Reduction of Thermal Expansion Coefficient of Electrodeposited Copper
Thermal expansion coefficient(TEC) mismatch between the chip and copper causes serious problems in 3D and PCB interconnections. However no one even ever tried to reduce the TEC of the copper wiring formed by electrodeposition. We are the pioneer to succeed in the TEC reduction of electrodeposited copper.
1.Figure 1 shows the thermal expansion results with and without the additive A. Without the additive A the sample is basically electrodeposited conventional copper and the thermal expansion is linear with the temperature(red line). This linearity holds with multiple numbers of annealing. With the additive A, this copper shrinks with raising the temperature for the first time anneal(green curve). Most materials expands with raising the temperature, and this copper shrinks. For the second time anneal, this copper shows about half of the TEC of conventional copper(blue curve) at 400°C.
2.There are no voids or seams and a perfect filling has been achieved of 4μm width and 25μm depth TSV. In-situ SEM observation of pumping with annealing will be presented at the meeting.
3.Next, let’s discuss the grain size distribution by EBSD. The electron back scattering diffraction(EBSD) observes the back scattering diffraction by irradiating electrons in the crystal sample.
The average grain size is 348nm. With the additive A at 400°C, the average grain size is 360nm and the grain size only grows 12nm. Without the additive A, with annealing, 45 nm growth have been observed(1).
The deference in grain size between room temperature and 400°C is only 12nm with the additive A. Additive A inhibits the grain growth at 400°C.
4.Symbols(⊥) indicate the dislocation(Fig.2). The impurity from the additive A segregates at the grain boundaries. This impurity which segregates at the grain boundary prevents the slipping transformation of piled up dislocations. However, without additive A, this impurity does not segregate at the grain boundaries and the slipping transformation of piled up dislocation is large. So the grain growth occurs without the additive A.