Polycrystalline silicon (polysilicon) based gauges

• Polysilicon is composed of many small single crystals or grains of silicon joined together by grain boundaries
• Many polysilicon properties are similar to that of single crystal silicon
• Other characteristics are strongly influenced by the grain boundaries rather than by grains themselves
• Piezoresistance coefficients
  • Large grained polysilicon: can approach 60-70% of single crystal silicon
  • Fine-grained, micromechanical polysilicon, p_L is less than 15 % of piezoresistance coefficient of single crystal silicon
    
• Advantages of polysilicon and especially of its fine grained form
  • High degree of thickness uniformity
  • Smooth surfaces
  • Low defect densities
  • Homogeneous, repeatable mechanical properties
  • Better linewidth control than with other, larger-grained films
    
• Disadvantages of polysilicon include
  • Temperature coefficient of expansion mismatches with substrate that can cause unwanted stress
  • Dependency of all physical properties on film morphology and processing history

• Calculation of expected p coefficients for polysilicon
  • Necessary to take into account the probability of orientations of single grains
  • Effect of grain boundaries must be included if more precise estimation is demanded

Basic types of piezoresistive sensors

• Cantilever beams sensors
  Made mostly for accelerometers
  
• Membrane sensors
  Manufactured mostly to measure pressure and flow