PC-based control simplifies wafer production

Edited by the Engineeringtalk editorial team Aug 12, 2003

Beckhoff PC-based control products are providing Thomas Swan Scientific with a secure platform for future developments of its market-leading semiconductor manufacturing equipment.

Beckhoff PC-based control products are providing Thomas Swan Scientific with a secure platform for future developments of its market-leading semiconductor manufacturing equipment, while also ensuring that the company's current products retain and enhance their competitive position in the market.

Based in Cambridge, Thomas Swan Scientific supplies 40% of the world market for metal-organic chemical vapour deposition (MOCVD) reactors which make the semiconductor wafers used in the manufacture of light-emitting diodes and similar devices.

The machines work by introducing various gases, supplied from a gas cabinet which forms part of the machine, into a reactor chamber which contains the substrate on which the wafer is deposited.

During the deposition process, the temperature and pressure within the chamber must be accurately controlled, as must be the flow rate of the gases.

Until recently, all control functions were provided by a PC, which handled the main control algorithms, linked via a Profibus connection to a PLC which provided I/O handling.

The Thomas Swan Scientific design team, however, wanted to separate the controls for the gas cabinet from those for the reactor, to simplify the manufacture and testing of the machines.

In addition, there was also a requirement to accommodate networked instruments, including mass-flow and temperature controllers.

An investigation showed that upgrading the existing control system to meet these requirements would almost double its price, but that implementing an upgraded system using Beckhoff PC-based products would halve the price of the existing system.

The current generation of machines, therefore, uses a PC which, in addition to providing supervisory functions, also provides two virtual (software-only) PLCs implemented with Beckhoff's TwinCAT software.

One of the virtual PLCs provides the gas cabinet control, while the other controls the reactor itself.

The machine currently has 160 analogue inputs and 60 digital inputs, along with similar numbers of each type of output.

All of these are handled by Beckhoff bus terminal products which communicate with the PC via Profibus, and which are mounted as remote I/O blocks at convenient points around the machine.

"Eliminating the conventional PLC hardware has yielded big cost savings, yet the design changes needed took less than two days to implement", said Chris Moorhouse, Thomas Swan Scientific's Chief Software and Control Engineer, "and using Beckhoff bus terminals has produced further big savings.

Not only is the amount of field wiring greatly reduced, we can now also connect PT100 temperature sensors and devices with RS232 interfaces direct to the I/O system without having to use costly and inconvenient signal convertors".

The Beckhoff implementation has also opened the way for future developments.

Engineers at Thomas Swan Scientific are planning to replace the present operator interface, which uses a conventional mimic panel, with a Beckhoff touch screen.

This will provide greater flexibility to accommodate changes, as well as providing the machine users with clearer and more comprehensive information.

A further step will be to implement the controls for the gas cabinet and the reactor in two separate CX1000 embedded micro-PCs from Beckhoff, thus allowing the cabinet and reactor sections of the machine to be built and tested separately.

As part of this upgrade process, a move will also be made to use instruments which communicate via the Profibus network, rather than by conventional analogue connections.

Beckhoff control products from Hayes Control Systems have proved so beneficial for Thomas Swan Scientific that the company's parent organisation, Aixtron in Germany, is now also considering making the change.

If this occurs, Beckhoff products will become virtually the worldwide de-facto standard for the control of MOCVD machines.