27th European Photovoltaic Solar Energy Conference and Exhibition ADVANCED PRODUCTION CHALLENGES FOR AUTOMATED ULTRA-THIN WAFER HANDLING

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1 ADVANCED PRODUCTION CHALLENGES FOR AUTOMATED ULTRA-THIN WAFER HANDLING Tim Giesen 1, Roland Wertz 1, Christian Fischmann 1, Guido Kreck 1, Jonathan Govaerts², Jan Vaes², Maarten Debucquoy² and Alexander Verl 1 1 Fraunhofer Institute for Manufacturing Engineering and Automation IPA, Stuttgart, Germany ²Interuniversity Microelectronics Centre IMEC, Leuven, Belgium tim.giesen@ipa.fraunhofer.de ABSTRACT: The handling of thin wafers in today s production lines demands high standards of the automation as well as complex investigations with a closer look on the actual and future needs for an economic and competitive production of solar cells. This paper describes the analysis and evaluation methods developed by the authors for ultrathin wafers (<100 µm) which are created by a newly developed wafering technique. A short overview about the new handling experiments within the automation test platform at the Fraunhofer Institute for Manufacturing Engineering and Automation (IPA) is subsequently followed by detailed ultra-thin wafer handling experiments. Different approaches were tested for the automated handling of the ultra-thin silicon foils. The difference in handling of curled and flat substrates, caused by the newly developed wafering method, will be pointed out as well as the difficulties for transportation with high-speed parameter settings. While looking for a reliable gripping advanced production challenges are given due to the requirement of certain cleanliness. Keywords: Crystalline, Substrates, Manufacturing and Processing 1 INTRODUCTION The test and demonstration of automated crystalline wafer handling has been widely researched recently in terms of the wafer integrity [1]. The results gave an outlook on what to expect and what we ll face in photovoltaic mass manufacturing when the substrates are getting thinner according to the international roadmaps [2]. New technological approaches require for the automation developers to look even beyond those aims of the roadmap, as the wafering of ultra-thin substrates is not utopistic and may be feasible for large-scale manufacturing in the foreseeable future [3]. Therefore, the scientific challenge of processing ultra-thin photovoltaic wafers with less than 100 µm thickness is paired with advanced requirements for the automation of ultra-thin wafer handling. Only automated solutions for new wafering techniques will be competitive in the future wafer-based photovoltaic cell market. The gripping and handling of ~50µm silicon foils needs to balance a high throughput capability while taking care of a very sensitive substrate and is therefore the key for an extensive exploitation of new wafering techniques. Crystalline photovoltaic wafers remain a very sensitive object to be handled as such. The EU-funded project SUGAR (Silicon Substrates from an Integrated Automated Process) researches and develops a process for wafering silicon ingots into ultra-thin foils. The researchers focus not only on the integrity and capability of the wafering process and cell processing, but also on the feasibility and applicability of an automated handling. Thus, the analysis and evaluation aims at the exploitation of the new technology results and will assist on bringing these project results into an industrial framework. 2 MATERIALS AND METHODS The automated handling of ultra-thin wafers is tested, demonstrated and evaluated with a newly developed prototype handling cell (cf. Figure 1) which is connected to different handling equipments within an industry-like environment at the Fraunhofer IPA`s test and demonstration centre. Figure 1: Prototype handling cell with SCARA-robot for integrated ultra-thin wafer assembly In this in-line production handling platform different opportunities of handling and automation solution are tested with thin and ultra-thin silicon-based photovoltaic substrates. For the development of new methods in automated ultra-thin wafer handling new and prototypic equipment was recently integrated. Thus several handling issues for the special scope of ultra-thin wafer handling can be investigated and demonstrated. Miniaturized gripper test and evaluation, position accuracy determination by digital image processing, a flexible process flow for a mini-module assembly, ultra-thin wafer carrier loading and conveyor transportation are some of the applied automation tests which are dealing with the challenges in the SUGAR project. 2.1 Ultra-thin wafer samples The ultra-thin wafers for which the automated handling is developed are produced in the laboratories of the IMEC by using the Stress induced Lift-off Method (SLiM-Cut). This method has been developed by the PV researchers at the IMEC [3] and makes particularly efficient use of bulk material by reducing kerf loss while 1165

2 producing wafers with a thickness of around 50 µm [4]. The different dimensioning of the investigated samples is depicted in Figure 2. While the standard 6 wafers do have edge lengths of around 156x156 mm the herein handled ultra-thin SLiM-cut wafers are conditioned to a size of 50x30 mm. Figure 2: (1) 1-euro coin, (2) SLiM-Cut-wafer format, (3) Standard 6 monocrystalline wafer 2.2 Handling Sequence Today, a wafer thickness of µm is processed into solar cells at industrial production sites. The handling issues, which were researched in the past [5] do still remain. Due to the reduced size in thickness while simultaneously demanding for increasing throughput rates, the challenges for achieving a high quality output in the production line need to be overcome by new handling concepts. Concerning the process flow of the SLiM-Cut method the handling issues are well beyond of today s handling issues. As depicted in Figure 3 the process sequence is different in comparison to the standard ingot wafering. First, a stress inducing layer (A) is deposited on a silicon substrate (e.g. metal). Certain temperatures recipe activates the stress (B) and the crack propagation (C) is subsequently carried out from a starting notch. The lift-off of a silicon substrate attached to the stress induced layer marks the start of the automated handling section. A curled sample (D) needs to be cleaned in an etching bath to become a flat silicon substrate. Finally, the flattened wafer can be further processed into a photovoltaic cell with (e.g.) IBC architecture. D E: handling of flat ultra-thin silicon foils from a liquid environment into a clean environment for cell processing. Thus, an automated handling method for curled and flat wafer handling in liquid (wet) and air (dry) environment has to be researched, developed and tested. The different geometries of the handling objects as well as the different opportunities for a gripper usage were considered first. The experimental development of the handling solution was then carried out on the handling test- and demonstration platform at Fraunhofer IPA (see chapter 3 for the description of the handling experiments). 2.3 Specimen Diversification According to the Slim-cut process flow the handling object appears in different shapes and geometries. In a first approach, thinned down 5 semi-squared wafers with a thickness of ~50 µm were taken into account to test and adapt the existing methods for automated handling. The existing and validated [5] methods consider a sample with µm thickness and an area of 156x156 mm. A second sample size was given with a thinned down 5 wafer which was diced into 30x50 mm wafers. These miniaturized wafer size was considered for handling test because the format of 30x50 mm was expected to be identical to one of the first wafers, which would be gained by the Slim-cut processing. According to the SLiM-cut mean substrate thickness of around 50 µm, the miniaturized wafers were also thinned down. For the general evaluation of the smaller sample size also 30x50 mm wafers were taken for rough handling investigations and first gripper parameter determination. These samples were diced from a 700 µm thick semiconductor wafer. The curled lift-off samples from the SLiM-cut process were also taken into consideration as a handling sample. These curled substrates (cf. Figure 9) were the most unknown handling objects in this row of investigations. The coated and curled wafer samples may appear in different shapes, depending on the bow strength due to the induced stress. 3 HANDLING EXPERIMENTS AND RESULTS To obtain a first overview on the feasibility of pick & place applications with ultra-thin wafers the application with the presented grippers was set up on a manipulating device. The Test procedure was performed as follows: Figure 3: Handling sequence in the SLiM-Cut process flow (green arrows), [4]. The advanced handling sequences may be separated into the following process sequences according to Figure 3: C D: handling of coated and curled wafer samples between lift-off and cleaning bath The cell/wafer is placed by hand symmetrically blow the gripper on two conveyor ring belts. The gripper is brought into a position right above the cell but without touching the substrate. The suction force generation will be started and the cell is supposed to be gripped. Vertical travelling of the gripper (z-axis) as well as a horizontal stroke (x-axis) back and forth (z-stroke: ~40 mm, x-stroke: ~900 mm). In summary the gripper travels a distance of ~80 mm on the z-axis and ~1800 mm on the x-axis while the wafer is attached. The handling experiments were performed with a 1166

3 successive raise of the acceleration, deceleration and velocity. 3.1 Handling Experiments with Bernoulli-gripper There are three main gripping technologies present in today`s industrial photovoltaic wafer production. Bernoulli grippers can handle the ultra-thin flat wafers. (cf. Figure 4). The pick-up process parameters in the shown sequence are set to 0 mm pick-up height, 3 bar operating pressure, 25 m/s² acceleration on 3 m/s top speed. The wafer is gripped with a waiting time of 200 ms and then accelerated vertically. Due to the absence of a wafer covering shield the wafer suffers heavy uncontrolled vibrations. The bending is not only caused by the air drag but also by the inertia forces of the wafer. Figure 4: Sequence of a thinned ~50 µm wafer pick-up with a Bernoulli gripper While Figure 4 shows the breakage-free vertical movement of the gripper and wafer pair, the horizontal transportation looks somewhat different. Figure 5 shows the sequence of a high-speed video during the horizontal transportation of an ultra-thin wafer. In detail, the sequence shows the deceleration in x-direction subsequently to a horizontal stroke of 800 mm. The extreme transportation parameters (velocity v=3 m/s, acceleration/deceleration a=25 m/s²) cause a heavy irritation to the wafer while being gripped with 3 bar Bernoulli operating pressure. The operating pressure of 3 bar was identified as being the optimal operating point for this gripper and standard ~180 µm 6 wafers during previous capability tests [5]. The ultra-thin wafer in Figure 5 is on the edge of getting completely detached from the gripper due to the fast movement. Furthermore, the stiffness of the ultra-thin wafer is much lower than compared to standard wafers. The brittle handling object suffers a huge deformation due to the air drag in horizontal direction. A breakage did not occur, unless the handled wafers had already a visible crack. A wafer handling with such a substrate deformation cannot be regarded as a qualitative good and valid handling method. It is still unknown, if micro structural defects in the wafer may appear due to such extreme handling parameters. Micro cracks <1 µm may be generated inside the wafer and will painfully grow during subsequent handling or thermal processes. Figure 5: Horizontal transportation of ~50 µm wafer with a standard Bernoulli gripper Today, there is no quality inspection tool for such wafer stresses and strains available. A reliably visual quality inspection of the crystalline handling object, even for standard wafers, requires still a sophisticated empiric study and instrumentation of the inspection tool. The implication of automated ultra-thin wafer handling needs to be further evaluated by using different approaches such as electrical performance characterization of the wafer after the transportation test runs. Nevertheless, it is feasible to handle the thinned ultrathin 5 wafers in a fully automated way with the presented Bernoulli-grippers. In some cases the handling of wafers with parameters for actual standard wafers ( µm thickness) could have been directly applied to the ultra-thin wafer handling. But by taking a closer look (Figure 4 captures 1-9) the handling research results require additional efforts for wafer protection. Large area support of the wafer during pick-up will be considered with e.g. a vacuum area gripper. 3.2 Handling Experiment with Vacuum Gripper The utilization of a vacuum gripper was also evaluated for the ongoing handling tasks. First, a perforation of the ultra-thin wafer surface was feared as a result of the punctual vacuum gripping of the substrate. Four vacuum suction cups were used for the gripping device. But a breakage-free transfer of the ultra-thin wafers was possible with the modified vacuum gripper. Due to the punctual support of the wafer the substrate suffers heavy uncontrolled vibrations (picture 3-7 in Figure 6). But the plate-type shielding above the suction cups avoid an even stronger vibration of the wafer. So this handling method is feasible for the tested thinned down ultra-thin wafers, but may not be the favorite one due to the below listed disadvantages. Figure 6: Sequence of a ~50 µm wafer pick-up and placement vacuum suction cups. 1167

4 3.3 Handling Experiment with Area Vacuum Gripper The third automated handling approach for the ultrathin wafers is the area vacuum gripper (Figure 7). The pick-up process is the calmest one among the investigated grippers (pictures 1-2 in Figure 7). But repeatable position accuracy is not possible with a smooth, thinned ultra-thin wafer: during the placement procedure the wafer needs to be blown off. Several observations showed an unsteady way of how the wafer is released from the gripper s surface. The placement requires a much longer waiting time than the placement with other grippers. In addition, the distance for the placement was set at a certain altitude. Otherwise the wafer would be heavily irritated and lifted when the gripper travels away vertically after the placement is completed due to a slipstream caused by the shape of the gripper. While the covering gripper area supports the wafer during the pick-up phase in a positive way the irritation during placement could harm the wafer and could cause chipping at the edges of the handled substrate. E.g. the wafer may hit the conveyor belts in an uncontrolled way. The placement takes extraordinary long comparing the application with today`s standard wafers. For the handling tests a waiting time for the placement procedure of up to 600 ms was considered, which has a huge effect on the overall cycle time for the handling. Figure 7: Sequence of a ~50 µm wafer pick-up and placement with an area gripper Nevertheless, the area gripper might be an adequate solution for the handling of large area ultra thin wafers in automated manufacturing lines. 3.4 Results of Handling Experiments Summarizing, the breakage-free transfer of thinned monocrystalline ultra-thin 5 wafers is generally possible while some restrictions need to be considered. Cup-based vacuum gripping of flat ultra-thin wafers induces punctual loads and heavy vibrations on the fragile crystalline substrate at the same time. It requires a relatively high effort to find the suitable parameter settings for a reliable pick-&-place application. The mass application of the vacuum gripping for volume handling would need to have a detailed look. Thus the cup based vacuum wafer handling is not considered for the further ultra-thin wafer handling method development. Bernoulli-gripping of flat thin wafers causes heavy vibrations during pick-up and placement of the handled object but the harmfulness of heavy vibrations is unknown as well as the limit of mechanical loads in general. Previous investigations results were inconclusive in terms of gripper dependant lowering of the mechanical integrity of the wafer. Handling caused a reduced mechanical strength. The question if micro-cracks do occur during wafer vibration needs still to be answered. The area gripping of flat thin wafers showed at first good results in flat wafer handling. But considering the SLiM-cut process flow with curled wafers the limitation of the handling capability is reached when it comes to manipulate handling objects with uneven or non-platelike surfaces. The slipping of the wafer has to be considered when an evaluation in term of reliable position accuracy is required. Long waiting times at placement due to a stick-effect (up to 600 ms) prolonging of cycle time essentially. An irritation of the ultra-thin and therefore very light wafer after placement could cause breakage, chipped edges or other losses of the working pieces quality. 3.5 Grippers for Miniaturized Wafer Handling The grippers for the accurate module assembly were tested in comparison to the grippers for standard 156x156 mm wafers. The gripping principles remain the same, such as Bernoulli-principle or conventional vacuum applications. But in terms of the substrate sizes the grippers were downsized and, if required, modified. In comparison to the grippers presented [5], the handling devices had to be reviewed and the downsizing was necessary for an accurate wafer placement. The selected grippers must not exceed the wafer`s area of 30x50 mm for specific processing reasons. Furthermore, there is no need for an implementation of a slipstream shield because the handling of the ultra-thin wafers is not performed with maximum parameter settings. The application of the Bernoulli-gripper for the substrate size had to be investigated as such. Bernoulligrippers do need to work with a certain substrate/object size for a reliable and accurate handling. When the handling substrate would be too small and therefore too light, the air stream based suction principle of a Bernoulli-gripper causes an irritation of the wafer before the gripper would pick the wafer. Thus an accurate wafer pick-and-place operation cannot be guaranteed for smaller sizes. Figure 8 shows the grippers for the wafer handling experiments with ultra-thin and miniaturized substrates. Figure 8: (1) Bernoulli, (2) composite gripper and (3) vacuum cup for small area gripping For the single grippers the optimal operating point had to be identified. The single grippers are: 1. Bernoulli gripper with diameter of 20 mm 2. Composite gripper for limp textile handling (d=30 mm) (miniaturized vacuum area gripper) 3. Vacuum suction cup I (d=7 mm) For a breakage-free transfer the gripping process of the downsized grippers was also observed with a high-speed camera. According to the first results the gripper was operated with 2 bar, 20 m/s² acceleration and 2 m/s top speed on both axes. The gripper touches the wafer 1168

5 punctually during the gripping phase. The high-speed records show no extreme deformation or other harmful behavior of the wafer. The gripper-wafer system seems to work properly. This procedure was validated by the position accuracy test, were the Bernoulli grippers came off well. The gripping of the miniaturized ultra-thin wafers with different vacuum suction cups was accompanied with stronger vibrations of the handling object. Also, the placement accuracy research of the vacuum grippers ended up in less accurate placement of the wafers. One reason may be the elastic behavior of the single contact point, the suction cup. While the wafer is vibrating and moving during the transportation, the suction cup may not grip strong enough to avoid a slipping of the wafer. A stronger gripping by applying a higher vacuum may harm the wafers microstructure or would even break it. Feasible vacuum operation parameters are -100 mbar and 200 mbar. 3.6 Curled/Bowed Wafers Grippers for PV-wafer handling applications are optimized for standard flat wafers. For providing a reliable transport of the wafers within production equipments the grippers aim at a minimum contact between gripper and wafer but also on evenly distributed contacts points in the contact area. For the new kind of handling objects, the bowed wafers, the gripping principles for standard flat wafers needed to be tested for a suitable application with the curled wafers in a first step. gripper in a liquid environment the gripper may suck a fluid due to its vacuum function and could cause a process error (e.g. destroyed vacuum, losing the attached wafer). A gripping on the silicon side of the curled wafer may be possible with a limited extend. It should be avoided, that the wafer is compressed between vacuum cup and ground for not breaking the silicon layer. Therefore, a sophisticated metrology would need to provide the information about position, height and shape of the wafer to the handling device. Table I: Tested handling parameters for curled wafers with available gripping principles (* B20, B30 and B40 are Bernoulli grippers (AL), VCup: standard suction cup (NBR), CGripper: Composite Gripper (POM). Gripper* Gripping Area [mm] Handling Pressure [bar] Speeds [m/s; m/s²] B20 20 Yes Min 0.1 2;20 B30 30 No - - B40 40 No - - VCup 7 Yes Min ;20 CGripper 40 No - - Among the tested Bernoulli grippers the B20 was the only one which was able to pick, transport and place a SLliM-cut wafer. A complication was noticed when the Bernoulli gripper`s area was increased (e.g. for the components B30 & B40) and the curled wafers area exceeded. No gripping was possible in this case. But even for the more or less successful gripping with the Bernoulli B20 a reliable handling could not be attested. Gripping on the silicon (convex) side of the curled wafer was not possible at all. The tests for gripping on the metalized surface caused a further complication. Due to the fact that the wafer is bowed towards the direction of the gripper the Bernoulli cannot approach the wafer as close as necessary. Figure 9: Curled specimen from SLiM-cut process Taking into consideration a prolonged transport distance of 600 mm for upcoming specifications in a wet bench environment, the handling parameters and used components are further listed in Table 1. The grippers are the same as for the miniaturized flat wafer handling test. Additionally, also the Bernoulli grippers with a body diameter of 30 and 40 mm were considered for a general benchmarking and feasibility study of bowed substrate handling. The composite gripper was taken into consideration as an area vacuum gripper for the ultra-thin wafer handling. Curled wafers could not have been gripped with this device. The gripper needs to approach the handling object s surface to a very close distance (~0 mm) for a reliable pick-up. This cannot be realized with the curled wafers. The maximum generated gripping force is insufficient for a distance gripping of the curled wafers. The vacuum cup is able to pick the wafer on its metal coated side with a very low vacuum pressure. But same as for flat wafers the suction cup generates a strong punctual force. This force generation is mainly compensated by the metal layer which protects the thin silicon layer in this case. But having in mind to use the Figure 10: Automated gripping of a curled slim-cut specimen with a Bernoulli gripper As a result, the curled wafer is gripped by the Bernoulli-effect but the contact between wafer and gripper is displaced. The usual contact points of small Bernoulli grippers are in the gripper s center or on the edge of the gripper surface. But the bended corners touch the gripper s outer aluminum edge in a tangential way and prevent the realization of the usual contact between gripper and handling object (Figure 10). Thus, a gripping is possible but yet a reliable handling of curled slim-cut wafers. When the gripper is accelerated for transportation, the attached substrate suffers heavy 1169

6 shaking and vibrations. Additionally the gripper tends to lose the curled wafers, when the transportation parameters were increased. A solution for automated handling of different curled wafer bows was elaborated and positively evaluated for future handling tasks. laboratory. Advanced challenges due to the decreased wafer thickness and the substrate`s shape diversification come along with an increased sensitivity for particular contamination. 4 ADVANCED AUTOMATED HANDLING ISSUES The presence of ultra-thin wafers comes coherently along with advanced manufacturing issues for the automated production. When implementing new technologies which make use of ultra-thin substrates and sensitive layers, the production environment needs to be reviewed in terms of a clean environment [6]. Most PV manufacturing lines today are already set up in factories with requirements for advanced cleanliness, e.g. for ISO 7/Class Contamination and Automated Handling The automated transportation and gripping of parts mostly causes contaminated surfaces of the handled object unless special arrangements for cleanliness are considered. The currently ongoing investigation in the department of ultraclean technology and micro manufacturing at the Fraunhofer IPA focuses besides automated handling also on the particle contamination on ultra-thin wafers due to handling by Bernoulli grippers. The experiments are carried out in an ISO class 1 cleanroom. In a test set up different operating modes of the grippers and measurements were carried out. In simple initial experiments the gripping devices were operated with clean air and the surfaces of handled wafers were subsequently inspected with a KLA Tencor surface scan. First results showed an increase of the contamination within a range from 0.6 µm 28 µm concerning the particle size. Due to the circular outflow of the compressed air for the Bernoulli-mechanism the sedimentation of the contaminating particles on the wafers surface was disturbed. A ring shaped area with fewer particles could have been noticed. But nevertheless, the surrounding area of the particle-free circle was heavily contaminated as well as the ring center. Upcoming researches will systematically investigate the handling-caused contamination of ultrathin wafers. The cleaning of contaminated surfaces will be evaluated in terms of applicability within an industrial process flow. New handling concepts and methods may be investigated for avoiding contamination by particles due to automated handling. 7 SUMMARY 4 ACKNOLEDGEMENT This work is co-funded by the European Commission within the 7 th Framework Program. Thanks to all partners of the SUGAR-project consortium for the contributed work and input. 5 REFERENCES [1] Koepge, R.; Schoenfelder, S.; Giesen, T. et al.: The Influence of Transport Operations on the Wafer Strength and Breakage Rate. In: Proceedings of the 26th European Photovoltaic Solar Energy Conference, 5-8 September 2011, Hamburg, Germany. Page [2] SEMI PV Group Europe International Technology Roadmap for Photovoltaic - Results rd Edition March 2012 [3] Dross, F. et al: Slim-Cut: A Kerf-Loss-Free Method for Wafering 50-µm-Thick Crystalline Si Wafers Based on Stress-Induced Lift-Off. In: Proceedings of the 23rd European Photovoltaic Solar Energy Conference and Exhibition, 1-5 September 2008, Valencia, Spain. Conference Proceedings Page [4] Gordon, I.; et al.: Three novel ways of making thinfilm crystalline-silicon layers on glass for solar cell applications. In: Solar Energy Materials and Solar Cells 95 (2011) S2-S7. [5] Fischmann, C. et al..: Automated Handling and Transport of Crystalline Photovoltaic Wafers. In: Proceedings of the 25th European Photovoltaic Solar Energy Conference and Exhibition, 6-10 September 2010, Valencia, Spain. Conference Proceedings Page [6] Bürger, F.; Wertz, R. and Verl, A. : Detection and avoidance of contaminations on solar wafer and cells. In: Proceedings of the 26th European Photovoltaic Solar Energy Conference, 5-8 September 2011, Hamburg, Germany. Page New technological approaches in PV request for the automation developers to look even beyond the aims of the roadmap, as the wafering of ultra-thin substrates requires adapted automation concepts. The proof of concept concerning automated ultra-thin wafer handling was investigated and demonstrated with thinned wafers. Miniaturized ultra-thin wafers and appropriate grippers were tested and evaluated for feasible industrial-scaled transportation. Automated gripping of curled or heavily bowed substrates was demonstrated in the handling 1170