Direct Silver to Copper Bonding Process

Abstract

A novel process of bonding silver (Ag) foils to copper (Cu) substrates has been developed. This direct bonding method does not use any intermediate layer in between. An important application of this process is electronic packaging where semiconductor device chips are bonded to Cu substrates or Cu electrodes fabricated on substrates. Cu is chosen as the major material for substrates and electrodes due to its high electrical and thermal conductivities, high strength, adequate rigidity, easiness in forging and machining, and low cost. On the other hand, Cu has a large mismatch in the coefficient of thermal expansion with most semiconductors, particularly with silicon. This makes it very difficult to bond large device chips to Cu substrates with a metallic joint. We thus design the Ag-cladded Cu structure to overcome this difficulty. Ag is quite soft and ductile. It can function as a strain buffer between the semiconductor chip and the Cu substrate. Ag also has superior physical properties. It has the highest electrical and thermal conductivities among all metals. In the beginning, we used an electroplating process to produce Ag-cladded Cu substrates. However, it is time consuming and costly to electroplate thick Ag layers. To obtain thick Ag layers (more than 200μm), this new laminating process is developed. The Ag foil is laminated to the Cu substrate directly with a static load of 1000psi at 250°C in a 50mtorr vacuum to suppress oxidation. No bonding medium is used. Scanning electron microscopy images on cross sections of bonded samples exhibit a perfect Ag–Cu bond.

Direct bonding for dissimilar metals assisted by carboxylic acid vapor

“…The joints thus obtained possessed an average shear strength less than 3 MPa. A similar bonding process using Ag foils has been proposed by Wang et al 9,10) The Ag foils was laminated to Cu substrate with a load of 6.89 MPa at 250°C under 5 © 10 ¹2 Torr. By doing so, the shear strength can be raised to 16.7 MPa.…”

Heat-resistant die-attach with cold-rolled Ag sheet

“…20) Sheet preforms have been also studied as a stress buffer for releasing stress caused by the coefficient of thermal expansion (CTE) mismatch between the chip and substrate. [21][22][23] In addition, it has been reported that hillocks can appear on surfaces of metallic films under compressive stress caused by the CTE mismatch between films and substrates. [24][25][26] Previous research revealed that the growth of hillocks on Ag thin films can promote interface bonding in the dieattachment structure, [27][28][29][30][31] which is referred to as stress migration bonding (SMB).…”

Abnormal drop in electrical resistivity with impurity doping of single-crystal Ag

“…Silver (Ag) is the best conductor at room temperature, with a resistivity of 1.59 μΩ · cm at 293 K, which may be expressed as 108% according to the International Annealed Copper Standard (IACS). The high conductivity of metals, especially Ag, makes them ideal for many industrial power applications, in which excellent printing, thermal, electrical, wire-bonding, and soldering properties are a prerequisite 2 3 4 5 6 . Many researchers have recently tried to develop advanced metal materials with greater power consumption and higher efficiency for the aerospace and medical industries 7 8 9 10 11 12 13 14 .…”