Frank Streller

Angle-resolved environmental X-ray photoelectron spectroscopy: A new laboratory setup for photoemission studies at pressures up to 0.4 Torr

The paper presents the development and demonstrates the capabilities of a new laboratory-based environmental X-ray photoelectron spectroscopy system incorporating an electrostatic lens and able to acquire spectra up to 0.4 Torr. The incorporation of a two-dimensional detector provides imaging capabilities and allows the acquisition of angle-resolved data in parallel mode over an angular range of 14° without tilting the sample. The sensitivity and energy resolution of the spectrometer have been investigated by analyzing a standard Ag foil both under high vacuum (10(-8) Torr) conditions and at elevated pressures of N(2) (0.4 Torr). The possibility of acquiring angle-resolved data at different pressures has been demonstrated by analyzing a silicon/silicon dioxide (Si/SiO(2)) sample. The collected angle-resolved spectra could be effectively used for the determination of the thickness of the native silicon oxide layer.

Scalable Production of Sensor Arrays Based on High-Mobility Hybrid Graphene Field Effect Transistors.

We have developed a scalable fabrication process for the production of DNA biosensors based on gold nanoparticle-decorated graphene field effect transistors (AuNP-Gr-FETs), where monodisperse AuNPs are created through physical vapor deposition followed by thermal annealing. The FETs are created in a four-probe configuration, using an optimized bilayer photolithography process that yields chemically clean devices, as confirmed by XPS and AFM, with high carrier mobility (3590 ± 710 cm2/V·s) and low unintended doping (Dirac voltages of 9.4 ± 2.7 V). The AuNP-Gr-FETs were readily functionalized with thiolated probe DNA to yield DNA biosensors with a detection limit of 1 nM and high specificity against noncomplementary DNA. Our work provides a pathway toward the scalable fabrication of high-performance AuNP-Gr-FET devices for label-free nucleic acid testing in a realistic clinical setting.

Next-Generation Nanoelectromechanical Switch Contact Materials: A Low-Power Mechanical Alternative to Fully Electronic Field-Effect Transistors.

The deficiency of existing electrical contact materials is currently a significant impediment to the commercialization of nanoelectromechanical (NEM) contact switches?a low-power ?beyond complementary metal-oxide semiconductor (CMOS) technology?. NEM switches using traditional metallic electrical contact materials, even those composed of inert, noble metals such as gold (Au) and platinum (Pt), demonstrate premature failure due to either their adhesiveness or catalytic activity, leading to a buildup of insulating interfacial contaminants. Commercially viable NEM switches demand novel contact materials along with efficient methods to evaluate the performance of these materials.

In situ oxygen plasma cleaning of microswitch surfaces-comparison of Ti and graphite electrodes

Ohmic micro- and nanoswitches are of interest for a wide variety of applications including radio frequency communications and as low power complements to transistors. In these switches, it is of paramount importance to maintain surface cleanliness in order to prevent frequent failure by tribopolymer growth. To prepare surfaces, an oxygen plasma clean is expected to be beneficial compared to a high temperature vacuum bakeout because of shorter cleaning time (<5 min compared to ~24 h) and active removal of organic contaminants. We demonstrate that sputtering of the electrode material during oxygen plasma cleaning is a critical consideration for effective cleaning of switch surfaces. With Ti electrodes, a TiO x layer forms that increases electrical contact resistance. When plasma-cleaned using graphite electrodes, the resistance of Pt-coated microswitches exhibit a long lifetime with consistently low resistance (<0.5 Ω variation over 300 million cycles) if the test chamber is refilled with ultra-high purity nitrogen and if the devices are not exposed to laboratory air. Their current–voltage characteristic is also linear at the millivolt level. This is important for nanoswitches which will be operated in that range.

Development and Assessment of Next-Generation Nanoelectromechanical Switch Contact Materials*

The deficiency of existing electrical contact materials is currently a significant impediment to the commercialization of nanoelectromechanical (NEM) contact switches - a low power “beyond CMOS technology”. NEM switches utilizing traditional metallic electrical contact materials, even those composed of inert, noble metals such as Au and Pt, demonstrate premature failure due either to their adhesiveness, or to catalytic activity leading to buildup of insulating interfacial contaminants. Commercially viable NEM switches demand novel contact materials along with efficient methods to evaluate the performance of these materials. This study highlights the development of one promising switch contact material: platinum silicide (PtxSi) thin films formed from a-Si and Pt precursors. Using CMOS-compatible fabrication methods, the stoichiometry of these silicide films was varied between PtSi and a Pt2Si/Pt3Si mixture by controlling the thickness of the precursor a-Si and Pt films. The stoichiometry had a significant impact on the mechanical and electrical characteristics of the films. Pt-rich silicides demonstrated metallic surface conductivity while electrical contact conductance was reduced with higher Si film content. We then present a novel, high-throughput electrical contact screening method using atomic force microscopy (AFM) to cycle nanoscale electrical contacts for up to two billion contact cycles under NEM switch-like conditions. The ability of this technique to resolve degradation of nanoscale Pt-Pt interfaces is demonstrated and, along with its ease of adoption, motivates future interrogation of PtxSi and other promising switch contact materials using this method.

Valence Band Control of Metal Silicide Films via Stoichiometry

The unique electronic and mechanical properties of metal silicide films render them interesting for advanced materials in plasmonic devices, batteries, field-emitters, thermoelectric devices, transistors, and nanoelectromechanical switches. However, enabling their use requires precisely controlling their electronic structure. Using platinum silicide (PtxSi) as a model silicide, we demonstrate that the electronic structure of PtxSi thin films (1 ≤ x ≤ 3) can be tuned between metallic and semimetallic by changing the stoichiometry. Increasing the silicon content in PtxSi decreases the carrier density according to valence band X-ray photoelectron spectroscopy and theoretical density of states (DOS) calculations. Among all PtxSi phases, Pt3Si offers the highest DOS due to the modest shift of the Pt5d manifold away from the Fermi edge by only 0.5 eV compared to Pt, rendering it promising for applications. These results, demonstrating tunability of the electronic structure of thin metal silicide films, suggest that metal silicides can be designed to achieve application-specific electronic properties.

Novel materials solutions and simulations for nanoelectromechanical switches

Nanoelectromechanical (NEM) switches are a candidate to replace solid-state transistors due to their low power consumption. However, the reliability of the contact interface limits the commercialization of NEM switches, since for practical purposes, the electrical contact should be able to physically open and close up to a quadrillion (1015) times without failing due to adhesion (by sticking shut) or contamination (reducing switch conductivity). These failure mechanisms are not well understood, and materials that exhibit the needed performance have not yet been demonstrated. This study presents the development of platinum silicide (PtxSi) as a promising NEM switch contact material. Using controlled solid-state diffusion of thin films of amorphous silicon and platinum, PtxSi was formed over a range of stoichiometries (1≤x≤3). The platinum-rich silicide phase (Pt3Si) may be a particularly ideal contact material for NEM switches due to its combination of mechanical robustness with metal-like conductivity. We then present a novel, high-throughput contact material screening method for NEM contact materials based on atomic force microscopy (AFM) that enables billions of contact cycles in laboratory timeframes for arbitrary material pairs. Self-mated Pt contacts showed more than three orders-of-magnitude increase in contact resistance after 2·109 cycles due to the growth of insulating tribopolymer. Finally, we present density functional theory (DFT) and molecular dynamics (MD) based studies to understand tribopolymer formation and growth. These calculations show that irreversible stress-induced polymerization processes are strongly affected by the ability of the molecule to displace laterally. Additionally, lower interaction energies between the model organic molecules and the PtxSi surface compared to Pt are found. This combination of experimental and theoretical methods in the framework of a materials genome effort aims to ultimately lead to accelerated discovery of suitable contact materials for NEM switches and to their commercialization.

Effectiveness of oxygen plasma versus UHV bakeout in cleaning MEMS switch surfaces

MEMS switch reliability studies have identified contamination of the electrical contact surfaces as an important source of failure. It has been previously reported that the formation of electrically insulating organic deposits degrades switch performance by increasing contact resistance. Previously, we performed a high temperature bakeout under ultra-high vacuum (UHV) to desorb possible contaminant sources from Pt electrodes prior to cycling tests in a MEMS switch. In this work, an in-situ plasma-cleaning method of Pt is assessed for its ability to actively remove adsorbed contaminants. Microswitch contact resistance is measured before and after UHV bakeout or plasma cleaning of the switches. While plasma cleaning is seen to reduce water contact angle, it raises contact resistance. The likely reason is the formation of a metal oxide on the switch surface associated with sputtering of the plasma electrode material.