Biomedical Engineering Reference
In-Depth Information
substrate. ALD is an analogue of the CVD technique, and is most
appropriate for binary compounds because a binary CVD reaction can
easily be separated into two half-reactions. In ALD, the film growth takes
place in a cyclic manner. The simplest case includes four steps: (1) exposure
of the first precursor; (2) purge or evacuation of the reactor chamber; (3)
exposure of the second precursor; (4) purge or evacuation of the reactor
chamber. The film thickness is therefore simply determined by the growth
rate per cycle and the number of cycles completed.
The unique self-limiting film growth mechanism allows a number of
advantageous features, such as excellent conformality and uniformity,
pinhole-free films, low impurity content, independence of line of sight,
simple and accurate film thickness control, and low processing temperature
(Suntola, 1992; George et al., 1996; Leskela and Ritala, 2002). This self-
limiting growth mechanism also ensures good reproducibility and relatively
straightforward scale-up. Depending on the coating material and process
conditions, one ALD cycle deposits
0.01-0.20 nm film thicknesses. This
slow growth rate provides for the control of the atomic-scale deposition.
This is also the major limitation of ALD processing, but this limitation is
losing its significance when weighed against the numerous benefits of ALD
relative to other techniques.
ALD processes are typically operated under some degree of vacuum. The
vacuum transport medium has the important advantages of a decreased
chance of contamination and clear access to the deposition surface. A large
number of materials can be prepared by ALD, including, but not limited to,
Al
2
O
3
(Ott et al., 1996; Yun et al., 1998; Jeon et al., 2002), ZnO (Ferguson
et al., 2005; Hamann et al., 2008; King et al., 2008a), TiO
2
(Ritala et al.,
1993; Kumagai et al., 1995; Ferguson et al., 2004), SiO
2
(Klaus et al., 1997a,
1997b; McCool and DeSisto, 2004), ZnS (Suntola and Hyvarinen, 1985;
Suntola, 1992), HfO
2
(Ding et al., 2003; Chang et al., 2004; Kukli et al.,
2004), AlN (Kim et al., 2009; Bosund et al., 2011), TiN (Ritala et al., 1995;
Satta et al., 2002; Elam et al., 2003), BN (Ferguson et al., 2002), Fe
2
O
3
(Scheffe et al., 2009; Martinson et al., 2011), CoO (Scheffe et al., 2010; De
Santis et al., 2011), MnO (Nilsen et al., 2003; Burton et al., 2009), W
(Wilson et al., 2008), Pt (Aaltonen et al., 2003; Zhu et al., 2007; King et al.,
2008b; Li et al., 2010) and Pd (Senkevich et al., 2003; Ten Eyck et al., 2005;
Elam et al., 2006; Lu and Stair, 2010). This vacuum environment also allows
operating thin film deposition in plasma. Plasma is a partially ionized gas
and contains a great deal of energy, which can activate film deposition
processes at lower temperatures. Plasma-enhanced CVD (Suchaneck et al.,
2001) and ALD (Yun et al., 2004; Ten Eyck et al., 2007) have been carried
out at low temperatures for film deposition on thermally sensitive
substrates. Reviews on the surface chemistry of ALD are given by George
et al. (1996), George (2010) and Puurunen (2005). For other aspects of ALD
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