Effect of interfacial SiO2 thickness for low temperature O2 plasma activated wafer bonding

It was experimentally demonstrated that bonding strength strongly depends on the total SiO₂ thickness near the bonding interface for a given O₂ plasma surface activation. Systematic experiments of Si/SiO₂ and SiO₂/SiO₂ wafer bonding are performed for analyzing the evolution of the bonding surface energy with the interfacial oxide thickness. Optimum plasma exposure time increases with the interfacial SiO₂ thickness to achieve the maximum bonding strength in SiO₂/SiO₂ or SiO₂/Si. An optimal process option for plasma activated SiO₂/SiO₂ wafer bonding is proposed.     

Low temperature wafer bonding for thin silicon film transfer

In this work, we investigate the low temperature (<200 °C) wafer bonding using wet chemical surface activation and we demonstrate high bonding strength sufficient to achieve the transfer of a thin silicon film of thickness less than 400 nm on top of another silicon wafer using spin-on-glass (SOG) film as an intermediate layer. The process developed is the first critical step that can enable three-dimensional (3D) integration and wafer level packaging of MEMS with electronic circuits.     

Low temperature silicon direct bonding for application in micromechanics: bonding energies for different combinations of oxides

Plain or structured hydrophillic silicon wafers covered with native oxide or with thermally grown oxide layers have been directly bonded at room temperature; afterwards, the samples were annealed at 100°C to 400°C. There is a significant difference in the observed bonding energy depending on the wafer pairing chosen. If one or both wafers are covered with a native oxide layer, high bonding strengths are reached even at low temperatures. This can be explained by the different diffusion behaviour of water molecules through a thick thermal oxide layer on one hand, and through a thin native oxide layer on the other hand. Two different methods for the activation of the wafer surfaces just prior to bonding are described.     

Wafer direct bonding with ambient pressure plasma activation

Ambient pressure plasma processes were applied for surface activation of semiconductor (Si, Ge and GaAs) and other wafers (glass) before direct wafer bonding for MEMS and engineered substrates. Surface properties of activated wafers were analysed. Caused by activation high bond energies were obtained for homogeneous (e.g. Si/Si) as well as for heterogeneous material combinations (for instance Si/Ge) after a subsequent low temperature annealing process at 200°C. The resulting bond energies are analogous or higher as obtained for low-pressure plasma activation processes. The advantages of the ambient pressure plasma processes are described; a technical solution is discussed demonstrating the low risk for contamination and radiation damage.     

Characterization of intermediate In/Ag layers of low temperature fluxless solder based wafer bonding for MEMS packaging

Low temperature fluxless solder for wafer bonding has received a lot of attention due to its great potential in hermetic MEMS packaging. Previous research activities mainly deploy solder alloy of eutectic composition to achieve low bonding temperature. We proposed new intermediate bonding layers (IBLs) of rich Ag composition in In–Ag materials systems. In this study, we investigated the intermetallic compounds (IMCs) at the bonding interface with respect to the bonding condition, post-bonding room temperature storage and post-bonding heat treatment. With this IBL, the IMCs of Ag₂In and Ag₉In₄ with high temperature resist to post-bonding process are derived under process condition of wafer bonding at 180 °C, 40 min and subsequent 120–130 °C annealing for 24 h. Low melting temperature IMC phase of AgIn₂ is formed in the interface after long term room temperature storage or 70 °C aging treatment. This low melting temperature IMC phase can be completely converted into high melting temperature IMCs of Ag₂In and Ag₉In₄ after 120 °C additional annealing. Based on our results, we can design the packaging process flow so as to get reliable hermetic packaged MEMS devices by using low temperature fluxless In–Ag wafer bonding.     

Characterising groundwater use by vegetation using a surface energy balance model and satellite observations of land surface temperature

This study presents a novel ‘model-data’ approach to detect groundwater-dependent vegetation (GDV), through differences in modelled and observed land surface temperatures (LST) in space and time. Vegetation groundwater use is inferred where modelled LST exceeds observed LST by more than a threshold determined from consideration of systematic and random errors in model and observations. Modelled LST was derived from a surface energy balance model and LST observations were obtained from Terra-MODIS thermal imagery. The model-data approach, applied in the Condamine River Catchment, Queensland, Australia, identified GDV coincident to existing mapping. GDV were found to use groundwater up to 48% of the time and for as many as 56 consecutive days. Under driest of conditions, groundwater was estimated to contribute up to 0.2 mm h⁻¹ to total ET for GDV. The ability to both detect the location and water-use dynamics of GDV is a significant advancement on previous remote-sensing GDV methods.     

Disaggregation of MODIS surface temperature over an agricultural area using a time series of Formosat-2 images

The temporal frequency of the thermal data provided by current spaceborne high-resolution imagery systems is inadequate for agricultural applications. As an alternative to the lack of high-resolution observations, kilometric thermal data can be disaggregated using a green (photosynthetically active) vegetation index e.g. NDVI (Normalized Difference Vegetation Index) collected at high resolution. Nevertheless, this approach is only valid in the conditions where vegetation temperature is approximately uniform. To extend the validity domain of the classical approach, a new methodology is developed by representing the temperature difference between photosynthetically and non-photosynthetically active vegetation. In practice, both photosynthetically and non-photosynthetically active vegetation fractions are derived from a time series of Formosat-2 shortwave data, and then included in the disaggregation procedure. The approach is tested over a 16 km by 10 km irrigated cropping area in Mexico during a whole agricultural season. Kilometric MODIS (MODerate resolution Imaging Spectroradiometer) surface temperature is disaggregated at 100 m resolution, and disaggregated temperature is subsequently compared against concurrent ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) data. Statistical results indicate that the new methodology is more robust than the classical one, and is always more accurate when fractional non-photosynthetically active vegetation cover is larger than 0.10. The mean correlation coefficient and slope between disaggregated and ASTER temperature is increased from 0.75 to 0.81 and from 0.60 to 0.77, respectively. The approach is also tested using the MODIS data re-sampled at 2 km resolution. Aggregation reduces errors in MODIS data and consequently increases the disaggregation accuracy.     

Separating surface emissivity and temperature using two-channel spectral indices and emissivity composites and comparison with a vegetation fraction method

The temperature-independent thermal infrared spectral indices (TISI) method is employed for the separation of land surface temperature (LST) and emissivity from surface radiances (atmospherically corrected satellite data). The daytime reflected solar irradiance and the surface emission at ∼3.8 μm have comparable magnitudes. Using surface radiances and a combination of day-night 2-channel TISI ratios, the ∼3.8 μm reflectivity is derived. For implementing the TISI method, coefficients for NOAA 9-16 AVHRR channels are obtained. A numerical analysis with simulated surface radiances shows that for most surface types (showing nearly Lambertian behavior) the achievable accuracy is ∼0.005 for emissivity (AVHRR channel-5) and ∼1.5 K for LST. Data from the European Centre for Medium-Range Weather Forecasts (ECMWF) is used for calculation of atmospheric attenuation. Comparisons are made over a part of central Europe on two different dates (seasons). Clouds pose a major problem to surface observations; hence, monthly emissivity composites are derived. Additionally, using TISI-based monthly composites of emissivities, a normalized difference vegetation index (NDVI)-based method is tuned to the particular study area and the results are intercompared. Once the coefficients are known, the NDVI method is easily implemented but holds well only for vegetated areas. The error of the NDVI-based emissivities (with respect to the TISI results) ranges between −0.038 and 0.032, but for vegetated areas the peak of the error-histogram is at ∼0.002. The algorithm for retrieving emissivity via TISI was validated with synthetic data. Due to the different spatial scales of satellite and surface measurements and the lack of homogeneous areas, which are representative for low-resolution pixels and ground measurements, ground-validation is a daunting task. However, for operational products ground-truth validation is necessary. Therefore, also an approach to identify suitable validation sites for meteorological satellite products in Europe is described.