Feature: Sensors
Confocal sensors are used to check the
parallel alignment of the mask; see Figure 7. T e gap between the glass and the substrate is detected using just one sensor. T e rectangular design of the sensors also allows installation in confi ned areas. In this version, the sensor requires only a minimum of installation space. To achieve maximum measured
resolution, white light interferometers are used, which generate absolute readings with sub-nanometer accuracy and enable accurate photomask alignment. Special evaluation algorithms and active temperature compensation ensure high signal stability. Similar to confocal sensors, white-
light interferometers are available in vacuum-compatible versions. High-precision mirror optics are
indispensable in lithography, used to achieve the highest possible imaging accuracy. To ensure their proper functioning, the mirrors, lenses and lens carriers must be positioned precisely. In the same way, the horizontal and vertical movement of the individual optical elements must be monitored. Alignment of the optics is measured
using confocal chromatic sensors. T is involves nanometer-precise detection of the respective lens tilt. For use on glass, the confocal chromatic sensors and white light interferometers are the ideal choice. Positioning of the lens carriers is
monitored using capacitive displacement sensors; see Figure 8. High precision capacitive sensors check the tilt to nanometer accuracy; see Figure 9. Several sensors around the carrier ensure exact positioning and therefore reproducible
36 February 2023
www.electronicsworld.co.uk
Figures 5 (top left) and 6 (above): Colour sensors are used to determine the concentration of the contained ozone, by monitoring the colour of the liquid
light projection onto the wafer. Non-contact eddy current sensors
also measure the position of lens elements. Depending on the lens system, displacement sensors from Micro-Epsilon are used to detect movement and position
in up to 6o of freedom. Due to the high
cut-off frequency, even highly-dynamic movements of the lens systems can be monitored. White light interferometers and eddy current sensors are used to monitor
Figure 7: Confocal sensors are used to check the parallel alignment of the mask
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