Differences between differential phase contrast and electron holographic measurements of a GaN p-n junction.

In this presentation we will compare differential phase contrast (DPC) [1] and off‐axis electron holography [2] for the measurement of electrostatic potentials in semiconductor devices. DPC uses the lateral shifts of a convergent electron beam to determine the field in the sample whereas for electron holography the changes in potential is encoded in interference fringes that are formed using a biprism [1]. To fairly assess the relative sensitivity of the different techniques on the same specimen, a symmetrically doped p‐n junction with a dopant concentration of 1 x 10 19 cm ‐3 has been measured as a function of reverse bias applied in situ in the TEM. Figure 1(a) and (b) shows maps acquired by DPC of the p‐n junction with 0V and 4V reverse bias applied. At 0V, it is difficult to see the presence of the junction whereas for a reverse bias of 4 V the space charge region is now visible. Profiles acquired from across the junction for various reverse bias voltages can be seen in Figure 1(c). Electron holograms were acquired and Figure 1(d) and (e) show reconstructed phase images of the junction at zero bias and 4 V reverse bias. Even though a low magnification has been used to obtain a large field of view at the expense of sensitivity, the junction is clearly visible in both of the phase images. Corresponding electric field profiles that have been calculated from the potential maps are shown in Figure 1(f). These results show that off‐axis electron holography has a significantly better sensitivity than DPC. However the advantages of using DPC is that a large field of view has been obtained and it is not necessary to examine a region close to vacuum. These techniques have also been applied to an InGaN/GaN system. Figure 2(a) shows a HAADF STEM image of the specimen. Figure 2(b) shows a potential map and (c) profile acquired by off‐axis electron holography with a spatial resolution of 5 nm. When using DPC, sub‐nanometer spatial resolution is expected and maps of the electric field in the InGaN layers can be observed in Figures 2(d) and (e). Here the specimen has been tilted onto and away from a zone axis and a large variation in the measured signal is observed which is visible in Figure 2(f). In this presentation will discuss the effects of diffracted beams on the measured DPC signal. We will also discuss the advantages and disadvantages of using DPC and electron holography for the measurement of electrostatic fields in a range of different doped and III/V semiconductor specimens and show improvements that have been applied.

In this presentation we will compare differential phase contrast (DPC) [1] and off‐axis electron holography [2] for the measurement of electrostatic potentials in semiconductor devices. DPC uses the lateral shifts of a convergent electron beam to determine the field in the sample whereas for electron holography the changes in potential is encoded in interference fringes that are formed using a biprism [1]. To fairly assess the relative sensitivity of the different techniques on the same specimen, a symmetrically doped p‐n junction with a dopant concentration of 1 × 10 19 cm −3 has been measured as a function of reverse bias applied in situ in the TEM. Figure 1(a) and (b) shows maps acquired by DPC of the p‐n junction with 0V and 4V reverse bias applied. At 0V, it is difficult to see the presence of the junction whereas for a reverse bias of 4 V the space charge region is now visible. Profiles acquired from across the junction for various reverse bias voltages can be seen in Figure 1(c). Electron holograms were acquired and Figure 1(d) and (e) show reconstructed phase images of the junction at zero bias and 4 V reverse bias. Even though a low magnification has been used to obtain a large field of view at the expense of sensitivity, the junction is clearly visible in both of the phase images. Corresponding electric field profiles that have been calculated from the potential maps are shown in Figure 1(f). These results show that off‐axis electron holography has a significantly better sensitivity than DPC. However the advantages of using DPC is that a large field of view has been obtained and it is not necessary to examine a region close to vacuum. These techniques have also been applied to an InGaN/GaN system. Figure 2(a) shows a HAADF STEM image of the specimen. Figure 2(b) shows a potential map and (c) profile acquired by off‐axis electron holography with a spatial resolution of 5 nm. When using DPC, sub‐nanometer spatial resolution is expected and maps of the electric field in the InGaN layers can be observed in Figures 2(d) and (e). Here the specimen has been tilted onto and away from a zone axis and a large variation in the measured signal is observed which is visible in Figure 2(f). In this presentation will discuss the effects of diffracted beams on measured DPC signal. We will also discuss the advantages and disadvantages of using DPC and electron holography for the measurement of electrostatic fields in a range of different doped and III/V semiconductor specimens and show improvements that have been applied.

Off‐axis electron holography can be used to measure the electrostatic and magnetic potentials in semiconductor devices with high‐sensitivity and nm‐scale resolution [1]. In this presentation we will show experimental results that have been obtained using combinations of electron holography, precession diffraction and differential phase contrast (DPC) on a range of different semiconductor devices. Deformation maps have been acquired using dark field electron holography on a variety of different device structures and these results have been compared to those obtained by precession electron diffraction (NPED). Figure 1 shows STEM images and results obtained by dark holography and NPED on a Si specimen containing 10‐nm‐thick SiGe layers with different Ge concentrations and also a recessed source and drain SiGe device. The deformation maps obtained by dark holography and shown here have a spatial resolution of 6 nm and a precision of 0.05 %. The maps obtained by precession electron diffraction have a spatial resolution of 2 nm and a precision of 0.02 %. In this presentation we will show the advantages and difficulties associated with the use of the different techniques [2]. We will also compare electron holography and DPC for dopant profiling on fully processed and electrically tested devices. Figure 2(a) and (b) shows STEM images that have been acquired from two different pMOS devices with different spacer widths. The spacers are used to prevent dopants from diffusing under the gate during the activation anneals. Figure 2(c) shows how the specimen is rotated to remove the top metal layers in the device, backside milling is then used to provide a high quality TEM specimen. Figures 2(d) and (e) show maps of the electrostatic potential in the devices that have been acquired using off‐axis electron holography. The spatial resolution in these maps is 5 nm and the difference in the potential distribution under the gate can be clearly seen. Potential profiles have been obtained from across the device and the parameters such as the electrical gate width can be measured. From the analysis of these real devices, the advantages and problems that are associated with electron holography and DPC can be discussed.

Probing nanometer scale electrostatic and mean inner potential (MIP) and establishing structure–properties relationship at this length scale in advanced functional materials are not only of fundamental interest but also of technological relevance and importance, especially for materials with application in the ever miniaturizing electronics. As pure phase object, these potentials can only be “seen” by phase of the wave of probing radiation/particle; and electron microscopy (EM) based phase contrast methods are the most suitable, if not the only, tool for this purpose. Here, MIP is, by definition, the local volume (or unit cell) average of the Coulomb potential of the sample, which can be theoretically calculated or accurately measured (cf. Ref [1]). While the measurable, the local scattering potential being probed by the high‐energy electrons, can be considered (assuming homogeneous potential in projection) superposition of MIP and electrostatic potential, which is the deviation of local Coulomb potential from the MIP of bulk from the perspective of measurement. Differential phase contrast (DPC) using electrons based on scanning transmission EM (STEM) has seen a renaissance of interest mainly due to the recent demonstration of probing electrostatic potential in real space at atomic resolution [3] , beside its demonstrated robustness in studying magnetic properties in the past decades, e.g. Ref. [2]. The DPC‐STEM signal composes the difference of intensity from opposite quadrants from a segmented annual bright field detector. In principle, a DPC‐STEM experiment effectively records, under kinematic approximation, a two dimensional vector of electron beam deflection (i.e., phase gradient) and an absorption/amplitude contrast signal (i.e., sum intensity of all quadrants) simultaneously at each probing position/pixel which raster over the specimen at desired field of view (FOV). Therefore, DPC‐STEM is expected to show advantages of 1) direct interpretability, 2) sharp features in focus, 3) flexible FOV and 4) simultaneous phase and amplitude contrast, compared to interference‐ (e.g. electron holography) and propagation‐ (e.g. Fresnel contrast) based phase contrast methods. Despite these simple descriptions, there are, however, very limited applications of DPC‐STEM in the quantitative and systematic study of electrostatic potential and MIP in crystals. Moreover, as STEM based method using convergent illumination, convergent beam electron diffraction (CBED) patterns under identical condition are indispensable to evaluate the DPC‐STEM results quantitatively. In this contribution, we focus on experimental studies of quantitative measurements of MIP from crystal wedges by compiling the results from DPC‐STEM and CBED raster arrays under identical diffraction conditions. The experiments were performed on a Titan Themis 3 TEM equipped with Cs correctors, working at 200 kV in μ‐probe STEM mode. The camera length and probe convergence angle are carefully calibrated and chosen to balance resolution (to about 1 nm) and detection sensitivity. Figure 1 shows the representative results of measuring MIP from a cleaved 90° Si wedge. Under quasi‐kinematic condition (cf. Fig. 1a), remarkable beam refraction is observed when the probe is moved from vacuum to inside sample and the refraction angle is constant to a considerablely large sample thickness. Meanwhile the total intensity decreases homogeneously within the beam disk and exponentially as a function of the local thickness, as expected. The refraction angle measured from a CBED raster array is quantified to sub‐pixel accuracy, which corresponds to a MIP of Si to be 12.52±0.21 V. The calibrated DPC‐STEM signals deliever very close mean value of the magnitude of refraction, but with much greater variance, due to the orders‐of‐magnitude shorter dwell time and thus noisier signal (Fig. 1g,h). The same measurements have been carried out with a 90° GaAs wedge, from which we derived the MIP of GaAs to be 14.10±0.33 V from CBED measurement and a very close mean value from DPC‐STEM data. The MIP values agrees very well with previous measurements based on electron holography and theoretical calculations [1] . Further examples on the application of the method for mapping electrostatic potentials in semiconductor nanostructures, as well as attempts to map piezo‐electric potentials will be presented at the conference.

In order to develop and manufacture semiconductor devices which are key components of the optical telecommunication products, such as the semiconductor laser diode, it is essential to confirm whether it is manufactured as designed. Electric potential distributions of the semiconductor devices are designed in nanoscale, so two dimensional methods to evaluate the electrical potential in the semiconductors with a high spatial resolution are necessary for product management. The observation of the gallium arsenide (GaAs) model specimen was carried out by using the electron holography and Lorentz microscopy [1]. Lorentz images and intensity profiles are shown in FIG 1 (a)‐(f). The p‐n junctions are clearly seen in both the 0.6 mm under‐focused and over‐focused images, but hardly any interfaces of different dopant concentration can be observed in the images. FIG 2 shows the electron holographic reconstructed phase image. The p‐ and n‐type regions are clearly seen as areas of dark and bright contrast, and some differences in the changing dopant concentrations can also be seen. A phase image of semiconductor laser diode by the electron holography is shown in FIG 3 [2]. In FIG 3(a), the interface region is approximately 5 μm. And the spacing between interface fringes is approximately 30 nm. Next, in order to observe the pn junction near the active layer in a high spatial resolution, the photograph was taken by changing the interference fringes conditions. The expanded phase image of a part of FIG 3(a), surrounded by a dotted line, is shown in FIG 3(b). Since the interference region is approximately 1.5 μm, and the interference fringe spacing is 5 nm, the spatial resolution is approximately 15 nm. As can be recognized from the phase image, we can understand that more detailed structure can be observed in the higher spatial resolution in comparison with the phase image in FIG 3(a). Here, the designed location of the pn junction was positioned at the dotted line, but it was found from the electron holography observation results that the pn junction did not exist in the original position. This semiconductor laser diode could not have the expected output characteristics. The structural defect of the pn junction, found out in this observation, is considered to be the cause. For other semiconductor electric voltage evaluation methods by TEM, electron diffraction microscopy [3] which is one method of phase reconstruction method, differential phase contrast [4] (DPC) which is one method of STEM are also effective and possible to be utilized complementarily with the electron holography. We will discuss about these methods applied for semiconductor in this presentation.

Journal Article A Phase Space Perspective on Electron Holography - Building Bridges Between Inline-, Off-axis Holography, Differential Phase Contrast and Diffractive Imaging Get access Axel Lubk, Axel Lubk Triebenberg Laboratory, Institute of Structure Physics, TU Dresden, 01062 Dresden, Germany Search for other works by this author on: Oxford Academic Google Scholar Falk Roder, Falk Roder Triebenberg Laboratory, Institute of Structure Physics, TU Dresden, 01062 Dresden, Germany Search for other works by this author on: Oxford Academic Google Scholar Hannes Lichte Hannes Lichte Triebenberg Laboratory, Institute of Structure Physics, TU Dresden, 01062 Dresden, Germany Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 21, Issue S3, 1 August 2015, Pages 2307–2308, https://doi.org/10.1017/S1431927615012313 Published: 23 September 2015

Quantitative electric field mapping of a p–n junction by DPC STEM

Differential phase contrast in TEM

Electron Holography and Lorentz Microscopy of Magnetic Materials

Electron microscopes have been used extensively to look at structure at the nanometer scale. Most of the information obtained from electron microscopes is amplitude information. Yet, the phase information of electron microscopy, which can be obtained from off-axis electron holography, provides unique information on electronic structure and structural changes in a wide variety of materials.For the semiconductor industry, junction profiling and strain mapping in Si at high spatial resolution provide information that is critical for further scaling of semiconductor devices. Because of the complex process conditions involved to control the junction position relative to the gate position, determination of junction position at high spatial resolution can help to reduce the cost and cycle time of development. Bright-field holography can measure the phase change of electrons traversing the materials, which is directly related to the mean inner potential of Si, indicating the junction position at the nanometer scale. Furthermore, in recent years stressors have been incorporated into devices to change the semiconductor lattice constant in the channel region and thereby enhance hole and electron mobility. Like the junction definition, the extra processing steps involved to add strain in a device have increased development and manufacturing costs. One way to minimize development cycle time is to monitor, at a nanometer scale, changes in channel deformation resulting from process changes. In 2008, Hytch et al. reported that dark-field holography can provide a promising path to nanometer scale strain mapping [1]. Cooper et al. reported using dark-field holography to measure strain related to different process conditions [2, 3].The requirements of electron holography to inspect the current generation of semiconductor devices are: (1) a fringe width (fringe overlap) in the range of about 100 to 800 nm for an adequate field of view (FOV), (2) fringe spacing between 0.5 and 10 nm for meaningful spatial resolution, (3) visibility of the fringe contrast (10–30%) for useful signal-to-noise ratio, and (4) adjust-ability of both the FOV and the fringe spacing relative to the sample. In previous papers and patent disclosures, we reported that we had developed a dual-lens electron holography method on a JEOL instrument to meet the above requirements, and later we implemented the same method on FEI instruments to provide a similar operational range for electron holography [4–6]. This dual-lens operation allows electron holography to be performed from low to high magnification and provides the FOV and fringe spacing necessary for two-dimensional (2D) junction profiling and strain measurements for devices with various sizes.In this article, we describe the electron optics for this method. We also describe several examples of junction profiling and strain mapping to show how to use dual-lens electron holography to resolve semiconductor device issues at high spatial resolution.

There is a great deal of activity in the development of new memory technologies that can be used to provide the required density and reliability for future generations of data storage [1, 2]. At this time there is a bewildering array of proposed systems each with advantages and disadvantages. One of the problems with the development of these types of materials systems is that it is not clear exactly how these devices function and as a consequence, it is difficult to select the best combinations of materials to provide the best overall performance. In this presentation we will present results that have been obtained on a range of different TaO and Ta2O5 OxRAM structures that have processed using reactive and RF deposition physical vapour deposition (PVD). The focus on this work is the mapping of oxygen and results will be presented that have been obtained by a range of different techniques including aberration‐corrected high‐resolution annular bright‐field (ABF) scanning transmission electron microscopy (STEM) imaging, EDS (Energy dispersive X‐Ray Spectroscopy) or Electron Energy Loss Spectroscopy (EELS) for the measurement of oxygen concentration (atoms) and electron holography and differential phase contrast (DPC) for the distribution of electrostatic potential caused by the distribution of the oxygen. An example is shown in Figure 1 where a (a) high resolution ABF STEM and (b) HAADF STEM image of a TaO/Ta2O5 stack with TiN top and bottom electrodes can be seen. Figures 1(c) and (d) show EELS spectra that have been acquired for the N, Ti, O and Ta regions. Figure 1(e) and (f) show quantitative maps and profiles respectively. Here the variations in the Ta and O concentrations can be observed across the active region of the device. In this presentation we will highlight the best techniques for measuring the variations of oxygen in the devices and discuss which stacks have the best stoichiometry for memory device applications. The key to the performance of these devices is the movement of oxygen during switching. As a consequence it is necessary to perform these observations in situ in the TEM to avoid problems with locating the conducting filaments during specimen preparation and additional issues such as device retention and other modifications that could occur during specimen preparation. In this presentation we will present results on specimens that have been switched using a dedicated in‐situ holder in the electron microscope. Figure 2 shows how a movable probe is placed onto a FIB‐prepared specimen with nm‐scale accuracy by using a Nanofactory biasing system. An electrical pulse is then used to switch the specimen in situ in the TEM. In this presentation we will show how to avoid common problems such as device heating and electrical shorting of the specimen from redeposition during preparation by focused ion beam milling. Finally in this presentation we will show results obtained on our TaO specimens that have been switched in situ in the microscope.

Electron Holography in Phase Space

A number of electron column techniques have been developed over the last forty years to permit visualization of magnetic fields in specimens. These include: Fresnel imaging, Differential Phase Contrast, Electron Holography and Lorentz STEM. In this work we have extended the LSTEM methodology using Position Resolved Diffraction (PRD) to quantitatively measure the in-plane electromagnetic fields of thin film materials. The experimental work reported herein has been carried out using the ANL AAEM HB603Z 300 kV FEG instrument 5. In this instrument, the electron optical column was operated in a zero field mode, at the specimen, where the objective lens is turned off and the probe forming lens functions were reallocated to the C1, C2, and C3 lenses. Post specimen lenses (P1, P2, P3, P4) were used to magnify the transmitted electrons to a YAG screen, which was then optically transferred to a Hamamatsu ORCA ER CCD array. This CCD was interfaced to an EmiSpec Data Acquisition System and the data was subsequently transferred to an external computer system for detailed quantitative analysis. In Position Resolved Diffraction mode, we digitally step a focused electron probe across the region of interest of the specimen while at the same time recording the complete diffraction pattern at each point in the scan.

Lorentz transmission electron microscopy (TEM) is a powerful tool to study the crystal structures and magnetic domain structures in correlation with novel physical properties. Nanometric topological magnetic configurations such as vortices, bubbles, and skyrmions have received enormous attention from the viewpoint of both fundamental science and potential applications in magnetic logic and memory devices, in which understanding the physical properties of magnetic nanodomains is essential. In this review article, several magnetic imaging methods in Lorentz TEM including the Fresnel and Foucault modes, electron holography, and differential phase contrast (DPC) techniques are discussed, where the novel properties of topological magnetic domains are well addressed. In addition, in situ Lorentz TEM demonstrates that the topological domains can be efficiently manipulated by electric currents, magnetic fields, and temperatures, exhibiting novel phenomena under external fields, which advances the development of topological nanodomain-based spintronics.

As there is growing interest in laterally confined, small magnetic particles in the sub‐µm and sub‐100 nm regime, researchers look for techniques which are capable to quantitatively measure magnetic properties in this range. Electron microscopy offers a technique which combines very high lateral resolution down to presently 5 nm and at the same time high sensitivity to the specimen's magnetic induction. However, to understand fully the present and future possibilities of ‘Lorentz’ electron microscopy, a brief introduction to the main techniques, such as Fresnel and Foucault imaging, differential phase contrast and electron holography has to be given. This more technical section is followed by various examples of what can be achieved today–measurement of hysteresis loops of individual particles, observation of magnetic configuration in the deep sub‐µm regime, observation of magnetic stray fields–and what will probably be possible in near future.