FILTRATION
21
Focusing on HMDS
Lens hazing is caused by salt deposition and by
siloxane contamination, which causes severe and
sometimes irreversible damage. To date the
concern in lens hazing has focused on siloxane
contamination from hexamethyldisilazane (HMDS).
HMDS is commonly used as a wafer treatment
to improve adhesion of the photoresist on to the
wafer, and the presence of HMDS has been linked
to lens hazing in the 193 nm lithography tools
2
.
Once the lens has become contaminated with haze
it must be cleaned or removed and replaced. This How HMDS converts to Figure 2: TMS and
type of repair requires the adjustment of the TMS/HMDSO NH
3
, from HMDS,
optical axis and involves extensive downtime. The Drawing on its filtration expertise and R&D removal with
detrimental effects of siloxane contamination are resources, Donaldson set out to analyse HMDS by activated carbon
not limited to lenses on lithography tools, but also running it through an acid catalyst to generate
reticles and masks, inspection tools, and other TMS in the lab. HMDS has been shown to
critical surfaces. Small amounts of siloxanes hydrolyse in aqueous solution to trimethylsilanol
normally found in indoor air are enough to cause (TMS) plus ammonia as shown in equation 1, or
problems, so these and additional siloxanes must directly to hexamethyldisiloxane (HMDSO) and
be eliminated. ammonia as shown in equation 2.
3
Through its lab tests and analysis, Donaldson
subsequently discovered the new breakthrough that
TMS undergoes acid catalysed dimerisation into
HMDSO as shown in Equation 3.
In order to protect expensive optics, it is
essential to effectively control not only HMDS, but
equally importantly, its decomposition by-products,
both ammonia and siloxanes (organics). As shown
in equations 1-3, TMS, NH
3
and HMDSO can
readily be found in the environment of these lens
systems.
Equation 1
((CH
3
)
3
Si)
2
NH + 2H
2
O > 2(CH
3
)
3
SiOH + NH
3
HMDS TMS
Equation 2
((CH
3
)
3
Si)
2
NH + H
2
O > ((CH
3
)
3
Si)
2
O + NH
3
HMDS HMDSO
Equation 3
2(CH
3
)
3
SiOH H+ > ((CH
3
)
3
Si)
2
O + H
2
O
TMS HMDSO
September 2008 www.euroasiasemiconductor.com
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