Page 79 - Sami Franssila Introduction to Microfabrication

58 Introduction to Microfabrication

Table 5.4 Properties of metals 5.8 DIELECTRIC THIN FILMS
Metal Resistivity CTE Thermal Melting Dielectric films have, just like metallic films, a plethora
◦
(µ -cm) (ppm/ C) conductivity point of applications in microdevices. The table below clas-
◦
(W/cm K) ( C) sifies dielectric film applications into three categories:
structural parts in finished devices, intermittent layers
Al 3 23 2.4 650
during wafer processing and protective coatings for fin-
Cu 1.7 16 4 1083
Mo 5.6 a 5 1.4 2610 ished devices. Surprisingly, many films can serve in all
W 5.6 a 4.5 1.7 3387 these roles.
Ta 12 a 6.5 0.6 3000
Ti 48 a 8.6 0.2 1660 Active, protective and sacrificial layers during wafer
Co 6.2 a 12.5 0.7 1500 processing
Ni 6.8 a 13 0.9 1455
Cr 13 a 6 0.7 1875 Mask for thermal Si 3 N 4
Pt 10 a 9 0.7 1769 oxidation
Au 1.7 14 3 1064 Diffusion and ion SiO 2 , Si 3 N 4
a Thin-film resistivity is much higher than bulk value: as a rule of thumb, implantation masks
1.5–2 times the bulk value can be used as an guestimate for thin-film Dopant evaporation barrier CVD oxide, SiN x
resistivity. Etch-stop layer in SiN x
polymer-based
inter-metal stacks
in solid. This correlation is seen in, for example, Window definition during CVD oxide
electromigration resistance. selective epitaxial
Electromigration is metal movement with the elec- growth
tron flow. Electrons transfer momentum to metal atoms, Etch masks in bulk CVD oxide,
which will consequently move and accumulate at the micromechanics
Si 3 N 4
positive end of the conductor and leave voids at Dopant sources PSG, BSG
the negative end (Figure 5.10). This effect is encoun- Spacers in MOS and CVD oxide,
tered in aluminum conductors when current densities bipolar transistors CVD nitride
approach the mega-ampere per square centimetre level, Sacrificial layers in PSG, resist
but copper and tungsten tolerate higher current den- surface micromechanics
sities. Electromigration will be discussed further in Gap fill materials Oxides, SODs
Chapter 24.

Voids Hillocks, whiskers

Electrons Current
(a) (b)
Figure 5.10 Electromigration: atoms are transported from the anode end of a wire towards the cathode with electron
wind. Voids are left at the anode end, and hillocks form towards the cathode end: (a) schematic. Figure courtesy Antti
Lipsanen, VTT; (b) SEM micrograph of Al lines (4 µm wide). Reproduced from Hu, C.-K. et al. (1993), by permission
of American Inst of Physics