Gate dielectrics for ultimate CMOS technologies – Limitations and alternative solutions

For nowadays CMOS technologies, the gate oxide thickness has reached a few nanometer range and will be lower than 2 nm for sub-0.1 μ m generations. This scaling of the gate dielectric thickness favors the onset of physical phenomena such as gate polysilicon depletion or quantum effects that limit the MOS device performance in terms of capacitance and leakage current. Moreover, these ultra thin oxide MOS structures are prone to new degradation processes that could reduce their operation lifetime. In this paper, the major limitations raised by the scaling of the gate dielectrics in CMOS technologies are briefly reviewed in terms of MOS capacitance, reliability and new materials issues. More specifically, we first focus on the limitations raised by physical phenomena inherent to MOS capacitors such as polysilicon depletion and quantum effects (carrier confinement and tunneling), impacting their performances. We then address the limitations related to the reliability concerns such as wearout, breakdown, quasi-breakdown, stress-induced leakage current, determining the device lifetime. Finally, the new materials currently envisaged, as replacement solutions in order to overcoming the difficulties due to the gate oxide scaling will be discussed. In particular, the possible solutions based on alternate high permittivity gate dielectrics and metallic gate materials will be emphasized.     

Electroluminescence of rock salt

The basic experimental results found in studies of the emission of thin single-crystal layers of rock salt in superhigh electric fields are classified and analyzed. The emission is shown to come from the salt layer and to be due to electronic processes in this dielectric. The basic characteristics of the emission are analyzed, and it is shown to be electroluminescence. The estimated concentration of luminescence centers, data from chemical analysis, and the independence of the spectral characteristics from the “past history” of the samples indicate that the luminescence centers are due to defects in the crystal matrix.     

A new analytical model of high voltage silicon on insulator （SOI） thin film devices

A new analytical model of high voltage silicon on insulator (SOI) thin film devices is proposed, and a formula of silicon critical electric field is derived as a function of silicon film thickness by solving a 2D Poisson equation from an effective ionization rate, with a threshold energy taken into account for electron multiplying. Unlike a conventional silicon critical electric field that is constant and independent of silicon film thickness, the proposed silicon critical electric field increases sharply with silicon film thickness decreasing especially in the case of thin films, and can come to 141V/μm at a film thickness of 0.1μm which is much larger than the normal value of about 30V/μm. From the proposed formula of silicon critical electric field, the expressions of dielectric layer electric field and vertical breakdown voltage (V_B,V) are obtained. Based on the model, an ultra thin film can be used to enhance dielectric layer electric field and so increase vertical breakdown voltage for SOI devices because of its high silicon critical electric field, and with a dielectric layer thickness of 2μm the vertical breakdown voltages reach 852 and 300V for the silicon film thicknesses of 0.1 and 5μm, respectively. In addition, a relation between dielectric layer thickness and silicon film thickness is obtained, indicating a minimum vertical breakdown voltage that should be avoided when an SOI device is designed. 2D simulated results and some experimental results are in good agreement with analytical results.     

Electrical breakdown in solid dielectrics

The mechanism of electrical breakdown in solid dielectrics is analyzed using the results of our investigations performed in this direction over a period of several decades. It is shown that the electrical breakdown in solid dielectrics involves interrelated prebreakdown processes, such as high-voltage polarization, defect formation, electron impact excitation and electron impact ionization of luminescence centers and ions in the host crystal lattice, etc. The electrical breakdown is initiated by electric-field and thermal generation of defects in the crystal. In turn, the generation of defects leads to the formation of defect regions and channels that provide an assisted transfer of charge carriers. Electron currents flow (and electrons are accelerated by the electric field to energies sufficient to induce impact ionization) in these regions of the crystal with a lattice distorted by defects. In this respect, the known approaches to the elaboration of the breakdown theory for alkali halide and other dielectric crystals on the basis of analyzing the motion and acceleration of electrons in an ideal crystal structure have appeared to be incorrect.     

Improved model of streamer propagation in liquid dielectrics

Breakdown and pre‐breakdown cause extensive damage in dielectric oils used in power systems subjected to high voltages. Consequently, several research works are devoted to these phenomena. Related investigations are made very difficult by the numerous influencing parameters and the large number of characteristics of these phenomena. The main goal of the present work was to establish a new electrical model that describes the propagation of streamers in liquid dielectrics for a point‐plane configuration subjected to high‐voltage fields. The novelty consists in the presence of an inductance in the equivalent electrical circuit, which represents the dielectric during the partial discharges characterized by fast variations of current. Criticisms were first made concerning previous electrical models that show some disadvantages. The developed model consists of a series RLC circuit whose element values change during propagation. In particular, the value of the resistance decreases in anticipation of breakdown. The simulation of the developed model gave satisfactory results, similar to experimental ones. For instance, the influence of electrical and geometrical parameters on the current pulses amplitude as well as on the streamers propagation velocity and the quantity of charge and the energy injected were studied. Good agreement was also obtained between the obtained results and experimental ones, which reinforces the validity of the model.     

Electrical breakdown and high-energy electron emission under dielectric charging

A Monte-Carlo calculation model for electron transport in crystalline dielectrics charged by irradiation is improved with allowance for impact ionization and cascading processes. The electron transport in SiO ₂ is simulated for high-strength electric fields. It is found that a breakdown in a dielectric can occur in the electric field strength range 11.5–12.5 MV/cm. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 52–58, March, 2008.     

Hot electrons in silicon dioxide: Ballistic to steady-state transport

Hot electron transport in silicon dioxide is examined with emphasis on current experimental and theoretical results. For oxide layers thicker than 100 Å, steady-state transport has been shown to control the carrier flow at all fields studied. The transition from a nearly thermal electron distribution at electric fields less than approximately 1.5 MV/cm to significantly hot distributions with average energies between 2 and 6 eV at higher fields of up to 16 MV/cm is discussed. The significance of nonpolar phonon scattering in controlling the dispersive transport at higher electric fields, thereby preventing runaway and avalanche breakdown, is reviewed. The transition from ballistic to steady-state transport on very thin oxides layers of less than 100 Å in thickness and the observation of single phonon scattering events are also discussed.     

α Decays in Superstrong Static Electric Fields

Superstrong static electric fields could deform Coulomb barriers between α clusters and daughter nuclei, and bring up the possibility of speeding up α decays. We adopt a simplified model for the spherical α emitter ²¹²Po and study its responses to superstrong static electric fields. We find that superstrong electric fields with field strengths|E|~ 0:1 MV/fm could turn the angular distribution of α emissions from isotropic to strongly anisotropic, and speed up α decays by more than one order of magnitude. We also study the influences of superstrong electric fields along the Poisotope chains, and discuss the implications of our studies on α decays in superstrong monochromatic laser fields. The study here might be helpful for future theoretical studies of α decay in realistic superstrong laser fields.     

Strong amplification of an electric field and electro-optical response in ultrathin heterostructures ferroelectric-linear dielectric

An electroded heterostructure consisting of a dye layer sandwiched between two polymer ferroelectric layers is discussed. The dye layer plays a role of the probe of the electric field measured by an electroabsorption technique. Using this new method the electric field in ferroelectric and dielectric layers can be measured separately. When an a.c. voltage is applied to the heterostructure, the electric field in the dye layer increases 2.2 times (up to 0.55 GV/m), whereas the field in the ferroelectric decreases 2 times with respect to the average field in the entire structure. Moreover, the dye layer sandwiched between the ferroelectric layers may stand without breakdown the fields 5–7 times higher than a neat reference dye layer confined between metal electrodes. Therefore, the performance of electro-optical, electromechanical and other field controlled devices may be improved considerably when their functional materials are placed between ferroelectrics layers.     

多层铁电陶瓷击穿分析及优化

建立了多层串联PZT95/5爆电换能组件3维数值模型,对固化封装条件下陶瓷介质击穿问题进行了计算分析,计算结果表明:在不改动器件外部结构尺寸条件下,采用等厚度PZT95/5叠片结构布局对进一步提高输出电压方面存在瓶颈。为克服上述影响以及降低爆电换能组件击穿概率,提出了PZT95/5铁电陶瓷非等厚度布局解决方案。为实现上述设想,通过引入不等式约束条件计算得到一组非等厚度优化布局,将爆电换能组件所用PZT95/5铁电陶瓷数量减至19片,同时有效实现该布局下,各片PZT95/5陶瓷电压均低于对应厚度击穿电压的优化目标。     

脉冲形成线用CaO-TiO2-Al2O3基介质陶瓷介电性能

设计了一种脉冲形成线用新型CaO-TiO2-Al2O3基介质陶瓷体系,采用传统固相法通过优化组分和制备工艺,调控材料的微结构,获得了介电性能优异的介质陶瓷。其介电常数在15~35之间可调,介电损耗小于0.002,频率稳定性好。在厚度为1 mm时,介电强度高达50 kV/mm。研究了厚度对CaO-TiO2-Al2O3基介质陶瓷介电强度的影响规律,当厚度从1 mm减小到0.1 mm时,介电强度呈非线性增大,从50 kV/mm（1 mm厚样品）提高到92 kV/mm（0.1 mm厚样品）,可见,CaO-TiO2-Al2O3基介质陶瓷的电击穿与其机械损坏具有相似性。结合CaO-TiO2-Al2O3基介质陶瓷的化学组分和微观结构,CaO-TiO2-Al2O3基介质陶瓷优越的电击穿特性可以用弱点击穿理论解释。     

Criterion of the incubation time in the problems of pulsed fracture and electric breakdown

The problems of a pulsed strength of continuum media are considered in terms of the structural-time approach that is based on the concept of the incubation fracture time. This approach makes it possible to describe phenomena that arise under high-velocity external effects. A limiting condition that determines the instant of rupture or breakdown is proposed on the basis of the structural-time approach. A way to interpret and to determine the incubation time is proposed. A phenomenological model of an electric breakdown of solid dielectrics is formulated. Examples are considered where the structural-time approach is applied to problems of spall fracture, crack initiation, and a pulsed breakdown of dielectrics. A procedure for describing the time dependence of the electric strength (volt-second characteristic) is described in detail. The results of the calculations are found to be in good agreement with experimental data.     

A simulation study of field plate termination in Ga_2O_3 Schottky barrier diodes

In this work, the field plate termination is studied for Ga_2O_3 Schottky barrier diodes(SBDs) by simulation. The influence of field plate overlap, dielectric material and thickness on the termination electric field distribution are demonstrated.It is found that the optimal thickness increases with reverse bias increasing for all the three dielectrics of SiO_2, Al_2 O_3, and HfO_2. As the thickness increases, the maximum electric field intensity decreases in SiO_2 and Al_2O_3, but increases in HfO_2.Furthermore, it is found that SiO_2 and HfO_2 are suitable for the 600 V rate Ga_2O_3 SBD, and Al_2O_3 is suitable for both600 V and 1200 V rate Ga_2O_3 SBD. In addition, the comparison of Ga_2O_3 SBDs between the SiC and GaN counterpart reveals that for Ga_2O_3, the breakdown voltage bottleneck is the dielectric. While, for SiC and GaN, the bottleneck is mainly the semiconductor itself.     

Plasma Processes Driven by Current Sheets and Their Relevance to the Auroral Plasma

Satellite and rocket observations have revealed a host of auroral plasma processes, including large dc perpendicular electric fields (E?) associated with electrostatic shocks, relatively weak parallel electric fields (E?) associated with double layers, upflowing ions in the form of beams and conics, downflowing and upflowing accelerated electron beams, several wave modes such as the electrostatic ion-cyclotron (EIC), lower hybrid (LH), very low frequency (VLF), extremely low frequency (ELF), and high-frequency waves and associated nonlinear phenomena. Recently, we have attempted to simulate the various processes using a two-dimensional particle-in-cell code in which the plasma is driven by current sheets of a finite thickness. Striking similarities between the observed auroral plasma processes and those seen in the simulations are found. In this paper we give a review of the plasma processes dealing with dc and ac electric fields, formation of ion beams and conics, and electron acceleration. Electrostatic shock-type electric fields (E?e) occur near the current sheet edges. Such fields arise because of the contact between the high-and low-density plasmas inside and outside the sheet, respectively. Double layers having upward electric fields form inside the sheet and they are distinguishable from the large perpendicular electric fields (E?e) only in wide sheets with thicknesses l >> ?i, the ion Larmor radius. Double layers with a reverse polarity form outside the sheet where downward currents flow. The most energetic ions are found to have pitch angles near 90°, implying a large perpendicular acceleration of the ions.     

Kinetics of failure of polycrystalline ferroelectric ceramics in mechanical and electric fields

The kinetics of mechanical failure and electrical breakdown in polycrystalline ferroelectric ceramics was studied under the simultaneous action of an electric field and a mechanical load. A kinetic approach is shown to be preferable as compared to the concepts that treat the failure and breakdown as critical phenomena. The mechanical failure and electrical breakdown are shown to be interrelated. It is found that a weak action of one of the fields retards failure caused by the other field and that the simultaneous action of these strong fields accelerates both the mechanical failure and electrical breakdown. Methods for determining the activation characteristics of both processes only from the failure kinetics in one of these processes are developed.     

A pure dielectric metamaterial absorber with broadband and thin thickness based on a cross-hole array structure

A pure dielectric metamaterial absorber with broadband and thin thickness is proposed, whose structure is designed as a periodic cross-hole array. The pure dielectric metamaterial absorber with high permittivity is prepared by ceramic reinforced polymer composites. Compared with those with low permittivity, the absorber with high permittivity is more sensitive to structural parameters, which means that it is easier to optimize the equivalent electromagnetic parameters and achieve wide impedance matching by altering the size or shape of the unit cell. The optimized metamaterial absorber exhibits reflection loss below -10 dB in 7.93 GHz-35.76 GHz with a thickness of 3.5 mm, which shows favorable absorption properties under the oblique incidence of TE polarization (±45°). Whether it is a measured or simulated value, the strongest absorbing peak reaches below -45 dB, which exceeds that of most metamaterial absorbers. The distributions of power loss density and electric and magnetic fields are investigated to study the origin of their strong absorbing properties. Multiple resonance mechanisms are proposed to explain the phenomenon, including polarization relaxation of the dielectric and edge effects of the cross-hole array. This work overcomes the shortcomings of the narrow absorbing bandwidth of dielectrics. It demonstrates that the pure dielectric metamaterial absorber with high permittivity has great potential in the field of microwave absorption.     

Luminescence of rock salt in very strong electric fields

The luminescence of thin single-crystal layers of NaCl in very strong electric fields was observed. Oscillograms of the brightness waves were obtained. The light flux and luminescence delay were measured as functions of the applied field strength amplitude. The reasons for the delay phenomenon and the drop in the electric strength of samples when electrodes with electron conductivity (metal or graphite) are used are discussed.The authors sincerely thank Prof. G. A. Vorob'ev for his guidance and P. E. Ramazanov for valuable observations.     

Nitridation of thin SiO2 films in N2 and NH3 plasmas

A low-temperature (700°C) plasma-enhanced nitridation process which improves the dielectric breakdown of thin silicon dioxide (SiO₂) layers is presented. It uses a new, production compatible, parallel plate plasma reactor working at low RF frequencies. Nitrided oxides produce less charge trapping under high field stress, higher breakdown charge and a tighter distribution of breakdown fields than pure SiO₂. More nitrogen is incorporated in films treated in a NH₃ plasma than in a N₂ plasma. However, the latter present better electrical properties.     

Design optimization of high breakdown voltage vertical GaN junction barrier Schottky diode with high-K/low-K compound dielectric structure

A vertical junction barrier Schottky diode with a high-$K$/low-$K$ compound dielectric structure is proposed and optimized to achieve a high breakdown voltage (BV). There is a discontinuity of the electric field at the interface of high-$K$ and low-$K$ layers due to the different dielectric constants of high-$K$ and low-$K$ dielectric layers. A new electric field peak is introduced in the n-type drift region of junction barrier Schottky diode (JBS), so the distribution of electric field in JBS becomes more uniform. At the same time, the effect of electric-power line concentration at the p-n junction interface is suppressed due to the effects of the high-$K$ dielectric layer and an enhancement of breakdown voltage can be achieved. Numerical simulations demonstrate that GaN JBS with a specific on-resistance ($R_{\rm on, sp}$) of 2.07 m$\Omega\cdot$cm$^{2}$ and a BV of 4171 V which is 167% higher than the breakdown voltage of the common structure, resulting in a high figure-of-merit (FOM) of 8.6 GW/cm$^{2}$, and a low turn-on voltage of 0.6 V.     

The electrical strength of oxide films on metals

Pulse breakdown of thin films is shown to be much the same as pulse breakdown in thin layers of gases and alkali halides, which means that collisional ionization is the basic process; but the relation of delay to thickness does not agree with current concepts on the breakdown mechanism in solids. It is supposed that secondary processes complicate the breakdown by collisional ionization.I am indebted to L. L. Odynets for direction in this work and to students V. Gavrilov and G. Savina for assistance in the experiments.