Team:Cambridge/Protocols/Substrate Preparation for Flow Coating and Spin Coating

Substrate preparation

Methods used to prepare the silicon substrates for flow coating and spin coating.

Theory

The silicon pieces were cut to size and treated with various methods to remove surface debris.

Note: A clean interface is essential for providing good contact for thin films.

Various methods were employed during our thin film making.

1. Acetone bath and sonication followed by either by O₂ plasma oven or high pressure jet wash with liquid CO₂ (CO₂ is used for its sublimation property at room temperature)
2. 'Piranha Solution', a potent bath of concentrated sulphuric acid and hydrogen peroxide

The methods are all either based upon cleaning up organic contaminants and debris via some chemical reaction like the introduction of oxygen radicals or by physical removal like the pressure wash.

Practice

Cutting Silicon to Size

1. Introduce a fine nick in surface of single-crystal silicon wafer
2. Gently bend upwards to precisely fracture the silicon in two
3. Repeat until desired dimensions reached

Notes: These silicon wafers are single crystal and have one orientation(100). Introduction of a 'directed' crack 'seeds' a defect into the surface by introducing a stress concentration. Subsequent Application of bending elicits fracture propagation along the defect direction.

Interface Cleaning

Silicon Substrates in the O2 Plasma Oven

Acetone Bath and Sonication

1. Place the substrates in a rack and submerge in beaker of acetone
2. Cover with foil to prevent outside debris and sonicate at standard settings for 10mins

O₂ Plasma

1. Following solvent cleaning place samples in plasma oven for 10 minutes. (time can be adjusted depending on purity levels)

Piranha Solution

1. Fill beaker with 50% HPLC grade sulphuric acid and 50% hydrogen peroxide HPLC grade
2. Place beaker on hot plate at 100^oc
3. Place all the prepared silicon wafers within the beaker (It does not matter if the silicon wafers overlap)
4. Leave for approximately 10 mins or when bubbling subsides.
5. When ready to spin take out desired silicon wafer for spinning, rinse with distilled water and blow dry with N₂ gas.

As explained to me by the lab researchers in the Thin Films Group absolutely nothing survives the piranha bath!

Safety

• Kept acetone in squirt bottles and piranha solution in the fume hood.
• Kept beaker containing substrates during sonication also in fume hood with foil to keep out debris
• The O2 plasma oven and CO₂ pressure washer were self contained units, operated and loaded by Dr Matthew Hawkeye.
• Always wear nitrile gloves and lab coat when handling such concentrated solutions of sulphuric acid and hydrogen peroxide.

Note of advice: Carry out all work in fume hoods even if it is safe not to do so. You are working with a lot of solvents and vapours which are potentially hazardous to health. Better safe than sorry!

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