Author: Mike Cooke, PhD, Chief Technology Officer, OIPT
Why have panels to enclose a machine? Apart from style, we think there are good reasons to care about the panels, many of them affecting machine safety.
The law often distinguishes between an operator who uses a machine and a skilled or informed person who services or maintains that machine. In a work setting, the machine must be safe for everyone in that area. The operator, using the controls provided with the panels fitted, must not be able to touch any hazardous live component. Anyone who gets inside the machine using a tool must be skilled, can understand the warnings posted on the tool and in the manual, and appreciate the hazards they find inside. Panels are an important contribution to the safe system of work of every machine user. Habitually leaving the panels off blurs the boundary between the usual machine users (who are often highly skilled) and others in the same room – including the cleaner.
A vacuum processing tool has several kinds of hazards inside, including:
It is a fundamental requirement that a machine is safe to operate, despite these hazards, and the panels play a role in assuring safety for all these hazard categories. Electric shock risks are clearly minimised by having the panels in place, especially if the panels are electrically grounded.
Two principal levels of concern for RF:
- RF can burn human tissue, so the radiation from the tool anywhere that can be touched is of the greatest importance
- RF can interfere with other equipment. This is part of the ‘electromagnetic compatibility’ (EMC) issue.
OIPT products are designed to be safe against RF burns to nearby people, even with the outer panels removed. However, reaching inside the tool there could be some hot spots at internal panel joints where the level exceeds a power density of 1 mWcm-2, which is a typical benchmark for RF safety. (Radiation from mobile phones is around this level). Skilled persons who appreciate the hazards associated with RF will be able to operate the tool with the outer panel removed (possibly at reduced power level) without endangering themselves or those nearby, for example when setting up RF matching units.
OIPT products normally have two sets of panels between the source of RF energy and the operator. Never turn on RF power with both sets of panels removed: there will be a significant risk of RF burns.
Electromagnetic compatibility
The panels also play a role at the second level of RF safety – electromagnetic compatibility. The test data underpinning the ‘CE’ certification of OIPT products is taken with the panels in place, and no guarantee of compliance can be given if they are removed. There is nearly always an increase in measurable RF interference broadcast from plasma tools when panels are removed. There is also the potential for interference with the machine from outside RF sources, and (more often) disruption from static sparks when a charged person touches the machine. An outer case which conducts such sparks away to ground before the energy can get into the machine wiring is the best way to handle the susceptibility side of EMC compliance.
Mechanical Hazards
The major mechanical hazard inside the tool is the turbomolecular vacuum pump. When operating an etch machine, you are standing close to an aluminium alloy rotor of at least 1kg, spinning at 30,000rpm or higher. While rotor crashes are rare (especially if the tool is used as recommended), they do occasionally occur. Most such incidents destroy only the pump, but recent stress analysis at OIPT using data supplied by Adixen shows that a worst case incident with a large turbo pump has the potential to disrupt vacuum envelope of the tool. Should this occur, the interlock system will immediately halt the flow of process gases, but the gases in the tool will begin to diffuse out, taking about one minute for concentrations to approach threshold limit values near the operator. It is clearly safer to the operator to have the outer panels installed to provide a physical barrier to the movement of both gases and broken parts.
The toxic gas hazard is most significant in the gas box, where the gases are piped in at pressures normally above atmospheric pressure, and are reduced to below atmospheric pressure before piping into the process chamber. The gas box is normally extracted if the gas hazard level is significant, and the panel plays an obvious role in maintaining local safety. This panel is normally interlocked, so that all gas valves are shut as the panel is opened.
Remember that there are always residual hazards in operating machinery. No-one can guarantee absolute safety, but as a responsible supplier OIPT keeps the risk of harm as low as reasonably possible. The panels are not just for the look of the machine; they are part of the safety and compliance design of the tool. If you take the panels off…PUT THEM BACK!
Information on RF safe levels is available at:
http://www.fcc.gov/Bureaus/Engineering_Technology/Documents/bulletins/oet65/oet65.pdf