Tuesday, April 21, 2009

Displacement Sensors Offer Nanometre Resolution


A supplier of nano-positioning systems for atomic force microscopy (AFM) is using non-contact capacitive displacement sensors from Micro-Epsilon to measure the travel distance of test items. Based in Munich, Attocube Systems specialises in the development, manufacture and distribution of nano-positioning systems. Founded in 2001, the company is a spin-off from the Center for Nanoscience in Munich, Germany.

The company's core competence is in ultra-high precision spatial positioning of objects - a prerequisite for most nanotechnology applications. Attocube's technology enables the positioning and scanning of objects with atomic resolution, but with travel distances in the centimetre range. Attocube has developed systems for a variety of extreme environments, including UHV applications, low temperatures and high magnetic fields.

Atomic force microscopes are used to image, measure and manipulate matter at the nano scale. The information is gathered by 'feeling' the surface with a mechanical probe. Piezoelectric elements that facilitate very small but accurate and precise movements on command enable very precise scanning of surfaces. Attocube is using two custom-designed CapaNCDT 6300 CSH1FL capacitive displacement sensors from Micro-Epsilon to measure the travel displacement of test items that need to be moved in the nanometre range, enabling surface topography data to be obtained.

Attocube has developed special actuator motors that are used to position the head of the microscope for these precise positioning tasks. The system uses a piezo-ceramic positioning unit, enabling movement of the X, Y and Z axis. The two sensors measure the positioning range, which is 1.2mm x 1.2mm. The complete head of the microscope is cooled using liquid helium to 4K above the absolute zero point of -273C. The traversing units operate at an ambient temperature of 4K in an ultra-high vacuum and under very strong magnetic fields.

In order to measure the X and Y-axis movements, Attocube has integrated two Micro-Epsilon CapaNCDT 6300 sensors with its actuator motors, positioned inside the head of the microscope. With a measuring range of 1mm, the sensors have a precision of less than 5nm and operate without contact. 'The extremely low ambient temperature and ultra-high vacuum were particular challenges here,' said Chris Jones, managing director at Micro-Epsilon (UK).

'In ultra-high vacuums, you tend to get degassing of sensor materials and components, both within the sensor itself and the cable. 'The solution had to operate without contamination in this environment and also experience no loss in signal quality when passing sensor signals through an ultra-high vacuum bulkhead connector,' he added. Because the sensors are expected to perform just as well at - 269C as they are at room temperature, Micro-Epsilon used special materials for the sensor and the cable, which due to their very low thermal expansion properties, provide very stable sensor measurements.

A complete sensor solution with vacuum bulkhead was delivered to the customer that required no modifications. The cable supplied was a triaxial design and the temperature stability of the complete system is 11ppm/C. 'We also had a very tight space envelope to work in and so we had to modify the sensor geometry to suit the installation,' said Jones. 'We also developed our own in-house low temperature test chamber that enables us to calibrate and test our sensors for these types of extreme application down to -200C.'.

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