Silicon Dioxide/Silica Glass (SiO2)
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The main reason of using silicon dioxide (SiO2) as dielectrics in conventional ICs is that microelectronics technology uses silicon (Si) as substrates. But SiO2 normally is not used in other semiconductor integrated chips. In Si technologies, SiO2 can be made from Si by thermal oxidation, while other semiconductors (Ge, GaAs, GaN, SiC. ..) have poor native oxides. Amorphous SiO2 has very few electronic defects and forms an excellent interface with Si. It is easy to etch and pattern SiO2 films to a nanometer scale. The main problem with SiO2 is that electrons and holes can easily tunnel across the SiO2 film if it is too thin (e.g. 1.4 nm for 45 nm node). Table 2075a lists the properties of amorphous SiO2.

Table 2075a. General properties of amorphous silicon dioxide (SiO2).

Static dielectric constant (K)
3.9
Band gap energy (eV)
9
Conduction band (CB) offset on Si (silicon) substrate
3.2
Work function (eV) of some compounds with SiO2
4.7 for NbSiN and NbN, 4.7-5.33 for MoN, 4.4-4.7 for TaN, 4.65-4.7 for HfN, 4.0-4.9 for ZrN, 4.36-5.1 for TiAlN, 4.15-5.10 for TiN

Table 2075b. Properties and deposition of different silicon dioxides.

Property/deposition
Thermal
PECVD

LTO
TEOS
LPCVD TEOS
PECVD TEOS
Ozone TEOS
HTO
Source/Reactants
O2
SiH4 + N2O
SiH4 + O2
TEOS + O2
TEOS
TEOS

TEOS and O3
SiCl2H2 + N2O

Growth temperature (°C)

900-1100

200

400-450

700

680-730

250-425
300-400
900

Growth rate

<25 nm/min
<800 nm/min
<200 nm/min

Typical composition in the films (Doping)

SiO2

SiO1.9(H)

SiO2(H)

SiO2

(TMP/TMB)
(TEOP/TMB)
SiO2(Cl)

Step coverage

Conformal

Nonconformal

Nonconformal

Conformal

Conformal

Thermal stability

Stable

Loses H

Densifies

Stable

Loses Cl
Density (g/cm3)
2.2
2.3
2.1
2.2
2.2

Stress (109 dyne/cm2)

3 (Compressive)

3 compressive to 3 tensile

3 (Tensile)

1 (Compressive)

3 (Compressive)

         *

PECVD: plasma-enhanced CVD; TEOS: tetra-ethyl-ortho-silicate; HTO: high temperature oxide; LTO: low temperature oxide.

Table 2075c. Chemical reactions used in CVD for SiO2 film growth.

Chemical reactions used in CVD for Si film growth

                                 * ACVD: Subatmospheric CVD.

Figure 2075a 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 TiSix 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 (TiSix, TiOx, 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 [1] 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.

several chemical reactions occurring during annealing of a transistor structure coated with Ti to form a low resistive SiTi2 layer

Figure 2075a. Several chemical reactions occurring during annealing of a transistor structure coated with Ti to form a low resistive SiTi2 layer.

Figure 2075b shows the dry and wet growth rates on <111> and <100> crystalline silicon (Si) substrates at different elevated temperatures. Two important properties are clearly presented here:
         i) The oxidation rates increase with temperature.
         ii) The wet oxidation rates are much faster than those in dry oxidation.

Dry and wet growth rates on <111> and <100> crystalline Si substrates

Figure 2075b. Dry and wet growth rates on <111> and <100> crystalline Si substrates.

Table 2075d. Etchants used in semiconductor manufacturing.

Film
Etchant
SiO2(Silicon oxide)
Dilute hydrofluoric acid (DHF)
Buffered HF (BHF)
Polysilicon
Alkaline hydroxide + organic
Si3N4(silicon nitride)-selective to SiO2
Boiling phosphoric acid (H3PO4)
Si3N4/SiO2 (non-selective)
Hydrofluoric acid + organic

Table 2075e. Etching selectivity of Si3N4:SiO2:Si in various solutions.

Etching selectivity of Si3N4:SiO2:Si
BHF
1:200:0
40% HF
1:≥110:0.1
H3PO4
10:1:0.3
CF4-4% O2 plasma
3:2.5:17

Si is easily oxidized in air during mechanical polishing for EM analysis. In some critical analyses (e.g. Si surface plasmon analysis) in EELS, Si TEM specimens are normally dipped in HF solution (e.g. 10% HF solution) to remove the surface oxide. However, a thin layer of SiOx may still exist due to exposure to air before loading into the TEM.

SiO2 can be in both amorphous and crystalline phases. Table 2075f lists the different forms of SiO2.

Table 2075f. Different forms of SiO2.

Form
Source
Amorphous

Quartz

Sandstone, sand, slate, granite

Cristobalite

Calcined amorphous silica
Tridymite
Calcined amorphous silica
Crystalline
Glass, diatomaceous earth, opal, fume silica

Silicates

SiO2 + Na, Mg, Al, K, Ca, and/or Fe

Kaolin, talc, mica, feldspar, fuller's earth, vermiculite

Table 2075g. Color chart for SiO2 films observed perpendicularly under daylight fluorescent lighting.

Film thickness (nm)
Color
Film thickness (nm)
Color

50

Tan
630
Violet-red
70
Brown
680
Bluish

100

Dark violet to red violet
720
Blue-green to green
120
Royal blue
770
Yellowish

150

Light blue to metallic blue
800
Orange
170
Metallic to very light yellow-green
820
Salmon

200

Light gold or yellow-slightly metallic
850
Dull light-red-violet
220
Gold with slight yellow-orange
860
Violet

250

Orange to melon colored dark pink
870
Blue-violet
270
Red-violet
890
Blue

300

Blue to violet-blue
920
Blue-green
310
Blue
950
Dull yellow-green

320

Blue to blue-green
970
Yellow to "yellowish"
340
Light green
990
Orange

350

Green to yellow-green
1,000
Carnation pink
360
Yellow-green
1,020
Violet-red

370

Green-yellow
1,050
Red-violet
390
Yellow
1,060
Violet

410

Light orange
1,070
Blue-violet
420
Carnation pink
1,100
Green

440

Violet-red
1,110
Yellow-green
460
Red-violet
1,120
Green

470

Violet
1,180
Violet
480
Blue-violet
1,190
Red-violet

490

Blue
1,210
Violet-red
500
Blue-green
1,240
Carnation pink to salmon

520

Green
1,250
Orange
540
Yellow-green
1,280
Yellowish

560

Green-yellow
1,320
Sky blue to green blue
570
Yellow to yellowish
1,400
Orange

580

Light-orange or yellow to pink borderline
1,450
Violet
600
Carnation pink
1,460
Blue-violet

Note that substantial local atomic order is still present in fully amorphous silica glass: Each silicon (Si) atom is tetrahedrally surrounded by four oxygen (O) atoms at 1.62 Å, and the oxygen–oxygen separation is typically 2.65 Å. This phenomenon is call short-range ordering.

 

 

 

 

 

[1] C. K. Lau, “Method of Forming Titanium Disilicide,” U.S. Patent 4,545,116, 1985.

 

 

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