Diffractive/Refractive Hybrid F-Theta Lens for Laser Drilling of Multilayer Printed Circuit Boards

摘要

A new type of f-theta lens has recently been developed for microvia laser drilling of multilayer printed circuit boards. It employs a diffractive/refractive hybrid lens which has a blazed surface-relief microstructure on an aspheric surface. By introducing that hybrid lens for CO_(2) laser system, and by stopping the use of germanium that is optically much sensitive to temperature, the f-theta lens that consists of all zinc selenide lenses is obtained with its optical performance stable on temperature. Achromatic properties against the wavelength fluctuations of actual lasers are also achieved. A prototype is fabricated through the development of single point diamond turning of hybrid surfaces. The performance of the lens is first examined by measuring wavefront errors with a tunable infrared interferometer. The results show diffraction-limited performance at all conditions, including different temperatures (up to 50°C) and wavelengths. The temperature dependence of the focal length of the lens is also measured and found to be 5 times as insensitive to temperature as that of a conventional one. Laser drilling experiments are performed for a polyimide film on copper foil. The result shows good uniformity of hole size and circularity all over the 50×50 mm~(2) scan field.