X-ray diffraction analysis of thermally-induced stress relaxation in ZnO films deposited by magnetron sputtering on (100) Si substrates

Residual stresses in sputtered ZnO films on Si are determined and discussed. By means of X-ray diffraction, we show that as-deposited ZnO films are highly compressively stressed. Moreover, a transition of stress is observed as a function of the post-deposition annealing temperature. After an 800 °C annealing, ZnO films are tensily stressed while ZnO films encapsulated by Si₃N₄ are stress-free. With the aid of in-situ X-ray diffraction under ambient and argon atmosphere, we argue that this thermally activated stress relaxation may be attributed to a variation of the stoichiometry of the ZnO films.     

In situ thermal residual stress evolution in ultrathin ZnO and Ag films studied by synchrotron x-ray diffraction

Residual-stress evolution in sputtered encapsulated ZnO/Ag/ZnO stack has been studied in-situ by synchrotron x-ray diffraction when heat treated. The ZnO/Ag/ZnO stack encapsulated into Si₃N₄ layers and deposited on (001) Si substrates was thermally heated from 25 °C to 600 °C and cooled down to 25 °C. X-ray diffraction 2D patterns captured continuously during the heat treatment allowed monitoring the diffraction peak shifts of both Ag (15 nm thick) and ZnO (10 nm and 50 nm thick) sublayers. Due to the mismatch between the coefficients of thermal expansion, the silicon substrate induced compressive thermal stresses in the films during heating. We first observed a linear increase of the compressive stress state in both Ag and ZnO films and then a more complex elastic-stress evolution starts to operate from about 100 °C for Ag and about 250 °C for ZnO. Thermal contraction upon cooling seems to dominate so that the initial compressive film stresses relax by about 300 and 700 MPa after thermal treatment for ZnO and Ag, respectively. The overall behavior is discussed in terms of structural changes induced by the heat treatment.     

Residual stresses in particle-reinforced ceramic composites using synchrotron radiation

Residual stresses were determined in particle-reinforced ceramic composites using synchrotron based x-ray diffraction. The baseline Si₃N₄ and the Si₃N₄-TiN composites were processed by turbomilling, pressure casting, and isopressing. They were then continuously sintered to full density, under a pressureless, flowing nitrogen atmosphere. The flexural strength, fracture toughness, and residual stress were measured for as-machined samples and following quenching in water from 1000°C, 1100°C, and 1200°C. The residual stresses for both the baseline Si₃N₄ and the Si₃N₄-TiN composites were determined from the (441) and (531) reflections, by applying the 2-sin² method. The measured residual stresses were compared with the flexural strength and fracture toughness results to determine the effects of residual stress and thermal shocking on the mechanical properties of each material. In both the baseline Si₃N₄ and Si₃N₄-TiN composites, after thermal shocking, the compressive residual stresses were developed in directions both parallel and perpendicular to the sample surface. The residual compressive stresses for the Si₃N₄-TiN composites were much higher than the baseline Si₃N₄. As a result, both fracture toughness and flexural strength of the Si₃N₄-TiN composites were improved. In addition, the addition of the TiN appears to improve both the strength and toughness of the baseline Si₃N₄.     

RF sputtering: A viable tool for MEMS fabrication

Fabrication of Micro-Electro-Mechanical-Systems (MEMS) requires deposition of films such as SiO₂, Si₃N₄, ZnO, polysilicon, phosphosilicate glass (PSG), Al, Cr-Au, Pt, etc. for use as structural, sacrificial, piezoelectric and conducting material. Deposition of these materials at low temperature is desirable for fabricating sensors/actuators on temperature-sensitive substrates and also for integrating MEMS structures on silicon in post-CMOS processing procedures. Plasma enhanced chemical vapour deposition (PECVD) and sputtering are amongst potential techniques for preparing films for MEMS fabrication at comparatively low temperatures. The sputtering technique has an added advantage that the process is carried out in an inert ambient (argon) and chemically sensitive substrate/sacrificial layers can be used in realization of MEMS. Furthermore, the same system can be used for depositing dielectric, piezoelectric and conducting materials as per requirement in the fabrication sequence. This enables rapid low-cost prototyping of MEMS with minimum fabrication facilities. In the present work, we report preparation, characterization and application of RF sputtered SiO₂, Si₃N₄ and ZnO films for MEMS fabrication. The effect of RF power, sputtering pressure and target-to-substrate spacing was investigated on the structural and other properties of the films. The residual stress in the films was obtained usingwafer curvature measurement technique. The deposition parameters are optimized to obtain low stress films of SiO₂ and Si₃N₄. The self-heating of the substrate during deposition was advantageously exploited to obtain highly c-axis oriented films of ZnO without any external heating. A variety of MEMS structures such as cantilever beams, micro-bridges, diaphragms, etc. are demonstrated using bulk, surface and surface-bulk micromachining techniques.     

Crystalline β-C3N4 synthesized by MPCVD

Carbon nitride films were grown on Si and Pt substrates by microwave plasma chemical vapor deposition (MPCVD) method. Scanning electron microscope (SEM) observations show that the films deposited on Si substrates consisted of densely populated hexagonal crystalline rods. Energy dispersive X-ray (EDX) analyses show that N/C ratios of the rods were in the range of 1.0 to 2.0 depending on deposition condition. X-ray diffraction experiments show that the films consisted of crystalline phase -C₃N₄. Comparison with films grown on Pt substrate show that the main X-ray diffraction peaks of -C₃N₄ are existed in films deposited on both substrate. XPS study showed that carbon and nitride atoms are covalent bounded to each other. IR results show that the film is predominantly C-N bonded. Raman measurement showed characteristic peaks of -C₃N₄ in the low wave number region. Temperature dependent growth experiments show that the amount of Si₃N₄ in the films grown on Si substrates can be significantly reduced to negligible amount by controlling the substrate temperature.     

Microstructure and hardening mechanisms in a-Si3N4/nc-TiN nanostructured multilayers

Amorphous/nanocrystalline Si₃N₄/TiN nanostructured multilayer films were fabricated by radio-frequency reactive magnetron sputtering. The microstructure and properties of these films were measured using an X-ray diffractometer, X-ray photoelectron spectroscope, high-resolution transmission electron microscopy and nanoindenter. The superhardness effect was found in Si₃N₄/TiN multilayers. The hardness of Si₃N₄/TiN multilayers is affected not only by modulation periods, but also by layer thickness ratio and deposition temperature. The hardness value is about 40% higher than the value calculated from the rule of mixtures at a deposition temperature of 500 °C and a layer thickness ratio (l_Si3N4/l_TiN) of 3/1. The hardening mechanisms in this system are discussed in the light of our experimental results. Results of calculation of the theoretical stress distribution in the multilayers suggests that alternating stress fields caused by thermal mismatching between Si₃N₄ and TiN is one of the main reasons for the superhardness effect observed in Si₃N₄/TiN multilayers.     

Effect of annealing temperature on the tribological behavior of ZnO films prepared by sol-gel method

The tribological behavior of zinc oxide (ZnO) films grown on glass and silicon (100) substrates by sol-gel method was investigated. Particularly, the as-coated films were post-annealed at different temperatures in air to investigate the effect of annealing temperature. Crystal structural and surface morphology of the films were measured by X-ray diffraction (XRD) and Atomic Force Microscopy (AFM). XRD patterns and AFM images indicated that the crystallinity and grain size of the films were enhanced and increased, respectively, with temperature. The tribological behavior of films was evaluated by sliding the ZnO films against a Si₃N₄ ball under 0.5 gf normal load using a reciprocating pin-on-plate tribo-tester. The wear tracks of the films were measured by AFM to quantify the wear resistance of the films. The results showed that the wear resistance of the films could be improved by the annealing process. The wear resistance of the films generally increased with annealing temperature. Specifically, the wear resistance of the films was significantly improved when the annealing temperature was higher than 550° C. The increase in the wear resistance is attributed to the increase in hardness and modulus of the film with annealing temperature.     

Microstructural investigation of alpha-beta yttrium sialon materials

Three yttrium sialon materials have been manufactured under similar conditions, but with varying amounts of the constituent phases. Scanning and transmission microscopy, electron probe microanalysis and X-ray diffraction have been used to characterize the microstructure. Precipitation of the alpha-sialon phase both increases significantly the hardness at all temperatures investigated and decreases the thermal diffusivity. The sialon particles frequently contained a core of unreacted Si₃N₄ raw material. This was always found to be alpha-Si₃N₄ in alpha-sialon particles and beta-Si₃N₄ in beta-sialon particles; thus the unreacted Si₃N₄ raw materials act as nuclei for sialon particles.     

Formation of SiC whiskers from silicon nitride

Attempts have been made to produce SiC whiskers through vacuum pyrolysis of Si₃N₄ without any addition of extraneous carbon. Vacuum pyrolysis of Si₃N₄ granules and powder compacts, has been carried out at 1550 and 1700°C using a graphite resistance furnace. The products of pyrolysis have been identified through XRD and SEM as SiC whiskers and particles. Small amounts of elemental silicon at 1550°C and free carbon at 1700°C have been detected through X-ray diffraction. Detection of elemental silicon through X-ray diffraction and solidified silicon droplets at the whisker tips in the SEM provide important clues regarding the mechanism of SiC_w formation, as the one involving the reaction 2Si(l) + CO(g) SiC(s) + SiO(g) Silicon carbide whiskers, 3–4 mm long, have been grown from Si₃N₄ compacts at 1550°C over a short period of 0.5 h. It has been shown in the present study that Si₃N₄ can be completely converted to SiC_w, when a loose bed of Si₃N₄ in the form of granules is pyrolysed in the presence of CO at about 1550°C.     

Activation of p-type conduction in ZnO:N films by annealing in atomic oxygen

ZnO:N epitaxial films have been grown by reactive magnetron sputtering. The effect of annealing in atomic oxygen on the structural and electrical properties of the ZnO:N films has been studied by X-ray diffraction, atomic force microscopy, Hall effect measurements, and X-ray photoelectron spectroscopy. By annealing at temperatures from 500 to 700°C, we have obtained p-type ZnO:N films with a resistivity of ～57 Ω cm, hole mobility of ～2.7 cm²/(V s), and hole concentration of ～6.8 × 10¹⁷ cm^?3. X-ray photoelectron spectroscopy results suggest that the p-type conductivity of the films is due to a decrease in the concentration of (N₂)_O and V _O donors.     

Formation and photoluminescence of Si nanocrystals in controlled multilayer structure comprising of Si-rich nitride and ultrathin silicon nitride barrier layers

The effect of ultrathin silicon nitride (Si₃N₄) barrier layers on the formation and photoluminescence (PL) of Si nanocrystals (NCs) in Si-rich nitride (SRN)/Si₃N₄ multilayer structure was investigated. The layered structures composed of alternating layers of SRN and Si₃N₄ were prepared using magnetron sputtering followed by a different high temperature annealing. The formation of uniformly sized Si NCs was confirmed by the transmission electron microscopy and X-ray diffraction measurements. In particular, the 1 nm thick Si₃N₄ barrier layers was found to be sufficient in restraining the growth of Si NCs within the SRN layers upon high annealing processes. Moreover, X-ray photoelectron spectroscopy spectra shown that films subjected to post-anneal processes were not oxidized during the annealing. X-ray reflection measurements revealed that high annealing process induced low variation in the multilayer structure where the 1 nm Si₃N₄ layers act as good diffusion barriers to inhibit inter-diffusion between SRN layers. The PL emission observed was shown to be originated from the quantum confinement of Si NCs in the SRN. Furthermore, the blue shift of PL peaks accompanied by improved PL intensity after annealing process could be attributed to the effect of improved crystallization as well as nitride passivation in the films. Such multilayer structure should be advantageous for photovoltaic applications as the ultrathin barrier layer allow better electrical conductivity while still able to confine the growth of desired Si NC size for bandgap engineering.     

Microstructure of brazed joints between mechanically metallized Si<Subscript>3</Subscript>N<Subscript>4</Subscript> and stainless steel

Brazing has been increasingly used to join metals to advanced ceramics. Brazing covalent materials requires either the use of active filler alloys or the previous metallization of the surface. To that end, a new and simple mechanical technique has been applied to metallize advanced ceramics, thus avoiding the use of costly Ti-based active filler alloys. The mechanical metallization of Si₃N₄ with Ti was employed as an alternative route to deposit active metallic films prior to brazing with stainless steel using 72% Ag--28% Cu or 82% Au—18% Ni eutectic alloys. The brazing temperatures were set to 40°C or 75°C above the eutectic temperature of each filler alloy. Ti-films of average thickness 4 μm produced adequate spreading of both filler alloys onto Si₃N₄ substrates, which were subsequently brazed to stainless steel. The interface of Si₃N₄/310 stainless steel basically consisted of a reaction layer, a precipitation zone and an eutectic microconstituent. Mechanically sound and vacuum-tight joints were obtained, especially upon brazing at relatively lower temperatures. Increasing the brazing temperature resulted in thermal cracking of the Si₃N₄, possibly due to increased thermal stress.     

Microstructure and mechanical properties of reactively sputtered Ti–Si–N nanocomposite films

A series of Ti–Si–N nanocomposite films with different Si content were deposited by reactive sputtering in a gas mixed with Ar, N₂ and SiH₄. Energy dispersive spectroscope, X-ray diffraction, transmission electron microscope and nanoindentation technique were employed to characterize the microstructure and mechanical properties of the films. The results reveal that Ti–Si–N nanocomposite films with different Si content can be easily obtained by controlling SiH₄ partial pressure in the mixed gas. With Si content ranging from 4 at.% to 9 at.%, the films are strengthened and reach the highest hardness and elastic modulus of 34.2 GPa and 398 GPa, respectively. With a further increase of Si content, the mechanical properties of films decrease gradually. The microstructure of Ti–Si–N films with high hardness shows the existence of TiN nanocrystals surrounded by Si₃N₄ interphase. The grain size of TiN is about 20 nm and the thickness of Si₃N₄ interphase is less than 1 nm.     

Spark plasma sintering of silicon nitride using nanocomposite particles

The spark plasma sintering (SPS) of silicon nitride (Si₃N₄) was investigated using nanocomposite particles composed of submicron-size α-Si₃N₄ and nano-size sintering aids of 5 wt% Y₂O₃ and 2 wt% MgO prepared through a mechanical treatment. As a result of the SPS, Si₃N₄ ceramics with a higher density were obtained using the nanocomposite particles compared with a powder mixture prepared using conventional wet ball-milling. The shrinkage curve of the powder compact prepared using the mechanical treatment was also different from that prepared using the ball-milling, because the formation of the secondary phase identified by the X-ray diffraction (XRD) method and liquid phase was influenced by the presence of the sintering aids in the powder compact. Scanning electron microscopy (SEM) observations showed that elongated grain structure in the Si₃N₄ ceramics with the nanocomposite particles was more developed than that using the powder mixture and ball-milling because of the enhancement of the densification and α-β phase transformation. The fracture toughness was improved by the development of the microstructure using the nanocomposite particles as the raw material. Consequently, it was shown that the powder design of the Si₃N₄ and sintering aids is important to fabricate denser Si₃N₄ ceramics with better mechanical properties using SPS.     

Preparation and properties of ZnO thin films deposited by sol-gel technique

Zinc oxide (ZnO) thin films were deposited on (100) Si substrates by sol-gel technique. Zinc acetate was used as the precursor material. The effect of different annealing atmospheres and annealing temperatures on composition, structural and optical properties of ZnO thin films was investigated by using Fourier transform infrared spectroscopy, X-ray diffraction, atomic force microscopy and photoluminescence (PL), respectively. At an annealing temperature of 400°C in N₂ for 2 h, dried gel films were propitious to undergo structural relaxation and grow ZnO grains. ZnO thin film annealed at 400°C in N₂ for 2 h exhibited the optimal structure and PL property, and the grain size and the lattice constants of the film were calculated (41.6 nm, a = 3.253 ? and c = 5.210 ?). Moreover, a green emission around 495 nm was observed in the PL spectra owing to the oxygen vacancies located at the surface of ZnO grains. With increasing annealing temperature, both the amount of the grown ZnO and the specific surface area of the grains decrease, which jointly weaken the green emission. Translated from Journal of Lanzhou University (Natural Science), 2006, 42(1): 67–71 [译自: 兰州大学学报 (自然科学版)]     

Design of Si<Subscript>3</Subscript>N<Subscript>4</Subscript>-based ceramic laminates by the residual stresses

Ceramic laminates with strong interfaces between layers are considered a very promising material for different engineering applications because of the potential for increasing fracture toughness by designing high residual compressive and low residual tensile stresses in separate layers. In this work, Si₃N₄/Si₃N₄-TiN ceramic laminates with strong interfaces were manufactured by rolling and hot pressing techniques. The investigation of their mechanical properties has shown that the increase in apparent fracture toughness can be achieved for the Si₃N₄/Si₃N₄-20 wt.%TiN composite, while further increase of TiN content in the layers with residual tensile stresses lead to a formation of multiple cracks, and as a result, a significant decrease in the mechanical performance of the composites. Micro-Raman spectroscopy was used to measure the frequency shift across the Si₃N₄/Si₃N₄-20 wt.%TiN laminate. These preliminary Raman results can be useful for further analysis of residual stress distribution in the laminate.     

Compressive creep behavior of hot-pressed Si3N4-CRE2O3-Al2O3 ceramics

In this work, yttrium-rare earth oxide solid solution, CRE₂O₃, produced at FAENQUIL-DEMAR at a cost of only 20% of pure commercial Y₂O₃, was used as sintering additive of hot-pressed Si₃N₄ ceramics. The objective of this work was to characterize and to investigate the creep behavior of these ceramics. The samples were sintered by hot-pressing at 1750 °C, for 30 min using a pressure of 20 MPa. Compressive creep tests were carried out in air, between 1250 and 1300 °C, for 60 h, under stresses of 200-300 MPa. The stress exponent under all conditions was determined to be about unity. The apparent activation energy obtained was around 460 kJ mol⁻¹, corresponding to the heat of solution of the Si₃N₄ in the glassy phase. Both the stress exponent n and apparent activation energy Q are within the range of values reported in other studies of the compressive creep of Y₂O₃-Al₂O₃-doped-Si₃N₄ ceramics. X-ray diffraction (XRD) characterization shows a global reorientation of the β-Si₃N₄ grains and SEM observations detected no grain growth after the creep tests. These results indicate that grain-boundary sliding controlled by viscous flow is the dominant creep mechanism observed in the present study. The creep resistance presented of this samples indicates that this additive CRE₂O₃ can be a cheap alternative in the fabrication of Si₃N₄ ceramics, resulting in promising mechanical properties.     

Ceramic composites: roles of fiber and interface

Results are presented that elucidate: (a) the effects of fiber coating on retained fiber strength and mechanical properties of Nicalon-fiber-reinforced SiC matrix composites; and (b) the role of residual stresses in the interfacial bond strength of SiC-fiber-reinforced reaction-bonded Si₃N₄ matrix composites. For Nicalon-fiber-reinforced SiC matrix composites that were fractured in a flexural mode, retained in-situ fiber strength, ultimate strength and work-of-fracture (WOF) of the composites increased with increasing thickness of the fiber coating and reached maximum values at a coating thickness of ≈0.3 μm. A direct correlation between the variation of in-situ fiber strength and the variation of ultimate strength and WOF of the composites clearly indicates the critical role of the retained in-situ strength of reinforcing fibers in composites. Fiber pushout tests performed on SiC-fiber-reinforced reaction-bonded Si₃N₄ matrix composites indicate that both debonding and frictional shear stresses decreased with increasing fiber content. These variations are consistent with the variation of residual radial stress on fibers, as measured by neutron diffraction, i.e. residual stresses decreased with increasing fiber content. Because fracture behavior is strongly controlled by interfacial bond strength, which is proportional to the residual radial stress, appropriate control of residual stresses is critical in the design of composites with desired fracture properties.     

Effect of pressure and substrate temperature on the deposition of nano-structured silicon-carbon-nitride superhard coatings by magnetron sputtering

A systematic investigation on the deposition of silicon-carbon-nitride (Si-C-N) films under varying deposition conditions such as pressure, substrate temperature and nitrogen content was carried out by radio frequency and direct current magnetron sputtering techniques. Significant role of the different deposition parameters on hardness and structure in the film was observed. It was observed that there was a certain range of nitrogen to argon partial pressure ratio (90:10 to 98:2) for which the particle size was reduced and the films were smooth with fine particle growth, beyond this limit the films had larger particle growth and roughness. The hardness of the deposited film varied between 4400 Hv and 473 Hv depending on deposition condition. Si-C-N film with hardness above 4400 Hv by reactive RF magnetron sputtering from SiC-C composite target in nitrogen-argon was obtained. X-ray diffraction studies revealed the amorphous nature of the deposited films, whereas nano-crystallinity of the particles was noticed during atomic forced microscopy observations. X-ray photoelectron spectroscopy analysis showed the presence of C-N and Si-N bonds in the harder films. It was found that the presence of β-C₃N₄, Si₃N₄ and graphite phases and the particle growth in the deposited films control the hardness of the film.     

Interaction of Si3N4 with Cr

The interaction of Si₃N₄ with chromium was investigated under an argon or nitrogen atmosphere at temperatures from 773–1573 K. Reaction rates were determined by thermogravimetric analysis, and reaction products were examined by X-ray diffraction. Cr₂N, Cr₃Si, Cr₅Si₃ and CrSi were produced under argon, while reaction products under nitrogen were Cr₂N, CrN and Cr₃Si. Possible reactions between Si₃N₄ and chromium have been discussed thermodynamically. The initial rate obeyed a linear rate law. When chromium particles were coated with the reaction layer, the rate was described by a parabolic rate law. The reaction mechanism and the rate-determining step have been proposed.