BOE (Buffered Oxide Etch) Storage & Tank Compatibility

Storing BOE (Buffered Oxide Etch)? Start Here

Buffered oxide etch (BOE), also called buffered HF or BHF, is an aqueous etchant used in microfabrication to remove silicon dioxide (SiO₂) and silicon nitride films. It is a formulation — not a single pure compound — built from hydrofluoric acid (HF), an ammonium fluoride (NH₄F) buffer, and water, sometimes with a surfactant. Common volume ratios such as 6:1, 7:1, 10:1, and 20:1 (NH₄F solution to HF) trade etch rate for control. The ammonium fluoride stabilizes pH and free-fluoride concentration, giving a slow, repeatable etch that is gentler on photoresist than raw HF. Material of construction matters intensely because the same chemistry that etches oxide wafers also etches glass and glass-reinforced plastics, and HF corrodes most metals. The wetted material must resist HF while tolerating fluoride and moderate acidity — which points squarely at polyethylene and fluoropolymers.

Polyethylene (HDPE / XLPE) Compatibility with BOE

Verdict: Compatible (S) — polyethylene is the recommended material for HF/BOE. Hydrofluoric acid is the classic case where plastic, not glass or metal, is required: HF attacks silica, so glass and glass-fiber FRP are unusable, while HDPE is rated excellent against hydrofluoric acid at both 20°C and 50°C in published resistance data and is the standard container for HF. For BOE storage, choose HDPE (or PP / PVDF / PTFE for wetted hardware). Two cautions: keep service temperature moderate — HDPE's practical ceiling is roughly 60-80°C, and resistance falls as temperature rises — and prefer HDPE over crosslinked polyethylene (XLPE), since some references note HF can slowly permeate XLPE. Always confirm fittings, gaskets, and vent hardware are HF-rated (e.g., fluoropolymer or Viton seals) and verify against the specific product SDS, as concentration and any surfactant change the picture.

Material compatibility at a glance

BOE is hydrofluoric acid buffered with ammonium fluoride, so the dominant compatibility driver is HF — it readily attacks silica (glass, borosilicate, glass-fiber FRP) and corrodes most metals. Polyethylene is the correct, industry-standard material: HDPE is rated excellent against HF and is the recommended container. Use HDPE, PP, or fluoropolymer (PVDF/PTFE); avoid glass, FRP, and steel. Keep service temperature moderate and prefer HDPE over XLPE.

Ratings: S suitable · C conditional / limited · U unsuitable. Verify against the cited resistance charts and your concentration/temperature before specifying.

The safety that actually matters

• HF systemic toxicity: Hydrofluoric acid penetrates skin and binds calcium — severe, deep, delayed burns and possible systemic effects (hypocalcemia, cardiac risk) even from small skin contact. Calcium gluconate antidote must be on hand.
• Toxic by multiple routes: Toxic if swallowed, in contact with skin, or if inhaled (H301/H311/H331).
• Severe corrosivity: Causes severe skin burns and eye damage (H314); attacks glass and many metals.
• Vapor / fume hazard: Releases corrosive, toxic hydrogen fluoride fumes; use only with fume extraction and proper PPE.
• Incompatibilities: Reacts dangerously with strong acids (liberates HF gas), strong bases, glass, and most metals (may generate hydrogen).
• Containment: Store in closed HDPE or fluoropolymer containers, away from incompatible materials, with secondary containment and HF-specific spill response.

Common questions

Can I store buffered oxide etch in an HDPE tank?

Yes. HDPE is the standard, recommended container for HF and HF-based formulations like BOE and is rated excellent for hydrofluoric acid service. Keep the temperature moderate (well below ~60°C), use HF-rated fittings and gaskets, and provide secondary containment. Always confirm against the specific product SDS.

Why can't BOE be stored in glass?

BOE contains hydrofluoric acid, which reacts with silica (SiO₂) — the main component of glass and borosilicate. The same reaction used to etch oxide wafers will etch and weaken a glass container, so glass, borosilicate, and glass-fiber FRP must be avoided.

What does BOE actually do, and what's in it?

BOE etches silicon dioxide and silicon nitride films in microfabrication. It is a mixture of hydrofluoric acid (HF) and an ammonium fluoride (NH₄F) buffer in water; the buffer stabilizes pH and free fluoride for a slow, controllable etch that is easier on photoresist than raw HF.

Is XLPE okay for BOE, or only HDPE?

HDPE is the safer choice. While polyethylene as a class resists HF well, some references note that hydrofluoric acid can slowly permeate crosslinked polyethylene (XLPE). For HF/BOE, prefer HDPE (or PP / fluoropolymer) and verify suitability against the product SDS and your supplier.