Page 169 - Semiconductor Manufacturing Handbook

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PLASMA ETCHING

12.18 WAFER PROCESSING

GaAs via wall slope versus photoresist slope
(at various GaAs: PR selectivities)
90 32 16 8

GaAs via wall angle (degrees) 60 (GaAs: PR) 4 2 1 Resist
75

Selectivity
45

30

15

0 GaAs
0 15 30 45 60 75 90
PR wall angle (degrees)
(a)

(b)
FIGURE 12.14 (a) Expected GaAs via wall angle as a function of resist slope
and etch selectivity, and (b) a SEM image showing a sloped GaAs via.

increases, the etch rate and selectivity to hard mask increase. Generally, at temperatures more than
150°C, a high etching rate, selectivity, and smooth morphology can be achieved. The HBr/CH
4
chemistry is used to minimize mask undercut and maintain a vertical profile. And out of concerns
over chamber contamination, the HBr/N chemistry can be used. The etch performance includes a
2
high etch rate (2.0 µm/min), good anisotropy (90 ± 1°), high selectivity (30:1), and excellent sur-
face smoothness (RMS <2.0 nm).

12.4.3 Etching of GaN Materials
Gallium nitride (GaN) is an important material used in an array of devices including light emitting
diodes (LEDs), transistors, and laser devices. Bulk GaN is not commercially available, it is epitaxially

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