Page 170 - Semiconductor Manufacturing Handbook

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PLASMA ETCHING

PLASMA ETCHING 12.19

RF bias power (ion energy)

ICP power (plasma density)
FIGURE 12.15 Pit/pillar formation and elimination in GaN etching using
Cl chemistry in an ICP system. Smooth surface morphology is obtained by
2
balancing the chemical-driven and ion-driven etching components.

grown on an appropriate lattice matched substrate material such as sapphire or SiC. RIE, ECR, and
ICP systems have been employed to etch GaN materials in different chemistries such as SiCl , HBr,
4
BCl , CH , and Cl . 94–99
3 4 2
The Ga-N bonds have high bond energies −8.9 eV versus 6.5 eV for Ga-As bonds. When GaN is
etched in Cl-based chemistry, for example, GaCl production is kinetically slow in the absence of
3
energetic ion bombardment so etching will not proceed. Also, defects in GaN appear to be particu-
larly sensitive to etching conditions and respond by etching faster or slower, ultimately forming pits
or pillars. Once again, the balancing effects with the ion-driven and chemical-driven components can
be applied to control etch performance. An example of etching GaN with Cl chemistry in an ICP sys-
2
tem is given in Fig. 12.15. 100 When ICP power is increased, GaN etching is driven into a more chem-
ical regime, resulting in pit formation. When the ion energy is increased by increasing RF bias power,
etching is more ion driven resulting in pillar formation. But balancing the chemical- and ion-driven
components leads to smooth etched surface.
The small wafer sizes, coupled with cost pressures for GaN based devices, promote demands on
high throughput capability in dry etching systems. One solution is to use an RIE process and run large
batches of wafers. However, there is reduction in etch rate associated with increased loading, as can be
understood from chemical mass balance. On the other hand, high-density ICP systems offer a capacity
for high etch rates, and thus high throughput. A comparison of RIE and ICP performance is shown in
Table 12.6.

TABLE 12.6 GaN Etching Comparison

Process metric Typical performance
RIE ICP
Etching rate (Å/min) 750 5000
Selectivity (GaN:hardmask) ≥ 5:1 ≥ 10:1
Nonuniformity percent ≤± 5 – 10 ≤± 3
Etched surfaces Smooth Smooth

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