Directing the incidence of light with precision.

Mirror steering or mirror deflection systems are present in a wide range of applications, from aerospace and solar systems to semiconductor lithography. Mirrors are used wherever light is reflected and directed to be focused or scattered at a specific point. Behind this is first of all a mechanism, a motor, which moves one or more parts, which in turn adjust the mirrors and align them appropriately to the light. In this way, the light is focused and brought to a point. This alignment is controlled by electronics and certain position sensors, which must transmit the status of the mirrors to the control system as precisely as possible. The elements to be aligned can be very different: e.g. huge photovoltaic elements in solar power plants, large-sized but also very small mirrors in space telescopes, where the deflection has to be performed in the nanometer range. Based on the three application examples mentioned - space telescope, solar power plant and semiconductor lithography - it will be shown which requirements and conditions have to be fulfilled for mirror deflection systems.

Space telescope: small distraction, great effect

Given the distances involved in using a space telescope, one might think that large mirrors and more extensive adjustment of them would be involved. In fact, this very example involves nanometer-scale deflection of the incident light beam. On the one hand, there are telescopes which are located on Earth and through which a tiny point in space is to be observed. The greater the mirror alignment and deflection here, the more this tiny point slips away. Behind this are simple angle laws. It is a matter of several mirrors that have to be aligned to this fixed point and precisely tuned to each other. In addition, there is the influence of the atmosphere on the earth. If, for example, there is moisture in the atmosphere, it acts like a lens and additionally deflects the light. Such atmospheric turbulence must be additionally compensated for by a further minimum deflection of the mirrors. For this reason, most reflecting telescopes are located in remote regions such as deserts and elevations in order to avoid disturbing terrestrial influences as best as possible. Nevertheless, the ELT (Extremely Large Telescope) in Chile's Atacama Desert, for example, uses 6,000 actuators to correct for atmospheric turbulence, performing about 1,000 actions per second.

For actual space telescopes such as the James Webb Telescope, which perform their services in orbit around the Earth, the harsh environmental conditions of space are a challenge for sensors and electronic elements: extreme temperatures, vacuum and UV radiation. Again, a few nanometers of position change are critical to align the various mirrors. Alignment here can even take several days or weeks. In addition, not only visible light is directed, but wavelengths up to the mid-infrared range, which must also be reflected precisely.

Solar thermal power plants: using solar radiation at the best angle

Solar thermal power plants use a variety of mirrors to reflect sunlight and concentrate it into a single point. These are huge plants in sunny regions that are used to generate electricity. One variant of the plants are solar tower power plants. All of the large-format mirrors are automatically tracked by the sun so that they are always oriented at the best possible angle to the sun's rays in order to "capture" as much sunlight as possible and focus it on the tower or absorber in the center of the collector array. The top of the tower heats to over 1,000 degrees Celsius and this heat is often used to drive a steam turbine to generate electricity. The deflection angles for solar reflectors are much larger than in the telescope example. What is possible is a combined system of a mechanical system that performs the large deflection of the mirrors to compensate for solar motion, while the point-by-point focusing on the absorber to compensate for wind effects with small deflections of the mirrors can be controlled by position sensors.

Challenging for position sensors and the electronics they contain are the extreme temperature differences that occur in sunny regions, often deserts: very high temperatures during the day (40 to 50 degrees Celsius), and temperatures can drop sharply at night (minus 10 degrees Celsius). It must be ensured that sensors function reliably and accurately over such a temperature range. Humidity must not have any influence on the measurement result, nor must contamination by dust, for example. Depending on the requirements, capacitive or inductive sensors can be used for distance measurements of the mirrors. In the case of humidity and contamination, inductive sensors are the better choice, as these conditions have no influence on the measurement result.

Semiconductor lithography: directing EUV light in ultra-high vacuum

Another field of application for mirror deflection systems is semiconductor lithography for the production of computer chips. Due to the miniaturization of computer chips with ever higher performance, extremely tiny structures have to be deposited on the silicon wafer. This is now done with very short-wave UV light - EUV, i.e. extreme ultraviolet light - which can only be deflected using particularly smooth special mirrors. Ordinary lenses would not be able to focus the UV light properly. The alignment of the mirrors must function with high precision in the nanometer range and places correspondingly high demands on the precision of the position sensors. As a rule, capacitive sensors are used here, which achieve highly accurate results in a very small measuring range and which cannot be influenced by the strong magnetic fields sometimes used in lithography - inductive sensors cannot be used in these cases.

In this example, the environmental conditions also pose special challenges. Wafer production takes place in an ultra-high vacuum, which electronics and circuits must be able to withstand in terms of low pressure. These must not outgas and thus introduce harmful substances into the system. The materials used must therefore be selected according to the vacuum requirements. It is critical, for example, if elemental hydrogen is produced. This can decompose the components themselves and damage the machine, as it is chemically very aggressive.

High-precision sensors for special requirements

When it is about high precision distance measurements, which are used in mirror steering systems for precise alignment of the mirror surfaces, the inductive and the capacitive measuring system are the measuring principles of choice. Inductive sensors are insensitive to dirt; dust and moisture have no effect on the measurement result. An electromagnetic field is generated into which a metallic counterpart is inserted, by which the field is weakened. The attenuation of the field can be measured and correlates with the object size, the object material and also with the distance. If the material and size are kept largely constant, but the distance is changed, the inductive sensor can be used as a distance sensor.

With the capacitive measuring system, a metallic counterpart is not necessarily required; non-metallic materials can also be measured with capacitive sensors - and with the smallest tolerances in the nanometer range. The setup consists of a surface to which charge is applied and a counter surface, which form the plates of an electrical capacitor. The capacitance of this assembly is measured. This is also distance dependent; as the distance decreases, the capacitance also changes. This measurement signal can be interpreted as distance. Tiny changes can thus be detected and used for the special application of mirror alignment.

Crucial to the functionality of the sensors are precise electronics that are temperature-stable and robust enough to withstand the harsh environmental conditions of desert, space and vacuum.

For these demanding applications, Micro-Hybrid's LTCC multilayer ceramic technology offers unique solutions.