Wüest M. 51 Wykes M. 82 Yamaguchi M. 17 Ybarra G. 129 Yubero F ...

JUNE 28 WEDNESDAY MORNING
WS-18-WeM-INV.9 PHYSICAL AND TECHNOLOGICAL ASPECTS OF MEMS VAC-
UUM PACKAGING. F. Völklein, A. Meier, FH Wiesbaden, University of Applied Sciences. Am
Brückweg 26, D-65428 Rüsselsheim, Germany
The realization of Micro Electro Mechanical Systems (MEMS) or Micro Opto Electro Mechanical
Systems (MOEMS) requires sofisticated fabrication technologies based on thin film deposition,
photolithography and etching techniques. MEMS fabrication can be divided into three
major groups:
i) fabrication with additive and subtractive processes on the wafer level
ii) packaging, involving processes such as bonding, lead attachment and encapsulation in a protective
body or in cavities with reduced gas pressure (vacuum)
iii) testing, including package leak test, electrical integrity and MEMS functionality.
The last two process groups incorporate the most costly steps. MEMS packaging is more difficult
and expensive than packaging of Integrated Circuits (IC) and may be totally different (e.g. for gas
sensors) from IC housing.
MEMS vacuum packaging is required for accelerometers in order to optimize the damping of the devices.
High-Q micro resonators might need a good vacuum. Mechnical vacuum sensors using piezoresistive
or capacitive measuring effects include small cavities (volumes in the order of 1 mm³)
with reduced reference pressure or vacuum.
Cavity sealing can serve as a batch-compatible packaging technique by encapsulating a chip feature
or whole chip at a time. Chip features can be sealed by surface micromachining using Polysilicon or
Silicon Nitride deposition techniques and sacrificial layer technology. Such micromachined surface
packages (microshells) are much smaller than typical bulk MEMS packages. Microshells can be
made by defining thin gaps (100 nm) between the substrate and the perimeter of the structural elements
by etching away a sacrificial layer sandwiched between the two and then sealing the resulting
gaps. In so-called reactive sealing thermal oxidation of the Polysilicon and Si-substrate seals the narrow
openings left after removal of the spacer phosphosilicate (PSG) sacrificial layer. Alternatively,
sealant films, such as oxides and nitrides, can be deposited over small etchant holes. The first commercial
absolut Polysilicon pressure sensor incorporates such a reactively sealed vacuum shell. Epitaxial
cavity sealing and HEXIL cavity sealing are alternative technologies. These and other lithography-defined
packages, such as those involving ultraviolet patternable polymers, might be an integral
part of the overall fabrication processs and lend to inexpensive batch solutions.
In bulk micromachining and Si fusion-bonded (SFB) surface micromachining, cavities are fabricated
by bonding, respectively, a glass plate (anodic bonding) or a Si wafer (fusion bonding) over etched
cavities in a bottom Si wafer. Field-assisted thermal bonding (anodic bonding, also known as electrostatic
bonding) can be established between a sodium-rich glass and Silicon at relatively low process
temperatures. This method is mostly applicable to wafer-scale chip bonding. SFB is performed
between flat Si wafers with slightly oxidized surfaces. When incorporating an intermediate layer between
two substrates, many thermal vacuum bonding techniques are feasible. Silicon microstructures
can be sealed together by eutectic bonding, e.g. Au-Si eutectic bonding at 363°C. The most important
problems of MEMS vacuum packaging are the very small volumes of cavities combined with
the outgassing during the sealing process. Long term stable vacuum in microcavities can be realized
by using thin getter layers and sealing materials with low permeation coefficients.
106