Page 159 - Sami Franssila Introduction to Microfabrication

138 Introduction to Microfabrication

12.4 ORGANIC CONTAMINATION Because sulphuric acid constitutes an environmental
concern and a safety hazard, other candidates have been
There are many sources of organic contamination in sought for organics removal. Ozonated DI-water with
the cleanroom. Table 12.3 below lists some of the most 10 to 100 ppm ozone has proven to be very effective for
usual ones.
some organic contamination. Furthermore, it is a room
◦
temperature process, versus 120 C SPM. The ultimate
12.4.1 Organics removal cleaning method for organic contamination is thermal
◦
oxidation: no organic compound can tolerate 1000 C in
Sulphuric acid peroxide mixture (SPM) removes organ- oxygen atmosphere. This provides a reference surface
ics by oxidizing decomposition. This is however, a for analytical methods, but of course it is not a practical
slow method, and other mechanisms are at work. Bond cleaning process.
breakage and subsequent formation of smaller molec-
ular mass fragments that are more soluble can explain
fast organics removal. SPM cleaning leaves difficult- 12.4.2 Measurement of organic contamination
to-remove sulphur residues, and RCA-1 step is often Organic contamination can be conveniently measured by
carried out immediately after SPM to turn sulphides into FTIR (Fourier transform infrared spectroscopy), which
soluble sulphates. identifies not only elements but also chemical bonds,
Oxidation of wafer surface by peroxide and the as shown in Figure 12.7. FTIR can be operated in
subsequent removal of this thin oxide by HF is shown attenuated total reflection mode (ATR-FTIR) to improve
in Figure 12.6. Organic films can prevent oxidation by sensitivity. XPS is very surface sensitive, and it can also
peroxide for some time, which leads to unequal oxide identify chemical bonds, which is often important in
thickness, and, after HF etching, to increased surface understanding the origin of the contamination.
roughness. Extended cleaning would remove organics Molecular surface contamination can be measured by
and lead to uniform oxide thickness and consequently thermal desorption spectroscopy (TDS). TDS consists
no roughness increase.
of a furnace connected to a mass spectrometer, and
desorption of contaminants is monitored as a function
Table 12.3 Sources of organic contamination of the furnace temperature. Silicon surface condition has
◦
also been clarified by TDS: at 340 C, water desorbs, at
– Liquid chemicals and vapours used in fabrication ◦
processes: HMDS, isopropyl alcohol (IPA), acetone 400 C, hydrogen-terminated silicon surface undergoes
1
◦
reaction SiH 2 → SiH + H 2 and at 500 C SiH → Si +
2
– Gases, for example according to reaction nCF 4 →
1
(CF 2 ) n + 2nF ∗ 2 H 2 . Baking can therefore be used as an in situ surface-
– Organic films (resist, spin-on polymers) cleaning method.
– Wafer holders and boxes
– Vacuum systems: pump oils, o-rings 12.5 METAL CONTAMINATION
– Cleanroom materials: sealants
– Intake air There are numerous sources of metals, even though

alternative materials like silicon, Teflon , SiC and quartz
are extensively used in making process equipment and
wafer-handling tools. Table 12.4 lists some common
sources of unwanted metals.
Table 12.4 Sources of metal contamination
(a) (b)
– Tool materials (shutter blades, collimators, chucks)
– System components (pipes, valves)
– Wafer handling (tweezers, robot arms, wafer holders)
– Impurities in chemicals (buffered HF, BHF, is a
(c) known source of copper)
– Chemicals themselves (some photoresist developers
Figure 12.6 Organics removal: (a) organic residue on are NaOH)
surface; (b) residue retards oxidation in H 2 O 2 and (c) – Human contribution (sodium from sweat, heavy
oxide removal in HF results in increased surface roughness. metals from cosmetics)
(Based on Hattori/Realize Inc.)