A versatile trench isolation technology for the fabrication of microactuators
E. Sarajlic, E. Berenschot, G. Krijnen, M. Elwenspoek
Transducer Science Technology Group, MESA+ Research Institute,
University of Twente
P.O. Box 217, 7500 AE Enschede, The Netherlands
Tel: [phone]. fax: [phone], e-mail: [e-mail]
Complex electromechanical micro-devices often require electrical isolation between
mechanically interconnected structures implemented within a minimum number of layers and
processing steps. A departure from the conventional in IC-fabrication widely used approach
of stacking of multiple dielectric and conductive layers connected by vertical conduction
paths is employment of (DRIE) etching of (deep) trenches and the successive refill with
dielectric (silicon oxide, silicon nitride) or poor conducting (undoped poly-silicon) materials
[1,2,3]. This paper extends the fully SOI compatible trench isolation technology [1] by adding
new functionalities while keeping the fabrication process rather simple (Fig. 1). By employing
trenches as a mold for a dielectric material and by applying an intermediate isotropic partial
etch of the sacrificial layer additional functions can be obtained. First, molded dielectric
structures can serve to prevent shorting between two movable electrodes. The non-conducting
stoppers maintain the end distance between electrodes regardless of a possible mask
misalignment (Fig.2). Second, isotropically etching of holes in the sacrificial oxide layer
before deposition of a dielectric material creates bumps (Fig.3) which not only reduce stiction
between moving parts and a substrate but also prevent the shorting due to the vertical
displacements. The trench isolation technology complemented with the above-mentioned
features becomes a promising platform for microactuator fabrication. Besides opportunities to
improve performance and simplify fabrication of existing actuators [4,5], the technology
allows designers to create new types of microactuators. A novel electrostatic microactuator
has been fabricated using this technology. The microactuator consists of a series of released 2
µm thick beams, spaced 2 µm from one another and electrically isolated from a neighboring
beams by trench isolation (Fig. 4). The actuator uses a mechanical transformation to obtain
large force and high-resolution step.
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Si-substrate Nitride Oxide Device layer
Figure 2: Molded silicon nitride structures acting
as stoppers by a gap closing actuator with two
Figure 1: Trench isolation process for fabrication
movable electrodes
of mechanical coupled/electrical isolated MEMS
components
Figure 3: Isotropic etching of a hole in sacrificial
layer preceding refilling create bumps to reduce
stiction and to prevent electrical shorting between
device layer and substrate
Figure 4: Novel electrostatic microactuator
employs trench isolation for electrical insulation
between neighboring polysilicon beams