Fundamental Research On Nano Thermocompression Bonding Technology

In order to meet the development of semiconductor technology,electronic packaging gradually moves into a new era of three-dimensional packaging and system-in-packaging.Bonding is an important process for three-dimensional packaging.Among many bonding methods,Cu-Cu bonding has huge advantages,such as high electrical and thermal conductivity,extraordinary strength,low cost and high electromigration resistance,which make it a very promising bonding technology for achieving three-dimensional packaging.However,a high bonding temperature required by normal Cu-Cu bonding does a lot damages to the package,which gives much importance to Cu-Cu low-temperature bonding technology.Considered as a potential bonding material,nanoporous copper has many unique properties,such as small size effect,high surface energy and low melting temperature.To prepare size-controllable nanoporous copper,dealloying method stands out from the rest for its simple process and low cost.In this paper,ceramic substrates were covered by thin nanoporous copper films,with which as bonding layers Cu-Cu low-temperature bonding was achieved.The main contents are listed as follows:Firstly,light was cast on the theoretical foundation for Cu-Cu low-temperature bonding with nanoporous copper as bonding layers.Properties like small size effect and good softness make nanoporous copper feasible to lower the temperature of Cu-Cu bonding.Secondly,nanoporous copper was fabricated by dealloying method,and the key process parameters affecting the nanoporous structure including annealing temperature,choice of etchant and precursor composition were studied.With all parameters above considered,ligament feature size of nanoporous copper was shrink as fine as 20 nanometers.Thirdly,bonding experiments with various process parameters were carried out.To analyze how much those parameters effect bonding,the bonding qualities were evaluated by three kinds of methods.An excellent bonding quality with a tensile strength as high as 8.4MPa was achieved by bonding nanoporous copper at 200? for 90 minutes,while the control groups with normal copper films failed to be bonded under the same conditions,which verified the feasibility of Cu-Cu low-temperature bonding with nanoporous copper as bonding layers.Last but not the least,in order to understand the micro-mechanism of bonding with nanoporous copper,molecular dynamics simulation was carried out to simulate the bonding process of two copper nano-particles.The key factors which contribute to a successful bonding with nanoporous copper were analyzed,too.