Page 128 - Sami Franssila Introduction to Microfabrication

10

Lithographic Patterns

We will now discuss photoresists. Resist chemistry and new resist formulation with different solid content must
resist working principles will be covered. In Chapter 9, be used. Viscosity is dependent on resist solid content
we treated resists as if they were digital on/off materials (which can vary from 20–80%) and temperature. The
that either react under exposure or do not; now we are solvent evaporation rate depends on ambient environ-
dealing with more realistic cases: resists have exposure ment, and a closed spinner bowl with saturated solvent
threshold energy, finite contrast and finite selectivity vapour and adjustable exhaust can be used to control
in developers. Resists are also optical materials and evaporation.
they are part of an optical system with reflections, On a planar surface, a 5 nm thickness variation across
interference and absorption. All these aspects become the wafer is standard for a 1 µm thick resist. Spin
more pronounced when resists go over topography;
processing over severe topography is difficult: liquid-
patterning on a planar surface is fairly straightforward. like film will fill grooves and crevasses, and a highly
Simulation of lithography will also be presented.
non-uniform resist thickness results (Figure 10.1). This
is a problem for textured solar cells (Figure 1.6) or
10.1 RESIST APPLICATION deep-etched MEMS structures (Figure 1.10). On the
other hand, this planarizing effect is sometimes used
The lithography process starts by a surface prepara- to advantage.
tion step like almost all microfabrication processes. In There are three more resist coating technologies: elec-
order to remove moisture, the wafers are baked. The
trochemical coating, spray coating and casting. Elec-
next step, wafer priming, also known as adhesion pro- trochemical coating requires special resist formulations,
motion, ensures known surface conditions. Hexamethyl spray is applicable to thin resists. Casting is suitable
disilazane vapour (HMDS, (H 3 C) 3 –Si–NH–Si–(CH 3 ) 3 ) for thick resists only. These techniques are especially
is applied at reduced pressure to form a monomolecular suited to applications in which resist coverage is needed
layer on the wafer surface, making the wafer hydropho- over severe topography, where spin coating is notori-
bic, which prevents moisture condensation. This is espe-
ously bad.
cially important for materials like metals, polysilicon
Thin resists are preferred for better resolution; but
and PSG, because resist adhesion to these materials is
poor. Adhesion promotion is also a guarantee against thinner resists are prone to particle defects, and pinhole
cleanroom humidity variations and an equalizer for density rapidly increases when resist thickness is scaled
wafers with different storage times. down. Spin-bowl cleaning is also a major particulate
Spin coating is the standard resist application method control issue: frequent cleaning prevents layer growth,
(recall Figure 5.9). A few millilitres of resist is applied and thus flaking of residual film from the walls.
on a static or a slowly rotating wafer. Acceleration to Even monolayer resists have been used in research
ca. 5000 rpm spreads the resist over the wafer, leaving applications. They can be used as etch masks for shallow
a very uniform layer. The remaining solvent evaporates etchings in the 10 nm range, or as electrodeposition
during soft bake, for example, 90 C, 30 min in an oven masks, but clearly are not general purpose resists.
◦
◦
or 90 C, 60 s on a hot plate. Monolayer resists are not spin coated: self-assembled
Spin speed can be used to tailor resist thickness over monolayers (SAMs) and Langmuir–Blodgett techniques
one decade, for example, 0.5 to 5 µm, but beyond that a are employed.
Introduction to Microfabrication Sami Franssila
 2004 John Wiley & Sons, Ltd ISBNs: 0-470-85105-8 (HB); 0-470-85106-6 (PB)