INTRODUCTION:
Buffered oxide etch (BOE) is used to remove SiO2. BOE is a very selective etch, meaning that it stops at the silicon and does not etch further. The etch may be used 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 immediately after exposure, HF may
do no harm. Also, immediately let the lab instructor know of any HF contact
(immediately after flushing the area with water). 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 using this etch. 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 the location of the nearest eye wash and safety shower and ALWAYS work
with a partner.
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 wafer holder.
B. The etch rate for BOE is about 1000Å/min. Estimate the appropriate etch time
required based 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 etched
regions with exposed silicon 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. Alternatively,
positive photoresist can be removed in acetone (possibly with ultrasonic agitation).
G. Using the Surface Profiler characterize the topography
of your wafers to ensure that your etch was complete.