Room Temperature Bonding of Electroformed Cu Based-Heat Spreader in Atmospheric Air
Authors: Matsumae Takashi , Kurashima Yuichi , Takagi Hideki 
Published: February 2020

With the rapid increase in the power density of power electronics devices, advanced thermal management systems are strongly required. Cu-based heat spreaders has received much attention because it allows for efficient cooling system (Numakura et al., 2016). For the integration of power devices and heat spreaders, solder bonding is a promising candidate because it is a relatively low-temperature process and tolerant to surface roughness. However, the thermal conductivity of the available solder materials is an order of magnitude lower than that of Cu, and the solder layer thickness is ~100 µm or more. Therefore, the solder layer degrades the heat dissipation from power devices to heat spreaders. On the other hand, thermo-compression bonding is expected to realize highly thermal-conductive device integration. However, the thermo-compression bonding of Cu requires relatively high-temperature processes over 250 °C (Fan et al., 2013) that induces large residual stresses between power devices and heat spreaders due to the thermal expansion mismatch. Thus, a low-temperature bonding technique without the use of soldering materials is preferable for the integration of power devices onto Cu based-heat spreaders.
Among available low-temperature bonding techniques, surface activated bonding (SAB) can realize strong bonding at room temperature. In SAB, the surfaces to be bonded are sputter cleaned using Ar fast atom bombardment or Ar plasma irradiation and then brought into contact with each other and pressed. The atoms on the cleaned surfaces are in reactive states and form chemical bonds, even at room temperature. Semiconductor materials such as Si (Takagi et al., 1996), Cu (Kim et al., 2003) and SiC (Mu et al., 2016) can be bonded in an ultra-high vacuum (UHV) by SAB or modified methods of SAB. Moreover, Au can be bonded even in atmospheric air because an oxide layer does not develop on the free surface of Au and Au atoms have a high self-diffusion coefficient. For successful bonding at room temperature in atmospheric air, substrates with atomically Au smooth surface is necessary (Higurashi et al., 2017).
To obtain a Cu based-heat spreader with such smooth Au surface, we have developed a electroforming technique using a Au thin film on an atomically smooth wafer. The overview of electroforming and bonding process flow is shown in the attached figure. Electroforming, which is a replication technique based on electrodeposition, can transfer nano-structures of master surfaces. A thermally oxidized Si (SiO2 thickness: 25 nm) wafer was used for the master surface because wafers with atomically smooth surface is easily available and the adhesion at the Au/SiO2 interface can be controlled to be weak (Kurashima et al., 2018). On the thermally oxidized Si wafer, a Cu/Ta/Au film (from top to bottom) in a 500/50/10 nm thick was sputter-deposited as a seed layer for electrodeposition, a diffusion-barrier layer, and the bonding layer, respectively. Then, a Cu plate in an ~200 µm thick was electrodeposited on the Cu/Ta/Au film in electrolyte that was 0.1 mol/L CuSO4 + 0.05 mol H2SO4. The deposited film on the Si wafer can be mechanically exfoliated without large deformation owning to the controlled adhesion between Au and SiO2. Without the Ta diffusion barrier layer, Cu diffused into the Au film and formed oxides on the surface, inducing surface roughness as root mean square (RMS) roughness was ~1.1 nm. On the other hand, atomically smooth surface was obtained as RMS roughness was ~0.3 nm in the case that the Ta diffusion barrier layer was deposited between Au and Cu layers. After high-pressure jet cleaning and activation using Ar plasma, the exfoliated Au surface can be bonded to a Si chip metalized with a Au/Ti layer in a thickness of 12/5 nm at room temperature in atmospheric air. We expected that SiC and GaN chips also can be bonded onto the heat spreader using this method.
Cu based-heat spreader with atomically smooth Au surface was obtained without planarization techniques, such as chemical mechanical polishing (CMP). Although the electrodeposited Cu surface was not smooth and has to be bonded to insulating substrates or heat sinks using solder or by thermo-compression bonding, we believe that the direct bonding allows for efficient heat dissipation from power devices to the Cu based-heat spreader.

Figure 1