Localized measurements of optical thickness variations in femtosecond trimmed structures

Abstract

Currently, the utilization of high power ultrafast lasers to induce optical changes in structures for the purpose of locally drawing patterns with dimensions inferior to the diffraction limit is well-established and controlled. Using this technique, we aim to modify the refractive index and/or the geometrical parameters of an optical interferential filter composed of successive thin layers. This local optimization will then allow the improvement or tuning of the performances of the optical filters. Thereafter, it is necessary to characterize these local modifications to achieve the final response of the expected filter. In our work, we developed a dedicated optical system, based on Fabry-Perot interferometry, to measure optical thickness, ranging from 10 -3 to 10 -4 , with a high spatial resolution (in the order of 5x5µm). We present here our preliminary results carried out on calibrated test samples.The use of powerful lasers is now well established in the industry to perform marking, cutting, drilling or welding operations on a wide range of materials and parts [1]. The quality of this laser machining is driven by the geometrical characteristics of the focused light beam (the sharpness of the "laser tool") as well as by the heat exchange phenomena between the machined area and the surrounding zone. It is one of the reason for which excimer lasers (which emit in the near UV part of the spectrum) can provide in some cases very attractive solutions [2,3]. The recent development of high power ultrafast lasers [4] (delivering energetic pulses in the range of few micro-joules with a duration of about one hundred of femtoseconds) has permitted to drastically improve the quality of this micromachining, first by increasing the peak power of the laser tool (typically the giga-watt), second by limiting the diffusion of the deposited energy around the focal point (the duration of the laser pulse is indeed much shorter than the heat diffusion time inside the material) and third by involving highly non linear laser-matter interaction phenomena [5][6][7]. This deterministic character of optical damage produced by femtosecond laser pulses [8,9] can be used to machine patterns whose dimensions are far below the diffraction limit.Obviously, the accuracy of this micro-machining is directly connected to the duration and energy stability of the used femtosecond laser pulses [10], but nowadays it seems possible to reach a reproducibility of few nanometers. Moreover, femtosecond laser pulses are also able, at lower fluence levels, to produce a localized change in the refractive index of transparent materials, which can be used to perform a direct laser writing of a waveguide in the bulk of a glass substrate [11][12][13][14] or to develop entirely new photonic device integration and fabrication procedures [15][16][17]. Besides, standard Nd:YAG lasers are today widely used in the semiconductors industry to adjust (or trim) thin film resistors or thin film capacitors to very tight electrical performance specifications [18,19], and to co...