Formation of a Buffer Layer for Graphene on C-Face SiC{0001}

Graphene films prepared by heating the SiC $ (000\bar{1}) $ surface (the C-face of the {0001} surface) in a Si-rich environment have been studied using low-energy electron diffraction (LEED) and low-energy electron microscopy. Upon graphitization, an interface with $ \sqrt {43} \times \sqrt {43} - R \pm 7.6^\circ $ symmetry is observed by in situ LEED. After oxidation, the interface displays $ \sqrt 3 \times \sqrt 3 - R 30^\circ $ symmetry. Electron reflectivity measurements indicate that these interface structures arise from a graphene-like “buffer layer” that forms between the graphene and the SiC, similar to that observed on Si-face SiC. From a dynamical LEED structure calculation for the oxidized C-face surface, it is found to consist of a graphene layer sitting on top of a silicate (Si₂O₃) layer, with the silicate layer having the well-known structure as previously studied on bare SiC $ (000\bar{1}) $ surfaces. Based on this result, the structure of the interface prior to oxidation is discussed.     

ICP刻蚀损伤对金属场板结构SiC肖特基二极管电学性能的影响

本文通过电感耦合等离子体（ICP）系统地研究了各种刻蚀参数对4H-SiC刻蚀的影响,并进一步研究了刻蚀损伤对金属场板结构4H-SiC肖特基二极管电学性能的影响。研究表明刻蚀速率和SiC表面形貌都会受到ICP功率、RF功率、压强和刻蚀气体流量的影响。在高的ICP偏压下,观察到了刻蚀损伤（刻蚀坑和刻蚀锥）的形成。更深入的研究表明,这些刻蚀损伤的形成和SiC自身的缺陷有关。这些刻蚀损伤的存在会导致SiC肖特基二极管正反向I-V性能发生恶化。在刻蚀损伤严重的情况下,对比正反向I-V测试结果发现,在0~50V的绝对电压范围内,正向电流甚至远小于反向电流。     

Effect of capping layer and post-CMP surface treatments on adhesion between damascene Cu and capping layer for ULSI interconnects

The effect of the type of capping layer and post-CMP surface treatments on the adhesion between damascene Cu and the capping layer was investigated. The CMPed-surface was treated by six methods divided into four groups which consisted of no surface treatment, cleaning by plasma with vacuum break, cleaning by plasma without vacuum break and cleaning by the wet chemical method. SiN_x and SiC were used to cap the surface after the post-CMP cleaning. The adhesion strength between Cu and the capping material was measured using a sandwiched structure constructed for the four point bending test. The X-ray photoemission spectroscopy analysis showed that the adhesion strength is related to the interfacial chemical bonds. The adhesion is influenced by the presence of contaminants and residual oxygen which inhibit the bonding of Si and Cu.     

Etching of silicon carbide for device fabrication and through via-hole formation

We have investigated the etching of SiC using inductively-coupled-plasma reactive ion etching with SF₆-based and Cl₂-based gas mixtures. Etch rates have been investigated as functions of bias voltage, ICP coil power, and chamber pressure. It will be shown, for the first time, that SiC surfaces etched in Cl₂-based plasmas yield better surface electrical characteristics than those etched in SF₆-based plasmas. We have also achieved SiC etch rates in excess of 1 μm/min which are suitable for micro-machining and via-hole applications. Through via-holes obtained in 140 μm thick SiC at an effective etch rate of 824 nm/min have been achieved. To the best of our knowledge, to date, this is the highest effective etch rate for a through via-hole etched with a masking process compatible with microelectronic fabrication.     

The role of oxygen in electron cyclotron resonance etching of silicon carbide

The electron cyclotron resonance (ECR) etching of silicon carbide (SiC) was studied using SF₆ + O₂ based plasma. The role of O₂ was studied by varying the O₂ flow rate while keeping the total gas flow constant. It was found that oxygen enhances the etch rate at low O₂ fraction through releasing more fluorine atoms, while lowers the etch rate at high O₂ fraction by diluting fluorine atoms and forming an oxide-like layer. The etched surface roughness was found to be affected by the surface oxidation and oxygen ion related physical ion bombardment. The role of oxygen in chemical etching of carbon was found to be insignificant. In general, the etched surface is smooth and free of micromasking effect that can arise from Al contamination and C rich layer.     

SF6分解气体在线红外光谱检测系统设计

SF6气体在放电或过热时会发生电离和分解,产生多种气体分解物。这些分解物会影响电气设备的绝缘性能。但现有的红外气体检测手段均无法对SF6分解气体进行现场在线测量。根据实际需求,本文根据光栅分光的原理设计了一种红外光谱检测系统。该系统可对SF6分解气体的成分和含量进行实时测量,可在电气设备带电运行的状态下进行在线测量。测量数据为分析设备的性能和安全状况提供有力的依据。     

Comparison of F2 plasma chemistries for deep etching of SiC

A number of F₂-based plasma chemistries (NF₃, SF₆, PF₅, and BF₃) were investigated for high rate etching of SiC. The most advantageous of these is SF₆, based on the high rate (0.6 μm·min⁻¹) it achieves and its relatively low cost compared to NF₃. The changes in electrical properties of the near-surface region are relatively minor when the incident ion energy is kept below approximately 75 eV. At a process pressure of 5 mtorr, the SiC etch rate falls-off by ∼15% in 30 μm diameter via holes compared to larger diameter holes (>60 μm diameter) or open areas on the mask. We also measured the effect of exposed SiC area on the etch rate of the material.     

A Study on Modified Silicon Surface after CHF3/C2F6 Reactive Ion Etching

The effects of reactive ion etching (RIE) of SiO₂ layer in CHF₃ / C₂F₆ on the underlying Si surface have been studied by X-ray photoelectron spectroscopy (XPS), secondary ion mass spectrometer, Rutherford backscattering spectroscopy, and high resolution transmission electron microscopy. We found that two distinguishable modified layers are formed by RIE : (i) a uniform residue surface layer of 4 nm thickness composed entirely of carbon, fluorine, oxygen, and hydrogen with 9 different kinds of chemical bonds and (ii) a contaminated silicon layer of about 50 nm thickness with carbon and fluorine atoms without any observable crystalline defects. To search the removal condition of the silicon surface residue, we monitored the changes of surface compositions for the etched silicon after various post treatments as rapid thermal anneal, O₂, NF₃, SF₆, and Cl₂ plasma treatments. XPS analysis revealed that NF₃ treatment is most effective. With 10 seconds exposure to NF₃ plasma, the fluorocarbon residue film decomposes. The remained fluorine completely disappears after the following wet cleaning.     

Surface contamination and damage from CF4 and SF6 reactive ion etching of silicon oxide on gallium arsenide

Two reactive ion etchants, CF₄ and SF₆, have been compared in terms of plasma characteristics, silicon oxide etch characteristics, extent of RIE damage, and formation of barrier layers on a GaAs surface after oxide etch. It was found that higher etch rates with lower plasma-induced dc bias can be achieved with SF₆ plasma relative to CF₄ plasma and that this correlates with higher atomic fluorine concentration in SF₆ plasma. RIE damage, measured by loss of sheet conductance in a thin highly-doped GaAs layer, could be modelled as a region of deep acceptors at a high concentration in the conductive layer. By relating the sheet conductance change to the modelled damaged layer thickness, it was found that the RIE-damaged thickness from both CF₄ and SF₆ plasmas had the same linear relation to plasma dc bias. Barriers to subsequent GaAs RIE were created during oxide overetch at the GaAs surface. The barriers were identified by XPS as ∼20 A of GaF₃ for CF₄ plasma and ∼30 A of GaF₃ on top of As_xS_y for SF₆ plasma. Ellipsometry was used to routinely determine the presence or absence of the barriers which could be removed in dilute ammonia.     

Effect of UV light irradiation on SiC dry etch rates

Inductively Coupled Plasma etching of 4H-SiC under ultraviolet illumination was examined for SF₆/Ar and Cl₂/Ar chemistries. Etch rate enhancements up to a factor of 8 were observed with UV light irradiation during Cl₂/Ar etching. The enhancement mechanism is related to photodesorption of SiCl_x and CCl_x species. Surface morphologies were unchanged as a result of the UV enhancement with Cl₂/Ar discharges. By contrast, there was no effect of UV irradiation on the SiC etch rates in SF₆/Ar plasmas, but the surfaces were typically smoother than those obtained without the ultraviolet illumination. In the SF₆/Ar chemistry the rate-limiting steps are either Si-C bond-breaking or supply of fluorine radicals to the surface, and not desorption of the SiF_x and CF_x etch products.     

CO2‐Laser‐Induced Growth of Epitaxial Graphene on 6H‐SiC(0001)

The thermal decomposition of SiC surface provides, perhaps, the most promising method for the epitaxial growth of graphene on a material useful in the electronics platform. Currently, efforts are focused on a reliable method for the growth of large‐area, low‐strain epitaxial graphene that is still lacking. Here, a novel method for the fast, single‐step epitaxial growth of large‐area homogeneous graphene film on the surface of SiC(0001) using an infrared CO₂ laser (10.6 μm) as the heating source is reported. Apart from enabling extreme heating and cooling rates, which can control the stacking order of epitaxial graphene, this method is cost‐effective in that it does not necessitate SiC pre‐treatment and/or high vacuum, it operates at low temperature and proceeds in the second time scale, thus providing a green solution to EG fabrication and a means to engineering graphene patterns on SiC by focused laser beams. Uniform, low–strain graphene film is demonstrated by scanning electron microscopy, X‐ray photoelectron spectroscopy, secondary ion‐mass spectroscopy, and Raman spectroscopy. Scalability to industrial level of the method described here appears to be realistic, in view of the high rate of CO₂‐laser‐induced graphene growth and the lack of strict sample–environment conditions.     

Low temperature cleaning of Si by a H2/AsH3 plasma prior to heteroepitaxial growth of GaAs by metalorganic chemical vapor deposition (MOCVD)

A method using a H₂/AsH₃ plasma to clean the Si surface before GaAs heteroepitaxy was investigated and the dependence of the effectiveness of this treatment on arsine partial pressure was studied. Thin GaAs-on-Si films deposited on the plasma-cleaned Si were analyzed using plan-view TEM, HRXTEM and SIMS. Although not optimized, this method of Si cleaning makes heteroepitaxial deposition of GaAs possible. Some roughening of the Si surface was observed and a possible explanation is offered. Using the results of this study, thick (2.5–3.0μm) epitaxial GaAs films were then deposited and their quality was evaluated using RBS, XTEM and optical Nomarski observation. All Si surface cleaning and GaAs deposition were carried out at temperatures at or below 650°.     

Plasma cleaning of the flex substrate for flip-chip bonding with anisotropic conductive adhesive film

Advanced integrated circuit packaging processes require good bondability and reliability between various mating surfaces. A key factor affecting this requirement is surface cleanliness. Plasma cleaning is the most suitable process for optimum surface cleanliness. An investigation of O₂, Ar, and O₂/SF₆ plasma cleaning was carried out on a flexible substrate to study the adhesion of anisotropic conductive adhesive film for flip chip bonding. Surface roughness was found to increase substantially after the plasma treatment. Adhesion strength was evaluated by 90° peeling tests both for untreated and plasma-treated flex. A higher adhesion strength of anisotropic conductive film (ACF) bond was observed after plasma cleaning. The surface morphology of plasma treated and untreated flex substrate before bonding, as well as the fracture surfaces after the peel test for both cases, was characterized by secondary electron image techniques of scanning electron microscopy (SEM). Based on the detailed SEM findings, extensive comparisons were made between the plasma treated and the untreated samples. Mechanical interlocking is found to be responsible for higher peel strength of the plasma treated flex bonding. It was also proposed to select the right flexible substrate for highly reliable, ACF bonded flip chip on flex substrate.     

Growth of Graphene-Like Structures on an Oxidized SiC Surface

Graphene-like structures were formed on an oxidized SiC (0001) surface following thermal annealing in a vacuum at high temperatures. The SiO₂/SiC structure was annealed at 1350°C in 10⁻⁵ Torr; the SiO₂ layer was vaporized, and two layer graphene-like structures were formed on the SiC surface. This method of fabricating graphene did not require an ultra-high vacuum. In the absence of the oxide layer, a film of vertical carbon nanotubes (CNTs) was grown on the SiC surface in the same temperature range at 10⁻⁵ Torr.     

Improved barrier and mechanical properties of Al2O3/acrylic laminates using rugged fluorocarbon layers for flexible encapsulation

Thin film encapsulation (TFE) with high barrier performance and mechanical reliability is essential and challenging for flexible organic light-emitting diodes (OLEDs). In this work, SF₆ plasma treatments were introduced for surface modifications of acrylic in Al₂O₃/acrylic laminates fabricated by atomic layer deposition (ALD) and ink-jet printing (IJP). It was found that micro-/nano-structures and surface fluoridation appeared on the surface of acrylic, and could be modulated by the discharge power and irradiation of plasma treatment. The water vapor transmission rates (WVTR) of Al₂O₃/acrylic multi-layers decreased evidently because of reducing surface polarity and strong cross-link of acrylic after plasma treatments. Furthermore, the rugged surface and relieved residual stress resulted from etching and heating of acrylic could enhance the mechanical property remarkably. The plasma treated Al₂O₃/acrylic multi-layers with only 3 dyads exhibited a low WVTR value of 1.02 × 10⁻⁶ g/m²/day and more stable mechanical property under 200 iterations blending test by comparative measurements, proving that the introduction of SF₆ plasma surface modifications could improve simultaneously the barrier performance and mechanical reliability prominently of the inorganic/organic multi-layers with no need of extra neutral axis design.     

Characteristics of germanium dry etching using inductively coupled SF6 plasma

Inductively coupled SF₆ plasma etching of germanium (Ge) was investigated at different inductively coupled plasma (ICP) power levels, the SF₆ flow rate, and the working pressure. The etch rate of Ge increases from 1007 to 2447 nm/min as the SF₆ flow rate increases from 10 to 60 sccm. Also, the etch rate of Ge increases from 265 to 1007 nm/min as the ICP power level increases from 100 to 400 W whereas the etch rate of Ge decreases from 552 to 295 nm/min as the working pressure increases from 5 to 20 mTorr. The etch profile is isotropic. As SF₆ flow, ICP power and working pressure decrease the surface roughness decreases. Optical emission spectroscopy was used to examine the gas phase species in the plasma, and emission from excited atomic S and F has been identified. Composition of the surface due to SF₆ plasmas has been obtained using X-ray photoelectron spectroscopy. Reaction layers on germanium due to inductively coupled SF₆ plasma etching are found to be a thin, layer with of G–-S, Ge–F and Ge–O bonded species.     

The electrical characteristics of 4H-SiC schottky diodes after inductively coupled plasma etching

The electrical characteristics of metal contacts fabricated on 4H-SiC have been investigated. Sputtered nickel ohmic contacts have been successfully produced on untreated 4H-SiC substrates and 4H-SiC surfaces cleaned with aggressive chemicals and ion sputtering. The current-voltage (I–V) characteristics of asdeposited contacts are observed to be nonohmic on all surfaces. After annealing at 1,000°C in a N₂ atmosphere, the contacts are seen to become ohmic with considerably less annealing time being required for the samples exposed to aggressive cleaning stages. Schottky diodes were then produced on the silicon carbide (SiC) surface after etching in an SF₆/O₂ inductively coupled plasma (ICP) for 3 min at varying substrate bias voltages and also on an untreated surface used as a control sample. The ideality factors of all diodes formed on the etched surfaces increased in comparison to the control sample. The highest ideality factor was observed for the diodes produced after etching at −0-V and −245-V bias voltage. A two-diode and resistor model was applied to the results that successfully accounted for the excess leakage paths. The defect density of each SiC surface was calculated using both the measured and the simulated ideality factors. An annealing stage was successful at reducing the ideality factors of all the diodes formed. The defect density calculated using the measured ideality factors of the annealed diodes was seen to have been reduced by an order of magnitude.     

Reactive ion etching of trenches in 6H-SiC

In this paper, we report the reactive ion etching (RIE) of trenches in 6H-silicon carbide using SF₆/O₂. The plasma parameters: etchant composition, gas flow rate, chamber pressure, and radio frequency power were optimized to obtain a maximum etch rate of 360Å/min. The etch rate of SiC was found to exhibit a direct correlation with the dc self bias except when the O₂ percentage was varied. Trenches were fabricated using the optimized conditions. It was found that the trench surface was extremely rough due to the aluminum micromasking effect. To overcome this effect, a Teflon^TM sheet was used to cover the cathode during the experiment. The trenches fabricated using this modification were found to have smooth etched surfaces and sidewalls. The angle of anisotropy of these trenches was approximately 80° which is suitable for device applications.     

Influence of different SiC surface treatments performed prior to Ni ohmic contacts formation

This work is dealing with the influence of surface treatment on ohmic contacts to hexagonal N-type SiC with medium doping level. The contact materials were Ni and Ni₂Si. The structures had to be annealed at high temperatures in order to reach ohmic behavior. A number of surface treatment methods were tested: wet cleaning, plasma etching, intentional oxidation with etching, H₂ annealing and their combinations. After some types of cleaning, the SiC surface was immediately analysed using the XPS method. The results of the analyses showed that the composition of the surface was not much influenced by these treatments. At lower annealing temperatures (approx. up to 850 °C) the prepared contacts showed Schottky behavior with large scatter of parameters. After annealing at approx. 960 °C, where the onset of ohmic behavior is expected, the structures were truly ohmic and of good parameters. Cleaning methods had just a negligible influence on the electrical parameters of the ohmic contacts. An explanation for these observed facts is suggested: Although - already on the basis of the XPS results - we could speak about a negligible influence of the cleaning onto the contact parameters, there might come across also other mechanisms coming from interaction of contact materials with SiC, which caused similar behavior of ohmic contacts on differently treated surfaces.     

SOI planar photonic crystal fabrication: Etching through SiO2/Si/SiO2 layer systems using fluorocarbon plasmas

Dry plasma etching of sub-micron structures in a SiO₂/Si/SiO₂ layer system using Cr as a mask was performed in a fluorocarbon plasma. It was determined that the best anisotropy could be achieved in the most electropositive plasma. A gas composition yielding the desired SOI planar photonic crystal structures was optimized from the available process gases, Ar, He, O₂, SF₆, CF₄, c-C₄F₈, CHF₃, using DC bias data sets. Application of the c-C₄F₈/(noble gas) chemistry allowed fabrication of the desired SOI planar photonic crystal. The average etching rates for the pores and ridge waveguide regions were about 71 and 97 nm/min, respectively, while the average SiO₂/Si/SiO₂ to Cr etching selectivity for the ridge waveguide region was about 33:1 in case of the c-C₄F₈/90%Ar plasma with optimized parameters.