Hermetic microdevice packages enjoy wide use in the semiconductor industry in applications where true hermeticity is required. Such hermeticity is required where any outside of the package ambient conditions or variations in the ambient conditions might affect device performance. Generally in the semiconductor industry hermetic passivation layers have been developed and applied to the surface of sensitive devices in order to give a primary level of defense against ambient conditions. Some of these layers are adequately hermetic. In other instances a hermetic package is required. Such hermetic packages are in common use for the sealing of semiconductor devices and other microdevices including the aformentioned MicroElectroMechanical (MEMS) devices.
Such hermetic packages consist of a package base commonly with electrical feedthrus insulated from such base for the purpose of extracting electrical signals from the active device inside the package. The sensitive active device is mounted on the package base and microwire bonds are made to connect the device output pads to the package base electrical feedthrus. Finally a cover or cap is attached to the base via a hermetic bonding technique which varies according the package material and its preparation. For convenience and high thruput all bonding techniques require some degree of heat application to insure a hermetic bond of cap to base. The bonding is required to be executed in the rarified atmosphere or vacuum that is required in the package after bonding.
The two most common cap to base bonding techniques utilized historically, and currently, involve cap welding or solder sealing. Cap welding is accomplished by passing a high weld current through a tip (often a small roller) which precesses around the rim of the cap/package assembly as it locally melts two metal members together. The solder sealing technique utilizes a solder preform (commonly gold/tin eutectic solder) placed between a gold plated cap and base followed by the application of a heated ring at nominally 320° C. to melt the solder and effect the hermetic seal. Both of these techniques result in a considerable amount of heat transmitted through the package base and into the active device. Although there are methods of reducing the amount of heat transfer to the active device it is not possible to eliminate the device heating altogether.
In addition to the active device heating described above heat generated at the sealing surfaces releases contaminants to the inside of the package which can affect the performance of unpassivated devices. This can be especially destructive to unpassivated MEMS devices where micro mechanical moving parts are fully exposed to released contaminants due to the sealing heat cycle.
MicroElectroMechanical devices which exhibit free standing micro mechanical structures have been hermetically packaged using both cap welding and solder sealing technology. However due to residual stress in free standing members and the extreme sensitivity of structure surfaces to contamination more complex MEMS devices cannot tolerate heat during the packaging operation. For such devices a room temperature package sealing process would be of great benefit. Room temperature hermetic sealing has been utilized in Ultra High Vacuum (UHV) technology for a number of years and is pervasive in the art of that technology.
U.S. Pat. No. 3,208,758, Carlson and Wheeler, describes a vacuum seal technique suitable for high temperature baking after a room temperature seal has been implemented. The patent is focused on large flanges used in UHV vacuum system assembly. A copper gasket seal is described wherein two mating vacuum parts structured with vertical and sloping cutting edges are swaged into the copper gasket to effect a vacuum seal. The high force required for the deformation of the copper is achieved by tightening a series of bolts and nuts around the periphery of a flange. A preferred shape of the cutting edge is disclosed although the force required to effect a vacuum tight seal is not disclosed. The assembly including the copper gasket and cutting edge shape has come to be known as a “conflat” type vacuum fitting and is in wide use in the vacuum equipment industry. It has not been applied to microdevice packaging.
Additional embodiments of the basic “conflat” sealing technique can be found in U.S. Pat. No. 3,217,992, Glasgow, and in U.S. Pat. No. 3,368,818, Asamaki, et. al. both describing alternative bolting attachment geometries to effect the metal seal. Neither patents address the possibility of applying the technique to seal MEMS or microdevice packages.
With recent rapid advances in MEMS technology leading to more sophisticated devices there has evolved a concerted effort to develop suitable packaging technology. The focus has been on both single MEMS die packaging and packaging at the wafer level. MEMS devices packaged at the wafer level is particularly attractive due to the unique fabrication technology involved. Virtually all MEMS devices end up as micro mechanical elements suspended in space. Thus during the fabrication process they must be supported by a sacrificial material usually through several levels of processing until the end of the fabrication sequence. At the end of the process the sacrificial material is removed leaving the micro mechanical members preserved in their design space. Clearly it is desirable from a cost point of view to remove (called release) the temporary support on a whole wafer rather than individual tiny die. However once release is performed the MEMS devices cannot be singulated without the individual mechanical parts being damaged or becoming stuck together (called stiction). The solution then to performing release at the wafer scale is to also package hermetically at the wafer scale prior to die singulation.
Recent development work in MEMS packaging at the wafer scale has focused on bonding directly to the silicon substrate that was used as the MEMS substrate. This includes anodic/fusion bonding using high electric fields, eutectic bonding using heating to form a eutectic bond between gold or aluminum to silicon and thermocompression bonding. The novel application of heat has been explored by using a polysilicon resistance heater element embedded directly into the MEMS devices. Such recent work has not included attempts to use the compression swaging technique disclosed herein.
U.S. Pat. No. 6,379,988 B1, Petersen and Conley describe a pre release plastic packaging of MEMS devices wherein the device is encapsulated in a plastic package prior to release. The plastic package can be perforated to allow release in the package using wet or dry etching processes. In a final step a cover lid is attached to the plastic package by various means common in prior art.
U.S. Pat. No. 6,400,009 B1, Bishop, et. al., discloses a MEMS package and bonding means employing a firewall to form a protective cavity for the MEMS device during heat sealing of top and bottom members of the package. Electrical feedthrus that penetrate the firewall are disclosed and may be made of polysilicon conductive material encapsulated with silicon dioxide. All structures disclosed are fabricated concurrently with the MEMS device. An integral plurality of solder bumps is utilized and claimed as a means of strengthening the solder bonded parts. The sealing means described is by heated solder sealing.
U.S. Pat. No. 6,627,814B1, David H. Stark, discloses a package with a continuous sidewall with a top surface prepared for solder sealing. A transparent window forms a top cover. The window is prepared with an outer metallic frame suitable for soldering to the base. The solder method requires the application of heat above the melting temperature of the solder.
U.S. Pat. No. 6,639,313 B1, Martin and Hamey, discloses a ceramic package with a recess for holding an optical MEMS mirror device. A glass window cover is disclosed which is heat solder sealed to the ceramic substrate by means of a flexible, folded metal interposer disposed peripherally around the edge of the glass window and ceramic base. Uniquely the folded metal interposer allows the difference in expansion and contraction between the window and the ceramic to be mitigated during heat cycling. Hermeticity is achieved by heat soldering.
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