Processes within society such as radio, television, industrial process controls, and telephone networks, have benefited tremendously from the discovery of the use of vacuum tubes as rudimental electronic components at the start of the 20th century. Since then however, solid-state devices powered by semiconductor technologies have made the use of vacuum tube-based electronics obsolete.
Now, the mass-production of semiconductor devices has grown into a multi-billion dollar industry with benefits such as being more eco-friendly, having a higher efficiency, an increased longevity, and being lower priced. However, more recent research has highlighted some limitations of semiconductor devices when using certain types of radiation that equipment in space is exposed to.
The race is on to find hardier components and systems in engineering space exploration companies  as well as other organizations that want to safeguard their electronic components and systems. But this comes at a price as such processes require a lot of time and money .
Attempts are being made to revisit the more outdated technology of vacuum tubes to provide a solution to finding a transistor technology that can obtain higher speeds and frequencies than any semiconductor device at nanoscale, whilst still leveraging a silicon-based fabrication protocol. The outcome is vacuum-channel transistor technology that has the potential to be produced on an industrial level using silicon fabs for solid-state transistor technologies, which is already available .
The investigation in this document used Scanning Capacitance Microscopy (SCM) to determine the nanoscale electrical characteristics of the latest vacuum-channel device. The aims of this investigation are to establish both its feasibility as a transistor and to provide evidence of the ability to control the method to fabricate its gate insulators.
A Park NX20 Atomic Force Microscopy (AFM) system was used to obtain information about the vacuum-channel device’s nanoscale electrical characteristics by running SCM scans of the device area containing the insulated gates, source-drain interface, and vacuum channel. Supporting topographical details at the scan area was recorded at the same time via contact mode AFM using the same probe.
SCM with AFM
SCM with AFM gives the user a method of characterizing both charge distribution and surface topography without destroying the sample and achieving it with a high spatial resolution and sensitivity , making it winning combination for researching transistor technologies. In SCM, there is a metal probe end and a very sensitive capacitance sensor, along with standard AFM hardware.
An electrical current is then is applied between the probe end and the sample, surface producing a pair of capacitors in series (in Metal-Oxide-Semiconductor devices). First, the insulating oxide layer on the device’s surface and second, the active depletion layer at the interfacial region between the oxide layer and doped silicon.
Following this, total capacitance is calculated by the thicknesses of the oxide layer produced, as well as the depletion layer which is affected by the how doped the silicon substrate is and the quantity of DC charge being applied.
The method of measuring capacitance in vacuum-channel technologies by SCM is almost identical. Insulation of the device is provided by a thin layer of oxide , but in vacuum-channel technologies the layer insulates the gate from the source-drain interface on the device’s surface. A DC current is then applied between the probe end and the sample surface as the probe end scans across different parts of the device.
The results of the identified differences in capacitance are supported by AFM data produced by measuring the deflections of the probe’s cantilever as the probe end interacts with the device surface . At the end, a light beam is reflected off the probe cantilever and onto a photodiode sensitive to the location. Following this, the differences in the laser’s position are handled with software to produce a rendering of the device’s surface topography.
Results and Discussion
The source-drain interface of the vacuum-channel device was captured in an area of 450 x 800 nm. It was discovered that the source and drain terminals are made into sharp ends by using contact mode AFM. This shape was chosen in order to magnify the electrical field created in this area . The topography information collected at this region also tells us the distance between the source and drain tips.
This distance includes the ends of the source-drain terminals and enclosed vacuum-channel, which is about 250 nm (Figure 1). To put this into context, the average free path of electrons and gas molecules under normal atmospheric pressure is approximately 200 nm . If the electrical current running through the equipment were maintained at a small level, the electrons transferring from source to drain would not be able to ionize surrounding gas molecules left behind in the channel .
Therefore, this device could theoretically function in the absence of an incident under normal air pressure. The purpose of the existing vacuum in the channel is as an additional safety measure to protect against ionized molecules from destroying the terminals of the source-drain interface.
Figure 1. Contact mode AFM topography image of the vacuum-channel device’s source-drain interface. The overlaid red line corresponds to the topography line profile displayed in Figure 2. Scan size: 450 x 800 nm.
Figure 2 shows that when taking a closer look at the topography line profile for this image, the length between the highest peaks and the deepest valleys in this vacuum-channel are greater than 5 nm. Something else worth noting is the presence of two peaks, thought to be the device’s quantum dots, which reflect the peak heights in the vacuum-channel and are found on either end of the span, close to the end of each terminal.
Figure 2. Line profiles of the AFM topography (red, left y-axis in nm) and the capacitance data (green, right y-axis in μV) of the device area scanned in Figures 1 and 3.
Device Capacitance with Topography
SCM information was also collected from the same 450 x 800 nm scan as shown above. In Figure 3, it can be seen that the device’s source-drain terminals are darker in color in comparison to other areas of the device. The variation in color of the terminals and closely located quantum dots indicates that these areas are more negatively charged than the rest of the scanned regions.
This is supported by the capacitance line profile seen in Figure 2 that illustrates a mean capacitance of approximately negative 1.4 μV in regions associated to the device’s source and drain terminals. It is also important to note that in areas where the quantum dots were adjacent to the ends of each terminal, slightly more negative values of -1.8 μV were measured.
The capacitance image also shows an area with greater positive values between each of the quantum dots of the device’s vacuum-channel between each of the quantum dots, with capacitance amounts as large as 2 μV being recorded.
About 175 nm long, this area is difficult to see using the topography image but is obvious in the SCM image, which has a higher contrast. The capacitance difference from the quantum dots located next to each source-drain terminal to the middle of the vacuum-channel is shown to be 3.8 μV, which is the biggest difference that was recorded along the selected line profile.
This alternating series of capacitance differences at these key properties gives the device promise of its capabilities of effective functionality as a transistor.
Figure 3. An SCM capacitance image of the region containing the device’s source-drain interface. Brighter colors correspond to relatively more positively charged areas on the device whereas darker colors correspond to relatively more negatively charged areas. The overlaid green line corresponds to the capacitance line profile displayed in Figure 2. Scan size: 450 x 800 nm.
SCM together with AFM successfully characterized both the spatial variations in capacitance and the topography of a newly developed vacuum-channel nanodevice. By examining the line profiles of the topography and capacitance data acquired down an identical path on the device’s source-drain interface, additional insight was gained into the relationship of key physical structures with changes in capacitance.
The device’s topography at its source-drain interface was imaged and revealed a vacuum-channel spanning 250 nm in length with valleys and peaks separated by approximately 5 nm. The electrical functionality of the device was assessed through the acquisition of a capacitance map.
This map revealed a relatively negatively charged (-1.4 to -1.8 μV) source-drain terminal and adjacent quantum dot followed by a relatively positively charged vacuum-channel (2 μV) and another dot-terminal structure (-1.4 to -1.8 μV) on the other end of the source-drain interface. This alternating series of capacitance changes at these key structures suggest the device can effectively function as a transistor.
G. Pascual, B. Kim, and K. Lee, Park Systems Inc., 3040 Olcott Street, Santa Clara, CA 95054, United States of America, J.W. Han, NASA Ames Research Center, Moffett Field, CA 94035, United States of America.
References and Further Reading
 A.S. Keys and M.D. Watson, Radiation Hardened Electronics for Extreme Environments, Huntsville, AL: NASA Marshall Space Flight Center, 2007.
 Vacuum Nanoelectronics: Interview with M. Meyyappan and Jin Woo Han, EEWeb, 2013. Retrieved from https://www.eeweb.com/blog/eeweb/interview-with-m.-meyyappan-and-jin-woo-han-on-vacuum-nanoelectronics
 Scanning Capacitance Microscopy (SCM), Park Systems, 2016. Retrieved from http://www.parkafm.com/index.php/park-spm-modes/94-electrical-properties/235-scanning-capacitance-microscopy-scm
 J.W. Han and M. Meyyappan, Introducing the Vacuum Transistor: A Device Made of Nothing, IEEE Spectrum, 2014.
 How AFM Works, Park Systems, 2016. Retrieved from http://www.parkafm.com/index.php/medias/nano-academy/how-afm-works
This information has been sourced, reviewed and adapted from materials provided by Park Systems Inc.
For more information on this source, please visit Park Systems Inc.