Abstract:
A capacitive-type sensor comprises a glass plate having an electrode formed thereon, and a micromachined structure formed from a semiconductor material and having an insulating rim formed thereon. A conducting seal is formed on the insulating rim and arranged to be bonded to the glass substrate to define an enclosed cavity containing the electrode, to thereby define a capacitive element, the conducting seal being arranged, in use, to have an electrical signal passed there through to determine a capacitance thereof which is indicative of the parameter to be determined by the sensor.

Description:
BACKGROUND 
     Silicon sensors are extensively used in a large and increasingly varied field, including important areas such as medical instrumentation, automotive applications such as engine control and tyre pressure monitoring, industrial process control and the avionics industry. The most commonly used conversion principles for silicon based sensors are capacitive detection and piezoresistive detection. 
     Piezoresistive sensors are generally considered to be more robust than capacitive sensors. Another advantage is that they give an output signal proportional to the input with good linearity. Capacitive sensors, on the other hand, have the advantage over the piezoresistive type in that they consume less power, but have a non-linear direct output signal and are more sensitive to electromagnetic interference. Capacitive silicon sensors can be made to be small in size and can easily be made by surface micromachining. However, they are not very robust and their pressure sensitive diaphragm needs to be protected against the pressure media by a gel or, other flexible material in most applications. This results in an increase in vibration sensitivity due to the mass added to the top of the diaphragm. Advanced and well proven methods of manufacturing silicon pressure sensors and inertial sensors are described in the patent publications EP-A-742581 and EP-A-994330, but these have the problems mentioned above. 
     SUMMARY 
     The present invention seeks to provide a capacitive silicon sensor arrangement that overcome the above mentioned problems. 
     According to the present invention there is provided a capacitive-type sensor comprising: 
     a glass plate having an electrode formed thereon; and 
     a micromachined structure formed from a semiconductor material and having an insulating rim formed thereon; and 
     a conducting seal formed on the insulating rim and arranged to be bonded to the glass substrate to define an enclosed cavity containing the electrode, to thereby define a capacitive element, the conducting seal being arranged, in use, to have an electrical signal passed there through to determine a capacitance thereof which is indicative of the parameter to be determined by the sensor. 
     This invention teaches a capacitive arrangement for the measurement of physical measurands such as pressure, flow and acceleration. The pressure sensor arrangement of this invention has a micromachined silicon diaphragm acting as the movable electrode in the capacitor, an on-chip vacuum reference volume sealed by anodic bonding acting as the gap in the capacitor and with the counter electrode of the capacitor on glass, connected to the outside of the sealed cavity by a conduction system consisting of metal interconnects on the glass, press contacts between the hermetically sealed cavity. The invention results in robust and reliable sensors with good media compatibility, obtained by having the measureand inlet towards the rear side of the silicon diaphragm. The process technology that is used results in low manufacturing cost, as is required for high volume applications such as in the automotive industry. Versions can be made that allow trimming of the capacitance value by forming the electrode system on glass with capacitors connected in parallel. Other versions may include integration of MOS-capacitors. 
     This invention is made possible by using silicon planar processing combined with silicon bulk micromaching processes such as dry etching, anisotropic and selective etching, thin-film metallization of glass and anodic bonding between glass and a thin-film layer. All of these techniques are well known within microsystem technology (MST) and micro-electro-mechanical systems (MEMS). 
     Although silicon is used as the material of choice in the description, the invention is not limited to silicon and can also be made by using other semiconductor materials such as III–V semiconductors such as GaAs or the high-temperature semiconductor SiC. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a good understanding of the invention and its features and advantages, reference is made to detailed descriptions and the drawings, in which: 
         FIG. 1  is a cross sectional view of a first and basic type of a capacitive absolute pressure sensor in accordance with the present invention; 
         FIG. 2  is a top plan view of the pressure sensor of  FIG. 1  through the line A—A; 
         FIG. 3  is a top plane view of a capacitive acceleration sensor in accordance with the present invention; 
         FIG. 4  shows details of one arrangement of the press contacts between metal electrode on glass and the metal of the seal-ring of  FIG. 1 ; 
         FIG. 5  shows details of a second arrangement of the press contacts between the metal on glass and the metal of the seal-ring of  FIG. 1 ; 
         FIG. 6  is a top plan view of a second example of a capacitive absolute pressure sensor that includes a trimmable metal-oxide-semiconductor capacitor in accordance with the present invention; 
         FIG. 7  is a top plane view of a third example of a capacitive pressure sensor with a trimmable electrode pattern in accordance with the present invention; and 
         FIG. 8  shows the separate silicon and glass parts of the capacitive pressure sensor shown in  FIGS. 1 and 2  before laminating the parts together by anodic bonding for forming of the sensor. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIGS. 1 and 2 , a sensor has a silicon part  100 , formed on a substrate with a rigid support rim  102  and a flexible microstructure which forms a thin flexible diaphragm  104 . The substrate is heavily doped in order to obtain low series resistance. A shallow recess is etched in the diaphragm  104 , and the support rim  102  has an electrically insulating layer  107  on the surface thereof. A conductive thin film layer  108  is formed on the electrically insulating layer  107  as a ring which surrounds the diaphragm  104 . 
     A first glass part  120  has a thin-film surface conduction system, formed with metal interconnects, on its surface, which constitutes a plate electrode  121  facing the silicon diaphragm  104 . The glass  120  is anodically bonded to the conductive thin film layer on the silicon part  100 , thereby forming a complete seal ring  108  at the interface  122 . 
     As shown in FIGS.  1  and  2 ,.the plate electrode  121  makes contact with the electrically conductive thin film seal ring  108  via electrical press contacts  109   a  and  109   b . In this way, the etched recess of the diaphragm  104  provides a sealed cavity  123  within the device. 
     The substrate  100  and the glass part  120  form a capacitive sensing device with the plate electrode  121 , on the glass  120 , acting as the first electrode. This electrode is electrically connected to a wire bonding pad  111   a , that is outside of the sealed cavity  123 , via the press contacts  109   a ,  109   b  formed between the electrode  121  and the metal seal ring  108 . The sealed cavity  123  acts as the electrical isolation gap in the capacitor. The flexible diaphragm  104  is the second electrode of the variable capacitor, and is electrically connected via the support rim  102  to an electrical contact pad  111   b  outside the sealed cavity. 
     Sensing function is provided by a change in capacitance when a force acts on the flexible structure of the diaphragm  104 , thereby pressing the diaphragm  104  in the direction towards the plate electrode  121  on the glass, giving a smaller gap  123  in the capacitor. 
     Preferably, insulating layer  107  is formed on a dielectric material and the seal-ring  108  also acts as a conductor to provide electrical contact between the plate electrode  121  on the glass  120  and the wire bonding pad  111   a  situated outside the cavity. 
     As described above, the basic sensing device shown in  FIGS. 1 and 2  comprises a flexible microstructure which can take the form of a thin diaphragm  104 . In this case, pressure acting on the diaphragm  104  presses the diaphragm  104  towards the plate electrode  121 , in the form of the thin-film conduction system. The gap  112  between the electrodes therefore decreases. The sensing device can therefore be used as a pressure sensor to detect changes in air or liquid pressures. 
     The sensor of  FIGS. 1 and 2  also includes a resilient centre boss  106 , and an electrically insulating mechanical overload protection component  110 . This component ensures that, under the influence of an extreme change in the parameter being sensed, the component  110  makes contact with the electrode  121  on the glass  120 , thereby ensuring the continued electrical isolation of the two electrodes. 
       FIG. 3  shows another example of the sensing device of the present invention, in the form of a capacitive accelerometer. The accelerometer  40  comprises a silicon part  400  built on a substrate  430 . The silicon part  400  has a rigid support rim  402  joined by a thin, flexible spring to seismic mass  406 , such that the mass  406  is supported at the end of the spring. The spring and mass constitute a thin, flexible diaphragm  404  which is electrically connected to an electrical contact pad (not shown) through the support rim  402 . 
     The silicon support rim  402  has an electrically insulating ring-shaped layer  407 , the surface of which has an electrically conductive, thin-film layer  408 . This thin-film layer acts an electrically conductive seal ring  408  which surrounds the diaphragm  404 , and is similar to the seal ring  108  of the pressure sensor of  FIG. 1 . 
     A glass part  420  of the capacitor accelerometer  40  has a thin-film conduction system, formed with metal interconnects, on its surface, which faces the silicon part  400 . The conduction system acts as a plate electrode in use. 
     The glass part  420  is anodically bonded to the silicon part  400  in order to form a complete seal at the interface  422  between the two parts. 
     As can be seen from  FIG. 3 , the seal-ring  408  has press contact areas  409 A,  409 B formed between the electrode  421  and the seal ring  408 , and is connected to the wire bonding area  411  that is situated outside the capacitive accelerometer, in order to provide an electrical connection out of and/or into the sensing device. Press contacts  409 A and  409 B form an effective electrical connection between the plate electrode  421  and the thin-film seal ring  408 . 
     A shallow recess is etched in the thin diaphragm  404  such that, when the glass and silicon parts are connected as described above, a vacuum reference volume, exists between the plate electrode  421  and diaphragm  404 . A sealed cavity  423  is therefore provided between the glass  420  and the silicon  400  parts of the sensing device. This seal cavity  423  acts as an electrical isolation gap of the capacitive accelerometer. 
     In use, the plate electrode  421  and seismic mass  406  of the silicon diaphragm  404  act as the first and second electrodes, respectively, of the capacitive accelerometer  40 . 
     The accelerometer  40  functions in a similar way to the capacitive pressure sensor  10  described above. An acceleration acting on the mass  406  forces the diaphragm  404  to deflect in a direction towards or away from the plate electrode  421 , thereby altering the size of the gap  412  between the electrodes, and hence the capacitance value measured. In order to maintain electrical isolation between the electrodes in the case where a relatively large acceleration is sensed, an electrically insulating mechanical overload protection component  410  is provided on the diaphragm  404 . 
     Referring to  FIGS. 4 and 5 , these show alternative configurations for forming the press contacts  109  of the sensing device.  FIG. 4  shows details of a press contact  109  as a individual component which links the metal electrode  121  of the glass  120  to the metal of the seal ring  108 . In  FIG. 5 , the metal of the electrode  121  and that of the seal ring  108 , contact one another directly. These press contact arrangements use the seal-ring  122  to incorporate the sealing function and electrical contact function into a single component, thereby allowing for a simplified device, with fewer parts. This incorporation also makes the device easier to manufacture. 
       FIG. 6  shows a further example of the invention in which a sensing device  20  is connected in series to one or more metal-oxide-semiconductor capacitors (MOS-capacitors)  212   a–d  via the substrate of the sensor. In the case of a plurality of MOS-capacitors, these are connected in parallel with one another. The MOS-capacitors should initially have a capacitance value higher than that of the sensor, and have interconnect  213   a–d  designed with areas which can be removed using a laser beam. By “trimming” the MOS-capacitor in this way, its capacitance value is reduced, and hence the overall capacitance value of the system, is increased. 
     By increasing the overall capacitance in this way the ratio at which the capacitor changes as a function of the detected parameter, the sensitivity of the device, can be set at a constant defined value that is not dependent on processing tolerances. 
     A further way of “trimming” the overall capacitance value is presented in the example of a sensor  30  shown in  FIG. 7 . Here, the electrode pattern on the glass part  320  of the sensor  30  comprises at least two electrodes. The electrodes  321   a–h  are connected in parallel within the cavity by a metal interconnect system on the glass part, ( 322   a–h ) and the silicon part, ( 323   a–d ). The metal line connecting the top capacitance electrode can be cut through the glass using a laser beam, and the overall capacitance value of the sensor can be reduced. 
       FIG. 8  illustrates a sequence of the manufacturing process of a device according to the present invention (as illustrated in  FIGS. 1 and 2 ). The silicon substrate should be heavily doped via a standard lithographic method, ion implantation and/or high temperature diffusion of dopants. The silicon part  100  of the sensor, including the rigid support rim  102 , can be manufactured by standard methods well-known within ths field of technology, such as silicon planar processors, double side photolithography, and wet and dry etching techniques. The recess is wet and/or dry etched on the silicon in two etching steps. The first of which creates press contacts  109   a  and  109   b , and the second of which creates the recess to provide a gap  112  in the fully manufactured sensor. The electrically insulating layer  107  may be thermally grown or vapour deposited on the surface of the rigid support rim  102 , and the seal ring  108 , in the form of a conductive thin-film layer, may then be formed on top of this. 
     The metal plate electrode  121  on the glass part  120  can be made by standard methods for the fabrication of thin-film structures on glass. The gap  112  and position of the press contacts  109   a  and  109   b  can also be created by etching the recess in the glass  120 , or by performing a combination of etching steps in both the glass parts  120  and the silicon part  100 . 
     Manufacture of this sensing device is completed by anodically bonding, in a vacuum, a glass substrate  120  with metal electrode  121  and thin film interconnects already formed thereon, to the silicon substrate  100 , resulting in a structure as shown in  FIGS. 1 and 2 , with the (anodic bonded) seal-ring  108  and the sealed cavity  123  formed by the recesses etched in the surface of the silicon substrate.