Elsevier

Applied Surface Science

Volume 254, Issue 4, 15 December 2007, Pages 997-1001
Applied Surface Science

Short pulse laser microforming of thin metal sheets for MEMS manufacturing

https://doi.org/10.1016/j.apsusc.2007.08.093Get rights and content

Abstract

Continuous and long-pulse lasers have been used for the forming of metal sheets for macroscopic mechanical applications. However, for the manufacturing of micro-electro-mechanical systems (MEMS), the applicability of such type of lasers is limited by the long-relaxation-time of the thermal fields responsible for the forming phenomena. As a consequence of such slow relaxation, the final sheet deformation state is attained only after a certain time, what makes the generated internal residual stress fields more dependent on ambient conditions and might make difficult the subsequent assembly process for MEMS manufacturing from the point of view of residual stresses due to adjustment.The use of ns laser pulses provides a suitable parameter matching for the laser forming of an important range of sheet components used in MEMS that, preserving the short interaction time scale required for the predominantly mechanic (shock) induction of deformation residual stresses, allows for the successful processing of components in a medium range of miniaturization but particularly important according to its frequent use in such systems.In the present paper, a discussion is presented on the specific features of laser interaction in the timescale and intensity range needed for thin sheet microforming with ns-pulse lasers along with relevant modelling and experimental results and a primary delimitation of the parametric space of the considered class of lasers for the referred processes.

Introduction

The increasing demands in MEMS fabrication are leading to new requirements in production technology. Especially the packaging and assembly require high accuracy in positioning and high reproducibility in combination with low production costs.

Conventional assembly technology and mechanical adjustment methods are time consuming and expensive. Each component of the system has to be positioned and fixed. Also adjustment of the parts after joining requires additional mechanical devices that need to be accessible after joining.

Accurate positioning of smallest components represents an up-to-date key assignment in micro-manufacturing. It has proven to be more time and cost efficient to initially assemble the components with widened tolerances before precisely micro-adjusting them in a second step.

As mounted micro-components are typically difficult to access and highly sensitive to mechanical forces and impacts, contact-free laser adjustment processes offers a great potential for accurate manipulation of micro-devices.

Laser forming, usually indicating laser thermal forming, is a flexible rapid prototyping and low-volume manufacturing process [1], [2], [3], [4], [5], which uses laser-induced thermal distortion to shape sheet metal parts without tooling or external forces. Laser thermal forming has many technological advantages compared to the conventional forming technologies, including design flexibility, production of complex shapes, forming of thick plates, and possibility of rapid prototyping. But it is hard for laser thermal forming to maintain material's properties of shaped metal parts because of thermal effects which will result in undesirable microstructure change including recrystallization and phase transformation even with no melting involved during the process [3]. Also, laser thermal forming may melt or burn the surface and even result in small crack on the surface.

Laser shock forming is a non-thermal laser forming method by using the shock wave induced by laser irradiation to modify the curvature of the target [6]. It has the advantages of laser thermal forming (non-contact, tool-free and high efficiency and precision). But its non-thermal process makes it possible to maintain material properties or even improve them by inducing compressive stress over the target surface, which is desirable since it is important in industry for shaped metal parts to resist cracks from corrosion and fatigue. In addition, such compressive stress will make the top surface to expand and produce a curvature to the material [6], [7].

However, possible interaction effects are [3]:

  • Changes in the materials microstructure could cause changes in density and volume and create stresses.

  • Chemical reactions of the irradiated surface, e.g. oxidation could take place and lead to stressed surface layers.

  • Thermal interactions, either caused by the laser's energy directly or by the laser generated plasma in the air could create stress conditions leading to the observable deformation.

  • Mechanical effects, especially shockwaves caused by the instant vaporization and the fast plasma expansion of the ablated material could initiate a microforming process.

In this paper, laser shock microforming is studied using both numerical and experimental methods for a thin metallic film in a one-side pinned configuration. The effect of laser spot position on deformation mechanism is investigated experimentally and data obtained from experiments is then used to validate the corresponding simulation model. The sample curvatures before and after laser micro-scale peen forming were measured using confocal microscopy to find the net bending effect of the process.

Section snippets

Model description

The developed calculational model is integrated by two principal modules conceived for the analysis of the problem of laser shock waves generation and propagation under two different but complementary approaches [7], [8], [9].

LSPSIM is a one-dimensional model intended for the estimation of the pressure wave applied to the target material in Laser Shock experiments [8], [9].

LSPSIM analyses the material-tamper gap assuming an only phase of evolution that can be extended to the end of the

Numerical results

The described model has been applied to study the effect of laser pulse energy and laser spot position on the net bending angle. With the aid of LSPSIM, the resulting plasma pressure is applied to the thin film (Fig. 2).

All the results presented in this paper refer to Stainless Steel 304, whose assumed mechanical properties are shown in Table 1.

Plastic deformation induced by the shock wave generates a residual stress distribution in the beam. Stress in the direction of the beam (S11) in the

Experimental setup

The microforming experiments reported in this paper were performed on AISI 304 alloy. The test piece is fixed on a holder by means of a computer controlled stage the laser spot position can be changed. A Q switched Nd:YAG laser at 1064 nm laser wavelength operating at 10 Hz and providing 9.4 ns FWHM, 1.05 J pulses laser light is then conducted to the interaction area by means of a reflecting mirror and a focussing lens. In order to obtain a smaller spot size and to reduce the energy, a mask (radius

Experimental results

In order to validate the numerical model, the sample was irradiated changing the laser spot position as shown in Fig. 8. Net bending angle increases as laser spot position is closer to 1/3 of the length and it decreases when laser spot is closer to the beam end.

Beam deformation is measured using reflection laser confocal microscopy. The observed experimental profiles are in good agreement with the numerical model predictions, as shown in Fig. 9.

Conclusions

The suitability of laser microbending of thin metal strips by means of ns pulsed lasers with average power in the range of several Watt has been experimentally demonstrated.

Simulations of single-end pinned targets show the presence of two opposite bending components:

  • Local bending at beam incidence position due to local plastic strain. This bending is produced in the direction of the laser beam.

  • Overall angular displacement from beam clamping due to shear stress in the beam. This bending is

Acknowledgements

Work partly supported by Spanish MEC Projects PSE020400-2006-1, PSE020400-2007-2 and CIT0205002005-11.

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