Biomedical Engineering Reference
In-Depth Information
500°C), and these sputtered films normally show good epitaxial film structure (Ishihara
et
al., 2004; Engelmark
et al., 2000). From an MEMS fabrication point of view, RF magnetron
reactive sputtering is one of the best methods, with good reproducibility and compatibility
with planar device fabrication technology. In this section, we will focus on the texture and
acoustic wave properties of the sputtered AlN films.
Cheung and Ong
(2004) have systematically studied the effect of substrate temperature,
RF power, and substrate materials for the formation of a broad range of amorphous, poly-
crystalline, and epitaxial AlN films. The growth dynamic or surface kinetic roughening
of AlN films grown on Si (100) substrates by sputtering has been thoroughly studied, and
a two-stage growth regime has been identified (Auger et al., 2005). In the first step, the
growth dynamic is unstable with significant sticking probabilities of the impinging parti-
cles. The films have a mixture of well-textured and randomly oriented crystals. In the sec-
ond regime, the films are homogeneous and well textured, and the growth is dominated
by the shadowing effect induced by the bombardment of impinging particles (Auger et al.,
2005). Based on this, a two-step deposition technique has been proposed to increase film
texture and reduce film stress by changing either gas pressure or plasma power during
deposition (Cherng
et al., 2008, 2009). For example, the first stage can be a low-power, high-
temperature deposition that provides high mobility to the surface atoms. The second stage
might be a high-power deposition at lower temperatures characterized by high deposition
rates and low residual stress.
For film growth, conditions are more critical for the AlN films than those for ZnO films.
Growing a thick AlN film is rather critical because of its tendency to present microcrack-
ing. Oxygen and argon could have significant influences for AlN film growth during sput-
tering and contamination due to residual oxygen or water can seriously interfere with
the formation of the AlN film structure. Growth rate of the AlN film will decrease with
increasing oxygen in the sputtering gas and their predominant polarity also changes from
Al polarity to N polarity with increases in the oxygen concentration (Cherng et al., 2008;
Vergara
et al., 2004). Increased oxygen concentration in sputtering gas also increases Al-O
bonding, as the bonding energy of Al-O (511 kJ/mol) is higher than that of Al-N (230 kcal/
mol) (Akiyama
et al., 2008), which is important as oxygen concentration significantly influ-
ences the PE response of AlN films.
AlN Film Texture and Substrate Effect
Films with strong crystallinity can have good PE coefficients, high electromechanical cou-
pling, and acoustic velocities approaching those of the single crystal AlN. The sputtering
process parameters significantly affect the orientation of the deposited AlN films. Okano
et al. (1992) identified that the
c
-axis orientation increases as the N
2
concentration in the
mixture of Ar and N
2
decreases, whereas Naik et al. (1999) showed that the c-axis orienta-
tion increases as the sputtering pressure is reduced. AlN films have been reported to show
preferred (002) growth orientation on silicon, quartz, glass, LiNbO
3
(Caliendo et al., 2003;
Lee et al., 2004), GaAs (Cheng et al., 1998), GaN/Sapphire (Kao et al., 2008; Xu et al., 2006),
SiC (Takagaki et al., 2002), and ZnO layer (Lim et al., 2001).
Commonly used electrode materials for AlN SAW devices include Mo, Ti, Al, Au, Pt, Ni,
and TiN. Al and Mo have low resistivity and high Q factors, with Mo being one of the most
widely reported electrodes in the AlN film-based acoustic devices (Akiyama et al., 2005;
Huang et al., 2004, 2005; Lee et al., 2003; Okamoto et al., 2008; Cherng et al., 2004). Ag, Al,
Co, Cr, Cu Fe, Nb, Ni, Zn, and Zr have also been reported as electrodes for the AlN-based
acoustic wave devices (Lee et al., 2004; Akiyama et al., 2004). For the AlN FBAR device, the
