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 for preparation all involving an initial cleaning step followed by a finer cleaning process

Initial preparation:

1. Wash substrates with acetone
2. Sonicate with standard settings

This step removes organic contaminants and loosens attached debris.

Finer preparation:

1. Bake in O₂ plasma oven for 10 mins

   or

2. High pressure jet of liquid CO₂ (CO₂ is used for its sublimation property at room temperature)

The first method introduces high energy oxygen radicals which oxidises surface impurities. The second physically displaces surface debris. This step should remove remaining contaminants

Note: The resulting surface though significantly improved is not defect-free but sufficient for our purposes.

It was found the most uniform films seemed to result from high pressure washing with liquid CO₂ (performed by Dr Matthew Hawkeye)

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

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)

Silicon Substrates in the O2 Plasma Oven

CO₂ Pressure Wash

1. Use vaccuum suction to hold down silicon wafer
2. Direct high pressure washer and pull trigger much like washing a car

Notes: This process was carried out in a fumehood to minimise all risk as there was solvent evaporation

Safety

• Kept acetone in squirt bottles 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.

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!