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|Analysis of Strain/Stress in Electroless Copper Films|
|Keywords: Electroless Copper, Strain/Stress, Adhesion|
|Polymer substrates were chemically coated with copper using various electroless copper baths and the strain/stress in the resulting copper films was studied during and after deposition as a function of the deposit thickness and the operation parameters of the electroless copper bath. These parameters included temperature and the concentration of bath components such as nickel and stabilizers. The deposit stress analyzer (DSA), the spiral contractometer (SC) and in situ X-ray diffractometry (XRD) were employed in these studies and a comparison of the three approaches revealed diverging measured strain/stress behaviors, which are attributable to the different substrate materials of each method (DSA: copper; SC: stainless steel; XRD: polymer). Ex situ grazing incidence XRD measurements were also performed in order to obtain spatially resolved stress profiles of the deposited copper films. Furthermore, the spatially resolved distribution of nickel in the copper deposits was determined by Auger electron spectroscopy (AES). The deposits do not display any texturing or any preference for a particular crystallite orientation. Appropriate variation of bath parameters could be applied to control film stress from compressive to tensile, over a range of about 100 MPa. An enhanced tendency towards blistering was observed in relaxed or compressively stressed deposits, which suggests that tensile stress is favorable for electroless copper film adhesion. A further observation made during these investigations was that cooling the deposited copper film from the bath operating temperature to the rinse water temperature can lead to a transformation of the stress in the deposit from tensile to compressive, which is indicative of substrate-induced thermal stress to the copper deposit.|
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