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 2036a 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 2036a (a). Unfortunately, the Si migrates into the Al, resulting in voids formed in the Si as shown in Figure 2036a (b). The Al can subsequently migrate into the formed voids, inducing spiking as shown in Figure 2036a (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 2036a (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 2036a (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 2036a (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 2036a (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 2036a. 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.

 

Figure 2036b shows the migration of silicide formation in semiconductor technology development.

Migration of silicide formation in semiconductor technology development

Figure 2036b. Migration of silicide formation in semiconductor technology development.

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)
Impurity precipitates
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)
Diffusion coefficient [cm2/s] Maximum solubility of metal on Si
[cm-3]
Space group
Structure
Lattice constant (nm)
Lattice mismatch
with Si
Strain induced
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
1x10-5 at 1100 °C 3x1016  
Cubic (CaF2)
0.537
1.2 %
Lower compressive strain than TiSi2
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
 
Cu3Si
                                    1x10-4 at 1100 °C 2x1018   η"-Orthorhombic                
α-FeSi2
 
>915
     
Tetragonal
0.9%
 
β-FeSi2
                                <915         Orthorhombic                
HfSi
530
550
 
2200
     
Orthorhombic
 
IrSi
930
300
 
     
 
MnSi
760
400
 
1275
     
Cubic
 
MnSi2-x
0.68-0.82
 
     
Tetragonal/FeS2
1.7%
 
MnSi2
                                    3x10-6 at 1100 °C 2x1015   Tetragonal/FeS2  
18.2%
           
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
4x10-5 at 1100 °C 7x1017  
Cubic/CaF2
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
TaB2, TaAs
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
TiB2, TiAs
1540
     
Higher compressive strain than CoSi2
TiSi2 (C54)
12-25
0.75-1.25
600
700-900
12
2.27
2.51
0.904
TiB2, TiAs
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
 

 

  Failure mechanisms of silicides, which play a key role in ESD (electrostatic discharge) failure of CMOS technology, are [2]:
         i) Silicide vertical penetration to the metallurgical junctions,
         ii) Silicide lateral penetration to the metallurgical junctions,
         iii) Silicide phase transformation,
         iv) Silicide removal.

 

 

 

 

 

 

 

 

[1] S. B. Herner, K. S. Jones, H.-J. Gossmann, J. M. Poate, and H. S. Luftman, Appl. Phys. Lett. 68, 1687 (1996).
[2] Steven H. Voldman, ESD: Failure Mechanisms and Models, 2009.

 

 

 

 

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