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Laser patterning of transparent electrode films: from solar panels through high-end displays
Picosecond lasers deliver better results than typical scribes By Ralf Hellmann and Colin Moorhouse The availability of an expanded range of laser pulsewidths and wavelengths enables patterning of transparent conductive oxides to be optimized for applications as disparate as solar cells and OLED displays. Introduction Thin films of transparent conductive oxides (TCOs) are used as electrodes in applications where light must pass through the electrode, such as on the front side of certain solar cell types as well as in flat panel displays. Often, these conductive films must be scribed through to create a pattern of electrically isolated areas. Until now, the Q-switched nanosecond laser has dominated these applications, but the advent of newer displays, particularly those based on OLED technology, is creating a demand for even higher quality scribes. The authors have investigated the use of a picosecond laser for this purpose. This article presents the results of this testing which shows that picosecond lasers can deliver results which are superior to typical scribes created by ultraviolet nanosecond lasers. Scribing: The need for patterned electrodes Thin films of materials such as indium tin oxide (ITO), gallium zinc oxide (GZO) and aluminum zinc oxide (AZO), demonstrate good conductivity, yet are essentially transparent at visible wavelengths. This makes TCOs very useful for devices which require electrodes placed on a front surface through which light must pass. Typical examples are thin film solar panels and touch screen displays. TCOs are typically formed as continuous films which must then be subsequently scribed through in order to form the required pattern of electrically isolated electrodes and interconnects. Originally, this was accomplished by photoetching using wet chemicals. However, laser scribing offers several advantages. It is a simpler, single step process that employs no wet chemicals, and which delivers high process flexibility. These advantages of the greener laser process have led to its adoption in most applications. Nanosecond lasers for solar and touchscreens Currently, most high volume TCO scribing applications rely on nanosecond, Q-switched, diode-pumped, solid-state (DPSS) lasers such as the Coherent AVIA and Matrix series. These lasers are available with a choice of infrared (1.064 µm), green (532 nm) or ultraviolet (355/266 nm) outputs. Despite TCO's transparency at visible wavelengths with tight focus and high laser fluence, visible, Q-switched lasers can scribe these films with similar results as infrared lasers with sufficient quality for current, commercially available flat panel displays. The highest quality scribes are created using ultraviolet lasers – see FIGURE 1. But today, most TCO scribing is actually performed using either infrared or green DPSS lasers as they both deliver the requisite scribe quality without incurring the higher cost of ultraviolet lasers and their more expensive beam delivery optics. The exact influence of laser wavelength on scribe quality depends on which TCO is being used, and for some, the increased quality available from ultraviolet lasers justifies their use. With regards to process geometry, some applications scribe from the TCO side of the panel whereas others reach the material by passing through the substrate side. The latter enables TCO scribing even after another layer has been deposited on top of it. But with this back-side processing configuration, care has to be taken to avoid damaging the substrate, since this may be exposed to high laser fluences. Picosecond lasers for OLED screens Some emerging, high volume applications, most notably OLED displays, are now putting higher demands on scribe quality beyond just electrical isolation, which in turn requires a more detailed examination of scribe geometry and hence quality. FIGURE 2 schematically illustrates the difference between the cross sections of an ideal scribe and a real world laser scribe. The ideal scribe is characterized by complete removal of the TCO layer for electrical isolation, a clean top surface with no shoulders or recast material, a flat bottom with no residual material and no damage sustained by the substrate. In addition, grooves with upright (vertical) sides are preferred as they deliver the highest electrical isolation for the narrowest overall scribe width. In contrast, real scribes created by nanosecond lasers may have shoulders, a round bottom and some recast debris. Even where these scribes exhibit complete electrical isolation, any high shoulders or ridges are unacceptable for emerging OLED applications, in part because of the dimensions of the top layer that is deposited over the TCO. Specifically, the organic active layer on top of the TCO in an OLED is often only 100 nm in total thickness. So TCO ridges approaching 100 nm in height create the risk of unacceptable shorts through this top layer. This situation is thus driving the need for an alternative laser scribing method that can produce more nearly perfect scribe characteristics. To this end, the authors research group at the University of Aschaffenburg has conducted extensive testing using a novel picosecond laser source; the Coherent Talisker ultrafast laser. This is a one-box, hybrid laser that combines a diode-pumped fiber laser oscillator with a diode-pumped free-space amplifier. This compact configuration provides the low cost and operational simplicity of a mode-locked fiber laser, while delivering the higher power/pulse energy and superior beam quality that can be uniquely generated by a free-space regenerative amplifier. Picosecond lasers can be expected to provide superior results as most of the undesirable groove imperfections are due to thermal effects. Even with ultraviolet lasers, the material being removed becomes hot before it is ejected. This is because the laser pulse primarily excites electrons in the TCO material. This electronic excitation is then converted to lattice vibration, i.e. lattice heating. With nanosecond and longer pulses, there is sufficient time for some of this heat to flow out of the localized laser interaction zone and cause peripheral thermal effects – the heat affected zone (HAZ). But with laser focal spots on the micron scale, the time for this heat flow is on the order of 10 picoseconds or more. So with laser pulse widths of 10 ps or less, the laser-driven material removal is completed before significant thermal energy out-flow can occur. reference: http://www.industrial-lasers.com/ Picosecond lasers;solar cell; touchescreen
 
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