The present invention is directed to the field of electronic devices, such as piezoelectric devices. The term “piezoelectric device” includes resonators, filters and surface acoustic wave devices. More particularly, the present invention is directed to a hermetic silicon package for such devices and a method of making a surface mountable electronic component utilizing such a package.
For over 50 years, the assignee of this patent application has provided piezoelectric devices such as quartz resonators, packaged in a glass housing, for use in aerospace applications. For this particular application, it is important that the piezoelectric device be housed in a vacuum chamber to prevent contamination of the quartz crystal since contaminants can cause deterioration of the accuracy of the resonator. While this deterioration is slow and typically involves changes in accuracy in the parts per billion per day range, for aerospace applications where precision accuracy is essential and the resonator needs to operate for 10-50 years on some exploratory missions, any deterioration is detrimental to the function of the resonator and, hence, to the space craft.
With the advent of solid state electronic circuitry, the number of companies manufacturing glass tubes or “bubbles” (of the type previously used for electronic vacuum tubes) has continued to dwindle, commensurate with the demand for their products. For some applications, the glass bubble can be replaced by a copper housing. However, for aerospace environment where the piezoelectric device may be exposed to temperatures exceeding 300° C., copper packages are subject to collapse as the metal reaches a softening point and the internal vacuum may draw contaminants into the cavity. This permits the piezoelectric device housed therein to be subject to whatever contaminants may be contained in the ambient environment, leading to degradation of its properties.
Within the past fifteen years, Sandia National Laboratory has performed some work on a silicon package which may be useful in housing piezoelectric elements for certain applications. This silicon package is taught in U.S. Pat. Nos. 5,198,716 and 5,339,051. Although these patents produce some of the benefits associated with silicon for housing an electronic component, the package taught therein is wholly inadequate for use in aerospace and related applications. A low temperature bonding technique is employed. When subjected to the rigorous environment of outer space, the bond will melt (fail) leading to unacceptable degradation of the performance of the electronic package. Further, the proposed device involves silicon-quartz-silicon bonding, a difficult proposition at best, given the dissimilarity of the materials, made all the more risky by doubling the extent of circumferential area subject to failure by doubling the number of bonds needed. For many applications, this bond failure may not significantly impact the performance of the housed components but, as has been noted, in the demanding environment of aerospace, even the slightest degradation must be resisted as intolerable.
It is, therefore, the intent of this invention to provide a silicon package which can replace the glass bubble and provide the electronic components housed therein a hermetic environment which will perpetuate initial performance, or as close to initial performance as possible, for the duration of the life of the component. Another aspect of the invention is a method of manufacturing a surface mountable electronic component which is capable of such enduring peak performance.
A hermetically packaged electronic device comprising a) an enclosure made of a plurality of elements each made of a material selected from a group consisting of silicon, titanium, zirconia, silicon carbide, sapphire, and tantalum, including a base element and a cover element, the cover element having contoured edges; b) means bonding the plurality of elements together; c) an Application Specific Integrated Circuit (ASIC) oscillator embedded in the enclosure; d) a piezoelectric device secured within the enclosure; e) a metallic layer bonded to an upper surface of the lower element, the contoured edges of the cover element exposing end portions of the metallic layer to permit easy connection to electronic circuitry outside the enclosure. Preferably the material is silicon. The vacated enclosure may be filled with a gas selected from a group consisting of hydrogen, nitrogen, helium, and other inert gases.
The plurality of elements forming the enclosure is secured together by thermo-compression bonding means performed at high temperature. The base member and the cover provide means of electrical contact to the piezoelectric device obviating need for leads and wires within and protruding through a wall portion of the package. Preferably, the cover is contoured to leave portions of the heating element exposed so that electrical connection can be made thereto. The base member and the cover are each comprised of single metallic silicon crystals. The heating element comprises a non-venting, electrically conductive element bonded to an upper surface of the base member which is preferably secured to the base member by the same bonding means used to secure the plurality of enclosure elements together. The heating element is made from a material selected from the group consisting of platinum and gold, most preferably platinum so that the same element may also function as a temperature sensing means. The thermo-compression bonding means is performed in a temperature range of between 300° C. and 800° C.
Various other features, advantages and characteristics of the present invention will become apparent to one of ordinary skill in the art after a reading of the following specification.