Indrajit Banerjee

High permittivity thin film nanolaminates

Thin (∼10 nm) films comprising of Ta2O5–HfO2, Ta2O5–ZrO2, and ZrO2–HfO2 nanolaminates were deposited and characterized for possible gate dielectric applications. These films were deposited on silicon substrates using atomic layer deposition. The dielectric constants of these films were in 12–14 range and the leakage currents in 2.6×10−8–4.2×10−7 A/cm2 range at 1 MV/cm electric field. It was found that as these films were made thinner, the value of their dielectric constant dropped compared to their bulk values. The dominant leakage current mechanism at low electric fields was determined to be Schottky emission, whereas Poole–Frenkel emission dominated at higher fields.

Microstructure of (Ba, Sr)TiO3 thin films deposited by physical vapor deposition at 480 °C and its influence on the dielectric properties

The orientation and microstructure of (Ba, Sr)TiO3 (BST) deposited via physical vapor deposition at 480 °C was studied using x-ray diffraction, atomic force microscopy, and transmission electron microscopy. Annealing Pt/BST (previously annealed at 400 °C) at 800 °C in O2 results in grain growth, enhancement of the {100} texture and a 20% increase in the dielectric constant. The 400 °C annealed films become more textured in the {100} orientation as film thickness is increased. Finally, it appears that an interfacial capacitance, rather than the “bulk” dielectric constant limits the total capacitance density of the films.

Anomalous diffusion of nitrogen in SiO2 under ion bombardment

Depth profiling was done by secondary ion mass spectrometry (SIMS) on SiO2/Si3N4/SiO2 (ONO) structures to determine if the nitrogen tail seen going into the silicon substrate was real or an artifact of ion bombardment. To determine this without an element of doubt, samples were thinned from the back side to the ONO layer and SIMS depth profiling was carried out on the exposed underside of ONO. It is determined that the layer of ‘‘nitride’’ at the Si/oxide interface is really an artifact of ion bombardment. Profiling from the back side shows there is no nitrogen tail in the silicon substrate and there is also no nitride at the oxide/Si interface. The interfacial nitride layer, and the apparent nitrogen tail extending into the underlying Si, seen from front‐side profiling, are due to anomalous nitrogen diffusion during ion bombardment. It is believed that nitrogen‐oxygen complexes are formed in the silicon substrate as a result of nitridation, and this adversely affects device performance. Though this may s...

Characterization of Chemical Vapor Deposited Amorphous Fluorocarbons for Low Dielectric Constant Interlayer Dielectrics

C-F materials, deposited by chemical vapor deposition (CVD) have been studied for their potential use as a low dielectric constant intermetal dielectric material for semiconductor applications. Though a dielectric constant of ∼2.4 has been determined, thermal stability of the material needs to be improved. It is shown that the presence of O and/or OH in the system causes a pyrolytic decomposition of the material causing CO and CO 2 to outgas from the material at low temperatures. This causes the C-F matrix to disintegrate and release CF x ions. Minimizing the O/OH content in the films improves thermal stability. Various structural properties of the films have been investigated using X-ray photoelectron spectroscopy and nuclear magnetic resonance spectroscopies.

NITROGEN REDISTRIBUTION IN SIO2 UNDER ION BOMBARDMENT

Secondary ion mass spectrometry (SIMS) and electron spectroscopy for chemical analysis were used to study SiO2/Si3N4/SiO2 (ONO) structures. The purpose was to determine if the nitrogen tail seen going into the silicon substrate was real or an artifact of ion bombardment. To determine this without an element of doubt, samples were thinned from the backside to the ONO layer and SIMS depth profiling was carried out on the exposed underside of ONO. Profiling from the backside shows there is no nitrogen tail in the silicon substrate and there is no nitride at the oxide/Si interface, as seen from front side profiling. The effects seen during profiling from the front side are due to anomalous diffusion of nitrogen caused by ion bombardment. It is believed that nitrogen–oxygen complexes are formed in the silicon substrate as a result of nitridation, and this adversely affects device performance. Though this may still be true, one needs to be cautious in interpreting SIMS and Auger depth profiles from the front si...

Testing of a rapid fault detection model for quality control: Borophosphosilicate glass thin films monitored by infrared absorption spectroscopy

Infrared absorption spectra of 108 borophosphosilicate glass (BPSG) thin films produced in a multiple-wafer low-pressure chemical vapor deposition (LPCVD) reactor were collected to enable the development and testing of a rapid and inexpensive method for determining if films are within the desired specifications. Classification of samples into good and bad product categories was made by applying principal component analysis to the spectra. Mahalanobis distances were used as the classification metric. The highest overall percentage of correct classification of samples based upon their spectra with two-step classification was 95%. The misclassified samples were, however, within the error of the reference methods that were used in making the original classification against which the infrared (IR) classification methods were tested. The classification errors are thus just as likely to be a result of misclassification by the reference method rather than errors by the IR classification. Although reference measur...

Dielectric relaxation and steady-state leakage in low-temperature sputtered (Ba, Sr)TiO3 thin films

Dielectric relaxation and steady-state leakage currents were studied over a range of bias voltages for Pt-electroded capacitors in which the 50-nm-thick (Ba, Sr)TiO3 dielectric layer was sputter deposited at 480 °C. A pronounced polarity dependence in the current–voltage characteristic of the capacitors was observed. Dielectric relaxation in the films showed a Curie–von Schweidler time dependence (J=J0t−n) for intervals of 3–200 s duration after application of a voltage step. The relaxation current density was found to depend linearly on electric field for fields up to 700 kV/cm and nonlinearly at higher fields. In addition to the Curie–von Schweidler relaxation currents, a time-dependent leakage current was detected under high voltage bias conditions. An empirical model developed to describe leakage currents in these films is presented.

Applications of Focused Ion Beams

Using a focused ion beam of Ga + ions at 25 keV, one can obtain usable beams as small as 50 nm in diameter. As a result of this, small and precise areas can be milled to obtain cross sections, small vias, and cut metal lines. Additionally, the ion beam can be used to induce metal deposition. This deposition allows nondestructive design changes directly on the device, and provides increased failure analysis capabilities. Examples on the use of the FIB for such applications are given and some drawbacks are discussed. Methods employed to overcome the drawbacks also are discussed

Use of advanced analytical techniques for VLSI failure analysis

Several new applications and techniques in the use of scanning electron microscopy (SEM), transmission electron microscopy (TEM), focused ion beam (FIB) microsurgery, and secondary ion mass spectroscopy (SIMS) for problem solving are presented. These tools, used in new ways, are playing a key role in identifying the sources of defects leading to potential device reliability problems, and are contributing to the elimination of their sources in the process line.<<ETX>>