绝缘膜负带电时的表面局部电场与二次电子返回特性

为分析IC多层版图扫描电镜(SEM)对准检测所利用的负带电成像原理,采用Mott弹性散射截面和修正的Bethe非弹性碰撞公式,对点照射入射电子在绝缘膜中的散射过程进行了Monte Carlo模拟,得到负带电绝缘物表面的局部电位分布.在此基础上计算了二次电子从表面出射后在局部场作用下的运动轨迹,获得了SEM像的二次电子信号电流.结果表明,在弱负带电条件下,照射点处表面电位越低,返回表面的二次电子就越多,对应的二次电子信号电流越弱.此结果与SEM实验中图像亮度随照射时间的变化规律相符.     

低能电子束照射集成电路芯片时的静态电容衬度分析

在研究低能电子束照射绝缘物时在二次电子返回特性的基础上,通过绝缘物表面照射微区和衬底之间的有效电容,获得了表面电位和二次电子信号电流在表面电荷积累过渡过程中随照射条件的变化关系,建立了电子束照射覆盖有绝缘膜的IC芯片时形成静态电容衬度的理论模型.从理论上分析了电子束照射条件和芯片内部形貌、材料参数对静态电容衬度的影响,解释了在扫描电镜实验中的最大衬度现象及其对应的最佳电子照射条件.     

低能电子束照射集成电路芯片时的静态电容衬度分析

在研究低能电子束照射绝缘物时在二次电子返回特性的基础上,通过绝缘物表面照射微区和衬底之间的有效电容,获得了表面电位和二次电子信号电流在表面电荷积累过渡过程中随照射条件的变化关系,建立了电子束照射覆盖有绝缘膜的IC芯片时形成静态电容衬度的理论模型.从理论上分析了电子束照射条件和芯片内部形貌、材料参数对静态电容衬度的影响,解释了在扫描电镜实验中的最大衬度现象及其对应的最佳电子照射条件.     

低能电子束照射电介质样品的二次电子特性

为阐明低能电子束照射下电介质样品的二次电子电流及产额的动态特性,将蒙特卡洛法和有限差分法相结合,建立了较为准确的电子散射、俘获、输运和自洽场等过程的数值计算模型;采用一个改进二次电子检测实验平台,准确测量了二次电子电流.模拟和实验结果表明,相对于电子束脉冲照射模式,电子束连续照射会导致二次电子产额明显降低.在连续照射模式下,随着电子束照射,二次电子电流和产额逐渐减小至一个稳定值.二次电子产额受入射电子束电流的影响较小,但随样品厚度的增大而增大.本文结果为提高扫描电镜成像质量、降低带电效应提供了理论指导,而且提供了依据二次电子特性研究样品参数的新思路.     

低能电子束照射电介质样品的二次电子特性简

 为阐明低能电子束照射下电介质样品的二次电子电流及产额的动态特性,将蒙特卡洛法和有限差分法相结合,建立了较为准确的电子散射、俘获、输运和自洽场等过程的数值计算模型;采用一个改进二次电子检测实验平台,准确测量了二次电子电流.模拟和实验结果表明,相对于电子束脉冲照射模式,电子束连续照射会导致二次电子产额明显降低.在连续照射模式下,随着电子束照射,二次电子电流和产额逐渐减小至一个稳定值.二次电子产额受入射电子束电流的影响较小,但随样品厚度的增大而增大.本文结果为提高扫描电镜成像质量、降低带电效应提供了理论指导,而且提供了依据二次电子特性研究样品参数的新思路.     

低入射能量下电介质样品出射电子特性

近年来,低入射能量下电介质样品出射电子特性研究在电子显微成像、电子束曝光等领域得到了广泛关注。结合数值计算和实验测量,研究和分析了石英样品出射电子电流的动态变化特性及相关因素的影响。结果表明,二次电子能谱的峰值半宽度比金属样品要小,几率最大的出射能量约为2 e V。随着电子束持续照射,表面电位逐渐下降并趋于稳定,二次电子电流逐渐增大至稳定值,背散射电子电流基本保持不变。稳态二次电子电流和背散射电子电流随入射能量的变化基本保持不变,二者随束流增大近似线性增大。样品台偏压越高,二次电子电流越小,背散射电子电流基本不变。     

电子束照射电介质厚样品的带电动态特性

采用较为准确的考虑电子散射、输运、俘获以及自洽场的数值计算模型,研究了电子束照射电介质厚样品的动态带电特性,并采用一个实验平台测试样品的电子产额。结果表明,由于电子的迁移和扩散,空间电荷呈现在散射区域内为正、区域外为负的分布特性。随着电子束照射,表面电位逐渐下降并趋于稳定值,电子总产额逐渐增大并趋近于1,暂态过程因此趋于平衡。表面电位随入射电子束能量的增大而下降,随入射电子束电流增大而略微下降。对于不规则表面样品,表面倾斜角度越大,表面电位越高。     

具有埋层结构电介质样品扫描电镜二次电子特性

采用较为全面的考虑电子散射、俘获、输运和自洽场等过程的数值模型,阐明了具有埋层结构电介质样品的扫描电镜检测机理及二次电子电流的动态特性。模拟结果表明,被沟槽界面俘获的电荷会影响空间电场分布,从而影响二次电子特性。随着电子束照射,样品表面沿着深度方向的电场强度增强,更多的二次电子返回表面,从而产生图像衬度。图像衬度随电子束能量的变化呈现极大值,而随电子束电流的增大而增大,模拟结果与实验结果基本一致。     

透过反射法的原理和应用

本方法针对SEM+EDX或SEM+WDX仪器的分析方法。本方法根据SEM的二次电子检测器所检测的信息并综合EDX信息加以实现的。令二次电子检测器检测的信息电流为I_s;则:I_s=i_1+i_2+i_3+i_4其中:i_1为入射束流激发样品时产生的纯二次电子电流。i_2为背散射电子电流在SE检测器所张立体角内的部分。i_3为背散射电子以样品表面逸出时产生的纯二次电子电流。i_4为背散射电子在真空壁上和样品支架上产生的纯二次电子电流。控制I_s和dI_s的大小和能量可以改变图像的特征和散射区域。因di_3和di_4远比di_1和di_2小,所以dI_s可以认为由di_1和di_2组成。我们可以用改变di_1/di_2来获取样品的物理、化学性质。综合EDX或WDX提供     

扫描电子显微镜中吸收电流像的应用

扫描电镜(SEM)具有多种成像模式,二次电子像和背散射电子像是最常用的模式。这两种像的衬度均与试样—检测器的几何位置有关,具有不同程度的阴影效应。吸收电流像与试样—检测器的几何位置无关,无阴影效应。对均匀的电惰性材料,吸收电流像与发射电流像(二次电子像和背散射电子像)的衬度互补。对半导体和磁性材料,没有这种互补关系。本文根据吸收电流像的特点,对均匀电惰性材料用吸收电流信号作为附加模式,用两次曝光技术对二次电子像中不易得到的形貌细节补偿,获得了清晰的图像。图1—4是对黄铜试样垫上的激光孔补偿前后的对比。我们还用吸收电流像观察了多相合金表面成份分布和半导体材料中的势垒电子伏特效应。实验结果表明吸收电流像主要有以下几种应用:     

Quantitative analysis of static capacitance contrast in scanning electron microscopy

A one-dimensional model is proposed for analysing static capacitance contrast (SCC) in scanning electron microscopy. For the large-scale integrated specimen covered by an insulating thin film, the imaging signal is calculated considering the redistribution of secondary electrons (SEs) and the charging process of the equivalent effective capacitance between the irradiation point and substrate. The calculated SCC as a function of the irradiation charge density is in good agreement with the experimental SCC. It is confirmed that the SCC arises from the redistribution of SEs and the difference in the effective capacitance of irradiation points under the condition of positive charging.     

Monte Carlo simulation of topographic contrast in scanning ion microscope

Topographic contrast of secondary-electron (SE) images in a scanning ion microscope (SIM) using a focused gallium (Ga) ion beam is investigated by Monte Carlo simulation. The SE yield of heavy materials, in particular, due to the impact of 30 keV Ga ions increases much faster than for the impact of electrons at < or =10 keV as a function of the angle of incidence of the primary beam. This indicates the topographic contrast for heavy materials is clearer in a SIM image than in a scanning electron microscope (SEM) image; for light materials both contrasts are similar to each other. Semicircular rods with different radii and steps with large heights and a small wall angle, made of Si and Au, are modeled for comparison with SE images in SEM. Line profiles of the SE intensity and pseudo-images constructed from the profiles reveal some differences of the topographic contrast between SIM and SEM. We discuss not only the incident-angle effect on the contrast, but also the effects of re-entrances of primary particles and SEs to the neighboring surface, the effect of a sharp edge on the sample surface, and the effects of pattern size and beam size.     

A scanning electron microscope study of electromigration in an Al-2% Cu thin film

Electromigration in an Al-27% Cu metallization thin film deposited on a silicon transistor structure has been studied in situ using the electron beam induced current mode (EBIC) of the scanning electron microscope. In this mode electrons which are transmitted through the metallization generate a signal which depends on the thin film thickness and its mass density. During the initial stages of the electromigration experiment the copper rich precipitates on the surface coarsened. Also, there was a preferential coarsening towards the positive end. Once the negative end became depleted of these surface precipitates grain thinning occurred. Voids nucleated and grew in from the sides of the metallization in these depleted regions finally causing film failure. However almost no voids were observed at grain boundaries or triple points. The copper rich precipitates in the interior of the metal film were stationary and showed no signs of coarsening or depleting any region. These observations suggest that surface diffusion is the predominant mode of atom migration in these alloys; grain boundary diffusion has been reduced compared to pure aluminum.     

平面薄膜场致发射的模型分析

该文系统地讨论宽带隙平面薄膜的场致电子发射(FEE)的机理。基本的理论模型是电子对表面势垒的隧穿效应,同时考虑到晶格的散射和薄膜势垒中微细贯穿通道的电子发射作用。分析结果表明,宽带隙平面薄膜结构用作场致电子发射阴极,具有发射电压的阈值低,发射电子的能量分布范围小等优点。另外这种结构制作简单、材料选择范围宽、理化稳定性好,是一种理想的场致发射电子源。     

Investigation of ion-electron emission in the process of reactive ion-beam etching of dielectric thin film heterostructures

This work presents a series of experimental studies to confirm the main theoretical aspects of ionelectron emission and it searches for the possibility of the practical implementation of the operative control method of reactive ion-beam etching processes of different dielectric thin film materials of electronic engineering. The series of experiments was carried out to study the electron emission on the specially formed thinfilm multilayer heterocompositions of Si₃N₄/Si, Ta₂O₅/Al/Si, and Al/TiO₂/Si. The evaluation of the effect of the induced surface potential in the dielectric film on the integral signal of the secondary electrons at reactive ion-beam etching was carried out. The dependence of the emission properties of thin dielectric films on the electric field generated in the dielectric by the surface potential induced by ion beam during reactive ionbeam etching was established. It is noted that the current level of secondary electrons from the surface of dielectric films deposited on the substrates of different materials differ in magnitude; i.e., it is determined by the substrate emission properties. It is shown that the electric field strength arising in the dielectric film under the influence of the induced potential creates the conditions for the emergence of Malter’s emission determined by the properties of its own dielectric and substrate.     

On the surface photovoltaic effect in a multivalley semiconductor in an external magnetic field

A theory of the photovoltaic effect in a semi-infinite multvalley semiconductor upon the absorption of polarized light at free carriers caused by the specular reflection and diffuse scattering of electrons at a film surface is developed by us. The kinetic Boltzmann equation in the approximation of time relaxation and boundary conditions, which determine the correlation between the distribution function of electrons reflected from a semi-infinite crystal surface and the distribution function of electrons incident onto a surface for the case of both specular reflection and diffuse scattering is used. The aforementioned takes into account the fact that the distribution function of electrons diffusely scattered from a surface depends only on their energy and is determined from the condition of total electron flux vanishing at the surface. Expressions for analyzing the spectral dependence of the current which is linearly dependent on the magnetic-field strength are obtained.     

The role of avalanching in cold emission devices

Nonthermionic emission of electrons into a vacuum is observed in Malter effect emission, MgO cold-cathode type emission, and thin film metal sandwich emission. These cold-cathode effects possess certain similarities which appear to be the result of a common basic emission mechanism. MgO cold-cathode type emission has been the most extensively studied of these three cold-cathode effects. In MgO cold cathodes an electric field of the order of 10⁶v/cm is developed across a relatively thin (∼7000Å) oxide layer. This electric field is indirectly developed as a result of hole migration through the MgO layer. A similar process of field enhancement probably occurs in Malter effect emission, while in thin film metal sandwich devices the required electric field is developed directly. Study of the detailed characteristics of MgO cold-cathode emission suggests strongly that electron avalanching is the basic mechanism responsible for self-sustained emission. In the self-sustained region, the IV characteristics are those of a simple avalanche process. Furthermore, the observed electron velocity distribution and the temperature dependence of emission follow from the requirement that electrons which are emitted come from a highly energetic electron "gas," presumably the terminal result of an avalanching process. Similarities between this process and the mechanism producing emission from thin film metal sandwich devices with insulating layers of 250Å-16,000 Å thickness are discussed.     

Computer simulation of electric field variations due to movements of electric charges

Simulations were carried out for the orbit of electron-induced secondary electrons around a charged microfibril of a sciatic nerve tissue. In order to set the parameters for the simulation, the shape of the microfibril was determined from a transmission electron microscopy image, while the electric potential on the surface of the charged microfibril was evaluated from a reconstructed phase image obtained with electron holography. On the other hand, the passing point and the angle of secondary electrons at the microfibril surface were determined from a reconstructed amplitude image. Eventually, simulation of orbits of secondary electrons was carried out by changing the kinetic energy of the secondary electrons. Under the given conditions, the orbit of secondary electrons with a kinetic energy of 29.6?eV fits the observations. If there are thin layers of electrons, the secondary electrons do not reach the surface but they go over it due to the repulsive Coulomb force resulting in successive revolving motion around the charged microfibril. Furthermore, the electric field variation due to the movement of the electric charges resulting from the specimen drift is also discussed briefly comparing it with electron holography data.     

Monte Carlo simulation of the energy dissipation profiles of 30, 50 and 100 keV incident beams in a layered structure in electron beam lithography

This paper presents a Monte Carlo simulation of the energy dissipation profiles of 30, 50 and 100 keV incident beams in thin film (0.4μm) of electron resist, polymethyl methacrylate (PMMA), on thick silicon substrate in electron beam lithography. The radial scattering and the energy loss of incident electrons (including backscattered electrons from the substrate) are simulated under the illumination of ideal point source and Gaussian round beam spot source, and the histories of 30000–50000 electrons are computed.