Method of fabricating anti-stiction micromachined structures

Details Method of fabricating anti-stiction micromachined structures Method of fabricating anti-stiction micromachined structures

2001-10-15

2004-11-09

Vinh, Lan (Department: 1765)

Semiconductor device manufacturing: process

Chemical etching

Vapor phase etching

C438S710000, C438S712000, C438S745000, C216S002000

Reexamination Certificate

active

06815361

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of fabricating micro-electromechanical system (MEMS) structures and, more particularly, to a microstructure fabricating method for preventing stiction between a micromachined structure and a substrate or adjacent structures after an etching process for releasing the micromachined structure.
2. Description of the Related Art
As shown in
FIG. 1
, an MEMS structure commonly referred to as a “microstructure” is fabricated by wet etching for releasing a portion of a microstructure
106
from a substrate
100
and creating a “suspended” microstructure having a space or gap
110
between a released portion
104
and the substrate
100
and one or more posts
102
attached to the substrate
100
. The microstructure
106
including the released portion
104
takes the form of a beam or plate having top and A; bottom surfaces which are suspended to be substantially parallel with the surface of the substrate
100
. Examples of a device having the suspended microstructure
106
may include accelerometers, pressure sensors, flow sensors, transducers, and microactuators. The term microstructure in this specification collectively refers to a structure fabricated by an MEMS fabrication technology including but not limited to photolithography, thin film deposition, bulk micromachining, surface micromachining, and etching.
A release etching method has two types of processes: a process to create a cavity in the substrate
100
(“bulk micromachining”) and a process to remove a sacrificial layer at an intermediate portion of the microstructure
106
and the substrate
100
(“surface micromachining”). In both micromachining processes, the released portion
104
of the microstructure
106
often permanently sticks to the substrate
100
or adjacent structures during post-etch cleaning and drying steps as shown in
108
of FIG.
1
.
More specifically, in the surface micromachining technique for creating the released portion
104
of the microstructure
106
, a sacrificial layer is removed by common wet etching. In this case, the substrate
100
is exposed to a chemical etching solution for decomposing only the sacrificial layer not affecting a material of the microstructure
106
and then cleaned with a cleaning solution. When removing the cleaning solution, the surface tension of the solution affects the released portion
104
of the suspended microstructure
106
to adhere the bottom surface of the released portion
104
to the substrate
100
or adjacent other structures as shown in
108
of FIG.
1
. The adhesion is called “stiction.” This stiction degrades the sensitivity of a sensor. Furthermore, severe stiction results in failure in device fabrication and consequently drops a micromachining process yield rate.
A variety of techniques have been developed to avoid stiction. One technique is to reduce the contact area between a released portion and a substrate as much as possible. Another technique is to solidify and sublimate an etching cleaning solution which is the cause of the surface tension between the released portion of the microstructure and the substrate. Similar to this technique, as disclosed in G. T. Mulhern et al., Proc. Int. Conf. Solid State Sensors & Actuators (Transducers '93), Yokohama, 1993 (IEEJ, Tokyo, 1993) p. 296, a pressure-controlled and temperature-controlled chamber is used to sublimate a cleaning solution in its supercritical state.
The above methods for preventing stiction suffer from drops in a yield rate and require additional fabrication steps and expensive equipment. Furthermore, the stiction due to surface tension forces may be basically removed by dry etching, but use of the dry etching may damage the material of a suspended microstructure such as polycrystalline silicon. Furthermore, none of the above methods prevents stiction due to an elastic force other than surface tension.
SUMMARY OF THE INVENTION
To solve the above problems, it is an aspect of the present invention to provide a method for fabricating a microstructure that can prevent stiction of a microstructure to a substrate during a microstructure fabrication process in a simple way.
Accordingly, the present invention provides a method of fabricating a micromachined structure suspended above a substrate using a sacrificial layer. The method includes the step of stacking an anti-stiction layer that can be removed by dry etching before or after stacking the sacrificial layer.
The anti-stiction layer, which may be formed of polymer, polycrystalline silicon or photoresist that can be removed by dry etching, is formed on the substrate. However, the anti-stiction layer may be formed after stacking the sacrificial layer.
If the anti-stiction layer is formed on the substrate before stacking the sacrificial layer, the sacrificial layer and the microstructure are sequentially formed on the anti-stiction layer. The sacrificial layer is removed by wet etching. In this case, the anti-stiction layer is removed by dry etching, thus providing a microstructure without stiction.


REFERENCES:
patent: 5638946 (1997-06-01), Zavracky
patent: 5683591 (1997-11-01), Offenberg
patent: 6096149 (2000-08-01), Hetrick et al.
patent: 6133670 (2000-10-01), Rodgers et al.
patent: 6276205 (2001-08-01), McNie et al.
patent: 6404028 (2002-06-01), Hetrick et al.

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