IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 58, NO. 4, APRIL2009 997 Progress Towards the Electron Counting Capacitance Standard at PTB Hansjörg Scherer, Sergey V. Lotkhov, Gerd-Dietmar Willenberg, and Benedetta Camarota Abstract—We report on the progress and new results in the setup of the electron counting capacitance standard (ECCS) at Physikalisch-Technische Bundesanstalt (PTB), Braunschweig, Germany. Considerable progress has been made in the opera- tion of the single-electron circuit and in the incorporation of a cryogenic capacitor into the millikelvin refrigerator setup. The single-electron tunneling (SET) circuit comprised a five-junction, single-electron R-pump that showed an electron retention time of more than 600 s, and the circuit was successfully used for the deterministic shuttle transfer of charge packets consisting of few electrons. In addition, the cryogenic capacitor was tested, and its performance was found to be suitable for a high-precision ECCS experiment. Index Terms—Capacitance, charge transfer devices, cryogenic electronics, tunnel effect, units (measurement). I. I NTRODUCTION T HE PRINCIPLE of the electron counting capacitance standard (ECCS) experiment, which was pioneered and—up to now—solely demonstrated by the National Institute of Standards and Technology (NIST) [1], [2], Boulder, CO, is to charge a capacitor by a well-known number of electrons and to measure the voltage across the capacitor electrodes by using a Josephson voltage standard. Relating the capacitor’s impedance to a quantum Hall resistance realizes the so-called quantum metrology triangle (QMT). This provides a consis- tency check for the three electrical quantum effects used in metrology, i.e., the Josephson, the quantum Hall, and the single- electron tunneling (SET) effect. A significant impact on the fundamental questions of quantum metrology will arise if the ECCS is performed with a relative uncertainty of a few parts in 10 7 or better [3], and this is currently being pursued at Physikalisch-Technische Bundesanstalt (PTB), Braunschweig, Germany. We have recently achieved considerable progress in the optimization and characterization of the two key compo- nents, namely, the SET circuit and the cryogenic capacitor (cryocap) [4]. We report experimental results from charac- terization measurements on the SET circuit and the cryocap. In addition, the measurement results from the cryocap setup allowed preliminary estimates of the corresponding uncertainty contributions. Manuscript received June 2, 2008; revised August 13, 2008. First published November 25, 2008; current version published March 10, 2009. This work was conducted within the EURAMET joint research project and supported by the European Community’s Seventh Framework Programme ERA-NET Plus under Grant Agreement 217257 and is part of Project “REUNIAM,” no. T1.J1.3. The Associate Editor coordinating the review process for this paper was Mr. Thomas Lipe. The authors are with Physikalisch-Technische Bundesanstalt, 38116 Braunschweig, Germany (e-mail: hansjoerg.scherer@ptb.de). Digital Object Identifier 10.1109/TIM.2008.2006959 Fig. 1. Layout scheme of the SET circuit. The sample chip contains a five- junction R-pump and a SET transistor. The pad island can electrically be contacted on-chip by closing a cryogenic needle switch. Fig. 2. Scanning electron micrograph of the R-pump section of the SET chip. The sample was prepared by three-angle evaporation technique. The positions of the Cr strip resistors and the five in-series Al-AlOx-Al tunnel junctions (arrows) of the pump are indicated. The junction area sizes were about 50 × 60 nm 2 (for the overlap of the microstrips, see the inset). II. SET CIRCUIT A. Sample Layout and Fabrication Our new SET circuits are comprised of a five-junction single- electron R-pump and a SET transistor (see Fig. 1 for the circuit scheme). The pump is equipped with on-chip Cr microstrip resistors on both sides (see also Fig. 2). These resistors (each R Cr ≈ 70 kΩ) suppress the unwanted cotunneling events and thus effectively increase the transfer accuracy in the R-pump [5]. In addition, they also enhance the electrical robustness of the SET circuit against damages that may occur during wire bonding or handling. One side of the pump is connected to a small circle-shaped pad island with a 90-μm diameter. The pad can be connected by a magnetically driven cryogenic needle switch. Thus, preliminary characterization and tuning of the pump can be carried out by measuring its I (V ) characteristics via the closed switch contact. In the final ECCS charging experiment, the switch will provide an electrical connection between the cryocap and the pump. 0018-9456/$25.00 © 2008 IEEE