PROJECT DESCRIPTION
BACKGROUND
The semiconductor industry is reputed to be a clean industry in comparison with others. However, it consumes huge amounts of energy, chemicals, technical gases and even water. Therefore it is affected by the growing demand for a reduced impact on the environment, especially in view of the rapidly growing market for IC. Reducing resource consumption and waste production are essential to allowing future growth of the semiconductor industry. Wolfram (WF6) is a product used as gas source for the Chemical Vapour Deposition (W-CVD) of Tungsten, an important step in the processing of submicron CMOS circuits. At least 50 % of this gas ends up unused in the exhaust. Until the beginning of the project, destructive abatement techniques, based on incineration of the waste gas stream with or without subsequent water spray, were used to avoid emission of this gas into the atmosphere. No effort has yet been made to develop a recycling system to recover the excess WF6.
OBJECTIVES
The objective of the project was to demonstrate a new, on line, cost effective means of reducing the escape into the atmosphere of the waste gas stream from CVD process equipment used in semiconductor manufacturing. This was to be done by means of a recovery technique (dry plasma scrubber) for Tungsten (very valuable) from W-CVD processing, where WF6 is used as a source gas. The project also included the construction of a Cost of Ownership model of the demonstrated recovery technique that was compared to the models of the classical abatement techniques. The expected impact of the project was to provide the European semiconductor industry with a cost-effective tool for the treatment of the waste gas from a W-CVD processing tool. If the system proved to be technically viable, it would result in a more efficient use of raw materials and at the same time reduce the impact of WF6 on the environment, especially in view of the expected strong increase in IC production volumes. An additional advantage would be the high market value of the recycled Tungsten: for a medium-sized wafer fabrication the "yield" theoretically amounts to EURO 50,000 per week.
RESULTS
The objective of trying to recover pure tungsten by means of a plasma scrubber was achieved. The purity of the tungsten was high but the resell value of the tungsten was less than expected, making the cost of ownership of the system less atractive. A significant difference was observed depending on what plasma frequency was used for the scrubber. When using a 100kHz power supply, an additional intermediate hydrogen plasma treatment was required in order to obtain a continuous high WF6 conversion efficiency (>98%) and a good deposited W-layer. A 40 kHz power supply gave a continuous conversion efficiency of >99.5%. No memory effect was observed at this frequency and therefore no intermediate hydrogen treatment was necessary. However, the arcing of the plasma scrubber is a serious lifetime limiting problem and requires further investigation. If the lifetime of the electrodes is high enough, the cost of ownership could be very reasonable. This system did not however abate all of the components present in the gas exhaust (i.e HF, silane and others). Therefore and additional abatement system is necessary.The objective of trying to recover pure tungsten by means of a plasma scrubber was achieved. The purity of the tungsten was high but the resell value of the tungsten was less than expected, making the cost of ownership of the system less atractive. A significant difference was observed depending on what plasma frequency was used for the scrubber. When using a 100kHz power supply, an additional intermediate hydrogen plasma treatment was required in order to obtain a continuous high WF6 conversion efficiency (>98%) and a good deposited W-layer. A 40 kHz power supply gave a continuous conversion efficiency of >99.5%. No memory effect was observed at this frequency and therefore no intermediate hydrogen treatment was necessary. However, the arcing of the plasma scrubber is a serious lifetime limiting problem and requires further investigation. If the lifetime of the electrodes is high enough, the cost of ownership could be very reasonable. This system did not however abate all of the components present in the gas exhaust (i.e HF, silane and others). Therefore and additional abatement system is necessary.
