Page 161 - Sami Franssila Introduction to Microfabrication

140 Introduction to Microfabrication

Segregation of contaminants between Si and SiO 2 has 12.5.3 Measurement of metallic contamination
a major impact on the effects of metallic contamina-
tion: during thermal oxidation, Al, Ca, Cr and Mg are Metal contamination surface concentrations range from
14
10
2
incorporated into the oxide and contribute to oxide qual- 10 to 10 atoms/cm , depending on technology gen-
eration, contamination-control strategies and particu-
ity problems, whereas Fe, Cu and Ni diffuse in silicon
lar process steps. Total reflection X-ray fluorescence
bulk.
(TXRF) uses a grazing incident angle to probe the wafer
Non-electronic devices are less sensitive to metal con-
surface to nanometre depth. It is most sensitive for
tamination, but metals cannot be completely ignored:
medium-mass atoms, and less sensitive towards both
metal contamination causes stacking faults in oxida-
ends of the mass range. Detection limit of TXRF is ca.
tion, and metals can catalyse peroxide decomposition,
2
9
10 atoms/cm . TXRF is a non-destructive method that
which leads to reduced particle-cleaning efficiency in
can be used on whole wafers.
RCA-1.
In vapour-phase decomposition (VPD) and wafer
surface analysis (WSA) methods, surface impurities
are first collected in oxide (native oxide or chemical
12.5.2 Metal removal
oxide), which is then decomposed by HF and collected
in a droplet. This concentrate is analysed by the
Acidic solutions HCl–H 2 O 2 and H 2 SO 4 –H 2 O 2 are the graphite furnace atomic absorption spectroscopy method
main methods for metal removal. Dilute HF, which (GFAAS) or by the inductive coupled plasma-mass
removes a thin oxide layer, will additionally remove spectrometer (ICP-MS), which can have sensitivities as
some metallic contaminants. Ammonia solutions (RCA- low as 10 cm .
8
−2
1) can also form complexes with metals and remove Metallic contaminants can be measured by their
Cu and Ni. effects on charge carriers. Minority carrier lifetime will
The cleaning efficiencies of HCl–H 2 O 2 and HF are be degraded by contamination. Surface photovoltage
very different, though. Both can reduce Fe and Ni
SPV and microwave photoconductivity decay (µPC)
levels below detection limit, but HF is much more
methods provide this information.
effective in removing Al, and HCl–H 2 O 2 in removing
Cu. Dilution of HF needs to be specified because various
workers use different concentrations. For aluminium 12.6 RINSING AND DRYING
removal, 0.1% DHF (by weight) is enough, but below
that the removal efficiency rapidly deteriorates. HCl Rinsing in DI-water and drying must be considered as
concentration in HCl–H 2 O 2 has to be at least 5% for essential parts of any cleaning process. As a general
it to remove iron. strategy, we should keep the wafer wet all along
The wet chemicals themselves contain metallic impu- the cleaning process and reduce the number of times
rities, and at the 10 ppb level their deposition on wafer when wafers are drawn from liquid to air. When
surface is of some concern. For example, iron at 1 ppb drying is required, there are a number of methods
level in RCA-1 solution results in a surface concen- available: spinning, nitrogen blowing, vapour drying,
2
tration of 10 12 atoms/cm . Metal removal after RCA-1 lamp drying, vacuum drying, and dry wafers can also
has to be performed. The use of higher-purity chemi- emerge from slow removal from hot DI-water. Spinning
techniques are prone to charging and particle adherence,
cals helps to reduce the need in the first place, but it
which are inherent in high-speed spinning equipment.
cannot be relied upon as the sole method because of
Various isopropyl alcohol (IPA) drying methods rely
statistical effects, both in manufacturing and in use (if
on low surface tension and good wettability of IPA.
RCA-1 bath is used several times, contamination from
In Marangoni drying, the wafer is drawn from water
previous batches remains in the solution). RCA-1 must
be accompanied by a cleaning step that removes metals into IPA-nitrogen atmosphere, and water is pulled back,
efficiently. However, both HF- and HCl-based solutions leaving a dry surface. IPA drying methods must be
lead to increased particle counts. considered for chemical consumption, hot vapours and
Newer cleaning solutions include HF:H 2 O 2 , which solvent accumulation.
has both oxidizing and metal-removal capabilities.
◦
It can be used at room temperature versus 70 C, 12.7 PHYSICAL CLEANING
which is typical of RCA-cleans. HF:H 2 O 2 seems to
increase surface roughness, so cleaning time needs to Three methods of physical removal of particles are
be optimized. widely used: