Patent Publication Number: US-5291572-A

Title: Article comprising compression bonded parts

Description:
FIELD OF THE INVENTION 
     This invention pertains to articles that compromise compression bonded parts, e.g., a &#34;lid&#34; that is compression bonded to a &#34;box&#34; such that a hermetically sealed enclosure results. 
     BACKGROUND OF THE INVENTION 
     In many fields of technology it is necessary to maintain a critical component or assembly in a controlled atmosphere. Exemplary of such technologies are optical communications and flat panel displays. See, for instance, U.S. Pat. No. 4,119,363, which discloses a hermetically sealed housing that contains a laser, with an optical fiber extending into the housing. See also U.S. Pat. No. 4,940,916, incorporated herein by reference, which pertains to field emission flat panel displays. Such displays require that the micropoint field emitters be located within a hermetically sealed enclosure. See also U.S. patent application Ser. No. 07/889,735, also incorporated by reference. 
     A known method of forming a hermetic enclosure involves solder bonding a piece part to another piece part (frequently to be called herein a &#34;lid&#34; and a &#34;box&#34;, respectively), such that the desired enclosure results. However, there are problems associated with this method. For instance, prior art boxes and lids frequently are relatively costly, manufacturing yield typically is determined by the uniformity of wetting of the solder, and the presence of a solder bond places an upper limit on the temperature of any subsequent manufacturing operation. 
     In view of the economic significance of articles that comprise a hermetically sealed enclosure, a method of forming such an enclosure that does not exhibit the drawbacks of the prior art method would be highly desirable. This application discloses such a method, and articles made by the method. 
     Compression bonding is a known bonding technique. See, for instance, U.S. Pat. No. 4,937,653, which discloses compression bonding of IC chips to a Si wafer carrier that comprises textured bonding regions. See also Japanese patent document 54-54572/1979, which discloses a Si element that is thermally bonded to a metal pedestal (via a gold or gold alloy layer applied to a &#34;rough&#34; section of the pedestal), with the &#34;roughness&#34; due to indentations formed in the pedestal surface by blades. 
     SUMMARY OF THE INVENTION 
     In a broad aspect the invention is embodied in an article that comprises a first body compression bonded to a second body. 
     More particularly, it is embodied in an article that comprises said first and second bodies, with at least one of the two bodies having a surface that comprises a textured bonding region, with the first body being compression bonded to the second body in the bonding region. Significantly, the textured bonding region surrounds an interior region wherein the first and second bodies are not bonded to each other, and comprises at least one raised feature (such raised features to be referred to herein as &#34;ridges&#34;). Said ridge is continuous and substantially surrounds the interior region. A currently preferred embodiment comprises at least two ridges that are continuous, substantially concentric and substantially surround the interior region, and further comprises optional ridges that extend between the at least two ridges, compartmentalizing the space between the two ridges. 
     In an exemplary embodiment, one of the two bodies is a box-like structure and the other body forms a &#34;lid&#34; for the &#34;box&#34;. The textured surface is coated with an appropriate metal (exemplarily gold or a gold alloy), and the mating surface region of the other body typically is coated with a (typically relatively thick) layer of an appropriate metal (exemplarily also gold or a gold alloy). Compression bonding the lid to the box can result in a hermetically sealed enclosure, with the one or more continuous ridges serving as barriers to the flow of gas. The presence of ridges that extend between two continuous ridges serves to further reduce the probability of a leakage path between the interior of the enclosure and the ambient, as those skilled in the art will appreciate. 
     Exemplarily, both the box and the lid are made from silicon by known lithography and etching techniques, and serve as hermetic housing for a semiconductor laser. Since hundreds (and even thousands) of such boxes or lids can readily be made by known methods in batch fashion from a single, commercially available silicon wafer, these piece parts can be produced at relatively low cost. Furthermore, manufacturing yield can be relatively high since solder wetting is not a consideration, and the compression bond in articles according to the invention is compatible with substantially any conceivable subsequent heat treatment that is compatible with all other aspects of the article. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 schematically depicts an exemplary article according to the invention, namely, a laser housing, prior to compression bonding; 
     FIGS. 2 to 5 schematically illustrate ridge formation; 
     FIGS. 6 and 7 are micrographs of an exemplary textured bonding region; and 
     FIG. 8 schematically depicts a laser package according to the invention in an optical fiber communication system. 
    
    
     DETAILED DESCRIPTION OF AN EXEMPLARY EMBODIMENT 
     A particular and commercially important embodiment of the instant invention is a hermetic enclosure for a semiconductor laser. FIG. 1 schematically depicts the &#34;box&#34; 10 (with attached lensed optical fiber 11) and &#34;lid&#34; 100 of an exemplary laser package according to the invention prior to bonding of the lid to the box. Surface 12 of the box comprises a recessed portion 13 which constitutes the interior of the box. In the recess is mounted laser 15 on submount 14, with electrical contact means (16, 17) provided, all in known manner. Gold (or other appropriate metal) &#34;O-ring&#34; 18 is electrically isolated from the laser contact strips (16, 17) by means of an appropriate insulating layer (not shown) and encloses the recess except for V-groove 19. An optional strip heater 190 facilitates solder bonding fiber 11 to the housing after alignment to the laser, exemplarily as described in U.S. patent application Ser. No. 07/914,782, filed by the instant applicants on Jul. 16, 1992. Optional contact stripes 191 and 192 facilitate electrical contact to optional monitor PIN diode 101 on lid 100. Rear wall 193 of recess 13 serves as turning mirror, directing radiation emitted by the laser in the back direction to monitor 101. See also U.S. Pat. No. 5,124,281. 
     Boxes and lids as shown are advantageously made from silicon, exemplarily (100)-oriented single crystal Si wafers, by known techniques that include anisotropic etching. Lid 100 comprises textured contact region 102 which surrounds interior region 103 except for V-groove 104, provided to facilitate placement and bonding of fiber 11. The nature of the texture will be described in more detail below. Generally speaking, the texture region comprises at least one (preferably two or more) ridge that is continuous and &#34;substantially&#34; (i.e., completely except for unavoidable gaps such as the due to the V-groove) surrounds the interior region. The textured region is covered with a relatively thin layer of gold (or other appropriate metal) such that a compression bond can be formed between O-ring 18 and the textured region. Such bonding will be referred to as a textured compression bonding (TCB). 
     Formation of a textured surface useful in the practice of the invention is schematically illustrated in FIGS. 2 and 3. On (100) surface 22 of Si body 20 are formed relatively narrow (e.g., about 1 μm) lines 21 of SiO 2  by known techniques, the lines to serve as mask during the subsequent selective etching step that results in ridges 30 as shwon in FIG. 3. Onto the textured region is then deposited a layer of metal selected such that by urging the textured surface against appropriately selected metal, a cold weld is formed. Gold is a currently preferred metal, but the invention is not restricted to the use of gold or gold alloys. 
     The invention is also not restricted to the use of ridges formed by selective etching, and FIGS. 4 and 5 schematically illustrate an alternative technique for ridge formation. On Si body 20 is relatively thick (e.g., 5 μm) SiO 2  layer 40 with patterned poly-Si layer 41 thereon. The combination is then etched in a known buffered oxide etchant until the poly-Si is substantially undercut, resulting in a texture substantially as shown in FIG. 5. After completion of texture etching, the poly-Si strips are removed by known means. 
     A significant aspect of the invention is the nature of the texture, and FIG. 6 shows a scanning electron microscope (SEM) micrograph of a relevant portion of a textured Si body. Numeral 60 refers to the recessed inner portion, 61 to untextured surface regions and 62 to the textured portion of the surface. FIG. 7 shows a SEM micrograph of a portion of the region within the circle in FIG. 6. As can be seen from FIGS. 6 and 7, the textured region compromises ridges of a first type that surround the recessed region, with ridges of a second type that extend between adjacent ridges of the first type. Ridges of the second type are optional but desirable, since they serve to reduce the probability that a single missing ridge would allow a leak between the inside of the box and the ambient. As those skilled in the art will recognize, a texture as shown in FIGS. 6 and 7 can be produced on a (100) Si wafer by known techniques. 
     Enclosures according to the invention can be used to provide a controlled environment not only for semiconductor lasers but also for other semiconductor devices, e.g., integrated circuits. They also have utility for, e.g., flat panel field emission displays. It is known that such displays typically require a high vacuum (e.g., 10 -9  Torr) to ensure long lifetime of the cathodes. Such a high vacuum requires a high temperature bake-out prior to sealing, and a reliable vacuum seal that can be made after the bake-out. TCB can advantageously be used since the bake-out does not affect the integrity of the relevant gold layers. 
     FIG. 8 schematically depicts an exemplary article according to the invention, namely, an optical fiber communication system that comprises hermetically sealed laser package 80. The package is, exemplarily, of the type illustrated in FIG. 1. The laser (not shown) emits radiation in response to an electrical signal, the radiation is coupled into optical fiber 81 and transmitted therethrough to receiver 83. FIG. 8 also shows optional optical amplifier 82 between the transmitter (which compromises the laser package) and the receiver. 
     EXAMPLE 
     A hermetic enclosure was formed as follows. Silicon &#34;boxes&#34; (3.6×6/4×0.5 mm 3 ) were made from a standard 5 inch (100)-oriented Si wafer by etching 2.9×5.9×0.1 mm recesses into the surface, gold plating to deposit a 10 μm thick &#34;O-ring&#34; surrounding each recess, and dicing the wafer, all by conventional methods. In particular, the etchant was a known diamine-based etchant. Si &#34;lids&#34; (3.4×7 mm) were made from the same wafer in substantially the same way as the boxes, except that a textured region was formed by selective etching. The texture consisted of a waffle pattern substantially as shown in FIG. 7, with 10×5 μm compartments. The pattern comprised 10 concentric and continuous ridges of the first type adapted to form, after TCB, ten separate barriers between the cavity and the ambient. The large number of ridges of the second type is adapted to further increase the barriers to leakage between the cavity and the ambient. The textured region was 6.0×3.3 mm, with a width of  150 μm, was etched by the technique of FIGS. 2 and 3 and was covered with 0.5 μm of gold. After dicing to separate the lids, a box and a lid were pressed together in a He atmosphere, using a press with a floating anvil. The floating anvil was used to ensure that the compression was uniformly applied over the bonding region. The mechanical advantage of the press was 8.5, and the applied force was about 190N. After sealing, the package was maintained in 50 atm. He for 14 hours. The package was then leak-tested in a He leak detector and was found to be leak free to a level of 10 -9  sccm, the sensitivity limit of the leak detector. 
     Although TCB can be carried out at room temperature or even below, it can also be carried out at elevated temperatures. In preferred embodiments, box and lid will be made of the same material (e.g., Si), eliminating thermal stress during temperature cycling. Thus, at least in preferred embodiments, TCB can be done at any desired point in the assembly process, without regard for soldering steps which follow or precede.