S. T. Yan

Electron Charging and Discharging Effects of Tungsten Nanocrystals Embedded in Silicon Dioxide for Low-Voltage Nonvolatile Memory Technology

Spherical and well-separated tungsten nanocrystals embedded in the SiO 2 layer are demonstrated for the low-voltage nonvolatile memory device. The tungsten dots are formed, based on the thermal oxidation of the tungsten silicide, with a mean size and aerial density of 4.5 nm and 3.7 × 10 1 1 /cm 2 , respectively. A pronounced capacitance-voltage hysteresis is observed with a memory window of 0.95 V under the 3 V programming voltage. Also, the endurance of the memory device is not degraded up to 10 6 write/erase cycles.

A Novel Approach of Fabricating Germanium Nanocrystals for Nonvolatile Memory Application

A nonvolatile memory device embedded with Ge nanocrystal dots is fabricated by the thermal oxidation of Si 0 . 8 Ge 0 . 2 combined with a rapid thermal annealing at 950°C in N 2 gas. The tunnel oxide in the nonvolatile memory is controlled to be 4.5 nm thick and embedded with 5.5 nm Ge nanocrystals. A low operating voltage, 5 V, is implemented and a significant threshold-voltage shift, 0.42 V, is observed. When the electrons are trapped in the Ge nanocrystals, the effect of Coulomb blockade prevents the injection and storage of more electrons and decreases the leakage current. Also, the retention characteristics are tested to be robust.

A distributed charge storage with GeO2 nanodots

In this study, a distributed charge storage with GeO2 nanodots is demonstrated. The mean size and aerial density of the nanodots embedded in SiO2 are estimated to be about 5.5 nm and 4.3×1011 cm−2, respectively. The composition of the dots is also confirmed to be GeO2 by x-ray absorption near-edge structure analyses. A significant memory effect is observed through the electrical measurements. Under the low voltage operation of 5 V, the memory window is estimated to ∼0.45 V. Also, a physical model is proposed to demonstrate the charge storage effect through the interfacial traps of GeO2 nanodots.

Quasisuperlattice storage: A concept of multilevel charge storage

A concept of the quasisuperlattice storage has been demonstrated in this study. Under suitably operated voltage, two apparent states of charge storage can be distinguished. The memory effects are due to the multilevel storage in the quasisuperlattice. Also, the a-Si quantum wells provide a feasible design for the 2 bit per cell nonvolatile memory devices. The operation of the 2 bit per cell needs to be performed by Fowler–Nordheim tunneling instead of conventional channel hot electron injection. Additionally, the dual read operation of the source and drain sides for conventional SONOS 2bit∕cell device is not necessary, which simplifies the circuit design engineering.

Moisture-Induced Material Instability of Porous Organosilicate Glass

The mechanism of leakage current of porous organosilicate glass (POSG) with O 2 plasma ashing is investigated. The O 2 plasma ashing often deteriorates the POSG film, causing moisture uptake. We find that the leakage-current mechanism of POSG transforms from Schottky emission into ionic conduction after O 2 plasma treatment. The mobile ions (H + ,OH - ) supported by absorbing moisture move easily when an external electric field is applied to POSG film, which leads to the ionic conduction leakage current across the moisture-absorbing POSG film. Hence, the leakage current density is drastically increased about four to five orders of magnitude compared with untreated POSG film.

CMP of low-k methylsilsesquiazane with oxygen plasma treatment for multilevel interconnect applications

The effect of oxygen plasma treatment on methylsilsesquiazane (MSZ) dielectric was investigated for chemical mechanical planarization (CMP) process. Oxygen plasma treatment was implemented before CMP. Experimental resultshave shown that the polishing rate of MSZ film with O 2 plasma pretreatment is increased as much as two order of magnitude more than that of MSZ without O 2 plasma pretreatment. Moreover, the electrical properties of post-CMP MSZ are close to those of an as-cured MSZ. These results indicate that the modified surfaces resulting from O 2 plasma treatment increase the polishing rate of MSZ. After polishing, the MSZ film still maintains a low-k duality.

Memory effect of oxide/SiC:O/oxide sandwiched structures

The memory effects of the oxide/oxygen-incorporated silicon carbide (SiC:O)/oxide sandwiched structure were investigated. The memory window is decreased with the increase of the oxygen content in the SiC:O film due to the reduction of dangling bonds. A concise model is proposed to explain the reduction of dangling bonds with increasing oxygen content. Also, a higher breakdown voltage is observed with less oxygen content in the SiC:O film, which is attributed to the high barrier height induced by electron trapping in the SiC:O film.

Study on SONOS nonvolatile memory technology using high-density plasma CVD silicon nitride

With the replacement of silicon nitride in an oxide/nitride/oxide (ONO) gate-stacked structure, trap-rich high-density plasma chemical vapor deposited (HDPCVD) SiN x shows a more significant threshold-voltage shift (memory window) than that of conventional low pressure (LP)CVD Si 3 N 4 . Also, low-temperature (200°C) deposited HDPCVD silicon nitride shows a good retention characteristic, the same as high-temperature (780°C) LPCVD Si 3 N 4 . With the optimization of thickness in the gate-stacked ONO structure, low-voltage and reliable operation, lower than 5 V. is realizable.

Memory effect of oxide/oxygen-incorporated silicon carbide/oxide sandwiched structure

The memory effects of the oxide/oxygen-incorporated silicon carbide ~SiC:O!/oxide sandwiched structure were investigated. The memory window is decreased with the increasing of the oxygen content in the SiC:O film due to the reduction of dangling bonds. A concise model is proposed to explain the reduction of dangling bonds with increasing oxygen content. Also, a higher breakdown voltage is observed with less oxygen content in the SiC:O film, which is attributed to the high barrier height induced by electron trapping in the SiC:O film.

Quasi-Superlattice Storage A Concept of Multilevel Charge Storage

In this work, a novel concept of quasi-superlattice storage (QS 2 ) is demonstrated. Under a suitable operating voltage, two apparent states of charge storage can be distinguished. The memory effects are due to the multilevel charge storage within the quasi-superlattice. The multilevel charge storage provides a feasible design for the 2-bit-per-cell nonvolatile memory devices. Also, the leakage behavior of the quasi-superlattice structure has also been characterized by current-voltage measurements at room temperature and low temperatures. The resonant tunneling-like leakage characteristic is observed at low temperatures. A concise physical model is proposed to characterize the leakage mechanism of tunneling for the quasi-superlattice structure, and this suggests that consideration of the operating voltage for the 2-bit-per-cell nonvolatile memory device needs to be taken into account.