
 330 has been opened) and thus to detect whether vessel 316 has been
 filled/emptied. The sensor 336 may be positioned in alternative locations
 which would likewise provide such feedback. For example, it may be
 positioned in a side tube extending from vessel 316 (similar to side tube
 310 of vessel 302) or it may be positioned in vessel 316 itself. It should
 be appreciated with this and the other sensors described herein that the
 sensors are not limited to any particular location so long as they are
 positioned to detect the condition for which they are used in the
 described process. In this case, sensor 336 is associated with vessel 316
 and its components to indicate whether vessel 316 has been filled/emptied.
 A side branch 334 connects outlet line 328 with an upper section of vessel
 316. Further downstream of side branch 334 is a dispensing line 338
 fluidly coupled with the vessel 340 into which the chemical is to be
 dispensed.
 There are four general steps involved during operation of chemical
 injection system 300. The first is the bulk fill step, in which chemical
 storage vessel 302 is filled with chemical from bulk supply 304. The
 second is timed secondary fill step, in which the amount of chemical
 needed to treat a batch of wafers is passed from chemical storage vessel
 302 into dispense vessel 316. The secondary fill step is accomplished by
 opening valve 322 for a period of time predetermined to cause the desired
 volume to be dispensed into vessel 316. Third, valve 330 is opened to
 allow the chemical from vessel 316 into line 338. As will be discussed in
 detail, this step is timed and utilizes sensor 336 to verify the accuracy
 of the secondary fill step. Finally, a dispensing step is carried out in
 which the chemical is carried from line 338 into the process tank by a DI
 stream passing into and through vessel 316.
 The bulk fill step is typically carried out when the volume of the chemical
 storage vessel 302 has decreased to a predetermined minimum level. Valve
 303 which lies between vessel 302 and bulk supply is opened, causing
 chemical to flow from the bulk supply into vessel 302. All other valves in
 the system remain closed throughout the bulk fill step.
 Fill sensor 312 is configured to provide feedback to controller 48
 indicating that the fluid level in chemical storage vessel 302 has reached
 a predetermined level. The level will preferably be selected to correspond
 to the volume of chemical needed to treat a predetermined number of wafer
 batches in vessel 340.
 Once fill sensor 312 detects that chemical storage vessel 302 has been
 filled to the desired volume, valve 303 is closed. Next, valve 322 is
 opened to initiate the secondary fill step into vessel 316. The system
 allows an accurate fill of vessel 316 by monitoring the time for which
 valve 322 has been opened. For example, the flow rate of the system may be
 such that it takes four minutes to dispense 200 ml into the vessel 316.
 Once valve 322 has been opened for the required duration, it is closed,
 thereby halting fluid flow into vessel 316. Reduced flow orifice 320
 causes fluids dispensed into dispense vessel 316 to flow slowly, so as to
 insure a high level of accuracy during the secondary fill step by
 minimizing the effect of the split second delay between issuance of the
 "close" control signal to valve 322 and the actual closing of the valve.
 It should be noted that the system is useful for applications in which
 successive runs of the system require different dispense volumes. Simply
 changing the amount of time for which valve 322 will be opened during the
 secondary fill step can change the volume of chemical dispensed.
 After valve 322 has been closed, valve 330 is opened to permit chemical to
 flow from dispense vessel 316 into dispense plumbing 338, which is
 preferably large enough to contain the entire dispense volume. Once line
 328 has been emptied, sensor 336 turns off, indicating that vessel 316 has
 been completely evacuated. The system registers the time lapsed between
 the opening of valve 330 and the turning off of sensor 336, which is the
 amount of time taken to empty vessel 316. The measured time is compared by
 the system to a value saved in the system's software correlating to the
 amount of time that it should take for the desired dispense volume to exit
 vessel 316 given the known rate at which fluid will flow from vessel 316.
 This step is done in order to verify the initial time dispense into vessel
 316. If the comparison reveals a possible error in the amount of chemical
 dispensed, remedial measures are taken before wafers are transferred into
 vessel 340. Such remedial measures may include disposing of the chemical
 via drain valve 339 and repeating the secondary fill step.
 It should be noted that if sensor 336 is positioned in vessel 316 or in a
 side tube extending from vessel 316 rather than in the position shown, it
 may also used for a volumetric verification (i.e. to verify that vessel
 316 has been filled above or drained below the location of the sensor) as
 well as for the time verification just described.
 Shortly afterwards, when it is time to dispense chemical into the vessel
 112, valve 324 is opened, causing DI water to flow from source 326, into
 dispense vessel 316, and then into plumbing 338 via lines 328 and 334.
 Because of the positioning of reduced flow orifice 332 in line 328, only a
 small portion of the DI water flows through line 328 where it serves to
 rinse chemical from the line. A larger percentage of the DI fills the
 vessel 316 and flows through side branch 334 into line 338, pushing the
 chemical in line 338 into vessel 340 while also rinsing vessel 316 and
 lines 334 and 338. Controller 48 causes valve 324 to close after the
 appropriate amount of DI water has been dispensed. Control over the volume
 of DI water dispensed can be carried out by keeping valve 324 opened for a
 predetermined amount of time know to result in dispensing of the required
 volume, or by closing valve 324 in response to feedback from a liquid
 level sensor in the vessel 340.
 FIG. 7A shows an eighth chemical injection system 400 particularly useful
 for dispensing chemical for use in processes for which the required
 dispense volume does not change for successive runs of the system. The
 eighth embodiment as shown also differs from the prior embodiments in that
 it dispenses chemical without mixing the chemical with another fluid such
 as DI water, although it may alternatively be configured to dispense a
 solution. As with all of the other embodiments, timing and control of the
 various valves utilized by the chemical injection system 400 are governed
 by process controller 48. The electronic coupling between these components
 and the controller is not represented in the drawings only for reasons of
 clarity.
 Chemical injection system 400 includes a chemical storage vessel 402
 coupled to a bulk supply of drying compound 404. A fluid line 410 extends
 between upper and lower portions of vessel 402. A liquid level sensor 412
 is positioned to monitor the liquid level in fluid line 410 and to provide
 feedback concerning the liquid level to system controller 48. A vent 414
 extends from a wall of vessel 402.
 A dispense vessel 416 is coupled to chemical storage vessel 402 by a system
 of plumbing formed of line 417, reservoir 418a, and lines 418b through
 418f. A reduced flow orifice 420 is positioned in line 417 and a valve 422
 is positioned downstream of orifice 420.
 The opening in reservoir 418a at its connection with line 418c is
 significantly smaller than the diameter of the pipe forming line 418c. For
 example, reservoir 418a may include a 1/2 inch diameter opening leading to
 a 1-inch diameter line 418c. Lines 418d and 418f have vents at their upper
 ends. The vent in line 418f prevents a pressure lock situation from
 occurring in the system in which bubbles form in the fluid and displace
 fluid volume. A sensor 436 is located in line 418d and a valve 437 is
 positioned below sensor 436.
 Vessel 416 and its associated plumbing 418a-f are proportioned to contain
 and precisely dispense the entire quantity of chemical needed for a single
 dispense operation. They are arranged such that detection of a fluid level
 by sensor 436 occurs when dispense vessel 416 and its associated plumbing
 has been filled with slightly more than the required volume of chemical
 for the process. Dispense vessels and plumbing of different volumes may be
 used to replace vessel 416 and its plumbing when different dispense
 volumes are needed.
 A dispensing line 428 extends from dispense vessel 416 and includes a valve
 430. Dispensing line 428 is fluidly coupled with a vessel 440 into which
 the chemical is to be dispensed.
 There are three general steps involved during operation of chemical
 injection system 400. The first is the bulk fill step, in which chemical
 storage vessel 402 is filled with chemical drying compound from bulk
 supply 404. The second is a secondary fill step, in which the amount of
 chemical needed for use in drying a batch of wafers is passed from storage
 vessel 402 into dispense vessel 416 and its plumbing.
 Third, valve 430 is opened to allow the chemical from vessel 416 and its
 plumbing into vessel 440.
 The bulk fill step is typically carried out when the volume of the chemical
 storage vessel 402 has decreased to a predetermined minimum level. Valve
 403 is opened, causing chemical to flow from the bulk supply into the
 vessel. Valve 422 remains closed throughout the bulk fill step.
 Fill sensor 412 is configured to provide feedback to controller 48
 indicating that the fluid level in chemical storage vessel 402 has reached
 a predetermined level. The level will preferably be selected to correspond
 to the volume of chemical needed to carrying out a predetermined number of
 drying procedures.
 Once fill sensor 412 detects that chemical storage vessel 402 has been
 filled to the desired volume, valve 403 is closed. Next, valve 422 is
 opened to initiate the secondary fill step into vessel 416. It should be
 noted that valve 437 in line 418d remains closed during the secondary
 fill.
 During the secondary fill, fluid flows through orifice 420, filling the
 portion of line 428 that lies upstream of valve 430, then filling vessel
 416, line 418b and then reservoir 418a. Next, fluid cascades from
 reservoir 418a into line 418c and into the portion of line 418d that sits
 above closed valve 437. Fluid also rises from vessel 416 into the portion
 of line 418d that lies below valve 437, and flows into lines 418e and
 418f. When sensor 436 detects a fluid level, the calibrated fluid volume
 has been achieved. In response, valve 422 is closed, thereby halting fluid
 flow into vessel 416. Shading in FIG. 7A represents the calibrated volume
 of fluid at the end of the secondary fill step.
 After valve 422 has been closed, valve 430 is opened to permit chemical to
 flow from dispense vessel 416 into vessel 440. It should again be noted
 that at this stage valve 437 remains closed.
 After valve 430 has been opened for a predetermined amount of time known to
 dispense the calibrated volume of chemical, it is closed. Because valve
 437 remains closed during the secondary fill, a small volume of fluid
 remains in line 418c and in the portion of line 418d that is above valve
 437. Valve 437 is next opened to allow this small volume of fluid to flow
 into vessel 416 where it will form a portion of the calibrated volume
 measured during the following secondary fill step. This small volume
 corresponds to the amount of volume over the required process volume that
 will enter the system as a result of the inability of valve 422 to close
 instantaneously when sensor 436 detects a liquid level.
 FIG. 7B shows an alternative configuration for the plumbing associated with
 vessel 416. The alternative configuration has geometry that will catch a
 larger overflow volume from reservoir 418a and keep that overflow volume
 separate from the volume ultimately dispensed into vessel 440. As
 discussed, the overflow volume results because valve 422 cannot close
 instantaneously when sensor 436 detects that the calibrated volume has
 been dispensed during the secondary fill. Naturally, the amount of
 overflow volume is greater when faster flow rates are used through line
 417. The geometry of the embodiment of FIG. 7B allows greater flow rates
 to be utilized for the secondary fill by providing plumbing that will
 accommodate a larger overflow volume. As a result, the secondary fill step
 can be carried out more quickly.
 Referring to FIG. 7B, plumbing associated with vessel 416 differs from that
 shown in FIG. 7A in that it includes additional line 418g extending from
 line 418c, and line 418h extending between lines 418g and 418d. Fluid
 sensor 436 is positioned to detect a fluid level in line 418g. Valve 437
 is positioned such that when opened the lines 418d, 418e and 418h are all
 fluidly coupled to one another, and that when closed it permits flow only
 between the upper portion of line 418d and line 418h, and between line
 418e and the lower portion of line 418d.
 During a secondary fill operation utilizing the system of FIG. 7B, valve
 437 is in the closed condition. Fluid flows through orifice 420, filling
 the portion of line 428 that lies upstream of valve 430, then filling
 vessel 416, line 418b and then reservoir 418a. Next, fluid cascades from
 reservoir 418a into line 418c and into line 418g. Since when valve 437 is
 closed fluid from line 418g can flow into the portion of line 418d above
 valve 437, fluid rises upwardly through line 418d. Fluid also rises from
 vessel 416 into the portion of line 418d that lies below valve 437, and
 flows into lines 418e and 418f. When sensor 436 detects a fluid level, the
 calibrated fluid volume has been achieved. In response, valve 422 is
 closed, thereby halting fluid flow into vessel 416. Shading in FIG. 7B
 represents the calibrated volume of fluid at the end of the secondary fill
 step.
 As with the embodiment of FIG. 7A, valve 430 is opened for a predetermined
 amount of time known to dispense the calibrated volume of chemical into
 tank 440, and is then closed. Since valve 437 is kept closed during the
 secondary fill, the overflow volume of fluid remains in lines 418c, 418d
 (upper portion), 418g and 418h. After valve 430 is closed, valve 437 is
 opened to allow this small volume of fluid to flow into vessel 416 where
 it will form a portion of the calibrated volume measured during the
 following secondary fill step.
 Several examples of embodiments utilizing principles of the present
 invention have been described herein. It should be appreciated that these
 embodiments are given for purposes of illustration only and are not
 intended to limit the scope of the invention. Many modifications may be
 made to these embodiments without departing from the scope of the
 invention.