Kavlico Corporation

M Salleh Ismail

Raffi M Garabedian

Gary J Pashby

Jeffrey K Wong

Max E Nielsen

A surface micro-machined sensor uses a pedestal in a cavity to support a flexible structure and reduce the span of the flexible structure. The reduced span per sense area allows larger sensor areas without permitting forces to permanently deform the flexible structure or cause the structure to touch an opposite wall of the cavity. The flexible structure bonded to the pedestal and an elevated region surrounding the pedestal defines a cavity between the flexible membrane and a lower plane region. Active regions can be formed in the lower plane region for capacitors or transistors. A pedestal can be of various shapes including a circular, ovoid, rectangular or polygonal shape. The lower plane region can be of various shapes including a ring or donut shape, ovoid, rectangular or polygonal shape with an inner dimension corresponding to the outer dimension of the pedestal. The elevated region can be of various shapes with an inner dimension corresponding to the outer dimension of the lower plane region. Alternative embodiments of the invention include cavities containing multiple pedestals which may be concentric. Additionally, a central pedestal may be hollow to increase sensor volume. The device can be a capacitive sensor with plates in the active region and the flexible structure. The device can be a piezoresistive sensor with the flexible structure containing piezoresistive elements. The device can also be a transistor with source, drain, and channel in the active regions of the substrate and the flexible structure containing a gate.

Surface micro-machined sensor with pedestal

A jig for a fusion bonding process includes a sealable chamber having a first station for a first wafer and a second station for a second wafer. The wafers are initially separated from each other while a vacuum is created in the chamber. In one embodiment of the invention, movably mounted spacers separate the wafers while the vacuum is formed. The spacers are then moved to allow the wafers to come into contact and form an initial bond. In another embodiment, wafers in the first and second stations are tilted away from each other so that gravity keeps the wafers separated while the vacuum is formed. After the vacuum is formed, the chamber is rotated so that gravity pushes the two wafers together. In either embodiment, a mechanical pushing system or vibrational energy can apply force to induce or improve the initial bond. The initial bond seals cavities formed between the wafers. The jig can be transparent to infrared radiation or visible light to allow inspection of the initial bond and the sealing of the cavities. If the cavities are sealed, the wafers are removed from the chamber for annealing which strengthens the bond between the wafers. The cavities between the wafers inherit from the chamber a vacuum that helps to keep the wafers together during annealing.

Process for wafer bonding in a vacuum

A multiple LOCOS (local oxidation) process shapes a surface of a substrate to form a series of planar regions which are vertically separated from each other. One exemplary process forms a hard mask layer for each LOCOS operation. Another exemplary process includes forming a silicon nitride mask layer and repeatedly changing the pattern of that mask layer. Each change in the pattern corresponds to a planar region to be formed; and after each change, oxide is grown in openings through the mask layer. The growth of oxide consumes part of the substrate and provides a vertical separation between the planar level corresponding to the pattern and a next higher planar level. Regions of the substrate once exposed by a mask pattern can remain exposed so that subsequent LOCOS operations maintain previously established separations between levels. A hard mask layer can include a polysilicon layer which protect a silicon nitride layer from conversion to oxide during the repeated LOCOS operations.

Multiple local oxidation for surface micromachining

A semiconductor device has a flexible structure bonded to a semiconductor substructure to form a cavity. The flexible structure is bonded over a conducting feed-through without the feed-through interfering with a hermetic seal formed by bonding. One embodiment of the device includes depressions that contain edges of a diffused feed-through so that imperfections at the edge of the diffusion do not interfere with bonding. The flexible structure is bonded to elevated areas thus hiding the imperfections. In one embodiment, a first elevated region is surrounded by a second elevated region, and diffusion for the feed-through extends from an active region in the cavity across the first elevated region with edges of the diffusion being between the first and second elevated regions. The flexible structure can thus bond to the first and second elevated regions without interference from the edge of the diffused feed-through. A via through the flexible structure to the first elevated region provides electrical contact with the active region. Another embodiment has either a surface or buried well in a semiconductor structure and extending from an active region in the cavity to a point outside the perimeter of the flexible structure. The well provides a conductive feed-through structure without creating imperfections that would interfere with the bonding that seals the cavity.

Fusion-bond electrical feed-through

Fabrication of semiconductor devices with movable structures includes local oxidation of a wafer and oxide removal to form a depression in an elevated bonding surface. A second wafer is fusion bonded to the elevated bonding surface and shaped to form a flexible membrane. An alternative fabrication technique forms a spacer having a depression on a first wafer and active regions on a second wafer, and fusion bonds the wafers together with the depression over the active regions. Devices formed are integrable with standard MOS devices and include FETs, capacitors, and sensors with movable membranes. An FET sensor has gate and drain coupled together and a drain-source voltage which depends on the gate's deflection. Selected operating current, channel length, and channel width provide a drain-source voltage linearly related to gate deflection. Alternatively, two transistors subjected to the same gate deflection provide a differential voltage related to the square root of the deflection if channel currents or channel widths differ. Transistors subjected to the different gate deflections provide a differential signal that cancels effects that are independent of deflection. A capacitive sensor includes a doped region underlying the center of a flexible membrane. The doped region is isolated from a surrounding region which is biased at the voltage of the membrane.

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Moorpark, CA, United States of America

M Salleh Ismail