Wet Oxide Etch

INTRODUCTION:

Buffered oxide etch (BOE) is used to remove SiO₂. BOE is a very selective etch, meaning thatit stops at the silicon and does not etch further. The etch may be used for in many steps, such as exposing the active region near the beginning of a process or defining contact holes at the end.

SAFETY:

BOE consists of HF acid at high concentration levels (about 10X greater than the oxide strip in the RCA clean).  HF acid is very dangerous and HF burns are particularly hazardous. An insidious aspect of HF burns is that there may not be any discomfort until long after exposure. These burns are extremely serious and may result in tissue damage. If you contact HF, flush the area well and be sure to work under and around your finger nails. Finger nails and cuticles are the classic areas where people receive burns, having washed off the HF without washing under their nails. If washed off within a few minutes of exposure, HF may do no harm. Remember, HF may not produce any burning sensation until after it has already done damage. All HF burns should be looked at by a physician.

Acid protective gear MUST be worn when working at this sink. A lab coat, goggles, acid-proof gloves (atop the normal clean room gloves), and an acid face mask (with the face shield DOWN) worn over safety glasses, are all required. ALWAYS know thelocation of the nearest eye wash and safety shower.

PROCEDURE:

I. Preparation of Solution  - Will be done by staff prior to lab.

Chemicals
A. DI Water (DIH₂O)
B. Hydrofluoric Acid (HF)
C. Ammonium Flouride (NH₃F)

A. Mix 400 g of Ammonium Flouride with 600 ml of water.
B. Carefully mix Ammonium Flouride Solution with HF in a 6:1 ratio.

Overall reaction for etching SiO₂ with BOE:
SiO₂ + 4HF ==> SiF₄ + 2H₂O
where a buffering agent, ammonium fluoride (NH₄F), is added to maintain HF concentration and to control pH (to minimize photoresist attack):
Buffering reaction:  NH₄F <==> NH₃ + HF.

II. Etching Procedure

A. Mount wafer on teflon wand.
B. The etch rate for BOE is about 1000�/min. Estimate the appropriate etch time required base on your known oxide thickness.
C. Immerse wafer in BOE for the desired time. When complete remove and rinse well with DI water, and blow dry with nitrogen.
D. If the etch was complete, then BOE and water should bead or "dewet" off the wafer backside. Why is that?
E.After the etch is complete, inspect the wafers under the microscope. The exposed patterned regions should appear to be silicon colored (white or metallic colored). If the exposed patterned regions still appeared colored, the wafers should be returned to the BOE. 
F. Strip off the photoresist using an oxygen plasma in the RIE.
G. Using the Surface Profiler characterize the topography of your wafers to ensure that your ecth was complete.

http://inside.mines.edu/fs_home/cwolden/chen435/wetoxide.html

MSDS : http://www.jtbaker.com/msds/englishhtml/B5636.htm