Abstract:
A lead-containing glass material of the type suitable for use in a wafer bonding process, wherein the moisture resistance of the glass material is increased by the presence of a lead phosphate coating on an outer exposed surface of the material, thereby acting as a barrier to reaction of moisture with the lead of the glass material. A source of reactive phosphate ions is applied to the glass material so as to spontaneously form the desired lead phosphate coating.

Description:
BACKGROUND OF INVENTION 
     1. Field of the Invention 
     The present invention generally relates to glass frit compositions. More particularly, this invention relates to lead-containing glass frit materials of the type suitable for use in wafer bonding processes, wherein the moisture resistance of the glass frit material after firing is increased by a lead phosphate coating formed on an exposed outer surface of the glass frit material. 
     2. Description of the Related Art 
     Within the semiconductor industry, there are numerous applications that require bonding a semiconductor wafer to a second wafer or glass substrate. As an example, a microelectromechanical system (MEMS) device formed in or on a semiconductor wafer (referred to herein as a device wafer) is often capped by a semiconductor or glass wafer (referred to herein as a capping wafer), forming a package that defines a cavity within which the MEMS device, such as a suspended diaphragm or mass, is enclosed and protected. Examples of MEMS devices protected in this manner include accelerometers, rate sensors, actuators, pressure sensors, etc. By the very nature of their operation, MEMS devices must be free to move to some degree, necessitating that the seal between the wafers is sufficient to exclude foreign matter from the cavity. Certain MEMS devices, such as absolute pressure sensors, further require that the cavity be evacuated and hermetically sealed. The performance of motion sensors and accelerometers with resonating micromachined components also generally benefit if the cavity is evacuated so that their micromachined components operate in a vacuum. A hermetical seal also ensures that moisture is excluded from the cavity, which might otherwise form ice crystals at low temperatures that could impede motion of the MEMS device. 
     In view of the above, the integrity of the bond that secures the capping wafer to the device wafer is essential to the performance and life of the enclosed MEMS device. Various bonding techniques have been used for the purpose of maximizing the strength and reliability of the wafer bond. Such techniques include the use of various intermediate bonding materials, including glass frit, as well as silicon direct and anodic bonding techniques that do not require an intermediate bonding material. As would be expected, each of these bonding techniques can be incompatible or less than ideal for certain applications. Silicon direct and anodic bonding methods require very smooth bonding surfaces, and therefore cannot produce a vacuum seal when trench isolation or unplanarized metal crossunders are employed on the device wafer, such as to electrically interconnect a MEMS device to bond pads outside the vacuum-sealed cavity of the package. In contrast, glass frit and other intermediate bonding materials are able to form suitable bonds with deposited layers, runners and other surface discontinuities often found on device wafers. 
     Glass frit bonding materials used for wafer bonding are often deposited by a screen printing technique, in which case the material is deposited as a paste that contains a particulate glass frit material, a thixotropic binder, and a solvent for the binder. The proportions of glass frit, binder and solvent are adjusted to allow screen printing of a controlled volume of the paste on a designated bonding surface of one of the wafers, typically on the capping wafer. After firing, the capping and device wafers are aligned and then mated so that the glass frit particles (bonded together as a result of the firing operation) contact a complementary bonding surface of the second (e.g., device) wafer. The wafers are then incrementally heated to completely remove the solvent and binder and finally melt the glass frit, so that on cooling the glass frit material resolidifies to form a substantially homogeneous glass bond line between the wafers. 
     The composition and size of a glass frit material used in a wafer bonding process are typically chosen on the basis of process and compositional considerations, including screening properties, process temperatures, etc. Suitable glass frit materials are usually a mixture of various oxides, such as litharge (PbO; also known as lead oxide, yellow and lead monoxide), boric acid (H 3 BO 3 ) which serves as a source for boron oxide (B 2 O 3 ), silicon dioxide (SiO 2 ; silica), aluminum oxide (Al 2 O 3 , alumina), titanium oxide (TiO 2 , titania), cupric oxide (CuO), manganese dioxide (MnO 2 ) or manganese carbonate (MnCO 3 ) as a source for manganous oxide (MnO), calcia (CaO), lithium oxide (Li 2 O), ceria (CeO 2 ), cobaltous carbonate (CoCO 3 ), and others. Glass frits that contain lead are widely used in the semiconductor industry to enable silicon wafer bonding at temperatures sufficiently low to reduce the risk of thermal damage. However, the percentage of lead required in a glass frit material to achieve high quality wafer-level sealing at low firing temperatures makes the resulting glass susceptible to attack by moisture. When a lead-containing glass is exposed to moisture, lead can be released from the glass and, in concert with moisture, migrate to exposed aluminum metallization on the wafer, resulting in galvanic corrosion of the metallization. This corrosion is a source of concern for both the wafer-level yield and the system-level reliability of a semiconductor device. 
     Prolonged exposure to moisture can also create holes in a glass bond line that are potential paths for the ingress of air, moisture and contaminants into a cavity sealed by the bond line. Glass frit used in wafer bonding processes is often exposed to attack by moisture as a result of the use of water to cool the wafers and remove debris during the dicing operation used to singulate dies from the wafers. During singulation, the glass bond line is generally covered by water for the duration of the dicing operation, making the glass-water interaction difficult to avoid. Following dicing, if the device is placed in a package that is not hermetically sealed, the glass bond line may be subjected to prolonged exposure to moisture during the operating life of the device. 
     In view of the above, it would be desirable if lead-containing glass frit materials of the type used in wafer bonding processes could be rendered more resistance to attack by moisture. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to lead-containing glass frit materials of the type suitable for use in wafer bonding processes, wherein the moisture resistance of the glass frit material is increased by the presence of one or more lead phosphate compounds. The lead phosphate compounds are preferably in the form of an outer coating that protects the surface of the glass frit material, thereby acting as a barrier to reaction of moisture with the lead contained by the material. 
     A suitable method for making and using the glass frit material of this invention comprises depositing the lead-containing glass frit material on a surface. The glass frit material, comprising glass frit particles, is then fired to melt and fuse (bond) the particles, after which a source of reactive phosphate ions is applied to the bonded glass frit so as to spontaneously form the desired lead phosphate coating on the exposed surface of the glass frit. 
     Glass frit materials protected with a lead phosphate coating in accordance with this invention are particularly well suited for bonding device and capping chips together, such as where the glass frit material is used to hermetically seal a cavity defined by and between the device chip and the capping chip. In such an application, the lead phosphate coating prevents the lead content of the glass frit material from reacting with moisture, such that lead is not released to react with aluminum metallization on the chips and create holes that would degrade the hermetical seal formed by the glass frit material. 
     Other objects and advantages of this invention will be better appreciated from the following detailed description. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a cross-sectional view of a MEMS device package that includes a capping wafer glass frit bonded to a device wafer in accordance with this invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 represents a portion of a MEMS device package  10  formed by glass frit bonding a device wafer  12  to a capping wafer  14 , such that a micromachined structure  16  (schematically represented in FIG. 1) is enclosed within a cavity  18  between the wafers  12  and  14 . The device wafer  12  is typically formed of a semiconductor material such as silicon, preferably monocrystallographic silicon, though it is foreseeable that other materials could be used. The capping wafer  14  may be formed of glass, ceramic, or a semiconducting material. The micromachined structure  16  can be of any desired type, such as a proof mass, resonating structure, diaphragm or cantilever that relies on capacitive, piezoresistive and piezoelectric sensing elements to sense acceleration, motion, pressure, etc., all of which are known in the art. 
     As is conventional, the micromachined structure  16  is electrically interconnected to metal bond pads  20  on the device wafer  12  by conductive runners  22 , often in the form of aluminum metallization. Through the bond pads  20 , the micromachined structure  16  and its associated sensing elements can be electrically interconnected with appropriate signal conditioning circuitry (not shown), which may be formed on the device wafer  12 , the capping wafer  14  or a separate chip. The runners  22  cross through a bonding surface  26  contacted by a glass bond line  30  that bonds a corresponding bonding surface  28  of the capping wafer  14  to the bonding surface  26  of the device wafer  12 . 
     According to common practice, the bond line  30  is formed by firing a paste composition containing a glass frit material and, typically, a binder and solvent. The binder and solvent are removed and the glass frit material is melted by sufficiently heating the paste, with the result that essentially only glass remains to form the bond line  30 . Suitable binders and solvents for the paste are those used in commercially-available glass frit paste mixtures, and have vaporization temperatures well below the softening point of glass frit materials suitable for the paste (e.g., about 355° C.). As known in the art, the proportions of the glass frit material, binder and solvent are chosen at least in part to achieve the desired deposition resolution for the paste, and consequently, the minimum width and thickness of the glass bond line  30 . A suitable deposition method is screen printing in accordance with known practices, by which the paste is deposited through a mask or screen to form a thick film on the bonding surface  26  or  28  of the device or capping wafer  12  or  14 . The wafer  12  or  14  is heated to remove the solvent and binder from the paste, and then soften or melt the surfaces of the glass frit particles to bond or tack the particles together. After cooling to resolidify the glass frit material, the capping and device wafers  12  and  14  are aligned and mated so that the glass frit material is between and contacts both bonding surfaces  26  and  28  of the wafers  12  and  14 . The wafers  12  and  14  are then heated to a temperature sufficient to remelt the glass frit material, after which the wafers  12  and  14  are cooled to resolidify the glass frit material and form the glass bond line  30  shown between the wafers  12  and  14  in FIG.  1 . 
     The present invention is concerned with glass bonds formed from glass frit compositions that contain lead, such as from lead oxide (PbO), also known as litharge, and yellow and lead monoxide. Various other materials may be present in the frit composition, including but not limited to boric acid (H 3 BO 3 ) which serves as a source for boron oxide (B 2 O 3 ), silicon dioxide (SiO 2 ; silica), aluminum oxide (Al 2 O 3 , alumina), titanium oxide (TiO 2 , titania), cupric oxide (CuO), manganese dioxide (MnO 2 ) or manganese carbonate (MnCO 3 ) as a source for manganous oxide (MnO), calcia (CaO), lithium oxide (Li 2 O), ceria (CeO 2 ), cobaltous carbonate (CoCO 3 ). However, it is the lead content in the glass bond line  30  formed by firing the glass frit that renders the bond line  30  susceptible to attack by moisture. As a result of exposure to moisture, elemental lead can be released from the bond line  30  and migrate to the aluminum runners  22  on the device wafer  12  to cause galvanic corrosion. Lead migration from prolonged exposure to moisture, such as during dicing when the device package  10  is singulated from a wafer stack, can result in the creation of holes in the bond line  30  that are paths for the ingress of air, moisture and contaminants into the cavity  18 , which is often required to be hermetically sealed by the bond line  30 . 
     To prevent or at least inhibit the depletion of lead from the bond line  30 , a source of reactive phosphate ions is applied to the bond line  30  (i.e., after firing of the glass frit paste), which results in the formation of a lead phosphate coating on the exposed outer surface of the bond line  30 . An example of a suitable source of phosphate ions is a surfactant commercially available from Union Carbide under the name Triton X-100, which contains a trace amount of phosphates, believed to be on the order of parts per million or less. The Triton X-100 surfactant has been successfully used in a diluted form by preparing a solution of about five parts by volume of the surfactant to about 95 parts of deionized water, though solutions containing as little as about 2.5 parts by volume of the surfactant, the balance water, are believed to be effective. While the Triton X-100 has been found to be a suitable source of reactive phosphate ions, other sources containing a sufficient amount of reactive phosphates—again, even amounts measured in parts per million (e.g., less than 0.001%) or less—could be used, as long as the source is able to deliver phosphate ions to the glass surface so that lead present in the glass is reacted to form a protective lead phosphate coating. 
     Phosphate ions are believed to bond with lead through several possible reactions, such as: 
     
       
         3Pb +2 +2(PO 4 ) −3 →Pb 3 (PO 4 ) 2   
       
     
     
       
         5Pb +2 +3(PO 4 ) −3 +H 2 O→Pb 5 (PO 4 ) 2 OH+H +   
       
     
     These reactions tie up lead on the surface of the bond line  30  by forming lead phosphates that are highly insoluble in water (log Ksp values of 44.36 and 76.8, respectively). Through these reactions, any lead-containing glass frit material (e.g., bond line  30 ) will develop a protective coating containing one or more lead phosphates. The lead phosphate coating acts as a moisture barrier to the bulk of the glass frit material, thereby inhibiting attack and greatly decreasing the probability of device failure due to moisture-induced degradation of the glass frit. 
     During an investigation leading to this invention, a solution was prepared containing the Triton X-100 surfactant and deionized water in a volume ratio of 5/95. Two identical accelerometer packages were obtained having an acceleration-responsive proof mass enclosed within a cavity hermetically sealed with a glass bond, similar to what is depicted in FIG.  1 . The glass bond was formed by firing a glass frit material containing lead oxide particles. Both packages also included runners and bond pads formed by aluminum metallization and contacted by the glass bond. One of the packages was treated in accordance with this invention by immersing the package in the surfactant solution for about ten minutes, such that the exposed portions of the glass bond were contacted by the solution. Both packages were then immersed in deionized water at a temperature of about 95° C. for about 2.5 hours. 
     Examination of the untreated package evidenced that a large amount of lead had been released from the glass bond, and that the aluminum metallizations were darkened by what was determined by energy dispersive x-ray (EDX) analysis to be a layer of lead oxide. In contrast, the glass bond of the treated package did not suffer any discernable loss of lead, and the surfaces of the aluminum metallizations were free of lead oxide contamination. Microphotographs of the glass bond lines of both packages evidenced that the untreated glass bond had a roughened, crystalline appearance with holes attributed to the loss of lead. In contrast, the bond line of the treated package had a smooth texture with a feathery “skin” identified by EDX as phosphorus-rich and attributed to lead phosphates formed by the surfactant treatment. The lead phosphate skin completely covered the exposed surface of the glass bond line. From these results, it was concluded that the lead phosphate coating was present in an amount sufficient to prevent the lead oxide of the glass bond line from reacting with the hot water bath. As such, it was concluded that treating a lead-containing glass frit material with a phosphate-containing source is capable of preventing the loss of lead from the frit material, and therefore capable of avoiding degradation of the glass frit bond and contamination of surrounding die surfaces. 
     While the invention has been described in terms of a preferred embodiment, it is apparent that other forms could be adopted by one skilled in the art. For example, glass frit material treated in accordance with this invention could be used in a variety of other applications, including displays, etc. Accordingly, the scope of the invention is to be limited only by the following claims.