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.
Enhanced laser-writing techniques for bimetallic grayscale photomasks
Under laser exposure, bimetallic thin films of Bi/In and Sn/In oxidize becoming transparent. By controlling the power, direct-write binary and grayscale photomasks have been produced with the mask's transparency (optical density, OD), ranging between ~3.0 (unexposed) to <0.22 (fully exposed). Precise 3D micro-optics require both high vertical accuracy, gray levels over large OD changes, and precise lateral pattern creation. To achieve this result, an OD measurement system has been developed that provides real-time measurements while the masks are being written. Using stationary exposures of Bi/In and Sn/In films with varying laser powers, the reduction in OD of the films is measured with respect to time. Using 1-minute exposures, the films reach a 'saturated' level within a second at 180 mW while at 50 mW their OD gradually reduces. The influence of film's thickness is examined with thicker films requiring a longer exposure time in order to reach a similar OD level. For mask-writing, the optimal line spacing is dependent upon the laser beam's power distribution profile. Using a line-spacing 3-5 times smaller than the effective spot-size, variations in the patterned mask caused by a Gaussian-distributed beam can be minimized at the cost of increasing the writing time of the mask by the same factor. The Gaussian-distributed beam at different laser powers is also found to create shifts in the OD measurements that are problematic for a closed-loop mask-writing system. The influence of the beam's power distribution is discussed along with solutions to eliminate the problems.
Measuring Sheet Resistances of Dielectrics Using Co‐Planar Hydrogel Electrochemical Cells with Practical Applications to Characterize the Protective Quality of Paints on Sculptures
Electrical properties of thin dielectric films at the solid-liquid phase boundary are an important performance characteristic of many devices, coatings and sensors. In this paper, co-polymeric hydrogels of polyacrylic acid co-sulfonic acid, swollen with a salt solution to act as the solid electrolyte, were used to assess interfaces using electrochemical impedance spectroscopy (EIS) in a co-planar geometry. Silane-modified glasses were characterized by the co-planar hydrogel EIS cell and found to be distinguishable based on their surface monolayer chemistry. EIS measurements were also made on primed and painted metal substrates, using both test panels and an outdoor sculpture, Tony Smith's Stinger. The panels were then exposed to accelerated and outdoor weathering and showed degradation on the surface paint layers, which was observable electrochemically using EIS and confirmed visually by SEM. Electrochemical spectral features were compared between data from a standard paint-test cell versus this co-planar hydrogel cell; both cell types were able to measure coating capacitance, providing useful information about the condition of the bulk coating. Yet, sheet resistance (Rs) was a spectral feature seen only by the co-planar hydrogel cell. Thus, it can be concluded that the use of such co-planar hydrogel cells can provide an earlier warning sign to coating degradation and such cells provide a new type of spectral information that is not assessable by the standard geometry.