Page 156 - Sami Franssila Introduction to Microfabrication

Wafer Cleaning and Surface Preparation 135

efficiency and roughness increase. Decomposition of
cleaning solutions and impurities can also catalyse sur-
face reactions leading to increased roughness.

12.2 WET CLEANING
Acid, base and solvent wet cleanings are the main
methods of cleaning. Dry cleaning by, for example,
vapours and plasmas offers some advantages that will
be discussed in Chapter 34. Wet cleaning is simple, it
has high throughput and it cleans both the front and the
back of the wafer simultaneously (see Figure 12.3). Wet
Figure 12.3 A wafer cassette with 25 wafers of 100 mm
benches are reliable tools, but chemical consumption can
diameter is being lowered into a cleaning bath. Photo
be high. There are two main approaches: either using
courtesy Paula Heikkil¨ a, Helsinki University of Technology
rather concentrated chemicals for cleaning many batches
before changing the chemicals or using dilute chemicals
and changing them after each and every batch. of work was done in uncovering the mechanisms of
From the end of the 1960s till the early 1990s, wet contamination and contamination removal.
cleaning relied on a few proven methods, which were, The standard clean, known as the RCA-clean
however, never studied in detail, and whose working (invented at RCA Laboratories), consists of a sequence
mechanisms were unknown. In the 1990s, a vast amount of different wet cleans. They are each effective in

Table 12.1 Wet-cleaning solutions: typical compositions and conditions
Name/alias Chemical composition Temperature/time
◦
RCA-1 NH 4 OH:H 2 O 2 :H 2 O (1:1:5) 50–80 C, 10–20 min
SC-1, standard clean; aka
APM; ammonia peroxide
mixture
◦
RCA-2 HCl:H 2 O 2 :H 2 O (1:1:6) 50–80 C, 10–20 min
SC-2; standard clean-2;
aka HPM, hydrogen
chloride-peroxide mixture
◦
SPM H 2 SO 4 :H 2 O 2 (4:1) 120 C, 10–20 min
Sulphuric peroxide mixture,
aka Piranha
DHF (dilute HF) HF:H 2 O (1:20 – 500) Room temperature, 1 min
Standard chemicals come in
the following
concentrations:
HCl 37%
H 2 SO 4 96%
H 2 O 2 30%
NH 4 OH 29%
HF 49%

Bath life: If the bath is used for more than one batch before changing, chemical concentration is monitored, and, for
example, ammonia evaporation or peroxide decomposition can be compensated by ‘spiking’, that is, refreshing the bath
with an injection of fresh chemicals.
Disposal: HF requires a separate disposal system because its health effects are different from other mineral acids, which
may all be collected in the same container. Sometimes, acids that contain heavy metals must be collected separately
(e.g., titanium or cobalt containing salicide etchants).