Bi/In bimetallic thermal resists for microfabrication, photomasks, and micromachining applications
Bilayer Bi/In thin films form thermal resists with many new microfabrication and micromachining applications due to their changed physical, chemical and optical characteristics after the laser exposures. Wavelength invariance has been shown from the results of both experiment and Airy Summation optical modeling. The modeling projects bimetallic resist sensitivity to be nearly constant at about ~7 mJ/cm2 from 248 to 13.4 nm and is still very sensitive at 1 nm in X-ray range. Two kinds of acid solutions were effective in developing the exposed films by removing the unexposed area. Both nitric acid mixture (HNO3:CH3COOH:H2O=1:3:6) and hydrochloride acid mixture (HCl:H2O2:H2O=1:1:48) give etching selectivity of exposed to unexposed area of larger than 60:1. The etch rate of unexposed area is about 2.6 nm/sec. The Bi/In resist can be stripped away by an RCA2 clean. Bi/In resist was successfully used as a mask layer for KOH anisotropic silicon etching process. Due to the unusual conductive property of its exposed and developed films, Bi/In has demonstrated that it can be used as a direct laser write electroplating resist material. Copper and nickel plating was carried out on developed Bi/In layers on various substrates such as Si wafers, glass slides, wet-oxidized wafers. Large optical transmission changes (OD>3.5 before exposure and OD<0.3 after exposure at I-line) indicate that Bi/In can be used as a direct-write photomask material.
BiIn: a sensitive bimetallic thermal resist
A class of DC-sputter deposited bimetallic thermal resists (BiIn, SnIn, BiSn) have been investigated with exposure sensitivities (7mj/cm2) near those of current organic photoresists. A prototype resist consists of stacked bismuth on indium layers (30-45nm thick) with thickness ratios matching the eutectic alloy points in BiIn binary phase diagram. Laser exposed areas have significantly different optical characteristics, with lower reflectivity and higher transparency, hence showing the imaged pattern before development, thus confirming correct exposures. Predicted exposure levels are almost wavelength invariant: <5% change between 248 and 157nm. Profilometery results and SEM images showed no signs of ablation in the exposed areas. The BiIn resist was wet developed using a HNO3:CH3COOH:H2O (1:3:6) acid solution which preferentially removed the unexposed areas leaving written patterns of alloyed lines seen both under profilometery and SEM, and stripped in a dilute HCl:H2O2:H2O bath. Shelf tests shows that the BiIn film is stable under 50#C/90% humidity conditions. XRD and TEM show that the film is polycrystalline. Both SnIn and BiSn exhibit similar resist property as BiIn that the exposed area converts to greater transparencies that unexposed areas, indicating these three bimetallic films can be a group of potential inorganic thermal resist.
Ignition and burning rates of segregated waste combustion in packed beds.
Recent developments in national recycling and re-use programmes for municipal waste have led to segregation of an increasing proportion of waste to enhance material recovery. Several of the segregated streams contain materials that can not viably be re-used or recycled but can be used for energy recovery. In this study, the combustion of cardboard and waste wood was investigated in a small-scale packed bed reactor in order to provide fundamental data for the design/operation of moving bed furnaces. Key parameters of combustion including the ignition and burning rates were evaluated for various air flowrates and compared to the modelling results. Two successive stages of combustion were identified for both samples: the propagation of ignition front into the bed and combustion of the fuel above the ignition front. The burning rate of cardboard reached a peak of about 300 kg/m(2)h at the air flowrate of 936 kg/m(2)h and decreased at higher air flowrates. For waste wood, both the ignition and burning rates increased in the tested range of the air flowrate up to 702 kg/m(2)h, of which the values were very close to those for the cardboard. The model prediction was in good agreement with the test results for waste wood. However, the burning rate for cardboard was under-predicted due to strongly irregular shapes of the fuel.