Abstract:
A semiconductor gas sensor device includes a first cavity that is enclosed by opposing first and second semiconductor substrate slices. At least one conducting filament is provided to extend over the first cavity, and a passageway is provided to permit gas to enter the first cavity. The sensor device may further including a second cavity that is hermetically enclosed by the opposing first and second semiconductor substrate slices. At least one another conducting filament is provided to extend over the second cavity.

Description:
PRIORITY CLAIM 
       [0001]    This application claims priority from Italian Application for Patent No. MI2014A001186 filed Jun. 30, 2014, the disclosure of which is incorporated by reference. 
       TECHNICAL FIELD 
       [0002]    The present disclosure relates to a semiconductor gas sensor device and the manufacturing method thereof. 
       BACKGROUND 
       [0003]    The thermal conductivity detector (TCD) is well known in the state of the art. A TCD is an environmental sensor device widely used for the measurement of the amount of gas in the environment. The operation is based on the fact that each gas has an inherent thermal conductivity and a filament (thermal resistor) changes its temperature as a function of the amount of gas that surrounds it. The most appropriate sensing element shape is that of a thin finger suspended, for which the temperature of the central part can locally reach even values of several hundred degrees. The feature that the finger is totally suspended allows for enhancing the amount of heat exchange with the gas in which it is immersed. The warming effect of the suspended finger is induced through an electrical stress of the sensor, for example by means of the flow of current through the finger. The sensor is able to better discriminate the gases whose conductivity is much different than normal air (roughly N 2  (79%), O 2  (19%), CO 2  (0.04%), plus other gases with negligible quantities: for example the CO is a few ppm). 
         [0004]    When a current flows through the finger, the value of the resistance of the finger changes. The measurement of the resistance value allows for measuring the conductivity of the gas mixture which depends on the molar fraction of the gas of interest. 
         [0005]    However, it is difficult in principle to discriminate which is the gas mainly responsible for the conductivity variation of the mixture of gas. For example, CO 2  has a lower thermal conductivity than dry air, therefore if its percentage increases inside the mixture, this will raise the temperature of the sensor with a consequent increase of the value of the measured resistance. 
         [0006]    The TCD sensor operates in accordance with the thermodynamic equilibrium among heat generated by the current flow, heat exchange with the material of which the sensor is made (e.g., polysilicon crystalline), and heat exchange with the gas mixture surrounding it. The ambient temperature determines the equilibrium value of the sensor in standard dry air. To take into account and compensate for the variation of ambient temperature, a Wheatstone bridge could be used as the sensor structure. The reference branches of the bridge are of the same nature and positioned in the vicinity of the sensor so as to be sensitive to the same way to changes in ambient temperature, with the difference that will not be exposed to the mixture of gas as the sensor. 
         [0007]    The Relative Humidity (RH) is the amount of water vapor (gas) present in the environment compared to a saturated environment in the same conditions of pressure and temperature. The thermal conductivity of water vapor is much larger than the dry air therefore an increase in relative humidity produces a lowering of the temperature of the sensor with a consequent reduction of the value of the measured resistance. The contribution of the RH value of the measured resistance could be 1/10 compared to the change of resistance in the presence of CO 2 , therefore, this is a parameter to measure and correct. Typically the correction is made by means of a dedicated sensor for the measurement of the RH. 
         [0008]    In view of implementation of space saving and low power consumption, a demand exists to further reduce the size of gas detectors for measuring the concentration of gas. In recent years, gas detection elements with greatly reduced sizes have been developed by the use of MEMS (Micro-Electro-Mechanical System) technology (also called the micromachining technique). A gas detection element formed by use of MEMS technology is configured such that a plurality of thin films are formed in layers on a semiconductor substrate (e.g., a silicon substrate). Examples of such a gas detection element include a thermal-conductivity-type gas detection element. The thermal-conductivity-type gas detection element has a heat-generating resistor and utilizes the phenomenon that, when the heat-generating resistor is energized and generates heat, heat is conducted to the gas. The conduction of heat causes a change in temperature of the heat-generating resistor and thus a change in resistance of the heat-generating resistor. On the basis of the amount of the change, the gas is detected. In the thermal-conductivity-type gas detection element, the resistance of the heat-generating resistor varies with the type or concentration of the gas. 
       SUMMARY 
       [0009]    One aspect of the present disclosure is to provide a semiconductor gas sensor device of simple architecture with respect to the known ones. 
         [0010]    One aspect of the present disclosure is a semiconductor gas sensor device comprising: one doped semiconductor substrate of a first semiconductor slice, a first insulating layer placed above said doped semiconductor substrate, a part of at least one first cavity formed inside said first insulating layer and said doped semiconductor substrate and extending inside said doped semiconductor substrate to a prefixed depth, at least one conductive filament placed over said part of the at least one first cavity in a bridge way, a conductive metal layer placed at the ends of at least one filament for contact it, another doped semiconductor substrate of a second semiconductor slice, said another doped semiconductor substrate comprising the other part of the at least one first cavity and being placed above said doped semiconductor substrate of the first semiconductor slice so as to form said at least one first cavity and close it, said another doped semiconductor substrate comprising at least one hole in correspondence of the first cavity for the inlet of gas to detect. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    For a better understanding of the present disclosure, a preferred embodiments thereof are now described, purely by way of non-limiting examples and with reference to the annexed drawings, wherein: 
           [0012]      FIG. 1A  shows a block diagram of a measurement apparatus comprising a measurement device and an integrated gas sensor device according to a first embodiment of the present disclosure; 
           [0013]      FIG. 1B  shows a block diagram of a measurement apparatus comprising a measurement device and an integrated gas sensor device according to a second embodiment of the present disclosure; 
           [0014]      FIG. 2A  shows a schematic layout of the integrated semiconductor gas sensor device according to a second embodiment of the present disclosure; 
           [0015]      FIG. 2B  shows a schematic layout of the sensing elements of the integrated semiconductor gas sensor device in  FIG. 2A ; 
           [0016]      FIGS. 3 and 4  are cross sectional views of a part of the semiconductor gas sensor device according to the first embodiment of the present disclosure; 
           [0017]      FIGS. 5-17  show the manufacturing process of the part of the semiconductor gas sensor device in  FIGS. 3 and 4 ; 
           [0018]      FIGS. 18-19  show the manufacturing process of the other part of the semiconductor gas sensor device according to the first embodiment of the present disclosure; 
           [0019]      FIG. 20  is a cross sectional view more in detail of the semiconductor gas sensor device according to the first embodiment of the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]      FIG. 1A  and  FIG. 1B  show a block diagram of a measurement apparatus comprising a gas sensor device  1  or a gas sensor device  50  according respectively to a first and a second embodiment of the present disclosure and a measurement device  100 . 
         [0021]    According to a first embodiment of the present disclosure, the integrated semiconductor gas sensor device  1  comprises at least one variable resistor R 2  exposed to the gas ( FIG. 1A ). The terminals of the resistor R 2  are connectable with a variable current generator  210  and ground GND; also the terminals of the resistor R 2  are connectable to the measurement device  100  able to measure the voltage across the variable resistor R 2 . According to the present disclosure the resistor R 2  is formed in a semiconductor substrate wherein at least a cavity  3  is performed which is coated by silicon but is open to the outside by means of one hole  35  so that the cavity  3  is exposed to the gases ( FIG. 20 ). The resistor R 2  is formed in the cavity  3  by means of suspended filament  30  preferably in polysilicon; the filament  30  is arranged in a bridge way. The sensitivity of the resistor R 2  depends on the resistivity of the filaments  30 , reducing the resistivity the sensitivity increases; for example a filament in polysilicon with size of 50×1×1 microns can be used. 
         [0022]    According to a second embodiment of the present disclosure, the integrated semiconductor gas sensor device is preferably a Wheatstone bridge  50  including a couple of reference resistors R 1  and a couple of variable resistors R 2  exposed to the gas ( FIG. 1B ); the use of a Wheatstone bridge allows for minimizing the dependence on the ambient temperature. The four connecting nodes A-D of the terminals of the resistances R 1  and R 2  of the Wheatstone bridge  50  are connectable respectively with a variable current or voltage generator  210 , to ground GND and to the measurement device  100  able to receive the voltage signal at the output of the Wheatstone bridge  50 . 
         [0023]    According to the present disclosure the Wheatstone bridge  50  is formed in a semiconductor substrate wherein two cavities  2  and  3  are formed which are both hermetically coated by silicon but wherein the only cavity  3  is open to the outside by means of two holes  35  so that the cavity  3  is exposed to the gases ( FIG. 2A ). The couple of reference resistors R 1  are formed in the cavity  2  by means of suspended filaments  20  while the couple of resistors R 2  are formed in the cavity  3  by means of suspended filaments  30  ( FIG. 2B ); the filaments  20  and  30  are formed preferably in polysilicon. The sensitivity of the Wheatstone bridge  50  depends on the resistivity of the filaments  30 , reducing the resistivity the sensitivity increases; for example a filament in polysilicon with size of 50×1×1 microns can be used. 
         [0024]      FIGS. 3 ,  4  and  20  show cross sectional views of a part of the integrated gas sensor device  1  formed in a semiconductor substrate; the sections show a doped semiconductor substrate  12  of a first semiconductor slice  40 , preferably a silicon slice preferably of the n type, and at least one insulating layers  10 ,  11  placed above said doped semiconductor substrate, but preferably a succession of a nitride  10  and oxide  11  layers are placed over the semiconductor substrate  12 . 
         [0025]    A lower part  301  of the cavity  3  is formed inside said doped semiconductor substrate  12  and the at least one insulating layer  10 ,  11  and extends inside said doped semiconductor substrate to a prefixed depth Dp, for example of 10 microns; at least one conductive filament  30 , preferably made in polysilicon, is placed inside the cavity  3  in a bridge way for forming the resistor R 2 , that is the conductive filament  30  is suspended over the lower part  301  of the cavity  3 . In the case of the integrated gas sensor device using a Wheatstone bridge  50 , two filaments  30  are placed inside the cavity  3  in a bridge way for forming the resistors R 2  and a lower part  201  of another cavity  2  is formed inside said doped semiconductor substrate  12  and the at least one insulating layer  10 ,  11  and extends inside said doped semiconductor substrate to the prefixed depth Dp, for example of 10 microns; two conductive filaments  20 , preferably made in polysilicon, are placed inside said lower part  201  of the cavity  2  in a bridge way for forming the resistors R 1 , that is the conductive filaments  20  are suspended over the lower part  201  of the cavity  2 . The conductive filaments  20 ,  30  are placed preferably at a distance of 100 micrometers. 
         [0026]    A second insulating layer  15 , preferably a nitride layer, is placed above and around the at least one conductive filament  30  except in the contact zones at the ends of the filament; a conductive metal layer  14  is placed on the ends of the filament for contact them. In the case of the integrated gas sensor device using a Wheatstone bridge  50 , the second insulating layer  15  is placed above and around said conductive filaments  30 ,  20  except in the contact zones at the ends of each filament; a conductive metal layer  14  is placed on the ends of the filaments for contact them so that the first pair of conductive filaments  20  represent the reference resistors R 1  of the Wheatstone bridge  50  while the second pair of conductive filaments  30  represent the variable resistors of the Wheatstone bridge  50 . 
         [0027]    A doped semiconductor layer  501  of a second semiconductor slice  45  which comprises the upper part  302  of the at least one first cavity  3  is placed above the first semiconductor slice  40  as to form the cavity  3  and to close said cavity  3 ; the doped semiconductor layer  501  presents at least one hole  35  for the inlet of gas to detect. In the case of the integrated gas sensor device  50 , the doped semiconductor layer  501  comprises the upper part  302  of the cavity  3  and the upper parts  202  of the cavity  2 ; the doped semiconductor layer  501  is placed above the first semiconductor slice  40  as to form the cavities  2  and  3  and to hermetically close the cavity  2  and close the cavity  3 . The doped semiconductor layer  501  present at least one hole  35  but preferably two holes  35  for the inlet of gas to detect in the cavity  3 . 
         [0028]    The process for the formation of the semiconductor gas sensor device  1  comprises a thermal oxidation of a part of a silicon substrate  12  of as first silicon slice  40 , preferably an n type silicon substrate, for forming silicon oxide layers  11  over and under the substrate  12  with a thickness of 0.5 micron and a deposition of insulating layers  10 , for example nitride layers with a thickness of 1000 angstrom, over the oxide layers  11  ( FIG. 5 ) formed over and under the silicon substrate  12  with a thickness of 725 microns. 
         [0029]    Successively a deposition of a polysilicon with a resistivity of 1.35 mΩ×cm occurs over the upper nitride layer  10  for forming a conductive layer  16 , preferably a polysilicon layer  16 , with a thickness of 1 micron ( FIG. 6 ). 
         [0030]    The filament  30  or the filaments  20 ,  30  in the case of forming the Wheatstone bridge  50  are then defined from the conductive layer  16  as is shown in  FIG. 7 ; this is obtained by placing a lithographic mask over the conductive layer  16  and successively effectuating a dry etching. 
         [0031]    In the next step an insulating layer  15 , preferably a nitride layer, is grown above and around the polysilicon filament  30  or the polysilicon filaments  20 ,  30  in the case of forming the Wheatstone bridge  50  and over the nitride layer  10  ( FIG. 8 ,  9  wherein the section across the filaments and the section along the filaments are respectively shown) and an activation step of the dopant of the polysilicon layer  16  is effectuated. 
         [0032]    Preferably successively RF sputtering deposition technique of metal, for example a palladium, is effectuated over the nitride layer  15  for forming the palladium layer  17  which is defined by placing a lithographic mask over the conductive palladium layer  17  and successively effectuating an etching so that the palladium layer  17  is maintained only over and around the polysilicon filament  30  or the polysilicon filaments  20 ,  30  in the case of forming the Wheatstone bridge  50  ( FIG. 10 ). The palladium layer  17  is optional and allows protecting the polysilicon filaments from weathering and making the polysilicon filaments controllable by process and stable over the time. 
         [0033]    Successively a silicon oxide deposition, preferably a tetraethylorthosilicate (TEOS) silicon oxide, over the nitride layer  15  for forming a silicon oxide layer  18  is effectuated ( FIG. 11 ). The thickness of the silicon oxide layer  18  is preferably of 3000 angstroms over the nitride layer  15  placed above the polysilicon filament  30  or the polysilicon filaments  20 ,  30  in the case of forming the Wheatstone bridge  50 . 
         [0034]    The next step is a definition of the contact zones  31  of the filament  30  or the contact zones  21 ,  31  (in the case of forming the Wheatstone bridge  50 ) of the filaments  20 ,  30  by placing a lithographic mask over the layer  18  and successively effectuating a dry etching ( FIG. 12 ) of the layers  15  and  18 . 
         [0035]    A RF sputtering deposition technique of metal, preferably Titanium and Aluminum, is typically used for the formation of the metal layer  41  ( FIG. 13 ). A successive definition of the metal contacts is effectuated by placing a lithographic mask over the layer  41  and successively effectuating a dry etching ( FIG. 13 ) of the layer  41 . Resist strips  42  are placed above the metal layer  41 . 
         [0036]    Then a deposition of a further silicon oxide layer  43  occurs, preferably by means of two LPCVD depositions (low pressure chemical vapor deposition), with a thickness of preferably of 6000 angstroms, by obtaining total oxide layer of with a thicknesses preferably of 9000 and 19000 angstroms over the nitride layer  15 . 
         [0037]    Successively the formation of the lower part  301  of the cavity  3  or the lower parts  201 ,  301  of the cavities  2  and  3  (in the case of forming the Wheatstone bridge  50 ) occur. This is obtained by placing a lithographic mask over the layer  43  and successively effectuating a wet etching to arrive to the nitride layer  15  in the area around the filament  30  or each filament  20 ,  30  ( FIG. 14 ), placing another lithographic mask over the nitride layer  15  and successively effectuating a dry etching of the nitride layers  15 ,  10  and the oxide layer  11  to arrive to the substrate  12  always in the area around the filament  30  or each filament  20 ,  30  preferably at a distance of 0.3 microns from the polysilicon filament  30  or each polysilicon filament  20 ,  30  ( FIG. 15 ) and placing a resist mask  44  over the substrate  12  so that the resist layer is placed at a distance of 0.8 microns from the polysilicon filament  30  or each polysilicon filament  20 ,  30  and 0.5 microns from the oxide layer  11  ( FIG. 16 ) and successively effectuating an anisotropic and isotropic dry etching of the substrate  12  of a depth Dp=10 microns ( FIG. 17 ) to etch even the portion of the silicon substrate  12  under the from the polysilicon filament  30  or the polysilicon filaments  20 ,  30 . 
         [0038]    Successively another dry etching of the oxide layer  11  occurs to arrive at the device in  FIGS. 3 ,  4 . 
         [0039]    The process for the formation of the semiconductor gas sensor device comprises placing a resist mask on a semiconductor substrate  501  of a second semiconductor slice  45 , preferably a n type silicon substrate, to define the upper part  302  of the cavity  3  or the upper parts  202 ,  302  of the cavities  2 ,  3  and successively effectuating an anisotropic and isotropic dry etching of the substrate  501  for a depth Dp=10 microns ( FIG. 18 ) to form the upper part  302  of the cavity  3  or the upper parts  202 ,  302  of the cavities  2 ,  3 . 
         [0040]    The process comprises even placing a resist mask  52  on a silicon substrate  50  after the definition of the upper part  302  of the cavity  3 , to define at least one hole  35  only in the upper part  302  of the cavity  3  and successively effectuating an anisotropic and isotropic dry etching of the substrate  50  to form the hole  35  in the upper part  302  of the cavity  3  ( FIG. 19 ). 
         [0041]    The first  40  and second  45  semiconductor slices are joined together so that the lower part  301  corresponds to the upper part  302  or the lower parts  301 ,  201  correspond respectively to the upper parts  202 ,  302 ; in this way the only cavity  3  or both the cavities  2  and  3  are formed. Preferably the first  40  and second  45  semiconductor slices are joined together by using an adhesive  60  such as glass frit or dry resist ( FIG. 20 ).