Fabrication of Biosensor Chips - From Bioimaging to Biosensors: Noble Metal Nanoparticles in Biodetection - page 105

6.1.1 Substrate Cleaning

In this section we introduce how to process the substrate materials
based on silicon and glass. Clean wafers are essential at all stages of
the fabrication process. To clean silicon, four methods are commonly
used and effective as below. Readers would try these methods
according to their different needs and wafer conditions.
(a) Removes organic contaminants by acetone: Organic
contaminants, usually produced in process steps of
semiconductor fabrication such as photoresist residue and
dry etched residue or exposed at organic molecule-contained
environment for a long time, are required removal during the
initial process. The simplest way is as follows: (i) immerse the
whole wafer in acetone. Use a swab to wash the wafer surface
or an ultrasonic bath would enhance the cleaning. (ii) Followed
by processing the wafer with methanol and then rinse with
water two to three times. (iii) Blow dry the wafer with N 2 gun
or spin the wafer to use the centrifuge force drying the wafer.
(b) Oxygen plasma etching: Plasma cleaning is also a common
way used to remove organic ilms and residues. Removing
residues or photoresist from a substrate through gentle
plasma cleaning is called descumming. Plasma is a cleaning
agent and alternative to traditional wet chemical cleaning
methods because it does not require using hazardous solvents.
However, the cost of plasma etcher is much expensive than
chemicals. If the equipment is available, the cleaning quality
and uniformity can be well-controlled by properly adjusting
the process parameters, such as oxygen pressure, plasma
power, etc.
(c) Piranha clean (hot sulfuric acid / hydrogen peroxide mixture):
Piranha is highly reactive. It attacks organic materials, such as
photoresist or oil stain, strongly and effectively but need to
use it carefully with full protective equipment. Usually when
the above two methods are not workable, piranha cleaning
could be a good try. The preparation of piranha is to mix 98%
H 2 SO 4 (sulfuric acid) and 30% H 2 O 2 (hydrogen peroxide) in
volume ratios of 2-4:1. Usually the samples are immersed
in the piranha solution (Heat to 100°C) for 10-20 min and
followed by water rinse for 15 min, and then blowing the
processed samples dry with nitrogen gun.