what is film stress and why does it matter

Deposited films are under internal STRESS — compressive (film wants to expand, pushes against substrate) or tensile (film wants to shrink, pulls). Why it matters: too much stress = WAFER BOW/WARP, film DELAMINATION (peeling), and adhesion failure. It's the #1 'the film looks fine but it peels or bows' complaint. Causes: deposition conditions (temperature, energy), film composition, lattice mismatch. Why GPS cares: stress problems show up as real field failures (delamination, broken wafers, yield loss), so a GPS engineer must understand it and how to control it (recipe/conditions). The key line: 'Stress is the hidden failure — a film can look perfect and still be peeling because it's under too much internal tension. Controlling stress is controlling yield.' Know the two directions and that they're managed by process conditions.