H. J. Wen

Dalian University of Technology

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R. Ludeke

University of Pretoria

E. Cartier

IBM (United States)

M. Teresa Cuberes

University of Castilla-La Mancha

Quan‐Zhi Zhang

Dalian University of Technology

S.-H. Lo

National Sun Yat-sen University

Andreas Schenk

ETH Zurich

D. M. Newns

IBM Research - Thomas J. Watson Research Center

G. Kaindl

Freie Universität Berlin

M. Prietsch

Freie Universität Berlin

Julian Schulze

Ruhr University Bochum

Cooperative Institutions

IBM (United States)

IBM Research - Thomas J. Watson Research Center

Freie Universität Berlin

Dalian University of Technology

IBM Research - Zurich

Ruhr University Bochum

Universidad Autónoma de Madrid

Zentrum für Sonnenenergie- und Wasserstoff-Forschung Baden-Württemberg

Essex Westford School District

Samsung (United States)

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Ballistic-electron emission microscopy study of the Au/Si(111)7×7 and Au/CaF2/Si(111)7×7 interfaces

Applied Physics Letters (1994)

M. Teresa Cuberes Andreas Bauer H. J. Wen M. Prietsch G. Kaindl

Ballistic-electron emission microscopy (BEEM) has been performed on Au/n-Si(111)7×7 and Au/CaF2/n-Si(111)7×7 in UHV. In both cases, the topography of the Au surface is characterized by ≊2.5 Å height terraces, stacked in several stages, with rounded shapes for Au/Si, and hexagonal shapes for Au/CaF2/Si. BEEM up to tip voltages of 8 V on Au/Si is not altering the ballistic transmissivity, in contrast to previous work on Au/Si interfaces which involved chemical preparations of the Si surfaces. The shape of the BEEM spectra on Au/CaF2/Si depends on spectral features of the density of states of the CaF2 thin film.

10.1063/1.111650

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Citations (21)

Stressing and high field transport studies on device-grade SiO2 by ballistic electron emission spectroscopy

Journal of Vacuum Science & Technology B Microelectronics and Nanometer Structures Processing Measurement and Phenomena (1996)

R. Ludeke H. J. Wen E. Cartier

The tip of a scanning tunneling microscope was used to inject hot electrons across the gate and into the oxide of a metal-oxide-semiconductor structure. This method, known as ballistic electron emission microscopy (BEEM), allows an arbitrary choice of the energy of the injected electrons, which may be further accelerated by the application of a gate bias. The high current densities and choice of energy make BEEM an attractive method to study hot electron transport and breakdown phenomena in dielectrics. The studies reported here were made on Pd/SiO2/Si(100) structures with a SiO2 layer thickness of 3.8 nm. Monte Carlo techniques were used to calculate the spreading of the electron beam as it traverses the oxide. A strong dependence of the spreading on the kinetic energy and oxide thickness were observed. Using the calculated beam spreads to determine current densities and injected charge densities, the charge-to-breakdown (Qbd) was measured for several breakdown sequences. The Qbd’s consistently exceeded by several orders of magnitude the values obtained by conventional Fowler–Nordheim (FN) tunnel injection under high field conditions. Most of the time, breakdowns could not be achieved for 3.8 nm oxides. It is concluded that impurity/defects still control all observed breakdowns; an intrinsic limit—although claimed to have been reached—has not yet been established. Changes in the BEEM spectra with injected electron charge are interpreted in terms of a three stage process to breakdown: (I) electron trap creation and filling at the SiO2–Si interface, (II) prebreakdown believed to occur through thinning of the oxide that starts at the oxide-Si interface, and (III) oxide punch-through, characterized by an injection threshold ≲1 V, close to that for Si. The role of hot hole injection into the SiO2 valence band was also assessed and deemed a negligible factor in the degradation process under the zero or low oxide biases used in the experiment reported here.

10.1116/1.588845

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Citations (32)

Atomic-scale studies of electron transport through MOS structures

Applied Surface Science (1998)

H. J. Wen R. Ludeke D. M. Newns S.-H. Lo E. Cartier

Atomic units

Rectangular potential barrier

10.1016/s0169-4332(97)00459-5

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Citations (6)

Investigation of existing defects and defect generation in device-grade SiO2 by ballistic electron emission spectroscopy

Journal of Vacuum Science & Technology B Microelectronics and Nanometer Structures Processing Measurement and Phenomena (1997)

H. J. Wen R. Ludeke

Degradation processes initiated by defect generation in device-grade SiO2 were studied by locally injecting hot electrons from a scanning tunneling microscope tip into Pd/SiO2/p-Si(100) metal–oxide semiconductor (MOS) structures. An analysis of the emerging collector current in the Si substrate, a technique known as ballistic electron emission microscopy, provides electron transport information, from which the oxide defect generation process was studied. The charging of the defects resulted in shifts of threshold energies for electron transport across the oxide. A novel sheet charge model was developed to assess the in-depth distribution and charge densities in the oxide from field-induced threshold shifts obtained from experiment. An as-fabricated MOS system with an oxide thickness of 71 Å was investigated and found to contain existing electron traps of charge densities in the range (0.7–2.8)×1013 e/cm2 that are distributed within a 30 Å region adjacent to the metal/oxide interface. Further stressing was performed at zero oxide bias with increasing tip voltages of up to −10 V. New electron traps characterized by charge densities of (1.9–3.6)×1013 e/cm2 and located within 40 Å of the SiO2/Si interface were generated when the kinetic energy of the electrons injected into the SiO2 conduction band exceeded 1.9 eV. This energy threshold is in very good agreement with the hydrogen-release energy that is frequently invoked to explain oxide degradation.

10.1116/1.589418

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Quantum interference in SiO2: A conduction-band mass reappraisal

Applied Physics Letters (1998)

R. Ludeke H. J. Wen Andreas Schenk

Quantum oscillations arising from interference in over-the-barrier injected electrons crossing a metal–oxide–semiconductor structure were observed for a 2.8 nm SiO2 layer. Model calculations that include image force effects are fitted to the data to obtain a conduction-band mass of mox=(0.63±0.09)m0. The field dependence of the oscillations was used to deduce the polarity and magnitudes of oxide charge induced by the high fluence of electrons injected with the scanning tunneling microscope during spectral acquisitions.

10.1063/1.122133

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Citations (14)

Ballistic-electron emission microscopy on the Au/n-Si(111)7×7 interface

Journal of Vacuum Science & Technology B Microelectronics and Nanometer Structures Processing Measurement and Phenomena (1994)

M. Teresa Cuberes A. Bauer H. J. Wen D. D. Vandré M. Prietsch

Ballistic-electron emission microscopy (BEEM), performed under ultrahigh vacuum conditions at the room-temperature-grown Au/n-Si(111)7×7 interface, allows a measurement of the BEEM current for tip biases up to ≊ 8 V without a noticeable change in ballistic transmissivity. The differences of the present results to previous reports, where either no BEEM current was observed or the transmissivity was modified when applying high tip voltages, can be explained by the absence of intermixing at the Au/Si interface. Scanning tunneling microscope images of ≊40-Å-thick Au films reveal a characteristic topography of the metal surface with ≊2.5 Å high circular terraces stacked in up to four stages.

Ballistic conduction

10.1116/1.587775

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Citations (33)

Kinetic behavior of secondary electrons in a magnetized voltage-driven discharge using combined rf/dc sources

Physica Scripta (2024)

H. J. Wen Jing‐Yu Sun Quan‐Zhi Zhang

Abstract This study employed particle-in-cell/Monte Carlo simulations, along with test particle methods, to examine the characteristics of secondary electrons (SEs) in a voltage-driven discharge using combined rf/dc sources and operates in the presence of a magnetic field. The behavior of SEs is significantly influenced by the magnetic field, leading to the emergence of complex branches in temporal electron energy probability distributions and spatiotemporal electron density distributions within the sheath. The number of branches is directly correlated to the cyclotron period. Moreover, the application of a direct current (dc) source thickens the sheath at the dc biased electrode while attenuating the sheath on the opposite side. This leads to an asymmetrical modulation of the kinetic behavior of SEs in the two sheaths, ultimately resulting in a substantial increase in electron energy on the side of the dc biased electrode.

Secondary electrons

Particle-in-cell

10.1088/1402-4896/ad3996

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Heating mode transitions in capacitively coupled CF₄ plasmas at low pressure

Plasma Sources Science and Technology (2024)

H. J. Wen Julian Schulze Quan‐Zhi Zhang

Abstract Capacitively coupled plasmas operated in CF 4 at low pressure are frequently used for dielectric plasma etching. For such applications the generation of different ion and neutral radical species by energy dependent electron impact ionization and dissociation of the neutral background gas is important. These processes are largely determined by the space and time dependent electron energy distribution function and, thus, by the electron power absorption dynamics. In this work and based on a particle-in-cell/Monte Carlo collision model, we show that the electron heating mode in such plasmas is sensitive to changes of the gap at a constant pressure of 3 Pa. At a gap of 1.5 cm, the dominant mode is found to be a hybrid combination of the Drift-Ambipolar (DA) and the α -mode. As the gap is increased to 2 cm and 2.5 cm, the bulk power absorption and ambipolar power absorption decreases, and the DA mode decays. When the gap reaches 3 cm, the α -mode becomes more prominent, and at a gap of 3.75 cm the α -mode is dominant. These mode transitions are caused by a change of the electronegativity and are found to affect the discharge characteristics. The presence of the DA-mode leads to significant positive electron power absorption inside the bulk region and negative power absorption within the sheaths on time average, as electrons are accelerated from the bulk towards the collapsed sheath. The heating mode transitions result in a change from negative to positive total electron power absorption within the sheaths as the gap increases. When accounting for secondary electron emission, the transition of the heating mode can occur at shorter gaps due to the enhanced plasma density and decreased electronegativity.

Ambipolar diffusion

Electronegativity

Electron temperature

10.1088/1361-6595/ad6b44

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Localized degradation studies of ultrathin gate oxides

Journal of Vacuum Science & Technology A Vacuum Surfaces and Films (1998)

H. J. Wen R. Ludeke

We present studies on the limits of oxide reliability on a local, microscopic scale, using scanning tunneling microscope (STM)-based ballistic electron emission microscopy/spectroscopy (BEEM/S). In these studies, electrons are injected from the STM tip into the conduction band of a SiO2 layer that is imbedded in a metal–oxide–semiconductor (MOS) structure. The electron energy is determined both by the tip bias that can be set up to −13 V and by the applied oxide bias. Combining the two biases can heat electrons to energies that are unreachable in thin oxides by conventional Fowler–Nordheim injection methods. Our studies indicate that breakdowns are difficult to achieve for 7.1 nm oxides. A local breakdown was not observed even for an injected charge dosage of 1.8×103 C/cm2 at equivalent Fowler–Nordheim stress fields of ∼25 MV/cm, although defect densities in the oxide were as high as ∼5×1013/cm2. Evidence of anode hole injection is also observed under high oxide biases ∼8 MV/cm. Therefore we conclude that trap creation and hole injection processes are not sufficient to cause breakdowns at arbitrary locations on the 7.1 nm oxides. Whereas electron trapping is dominant during electron injection for 7.1 nm oxides, only a positive charge buildup was observed in the 2.8 nm oxides while stressing with only 1 eV electrons. For 2.8 nm oxides, a local breakdown did not occur for dosages of 3.1×103 C/cm2 at equivalent fields >43 MV/cm. The observed breakdowns were accompanied by gate metal failure and are hence believed to occur at weak spots in MOS capacitors. We conclude that an intrinsic breakdown limit of SiO2 has not yet been reached.

10.1116/1.581293

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Citations (22)

Current oscillations in thin metal–oxide–semiconductor structures observed by ballistic electron emission microscopy

Journal of Vacuum Science & Technology B Microelectronics and Nanometer Structures Processing Measurement and Phenomena (1998)

H. J. Wen R. Ludeke Andreas Schenk

Quantum interference oscillations of electrons in a thin SiO2 layer were observed by ballistic electron emission microscopy (BEEM). With BEEM, electrons are injected across the gate of a metal–oxide–semiconductor (MOS) structure and directly into the conduction band of the SiO2. The MOS capacitor consisted of a 5 nm thick Pd film deposited on a 2.8±0.2 nm oxide thermally grown on Si(100). Oscillations with up to four peaks in an energy range of 0–3 eV above the injection threshold were noted. Their magnitude is of the order of 30% of the underlying BEEM current. The oscillations were most salient and their energy location repeatable at points of the sample that were previously not exposed to the electron beam. Even modest exposures caused a buildup of positive charge. This charge resulted in energy shifts, as well as a weakening of the oscillations, both of which are a consequence of the added scattering and local field inhomogeneities associated with the random distribution of the positive charge. Solutions of the Schrödinger equation that included a built-in oxide potential of 0.20 V and image force effects at both interfaces gave excellent fits to the experimental data for an effective electron mass in the oxide mox=0.63±0.09mo. The uncertainty in mox arises from an uncertainty of ±0.2 nm in the determination of the oxide thickness by ellipsometric methods. Nevertheless, the obtained value is well above the generally accepted value of 0.5mo.

10.1116/1.590164

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Citations (17)