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.
Single step direct-write photomask made from bimetallic Bi/In thermal resist
A new single step direct-write photomask process has been proposed by using Bi/In bimetallic thermal resist which turns almost transparent with high energy laser exposure. The Bi over In metallic films, each layer approximately 40 nm thick, were DC-sputtered onto quartz mask plate substrates in a single pump-down chamber. Before laser exposure the Bi/In had 2.91 Optical Density. Bi/In is a bimetallic thermal resist and hence shows near wavelength invariance exposure sensitivity from Near IR to UV light. For Bi/In exposure, up to 0.9 W Argon laser (514 nm) beam was focused by an f = 50 mm lens to a 10 micron spot. When writing a mask the Bi/In coated sample was placed on a computer-controlled high accuracy X-Y table and the pattern was raster-scanned by the laser at 10 mm/sec. After exposure the Bi/In film became nearly transparent (0.26 OD) at I-line (365 nm) wavelength, and remained conductive. Bi/In photomasks have been used together with a standard mask aligner to pattern the oxide and Al layer during the manufacturing of test solar cell devices in the lab. Experiments also showed that annealing the as-deposited films at 90°C before laser exposure increase the Bi/In transparency.
A review of excimer laser projection lithography
Excimer‐laser projection lithography now appears to be in a position to extend production optical techniques to dimensions approaching 0.25 μm. Such methods could well be the basis for the bulk of the advanced manufacturing capability in microelectronics over the next decade. This technology is reviewed with an eye to the state of the art and to the optical‐, resist‐, and materials‐engineering issues that it presents.