Method to reduce release time of micromachined devices

Details Method to reduce release time of micromachined devices Method to reduce release time of micromachined devices

2001-04-18

2003-02-25

Abraham, Fetsum (Department: 2826)

Active solid-state devices (e.g., transistors, solid-state diode

Field effect device

Responsive to non-optical, non-electrical signal

C438S050000

Reexamination Certificate

active

06525352

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to micromachined devices. More particularly, the present invention relates to a method to reduce the release time of micromachined devices.
Common processing techniques that are used to sculpt micromechanical structures include bulk micromachining, surface micromachining, and high-aspect-ratio micromachining. Bulk micromachining is a technique applied to a variety of etching procedures that selectively remove material, typically with a chemical etchant whose etching properties are selected depending on the crystallographic structure of the bulk material. High-aspect-ratio micromachining is a technique allowing the fabrication of thick (usually greater than hundreds of microns and up to millimeters thick), precision, high-aspect-ratio microelectromechanical systems (MEMS) structures, i.e. structures with near-vertical sides.
In surface micromachining (SMM), alternating layers of structural, usually polysilicon, and sacrificial material, usually silicon dioxide, are deposited and etched to form the shape required. Surface micromachining enables the fabrication of free-form, complex and multicomponent integrated electromechanical structures, giving freedom to fabricate devices and systems without constraints on materials, geometry, assembly, and interconnections. That is the source for the richness and depth of MEMS applications that cut across so many areas.
Micromachined devices are constructed of structural and sacrificial layers. To make such devices operable, the sacrificial layers are removed. This removal step is referred to as a “release.” The release of microdevices typically involves a chemical reaction. In surface micromachining and in high-aspect-ratio silicon-on-insulator (SOI) technologies, the sacrificial layer is generally silicon dioxide and the release involves use of hydrofluoric acid (HF). Long exposure to HF can result in problems to the structural layers. For example, it has been reported that HF attacks the grain boundaries of polycrystalline silicon, making this structural material weaker.
The techniques disclosed in the prior art directed towards improving on the release time present many drawbacks. Indeed, one prior-art technique involves the introduction of etch holes into the structural layers. These holes allow the etchant to undercut the structural layers more readily and hence reduce etch time. However, the introduction of etch holes into the structural layers reduces the surface area that can be available in applications such as the fabrication of a micromirror. The holes interfere with the optical surface, thus posing problem in some applications.
Another prior-art approach to reduce release time involves the use of highly doped sacrificial oxide layers. The doped oxide etches faster in hydrofluoric acid (HF) than undoped oxide and hence reduces the required release time to dissolve the sacrificial oxide film. However, in surface micromachining, the dopants in the sacrificial layer have a tendency to migrate into the structural layers during high-temperature processing steps. As a result, the dopant distribution in the structural layers is non-uniform causing an undesirable stress gradient leading to a warping of the mechanical structure. In silicon-on-insulator (SOI) technology a doped oxide film cannot be used as a sacrificial material because this technology requires bonding of two structural substrates. This bonding is best accomplished with undoped thermal oxide film having the required characteristics of being smoother and more uniform.
Therefore, it would be highly desirable to make the release step of as a short duration as possible without interfering with the chemical and physical properties of the structural layers.
SUMMARY OF THE INVENTION
The present invention solves the above and other problems in the prior art by reducing the duration of the etch process without sacrificing the electromechanical properties of the structural layers in the micromachined devices. According to the present invention a method for reducing the etch duration is disclosed. In the present invention, reducing the etch time involves the introduction of a fast etch path within slower-etching higher-quality sacrificial layers. Indeed, by burying the fast etch channel in the sacrificial layer the impact of this channel on the structural layers in the microdevice is negligible. Therefore, the sacrificial and structural layer can be engineered for optimal electromechanical performance without consideration for the release time.
In surface micromachining and silicon-on-insulator technologies, a fast etch path is, preferably, introduced by depositing the sacrificial layers in three sequential steps. The first step, preferably, involves the deposition of an undoped oxide film, such as but not limited to, tetraethylorthosilicate (TEOS), thermal oxide, low-temperature oxide (LTO), and plasma-enhanced chemical-vapor-deposition (PECVD) oxide. The second step, preferably, involves the deposition of a thin doped oxide layer, such as but not limited to, phosphosilicate glass (PSG), borosilicate glass (BSG), and borophosphosilicate glass (BPSG). Preferably, the thickness of this layer is only slightly higher than the size of the etchant molecule. Preferably, the etchant molecule is hydrofluoric acid (HF). Preferably, the third step involves a second deposition of the undoped film. Preferably, this undoped film is an oxide film. This undoped oxide film can either be selected from the same material of the first undoped film or from another oxide.
The undoped films essentially cap the doped layer and thus constrain the dopants to the center of the sacrificial sandwich. Therefore, the dopants are prevented from interacting with the structural layers, yet they provide a fast etch channel.
During the release step, the etchant quickly dissolves the center of the sacrificial film composed of the thin doped oxide layer, then proceeds to dissolve the rest of the sacrificial film. Hence, this method significantly reduces the over etch release time compared to a single undoped sacrificial layer, which is a slower-etching material.
The obtained micromachined device can be used as part of a mechanical or optical system. For instance, it can be incorporated as a part of a wavelength router.
The present invention not only provides a method for making a micromachined device, but further provides a method for reducing the release time of micromachined devices by using a novel technique for allowing a fast etching of the sacrificed without altering the structural layer of the micromachined device. Other features, objects and advantages of the invention will become apparent from the following description when read in conjunction with the accompanying drawings.


REFERENCES:
patent: 6276205 (2001-08-01), McNie et al.
Ashruf, C.M.A. et al. “Galvanic porous silicon formation without external contacts”, Sensors and Actuators 74, Jun. 1999, pp. 118-122.
Torcheux, L. et al. “Electrochemical Coupling Effects on the Corrosion of Silicon Samples in HF Solutions,” J. Electrochem. Soc., vol. 142, No. 6, Jun. 1995, pp. 2037-2046.
Keller, Christopher Guild, “Microfabricated Silicon High Aspect Ratio Flexures for In-Plane Motion,” dissertation submitted in the graduate division of the University of California, Berkeley, Fall 1998.
Muller, Lilac, “Gimballed Electrostatic Microactuators with Embedded Interconnects,” dissertation submitted in the graduate division of the University of California, Berkeley, Springs 2000.

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