Method and apparatus for gas flow control

Abstract

A method and apparatus for self-calibrating control of gas flow. The gas flow rate is initially set by controlling, to a high degree of precision, the amount of opening of a flow restriction, where the design of the apparatus containing the flow restriction lends itself to achieving high precision. The gas flow rate is then measured by a pressure rate-of-drop upstream of the flow restriction, and the amount of flow restriction opening is adjusted, if need be, to obtain exactly the desired flow.

Claims

1. A gas flow control valve, comprising: a first body maintained static in space; a second body movably situated inside the first body; a flow restriction valve formed by a flow restriction surface provided on the first body and a complementary flow restriction surface formed on the second body; a lower flexure part extending from the second body and coupling the first body and the second body and a second flexure part positioned above the lower flexure part and coupling the first body and the second body, wherein the lower flexure part and the second flexure part limit relative motion between the first body and the second body, thereby permitting only uniaxial motion between the first body and the second body; a lever connected to the second body; an actuator provided between the lever and the first body, such that when the actuator expands, it raises the lever so as to raise the second body and elastically flex the lower and second flexure parts thereby inducing displacement between the first body and the second body; and, a displacement sensor installed to measure the displacement between the first body and the second body.

2. The gas flow control valve of claim 1, further comprising an annular extension machined to form a perfect seal between the flow restriction surface and the complementary flow restriction surface.

3. The gas flow control valve of claim 1, further comprising first cavity and a second cavity formed between the flow restriction surface and the complementary flat flow restriction surface, and wherein when the actuator is not energized, the flow restriction surface and the complementary flat flow restriction surface prevent fluid flow between the first cavity and second cavity.

4. The gas flow control valve of claim 1, further comprising inlet piping and outlet piping, and wherein the first cavity is coupled to the inlet piping and the second cavity is coupled to the outlet piping.

5. The gas flow control valve of claim 1, wherein the actuator is configured to induce displacement between the first body and the second body in a direction separating the flow restriction surface and the complementary flow restriction surface.

6. The gas flow control valve of claim 1, wherein the displacement sensor can measure linear displacements on the order of one nanometer.

7. The gas flow control valve of claim 1, wherein the displacement sensor is a capacitance position sensor.

8. The gas flow control valve of claim 1, further comprising a lookup table correlating required displacement and fluid flow through the flow control valve.

9. The gas flow control valve of claim 1, further comprising a closed loop control circuit formed with an output of the displacement sensor and action of the actuator.

10. The gas flow control valve of claim 1, further comprising a controller that measures output of the displacement sensor, and using values stored in a computer readable storage medium, determines an amount of flow restriction opening, and controls the linear actuator to move the second body until a value indicated by the displacement sensor is consistent with a desired opening.

11. The gas flow control valve of claim 10, wherein the controller executes proportional-integral-derivative control.

12. The gas flow control valve of claim 1, wherein when the actuator expands it causes deformation of the lower flexure part and the second flexure part, thereby causing uniaxial motion of the second body in a direction perpendicular to plane of the complimentary flow restriction surface.

13. The gas flow control valve of claim 1, wherein the actuator comprises a piezoelectric actuator.

14. The gas flow control valve of claim 1, further comprising a controller determining a required displacement using a lookup table that is predetermined by measuring gas flow rates for a wide range of values of input gas pressure, gas temperature, and displacement signal from the displacement sensor.

15. The gas flow control valve of claim 1, wherein the lower flexure part and second flexure part are configured to elastically deform when the actuator induces displacement between the first body and the second body.

16. The gas flow control valve of claim 1, wherein the displacement sensor measures the uniaxial motion with resolution of at least 1 nanometer.

17. The gas flow control valve of claim 1, wherein the actuator is configured to induce displacement between the first body and the second body with a resolution of at least 0.1 nanometer.

18. The gas flow control valve of claim 1, wherein the lower flexure part forms a seal shaped as a round disk.

19. The gas flow control valve of claim 1, wherein either the first or the second flexure part forms a seal.

20. The gas flow control valve of claim 1, wherein the actuator is provided between the lever and a top portion of the first body and the displacement sensor is installed on the first body.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The accompanying drawings, which are incorporated in and constitute a part of this specification, exemplify the embodiments of the present invention and, together with the description, serve to explain and illustrate principles of the invention. The drawings are intended to illustrate major features of the exemplary embodiments in a diagrammatic manner. The drawings are not intended to depict every feature of actual embodiments nor relative dimensions of the depicted elements, and are not drawn to scale.

(2) FIG. 1 is a simplified schematic diagram of an embodiment of an apparatus in accordance with the present invention for self-calibrating gas flow control.

(3) FIG. 2 is a simplified schematic diagram of an embodiment of an apparatus in accordance with the present invention for a high precision controllable flow restriction, while FIG. 2A illustrates an alternative embodiment.

(4) FIGS. 2B-2D illustrate examples for modifications of the embodiment of FIG. 2, while FIG. 2E illustrates an enlarged view of details of FIG. 2D.

(5) FIGS. 2F and 2G illustrate another embodiment of the invention.

(6) FIG. 3 is a simplified schematic diagram showing the details of the flow restriction.

(7) FIG. 4 is a simplified schematic diagram showing the embodiment of FIG. 2 with the flow restriction opened, where the amount of opening is designated by h.

(8) FIG. 5 is a graph showing the relationship between flow and the amount of flow restriction opening, h, for the specified inlet pressure and flow restriction radial dimensions.

(9) FIG. 6 is a simplified schematic diagram of an embodiment of an apparatus in accordance with the present invention for a high precision controllable flow restriction, including a processor and computer readable storage medium to control automatically the amount of flow restriction opening.

DETAILED DESCRIPTION

(10) Embodiments of the present invention provide for a controllable flow restriction in which the dimensions of the flow restriction are measurable and controllable to a very high degree of precision. The measurement and control of the dimensions are precise enough that they can be used to accomplish the self-calibrating gas-flow-control scheme shown in FIG. 1 with the flow accuracy required by the semiconductor industry.

(11) In various embodiments of the present invention, this level of precision is obtained by incorporating the following characteristics: 1. Uniaxial motion of the two opposing faces of the flow restriction, where transverse and/or rotational motion in the other two axes is limited to less than approximately 1 nm; 2. Measurement of motion in the uniaxial dimension to a precision of approximately 1 nm; 3. Actuation of motion with resolution of approximately 0.1 nm.

(12) An illustrative embodiment of the invention, shown in FIG. 2, consists of two adjacent bodies 201 and 202 with a planar contacting area that forms the flow restriction valve 211. That is, the first block 201 has a flow restriction surface 213 and the second block 202 has a complementary flow restriction surface 212. The flow restriction surface 213 of the first block 201 cooperates with the complementary flow restriction surface 212 of the second block 202 to thereby form flow restriction valve 211. In the embodiment of FIG. 2, the flow restriction valve 211 is formed by an annular extension 215 formed on the flow restriction surface 213 of block 201, thereby defining hole 216 (see, FIG. 3). Conversely, the complementary flow restriction surface 212 is machined to be flat so as to form a perfect seal when urged against the annular extension 215.

(13) The first body or block 201 is static in space, and the second body or block 202 is coupled to the first with a cantilever 203. The cantilever is positioned so that the motion of the second body with respect to the first at the planar contacting area is essentially uniaxial and very predictable and reproducible. The planar faces of the bodies are patterned to form two separate cavities 204 and 205 that are isolated from each other when the two bodies are contacting, but are coupled by a flow restriction valve 211 when the bodies are displaced from each other.

(14) In FIG. 2, bodies 201 and 202 are machined from a single piece of material, such as, e.g., stainless steel. Of course, any material that is compatible with the gas being used and allows reliable and repeatable flexure at the cantilever could be used. For example, alternative materials include other types of steel, Inconel, Hastelloy, etc. It is noted, however, that when made from one solid piece of material, it would be difficult to machine the valve surfaces. Therefore, FIG. 2A illustrates an alternative embodiment, wherein bodies 201 and 202 are fabricated from two pieces, so that the machining becomes very straightforward. Of course, the one or both bodies 201 and 202 may be made from more than one single piece. The most typical way to fasten the pieces together is with fastener 220, such as, e.g., bolts, but they could also be glued or welded together. The main requirement for the fastening is that there should be no movement at the location of the fastening, but it should allow for uniaxial movement at the flow restriction area. As both embodiments of FIGS. 2 and 2A would operate the same, the following description proceeds as applicable to either embodiment.

(15) An actuator 206 is installed in the first body 201 which acts on the second body 202 to induce displacement of the second body, and therefore change the flow restriction dimension. That is, as the actuator expands or contracts, it causes an elastic flexure in body 202 about the cantilever 203. This is similar to what is sometimes referred to as flexure bearing, wherein the motion is caused by elastic flexure or deformation of the material forming the flexure bearing. Since the motion is elastic deformation, it is very precise and controllable. Also, when relaxed, the apparatus inherently assumes its natural position due to the elastic nature of the deformation. The displacement sensor 207 is installed in the first body to measure this displacement. In one embodiment, this is accomplished using a capacitive measuring device, or displacement sensor, which can measure linear displacements on the order of one nanometer.

(16) A closed loop control circuit is formed with the output of the sensor 207 and the action of the actuator 206 to accomplish control of the flow restriction 211 dimensions, and consequently, the flow conductance coupling the two cavities. Piping 208 and 209 is incorporated into the system such that gas flow is directed through hole 218 into one cavity and out of the other cavity 205 through hole 219 to pipe 209, such that all flow must pass through the flow restriction valve 211 defined by the two bodies.

(17) By coupling the two bodies that form the flow restriction with a cantilever, as opposed to mechanical hinges or sliding assemblies, mechanical play and hysteresis are eliminated because friction sources are eliminated. Also, during actuation, there is negligible elastic deformation within the two bodies; elastic deformation is isolated to the cantilever coupling the two bodies. Both planes which define the flow restriction, therefore, are rigid.

(18) As depicted in FIGS. 2 and 2A, body 202 is positioned as close as possible to body 201, thus closing the flow restriction 211. According to one embodiment, the two bodies 201 and 202 are constructed such that when they are coupled together via the fastener, the two bodies are urged against each other so as to close the flow restriction 211. A flexible seal 210 is provided about the flow restriction 211, so as to prevent gas flow to the atmosphere. Also, according to one embodiment, the seal 210 is constructed such that it is in tension and serves to pull the two bodies 201 and 202 together to cause the flow restriction 211 to be closed. In this particular figure, the linear actuator 206, which is secured in body 201 and pushes against body 202, is in its relaxed state. When the linear actuator 206 is activated, it pushes against the tension of the flexible seal 210, moving body 202 away from body 201, forcing body 202 to pivot on the cantilever 203 and consequently allowing the flow restriction 211 to open up. It should be appreciated that the actuator 206 may be attached to body 202 and press against body 201.

(19) This flow restriction 211, as viewed from the top, forms a circle as shown in FIG. 3, where the inside dimension of the flow restriction 211 is r.sub.1 and the outside dimension is r.sub.2. Gas flow is from the inside of the flow restriction 211, across the flow restriction 211, to the outside of the flow restriction 211. That is, with reference to FIG. 4, gas flows from inlet piping 208, to cavity 204, to cavity 205 (when the flow restriction 211 is open) and, since it's blocked by seal 210, proceeds to outlet piping 209. Consequently, when the two bodies 201 and 202 are closed against each other as shown in FIG. 2, the flow restriction 211 is closed, and no gas can flow. When the linear actuator 206 is activated, the flow restriction 211 opens up, and gas can flow from the inlet 208 to the outlet 209. In general, the flow of gas will increase as the flow restriction opens.

(20) Since both body 201 and body 202 are rigid, and the only motion that can occur in the apparatus is flexure of the body at the cantilever 203, the movement of body 202 with respect to body 201 is very well defined. For small movements, where the opening of the flow restriction 211 is on the order of micrometers, which is much smaller than the distance between the flow restriction and the flexure, the movement of body 202 with respect to body 201 at the flow restriction will be essentially uniaxial in a direction perpendicular to the plane of the flow restriction 211. This well defined movement is critical for reproducible gas flow characteristics of the apparatus.

(21) As can be appreciated, the embodiment of FIG. 2 is provided only as one illustration and it may be varied without detracting from its effectiveness. For example, FIG. 2B illustrates an embodiment wherein one body, here body 201, includes the hole for the gas inlet, while the other body, here body 202, includes the hole for the outlet. Of course, the reverse can also be done with the same result. FIG. 2C illustrates an embodiment wherein the two bodies are not connected to each other. Rather, body 201 is anchored and does not move, while body 202 is anchored independently via a cantilever arrangement, such that it can be elastically flexured to control the opening of the flow restriction.

(22) Additionally, FIGS. 2D and 2E illustrate how the seal can be implemented such that it is also controlling the amount of flow through the valve. As shown in FIG. 2D and in the detail view of FIG. 2E, seal 210 is provided about cavities 204 and 205. Its periphery is fixedly attached to stationary body 201, while its central region is fixedly attached to flexure body 202. In this manner, when actuator 206 is actuated, it pulls on body 202, which in turn pulls on seal 210. Consequently, seal 210 elastically deforms such that it creates an opening of height h to enable gas to flow from cavity 204 to cavity 205.

(23) FIGS. 2F and 2G illustrate yet another embodiment of the invention, wherein FIG. 2F illustrates the closed, i.e., no flow condition, and FIG. 2G illustrates the open position. As shown, body 202 is joined to body 201 via flexures 221. In one embodiment, body 201 and body 202 are cylindrical and the flexure parts 221 are round disks extending from body 202 and may be machined from the same block as body 202 or maybe simply attached to body 202 by, e.g., welding. While other shapes are possible, circular shapes would provide uniform and balanced movement. In this embodiment, the lower flexure part 221 also functions as the seal 210, although it is clearly possible to provide a separate seal, such as with the other embodiments. Linear actuator 206 is provided between lever 240 and the top portion of body 201, such that when the actuator 206 expands, it raises the lever so as to raise body 202 and elastically flex the flexure parts of body 202, as illustrated in FIG. 2G. In the elevated position, the bottom surface of body 202, which forms the flow restriction surface, is raised a distance h from the complementary flow restriction surface of body 201, to thereby allow controlled fluid flow through the flow restriction valve 211. In this embodiment the two cylindrical flexures would limit relative motion between the bodies 201 and 202 to one degree of freedom (vertical), and would restrict rotation of the bodies with respect to each other in the plane of the page. This enables high accurate control of the fluid flow through the flow restriction 211.

(24) If we quantify the amount of flow restriction opening as h, as shown in FIG. 4, we can write the following equation for the flow of gas as a function of the opening, h:
Flow=2P.sub.in.sup.2h.sup.3/3RTln(r.sub.1/r.sub.2)Equation (1)
where P.sub.in is the pressure of the gas at the inlet 208 R is the universal gas constant=1.986 calories per mol per K T is the absolute temperature in K is the viscosity of the gas and h, r.sub.1, and r.sub.2 are the dimensions shown in FIGS. 3 and 4.

(25) For most gas flow applications, Equation (1), which describes laminar flow through the flow restriction, will provide a sufficiently accurate answer; however, for those cases where the downstream pressure, i.e., the pressure of the gas at the outlet 209, P.sub.out, is sufficiently high compared to the pressure, P.sub.in, at the inlet 208, the flow determined in Equation (1) must be multiplied by cos(arcsin(P.sub.out/P.sub.in).

(26) FIG. 5 shows the gas flow for an inlet pressure of 0.2 MPa (approximately 30 psi), absolute. One of the advantages of the configuration of the restriction is that the flow is a function of the cube of the restriction opening, h. This means that one order of magnitude change in the amount of flow restriction opening can control three orders of magnitude of flow, giving the apparatus a very large range of flow rate control.

(27) The linear actuator 206 can be of various types, such as a solenoid or piezoelectric actuator. A typical example is a piezoelectric actuator, part number P830.30, from Physik Instrumente, GmbH of Karlsruhe/Palmbach, Germany. The displacement sensor can also be of various types, such as a strain gauge or capacitance position sensor. A typical example is a capacitance position sensor, part number D510.050, also from Physik Instrumente.

(28) To be useful as a gas flow controller, the apparatus of FIG. 2 must have some means to control the amount of the flow restriction opening, h. FIG. 6 shows such an embodiment, with a controller 601 that measures the output of the displacement sensor, and using values stored in the computer readable storage medium, determines the amount of flow restriction opening, h. The controller then controls the linear actuator to move body 202 until the value indicated by the displacement sensor is consistent with the desired opening, i.e., the position set point. This control can be carried out with a standard control loop, such as a PID (proportional-integral-derivative) controller.

(29) As indicated by Equation (1), in addition to the known values of h, r.sub.1, and r.sub.2, effective control of the gas flow rate also requires that P.sub.in and T be known. The determination of these parameters can be carried out with the apparatus shown in FIG. 1. In this embodiment, the apparatus 600 of FIG. 6 is represented by the control valve 108 of FIG. 1. The controller 601 of FIG. 6 is part of the control valve 108 of FIG. 1 and represents a control loop that is nested within the control loop of controller 120 of FIG. 1.

(30) The controller 120 of FIG. 1 has stored within its computer readable storage medium the values that allow it to determine the required amount of flow restriction opening, h, that is necessary to obtain the desired flow rate for a given gas pressure and temperature. The determination of the required opening can be carried out using an equation such as Equation (1) or alternatively, using a lookup table that is determined ahead of time by measuring the gas flow rate for a wide range of values of P.sub.in, T, and h.

(31) The gas flow controller of FIG. 1 has a sufficient number of observable and controllable parameters to be able to perform self-diagnostics and self-calibration. Furthermore, these self-diagnostics and self-calibration can take place while the gas flow controller is delivering gas at a desired flow rate to a process chamber.

(32) As shown in FIG. 1, the apparatus comprises a gas line 101 having an inlet 103 in fluid communication with a gas source 104, and an outlet 105 in fluid communication with a process chamber (not shown). Under standard process conditions, the valve 106 would be open and gas would be flowing through the volume 110, through the control valve 108, and then ultimately into the process chamber.

(33) The volume 110 represents the total fixed volume between the valve 106 and the control valve 108. A pressure transducer 112 is configured to measure the pressure in this volume V 110. A temperature sensor 114 is positioned to measure the temperature of the components. In certain embodiments, the sensor 114 may be a specialized sensor in direct thermal communication with one or more components. In other embodiments, where the environment is temperature-controlled and it is not expected that the temperature will vary greatly from place to place or time to time, a thermometer positioned near the gas delivery system will provide sufficient information regarding the temperature of interest.

(34) The procedure for testing the flow of gas through the control valve 108 may be summarized as follows: 1. The control valve 108 is set to a desired flow rate, and a flow of gas is established. 2. The valve 106 is closed. 3. While the valve 106 is closed, the pressure is measured at regular periods, typically ranging from 1 to 100 milliseconds, by the pressure transducer. 4. After the pressure has dropped by some amount (typically 1-10% of the starting value), the valve 106 is opened, and the testing procedure concluded. 5. At some point during this measurement, the reading of the temperature sensor 114 is noted.
There is some amount of flexibility in the ordering of these steps; for example, steps 1 and 2 can be interchanged. Step 5 can be done at any time during the testing procedure.

(35) Some elaboration on the valve 106 is warranted. In its simplest form, valve 106 would be an on/off shutoff valve. A potential disadvantage of this type of valve is that in step 4, when the valve is opened, there will be a rapid rise in pressure inside the volume V 110. This rapid rise in pressure might make it difficult for the control valve 108 to change the amount of flow restriction opening sufficiently fast to keep a constant flow of gas flowing to the process chamber. A good alternative to the shutoff valve is a metering valve (as indicated in FIG. 1), which is a valve designed to provide varying gas flow rates over a range of settings. When metering valve 106 is opened at the end of the measurement period, the controller controls the amount of valve opening such that the rise in pressure, as determined with pressure transducer 112, is maintained at a certain rate that is sufficiently low so that the flow through the control valve 108 is not perturbed. In other words, the opening of metering valve 106 is performed gradually rather than abruptly, so that the gas flow is not perturbed. Alternatively, rather than raising the pressure at all during the process step, the pressure could be held constant at the end of the measurement period and then raised once the process step was terminated. This approach would have the least effect on any perturbation of the flow rate through the control valve 108.

(36) According to the ideal gas equation, the amount of gas in the volume V 110, is given by:
n=PV/RT,Equation (2)
where n=amount of gas (measured in moles) P=pressure measured by the pressure transducer V=volume of gas R=ideal gas constant=1.987 calories per mol per K T=absolute temperature in K.

(37) To some extent, all real gases are non-ideal. For these non-ideal gases, Equation (2) can be rewritten as:
n=PV/ZRT, whereEquation (3) Z=compressibility factor.

(38) The compressibility factor can be found in various handbooks or it can be determined from experimental measurements for any particular gas, and is a function of temperature and pressure.

(39) The flow rate of a gas can be written as the change in the amount of gas per unit time; i.e.:
flow rate=n/t,Equation (4) where t=time.

(40) Substituting into Equation (4) from Equation (3), yields:
flow rate=(P/t)V/ZRT.Equation (5)

(41) The first factor (P/t) is merely the slope of the pressure measurements as a function of time taken in step 3 of the procedure above. Thus, taking these pressure measurements in conjunction with the volume, temperature, and the compressibility factor, the actual rate of flow of the gas through the control valve 108 can be determined according to embodiments of the present invention, thus providing two independent measurements of the gas flow rate into the process chamber.

(42) One or more steps of the various embodiments of the present invention could be performed with manual or automatic operation. For example, the steps of opening/closing valves and taking pressure readings could be conducted automatically according to computer control. Alternatively, one or more of the various valves could be actuated manually, with the resulting flow rate calculated automatically from the detected pressure drop. Automatic operation of one or more steps could be accomplished based upon instructions stored in a computer readable storage medium, utilizing communication through control lines as indicated in FIG. 1.

(43) Another benefit of this measurement system is that if a discrepancy is found between the desired flow rate and the measured flow rate, the setting of the control valve 108 can be changed to correct for the discrepancy and provide the desired flow rate. This type of correction is particularly appropriate considering that the pressure rate-of-drop measurement provides a primary calibration standard. This correction can be done in the same process step or in a subsequent process step. This type of correction is greatly simplified if the system is under computer control.

(44) It should be understood that processes and techniques described herein are not inherently related to any particular apparatus and may be implemented by any suitable combination of components. Further, various types of general purpose devices may be used in accordance with the teachings described herein. It may also prove advantageous to construct specialized apparatus to perform the method steps described herein.

(45) The present invention has been described in relation to particular examples, which are intended in all respects to be illustrative rather than restrictive. Those skilled in the art will appreciate that many different combinations of hardware, software, and firmware will be suitable for practicing the present invention. Moreover, other implementations of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.