Materials for Micromachining

An Introduction to MEMS 

 

 

Prime Faraday Technology Watch – January 2002 

19

 

iii)  high definition and reproduction of silicon device shapes using photolithography are 

perfect for high levels of MEMS precision 

 

iv)  silicon microelectronics circuits are batch fabricated (a silicon wafer contains 

hundreds of identical chips not just one) 

 

Other crystalline semiconductors including germanium (Ge) and gallium arsenide (GaAs) are 

used as substrate materials due to similar inherent features, but silicon is distinguished from 

other semiconductors in that it can be readily oxidized to form a chemically inert and 

electrically insulating surface layer of SiO

2

 on exposure to steam.   

 

The homogeneous crystal structure of silicon gives it the electrical properties needed in 

microelectronic circuits, but in this form silicon also has desirable mechanical properties.  

Silicon forms the same type of crystal structure as diamond, and although the interatomic 

bonds are much weaker, it is harder than most metals.  In addition, it is surprisingly resistant 

to mechanical stress, having a higher elastic limit than steel in both tension and compression.  

Single crystal silicon also remains strong under repeated cycles of tension and compression. 

 

The crystalline orientation of silicon is important in the fabrication of MEMS devices because 

some of the etchants used attack the crystal at different rates in different directions (Figure 

18). 

 

Figure 18.  Low crystallographic index planes of silicon [29]. 

 

Silicon is dominant as a substrate for MEMS but research and development is ongoing with 

other non-semiconductor substrate materials including metals, glasses, quartz, crystalline 

insulators, ceramics and polymers.  The ability to integrate circuitry directly onto the substrate 

is currently the underlying issue with today’s MEMS substrate materials; hence the success of 

silicon. 

 

 

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