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Explain in detail fabrication steps for MEMS microheater.
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  • Fabrication of micro heater on silicon oxide membrane. two inch double sided polished [100] P-type wafer is used to fabricate this micro heater.

  • Microheater fabrication consists following steps.

  1. Wafer cleaning
  • Switch the exhaust of chemical hood. wear protective gown, google, mouth mask, head cap just before starting work on wet bench. Take 2 quartz beakers, 1 measuring cylinder, wafer holder and wash with DI [deionised] wafer.

  • Clean the wafer with RCA - 1 solution. The RCA - 1 cleaning removes surface contaminants such as dust, grease and silicon gel from the wafer.

  • The RCA -2 cleaning removes metallic contaminants from the wafer.

  1. WET oxidation
  • Switch on the mains of thermal wet oxidation furnace to grow 1000 nm oxide layer. Ramp up furnace temperature to 500 degree c and pass nitrogen gas [0.5 liter/min] into the furnace to create inert nitrogen atmosphere.

  • Then load the wafers into furnace and ramp up furnace temperature from 500 degree c to 1100 degree c

  • Now pass oxygen gas [ 1 liter/min ] directly into the furnace for 10 minutes for dry oxidation. after 10 minutes stop direct supply of oxygen gas into the furnace and pass oxygen through the water bubbler for 3 hrs for wet oxidation.

  • After 3 hrs, stop oxygen supply and pass nitrogen gas [ 0.5 liter/min ] Ramp down the furnace temperature at 37 degree c and unload the wafers. this oxide is used as a hard mask during the KOH etching process.

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  1. Top layer lithography
  • Top side of oxidized wafer is coated with SU 8 photoresist at 4000 rpm for 40 sec and pre baked at 125 degree c for 1 min for top layer pattern transferring double sided EVG 620 mask aligner was used.

  • The UV bulb is switched on for 10 min for stabilization. mast is loaded according to procedure.

  • Wafer is loaded and aligned with the mask the mask pattern is transferred onto the coated photoresist by UV exposure

  • The pattern is then developed in a ME2CA developer solution for 1 min and rinsed with DI water followed by nitrogen drying.

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  1. Platinum deposition and lift off: 200 nm platinum was deposited after lithography process by sputtering method.

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  1. Silicon nitride deposition
  • 80 nm silicon nitride was deposited as a hard mask for bulk silicon etching

  • After deposition nitride was annealed in a polymer annealing furnace at 7000 degree c for 45 min in nitrogen ambient flow rate of nitrogen was 0.5 lit/min

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  1. Back side membrane patterning and oxide etching
  • Back side of oxidized wafer was coated with SU 8 photoresist at 4000 rpm for 40 sec and pre baked at 125 degree c for 1 min

  • For membrane pattern was aligned with respect to top layer position pattern by double/sided EVG620 mask aligner

  • The UV bulb is switched on for 10 min for stabilization. mask is loaded according to procedure.

  • Wafer is loaded and aligned with the mask

  • The mask pattern is then developed in a MF 26 A developer solution for 1 min and rinsed with DI water followed by nitrogen drying

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  • After lithography silicon oxide was etched using 1:3 buffered HF [ BHF ] solution followed by DI water rinse and nitrogen drying photoresist was removed by dipping in acetone for 1 min followed by IPA dip and nitrogen drying.

  • Etch rate of oxide in 1:3 BHF is 300 nm/min

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  1. Silicon etching wing KOH + IPA nad TMAH solution
  • Etch rate of silicon in KOH + IPA is 50 micron/hour and etch rate of silicon oxide is 150 nm/hour

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  1. Silicon nitride etching using BHF solution
  • Silicon nitride was etched using 1:3 BHF solution by dipping for 10 secs

  • After etching of silicon nitride layer wafer was thoroughly rinsed with DI water and dried with nitrogen.

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  • Top view of the micro - heater after fabrication:

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