Among many implementations of quantum technology under investigation, there is a particular interest in solid-state quantum devices. These require fabrication processes with accuracy and resolution well beyond the capability of the commercial tools presently used in the semiconductor industry, or even the best available research tools ever reported in the scientific literature. To meet the requirements of solid-state quantum device fabrication, the Quantum Center at UT Dallas brings together a collaborative team of researchers to develop atomically precise fabrication technologies, based on the Hydrogen Depassivation Lithography (HDL) method. This approach promises orders of magnitude improvements in resolution and precision over existing technologies.
The technology pursued by the Center researchers is the only known route to 3D atomistic control of dopant position and numbers in silicon and thus to building silicon nanoelectronics with full 3D atomic precision. The procedure is graphically shown in Fig. 1, below [1]. It starts by etching alignment features into the silicon substrate that are used to align subsequent process layers. The sample is then moved to a UHV chamber to prepare an H-terminated Si surface for lithography. A STM tip is used to break Si-H bonds by injecting current to selectively remove H atoms from the surface. This creates a template of highly reactive Si dangling bonds. A doping precursor adsorbs electively on the exposed Si pattern, meaning that H-terminated regions function as a resist in this process. After annealing dopant atoms are incorporated in electrically active substitutional sites. The next step involves burying the dopant structure in epitaxial Si allowing backend processes such as aligning contacts to the buried dopants to be performed using conventional microfabrication techniques.
There are numerous challenges associated with this approach, many of them arising from the single existing tool for atomic precision lithography, i.e. the scanning tunneling microscope (STM). The STM is designed to function as a microscope. It was never intended to be a lithography tool. The STM is slow and has limited throughput due to its single-tip design. Its feedback control loop tends to become unstable during lithography resulting in the tip crashing into the surface. Furthermore, spurious H depassivation commonly occurs in HDL. Our research efforts are aimed at addressing these and many other issues related to HDL.
![Figure 1 Atomic precision positioning of dopant precursors in silicon [1]](https://qdc.utdallas.edu/files/2021/09/atomic-precision-positioning-of-dopant-precursors-in-silicon.png)
MEMS STM: In order to address the issue of low throughput in STM, we are building active STM tips with SOI-MEMS technology. These MEMS devices replace the z-axis of STM, i.e. rastering is performed by the piezoelectric tube scanner, but the z-axis feedback loop is closed on the MEMS device. These MEMS STMs are designed to have significantly faster resonances compared with piezotubes and thus offer higher bandwidths compared with conventional STMs, enabling faster imaging and lithography. Furthermore, due to their small dimensions they can be packaged to work in parallel in an array, enabling a higher throughput than conventional STM.
Fig.2 shows the SEM images of a SOI-MEMS device that was fabricated at the UT-Dallas Cleanroom [2]. Fig. 3 and Fig. 4 demonstrate its integration into a Scienta Omicron VT-STM. Fig. 5 shows three sets of images that were obtained with three different MEMS-STMs in UHV.
Adaptive control of STM: A common cause of tip-sample crash in a Scanning Tunneling Microscope (STM) operating in constant current mode is the poor performance of its feedback control system. Our research has established that there is a direct link between the Local Barrier Height (LBH) and robustness of the feedback control loop [3,4]. A technique known as the “gap modulation method” was proposed in the early STM studies for estimating the LBH. We have been able to show that the measurements obtained by this method are affected by controller parameters and subsequently proposed an alternative method which we were able to prove to produce LBH measurements independent of the controller dynamics. We use the obtained LBH estimation to continuously update the gains of a STM proportional-integral (PI) controller and show that while tuning the PI gains, the closed-loop system tolerates larger variations of LBH without experiencing instability. Improved feedback stability is believed to help in avoiding the tip/sample crash in STMs. Experimental results obtained with this self-tuning PI controller confirm the improved STM performance compared to the conventional fixed-gain PI controller; see Fig. 5 and Fig.6. These experiments confirm effectiveness of the proposed method in extending the tip lifetime by lowering the chance of a tip/sample crash.
High-precision HDL: Formation of spurious dangling bonds during conventional HDL is regularly observed by researchers and practitioners. We propose a lithography method that allows the STM to operate under negative bias imaging conditions and simultaneously desorb H atoms as required [5]. A high frequency signal is added to the negative bias voltage to deliver the required energy for hydrogen removal. The resulting current at this frequency and its harmonics are filtered to minimize their effect on the operation of the STM’s feedback control loop. We show that the chance of tip-sample crash during the lithography process is reduced by employing this method. We also demonstrate that this approach offers a significant potential for controlled and precise removal of H atoms from a H-terminated silicon surface and thus may be used for the fabrication of practical silicon-based atomic-scale devices; see Fig. 7.
References
[1] Bussmann, E.; Butera, R. E.; Owen, J. H. G.; Randall, J. N.; Rinaldi, S. M.; Baczewski, A. D.; Misra, S. Atomic ‑ Precision Advanced Manufacturing for Si Quantum Computing. MRS Bull. 2021, 46, 1–9.
[2] A. Alipour, S. O. R. Moheimani, J. H. G. Owen, E. Fuchs, J. N. Randall. Atomic precision imaging with an on-chip STM integrated into a commercial UHV STM system. Journal of Vacuum Science & Technology B, 39 (4), 2021.
[3] F. Tajaddodianfar, S. O. R. Moheimani, J. Owen, J. N. Randall. On the effect of local barrier height in scanning tunneling microscopy: Measurement methods and control implications.
Review of Scientific Instruments, 89 (1), pp. 013701, 2018.
[4] F. Tajaddodianfar, S. O. R. Moheimani, J. N. Randall. Scanning Tunneling Microscope Control: A Self-Tuning PI Controller Based on Online Local Barrier Height Estimation. IEEE Transactions on Control Systems Technology, 27 (5), pp. 2004 – 2015, 2019. [5] H. Alemansour, S. O. R. Moheimani, J. H. G. Owen, J. N. Randall, E. Fuchs. Controlled removal of hydrogen atoms from H-terminated silicon surfaces. Journal of Vacuum Science & Technology B , 38 (4), pp. 040601, 2020.