Silicides/silicidation
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In integrated circuits (ICs), conductive lines provide electrical interconnection among different parts of the ICs, devices, and the outside. The main applications of metallization are classified by gate, contact, and interconnection. Polysilicon and silicides are commonly applied as gates and interconnects in MOS devices.

Figure 2036 shows the historical view of Ohmic contact developments in Si(silicon)-based ICs. Ideally, Al (aluminum) would be deposited directly onto Si with perfect interfaces between Al and Si as shown in Figure 2036 (a). Unfortunately, the Si migrates into the Al, resulting in voids formed in the Si as shown in Figure 2036 (b). The Al can subsequently migrate into the formed voids, inducing spiking as shown in Figure 2036 (c). The big spike leads to junction shorts between n+ and p layers. Because the solubility of Si in Al is small (e.g. 0. 5 wt% at 450 °C), low Si doping in Al (1 to 3 wt% Si) suppresses spike formation significantly. However, other problems can occur subsequently, for instance, Al-doped p+ Si can precipitate out or p+ Si epitaxial layer can be formed between the original Si surface and the Al film as shown in Figure 2036 (d). Note that here Al is a p-type of dopants in Si. Furthermore, a challenge is that the formation probability of such epitaxial films is higher for (100)- than for (111)-oriented Si substrates. This can be more severe problem when the contact is scaled further and thus it only contains one or few grains. Note that titanium silicidation generates less silicon vacancies in the silicon substrate than by cobalt silicidation. [1]

Silicides were introduced to solve the precipitation problem. As shown in Figure 2036 (e), a silicide is formed by depositing a metal onto Si, followed by heating the sample. Here, the Al does not have to be doped with Si. However, the Al above the silicide can still migrate through the silicide along the grain boundaries of the silicide, resulting in formation of Al/Si contacts as shown in Figure 2036 (f). This migration can be enhanced by the high compressive stress in the Al induced by its high CTE than that of Si (CTEAl>> CTESi) at high temperatures. Recently, barrier layers are introduced to address the migration problem as shown in Figure 2036 (g). These barrier layers have the properties of low contact resistance, and good chemical, electrical, and mechanical stabilities.

Historical Development of Ohmic Contacts in Si-based ICs

Figure 2036. Historical view of Ohmic contact developments in Si-based ICs: (a) Al deposited directly onto Si; (b) Voids formed in Si; (c) Al spiking; (d) p+ Si formation; (e) Silicides; (f) Formation of Al/Si contacts; and (g) Barrier layer/silicide.

Refractory metal silicides such as TiSi2, WSi2, TaSi2, and MoSi2 have been widely applied in Si technology because of their low resistivity and high temperature stability. TiSi2 among those silicides shows the lowest electrical resistivity (~12.4 µΩ cm).

Table 2036a. Metallization selections in ICs.

Application
Selection
Gates, interconnection, and
contacts
Polysilicon, refractory metal silicides (e.g. MoSix, TaSix, WSix, and TiSix), nitrides,aluminum, copper, and/or refractory metals.
Diffusion barrier layer
Ti, TiN, Ta, TaN, Ti-W alloy, and/or silicides
Top level
Aluminum, and/or copper
Metallization on silicon
Silicides, tungsten, aluminum, and/or copper

In high k dielectric MOS structures in ICs, the metal gate oxides must not react with Si (silicon) to form either SiO2 or a silicide,

         MO2 + Si = M + SiO2 ------------------------------- [2036a]
         MO2 + 2Si = MSi + SiO2 ------------------------------- [2036b]

where,
         M -- The metal element.

Chemical reaction Equation 2036a results in a SiO2 layer that has a lower dielectric constant than the high k material. Equation 2036b forms a silicide, resulting in electrical short failure.

Some silicides can cause problems in IC devices. For instance, copper migrating into silicon active areas can form copper silicide precipitates resulting in leakage current at source and drain shallow junctions.

Table 2036b. Silicide parameters.

Silicides
Resistivily (µΩ-cm)
Sheet resistance for a 200-nm thick film (Ω/□)
Direct band gap (eV)
Indirect band gap (eV)
Barrier height on n-Si (mV)
Sintering
temp
(°C)

Bonding energy (eV)

Thermal expansion (ppm/°C)
nm of Si
consumed
per nm of
metal
nm of
resulting
silicide
per nm of
metal
Si consumption (nm Si/nm Silicide)
Density gm/cm3
Room Temp. Fracture Toughness MPa. m1/2
Melting point (°C)
Reaction
with Al at
(°C)
Stable on
Si up to
(°C)
Stable on
Al up to
(°C)
Space group
Structure
Lattice constant (nm)
Lattice mismatch
with Si
Barrier
height to
n-Si (eV)
Molecular weight
Selective etchant
Chemical resistance
Major applications
Co2Si
~70
300-500
0.91
1.47
 
CoSi
100-150
680
400-600
1.82
2.02
1460
 
Cubic
CoSi2
14-28
640
450-800
~10
3.64
3.52
1.03
4.98
1325
400
~950
 
Cubic (CaF2)
0.537
1.2 %
0.65
HCl/H2O2
CoSi/Co2Si
350, 500
 
Cr3Si
10.5
6.46
1773
 
Cubic Al5
CrSi2 (C11b)
0.9
0.35
570
450
0.15
1475
 
Tetragonal
CrSi2 (C40)
0.9
0.35
0.05
4.63
1477
 
Hexagonal
FeSi2
 
Tetragonal
0.9%
HfSi
530
550
2200
 
Orthorhombic
IrSi
930
300
 
MnSi
760
400
1275
 
Cubic
MnSi2-x
0.68-0.82
 
Tetragonal
1.7%
Mn11Si19
720
800
1145
 
Tetragonal
Mo5Si3
8.24
2160
 
Tetragonal
MoSi2 (C11b)
40-110
550
1000
0.20
8.25
6.24
5-8
1980 - 2030
I4/mmm
a = 0.32056(3);
c = 0.78450(4)
MoSi2 (C40)
0.12
 
Hexagonal
MoSi2
20-110
2-5.5
0.05
570
800-1000
~8
2.56
2.59
0.99
1980
500
~1000
500
 
0.64
NH4OH/H2O2
Very good
Nb/Nb5Si3
10-22
1883
 
(Eutectic)
Nb5Si3 (D8b)
7.16
2484
 
Hexagonal
NbSi2
5.66
1930
 
Hexagonal
NbSi2 (C11b)
0.18
 
Tetragonal
NbSi3
7.16
2480
 
Tetragonal
Ni2Si
700 - 750
200
1318
 
Orthorhombic
NiSi
14-20
660 - 750
400-600
1.83
2.34
992
~650
 
Orthorhombic
NiSi2
40-50
700
600-800
3.65
3.63
993
 
Cubic
0.4%
0.66
Pd2Si
30-35
1.5-1.75
720 - 750
200 - 400
0.68
1.44
1330
700
300
 
Hexagonal
KI/I2
Medium (soluble in HNO3 and HF+HNO3)
PtSi
28-35
1.5-1.75
840 - 880
250-500
1.22
2.04
1229
250
~750
300
 
Orthorhombic
9.5%
0.84
aqua regia
Very good
ReSi2 (C11b)
6.6
 
Tetragonal
RhSi
690
300
 
Cubic
Ta5Si3 (C40)
α1 = 5.5
α3 = 8
 
Hexagonal
Ta5Si3
13.4
2500
 
Tetragonal
TaSi2(C40)
10-70
1.75-3.5
600
750 - 1000
8-11
2.21
2.41
0.92
9.08
2200
500
~1000
500
 
Hexagonal
0.59
Good
Ti5Si3 (D8m)
α1 = 3.05
α3 = 10.7
4.32
2130
 
Ti5Si3
4.32
2130
 
Hexagonal
Ti/Ti5Si3
11
1332
 
(Eutectic)
TiSi2 (C49)
60-70
600
500-700
2.27
2.51
1540
 
TiSi2 (C54)
12-25
0.75-1.25
600
700-900
12
2.27
2.51
0.904
1540
450
~900
450
 
0.58
NH4OH/H2O2
Poor (soluble in dilute HF)
V3Si
12.5
5.71
1973
 
Cubic Al5
VSi2 (C11b)
0.17
 
Tetragonal
VSi2(C40)
0.074
P6422 (181)
W5Si3
14.5
2370
I4/mmm
a = 0.32138(2);
c = 0.78299(3)
WSi2 (C40)
0.44
0.07
0.22
 
Hexagonal
240.01
WSi2 (C11b)
30-120
1.5-5
650
650 - 1000
0.21
6-8
2.53
2.58
0.98
2165
500
~1000
500
 
Tetragonal
0.67
240.01
K3Fe(CN)6/KOH
Very good
Contact in ICs
ZrSi2
550
600
1520
 
Orthorhombic

 

 

 

 

 

[1] S. B. Herner, K. S. Jones, H.-J. Gossmann, J. M. Poate, and H. S. Luftman, Appl. Phys. Lett. 68, 1687 (1996).

 

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