Discrete nitride program charge loss in a small-area SONOS Flash memory cell during retention is observed. Our measurement shows that a retention V t of a programmed SONOS cell exhibits a stepwise evolution with retention time. Individual single-program charge-loss-induced threshold voltage shifts (ΔV t ) are characterized. We find the following: 1) The magnitude of ΔV t exhibits an exponential distribution, which is believed due to a current-path percolation effect caused by random program charges and substrate dopants, and 2) program-state V t retention loss has large variations in different cells and P/E cycles due to the percolation effect. We develop a Monte Carlo analysis to take into account the distribution of ΔV t and a tunneling front model to study the spread of a retention V t distribution in a SONOS Flash memory.
Statistical characterization of two-level random telegraph noise (RTN) amplitude distribution in a hafnium oxide resistive memory has been performed. We find that two-level RTN in HRS exhibits a large amplitude distribution tail, as compared to LRS. To investigate an RTN trap position in a hafnium oxide film, we measure the dependence of electron capture and emission times of RTN on applied read voltage. A correlation between an RTN trap position and RTN amplitude is found. A quasi-two-dimensional RTN amplitude simulation based on trap-assisted electron sequential tunneling is developed. Our study shows that RTN traps in a rupture region of a hafnium oxide film are responsible for an RTN large-amplitude tail in HRS mostly.
Abstract-A V t retention distribution tail in a multitimeprogram (MTP) silicon-oxide-nitride-oxide-silicon (SONOS) memory is investigated. We characterize a single-program-chargeloss-induced ΔV t in NOR-type SONOS multilevel cells (MLCs). Our measurement shows the following: 1) A single-charge-lossinduced ΔV t exhibits an exponential distribution in magnitudes, which is attributed to a random-program-charge-induced currentpath percolation effect, and 2) the standard deviation of the exponential distribution depends on the program-charge density and increases with a program V t level in an MLC SONOS. In addition, we measure a V t retention distribution in a 512-Mb MTP SONOS memory and observe a significant V t retention tail. A numerical V t retention distribution model including the percolation effect and a Poisson-distribution-based multiple-charge-loss model is developed. Our model agrees with the measured V t retention distribution in a 512-Mb SONOS well. The observed V t tail is realized mainly due to the percolation effect.Index Terms-Model, percolation, silicon-oxide-nitride-oxidesilicon (SONOS), V t retention distribution.
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