Metal, e.g. copper or aluminum, is difficult to be firmly bonded with silicon if only thermally treated at room temperature, because the surface status of the silicon substrate is usually in covalent bonds.

However, the low temerature Cu/Si bonding structure is usually achieved by pre-depositing Cu layer on the surfaces of the silicon at relative high temperature prior to bonding. In practice, this bonding is one type of copper-to-copper bonding. In this kind of bonding case, annealing temperature is normally at several hundred °C as well. Another way is to employ a so-called “low temperature” (400°C or below) thermo-compression bonding technique to create the Si/Cu structure. The first step is to deposit a Cu layer on the surface of blanket silicon wafer. Then, in practice, the two Cu surfaces were joined together with mechanical pressing and heating simultaneously for tens of minutes.

In the study, we demonstrated the copper-silicon bonding pair formed via an electrochemical processing at room temperature. The silicon substrates were double-side polished, phosphorus- doped, N-type (100) silicon wafer with resistivity of 1−10 Ω cm. The surface of copper plate was also polished. Prior to electrochemical processing, all the silicon specimens were cleaned at 80 °C in the following solutions: NH₄OH:H₂O₂:H₂O = 1:1:5 and HCl:H₂O₂:H₂O = 1:1:6. Then silicon specimens were dipped in 0.5% HF solution for 30 seconds to remove all native oxide layer. Via a hydrophobic wafer bonding processing with HF dipping, the silicon specimen was bonded with the polished copper plate which is also as an anode electrode. The electrochemical anodization was performed in darkroom at a constant current density of 50 mA/cm² with a platinum cathode in the electrolyte. After the electrochemical processing, we found copper was teared off and then firmly attached on the silicon surface when we tried to separate the bonded pair. We proposed that the carrier-mediated rectification through the Schottky junction of metal-semiconductor contact allows electrons drain to the copper electrode, thereby greatly increasing the hole density at the interface on the side of silicon substrate, i.e. activating the silicon dangling bonds. Once the electrons drained off to the copper electrode from silicon surface, the bonds formed on the Cu surface were also greatly activated to be bonded to silicon bonds. This kind of bonding technique of copper/silicon or metal/ceramic materials may be applied to the additive manufacturing for 3-D microstructure.

Figure 1: The copper layer was directly teared from bulk copper and then transferred onto silicon after electrochemical processing at room temperature for two hours. It appears the bonding strength between copper and silicon is stronger than the bond strength of copper.