Abstract:
An electronic microbalance made in a semiconductor body accommodating an oscillating circuit adjacent to a diaphragm. A stack formed by a first electrode, a second electrode, and a piezoelectric region arranged between the first and the second electrode extends above the diaphragm. Any substance that deposits on the stack causes a change in the mass of the microbalance and thus in the resonance frequency of a resonator formed by the microbalance and by the oscillating circuit and can thus be detected electronically. A chemical sensor is obtained by forming a sensitive layer of a material suitable for binding to target chemicals on the stack. The sensitivity of the microbalance can be increased by making the first electrode of molybdenum so as to increase the piezoelectric characteristics of the piezoelectric region.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to Italian Application No. TO2008A001012 filed on Dec. 30, 2008, incorporated herein by reference in its entirety. 
       FEDERALLY SPONSORED RESEARCH STATEMENT 
       [0002]    Not applicable. 
       REFERENCE TO MICROFICHE APPENDIX 
       [0003]    Not applicable. 
       FIELD OF THE INVENTION 
       [0004]    The present invention relates to an integrated electronic microbalance, in particular an integrated chemical sensor for detecting odorous matters. 
       BACKGROUND OF THE INVENTION 
       [0005]    As is known, the request for microsensors of small dimensions has led to the study of integrated solutions that apply the techniques and know-how acquired in the sector of the manufacture of semiconductors. In particular, integrated sensors for detecting acoustic waves have been studied that use piezoelectric layers, wherein a piezoelectric material layer, arranged between two electrode layers, overlies a cavity and forms an acoustic resonator (see, for example, “Bulk Acoustic Wave Theory and Devices” Joel F. Rosenbaum Artech House Inc., 1988). 
         [0006]    These electro-acoustic resonators have been proposed for producing sensors of different types, such as force sensors, pressure sensors, acceleration sensors, weight sensors, and sensors for detecting chemicals, which exploit the variation of the oscillating frequency of the acoustic resonator following a variation of its mass and/or of its geometrical configuration. In practice, the resonator forms an integrated microbalance based upon the piezoelectric effect. 
         [0007]    Recently, the use of microbalances as chemical sensors (electronic noses) has awakened particular interest. These sensors find in fact application in the foodstuff sector, where they can be used for controlling the degree of freshness of foodstuffs in the fishery industry (fish, molluscs, etc.), for assessing the degree of seasoning of cheese, for controlling the suitability of packaging, for controlling cooking of foodstuffs, for assessing the quality of beer, liqueurs and spirits. Integrated chemical sensors can moreover be used also in the cosmetics and pharmaceutical industry for controlling perfumes and aromas. The sector of environmental monitoring and that of medicine represent, instead, emerging markets for electronic noses. In both of these last fields they can be used for detecting chemical species produced by bacteria, for example, in environmental applications, for detecting cyanobacteria present in lakes and rivers, or in the medical field for detecting the presence of Escherichia coli. Finally, a market that represents an outlet that is very promising from the economic standpoint for electronic noses or, more in general, for automatic gas-detection systems is represented by the automotive sector. In this field, manufacturers are interested in controlling the quality of the air in the passenger compartment of vehicles and in controlling the exhaust gases. 
         [0008]    For application as an electronic nose, an apparatus has been proposed that comprises a plurality of quartz chemical sensors, each formed by a quartz region having a surface covered by an adsorbent layer, which is able to bind in a non-selective way with the volatile substances present in the environment (ITRM2001A000455). In practice, the quartz forms, with an associated oscillating circuit, an electrical resonator having a natural resonance frequency comprised between 1 MHz and 20-30 MHz. Each sensor is provided with a different adsorbent layer. When the chemical substances in the environment (analytes) are adsorbed by one or more chemical sensors, the latter increase in weight, varying their own masses, and thus their own oscillating frequency. Alternatively, the relaxation time of the oscillations is measured. 
         [0009]    A processing electronics connected to the chemical sensors processes the generated signals and compares them with known configurations in order to recognize the chemicals. 
         [0010]    Known quartz sensors may, however, undergo improvement, in particular as regards sensitivity and the overall dimensions, which do not enable use thereof in many applications. The use of quartz renders moreover production complex and burdensome. 
         [0011]    Sensors on silicon substrates have moreover been proposed, having cavities obtained by “bulk micromachining” using tetramethyl-amonium hydroxide (TMAH) (see for example “Sensors and Microsystems: Proceedings of the 10th Italian Conference” A. G. Mignani, R. Falciai, C. Di Natale, A. D′Amico, World Scientific Publishing Company, July 2008). This solution envisages deposition, on a surface of a silicon wafer, of a silicon nitride layer, operating as an etch stop, a first aluminium layer (bottom electrode), an aluminium nitride layer (piezoelectric material), and a second aluminium layer (second electrode). Then, an anisotropic back etching is performed, which is stopped on the silicon nitride layer, and the wafer is diced. In each die thus obtained, the stack of layers on the front defines a diaphragm, whereon a thin layer of a sensitive material, such as porphyrin, is deposited. 
         [0012]    In this process, the required type of machining is costly and the use of TMAH is not usual in present production lines for integrated circuits. In addition, the etching procedure causes the formation, in the substrate, of a cavity with a trapezium cross-section having a minor base formed by the diaphragm and sides inclined by 45°-50°. Given that the thickness of the substrate is generally 675-700 μm, the major base of the cavity occupies an area having a side or diameter of 1.2-1.4 mm added to the width of the diaphragm. The minimum total area required by each microbalance is thus much greater than that due to the oscillating region alone. The microbalance thus has large overall dimensions, which reduces the possibility of integration thereof. 
         [0013]    Consequently, in general, the known sensors do not provide the desired sensitivity, involve complex manufacturing processes, present high costs and dimensions such as not to enable a wide application thereof. 
       SUMMARY OF THE INVENTION 
       [0014]    The aim of the present invention is to provide a microbalance structure that overcomes one or more of the drawbacks of the known art. 
         [0015]    An electronic microbalance comprises a body of semiconductor material; an electronic high-frequency circuit integrated in the body; a diaphragm arranged laterally to the electronic high-frequency circuit; a first electrode arranged above the diaphragm; a second electrode arranged above the first electrode; and a piezoelectric region, arranged between the first and second electrodes. 
         [0016]    A process of manufacturing a microbalance comprises the steps of providing a wafer of semiconductor material having a surface; integrating an electronic circuit in the wafer; forming a diaphragm laterally to the electronic circuit; forming a first electrode above the diaphragm; forming a piezoelectric region on the first electrode; and forming a second electrode above the piezoelectric region. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    For a better 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: 
           [0018]      FIG. 1  is a cross-section of a silicon wafer integrating an embodiment of the present electronic microbalance; 
           [0019]      FIG. 2  shows a cross-section of a semiconductor substrate, in a manufacturing step of the microbalance of  FIG. 1 ; 
           [0020]      FIG. 3  is a perspective view of the structure of  FIG. 2 ; 
           [0021]      FIG. 4  shows a cross-section of the semiconductor substrate of  FIG. 2 , in a successive manufacturing step; 
           [0022]      FIG. 5  is a partially sectioned perspective view of a sensor formed by a plurality of microbalances of  FIG. 1 ; 
           [0023]      FIG. 6  is a block diagram of an apparatus for detecting chemical substances; 
           [0024]      FIGS. 7-9  are different embodiments of the instant microbalance; 
           [0025]      FIGS. 10   a  and  10   b  show a different embodiment of the present microbalance, in two subsequent manufacturing steps; 
           [0026]      FIG. 11  shows a top view of an embodiment of the present electronic microbalance. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0027]      FIG. 1  shows a microbalance  1  integrated in a body  2  of semiconductor material, for example, monocrystalline silicon, and having a buried cavity  3 , which delimits at the bottom a diaphragm  18 , which is also of monocrystalline silicon. 
         [0028]    A buffer layer  5 , for example, of aluminium nitride (AlN), extends above the diaphragm  18 , and a bottom electrode  10 , for example, of molybdenum, extends on the buffer layer  5 . In the embodiment considered, the buffer layer  5  has a thickness comprised between 30 and 100 nm, for example 50 nm, and the bottom electrode  10  has a thickness of 50-150 nm, for example 100 nm. 
         [0029]    A piezoelectric region  11  extends on the bottom electrode  10  and has here an area smaller than the electrode  10  so as to enable electrical connection of the bottom electrode  10 , as represented by the wire  12 , to a ground potential. The piezoelectric region  11  has a thickness of 1-3 μm, for example, approximately 2 μm. 
         [0030]    A top electrode  15 , which is, for example, also of molybdenum and has a thickness of 50-150 nm, for example, 100 nm, extends on the piezoelectric region  11 . The top electrode can have the same area as, or an area smaller than, the piezoelectric region  11  and is connected, for example, via a wire  17  to an oscillator  19 , of a known type and not shown in detail. 
         [0031]    Finally, a supporting region  16  extends on the top electrode  15 . If the microbalance  1  forms a chemical sensor, the supporting region  16  is a sensitive region of a material such as to bind with the chemical to be detected, in particular, a metal-porphyrin having affinity with this chemical. 
         [0032]    Operation of the microbalance  1  of  FIG. 1  is illustrated hereinafter. The circuit formed by the piezoelectric region  11  and by the oscillator  19  forms an electrical resonator having a natural oscillation frequency. When a mass is deposited on the supporting region  16  or an analyte binds thereto, the resonator undergoes a variation of the oscillating frequency Δf according to the Sauerbray equation: 
         [0000]    
       
         
           
             
               
                 
                   
                     Δ 
                      
                     
                         
                     
                      
                     f 
                   
                   = 
                   
                     
                       - 
                       
                         
                           2 
                            
                           
                             f 
                             0 
                             2 
                           
                         
                         
                           A 
                            
                           
                             μρ 
                           
                         
                       
                     
                      
                     Δ 
                      
                     
                         
                     
                      
                     m 
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where Δm is the mass variation, f o  is the natural oscillation frequency of the resonator, μ is the shear modulus, ρ is the density, and A is the area of the oscillating surface (area of the base of the parallelepiped or in general of the solid formed by the piezoelectric region  11 ). 
         [0033]    The microbalance  1  has a high sensitivity, thanks to its reduced dimensions and to its structure, obtainable by virtue of the microelectronic techniques. 
         [0034]    The sensitivity S of the balance  1  can be calculated as: 
         [0000]    
       
         
           
             
               
                 
                   S 
                   = 
                   
                     
                       
                         Δ 
                          
                         
                             
                         
                          
                         f 
                       
                       
                         Δ 
                          
                         
                             
                         
                          
                         m 
                       
                     
                     = 
                     
                       - 
                       
                         
                           2 
                            
                           
                             f 
                             0 
                             2 
                           
                         
                         
                           A 
                            
                           
                             μρ 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
         [0035]    The microbalance  1  of  FIG. 1  can be made using the technique described in EP-A-1 324 382, for manufacturing an SOI wafer, and the process described in EP-A-1577656, for manufacturing a pressure sensor. 
         [0036]    Initially ( FIG. 2 ), using a mask  40 , a dry anisotropic chemical etching is carried out of a substrate  41  of semiconductor material, for example silicon, so as to form a plurality of trenches  42  having a width of, for example, 1 μm, which communicate with one another and delimit a plurality of columns  43  having a substantially hexagonal cross-section, as may be seen in  FIG. 3 . For example, the trenches can have a depth of 11 μm±1 μm, and the columns  43  can have a diameter of 2 μm. 
         [0037]    Next, an epitaxial growth is performed in a known way. Consequently, an epitaxial layer  44  grows above the columns  43  and closes the trenches  42  at the top. 
         [0038]    A thermal annealing is then performed, for example for 25-30 minutes at approximately 1200° C., preferably in hydrogen atmosphere. Alternatively, nitrogen can be used, with longer times and using protection layers. 
         [0039]    As discussed in the aforementioned patent applications, the annealing causes a migration of the silicon atoms, which tend to go into a lower-energy configuration. Consequently, by suitably selecting the width of the trenches  42 , it is possible to cause the silicon atoms to migrate completely from the portions of the columns  43 , forming the buried cavity  3 , as shown in  FIG. 4 . A silicon layer remains on the buried cavity  3  and has a thickness of, for example, 20 μm, formed in part by silicon atoms grown epitaxially and in part by silicon atoms that have migrated and form the diaphragm  18 . The body  2  is thus formed. 
         [0040]    On a wafer thus obtained, after the possible integration of electronic components  38  ( FIG. 5 ) of the oscillator and of the other associated electronic circuits, the buffer layer  5  is deposited, preferably by RF sputtering. The buffer layer  5 , for example of AlN, has the aim of optimizing the quality of the molybdenum layer forming the bottom electrode  10  and consequently the subsequent growth of the piezoelectric layer intended to form the piezoelectric regions  11 . The buffer layer  5  can be left over the entire surface  4  of the wafer, at least in the area of the microbalances, or defined so as to form a plurality of buffer regions  5 , one for each microbalance  1 . 
         [0041]    Next, a first electrode layer is deposited. For example, molybdenum is deposited. The use of molybdenum has proven particularly advantageous for the performance of the microbalance  1 . In fact, the intensity of the piezoelectric effect of a given piezoelectric layer is the greater the more the piezoelectric layer is oriented along the crystallographic axis, perpendicular to the plane of growth. Markedly oriented polycrystalline piezoelectric layers have piezoelectric properties similar to those of the epitaxial layers, with the advantage that they can be deposited at lower temperatures. Studies conducted by the present applicant have shown that molybdenum is more effective than aluminium for forming piezoelectric layers of AlN that are markedly oriented along the axis c; thus the use of the indicated materials contributes to obtaining a high sensitivity of the microbalance  1 . 
         [0042]    The first electrode layer can coat the entire surface of the wafer, at least in the area of the microbalances  1 , since it defines a ground electrode, common to all, and can possibly be used as level metallization for the associated circuitry, connected to the standard aluminium tracks. 
         [0043]    Next, a piezoelectric layer, preferably AlN, is deposited by sputtering and defined so as to obtain one piezoelectric region  11  for each microbalance  1 . Alternatively, it is possible to have a piezoelectric layer not defined, common to all the microbalances  1  so as to prevent the presence of a step during deposition of the subsequent top-electrode layer. After deposition, the top-electrode layer, preferably of molybdenum, is defined via current photolithographic techniques so as to form the top electrode  15 , one for each microbalance  1 . 
         [0044]    As an alternative to the above, it is possible to define the layers of the top and bottom electrodes and the piezoelectric layer in sequence, only after depositing the top-electrode layer. 
         [0045]    Finally, the sensitive region  16 , for example, an appropriate metal-porphyrin, according to the chemical to be detected, is deposited on the top electrodes  15 , for example by “spotting”, so as to obtain the structure of  FIG. 1 . In addition, a passivation layer (not shown) may be deposited and opened for forming contacts (not shown). 
         [0046]    The microbalance  1  of  FIG. 1  can be integrated in a same chip with other microbalances  1  sensitive to other chemicals.  FIG. 5  shows, for example, a chip  20 , which integrates a plurality of chemical sensors  21 , for example eight (only three whereof are visible). The chemical sensors  21  have the structure shown in  FIG. 1  and are represented schematically, with a surface region  22  representing the set of the regions  11 ,  15  and  16 . In the shown embodiment, the bottom electrode  10  covers all the surface shown of the chemical sensors  21  area, and the wires  17  are connected to appropriate external areas. Alternatively, the bottom-electrode layer  10  can be defined so as to form contact pads and interconnection lines to the associated circuitry ( FIG. 6 ). 
         [0047]    In practice, the chemical sensors  21  are arranged in an array and are for example made so as to bind to different chemicals; the different signals generated can then be used and compared with known distributions for recognition of chemical substances. 
         [0048]      FIG. 6  shows, for example, an apparatus for recognition of odours or “electronic nose”  25  comprising a board  35 , carrying the chip  20 , which integrates an array of eight chemical sensors  21 , and an associated electronics, which includes an oscillator stage  26 , including a plurality of oscillators  19 , one for each microbalance  21 , and a converter  27 , for example a counter. The oscillator stage  26  drives the array of chemical sensors  21  and generates a plurality of periodic signals (pulse trains) having a frequency equal to the frequency f of the resonators. The counter  27  counts the number N of pulses within a fixed time interval T and generates digital signals that encode their oscillation frequencies f, the value whereof depends upon the possible bond with respective analytes bound thereto. 
         [0049]    The output of the chip  20  is connected to a microcontroller  30  connected to a memory  31 , for example, of an EEPROM type, to a power-supply stage  32  and to an input/output stage  33 , for example for displaying the results. The microcontroller  30  is then able to detect the possible differences of frequency Δf for each chemical sensor  21  and, according to patterns stored in the memory  31 , to recognize the chemicals present in the environment. 
         [0050]    The odour-recognition apparatus  25  can be housed in an external casing so as to protect the chemical sensors  21 . In this case, it can be provided with pneumatic actuators and corresponding input/output connectors, valves and control stages, for introducing and venting the gases to be analysed, as well as possibly input/output stages and electrical connectors towards an external personal computer (PC). 
         [0051]      FIG. 7  shows an embodiment of a detector wherein the cavity underlying the diaphragm is not buried. In detail, here the substrate  2  has a trench  100  obtained using the technique of bulk micromachining by selectively removing a portion of the silicon substrate  2  from the back using a reactive ion etching plasma tool. A dielectric layer  105 , e.g., thermally grown silicon dioxide, extends on the surface of the substrate  2  and defines the diaphragm  18 . A protection or passivation layer  37  extends on the surface of the microbalance  1 , except at the sensitive region  16  and at contacts  106 - 108 . The protection layer  37  may be SiN and the supporting region  16  may not be present. A circuitry  38  extends laterally to the microbalance area; the contacts  106 - 108  connect the circuitry  38 , the bottom electrode  10  and the upper electrode  15  to the other components of the “electronic nose”  25  of  FIG. 6 . Here, the substrate  2  is a standard substrate having a thickness of 725 μm and a thermal oxide  110  having a thickness of 1 μm extends on the bottom surface of the substrate  2 . 
         [0052]    The substrate  2  has here a trench  100  for each microbalance  1 . 
         [0053]    In  FIG. 8 , the diaphragm  18  is formed by a multilayer including a bottom dielectric region  111  (e.g., of silicon dioxide), a polysilicon portion  112  and an upper dielectric layer  113  (e.g., of silicon dioxide). Such a structure may be obtained, before forming the components of the circuitry  38 , during the epitaxial growth. In detail, before growing an epitaxial layer, the bottom dielectric region  111  is thermally grown or deposited on the sensing portions of the wafer, so that during the epitaxial growth, the polysilicon portion  112  grows over the bottom dielectric region  111 . Elsewhere, the epitaxial layer is monocrystalline, to allow integration of electric components. Then, after the integration of the circuitry  38 , the upper dielectric layer  113  is grown or deposited and covers the entire surface of the substrate  2 . In the alternative, the bottom dielectric layer  111  and the polysilicon layer  112  may be grown or deposited on the upper surface of the substrate  2 , before or after forming the components of the circuitry  38 . 
         [0054]      FIG. 9  is an embodiment wherein the detector uses the reflecting ability of a Bragg mirror to contain the acoustic waves in the overlying piezoelectric region  11  and thus is functionally equivalent to the buried cavity  3  or the trench  100 . In detail, here, the substrate  2  is covered by a thick dielectric layer  120  which, in each sensor portion thereof, accommodates a mirror  121 . The mirror  121  is formed by a stack of alternating high acoustic impedance layers  122  and low acoustic impedance layers  123  as described, e.g., in U.S. Pat. No. 6,448,695 or U.S. Pat. No. 6,933,807. As an example, the low acoustic impedance layers  123  may be of silicon dioxide and the high acoustic impedance layers  122  may be of tungsten. 
         [0055]      FIGS. 10   a  and  10   b  refer to an embodiment wherein the cavity  100  is not obtained by bulk micromachining, but by using the process described in U.S. Pat. No. 7,294,536 or in U.S. Pat. No. 7,071,073. In particular, before forming the components of the circuitry  38 , a buried oxide layer  150  is formed in the substrate  2 ,  FIG. 10   a . At the end of the process, after forming the circuitry  38  and the microbalance regions  10 ,  11 ,  15 ,  16 ,  37  on the surface of the substrate  2 , the substrate  2  is etched from the back to selectively remove silicon using, e.g., a reactive ion etching plasma tool, until reaching the buried oxide layer  150 . The buried oxide layer  150  is removed as well, forming the cavity or trench  100  under a silicon layer  151  under the substrate upper surface. The remaining portion of the substrate  2  over the trench  100  forms a diaphragm  152 . Here, the substrate  2  may have a thickness of 308 μm. 
         [0056]    In an embodiment, an integrated heater is formed using the aluminum metallization, as shown in the layout of  FIG. 11 , relative to the microbalance area. Here, the chip  20  comprises eight microbalances  1  made according to any of  FIGS. 1 ,  7 - 10 . Each microbalance  1  has its own top electrode  15  connected to an contact  107  and overlying a diaphragm  18 , each defined by an underlying cavity  3  or trench  100  or Bragg mirror  121 . The bottom electrodes  10  of the microbalances  1  are connected together through a connecting line  130  connected to contacts  108 . At least one heater  135  is formed in the microbalance area, e.g. in a central portion thereof. The heater  135  is for example formed of an aluminum coil, in the same metal level as the contacts  107 - 108 . Temperature sensors  136  are also formed laterally to the microbalances area. They may also be formed of aluminum, in the same metal level as the heater  135 . Thereby, forming the heater and the temperature sensors  136  does not require additional masks. 
         [0057]    The presence of a heater and a temperature control (including the temperature sensors  136 ) allows a reduction in the cleaning times of the microbalances  1 . In fact, after each olfactory operation, the microbalances  1  are to be brought back to the starting conditions, by cleaning the chemically interactive materials, e.g. the metal-porphyrins. This operation is generally performed by causing dry air to flow on the chemically interactive materials. The presence of heaters allows a heating of the flowing air, helping the “desorption” processes and thus releasing of the substances adsorbed during the previous olfactory operation. To this end, the apparatus may comprise electric controlled valves that allow introduction of a stream of gas or air to be sensed, and then introduction of a cleaning and calibration stream of air, to reset the initial conditions. 
         [0058]    Thereby, the costly Peltier cell can be eliminated, and the apparatus may have much lower cost, smaller dimensions and reduced weight. 
         [0059]    The microbalance described herein has numerous advantages. 
         [0060]    In particular, it has a high sensitivity, thanks to its integration in an integrated device of small dimensions, and to the use of molybdenum in contact with the piezoelectric region  11 . 
         [0061]    The definition of the diaphragm  18  by means of the buried cavity  3  causes the area of the microbalance to correspond substantially to that of the buried cavity. Consequently, the microbalance  1  has small dimensions, which enable integration of a number of microbalances and possibly of the driving and control circuits in a same chip. 
         [0062]    The integration of a heater  135  and any temperature sensor in the chip  20  allows the obtainment of a detecting device of very small dimensions (pocket size), low weight, small power consumption, low cost. Thus, the device may be mounted in portable apparata, such as a cell phone, and may be easily connected to an external unit (e.g., a portable computer) for data processing. 
         [0063]    Finally, it is clear that modifications and variations can be made to the microbalance described and illustrated herein, without thereby departing from the scope of the present invention, as defined in the attached claims. 
         [0064]    For example, part of the electronics for processing the digital electrical signals provided by the converter can be integrated in the same chip as the sensors.