Optoelectronic Semiconductor Devices Principals and Characteristics

Figure 39.: A periodic multilayer (highly reflective film). 
[4]
Typical pairs for highly reflective films used in optoelectronic devices Si/SiO
2
(3.4/1.4) and TiO
2
/SiO
2
(2.5/1.4). One of two pairs is usually coated onto the semiconductor surface such as facets of laser diodes, 
and the material with the lower refractivity is deposited first. For example, in air (n
r2
=1) one pair of 
amorphous Si/SiO
2
layers and one pair of TiO
2
/SiO
2
layers deposited on AlGaAs and InGa AsP (n
r1
≈3.5) 
give reflection of a 83% and 70%, respectively. 
8.2.3 
LITHOGRAPHY AND CHEMICAL AND PHYSICAL ETCHING
 
The photolithography is carried out as follows: 
• 
formation of a photoresist (PR) film on the dielectric or metal film deposited on the 
semiconductor; 
• 
exposure of the PR through a patterned mask by photoexposure or by electron-beam exposure 
without any mask (direct writing); 
• 
developing of the PR; 
• 
rinsing of the PR. 
The dielectric (or metal) film is then chemically or physically etched through the patterned PR. 
There are two types of etching: wet and dry. 
The etching rate and shape of the etched pattern depends on combination of semiconductor materials and 
etchants. The etched shapes and patterns for the wet etching are determined by the combination and 
depend on the crystal orientation of the semiconductor. There is little process damage and the defect 
density in the processed devices is low. 

---

Physical etching can be used to form relatively fine patterns. Typical kind of physical etching is dry 
etching, in which semiconductor materials are sputtered by collisions of particles. But a damaged layer is 
formed and the etched surface contains a large number of defects. (See Figure 
40.
). This damage affects 
the device performance and reduces reliability. After dry etching, the etched surface is therefore lightly 
etched chemically to eliminate the damaged layer.