The Constructions Of High Precision Scanning Tunneling Microscopes And Atomic Force Microscopes

The scanning tunneling microscope (STM) and the atomic force microscope (AFM) are the most widely applied two types of microscopes in the scanning probe microscope (SPM) family. Although they both possess the capabilities of atomic resolution, atomic manipulation, and nano-fabrication, their working principles are different, leading to different surface information they can obtain from the sample. What an STM measures is the surface density of states (DOS) of the electrons in the sample. It has atomic resolution, but an STM can not obtain the true atomic structure of the sample.However, what an AFM measures is the information of the interaction forces between the tip and the sample, hence the AFM images will reflect the true atomic arrangement of the sample surface, that is, the sample's true structure. An AFM is nevertheless unable to obtain the DOS information which is important in the sense it can be compared with theory. Thus, the STM and AFM are complementary.Currently, there are many commercial and home-made STMs and AFMs in the world, which are very mature too. But, they need high voltages (higher than 100V) to operate, because the piezoelectric motors they use for the coarse approach need high voltage to work. Using high voltages means high voltage devices must be used such as high voltage operational amplifiers and transistors, which brings the severe issues of large leakage currents, low precisions, big noises and drifts, etc. In the meanwhile, the use of high voltages also leads to high costs and safety problems. And, the high voltage piezoelectric motors are in general large in size and complicated in structure and operation, causing the STM head to be big and complicated, which does not favor a highly integrated STM and makes it difficult to put the STM into extreme physical conditions.To solve the above problems, we have, for the first time, proposed the concept of"fully low voltage STM"and invented the 90o-flipped lateral junction regulation technology, in which high voltages are completely abandoned, thus avoiding all the drawbacks and issues associated with use of high voltages. As we know, traditional STMs utilize the axial displacement (small for unit applied voltage) of a piezoelectric tube to generate inertial stepping for coarse approach, which needs high voltage to produce an inertial force sufficiently large to overcome the friction force. To implement the coarse approach (inertial stepping) under low voltages (lower than the industrial standard±15V power supply voltages for low voltage devices), we take advantage of the fact that the lateral displacement of a piezoelectric tube scanner is much larger than its axial displacement, and flipped the traditional axially regulated junction by 90o so as to regulate the tip-sample gap laterally (by the scanner's lateral displacement). Based on this technology, we have successfully built a piezoelectric inertial motor that can step move under 4V low voltage, and realized the first fully low voltage STM (applicable to other types of scanning probe microscopes) in the world. Using just hand cut Pt/Ir wire as the STM tip, we have obtained extremely clear atomic resolution images (raw data) of graphite in air and room temperature with the image quality being comparable with the images obtained from a low temperature ultrahigh vacuum STM with a chemically etched tungsten tip. Based on the 90o flipped junction STM of which I am in charge, Zongqiang Pang has also built the inertial motor that can step move in two dimensional plane, which gives rise to an STM that can search scarcely distributed defects (or atoms or devices) across the entire sample surface by taking atomic resolution images continuously (gapless) while the tip travels across the sample surface.Here is how the 90o flipped junction STM works: two tube scanners are mounted on the base in parallel with the gap between the scanners being less than 1 mm. Because the two scanners can be identical, the thermal drifts in the axial direction will cancel, and other external interferences (such as vibrations) will cancel too, resulting in a very stable junction structure. The thermal drift (shrinking or expansion) of the base may impact the junction gap, this impact is small since the two scanners are very close to each other (the gap is less than 1 mm) and can be even further reduced by using low drift material to make the base (such as sapphire as we used). So, the junction structure in our STM is highly stable, which is well suited to temperature varying conditions. Furthermore, the STM is very compact and can be put in various extreme conditions. This technology is currently being used in building our ?52mm 20T STM which will be the strongest magnetic field STM in the world.For the STM, the highest measurement resolution of the tunneling current that can be found in published papers is 50fA. This low resolution has limited the applications of STM in studying low conducting or insulating samples. To this end, we have invented and developed a ultra-low noise ultra-high precision two-stage bias deducted trans-impedance amplifier (ULN-UHP-TIA), in which the tip is truly grounded (thus reducing the stray field from the tip) and the bias removal from the output of the first stage is done by a second stage subtractor with extremely high input impedance (thus minimizing the signal distortion). Combined with the above mentioned 90o flipped junction STM, we have achieved a record breaking 10fA current resolution. This will help expand the STM into the measurements of weak conductors (such as biological samples) or even insulatorsWe have also patented an interference-resistant double channel differential current amplifying circuit which is a symmetric circuit with very high capability of canceling and reducing electromagnetic interferences (EMI). It can detect picoamp (even femptoamp) weak current, which is not seen published anywhere else. It may have broad applications, especially in enhancing STM's ability to withstand EMI. For the controller, we (in charge by Jihui Wang and I have participated in the testing) use Labview programming language in the PXI real time system to develop the SPM controlling system. Compared with the commercial controller, ours is easier, more versatile and much cheaper, making the software development simplified significantly.On the basis of our lateral junction regulation method, we are currently constructing a frequency modulated AFM (FM-AFM) which is as compact as the above mentioned 20T-STM. We have abandoned the conventional reflected laser scheme for force sensing. In stead, we adopt the qPlus quarts tuning fork as the force sensor, which uses the piezoelectric effect to detect the bending of the fork. We have tried two different automatic gain control (AGC) circuits in the oscillation circuit, both leading to a very stable oscillation of the qPlus probe. To convert the oscillation frequency into a voltage signal, we have successfully developed a 3 mHz frequency resolution phase locked loop (PLL, built by Yizhi Shi and tested by me on my AFM), which is better than the commercial 5 mHz resolution Nanosurf EasyPLL. At present, the whole AFM is already completed and is being tested in liquid helium environment. Although we have not got the atomic resolution images yet, we have nevertheless measured the correct force gradient vs. tip-sample distance curves.To better isolate the vibration influence, which is critical in achieving atomic resolution, we have referenced some of the vibration isolation techniques used in gravity wave detection and designed a multi stage damped long pendulum vibration isolation platform. It consists of stacks of alternate steel block layer and damping rubber ball layer with the table top being suspended from the tops of the steel block stacks. We have also designed the rubber ball recipe ourselves. The rebound rate of the rubber balls is less than 5%. The final vibration isolation result is rather excellent. As the consequence of exploiting the above quite a few techniques we develop, we are successful in implementing the world's first all low voltage and better than 10 fA current resolution STM (the result of an official novelty search is provided in the appendix), which has demonstrated very high stability and precision and has produced extremely clear atomic resolution images (raw data) for graphite samples in ambient conditions (comparable or better quality images have not seen previously under the same measurement conditions).