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Describe the representative process flow for fabricating the micro-heater. Also explain the operating principle of this MEMS device in detail with its analytical expression.

Mumbai University > Electronics Engineering > Sem 8 > MEMS Technology

Marks: 10M

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Fabrication steps of Micro-heater

Step 1 - A polysilicon is taken and deposited with silicon nitride(Si3N4) film of thickness 1 μm using the low pressure chemical vapor deposition (LPCVD) process.

Step 2 – On this, a polysilicon thin film (2000 Å) is deposited again using LPCVD process and phosphorous doping is done using ion implantation to control the resistance.

Step 3 – After this Pt is deposited with a thickness of 2000 Å by sputtering to make the electrical contacts.

Step 4 - In order to prevent oxidation of the poly-Si during heating, an SiO2 passivation layer is deposited having a thickness of 2,000 Å using plasma enhanced chemical vapor deposition (PECVD), followed by partial etching using the buffered oxide etchant (BOE). and the membrane is fabricated by anisotropic etching of the Si substrate using deep RIE process.

Fabrication process

General Characteristics of micro-heater

• Micro-heaters are small high power heaters with precise coil that can offer temperature ranges of 1000°C - 1900°C.

• Micro-heater consists of 2 suspended and interdisitated Platinum coils, heater and a sensor.

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• The low thermal mass allows for high speed heating and cooling within milliseconds.

Operational Characteristics of micro-heater

• Resistance of heater (25°C) ͢ 200Ω

• Resistance of sensor (25°C) ͢ 160Ω

• TCR ͢ 0.0026/°C

• Thermal Rise time(25°C - 500°C) ͢ 4ms

• Thermal Fall time(500°C - 25°C) ͢ 6ms

• Temperature Sensing I is defined as -

$ I = \frac{\bigg( \frac{R_T}{R_{2S}} - 1 \bigg)}{ \alpha } $

$\begin{aligned} \text{Where,} R_r & \rightarrow \text{Resistance at temp T} \\ R_{2S} & \rightarrow \text{Resistance at } 25^\circ C \\ \alpha & \rightarrow \text{Temp coefficient} \end{aligned}$

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