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Liquid crystal polymer (LCP) and Cu thin film cleaned with radio frequency (RF) plasma in a vacuum pressure of Pa followed by Cu deposition on LCP were bonded at room temperature. Immediately after bonding, the samples were unloaded, heated in air, argon, nitrogen, hydrogen, and oxygen gases, and subjected to peel strength measurements before and after heating to investigate the adhesion enhancement and the bonding mechanism of Cu/LCP. The interfacial adhesion was evaluated in terms of peel strength (180 ) measured by a peel tester (AGS—1 kNG, Shimadzu Corporation). The adhesion strength after the heat treatment depended on the heating environment and increased in the order of oxygen, air, nitrogen, and argon gas environments, respectively, but significantly declined in further heating in O gas, approximating the level prior to heating. Visual and optical inspections on the delaminated samples showed bulk fractures or partial bulk fractures in LCP heated in all gas environments except in O and its sequential treatments. X-ray photoelectron spectroscopy (XPS) analyses showed a carbon-rich LCP surface with a reduced amount of oxygen after sputter cleaning with RF plasma, which favorably reacted with the deposited Cu. Heating of bonded samples in various environments might enhance or suppress the interfacial reaction resulting in strong adhesion or poor adhesion depending on the heating environments. Depth profile on peeled LCP sample (which was heated first in Ar and then in oxygen prior to delamination) revealed thicker Cu oxide layers on delaminated LCP than natural Cu oxide layers. The new oxidized Cu layers produced across the interface were likely a result of the reaction of diffused oxygen in LCP with deposited Cu layers.