Electrodeposition of low stress gold for x-ray mask

In order to maintain the dimensional accuracy of an x-ray mask, the residual stress of the absorber must be kept low (in the low range of 108 dyn/cm2 ). The absorber material studied in this paper is pure gold electroplated from a proprietary plating solution. Stress of the gold deposits is measured by holographic interferometry of bending disk/beam experiments. The effects of temperature, current density, thickness, pH, and pulse plating on the stress of the gold are investigated. The aging behavior of the plating solution is also discussed. The stress of electroplated gold does not show a strong dependence on the current density (1–5 mA/cm2 ). However, aging of the plating solution changes the stress of the gold deposits, especially for those obtained at a current density of ∼1 mA/cm2. Pulse plating produces stress levels similar to those given by dc plating, over a wide range of on-current-density, on-time, and off-time. Moreover, the pH of the solution does not affect the stress of gold when the current density is greater than 1 mA/cm2. Hence a plating solution and process has been found that produces low stress gold (≤4×108 dyn/cm2) over a wide range of operating conditions and shows no adverse effects upon aging over an extended period of time.