Metallic strain-absorbing layer for improved fatigue resistance of solder-attached devices

A metallurgical interconnection attaching an electronic device having first coefficient of thermal expansion to an outside body having second coefficient of thermal expansion, comprising:

• an electronic device having a plurality of contact pads exposing the device metallization;
• each of said contact pads having a layer of nickel/titanium alloy deposited over said device metallization, followed by a solderable nickel barrier layer;
• a solder bump attached to said solderable layer of each contact pad;
• said outside body having a plurality of terminal pads located in positions respective to said device contact pads, said terminals exposing the metallization of said outside body;
• said device attached to said outside body so that said contact pads are aligned with said respective terminal pads, whereby a plurality of metallurgical interconnections are formed operable to absorb, in said nickel/titanium layer, strain caused by thermomechanical stress due to the difference between said first and second coefficients of thermal expansion.

A metallurgical interconnection attaching an outside body having first coefficient of thermal expansion to an electronic device having second coefficient of thermal expansion, comprising:

• an outside body having a plurality of terminal pads exposing the body metallization;
• each of said terminal pads having a layer of nickel/titanium alloy deposited over said body metallization, followed by a solderable barrier layer;
• a solder bump attached to said solderable layer of each terminal pad;
• said electronic device having a plurality of contact pads located in positions respective to said body terminal pads, said contacts exposing the metallization of said device;
• said device attached to said outside body so that said contact pads are aligned with said respective terminal pads, whereby a plurality of metallurgical interconnections are formed operable to absorb, in said nickel/titanium layer, strain caused by thermomechanical stress due to the difference between said first and second coefficients of thermal expansion.

A method of fabricating integrated circuit chips having a plurality of contact pads to be connected by reflow attachment to outside parts, comprising the steps of:

• depositing a layer of nickel/titanium alloy composed of 55.5±0.5 weight % nickel and 44.5±0.5 weight % titanium in the thickness range of 0.3 to 6.0 μm;
• recrystalling said alloy layer at 500° C. for about 5 min, creating a crystalline structure operable in reversible phase transitions under thermo mechanical stress, whereby mechanical strain is absorbed by said alloy layer;
• depositing a layer of solderable metal on said alloy;
• and
• patterning said deposited layers.