Gerd Jäger

An optical standing-wave interferometer for displacement measurements

Laser interferometers have become an important instrument for the measurement of displacement. In future, there is likely to be ever greater importance set on those measuring tasks which use sensors taking up very little space. One solution, proposed here, is a standing-wave interferometer, with a novel photoelectric detector. The latter is partially transparent and photoelectrically active, and scans the intensity profile of an optical standing-wave pattern. Arranging two of these transparent photoelectric detectors on the optical axis of the standing wave with the phase shifted permits bidirectional fringe counting. The transparent photoelectric detectors are basically pin-photodiodes with transparent contacts. Two sets of measurements were obtained, the first on a single transparent photoelectric detector and the second on a pair of transparent photoelectric detectors in the standing-wave pattern, and the results are discussed. It is shown that two phase-shifted photoelectric signals (sine and cosine) from two transparent photodetectors in different spatial positions on the optical axis of the standing wave can be received, and that the phase relation between the signals is a function of the distance.

Standing-wave interferometer

An interferometric position sensor was developed using the concept of sampling a standing wave. Interference of a standing wave created in front of a plane mirror can be detected by thin, partly transparent sensors based on amorphous silicon. The optical thickness of the absorption layer is thinner than the wavelength λ of the incident light. Detection of minima and maxima of the standing wave can be used to determine the relative displacement of the plane mirror and the detector. For determination of bidirectional fringe counting, two detectors with a certain phase shift were introduced into the standing wave. An integrated solution of two stacked n-i-p diodes and a phase shifter will be presented. The operation principle of the device will be demonstrated by measured Lissajous figures.

Advanced three-dimensional scan methods in the nanopositioning and nanomeasuring machine

The nanopositioning and nanomeasuring machine developed at the Ilmenau University of Technology was originally designed for surface measurements within a measuring volume of 25 mm ? 25 mm ? 5 mm. The interferometric length measuring and drive systems make it possible to move the stage with a resolution of 0.1 nm and a positioning uncertainty of less than 10 nm in all three axes. Various measuring tasks are possible depending on the installed probe system. Most of the sensors utilized are one-dimensional surface probes; however, some tasks require measuring sidewalls and other three-dimensional features. A new control system, based on the I++ DME specification, was implemented in the device. The I++ DME scan functions were improved and special scan functions added to allow advanced three-dimensional scan methods, further fulfilling the demands of scanning force microscopy and micro-coordinate measurements. This work gives an overview of these new functions and the application of them for several different measurements.

Interference signal demodulation for nanopositioning and nanomeasuring machines

The nanopositioning and nanomeasuring machine NMM-1 developed at the Ilmenau University of Technology was designed for measurements within a measuring volume of 25 × 25 × 5 mm3. The interferometric length measuring and drive systems make it possible to move the stage and corner mirror with a resolution of 0.1 nm in all three axes. The object being measured is placed on the corner mirror and can be measured with different probe systems. The high precision of the machine can be attributed to several factors. The most important is the accuracy of the interferometric measuring systems. Starting with a short description of NMM-1 and an improved equation for length calculation, this paper describes a small detail of the measurement uncertainty analysis for a displacement measurement using two positions of the measuring mirror. The overall 3D uncertainty for measurements carried out with the machine depends on the machine itself and the probe system in use as well as the specific measuring task. In particular, this paper discusses only the influence of the interference signal demodulation for homodyne interferometers.

Novel control scheme for a high-speed metrological scanning probe microscope

Some time ago, an interferometer-based metrological scanning probe microscope (SPM) was developed at the Institute of Process Measurement and Sensor Technology of the Ilmenau University of Technology, Germany. The specialty of this SPM is the combined deflection detection system that comprises an interferometer and a beam deflection. Due to this system it is possible to simultaneously measure the displacement, bending and torsion of the probe (cantilever). The SPM is integrated into a nanopositioning and nanomeasuring machine (NPM machine) and allows measurements with a resolution of 0.1 nm over a range of 25 mm ? 25 mm ? 5 mm. Excellent results were achieved for measurements of calibrated step height and lateral standards and these results are comparable to the calibration values from the Physikalisch-Technische Bundesanstalt (PTB) (Dorozhovets N et al 2007 Proc. SPIE 6616 661624?1?7). The disadvantage was a low attainable scanning speed and accordingly large expenditure of time. Control dynamics and scanning speed are limited because of the high masses of the stage and corner mirror of the machine. For the vertical axis an additional high-speed piezoelectric drive is integrated in the SPM in order to increase the measuring dynamics. The movement of the piezoelectric drive is controlled and traceable measured by the interferometer. Hence, nonlinearity and hysteresis in the actuator do not affect the measurement. The outcome of this is an improvement of the bending control of the cantilever and much higher scan speeds of up to 200 ?m?s?1.

A focus sensor for an application in a nanopositioning and nanomeasuring machine

A focus sensor on the basis of a hologram laser unit was developed and successfully tested in a nanopositioning and nanomeasuring machine as a zero indicator. The high resolution of the focus sensor is due to a high-precision optical adjustment and special solutions incorporated into the electronic parts. Thus, any sensor malfunctions caused by back-reflected light inside of the assembly could be completely avoided by means of the special high-frequency modulation and laser power stabilization. Common mode noise reduction provides the high SNR of the output signals. The measurements were made according to a dynamic principle by permanent difference formation between the output signal of the focus sensor and the length value of the z-interferometer of the nanopositioning and nanomeasuring machine. The measuring results are presented, and further possibilities of application are outlined.

Metrological scanning probe microscope

Todays technological progress calls for metrologically accurate object measurement, positioning and scanning with nanometre precision and over large measuring ranges. In order to meet that requirement a nanopositioning and nanomeasuring machine (NPM machine) was developed at the Institute of Process Measurement and Sensor Technology of the Technische Universitaet Ilmenau. This device is capable of highly exact long-range positioning and measurement of objects with a resolution of less than 0.1 nm. Due to the structure of the machine many different probe systems can be installed, including scanning probe microscopes (SPMs). A few SPMs have outstanding metrological characteristics and many commercial microscopes only perform as image acquisition tools. Commercial SPMs use piezoelectric actuators in order to move either the sample or the probe. The position measurement sometimes results from the applied voltage to the piezoelectric actuators or from the strain gauge or capacitive displacement sensor data. This means that they suffer from hysteresis, creep, nonlinear characteristics and Abbe offsets. For an accurate measurement the position of the cantilever must be measured in addition to the torsion and bending. The best solution is a combined detection system with a single laser beam. This system has been realized with a special interferometer system, in which the measuring beam is focused on the cantilever backside using a lens. The reflected beam is split with a part being detected by a quadrant photo-diode and the other part being fed back into the interferometer for position measurement. The quadrant photo-diode is used to detect the cantilever torsion and bending.

Measuring value correction and uncertainty analysis for homodyne interferometers

The Nanopositioning and Nanomeasuring Machine (NMM-1) developed at the Ilmenau University of Technology is equipped with three homodyne plane-mirror miniature interferometers for the measurement of the displacement of a movable corner mirror. The object being measured is placed on the corner mirror, which is positioned by a three-axis drive system. The uncertainty of interferometric length measurements is dependent on several factors. Starting with an improved equation for calculating length, this paper describes a part of the determination of the measurement uncertainty for a displacement measurement using two positions of the measuring mirror. The new approach separates the correlated influencing factors for three positions of the measuring mirror: for the position where the interferometer counter is reset to zero and the other two positions of two measured points. Additionally, the work takes into account the influence of the dead path length and its determination.

Miniature interferometers for applications in microtechnology and nanotechnology

An initial description of the design and operation of compact miniature interferometers that employ fiberoptic lightguides for all of their optical couplings and are suitable for general-purpose use is followed by a metrological analysis of their mode of operation and examples of their broad applicability, based on several typical instrumental setups.

Long-range nanopositioning and nanomeasuring machine for application to micro- and nanotechnology

The paper describes the operation of a high-precision long range three-dimensional nanopositioning and nanomeasuring machine (NPM-Machine). The NPM-Machine has been developed by the Institute of Process Measurement and Sensor Technology of the Technische Universität Ilmenau. The machine was successfully tested and continually improved in the last few years. The machines are operating successfully in several German and foreign research institutes including the Physikalisch-Technische Bundesanstalt (PTB). Three plane mirror miniature interferometers are installed into the NPM-machine having a resolution of less than 0,1 nm over the entire positioning and measuring range of 25 mm x 25 mm x 5 mm. An Abbe offset-free design of the three miniature plane mirror interferometers and applying a new concept for compensating systematic errors resulting from mechanical guide systems provide extraordinary accuracy with an expanded uncertainty of only 5 - 10 nm. The integration of several, optical and tactile probe systems and nanotools makes the NPM-Machine suitable for various tasks, such as large-area scanning probe microscopy, mask and wafer inspection, nanostructuring, biotechnology and genetic engineering as well as measuring mechanical precision workpieces, precision treatment and for engineering new material. Various developed probe systems have been integrated into the NPM-Machine. The measurement results of a focus sensor, metrological AFM, white light sensor, tactile stylus probe and of a 3D-micro-touch-probe are presented. Single beam-, double beam- and triple beam interferometers built in the NPM-Machine for six degrees of freedom measurements are described.