Patent Publication Number: US-2011076422-A1

Title: Curved microwave plasma line source for coating of three-dimensional substrates

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This patent application is a non-provisional of and claims the benefit of U.S. Provisional Patent Application No. 61/224,224, entitled “High Efficiency Low Energy Microwave Ion/Electron Source,” filed Jul. 9, 2009, the entire disclosures of which are incorporated herein by reference for all purposes. 
     This patent application is a non-provisional of and claims the benefit of U.S. Provisional Patent Application No. 61/224,234, entitled “Curved Surface Wave Fired Plasma Line for Coating of 3 Dimensional Substrates,” filed Jul. 9, 2009, the entire disclosures of which are incorporated herein by reference for all purposes. 
     This patent application is a non-provisional of and claims the benefit of U.S. Provisional Patent Application No. 61/224,371, entitled “Simultaneous Vertical Deposition of Plasma Displays Layers,” filed Jul. 9, 2009, the entire disclosures of which are incorporated herein by reference for all purposes. 
     This patent application is a non-provisional of and claims the benefit of U.S. Provisional Patent Application No. 61/224,245, entitled “Microwave Linear Deposition of Plasma Display Protection Layers,” filed Jul. 9, 2009, the entire disclosures of which are incorporated herein by reference for all purposes. 
     This patent application is a continuation-in-part application of International Application No. PCT/US2008/052383, entitled “System and Method for Microwave Plasma Species Source,” filed 30 Jan. 2008, the entire disclosures of which are incorporated herein by reference for all purposes. 
    
    
     BACKGROUND OF THE INVENTION 
     Typical applications of coaxial plasma line sources use an array of linear antenna and tubes that are arranged in parallel with a half wavelength spacing between two of the tubes. This arrangement may be used for coatings over substrate with a simple geometry. But many substrates that need coating have complex geometries and can have large variations in a vertical direction perpendicular to the substrate. For example, the substrate may be three-dimensional with a large area. 
     Substrates of complex geometries, for example, sun roofs, automotive lamps and reflectors, are coated by other methodologies than using a straight antenna coaxial line technology. Sun roofs, for example, may be made of polycarbonate as a replacement to glass components. However, polycarbonate is susceptible to scratching and degradation from UV light. Highly transparent protective coating is needed for both scratch resistance and UV light absorption. 
     One of the common coating methodologies utilizes liquid based lacquers to coat a large substrate of complex geometry. The lacquer-based coating can be sprayed on substrates such as polycarbonate and then thermally cured to provide a hard coating that also blocks UV light. Such a coating is typically in the range of 2-10 μm thick. The cost associated with this lacquer-based technology is so high that it limits applications. 
     Another coating method includes forming a soft UV blocking layer of benzyl phenon on a polycarbonate substrate. A hard organo-silicon coating can then be formed on the coated polycarbonate substrate by using plasma enhanced chemical vapor deposition (PECVD). The two-layer coating is normally 2-10 μm thick. Such an organo-silicon coating provides a much harder coating than the lacquer based system. However, the UV blocking layer beneath the organo-silicon coating is degraded from UV light attack over time. As a result of consumption of UV absorbers, the organo-silicon coating may shrink or crack after about 3000 to 5000 hours, which will shorten the lifetime of the scratch resistance coating. 
     For thin film deposition, it is often desirable to have a high deposition rate to form coatings on large substrates, and flexibility to control film properties. Higher deposition rates may be achieved by increasing plasma density or lowering the chamber pressure. For plasma etching, higher etching rates can be helpful for shortening processing cycle time. And a high plasma density source can be desirable. 
     In chemical vapor deposition (CVD), a film is formed by chemical reaction near the surface of a substrate. Typically, reactive gases are introduced into a processing chamber. The reactive gases may decompose from heat to form plasma. And a chemical reaction may occur on the surface of a substrate forming a film over the substrate. Volatile byproducts may be produced and transported away from the processing chamber. Examples of common CVD technologies include thermal CVD, low pressure CVD (LPCVD), plasma-enhanced CVD (PECVD), microwave plasma-assisted CVD, atmospheric pressure CVD, and the like. LPCVD uses thermal energy for reaction activation. The chamber pressure ranges from 0.1 to 1 torr, where temperature may be controlled to be around 600-900° C. by using multiple heaters. PECVD uses radio frequency (RF) plasma to transfer energy into the reactive gases and form radicals. This process allows a lower temperature than does LPCVD. 
     Using a microwave frequency sour can also provide an increase in plasma density. Microwave plasma PECVD inputs microwave power into the reactive gases at a microwave frequency, for example, commonly at 2.45 GHz, which is much higher than the RF frequency of 13.56 MHz. It is well known that at low frequencies, electromagnetic waves do not propagate in a plasma, but are instead reflected. However, at high frequencies such as at typical microwave frequencies, electromagnetic waves effectively allow direct heating of plasma electrons. As the microwaves input energy into the plasma, collisions can occur to ionize the plasma so that higher plasma density can be achieved. Typically, horns are used to inject the microwaves or a small stub antenna is placed in the vacuum chamber adjacent to the sputtering cathode for inputting the microwaves into the chamber. However, this technique does not provide a homogeneous assist to enhance plasma generation. It also does not provide enough plasma density to sustain its own discharge without the assistance of the sputtering cathode. Additionally, scale up of such systems for large area deposition is limited to a length on the order of 1 meter or less due to non-linearity. 
     BRIEF SUMMARY 
     Embodiments of the invention includes a deposition system for dynamically coating surfaces with complex geometries. The system can include a processing chamber, a non-linear coaxial microwave source comprising an antenna within the processing chamber. The system can also include a substrate support member disposed inside the processing chamber that can hold a non-planar substrate, wherein the non-planar substrate can comprise a first contour along a first direction and a second contour along a second direction orthogonal to the first direction. The system can also include a carrier gas line for providing a flow of sputtering agents inside the processing chamber, and a feedstock gas line for providing a flow of precursor gases. 
     According to some embodiments, the deposition system for dynamic coating can include a stage coupled to the non-linear coaxial microwave source The stage can be configured to be movable relative to the non-planar substrate. In some embodiments, the deposition system for dynamic coating may include a stage coupled to the non-planar substrate that is configured to be movable relative to the coaxial microwave line source. 
     According to some embodiments, a deposition system for static coating includes a processing chamber, a substrate support member disposed inside the processing chamber, the substrate support member being configured to hold a non-planar substrate. The non-planar substrate can have a first contour along a first direction and/or a second contour along a second direction orthogonal to the first direction. The deposition system may include an array of curved coaxial microwave sources within the processing chamber. In some embodiments, each of the curved coaxial microwave sources can include a respective antenna and be formed in a respective shape. The curved coaxial microwave sources can be spaced along the second direction to cover the substrate. The deposition system can also include a carrier gas line for providing a flow of sputtering agents inside the processing chamber, and a feedstock gas line for providing a flow of precursor gases. 
     In some embodiments, a method for dynamically coating a non-planar substrate is disclosed. The method can include loading a non-planar substrate into a processing chamber. The non-planar substrate can have a first contour along a first direction and a second contour along a second direction orthogonal to the first direction. The method can also include providing a curved coaxial microwave source comprising an antenna and generating microwaves with the antenna. The method can also include flowing precursors into the processing chamber, forming a plasma from the precursors with the generated microwaves, and depositing coating over the non-planar substrate at a first position of the curved coaxial microwave source. Furthermore, the method can also include moving the curved coaxial microwave source to a second position along the second direction and forming coating over the substrate at the second position. 
     In some embodiments, a method for statically coating a non-planar substrate is disclosed. The method can include loading a non-planar substrate into a processing chamber. The non-planar substrate can have a first contour along a first direction and a second contour along a second direction orthogonal to the first direction. The method can also include providing an array of curved coaxial microwave sources, each of the curved coaxial microwave sources including a respective antenna. The curved coaxial microwave sources can be spaced along the second direction to cover the substrate. The method can also include generating microwaves with the curved coaxial microwave sources, flowing precursors into the processing chamber, forming plasma from the precursors with the generated microwaves, and/or depositing coating over the non-planar substrate. 
     Additional embodiments and features are set forth in part in the description that follows, and in part will become apparent to those skilled in the art upon examination of the specification or may be learned by the practice of the invention. A further understanding of the nature and advantages of the present invention may be realized by reference to the remaining portions of the specification and the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a simplified diagram of a coaxial microwave-assisted chemical vapor deposition (CVD) system without a containment shield according to some embodiments of the invention. 
         FIG. 2  shows a simplified deposition system with a containment shield partially surrounding an antenna and having a generally circular cross section according to some embodiments of the invention. 
         FIG. 3  shows a schematic of a system including an array with curved coaxial microwave sources and a curved substrate according to some embodiments of the invention. 
         FIGS. 4A-4C  illustrate embodiments of a curved coaxial microwave plasma source with recombination shielding that provides dynamic coating over a 2-dimensional curved substrate according to some embodiments of the invention. 
         FIGS. 5A-5B  illustrate another embodiment of a curved coaxial microwave plasma source that provides dynamic coating over a 2-dimensional curved substrate according to some embodiments of the invention. 
         FIGS. 6A-6C  illustrate one embodiment of an array with curved coaxial microwave sources that provides static coatings over a three-dimensional curved substrate according to some embodiments of the invention. 
         FIG. 7  is a flow diagram illustrating steps that may be used to dynamically form a film on a curved substrate according to some embodiments of the invention. 
         FIG. 8  is a flow diagram illustrating steps for static coating over a curved substrate according to some embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In accordance with embodiments of the invention, a substrates of complex geometries can be coated using curved coaxial microwave sources to match the complex geometries of the substrates. The coaxial microwave source can include an antenna that is a metallic waveguide with a microwave source. The coaxial microwave source may also include an isolation dielectric layer, such as quartz and a containment shield outside the antenna with or without the isolation dielectric layer. To accommodate large areas of a substrate, either the substrate is moved relative to the curved coaxial microwave source, or the curved coaxial microwave source is moved relative to the substrate. In some embodiments, either coating method can achieve coatings on 3 dimensional substrates. 
     Overview of Microwave PECVD 
     Microwave plasma deposition has been developed to achieve higher plasma densities (e.g. ˜10 12  ions/cm 3 ) and higher deposition rates, as a result of improved power coupling and absorption at 2.45 GHz when compared to a typical radio frequency (RF) coupled plasma sources at 13.56 MHz. One drawback of using RF plasma is that a large portion of the input power is dropped across the plasma sheath (dark space). By using microwave plasma, a narrow plasma sheath can be formed and more power can be absorbed by the plasma for creation of radical and ion species. This can increase the plasma density with a narrow energy distribution by reducing collision broadening of the ion energy distribution. 
     Microwave plasma can have other advantages, such as lower ion energies with a narrow energy distribution. For instance, microwave plasma may have a lower ion energy, for example, of about 0.1-25 eV, which can lead to lower damage when compared to processes that uses RF plasma. In contrast, standard planar discharge could result in high ion energy of 100 eV with a broader ion energy distribution, which can lead to higher damage as the ion energy exceeds the binding energy for most materials of interest. This can inhibit the formation of high-quality crystalline thin films through the introduction of intrinsic defects. With low ion energy and/or narrow energy distribution, microwave plasma, for example, can help in surface modification and can improve coating properties. 
     In addition, a lower substrate temperature (e.g., lower than about 200° C. or about 100° C.) can be achieved as a result of increased plasma density at lower ion energy with narrow energy distribution. Such a lower temperature can allow better microcrystalline growth in kinetically limited conditions. Also, standard planar discharge without a magnetron can normally require a pressure greater than about 50 mtorr to maintain self-sustained discharge, as plasma becomes unstable at pressures lower than about 50 mtorr. The microwave plasma technology, described herein, however, can allow the pressure to range from about 10 −6  torr to 1 atmospheric pressure. Thus, the processing window in temperature and pressure can be extended by using a microwave source. 
     In the past, one drawback associated with microwave source technology in the vacuum coating industry was the difficulty in maintaining homogeneity during scale up from small wafer processing to very large area processing. Microwave reactor designs in accordance with embodiments of the invention address these problems. Arrays of coaxial plasma linear sources have been developed to deposit substantially uniform coatings of ultra large area (e.g., greater than 1 m 2 ) at high deposition rate to form dense and thick films (e.g., 5-10 μm thick). 
     An advanced pulsing technique has been developed to control the microwave power for generating plasma, and thus to control the plasma density and plasma temperature. This advanced pulsing technique may reduce the thermal load disposed over the substrate, as the average power may remain low. This feature can be relevant when the substrate has a low melting point or a low glass transition temperature, such as in the case of a polymer substrate. The advanced pulsing technique can allow for high power pulsing into plasma with off times in between pulses, which reduces the need for continuous heating of the substrate. Another aspect of the pulsing technique is significant improvement in plasma efficiency compared to continuous microwave power. 
     A Sample Deposition System 
       FIG. 1  shows a diagram of a coaxial microwave-assisted chemical vapor deposition (CVD) system  100  without a containment shield according to some embodiments of the invention. Multiple-step processes can be performed on a single substrate or wafer without removing the substrate from the chamber. The substrate may have a complex geometry that is either planar or non-planar. The major components of the system include, among others, a processing chamber  124  that receives precursors from feedstock gas line  104  and carrier gas line  106 , a vacuum system  132 , a coaxial microwave source  126 , a substrate  102 , and a controller  132 . 
     The coaxial microwave source  126  includes, among others, an antenna  112 , a microwave source  116 , an outer envelope surrounding the antenna  112  made of dielectric material (e.g. quartz). The dielectric material, for example, can serve as a barrier between the vacuum pressure  108  and atmospheric pressure  114  inside the dielectric layer  110 . The microwave source  116  can input the microwave into the antenna  112 . The atmospheric pressure can be used to cool the antenna  112 . Electromagnetic waves are radiated into the chamber  124  through the dielectric layer  110 . Plasma  118  may be formed over the surface of the dielectric material. In a some embodiments, the coaxial microwave source  126  may be curved. The coaxial microwave source  126  may be an array of the coaxial microwave sources. 
     In some embodiments, the feedstock gas line  104  may be located below the coaxial microwave source  126  and above the substrate  102  which is near the bottom of the processing chamber  124 . in some embodiments, the carrier gas line  106  may be located above the coaxial microwave source  126  and near the top of the processing chamber  124 . Through the perforated holes  120  and  122 , the precursor gases and carrier gases can flow into the processing chamber  124 . The precursor gases are vented toward the substrate  102  (as indicated by arrows  128 ), where they may be uniformly distributed radically across the substrate surface, typically in a laminar flow. After deposition is completed, exhaust gases exit the processing chamber  124  by using vacuum pump  132  through exhaust line  130 . 
     The controller  134  can controls activities and/or operating parameters of the deposition system, such as the timing, mixture of gases, chamber pressure, chamber temperature, pulse modulation, microwave power levels, and other parameters of a particular process. 
       FIG. 2  shows deposition system  200  with a containment shield  202  partially surrounding an antenna with a generally circular cross section. The antenna can include a waveguide  206  and a dielectric tube  204  as a pressure isolation barrier. Air or nitrogen can be filled in the space between the dielectric tube  204  and waveguide  206  for cooling the antenna. The first pressure inside the dielectric tube  204  may be one atmospheric pressure. The circular containment shield  202  can be placed outside the dielectric tube  204  for containing plasma  216  that is formed from sputtering agents coming from a carrier gas line  208  located on a centerline  212 . The plasma  216  can come through an aperture  214  near the bottom of the containment shield  202  to collide with reactive precursors from a feedstock gas line  224 . Radical species generated by the plasma  216  disassociate the reactive precursors to form a film on a substrate  220  that is held by a substrate support member  222 . The second pressure inside the containment shield  202  may be higher than the third pressure inside a processing chamber  226 . The dielectric tube  204  may comprise a quartz to form a pressure isolation barrier and still allow microwaves to leak through. 
     For illustration purpose, only circular containment shield is shown. Other shapes of containment shield may be used. Details are included in U.S. patent application Ser. No. 12/238,664, entitled “Microwave Plasma Containment Shield Shaping” by Michael Stowell, the entire contents of which are incorporated herein by reference for all purposes. 
     A feedstock gas line  224  can be located outside the containment shield  202  and proximate the substrate  220  to be coated as shown in  FIG. 2 . The feedstock gas line  224 , for example, can be placed here because the radical density may be so high that some of the radicals may deposit over the inner wall of the containment shield  202 . The feedstock gas can provide one or more of the atoms or molecules to produce desired dielectric coatings such as SiO 2 , where a silicon containing gas, for example, hexamethyldisiloxane (HMDSO), can be left in the feedstock gas line  224 . The position of the feedstock gas line may be adjusted to control the film chemistry. There are also exceptional cases where a reactive gas may be included among the carrier gases, such as ammonia that may be used to form nitride. 
     The containment shield  202  may comprise a dielectric material, for example, Al 2 O 3 , quartz, or pyrex. A pressure difference may be present between the internal pressure of the containment shield and the external pressure of the containment shield, with the internal pressure being higher than the external pressure. This allows more processing flexibility than without using the containment shield. With increased pressure inside the containment shield, plasma species or radicals may have more collisions and thus higher radical density. With lower pressure outside the containment shield, the chamber pressure may be lower. As a result of lower chamber pressure, the mean free path can increase for plasma species or radicals and thus deposition rates can be increased. 
     Furthermore, the plasma containment shield may help increase radical density and/or can help form a homogeneous plasma. The shield can help increase the collisions among the radicals by confining the radicals within the containment shield without losing the radical species. As a result of using the plasma containment shield, radical density can be increased and homogeneity is improved, particularly in radical direction. 
     In addition, by using a containment shield, the volume of the gas inside the plasma containment shield may be more fully ionized and thus may produce more radicals so that ionization efficiency may be improved. For example, the inventors performed experimental tests to demonstrate that the ionization efficiency may be improved from 65% to 95% by using a plasma containment shield. 
     Film properties requirements can be achieved by varying process conditions during deposition, including the power levels, pulsing frequency and duty cycle of the source. To achieve the required film properties the structure and structural content of the deposited film may be controlled; for example, by varying the radical species content, among other processing parameters. The radical density is controlled primarily by the average and peak power levels into the plasma discharge. 
     Multiple antenna and plasma pipes may be used in this fashion to produce a large array for static or dynamic coatings.  FIG. 3  shows a schematic of a simplified system including an array  302  comprising 4 curved coaxial microwave sources  310 , a curved substrate  304 , a cascade coaxial power provider  308 , and an impedance matched rectangular waveguide  306 . In the curved coaxial microwave source  310 , microwave power is radiated into a processing chamber in a transversal electromagnetic (TEM) wave mode. A curved tube can be made of dielectric material, such as quartz or alumina having high heat resistance and a low dielectric loss, which can act as the interface between the waveguide having atmospheric pressure and the vacuum chamber. 
     A cross sectional view of a coaxial microwave source  300  can illustrate the cross section of a curved conductor (e.g., antenna)  326 . This curved conductor  326  can be used, for example, to radiate microwaves at a frequency of 2.45 GHz. The radial lines represent an electric field  322  and the circles represent a magnetic field  324 . The microwaves can propagate through the air to the curved dielectric layer  328  and then leak through the dielectric layer  328  to form an outer plasma conductor  320  outside the dielectric layer  328 . Such a wave sustained near the coaxial microwave source can be a surface wave. The microwaves can propagate along the curved conductor  326  and go through a high attenuation by converting electromagnetic energy into plasma energy. Another configuration that may be used is without quartz or alumina outside the microwave source. 
     The curved antenna  326  can be surrounded by a curved dielectric layer  328  forming a pressure isolation barrier between the atmospheric pressure of the antenna cooling from the internal lower pressure of the processing chamber. Electromagnetic radiation can radiate into the processing chamber through the dielectric envelope, and plasma is formed on the outside surface of the quartz tube. A support gas pipe provides gases used to form plasma and produce radicalized species used in the deposition process. The support gas may include more than one gas for this purpose. The feedstock gas can be the precursor containing one or more of the atoms and molecules necessary to produce the desired film properties. This feedstock gas pipe can be located, for example, near the surface of the substrate to be coated. The position of this pipe may be tuned to provide desired film chemistry. 
     The plasma produces radicalized species that reacts with the feedstock gas inside the processing chamber, near the surface of the substrate. These radicals, for example, can recombine in the gas volume and become unusable to produce required fractional components and to form desired films. Typically, the radicals may be pumped out of the system and do not contribute to forming the desired films. The plasma produced radical species can have multiple loss mechanisms, including, among others, recombination, pumping, fractionalization of precursor gas, inclusion into the growing film. The gas ionization efficiency or plasma efficiency is typically not 100%. Hence, reducing the loss of radicals and or increasing the amount of radicals produced for a given power level can be beneficial in growing films. 
       FIGS. 4A-4C  illustrate one embodiment of a curved coaxial microwave plasma source with recombination shielding to provide dynamic coating over a large curved substrate, such as a car sun roof. As illustrated in  FIG. 4A , a curvature of the coaxial microwave plasma source  402  may be substantially matched with shape of the curved substrate  404  such that the distance between the microwave source  402  and the substrate  404  remains substantially constant in a cross sectional view.  FIG. 4B  shows a top view of the curved substrate  404  that has a dimension remaining unchanged along a horizontal direction as shown by x-axis. Therefore, a homogeneous coating would be obtained over a large area by moving the curved coaxial microwave plasma source  402  along the x-axis relative to the substrate  404 .  FIG. 4C  illustrates a three-dimensional view of the substrate having a curved contour in a sectional view perpendicular to the x-axis. 
     The curved coaxial microwave plasma source  402  may also be moved in the vertical axis to be closer or away from the substrate, depending upon film chemistry. For example, a typical spacing between the microwave source and substrate may be 15 cm for depositing silicon oxide, but may be approximately 5-15 cm for depositing magnesium oxide over the substrate. 
     In some embodiments of the invention, the coaxial microwave source may be moved along a horizontal direction perpendicular to the x-axis. This can be done, for example, to coat a large substrate. For example, if the substrate has a dimension of 16 feet long, 3-4 feet wide and 3-4 feet tall, the coaxial microwave source may need to be moved along the length of the substrate. However, if the substrate has a dimension of 16 feet long, 16 feet wide and 3-4 feet tall, the coaxial microwave source may need to be moved along both the length and the width of the substrate in order to form coatings over the large substrate. Large substrates can include, for example, automotive parts, aircraft parts, maritime parts, etc. 
       FIGS. 5A-5B  illustrate another embodiment of a curved coaxial microwave plasma source that can provide dynamic coatings over a large curved substrate. As illustrated, a curvature of the coaxial microwave plasma source  502  may be substantially matched with the shape of the curved substrate  504  such that the distance between the microwave source  502  and the curved substrate  504  remains substantially constant in a cross sectional view. The geometries of this substrate and the microwave source are different from that shown in FIGS.  4 A-C. However, curvatures in both  FIG. 5A  and  FIG. 4A  are smooth, such that a first derivative of the curvature would show continuous curvatures. Such a smooth curvature would be beneficial to forming a homogeneous coating over the substrate. Deposition can occur on substrates with various curvatures. 
     For illustration purposes,  FIGS. 6A-6C  show an array of 6 curved coaxial microwave sources for providing static coatings over a three-Dimensional substrate.  FIGS. 6A-6B  are a sectional view and a front view of the arrangement of curved coaxial microwave source  602  and curved substrate  604 , respectively. Note that the curvatures of the coaxial microwave source  602 A-F are roughly matched with the curvatures of the substrate  604  at positions A-F, respectively. The curvatures of the coaxial microwave plasma sources  602 A-F may vary from the corresponding positions A-F on the substrate. Each of the curved coaxial microwave plasma sources  602 A-F may have a distance between each of the curved coaxial microwave sources and the substrate. The distance may vary from the positions A-F of the sources  602 A-F such that the array of 6 curved coaxial microwave sources provides coverage of surface area of a three-dimensional substrate of any complex geometry. As illustrated in  FIG. 6B , the positions of the curved coaxial microwave plasma sources  602 A-F are arranged on a curve which approximately matches with the curvature of the substrate  604 .  FIG. 6C  shows a top view of the array of the curved coaxial microwave sources and the substrate. Note that the coaxial microwave sources are spaced out to cover the substrate. The distance between each two neighboring sources of  602 A-F may be half wavelength of the microwave. The length of each coaxial microwave source may be up to 3 m in some embodiments. 
     Substrate preheating treatment can be achieved by utilizing many techniques and heater arrangements. It is common to heat the substrate using a direct heater such as a resistor heating plate in thin film deposition processes. By using a direct heating plate, the substrate temperature may be heated up to approximately 700° C. With microwave-assisted CVD, the substrate temperature may be lowered to below 200° C. In the case of lower substrate temperature, indirect heating sources may be used, such as a resistor heating source, a lamp, or a flash heater. Flash heaters have been developed to significantly reduce cycle times and increase productivity in rapid thermal processing. Flash heaters are used in many applications, such as repairing damage and annealing surface and so on. 
     One of the challenges in thin film deposition on plastic substrates is the difficulty in maintaining structural integrity of plastic substrates. Plastics have a much lower softening temperature, such as melting point or glass transition temperature, than glasses or ceramics. When a plastic substrate is heated near the softening temperature prior to thin film deposition or etching, the plastic substrate often reaches the melting point or glass transition temperature with the additional heat generated from the thin film deposition process. Therefore, the plastic substrate may experience structural distortion as a result of overheating during the thin film deposition or etching process. 
     A source of IR radiation, such as an infrared heater, can heat a plastic substrate in a fast fashion in a processing chamber, where the processing chamber is configured to preheat the plastic substrate and to perform thin film deposition, such as chemical vapor deposition (CVD). One advantage of using the source of IR radiation is to preheat only the surface of the plastic substrate while the core of the plastic substrate remains substantially unheated and the structure of the plastic substrate may remain unchanged. Meanwhile, the surface properties of the plastic substrate may be modified after the preheating treatment. 
     The source of IR radiation can be selected at a wavelength that substantially matches the absorption wavelength of the plastic substrate. This can optimize the energy absorption of the surface of the plastic substrate. Another aspect of the fast preheating treatment is that the source of IR radiation can be powered on continuously while the plastic substrate moves through the heat flux zone generated by the source of IR radiation at a controllable speed. Such a preheating treatment allows the plastic substrate to be heated substantially uniform in a few seconds. The plastic substrate may be preheated near a critical temperature that allows a change in surface morphology or surface structure to occur. Examples are included in U.S. patent application Ser. No. 12/077,375, entitled “Surface Preheating Treatment of Plastic Substrate” by Michael W. Stowell et al, the entire contents of which are incorporated herein by reference for all purposes. 
     The source of IR radiation may be configured to move relative to the substrate such that the movement of the source of IR radiation corresponds to the movement of the coaxial microwave source to provide local heating of a large substrate and dynamic coating over the large substrate. 
     Fabrication of Curved Coaxial Microwave Sources 
     According to one embodiment of the present invention, the antenna includes a conductive waveguide. The conductive waveguide may experience thermal distortion due to heating of the antenna in radiating electromagnetic radiation. Material selection of the waveguide may vary with the need to have both good electrical conductivity and good thermal resistance to warp or distortion. In a specific embodiment, the waveguide may be made of titanium coated with gold, where titanium provides good thermal resistance while gold is a very good conductor. In another embodiment, the waveguide may be made of aluminum, stainless steel, copper coated with silver. Different materials may have various electrical conductivity, various resistance to thermal stress or thermal distortion, and cost variation associated with material and fabrication. 
     For example, the waveguide may have an outer diameter of a few millimeters, such as 6 mm with a wall thickness of 1 to 1.5 mm. The isolation barrier tube may have a larger diameter than the waveguide, for example, an outer diameter of 38 mm with a wall thickness of 3 mm. There may be different ways of making the dielectric tube. In a specific embodiment, the isolation barrier tube may be fabricated by using a sheet of glass having a desired wall thickness. The sheet of glass may be heated by using a flame heater to bend and wrap around a mandrel to form a curved tube of any desired shape. The mandrel may be a metal that can be formed to have the desired shape. 
     The containment shield may have a relatively larger diameter to provide space for containing plasma inside. In some embodiments of the invention, an outer diameter of the containment shield may be 6 inches with a wall thickness of approximately 0.2 inches. The containment shield may be made of quartz, alumina or a borosilicate glass with low coefficient of thermal expansion such as Pyrex. One of the common fabricating methods is to cast the containment shield in a mold to obtain any desired shape. The containment shield may be further annealed to increase density to achieve required properties or performance. 
     The waveguide, quartz tube, and containment shield may be integrated together by common technologies known in the art after each of the component is fabricated to the desired shape which matches with any desired shape of the substrate. 
     Deposition Process 
     For purposes of illustration,  FIG. 7  provides a flow diagram of a process that may be used to form a film on a curved substrate in a dynamic coating according to some embodiments of the invention. The process begins with loading a curved substrate into a processing chamber at block  702 . The substrate may have smooth curvature, for example, as illustrated in  FIGS. 4A-C  or  FIGS. 5A-5C . Next, the process can provide a curved coaxial microwave source to the processing chamber at block  704 . The curved coaxial microwave source can be configured to move relative to the substrate, or the substrate is configured to move relative to the curved coaxial microwave source within the processing chamber. The process followed by generating a microwave with the microwave source at block  706 . 
     Film deposition can be initiated by flowing gases, such as sputtering agents or reactive precursors at block  708 . For deposition of SiO 2 , such precursor gases may include a silicon-containing precursor such as hexamthyldisiloxane (HMDSO) and oxidizing precursor such as O 2 . For deposition of SiO x N y , such precursor gases may include a silicon-containing precursor such as hexmethyldislanzane (HMDS), a nitrogen-containing precursor such as ammonia (NH 3 ), and an oxidizing precursor. For deposition of ZnO, such precursor gases may include a zinc-containing precursor such as diethylzinc (DEZ), and an oxidizing precursor such as oxygen (O 2 ), ozone (O 3 ) or mixtures thereof. The reactive precursors may flow through separate lines to prevent them from reacting prematurely before reaching the substrate. Alternatively, the reactive precursors may be mixed to flow through the same line. 
     The carrier gases may act as a sputtering agent. For example, the carrier gas may be provided with a flow of H 2  or with a flow of inert gas, including a flow of He or even a flow of a heavier inert gas such as Ar. The level of sputtering provided by the different carrier gases is inversely related to their atomic mass. Flow may sometimes be provided using multiple gases, such as by providing both a flow of H 2  and a flow of He, which mix in the processing chamber. Alternatively, multiple gases may sometimes be used to provide the carrier gases, such as when a flow of mixed H 2 /He is provided into the processing chamber. 
     As indicated at block  710 , a plasma is formed from the gases by microwave at a frequency ranging from 1 GHz to 10 GHz, for example, commonly at 2.45 GHz (a wavelength of 12.24 cm). In addition, a higher frequency of 5.8 GHz is often used when power requirement is not as critical. The benefit of using a higher frequency source is that it has smaller size (about half size) of the lower frequency source of 2.45 GHz. In some embodiments, the plasma may be a high-density plasma having an ion density that exceeds 10 12  ions/cm 3 . The process continues by depositing dynamic coating over the curved substrate at block  712  and moving the coaxial microwave plasma source to a next position at block  714 . Assuming that a width of the coaxial microwave source is longer than a width of the substrate, the movement of the source is along a longitudinal direction perpendicular to the width of the substrate. The process proceeds by further depositing coating over the substrate at the next position at block  716 . 
       FIG. 8  is a flow diagram illustrating steps for static coating over a curved substrate according to some embodiments of the invention. Similar to the dynamic coating over a curved substrate, the process can start with loading a curved three-dimensional substrate into a processing chamber at block  802 . The process can also provide an array of curved antenna into the processing chamber at block  804  and can generate microwaves at block  806 . The array of curved antenna is arranged such that a homogeneous static coating may be formed over the curved three-dimensional substrate, for example, as shown in  FIGS. 6A-6C . Again, like the dynamic coating process illustrated in  FIG. 7 , the process can continue by flowing precursors into the processing chamber at block  808  and forming plasma from the precursors with the generated microwaves at block  810 . The process proceeds by depositing static coating over the curved substrate at block  812 . 
     While the above is a complete description of specific embodiments of the present invention, various modifications, variations and alternatives may be employed. Examples of the possible parameters to be varied include but are not limited to the temperature of deposition, the pressure of deposition, and the flow rate of precursors and carrier gases. 
     Those of ordinary skill in the art will realize that specific parameters can vary for different processing chambers and different processing conditions, without departing from the spirit of the invention. Other variations, among others, including shapes or geometry of curved coaxial microwave sources or non-planar substrates, configuration of the array of curved coaxial microwave sources relative to the substrates, types of source of IR radiation, configuration of moving stages for either the sources or substrate, will also be apparent to persons of skill in the art. These equivalents and alternative are intended to be included within the scope of the present invention. Therefore, the scope of this invention should not be limited to the embodiments described, but should instead be defined by the following claims.