Dopant profiling based on scanning electron and helium ion microscopy

A Scanning Helium Ion Microscope (SHIM) is a high resolution surface observation instrument similar to a Scanning Electron Microscope (SEM) since both instruments employ finely focused particle beams of ions or electrons [1]. The apparent difference is that SHIMs can be used not only for a sub-nanometer scale resolution microscopic research, but also for the applications of very fine fabrication and direct lithography of surfaces at the nanoscale dimensions. On the other hand, atomic force microscope (AFM) is another type of high resolution microscopy which can measure a three-dimensional surface morphology by tracing a fine probe with a sharp tip apex on a specimen's surface.In order to measure highly uneven and concavo-convex surfaces by AFM, the probe of a high aspect ratio with a sharp tip is much more necessary than the probe of a general quadrangular pyramid shape. In this paper we report the manufacture of the probe tip of the high aspect ratio by ion-beam induced gas deposition using a nanoscale helium ion beam of SHIM.Gas of platinum organic compound was injected into the sample surface neighborhood in the vacuum chamber of SHIM. The decomposition of the gas and the precipitation of the involved metal brought up a platinum nano-object in a pillar shape on the normal commercial AFM probe tip. A SHIM system (Carl Zeiss, Orion Plus) equipped with the gas injection system (OmniProbe, OmniGIS) was used for the research. While the vacuum being kept to work, we injected platinum organic compound ((CH3)3(CH3C5H4)Pt) into the sample neighborhood and irradiated the helium ion beam with the shape of a point on the apex of the AFM probe tip. It is found that we can control the length of the Pt nano-pillar by irradiation time of the helium ion beam. The AFM probe which brought up a Pt nano-pillar is shown in Figure1. It is revealed that a high-aspect-ratio Pt nano-pillar of ∼40nm diameter and up to ∼2000 nm length can be grown. In addition, for possible heating by the helium ion beam, it was observed that an original probe shape was transformed. AFM measurement of a reference sample (pitch 100-500 nm, depth 100 nm) of the lines and spaces was performed using the above probes. The conventional probes which did not bring up platinum was not able to get into the ditch enough. Therefore it was found that a salient was big and a reentrant was shallow. On the other hand, the probe which brought up platinum was able to enter enough to the depths of the ditch.jmicro;63/suppl_1/i30-a/DFU075F1F1DFU075F1Fig.1.SHIM image of the AFM probe with the Pt nano-pillar fabricated by ion-beam induced deposition.

Surface organic residues inhibit the extraordinary electronic properties of graphene, hindering the development of graphene electronics. However, fundamental understanding of the residue morphology is still absent due to a lack of high-throughput and high-resolution surface characterization methods. Here, we demonstrate that secondary electron (SE) imaging in the scanning electron microscope (SEM) and helium ion microscope (HIM) can provide sub-nanometer information of a graphene surface and reveal the morphology of surface contaminants. Nanoscale polymethyl methacrylate (PMMA) residues are visible in the SE imaging, but their contrast, i.e. the apparent lateral dimension, varies with the imaging conditions. We have demonstrated a quantitative approach to readily obtain the physical size of the surface features regardless of the contrast variation. The fidelity of SE imaging is ultimately determined by the probe size of the primary beam. HIM is thus evaluated to be a superior SE imaging technique in terms of surface sensitivity and image fidelity. A highly efficient method to reveal the residues on a graphene surface has therefore been established.

Abstract

Secondary electron (SE) imaging techniques, such as scanning electron microscopy and helium ion microscopy (HIM), use electrons emitted by a sample in response to a focused beam of charged particles incident at a grid of raster scan positions. Spot size—the diameter of the incident beam's spatial profile—is one of the limiting factors for resolution, along with various sources of noise in the SE signal. The effect of the beam spatial profile is commonly understood as convolutional. We show that under a simple and plausible physical abstraction for the beam, though convolution describes the mean of the SE counts, the full distribution of SE counts is a mixture. We demonstrate that this more detailed modeling can enable resolution improvements over conventional estimators through a stylized application inspired by semiconductor inspection: localizing the edge in a two-valued sample. We derive Fisher information about edge location in conventional and time-resolved measurements (TRM) and also derive the maximum likelihood estimate (MLE) from the latter. Empirically, the MLE computed from TRM is approximately efficient except at very low beam diameter, so Fisher information comparisons are predictive of performance and can be used to optimize the beam diameter relative to the raster scan spacing. Monte Carlo simulations provide an example of the MLE giving a 5-fold reduction in root mean-squared error (RMSE) of edge localization as compared to conventional interpolation-based estimation. The RMSE is substantially below both the beam diameter and the raster scan spacing and thus sub-pixel localization is demonstrated. Applied to three real HIM datasets, the average RMSE reduction factor is 5.4.

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 is an efficient approach 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 atnanoscale. With passive voltage contrast (PVC) in HIM images, the defect localization for failure of conductive structures can be rapidly and easily detected with a 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 a 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 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.

Helium ion milling of suspended silicon nitride thin films is explored. Milled squares patterned by scanning helium ion microscope are subsequently investigated by atomic force microscopy and the relation between ion dose and milling depth is measured for both the direct (side of ion incidence) and transmission (side opposite to ion incidence) regimes. We find that direct-milling depth varies linearly with beam dose while transmission-milling depth varies with the square of the beam dose, resulting in a straightforward method of controlling local film thickness.

Microscopy is the art and science of visually observing small objects. Electrons are electrically charged subatomic particles typically responsible for the flow of electrical current. Scanning is a method of observation where the focus of attention is moved in a chosen pattern and desired information is gained by assembling observations in sequence. In a scanning electron microscope (SEM), a beam of electrons is scanned in a regular pattern over a small object, and the image of the object appears on a screen by displaying the image points in synchrony with the movements of the electron beam on the object.A familiar example of image scanning is a television receiver, where a picture is created on a screen by rapidly scanning an electron beam across the phosphor face of the television tube. The beam striking the phosphor produces light, which is seen as a picture. The signal that modulates the electron beam in a television set is created in a television camera, which is scanned in the same way that the television screen is scanned. In a SEM, the signals come from special detectors in the microscope, not from an optical camera. Otherwise, the image display in a SEM is very similar to television. Although it is typical for SEM images to be scanned at a slower rate than are television signals, some SEMs display images at the same speed as television.The most common signal for detection in a SEM is the secondary electron signal. Secondary electrons are low‐speed electrons (having energies less than about 50 eV) that result from the interaction of the primary (i.e. beam) electrons with the sample. The secondary electrons are detected, usually by a scintillator/photomultiplier detector, and the resulting signal modulates the display screen.Secondary electrons are popular because they provide a kind of shading that gives a three‐dimensional appearance to an image, making it easy to visualize the structural relationships of parts of the sample (Figure 1).Other signals provide information of a more analytical nature. The signal produced by backscattered electrons is directly (although not exactly proportionally) related to the atomic number of the material being struck by the primary beam. Diffracted electrons provide information about crystal spacings and orientations in the sample. Induced‐current signals are particularly interesting for studying the response of active electronic components.The resolution of a SEM image depends on the quality of the optical system that forms the electron beam, including the nature of the electron source. Most SEMs can typically resolve objects as small as 10 nm. The resolution of specialized SEMs is better than 1 nm. In contrast, the resolution of light microscopes is limited by the wavelength of light to about 500 nm.The magnification of a SEM is not directly related to the optical system, very unlike a light microscope. The magnifying power of a SEM is related to the ratio between the size of the scan on the object compared to the size of the scan on the image display. In this way a SEM functions like a mechanical pantograph that was used, typically in older days, for copying drawings. If the arms of the pantograph were of equal length, the drawing would be copied the same size. If the arms were adjusted to different lengths, the drawing could be magnified or reduced.The primary advantage of a SEM is that it provides familiar‐looking images of surfaces with higher resolution and greater depth‐of‐field than an optical microscope. Although the images are typically familiar looking, the contrast mechanism in a SEM is not the same as the contrast provided by light, and an observer expecting always to interpret a SEM image the same way as a light image could easily be fooled.As the electron beam consists of electrically charged particles, the sample for SEM observation must either be inherently electrically conducting or must be made electrically conducting to yield good images. Otherwise, electrical charge builds up on the sample, the yield of secondary electrons is changed, and the scanning beam can become erratic. Low‐voltage or low‐vacuum SEMs can be used to reduce the charging problem.

Hexagonal boron nitride is a promising material of modern optoelectronics. Point defects in this material can serve as single-photon sources. In this paper we investigate the modification of the luminescent properties of hexagonal boron nitride by means of local irradiation with focused gallium and helium ion beams. It is demonstrated that the intensity of band-to-band cathodoluminescence monotonically decreases with increasing ion fluence for both gallium and helium. The luminescence band of about 2 eV may become more intense after exposure to He ions with certain ion fluence. The effect of complete quenching of luminescence after gallium irradiation is used to estimate the diffusion length of excess charge carriers. Keywords: point defects, cathodoluminescence, scanning helium ion microscope, excess charge carriers.

The purpose of this one-year LDRD was to investigate the use of the helium ion microscope (HeIM) for imaging dopant profiles in silicon relevant to integrated circuit technologies. HeIM is a new technology that offers improved spatial resolution over scanning electron microscopy and different beam-solid interaction physics which leads to unique contrast mechanisms. Two parallel thrusts were pursued: 1) traditional imaging via the secondary electron signal and 2) a novel topographical approach. To obtain the experimental details and results, please refer to the classified report from the project manager, Ed Cole, or the Cyber IA lead, Justin Ford.

Helium ion beam induced electron emission from insulating silicon nitride films under charging conditions

The theories, modeling and experiments of the processes of secondary electron (SE) generation and SE usage in helium ion microscopy (HIM) are reviewed and discussed. Conventional and recently introduced SE imaging modes in HIM utilizing SE energy filtering and ion-to-SE conversion, such as scanning transmission ion microscopy and reflection ion microscopy, are described.

Chemical characterization and ultrastructure study of pulp fibers

As the miniaturization of semiconductor devices continues, characterization of dopant distribution within the structures becomes increasingly challenging. One potential solution is the use of the secondary electron signal produced in scanning electron (SEMs) or helium ion microscopes (HeIMs) to image the changes in electrical potential caused by the dopant atoms. In this article, the contrast mechanisms and resolution limits of secondary electron dopant contrast are explored. It is shown that the resolution of the technique is dependent on the extent of electrical potential present at a junction and that the resolution of dopant contrast can be improved in the HeIM after an in-situ plasma cleaning routine, which causes an oxide to form on the surface altering the contrast mechanism from electrical potential to material contrast.

High resolution secondary electron (SE) images can be obtained in a scanning transmission electron microscope (STEM) by collecting the type I secondary electrons generated directly by the incident electron beam. The high resolution SE signal is, in general, localized to about 1 nm in the generation process. The escape depth of the secondary electrons, however, varies from about 1 nm for most metals to about 10 nm for some insulating materials. Surface steps can be imaged by collecting high resolution secondary electrons Among other possible contrast mechanisms for SE imaging of surface steps, topographic contrast plays an important role. The topographic contrast of surface steps depends on: (1) the incident electron probe size; (2) step height to SE escape depth ratio; (3) localization of the generated SE signals and (4) the incident beam angle relative to the specimen surface.In our UHV STEM (MIDAS) instrument, both the entrance and exit surfaces of the sample can be imaged by collecting secondary electrons. For thin samples, the resolution is < lnm for SE images of entrance and exit surfaces but for thicker samples the resolution of the SE image obtained from the exit surface will be degraded by the beam broadening effects. For studying steps on thick or bulk crystal surfaces, secondary electrons emitted from the entrance surfaces are, in general, collected to form images with high contrast and high resolution. In this report, all the SE images are obtained, in MIDAS, by collecting secondary electrons emitted from the entrance surfaces of the samples. Figure 1 is a SE image of a MgO smoke crystal revealing the surface growth steps with high contrast. The parallel straight steps, along <100> directions, constitute the edges of unfinished atomic planes on the MgO {100} crystal surfaces. This image indicates that the {100} surfaces of MgO smoke crystals are flat surfaces. Figure 2 shows a SE image of a MoO3 crystal with surface growth steps spiraling from an end-on screw dislocation (the black dots in the SE image represent electron beam induced reduction products). The dark lines (indicated by D) represent up-steps and the bright lines (indicated by B) represent down-steps. It is interesting to note that the two-dimensional crystal growth around the dislocation is anisotropic, forming terraces with long edges parallel to [010] direction.

ABSTRACTThe gas field ion source (GFIS) is able to generate tightly focused ion beams, which can be used to image or modify a specimen. Among the beam species, helium offers extremely high resolution, however, low sputter yield and sub-surface bubble formation are limiting factors in some applications. Therefore, heavier ions such as neon or nitrogen are used as well. In addition to being a suitable choice for lithographic mask editing, secondary electron (SE) generation by nitrogen beams has been recently shown to be affected by certain types of samples, providing additional contrast compared to helium ions. Here, we report our progress on the study of SE imaging differences between the nitrogen ion microscopy (N2IM) and helium ion microscopy (HIM). SE images of two nano-patterned samples comprising insulator, metal and carbon regions have been imaged by nitrogen and helium ions in two fundamentally different GFIS microscopes. The results corroborate previous reports of significant contrast differences in certain samples caused by the different ion species.