Getting Started on Home Brewing an STM

A scanning tunneling microscope (STM) can image surfaces at atomic resolutions and may even be used to manipulate atoms and molecules. If you're interested in getting some hands-on experience with nanotechnology, then an STM appears to be a good tool. An STM certainly seems like a simple machine, and there ae few competitors to it for manipulating matter one molecule at a time - which some consider a prerequisite to building the first pieces of a molecular machine. But if you've become interested in owning your own STM to do some personal experimentation and you've checked out the commercial units, then you've no doubt discovered just how costly they are. But when you consider building your own "home brew" STM, you discover that almost no one outside academia or industry, it seems, has carried a home built STM project to completion. If you check the research journals, you'll find a great deal of information on theory and operation, but not nearly as much information telling you how to build one. The information on design and construction is scattered about and it is difficult to tell what designs are worth pursuing, and what design principles you need to follow. You are probably unsure about where to start, what approach to take, and what the major problems are. Those are issues I hope to provide some answers to in this article, based on my own investigations.

First, for experimenters on a tight budget (just about everyone!), the bad news is that most commercial STMs are in the "if you have to ask, you can't afford it" price range. One "low cost" system, Nanosurf's easyScan ^TM STM (available from several distributors, such as RHK Technology in the U.S.), starts around $8,000 (all prices in this article are quoted in U.S. dollars). Another published price is $9,500 for an "STM for Students". But beyond those lone examples, I have been unable to find published price lists from the leading manufacturers. I needed to give their distributors and sales representatives a call. Vendors quoted me prices anywhere from from $30,000 to $150,000.

A home-brewed STM, by comparison, should cost well under $2,000 in parts and optional hired labor, excluding the cost of a computer needed for data acquisition and display. Additional effort and ingenuity should bring the cost to under $1,000. At least a couple hundred man-hours of effort should be anticipated.

As far as I have been able to determine, many past attempts at home-brew STMs have failed not for technical reasons, but because the builders didn't follow through. The literature search, the design, the search for suppliers and ordering of parts, and the actual construction all conspire to make the project last longer than expected. So you need to adjust your expectations, but mostly you need to maintain your motivation.

I believe one way to do that is to enlist others in the project. A person working alone must rely on internal motivation, which can fade, but a person working in a team has multiple external motivators. These tend to be mutually reinforcing. I believe that mutual motivational support is a characteristic that should not be underestimated. In addition, designing an STM requires knowledge of several engineering disciplines, so a team of (possibly novice) specialists working in parallel will generally work faster than a single generalist. Fortunately several major subsystems can be developed almost independently. And you'll find a team comes in handy during debugging, testing, and operation. And last but not least, costs can be shared. Just remember though, that once you have a successful system running, you'll probably want to build a couple more copies for use by the group!

To get an STM built requires a plan that, ideally, lists the major steps that must be performed in a reasonable order. Here is my suggestion of the major steps and a proposed order in which they should be tackled:

Build Your Team

If you are going to go it alone, naturally you can skip this step! But if you have decided that a team is a good idea, you should build the team first before building the STM. However, I suspect that too large a team can itself be dangerous! I suggest that you have a team lead to coordinate actions and act as arbiter in the case of minor disputes. Either a simple nomination process and election by a show of hands, or even a dictate by the instigator of the project, is normally sufficient for establishing a lead. So long as the team lead's responsibilities and authority are limited you should see some benefit and few problems from having someone in such a position.

You'll need computer programming expertise, including low level interfacing and graphical user interface development experience. I've been surprised to discover that data acquisition, computer interfacing, and software development have been common (and unexpected) stumbling blocks, so don't take these for granted. Fortunately there is now some open source GPL code available in the form of the GXSM project noted later in this article. You'll also need someone with electronics experience, including of course computer interfacing skills. And you'll need someone with mechanical skills. And each team member will need appropriate tools and a working area suited to their work. I suspect a team of three or four would be optimum. Team members should be physically near each other, but some of the software development might be accomplished over the Internet by more distant developers.

Select an Approach from the Literature

To avoid excessive research costs and blind alleys, you should base your design on something that has already been shown to work. In the hope of saving readers some time and effort, I've looked over many design ideas and done some winnowing, searching through a number of journals, books, and the web, with the journal Review of Scientific Instruments being my primary source. In-air operation, simplicity, and (relatively) easy access to parts were part of my filtering criteria. Because of the cost and complexity, vacuum and cryogenic systems were generally dropped from consideration. While such regimes may be critical to some nanotechnology research, I suggest that such systems be tackled as a second follow-on project for those interested. The old adage of learning to walk before you run seems applicable!

The first design that I suggest you should consider as a base is the one described in Sang-il Park, C. F. Quate, "Scanning tunneling microscope" Rev. Sci. Instrum. 58(11) 2010-2017 (1987). This is an early design, but the article is well written and provides some general design rules. It provides circuit diagrams for the preamplifier and Z-axis control loop, X and Y axes drive, and stepper motor control. Park went on to found Park Scientific, an early vendor of commercial STMs.

The second rather elegantly simple design approach is the one described in S. Kleindiek, K. H. Herrman, "A miniaturized scanning tunneling microscope with large operation range" Rev. Sci. Instrum. 64(3) 692-693 (1993). Because of its extreme simplicity, it should be given some serious consideration. It employs what is known as stick-slip operation for coarse approach. This allows it to use the same piezo actuators for both coarse and fine motion, which makes for a simple mechanical design. The article also provides some basic circuit diagrams that use low-cost common components. The design has been commercialized. Acquiring the tubes and making the stick-slip operation work reliably may be the biggest difficulties.

The third recommended design approach is the remarkable amateur effort of Jürgen Müller. He has meticulously described his project on his web site, STM, a project by Jürgen Müller . He provides pointers to many other resources, including books, articles, commercial STM builders, and various suppliers. His site cannot be recommended highly enough.

The last design approach I would recommend looking at is the Simple STM Project developed by John D. Alexander. As designed, the project appears to cost under $100! But it lacks a decent coarse approach mechanism and an interface to a computer or any other output device (unless you have a storage oscilloscope handy). Once you throw proper display interfaces and a more reliable coarse approach mechanism into the picture, the cost will climb. And it seems unlikely that it could reach atomic resolution, but it certainly is low enough in cost to give it a try.

You'll find that the above articles generally do not provide enough background to understand why the designers made all their decisions, or they lack critical details in one or more places. To get a comprehensive understanding of the design aspects left un-addressed, I strongly suggest the text Introduction to Scanning Tunneling Microscopy (Oxford Series in Optical and Imaging Sciences, by C. Julian Chen, ISBN 0195071506 (1993). The instrumentation section is fairly complete, indispensable, and still relevant after 10 years. Another text worth investing in is Scanning Tunneling Microscopy (Methods of Experimental Physics, Vol 27) by William J. Kaiser (Editor), Joseph Stroscio (Editor), ISBN 012674050X (1993, reprint 1997). Chapter 2, "Design Consideration for an STM System" by Sang-il Park and Robert C. Barrett are of value in that they not only discuss design considerations, but also provide a section on troubleshooting common problems.

High Level Design

An STM consists of a set of relatively independent subcomponents separated by interfaces and interactions that can be specified in quite good detail without having to give much consideration to the interior of the subcomponents. So an STM is a good candidate for design by functional decomposition. Concentrate on specifying interfaces, both physical and data. I've listed below some subcomponents. Additional decomposition should be feasible, should it be desired.

Interface Electronics Design and Construction

The X, Y, and Z tip fine motion control signals can be generated using dedicated circuits, but for flexible control, you'll want to generate the signals using a computer via digital-to-analog (D/A) converters. Control of the Z axis (moving the tip toward and away from the sample surface) is fundamentally different from the X and Y axis control. The Z axis control generally requires realtime control in a feedback loop, while X and Y do not. The range and speed of motions, as well as the mechanics are also distinctly different.

In addition to the electronic control circuits listed in the suggested base designs above, the article Raul C. Munoz, Paolo Villagra, German Kremer, Luis Moraga, Guillermo Vidal "Control circuit for a scanning tunneling microscope" Rev. Sci. Instrum. 69(9) 3259-3267 (1998) should be referenced. The article goes into detail on control circuits that allow digital or analog feedback control of the Z axis and digital control of the scan, with relatively good noise immunity.

Tunneling Current Data Acquisition and Feedback Control

The computer analog-to-digital (A/D) interface that acquires the tunneling current or feedback voltage (if using an analog feedback circuit) value is somewhat self contained from the other systems. It interfaces to the Z fine motion control by way of the feedback mechanism, which can be implemented in analog circuitry or digital circuitry. The rather short article, B. A. Morgan, G. W. Stupian, "Digital feedback control loops for scanning tunneling microscopes" Rev. Sci. Instrum. 62(12) 3112-3113 (1991), sketches out the background theory of digital feedback loops applied to STMs.

Software for Data Acquisition, Control, and Rendering

Tunneling Current Amplifier Design and Construction

Tunneling current pre-amp circuits use one of two general approaches: a feedback picoammeter or an electrometer current amplifier. Most of the previously mentioned articles use feedback picoammeter pre-amps, because of their alleged lower frequency response. But an article that describes a suitably optimized electrometer approach that appears to perform very well is in Y. P. Chen, A. J. Cox, M. J. Hagmann, H. D. A. Smith, "Electrometer preamplifier for scanning tunneling microscopy" Rev. Sci. Instrum. 67(7) 2652-2653 (1996).

Coarse Approach Mechanics Design and Construction

Coarse approach is the term used to describe moving the tip from ~1 mm from the sample to ~10 Angstroms. That is too large a motion range for typical piezo scanners, so other mechanisms are employed. In order to maintain stiff mechanical coupling between the tip and sample, you'll need to design the mechanical assembly and coarse approach together. You'll find a number of approaches have been suggested or tried, with some of the better ones to be found in the previously suggested base design articles. Yet another one is a piezotube walker, outlined in the article Anjan K. Guta, K.-W. Ng, "Compact coarse approach mechanism for scanning tunneling microscope" Rev. Sci. Instrum. 72(9) 3552-3555 (2001).

Mechanical Design and Construction

You'll probably find that you can't do the mechanical design until you've chosen and designed the coarse approach mechanism. Some flexibility may be realized by using other materials. Besides metal, you should consider using materials that are easier to work, such as wood, plastic, oven baked polymer clays (such as Sculpey), and other materials where it makes sense. Keep an open mind on ways of connecting things together. You can mount tips and samples using conductive tape, for example. Two good sources of hard to find materials in small quantities for amateur experimenters (for U.S. readers at least) are Small Parts Inc and Structure Probe Inc..

Vibration Isolation Design and Construction

Passive vibration isolation is covered in some of the references given and it is unlikely you'll find any alternative that is as inexpensive. This subsystem can be designed and built once the approximate size and mass of the STM is known. One interesting one-dimensional vibration isolation system I might suggest you look at is the one described in the article Jiangfeng Liu, John Winterflood, David G. Blair, "Transfer function of an ultralow frequency vibration isolation system" Rev. Sci. Instrum. 66(5) 3216-3218 (1995). In fact several interesting vibration isolation systems have been proposed for gravity wave detection systems - most of which I've had to omit, since they seem to be overkill or inappropriate for STM operation.

Tips

Good tips are a necessity, and several materials have been used, as have several mechanisms for making the points. The classic technique for making tips from Platinum-Iridium (Pt-Ir) wire is to snip them at an angle. Another technique for consistently making good tips from Pt-Ir wire is outlined in the article B. L. Rogers, J. G. Shapter, W. M. Skinner, K. Gascoigne, "A method for production of cheap, reliable Pt-Ir tips" Rev. Sci. Instrum. 71(4) 1702-1705 (2000). The chemicals used to etch the tips, while slightly hazardous, are much less so than the chemicals called for in some of the other techniques I've seen. Another technique for making tips is outlined in the article Liu Anwei, Hu Xiaotang, Liu Wenhui, Ji Guijun, "An improved control technique for the electrochemical fabrication of scanning tunneling microscopy microtips" Rev. Sci. Instrum. 68(10) 3811-3813 (1997).

Operation

Testing and debugging should be done during construction as much as possible. There isn't much I can guide you on those aspects, since they are dependent on what approach you choose. Your goal should be more than just atomic resolution imaging - though that is no trivial feat! Since I'm assuming molecular nanotechnology research is your intent, getting the STM built and running is just the starting point of your efforts, not the end point.

The last thing I should point out is that you should be careful not to run afoul of the patents that are held by SPM companies, some of which may be very zealous in protecting any perceived infringement on their intellectual property. I doubt any are anal-retentive enough to hassle a lone experimenter, but you never can tell. IBM actually patented the STM when it was first developed, but because they specified operation in a vacuum, their original patents are allegedly not applicable to STMs that are operated in air. Should you decide your efforts are worth commercializing or even spreading freely to others, you should take some precaution and do a basic patent search.

Good luck in your efforts!