PHYSIKALISCH-TECHNISCHE BUNDESANSTALT. The Clean Room Center of PTB

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1 info sheet PHYSIKALISCH-TECHNISCHE BUNDESANSTALT The Clean Room Center of PTB

2 Clean Room Center The Clean Room Center of PTB is used to fabricate micro- and nanoelectronic circuits for electrical quantum metrology, to advance microand nanometrology, to perform ultra-precise angle and coordinate measurements, and to push the limits of optical metrology. Electrical precision measurements increasingly rely on the use of quantum standards whose precision is not limited by fabrication tolerances. Using quantum standards, electric units can be traced to the fundamental constants of nature, which are independent of time and space. Thus, quantum standards are universal gauges. Already today, the national metrology institutes use electrical quantum standards routinely to reproduce the units of voltage and resistance. Currently, quantum voltage and resistance standards are also being developed for industrial use. Quantum standards of the electric current are intensively investigated, driven by the intention of the Meter Convention to redefine the SI base unit ampere by linking it to the elementary charge. Quantum sensors, such as single-charge detectors and SQUIDs, are important measuring instruments due to their high sensitivity. The basis for all these metrological applications is laid by state-ofthe-art clean-room technology required for the fabrication of the micro- and nanoelectronic circuits that underpin today s electrical metrology. n Highly integrated Josephson Voltage Standards The Josephson voltage standard is the quantum standard for the representation of the volt. It consists of a series circuit of Josephson tunneling junctions which are fabricated by the deposition and patterning of superconducting thin films in the Clean Room Center. If a microwave is coupled into the Josephson junctions at low temperatures, quantized voltage steps are generated with values that only depend on fundamental constants and the frequency of the microwave. The quantized voltages have a relative uncertainty smaller than Series circuits with junctions for the generation of voltages of up to 10 V can be fabricated in the Clean Room Center. n Programmable Josephson voltage standard with junctions fabricated using SNS technology. The circuit generates an output voltage of 10 V. Quantum Hall Devices Quantum Hall devices are used to reproduce the unit of electrical resistance. The devices consist of a semiconductor hetero-structure fabricated by molecular beam epitaxy in the clean room centre. The two-dimensional electron gas in these structures shows quantized resistance values h / (ie 2 ) in high magnetic fields and at low temperatures (h Planck s constant, e elementary charge, i integer number). Resistance values can be reproduced with an uncertainty of approx in direct currents measurements. Moreover, if appropriate shielding is implemented, quantum Hall devices can also be used for highly precise ac measurements. Combined with optimized impedance bridges the unit of capacitance can be reproduced with an uncertainty below 10 8 using shielded quantum Hall devices. n Quantum Hall device.

3 Single-electron circuit with superconducting (S) leads and a normal conducting (N) charge island. Metallic single-electron Circuits The electric current can be linked to the elementary charge if single electrons are moved through a conductor in a controlled way. In the Clean Room Center, very small metallic tunneling junctions can be fabricated having sizes below 100 nm. At extremely low temperatures in the mk range, such structures exhibit the Coulomb blockade effect, which allows single electrons to be transferred at a selected frequency. By combining normal conductors and superconductors, unwanted transport processes can be suppressed to reduce the transfer error rate. n Semiconductor single-electron Pumps Semiconductor quantum circuits can be fabricated using molecular beam epitaxy and nanopatterning. Such circuits can be operated as single-electron pumps, in which an integer number of electrons is transferred in each cycle of a radiofrequency drive signal. Pumping operation can be realized at GHz frequencies generating comparatively large quantized currents in the na range. Semiconductor single-electron pumping is a promising concept for a scalable quantum standard of current. n Electron microscope image of a semiconductor single-electron pump: transport channel (colored green), gate electrodes (violet, yellow). Integrated Quantum Circuits Conventional measuring techniques are not able to determine the precision of single-electron transport in quantum circuits. Therefore, quantum sensors are used, which need to be integrated with single-electron sources on the same chip. If several single-electron sources are connected in series, the source of possible transfer errors can be identified. With the nanotechnology available in the Clean Room Center, integrated quantum circuits with several semiconductor single-electron pumps and metallic single-electron detectors can be fabricated. In order to increase the current, parallel circuits of singleelectron sources can be assembled. n Four semiconductor single-electron pumps connected in series. The charge transport is monitored by three single-electron detectors.

4 Reference Masks for Lithography The continuous miniaturization and increasing complexity of structures in the lithographical production process of highly integrated electronic components is a demanding challenge for the traceability of the dimensional measurements of such structures. This is particularly true for the characterization of structures on lithography masks. Calibrations of the widths and coordinates of structures on the masks require uncertainties in the nanometer range. With the help of PTB s electron optical metrology system (EOMS), e. g. calibrations of linewidth standards are performed for the mask industry. These measurements are refined by additional optical and scanning probe microscopy as well as by Monte Carlo simulations of the contrast of the EOMS signal at the line edges. n Optical Measurement Methods The ongoing quest for miniaturization in electronics, optics and mechanics requires an adequate measurement technology that is able to cope with the increasing challenges. Optical techniques for the dimensional measurement of structural parameters, e. g., width and pitch, have basic advantages because they are fast, contact-free and nondestructive. In the Clean Room Center, different optical measuring devices are provided, e. g., a UV microscope and an optical diffractometer, capable of performing dimensional measurements in the micrometer and submicrometer regimes. The sensitivity to contamination of the delicate samples requires the measurements to be performed in a clean room. n Layer Thickness and Surface Texture Metrology Surfaces of diamond-turned or super-polished metals, semiconductors and glass ceramics of, e. g., laser mirrors, wafer substrates and X-ray optics can be produced with nanometer surface roughness values. The measurement of such surfaces requires the application of three-dimensional measurement methods with improved lateral (area-based) and vertical resolution. In addition to standard interference microscopes, which allow the traceability to be realized to the unit of length, scanning-probe-based measurement methods and light-scatteringbased methods are applied. Furthermore, a spectroscopic ellipsometer with variable angle range is used for the calibration of layer thickness standards. For these calibrations, customers often require the work to be done under clean-room conditions. n Calibration of Micro- and Nanostructures Scanning probe microscopes have been established as important tools in different areas of nanotechnology. To use these instruments for purposes that go beyond mere imaging, i. e. as quantitative measurement instruments, a calibration of their scanning axes as well as exact knowledge of the effective interaction forces between the probe tip and the sample is necessary. In this context, different metrological scanning probe microscopes and special probe tips were developed which are operated in the Clean Room Center. Instruments of different designs and with different probing methods (contact and non-contact mode, probes for 2.5D and 3D probing) and scanning ranges are available. These measuring instruments are also increasingly applied for the calibration of standards which are necessary for quality control in production processes in micro- and nano-technology. On standards with high quality, uncertainties of U (k = 2) < 1 nm for step height calibrations for h = 2 µm and U (k = 2) < 10 pm for pitch calibrations can be obtained. n UV microscope for measurement of the structures on photo masks.

5 Coordinate Metrology For calibrations of the dimensions and form deviations of precision engineering components like, e. g., parts from fuel injection pumps, a high temperature stability and a close realization of the reference temperature for length measurements at 20 C is necessary. These conditions are available in the Clean Room Center. The reference standards are calibrated with small uncertainties and, in this way, make quality improvements in automobile supply industries feasible. Indirectly, they thus also contribute to a reduction of the CO 2 emissions in road traffic. Another important field of application is the dimensional calibration of primary standards like, e. g., piston-cylinder pressure standards. Dimensional measurements on prismatic and cylindrical standards can be realized with the measuring instruments operated in the Clean Room Center with the required measurement uncertainties. Many important standards are made of non-conducting and, thus, electrostatically sensitive materials. Measurements on these materials benefit in particular from the clean-room environment. n Precision Measurement of Optical Radiation In photometry and applied radiometry, one of the most urgent requests from customers is the simultaneous determination of the very different sensitivities of single-element and matrix detectors. This has to be accomplished by laser-based methods. The dust-free atmosphere of the Clean Room Center guarantees a perturbation-free, stable propagation of laser beams inside and outside of optical resonators, windows, lenses or the micrometer-sized surfaces of optical fibers used as lightguiding elements. The superb conditions are prerequisites for precision measurements of the optical radiative power and the generation of power-stabilized laser radiation fields. n Wavelength-tunable Ti:Sa laser with frequency doubling unit in the TULIP facility for the calibration of the irradiance-based sensitivity of photometers and radiometers in the short and long wavelength spectral region. Comparator MFU 110 for the precise determination of dimensional measurands on cylindrical standards. Angle Metrology For measurement and positioning tasks in precision manufacturing and in precision engineering, the reduction of the uncertainty for the determination of the plane angle is of increasing importance. Incremental angle measuring systems are the most accurate measuring systems applied for this purpose in industry. These systems, as well as the calibration of high-resolution electronic autocollimators, require measurements with highest accuracy. The precision angle comparator WMT 220 was installed for these tasks in the Clean Room Center and is applied for challenging calibration tasks, industrial cooperation projects and also for international comparison measurements. The comparator performance benefits from the very good temperature stability in the Clean Room Center and allows calibrations of arbitrary plane angles to be realized in the range up to 360 with uncertainties of only n Chip-scale Traps for Quantum Information and quantum-optical Precision Measurements Miniaturized traps for neutral atoms and single ions have found widespread application in the past few years, e. g. for quantum information processing, atom interferometry and for the investigation of novel quantum many-particle systems. In the framework of the Cluster of Excellence QUEST, chip scale traps are produced in the Clean Room Center which are used in the QUEST Institute at PTB and at the Leibniz Universität Hannover for the quantum information processing with single trapped ions. The metallic structures produced by means of optical lithography are distinguished by the integration of microwave conductive structures for the realization of novel control mechanisms for trapped ions. With the same technology, neutral atom chips are developed for atom-interferometric experiments at QUEST. n Test sample of an ion trap structure produced in the Clean Room Center. This structure has been devised by C. Ospelkaus at the National Institute of Science and Technology (NIST), USA, and is used here to characterize the fabrication process.

6 The Building Infrastructure and Contacts The Clean Room Center of PTB provides a well-suited environment with clean-room conditions, reduced mechanical vibrations, and a low audio noise level. The temperature and humidity are kept stable. The central clean room, 800 m 2 in size, can be divided in ten segments with different clean-room classes and environmental conditions. Central facilities have been established for the fabrication of micro- and nanostructures. Various electron beam and optical lithography systems exist, which allow direct writing of ultra-small structures and the writing of masks with feature sizes as small as 10 nm. Metal, semiconductor, and insulator thin films can be deposited, e. g., using a molecular beam epitaxy machine that allows the growth of ultra-clean semiconductor heterostructures. Lateral patterning is performed using various etching techniques. For the analysis of the micro- and nanostructures, high-resolution scanning electron and scanning force microscopes are available. The work in the Clean Room Center is performed in collaboration with universities, research institutes, other national metrology institutes, and industry partners located across Europe and around the globe. The joint work is funded by the European Union, the German federal government, the German National Science Foundation, the Volkswagen Foundation, and other national and international research funding organizations. The Clean Room Center is intensively used to carry out projects of the European Metrology Research Programme, in which national metrology institutes from Europe have joined forces to address the metrology needs of Europe in a coordinated effort. In the frame of PTB s mission, the Clean Room Center with its state-of-the-art technical infrastructure is available for joint work with external partners now and in the future. n Technical Infrastructure, basic Facts n Clean-room size 48 m x 17 m n Clean-room class 100 und 1000 n Air circulation 200/h n Average air velocity 20 cm/s n Temperature stability 0.1 K n Available temperature range 20 C to 23 C n Relative humidity 40 % to 55 % n Audio noise level < 45 db(a) n Oscillation amplitude of the clean-room floor < 0.3 μm for > 2 Hz < 0.5 μm for > 100 Hz Central Utilities n Ultra-clean water n Cooling water n Dust collection system n Handling vacuum n Ultra-clean gases n Ultra-clean aceton n Liquid nitrogen n Pressurized nitrogen n Ultra-clean pressurized air n Helium recovery n Emergency power supply Division 2: Electricity Head of Division: Dr. Uwe Siegner Phone: uwe.siegner@ptb.de Division 4: Optics Head of Division: Dr. Fritz Riehle Phone: fritz.riehle@ptb.de Division 5: Precision Engineering Head of Division: Dr. Harald Bosse Phone: harald.bosse@ptb.de Physikalisch-Technische Bundesanstalt Press and Information Office Bundesallee Braunschweig As of: March 2012