Assignment:Piezoresistor

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Hands-on design training for MultiMEMS
D. Lapadatu and P. Ohlckers

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| 2. Assignment:Piezoresistor

The goal of this assignment is to design a piezoresistor that we will use in Assignment:Pressure sensor . The resistor is to have a value about 8 kOhm and be as small as possible to maximize the sensitivity of the sensor.
Starting point
Before we start drawing, we give a brief introduction to how to design a resistor. We will follow a simple recipe:

- Find the number of squares needed
- Find nominal dimensions for a resistor on the SURE layer
  - Minimum line width: 4 micron
- Calculate the resistor value taking lateral diffusion into account.

Number of squares needed
With a target of 8 kOhm and a sheet resistance of 1210 Ohm/sq, we have that 8000 Ω /1210 Ω/sq=6.61 sq.
Nominal dimensions
With a minimum linewidth of 4 μm on the SURES layer, we have that the nominal length must be 4 micron * 6.61 = 26.44 micron.
Resistor values with lateral diffusion
Knowing that SURES diffuse 0.3 micron in all directions, we must adjust the length correspondingly. 4micron + (2*0.3 micron)=4.6 micron.
4.6 μm * 6.61 μm= 30.41 μm
Not only the SURES layer diffuse laterally, but also the SUCON. The lateral diffusion of the SUCON layer is 0.7 micron. This must be compensated for in the layout.
The length we need to draw in the layout software is therefore
30.41 μm+(2*0.7 μm)=31.91μm=32μm
Nominal resulting resistance
The nominal resistance can now be found to be
30.6/4.65*1210=8049 Ohm
Geometry to the drawn
Layers needed
For this assignment we will use the following layers:

- SUCON: SUrface CONductor
  - Will be used to connect to the resistor
- SURES: SUrface REsistor
  - The implanted regions that function as piezoresistors.
- NOSUR:NO SURface implantation
  - Areas where there will be no additional surface implantation. Goes everywhere where you have implanted regions near the surface of the wafer.
- NOBOA:NO BOnding Area
  - Areas where the surface will not be removed. Typically used over surface implanted areas to improve stability of resistors.

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Last updated: June. 2009 24.06.2009