Page 40 - MEMS Mechanical Sensors

2.3 Fabrication Techniques 29

of rinses such that the final rinse is in a hydrophobic liquid such as hexane or tolu-
ene [24]. Another approach relies on changing the phase of the liquid in which the
wafers are finally rinsed, either by freezing or heating the liquid into a supercritical
state. T-butyl can be frozen solid and sublimed at low vacuum pressures [25]. In the
supercritical drying method the final rinse is done in a pressure vessel in liquid CO ,
2
which is then raised into a supercritical state. In this state, the interface between the
liquid and gas phases is indistinguishable and there are no surface tension forces
[26]. Thus, the CO gas can be vented without affecting the structures. Other meth-
2
ods involve dry release of the structures. One such method is etching the oxide in an
HF vapor [27], and another entails rinsing the wafers in acetone then adding photo-
resist, which fills the gaps beneath the structures after the acetone has evaporated.
The resist can then be removed in an oxygen plasma. There are yet other methods,
which have the added benefit of preventing the stiction of the structures when
they are in use, that rely on modifying the surfaces of the structures using self-
assembled monolayers formed, for example, from DDMS [(CH ) SiCl ] or ODTS
3 2 2
[(CH (CH ) SiCl ] [28, 29]. In-use stiction can also be prevented by coating released
3 2 17 3
structures in a fluorocarbon by PECVD [30].

2.3.5 Wafer Bonding
There are many wafer bonding processes currently available, and the choice of
which is most suitable depends on the particular application and the materials
involved. Bonding processes are as likely to be used at the beginning of a process
sequence as at the end. For example, bonding is used in the fabrication of SOI
wafers, but also in device fabrication processes, such as the bonding together of
wafers to form the vacuum cavity of an absolute pressure sensor, as well as at the
end of processes to package devices. In all the bonding processes described here, sur-
face cleanliness is of paramount importance. Particulates trapped between wafers
can lead to the formation of voids and ultimately failure of the bond. Also, as with
most micromachining processes, attention has to be paid to the stress created by the
process and this is particularly relevant to mechanical sensors. For this reason the
materials bonded together and the material, if any, used to bond them should have a
minimal thermal mismatch, otherwise temperature changes will result in strain
being applied to devices. Bonds should also be stable over the life of the device. Any
plastic flow, or creep, may alter the output of a device affecting its calibration and
long-term stability. The bond should also be strong enough to withstand any strain
the device is likely to be subjected to. If bonding is used for packaging devices it
should provide, if possible, some degree of strain relief for the device.

2.3.5.1 Silicon Fusion Bonding

Silicon fusion bonding is a direct silicon-to-silicon bonding technique that does not
require any melting alloys, glass layers, or polymer glues. As a result little or no stress
due to thermal mismatch is introduced into the assembly, and the perfectly matched
thermal expansion coefficients of the two wafers ensure that this low stress condition
is preserved. The process requires the surfaces to be planar, clean, and hydrated. The
hydration step can be carried out in a number of ways, either by boiling in nitric acid
or ammonium hydroxide or simply by performing a standard RCA clean (so called