X-ray Topographic Observation Research Articles

Proposal of Damage-Free SiC Wafer Dicing Using Water Jet Guided Laser

A damage-free SiC wafer dicing method has been strongly required for practical applications of power devices. In this research, we propose water jet guided laser processing as a novel dicing method. Water jet guided laser processing, which uses a high-pressure fine water jet as waveguide, could generate no cracks or dislocations in crystal. In this paper, water jet guided laser grooving quality was evaluated to demonstrate there should be no chippings and basal plane dislocations. Scanning electron microscopic and X-ray topography observations were conducted. The results indicated the superiority of water jet guided laser dicing to a conventional dicing method.

Development of High-Resolution Nuclear Emulsion Plates for Synchrotron X-Ray Topography Observation of Large-Size Semiconductor Wafers

Characterization of defects in semiconductor wafers is essential for the development and improvement of semiconductor devices, especially power devices. X-ray topography (XRT) using synchrotron radiation is a powerful methods used for defect characterization. To achieve detailed characterization of large-size semiconductor wafers by synchrotron XRT, we have developed nuclear emulsion plates reaching a high-resolution and wide dynamic range. We have shown that higher-resolution XRT images could be obtained using emulsions with smaller iodobromide crystals, and demonstrated clear observation of threading edge dislocations in a SiC epitaxial layer having small contrast. Furthermore, we demonstrated XRT image acquisition for almost all of a 150-mm SiC wafer with one plate. Our development will contribute to advances in electronic materials, especially in the field of power electronics, in which defect characterization is important for improving the performance and yield of devices.

Characterization of dislocation of halide vapor phase epitaxial (001) β-Ga2O3 by ultrahigh sensitive emission microscopy and synchrotron X-ray topography and its influence on Schottky barrier diodes

This study elucidates the dislocation responsible for the leakage current in a halide vapor phase epitaxial (001) β-Ga2O3 Schottky barrier diode. A high reverse leakage current of −0.98 μA was observed at −100 V as an emission pattern via ultrahigh-sensitive emission microscopy. This was attributed to the presence of dislocation. Moreover, the Burgers vector of this dislocation was determined to be ⫽ 〈12〉 from the g·b invisibility criteria via synchrotron X-ray topography observation. This dislocation was speculated to be induced by the relaxation of the strain field surrounding the void and subsequently propagated to the surface. Cross-sectional scanning transmission electron microscopy revealed multiple threading dislocations along the (100) plane.

X-ray topographic observation of dislocations in β-Ga2O3 Schottky barrier diodes and their glide and multiplication under reverse bias

We observed dislocations in β-Ga2O3 Schottky barrier diodes (SBD) using synchrotron X-ray topography (XRT) and studied their behaviors under forward or reverse bias. Several representative dislocation types were identified by observing the dislocation lines and their Burgers vectors. After comparing the XRT images taken before and after the bias was applied, we found that the dislocations were not static but could glide and multiply under a reverse bias of −140 V for 5 min. This was frequently observed near the dislocation bundles emanating from the end of a screw-type b-axis dislocation that had a Burgers vector of b∥[010]. The glide belonged to the 〈010〉{001} slip system. The glide and multiplication occurred even when the dislocations were not directly located in the SBD area being electrically stressed. It is considered that the b-axis screw dislocations acted as pathways of leakage current that was the driven force for dislocation glide and multiplication.

Seed surface orientation dependence of the defect formation at the initial stage of physical vapor transport growth of 4H-SiC crystals

The influence of the seed crystal surface orientation on the defect formation at the initial stage of physical vapor transport (PVT) growth of 4H-SiC crystals was investigated using X-ray topography and Raman scattering microscopy. To examine the detailed influence of the seed surface orientation, a seed crystal having a groove on the growth surface was used for PVT growth; the seed crystal with a groove provides a continuously varying seed surface orientation from (0 0 0 1¯) (on-axis c-face growth) to (1 1 2¯ 0) (a-face growth), and using the seed crystal, the effect of the seed surface orientation can be examined in a single growth experiment. X-ray topography observations revealed that a number of threading screw dislocations (TSDs) were formed at the bottom corners of the groove, while their formation was largely suppressed at the bottom of the groove, indicating that an off-oriented (0 0 0 1¯) seed crystal surface tends to generate TSDs. Characteristic dopant striations were also observed in the crystal portions grown on the off-oriented seed crystal surface, and the generated TSDs were found to propagate along the boundaries of these dopant striations. Based on these results, we discuss the TSD formation mechanism on an off-oriented (0 0 0 1¯) seed crystal surface at the initial stage of PVT growth of 4H-SiC crystals and suggest an important role of macrostep formation on the surface.

Suppression of stacking fault expansion in a 4H-SiC epitaxial layer by proton irradiation

SiC bipolar degradation, which is caused by stacking fault expansion from basal plane dislocations in a SiC epitaxial layer or near the interface between the epitaxial layer and the substrate, is one of the critical problems inhibiting widespread usage of high-voltage SiC bipolar devices. In the present study, we investigated the stacking fault expansion behavior under UV illumination in a 4H-SiC epitaxial layer subjected to proton irradiation. X-ray topography observations revealed that proton irradiation suppressed stacking fault expansion. Excess carrier lifetime measurements showed that stacking fault expansion was suppressed in 4H-SiC epitaxial layers with proton irradiation at a fluence of 1 × 1011 cm−2 without evident reduction of the excess carrier lifetime. Furthermore, stacking fault expansion was also suppressed even after high-temperature annealing to recover the excess carrier lifetime. These results implied that passivation of dislocation cores by protons hinders recombination-enhanced dislocation glide motion under UV illumination.

In-operando x-ray topography analysis of SiC metal–oxide–semiconductor field-effect transistors to visualize stacking fault expansion motions dynamically during operations

We developed an in-operando x-ray topography method for dynamically visualizing single Shockley-type stacking fault (1SSF) expansion motions in silicon carbide (SiC) metal–oxide–semiconductor field-effect transistors (MOSFETs) during their operations and investigated the effect of the operating condition applied to the body diodes in SiC MOSFETs on dislocation glide velocity. In-operando x-ray topography observations were carried out in reflection geometry, and a high-resolution x-ray camera was used as a detector to record topographies dynamically. The sequence of 1SSF expansion motions in the SiC MOSFETs was observed at a high resolution of 1 s in x-ray topographies, which is sufficient to analyze the dislocation glide velocity of a 1SSF expansion. The observation results of changing the forward current density applied to the body diodes in SiC MOSFETs revealed that each triangular and bar-shaped 1SSF expands at different forward current densities. The 1SSF expansion timings also differed, even in the same chip under the same current density. The dislocation glide velocity of each expanded 1SSF in SiC MOSFETs was extracted, and it increased with the forward current density. Our method enables the dynamic visualization of bipolar degradation in SiC MOSFETs during their operations, and we can accurately obtain the information of when, where, and which 1SSF expands in a SiC MOSFET.

Slip planes in monoclinic β-Ga2O3 revealed from its {010} face via synchrotron X-ray diffraction and X-ray topography

Dislocations and their arrays in a {010}-oriented β-Ga2O3 substrate were observed using synchrotron X-ray diffraction (XRD) and X-ray topography (XRT). The XRD measurements used spatially-resolved ω-rocking curves to show that dislocation arrays have a linear contrast with a lower diffraction intensity and a broader full-width at half maximum. Further XRT observations at three different vectors resolved these lines as dot contrasts aligned along the four different directions and which correspond to the outcrops of threading dislocations that lay in four different slip planes. The Burgers vectors for these dislocation arrays were determined using the ∙ invisibility criterion. The results indicate there are four slip systems of (1) {001}, (2) {01}, (3) {10}, and (4) {100}. The slip system {10} has not been previously reported.

Analysis of Dislocations in PVT-Grown 6H-SiC through Grazing-Incidence X-Ray Topographic Images and Ray-Tracing Simulation

The superior electronic and physical properties of silicon carbide (SiC), such as wide band-gap, high saturated electron velocity, high breakdown electric field strength, and high thermal conductivity, enable successful applications in modern power electronic devices with high power, high frequency, and high temperature operation requirements. [1,2] Therefore, understanding the nature and behavior of different dislocations is of great significance towards crystal quality enhancement. Synchrotron X-ray has been widely adopted for generating topographic images of threading screw dislocations (TSDs) as well as threading edge dislocations (TEDs) and basal plane dislocations (BPDs). Besides, recent study in 4H-SiC revealed the presence of threading mixed dislocations (TMDs) with Burgers vectors nc+ma (where m and n are integers). [3] Since direct TMDs observation methods such as transmission electron microscopy (TEM) and synchrotron white-beam X-ray topography (SWBXT) are not suitable for commercially preferred c-plane-oriented wafers, ray-tracing simulated images of possible dislocation types are compared with X-ray topographs in order to identify the TMDs non-destructively. Except 4H-SiC, ray-tracing simulation has also been proved as an effective approach for dislocation characterization in various materials such as GaN [4], ZnO [5], and AlN [6]. The same principle is implemented in this study for the investigation of dislocations in 6H-SiC wafer.Synchrotron monochromatic beam X-ray topography (SMBXT) analysis was carried out on a PVT grown 6H-SiC wafer for recording its grazing-incidence images of all six different 11-212 reflections (Fig. 1). The distribution of TSDs, TEDs, TMDs, and BPDs are characterized, and the determination of their Burgers vectors is achieved by correlating the simulated results obtained through ray-tracing simulation with real X-ray topographic observations (Fig. 2). This study provides a clear understanding of dislocations in 6H-SiC, which is crucial for quality improvement of the crystal.Reference:[1]: C. Codreanu, et al, Comparison of 3C-SiC, 6H-SiC and 4H-SiC MESFETs Performances, Materials Science in Semiconductor Processing 3 137 (2000).[2]: J. W. Sun, et al, Shockley-Frank Stacking Faults in 6H-SiC, Journal of Applied Physics 111, 113527 (2012).[3]: J. Q. Guo, et al, Direct Determination of Burgers Vectors of Threading MixedDislocations in 4H-SiC Grown by PVT Method, Journal of Electronic Materials 45, 4 (2016).[4]: B. Raghothamachar, et al, X-ray Topography Characterization of Gallium Nitride Substrates for Power Device Applications, submitted for publication.[5]: T. Y. Zhou, “Threading Dislocation Characterization and Stress Mapping Depth Profiling via Ray Tracing Technique,” Ph.D. Thesis, SUNY Stony Brook, Advisor: Michael Dudley (2015).[6]: T. Y. Zhou, et, al, Characterization of Threading Dislocations in PVT-Grown AlN Substrates via x-Ray Topography and Ray Tracing Simulation, Journal of Electronic Materials volume 43, 838 (2014). Figure 1

Formation and multiplication of basal plane dislocations during physical vapor transport growth of 4H-SiC crystals

The distribution of basal plane dislocations (BPDs) in physical vapor transport (PVT) grown 4H-SiC crystals has been investigated using Raman microscopy and X-ray topography. X-ray topography observations of (1 1 2¯ 0) wafers vertically sliced along the growth direction from the crystals revealed that there existed almost periodically arranged layers with a high density of BPDs (bunched BPDs) in PVT-grown 4H-SiC crystals. These layers were characterized using Raman microscopy and high resolution X-ray diffraction, and it was found that large tensile and compressive stresses were associated with the layers. The stresses showed characteristic variations along the basal plane as well as the growth direction. Based on these results, the formation and multiplication processes of BPDs during PVT growth of 4H-SiC crystals are elucidated.

Observation of Threading Dislocations in Ammonothermal Gallium Nitride Single Crystal Using Synchrotron X-ray Topography

Synchrotron monochromatic-beam x-ray topography observation has been performed on high-quality ammonothermal gallium nitride single crystal to evaluate threading dislocations (TD) in a nondestructive manner. Asymmetric diffractions with six equivalent g-vectors of 11–26, in addition to a symmetric diffraction with g = 0008, were applied to determine the Burgers vectors (b) of dislocations. It was found that pure edge-type TDs with \( {\varvec b} = \left\langle {11 - 20} \right\rangle /3 \) did not exist in the sample. A dominant proportion of TDs were of mixed type with \( {\varvec b} = \left\langle {11 - 20} \right\rangle /3 + \left\langle {0001} \right\rangle \), i.e., so-called c + a dislocations. Pure 1c screw dislocations with \( {\varvec b} = \left\langle {0001} \right\rangle \) and TDs with c-component larger than 1c were also observed.

Conversion of Basal Plane Dislocations to Threading Edge Dislocations in Growth of Epitaxial Layers on 4H-SiC Substrates with a Vicinal Off-Angle

We have investigated a conversion of basal plane dislocation (BPD) to threading edge dislocation (TED) in growth of epitaxial layers (epi-layers) on 4H-SiC vicinal substrates with an off-angle of 0.85° at low C/Si ratio of 0.7 by using deep KOH etching and X-ray topography observations. Deep KOH etching indicated that BPDs in the substrates converted to TEDs in the epi-layers. X-ray topography observations suggested that the conversion occurred during epitaxial growth when the thickness of epi-layers was less than 1.5 μm. We found that the conversion ratio obtained from counting deep KOH etch pits was over 99%.

Direct Observation of Stacking Fault Nucleation from Deflected Threading Dislocations with Burgers Vector c+a in PVT Grown 4H-SiC

ABSTRACTIn our previous studies [1-3], four kinds of stacking faults in 4H-SiC bulk crystal have been distinguished based on their contrast behavior differences in synchrotron white beam x-ray topography images. These faults are Shockley faults, Frank faults, Shockley plus c/2 Frank faults, and Shockley plus c/4 Frank faults. Our proposed formation mechanisms for these stacking faults involve the overgrowth of the surface outcrop associated with threading screw dislocations (TSDs) or threading mixed dislocations (TMDs) with Burgers vector of c+a by macrosteps and the consequent deflection of TSDs or TMDs onto the basal plane. Previous synchrotron x-ray topography observations were made in offcut basal wafers using transmission geometry. In this paper, further evidence is reported to confirm the proposed stacking fault formation mechanism. Observations are made in axially cut slices with surface plane {11-20}. Several kinds of stacking faults are recognized and their contrast behavior agrees with the four kinds previously reported. Direct observation is obtained of a Shockley plus c/4 Frank stacking fault nucleating from a TMD deflected onto the basal plane. The contrast from stacking faults on the basal plane in the axial slices is enhanced by recording images after rotating the crystal about the active -1010 reflection vector enabling a broader projection of the basal plane.

X-ray Topographic Observations of Dislocations in Diamond(<Special Issue>Diamond Growth)

X-ray Topographic Observations of Dislocations in Diamond(<Special Issue>Diamond Growth)

Correlation between etch pits formed by molten KOH+Na2O2 etching and dislocation types in heavily doped n+-4H–SiC studied by X-ray topography

X-ray topography (XRT) has been carried out to examine the accuracy of KOH+Na2O2 etching (KN etching) technique which was recently developed for identifying dislocation types in n+-4H–SiC (n>1019cm−3). Precise one-to-one correlation have been found between the position of etch pits and dislocations revealed by XRT. More importantly, XRT observation has also confirmed the validity of using the pit sizes formed by KN etching to distinguish threading screw dislocations from threading edge dislocations.

Correlation Between the V-I Characteristics of (0001) 4H-SiC PN Junctions Having Different Structural Features and Synchrotron X-ray Topography

ABSTRACTIn practical use of the SiC power MOSFETs, further reduction of the channel resistance, high stability under harsh environments, and also, high product yield of large area devices are indispensable. Pn diodes with large chip area have been already reported with high fabrication yield, however, there is few reports in terms of the power MOSFETs. To clarify the difference between the simple pn diodes and power MOSFETs, we have fabricated four pn-type junction TEGs having the different structural features. Those pn junctions are close to the similar structure of DIMOS (Double-implanted MOS) step-by-step from the simple pn diodes. We have surveyed the V-I characteristics dependence on each structural features over the 2inch wafer. Before their fabrication, we formed grid patterns with numbering over the 2inch wafer, then performed the synchrotron x-ray topography observation. This enables the direct comparison the electrical and spectrographic characteristics of each pn junctions with the fingerprints of defects.Four structural features from TypeA to TypeD are as follows. TypeA is the most simple structure as same as the standard pn diodes formed by Al+ ion implantation (I/I), except that the Al+ I/I condition conforms to that of the p-well I/I in the DIMOS. The JTE structure was used for the edge termination on all junctions. While the TypeA consists of one p-type region, TypeB and TypeC consists of a lot of p-wells. The difference of Type B and C is a difference of the oxide between the adjacent p-wells. The oxide of TypeB consists of the thick field oxide, while that of TypeC consists of the thermal oxide corresponding to the gate oxide in the DIMOS. In the TypeD structure, n+ region corresponding to the source in the DIMOS was added by the P+ I/I. The TypeD is the same structure of the DIMOS, except that the gate and source contacts are shorted. The V-I measurements of the pn junctions are performed using the KEITHLEY 237 voltage source meters with semi-auto probe machine. An active area of the fabricated pn junctions TEGs are 150um2 and 1mm2. Concentration and thickness of the drift layer are 1e16cm−3 and 10um, respectively.In order to compare the V-I characteristics of fabricated pn junctions with their defects information that obtained from x-ray topography measurements directly, the grid patterns are formed before the fabrication. The grid patterns were formed over the 2inch wafer by the SiC etching. The synchrotron x-ray topography measurements are carried out at the Beam-Line 15C in Photon-Factory in High-Energy-Accelerator-Research-Organization. Three diffraction conditions, g=11-28, -1-128, and 1-108, are chosen in grazing-incidence geometry (improved Berg-Barrett method).In the presentation, the V-I characteristics mapping on the 2inch wafer for each pn junctions, and the comparison of V-I characteristics with x-ray topography will be reported.

Solution growth of high-quality 3C-SiC crystals

Abstract 3C (cubic)-silicon carbide (SiC) bulk crystals that are stable at relatively low temperatures are difficult to grow by the sublimation method, which requires relatively high growth temperatures. This study focused on growth from the solution, which has the advantage that low growth temperatures are applied and the resulting crystals are mostly of high crystalline quality. Firstly, a large number of 3C-SiC crystals were grown by solution growth via spontaneous nucleation in a large temperature gradient. A relatively high yield of 6.4% was achieved from the charged Si solvent. Transmission electron microscopy and X-ray topography observations indicated that the stacking fault density of these crystals was much lower than that of the substrates grown by chemical vapor deposition methods. A dipping solution technique was then applied for 3C-SiC growth, using 3C-SiC seed crystals grown via spontaneous nucleation. Growth was performed on the (1 1 1)Si or (1¯ 1¯ 1¯)C faces. As a result, 3C-SiC grew on the (1¯ 1¯ 1¯)C face, but 6H-SiC crystals grew on the (1 1 1)Si face. For the stable growth of 3C-SiC crystals without 6H-SiC precipitation, growth on the (1¯ 1¯ 1¯)C faces is necessary.

An x-ray topographic study of diamond anvils: Correlation between defects and helium diffusion

X-ray topographic images of several dozens of diamonds have been recorded before and after their use as anvils in a diamond anvil high pressure cell. The aim of this study is to better understand and prevent the breakage of diamond anvils when they are used in contact with a helium sample. Indeed, helium is a good hydrostatic medium for very high pressure studies but many anvils are subjected to helium embrittlement. X-ray topographic observations suggest that helium atoms enter the diamond anvil by a diffusion process enhanced by subsurface strains and/or extended preexisting defects and nonhydrostatic stress. Then these atoms open cracks during nonhydrostatic load of the diamond anvil. The diamond anvils can be classified into three types depending on the nature of the defects. An associated criterion for the selection of stones resistant to helium is proposed.

Some Bristol–Prague explorations in x-ray topography

This paper briefly chronicles a long-standing and productive collaboration between the Institute of Physics, Czech Academy of Science and the H H Wills Physics Laboratory. It began in early 1962 with a brief visit to Bristol by Milena Polcarová. The initial aim, successfully achieved, was the mapping by transmission topography of dislocations in melt-grown single crystals of a Fe–Si alloy. A novel by-product was the x-ray topographic observation of internal magnetic domain structures in specimen plates prepared in both (110) and (112) orientations. In the alloy studied, which contained about 3 wt% Si, the directions of easy magnetization are ⟨ 100 ⟩, and domain boundaries are either 180° or 90° Bloch walls. The latter walls can generate strong x-ray diffraction contrast, but no contrast from 180° walls is expected. In the (110) plates x-ray topography revealed complex internal domain structures containing 90° walls, some previously unsuspected on evidence of optical micrography of colloid patterns (Bitter patterns). Certain details of these structures remain a puzzle to this day! In (112) specimens, in which no direction of easy magnetization lies in the plate surface, the specimen is filled with a hierarchy of domains, diminishing in scale towards the surfaces in order to minimize magnetostatic energy due to free poles. However, in (112) plate thicknesses less than ∼20 µm, x-ray topographs recorded internal domain structures sufficiently uncomplicated for their main features to be interpreted. This was achieved by F C Frank in the early 1960s, but not published till 1993! During a 1968 visit to Bristol by Polcarová it was discovered that under appropriate diffraction conditions x-ray topographic contrast from 180° Bloch walls was just detectable. This finding was not published till 1991. More recent work with Prague specimens has applied synchrotron x-ray reticulography at Daresbury, showing that this technique can be informatively used with lightly-deformed Fe–Si bicrystals.A very different enterprise was the Bristol method of producing x-ray moiré patterns by superposing one crystal plate upon another, achieved in 1967. This project gained from Czech collaboration through the skilful participation of J Brádler, on leave from Prague.

Plane-wave synchrotron x-ray topography observation of grown-in microdefects in a slowly pulled CZ-silicon crystal

Plane-wave x-ray topography was used to characterize grown-in defects in silicon crystals grown by the Czochralski method at a slow speed. The 220 reflection rocking curves of the sample crystal were measured in a symmetric Laue case. Oscillatory profiles were clearly observed in the rocking curves and were in good agreement with those calculated from the dynamical theory of x-ray diffraction. Images of the grown-in defects were observed in the plane-wave x-ray topographs of the 220 reflection. The density of the defects was determined as 300 cm−3. The lattice strain around the defects was evaluated as 10−6 from the dependence of the defect image contrast on the x-ray incident angles. The defect images are discussed using a grown-in defect model of entangled dislocation loop clusters.