4.3 Memories of the Scanning Electron Microscope at the Cambridge Instrument Company

ADVANCES IN IMAGING AND ELECTRON PHYSICS, VOL. 133

4.3 Memories of the Scanning Electron Microscope at the Cambridge Instrument Company D. J. UNWIN Formerly at: Cambridge Instrument Co. Ltd.

I. Beginnings Ever since an early age the name ‘Scientific’ has been known to me. When my mother was referring to our next door neighbour, Will, she would point out that he had a good job as he worked at ‘The Scientific’, the name by which older local people knew the Cambridge Instrument Company. From the age of eight, on my way to Milton Road School I used to pass that short side-road, once called Camford Road, oV Chesterton Road, and see the big double gates with ‘Cambridge Instrument Company’ in large letters across the top. Of course it did not mean much to me except that it was where ‘Will’ worked. When you walked up Carlyle Road, an intriguing hum could be heard emanating from the double-storey building on the right, while at the gates there were always horse-drawn railway lorries being loaded with wooden boxes. The environment in which I grew up was one of making things, as my father, who was an engineer of the old school, would tackle anything from building a house, setting up a 16-ton machine, to mending a watch or clock. In fact, he was the ‘King of Do It Yourself’ (DIY) before the term was invented. I remember seeing the tiny picture of a woman on the Baird television set he made in 1928! He had a well-equipped home workshop with a treadle lathe in addition to a comprehensive kit of woodworking, building and engineering tools. From an early age every encouragement was given to me, including being taught how to use all this equipment, supplemented by an ever-increasing amount of that greatest of engineering toys, Meccano, which I still have and use for rig-ups, trials of ideas and many other tasks. Reading the several practical journals he took, and the Meccano Magazine for me, backed this up. However, being a self-taught man, father did not consider it necessary for me to go to university. To make up for this I have continued private study throughout my life. 339 Copyright 2004, Elsevier Inc. All rights reserved. ISSN 1076-5670/04

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The Central School followed Milton Road School, and then the Technical School in Collier Road for the engineering course, something I thoroughly enjoyed. The nearest I ever got to the Cambridge Instrument Company (CIC) was when Ikey Buckerfield, engineering instructor at the Tech, sent me to collect some silver solder from Joe Wilderspin who worked there. It was to enable me to hard-solder the boiler of the model locomotive I was building. I got no farther than the entrance room next to the telephone switchboard, but I could hear the tantalizing hum of machines.

A. Starting Work My great friend from early school days, Denis Gravestock, and I both went to the Tech, but when we left he joined the CIC drawing oYce and I went as an apprentice to Unicam Instruments. When I went for my interview, I took the 2 12-inch gauge steam locomotive I had just built, which suitably impressed them. Incidentally I had been awarded Highly Commended for it at the London Model Engineering Exhibition—not bad for a 16-year-old. Unicam was a small outfit, with only about 20 employees, run by the boss Sidney W. J. Stubbens, an ex-CIC Shop H foreman, and his two brothers, Ernest and John, also ex-CIC. Other ex-CIC men were Bill Varley and Ernie Marsh, who always started the answer to a question by ‘When I was at the Company we used to . . .’. He did this for many years until he cottoned on that we were making fun of it; then he attempted to vary it a bit. The directors of Unicam were a Mr Pretty, a director of Robert Sayles, Winton Smith the pork-pie maker, Mr Slater a solicitor, and W. G. Collins, ex-works manager at CIC, who had obviously helped Sid considerably when he was starting up. When I first joined them, I noticed that many of the tools were stamped CIC, so I wrongly assumed that it must be associated in some way! My starting rate was a half-penny per hour more than the normal starting rate of one penny per hour because I had been to the Tech—4 shillings and 8 pence less deductions that included one penny for the hospital! The workshop was in Barrett’s yard next door to the Bun Shop in St Tibbs Row, a rickety building, equipped with mostly old machines, all driven by line shafting powered by an old electric motor which, when starting up, had to be helped by pulling on the belt. I had been there only about a year, during which time I learnt a lot, including gas welding, when Sid Stubbens asked me to go to his new works site in Arbury Road to help fit it out. When I got there it was just a roof, walls still being built and a dirt floor! However, standing on two paving slabs was a Seneca Falls 6-inch centre lathe, secondhand of course but in good order. My first job was to fit up the countershaft and a motor on to the girder work of the building structure. I was completely

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on my own, with only the builders for company, although Sid used to come about once a day to see how I was getting on. After the lathe was operational, I had to make all the fittings for the line shafting and then erect it; fortunately, he sent a young lad to help with the lifting. By this time the walls, windows and floor were completed and Sid and the secretary, his niece, had taken up residence in an already existing timber building on the site. John Mellenby, the electrician who was wiring up the site, also had his store and workshop there. Eventually, the place was ready for occupation and I got involved in moving and installing the plant. I received considerable advice in this from my father, who was the foreman erecting engineer for Chivers at Histon and who considered 15 tons just a nice weight to handle! From this time on I never had production jobs, always specials, usually for Sid’s many university clients. The Seneca Falls lathe became ‘my lathe’, a machine I liked very much indeed. The business must have been doing quite well, as it was not long before an extension was announced. However, soon after the two steel erectors had started work, one of them put his hand through a window and then there was only one! He asked Sid if he could borrow a couple of men to help; guess who was one of them detailed to help. Yes, I was one of them, climbing up stanchions, drilling 3/4-inch holes in girders by hand using a ratchet brace, oxy-acetylene cutting and welding, all in freezing cold weather. I thoroughly enjoyed every minute of it and it was a wonderful and valuable experience. After a spell installing equipment in the extension it was back to instrument making, involving experimental work this time. As most of this was directly for Sid Stubbens, my bench was just outside his oYce and he gave me a young apprentice to help although I was also still an apprentice myself! He also gave me, in addition to the Seneca Falls lathe, a drilling machine and a new 5-inch Atlas lathe. After a period working on various special projects, he obtained several defence contracts as a result of the period of rearmament and it was this work that increasingly occupied my time. Not all was experimental work: much involved special tooling, fixtures and even simple special-purpose machines, most of which I had to scheme out myself. Then the war started and I received my call-up only to have it cancelled due to being in a reserved occupation. Soon after I was in Sid’s oYce and told that, to increase production, two shifts were to be operated in the machine shop, and he wanted me to take charge of one of them. I was transferred to staV and allocated a bright chap to be toolmaker, setter and anything else that needed doing. All this was just after my 21st birthday. We were to work 12-hour shifts, alternate weeks of days and nights. On the night shift we were the only two non-productive people, and I was totally responsible for everything. The operatives were from every walk of life, male and female, some quite young, others old enough to be my grandparents, some very

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intelligent, some dim, mostly unfamiliar with machine work and working in a factory. It was a traumatic but valuable experience for me and I learnt a great deal from it, not least how to deal with people. Sid Stubbens was a very good governor and gave every encouragement to any young person who showed promise. He would support new ideas, suggestions and innovations for production, designs, procedures or organization that resulted in a forward-looking manufacturing unit. I must have been one of these ‘bright boys’ as during the 12 years working at Unicam I had 14 diVerent assignments following that first staV appointment. He would call me into his oYce and say ‘I want you to start up a tool room’, or ‘We have got to make our own optical components, I want you to set this up’. Then as I was leaving the oYce he would say ‘I’ll see that your money is all right’—and he did! During my whole time with Unicam I never had to ask for a rise: it always came after every new assignment.

II. A Job at the Cambridge Instrument Company Then one day early in 1946, as I was cycling home, my friend Denis Gravestock, who had just returned from the forces, met me outside CIC. He said ‘Stallen is retiring and Dr Marsh, Head of Research, would like you to come in and have a chat.’ This I did and was oVered a job as experimental research engineer, to take over the work of William Stallen. It appears that Dr Marsh had been following my activities over the years and decided that I was the person for the job. Of course I had to see the managing director but the interview was very short and painless. This turned out to be a most fascinating job, involving not only engineering but a great deal of physics as well: Dr Marsh, who was an eminent physicist, provided me with encouragement and excellent tuition. I started on 1 April 1946. Compared with Unicam the place seemed to me somewhat old-fashioned in certain areas: machine tools, many quite elderly, were still being driven by belts from overhead shafting, a system we had abandoned at Unicam six years before. However, on press work and the use of die-castings CIC were more advanced. It wasn’t long before I became interested in the history of the company and started to build up my own collection of archive material, and although I was headhunted away in 1974, the process has continued to the present day. The company dealt with such a huge range of instruments: over 2000 diVerent types, not including detail variations, were manufactured at the Cambridge works alone. Products were made at other sites such as Muswell Hill in London and, after 1949, at the Finchley works as well.

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My 28 years at CIC were full of interest and challenges: there were few of the 2000 product lines in which I did not in some way become involved, either on design, development or in a problem-solving role. I also had considerable contact with the other sites as well. Perhaps the work that gave me the most satisfaction was helping young people to develop their potential skills. My first taste of this came when Dr Marsh asked me to start up a ‘first year oV-the-job training’ school for the apprentice intake. It was to be additional to my normal work and expected to absorb about 10% of my time. I was to set the curriculum, choose the plant, fix the layout of the working area, arrange for lectures by various members of the staV and find a suitable instructor, in fact everything except the daily running of the school. Right from the start I decided that all the training workpieces made by the apprentices were to be of use to them after they left the school and went into the factory—no filing of squares to fit holes and suchlike useless and demoralizing activities. Then came my first problem. Albert Barker, the Tool Room Foreman thought he knew all about training apprentices and said that his was the way it should be done. Needless to say, it was done my way and later I had the satisfaction of advising the newly set up Engineering Industries Training Board on devising their curriculum, and they included several of our exercises in their programme. One day, while I was talking to the instrument shop foreman about the new school, he suddenly stopped and pointed to one of the older instrument makers who was passing by. ‘There’s the man you want as instructor, a good craftsman and a fatherly type of person’, he said. That is how the first Instructor, Harry Speechley, was chosen and what a good choice it turned out to be. He was excellent and well respected by everybody. An apprentice committee, chaired by Dr Marsh and of which I was a member, interviewed all new applicants. It must have been quite a traumatic experience for the interviewee confronted by three or four old fogies asking silly questions such as what their hobbies were! Later I took over chairmanship from Dr Marsh and continued for many years. At the time the Physical Society held a yearly competition for apprentice work and we decided to send an entry from the first year of the school. We chose the lad showing the most promise, Michael Willis, and got him to make a simple dividing-head for use in the school. We were delighted when he was awarded ‘Very Highly Commended’ and we continued to enter every year until 10 years later when the competition was stopped. The reason it stopped was that every year after that first year, a Cambridge apprentice gained the First Prize! In later years this interesting and satisfying activity developed into dealing with sandwich students and their tutors, and with new young graduates entering the Research Department. My section of the department was known as the Experimental Department. Although at first my team included only two skilled

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instrument makers, it was soon augmented by an apprentice, each staying for six months. As the workload increased, the number of skilled men and the amount of machinery increased.

III. H. C. Pritchard—the New Broom In 1958, a new managing director, H. C. Pritchard, was appointed and the construction of the new four-floor research building facing on Chesterton Road was started. I was asked to act as the company representative engineer to liase with the contractors for all the R&D services. The building was occupied in 1959; on the top floor was the Drawing OYce and Board Room, the next floor down the Chemistry/Gas Analysis Department. Below this was the Physics Laboratory with electronics, and the ground floor area was designated the Mechanical Laboratory, but known colloquially as the ‘Mech Lab’. In addition to a very well-equipped workshop it had facilities for a wide variety of mechanical research and development projects, a controlledtemperature room and vibration testing equipment, for example. It provided a service to all the other research departments including a well-stocked store. A. Fields New—Microscan and Stereoscan Pritchard was looking for new projects to update the company image and which could be made under licence. The first of these was the Ultra Microtome designed by Dr A. F. Huxley. Then by a chance coincidence he found what he was looking for. At the Tube Investments Research Laboratories at Hinxton Hall, near Cambridge, in March 1959 he was shown an X-ray Microanalyser that had been developed by Dr D. A. Melford and Dr P. Duncumb as a research tool (see Chapters 3.3A and 3.4). One week after Pritchard’s visit to view the instrument in March 1959, Steve Bergen, the CIC Chief Development Engineer, also visited Hinxton. By that summer, the company was publicly declaring its intention to manufacture the ‘‘Microscan’’. Dr Melford’s design sketches were used to prepare some 700 production drawings. In parallel, the research workshop made a prototype instrument in about six months—in time in fact, to be demonstrated at a private exhibition at the company’s London oYce while the Physical Society Exhibition was being held in January 1960. By the end of that year, customers’ instruments were already being delivered. Dealing with the prototype of the Microscan was my first experience of electron-probe technology, and it involved many new technologies such as vacuum systems and mechanical components unfamiliar to us. Although it was really only a ‘copy’ job, this

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work enabled us to develop many new manufacturing techniques, some of which stood us in good stead when we started work on the development of the next project, the scanning electron microscope. Not long after the new research building was opened, Pritchard had made a tentative arrangement with Professor C. W. Oatley of the Cambridge University Engineering Department (CUED) to develop for commercial manufacture the SEM he and his team were working on. This was a very diVerent project from the Microscan, as much more development work was to take place at the CIC. As a result, the Physics Laboratories under Michael Snelling, and the Mechanical Laboratories under myself, became deeply involved. By this time the staV of the Mech Lab had grown considerably and included graduate engineers, physicists and hand-picked men who had a flair for design work, able to develop processes and lead other instrument makers. It seemed to us that the obvious point at which to start development of the SEM was to use the basic Microscan column as this provided a readymade electron optical system, though it needed some modifications to suit the diVerent requirements of the SEM. The Physics Laboratory under Mike Snelling was responsible for the overall planning, with the Mech Lab and the Drawing OYce closely involved also. My particular responsibility was the mechanical design. There was considerable liaison between the CIC staV and Professor Oatley’s team at CUED, in particular Garry Stewart who later joined CIC. The research department was able to make a practical start in 1961 after the appropriate parts had been extracted from the Microscan production. Modifications were made to the final lens that enabled a satisfactory resolution of 50 nm to be achieved by the experimental instrument, which was completed in 1962. One of the pieces of equipment in the Mech Lab was the evaporating plant. During the SEM development it was used to evaporate a conducting film on to nonconducting specimens to enable them to be examined. One member of the Mech Lab team was a bright young applied physics graduate, Anthony Randall, and he took this work under his wing. I well remember him coming into my oYce excitedly waving a photograph of a diatom, a tiny crustacean rather like a little rowing boat, the first picture taken on the experimental SEM. Although the resolution was not all that good, it was better than that of an optical microscope and the great depth of focus possible was very apparent. Not long after he was seen catching flies, wasps, butterflies and the like to enable the techniques for examining biological specimens to be developed. Needless to say, there were a number of serious problems to be solved, of which vibration was one. Any relative movement between the column and the specimen degrades the resolution. The rigidity this demands, together

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with support for a specimen 12 mm in diameter having X, Y and Z linear movements plus rotation, posed considerable design problems on the specimen stage. An experimental stage was designed and constructed in the Mech Lab, but while it was adequate for experimental work it was quite unsuitable for a commercial instrument, partly due to the limitations imposed by the design of the Microscan specimen chamber we were using. The vacuum system consisted of a backing pump and oil diVusion pump which, although coupled to the column by a flexible bellows, was fitted within the plinth on which the instrument was mounted. Both pump and ground vibrations were transmitted to the column and badly degraded the performance. The Microscan specimen chamber was rectangular and, being of brass, provided inadequate screening. Other designs that provided more convenient side access ports were considered: one of my original free-hand sketches made at a design discussion is reproduced in Fig. 1(a). The final form of the

Figure 1. Reproductions of original ‘blunt pencil’ free-hand sketches made at early design discussions. (a) Proposal for Stereoscan specimen chamber. (b) Proposal for Stereoscan final lens.

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specimen chamber was made from a square slab of mild steel flame-cut to shape and machined all over. It had a large circular port in each face and  each of the four corners was angled oV at 45 , with two smaller round, ports one above the other. It was immediately given the nickname ‘threepenny bit’, a name which, like the design, remained unchanged throughout the production life of the instrument. This specimen chamber design was larger than our equipment could deal with, so production was subcontracted at first to Pope and Meads at Ware. We gave them an order for one only, to enable us to assess their capabilities. They proved to be excellent and were chosen later to be the supplier of production components. However, to reduce delays we soon installed a larger lathe and milling machine so that we could make our own large components. During the early development stages of the SEM, we in the Mech Lab had to devise new methods to provide the accuracy needed. One of the objections to the Microscan column was the need to realign the components each time it had been taken down for cleaning. There was a fairly frequent occurrence of deposits, such as from the oil vapour from the pumps, that contaminated the bores and caused deterioration in performance. The realigning operation was irritating and time-consuming and required a considerable knack on the part of the operator. Experience with the experimental instrument showed us that the more stringent requirements of the SEM made it essential that we find a solution to this problem. Designing the lenses involved a considerable amount of calculation, all executed on a desk calculator as computers did not become available until some years later. The physicists defined the requirements that enabled us to fix the tolerances of the components of the prealigned column. Because there were three or four interfaces, the dimensional and circularity tolerances on each component had to be very small. As the pole pieces of the lenses formed part of an electron-optical system, the quality of the spot was dependent upon the accuracy of the components in the same way as in a light-optical system. This imposed geometric tolerances on machined components comparable with those used on glass optical components, and the soft-iron parts had to be designed in a form that would enable the desired accuracy to be achieved. An example is the final lens: the sketch in Fig. 1(b) is another of my infamous blunt pencil eVorts. This design continued to be used on the SEM or Stereoscan, as it was named, until the mid-1970s. The central zone of the bottom plate needed to be flat to within a few Newton rings, while the bore had to be less than 0.3 mm out of round. Similarly, the upper pole-piece needed to be of the same order and also mounted precisely with respect to the lower plate. When I approached various manufacturers for advice about lapping surfaces to these degrees

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of accuracy, we were inundated with representatives all anxious to show us how to do it. Unfortunately, they were all very experienced in producing surfaces of this type on hard materials, but electron-optical lenses have to be made of soft iron and their techniques would not produce the desired results. They all departed despondent and unable to help us. Left on our own we were able to devise a means of producing a lapping technique on the soft iron by using carefully chosen laps and certain types of lapping compounds. However, producing the finish was only half the problem as this had to be combined with circularity, straightness and flatness. Measuring the flatness of the small central zone of the plate was relatively easy using glass proofplanes, although the larger surfaces were a bit of a problem, but measuring the circularity of the bores was more diYcult. At that time the Rank Taylor Hobson (RTH) ‘Talyrond’ was the best tool available for checking circularity, but it was expensive and had an extended delivery time. It was a precision metrological instrument designed and made by RTH for accurately measuring circularity and flatness of a surface to the order of a millionth of an inch. The nearest machine to Cambridge was at the RTH works at Leicester, so we had to take our work 80 miles each time we needed to check the result of a test lapping. This made progress slow and tedious. When Mr Pritchard, who incidentally always gave me a great deal of backing and encouragement, heard of the problem he told me to go to RTH with an order in my pocket to try to twist their arm and see if I could get a Talyrond quickly, and if so to order it on the spot. It is amazing what the attraction of a firm immediate order will do! They soon found a machine to ‘divert’ to us and delivered it within a few days. After delivery it was quickly installed and commissioned in the Mech Lab constant-temperature room. The value of being able to observe frequently the eVects of a change of method was immediate: the problem of lapping both circular bores and flat surfaces in soft iron was solved within a week! When the trade got wind of the fact that we had solved the problem, they came back to try to find out how it was done: needless to say, we were not telling! To enable us to solve the prealigned column problem, the physicists calculated the maximum error that could be tolerated between the top and final lens. We then calculated the maximum tolerance that could be permitted at each component interface. By choosing very tight geometric tolerances for column components and the mating faces, it was possible to design a prealigned column that would need no adjustment after reassembly. The clamping rings of the Microscans were unnecessary as the column was held rigid when under vacuum by the atmospheric pressure. Simple clamps were fitted to the outside to hold the joints suYciently close to seal the O-rings during pump-down. Production instruments were always despatched in a partially pumped-down condition to ensure that the column was rigid. To

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maintain the geometric concentricity, parallelism, circularity and flatness of the components, the development of special manufacturing techniques in the laboratory workshop was required for the first prototype. The lathes used for this work were first checked to make sure that they turned circular and flat, that is that the mandrel was free of cam action or end movement, by testing specially made test pieces on the Talyrond. Before any new machine was purchased the manufacturer was required to allow us to turn a test piece and check it on our Talyrond. It was interesting to discover that it was not the most expensive lathes that were best in this respect. When I asked one manufacturer of a high-class lathe what the circular error was and the amount of cam action, he replied ‘negligible’. My reply was that we would decide what was ‘negligible’ and required a test piece turned to our specification. Although good, the lathe was not good enough. Another managing director was most insulted when his lathe was rejected: ‘Nobody has ever criticized my machines like this before’ he stormed. However, his chief designer was more realistic and agreed with my suggested modifications that would make the lathe acceptable to us. They incorporated the modifications and we purchased what proved to be an excellent machine. It became obvious to us that the machining accuracy and surface finishes we had found to be necessary, and had developed methods of achieving, would eventually be needed in production. These requirements were far ahead of anything being undertaken in the factory, and it would certainly be said that the requirements were impossible to meet. To prove that others than the picked Mech Lab team could meet the demands, we decided to have a set of parts manufactured by an outside subcontractor: Pope and Meads, the firm who had previously allowed us to check their machines before placing an order for the early specimen chamber, was chosen. Before this could happen, however, the drawing oYce had to learn a lot about precision dimensioning, specifying circularity, parallelism and flatness tolerances and surface finishes. After the drawings had been produced they were sent to Pope and Meads, who made a set of machined components well within specification, showing that the choice had proved an excellent one. They continued to be the main supplier of column components for many years with very few rejects, all of which they corrected without question. Ever since the inception of CIC it had been the practice to dimension drawings in metric units as far as possible. However, metric-size materials were not always available, so the imperial dimension was used on the drawing, resulting in hybrid dimensioning such as 1/2-inch diameter  1-mm pitch! As all our Mech Lab machinery had imperial scales, we made the components to imperial dimensions. When they were being drawn in the drawing oYce, all the imperial dimensions we had used were converted to metric. Then it was found that the production workshops were converting back to imperial

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because most of their machines had imperial scales. This was disastrous as two lots of conversion errors resulted in unacceptable departures from the accuracy required. As a result, all the original SEM designs were made to imperial dimensions. It quickly became apparent to us that vibration isolation of the column from ground and pump vibrations was essential. To isolate the column and specimen chamber, the whole assembly was mounted on the top plate of the plinth. This plate, which was very stiV, was not coupled directly to the plinth framework but by large low-natural-frequency coil springs. These were compressed to the correct working position by the downward force exerted by the flexible vacuum connection located in the centre of the bottom of the specimen chamber. To design this system, knowledge of the amplitude and frequency of the disturbing vibrations was needed. For many years CIC had been making a successful recording universal vibrograph that was well suited to measuring frequencies up to about 100 Hz, the range in which we were interested. The instrument was used to provide data necessary to design the isolation system. Later, when the instruments were in production, the service department engineers used a vibrograph to test customers’ proposed sites and advise on the suitability for the installation of an electron-probe instrument. To assist with the vibration problems of the SEM a 125-lb thrust vibration generator was installed and the behaviour of almost all subsections of the instrument, including specimen stages, was investigated. This work became the province of another young graduate engineer, Neil Dunlop, who became an expert on vibration isolation. He later cooperated with the Computer Aided Design Centre on Madingley Road to introduce a computer terminal to the research department. Development of specimen stages with traverses of the specimen in X–Y–Z, able to work in a vacuum and free from vibration problems, became one of the areas of expertise of Mech Lab experimental engineers. Learning from the mistakes of the experimental specimen stage, we designed and made a new one to fit the steel chamber. To allow the specimen to be changed, the stage had to be easily withdrawn from the chamber, and orthogonal traverses of 12 mm in each direction with 10-mm adjustment vertically were required. In addition, the specimen had to be able to be tilted from horizon tal to vertical and have 360 rotation about its axis—all of these movements to be operated from the outside while the stage was under vacuum. Backlash-free movements were essential and large-diameter micrometer heads enabled the specimen to be positioned to 0.01 mm. The specimens were mounted on simple stubs 12.5 mm in diameter which slipped into a socket with kinematic locations on the tilting cradle of the stage. To ensure that there was no relative movement between the stage and the final lens, the

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specimen supporting assembly was clamped directly to the underside of the lens. After the first batch of instruments, it was found that the new stiV specimen chamber made this an unnecessary complication, so it was dropped in favour of a much simpler form of clamping to the bottom plate of the chamber. Since Horace/Darwin’s time, the company had used for precision apparatus Maxwell’s principles of kinematic design and controlled constraints (Unwin, 1990). I soon became an ardent disciple of the system, encouraging its use and frequently lecturing about it to design staV. Maxwell’s rule stated that the number of degrees of freedom plus the number of constraints must equal 6. If more, then there would be strain. When discussing a design with anyone, my first action, therefore, was to ensure that constraints plus degrees of freedom conformed to Maxwell’s rule. Some of the Mech Lab team were academically relatively unqualified but had the ability to visualize a solution to a mechanical problem and, more importantly, were able to guide others to make the parts, so reducing the development time. One group became very skilled in stage design and continued to develop the basic stage to incorporate many extra degrees of freedom for special requirements. I remember one experiment with a sad end. We were trying a ball slide with vee tracks running on sapphire balls. It worked well until it was put down rather sharply and all the balls were found to have sheared across a diameter. We had discovered that sapphire is very weak in shear! One of the problems that consumed a lot of eVort was the assessment of bearing materials suitable for use in a vacuum. I got some very useful advice from NASA, but there were significant diVerences between the space application and the electron-probe system. The probe system has limited pumping capacity; outgassing can increase pump-down times unacceptably and contamination degrades the beam, involving unnecessary cleaning of the column. Space applications have none of these limitations: they have a vacuum ‘pump’ of infinite capacity and contamination is not a problem. Some of the materials they suggested proved successful but were new to us and the techniques of machining, moulding and manipulation had to be mastered, and the methods transferred to the production departments. All brass components in the vicinity of the electron beam had to be nonmagnetic. This needed the introduction of special brasses with no ferrous inclusions. We had to develop processes to clean the finished components of ferrous particles picked up from the tools during machining, and devise tests for the initial acceptance of the raw material and acceptance of the finished parts. An interesting development was the aperture holder. Apertures consisted of three small platinum discs 3 mm in diameter with central holes a few micrometres in diameter. They were located in the small space between the

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upper and lower pole pieces of the final lens. In addition to the three positions, the slide had a fourth, blank, position that provided means to seal oV the column so that it was only necessary to let the specimen chamber up to atmosphere when changing specimens. The slide was precisely located in any of the four positions and moved by an external lever. Two external knobs provided a small amount of orthogonal movement to the aperture discs, which were held in kinematic recesses in the holder slide. The whole assembly had to be easily withdrawn from the column to enable apertures to be replaced. Platinum apertures were originally purchased from outside suppliers but, as the numbers required increased, supply became a problem. We devised a method of coining these from sheet platinum, thus easing the supply. Fortunately, the scrap price of the platinum waste was almost as high as the new sheet, so that oV-cuts and work spoilt during the experimental activities could be sold at little loss. The phosphor tip of the scintillator required some development to produce the shape and polish, and work was also required to select the adhesive to fix the tip to the Perspex light-pipe. Adhesives used had to be suitable for working in a vacuum and have a refractive index to match that of the lightpipe material. Due to the extreme sensitivity to light of the photomultiplier and the high voltages used, the housing involved elements of design not familiar to us. This, as in many other areas of the work, meant that the team had to learn many new ‘tricks of the trade’. It quickly became clear that our concepts of light traps had to be abandoned and the photomultiplier housing designed as a hermetically sealed chamber. Laboratory involvement did not finish when full production of the instrument had started. Electroplating or painting could only be used as protection on nonfitting surfaces because of thickness variations. The need to keep the unprotected mild steel and soft iron surfaces scrupulously clean permitted corrosion to take place. We were able to find and introduce a phosphornickel chemical plating system that enabled us to overcome the problem on many of the surfaces. Unlike electroplate, the thickness deposited was uniform over all surfaces to within 10 per cent and the plater could guarantee a deposit thickness to within 10 per cent of that specified. This allowed closetolerance surfaces to be machined a specified amount over or undersize, then plated up the correct amount to bring the surface to the required dimension and still maintain the tolerance. All mild steel and soft iron components except a few pole pieces were subsequently plated in this manner. Another associated production problem we had to solve involved corrosion arising from the finger marks of assemblers. We found that the perspiration of young men up to the age of about 24 and of women at certain times was very corrosive. Providing people in these groups with cotton gloves to wear when assembling critical column components cured the trouble.

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As the electron-optical performance of the instrument was improved, the slag inclusions in the mild-steel column components began to become significant. The lens bodies were machined from billets of commercial-quality mild steel in which quite large, randomly distributed, slag inclusions occurred. A component could not be verified as satisfactory until it was completely machined, by which time it was expensive to scrap. I found that ultrasonic or X-ray examination did not prove satisfactory in revealing the smaller inclusions. Discussions with Firth Brown, a specialist steel producer at SheYeld, enabled us to get a sample billet of a vacuum-remelted steel in which the inclusion size did not exceed 25 mm. We were able to make a trial lens body of this that seemed perfectly satisfactory. It was considerably more expensive than ordinary mild steel and less easy to machine, but its use stopped the costly scrapping of finished lens bodies used on the production instruments. It was only possible to adopt this steel when the quantities required were suYcient to justify the purchase of a complete billet of steel. Somewhat later, when quantities were even larger, the steel maker suggested using shaped drop-forged billets of vacuum-remelted steel. Not only did the process of drop forging break up any inclusions, the billets were shaped so that the amounts of machining and scrap generated were significantly reduced. An interesting example of the combined eVorts of the physicist and a Mech Lab experimental engineer, Colin Nordon, was the development of the three coil assemblies incorporated in the column. These consisted of beam-bending coils, stigmator coils and scanning coils. All the completed assemblies had to be encapsulated for use in the vacuum. It was important that the encapsulating material exerted no distorting forces during curing, or subsequently, and had no plasticizer in it to leach out when under vacuum. The experimental engineer devised and made jigs, tools, fixtures and methods to machine from solid the specially shaped ferrite formers, wind them with the wire coils and encapsulate the whole assembly. Scanning coils were wound and preformed on a custom made tool. The core was a very thin nonmagnetic brass tube with the coil-locating lugs of epoxy resin cast on the circumference in a specially developed mould. Again after mounting and fitting connectors the whole was encapsulated to prevent any movement of the wire coils.

B. The Geoscan While the Stereoscan work was going on, microanalyser development was also proceeding. Pritchard had come to an arrangement with Dr J. V. P. Long of the Department of Mineralogy and Petrology, University of

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Cambridge, for the company to work with him on the development of a microanalyser designed primarily for geological specimens. This instrument became known as the Geoscan. It was a very diVerent instrument from the relatively simple Microscan, having two programmable precision fully focusing X-ray spectrometers, prealigned column, variable-speed specimen movements, automatic standard selection and other advanced facilities. The specimen chamber volume was small, which permitted rapid pumpdown after changing the large interchangeable combined specimen and standard holder. The first prototype was completed in the Mech Lab during 1964. Some of the mechanical development work was of a similar nature to that of the Stereoscan and went on in parallel, often being undertaken by the same experimental engineers. Possibly the most demanding project was the fully focusing spectrometer. The required Bragg angles had to be preselectable, repeatable to within 6 seconds of arc and within 2 minutes of arc absolute accuracy. A precision angle-measuring tool called the Angle Dekkor was purchased for the purpose. Three crystals had to be quickly interchangeable without letting the chamber vacuum up to atmosphere. I suggested to Dr Long that the way to provide the movements of the counter carriage was in the same manner that the level luYng crane maintains its load at a constant height while luYng. Various movements of the two carriages and the crystal, which maintain the linear and angular relationships, were controlled by a number of interconnected flexible beryllium–copper tapes. Kinematic or semi-kinematic design principles were used throughout to ensure continued accuracy during long use. While the first experimental model spectrometer components were fabricated of brass, light alloy castings were used for the first prototype and subsequent instruments. To maintain the necessary accuracy, the castings had to be stabilized after rough machining, then finish-machined at a constant known temperature to ensure precise dimensions. As accurate positioning of the movements was needed, a 400-Hz AC servo-system was used, a positional encoder driven from a precision-geared quadrant providing angular position. Checking component parts, straightness of tracking, repeatability and absolute angles involved the adaptation and development of optical measuring devices as lasers were not available at that time. A spectrometer command unit of comparable precision was also needed. This had means to preset Bragg angles, and sweep- or step-scanning over a range of angles and speeds. While the first experimental models were designed and made in the Mech Lab using in-house cut gearing, the spectrometer command units were ultimately manufactured by a firm specializing in high-precision gearing who also provided the precision quadrant for the spectrometer. An interesting problem came to light some years later when the Geoscan was being upgraded to the Microscan Mk5. To permit computer control we

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were replacing the spectrometer analog positioning system by a digital system using a circular grating and stepper motors. When modified, the spectrometer would no longer repeat to within the 6 seconds of arc that had been easily achieved with the AC servo-system. We found that the 400-Hz servomotors had induced just suYcient vibration in the mechanical system to overcome static friction and so reduce hysteresis. This was solved by artificially introducing a very small high-frequency vibration. The orthogonal traverses of the specimen stage were to be operated in both the manual and automatic modes covering the relatively large area of 45 mm  80 mm. As the automatic system had to have constant- and highvelocity modes and positional mode, a DC servo-system with tachofeedback and precision screws was used. Hysteresis had to be kept to an absolute minimum, so considerable ingenuity went into the anti-backlash mechanical design. The experimental engineer responsible for this work, Ralph Kerley, was originally a watch and clock maker but had a flair for designing complicated gearing. He eventually moved to the design oYce and became an authority on specimen stages in addition to gearboxes. The proportional counters were fabricated of stainless steel with Kovar glass-to-metal seals at each end. To avoid problems with flux residues we installed a vacuum brazing furnace. This was later transferred to the production department in the same manner as much of the other specialized equipment we had obtained for development purposes. One counter was gas-filled and sealed while the other had gas passing through it. Both had thin Mylar windows that had to be vacuum-sealed—more of a problem than had been anticipated. The gas supply tubes to the flow counter had to be of very flexible plastic to avoid biasing the spectrometer position. No plasticizer could be permitted in the tube material as this would leach out when under vacuum, causing stiVening. This happened on one occasion when an incorrect batch of tube had been supplied. Unlike the Microscan and Stereoscan, which had relatively small vacuum chambers, the greater size of the Geoscan main frame and spectrometer chamber necessitated the use of light alloy castings. Finding an economical method of making them vacuum tight and with a smooth surface on the vacuum side proved to be diYcult. Vacuum impregnation was not wholly reliable; certain solvent-free epoxy paints were more successful. The Geoscan oil-filled electron gun was of diVerent design from that used on the Microscan and Stereoscan, which were air-insulated. It used a large ceramic insulator designed in cooperation with the ceramics manufacturer Worcester Porcelain. Providing the accurate surfaces for the O-ring seals proved diYcult and required a considerable amount of development work. Problems of oil leakage were also experienced with the filament holders, which were a casting of filled epoxy resin. These were cast in moulds and

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then a groove was machined to receive the O-ring seal. The leak occurred where the filler particles were exposed by the machining operation. The problem was not cured until we had found means to accurately mould the O-ring groove and eliminate all machining. All the guns used a standard tungsten wire filament or ‘hairpin’. For many years these were a bought-in item, but supply diYculties prompted us to investigate the possibility of manufacturing our own. A Kovar seal base was designed and a source was located while bending jigs were made to shape the tungsten wire to form the filament. Spot welding of wires of special materials had been a process used at CIC for many years and the equipment was adapted to weld the ‘hairpins’ to the base wires. Some years later, the Mech Lab became involved in developing techniques for machining lanthanum hexaboride pins, which were to replace tungsten filaments in a new generation of guns. The Royal Radar Establishment (RRE) Malvern provided considerable assistance with this work. They also supplied us with some specialized equipment that they had designed. After the laboratory technique had been developed, the experimental engineer then had to transfer it into a production process for use in the factory. In addition to examining the specimen under the influence of the electron beam, a sophisticated light-optical viewing system was developed, working in cooperation with R & J Beck Ltd. This provided means of viewing both sides of transparent specimens, and also viewing opaque specimens from the beam side, all with the choice of high- or low-power objectives. Simultaneous viewing of the specimen surface for probe-induced luminescence was also provided. The high- and low-power objective lenses were carried on a slide mounted between the final electron lens and the specimen. This slide, which could be operated from outside the chamber, had a third position that carried the scanning coils for use in the probe mode. Limitations of space, long light paths and diYculties of vacuum sealing contributed to a very demanding project ultimately carried to a very satisfactory conclusion. An amusing incident occurred in the Mech Lab when the first experimental instrument was being assembled and tested. Unlike the Stereoscan, the Geoscan beam is horizontal, about 0.75 m from the ground. The partially assembled but working instrument was being demonstrated to a party of international company representatives who were gathered around. As an aside the technician demonstrating said that exposure to the beam in the region of the groin was believed to cause males to become sterile. It was interesting to observe those of the party who moved away and those who moved in closer! Actually there was no risk as the column was adequately screened. One of the experimental engineers was involved in devising methods of preparation of the special large crystals for the fully focusing spectrometer.

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These had to be cut, bent and cemented to the curved holder, then ground to a precise curvature. Some of the crystal materials were very diYcult to manipulate, requiring considerable expertise. After we had been working on the project for a while, we asked an expert from the Warren Spring Laboratory for advice. When he left us, he complimented our engineer and said that he felt that there was little he could teach us. In fact we had shown him a trick or two. Special crystal cutting equipment was obtained, and we designed and made a machine for grinding the curved surfaces of the holder and the mounted crystal in the Mech Lab. The equipment and techniques were ultimately transferred to the production departments. Making the stearate crystals was a challenge to both the chemist and the experimental engineer, who had to devise a special bath for producing the film and depositing it on the mount. Dr Long used the first Geoscan to examine some of the rock brought back from the first Moon landing. After the successful launching of the Stereoscan and Geoscan MkI, electron probe development work continued. However, the work was one of improvement and devising new versions that were variants of the original designs, so lacking the challenges and problems of breaking new ground and the satisfaction of creating the first series production commercial scanning electron microscope in the world. Around 1973/1974 saw another period of redundancies and reorganization within the company. At the same time, the decision was made to move from the Chesterton Road site to a leased site at Rustat Road, on the other side of Cambridge. I was asked to lead a Feasibility Group to look at new design possibilities. Although the work was of considerable interest, I was not very happy and so was delighted to be invited to consider three posts from other organizations, two in Cambridge and the other in Hertfordshire. As both of my children had graduated and left home, my wife and I decided to take the Hertfordshire oVer and see another part of the country at someone else’s expense. IV. A New Career The post was that of Chief Engineer of the testing laboratories of the Consumers Association (CA) at Harpenden, Hertfordshire and at Gosfield, Essex. I started on 1 September 1974 and it proved to be a most demanding, varied and fascinating job in which I was able to use almost everything I had ever learned in the past and learn a lot more besides. Changing to this new job was probably the best day’s work I had ever done; I enjoyed every minute of the time I worked for CA.

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A. Retirement I retired from CA at 65 in October 1983 on a Friday but started back again on the following Wednesday. From my point of view the arrangement was excellent, as I was able to shed all the uninteresting administration and concentrate on the interesting technical work. While with CA I represented them on several British Standards technical committees, a duty I continued to carry out until complete retirement and moving back to Cambridge in January 1996, shortly after my wife had died of cancer. Since retirement, I have built replicas of the planetarium of Giovanni de Dondi (1347); the 14th-century astronomical clock of Richard of Wallingford, Abbot of St Albans; an 18th-century Grand Orrery; and the sea clock H3 made by John ‘Longitude’ Harrison in 1757. These are now on public display in the Manor House Museum, Bury St Edmunds. In 1947 the first of my published articles appeared in the Model Engineer and I continue to write for various journals; also contributed a chapter to a book of physicists’ reminiscences (Unwin, 1990). Recently I have written a brief history of the Cambridge Instrument Company entitled ‘The Scientific’ (Unwin, 2002). All this amounts to a pretty full and interesting life.

References Unwin, D. J. (1990). Development of mechanical parts of electron probe instruments, in Physicists Look Back, edited by J. Roche. London: Adam Hilger Ch. 14. Unwin, D. J. (2002). ‘ ‘‘The Scientific’’ The Story of the Cambridge Instrument Company.’ 2nd ed. Cambridge, England: Cambridge Industrial Archaeology Society.