Robert Gordon Ridgeway

Life-Cycle Nitrogen Trifluoride Emissions from Photovoltaics

Amorphous- and nanocrystalline-silicon thin-film photovoltaic modules are made in high-throughput manufacturing lines that necessitate quickly cleaning the reactor. Using NF₃, a potent greenhouse gas, as the cleaning agent triggered concerns as recent reports reveal that the atmospheric concentrations of this gas have increased significantly. We quantified the life-cycle emissions of NF₃ in photovoltaic (PV) manufacturing, on the basis of actual measurements at the facilities of a major producer of NF₃ and of a manufacturer of PV end-use equipment. From these, we defined the best practices and technologies that are the most likely to keep worldwide atmospheric concentrations of NF₃ at very low radiative forcing levels. For the average U.S. insolation and electricity-grid conditions, the greenhouse gas (GHG) emissions from manufacturing and using NF₃ in current PV a-Si and tandem a-Si/nc-Si facilities add 2 and 7 g CO₂(eq)/kWh, which can be displaced within the first 1-4 months of the PV system life.

Power coupling and utilization efficiencies of silicon-depositing plasmas in mixtures of H2, SiH4, Si2H6, and Si3H8

Adding Si2H6 or Si3H8 additives to SiH4/H2 discharges increases the growth rates for thin films of microcrystalline and amorphous silicon, but the reasons for this increase are not well understood. To better distinguish the chemical and physical from electrical effects of these additives, a comprehensive electrical study was performed for mixtures of H2, SiH4, Si2H6, and Si3H8. The power coupling efficiency, power utilization efficiency, voltage, current, impedance, and phase were measured as a function of total pressure, electrode gap, gas mixture, rf power, and time. The measurements identified a regime of pressure and gap in which the electrical behavior is optimized. In this regime, the power coupling efficiency is quite high and insensitive to gas mixture, and the power utilization efficiency also does not vary dramatically with mixture. Therefore, in this regime, chemical or physical effects of additives on growth rates predominate over electrical effects. Impedance models of the plasma and sheaths provide explanations for the optimized regime and its correlation with impedance phase. In addition, electrical signals were identified that can be used to detect a transient in the gas-phase density of silicon-containing molecules during deposition as well as other transient phenomena. The signals show promise for use in process monitoring and control.

Reduction of PFC emissions to the environment through advances in CVD and etch processes

One the most focused environmental health and safety (EHS) goals for the semiconductor industry has been to reduce perfluorocompound (PFC) emissions because of their high global warming potentials and long residence times in the atmosphere. During the last decade, significant achievements have been reached in attaining this goal. Chemical vapor deposition (CVD) chamber cleaning and plasma etch are two processes that use PFCs in which studies have been conducted to reduce emissions. Two successful strategies for reducing PFC emissions and enhancing process performance are described.