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EDM texturing of multicrystalline silicon wafer and EFG ribbon for solar cell application
International Journal of Machine Tools & Manufacture 42 (2002) 1657–1664
EDM texturing of multicrystalline silicon wafer and EFG ribbon for solar cell application J. Qian, S. Steegen, E. Vander Poorten, D. Reynaerts ∗, H. Van Brussel Division PMA, Department of Mechanical Engineering, Katholieke Universiteit Leuven, Celestijnenlaan 300B, B-3001 Heverlee, Belgium Received 8 April 2002; accepted 22 July 2002
Abstract This paper presents a novel electrical discharge machining (EDM) texturing method for roughening mc-Si wafers and EFG ribbons for solar cell application. Experiments were carried out on an EDM die-sinker using a specially designed conductive and soft magnetic brush to texture the workpiece. The textured substrates were investigated and analysed using scanning electron microscope, and solar cells were made on textured samples to evaluate the effect of this method. Preliminary experimental results show that the throughput of this method can be over 1000 mm2 minute with a brush of 100 mm diameter. Solar cells made on textured substrates give reasonable output. 2002 Elsevier Science Ltd. All rights reserved. Keywords: Electrical discharge machining; EDM; Texturing; Multicrystalline silicon; EFG ribbon; Solar cell
1. Introduction A recently finished MusicFM study of the European Commission has clearly demonstrated that the increasing market size towards 500 MWp/year will lead to a drastic crystalline silicon photovoltaic (PV) module price reduction below 1 ECU/Wp [1], if the solar cell process is based on printing metallization and thin large-area substrates from silicon sheets, ribbons or multicrystalline wafers. Although laboratory and industrial production processes have already demonstrated required efficiency levels, there are still severe barriers and bottlenecks in industrial production lines with respect to throughput, yield, investment cost and energy consumption for the available production equipment. The reason for this is the lack of specially designed and developed equipment for the PV industry. Almost all of the equipment in use today were originally made for the ‘High-Tech’ semiconductor or hybrid industry according to their specific requirements, with only minor adjustments to solar cell production [2]. ∗ Corresponding author. Tel: +32-16-32-26-40; fax: +32-16-3229-87. E-mail address: [email protected] (D. Reynaerts).
While a wide variety of semiconductor materials have been examined and are still currently under development for PV modules, most solar-cell modules manufactured today utilize monocrystalline silicon (cSi), although there has recently been some success with amorphous silicon solar cells [3]. Monocrystalline silicon solar cells are fabricated using the same techniques as commercial silicon integrated circuits. Most solar-cell substrates start as 250–350 µm wafers sliced from a silicon ingot. Despite the relative maturity of cSi PV technology, industry continues to make improvements in its manufacturing processes and module design to reduce manufacturing cost and increase throughput. One novel approach for creating solar-cell substrates, the edgedefined film-fed growth (EFG) technique, grows the multicrystalline silicon by extracting the crystallizing silicon which is melt through a graphite die. By this technique, ribbons of multicrystalline silicon can be grown as octagonal tube. The tube is later cut into sheets, reducing the amount of silicon lost in the sawing process. After sawing, the silicon is doped to form n and p regions, followed by the deposition of metal contacts on both the top and bottom of the wafer. The top metal contact is deposited selectively to allow light to enter the cell. Although the multicrystalline substrates and ribbons
0890-6955/02/$ - see front matter. 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 8 9 0 - 6 9 5 5 ( 0 2 ) 0 0 1 1 6 - 5
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are cheap, they are characterized by high reflection and high defect density, which need to be improved by other processes such as surface texturing, etc. At present, high throughput processes and equipments for front surface texturing or effective bulk passivation are not available. Besides, multicrystalline cast silicon and ribbon materials, such as edge-defined file-fed growth and ribbon growth on substrate (RGS), can not be surface textured as efficiently as monocrystalline silicon wafers with ⬍100⬎ orientation and suffer from uneven surfaces as compared to sliced silicon wafers. This leads to enhanced reflectance losses and later is an obstacle in the application of screen printing for contact formation. Isotropic chemical texturization method has already been tried by few research groups and it gives much lower surface reflection than standard NaOH texturization process [1]. But this process has a significant disadvantage in that it produces a large amount of chemical waste. Therefore, new texturing methods that do not introduce mechanical stress to the wafers/ribbons are pursued to overcome these obstacles. In this research, the electrical discharge machining (EDM) method is investigated in order to verify its feasibility of roughening mc-Si wafers and EFG ribbons and provide know-how support for equipment development based on this technology. According to the thermo-electric model in electrical discharge machining, the EDM is a thermal erosion process in which material is removed by a series of recurring electrical discharges between a cutting tool acting as an electrode and a conductive workpiece, in the presence of a dielectric fluid. This discharge occurs in a voltage gap between the electrode and workpiece. Heat from the discharge vaporizes minute particles of workpiece material, which are then washed from the gap by the continuously flushing dielectric fluid. The EDM process has been for some time classified as a non-traditional machining method and used to deal with hard-tomachine and conductive workpiece. The implication and application of EDM have been expanded from its original shaping workpiece to other areas during the last decade. Some research on EDM focus on using it as a method for surface modification [4], including surface treatment, colouring and texturing. Using EDM to texture cold-rolled steel and aluminium sheet/strip has been widely applied in the automotive industry on a worldwide basis, in order to improve formability through the retention of a lubricant film and the appearance of the painted product. Accordingly, it has attracted a lot of research effort [5,6]. The silicon material, mainly in the form of monocrystalline, usually acts as the workpiece material in EDM, and it is always used to fabricate microstructures for sensor applications [7,8]. Silicon powder is also used in some EDM applications as an additive to improve the surface quality (mainly surface roughness) [9,10]. However, using EDM to texture
multicrystalline silicon materials has not been reported in the literature.
2. Experimental 2.1. Device Experiments were carried out on an AGIE die-sinking EDM machine—AGIE INNOVATION II, which is equipped with a micro generator SBOX using a RC relaxation circuit to generate very narrow discharge current pulse. There is an auxiliary rotary device attached to the spindle of the EDM machine, with which the electrode can rotate at high speed. Instead of using the machine’s dielectric system, a plastic box was put on the machine saddle as the dielectric tank for the sake of reducing dielectric consumption in the experiments, and the dielectric was renewed periodically. Detail of the EDM texturing set-up inside the dielectric box is shown in Fig. 1. An aluminium frame for holding magnetic devices is connected to the spindle quill, which is fixed on the rotary device, and several magnetic components are inserted in the holes on it. When the device works, metallic (iron) powder hangs below these magnetic components and it functions like a conductive soft brush. The workpiece is fixed on a stainless steel plate and then submerged into the dielectric tank. The stainless steel plate is connected to the power supply of the EDM machine. 2.2. Materials Two kinds of substrates, multicrystalline silicon (mcSi) wafers and EFG ribbons, were tested in this investigation. The mc-Si wafers are sliced from ingot and rela-
Fig. 1.
Detail of magnetic brush.
J. Qian et al. / International Journal of Machine Tools & Manufacture 42 (2002) 1657–1664
tively flatter than the EFG ribbons. Fig. 2(a) shows a detailed view of the wafer surface. The size of the mcSi wafers is 10 cm by 10 cm and its thickness is around 300 µm. Before EDM texturing, these wafers have been doped to form P-type region and their conductivity is 1 ⍀cm, and the surface roughness is Ra ⫽ 1.1 µm, and Rt ⫽ 11.4 µm. The EFG ribbons are cut from octagonal tube and much more brittle compared to the mc-Si wafer. The size of EFG ribbon is also 10 cm by 10 cm and its thickness is around 300–400 µm. As shown in Fig. 2(b), there are small ripples on the ribbon surface. These ripples are formed in the process of crystalline growth and their maximum peak-to-valley value is about 50 µm. Normally, the sparking gap between the workpiece and tool electrode in EDM is less than 30 µm, therefore, it is quite difficult to spark both of the peak and valley with an ordinary shaped electrode at one time. With the soft brush introduced in this paper, it is possible to texture the peak and valley simultaneously.
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2.3. Method At the beginning of this research, EDM texturing with a rod electrode was first carried out to verify the possibility of this method (Fig. 3). The workpiece was vertically clamped to a metal plate and a tungsten copper electrode was used to texture the workpiece by scanning over it. The major disadvantages of this method are its low efficiency and the difficulty to texture the whole surface in one pass due to aligning and mounting problems encountered during the experiment. Furthermore, especially in the case of EFG ribbons, it is impossible to texture the grooves on the surface in the same time. Therefore, a new approach with a magnetic brush was developed and applied in this experiment. As shown on the right of Fig. 3, the magnetic brush rotates and moves horizontally to scan over the workpiece. Metallic (iron) powder with average diameter of 450 µm is kept below those magnetic components due to magnetic force (Fig. 1). In the process of texturing, the brush rotates at a low speed of about 1 Hz while it scans over the workpiece surface together with the movement of the spindle. Electrical sparks take place in between the metallic powder on the brush and the workpiece. The whole surface is roughened when the brush passes through it. The experimental procedure is as follows: 앫 machine preparation, such as machine start-up, dielectric check, etc.; 앫 fix the workpiece on the workpiece holder and submerge it into the dielectric tank; 앫 magnetic brush preparation and mounting; 앫 lower down the magnetic brush to a preset position; 앫 start EDM texturing; 앫 cleaning; 앫 reflectivity measurement;
Fig. 2. Substrates before texturing. (a) mc-Si wafer. (b) EFG ribbon.
Fig. 3. EDM texturing mode.
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Fig. 4.
Surface with a few sparking craters.
앫 solar cell processing; 앫 IV measurement/evaluation. Isotropic chemical texturization is used as a reference process in this investigation.
3. Result and analysis 3.1. Topography of EDM textured workpiece After EDM texturing, the textured surface has a typical surface roughness of Ra ⫽ 3 µm and Rt ⫽ 30 µm with a current setting of 3.2A. The microscope picture Fig. 4 shows the edge of the textured area of an EFG ribbon, on which individual spark craters can be clearly recognized. In Fig. 5, which was taken at the position that was visually recognized as fully textured area, a few un-sparked islands (white parts) are still visible under the microscope. When the
Fig. 5. Textured surface with un-sparked islands.
brush’s scanning speed increases, the size and number of theses islands increase as well. The un-sparked islands left on the sample did not bring about any serious problem in the following solar cell processing procedure, but it can be foreseen that too many un-sparked islands will reduce the effect of texturing. Those islands were diminished in the experiment when the workpiece was textured twice by the two half arcs of the brush or servo gain was set at a low value, which means longer brush travel path and machining time. So the occurrence of these islands is, to some extent, an obstacle to increasing the texturing speed. This situation may be improved by using new structure of the brush, such as parallel chains, or finer metallic powder to increase the number of small electrodes and hence the probability of spark ignition in the same area. According to the principle of EDM machining, spark occurs when the two electrodes under a certain voltage go close. In the EDM texturing method introduced here. The tool consists of a great amount of fine particles and these particles act as a cluster of small electrodes during the texturing process. The spark occurs randomly when the brush rotates and scans over the workpiece. Each spark leaves a crater on the workpiece (Fig. 4), and the final surface is the result of accumulated craters. If the brush moves forwards too quickly, sparking craters cannot cover the whole surface and there will be un-sparked islands left on the workpiece. Fig. 6 shows a typical instance of such situation. After 60 min etching, the exact shape of each crater shows up and a flat un-sparked island remains as shown in the middle of the picture. Fig. 6 also indicates that the shape of these craters left by EDM is different from what can be expected on an EDMed metallic material. The reason for this can be explained by the different material removal mechanism when EDMing silicon material and traditional metallic material. Melting and evaporating of material occurs on metallic material, therefore the spark crater is relatively
Fig. 6.
EFG ribbon surface after 60 min etching.
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smooth and continuous. In the case of silicon material, the shape of the crater becomes much more irregular, because not only melting and evaporation but also spallation of material occurs due to its physical characteristics. Big flat craters are not good for effectively capturing light and heavy sparking has proved harmful to solar cell efficiency, it is necessary to reduce the spark energy to reduce the size of spark crater. According to the energy equation (Eq.1) [8], the spark energy can be further reduced by using smaller capacitance. e ⫽ 1 / 2CU2
(1)
It is observed both on EDM textured mc-Si wafer and EFG ribbon that silicon balls appear on the textured substrates. As shown in Fig. 7, it is visible that substrate textured by spark erosion contains a lot of redeposited silicon, mostly presented in the form of silicon balls. Those balls may be amorphous and will have a bad effect on the emitter doping, so they should be removed by following etching process. 3.2. Texturing effect The reflectance of light on the textured samples was measured to evaluate the effect of this texturing method. Fig. 8(a) and (b) show the reflectance on mc-Si wafer and EFG ribbon respectively. Low reflectivity less than 20% can be obtained on both mc-Si wafer and EFG ribbon. In the area with more redeposited silicon balls, the reflectivity is lower than that in the place with less silicon balls. Therefore, low reflectivity is not equivalent to good efficiency, because with the presence of redeposited silicon balls the light absorbed by them cannot be converted to useful solar cell energy. In the case of EFG ribbon, the surface reflectivity is still below 20% even after 120 min of etching. The reason for this is
Fig. 7.
mc-Si wafer agfter EDM texturing.
Fig. 8. Reflectivity characteristic of textured substrates.
supposed to be the relatively low effectiveness of this kind of etchant to the EFG ribbon. To evaluate the effect of the EDM texturing, an untextured mc-Si wafer was etched in an isotropic etchant H1 for 3 min an EDM textured sample for 6 min. Figs. 9 and 10 show the section views of the two etched sub-
Fig. 9.
Section view of isoetched surface.
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Fig. 10.
Section view of textured surface after etching.
strates. Reflectivity measurement shows that both of them give reflectivity less than 20%. Small and deep holes remain on the textured surface after etching. Relatively even and uniform surface is available on the untextured one. Several EDM textured mc-Si wafers and EFG ribbons were further processed into solar cells for efficiency evaluation and parameter optimisation. These EDM textured mc-Si samples were first cleaned and then etched in a specially developed etchant. After processing them into solar cells, IV measurement was carried out on them. Fig. 11 shows typical efficiencies of solar cells made on the textured wafers and ribbons. The REF value is the standard reference value for efficiency comparison and it is measured on a sample of the same material. The solar cell efficiency of each material is a bit lower than its reference value. This is due to the surface topography after EDM texturing, especially the cavities with flat valley, is not the ideal for capture light for energy conversion. Compared to current efficiency of
Fig. 12.
Effect of current settings on mc-Si wafer.
solar cell made on mc-Si wafer and EFG ribbon, the efficiencies of these solar cells made on textured samples are reasonable. 3.3. Effect of texturing parameters In accordance with the principle of EDM, different surface topographies appear when different parameter settings were utilized. When a higher sparking current is chosen, the spark energy becomes larger and it melts and removes more material from both electrodes, thereby leaves bigger spark crater on them. Fig. 12 illustrates textured surface with two current settings on mc-Si wafer. The sample was first sparked with a current setting of 3.2 A and then at 6.4 A. With the latter setting, the spark craters are obviously larger than that left by sparking at 3.2 A. So the higher the current setting be, the rougher the surface become. Even after removing the textured structure by etching, obvious fine cracks induced by spark erosion remain on the textured surface (Fig. 13).
Fig. 11. Solar cell efficiency made on textured mc-Si wafer and EFG ribbon. Fig. 13.
Cracks on textured surface with high current settings.
J. Qian et al. / International Journal of Machine Tools & Manufacture 42 (2002) 1657–1664
Fig. 14(a) gives the detail of the cross-section of a textured mc-Si wafer with a current setting of 6.4 A. The peak-valley value on the obtained structure is around 5 µm. The surface structures left by the heavy spark are not suitable for solar cell process. Although it gives a satisfactory reflectivity, such structure needs to be removed by following etching procedure, since no light captured by it can be converted to effective electricity. Fig. 14(b) illustrates that some of the worm-like craters may extend fairly deep into the silicon surface at such a current setting. Fig. 15 shows the efficiency of solar cells made on a set of wafers textured by two current settings, 3.2 and 6.4 A respectively. Generally speaking, low current setting offers better solar cell efficiency. The spark induces less surface defect, and less and smaller redeposited silicon balls will appear on the textured surface with a small current setting. Therefore, current setting, or spark energy in other words, is a crucial parameter in EDM texturing of such material.
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Fig. 15. Efficiency solar cell made on wafers textured by different current settings.
3.4. Throughput The EDM machine on which to carry out the experiment is a commercial product for normal die-sinking EDM process. The servo system and other functions are built in with the machine and the traverse rate of each axis cannot be adjusted independently by the operator. The sparking mode used in our experiment is unique and different from what is usually applied on such a machine. In our mode, the sparking takes place vertically in the gap between the brush and the workpiece, while the brush moves in the horizontal direction, in which the servo system is supposed to work (Fig. 3). So the servo mode is not strictly working in a right way. The only way to adjust the feedrate of the spindle is to tune the servo gain parameter. When the servo sensitivity is changed, the texturing speed varies. Fig. 16 shows the time the brush took to scan over and fully texture the whole workpiece. When a servo gain value, bigger than 20, was used the brush moved forwards so fast that the surface was not fully textured. The mean feed-rate of the brush can be up to 12 mm
Fig. 14. Surface topography of EDM textured mc-Si wafer.
Fig. 16.
Texturing time versus gain setting.
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per min. With the diameter of the brush being 100 mm in this experiment, the throughput of this set-up is over 1000 mm2 minute. If necessary, the throughput can be further upgraded by adapting other configuration, such as using multi-head/brush.
Energy Programme JOULE III (JOR3-CT98-0223). Special thanks go to project consortium partners, especially IMEC in Belgium, and ASE in Germany.
References 4. Conclusions Texturing of mc-Si wafers and EFG ribbons by EDM with magnetic brush was successfully carried out. Based on the experimental results introduced above, some conclusions may be drawn as follows: 앫 EDM texturing with magnetic brush is feasible for roughening mc-Si solar cell substrates. 앫 Both EDM textured mc-Si wafer and EFG ribbons can be made into solar cells of reasonable efficiency. 앫 As for mc-Si wafer, the textured workpiece needs further etching process to remove re-deposited silicon balls, and surface defects occur when the current setting is high. 앫 In the case of EFG ribbon, this method is possible to texture the whole surface with fine grooves in one path. 앫 Throughput of this method can be above 1000 mm2 per minute when a 100 mm diameter brush is utilized. 앫 Further parameter optimisation is necessary for industrial application. Acknowledgements This research has been funded in part by the European Commission in the framework of the Non Nuclear
[1] T.M. Bruton, et al. Multi-megawatt upscaling of silicon and thinfilm solar cell and module manufacturing—MUSIC FM, in: Eur. Conf. Renewable Energy Development, Venice, Italy, November, 1995, pp. 22–25. [2] W. Schmidt, F. Schomann, G. Wahl, High throughput crystalline silicon solar cell key manufacturing technologies, HIT project mid-term assessment report, Achen, Germany, September, (2000). [3] J. Szlufcik, S. Sivoththaman, J.F. Nijs, R.P. Mertens, R. Van Overstraeten, Low-cost industrial technologies of crystalline silicon solar cells, Proceeedings of the IEEE 5 (85) (1997) 709–730. [4] N. Mohri, H. Takezawa, K. Furutani, Y. Ito, T. Sata, A new process of additive and removal machining by EDM with a thin electrode, Annals of the CIRP 49 (1) (2000) 123–126. [5] J.A. McGeough, H. Rasmussen, A theoretical model of electrodischarge texturing, Journal of Materials Processing Technology 68 (2) (1997) 172–178. [6] J. Simao, D.K. Aspinwall, F. El-Menshawy, K. Meadows, Surface modification/alloying of electrical discharge textured (EDT) mill rolls using PM tool electrodes and various coatings, proceedings of ISEM XIII, Bilbao, Spain 89 (2001) 3–902. [7] D. Reynaerts, P.H. Heeren, H. Van Brussel, Microstructuring of silicon by electro-discharge machining (EDM)-1-Theory, Sensors and Actuators, A-Physical 60 (1-3) (1997) 212–218. [8] X. Song, D. Reynaerts, W. Meeusen, H. Van Brussel. A study on the elimination of micro-cracks in a sparked silicon surface, Sensors and Actuators(A-PHYSICAL), 1-3/92 (2001) 286–291. [9] L. Stevens. Improvement of surface quality in die-sinking EDM, Ph.D. Dissertation, KULeuven, 1998. [10] Y.S. Wong, L.C. Lim, I. Rahuman, W.M. Tee, Near-mirror-finish phenomenon in EDM using powder-mixed dielectric, Journal of Materials Processing Technology 79 (1998) 30–40.
EDM texturing of multicrystalline silicon wafer and EFG ribbon for solar cell application J. Qian, S. Steegen, E. Vander Poorten, D. Reynaerts ∗, H. Van Brussel Division PMA, Department of Mechanical Engineering, Katholieke Universiteit Leuven, Celestijnenlaan 300B, B-3001 Heverlee, Belgium Received 8 April 2002; accepted 22 July 2002
Abstract This paper presents a novel electrical discharge machining (EDM) texturing method for roughening mc-Si wafers and EFG ribbons for solar cell application. Experiments were carried out on an EDM die-sinker using a specially designed conductive and soft magnetic brush to texture the workpiece. The textured substrates were investigated and analysed using scanning electron microscope, and solar cells were made on textured samples to evaluate the effect of this method. Preliminary experimental results show that the throughput of this method can be over 1000 mm2 minute with a brush of 100 mm diameter. Solar cells made on textured substrates give reasonable output. 2002 Elsevier Science Ltd. All rights reserved. Keywords: Electrical discharge machining; EDM; Texturing; Multicrystalline silicon; EFG ribbon; Solar cell
1. Introduction A recently finished MusicFM study of the European Commission has clearly demonstrated that the increasing market size towards 500 MWp/year will lead to a drastic crystalline silicon photovoltaic (PV) module price reduction below 1 ECU/Wp [1], if the solar cell process is based on printing metallization and thin large-area substrates from silicon sheets, ribbons or multicrystalline wafers. Although laboratory and industrial production processes have already demonstrated required efficiency levels, there are still severe barriers and bottlenecks in industrial production lines with respect to throughput, yield, investment cost and energy consumption for the available production equipment. The reason for this is the lack of specially designed and developed equipment for the PV industry. Almost all of the equipment in use today were originally made for the ‘High-Tech’ semiconductor or hybrid industry according to their specific requirements, with only minor adjustments to solar cell production [2]. ∗ Corresponding author. Tel: +32-16-32-26-40; fax: +32-16-3229-87. E-mail address: [email protected] (D. Reynaerts).
While a wide variety of semiconductor materials have been examined and are still currently under development for PV modules, most solar-cell modules manufactured today utilize monocrystalline silicon (cSi), although there has recently been some success with amorphous silicon solar cells [3]. Monocrystalline silicon solar cells are fabricated using the same techniques as commercial silicon integrated circuits. Most solar-cell substrates start as 250–350 µm wafers sliced from a silicon ingot. Despite the relative maturity of cSi PV technology, industry continues to make improvements in its manufacturing processes and module design to reduce manufacturing cost and increase throughput. One novel approach for creating solar-cell substrates, the edgedefined film-fed growth (EFG) technique, grows the multicrystalline silicon by extracting the crystallizing silicon which is melt through a graphite die. By this technique, ribbons of multicrystalline silicon can be grown as octagonal tube. The tube is later cut into sheets, reducing the amount of silicon lost in the sawing process. After sawing, the silicon is doped to form n and p regions, followed by the deposition of metal contacts on both the top and bottom of the wafer. The top metal contact is deposited selectively to allow light to enter the cell. Although the multicrystalline substrates and ribbons
0890-6955/02/$ - see front matter. 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 8 9 0 - 6 9 5 5 ( 0 2 ) 0 0 1 1 6 - 5
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are cheap, they are characterized by high reflection and high defect density, which need to be improved by other processes such as surface texturing, etc. At present, high throughput processes and equipments for front surface texturing or effective bulk passivation are not available. Besides, multicrystalline cast silicon and ribbon materials, such as edge-defined file-fed growth and ribbon growth on substrate (RGS), can not be surface textured as efficiently as monocrystalline silicon wafers with ⬍100⬎ orientation and suffer from uneven surfaces as compared to sliced silicon wafers. This leads to enhanced reflectance losses and later is an obstacle in the application of screen printing for contact formation. Isotropic chemical texturization method has already been tried by few research groups and it gives much lower surface reflection than standard NaOH texturization process [1]. But this process has a significant disadvantage in that it produces a large amount of chemical waste. Therefore, new texturing methods that do not introduce mechanical stress to the wafers/ribbons are pursued to overcome these obstacles. In this research, the electrical discharge machining (EDM) method is investigated in order to verify its feasibility of roughening mc-Si wafers and EFG ribbons and provide know-how support for equipment development based on this technology. According to the thermo-electric model in electrical discharge machining, the EDM is a thermal erosion process in which material is removed by a series of recurring electrical discharges between a cutting tool acting as an electrode and a conductive workpiece, in the presence of a dielectric fluid. This discharge occurs in a voltage gap between the electrode and workpiece. Heat from the discharge vaporizes minute particles of workpiece material, which are then washed from the gap by the continuously flushing dielectric fluid. The EDM process has been for some time classified as a non-traditional machining method and used to deal with hard-tomachine and conductive workpiece. The implication and application of EDM have been expanded from its original shaping workpiece to other areas during the last decade. Some research on EDM focus on using it as a method for surface modification [4], including surface treatment, colouring and texturing. Using EDM to texture cold-rolled steel and aluminium sheet/strip has been widely applied in the automotive industry on a worldwide basis, in order to improve formability through the retention of a lubricant film and the appearance of the painted product. Accordingly, it has attracted a lot of research effort [5,6]. The silicon material, mainly in the form of monocrystalline, usually acts as the workpiece material in EDM, and it is always used to fabricate microstructures for sensor applications [7,8]. Silicon powder is also used in some EDM applications as an additive to improve the surface quality (mainly surface roughness) [9,10]. However, using EDM to texture
multicrystalline silicon materials has not been reported in the literature.
2. Experimental 2.1. Device Experiments were carried out on an AGIE die-sinking EDM machine—AGIE INNOVATION II, which is equipped with a micro generator SBOX using a RC relaxation circuit to generate very narrow discharge current pulse. There is an auxiliary rotary device attached to the spindle of the EDM machine, with which the electrode can rotate at high speed. Instead of using the machine’s dielectric system, a plastic box was put on the machine saddle as the dielectric tank for the sake of reducing dielectric consumption in the experiments, and the dielectric was renewed periodically. Detail of the EDM texturing set-up inside the dielectric box is shown in Fig. 1. An aluminium frame for holding magnetic devices is connected to the spindle quill, which is fixed on the rotary device, and several magnetic components are inserted in the holes on it. When the device works, metallic (iron) powder hangs below these magnetic components and it functions like a conductive soft brush. The workpiece is fixed on a stainless steel plate and then submerged into the dielectric tank. The stainless steel plate is connected to the power supply of the EDM machine. 2.2. Materials Two kinds of substrates, multicrystalline silicon (mcSi) wafers and EFG ribbons, were tested in this investigation. The mc-Si wafers are sliced from ingot and rela-
Fig. 1.
Detail of magnetic brush.
J. Qian et al. / International Journal of Machine Tools & Manufacture 42 (2002) 1657–1664
tively flatter than the EFG ribbons. Fig. 2(a) shows a detailed view of the wafer surface. The size of the mcSi wafers is 10 cm by 10 cm and its thickness is around 300 µm. Before EDM texturing, these wafers have been doped to form P-type region and their conductivity is 1 ⍀cm, and the surface roughness is Ra ⫽ 1.1 µm, and Rt ⫽ 11.4 µm. The EFG ribbons are cut from octagonal tube and much more brittle compared to the mc-Si wafer. The size of EFG ribbon is also 10 cm by 10 cm and its thickness is around 300–400 µm. As shown in Fig. 2(b), there are small ripples on the ribbon surface. These ripples are formed in the process of crystalline growth and their maximum peak-to-valley value is about 50 µm. Normally, the sparking gap between the workpiece and tool electrode in EDM is less than 30 µm, therefore, it is quite difficult to spark both of the peak and valley with an ordinary shaped electrode at one time. With the soft brush introduced in this paper, it is possible to texture the peak and valley simultaneously.
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2.3. Method At the beginning of this research, EDM texturing with a rod electrode was first carried out to verify the possibility of this method (Fig. 3). The workpiece was vertically clamped to a metal plate and a tungsten copper electrode was used to texture the workpiece by scanning over it. The major disadvantages of this method are its low efficiency and the difficulty to texture the whole surface in one pass due to aligning and mounting problems encountered during the experiment. Furthermore, especially in the case of EFG ribbons, it is impossible to texture the grooves on the surface in the same time. Therefore, a new approach with a magnetic brush was developed and applied in this experiment. As shown on the right of Fig. 3, the magnetic brush rotates and moves horizontally to scan over the workpiece. Metallic (iron) powder with average diameter of 450 µm is kept below those magnetic components due to magnetic force (Fig. 1). In the process of texturing, the brush rotates at a low speed of about 1 Hz while it scans over the workpiece surface together with the movement of the spindle. Electrical sparks take place in between the metallic powder on the brush and the workpiece. The whole surface is roughened when the brush passes through it. The experimental procedure is as follows: 앫 machine preparation, such as machine start-up, dielectric check, etc.; 앫 fix the workpiece on the workpiece holder and submerge it into the dielectric tank; 앫 magnetic brush preparation and mounting; 앫 lower down the magnetic brush to a preset position; 앫 start EDM texturing; 앫 cleaning; 앫 reflectivity measurement;
Fig. 2. Substrates before texturing. (a) mc-Si wafer. (b) EFG ribbon.
Fig. 3. EDM texturing mode.
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Fig. 4.
Surface with a few sparking craters.
앫 solar cell processing; 앫 IV measurement/evaluation. Isotropic chemical texturization is used as a reference process in this investigation.
3. Result and analysis 3.1. Topography of EDM textured workpiece After EDM texturing, the textured surface has a typical surface roughness of Ra ⫽ 3 µm and Rt ⫽ 30 µm with a current setting of 3.2A. The microscope picture Fig. 4 shows the edge of the textured area of an EFG ribbon, on which individual spark craters can be clearly recognized. In Fig. 5, which was taken at the position that was visually recognized as fully textured area, a few un-sparked islands (white parts) are still visible under the microscope. When the
Fig. 5. Textured surface with un-sparked islands.
brush’s scanning speed increases, the size and number of theses islands increase as well. The un-sparked islands left on the sample did not bring about any serious problem in the following solar cell processing procedure, but it can be foreseen that too many un-sparked islands will reduce the effect of texturing. Those islands were diminished in the experiment when the workpiece was textured twice by the two half arcs of the brush or servo gain was set at a low value, which means longer brush travel path and machining time. So the occurrence of these islands is, to some extent, an obstacle to increasing the texturing speed. This situation may be improved by using new structure of the brush, such as parallel chains, or finer metallic powder to increase the number of small electrodes and hence the probability of spark ignition in the same area. According to the principle of EDM machining, spark occurs when the two electrodes under a certain voltage go close. In the EDM texturing method introduced here. The tool consists of a great amount of fine particles and these particles act as a cluster of small electrodes during the texturing process. The spark occurs randomly when the brush rotates and scans over the workpiece. Each spark leaves a crater on the workpiece (Fig. 4), and the final surface is the result of accumulated craters. If the brush moves forwards too quickly, sparking craters cannot cover the whole surface and there will be un-sparked islands left on the workpiece. Fig. 6 shows a typical instance of such situation. After 60 min etching, the exact shape of each crater shows up and a flat un-sparked island remains as shown in the middle of the picture. Fig. 6 also indicates that the shape of these craters left by EDM is different from what can be expected on an EDMed metallic material. The reason for this can be explained by the different material removal mechanism when EDMing silicon material and traditional metallic material. Melting and evaporating of material occurs on metallic material, therefore the spark crater is relatively
Fig. 6.
EFG ribbon surface after 60 min etching.
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smooth and continuous. In the case of silicon material, the shape of the crater becomes much more irregular, because not only melting and evaporation but also spallation of material occurs due to its physical characteristics. Big flat craters are not good for effectively capturing light and heavy sparking has proved harmful to solar cell efficiency, it is necessary to reduce the spark energy to reduce the size of spark crater. According to the energy equation (Eq.1) [8], the spark energy can be further reduced by using smaller capacitance. e ⫽ 1 / 2CU2
(1)
It is observed both on EDM textured mc-Si wafer and EFG ribbon that silicon balls appear on the textured substrates. As shown in Fig. 7, it is visible that substrate textured by spark erosion contains a lot of redeposited silicon, mostly presented in the form of silicon balls. Those balls may be amorphous and will have a bad effect on the emitter doping, so they should be removed by following etching process. 3.2. Texturing effect The reflectance of light on the textured samples was measured to evaluate the effect of this texturing method. Fig. 8(a) and (b) show the reflectance on mc-Si wafer and EFG ribbon respectively. Low reflectivity less than 20% can be obtained on both mc-Si wafer and EFG ribbon. In the area with more redeposited silicon balls, the reflectivity is lower than that in the place with less silicon balls. Therefore, low reflectivity is not equivalent to good efficiency, because with the presence of redeposited silicon balls the light absorbed by them cannot be converted to useful solar cell energy. In the case of EFG ribbon, the surface reflectivity is still below 20% even after 120 min of etching. The reason for this is
Fig. 7.
mc-Si wafer agfter EDM texturing.
Fig. 8. Reflectivity characteristic of textured substrates.
supposed to be the relatively low effectiveness of this kind of etchant to the EFG ribbon. To evaluate the effect of the EDM texturing, an untextured mc-Si wafer was etched in an isotropic etchant H1 for 3 min an EDM textured sample for 6 min. Figs. 9 and 10 show the section views of the two etched sub-
Fig. 9.
Section view of isoetched surface.
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Fig. 10.
Section view of textured surface after etching.
strates. Reflectivity measurement shows that both of them give reflectivity less than 20%. Small and deep holes remain on the textured surface after etching. Relatively even and uniform surface is available on the untextured one. Several EDM textured mc-Si wafers and EFG ribbons were further processed into solar cells for efficiency evaluation and parameter optimisation. These EDM textured mc-Si samples were first cleaned and then etched in a specially developed etchant. After processing them into solar cells, IV measurement was carried out on them. Fig. 11 shows typical efficiencies of solar cells made on the textured wafers and ribbons. The REF value is the standard reference value for efficiency comparison and it is measured on a sample of the same material. The solar cell efficiency of each material is a bit lower than its reference value. This is due to the surface topography after EDM texturing, especially the cavities with flat valley, is not the ideal for capture light for energy conversion. Compared to current efficiency of
Fig. 12.
Effect of current settings on mc-Si wafer.
solar cell made on mc-Si wafer and EFG ribbon, the efficiencies of these solar cells made on textured samples are reasonable. 3.3. Effect of texturing parameters In accordance with the principle of EDM, different surface topographies appear when different parameter settings were utilized. When a higher sparking current is chosen, the spark energy becomes larger and it melts and removes more material from both electrodes, thereby leaves bigger spark crater on them. Fig. 12 illustrates textured surface with two current settings on mc-Si wafer. The sample was first sparked with a current setting of 3.2 A and then at 6.4 A. With the latter setting, the spark craters are obviously larger than that left by sparking at 3.2 A. So the higher the current setting be, the rougher the surface become. Even after removing the textured structure by etching, obvious fine cracks induced by spark erosion remain on the textured surface (Fig. 13).
Fig. 11. Solar cell efficiency made on textured mc-Si wafer and EFG ribbon. Fig. 13.
Cracks on textured surface with high current settings.
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Fig. 14(a) gives the detail of the cross-section of a textured mc-Si wafer with a current setting of 6.4 A. The peak-valley value on the obtained structure is around 5 µm. The surface structures left by the heavy spark are not suitable for solar cell process. Although it gives a satisfactory reflectivity, such structure needs to be removed by following etching procedure, since no light captured by it can be converted to effective electricity. Fig. 14(b) illustrates that some of the worm-like craters may extend fairly deep into the silicon surface at such a current setting. Fig. 15 shows the efficiency of solar cells made on a set of wafers textured by two current settings, 3.2 and 6.4 A respectively. Generally speaking, low current setting offers better solar cell efficiency. The spark induces less surface defect, and less and smaller redeposited silicon balls will appear on the textured surface with a small current setting. Therefore, current setting, or spark energy in other words, is a crucial parameter in EDM texturing of such material.
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Fig. 15. Efficiency solar cell made on wafers textured by different current settings.
3.4. Throughput The EDM machine on which to carry out the experiment is a commercial product for normal die-sinking EDM process. The servo system and other functions are built in with the machine and the traverse rate of each axis cannot be adjusted independently by the operator. The sparking mode used in our experiment is unique and different from what is usually applied on such a machine. In our mode, the sparking takes place vertically in the gap between the brush and the workpiece, while the brush moves in the horizontal direction, in which the servo system is supposed to work (Fig. 3). So the servo mode is not strictly working in a right way. The only way to adjust the feedrate of the spindle is to tune the servo gain parameter. When the servo sensitivity is changed, the texturing speed varies. Fig. 16 shows the time the brush took to scan over and fully texture the whole workpiece. When a servo gain value, bigger than 20, was used the brush moved forwards so fast that the surface was not fully textured. The mean feed-rate of the brush can be up to 12 mm
Fig. 14. Surface topography of EDM textured mc-Si wafer.
Fig. 16.
Texturing time versus gain setting.
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per min. With the diameter of the brush being 100 mm in this experiment, the throughput of this set-up is over 1000 mm2 minute. If necessary, the throughput can be further upgraded by adapting other configuration, such as using multi-head/brush.
Energy Programme JOULE III (JOR3-CT98-0223). Special thanks go to project consortium partners, especially IMEC in Belgium, and ASE in Germany.
References 4. Conclusions Texturing of mc-Si wafers and EFG ribbons by EDM with magnetic brush was successfully carried out. Based on the experimental results introduced above, some conclusions may be drawn as follows: 앫 EDM texturing with magnetic brush is feasible for roughening mc-Si solar cell substrates. 앫 Both EDM textured mc-Si wafer and EFG ribbons can be made into solar cells of reasonable efficiency. 앫 As for mc-Si wafer, the textured workpiece needs further etching process to remove re-deposited silicon balls, and surface defects occur when the current setting is high. 앫 In the case of EFG ribbon, this method is possible to texture the whole surface with fine grooves in one path. 앫 Throughput of this method can be above 1000 mm2 per minute when a 100 mm diameter brush is utilized. 앫 Further parameter optimisation is necessary for industrial application. Acknowledgements This research has been funded in part by the European Commission in the framework of the Non Nuclear
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