Focused Ion Beam sample preparation for electron microscopy often requires large volumes of material to be removed. Prior efforts to increase the rate of bulk material removal were mainly focused on increasing the primary ion beam current. Enhanced sputtering yield at glancing ion beam incidence is known, but has not found widespread use in practical applications. In this study, etching at glancing ion beam incidence was explored for its advantages in increasing the rate of bulk material removal. Anomalous enhancement of material removal was observed with single raster etching in along-the-slope direction with toward-FIB raster propagation at glancing ion beam incidence. Material removal was inhibited with raster propagation away from FIB. The effects of glancing angle and ion dose on depth of cut and volume of removed material were also recorded. We demonstrated that the combination of single-raster etching in along-the-slope direction by raster propagating toward-FIB at glancing incidence and a “staircase” type of etching strategy holds promise for reducing the process time for bulk material removal in FIB sample preparation applications.

Widespread adoption and significant developments in Focused Ion Beam technology has made FIB/SEM instrumentation a commonplace sample preparation tool. Fundamental limitations inherent to Ga ion species complicate usage of Ga+ FIB instruments for the modification of semiconductor devices on advanced technology nodes. Said limitations are fueling interest in exploring alternative primary species and ion beam technologies for circuit edit applications. Exploratory tests of etching typical semiconductor materials with Xe ion beams generated from two plasma ion sources confirmed advantages of Xe+ as a potential ion species for gas-assisted etching of semiconductor materials, but also revealed potential complications including, swelling of metal and Xe+ retention within the material arising from excessive Xe ion beam current density.

Despite commercial availability of a number of gas-enhanced chemical etches for faster removal of the material, there is still lack of understanding about how to take into account ion implantation and the structural damage by the primary ion beam during focused ion beam gas-assisted etching (FIB GAE). This paper describes the attempt to apply simplified beam reconstruction technique to characterize FIB GAE within single beam width and to evaluate the parameters critical for editing features with the dimensions close to the effective ion beam diameter. The approach is based on reverse-simulation methodology of ion beam current profile reconstruction. Enhancement of silicon dioxide etching with xenon difluoride precursor in xenon FIB with inductively coupled plasma ion source appears to be high and relatively uniform over the cross-section of the xenon beam, making xenon FIB potentially suitable platform for selective removal of materials in circuit edit application.

Traditional approaches to navigation in focused ion beam (FIB) circuit edit include blind CAD navigation based on GDSII data from the manufacturer and navigation assisted by the in-situ optical microscope (OM). These approaches are difficult to apply in security audit and reverse engineering fields, where CAD data are unavailable and objects of interest are either too small, or located in an array that is too dense for imaging by in-situ OM. To address this issue, this article presents a methodology which is based on establishing a chip-specific system of coordinates and determination of precise locations of the objects of interest within the device. The work was performed on a Vectra 986 FIB system from FEI Company and a proprietary system for optical scanning of semiconductor devices. Auxiliary techniques allowing enhancement of navigational accuracy, developed for this application, are equally applicable to the general navigation procedures during generic FIB circuit modification.

Gas Assisted Etching (GAE) is the enabling technology for High Aspect Ratio (HAR) circuit access via milling in Focused Ion Beam (FIB) circuit modification. Metal interconnect layers of microelectronic Integrated Circuits (ICs) are separated by Inter-Layer Dielectric (ILD) materials, therefore HAR vias are typically milled in dielectrics. Most of the etching precursor gases presently available for GAE of dielectrics on commercial FIB systems, such as XeF2, Cl2, etc., are also effective etch enhancers for either Si, or/and some of the metals used in ICs. Therefore use of these precursors for via milling in dielectrics may lead to unwanted side effects, especially in a backside circuit edit approach. Making contacts to the polysilicon lines with traditional GAE precursors could also be difficult, if not impossible. Some of these precursors have a tendency to produce isotropic vias, especially in Si. It has been proposed in the past to use fluorocarbon gases as precursors for the FIB milling of dielectrics. Preliminary experimental evaluation of Trifluoroacetic (Perfluoroacetic) Acid (TFA, CF3COOH) as a possible etching precursor for the HAR via milling in the application to FIB modification of ICs demonstrated that highly enhanced anisotropic milling of SiO2 in HAR vias is possible. A via with 9:1 aspect ratio was milled with accurate endpoint on Si and without apparent damage to the underlying Si substrate.

Precision detection of endpoint after the milling has reached targeted conductor during circuit modification by focused ion beam system is important. While the sensitivity of the endpoint detection can be enhanced by improved secondary electron collection and sample absorbed current monitoring, a detailed understanding of the endpoint signal distribution within a high aspect ratio (HAR) via is of great interest. This article presents an alternative model of HAR via milling endpointing mechanism in which a phenomenon of spatial distribution of the endpoint information within the HAR via is explained based on sputtering of the material from the targeted metal line and redeposition of the spattered material on the via sidewalls. Increased emission of the secondary electrons, resulting from the subsequent bombardment of this conductive re-deposition by the primary ion beam, is detected as the endpoint. A methodology for the future experimental verification of the proposed model is also described.

Secondary electron signal is widely used in Focused Ion Beam (FIB) systems for imaging and endpointing. In the application of integrated circuit modification, technology has progressed towards smaller dimensions and higher aspect ratios. Therefore, FIB based circuit modification processes require the use of primary ion beam currents below 10 pA and Gas Assisted Etching (GAE). At low beam currents, short pixel dwell times and high aspect ratios, the level of available secondary electrons for detection has declined significantly. FIB GAE and deposition requires delivery and release of a gaseous agent near the beam scanning area, and involves insertion of a gas delivery nozzle made of conductive material and grounded for charge prevention purposes. The proximity of a grounded gas delivery nozzle to the area being milled and/or imaged creates a “shielding” effect, further lowering secondary electron signal level. The application of a small positive bias to the gas delivery nozzle provides an effective way of reducing the “shielding” effect. Depending on the geometrical arrangement of the gas delivery system and other conductive objects in the chamber, an optimized nozzle bias potential can create conditions favorable for enhanced extraction and collection of secondary electrons. The level of the secondary electron image signal, collected in an FEI Vectra 986+ system, from a grounded copper sample with the nozzle extended and biased can be enhanced as much as six times as compared to the grounded nozzle. Secondary electron intensity endpoint is improved on backside samples, however shielding of the nozzle field by the bulk silicon substrate limits the electron extraction effect from within a via. For front side edits the improvement of endpoint signal level can be dramatic. Lateral image offset induced by the electrostatic field of a biased nozzle, can be removed by software position compensation.

As semiconductor device manufacturing technologies move below the 100 nm node constrains on using Focused Ion Beam (FIB) systems to perform circuit edit operations tighten dramatically. Phenomena associated with via milling and deposition processes, considered minor side effects in the past, may become performance-limiting factors. Obstacles, associated with editing deep sub micron technologies beyond 100nm node, which include navigational accuracy, beam placement stability, and small via milling and filling processes, cannot be completely overcome without advances in overall FIB system performance and operation. We present a detailed technical overview of the challenges, associated with silicon microsurgery on devices, manufactured with sub 100 nm process technology and describe recent advancements in FIB technology and techniques which address these areas and allow successful modification of today’s most advanced designs.

Advances in FIB (focused ion beam) chemical processes and in the Ga (gallium) beam profile are discussed; these advances are necessary for the successful failure analysis, circuit edit and design verification of advanced, sub-0.13µm Cu devices. Included in this article are: a novel FIB method (CopperRx) for smoothly milling thick, large grained Cu lines; H2O and O2 processes for cleanly cutting thin, smaller grained Cu lines, thereby forming electrically open interconnects; a XeF2 GAE (gas assisted etching) process for etching low k, CVD dielectrics such as F and C doped SiO2; H2O and XeF2 GAE processes for etching low k, spin-on, organic dielectrics such as SiLK; a recently developed recipe for the deposition of SiO2 based material with intermediate resistivity (10 6 µohm·cm) which is useful in the design verification of frequency sensitive, high speed analog and SOC (system on chip) circuits; an improved, more Gaussian Ga beam with less current density in the beam tails (VisION column) which provides higher resolution, real time images needed for end-point detection on sub 0.13µm features during milling.