GB2419420A

GB2419420A - Mass flow controller with dual processor controller

Info

Publication number: GB2419420A
Application number: GB0526341A
Authority: GB
Inventors: Ali Shajii; Nicholas Kottenstette; Jesse Ambrosina; John A Smith
Original assignee: MKS Instruments Inc
Current assignee: MKS Instruments Inc
Priority date: 2002-06-24
Filing date: 2003-06-24
Publication date: 2006-04-26
Anticipated expiration: 2023-06-24
Also published as: GB0526346D0; GB2419958B; GB2419957A; GB0526243D0; GB2419422B8; GB2419957B; GB0526341D0; GB0526343D0; GB0526337D0; GB2404028B8; GB2419958B8; GB2419958A8; GB2419676B; GB2419957A8; GB2419422B; GB2419676A8; GB2419677A; GB0526348D0; GB2419420B8; GB2419422A

Abstract

A mass flow controller (200) comprises: a mass flow sensor (202) configured to produce a mass flow signal representative of a gas flow through the mass flow controller (200); and an electronic controller (210) configured to produce a closed loop control signal for an outlet control valve(220), based on the mass flow signal. The electronic controller (210) comprises a dual processor controller configured to provide a network interface, for example a web server (602), that permits the execution of mass flow controller active diagnostics from a device connected to the network. The electronic controller (210) may comprise a dual processor arrangement consisting of a deterministic 1002 and a non-deterministic 1004 processor, where the deterministic 1004 processor is configured to produce the close loop signal for the outlet control valve (220). This arrangement may enable on-line monitoring and manipulation of mass flow controller signals.

Description

24 1 9420

MASS FLOW CONTROLLER

The present invention relates to mass flow sensing and control systems.

Capillary tube tlerrnal mass flow sensors exploit the fact plot lariat trar.sfer to a fluid flowing in a laniar tube frown the tube walls is a function of: mass flow rate of Lhe fluid, tle difference between Lhe fluid temperature and the wall temperature, and the specific heat of the fluid. Mass flow controllers employ a 1' variety of Class flow sensor configurations. For example, ogle type oLcolstnction involves a stainless steel flow sensor tube with one, and more typically two or snore, resistive elements in thermally conductive conflict Title the sensor tube.

rl:lc resistive elements are typically composed of a material having a slimly tcngeratuie coefficient of r esisLa.nce Each of t.lc elements call act as a lcai.er, a detector; or l.,otl. One or more of Lhe elements is elergizecl evils electrical current to sunup])/ labial Lo Ale fluid stream L}-otgl1 Lhe tube. If the healers are supplied Title cosLanl: cui- renL, the rate of fluid mass flow tl-ough i:lle tube can be derived fi- orn tenperaLure diIferences in Lhe elenenLs Fluid mass flow rates can also be cleri iced by Nondrying tulle current: Trout the leal.ers Lo Sinai a constant telperatue profile.

Sucl thermal Miss llov censors nary be Bleached as.q. 1:arl: of a mass BOND o0 coiLolJer, with fluid O-on flee controller's ingrain cilanel feeding tile capillary tube (also referred to herein as the sensor tube). The portion of the main channel to which the inlet and outlet of Lhe sensor tube are attached is often referred to as the Bypass" of the flow sensor. Many applications envoy a plurality of mass BONG cotrol]es to regulate the supply offluid through a supply line, and a plurality of the supply lines may be "tapped off?' a nail fluid supply line. A sudden charge in flow to ogle of the controllers may create pressure fluctuations at the inlet to one or more of the other controllers tapped off the main supply line. Such pressure fluctuations create differences between tile flow rate at the inlet Gild outlet of an affected mass flow controller Because thermal mass how sensors measure ONV - 2 at the inlet of a mass flower controller, but outlet flow from the controller is the critical parameter for process control, such inlet/outlet flow discrepancies can lead to significant process control errors.

In a semiconductor processing application, a process tool may include a plurality of chambers with each chamber having one or more mass flow controllers controlling the flow of gas into the chamber. Each of the mass flow controllers is typically re-calibrated every two veel.s. The recalibration process is described, for example, in U.S. Patent 6,332,348 Bl, issued to YelverLon et al. December 25, 2001, which is hereby incorporated by reference. In the course of such an "In Situ" calibration, conventional methods require a technician to connect a class f1Ov meter in dine with each of the mass flow controllers, flow gas tl-ough the nears flow uniter and mass flow controller, compare the snags flow controller reading to that of the mass flow meter and adjust calibration constants, as necessary. Such painstaking operations can require a great deal of tinge and, due Lo labor costs and the unavailability of process tools, with wl1ich the mass flow controllers operate, can be very costly.

A mass how sensor that substantially eliminates sensitivity to pressure variations would therefore be highly desirable. A convenient calibration method and apparatus for mass flow controllers would also be higl1ly desirable. More flexible access to a mass flow controller would also be highly desirable.

Apparatus and method for increasing the control perfonnance of a mass flow controller would also be highly desirable.

Sul11l11ary of the Invelllioll According to the present invention there is provided a mass flow controller and a system, defining a flow path between a gas inlet and an outlet of the system as set forth in the appended claims.

Brief Description of Drawings

Figure 1 is a block diagram of a system that includes a mass flow sensor; Figure 2 is a sectional view of a mass flow controller that employs a mass flow sensor.

Figure 3 is a sectional view of an illustrative thermal mass flow sensor as used in conjunction with a pressure sensor to produce a compensated indication of mass flow through a mass flow controller; Figure 4 is a block diagram of the control electronics employed by an illustrative embodiment of a mass flow sensor.

Figure 5 is a flow chart of the process of compensating a teal mass flow sensor si anal.

Figure 6 is a conceptual block diagram of a web-enabled mass How controller.

Figure 7 is a conceptual block diagram of calibrator such as may be employed with a mass now controller.

Figure 8 is a block diagram of a self-calibrating mass flow controller.

Figure is a graphical rcprcscutation of flow and pressure curves corresponding to the process of calibrating a mass flow controller.

Figure 10 is a conceptual block diagram of a dual processor configuralion such as may be used in a mass flow controller.

Figure 11 is a flow chart of the general operation of a mass flow controllers non detenninistic processor.

Figures 12A and 12B are flow charts of the general operation of a mass flow controller's deterministic processor, and Figures 1 3A through 1 SE are screen shots of web pages such as may be employed by a Web server embedded within a mass flow controller.

Detailed Description of Disclosure

A mass flow sensor employs a thermal mass flow sensor to sense and provide a measure of the flow of fluid into an inlet of a fluid flow device, such as a mass flow controller. The mass flow sensor uses a pressure sensor to compensate the inlet flow measure provided by the thermal mass flow sensor to thereby provide an indicator that more accurately rejects the fluid flow at the outlet of the associated mass flow controller.

A system 100 that benefits from and includes the use of a mass flow sensor is shown in the illustrative block diagram of Figure I. A plurality of mass flow controllers MFC1, MFC2, ... MFCn receive gas from main gas supply lines 102, 103. The mass Dow controllers, MFCI, MFC2, MFCn are respectively connected through inlet supply lines 104, 106, ... 1()9 to a main gas supply line ]02, 103 and through respective outlet supply lines 110, 112, ...115 to chambers C], C2, Cn. In this illustrative embodiment, the term "chamber" is used in a broad sense, and each ol the chambers may be used for any of a variety of applications, including, but not limited to, reactions involved in the production of semiconductor components. Generally, users of the chambers are interested in knowing and controlling the account of each gas supplied to each of the chambers Cl, C2, ...Cn. Each chamber C1, C2, ... Cn may also include one or snore additional inlet lines for the supply of another type of gas. Outflow fron1 the chambers Nay be routed through lines (not shown) for recycling or disposal.

The mass flow controllers, MFC1, MFC2, MFCn, include respective mass flow sensors MFS1, MFS2, ...MFSn, electronic controllers ECI, EC2, ...Ecn and outlet control valves OCV1, OCV2, ...OCVn. At least one of the mass flow sensors is, and, for ease of description, assume all are, compensated mass flow sensors. Each mass flow sensor senses the mass of gas flowing into the mass flow controller and provides a signal Indicative of the sensed value to a corresponding electronic controller. The electronic controller compares the indication of mass flow as indicated by the sensed value provided by the mass flow sensor to a set point and operates the outlet control valve to minimize any difference between the set point and the sensed value provided by the mass flow sensor.

Typically, the set point may be entered manually, at the mass flow controller, or downloaded to the mass flow controller. The set point may be adjusted, as warranted, through the intervention of a human operator or automatic control system. Each of the inlet supply lines 104, 106, ... 109 may be of a different gauge, and/or may handle any of a variety of flow rates into the mass flow controller. A single electronic controller, such as electronic controller EC I, may be linked to and operate a plurality of mass flow sensor/outlet control valve combinations. That is, for example, any number of the illustrated electronic controllers EC2 through Ecn may be eliminated with the corresponding mass flow sensors and outlet control valves linke:1 to the electronic controller ECI for operation.

An abrupt change of flow rate, due to a change in set point for example, into any of the mass flow controllers may be reflected as an abrupt pressure change at the inlet of one or more of the other mass flow controllers. This unwanted side effect may be more pronounced in a relatively low flow rate mass flow controller if the abrupt change occurs in a high flow-rate Glass flow controller. Because the mass flow sensors In this illustrative embodiment are thermal mass flow sensors positioned to sense flow in the mass flow controller at the inlet to the mass flow controller, the mass flow sensed by the thennal mass flow sensor may not accurately reflect the flow at the outlet of the controller. In order to compensate for this discrepancy, a mass flow sensor includes a pressure sensor positioned to provide an indication of the pressure within the volume between the inlet and outlet of the mass flow controller. In an illustrative embodiment, the pressure sensor is located in the "dead volume" between the thermal mass flow sensor's bypass and the outlet control valve. An electronic controller employs the indication of pressure provided by the pressure sensor to Compensate the measure of mass flow provided by the thermal mass flow sensor.

The resultant, a compensated mass flow indication, more accurately reflects the flow at the outlet of the mass flow controller and, consequently, this indication may be employed to advantage by a mass flow controller in the operation of its outlet control valve. A display may be included to display the sensed pressure. The display may be local, attached to or supported by the mass flow controller, or it may be remote, at a gas box control panel, for example, connected to the mass flow controller through a data link.

In a semiconductor processing application, a process tool may include a plurality of chambers with each chamber having a plurality of mass flow controllers respectively controlling the flow of constituent gases into the chamber. Each of the mass flow controllers is typically re-calibrated every two weeks. The re-calibration process is described, for example, in US Patent 6,332,348 Bl, issued to Yelverton et al. December 25, 2001, which is hereby incorporated by reference. In the course of such an "In Situ" calibration, conventional methods require a technician to connect a mass flow meter in line with each of the mass flow controllers, flow gas through the mass flow nester and mass flow controller, compare the mass flow controller reading to that of the mass flow meter and adjust calibration constants, as necessary. Such painstaking operations can require a great deal of time and, due to labour costs and the unavailability of process tools with which the mass flow controllers operate, can be very costly. In an illustrative embodiment described in greater detail in the discussion related to Figure 7, a mass flow controller includes a self-calibrating mechanism that substantially eliminates such tedious and costly chores.

The sectional view of Figure 2 provides an illustration of a mass flow controller 200 that employs a mass flow sensor 202. The mass flow sensor 302 includes a thermal mass flow sensor 204, a pressure sensor 206, a temperature sensor 208 and an electronic controller 210. A laminar flow element 212 establishes a pressure drop across the capillary tube of the thermal mass flow sensor 204, as will be described in greater detail in the discussion related to Figure 3. In operation, a fluid that is introduced to the mass flow controller 200 through the inlet 214 proceeds through the bypass channel 216 containing the]aminar flow element 2]2. A relatively small amount of the fluid is diverted through the thermal mass flow sensor 204 and re-enters the bypass channel 216 downstream of the laminar flow element 212. The electronic controller 210 provides a signal to the control valve actuator 218 to thereby operate the outlet control valve 220 in a way that provides a controlled mass flow of fluid to the outlet 222. The pressure sensor 206 senses the pressure within the volume within the bypass channel 216 between the laminar flow clement 212 and the outlet control valve 220, referred to herein as the "dead volume 216a".

As will be described in greater detail in the discussion related to Figure 5, the electronic controller 210 employs the pressure sensed within the dead volume 21Ga by the sensor 206 to compensate the inlet flow rate sensed by the thennal mass flow sensor 204. This compensated inlet flow rate figure more closely reflects the outlet flow rate, Chicle is the ultimate target of control. In particular, a mass flow sensor is a combination sensor that employs the tin1e rate of change of pressure within a known volume 216a to provide a precise measure of mass flow during pressure transients and a thermal mass Dow sensor that may be "corrected" using the pressure-derived mass flow n1easuren1ent. Both the thermally-sensed and pressure derived mass flow measurements are available for processing. The temperature sensor 208 senses the temperature of the fluid within the dead volume. In an illustrative embodiment, the temperature sensor 208 senses the temperature of a wall of the controller, as an approximation of the temperature of the fluid within the dead volume 2 16a.

The volume 216a of the dead volume is determined, during manufacturing or a calibration process, for example, and may be stored or downloaded for use by the electronic controller 210. By taking sequential readings from the pressure sensor output 206 and operating on that data, the electronic controller 210 determines the time rate of change of pressure within the dead volume 216a. Given the dead volume, the temperature of the fluid within the dead volume, the input flow rate sensed by the thermal mass flow sensor 204, and the time rate of change of pressure within the dead volume, the electronic controller 210 approximates the fluid flow rate at the output 222 of the mass flow controller 200. As previously noted, this approximation may also be viewed as compensating the mass flow rate figure produced by tle tlennal mass How sensor 204.

The electronic controller 210 employs this computed output fluid flow rate in a closed loop control system to control l.lle opening of the mass Cow controller outlet control valve 220.

In an illustrative embodiment, the value of the pressure sensed by the pressure sensor 206 may also be displayed locally, (that is, at the pressure sensor) and/or remotely (at a control panel or through a network interface, for example). In a self-calibratig process described hereinafter in the discussion related to Figure 7, the electronic comptroller 210 may take the time derivative of the pressure signal Helen the flovrate varies in the mass flow controller arid thereby derive the actual flov rate into the mass flow cor.troller.

The actual flow rate may then be used to calibrate the mass flow controller.

The sectional view of Figure 3 provides a snore detailed view of a thermal mass flow sensor 204, such as may be employed in conjunction with a pressure sensor to produce a corngensated mass flow indication that is, in a digital implementation, a multi-bit digital value. The multi-bit digital value provides a closer approximation to the actual mass flow at the outlet of a mass flow controller than an uncompensated mass flow sensor would, paticular]y during pressure transients on the mass flow controller inlet lines. The thermal mass flow sensor 204 includes larninar Bow clement 212, which rests within the bypass (channel 216 and provides a pressure drop across tile bypass channel 216 for the thermal mass flow sensor 204 and drives a. portion of the gas through the sensor capillary tube 320 of the thermal mass flow sensor 204. The mass flow sensor 202 includes circuitry that senses the rate of flow of gas through the controller 00 and controls operation of tle control valve 220 accordingly. The thermal mass flow sensor a.ssenbly 204 is attached to a wall 322 of the mass flout c.nutrller 200 that fores a boundary of the bypass channel 2]6 Input 324 and output 326 apertures in the wall 322 provide access to the thermal mass flow sensor assembly 20z} for a gas travelling tl-ough tle thermal mass flow controller and it is the portion of this passageway between the input and output that typically defines the bypass channel. In this illustrative embodiment the mass flow sensor assembly 204 includes a baseplate 338 for attachment to the wall 322. The baseplate 328 may de attached to the wall and to the remainder of the sensor assembly using threaded hole and mating bolt combinations, for example.

Input 330 and output 332 legs of the sensor tube 320 extend through respective input 334 and output 336 apertures of the baseplate 398 and, through apertures 324 and 326, the mass flow controller wall 3Q2.

The mass flow sensor assembly preferably includes top 338 and bottom 340 sections that, plan joined, form a thermal clamp 341 that holds both ends of the sensor tube 320 active area (that is, the area defined by the extremes of resistive elements in thermal cor.tact with the sensor tube) at substantially the same temperature. The thermal clamp also forms a chamber 342 around the active area of tle sensor tube 320 That is, the segment of the mass flow sensor tube within tle clamber 342 is in thermal communication with two or more resistive elements 344, 346, each of oldish may act as a heater, a detector, or both One or more of the elements is energized Title electrical current to supply heat to the fluid as it streams through the tribe 320. The thermal clamp 34], which is Lypicall,v fabricated fro rn a material characterized by a high L1ermal conductivity relative to the thermal conductivity of the sensor tube, makes good thernallyr conductive contact with tile portion old the sensor tube just clownstrcarn Irons the resistive element 344 and with the portion of the sensor tube just upstream Tom the resistive elei1cnL 346. The tlermal clamp thereby encloses and protects the resistive element 344 and 346 and the sensor tube 370 Additionally, the tlerma.l clamp 341 thermally "anchors" those portions of the sensor tube with which it Goalies contact at, or near, the ambient tcnperature. In order to eliminate even minute errors due to Lemperatre dtrentials, bile sensor tube may be moved viLhin the thermal clamp lo insure that any difference between the resistance of the two coils is due to fluid floral- through the sensor tube; not to temperature gradients imposed Upon the coils from the environment.

In this illustrative embodiment, each of the resistive elements 344 and 346 includes a thermally sensitive resistive conductor that is wound around a respective portion of the sensor tube 320. Each of the resistive e]emenLs extends along respective portions of the sensor tube 320 along an axis defined by the operational segment of the sensor tube 320. Downstream resistive element 346 is disposed downstream of the resistive elencr1t 344 The elements abut one another or are separated by a sn1a1l gap for manufacturil?g convenience and are preferably electrically connected at the center of the tube. Eacll resistive element 344,346 provides an electrical resistance that Diaries as a function of its temperature. The temperature of each resistive element varies as a function of the electrical current flowing through its resistive conductor and the mass flow rate within the sensor tube 320. In this way, each ofthe resistive elemel.ts operates as both a heater and a sensor. That is, the element acts as a heater that generates heat as a function of the (;urrent through the element and, at the saline time, the element acts as a sensor, allowing the temperature of the element to be measured as a function of its electrical r esistance The thermal mass flow sensor 904 may employ any of a variety of electronic circuits, typically in a Wheatstone bridge arrangement, to apply energy Lo the resistive elements 346 and 344, to measure iDe temperature dependent resistance changes in the elect and, thereby, the mass Ilov rate of fluid glassing through flee sensor tube 320. Circuits employed for this purpose are disclosed, for example, in U.S. Ptc't 5,461,913, issued to Hild;le et al and U.S. Patent 5,41O,9]2 issued to SUZUI;:i, both of which are hereby incorporated by reference ifs their entirety.

I:n operation, fluid flows from the inlet 214 to the outlet 222 and a portion of the fluid flows through the restrictive laminar flow element 212. The remaining and proportional amount of florid flows 1.hrougl1 the sensor tube 320.

The cirC.lit (not shown here) causes an electrical current to flow through the resistive elements 344 and 346 so that the resistive elements 344 and 346 generate and apply heat to the sensor tube 320 and, thereby, to the fluid flowing through the sensor tube 320. Because the upstream resistive element 346 transfers heat to the fluid before the fluid reaches the portion of tile sensor tube 390 enclosed by flee dovi1stream resistive element 344, the fluid conducts more heat away frown the upstream resistive element 346 than it does from the downstream resistive element 344. The difference in the amount of heat conducted away from the two resistive elements is proportional to the mass flow rate of fluid within the sensor tube and, by extension, the total mass flow rate through the mass flow rate controller 200 from tile input port 214 through the output port 222. The circuit measures this difference by sensing the respective electrical resistances of resistive elements 344, 346 and generates all output signal that is representative of the mass flout rate through tile sensor tube 320.

The conceptual block diagram of Figure 4 illustrates the architecture of an electronic controller 400 such as may be used in a mass flow sensor. In this illustrative embodiment, the controller 400 includes sensor 402 and actuator 404 interface Among the sensor interfaces 402, a {low sensor interface 408 operates in conjunction with a mass flow sensor to produce a digital representation of the rate of mass flow into an associated mass flow controller. The controller 400 may include various other sensor interfaces, such as a pressure sensor interface 410 or a temperature sensor interface 411. One or more actuator drivers 412 are employed by the controller 400 to control, for example, the opening of an associated mass flow controller's output control valve. The actuator may be any type of actuator, such as, for example, a current-driven solenoid or a voltagedriven piezoelectric actuator.

The controller 400 operal:es in conjunction witl1 a mass flow controller to produce a digital representation of the rate of mass Shiv into an associated mass flow controller. A thermal mass pONV controller, sucl1 as described in the discussion related Lo Figure 3, may be employed to produce the mass flow neasureinent. The controller 4()0 may employ a pressure sensor inl.erface 410 to monitor the pressure of iRuid wi:llin an associated mass flow controller. In an illustrative embodiment, a pressure sensor, such as the pressure sensor 706 of Figure 2', provides a measure of the pressure within the mass flow controller.

More specifically, in this illustrative embodiment, the sensor measures flee pressure within dead Volume of the mass flow controller. he an illustrative embodiment, the mass flow controller pressure thus measured may be displayed, at the pressure sensor 206 or at the controller housing, for entangle, or some other location The controller 400 may convert the pressure measurement to digital form and employ it in arialysis or other Unctions. For example, if the mass flow controller employs a thermal mass pow sensor, the controller 400 may use the mass flo;v controller pressure measurement to compensate for inlet pressure transients. Although a temperature sensor interface may be used to obtain a temperature reading from a temperature sensor attached, for example, to the wall of a mass flow controller, a separate temperature sensor may not be required for each mass ilov controller. For example, Gauss flow controllers are often employed, as described in greater detail in the discussion related to Figure 1, in O conjunction with a semiconductor processing tool that includes a nunnber of mass flow controllers and other devices that are all linked to a controller, such as a vorkstatio. The processing tool i s operated within a careful Iy controlled environment that features a relatively stable temperature. Because the temperature of Lyle fluid within the mass Bow controller is very nearly eclual to that of the wall of the enclosure and the wall of Life enclosure is very nearly the temperature of the room within Chicle the tool is housed, a temperature measurement fiom, for example, the wor|:staLion that controls flee tool, may provide a suff;oJenty accurate estimate of the gas temperature within the mass flow controller. Consequently, in addition to, or instead of, employing a separate temperature sensor on each mass f10NV controller, the temperature may be obtained fi-on another sensor within the same environment as the mass flo,v controller: of e located at a wo].siation, for example.

The controller 400 includes a local user interface 416 that may be used with one or more input devices, such as a keypad, I.eyboard, mouse, trackball, joy stick, buttons, touch screens, dual inline packaged (DIP) or tlunl'-heel switches, for example, to accept input from users, sucl, as technicians who operate a mass flow controller. The local user interface 414 may also include one or more outputs suitable for driving one or more devices, such as a display, which may be an indicator light, a character, alphanumeric, or graphic display, or an audio output device used to communicate infornatio D-orn a mass flout controller to a user, for example. A communications interface 416 permits a mass flow controller to communicate with one or more other instruments, and/or with a local controller, such as a worl.station that controls a tool that employs a plurality of mass flow controllers and/or other devices in the production of integrated circuits, for example.

In tliis illustrative example, the communications interface 414 includes a DeviceNet interface. DeviceNet is known and discussed, for example, in U.S. Patent No. 6,343,617 B1 issued to Tinsley et al. February 5, 0009, which is hereby incorporated by reference. The controller 400 also includes storage 418 in the form, for example, of electrically, erasable pro=,rainnnable read only Emory (EEPROM! that may be used to store calibration data, mass flow controller identifictLion, or code for operating the mass flow controller, for example Various other fonns of storage, such as random access memory (RAM) , Nay he empkyed. he storage can take many forms, and, for example, may be distributed, with portions physically located 01 a controller "chip" (integrated circuit) and oilier portions located off-chip. The controller 40() employs a data processor 420, which nigl1t take the form of an arithmetic logic unit (ALU) in a general purpose microprocessor, for example, to reduce data. For example, the data processor 420 may average readings received at the sensor inputs, determine l.he nun1her of times a sensor reading has exceeded one or more ills eshold values, record the tinge a sensor reading remains beyond a ti-eshold value, or yefuiii otl1er fornts of data logging.

Pressure tallsients on the inlet supply line Lo a mass flow conL-ol]er 200 that employs a il1ermal mass flow sensor 204 may create erroneous massflow o5 readings. Erroneous mass flow readings may lead, in turn, to improper control of a mass flow controllers outlet valve, which could damage or destroy articles being processed witl1 gasses under control of the mass flow controller. The digital representation of mass flow may talkie the form of one or more data values and is subject to fluctuations due to pressure trarsients on the inlet line oftle mass flow sensor. h1 an illustrative ernbodinent, the controller 400 employs data obtained at the pressure sensor interface 410 to compensate for fluctuations induced in a thermal mass flow sensor 204 by pressure transients on the mass flow sensor inlet line 214. Ire this illustrative embodiment, the controller 400 obtains temperature information through a temperature interface 411. The controller 400 employs the temperature, pressure, arid mass flow readings obtained from the respective interfaces, to produce a connpensated mass flow reading that more closely reflects the mass flow at the outlet of the mass flow sensor than a reading front the thermal mass flow sensor alone provides. The controller 400 also provides control to sensors, as necessary, through floral sensor interface, pressure servitor JO inl:erce, and temperature interfaces, 408, 41O, and 411, respectively.

The controller 400 also includes a. valve actuator interface 404, which the controller zlOO employs to control the position of a valve, such as the valve 220 of Figure 2, to thereby control the rate of fluid flow Llougl a mass flow controller, such as the mass flow controller 200, in a closed -loop control process. The valve acl:uator may be a solenoiddriven actuator or peizo-electric actuator, for example. The controller 400 must be capable of operating with suffcielt speed to read tile various sensor outputs, compensate as nec.essa.ry, and adjust the mass flow controller outlet control valve 220 to produce a predetermined flow rate.

The fiord rate is predetermined in the sense that it is "desirecl" in some sense. It is not predetermined in the sense that it must be a static setting. That is, the predetermined pow rate may be sot by an operator using a mecl.anica.l meals.,, SUCH as a. dial setting, or may be downloaded Fom another controller, such as a workstation, for example, and may be updated.

In an illustrative ernbodirnent, Llie controller 400 employs readiness on} the pressure interface 410 to compensate flow neasurenJents obtained at the mass flow interface 408 from thermal mass flow sensor 204 that senses mass flow at the inlet 214 to mass flow controller:00 The compensated flow measurement more accurately depicts the float at the outlet 222 of the mass flow controller 200.

This outlet flow is the flow beirig directly controlled by the mass flow controller o00 and typically is the flow of interest to end users. Employing, a pressure compensated flow measurement in accordance Edith the principles of the present inetion improves the accuracy of a mass flow sensor's outlet flow reading and thereby permits a mass flow controller to more accurately control the flow of fluids. That is, at equilibrium, mass flop at a mass flower controller's inlet is equal to the mass flow at the outlet of the mass flow controller, but during inlet or outlet pressure transients, the pow rates differ, sometimes significantly. As a result, a mass flow controller Slat provides closed loop control using its inlet flow to control its outlet flow may connnit substantial control errors.

The steady state mass [low ire the capillary sensor tube 320 of a thermal mass flow sensor such as described in the discussion related to Figure 3 is generally described by the following equation: ARC= C _r i 32! PR Lo) where: do = Papillary tube inside diameter Lc = capillary tube length pi = the density, of the gas at the inlet pR = the clensit:y of the gas at sl.andard temperature and pressure jl = the;a.s -viscosity Pi = the pressure at the inlet of the mass flow controller o0 Po -- I'le pressure at the outlet of the mass flow controller P = the pressure in the dead voluTne of the Nash flow controller The total flow through tle mass flow controller is related to that through the capillary sensor tube 320 through a split ratio: a- QBP/Qc Where QBP is the flow through tile bypass cham1el 216 and Qc is the flow through the capillary'tube 320. The total flow Ql at the mass flow Qcontroller inlet 214 is: Qi = QBP +Qc = (1+ a)Qc If flow remains laminar in both the bypass and capillary, the split ratio Will remain constant. When the inlet pressure varies with tine, the nature of the inlet pressure transient and the pressurization of tile dead volume govern the flow at tile inlet. Assuming blat all thermodynamic events within the dead volume occur at a constant temperature that is equal to the temperature of the enclosure lO blat forms a partial receptacle around the dead volume, flee mass coserva Lion villain the dead volume Inlay be described by: (A =g, _ Tale cIP Where: Pit - pressure at standard temperature and pressure (760 Torr.) TR --= temperature at standard temperature and pressure (273 l() Tw = wall temperature (temperature of the wall of l:he mass how controller) V = volume of the dead volume Q. inlet flow to the mass flow controIier QO = outlet flow from the mass flow controller A mass flow sensor employs the relationship of equation (2) to compensate a thermal mass flow sensors mass how signal and to thereby substantially reduce errors In mass flow readings during pressure transients.

The flow chart of Figure 5 depicts the process of compensating a thermal mass flow sensor reading.

The process begins in step 500 and proceeds from there to step 502 where a mass flow sensor's controller, such as the controller 400 of Figure 4, obtains a mass flow reading. This reading may be obtained from a thermal mass flow sensor through a flow interface, such as interface 408 of Figure 4, for example.

This flow measurement reflects the rate of mass flow at the inlet of a mass flow controller and, as previously described, may not adequately represent the mass flow rate at the outlet of the mass flow controller. The mass flow rate at the outlet of a mass flow controller is generally the rate of interest for use in control applications Consequently, a mass flow controller compensates for the inaccuracy inherent in assuming that the inlet flow rate to a mass flow controller is equal to the outlet flow rate from the mass flow controller. From step 5()2 the process proceeds to step 504 where the sensor controller 400 obtains the temperature of the flow within the bypass channel. The temperature could be obtained through a temperature interface such as interface 412 of Figure 4, or it may be 1ownloadcd to the compensated mass flow sensor. The compensation process may safely assume that the gas temperature is equal the temperature of the enclosure of the mass flow controller. Additionally, in most applications, the temperature will remain relatively stable over a long period of time, so that a stored temperature value may be employed, with updates as necessary.

Skirter obtaining tl,e gas 1:empcraturc in step 50-4 the process proceeds to step 506 vl-ere the sensor controller obtains the volume of the dead volume. This value may have been stored during manufacturing, for example. From step 506 the process proceeds to step 508 where the pressure within the dead volume is obtained cater a period of time The iumbcr of Measurements and the time over which tile measurements are made depend Upon the speed and duration of transients at tile inlet of the mass flow controller. In step 510 the processor employs the pressure measurements made in step 508 to compute tile time rate of change of pressure within the dead volume. After computing the tithe rate of change of pressure viLhin the dead volume, the process proceeds to step 512 inhere a compensated outlet flow value is computed according to equation (a).

: Simplifications may be made in the computational process. For example, the volume of the dead volume, standard temperature, and standard pressure may all be combined into a single constant for use with the inlet flow measurement and time rate of change of pressure within the dead volume to compute a compensated outlet flow approximation. This simplification would yield an equation ofthe form: Qo = Qi - Cl(V/T) (dP/dt) (3) where: Qo = the compensated sensed outlet floral rate, Qi = the sensed inlet flow rate, C1 = a normalizing constant re]atin the temperature and pressure to standard temperature and pressure V = the volume between the sensor bypass and the outlet Bow control valve, T = the l.emperature of the Bui d within the volume, dP/dt - tickle rate of change of pressure within the volume.

o0 As previously noted, the. ;olurne Bier could oc 1rldcd into the c; stant I. From step 51Q the process proceeds to step 514 where it continues, with tile how sensor's controller obtaining pressure, temperature, and flow readings and computing a compensated outlet flow estimate, as described The process proceeds from step 514 to end in step 516, for example, when the mass flow sensor is slant down.

Returning to the block diagram of Figure 4, in this illustrative embodiment, tlae controller 400 includes a diagnostic interface 422 that pennits art operator, such as a tecllician for example, to not only initiate, hut conduct diagnostic tests on the mass flow controlled-. Furtllerinore, the interface 422 permits the operator to conduct the diagnostics in a manner that requires no input from the local system controller, Which may be a Workstation, that otherwise normally controls the mass Dow controller. Such diagnostics are transparent to the local system controller, which may not even be made aware of the diagnostics being performed and may, consequently, continue its operations unabated. The diagnostic interface provides access to mass flow controller sensor neasuremerlts, control outputs and mass flow controller diagnostic inputs and outputs. These various inputs and outputs may be exercised and measured through the diagnostic interface With very little delay. In an illustrative dual processor embodiment described in greater detail in the description related to the discussion of Figure9, a deterministic processor may modify outputs and/or monitor iraputs, from sensors or test points, for example. During the execution of online diagnostics' the controller continues to execute its process control functions, unimpeded, while, at tile sang Lime, the cctroller may Provide real tinge interaction faith a technician (i e., interactions wherein the delays are imperceptible to a human operator) either locally or through a telccommuni cations connection.

Using the diagnostic ir.terface 422, an operator can adjust control values, such as the set point, used to determine the mass f10v controller's operation.

Additionally, the operator may modify sensor output values in order to lest the ma.s.s flow c.onLr-oller's response to specified seriOor readings. Fl,at is, act vpuZaLL>1 can modify the sensor readings a mass flow controller employs to control the flow of gasses thi-ougl its outlet valve and, thereby, exercise the controller for diagnostic purposes. An operator may read all sensor and test point inputs as well as information stored regarding control (stored by tle deterllilisLic controller in the dual processor elbodinent), read all sensor values, read test point values, read control information, sulk as the desired set point. Additionally, the operator relay write to control outputs and test points and overwrite stored values, such as sensor readings or set point information in order to hilly test the controller O through the diagnostic post.

In an illustrative embodiment, a mass flow controller may include a web server. Such a web server may be included within the diagnostic interface, for example. In such an embodiment, the diagnostic interface includes a web-server that permits the mass flow controller to be used in a system such as illustrated in the block diagram of Figure 6. In such a system, a user, such as a technician, may employ a web-enabled device 600 such as a personal computer, personal digital assistant, or cellular telephone that runs a web Browser (e.g. Netscape (RTM) or Explorer (RTM) to communicate with a server 602 embedded in the mass How controller 604. The server 602 includes web pages that provide an interface for the user to the mass flow controller 604. The discussion related to Figures 13A through 13E provide greater letai] related to the web server capability embedded in an illustrative embodiment of a mass flow controller.

Mass flow sensors are typically calibrated during their manufacturing process. Because a mass [low sensor is usually incorporated into a mass flow controller, this discussion will center or mass flow controllers, but the methods and apparatus discussed herein are applicable to ''standalone'' mass flow sensors as well. The calibration process requires a teclmician to supply a gas at a 1cnovn flow rate to the mass flow controller and correlate iDe mass flow sensor's iron si,,n! to the known flow rate. For cxarllple, in tl,e case of a class Bc,' sensor that provides a voltage output corresponding to flow, the technician maps the voltage output from the sensor into 1.he actual flow rate. Allis process may be repeated for a plurality of [lows in order to develop a set of voltagelflovr correlations: for example, a 457olt output indicates a 40 standard cubic centimeter per nilluLe (scam) flow, a 5 Volt output indicates a 50 scorn flow, etc. Flow rates that fall between calibration points may be interpolated using linear or polynomial interpretation techniques, for example. Allis process may be repeated for several gases Correlation tables Blat relate the signal frorL1 tile mass flow sensor (wllich may be a voltage) to flow rates for various gases may thus be developed and stored. Such tables may be downloaded to a mass flow controller for use "in the field", or may be stored within a mass flow controller. Often, technicians calibrate a mass Dow controller using a relatively innocuous gas, such as N2, and provide calibration coefficients that may be used to correlate the flow of another gas to the calibration gas. These calibration coefficients may then be used in the field when a known gas is ''flowed" tlougrh the mass flow controller to compute the actual flow from the apparent flow. That is, the apparent flo,v may be a flout con-elated to N2 and, if Arsine gas is sent tlrou,gh the mass flow controller, the mass flow controller multiplies the apparent flow by an Arsine gas calibration coefficient to obtain the actual flow. Additionally, once in the field, mass flow controllers may be re-calibrat;ed on a regular 1: asis to accommodate "drift", orientation, water content of a gas flee flow of which is being controlled, or to compensate for other factors. U.S. Patent 6,332, 34cS B 1, issued on December 2S, 2001 to Yelverton et al., wl1ich is hereby incorporated by ]S reference, discusses these factors, and the unwieldy processes and equipment required to carry out these in-the-field calibrations in greater detail.

A calibration method and apparatus will be described in the discussion related to the conceptual block diagram of Figure 7. This calibration system and method nay he employed in a manufacturing setting, or, in an illustrative embodiment, may be incorporated into a self-calibrating mass flow controller. The nass flow controller 700 Includes a mass flow sensor 702 and an electronic controller 7()4 that receives a flow signal from the mass flow sensor 702. A calibrator 706 includes a variable flow gas source 7()8, a receptacle of predetermined volume 710, and a pressure differentiator 712. It should be noted that the lines separating different functional blocks are somewhat fluid. That is, n1 different embodiments, the function associated with one block may be subsumed by one or more other blocks. For example, in an Illustrative embodiment, the pressure differentiator 712 is implemented all, or in part, by the execution of code within the electronic controller 704. The variable flow gas source 708 provides a gas at proportional rates to both the receptacle of predeterrnied volume and the mass flow sensor.

The flow rate to the mass flow sensor 702 may be equal to the flow rate to the receptacle of predetermined volume 7IO: i e., a proportionality constant of 1, for exainple. The mass flow sensor 702 is configured to produce a mass flow signal indicative of the flow that it senses and, in this illustrative embodiment, this signal is sent to the electronic controller 704. The pressure differe1tiator 712 produces a signal correlated to the flow from else variable flow source 708 into the receptacle of predetermined volume 710 according to the relationship of equation 4: O Qo = Qi - C 1 (V/T)(dP/dt) (I) where Qo = the outlet flow rate in standard cubic centimeters per minute, Qi = the inlet flow rate in standard cubic centimeters per minute, C1 = a normalizing constant relating, Lee temperature and pressure to standard temperature and pressure V = the predetermined volume of the receptacle in liters, T = the Kelvin tcmperatw-e of the fluid within the receptacle, dP/dt = time rate of change OZr2 pressure within tile receptacle in Torr/second In an illustz-ative eznbodirnent, tile receptacle is closed and gas Lows into the receptacle until the pressure within the receptacle equals that of gas supplied by the variable flow source 708 In such an illustrative eznbodiincnt, the variable Now source may be a constant-pressure source that, as pressure within the receptacle builds, supplies gas at an exponentially decreasing flow rate In such a case, the outlet [low Qo = 0, and the inlet flow, 2iis given by: (:i = C 1 (NflT)(dP/dt) (a) The pressure differentiator 712 takes the time derivative of the pressure within the receptacle 710 and, given the normalizing constant C1, the predetermined volume V, and the gas temperature within the receptacle, the differetntia.tor (and/or the electronic controller 704) may determine the actual flow into the receptacle 710. Because the flow into the receptacle is proportional to the DONV into tile thermal mass flow sensor 70Q, Llle actual flow into the thermal mass flow sensor 702 may also be determined by a rnultip]ying the actual flow into the receptacle by a proportionality constant (e.g., the proportionality constant is 1 if the flows are equal). The signal from the mass flow sensor is then correlated, by t:l.le electronic controller 704 for example, to tile actual flow, determined as just described. Such correlation relates one or more signal levels from the mass flow sensor to the actual flows. flee pressure difterentiator 712 may include analog differentiator circuitry, for example, that talces tile time derivative of the pressure signal. The dif ferentiator output signal, a signal representative of the tinge derivative of the pressure within the receptacle dP/dt, may be sampled by an analog-lo-digital converter (not shown) Lo permit the electronic controller 704, whicl1 may include a microprocessor, IDS- chip, or dual processors, for example to operate on the time derivative signal Alternatively, the pressure differentiator 712 may convert the pressure signal to digital fond for processing by the electronic controller 704, vllicll takes tile time derivative of the pressure signal.

In such an embodiment, the electronic controller, in combination with diffcrcntiaior cDdc, operates as tile dilferentiator. Tle comptroller employs at least two pressure differences divided by corresponding time intervals l.o compute the derivative. Tile gas may be supplied in parallel to the receptacle and mass flow :5 sensor, or it may be supplied in scrics, as will be described in greater detail in the following discussion related to a self-calibrating mass flout controller.

In operation, a mass flow controller nay be calibrated as just described, using a plurality' of gases, Title tile correlation values (mappings of sensor output to actual flow) stored in tables. Calibration coefficients, relatin;, flow measurements of one gas to another may also be developed and stored The tables and/or coefficients may be downloaded to a mass flow controller in the field for use by the controller in controlling the flow of a gas. Various lcnown interpolation techniques, such as lir.ear or polynomial interpolation may be employed in conjunction with the calibratior. tables and/or coefficients. Additionally, such stored calibration tables and/or coefficients nosy be used as default values in a self-calibrating n1a.ss flow controller in accordance with the principles of the present invention. A self-calibratincr mass flow controller in accordance Title the principles of the present invention includes a calibrator 706 and a mass flow sensor 702 which may be employed to calibrate the mass flow controller in a manner as just: described In the case of a self-calibrating mass flow controller, though, the calibration can be performed, In Situ, in the field just as readily as in a manufacturing setting.

Once installed in flee field, on a semiconductor processing tool as in the system 100 of Figure 1, for example, the mass flow controller can calibrate itself using the gas that is to be used during the semiconductor processing. By using the gas that is to l:'e used in processing, tle mass flow controller may provide a more accurate flow measurenient, because it Still aul.orn.-tically accommodate variations, such as moisture content, for example. Additionally, a new prccessing As may be used just as readily as a conventional gas, since the self calibrating' o0 mass Mow c-,ntrollcr nay calibrate itself (that is' correlate inns Mow signal levels to actual flow levels detemlined by the i'essue differentiator), on:-he gas to be used, not in relation to another, standard gas, such as No. Because the mass flow controller is calibrated in else orielta.tion in which it will be used, discrepancies due to re-orientatioi1 of the mass flow controller in the field relative to the position in wllicl, it was ca1ibraLed during manufacturirg will be substantially eliminated. All the mass flow controllers Violin a system such as system 100 of: Figure 1 may be calibrated automatically and simultaneously, within monuments.

This is in contrast to the cumbersome, painstaking process employed for conventional mass flow controllers, which are typically individually calibrated by a tactician en-ployiilg multiple mass flow meters, groin" from mass flow controller to mass flow controller. As will be described in greater detail in the description related to the discussion of Figure 8, a mass flow controller that includes a thermal mass flow sensor alla a pressure transducer may shut its outlet valve to create a varying gas flow into it's dead volume. By taking the time derivative of the pressure the actual flow into the dead volume receptacle may be determined. The mass flow controller's correlations of the actual value of the floor to the thermal mass flow sensor signal acts as the mass flow controller's calibration.

Figure 8 is a conceptual block diagram of a self-calibrating mass flow controller 800. In this illustrative, series-flow, embodiment, a gas flows through a thermal sensor 802 into a receptacle of predetermined volume 804, then through an outlet valve 806.

The outlet flow Qo would nonnally be a controlled flow into a chamber, such as a chamber violin an integrated circuit processing tool. An electronic controller 8()8, which, in. this illustrative embodiment, executes code to perform the differentiation required Lo obtain actual flow, as described in the discussion related Lo Figure 7, is in conmunica.tion with the thermal sensor 802, pressure sensor of SOS and the outlet valve S06. In an illustrative process, the electronic controller 808 operates in conjunction with the outlet valve 806 Lo forth a variable-flow gas supply. That is, the electronic controller shuts the outlet valve, whicl1 causes Ll1e flow to decrease exponenLil]y Tlie pressure within the dead volume increases, and the electronic controller differentiates this signal a number of times in order to obtain actual flow readings to correlate Lo the mass flow sensor signal values over a relatively broad range of flows. Additionally, in order Lo emend tl-e period of time durin,, which the low is Wearying and l:o obtain actual lion values for correlation with the thermal mass flow signal values over a broad range, the electronic controller may open the outlet valve to a fully open position before closing it The pressure and f low profiles associated with such a process are illustrated conceptually in the graph of Figure 9 At an initial time to the pressure difference between gas at the inlet to the mass flow controller Pin and the pressure Pr downstream in the receptacle 804 forces gas to flower through the mass flow controller at a rate Qin. Ire this example, the inlet pressure Pa,, pressure within the receptacle PR, and flow througl, the input of the mass flow comptroller Qin are constant. At time tSo the controller slants the outlet valve, thereby reducing outlet flow Qo to zero. Gas continues to flow info the receptacle as long as there is a pressure difference between the receptacle and the inlet. As the pressure PR within Lhe receptacle rises exponentially toward an equilibrium state of equality with the inlet pressure Pin, tile inlet flONV Q.,, decreases By talking the derivative of the pressure change Within the receptacle (also referred to herein as "dead volume" in association with an illustrative embodiment of Lhe invention), the elec tronic controller may determine the actual 110NV into the receptacle, as previously described.

AL he electronic c ontrol]er may correlate a plurality of simultaneous readings produced by the thermal mass flow sensor, l:o thereby calibrate the mass flow sensor rlhat is, once this process is completed for a specific gas, flow readings from the thermal mass flow sensor may be correlated to actual loom rates Tle results may be employed by the electronic controller SOS to control the opening, of the valve,06 in a closed loop control system in order to deliver a 0 sulecLed flow dowilLrtam In order to increase Lhe period of. time to frown Alan the controller S)JUtS the Bivalve, to the,:ime at vllicll tle t1ov become, undetectable, and to tl-ereby increase the number and precision of pressure measurements that may be made, the controller may open the valve completely before shutting it at time tso. Additionally, one or more flow restrictors nay be Q5 placed in the flow path bets tlie inlet to the Llernal mass flow sensor and tle inlet to the r eceptacle S04.

Tl1e conceptual block diagram of Figure 10 illustrates the architecture of a dual-processor embodiment of an electronic controller 1000 such as may be used in a mass flow sensor in a.ccordace Title the principles of the present invention In this iliusLra.tive embodiment, the controller includes two processors 1002, 1004. One of the processors 1002 is dedicated to "real time" processes and the other processor 1004 is dedicated to nor.-real time processes. By "real time" we mean processes that require a specified level of service within a bounded response time. In this sucrose, the processes are deterministic and the processor 1009 will be referred to herein as the deterministic processor. The objective of the dual processor arclitecture is to reduce the number of interrupts and manage asynchronous event responses in a predictable way. The non-deten1inistic processor 1004 may handle event-driven interrupts, such as responding to input from a user. The deterministic processor 1009 handles only frame-driven, that is, regularly scheduled, interim pts. In an illustrative embodiment, Lhe non deterministic processor is a general purpose processor 1004, suited for a variety of tasks, such as userinterface, and other, miscellaneous tasks, rather than a specialized coprocessor, such as a math- or communications - coprocessor. In particular, a TMS320VC5471, available from Texas Instruments, inc., snap be employed in a dual-processor embodiment in accordance with the principles. The fMS390VCS47l is described in a data manual, available at http://ww s t.icon/sc/ds/tms320vc547]. .pdf, which is l1ereby incorporated by reference.

A processor interface l 006 provides for inter-lrocessor communications.

Else deterninistic processor 1002, includes sensor and actuator interfaces. Amorg o0 Lhe sensor inLerfices, a Bow sensor interface 1005 operates in conjunction Judith a mass Lk'nv sour Lo puduc;e a:iigiLil representation oftLe rate of Class flG-V in an associated mass flow controller One or more actuator interfaces 1010 are employed by the deterministic processor 1002 to contiol Lhe opening of an associated mass flout controller's output control valve or drive a diagnostic test point, fur exanple. The acLLat.or may be a curent-driven solenoid or a voltage dri:en piezo-electric actuator, for example. As will be described in greater detail in the discussion related to the flow chart of Figure 9, after iniLializatiorr, the deterministic processor 1002 loops through a control sequence, gathering sensor data, gathering setting information (for example, a desired mass flow setting), providing status inforatic,i, and Scrolling the sLaLe calf Lhe outlet Bivalve Because non-deterministic tasks are offloaded to the non-deterministic processor 1004, the deterministic processor's control loop nnay be very compact.

Consequently, control tousles nary be executed within a minimal period of time and control readings and drive signals may be updated more frequently than possible if time were set aside for servicing non- deterministic tasks.

The controller 1000 operates in conjunction witl, a thermal mass flow sensor as generally described in the discussion related to Figure 3 to produce a digital representation of the rate of mass flow into an associated mass flow controller. The digital representation may take the form of one or snore data values and is subject to fluctuations due to pressure transients at the input of the mass flow sensor. The controller 1000, and more specifically, the deterministic processor] 002 may employ data obtained at the pressure sensor interface 1006 to compensate for fluctuations induced in the thermal mass llo,v sensor by pressure transients on the snags flow sensor inlet line. In this illustrative embodiment, the deterministic processor 1002 employs the temperature, pressure, and mass flow readings ob t.ained from the espective 008, 1007, and 1005 interfa ces, to produce a compensated mass flow readiig that more closely reflects the mass flow at the outlet of the mass flow sensor than a reading from the thermal mass flow sensor alone. The deterministic processor 1002 also provides control to sensors, as neccssay, through thermal flow 1005, pressure 1007, and toniperature 10(). .senscr interfaces Tl' compensation process will be described in Breather detail in the discussion related to Figure 11. The deterministic processor 1002 also includes a valve actuator interface 1010, which the deterministic processor employs to control the position of a valve, such as the valve 220 of Figure 9, to Lleeby control the rate of fluid flow tl- uugh a mass flow controller, such as the mass flow controller 200, in a closed -loop control process.

The deterministic processor 1000 is devoted to the closed-loop valve control process, and, consequently, must be capable of operating Title suLi:;cient speed to read the various sensor outputs, compensate as necessary, and a.cljtst the valve to produce a predetermined flow race. Tlle flora rate is predetermined ifs the sense that it is "desired" in some sense, alla it need not be a static setting. That is, the predeterh1ined flow rate may be set by an operator using a mechanical means, such as a dial setting, or may be downloaded from another controller, such as a workstation, for example, and updated frequently. Typically, gas'flov control, and in this case, compensated gas Dow control, requires relatively hirh-speed operation. Various types of processors, such as reduced instruction set (DISC), matI1 coprocessor, or digital signal processors (DSPs) may be suitable for such high-speed operation The computational, signal conditioning, and interfacing capabilities of a DSP make it particularly suitable for operation as the ]0 deterministic processor 1002. As will be described in greater detail in the description of the control process related to Else discussion of Figure 9, the function performed by the deterministic processor 1002 is delcrmin.istic in the sense that certain operations are completed in a timely and regular manner in order to avoid errors, and possible instabilities, ifs the control process. The dDi.erninistic 1002 and nondeterministic 1004 processors communicate via the inl.er-processor interface 1006 in a manner that does not impede Else deterministic operation of the deterministic processor lOO9. In1-er-processor communications are discussed in greater detail in tle discussion rebated to Figure 9.

The non-deterministic processor 1004 includes a local user interface 1016 that may be used Title one or more input devices, such as a keypad, keyboard, Denise., trackhall' jo,r stick, buttons' touch screens; dual iTlliTe paclarrcd (DlT') or thumb-wheel switches, for example, to accept input from users, such as technicians who operate a mass flow controller associated with the non- dienninistic processor 1004. The local user interface 1016 also includes one or more outputs suitable for driving one or more devices, such as a display, Vehicle may be a character, alphanumeric, or graphic display, for example, indicator Light, or audio output device used to communicate infiorllation from a mass flow controller to a user. A communications interface 1018 permits a mass flow controller to communicate with one or more other instruments, and/or with a local controller, such as a N. orksta.tion that controls a tool that employs a plurality of mass floral controllers and/or other devices in.the production of integrrated circuits, for example. In this illustrative example, the communications interface IO18 includes a DeviceNet interface. A diagnostic interface 1090 provides an interface for a tecllician to run diagnostics, as previously described in relation to tile diagnostic interface 422 of Figure 4. In au illustrative embodiment, the diagnostic interface includes an Ethernet interface and a web server.

The compactness oicode for the deterministic processor 1002 permits the deterministic processor to be highly responsive to input changes and to quickly modify actuator signals in response to those changes. This partitiorang of JO operations between deterministic and non-deterministic processors also eases the initial development of code, for both the deterministic and non-deterninistic processors For example, the detenninisLic code needn't respond to unscheduled events, such as "mirroring" a user's requests on a display at a user interface, and the non-determinisl:ic code needn't brealc away from providing such user feedback in order to adjust an outlet valve control setting every fifty bus cycles. The parLit:ioning between deterministic and non-deterministic also permits relatively simple revision.q and upgrades If the code for one processor must be revised or upgraded, the code for the other may require no revisions or only minor revisions.

In particular, the code for the deterministic processor may be more "mature", or fixed than that for the non-deterministic processor; user interfaces, comnunicaions and other similar functions tend to be upgraded more liecluently than the deterministic, mass flow control, functions.

Using this illst-atife dual-processor enbodinenL, a user interface may be updated without any impact on the control function code, for example. Revision and naiteiance of miied-mode code (deterministic and nondeterninistic code) would be a much more complicated and costly proposition than code partitioned in a rewarmer in accordance faith the principles of the present invention. In an illustrative embodiment the dual-processor controller 1000 may by a hybrid processor that incorporates two processors 021 one integrates circuit An integrated circuit such as the TMS320C5471 hybrid processor available Tom Texas Instruments (RIM) may be employed as the dual processors ire accordance with the principles of the present invention Tle digital signal processing (DSP) subsystem of the chip, due to its math capabilities would be more suitable as the cleterrtlinistic processor in sulk an applications. The IC's dual-ported memory may be employed as the inter- processor interface, with the processors writing to and reading from memory locations set aside to act as "mail boxes" for the transfer of inforination, including data, commands, and command responses.

Such an inter-processor interface permits the deterministic processor tot continue operating in a frame-driven mode while, at the same tinge, allowing the deterministic processor to play a role in diagnostics and calibration. AJ]Y request for sensor data from. the non-deterrinistic processor may be picked up fro na the mailbox on one pass of the deterministic processor's control loop' with the readings deposited in the mailbox the very next time through the loop. Diagnostic outputs lacy be modified similarly. The deterministic processor may also operate in other, non-process oriented modes. For exarnp]e, during a self-calibratio process such as previously described, the deterministic processor would no longer operate to maintain a set flow through the mass Bow controller. In such a mode the deterministic processor would be occupied by shutting the mass flow c.ontroller's outlet valve, taking a plurality of time derivatives of the pressure within the dead vo]urne, computing the correspondir.g actual flow in the mass flow controller, and correlating the actual flow to the floral signal produced by thermal mass flow sensor.

The flow chart of Figure 11 outlines the process of sensing and controlling the flow of gas through a dual processor mass Cow controller.

The process begins in step 110() and proceeds from there to step 11()2 where the controller is initialized. This initialization step may include the uploading of calibration values or a calibration sequence itself. Additionally, operating code for both the deterministic and nondetemlinistic processors 1004 may be uploaded at this point. In an illustrative embodiment, the non-deterministic processor 1004 may upload its own code and begil1 operating, then upload code for the deterministic processor 1002. Ire the process of uploading code for the deterministic processor 1002, the non-deterministic processor 1004 may select among a plurality of executable code sets to upload'to the deterministic processor 1002, thereby tailoring the operation of the deterministic processor 1002. Tle non-deterministic processor 1004 may base this selection on switch settings, commands from a local comptroller (e.g., a Workstation controlling the operation of a semiconductor process tool), or settings stored in nonvolatile storage, for example. Such a selection pennits a mass flow controller to be tailored to different flow control operations For example, a technician may, by selecting among code sets, choose to operate the controller in a "pressure ccnLroller" mode rather than a "mass flow conl:rolleri' node, and this selection may be made locally or remotely (i e., through a te] ecommur.icatios lit).

In step 1104 the non-deterministic processor 1004 passes operating code and initial control settings Lo the deterministic processor 1002 which then begins operating in a mamler described generally in connection with the flow chart of Figure 12. From step 1104, the process proceeds to step 1106 where the non- determirlistic processor 1()04 services the local inlut/out interface. Such servicing may include reading various inputs, including keyboard, switch, or mouse inputs, o0 and displaying information locally, Hugh LEDS, alphanumeric displays, or grapliGfl displays. Prom step 110 the process proceeds to step 1lt)S inhere the non-determilistic processor]004 services the communications interface. I'his servicing may include tile steps of uploading control and sensor data to a vorlcstation that operates as the local controller of a semiconductor process tool, for example. Additionally, tile non-deterministic processor 1001 may download updated settings from the local controller.

From step 1108 the process proceeds to step 1110 where the nondeterministic processor 1004 services the diagnostic interface. Various diagnostic operations, sucl1 as set forth in the description related to the discussion of Figure 4, may be performed in this step. In an illustrative embodiment, the mass flow controller includes a web server, which permits an operator to run diagnostics through a network such as the "world wide web. " Frorn step 11 10 the process proceeds to step 1112 Moliere the no-deterministic processor 1004 services the inter-processor iterface 1006. During ''normal", non-diagnosti operation, the nordeterministic processor 1004 obtains readings from the deterministic processor 1002 and passes control information, such as a CONY setting obtained thrcugh the communications interface, to the deterministic processor From step 1112, the process proceeds to continue the processes just set forth in step 1114. The process proceeds to end in step 1116 when the mass CONY controller iS turned off, for example.

As previously noted, the steps set forth in this and other flow charts herein need not be sequential and, in fact, a number of functions performed by the non- deterministic processor 1004 may be event-interrupt-driven and no yredictab]e sequence may be ascribed to the non-deterministic processor's operation. Other processes, such as data-logging may be performed at regular intervals. The non(leLerministic processor can support a two-way soclcet connection to the deterministic processor tl-ough an Ethernet network interface, for example, to provide a relatively direct connection betNvcen a remote user and the deterministic processor.

The flow chart of Figure 12A-12B depicts the operation of the deterministic processor of a dual processor mass flow controller. In the context of this flow chart, it is assumed that an initialization process has taken place and that the deterministic processor is cycling through its control loop. The process begins in step 1200, Figure]2A, and proceeds from there to step 1202 where the deterministic processor determines whether it is to operate in its "normal" control capacity or whether it is to operate in an alternative mode, such as a manual diagnostic mode or an automatic diagnostic mode, for example.

The deterministic processor bases this decision on information it obtains from the inter processor interface 1006. The deterministic processor services frame-driven, rather than even-driven interrupts, consequently, it regularly polls the inter-processor interface to obtain infonnation such as this.

If the deterministic processor is to operate in its normal mode, the process proceeds from step 1202 to step 1204, blare the deterministic processor obtains information from the inter-processor interface regarding the desired control settings. This information may be in tl1e form of a desired flow rate received from a local controller, f om a front panel user interface, or through the diagnostic port 1020 for example. The deterministic processor may also transfer information, such as sensor data, for example, to the non-deterministic processor through the inter-processor interface during this step. From step 1204 the process proceeds to step 1206 where the deterministic processor gathers data, fro no a variety or sensors for example. The sensors from which the deterministic processor obtains data may include a mass flow sensor (thermal or other type), a temperature sensor, or a pressure sensor, for example.

] From step 1206 the process proceeds to step 1208, where the :leterministic processor computes the flow rate of material through the mass flow controller. In an illustrative embodiment, the mass pONV controller includes a thermal mass [low sensor and a pressure sensor configured to measure tle pressure Wichita the dead volume between the thermal mass flow sensor's bypass and the mass flow controller outlet valve, step 1212. In this embodiment, the dctermiisLic processor may employ the net.lc,d described in relation to the discussion of Figure 5 to compensate a flow rate measured by a thermal mass flow sensor at the inlet of the controller Lo more closely approximate the GONE rate at the outlet of the controller. In an embodied in vlich the flow rate obtained from tile sensor is not compensated, the process Would proceed directly front step 1006 to step 1210, skipping the computational process of step]20S.

In step 1210 the deterministic processor determines vhetler the flow rate computed in step 120S (or read in step 1206) is equal to the desired flow rate indicated by the setting information obtained from the nondeterninistic processor via the inter-processor interface in step 1204. If the values are equal the deterministic processor continues the operation as just described, as indicated by the "continue" blocl: 1214 (i.e., the deterministic processor retunes to step 1202 and continues to cycle through the loop). If the values a're not equal, the deterministic processor computes an error signal and employs the error signal to adjust the drive signal to the mass flow controller's outlet Bivalve. From step 1212 the process proceeds to continue in step 1214. The process proceeds from step 1014 to end in step 1716 when the mass flow controller is shut down or reset, for

example.

10If; in step 1202 the deterministic processor concludes that it is not to operate in the normal mode, the Process proceeds through connecting box A to step 1218, [Figure 12B, where the deterministic processor determines whether it is to operate in a diagnostic mode. The deterministic processor may obtain this inforntion from the inter- processor interface. If the deterministic processor is to 15operate in a diagnostic mode, the process proceeds to stop 020. In step 12'0 tile determiist.ic processor detenni:es vl1ich diagnostic mode it is Lo operate it?.

Once again, tlis information may be passed to the deterministic processor through the inter-processor interface. In an 'tutomaLic" mode, the deterministic processor acquires a sequence of diagnostic values front the inter-processor interface The sequence of values is available at the iterfa.ce for acquisition by the deterministic processor. The dia,n1otic valucs,nay be control outputs, fur selling the opening of tile class flow controller outlet valve or for setting test point drive values, for example, or the diagnostic values may indicate desired sensor readings or readings from test points, for example. The diagnostic values ?) may also indicate the sequence in wlich tle Values are to be employed, in order to set test point driver values, then read test point outputs, for example. In a manual mode, diagnostic values are node assailable to the deterministic processor tl-ou,h the inter-processor interface one at a time In an embodiment in Chicle the mass flow controller includes a web server, a technician may use a,veb enabled workstation to contact the server ilk the mass Cow controller. Once linked hi, to the server, the technician may enter a valve setting command, by typing, selecting from a pull down menu or clicking on icon, for example. This single, setting, command would be received by the non-deterministic processor through its diagnostic port and passed to the deterministic processor through the inter processor interface.

In the manual diagnostic node the deterministic processor executes through whatever diagnostic values al e available at the inter-yrocessor interface, then returns to it's normal control loop. Allis could "override" a single control loop cycle if, for example, a single diagnostic value, such as a test point drive ]O value, is presented to the deterministic processor or, if a sequence of diagnostic Agues is presented to the deterministic processor, a number of control loop cycles may be overridden. In the automatic djacrrostiC mode a number of dia.g,rnostic values may be exchanged through the inter- processor interface in a period corresponding to a few control loop cycles, with a substantial number, on the order of at lest ten times as many, control loop cycles intervening between automatic diagnostic exchanges. Diagnostic nodes may be combined, for example, to produce an automatic active on-line diagnostic mode, for example. In an illustrative enbodirnent, a mass flow controller in accordance with the principles of the present invention operates on a one-millisecond control loop cycle, during' which it provides one percent of full-scale accuracy.

Keeping, Ike various diagnostic modes in mind, arid keeph in mind that processes illusLr.ted through the use of low charts may not be strictly linear processes and alternative Nazis n1ay be implemented within the scope of the invention, the diagnostic process will be described generally in re]aLion Lo steps 1220 tl-ough 1226. In step 1220 the deterministic processor acquires diagnostic values from the inter-processor interface. As previously noted, these values may be for the deterministic processor to use as control outputs or they may indicate data that is to be acquired by the deterministic processor, from a sensor, for example. Prom step l 220 the process proceeds to step 1292 where the deterministic processor processes the values acquired in step 12QO, by changing an outlet valve actuator drive signal or transferring a sensor reading to the interprocessor interface, for example.

From step 1222 the process proceeds to step 1224 where the deterministic processor determines whether it has completed its diagnostic tasks. If it leas not completed its diagnostic tasks, for example if it is operating in the automatic diagnostic mode and there are more values in a sequence of values to be retrieved frown the inter-processor interface, the process retunes to step 1202 and from there as previously described. If, in step 1224 the deterministic processor concludes that it has completed its diagnostic task, the process returns through connecting 10box B to step 1914 of Figure 12A. If the deterministic processor determines that it is not to operate in a diagnostic mode, the process proceeds from step 1718 Were processor Preforms functior.s such as routine bacl,rround operations, then proceeds to return tlrougl connecting block B to step 1214 and from there as previously described.

15The screen slots of Figures j3A through 13E illustrate a user interIdoe such as Nay be made assailable for access to a mass ilov controller in accordance with the principles of the present invention that includes a web server interface, such as the interface 608 of Figure 6. In an illustrative embodiment flee mass flow controller includes a web server, such as the server 602 of Figure 6. A user may employ the server locally, tl-ougl a local controller, or remotely, from a web- enalled device, such as the device 600 of Figure 6. In this namer, the shine user interface may be employed for both remote and local interactions with the mass flow controller. Detailed information regarding a mass floor controller, such as mode] number, range, and manufacturing,, setup parameters, may be displayed Lo a user and user- changeable setup pErraneters may be displayed as well. Different display techniques niay be employed If there are only a limited number of acceptable values, they may be displayed and chose from a pulldown menu, for e';anple. As previously described, a user, such as a technician can change set point values, open or close a valve, or monitor flow output, for example, through this interface Additionally, while the mass fiord controller is operating under a process control application, a user relay induce the server to plot arid log parameter values obtained from the mass flow controller.

The screen shot of Figure 13A illustrates the display a user may encounter when first accessing a mass flow controller in accordance with the principles of the present invention over the web The display prompts the user to choose a communications protocol through use of the pulldown window 300. The "query devices" link 1302 allows the user to initiate a process hereby his browser attempts to locate all devices that it recognizes.

Basic information may be do:vnloaded through the server. Information related to the mass f:lov controller are displayed in the screen of figure 13B. Such screens may be expanded or collapsed A user mast choose to view information related to a subset of the displayed mass flow controllers. Based on the model nunber, serial slumber and ir.tenally stored codes, product specifications for the mass flow controller are displayed along with user-selectable parameters, which may be displayed i', a list, for exa.mple. BY user may ernp]oy this screen to download calibration data to or from a mass flow controller arid to enter calibration tables. A user may also alter set points through this interface and monitor flee reported flow tl-ough the con espondinu mass flow controllers Additionally, a user may override settings and open or close a mass flow controller's outlet control valve. Each mass flow controller's speci:fications may be viewe::l, as illustrated by the screen of Figure 13C. Illustrative user-selectable parameters are displayed in ale screen shot of Figure 13D and calibration data such as a user nary download from a mass flow controller is illustrated in the screen sl1ot of Figure 13E.

A software implementation of the above descritcd embodiment(s) may comprise a series of computer instructions either fixed on a tangible medium, such as a computer readable media, em. diskette, CD-ROM, ROM, or fixed disc, or transnittable to a computer system, via a modem or other interface device, such as con1rnunications adapter corrected to the network over a medium.

Medium can be either a tangible medium including but not limited to, optical or analog communications lines, or may be implemer.ted with wireless techniques, including but not limited to nicrowae, infrared or other transmission techniques.

The series of computer instructions embodies all or part of the functionality previously described herein Title respect to the invention. Those sl;illed in the art will appreciate that such computer instructions can be written in a number of progrramming languages for use With many computer architectures or operating systems Further, such istructiors may be stored using any memory technology, present or future, including, but not limited to, semiconductor, magnetic, optical or other memory devices, or transmitted USiHg any communications teclmology, present or future, including but not linaiLed Lo optical, infrared, microwave, or other transmission tech1olDcries It is contemplated that such a computer program product may be distributed as a removable media with accompanying printed or electronic documentation, e gr shrink wrapped software, preloa. ded with a computer system, e g., on system ROM or fixed disc, or distributed from a server or electronic bulletin board over a network, e g., the Internet or World Wide Web Although various exemplary embodiments of the invention have been disclosed, it will be apparent to those skilled in the art that various changes and modifications can be ma.dc whicl1 will achieve some of the advantages of tle invention without departing from the scope of the invention. It will be apparent to those reasonably skilled in the art that other components perking the same functions may be suitably substituted. Further, the methods of the invention may be achieved in either all software implementations, using the appropriate object or processor instructions, or in hybrid implementations Blat utilize a combination of hardware logic, software logic and/or firmware to achieve the sank results. Processes illustrated through the use of flow charts may not be strictly linear processes and alternative flows may be implemented within the scope of the invention The specific configuration of logic and/or instructions utilized to achieve a particular function, as well asother modifications to the inventive concept are intended to be covered by the appended claims.

The foregoing description of specific embodiments of the invention 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 forms disclosed, and many modifications and variations are possible ill light of the above teachings. The enabodinents were chosen and described to best explain the principles of the invention and its practical application, and to thereby enable others skilled in the art to best utilize the invention. It is intended that the scope of the invention be limited only by the claims appended hereto.

Claims

1. A mass flow controller comprising: a mass flow sensor configured to produce a mass flow signal representative of a gas flow through the mass flow controller; an electronic controller configured to produce a closed loop control signal for an outlet valve based on said mass now signal, wherein said electronic controller comprises a dual-processor controller that is configured to provide a network interface that permits the execution of mass flow controller active diagnostics from a device connected to the network.

2. The mass flow controller of claim 1, wherein active diagnostics code operational with the mass flow controller's electronic controller includes code to enable the manipulation of operational mass flow controller signals from a device connected to the network.

3. The mass flow controller of claim 1, wherein active diagnostics code operational with the mass flow controller's electronic controller includes code to enable the manipulation of diagnostic mass flow controller signals from a device connected to the network.

4. ':[he mass flow controller of claim 1, wherein active diagnostics code operational with the mass flow controller's electronic controller includes code to enable the monitoring of operational mass flow controller signals from a device connected to the network.

5. The mass flow controller of claim 1, wherein active diagnostics code operational with the mass flow controller's electronic controller includes code to enable the monitoring of diagnostic mass flow controller signals from a device connected to the network.

6. The mass flow controller of claim 1, further comprising: on-line diagnostics code operational with the mass flow controller's electronic controller that enables on-line monitoring of mass flow controller signals from a device connected to the network.

7. The mass flow controller of claim 1, further comprising: on-line diagnostics code operational with the mass flow controller's electronic controller that enables on-line manipulation of mass flow controller signals from a device connected to the network.

8. The mass flow controller of claim 1, wherein said dual processors include a processor configured to operate in a deterministic mode and a processor configured to operate in a non-deterministic mode and the deterministic processor is configured to produce said closed loop control signal for an outlet valve.

9. The mass flow controller of claim 2, wherein the deterministic processor is configured to acquire one or more sensor readings.

10. The mass flow controller of claim 3, wherein the mass flow sensor is a thermal mass flow sensor, including a sensor bypass, configured to sense the Dow of fluid into the inlet of the controller and the mass flow controller further comprises a pressure sensor configured to sense the fluid pressure in the volume between the thermal mass flow sensor bypass arid the control valve, and said deterministic processor is configured to acquire a pressure signal produced by said pressure sensor and to compensate the sensed inlet flow rate sensed by said mass flow sensor using the pressure signal to thereby produce a compensated measure of the rate of fluid flow out of the controller.

11. The mass flow controller of claim 10, wherein the deterministic processor is configured to compute the time rate of change of pressure within the volume between the sensor bypass and the outlet control valve, and to use this time rate of change of pressure to produce the compensated measure of the rate of fluid flow out of the controller.

12. The mass flow controller of claim 1, wherein the network interface comprises a web server that enables a web-enabled device connected to the world wide web to execute on-line diagnostics from the web-enabled device.

13. The mass flow controller of claim 1, wherein the mass flow controller is self- calibrating.

14. A mass flow controller comprising: a thermal mass flow sensor, including a sensor bypass, configured to sense the flow of fluid into the inlet of the controller and to produce a mass flow signal representative of said flow; a pressure sensor configured to sense the fluid pressure in the volume between the thermal mass flow sensor bypass and the control valve; a display configured to display the pressure sensed by said pressure sensor; and an electronic controller configured to produce a closed loop control signal for an outlet valve based on said mass flow signal, wherein said electronic controller comprises a dual-processor controller that is configured to provide a network interface that permits the execution of mass flow controller active diagnostics from a device connected to the network.

15. The mass flow controller of claim 14, wherein active diagnostics code operational with the mass flow controller's electronic controller includes code to enable the manipulation of operational mass flow controller signals from a device connected to the network.

16. The mass flow controller of claim 14, wherein active diagnostics code operational with the mass flow controller's electronic controller includes code to enable the manipulation of diagnostic mass flow controller signals from a device connected to the network.

17. The mass flow controller of claim 14, wherein active diagnostics code operational with the mass flow controller's electronic controller includes code to enable the monitoring of operational mass flow controller signals from a device connected to the network.

18. The mass flow controller of claim 14, wherein active diagnostics code operational with the mass flow controller's electronic controller includes code to enable the monitoring of diagnostic mass flow controller signals from a device connected to the network.

19. The mass flow controller of claim 14, further comprising: on-line diagnostics code operational with the mass flow controller's electronic controller that enables on-line monitoring of mass flow controller signals from a device connected to the network.

20. The mass flow controller of claim 14, further comprising: on-line diagnostics code operational with the mass flow controller's electronic controller that enables on-line manipulation of mass flow controller signals from a device connected to the network.

21. The mass flow controller of claim 14, wherein said dual processors include a processor configured to operate in a deterministic mode and a processor configured to operate in a non-deterministic mode and the deterministic processor is configured to produce said closed loop control signal for an outlet valve.

22. The mass flow controller of claim 21, wherein the deterministic processor is configured to acquire one or more sensor readings.

23. The mass flow controller of claim 22 wherein the mass flow sensor is a thermal mass flow sensor, including a sensor bypass configured to sense the flow of fluid into the inlet of the controller and the mass flow controller further comprises a pressure sensor configured to sense the fluid pressure in the volume between the thermal mass flow sensor bypass and the control valve, and said deterministic processor is configured to acquire a pressure signal produced by said pressure sensor and to compensate the sensed inlet flow rate sensed by said mass flow sensor using the pressure signal to thereby produce a compensated measure of the rate of fluid flow out of the controller.

24. The mass flow controller of claim 23, wherein the deterministic processor is configured to compute the time rate of change of pressure within the volume between the sensor bypass and the outlet control valve, and to use this time rate of change of pressure to produce the compensated measure of the rate of fluid flow out of the controller.

25. The mass flow controller of claim 14, wherein the network interface comprises a web server that enables a web-enabled device connected to the world wide web to execute on-line diagnostics from the web-enabled device.

26. The mass flow controller of claim 14, wherein the mass flow controller is self- calibrating.

27. A mass flow controller comprising: a mass flow sensor configured to produce a mass flow signal representative of a gas flow through the mass flow controller, and an electronic controller configured to produce a closed loop control signal for an outlet valve based on said mass flow signal, wherein said electronic controller comprises a dual-processor controller.

28. The mass flow controller of claim 27, wherein said dual processors include a processor configured to operate in a deterministic mode and a processor configured to operate in a non-deterministic mode.

29. The mass flow controller of claim 28, wherein the deterministic processor is configured to produce said closed loop control signal for an outlet valve.

30. The mass flow controller of claim 28, wherein the deterministic processor is configured to acquire one or more sensor readings.

31. The mass flow controller of claim 30, wherein the deterministic processor is configured to acquire temperature readings from a temperature sensor.

32. The mass flow controller of claim 31, wherein the mass flow sensor is a thermal mass flow sensor, including a sensor bypass, configured to sense the flow of fluid into the inlet of the controller.

33. The mass flow sensor of claim 32, further comprising; a pressure sensor configured to sense the fluid pressure in the volume between the thermal mass flow sensor bypass and the control valve.

34. The mass flow controller of claim 33, wherein said deterministic processor is configured to acquire a pressure signal produced by said pressure sensor and to compensate the sensed inlet flow rate sensed by said mass flow sensor using the pressure signal to thereby produce a compensated measure of the rate of fluid flow out of the controller.

35. The mass flow controller of claim 34, wherein the deterministic processor is configured to compute the time rate of change of pressure within the volume between the sensor bypass and the outlet control valve, and to use this time rate of change of pressure to produce the compensated measure of the rate of fluid flow out of the controller.

36. The mass flow controller of claim 35, wherein the deterministic processor is configured to compare the compensated measure of the rate of fluid flow out of the controller to a set value and to adjust the outlet control valve to minimise the difference between the set value and the compensated measure of the rate of fluid flow out of the controller.

37. The mass flow controller of claim 36, wherein the deterministic processor is configured to compensate the controller's sensed inlet flow rate, Qi, by calculating the compensated sensed inlet flow rate Qo, according to: Qo = Qi - Cl(V/T)(dP/dt), where: Qo = the compensated sensed inlet flow rate, Qi = the sensed inlet flow rate, C1 = a normalizing constant, V = the volume between the sensor bypass and the outlet flow control valve, T= the temperature of the fluid within the volume, C1 is the resultant of the temperature at standard temperature and pressure divided by the pressure at standard temperature and pressure, and (dP/dt) = time rate of change of pressure within the volume.

38. The mass flow controller of claim 28, wherein the deterministic processor is a digital signal processor (DSP).

39. The mass flow controller of claim 28, further comprising one or more diagnostic outputs, wherein said deterministic processor is configured to drive at least one of said diagnostic outputs.

40. The mass flow controller of claim 28, further comprising one or more diagnostic inputs, wherein said deterministic processor is configured to read at least one of said diagnostic inputs.

41. The mass flow controller of claim 28, further comprising an interprocessor interface configured for communication between said deterministic and non-deterministic processors.

42. The mass flow controller of claim 41, wherein the inter-processor interface is a dual-ported memory with one or more locations arranged as mailboxes for the processors.

43. The mass flow controller of claim 28, wherein the non-deterministic processor is configured to provide a user interface to the mass flow controller.

44. The mass flow controller of claim 43, wherein said user interface includes a display.

45. The mass flow controller of claim 43, wherein said user interface includes an input device.

46. The mass flow controller of claim 28, wherein the non-deterministic processor is configured to provide a communications interface.

47. The mass flow controller of claim 46, wherein the communications interface is a Device Net communications interface.

48. The mass flow controller of claim 47, wherein the non-deterministic processor includes a network interface.

49. The mass flow controller of claim 48, wherein the network interface is an Ethernet network interface.

50. The mass flow controller of claim 48, wherein the network interface includes a web server.

51. The mass flow controller of claim 47, wherein the non-deterministic controller is configured to set up diagnostics through the network interfaces and exchange diagnostic information with the deterministic processor through the inter-processor interface and the deterministic processor is responsive to commands from the non-deterministic processor to perform diagnostic operations.

52. The mass flow controller of claim 51, wherein the deterministic processor is configured to run on-line diagnostics.

53. The mass flow controller of claim 51, wherein the network interface includes a web server and the web server is configured to set up said diagnostics.

54. The mass flow controller of claim 53 wherein the deterministic processor is configured to run on-line diagnostics.

55. The mass flow controller of claim 27 wherein the mass flow sensor is a thermal mass flow sensor, including a sensor bypass, configured to sense the flow of fluid into the inlet of the controller and further comprising: a mass flow calibrator operative to produce an electronic signal representative of mass flow in the mass flow controller independent of the mass flow sensor flow signal; and an electronic controller configured to correlate the mass flow signal from the thermal mass flow sensor to that of the mass flow calibrator.

56. The mass flow controller of claim 35 further comprising: a valve operative to control the flow of gas in the mass flow controller under control of the electronic controller.

57. The mass flow controller of claim 35 wherein the mass flow sensor is a thermal mass flow sensor.

58. The mass flow controller of claim 35, wherein the deterministic processor is configured to employ the correlated mass flow sensor signal to control the outlet valve during non-calibration operation.

59. A mass flow controller as in claim 35, wherein the mass flow calibrator comprises: a variable flow gas source; a receptacle of predetermined volume configured to receive gas from the variable flow gas source, the variable flow gas source configured to provide proportionate flow to the mass flow sensor and to the receptacle; and a pressure differentiator configured to produce an electronic signal representative of the time derivative of gas pressure within the receptacle of predetermined volume, said time derivative signal being proportional to the mass flow signal of the mass flow calibrator.

60. The mass flow controller of claim 59, wherein the gas flow source is configured to supply the same gas at the same flow rate to both the mass flow sensor and the receptacle of predetermined volume.

61. The mass flow controller of claim 59, wherein the differentiator includes: a pressure transducer configured to produce an electronic signal representative of the pressure within the receptacle; analog differentiator circuitry configured to produce an electronic signal that is representative of the time derivative of said electronic signal representative of the pressure within the receptacle; and an analog to digital converter configured to convert one or more values of the analog time derivative signal to digital samples of the time derivative.

62. The mass flow controller of claim 59, further comprising: storage for storing one or more samples of the signal representative of the time derivative of gas pressure.

63. A mass flow controller comprising: a mass flow sensor configured to produce a mass flow signal representative of a gas flow through the mass flow controller; an electronic controller configured to produce a closed loop control signal for an outlet valve based on said mass flow signal, wherein said electronic controller comprises a dual-processor controller that is configured to provide a web server that permits the execution of mass flow controller active diagnostics from a device connected to an interworking network employing active diagnostics code operational with the electronic controller.

64. The mass flow controller of claim 63, wherein active diagnostics code operational with the mass flow controller's electronic controller includes code to enable the manipulation of operational mass flow controller signals from a device connected to the interworking network.

65. The mass flow controller of claim 63, wherein active diagnostics code operational with the mass flow controller's electronic controller includes code to enable the manipulation of diagnostic mass flow controller signals from a device connected to the interworking network.

66. The mass flow controller of claim 63, wherein active diagnostics code operational with the mass flow controller's electronic controller includes code to enable the monitoring of operational mass flow controller signals from a device connected to the interworking network.

67. The mass flow controller of claim 63, wherein active diagnostics code operational with the mass flow controller's electronic controller includes code to enable the monitoring of diagnostic mass flow controller signals from a device connected to the interworking network.

68. The mass flow controller of claim 63, further comprising: on-line diagnostics code operational with the mass flow controller's electronic controller that enables on-line monitoring of mass flow controller signals from a device connected to the interworking network.

69. The mass flow controller of claim 63, further comprising: on line diagnostics code operational with the mass flow controller's electronic controller that enables on-line manipulation of mass flow controller signals from a device connected to the interworking network.

70. The mass flow controller of claim 63, wherein said dual processors include a processor configured to operate in a deterministic mode and a processor configured to operate in a non-deterministic mode and the deterministic processor is configured to produce said closed loop control signal for an outlet valve.

71. The mass flow controller of claim 70, wherein the deterministic processor is configured to acquire one or more sensor readings.

72. The mass flow controller of claim 71, wherein the mass flow sensor is a thermal mass flow sensor, including a sensor bypass, configured to sense the flow of fluid into the inlet of the controller and the mass flow controller further comprises a pressure sensor configured to sense the fluid pressure in the volume between the thermal mass flow sensor bypass and the control valve, and said deterministic processor is configured to acquire a pressure signal produced by said pressure sensor and to compensate the sensed inlet flow rate sensed by said mass flow sensor using the pressure signal to thereby produce a compensated measure of the rate of fluid flow out of the controller.

73. The mass flow controller of claim 72, wherein the deterministic processor is configured to compute the time rate of change of pressure within the volume between the sensor bypass and the outlet control valve, and to use this time rate of change of pressure to produce the compensated measure of the rate of fluid flow out of the controller.

74. The mass flow controller of claim 63, wherein the mass flow controller is self- calibrating.

75. A mass flow controller comprising: a mass flow sensor configured to produce a mass flow signal representative of a gas flow through the mass flow controller; and an electronic controller configured to produce a closed loop control signal for an outlet valve, based on said mass flow signal, wherein said electronic controller is also cony gured to provide an interface for on-line diagnostics.

76. The mass flow controller of claim 75, wherein said electronic controller is configured to provide a network interface that permits the execution of mass flow controller active diagnostics.

77. The mass flow controller of claim 76, wherein active diagnostics code operational with the mass flow controller's electronic controller includes code to enable the manipulation of operational signals.

78. The mass flow controller of claim 76, wherein active diagnostics code operational with the mass flow controller's electronic controller includes code to enable the manipulation of diagnostic signals.

79. The mass flow controller of claim 76, wherein active diagnostics code operational with the mass flow controller's electronic controller includes code to enable the monitoring of operational signals.

80. The mass flow controller of claim 76, wherein active diagnostics code operational with the mass flow controller's electronic controller includes code to enable the monitoring of diagnostic signals.

81. The mass flow controller of claim 76, further wherein on-line diagnostics code operational with the mass flow controller's electronic controller enables on-line monitoring of operational signals.

82. The mass flow controller of claim 76 wherein on-line diagnostics code operational with the mass flow controller's electronic controller enables on-line monitoring of diagnostic signals.

83. The mass flow controller of claim 76, wherein on-line diagnostics code operational with the mass flow controller's electronic controller enables on-line manipulation of diagnostic signals.

84. The mass flow controller of claim 76, wherein on-line diagnostics code operational with the mass flow controller's electronic controller enables on-line manipulation of operational signals.

85. A mass flow controller comprising: a mass flow sensor configuecd to produce a mass flow signal representative of a gas flow through the mass flow controller; an electronic controller configured to produce a closed loop control signal for an outlet valve based on said mass flow signal, wherein said electronic controller comprises a dual-processor controller that is configured to provide an interface for on-line diagnostics.

86. The mass flow controller of claim 85, wherein active diagnostics code operational with the mass flow controller's electronic controller includes code to enable the manipulation of operational mass flow controller signals.

87. The mass flow controller of claim 85, wherein active diagnostics code operational with the mass flow controller's electronic controller includes code to enable the manipulation of diagnostic mass flow controller signals.

88. The mass flow controller of claim 85, wherein active diagnostics code operational with the mass flow controller's electronic controller includes code to enable the monitoring of operational mass flow controller signals.

89. The mass flow controller of claim 85 wherein active diagnostics code operational with the mass flow controller's electronic controller includes code to enable the monitoring of diagnostic mass flow controller signals.

90. The mass flow controller of claim 85, wherein said dual processors include a processor configured to operate in a deterministic mode and a processor configured to operate in a non-deterministic mode and the deterministic processor is configured to produce said closed loop control signal for an outlet valve.

91. The mass flow controller of claim 90, wherein the deterministic processor is configured to acquire one or more sensor readings.

92. The mass flow controller of claim 91, wherein the mass flow sensor is a thermal mass flow sensor, including a sensor bypass, configured to sense the flow of fluid into the inlet of the controller and the mass flow controller further comprises a pressure sensor configured to sense the fluid pressure in the volume between the thermal mass flow sensor bypass and the control valve, and said deterministic processor is configured to acquire a pressure signal produced by said pressure sensor and to compensate the sensed inlet flow rate sensed by said mass flow sensor using the pressure signal to thereby produce a compensated measure of the rate of fluid flow out of the controller.

93. The mass flow controller of claim 92, wherein the deterministic processor is configured to compute the time rate of change of pressure within the volume between the sensor bypass and the outlet control valve, and to use this time rate of change of pressure to produce the compensated measure of the rate of fluid flow out of the controller.

94. The mass flow controller of claim 85, wherein the network interface comprises a web server that enables a web-enabled device connected to the world wide web to execute on-line diagnostics from the web-enabled device.

95. The mass flow controller of claim 85, wherein the mass flow controller is self- calibrating.