Process for fabricating micromechanical devices

Details Process for fabricating micromechanical devices Process for fabricating micromechanical devices

2000-04-07

2001-10-09

Bowers, Charles (Department: 2813)

Semiconductor device manufacturing: process

Making device or circuit responsive to nonelectrical signal

Physical stress responsive

C438S415000, C438S723000, C438S745000, C438S747000, C438S924000

Reexamination Certificate

active

06300156

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to processes for fabricating micromechanical devices, and more particularly, micro-electro-mechanical systems (MEMS) that are components of optical communication systems.
DESCRIPTION OF THE RELATED ART
MEMS devices are miniature mechanical devices that are controlled by electrical or optical signals. Such devices have dimensions that are typically less than one millimeter, with some dimensions being on the order of one micron or less. In order to fabricate mechanical devices on this size scale, microfabrication techniques have been developed.
Microfabrication techniques for MEMS devices utilize the lithographic techniques used to fabricate semiconductor integrated circuits. In integrated circuit fabrication, a layer or layers of material are deposited on a substrate (typically a semiconductor substrate with one or more layers of material formed thereon). Lithography is then used to pattern the layer or layers into the desired shape. In lithography, a pattern is defined in an energy sensitive material that is formed over the layer or layers to be patterned. An image of the pattern is then delineated in the energy sensitive material. The image is delineated using techniques such as projection lithography (i.e. projecting radiation through a mask that patterns the radiation and transfers an image of the mask into the energy-sensitive material) or direct write lithography (a beam or radiation is used to write the pattern into the energy-sensitive resist material). After delineation, the image is developed into a pattern, and the pattern is transferred into the underlying layer using expedients such as plasma etching or reactive ion etching (RIE).
In MEMS fabrication, a plurality of polycrystalline silicon (polysilicon) layers are deposited on a single crystal substrate (e.g. a silicon substrate). The structure also has a layer or layers of silicon oxide or silicon nitride incorporated therein. The materials are selected because certain etch expedients are selective for one or the other of these materials. That is, expedients that quickly etch polysilicon, etch silicon oxide or silicon nitride more slowly and vice-versa. This etch selectivity is exploited to selectively remove certain portions of one layer of material while a portion of another layer remains intact.
In MEMS fabrication, the three-dimensional structures are frequently defined and delineated using a technique known a surface-micromachining. The technique is described in Pister et al., “Microfabricated Hinges,”
Sensors and Actuators
, Vol. A33, pp. 249-256 (1992). In surface-micromachining, a member is delineated and defined in a layer or layers of material formed on a substrate. The member is in hinged connection with a support layer. The member is then released from the substrate by removing a sacrificial layer of silicon oxide that tethers the member to the substrate. Since the member is in hinged connection with the support layer, it is then capable of being pivoted out of the plane of the support layer after release. Thus, such members, upon release, are pivoted out of the plane of the layer in which they are fabricated to assemble three-dimensional structures. With surface-micromachining of hinged members, higher resolution (i.e. a more precise delineation and definition) of the members that form the three-dimensional structure is obtained than if the three dimensional structure was fabricated using a three-dimensional fabrication process. The higher resolution derives from the fact that high vertical resolution (i.e. resolution in the direction normal to the substrate surface) is more difficult to obtain than planar resolution (i.e. resolution in the plane of the layer). Surface-micromachining of hinged members combines the advantage of high planar resolution of the members with the ability to assemble three-dimensional structures from the members after they are released from the substrate.
A self-assembling micro-mechanical device formed by surface micromachining is described in U.S. Pat. No. 5,994,159 to Aksyuk et al. The device has a hinged plate and a beam having one end that is moveable, upon actuation, in an upwardly directed arc away from a support surface. The beam and hinged plate are defined and delineated to lie flat on the surface of a supporting substrate. The hinged plate is released from the substrate surface in the previously described manner. Upon release, it is desired for the plate to pivot out of the plane of the substrate and into a plane orthogonal to the substrate.
One problem with manufacturing such MEMS devices is that, occasionally, a plate does not release (i.e. pivot away from the substrate) after the sacrificial underlying layer of silicon dioxide has been removed. If the plates fail to pivot, then the device will not operate in its intended manner. Therefore a process for fabricating MEMS devices that ensures the release of hinged components when the underlying layer is removed is sought.
SUMMARY OF THE INVENTION
The present invention is directed to a process for fabricating a MEMS device. In the process, at least one hinged member is lithographically defined in a top silicon layer of a silicon on insulator (SOI) semiconductor substrate. The insulator is a silicon compound such as silicon dioxide or silicon nitride. The substrate is then placed face down and immersed in an etchant that etches the insulator at a much faster rate than it etches the top silicon layer. Aqueous hydrofluoric acid is one example of a suitable etchant. The substrate is placed face down to ensure that the hinged member pivot away from the substrate in the intended manner when the insulator is etched away from the hinged member. When the substrate is free to pivot away from the substrate, the hinged member is referred to herein as having been released. It is advantageous if agitation is applied to the etchant in which the hinged member is placed.
As a result of being placed face down in the etchant, after the portion of the insulator layer underlying the hinged member is removed, the hinged member pivots away from the substrate to the desired degree. The hinged member is then rinsed in a solvent and dried while face down.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and do not serve to limit the invention, for which reference should be made to the appended claims.


REFERENCES:
patent: 4285892 (1981-08-01), Betsuda et al.
patent: 4871418 (1989-10-01), Wittlinger et al.
patent: 5178725 (1993-01-01), Takeno et al.
patent: 5994159 (1999-11-01), Aksyuk et al.
Pister et al.,Sensors and Actuators, “Microfabricated Hinges”, vol. A33, pp. 249-256, 1992.
Xiao et al.,Journal of Micromechanics and Microengineering, “A New Release Process for Polysilicon Surface Micromachining Using Sacrifice”, pp. 300-304, 1999.

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