Abstract:
A method of manufacturing an ignition device is provided. The method includes patterning a plurality of resistors on a membrane to form heating elements and thermally isolating the heating elements from an external environment via a cavity disposed adjacent to the heating elements.

Description:
BACKGROUND 
   The invention relates generally to gas appliances, and more particularly to ignition devices for igniting a flow of gas in gas appliances and other gas-fired equipment. The invention may be applied to any application where ignition of a fuel air mixture is required. 
   Conventional gas appliances, such as those found in households, have one or more burners in which gas is mixed with air and burned at a cooktop or in an enclosed space, such as an oven. Various types of igniters are employed in such gas appliances for igniting the flow of gas. For example, in some systems spark igniters are employed that generate a spark to ignite the gas flowing to the burner. In certain other systems, ceramic hot surface igniters are employed that include heating elements for generating sufficient heat to ignite the gas supplied to the burner. 
   In certain systems, silicon carbide or silicon nitride hot surface igniters are employed for igniting the gas flow. Some of the problems with these conventional igniters are that they are porous, fragile, power hungry, relatively expensive and are fairly slow to reach ignition temperature. In addition, the resistance versus temperature characteristics of these conventional silicon carbide igniters may alter or drift over time, thereby adversely affecting their reliability. 
   Unfortunately, existing hot surface igniters need substantially high power for operation and can require an unacceptably long time to reach the required temperature for ignition. Further, heating elements of the igniters are exposed to the environment, resulting in accelerated failure of such elements due to degradation and contamination of the elements. Additionally, such igniters are often subjected to impacts from an operator during routine cleaning and maintenance, which may cause the igniter to break. Furthermore, such igniters require precise control of the voltage supplied to the heating elements. For example, a relatively high voltage may result in premature failure of the heating elements. Similarly, an applied voltage less than the required voltage may result in poor performance of the igniter. 
   Accordingly, it would be desirable to develop an ignition device for a gas appliance that has reduced power and voltage requirements. It would also be advantageous to develop an ignition device that requires relatively less time to reach the required ignition temperature, and is more robust and reliable. 
   BRIEF DESCRIPTION 
   Briefly, according to one embodiment a method of manufacturing an ignition device is provided. The method includes patterning a plurality of resistors on a membrane to form heating elements and thermally isolating the heating elements from an external environment via a cavity disposed adjacent to the heating elements. 
   In another embodiment, a method of manufacturing an ignition device is provided. The method includes depositing a thermal oxide layer on front and back sides of a substrate and depositing an electrically conductive material on the front and back sides of the substrate. The method includes etching the electrically conductive material on the front side of the substrate to form heating elements on the substrate and depositing a non-electrically conductive material adjacent to the electrically conductive material. The method also includes etching the non-electrically conductive materials on the front side of the substrate to form contact pad openings to the electrically conductive material. 

   
     DRAWINGS 
     These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
       FIG. 1  is a diagrammatical illustration of a gas range having an ignition device in accordance with aspects of the present technique; 
       FIG. 2  is an exploded perspective view of a gas burner employed in the gas range of  FIG. 1  in accordance with aspects of the present technique; 
       FIG. 3  is a top plan view of the gas burner of  FIG. 2  in accordance with aspects of the present technique; 
       FIG. 4  is a diagrammatical illustration of an ignition device incorporated in the gas range of  FIG. 1  in accordance with aspects of the present technique; 
       FIG. 5  is a cross-sectional view of an exemplary configuration of ignition device employed in the cooktop of  FIG. 4  in accordance with aspects of the present technique; 
       FIG. 6  is a top plan view of the ignition device of  FIG. 5  in accordance with aspects of the present technique; 
       FIG. 7  is a diagrammatical representation of an exemplary process for manufacturing the ignition device of  FIGS. 5 and 6  in accordance with aspects of the present technique; 
       FIG. 8  is a diagrammatical representation of another exemplary process for manufacturing the ignition device of  FIGS. 5 and 6  in accordance with aspects of the present technique; 
       FIG. 9  is a diagrammatical illustration of an exemplary configuration of the ignition device of  FIG. 4  in accordance with aspects of the present technique; 
       FIG. 10  is a graphical representation depicting change in resistance of the ignition device of  FIGS. 5 and 6  with temperature in accordance with aspects of the present technique; 
       FIG. 11  is a graphical representation depicting the time response of the ignition device of  FIGS. 5 and 6  to reach an ignition temperature in accordance with aspects of the present technique; 
       FIG. 12  is a cross-sectional view of another exemplary configuration of ignition device employed in the cooktop of  FIG. 4  in accordance with aspects of the present technique; and 
       FIG. 13  is a cross-sectional view of another exemplary configuration of the ignition device employed in the cooktop of  FIG. 4  in accordance with aspects of the present technique. 
   

   DETAILED DESCRIPTION 
   As discussed in detail below, embodiments of the present technique function to provide an ignition device for gas range and cooktop applications. Although the present discussion focuses on ignition devices for a gas range, the ignition devices may be employed in other applications, such as gas heater devices, gas ovens, gas boils, gas kilns, and so forth. Turning now to the drawings and referring first to  FIG. 1 , an exemplary gas range  10  is illustrated. The gas range  10  includes a body  12  and a cooktop  14 . Further, the gas range includes an oven  16  positioned below the cooktop  14 . A range backsplash  18  extends upwards of the cooktop  14  and may include control features for controlling the operational parameters of heating elements for the cooktop  14  and the oven  16 . 
   In the illustrated embodiment, the gas range  10  includes four gas burner assemblies  20  positioned in the cooktop  14  and configured to receive a flow of gas for combustion. However, a greater or lesser number of the gas burner assemblies  20  may be envisaged. Further, each burner assembly  20  extends upwardly and a grate  22  is positioned over each burner assembly  20 . In the present embodiment, each of the grates  22  includes a flat surface thereon for supporting the cooking utensils over the burner assembly  20 . In the illustrated embodiment, an ignition device is disposed adjacent each burner assembly  20  and is configured to ignite the gas flow received by the gas burner assembly  20 . The ignition device employed in the gas range  10  will be described in a greater detail below. 
     FIG. 2  is an exploded perspective view of a gas burner assembly  30  employed in the gas range  10  of  FIG. 1 . In a presently contemplated configuration, the gas burner assembly  30  includes a burner body  32  and a base portion  34 . Further, the gas burner assembly  30  also includes a sidewall  36  extending from the base portion  34 . The gas burner assembly  30  receives a gas flow from a gas conduit  38  having an entry area  40  and a burner throat region  42 . As will be appreciated by those skilled in the art, the gas flow refers to a combustible gas or a gaseous fuel-air mixture. In this embodiment, the gas burner assembly  30  is disposed on a support surface  44 , such as cooktop  14  (see  FIG. 1 ) of a gas cooking appliance. In addition, a burner cap  46  is disposed over the top of the burner body  32  and defines a main fuel chamber  48 . A toroidal shaped upper portion  50  of the burner body  32  in combination with cap  46  defines an annular diffuser region. The burner assembly  30  also includes at least one ignition device extending through an opening in the base portion  34 .  FIG. 3  is a top plan view  52  of the gas burner of  FIG. 2 . As illustrated, the gas burner  52  includes an ignition device  54  positioned adjacent to the gas burner assembly for igniting the flow of gas. The operation of the ignition device in the burner assembly  30  will be described below with reference to  FIG. 4 . 
     FIG. 4  is a diagrammatical illustration of a cooktop  56  of the gas range  10  of  FIG. 1  having the ignition device of  FIG. 3 . In the illustrated embodiment, the cooktop  56  includes a supporting base  58 , which in turn includes four burners  60 ,  62 ,  64  and  66  disposed on the base  58 . In addition, the cooktop  56  includes ignition devices  70 ,  72 ,  74  and  76  coupled to the burners  60 ,  62 ,  64  and  66  for igniting the gas flow received by the burners. In the illustrated embodiment, the gas burners  60 ,  62 ,  64  and  66  receive a flow of gas such as natural gas, or propane from a gas source  78  via a gas conduit  80 . Further, the flow of gas to the burners  60 ,  62 ,  64  and  66  is controlled by a valve  82  disposed upstream of the burners. In a presently contemplated configuration, a power source  84  is coupled to the ignition devices  70 ,  72 ,  74  and  76  to apply a voltage to the ignition devices for heating them to achieve a flame ignition temperature. Further, a controller  86  may be coupled to the ignition devices  70 ,  72 ,  74  and  76  to control the amount of voltage applied to them for ignition of the gas flows. In the illustrated embodiment, the ignition devices  70 ,  72 ,  74  and  76  include hot surface igniters that will be described below with reference to  FIGS. 5 and 6 . 
     FIG. 5  is a cross-sectional view of an exemplary configuration  90  of ignition devices  70 ,  72 ,  74  and  76  employed in the cooktop  56  of  FIG. 4 . In the illustrated embodiment, the ignition device  90  includes a membrane  92  and a plurality of heating elements embedded in the membrane  92  such as represented by reference numeral  94 . In one embodiment, a thickness of the membrane  92  is about 4 micrometers to about 5 micrometers. The heating elements  94  are configured to heat the membrane  92  on application of voltage through the heating elements  94 . In the illustrated embodiment, the heating elements  94  include a plurality of microscale resistors. In certain embodiments, the heating elements  94  are disposed at a pre-determined distance from the membrane  92 . In this embodiment, the ignition device  90  also includes contact pads, such as represented by reference numeral  96  for facilitating electrical connection of the ignition device  90 . Further, the ignition device  90  includes a cavity  98  disposed adjacent to the heating elements  94  and configured to provide thermal isolation of the heating elements  94 . Moreover, the cavity  98  is sealed in vacuum or in an inert environment to substantially prevent degradation of the heating elements  94  by oxidation. Advantageously, the membrane structure of the ignition device with a vacuum or inert environment substantially reduces power consumption of the ignition device  90 . In one embodiment, the power consumption by the ignition device  90  is in a range of about 1 watt to about 3 watts. The ignition device  90  is bonded to a silicon wafer  100  disposed adjacent to the cavity  98  for sealing the ignition device  90 . In the illustrated embodiment, a non-conducting layer  104  is disposed below the doped layer  96 . As will be appreciated by those skilled in the art, due to the thermal isolation of the structure of the ignition device  90 , the hot surface exposed to the combustion gases will attain the temperature of the heating elements  94  in the device  90 . Furthermore, the membrane  92  enables the device  90  to reach the desired temperature in a relatively short time. In certain embodiments, the time taken to reach the desired temperature is less than 2 seconds. 
   In the illustrated embodiment, the membrane  92  includes a non-electrically conductive high temperature material. Examples of the non-electrically conductive high temperature material include un-doped silicon carbide, silicon nitride, boron nitride, or other suitable ceramic materials. Further, the heating elements  94  include a high temperature electrically conductive material that is compatible with the membrane  92 . Examples of such materials include doped ceramics and metallic materials. In the illustrated embodiment, the heating elements  94  and contact pads  96  include doped silicon carbide. In other embodiments, the heating elements  94  may include other conductive high temperature materials such as platinum, titanium, doped polysilicon, or other metals. In certain embodiments, the membrane  92  may include a plurality of layers of doped and un-doped silicon carbide to provide a gradient of coefficient of thermal expansion for substantially reducing thermal stresses. In certain other embodiments, the membrane  92  may be coated with materials that will provide a gradation in thermal properties of the device  90 . In operation, a voltage is applied to the heating elements  94  via the voltage source  84  (see  FIG. 4 ) and a gas flow  102  is ignited via hot surface ignition by the heating elements  94 . Particularly, the surface of the ignition device  90  will attain the temperature of the heating elements  94  due to heat transfer from the heating elements  94  to the surface. 
     FIG. 6  is a top plan view of the ignition device  90  of  FIG. 5 . As illustrated, the ignition device  90  includes a two dimensional microplate. The microplate  90  includes the plurality of heating elements  94  embedded within the membrane  92 . The heating elements  94  are configured to heat the microplate on application of voltage. In the illustrated embodiment, the heating elements  94  include a doped silicon carbide material that can sustain substantially high temperatures and harsh environments. However, other materials having similar properties may be envisaged. The microplate  90  also includes contact pads  106  and  108  for facilitating the electrical connections for the ignition device  90 . In this embodiment, the contact pads  106  and  108  include doped silicon carbide. In certain embodiments, the contact pads  106  and  108  include other suitable metals. Further, a contact pad material may be deposited on the silicon carbide contact pads  106  and  108 . Examples of such materials include titanium, tungsten, gold, nickel and combinations thereof. 
   The ignition device  90  described above may be manufactured through a batch semiconductor fabrication process.  FIG. 7  is a diagrammatical representation of an exemplary process  112  for manufacturing the ignition device of  FIGS. 5 and 6 . Furthermore, various operations may be described as multiple discrete steps performed in a manner that is helpful for understanding embodiments of the invention. However, the order of description should not be construed as to imply that these operations always need be performed in the order they are presented, nor that they are even order dependent. The process begins at step  114  where a silicon wafer  116  is provided as a substrate. In this embodiment, the silicon wafer  116  includes a double-sided polished (DSP) wafer. Further, the resistivity of the silicon wafer  116  is in a range of about 1 ohm-cm to about 10 ohm-cm. At step  118 , the substrate  116  is etched to create the silicon etch mask alignment marks such as represented by reference numerals  120  and  122 . Further, at step  124 , electrically insulative layers  126  and  128  are deposited on both sides of the silicon wafer  116 . The electrically insulative layers  126  and  128  substantially prevent current flow into the wafer  116  so that the current flows through the resistive elements on the membrane  92 . In this embodiment, the thickness of the silicon wafer  116  is about 300 micrometers to about 600 micrometers and the thickness of the electrically insulative layer may be from about 1 micrometers to about 3 micrometers. Examples of the electrically insulative layers include silicon dioxide, low pressure chemical vapor deposited silicon dioxide, silicon nitride and undoped silicon carbide. In one embodiment, the electrically insulative layers  126  and  128  are grown on the silicon wafer  116 . In certain other embodiments, the electrically insulative layers  126  and  128  may be deposited on the silicon wafer  116  via techniques such as plasma enhanced chemical vapor deposition (PECVD), low temperature oxide (LTO) and high temperature oxide (HTO) deposition techniques. 
   At step  132 , a layer of electrically conductive material such as doped poly-silicon carbide is deposited on either sides of the silicon substrate  116  as represented by reference numerals  134  and  136 . In this embodiment, the thickness of the doped poly-silicon carbide layers  134  and  136  is about 1 micrometers and the resistivity of the doped poly-silicon carbide is about 0.01 ohm-cm to about 0.2 ohm-cm. Further, at step  138 , the doped poly silicon layer  134  on the front side of the substrate  116  is etched to create heating elements  140  and contact pads  142  on the substrate  116 . As previously described, the heating elements  140  may be coupled to a power source for applying a voltage to the heating element  140  for heat generation. In the present embodiment, the doped poly-silicon carbide layer  134  is masked via a photoresist masking technique, and is subsequently etched via inductively coupled plasma (ICP) etching technique. However, other etching techniques may be employed. 
   At step  144 , an electrically insulative material such as undoped poly-silicon carbide layers  146  and  148  are disposed on the doped poly-silicon carbide layers  140  and  136 . In this embodiment, a thickness of the undoped poly-silicon carbide layers  146  and  148  is about 1 micrometers to about 5 micrometers and a resistivity of the layers  146  and  148  is about 2 ohm-cm to about 20 ohm-cm. Subsequently, at step  150 , the undoped silicon layer  146  is etched to form contact pad hole  152 . In this embodiment, the undoped silicon layer  146  is etched via photoresist masking and ICP etching techniques. Moreover, the silicon carbide layers  136  and  148  are dry etched on the backside as represented by step  154 . A layer of silicon nitride  156  is deposited on the backside of the substrate  116  via plasma enhanced chemical vapor deposition (PECVD) technique, as represented by step  158  to serve as an etch mask for step  160 . However, other materials such as silicon carbide may be employed as an etch mask. In certain embodiments, the nitride layer  156  may be deposited via low pressure chemical vapor deposition (LPCVD) technique. 
   Further, at step  160  the oxide layer  128  is patterned and etched to form patterned oxide  162  and  164  to expose the silicon for etching. In this embodiment, a cavity  166  is formed by wet etching, such as by employing potassium hydroxide (KOH). In certain other embodiments, the cavity  166  may be formed using Deep Reactive Ion Etching. Further, at step  168 , the silicon nitride layer  156  and silicon dioxide  162  and  164  are removed by employing a combination of wet and dry etch techniques, as represented by reference numeral  168 . Moreover, a silicon wafer  170  is bonded in vacuum adjacent the cavity  166  as represented by step  172  to form the ignition device. 
     FIG. 8  is a diagrammatical representation of another exemplary process  180  for manufacturing the ignition device of  FIGS. 5 and 6 . In the illustrated embodiment, a p doped silicon substrate  182  is etched to form a cavity  184 , as represented by reference numeral  186 . Next, at step  188 , silicon dioxide is deposited and patterned to form lines in and adjacent the cavity  184  as represented by reference numerals  190  and  192 . Further, the patterned lines  190  are etched to a required depth  194  (step  196 ). At step  198 , thin sidewall oxide layer  200  is grown or deposited. Moreover, at step  202 , the silicon etch is extended to a desired depth as represented by reference numeral  204 . Next, the silicon is isotropically etched and the oxide is stripped to form the microwires  208  within the cavity, that function as heating elements of the ignition device (step  206 ). In one embodiment, the plurality of microwires  208  are coupled in a series arrangement. Alternatively, the microwires  208  may be coupled in a parallel arrangement. In the illustrated embodiment, the number of microwires employed in the ignition device is determined based upon a resistivity of wires  208 , geometry of wires  208 , an applied voltage, and the desired temperature of the device when energized. In the illustrated embodiment, the microwires  208  are made of silicon. In certain embodiments, the microwires  208  include tungsten, or molybdenum disilicide. In one embodiment, the microwires  208  include a material having a melting point that is greater than about 1200° C. Further, at step  210 , a silicon carbide or a platinum coated wafer  212  is bonded adjacent the cavity  184  in vacuum to form the ignition device. 
     FIG. 9  is a diagrammatical illustration of an exemplary configuration  214  of the ignition device of  FIG. 4 . In the illustrated embodiment, the ignition device  214  has a U-shaped configuration comprising two parallel legs and a central zone. Heating elements  216  are disposed in the central zone and contact pads  218  and  220  are disposed on opposite sides of the heating elements. As described earlier, the heating elements  216  include a doped silicon carbide material that can sustain substantially high temperatures and harsh environments. However, other materials having similar properties may be envisaged. Further, contact pads  218  and  220  facilitate electrical connections for the ignition device  214 . In this embodiment, the contact pads  218  and  220  include doped silicon carbide, nickel, gold, platinum, tungsten and combinations thereof. The ignition device  214  may be manufactured by exemplary processes described above with reference to  FIGS. 7 and 8 . 
     FIG. 10  is a graphical representation  222  depicting change in resistance of the ignition device of  FIGS. 5 and 6  with respect to change in temperature of the ignition device. As illustrated, the ordinate axis represents temperature  224  and the abscissa axis represents a resistance  226  of the heating element of the ignition device. The profile of the change in temperature at varying resistance is represented by reference numeral  228 . In the illustrated embodiment, the resistance  226  varies as a function of temperature  224 , as indicated by the profile  228 . Those skilled in the art will recognize that this relationship offers the potential to use the same general structure, made by the methods described above, as a temperature sensor to sense the temperature based upon a measured resistance of the heating elements (e.g., by application of a test voltage that would result in a known voltage drop as a function of the resistance). 
     FIG. 11  is a graphical representation  230  depicting the time response for reaching an ignition temperature in the ignition device  90  of  FIGS. 5 and 6 . In this embodiment, the voltage applied to the ignition device  90  is represented by profile  232  and the temperature of the heating elements  94  of the ignition device  90  is represented by reference numeral  234 . Initially, the ignition device  90  is at a room temperature, as indicated by numeral  236 . Further, the time required by the ignition device to attain an ignition temperature  238  from the room temperature  236  is represented by reference numeral  240 . It should be noted that the time  240  required by the ignition device to achieve the ignition temperature  238  is substantially less as compared to existing ignition devices. In this embodiment, the time  240  taken to attain the ignition temperature is about 100 ms. 
     FIG. 12  is a cross-sectional view of another exemplary configuration  250  of the ignition device of  FIG. 5 . In the illustrated embodiment, the ignition device  250  includes the membrane  92  and the plurality of heating elements  94  embedded in the membrane  92 , as described earlier with reference to  FIG. 5 . The membrane  92  includes un-doped silicon carbide and the heating elements  94  include doped silicon carbide. Further, the ignition device  250  also includes contact pads  96  to facilitate the electrical connection of the ignition device  250 . In addition, the cavity  98  is disposed adjacent to the heating elements. In this embodiment, a layer of un-doped silicon carbide  252  is disposed adjacent to the heating elements  94  for substantially preventing the heating elements  94  from oxidation and changing resistance. Advantageously, the additional layer of un-doped silicon carbide may eliminate the need for sealing the cavity  98  in vacuum for reaching a desired ignition temperature. 
     FIG. 13  is a cross-sectional view of another exemplary configuration  254  of the ignition device of  FIG. 5 . In the illustrated embodiment, the ignition device  254  includes the un-doped silicon carbide layer  252  disposed adjacent to the heating elements  94 . Additionally, the ignition device  254  also includes a first oxidation resistance layer  256  disposed above the membrane  92  and a second oxidation resistance layer  258  disposed adjacent to the un-doped silicon carbide layer  252  for substantially preventing the membrane  92  and the un-doped silicon carbide layer  252  from oxidation and changing mechanical properties. As will be appreciated by one skilled in the art the ignition devices  250  and  254  may be manufactured by exemplary processes illustrated above with reference to  FIGS. 7 and 8 . 
   The various aspects of the structures and methods described hereinabove have utility in gas appliances and heating equipment, used in various applications. In particular, the ignition devices described above may be employed in gas fuel ignition applications, such as furnaces and cooking appliances, as well as in various industrial and commercial settings, such as on boilers, water heaters, industrial furnaces, and so forth. As noted above, the ignition device needs substantially less power for operation and attains the required ignition temperature within a relatively short period of time. Further, the reduction in power consumption allows for a continuous operation of the ignition device and provides the ability to maintain an energizing signal to the device while gas is flowing, so as to automatically reignite the flame if it is extinguished. Additionally, the heating elements of the ignition device are not directly exposed to the environment, thus resulting in a more robust device. 
   It should be noted that, as described and claimed herein, the invention offers improved structures and methods for gas appliances generally. That term is intended to be understood broadly to include both consumer appliances, as well as other gas-burning devices and systems of the types mentioned above. 
   While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.