Abstract:
The present invention relates to a method of bonding a first member ( 110, 210, 130, 230, 410, 430, 510, 530, 610 ) to a second silicon member ( 120, 220, 420   a,    420   b,    600 ) through anodic bonding. The method comprises the steps of selectively depositing on said first member bondable sections ( 170   a,    170   b,    270, 470   a,    470   b,    470   c,    570, 620 ) before bringing said first and second members together for anodic bonding.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention relates to a method of bonding at least a first member to a second silicon member through anodic bonding. 
     BACKGROUND OF THE INVENTION 
     EP 742 581, for example, relates to a method of making sealed cavities on silicon wafer surfaces by anodic bonding and with electrically insulated conductors through the sealing areas to connect functional devices inside the cavities to electrical terminals outside said cavities. The conductors are provided through the use of doped buried crossings in a single crystal silicon substrate, thereby also allowing production of different kinds of integrated silicon devices, e.g. sensors. 
     The technique implies that Borosilicate glass plates are bonded onto a silicon (Si) wafer using so-called anodised bonding. A plate of glass is arranged on a Si wafer under an amount of pressure. The Si wafer and the glass are then heated up to some hundred degrees and a voltage is applied across the plates (glass and wafer) whereby the glass, which contains sodium (Na) ions migrate into the Si wafer, and a hermetic junction is obtained. 
     When two Si wafers are to be connected, a similar method as mentioned above is carried out, however, one of the wafers is coated with sputtered borosilicate glass and an anodised bonding is performed. 
     Generally, to be able to achieve an anodised bonding the object intended to be bonded to Si must contain Na-ions, usually through doping a glass with Soda lime glass. The reason for using borosilicate glass is that it matches the Si wafer characteristics, specially with respect to coefficient of expansion. 
     One major problem related to above mentioned and similar methods is the possibility of providing a conducting arrangement through the glass or wafer. In above mentioned European Patent No. 742 581, for example, the electrical connection path is from one of an outside wire bonding area via a first contact diffusion down to a buried conductor which crosses below the sealing area of the cavity, and via a second contact diffusion to a second aluminium interconnection line which establishes connections to two piezo-resistors. 
     SUMMARY OF THE INVENTION 
     The main object of the present invention is to provide a method of providing a hermetical sealing between a first substrate and a silicon substrate. 
     Another object of the present invention is to achieve a hermetical sealing through selective deposition of bondable surfaces on a first substrate and the silicon substrate. Yet another object of the invention is to provide electrical connection between the sealed space and the outside environment through or under the sealing section. 
     For these reasons, the initially mentioned method comprises the steps of selectively depositing on said first member at least one bondable section before bringing said first and second members together for anodic bonding. 
     Preferably, the first member is a glass wafer, specially a borosilicate glass wafer and said second wafer is a silicon wafer or the first member is a carrier wafer specially one of glass, ceramics or glass composite, such as LTCC (Low Temperature Cofired Ceramic) and said second wafer is a silicon wafer. The bondable section comprises a paste containing Na ions. 
     Preferably, the selective deposition is provided through screen printing or photo image forming. 
     Most preferably, depending on the function of the circuitry the bonding is hermetical. 
     Preferably, said first member comprises of a cover, that said second silicon member is a carrier for a functional device and said first member bonded to said second member provides a sealing for said functional device. Moreover, a third member is arranged as a carrying member for supporting said second member. 
     In one embodiment, electrical connections are arranged out of said cover through said bonding sections and/or said third supporting member. 
     Preferably, said connections through said bonding sections are arranged on one of said first, second or third members before applying the bonding paste. The bonding sections are provided on said first member. 
     According to the invention, a method of selectively bonding a first member to a second silicon member through anodic bonding is provided, wherein the method comprises the steps of: providing the said first and second members, arranging said first member with bonding sections in predetermined sections, arranging said first and second members in a contacting position, pressing and heating said first and second members in said contacting position, and applying a voltage to said first and second members. Preferably, the second member is a silicon wafer comprising one or more active sections. The first member is a glass wafer provided with frames corresponding to said active sections. 
     The invention also concerns a sensor comprising a lid, a silicon substrate and a carrying substrate, wherein said lid, silicon substrate and carrying substrate are bonded through the method f the invention. 
     The invention also concerns a biological circuit hermetically connected to a substrate using the method f the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the following, the invention will be further described in a non-limiting way with reference to the accompanying drawings in which: 
         FIG. 1  schematically illustrates an arrangement produced according to the teachings of the invention, 
         FIGS. 2   a ,  2   b  shows the wafers for anodic bonding process according to the invention in plan view, 
         FIG. 3  shows a cross-section along line III—III in  FIGS. 2   a  and  2   b , in a preassembled form, 
         FIGS. 4   a ,  4   b ,  4   c  are cross-sections through different schematic embodiments showing wiring according to the invention, 
         FIG. 5  is a cross-section through yet another embodiment, and 
         FIG. 6  is a schematic cross-section through a device bonded according to another aspect of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     According to one preferred embodiment paste, e.g. of thick or thin film is through, e.g. screen printing or photo image forming, with doping containing Na ions, provided with conductive and non conductive sections which are bondable through anodic bonding. The carrier section may be one of glass, ceramics or glass composite, such as LTCC (Low Temperature Cofired Ceramic). 
     There are a number of different thick-film pastes with different glass mixtures. It is also possible to produce pastes with sodium or soda-lime content, both as dielectric and conductive pastes. However, the object of these is to provide a glass composition which matches the substrate to be printed. 
       FIG. 1  is a cross section through a device  100 , e.g. a sensor according to above mentioned sensor of EP 742 581. The device comprises a cover or lid  110 , e.g. of borosilicate glass or other glass composition, a semiconducting wafer  120 , a substrate  130 , preferably a multi layer substrate including conductors  140  and vias  150  arranged therein and solder pads  160 . The lid  110  is bonded to the Si wafer  120  through bonding areas  170   a , provided in accordance with the teachings of the present invention. The substrate  130  is also bonded to the Si wafer  120  through bonding areas  170   b , provided in accordance with the teachings of the present invention. The bonding areas  170  and  170   b  are provided as a paste on the lid  110  and carrier substrate  130 , respectively, as closed frames through screen printing and/or photo image forming or the like. 
     The electronic circuitry or functional devices  180  arranged on the silicon (Si) wafer  120  are connected to the conductors  140 , e.g. through connections  185  via the Si wafer. It is also possible to arrange connections that pass the bonding paste of the connection areas  170   a  and/or  170   b , which will be exemplified in the following embodiments. The electronic circuitry  180  is further connected to other circuits through solder pads  160 . 
     In a sensor, filter or similar applications both the lid  110  and the substrate  130  can be provided with cavities  190   a  and  190   b , respectively. 
       FIG. 2   a  is a plane view of lid wafer  210  of glass on which a number of sealing frames  270  of a paste material containing Na-ions are printed, e.g. through screen printing. The frames provide a closed space building the cavities  290 . On the other side, i.e.  FIG. 2   b , functional devices  280  are realised on a Si wafer  220 , which can be arranged on a carrying substrate  230  ( FIG. 3 ), which also is provided with bonding frames or sections  270   b.    
       FIG. 3  illustrates the moment before the glass wafer  210  of  FIG. 2   a  is bonded onto the Si wafer  220  of  FIG. 2   b . After the bonding process packaged units are formed, and each unit is cut out later in a suitable way well known for a skilled person. 
     In  FIG. 3 , the functional devices  280  may also be countersunk in the substrate  220  through micro-machining or the like depending on the application and/or the material of the substrate. 
     The bonding process is performed in a known way, i.e. the Si wafer  220  and the lid glass wafer  210  or carrier  230  are combined and exposed to a pressure and heat up to a specific level, for example 350° C. (not limited) and then a voltage, e.g. 800 V (not limited), is applied through the stack comprising the Si wafer and the lid wafer and/or the carrier. 
     Here, it is possible to provide different ways of electrical connections out of the functional devices arranged inside the sealed area on the semiconductive material  230 : Firstly, according to  FIG. 1 , i.e. through the carrying substrate  130  and secondly through the sealing frame  270 / 270   b . In the embodiment of  FIG. 4   a , a substrate  420   a  is provided with conductors  440   a , e.g. through etching or the like. Then the paste  470   a  (thin film paste) applied onto the glass  410  is pressed on the substrate  420   a.    
     In  FIG. 4   b  a thick-film paste  470   b  is applied through, e.g. screen printing onto the glass  410 . Conductors  440   b  having substantially the same thickness as the paste are arranged through a suitable method on the substrate  430   b , e.g. alumina. 
     Clearly, other embodiments are also possible as shown in  FIG. 4   c , in which a substrate  430  of LTCC is used and into which conductors  440   c  are immersed. Paste  470   c  is applied onto the glass  410  before bonding. 
     As shown in  FIG. 5 , it is also possible to countersink the pastes  570  into the substrate  530  of LTCC so that the upper surface of the paste comes into a substantially same level as the upper surface of the LTCC. The glass is denoted with  510 . 
     Additionally, the bonding according to the invention can be used as a sealing in further applications. In  FIG. 6 , for example, a so-called biological circuit  600  is connected to a substrate  610 . The biological circuit comprises a conduit  601  for transporting fluid or gas. It is possible to connect and seal the circuit to an external substrate  610  of, e.g. LTCC of another circuit likewise provided with a conduit  611  using the teachings of the invention, i.e. arranging a ring shaped paste bonding means  620  and anodically bonding the circuit to the substrate or other circuits. 
     The invention is not limited the shown embodiments but can be varied in a number of ways without departing from the scope of the appended claims and the arrangement and the method can be implemented in various ways depending on application, functional units, needs and requirements etc.