Abstract:
A sensor assembly is formed by attaching a micromechanical semiconductor sensor in a housing. The micro-mechanical sensor is secured to the housing by a gel, which leads to a particularly favorable isolation between the micromechanical sensor and the housing.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention lies in the mechanical arts. More specifically, the invention relates to a method for attaching a micromechanical semiconductor sensor in a housing and a sensor assembly having a micromechanical sensor arranged in a cavity in a housing. 
     2. Description of the Related Art 
     European patent EP 0 548 470 B1 discloses a sensor assembly with a micromechanical pressure sensor. The pressure sensor is protected by a corrosion-proof cover formed of several layers. A silicone gel serves as the first layer of the cover and as a pressure transmitting medium. 
     U.S. Pat. No. 4,866,989 discloses a pressure transducer which has a chip on the underside of a wall of a sensor attachment. The chip is attached to the wall with a sealing ring made of silicone. The chip and the sealing ring are covered by a silicone gel. The silicone gel protects the chip against a fluid whose pressure is being measured. 
     German published patent application DE 137 03 206 A1 relates to a pressure sensor component with a hose attachment. A semiconductor chip is bonded to a chip carrier of a base body. The chip carrier is filled with silicone gel in order to cover the semiconductor chip. At the same time, by filling in the chip carrier, the base body is connected to a connecting element and sealed. 
     U.S. Pat. No. 4,655,088 discloses a pressure transducer with a one piece housing and a semiconductor chip which is secured in a defined position by means of a bonding material. The semiconductor chip is additionally protected against environmental influences by a protective material. 
     European patent EP 0 400 074 B1 discloses a method for fabricating a protected pressure sensor assembly. An interior space around a pressure sensor element is at least partially filled with a pressure transfer material and is closed off by a flexible, pourable diaphragm material. 
     European patent EP 0 568 781 B1 discloses a pressure sensor assembly with a receptacle which comprises a first recess and a second recess. A semiconductor chip is arranged on a glass substrate in the first recess cavity. Both the first recess and the second recess are provided with gelatinous material. The second recess serves as a working area from which excess gelatinous material can be removed from the first recess. 
     Micromechanical semiconductor sensors react sensitively to mechanical or thermal stresses in the silicon chip. In the prior art micromechanical sensors, the unhoused sensor chip is bonded to a surface. As a result, interactions with the underlying surface occur. The underlying surface expands differently than the chip, the bonding expands as a result of moisture, the mechanical clamping of the entire housing exerts stresses on the chip and the bonding agent responds to temperature changes by plastic deformation. In addition the plastic housing shrinks after the injection. 
     In order to reduce the interactions between the sensor chip and its underlying surface, the sensor chip is frequently glued, anodically bonded, or soldered with glass solder onto a ceramic carrier, another silicon component or Pyrex glass. Such a carrier is then bonded into the plastic housing so that two connection points, which are relevant for quality, are produced. Both connection points have effects on the characteristic curve of the sensor. Even minute air bubbles in the bonding agent lead to quality problems. 
     SUMMARY OF THE INVENTION 
     It is accordingly an object of the invention to provide a method of attaching a micromechanical semiconductor sensor in a housing and a sensor assembly, which overcome the above-mentioned disadvantages of the heretofore-known devices and methods of this general type, and which permit particularly good isolation of a semiconductor sensor from its underlying surface. 
     With the foregoing and other objects in view there is provided, in accordance with the invention, a method of mounting a micromechanical semiconductor sensor in a housing, which comprises the following steps: 
     positioning a micromechanical sensor on a supporting surface in a recess of a housing; 
     electrically connecting the micromechanical sensor with an electrical contact on the housing and securing the micromechanical sensor to the housing during the connecting step by generating a partial vacuum and holding the micromechanical sensor with the partial vacuum via a duct in the housing; and 
     filling gel into the recess for attaching the micromechanical sensor to the housing, so that the micromechanical sensor is covered and held by gel. 
     In other words, while the electrical connection is produced, the semiconductor sensor is secured by generating a partial vacuum or a flow of air on its supporting surface. 
     Dispensing with an essentially rigid connection between the semiconductor sensor and its supporting surface brings about an isolation of the sensor from the housing. The semiconductor sensor is essentially secured by means of a gel which simultaneously protects the sensor against environmental influences. A fluorized silicone gel, which has very stable behavior with respect to chemical influences, is particularly suitable for this. The viscous or soft, elastic properties of the fluorized silicone gel prevent any interaction between the semiconductor sensor and the housing and thus prevent the sensor characteristic curve from being influenced. The electrical connection between the semiconductor sensor and an electrical contact on the housing can serve as an additional securing means. 
     It is possible to dispense with the attachment of a carrier to the semiconductor sensor and with bonding in the housing. As a result the fabrication of a sensor assembly according to the invention is made simpler and cheaper. A special diaphragm for protecting the semiconductor sensor is not necessary either. 
     In accordance with an added feature of the invention, gel is aspirated via the duct. 
     In accordance with an additional feature of the invention, a layer of gel is formed between the micromechanical sensor and the supporting surface by aspirating the gel via the duct. 
     In accordance with another feature of the invention, a suction device is applied to a side of the housing in which the recess is formed. 
     In accordance with a further feature of the invention, the duct is a groove in the housing and the duct is covered with a suction device during the aspirating process. 
     In accordance with another added feature of the invention, a sealing lip of a suction device is placed in an area of movement of a bonding device bonding the electrical connections to the micromechanical sensor, and wherein the sealing lip is at least partially countersunk in the housing. 
     With the above and other objects in view there is also provided, in accordance with the invention, a sensor assembly, comprising: 
     a housing having a recess formed therein with a supporting surface; 
     a micromechanical sensor disposed on the supporting surface; 
     a gel covering the sensor and exclusively securing the sensor; and 
     the housing having a duct formed therein starting at a side of the recess and extending into the supporting surface such that the micromechanical sensor at least partially covers the duct. 
     In accordance with another added feature of the invention, the duct is a groove formed in the housing. In a further preferred embodiment, the duct has an opening formed on the same side of the housing as the recess. 
     In accordance with a concomitant feature of the invention, the recess is formed with lateral walls defining a spacing gap of from 0.05 mm to 1 mm between the micromechanical sensor and the lateral walls. 
     In the preferred embodiment of the invention, therefore, the sensor assembly not only has the recess but also has a duct which extends to the supporting face of the micromechanical sensor, so that an ambient pressure acts on both planar sides of the micromechanical sensor. As a result, even at a low ambient pressure a pressure differential is prevented from lifting the micromechanical sensor from its underlying surface, in which it is secured solely by the gel or at least essentially by the gel. Damage to the electrical connections to the micromechanical sensor is thus prevented. 
     Suitable fields of application of the invention are, for example, acceleration sensors, rotational speed sensors and chemical sensors for determining the composition of mixtures of gases. A particularly preferred field of application of the invention is the field micromechanical pressure sensors such as are used, in particular, in automotive engineering. Such pressure sensors are produced, for example, in surface micromechanics or bulk micromechanics. 
     Other features which are considered as characteristic for the invention are set forth in the appended claims. 
     Although the invention is illustrated and described herein as embodied in a method for attaching a micromechanical sensor in a housing and sensor assembly, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. 
     The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a section taken through a sensor assembly; 
     FIG. 2 is a section taken through a housing onto which a suction device is fitted; and 
     FIG. 3 is a plan view of the configuration of FIG.  2 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the figures of the drawing in detail and first, particularly, to FIG. 1 thereof, there is seen a sensor assembly  1  for measuring an absolute pressure with a housing  11  and with a recess cavity  111  formed in the housing  11 . Arranged in the recess  111  is a supporting surface  112  for a micromechanical semiconductor pressure sensor  12  and a surrounding spacing gap  113  between the wall which bounds the recess  111  and the micromechanical sensor  12 . The recess is essentially filled completely by a gelatinous compound or gel  13  and, more precisely, by fluorized silicone gel. 
     The micromechanical sensor  12  is a silicon disk which has been treated with surface micromechanics. It is an unhoused chip. In the micromechanical sensor  12  there is a pressure chamber in which a reference pressure prevails. In addition, an evaluation circuit, which evaluates the effect of an environmental variable, that is to say in this example a deflection of the surface of the micromechanical sensor caused by pressure, is integrated in the micromechanical sensor. 
     The result of the evaluation is transmitted to an electrical contact  15  via an electrical connection  14 , a bonding wire. The electrical contact  15  is connected to a plug-type connector via which the measurement results are transferred to a control unit of a motor vehicle, for example. The bonding wire does not assume any securing functions for the micromechanical sensor  12  whatsoever. The sensor  12  is secured in position solely by the gel  13 , so that no forces act on the electrical connection  14 . 
     The spacing gap  113  between the wall of the recess and the micromechanical sensor  12  is dimensioned in such a way that when the gel  13  is filled into the recess  111 , the gel  13  can overcome the capillary forces in the gap  113 . The gap must not be too narrow, so that no undesired interaction can occur between the micromechanical sensor  12  and the housing  11 . However, the gap must not be too wide since otherwise the gel  13  would no longer be capable of sufficiently securing the micromechanical sensor  12 . Vibration of the micromechanical sensor could lead to damage to the electrical connection  14 , in particular to rupturing of bonding wires. 
     A suitable width of the gap  113  would be approximately between 0.05 millimeters and 1 millimeter. For a micromechanical sensor with a diameter of approximately 5 millimeters, a gap width of 0.1 millimeter to 0.3 millimeters is particularly advantageous. 
     The supporting surface  112  can be roughened or provided with a thin vulcanization in order to prevent the micromechanical sensor from slipping when one or more electrical connections  14  are attached. 
     In order to make it easier to center the micromechanical switch  12  in the recess  113  when it is being inserted, the surrounding gap  113  can be interrupted by very narrow projections which point “radially” in the direction of the inserted micromechanical sensor  12 . In order to be able to absorb particularly powerful impacts, the micromechanical sensor  12  may be additionally snapped into the recess  111 . However, such measures are not necessarily for customary vibration stresses which occur in motor vehicles. 
     The supporting surface  112  is interrupted in a central region by a duct  114 . The duct  114  extends from the micromechanical sensor  12  to an opening  115  which is located on the same side of the housing  114  as the recess  111 . 
     Although the housing  11  is simpler to produce if the duct  114  extends from the recess  111  to that surface of the housing  11  which lies opposite the recess, it is more favorable for the fabrication of the sensor assembly if the opening  115  of the duct  114  is arranged on the side of the housing  11  from which the micromechanical sensor  12  can be inserted into the housing. In the latter case, all the production processes during the fabrication of the sensor assembly  1  can be carried out from a single side. 
     In order to fabricate the duct, an encapsulation preform is fabricated in that plug-contacts which are intended to connect the sensor assembly  1  to an external connection are encapsulated with plastic together with a tubelet, which is later intended to be arranged parallel to the supporting surface  112  in the housing  11 . Then, the encapsulation preform is inserted into an injection molding tool in order to fabricate the housing  11  in plastic. In the process, in each case the recess ill and the opening  115  are fabricated by means of two slide bars. The two slide bars close the tubelet during the injection molding process so that, after the end of the production process, a continuous duct has been produced from the supporting surface  112  as far as the opening  115 . 
     The duct  114  ensures that the micromechanical sensor  12  is subjected to essentially the same pressure conditions at its two planar sides. The securing forces which are to be applied by the gel  13  therefore only need to be relatively small. 
     While the electrical connections  14 , usually bonded connections, are being fabricated, the micromechanical sensor  12  can be secured over the duct  114  by producing a partial vacuum or a vacuum. The micromechanical sensor  12  is prevented from slipping during the suction process by the vulcanization, already described, or by a supporting surface  112  with a high coefficient of friction. Alternatively, the supporting surface  112  may have a very narrow surrounding web on which the micromechanical sensor  12  rests and which surrounds the opening of the duct  114  which opens into the supporting surface  112 . During the suction process, the sensor  12  is pushed against the web so that the latter deforms. As a result, optimum sealing of the sensor  12  with respect to the duct  14  is achieved and the sensor is reliably prevented from moving laterally during the production process. 
     Therefore, the duct  114  performs both the function of a pressure equalization duct and the function of a suction duct. After the electrical connections  14  have been fabricated, the recess  111  is filled with gel  13  which finally fixes the micromechanical sensor  12  in position and protects it against environmental influences. The suction process for securing the micromechanical sensor  12  can be continued in order to fill in the gel  13  so that penetration into the gap  113  is supported. In addition, gel can be pulled between the supporting surface  112  and the micromechanical sensor  12  by the suction. In this case, the micromechanical sensor  12  is completely isolated from the housing  11 . In order to support the sucking in of the gel, grooves, which extend from the gap  113  to the duct  114 , may be provided in the supporting surface  112 . 
     FIG. 2 illustrates the fabrication of a sensor assembly according to a further exemplary embodiment of the invention. 
     The duct  114  is embodied as a groove which extends from outside the recess  111  into it and deepens it. The duct  114  therefore starts to the side of the recess  111 . The duct  114  is accessible from the surface of the housing  11 . It extends at least as far as a central region of the supporting surface  112 . 
     A suction device  2  is formed with sealing lips  21  for covering and sealing the opening  115  of the duct  114 . A sealing lip  21  covers that region of the duct  114  which is located outside the recess  111 . The sealing lip  21  can project into the recess  111  and, in doing so, cover the gap  113 . When the suction device is actuated, the micromechanical sensor is pulled against the edge of the sealing lip  21  and thus aligned in the optimum position. If the suction device is placed against the housing  11  before the insertion of the micromechanical sensor  12 , it can support the centering of the micromechanical sensor  12  in the recess  111 . 
     The part of the sealing lip  21  which faces the micromechanical sensor  12  is completely countersunk in the duct  114 . Therefore, electrical connections  14  between the micromechanical sensor  12  and electrical contacts  14  can be bonded without difficulty although the sealing lip  21  lies in a bonding shadow, that is to say a movement region of a bonding device  3 . 
     FIG. 3 illustrates the covering of the duct  114  by the sealing lip  21  of the suction device  2 . The broken line signifies the course of the duct in the supporting surface  112  for the micromechanical sensor  12 .