Mumbai University > Electronics Engineering > Sem 8 > MEMS Technology

Marks: 10M

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.

2.WET oxidation

• Switch on the mains of thermal wet oxidation furnace to grow 1000 nm oxide layer. Ramp up furnace temperature to $500^oc$ 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^oc$ to $1100^oc$

• 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 $37^oc$ and unload the wafers. This oxide is used as a hard mask during the KOH etching process.

3.Top layer lithography

• Top side of oxidized wafer is coated with SU 8 photo-resist at 4000 rpm for 40 sec and pre baked at $125^oc$ 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. Mask is loaded according to procedure.

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

• The pattern is then developed in a MF2CA developer solution for 1 min and rinsed with DI water followed by nitrogen drying.

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

5.Silicon nitride deposition

• 80nm silicon nitride was deposited as a hard mask for bulk silicon etching

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

6.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 $125^oc$ 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 MF26A developer solution for 1 min and rinsed with DI water followed by nitrogen drying

• After lithography silicon oxide was etched using 1:3 buffered HF [ BHF ] solution followed by DI water rinse and nitrogen drying photo-resist 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

7.Silicon etching using 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

8.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.

• Top view of the micro - heater after fabrication: