Page 68 - Sami Franssila Introduction to Microfabrication

5

Thin-film Materials and Processes

Thin-film processes are needed to make metal wires and Note on notations
to insulate those wires, to make capacitors, resistors,
inductors, membranes, mirrors, beams and plates, and to <Si> Single-crystal material
protect those structures against mechanical and chemical c-Si Single-crystal material
damage. Thin films have roles as permanent parts of α-Si Amorphous material
finished devices, but they are also used intermittently a-Si:H Amorphous material with imbedded
during wafer processing as protective films, sacrificial hydrogen (at% usually given)
layers and etch and diffusion masks. nc-Si Nanocrystalline (grain size a few
Metallic, semiconducting and insulating films are nanometres)
employed (Table 5.1) in microfabrication. Films are µc-Si Microcrystalline material (grain size
often used, however, not because of their metallic, in the range of tens of nanometres)
semiconducting or dielectric properties, but for other mc-Si Multicrystalline (large-grained,
features. For example, doped single-crystalline silicon polycrystalline, grain size ≫ film
carbide is a semiconductor, but amorphous SiC thin thickness)
films are insulators for all practical purposes. SiC Al-0.5%Cu Alloy with 0.5% copper
is frequently used as a structural material in high- W 2 N, Si 3 N 4 Stoichiometric compounds
temperature/corrosive ambient microdevices because of SiN x , x ≈ 0.8 Non-stoichiometric compound
its excellent mechanical and chemical stability. Simi- W:N Stuffed material, nitrogen at grain
larly, silicon is used not only for its electronic properties boundaries (non-stoichiometric)
but also for its mechanical strength (micromechanics), WF 6 (g) Material in gas phase
optical absorption in visible wavelengths (solar cells, W (s) Material in solid phase
photodetectors), low absorption in infrared (waveguides TiW Exception: TiW is not a compound
for 1.55 µm optical telecom applications), high See- but pseudoalloy with 30 atom% Ti
beck coefficient (thermoelectric devices) and because Si/SiO 2 /Si 3 N 4 Film stacks are marked with substrate
of special properties of certain silicon microfabrication or bottom film on the left
processes. Silicon nitride is used for free-standing thin
membranes as etch and oxidation mask, as an etch-stop
and polish-stop layer and as a passivation material that
protects from mechanical and chemical damage. effects. The size scale for quantum effects is estimated
by Debye lengths, which are of the order of 10 to 100 nm
at room temperature.
5.1 THIN FILMS VERSUS BULK MATERIALS
The density of thin films is often very low compared
In thin films, at least one dimension of the material, the to bulk materials. Sputtered tungsten films can have a
3
thickness, is small. For narrow lines, two dimensions density as low as 12 g/cm compared to the bulk value
3
are small, and for dots all three dimensions are small. of 19.5 g/cm . Thin films are often porous, which results
This gives rise to prominence of surface effects like in long term instability: humidity can be absorbed in
surface scattering of electrons, leading to size-dependent the film, and high surface-area porous films oxidize and
resistivity, or at very small dimensions, to quantum corrode readily.

Introduction to Microfabrication Sami Franssila
 2004 John Wiley & Sons, Ltd ISBNs: 0-470-85105-8 (HB); 0-470-85106-6 (PB)