Phosphoric acid

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Phosphoric acid is a weak acid with the chemical formula H₃PO₄. It is often used to etch Silicon nitride. Phosphoric acid is commonly encountered as an 85% aqueous solution. In appearance, it is a colourless, odourless, and non-volatile viscous liquid. Phosphoric acid will severely irritate the skin and damage the eyes.

Equipment

Phosphoric acid can be used at the following benches

• Acid Bench 02 - tank
• Acid Bench 73 - hotplate
• Acid Bench 92

Storage Locations

Phosphoric acid is stored in the passthroughs in

• 1436
• 1440A
• 1480C

Processes

• Hot Phosphoric Acid for Etching Silicon Nitride

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Process SOPs

• Acid Bench 02 SOP has information about the tank of phosphoric acid in the lab.
  • Standard Operating Procedure
• According to "Etch Rates for Micromachining Processing Part II" by Williams, phosphoric acid at 160C will have an etch rate of 4.5 nm/min.
  • KR Williams, K Gupta, M Wasilik, "Etch Rates for Micromachining Processing - Part II", JMEMS vol. 12 No 6, Dec 2003

Hazardous Waste

Never aspirate or drain a liquid that is heated. Wait til the solution has cooled to room temperature before disposing. Phosphoric acid can go down any drain that has an acid waste neutralization system.

Technical Data Sheet

• There are currently no data sheets for this chemical.

Safety Data Sheet

• https://drive.google.com/file/d/0B8WeTTMTotIfa0Z0Ry0xT1pRbWF1M3o5bXVfMFR2UQ/view?usp=sharing

References

• KR Williams, K Gupta, M Wasilik, "Etch Rates for Micromachining Processing - Part II", JMEMS vol. 12 No 6, Dec 2003
• Vossen, John L., and Werner Kern. Thin Film Processes. Academic Press, 1991.
• https://www.microtechprocess.com/wp-content/uploads/2018/04/MTS_Nitride.pdf