New aspects of simulation in hot embossing

New aspects of simulation in hot embossing M.Worgull,M.Heckele

Abstract Hot embossing is especially well suited for manufacturing small and medium-volume series.How-ever,wider diffusion of this process currently is seriously hampered by the lack of adequate simulation tools for process optimization and part design.This lack of simu-lation tools is becoming critical,as the dimensions of the microstructures continuously shrink from micron and sub-micron to nano scales and as productivity require-ments dictate the enlargement of formats to process larger numbers of devices in parallel.Having no macroscopic equivalent,the micro hot embossing process cannot be described by simple downscaling of existing software tools like in injection molding.In this paper afirst survey is given of how numerical simulation can also be applied to the hot embossing process.

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Introduction

Microsystems technology has been developing rapidly over the past ten years,resulting in a variety of novel products for applications in the automotive sector,com-puter peripherals,health care,and many more.A recent major MST focus is on polymer-based products for emerging new markets,such as dosing systems for medical care,pharmaceutical applications,and analysis systems (‘‘lab-on-a-chip’’)for genes and proteins.

Well-known injection molding and reaction injection molding processes were adapted to the microreplication scale.Another replication process–hot embossing–, however,turned out to be a promising way of fabricating fragile microstructures with high aspect ratios and small distortions[1].

The vacuum hot embossing process is an open tool technique,where a semi-finished polymer sheet is put in between the upper and the lower molding tool.Then,the complete tool is evacuated in order to ensure complete

filling of the cavities of the microstructured tool,and the polymer is heated up above its softening temperature (melting temperature or glass transition temperature, depending on the polymer class).The softened polymer is pressed into the microstructured cavities.After moldfill-ing,the polymer is cooled down below the softening tem-perature,while maintaining the applied force in order to avoid shrinkage and sinking marks.Finally,the machine is opened and the microstructured part can be demolded.

Lowflow rates and small molding speeds ensure that even the smallest details in the nanometer range are rep-licated perfectly.Hot embossing is especially well suited for small and medium series production,because the mold insert can be exchanged within a rather short period of time.Another advantage is the discrete material supply.In this way,polymers can be changed from one molding cycle to the next without any hardware modification being re-quired.The resultant applications are prototyping and material screening.

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New approach to simulation of the hot embossing process Today,a number of microstructures are being replicated by hot embossing[2].Due to future requirements,how-ever,a better theoretical understanding of the hot embossing cycle is necessary.Especially microstructures with high aspect ratios on a large embossing area are susceptible to damage.To avoid defects and distortions, simulation of the whole process–molding and demolding –is needed.Simulation of molding processes is state of the art in the macroscopic world,e.g.for injection molding[3, 4].Now,these processes are being adapted to microre-plication[5].For hot embossing,however,this cannot be done.The intermediate state between stamping and molding makes it difficult to describe the process with available models.Therefore,this paper is aimed at devel-oping reliable computer models and simulation tools for the hot embossing process.As there is no macroscopic tool to describe the entire process,the latter was split up into two sub-processes(Fig.2).

For moldfilling,the simulation software MOLDFLOW was applied using the injection compression molding feature.Qualitative results like theflow behavior of the melt,thefilling of cavities,regions of high shrinkage, pressure distribution,and the thickness of the residual layer are obtained by simulation.

Modeling and simulation of the demolding behavior using the commercialfinite element system ANSYS.The model considers a viscoelastic material behavior.With this model,damage of structures can be predicted and process parameters are obtained to optimize the de-molding behavior and in particular to reduce the de-molding forces.

Microsystem Technologies10(2004)432–437ÓSpringer-Verlag2004

DOI10.1007/s00542-004-0418-z

Received:18June2003/Accepted:12November2003

M.Worgull(&),M.Heckele

Forschungszentrum Karlsruhe,

Institut fu¨r Mikrostrukturtechnik,

Postfach3640,76021Karlsruhe,Germany

e-mail:Matthias.worgull@imt.fzk.de

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