Franz Kreupl

Carbon nanotubes in interconnect applications

Carbon nanotubes with their outstanding electrical and mechanical properties are suggested as an interconnect material of the future. In this paper we will introduce nanotubes, compare their electrical properties with equivalent metal wires made of gold and describe our progress in process integration. Multi-walled carbon nanotubes are grown on 6-inch wafers in a batch process. The resulting nanotubes are evaluated with respect to their conductance as single multiwalled nanotubes and in their implementation as interconnects in vias and contact holes.

On the Applicability of Single-Walled Carbon Nanotubes as VLSI Interconnects

This paper presents a comprehensive study of the applicability of single-walled carbon nanotubes (SWCNTs) as interconnects in nanoscale integrated circuits. A detailed analysis of SWCNT interconnect resistance (considering its dependence on all physical parameters, as well as factors affecting the contact resistance), the first full 3-D capacitance simulations of SWCNT bundles for realistic very large scale integration (VLSI) interconnect dimensions, and a quantitative evaluation of the importance of inductive effects in SWCNT interconnects are presented. The applicability of carbon nanotube (CNT) based vias (vertical interconnects)-the most realizable CNT interconnects in the current state of the art-is addressed for the first time. It is shown that CNT interconnects can provide 30%-40% improvement in the delay of millimeter-long global interconnects. The applicability of CNT monolayers as local interconnects is found to be much more limited than that reported in the prior literature. Dense CNT bundle global interconnects are shown to offer a 4times reduction in power dissipation while achieving the same delay as optimally buffered Cu interconnects at the 22 nm node. This power saving increases to 8times at the 14 nm node. Furthermore, 3-D finite-element electrothermal simulations show that CNT bundles used as vias in between Cu metal layers can provide large improvement in metal interconnect lifetime by lowering the temperature of the hottest interconnects.

Carbon-based resistive memory

We propose carbon as new resistive memory material for non-volatile memories and compare three allotropes of carbon, namely carbon nanotubes, graphene-like conductive carbon and insulating carbon for their possible application as resistance-change material in high density non-volatile memories. Repetitive high-speed switching and the potential for multi-level programming have been successfully demonstrated.

A Perpendicular Spin Torque Switching based MRAM for the 28 nm Technology Node

We report on a novel spin torque select MRAM with perpendicular anisotropy (P-ST-MRAM). The P-ST concept offers superior scalability performance at the 28 nm technology node compared to the conventional in-plane spin torque MRAM (I-ST-MRAM). The critical programming currents (~ 30 muA) are low, allowing a cell layout as small as 6 F2. In addition, data retention is significantly better for P-ST enabling a non-volatile, high density product for the 28 nm node. Estimations on write performance promise high write endurance and high write speed. Circuit simulations with improved read circuit demonstrate array read access time ~ 30 ns at sensing currents ~ 10 muA.

Carbon / high-k Trench Capacitor for the 40nm DRAM Generation

Carbon is proposed as a new FEOL material with high conductivity and thermal stability for CMOS integration. Here the application of carbon-based electrodes for future DRAM cell capacitors is presented. Trench capacitors with high-k dielectrics have been realized, fulfilling the requirements for serial resistance, capacitance, leakage, reliability and temperature stability beyond the 40 nm technology node.

Reconfigurable Nanowire Electronics-Enabling a Single CMOS Circuit Technology

Reconfigurable nanowire transistors are multifunctional switches that fuse the electrical characteristics of unipolar n- and p-type field effect transistors (FETs) into a single universal type of four-terminal device. In addition to the three known FET electrodes the fourth acts as an electric select signal that dynamically programs the desired polarity. The transistor consists of two independent charge carrier injection valves as realized by two gated Schottky junctions integrated within an intrinsic silicon nanowire. The transport properties that provide unipolar n- and p-type behavior will be elucidated. Further, solutions to the major device challenges toward the implementation of these novel transistors at the circuit level are proposed, by exploiting specific nanowire geometries and dimensions. These include methods that deliver equal on-currents and symmetric transfer characteristics for n- and p-type, and that eliminate supra-linear output characteristics at low source-drain biases. We will further show that circuits built of these symmetric transistors successfully exhibit complementary operation. Finally, the prospects in building reconfigurable circuits and systems will be briefly summarized.

Technology and circuit optimization of resistive RAM for low-power, reproducible operation

Low-power, reproducible operation of Resistive RAM (RRAM) requires control of capacitive surge currents during write. We propose a fab-friendly TiN/conductive TaOx/HfO2/TiN RRAM with a built-in surge current reduction layer. It reduces worst case write current by 33% and fail bit count by 23× compared to conventional RRAM. A novel circuit to control surge current is demonstrated that improves write current by 40% and endurance by 63%. Switching, endurance and retention data for a 256kb chip with these concepts is presented.

High Performance X-Ray Transmission Windows Based on Graphenic Carbon

A novel x-ray transmission window based on graphenic carbon has been developed with superior performance compared to beryllium transmission windows that are currently used in the field. Graphenic carbon in combination with an integrated silicon frame allows for a window design which does not use a mechanical support grid or additional light blocking layers. Compared to beryllium, the novel x-ray transmission window exhibits an improved transmission in the low energy region ( 0.1 keV-3 keV) while demonstrating excellent mechanical stability, as well as light and vacuum tightness. Therefore, the newly established graphenic carbon window, can replace beryllium in x-ray transmission windows with a nontoxic and abundant material.

Design and properties of low energy x-ray transmission windows based on graphenic carbon

X-ray transmission windows for the low energy range, especially between 0.1 and 1 keV have been designed and fabricated based on graphenic carbon (GC) with an integrated silicon frame. A hexagonal and a bar grid support structure design have been evaluated. The bar grid design offers higher X-ray transmission and better visible light rejection than polymer-based windows, and allows vacuum encapsulation of silicon drift detectors (SDD). The high mechanical resilience of graphenic carbon is demonstrated by pressure cycle tests, yielding over 10 million cycles without damage. The data are complemented by bulge tests to determine a Youngs modulus for graphenic carbon of approximately 130 GPa. Additional finite-element simulation and Raman studies reveal that the mechanical stress is not homogeneously distributed, but reaches a maximum near the anchoring points of the free standing graphenic carbon membrane

Robust valley polarization of helium ion modified atomically thin MoS2

Atomically thin semiconductors have dimensions that are commensurate with critical feature sizes of future optoelectronic devices defined using electron/ion beam lithography. Robustness of their emergent optical and valleytronic properties is essential for typical exposure doses used during fabrication. Here, we explore how focused helium ion bombardement affects the intrinsic vibrational, luminescence and valleytronic properties of atomically thin MoS 2 . By probing the disorder dependent vibrational response we deduce the interdefect distance by applying a phonon confinement model. We show that the increasing interdefect distance correlates with disorder-related luminscence arising 180 meV below the neutral exciton emission. We perform ab-initio density functional theory of a variety of defect related morphologies, which yield first indications on the origin of the observed additional luminescence. Remarkably, no significant reduction of free exciton valley polarization is observed until the interdefect distance approaches a few nanometers, namely the size of the free exciton Bohr radius. Our findings pave the way for direct writing of sub-10 nm nanoscale valleytronic devices and circuits using focused helium ions.