POWERMANAGEMENT
manufacturing. The magnetrons typically employ DC power supply technology for the conductive material deposition. The targets, consisting of either pure metals or ceramic compounds, are used to deposit the layers that form the front contact and back contact layers of a thin-film solar cell. The metal layers make up the back contact electrical connection, so that light is reflected and resistance is minimized.
P
The ceramic materials are transparent conductive oxides (TCOs) used for the front contact electrical connection so that light is transmitted and resistance is relatively low.
In any DC PVD process, charge build up can form around inclusions or oxides on the target surface, which may result in plasma instability. An arc is simply a charge build up on a small area of the target with sufficient potential to cause an electrical breakdown. These low-voltage, high-current breakdowns eject fragments of the target material, which can manifest as defects on the substrate, leading to particle formation, pinholes, rate reduction, short circuits, or other undesirable process disturbances and loss of cell performance.
Due to the reactive nature of the process, arcing is a common occurrence in thin-film solar panel deposition of TCOs. For example, aluminum-doped zinc oxide (AZO)—a transparent conductor in many solar cell applications—is known to arc hundreds or even thousands of times per second. It’s important nevertheless to maintain a high level of process control when depositing quality films to ensure deposition rate and film quality are preserved. High arc rates inhibit the creation of defect-free films and may prevent consistent deposition rate. Effective arc management is therefore a central topic of interest for solar cell process engineers performing thin-film deposition.
Power supplies must effectively manage arc events to minimize process disturbances and eliminate persistent arcs that damage the target or substrate. To extinguish an arc, the power supply needs to shut down or remove power from the output as fast as possible to limit the energy provided to the
Fig 2. A power supply that stores high energy will contribute to damage on the target or substrate surface. Next-generation DC power supplies store minimal energy to help prevent such damage
arc. Traditionally, power supplies used only voltage to detect an arc event, and the arc events were categorized as micro arcs or hard arcs. Previous- generation power supplies typically respond to an arc with this two-step approach.
The initial response (micro arc) is very short, typically tens of micro-seconds. After the micro arc shut-down time, power is re-applied to the magnetron. If the arc persists, this is treated as a hard arc and the power supply shuts off for a much longer duration. The response to either step is as fast as possible, the only difference being the length of shut-down time. The energy delivered to the arc is dictated by the energy stored in the
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VD (sputtering) is a common deposition method for thin-film solar cell
Fig 1 Diagram of a PVD system
www.solar-pv-management.com Issue VI 2010
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