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Kerst Griesbach, Andreas Bertz, Klaus Wolf, Jürgen Wibbeler, Matthias Küchler, Thomas Geßner:
Single Crystal Silicon Vertical Actuators based on Surface Micromachining
Proceedings of the 8th International Fair and Congress for Sensors, Transducers & Systems, Nürnberg, 1997, vol. 3, pp. 7 - 11
ABSTRACT
In-plane movement of sensors and actuators has been realized advantageously using surface micromachining.
In order to achieve an additional z-drive several techniques were taken into consideration.
The application of piezo elements, thermal stimulation and the use of electrostatic forces are possible.
Xu et al. [1] have published first Micro-Scanning Tunneling Microscopes based on the SCREAM (Silicon Reactive Etching and Metallization)
process [2] including capacitor drives in z-direction.
On the one hand the SCREAM method in combination with an electrostatic attraction is a very interesting
approach but on the other hand also risks are included.
Due to the limitation of the release etch only by time (no etch stop layer exists), the distance and the
geometry between movable beams and the bottom electrode could vary from wafer to wafer and across the wafer.
Thus, an influence on the electrical behaviour of the system is expected.
Additionally, any metallization asymmetry by shadowing etc. could result in an asymmetric bar attraction
followed by distortions of the device function.
In this paper two methods of capacitive z-drives, as shown in Fig. 1, will be studied.
As illustrated in Fig. 1b, a symmetrical adjustment by electrostatic forces and a local self-limitation
of the bar motion can be achieved.
The different fabrication technologies will be discussed.
Wet etching steps during or after release etch should be avoided because of the sticking effect.
In order to get a better definition of the bottom geometry, an advanced dry etching method has been developed.
This method includes a crystal plane dependent etch anisotropy, which is already known from wet etching,
using e.g. a KOH solution. Fig. 2 shows test patterns for the qualification of the SCREAM process steps.
An overview of released beams with an isotropic bottom profile is given in Fig. 2a while Fig. 2b demonstrates
the potential of the crystal plane dependent dry etch process.
The different release technics are examined on an actuator (Fig. 3) which is movable by a generated
electrostatic force as illustrated in Fig. 1a.
Moreover, results of electrical measurements will be presented.
REFERENCES
[1] Yang Xu, Scott A. Miller, Noel C. MacDonald: Microelectromechanical scanning tunnelling microscope.
The 8th International Conference on Solid-State Sensors and Actuators, and Eurosensors IX. Stockholm, Sweden, 1995, TRANSDUCERS '95, pp. 640 - 643
[2] Kevin A. Shaw, Z. Lisa Zhang, Noel C. MacDonald: SCREAM I: a single-crystal silicon, reactive ion etching process for microelectromechanical structures.
Sensors and Actuators A, 40 (1994) pp. 63 - 70
Fig. 1: Schematic drawing of the two principles for electrostatic driven bars, which are movable in -z (a) and +z (b) direction
Fig. 2: Test structures using isotropic (a) and crystal plane dependent (b) dry etching
Fig. 3: Fabricated torsional bar actuator
MK, 30. Dezember 1999