Abstract:
A microsystem, e.g., a micromechanical sensor, has a first cavity which is sealed off from the surroundings and a second cavity which is sealed off from the surroundings. The first cavity is bounded by a first bond joint and the second cavity is bounded by a second bond joint. Either the first bond joint or the second bond joint is a eutectic bond joint or a diffusion-soldered joint.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a microsystem, e.g., micromechanical sensor structure, and to a method for the production of such a microsystem. 
         [0003]    2. Description of the Related Art 
         [0004]    Micromechanical sensor structures are known from the related art. Micromechanical rotation rate sensors for determining rates of rotation about one or more axes are used, for example, in the automotive sector and in entertainment electronics for navigation, image stabilization and detection of movement. Such rotation rate sensors have a movable micromechanical element enclosed in a cavity. To obtain low damping and thus a high mechanical Q factor, the micromechanical element is typically enclosed at a very low gas pressure, for example at from 1 to 5 mbar. Micromechanical acceleration sensors serve to determine accelerations in one or more directions in space and are used, for example, for electronic stabilization programs, for airbag release and for attitude detection. Such acceleration sensors also have a movable micromechanical element enclosed in a cavity. To obtain a critical damping and thus a rapid settling of the movable element, the micromechanical element is typically enclosed in a cavity with a relatively high gas pressure, for example of around 800 mbar. It is also known for a plurality of rotation rate sensors and acceleration sensors to be combined in an inertial navigation system which makes it possible to track position and orientation by time integration of the individual signals. 
         [0005]    The enclosure of the micromechanical elements in the cavities is done by wafer-level encapsulation, for example by seal glass bonding or eutectic bonding. In that procedure, the pressure used during bonding is enclosed in the interior of the cavity. If a plurality of chips is implemented on one chip, all of the encasing cavities have the same internal pressure. When seal glass bonding is employed, the bond pressure is increased in addition by solvent evaporation from the seal glass. 
       BRIEF SUMMARY OF THE INVENTION 
       [0006]    An object of the present invention is to provide a microsystem having a first cavity and a second cavity which are sealed by different bond joints. It is a further object of the invention to provide a method for the production of such a microsystem. 
         [0007]    A microsystem according to the present invention has a first cavity which is sealed off from the surroundings and a second cavity which is sealed off from the surroundings. The first cavity is bounded by a first bond joint and the second cavity is bounded by a second bond joint, wherein either the first bond joint or the second bond joint is a eutectic bond joint or a diffusion-soldered joint. Advantageously, different internal pressures may be obtained in the cavities of that microsystem. That allows different micromechanical sensors to be integrated on one chip. In that manner it is possible to obtain more highly integrated micromechanical sensor systems, which are cheaper and take up less space. 
         [0008]    Preferably, in the first cavity there is a first pressure and in the second cavity there is a second pressure, wherein the first pressure and the second pressure are of different magnitudes. Advantageously, it is possible to arrange different sensors in the cavities, with the optimum internal pressure for operation of the respective sensor being provided for in each cavity. 
         [0009]    In one embodiment, the first bond joint has aluminum and gold, and the second bond joint has aluminum and silicon. Advantageously, the first bond joint may then be closed by thermocompression bonding at a low temperature and the second bond joint may be closed by eutectic bonding at a higher temperature. In an alternative embodiment, the first bond joint has copper and the second bond joint has copper and tin. Those two bond joints also may advantageously be closed at differing temperatures and pressures. 
         [0010]    Preferably, a first sensor structure is disposed in the first cavity and a second sensor structure is disposed in the second cavity. In that manner, it is advantageously possible to obtain highly integrated sensor components having a number of functions. 
         [0011]    In a method according to the present invention for the production of a microsystem, a substrate and a capping wafer are joined to each other by bonding. In a first method step, the substrate and the capping wafer are joined to each other in a first region by a first bonding process at a first temperature and at a first ambient pressure, and in a further method step are joined to each other in a second region by a second bonding process at a second temperature and at a second ambient pressure, wherein either the first bonding process or the second bonding process is a eutectic bonding process or a diffusion-soldering process. That method advantageously allows the first ambient pressure and the second ambient pressure to be selected such that they differ. 
         [0012]    In an example implementation of the method, method steps are additionally carried out beforehand to provide the substrate with a first surface, on which a first bonding frame and a second bonding frame are disposed, to provide the capping wafer with a second surface, on which a first further bonding frame and a second further bonding frame are disposed, wherein the second bonding frame and/or the second further bonding frame are/is interrupted by at least one opening, and for arranging the substrate and the capping wafer in such a manner that the first surface faces the second surface and the first bonding frame comes into contact with the first further bonding frame and the second bonding frame comes into contact with the second further bonding frame. The at least one opening in the second bonding frame or in the second further bonding frame advantageously allows the pressure in the second region to be adapted to the second ambient pressure before the capping wafer and the substrate are joined to each other in the second region by the eutectic bonding process or the diffusion-soldering process. 
         [0013]    The second bonding frame and/or the second further bonding frame advantageously melt(s) briefly during the eutectic bonding or the diffusion-soldering, thereby closing the at least one opening in the second bonding frame and/or in the second further bonding frame. Advantageously, the second ambient pressure is then enclosed in the region that has been sealed off by the eutectic bonding process. 
         [0014]    The first temperature is advantageously lower than the second temperature. That advantageously ensures that the bond joints may be produced one after the other. 
         [0015]    Preferably, the first bonding frame has aluminum, the second bonding frame silicon, and the first and second further bonding frames have gold. In that case, the first temperature is above 300° C. and below 363° C. and the second temperature is 363° C. or above. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1  is a plan view of a substrate. 
           [0017]      FIG. 2  is a plan view of a capping wafer. 
           [0018]      FIG. 3  shows a section through a microsystem at a first processing stage. 
           [0019]      FIG. 4  shows a section through the microsystem at a second processing stage. 
           [0020]      FIG. 5  shows a section through the microsystem at a third processing stage. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0021]      FIG. 1  is a plan view of a substrate  110  shown schematically. Substrate  110  may, for example, be a silicon substrate. Substrate  110  may be a complete wafer or part of a wafer. Disposed on substrate  110  are a first sensor structure  200  and a second sensor, structure  300 . Sensor structures  200 ,  300  may be micromechanical sensor structures, for example rotation rate sensors or acceleration sensors. For example, first sensor structure  200  may be a rotation rate sensor and second sensor structure  300  may be an acceleration sensor. 
         [0022]    First sensor structure  200  is bounded by a first lower bonding frame  220  disposed on the surface of substrate  110 . In the example of  FIG. 1 , first lower bonding frame  220  has the shape of an approximately square frame. First lower bonding frame  220  may, however, also have the shape of a circular ring or any other closed shape surrounding first sensor structure  200 . First lower bonding frame  220  may, for example, consist of aluminum deposited on the surface of substrate  110 . The height of first lower bonding frame  220  perpendicular to the surface of substrate  110  may in that case be, for example, from ½ μm to 10 μm. The width of first lower bonding frame  220  parallel to the surface of substrate  110  may, for example, be from 10 μm to 500 μm, preferably 100 μm in size. 
         [0023]    Second sensor structure  300  is bounded all round by a second lower bonding frame  320 . Second lower bonding frame  320  has four openings  325  at which second lower bonding frame  320  is interrupted. It is also possible for fewer or more than four openings  325  to be provided, but at least one opening  325  is provided. In the example illustrated, second lower bonding frame  320  also has the shape of an approximately square frame. As with the shape of first lower bonding frame  220 , however, a different shape may be selected for second lower bonding frame  320 . The dimensions of second lower bonding frame  320  approximately correspond to those of first lower bonding frame  220 . Second lower bonding frame  320  may, for example, consist of silicon. 
         [0024]      FIG. 2  is a plan view of a capping wafer  120  shown schematically. Capping wafer  120  serves to encapsulate sensor structures  200 ,  300  disposed on substrate  110 . Capping wafer  120  may consist, for example, of silicon or glass. Capping wafer  120  may be a complete wafer or part of a wafer. Disposed on the surface of capping wafer  120  are a first upper bonding frame  230  and a second upper bonding frame  330 . The position and size of first upper bonding frame  230  and second upper bonding frame  330  are so selected that first upper bonding frame  230  may be brought into registration with first lower bonding frame  220  on substrate  110  and second upper bonding frame  330  may be brought into registration with second lower bonding frame  320  on substrate  110 . First upper bonding frame  230  and second upper bonding frame  330  may, for example, consist of gold deposited on capping wafer  120 . Like second lower bonding frame  320 , second upper bonding frame  330  may have openings at which second upper bonding frame  330  is interrupted. If second upper bonding frame  330  has such openings, openings  325  in second lower bonding frame  320  may optionally be omitted. 
         [0025]    Substrate  110  and capping wafer  120  may be joined to each other to enclose or encapsulate first sensor structure  200  and second sensor structure  300 . For this, first lower bonding frame  220  has to be joined to first upper bonding frame  230  and second lower bonding frame  320  has to be joined to second upper bonding frame  330 . In so doing, first sensor structure  200  is enclosed in a first cavity  210  and second sensor structure  300  is enclosed in a second cavity  310 . If first sensor structure  200  and second sensor structure  300  make different demands on the ambient pressure, different internal pressures have to be enclosed in first cavity  210  and in second cavity  310 .  FIGS. 3 to 5  show different processing steps for the production of such a joint between substrate  110  and capping wafer  120 . 
         [0026]      FIG. 3  shows in a sectional view that the surface of substrate  110  exhibiting lower bonding frames  220 ,  320  and the surface of capping wafer  120  exhibiting upper bonding frames  230 ,  330  are first arranged facing each other in such a manner that first lower bonding frame  220  is opposite first upper bonding frame  230  and second lower bonding frame  320  is opposite second upper bonding frame  330 . Substrate  110  and capping wafer  120  are then brought into contact with each other at a first pressure in a bonding system. Depending on the pressure obtainable in the bonding system, first pressure  215  may, for example, be from 10 −3  mbar to over 1000 mbar. 
         [0027]    There then follows a first phase of the bonding process, which is illustrated schematically in  FIG. 4 . The ambient temperature in the bonding system is increased to a first temperature, at which first lower bonding frame  220  and first upper bonding frame  230  are joined to each other by thermocompression bonding. The first temperature is typically above 300° C. and less than 363° C. Preferably, the first temperature is approximately 350° C. While first lower bonding frame  220  and first upper bonding frame  230  are being joined to each other by thermocompression bonding, a first bond joint  240  is produced which encloses a first cavity  210  surrounding first sensor structure  200 . In that operation, first pressure  215  is enclosed in first cavity  210 . First cavity  210  is enclosed, therefore, by substrate  110 , capping wafer  120  and first bond joint  240  and is so leak-tight that first pressure  215  in first cavity  210  is maintained. 
         [0028]    Second lower bonding frame  320  and second upper bonding frame  330  are also in contact with each other, but have not yet been joined to each other. By virtue of openings  325  in second lower bonding frame  320  and/or in second upper bonding frame  330 , it is possible for pressure equalization to take place between the region surrounding second sensor structure  300  and the ambient environment of substrate  110  and capping wafer  120 . Thereafter, the ambient pressure in the bonding system is changed to a second pressure  315 . Second pressure  315  may be higher or lower than first pressure  215  and similarly, depending on the capabilities of the bonding system, may be from 10 −3  mbar to over 1000 mbar. Via openings  325 , second pressure  315  is also established in the ambient environment of second sensor structure  300 . The temperature in the bonding system is then increased to a second temperature, which is preferably equal to or greater than 363° C. First bond joint  240  is not adversely affected by the increase in temperature to produce second bond joint  340 . At a temperature of 363° C., a eutectic bonding process occurs between second lower bonding frame  320  and second upper bonding frame  330 . In that process, the silicon of second lower bonding frame  320  and the gold of second upper bonding frame  330  are joined to each other and briefly melt, thereby producing second bond joint  340 . During the melting process, openings  325  in second lower bonding frame  320  and/or in second upper bonding frame  330  are closed. That produces in the area surrounding second sensor structure  300  a second cavity  310  which is bounded by substrate  110 , capping wafer  120  and second bond joint  340  and in which second pressure  315  is enclosed. Second cavity  310  also is so leak-tight that second pressure  315  is retained.  FIG. 5  shows the completed microsystem  100  in schematic section. 
         [0029]    Instead of using the above-mentioned material systems for lower bonding frames  220 ,  320  and upper bonding frames  230 ,  330 , other bonding materials may also be used. It is important merely that first bond joint  240  is produced at a lower temperature than second bond joint  340  and that second bond joint  340  is a eutectic bond or another bond during the production of which a brief liquefaction of the bonding materials occurs which results in openings  325  being fused closed. As an alternative material system, first lower bonding frame  220  and first upper bonding frame  230  may, for example, both have copper, and second lower bonding frame  320  and second upper bonding frame  330  may have copper and tin. In that case, the second bond joint is formed from tin and copper by what is called a solid-liquid interdiffusion (SLID) bonding process. That process is also referred to as diffusion soldering. In that case also, a brief liquefaction of the bonding materials occurs.