In-Situ Atom Probe Specimen Preparation with a Planar Reigon of Interest
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IN-SITU ATOM PROBE SPECIMEN PREPARATION WITH A PLANAR REIGON OF INTEREST by Scott E. Allen © Copyright by Scott E. Allen, 2013 All Rights Reserved A thesis submitted to the Faculty and the Board of Trustees of the Colorado School of Mines in partial fulfillment of the requirements for the degree of Master of Science (Applied Physics). Golden, Colorado Date Signed: Scott E. Allen Signed: Dr. Brian Gorman Thesis Advisor Golden, Colorado Date Signed: Dr. Thomas E. Furtak Professor and Head Department of Physics ii ABSTRACT Techniques for rapid preparation of atom probe specimens extracted from the surface of a bulk crystal sample are further developed with regards to samples having a planar region of interest. An atom probe specimen is a needle-shaped structure several microns long and approximately 100 nm in diameter. The conventional method for preparing these specimens maintains the sample orientation throughout the process of extracting sample segments from the bulk crystal, mounting them onto microtips, and shaping them into specimens. For samples having a planar region of interest parallel to the surface, that method produces a small disc-shaped region of interest oriented perpendicular to the specimen axis (horizontally). If the planar region of interest of the sample is composed of atoms susceptible to migrating along the curved surface of the specimen tip during atom probe tomography, this horizontal orientation makes it difficult to quantify and account for any such surface migration. By reorienting the sample with a 90 degree rotation, specimens were produced with the planar region of interest oriented parallel to the specimen axis. This vertically oriented plane is up to 5 times larger than the horizontally oriented planes produced with previous specimen preparation methods. According to the model of field-driven surface migration proposed herein, this orientation also makes it possible to quantify and account for surface migration leading to more accurate atom probe data. iii TABLE OF CONTENTS ABSTRACT . iii LIST OF FIGURES . vi LIST OF ABBREVIATIONS . x ACKNOWLEDGMENTS . xi DEDICATION . xii CHAPTER 1 INTRODUCTION . 1 CHAPTER 2 BACKGROUND . 4 2.1 Two-Dimensional Electron Gasses . 4 2.2 Delta Doping . 5 2.3 Solid State Quantum Computing . 6 2.4 Focused Ion Beam . 9 2.5 Atom Probe Tomography . 12 2.6 Previous Atom Probe Experiments on Delta-Doped Silicon . 17 CHAPTER 3 MODELING SURFACE MIGRATION . 21 CHAPTER 4 EXPERIMENT . 24 4.1 Experimental Apparatus . 24 4.2 Conventional Sample preparation . 25 4.2.1 Feature-non-specific extration and mounting . 26 4.2.2 Feature-specific extraction and mounting . 33 4.2.3 Sharpening and low-energy cleanup . 33 iv 4.3 Vertical Sample Preparation . 34 4.3.1 Mounting . 34 4.3.2 Sharpening . 36 CHAPTER 5 RESULTS . 37 5.1 Reinvestigating previous APT data for signs of field-driven surface migration . 37 5.2 Augmenting chemical resolution . 38 CHAPTER 6 CONCLUSION . 42 REFERENCES CITED . 43 v LIST OF FIGURES Figure 1.1 Resolution and field of view for various tomographic techniques. This shows that although APT has a small field of view, it has greater resolution than other existing 3-D tomography technologies[24]. 2 Figure 1.2 Facsimile of relative orientations of the planar ROI within an APT specimen . 3 Figure 2.1 Quantum well formed by a double heterojunction [28]. 4 Figure 2.2 STM Lithography [37] . 6 Figure 2.3 Low dimensional features made possible by STM lithography and low temperature molecular beam epitaxy [33] . 6 Figure 2.4 The state of a single qubit is represented as a vector on the surface of the Bloch Sphere because it is in a superposition of the 1 and 0 states and contains relative phase information [63]. 7 Figure 2.5 Kane quantum computer with 'A' gates to tune individual phosphorus nuclei to respond to control pulses and 'J' gates to perform 2-qubit logic operations[72]. 9 Figure 2.6 Simplified graphic of ion sourcing, accelerating, and focusing in the FIB [76]. 10 Figure 2.7 Penetration depth of gallium ions in silicon and platinum at various beam energies [77]. 11 Figure 2.8 Various molecular processes occur during FIB deposition. Most of these are adsorption of organic precursor onto the sample surface, breakdown of precursor by secondary electrons, and deposition of platinum composite. Some sputtering and Ga implanation are undesirable yet inevitable during deposition operations [78]. 12 Figure 2.9 Trajectories in (a) Aluminum at 10 keV (b) Aluminum at 30 keV (c) Gold at 10 keV and (d) Gold at 30 keV. In each the large interaction volume on the left is for a beam of electrons and the smaller one to the right is for a beam of FIB ions. Increasing interaction volume is shown at incresing beam energy [79]. 13 vi Figure 2.10 This example atom probe tomogram shows layers of GaAs and GaInP with planar interfaces. This reconstruction shows Ga in blue, As in yellow, In in green and P in red [25]. 14 Figure 2.11 Example atom probe specimen with needle-like geometry [21]. 15 Figure 2.12 The projection of the sample surface onto the position sensitive detector is much larger than the sample tip. The trajectories of the evaporated atoms yields the high magnification needed for atomic resolution [85]. 15 Figure 2.13 Example specimen before and after APT shows an increase in end radius due to shank angle[86]. 16 Figure 2.14 Local electrode geometry augments time-of-flight mass spectroscopy by dominating over ion-ion interactions and other sources of energy defects [89]. 17 Figure 2.15 (a) APT microtip array formed by DRIE and (b) bird's-eye-view of a sample segment mounted atop a microtip [77] . 18 Figure 2.16 Depth profile of phosphorus concentration in delta-doped Si:P [92] . 18 Figure 2.17 The magnitude of the electric field normal to the specimen surface modeled with COMSOL 4.3 by David Diercks shows that the field is maximized at apex of the specimen tip. This is one possible explaination for the anisotropic migration of phosphorus dopant atoms in delta-doped silicon. 20 Figure 3.1 Facsimile of relative orientations of the planar ROI within an APT specimen . 21 Figure 3.2 If bulk migration is not a factor, the dopant atoms will be constrainted to move on the surface of the specimen tip. As they travel toward the apex of the tip they will move up in z and toward the z-axis. 22 Figure 3.3 This top-down view of the dopant plane within the APT specimen shows how field driven surface migration may cause dopant atoms to move toward the z-axis of the specimen. On the left a dopant plane which is not centered in the specimen appears to bend inward. On the right a dopant plane centered in the specimen appears to narrow in the middle. These anomalies of field driven surface migration would not be visible when the dopant plane is oriented horizontally within the APT specimen. 23 vii Figure 4.1 Photograph from inside the chamber of the Helios 600i FIB showing the ion beam at 52º from vertical. 25 Figure 4.2 Temperature of an APT specimen as a function of time under a single laser pulse for two energies modeled with COMOSL 4.3 by David Diercks. This shows that a 100 pJ pulse heats the specimen to a much higher temperature than a 1 pJ pulse. 26 Figure 4.3 Protective platinum composite cap over bulk silicon sample [77] . 27 Figure 4.4 Trangular cross-section of sample bar formed by trench milling [77] . 27 Figure 4.5 (a)First release cut and (b&c) using the manipulator for the cantilever test [77] . 29 Figure 4.6 Extracting the wedge-shaped sample liftout with the manipulator [77] . 30 Figure 4.7 (a) Aligning the end of the liftout over a microtip and (b) slicing off a segment [77] . 31 Figure 4.8 This side view of a sample segment mounted on a mirotip shows the planar junction between the protective platinum cap and the sample surface on the top of the sample segment. The conventional extraction and mounting methods maintain the horizontal orientation of this plane. 32 Figure 4.9 This side view of a sample segment mounted on a microtip show the planar junction between the protective platinum cap and the sample surface on the left side of the sample segment. By rotating the sample liftout 90º in-between extraction and mounting, this plane which was horizontal is now vertical. 35 Figure 5.1 Shown is a close up view of a vertically prepared specimen containing a visable junction between two dissimilar materials. This junction is seen as a faint vertical line down the center of the specimen. 37 Figure 5.2 We divided the tomogram from previous characterizations into vertical columns and made 1-D concentration plots as a function of depth for each. The plot shown is the depth profile of the column in the center about the delta-plane. Silicon is shown in grey and phosphorus is shown in pink. 39 viii Figure 5.3 Top-down view of the tomogram shown in Figure 5.2 superimposed with peak dopant concentration of each segment in atomic percent. This shows an apparent congragation of phosphorus toward the center of the specimen. 39 Figure 5.4 Minimum detectable dopant concentration for a 200 nm long tomogram with Si:P ratio averaged over 1 nm thick slices. This shows that due to increased ROI area, specimens prepared with thier planar ROI oriented vertically yield tomograms wich can show much smaller dopant concentrations.