Contact AFM Nanolithography Based on Anodic Oxidation
Armando Melgarejo, Ben Schoenek, Jiali Zhang, and Byong Kim* Park Systems, Inc., Santa Clara, CA
*
byong@parksystems.com
Abstract: In this report, atomic force microscopy (AFM) nanolithography and electrochemical anodic oxidation using bias- assisted lithography are used to created oxide patterns on the surface of a silicon substrate. Non-contact mode imaging was conducted after the lithography process to confirm the successful fabrication of oxide patterns on the surface as well as to distinguish the surface difference between the oxide layers and silicon substrate. With only a few seconds of run time, oxide patterns as narrow as 35 nm in width were created illustrating that the bias mode in the Park Scientific SmartLitho software can be used to generate well-defined nanoscale patterns and features.
Keywords: nanotechnology, lithography, anodic oxidation, atomic force microscopy
silicon,
Introduction Te field of nanotechnology has diversified into different
areas of research, from materials science to biotechnology. Many of these applications are based on the ability to fabricate or manipulate nanostructured materials [1]. One convenient technique for structuring, manipulation, and fabrication at
the nanometer scale is atomic force microscopy (AFM)
nanolithography. All AFM nanolithography techniques can be classified into two general groups in terms of their operational principles: force-assisted nanolithography and bias-assisted nanolithography [2]. Te force-assisted method consists of applying a large force to a sharp tip to mechanically modify the surface atoms or molecules of the sample and produce trenches on the surface. In this case, the interaction between the tip and the sample is purely mechanical [2]. On the contrary, the bias- assisted method entails applying a voltage between the AFM tip and the substrate in contact with the sample. Te tip-sample voltage induces an electrochemical reaction that produces oxide on the surface of the substrate [1]. Tis article demonstrates an electrochemical process called
anodic oxidation using the bias-assisted lithography method to create oxide patterns on the surface of a silicon substrate. Te success of this technique relies on using the AFM tip as a biased cathode to the sample surface. Also, the water meniscus around the sample acts as an electrolyte for the chemical reaction. Te environment humidity directly influences the size of the meniscus [1]. Below we report nanoscale oxide line formation using AFM anodized lithography.
Materials and Methods For this experiment, a Park NX10 AFM was used to perform
bias-assisted nanolithography to draw nanopatterns onto a bare silicon substrate. Te oxide patterns were formed using Park SmartLitho™ soſtware [3]. Non-contact mode imaging was conducted aſter the lithography process to confirm the success- ful fabrication of the oxide patterns on the surface as well as to distinguish the surface difference between the oxide layers and
12 doi:10.1017/S1551929520001571
silicon substrate. A conductive AFM cantilever probe (Multi 75G) with a nominal spring constant (k = 3 N/m) and resonance frequency (f = 75 kHz) was used in these experiments. Te Park SmartLitho soſtware consists of five windows
and panels that allow the operator to control the complete lithography process (Figure 1). In the first step, the appropriate alignment of the super-luminescent diode (SLD) and position- sensitive photodetector (PSPD) is verified (Figure 1A). Aſter confirming the correct alignment,
the operator selects the
control mode; in this case, the setpoint mode was selected. For the anodic oxidation, a bias has to be applied; in this case, the tip was chosen as the desired channel (Figure 1B). In the design area (Figure 1C), the operator can insert the desired figures and shapes using a previous AFM image as a baseline. All embedded figures appear in the object list (in this case, just one line) (Figure 1D). Te operator can change the formation order, and the estimated lithography time for each figure is provided. Finally, for any desired feature, parameters can be changed in the object edit panel (Figure 1E). Parameters such as voltage, load force, stroke speed, extend speed, or liſt speed can be modified to obtain different features within the same designed figures. To start a lithography or manipulation process, a baseline image is needed. In this case,
the baseline image already
contains one previously drawn line to compare with the line about to be drawn (Figure 1C). Te image is 1.5 μm by 1.5 μm. Te sample features small nanoparticles on its surface, according to the image taken using non-contact AFM mode. Te lithography process is planned by drawing a shape on
the surface using the soſtware. Te AFM operating soſtware, Park SmartScan™, provides the operator with offset and easy positioning features. Te DC bias lithography process is done in contact mode, and in this case, a bias of negative 5.0 volts is applied between the tip and sample. For this experiment, a line was drawn at 0.2 μm/s with a 200 nN load force. Te drawn line is approximately 1.8 μm in length (Figure 2, top leſt). Te process takes a couple of seconds to complete. As the cantilever is compressed to the loading force of 200 nN, the PSPD signal moves up, which visually verifies the applied pressure.
Results and Discussion Following the lithography process and using Park
SmartScan, the system is switched back to non-contact mode. An AFM cantilever probe is mounted on a piezoelectric bimorph shaker, attached to the AFM head Z piezoelectric scanner. Te shaker vibrates the AFM cantilever probe at its resonant frequency, which is automatically selected along with the vibration amplitude. As the probe nears the sample surface, the vibration amplitude decreases. Likewise, as the probe moves away from the surface, the amplitude increases. Te Z piezo
www.microscopy-today.com • 2020 November
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