Page 154 - Sami Franssila Introduction to Microfabrication

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Wafer Cleaning and Surface Preparation

Microfabrication takes place under highly controlled integrity (low interface trap density, low oxide charge,
conditions: all materials for cleanroom construction, and small leakage current).
processing equipment and wafer-handling tools are
carefully selected to minimize particle, molecular or 12.1 CONTAMINATION FORMS
ionic contamination. Water, gases and chemicals are
purified of contaminants and filtered of particles. These Contamination comes in various forms, which have dif-
are, however, passive precautions, and active wafer ferent sources, effects on device and cleaning methods.
cleaning must be undertaken before practically every The main classes of contamination are
major process step. Wafer-cleaning steps can account
for up to 30% of all process steps. – particles
Wafer cleaning is about contamination control, but – metals
it is also about leaving the surface in a known and – organics
controlled condition. This means damage removal, sur- – volatile inorganic contamination
face termination (hydrophobicity/hydrophilicity control) – native oxide
and prevention of unwanted adsorption. Therefore, many – microroughness.
people prefer to call this activity surface preparation.
The main sources of contamination are the fabrication Particle-size monitoring is becoming a problem in
processes themselves. Air cleanliness in an advanced advanced integrated circuits; in 130 nm processes, par-
cleanroom is so good that airborne particles are not ticles greater than 65 nm are monitored. A few decades
the main contamination source anymore, but airborne ago, particles of the size 1/10 of minimum linewidth
gaseous contaminants need careful attention. The human could be detected (with reasonable throughput), and
contribution has also been reduced significantly with more recently, particle detection at one-third of mini-
correct gowning and working procedures or by factory mum linewidth was the norm. As scaling continues, it
automation. These matters are dealt in more detail in may be that monitored particle size will be identical to
Chapter 35. minimum linewidth. Particles are also a major concern
The purity of starting materials is important: liquid in wafer bonding (Chapter 17), irrespective of linewidth.
chemicals for advanced IC processes come with 1 Metal contamination cannot be avoided as long
or 0.1 ppb (parts per billion) impurity specifications. as machine parts are made of metals; so, metal
Sputtering target purities are, for example, 99.999%. contamination has to be controlled by cleaning. Metal
Similar ‘5Nine’ purities are typical for many process contamination on the surface can spread into the silicon
gases, but some applications need 99.99999% (7N) bulk, and dissolved metals and metal precipitates in the
purity. Water purity is measured by resistivity: typical bulk act as recombination centres for charge carriers.
requirement is 18 Mohm-cm. This de-ionized water Precipitates at silicon/oxide interface or in the critical
(DIW) is also known as UPW, for ultra pure water. areas of the device are detrimental because they affect
Because of device-size downscaling, contamination diffusion profiles via their effect on crystal defects. If
becomes even more critical. Finer patterns demand metals segregate into the oxide during oxidation, they
control of finer particles, and ultra-thin gate oxides can prevent, retard or degrade oxide film growth, and
necessitate low metal contamination levels for good result in poor-quality oxides.

Introduction to Microfabrication Sami Franssila
 2004 John Wiley & Sons, Ltd ISBNs: 0-470-85105-8 (HB); 0-470-85106-6 (PB)