Abstract:
Method for manufacturing a semiconductor pressure sensor, wherein, in a silicon substrate, trenches are dug and delimit walls; a closing layer is epitaxially grown, that closes the trenches at the top and forms a suspended membrane; a heat treatment is performed so as to cause migration of the silicon of the walls and to form a closed cavity underneath the suspended membrane; and structures are formed for transducing the deflection of the suspended membrane into electrical signals.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The subject of the present invention is a method for manufacturing a semiconductor pressure sensor.  
         [0003]     2. Description of the Related Art  
         [0004]     As is known, a pressure sensor is a device that converts a variation in pressure into a variation of an electrical quantity (a resistance or a capacitance). In the case of a semiconductor sensor, the pressure variation is detected by a membrane of semiconductor material, which overlies a cavity and is able to undergo deflection under mechanical stress.  
         [0005]     Pressure sensors using semiconductor technology typically find their application in medicine, in household appliances, in consumer electronics (cell-phones, PDAs-Personal Digital Assistants-), and in the automotive field. In particular, in the latter sector, pressure sensors are used traditionally for detecting the pressure of the types of motor vehicles, and are used by the control unit for alarm signaling. Pressure sensors are, on the other hand, also used for monitoring air-bag pressure, for controlling the breakdown pressure of the ABS, and for monitoring the pressure of oil in the engine, the pressure of injection of the fuel, etc.  
         [0006]     Currently existing sensors manufactured using the semiconductor technology are basically of two types: piezoresistive and capacitive sensors.  
         [0007]     Operation of piezoresistive sensors is based upon piezoresistivity, i.e., the capability of some materials to modify their resistivity as the applied pressure varies. Piezoresistors are normally formed on the edge of a suspended membrane (or diaphragm) and are connected to one another in a Wheatstone-bridge configuration. Application of a pressure causes a deflection of the membrane, which in turn generates a variation in the offset voltage of the bridge. By detecting the voltage variation with an appropriate electronic circuit, it is possible to derive the desired pressure information. An example of a piezoresistive sensor of the above type is described in U.S. Pat. No. 6,131,466.  
         [0008]     Sensors of a capacitive type are based upon the change in capacitance that occurs when a pressure is applied on a flexible membrane suspended above a support and separated therefrom by a region that is empty or filled with gas (air gap). Two examples of silicon sensors of a capacitive type are described in “A MEMS-Based, High-Sensitivity Pressure Sensor for Ultraclean Semiconductor Applications”, A. K. Henning, N. Mourlas, S. Metz published on: http://www.redwoodmicro.com/Papers/ASMC. pdf and “Application of High-Performance MEMS Pressure Sensors Based on Dissolved Wafer Process” A. Tadigadapa, S. Massoud-Ansari, published on: hftp://Iwww.mems-issys.com/pdf/issystech2.pdf.  
         [0009]     In the case where the dielectric present between the two electrodes is a vacuum, an absolute pressure sensor is obtained, whereas, if gas is present, which is generally introduced hermetically at a known reference pressure, the detected capacitance variation is linked to the difference between the external pressure and the internal pressure, and, consequently, a relative pressure sensor is obtained.  
         [0010]     Application of a pressure causes a deflection of the membrane with consequent reduction in its distance from the bottom electrode. In this way, the capacitance of the pressure sensor increases. By measuring the difference between the capacitance thus obtained and the rest capacitance (i.e., in the absence of stress), the pressure variation detected by the sensor is obtained.  
         [0011]     Also in this case, a circuit for processing the electrical signals generated by the capacitive sensor provides the information of pressure sought.  
         [0012]     In general, capacitive technology presents a lower current consumption than does piezoresistive technology. Consequently, capacitive sensors are preferable in those applications where power consumption is an important parameter, for example, in the automotive field, wherein accurate control of the load power is required. Moreover, capacitive pressure sensors present smaller overall dimensions and lower costs, as is required in numerous applications.  
         [0013]     Both piezoelectric sensors and capacitive sensors hence call for the construction of a cavity underneath the flexible membrane.  
         [0014]     Currently, various solutions have been proposed: 
        1. use of silicon-on-insulator (SOI) substrates; for instance, pressure sensors using this solution are described in U.S. Pat. Nos. 5,369,544; 5,510,276 and 6,131,466;     2. use of porous silicon (see, for example, U.S. Pat. No. 5,242,863);     3. wet etching from the front (see, for example, U.S. Pat. No. 4,766,666);     4. wet etching from the rear, using tetramethyl ammonium hydroxide (TMAH);     5. other methods (see, for example, U.S. Pat. No. 4,744,863).        
 
         [0020]     In all known solutions, the use of semiconductor technology for making cavities underneath suspended structures and layers calls for processes that are complex, costly and, in some cases, far from compatible with the manufacturing steps currently used in the semiconductor industry for manufacturing integrated circuits.  
       BRIEF SUMMARY OF THE INVENTION  
       [0021]     One embodiment of the present invention is hence to provide a manufacturing method which will overcome the disadvantages of known solutions. The method comprises: providing a wafer comprising a bulk region of semiconductor material; forming a membrane above and at a distance from said bulk region; forming a closed cavity between said membrane and said bulk region; and forming structures for transducing the deflection of said membrane into electrical signals, wherein, the step of forming a membrane includes: digging a plurality of first trenches in said bulk region, said first trenches delimiting a plurality of first walls of semiconductor material; epitaxially growing, starting from said first walls, a closing layer of semiconductor material, said closing layer closing said trenches at the top and forming said membrane; and carrying out a heat treatment, thereby causing migration of the semiconductor material of said first walls and forming a closed cavity.  
         [0022]     Another embodiment of the present invention provides a pressure sensor comprises: a bulk region of semiconductor material; a buried cavity overlying a first part of the bulk region; and a membrane suspended above said buried cavity, wherein, said bulk region and said membrane are formed in a monocrystalline substrate, and in that said monocrystalline substrate carries structures for transducing the deflection of said membrane into electrical signals. 
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
       [0023]     For an understanding of the present invention, preferred embodiments thereof are now described purely by way of non-limiting example, with reference to the attached drawings, wherein:  
         [0024]      FIG. 1  is a cross-section through a wafer of semiconductor material in an initial manufacturing step;  
         [0025]      FIG. 2  is a top view of the wafer of  FIG. 1 ;  
         [0026]      FIG. 3  is a cross-section of details of  FIG. 2 , at an enlarged scale;  
         [0027]      FIGS. 4-9  are cross-sections through the wafer of semiconductor material of  FIG. 1 , in subsequent manufacturing steps, for a pressure sensor of capacitive type; and  
         [0028]      FIG. 10  is a cross-sectional view through a wafer of semiconductor material for a pressure sensor of piezoelectric type. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0029]     Hereinafter, an embodiment of a process for manufacturing a semiconductor material sensor of capacitive type is described. The present process is based upon the process disclosed in U.S. application Ser. No.  10 / 327 , 702  for manufacturing a SOI wafer, and, more precisely, refers to the second embodiment shown in  FIGS. 11-14  of said document.  
         [0030]      FIG. 1  shows a wafer  1  of semiconductor material, preferably monocrystalline silicon, comprising an N-type substrate  2 , designed to form the bulk of the device. A resist mask  3  (visible in the enlarged detail of  FIG. 3 ) is formed on the top surface of the substrate  2 . The mask  3  has two circular areas, designated by  4   a  and  4   b  and hereinafter referred to as sensor area and reference area, respectively. In each of these areas, a honeycomb lattice is defined, the two lattices being of different sizes.  
         [0031]     In particular, as appears in the enlarged detail of  FIG. 2 , the sensor area  4   a  has mask regions  5   a  with an hexagonal shape arranged close to one another (see also the cross-section of  FIG. 3 ), while the reference area  4   b  has mask regions  5   a  that are more widely spaced. For example, the distance t between opposite sides of the mask regions  5   a  and  5   b  may be 2 μm, the distance d 1  between sides facing adjacent mask regions  5   a  may be 1 μm, and the distance d 2  between sides facing adjacent mask regions  5   b  may be 2 μm.  
         [0032]     Using the mask  3 , trench etching of silicon of the substrate  2  is performed, so forming a sensor trench  6   a  and a reference trench  6   b  at the sensor area  4   a  and at the reference area  4   b , respectively. The channels of the sensor and reference trenches  6   a ,  6   b  may have, for example, a depth of approximately 10 μm, are of different width, as may be seen in  FIG. 3 , and delimit silicon columns  7   a  and  7   b , respectively, which are identical and have a shape at the cross section corresponding to that of the mask regions  5   a  and  5   b.    
         [0033]     Next (see  FIG. 4 ), the mask  3  is removed and an epitaxial growth is performed in a deoxidizing environment (typically, in an atmosphere with a high concentration of hydrogen, preferably using trichlorosilane SiHCl 3 ). Consequently, an epitaxial layer  10  (shown only in  FIG. 4  and hereinafter not distinguished from the substrate  2 ) of N type, grows on top of the silicon columns  7   a  and  7   b  and closes, at the top, the sensor and reference trenches  6   a ,  6   b , trapping the gas present therein (here, molecules of hydrogen H 2 ). The thickness of the epitaxial layer  10  may be, for example, 9 μm.  
         [0034]     An annealing step is then carried out, for example for 30 minutes at 1190° C.  
         [0035]     As discussed in the aforementioned U.S. application Ser. No. 10/327,702, annealing causes a migration of the silicon atoms, which tend to arrange themselves in lower-energy position. Consequently, at the sensor trench  6   a , where the columns  7   a  are arranged close together, the silicon atoms migrate completely and form a sensor cavity  11 , closed at the top by a membrane  13 . On account of the presence of the sensor cavity  11  (having, for example, a diameter of 600 or 200 μm, according to the pressure to be applied), the membrane  13  is flexible and can be deflected under external stresses.  
         [0036]     On the other hand, at the reference trench  6   b , where the columns  7   b  are arranged at a bigger distance from one another, the migration of silicon atoms causes only a thinning of the columns  7   b , hereinafter indicated as pillars  15 . In practice, a labyrinthine cavity  12  is formed, wider than the reference trench  6   b . Furthermore, the pillars  15  in the labyrinthine cavity  12  prevent any movement to the overlying region, hereinafter referred to as electrode region  14 .  
         [0037]     Preferably, annealing is performed in an H 2  atmosphere so as to prevent the hydrogen in the sensor and reference trenches  6   a ,  6   b  from escaping through the epitaxial layer  10  to the outside and so as to increase the concentration of hydrogen in the cavities  11  and  12 , should the hydrogen trapped inside during the epitaxial growth be not sufficient. Alternatively, annealing can be carried out in a nitrogen environment.  
         [0038]     The crystallographic quality of the membrane  13  is excellent, as is evident from tests carried out by the present applicant.  
         [0039]     Next (see  FIG. 5 ), the membrane  13  and the electrode region  14  are doped via implantation of P-type dopant species, for example boron. Subsequently (see  FIG. 6 ), an access trench  20  is dug just in the electrode region  14 , from the surface of the wafer  1  to reach as far as the labyrinthine cavity  12 . The access trench  20  preferably has the shape shown in  FIG. 6 , and hence extends, by stretches, near the periphery of the area occupied by the labyrinthine cavity  12 .  
         [0040]     Thermal oxidation of the columns  7   b  is then carried out so as to form an oxidized region  21  underneath the electrode region  14 . The necessary oxygen is fed to the labyrinthine cavity  12  through the access trench  20 . In this step, there is a gradual growth of the oxidized region  21  at the expense of the columns  7   b  and of the silicon of the substrate  2  surrounding the access trench  20  and the labyrinthine cavity  12 . In particular, the columns  7   b  are completely oxidized and increase in volume. As shown in  FIG. 7 , the labyrinthine cavity  12  and the access trench  20  are filled in part with thermal oxide, but remain partially open (remaining portions  12 ′ and  20 ′ of the labyrinthine cavity and of the access trench).  
         [0041]     Next (see  FIG. 8 ), the remaining portions  12 ′ and  20 ′ of the labyrinthine cavity and of the access trench are filled with insulating material  22 , for example TEOS, forming, as a whole, an insulating region  24 . In  FIG. 8 , for clarity, the demarcation line between the insulating material  22  and the oxidized region  21  is represented by a dashed line. As an alternative, the labyrinthine cavity  12 ′ can remain empty of insulating material, thus avoiding the filling step.  
         [0042]     A P-type implantation, an N-type implantation and respective diffusion steps are then carried out in order to form contact regions  25   a ,  25   b  of P + -type above the membrane  13  and the electrode region  14  as well as contact regions  25   c ,  25   d  of N + -type above the substrate  2  (see  FIG. 9 ). The contact regions  25   c ,  25   d  preferably have an annular shape and extend, respectively, around the membrane  13  and around the electrode region  14 . Next, metal contacts  26   a ,  26   b ,  26   c  and  26   d  are formed and contact the contact regions  25   a  to  25   d , respectively.  
         [0043]     In practice, the structure of  FIG. 9  forms two capacitors, designated by C 1  and C 0 , which have, as first electrode, the membrane  13  and the electrode region  14 , respectively; as second electrode, the bulk region underlying the membrane  13  and the bulk region underlying the electrode region  14 , respectively; and as dielectric, the sensor cavity  11  and the insulating region  24  (or the oxidized region  21  and the labyrinthine cavity  12 ′), respectively.  
         [0044]     The capacitor C 1  (referred to also as sensing capacitor) represents the element sensitive to the pressure that is applied on the membrane  13 , while the capacitor C 0  (reference capacitor) represents the reference element, which provides the rest capacitance. Since the areas of the P/N junctions of the sensing capacitor C 1  and of the reference capacitor C 0  are equal, these capacitors have the same junction capacitance and the same leakage currents. In addition, the reference capacitor C 0  undergoes a trimming step at the wafer level, using one or more known capacitors arranged in parallel and using a one-time programmable (OTP) device.  
         [0045]     If so desired, prior to forming the contact regions  25   a - 25   d , it is possible to integrate the electronic components making up the control circuitry on the same chip of the pressure sensor.  
         [0046]     Finally, in a way not shown, the wafer  1  is cut into dice, each containing a sensing capacitor C 1  and a reference capacitor C 0  (as well as, if envisaged, the control circuitry), and the dice are encapsulated in such away that the membrane  13  is accessible from the outside.  
         [0047]     Operation of the pressure sensor is described hereinafter.  
         [0048]     If a pressure is applied on the membrane  13 , the latter is deflected, reducing its distance from the bottom electrode (substrate  2 ). Consequently, the capacitance of the sensing capacitor C 1  increases. If the difference between the signal supplied by the sensing capacitor C 1  and the signal supplied by the reference capacitor C 0  is measured via an electronic circuit for signal processing of the “fully differential” type, there is rejection of the common-mode components and amplification of the differential ones, and hence an indication of the pressure applied is obtained.  
         [0049]     The advantages afforded by the described pressure sensor emerge clearly from the foregoing description. In particular, thanks to the described manufacturing process, the silicon pressure sensor is of low cost and reduced dimensions, and hence can be used in numerous applications where these requirements are important.  
         [0050]     If so desired, it is possible to integrate on the same chip the sensing capacitor C 1  and the reference capacitor C 0  and the relative control circuitry, thus reducing the manufacturing and assembly costs, as well as the overall dimensions of the device.  
         [0051]     Finally, it is clear that numerous modifications and variations may be made to the pressure sensor described and illustrated herein, all falling within the scope of the invention, as defined in the annexed claims.  
         [0052]     In particular, the described technique can be used, with just a few modifications, for producing a pressure sensor of a piezoelectric type. In this case, in fact, it is sufficient, during the final manufacturing steps, for example, simultaneously with the manufacture of the components of the control circuitry, to form piezoresistive elements near the periphery of the membrane  13 . Preferably, the piezoresistive elements are of P-type, and the membrane is of N-type. In case of a pressure sensor of piezoelectric type, however, it is not necessary to provide a reference element, and hence all the steps necessary to form the labyrinthine cavity  12  and the insulating region  24 , as well as boron implantation, are omitted. An embodiment of a pressure sensor of piezoelectric type is shown in  FIG. 10 , wherein the elements in common with the embodiment of  FIG. 9  are designated by the same reference numbers.  
         [0053]     As is known, in  FIG. 10  no insulating region  24  is present, and resistors  30 , implanted or diffused, here of a P-type, are formed on the periphery of the membrane  13  (of an N − -type). In  FIG. 10 , only three resistors  30  can be seen, but it must be understood that a fourth resistor is formed in the non-visible part of the wafer  1  and is connected to the visible resistors  30  in a bridge configuration. In  FIG. 10 , the interconnections between the resistors  30  (typically metal regions extending above an insulating layer, not shown) are represented schematically.  
         [0054]     As an alternative, the resistors  30  may be made of polysilicon above the membrane  13 .  
         [0055]     For a capacitive sensor, the membrane may be of any shape, for example square or generically polygonal, even though the preferred shape is circular, since it prevents any stress concentration.  
         [0056]     The contact of the capacitors C 0  and C 1  with the second electrode can be made, instead of on the front of the device, on the rear, as indicated by the dashed line  35  in  FIG. 9 ; in this case, it is expedient to use a substrate  2  of N + -type in order to reduce the access series resistance. Also in this case, the epitaxial layer is of N − -type.  
         [0057]     The shape of the columns  7   a ,  7   b  may vary with respect to what is illustrated. For example, they can be replaced by diaphragms of semiconductor material with reduced thickness, or, in general, by other thin structures (referred to also as walls) capable of enabling migration of silicon during the annealing step and forming the sensor cavity  11  and the labyrinthine cavity  12  (for a capacitive implementation).  
         [0058]     All of the above U.S. patents, U.S. patent application publications, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referred to in this specification and/or listed in the Application Data Sheet, are incorporated herein by reference, in their entirety.  
         [0059]     From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims.