Kim Y. Lee

Electromigration and stress-induced voiding in fine Al and Al-alloy thin-filmed lines

Physical phenomena underlying failure due to electromigration and stress-induced voiding in fine Al and Al-alloy thin-film conducting lines are examined in the context of accelerated testing methods and structures. Aspects examined include effects due to line isolation (the absence of reservoirs at conductor ends), solute and precipitate phenomena, conductor critical (Blech) length, microstructure, film deposition conditions, and thermal processing subsequent to film deposition. Emphasis is on the isolated, submicron-wide, Al(Cu)-based thin-film interconnection lines of IBM VLSI logic and memory chips.

Electron-beam microcolumn technology and applications

A fully functional electron beam microcolumn, 3.5 mm in length, demonstrating a probe size of 10 nm and beam current >= 1 nA at 1 keV has been successfully developed. This paper presents its current status and future directions. Potential applications ranging from low cost scanning electron microscopy to arrays of such microcolumns for lithography, metrology, testing etc. will be discussed.

Copper Migration and Precipitate Dissolution in Aluminum/Copper Lines During Electromigration Testing

We have used transmission electron microscopy to examine in detail how microstructural changes take place in aluminum/copper lines adjacent to tungsten contacts. A structure that could be powered in a probe station was deposited directly on an electron transparent silicon nitride window to allow direct observation of the structure of the line after different intervals of electromigration testing. Observations made on aluminum-4% copper lines show that significant changes in the grain structure surrounding precipitates occur during testing even under conditions where no normal grain growth is observed in the lines. Elecromigration testing at 260°C resulted in grain coarsening and the formation of new diffusion blocking structures in the lines. By making measurements of the volume of individual precipitates as a function of time, the flux of copper through specific segments of the microstructure of the line could be measured. The results indicate that the presence of a single grain boundary can increase the electromigration flux of copper along the line by an order of magnitude. In line segments where a bamboo grain occurred copper diffusion appears to be controlled by diffusion along the Al/oxide interface at the surface of the lines. It is also shown that, in polycrystalline lines, the rate of copper depletion frorr the cathode is largely determined by the rate of dissolution of the precipitates. In mixed structure lines the migration of copper can become quite complex as changes in diffusion paths can occur as precipitates dissolve or grow.

Negative-tone resist system using vinyl cyclic acetal crosslinker

Most high performance negative tone resists are chemically amplified systems. The chemistry involves a creation of acid during photo-exposure and subsequent crosslinking of the polymer matrix during post-exposure bake. The commonly used crosslinkers are epoxies, melamines, benzyl alcohol and benzyl acetates. In light of the high reactivity of vinyl group on vinyl ether type compounds, literature has suggested that photochemical addition reaction of a polymer- bearing pendant vinyl ether with various thiol compounds can potentially be highly sensitive negative-type photoresists. Recently, bis-dihydropyrane derivative has been used for the first time to develop high performance negative tone resists for DUV, E-beam and x-ray applications. A cyclic acetal system based on acetal blocked aromatic aldehyde has also been demonstrated to be a good crosslinker for negative DUV resist. In order to take advantage of the above chemistries, we have investigated a crosslinker, 3,9-divinylspirobi(M-dioxane) (DVSDO), which contains both cyclic acetal groups and vinyl groups. Different loadings of DVSDO from 8% to 17% were formulated in combination with triphenyl sulfonyl triflate and N-sulfonyl triflate derivatives in polyhydroxystyrene matrix. One composition contains 8% N- sulfonyloxy derivative, 12% DVSDO in 20% solid of polyhydroxystyrene has shown resolution to 0.35 micrometer from Canon 0.37 NA DUV stepper. It also shows promising resolution in E-beam lithography. Varying the post apply bake (PAB) temperatures and post exposure bake temperatures (PEB) demonstrate a great dependency of sensitivity to baking temperature. The sensitivity increases with decreasing PAB, while decreases with decreasing PEB. Insufficient baking time (less than 4 - 5 minutes) at lower PEB temperature 90 degrees Celsius causes significant film loss after development in 0.14N TMAH for 60 - 75s. On the other hand, when the PEB temperature is too high (greater than 120 degrees Celsius), the resists resolution is degraded. Recent literature report has shown that high resolution can be achieved on negative resist system by raising PAB temperature. However, most of the known resist systems cannot tolerate high temperature, due to background crosslinking. The crosslinker, DVSDO, introduced in this paper seems to have higher heat resistance toward thermal induced background crosslinking. The resist formulated here can tolerate 140 degrees Celsius bake without seeing residues on the unexposed area. In this paper, we also discuss some IR studies on the crosslinking mechanism in addition to the lithographic evaluation.

Benzyloxypropene-protected PHS resist system for e-beam applications

E-beam lithography has recently emerged as a critical technology for the future semiconductor industry in the aspects of mask making, direct write and projection printing. Complexities in phase shifted and optical proximity corrected mask fabrications require high resolution, thermal bake insensitivity and RIE etch resistance. Direct write and projection also need high speed to address the throughput issue. These stringent requirements have rendered chemically amplified resists the strong candidates for the next generation E-beam lithography. Previously, we reported on a methoxypropene protected polyhydroxystyrene (PHS) system as a high performance chemically amplified E-beam resist. This system required weak aqueous base developer and was more sensitive to high temperature bake. The need for higher thermal stability to endure high temperature device fabrication process has prompted us to investigate other protecting groups. Benzyloxypropene protected PHS resist system was one of the earliest systems investigated. 100nm Cu lines fabricated with this resist formation through metallization, Damascence and lift off process have been demonstrated. In this paper, we will discuss the chemistry and properties of this resist system in conjunction with its E-beam lithography and metallization application.

An electron beam microcolumn for multi-beam applications

Summary form only given. Highly miniaturized electron beam columns based on a field emission source and microfabricated electron optical components have been developed. A 1 keV microcolumn operating with a miniaturized Zr/O/W Schottky emitter has been successfully evaluated for the first time. This paper will present the results obtained with the new microcolumn and discuss present efforts to further improve column performance with respect to resolution, beam current, and deflection field size.