Low-damage direct patterning of silicon oxide mask by mechanical processing
© Miyake and Yamazaki; licensee Springer. 2014
Received: 4 April 2014
Accepted: 17 May 2014
Published: 29 May 2014
To realize the nanofabrication of silicon surfaces using atomic force microscopy (AFM), we investigated the etching of mechanically processed oxide masks using potassium hydroxide (KOH) solution. The dependence of the KOH solution etching rate on the load and scanning density of the mechanical pre-processing was evaluated. Particular load ranges were found to increase the etching rate, and the silicon etching rate also increased with removal of the natural oxide layer by diamond tip sliding. In contrast, the local oxide pattern formed (due to mechanochemical reaction of the silicon) by tip sliding at higher load was found to have higher etching resistance than that of unprocessed areas. The profile changes caused by the etching of the mechanically pre-processed areas with the KOH solution were also investigated. First, protuberances were processed by diamond tip sliding at lower and higher stresses than that of the shearing strength. Mechanical processing at low load and scanning density to remove the natural oxide layer was then performed. The KOH solution selectively etched the low load and scanning density processed area first and then etched the unprocessed silicon area. In contrast, the protuberances pre-processed at higher load were hardly etched. The etching resistance of plastic deformed layers was decreased, and their etching rate was increased because of surface damage induced by the pre-processing. These results show that etching depth can be controlled by controlling the etching time through 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.
KeywordsNanofabrication Mechanochemical processed layer Atomic force microscopy Natural oxide layer
In nanotechnology, nanoelectric devices and nanomachines can be manufactured by manipulating atoms and molecules. Nanofabrication is one of the most important aspects in the development of nanotechnology. Scanning probe microscopy (SPM) is useful for the nanofabrication of nanometer-scale engineering materials and devices and can be used to realize atomic-scale fabrication. Various attempts have also been made to use SPM techniques for the local modification of surfaces[2–4]. In particular, the local oxidation technique is expected to allow the fabrication of electric devices on the nanometer scale[5–7]. The oxide layers formed by this technique can function as a mask during the etching step or can be used directly as an insulating barrier. In this method, oxidizing agents contained in surface-adsorbed water drift across the silicon oxide layer under the influence of a high electric field, which is produced by application of a voltage to the SPM probe.
Mechanical processing methods that transcribe a tool locus can produce three-dimensional nanoprofiles with high precision by exploiting the tribological properties of the tool geometry and workpiece[8, 9]. If profile processing using mechanical action can be achieved at nanometer scales, the degrees of freedom of the materials that can be used and the range of profiles and sizes of the objects that can be processed will be greatly increased[10–13]. Therefore, the applications of nanofabrication can be expected to be significantly extended through such novel processes[8–13].
Meanwhile, processing methods combining both mechanical and chemical actions have been widely used to machine high-quality surfaces with high precision. Mechanochemical polishing (MCP) uses mechanical energy to activate chemical reactions and structural changes. The processing of highly flat surfaces with few defects has been made possible by this method. Recently, the so-called chemical-mechanical polishing (CMP) has been applied to the fine processing of electronic devices. Further, a complex chemical grinding approach that combines chemical KOH solution etching and mechanical action has been studied. These combined mechanochemical processing methods can achieve high-precision and low-damage machining, simply by using mechanical action to promote reactions with atmospheric gas and surface adsorption layers.
Atomic force microscopy (AFM) is a useful technique for mechanical nanofabrication[8–10]. Mechanical friction methods have been used for the fabrication of silicon nanostructures on H-passivated Si (100) substrate, and the so-called maskless[18, 19] or friction-induced nanofabrication[20–22] has also been proposed. However, the mask patterns formed by these methods are mechanically produced at higher load and stress, damaging the mask surfaces and creating an oxidation layer that decreases the etching rate achieved with KOH solution. As a result, these damages remain on the processed surfaces[18–22].
In our previous study, we proposed a lower damage direct patterning of oxide layers by mechanical processing. Sliding of an AFM diamond tip on a silicon surface forms protuberances under ambient conditions[23–25]. Proper mechanical action without plastic deformation by a sliding diamond tip on a silicon surface results in local mechanochemical oxidation with low damage[23–26]. The resulting oxide masks can be used for pattern transfer during selective wet etching processes[24–28].
Subsequently, by changing the diamond tip sliding scanning density, we realized the control of the etching rate of a silicon surface by KOH solution. We also evaluated the dependence of etching depth on KOH solution etching time. An approach combining mechanical and electrical processes, such as an AFM technique that simultaneously uses a mechanical load and bias voltage, could be developed in the future. Reports on electrical and mechanical nanoprocessing have indicated that this complex approach can produce more electrically resistant layers.
In this study, we attempted to fabricate a nanometer-scale etching mask pattern with low damage and evaluate the chemical resistance properties of the mechanically processed areas. First, we removed the natural oxide layer by diamond tip sliding at low load and then increased the etching rate with KOH solution. Then, at higher load, we formed an etching resistance layer using mechanochemical oxidation. We fabricated protuberances with and without plastic deformation by mechanical processing. Finally, the surfaces were processed at low load and scanning density to remove the natural oxide layer. The dependence of the KOH solution etching depth of these processed areas on etching time was also investigated.
The specimens were n-type Si (100) wafers. The samples were exposed in a clean atmosphere to allow their surfaces to become covered with a natural oxide layer less than 2 nm thick. First, mechanical processing was performed using diamond tip sliding with an AFM under atmospheric conditions at room temperature and humidity ranging between 50% and 80%.
Dependence of KOH solution etching on load and scan density of mechanical pre-processing
KOH solution etching of the pre-processed silicon substrate with 10 wt% KOH solution at 20°C ± 3°C was performed on the AFM apparatus. After etching, the specimen was washed with distilled water, and the profile changes caused by the etching were then evaluated at the same positions using the same diamond tip as the processing tool.
Dependence of additional KOH solution etching on etching time
Results and discussion
Dependence of KOH solution etching on mechanical pre-processing owing to the removal of the natural oxide layer
Therefore, with 256 scanning cycles, mechanical pre-processing at a load of 1 to 4 μN was effective in increasing the etching rate. Over 8-μN load, mechanical pre-processing was effective in forming an etch-resistant layer on the Si surface.
To clarify the mechanism of the mechanical removal and formation of this etch-resistant layer, the surface contact stress was evaluated using the boundary element method. The dependences of the maximum principal and shear stresses on load were estimated for 100-nm-radius diamond tips. The 1- to 4-μN-load range corresponds to a contact pressure of 6.9 and 10.9 GPa. Therefore, it can be concluded that this contact pressure range is suitable for the removal of the natural oxide layer on a silicon surface at low-density scanning.
Silicon fractures under tensile stress at a certain load. In maximum tensile stress areas, silicon bond breakage appears to stem from tensile stress caused by diamond tip friction. Therefore, the reaction of silicon may take place at the rear edge of the sliding contact area where the elongation stress is the highest. At loads of over 8 μN, protuberance height increased rapidly at 13.8-GPa contact pressure and 1.8-GPa tensile stress. Therefore, this protuberance-related phenomenon occurred through a mechanochemical reaction where adsorbates, such as water and oxygen, reacted with the silicon. The local destruction of interatomic bonds seems to increase at over 6 μN because of the concentrated stress and reaction of the newly formed surface with surrounding materials. This boundary load that increases and decreases the etching depth is nearly 6 μN. At this load, the contact pressure and tensile stress are 12.5 and 1.5 GPa, respectively.
Additional KOH solution etching of processed protuberances with and without plastic deformation
As mechanical pre-processing, protuberances with and without plastic deformation were processed at 10- and 40-μN loads. It was found that less surface damage occurred than that due to plastic deformation during the nanoprocessing on Si. The shear stress was evaluated to estimate the plastic deformation of the silicon, and the effect of the evaluated contact stress on protuberance height and groove depth was studied[27, 28].
To understand the dependence of the relative etching depth on etching time, the pre-processed and unprocessed areas were etched with KOH solution for 10, 15, 20, 25, 30, and 40 min. No significant change in the topography of the surface was observed even after 10- and 15-min etching. The heights of the protuberances were slightly increased to 2.3 and 3.4 nm at 10 and 40 μN, respectively.
From 35 to 40 min, the etching depths of both the unprocessed and 1.5-μN-load pre-processed areas were larger than those of the areas processed at higher load. The area mechanically pre-processed at higher load exhibited resistance to etching owing to mechanochemical oxidation layer formation.
To realize the nanofabrication of a Si substrate, the etching depths obtained with KOH solution were controlled using mechanical pre-processing under various loads and scanning density conditions. Removal and formation of the oxide etching mask was performed on silicon surfaces using atomic force microscopy.
Areas mechanically pre-processed at 1- to 4-μN load exhibited an increased KOH solution etching rate due to the removal of the natural oxide layer by the mechanical action. The dependence of etching depth on pre-processing load and scanning density was clarified. At every scanning density, there were certain load ranges within which the etching depth increased. In contrast, protuberances with a thick oxide layer produced by mechanical pre-processing at higher load suppressed etching. This mechanochemical oxide layer had superior etching resistance to that of the natural oxide layer.
Protuberances were processed on the Si surfaces under stress conditions both lower and higher than that where plastic deformation occurs. These processed areas were hardly etched by the KOH solution. For protuberances with plastic deformation, the damaged layers were more easily etched than those without plastic deformation. Protuberance formation without plastic deformation by mechanical pre-processing can realize less damaged mask patterning. Additionally, areas at pre-processed low load and scanning density were easily etched. This implies that the various profiles obtained were possibly fabricated by the changing load and scanning density of the mechanical pre-processing and by additional KOH solution etching. With the removal of the natural oxide layer and formation of a mechanochemical oxide layer without plastic deformation, the etching depth can be controlled by changing the etching time. This therefore allows us to fabricate low-damage grooves of various depths.
This research was performed with the help of our graduate students at Nippon Institute of Technology.
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