Patent Publication Number: US-2009232983-A1

Title: Substrate temperature control for combustion chemical vapor deposition

Description:
A related application Ser No. ______ (applicants&#39; assignee&#39;s docket no. US040543) is filed contemporaneously with this application. 
    
    
     The invention relates to deposition of a thin film on a substrate by a process of combustion chemical vapor deposition. 
     Combustion chemical vapor deposition (C-CVD) is a relatively new technique for gas phase deposition of films and coatings on a substrate at atmospheric pressure. In C-CVD, gaseous chemical reactants (“precursors”) are activated in a combustion flame before they reach the substrate surface. As a result, the substrate temperature may be significantly lower in C-CVD than in conventional (thermal) CVD processes, where only the substrates arc heated. The combination of atmospheric pressure (“open air”) and low temperature processing make C-CVD a promising technique for various applications in which high throughput coating is required, with inexpensive equipment, on temperature-sensitive substrates. 
     A number of publications on the C-CVD technique have appeared in the literature. For example, “Combustion chemical vapor deposition: A novel thin-film deposition technique,” A. T. Hunt et al.,  Applied Physics Letters  63 (1993) 266-268 discloses a C-CVD technique in which a flame provides an environment for deposition of a dense film whose elemental constituents are derived from solution, vapor or gas sources. A number of patents on C-CVD techniques have issued (e.g., U.S. Pat. No. 5,652,021 (1997) corresponding to WO 94/21841). The prior art discloses that a number of different materials can be deposited on various types of substrate materials such as ceramics, glass, metals, and polymers. Although many applications are being envisaged, no current industrial applications are known and it is believed that C-CVD is still at a development stage. 
     Several methods for maintaining substrates at deposition temperatures that are desirable for certain processes have been reported in the art. See, for example, US 2003/0113479 A1 in which, in a treatment process with a plasma at atmospheric pressure, the temperature of a ground electrode surface of metal base material (substrate) is controlled by supply of chilled water to the interior of the ground electrode. 
     U.S. Pat. No. 5,135,730 to Suzuki et al. discloses a process to synthesize diamond by combustion in which a flame contacts a surface of a substrate with a temperature maintained from 300° C. to 1200° C. by cooling water flow through a substrate holder, by cooling water and air flow through a substrate holder or by cooling gas directed against the back of the substrate. 
     In a plasma jet deposition process a substrate may be mounted on a cooling block with a gap between the substrate and a surface of the cooling block being filled with a gas to improve heat transfer, as disclosed in published European application no. EP 0747505A2. 
     U.S. Pat. No. 5,085,904 to Deak et al. discloses multi-layer structures suitable for food packaging in which barrier layers of SiO and SiO 2  are successively vacuum deposited on a polyester or polyamide resin substrate such as polyethylene terephthalate (PET) film. 
     A flexible display can be achieved by a structure in which thin film transistors (TFT&#39;s) are formed on a flexible substrate, in particular a polymer substrate, as components of display elements or pixels of an active matrix. These structures typically comprise several layers, including semiconductor, dielectric, electro-conductive and barrier layers. 
     The combustion flame in C-CVD must, in general, be in close proximity to the substrate (typically a few centimeters). As a result, heating of the substrates by the flame may be a serious problem, especially if the substrates (e.g. polymers) are sensitive to high temperature. The methods to prevent excessive heating up of substrates, which are described in the literature, are rather inefficient The prior art includes blowing of cold air on the back of the substrate, and/or moving (“sweeping”) the burner over the substrate surface, and cooling a substrate holder by air or water flow or by moving the substrate past the flame. Otherwise, no special arrangements are disclosed in the existing publications on C-CVD to prevent excessive heating up of substrates. Many plastic substrates, especially foils, deteriorate if subjected to conventional procedures, making them unsuitable for some applications, such as processing of flexible foils to be used in displays. 
     A solution for the above problems has been found in substrate foil handling by means of a combination of 
     1) providing suction to maintain contact between a foil to be coated and a susceptor (substrate support plate or holder); and 
     2) cooling the susceptor using an appropriate cooling fluid. 
     Accordingly, it is desirable to provide a C-CVD apparatus and method for deposition of a thin film on a temperature sensitive substrate. 
     It is also desirable to provide a cooling apparatus for a substrate during deposition of a thin film by C-CVD. 
     It is further desirable to provide a method and apparatus for combustion chemical vapor deposition of a dense barrier film on a temperature sensitive substrate. 
     It has been found that the substrate temperature may be low, but at least 50° C., preferably above 70° C., to prevent condensation of water generated by the combustion flame, and below the temperature at which the substrate deteriorates. Typically, for a polymer foil, this deterioration begins when the substrate temperature reaches the glass transition temperature of the polymer. The gas transition temperature depends on the type of material. The present invention allows C-CVD to be done at temperatures at or below the glass transition temperatures of a wider range of substrates than are available using processes in the prior art. 
     One application of the apparatus and method of the present invention is in manufacturing of flat and flexible displays. Silica (SiO 2 ) layers deposited on a substrate by C-CVD may, in particular, serve as barrier layers and/or dielectric layers. Barrier layers are layers which are required to prevent permeation of oxygen and moisture. The C-CVD silica layer may be part of a multilayer stack, with other inorganic and/or organic layers. In one embodiment the present invention concerns a C-CVD technique for deposition of films on flexible (plastic/metal foil) and/or temperature sensitive substrates specifically for display technologies. 
     These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter. 
    
    
     
       An embodiment of the invention will be described, by way of example only, with reference to the drawings, in which: 
         FIG. 1   a  shows an exemplary embodiment of a combustion chemical vapor deposition apparatus of the present invention. 
         FIG. 1   b  shows a second view of the combustion chemical vapor deposition apparatus of  FIG. 1   a  of the present invention 
         FIG. 2  is a graph showing a relation between coating thickness and oxygen transmission rate (OTR) on a polymer substrate. 
     
    
    
     Referring to  FIG. 1   a  substrate  101 , e.g. a piece of flexible polymer or metal foil, or a sheet of glass, is kept on a substrate holder  102  by means of suction (connected to a vacuum line  103 ) which maintains a vacuum connection to vacuum openings  113  in a surface of the substrate holder  102 . The suction is a means for maintaining conductive heat transfer between the substrate  101  and substrate holder  102 . The substrate holder  102  has temperature control means such as a coolant inlet  111  and coolant outlet  112  and coolant channels  104  for temperature control using a coolant, in this embodiment, water, from a temperature control system including a heater/cooler circulator (not shown). The heater/cooler circulator may, for example, be a system that includes a microprocessor, program of instructions, and data storage device, temperature sensor, control valve(s), heat exchanger, etc. and be capable of sensing the temperature of the substrate  101  or substrate holder  102  and adjusting coolant flow and/or temperature to maintain a desired temperature of the substrate  101  or substrate holder  102 . Control systems of this kind are well known to those of ordinary skill in the art. 
     In this embodiment, the vacuum line  103  is connected to vacuum channels (not shown) in the substrate holder  102  which connect to vacuum openings  113  which are on a surface of the substrate holder  102 . The vacuum openings  113  are in a rectangular groove  114  which extends around and is outside the periphery of a frame opening  106  (shown in  FIG. 2 ). Alternatively, the vacuum openings  113  may be arranged in any desired pattern or randomly. A porous material may be used as all or part of the flat surface of the substrate holder  102  with pores opening on the flat surface serving as vacuum openings  113 . 
     An aluminum frame  105  is placed on top of the substrate  101  and holder in order to protect the edges of the flexible substrates. The coated area on the substrate  101  corresponds to the frame opening  106 . The substrate holder  102  is mounted for linear movement (in an x-direction along an axis  107 ). The C-CVD burner holder is height adjustable, and mounted for linear movement (in a z-direction along an axis  108 , i.e. perpendicular to substrate  101  movement), in order to achieve improved uniformity. In other embodiments, the burner  109  may be movable in a y-direction along an axis  115  perpendicular to axes  107  and  108  or in both directions, a third direction or all three directions. 
     The burner  109 , which in this embodiment, has a linear shape, and is fed with a gas feed  110  of a common combustible gas such as propane or natural gas, and an oxidizing gas such as pure oxygen or air. The burner  109  gases may be pre-mixed or surface-mixing. Nitrogen may be added to adjust the temperature and shape of the flame. Part of the nitrogen flow may be passed through a so-called bubbler, in which it is saturated with the vapor of coating precursor, for example, tetra-ethoxy-silane (TEOS). Alternatively, TEOS or another precursor may be mixed with nitrogen, an inert gas or the oxidizing gas using a mixing valve, nebulizer, aspirator or similar device. TMOS (tetramethylorthosilicate) and HMDSO (hexamethyldisiloxane), for example, as well as TEOS, arc common CVD precursors for silica coatings in conventional (thermal) CVD processes and may also be used in the present invention. Other metal oxide materials such as lanthanum oxide, chromium oxide, tungsten oxide, molybdenum oxide, vanadium oxide, and copper oxide may be used. 
     In this embodiment, the TEOS concentration is 0.01-0.05 mol % in the total gas stream (i.e. the mixture of combustion gas, oxidant gas, inert carrier/diluent gas and precursor gas). Substrate temperature is kept about 70° C. The substrate is drawn through the burner  109  along the x-direction axis  107 . The distance along the axis  108  (z-direction) from the burner  109  to-the substrate  101  is maintained constant. A deposition rate of 1-20 nm per pass is achieved. The number of passes determines the final thickness of the coating. 
     A substrate temperature of at least 50° C., and preferably above 70° C. prevents condensation of water generated by the combustion flame. Condensation of water prevents the growth of a continuous coating. Condensation generated by the combustion flame is affected by, among other things, the amount of nitrogen or other non-oxidizing gas used to dilute the feed to the burner, with a higher amount of diluent allowing a lower substrate temperature. 
     The upper limit of the substrate temperature depends on the type of substrate material, rather than being determined by the C-CVD process. For polymer substrates the upper limit depends on, among other factors, the glass transition temperature (Tg) of the polymer material and is, typically, lower (in the range 80-200° C.) than for, for example, glass (to 600° C.) or metal substrates. Substrates such as polynorbornene (Tg of 340° C.), polyimide (275° C.), polyethersulphone (220° C.), polyarylate (215° C.), high temperature polycarbonate (205° C.), polycarbonate (150° C.), polyethylenenapthalate (120° C.) and PET (68° C.) are advantageously used in the present invention. The film material itself is usually more stable than the substrates, typically to at least 1000° C. 
     In this example, SiO 2  coatings have been deposited using C-CVD on sheets of AryLite™, a polyarylate (PAR) substrate for flexible displays manufactured by the company Ferrania S.p.A. The substrate may, however, be of any suitable material. Polymeric materials suitable for use as substrates include, but are not limited to, polycarbonate (PC), polyethersulfone (PES), polynorbonene (PNB), PET, polyethylenenapthalate (PEN), epoxide, polymethylmethacrylate (PMMA), polyurethane (PUR), polyethylene (PE), polypropylene.(PP) and polyimide (PI). Different materials may be suited for different uses and are known to one skilled in the art. The substrate may be of an organic compound, or an at least partly inorganic compound arranged with a organic surface. Other substrates that can be used are glass or metal (foils) with or without device structures, that require a barrier coating or dielectric coating. 
     The apparatus and method of the present invention allow deposition of a film with good properties for a barrier layer in a flexible display screen, in particular, a clear, flexible and dense film (one that has a bulk density that is close to the bulk density of quartz) of silica can be obtained. 
     The barrier properties of coatings of various thicknesses obtained in this embodiment of the present invention have been determined using standard oxygen permeation (Mocon test) measurements conducted at Dow Coming Plasma Solutions. Table 1 shows the variation of Oxygen Transmission Rate (OTR) with coating thickness for the different samples. There is a significant improvement in OTR for the coated films relative to the uncoated. As the coating thickness increases, the barrier performance is improved. The results are displayed graphically in  FIG. 2 . In  FIG. 2  the x-coordinate  201  is coating thickness, and the y-coordinate  202  is OTR. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Measured OTR for coated and uncoated samples 
               
            
           
           
               
               
               
            
               
                   
                 coating thickness 
                 OTR 
               
               
                 sample 
                 (nm) 
                 cc/(m 2  · day) 
               
               
                   
               
            
           
           
               
               
               
            
               
                 1. 
                 100 
                 3.03 
               
               
                 2. 
                 50 
                 4.13 
               
               
                 3. 
                 25 
                 12.8 
               
               
                 Uncoated 
                 0 
                 3766 
               
               
                   
               
            
           
         
       
     
     In another embodiment of the present invention, the same properties arc achieved by using a nebulizer to create micron-sized TEOS droplets which are introduced into the flame. 
     If a polymer substrate is used, it may be flexible. Some of the polymeric test substrates, that may be used in the present invention are described in the article “Flexible active-matrix displays and shift registers based on solution processed organic semiconductors,” G. H. Gelinek et al,  Nature Materials,  2004, 3(2), pages 106 to 110, which is incorporated herein by reference. Such substrates may comprise a support with a foil on top, then a planarisation layer, structured gold as gate electrode, a polymer such as the commercially available epoxy based negative resist SU8 as a gate dielectric, typically SU8 and gold source and drain electrodes. 
     Because of its precursors are easily and inexpensively produced and applied, silica is advantageously used to form barrier layers. Other materials, including, but not limited to inorganic metal oxides of magnesium, zinc or zirconium, may also be suitable, in particular, as barrier layers, depending on the application. 
     The invention is not limited to barrier and dielectric layers, but may advantageously be used for other layers, including, without limitation, conducting layers such as a transparent conducting layer of, e.g. indium-tin-oxide (ITO) or doped zinc oxide. Deposition of Al-doped zinc oxide by C-CVD for solar cell applications is known from the prior art. 
     Finally, the above-discussion is intended to be merely illustrative of the present invention and should not be construed as limiting the appended claims to any particular embodiment or group of embodiments. Each of the systems utilized may also be utilized in conjunction with further systems. Thus, while the present invention has been described in particular detail with reference to specific exemplary embodiments thereof, it should also be appreciated that numerous modifications and changes may be made thereto without departing from the broader and intended spirit and scope of the invention as set forth in the claims that follow. The specification and drawings are accordingly to be regarded in an illustrative manner and are not intended to limit the scope of the appended claims. 
     In interpreting the appended claims, it should be understood that: 
     a) the word “comprising” does not exclude the presence of other elements or acts than those listed in a given claim; 
     b) the word “a” or “an” preceding an element does not exclude the presence of a plurality of such elements; 
     c) any reference numerals in the claims are for illustration purposes only and do not limit their protective scope; 
     d) several “means” may be represented by the same item or hardware or software implemented structure or function; 
     e) each of the disclosed elements may be comprised of hardware portions (e.g., discrete electronic circuitry), software portions (e.g., computer programming), or any combination thereof; 
     f) hardware portions may be comprised of one or both of analog and digital portions; 
     g) any of the disclosed devices or portions thereof may be combined together or separated into further portions unless specifically stated otherwise; and 
     h) no specific sequence of acts is intended to be required unless specifically indicated.