Scanning Electron Microscope

I have always wanted to use a Scanning Electron Microscope (SEM) and my chance came last week at Cambridge University.  What a thrill.  I received an optical microscope for Christmas one year as a child and had endless fun and excitement with it, but the SEM is a whole new ball game and has a resolution down to 1.4 nm.   That’s a thousandth of a micron or a millionth of a millimeter. Hard to imagine.  To add to my excitement, it also had a focused ion beam (FIB) microscope adjacent to it that we used.  I didn’t even know what they were until a few days before we used it.  Its similar to the SEM microscope but fires ion instead of electrons which are much bigger and heavier so can inflict more damage.  When the current is turned up, you can machine or mill the sample in a very controlled and accurate way.  We also injected gas molecules (organic platinum) between the sample and the FIB.  If the current is just right, it won’t machine the surface but rather split the platinum from the organic part of the molecules and deposit them on the surface of the sample.  Because you have the SEM pointing at the same place you can look at what you have done.  I want one!

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SEM with FIB at 54 degrees

You don’t think of a microscope firing a current, but that’s exactly what a current is, a flow of electrons.  To get an idea of how many electrons are being fired at the sample; A Coulomb = 1 Amp per second.  The charge of a single electron is around 1.6 x 10-19 coulombs so there are 1 / 1.6 x 10-19 = 6.25 x 1018 electrons per second for 1 Amp.  We have a typical probe current of 200 pA so there will be 6.25 x 1018 x 200 x 10-12 = 1250 million electrons per second or 1250 electrons every μs bombarding the sample.  Just shows how unimaginably small they are and how many of everything must move before we can even notice it.  Of course, the FIB also fires a current, just not the negative electrons rather the positive part of the atoms that are left once some electrons have been stripped.  They usually use Gallium ions because it melts below 30°C.  Its solid at room temperature but melts in your hand, unlike treats (now called M&M’s).  A proton weighs 1.6727 x 10-24, a neutron about the same at 1.6750 x 10-24 and an electron 9.110 x 10-28 so a proton is 1837 times heavier than an electron.  Gallium normally has 31 protons and normally 39 neutrons in its nucleus.  It therefore normally has 31 electrons in orbitals around the nucleus but if the electrons are stripped from the outermost shell (valance) it will become a little smaller and be positively charged.  Because only one electron is in the outermost shell of gallium it will have exactly the same charge as an electron but be positive rather than negative (1.6 x 10-19 coulombs).  So, each ion in the FIB will carry the same charge as each electron in the SEM but weigh about 1837 x 70 = 128,590 times as much.  So, you can imagine how much more momentum and damage a beam of ions can do to the sample surface compared to electrons.  That’s the same difference as an average man (70 kg) compared to a freight train with an engine and 63 loaded rail cars.

We started by measuring the features on a DMD (Digital Micromirror Device) using the SEM.

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Image of DMD mirrors

The mirrors are about 13 μm square with a Ø1.5 μm hole and the gaps between the mirrors are about 1.1 μm.  So surprisingly we could get an array of 100 x 100 or 10,000 mirrors in a square of 1.3 mm on the side.

Well a DMD chip has several hundred thousand microscopic mirrors arranged in a rectangular array on its surface; each of which correspond to a pixel in the image displayed.  Each mirror can be individually rotated by around ±10° which represents an on or off state.  When on, light from the projector is reflected off the mirror, through a lens onto the screen as a bright pixel.  When off, the light is directed to a heatsink, so the same pixel appears dark.  To produce grey, the mirror is toggled on and off very quickly by pulse width modulation, so the shade of grey corresponds to the ratio of time on to time off.  To produce colour, three coloured projectors and three mirrors are required for each pixel and the ratio of on to off for each colour determines the pixel colour brightness.  The mirrors are made from aluminium and are mounted on a yoke which is connected to support posts by torsion hinges.  Because of the small scale, hinge fatigue does not usually cause a problem.

We did a few more experiments, carrying the acceleration voltage and the probe current etc, and then changed the sample for an EPROM and set up the FIB.  If you set the distance an M stage tilt correctly, its possible to be able to view the same part of the sample with the SEM and the FIB without changing the focus or moving the sample.  This is called the Eucentric point.  Below is the same image using both devices.Compare

TrapYou can see that the SEM is viewing the sample at an angle but there is a tilt correction mode if you like.  The resolution and contrast of the SEM is so much better than with the FIB.  Next we drew a trapezium on the sample that we were to mill out by setting the accelerating voltage and current just right we got a really nice cut of 3 microns deep.D200pSEM

We found the optimum current to be around 200 pA.  Its amazing how accurate and easy it is to machine the sample; this image is taken at 4000 times magnification.  The maximum optical lens you can use is about 1000 times.  This is just run of the mill, its capable of far greater magnification if you like.  When looking at biological samples, they have to be stained with a rigorous process prior to viewing so that the different tissue types stand out and can be seen.  When looking at anything else, you just put it in and focus; and the images are so much better than the ones I could get on my optical microscope as well.

GISOur next step was to inject a gas between the FIB and sample when it was scanning.  The gas we used was made from platinum organic molecules.  If everything is set correctly it will split the gas and deposit the platinum part on the sample while the organic part is vented away.  This time we drew a small rectangle on the sample as it takes a while.

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Platinum deposit on EEPROM

We then decided to do the same cut, half on the deposit and half off.108

We noticed some funny furry deposit at the front of the cut so zoomed in for a closer look.Fur

This view was taken at 8200 time magnification and we could have gone in much further.  We then moved onto our last part where we analysed the surface at different places to do a spectral analysis and find out which elements were present.

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Spectrum of area around the cut out and deposit
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Summary of elements at various places

The redeposit column was taken on the furry deposit which was found to be made up of platinum, silicon and carbon.  Basically, everything that was blasted away when the cut was made.  Its interesting that there is so much carbon around; I think the only place that could come from is the organic part of the gas which should have been vented away.  Not quite sure how it gets vented away anyway as everything happens in a vacuum and to vent away you need a lower pressure than where it is.  Well, you don’t get much lower pressure than in a vacuum.

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Image inside vacuum chamber

Sorry its been so long since my last post but I have been so busy I cant tell you.  We have lectures all next week at Cranfield then the following week at Cambridge and then no more!  So once I have written up the reports for those I will only be doing my thesis so should have more time to catch up with everything else that has happened.  I will get there in the end.

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Cambridge Week 2

Again, we set of about 7am and again we hit traffic miles from Cambridge.  It was even worse this week so we didn’t get there until 9am, just time for a cup of tea and for everyone to copy my data stick of data and pictures from last week.

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Tea in the canteen before we start
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Katjana Lange

We started the day by trying to catch up with what we missed last week.  We eventually got through it all but it was already lunchtime.  Now only the afternoon to complete the entire days’ work.  Lilly helped us again with our catch-up work in the morning and Daniel for a while in the afternoon.

Daniel brought Katjana in to talk to us as her PhD is largely

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Actual laser cut

about laser ablation which we are working on.  Very enlightening – Kat’s short talk enabled us to better understand what we were supposed to be doing and to get on efficiently.  Today’s work was about measuring the laser cuts we had done last week so we could establish various properties about the laser power and focus etc.

 

 

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Catch up with Lilly

It was a very long day, especially as I had awoken at 4am and got up at 4.30.  We left IfM at Cambridge University around 6.45 and I arrived home at 20.10.  I had had to fill the car with petrol on the way home as tomorrow I need to take the hire car back and drive to Bedford to pick up my new car before driving to Cranfield University to continue lectures there.

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My new car

Cambridge University

Probably the best Science University in the World?

We decided to set off early as I know that traffic into Cambridge can be challenging.  I picked up Mohamed from Newport Pagnell first at 7am and went onto picking up Rita from her shared house in Cranfield.  Rita’s real name is, “Junguo Zhao”, but has chosen an English name because none of us can pronounce it.  Ren (Ren Guicun) decided to travel to Cambridge on Wednesday evening, to give himself a little tourist time.  Although the traffic was very heavy, we still arrived early at around 8.45am and parked on double yellow lines at IfM (Institute for Manufacturing) as nothing else was available.  We wrote a note which was left in the window giving my mobile number etc. and had a quick walk around the campus to take some photo’s.

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There are some very modern and colourful buildings along the road fronts with the old buildings behind.

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The Cavendish Laboratories are just down the road; this is Rita and Mohammad walking towards them.  When we got to the end of the building we bumped into Ran so our quorum was complete.

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Rita and Mohammad walking towards Cavendish Labs.

There were some notable street names, like J.J. Thompson Avenue and Charles Babbage Road etc.  Of Course, The Cavendish Laboratories is where Crick and Watson worked when they discovered the form and shape of DNA (National Association for Dyslexics).  There were too many famous scientists to mention but a couple, James Clerk Maxwell and Ernest Rutherford.  There were also 29 Nobel Laureates from these laboratories.

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Dan Gortat

We still arrived early and in time for a cup of tea and we were off for our first proper lectures.  I was also allocated a parking space so moved my car.  Today we have two laboratories about measurement (metrology) and we started by being introduced to two microscopes by Daniel, a Cambridge PhD student.  I met Daniel on our day out as I sat on the same table for dinner.  Daniel is at the end of his PhD and seems to be getting some stick from his peers as he is yet to start writing his thesis.

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Ran at one of the microscopes

I used to own a microscope when I was young as I pleaded with my parents to buy me one for a combined Christmas and Birthday one year.  They have changed so much since those days mainly because the lighting is so much better and we now have CCTV cameras so the image can be shown on a screen and manipulated by computers.  We didn’t even have calculators when I had mine.  The sample can be illuminated from the front with lights shining around the viewing lens or from behind like I had.  There is also an option to have the light at an angle to show cast shadows.

DSCF3081The post processing option opens a whole new world, for example, you can view samples in 3D by focusing on the top of the sample, then refocusing progressively lower saving each image.  The processor knows the focal heights and can stitch the images together enabling you to view the sample from any angle.

We over ran the morning session and therefore our canteen was closed so we had to walk to the West Café, just down the road towards the Cavendish.

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Lilly Delimarta

We were under pressure from the start in the afternoon when we were introduced to interferometry by Lilly a Cambridge PhD student.  I met Lilly on our day out at Cambridge because I sat next to Kryste, her thesis supervisor.

An interferometer microscope is for examining and measuring surfaces.  It works by splitting the focal beam, sending one beam directly to the viewing lens and the other via an adjustable mirror.  The mirror can be adjusted until interference bands can be viewed caused by diffraction.  As all setting are known, measurements can be calculated and interesting images and histograms created.

We were only about half way through our samples when we encountered a problem where the interferometer microscope seemed to view and measure the interference pattern rather than the sample.  Lilly couldn’t resolve the problem and brought in an expert, Andy Payne who I also met on our day.  Anyway, try as they might our problem remained unsolved when it was late and we all had to go home.

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Andy Payne

Andrew received his MPhys in Physics from the University of Kent in 2012 where his master’s year research was in Fourier domain optical coherence tomography. Subsequently Andrew joined the Centre for Doctoral Training in Photonic Systems Development and received his MRes from the University of Cambridge after conducting research into the coherence properties of liquid crystal lasers at Cambridge and the registration of 3D laser scanned point clouds at UCL.

We were booked into a Premier Inn only a mile or two away so we headed off and checked in.

DSCF3097To save money, Mohammad and myself were sharing in a “quad” room”, so we unpacked and tidied ourselves up and headed off to the restaurant attached.  Rita decided she wasn’t ready to eat and fancied a walk into the centre to have a look around.  By the time we had eaten it was 20.30 and we had no response from Rita when we knocked so went to our room to catch up on emails etc.  I emailed my computer written notes to the others.  I also took my new camera and will share the pictures once I remember a USB stick.

DSCF3098Our first night in Cambridge happened to be Harvest Moon so I took a picture of the

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Harvest Moon

Moon but it was drowned out by the street lamp.  Harvest Moon is the full moon that takes place closest to the Autumn equinox.  It used to be used to indicate when the crops should be gathered in to prepare for the winter months.  The full moon meant that work could go on later because of the extra light before we had street lights.

 

We all met for breakfast around 7.30 and could have as much as we liked from any menu.

DSCF3101We did our best and headed off to the University.  Today we were introduced to lasers by Dr Martin Sparks.

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Dr Martin Sparks

Martin is part of the Cambridge core team and is a senior research assistant.

Again, we all struggled, Martin talked and went through procedures at a tremendous pace and we all struggled to keep up while trying to take pictures and notes in unfamiliar topics.  As something had happened to the alignment of the equipment, we had to readjust the mirrors and equipment to get back in alignment again.  I am pleased that we had to go through this process because the alignment experience was a real help to understand the process.  Starting from the laser source we had to work our way through the system following the beam route adjusting as we went.  This meant that we had to wear special goggles, have the covers open and laser on while making these adjustments.

DSCF3112The main purpose of this Lab was to lead us into understanding the importance of documenting everything, thinking about things before acting and, “leaving no stone unturned”.  In research it is so important to document everything so that your work can be criticised and repeated to verify if necessary.  If anything is left out it leaves doubts over the validity of your work.  A good example of this was the research done into room temperature cold fusion.

DSCF3113This day was extremely stressful to us all as Martin left us to work so much out for ourselves.  At first, problems seemed insurmountable but gradually, maybe with hints, we got there in the end.  The harder things are to solve, the more fulfilling when they eventually are.

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Laser Goggles

In the afternoon, we had a sample that we could blast with the laser, doing experiments, varying power and other things so that these could all be examined using the microscope and interferometer next week.  We used G programming language to control the laser cutting process.  Two programs were given to us that we had to run, changing various variable, power etc. until the images of the cuts appeared optimum.  Again, we had two cameras focused on the workpiece that we initially set up.  One optical and one CCTV and both could be viewed on our computer screen.  The second program took moved the laser in a snake shape up the workpiece incrementally cutting deeper in variable steps.

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Snake

Once we had attenuated the power sufficiently that the image appeared optimal, we had to modify the program to cut in all four directions to enable us to compare the results of directional change next week when we measure these cuts.  My previous MSc experience came in handy here and I was quickly able to write a new section of code that would achieve this.  Again, we had to change variables and power etc. to optimise the laser cuts making it easier for us to measure next week.  The dots are where the cut height is changed and the red is the extra program cuts that we added starting at the deepest cut, incrementally raising so that adjacent cuts are the same height for comparison.

This time we finished ahead of time, but instead of heading home we decided to talk Lilly into helping us go through our missed interferometer lab.  She kindly agreed but we only had time to get through the third sample before she had to head off home.  We can go through our final sample before we start our lab next week.

I think I can speak for the others when I say that we all found this experience very stressful but once we were finished, quite a relief but fulfilling.  I feel so much better about this experience this morning following a great sleep.  For the past week, my sleep patterns have not been so good because of my excitement and anticipation of this course.  I now feel grounded and ready to go.  Before I can start though, I really do need to find and buy a car this weekend.  Also, it’s the Japanese Grand Prix.  Lewis just broke the, “total number of poles” record again which means that he has now won pole position at all the GP’s on this year’s calendar.  See you next Tuesday Cambridge!