206
T. Suga et al.
8.4 Thermo-Compression Bonding (Example of Diffusion
Bonding)—Material Fundamentals and Microstructure
Effects
The basis of Cu–Cu thermo-compression bonding (TCB) is inter-diffusion and selfdiffusion at elevated temperatures and under an external compression, depending
on the cleanness of the mating Cu surfaces and potentially additional passivation
or capping layers. The compression pressure, which is mainly dependent on the
surface topography and roughness, can be in order of 100–150 MPa for as-plated Cu
films/pillars, and <2.5 MPa for thin smooth Cu films or CMP (Chemical-Mechanical
Polishing) polished Cu films. Typically, Cu–Cu TCB is performed at 300–400 °C
in vacuum or protecting/reducing gas environment (or preceded by plasma clean),
followed by a post-bonding annealing at 300–400 °C to improve the bond strength
[12]. However, such high process temperatures and high heating/cooling rates may
cause large thermal expansion and stress, which degrade or even damage thermaland stress-sensitive materials and devices. Moreover, formation of interfacial voids
were observed when bonding temperature is higher than 300 °C [13].
In order to lower the bonding temperature, surface treatments such as wet chemical cleaning and gas/vapor-phase thermal treatments have been researched and
employed. For instance, forming gas (H 2 + Ar or N 2 ) treatment has been studied
for Cu–Cu bonding at below 200 °C. After treatment at 175 °C for 30 min, wafers
were bonded with thermo-compression at 175 °C for 30 min followed by 200 °C 1 h
annealing [14, 15]. W. Yang et al. [16, 17] at the University of Tokyo studied Cu–Cu
bonding by using formic acid (HCOOH) vapor treatment combined with Pt-catalyzed
dehydrogenation for in situ generation of hydrogen (H) radicals/molecules. In this
combined process, H radicals/molecules are generated by heating Pt foils exposed
to HCOOH vapor through the following reaction.
HCOOH
Pt
→ CO 2 + 2H
(8.1)
The generation of H contained in the HCOOH vapor was confirmed by ex situ gas
chromatography analysis [16]. The detected H 2 peak area was almost the same as
standard 0.1% H 2 spectra. Compared to the forming gas, the H-containing HCOOH
vapor appears to be more effective for Cu oxide reduction because of the presence
of highly reactive H radicals, resulting in strong Cu–Cu bonding (shear strength of
above 10 MPa) at 200 °C. As summarized in Table 8.1, comparing to the forming
gas treatment, the HCOOH vapor treatment enables Cu–Cu bonding with very short
treatment and bonding time and without the need of post-bonding annealing.
Most of the existing studies on Cu–Cu TCB were based on diffusion between
randomly oriented Cu films. Diffusion, as known from materials fundamentals, is
not only temperature-dependent but also microstructure-dependent. Cu–Cu bonding
by using Cu layers that have special microstructures, such as Cu nanorod array [18],
Cu particles [19], and highly (111)-oriented nano-twinned Cu films [20], has also
been investigated in order to lower the bonding temperatures. Table 8.2 summarizes
T. Suga et al.
8.4 Thermo-Compression Bonding (Example of Diffusion
Bonding)—Material Fundamentals and Microstructure
Effects
The basis of Cu–Cu thermo-compression bonding (TCB) is inter-diffusion and selfdiffusion at elevated temperatures and under an external compression, depending
on the cleanness of the mating Cu surfaces and potentially additional passivation
or capping layers. The compression pressure, which is mainly dependent on the
surface topography and roughness, can be in order of 100–150 MPa for as-plated Cu
films/pillars, and <2.5 MPa for thin smooth Cu films or CMP (Chemical-Mechanical
Polishing) polished Cu films. Typically, Cu–Cu TCB is performed at 300–400 °C
in vacuum or protecting/reducing gas environment (or preceded by plasma clean),
followed by a post-bonding annealing at 300–400 °C to improve the bond strength
[12]. However, such high process temperatures and high heating/cooling rates may
cause large thermal expansion and stress, which degrade or even damage thermaland stress-sensitive materials and devices. Moreover, formation of interfacial voids
were observed when bonding temperature is higher than 300 °C [13].
In order to lower the bonding temperature, surface treatments such as wet chemical cleaning and gas/vapor-phase thermal treatments have been researched and
employed. For instance, forming gas (H 2 + Ar or N 2 ) treatment has been studied
for Cu–Cu bonding at below 200 °C. After treatment at 175 °C for 30 min, wafers
were bonded with thermo-compression at 175 °C for 30 min followed by 200 °C 1 h
annealing [14, 15]. W. Yang et al. [16, 17] at the University of Tokyo studied Cu–Cu
bonding by using formic acid (HCOOH) vapor treatment combined with Pt-catalyzed
dehydrogenation for in situ generation of hydrogen (H) radicals/molecules. In this
combined process, H radicals/molecules are generated by heating Pt foils exposed
to HCOOH vapor through the following reaction.
HCOOH
Pt
→ CO 2 + 2H
(8.1)
The generation of H contained in the HCOOH vapor was confirmed by ex situ gas
chromatography analysis [16]. The detected H 2 peak area was almost the same as
standard 0.1% H 2 spectra. Compared to the forming gas, the H-containing HCOOH
vapor appears to be more effective for Cu oxide reduction because of the presence
of highly reactive H radicals, resulting in strong Cu–Cu bonding (shear strength of
above 10 MPa) at 200 °C. As summarized in Table 8.1, comparing to the forming
gas treatment, the HCOOH vapor treatment enables Cu–Cu bonding with very short
treatment and bonding time and without the need of post-bonding annealing.
Most of the existing studies on Cu–Cu TCB were based on diffusion between
randomly oriented Cu films. Diffusion, as known from materials fundamentals, is
not only temperature-dependent but also microstructure-dependent. Cu–Cu bonding
by using Cu layers that have special microstructures, such as Cu nanorod array [18],
Cu particles [19], and highly (111)-oriented nano-twinned Cu films [20], has also
been investigated in order to lower the bonding temperatures. Table 8.2 summarizes