Nobuyuki Nakagiri

Potential shielding by the surface water layer in Kelvin probe force microscopy

Kelvin probe force microscopy (KFM) was applied to two-dimensional profiling of silicon pn-structures covered with a 2 nm-thick oxide layer. The surface potential contrast between the p- and n-type regions depended on the hydrophobicity of the oxide surface when KFM imaging was conducted in air with a relative humidity of more than 50%. By decreasing the density of surface hydroxyl groups on the oxide layer through thermal annealing, the potential contrast between the p- and n-type regions increased. While there was no detectable contrast on samples covered with hydrophilic oxide with a water contact angle of almost 0°, contrast increased to greater than 50 mV on the samples covered with hydrophobic oxide with a water contact angle of about 80°. However, when KFM imaging was conducted in a dry nitrogen atmosphere with relative humidity less than 0.6%, a clear potential contrast of about 50 mV could be acquired even on samples covered with the hydrophilic oxide layer. Since samples with less adsorbed water...

Chemical Approach to Nanofabrication: Modifications of Silicon Surfaces Patterned by Scanning Probe Anodization.

Scanning probe microscope-induced local oxidation of a material surface with adsorbed water is a recent nanolithographic technology. We applied this technique to the nanoscale patterning of hydrogen-terminated silicon (Si-H) surfaces. Using the silicon oxide (SiO x ) patterns as masking, examples of two types of pattern transfer method through area-selective chemical modification were demonstrated. Nanostructures of substrate Si or deposited gold were fabricated by wet chemical etching or electroless plating, respectively. These area selectivities arose from the difference in surface chemical reactivities between anodic SiO x and the surrounding Si-H. The oxidation chemistry is discussed in terms of anodization.

Nanoscale patterning of an organosilane monolayer on the basis of tip‐induced electrochemistry in atomic force microscopy

An organosilane trimethylsilyl (TMS) monolayer prepared on silicon (Si) substrate by chemical vapor deposition was successfully applied as a self‐developing resist for atomic force microscope (AFM) lithography. The thickness of the monolayer was less than 1 nm. This resist was locally degraded due to electrochemical reactions induced in the junction between a conductive AFM probe and a Si–TMS sample. The generated pattern on the sample was then transferred to the Si substrate by chemical etching using the degraded region as an etching window. Degradation of the monolayer proceeded with both positive and negative sample biases. However, the absolute values of the voltage at which the probe‐scanned region began to show etching were +3.0 for Vs>0 and −5.0 V for Vs<0, in a 60% relative humidity air atmosphere. Faster patterning was achieved through increased current flow by applying a higher bias voltage. A 500 μm/s line drawing at Vs=+20.0 V with 2–3 nA was obtained. The number of injected electrons was esti...

Scanning probe anodization: Nanolithography using thin films of anodically oxidizable materials as resists

We report scanning probe microscope (SPM) nanolithography that uses thin films patternable by scanning probe anodization as resists. An organosilane monolayer composed of trimethylsilyl [−Si(CH3)3] groups was prepared by chemical vapor deposition on a Si substrate. It was then patterned through the localized degradation of the monolayer as a result of anodic reactions induced beneath a SPM tip. The fabricated patterns on the resist were transferred into the substrate Si by area‐selective chemical etching in an aqueous NH4F/H2O2 solution. Consequently, grooves narrower than 100 nm were successfully fabricated. We have also developed another resist system for SPM lithography that is compatible with a wide range of surfaces, including insulators. The resist consisted of a thin Ti film covered with a fluoroalkylsilane monolayer. The monolayer was degraded along a tip‐scanning trace. This patterned resist was etched in a dilute HF solution. The chemical etching of the underlying Ti film proceeded selectively i...

Measurement of the Resistance of Manganin under Liquid Pressure to 100 kbar and Its Application to the Measurement of the Transition Pressures of Bi and Sn

The electrical resistance of manganin wire and the X-ray diffraction of NaCl were measured simultaneously under hydrostatic pressures up to 100 kbar using a cubic-anvil type pressure apparatus at room temperature. The solidification pressures of 1: 1 pentane-isopentane and isopropanol were determined by taking advantage of the sensitivities of the resistance of manganin wire to the local stress. The transition pressures for BiIII↔V and SnI↔II were measured in both loading and unloading by detecting the pressure where the high-pressure phase and the low-pressure phase coexist.

AFM lithography in constant current mode

An organosilane monolayer consisting of trimethylsilyl ([ - ) groups prepared on the native oxide of a silicon substrate effectively served as a resist material for AFM-based nanolithography. The patterning of this resist was performed through its electrochemical degradation locally induced around the contact point of a conductive AFM probe while biasing the sample substrate positively. In the region where the probe passed, the monolayer resist was degraded and the underlying silicon oxide surface was selectively uncovered. The number of electrons injected into the probe-scanned region was controlled by conducting the AFM lithography in constant current mode. By means of this constant current AFM lithography a sufficient amount of electrons could be injected even at high probe-scan rates faster than . It was demonstrated that the TMS monolayer resist was sensitive enough to allow line drawing at a probe scan rate of .

Maskless patterning of silicon surface based on scanning tunneling microscope tip‐induced anodization and chemical etching

A microprocessing method for silicon (Si) without photolithography is proposed. The method consists of only two processes. Hydrogen‐terminated Si surfaces (Si‐H) were first locally anodized using scanning tunneling microscopy (STM). The non‐anodized surfaces were then etched chemically in potassium hydroxide solution. The anodic oxide produced with the first process performed as an etching mask. The height of the etched pattern of approximately 50 nm was much larger than the thickness of the anodic oxide. Humidity effect on STM tip‐induced anodization of Si‐H is also shown. The area of the anodization was enlarged with increasing humidity, and the spatial resolution became worse.

Area‐selective formation of an organosilane monolayer on silicon oxide nanopatterns fabricated by scanning probe anodization

An organosilane monolayer was formed from a precursor vapor (trimethylchlorosilane, TMCS) onto the surface of silicon oxide (SiOx) nanopatterns surrounded by hydrogen‐terminated silicon (Si–H). The nanopattern was fabricated with a resolution of 20 nm by scanning probe anodization, that is, localized anodization induced beneath the tip of a scanning probe microscope. The area‐selectivity arises from the difference in the chemical reactivity of the vapor between the SiOx and Si–H surfaces. The TMCS‐coated SiOx patterns showed a resistivity to chemical etching with an aqueous solution of ammonium fluoride and hydrogen peroxide. Lateral force microscopy with an organosilane‐coated probe also indicated the presence of the monolayer on SiOx through a friction force contrast between the monolayer‐coated and uncoated regions.

Surface potential contrasts between silicon surfaces covered and uncovered with an organosilane self-assembled monolayer.

Surface potentials of Si substrates covered with a organosilane self-assembled monolayers (SAMs) were measured with reference to the substrate uncovered with the SAM using Kelvin probe force microscopy. Based on a photolithographic technique, the reference surface was prepared in a micrometer scale on each of the samples. SAMs were prepared from n-octadecyltrimethoxysilane [ODS: CH3(CH2)17Si(OCH3)3], 3,3,3-trifluoropropyltrimethoxysilane [FAS3: CF3(CH2)2Si(OCH3)3], heptadecafluoro-1,1,2,2-tetahydro-decyl-1-trimethoxysilane [FAS17: CF3(CF2)7(CH2)2Si(OCH3)3] or n-(6-aminohexyl) aminopropyltrimethoxysilane [AHAPS: H2N(CH2)6NH(CH2)3Si(OCH3)3) by chemical vapor deposition. Potentials of the surfaces covered with ODS-, FAS3- and FAS17-SAMs became more negative than the uncovered Si substrate, while the surface covered with AHAPS-SAM showed a more positive surface potential than the reference. The potential contrasts of these SAMs to the reference were -25, -170, -225 and +50 mV for ODS-, FAS3-, FAS17- and AHAPS-SAMs, respectively. These results almost agreed with potentials expected from the dipole moments of the corresponding precursor molecules estimated by ab initio molecular orbital calculation, except for FAS3-SAM. Despite FAS3 molecule having a larger dipole moment than FAS17 molecule, the surface potential contrast of FAS3-SAM was smaller than that of FAS17-SAM, since surface coverage of FAS3-SAM was relatively incomplete compared with the other SAMs.

Kelvin Probe Force Microscopy Images of Microstructured Organosilane Self-Assembled Monolayers.

A difference in surface potentials between organosilane self-assembled monolayers (SAMs) was measured by means of Kelvin probe force microscopy (KFM). The SAMs were deposited on a single Si substrate from alkylsilane, that is, octadecyltrimethoxysilane (ODS) [H3C(CH2)17Si(OCH3)3], and fluoroalkylsilane (FAS), that is, heptadecafluoro-1,1,2,2-tetrahydro-decyl-1-trimethoxysilane [F3C(CF2)7(CH2)2Si(OCH3)3]. The locations of these SAMs on the substrate were defined by means of a photolithographic technique. The regions terminated with ODS and FAS were clearly distinguished by KFM with the surface potential difference between the ODS- and FAS-terminated surfaces under optimized imaging conditions. The surface potential of the FAS-terminated region was 170~180 mV lower than the potential of the ODS-terminated surface. The origin of such a low surface potential of FAS-SAM was ascribed to the larger dipole moment of the FAS molecule induced by the electron negativity of F atoms as estimated from a molecular orbital calculation.