Practical Electron Microscopy and Database

TiO₂ TEM specimens can be prepared by crushing bulk single crystals into fine fragments in an agate mortar, dispersing in ethyl alcohol and finally by putting drops of ethyl alcohol on a carbon-film grid. [1]

Figure 2071a shows several chemical reactions occurring during annealing of a transistor structure coated with Ti to form a low resistive TiSi₂ layer at the source, gate, and drain. When silicon and titanium are reacted, Ti and Si can diffuse laterally over the isolation regions and form thin filaments of TiSi_x that can electronically connect a gate electrode and source/drain regions and can cause failure of the device. This phenomenon is called "bridging". On the other hand, those by-products (TiSi_x, TiO_x, and TiN) form over the isolation regions and cannot easily be etched in conventional salicide etching solutions. The use of a nitrogen atmosphere during thermal annealing is essential in preventing this failure mechanism [3] because nitrogen quickly diffuses into the grain boundaries and significantly suppresses any long-range silicon diffusion in the titanium layer. Furthermore, to avoid the by-product formation, the annealing temperature cannot be too high.

Figure 2071a. Several chemical reactions occurring during annealing of a transistor structure coated with Ti to form a low resistive SiTi2 layer.

Table 2071b. Properties of main crystalline TiO₂.

Structure	Rutile (Tetragonal)	Anatase (Tetragonal)	Brookite (Orthorhombic)	TiO2 (B)	TiO2, II	TiO2, (H)	Perovskite
Space group	P42/mnm	I41/amd	Pbca	C2/m	Pbcn	I4/m	Pnma
Lattice parameters (nm)	a = 0.459; c = 0.296 [1]	a = 0.379; c = 0.951	a = 0.917; b = 0.546; c = 0.514	a = 1.216; b = 0.374; c = 0.651, β = 107.29°	a = 0.452; b = 0.550; c = 0.494	a = 1.018; c = 0.297	a = 0.537; b = 0.764; c = 0.544
Unit cell
Density (g/cm3)	4.1	3.7	3.99	3.64	4.33	3.46	4.03
Specific weight	4.2	3.9	2.87-4.01
Rockwell hardness	6 - 6.5	5.5 - 6	5.5 - 6
Habitus	Ditetragonal bipyramidal	Tetragonal bipyramidal tabular	Bipyramidal rhombic
Existing condition	Stable: <1845 °C	Metastable	Metastable, rare
Molecules per cell ( Z)	2	4	8
Ti coordination number	6	6	6
Ti-O distance (Å)	2 at 1.946 4 at 1.984	2 at 1.937 4 at 1.964	2 at 1.993 1 at 1.865 1 at 1.919 1 at 1.945 1 at 2.040
Motif (basis)	Ti: 0, 0, 0; 1/2, 1/2, 1/2. O: 0.3, 0.3, 0; 0.8, 0.2, 1/2; 0.2, 0.8, 1/2; 0.7, 0.7, 0.
Melting point (°C)	1870
Boiling point (°C)	2500-3000	2500-3000
Model of stacking fault	Oxygen deficiencies [2]
Relaxation width of lattice at stacking faults	1 nm [1]
Displacement vector R of stacking faults in (-101) plane	-0.25±0.00, 0.00±0.03, -0.27±0.01
Band gap (eV)	3.02	3.2
Conduction band (CB) offset on Si substrate		0
Static dielectric constant		80
Absorption onset (nm)	410	380
Molar mass	79.9	79.9

Table 2071c. Example of compounds with similar structures to BCC.

Figure 2071b. Ti-O phase diagram.

Table 2071d. Phases and space groups of TiO_x.

Table 2071e. Other characteristics of TiO₂ materials.

[1] Susumu Yamada and Michiyoshi Tanaka, Structure of a stacking fault in the (-101) plane of TiO₂, Journal of Electron Microscopy 1: 67-74 (1997).
[2] Yamada S and Tanaka M (1995) Determination of the displacement vector at a stacking fault in TiO₂ by convergent-beam electron diffraction. J. Electron Mkrosc 44: 212-218.
[3] C. K. Lau, “Method of Forming Titanium Disilicide,” U.S. Patent 4,545,116, 1985.