We report selective patterning process, laser ‘rail-roading’ scribing method, of which operating principle is based on transient force balance between the material properties including cohesion and adhesion forces subjected to underlying substrate and laser-induced shock compression and shear forces. By using dual fs-laser beam lines with an interval larger than laser spot size, we provide a proof of the concept by patterning the photovoltaic modules based on CIGS (Cu(In,Ga)Se2) coated on Mo electrode. With varying the interval between the two laser beam tracks, we can provide intact Mo back contact surface without any residues in a manner of more facile, high-speed and high scribing efficiency. We have interpreted the effect of the ambient gases and grooving width on the scribing performance in terms of the cohesion forces between the grains of CIGS thin films as well as adhesion force between underlying Mo layer and CIGS, which are mainly governed by local laser ablation and peening process followed by laser-induced shock compression, respectively.
© 2011 OSA
Selective patterning for functional thin films including metal and transparent conductive electrode without damage on underlying layers plays an important role in processes for modern devices. Various techniques have been introduced to provide desired patterns. For example, photolithographic technique can be applied to result in high-resolution pattern. Also, mechanical method utilizing solid needles can provide tools of rather high-speed processing for low-cost mass production. The limitations arising from the use of photolithography, however, are numerous, owing to high-cost facilities and a number of process steps and low speed . For mechanical tip method, it is also difficult to overcome the chipping and delaminating at the edge as well as limitations in decreasing the pattern width . More recently [3–8], laser direct patterning are actively investigated to find a solution for high performance process with high speed, yet this method is not completely free from the residuals inside patterns like stitch lines, which is a visible part observed in and/or between two-successive laser spot on the processed surface, and the deterioration of electrical properties like contact resistance as well as shunt resistance. Successful results were already achieved for patterning with a local ablation of transparent conductive thin-film with ultrafast lasers by providing minimized heat affected zone around the scribing edges [9,10].
Efficiency loss of thin-film solar cells due to imperfect patterning is one of the major challenges facing photovoltaic (PV) cell manufacturing . In laboratory scaling for PV, an improvement of conversion efficiencies in thin-film solar cells and modules resulted in a 19.9% efficient device of ZnO/CdS/Cu(In,Ga)Se2 (from now on CIGS) with photolithographic patterning and e-beam evaporated Ni/Al grid in 2008 . In 2010, researchers from the Germany Center for Solar Energy and Hydrogen Research (ZSW) have reported a new efficiency record of 20.3% with 0.5 cm2 thin-film CIGS PV cells. This new record is only a fraction less than the best multi-crystalline cells. Meanwhile, CIGS cells typically reach only about 10 ~14% when manufactured in mass-production. Due to low processing speed, photolithographic isolation method has a limitation in industrial application. To overcome this rather large discrepancy in the efficiency between laboratory and mass production is to achieve reliable and reproducible PV cell patterning, which must have low series resistance and high shunt resistance, and a minimum of dead area between cells [13,14].
In this work, we report femtosecond laser scribing methods with dual beam exposing, so called, laser ‘rail-roading’ scribe technology, which might be applicable in back-to-front via opening and front contact isolation steps of thin film PV modules. This method was found to provide an intact Mo surface by selectively removing CIGS films inside laser beam tracks with femtosecond laser. We have further investigated the effect of processing condition including ambient gases and grooving width on the processing performance. We have interpreted the observations in terms of cohesion forces among the grains of CIGS as well as adhesion force between underlying Mo and CIGS layer. The former is mainly affected by the extent of ablation depth and the latter by local laser peening on the exposed area followed by laser-induced shock pressures.
Multi-layered structure of CIGS/Mo/Glass substrate was fabricated with a sputtering of molybdenum on glass substrate and the deposition of CIGS by coevaporation in an in-line evaporation chamber. Schematic diagram for the laser rail-roading scribing are shown in Fig. 1 . The regenerative amplified femtosecond laser system (Light Conversion, PHAROS) produces 30 μJ/pulse at the wavelength of 1030 nm with a repetition rate of 200 kHz. The repetition rate can be changed by pulse picker combined with a digital pulse divider. The laser beam is divided by 50:50 beam splitter. The two laser beams are focused on the CIGS film surface by using an objective lens (x 5, N.A. = 0.15). The laser polarization is set to be parallel to the processing direction by a variable wave plate (Newfocus). The scribing area was blown with 4.5 atm of gas through a nozzle with about 50 μm in inner diameter at right angle to the processing direction. The sample is mounted on XY translation stage with a maximum speed of 1000 mm/sec. The surface of processed CIGS/Mo substrate was characterized by scanning electron microscopy (SEM) and optical microscopy.
3. Results and discussions
Figure 2 shows SEM image of 2.2 μm thick CIGS thin film on Mo-coated glass patterned by fs-laser ‘rail-roading’ scribe with a stage speed and pulse repetition rate of 150 mm/sec and 33.3 kHz, respectively. The number of laser pulses exposing the same area is about 3. The scribing area was blown with 4.5 atm of N2 gas. With a laser fluence of 10.6 J/cm2 (approximately 17 times the damage threshold of CIGS thin films), the ablation depth is ca. 1.2 μm, which is much lower than CIGS film thickness of 2.2 μm. For the interval between the two laser lines larger than 110 μm, no removal of the thin films between the scribing lines is observed. (Fig. 2(A)) With decreasing the interval to about 80 μm, appreciable portion of completely scribed area is observed as shown in Fig. 2(B). The interval of 50 μm results in complete remove of the CIGS films inside two laser scribing lines to provide clean Mo surface (see Fig. 2(C)). When patterning the CIGS films with one laser line followed to additional scribing with the same intervals, however, no any remove of CIGS films inside the beam tracks can be observed as shown in Fig. 2(D). It should be noted that there is no direct optical interactions between two laser beams since the beam spot size in diameter is about 12 μm.
We have further studied the effect of ambient conditions on fs-laser ‘rail-roading’ scribe of CIGS films coated on Mo electrode (see Table 1 ). Without blowing gas, there is no desired patterning of CIGS film even though the interval between the two laser beams is 30 μm. With increasing the back pressure of blowing gas, the probability for the removal of CIGS films inside two laser beam lines increases. The ablation depth of CIGS was found to be invariant on the back pressure. For removal of CIGS films, the nitrogen gas is more efficient than Argon gas as shown in Table 1. If the role of gas blowing in the current scribing is just mechanical pressure source, then the CIGS film removal efficiency should be invariant with changing the gas species from N2 to Ar since the difference of the force exerted by the two different gases even at the nozzle exit is negligible in case of isentropic expansion of the gas from sonic nozzle. However, the performance is dependent on the species of blowing gases with keeping the back-pressure of the gas constant. Furthermore, 4.5 atm N2 gas blowing on the previous patterned area has no effect. This suggests that the gas blow during the laser scribing should act as a cooling tool. Even though no any conclusive idea to explain the effect of gas blowing on the current laser scribing is available from the current work, high-pressure gas blowing should play an important role as a cooling the thin films as well as a mechanical pressure for film detachment.
In the formation of a trench with a width larger than beam spot size, the mechanical strength of the films and its adhering force to the substrate should be important. For example, when the film has finite-sized grains, the resulting mechanical strength will be weaker at the grain boundary than that within the grain. We have examined the microstructure of CIGS bottom surface at CIGS/Mo interface after mechanical exfoliation by using carbon tape. Figure 3(A) and 3(B) exhibit the SEM images of CIGS bottom surface before and after laser exposing, respectively. Intact CIGS surface at the interface exhibits two different grain features with a mean size of several nanometers as well as hundreds of nanometers, respectively, in addition to a number of pores with a diameter of about 100 nm. The former grain should be responsible for the contact of CIGS films to rather flat Mo surface while the latter is for the polycrystalline CIGS. So, it is reasonable to suppose that the laser induced external force, if applied, may easily shear a block of material along the grain boundaries. To test this supposition, we have studied single-line ablation of CIGS films with varying the laser fluence. As shown in Fig. 4(A) , laser exposing to CIGS surface with a fluence of 7.1 J/cm2 result in only the scribing of CIGS thin films. However, with increasing the fluence to 14.1 J/cm2 apparent crack of the films was formed near the scribing lines on the thin films. (Fig. 4(B) As shown in Fig. 4(C), CIGS films randomly peeled off from Mo surface near the scribing line if the laser beam with rather high fluence of 21.2 J/cm2 (approximately 35 times the damage threshold) was used. It should be noted that both the cracks and chipping is happened in both side along the laser scribing line. This result suggest that the force applied by fs-laser laser with a current range of laser fluence can provide an appreciable force to affect the nature of bonding of CIGS films to Mo surface near the scribing line as well as to crack the CIGS films. To interpret this result, we have supposed that the grains of polycrystalline CIGS films bonded to one another by a cohesive forces, Fc,CIGS, and the grains in the bottom are attached to the substrate by an adhesive forces, Fa,CIGS/Mo. Even if Fc,CIGS and Fa,CIGS/Mo should be strongly dependent on the local position due to the inhomogeneity of the films, we can suggest that the force near the scribing line applied by laser exposure with a laser fluence of 14.1 J/cm2 should be comparable to Fc,CIGS. Meanwhile, the adhesion force, Fa,CIGS/Mo, should be higher than Fc,CIGS since chipping of the films happened under higher laser fluence of 21.2 J/cm2.
The phenomena of the mechanical forces to provide material changes after ultrafast laser exposing can be explained by a two-stage process including an instantaneous uniaxial elastic compression, followed by a slower relaxation to a plastically deformed state exhibiting a hydrostatic pressure. The former should provide a laser peening to increase the bonding strength of the films to back surface just below the laser exposing area, while the latter might result in the shear forces on the films near the scribing lines. The laser peening of the films to back substrate in addition to a phase transformation of the materials resulted from the laser induced shock followed by an ablation of the materials . Normally, one can produce shock waves yielding peak pressures of about several GPa when ultrafast laser pulses with a fluence of ~J/cm2 was applied to ablate absorbing layers . An evidence for the increment of adhesion force is provided by examining the bottom surface of CIGS films after laser exposing (Fig. 3(B)). The surface exhibits an appreciable decrease in the number of pores as well as grains followed by laser exposing, which eventually increase the bonding strength of the films to underlying substrate.
To see how different states of the films happened under the current fs-laser ‘rail-roading’ scribing method, a model has been proposed (see Fig. 5(A) ). For intact CIGS/Mo interface, one can suppose that Fa,CIGS/Mo should be higher than Fc,CIGS since the chipping process happened at the laser fluence higher than the formation of film crack (see the results observed from single laser beam scribing with varying the laser fluence shown in Fig. 4). Meanwhile, Fc,CIGS (x = -X0 and X0) should decrease since the effective thickness of CIGS is lowered due to the laser ablation if the two laser beams are applied to the CIGS/Mo structure at the position of -X0 and + X0. However, Fa,CIGS/Mo (x = -X0 and X0) increases due to the laser peening, which should enhance the adhering forces of CIGS films to Mo electrode followed by laser exposing. We tentatively assume that the changes of Fc,CIGS and Fa,CIGS/Mo induced by laser exposure follows a Gaussian profiles with a width of laser spot size of 12 μm. We also assume that the external shear force, Fshock, induced by laser exposure on CIGS films follows Gaussian profile with width of 80 μm, which is the interval between the scribing lines to provide partial detachment of CIGS films. Since the laser fluence used in the current ‘rail-roading’ scribing is 10.6 J/cm2, the maximum external force applied by single laser beam exposure, Fshock should be lower than both Fc,CIGS and Fa,CIGS/Mo for intact CIGS films .
Since the two laser beam is not overlapped, the above Fc,CIGS and Fa,CIGS/Mo should be invariant with respect to the intervals between the laser beams as shown in Figs. 5(B)–5(D). However, an external force of Fshock caused by the two laser beams should be additive if the plane stress conditions is applied in the current models. Actually, the CIGS layer inside the two laser beam spots have experienced higher effective stress since a part of the instantaneous plain compressive waves generated by the two laser spots reflects on CIGS and Mo interface and eventually act as a shear stress. If we ignore any possibility of an interference between the waves generated by the two laser beams, a necessary conditions for the confined detachment of CIGS films inside the beam tracks is that the sum of Fshock1 and Fshock2 generated by the two beams be larger than Fc,CIGS and Fa,CIGS/Mo. The force exerted by the two laser lines should be high enough to overcome the cohesive and adhesive forces and make a local detachment of the materials (Fig. 5(B)). And also a partial detachment of the films from the substrate could be also observed if the external forces are comparable to the adhesion and cohesion forces (Fig. 5(C)). When Fc,CIGS > Fa,CIGS/Mo> Fshock, however, there is no detachment of CIGS from Mo (Fig. 5(D)).
The proposed model can be used to explain the experimental observations from this work. The detachment is not successful when the two laser beams did not scanned simultaneously even if the intervals and other scribing conditions including laser fluence, laser repetition rate, stage speed, and blowing conditions is set to fully scribing the CIGS films. Even if the external force caused by single-line scribing is not larger than Fc,CIGS and Fa,CIGS/Mo, however, the sum of the two external force provided by dual-line scribing should be high enough to remove the CIGS layers inside the beam tracks. This interpretation can be further utilized to explain why the scribed area is confined under current laser scribing conditions without any chipping outside the laser beam tracks. In other words, the external force inside the dual beam tracks is higher than Fc,CIGS and Fa,CIGS/Mo, but the external force is not enough to alter the nature of the films outside the tracks. The effect of ambient conditions like blowing gas species on the rail-roading scribe of CIGS can be also understood in terms of accumulated thermal effect by a successive laser pulses. Since Ar gas blowing (even no blowing) should be less effective to cooling down the substrate temperature compared to nitrogen gas due to the difference in their heat capacities, the mechanical properties of CIGS films during the processing should be different from each other. Because so, the detailed force balance applicable to the case of Ar gas blowing should be different from that of nitrogen gas. Since there is no any conclusive idea to explain the exact role of blowing gas as either mechanical pressure source or cooling agent, no clear statement can be made. It should be necessary to investigate the effect of the ambient gases on the current laser scribing method in more detailed.
Finally, it should be noted that the current method is energy-effective way. A width of the trench formed by current laser ‘rail-roading’ method exceeds the laser beam spot size. In other words, we have used only the laser energy to ablate very much small portion of the total scribing area. The ablated cross-sectional area estimated with ablated depth and width from the data shown in Fig. 2(D) is only 21.6 μm2, which is much lower than the total ablated cross-sectional area of 110 μm2 in Fig. 2(C). This suggests that we can improve scribing efficiency more than 5 times by applying laser ‘rail-roading’ method with keeping the laser processing quality. This might be resulted from the fact that a part of laser energy responsible for the laser-induced shock force, which is usually dissipated into environment and even cause detrimental effect like generation of cracks and chipping near the scribing area in single line scribing method, was further utilized to detach CIGS thin films from the MO back contact surface. This should be much interesting from technical point of view. The laser powers of ultrafast pulsed system were not sufficient to meet industrial efficiency requirement due to its low ablation rate compared to that of pulsed nanosecond laser. And also, it is necessary to keep the laser powers close to the ablation threshold in order to maintain high-quality of processing with less thermal and mechanical damage and non-melting scribing. So, it is very important to design an energy-effective processing technique for high efficiency and minimized damages as well as no melting of material during the scribing. Laser ‘rail-roading’ method proposed from this work, we think, should be one of the applications to resolve the conflict between laser processing quality and efficiency in ultrafast laser microstructuring to fulfill the industrial requirement.
In summary, we have designed more facile, high-speed, and energy-effective patterning of CIGS thin films coated on Mo electrode by applying fs-laser ‘rail-roading’ scribe method. By utilizing the combined effect of the direct ablation of CIGS film, its local peening into Mo surface, and laser induced shear force, we can control the force balance between Fc,CIGS, Fa,CIGS/Mo and Fshock to provide a clean pattern for back-to-front via opening step of the photovoltaic modules based on CIGS thin films coated on Mo electrode. It should be also noted that the current method is energy-effective process since we used only the laser energy to ablate very much small portion of the total scribing area and the energy dissipated into mechanical forces was further utilized for the detachment of the thin films from the Mo back contact surface.
This work was financially supported by KRISS program (KRISS-11-10071006).
References and links
1. F. Kessler, D. Herrmann, and M. Powalla, “Approaches to flexible CIGS thin-film solar cells,” Thin Solid Films 491, 480–481 (2005).
2. P. O. Westin, U. Zimmermann, and M. Edoff, “Laser patterning of P2 interconnect via in thin-film CIGS PV modules,” Sol. Energy Mater. Sol. Cells 92(10), 1230–1235 (2008). [CrossRef]
3. P. M. Harrison, N. Hay, and D. P. Hand, “A study of stitch line formation during high speed laser patterning of thin film indium tin oxide transparent electrodes,” Appl. Surf. Sci. 256(23), 7276–7284 (2010). [CrossRef]
4. H. Yoo, H. Shin, B. Sim, S. Kim, and M. Lee, “Parallelized laser-direct patterning of nanocrystalline metal thin films by use of a pulsed laser-induced thermo-elastic force,” Nanotechnology 20(24), 245301 (2009). [CrossRef] [PubMed]
5. P.-O. Westin, U. Zimmermann, M. Ruth, and M. Edoff, “Next generation interconnective laser patterning of CIGS thin film modules,” Sol. Energy Mater. Sol. Cells 95(4), 1062–1068 (2011). [CrossRef]
6. P. Gečys, G. Račiukaitis, E. Miltenis, A. Braun, and S. Ragnow, “Scribing of thin-film solar cells with picosecond laser pulses,” Phys. Proc. 12, 141–148 (2011). [CrossRef]
7. Y. Hernandez, A. Bertrand, S. Selleri, F. Salin, L. Leick, M. Hueske, R. Petkovsek, F. Ferrario, and N. Lichtenstein, “Recent progress on the ALPINE (advanced lasers for photovoltaic industrial processing enhancement) FP7 integrated project,” in Fiber Laser Applications OSA Technical Digest FThB1 (2011).
8. N. G. Dhere, “Scale-up issues of CIGS thin film PV modules,” Sol. Energy Mater. Sol. Cells 95(1), 277–280 (2011). [CrossRef]
9. M. Park, B. H. Chon, H. S. Kim, S. C. Jeoung, D. Kim, J.-I. Lee, H. Y. Chu, and H. R. Kim, “Ultrafast laser ablation of indium tin oxide thin films for organic light-emitting diode application,” Opt. Lasers Eng. 44(2), 138–146 (2006). [CrossRef]
10. R. R. Gattass and E. Mazur, “Femtosecond laser micromachining in transparent material,” Nat. Photonics 2(4), 219–225 (2008). [CrossRef]
12. I. Repins, M. A. Contreras, B. Egaas, C. DeHart, J. Scharf, C. L. Perkins, B. To, and R. Noufi, “19.9%-efficient ZnO/CdS/CuInGaSe2 solar cell with 81.2% fill factor,” Prog. Photovolt. Res. Appl. 16(3), 235–239 (2008). [CrossRef]
13. M. D. Abbott, T. Trupke, H. P. Hartmann, R. Gupta, and O. Breitenstein, “Laser isolation of shunted regions in industrial solar cells,” Prog. Photovolt. Res. Appl. 15(7), 613–620 (2007). [CrossRef]
14. L. Zhang, H. Shen, Z. Yang, and J. Jin, “Shunt removal and patching for crystalline silicon solar cells using infrared imaging and laser cutting,” Prog. Photovolt. Res. Appl. 18(1), 54–60 (2010). [CrossRef]
15. B. Wu, S. Tao, and S. Lei, “Numerical modeling of laser shock peening with femtosecond laser pulses and comparisons to experiments,” Appl. Surf. Sci. 256(13), 4376–4382 (2010). [CrossRef]
16. J. S. Wittenberg, M. G. Merkle, and A. P. Alivisatos, “Wurtzite to rocksalt phase transformation of cadmium selenide nanocrystals via laser-induced shock waves: transition from single to multiple nucleation,” Phys. Rev. Lett. 103(12), 125701 (2009). [CrossRef] [PubMed]
17. The shock provided by successive pulse trains in addition to that by individual laser pulse exposure should contribute to the overall external forces if the pulse repetition rate is fast than the relaxation rate of shock pressure. And also, we cannot completely rule out the possibility of the contribution of high-pressure gas blowing to the material removal accompanied with laser-induced shock propagation along CIGS/Mo interface. However, we tentatively neglect the contribution of gas blowing since it is so complicated to analyze the external force applied both by shock pressure from high-repetition rate laser pulses and by gas pressure in quantitatively with current experimental observation.