By Keming Du

Fig. 1: Typical process for laser based thin film removal, illustrated at the example of a thin-film solar cell structure.

In the past years, a strongly increasing interest has developed in laser ablation processes for the removal of thin films, for instance the production of photovoltaic and display devices. The principal advantages of a laser in comparison with other processes are the high flexibility in combination with precision and quality of the processed area. In the production of thin film solar cells for example, two laser ablation processes are currently being used: scribing via selective ablation and edge isolation via deletion (Fig. 1) [1,2,3].

 

Fig. 2: Microscope image of the processed zone. Laser wavelength 1064 nm.

The laser based ablation processes used today are commonly performed by using classical commercial lasers such as solid-state lasers with a fundamental wavelength around 1 µm and with Gaussian beam profiles in combination with beam scanners and scanner optics. However, for ablation processes a rotational symmetric Gaussian intensity profile is not the best solution: the processing of large areas requires an appropriate side by side application of the individual pulses, preferably without overlap which is not possible with Gaussian beams. And the process itself has a threshold character, which causes energy losses for intensity profiles such as a Gaussian, as the parts of the Gaussian beam with intensities below threshold do not contribute to the process. An inhomogeneously processed zone such as illustrated in Fig. 2 is the result. A beam of rectangular shape and top-hat intensity profile however would perfectly match the process requirements of such an ablation process.

Fig. 3: Increased quality and processing speed by using a two stage process with different laser wavelengths (532 nm, green (top) and 355 nm, UV (bottom)).

In addition to this problem with the beam shape, it is not preferable to process with one single laser wavelength as the absorption of the different materials layers in Fig. 1 cannot be matched optimally to one single wavelength. If two wavelengths are used and each wavelength is adapted to the absorption maximum of the individual layer (Fig. 3), the efficiency of the process and thus the processing speed can be considerably increased.

Fig. 4: Schematic set-up (top) and commercial example (bottom) for a high-power INNOSLAB-laser.

In order to activate all these prospective advantages, it is necessary to use a laser, which inherently has properties, being advantageous for the creation of a rectangular beam with top-hat profile and with which a laser beam of different wavelengths can be produced efficiently. The laser concept shown in Fig. 4 has all of these advantages. The active laser medium is slab-shaped and therefore perfectly matched to produce a beam with rectangular cross section and by proper adaption of the laser resonator the cross-section of the beam can be made top-hat shaped as illustrated in Fig. 5. With such a beam shape, it is easy to create exactly and homogeneously the laser intensity required by the process and the beams of the individual laser pulses can perfectly be arranged side by side, assuring high process efficiency.

Fig. 5: Top-hat intensity profile of a specially adapted Innoslab-resonator, the beam has near-diffraction limited beam quality.

The second advantage of the laser concept becomes apparent when multi-wavelength operation is required. Commercial versions of the laser [4] are supplied with resonator-external nonlinear frequency conversion units (frequency doubling and sum-frequency mixing units), which allow the selection of different wavelengths without any mechanical modifications or adjustments. Here again, the big advantage of this type of laser is the rectangular beam profile in combination with high beam quality and short pulse length. With the high beam quality and short pulse length very high laser intensity can be produced at the position of the crystals for nonlinear frequency conversion. This ensures a high efficiency of the frequency conversion process. On the other hand the laser beam in the frequency conversion crystals has a rectangular shape with one dimension, which can be chosen very small, thus ensuring a good cooling of the crystals and thus a high damage threshold, needed for reliable laser operation.

Fig. 6: Laser ablation of thin films according to Fig. 1, with a dual-wavelength (532 nm + 355 nm) slab laser.

Fig. 6 shows the ablation result achieved with this laser: a very clean processed area can be seen and the two-wavelength process requires only about 10% of the laser energy of a process solely performed with the fundamental infrared wavelength.

References:

  1. S. Engelhart, S. Hermann, T. Neubert, R. Grischke, N.-P. Harder, R. Brendel, “Laser processing for high efficiency solar cells,” Proceedings of 17th NREL Workshop, 1 903 (2007).
  2. S. Eidelloth, T. Neubert, T. Brendemühl, S. Hermann, P. Giesel, and R. Brendel, “High speed laser structuring of crystalline silicon solar cells,” 34th IEEE Photovoltaic Specialists Conference (PVSC), Philadelphia, June 7-12, 2009.
  3. S. Haas, S. Ku, G. Schöpe, K. M. Du, U. Rau, H. Stiebig, “Patterning of thin-film silicon modules using lasers with tailored beam shapes and different wavelengths,” Proceedings of the 23rd European Photovoltaic Solar Energy Conference, Valencia, Spain, September 2008.
  4. www.edge-wave.com