Combining X-Ray Diffraction and Substrate Deflection Analysis to Understand Internal Stress in Electroless Copper Films
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Combining X-Ray Diffraction and Substrate Deflection Analysis to Understand Internal Stress in Electroless Copper Films Tanu Sharma1, Ralf Brüning1, Delilah A. Brown1, Simon Bamberg2,3, Michael Merschky3 and Frank Brüning3 1 Physics Department, Mount Allison University, Sackville NB, Canada E4L 1E6 2 Atotech Deutschland GmbH, Erasmusstrasse 20, D-10553 Berlin, Germany 3 Department of Chemistry, Chemical Engineering Division, Technical University Berlin, Strasse des 17. Juni 124, 10623 Berlin, Germany ABSTRACT Electroless copper films are usually the first conducting layer on the insulating substrates of printed circuit boards. For this and other emerging applications, the internal stress of the copper layer is an important consideration both for film adhesion and film-substrate interaction. We have combined stress/strain analysis based on X-ray diffraction, which is sensitive to the strain of the copper crystallites, with a conventionally used technique that analyses the bending of the substrate (Deposit Stress Analyzer). Both techniques were implemented in such a way that the stress could be monitored continuously during the deposition of the films from the electroless plating bath as well as afterwards. These tests were carried out for three chemical formulations and the results from both techniques agree qualitatively. For one bath, the substrate bending method detects a 60 nm region of local stress at the film-surface interface. INTRODUCTION During roll-to-roll (R2R) processing on flexible substrates such as polyethylene terephthalate (PET) and polyethylene naphthalate (PEN), stresses are generated and compounded with the internal stress of electroless copper interconnects [1-3]. For this and other applications, industrial users increasingly specify limits for internal stress in electroless copper films. A number of methods have been utilized to determine the stress in the thin films including X-ray diffraction and substrate curvature method. The first method detects the elastic strain of the Cu crystallites and determines stress using X-ray elastic constant of a material. Substrate curvature, on the other hand, determines the overall average macroscopic stress, assuming film thickness is small in comparison to the substrate. The aim of this paper is to determine in-situ stress evolution in copper films and to compare the internal stress behavior using these two different techniques. EXPERIMENTAL DETAILS Table I lists characteristic properties of the three chemical copper baths. The operating temperatures were selected for optimal performance for each bath type. The substrate curvature based average film stress was determined by using dual leg test strips made from alloy 42 (Ni-Fe material) supplied by Specialty Testing and Development Co., York, Pennsylvania (Deposit Stress Analyzer, DSA). The legs are coated with varnish on opposite sides such that the Cu film can grow only on one side, and film stress will bend the strips in opposite directions. The dual leg test strips were activated with 10% sulphuric acid
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