Microelectromechanical timer

Details Microelectromechanical timer Microelectromechanical timer

1998-07-23

2001-10-23

Roskoski, Bernard (Department: 2859)

Horology: time measuring systems or devices

Display trains

Reexamination Certificate

active

06307815

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to microelectromechanical (MEM) devices, and in particular to a microelectromechanical timer having an optical readout.
BACKGROUND OF THE INVENTION
Polysilicon surface micromachining adapts planar fabrication process steps known to the integrated circuit (IC) industry to manufacture microelectromechanical or micromechanical devices. The standard building-block processes for polysilicon surface micromachining are deposition and photolithographically patterning of alternate layers of low-stress polycrystalline silicon (also termed polysilicon) and a sacrificial material (e.g. silicon dioxide). Vias etched through the sacrificial layers at predetermined locations provide anchor points to a substrate and mechanical and electrical interconnections between the polysilicon layers. Functional elements of the device are built up layer by layer using a series of deposition and patterning process steps. After the device structure is completed, it can be released for movement by removing the silicon dioxide layers using a selective etchant such as hydrofluoric acid (HF) which does not attack the polysilicon layers.
The result is a construction system generally consisting of a first layer of polysilicon which provides electrical interconnections and/or a voltage reference plane, and up to three or more additional layers of mechanical polysilicon which can be used to form functional elements ranging from simple cantilevered beams to complex systems such as an electrostatic motor connected to a plurality of gears. Typical in-plane lateral dimensions of the functional elements can range from one micron to several hundred microns, while the layer thicknesses are typically about 1-2 microns. Because the entire process is based on standard IC fabrication technology, a large number of fully assembled devices can be batch-fabricated on a silicon substrate without any need for piece-part assembly.
The present invention relates to a microelectromechanical (MEM) timer formed from silicon micromachining technology using MEM electrostatic motors of the type disclosed by Garcia et al in U.S. Pat. No. 5,631,514 which is incorporated herein by reference. In the present invention, a first MEM electrostatic motor is used to intermittently wind a mainspring of the MEM timer. The MEM timer drives a set of meshed timing gears that are encoded so that timing information that can be optically read out. The present invention can also include a second electrostatic motor for starting and stopping the MEM timer.
An advantage of the present invention is that a compact and rugged MEM timer can be formed which, once activated, provides accurate timing information through an optical readout and retains the timing information even though electrical power to the device may be temporarily interrupted.
Another advantage of the present invention is that the MEM timer can provide millisecond timing resolution, and a running time of up to an hour or longer depending upon the number of timing gears provided in a mechanically-driven gear train and how often the mainspring is rewound.
Yet another advantage of the present invention is that the MEM timer provides an optical readout of timing information that can be accessed optically by a plurality of light beams, including light-emitting-diode (LED) or laser beams, transmitted through free space or optical fibers.
Still another advantage of the present invention is that preferred embodiments of the MEM timer can be fabricated without the need for piece part assembly.
These and other advantages of the method of the present invention will become evident to those skilled in the art.
SUMMARY OF THE INVENTION
The present invention relates to a microelectromechanical (MEM) timing apparatus (i.e. a MEM timer) formed on a silicon substrate by surface micromachining processes. The MEM timer includes a main gear formed on the substrate; and a coiled mainspring operatively connected to the main gear to supply mechanical power thereto. A plurality of meshed timing gears is formed on the substrate, and driven by mechanical coupling to the main gear. Rotation (i.e. rotary motion) of each of the meshed timing gears is controlled by a verge escapement mechanism operatively connected to one of the timing gears (e.g. a last-driving timing gear). An optical readout is provided for recovering timing information from the rotary motion of one or more of the timing gears. The mainspring, main gear, and timing gears can all be formed, for example, from deposited and patterned polycrystalline silicon.
The present invention preferably further includes a MEM electrostatic motor for winding the mainspring. The electrostatic motor can be mechanically coupled to the mainspring by a reduction gear train, and by a ring gear attached to one end of the mainspring. Idler gears can be provided for lateral constraint of the ring gear, thereby allowing the ring gear to be formed as an annulus. Additionally, one or more counter-rotation pawls can be provided to limit rotation of the ring gear to single direction as required for winding of the mainspring.
A start/stop switch is also preferably provided for starting and/or stopping operation of the MEM timer. The start/stop switch can be formed by providing a second MEM electrostatic motor that operates to move a catch into or out of engagement with a verge (i.e. the verge escapement mechanism) to stop or enable motion of the timing gears, respectively.
Timing information can be optically read out of the MEM timer by providing an optical encoder (e.g. a binary or gray-scale optical encoder) on each timing gear (e.g. on an upper surface of each timing gear) for determining the rotary position of each timing gear over time. The optical encoder can comprise, for example, a plurality of annular trenches or slots formed into each timing gear. Read out of the timing information from the MEM timer can be accomplished using one or more light beams incident on each timing gear containing an optical encoder so that the light beams are either transmitted through each timing gear (e.g. transmitted through optical encoder slots formed through the timing gears), or alternately reflected or scattered from each timing gear (e.g. reflection or scattering of light from annular trenches formed in each timing gear). The transmitted, reflected or scattered light becomes encoded with timing information that can then be recovered by detecting the light to generate an electrical signal containing the timing information. Each light beam can be, for example, a laser beam or a beam from a light-emitting-diode (LED). The incident light beams and detected light can be coupled into and out from the MEM timer, respectively, by free-space or optical fiber coupling.
Additional advantages and novel features of the invention will become apparent to those skilled in the art upon examination of the following detailed description thereof when considered in conjunction with the accompanying drawings. The advantages of the invention can be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.


REFERENCES:
patent: 4420263 (1983-12-01), Besson et al.
patent: 4645357 (1987-02-01), Algaier et al.
patent: 5231612 (1993-07-01), Allgaier et al.
patent: 5270993 (1993-12-01), Besson et al.
patent: 5378583 (1995-01-01), Guckel
patent: 5566140 (1996-10-01), Kohata et al.
patent: 5631514 (1997-05-01), Garcia
patent: 5930205 (1999-07-01), Baba et al.
U.S. application No. 08/874,815, Garcia, filed Jun. 13, 1997.

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