Low-energy Electron-beam Proximity-projection Lithography Research Articles

Low energy electron beam proximity projection lithography

The low energy electron beam proximity projection lithography (LEEPL) system consists of three properties: low energy electron beam, a parallel beam, and proximity projection. The low energy electrons increase the effective resist sensitivity and greatly minimize the proximity effect. Over a 20 µm depth of focus is achieved by the parallel beam on the proximity projection. The subdeflection system of the LEEPL system is useful in correcting the mask distortion and chip distortion on the wafer by a correction data map corresponding to the field, because of the space (>30 µm) between the wafer and the mask. The overlay accuracy of the machine itself is less than 14 nm and that of mix and match is less than 25 nm. This implies that the overlay between the LEEPL system and an ArF scanner in both the x and y directions are obtained. This machine shows the 48 nm CH resist patterns as the ultimate resolution. The cost of ownership (CoO) of the LEEPL system for a 65 nm node device will be approximately less than $25/wafer/layer and the value is lower than that of an ArF scanner.

Impacts of 30-nm-thick resist on improving resolution performance of low-energy electron beam lithography

Exploring the resolution performance of various electron beam lithography with the acceleration voltage of 2–100kV showed that line edge roughness (LER) and resolution limit of resist patterns was in linear relation with blur of latent image profile. Reducing resist thickness is effective in decreasing the blur of low-energy electron-beam proximity projection lithography (LEEPL) because 47% of the blur is due to scattering of 2keV electrons in 70-nm-thick resist film. The impact of thinning the resist thickness on the LER and the exposure latitude in LEEPL was investigated by simulation and experiment. A Monte Carlo simulation shows that the LER of a 100-nm-width space pattern decreases to 5.5 from 10.4nm with reducing the resist thickness to 20 from 70nm. Exposing the photosensitive polyorganosilazane resists (Clariant (Japan) K.K.) by LEEPL-3000 underpinned that latent image quality was improved by thinning the resist. The exposure latitude of 80-nm-diameter dense contact holes increased to 25 from 16% with reducing the resist thickness to 30 from 50nm. It is necessary for taking an advantage of the improved latent image of the 30-nm-thick resist in resolving the 90-nm-pitch patterns to mitigate the edge roughness probably caused by the properties of a highly interfacial resist.

Progress in proximity electron lithography: demonstration of print and overlay performance using the low-energy electron beam proximity-projection lithography β tool

The lithographic performance of the low-energy electron-beam proximity-projection lithography (LEEPL) tool is demonstrated in terms of printability and overlay accuracy to establish the feasibility of proximity electron lithography (PEL) for the 65-nm technology node. The CD uniformity of 5.8 nm is achieved for the 1× stencil mask, and the mask patterns are transferred onto chemically amplified resist layers, coupled with a conformal multilayer process with the mask-error enhancement factor of nearly unity. Meanwhile, the overlay accuracy of 27.8 nm is achieved in the context of mix and match with the ArF scanner, and it is also shown that real-time correction for chip magnification, enabled by the use of die-by-die alignment and electron beam, can further reduce the error down to 21.3 nm. On the basis of the printability of programmed defects, it is shown that the most critical challenge to be solved for the application to production is the quality assurance of masks such as defect inspection and repair.

Resolution-limiting factors in low-energy electron-beam proximity projection lithography: Mask, projection, and resist process

The resolution-limiting factors in low-energy electron-beam proximity projection lithography were analyzed quantitatively using the blur of a Gaussian-shaped latent image (σQBP) as the resolution index. σQBP is the square root of the sum of squares of the factors, such as electron scattering and resolution performance of resist. The resolution limit of 45 nm for isolated patterns and the resolution of 70 nm for practically used periodic patterns with 10% exposure latitude were achieved at σQBP of 49 nm. Eliminating a crossover in the electron optics decreased the factor depending on the gap between a mask and a wafer to 19 nm at a 40 μm gap. Because of the intensive studies on multilayer processes, the factor attributed to the resolution performance of thin resist dropped from 58 to 26 nm. Reduction in the blur due to electron scattering, 34 nm in the case of a 70-nm-thick resist film and 2 keV electrons, must be considered for the 45 nm technology node.

Challenges to Ultra-thin Resist Process for LEEPL

An ultra-thin resist process is indispensable for low-energy electron-beam proximity projection lithography (LEEPL) because it uses 2-kV-accelerated electrons with small penetration depth. 70- nm-thick chemically-amplified resists for a tri-layer process were developed with the consideration of the interaction of a polymer with a spin-on-glass material, showing the resolution of 140-nm- pitch contact holes. Application of the tri-layer process developed for LEEPL to making via holes in a 90-nm-node back-end-of-line process proved that the ultra-thin resist was lithographically useful in terms of resolution and etching tolerance. Exploring the resolution performance of electron beam lithography showed that line edge roughness and resolution limit of resist patterns was in linear relation with blur of latent image profile. Reducing the resist thickness is effective in enhancing the resolution of LEEPL because 47 % of the blur is attributed to electron scattering. A Monte Carlo simulation shows that the blur caused by the electron scattering decreases 41 %, to 20 nm from 34 nm, by reducing the resist thickness to 30 nm from 70 nm.

Stencil Mask Technology for Electron-Beam Projection Lithography

The development of silicon stencil masks for cell projection lithography (CPL), electron projection lithography (EPL), low-energy electron-beam-proximity-projection lithography (LEEPL), and low-energy electron-beam lithography (LEB) is described. To satisfy the required pattern accuracy of the stencil masks, such as critical dimension and image placement, we developed a high-aspect stencil pattern formation technique to achieve a side-wall angle of over 90 deg and a membrane stress control technique using a stress-correction layer. In addition, for fabricating low-magnification stencil masks, we developed a new mask substrate with a sputtered scattering silicon membrane and a sputtered intermediate stopper layer as an etching stopper. We selected CrNx as the intermediate layer material, which demonstrated high performance in stencil mask fabrication. By improving the CrNx film qualities, its durability to dry and wet etching of backside silicon was improved. In order to improve the pattern image placement accuracy caused by membrane stress changes during stencil pattern formation, we developed a new and simple technique using a stress correction layer, which also functions as a silicon deep-etching mask. By using this functional layer, the total stress change in the pattern field in a mask pattern formation process was reduced to within 5 N/m, which corresponds to less than a 10 nm pattern image placement (IP) error.

Complementary masking approach for proximity electron lithography

We have devised a new mask format for low-energy electron-beam proximity-projection lithography (LEEPL) that enables faster and more accurate image transfer from the mask to a wafer, in comparison with the single-membrane mask as originally proposed. Simultaneous exposure over four adjacent complementary quadrants of the mask, synchronized with step-and-repeat motion of the wafer stage, yields one complete pattern with the throughput of ∼30 wafers/h. Mechanical analysis has demonstrated that the new format also offers better controllability of image placement owing to smaller membranes, even though the gravitational deformation of the mask, for example, must still be corrected for.

Present Status of Exposure Tool Development for Low Energy Electron-beam Proximity Projection Lithography.

A s-tool for Low Energy Electron-beam Proximity Projection Lithography (LEEPL)1-2 has been developed for proof of lithography (POL) of mass production tool, which is applied to required performance in 100-nm and 70-nm technology node. Major features of system design are an acceleration voltage of 1-5kV, a beam current of 3-20μA, a maximum field size of 40mm x 40mm (effective field size of 25mm x 25mm), a die-by-die alignment system and automatic loader systems for masks and wafers. The throughput is estimated over 20 wafers of 300mmφ per hour. As examples of initial evaluation results, resolution of 45-nm L/S patterns and 48-nmφ hole patterns in resist image were obtained. Alignment experiments are on going now, and overlay accuracy around 20-nm (3σ) over effective area of 8-in. wafer is being expected.

Comparative study of resolution limiting factors in electron beam lithography using the edge roughness evaluation method

Resolution capabilities of a Nikon 100 kV experimental column of electron beam (EB) projection lithography, a proof of concept column of low energy electron beam proximity projection lithography (LEEPL), and a variable-shaped EB direct writer were compared using a negative-type chemically amplified resist. Line edge roughness and resolution limit of an isolated line were 2.8 and 40 nm for the Nikon experimental column, and were 4.9 and 52 nm for the EB direct writer at the resist thickness of 250 nm, while they were 10.7 and 61 nm for LEEPL, and were 9.6 and 57 nm for the EB direct writer at the resist thickness of 70 nm. Quantitative influences of resolution limiting factors such as electron-optical beam blur and resist performance were analyzed using the edge roughness evaluation method. The analysis shows that the Nikon experimental column has an ability to resolve 35 nm patterns if resolution performance of the resist is 10% improved. In the high-throughput EB stepper system with a large beam current, it is necessary for resolving 50 nm patterns to keep the electron-optical beam blur under 36 nm. It is known about LEEPL that a necessary and sufficient condition for realizing 50 nm patterns is to develop a 50 nm thick resist having the same resolution performance as the conventional resist dedicated for the use with thicker film thickness, and to obtain a 50 nm pattern mask having the same aperture roughness as the conventional mask with 70 nm patterns.