Patent Publication Number: US-6663713-B1

Title: Method and apparatus for forming a thin polymer layer on an integrated circuit structure

Description:
This application is a continuation-in-part of U.S. patent application Ser. No. 08/583,888, filed Jan. 8, 1991 now U.S. Pat. No. 5,958,510. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to a method and apparatus for forming a thin polymer layer on a substrate. More particularly, this invention relates to a method and apparatus for depositing a layer of a polymeric or polymerizable material having a low dielectric constant, such as parylene, on or between metal layers on a substrate containing an integrated circuit structure. 
     BACKGROUND OF THE INVENTION 
     In the construction of integrated circuit structures, device geometries are constantly shrinking, resulting in an increase in parasitic capacitance between devices. Parasitic capacitance between metal interconnects on the same or adjacent layers in the circuit can result in crosstalk between the metal lines or interconnects and in a reduction of the response time. Lowering the parasitic capacitance between metal interconnects separated by dielectric material can be accomplished by either increasing the thickness of the dielectric material or by lowering the dielectric constant of the dielectric material. Increasing the thickness of the dielectric materials is, however, contrary to the goal of reducing device and structure geometries. 
     As a result, to reduce the parasitic capacitance between metal interconnects on the same or adjacent layers, one must change the material used between the metal lines or interconnects to a material having a lower dielectric constant than that of the materials currently used, i.e., silicon dioxide (SiO 2 ). 
     Jeng et al. in “A Planarized Multilevel Interconnect Scheme with Embedded Low-Dielectric-Constant Polymers for Sub-Quarter-Micron Applications”, published in the Journal of Vacuum and Technology in June 1995, describes the use of a low dielectric constant polymeric material, such as parylene, as a substitute for silicon dioxide (SiO 2 ) between tightly spaced conductor lines or other strategically important areas of an integrated circuit structure. Parylene, a generic name for thermoplastic polymers and copolymers based on p-xylylene and substituted p-xylylene monomers, has been shown to possess suitable physical, chemical, electrical, and thermal properties for use in integrated circuits. Formation and deposition of such polymers by the initial decomposition of a stable dimer, followed by deposition and polymerization of the resulting reactive monomer, is discussed by Ashok K. Sharma in “Parylene-C at Subambient Temperatures”, published in the Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 26, at pages 2953-2971 (1988). Properties of such polymeric materials, including their low dielectric constants, are further discussed by R. Olson in “Xylylene Polymers”, published in the Encyclopedia of Polymer Science and Engineering, Volume 17, Second Edition, at pages 990-1024 (1989). 
     However, changing the dielectric material in the construction of integrated circuit structures from conventional silicon dioxide (SiO 2 ) to polymeric materials cannot be accomplished using conventional methods and apparatus used to form SiO 2  dielectric materials between adjacent metal lines or interconnects on a single layer or between adjacent layers of metal interconnects because the polymer deposition process involves many additional variables which must be controlled. 
     As a result, there is a need in the art for a controllable process and corresponding apparatus for depositing a polymer on a substrate such as an integrated circuit. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method and apparatus for forming a polymer layer between metal interconnects and between layers of metal interconnects on a substrate. In particular, an apparatus and method is provided for the deposition of polymeric or polymerizable material preferably having a dielectric constant lower than that of silicon dioxide. The method and apparatus specifically provide for continuous introduction of a reactive xylylene monomer and optionally a carrier gas into a deposition chamber operated at a total pressure from 30 milliTor to 5 Torr such as a chemical vapor deposition (CVD) chamber and condensation of the monomer onto a work piece to form a parylene layer thereon. A carrier gas is preferably bubbled through a vaporizer for vaporizing a xylylene compound and the combined carrier gas and xylylene compound are passed through a furnace to decompose the xylylene into a reactive monomer. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of a polymer deposition apparatus of the present invention; 
     FIG. 2 is a partially sectioned view of a portion of the apparatus of FIG. 1, showing a vaporizer, a decomposition chamber, and a manifold for feeding reactive vapors into the deposition chamber; 
     FIG. 3 is a horizontal cross-sectional view of the decomposition chamber shown in FIGS. 1 and 2, showing the hollow tubes placed within the decomposition chamber to increase the surface area in contact with the vapors and gases passing through the chamber; 
     FIG. 4 is a vertical cross-sectional view of an alternate structure for the decomposition chamber of FIGS. 1 and 2, using a series of discs with non-aligned holes therein; 
     FIG. 5 is a top view of one of the discs shown in FIG. 4, showing the misalignment of the opening therein with the opening of an underlying disc; 
     FIG. 6 is a cross-sectional diagram of an exemplary CVD processing chamber used according to one embodiment of the present invention; 
     FIG. 7 is a diagram of the system monitor of the exemplary CVD processing chamber of FIG. 6; 
     FIG. 8 is a flowchart of a process control computer program product used in conjunction with the exemplary CVD processing chamber of FIG. 6; 
     FIG. 9 is a top view of the wafer support mechanism of FIG. 6; 
     FIG. 10 is a vertical cross-sectional view of a portion of the apparatus generally shown in FIG. 1, showing the processing of gases/vapors exiting from the processing chamber; 
     FIG. 11 is a flow sheet illustrating a process according to one embodiment of the present invention; and 
     FIG. 12 is a schematic view of a carrier gas delivery system for transporting a polymerizable material from a vaporizer to a deposition chamber. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention generally comprises a method and apparatus for forming thin polymer films on the surface of a work piece. A multi-purpose processing chamber is provided wherein vapor deposition of a polymer onto a substrate, plasma etching and in situ plasma cleaning can be accomplished. The method and apparatus of the present invention is computer controllable and can be utilized in a computer controlled multi-chamber integrated processing system. 
     The present invention further provides a method and apparatus for forming a polymer layer between metal interconnects and between layers of metal interconnects on a substrate. In particular, an apparatus and method is provided for the deposition of polymeric or polymerizable material preferably having a dielectric constant lower than that of silicon dioxide. The apparatus of the invention may be used in connection with deposition (and etching) on a substrate of any polymeric or polymerizable dielectric material, including, for example, xylylene, tetrafluoroethylene, polytetrafluoroethylene, naphthalene, or polynaphthalene. It is preferred that the polymer have a dielectric constant lower than silicon dioxide as well as suitable physical, chemical, electrical and thermal properties for use on integrated circuits in accordance with the present invention. In one embodiment, the method and apparatus specifically provide for continuous introduction of p-xylylene and a carrier gas into a processing chamber operated at a total pressure from 30 mmtorr to 5 Torr such as a CVD or plasma etch chamber, and condensation of reactive p-xylylene onto a work piece to form a parylene layer thereon having a thickness from 0.05 micron to 150 microns. 
     The apparatus specifically comprises a deposition chamber for depositing a thin polymer layer on an object, and a vaporizer for vaporizing a polymerizable material, the vaporizer optionally having bubbling means for bubbling a carrier gas through a non-vaporized polymerizable material in the vaporizer and pumping means for pumping the carrier gas and vaporized polymerizable material from the vaporizer into the deposition chamber. 
     The deposition chamber generally comprises a gas inlet for providing reactive material into the chamber, a gas outlet for exhausting the chamber and controlling the pressure within the chamber and a movable substrate support member disposed in the lower portion of the chamber to position a substrate relative to the gas inlet. The chamber preferably includes a DC bias for generating an E field within the chamber to enhance the deposition rate of the polymer and a RF bias to facilitate generation of a plasma to assist in thermal cracking of the polymerizable material, and to enable the chamber to be utilized in etching and other fabrication processes. A magnet assembly provides a B field at the surface of a substrate within the chamber which is also believed to enhance the deposition rate of the polymerizable material. 
     The method and apparatus preferably include vaporization or sublimation of a monomer such as the stable dimer of p-xylylene, and, when appropriate, the pyrolytic conversion of the stable dimer into reactive p-xylylene monomer. The method and apparatus also include vaporization of comonomers and derivatives of the p-xylylene monomers. 
     The apparatus and method further include the distribution of the reactive monomer into the deposition chamber, cooling the substrate down to an acceptable deposition temperature, heating the walls of the deposition chamber to inhibit accumulation of residues thereon, and controlling the pressure and residence time of the gaseous reactants in the chamber. The apparatus and method may further include recapturing undeposited vapors, such as unreacted monomeric vapors, exiting the deposition chamber. 
     The apparatus is preferably incorporated into a computer controlled multi-chamber integrated processing system wherein etching of the substrate and in situ plasma cleaning can be accomplished in the same deposition chamber. An RF bias provided in the chamber enables in situ plasma cleaning, enhanced thermal cracking of the precursor polymerizable material provided in the chamber, enhanced heating of the chamber walls, and reduces the degree of polymerization occurring in the gas phase. 
     It is contemplated that in the present invention a continuous supply of reactive polymerizable material may be provided into the chamber through the gas inlet. An inert carrier gas such as helium or argon is preferably used to supply the reactive polymerizable material into the chamber. This inert gas and the RF bias may be used to form a plasma within the processing chamber in some applications. These and other aspects of the present invention will be described in more detail below. 
     As used herein, the term “parylene” is the generic name for thermoplastic polymers or copolymers based on p-xylylene (CH 2 C 6 H 4 CH 2 ) or derivatives of p-xylylene. The non-substituted p-xylylene polymers have the formula: 
     
       
         —(CH 2 —C 6 H 4 —CH 2 —) n — 
       
     
     wherein n is the number of monomer units in a molecule, preferably n averages about 5,000 giving the parylene a preferred number average molecular weight of about 500,000. The polymer grows by addition of monomer on both ends and the parylene molecules have end groups which are not easily identified given the large molecular weight. It is believed that the end groups have no influence on properties. The term “parylene” also includes chlorinated or fluorinated forms of the parylene polymers produced by halogenating the monomers or the polymers. 
     The typical starting material for making parylene polymers is a stable cyclic dimer, di-p-xylylene, or halogenated derivative, which is available in solid form. The dimer must be vaporized or sublimed, and then decomposed to the reactive monomeric for the polymerization to proceed. The dimer is commercially available from companies such as Union Carbide. Usually the solid dimer is available in particulate form, e.g., in powder form, for ease of handling. However, it is contemplated by the present invention that dimer pellets may be used in conjunction with a packed bed or that the solid precursor material may be liquefied or dissolved in a carrier fluid to facilitate continuous delivery of the dimer. 
     Referring to FIG. 1, in one embodiment, a vaporizer  10  is provided to heat and vaporize or sublime a monomer such as di-p-xylylene or substituted di-p-xylylene. A heated pressure gauge (not shown), such as one available from Baratron, is located in the vaporizer to monitor the pressure of the vaporizer to insure that a continuous feed of particulate solid or liquid dimer is provided to the vaporizer  10 . The pressure gauge is preferably heated so that material will not deposit on the gauge and render the gauge inoperable. 
     The vaporized dimer, such as di-p-xylylene, or optional mixture of vaporized dimer and a carrier gas, then passes from vaporizer  10  through a gate valve  20  to a pyrolysis or decomposition chamber  30  where the vaporized dimer is at least partially decomposed to a reactive monomer, such as p-xylylene. It should be recognized that when the starting polymerizable material is a monomer or oligomer that does not require vaporization or decomposition to produce a reactive species, then the vaporization and decomposition chambers may be removed or bypassed. 
     Referring now to FIGS. 1 and 2, one embodiment of a vaporization chamber or vaporizer  10  is shown heating the starting material to vaporize or sublime a liquid or solid polymerizable material before introducing it into the deposition chamber or blending it with another monomer. Vaporizer  10  may comprise a metal cylinder comprising stainless steel or aluminum, having metal flanges  12   a  and  12   b  thereon. Metal flange  12   a  has a cover  14  thereon which is provided with a gas inlet port  16  to permit the flow of a non-reactive gas into vaporizer  10 , as will be described below. Flange  12   b , which comprises the exit port of vaporizer  10 , is bolted to a matching flange  22   a  of gate valve  20  which separates vaporizer  10  from decomposition chamber  30 , as will be described below. 
     Within vaporizer  10 , as shown in FIG. 2, is a containment vessel  18  for placement of polymerizable starting material, such as di-p-xylylene. Containment vessel  18 , which may rest on the inner surface of the vaporizer  10 , is made of a non-reactive material, usually a ceramic material, and preferably comprises quartz. As an option, containment vessel  18  may be further provided with a number of openings (not shown) in the upper half of the vessel to facilitate flow of heated gases into and out of containment vessel  18  to assist in the vaporization of the solid p-xylylene dimer and entrainment of the dimer vapors in the carrier gas flow. 
     The pressure in vaporizer  10  may be maintained at atmospheric pressure. However , the entire apparatus (vaporization, decomposition, and deposition chambers) is preferably maintained at a pressure from 30 milliTorr to about 5 Torr. For non-substituted di-p-xylylene, the pressure will preferably range from about 100 milliTorr to about 1 Torr. For other monomers and polymers, the total pressure will range preferably from Torr to about 5 Torr. The increase in total pressure up to 5 Torr increases the deposition rate of the polymers and allows better control of the amount of monomer or polymer that is provided to the deposition chamber. The carrier gas can be any inert gas, preferably helium, argon, or nitrogen, most preferably helium. 
     Referring to FIGS. 1 and 2, a vaporizer  10  may be heated by any convenient means such as, for example, a heating coil  15  which may be wrapped around the vaporizer  10  to heat the same. The heating coil, in turn, is connected to an external electrical power source  11 , which is adjustable to provide sufficient heat to vaporizer chamber  10  to heat it to the vaporization temperature of the polymerizable material therein, but below a temperature at which the material will decompose into the reactive monomer. An external heat controller, such as a Watlow 965 Temperature Controller, may be used in connection with the heating coil to maintain the desired temperature. 
     The temperature of vaporizer  10 , when operated within the previously described pressure ranges, will usually vary from a minimum temperature below which the material will not vaporize, at the pressure required for deposition, up to a maximum temperature below the temperature at which the vaporized material will decompose, at the operative pressure. While the operating temperature of the vaporizer will vary according to the material to be vaporized, the temperature is preferably maintained between about 100° C. and about 200° C. 
     Non-reactive carrier gases such as nitrogen, argon, or helium are optionally introduced into the vaporizer  10  through gas inlet port  16  in cover  14 , and are then heated by the heat radiating or being conducted from the vaporizer  10  to thereby vaporize the monomer which then leaves chamber  10  and passes through gate valve  20  to decomposition chamber  30 . However, it should be recognized that the process may be carried out using only the vaporized reactant, e.g., parylene dimer, without the use of an additional gas such as a carrier gas. 
     An alternative embodiment for the vaporizer  10  is shown in FIG. 12 wherein a thermostatic oven  306  heats the vaporizer  10  which contains, for example, non-vaporized di-p-xylylene and vaporized di-p-xylylene. The carrier gas is passed through a flow controller  302  such as a metering pump or a needle valve and is bubbled through the non-vaporized di-p-xylylene in the vaporizer  10 . The combined carrier gas and vaporized di-p-xylylene are then passed to the decomposition chamber  30  and a deposition chamber  60  by a rough pump  150 . A mass flow meter (not shown) can be placed anywhere between the vaporizer  10  and the deposition chamber  60  to measure the total mass leaving the vaporizer  10 . The flow rate of di-p-xylylene can then be calculated by subtracting the mass of carrier gas sent to the vaporizer  10  from the mass of material leaving the vaporizer  10 . 
     A first valve  20  following the vaporizer  10  may be manually operated, but preferably will be automatically operated and connected to a valve controller  21  which will sense the temperature and pressure in vaporization chamber  10  and open valve  20  only after vaporization chamber  10  has reached a temperature at which the polymerizable material will vaporize so that gases flowing from vaporization chamber  10  through the first valve  20  will contain vaporized polymerizable material, as well as the optional non-reactive carrier gases flowing through vaporization chamber  10 . A second flange,  22   b , connects first valve  20  to a first flange  24   a  on a conduit  26  having a second flange  24   b  at its opposite end connected to a first flange  32   a  of decomposition chamber  30 . 
     Flanged metal conduit  26 , is preferably heated by an external heater such as heating tape wrapped around conduit  26  to maintain the reactive monomer at a temperature sufficiently high so that it will not begin to polymerize. Typically, this will be a temperature of at least about 120° C. 
     The gas/vapor flow containing the active monomer then passes out of decomposition chamber  30  to a tee  44  having flanges  42   a-c  where the vapors are optionally blended with a comonomer in vaporized form from conduit  46  (shown in FIG.  1 ). The vaporized monomer and optional comonomer then flow through a second gate valve  40  having flanges  40   a ,  40   b  to a conduit  48  which connects valve  40  with an entrance port  50  to a substrate processing chamber  60  where the monomer deposits and polymerizes on an object therein, such as a semiconductor substrate, which is preferably temperature controlled by a support member  180  which is connected to a chiller  184 . Where further vaporization and/or decomposition of the polymerizable material is not necessary, polymerizable material is introduced at tee  44  for direct communication to the chamber  60  and the vaporizer  10  and decomposition chamber  30  may be eliminated. The decomposition chamber  30  will typically be needed when a carrier gas is present. 
     It is preferred that the walls of deposition chamber  60  be maintained at a sufficiently high temperature to prevent deposition and polymerization of the vaporized polymerizable material. In one embodiment, the chamber wall temperature is maintained by a heater  70 , under the control of heater controller  71 . The remaining gas/vapor mixture then passes from deposition chamber  60  through a throttle valve  80 , under the control of valve controller  81 , which regulates the pressure in chamber  60 , and then passes through a cold trap  90  connected to a chiller  100 . The remaining gases then pass through a gate valve  120 , controlled by valve controller  121 , to a rough pump  150 . It is anticipated that the chamber walls may be heated by any other heating means, including the use of a plasma generated within the chamber itself. 
     When it is desired to use chamber  60  for etching or when in situ cleaning of chamber  60  is performed, the gases from optional cold trap  90  may instead be routed through gate valve  110  (with gate valve  120  shut off), controlled by valve controller  111 , to turbo pump  130  and then through isolation valve  140 , controlled by valve controller  141 , to rough pump  150 . During deposition in chamber  60 , valves  110  and  140  are typically shut. 
     In one embodiment, the apparatus may be provided with an RF generator  61  which is coupled to chamber  60  through an RF network  63  to permit generation of a plasma within chamber  60 . The plasma may be used to enhance the decomposition of stable precursors by generating enough heat to convert the stable dimer into the reactive species. The plasma may also provide sufficient heating of the chamber walls to prevent polymerization thereon and/or sufficient heating of the process gases to prevent polymerization in the gas phase. In addition, the RF generator enables integration of the chamber so that either etching of the substrate or in situ cleaning of chamber  60  can be performed. 
     It is contemplated that the chamber may include an electric bias to provide an electric field to enhance the deposition rate of the polymerizable material onto the substrate. Preferably, the chamber walls are grounded and a DC bias is applied to the substrate support member  180 . In addition to the electric field, a B field may be created in the chamber by placement of magnetic field coils or permanent magnets within or about the walls of the chamber. It is also believed that both the E field and the B field enhance the deposition rate of the polymerizable material onto the substrate because the reactive polymerizable material may be polarized. It is theorized that the E field and/or the B field assist in the polymerization process by aligning the reactive material so that the polymerization reaction is facilitated. 
     It should be noted that, in accordance with the invention, the substrate  200  is preferably processed in a closed system. Thus, after the deposition, the substrate may be removed from the deposition chamber  60 , placed in a storage area in a load lock chamber between processing chambers, and allowed to reach ambient temperature. This permits the substrate to reach ambient temperature without either exposing the substrate to the ambient atmosphere during the warmup (which would result in unacceptable moisture pickup), or leaving the coated substrate in the deposition chamber to warm up (which would reduce the throughput rate in the parylene deposition chamber). Alternatively, the substrate may be moved from the load lock chamber to other chambers for subsequent processing without removing the substrate from the vacuum environment, e.g., moved to another chamber for PECVD formation of a silicon dioxide (SiO 2 ) film thereon. 
     When decomposition of the vaporized polymerizable starting material is necessary to form a reactive monomer, such as when using di-p-xylylene, the vapors from the vaporization chamber are preferably sent to a decomposition chamber. While the decomposition chamber may be constructed in many ways, it is preferred that the chamber have a large surface area to heat the vaporized material rapidly and evenly. In one embodiment, the decomposition chamber  30  comprises a metal cylinder wall  32  terminating, at one end, in first flange  32   a  through which it is connected, via conduit  26 , to first gate valve  20 , which is used to control the flow of the vapors of the dimer entering the decomposition chamber  30 . The inner surface of metal cylinder  32  may be optionally lined with quartz, as shown at  33 , to avoid contamination of the reactive p-xylylene vapors at the temperature used to decompose the dimer. 
     Surrounding metal cylinder  32  is a cylindrical ceramic furnace  34 , having heater wires  202  embedded therein to heat cylinder  32 . An outer layer of perforated metal  204  may optionally surround ceramic furnace  34  to serve both as a shield to avoid contact, by the operator, with the hot ceramic furnace, as well as to permit the flow of air between the metal shield and ceramic furnace  34 . 
     The heater wires of the ceramic furnace  34  are connected to an external power supply, temperature controller  31 , and computer control means  340  to maintain a temperature between 400° C. and about 900° C., preferably above 700° C. A temperature above 400° C. and preferably above about 700° C., is necessary to assure sufficient decomposition of the stable dimer into the reactive monomer, while the maximum temperature should not exceed about 900° C. to avoid decomposition of the monomer formed in the decomposition chamber  30 . It should again be recognized that the decomposition temperature will vary according to the dimer material being used. 
     It is preferred that the decomposition chamber  30  decompose a sufficient amount of the dimer during its passage through cylinder  32  to form the reactive monomer to prevent the deposition of unwanted particles on the substrate surface or the formation of lumps in the deposited coating. Dimer that has not been decomposed will not polymerize, and may, therefore, cause lumps in the coating as it deposits on the substrate, cause unwanted particles on the surface, or pass through the deposition chamber and clog the cold trap mechanism  90  located downstream of deposition chamber  60  ahead of rough vacuum pump  150 . 
     It should be noted that the use of first gate valve  20  and second gate valve  40 , both preferably computer controlled, allows both the vaporizer  10  and decomposition chamber  30  to be isolated from deposition chamber  60  while the substrate is removed from the deposition chamber. That is, the whole system need not be shut down during movement of a substrate into the deposition chamber to be coated, or out of the deposition chamber after the deposition. The operation of the entire system, including the heater power sources  11 ,  31 , valve controllers  21 , 41 ,  81 ,  111 ,  121 ,  141 , heater controller  71 , and chiller controllers  101 ,  181 , is preferably controlled by computer control means  340 . 
     To ensure a high level of decomposition of the stable dimer vapors, it is preferred that the dimer vapor be sufficiently heated in the decomposition chamber  30 . This can be accomplished either by increasing the surface area of cylinder  32  in decomposition chamber  30  in contact with the vaporized dimer, or by extending the residence time of the vaporized dimer in decomposition chamber  30 , or by a combination of both. Typically, the residence time of the vaporized dimer in the decomposition chamber is 1-5 minutes. These operations also can all be controlled by control means  340 . To enhance decomposition of the dimer to reactive monomer, a plasma may be established in the deposition chamber to provide sufficient heat to decompose any stable precursor material into reactive material for subsequent deposition and polymerization on a substrate. 
     Referring now to FIG. 3, a cross-sectional view of the preferred decomposition chamber of FIG. 2, shows that metal cylinder  32  has been loaded or packed with a series of hollow tubes  36 , each having its axis parallel to the center axis of metal cylinder  32 . Tubes  36  are packed sufficiently tightly within cylinder  32  so as to be in thermal communication with one another so that each tube  36  is at the temperature of the wall of cylinder  32 . The presence of tubes  36  within cylinder  32  therefore serve to greatly increase the surface area within cylinder  32  in contact with the vaporized monomer. Thus, the vaporized dimer is channeled through or in between tubes  36  so that the vaporized dimer is uniformly heated to the decomposition temperature, thus maximizing the amount of the dimer decomposed to the reactive monomer. Preferably, each hollow tube  36  is made of quartz, or at least has quartz surfaces thereon. Preferably when the inner diameter (ID) of cylinder  32  ranges from about 1.5 to about 2 inches, small tubes  36  will each have an outer diameter (OD) of from about 0.3 to about 0.5 inches, and a wall thickness of about 0.050 inches, resulting in an ID of from about 0.2 to about 0.4 inches. 
     As an alternative or supplement, as mentioned above, the residence time of the vaporized dimer within decomposition chamber  30  may also be increased. Extension of the residence time in the decomposition chamber may be provided by regulating the flow rate of vaporized dimer into decomposition chamber  30 , such as by regulating the flow of carrier gas into vaporization chamber  10 ; or by throttling gate valves  20  and  40 ; or by a combination of such valve throttling and carrier gas flow rate control, under the control of control means  340 . The residence time can also be controlled by the length of decomposition chamber  30 , i.e., by lengthening metal cylinder  32 , and/or by increasing the mean path length within cylinder  32  by the placement of flow redirecting elements within cylinder  32 , as will be described below. 
     Referring now to FIGS. 4 and 5, another embodiment of a decomposition chamber is shown comprised of a series of circular discs  38   a-d  placed in spaced apart relationship in cylinder  32 , with the plane of each disc perpendicular to the center axis of cylinder  32 . Each disc is provided with an opening through which the carrier gas/vaporized dimer mixture flows. However, as best seen in FIG. 4, openings in adjacent discs are deliberately misaligned to extend the length of travel of the gaseous mixture through cylinder  32 . Thus, opening  39   b  in disc  38   b  shown in FIG. 4 is axially misaligned to the underlying opening  39   a  in disc  38   a  beneath disc  38   b . This embodiment actually acts to both increase the residence time (by increasing the length of the path of flow), and also to increase the contact area within cylinder  32  which comes in contact with the gaseous mixture, since the surface of each disc  38   a-d , which will be at the same temperature as cylinder  32 , will also be in contact with the gaseous mixture. 
     As further shown in FIGS. 1 and 2, the gases flowing from decomposition chamber  30  pass into a metal tee  44 , wherein the reactive monomer gases may be optionally blended with other copolymerizable materials, i.e., monomers or dimers with which the reactive parylene monomers will react and polymerize in the deposition chamber, as will be described below. A second flange  32   b  on the opposite end of cylinder  32  is connected to a first flange  42   a  of tee  44  to provide the connection between decomposition chamber  30  and tee  44 . A second flange  42   b  of tee  44  is secured to a first flange  40   a  on second gate valve  40 , which is operated by valve controller  41  and computer control means  340  to control the flow of vapors into deposition chamber  60 , as will be described below. 
     Third flange  42   c  of tee  44  is either secured to a flange (not shown) of conduit  46  (shown in FIG. 1) leading to a vaporized source of a copolymer to be mixed in tee  44 , with the vaporized parylene monomer or, as shown in FIG. 2, a cap or cover  47  may be secured to flange  42   c  when no separate source of copolymer vapors are mixed with the reactive monomer from the decomposition chamber. 
     Tee  44 , like flanged metal conduit  26 , is preferably heated by an external heater such as heating tape wrapped around tee  44  to maintain the reactive monomer at a temperature sufficiently high so that it will not begin to polymerize. Usually this will comprise a temperature of at least about 150° C. When a copolymerizable source is used, a second vaporization chamber, similar to the previously described vaporization chamber  10 , can be used to vaporize the copolymerizable material. If necessary, further apparatus forming a decomposition chamber similar to previously described decomposition chamber  30  may also be used. In either case, the apparatus used to provide such a copolymerizable material in gaseous form may then be connected to flange  42   c  of tee  44  to thereby permit the respective gaseous reactive copolymerization sources to blend together in tee  44  prior to introduction into deposition chamber  60 . 
     In a further embodiment, other polymerizable gases, such as monomers, dimers, and other oligimers, etc., may be blended with the gaseous p-xylylene monomer in heated tee  44  if it is desired to form and deposit a copolymer on the surface of the substrate in the deposition chamber. Such other polymerizable materials might include, for example, a vinyl biphenyl monomer, such as available from the Lancaster Synthesis Company. One reason for copolymerizing the reactive p-xylylene monomer with such a monomer is to increase the temperature stability of the film to be formed on the substrate during the deposition from about 400° C. (polymerized p-xylylene alone) up to between about 500° C. and 550° C. for the copolymer. The amount of copolymerized monomer blended with the gaseous flow of p-xylylene reactive monomer and carrier gas may range from about 5% by wt. to about 25% by wt. of the total mixture of monomers, but preferably will range from about 5% by wt. to about 15% by wt., with the typical amount of copolymerizable monomer added usually comprising about 10% by wt. of the monomer mixture total. 
     Referring again to FIGS. 1 and 2, tee  44  is connected to second gate valve  40 , through flanges  42   b  and  40   a , respectively, and second gate valve  40  is connected to a further heated conduit  48  through flange  40   b  on gate valve  40  and flange  48   a  on conduit  48 . As described above, with respect to conduit  26  and tee  44 , conduit  48  is preferably heated, for example by heating tape, to avoid condensation therein. Heated conduit  48  is, in turn, connected via flange  48   b  to entrance port  50  of processing chamber  60 . 
     Processing chamber  60 , may be a standard commercially available vacuum processing chamber, with some modifications as will be described below. Such a vacuum processing chamber is available from Applied Materials, Inc., such as an oxide etch chamber, which may be connected, via a load lock mechanism to other vacuum or closed chambers in a semiconductor substrate processing apparatus, such as the multi-chambered 5000 Series processing apparatus available from Applied Materials, Inc., without exposing the substrate to the atmosphere. Such an apparatus is disclosed and described in Maydan et al. U.S. Pat. No. 4,951,601, the disclosure of which is hereby incorporated by reference. For parylene deposition, the apparatus is maintained or a temperature above the polymerization temperature of the reactive parylene monomer, i.e., at a temperature above 200° C., but below a temperature at which further decomposition of the reactive monomer might occur, i.e., at a temperature below about 750° C. Typically, the temperature of the apparatus will be maintained within a range of from about 200° C. and about 300° C. 
     One suitable CVD processing chamber in which the method of the present invention can be carried out is shown in FIG. 6, which is a vertical, cross-sectional view of a simplified, parallel plate chemical vapor deposition processing chamber  60  having a vacuum chamber  62 . Chamber  60  contains a gas distribution manifold  52  for dispersing process gases through perforated holes in the manifold to a wafer (not shown) that rests on a substrate support member  180 . Generally, the process gases supply lines for each of the process gases include (I) safety shut-off valves (not shown) that can be used to automatically or manually shut-off the flow of process gas into the chamber, and (ii) mass flow controllers (also not shown) that measure the flow of gas through the gas supply lines. When toxic gases are used in the process, the several safety shut-off valves are positioned on each gas supply line in conventional configurations. 
     Substrate support member  180  is highly thermally responsive and is mounted on support fingers  54  so that support member  180  (and the wafer supported on the upper surface of support member  180 ) can be controllably moved between a lower loading/off-loading position and an upper processing position  64 , which is closely adjacent to manifold  52 . A centerboard (not shown) includes sensors for providing information about the position of the wafers. 
     When support member  180  and the wafer are in processing position  64 , they are surrounded by a baffle plate  160  having a plurality of spaced holes  162  which exhaust into an annular vacuum manifold  164 . During processing, gas inlet to manifold  52  is uniformly distributed radially across the surface of the wafer as indicated by arrows  166 . An exhaust system then exhausts the gas via ports  162  into the circular vacuum manifold  164  by a vacuum pump system shown in FIG.  1 . Before reaching manifold  52 , deposition and carrier gases are input through the gas lines  168  into a mixing system  170  where they are combined and then sent to manifold  52 . 
     Referring now to substrate support member  180 , the movable substrate support member may need to be heated or cooled during the process. Where the desired polymer is to be formed on the substrate such as parylene, the substrate support  180  should be maintained at a temperature below the condensation temperature of the monomer, e.g. for p-xylylene the substrate support  180  should not exceed about 40° C., and preferably within a range of from about −40° C. to about +25° C., using chiller  184 , under control of chiller controller  181 . When the gaseous mixture contacts the cooled surface of, for example, semiconductor substrate  200 , polymerization of the reactive parylene monomers commences, as well as copolymerization with other reactive polymerizable materials (if present) resulting in the formation of the desired dielectric film of parylene or parylene copolymer on the surface of the substrate, e.g., on the surface of semiconductor wafer  200 . 
     A temperature control system may be provided to cool substrate support  180 , or support  180  may be provided with a hollow passageway through which a fluid may be circulated, for example, from chiller  184 , under control of chiller controller  181 , to control the temperature of substrate support  180  and a substrate mounted thereon. The substrate may be retained on substrate support  180  by any conventional substrate retention means such as a bipolar or monopolar electrostatic chuck  210 , portions of which are shown in FIG. 9. A backside gas such as helium is preferably flown through channels provided in the upper surface of the electrostatic chuck to facilitate heat transfer between the substrate support member  180  and a substrate located thereon for processing and to prevent deposition of the reactive materials onto the edge and backside of the substrate. 
     When desired, heat can be distributed to the substrate support  180  by an external lamp module  260  that provides a collimated annular pattern of light  270  through a quartz window  280 . 
     The deposition process performed in chamber  60  can be either a thermal process or plasma enhanced thermal process. In a plasma process, a controlled plasma is formed adjacent to the wafer by RF energy applied to distribution manifold  52  from RF power supply  61  (with substrate support member  180  grounded). Gas distribution manifold  52  is also an RF electrode, while substrate support member  180  is grounded. RF power supply  61  can supply either single or mixed frequency RF power to manifold  52  to enhance the decomposition of reactive species introduced into chamber  60 . A mixed frequency RF power supply typically supplies power at a high RF frequency (RF 1 ) of 13.56 MHZ. And at a low RF frequency (RF 2 ) of 360 KHZ. 
     Typically, any or all of the chamber lining, gas inlet manifold faceplate, support fingers  54 , and various other reactor hardware is made out of material such as aluminum or anodized aluminum. An example of such a CVD apparatus is described in U.S. Pat. No. 5,000,113, entitled “Thermal CVD/PECVD Reactor and Use for Thermal Chemical Vapor Deposition of Silicon Dioxide and In-situ Multi-step Planarized Process,” issued to Wang et al, and assigned to Applied materials, Inc., the assignee of the present invention. 
     Gas mixing system  170  and RF power supply  61  are controlled by the computer control means  340  over control lines  360 . The chamber includes analog assemblies such as mass flow controllers (MFCs), RF generators, and lamp magnet drivers that are controlled by the control means  340  which executes system control software stored in a memory  380 , which, in the preferred embodiment, is a hard disk drive. 
     Now referring to FIG. 1, after the mixture of vaporized gases and optional carrier gases flow into chamber  60  a parylene polymer, for example, is deposited on the surface of substrate  200  by condensation and polymerization of the reactive p-xylylene monomers. The remainder of the optional carrier gases, and unreacted monomer vapors, then pass out of chamber  60  through an exit port  66  (see FIG. 10) and then through a throttle valve  80  to a cold trap  90 . The purpose of throttle valve  80  is to maintain the desired pressure within chamber  60 . The deposition/polymerization reaction is usually carried out while maintaining a pressure within deposition chamber  60  of from about 30 milliTorr (mmTorr) to 5 Torr. When the monomer is non-substituted p-xylylene, the pressure is maintained between 30 mTorr and 1 Torr since a pressure above about 1 Torr will result in deposition of a low crystallinity film, including unreacted monomer. When the pressure in deposition chamber  60  deviates from the set pressure, throttle valve  80 , which is connected to a pressure sensor, either opens to cause the pressure to drop, or closes to cause the pressure to rise. 
     Now referring to FIG. 10, throttle valve  80  may be modified, if desired, to permit a nonreactive gas, e.g., argon, helium, or nitrogen, to be added to the gaseous stream flowing from chamber  60  through throttle valve  80  to cold trap  90 . Typically, this additional gas flow into cold trap  90  will comprise a flow of about  50  standard cubic centimeters per minute (sccm), depending on the chamber volume. This flow will also be under the control of controller  340 . The purpose of the added non-reactive gases is to control and slow down the flow of the gaseous stream of carrier gas and reactive monomer through deposition chamber  60 , i.e., to increase the residence time, to permit more complete extraction of the heat from the gaseous stream flowing through chamber  60  and to provide for more complete reaction of the polymerization, i.e., to further minimize the amount of unreacted polymerizable material leaving chamber  60  via exit port  66  which must be extracted in cold trap  90 . 
     The vapors and gases passing through throttle valve  80  then enter cold trap  90  which, in turn, is connected to a vacuum pump  150  (see FIG. 1) which is capable of maintaining chamber  60  at subatmospheric pressure. It is important, however, that unreacted monomer and other copolymerizable materials not enter vacuum pump  150 , but rather be removed from the gas stream in cold trap  90 . Cold trap  90  may comprise any conventional commercial cold trap, such as, for example, a standard Norcal cold trap, which is connected to the downstream side of throttle valve  80  to trap and remove any monomers or polymers from the gas stream. 
     Connected to the downstream side of cold trap  90  is gate valve  120  through which the remaining gases in the gas stream pass to rough vacuum pump  150  to maintain the desired low pressure. As shown in FIGS. 1 and 10, cold trap  90  is also connected through gate valve  110  to a turbo pump  130  and then through an isolation valve  140  to rough vacuum pump  150 . When chamber  60  is used as a deposition chamber, such as for the previously discussed polymeric deposition of reactive p-xylylene monomer, valves  110  and  140  are shut and valve  120  is opened to connect rough vacuum pump directly to cold trap  90 . However, if the same chamber is to be used as a plasma etch chamber or for any other processing requiring high vacuum, such as for in situ plasma cleaning of the chamber, as previously discussed, gate valve  120  may be shut off and both gate valve  110  and isolation valve  140  opened to place high vacuum turbo pump  130  in the stream between cold trap  90  and rough vacuum pump  150 . 
     To clean the chamber following deposition of reactive monomer, ozone is flown into the chamber at a rate of 1000 sccm. It is believed that the reactive ozone reacts with the parylene to facilitate removal of the parylene from the chamber. In addition to ozone, oxygen can be introduced into the chamber at a rate of 100-1000 sccm and an RF bias of 750-1200 watts applied to the support member to effectuate cleaning of the chamber. It is believed that the oxygen reacts with the parylene in a manner similar to the reaction of ozone with parylene. 
     The computer control means  340  controls all of the activities of the CVD chamber and a preferred embodiment of the control means  340  includes a hard disk drive, a floppy disk drive, and a card rack. The card rack contains a single board computer (SBC), analog and digital input/output boards, interface boards and stepper motor controller boards. The system controller conforms to the Versa Modular Europeans (VME) standard which defines board, card cage, and connector dimensions and types. The VME standard also defines the bus structure having a 16-bit data bus and 24-bit address bus. 
     The control means  340  operates under the control of a computer program stored on the hard disk drive  380 . The computer program dictates the timing, mixture of gases, RF power levels, substrate support member, and other parameters of a particular process. The interface between a user and the system controller is a via a CRT monitor  342  and light pen  344  which is depicted in FIG.  7 . In the preferred embodiment two monitors  342  are used, one mounted in the clean room wall for the operators and the other behind the wall for the service technicians. Both monitors  342  simultaneously display the same information, but only one light pen  344  is enabled. The lightpen  344  detects light emitted by CRT display with a light sensor in the tip of the pen. To select a particular screen or function, the operator touches a designated area of the display screen and pushes the button on the pen  344 . The touched area changes its highlighted color, or a new menu or screen is displayed, confirming communication between the light pen and the display screen. 
     The process can be implemented using a computer program product  400  that runs on, for example, the computer control means  340 . The computer program code can be written in any conventional computer readable programming language such as for example 68000 assembly language, C, C++, or Pascal. Suitable program code is entered into a single file, or multiple files, using a conventional text editor, and stored or embodied in a computer usable medium, such as a memory system of the computer. If the entered code text is in a high level language, the code is compiled, and the resultant compiler code is then linked with an object code of precompiled windows library routines. To execute the linked compiled object code, the system user invokes the object code, causing the computer system to load the code in memory, from which the CPU reads the executes the code to perform the tasks identified in the program. 
     FIG. 8 shows an illustrative block diagram of the hierarchical control structure of the computer program  400 . A user enters a process set number and process chamber number into a process selector subroutine  420  in response to menus or screens displayed on the CRT monitor by using the lightpen interface. The process sets are predetermined sets of process parameters necessary to carry out specified processes, and are identified by predefined set numbers. The process selector subroutine  420  identifies (I) the desired process chamber, and (ii) the desired set of process parameters needed to operate the process chamber for performing the desired process. The process parameters for performing a specific process relate to process conditions such as, for example, process gas composition and flow rates, temperature, pressure, plasma conditions such as RF bias power levels and magnetic field power levels, cooling gas pressure, and chamber wall temperature and are provided to the user in the form of a recipe. The parameters specified by he recipe are entered utilizing the lightpen/CRT monitor interface. 
     The signals for monitoring the process are provided by the analog input and digital input boards of the control means  340  and the signals for controlling the process are output on the analog output and digital output boards of the control means  340 . 
     A process sequencer subroutine  430  comprises program code for accepting the identified process chamber and set of process parameters from the process selector subroutine  420 , and for controlling operation of the various process chambers. Multiple users can enter process set numbers and process chamber numbers, or a user can enter multiple process set numbers and process chamber numbers, so the sequencer subroutine  430  operates to schedule the selected processes in the desired sequence. Preferably the sequencer subroutine  430  includes a program code to perform the steps of (I) monitoring the operation of the process chambers to determine if the chambers are being used, (ii) determining what processes are being carried out in the chambers being used, and (iii) executing the desired process based on availability of a process chamber and type of process to be carried out. Conventional methods of monitoring the process chambers can be used, such a polling. When scheduling which process is to be executed, the sequencer subroutine  430  can be designed to take into consideration the present condition of the process chamber being used in comparison with the desired process conditions for a selected process, or the “age” of each particular user entered request, or any other relevant factor a system programmer desires to include for determining scheduling priorities. 
     Once the sequencer subroutine  430  determines which process chamber and process set combination is going to be executed next, the sequencer subroutine  430  causes execution of the process set by passing the particular process set parameters to one of several chamber manager subroutines  440  which controls multiple processing tasks in a process chamber  60  according to the process set determined by the sequencer subroutine  430 . The chamber manager subroutine  440  controls execution of various chamber component subroutines which control operation of the chamber components necessary to carry out the selected process set including the vaporizer  10  decomposition chamber  30 , and cold trap  90 . Examples of chamber component subroutines are vaporizer control subroutine  450 , process gas control subroutine  460 , pressure control subroutine  470 , heater control subroutine  480 , and decomposition control subroutine  490 . Those having ordinary skill in the art would readily recognize that other chamber control subroutines can be included depending on what processes are desired. In operation, the chamber manager subroutine  440  selectively schedules or calls the process components subroutines in accordance with the particular process set being executed. The chamber manager subroutine  440  schedules the process component subroutines similarly to how the process sequencer  430  schedules which process equipment and process set is to be executed next. Typically, the chamber manager subroutine  440  includes steps of monitoring the various chamber components, determining which components needs to be operated based on the process parameters for the process set to be executed, and causing execution of a chamber component subroutine responsive to the monitoring and determining steps. 
     The process gas control subroutine  460  has program code for controlling process gas composition and flow rates. The process gas control subroutine  460  controls the open/close position of the safety shut-off valves, and also ramps up/down the mass flow controllers to obtain the desired gas flow rate. The process gas control subroutine  460  is invoked by the chamber manager subroutine  440 , as are all chamber component subroutines, and receives from the chamber manager subroutine process parameters related to the desired gas flow rates. Typically, the process gas control subroutine  460  operates by opening the gas supply lines, and repeatedly (I) reading the necessary mass flow controllers, (ii) comparing the readings to the desired flow rates received from the chamber manager subroutine  440 , and (iii) adjusting the flow rates of the gas supply lines as necessary. Furthermore, the process gas control subroutine  460  includes steps for monitoring the gas flow rates for unsafe rates, and activating the safety shut-off valves when an unsafe condition is detected. 
     In some processes, an inert gas such as argon is flowed into the chamber  60  to stabilize the pressure in the chamber before reactive process gases are introduced into the chamber. For these processes, the process gas control subroutine  460  is programmed to include steps for flowing the inert gas into the chamber  60  for an amount of time necessary to stabilize the pressure in the chamber, and then the steps described above would be carried out. Additionally, when the process gas is to be generated in the vaporizer  10 , for example di-p-xylylene, the process gas control subroutine  460  can be written to obtain the carrier flow from the vaporizer control subroutine  450 . 
     The pressure control subroutine  470  comprises program code for controlling the pressure in the chamber  60  by regulating the size of the opening of the throttle valve  80  in the exhaust system of the chamber. The size of the opening of the throttle valve  80  is set to control the chamber pressure to the desired level in relation to the total process gas flow, size of the process chamber, and pumping set point pressure for the exhaust system. When the pressure control subroutine  470  is invoked, the desired, or target, pressure level is received as a parameter from the chamber manager subroutine  440 . The pressure control subroutine  470  operates to measure the pressure in the chamber  60  by reading one or more conventional pressure manometers connected to the chamber, compare the measure value(s) to the target pressure, obtain PID (proportional, integral, and differential) values from the stored pressure table corresponding to the target pressure, and adjust the throttle valve  80  according to the PID values obtained from the pressure table. Alternatively, the pressure control subroutine  470  can be written to open or close the throttle valve  80  to a particular opening size to regulate the chamber  60  to the desired pressure. 
     The heater control subroutine  480  comprises program code for controlling the temperature of the chamber  60 . The heater control subroutine  480  is invoked by the chamber manager subroutine  440  and receives a target, or set point, temperature parameter. The heater control subroutine  480  measures the temperature by measuring voltage output of thermocouple located in the chamber  60 , compares the measured temperature to the set point temperature, and increases or decreases current applied to the lamp module  260  and other heating components to obtain the set point temperatures. The temperature is obtained from the measured voltage by looking up the corresponding temperature in a stored conversion table, or by calculating the temperature using a fourth order polynomial. 
     The above CVD system description is mainly for illustrative purposes, and other CVD equipment may be employed. Additionally, variations of the above described system such as variations in substrate support design, heater design, location of RF power connections and others are possible. 
     EXAMPLE 1 
     To further illustrate the process of the invention, using an apparatus of the invention, an eight inch diameter silicon wafer was mounted on a fixed substrate support maintained at a temperature of about 10° C. in the deposition chamber. Three to four grams of particulate di-p-xylylene were loaded into the vaporizer  10  and the vaporizer was then heated to 150° C. After about 5 minutes of heating, the temperatures throughout the apparatus reached equilibrium and the gate valve  20  separating the vaporizer  10  from the decomposition chamber  30  was opened and the vapors of dimer were allowed to flow into the decomposition chamber  30  which was preheated to a temperature of 600° C. The exit gate valve  40  of the decomposition chamber  30  was then opened and the vaporized reactive p-xylylene formed in the decomposition chamber  30  then flowed from the decomposition chamber through the heated conduit to the deposition chamber  60 . The lid of the deposition chamber  60  was maintained at a temperature of about 150° C., and the walls of the chamber were maintained at about 100° C. About 50 sccm of nitrogen was flown into the cold trap  90  during the deposition and the valve  80  was set to maintain a pressure of about 50 mTorr in the deposition chamber. The reactive p-xylylene monomer vapors contacted the cooled silicon wafer  200  and polymerized thereon. After about 15 minutes, the flow of reactive monomer vapors was shut off by first shutting the gate valve  20  between the vaporizer  10  and the decomposition chamber  30 , and then, after pumping out the decomposition chamber  30  to remove all monomer vapors from that chamber, shutting off the gate valve  40  between the decomposition chamber and the deposition chamber. The wafer  200  was then removed from the chamber  60 , after being allowed to warm to ambient temperature, and examined. About 5000 Angstroms of a parylene polymer film was found to have formed over the wafer. The dielectric constant of the film was tested and found to be 2.7. 
     Thus, the invention comprises apparatus and process for forming a low dielectric constant film on a semiconductor substrate comprising parylene or a copolymer formed using reactive parylene monomer and a second copolymerizable material.