Zengtao Liu

Metal nanocrystal memories. I. Device design and fabrication

This paper describes the design principles and fabrication process of metal nanocrystal memories. The advantages of metal nanocrystals over their semiconductor counterparts include higher density of states, stronger coupling with the channel, better size scalability, and the design freedom of engineering the work functions to optimize device characteristics. One-dimensional (1-D) analyses are provided to illustrate the concept of work function engineering, both in direct-tunneling and F-N-tunneling regimes. A self-assembled nanocrystal formation process by rapid thermal annealing of ultrathin metal film deposited on top of gate oxide is developed and integrated with NMOSFET to fabricate such devices.

Metal nanocrystal memories-part II: electrical characteristics

This paper describes the electrical characteristics of the metal nanocrystal memory devices continued from the previous paper [see ibid., vol. 49, p. 1606-13, Sept. 2002]. Devices with Au, Ag, and Pt nanocrystals working in the F-N tunneling regime have been investigated and compared with Si nanocrystal memory devices. With hot-carrier injection such as the programming mechanism, retention time up to 10/sup 6/ s has been observed and 2-bit-per-cell storage capability has been demonstrated and analyzed. The concern of the possible metal contamination is also addressed by current-voltage (I-V) and capacitance-voltage (C-V) characterizations. The extracted inversion layer mobility and minority carrier lifetime suggest that the substrate is free from metal contamination with continuous operations.

Charge-based chemical sensors: a neuromorphic approach with chemoreceptive neuron MOS (C/spl nu/MOS) transistors

A novel chemoreceptive neuron MOS (C/spl nu/MOS) transistor with an extended floating-gate structure has been designed with several individual features that significantly facilitate system integration of chemical sensing. We have fabricated C/spl nu/MOS transistors with generic molecular receptive areas and have characterized them with various fluids. We use an insulating polymer layer to provide physical and electrical isolation for sample fluid delivery. Experimental results from these devices have demonstrated both high sensitivity via current differentiation and large dynamic range from threshold voltage shifts in sensing both polar and electrolytic liquids. We have established electrochemical models for both steady-state and transient analyses. Our preliminary measurement results have confirmed the basic design and operations of these devices, which show potential for developing silicon olfactory and gustatory units that are fully compatible with current CMOS technology.

Novel electrostatic repulsion forces in MEMS applications by nonvolatile charge injection

This paper demonstrates, for the first time, electrostatic repulsion forces between two isolating beams as a viable actuation mechanism in MEMS applications by nonvolatile charge injection. Devices integrating MEMS beams and EEPROM structures have been fabricated and an actuation force of /spl sim/0.2 /spl mu/N has been recorded across a 3 /spl mu/m gap for beams 360 /spl mu/m in length. Larger actuation forces can be achieved through smaller gaps. A capacitor-network model is presented for analyzing such systems. This scheme holds promises in complimenting attractive electrostatic actuation and also finds valuable applications in achieving wear-free micro-bearings, hinges and turbines.

A novel quad source/drain metal nanocrystal memory device for multibit-per-cell storage

Based on the 2-bit-per-cell metal nanocrystal memories, a novel quad source/drain device capable of 4 bits per cell data storage is demonstrated. Along with the new device structure, a reliable parallel read scheme with low V/sub DS/ is also proposed and verified for 4-bit-per-cell operations. The proposed read scheme requires 1.125 read operations on average to read out the 4 bits stored in a cell, while minimizing the read disturb and interference between the different storage bits.

Low programming voltages and long retention time in metal nanocrystal EEPROM devices

Conventional Si/Ge nanocrystal EEPROM devices have several advantages over conventional EEPROM with continuous floating gate. However, to maintain good retention characteristics the programming voltages (write and erase) cannot be scaled down easily. Interface and dopant fluctuations at the Si/Ge nanocrystal interface also cause device design difficulty. We demonstrate for the first time that metal nanocrystal EEPROMs can have much lower programming voltages by careful choice of the metal floating-gate workfunction. The interface fluctuations are at a minimum due to the metal density of states. The retention characteristic for metal nanocrystals is similar to that of Si nanocrystals in view of the Coulomb blockade. Process integration of the metal floating gate on thin tunneling oxide is greatly improved by the self-assembly step of nanocrystal formation. Energy minimization during self assembly will relax the interface stress and stabilize the metal structure to avoid penetration and contamination. Together with Schottky contact injection improvement, the introduction of metal nanocrystals can push deep submicron CMOS and EEPROM scaling for more technology generations.

Investigation on Process Dependence of Self-Assembled Metal Nanocrystals

We report the systematic characterization of metal nanocrystal formation on ultra-thin tunnel gate oxide (2∼3nm) for memory applications. To get a high density and small average size of nanocrystals, the process parameters including annealing temperature, initial film thickness, and substrate doping are investigated for Au, Ag, and Pt nanocrystal formation with Si nanocrystal structure as control samples. The observation of nanocrystal formation by scanning electron microscope (SEM) shows that annealing below melting temperature of deposited film contributes to the reshaping of nanocrystals, while the initial film thickness to actual nanocrystal growth. In addition, the Schottky charge effect from substrate doping is not negligible if the tunnel oxide is thin. Controlling the process parameters, Au, Ag, and Pt nanocrystals of 4.0×10 11 cm -2 , 2.8×10 11 cm -2 , and 2.4×10 11 cm -2 can be formed with mean size of 6.2nm, 6.6nm, and 8.0nm, respectively. The observation of nanocrystals by scanning transmission electron microscope (STEM) shows that nanocrystals are spherical and crystalline. Metal contamination to the Si/SiO 2 interface is also closely monitored with many process recipes of metal nanocrystal formation on 2∼3nm oxide showing atomically clean interface. Electrical evaluation of nanocrystal formation is carried out by C-V measurements of metal-oxide-semiconductor (MOS) capacitors with embedded metal nanocrystals.

The chemoreceptive neuron MOS transistors (C/spl nu/MOS): a novel floating-gate device for molecular and chemical sensing

We propose a novel chemoreceptive neuron MOS transistor (C/spl nu/MOS) with distinctive features that are crucial to molecular sensing system integration. We have fabricated C/spl nu/MOS transistors with generic molecular receptive areas and experimentally demonstrated high sensitivity from channel current changes and large dynamic range from subthreshold slope shifts in sensing polar and electrolytic liquids. Preliminary measurements have validated most of our assumptions about the performance of these devices and have indicated the feasibility in developing a micropower neuromorphic electronic interface based on existing CMOS technology.