Facilities

Introduction

Our lab provides two modern tomographic atom probes.

The LEAP 4000 HR (Local Electrode Atom Probe with UV-Laser and reflectron) and the LAWATAP (Laser Assisted Wide Angele Tomographic Atom Probe). Both machines are suitable for obtaining 3D chemical analysis with near atomic resolution.


Additionally, we have the opportunity to use the scanning electron microcopy (SEM),  focused ion beam (FIB) and transmission electron microscopy (TEM) facilities at the Core Lab for Imaging and Characterization.

 

Atom Probe Tomography in a Nutshell

Atom Probe Tomography  (APT) is a destructive method which relies on samples which are prepared in the form of a very small needle. The diameter of the tip of this needle has to be smaller than 200 nm. This is the length which you get when you place about 500 atoms next to each other. The small needle can be either a wire quenched into a copper tube (LEAP, LAWATAP) or a small post on a Silicon coupon (LEAP).

The atom probe utilizes Ultra High Vacuum. A high Voltage (several kV) is applied on the needle. This creates a field which, directly at the surface of the tip, is almost high enough to rip out (field evaporate) atoms from the tip. In this situation  an additional voltage pulse or a laser pulse is used to change the situation such as that a low probability for the atoms to evaporate arises. The probability can be adjusted by either changing the base voltage or the pulse voltage or the laser intensity. It is typically in the range of <5% per pulse. In this pattern the tip is taken apart atom by atom.

The evaporated ion is accelerated in the electric filed and then detected on a 2D detector (see sketch). The time of the evaporation is known, because it happened at the time, when the pulse was applied. The time of the arrival is also known. Thus the flight time can be calculated and used to determine the chemical identity (e.g. Aluminum, Iron, Copper, ...)  of the ion.

The 3D-recontruction is calculated by considering, that atoms, which arrived at a pulse that was applied early in the measurement, must have been closer to the top than atoms which arrived during later pulses.

APT scematic.jpg
On the left side the tip of the needle is displayed. The different colored circles represent atoms. These atoms are evaporated in the high field by a voltage pulse and accelereated to the detector, where their impact is detected. The time of flight and 2D-information is analyzed by an algorithm that reconstruct a 3D image, in which again each circle represents a single atom.



LAWATAP

The LAWATAP (Laser Assisted Wide Angle Tomographic Atom Probe) was developed by CAMECA in Rouen France. It has a traditional electrode that be used for wire samples.

LEAP 4000 HR
The LEAP 4000 HR (Local Electrode Atom Probe) was originally developed by IMAGO (now a part of CAMECA)  in Madison, USA. The LEAP provides a local electrode with a diameter of 25 microns, which allows the analysis of smaller samples, especially micro tips on coupons. Nevertheless it is still possible to analyze wire shaped samples.