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Excimer Laser Machining of Fired LTCC for Selectively Metalized Open Micro-channel Structures
Keywords: LTCC, Excimer laser, Electroless plating
The recent research of Low Temperature Co-fired Ceramic (LTCC) as an electronic substrate material has drawn much attention for its use in the micro and millimetre wave frequency range as it gives low dissipation factors and suitable dielectric properties. LTCC has been established as a multilayer technology and the state of the art of this technology provides for channel structures and cavities inside the ceramic body. These cavity structures and micro-channels permit the application of embedded active and passive components and become very attractive when improving miniaturized electronic devices. So far, laser machining of green state LTCC is widely used when realising such cavities and channels. However, even though the green state LTCC layers are carefully aligned during lamination, deformation of these structures can occur during the firing process. In addition, screen printed conductor lines on green tapes can also lead to deformation when firing. As an alternative approach, in this work, a KrF excimer laser (wavelength 248 nm) was employed to machine fired LTCC to create channel structures. Fired LTCC tapes (DuPont 9k7) 40mm×20mm were used in the experiments, for which the UV-VIS absorption spectrum at 248 nm indicated that around 57 % of the laser radiation was absorbed, making it suitable for machining. Single tracks were machined on fired LTCC using a square mask. Different laser parameters such as repetition rate, feed rate, and number of passes of the laser beam were used to optimise the machining process. The surface roughness of the machined lines was especially taken in to account as this is a crucial issue in signal propagation at high frequency. The machined channels were also characterized for depth, width and uniformity of the machining process. Selective electroless Cu metallisation of machined tracks was investigated. The key challenge was to catalytically activate the surface with Sn/Pd catalyst only on the machined areas. Application of a dry film photo resist before laser processing was used to overcome this issue. Characterization of electroless Cu deposits in terms of surface roughness and electrical performance was also conducted. The excimer laser machining process combined with electroless copper plating was successfully employed to create selectively metallized channel structures on fired LTCC substrates. Future work involves adhesion measurements of deposited Cu and realizing finer conductive structures with high resolution. The process shall be further extended to complete circuit patterns and planar passive components such as inductors and capacitors on fired LTCC.
Dilshani Rathnayake-Arachchige,
Loughborough university
Loughborough, Leicestershire

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