Abstract:
Applicant has discovered that articles comprising inorganic surfaces that are difficult to bond can be more effectively soldered or brazed with a solder or braze containing rare earth elements where the rare earth (RE) elements are substantially kept from contact with air at soldering temperatures, i.e. the RE elements are exposed to air for no more than a few seconds at soldering temperature. This can be efficiently accomplished in several ways. The result is efficient, strong bonding of materials previously considered difficult to bond.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates to methods of soldering or brazing and, in particular, to a method of soldering or brazing surfaces that are difficult to bond, such as surfaces comprising inorganic materials. It also includes novel articles made by the method.  
       BACKGROUND OF THE INVENTION  
       [0002]     Bonding using solder or braze is highly important in the fabrication of a variety of important optical, electronic and micro-electro-mechanical (MEMs) devices. Solders comprise low melting compositions composed of elemental metal or metal alloy. They typically melt at temperatures lower than about 450° and are very useful in bonding together surfaces to which the solder adheres. Brazes are similar materials of higher melting temperature and are used to form more thermally resistant bonds. Solder and brazes are used, for example, assembling lasers, bonding optical fibers to assembly substrates, connecting electronic components to assembly boards, and to assembling MEMs chips.  
         [0003]     While solders and brazes are generally very effective in bonding to many surfaces, they have been considerably less effective in bonding to stable inorganic surfaces such as oxides, nitrides, selenides, silicon, GaAs, GaN, other semiconductors, fluorides, diamond, and stable metals. These materials, which are increasingly used in high performance optical, electronic and MEMs devices, form relatively stable surfaces that have little tendency to chemically react with molten solder material. The result is low adherence and a weak bond.  
         [0004]     Solder bonding and brazing of these stable, inorganic materials can be enhanced by pre-treating the surfaces with multilayer metallization to present a more bondable surface, e.g. the well-known Ti/Pt/Au sputter-deposited metallization. But multiple coatings complicate production, add costs and introduce additional reliability concerns.  
         [0005]     Another approach is to make the solder or braze more reactive, as by adding reactive rare earth elements (RE elements). The resulting more reactive solders are known as universal solders. The difficulty is that universal solders which react with stable inorganic materials also react with less stable ambient materials, with deleterious consequences to the solder braze or bond. Accordingly there is a need for improved methods of soldering or brazing articles having surfaces that are difficult to bond.  
       SUMMARY OF THE INVENTION  
       [0006]     Applicant has discovered that articles comprising inorganic surfaces that are difficult to bond can be more effectively soldered or brazed with a solder or braze containing rare earth elements where the rare earth (RE) elements are substantially kept from contact with air at soldering temperatures, i.e. the RE elements are exposed to air for no more than a few seconds at soldering temperature. This can be efficiently accomplished in several ways. The result is efficient, strong bonding of materials previously considered difficult to bond. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]     The nature, advantages and various additional features of the invention will appear more fully upon consideration of the illustrative embodiments now to be described in detail in connection with the accompanying drawings. In the drawings:  
         [0008]      FIG. 1  is a schematic flow diagram of a method of brazing or soldering articles in accordance with the invention;  
         [0009]      FIGS. 2A through 2D  illustrate various configurations of universal solder bodies that can be used in the process of  FIG. 1 ;  
         [0010]      FIG. 3  shows vacuum bonding;  
         [0011]      FIGS. 4A and 4B  illustrate rapid application and bonding;  
         [0012]      FIGS. 5A and 5B  show bonding by mechanical collapse of a preform body;  
         [0013]      FIGS. 6A and 6B  show bonding by local heating collapse of a preformed body;  
         [0014]      FIG. 7  illustrates a MEMs multilayer structure bonded in accordance with the invention;  
         [0015]      FIG. 8  shows an assembly for a MEMs device hermetically packaged in accordance with the invention;  
         [0016]      FIG. 9  illustrates an optical fiber/laser assembly bonded in accordance with the invention; and  
         [0017]      FIGS. 10 and 11  illustrate fiber grating devices bonded in accordance with the invention. 
     
    
       [0018]     It is to be understood that the drawings are for purposes of illustrating the concepts of the invention and are not to scale.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0019]     Applicant has observed that the very reactive rare earth elements used in universal solders easily oxidize and solders or brazes containing them form oxide skins with high melting points (e.g., −2300° C.) when heated or melted. Rapid oxidation of rare earth elements on the surface of molten universal solders tends to deteriorate the solder wetting characteristics. Universal solder bonding processes conducted in oxygen-containing atmospheres, such as the air, offer only a short window for wetting and joining before oxidation. Once oxidation begins, an undesirable rare-earth-rich, gray oxide skin is formed on the surface of the universal solder that prevents the universal solder from wetting surfaces to be bonded. The oxide skin also impairs the diffusion of rare earth atoms from the interior of the solder to the interface to be bonded and prevents the universal solder from forming a strong solder bond.  
         [0020]     In an effort to ameliorate this problem applicant has previously proposed various approaches to modify the composition and/or structure of universal solders to isolate and effectively bury the RE components underneath the solder surface. The approaches include jacketing the universal solder with regular solder, coating the universal solder with noble metal or ion implanting of RE elements beneath the surface of regular solder. See published United States Patent Application No. 2002/0106528 filed by S. Jin et al. While the approach of burying the RE components has improved the bonding of universal solders, additional improvement is desired for use in bonding stable inorganic surfaces. Specifically, applicant has discovered that soldering or brazing such surfaces are substantially improved by wetting and bonding with universal solder under substantially oxygen-free conditions.  
         [0021]      FIG. 1  is a schematic flow chart of a method of brazing or bonding two or more articles in accordance with the invention. The first step shown in Block A, is to provide two or more articles having respective surfaces to be bonded. The invention is particularly valuable when one or more of the bonding surfaces is a stable inorganic surface such as oxide, nitride, selenide, silicon, GaAs, GaN or other semiconductor, fluoride diamond or stable metal.  
         [0022]     The next step, shown in Block B, is to dispose between the bonding surfaces a universal solder or braze, advantageously in the form of a body comprising the solder or braze. By the term “universal solder” is meant a low melting temperature solder doped with at least one rare earth element. Advantageously the low melting temperature solder comprises 0.1 to 10% by weight of one or more rare earth elements. Suitable low-melting temperature solders for use in the universal solder include, but are not limited to, Sn—Sb, Bi—Sn, In—Sn, In—Ag, Pb—Sn, Sn—Ag, and eutectic Au—Sn. Suitable rare earth dopants include, but are not limited to, Lu, Er, Ce, Y, Sn, Gd, Th, Dy, Tm and Yb. Brazes are similar compositions with propositions chosen for higher melting temperatures.  
         [0023]     The universal solder can be in the form of a simple alloyed universal solder of the components described above, such as: Sn—Ag-RE, Au—Ag-RE, Sn—Sb-RE, Bi—Sn-RE, In—Sn-RE, In—Ag-RE, Sn—Ag-RE. However the preferred form is a body configured, as set forth in U.S. published patent Application No. 2002/0106528, to bury the RE elements within the interior of the solder body.  
         [0024]      FIGS. 2A-2D  illustrate various configurations of universal solder bodies that can be used.  FIG. 2A  shows a universal solder body  20 .  FIG. 2B  illustrates a protective coating on film  21  of noble metal covering a universal solder core  22 .  FIG. 2C  shows a universal solder core  22  with regular solder jacket  23 , and  FIG. 2D  illustrates a universal solder paste comprised of solder particles  25  in a paste  26  matrix. The particles  25  can comprise universal solder particles coated with noble metal.  
         [0025]     Referring back to  FIG. 1 , the third step in the process is to wet and bond the surfaces under substantially oxygen-free conditions. This can be efficiently accomplished in at least four different ways:  
         [0026]     1) vacuum bonding;  
         [0027]     2) rapid application of molten universal solder;  
         [0028]     3) controlled collapse joining with universal solder, and  
         [0029]     4) rapid and localized heating by deposition of a resistive heating element. Each of these approaches are exemplified below.  
       EXAMPLE 1  
     Vacuum Bonding  
       [0030]     One method of minimizing the universal solder&#39;s exposure to an oxygen-containing atmosphere is by conducting the bonding in a vacuum. Solder bonding in a vacuum offers a viable batch-type packaging process, especially suited for hermetically sealing MEMS devices, optical devices and/or electronic devices.  
         [0031]      FIG. 3  schematically illustrates the step of wetting and bonding the surfaces  30  of an assembly  31  of two articles  32 ,  33  (e.g. MEMs upper ( 32 ) and lower ( 33 ) parts) in a substantially oxygen-free ambience. Here the articles  32 ,  33  have bonding surfaces (contact pads)  34  and bodies  35  comprising universal solder are disposed between contact pads of the respective parts. The assembly  31 , in turn, is disposed within a vacuum chamber  36  including a heater (not shown) and in communication with a vacuum pump. The chamber is advantageously evacuated to a pressure of 10 −6  torr or less, preferably 10 −7  torr or less, and even more preferably to 5×10 −8  torr or less. The assembly is heated and pressed together under vacuum to effect wetting and bonding without the presence of ambient atmospheric oxygen.  
         [0032]     The vacuum bonding process using a universal solder described herein is suited for use in fabricating MEMS devices, which are micromachines of small dimensions. For example, many MEMS devices to be bonded with a universal solder may be arranged, using automated assembly processing, on each of a multitude of shelves and placed in a vacuum chamber equipped with a capability to render either global or local heating. Each packaging assembly would have a lower device or substrate, preforms of a universal solder placed on contact pads or hermetic seal pads, and the upper device placed over the universal solder preforms with appropriate alignment and convenient fixturing array to maintain the alignment The preform can be either bulk solder or thin film deposited solder.  
       EXAMPLE 2  
     Rapid Application of Molten Universal Solder  
       [0033]     For universal solder bonding to produce successful solder bonding in oxygen-containing atmosphere such as the air, there is a time window of a minute or less and preferably less than 10 seconds to accomplish the wetting and joining before the oxidation of the universal solder takes place and an undesirable rare-earth-rich, gray colored oxide skin is formed that impedes further wetting. One way of carrying out desirably rapid solder bonding is by introducing controlled and rapid application of molten solder, preferably by using rapid automated processes. Carrying out such an inventive process is preferably done in an inert gas atmosphere, although this is not an absolute requirement.  
         [0034]      FIG. 4A  illustrates such a rapid application step using a hot metal brush  40  to pick up a volume of molten universal solder  41  from a molten bath (not shown) and, quickly coat a bonding surface  42  such as a hermetic seal pads (pre-heated if necessary). An upper device (not shown) is then quickly placed and pressed on top of the molten solder  41  to form a joint. Natural air cooling or an air blast may be used to initiate the solidification of the solder joint. The time from the brushing to the formation of the solder joint should be a minute or less and preferably is 10 seconds or less.  
         [0035]      FIG. 4B  shows an alternate rapid application step using a metallic doctor blade trailing a wire solder depositing brush (not shown) to produce a uniform thickness solder layer  41 . The upper device is placed and pressed on the bladed solder.  
       EXAMPLE 3  
     Controlled Collapse Joining with Universal Solder  
       [0036]     If the undesirable oxide skin formed on the surface of molten universal solder can be broken off, fresh universal solder can be released into immediate contact with the bonding surface. In such case, the contact areas are sealed against ambient oxygen by surrounding molten solder and desired universal solder bonding can be achieved in air.  
         [0037]      FIGS. 5A and 5B  illustrate the step of wetting and bonding using a mechanical disturbance to break the skin off molten solder so that the solder/surface contact is self-sealed from ambient oxygen. Specifically, relatively tall universal solder preform bodies  50  placed between the surfaces  51  to be bonded are melted and then the upper device  52  and lower device  53  are pressed together to collapse the molten solder  54 . The collapse breaks the oxide skin and allows fresh solder to wet and bond the device surface. Small spacer bumps  55  can be dimensional and placed to pre-set the solder joint thickness.  
       EXAMPLE 4  
     Rapid and Localized Heating by Deposition of a Heating Element  
       [0038]     Another way to minimize oxidation of the molten solder surface is to melt the solder rapidly and thus minimize the time of oxidation.  
         [0039]      FIGS. 6A and 6B  schematically illustrate an exemplary rapid heating step. Here resistive heating elements  60  such as resistive films of Mo or W are disposed in thermal contact with universal solder bodies  61 . An electrical current passed through elements  60  rapidly melts the bodies  61 . The heating elements, if deposited on the pads can remain as a buried part of the solder joint because the universal solder bonds well to the resistive materials.  
         [0040]     We now describe several exemplary advantageous applications of the method of  FIG. 1  to make articles.  
       Articles Fabricated Using the Bonding Methods of the Invention  
       [0041]     The universal solder materials and bonding techniques described here can useful for a variety of applications for assembling various MEMS, optical devices and electronic devices, especially for creating reliable hermetic sealing and for permitting flip-chip assembly without introducing complicated metallizations of various surfaces to be bonded.  
         [0042]      FIG. 7  illustrates a MEMS multilayer structure  70  bonded in accordance with the invention comprising light-reflecting mirror layer  71 , an electrode layer  72 , a spacer layer  73 , and a stiffening frame  75  to hold the components together. See R. Ryf, et al, “1296-Port MEMS Transparent Optical Crossconnect with 2.07 Petabits/s Switch Capacity”, OFC&#39;2001 (Optical Fiber Conference), Paper No. PD-28, Mar. 17-22, 2001, Anaheim Calif., USA. Universal solder bonds  74 , made in accordance with the method of  FIG. 1 , can be used to hold the components together within the stiffening frame  75 .  
         [0043]      FIG. 8  shows an assembly  80  for an optical MEMs device  81  hermetically packaged in accordance with the invention. The device  81  is sealed on substrate  84  by a transparent window  82  on a spacer  83 . Universal solder bonds  85 , made in accordance with the method of  FIG. 1 , can hermetically seal the spacer/window enclosure to the substrate  84 .  
         [0044]     Yet another example is the bonding of optical fiber devices. The universal solders are directly solderable to optical fibers, and hence are technically useful for a variety of applications in optical communication devices.  
         [0045]      FIG. 9  illustrates an assembly  90  optically coupling a semiconductor laser  91  in alignment with an optical fiber  92 . The laser  91  is mounted on a heat spreader  93 , and the fiber  92  is mounted on a standoff  94  in precise optical alignment with the laser output. Creep resistant bonding is essential for securing and maintaining alignment between the laser and the fiber. Tight micrometer tolerance in dimensional stability is required. The critical bonds  95  can be made using the method of  FIG. 1  and creep resistant solders such as those based on Sn—Ag-RE or Au—Sn-RE eutectic solder.  
         [0046]     Fiber gratings are SiO 2  based optical fiber devices with internal periodic refractive index perturbations along the fiber length corresponding to specific Bragg reflections for a certain wavelength of optical signals. They are frequently used for filtering specific, designated wavelength channels in wavelength-division-multiplexed optical communication systems. They need to be temperature-compensated to eliminate the fluctuation of refractive index of the grating with ambient temperature. One way of accomplishing this is to attach a negative CTE (coeffecient of thermal expansion) material. See, H. Mavoori and S. Jin, “Low Thermal Expansion Copper Composites via Negative CTE Metallic Elements”, JOM 50(6), 70 (June, 1998); A. W. Sleight, A. W.  Nature  389 (6654), 923 (1997).  
         [0047]      FIG. 10  illustrates a temperature compensated fiber grating device  100  assembled by bonding of a negative thermal expansion material  101  such as Ni—Ti or Zr-Tungstate on an elastically pre-strained fiber grating  102  such that when the ambient temperature rises, the strain in the grating  102  is reduced by the attached negative CTE material  101 . The fiber and the negative CTE material  101  are attached by universal solder bonds  103  made in accordance with the method of  FIG. 1 .  
         [0048]     The rare-earth containing solders can also be useful for convenient assembly of wavelength-tunable fiber gratings, such as those described in an article by S. Jin, et al., “Broad-range, latchable reconfiguration of Bragg wavelength in optical gratings”, Appl. Phys. Lett. 74 (16), 2259 (1999). Other examples include hermetic sealing of RF relay MEMS switches [see an article by J. Kim, et al., “Integration and Packaging of MEMS Relays”, SPIE Conf. Proc. on MEMS, May 2000, Paris, France] which can be useful for management of electronic data in automated test systems or control of communication information flow. The speed of movement of MEMS membranes, and hence the switching speed, is significantly reduced by air damping. For higher speed operations of such MEMS switches, hermetic sealing with vacuum environment is desirable. Hermetic sealing of such MEMS devices may involve simultaneous bonding to various surfaces such as Si, insulators like SiO 2 , SiN x , and electrical wiring made of poly Si or Al lines. Universal solders have desirable characteristics of being able to bond to all these different surfaces simultaneously during hermetic sealing.  
         [0049]      FIG. 11  illustrates such a latchable, tunable fiber grating  110  comprising an optical fiber  111  having a grating  112  attached to a guiding tube  113  and a programmable latchable magnet  114 . Key bonds  115  involving difficult to bond surfaces can be made using universal solders using the method of  FIG. 1 .  
         [0050]     Although the present invention has been described in considerable detail with reference to certain preferred embodiments and versions, other versions and embodiments are possible. For example, while the examples are discussed in relation to bonding using solder, they could equally well be applied to brazes. Therefore, the spirit and scope of the appended claims should not be limited to the description of the versions and embodiments expressly disclosed herein.