Focused Ion Beam (FIB) System:Introduction to System Functions and Common Applications

Focused ion beam function

In the FIB system, a wide range of applications can be achieved. However, in terms of the main functions of the ion beam, there are three: imaging, sputtering, and deposition.

Figure 1:The three working modes of FIB are: (a) imaging, (b) sputtering, and (c) deposition

Imaging:The focused ion beam can perform progressive scanning on the micro-area of the sample surface like an electron beam. During this process, the ion beam interacts with the atoms on the surface layer of the material to generate secondary electrons and secondary ions. These electrons or ions are collected by the corresponding detector and can then image the material surface. Compared with SEM, the penetration ability of the ion beam along different crystal directions is different. Therefore, ion beam imaging can be used to analyze the grain orientation, grain boundary distribution and grain size distribution of polycrystalline materials. Ion beam imaging also has the advantage of reflecting the detailed morphology of the material's surface layer more truly. When gallium ions bombard the sample, positive charges will preferentially accumulate in the insulating area or discrete conductive area, suppressing the excitation of secondary electrons. Therefore, the insulating area and discrete conductive area on the sample will be darker in color on the ion image, while the grounded conductor will be brighter, thus increasing the contrast of the ion imaging. Use of ion channel effect, polycrystalline samples under the ion beam imaging, there will be obvious contrast effect channel effect ion channels. The channel effect refers to the fact that grains with different orientations have obvious contrast differences. Due to the weight of the ion beam, the penetration depth of ions varies for grains with different orientations. The secondary electron signal quantities excited by the ion beam of grains with different orientations are different, forming the contrast of grains with different orientations. Based on this, the size of crystal particles can be determined.

Figure 2:Cross-sectional images of copper samples: (a) images obtained by electron beam scanning, (b) images obtained by ion beam scanning

Sputtering:Sputtering is the most important function of FIB processing. Sputtering is a phenomenon where incident ions transfer energy to the atoms of a solid target material, enabling these atoms to gain sufficient energy and escape from the solid surface. Ion sputtering is not a one-to-one process. When an ion beam bombards a target material, it generates a large number of rebounding atoms. These rebounding atoms further transfer their energy to the surrounding atoms, forming more rebounding atoms. Among them, some rebounding atoms close to the material surface may gain sufficient kinetic energy to break free from the constraints of surface energy and become sputtering atoms. One of the most important parameters of ion sputtering is the sputtering yield (sputtering yield), which also determines the processing efficiency of FIB. The yield of ion sputtering is not only related to the energy of the incident ions, but also to the incident Angle, the atomic density and mass of the target material, and the crystallographic orientation. In practice, for gallium ion beams, the sputtering yield with energy above 30 keV no longer shows significant changes. Therefore, the general commercial focused ion beam system usually operates within 30 keV. Sputtering is usually accompanied by the phenomenon of atomic redeposition. As the processing depth increases, more and more sputtered atoms will deposit on the processing side walls. This phenomenon can be reduced by decreasing the residence time. In addition, you can pass into the auxiliary Gas, Gas Assisted Etching (Gas Assisted Etching), which can realize the Etching speed significantly increase and decrease to some material deposition effect. There are two situations of GAE: One is to use a gas that does not react with the etched sample material. The role of this gas is only to form a surface gas flow during the etching process; The second method is to use reactive gas etching (CL2, I2, Br2, XeF2), which means that during direct physical sputtering etching, the gas can simultaneously react with the sputtering products of the sample, thereby effectively suppressing the redeposition effect.

Figure 3:Schematic diagram of redeposition formation

Deposition:In addition to achieving the etching function through the sputtering effect of the ion beam, the energy of the ion beam triggers chemical reactions to deposit metallic materials (such as Pt, W, Au, etc.) and non-metallic materials (such as C, SiO2, etc.), thereby realizing induced deposition. A gas injection system is added to the FIB system. The precursor gas is generated by heating and introduced into the sample surface. When the ion beam is focused on this area, the energy of the ion beam induces the precursor gas to react, generating solid components that remain on the sample surface, while the remaining volatile components are removed by the vacuum system. During the deposition process, the ion beam is constantly bombarding the material surface. Therefore, ion sputtering and molecular deposition processes coexist and compete with each other. It is necessary to carefully adjust the ion energy, dosage, and the pressure and flow rate of the gas introduced to ensure that the deposition rate is greater than the sputtering rate, thereby continuously thickening the deposited film.During the induced deposition process, the ion beam is constantly bombarding the material surface. Therefore, the ion sputtering and molecular deposition processes coexist and compete with each other. It is necessary to carefully adjust the ion energy, the dose per unit time, the pressure and flow rate of the gas introduced to ensure that the deposition rate is greater than the sputtering rate, thereby continuously thickening the deposited film.

Figure 4:Metal Pt patterns induced by ion beams for deposition

Introduction to Comprehensive Application

Sample preparation by transmission electron microscopy (TEM):The sample preparation for transmission electron microscopy plays a very important role in the research of electron microscopy. TEM sample preparation requires the preparation of very thin samples so that electrons can penetrate the samples and form electron diffraction images. The general required thickness should be below 100nm. In the past, the preparation of transmission electron microscope samples was done by manual grinding and ion sputtering thinning, which was time-consuming, labor-intensive and had a low success rate. It was generally used for the preparation of transmission electron microscope samples made of block materials, and it was difficult to precisely locate the samples. The focused ion beam technology has successfully solved the problem of preparing samples with precise positioning for transmission electron microscopy. FIB can deposit a protective layer at a fixed point in the required area. After processing, it uses a focused ion beam to process and dig pits from both the front and back directions of the slice area. Then, it uses a nano-manipulator to transfer the sample to a copper mesh for final thinning to form a thin sheet with a thickness of less than 100nm as the TEM observation sample.

Figure 5:TEM samples prepared with FIB

Characterization and analysis of cross-sectional cutting:The sputtering etching function of FIB can be utilized to perform fixed-point cutting on the sample, observe its cross-section to characterize the cross-sectional morphology and size. Meanwhile, it can be equipped with a combined elemental analysis (EDS) system, etc., to analyze the cross-sectional composition. It is generally used in the field of failure analysis for chips, leds, etc. When problems occur during the processing of general IC chips, the FIB system can quickly and precisely analyze the cause of the defect and improve the process flow. The FIB system has become an indispensable device on modern integrated circuit process lines.

Figure 6:FIB conducts cross-sectional cutting analysis on the defect areas of the sample

Chip repair and circuit editing:In IC design, it is necessary to verify, optimize and debug the design changes of the formed integrated circuits. When problems are found, these defective parts need to be repaired. The current integrated circuit manufacturing process is constantly shrinking. The number of lines is also constantly increasing. By using the sputtering function of FIB, the connection at a certain point can be disconnected, or by taking advantage of its deposition function, the originally unconnected parts at a certain location can be connected, thereby changing the direction of the circuit connection. It can detect and diagnose circuit errors and directly correct these errors on the chip, reducing R&D costs and accelerating the R&D process. Because it saves the time and cost of preparing the prototype and changing the mask.

Figure 7:Re-edit the line using FIB

Preparation of micro-nano structures:The FIB system does not require a mask and can directly engrave or deposit the required graphics under a GIS system. With the FIB system, complex functional structures at the micro and nano scales can already be fabricated, including nano-quantum electronic devices, subwavelength optical structures, surface plasmonic components, photonic crystal structures, etc. Through reasonable methods, not only the two-dimensional planar graphic structure can be achieved, but even the preparation of complex three-dimensional structural graphics can be realized.

Figure 8:Complex three-dimensional structural patterns prepared by FIB deposition and sputtering

Repair of lithography masks:In ordinary optical lithography, the mask plate is the origin of the pattern. However, after long-term use, the pattern on the mask plate may be damaged, resulting in pattern defects after lithography. The mask plate is expensive. If a small pattern defect on the mask plate causes the entire mask plate to fail and a new mask plate to be prepared, the cost will be high. The defects of the mask plate can be repaired at specific points by using the FIB system. The method is simple and the operation is simple and rapid. For defect repair in the light-transmitting area, ion deposition can be used, and deposition C is selected as the repair material for the mask. In the defect repair of the light-blocking area, ion sputtering is used to etch off the light-blocking defects. However, the biggest problem with using FIB to repair the mask is that it can cause Ga ion contamination, altering the glass's light transmittance and resulting in residual defects. This issue can be resolved by using RIE combined with cleaning to etched and remove the surface layer of glass with Ga ion injection, thereby restoring the glass's light transmittance.

Three-dimensional reconstruction analysis:3D imaging analysis of material three-dimensional reconstruction using FIB is also a field that has grown rapidly in recent years. This method is mostly used in disciplines such as materials science, geology, and life science. The main purpose of three-dimensional reconstruction analysis is to alternate between FIB layer-by-layer cutting and SEM imaging controlled by software, and finally conduct three-dimensional reconstruction through software. The effective combination of FIB three-dimensional reconstruction technology and EDS enables researchers to characterize the structural morphology, composition and other information of materials in three-dimensional space. Combined with EBSD, it can characterize the structure, orientation, grain morphology, size, distribution and other information of polycrystalline materials in the spatial state.

Figure 9:Three-dimensional reconstruction effect diagram of shale gas

Preparation of atomic probe samples:Atomic probes (AP) can be used for three-dimensional imaging (Atom Probe Tomography, APT), and can also quantitatively analyze the chemical composition of samples at the nanoscale. An important condition for achieving this application is to prepare a probe with a large aspect ratio and sharpness, and the size of the needle tip should be controlled at about 100 nm. The preparation requirements for atomic probe samples are very close to those for TEM thin slice samples, and the methods are also similar. First, select the sampling position of interest, dig V-shaped grooves on both sides, cut the bottom, and then use a nano-manipulator to take out the sample. Transfer to the fixed sample support, weld with Pt and cut from the large sample. Continuously remove the peripheral part from the outside to the inside to form a sharp needle tip. Finally, the sample is ultimately polished with an ion beam at a low voltage to eliminate the amorphous layer and the areas with more ion implantation.

Figure 10:Atomic probes of metallic silicon prepared by FIB

Ion implantation:The research on ion beam injection modification is also a fundamental research topic in FIB processing. For instance, when high-energy ion beams are used to bombard the surface of monocrystalline silicon, if the injection volume is sufficient, the ion bombardment will introduce vacancies and cause amorphous plasma bombardment damage on the surface of the sample. During this process, the injected ions collide with the orderly arranged Si atoms inside the material and generate energy transfer, causing the originally orderly arranged Si atoms to become disordered and form an amorphous layer beneath the surface. The injected ions lose energy during the collision process and eventually remain in an area at a certain depth from the surface.