High Voltage Scanning Electron Microscope Research Articles

Flash memory hole etch profile monitoring using x-ray and optical scatterometry

Memory hole (MH) etching may be one of the most critical and challenging processes in three-dimensional flash processing and integration. As the holes get deep, hole critical dimensions (CDs) can vary significantly during the top-down etch processing, the hole shape can deviate from a round hole, and the centerline of the hole can deviate from a vertical line. These and other complex behaviors depend on not only the process conditions (chemistry, plasma power, bias, temperature, etc.) but also the process chamber history [e.g., radio frequency (RF) hours from preventative maintenance] and how the process chambers are conditioned. In addition, the etch behavior depends on the tier thickness (ONON or OPOP), materials, deposition conditions, and overall wafer topography and shapes. The etch behavior is further related to the die positions on the wafer (center or middle versus edge dies) and the position in the memory cell (close to the edge of the cell or near the center of the cell). To monitor and control the etch behavior, many metrology methods have been developed; these include the etch-back top-view scanning electron microscopy (SEM), tilt SEM, transmission electron microscopy (TEM), and high voltage scanning electron microscopy (HVSEM). These methods are time consuming, and some are destructive, but they have been deemed the ground truth for determining variations in MH etch processes. We report a methodology that combines optical scatterometry (SCD) and small-angle x-ray scatterometry measurements to provide reliable CD profile measurements while meeting the fast-turnaround sampling requirements of high-volume manufacturing.

Unavoidable renaissance of electron metrology in the age of high numerical aperture extreme ultraviolet lithography

Things are drastically changing in the field of metrology. The main reason for that is related to the daunting specification requirements for metrology imposed by high numerical aperture extreme ultraviolet lithography (high NA EUVL). We observe a variety of generation e-beam tools proliferating in imec unique ecosystem, from in-line transmission electron microscope (TEM) to voltage contrast (VC) overlay tools, from die to database (D2DB) large area scanning electron microscope (SEM) to high-voltage SEM, from artificial intelligence (AI)-based inspection tools to massive data acquisition e-beam system. We are facing a renaissance of e-beam metrology. We are going to describe the challenges as well as the latest evolutionary developments of e-beam metrology in the semiconductor industry.

High-resolution planar electron beam induced current in bulk diodes using high-energy electrons

Understanding the impact of high-energy electron radiation on device characteristics remains critical for the expanding use of semiconductor electronics in space-borne applications and other radiation harsh environments. Here, we report on in situ measurements of high-energy electron radiation effects on the hole diffusion length in low threading dislocation density homoepitaxial bulk n-GaN Schottky diodes using electron beam induced current (EBIC) in high-voltage scanning electron microscopy mode. Despite the large interaction volume in this system, quantitative EBIC imaging is possible due to the sustained collimation of the incident electron beam. This approach enables direct measurement of electron radiation effects without having to thin the specimen. Using a combination of experimental EBIC measurements and Monte Carlo simulations of electron trajectories, we determine a hole diffusion length of 264 ± 11 nm for n-GaN. Irradiation with 200 kV electron beam with an accumulated dose of 24 × 1016 electrons cm−2 led to an approximate 35% decrease in the minority carrier diffusion length.

Virtual scanning electron microscope. 5. Application in nanotechnology and in micro- and nanoelectronics

The provided examples demonstrate the application of a simulator-based virtual scanning electron microscope (SEM) in certification of test object sizes on a low-voltage SEM and in calibration of a high-voltage SEM operating in the slow secondary electron detection mode. Using the virtual SEM, the problem of comparing different SEM calibration techniques is solved.

Heavily nitrogen-doped 4H-SiC homoepitaxial films grown on porous SiC substrates

Heavily nitrogen-doped 4H-SiC homoepitaxial films were grown on normal p-type and porous 4H-SiC (PSC) substrates. The nitrogen doping concentration and resistivity of the epitaxial films, measured by Hall measurement, were about 10 20–10 21 cm −3 and 0.002–0.006 Ω cm, respectively. From X-ray analysis, it was observed that the crystallinity of the epilayer grown on porous 4H-SiC substrates is better than that grown on p-type 4H-SiC substrates. In spite of the heavy doping, the full-width at half-maximum (FWHM) value of the epitaxial film, which was grown on the porous substrate, was about 13 arcsec, proving its excellent crystal quality. In contrast, the heavily nitrogen-doped epilayer, which was grown on the normal substrate, had a FWHM value of 16.9 arcsec. High-voltage transmission electron microscope (HVTEM) and scanning electron microscope (SEM) observations made clear that the enhancement of the crystallinity was caused by stress relaxation near pore boundary and defect terminations in the porous layer.

Stress-induced and electromigration voiding in aluminum interconnects passivated with silicon nitride

Void nucleation in aluminum interconnects passivated with silicon nitride was studied using a high voltage scanning electron microscope. Extensive stress-induced voiding was observed in these interconnects independent of the passivation thickness. Some of the stress-induced and electromigration-induced voids formed away from the interconnect sidewall in contrast to results from other studies of void nucleation in passivated aluminum lines. Nuclear reaction analysis measured large amounts of hydrogen in the aluminum films passivated with silicon nitride. Transmission electron microscopy showed a high density of nanometer-sized bubbles in the aluminum. These bubbles, which are thought to have formed from hydrogen that evolved from the silicon nitride layer during processing, served as nucleation sites for stress- and electromigration-induced voids.

Effects of dielectric materials on electromigration failure

The effects of dielectric layers on electromigration failure were studied in situ using a high-voltage scanning electron microscope and at the wafer level using conventional accelerated testing. Several different passivation layers were deposited on wafers with A1 interconnect test structures. Prior to the deposition of the final dielectric, the wafers were processed identically and, whenever possible, simultaneously. Interconnects encapsulated with compliant polymer and very thin (0.1 μm) SiO2 layers demonstrated substantial lifetime extensions over those with more rigid (1 μm thick) SiO2 layers. Unpassivated lines behaved dramatically differently and failed much sooner than those covered with only 0.1 μm of SiO2. As expected, increasing the passivation thickness from 0.5 to 4 μm increased the electromigration lifetime for SiO2 covered specimens. The fabrication of silicon dioxide dielectrics using electron-cyclotron-resonance chemical-vapor deposition (CVD) and silicon nitride dielectrics via plasma-enhanced CVD damaged the interconnects. This damage nearly completely removed the barrier to void nucleation during electromigration.

A high-voltage scanning electron microscopy system for in situ electromigration testing

An apparatus has been constructed to conduct electromigration tests on realistic specimens while simultaneously observing them at relatively high magnification. A scanning transmission electron microscope has been converted into a high-voltage scanning electron microscope (HVSEM) with a large specimen chamber. By imaging with high-energy electrons (120 keV) and detecting backscattered electrons, voids in metal lines can be viewed through passivation layers. The HVSEM has a resolution of 50 nm through 1 μm of passivation. We have constructed instrumentation to heat and pass current through interconnect structures, while they are inside the electron microscope. Presently, the specimen temperature can be as high as 350 °C and is maintained constant to within 0.1 °C. The resistances of interconnects are measured with a precision of 0.05% during an experiment. Testing the lines at moderately accelerated conditions requires great stability of the microscope and instrumentation as well as full automation of the data collection. These requirements have been met, and metallization lines can be tested for several weeks with minimal operator intervention. Digital images of an entire 300-μm-long test structure as well as electrical data are stored automatically every few minutes during a test. The hundreds to thousands of pictures are analyzed using digital image processing techniques to extract void positions and sizes as a function of time. We use this system to characterize electromigration failure in advanced interconnect structures and to test existing theories on electromigration phenomenon.

The evolution of the resistance of aluminum interconnects during electromigration

Resistance monitoring is a traditional method to investigate electromigration failure. It is important to understand how much information can be extracted from the data generated by these experiments. To this end, precision resistance measurements were included as part of accelerated electromigration tests performed inside of a high voltage scanning electron microscope (HVSEM). Twenty-two passivated Al interconnects were tested at 30 mA/μm 2 and at two temperatures, half at 212°C and half at 269°C. During every test, our automated apparatus stored images of each 300 μm long structure several times per hour. The resistance of each line was also precisely measured and recorded. Changing the temperature affected only the time scale of the resistance evolution. There were resistance changes before voids formed that were neither due to temperature fluctuations nor solute effects. In most cases, the nucleation of the first void to form in a line was signaled by an increase in the time derivative of the resistance. Due to the strong effect of void shape, the void volume could not be determined by the magnitude of the resistance change. The width of a void (transverse to the line) rather than the volume largely determined the resistance change.

Void nucleation on intentionally added defects in Al interconnects

Void nucleation in passivated aluminum interconnects was studied using high voltage scanning electron microscopy. To test theories about stress-induced and electromigration void nucleation, Ar ions were implanted into Al specimens. The Ar atoms precipitated and formed bubbles that served as nucleation sites with high surface energy. In the implanted samples, voids formed away from the interconnect sidewalls, in contrast to voids in ordinary passivated Al interconnects. The evolution of the void volume was also affected by the reduction in the nucleation barrier. These results strongly support the theory of void nucleation on interface flaws in Al interconnects.

A High Voltage Scanning Electron Microscope for the In-SituObservation and Recording of Electromigration Voids in Metal Lines on Integrated Circuits.

The passing of current through thin metal lines results in the formation of voids which will eventually cause the line to fail electrically. Gathering of experimental data on electromigration has been limited by two facts: The dimensions of the metal lines on modern integrated circuits are on the order of the resolution limit of light microscopy; and voids in these lines behave quite differently when they are surrounded by a protective film of SiO2 or other dielectric. This dielectric protects the metal surface and constrains the line, pushing back on the metal displaced by void formation. Several additional factors further complicate experimental analysis: the sample test current must be precisely controlled during observation; the sample must be accurately heated to several hundred degrees; the metal lines must be long and narrow (typically 0.2-3.0 (am wide by 300 μm long); testing is often lengthy, sometimes extending over several days even under conditions selected to minimize the time to failure; and electromigration failures are characterized by long incubation periods during which no changes are observed followed by rapid and complex behavior.

Observation of Electromigration Voiding in Cu Lines

ABSTRACTCu film characteristics and in-situ observation of electromigration voiding in passivated Cu lines are reported. Measured grain size distribution show a median grain size of 0.4 μm. A 3 μm wide, 800 μm long line passivated with 5000 Å of SiO2 was tested in a high voltage scanning electron microscopy (HVSEM), enabling dynamic in-situ events to be recorded. Electromigration voiding in this line showed some similarity to that previously observed in polygranular Al lines. Transmission electron microscopy was used to study the microstructure of the passivated Cu lines.

Magnetic properties and dynamic domain behavior in grain-oriented 3% Si-Fe

The magnetic properties of soft magnetic materials are based on dynamic behavior of magnetic domain walls. Many domain structure studies have contributed to the progress of magnetic materials. In this paper, the present state of new development in advanced grain-oriented silicon steel is introduced through the aid of dynamic domain observation of real materials with forsterite film. The basic magnetic phenomena of material, such as the dynamic behavior of surface closure domains, domain wall pinning, and domain structure under rotating magnetization, are explained mainly through the observation of static and dynamic domain wall movements under a high-voltage scanning electron microscope. A few techniques for manufacturing the latest grain-oriented silicon steel, such as improvement in alignment with (110) [001] orientation, increase in the number of mobile domain walls, and relaxation of domain wall pinning, are also described through the observation of dynamic domain patterns. Examples of core losses are given for ideal grain-oriented silicon steel as industrial material, and future developments are predicted.

Measurement and interpretation of strain relaxation in passivated Al–0.5% Cu lines

X-ray diffraction has been used to measure the strain relaxation in passivated Al–0.5% Cu lines at 200 °C after cooling directly from an anneal at the passivation deposition temperature of 380 °C. Fits to the measured X, Y, and Z components of strain are summed to obtain the hydrostatic component, which exhibits a decay over time. Three mechanisms are considered to explain the decay of the hydrostatic strain in the metal line: Cu precipitation from the solid solution, the presence and growth of voids in the lines, and time-dependent deformation of the passivation. Calculations of the effect of Cu precipitation from the solid solution demonstrate that it plays an insignificant role in the relaxation. A high-voltage scanning electron microscope is used to image the presence and growth of voids through the passivation. The time scale of the growth of stress-induced voids is not the same as the hydrostatic relaxation, indicating that voiding is not solely responsible for the observed relaxation. The relaxation of the line is modeled using a time-dependent finite element model, allowing elastic compliance of the passivation. The magnitude of the calculated relaxation agrees with the measurements. It is suggested that a combination of voiding and passivation compliance is responsible for the measured hydrostatic strain relaxation in the metal line.

In situ observations of electromigration induced void behavior

Electromigration and stress voiding have been active areas of research in the microelectronics industry for many years. While accelerated testing of these phenomena has been performed for the last 25 years[1-2], only recently has the introduction of high voltage scanning electron microscopy (HVSEM) made possible in situ testing of realistic, passivated, full thickness samples at high resolution.With a combination of in situ HVSEM and post-testing transmission electron microscopy (TEM) , electromigration void nucleation sites in both normal polycrystalline and near-bamboo pure Al were investigated. The effect of the microstructure of the lines on the void motion was also studied.The HVSEM used was a slightly modified JEOL 1200 EX II scanning TEM with a backscatter electron detector placed above the sample[3]. To observe electromigration in situ the sample was heated and the line had current supplied to it to accelerate the voiding process. After testing lines were prepared for TEM by employing the plan-view wedge technique [6].

Strain Measurement and Calculation in Passivated Cu Lines Deposited by Three Different Methods

AbstractPassivated Cu lines deposited by CVD, electroplating, and sputter-reflow were investigated using x-ray diffraction. Blanket films of the three types were measured for strain and texture post-deposition and after an anneal step to mimic the passivation temperature step. Texture in the CVD films was random, while the electroplated and sputtered films showed a strong {111} texture. Lines were then measured of each type. The measured strain was modeled using finite element calculations. While the strain in Cu was high compared to Al lines of similar geometry, no stress voiding was observed using high voltage scanning electron microscopy.

Observation of voids induced by mechanical stress and electromigration in passivated Al lines deposited at different purity levels

Two pure Al passivated line samples were prepared under different metal deposition conditions, and were electromigration tested in a high voltage scanning electron microscope (HVSEM). The sample prepared under ultrahigh purity conditions showed a high resistance to void initiation and a very large grain size, while the more conventionally deposited metal displayed comparatively poor electromigration resistance. The sample prepared at lower purity conditions showed initial stress voids, but these were not the site for electromigration damage.

Domain structures and magnetic properties of advanced grain-oriented silicon steel

The dynamic behavior of surface closure domains governing the soft magnetic properties of grain-oriented 3% silicon-iron and the basic magnetic phenomena, such as domain wall pinning, are explained mainly through the observation of static and dynamic domain wall movements under a high-voltage scanning electron microscope. Techniques for manufacturing the latest grain-oriented silicon-iron, such as improvements in the alignment of the (110)[001] orientation, increases in the number of mobile domain walls and relaxation of domain wall pinning, are also described through the observation of domain patterns.

Low voltage scanning electron microscopy in membrane research

The sample preparation technique in electron microscopy is very important but often overlooked. Because the preparation technique can influence the actual structure, care must be taken in order to avoid misinterpretation of images. This paper highlights various important factors in scanning electron microscopy (SEM) that must be considered in arriving at an optimal choice of condition for imaging in the study of polymer membranes. The same coating technique can produce different quality images depending upon the types of specimen. For example, for unsuded membranes platinum (Pt) coating gave a superior quality image to chromium (Cr) coating. However, for protein-fouled membranes Cr coating produced a superior image with more surface details. The advantages and disadvantages of low voltage SEM (LVSEM) is discussed in comparison with high voltage SEM.

In Situ Observations of Voiding in Metal Lines Under Passivation

ABSTRACTMovies showing high voltage scanning electron microscope (HVSEM) in situ observations of void motion in passivated metal lines were produced. By taking pictures of the line when the void morphology changes and combining these images digitally with a technique called morphing, a time-lapse movie is constructed with the minimum amount of stored images. The sequence of still images from which a movie is constructed are shown in this paper. This sequence demonstrates that wedge voids do not cause failure in near-bamboo pure Al lines; rather the void will move until it encounters a grain which it can grow across in a slit-like manner.