8 Direct Cu to Cu Bonding and Alternative Bonding Techniques …
207
Table 8.1 Thermal budget of
Cu–Cu bonding by forming
gas and formic acid vapor
surface treatments
Process/references
Cu treatment (T,
time)
Bonding and
annealing (T, time)
Forming gas
(H 2 + Ar or N 2 )
[14, 15]
175 °C 30 min
175 °C 30 min
+ 200 °C 1 h anneal
Formic acid vapor
[16, 17]
200 °C 1–10 min 200 °C 5 min
without annealing
Table 8.2 Cu surface
diffusion coefficients on 3
crystal planes as function of
temperature [20]
Temperature
(°C)
Cu surface diffusivity, D surface (m 2 /s)
(111)
(100)
(110)
150
6.85 × 10 −10 2.15 × 10 −14 6.61 × 10 −16
200
9.42 × 10 −10 1.19 × 10 −13 5.98 × 10 −15
250
1.22 × 10 −9
4.74 × 10 −13 3.56 × 10 −14
300
1.51 × 10 −9
1.48 × 10 −12 1.55 × 10 −13
data of Cu surface diffusion coefficients, D surface [m
2 /s], on 3 crystal planes as a function of temperature in the range of 150–300 °C [20]. Because of the larger surface
diffusivity on (111) plane than (100) and (110) planes, the Cu–Cu bonding temperatures can be reduced by using (111)-oriented Cu surfaces. Liu et al. [20] studied the
Cu–Cu bonding of highly (111)-oriented nano-twinned Cu films and good bonding
was obtained under thermo-compression at 150–250 °C for 10–60 min. Figure 8.3a
presents TEM image of the cross-sectional Cu–Cu interface at 200 °C for 30 min,
showing a void-free bonding interface. Figure 8.3b presents an electron backscattered diffraction (EBSD) orientation image of the bonding interface, showing all
the Cu grains near the bonding interface were oriented in the < 111 > direction.
The surface diffusion also occurs even at room temperature if the surfaces are free
of oxides. Shimatsu et al. demonstrated Cu–Cu bonding between clean nanocrystalline Cu films in UHV (Ultra High Vacuum) immediately after sputtering deposition
[21]. TEM result showed that the bonding interface become invisible, as shown in
Fig. 8.3 Bonding between two electroplated (111)-oriented Cu films at 200 °C for 30 min: a TEM
cross-sectional image and b electron backscattered diffraction (EBSD) orientation image [20]
207
Table 8.1 Thermal budget of
Cu–Cu bonding by forming
gas and formic acid vapor
surface treatments
Process/references
Cu treatment (T,
time)
Bonding and
annealing (T, time)
Forming gas
(H 2 + Ar or N 2 )
[14, 15]
175 °C 30 min
175 °C 30 min
+ 200 °C 1 h anneal
Formic acid vapor
[16, 17]
200 °C 1–10 min 200 °C 5 min
without annealing
Table 8.2 Cu surface
diffusion coefficients on 3
crystal planes as function of
temperature [20]
Temperature
(°C)
Cu surface diffusivity, D surface (m 2 /s)
(111)
(100)
(110)
150
6.85 × 10 −10 2.15 × 10 −14 6.61 × 10 −16
200
9.42 × 10 −10 1.19 × 10 −13 5.98 × 10 −15
250
1.22 × 10 −9
4.74 × 10 −13 3.56 × 10 −14
300
1.51 × 10 −9
1.48 × 10 −12 1.55 × 10 −13
data of Cu surface diffusion coefficients, D surface [m
2 /s], on 3 crystal planes as a function of temperature in the range of 150–300 °C [20]. Because of the larger surface
diffusivity on (111) plane than (100) and (110) planes, the Cu–Cu bonding temperatures can be reduced by using (111)-oriented Cu surfaces. Liu et al. [20] studied the
Cu–Cu bonding of highly (111)-oriented nano-twinned Cu films and good bonding
was obtained under thermo-compression at 150–250 °C for 10–60 min. Figure 8.3a
presents TEM image of the cross-sectional Cu–Cu interface at 200 °C for 30 min,
showing a void-free bonding interface. Figure 8.3b presents an electron backscattered diffraction (EBSD) orientation image of the bonding interface, showing all
the Cu grains near the bonding interface were oriented in the < 111 > direction.
The surface diffusion also occurs even at room temperature if the surfaces are free
of oxides. Shimatsu et al. demonstrated Cu–Cu bonding between clean nanocrystalline Cu films in UHV (Ultra High Vacuum) immediately after sputtering deposition
[21]. TEM result showed that the bonding interface become invisible, as shown in
Fig. 8.3 Bonding between two electroplated (111)-oriented Cu films at 200 °C for 30 min: a TEM
cross-sectional image and b electron backscattered diffraction (EBSD) orientation image [20]