Review: Developments in micro/nanoscale fabrication by focused ion beams

Focused ion beam (FIB) technology has become increasingly attractive for the fabrication of micro/nano structures for the purpose of the demands in industry and research. In this paper, various efforts to fabricate micro/nanoscale structure and geometrically complex structure are described, and their efficiency and structural stability are discussed. In order to fabricate such complex structures with micro/nanoscale features, the capability of the FIB is directly related to its destructive and constructive processes. It can also be used for modification and imaging in four basic modes. The fabrication processes, including both milling and deposition, are related to the precision fabrication of samples at the micro/nanoscale. By taking into account material processing rates, surface morphologies with nanoscale effects can be explained in detail. Finally, very recent developments using FIB will be reviewed.     

Evaluation of annular pupil for scanning transmission electron microscope formed by focused ion beam technique

Annular pupils for electron optics were developed using a focused ion beam (FIB) technique to realize an increase in the depth of focus, aberration-free imaging and separation of amplitude and phase images under scanning transmission electron microscopy (STEM). A tantalum plate 30 μm thick was used as the annular pupil material in the present experiment. The annular pupils were designed with various outer diameters from ?120 μm to ?40 μm. The inner diameter was designed at 60 to 80% of the outer diameter. The fabricated annular pupils were inspected by scanning ion beam microscopy and scanning electron microscopy. Annular pupils were successfully obtained at the designed size, although the slits of the pupils were slightly tapered by the ion beam etching process. These annular pupils were loaded on a STEM and confirmed to display no charge-up phenomenon by observation of the projection image on a scintillator using a CCD camera. We confirmed the image taken by annular pupil with narrow width was able to suppress the influence of the normal illumination.     

Formation of square matrix pores on Al film utilizing focused ion beam milled indent

We successfully fabricated a square pattern of anodic alumina oxide (AAO) template in thin Al film grown on Si substrate utilizing focused ion beam (FIB) milled indent. The pore size and its period of the square matrix were about 50 nm and 100 nm, respectively. We found that the minimum indented depth is required to satisfy the critical electric field which is a vertical component toward downward. In addition, we found that the anodizing voltage plays an important role in determining the pore shape and uniformity. The higher anodizing voltage not only reduces the minimum indent depth that is required to create a matrix array but also improves the uniformity of pore shapes in the matrix array.     

电子-离子双束纳米工作站

现代微电子工业迅猛发展,对电子设备的精度和功能也提出了新的要求。电子-离子双束纳米工作站是扫描电子束和聚焦离子束技术的独特融合,突破了只能对表层成像和分析的局限,它可以对样品进行三维的、表面下的观察和分析,也可以切割、研磨样品材料和沉积特定的材料,用它可以获得以前无法得到的样品信息。该工作站为研究人员和制造人员提供了一种对多种样品在纳米尺度进行修改、制作和分析的有效工具。     

Focused ion beam production of nanoelectrode arrays

We present a method for the production of nanoelectrodes using focussed ion beam techniques (FIB). The electrodes utilise nanometric holes milled in a silicon nitride based pasivation layer, followed by wet etching of a silicon oxide based pasivation layer, to expose an underlying gold electrode. After functionalisation using a surface assembled monolayer and an electrochemically grown polypyrrole, these gold nanoelectrodes have been tested, via cyclic voltammetry, in the detection of [Fe(CN)₆]^4−/3− ions. The nanoelectrodes will be used to investigate the electrical properties of nanometric biological specimen.     

Interfacial microstructure and defect analysis in Cu(In,Ga)Se([sub]2)-based multilayered film by analytical transmission electron microscopy and focused ion beam

Interfacial microstructures of Cu(In,Ga)Se₂(CIGS)-based multilayered film are closely characterized by TEM (transmission electron microscopy), SEM (scanning electron microscopy) and FIB (focused ion beam). A cross-sectional TEM, energy dispersive X-ray spectroscopy and energy-filtered TEM reveal that a pronounced Cu diffusion occurs across the interface of the CdS/CIGS, which leads to a large amount of Cu rich in the CdS layer and a Cu-deficient sub-surface in the CIGS layer as well as a rough interfacial structure. TEM studies further reveal that the interface microstructures in the multilayered film are dissimilar, both ZnO/CdS and CdS/CIGS interfaces are strongly bonded whereas the CIGS/Mo interface is weakly bonded and interface separation occasionally occurs. Mo back contact layer shows a well adhesion to glass substrate.Detailed observation on defects in the CIGS-based multilayered film is carried out by 3D (3-dimensional) FIB and SEM techniques. Sequential 2D (2-demensional) cross-sectioning shows that dominant growth-defects in the CIGS and top SiO₂ layers are micro-scale crack, appearing as diversified morphologies. The micro-scale crack in the CIGS layer is possibly released by propagating into the adjacent layer while the crack in the SiO₂ layer is relieved usually by forming a small particle behind. It is noted that in the multilayered film the interface frequently acts as crack initiation sites due to distinct thermal expansion coefficients.     

Carbon coatings on silica glass optical fibers studied by reflectance Fourier-transform infrared spectroscopy and focused ion beam scanning electron microscopy

Carbon coatings applied on optical fibers via chemical vapor deposition were characterized by a resistance technique, focused ion beam/scanning electron microscopy (FIB/SEM), and reflectance Fourier-transform infrared spectroscopy (FTIR). The resistance technique measures the thickness of carbon film by measuring the resistance over a section of optical fiber, and backing out the film thickness. The FIB/SEM system was used to remove a cross section of the optical fiber and carbon coating and using a scanning transmission electron detector the thickness was measured. The FTIR approach is based on the fact that the wavelength of the light in the mid-infrared region (~ 10 μm) is significantly larger than the typical thickness of the carbon coatings (< 0.1 μm) which makes the coating “semi-transparent” to the infrared light. Carbon coating deposition results in significant transformations of the band profiles of silica in the reflectance spectra that were found to correlate with the carbon coating thickness for films ranging from 0.7 nm to 54.6 nm. The observed transformations of the reflectance spectra were explained within the framework of Fresnel reflection of light from a dual-layer sample. The advantage of this approach is a much higher spatial resolution in comparison with many other known methods and can be performed more quickly than many direct measurement techniques.     

Effect of molecular adsorption on the electrical conductance of single au nanowires fabricated by electron-beam lithography and focused ion beam etching

Metal nanowires are one of the potential candidates for nanostructured sensing elements used in future portable devices for chemical detection; however, the optimal methods for fabrication have yet to be fully explored. Two routes to nanowire fabrication, electron-beam lithography (EBL) and focused ion beam (FIB) etching, are studied, and their electrical and chemical sensing properties are compared. Although nanowires fabricated by both techniques exhibit ohmic conductance, I-V characterization indicates that nanowires fabricated by FIB etching exhibit abnormally high resistivity. In addition, the resistivity of nanowires fabricated by FIB etching shows very low sensitivity toward molecular adsorption, while those fabricated by EBL exhibit sensitive resistance change upon exposure to solution-phase adsorbates. The mean grain sizes of nanowires prepared by FIB etching are much smaller than those fabricated by EBL, so their resistance is dominated by grain-boundary scattering. As a result, these nanowires are much less sensitive to molecular adsorption, which mediates nanowire conduction through surface scattering. The much reduced mean grain sizes of these nanowires correlate with Ga ion damage caused during the ion milling process. Thus, even though the nanowires prepared by FIB etching can be smaller than their EBL counterparts, their reduced sensitivity to adsorption suggests that nanowires produced by EBL are preferred for chemical and biochemical sensing applications.     

Microstructural characterization of a carbonitrided heat resisting alloy using focused ion beam-based techniques

Abstract

An austenitic Fe–25Cr–20Ni (wt.%) alloy was first nitrided in N₂–H₂ then carburised at 1000°C. The nitridation produced lamellar internal precipitates of Cr₂N which grew by discontinuous precipitation at a recrystallised austenite boundary formed at the precipitation front. Subsequent carburisation converted the Cr₂N to a mixture of chromium-rich M₇C₃+CrN, and released nitrogen into the austenite matrix. The submicron CrN precipitates were stabilized by a matrix supersaturated with nitrogen and exhibited a strong orientation relationship with the surrounding austenite. The rejected nitrogen diffused deeper into the alloy to form new, more finely spaced Cr₂N lamellae. Carbon diffusion overtook the nitride structure and precipitated finely spaced, lamellar M₂₃C₆ by a discontinuous precipitation process. This process is available to the carbides only when a prior boundary is present. This phenomenon, and the ability of nitridation to form a boundary and the inability of carburisation to do so, are interpreted using the energetics of nucleation.     

Focused ion beam ring drilling for residual stress evaluation

Focused Ion Beam (FIB) milling of ring-shaped trenches (ring drilling) induces controlled gradual strain relief at the surface of residually stressed samples, with the central “island” approaching the unstressed state. Strain change was measured at sample surface via Digital Image Correlation analysis of SEM micrographs of a regular pattern of dots deposited on the sample surface prior to ring drilling. FE modeling of strain evolution with ring drilling depth agrees with experimental observations. The technique proposed represents a substantial improvement over prior attempts, constituting an efficient semi-destructive method for accurate residual stress evaluation at the (sub)micron scale.     

Investigation of electrical transport characteristics of nanoscale stacks fabricated on thin graphite layer

We investigated the electrical transport characteristics of nanoscale stacks of thin graphite layer fabricated using focused ion beam (FIB) three dimensional (3D) etching. By varying the stack height, we fabricated nanostacks with the dimensions of W = 1 μm, L = 1 μm, H = 100 nm and W = 1 μm, L = 1 μm, H = 200 nm. The nanostack contains lot of elementary junctions between the graphene layers with interlayer distance of 0.34 nm (along c-axis). We observed a typical semiconducting characteristic until 50 K and a metallic behavior below 50 K for all the fabricated nanostacks, which was well agreed with an identical c-axis characteristics of graphite. From I-V characteristics, we found an ohmic-like characteristic at 300 K for both low and high current biasing; however at low temperature, this behavior was turned into nonlinear characteristics when it was highly biased. The temperature dependent differential conductance measurements for the fabricated stacks were carried out and found that at 300 K, the stack showed larger conductance rather than at any other lower temperatures. However, the stack with 100 nm height showed larger conduction values of 36.3 mS and 16.95 mS for 300 K and 20 K respectively, which are comparatively smaller values of stack with 200 nm height. We also studied conductance fluctuations of nanostacks experimentally and the fluctuations are appearing on increasingly fine when the temperature goes down; however the amplitude of fluctuation is suppressed at low temperature.     

浅析离子辅助光学薄膜镀膜机的特性

真空镀膜产品的质量问题，一直是应用镀膜技术人员探索的课题。人们经过长期探索，采用离子束辅助沉积镀膜技术，改善了镀膜层牢固性，提高了反射率。本技术在通讯技术、太阳能应用、汽车、装潢等领域应用十分广泛。本文介绍了真空离子束辅助沉积镀膜设备特性。     

Application of focused ion beam (FIB) microscopy to the study of crack profiles

Focused ion beam (FIB) microscopy is used to form an image similar to that formed in a conventional scanning electron microscope (SEM), but with greatly enhanced grain contrast due to the use of a primary ion beam as opposed to a beam of primary electrons as found in the SEM. In addition, by increasing the current of the positively charged gallium ion beam, it can be used as a precision, essentially stress-free milling machine on a scale of tens of nanometres to several hundred microns. In this paper, the application of FIB microscopy to the study of crack profiles is reported. Samples of a Grade 448 (X-65) pipeline steel were cyclically loaded in a dilute simulated groundwater solution of near neutral pH. Cracks that initiated on these samples were imaged in plan view and were locally cross-sectioned using a high-current focused gallium ion beam at different locations along the cracks. Samples were then tilted in situ to permit imaging of the crack profile, the grain structures around the crack tip and accurate measurement of the crack depth using the same ion beam at lower currents. The transgranular crack path and other microstructural features associated with crack development were clearly illustrated, and crack aspect ratio (depth/length) could be obtained. The capability of the FIB for enhanced imaging of grain structures, combined with its precision-sectioning capability, make it a novel new tool for the study of cracks.     

Observation of submicronic contacts and vias by scanning ion and electron microscopy

Scanning ion and electron microscopy of focused ion beam cross-sections has been used to observe 0·6 μm contacts and 0·7 μm vias of a 0·5 μm CMOS technology. Three different kinds of cross-sections have been tested to improve the observation quality. The comparison of images obtained with ions and electrons provides information on ion and electron contrast mechanisms. This technique has been used to identify very small defects in interconnection structures and to evaluate the 0·5 μm CMOS metallization process.     

双离子束溅射法制备铁氮薄膜

使用双离子束溅射法制得了铁氮薄膜,随着基片温度的变化,薄膜的成分是ε－Fe2-3N,γ′－Fe4N或是二相的混合物,薄膜的晶粒尺寸随基片温度的升高而增大。以振动样品磁强计（VSM）测定了薄膜的磁性能。另外,研究了在基片温度为160℃时,改变主源中通入N2／Ar的比例对薄膜成分的影响。     

低能离子束方法制备磁性Fe-Si合金薄膜

利用质量分离的低能离子束技术．获得了磁性Fe-Si合金薄膜。利用俄歇电子能谱法(AES)、X射线衍射法(XRD)以及交变梯度样品磁强计(AGM)测试了样品的组分、结构以及磁特性。测试结果表明在室温下制备的Fe-Si合金是Fe组分渐变的非晶薄膜,具有室温铁磁性。当衬底温度为300℃时制备的非晶Fe-Si薄膜中有Fe硅化物FeSi相产生．样品的铁磁性被抑制。     

离子束刻蚀机的电源系统

离子束刻蚀机的性能,除主机外,在很大程度上取决于供电电源系统。为了提高离子束刻蚀机整机工作的稳定性,在以往的配套电源的基础上,研制了一套电源系统,用于离子束刻蚀机中。整机工作稳定度达±0.8%/h,每次连续刻蚀器件8h以上,连续运转3年多,电源系统稳定可靠。是国内离子束刻蚀机电源系统性能较好的供电电源系统。     

离子束辅助脉冲激光沉积硼碳氮薄膜

以烧结B4C为靶材料、在氮离子束辅助下用脉冲激光沉积方法制备了三元化合物硼碳氮（BCN）薄膜.用X光电子谱和傅立叶变换红外谱方法表征了制备的薄膜.结果表明,膜层中包含B-C、N-C、B-N键等复合结构,以B-C-N原子杂化的形式结合成键,而并非各种成分的简单混合.还探讨了成膜过程和相关机理,离子束中的活性氮有效地和脉冲激光对B4C靶烧蚀产生的硼和碳结合成键,氮离子束的辅助还能在一定程度上抑制氧杂质进入膜层,给衬底适当加温有利于提高氮的含量并影响薄膜的化学结构.     

离子束溅射生长Ge纳米薄膜的表面形貌观察

采用离子束溅射技术并按正交试验方案生长了不同厚度以及在不同条件下退火的Ge纳米薄膜,用AFM图谱对薄膜的表面形貌进行了表征.结果表明厚度为2.8nm的Ge膜在600℃下退火10min,出现了高4nm、直径50nm左右的Ge岛,而10nm厚的Ge膜在720℃下退火120min,岛的数量较多且分布比较均匀.通过离子束溅射机理和沉积原子之间的扩散运动,对这些现象进行了较为合理的解释.