Prasad Chaparala

Low electric field breakdown of thin SiO/sub 2/ films under static and dynamic stress

A comprehensive study of Time-Dependent Dielectric Breakdown (TDDB) of 6.5-, 9-, 15-, and 22-nm SiO/sub 2/ films under dc and pulsed bias has been conducted over a wide range of electric fields and temperatures. Very high temperatures were used at the wafer level to accelerate breakdown so tests could be conducted at electric fields as low as 4.5 MV/cm. New observations are reported for TDDB that suggest a consistent electric field and temperature dependence for intrinsic breakdown and a changing breakdown mechanism as a function of electric field. The results show that the logarithm of the median-test-time-to failure, log (t/sub 50/), is described by a linear electric field dependence with a field acceleration parameter that is not dependent on temperature. It has a value of approximately 1 decade/MV/cm for the range of oxide thicknesses studied and shows a slight decreasing trend with decreasing oxide thickness. The thermal activation E/sub a/ ranged between 0.7 and 0.95 eV for electric fields below 9.0 MV/cm for all oxide thicknesses. TDDB tests conducted under pulsed bias indicate that increased dielectric lifetime is observed under unipolar and bipolar pulsed stress conditions, but diminishes as the stress electric field and oxide thickness are reduced. This observation provides new evidence that low electric field aging and breakdown is not dominated by charge generation and trapping.

Optimized temperature-pulse sequences for the enhancement of chemically specific response patterns from micro-hotplate gas sensors

Abstract Microfabricated solid-state gas sensors have been of continuing interest because of the potential for arrays of devices with low power consumption. Devices based on a micromachined ‘hotplate’ offer the additional advantage of a wide operating temperature range with a rapid thermal time constant of order 1 ms. By operating the device in a temperature-programmed mode, reaction kinetics on the sensing film surface are altered, producing a time-varying response signature that is characteristic of the gas being sensed. Approaches to optimizing such temperature programs to maximize the differences in response signatures for gases of interest or to enhance the sensitivity of the device are discussed.

Field and temperature acceleration of time-dependent dielectric breakdown in intrinsic thin SiO/sub 2/

Time-Dependent Dielectric Breakdown (TDDB) data are presented for 15- and 22.5-nm oxides collected over a wide range of electric fields and temperatures. The results indicate that it is necessary to obtain data over this range to distinguish between the two field acceleration models and to quantify the electric field and temperature dependencies of the thermal activation energy and the field acceleration factor, respectively. We also report on the TDDB characteristics of thin SiO/sub 2/ films at temperatures as high as 400/spl deg/C and demonstrate the use of these temperatures to accelerate TDDB.<<ETX>>

Growth of SnO2 films on micromachined hotplates

Arrays of micromachined hotplates have been used for materials processing on a microscopic scale. The temperature of individual elements ‘‘micro‐hotplates’’ of an array is controlled by addressing a given element with a specified current and measuring the temperature from a resistance change. This unique temperature control capability has been exploited to deposit SnO2 overlayers onto micro‐hotplates with individually controlled temperatures using reactive sputter deposition and organometallic chemical vapor deposition. Post‐deposition heating in vacuum was used to alter the stoichiometry of films. The result is an array of separately, but simultaneously, processed films. The micro‐hotplates have excellent thermal isolation from other devices (transistors, logic elements) on the chip. Electrical contact pads allow for in situ electrical characterization of the films. The use of micro‐hotplates allows high‐temperature growth to occur on portions of a silicon substrate, while other portions remain at room t...

Electric field dependent dielectric breakdown of intrinsic SiO/sub 2/ films under dynamic stress

Time-dependent dielectric breakdown (TDDB) characteristics are reported for 6.5 nm, 9 nm, 15 nm, and 22 nm intrinsic silicon dioxide films stressed under dc and bipolar pulsed bias conditions for a wide range of electric fields and temperatures. Our results show that the increased lifetime observed under bipolar pulsed stress conditions diminishes as the stress electric field and oxide thickness are reduced. Similar electric field and temperature dependencies of TDDB are observed under both static and dynamic stress conditions. It is observed that lifetime enhancement only occurs for electric fields and thicknesses where charge trapping is significant. Contradictory to the conventional notion, TDDB tests on intrinsic thin oxides indicate that static stress testing cannot be considered as a conservative test of bipolar stressing for estimating oxide reliability. These results also confirm the existence of two separate failure mechanisms for TDDB that are functions of electric field and oxide thickness.

A new oxide degradation mechanism for stresses in the Fowler-Nordheim tunneling regime

In this study, voltage and current stress measurements in the Fowler-Nordheim regime, performed on gate oxides (9 nm-28 nm), indicated that a ramped pre-stress prior to a constant stress can increase the time to breakdown in some cases. In the literature oxide breakdown is said to be related to a fixed amount of trapped oxide charge or to a fixed amount of generated traps in the oxide. However, these models cannot explain our experimental observations. Current-time, current-charge, voltage-time characteristics and results of high frequency pre-stresses have been extensively studied in order to gain information about the charge trapping properties of the virgin and pre-stressed oxides. It is concluded from experimental results that the rate of initial positive charge build up in the oxide during the constant stress is a key factor for oxide degradation and breakdown.

Characterization of Time-Dependent Dielectric Breakdown in Intrinsic Thin SiO2

Abstract Time-dependent dielectric breakdown data collected from 6.5-, 9-, 15-, 20- and 22.5-nm-thick SiO2 films are presented. The failure distributions are of single mode with no apparent extrinsic population. The logarithm of the median-test-time-to failure, log(t50), is described by a linear electric field dependence. Contrary to reports in earlier studies, the field acceleration parameter is observed to be insensitive to temperature and has a value of approximately 1.0 decade MV−1 cm−1 for the range of oxide thicknesses studied. Capacitance-voltage studies indicate that there is no strong correlation between oxide trapped charges and time to failure under constant voltage stress conditions.

OBIC analysis of stressed, thermally-isolated polysilicon resistors

High gain Optical Beam Induced Current (OBIC) imaging has been used for the first time to examine the internal structural effects of electrical stress on thermally-isolated polysilicon resistors. The resistors are examined over a wide range of current densities, producing Joule heating up to /spl sim/1200/spl deg/C. Throughout this current density range, the OBIC images indicate a clustering of dopant under dc stress and a more uniform distribution under ac conditions. The OBIC images also reveal areas that are precursors to catastrophic resistor failure. In addition to OBIC imaging, conventional electrical measurements were performed, examining the polysilicon resistance degradation and time-to-failure as a function of electrical stress. The electrical measurements show a monotonic increase in polysilicon resistor lifetime with frequency (up to 2 kHz) when subjected to a bipolar ac stress. The enhanced lifetime was observed even under high temperature (from Joule heating) stress conditions previously reported to be electromigration-free. The dopant redistribution indicated by the OBIC images is consistent with an electromigration stress experienced by the polysilicon resistors. The implications for thermally-isolated polysilicon resistor reliability are examined briefly.

Time-dependent dielectric breakdown of intrinsic SiO/sub 2/ films under dynamic stress

We present time-dependent dielectric breakdown (TDDB) characteristics for 9, 15, and 22 nm silicon dioxide films stressed under DC, unipolar, and bipolar pulsed bias conditions. Our results indicate that the increased lifetime observed under pulsed stress conditions diminishes as the stress electric field and oxide thickness are reduced. TDDB data under pulse bias conditions exhibit similar field and temperature dependencies as under static stress. C-V measurements indicate that lifetime enhancement only occurs for electric fields and thickness where charge trapping is significant.

Time-Dependent Dielectric Breakdown in Thin Intrinsic SiO2 Films

Time-Dependent Dielectric Breakdown studies were performed on 6.5-, 9-, 15-, 20-, and 22.5- nm thick SiO2 films over a wide range of stress temperatures and electric fields. Very high temperatures (400 °C) were used to accelerate breakdown so that stress tests could be performed at low electric fields close to those used for device operating conditions. The results indicate that the dependence of TDDB on electric field and temperature is different from that reported in earlier studies. Specifically, the electric-field-acceleration parameter is independent of temperature and the thermal activation energy was determined to be between 0.7 and 0.9 eV for stress fields below 7.0 MV/cm.n Failure distributions of high-quality current-generation oxide films are shown to be of single mode and have dispersions that are not sensitive to stress electric field or temperature, unlike distributions observed for oxides examined in earlier studies. These results have implications on the choice of the correct physical model to describe TDDB in thin films. The data also demonstrate for the first time the reliability of silicon dioxide films at very high temperatures.