Mold-free fabrication of 3D microfeatures using laser-induced shock pressure

Abstract

This paper reports on the fabrication of microfeatures on metallic foils using laser-induced shock forming without the assistance of micromold patterns. A mold-free laser shock forming technique, Flexible Pad Laser Shock Forming (FPLSF) has been developed and demonstrated to fabricate near-spherical microcraters on thin copper foils through the laser-generated plasma shock inducing plastic deformation on the copper foil. It is found that the crater formation strongly depends on the laser energy fluence applied to ablate an ablative overlay which is on top of the copper foil for plasma shock generation. Microfeatures with deformation depth of 80 μm to130 μm and radius of 485 μm to 616 μm were formed on 25 μm thick copper foils for the laser fluence of 7.3 J/cm² to 20 J/cm² while using aluminum foil as the ablative overlay and silicone rubber as a flexible support instead of a micromold. Fabrication of crater arrays on copper foils was also demonstrated successfully.

Introduction

The demand in manufacturing of microdevices used in various sectors including electronics, automobile, medical equipments, sensor technology and optics has been rapidly growing over the past decades. In general, the components of the microdevices or microsystems can be fabricated using several methods: lithographic techniques, micromachining, microforming, material deposition processes, etc. However, considering some process issues, such as limitations in the fabrication of high aspect ratio features, microtool fabrication, material compatibility, process flexibility, etc., it would be preferable to develop the fabrication methods which can compensate the process disadvantages.

Laser technology is one of the methods for the fabrication of microcomponents. So far, it has been used extensively in microcutting, microwelding, microforming, material deposition and surface patterning due to the localized laser beam control, process flexibility, and reliability [1]. In the recent years, fabrication of microfeatures on metal sheets using pulsed-laser driven deformation force has been accomplished [2], [3], [4], [5], [6]. A process for deforming 0.3–0.9 mm thick austenitic and ferritic stainless steel sheets using laser-induced shock waves has been demonstrated [7], [8]. Laser dynamic forming to deform metallic foils into a micromold with width ranging from 200 μm to 300 μm has been achieved, in which the geometry of the formed features on the foils was in conformance with that of the micromold [9]. Laser deep drawing is another technique developed by Vollertsen et al. to plastically deform 20 μm thick copper, aluminum and stainless steel sheets into spherical cups of 1 mm height using a TEA-CO₂ laser [3]. Very recently, microchannels with the dimension of 260 μm × 59 μm have been produced on 10 μm thick copper foils by laser shock embossing the copper foil on a micromold [5]. In spite of the differences in the reported processes above, the use of master micromold is necessary in all the processes for the fabrication of microfeatures on metal sheets or foils. It is well known that micromold fabrication is not only expensive but also limited in some complex microstructures, whereas the mold fabrication is achieved by Ultra-Precision Micromachining (UPM), micro-Electrical Discharge Machining (EDM), LIGA or laser ablation [10], [11]. Therefore, it is attractive to develop laser techniques which can produce microfeatures on metal sheets or foils without the need of master micromold in the manufacturing of microdevices or microsystems.

In laser-induced shock forming processes, it has been noted that the geometry of the fabricated microfeatures is dependent not only on the micromold geometry but also on the laser process parameters such as the laser fluence, pulse duration, spot size, and the number of pulses [12], [13]. By varying the laser beam energy, different deformation depths of a microfeature have been achieved with the same micromold [2], [4], [14]. As the micromold is not the only factor to determine the microfeature geometry formed on metal sheets, it will be interesting to investigate the metal sheet deformation behavior under laser shock pressure using a flexible pad instead of using a micromold.

Therefore, this study focuses on developing a microfabrication technique to form microfeatures on metal foils using laser-generated plasma shock without the assistance from any specific master micromold patterns. Of particular interest is the study of microcrater formation on metal foils using the flexible pad laser shock forming (FPLSF), i.e. using pliable support as a flexible pad instead of using master mold pattern during the laser shock forming. Special effort has been made to determine the crater size dependence on the laser fluence applied to ablate the ablative overlay over the metal foil.