Pragya R. Shrestha

Electron spin resonance scanning probe spectroscopy for ultrasensitive biochemical studies.

Electron spin resonance (ESR) spectroscopys affinity for detecting paramagnetic free radicals, or spins, has been increasingly employed to examine a large variety of biochemical interactions. Such paramagnetic species are broadly found in nature and can be intrinsic (defects in solid-state materials systems, electron/hole pairs, stable radicals in proteins) or, more often, purposefully introduced into the material of interest (doping/attachment of paramagnetic spin labels to biomolecules of interest). Using ESR to trace the reactionary path of paramagnetic spins or spin-active proxy molecules provides detailed information about the reactions transient species and the labels local environment. For many biochemical systems, like those involving membrane proteins, synthesizing the necessary quantity of spin-labeled biomolecules (typically 50 pmol to 100 pmol) is quite challenging and often limits the possible biochemical reactions available for investigation. Quite simply, ESR is too insensitive. Here, we demonstrate an innovative approach that greatly enhances ESRs sensitivity (>20000× improvement) by developing a near-field, nonresonant, X-band ESR spectrometric method. Sensitivity improvement is confirmed via measurement of 140 amol of the most common nitroxide spin label in a ≈593 fL liquid cell at ambient temperature and pressure. This experimental approach eliminates many of the typical ESR sample restrictions imposed by conventional resonator-based ESR detection and renders the technique feasible for spatially resolved measurements on a wider variety of biochemical samples. Thus, our approach broadens the pool of possible biochemical and structural biology studies, as well as greatly enhances the analytical power of existing ESR applications.

Characteristics of Resistive Memory Read Fluctuations in Endurance Cycling

We report on new fluctuation dynamics of the high resistance state of Hafnia-based RRAM devices after RESET. We observe that large amplitude fluctuations occur more frequently immediately after programming and their frequency of occurrence decays in the tens of microseconds. The fluctuation amplitude, on the other hand, does not decay noticeably over the entire millisecond read time. While post-programming instability and post-programming resistance dispersion have both been reported in the literature, the relaxation in the frequency of occurrence without a commensurate amplitude decay is new. Since picosecond pulses were used for our RESET operation, post-programming thermalization occurs on the nanosecond time scale. This clearly eliminates a thermally driven mechanism as the cause of the observed fluctuation behavior. Furthermore, reducing the READ voltage by tenfold does not have any effect and also eliminates read disturb as possible cause.

Energy control paradigm for compliance-free reliable operation of RRAM

We demonstrate reliable RRAM operation by controlling the forming energy via short voltage pulses (picosecond range) which eliminates the need for a current compliance element. We further show that the dissipated energy during forming and SET/RESET processes plays a critical role. The SET/RESET cycling endurance of thus formed devices is shown to also be dependent on the SET/RESET energy. Multiple-pulse forming is also investigated as a method to further tighten the control of forming energy with promising endurance results.

Dependence of the filament resistance on the duration of current overshoot

The characteristics of a conductive filament in HfO2 RRAM is shown to be dependent on the duration of the current compliance overshoot, which may occur during the filament formation process. In addition to the overshoot amplitude, the filament resistance is found to be affected by the duration of the overshoot caused by the parasitic capacitance.

Accurate Fast Capacitance Measurements for Reliable Device Characterization

The performance and reliability of highly scaled devices are becoming increasingly dominated by transient phenomena. Recently, fast capacitances versus voltage (CV) measurements have been gaining attention as a promising measurement tool to characterize the transient phenomena. However, fast CV has mainly been limited to monitoring stress-induced deviations in accumulation capacitance due, at least in part, to the inability to accurately measure the full CV. In this paper, we identify and mitigate the measurement considerations required to obtain a remarkably accurate correspondence between a complete fast CV measurement, from accumulation to inversion, and a conventional CV measurement on the same device. The results indicate that fast CV can be a potentially powerful tool for device characterization and reliability measurements.

Comparison of Nanomechanical Behavior of the Amorphous and Crystalline Phases of ALD HfO2

Integrating high-k HfO2 as gate insulator replacement into a state-of-the-art CMOS processes is subjecting the HfO2 film to multiple thermal cycles. Although ALD HfO2 is initially deposited in the amorphous phase at 250C, these multiple thermal cycles inherent to CMOS device processing cause a phase change and render the HfO2 film polycrystalline. Process integration engineering has to take into account these different phases of HfO2 and how they affect reliability and performance. Little is known about the different nanomechanical properties of the various phases of ALD HfO2. In this paper we report detailed nanoindentation measurements on the pure amorphous, mixed-phase and completely recrystallized phase of HfO2. We use nanoindentation for determining the important mechanical properties such as hardness and effective modulus and transmission electron microscopy (TEM), X-ray diffraction (XRD) and atomic force microscopy (AFM) for the analysis of the micro structural properties of the various phases of these ALD HfO2 films. The modulus and hardness are calculated from the loaddepth curve (1,2) and the contact area of the indent (3). A series of different film thicknesses were deposited using a Cambridge Nanotech Savannah 100 ALD reactor. Tetrakis(dimethylamido)hafnium (IV) was used as the precursor for hafnium and water was used as the oxidation source. The films were deposited at 200C, 185C and 120C. A portion of the wafers were subdivided by cleaving and annealed at 400C and 600C in nitrogen using the Solaris 150 rapid thermal annealing (RTA) system from SSI. The summary of the sample preparation is given in Table I. Using a Hysitron Triboindenter® equipped with a Berkovich tip, 20 indents were placed on each thin film. The load function consisted of a 2 second loading segment, 5 second hold, 1 second unload to 25% of the maximum load, 30 second hold to account for thermal drift, 1 second final unload and 2 second hold at zero load. The loads ranged from 5 to 10,200 μN and contact areas were measured from images obtained from a carefully calibrated Quesant® AFM incorporated into the Hysitron Triboindenter®. AFM micrographs of indents on a recrystallized sample are shown in Fig.1. The experimental results are shown in Fig 2 and the results were validated using simulations (4). Our measurements clearly demonstrate that the initially amorphous ALD HfO2 behaves very differently compared to its final polycrystalline phase. The amorphous HfO2 films are much harder than the crystallized HfO2 films. After high temperature annealing, the hardness and modulus drop significantly from the as-deposited amorphous films to the annealed recrystallized films. The effective modulus was 370 GPa for amorphous low temperature ALD HfO2 and decreased to 240 GPa after crystallization with RTA annealing. Similarly, the hardness measurements reveal a high value of 18 GPa for amorphous HfO2 and a decrease to 15 GPa following the transition temperature to polycrystalline HfO2. The exact nature of the amorphous or polycrystalline phase was corroborated by detailed TEM cross-section analysis.

Accurate RRAM transient currents during forming

The magnitude of overshoot current during forming has been shown to be a serious issue. Recently we showed that the overshoot duration is equally important in impacting device performance. Shorter duration overshoot in the range of ns yields better performance, suggesting extremely short forming pulse to be desirable. But investigation of such short forming transients is severely limited experimentally due to parasitic. In this study we demonstrate a technique to accurately de-embed these parasitic components yielding accurate forming current transients in the ps range, paving the road to careful study of the forming process.

Experimental Study of ALD HfO2 Deposited on Strained Silicon-on-Insulator and Standard SOI

HfO 2 films of 4 nm were deposited by atomic layer deposition on silicon-on-insulator (SOI) substrates with various amounts of intentionally introduced lattice strain and several film thicknesses. After postdeposition annealing (PDA), the samples were studied by Rutherford backscattering spectroscopy, Raman spectroscopy, high-resolution transmission electron microscopy (HRTEM), electron energy-loss spectroscopy (EELS), and X-ray photoelectron spectroscopy (XPS). The as-deposited HfO 2 film showed a good stoichiometry and thickness uniformity. The strain in strained SOI (sSOI) layers remained after high-temperature PDA at 1100°C. HRTEM images showed that, while HfO 2 films on standard nonstrained SOI became polycrystalline after PDA at 600°C, HfO 2 films on sSOI remained amorphous. The strain in the sSOI layer also suppressed the interlayer (IL) growth during PDA. The EELS and XPS results confirmed the interdiffusion across the HfO 2 /Si interface. The XPS data also showed that the formation of Hf-O-Si bonds depends on the SOI lattice strain and thickness. The SOI thickness is critical to reduce the formation of silicate in the IL.

Impact of RRAM Read Fluctuations on the Program-Verify Approach

The stochastic nature of the conductive filaments in oxide-based resistive memory resistive random access memory (RRAM) represents a sizeable impediment to commercialization. As such, program-verify methodologies are highly alluring. However, it was recently shown that the program-verify methods are unworkable due to strong resistance state relaxation after SET/RESET programming. In this letter, we demonstrate that resistance state relaxation is not the main culprit. Instead, it is fluctuation-induced false-reading (triggering) that defeats the program-verify method, producing a large distribution tail immediately after programming. The fluctuation impact on the verify mechanism has serious implications on the overall write/erase speed of RRAM.

Nanomechanical Properties of High-k Dielectrics Grown by Atomic Layer Deposition

K. Tapily, J. Jakes, D. S. Stone, P. Shrestha, D. Gu, H. Baumgart and A.A. Elmustafa Department of Electrical Engineering, Old Dominion University, Norfolk, Virginia 23529, USA Applied Research Center, Jefferson National Accelerator Facility, Newport News, Virginia, 23606, USA Department of Materials Science and Engineering, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA USDA Forest Products Laboratory, Madison, Wisconsin 53726, USA Department of Mechanical Engineering, Old Dominion University, Norfolk, Virginia 23529, USA