BHF Solution Research Articles

Guideline to atomically flat TiO2-terminated SrTiO3(001) surfaces

The use of SrTiO3(001) substrates with single TiO2- or SrO-terminated surface is often required in order to grow high quality epitaxial thin films. Many surface preparation recipes can be found in the literature, based on different etching solutions (acid, neutral and basic pH) and combined with different annealing treatments. In this work, we compare three different methods usually used to obtain atomically flat TiO2-terminated SrTiO3 surfaces in which water, aqua regia (HCl-HNO3) and BHF (NH4F-HF) solutions are used as reactive agent. Atomic force microscopy measurements (recorded in topography, phase detection and Kelvin-probe force modes) show that using the BHF solution leads to a reliable selective dissolution of SrO on the substrate surface, thus allowing to efficiently obtain atomically flat single TiO2-terminated surfaces. The aging of the resulted surfaces as well as the possibility of layer-by-layer deposition atop is also discussed.

Improved PECVD SixNy film as a mask layer for deep wet etching of the silicon

Although plasma enhanced chemical vapor deposition (PECVD) silicon nitride (SixNy) films have been extensively investigated by many researchers, requirements of film properties vary from device to device. For some applications utilizing SixNy film as the mask Layer for deep wet etching of the silicon, it is very desirable to obtain a high quality film. In this study, SixNy films were deposited on silicon substrates by PECVD technique from the mixtures of NH3 and 5% SiH4 diluted in Ar. The deposition temperature and RF power were fixed at 400 °C and 20 W, respectively. By adjusting the SiH4/NH3 flow ratio, SixNy films of different compositions were deposited on silicon wafers. The stoichiometry, residual stress, etch rate in 1:50 HF, BHF solution and 40% KOH solution of deposited SixNy films were measured. The experimental results show that the optimum SiH4/NH3 flow ratio at which deposited SixNy films can perfectly protect the polysilicon resistors on the front side of wafers during KOH etching is between 1.63 and 2.24 under the given temperature and RF power. Polysilicon resistors protected by the SixNy films can withstand 6 h 40% KOH double-side etching at 80 °C. At the range of SiH4/NH3 flow ratios, the Si/N atom ratio of films ranges from 0.645 to 0.702, which slightly deviate the ideal stoichiometric ratio of LPCVD Si3N4 film. In addition, the silicon nitride films with the best protection effect are not the films of minimum etch rate in KOH solution.

Impact of N2O/NH3/N2 Gas Mixture on the Interface Quality of Germanium MOS Capacitors

The development of high quality germanium based devices has been generating a lot of interest due to the excellent optical and electrical properties of germanium [1-4]. From an optical perspective germanium has high index of refraction, low optical dispersion, and the possibility to tune the silicon physical properties and reduce the band gap by controlling Ge content which makes it very useful for photo-detectors application. Furthermore, the bulk hole and electron mobility of Ge are approximately four and two times higher than conventional Si channel, respectively, which makes it an excellent candidate for future high performance complementary metal-oxide-semiconductor (CMOS) devices. The major weaknesses in Ge technology is germanium oxide (GeO and GeO2) stability and water solubility, the poor quality of Ge/ high κ-dielectric interfaces and the difficulty in forming a low-Dit (density of traps) Ge metal-oxide-semiconductor interface. However, a superior high-k/Ge gate stack satisfying low Dit and thin EOT simultaneously is mandatory for high performance Ge-channel devices. Several techniques have been adopted for Ge interface passivation, among them the oxygen-based passivation such as GeOx which has been proved to be thermally unstable, while nitrogen-based passivation has better thermal stability. Furthermore, the energy needed to break the nitrogen-oxygen bond is smaller than that of oxygen-oxygen bond in O2 plasma which makes nitration-based passivation a low temperature technique and compatible with Ge MOSFET fabrication. In this paper a mixture of N2O, NH3 and N2passivation on Ge is performed using RF-PECVD reactor and extensively studied in terms of Ge MOS capacitor. In order to fabricate Ge MOS capacitor a P-type (100) Ge substrate with a resistivity of (5-10) mΩ.cm is cleaned by diluted 1:10 BHF solution. After being cleaned the samples is loaded immediately to an Oxford Instruments System 100 PECVD tool at RF frequency of 13.56MHz.The plasma power was fixed at 20W and a 710 sccm flow of N2O for 2min is used in all treated samples as a pre plasma clean step. After this 380 sccm of N2 and 30 sccm of NH3 were performed at 285 ºC and pressure of 1T at different exposure time on top of the Ge surface. A sample without passivation is fabricated as a control sample. A 5nm thin film of HfO2 was deposited by plasma- Atomic Layer Deposition (ALD) at 200 ºC, followed by Al deposition using e-beam evaporator through a shadow mask to define the gates. Finally, post dielectric annealing was carried out at 400 ºC for 10min under forming gases. Capacitance-Voltage (C-V) measurements are carried out using Agilent B 1505 A curve tracer + Signatone manual 1160 prober. Fig. 1 shows a schematic of the fabricated gate stack and Fig. 2 shows the typical C–V characteristics of nMOS capacitors without (a) and (b), (c) and (d) with N2O/NH3/N2 treatment at different exposure time. As can be seen from Fig.2 the hump is invisible for the samples passivated by N2O/NH3/N2, indicating that the formed GeOxNy IL may block the impurities from diffusing from the gate dielectric into the Ge substrate. Furthermore, the hysteresis of metal–oxide–semiconductor (MOS) capacitor with HfO2 serving as gate dielectric is reduced to ~150 mV, compared with ~400mV mV of the untreated one. Moreover, this passivation technique can act as an effective method for high performance NMOSFET devices. 1. R.Cariou, R.Ruggeri, X.Tan, Giovanni Mannino, J.Nassar, and P.Roca I Cabarrocas, structural properties of relaxed thin film germanium layers grown by low temperature RF-PECVD epitaxy on Si and Ge (100) substrate, AIP Advances 4,077103 (2014). 2. Ammar Nayfeh, Fabrication of high-quality p-MOSFET in Ge grown heteroepitaxially on Si, IEEE ELECTRON DEVICE LETTERS, 26 (9), 2005. 3. Ali K Okyay, Ammar M Nayfeh, Krishna C Saraswat, Takao Yonehara, Ann Marshall, Paul C McIntyre, High-efficiency metal–semiconductor–metal photodetectors on heteroepitaxially grown Ge on Si, Optics Letter, 31(17), 2006. 4. Mahmoud S Rasras, Douglas M Gill, Mark P Earnshaw, Christopher R Doerr, Joseph S Weiner, Cristian Bolle, Young-Kai Chen, CMOS silicon receiver integrated with Ge detector and reconfigurable optical filter,IEEE photonic Technology Letters, 22(2), 2010. Figure 1

Surface preparation of ZnO single-crystal substrate for the epitaxial growth of ZnO thin films

The surface treatment method using chemical etching for Zn-polar zinc oxide (ZnO) single-crystal substrates without annealing is proposed. The wet-chemical etching of Zn-polar ZnO had a serious issue that led to the formation of a large number of etch pits at the surface together with the formation of step structure. After various experiments, the pH control of the solution during wet-etching at first stage of the etching is found to be very important to obtain the straight step-edges without formation of etch-pit. Atomically flat substrates without etch pits were able to be obtained by controlling the pH in BHF solution. Eventually, homoepitaxial ZnO thin films with atomically flat surface were formed as well.

Apparent and steady-state etch rates in thin film etching and under-etching of microstructures: II. Characterization

The apparent and steady-state etch rates of PECVD SiO2, HDP SiO2 and PECVD Si3N4 are measured both in a single thin film and a stacked film configuration. This is done for a HF:H2O/1:1, a HF:IPA/1:1 and a BHF solution. It is shown that etch rates vary with the used etch time, confirming the influence of both an incubation and a rinsing period on the average etch rate when performing typical ex situ etch rate experiments. Hence, this second part of a set of two papers provides the experimental evidence for part I where a general etch rate model was proposed. Furthermore this work shows that the etch rate varies whether it is determined on a single layer, in a stacked configuration or while under-etching a structural layer. This confirms the need of a straightforward characterization method for under-etching measurements at the sacrificial release stage of MEMS fabrication processes. Therefore, a new characterization method, using a suspended beam array and a surface profilometer, is proposed to determine the amount of under-etch after sacrificial release of surface micromachined devices.

PECVD oxide as intermediate film for wafer bonding: Impact of residual stress

The deposition rate, the etch rate in a HF-based solution and the residual internal stress of PECVD oxides are systematically analysed for various deposition conditions and post-anneal treatments. Rapid thermal anneal (RTA) at a temperature over 900 °C for 15 s is proven to be the most efficient to reduce the residual stress in the film and its etch rate in BHF solution, as well as to enhance its long term stability. The reduction of the internal stress in PECVD oxide is mandatory to minimize the wafer bow which degrades the wafer bonding quality. Bonded samples show that the resulting surface energy tends to vary inversely with the elastic energy stored by the conformation of the wafers during the direct bonding. About 45 μm wafer bow (3 inch wafer, 380 μm-thick) comes out as an upper bow limit, preventing direct bonding to occur. The use of a RTA step following the PECVD oxide layer deposition is demonstrated to be an efficient technological solution to minimize the wafer bow and thus maximize the bonding surface toughness. The experimental results presented in this paper highlight the importance of monitoring the residual stress in intermediate oxide layers to assure high quality and reliable bonding and thus future three-dimensional integration.

광섬유 국소화 표면 플라즈몬 공명 센서를 위한 광섬유 표면상의 금속 나노 입자 형성

Various etching methods of optical fiber and formation of metal nano particles on the optical fiber have been proposed for fabrication of fiber optic localized surface plasmon resonance (FO LSPR) biosensors. Different types of etched optical fiber are possible by removing the cladding of optical fiber using HF (hydrofluoric acid) solution and BHF (buffered hydrofluoric acid) solution, which results in improved surface roughness when BHF solution is used. Localized surface plasmon can be formed and measured by formation of silver and gold nano particles on the etched optical fiber. The characteristics of the etched optical fiber and metal nano particles on the etched surface of the optical fiber play a key role in dictating the sensitivity of the LSPR sensors, so that the proposed results can be expected to be applied for related research on fiber optic based biosensors.

Ion beam lithography by using highly charged ion beam of Ar

The ion beam lithography (IBL) method is one of the promising techniques that can fabricate 3D nano-structures. In order to develop IBL further, highly charged ion (HCI) beams have been applied to IBL method in our research group. Considering a high reactivity of HCI beams in an irradiated material, it is expected that HCI beams enhance a productivity of damage in irradiated materials compared with singly charged ion beams. This effect will be observed as a change in etching speed of IBL process. In the present study, the HCI effect on IBL was investigated by irradiating Ar9+ and Ar1+ with the fluence of 6.3×1012 to 4.7×1014ptcl./cm2, which is lower than that in our previous study (6.3×1014 to 3.1×1015ptcl./cm2). The Ar9+ and Ar1+ ion beams with E=90keV, which were prepared by an irradiation facility of HCI beams at Kochi University of Technology, were irradiated onto spin-on-glass (SOG) through a stencil mask. In order to investigate an etching process by using BHF solution, the fabrication depth of SOG surface was measured as a function of an etching time. The depth measurements show that an irradiation of HCI beams enhances an etching speed and the fabrication depth. For example, the fabrication depth with Ar9+ beams, which is at least 100nm deeper than that with Ar1+ beams, was achieved. The present result shows a priority of HCI beams to fabricate deeper 3D-structures and gives information required to optimize the fabrication process with keeping a good precision.

Effect of surface preparation on Ni Ohmic contact to 3C-SiC

The effect of roughness and chemical treatment of 3C-SiC film surface on Ni ohmic contact was studied in this work. 3C-SiC(1 1 1) film was grown on Si(1 1 1) in a chemical vapor deposition reactor. The 3C-SiC surface was polished using a chemical mechanical polishing (CMP) technique to get a smooth and flat surface. The polished surface was oxidized and then was etched in BHF solution to remove subsurface damages formed during the CMP process. The morphology of thus prepared silicon carbide (SiC) surfaces was investigated using SEM and AFM. Ni contact resistance to the 3C-SiC films was evaluated using linear transmission line method pattern. The formation of good ohmic contact characteristics was observed from Ni contact to all the tested SiC samples. After the CMP process, it was found that the RMS roughness of 3C-SiC surface apparently reduces and the specific contact resistance to 3C-SiC decreases as well, in proportion to the SiC surface roughness. The sacrificial oxidation and etching of the polished SiC surface abruptly decrease the contact resistance to be 3.7×10 −4 Ω cm 2. It was shown that the surface morphology and subsurface damage concentration of 3C-SiC films are important factors to give a great effect on the contact characteristic of the 3C-SiC films. However, it was considered that the reduction of subsurface damage concentration is essential to get better contact resistance to 3C-SiC surface.

A comparative Raman spectroscopy study on silicon surface in HF, HF/H 2O 2 and HF/NH 4F aqueous solutions

The nature of Si(100) surfaces during immersion in dilute hydrofluoric acids (DHF), HF/H 2O 2/H 2O mixture and buffered hydrofluoric acids (BHF) has been comparatively investigated using confocal Raman spectroscopy. In DHF solution, silicon surfaces are covered mainly with silicon trihydrides (SiH 3) at the beginning of etching. As the etching goes on, silicon dihydrides (SiH 2) become main surface bonds, and silicon monohydride (SiH) signal appears clearly. In HF/H 2O 2/H 2O solution, silicon surfaces are terminated with hydrides, oxides and hydrogen-associated silicon fluorides. In BHF solution, silicon surfaces are covered with hydrides and hydrogen-associated silicon fluorides.

Fabrication of field emission Si-tip array using reduced submicron masks generated by isotropic etching of mask patterns

We have developed a novel technique for fabricating submicron size mask patterns using a wet etching technique of SiO 2 . The overetching of the SiO 2 mask layer would present reduction of the diameters of the mask. The reduction of 2 μm SiO 2 mask down to 0.5 μm diameter will be presented and utilized for the fabrication of nano-size Si-tip array. The 4 inch Si (100) wafer was thermally oxidized and followed by the pattern transfer of original mask patterns on the thermally grown oxide using conventional photographic contact printing method. The coated photoresist (PR) layer with 2 μm thickness was exploited as an etch mask layer for following mask reduction procedure. The mask reduction procedure was performed using 7:1 buffered hydrofluoric acid (BHF). Initially, the patterned PR mask layer was dipped into BHF solution for about 300 seconds in order to remove the SiO 2 layer until the Si substrate would be exposed. Additional ∼ 300 second extra-dipping in BHF solution created considerable overetching and resulted in the smaller diameter than that of the original one. The repetition of the dipping process with precise control provided proper size of mask layers. The shape of the mask layer is found to have smaller diameter at the top and greater diameter at the bottom of the mask layer. Further experiments resulted in the various submicron size SiO 2 mask patterns. The diameters of the reduced masks were measured by scanning electron microscopy (SEM) and observed, for example, to be 0.3, ∼ 0.2 μm on the top and 0.7, 0.5 μm at the bottom, respectively. The isotropic and anisotropic Si reactive ion etching (RIE) was performed with this patterned wafer in order to provide sharp Si tip on the ∼ 2 μm height Si-post arrays with the post diameter down to ∼ 0.2 μm. The sharpening oxidation followed by oxide etching resulted in the successful fabrication of the nanosize Si-tip arrays.

Wet Etching of Doped and Nondoped Silicon Oxide Films Using Buffered Hydrogen Fluoride Solution

Wet etching of various silicon oxide films using BHF solution was studied as a means of post‐RIE wet cleaning to form small contact holes with the designed shape. Measurements show a dependence of etching rate on concentration for thermal and CVD oxide films. Thermal and ECR‐CVD oxide films showed lower etching rates than doped/nondoped APCVD, LPCVD, and PCVD oxide films showed. The etching rate dependence on concentration was linear for the lower etching rate films but nonlinear for the higher etching rate films. This suggests that, for etching of the higher etching rate films, the dominant etching species is , while plays only a small role.

Deposition Characteristics and Properties of SiO2 Films Prepared by Reactive Sputtering in Hydrogen-, Oxygen- and Nitrogen-Argon Mixtures

Films are deposited by reactive sputtering in hydrogen-, oxygen- and nitrogen-argon mixtures from an SiO2 target in order to obtain device quality SiO2 films at low temperature. Deposition characteristics and film properties of SiO2 films sputtered in the mixtures are examined. Oxygen and nitrogen mixing with the sputtering gas leads to a large decrease in deposition rate compared with argon-only sputtering gas and hydrogen mixing. The SiO2 film properties of the etching rate in BHF solution, surface morphology and resistivity are found to be greatly affected by mixing these gases with sputtering gas. Mixing of oxygen and a small amount of hydrogen provides excellent SiO2 films, whose properties are superior to thermally grown films, even at low temperatures. These SiO2 films have been used to successfully fabricate silicon electronic devices. The effects of mixing the reactive gases with the sputtering gas are also discussed qualitatively through plasma-surface interaction on growing film surfaces.