Micromechanical structures for sensing a physical quantity such as acceleration, vibration or electrostatic potential are useful in many applications, including air bag deployment and active suspension in automobiles, and guidance systems in military weapons, among others.
One method of fabricating suspended microstructures is generally termed bulk-micromachining. In bulk-micromachining, a block of material, such as silicon, for example, is subtractively etched to remove material, leaving behind the desired microstructure shape suspended from the remainder of the substrate by very thin resilient connectors. Accordingly, in bulk-micromachining, the microstructure, the supporting portion of the substrate, and the thin connectors are monolithically constructed of the same material. U.S. Pat. No. 4,711,128 (Boura) discloses one such bulk-micromachined suspended microstructure.
Another method of fabricating chips with suspended micromachined microstructures is generally termed surface-micromachining. Surface-micromachining involves additive forming of the microstructure over a substrate. For instance, a sacrificial oxide spacer layer such as silicon dioxide is deposited over the surface of a substrate of a wafer. The sacrificial spacer layer is selectively etched to open up holes in the spacer layer, down to the substrate, in which anchors for supporting the microstructure will be formed. A thin film microstructure material, such as polysilicon, is deposited over the sacrificial layer. The microstructure material fills in the holes where the sacrificial layer had been etched down to the substrate and contacts the substrate to form anchors for supporting the microstructure. Enough microstructure material is deposited to fill in completely the holes as well as to form a uniform layer over the top of the sacrificial-layer. The microstructure material is then patterned into a desired shape by photolithography. Finally the sacrificial layer is removed (i.e., sacrificed) by, for instance, wet etching, thus leaving behind a microstructure suspended above the substrate by the anchors. International patent application publication No. WO93/25915, entitled MONOLITHIC CHIP CONTAINING INTEGRATED CIRCUITRY AND SUSPENDED MICROSTRUCTURE and assigned to the same assignee as the present application, discloses in detail one such method for manufacturing a surface-micromachined suspended microstructure.
FIG. 2 is a top plan view of an exemplary suspended microstructure. The microstructure comprises a bridge 112 suspended above a substrate 114 by four corner anchors 116. The bridge comprises a central beam 118 having a plurality of fingers 120 extending transversely therefrom. A suspended polysilicon stationary finger 122 is positioned parallel and adjacent to each finger 120 of the bridge 112. Stationary fingers 122 are also suspended o anchors and cantilevered over the substrate, but are substantially stationary because of their smaller mass and shorter length of extension beyond the anchor. FIG. 1 is a side view of the bridge 112 in which the stationary fingers 122 have been removed from the view in order not to obfuscate the illustration. FIG. 1 helps illustrate the anchors 116 and the elevation of the bridge 112 above the substrate 114. The polysilicon of the stationary fingers 122 and the bridge 112 is electrically conductive. The stationary fingers are connected via conductors embedded (i.e., formed) in the substrate to comprise two electrical nodes. In particular, the stationary fingers which are to the left of the corresponding moveable fingers form a first node which is charged to a first voltage and the stationary fingers which are to the right of the corresponding moveable fingers form a second node which is charged to a second voltage. The beam 112, including the moveable fingers, is a third node that is charged to a third voltage between the first and second voltages.
The first and second sets of stationary fingers and the moveable fingers form two capacitors. The two sets of stationary fingers form the first plate of first and second capacitors, respectively, and the moveable fingers form the second plate of both of the capacitors. When the chip is subjected to a force, the beam 112 moves relative to the stationary fingers 122, thus altering the capacitance between each stationary finger 122 and its corresponding moveable finger 120. Circuitry measures the change in aggregate capacitance, which is directly indicative of the acceleration to which the bridge is subjected. Preferably, the circuitry forms a closed loop including the beam, to provide a feedback signal which re-centers the beam when it is offset from its equilibrium position by acceleration. During both fabrication and normal use after fabrication, it is possible for the suspended moveable portion of a microstructure, such as beam 112, to be subject to a force which will cause a portion of the beam 112 (e.g., one of the moveable fingers) to come in contact with another portion of the chip. For instance, a moveable finger may contact a stationary finger, if subjected to a strong lateral force, or the bottom surface of the beam 112 may come in contact with the substrate, if subjected to a strong vertical force. Contact also may occur due to electrostatic attraction or, during fabrication, due to liquid surface tension during drying after a wet etch step. Such contact is undesirable since sticking between contacting surfaces, particularly when one or both of the surfaces is polysilicon, is likely. Once a portion of the suspended microstructure becomes stuck to another portion of the unit, it is very difficult to separate the two. Accordingly, sticking typically results in failure of the sensor.
International patent application publication No. WO93/25915 discloses a method for fabricating a surface-micromachined suspended microstructure in which bumps are formed on the bottom surface of the suspended microstructure. The bumps are formed by placing small hollows in the top surface of the spacer layer over which the microstructure material is deposited during fabrication by means of standard photolithography. When the microstructure material is deposited over the sacrificial layer, the microstructure material will fill in the hollows, forming bumps of microstructure material on the bottom surface of the microstructure. When the sacrificial layer is removed, the bumps remain on the bottom surface of the microstructure and serve to minimize the area of contact, between the microstructure and the substrate. If and when the bottom surface of the microstructure comes into contact with the substrate due to electrostatic forces, vertical acceleration, or liquid surface tension, only the bumps will contact, the surface. By thereby minimizing the area of contact the likelihood of sticking is reduced. Also, by minimizing contact area, the magnitude of any electrostatic attraction is lessened, thus decreasing the likelihood of sticking even if contact occurs.
Generally, it is desirable to make the bumps as small as possible in order to minimize the contact area of the bumps and also to allow a large number of bumps to be placed throughout the bottom surface of the suspended microstructure without significantly affecting the mass or geometry of the structure. The minimum bump size which can be reliably achieved using standard photolithographic procedures, as discussed in international patent application publication WO93/25915, is limited by the process in use at the foundry where the chip is fabricated. Presently, a typical fabrication process line can achieve a minimum bump size of about 1 micron diameter.
Today, typical suspended microstructure geometries employ a minimum dimension of around a micron or larger. Accordingly, such chips are commonly fabricated in foundries with minimum size capabilities of 1 micron or larger. While there are foundries which can achieve higher resolution, e.g., minimum sizes of half a micron or possibly less, creating such a foundry is a significant expense and typically is not justifiable solely for the purpose of producing smaller bump sizes, if a less fine process is adequate in all other respects for the chip being produced.
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