Study of particle emission in vacuum from film deposits

This study was carried out to determine how much material can be accumulated on vacuum process chamber walls and shields before excessive particle generation occurs, and how the chamber wall materials influence particle generation. Samples of aluminum, stainless steel, titanium, and anodized aluminum were coated with successive layers of sputtered SiO₂. Between each layer deposit, samples were transferred in vacuum to proximity with a particle counter and allowed to cool down. Particles were found to be emitted from the surfaces for times as long as 30 min, typically, before emission rates dropped to zero. The total numbers of particles emitted in a fixed time period increased with the accumulated film layers, until massive delamination was observed. Titanium was found to give the lowest particle rates for a given accumulated thickness, followed by stainless steel, and then aluminum. Anodized aluminum was found to give extremely high particle emissions. Residual stress measurements were made by measuring the bending of the samples. Stress was found to be in the range of 100 to 400 MPa compressive for all the samples. Particle data were taken every 20 s, and size distribution was calculated by the instrument, from a minimum particle size of 0.5 μm. In most cases, the particle size distribution peaked in the 2–5 μm range. The reason for this is not clear. Stress calculated for the titanium samples was somewhat higher than for the stainless steel (390 versus 120 MPa) but the particle emission rates were lower, suggesting better interfacial adhesion on titanium.