Cyclic Voltammetric Stripping Controls for Vacancy Injection Rate Suppression
Cyclic voltammetric stripping regulates organic additive ratios to suppress non-equilibrium vacancy injection during high-aspect-ratio copper microvia plating.
Non-Equilibrium Point Defects
Meaning
Crystallographic imperfections formed during high-rate electrodeposition processes exist in concentrations far exceeding thermodynamic equilibrium values. These non-equilibrium point defects, primarily consisting of interstitial atoms and vacancies, are frozen into the metal during the rapid arrival of copper ions. Their presence increases the internal energy of the plated layer and drives subsequent structural changes like recrystallization.
Point Defect
When electrodeposition occurs at high current densities, copper atoms are deposited faster than they can rearrange into a perfect crystal lattice. This rapid deposition creates non-equilibrium point defects that distort the metal structure and generate high internal tensile stress. These defects are highly unstable and seek to minimize their free energy by migrating or coalescing.
Their concentration depends on the plating bath temperature and the presence of organic additives.
Strain Relaxation
Over time, or when exposed to elevated temperatures, the plated copper undergoes recrystallization to release this trapped energy. The non-equilibrium point defects migrate out of the grains, which reduces the internal stress but can cause microscopic voids to grow. This grain growth change is often referred to as self-annealing.
An unmanaged transition can alter the mechanical properties of the copper film, making it more brittle.
Stress Management
Manufacturing plants utilize controlled thermal annealing steps to stabilize the plated copper before subsequent manufacturing steps. Annealing allows the metal to release its internal stress in a controlled manner, reducing the risk of trace cracking. This thermal treatment is a standard step in advanced PCB production lines to ensure product reliability.