The Difficult Birth of the Scanning Electron Microscope

The Difficult Birth of the Scanning Electron Microscope

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It was a device that virtually nobody believed in. The scanning electron microscope  or SEM is famous for its amazing,   three-dimensional photographs. It is hard  to imagine a world without such a machine. Not only for eliciting the curiosities  of children and nerds like me,   but also for diagnosing semiconductor issues.

Despite its usefulness, we wouldn't have the  commercial SEM today as we know it without   the quiet, persistent efforts of a single man. In this video, we cover the long and difficult  birth of the scanning electron microscope. ## Beginnings Charles Oatley was born in 1904 to the  owner of a bakery and schoolteacher.

His father did not have much in terms  of an education, but passed on a love   for science and technology. One of  Charles' early toys was a microscope. Oatley grew up to be a formal and  soft-spoken man - a consummate English   gentleman. He graduated from Cambridge  and worked in industry, building radios. This got him into radar  development during World War II. After the War, Oatley was appointed a  lecturer at the Cambridge University   Engineering Department. His goal  there was to help upscale the   university's research effort in the new  and bustling world of applied physics.

He tried to find projects for his  students to do - speculative projects   that trained them and exercised their  creativity but can still be useful. In his view, such university projects can  be different from those in industry. PhD   students cost less. The stakes of  failure are far lower, because the   student is judged on the excellence of  their work rather than the end result.

So you can research more wild and wooly  things, knowing that the end goal isn't   necessarily a commercial product. This search  eventually took him to the electron microscope. ## The Electron Microscope In 1926, a German physicist named Hans Walter  Hugo Busch showed that you can use magnetic   and electric fields to guide the electrons  of an electron beam in a certain direction. These principles enabled the  possibility of a "lens" that can   act on beams of electrons like how  optical lens act on visible light. Electron wavelengths are almost five  orders of magnitude smaller than that   of optical light. In theory, an  electron-based microscope could   produce resolutions far superior  to that of optical microscopes.

Inspired by Busch's discovery, a team of two  men in Berlin set out to build such a machine. In 1931, Max Knoll and Ernst  Ruska - built the first   conventional transmission electron microscope. The word “transmission” is in the name because  the machine sends a beam of electrons through an   ultra-thin slice of the specimen. After that,  we focus the beam and turn it into an image. Unfortunately at the time, it was difficult  to slice the specimens thin enough in order   to get a good image. This lasted until 1940  when Hans Mahl introduced "replicas" - a   method of using plastic and metals to  prepare a specimen for TEM imaging. ## Another Microscope Knoll and Ruska knew that a second  type of electron microscope was   possible - one that would work more  like a traditional optical microscope.

After his work on the  Transmission Electron Microscope,   Knoll joined the Telefunken company  to build TV camera tubes. In 1935,   he built an electron device based on this second  idea in order to study a component of his tubes. He put the sample into one end of a glass tube,  with an electron gun on the other side. The   electron beam is scanned across the sample.  Electrons reflected back or emitted from the  

sample are collected, amplified, and then used to  create a magnified image of the sample surface. Knoll called his device "der Elektronen Abtaster",   which means "electron scanner" in German.  Sounds cooler in German, if you ask me. Knoll's machine could magnify about 10x  but not much more due to the width of its   electron beam probe. Interestingly, he did not  use electron optics to try shrinking that width. He certainly must have considered it. Maybe  he passed because the current setup already  

met his needs and sufficiently sensitive  electron detectors were not available. Ruska later won the Nobel Prize for Physics in  1986 for his work in electron optics. I reckon   Knoll would have won part of that Nobel  as well had he not passed away in 1969.

## Von Ardenne Meet the German scientist  Manfred baron von Ardenne. He was born into a wealthy family, which let  him run his own lab. Brilliant and self-taught,   he received his first patent at the age of 15. In 1931, he demonstrated the first fully  electronic television system. It used   cathode-ray tubes to produce  images. Which is pretty cool.

In 1936 Siemens contracted Ardenne to design  and build a new electron microscope to avoid   an issue in existing TEMs. In an electron  stream, not all electrons are made equal.   Some have more energies than others,  causing them to react differently   as they accelerate and pass through the  lens - creating aberrations in the image. So Ardenne built a microscope that  scanned an electron beam across the   sample line by line. Today we call  this a Scanning Transmission Electron   Microscope. "Scanning" because of how  the machine scans across line by line.   "Transmission" because the electron  beam still goes through the sample. Ardenne only spent two years on  this. His machine was sophisticated,  

but also limited. The beam was not  powerful enough and Ardenne did   not have sufficiently sensitive electron  sensors - resorting to photographic film. So it took 20 minutes to record an image. And  since you can't see if the image needed focusing   until after developing the film, you had to sort  of wing that part. No images were ever published. A brilliant and creative mind, Ardenne  probably would have been able to build   something had there been time. He had all  the essential principles down- including  

a correct theoretical model of the  sample’s interaction with the beam. When an electron beam hits the sample, two  things happen. First, primary electrons from   the beam are back-scattered - as in they bounce  back off the surface like ping pong balls. And then there is the secondary  reaction. This is where atoms in   the sample get energized by the primary beam  and start emitting electrons of their own. Ardenne knew this and so he was on the  right path. But then World War II broke out,  

and Ardenne abandoned the project to work on a  cyclotron for the German atomic bomb project. His electron microscope was destroyed during an  air raid in Berlin in 1944. Later, Oatley calls   him the "true father of the scanning microscope".  The ideas were right. But the times failed him. After the war, Ardenne worked for the Soviet  atomic bomb program. Then he settled in East   Germany and founded a research institute.  He passed away in 1997 at the age of 90  

with 600 patents to his name. A life  of discovery and invention, well lived. ## RCA There is one last scanning electron  microscope project that we should mention. The American radio company  RCA did work on a scanning   electron microscope of their own for a few years. The project was initiated and  led by the famed inventor of   the television camera tube - Vladimir Zworykin. The project started as something to  build a new type of microscope. But  

then in 1940 the aforementioned Hans  Mahl introduced his replica method.   Now suddenly the Transmission Electron  Microscope became a formidable competitor,   and the RCA team had to adjust  their system to accommodate. They focused on building a machine  that can work on samples that we   can't cut into thin pieces. In  it, a tube generates an electron   beam that hits the sample. Electrons then  reflect off or are emitted by the sample.

Those electrons are accelerated back up  through another set of electron lenses and an   electron multiplier tube to intensify the image  before hitting a detector screen for imaging. This SEM did not image smaller features  than existing TEMs. At the same time,   the downsides were substantial. It was  expensive. It took 10 minutes to make   an image. And noise often clouded the  images. RCA rightly decided to move on   and focus on their transmission  electron microscope products. There was little more work done  on scanning electron microscopes   for several years, when Charles Oatley came along.

## A Hunch Oatley first came onto the scanning microscope idea through a pre-existing  interest in electron optics. Soon after he started, he asked one of his  PhD students - K. F. Sander - to try building   a TEM for their project. Sander started on  it before abandoning it for something else. Sander's work was not revived because soon after  that, Oatley's colleague Vernon Cosslett began   some work in TEMs and the gentlemanly Oatley  did not want to step on his colleague's toes.

But Oatley did feel that a scanning  electron microscope might be a good   alternative. The previous body of  work done at RCA had shown that its   scientific principles were fundamentally sound. There also seemed a pathway towards solving the  RCA SEM's long recording time and noise issues.   The two problems were related. The longer it took  to record the image, the more noise there was. Oatley reasoned that if they can somehow collect  more of the electrons coming back from the sample,   then you can both cut the recording time and  improve the noise. You might even be able  

to put the imaged result on a cathode  ray tube screen for more convenience. And how can you collect more electrons? Well,  if you recall with the original RCA SEM,   electrons from the sample are run through  lens and accelerators before hitting the   multiplier and then the detector. We lose  some of them by doing this. So what if we   just send them right into the multiplier instead? There were also possible improvements  using technologies developed during the   war. Most notably, an electron multiplier  that Oatley felt might be better than   RCA's. It was built by his colleague  at Cavendish Laboratory A.S. Baxter. This was all hunch. And no clear-thinking  manager at a company would authorize a R&D  

project on such thinking. But like as I said, a  PhD project was different. The stakes were lower. So Oatley assigned Dennis McMullan, a student  uniquely suited for the task - with analog,   tube and radar experience from during  the war. The project received a small   grant and had a bunch of valves, tubes,  and other components at their disposal. People had low expectations. If RCA and its  brilliant minds failed to get this work,   how can a random Cambridge student with a  thousand pounds and some tubes do better? ## Cambridge Over the time he spent on this  project from 1948 to 1953,   McMullan produced a working  proof of concept machine.

He took over the uncompleted TEM left behind by  Sander and added a power supply, electron lens,   and cathode ray display unit he built  with his own hands. This machine had   a more powerful electron probe, as well as a tilt. In an attempt to get better  contrast on the SEM's images,   McMullan tried tilting the sample at a  far higher angle relative to the beam   than before tried - 25 to 30 degrees  as compared to the previous 2 degrees. What you got were the beautiful three-dimensional  images that scanning electron microscopes are so   famous for. The tilt also helped get more  electrons into the electron multiplier,   enough to put some images on a cathode  ray-tube display for a few seconds.

The results were promising enough  that Oatley continued the project   after McMullan left the lab in 1953  - handing it over to Ken Smith. Smith contributed several improvements to  the machine. The most important of which   was to collect the low-energy electrons emitted  from the sample as part of secondary reactions. As I mentioned, Ardenne had pointed this  out before. But when McMullan was building  

his machine, he did not collect those  secondary electrons because he thought   they would hurt the image quality. As it turns  out, collecting those secondary electrons - as   well as emitted light and X-rays from the sample  - boosted image quality by a factor of fifty. ## Commercialization Oatley's subsequent graduate students  like OC Wells, Garry Stewart,   and Thomas Everhart locked down the design and  improved components for collecting backscattered   and secondary electrons. By 1960, Oatley and  his team had largely completed the machine. Oatley oversaw their efforts, checking  in frequently with questions, discussion,   and suggestions to move the  research forward. Together,   they developed techniques for using the  device. Some of the science made with the   device was published in scientific  journals, though not all of them.

Some academics from Canada's Pulp and  Paper Research Institute saw the device   in action and asked for their own.  It wasn't being sold commercially,   so the Cambridge team custom built  one and shipped it to Canada. Since 1955, Oatley had tried to  convince a company to produce   the device for customers. But at the start,  people questioned its commercial viability. The Scanning Electron Microscope offered far  less resolution than its sibling the Transmission   Electron Microscope - hundreds of angstroms  rather than the latter's 10 or 20 angstroms.

On the other hand, the SEM did  not require the long and tedious   preparation of replicas. This not  only made it more convenient to use,   but also suited items like fibers - often  susceptible to burning whilst being prepared. Furthermore, SEM images have that  fantastic and distinctive depth of   field. Can't dispute that these  SEM images just look amazing. Nevertheless, experts initially assessed the  world market for the SEM device to be just   15 units. This disinterest lasted until  Oatley approached the managing director   of the Cambridge Instrument Company with his idea. ## Cambridge Instruments The Cambridge Instrument Company  itself has a fascinating history.

The company was founded in 1881 as a  partnership between a wealthy student   Albert George Dew-Smith and Horace Darwin -  the youngest surviving son of Charles Darwin. Yes, that Charles Darwin. The guy forever remembered for collecting  the first fossils of a 9 foot long capybara. And some other stuff about evolution or whatever. Anyway, Horace Darwin was a competent  mechanical engineer, and the company he   co-founded produced equipment for  physiological labs. For example,   a breathing mechanism that pumped chloroform  into an animal while it was being operated on.

Later Darwin took sole control over  the instruments part of the business,   and continued producing instruments like  thermometers, galvanometers, and thermographs   over the years. It became publicly traded,  and continued on doing what it was doing. By the late 1950s, the company had gotten to be  in need of a little fresh blood. So they hired   Harold Pritchard, an Oxford mathematician, to  reboot R&D and get some new products out there. ## The Stereoscan When meeting Oatley, Pritchard had actually  been interested in a different product. That was the Microscan, which today we call the  electron microprobe. It fires an electron beam  

at a sample in such a way that causes them  to emit X-rays, which the instrument reads. The Microscan sold well - it did something  no other instrument can do. And that finally   convinced Cambridge to take a chance  on Oatley's scanning microscope idea. In 1962 Du Pont's Canada team saw  a prototype of what was called the   Stereoscan SEM at the Institute  of Physics and Physical Society   Exhibition. And having used the SEM  previously built for the Canadian Pulp   and Paper Institute - they put in a solid  order for the first production machine.

The SEM had many parts in common with  the electron microprobe so in theory it   should have been relatively easy. But an  issue came up that when someone tried to   observe at a resolution of about 50 nanometers,  extremely small vibrations ruined the image. This first machine was delayed for literally  years in order to deal with this problem,   which annoyed Du Pont. Finally, Cambridge  shipped its development machine over to  

them - dropping it down from a forklift  onto a concrete sidewalk in New York. The UK government then chipped in.  They offered to fund the purchase of   Cambridge's first few machines by  British universities. So in 1965,   Cambridge managed to build five  commercial machines - which sold well.

That was the turning point. A publicity campaign  hauled in orders for twenty more machines and   that was that. By 1971, 520 Stereoscans had  been shipped to customers. We have a hit. ## Semiconductors The scanning electron microscope has influenced virtually every part of the  science and technology worlds. But the semiconductor industry  has especially benefitted from   its bounty of gifts. The fact that you  did not have to prepare replicas for   samples made it far more convenient than  the transmission electron microscope. Oatley realized this early on, and assigned  one of his students Garry Stewart to add an   ion beam to the SEM for additional measurements.  Though filters had to be added to remove oxygen  

ions from the beam - they were causing oxides  to form on the surfaces of the etched chips. This convenience as well as the microscope's high  contrast images made it easy for technicians to   look at malfunctioning chips to see if  they might have a broken connection,   an invasive particle on it or some other flaw. As integrated circuits have  gotten denser over the years,   this has pushed the SEM industry  to keep up. This includes the use   of digitization and software to improve  imagery and shrinking the resolution.

## Carl Zeiss I want to thank an anonymous friend of the channel  who works at Carl Zeiss for suggesting this topic. Unfortunately, Cambridge's historic role in  the development of the SEM did not save it,   financially. Persistently high  R&D and manufacturing costs as   well as a few bad acquisitions  later cost Pritchard his job.

As it turned out, SEM-like devices had  been invented intermittently in Japan,   France, and the Soviet Union. The British  firm Metropolitan-Vickers/Associated   Electrical Industries even sold one SEM-ish  unit back in 1959 but it failed to work well. But when Cambridge demonstrated the  instrument's commercial viability,   others rushed in. Fierce competition ensued  particularly from the Japanese - the first  

Japanese SEM came just six months  after the Stereoscan's debut. In 1968, Cambridge faced a hostile takeover  from another firm - forcing them to sell   themselves to George Kent Limited,  another British instruments maker. A few years later in 1974, George Kent was itself  acquired by the Swiss electronics company Brown   Boveri. In doing so, Cambridge was spun  off once more as an independent company. Some time after that, Cambridge becomes part  of the famed German company - Carl Zeiss. ## Conclusion Charles Oatley retired from Cambridge in 1971,  but continued to spend his retired life on   the development of his microscope.  Three years later, he was knighted.

In 1982, he published an iconic, highly  cited paper on his work - humbly titled   as always "The early history of the  scanning electron microscope". It was   a critical source for this  video, and I recommend it. Oatley spent the rest of his life tending to  his garden, passing away at the age of 92. His   students remember him as a quiet, humble  man who paved the way for his students. And many of his students indeed went  on to great things. For instance,  

Ian Ross was a transistor pioneer, and  went on to be the president of Bell Labs. Thomas Everhart became the fifth president  of the California Institute of Technology. And Alec Broers is an e-beam pioneer, and became   Cambridge's vice-chancellor  - which delighted Oatley. Carl Zeiss Microscopy continues to honor the  legacy of Oatley and his team at Cambridge   Engineering, with various pages explaining his  story. A quiet man, who made a deep impact on  

his students and through persistence opened up  an entire new world for all of us. What a legacy.

2024-05-11 17:06

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