Patent Application
20050077342
The present invention relates to devices, and in particular to securing a cover for the devices.
Micro-electromechanical system (MEMS) devices are very small and fragile. They need to be protected from physical harm and contamination. Some MEMS devices require a special environment, such as a gas or liquid fluid, in which to operate. Prior attempts to provide such protection involve the use of a cover, such as a window or plate fixed over the MEMS device to protect it. Such windows or plates may be fixed on an annular ring of a polymer extending above and around the MEMS device. Polymers may not be compatible with fluids required for proper operation of certain MEMS devices. Alternatives include the use of solder paste containing flux, which becomes a source of contamination for the MEMS device. It may be difficult to place and bond the window to the ring without damaging the MEMS device. Some materials require a high temperature to bond, or bond at lower temperatures with high forces that may adversely impact the MEMS device. A seal between the bond ring and the window may also need to be better than that obtained with polymer bond rings.
In the following description, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that structural, logical and electrical changes may be made without departing from the scope of the present invention. The following description is, therefore, not to be taken in a limited sense, and the scope of the present invention is defined by the appended claims.
A cover 100 for a micro-electromechanical system (MEMS) device has an annular cover bond ring 105 formed thereon. In one embodiment, the cover comprises a window 110 formed of a transparent material such as glass. The window may be formed of different materials as desired or dictated by the type of MEMS device it is designed to protect. Some typical materials include silicon, gallium arsenide, sulfides and others. When an optical type MEMS device is to be protected, a transparent window is used. In other embodiments the window may be opaque, or formed of a material compatible with maintaining a hermetic seal over the MEMS device, or for maintaining a desired operating environment for the MEMS device, such as oil, gas such as argon or neon, or other types of fluid.
Cover bond ring 105 comprises a cover bond ring layer 120, an intermediate tacking layer 130, and optionally, a antioxidation layer 140, such as thin layer of noble metal covering the tacking layer 130 to prevent oxidation of the tacking layer 130. Other materials may be used for this purpose, but in one embodiment, Au is used.
The cover bond ring layer 120 is formed of an inorganic material such as gold, or a gold alloy in one embodiment, such as AuSn or AuGe. Sn containing solders, silver and copper and other materials may also be utilized. The intermediate tacking layer 130 comprises a thin layer of a soft low-melting point material such as In in one embodiment. Further materials include Bi, Sn or In and Bi alloys. In one example, the intermediate tacking layer 130 is between 100 A and 50 um thick. The cover bond ring 105 may also be formed in shapes other than annular, such as square, oval, or any other desired shape suitable for properly supporting the window and allowing full operation of the MEMS device. In one embodiment, the layers are deposited and patterned using lithographic processes, or other suitable processes. Depositing techniques include but are not limited to plating or vacuum deposition.
In further embodiments, the tacking layer 130 can be treated with argon sputtering, or a plasma treatment such as CHF3 or SF6 to activate the tacking layer 130.
In an alternative embodiment, a tacking layer and optional antioxidant layer are formed on the bond ring 210 formed on substrate 200. In this embodiment, cover bond ring layer 130 need not have the tacking layer, or may also have a tacking layer if desired. In one embodiment, the bond ring 210 is formed of AuSn, tacking layer 130 is formed of Sn, and antioxidant layer 140 is formed of Ag.
The tacking layer 130 is absorbed into the bond ring during the staking to form final bond rings 445, 450 and 455. The amount of In or other material used in tacking layer 130 is such that mechanical properties of the final bond rings 445, 450 and 455 are not adversely degraded. In one embodiment, the final bond rings comprises AuSn with a trace amount of In. The final bond rings may be between approximately 0.5 um to 60 um high in one embodiment. The actual height may be varied based on the type of MEMS device to be protected. Following staking, individual MEMS devices with covers may be saw cut from a wafer. Such individual MEMS devices are supported by cut sections of substrate 410.
In one embodiment, the bond rings on the substrate and the cover have a combined height to ensure that the cover has a sufficient height above the height of the MEMS structures, allowing such structures to operate properly when the cover is staked. The rings may be the same height, or one ring may be taller than the other. The bond rings may be formed by a process selected from the group consisting of physical vapor deposition, sputtering, evaporation, plating, or chemical vapor deposition.
In one embodiment, the bond ring 210 on substrate 200 is formed by use of a method described in United States Application entitled: “Bond Ring for Micro-electromechanical System” docket number 200308958-1, filed on the same date herewith, which is incorporated herein by reference. At least one sacrificial layer is used to form a MEMS device. The sacrificial layer also serves to protect the MEMS device during deposition of bond ring material, which covers both a bond ring area, and the sacrificial layer. The bond ring material is formed to a desired depth in one of many ways compatible with the sacrificial layer. When the sacrificial layer is photoresist, the temperature of the process used to form the bond ring material is low enough to avoid burning the photoresist. The sacrificial layer is then etched, releasing the MEMS device.
This application is related to U.S. application entitled: “Bond Ring for Micro-electromechanical System” docket number 200308958-1, filed on the same date herewith, which is incorporated herein by reference.
