Patent Publication Number: US-8115272-B2

Title: Silicon dioxide cantilever support and method for silicon etched structures

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a divisional application that claims priority from U.S. patent application Ser. No. 12/454,257, filed May 14, 2009, which is hereby incorporated by reference for all purposes. 
    
    
     BACKGROUND 
     The present invention relates generally to structures and methods for strengthening/supporting dielectric membranes which extend over the over openings/cavities in integrated circuit devices, and more particularly to deep trench oxide post/support structures extending through openings/cavities to support various parts of the membranes. 
     This invention is related to the structures and methods disclosed in the assignee&#39;s pending application “INFRARED SENSOR STRUCTURE AND METHOD” by Walter B. Meinel and Kahn V. Lazarov, Ser. No. 12/380,316, filed Feb. 26, 2009, and incorporated herein by reference. 
     Integrated semiconductor sensors can make use of “cantilevered” silicon dioxide membrane structures which extend from, or “overhang” from, a silicon base or the like. Such membrane structures tend to be fragile and therefore susceptible to damage during assembly operations that occur after formation of the membrane structures. For example, to achieve maximum sensitivity, the cavity openings covered by the SiO2 membrane in the infrared sensors described in the above mentioned Meinel et al. application should be as large as possible. The larger the cavity opening is, the more fragile the SiO2 membrane will be. 
     The closest prior art is believed to include the article “Investigation Of Thermopile Using CMOS Compatible Process and Front-Side Si Bulk Etching” by Chen-Hsun-Du and Chengkuo Lee, Proceedings of SPIE Vol. 4176 (2000), pp. 168-178, incorporated herein by reference. Infrared thermopile sensor physics and measurement of IR radiation using thermopiles are described in detail in this reference. Prior Art  FIG. 1A  herein shows the CMOS-processing-compatible IR sensor integrated circuit chip in  FIG. 1  of the foregoing article. “Prior Art”  FIG. 1A  herein is similar to that drawing, and Prior  FIG. 1B  herein shows the top perspective view of the same IR sensor integrated circuit chip illustrated in  FIG. 2  of the foregoing article. 
     Referring to Prior Art  FIG. 1A  herein, the IR sensor chip includes a silicon substrate  2  having a CMOS-processing-compatible dielectric (SiO2) stack  3  thereon including a number of distinct sub-layers. A N-type polysilicon (polycrystalline silicon) trace  11  and an aluminum trace M 1  in dielectric stack  3  form a first “thermopile junction” where one end of the polysilicon trace and one end of the aluminum trace are connected. Additional oxide layers and additional metal traces also may be included in dielectric stack  3 . An oxide passivation layer  12 A is formed on top of dielectric stack  3 , and a nitride passivation layer  12 B is formed on oxide passivation layer  12 A. A number of silicon etchant openings  24  extend through nitride passivation layer  12  and dielectric stack  3  to the top surface of silicon substrate  2  and are used to etch a cavity  4  in silicon substrate  2  underneath the portion of dielectric stack  3  in which the thermopile is formed, to thermally isolate it from silicon substrate  2 . 
     Prior Art  FIG. 1A  is taken along section line  1 A- 1 A of Prior Art  FIG. 1B , which is essentially similar to  FIG. 2  of the above mentioned Du and Lee reference. Cavity  4  is etched underneath SiO2 stack  3  by means of silicon etchant introduced through the various etchant openings  24 , which are relatively large and irregular.  FIG. 1B  shows various metal-polysilicon strips MP 1  each of which includes an aluminum strip M 1  and a polysilicon strip  11  which makes electrical contact to the aluminum strip M 1  as shown in  FIG. 1B . The metal strips M 1  run parallel to the polysilicon strips  11  and, except for the electrical contact between them as shown in  FIG. 1A , are separated from polysilicon strips  11  by a sublayer of SiO2 stack  3 . Although not shown in  FIG. 1A , the dielectric material directly above metal strips M 1  actually has corresponding steps which are indicated by reference numerals MP 2  in  FIG. 1B . The relatively large etchant openings  24  and their various angular shapes cause the “floating” membrane consisting of the various metal-polysilicon strips MP 1  and the central section  3 A of SiO2 stack  3  supported by metal-polysilicon strips MP 1  to be very fragile. Such fragility ordinarily results in an unacceptably large number of device failures during subsequent wafer fabrication, subsequent packaging, and ultimate utilization of the IR sensor of  FIGS. 1A and 1B . 
     A second thermopile junction (not shown) is also formed in dielectric stack  3  directly over a silicon substrate  2  and is not thermally isolated from silicon substrate  2 , and therefore is at essentially the same temperature as silicon substrate  2 . The first and second thermopile junctions are connected in series and form a single “thermopile”. The various silicon etchant openings  24  are formed in regions in which there are no polysilicon or aluminum traces, as shown in the dark areas in  FIG. 2  of the Du and Lee article. 
     Incoming IR radiation indicated by arrows  5  in Prior Art  FIG. 1A  impinges on the “front side” or “active surface” of the IR sensor chip. (The “back side” of the chip is the bottom surface of silicon substrate  2  as it appears in Prior Art  FIG. 1A .) The incoming IR radiation  5  causes the temperature of the thermopile junction supported on the “floating” portion of dielectric membrane  3  located directly above cavity  4  to be greater than the temperature of the second thermopile junction (not shown) in dielectric membrane  3  which is not thermally insulated by cavity  4 . 
     It is well-known that the upper limit of the operating frequency of an integrated circuit is often determined by the amount of parasitic capacitance associated with circuit elements such as resistors and/or capacitors and/or inductors therein. It would be very beneficial to be able to substantially lower such parasitic capacitance. 
     It would be highly desirable to provide integrated circuit devices which include fragile “cantilevered” dielectric membranes and which are more economical and more robust than those known in the prior art. It also would be highly desirable to provide robust integrated circuits that are operable at higher RF frequencies than previously have been economically achievable. 
     There is an unmet need for integrated circuit devices which include fragile “cantilevered” dielectric membranes and which are more robust than those known in the prior art. 
     There also is an unmet need for an IR radiation sensor which includes a fragile “cantilevered” dielectric membrane and which is more robust than those known in the prior art. 
     There also is an unmet need for a CMOS-processing-compatible IR radiation sensor chip which is substantially more robust than those of the prior art. 
     There also is an unmet need for an improved method of fabricating an IR radiation sensor. 
     There also is an unmet need for a robust, economical integrated circuit that is operable at higher RF frequencies than have been previously achievable for similar integrated circuits. 
     There also is an unmet need for a way of providing a circuit component having reduced parasitic capacitance in an integrated circuit. 
     There also is an unmet need for a way of providing a resistor and/or a capacitor and/or an inductor having reduced parasitic capacitance in an integrated circuit. 
     SUMMARY 
     It is an object of the invention to provide integrated circuit devices which include fragile “cantilevered” dielectric membranes and which are more robust than those known in the prior art. 
     It is another object of the invention to provide an IR radiation sensor which includes a fragile “cantilevered” dielectric membrane and which is more robust than IR radiation sensors known in the prior art. 
     It is another object of the invention to provide a more accurate IR radiation sensor which includes a fragile “cantilevered” dielectric membrane and which is more robust than those known in the prior art. 
     It is another object of the invention to provide a CMOS-processing-compatible IR radiation sensor chip which is substantially more robust than those of the prior art. 
     It is another object of the invention to provide an improved method of fabricating an IR radiation sensor. 
     It is another object of the invention to provide a robust, economical integrated circuit that is operable at higher RF frequencies that have been previously achievable in similar integrated circuits. 
     It is another object of the invention to provide a circuit component having reduced parasitic capacitance in an integrated circuit. 
     It is another object of the invention to provide a resistor and/or a capacitor and/or an inductor having reduced parasitic capacitance in an integrated circuit. 
     Briefly described, and in accordance with one embodiment, the present invention provides a semiconductor device including a semiconductor layer ( 2 ) having therein a cavity ( 4 ). A dielectric layer ( 3 ) is formed on the semiconductor layer. A plurality of etchant openings ( 24 ) extend through the dielectric layer for passage of etchant for etching the cavity. An SiO2 pillar ( 25 ) extends from a bottom of the cavity to engage and support a portion of the dielectric layer extending over the cavity. In one embodiment, a cap layer ( 34 ) on the dielectric layer covers the etchant openings. 
     In one embodiment, the invention provides a semiconductor device including a semiconductor layer ( 2 ), a dielectric layer ( 3 ) disposed on the semiconductor layer ( 2 ), and a portion of the semiconductor layer ( 2 ) extending over a cavity ( 4 ) in the semiconductor layer ( 2 ). A plurality of etchant openings ( 24 ) extend through the dielectric layer ( 3 ) for passage of etchant for etching the cavity ( 4 ). A pillar ( 25 ) of dielectric material extends from a bottom of the cavity ( 4 ) to engage and support the dielectric layer ( 3  of the portion of the dielectric layer ( 3 ) extending over the cavity ( 4 ), to thereby increase robustness of the semiconductor device. In a described embodiment, the pillar ( 25 ) is composed of SiO2, and the semiconductor layer ( 2 ) is composed of silicon. A cap layer ( 34 ) over the dielectric layer ( 3 ) covers the etchant openings ( 24 ). A passivation layer ( 12 ) is disposed on the dielectric layer ( 3 ), and the etchant openings ( 24 ) extend through the passivation layer ( 12 . In a described embodiment, the cap layer ( 34 ) is composed of roll-on epoxy film in the passivation layer ( 12 ). 
     In a described embodiment, a first thermocouple junction ( 7 ) is included in a membrane portion of the dielectric layer ( 3 ) extending over the cavity ( 4 ), and a second thermocouple junction ( 8 ) is included in another portion of the dielectric layer ( 3 ) disposed directly on the semiconductor layer ( 2 ), the first ( 7 ) and second ( 8 ) thermocouple junctions being coupled to form a thermopile ( 7 , 8 ). 
     In another described embodiment, a passive component ( 87 , 88 , 94 ) is included in a membrane portion of the dielectric layer ( 3 ) extending over the cavity ( 4 ) to provide low parasitic capacitance associated with the passive component. Material, such as gas, in the cavity ( 4 ) has a low dielectric constant to provide the low parasitic capacitance. The passive component may include a resistor ( 87 ), a capacitor ( 88 ), and/or an inductor ( 94 ). 
     In one embodiment, the semiconductor layer ( 2 ) is disposed on a silicon-on-insulator (SOI) structure and the pillar ( 25 ) extends from the insulator thereof to the membrane portion of the dielectric layer ( 3 ). 
     In one embodiment, the invention provides a method for making a semiconductor device, including forming a deep trench in a semiconductor layer ( 2 ), filling the deep trench with SiO2( 25 ), providing a dielectric layer ( 3 ) on the semiconductor layer ( 2 ) and the SiO2( 25 ), forming a plurality of etchant openings ( 24 ) through the dielectric layer ( 3 ), the etchant openings ( 24 ) being proximate to the SiO2 ( 25 ), and introducing etchant through the etchant openings ( 24 ) to etch a cavity ( 4 ) in the semiconductor layer and thereby expose the SiO2 as a SiO2 pillar ( 25 ) extending from a bottom of the cavity ( 4 ) to engage and support a portion of the dielectric layer ( 3 ) extending over the cavity ( 4 ), to thereby increase robustness of the semiconductor device. 
     In one embodiment, the method includes providing a cap layer ( 34 ) above the dielectric layer ( 3 ) to cover the etchant openings ( 24 ). 
     In a described embodiment, the invention includes providing a first thermocouple junction ( 7 ) in a portion of the dielectric layer ( 3 ) extending over the cavity ( 4 ), and providing a second thermocouple junction ( 8 ) in another portion of the dielectric layer ( 3 ) disposed directly on the semiconductor layer ( 2 ), and coupling the first ( 7 ) and second ( 8 ) thermocouple junctions to form a thermopile ( 7 , 8 ). In another described embodiment, the invention includes providing a passive component ( 87 , 88 , 94 ) in a portion of the dielectric layer ( 3 ) extending over the cavity ( 4 ) to provide low parasitic capacitance between the passive component and the semiconductor layer ( 2 ). 
     In one embodiment, the invention provides a semiconductor device including a semiconductor layer ( 2 ) having therein a cavity ( 4 ), a dielectric layer ( 3 ) on the semiconductor layer ( 2 ), a plurality of etchant openings ( 24 ) extending through the dielectric layer ( 3 ) for passage of etchant for etching the cavity ( 4 ), and support means ( 25 ) extending from a bottom of the cavity ( 4 ) for engaging and supporting a portion of the dielectric layer ( 3 ) extending over the cavity ( 4 ). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
         FIG. 1A  is a section view diagram of a prior art IR radiation detector supported in a membrane formed in a CMOS-processing-compatible process, taken along section line  1 A- 1 A of  FIG. 1B . 
         FIG. 1B  is a top perspective view of the prior art IR radiation detector shown in  FIG. 1A . 
         FIG. 2A  is a section view of a CMOS-processing-compatible IR sensor chip according to the present invention. 
         FIG. 2B  is a section view of a CMOS-processing-compatible IR sensor chip according to the present invention, formed on an SOI (silicon-on-insulator) substrate. 
         FIG. 3  is a transparent plan view of the IR sensor chip shown in  FIG. 2A  showing the relative alignment of various parts therein. 
         FIG. 4  is a more generalized section view diagram of the IR sensor of  FIG. 2A , indicating various minimum dimensions of one embodiment thereof. 
         FIGS. 5A-5I  show a sequence of section view diagrams of the IR sensor of  FIG. 3A  or  FIG. 3B  as it is being fabricated. 
         FIG. 6A  is a section view of an integrated circuit of the present invention including a polycrystalline silicon resistor having very low resistor-to-substrate parasitic capacitance. 
         FIG. 6B  is a transparent plan view of the integrated circuit shown in  FIG. 6A  showing the relative alignment of various parts therein. 
         FIG. 7A  is a section view of an integrated circuit of the present invention including a capacitor having very low capacitor-to-substrate parasitic capacitance. 
         FIG. 7B  is a transparent plan view of the integrated circuit shown in  FIG. 7A  showing the relative alignment of various parts therein. 
         FIG. 8A  is a section view of an integrated circuit of the present invention including an inductor having very low inductor-to-substrate parasitic capacitance. 
         FIG. 8B  is a transparent plan view of the integrated circuit shown in  FIG. 8A  showing the relative alignment of various parts therein. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention provides one or more deep trench oxide pillars  25  to act as a support structure under a portion of a subsequently described SiO2 membrane which extends over a subsequently described cavity  4  etched in a silicon layer  2  ( FIG. 2A ). The deep trench oxide is strong and resistant to the silicon etching process and therefore the oxide pillar is exposed as the cavity is etched into the silicon layer. Ordinary forces on the top surface of the fragile SiO2 membrane therefore will not damage it. Consequently, the present invention makes it more feasible to use standard low cost packaging (WCSP) for the IR sensor disclosed in the above mentioned pending Meinel et al. application. As subsequently explained, the present invention also makes it more feasible to provide greatly reduced parasitic capacitances for resistors and/or capacitors and/or conductors in an integrated circuit. 
       FIG. 2A  shows a cross-section of an integrated circuit IR sensor chip  1 A which includes silicon substrate  2  and an etched cavity  4  therein. Silicon substrate  2  in  FIG. 2A  typically includes a thin layer (not shown) of epitaxial silicon into which cavity  4  is etched, and also includes a silicon wafer substrate on which the original epitaxial silicon layer is grown. IR sensor chip  1 A includes dielectric stack  3  (also referred to as SiO2 stack  3 ) formed on the upper surface of silicon substrate  2 . SiO2 stack  3  includes multiple oxide layers  3 - 1 ,  2  . . .  6  as required to facilitate fabrication of N-doped polysilicon (polycrystalline silicon) layer  13 , titanium nitride layer  15 , tungsten contact layers  14 - 1 ,  14 - 2 ,  15 - 1 ,  15 - 2 , and  17 , first aluminum metallization layer M 1 , second aluminum metallization layer M 2 , third aluminum metallization layer M 3 , and various elements of CMOS circuitry (not shown), all within SiO2 stack  3 . 
     The various layers shown in dielectric stack  3 , including polysilicon layer  13 , titanium nitride layer  15 , aluminum first metallization layer M 1 , aluminum second metallization layer M 2 , and aluminum third metallization layer M 3 , each are formed on a corresponding oxide sub-layer of dielectric stack  3 . Thermopile  7 , 8  is formed within SiO2 stack  3 . Cavity  4  in silicon substrate  2  is etched directly beneath thermopile junction  7 , and therefore thermally insulates thermopile junction  7  from silicon substrate  2 . However, thermopile junction  8  is located directly adjacent to silicon substrate  2  (and therefore is at essentially the same temperature as silicon substrate  2 ). A relatively long, narrow polysilicon trace  13  is disposed on a SiO2 sub-layer  3 - 1  of dielectric stack  3  and extends between tungsten contact  14 - 2  (in thermopile junction  7 ) and tungsten contact  14 - 1  (in thermopile junction  8 ). Titanium nitride trace  15  extends between tungsten contact  15 - 1  (in thermopile junction  8 ) and tungsten contact  15 - 2  (in thermopile junction  7 ). Thus, polysilicon trace  13  and titanium nitride trace  15  both function as parts of thermopile  7 , 8 . 
     The right end of polysilicon layer  13  is connected to the right end of titanium nitride trace  15  by means of tungsten contact  14 - 2 , aluminum trace  16 - 3 , and tungsten contact  15 - 2  so as to form “hot” thermopile junction  7 . Similarly, the left end of polysilicon layer  13  is connected by tungsten contact  14 - 1  to aluminum trace  11 B and the left end of titanium nitride trace  15  is coupled by tungsten contact  15 - 1 , aluminum trace  16 - 2 , and tungsten contact  17  to aluminum trace  11 A, so as to thereby form “cold” thermopile junction  8 . The series-connected combination of the two thermopile junctions  7  and  8  forms thermopile  7 , 8 . 
     Aluminum metallization interconnect layers M 1 , M 2 , and M 3  are formed on the SiO2 sub-layers  3 - 3 ,  3 - 4 , and  3 - 5 , respectively, of dielectric stack  3 . A conventional silicon nitride passivation layer  12  is formed on another oxide sub-layer  3 - 6  of dielectric layer  3 . A number of relatively small-diameter etchant holes  24  extend from the top of passivation layer  12  through dielectric stack  3  to cavity  4 , between the various patterned metallization (M 1 , M 2  and M 3 ), titanium nitride, and polysilicon traces which form thermopile junctions  7  and  8 . As subsequently explained, silicon etchant is introduced through etchant holes  24  to etch cavity  4  into the upper surface of silicon substrate  2 . Note, however, that providing the etchant openings  24  is not conventional in standard CMOS processing or bipolar integrated circuit processing, nor is the foregoing silicon etching conventionally used in this manner in standard CMOS processing or bipolar integrated circuit processing. 
     In accordance with the present invention, one or more “deep trench” SiO2 pillars  25  are provided within cavity  4 , extending from the bottom of cavity  4  to engage and support the bottom of the portion of dielectric membrane  3  extending over cavity  4 . The method of forming SiO2 pillars  25  is subsequently described. 
     The small diameters of round etchant holes  24  are selected in order to provide a more robust floating thermopile membrane, and hence a more robust IR radiation sensor. The diameters of the etchant hole openings  24  may vary from 10 microns to 30 microns with a spacing ratio of 3:1 maximum to 1:1. The spacings between the various etchant openings  24  may be in a range from approximately 10 to 60 microns. A smaller spacing ratio (i.e., the distance between the edges of the holes divided by the diameter of the holes) has the disadvantage that it results in lower total thermopile responsivity, due to the packing factor (the number of thermopile junctions per square millimeter of surface area) of the many thermopile junctions of which thermopile junctions  7  and  8  are composed, respectively. However, a smaller spacing ratio results in a substantially faster silicon etching time. Therefore, there is a trade-off between the robustness of the membrane and the cost of etching of cavity  4 . The use of SiO2 pillars  25  in accordance with the present invention allows the use of larger spacing between the etchant openings  24 , due to the additional membrane robustness achieved by use of SiO2 pillars  25 . The use of SiO2 pillars  25  also allows the maximum size of cavity  4  to be increased. 
     The IR sensor  1 B shown in  FIG. 2B  is the same as the IR sensor  1 A shown in  FIG. 2A  except that the structure  1 B in  FIG. 2B  is formed on an SOI (silicon-on-insulator) substrate structure instead of on a thick silicon wafer substrate. SOI technology is well-known and therefore is not described in detail, except to note that in  FIG. 2B  the silicon layer  2  is supported on an insulative layer  26  which provided on a suitable substrate  28 . Cavity  4  is etched through silicon layer  2  all the way to the top surface of insulative layer  26 , exposing the previously deposited SiO2 material into a deep trench in silicon layer  2 . SiO2 pillar  25  extends from the upper surface of insulative layer  26  to the top surface of silicon layer  2 , as subsequently described in more detail. 
     A roll-on epoxy film  34  may be provided on nitride passivation layer  12  to permanently seal the upper ends of etch openings  24  and to help reinforce the “floating membrane” portion of dielectric layer  3 . However, the increased robustness of the membrane portion of SiO2 stack  3  over cavity  4  achieved by use of SiO2 pillars  25  may allow epoxy film  34  to be omitted in certain applications. In some applications, a thinner epoxy film  34  might be used. Although there may be some applications of the invention which do not require epoxy cover plate  34 , the use of epoxy cover plate  34  may be an important aspect of providing a reliable WCSP package configuration of the IR sensors of the present invention. In an embodiment of the invention under development, epoxy cover plate  34  is substantially thicker (roughly 16 microns) than the entire thickness (roughly 6 microns) of dielectric stack  3 . 
       FIG. 3  shows an approximate “transparent” plan view of the portions of the IR sensors  1 A and  1 B shown in  FIGS. 2A and 2B , respectively, to illustrate the relative locations of a number of the SiO2 pillars  25 , a number of the etchant openings  24 , thermopile junction  7 , and thermopile junction  8 . In this example, six SiO2 pillars  25  are symmetrically located around thermopile junction  7 , extending from the bottom of cavity  4  to six corresponding areas of the bottom surface of dielectric layer  3 . Typically, it would be more convenient to apply cover plate  34  to the entire top surface of passivation layer  12 , although it could be applied to cover just the area above cavity  4  as shown in subsequently described  FIG. 6A . 
       FIG. 4  illustrates minimum dimensions, in microns, of the various features of cavity  4  and the “floating” membrane portion of dielectric layer  3  which supports thermopile junction  7  above cavity  4  for an embodiment of the invention presently under development. In that embodiment the etchant openings  24  are at least 10 microns (μ) in diameter and are spaced at least approximately 10 microns apart. The span of cavity  4  is typically 400 microns, and its depth is at least 10 microns. The diameters of SiO2 pillars  25  are at least 0.5 microns in diameter, and are spaced at least 10 microns from any of etchant openings  24  and at least 10 microns from thermopile junction  7 . 
     The differential voltage Vout generated between (−) conductor  11 B and (+) conductor  11 A can be applied to the input of CMOS circuitry (not shown). 
     The presence of cover plate  34  in  FIGS. 2A and 2B , the thickness of which may be comparable to or greater than the thickness of dielectric layer  3 , further strengthens the floating membrane portion of dielectric stack  3 . SiO2 pillars  25  prevent deflection of points of SiO2 membrane  3  supported by SiO2 pillars  25 . This reduces the total deflection of SiO2 membrane  3  and therefore reduces the associated stresses therein when various forces, e.g. due to subsequent fabrication steps, are applied to SiO2 membrane  3  over cavity  4 . 
       FIGS. 5A-5I  show a sequence of section view diagrams of the IR sensor structures generated according to the process for fabricating the IR sensor chips of  FIGS. 2A and 2B .  FIG. 5A  shows a “deep trench SiO2 pillar”  25  formed by etching a deep trench in silicon layer  2  in a conventional manner and then depositing SiO2 to fill the trench. The deposited SiO2 eventually becomes SiO2 pillar  25 . 
     Essentially the same procedure can be performed if silicon layer  2  is supported on a SOI layer  26  which in turn is supported on a suitable substrate  28 , as shown in  FIG. 5B  wherein the deep trench is etched all the way through silicon layer  2  so that SiO2 pillar  25  extends all the way to down SOI insulating layer  26 . An advantage of using SOI layer  26  is that it defines a specific cavity depth which can be easily controlled by timing of the silicon etching process. 
       FIG. 5C  shows providing an SiO2 layer  3 - 1  of dielectric layer  3  on the upper surface of silicon substrate  2  in either  FIG. 5A  or  FIG. 5B , and then depositing a layer of polysilicon on the upper surface of SiO2 sublayer  3 - 1  (see  FIGS. 2A and 2B ). The layer of polysilicon then is patterned so as to provide the polysilicon trace  13  required to fabricate thermopile  7 , 8 . Then, as indicated in  FIG. 5D , another SiO2 sub-layer (see sub-layer  3 - 2  in  FIGS. 2A and 2B ) is deposited on the polysilicon traces  13 , and then titanium nitride layer  15  is deposited on that sublayer and then patterned to provide the titanium nitride trace  15  as required to make thermopile  7 , 8 . Then suitable via openings are provided through the SiO2 sub-layers  3 - 2  and  3 - 3  and conventional tungsten contacts are formed in the via openings. Next, a first metallization layer M 1 , which may be aluminum, is deposited on the SiO2 sub-layer  3 - 2  and patterned as needed to provide connection to the tungsten contacts and any CMOS circuitry (not shown) that is also being formed on infrared sensor chip  1 . 
     Then, as indicated in  FIG. 5E , another SiO2 sub-layer  3 - 4  (see  FIGS. 2A and 2B ) is deposited on the first metallization M 1 . Suitable via openings then are formed in SiO2 sublayer  3 - 4 , and tungsten vias are formed in those openings. Then a second aluminum metallization layer M 2  is deposited on SiO2 sub-layer  3 - 4  and patterned as necessary to complete the formation of thermopile  7 , 8  and also to make connections that are required for any CMOS circuitry (not shown) also being formed on the integrated circuit chip. Next, as indicated in  FIG. 5F , another SiO2 sub-layer  3 - 5  is deposited on the aluminum metallization M 2 . A third aluminum metallization layer M 3 , designated by reference numeral  19 , is formed on sub-layer  3 - 5  and patterned as needed. Then a final SiO2 sub-layer  3 - 6  is deposited on the M 3  metallization to complete the structure of SiO2 dielectric stack  3 . Then, a silicon nitride passivation layer  12  is formed on dielectric sub-layer  3 - 6  (see  FIGS. 2A and 2B ). 
     Next, as indicated in  FIG. 5G , silicon etchant openings  24  are formed, extending from the upper surface of silicon substrate  2  to the top surface of passivation layer  12 . Then, as indicated in  FIG. 5H , a conventional isotropic silicon etchant is introduced into etchant openings  24  in order to etch cavity  4  in the upper surface of silicon substrate  2  so that cavity  4  has a shape determined by the locations of the various etchant openings  24 . Since the silicon etchant used to etch cavity  4  does not attack SiO2, more and more of the SiO2 material  25  is exposed as the silicon etching progresses. Therefore, after cavity  4  has been completely etched, deep trench SiO2 pillar  25  extends from the bottom of cavity  4  all the way up to engage the bottom of dielectric layer  3 . The portion of thin dielectric stack  3  extending over cavity  4  and containing etchant openings  24  and thermopile junction  7  thus becomes a more robust “floating” thermopile membrane which is thermally isolated by cavity  4  from silicon substrate  2 . 
     Finally, as indicated in  FIG. 5I , a relatively thick roll-on epoxy film or other suitable permanent cap layer  34  is provided on at least part of the upper surface of silicon nitride passivation layer  12  in order to permanently seal cavity  4  and etchant openings  24  and also, in some cases, to substantially strengthen the “floating” portion of dielectric membrane  3 . This is desirable because a very vigorous water stream may impinge on the surface of IR sensor chip  1  during subsequent wafer sawing operations and would tend to crush the “floating” thermopile membrane over cavity  4 . Cap layer  34  also prevents silicon residue generated by the wafer sawing operations from entering cavity  4 . 
     The size, shape, spacing between, and number of etchant openings  24  may be selected to optimize the strength of the “floating” thermopile membrane above cavity  4 , so as to provide a more robust IR radiation sensor device. The improvement in membrane strength is inversely proportional to the decrease in distance or span between SiO2 pillars  25 . (For example, if the span is reduced by a factor of 2, the membrane strength is doubled. The number and spacing of SiO2 pillars  25  will be determined by the amount of thermal conductance allowable for an IR sensing device, or by the allowable substrate capacitance (e.g., substrate capacitance of a resistor, capacitor, and/or inductor) for high frequency applications. Epoxy cover plate  34  preferably is placed over at least the IR sensor to seal cavity  4  and strengthen the floating membrane portion of dielectric layer  3  containing thermopile junction  7 . 
       FIGS. 6A and 6B  show simplified section view and plan view diagrams, respectively, of an embodiment of the present invention wherein the parasitic capacitance between a polysilicon resistor  87  and an epitaxial silicon layer or substrate  2  of an integrated circuit chip  1 C is greatly reduced by providing cavity  4  in silicon layer  2  directly underneath resistor  87 . In  FIGS. 6A and 6B , silicon layer  2 , cavity  4 , SiO2 pillar  25 , a dielectric layer  3 , passivation layer  12 , etchant openings  24 , and cover plate  34  may be substantially the same as in above described  FIGS. 2A and 3  except that thermopile junctions  7  and  8  are omitted and instead an aluminum conductor  85 - 1  of the M 1  metallization layer is connected by a tungsten contact  86 - 1  to one end of resistor  87  located directly above cavity  4 . The other end of resistor  87  is connected by tungsten contact  86 - 2  to aluminum conductor  85 - 2  of the M 1  metallization layer. 
     Since cavity  4  is filled with gas or other material having a much lower dielectric constant than SiO2, the parasitic resistor-to-substrate capacitance between resistor  87  and substrate  2  is much lower than would be the case if cavity  4  were not provided between resistor  87  and silicon substrate  2 . RF circuitry (not shown) containing polysilicon silicon resistor  87  therefore may be operable at much higher frequency than if cavity  4  is not present (if the parasitic capacitance is the factor which actually limits the maximum RF frequency). As in the case of  FIGS. 2A and 2B , the presence of SiO2 pillars  25  results in a much stronger, more robust SiO2 membrane extending over cavity  4 . 
       FIGS. 7A and 7B  show simplified section view and plan view diagrams, respectively, of an embodiment of the present invention wherein the parasitic capacitance between the lower plate of a capacitor  88  and the substrate  2  of integrated circuit  1 D is greatly reduced by providing cavity  4  directly underneath capacitor  88 . In  FIGS. 7A and 7B , silicon layer  2 , cavity  4 , SiO2 pillar  25 , dielectric layer  3 , passivation layer  12 , etchant openings  24 , and cover plate  34  may be substantially the same as in above described  FIGS. 6A and 6B  except that resistor  87  is omitted and instead is replaced by capacitor  88 . Capacitor  88  includes an aluminum trace  89  of the M 1  metallization layer connected to an enlarged aluminum lower capacitor plate  89 A that is also formed in the M 1  metallization layer. Capacitor  88  also includes an aluminum trace  90  of the M 2  metallization layer connected to an enlarged aluminum upper capacitor plate  90 A that is also formed in the M 2  metallization layer. Lower capacitor plate  89 A and upper capacitor plate  90 A both are located directly above cavity  4 . Therefore, the parasitic capacitor-to-substrate capacitance between capacitor  88  and silicon layer  2  is much lower than would be the case if cavity  4  were not provided between capacitor  88  and silicon substrate  2 . RF circuitry (not shown) containing capacitor  88  therefore may be operable at much higher frequency than if cavity  4  is not present (if the parasitic capacitance actually is the factor which limits the maximum RF frequency). 
       FIGS. 8A and 8B  show simplified section view and plan view diagrams, respectively, of an embodiment of the present invention wherein the parasitic capacitance between an inductor  94  and the substrate  2  of integrated circuit  1 E is greatly reduced by providing cavity  4  directly underneath inductor  94 . In  FIGS. 7A and 7B , silicon substrate  2 , cavity  4 , SiO2 pillar  25 , dielectric layer  3 , passivation layer  12 , etchant openings  24 , and cover plate  34  may be substantially the same as in above described  FIGS. 6A and 6B  except that resistor  87  is omitted and instead is replaced by inductor  94 . Inductor  94  includes an aluminum trace  92  of the M 1  metallization layer having an outer end connected to a first terminal of inductor  94  and an inner end connected by a tungsten contact  91  to an inner end of an aluminum trace  93  of the M 2  metallization layer. Aluminum trace  93  forms one or more spiral loops and an outer end of aluminum trace  93  extends to a second terminal of inductor  94 . Inductor  94  is located directly above cavity  4 . Therefore, the parasitic capacitor-to-substrate capacitance between inductor  94  and substrate  2  is much lower than would be the case if cavity  4  were not provided between capacitor  88  and silicon substrate  2 . RF circuitry (not shown) containing inductor  94  therefore may be operable at much higher frequency than if cavity  4  is not present (if the parasitic capacitance actually is the factor which limits the maximum RF frequency). 
     While the invention has been described with reference to several particular embodiments thereof, those skilled in the art will be able to make various modifications to the described embodiments of the invention without departing from its true spirit and scope. It is intended that all elements or steps which are insubstantially different from those recited in the claims but perform substantially the same functions, respectively, in substantially the same way to achieve the same result as what is claimed are within the scope of the invention. For example, the invention is applicable to bipolar integrated circuit technology as well as CMOS integrated circuit technology.