Abstract:
Precise control over gas delivery is achieved at the micro and nanobar mass levels by incorporating blocks of aligned carbon nanotubes into valves and finely adjusting the flow through the block by controlling a compressing force applied to the block. A valve for controlling gas flow includes: a valve housing; a block of aligned carbon nanotubes, the block and the valve housing being configured to direct the gas through the carbon nanotubes in the block; and a device configured to apply a force to the block in order to compress the block, wherein the block is compressed perpendicular to the walls of the carbon nanotubes in the block; whereby the application of the force to the walls restricts the flow of the gas through the valve. The valve may further comprise an electrical device for monitoring the electrical properties of the carbon nanotube block. This monitoring provides information on the state of compression of the carbon nanotube block and/or the gas that is flowing through the valve.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates generally to gas valves, and more particularly to gas valves comprising carbon nanotubes. 
       BACKGROUND OF THE INVENTION 
       [0002]    There are a multitude of different gas valves available for use in semiconductor process control. The following valves and related flow control devices are used in circumstances where the amount of gas to be delivered is very small (at the micro or even nano mass level). 
         [0003]    Vacuum leak valves (sometimes referred to as a type of vacuum throttle valve) are widely used for controlling the flow of gases at very low pressures. A typical leak valve comprises a knife edge seal and is actuated by controlling the pressure applied to a metal diaphragm. Commonly, the actuator is a finely threaded screw in the valve body which applies pressure to the metal diaphragm. (A variable leak valve of this type is available from MDC Vacuum Products, LLC as part number 315002.) Other vacuum leak valves comprise an optically flat sapphire that meets a captured metal gasket to form the seal. The pressure with which the sapphire is held against the gasket is controlled by a movable piston attached to a lever arm mechanism with a mechanical advantage of the order of 10,000 to 1. (A variable leak valve of this type is available from Varian, Inc. as part number 9515106.) The actuator may be manual or computer controlled. 
         [0004]    A mass flow controller (MFC) is a device used to measure and control the flow of gas using flow sensors and valves. Proportional valves are often used in MFCs. A proportional valve typically comprises a proportional solenoid or an on/off solenoid operated in a dithering mode. However, MFCs are only available for gas flow control down to the micro mass level, and not for nano mass flow control. 
         [0005]    A gas pressure regulator is a valve that automatically stops the flow of gas when a preset pressure is reached on the output side of the valve. A regulator is often installed between the gas source (such as a cylinder of compressed gas) and a MFC or a leak valve (which controls the flow of gas into a process chamber). 
         [0006]    For processes such as atomic layer deposition, a controlled pulsed delivery of precursor gases at the micro and nano mass levels will be advantageous. 
         [0007]    Therefore, there remains a need in the semiconductor industry for a means of precisely controlling the flow of gases and for delivering gas at micro and even nanobar mass levels. Furthermore, there remains a need for a means of providing a pulsed delivery of gases at micro and nanobar partial pressures. 
       SUMMARY OF THE INVENTION 
       [0008]    The concepts and methods of the invention allow for precise control of gas delivery at the micro and nanobar mass levels. This level of control over gas delivery is advantageous to semiconductor processes such as atomic layer deposition and to medical applications such as anesthetic delivery. Precise control over gas delivery is achieved by incorporating blocks of aligned carbon nanotubes into valves and finely adjusting the flow through the block by controlling a compressing force applied to the block. According to aspects of the invention, a valve for controlling gas flow includes: a valve housing; a block of aligned carbon nanotubes, the block and the valve housing being configured to direct the gas through the carbon nanotubes in the block; and a device configured to apply a force to the block in order to compress the block, wherein the block is compressed perpendicular to the walls of the carbon nanotubes in the block; whereby the application of the force to the walls restricts the flow of the gas through the valve. The device configured to apply a force to the block may comprise: parallel planar walls attached to the block, the walls being parallel to the long axes of the carbon nanotubes in the block; and a mechanical device, configured to apply a force to at least one of the walls, wherein the block is compressed perpendicular to the walls on the application of the force to the walls. 
         [0009]    Furthermore, the device configured to apply a force to the block may comprise: parallel planar electrodes attached to the block, the electrodes being parallel to the long axes of the carbon nanotubes in the block; and a voltage supply electrically connected to said parallel plates, said voltage supply being configured to apply a potential difference across said block, wherein said block is compressed perpendicular to the walls of the carbon nanotubes in said block on the application of said potential difference across said block. The valve may further comprise an electrical device for monitoring the electrical properties of the carbon nanotube block. This monitoring provides information on the state of compression of the carbon nanotube block and/or the gas that is flowing through the valve. 
         [0010]    According to further aspects of the invention, a method is provided for controlling gas flow including the following steps: maintaining gas at constant pressure on the intake side of a gas valve; restricting gas to flow through a block of aligned carbon nanotubes in the gas valve, wherein the direction of flow is parallel to the long axes of the carbon nanotubes in the block; and controlling a compressive force applied to the block, the compressive force being applied approximately perpendicularly to the long axes of the carbon nanotubes in the block, whereby the application of the force to the block restricts the flow of the gas through the valve. Furthermore, the compressive force may be a pulsed compressive force. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    These and other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures, wherein: 
           [0012]      FIG. 1A  shows a cross-section in a first plane of a first embodiment of the valve of the invention; 
           [0013]      FIG. 1B  shows a cross-section in a second plane of the valve of  FIG. 1A , where the second plane is perpendicular to the first plane; 
           [0014]      FIG. 2  shows a vise-type device of the invention, for compressing the carbon nanotube block in the valve; 
           [0015]      FIG. 3A  shows a cross-section in a first plane of a second embodiment of a valve of the invention; 
           [0016]      FIG. 3B  shows a cross-section in a second plane of the valve of  FIG. 3A , where the second plane is perpendicular to the first plane; 
           [0017]      FIG. 4  is a schematic diagram of a diagnostic circuit for use with the valve of the invention; 
           [0018]      FIG. 5  shows a schematic diagram for an electrostatically operated valve of the invention; and 
           [0019]      FIG. 6  shows a representation of a vacuum processing system incorporating a valve of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0020]    The present invention will now be described in detail with reference to the drawings, which are provided as illustrative examples of the invention so as to enable those skilled in the art to practice the invention. Notably, the figures and examples below are not meant to limit the scope of the present invention to a single embodiment, but other embodiments are possible by way of interchange of some or all of the described or illustrated elements. Moreover, where certain elements of the present invention can be partially or fully implemented using known components, only those portions of such known components that are necessary for an understanding of the present invention will be described, and detailed descriptions of other portions of such known components will be omitted so as not to obscure the invention. In the present specification, an embodiment showing a singular component should not be considered limiting; rather, the invention is intended to encompass other embodiments including a plurality of the same component, and vice-versa, unless explicitly stated otherwise herein. Moreover, applicants do not intend for any term in the specification or claims to be ascribed an uncommon or special meaning unless explicitly set forth as such. Further, the present invention encompasses present and future known equivalents to the known components referred to herein by way of illustration. 
         [0021]    In general, the present invention contemplates incorporating blocks of aligned carbon nanotubes into valves and finely adjusting the flow through the block by controlling a compressing force applied to the block. Although the examples provided herein are in the context of semiconductor processing using gases, there is no intention to limit the invention to devices and methods for semiconductor processing or gas flow control. For example, valves comprising blocks of aligned carbon nanotubes may be used to control delivery of medical anesthetics, and to control delivery of liquids. 
         [0022]    Carbon nanotubes are nanometer-scale cylinders with walls formed of graphene—single atom thick sheets of graphite. Nanotubes may be either single-walled (cylinder wall composed of a single sheet of graphene, referred to as SWNTs) or multi-walled (cylinder wall composed of multiple sheets of graphene, referred to as MWNTs). Nanotubes have diameters as small as one nanometer, for a SWNT, and length to diameter ratios of the order of 10 6 . Blocks of carbon nanotubes, where the nanotubes are aligned parallel to each other along their lengths, can be grown using chemical vapor deposition techniques. These blocks may have end areas of the order of 1 to 100 square millimeters, lengths (measured along the direction of the aligned nanotubes) of 0.2 to 4 millimeters, porosities of 87-92%, and contain millions of carbon nanotubes. See Cao et al., Super-compressible Foamlike Carbon Nanotube Films, Science vol. 310, 1307-1310 (2005). The center-to-center spacing of nanotubes in a block can be predetermined in the range of 10 to 200 nm by the spacing of catalyst particles. See Wei et al., Microfabrication Technology: Organized Assembly of Carbon Nanotubes, Nature vol. 416, 495 (2002); and Andrews et al., Continuous Production of Aligned Carbon Nanotubes: A Step Closer to Commercial Realization, Chem. Phys. Lett. vol. 303, 467-474 (1999). 
         [0023]    Nanotubes exhibit extraordinary mechanical properties, including supercompressibility (greater than 33%), and high bending and compressive strengths (14 GPa and 100 GPa, respectively, for MWNTs). See Saito et al., Physical Properties of Carbon Nanotubes, Imperial College Press, London (1998); Tombler et al., Nature vol. 405, 769 (2000); Cao et al., Science vol. 310, 1307 (2005); Qian et al., Appl. Mech. Rev. vol. 55, 495 (2002); Iijima et al., J. Chem. Phys. vol. 104, 2089 (1996); Sazonova et al., Nature vol. 431, 284 (2004); and Min-Feng et al., Science vol. 287, 637 (2000). These physical properties are advantageous for use of carbon nanotubes in valves. The present invention contemplates using blocks of SWNTs and/or MWNTs to controllably restrict flow through a valve. 
         [0024]    Furthermore, blocks of carbon nanotubes have electrical and mechanical properties that are advantageous in the use of carbon nanotube blocks for controlling gas flow. For example, the electrical properties of carbon nanotubes allow the presence of certain chemical species in the gas flowing through the block to be detected. Also, carbon nanotube blocks exhibit large changes in conductivity in response to strain (0.02 Siemens per centimeter change in conductivity per 1% change in compressive strain), which provides feedback to determine the extent to which the block is being compressed, and therefore the gas conductance of the block. See Suhr et al. Nature-Nano. vol. 2(7), 417 (2007). 
         [0025]    Nanotubes are also hydrophobic and the structural integrity of a nanotube block is unaffected by water. This is advantageous when nanotubes are used in gas delivery systems that may have residual moisture. 
         [0026]      FIG. 1A  shows a cross-section in a first plane of a first embodiment of the valve of the invention.  FIG. 1B  shows a cross-section in a second plane of the valve of  FIG. 1A , where the second plane is perpendicular to the first plane. The valve  100  comprises a housing  110 , a carbon nanotube block  120  and an actuator  130 . An actuator is a device which translates an input signal, such as an electrical signal, into motion. Here, the actuator  130  may be a manual actuator, such as a threaded screw or an electrical actuator such as a piezoelectric drive. The carbon nanotube block  120  is generally cuboid (a solid bounded by 6 rectangular faces) and is fixed in place by an adhesive material  122 . The adhesive material  122  also serves to provide a seal between the block  120  and the wall of the valve housing  110 , so that the gas flowing through the valve  100  must all pass through the block  120 . The adhesive material  122  may need to be: vacuum compatible; be able to withstand heating to 450° C.; and be sufficiently compliant to allow for significant compression of the block  120 . The carbon nanotubes in the block  120  are aligned with their long axes parallel to each other and the block is orientated in the valve housing so that the nanotubes are aligned with the flow of gas, indicated by arrows  140 . Parallel walls  124  are on opposite faces of the block  120 , the walls  124  are parallel to the alignment of the carbon nanotubes in the block  120 , and the walls  124  may be used to maintain the mechanical integrity of the block  120  and/or to make electrical contact to the block  120 . Some embodiments of the valve (not shown) do not comprise walls  124 . The actuator  130  is positioned between the wall of the valve housing  110  and the block  120 . When the actuator  130  is activated, a force  132  is applied to the block  120  to compress the block. (The force is transferred through the wall  122 ). When the block  120  is compressed the flow through the valve  100 , shown by arrows  140 , is reduced. Due to the supercompressibility of the block of carbon nanotubes, the flow through the block  120  can be varied substantially. Furthermore, due to the elasticity of carbon nanotubes the block  120  can be repeatedly compressed and yet recover its original shape. 
         [0027]    In  FIG. 1 , the actuator  130  may include a mechanical or electromechanical device for applying a compressive force to the carbon nanotube block  120 . It will be appreciated by those skilled in the art that there are many variations on the actuator  130 . For example, the actuator  130  may include a vise, as described below in reference to  FIG. 2 , a piston, and/or a piezoelectric drive. The compressive force may be applied directly to one side of the block  120 , as shown in  FIG. 1A , or to both sides of the cuboid block  120 . Furthermore, the actuator  130  may be manually operated or electrically operated. 
         [0028]      FIG. 2  shows a vise-type device of the invention, for compressing the carbon nanotube block in the valve. The valve  200  comprises a housing  210 , a carbon nanotube block  120  and a mechanical actuator  230 . The mechanical actuator  230  comprises a threaded bolt  232  with a rounded end  234  for pushing against the wall  224  and a control knob  236  for screwing the bolt into and out of the valve housing  210 . The valve housing  210  contains a circular threaded aperture for receiving the threaded bolt  232 . As the bolt  232  is screwed into the valve housing  210  the end  234  pushes against the wall  224  compressing the carbon nanotube block  120 . It will be appreciated by those skilled in the art that there are many variations on the mechanical actuator  230 , including the addition of a lever arm to provide mechanical advantage and the addition of a motor to screw the bolt into and out of the valve housing. 
         [0029]      FIG. 3A  shows a cross-section in a first plane of a second embodiment of a valve of the invention.  FIG. 3B  shows a cross-section in a second plane of the valve of  FIG. 3A , where the second plane is perpendicular to the first plane. The valve  300  comprises a carbon nanotube block  320  and a valve housing with a first part  312  and a second part  314 . The first and second parts are hollow cylinders with threaded ends which are configured to screw together and compress the block  320 . The force applied to the block is indicated by arrows  332  in  FIG. 3B  and is seen to be radially uniform. Arrow  316  shows the direction in which the second part  314  is turned in order to compress the lower end of first part  312 , and therefore the block  320 . The first part  312  has slits  313  at its lower end to allow the first part  312  to be screwed into the second part  314 . The block  320  is cylindrical in shape and is fixed in place by an adhesive material  322 . The adhesive material  322  also serves to provide a seal between the block  320  and the wall of the first part of the valve housing  312 , so that the gas flowing through the valve  300  must all pass through the block  320 . The carbon nanotubes in the block  320  are aligned with their long axes parallel to each other and the block is orientated in the valve housing so that the nanotubes are aligned with the flow of gas, indicated by arrows  340 . It will be appreciated by those skilled in the art that there are many variations of the valve housing shown in  FIGS. 3A and 3B , which will produce a radially uniform compressive force on the carbon nanotube block  320 . 
         [0030]      FIG. 4  is a schematic diagram of a diagnostic circuit  400  for use with the valve of the invention. Diagnostic circuit  400  comprises a cuboid carbon nanotube block  120 , conductive parallel plates  422  on opposing sides of the block, where the plane of the plates is parallel to the aligned carbon nanotubes in the block, and diagnostic electronics  450  electrically connected to the plates  422  for characterizing the electrical properties of the block  120 . The electrical properties of the block  120  may change due to adsorption and even chemical bonding of gas molecules on the surface of the carbon nanotubes. The electrical characteristics may be used to identify particular gas species. For example, the presence of NO 2  gas may be determined. See http://www.nasa.gov/centers/ames/research/technology-onepagers/gas_detection.html (last visited Jun. 26, 2008). Furthermore, the diagnostic circuit  400  can be used to monitor the conductivity of the carbon nanotube block  120 , which provides feedback to determine the extent to which the block is being compressed, and therefore the gas conductance of the block  120 . 
         [0031]      FIG. 5  shows a schematic diagram for an electrostatically operated valve  500  of the invention. The electrostatically operated valve  500  comprises a nanotube block  120 , conductive parallel plates  422  on opposing sides of the block, where the plane of the plates is parallel to the aligned carbon nanotubes in the block, and a voltage supply  560 . Depending on the electrical properties of the block  120 , insulating layers (not shown) may be required between the block  120  and the plates  422 . The voltage supply  560  may be a direct current supply or a pulse signal generator. The electrostatically operated valve works by building up opposite charges on the opposing plates  422 . The oppositely charged plates  422  attract and compress the block of carbon nanotubes  120  between them, thus reducing the flow of gas through the block  120 . The compressive force per unit area is determined roughly by the potential difference between the plates  422 , the spacing between the plates and the dielectric constant of the carbon nanotube block  120 . Reducing the potential difference between the plates  422  results in a reduction in the compressive force on the block  120 . Providing approximately  80 % strain is not exceeded, the block  120  will return to its original size when the compressive force is removed. It is expected that the block  120  will respond mechanically to the application of a voltage pulse within a few milliseconds. 
         [0032]    A further embodiment of the valve of  FIG. 5  includes a block  120  in which one or more walls of the block are coated with an insulating, piezoelectric polymer, such as polyvinylidene fluoride (PVDF), such that PVDF exists between one or more of plates  422  and block  120 . (The piezoelectric polymer layer is not shown in the figure.) When a potential difference is applied across the plates  422  the piezoelectric PVDF will contribute to the compressive force on the block  120 . Alternatively, the block may be infiltrated with PVDF polymer, such that there is sufficient PVDF to have a compressive effect when a potential difference is applied, and yet sufficient porosity of the block to allow for gas to flow through the block. 
         [0033]    Furthermore, the diagnostic circuit of  FIG. 4  may be readily integrated with valve  500 , as shown in  FIG. 5 . 
         [0034]      FIG. 6  shows a representation of a vacuum processing system  600  incorporating a valve of the invention. Gas flows from a gas source  670  through a pipe to the vacuum processing chamber  672 . The carbon nanotube valve  625  provides very accurate control of the flow of gas through the pipe into the chamber  672 . The chamber contains a substrate  674  held on a platen  676  and situated below a gas distributor  678 . The gas source  670  will generally include a gas regulator and the gas pressure at the inlet to the valve  625  will be held constant. Another valve (not shown) may also be included in series with the carbon nanotube valve  625  for shutting off the gas supply to the chamber  672 . (The function of the carbon nanotube valve  625  is to control gas flow, and not to act as a shut-off valve). 
         [0035]    The process chamber  672  in  FIG. 6  may be an atomic layer deposition chamber and the carbon nanotube valve  625  may be utilized to provide very fine control of a precursor gas as it is leaked into the process chamber  672  through the gas distributor  678 . Furthermore, the valve  625  may be operated in a pulsed mode to allow controlled pulses of precursor gas into the chamber  672 . It will be appreciated by those skilled in the art that there are many variations on the process system  600  shown in  FIG. 6 , including a system with multiple gas sources  670  controlled by multiple carbon nanotube valves  625 . 
         [0036]    Although the present invention has been particularly described with reference to the preferred embodiments thereof, it should be readily apparent to those of ordinary skill in the art that changes and modifications in the form and details may be made without departing from the spirit and scope of the invention. It is intended that the appended claims encompass such changes and modifications.