Specimen Preparation of Metals and Alloys for Atom Probe Tomography by Electropolishing

Transcription

1 Specimen Preparation of Metals and Alloys for Atom Probe Tomography by Electropolishing R. Prakash Kolli, Ph.D., PE July 24, st Atom Probe Pre-meeting Congress Organized by the MSA AP FIG 1

2 Atom Probe Tomography (APT) Field evaporation technique that permits 3D atomby-atom reconstructions Specimens in the form of sharp needles or microtips fabricated by electropolishing or by a dual-beam focused ion beam (FIB) scanning electron microscope (SEM) instrument h"p:// leap-si.aspx nm 3 analysis volume torr UHV K specimen temperature 200 khz voltage pulsing 1,000 khz laser pulsing ~ nm depth spatial resolution ~ nm lateral spatial resolution Equally sensitive for all chemical species 2

3 Overview Specimens in the shape of sharp needles or microtips Apply a positive DC voltage Apply rapid AC voltage pulses or laser pulses Ions are field evaporated and detected x- and y-position determined by position sensitive detector z-position given by pulse sequence mass-to-charge state (m/n) ratio determined by time-of-flight (TOF) mass spectrometry 3

4 Specimen Preparation Overview APT requires a sufficiently high electric field at the specimen tip of ~10 50 V nm -1 Sharp specimens with small radii can attain such a field r tip ~50 nm Shank half-angle ~10 Specimen fabrication for APT is a combination of science and art Researcher skill is a significant factor in the rate of successful specimen preparation Successful specimen preparation leads to more successful data acquisition and data analysis 4

5 Specimen Preparation Techniques Electropolishing Dual-beam Focused Ion Beam (FIB) Scanning Electron Microscope (SEM) instrument Other fabrication techniques 5

6 Electropolishing Conventional method that relies on conductivity of the specimen to be analyzed Electrochemical process that is often performed at room temperature Manual or Automated Initial (Rough) Polishing, Final Polishing, Pulse Polishing Electrolytes Artifacts Advantages and Disadvantages 6

7 Manual Electropolishing A two-step electrochemical method that shapes conductive specimen blanks into sharp needle-shaped specimens for APT analysis Specimen blanks are wire or small rectangles ( x ~ mm 3 ) taken from bulk material Specimen blank acts as an electrode, requires a counter electrode, an electrolyte, and a controllable DC voltage and current Assisted by a low power optical microscope or steromicroscope Cut Electropolish APT Needle-Shaped Specimen Bulk Schema9c courtesy of Mr. Richard Martens (U. Alabama) 7

8 Specimen Blanks Blanks from a thin ribbon or sheet Miller and Smith (1989) Wires or Whiskers Larson, et al. (2013) Blanks cut from the bulk by low-speed saw Gault, et al. (2012) 8

9 Manual Electropolishing Station Light Source Stand Suspended Specimen Blank Fume Exhaust Vent Counter Electrode CuveDe w/ Electrolyte VerFcal TranslaFonal Control Power Supply and Control Images courtesy of Dr. Dieter Isheim (NUCAPT) Stereomicroscope 9

10 Other Manual Electropolishing Configurations Manual ElectropointerTM by Simplex Scien9fic Standardized Configura9on Power Supply and Control Specimen Blank and Loop Counter Electrode OpFcal Microscope Adjustable Actuators Larson, et al. (2013) 10

11 Manual Electropolishing Procedure Initial or Rough Polishing thin the wire or specimen blank into a necked shape by using the meniscus of the electrolyte that is in the cuvette while applying a DC voltage Final Polishing continue thinning the necked wire or specimen blank in a controllable manner until separation by using a lower strength electrolyte and a lower DC voltage Pulse Polishing uses a wire loop with a thin film of electrolyte to resharpen a blunt or fractured needle and short voltage pulses 11

12 Manual Electropolishing Method Blank IniFal Polish Electrolyte Needle Final Polish Miller and Smith (1989) Original schematic courtesy of Dr. Matt Bender (Northwestern) 12

13 Manual Electropolishing Method NUCu Aged o C 13

14 Automated Electropolishing Station ElectropointerTM by Simplex Scientific Automated TranslaFonal Stepper Motor Computer Control Monitor and Control Voltage Beaker w/ Electrolyte Suspended Specimen Blank Loop Counter Electrode Images courtesy of Mr. Richard Martens (U. Alabama) Kostrna, et al. Microsc. Microanal. (2006) 14

15 Electrolytes Acids in solvents similar to those used in TEM specimen preparation These acids are often toxic, corrosive, and flammable check Material Safety Data Sheet (MSDS) Different metals and alloys require different electrolytes and voltages to produce controlled electropolishing Too high a voltage for a particular electrolyte will cause pitting 15

16 Example Electrolytes Steel & Nickel-Based Superalloys ~2-10% perchloric acid in acetic acid or butoxyethanol or methanol Aluminum Alloys ~20-30% nitric acid in methanol; ~2-10% perchloric acid in butoxyethanol or methanol Copper Phosphoric acid Magnesium Alloys 5% perchloric acid in butoxyethanol Titanium Alloys 6% perchloric acid in methanol References Miller and Smith (1989) Miller, et al. (1996) Gault, et al. (2012) Larson, et al. (2013) Miller and Forbes (2014) Perchloric acid is hazardous and requires a dedicated and certified fume hood. 16

17 Electropolishing Artifacts Different phases can polish at different rates Iron-based bulk metallic glass that exhibits non-uniform cross-secfon and mulfple phases Miller and Russell Ultramicrosc. (2007) Tip may bend during final polishing Electrolyte residue Pitting 17

18 Advantages Inexpensive equipment Can easily repolish and resharpen blunt or fractured needles Can rapidly make specimen blanks Can rapidly make multiple needles No Ga + ion implantation No embrittlement 18

19 Limitations Limited to conductive metals and alloys Only one to four needles per puck Oxides can form on some metals such as titanium Site-specific sample preparation is more difficult 19

20 Questions? 20

21 Backup Slides 21

22 Focused Ion Beam (FIB) Instrument Focused Ion Beam (FIB) instrument Use a FIB instrument or dual-beam FIB instrument to fabricate needleshaped specimen tips for APT analysis Dual-beam FIB instrument is more advantageous since one can alternate between the gallium + (Ga + ) ion beam and the e-beam for milling and imaging to check progress Can fabricate conductive, semi-conductive, or insulator materials into tips Permits site-specific specimen fabrication Multi-step method that is derived from TEM specimen fabrication methods 22

23 Focused Ion Beam (FIB) Instrument In situ moat method Material surrounding the region of interest (ROI) is removed by Ga + ion milling that is then annularly milled into a needleshaped post for APT analysis In situ lift-out method Material surrounding the ROI is milled and then extracted using a micromanipulator and mounted on a pre-fabricated post. It is then annularly milled into a needle-shaped post for APT analysis Image from Google Images. 23

24 Focused Ion Beam (FIB) Instrument In situ Moat Method Circumferential milling Annular milling Miller and Russell (2007) 24

25 Focused Ion Beam (FIB) Instrument In situ Lift-Out Method Milling of ROI Creation of Specimen Blanks Annular Milling 25

26 Focused Ion Beam (FIB) Instrument 26

27 Focused Ion Beam (FIB) Instrument Focused Ion Beam (FIB) Instrument Works very well almost all materials including odd geometries, such as flakes, powders, sheet, etc. Easier to control location and final geometry Ga + ion implantation in specimen tip will affect mass spectrum User skill is critical for fabricating successful specimens References M. K. Miller and G. D. W. Smith, Atom Probe Microanalysis: Principles and Applications to Materials Problems, Materials Research Society, M. K. Miller, Atom Probe Tomography: Analysis at the Atomic Level, Springer, B. Gault, M. P. Moody, J. M. Cairney, S. P. Ringer, Atom Probe Microscopy, Springer

28 Focused Ion Beam (FIB) Instrument References G. B. Thompson, M. K. Miller, and H. L Fraser, Some aspects of atom probe specimen preparation and analysis of thin films, Ultramicroscopy, 100 (2004) M. K. Miller, K. F. Russell, and G. B. Thompson, Strategies for fabricating atom probe specimens with a dual beam FIB, Ultramicroscopy, 105 (2005), K. Thompson, D. Lawrence, D. J. Larson, J. D. Olson, T. F. Kelly, and B. Gorman, In situ site-specific specimen preparation for atom probe tomography, Ultramicroscopy, 107 (2007), M. K. Miller and K. F. Russell, Atom-probe specimen preparation with a dual beam SEM/FIB miller, Ultramicroscopy, 107 (2007), Some pre-2004 references by Larson. 28

29 In situ Fabrication In situ fabrication Fabrication of tips through catalytic growth methods, e.g. nanowires 29