Abstract:
In a mass flow sensor having a layered structure on the upper side of a silicon substrate ( 1 ), and having at least one heating element ( 8 ) patterned out of a conductive layer in the layered structure, thermal insulation between the heating element ( 8 ) and the silicon substrate ( 1 ) is achieved by way of a silicon dioxide block ( 5 ) which is produced beneath the heating element ( 8 ) either in the layered structure on the silicon substrate ( 1 ) or in the upper side of the silicon substrate ( 1 ). As a result, the sensor can be manufactured by surface micromechanics, i.e. without wafer back-side processes.

Description:
FIELD OF THE INVENTION 
   The invention relates to a micromechanical mass flow sensor using silicon technology. The present invention furthermore relates to a method for manufacturing such a sensor. 
   BACKGROUND OF THE INVENTION 
   Silicon micromechanics may allow economical mass production of small sensor chips using semiconductor technology methods. Mass flow sensors having dimensions on the order of a few hundred micrometers are used, for example, in the automotive sector to control air delivery. 
   Mass flow sensors using silicon micromechanics from other systems, for example as discussed in German Published Patent Application No. 195 27 861, encompass a heating element and typically several temperature measurement elements, all of which are patterned out of a metal resistor layer and are positioned on a thin membrane made of dielectric material. The membrane spans a recess in a silicon substrate. 
   When a flow of medium, in particular an air flow, flows along the upper side of the membrane, that flow of medium causes cooling of the membrane. That cooling can be evaluated, in particular, using one temperature measurement element placed upstream (relative to the heating element) and a further temperature measurement element placed downstream, the measurement element placed upstream being more greatly cooled than the one placed downstream. Alternatively, the cooling of the membrane may be determined by measuring the resistance of the heating element. The temperature measurement elements are also resistors, which are made of a material whose resistance is temperature-dependent. One material that is suitable for the heaters and the temperature measurement elements is platinum. 
   German Published Patent Application No. 195 27 861 discusses the manufacture of a membrane sensor of this kind, to begin with a silicon substrate and firstly to deposit on its upper side a membrane layer made of silicon oxide, silicon nitride, or similar materials. A metal layer is then applied onto the entire surface of the membrane layer, and the measurement and heating elements, conductor paths, etc. are patterned out it by photolithography and etching. In a further process step the recess is etched in, proceeding from the back side of the silicon substrate, by manner of an etching frame, so that only a frame made of silicon, with the membrane spanned within it, remains. The underside of the membrane layer is then open (except where it covers the frame), i.e. it is bounded by air. 
   This membrane construction produces a thermal insulation, between the resistors patterned out of the metal layer and the substrate, that is ensured by the recess and the air present in it (the thermal conductivity of air or SiO 2  is approximately two orders of magnitude less than that of silicon). Without a recess, a substantial portion of the heat generated by the heating element would not be dissipated by the medium flowing past, but rather would flow away through the membrane layer to the silicon substrate. This heat flowing away laterally and vertically into the substrate would be problematic simply because a predefined working temperature that is also important in terms of measurement sensitivity would be achievable, at best, only by increasing the energy consumption of the heater. 
   A lateral heat outflow would be problematic in terms of the measurement elements that are typically positioned laterally next to the heater, since their function is based on the existence of a temperature gradient. This gradient exists between the center of the membrane (heated by the heating element) and the edge of the sensor (which should be as close as possible to ambient temperature). With a lateral heat outflow, to which the direct thermal coupling via the membrane itself does not make a substantial contribution, the temperature measurement elements would also be heated via the substrate. The decreased temperature gradient between heater and measurement elements would result directly in decreased sensor sensitivity. Thermal decoupling in the lateral and vertical directions thus appears indispensable. 
   As discussed, however, the membrane sensors require for their manufacture a volume-based micromechanical process from the back side of the substrate. This patterning of the membrane may be accomplished via a KOH etching process in which the wafer is inserted into an etching box. The manufacturing method using bulk silicon micromechanics with an integrated back-side process is very complex and therefore associated with high cost. In addition, the thin membrane, with typical thicknesses of 1 to 2 um, has a tendency toward undesired vibrations due to its low mechanical stability. 
   SUMMARY OF THE INVENTION 
   An exemplary embodiment of the present invention provides a micromechanical mass flow sensor that may be manufactured with less complexity than before, and that has sufficient sensitivity and mechanical stability. The present invention is also provides a corresponding manufacturing method for a sensor of this kind. 
   This may be achieved by manner of a sensor according to the present invention and a method for manufacturing that sensor according to the present invention. 
   The exemplary method produces a micromechanical mass flow sensor including a layered structure on the upper side of a silicon substrate, the layered structure encompassing an electrically conductive layer out of which at least one heating element is patterned; and including a thermal insulation between the heating element and the silicon substrate, the thermal insulation encompasses a silicon dioxide block, the silicon dioxide block is produced beneath the heating element either in the layered structure on the silicon substrate or in the upper side of the silicon substrate. 
   The exemplary embodiment and/or exemplary method of the present invention provides for the thermal insulation to be arranged between the (platinum) resistors and the silicon substrate substantially by manner of a sufficiently thick silicon dioxide layer that also has adequate mechanical stability. This silicon dioxide layer is embodied as an oxide block, and either is patterned directly into the silicon substrate from the upper side, or is manufactured on the silicon substrate in a corresponding layered structure. In the latter case, a silicon oxide layer is applied on the silicon substrate, and a silicon layer is applied above that, and the silicon dioxide block is produced in the silicon layer. 
   The sensor according to the present invention provides that only surface micromechanical processes, i.e. only front-side processes, are necessary for manufacture, so that complex back-side processes can be omitted. Elimination of the KOH etching step from the back side moreover may allow a miniaturization of the component. Also, the sensor provides for reduction in scratches and particles on the front side of the wafer, which are almost unavoidable in conjunction with handling in the context of back-side processes. 
   One exemplary embodiment of this sensor lies in the fact that a plurality of vertical, crevice-shaped cavities are formed in the oxide block. Since the cavities contain air or a vacuum, the thermal insulation is considerably increased by an oxide block patterned in this fashion. 
   Configuration of the oxide block in a layered structure, as mentioned previously, opens up the possibility of removing the silicon oxide layer beneath the oxide block and configuring there, toward the silicon substrate, a horizontal cavity that then contributes substantially to the thermal insulation. 
   At least one pillar, adjoining the silicon dioxide block at the top and the silicon substrate at the bottom, may be positioned in the horizontal cavity and further improves mechanical stability. 
   The present invention furthermore produces a method for manufacturing a sensor where after patterning of an etching mask, which either is applied on a silicon layer that is applied above a silicon oxide layer on the silicon substrate, or is applied directly on the silicon substrate, approximately vertical grooves are produced by etching in the silicon material located below the etching mask, lands of silicon remaining behind between the grooves. The lands are then completely oxidized by thermal oxidation. The silicon dioxide block produced in this manner by surface micromechanics is covered by deposition of a silicon dioxide sealing layer. Before deposition and patterning of the electrically conductive layer, a leveling step is performed to produce a a smooth surface on the silicon dioxide block. 
   The method according to the present invention permits easy manufacture of the sensor, using only methods from other systems in semiconductor technology. This may allow for the production of a reproducible layer thickness and structure for the oxide block. In particular, in the interest of greater porosity and thus insulation of the oxide block, the silicon layer or the silicon substrate may be undercut beneath the etching mask by isotropic etching. If the etching mask remains on the lands after etching and is later also covered, wider grooves with thinner lands may be generated. In general, only a few layers and photolithography steps are necessary according to the present invention. 
   One exemplary embodiment of the method according to the present invention lies in depositing a silicon oxide layer for planarization and then planarizing by chemical-mechanical polishing (CMP). This can be performed by manner of a silicon oxide auxiliary layer or also with the use of the silicon dioxide sealing layer. The step of leveling the topography, which is necessary due to the thin, sensitive, electrically conductive layer that is later applied, thereby becomes efficient. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a step in the manufacture of a first exemplary embodiment. 
       FIG. 2  shows a steps in the manufacture of a first exemplary embodiment. 
       FIG. 3  shows a step in the manufacture of a first exemplary embodiment. 
       FIG. 4  shows a step in the manufacture of a first exemplary embodiment. 
       FIG. 5  shows a step in the manufacture of a first exemplary embodiment. 
       FIG. 6  shows a step in the manufacture of a first exemplary embodiment. 
       FIG. 7  shows a step in the manufacture of a first exemplary embodiment. 
       FIG. 8  shows a step in the manufacture of a second exemplary embodiment. 
       FIG. 9  shows a step in the manufacture of a second exemplary embodiment. 
       FIG. 10  shows a step in the manufacture of a second exemplary embodiment. 
       FIG. 11  shows a further exemplary embodiment. 
       FIG. 12  shows a further exemplary embodiment. 
       FIG. 13  shows a further exemplary embodiment. 
       FIG. 14  shows a further exemplary embodiment. 
       FIG. 15  shows an exemplary embodiment of the manufacturing method that is usable for all the exemplary embodiments. 
       FIG. 16  show a step of two exemplary embodiments of the manufacturing method usable for leveling. 
       FIG. 17  shows a step of two exemplary embodiments of the manufacturing method usable for leveling. 
       FIG. 18  shows a step of two exemplary embodiments of the manufacturing method usable for leveling. 
       FIG. 19  shows a step of two exemplary embodiments of the manufacturing method usable for leveling. 
       FIG. 20  shows a step of two exemplary embodiments of the manufacturing method usable for leveling. 
   

   DETAILED DESCRIPTION 
     FIG. 1  shows a silicon substrate  1  in which grooves  2  have already been produced by etching from the upper side. Similar grooves, albeit only up to approx. 20 μm deep and produced for other purposes, may be referred to in other surface micromechanics systems. The width of lands  3  separating grooves  2  is selected so that upon subsequent thermal oxidation, lands  3  are in all cases completely oxidized. The state achieved thereafter is shown for the situation in which the initial width of grooves  2  was selected so that after complete oxidation of lands  3 , there remains between them a crevice  4  shown in  FIG. 2 . The initial width of grooves  2  may also be selected so that grooves  2  are completely closed up (see  FIG. 3 ). 
   In order to produce additional cavities, additional grooves may be etched, from the outset, transversely to grooves  2 . In this case lands  3  exhibit interruptions and are made up, strictly speaking, of columnar land elements. 
   Thermal oxidation is a relatively slow surface process that nevertheless results in conformal growth of the oxide even in the grooves. In all cases, the process must be continued until lands  3 , typically 1 to 5 μm wide, are each constituted entirely of SiO 2 . 
   In the state as shown in  FIG. 2  or  3 , the actual oxide block  5  having the desired low thermal conductivity has already been produced. The discussion hereinafter will always be of the exemplary embodiment including crevices  4  as shown in  FIG. 2 , since oxide block  5  constructed in this fashion yields insulation advantages. The necessary thickness of oxide block  5  is on the one hand to be selected in accordance with requirements, and on the other hand can be decreased by manner of vertical and/or horizontal cavities. Typically, a thickness of up to 100 μm will be sufficient. As the thickness of oxide block  5  increases, so does the mechanical stability of the sensor. 
   As the next step, an oxide sealing layer  6  is deposited over the entire surface of substrate  1 , and over that a nitride layer for moisture rejection, resulting in the state shown in  FIG. 4 . In the step thereafter, the platinum layer for resistors  8  (heating and measurement elements), pads, etc. is deposited and patterned (cf.  FIG. 5 ). The spacings of resistors  8  are typically only a few um, so that many of them may, if applicable, be positioned next to one another. Instead of platinum, another electrically conductive layer, made in particular of metal, polysilicon, or germanium, can be produced and patterned. Lastly, a protective oxide layer  9  having a thickness of only a few nm can be deposited and patterned (cf.  FIG. 6 ). Deposition and patterning of aluminum conductor paths  10  are performed last, yielding the finished sensor as shown in  FIG. 7 . 
     FIGS. 8 and 9  show a exemplary embodiment of the sensor in which oxide block  5  is produced not in substrate  1  itself, but in a suitable layered structure on its upper side. As shown in  FIG. 8 , the procedure begins with a substrate  1  on which a (buried) oxide layer  11  and a silicon layer  12  (for example, epitaxially grown polysilicon) are applied. Oxide layer  11  serves as an etching stop during production of silicon lands  3  and grooves  2 .  FIG. 9  shows, by analogy with  FIG. 2 , oxide block  5  generated by complete oxidation of silicon lands  3  by manner of thermal oxidation. The further method steps may be completed by analogy with the steps already shown in  FIGS. 4 through 6 , so that ultimately the final result shown in  FIG. 10  is obtained. 
   The exemplary embodiment just shown may be used as a starting point for a further exemplary embodiment, in which firstly a gas-phase etching step is performed that results in partial removal of oxide layer  11  beneath the patterned silicon layer. A horizontal cavity  13  is thus generated there (cf.  FIG. 11 ), providing additional thermal insulation between platinum resistors  8  and substrate  1 . Oxide block  5  is then once again produced in silicon layer  12  by analogy with  FIGS. 8 and 9 , resulting in the finished state as shown in  FIG. 12 . The further steps may occur in the manner already described, resulting in the final state shown in  FIG. 13 . The necessary thickness of oxide block  5  may be reduced by, or to, a fraction of its value by manner of the additional cavity  13 . 
     FIG. 14  shows a completed sensor  1  that conforms to the exemplary embodiment shown in  FIG. 13 , except that at least one pillar  14 , adjoining oxide block  5  at the top and substrate  1  at the bottom, is additionally positioned in horizontal cavity  13 . In terms of the process sequence, this requires that oxide layer  11  be appropriately patterned before the application of silicon layer  12 . Pillar  14  may allow for greater mechanical stability for this sensor configuration, so that oxide block  5  may not be caused to vibrate by the air flow being measured. 
     FIG. 15  shows a favorable procedure for producing wider grooves  2  with thinner lands  3 . In this, etching mask  15  (a hard mask, in particular SiO 2 ) that remains behind after the etching of grooves  2  is not removed, but instead remains on substrate  1  during subsequent thermal oxidation. Trenching processes with stronger undercutting caused by isotropic etching, as evident in  FIG. 15 , may be effective in this context. In this case, etching mask  15  persists after the trenching process, with an opening that is smaller than the groove width. This may fllow for, after oxidation of lands  3 , seal up of even wide grooves  2  or crevices  4 , for example using an SiO 2  sealing layer. The wide grooves  2  or crevices  4  may allow a greater porosity, with the corresponding insulation advantage, to be produced. 
   As shown in  FIG. 16 , covering oxide block  5  e.g. with oxide sealing layer  6  does not immediately yield a flat surface. As has been ascertained in SEM investigations, depressions  16  occur above crevices. This undesirable topography of only a few micrometers is nevertheless highly problematic in terms of the metal layer that must be applied later. The typical thickness of the platinum layer is approximately an order of magnitude thinner than the topography, so that a considerable risk of breakage would exist. 
     FIG. 17  shows one possibility for leveling the topography shown in  FIG. 16 . Firstly a nitride layer  7  is deposited onto oxide sealing layer  6 ; it is evident that this may allow depressions  16  to persist. An SiO 2  auxiliary layer  17  is then applied onto nitride layer  7  for subsequent planarization. Planarization of auxiliary layer  17  is accomplished, by manner of the chemical-mechanical polishing (CMP) method of other systems, down to nitride layer  7  which here serves initially as a planarization stop. A residual oxide  18  remains behind in depressions  16 , and planarizes the surface as is evident from  FIG. 18 . Platinum structures, protective oxide layers, and aluminum conductor paths can then be deposited and patterned as already described above. 
   Another possibility for leveling is shown in  FIG. 19 . In this exemplary embodiment, nitride layer  7  is deposited before oxide sealing layer  6 . This eliminates the need for subsequent deposition of oxide auxiliary layer  17  described above. 
     FIG. 19  shows the deposited nitride layer  7  and oxide sealing layer  6  deposited thereonto, which once again includes depressions  16 . Planarization of the structure down to nitride layer  7 , which serves as a planarization stop, can once again be accomplished, as described above, by manner of CMP. Residues  18  of oxide sealing layer  6  then remain behind at those points at which the critical depressions  16  were previously located (cf.  FIG. 20 ). To improve adhesion of the platinum structures, nitride layer  7  can be converted back into an oxide layer at the surface, in a manner referred to in other systems, using a reoxidation process. Platinum structures, protective oxide layers, and aluminum conductor paths may then be deposited and patterned as described above. 
   For leveling, as an alternative to CMP planarization, a passivation layer can be applied onto the surface using the spin-on-glass method. The passivation layer flows and fills depressions  16 . A thin oxide layer can then be deposited for adhesion of the platinum structures. 
   As further leveling alternatives, BPSG or PSG can be applied using a flow-glass method (like the spin-on-glass method) from other systems in the technology of highly integrated circuits. Depressions  16  are filled by the flow process, and a smooth surface forms. A thin oxide layer may then be deposited. 
   The topography may also be sufficiently decreased by CVD deposition methods of other systems. With suitable deposition parameters, only depressions  16  are filled with SiO 2 , and an almost planar surface is established.