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

Silicon substrates are cleaved using a diamond scribe to introduce a fine nick in the surface of the single silicon crystal. Then upward pressure at the nick and downward pressure at the edges of the crystal an be used to gently and precisely fracture the silicon in two.

Acetone bath and sonication is achieved by placing the substrates in a rack and placing in a beaker filled with enough acetone to cover them. This is covered with foil and placed in a sonicator for 10 minutes. Following this the O2 plasma oven can be used to burn off contaminants, we used 10 minutes in this oven to make sure our samples were clean.

Silicon Substrates in the O2 Plasma Oven

CO2 pressure washing was not performed by us, and is as simple as pointing the pressure washer and pulling the trigger. This is performed in a fumehood and uses a strong vacuum to stabilize the sample during washing.

Safety

Acetone was kept in squirt bottles in the fume hood, as was the beaker containing the substrates during sonication. Foil was used to keep debris out and acetone in the beaker. The O2 plasma oven was a self contained unit, operated and loaded by Dr Matthew Hawkeye, as was the CO2 pressure washer.