Biomedical Engineering Reference
In-Depth Information
bottom metal layer significantly affects the texture of AlN films and, hence, its electro-
acoustic properties. AlN films deposited on the materials with fcc lattice structure show a
high c-axis orientation, especially for Au and Pt (Tay et al., 2005). Ti has a hexagonal struc-
ture similar to that of AlN (Lee et al., 2004; Chou et al., 2006). W has a low acoustic attenua-
tion, small mismatch in the coefficient of thermal expansion and high acoustic impedance
with AlN, and is thus a good electrode material for AlN devices. Ni was often chosen
because of its good surface smoothness, but the AlN film texture on Ni is not as good as
that on the other fcc metals. Tantalum (Hirata et al., 2005) and iridium (Clement et al., 2009)
have also been reported to be used as electrodes for AlN film growth. The thickness ratio
of AlN and top electrodes has been reported to have a significant influence on PE effect of
AlN films (Akiyama et al., 2004; Huang et al., 2005).
Sputtered AlN films normally show a (002) film texture, which results in longitudinal
(or Rayleigh) wave modes and is therefore good for sensing in air or gas. However, as
explained before, if liquid exists on the sensing surface, excessive damping and attenua-
tion of the propagating wave occurs when the longitudinal mode couples into the liquid.
This problem can be solved by generating a SH-SAW, which propagates on a piezo-material
by an in-plane SH motion (Wingqvist et al., 2007), and dramatically reduces SAW coupling
into a liquid medium (Mchale, 2003). However, the commonly observed (002) texture in
the sputtered AlN films is unsuitable for generating SH-SAWs. Besides this, using a pure
shear wave is not efficient in driving liquid droplets forward. A good approach to solving
the problem is to develop AlN films in which the
c
-axis is inclined relative to the surface
normal, thus allowing both longitudinal and shear wave modes to be generated (Webber,
2006). These two modes will have different frequencies and, thus, can be individually con-
trolled for either pumping or sensing purposes. To the best of our knowledge, there are no
reports of the application of both the functions (microfluidics and biosensing) on a
c
-axis
inclined AlN-based SAW device in liquid conditions. The techniques for the deposition
of the inclined AlN film include (1) using a tilted substrate (up to 45°) with a controlled
position under the sputter-target; (2) using a high-energy nitrogen ion beam aimed at the
desired angle with respect to the substrate surface normal (Yanagitani and Kiuchi, 2007).
Obtaining the inclined AlN films strongly depends on the sputtering pressure, tempera-
ture, and the oblique incidence of particles (Yang et al., 2009).
c
-Axis-inclined AlN films
have been deposited on silicon substrate and diamond substrate (Fardeheb-Mammeri et
al., 2008).
During sputtering, the particle bombardment could induce large film stress (Iborra et
al., 2004). Films with large compressive stress can cause buckling-induced delamination in
the deposited films and fracture in the released devices. In Ar/N
2
-based deposition system
for AlN film deposition, the energy of Ar ions colliding with the substrate controls the
preferred orientation of the AlN films; moreover, directionality and the energy of the ions
determine the residual stress levels. The energy of the Ar bombardment can be adjusted by
the substrate bias voltage during sputter deposition. Thermal annealing is a good method
for posttreatment to reduce film stress and improve coating quality (Hung and Chung,
2009).
PE Properties of Sputtered AlN Films
There are two key issues for the PE properties of the AlN acoustic wave device: the elec-
tromechanical coupling coefficient and the quality factor Q. The quality of the AlN film,
such as film texture and stress, strongly affects the resonant frequency. For the AlN-based
acoustic wave device, parameters of Q factor, resonant frequency, and effective coupling
