Mikrointerferometer auf Basis von interferenzoptischen Stehende-Welle-Sensoren

Seit dem Michelson-Morley-Experiment im Jahr 1887 werden Interferometer erfolgreich in Forschung und Industrie für verschiedenste Aufgaben eingesetzt. Laserinterferometer sind heute hochentwickelte und enorm leistungsfähige Geräte mit beachtlichen Parametern hinsichtlich Messauflösung und Messunsicherheit. Diese Leistungsfähigkeit jedoch beruht auf einem äußerst komplexen Aufbau mit einer großen Anzahl optischer Präzisionskomponenten, weshalb klassische Laserinterferometer kostenintensive Messmittel nahezu ausschließlich für Aufgaben der Präzisionsmesstechnik mit höchsten Anforderungen darstellen. Gemeinsam mit der begrenzten Miniaturisierbarkeit von diskret aufgebauten Interferometern resultiert daraus eine Einschränkung der möglichen Einsatzgebiete. Das Stehende-Welle-Interferometer stellt einen neuen Interferometeransatz dar, mit dem die genannten Einschränkungen überwunden werden können. Das Konzept basiert auf einer optischen stehenden Welle, welche im Raum vor einem Spiegel bei senkrechter Reflexion eines Laserstrahls in sich selbst entsteht. Die Intensitätsminima und -maxima der stehenden Welle sind räumlich an den Spiegel gekoppelt und können mit einem dünnen, transparenten Photosensor detektiert werden. Eine Zählung der den Sensor bei einer Spiegelverschiebung durchlaufenden Extrema ermöglicht bei bekannter Wellenlänge der Laserquelle eine Bestimmung des Verschiebewegs des Spiegels. Da sich der genannte Sensor im optischen Strahlengang befindet, beeinflusst dieser direkt die stehende Welle. Für den Sensor existieren daher besondere Anforderungen hinsichtlich dessen Dicke, Transparenz, Reflexionsgrad und Ebenheit. Im Rahmen dieser Arbeit werden entsprechende Stehende-Welle-Sensoren für hochdynamische Messungen und verschiedene optische Aufbauten entwickelt und untersucht. Die Sensoren basieren auf kommerziellen SOI-Wafern und können mit üblichen Halbleitertechnologien hergestellt werden. Bei der Entwicklung liegen die Schwerpunkte auf einer hohen Grenzfrequenz, auf der Entspiegelung der Sensoren und auf Verfahren zur mechanischen Stabilisierung der äußerst dünnen photoaktiven Schicht. Die elektrischen, optischen und elektrooptischen Eigenschaften der Sensoren werden umfangreich untersucht und deren Einsatz in Homodyn-, Heterodyn - und Interferometeraufbauten mit Phasenmodulation nachgewiesen.Since the Michelson-Morley-experiment in the year 1887, interferometers are successfully used in research and industry for various tasks. Today, laser interferometers are highly developed and enormously powerful instruments with considerable parameters in terms of measurement resolution and measurement uncertainty. This performance, however, is based on an extremely complex structure with a large number of optical precision components Therefore, classical laser interferometers are cost-intensive measuring instruments, almost exclusively for precision metrology tasks with the highest requirements. In combination with the limited potential for miniaturization of discretely constructed interferometers, this results in a limitation of the possible fields of application. The standing-wave-interferometer represents a new interferometer approach which can overcome the mentioned limitations. The concept is based on an optical standing wave, arising in the space in front of a mirror when a laser beam is reflected perpendicularly in itself. The intensity minima and maxima of the standing wave are coupled to the mirror surface and can be detected with a thin, transparent photosensor. Counting the extremes passing through the sensor during a mirror displacement enables the determination of the mirror displacement as long as the wavelength of the laser source is known. Since the sensor is located in the optical path, it directly influences the standing wave. Therefore, the sensor has to meet special requirements regarding its thickness, transparency, reflectance and flatness. Within the scope of this work, corresponding standing-wave-sensors are developed and investigated, which enable highly dynamic measurements in different optical setups. The sensors are based on commercial SOI-wafers and can be manufactured with common semiconductor technologies. The development focuses on a high cut-off frequency, antireflection coating of the sensors and methods for a mechanical stabilization of the extremely thin photoactive layer. The electrical, optical and electro-optical properties of the sensors are extensively investigated and their use in homodyne, heterodyne and phase modulated interferometer setups is proven

Processing and analysis of long-range scans with an atomic force microscope (AFM) in combination with nanopositioning and nanomeasuring technology for defect detection and quality control

This paper deals with a planar nanopositioning and -measuring machine, the so-called nanofabrication machine (NFM-100), in combination with a mounted atomic force microscope (AFM). This planar machine has a circular moving range of 100 mm. Due to the possibility of detecting structures in the nanometre range with an atomic force microscope and the large range of motion of the NFM-100, structures can be analysed with high resolution and precision over large areas by combining the two systems, which was not possible before. On the basis of a grating sample, line scans over lengths in the millimetre range are demonstrated on the one hand; on the other hand, the accuracy as well as various evaluation methods are discussed and analysed

Phase-modulated standing wave interferometer

Standing wave interferometers (SWIs) show enormous potential for miniaturization because of their simple linear optical set-up, consisting only of a laser source, a measuring mirror and two standing wave sensors for obtaining quadrature signals. To reduce optical influences on the standing wave and avoid the need for an exact and long-term stable sensor-to-sensor distance, a single-sensor set-up was developed with a phase modulation by forced oscillation of the measuring mirror. When the correct modulation stroke is applied, the harmonics in the sensor signal can be used for obtaining quadrature signals for phase demodulation and direction discrimination

Heterodyne standing-wave interferometer with improved phase stability

This paper describes a standing-wave interferometer with two laser sources of different wavelengths, diametrically opposed and emitting towards each other. The resulting standing wave has an intensity profile which is moving with a constant velocity, and is directly detected inside the laser beam by two thin and transparent photo sensors. The first sensor is at a fixed position, serving as a phase reference for the second one which is moved along the optical axis, resulting in a frequency shift, proportional to the velocity. The phase difference between both sensors is evaluated for the purpose of interferometric length measurements

Integration of an ultraviolet direct write laser and its red differential confocal probe

The maskless photolithography method direct laser writing (DLW) can achieve sub-100-nm writing resolution if the photoresist is kept well on the plane of best focus of the optical system. Deviation from this plane leads to larger than intended or loss of developed areas. Here we present an approach to account for this problem through a Bessel-Gauss beam for the exposure

FPGA-based signal processing of a heterodyne interferometer

A heterodyne interferometer and a data acquiring algorithm have been developed to measure the movement of a mirror in one dimension, as well as its rotation around two axis. The interferometer uses spatially separated beams to reduce periodic optical non-linearities, furthermore the optical set-up was designed for low drift, few number of optical elements and easy adjustment. The FPGA-based signal processing is based on an undersampling technique with the aim to minimise the calculation effort. The working principles of the interferometer and the electronics are described and their remaining non-linearities are investigated. Finally, the z-position, the tip and tilt angle of a planar stage are measured with the described system as an example of use

A heterodyne interferometer with separated beam paths for high-precision displacement and angular measurements

As standard concepts for precision positioning within a machine reach their limits with increasing measurement volumes, inverse concepts are a promising approach for addressing this problem. The inverse principle entails other limitations, as for high-precision positioning of a sensor head within a large measurement volume, three four-beam interferometers are required in order to measure all necessary translations and rotations of the sensor head and reconstruct the topography of the reference system consisting of fixed mirrors in the x-, y-, and z-directions. We present the principle of a passive heterodyne laser interferometer with consequently separated beam paths for the individual heterodyne frequencies. The beam path design is illustrated and described, as well as the design of the signal-processing and evaluation algorithm, which is implemented using a System-On-a-Chip with an integrated FPGA, CPU, and A/D converters. A streamlined bench-top optical assembly was set up and measurements were carried out to investigate the remaining non-linearities. Additionally, reference measurements with a commercial homodyne interferometer were executed

Carrier mobility in semiconductors at very low temperatures

Carrier mobilities and concentrations were measured for different p- and n-type silicon materials in the temperature range 0.3–300 K. Simulations show that experimentally determined carrier mobilities are best described in this temperature range by Klaassen’s model. Freeze-out reduces the carrier concentration with decreasing temperature. Freeze-out, however, depends on the dopant type and initial concentration. Semi-classical calculations are useful only for temperatures above 100 K. Otherwise quantum mechanical calculations are require