Abstract:
A method for adjusting resistivity of a film heater on a substrate for use in process fluids employed in the semiconductor-processing industry as part of a clean, particle-free, nonreactive, non-trapping, ultra-pure, thermally tolerant, sealed system. In one arrangement, the method includes the steps of selecting a heating rate, selecting an electrical resistance value in accordance with the heating rate, selecting a resistive material for coating a substrate to produce resistance heating consistent with the electrical resistance value, selecting dimensions for a film of the resistive material selected to balance effects of conductivity, resistivity, length, and area against effects of the heating rate, and forming the film by conformally coating a surface of the substrate with the film at the selected dimensions.

Description:
RELATED APPLICATIONS 
     This Patent Application is a continuation in part of U.S. Provisional Patent Application Ser. No. 60/179,541 filed on Feb. 1, 2000. 
    
    
     BACKGROUND 
     1. The Field of the Invention 
     This invention relates to semiconductor processing technology and, more particularly, to novel systems and methods for heating fluids and making heaters carrying ultra-pure fluids for processing operations. 
     2. The Background Art 
     The semiconductor manufacturing industry relies on numerous processes. Many of these processes require transportation and heating of de-ionized (DI) water, acids and other chemicals. By clean or ultra-pure is meant that gases or liquids cannot leach into, enter, or leave a conduit system to produce contaminants above permissible levels. Whereas other industries may require purities on the order ofparts-per-million, the semiconductor industry may require purities on the order of parts-per-trillion. 
     Chemically clean environments maintained for handling pure de-ionized (DI) water, acids, chemicals, and the like, must be maintained free from contamination. Contamination in a process fluid may destroy hundreds of thousands of dollars in value by introducing contaminants into a process during a single batch. Several difficulties exist in current systems for heating, pumping, and carrying process fluids (e.g. acids, DI water, etc.). Leakage into or out of a liquid must be eliminated. Moreover, leaching and chemical reaction between any contained fluid and the carrying conduits must be eliminated. 
     Elevated temperatures in semiconductor processing are often over 100° C., and often sustainable over 120° C. In certain instances, temperatures as high as 180° C. may be approached. It is preferred that all heating and carrying of process fluids include virtually no possibility of contact with any metals regardless of the ostensibly non-reactive natures of such metals, regardless of a catastrophic failure of any element of a heating, transfer, or conduit system. 
     Conventional immersion heaters place a heating element, typically sheathed in a coating, directly into the process fluid. The heating element and process fluid are then contained within a conduit. Temperature transients in immersion heaters may overheat a sheath up to a melting (failure) point. A failure of a sheath may directly result in metallic or other contamination of the process fluid. Meanwhile, temperature transients in radiant heaters may fracture a rigid conduit. 
     A heating alternative is needed that does not have the risks associated with conventional radiant and immersion-heating elements. A system is needed that is both durable and responsive for heating process fluids. Failure that may result in fluid contamination is an unacceptable risk. 
     BRIEF SUMMARY AND OBJECTS OF THE INVENTION 
     In view of the foregoing, it is a primary object of the present invention to provide a heater for handling process fluids at elevated temperatures in the range of 0° C. to 180° C. It is an object of the invention to provide a heater having electrical resistance in close proximity to a process fluid for heating by conduction and convection without exposing process fluids to a prospect of contamination, even if electrical failures or melting of conductive paths should occur within a heater. 
     Consistent with the foregoing objects, and in accordance with the invention as embodied and broadly described herein, a method and apparatus are disclosed in one embodiment of the present invention as including a heater comprising one or more tubes of quartz. Tubes may be abutted end-to-end with an adaptor (e.g. fluorocarbon fitting) fitted to transition between two tubes in a series. One pass or passage, comprising one or more tubes of quartz in a series, may be fitted on each end to a manifold (e.g. header/footer) comprised of a fluorocarbon material properly sealed for passing liquid into and out of the individual passage. 
     Individual tubes or conduits may improve the temperature distribution therein by altering the internal boundary layer of heated fluids passing therethrough. In one embodiment, a baffle tube, within the outer tube, may have a plug serving to center the baffle in the heating tube. The plug may restrict flow, such that the fluid inside the baffle does not change dramatically. Thus an annular flow between the baffle tube and the outer heating tube may maintain a high Reynolds number in the flow, enhancing the Nusselt number, heat transfer coefficient and so forth. Moreover, the temperature distribution may be rendered nearer to a constant value across the annulus, rather than running with a cold, laminar core. 
     In one embodiment, a heater may be manufactured by electroless nickel plating on a roughened (textured) surface. A resistive, conductive layer may extend along most of the length of a rigid (e.g. quartz) tube. The resistive coating may be configured to connect in series or to multi-phase power along the length of a single tube. Accordingly, a quartz tube may be roughened, etched, dipped, coated, and protectively coated. The quartz tube need not be heated to sinter the conductive layer, which may be plated as a continuous ribbon of well-adhered, resistive, conducting, metallic material. 
     The electrical length of the heated portion may be adjusted by application of an end coating for distributing current around a conduit tube. Conductive material and mechanical fasteners may be added to provide electrical connections between the end coating and power delivery lines. For example, braided cables or straps may be clamped around a soft, conductive interface material surrounding each end of a plated section of a conduit. Mechanical clamps may maintain normal forces against the surface, while accommodating expansion with temperature, without harming mechanical bonds between the conductive/resistive coating and the conduit (substrate). 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other objects and features of the present invention will become more fully apparent from the following description and appended claims, taken in conjunction with the accompanying drawings. Understanding that these drawings depict only typical embodiments of the invention and are, therefore, not to be considered limiting of its scope, the invention will be described with additional specificity and detail through use of the accompanying drawings in which: 
     FIG. 1 is a side elevation view of a heater unit in accordance with the invention; 
     FIG. 2 is a front elevation view of a heater assembly including multiple units of the apparatus illustrated in FIG. 1; 
     FIG. 3 is a perspective view of one embodiment of a coated conduit in accordance with the invention; 
     FIG. 4 is a schematic, side, elevation, cross-section view of a portion of the apparatus of FIG. 3, illustrating the comparative positions of the substrate, resistive coating, end plating (coating), and connection scheme for introducing electricity to the apparatus; 
     FIG. 5 is a block diagram of one embodiment of a process for making a heating unit in accordance with the invention; 
     FIG. 6 is a graph illustrating a relationship between a bath time in a plating composition, illustrating the effect of normalized resistance per square in ohm-inches per inch; 
     FIG. 7 is a graph illustrating a comparison between terminated resistance and watt density in a heater in accordance with the invention as a function of the cured resistance of a coating in accordance with the invention, further illustrating typical termination resistance adjustment depending upon the cured resistance of a conductive and resistive coating; and 
     FIG. 8 is a chart illustrating a change in heating area (function of termination distance), in order to correct for variations in cured (heat treated) resistance values in a resistive coating of an apparatus in accordance with the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     It will be readily understood that the components of the present invention, as generally described and illustrated in the Figures herein, could be arranged and designed in a wide variety of different configurations. Thus, the following more detailed description of the embodiments of the system and method of the present invention, as represented in the Figures, is not intended to limit the scope of the invention, as claimed, but is merely representative of the presently preferred embodiments of the invention. 
     The presently preferred embodiments of the invention will be best understood by reference to the drawings, wherein like parts are designated by like numerals throughout. Those of ordinary skill in the art will, of course, appreciate that various modifications to the detailed schematic diagram may easily be made without departing from the essential characteristics of the invention, as described in connection with the Figures. Thus, the following description of the Figures is intended only by way of example, and simply illustrates certain presently preferred embodiments consistent with the invention as claimed herein. 
     Referring to FIGS. 1-3, an apparatus  10  may be created for heating or otherwise handling process fluids such as those used in the semiconductor industry. The semiconductor-processing industry requires ultra-pure, de-ionized (DI) water, acids, and the like. A conduit  12  may be formed of a comparatively rigid material such as quartz. 
     Fused quartz has been found to resist distortion with temperature and time, providing dimensional stability and repeatable structural properties. Meanwhile, quartz has been found to be sufficiently non-reactive with processing fluids to maintain better than parts-per-billion (or even trillion) purity requirements in acids and water, such as de-ionized water. 
     Fittings  14 ,  16  may support the conduit  12  and apply force  18  from a pressure plate  32 , loader (e.g. spring)  34 , baseplate  36  and adjuster  38  to support a suitable seal  20 . An inlet  22  and outlet  24  may convey fluid along the length  45  of the apparatus  10  from a manifold  46 . A plurality of the individual apparatus  10  may be assembled as a heater  47  in a cabinet  48  or outer frame  48  enclosing an outer envelope  49 . 
     The heater  47  does not expose metals to the process fluid inside the conduits  12 . In one presently preferred embodiment, a resistive coating on the conduit  12  heats the conduit  12 . The heat passes through the wall of the conduit  12  into the process fluid therein. 
     Referring to FIG. 3, a conduit  12  may be formed of a crystalline material such as fused quartz. In general, a conduit  12  may be of any suitable shape. For example, a flat plate may be fitted, as a window, or the like, against a structure suitable for sealing the window. A coating may be applied to such a substrate. Accordingly, the term conduit  12 , may include any substrate, of any shape, suitable for receiving a coating for generating electrical resistance heating. 
     The conduit  12  may define an axial direction  50   a  and radial directions  50   b . A wall  52  of the conduit  12  may extend in an axial direction  50   a  and circumferentially  50   c . The wall  52  may define, or be defined by, an outer surface  54  and an inner surface  56 . 
     In selected embodiments, an outer surface  54  may be treated, such as by mechanical etching to provide a portion of roughened surface  58 . The textured surface  58  may be prepared by a mechanical abrasive action, such as grit blasting, bead blasting, or sandblasting. Accordingly, in a crystalline material, such as quartz, small crystalline chunks may remove from the surface  54 , leaving small, angular, crystalline inclusions in the surface  54 . 
     What is true for the outer surface  54 , may be true for the inner surface  56  in alternative embodiments. For example, due to the processes by which a surface  54  may be coated with a resistive, conducting coating  60 , the wall  52  may be treated to provide a textured surface  58 , at the outer surface  54 , or the inner surface  56 . Since fluids (typically liquids) are transferred between devices, through heaters  10 , and so forth, one practical embodiment contains a fluid flow  78  within a conduit  12 , exposed to a non-reactive, ultra-pure, inner surface  56 . 
     The coating  60  may typically be a substantially continuous film  60  extending axially  50   a  and circumferentially  50   c  about the surface  54 . An end coating  62 , applied over the basic coating  60 , may be formed of the same material, or a different one. Since a major consideration in construction of the heater  10  is the mechanical integrity of the attachment of the coating  60  to the textured surface  58 , the end coating  62  may be of any suitable material. In certain embodiments, the end coating  62  may be applied by a method very different from that of the coating  60 . In alternative embodiments, the end coating  62  may simply be additional material, identical to the coating  60 . The end coating  62  may decrease the resistance of the coating  60  by providing increased cross-sectional area along a portion of the length. Thus, the end coating  62  effectively shortens the resistive coating  60 . 
     The end coating  62  provides less resistance along a circumferential direction  50   c  than does the resistive coating  60  in an axial direction  50   a  or a circumferential direction  50   c . That is, the end coating  62  may include more material per unit of area in order to distribute electricity from a connector lug  64  in an axial  50   a  and a circumferential direction  50   c . Thus, the end coating  62  becomes a distributor or a manifold for electricity provided to a lug  64  or connector  64  suitable for receiving a wire delivering current to the resistive coating  60 . 
     A protective coating  66  of some suitable, conformal material may reduce scratch, wear, and chemical reaction of the resistive coating  60 . The surfaces  54 ,  56  are not necessary uniform from end  68  to end  70  of the conduit  12 . A distance  72  or smooth surface  54  may remain in order to support sealing of the ends  68 ,  70  as described herein. Smooth, fired, quartz formed in a lip  30  provides distinct advantages. 
     A distance  74  from each end  68 , 70 , a lug  64  or band  64  may serve as a base for connections  65  to power inputs. A distance  75  from each end  68 , 70 , an end coating  62  of conductive material may feed electricity into the resistive coating  60 . 
     Electricity travels between the bands  64  and end coatings  62  along a resistance length  76 . Power dissipation for heating requires current and a resistance. The coating  60  is both resistive and conductive along the length  76  in order to carry sufficient current to provide the electrical power (wattage) required. Accordingly, the coating  60  is sized in thickness and length to provide the proper combination of conductivity and resistance along the length  76 . 
     The coating  60  is designed and applied within parameters engineered to balance several factors. For example, if the textured surface  58  is too rough, the conduit  12  may fail under test pressures and burst. If not sufficiently rough, the textured surface  58  may provide inadequate adhesion forces between the resistive coating  60  and the outer surface  54  of the conduit  12 . 
     Likewise, the resistive coating  60  requires uniformity and conductive, cross-sectional area along the length  76  in an axial direction  50   a . However, too much of the coating  60 , may provide so much strength within the coating that the resistive material  60  separates mechanically from the textured surface  58 , due to a superior bond to itself during thermal expansion at elevated temperatures. 
     Ceramics and many materials, such as quartz, provide comparatively little or no expansion with increased temperature. By contrast, most metals provide substantial expansion with increased temperature. Accordingly, at elevated temperatures, the coating  60  tends to expand and separate as a continuous annulus surrounding the conduit  12 . 
     At a microscopic level, the coating  60  tends to shear away from the microscopic inclusions developed in the textured surface  58 . Thus, a balance in application of the coating  60  is required to balance the forces due to the coefficient of thermal expansion with the mechanical bond between the coating  60  and the inclusions in the textured surface  58 . 
     The effective resistance of the coating  60  changes as the coating  60  is heat treated. Heat treatment does not melt the deposited coating  60 . Nevertheless, metallurgical grain boundaries form, grow, and affect electrical conductivity in the coating  60 . If the effective resistance is too high, yet in the range of the design point, the heater  10  does not provide sufficient energy input through the wall  52  into a fluid flow  78 . If the resistance is too low, but close to the design point, the heater  10  provides too much output, and may be outside the desired range of control. In some apparatus, too high a heating rate can damage equipment, including fracturing solids due to differential expansion. 
     The end coating  62  or band  62  if applied too thickly may overcome the adhesion or other bonding between the end coating  62  and the resistive coating  60 . Alternatively, the end coating  62  may maintain a sufficient bond with the coating  60 , but separate the coating  60  from the textured surface  58  if either  60 ,  62 , or their combination is too thick and mechanically rigid. Similarly, as with the resistive coating  60 , applying the end coating  62  too thinly, tends to reduce the average number of atoms at any site, yielding poor uniformity, and inadequate process control for reliable currant conduction. 
     Too high a resistance in the end coating  62  may generate too much heat. Excessive heat may destroy the connection between the end coating  62  and the base resistive coating  60 , or separate both from the textured surface  58 . The types of difficulty that may arise with excessive heat generation may result from too high a resistance in the end coating  62 . 
     A lug  64  or connector band  64  needs to be secured with the same considerations required for the coatings  60 ,  62 , too much material may provide too high strength. Too little material may raise local heating issues as a result of inadequate conductivity. Materials may be selected to provide flexibility or malleability. 
     Referring to FIG. 4, a wall  52  may be thought of as a substrate  80 . Thus, a substrate  80  may generalize a conduit  12  into any particular shape, open, closed, and so forth. As discussed, a thickness  82  of a substrate  80  provides mechanical integrity in a conduit  12 . That is, a thickness  82  of a wall  52  provides mechanical strength. However, the conduits  12  must typically sustain some pressure load. Accordingly, excessive thickness  82  may actually cause a stress distribution between the inner surface  56  and the outer surface  54 . Another concern with the thickness  82  is the effect of the inclusions in the textured surface  58 . The thickness  82  may benefit from being sufficiently large that the inclusions of the textured surface  58  lack sufficient influence to propagate cracks therethrough. 
     The thickness  73  of the resistive coating  60  is precisely controlled. The thickness  73  may be on the order of numbers of atoms in dimension up to some few millions of an inch. At a microscope level, the thickness  73  may be of an order of magnitude the same as of the size of inclusions in the tenured surface  58 , or less. Accordingly, the coating  60  may appear like a crepe material. This crepe may be a thin, crinkly film following the peaks and valleys of the textured surface  58 . 
     Thermal expansion with a rise in temperature may be easily accommodated by localized bending of portions of the coating  60 . However if the thickness  73  becomes too great, the coating  60  behaves as a beam extending in the circumferential direction  50   a  and the axial direction  50   a . Accordingly, the beam may change diameter, applying comparatively large radial forces withdrawing the small irregularities from their places filling the inclusions in the textured surface  58 . 
     Excellent thermal contact between the coating  60  and the conduit  12  requires superior adhesion by balancing the thickness  73 . The value of the thickness  73  may be successfully selected to provide mechanical compliance with the textured surface  58  while providing uniformity. Thus, material selection and selection of the thickness  73  along with selection of the size of the conduit  12  can be used to control the beat input at a desired level for a fluid flow  78  while maintaining mechanical integrity and thermal conductivity. 
     The thickness  77  of the end coating  62  is selected according to similar parameters, as discussed above. Although a solder  78  may be selected from a softer material than the coating  60 , as may the end coating  62 , mechanical mass eventually provides compressive strength. Accordingly, expansion of the band  64  or end coating  62  with an increase in temperature may cause the separation of metals from the inclusions by which capture is maintained. Selecting materials that are comparatively malleable and thin, while having comparatively higher electrical conductivity than the coating  60 , can produce suitable mechanical and electrical integrity. 
     The roughness height  90  is detectable by its effect on light. Visual inspection serves very well, since the roughness height  90  dramatically affects the sheen of the outer surface  54 , even with comparatively slight roughness heights  90 . Thus, the adequacy of the roughness height  90  may be reasonably well detected from a visual inspection. 
     Excessive roughness height  90  may result from removing too much of the wall  52  from the textured surface  58 . A grit size (e.g. bead size), and a time for application of uniform grit blasting may provide a suitable roughness height  90 . The roughness height  90  should accommodate mechanical lodgment of metal atoms within inclusions in the surface. Thus, micro-mechanical anchors grip the thin coating  60  against the outer surface  54 . 
     The roughness height  90  is significant, not for its size alone, which need only accommodate a few atoms of metal, but in the crystalline sharpness and angularity of the inclusions. Because the spalling of material from the outer surface under the influence of grit, bead, or sand blasting will tend to break along crystal boundaries, a fully randomized set of inclusions, including concavities overhung by sharp crystalline comers, may securely capture pockets of metallic atoms of the coating  60 . 
     Likewise, the resistive path of the coating  60  may be affected by the roughness height  90  compared to the thickness  73 . For example, a smooth outer surface  54  tends to provide a rather direct path. A textured surface  58 , provides a circuitous path over hills and valleys. Thus, providing too great thickness  73  may also decrease resistivity reducing the heating wattage below a designed value. 
     Referring to FIG. 5, one embodiment of a method for manufacturing the heaters  10  may include providing  102  the conduit  12  or other substrate  80 , followed by suitable masking  104  and texturizing  106 . Texturizing  106  may include bead blasting, sand blasting, sand blasting, grit blasting, or etching by other means. The texturizing  106  is important for providing mechanical grip, as discussed above. Nevertheless, texturizing  106  should not compromise the mechanical integrity of the conduit  12  under operational pressures. Thus the roughness height  90  is balanced in that it does not create inclusions that will compromise the mechanical integrity of the conduit  12 . 
     Likewise, the wall thickness  82  is selected to balance heat transfer demands for energy transfer per unit area, against surface temperatures and thermal gradients. Thermal gradients are considered in view of the thickness  82  and thermal stresses created. 
     A thin film  60  is applied in a plating process  108 . In one embodiment, electroless nickel plating has been found effective. The plating process is continued for a time selected to provide a thickness  73  that balances current-carrying capacity of the film, mechanical stiffness and strength limits required to maintain adhesion, and coating uniformity (related to both other factors). 
     By balance is meant adequacy and uniformity of performance, either mechanically, thermally, electrically, or a combination thereof. If the coating  60  on a conduit  12  or other substrate  80  is adequate, it may be heat treated  110 . 
     In one embodiment, the heat-treating process  110  involves a metallurgical heat treatment  110 . Such a process  110  does not elevate temperatures sufficiently to melt the metallic coating  60 . Rather, temperatures are sufficiently high during the process  110  to raise the energy level of various atoms within the composition of the coating  60 , encouraging migration of interstitial materials. Migration of interstitial materials fosters growth of various grain boundaries. Growth of grain boundaries affects the binding of electrons into orbitals of various atomic or molecular structures. Thus, the heat-treating process  110  may substantially affect electrical conductivity. Accordingly, the time and temperature of the heat treatment process  110  provide a certain element of control over the effective electrical resistivity of the coating  60 . 
     Heat treating  110  may include a surface treatment. In one embodiment, application  111  or deposition  111  (e.g. vapor deposition) of a surface-protecting layer may include adding a composition (e.g. a silicate, in one embodiment) to the heat-treatment environment (e.g. oven). The application process  111  may include masking portions of the coating  60  that will later be coated with additional conductive materials. The protective process  111  provides a non-reactive coating or passivating coating to reduce oxidation of the resistive coating  60  during heat treating  110 . 
     Following the heat-treating process  110 , and if resistance is satisfactory in the coating  60 , a termination process  112  provides end coatings  62 , and so forth. The termination process  112  may include, among other steps, application  114  of a termination coating  62  or end coating  62  to reduce the resistance that would be available in the coating  60 . Resistance is typically lowered by half an order of magnitude. The thickness  77  of the end coating  62  must be balanced to provide good current distribution while not compromising the mechanical integrity of the bond between the conductive-resistive materials and the conduit  12  or substrate  80 . 
     The termination process  112  may involve application  114  of a end coating  62  having a specific length  75  calculated to provide a precise power delivery in the heater  10 . Similarly, a soft, compliant, conductive material  63  may be added  116  over a portion of the end coating for receiving a connector  64 . The connector  64  may be a suitable braided conductor  64 , applied  118 , and then mechanically clamped  120  by a clamping mechanism  67 . 
     Chemical bonds have been found unsatisfactory in many instances, as they add mechanical thickness and stiffness of materials. Thus, the compliant material  63 , yielding under the load of a braided conductor  64 , at the urging of a clamping mechanism  67 , provides sufficient compliance that strength and stiffness of the film  60  are not significantly affected. Therefore, mechanical bonding of the coating  60  to the conduit  12  (e.g., substrate  80 ) is not compromised. A protective, conformal coating  66  may be applied  122  following, or as part of, the termination process  112 . 
     The plating process  108  may be one of several types, including vapor deposition, sputtering, painting, sintering, powder coating, and electroless plating. In electroless plating, such as electroless nickel plating, application  109  of a surfactant may greatly improve the quality of the coating  60 . Application  109  of a surfactant may actually involve a surfactant scrub  109  in which vigorous application of force breaks down any pockets of gas that might adhere to concavities in the textured surface  58 . Thereafter, the coating  60  may form, maintaining a continuous mechanical structure about the inclusions of the textured surface  58 . 
     As a texturing method, bead blasting has provided considerable uniformity in the fracture mechanics of forming inclusions. Also, pressure tests show that mechanical integrity may be maintained thereby. 
     Referring to FIG. 6, a graph  130  having a time axis  132  and resistance axis  134  illustrates various data points  136  from tests. The values  136  characterize the effect of time, during plating, on the initial resistance  134  of the coating  60 . The scales are logarithmic. Thus, the process results in resistance being dependent upon a power of time. However, the relationship does not appear to change dramatically at any point on the graph  130 . 
     Referring to FIG. 7, a chart  140  of a resistance in a range  204  corresponds to a value of heat-treat temperature in a domain  144  of temperatures for the coating  60 . The values  148  reflect the adjustment of resistance in ohm-inches per inch, due to a particular temperature during heat treating of the coating  60 . The resistance of the coating  60  may vary due to variations in controlled parameters, such as the time and temperature associated with heat treatment. Parametric controls may vary during the plating process, and the heat-treating process  110 . Thus, FIG. 7 reflects an ability to adjust the effective resistance of the apparatus  10  according to the heat-treat temperature. 
     Referring to FIG. 8, a graph  150  shows both a percentage  152  of available surface area heated by the coating  60  and a watt density  154  as a function of resistance per square  156 . The graph  150  shows the correction ability for any given resistivity resulting from the heat-treat process  110 . That is, given a particular value of the cured resistance  156 , a final percentage  152  of area to be heated (powered) may be determined. Thus, the exact locations of the end coatings may be designed to obtain the desired heated area. Similarly, for a particular cured resistance  156 , a watt density  154  may be determined. These results are typical of the influence that the end termination process  112  can have on correcting the overall value of resistance of the coating  60  in an apparatus  10 . 
     From the above discussion, it will be appreciated that the present invention provides apparatus and methods for heating ultra pure fluids in a hyper-clean environment. Power densities are very high, while heater reliability is superior. Meanwhile, manufacturing adjustments are available to produce high yields of highly predictable product. 
     The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative, and not restrictive. The scope of the invention is, therefore, indicated by the appended claims, rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.