Defect Localization and Nanofabrication for Conductive Structures

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Surfaces, Interfaces, and Applications

Defect Localization and Nanofabrication for Conductive Structures with Voltage Contrast in Helium Ion Microscopy Deying Xia, Shawn McVey, Chuong Huynh, and Wilhelm Kuehn ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b18083 • Publication Date (Web): 15 Jan 2019 Downloaded from http://pubs.acs.org on January 19, 2019

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ACS Applied Materials & Interfaces

Defect Localization and Nanofabrication for Conductive Structures with Voltage Contrast in Helium Ion Microscopy Deying Xia*, Shawn McVey, Chuong Huynh and Wilhelm Kuehn.

Carl Zeiss SMT Inc, PCS Integration Center, One Corporation Way, Peabody, MA 01960

ACS Paragon Plus Environment

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ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Corresponding Author *E-mail: [email protected]


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ABSTRACT

As the dimensions of feature sizes in electronic devices decrease to nanoscale, an easy method for failure analysis and evaluation of processing steps is required. Gallium focused ion beam (Ga-FIB) or scanning electron microscope (SEM) are efficient approaches to detect voltage contrast for addressing failure analysis in semiconductor devices and processing. However, Ga-FIB may cause damage or implantation to the surface of the analyzed area, and its resolution is low. Helium ion microscopy (HIM) uses a light ion beam (helium or neon) for imaging and fabrication in nanoscale. With passive voltage contrast (PVC) in HIM images, the defect localization for failure of conductive structures can be rapidly and easily detected with sufficient voltage contrast. Furthermore, a defect gap as narrow as sub 10-nm can be investigated with HIM imaging. PVC with HIM is an efficient method for defect localization at nanoscale with minimal damage to the analyzed area. For circuit edit and failure analysis, it may be necessary to intentionally cut the conductive connection. In this circumstance, final results can be easily verified using PVC imaging with HIM. With XeF2 gas assistance, both helium and neon ion beams


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can be used to perform nanofabrication for metal disconnection. XeF2 gas plays an important role in preventing deposition of conductive materials on etching region and enhancing material removal rates to achieve electrically isolated structures. The etching rate with a neon ion beam is much faster than that of a helium ion beam. PVC in HIM images with controllable operation and dimensions using a helium ion beam with XeF2 gas assistance could also be used to localize a hidden defect for a single-location-defect situation. With neon ion beam irradiation on a defective location, PVC can be used to find the defect locations in the case of a series of defects.

KEYWORDS: nanofabrication, voltage contrast, defect, helium ion microscopy, etching, XeF2, focused ion beam.


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Introduction

The increase of computing power and energy efficiency has pushed semiconductor processing technology advancement in recent years. New products with low cost, high computing speed and low energy consumption will need a combination of reduced feature and cell size, novel device architectures and new materials in semiconductor process technology.

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The dimensional feature sizes (transistor fin, gate, interconnect pitch etc.)

in electronic devices approach a near single-digit nanometer scale as Moore’s law predicted. The number of transistors and memory arrays, the complexity of architectures and the number of structure layers will be increased to ensure the advancement of integrated circuit performance. As a result, failure analysis becomes increasingly critical not only for trouble-shooting and problem-solving, but also for evaluation of new processing techniques and novel materials, to ensure on-time delivery of new generation products to market. Voltage contrast effect is an effective way for failure analysis for a conductive connection.


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Passive voltage contrast (PVC) localization uses the greyscale differences in the focused ion beam (FIB) or scanning electron microscopy (SEM) images of conductive structures in the semiconductor circuit to identify failure as opens or shorts.

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The active voltage

contrast method uses the application of external bias to certain structures to produce local voltage contrast images, in order to check the potential failure locations. PVC is more direct and easier for initial fast failure analysis and process evaluation. Failure analysis for localization of defects using PVC is based on the principle that floating structures are charging up under the influence of the primary electron or ion beams in FIB and SEM. In general, the charging is positive and dependent on the beam current with the FIB, while it can be positive or negative dependent on beam energy for SEM.2 The accumulated charging in a floating structure causes the image to darken due to less electrons reaching the secondary electron detector while the ground structure appears to be brighter due to no charging effect in most cases. The Ga-FIB is a very important tool in the field of semiconductors for failure analysis, mask repair, transmission electron microscope (TEM) sample preparation and backside circuit edit.1,3,4 PVC using a Ga-FIB always has stronger charging effect due to its high


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beam current. However, the low image resolution and high sputtering rate make the GaFIB inappropriate for feature sizes below 100 nm in semiconductors. PVC using SEM can create high resolution images; however, its voltage contrast is more dependent imaging parameters such as beam energy, dwell time etc, and relatively low contrast for fast defect localization. 5 It is necessary to have a technology with a combination of high resolution, low sputtering rate and high contrast for defect localization in failure analysis using voltage contrast as the feature size approaches 10 nm in semiconductors. Focused helium ion microscopy (HIM) techniques have been developed in the past decade for high resolution imaging, nanofabrication and analysis in a wide range of applications. well,

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6,7

In terms of semiconductor applications, the HIM has been explored as

such as in dopant contrast mapping,

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nanolithography

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and devices.

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More

importantly, the combination of superior resolution, high secondary electron yield and relatively low sputter rate of the light element ions provides a great opportunity for HIM to be used for failure analysis using voltage contrast in semiconductor devices.

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Initial

results with high contrast using HIM imaging to investigate a fully processed dynamic random-access memory (DRAM) sample have been demonstrated. 1,11 Voltage contrast


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using HIM imaging is also used to visualize the nanofabrication of graphene patterns via conductivity properties.

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Active voltage contrast using HIM images for in-situ voltage-

application has been explored as well, to investigate the electrical potential distribution in nanoscale on a multilayer ceramic capacitor. 13, 14 More deep investigations in the use of PVC in HIM images are needed to explore the wide applications in failure analysis of different kinds of defects and nanofabrication of conductive structures. In this work, we use HIM based voltage contrast imaging to localize defects in dense and complex patterned conductive nanostructures. The voltage contrast images in this study illustrate passive voltage contrast. A Ti3N4 metal layer was chosen as the model materials for this work and is being widely used in current semiconductor processing. Using PVC in HIM images, it has been found that it is easy to find the locations of defects, and that higher magnification imaging can be used to find the broken points in the metal connection in sub 10-nm scale with less damage on patterned structures. Furthermore, artificial defects have been fabricated with ion beam assisted etching processing. XeF2 gas is used for this purpose. The artificial defect is helpful in identifying the hidden defect location using voltage contrast imaging in single-location defect cases. Finally, neon ion


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beam irradiation has been used to localize the positions of hidden defects when there is a series of defects.

Results and Discussions

The tested sample has layered structures as shown in Figure 1.A. The 25-nm patterned Ti3N4 nanostructures are on a 20 nm SiO2 layer on Si substrate. HIM images can be used to check patterned metal nanostructure as shown in Figure 1.B-D. The images of metal stripes of complicated nanostructures exhibit clearly the connected patterns of line widths as narrow as 10 nm both in a large area view and a small area view. The width of a stripe is ~340 nm and the gap between stripes is ~ 80 nm. From tilted images, it is easy to view the raised layer of metal nanostructures on the substrate (Figure 1.D). This tested sample is designed to test the process for large area uniformity and processing control in semiconductors. Images in Figure 1.B-D show the good metal connection (defect free) and a desirable processing region.


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On this sample, there is disconnection of metal nanostructures in some regions. Passive voltage contrast in HIM imaging is an effective way to perform defect localization for conductive connection structures. Figure 2.A shows the defective stripes (black) and wellconnected stripes (bright) over a large field of view. When moving the HIM image view along black stripes, one can find the starting points of the defective stripe as shown in Figure 2.B. Looking closely at the starting point of the defective stripe in the middle of the image in Figure 2.B, the three broken points as indicated by white arrows are easy to locate as shown in Figure 2.C. These three broken points make the left end of the stripe fully isolated as a floating structure from the right end. The broken gap is as small as 10 nm. The requirement for voltage contrast images is that the stripes are completely broken on both ends of the stripes, which allows the stripe segment (black) to charge and float away from ground or any other connection to the bulk structures. Therefore, we localize the end points of the defective stripe by moving along the image to the right of the stripe. In some cases, the defective stripe segments (black) could be tens or hundreds of micro meters in length. The image in Figure 2.D shows five long defective stripes and one short defective stripe. The short defective stripe segment is only 7 µm long with obvious broken


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gaps on both the starting point and the end point while the other five defective stripes are too long and beyond the field of view of the image. Therefore, using HIM voltage contrast images, it is quick and easy to localize defective stripes on patterned structures, and then find defect locations at the starting point and end-point of a defective stripe with a moving view of the sample. Compared to voltage contrast in SEM (Figure S1), the image contrast of PVC in HIM is comparable or a little higher due to the high charging effect in HIM.

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The voltage contrast in HIM is also less dependent on beam energy and the voltage contrast in SEM is heavily dependent on beam energy and detector.

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Another

advantage of voltage contrast in HIM images is a lower amount of damage to the surface of the sample. With the neon ion beam, the voltage contrast image is also visible on defective stripes (Figure S2), but the use of neon ion images is not recommended for voltage contrast for investigation of failed connections in nanoscale and thin metal structure because the neon ion beam may cause damage to the sample surface and the resolution is not as high as helium ion images for voltage contrast.


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In some circumstances, it is necessary to create an artificial defect for circuit edit or analysis of structure connection. Ion beam etching is an alternative approach for such nanofabrication beyond electron beam nanofabrication. Direct ion beam etching or milling is an easy way for nanofabrication of nanostructures (nanopore, nanogap) in applications in biosensors and plasmonic devices.

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A helium ion beam is first used to etch the

metal connection on the tested sample. The image in Figure 3.A shows two pairs of cuts using a helium ion beam, indicated by white arrows. On each pair of cuts, both ends of the stripes were etched with a small box of 80 nm × 10 nm to separate the narrow lines at the edge of the stripes, using a dosage of 3.7×1018 ions/cm2. From the HIM image, the cut of the metal connection seems to be successful; however, the image doesn’t exhibit the voltage contrast effect. An increased dose was then applied to form a wider cut. In Figure 3.B, on the middle bright stripe (connected stripe), the narrow helium ion beam cut (9-10 nm) on both edges of the stripe was applied on the left end with a beam current of 0.7 pA and dosage of 7.5×1018 ions/cm2, while the right end of stripe was cut with a high dosage of 5.0×1019 ions/cm2 using same beam current as the left end. Voltage contrast is still not observed, and the right end of the stripe is swollen and deformed in the area of


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the cut. As previous research results, the swelling of the surface using the helium ion beam is caused by formation of amorphization and helium bubbles with helium ion interaction on Si substrate. 17,18 Even on the large pattern with high dosage of helium ion beam etching, there is still no voltage contrast and the swelling effect is obvious (Figure S3). This is also a good example of voltage contrast in HIM images for micro-patterns. Therefore, voltage contrast with HIM imaging also has applications in failure analysis and process control for conductive structures of microelectromechanical systems (MEMS) devices.

Neon ion beam etching has been investigated as well for direct etching of the tested sample. Both low landing energy and high dose etching were tested. In Figure 3.C and Figure 3.D, a high dosage neon ion beam (~1.2×1019 ions/cm2) using a 29 keV landing energy was applied on long line structures to avoid a neighborhood effect from dense arrays of nanostructures. The long metal band is ~40 µm in length. The cutting line is wide enough for HIM image observation, and in fact, there is still no voltage contrast for such neon separated stripes. With a low landing energy for the neon ion beam, the


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damage and swelling effect should be minimal. 19 In another test, no voltage contrast was observed on stripes after neon ion beam cutting with low beam energy of 10 KeV (Figure S4). Therefore, both helium and neon ion beams could cut the metal connection on this tested sample and no voltage contrast image would be observed on the separated stripes in these etching/milling conditions. The main reason is the deposition of conducting materials on the surrounding surface of the cutting line.2 The deposition of conducting materials could be re-deposition of metal material from Ti3N4 film and deposition of carbon material from residual hydrocarbon in HIM chamber and sample preparation. With optimized ion beam and patterning parameters, it is possible to minimize the re-deposition issue. The deposition could occur around milling gap structures (top layer and sidewall) causing a connection with bulk metal structures, or ground to the substrate, leading to no charging effect. With chamber cleaning processing in HIM and careful sample preparation, the carbon deposition from contamination can be minimized or avoided. Especially, with neon ion beam, the contribution of carbon deposition from the chamber would be much less. With change of parameters of line milling conditions and after chamber cleaning, the segment between two cuts still has no voltage contrast on this


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sample (Figure S5). This test further confirmed that the re-deposition of conductive materials causes the short-circuit to substrate (silicon) for such thin insulating layer (SiO2, 20 nm) structures. It is possible to have voltage contrast with ion beam cutting for conductive connection on thick insulating (e.g. >200 nm) layer structure. More details about morphological change and re-deposition issues in the process of milling gap will be investigated in future experiments with cross-section images and elemental analysis. HIM-based secondary ion mass spectrometry (SIMS) is an alternative method beyond energy dispersive X-ray spectrometry (EDX) for elemental analysis with advantages of in situ processing and high lateral resolution. 20

With XeF2 gas assistance, etching of metal connection with an ion beam would be effective and fast. The precursor gas XeF2 is delivered to the HIM chamber with controlled pressure and temperature using a gas injection system (GIS). XeF2 helps to remove the metal layer completely

by the formation of a volatile byproduct, TiF4, thus forming a

complete isolation of the conductive connection.17 Using HIM images, it would be easy to capture voltage contrast images. With XeF2 gas assistance, the etching rate would be


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enhanced. To compare the etched stripes with and without XeF2 gas assistance, both helium and neon ion beams cuts were made on an area of long line structures as shown in Figure 4. In Figures 4.A-B, from top to bottom, the 1st and 3rd lines were etched with a helium ion beam only while the 2nd and 4th lines were etched with both a helium ion beam and XeF2 gas using an etching dosage 3.7×1018 ions/cm2. In the 1st and 2nd lines, shortsegments were cut while in the 3rd and 4th lines, long-segments were cut (almost 40 µm). On the 1st line, the length of the segment of the separated strip is 5.2 µm on right end of the long lines (Figure 4.B). Even though there is a barely visible cutting gap in the HIM image as shown the white arrows, voltage contrast doesn't exist due to the deposition of conductive materials on the cutting gap location as discussed above. On the 2nd line, the short length of the separated stripe segment is 4.2 µm on right end of the long line (Figure 4.B). The high voltage contrast is observed on this stripe, cut using a helium ion beam with the assistance of XeF2 gas. Similar results are observed on the long, separated stripe segments on the 3rd and 4th lines, as shown in Figure 4.A and 4.B. With the same helium ion beam dosage, the increase of the width of cut line with XeF2 gas assistance is at least double compared to that without XeF2 gas as shown in Figure 4.C. It is also very easy to


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cut the micro-scale pad to form isolation patterns with a long fine line using helium ion beam assisted with XeF2 gas and the difference in voltage contrast due to the assistance of XeF2 can be easily observed on micro-patterned structures (Figure S6).

The etching rate with a neon ion beam is much higher than that with a helium ion beam. 12

From our experiments, it is determined to be at least a factor of 3 greater. In Figure

4.D-F, the 1st and 3rd lines are etched with neon beam only, using a dosage of 1.3×1018 ions/cm2, while the 2nd and 4th lines are etched with a neon ion beam assisted with XeF2 gas, using a dosage of 3.1×1017 ions/cm2, only ¼ of dosage used in cutting for 1st and 3rd lines. In the 5th line, using a neon ion beam and XeF2 gas, the left end is cut with a dosage of 3.1×1017 ions/cm2 and the right end is cut with a dosage of 1.3×1018 ions/cm2. The increase in the width of the cut gap with XeF2 gas assistance is at least a factor of 5 compared to that without XeF2 gas as shown in Figure 4.F. Without XeF2 gas assistance, no voltage contrast is observed in the image, as expected, for the 1st and 3rd lines. With XeF2 gas assistance, the voltage contrast in the images is easy to observe for both the long, separated stripe segment (~40 µm, 4th line) and the short stripe segment (~3.5 µm,


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2nd line). It is noticeable that there are some black halos around the cutting gaps as shown in Figure 4. B and 4.F. The black halos are most likely caused by simultaneous etching of the Si layer underneath the thin metal layer and SiO2 layer by XeF2 gas.

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The black

halo issue depends on dosage (etching depth and width), sequence of etching pattern, time for withdrawal of the GIS needle and shutoff of the XeF2 gas after etching is completed. The issues of black halos in Figure 4.C and 4.D is minimum and neglectable due to this being the last etch in the sequence. With the assistance of XeF2 gas, the conductive structure is electrically separated and well isolated. The etching rate of a neon ion beam with XeF2 gas is so fast that it is not suitable for short nano-gap cutting on dense arrays of nanostructures from the aspect of control of uniformity of gap size over the total arrays of gaps, because the very small fluctuation in a neon ion beam current causes a change of gap size with assigned average beam current and dosage due to very short milling time for short nano-gap cutting. The use of a neon ion beam with XeF2 gas is more applicable for micro-fabrication or long nano-gap cutting.


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In order to compare the effect of ion beam and assistance of GIS gas XeF2 on etching rate and voltage contrast, Table 1 gives the summary of results for experiments in Figure 4. The patterned etching box is designed to be 400 nm × 5 nm with different dosage for helium or neon ion beam. The resultant width of cutting box is always larger than 5 nm. The width of milled line given in table is an average width of line width. With XeF2 assistance, the enhancement of milling line is about 3 for helium ion beam and 7 for neon ion beam for these etching parameters and GIS precursor conditions on this tested sample.

Images in Figure 5 show the voltage contrast images with helium ion beam etching using a dosage of 3.7×1018 ions/cm2 with XeF2 gas assistance (8×10-6 Torr) for complex and dense patterned stripe nanostructures. The pairs of cutting lines on one end of the stripes have small dimensions of 80 nm × 10 nm. This further confirms that it is easy to control isolation in such a nanoscale on such dense arrays of nanostructures using helium ion beam and XeF2 gas assistance. With such a method, it is also useful for diagnosis of the invisible defects in initial voltage contrast images. If the long stripe is well connected,


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isolation on only one end by helium ion beam etching with XeF2 assistance will not produce voltage contrast because the other end of the stripe is still connected to ground or bulk structure, not a floating structure. That is also true for a natural defect, as one defective location (broken point) along the stripe does not cause voltage contrast image to show up, and the location of such a lone defect is not easy to localize with normal HIM image. If one suspects that such a defect exists on a stripe (bright stripe), one can use helium ion beam etching with XeF2 gas assistance to localize the hidden defect. Once artificial isolation is formed in such a stripe, voltage contrast will show up on either the left or right side of the artificial isolation position depending on the natural defect location. One could check for voltage contrast image starting from the artificial isolation point, and then along the segment to the end point to find the natural defect location. This diagnosis method is an easy and direct method. For the first artificial isolation (white arrow) on 2nd stripe, the HIM image displays voltage contrast as shown in Figure 5.B. That means there is another natural hidden defect on the right side of the artificial defect. One can find the location of such a defect by moving through the voltage contrast image to the end point. In another case, there is no voltage contrast after the first artificial isolation (white arrow)


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on the middle stripe as shown in Figure 5.C. That means this stripe has no hidden defect and is completely connected before cutting. Voltage contrast appears after the 2nd artificial isolation (black arrow), as expected, as shown in Figure 5.D. With relatively long gap isolation such as 400 nm × 10 nm on the stripe, this method also works easily for checking hidden defects.

There is another case for the use of voltage contrast images for a series of defects. In this case, it is easy to localize the defects at left-most and right-most positions in the series of defects from the voltage contrast image. It is not easy to localize the defect locations for intermediate defects, especially for a long stripe of voltage contrast, using quick HIM image scanning. In principle, one can repair the defect on one side (e.g., the defect furthest to the left in the voltage contrast image) with metal deposition (Pt, Co) 7,21 using GIS metal precursors. After repairing the defect furthest to the left, the beginning point of voltage contrast moves to right to the next defect location in the series of defects. One can then localize this hidden defect location. However, there is an easy way without using GIS. Since neither helium nor neon ion beam could effectively etch metal


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connections due to re-deposition of metal or conductive carbon deposition from residual hydrocarbons in the chamber, one can apply a helium or neon ion beam on the region of defects (e.g. defect furthest to the left in a voltage contrast image) to achieve the goal described above. Images in Figure 6 show such an effect using a neon ion beam. The image in Figure 6.A shows voltage contrast for defective stripes with a very short defective segment (only ~0.6 µm long) as indicated by the white arrow, at the starting point of one defective strip segment as indicated by P1, and one extra longer defective segment as shown by the black arrow. In order to repair the defective region well and have easy operation processing, the 400 nm × 1000 nm box is used for neon ion beam irradiation of stripe P1 as shown in Figure 6.B, using a beam current of 1.8 pA and dosage of 2.5×1017 ions/cm2. After applying neon ion irradiation at the starting point of the voltage contrast stripe for 1.3 min, the voltage contrast disappears from that point as shown in Figure 6.B in the defective area. Images in Figure 6.C and 6.D show another example of irradiating defects for stripe P2 at the bottom right of the image beyond the repair on P1 above. The obvious effect is observed as well in Figure 6.D. In order to localize the next defect position, the HIM image is moved to the right to find the defect location in a series


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of defects. The image in Figure 7 shows a combination of several HIM images for moving images. For stripe P1, there is no more voltage contrast for a distance of 50 µm, as shown in Figure 7. In fact, even though the image is moved further right, no voltage contrast is observed for stripe P1. That means that the defective stripe P1 had only one pair of defects, located at the place that was irradiated. For stripe P2, the new starting point of voltage contrast is found, as indicated by the middle white arrow. This is the closest defect location from the initial starting point, as shown in the bottom right of Figure 6.C and the left white arrow in Figure 7. The end point is the same as before, as indicated by the right white arrow in Figure 7. This series failure has three defect locations. Therefore, with neon ion beam irradiation, it is easy to localize the defects in a series of defects with voltage contrast using HIM images. With a helium ion beam, this method should also work. It requires a relatively long dosage for irradiating defective regions to localize the closest defect location using a helium ion beam (Figure S8). It is obvious that the starting point of voltage contrast segment disappears after helium ion beam irradiation. It is interesting that the metal nanostructure patterns on the swelling region (irradiated region) maintained their original shapes with uniform deformation to some degree. This is


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attributed to a combination of Ti3N4 superplastic-like behavior and local volumetric swelling of the Si substrate.

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With a high dosage of helium ion irradiation on patterned

metal structures on Si substrate, novel structures can be created for some other possible applications. Another possible way to repair defects with helium ion beam is the formation of conductive carbon pattern on defective region with helium ion irradiation in contaminated chamber or sample situation, which is not so controllable and repeatable. Ga-FIB images could also provide PVC images with low resolution and high sputtering to the surface of the test sample. 2 Similarly, a gallium ion beam could be used to irradiate the defects and roughly diagnose the defective locations in a series of defects. However, it is difficult to focus on a region to a single stripe and obtain a high-resolution image, thus difficult to check the broken gaps in nanoscales (Figure S9).

Even though only a Ti3N4 metal layer has been tested for defect localization analysis with voltage contrast using HIM images, this approach should be applicable to other conductive materials. More importantly, voltage contrast imaging with HIM provides good contrast, makes it easy to localize the defects and is less damaging to the test surface.


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The simple neon ion beam irradiation approach can be extended to localize more defect positions for a series of defects. With XeF2 gas assistance, a helium ion beam can be used to form artificial defects, and diagnose the singe-defect situation. It is obvious that easy defect localization, nanofabrication of metal connection and further localization of hidden defects could be performed on a sub 10-nm scale with dense arrays of nanostructures and long-distance stripes. It is also possible that voltage contrast imaging can be performed on other kinds of conductive materials with direct helium or neon ion beam etching without XeF2 gas assistance because different material systems have different material layers, etching rate and different degree of re-deposition. Voltage contrast imaging also provides good indication and direct visualization for making patterns with one-dimension materials (e.g. carbon nanotube) and two-dimensional materials (e.g. graphene) in nanofabrication processing. As semiconductor device transistor approaches three-dimensional architectures in nanoscale, it is possible to use this method to diagnose the defect locations for metal connection with reference of defect-free structures. This method can be further to extend to construct three-dimensional image structures to


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localize the defects in different connection layers with combination neon ion beam milling and imaging.

Conclusions

In summary, successful use of passive voltage contrast with helium ion microscopy imaging for defect localization and nanofabrication in conductive materials has been demonstrated in this work. First, the voltage contrast using this method has high resolution, high contrast and low damage to the surface. A broken gap as narrow as sub10 nm can be easily localized with the voltage contrast stripe and motion of imaging. Secondly, with a helium or neon ion beam and XeF2 gas assistance, the effective and clean nanofabrication of metal connection gaps was successively demonstrated with voltage contrast indication. Voltage contrast in helium ion images can also indicate floating structures as ultra-long distance isolated stripes and isolated large micro pads formed by ion beam induced XeF2 etching. XeF2 precursor for ion beam etching helps to enhance materials removal rate, reduce substrate swelling and avoid re-deposition.


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Helium ion beam etching with XeF2 gas could be also used to diagnose the invisible defect position for a single-location defect on a long stripe. Finally, neon ion beam irradiation for defective locations is introduced to identify the hidden defect locations in series defect cases. These results confirm that the helium ion beam technique is an attractive method for defect localization for different types of defects in conductive materials using voltage contrast images.


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Experimental Section

Helium and Neon Ion Beam. A Zeiss ORION NanoFab multi-ion beam system was used for imaging and etching experiments. The base pressures in the system for chamber and gun were ~2.0 × 10-7 Torr and 4.0 × 10-10 Torr respectively. Both helium and neon ion beams can be used to etch the sample. The beam energy of 25-30 keV with a beam current at the range of 0.4 to 2.0 pA was used for most experiments for He and Ne beam. A Fibics NanoPatterning and Visualization Engine (NPVE) was used to generate patterns. With an appropriate beam current, some dosage (ions/cm2) was applied on NPVEdefined patterns to produce a narrow cut on a metal connection. In general, the parameters for the NPVE pattern were serpentine, dwell time 10 µs or 1 µs, pixel size of 1 nm or 0.5 nm. For cutting two sides of patterned stripes or line structures on test samples, the etching box was 10 nm × 80 nm or 5 nm × 400 nm.

Gas Injection System. An Oxford Instruments OmniGIS II gas injection system (GIS) was used to deliver XeF2 gas into the chamber with the needle close to the sample surface. The needle to sample surface was ~ 80 µm and ~300 µm from the center of the ion beam


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field of view at an angle of 36o between the needle and the sample surface. The temperature for XeF2 precursor was room temperature (28oC-30oC). The precursor XeF2 pressure was ~ 8×10-6 Torr. Once etching with the precursor was finished, the precursor was turned off and the needle was withdrawn from the sample surface to avoid further etching on the sample.

ASSOCIATED CONTENT

Supporting Information. The Supporting Information is available free of charge on the ACS publication website at DOI: xxxx. It provides additional discussions and images for comparison of HIM images and SEM images, neon ion images for defect localization, direct He/Ne ion beam etching, gas-assisted helium ion beam etching and He/Ga ion beam irradiation on the defects.


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ACKNOWLEDGMENT The authors would like to thank Dr. John Notte and Dr. Brett Lewis at Carl Zeiss SMT, Inc., for helpful discussion and support. The authors also would like to thank Dr. JennsTimo Neumann at Carl Zeiss SMT, Inc. and Dr. Sandip Halder from IMEC for sample support and imaging analysis.


30

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A

25nm Ti3N4

B

20nm silicon oxide Silicon

500nm

C

100nm

D

200nm


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Figure 1. Helium ion microscopy images of good connection regions: (A) illustration of sample layer structures; (B) top view over a large area; (C) high resolution image; (D) tilted (45o) view.


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A

B

2µm

500n m

C

D

100n

1µm

Figure 2. Helium ion microscopy images displaying voltage contrast over defective regions: (A) large field of view; (B) small field of view; (C) enlarged view of one starting point of the defective stripe in the middle of image from (B); (D) image of five long defective stripes and one short defective stripe.


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B

A

500nm

200nm

D

C

500nm

500nm


34

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Figure 3. Helium ion microscopy images of He or Ne ion etching: (A)-(B) helium ion beam etching; (A) 3.7×1018 ions/cm2; (B) left: 7.5×1018 ions/cm2; right: 5.0×1019 ions/cm2; (C)(D) neon ion beam etching with 1.2×1019 ions/cm2.

A

500nm

D

B

C

500nm

200nm

E

F

ACS Paragon Plus Environment 500nm

500nm

500nm

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Figure 4. Helium ion microscopy images for He or Ne ion etching with XeF2 gas over simple 40 µm long line structures: (A)-(C) helium ion beam etching with dose of 3.7×1018 ions/cm2, 1st and 3rd lines without XeF2, 2nd and 4th lines with XeF2; (A) left end of lines; (B) right end of lines; (C) enlarged view of left end for 3rd and 4th lines; (D)-(F) neon ion beam etching, 1st and 3rd lines using dose of 1.3×1018 ions/cm2 without XeF2, 2nd and 4th lines using dose of 3.1×1017 ions/cm2 with XeF2, 5th line using dose of 3.1×1017 ions/cm2 at left end and dose of 1.3×1018 ions/cm2 at right end with XeF2; (D) left end of long lines; (E) middle of long lines; (F) right end of long lines.


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A

B

500nm

500nm

C

D

500nm

500nm


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Figure 5. Helium ion microscopy images of He ion etching using dosage of 3.7×1018 ions/cm2 with XeF2 gas assistance over complex and dense nanostructure regions: (A) before etching; (B) after one side etching on one end of top stripe; (C) after etching one end of middle stripe; (D) after pair of etching on middle stripe.


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B

A

P1

500nm

500nm

C

D

P2 2µm

2µm


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Figure 6. Helium ion microscopy images using neon irradiation on the defects (A) before neon irradiation; (B) after neon irradiation on P1; (C) large field of view before neon irradiation; (D) large field of view after neon irradiation on defects on P1 and P2 locations.

P1

2µm

P2


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Figure 7. Combination of helium ion microscopy images using neon irradiation on the defects over a long distance.


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Table 1. Comparison of Helium and Neon Ion Milling Metal Line with and without XeF2 Ions, GIS gas

dose (ions/cm2)

milled line width

voltage contrast

(nm) He+

3.7×1018

6

No

He+, XeF2

3.7×1018

18

Yes

Ne+

1.3×1018

10

No

Ne+, XeF2

3.1×1017

24

Yes

Ne+, XeF2

1.3×1018

74

Yes


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REFERENCES [1] Tan, S. & Livengood, R. Applications of GFIS in Semiconductors, in Helium Ion Microscopy; Hlawacek. G; Golzhauser, A. Springer International Publishing, Switzerland, 2016, 471-498. [2] Rosenkranz, R. Failure Localization with Active and Passive Voltage Contrast in FIB and SEM. J. Mater. Sci: Mater Electron 2011, 22, 1523-1535. [3] Volinsky, A. A.; Rice, L.; Qin, W. & Theodore, N. D. FIB Failure Analysis of Memory Array. Microelectron Eng. 2004, 75, 3-11. [4] Xia, D.; Notte, J.; Stern, L. & Goetze, B. Enhancement of XeF2-Assisted Gallium Ion Beam Etching of Silicon Layer and Endpoint Detection from Backside in Circuit Editing. J. Vac. Sci. Technol. B, 2015, 33, 06F501.


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[5] O’ Connel, P.; Chen, Y.; Zhang, H.; Zhou, Y.; Fox, D.; Maguire, P.; Wang, J.J.; & Rodenburg, C. Comparative Study of Image Contrast in Scanning Electron Microscope and Helium Ion Microscope. J. Microsc. 2017, 268, 313-320. [6] Hlawacek, G.; Veligura, V.; van Gastel, R. & Poelsema, B. Helium Ion Microscopy. J.

Vac. Sci. Tech. B 2014, 32, 020801. [7] Stanford, M. G., Lewis, B. B., Mahady, K., Fowlkes, J. D. & Rack, P. D. Review Article: Advanced Nanoscale Patterning and Material Synthesis with Gas Field Helium and Neon Ion Beams. J. Vac. Sci. Technol. B, 2017, 35, 30802. [8] Jepson, M. A. E.; Inkson, B. J.; Rodenburg, C. & Bell, D. C. Dopant Contrast in the Helium Ion Microscope. Europhys. Lett. 2009, 85, 46001. [9] Winston, D.; Manfrinato, V. R.; Nicaise, S. M.; Cheong, L. L.; Duan, H.; Ferranti, D.; Marchman, J.; McVey, S.; Stern, L.; Notte, J. & Berggren, K. K. Neon Ion Beam Lithography (NIBL). Nano Lett. 2011, 11, 4343-4347. [10] Lemme, M.C.; Bell, D. C.; Williams, J. R.; Stern, L. A.; Baugher, B. W.; Jarilloherrero, P. & Marcus, C. M. Etching of Graphene Devices with a Helium Ion Beam. ACS Nano 2009, 3, 2674-2676. [11] Scipioni, L.; Sanford, C. A.; Notte, J.; Thompson, B. & McVey, S. Understanding Imaging Modes in the Helium Ion Microscope. J. Vac. Sci. Technol. B, 2009, 27, 3250-3255.


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[12] Iberi, V.; Vlassiouk, I.; Zhang, X. G.; Matola, B.; Linn, A.; Joy, D. C. & Rondinone, A. J. Maskless Lithography and in situ Visulization of Conductivity of Graphene using Helium Ion Microscopy. Sci. Rep. 2015, 5, 11952. [13] Sakai, C.; Ishita, N.; Masuda, H.; Nagano, S.; Kitahara, M.; Ogato, Y. & Fujita, D. Active Voltage Contrast Imaging of Cross-sectional Surface of Multilayer Ceramic Capacitor using Helium Ion Microscopy. Appl. Phys. Lett. 2016, 109, 051603. [14] Sakai, C.; Ishita, N.; Masuda, H.; Nagano, S.; Onishi, K. & Fujita, D. In situ Voltageapplication System for Active Voltage Contrast Imaging in Helium Ion Microscope. J. Vac. Sci. Technol. B, 2018, 36, 042903. [15] Xia, D.; Huynh, C.; McVey, S.; Kobler, A.; Stern, L.; Yuan, Z. & Ling, X. S. Rapid Fabrication of Solid-State Nanopores with High Reproducibility over a Large Area using a Helium Ion Microscope. Nanoscale, 2018, 10, 5198-5204. [16] Abbas, A.; Liu, G.; Liu, B.; Zhang, L.; Liu, H.; Ohlberg, D.; Wu, W. &Zhou, C. Patterning, Characterization, and Chemical Sensing Applications of Graphene Nanoribbon Arrays down to 5nm using Helium Ion Beam Lithography. ACS Nano 2014,

8, 1538-1546.


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[17] Stanford, M. G.; Mahady, K.; Lewis, B. B.; Fowlkes, J. D.; Tan, S.; Livengood, R. H.; Magel, G. A.; Moore, T. M.; & Rack, P. D. Laser Assisted Focused He+ Ion Beam Induced Etching with and without XeF2 Gas Assist. ACS Appl. Mater. Interfaces 2016, 8, 29155-29162. [18] Aramesh, M.; Mayamei, Y.; Wolff, A.& Ostrikov, K. Superplastic Nanoscale Pore Shaping by Ion Irradiation. Nat. Commun. 2018, 9, 835. [19] Livengood, R.; Tan, S.; Greenzweig, Y.; Notte, J. & McVey, S. Subsurface Damage from Helium Ions as a Function of Dose, Beam Energy and Dose Rate. J. Vac. Sci. Technol. B, 2009, 27, 3244-3249. [20] Dowsett, D. & Wirtz, T. Co-Registered In Situ Secondary Electron and Mass Spectral Imaging on the Helium Ion Microscope Demonstrated using Lithium Titanate and Magnetism Oxide Nanoparticles. Anal. Chem. 2017, 89, 8957-8965. [21] Wu, H.; Stern, L.A; Xia, D., Ferranti, D.; Thompson, B.; Klein, K. L.; Gonzalez, C. M. & Rack, P. R. Focused Helium Ion Beam Deposited Low Resistivity Cobalt Metal Lines with 10nm Resolution: Implications for Advanced Circuit Editing. J. Mater. Sci: Mater Electron 2014, 25, 587-595.


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For Table of Contents Only

He++XeF2 defect

defect He+ 200 nm

1000 nm


47

Defect Localization and Nanofabrication for Conductive Structures

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