Repeatable change in electrical resistance of Si surface by mechanical and electrical nanoprocessing
© Miyake and Suzuki; licensee Springer. 2014
Received: 14 July 2014
Accepted: 23 August 2014
Published: 31 August 2014
The properties of mechanically and electrically processed silicon surfaces were evaluated by atomic force microscopy (AFM). Silicon specimens were processed using an electrically conductive diamond tip with and without vibration. After the electrical processing, protuberances were generated and the electric current through the silicon surface decreased because of local anodic oxidation. Grooves were formed by mechanical processing without vibration, and the electric current increased. In contrast, mechanical processing with vibration caused the surface to protuberate and the electrical resistance increased similar to that observed for electrical processing. With sequential processing, the local oxide layer formed by electrical processing can be removed by mechanical processing using the same tip without vibration. Although the electrical resistance is decreased by the mechanical processing without vibration, additional electrical processing on the mechanically processed area further increases the electrical resistance of the surface.
KeywordsElectrical resistance Atomic force microscopy Nanoprocessing Mechanical processing
In the future, a wide variety of nanoelectronic devices and nanomachines will be manufactured using nanofabrication techniques. Scanning probe microscopy (SPM) techniques are expected to be effective for the nanofabrication of nanometer-scale materials and devices via the atomic-scale processing of nanostructures. Various attempts have also been made to use SPM techniques for the local modification of surfaces[2–5]. The so-called local oxidation technique is being investigated for the fabrication of devices on the nanometer scale[6–8]. In this method, the oxidizing species contained in the water layer adsorbed on the surface drift across the oxide layer under the influence of a high electric field produced by a voltage applied to the probe. This SPM-generated oxide layer can function as a mask for etching or can be used directly as an insulating barrier.
Nanometer-scale protuberances and grooves can be produced on a silicon surface by diamond tip sliding in the atmosphere using an atomic force microscopy (AFM). In some cases, when a diamond tip is slid on Si, the surface protuberates. This upheaval phenomenon has been applied to processing, and the nanoprotuberance characteristics and formation mechanism were evaluated[9, 10]. Areas on a silicon (100) surface nanoprocessed by mechanical local oxidation through diamond tip sliding without a bias voltage have been found to act as an etching mask towards aqueous potassium hydroxide (KOH) solution[11–13]. Mechanical processing-induced mask patterns formed of such plastically deformed damaged layers are able to withstand selective wet etching processes for pattern transfer, resulting in the so-called maskless patterning[14, 15] or friction-induced fabrication[16–18], and their formation mechanisms have been evaluated.
In our previous work investigating the fabrication of silicon nanostructures by mechanical local oxidation, silicon (100) specimens were processed by diamond tip sliding at low and high scanning densities to control their subsequent rate of etching by KOH solution. Processing at a low scanning density resulted in the removal of the natural oxide layer by the mechanical action. The thick mechanochemically oxidized layer formed suppressed etching by the KOH solution and decreased the etching rate without plastic deformation[19, 20]. These results showed that etching depth can be controlled using etching time via natural oxide layer removal and mechanochemical oxide layer formation. These oxide layer removal and formation processes can be exploited to realize low-damage mask patterns.
An understanding of the electrical properties of areas processed by various mechanical and electrical processing methods is important to investigate their mechanism and to improve nanoprocessing techniques. Conductive atomic force microscopy (CAFM) can be used to obtain the local conductivity distribution of a surface. We previously evaluated the profiles and electrical properties of mechanically and electrically processed areas on silicon using an electrically conductive diamond tip with and without electric voltage. A dense oxide layer was obtained after complex processing involving both mechanical and electrical processing, and the resulting decrease in current was found to be more significant than that caused by mechanical or electrical oxidation alone. If repeatable change in the electrical resistance of Si surfaces were to be performed by combining such mechanical and electrical processing techniques, the method would find applications in the future nanodevice fabrication processes and nanolithography technology. For example, reproducible repeatedly recordable AFM memory could be fabricated, and direct formation and correction of circuit patterns for various electronic devices could be performed.
In this study, we attempted to alter the electrical resistance of a Si surface by forming and removing its oxide layer using electrical and mechanical processing. The modifications were performed by scanning the Si surface with an electrically conductive diamond tip using the mechanical and electrical methods with and without vibration. The profile, electric current, and friction distributions of the mechanically and electrically processed areas were characterized. The effects of vibration on (changes in) the topography, electric current, and friction distributions of the processed areas were discussed. Using the results, we used sequential electrical and mechanical processing to realize the change of electrical resistance due to the formation and removal of the oxide layer.
Results and discussion
Effect of load on mechanical processing
Effect of applied voltage on electrical processing
From these results, mechanical processing allows the removal and formation of oxide layers on the surface of Si. The electrical resistance of the surface can be changed by mechanical processing without and with vibration.Figure 6b shows the average difference of currents calculated from the mean current of each processed area minus the current of the unprocessed area. The increase in the current of the areas mechanically processed without vibration was as large as 10 to 25 nA; the current fluctuated and was unstable. In contrast, the mean current change of the electrically processed area was nearly -1.3 nA. The decrease in the current of the area mechanically processed with vibration was -2.5 nA, larger than that of the areas electrically processed with the same vibration amplitude.
Sequential mechanical processing of electrically processed areas
The thickness of the reacted oxide layer can be estimated. Assuming that the electrical processing produced a uniform SiO2 layer, the ratio of the thickness of the SiO2 and Si layers should be equal to the volume ratio of the SiO2 and Si molecules per 1 mol. Therefore, an x/0.44-nm-thick SiO2 layer is generated from an x-nm-thick Si substrate. The total thickness of the SiO2 layer is also equal to the sum of the protuberance height and the thickness of the reacted Si. From the average wear depth shown in Figure 7a, the height of the protuberance was nearly 0.54 nm. Thus, the thickness of the SiO2 layer should be nearly (0.54 + x) nm, where 0.44 (0.54 + x) = x. The thickness of the reacted Si layer x was thereby determined to be 0.42 nm. This 0.42-nm-thick Si layer was then converted into a 0.96-nm-thick SiO2 layer by anodic oxidation. In fact, the layer formed as the result of the anodic oxidation contained SiO and damage; therefore, the actual thickness of this layer appeared to be deeper than 0.42 nm. In contrast, with additional mechanical processing, the depth of the electrically processed area became nearly 0.74 nm, thinner than that of the reacted SiO2. Thus, it can be concluded that the diamond tip did not remove the total reactive layer. This means that the oxide layer formed by the electrical processing remained even after mechanical processing at 2,000 nN load. In contrast, the removal of the natural oxide layer of unprocessed areas by additional low-load mechanical processing increases the surface current.
It was confirmed that electrically processed areas can be deeply removed by subsequent mechanical processing without vibration. Additional electrical processing caused protuberance at the center of the mechanically processed area; however, the height was as low as <1 nm. It is clear that protuberances formed by electrical processing can be removed by mechanical processing and then reformed by subsequent additional electrical processing. The low height protuberance created inside the mechanically processed area was formed by anodizing.
The currents measured after the electrical processing in the first and third steps were decreased compared with that of the unprocessed surface owing to the anodic oxidation layer formed by the electrical processing. In contrast, the current of the area mechanically processed without vibration was remarkably higher than that of the unprocessed area. Thus, the oxidation layer formed during the first electrical processing was removed by the mechanical processing without vibration, increasing the current. The subsequent electrical processing carried out at the center caused an oxidation layer to reform and decreased the current. Thus, a layer of high electrical resistance can be repeatedly formed and removed by the present processing methods.The friction force increased rapidly at the edge of the groove of the mechanically processed area as shown in Figure 8c, similar to that in Figure 3c. The diamond tip slid at the edge part of the groove, and it is thought that the friction increased owing to the sudden change in shape. The friction of the electrically processed area increased with its detail, and the friction force of the last electrically processed area of the central part was the highest.
Nanometer-scale groove and protuberance can be processed by mechanical processing with a sharp and conductive diamond tip without and with lateral vibration, respectively. By mechanical processing without vibration, grooves are formed because of plastic deformation. The natural silicon oxide layer on the surface is removed by the mechanical processing without vibration. Therefore, electric resistance on the surface decreases. In contrast, an oxide layer is formed on the Si surface because of lateral vibration by reacting water or oxygen in the surrounding environment by the mechanochemical reaction. The processed surface protuberates, and their electric resistance increases. Removal and additional oxidization of the surface by mechanical processing can be realized.
By electrical processing, the processed surface protuberates and the current decreases by the formation of the oxide layer because of anodization by applying a positive voltage to the silicon. The protuberance height increases with the increase of the applied voltage and the amplitude of the lateral vibration. Decreasing rate in the current of processed surface that was anodized by electrical processing saturated at a certain applied voltage.
For sequential processing, the resistivity increases by electrical processing because of anodic oxidation. Then, it is possible to recover the conductivity of the electrically processed area because of removal of the oxide layer by mechanical processing without vibration. The protuberance composed of the oxidation layer processed by the electrical processing can be removed by an appropriate mechanical processing without vibration. The electric resistance of the mechanically processed area can then be increased because of oxidization by additional electrical processing. Repeatable changes of electrical resistance can be performed.
This research was performed with the help of the graduate students at Nippon Institute of Technology and was supported by the Storage Research Consortium (SRC) of Japan.
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