Displacement sensors operate at extremely low temperatures

Based in Munich, Attocube Systems AG specialises in the development, manufacture and distribution of innovative nano-positioning systems for Atomic Force Microscopy (AFM). Founded in 2001, the company is a spin-off from the CeNS Munich (Center for NanoScience) in Germany. The company's core competence is ultra-high precision spatial positioning of objects - a prerequisite for most nanotechnology applications. Attocube's technology enables objects to be positioned and scanned with atomic resolution, but with travel distances in the centimetre range.

Attocube has developed systems for use in a variety of extreme environments, including UHV (ultra-high vacuum) applications, low temperatures and high magnetic fields. These systems have enabled pioneering investigations in science and industry, opening up new markets in semiconductors, biotechnology, material science, medicine, chemistry and aerospace.

Atomic force microscopes are used to image, measure and manipulate matter at the nanoscale. 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 surfaces to be scanned very precisely.

Custom capacitive sensors

To measure the travel displacement of test items that need to be moved in the nanometre range, Attocube is using two custom-designed capaNCDT 6300 CSH1FL capacitive displacement sensors from Micro-Epsilon. These sensors enable the 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 axes. The two sensors measure the positioning range, which is 1.2mm by 1.2mm. The complete head of the microscope is cooled using liquid helium to 4K above absolute zero. The traversing units operate at a 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 better than 5nm and operate without contact.

Chris Jones, Managing Director at Micro-Epsilon (UK), comments: "The extremely low ambient temperature and ultra-high vacuum were particular challenges here. 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."

Excellent temperature stability

Because the sensors are expected to perform just as well at -269degC as they are at room temperature, Micro-Epsilon therefore 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 with vacuum bulkhead was delivered to the customer that required no modifications. The cable supplied is a triaxial design and the temperature stability of the complete system is 11ppm/K (most alternative sensors are said to offer 100-200ppm at best).

Jones continues: "We also had a very tight space envelope to work in and so we had to modify the sensor geometry to suit the installation. 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 -200 degrees Centigrade."

The capaNCDT 6300 CSH1FL is now part of the standard Micro-Epsilon range of capacitive displacement sensors.

Follow the link for more information on Micro-Epsilon's capacitive displacement sensors.