Scott P. Doran

Line-edge roughness in sub-0.18-μm resist patterns

We have characterized line-edge roughness in single-layer, top-surface imaging, bilayer and trilayer resist schemes. The results indicate that in dry developed resists there is inherent line-edge roughness which results from the etch mask, resist (planarizing layer) erosion, and their dependence on plasma etch conditions. In top surface imaging the abruptness of the etch mask, i.e., the silylation contrast, and the silicon content in the silylated areas are the most significant contributors to line-edge roughness. Nevertheless, even in the case of a trilayer, where the SiO2 layer represents the near ideal mask, there is still resist sidewall roughness of the planarizing layer observed which is plasma induced and polymer dependent. The mechanism and magnitude of line-edge roughness are different for different resist schemes, and require specific optimization. Plasma etching of silicon, like O2 dry development, contributes to the final line-edge roughness of patterned features.

Supplemental multilevel interconnects by laser direct writing: Application to GaAs digital integrated circuits

A nonperturbing sequence of laser direct‐writing processes is described for applying supplemental multilevel interconnects on partially or fully fabricated circuits. The approach adds multiple levels of laser direct‐written tungsten metallization and is demonstrated here for the assembly of a simple latch circuit on a standard GaAs digital test chip. Principal applications are for subsystem verification, device testing, restructuring, and fault avoidance on Si and GaAs circuits.

Polymer photochemistry at advanced optical wavelengths

As lithography is extended to 157 nm, the molecular absorptivity becomes high for most organic polymers. Polymer photochemistry depends on photon absorption, and the higher energy associated with 157 nm light should lead to higher quantum yields of photoproducts. Polymers representative of those commonly employed in 193 or 248 nm resists were selected for this study. A gel permeation chromatography based method was developed to determine quantum yields for chain scission and crosslinking on thin polymers films coated on silicon wafers. This method was applied to determine the ΦS and ΦX of a number of lithographically significant homopolymers and copolymers at both the 157 and 248 nm wavelengths. It was found that polymers containing hydroxystyrene only undergo crosslinking while acrylate and methacrylate polymer only undergo chain scission. The film loss of 157 nm exposed poly-t-butyl acrylate and polymethyl methacrylate was found to be very high and attributed primarily to side chain cleavage of the este...

Infrared frequency selective surfaces fabricated using optical lithography and phase-shift masks

A frequency selective surface (FSS) structure has been fabricated for use in a thermophotovoltaic system. The FSS provides a means for reflecting the unusable light below the band gap of the thermophotovoltaic cell while transmitting the usable light above the band gap. This behavior is relatively independent of the light’s incident angle. The fabrication of the FSS was done using optical lithography and a phase-shift mask. The FSS cell consisted of circular slits spaced by 1100 nm. The diameter and width of the circular slits were 870 and 120 nm, respectively. The FSS was predicted to pass wavelengths near 7 μm and reflect wavelengths outside of this pass band. The FSSs fabricated performed as expected with a pass band centered near 5 μm.

Laser‐direct‐writing processes: Metal deposition, etching, and applications to microcircuits

The laser‐directed discretionary deposition of metal films broadens the scope of electronics fabrication and provides new capabilities for building and restructuring microelectronic devices. When such depositions are combined with local laser‐guided etching processes, multilevel modifications are enabled. This mode of processing allows the modification of individual circuits on a micrometer scale, as well as permitting the interconnection of larger circuits composed of multiple devices, both on a single substrate (‘‘wafer‐scale’’ integration) and in multichip hybrid configurations. In this paper, recent advances in the interconnect deposition processes useful for these applications are described, as well as some principal demonstrations on functioning circuits.

Prospects for using existing resists for evaluating 157-nm imaging systems

Lithography at 157 nm represents the next evolutionary step in the Great Optical Continuum and is currently under investigation as a possible successor to 193-nm lithography. If successful, the photoresists used for this technology must be initially capable of 100-nm resolution and be extendable to less than 70 nm. Unfortunately, as with the transition to shorter wavelengths in the past, the photoresist materials developed for longer wavelengths appear to be too absorbent for practical use as a traditional high resolution single layer resist imageable with 157 nm radiation. Until new photoresist materials are developed that are sufficiently transparent to be used as single layer resists, the existing need for a resist to be used to evaluate 157 nm imaging systems, such as the prototype steppers now under development, will have to be met by employing existing resists. We have surveyed the commercial resist market with the dual purpose of identifying the general categories of commercial resists that have potential for use as tool evaluation resist and to baseline these resists for comparison against future 157 nm resist candidates. Little difference was observed in the 157- nm absorbance between different classes of resists with most resists having an absorbance between 6 and 8 per micron. Due to the high absorbance at 157 nm of polyhydroxystyrene, polyacrylate, and polycyclic copolymer based resists, the coated resist thickness will need to be under 100 nm. All four commercial resists evaluated for imaging at 157 nm showed that they are capable of acting as a tool testing resist to identify issues attributed focus, illumination, and vibration. Finally, an improved tool testing resist can be developed within the existing resist material base, that is capable of 100 nm imaging with a binary mask and 70 nm imaging with a phase shift mask. Minor formulation modification can greatly improve resist performance including improved resolution and reduced line edge roughness.

High resolution fluorocarbon based resist for 157-nm lithography

Lithography at 157nm represents the next evolutionary step in optical lithography and is clearly seen as the likely successor to 193nm lithography. If successful, the photoresists used for this technology must be initially capable of 100nm resolution and be extendable to less than 70nm. As with the transition to shorter wavelengths in the past, the photoresist materials developed for longer wavelengths appear to be too absorbent for practical use as a traditional high resolution single layer resist imageable with 157nm radiation. The high 157nm absorbance of polyacrylate, polycyclic, and polyhydroxystyrene copolymer resists, will force the coated resist thickness to be under 100nm. It has been shown that some fluorine-functionalized polymers are more transparent in this spectral region than pure hydrocarbon polymers. This has led us to investigate the use of fluorocarbon polymers in resists specially designed for 157nm lithography. We have synthesized and evaluated a number of unique 4-hexafluoroisopropanol1 styrene based polymer systems that yield resists in which the 157nm absorbance ranges from 3.0 to 4.0micrometers . Resists of this type are potentially capable of imaging at resist thickness of 150nm. Examples of the high performance imaging capability of our resist design are shown to have imaging capability of 150nm with 0.50NA microstepper and 40nm employing interference lithography.

Encapsulated inorganic resist technology

Resolution in traditional single layer organic resists has been limited by the inability to image at aspect ratios (resist height to image width) of much greater than 3:1. Unless plasma etch selectivity increases several fold (an unlikely event with organic based resists) single layer resist chemistry will cease to be practical at sub-100-nm resolution. Multilayer resist schemes offer the capability of increased aspect ratio, but they add to the process complexity and cost. Encapsulated inorganic materials as resist components will be ultimately capable of sub-100-nm resolution with sufficient plasma etch selectivity. The encapsulated inorganic resist technology (EIRT) resist will act as a single layer hard mask compatible with existing resist processing steps. Material evaluation showed that encapsulated inorganic materials have properties compatible with current resist technology. Lithographic evaluations have been performed with electron beam, and with 248 nm and 157 nm projection systems. It was shown that 150-nm imaging is possible with resists having high inorganic material content. In all cases the EIRT resists have shown lithographic performance equivalent to control resists containing no SiO2. Reactive ion etch (RIE) etch rates in oxygen and chlorine plasmas are significantly reduced for resists containing SiO2 nanoparticles as compared to a commercial resist providing a proof of concept that EIRT resists can dramatically improved plasma etch rates.

Encapsulated inorganic resist technology applied to 157-nm lithography

In order to increase plasma etch selectivity in traditional single layer organic resists SiO2 nanoparticles have been added to typical 248nm resist formulations. Formulation modifications are necessary due to the dissolution acceleration effect of the particles. Surface functionalization of the nanoparticle surfaces with organic groups lessens this effect and allows the inclusion of acid labile groups. This allows for a wider formulation window and limits unexposed film thickness losses (UFTL). Both t- butyl ester groups and poly(t-butyl acrylate) have been used to achieve this effect. Encapsulated inorganic resist technology (EIRT) can be used as a single layer hard mask compatible with existing resist processing steps and replace complex and costly multilevel resist approaches. Lithogrpahic evaluations have been performed with electron beam, and with 248nm and 157nm projection systems. Greater transparency at 157nm is achieved by the addition of these materials, thus enabling the use of thicker films. High resolution imaging is demonstrated at these wavelengths.

Performance of excimer lasers as light sources for 193-nm lithography

The performance of argon fluoride excimer lasers is an important issue in determining the practical feasibility of 193-nm exposure systems. This paper presents a summary of the experience gained at MIT Lincoln Laboratory regarding the long-term performance of 193-nm lasers, used under conditions similar to those expected in production-type lithographic systems.