Figure 1. Schematic illustration of Bias-Assisted AFM Nanolithography [4, 5].
John Paul Pineda, Charles Kim, Cathy Lee, Byong Kim, and Keibock Lee, Park Systems Inc., Santa Clara, CA USA
Nanotechnology has received increased attention within the scientific community due to its application in a number of fields, ranging from electronics to biomedical technologies . Progress in many of these applications depends mainly on the capability to fabricate nanostructured materials that include polymers and semiconductors, among others [2, 3]. Several methods have been introduced for the fabrication of nanostructures; the more common ones are electron beam lithography and focused ion beam lithography. However, these methods are not straightforward and expensive to operate. One powerful method designed to overcome these problems is atomic force microscopy (AFM) nanolithography [1-3]. This technique is simple and less expensive . AFM nanolithography is divided into two general groups based on their mechanistic and operational principles: bias-assisted and force-assisted AFM lithography . In the bias-assisted method, a bias voltage is applied to the tip to generate an oxide pattern on a metallic or semiconductor substrate [4, 5]. Whereas, the force-assisted method applies a large force to the tip to produce fine grooves on the surface of polymer samples by mechanically scratching, pushing or pulling the surface atoms and molecules with a sharp tip; the interaction between the tip and the sample is mainly mechanical [2, 3].
In this technical note, we demonstrate the capability of AFM nanolithography to generate nanopatterns by utilizing a Park NX10 Atomic Force Microscope. The tip bias mode is used to create oxide patterns on the silicon substrate. Successful pattern fabrication is confirmed by an AFM topography measurement. The integration of lithography with AFM enables to fabricate nanopatterns and acquire the sample topography. The results obtained in this experiment confirms the fabricated oxide nanopatterns are successfully executed.
A Park NX10 AFM was used for AFM nanolithography to draw nanopatterns onto a silicon substrate. AFM topography imaging was conducted after the lithography process to confirm the successful fabrication of the oxide patterns on the surface. A PPP-CONTSCPt cantilever with a nominal spring constant (k = 0.2 N/m) and resonance frequency (f = 25 kHz) was utilized in the experiment.
Results and Discussion
Bias-Assisted AFM Nanolithography
Bias-Assisted AFM Nanolithography is a technique described as material modification (oxidation) through the application of a bias voltage to the AFM tip. The applied bias on the tip results in the generation of an electric field between the tip and the sample. The field ionizes water molecules formed between the tip and the sample, which leads to local anodic oxidation (LAO) used to fabricate nanoscale oxide features on the surface. In the lithography process, the tip acts a nanoscale electrode for charge injection or collection [1, 2]. The thickness of the oxide layer formed on the surface depends on the magnitude of the applied bias and the humidity. A previous study  reported that the thickness of a pre-designed nanoscale oxide pattern of a device structure increased due to the increment of applied tip voltages.
Therefore, the authors concluded the tip voltage influences the amount of oxide formed in the surface. Figure 1 shows a schematic illustration of bias-assisted AFM nanolithography. In this study, Park lithography software was used to design and apply nanolithography to the sample surface. A silicon substrate was used as sample. The application of –10V resulted in a high electric field around the tip, which led to the cleavage of water molecules into H+-, OH– and O– ions . The OH- and O- ions are repulsed by the negatively biased tip and react with silicon to form SiO2. The nanoscale oxide pattern forms on the surface of the silicon substrate along the scan area of the lithography scan.
Figure 2. (a) 3D View, (b) Topography image
The AFM topography image (Figure 2a and b) revealed the nanolithography design is a Christmas ball pattern with diameter of approximately 17 µm and composed of multiple smaller structures with heights ranging from 0.2–1.5 nm.
Here we have demonstrated the use of AFM nanolithography to generate nanoscale oxide patterns on a silicon substrate using a Park NX10 system. The Bias-Assisted technique was used in the AFM nanolithography process. The application of –10 V voltage bias on the tip led to successful fabrication of nanoscale oxide features on the surface. Overall, AFM nanolithography described in this study is an effective method to fabricate next generation materials and devices with nanoscale features.
1. A. Pimpin, et al., Review on Micro- and Nanolithography Techniques and their Applications. Engineering Journal 16(1), 2011: 37-56.
2. S. Hutagalung, et al., Nanoscale Patterning by AFM Lithography and its Application on the Fabrication of Silicon Nanowire Devices. Sains Malaysiana 43(2), 2014: 267–272.
3. R. Garcia, et al., Advanced scanning probe lithography. Nature Nanotechnology 9, 2014: 577–587
4. J. Voves, Nanoelectronics and nanolithography. NanoCon 2009, https://www.researchgate.net/publication/228859246_Nanoelectronics_and_nanolithography
5. A. Bernal et al., Local anodic oxidation on silicon (100) substrates using atomic force microscopy. Dyna 79(174), 2012: 58-61