Laser Profile Uniformity at the Image Plane: Comparing Hard and Digital Apertures Illuminated by a Diffractive Optical Beam Shaper

摘要

Imaging based beam delivery systems are used for precision laser material processing applications such as microvia drilling, wafer dicing/scribing and micro-machining. A critical element within the beam delivery system is the aperture or mask which is employed to form the desired spot or image on the surface of the material to be laser processed. Laser beams for precision laser processing are shaped into uniform beam profiles, so the aperture is illuminated with a plane wave and subsequently imaged with a projection lens, forming a flat top spot on the target material. Apertures, in conjunction with illumination by diffractive optical beam shapers can produce ringing or noise at the image plane, causing undesirable effects to the features being formed into the material being processed. These effects become even more severe as the diameter of the aperture decreases (Diffraction effects increase) and/or when the de-magnification is increased (Long path lengths). Since the apertures follow the general principle of diffraction of a circular aperture it is possible to model these effects optically. However, beam shapers have unique attributes that can not be adequately realized in an optical ray tracing model. This paper investigated the use of digital apertures as an alternative to hard apertures to improve the uniformity of the imaged spot, with an emphasis on better uniformity.