UC Irvine

Rapid evolution of electronics industry has made the adoption of high performance chips more urgent. In fact, those chips cannot be used unless high temperature die attach materials and methods are available. The conventional die attach methods typically require a processing temperature 20-30 °C above the melting temperature of bonding material, which melts and reacts with to-be–bonded parts to form the bonding joint. For high temperature applications (> 400 °C), the processing temperature will sharply increase along with the increased melting temperature to reach the molten phase. However, such high processing temperature will damage nearly all electronic components. To respond to such challenge, the metal bonding is considered to be a possible solution.

In this dissertation, the solid-state bonding technique is employed for various bonding designs by electroplating Ag-based and In-based systems as bonding media. No molten phase or flux is involved. For most cases, the bonding conditions are conducted at 300 °C with 6.89 MPa (1,000 psi) pressure for a dwell time of 3 min in vacuum. The bonding time, 3 min, is constrained by the furnace. It is worth mentioning that this pressure is less than 10% of what is used in industrial thermo-compression processes.

To begin with, silver (Ag) was chosen as a bonding medium because of its exceptional properties and reasonable price. There are two designs for the Ag layer. For the first design, the 50 μm Ag layer plated on the copper (Cu) substrate is initially annealed at 400 °C for 5 h to increase Ag grain sizes, thereby making it easier to deform during bonding. For the second design, the 10 μm Ag layer is plated on the substrate and followed another 5 μm Ag with cavities. The fundamental concept is to release the thermal induced stress by creating cavities in the Ag layer to allow easier plastic deformation for the bonding medium. The resulting Ag layers are then bonded to the Cu chips. For both designs, all samples are bonded well and pass MIL-STD-883J method 2019.9. The Ag layer is also applied to the Cu wire bonding. To overcome Cu oxidation issue, the bonding surface on the 1 mm Cu wire is plated a 50 μm Ag layer. An annealing step is followed to facilitate the Ag layer easier to deform and conform to the Cu or Si bonding surfaces. In-plane pull test and vertical pull test are performed to measure the breaking force of the wire bonds. For wire-bonds made on the Cu substrate, the breaking forces on in-plane pull test are greater than 20 kg. The breaking forces on vertical pull test are approximately one-half of in-plane pull test results. For wire-bonds made on Si chip, breaking forces are approximately 80% of those made on Cu substrate.

Next, the novel method is to bond the Si chips to Cu substrates directly. This structure design can provide low-resistance paths for electricity and heat. There is no specific joint used in between. The basic concept is to provide room for Cu and Si to deform without restriction by producing trenches inside the Cu substrate. When the bond between two surfaces is formed, the trenches inside the Cu substrate could release the part of thermal stress from the coefficient of thermal expansion CTE mismatch. Therefore, the bond could deal with the large CTE mismatch between Si (3 ppm/°C) and Cu (17 ppm/°C). The cross-sectional images show that the Si chips are well bonded to the Cu substrates without visible voids and cracks. After the bonding process, the simple shear tests are conducted to evaluate the bond strength, while the Si chips are all broken first. It demonstrates that the bond is stronger than the Si chip itself.

Finally, the Ag-rich Ag-In solid solution layers have successfully developed on Si and silicon carbide (SiC) chips. Ag has definite advantages among metals, but it still has its own weaknesses, which can actually or potentially lead to failure. That is Ag can be tarnished not only when exposed to certain corrosive gases such as hydrogen sulfide or sulfur gas but also under normal atmospheric conditions for a long period of time. Ag-In solid solution is further studied while Ag-In binary system has been demonstrated with an exceptional anti-tarnish property. To fabricate the single phase Ag-In solid solution, the Ag/In/Ag multilayers are electroplated on the chips. Importantly, the bottom Ag layer is initially annealed at 350 °C to increase its grain sizes and to reduce grain boundaries, inasmuch as the reaction rate between Ag and In is subject to the microstructure of Ag layer. The In/Ag layers are followed to be plated onto Si chip. To decompose AgIn2 and Ag2In, the two-step annealing process is then performed in a vacuum environment at 180 °C and 350 °C, respectively. After the bonding process, the compositions of resulting joints are measured by SEM/EDX, which is a homogeneous Ag-rich Ag-In solid solution.

In this study, five different bonding designs are reported. All bonds made between the chips and the substrates are fabricated at relatively low processing temperatures. The bonding structures are designed to be used for high operating temperatures. The results are encouraging. These novel methods may bring a chance for those who need die-attach materials for high performance electronic devices under severe environmental conditions in industries.