Jfc Van Gurp
email: j.f.c.vangurp@tudelft.nl

M Tichem
email: m.tichem@tudelft.nl

U Staufer
email: u.staufer@tudelft.nl

Design, Fabrication and Testing of Assembly Features for Enabling Sub-micron Accurate Passive Alignment of Photonic Chips on a Silicon Optical Bench

Keywords: Micro-Assembly, Passive Alignment, Photonics, Flip-Chip, Submicron Accuracy, Silicon Optical Bench (SiOB)

In this paper, we report on passive alignment with sub-micron precision of two photonic chips on a silicon optical bench. An effective design principle to minimize the tolerance chain is presented and applied to a case study. The chips have been successfully manufactured and individual characterization of the chips revealed that all critical dimensions were within or close to specs. Sub-pixel analysis of images of assembled chips showed that a repeatability of 0.3 µm from a single photonic chip to the silicon optical bench can be achieved. Moreover, it was demonstrated that passive alignment features defined in the waveguiding layers are robust enough to function as mechanical endstops.

Introduction

Passive alignment of photonic components is a high-volume assembly method. However, its precision performance relies completely on the dimensional accuracies of the geometrical alignment features. Passive alignment of single mode fibers in V-grooves has been researched for several decades [START_REF] Basavanhally | Optoelectronic packaging[END_REF]. Accuracies down to 1 µm have been demonstrated [START_REF] Biao | Optimizing fiber coupling with a quasipassive microoptical bench[END_REF], which is usually sufficient compared to the approximate 8 µm mode field diameter for a single mode fiber. For chip to chip coupling, the required alignment accuracy can be as precise as 0.1 µm [START_REF] Takigawa | Passive Alignment and Mounting of LiNbO3 Waveguide Chips on Si Substrates by Low-Temperature Solid-State Bonding of Au. Selected Topics in Quantum Electronics[END_REF]. However, with current passive alignment methods, 0.5 -1 µm accurate waveguide to waveguide alignment is about the best that can be achieved [START_REF] Takigawa | Passive Alignment and Mounting of LiNbO3 Waveguide Chips on Si Substrates by Low-Temperature Solid-State Bonding of Au. Selected Topics in Quantum Electronics[END_REF][START_REF] Krishnamoorthy | Optical Proximity Communication With Passively Aligned Silicon Photonic Chips[END_REF][START_REF] Hayashi | An innovative bonding technique for optical chips using solder bumps that eliminate chip positioning adjustments[END_REF]. Frequently, the more time-consuming alternative of active alignment is used when sub-micron accuracy is required [6,[START_REF] Karioja | presented at the Electronic Components and Technology Conference[END_REF][START_REF] Morgan | Automated Two-Axes Optical Fiber Alignment Using Grayscale Technology[END_REF][START_REF] Weiland | presented at the Electronics Materials and Packaging[END_REF]. In this paper, we propose design rules to achieve sub-micron accurate passive alignment. Minimizing the tolerance chain is crucial [START_REF] Gurp | Systematic approach for tolerance analysis of photonic systems[END_REF]. The way to do that is to define the alignment features in the same masks, the same layers and the same processes as the waveguides. We report about the application of this principle to the design and manufacturing of such features for 0.1 µm accurate passive waveguide to waveguide alignment for a specific case study. Then, the critical dimensions of the individual chips are characterized. After that, the alignment performance in terms of absolute accuracy and repeatability is evaluated with the aid of image processing.

Case study

The object of the case study (figure 1) consists of two photonic chips, flip-chip mounted on a common silicon substrate. An indium phosphide (InP) chip forms the photonic heart of the assembly. InP as a photonic platform offers a wide variety of possibilities for integration of active optical components [START_REF] Smit | presented at the Indium Phosphide & Related Materials[END_REF]. The major drawback is, however, the mismatch in spotsize between the InP waveguides and a single-mode fiber. Therefore, an interposer chip named TriPleX [START_REF] Morichetti | Box-Shaped Dielectric Waveguides: A New Concept in Integrated Optics? Lightwave Technology[END_REF] is used. Both photonic chips are mounted with their active side facing down (flip-chip) to a silicon optical bench (SiOB) that functions as common substrate. This case study focuses on the passive alignment of the InP chip with respect to the TriPleX interposer. For that purpose the two photonic chips have been equipped with passive alignment features, while the SiOB is populated with corresponding counter features.

InP PIC TriPleX interposer Fibers

Silicon Optical Bench

Fig. 1. Case Study

The photonic platform TriPleX (figure 2) consists of a silicon wafer with 8 µm thermally grown oxide. A silicon nitride box was formed on this oxide using one single lithography and several deposition and etching steps . This box is again filled with silicon dioxide. The topcladding is formed by deposited oxide. The effective index of the core can be tuned by changing the nitride and oxide layer thicknesses. The waveguides for this project consist of 550 nm oxide, surrounded by 170 nm nitride.

Different types of waveguides are available for the photonic platform of indium phosphide (figure 2). The type of waveguide as used in this project is a slab waveguide.

Design of passive alignment features

Fabricating the passive alignment features on the TriPleX and InP chips was conceived such that they fit the process steps, which already existed for these chips, while the SiOB was designed from scratch.

TriPleX

As shown in figure 2, a TriPleX waveguide consists of a silicon nitride box, both filled and surrounded by silicon dioxide. The thicknesses of the several layers and therefore the height of the waveguides are more or less fixed. Altering layer thicknesses will change the optical properties of the waveguide. However, the width of these boxes can easily and individually be varied in the mask in order to build passive alignment features within the same layers as the waveguide were made of. One drawback is that the waveguides are buried in the topcladding. Exposing the features therefore requires an additional lithography and etching step. The feature in the TriPleX is shown in figure 3. A surface with a square hole is made out of the same silicon nitride box as the waveguides. The alignment feature will therefore have the same height (thickness) as the waveguides, but is of course much larger in the in-plane (IP) directions.

InP

The thickness and etch depth of the InGaAsP layer together with the thickness of the silicon nitride pad determine the out-of-plane (OOP) position of the waveguide centre, while the width and location of the InP slab defines the in-plane position. The passive alignment features on the InP chip are made from an InP slab with a concave shape as shown in figure 4. The concave slab assures alignment in the IP directions. For the OOP direction, the positioning is done by a silicon nitride pad with a well defined height. This latter layer requires an additional mask and a few additional process steps.

Silicon Optical Bench

The SiOB only contains the counter parts of the InP and TriPleX features. The substrate with the two photonic chips assembled on top, is shown in figure 5. The Tri-PleX counter feature consists of two pedestals with different heights, while the InP counter feature is just one pedestal. For the OOP alignment, it is important that the lower pedestal of the TriPleX counter feature is at the same height as the pedestal for the InP. For the IP alignment, it is essential that the sidewalls of both counter features are accurate with respect to each other, i.e. defined in the same process steps. Moreover, there should be a height difference between the higher tower and the lower tower of 1 -1.5 µm.

Fig. 5. SiOB design

Processing of the substrate is shown in figure 6. It starts with a normal p-type, primegrade silicon wafer [START_REF] Basavanhally | Optoelectronic packaging[END_REF]. After making the zero-layer markers, a 4 µm silicon oxide layer is deposited [START_REF] Biao | Optimizing fiber coupling with a quasipassive microoptical bench[END_REF]. Next, the silicon oxide is etched down to approximately 2.5 µm almost everywhere, except where the higher tower will come (3). Afterwards, the whole wafer is covered with 500 nm thick silicon nitride (4). Now, the lower tower and the final shape of the higher tower are patterned [START_REF] Hayashi | An innovative bonding technique for optical chips using solder bumps that eliminate chip positioning adjustments[END_REF]. To do so, a 6 µm resist coating is applied, because it will also be used for the next step. The oxide and the nitride are etched. Overetching is not a problem, since it will be immediately followed by etching 65 µm Si by means of the Bosch process ( 6) that uses the same mask and the same resist.

Manufacturing Results

Directly after manufacturing and dicing, the alignment features were inspected with scanning electron microscopy (SEM). An SEM image of the manufactured TriPleX features as well as the corresponding SiOB counter feature is shown in figure 7. The alignment features of the TriPleX chip are positioned near the edge of the chip. Hence, the surface of the SEM sample holder is also visible on the picture. The edges of the inner hole are used to align against, while the outer hole was necessary to expose the feature itself, since TriPleX waveguides are buried in the cladding. For the SiOB counter feature, the inner and outer tower are visible. The height difference between the towers as well as their nominal height are important. Secondly, the critical features and dimension of the chips (figure 9) were individually characterized by means of atomic force microscopy and white light interferometery.

The results are listed in table 1. It can be concluded that all critical dimensions were manufactured within specifications, except for one (parameter d in the table, representing the SiN x layer thickness on the InP chip). This deviation will result in a systematic misalignment of 85 nm in the out-of-plane direction.

Assembly results

After having individually characterized both the TriPleX and the InP chip, they were assembled on the SiOB, which was monitored with a video camera. Initially, the SiOB was placed on a holder in the field of view of the camera. Then, the TriPleX or InP chip was placed upside-down (flip-chip) on the SiOB, after which it was gently pushed towards its respective intended position. Afterwards, image processing was used to compare the actual position of the chips with there nominal intended positions (figure 10).

At this stage of research, repeatable assembly was achieved only with the InP chip. (See discussion). Therefore, further image analysis has been conducted solely with the InP chip. Image analysis with Matlab was performed to compare the InP's final position with its nominal position. The assembly shown in figure 10 was repeated several times with the same chips. 9 out of 9 times, the edges of the InP chip were exactly found at the intended pixels or position respectively. This proofs that an edge of no more than 2 µm high can be used as a mechanical endstop. However, since one pixel represents about 3.4 µm, an absolute positioning performance more accurate than 3.4 µm cannot be concluded from just these images. Therefore, also the relative positioning of the InP chip (i.e. repeatability) was evaluated with sub-pixel analysis, the method described below.

First, two clearly distinguishable features were identified; in this case the edge of the chip and the edge of an alignment feature (figure 11a). At the edge of the alignment feature, the grey-value changes suddenly from white (255) to black (0) (figure 11b). A similar transition can be observed at the edge of the chip. The edges are then arbitrarily defined at the location where it has a specific grey-value (chosen threshold value of 150 in our case). This interpolated position is computed for a large amount of rows along both edges, resulting in the distribution shown in figure 11c. This computation is repeated for all 9 pictures or experiments resp. and the mean value for every experiment is taken. These mean values finally define the deviation between the several pictures, i.e. the repeatability of the assembly (figure 11d). From this graph, it can be concluded that the assembly is repeatable within ±0.09 pixel, which corresponds to about ±300 nm. Due to the rather large variation in grey-scale intensity on the surface of the InP chip, the edge of the InP chip was difficult to define by the method described above. Figure 15 top-left already indicated that there was only a small part of the vertical edge suitable for this analysis. For the horizontal edge, however, it appeared impossible to find a clearly defined edge of sufficient length, most probably caused by the illumination direction. Therefore, the analysis has not been conducted along the horizontal edge.

Discussion

Based on various measurements during and after the manufacturing of the individual chips, it was concluded that all critical dimensions of the alignment features were manufactured within specs, except for the thickness of the nitride layer on the InP chip. It is 330 nm thick, while it should have been 85 nm. This gives a systematic error (misalignment) of 85 nm in the out-of-plane direction.

The assembly of the TriPleX chip on the SiOB was unsuccessful in the sense that the TriPleX chip did not always end on the intended position. In many occasions, the TriPleX chip hits its counter feature not with the edge of the square silicon nitride "donut", but with the edge of the exposure hole instead. This effect can be prevented by performing a more accurate initial placement of the chip. While the initial placement of the InP chip was found to be not critical, the TriPleX chip should be placed coarsely within ±20 µm before pushing it towards its endstops.

The assembly of the InP chip was successful in 9 out of 9 trials. Every time, the edges of the chips were found at the intended pixels. However, since one pixel covers 3.4 × 3.4 µm in the object plane, the absolute positioning could not be verified to an accuracy of better than 3.4 µm. The alignment itself is probably more accurate, but it can just not be concluded from these images. Moreover, we have looked at the edge of the chip, which is not very accurately defined with respect to the actual waveguides on the chip. Therefore, the absolute alignment performance will be evaluated by means of optical coupling measurement in the near future.

Nonetheless, the relative positioning, or the repeatability, can be evaluated more accurately by means of sub-pixel analysis. It turned out that the InP chip was positioned within ±0.3 µm every time. Although it is worse than the intended 0.1 µm, it is considered promising. So far, the chips have been pushed against their end stops with a pair of tweezers. Positioning the chips with vacuum chucks mounted on an xyz-stage is likely to yield an even better repeatability. Unfortunately, the sub-pixel analysis could only be conducted in one direction due to the image quality. Nonetheless, the alignment feature for the other in-plane direction is exactly the same and there is therefore no reason to assume any different performance.

The alignment in the out-of-plane direction is also very important. Due to the limitations of the assembly setup, it was not possible to evaluate the performance in this direction. This can be done by measuring the optical coupling [START_REF] Krishnamoorthy | Optical Proximity Communication With Passively Aligned Silicon Photonic Chips[END_REF].

Conclusion

This paper reports on the design, fabrication and testing of assembly features for submicron accurate passive alignment of photonic chips on a silicon optical bench. From the various results and discussions, the following conclusions can be drawn.

First of all, the alignment edges on the various counter features on the SiOB could be manufactured within the same lithography and etching steps as the actual waveguides. Also, it was demonstrated that edges being not more than 2.0 µm high can be used as mechanical endstops for passive alignment. This proofs the principle that it is possible to create sufficiently robust and reliable alignment features in the same lithographic and etching steps as the waveguides. Furthermore, an absolute positioning accuracy (in-plane) of at least 3.4 µm (1 pixel) of the InP chip with respect to the SiOB was achieved. Finally, an in-plane position repeatability of 0.3 µm for the InP chip on the SiOB was achieved.

Fig. 2 .

 2 Fig. 2. TriPleX and InP waveguides

Fig. 3 .

 3 Fig. 3. Design of alignment feature in TriPleX

Fig. 4 .

 4 Fig. 4. InP Design

Fig. 6 .

 6 Fig. 6. SiOB manufacturing steps

Fig. 7 .

 7 Fig. 7. SEM image of the TriPleX alignment features (left) and the SiOB counter feature (right)

Fig. 8 .

 8 Fig. 8. SEM images of an InP alignment feature (left) and the SiOB counter feature (right)

Fig. 9 .

 9 Fig. 9. Schematic overview of the critical dimensions in the several chips

Fig. 10 .

 10 Fig. 10. Assembly sequence of the InP on the SiOB. One such square, black-colored alignment feature measures 400 × 400 µm.

Fig. 11 .

 11 Fig. 11. Sub-pixel analysis of assembly performance

Table 1 .

 1 Measurement of critical dimensions

	Chip	Critical Dimension	Required value	Measured value	Measurement method
	TriPleX	a b	none > f	5 µm 2 µm	AFM * AFM
	InP	c d	1.9 µm 245 nm	1.9 µm 330 nm	WLI ** AFM
	SiOB	e f	50 -70 µm < b	65 µm 1.1 -1.7 µm	WLI WLI
	* AFM = Atomic Force Microscopy
	** WLI = White Light Interferometry

Acknowledgements

The authors gratefully acknowledge the support of the Smart Mix Programme of the Netherlands Ministry of Economic Affairs and the Dutch Ministry of Education, Culture and Science. (http://www.smartmix-memphis.nl) Moreover, the authors would like to thank the consortium partners Jing Zhao and Xaveer Leijtens from Eindhoven University of Technology, group of Opto-Electronic Devices and Arne Leinse from LioniX BV for providing the InP and TriPleX chips respectively.


Matthias Burgard
email: matthias.burgard@ipa.fraunhofer.de

Norbert Schläfli

Uwe Mai
email: uwe.mai@ipa.fraunhofer.de

Processes for the Self-assembly of Micro Parts

Keywords: handling, automation, manufacturing, assembly, self-assembly, surface tension, fluid, roll-to-roll

.

Introduction

The roll-to-roll manufacturing technology is used for a high throughput production and is widely-used in the printing industry. This technology is getting more and more important also for the electronic industry, especially for the manufacturing of organic and printed electronics and the manufacturing of microsystems [START_REF] Hecker | Organic and Printed Electronics[END_REF]. The technology is based on a layer by layer approach to avoid serial processes, which are slowing down the production speed. Therefore, an integration of devices during the roll-to-roll manufacturing is not advisable nowadays. But the combination of conventional devices (e.g. semiconductor circuits, LEDs, sensors) with a roll-to-roll organic and printed electronics could take specific advantages of both technology platforms and could lead to new products with high functionalities [START_REF] Gourlay | Light-Rolls: High throughput manufacture for LED Lighting and Displays[END_REF]. Also the long lasting trend of miniaturization leads to increasing functionality but also to continuously shrinking devices. These small devices have to be automatically handled in assembly processes [START_REF]MINAM -Roadmaps[END_REF]. LEDs (light emitting diode) are reaching nowadays a feed size of 300µm. The automatic handling and positioning of devices is getting more complex with its miniaturization. Surface forces play an increasing role and affect the common handling process [START_REF] Morris | Self-Assembly for Microscale and Nanoscale Packaging: Steps Toward Self-Packaging[END_REF]. These effects are occurring at device sizes of approx. 500µm and smaller [START_REF] Boufercha | Influences of Nanostructured Interfaces on Microfluidics[END_REF]. The self-assembly process is using these surface forces for a precise handling and positioning of small devices and provides a high-throughput and cost efficient assembly for an inline integration of components in a roll-to-roll manufacturing.

Self-assembly processes are a known method for a highly parallel assembly of small parts [START_REF] Fang | Wafer-Level Packaging Based on Uniquely Orienting Self-Assembly (The DUO-SPASS Process)[END_REF], [8]. The described processes chain in [START_REF] Fang | Wafer-Level Packaging Based on Uniquely Orienting Self-Assembly (The DUO-SPASS Process)[END_REF] using the self-assembly effect have a sequential arrangement. The fluidic self-assembly for the integration of GaAs devices that utilizes fluid transport and shape differentiation for placement and orientation requires well defined structures as well on the device as on the substrate [8]. For a roll-to-roll manufacturing a seamless processing has to be provided and to keep the manufacturing cost of devices as low as possible, their design restrictions should be minimized.

The Overall Process

The surface, on which the device should be positioned, is functionalized in hydrophobic and hydrophile areas. The hydrophile areas are surrounded by a hydrophobic surface and have the same shape and dimension as the device to be positioned. The hydrophile area is wetted with ultrapure water and caught by the hydrophobic surroundings. The device is deposited on this droplet and due to surface tension aligned according the hydrophile shape.

Surface Functionalization

The requirements of the functionalized areas are:

• Sharp transition from hydrophobic to hydrophile characteristic • Same shape and size of functionalized area and device e.g. 500 x 500 µm Fig. 2. Schematic of the functionalized area.

The surface functionalization can be realized by different methods. Investigations have shown that atmospheric plasma and the technology for the manufacturing of offset-printing plates can be used to realize the required functionalized areas with an edge length below 500µm.

For the atmospheric plasma functionalization a polymer is used as substrate. An SMD (Surface Mounted Device)-mask with apertures of e.g. 500µm is placed on the polymer. The plasma nozzle with exhausting atmospheric plasma is moved across the mask apertures. The plasma increases the surface energy where it is hitting the surface and so hydrophilizing the substrate through the mask apertures. The results can be seen in figure 3 right, where droplets are positioned on the hydrophile areas to show the effect. Hydrophile areas with 500µm edge length have been realized with a sharp transition, which consequences the square shaped droplets. Also the realized 100µm gap between two droplets is an indicator for a sharp transfer of the shape of the mask apertures to the surface. Also good results have been achieved with the established technology for the manufacturing of offset-printing plate. A Teflon covered aluminum plate is structured with a laser to realize hydrophile and hydrophobic surfaces. In figure 4, the water droplet on the functionalized area shows even better characteristics as the atmospheric plasma functionalization. But the disadvantage of the adapted substrate material has to be mentioned. Structures with 200µm size can be realized easily with this method.

Droplet Deposition

Ultrapure water is deposited onto the hydrophile areas. The water evaporates residuefree, so that a contamination of the devices can be excluded. For a 500µm square a droplet with a diameter of approx. 400µm is needed. This is realized with an airpowered dispenser and a passive magnetic valve developed at Fraunhofer IPA, the so called IPA.VALVE [9]. The valve supports a precise volume control, which is an important parameter for a stable process. The valve can be miniaturized and integrated into capillaries as shown in figure 5. A parallel applying of droplets for a high throughput production can be achieved by an application specific arrangement of capillaries with integrated valves.

Device Placement

The transfer of the device onto the droplet is realized without using a gripper. For that, the devices are arranged in a channel one after the other and moved in such a manner, that the last device protrudes from the magazine edge. The substrate with the droplet is moving underneath. As soon as the droplet, which is slightly higher as the substrate itself, is touching the device, it is grabbed by it and pulled out of the channel onto the droplet. The device gets positioned onto its target position according the functionalized surface due to the surface tension.

Integration into a Roll-to-Roll Process

For a continuous process, as it is needed for a roll-to-roll process, the devices have to be deposited in an array onto a moving substrate. In figure 7 the moving substrate is described by a rotating cylinder. An offset printing plate is bended on the cylinder with functionalized squares of 500µm edge length. The droplets and devices are deposited in parallel with arranged capillaries and channels

Conclusion

The processes for taking advantage of the self-assembly effect for the positioning of devices with an edge size of 500µm are described. The processes are adapted for a continuous roll-to-roll manufacturing. The requirements for a parallel positioning of devices within a roll-to-roll manufacturing are fulfilled with the above described method. Future work will be focused on the automation of the processes to meet the requirements of production equipments.

Fig. 1 .

 1 Fig. 1. Schematic diagram of the process steps for the self-assembly of micro parts

Fig. 3 .

 3 Fig. 3. Left: SMD-mask on polymer substrate and plasma nozzle above. Right: Droplets on functionalized surface with 500µm edge length and minimal spacing of 100µm

Fig. 4 .

 4 Fig. 4. Left: 500µm hydrophile square on an offset-printing plate. Centre: Inked water (for better visualization) on hydrophile area. Right: Self-assembled chip.

Fig. 5 .

 5 Fig. 5. Left: Miniaturized IPA.VALVE in a 1mm tube. Right: 400µm droplet.Ø

Fig. 6 .

 6 Fig. 6. Left: Top view onto the channel with a single chip protruding from the edge. Right: Side view with moving substrate and droplet underneath the chip.

Fig. 7 .

 7 Fig. 7. Cylinder with functionalized surface and parallel deposition of devices

Acknowledgments. The work leading to these results has received funding from the European Community's Seventh Framework Program under grant agreement n° CP-TP 228686. www.light-rolls.eu


Matthias Mohaupt
email: matthias.mohaupt@iof.fraunhofer.de

Erik Beckert

Thomas Burkhardt

Marcel Hornaff

Christoph Damm

Ramona Eberhardt

Andreas Tünnermann

Hans-Joachim Döring

Klaus Reimer

Precisely Assembled Multi Deflection Arrays -Key Components for Multi Shaped Beam Lithography

Keywords: multi shaped beam lithography, multi deflection array, MEMS assembly, precision alignment

ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Multi Shaped Beam Lithography (MSB) is a promising approach for high throughput mask and direct writing that addresses the shot count/writing time bottleneck [START_REF] Slodowski | Coulomb blur advantage of a multi-shaped beam lithography approach[END_REF], [START_REF] Slodowski | Multi-shaped-beam (MSB): an evolutionary approach for high throughput e-beam lithography[END_REF] for the 22 nm node and beyond. Multiple apertures and electrodes necessary for a beam deflection can be manufactured by new developed MEMS micromachining technologies. Nevertheless, for integration into the electron beam column a hybrid assembly of several MEMS Multi Deflection Arrays (MDA) with an aperture overlaying accuracy of less than 1 µm, thermal stability up to 100 °C and low out gassing rates must be accomplished. The development of the whole process chain of manufacturing, assembly and integration was necessary for realization of a new lithography tool for MSB.

Multi Deflection Array -assembly requirements

The x-and y deflection functionality of a Multi-Beam Deflector (MBD) requires the face to face alignment and mounting of two individual deflection arrays that deflect the beam either in the x-or y-direction only. Figure 1 shows the beam forming and deflection principle used in Vertical shaped beam (VSB) and Multi shaped beam (MSB) lithography. Between the two arrays a small distance is desired in order to keep deflection aberrations low. Due to this for the implemented assembly it was chosen to mount each MDA chip on an individual LTCC carrier and afterwards align these sub-assemblies with respect to each other. The alignment has to be done in the x-and y-direction with less than 1 µm accuracy. Tip (Rx) and tilt (Ry) degrees of freedom are given by the contact of the LTCC carriers; the rotation around z (Rz) has to be aligned with an accuracy of less than 17 mrad. Because of the MBD operation in an electron beam environment under vacuum conditions the used materials have to be non magnetic, radiation resistant and vacuum suitable with low outgassing rates. To maintain the cleanliness of the apertures of the MDA chip the assembly has to take place under clean room conditions (class 5 ISO 14644-1). A temperature stability of the assembly device of < 1 K is necessary to reach the high alignment accuracy and measurement uncertainty.

Multiple apertures and electrodes MEMS technology

In MEMS surface micromachining technology based on thick poly silicon layers is a common method for the fabrication of sensors and actuators. This technology has been developed for production of MEMS gyros and MEMS accelerometers with a layer thickness of about 11 µm. Fraunhofer ISIT established this kind of process named PSM-X2 [START_REF] Merz | PSM-X2: Polysilicon surface micromachining process platform for vacuum-packaged sensors[END_REF] during the last years supporting automotive sensor production. Using this technology, enhancing EpiPoly thickness up to 85 µm with an aspect ratio >10 for the deflectors and combining it with double layer Buried Poly wirings Multi Deflection Arrays (MDA) within specifications for multi shape beam writing application were manufactured. A cross section of this modified PSM-X2 process is drawn in fig. 2. The challenge of precise electrode shaping for accurate deflection control and low resistivity wiring for small time constants for beam deflection could be fulfilled. Figure 3 shows 65 µm high, well shaped deflection electrodes close to the aperture opening for the electron beam. Additional alignment marks are structured during the aperture etching process to guarantee lithographic accuracy of the mark positions with respect to the beam apertures. These precise marks will be used for the assembly process.

LTCC carrier system platform

The carrier system platform, a multi layer printed circuit board, is made of "Low Temperature Cofired Ceramics" (LTCC). Within this carrier cavities and geometrical structures are located for passive pre alignment of the MDA chip. Screen printed wirings provide signal routing and localized land patterns for contacting the MDA chip by soldering, furthermore even passive components such as thermistors and sensors could be integrated. The LTCC carrier provides a thermally stable system platform that's CTE is matched to the silicon based MDA and has a close to zero out gassing rate for usage in the electron optical column (EOC) (see fig. 4).

The design of the LTCC carriers consists of seven layers of LTCC tape, using three layers with 90 µm thickness and four layers with 200 µm thickness for increased mechanical strength. The design utilizing screen printed thick film metallization comprises two outer contacting layers, two inner routing layers and two internal shielding layers. Additional thin film metallization of the outer layers prevent electric charging of the assembly in the EOC.

Solder bumping fixation technology

A fine pitch Solderjet Bumping technology was applied for fixation and contacting of the MDA with respect to the LTCC carrier and second for the fixation of two carriers after alignment. Solderjet Bumping is a flux free laser-based soldering process that places bumps of a wide variety of soft solder alloys, e.g. tin-based lead free solders or eutectic gold-tin (Au80Sn20) onto different substrates, such as flip chip or MEMS/MOEMS devices. It allows the hybrid integration of micro-optical and micro-mechanical components on smart system platforms and provides localized and time restricted energy input, minimizing potential damage to the components. The solder bumping device integrates the process steps of handling of the spherical solder preforms with a diameter ranging from 80 µm to 760 µm, the reflow and the application of the solder to the joining geometry, thus increasing repeatability and enabling for a high degree of automation [START_REF] Beckert | Solder Jetting -A Versatile Packaging and Assembly Technology for Hybrid Photonics and Optoelectronical Systems[END_REF], [START_REF] Burkhardt | Precision photonic packaging using laser-based Solderjet Bumping[END_REF], [START_REF] Burkhardt | Parametric investigation of solder bumping for assembly of optical components[END_REF]. The basic principle of solder bumping is shown in figure 6. Spherical solder preforms get singularized and transferred from a reservoir to the placement capillary that has an inner tip diameter which is conical at the end and is slightly smaller then the diameter of the solder spheres. Next to the joining area a laser pulse heats and melts the solder sphere and the nitrogen pushes the solder volume out of the capillary accelerating it towards the components to be joined. The liquid solder hits the wetting surfaces and forms the solder joint by partially transferring its stored thermal energy to the substrate and component to be joined and creating the intermetallic phases during re-solidification. Solder Spreading and re-solidification takes about 10 ms and is mainly influenced by the thermal conductivity of the joined materials. The solder joints provide mechanical fixation of the components as well as a good thermal and electrical connectivity between the MDA chip and the LTCC carrier. Furthermore the joints provide a good long term, thermal and radiation stability and humidity resistance during usage in the EOC. Most promising for the MBD assembly is that the de-alignment during solder bumping fixation can be minimized to less than 1 µm [START_REF] Burkhardt | Precision photonic packaging using laser-based Solderjet Bumping[END_REF]. Ultimate shear forces of up to 17 N were achieved by soldering optical components to LTCC and alumina platforms using two 400 µm solder spheres with a dominant failure mode of rupture of the glass component [START_REF] Burkhardt | Parametric investigation of solder bumping for assembly of optical components[END_REF].

The shear strength of the solder joints can easily exceed the bulk material strength of typical materials in photonic packaging.

Assembly of the MBD

The assembly of an MBD can be divided into two assembly steps. First, a MDA chip is aligned and fixed to the LTCC system carrier. During this assembly step the alignment criteria is the positioning of the MDA contact pads with respect to the contact via holes of the LTCC carrier, requiring an alignment accuracy in the range of 10 µm to 20 µm. This requirement results from the necessity of the mechanical and electrical contact of the MDA chip and the LTCC carrier. The MDA chip is placed at a fixed position by the first gripper of the assembly device (fig. 8).

The LTCC carrier then gets positioned by an additional gripper, which can be moved in the x-, y-and Rz direction for the alignment of the LTCC carrier to the fixed MDA chip. In the next assembly step the LTCC carrier is placed onto the fixed MDA chip by the assembly device.

The initial position for the alignment process of the LTCC carrier with respect to the MDA chip is the planar contact of the upper surface of the MDA chip and the lower surface of the LTCC carrier. The pressing force given by the planar contact of the LTCC carrier and the MDA chip should be less than 1 N and is realized by a flat spring within the assembly device that can balance the tip and tilt mismatch between LTCC carrier and MDA chip. The contact pads of the MDA chip are detected by a microscope equipped with a reflected light illumination and a CCD imaging camera, focusing the pads through the vias of the LTCC carrier (fig. 9). During the alignment process the position of the contact pads are detected by an image processing algorithm. If all 76 contact pads of the MDA chip are positioned within the centre of their respective via hole in the LTCC carrier the MDA chip can be fixed to the LTCC carrier by solder bumping AuSn spheres with a diameter of 200 µm into the holes. All solder joints of the MDA chips fixed to the LTCC carriers have to be inspected by an optical and electrical inspection before they subsequent assembly. The solder joints provide both mechanical fixation and electrical contacts with a resistance of less than 1.5 Ω per contact. Thermal cycling up to 80 °C peak temperature shows no significant change in electrical behavior. Shear force measurements show an average ultimate strength of 10.5 N per solder joint. The principal failure mode during testing is the disruption of the LTCC rather than the breakup of the solder alloy.

In the second assembly and alignment step, two LTCC/MDA sub-assemblies are aligned and mounted with respect to each other. For this assembly task lithographically structured alignment marks within the MDA chip are used for alignment. An overlay accuracy of better than 1 µm has to be reached to guarantee the performance of the MSB device, including the whole error budget of alignment and mounting.

The device for the assembly of the two LTCC/ MDA sub-assemblies is similar to that for the first assembly step. The lower LTCC carrier is fixed within the assembly device by a first gripper; the upper LTCC carrier is handled by an adjustable gripper. The LTCC carriers are put together face to face with mechanical contact of the LTCC carriers. The electrode structures of the MDA chip are protected by the lowered design of the MDA chip support of the LTCC carrier.

An assembly procedure starts with the detection of the alignment marks of the lower MDA chip. Overall eight alignment marks are detected, their centre of gravity being calculated and saved with respect to the field of view and the position of the x-y-positioning system for later calculation of the alignment status. After the detection of the alignment marks of the lower MDA chip the upper MDA chip gets positioned in the assembly device. In this status of assembly the alignment marks of the lower MDA chip are masked by the upper MDA chip. To check the position of the lower MDA chip with respect to the upper MDA chip during the assembly two additional alignment marks are used. These alignment marks are designed having two larger apertures allowing the detection of two reference marks of the lower MDA chip looking through the marks of the upper MDA chip. By this the position of the two MDA chips compared to each other can be calculated and any movement of the lower LTCC carrier during the alignment process can compensated. The alignment position of the upper alignment marks is also measured by mark detection and image processing. The alignment status of the upper MDA chip with respect to the lower chip then can be calculated by comparing the lower and the upper mask position. After the calculation the upper LTCC carrier is aligned to the calculated position. The alignment process iterates until the position failure is less than 500 nm. The control of the assembly process is implemented by use of the Software LabView ® combining the control of the positioning system, the image processing and the calculation algorithms (see fig. 10). For the evaluation of the assembly process the alignment positions of all assembly steps are detected and saved, enabling for a deviation evaluation of the MBD assembly during the fixation by solder bumping. The measured misalignment of the MBD assemblies is less than 1 µm (see fig. 12), dependant from the x-and y-direction. The alignment status was detected and calculated for each of the four alignment marks. Before soldering it exhibits a maximum error of 500 nm with respect to the target position of the first integrated MDA chip. After soldering a misalignment of about 100 nm to 500 nm in x-direction was calculated. In y-direction a misalignment of 200 nm to 600 nm in y-direction was calculated. The difference of the misalignments of the different alignment marks positions depends on the measurement uncertainty during the mark detection because of the different quality of the alignment marks. During further evaluations using a redesigned LTCC carrier and optimized alignment marks an overlap accuracy of about 300 nm at MBD assemblies #6 to #8 was reached.

Figure 12: alignment status of MBD assemblies in x-and y-direction

Summary

The developed LTCC carrier platform is the basic component for the integration of the MDA chip. The solder bumping technology has been successfully used for the MDA assembly and alignment process during MBD manufacturing. An assembly accuracy of less than 1 µm was reached during the whole process chain of MBD manufacturing.

The manufacturing and evaluation of several prototypes of MBDs prove an overlay accuracy of 1 µm. The functionality of beam deflection using these assemblies was demonstrated by VISTEC Electron Beam GmbH. Figure 13 shows resolution test structures which have been exposed with the MSB 3050 test tool on silicon. The used resist is HSQ XR1541 (Dow Corning) at a thickness of 40 nm. As can be seen, lines down to a resolution of 25 nm can be well resolved using existing variable shaped beam electron optics with integrated MSB deflection arrays. The promising deduction of this result is, that MSB is both well aligned with the resolution enhancement potential of variable shaped beam electron optics and flexible in that respect concerning future technology nodes.

The next steps in MBD assembly activities will be the further automation of the assembly and alignment processes for higher efficiency of manufacturing.

Figure 1 :

 1 Figure 1: Schematic comparison of VSB (left) and MSB (right) exposure principle

Figure 2 :

 2 Figure 2: Cross section of a poly silicon MDA

Figure 4 :

 4 Figure 4: LTCC carrier Figure 5: schematics of the LTCC carrier layer stack

Figure 6 :

 6 Figure 6: Simplified schematics of the solder bumping bond Figure 7: bond head in assembly head environment

Figure 8 :

 8 Figure 8: Assembly device Figure 9: MDA contact pad inside a via hole of the LTCC carrier

Figure 10 :

 10 Figure 10: Alignment control user interface Figure 11: MBD assembly

Figure 13 :

 13 Figure 13: Resist image of resolution test pattern (1:1 lines and spaces) exposed with the MSB 3050 test tool in HSQ resist on silicon wafer: lower left 40 nm, lower right 35 nm, upper left 30 nm, upper right 25 nm line width.


Timo Prusi
email: timo.prusi@tut.fi

Riku Heikkilä
email: riku.heikkila@tut.fi

T H Ha

J Y Song

C W Lee

Reijo Tuokko
email: reijo.tuokko@tut.fi

Flexible Gripper System for Small Optical Assemblies -Final Tests and Findings

Keywords: Miniaturized gripper, Assembly, Small optics

This paper presents our work on developing a flexible, adaptive and multifunctional gripper system for the assembly of camera phone lens modules. Key features of the system include tool change system for different end tools and visual position measurement of the component after grasping. This paper presents the development work and discusses the findings of system tests carried out in order to validate the developed gripper system.

Introduction

The number of handheld devices with integrated cameras has increased exponentially during past years. Every camera needs optical components to focus the image on camera detector. Therefore also the production capacity and production techniques for small lens modules need development. This paper introduces an industrial case study for developing a gripper system capable of assembling lens modules consisting of small and fragile optical components. The developed gripper system is flexible, adaptive and multifunctional with a modular tool change mechanism and integrated visual position measurement of the component after grasping.

Earlier publications from this study [1 -3] describe our previous work. This paper presents results from final tests and, most importantly, presents and discusses our findings during the development work and tests. Although this paper only focuses on this specific case, the ideas we present can be generalized to other applications in the field of gripping and assembling small size parts. The developed gripper system was mainly designed for the case product presented in this chapter. However, because of the modular structure and easy tool change mechanism, the gripper can be used to handle any component that has similar dimensions as the components in our case product.

The case product is a mobile phone lens module consisting of nine parts shown in As Fig 1 shows, the lenses have three dimensional shapes that in some cases make grasping more difficult. Furthermore, the optically active area of the lenses cannot be scratched. The shape and small wall thickness of the lock ring make it difficult to grasp in a way that it can still be assembled inside the base part on top of lens 4.

Looking at the parts in Fig 1, it is clear that redesigning the parts would make automatic handling easier. One small but important change would be to modify the parts to have identical gripping area at the outer edge which is not optically active. Another small but helpful change would be modifying the base part (barrel) to have (two or) three vertical "grooves" so that there would be a small space between the lens or spacer ring and the barrel. In this way it would be possible to use finger gripper. Both of these modifications would significantly reduce the number of end tools needed. However, designing or adding such changes was not in the scope of this project.

Gripper System

Because the gripper design is described in detail in [1 -3], this chapter only describes the design on a coarse level. Shortly, the basic idea is to suspend the tool coaxially below an integrated camera. One vacuum locks the end tool to tool body and a second vacuum line sucks the part into the gripper. After that, the integrated camera can measure the exact position and orientation of the part relative to the gripper. Part rotation is compensated by rotating the see-through secondary body. Therefore any 3 degree-of-freedom XYZ manipulator can do the assembly movements. Since there is no space between the lens and the base part and because gripper is not allowed to touch the optically active center part of the lens, the only available gripping surface is the outer edge of the upper surface. In our opinion, vacuum gripping is the best possible method for this case even though there are other quite exotic methods, such as, ice gripping. a) b)

End Tools

Designed gripper can easily adapt to different parts using changeable end tools. With our case product we needed five different end tools for the eight parts. Designing the end tools was straightforward as we only needed to consider the shape of the surface and available flat grasping areas. However, the manufacturing of these small end tools with small diameter drillings proved to be very difficult.

Integrated Camera Systems

The main task of the integrated camera system is to a) verify that a lens or spacer ring was successfully picked from the feeder and b) locate it relative to gripper center. After locating the part in gripper, we can modify the robot assembly movement into fixed base part position. By selecting suitable background and illumination, the camera suspended coaxially above the tool produces images such as the leftmost image in Fig 4 . From these images, it is relatively easy to find and locate the parts based on their outer edges. Soon we noticed that if we capture an image over the base part, we can see and locate both the base part and lens at the same time. In this way it is possible to adapt to small changes in how the part is grasped and what is the exact location of the base part. Because the parts already have some (and could have much more) self-aligning features, the positioning can be relatively coarse which easies image processing.

We decided to integrate the camera to gripper mainly in order to save cycle time: now we can locate the part in gripper during robot movement instead of slowing down or stopping for capturing the image with a stationary camera. Also seeing the base part and part at the same time would be impossible with stationary camera(s). Therefore we consider that integrated camera system is a better choice even though stationary camera(s) would simplify many issues in gripper design. In the current design camera optic's working distance is the main factor defining the length of the gripper. If we want to make a shorter gripper, we have to get the camera closer to end tool. By using suitable optics this is possible; however, bringing camera closer to the end tool creates new problems. The main problem would be that the closer the camera is to the end tool, the bigger area of the image is blocked and blurred by the tool body and end tool thus making locating small diameter parts impossible. Fig 5 uses simple pin-hole model to illustrate how the portion of the (constant size) camera field-of-view blocked by tool body increases if the camera is closer to the end tool. This problem could be avoided by using telecentric optics. However, we did not find small enough commercially available telecentric lenses. Other way to minimize the area blocked by the tool body is to make the secondary body smaller and the tool body shorter. Unfortunately this would make the manufacturing of the gripper even more difficult.

During the tests we also found out that vacuum lines for locking the end tool and grasping the part suck dirt inside the secondary body between the glasses. Dirt creates fuzziness into the images and makes part location less robust. Also dust, dirt or burr on lens/spacer edges causes major problems to our current image processing algorithms. In order to increase the reliability of this gripper system these problems have to be fixed.

Tests and Findings

In order to validate the functionality of the gripper, we carried out some tests.

Test Setup

In our tests we used normal 6 degree-of-freedom industrial robot to carry the gripper system. Because part can be rotated around Z axis with the gripper's rotating secondary body, our robot is unnecessary large and complicated as a simple 3 degreeof-freedom XYZ Cartesian manipulator would be sufficient. However, we decided to use it as it was available and interfacing with it was easy. Fig 6 shows the test setup.

In addition to robot, also the layout of the lens trays, spacer "feeder" and assembly jig are not optimized. Since the purpose of these tests, however, was only to validate the functionality of the gripper system and, for that purpose, the test setup is adequate.

Test Results -Assembly Cycle Times

When assembling lens modules one at the time, the current case product needs a tool change after assembling each part. Complete assembly cycle of assembling all eight parts therefore consists of 24 tasks (pick tool 1, assemble lens 1, release tool 1, pick tool 2, assemble spacer 1, release tool 2, etc.). Each task was then divided into four steps: 1) move above pick position, 2) pick tool/part, 3) move above release/assembly position, and 4) release/assemble tool/part. We videoed several assembly cycles and, from the videos, measured the durations of each phase of the work cycle. When running the unoptimized assembly work cycle with low accelerations and slow speeds, one full assembly cycle took 85 seconds.

Considering that the lenses are fed to robot in trays of 185 pieces, assembling lenses in the way described above is not feasible. More realistic way would be to first assemble all 185 lenses of type 1 to a matrix of 185 base parts, then change the tool, assemble 185 spacers, change the tool, etc. This would lower the number of needed tool changes considerably. Using the previously measured step durations, the calculated assembly time would lower to 47 seconds / lens which is 55% of the cycle time of batch size 1.

Even more realistic scenario would be to have a dedicated robot for assembling each part. In this scenario tool changes would be unnecessary and end tools could be static. This would enable higher accelerations and speeds and therefore considerably faster assembly times. In this scenario and utilizing on-the-fly imaging, the limiting factor would probably be the needed image processing time and communication delays between machine vision system and robot movement control.

Test Results -End Tools

The optically active area in the center part of the lenses cannot be scratched or otherwise damaged during handling and assembly. We were worried that the end tool's steel surface would damage the lens and therefore we did 100 pick-and-place (from and to lens tray) cycles for one single lens. Fig 7 shows lens (type 2) before and after this test. Carefully looking there is some fuzziness on the lens surface where end tool has been in contact with the lens. To prevent damage to lenses, we thought about and tried coating the contact surface of end tool with rubber. Applying and bonding thin enough rubber coating to such a small area of the end tool proved to be very difficult. However, since the end tools do not touch the optically active areas of the lenses, we think that rubber coating is not necessary.

Discussion, Conclusions and Summary

Based on our tests presented and discussed above, we can conclude that the developed gripper system is capable of assembling the case product. Considering industrial use there are, however, a few challenges. These are: 1) End tools, 2) tool body structure and 3) maintenance and cleaning. End tools are not mechanically locked and the vacuum force attaching them to tool body is small. Therefore end tools occasionally dropped during fast and high acceleration robot movements. Since end tools are small in size, they are very easy to lose. Small size and needed small vacuum channels make the end tools difficult to manufacture and therefore they are expensive.

The lower tip and edge of the tool body is fragile and it will be damaged in collisions. Since the tool body is glued to the class plates, the whole glass assembly has to be changed in case tool body gets damaged. This is difficult operation and the long and thin holes (D = 0.5 mm, L = 10 mm) in the tool body make also this part difficult and expensive to manufacture.

For maintaining and cleaning the glass assembly, it would be important to be able to remove the dirt that vacuum lines suck between the glasses. In the current design this practically impossible. To overcome these challenges, we designed a new version of the gripper with dismountable glass suspension parts, integrated optics and a smaller board level camera. We also left out the rotation of the tool since assembly type operations are usually done with robots with rotation, e.g. Scara type robots. In this version the tool body is glued to the thickest glass plate. In addition, the upmost glass plate is glued to gripper body. All other components and glass plates are fastened with screws to gripper body so that they can be removed and cleaned or replaced easily when needed. The dimensions of specified board level camera (32 x 32 x 7 mm) and relay lens (6.25 x 12.5 mm) are from commercially available components. The length of the gripper is now only 60.2 mm and this setup would create a blurred area of 3.8 mm in diameter. To summarize, we have shortly presented the development of a flexible gripper system for small optical assemblies. We also presented the tests we made in order to validate the functionality of the gripper and discussed the findings of our tests and necessary improvements before this gripper could be used in industry.

Fig 1 (

 1 a) where the parts are in assembly order from right to left: base part (barrel), lens 1, spacer 1, etc. until the lock ring which is the leftmost part in Fig 1 (a). The diameters of the parts range from 4 to 7 mm

Fig 1 . 3 . 1

 131 Fig 1. Components of the lens module, cross section and exploded view of the assembled lens module

  Fig 2 b) shows the end tools: tool L1 for lenses 1 & 2; tool S2 for spacers 1 & 2 & 3; tools L2 and L3 for lenses 3 and 4 (respectively); and finally tool S2 for lock ring. Fig 3 shows images of the tools.

Fig 2 .

 2 Fig 2. a) Image and cross section of the gripper. b) Five different end tools for grasping eight parts

Fig 4 .

 4 Fig 4. Images taken with integrated camera. On left, lens with black background. On right, lens and base part on the background.

Fig 3 .

 3 Fig 3. Five end tools. From left to right: S2, L3, L2, S1, L1.

  Fig 5. Pin-hole model illustrates how bringing camera closer increases the portion of camera field-of-view blocked by tool body

Fig

  Fig 6. Test setup

Fig 7 .

 7 Fig 7. Lens before (left) and after (right) 100 pick-and-place cycles.

Fig 8 .

 8 Fig 8. Redesigned gripper


Prof Rainer Müller

Dr Martin Esser

Christian Janßen

Matthias Vette

Stefan Quinders
email: quinders@wzl.rwth-aachen.de

M Mueller

C Esser

M Janssen

S Vette

Tolerance Management for Assembly -Not a Matter of Product Size

Keywords: tolerance, key characteristics, tolerance chain, assembly

Today's industrial assembly systems have to enable individualized production as well as to feature a high degree of automation to cope with both quality requirements and increasing cost pressure. An overall tolerance concept is necessary to allow processes under the above referenced conditions. Reducing costs and the complexity of assembly processes can be achieved by an assembly product design based on the information provided by a tolerance analysis. This paper describes industrial standards and innovative concepts of a holistic tolerance management that are independent of product size.

Introduction

Almost all of today's industries, independent of the product's size or costs, are affected by the progressive globalization, a fast technological development, changing resource conditions, growing product complexity, increasing product variants and a shorter product life cycle. Assembly systems and processes are particularly exposed to the pressure to reduce production costs because they play an important role regarding the value in the manufacturing process. To meet the referenced requirements, strong tolerance management methods are necessary. Otherwise the adaption of an assembly system to continuously changing tasks in minimum machine downtime will not be possible.

To provide examples, tolerance management approaches for aviation and electronic industries are shown. Although there is a large difference in product size, both industries show numerous parallels regarding the requirements, like a quick integration of new products and a growing number of product-variants. The lead approach is to cut down assembly costs by reducing the complexity of the assembly processes and the equipment by using an intelligent tolerance management system which offers the possibility to select simple processes that can be performed with low cost assembly systems.

Basics of Tolerance Management for Assembly Processes

The development of an assembly system is based on a stringent planning logic as shown in figure 1. To satisfy the customer´s requirements high-quality products are needed. The customers' requirements and/or the globalized market exert the cost and quality pressure on the product. The assembly process is defined on the basis of the product analysis. A process analysis determines the specification of the assembly equipment. For optimization of the product design, the assembly processes and the equipment a feedback loop is needed. In the following section, the four major steps, "Product Analysis", "Design for Assembly", "Process Analysis" and "System Optimization" will be explained while considering tolerance management concepts.

Assembly Equipment Assembly Processes Product

Customer Product Analysis

System Optimization

Design for Assembly

Process Analysis

Fig. 1: Logic approach for the development of new assembly equipment As already mentioned above, this paper demonstrates the topic with examples from aviation and electronic industries. There is an airplane which is assembled in large jigs and assembly stations versus a slab-lens marking laser (80mm x 200mm) which is assembled in a reconfigurable and fully automated assembly system. The challenge in laser assembly is to position all lenses on one level for optimal laser-beam characteristics. The relation between the product's size and the assembly accuracy is similar.

Product Analysis

At first the products precedence graph has to be set up. The precedence graph indicates the sequence requirements so that one possible order of the assembly processes is given. Out of the product specifications a large amount of tolerances and productcharacteristics are given for manufacturing and assembly processes. But not all tolerances are important for all steps in assembly. Therefore, identifying the most important tolerances for the assembly processes is one of the major challenges of a strong tolerance management. These important tolerances are the Key Characteristics of the product and can be identified by the use of the Key Characteristic Flow-Down method which is described in the next chapter. The term Key Characteristic (KC) describes a quantifiable characteristic of a product, part, or part of a process whose deviation from the specified set point results in unacceptable impacts on the product's costs, function or safety [START_REF] Thornton | Variation Risk management. Focusing Quality Improvements in Product Development and Production[END_REF]. To avoid these effects, the deviations of the KCs are limited by tolerances, and the compliance of the tolerances can be implemented within the assembly process. There are a lot of tolerances within the product specification for the final product and for all single components. A consideration of all tolerances generates high costs for the assembly process. Therefore, the challenge for the product analysis is to decide which tolerances are essential for the assembly process and must be identified as KCs. Furthermore, the traceability of tolerances back to why they are needed is important for an understanding of the assembly process definition and optimization.

Approach to Identify Admissible Tolerances

An essential part of the method of KCs is the Key Characteristic Flow-Down, where the relation between the customer's requirements and the components or module KCs is shown [START_REF] Thornton | Variation Risk management. Focusing Quality Improvements in Product Development and Production[END_REF]. Developing the KC Flow-Down follows the top-down approach from the customer's demands to the point where these demands are broken down into individual and measureable KCs [START_REF] Whitney | Mechanical Assemblies. Their Design, Manufacture, and Role in Product Development[END_REF]. From these components KCs relevant assembly processes are derived [START_REF] Thornton | Variation Risk management. Focusing Quality Improvements in Product Development and Production[END_REF]. In case of the slab-lens marking laser, for example, the tolerance of the lens' position relative to the crystal is identified as a relevant KC for the assembly process, because of its direct influence on the quality of the laser beam. On the other hand, there are tolerances which are not taken into account because they are irrelevant for the execution of the function. By applying this method, the number of specified tolerances could be reduced from initially 36 to 12, so that the effort in planning the assembly system was significantly reduced. The described method for focusing on the relevant product functionality helps to achieve this result [START_REF] Müller | Flexibel automatisierte Montagesysteme -Toleranzoptimierte Montage von miniaturisierten Produkten[END_REF]. Focusing on the special KCs during the assembly processes helps to reduce the considered product characteristics and helps to simplify the assembly processes.

Evaluation and Classification of Key Characteristics

An example of the aircraft assembly (figure 2) helps to explain the context of the evaluation and classification of relevant KCs. There are KCs for all kind of product characteristics. For this example the geometric KCs are focused. These are classified in three different groups. Performance Key Characteristics (PKCs) are related to the fundamental and top-level KCs who determine the overall design and aircraft performance. Assembly Key Characteristics (AKCs) guarantee the assembly capacity in the following process. Manufacturing Key Characteristics (MKCs) are part of the physical feature of each elementary part or tool [START_REF] Schwanzar | Prozessoptimierung in der Flugzeug-Rumpfschalenmontage[END_REF]. Figure 2 shows the application of the referenced classification for an aircraft assembly beginning with a single part and ending in the final assembly line. At the lowest level there are only MKCs due to the manufacturing accuracy of single parts. The AKCs are quality gates which must be reached to enter the next stage within the assembly process. The PKCs directly influence the flight performance and the cockpit ergonomics. Using the method of classifying the KCs helps to figure out the reason why PKCs cannot be hold in a late stage of the assembly process. The diagram guides the user directly to all reasonable KCs in former assembly steps and helps to find the critical assembly process or a single part which is out of the tolerance. But this traceability of tolerances can only be done if there is an exact understanding for the assembly processes and the used technologies.

A feedback loop for product optimization may be titled "Design for Assembly". It will not be in focus here but is based on the knowledge of the assembly processes and technology and the "Product Analysis". Concepts and design guidelines can be found in literature [START_REF] Pahl | Konstruktionslehre, Grundlagen erfolgreicher Produktentwicklung, Methoden und Anwendungen[END_REF][START_REF] Hesse | Montage-Atlas, Montage-und automatisierungsgerecht konstruieren[END_REF][START_REF] Konold | Praxis der Montagetechnik. Produktdesign, Planung, Systemgestaltung[END_REF].

Process Analysis

Deviations in assembly processes may result from manufacturing inaccuracies, the used technology and from assembly equipment. To recognize the deviations in an assembly process, a visualization of the processes is absolutely necessary. Tolerance chains offer a comprehensible tool for the visualization of tolerance problems in the process analysis and equipment planning. Figure 3 shows the tolerance chain for an aircraft section assembly. All section components (lower, upper, side shells and the floor grid) are positioned by the assembly equipment. The vertical section diameter is one of the main AKCs and will be decomposed as shown in the tolerance chain. Each knot of the tolerance chain symbolizes one datum of the product or the assembly equipment, thus the consideration includes both the component tolerances caused by the manufacturing process and the assembly tolerances by the equipment. The links describe the uncertainty or the deviation of specific values between two knots. This method splits the complete assembly process in single subsystems. Thus, the visualization of tolerance chains enables an easy overview in complex processes by breaking down the entire system into subsystems. The tolerance chain starts at the measuring point on the upper shell. Then it reaches the jig of the upper shell and the assembly station framework. The next steps are the foundation, the jig of the lower shell and the measuring point on the lower shell. From the derived information, external measurements and expert knowledge it is possible to estimate the accuracy of each link. Now, it is possible to assess the assembly-ability of the assembly process. Therefore a tolerance analysis for the entire tolerance chain is carried out. To determine the maximum/ minimum deviation of the tolerance chain, the chain's links with their maximum/ minimum tolerances are added. [START_REF] Hesse | Montagegerechte Produktgestaltung[END_REF] maximum of the tolarance chain minimum of tolarance chain maximum tolarance of the component i minimum tolarance of the component i number of components within the assembly When a mounting capability is given by this maximum/minimum method, a complete replacement of the individual components is possible and the assembly will always be successful [START_REF] Konold | Praxis der Montagetechnik. Produktdesign, Planung, Systemgestaltung[END_REF]. Assuming, however, that the existence of individual tolerances is random, and that they do not represent a specific trend or a systematic error within the assembly process, a worst-case scenario would lead to unnecessary costs and complexity. Therefore, probabilistic methods to determine the tolerance chain should be applied [START_REF] Konold | Praxis der Montagetechnik. Produktdesign, Planung, Systemgestaltung[END_REF]. Under the premise of a normal distribution of each individual tolerance the common method of Root-Sum-Square-Calculation (RSS) is applied [START_REF] Rod | Root-Sum-Square (RSS) Calculations of Digital Timing Delays[END_REF]:

sum of the tolerance chain tolerance of component i
The key characteristic vertical section diameter (missing link of the chain in figure 3) can be estimated by the root sum square (RSS) of all link tolerances. Experience has shown that genuine tolerance values are closer to the result of the RSS-method than to the Min-Max-method. [START_REF] Conrad | Taschenbuch der Konstruktion[END_REF]

System optimization

In some cases the analysis of the tolerance chain or the experience with the assembly process shows that the allowed functional tolerance does not lead to a stable assembly result. There are two options to solve this problem. The first option is to replace the equipment with one offering higher accuracy. But this goes hand in hand with higher equipment costs. Optimizing only the critical parts of the system based on the tolerance chains' analysis is a smarter option.

Fig. 4: Methods for optimizing the tolerance chain

Figure 4 shows four possibilities to optimize a tolerance chain. Shortening single links (4, 1) means to reduce process deviations or uncertainties. Also the number of chainknots can be reduced (4, 2) by using an additional measurement system. Reciprocal changes (4, 3) are an option, if the shortening of one link does not increase costs that much, and the extension of another link safe a lot of investment. The combination of all three methods (4, 4) generates maximum savings. Another important aspect of the tolerance chain method is that the user is forced to consider all factors of the assembly process, which contribute to a deviation in the product. This ensures that the entire tolerance chain is examined and only those variables are changed, which will optimize the cost-benefit ratio [START_REF] Müller | Umfassendes Toleranzmanagement, Eine Notwendigkeit für wirtschaftliche Montageprozesse, wt Werkstatttechnik online[END_REF]. This is the theoretical approach to benefit from the referenced tolerance chain analysis combined with a system optimization. Figures 5(a,b) shows the assembly of a miniature slab-lense marking laser (micro-slab). The task is to position several lenses in a specific order to achieve an optimal Performance Key Characteristic "Quality of the Laser-beam". The same process is shown in both figures 5(a,b), but different equipment is used. Figure 5(a): A robot with a gripper places an optical component (lens) on the laser base plate. The base plate is fixed on a work piece carrier on the installation surface. To program the referenced process offline, the relative positions of the robot, the assembly equipment and the location of the base plate on the installation surface have to be determined in advance. For the positioning of the optical component the accuracy of the robot is important. Only if the component can be supplied in the required quality, the assembly process is feasible. These influences are shown in the tolerance chain below. [START_REF] Müller | System identification of assembly cellsincreased accuracy and demand-driven reconfiguration[END_REF] The system optimization can be executed in several steps to cut down the long tolerance chain. With the help of the system identification, the individual tolerance chain links can be shortened by metrologically identifying the existing values without using the manufacturing and assembly tolerances. Accuracy can also be enhanced during the ongoing process by using system identification. The first idea is the integration of an additional vision system which determines the position of the optical component inside the gripper after each gripping process. The location of the base plate can also be inspected before positioning the component and therefore be included in the path planning. Thus, during the process the conditions for part and assembly tolerances are replaced by only one condition, i. e. measurement uncertainty. In some cases, the accuracy of the system may not be enough and the process can not be realized as required. Besides the system identification, there still exist additional possibilities to achieve a further optimization of the system. For idea two an in-line measurement is used to monitor the robot-position during critical processes in order to compensate possibly occurring position errors that are not modeled in advanced. Thus, the condition describing the position of the robot, is replaced by a more specific one that describes the measurement accuracy of the inline measurement system. The accuracy of the robot itself only has a minor influence on the system. A positive side effect is that less precise robots can be used which are considerably cheaper than high-precision designs. If the increase of accuracy through system identification and inline measuring does not lead to the required quality, the assembly process needs to be changed. The third idea is to use the product itself within the operation during the assembly process. As a result, the quality of the product's features can be controlled immediately. The idea is to place sensors as an adjustment mechanism in the product. Upon commissioning, the product's features can be adjusted immediately. This process will completely eliminate the influence of the assembly equipment and reduce the tolerance chain to a minimum. The tolerance chain, as described in the third idea, is shown in figure 5(b) [START_REF] Müller | System identification of assembly cellsincreased accuracy and demand-driven reconfiguration[END_REF].

Conclusions

In practice it is observed that, in order to achieve functional tolerances, complex assembly processes are planned, independent of the product's size and costs. This paper shows that a consistent tolerance management will ensure a defined installation process, thus an efficient implementation can be performed and the desired functionality and quality of the product will be achieved. This planning process allows a noticeable reduction of the start-up time after a product changeover and therefore enables a more economical automated assembly. The concepts of tolerance management hold true independent of product size or the industry branch. Additionally, a strong tolerance management provides the user with a better understanding of the assembly system, as certain parameters are known in advance, which again is helpful for further production processes. In particular for reconfigurable and flexible assembly systems an exact planning is absolutely necessary to achieve short commissioning times and to guarantee an error-free product launch. The main methods of "Product Analysis", "Product Design", "Process Analysis" and "System Optimization" guide the user towards a strong assembly systems' tolerance management in every branch of global industries.

Fig. 2 :

 2 Fig.2: Classification of the Key Characteristics in aircraft assembly[START_REF] Schwanzar | Prozessoptimierung in der Flugzeug-Rumpfschalenmontage[END_REF]

Fig. 3 :

 3 Fig. 3: Tolerance chain for an aircraft fuselage section assembly

1 Figure 5 (

 15 Figure 5(b):In the following, three ideas for shortening and optimizing the referenced tolerance chains are presented. The first idea is the integration of an additional vision system which determines the position of the optical component inside the gripper after each gripping process. The location of the base plate can also be inspected before positioning the component and therefore be included in the path planning. Thus, during the process the conditions for part and assembly tolerances are replaced by only one condition, i. e. measurement uncertainty. In some cases, the accuracy of the system may not be enough and the process can not be realized as required. Besides the system identification, there still exist additional possibilities to achieve a further optimization of the system. For idea two an in-line measurement is used to monitor the robot-position during critical processes in order to compensate possibly occurring position errors that are not modeled in advanced. Thus, the condition describing the position of the robot, is replaced by a more specific one that describes the measurement accuracy of the inline measurement system. The accuracy of the robot itself only has a minor influence on the system. A positive side effect is that less precise robots can be used which are considerably cheaper than high-precision designs. If the increase of accuracy through system identification and inline measuring does not lead to the required quality, the assembly process needs to be changed. The third idea is to use the product itself within the operation during the assembly process. As a result, the quality of the product's features can be controlled immediately. The idea is to place sensors as an adjustment mechanism in the product. Upon commissioning, the product's features can be adjusted immediately. This process will completely eliminate the influence of the assembly equipment and reduce the tolerance chain to a minimum. The tolerance chain, as described in the third idea, is shown in figure5(b)[START_REF] Müller | System identification of assembly cellsincreased accuracy and demand-driven reconfiguration[END_REF].

Fig. 5 (Fig. 5 (

 55 Fig. 5(a):Assembly of a slab-lens marking laser[START_REF] Müller | System identification of assembly cellsincreased accuracy and demand-driven reconfiguration[END_REF]