Page 77 - Sami Franssila Introduction to Microfabrication

56 Introduction to Microfabrication

Resist dispensing Acceleration Final spinning 5000 rpm
(a few millilitres) (resist expelled) (partial drying via evaporation)
Figure 5.9 Spin-coating process

on the wafer size and desired film thickness, a drop and grow by condensation reaction,
3
of 1 to 10 ml (cm ) is dispensed at the wafer centre.
Acceleration to ca. 5000 rpm spreads the liquid towards (OR) n M–OH + HO–M(OR) n −→
the edges. Half of the solvent can evaporate during (OR) n M–O–M(OR) n + H 2 O
the first few seconds, so rapid acceleration is a must
because viscosity changes with solvent content, and A great variety of simple methods can be used for
radially non-uniform thickness will result from viscosity sol–gel processing: for example, dipping, spraying
differences. Spin speed can be controlled to ca. ±1 rpm, and spinning. Compositional variation (by changing
and an error of ±50 rpm will result in 10% thickness alkoxides ratios) is easy. Thickness can be tailored not
differences. Turbulence (both from the spin process itself only by spin speed but also by chemical modifications
and from cleanroom airflows) and ambient humidity in the organic side chain R. Film thicknesses of
(which is affected by exhaust from the spinner bowl and hundreds of micrometres are possible for both glassy
the cleanroom environmental control) affect evaporation SiO-type materials and ceramics like lead–zirconium
rate, and consequently, film thickness. Pinhole defects in titanate (PZT).
spin-coated films are thickness-dependent: thinner films Drying of gel leads to drastic volume shrinkage
are more defective. Pinholes can be caused by particles (easily by a factor of 10), and the resulting material
on the wafers, and also by particles in the dispensed is known as xerogel. Supercritical drying eliminates
fluid, even though all chemicals in microfabrication have capillary forces and collapse of the gel, leading to
been filtered with submicron filters. Air bubbles formed aerogels, which can be 99% void with only 1% solid
during dispensing (caused by e.g., an unclean dispense material. Such a material could be the ultimate dielectric,
tip) can cause either pinholes or large bubbles, in the with a dielectric constant ε close to unity. Application of
millimetre range. these materials as structural parts in microdevices will
Spin-coated films fill cavities and recesses because be difficult, but as sacrificial materials they could be
they are liquids during spin coating. This is advan- easily removable.
tageous for gap filling and smoothing, but if uniform
thickness over the topography is desired, spinning is not
ideal. Room temperature spinning is always accompa- 5.7 METALLIC THIN FILMS
◦
nied by baking in the range 100 to 250 C.
Metallic thin films have various applications in micro-
fabricated devices.
5.6.4 Sol–gel
A sol is a colloidal suspension of small (1–1000 nm) Conductors: Resistivity is the main consideration: alu-
particles in a liquid. A gel is 3D solid network that minum and copper are main choices for most appli-
forms in a colloidal liquid. A typical sol–gel process cations, and gold is often used in RF devices, like
uses metal alkoxides M–(O–CH 3 ) n in organic solvents. inductor coils, to minimize resistive losses. Doped
Alkoxides hydrolyze according to silicon (and polycrystalline silicon) can be used as a
conductor, but its resistivity is very high compared
M(OR) n + xH 2 O −→ M(OH) n + xROH to metals.