HF-treated Silicon Research Articles

Resistless EUV lithography: Photon-induced oxide patterning on silicon

In this work, we show the feasibility of extreme ultraviolet (EUV) patterning on an HF-treated silicon (100) surface in the absence of a photoresist. EUV lithography is the leading lithography technique in semiconductor manufacturing due to its high resolution and throughput, but future progress in resolution can be hampered because of the inherent limitations of the resists. We show that EUV photons can induce surface reactions on a partially hydrogen-terminated silicon surface and assist the growth of an oxide layer, which serves as an etch mask. This mechanism is different from the hydrogen desorption in scanning tunneling microscopy-based lithography. We achieve silicon dioxide/silicon gratings with 75-nanometer half-pitch and 31-nanometer height, demonstrating the efficacy of the method and the feasibility of patterning with EUV lithography without the use of a photoresist. Further development of the resistless EUV lithography method can be a viable approach to nanometer-scale lithography by overcoming the inherent resolution and roughness limitations of photoresist materials.

Surface-dependent chemical properties of silicon nanowires: The acceleration of copper oxidation

HF-treated silicon nanowires exhibited surface-dependent chemical property and accelerated the oxidation rate of copper with the enhancement factor of 10 000, confirmed by x-ray diffraction semi-quantitative analysis. This unexpected oxidation characteristic would make HF-treated silicon nanowires valuable for catalysis applications.

Spectroscopic Changes of Silicon Nanowires Induced by Femtosecond Laser Pulses

Abstract When HF-treated silicon nanowires (SiNWs) were irradiated by femtosecond laser pulses, three new energy transitions appeared in the UV region, which might be attributed to defect-related deep levels, especially oxygen and silicon vacancies rather than their morphology and structure changes.

On the growth and microstructure of carbon nanotubes grown by thermal chemical vapor deposition.

Carbon nanotubes (CNTs) were deposited on various substrates namely untreated silicon and quartz, Fe-deposited silicon and quartz, HF-treated silicon, silicon nitride-deposited silicon, copper foil, and stainless steel mesh using thermal chemical vapor deposition technique. The optimum parameters for the growth and the microstructure of the synthesized CNTs on these substrates are described. The results show that the growth of CNTs is strongly influenced by the substrate used. Vertically aligned multi-walled CNTs were found on quartz, Fe-deposited silicon and quartz, untreated silicon, and on silicon nitride-deposited silicon substrates. On the other hand, spaghetti-type growth was observed on stainless steel mesh, and no CNT growth was observed on HF-treated silicon and copper. Silicon nitride-deposited silicon substrate proved to be a promising substrate for long vertically aligned CNTs of length 110–130 μm. We present a possible growth mechanism for vertically aligned and spaghetti-type growth of CNTs based on these results.

Excellent Photocatalysis of HF-Treated Silicon Nanowires

HF-treated silicon nanowires exhibited excellent photocatalysis, which were even better than some noble metal-modified ones, such as palladium, gold, silver, and rhodium. This phenomenon was critical in the application of silicon-related materials as they are normally employed as a catalyst carrier. These HF-treated silicon nanowires were also stable in solution over 1 week; consequently, a possible explanation for the stability was proposed.

Surface functionalization of HF-treated silicon nanowires

Versatile methods were employed to investigate the chemical reactivity of hydrogenterminated surface of silicon nanowires. The experimental results showed that coupling reaction took place when silicon nanowires reacted with 2,2,2-trifluoroethyl acrylate, and reductive deposition reaction occurred in the presence of inorganic salt such as HgCl2, and also co-reduction reaction took place in a solution containing both AuCl3 and PdCl2. The possible reaction mechanisms were studied and this study would be expected to favour the homogeneity, selectivity, reproducibility, and stability of SiNW devices or sensors.

Influence of Electrostatic Charge on Recombination Lifetime and Native Oxide Growth on HF-Treated Silicon Wafers

This paper reports on the influence of electrostatic charge on the time-dependent change in the hydrogen-terminated surface of HF-treated silicon wafers. The change in the hydrogen-terminated silicon surfaces were evaluated from the changes in recombination lifetime and the rate of native oxide growth. It is known that electrostatic charges are generated on silicon wafers by wet processing, such as HF treatment. This study notes that the electrostatic charge on wafers is modified over time to achieve an equilibrium, resulting from the interactions between charged wafers and the ambient. Using these characteristics of the electrostatic charge, the conditions under which changes in recombination lifetime and native oxide growth could be controlled were found. Furthermore, this mechanism was considered to influence the electrostatic charge on the band structure of p-type silicon. Using this phenomenon, native oxide growth on a silicon surface could be suppressed and HF treatment would provide a simple surface passivation method for recombination lifetime measurement.

Covalent Attachment of Organic Monolayers to Silicon Carbide Surfaces

This work presents the first alkyl monolayers covalently bound on HF-treated silicon carbide surfaces (SiC) through thermal reaction with 1-alkenes. Treatment of SiC with diluted aqueous HF solutions removes the native oxide layer (SiO2) and provides a reactive hydroxyl-covered surface. Very hydrophobic methyl-terminated surfaces (water contact angle theta = 107 degrees ) are obtained on flat SiC, whereas attachment of omega-functionalized 1-alkenes also yields well-defined functionalized surfaces. Infrared reflection absorption spectroscopy, ellipsometry, and X-ray photoelectron spectroscopy measurements are used to characterize the monolayers and show their covalent attachment. The resulting surfaces are shown to be extremely stable under harsh acidic conditions (e.g., no change in theta after 4 h in 2 M HCl at 90 degrees C), while their stability in alkaline conditions (pH = 11, 60 degrees C) also supersedes that of analogous monolayers such as those on Au, Si, and SiO2. These results are very promising for applications involving functionalized silicon carbide.

Atomic Level Analysis of Carbon and Silicon by a Scanning Atom Probe

Carbon nanotubes, chemical vapor deposition (CVD) diamond, high-temperature-high-pressure (HPHT) diamond, vitreous carbon, graphite and silicon are analyzed by a scanning atom probe (SAP). All specimens contain a large amount of hydrogen. The CVD diamond grown in hydrogen exhibits the highest hydrogen concentration with a ratio of hydrogen to carbon atoms (H/C) of 2.02, and vitreous carbon has the lowest hydrogen concentration with a H/C ratio of 0.25. Significant difference is noticed in the hydrogen desorption energy among analyzed CNTs. The [111]-oriented silicon wafers are chemically etched by HF and NH4F. Although a few analyzed areas are hydrogen-covered and stay clean even when exposed to air, most areas are contaminated with hydrogen, oxygen and carbon. The carbon concentration in the HF-treated silicon is found to be slightly higher than that in the NH4F-treated silicon. An unexpected finding is the shift of the most abundant Si-H clusters from Si4H+ to Si2H+ with the depth of the analyzed area of the NH4F-treated silicon.

Effect of the oxidation process on the luminescence of HF-treated porous silicon

Abstract Porous silicon (PS) films were prepared by lateral anodization of crystalline silicon in HF based solutions at different current densities. The oxidation mechanism of the HF-treated PS films has been monitored by means of photoluminescence (PL) and Fourier transform infra-red vibrational studies. HF treatment of PS films for a short duration of ∼10 s resulted in the increment of polysilane/hydride species on the surface of PS rather than any change in the dimensions of the silicon-crystallites. It has been found that HF-treated PS surfaces are relatively stable against oxidation as compared to untreated PS films. Upon oxidation of the HF-treated PS films, the PL intensity initially increases as a result of reduction in crystallite size to exhibit quantum size effects and then decreases owing to loss of luminescing structures due to over-oxidation of the Si-columns. We infer that the surface passivation either by hydrogen or oxygen is one of the requisite conditions for obtaining strong PL efficiency in PS. It seems that more than one emission mechanisms are responsible to explain the luminescence properties of PS.

Local modification of HF-treated silicon (100) surface and its characterization by scanning tunneling microscopy and spectroscopy

The modification of silicon (100) surfaces on samples etched with HF and without rinsing in de-ionized H2O is produced and studied by scanning tunneling microscopy and spectroscopy. The dependence of the size of the modification with the tunnel current and the voltage between tip and sample is explained from the evaluation of the electrical field between the tip and the silicon surface, starting from the planar metal–insulator–semiconductor theory. Current image tunneling spectroscopy data allow to clarify the nature of the modification, concluding that an electrochemical change on the surface of the sample has been produced.

X-ray photoelectron spectroscopic studies of the chemical nature of as-prepared and NaOH-treated porous silicon layer

The effect of various surface treatments of a porous silicon layer (PSL) including etching in NaOH solution on the chemical nature of the surface was studied using x-ray photoelectron spectroscopy. As-prepared PSL, which is formed by anodic oxidation of silicon in ethanolic HF solution, is covered with silicon oxide. NaOH treatment removes the surface oxide almost completely. Chemical states of surface silicon at the NaOH-treated PSL surface are very close to that at the HF-treated silicon surface. Surface oxygen on the NaOH-treated PSL surface seems to be in the form of Si-OH.

The Effect of Illumination on the Contact Angles of Pure Water on Amorphous Silicon

Contact angle determinations were made to allow us to study the effect of the wavelength of illumination on the surface of hydrogenated amorphous silicon thin films. The films were prepared by the PECVD technique from pure silane under conditions which give good quality material. Advancing and receding contact angles of water drops, on hydrofluoric acid etched and nonetched silicon surfaces, were measured as a function of drop volume. Large differences in the contact angles were observed depending on the wavelength of the incident light. The contact angles on the HF-treated silicon surface under violet light yielded the lowest values, while green light yielded the highest values. However, in the case of nontreated silicon surfaces, green light yielded the lowest contact angles and red light the highest values. These surface phenomena, related to the surface photovoltage effect, were seen to be completely reversible.

Modification of HF-treated silicon (100) surfaces by scanning tunneling microscopy in air under imaging conditions

The modification of HF-etched silicon (100) surface with a scanning tunneling microscope (STM) operated in air is studied for the first time in samples subjected to the standard HF etching without the follow-up rinsing in H2O. The modifications are produced in air under normal STM imaging conditions (Vt=−1.4 V and It=2 nA). The simultaneous acquisition of topographical, current image tunneling spectroscopy and local barrier-height images clearly shows that the nature of the modification is not only topographical but also chemical. The features produced with a resolution better than 25 nm are attributed to a tip-induced oxidation enhanced by the presence of fluorine on the surface.

Hydrogen Desorption Behaviors of HF-Treated Silicon Wafer

Hydrogen desorption behaviors of HF-treated silicon wafer and decomposition reaction of water vapor on an HF-treated silicon surface under UHV were investigated by means of thermal desorption spectroscopy (TDS). When an HF-treated silicon surface was annealed in high vacuum, silicon trihydride species (SiH3) desorbed from the HF-treated silicon surface at 580°C, and SiH2F species which had probably the same structure as SiH3 on an HF-treated silicon surface desorbed and then recombined to form volatile SiH3-F with adjacent hydrogen at 580°C in addition to H2 desorption at about 400, 500 and 600°C. The decomposition reaction of water vapor was found to proceed in two steps at 500 and 750°C on a dehydrogenated silicon surface. The reaction made background hydrogen increase at 500 and 750°C in H2+ spectra from HF-treated silicon wafer.