Air Gap Formation Research Articles

Measurements and modeling of air gap formation in iron-base alloys

The formation of an air gap has been experimentally studied during solidification of several iron-based alloys. Air gap widths and temperature distribution have been measured during solidification in a cylindrical water-cooled Cu-mold. Mathematical modeling has been performed to increase the understanding of the solidification process and the air gap formation. A model, developed earlier for Al- and Cu-based alloys, for description of air gap formation in alloys solidifying with varying solidification intervals was tested for Fe-base alloys. The model includes the effect of formation and condensation of lattice defects on the solidification process and the air gap formation. The calculated shrinkage using this model shows good agreement with the experimental data.

A coupled thermomechanical, thermal transport and segregation analysis of the solidification of aluminum alloys on molds of uneven topographies

A coupled thermomechanical, thermal transport and segregation analysis of aluminum alloys solidifying on uneven surfaces is presented here. Uneven surfaces are modelled as sinusoids with different wavelengths and amplitudes. Effects of various coupling mechanisms between the solid-shell deformation, air-gap formation, heat transfer, fluid flow and segregation, near the mold-metal interface, are observed for different mold topographies during the early stages of solidification of an aluminum alloy. The role of inverse segregation, arising from shrinkage driven flow in the melt, melt superheat and varying mold surface topography on nucleation of air-gaps and evolution of stresses in the solidifying shell is examined. The numerical model consists of a volume-averaged solidification model coupled with a small-deformation model combining elasto-viscoplastic deformation in the solidifying shell with air-gap nucleation and imperfect contact at the metal/mold interface. Heat transfer at the mold-metal interface is either contact pressure or air-gap dependent and is modelled using the actual contact pressure or air-gap size obtained from the contact sub-problem at the metal-mold interface. Variation in heat transfer leads to variations in fluid flow, segregation and stresses developing in the solid and mushy-zone, which in turn affect the morphology of the growing solid-shell. A wavelength range that leads to a reduction in equivalent stresses, segregation and growth front morphology unevenness, in the evolving solid-shell, is obtained for varying solute concentrations. One of the main objectives of the current analysis is to seek optimal mold surface topographies that minimize surface defects leading to desired cast surface morphologies.

Measurements and modelling of air gap formation in Cu-based alloys

The formation of an air gap has been experimentally studied during solidification of pure Cu, Cu–Te and Cu–6%Sn in a cylindrical mould. The displacements of the casting and the mould causing an air gap have been measured during solidification and cooling of the casting. The temperature distribution was measured simultaneously. Mathematical modelling has been performed to increase the understanding of the solidification process and the shrinkage of the casting leading to air gap formation. A model, which has been tested in earlier work showing good results for aluminium based alloys, has been applied here to describe air gap formation during solidification of copper based alloys. The model includes the effect of the formation and condensation of vacancies on the solidification process as well as on the material shrinkage resulting in air gap formation. The results from the modelling show a reasonable agreement with the experimental measurements.

Measurements and modelling of air gap formation in aluminium based alloys

The formation of an air gap has been experimentally studied during solidification of several aluminium based alloys. Air gap widths and temperature distribution have been measured during solidification in a cylindrical mould. The effect of grain refinement on heat transfer and air gap formation has been studied. Mathematical modelling has been performed to increase the understanding of the solidification process and air gap formation. A model was developed for description of air gap formation in alloys solidifying with varying solidification intervals. The model includes the effect of formation and condensation of lattice defects on the solidification process and air gap formation. The calculated shrinkage using this model shows good agreement with the experimental data.

Comparison of numerical simulation models for predicting temperature in solidification analysis with reference to air gap formation

As a result of its influence on heat transfer between cast part and mould, air gap formation is an important problem for many casting processes. The general explanation for gap formation is that, as a result of stresses and distortions that are created from inhomogeneous cooling, shrinkage of the casting and expansion of the mould occur. In this paper, different thermomechanical approaches are applied to a well defined casting process using three commercial and one inhouse codes and their predictions are compared with experimental findings. The experimental data were obtained from the solidification and subsequent cooling of cylindrical castings of eutectic Al–13%Si and ternary Al–7%Si–0.3%Mg alloys. Based on these findings, the major differences between the predictions of the models and the actual formation of the air gap are discussed.

Hydrophobic/hydrophilic surface modification within buried air channels

Recently, a method for fabricating air channels using a photodefinable sacrificial material (Unity 2203P) with acid-catalyzed degradation at low temperature was reported [J. P. Jayachandran et al. J Microelectromech. Syst. 12, 147 (2003)]. The acid is created via a ‘photoacid’ generator (PAG) either photolytically (when exposed to UV irradiation) or thermolytically (when heated to the decomposition temperature of the PAG). This approach to the fabrication of micro air-channel structures using low-temperature decomposable sacrificial materials has applications to air-gap formation for electrical/optical interconnects, microelectromechanical systems, microfluidics, and microreactors. In this study, the surface characteristics of the silica surface after the decomposition of Unity 2203P was explored. It was found that the surface inside the air-channel after low-temperature Unity 2203P decomposition was hydrophilic and was then converted to hydrophobic after higher-temperature treatment. The modification of the silicon surface using silane-based chemistries and a method for creating alternating hydrophobic/hydrophilic textures within a buried air channel has been demonstrated. Trifluoropropyl dimethylchlorosilane was the most effective surface treatment for creating hydrophobic channels. The hydrophobic/hydrophilic nature of the silicon surfaces is shown by the contact angle measurements and x-ray photoelectron spectroscopy analyses.

Thermo-elasto–viscoplastic numerical model for metal casting processes

The finite element method is used to model the thermomechanical behaviour of ductile cast iron using metallic moulds. Heat conduction is assumed for the heat transfer analysis and an elasto-viscoplastic model is employed to predict the development of thermal stresses and strains. Special finite elements with coincident nodes are used to model the heat transfer and the mechanical contact at the metal–mould interface. The local heat transfer coefficient between the casting and the mould may be dependent on the air-gap formation. The latent heat evolution effect is modelled by the use of the enthalpy method. An iterative procedure is required to take into account the material and the contact non-linearity. A real casting has been modelled and numerical results are compared with the experimental measurements.

A new in situ sensor for large-scale snow-cover monitoring

AbstractA new in situ sensor for the simultaneous measurement of snow water equivalent, snow density and liquid-water content is presented in this paper. The system consists of radio frequency transmission lines of up to 25 m length cast in a flat PVC band, which can be set up either horizontally to monitor single snow-layer properties or sloping from a mast to the soil surface to determine vertical snowpack properties. The dielectric coefficient along the flat-band cable is measured with a time-domain reflectometer at high frequencies, and with a low-frequency impedance analyzer. The performance of the sensor system was tested during two winter seasons (2001–03) at the high-alpine test site Weissfluhjoch, Davos, Switzerland. The cable suspension and set-up of the sloping cable was shown to be critical with regard to stability and the formation of unwanted air gaps along the cable. Overall, the sensing system proved quite robust and produced results in agreement with manual snowpack observations.

Selective Sidewall Airgap Integration for Deep Submicrometer Interconnects

A novel process for narrow, ca. 100 nm wide, airgap formation based on chemical selectivity is presented. First, chemically modified sites are induced at the Ta(N)/SiC interface during patterning operations. Then, the SiO 2 material lying between copper lines is locally removed by vapor HF exposure, leading to air cavity formation between closely packed interconnects. Based on simulations this sidewall airgap concept has a promising potential for very narrow interline dielectrics in future interconnect generations.

Formation of “air‐gap” structure at a GaN epilayer/substrate interface by using an InN interlayer

We propose a new technique for “air-gap” formation at a GaN/sapphire interface by using an InN interlayer. This is aimed to grow epitaxial GaN films with reduced stress and cracks. First, an InN interlayer of about 0.2 μm thick is grown at 600 °C in atmospheric pressure. Then a 30 nm-thick GaN buffer layer is grown on the InN layer at 550 °C. The substrate temperature is ramped up to 1000 °C in the NH3 flow, and finally a 1.5 μm-thick GaN epilayer is grown on the annealed GaN buffer layer using nitrogen carrier gas. Consequently, an “air-gap” structure is naturally formed close to the substrate surface. During the ramping period of substrate temperature, the InN layer decomposes due to its thermal instability and metallic In is formed. It is found that metallic In drops as a result of InN decomposition contribute to the air-gap formation. No cracks are found on the GaN surface and a reduced stress in the layer is confirmed by PL and Raman shift measurements. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Integration of SiOC air gaps in copper interconnects

The formation of air gaps by means of a non-conformal chemical vapor deposition (CVD) on patterned wafers was successfully demonstrated using SiOC ( K=2.9) as inter-level metal dielectric. This paper presents the results on physical characterization and electrical performance evaluation of integrated SiOC air gaps in a three-metal level Cu/SiOC interconnect module. The influence of the air gaps shape on parasitic coupling capacitance between copper lines was also discussed in accordance with mask design rules and processing steps.

Study of inhomogeneity in thickness of LR 115 detector with SEM and Form Talysurf

Long-term measurements of radon progeny concentrations using Solid-State nuclear tract detector are being actively explored. These measurements depend critically on the thickness of the removed layer during etching. Scanning electron microscope (SEM) observations have identified irregularities in etched LR 115 detectors, such as detachment of the active layer from the substrate and formation of air gaps in the substrate. After discarding these irregularities, by using “Form Talysurf” surface profile measurements, the thickness of the active layers for the LR 115 detector are found to be 11.8±0.2 and 5.0±0.4 μm before and after 2 h of etching, respectively. The coefficient of variation has thus risen from 1.7% to 8.0% on etching. The increased inhomogeneity is explained by the formation of track-like damages, which have been observed using Form Talysurf, SEM, optical microscope and atomic force microscope. With this relative large coefficient of variation, the thickness of the active layer in the LR 115 detector cannot be assumed to be homogeneous in general, and the associated uncertainties should be considered carefully when the detector is used for alpha spectroscopy.

Air-channel fabrication for microelectromechanical systems via sacrificial photosensitive polycarbonates

This research involves the fabrication of encapsulated air-channels via acid-catalyzed degradation of photosensitive polycarbonates (PCs). There is a need for lower-temperature, degradable polymeric materials to fabricate buried air-channels for microelectromechanical systems (MEMS), microfluidic devices, and micro-reactors. Some polycarbonates undergo thermolytic degradation in the temperature range of 200 to 350°C. These polycarbonates are also known to undergo acid-catalyzed decomposition in the presence of catalytic amounts of acid. A small percentage of an acid in the polycarbonate formulation can greatly reduce the onset of decomposition temperature to the 100 to 180°C temperature range. The photoacid and thermal acid induced degradation behavior of PCs and its use as a sacrificial material for the formation of air-gaps have been studied in this work. The decomposition of several polycarbonates with the aid of in situ generated photo-acid has been demonstrated and applied to the fabrication of micro air-channels. Based on FT-IR, mass spectrometry, and thermogravimetric analysis (TGA), a degradation mechanism was proposed.

Conjugate Heat Transfer and Effects of Interfacial Heat Flux During the Solidification Process of Continuous Castings

Multiphase fluid flow involving solidification is common in many industrial processes such as extrusion, continuous casting, drawing, etc. The present study concentrates on the study of air gap formation due to metal shrinkage on the interfacial heat transfer of a continuous casting mold. Enthalpy method was employed to model the solidification of continuously moving metal. The effect of basic process parameters mainly superheat, withdrawal velocity, mold cooling rate and the post mold cooling rate on the heat transfer was studied. The results of cases run with air gap formation were also compared with those without air gap formation to understand the phenomenon comprehensively. The current study shows that there exists a limiting value of Pe above which the effect of air gap formation on the overall heat transfer is negligible.

Cooperation within MEBSP on the Subject of Air Gap Formation During a Casting Process

The goal of quantitatively predicting air gap growth in casting processes is still not reached. Subgroup IV of the international research platform MEBSP is tackling the problem with both an experimental approach (see Figure for a layout currently used at the Royal Institute of Technology) and a variety of simulation programs, being used by different participating institutes.

Analysis of Gap Formation at Mold-Shell Interface during Solidification of Aluminum Alloy Plate

Thermal contraction of the casting and the thermal expansion of the mold result in gap formation at the mold-casting interface, which affects greatly the heat transfer between the mold and casting. Information on the movements of the mold and casting during solidification and cooling is also the basis for the investigation of the casting deformation.Movements of the casting and mold were analyzed simultaneously using coupled thermo-mechanical contact boundary model for the plate casting of aluminum alloy. The calculated displacement and temperature distributions of the casting and mold were compared with the previous measured data. It was confirmed that the mold and the casting moved together until the air gap formed by the calculation. Comparing the air gap curve with the displacement curve of the casting just before and after air gap formation made it apparent that the growth of the air gap was mainly due to contraction of the casting because the change in the mold displacement was not significant. Gap formation time increased with increase in initial mold temperature.

Mathematical Modelling of Aluminiun Surface when Dipped in Molten Metal

A mathematical model is presented to describe the undulating surface of aluminium casting during an industrial process involving the dipping of the mould, at a particular velocity, into the molten metal. The problem of air gap formation between the mould and the casting was also considered. Below certain value of the mould velocity the shape of the casting as well as its thickness remain practically unchanged with changes in mould velocities. The undulating surface disappears when the mould temperature is in excess of 120oC. (Nigeria Journal of Pure and Applied Physics: 2003 2 (1): 36-39)

Air gap formation during solidification in cylindrical castings of pure aluminium and eutectic Al-Si

Casting of aluminium and eutectic Al-Si in a cylindrical mould has been performed. The air gap formed between the inner mould wall and the solidified shell was measured throughout the solidification process. Simultaneously the temperature distribution in the metal and the mould was measured. Calculations of the shrinkage of the metal were performed. The shrinkage due to thermal contraction was found to be too small to fit the measurements. A new model for the solidification process and the air gap formation was used, where the effect of the formation and condensation of lattice defects was considered. The condensation of lattice defects was used to explain the shrinkage and the air gap found experimentally.

Estimation of air gap and heat transfer coefficient at different faces of Al and Al–Si castings solidifying in permanent mould

In the casting processes, the heat transfer coefficient at the metal/mould interface is an important controlling factor for the solidification rate and the resulting structure and mechanical properties. Several factors interact to determine its value, among which are the type of metal/alloy, the mould material and surface conditions, the mould and pouring temperatures, casting configuration, and the type of gases at the interfacial air gap formed. It is also time dependent. In this work, the air gap formation was computed using a numerical model of solidification, taking into consideration the shrinkage and expansion of the metal and mould, gas film formation, and the metallostatic pressure. The variation of the air gap formation and heat transfer coefficient at the metal mould interface are studied at the top, bottom, and side surfaces of Al and Al–Si castings in a permanent mould in the form of a simple rectangular parallelepiped. The results show that the air gap formation and the heat transfer coefficient are different for the different casting surfaces. The bottom surface where the metallostatic pressure makes for good contact between the metal and the mould exhibits the highest heat transfer coefficient. For the sidewalls, the air gap was found to depend on the casting thickness as the larger the thickness the larger the air gap. The air gap and heat transfer coefficient also depend on the surface roughness of the mould, the alloy type, and the melt superheat. The air gap is relatively large for low values of melt superheat. The better the surface finish, the higher the heat transfer coefficient in the first few seconds after pouring. For Al–Si alloys, the heat transfer coefficient increases with increasing Si content.

Thermal stresses and distortion in dies of die casting processes:a new normalized approach

A general approach to thermal stresses in die casting dies has been attemptedwith a simple slab geometry. The approach consists of finding the propernormalization parameters, which enables one to summarize the thermal andthermo-elastic equations into a single expression whatever the variables, suchas the thermal conductivity of the die, its Young's modulus, its expansioncoefficient, its thickness, the maximum heat flux density transferred throughthe moulding surface and the duration of this maximum heat transfer.Provided that the heat flux density entering the die can be seen as a squarefunction of maximum ϕmax and duration τ, corresponding to the timespent between filling the cavity and the air gap formation, the evolution ofthe normalized thermal stresses on the moulding surface versus the normalizedtime can then be summarized with one single figure, instead of a chart ofcurves. This shows that the heat flux density is a parameter for the thermalstress problems, which replaces the more classical pair (heat transfercoefficient and temperature difference). The same trend applies for theevolution of the normalized distortion of the die (expressed in terms of aradius of curvature). These two curves follow a reference curve that wasobtained from a step function from zero to the maximum heat transfer.The duration τ appears to be the key process parameter, along with theheat flux density, that controls the normalized maximum stresses andtemperature. In contrast, the heat transfer coefficient at the rearsurface has an insignificant influence on the maximum temperature andstresses. In a complex shaped die the key parameter varies from one locationto another. Therefore a sensitivity study has been performed fromhigh-pressure die casting data. The sensitivities are at their highest whenthe duration τ is short. It is possible for the stress sensitivity toreach zero. The temperature sensitivity then tends towards a low asymptoticvalue. These conditions correspond to a situation where the heat wave hasenough time to reach the rear face of the die. It illustrates the closeinteraction between the transient heat transfer and thermal stresses.