Smooth sensation

A sensor system claimed to outperform traditional alternatives could make surface detection across a range of industries more efficient.

A sensing technology that is claimed to outperform its traditional alternatives has been developed by Qcept, which says the system will become an invaluable tool in the semiconductor industry.

The Scanning Contact Potential Difference Sensor (CPD) has taken over 10 years to develop and can detect minute changes to the physical and chemical features of a range of surfaces.

As well as the semiconductor industry, the system can be used in industrial machine monitoring applications, such as sensing critical conditions in shafts, rotors and bearings.

At present, different techniques are used for different applications, but CPD threatens to oust them all.

This is put down to Its anticipated low cost, the high quality of the information it provides, and its ability to sense geometrical and chemical changes, over a moving surface, in a high-speed, non-contact manner.

Allen Vance, sales and marketing vice-president, explained the principles behind thesensor’s operation.

In 1898 William Thomson (Lord Kelvin) described the idea that different materials have different ‘work functions’, which is directly related to the amount of energy it takes to remove electrons from them.

When two materials are brought near each other and connected in a circuit, a current flows briefly to equalise the electric potential due to the ‘contact potential difference’. In other words, the electric potential is the difference, or D, in CPD’s name.

If one material is the surface being inspected, and the other material is the surface of the Qcept probe, then changes in the geometry of the surfaces cause capacitance to alter. This is because of the changing difference in distance between them. Moving the probe up and down over a spot allows the work function to be measured indirectly, and thus characterise the material. This is known as the Kelvin-Zeissman probe, or Vibrating Kelvin Probe, invented in the 1930s.

Qcept’s innovation is to scan the probe over the surface, without vibrating it, and to measure the chemically-related work function change as well as the geometrically-related change in capacitance. Over almost any surface, the chemical nature varies as the probe moves. This, claimed Vance, is unique to Qcept.

He added that the system also has a technique that can separate the physical from the chemical information.

During operation the sensor is typically dealing with a rapidly- moving surface. A semiconductor wafer, for instance, is spun during inspection, and a machine tool shaft spins in operation.

The sensor is mounted over and close to the surface. The varying voltage that comes from the probe’s electronics is turned into digital data and stored, processed, and turned into a real-time image, by software also developed by Qcept.

The user then gets useful information about physical features. For example, it may be any movement like vibration or displacement of a machine tool shaft, or chemical uniformity information on a semiconductor wafer that is being examined for quality control purposes.

Vance added that the system is currently being evaluated by an undisclosed global industrial company that has applications in both the machine monitoring area and in semiconductor analysis.

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