TECHNICAL PAPER

Robert Schmitt Æ Sascha Driessen Æ Bastian Engelmann

Controlling the assembly of micro systems by image processing

Received: 28 June 2005 / Accepted: 7 November 2005 / Published online: 7 February 2006� Springer-Verlag 2006

Abstract The following article presents two approachesfor the observation and control of assembly processesfor hybrid micro systems. To meet the demands of aproduction of huge quantities of pieces, the control ofthe assembly of a micro mixing by image processing ispresented. Finally a flexible fiber based sensor for han-dling devices is presented.

1 General introduction

The automation of micro assembly is becoming moreand more important in micro technology. Especially thedemands concerning handling and process observationof hybrid micro systems with optical and mechanicalfunctions are quite high. Thus, process monitoring isinevitably necessary for a better understanding of whatis really happening in the micro world. Process visuali-zation is very important in MEMS but does not guar-antee an operative micro system. Therefore, instrumentsfor quality control are required to secure a safe andreproducible assembly. The hybrid assembly of microsystems requires a fail-safe quality control and processobservation.

The aim of this article is to present two typical tasksof the Collaborative Research Center 440 ‘‘Assembly ofhybrid Micro Systems’’ in the field of process observa-tion and process control of the assembly of hybrid microsystems.

In the first part of the article the application ofclassical image processing techniques is portrayedexplaining the assembly control of a micro mixer. In thesecond part of the article the classical image processingtechniques are enhanced by concepts for the acquisitionof 3D data. In this context a flexible handling device formicro gripper units using fibre-based fringe projection ispresented.

2 Assembly control of a micro mixer

The micro mixer is a micro fluid reactor that is com-monly used to mix micro-dosed fluids [1]. They are ap-plied in chemical, pharmaceutical and biologicalapplications. The basic idea of these reactors is to usethe effect that with decreasing dimensions the gradientsof the physical parameters are increasing.

The present micro mixer consists of three componentsin sandwich-like layering (Fig. 1). The topmost layer ofPyrex contains three ultrasonic-drilled holes for twoinputs and the output. The middle layer is a double-sided structured silicon wafer die, containing three pas-sages to the underside and the mixing structure. Due tothe double sided mixing, the micro mixer must be sealedwith a bottom die of Pyrex.

There are basically three steps necessary to assemblethe micro mixer: pick-up and handling, positioning andfinally joining of the parts. Main difficulty is the piling ofthe dies while aligning the drilled holes in the topmostPyrex and the silicon die exactly.

The following text describes how an exact positioningof the topmost Pyrex die and the silicon wafer can beachieved by using image-processing methods.

2.1 Setup

The test rig consists of an illumination source, an objectdeposit for Pyrex and silicon die, micro actuators and amini camera with a mounted microscope optic (Fig. 2).

R. Schmitt Æ S. Driessen Æ B. Engelmann (&)Laboratory for Machine Tools and Production Engineering(WZL), Chair of Metrology and Quality Management,Aachen University of Technology, 52056 Aachen, GermanyE-mail: b.engelmann@wzl.rwth-aachen.deTel.: +49-241-8024782Fax: +49-241-8022671

Microsyst Technol (2006) 12: 640–645DOI 10.1007/s00542-006-0097-z

While the Pyrex die is fixed, the silicon die can be movedin x, y-direction and turned around the z-axis in / usingthree micro actuator axes.

The parts are illuminated verso by a LED with jointdiffuser. The image acquired by the camera is depicted inFig. 3. A commercial image processing software calcu-lates the direction and the length of the necessary cor-rection using the resulting intersection image of thedrilled holes in the Pyrex and the silicon die (Dx, Dy,Dj)

T. This vector is used to direct the actuator.

2.2 Process observation and process control

To calculate the correction vector, the center of thePyrex holes must be known e.g. by prior image pro-cessing. If the dies lay one upon the other, only the areaof the silicon die, which is within the Pyrex die, isvisible (Fig. 4). By the image processing software it ispossible to calculate the center of gravity of the inter-section area between silicon and Pyrex hole (d). Thiscenter of gravity is on the imaginary line that connectsthe centers of the Pyrex and the silicon die. Due to thischaracteristic the direction of the necessary correctionis now known.

Fig. 1 Structure of micro mixer(left), silicon mixing structure(right)

Fig. 2 Setup of the test rig

Fig. 3 Resulting intersection areas

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The following variables are of interest to solve thecorrection task:

d Position of the center of gravity of the inter-section area

RP Radius of the hole in the Pyrex dieRW Radius of the hole in the siliconD Distance between the silicon die and the Pyrex dieDP Distance between the hole of the Pyrex die and

the vertical line through the intersection pointsof the two holes

DW Distance between the hole of the silicon die andthe vertical line through the intersection pointsof the two holes

AP Area segment of the hole of the Pyrex dieAW Area segment of the hole of the silicon diesP Center of gravity of the area segment of the hole

in the Pyrex die

sW Center of gravity of the area segment of the holein the silicon die

MP Center of the hole of the Pyrex dieMW Center of the hole of the silicon die

To calculate the missing length correction D the fol-lowing values must be already known:

(1) Radius of the Pyrex die (RP)(2) Radius of the silicon die (RW)(3) Distance between the center of drilled hole of the

Pyrex die and the center of gravity of the intersectionarea (d)

The center of gravity of the intersection area d can beexpressed as the sum of each center of gravity of the areaAP and AW. The resulting expression d(D) contains bothD and arcsin(D) so that this equation can be only solvedto D by the following steps:

Fig. 4 Geometrical character ofthe intersection area of thedrilled holes

Fig. 5 Principle of the fringe-projection

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– Calculating a table containing values of d(D) of dis-crete points

– Constructing the inverse function D(d) by a polyno-mial using the calculated discrete points

2.3 Conclusion

This part of the article shows that for particular tasksimage processing methods are well suited to detect nec-essary assembly attributes efficiently though just 2Dinformation is processed. Nevertheless it should be as-serted that despite of the presented advantages 2D imageprocessing applications are strictly limited to the mea-surement task. [2]

3 3D image acquisition by fiber-based fringe projection

Nowadays, the assembly ofmicro-systems in a small batchprocess is mostly donemanually [3]. To solve the problemsthat entail with this fact relating to quality and cost, it is

very important to automate this process [4]. A flexibleconcept of handling along with a secure process controlare essential conditions to fulfil the manifold requirementsof the assembly in the field of micro-technology.

To reach that goal in a first step a flexible fiber-scopehas been integrated into the center of the tweezers of aprecision handling system [5]. Accordingly, a direct viewonto a micropart is obtained that can be used for iden-tification and classification.

E.g. the positioning procedure of the precision han-dling system in order to grip a SMD-part can be ob-served. Nevertheless, with this identification method it isstill only possible to handle the part semi-manually byfollowing the movements on a monitor screen. To get toan automated handling process it is necessary to obtain3D-data from the object which is not possible on thebasis of a 2D camera image [6, 7]. Furthermore, it isnecessary to stick to a flexible concept for the visuali-zation and detection of the micro-parts in order to be asfree as possible in using the sensor on different systems[8]. Additionally, a sensor for that purpose must be verysmall in order to fit into the very limited space that isavailable in the micro-assembly scene.

Fig. 6 Fiber-based fringeprojection system

Fig. 7 Micro gear-wheel takenby the new system

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3.1 New solution

In macroscopic production processes several measuringmethods are used to obtain the geometry and to assurethe quality of 3D parts. Definitely, one of the mostwidespread and well known techniques is the fringeprojection. Furthermore, the use of flexible fiber-scopesis a very suitable means for the observation of micro-assembly processes. Therefore, a combination of thefringe projection method with fiber-optic devices prom-ises to be a new and highly flexible system that will openup new possibilities in the automation of hybrid microsystems.

3.2 Fringe projection

In Fig. 5 the principle of the fringe projection method isshown. A number of fringe patterns are projected incertainly timed intervals onto a sample. Under a specificangle those patterns are detected by a camera with thesame frequency. Depending on the shape of the sampleand the camera-angle the projected patterns appear tobe deformed in a certain way. From the deformedcamera images the original shape of the sample can beconcluded.

An example of a sphere next to a flat surface bothilluminated by the same pattern of fringes is given inFig. 5, too. It can be seen how the fringes on the surfaceof the sphere are deformed.

Thus, the fringe projection is a fast and accuratemethod to receive the topography of a part as a com-puter file which can be processed in further steps, e.g. tocompare the measured part to a given CAD file.

3.3 Concept for the new system

The new system mainly consists of two sections. In thefirst section the fringe patterns are generated as well asprojected onto the micro-part. In the second section thedeformed image is received and processed (Fig. 6).

3.4 Generating unit

The fringes are generated by the Mini-ROT projector.A cold light source is connected by a light guide to theprojector. Inside the projector is a transparent rotatingdisk which has three different fringe patterns on it.The revolving speed of the disk can be controlled by aPC. Using the C-mount connector a standard micro-scope lens can be used in order to project the patternonto the front-end of a fiber-scope. The image guideserves as a transmitter for the fringe patterns. A mi-cro-lens is attached to the outlet of the fiber-scope inorder to focus the fringe pattern onto the surface ofthe micro-part.

3.5 Acquiring unit

Another fiber-scope is used to observe the micro part. Itis mounted under an angle relating to the illuminatingimage guide so that the triangulation condition is ful-filled. With a CCD camera connected to the fiber-scopethe projected fringes are recorded and those data isanalysed by an image processing system. The result is athree dimensional cloud of points describing the surfaceof the micro part. [9]

Fig. 7 shows the results of measuring a micro gear-wheel. On the left-hand side a picture taken by a CCDcamera is given. On the right-hand side the surface ofthat micro gear-wheel is shown.

3.6 Conclusion

The sensor concept presented, shows how the use ofminiaturized optical components can substantiallyincrease the efficiency of known measuring techniques.The developments open up a considerably more exten-sive area of application for the fringe projection method.

In further steps it will be necessary to examine theprototype-system in detail. Thus, the limits of the systemconcerning measuring accuracy as well as repeatabilityhave to be tested. It has to be analysed in which waythose system specifications depend on the number ofsingle fibers the fiber-scope consists of.

4 Overall summary

The article confirms that image processing methods arewell suited for an application in a micro productionenvironment. In comparison to classical metrology de-vices like the white light interferometer or the confocallaser scanning microscope, image processing methodsboth 2D and 3D show several advantages.

These systems are small and easy to integrate andallow fast measurements. By easily adapting compo-nents of image processing systems e.g. optics, lowworking distances and high resolution are achievable. Incontrast to classical interferometer based applicationsthe calibration of image processing systems is easy andstandardized.

Acknowledgements The authors gratefully acknowledge the finan-cial support of the Deutsche Forschungsgemeinschaft (DFG)within the Collaborative Research Center (SFB) 440 ‘‘Assembly ofhybrid microsystems’’.

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