A new type of f-theta lens has been recently developed for state-of-the-art microvia laser drilling of multilayer printed circuit boards. It employs a diffractive / refractive hybrid lens element which has a blazed surface-relief microstructure on an aspheric surface. By applying this hybrid technique to a multi-element f-theta lens for a CO2 laser system, and by discontinuing the use of germanium, which is optically highly sensitive to temperature variations, an f-theta lens consisting of all zinc selenide elements is obtained with a temperature-stable optical performance. Achromatic properties against the wavelength fluctuations of actual lasers are also achieved. A prototype was fabricated through the development of single point diamond turning of hybrid surfaces. The performance of the lens was first examined by measuring wavefront errors with a tunable infrared interferometer. The results showed diffraction-limited performance under various conditions, including different temperatures (20 to 50 °C) and wavelengths (9.3 and 9.6μm). The temperature dependence of the focal length of the lens was also measured and found to be five times less sensitive to temperature variations than that of a conventional one. Laser drilling experiments were performed on a polyimide film on copper foil. The results showed good uniformity of hole size and circularity over the entire 50 × 50mm2 scan field.

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