ANU conducts state-of-the-art research in the field of laser processing for photovoltaics. Key fundamental areas of laser research include both n-type and p-type laser doping of silicon, selective ablation of dielectric materials and micromachining of silicon surfaces and wafers. With access to several laser sources and beam delivery options, the laser processing research team are developing laser processes that can be integrated into industrially feasible solar cell fabrication sequences.
The general goal of laser processing techniques is to replace the more traditional complex and expensive manufacturing options (such as lithography and furnace diffusions) with laser processes that offer greater flexibility, thus allowing more complex solar cell designs to be implemented industrially. For example, laser doping and selective dielectric ablation could be readily implemented in selective emitter cells, rear passivated and locally contacted cells, and rear junction point contact cells amongst others.
Characterisation of laser processed samples is a critical part of the research focus, being required in particular in order to optimise laser parameters and ensure desired characteristics (eg deep, homogenous doping with easily contacted surfaces) with minimal induced silicon damage can be achieved. A range of characterisation methods are utilised, including sheet and contact resistivity analysis, SIMS and ECV for dopant profiling, SEM for surface and dopant imaging, photoconductance lifetime analysis and photoluminesence imaging (PL) for extraction of surface quality parameters, along with and micro-PL techniques.
ANU Researcher demonstrating laser equipment to the Prime Minister.
Laser Laboratory Capabilities
530nm laser beam contained within 50µm diameter acid jet in Laser Chemical Processing (LCP) system.
- JPSA precision micromachining XYZ-? stage, dry optics beam delivery
- Laser Chemical Processing (LCP) System, precision XY stage, 30 – 150um diameter & 50 – 400 bar pressure liquid micro-jet coupled beam delivery
- DPSS 532nm Q-switched 20W laser 10 – 40nS pulses
- Excimer KrF 248nm laser, 400mJ / pulse, 20nS pulses
- Pyroflex 1064 / 532 nm programmable pulse width laser, 2 – 500nS pulse
- 530 nm pulsed fiber laser
- IR / Green advanced beam-shaping optics
Laser doping using 532nm DPSS laser and ‘dry’ optics.
Melted and recrystallised laser doped line.
Precisely placed localised doping of silicon can be easily and rapidly achieved without any need for masking films, photolithography or high temperature diffusion processes. By scanning a laser beam across the silicon surface, a thin dielectric layer can be removed simultaneously while a local surface layer of silicon (to a typical depth of 1 to 2 micron) is melted and recrystallised. Dopants, for example boron, phosphorus or aluminium, can be easily introduced from the wafer surface and into the melt to form a doped region in the laser affected area.
Doping profiles that are achieved by laser doping are comparable to those obtained from traditional high temperature diffusions. The optimal set of laser parameters for laser doping will yield regions of deep, homogenous doping with minimal damage, low surface recombination and suitability for making low resistance ohmic contacts.
3-dimensional depth profile produced using a high-resolution optical profilometer.
2-D cross section, produced by excimer 248nm laser ablation of PECVD silicon nitride.
Selective removal of dielectric materials from silicon for opening up regions for local contacts or local doping windows is achieved simply and elegantly by laser ablation. The ideal ablation process is a mimic of photolithography but is by contrast a highly industrially applicable process.
Green, IR or UV lasers can all be used to remove materials with minimal or no damage to the underlying silicon. However, for many of the materials of interest to solar cell manufacturing, UV laser ablation seems the most promising option – offering the ability to remove thin layers of different dielectrics with each laser pulse while resulting in minimal change to surface electronic quality. Dielectric materials currently under investigation include silicon dioxide, silicon nitride, aluminium oxide, and titanium dioxide.
Investigations into the application of lasers for micromachining of silicon are also being carried out, including the drilling and scribing of deep vias and trenches and the initiation of crystal plane selective silicon etching. Rapid creation of deep features with minimal micro-cracking or laser induced damage is relevant to various complex solar cell designs, and also allows for the utilisation of various test structures that can be used for characterision of laser and other solar cell fabrication processes.