Piranha Clean

Standard Process: Piranha Clean

A completely dry glass container should be used.
Add 3 parts Sulfuric Acid (H₂SO₄).
Add 1 part Hydrogen Peroxide (H₂O₂).
Allow the solution to work for 5-10 minutes.
With sink running and while flushing with large amount of water, slowly pour used Piranha solution down drain.

Video SOP (for degreasing substrates only, not for metal ion sensitive processes)

This video shows cleaning using metal tweezers, which can cause metal ion contamination. If doing wafer processing use solid Teflon tweezers or dippers.)

Background

Piranha is a strong oxidizing agent that will strip surfaces of organic (carbon containing) matter and hydroxylate surfaces (add OH groups, making the surface hydrophilic). Note that Piranha will not remove inorganic surface contamination such as metals (Reinhardt 2008, p. 24) or particles. Piranha is self heating, and mixtures are typically used at the self heating temperature for 10 minutes. A more complete clean can be had from heating the mixture on a hotplate to 100-130 C for 10-15 minutes (Reinhardt 2008, p. 23).

Samples should be free of large areas of organics. Piranha should not be used for removing a layer of PR off of a sample, a solvent clean should be used for large amounts of organics. Piranha is appropriate for cleaning off any remaining organic residue after a solvent clean, or for cleaning a new wafer. Attempting to strip large amounts of PR may result in a hardening of the PR layer (presumably due to the formation of undissolved elemental carbon), which will then be extremely difficult to remove.
Piranha may roughen your substrate. Piranha is generally safe for use with inorganic substrates, but may roughen the surface on the nanoscale.
Piranha will/may etch metals. Check Williams 2003 for more info.
Ratios of H₂SO₄ and H₂O₂are important. Getting close to the 3:1 ratio (3 parts H₂SO₄: 1 part H₂O₂) is important. Ratios down to 7:1 will still give powerful cleaning, but something within the range of 3:1 to 7:1 should be used. Adding more peroxide (e.g. a 1:1 mixture) both presents a safety hazard and weakens the strength of the solution, and should not be done.
Contrary to the conventional rule of "Always Add Acid" in chemistry, Piranha should always be prepared with acid first, and peroxide slowly added second. Improperly prepared Piranha may bubble violently when adding the chemicals, this is avoided by adding chemicals slowly and adding the sulfuric acid first.
Water should NOT be added to Piranha solution and ALL beakers, tweezers and samples added to Piranha must be completely dry. Piranha is self heating, and can easily reach temperatures above 100 C. The addition of water can cause violent boiling.
Piranha solution should be left uncovered, and under no circumstances should it be sealed. The oxygen and oxidation byproducts produced by the reaction are flammable, and sealing the container can cause them to explode.
Piranha etches standard metal acid tweezers and will react explosively with "plastic" tweezers.
- Standard metal tweezers may be used safely with Piranha solutions, but the tweezers will be etched, and this etched iron will/may contaminate glassware and your sample.
- Carbofib tipped tweezers should NOT be used with Piranha. Carbofib is PEEK reinforced with carbon fiber, and will not be inert to Piranha, or other strong acids.
- "Plastic" tweezers should absolutely NOT be used with Piranha (or any metal including tweezers).
- Teflon coated metal tweezers (typically green) should not be used (or inspected often for damage), as Piranha will remove the outer coating and expose the bare metal underneath.
- The preference is for solid Teflon tweezers (PTFE) will be both chemically resistant to Piranha and will not present any contamination issues, but these are hard to hold samples with.

Piranha alternative: Nanostrip

Birck supplies Nanostrip, a stabilized Piranha alternative. Standard Nanostrip consists of 90% sulfuric acid (H₂SO₄), 5% peroxymonosulfuric acid (H₂SO₅), 5% water (H₂O), and <1% hydrogen peroxide (H₂O₂). A more powerful version, Nanostrip 2X, consists of 85% sulfuric acid (H₂SO₄), 10% peroxymonosulfuric acid (H₂SO₅), 5% water (H₂O), and <1% hydrogen peroxide (H₂O₂).

Nanostrip can be used at room temperature, but to be an effective cleaner, it should be heated to 60-80 C. More information can be found here: https://web.archive.org/web/20161023223906/http://cyantek.com/nano-strip.php

Room temperature Nanostrip is discouraged compared to either heated Nanostrip (to 60-80 C) or a freshly mixed Piranha solution due to:

Decreased cleaning power.
Difficulty of cleaning and contamination concerns. Room temperature Nanostrip is highly viscous (being 90% concentrated sulfuric acid), and is far more difficult to wash off than either heated Nanostrip or freshly mixed Piranha. While any "type" of Piranha presents potential sulfur contamination issues, room temperature Nanostrip is especially prone to contamination due to the difficulty in washing. Nanostrip residue may be left, which will not only contaminate your sample, but may also outgas in high temperature or vacuum systems, eating away the tool.

Oxide Growth and Removal

On silicon, Piranha will grow a contaminated oxide layer. This should be removed with a quick dip in HF/BOE, followed by thorough rinsing. As always with HF/BOE, all appropriate safety procedures and precautions MUST be followed.