Eric Gerardus Theodoor Bosch

Phase contrast STEM for thin samples: Integrated differential phase contrast.

It has been known since the 1970s that the movement of the center of mass (COM) of a convergent beam electron diffraction (CBED) pattern is linearly related to the (projected) electrical field in the sample. We re-derive a contrast transfer function (CTF) for a scanning transmission electron microscopy (STEM) imaging technique based on this movement from the point of view of image formation and continue by performing a two-dimensional integration on the two images based on the two components of the COM movement. The resulting integrated COM (iCOM) STEM technique yields a scalar image that is linear in the phase shift caused by the sample and therefore also in the local (projected) electrostatic potential field of a thin sample. We confirm that the differential phase contrast (DPC) STEM technique using a segmented detector with 4 quadrants (4Q) yields a good approximation for the COM movement. Performing a two-dimensional integration, just as for the COM, we obtain an integrated DPC (iDPC) image which is approximately linear in the phase of the sample. Beside deriving the CTFs of iCOM and iDPC, we clearly point out the objects of the two corresponding imaging techniques, and highlight the differences to objects corresponding to COM-, DPC-, and (HA) ADF-STEM. The theory is validated with simulations and we present first experimental results of the iDPC-STEM technique showing its capability for imaging both light and heavy elements with atomic resolution and a good signal to noise ratio (SNR).

A quantitative model for doping contrast in the scanning electron microscope using calculated potential distributions and Monte Carlo simulations

This paper describes the use of a Monte Carlo model incorporating a finite-element method computing the electrostatic fields inside and outside a semiconductor, plus a ray-tracing algorithm for determining the doping contrast observed in a scanning electron microscope (SEM). This combined numerical method also enables the effects on the doping contrast of surface band-bending to be distinguished from those of external patch fields outside the specimen, as well as any applied macroscopic external fields from the detection system in the SEM. Good agreement of our new theory with experiment is obtained. The contrast characteristics in energy-filtered secondary electron images are also explained. The results of this work lead to a more advanced understanding of the doping contrast mechanisms, thereby enabling quantitative dopant profiling using the SEM.

Analysis of HR-STEM theory for thin specimen.

A compact mathematical model of the STEM imaging process including bright field (BF) and dark field (DF) is derived. This description is valid for thin samples, does not rely on the weak phase approximation and does not require time-consuming simulation of the scanning process. It is well-known that STEM imaging is a nonlinear technique and therefore cannot be described in terms of a sample-independent linear contrast transfer function (CTF). In this work we derive a nonlinear description showing that a STEM image can in fact be described with two terms. Both terms are cross-correlations between a function that is independent of the sample and a function that depends only on the sample. The latter two can be seen as two different objects. These objects directly correspond to two specific cases: the weak phase approximation (WPA) and annular dark field (ADF) imaging, which are known from the literature. We clarify the need for recognizing and understanding what the object is of any particular STEM technique. The model was validated using simulated STEM images and an excellent agreement as well as a reduction in computation time of 3 orders of magnitude was found.

Integrated Differential Phase Contrast (iDPC)–Direct Phase Imaging in STEM for Thin Samples

Imaging the phase of the transmission function has always been the ultimate goal of any (S)TEM imaging technique as it is, for thin samples, directly proportional to the projected potential in the sample. Customarily this information is obtained using Holography [1] or by performing focus series reconstruction in TEM (FSR-TEM) [2], recently also in combination with Phase Plates (PP) [3] and/or image Cs correction. Ptychographic reconstruction has also been considered as an alternative [4].

Quantitative Phase Imaging of Ba 2 NaNb 5 O 15

High-Resolution Transmission Electron Microscope (HRTEM) is one of the most powerful imaging tools to study the structure of materials at atomic resolution. However, HRTEM images, generally speaking, are not directly interpretable and also suffer from aberrations introduced by the objective lens. Last decade the resolution of HRTEM images is greatly improved by utilization of spherical aberration correctors [1]. Direct interpretation of HRTEM images can be achieved by restoring the amplitude and phase of the specimen exit-wave function using electron holographic met such as Through Focus Series Reconstruction (TFSR) and off-axis electron holography [2, 3]. Such reconstruction methods provide an aberration free complex exit-wave function and reconstructed exit waves also benefits from improved signal to noise ratio compared to a single electron micrograph, making direct imaging of atom columns with high precision and accuracy [4]. The phase and amplitude of the reconstructed exit wave can also be used to quantitatively analyze the atomic column composition and positions, respectively [4].

Integrated differential phase contrast (iDPC) STEM

In this talk we present Integrated Differential Phase Contrast (iDPC) STEM [1-3], an electron microscopy (EM) technique that directly images the electrostatic potential field produced by charged particles forming the sample. The electrostatic potential field of the sample has clear maxima at the atomic core positions. Therefore it represents an ideal sample map and is the ultimate goal for any EM imaging technique. For thin samples, the phase of the transmission function of the sample is directly proportional to the projected electrostatic potential field. The iDPC-STEM is therefore also a direct phase imaging technique. For non-magnetic samples we know from basic electrostatics that the electric field of the sample (which is a conservative vector field) is the gradient (differential) of the electrostatic potential field of the sample (a scalar field). An electron passing the sample is influenced by this electric field. If the sample is thin, the electric field at the impact point deflects the electron proportionally to its inplane component. This deflection can be measured by detecting the position of the electron at the detector in the far field. Measurement of the electron (beam) deflection is the subject of Differential Phase Contrast (DPC) techniques [4, 5]. In reality, the motion of the electron is described quantum mechanically with its electron wave function. By focusing the electron wave (the probe) we increase the probability that the electron passes at a certain position and is influenced by the electric field at that position. In the far field, at the detector plane, we obtain a corresponding convergent beam electron diffraction (CBED) pattern, a result of many electrons passing the sample. It was indicated [5] and strictly proven [2] that the mathematical expectation of the electron position in the detector plane, in other words, the center of mass (COM) of the CBED pattern preserves a linear relation to the local electric field at the position of the probe. By scanning the probe, the full COM vector field can be obtained as a linear measure of the electric vector field of the sample. An ideal DPC technique should therefore acquire the COM vector field. A straightforward way of performing this is to use a camera. By recording each CBED pattern COM components can be computed directly [2, 6]. Because this requires fast readouts and stable drift-free samples, in practice COM components are measured using detectors with only a few segments [2, 3]. These methods are fast and enable live imaging, as in (A)BFand (HA)ADF-STEM. By integrating the measured COM vector field we obtain the iCOM scalar field, a linear measurement of the electrostatic potential field of the sample [1-3]. iDPC-STEM is a practical method of obtaining iCOM. In this talk iDPC-STEM using a 4 quadrant segmented detector will be explained. Various experimental results and applications will be presented and compared to standard (S)TEM imaging results. It will be demonstrated that iDPC-STEM is capable of imaging light and heavy elements together, has full low frequency transfer and is a low dose technique.

Integrated Differential Phase Contrast (iDPC) STEM: A New Atomic Resolution STEM Technique To Image All Elements Across the Periodic Table

A new, recently introduced Integrated Differential Phase Contrast (iDPC) STEM imaging technique [1] is enabling live imaging of the phase of the transmission function of thin samples. One of the first striking advantages of this new technique is that it is able to image light (C, O, N ...) and heavier elements (Sr, Ti, Ga ...) together in one image whereas a standard (HA)ADF-STEM image shows only the heavier atoms. Figure 1 shows an example of SrTiO3 imaged using the conventional ADF-STEM technique vs. our new iDPC-STEM technique. The oxygen columns and carbon contamination (low frequency information) are clearly visible in the latter and missing in the former.

A New In-situ Broad Ion Beam, With Energy Range 1 – 500 eV

The interest in low energy ion beams (typically Ar at 50 – 500 eV), is increasing and finds applications in surface clean up, such as removal of hydrocarbons and oxide layers and in fields related to reactive ion etching. Also the removal of Ga + ions and the amorphous layer in a TEM lamella prepared with FIB is interesting. Within the environment of an SEM or DualBeam a new ion source has been constructed. The ion source is based on a narrow gas channel, in which the atoms are converted into ions using direct ionization by the primary electron beam of the system. This local ionization is primarily driven by the electron ionization cross-section of the gas involved, as a function of the primary electron beam energy. Opposite the channel is a surface at potential -V and at gap distance d (Figure 1). The resulting field (V/d) between this surface and the channel-output, will induce ion acceleration towards the surface: in this way a stationary broad beam of ions with well-defined energy is created. In this set-up the ion energy and the ion current are decoupled parameters and hence can be chosen each within their practical boundaries. In case the gas type is changed, the ionization only scales with the respective cross-section of the applied gas. The behavior of ion source has been simulated with both Opera simulation software and GEANT4 [1], using the actual geometrical set up and physical data as input. This allows to study the influence of the most relevant parameters, including the geometry and to compare it to measured values -using a method described belowwith the aim to optimize the source for its application.