Page 263 - Semiconductor Manufacturing Handbook

Geng(SMH)_CH18.qxd 04/04/2005 19:58 Page 18.4

WET CLEANING

18.4 WAFER PROCESSING

TABLE 18.1 Typical Wet Cleaning Chemistries
Chemistry Common name Purpose or removal of
NH OH/H O /H O RCA-1, SC-1, APM Light organics, particles, and metals
4 2 2 2
HCl/H O /H O RCA-2, SC-2, HPM Heavy metals, alkalis, and
2 2 2
metal hydroxides
H SO /H O Piranha, SPM, “Caros acid” Heavy organics
2 4 2 2
HF/H O HF, DHF (dilute HF) Silicon oxide
2
HF/NH /H O BOE (buffered oxide etch), BHF Silicon oxide
4 2
HNO Nitric Organics and heavy metals
3
+
(CH ) N CH CH OH⋅OH Choline, Trimethyl (2-hydroxy-ethyl) Metals and organics
3 3 2 2
ammonium hydroxide
(CH ) N CH CH OH⋅OH/ Choline/peroxide Heavy metals, organics, particles
+
3 3 2 2
H O /H O
2 2 2
(NH ) SO /H SO SA-80 Organics
4 2 4 2 4
H S O /H SO PDSA, “Caros acid,” Piranha Organics
2 2 8 2 4
O /H O Ozonized water Protective oxide regrowth, organics
3 2
H SO /O /H O SOM (sulfuricozone mix) Organics
2 4 3 2
HF/HNO Hydrofluoric acid/nitric acid Slight Si etch; metals
3
HF/H O Hydrofluoric acid/hydrogen peroxide Slight Si etch; metals
2 2
Source: Burkman D., D. Deal, D. Grant, C. Peterson, Handbook of Semiconductor Wafer Cleaning Technology, W. Kern
(ed.), p. 121, Noyes Publications, New Jersey (1993).
of organics, alkali ions, and metals. The choice of chemicals is based on their ability to selectively react
with contaminants to either dissolve them (or solubilize) or to cause them to be dispersed in the liquid. 8
The most common chemicals used in wet chemical cleaning are listed in Table 18.1.
In the front end-of-line the most commonly used wet clean approach to remove particles and met-
als from the wafer surface is the Radio Corporation of America (RCA) method via immersion of a
batch of wafers in a tank. This approach is well characterized and has thus far proved successful for
the existing technology requirements of the industry. Some advantages of batch wet chemical clean-
ing are the removal of metal ions and soluble impurities and the cost of the process. Disadvantages
include the requirement of additional process steps such as deionized (DI) rinsing and wafer drying.
Furthermore, in the case of immersion, there is the potential for redistribution of particles within the
wafer batch, from a wafer backside to a front side surface.
Scrubbing. Scrubbing involves the use of rotating brushes that move across the surface of a wet
wafer as shown in Fig. 18.1. The brushes are typically made of nylon or polypropylene or some other
organic polymer. In theory, these brushes never touch the surface of the wafer because the materials
are hydrophilic and therefore have a boundary layer of liquid that is always between the brush and
the wafer surface. A mathematical model of this interaction utilizes the critical particle Reynolds

Rotating brush Brush motion

Fluid
Wafer
FIGURE 18.1 Typical scrubbing system.

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