Mechanical Study of Copper Bonded at Low Temperature Using Spark Plasma Sintering Process

Bonding of high purity polished copper was investigated using the Spark Plasma Sintering technique (SPS) showing the effect of SPS parameters (surface roughness, time, temperature and pressure) on the bonding strength behaviour. Mechanical characterization of the bonded samples was performed at room temperature using tensile test. Two surfaces roughnesses were studied (un-polished and polished samples). It was found that the bonding strength varied from 50 MPa to 233 MPa for un-polished and polished surfaces respectively The tensile strength of the used bulk copper-rod was found to be 365 MPa, while most results are over 122 MPa (a third of the bulk value). Introduction The work presented here is aimed at developing a new interconnect technology for power electronic devices. Such new technology must provide a very low electrical and thermal resistance. Furthermore, the manufacturing conditions must be compatible with the semiconductor dies (temperature, pressure) and with the industrial constraints (time). Therefore, we present here a technique in which pieces of copper are joined together at temperature lower than 300 °C, with a short turnaround cycle (30 min max) and a relatively low pressure (16-77 MPa). Several experiments on Cu/Cu direct bonding have been reported [1-3]. Although the methods are simple, they require either high external pressure and annealing or toxic chemical cleaning processes to achieve a reasonable bonding strength. For example, Cu wafers exhibit good bonding properties when Cu/Cu contacts are carried out at 400 °C and 4 bar for 30 min, followed by an annealing cure at 400 °C for 30 min in N2 atmosphere [4]. Another solution, the so-called Surface Activated Bonding (SAB) process was used to bond Cu-coated wafers at room temperature without any wet chemical process. SAB is a process that joins similar or dissimilar materials by means of the adhesion force between atoms of two atomically clean surfaces in an ultrahigh vacuum UHV at room temperature [5,6]. The same authors also reported that the bonding strength of Cu/Cu interface is approximatively 6.47 MPa [7]. J. W. Elmer & al [8] have presented a diffusion bonding of high purity copper using a conventional furnace. A series of diffusion bonds was done to determine the relationship between bond strength and bonding parameters in order to determine a range of bonding conditions. Bonding strengths of 65 MPa and 190 MPa were found using uniaxial tensile testing for samples bonded at 400 and 800°C respectively (bonding time 60 min). In general, high temperature annealing (>400°C) is required to increase the bonding strength. However, semiconductor devices have been designed to meet the requirement of the existing soldering technology (a few seconds above 300°C). Therefore, an increase in the process temperature could require heavy modification in the dies manufacturing parameters to avoid possible degradation. In this paper, we report the Cu/Cu direct bonding using Spark Plasma Sintering (SPS) where we focus on the bonding issue by evaluating the bonding strength as a ha l-0 07 99 89 9, v er si on 1 12 M ar 2 01 3 Author manuscript, published in "Advanced Materials Research 324 (2011) 177-180" DOI : 10.4028/www.scientific.net/AMR.324.177