Due to the constant progress in the field of nanotechnology with ever smaller structures, the need to detect, measure and manipulate the smallest structures has arisen in recent decades. For this purpose, high-resolution methods have been available since the 1980s (STM, AFM), which, however, can usually only be applied to very small areas and which only allow metrologically traceable measurements at great expense. In order to be able to measure large objects with nanometer resolution, the nanometer measuring machine NMM-1 was developed at the IPMS. This offers a previously unattained measuring range of 25 mm x 25 mm x 5 mm with a resolution of 0.1 nm.
A special feature of all nanomeasuring machines at the IPMS is the arrangement of the miniature interferometers for length measurement. Their measuring axes intersect virtually at the point of contact of the probe sensor with the measured object, which ensures an error-free measurement in all three coordinate axes.
The NMM-1 can be used for a wide variety of measurement tasks, including
- the positioning, manipulation, processing and measurement of objects in microelectronics, micromechanics, optics, molecular biology and microsystems technology
- the measurement of precision parts membranes and microlenses
- the calibration of step height standards and pitch standards
The NMM-1 has been successfully used for a long time in various national metrological institutes for the calibration of micro and nano metrology standards. However, due to the growing requirements, it became necessary to develop a new nanopositioning and nanometrology machine NPMM-200. This machine has a measuring and positioning range of 200 mm x 200 mm x 25 mm and a resolution of 0.02 nm. The functional characteristics reflect the consistent application of the greatly improved metrological concept of the NMM-1.
To further minimize length measurement errors in all three coordinate axes compared to the NMM-1, the NPMM-200 permanently measures the angular deviations of the machine table by additional laser interferometers and keeps the angular orientation constant by a complex control system. This allows the errors of the mechanical system of the NPMM-200 to be almost completely eliminated by continuous control.
External disturbances have an enormous influence on the running measurement in the nanometer range. For this reason, the NPMM-200 is located in a closed and temperature controlled chamber, which realizes the functions of temperature control as well as acoustic and thermal isolation. In addition, a vacuum can be created in the chamber, which means that influences due to airborne noise, air currents and refractive index changes can be practically completely suppressed. Therefore, a key constraint in implementing the design of the NPMM-200 was the vacuum capability of all machine components.
In the optical industry, optical components with free-form surfaces play a central role in various applications, as they make optical systems significantly more powerful.
The manufacturing process of free-form optics often consists of successive measuring and processing steps. During the measurements, it is necessary that the sensor system is always aligned perpendicular to the surface of the optics, otherwise additional measurement deviations occur. At the same time, this means that the sensor system is not rigidly mounted on the nanomeasuring machine, but must be swiveled according to the local inclination of the measurement object. For this purpose, the Nanomeasuring Machine 1 (NMM-1) was extended by two rotational axes of motion within the framework of the research training group NanoFab, so that the sensor can be aligned orthogonally to the measurement object surface at any time. Thus, the excellent metrological properties of the NMM-1 could be extended to strongly curved surfaces. In order to take into account the imperfections of the mechanical swivel axes in the measurement, a reference measurement system is integrated. This allows the systematic and random path deviations of the rotation systems to be detected and the resulting undesired displacement of the measuring point to be corrected.
A major challenge in precision length metrology, due to the ever increasing size of measurement objects, lies in the growing measurement volumes combined with ever increasing precision requirements in the nanometer and subnanometer range. However, the scanning-stage principle typically used for these applications, as employed in the NMM-1 and NPMM-200, encounters fundamental limitations in the future measurement ranges due to two conflicting requirements: Measurement objects with increasing size and mass must be moved highly dynamically and precisely, together with a correspondingly large and massive machine table (approx. 450 kg for a measurement range of 700 mm x 700 mm x 100 mm). At the same time, the dynamics of the measurement must be increased compared to today's typical parameters in order to enable measurement times within an acceptable range. To address this problem, a concept for an NPMM with an inverse kinematic concept is being pursued at IPMS.
The principle approach consists of an inverse kinematic setup, where the large and heavy precision mirrors as well as the large and heavy measurement object are fixed in space. The measuring object is probed with a movable probe system whose position is measured with several laser interferometers. This measurement concept allows the moving mass to be reduced from over 400 kg (mirror + measurement object) to about 1 kg (miniature interferometer + probe). Thus, a very large measurement dynamic and positioning capability can be achieved.
High-precision distance measurement and positioning technology as a key component of precision manufacturing is constantly challenged by growing measuring ranges with simultaneously increasing accuracy requirements. High-precision interferometric length measurements are significantly influenced by the stability and accuracy of the underlying laser frequencies. These are subject to short-term ("frequency noise") and long-term fluctuations. At the nanopositioning machines of the TU Ilmenau, He-Ne lasers with a relative long-term stability in the range 10-8 - 10-9 are currently used as wavelength standards in interferometric length measurement. The IPMS has many years of experience in the development of frequency-stabilized He-Ne laser sources. Current research involves the development of low-noise dual-frequency laser sources and the traceability of the wavelength to the SI meter definition by using frequency comb technology.
Unlike continuous He-Ne laser sources, optical frequency combs (OFC) are mode-locked pulsed lasers, typically based on fiber lasers, that can be stabilized to a highly stable reference in the optical or radio frequency range (e.g., atomic clock or hydrogen maser). Lately commercially available optical frequency combs have opened up new possibilities in tracing back length measurements to the SI-meter definition. Frequency combs allow highly accurate frequency measurements over a wide spectral range. Furthermore the frequency of a metrology laser can be directly traced back to a primary standard (e.g. atomic clock) by creating a permanent link between this lasers and the frequency comb. Thus, the stability of the timing device (e.g., atomic clock via a GPS disciplined oscillator (GPSDO) with a relative frequency stability in the range of 10-12 ) can be transferred to a He-Ne gas laser by means of the frequency comb.
The "Metrobase" project, funded by the Carl Zeiss Foundation, intends to create a new metrological basis based on the frequency comb technology at the TU Ilmenau that will reduce the current measurement uncertainty of the nanomeasurement machine laser interferometers by up to three orders of magnitude. This is accomplished by stabilizing the helium-neon lasers of the NPMM-200 to a single comb line. Thus a new quality in the traceability of the unit of length will be achieved. The combination of frequency comb and nanopositioning technology will altogether lead to a higher level in the field of dimensional metrology at the TU Ilmenau and open up novel concepts for the coupling of frequency comb and nanometrology.
Coupling the metrology lasers of the NPMM-200 to an RF-referenced frequency comb enables highly accurate frequency determination and an enormous increase in the long-term stability of the He-Ne lasers. However, the frequency noise of the metrology lasers can only be inadequately captured by this approach. In the field of nanometrology, however, these short-term frequency fluctuations also play an important role in the uncertainty of length measurements.
The Institute for Process Measurement and Sensor Technology is therefore equipped with a so-called "Optical Reference System" (ORS). This laser system is based on a diode laser locked to an ultra-stable cavity. The ORS provides ultranarrow linewidths in the sub-Hz range with a simultaneous output power of 10 mW and relative frequency stabilities of better than 2∙10-15 in the range ≤ 1s. The ORS system can thus be used as a highly stable optical reference for the frequency comb but also as a stand-alone system for the interferometric measurement systems. In the future, a combination of both systems for nanometrological applications should equally ensure traceability and highest frequency stability.
Nano- and micro-coordinate measuring machines have been developed for the precise measurement of complex microcomponents. To detect the smallest structures, the probes used by these devices require probing spheres with diameters smaller than 300 µm. A significant uncertainty contribution in such measuring devices is caused by the shape of the probing sphere. At present, however, there is - with regard to the required uncertainty - no suitable method for characterizing such spheres.
Therefore, strategies for the precise measurement of microspheres are currently being developed and investigated at the institute. Among other things, a method based on surface scans with atomic force microscopes has been developed. With this, a precise and high-resolution characterization of the radius and roundness of a large circle of spheres with diameters smaller than 300 µm is possible. Further work is focused on the extension of the method for the characterization of the complete sphere, as well as the development of alternative methods.
For more than 40 years, research has been carried out at the Institute of Precision Metrology under the decisive leadership of Professors G. Jäger and E. Manske, research has been carried out on the development of fiber-coupled interferometers. The interferometers are supplied with laser light and the interferograms are scanned by means of optical fibers. In this way, only minimal power is transmitted to the measurement environment. At the same time, there is almost no interference of the measurement by external electromagnetic fields. Fiber optic scanning of the interferograms requires the generation of interference fringe patterns. For this purpose, one of the overlapping wavefronts is tilted in the millirad range. Technically, this is implemented by the special geometric design of the optical interfaces. As an alternative to fiber scanning, high-resolution scanning of the interference patterns is possible using matrix or line scan cameras. By algorithmic evaluation, measurement path resolutions and reproducibilities in the range of a few femtometers are thus possible. Current research work is concerned with the development of miniature interferometers that are modular in design and in which functionality is achieved by a minimal number of optical components. For this purpose, state-of-the-art optical component manufacturing processes are used.
The Nanofabrication Machine 100 serves as an important experimental platform for fundamental research in the field of tip- and laser-based nanofabrication for sub-10nm patterning on surfaces up to Ø100 mm. This platform enables the graduate program 'NanoFab' to push into new, forward-looking frontiers of nanofabrication with sub-nanometer reproducibility and uncertainty. The currently assembled AFM system can be used simultaneously for field emission scanning probe lithography (FESPL), enabling new nanostructuring methods to be investigated on the machine. It will support the research work of the DFG Graduate School Nano-Fab (GRK 2182/1) of 13 PhD students over three generations (9 years). The device is internationally unique and unprecedented and is based on many years of research of the SFB 622 (2002-2013).
Direct laser writing enables the patterning of wafers for electronic circuits, electronic and microsystem technology components but also the fabrication of components with micro to submicrometer dimensions . The advantage here is the elimination of the costly production of masks, which makes the manufacturing process less expensive and more ﬂexible .
In direct laser writing (DLW), the focused laser beam is directly directed onto a photoresist-coated substrate, and a positioning system can be used to induce relative motion between the laser beam and the sample . In this way, the specified structures can be exposed, resulting in structures as geometric shapes at the exposed locations, which consist of cured photoresist or trenches. Through a subsequent etching process, the structures are transferred from the photoresist to the substrate in the form of etch trenches .
To obtain a reaction in the photoresist of the coated substrate, a photoreaction, a certain amount of energy is required, which depends on the energy diﬀerence of the electronic ground state to the excited state in the initiator molecule of the photoresist. The energy required for this can be contributed by a photon . The energy can be provided by the absorption of one (Figure a) ) or by quasi-simultaneous absorption of two photons (Figure b) )with sufficient energy .
 Kombination von zweiphotonenbasiertem direktem Laserschreiben mit großflächiger und hochpräziser Nanopositionierung, Laura Weidenfeller.
 Kenneth H. Church, Charlotte Fore, und Terry Feeley. Commercial applications and review for direct write technologies. MRS Online Proceedings Library Archive, 624, 2000.
 Robert W. Boyd. Nonlinear Optics. Academic Press, 2003.
 Shoji Maruo, Osamu Nakamura, und Satoshi Kawata. Three-dimensional microfabrication with two-photon-absorbed photopolymerization. Optics Letters, 22 (2):132–134, 1997.