BiIn: a sensitive bimetallic thermal resist

Abstract

A class of DC-sputter deposited bimetallic thermal resists (BiIn, SnIn, BiSn) have been investigated with exposure sensitivities (7mj/cm 2 ) 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 HNO 3 :CH 3 COOH:H 2 O (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:H 2 O 2 :H 2 O bath. Shelf tests shows that the BiIn film is stable under 50ûC/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.

Wavelength invariant Bi/In thermal resist as a Si anisotropic etch masking layer and direct-write photomask material

“…The three plots in Figure 2 demonstrate this with simulated Reflection, Transmission and Absorption (RAT) curves versus film thickness at wavelengths 248 nm (current exposure systems), 13.4 nm and 1 nm (X-ray). As reported previously [2,3], Bi/In experimental results and optical simulation suggest that Bi/In varies little from 533 nm to 248 nm. Simulation suggests exposure is almost unchanged from 248 nm (Figure 2 (a)) down to 100 nm, a range that includes the proposed 157 nm exposure tools.…”

“…Simulation suggests exposure is almost unchanged from 248 nm (Figure 2 (a)) down to 100 nm, a range that includes the proposed 157 nm exposure tools. Experimental results [1,2] showed 4 nsec pulses at 266 nm required ~7 mJ/cm 2 .When these simulations are extended from 248 nm to 13.4 nm the absorption increases 18% at the thickness 20 nm of each layer, and reflectivity becomes very small (<1%), indicating a better sensitivity. Indeed there is very little change in the 50 nm to 10 nm range.…”

“…In the resist the ratio of the two films are chosen to match these eutectic compositions. When exposed to a short laser pulses of ~7 mJ/cm 2 [2] under a photomask or direct laser writing (Figure 1(b)) the exposed area will absorb the light energy and with sufficient power heat up to above the Bi/In reaction temperature. At the end of the laser pulse, the resist layers will cool down rapidly (<1 nsec), and the physical and chemical property of the material is changed.…”

“…An optical model [1,2] was established based on an Airy Summation to calculate the total amount of light Reflected from the top surface (R), Transmitted through the bottom surface (T), and Absorbed through the total thickness of the imaging layers (A), which vary with: the laser wavelength, the resist thickness, the index of refraction n and the absorption index k of different materials.…”

“…Our previous research has found that metallic bilayer Bismuth on Indium film creates an inorganic thermal resist with many interesting properties [1,2,3]. When optical exposure short laser pulses (4-20 nsec.)…”