Marching of the microlithography horses: electron, ion, and photon: past, present, and future

Abstract

Microlithography patterning employs one of three media; electron, ion, and photon. They are in a way like horses, racing towards the mainstream. Some horses such as electrons run fast but repel each other. Ion beams behave like electron beams but are less developed. The photon beam is the undisputed workhorse, taking microlithography from the 5-µm minimum feature size to 32-nm half pitch. This paper examines the history of microlithography in pattern generation, proximity printing, and projection printing, then identifies the strong and weak points of each technology. In addition to ion-beam and e-beam lithography, the coverage of optical lithography spans the wavelength from 436 to 13.5 nm. Our learning from history helps us prevent mistakes in the future. In almost all cases, making or using the mask presents one of the limiting problems, no matter the type of beams or the replication method. Only the maskless method relieves us from mask-related problems. A way to overcome the low throughput handicap of maskless systems is to use multiple e-beam direct writing, whose imaging lens can be economically and compactly fabricated using MEMS techniques.In a way, the history of microlithography parallels that of aviation. Proximity printing is like the Wright-Brothers' plane; 1X projection printing, single-engine propeller plane with unitized body; reduction step-and-repeat projection printing, multi-engine commercial airliner; scanners, jet airliners. Optical lithography has improved in many ways than just increasing NA and reducing wavelength just as the commercial airliners improving in many other areas than just the speed. The SST increased the speed of airliners by more than a factor of two just as optical resolution doubled with double exposures. EUV lithography with the wavelength reduced by an order of magnitude is similar to the space shuttle increasing its speed to more than 10 times that of the SST. Multiple-beam direct write systems are like helicopters. They do not need airports(masks) but we need a lot of beams to carry the same payload.Keywords: Microlithography, e-beam lithography, ion-beam lithography, optical lithography, x-ray lithography, EUV lithography, microlithography history, microlithography outlook IntroductionMicrolithography, a specific technology used to pattern IC circuits as early as 1958, is coming to its 50 th anniversary. Initially, it was as straightforward as ancient-day lithographers replicating patterns carved in stone masks. However, as circuit dimensions continued to shrink, more care had to be taken for microlithography. Very early on, people recognized that there was a limit to photon-based patterning techniques, higher energy particles or shorter wavelengths such as electrons and ions were proposed as the patterning carrier. "Optical techniques for fabricating devices are generally limited to > 5 µm linewidths in practical production by the diffraction effects", according to Herriot 1 in 1975. He must have in mind proximity printing at a large mask-to-wafer gap using near-uv...

Challenges of 22 nm and beyond CMOS technology

“…Multiple e-beam maskless exposure tool is very attractive in that it saves the cost of the mask set and reduces the design to product cycle time, which can become a considerable factor in terms of profitability for a product at 22 nm node. The direct writing nature and the required data transfer rate for enormous mask data from database to writing machine could be its limiting factors for the throughput [5,6] . Nano imprint lithography has demonstrated its resolution capability for 22 nm L/S structures with good line edge roughness, the defect control during imprinting and the availability of defect free template remain as its main challenges [7,8] .…”

“…Since most of the materials have refractive index around unity at 13.5 nm wavelength, the projection imaging system can no longer be based on refractive lens elements, instead, it is based on reflective mirrors. Each reflective mirror has around 40 pairs of Mo/Si layers [5] . The reflectivity from each mirror is around 67%.…”

OPC simplification and mask cost reduction using regular design fabrics

“…Figure 6 summarizes the tradeoffs between mask cost and yield for products with varying volumes. Lithography costs per layer was determined based on model proposed by Menon et al [13] and lithography cost data reported by Lin [14]. A 5% improvement in yield reduces the lithography cost per layer by 5% over all product volume.…”

PML2: the maskless multibeam solution for the 22nm node and beyond

“…1 It is predicted, however, that ML2 will have the lowest cost of ownership of all lithographic techniques at the 32nm-hp node and beyond if it can achieve a throughput of 15 wafers (300mm) per hour. 2 Projection Mask-Less Lithography (PML2) 3 is the multibeam maskless solution developed by IMS Nanofabrication and is based on its "charged particle large field projection optics" technology. The main strength of PML2 lies in the fact that pattern transfer is realized using an array of several hundreds of thousands individually addressable electron beams, thereby pushing the potential throughput from hours per wafer into the wafers per hour regime.…”