Magnetic levitation ensures contactless transport for microchips
07 June 2015
A major issue in the manufacture of semiconductors and microchips is particle contamination. Paul Streatfield examines a potential solution to contamination problems, which combines linear drives and magnetic levitation.
The manufacture of semiconductors and microchips demands an ultra-clean production environment; an in-line vacuum deposition process is one example. However, the process of production, even in a vacuum, all too often lends to the creation of tiny contaminant particles, which can affect product quality and reduce the operational life of the end product.
Particles are generated by metal-to-metal, or metal-to-grease contact that is commonly encountered with conventional methods of in-line transport. Electronics manufacturers are currently using a variety of methods, from chain drives to conveyor belts with linear motors, that are complex, expensive and – crucially - particle generating.
Typically, the layout includes a vacuum-sealed process chamber with the carrier inside a vacuum. The problem with this method is that the bearings are also inside the vacuum, resulting in metal-to-metal contact and the potential for particle contamination. And particles are not the only problem. This type of production is neither scalable nor flexible. Changes in production schedules cannot be quickly accommodated and the line will need extensive service and maintenance.
So, how can contact with these components be avoided during the manufacturing process? A potential solution to this problem - one that is currently being extensively tested - is magnetic levitation. One company working in this area is Bosch Rexroth whose LeviMotion concept combines inverted linear motion technology with a completely contactless transportation system.
The standard transportation system comprises a linear motor with one moving coil, the motion of the attached carrier being controlled by the switching of the current through the coil. LeviMotion, on the other hand, uses an inverted linear motor with magnets beneath the drive carrier, and with the coil units mounted outside the process chamber. This configuration enables large air gaps to be maintained between magnets and carrier, so the latter is able to levitate above the permanent magnet tracks.
A dual Hall sensor arrangement is used to control the carrier location. Magnets moving over the sensors create a sinusoidal wave (the sensors being spaced to ensure the phase difference is 90 degrees). Interpolation of the two sensor signals provides the carrier position with a high degree of precision. The carrier is also equipped with an automatic alignment and control system for the bearings’ air gap, which enables movement in five axes, including pitch, roll and yaw.
This type of system has two advantages. Firstly, a series of coils can be constructed and up to 32 carriers can be used, rather than just the single carrier as is the case with the standard linear motor. Secondly, with the coils mounted beneath the carrier, contaminant particles are more likely to fall away, rather than onto, the carrier.
And as the inverted linear motor has no active parts (the bearings being located in fixed positions) there is much less potential for particle ingress. What’s more, this method of transport is frictionless with no bearing related disturbances like sticking or slipping or fluctuating stiffness. Only passive or sealed components are located in the process chamber.
The combination of linear motion and magnetic levitation can also deliver operational benefits. This type of transportation system could provide high production line speeds that are constant with low ripple, as well as ensuring high positioning accuracy. What’s more, testing has shown excellent planarity over long transportation distances, thanks to the automatic alignment feature mentioned above.
In terms of production throughput, the carriers can achieve speeds of up to 5cm/s and can carry loads from 1kg to 1,000kg. Of significant importance to the microelectronics fabrication industry, the carriers are also capable of repeat positioning within the range 10-20µm, along with minimal velocity ripple.
Whilst this combination of Bosch Rexroth’s NYCe4000 LMS drive system and Mecatronix’s magnetic levitation is only currently at the testing stage, it has already gained significant interest from electronics manufacturers seeking to minimise contamination issues in their manufacturing processes.
Paul Streatfield is with Bosch Rexroth
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