I’m ashamed to admit that I don’t know as much about the topic of test and measurement (T&M) as perhaps I should. My only excuse is that there’s so much to learn and I am a bear of little brain (and my little bear brain is pretty much full).

There are lots of things I’ve not done in my life. One of them is to build my own digital multimeter (DMM) comprising a voltmeter (to measure voltage), an ammeter (to measure current), and an ohmmeter (to measure resistance).

I haven’t given this much thought (to be honest, I haven’t given it any thought), but I dare to say that I could probably throw something together reasonably quickly. This would be a hand-ranging device, requiring the user to set the desired range of the selected measurand (that is, the physical property—voltage, current, or resistance—to be measured).

How accurate would it be? “Ay, there’s the rub,” as Prince Hamlet might say. It would probably be accurate enough for many of my rough-and-ready hoppy projects, but I wouldn’t use it for any form of professional application.

As an aside, have you seen those rinky-dinky component testers for resistors, capacitors, inductors, diodes, transistors, etc.? These can be rather useful on occasion, and they are easy to use—just stick the component into the socket, press the “Test” button. and it will automatically determine which pins are which and then present you with any relevant information. I just saw this one on Amazon for only $17. I’ve often thought that building one of these would be a great project to teach fundamental electronic and computing topics to beginners (one more thing to add to the list).

So, how would one go about creating a T&M device that could measure picovolts and femtoamps? I don’t have a clue. Your next question might be, “Why would you want to measure picovolts and femtoamps?” That’s a good question. It shows you are paying attention. I’ll answer it in a moment.

Are you familiar with the term “cryotronics”? This refers to the branch of electronics that involves the design, development, and application of electronic devices and circuits operating at very low temperatures, often near absolute zero. This field leverages the unique properties of materials and phenomena that occur under cryogenic conditions.

Cryotronics is applicable to superconductivity and low-temperature physics. Applications include quantum computing, medical imaging, sensitive measuring equipment, and high-speed electronics.

The ‘cryo’ portion of ‘cryotronics’ comes from the Greek word kryos, meaning ‘cold’ or ‘frost.’ Until recently, I could have written all I knew about cryotronics on the tip of my (frostbitten) pinky finger. Today, by comparison, I know enough to be highly dangerous—well, slightly perilous—because I was just chatting with Chuck Cimino, who is Senior Product Manager at Lake Shore Cryotronics.

Founded in 1968, the folks at Lake Shore Cryotronics have built an enviable reputation as being at the forefront of cryotronic T&M equipment. The reason this is of interest to anyone working in the semiconductor industry is that the guys and gals at Lake Shore Cryotronics have recently started to bring their deep expertise in ultra-low-level measurements and test equipment technology out of physics laboratories and into engineering environments, like semiconductor foundries, for example. 

At first, I must admit to being a tad confused (I was playing to my strengths, because being confused is one of the things I do best). I do have some experience in testing, but predominantly in testing the logical functionality of things like silicon dice and printed circuit boards. So, when Chuck started to talk about their test equipment in the context of silicon wafers, I was thinking in terms of testing entire dice, which didn’t make sense.

Of course, I was heading in completely the wrong direction. What Chuck was talking about is the ability to characterize pieces of material or individual components (resistors, diodes, transistors) on the wafer. While many devices continue to be fabricated at the 10nm, 14nm, and higher nodes, there is widespread adoption of the 7nm and 5nm nodes. Cutting-edge companies are currently working with the 3nm node; the 2nm node is expected to move into production in the 2025-2026 timeframe; and the 1.5nm and 1.0nm nodes will follow at some stage in the future.

Using special test structures on the wafer, picovolt and femtoamp measurements are of use when characterizing a new process and bringing up a new production line. They continue to be of use when monitoring process variations once the line is in full swing.

Consider the image shown below. Let’s start with the MeasureReady M81-SSM (Synchronous Source and Measure) system on the left. The M81-SSM is designed to eliminate the complexity of multiple function-specific instrumentation setups, combining the convenience of DC and AC sourcing with DC and AC measurement, including a lockin’s sensitivity and measurement performance. This extremely low-noise system ensures synchronized measurements from 1 to 3 source channels and from 1 to 3 measure channels.

The M81-SSM (left) with one SMU-10 module (right) (Source: Lake Shore Cryotronics)

As exciting as this is, the really exciting part is the new SMU-10 on the right, where ‘SMU’ stands for “Source Measure Unit.” The combination of the M81-SSM and up to three SMU-10s enables engineers to easily source and measure picovolt and femtoamp signals that would otherwise be swamped by electrical noise.

The M81-SSM system with SMU-10 module is useful in any low-power test applications that have challenging signal-to-noise ratios (SNRs). Primary applications include I-V characterization of transistors used in specialized sensors, nanoelectromechanical systems (NEMS), quantum computer readout electronics, and emerging specialized and integrated circuit nanoscale semiconductor-based devices.

To avoid noise and extract very small usable signals, the M81-SSM system employs frequency and phase-sensitive technology—also known as lock-in amplifier technology—which has been employed by research scientists for decades. The source signal is set for AC at a user-determined amplitude and reference frequency, and the measurement processor “locks in” on only those signals while ignoring any other offsets and noise signals not at the exact reference frequency. The M81-SSM system with the new SMU-10 module is the first I-V characterization system to combine both AC and DC capabilities with synchronous AC detection measurements.

The M81-SSM allows testing across a wide range of user-selectable frequencies (including DC) to optimize results by avoiding power line and other known interfering signals. In addition, the well-proven ultra-low sensitivity of such synchronous AC measurements significantly reduces the stimulus signal power applied to devices, further eliminating unwanted thermal effects that could alter the device’s baseline characteristics. Because the system and all modules provide both AC and DC signals, a user can easily switch between or combine DC and/or AC measurement modes—all without changing the physical setup.

The M81-SSM system and SMU-10 module’s source noise performance is several orders of magnitude below that of conventional DC SMUs. The measurement sensitivity is also decades below that of conventional DC SMUs.

Each M81-SSM instrument can control up to three SMU-10 modules for synchronous and independent control of the gate, drain, and source of a FET or other complex semiconductor devices. As an example, the M81-SSM system can sweep gate voltage while monitoring gate current and measuring channel current as a function of gate voltage.

Just how good is this setup as compared to “the others” (they know who they are)? Well, consider the left-hand portion of the image below. This shows an SMU-10 sourcing a 10mV sine wave signal (red) vs. a popular SMU sourcing a 0V DC signal (blue). Observe that the source noise of the SMU-10 (the thickness of the red sine trace) is less than 1/10 the width of the source noise of the traditional low noise DC SMU.  Also observe that the blue trace contains many frequencies from DC to 20MHz that can affect the transistor and cause errors.

Some examples (Source: Lake Shore Cryotronics)

Now turn your attention to the right-hand portion of the image above. In this case, an industry-leading SMU measuring DC current in the 1nA range (the blue trace) is compared to an M81+SMU-10 (the red trace).

To be honest, we haven’t even touched on the super exciting part yet. First, we have carbon nanotubes, which can be coerced to act as transistors. Although the discovery of these structures is often credited to Sumio Iijima of NEC in 1991, L.V. Radushkevich and V.M. Lukyanovich published clear images of 50-nanometre diameter tubes made of carbon in the Journal of Physical Chemistry of Russia as far back as 1952.

More recently, we have the first single-atom-layer material ever discovered—graphene—which was isolated at the University of Manchester, England, in 2003 and formally announced in 2004. Graphene can also be coerced to act as transistors.

The future may be carbon nanotube and graphene-based (Source: Lake Shore Cryotronics)

A bandgap is the energy barrier that electrons must overcome to move from the valence band to the conduction band. It plays a critical role in determining whether a material is a conductor, semiconductor, or insulator. Graphene, in its pristine form, is a zero-gap semiconductor—meaning it conducts electricity freely with no way to easily switch on and off the flow of electrons.

Although graphene doesn’t have a bandgap per se, recent developments in graphene research involve twisted bilayer graphene (TBG) in which two layers of graphene are stacked one on top of the other but rotated by a small angle (typically around 1.1 degrees), which is known as the “magic angle.” When two layers of graphene are twisted at this specific angle, it creates a moiré pattern (a type of interference pattern) between the layers. The resulting electronic properties are significantly altered, leading to the emergence of a bandgap in a material that would otherwise have none.

If you’d asked me yesterday about the possibility of carbon nanotube and graphene-based devices replacing silicon chips, my reply would probably have been along the lines of: “This isn’t likely to happen in the near term.”

So, you can only imagine my surprise when Chuck told me that, about a year ago as I pen these words, he visited a facility in Germany where he saw a 300mm fab producing graphene-based circuits (I’m assuming we are talking about multiple layers of graphene deposited on a silicon wafer substrate, but I may be wrong). Chuck says he has the impression these circuits were switching in the terahertz range (but he may be wrong).

And what was Chuck doing at a graphene wafer fab? Well, can you guess who might want to measure things in the picovolt and femtoamp range?

All I can say is that we truly do live in interesting times. If I hear more about these graphene-based wafers, I will report back in a future column. In the meantime, do you have any thoughts you’d care to share about anything you’ve read here?