Page 98 - Sami Franssila Introduction to Microfabrication

Thin-film Growth and Structure 77

Table 7.1 Sputtered titanium nitride (TiN) film characterization: collimated vs. standard
Film property Analytical technique Collimated TiN Standard TiN
−3
Thickness (nm) RBS (density = 4.94 g/cm ) 81 nm 161 nm
TEM cross section 82 nm 178 nm
Sheet resistance Four-point probe 13.7 ohm/sq 7.4 ohm/sq
Rs uniformity Four-point probe 3.3% 5%
Resistivity (µohm-cm) Rs by four-point probe, 112 132
Thickness by TEM
Density Thickness by TEM & RBS, 4.88 g/cm −3 4.47 g/cm −3
Density by RBS 93% of bulk 86% of bulk
Stoichiometry (Ti/N) RBS 1.31 1.00
Phase Glancing angle XRD TiN (38–1420) TiN (38–1420)
(JCPDS card #) Electron diffraction TiN (38–1420) TiN (38–1420)
Preferred orientation θ − 2θ XRD (220) (220)
Electron diffraction
Net stress Gpa Wafer curvature 2.7 3.1
(tensile) (tensile)
Grain structure Cross-section TEM Columnar Columnar
Plane view TEM 2D equiaxial 2D equiaxial
Average grain size TEM 19.2 nm 18.3 nm
Average roughness AFM 0.43 nm 1.23 nm
Min/max roughness 8 nm 18.7 nm
Specular reflection Scanning UV 248 nm: 142% 145%
(% of Si reference) 365 nm: 55% 95%
440 nm: 57% 123%
Impurities Auger O < 1% O < 1%
(atom %) C < 0.5% C < 0.5%
Source: Wang, S.-Q. & J. Schlueter: Film property comparison of Ti/TiN deposited by collimated and uncollimated physical
vapor deposition techniques, J. Vac. Sci. Technol., B14(3) (1996), 1837.

7.3 CVD-FILM GROWTH AND STRUCTURE grain-size variation can be seen as a function of
temperature: at 630 C large grains (of the order of
◦
CVD reactions have much lower sticking coefficients 100 nm) are formed, below 600 C the grain size is
◦
than PVD reactions. CVD processes are diffusive reduced and at 570 C the film is amorphous.
◦
processes, whereas PVD processes are line-of-sight CVD films can be either amorphous, polycrystalline
processes (in the first approximation). This means that or single crystalline (epitaxial) as deposited. Epitaxial
deposition around corners, and even under overhang films remain single crystalline during annealing; poly-
structures, is possible in CVD but impossible in PVD.
CVD temperatures are high compared to PVD processes, crystalline films experience grain growth and even phase
transitions. Amorphous films either stay amorphous or
which means that the adatoms have high surface
crystallize. Silicon dioxide and aluminium oxide are
mobilities, which also enhances step coverage.
The main parameters in CVD processes are flow rates, exceptional amorphous films because they remain amor-
flow-rate ratio of reactants, temperature and pressure. phous throughout typical microfabrication temperatures.
In PECVD, RF power plays an important role. In Pictured below are Al 2 O 3 and SrTiO 3 films: aluminium
Figure 7.4, PECVD silicon grain sizes are recorded oxide is amorphous and strontium titanate is polycrys-
as a function of SiH 4 /(SiH 4 + H 2 ) flow ratio. High- talline (Figure 7.5).
frequency (70 MHz) PECVD was employed, and glass Dielectric films have a number of measurements
wafers were used as substrates at 225 C. Keeping different from metallic films. One special feature is the
◦
all other deposition parameters constant, a change in use of etch rate as a quality criterion. With dielectrics,
the gas ratio has resulted in enormous grain-size and thermal SiO 2 acts as a reference film that can always be
surface-roughness variation. In LPCVD, polysilicon used to eliminate etchant concentration or temperature
deposition using SiH 4 as a source gas, a similar effects. Boron nitride is a new material that has been