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  Method of fabricating a vacuum sealed microdevice package with gettersRelated Patent Categories: Semiconductor Device Manufacturing: Process, Packaging (e.g., With Mounting, Encapsulating, Etc.) Or Treatment Of Packaged Semiconductor, Including Contaminant Removal Or MitigationMethod of fabricating a vacuum sealed microdevice package with getters description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20060148132, Method of fabricating a vacuum sealed microdevice package with getters. Brief Patent Description - Full Patent Description - Patent Application Claims
CROSS REFERENCE TO RELATED APPLICATIONS [0001] This application is a divisional of application Ser. No. 10/797,322 filed Mar. 9, 2004 for Vacuum Sealed Microdevice Packaging With Getters. BACKGROUND OF THE INVENTION [0002] 1. FIELD OF THE INVENTION [0003] Various embodiments of the invention pertain to sealed chip packaging. More particularly, one embodiment of the invention pertains to a system, device, and method for incorporating getters into hermetically sealed silicon microdevices, in particular silicon gyroscope chips. [0004] 2. DESCRIPTION OF RELATED ART [0005] Certain silicon microdevices, such as vibratory gyroscopic rate sensors, typically require very low gas damping (high Q) to operate. This is often accomplished by operating the device in a vacuum, reducing gas damping to a low level. At the present, devices required to be operated in vacuum are typically mounted in packages that can then be sealed in vacuum. It is also important to maintain that same low pressure over the operating period of the gyroscope, since a change in pressure over time can cause performance errors. Silicon microdevices can be made and sealed hermetically. This is usually accomplished at the wafer level by such methods as silicon direct bonding, anodic bonding, metal sealing, glass frit bonding, or polymeric adhesion. In most applications for wafer level packaging or encapsulation, microdevices are sealed at atmosphere pressure. Although microdevice cavities can be sealed in this fashion in vacuum, the vacuum level quickly degrades due to outgassing, permeation, and/or virtual leaks. [0006] Incorporation of one or more active getters within the cavities of a microdevice chip serves to stabilize the vacuum level, as gasses are constantly sorbed to create a steady state pressure. To incorporate a gettering capacity within a cavity, a getter must be activated or fired (depending on the type) at high temperatures, usually at or higher than about 400.degree. C. This must be done in vacuum or an inert gas (e.g., helium, neon, argon, etc.) and in situ immediately prior to sealing, or the getter loses its capacity due to re-sorption of active gasses (hydrogen, oxygen, nitrogen, etc.). Thus, to incorporate getters into cavities defined by bonding silicon wafers together, the getters must be positioned in the cavities before the bonding process. They must then be activated within a bonding chamber that includes vacuum and a bonding mechanization to bring wafers together to form the bond. The bonding process can include parameters such as pressure, heat, electrical current, or any other parameter that would normally be used to bond wafers with the selected technique. [0007] An additional requirement for fabrication of certain microdevices, such as a silicon gyroscope chips, is that the wafers be precisely aligned, to within several microns or less, at the time of bonding. In effect, this requires that getters be inserted into a first wafer or wafer assembly prior to alignment of a second wafer or wafer assembly so as to effect and maintain the required level of alignment precision throughout the bonding process. [0008] Typically, fabrication of such microdevices also requires the stable positioning and bonding of the getter itself to the interior of the cavity. Securing the getters to the interior cavity prevents the getters from moving within the cavity during operation and interfering with device motion or electrical integrity. [0009] Another common requirement for fabrication of such microdevices is the application of a high vacuum, with pumping, during the getter activation time and immediately thereafter until the wafers are brought together and sealed. SUMMARY OF THE INVENTION [0010] One embodiment of the invention relates to a microdevice package containing getters for maintaining a constant vacuum level within the sealed microdevice package. A stacked wafer assembly, containing a plurality of microdevice packages, is formed by aligning a bottom cover wafer with a center wafer. The bottom cover wafer includes one or more bond pads to receive one or more getters. The center wafer includes one or more vias substantially aligned and corresponding to the one or more bond pads. One or more getters are inserted into the one or more vias. The stacked wafer assembly is completed by aligning a top cover wafer opposite the bottom cover wafer to sandwich the center wafer in between. [0011] A constant vacuum level is maintained inside the microdevice packages by aligning the wafers, activating the getters, and sealing the microdevice packages in a given sequence. The bottom cover wafer and the center wafer are aligned. Getters are then inserted in the vias of the center wafer. In one alternative implementation, the bottom cover wafer is bonded to the center wafer prior to inserting the one or more getters into the one or more vias. The bottom cover wafer and the top cover wafer are then aligned. [0012] The wafer stack assembly is placed in a vacuum chamber for bonding and getter activation. The wafer stack assembly is heated in a vacuum to a temperature sufficient to re-flow solder on the one or more bond pads. The wafer stack assembly is then cooled to solidify the solder and secure the one or more getters to their corresponding bond pads. The vacuum chamber is then purge of air and contaminants and, optionally, a non-gettable gas is inserted into the vacuum chamber. The getters are then heated to an activation temperature that is less than the solder re-flow temperature. The vacuum chamber is pumped to achieve a desired vacuum level. The bottom cover wafer, center wafer, and top cover wafer are then bonded to form a sealed package having a substantially constant vacuum therein. BRIEF DESCRIPTION OF THE DRAWINGS [0013] FIG. 1 illustrates a wafer assembly in which a center wafer is sandwiched between a top cover wafer and a bottom cover wafer according to one embodiment of the invention [0014] FIG. 2 illustrates a microdevice chip that may be formed in a stacked wafer assembly according to one embodiment of the invention. [0015] FIG. 3 illustrates a corner portion of a microdevice chip bottom cover layer according to one embodiment of the invention. [0016] FIG. 4 illustrates a corner portion of a microdevice chip center layer according to one embodiment of the invention. [0017] FIG. 5 illustrates a method for inserting getters into hermetically sealed silicon microdevices to maintain the microdevice at a substantially constant vacuum level according to one embodiment of the invention. DETAILED DESCRIPTION [0018] Methods and systems that implement the embodiments of the various features of the invention will now be described with reference to the drawings. The drawings and the associated descriptions are provided to illustrate embodiments of the invention and not to limit the scope of the invention. Reference in the specification to "one embodiment" or "an embodiment" is intended to indicate that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least an embodiment of the invention. The appearances of the phrase "in one embodiment" or "an embodiment" in various places in the specification are not necessarily all referring to the same embodiment. Throughout the drawings, reference numbers are re-used to indicate correspondence between referenced elements. In addition, the first digit of each reference number indicates the figure in which the element first appears. [0019] In the following description, certain terminology is used to describe certain features of one or more embodiments of the invention. For instance, the term "microdevice" includes any electronic, electromechanical, mechanical, and/or storage device that may be mounted within a chip package. Continue reading about Method of fabricating a vacuum sealed microdevice package with getters... 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