GB2536853A

GB2536853A - Process improvement for creating TEM sections

Info

Publication number: GB2536853A
Application number: GB1411434.2A
Authority: GB
Inventors: Lloyd Peto Neil
Original assignee: Individual
Current assignee: Individual
Priority date: 2014-06-26
Filing date: 2014-06-26
Publication date: 2016-10-05
Also published as: GB201411434D0

Abstract

A method of making Transmitting Electron Microscopy (TEM) sections using a Focussed Ion Beam (FIB) wherein when thinning the section, three cuts 3 are made, framing the area of interest and leaving the section attached to the sample at its top two corners. One side of the section is cut away 4 so that the section is connected to the bulk sample by two thin flexible spring-like elements 5 that allow the section to move whilst providing support to help prevent damage due to deformation of the section during the milling/polishing process.

Description

Process improvement for creating TEM sections This invention relates to a process improvement used when making TEM sections using a Focused Ion Beam system (FIB).

FIB systems can make very thin sections from a piece of a material. There are many ways of polishing samples to make them thin but the FIB approach has many advantages and so is becoming increasingly important.

The FIB system can cut away small amounts of a sample under very controlled conditions and the process of making a TEM section relies on this capability. Firstly 2 parallel trenches are milled into the top surface of the sample (with or without surface protection), and the wall that separates these trenches is then polished with the FIB to make a very thin vertical wall. This wall or section can be extracted from the sample in several ways, but ultimately it will be thinned by the FIB until it is less than 120 nano-meters thick, and often much thinner, and will then be a TEM section.

When FIB or FIB-SEM systems are used to create TEM sections from bulk samples, the sections often distort or deform during the final stages of this thinning process. This deformation reduces the effectiveness of the FIB process by making the section more difficult to polish evenly, and this makes the resulting section perform less well and offer poorer results in the subsequent TEM work which normally follows.

The process of using a FIB system to make TEM sections offers many advantages for researchers looking at complex materials, but these materials are often the most likely to deform in the ways stated during this FIB polishing process.

The conventional method for reducing these distortions deformations is to release one end of the section at the time of the distortion prior to the polishing process being complete. This release sometimes permits the section deformation to be reversed, but is in many cases ineffective, and often it introduces a new form of deformation as the section is now able to move more freely. Deformation may take several forms, the most common of which is referred to as bowing. Other forms of deformation may be referred to as folding, flapping, corner curling, and rolling.

In order for the deformations in the section to be better controlled this invention modifies the thinning process so that the section can still be held in the optimal polishing position but is also able to move sufficiently to release any deformations that occur as part of the materials response to the polishing process. This is achieved by using the FIB to create soft 'springs' at one side of the section as it is being polished and modifying these springs so that their characteristics are always optimised for supporting further section polishing. These fabricated springs have certain characteristics that ensure they do not adversely affect the FIB process at all, but are purely complimentary to it.

The structure of the springs and their location during the FIB process will be described in figures 1,2 and 3.

Figure 1 shows the 2 FIB milled trenches as seen from directly over the sample. These 2 deep trenches are indicated as item 1. There is normally a protective metal top coat put down by the FIB to protect the top surface of the sample and this is in a line between the two deep trenches and some of this is removed by the trench digging process so that a square tab remains on each side of the section, and a thin line of this metal remains on the top edge of the section, identified as item 2 and item 5. Item 4 is the location of the cut that is normally placed to partially release the section should it deform during polishing.

Figure 2 shows the same sample location but viewed at 45 degrees to the flat surface of the sample so that the view into the trench can show the vertical section hanging from the top surface. The left, right and bottom edges of the section has been cut away as shown as item 3. Only 2 small bridges (or tabs) at the top of the left and right hand sides still connect the vertical section to the bulk sample there, as identified as items 3 and 4 on figure 1.

Should section deformation occur during thinning the conventional process would demand that the top right tab (figure 2 item 5) be cut away removing all connection between the section and the bulk material on this side and allowing the section to be held with only the remaining tab on the far left (Figure 2 item 4). This removal of a second anchorage point has the effect that as the section becomes thinner a new failure mode is introduced which is that the section starts to 'flap' with the last remaining tab in the top left hand corner as the 'hinge' and this prevents optimal final polishing of the section.

The invention changes this process so that instead of cutting away the right hand side of the section and leaving only a tab of rigid material left, that instead at the right side of the section (Figure 3, item 5) the section has been trimmed into a specific pattern leaving two angled struts connecting the right side of the section to the bulk sample still.

These struts can vary in size but are each of the order of 1 00nm's in width. These struts have been thinned to the same thickness as the section but in addition have been damaged by the FIB's ion beam during the cutting process, so that they are 'mostly' amorphous and so have low stiffness but still some mechanical strength. It is these 'soft struts' that act as a spring to hold the section both flat, and in its optimal polishing location while also permitting the section to expand or move sufficiently to release any deformations which may have occurred during polishing.

These 'soft struts' are created by using the FIB to mill out 3 specific polygon shapes from the right hand side of the tilted section before the final polishing is completed.

This invention allows the FIB process to be applied to a much wider variety of materials systems which may be more prone to deformation during FIB section preparation, and ensures that better TEM sections may be more routinely produced from all materials.

This new 'soft strut' spring process step is equally applicable to the process known as ex-situ' section (or foil, or lamellae) liftout (or extraction) as it is to in-situ section liftout when the section is attached between 2 parts of the section carrying grid. This is because it can be equally applied to both, and is not dependant on the extraction method, only on how the section is anchored during the final polishing process. This means that this invention works equally well on both these process embodiments and is applicable to both.

It works equally well on a single beam FIB system as on a FIB-SEM or DualBeam system and so is equally applicable to both these embodiments.