Patent Publication Number: US-6987033-B2

Title: Method for making electronic devices including silicon and LTCC and devices produced thereby

Description:
This application is a divisional of Ser. No. 09/741,754 filed on Dec. 19, 2000, now U.S. Pat. No. 6,809,424 the disclosures of which are hereby incorporated by reference in their entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to the field of electronic devices and manufacturing methods, and, more particularly, to methods for making and devices such as including packaged integrated circuits. 
     BACKGROUND OF THE INVENTION 
     Integrated circuits are widely used in many types of electronic equipment. An integrated circuit may include a silicon substrate in which a number of active devices, such as transistors, etc., are formed. It is also typically required to support one or more such integrated circuits in a package that provides protection and permits external electrical connection. 
     As the density of active devices on typical integrated circuits has increased, dissipation of the heat generated has become increasingly more important. Designers have developed cooling techniques for integrated circuits based on micro-electromechanical (MEMs) technology. 
     For example, as shown in  FIG. 1 , a prior art electronic device  10  includes a package  11  including a first member  12  comprising silicon, and a second member  14  comprising a low temperature co-fired ceramic (LTCC) material. The first member  12  may include several stacked silicon substrates  12   a ,  12   b  having various components of a micro-fluidic cooler formed therein. For example, as shown in the illustrated embodiment, an evaporator  16  and condensor  17  may be provided and interconnected via one or more micro-fluidic channels or passageways  21  formed between the silicon substrates  12   a ,  12   b . One or more MEMs pumps, not shown, may circulate the cooling fluid. 
     The second member  14  may also include several LTCC layers  14   a ,  14   b  laminated together as shown in the illustrated embodiment. The second member  14  also illustratively carries an integrated circuit  22 , such as an insulated gate bipolar transistor (IGBT) or other integrated circuit that may typically generate substantial waste heat. The second member  14  also includes external connections  23  which are connected to the electrical connections  24  of the integrated circuit  22  via the illustrated wires  25 . 
     As shown in the enlarged view of  FIG. 2 , the integrated circuit  22  is carried by a receiving recess  27  in the second member  14 . A series of micro-fluidic passageways  30  may be provided through the LTCC member  14  adjacent the integrated circuit  22  to deliver cooling fluid thereto. 
     Typically, the LTCC member  14  and the silicon member  12  are adhesively joined together as schematically illustrated by the adhesive layer  31 . Thermoplastic and/or thermosetting adhesives are commonly used. Metal layers may also be used. Unfortunately, the adhesive layer  31  has a number of shortcomings. The adhesive layer  31  may not typically provide a hermetic seal at the interface between the silicon and LTCC, thus, cooling fluid may be lost. In addition, the adhesive layer  31  may also provide yet another layer through which the heat must pass. Of course, it may be difficult to provide an adhesive layer  31  which is uniform and which does not protrude into the interface or otherwise block or restrict the flow of cooling fluid. In other words, such an adhesive layer  31  unfortunately provides only non-hermetic and non-uniform bonding the members. 
     U.S. Pat. No. 5,443,890 to Ohman discloses a leakage resistant seal for a micro-fluidic channel formed between two adjacent members. A sealing groove is provided and filled with a fluid sealing material which is compressed against adjacent surface portions of the opposing member. The provision for such a sealing structure requires additional manufacturing steps and may not be suitable for many applications. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing background, it is therefore an object of the invention to provide a method and associated electronic device wherein LTCC and silicon members are bonded together to form a hermetic seal with uniform bonding. 
     This and other objects, features and advantages in accordance with the present invention are provided by a method for making an electronic device comprising positioning first and second members so that opposing surfaces thereof are in contact with one another, the first member comprising silicon and the second member comprising a low temperature co-fired ceramic (LTCC) material. The method also includes anodically bonding together the opposing surfaces of the first and second members to form a hermetic seal therebetween. The anodic bonding provides a secure and uniform bond between the members. 
     The first and second members may have substantially planar major opposing surfaces. The anodic bonding provides a uniform bond across these surfaces to reduce possible stress effects which may otherwise occur due to the difference in thermal coefficients of expansion of the two different materials. 
     Anodically bonding may comprise applying a voltage across the first and second members, applying pressure to the opposing surfaces of the first and second members, and/or heating the first and second members. The method may also include cleaning the opposing surfaces of the first and second members prior to anodically bonding the members. 
     The method may further include forming at least one cooling structure in at least one of the first and second members. More particularly, the least one cooling structure may comprise at least one first micro-fluidic cooling structure in the first member, and at least one second micro-fluidic cooling structure in the second member aligned with the at least one first micro-fluidic cooling structure. The at least one first micro-fluidic cooling structure may comprise an evaporator and the at least one second micro-fluidic cooling structure may comprise at least one micro-fluidic passageway. Anodic bonding permits a hermetic seal between the two members, and significantly reduces or eliminates the loss of cooling fluid at the interface between the two members which could otherwise occur. 
     The method may also include positioning at least one integrated circuit adjacent the at least one cooling structure, such as adjacent the at least one micro-fluidic cooling passageway in the second member. The at least one integrated circuit may comprise electrical connections, and the second member may carry external electrical connections connected to the electrical connections of the at least one integrated circuit. 
     For typical electronic devices, the anodically bonding may comprise applying a voltage in a range of about 500 to 1000 volts across the first and second members. Similarly, the anodically bonding may comprise applying pressure in a range of about 1 to 20 psi to the opposing surfaces of the first and second members. Continuing along these lines, the anodically bonding may comprise heating the first and second members to a temperature in a range of about 100 to 150° C. 
     Another aspect of the invention relates to an electronic device, such as a multi-chip module (MCM) or other similar packaged integrated circuit, for example. The electronic device may comprise a first member comprising silicon, and a second member comprising a low temperature co-fired ceramic (LTCC) material. Moreover, the first and second members have opposing surfaces thereof anodically bonded together to form a hermetic seal therebetween. The first and second members may have opposing generally planar major opposing surfaces, for example. 
     At least one of the first and second members may comprise at least one cooling structure. For example, the first member may comprise at least one first micro-fluidic cooling structure therein, such as an evaporator. In addition, the second member may further comprise at least one second micro-fluidic cooling structure aligned with the at least one first micro-fluidic cooling structure of the first member. For example, the at least one second micro-fluidic cooling structure may comprise at least one micro-fluidic passageway. 
     The electronic device may also include at least one integrated circuit adjacent the at least one second micro-fluidic cooling structure of the second member. The at least one integrated circuit may also comprise electrical connections. Accordingly, the second member may comprise external electrical connections connected to the electrical connections of the at least one integrated circuit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic cross-sectional view of an electronic device according to the prior art. 
         FIG. 2  is a greatly enlarged view of a portion of the electronic device as shown in  FIG. 1 . 
         FIG. 3  is a schematic cross-sectional view of an electronic device in accordance with the present invention. 
         FIG. 4  is a greatly enlarged portion of the electronic device as shown in  FIG. 3 . 
         FIG. 5  is a schematic diagram of the electronic device as shown in  FIG. 3  being made in an apparatus in accordance with the invention. 
         FIG. 6  is a schematic diagram of the anodic bond interface as in the electronic device shown in  FIG. 3 . 
         FIG. 7  is a flowchart illustrating the method in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will now be described more fully hereinafter with reference to the accompanying drawings in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the illustrated embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout. 
     Referring now initially to  FIGS. 3–7  the electronic device and method for making the device in accordance with the invention are now described. In particular, as shown in  FIGS. 3 and 4 , an illustrated embodiment of an electronic device  110  in accordance with the invention is shown. The electronic device  110  differs from the prior art device shown in  FIGS. 1 and 2  in that the conventional adhesive layer  31  is replaced by an anodically bonded interface  135  as will be described in greater detail herein. 
     The electronic device  110  illustratively mounts a single integrated circuit  122  in the package  111  although those of skill in the art will recognize that the invention is also applicable to other electronic devices as well. For example, the electronic device may also be an MCM, or other similar device including one or more integrated circuits  122  contained in a similar mounting package. The electronic device  110  illustratively includes a first member  112  comprising silicon, and a second member  114  comprising a low temperature co-fired ceramic (LTCC) material. The first and second members  112 ,  114  have opposing surfaces thereof anodically bonded together to form a hermetic seal at the interface  135  therebetween. 
     As shown in the illustrated embodiment, the first and second members  112 ,  114  have opposing generally planar major opposing surfaces being anodically bonded together. At least one of the first and second members  112 ,  114  may comprise at least one cooling structure therein as will be appreciated by those skilled in the art. For example, as shown in the illustrated electronic device  110 , the first member  112  includes at least one first micro-fluidic cooling structure therein, such as the illustrated evaporator  116 . 
     The second member  114  may include at least one second micro-fluidic cooling structure aligned with the at least one first micro-fluidic cooling structure of the first member  112 . For example, and as shown in the illustrated embodiment of the electronic device  110 , the at least one second micro-fluidic cooling structure may comprise at least one micro-fluidic passageway  130 . 
     The electronic device  110  also illustratively includes an integrated circuit  122  adjacent the micro-fluidic passageways  130  of the second member  114 . Of course, in other embodiments, more than one integrated circuit may be mounted within the package  111 . In addition, an optical/electronic device may also be mounted and cooled as described herein as will be appreciated by those skilled in the art. The integrated circuit  122  also illustratively includes electrical connections  124  which are brought out to the external electrical connections  123  using conventional techniques as will be appreciated by those skilled in the art. 
     As will also be appreciated by those skilled in the art, in other embodiments of the invention, the integrated circuit  122  may include a back contact layer, not shown, which is also connected to an external electrical connector carried by the second member. In addition, the integrated circuit  122  may be mounted using flip chip bonding techniques in other embodiments. 
     The other elements of the illustrated electronic device  110  of the invention are indicated with reference numerals incremented by one hundred as compared to the similar elements of the electronic device shown in  FIGS. 1 and 2 . Accordingly, these common elements need no further discussion herein. 
     Referring now more particularly to  FIGS. 5–7 , method aspects of the invention are now described in greater detail. The method is for making an electronic device  110 , such as described above. As seen in the flowchart of  FIG. 7 , from the start (Block  150 ) the method may include cleaning and preparation of the opposing surfaces of the first and second members  112 ,  114  at Block  152 . Preparation may include polishing or other techniques to ensure that the surface roughness of each opposing surface is within a desired range. 
     At Block  154  the method includes positioning first and second members  112 ,  114  so that opposing surfaces thereof are in contact with one another. As described above, the first member  112  comprises silicon and the second member  114  comprises an LTCC material. At Block  156  the opposing surfaces of the first and second members  112 ,  114  are anodically bonding together. 
     Referring now briefly to the schematically illustrated apparatus  140  of  FIG. 5  an embodiment of anodic bonding is further described. The first and second members  112 ,  114  may be aligned between the top electrode  142  and the bottom electrode  141  of the apparatus  140 . The bottom electrode  141  is also carried by a heated support  144 . A voltage source  143  is connected to the top and bottom electrodes  142 ,  141 . The apparatus  140  can provide the necessary voltage, pressure and temperature ranges for efficient anodic bonding of the first and second members  112 ,  114 . 
     For typical electronic devices such as the illustrated electronic device  110  or MCMs, for example, the voltage source  143  may apply a voltage in a range of about 500 to 1000 volts across the first and second members  112 ,  114 . Similarly, the apparatus may also apply a force such that the pressure between the opposing surfaces is in a range of about 1 to 20 psi. Additionally, the heated support may heat the first and second members  112 ,  114  to a temperature in a range of about 100 to 150° C. Of course, other voltages, pressures and temperatures are contemplated by the invention and may be used for other devices as will be appreciated by those skilled in the art. After the anodic bonding (Block  156 ), the bonded first and second members  112 ,  114  may be cleaned and further processed before stopping (Block  160 ). 
     As described above, the first and second members  112 ,  114  may have substantially planar major opposing surfaces, so that anodic bonding provides a uniform bond across these surfaces to reduce possible stress effects which may otherwise occur due to the difference in thermal coefficients of expansion of the two different materials. The anodic bonding provides a secure and uniform hermetic seal between the members  112 ,  114  and while overcoming the disadvantages described above resulting from using an adhesive. 
     The method may further include forming at least one cooling structure in at least one of the first and second members  112 ,  114 . These may be formed before or after anodic bonding, or they may be formed both before and after anodic bonding. The method may also include positioning at least one integrated circuit  122  adjacent the at least one cooling structure, such as adjacent the at least one micro-fluidic cooling passageways  130  in the second or LTCC member  114 . 
     Anodic bonding advantageously provides a hermetic seal between the two members, and significantly reduces or eliminates the loss of cooling fluid at the interface between the two members which could otherwise occur. It is believed without applicants wishing to bound thereto that the anodic bonding causes a coordinate covalent matrix to form at the interface  135  between the first and second members  112 ,  114  as perhaps best shown in the schematic view of  FIG. 6 . 
     Many modifications and other embodiments of the invention will come to the mind of one skilled in the art having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Accordingly, it is understood that the invention is not to be limited to the embodiments disclosed, and that other modifications and embodiments are intended to be included within the spirit and scope of the appended claims.