Low Energy Electron-beam Proximity Research Articles

Performances by the electron optical system of low energy electron beam proximity projection lithography tool with a large scanning field

An electron optical system has been developed for a low energy electron beam proximity projection lithography tool that is capable of scanning a field of 40mm square with a beam current up to 20μA. By using a beam current of 18μA the system has successfully resolved a dense 46nm hole pattern with 90nm pitch near one corner of the field. This article introduces a double-pinhole technique to monitor and measure the telecentricity of the electron beam (or e-beam) of such a system. Other related areas covered in the article are the simulations of space charge effect and of deflection aberration: also discussed here is the linearity of the subdeflection system used for correcting the mask distortion.

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.

Optimization of Low-Energy Electron Beam Proximity Lithography Stencil Mask Structure Factors by Monte Carlo Simulation

Low-energy electron beam (e-beam) proximity lithography (LEEPL) is a notable next-generation lithography (NGL) technology. This study was performed to search for the most optimized structure of the LEEPL mask. The acceleration voltage of the e-beam and the angle of the pattern wall were taken as variables for our simulation. We calculated the energy and the coordinate of each electron with various possible combinations of these variables. Using this procedure, the count and angular distribution of electrons transmitted through a Si mask were analyzed. For the 50-nm-linewidth processing, the dependencies of the electron count distribution and the electron angular distribution on the pattern wall angle were very high in the case of using relatively high acceleration voltage such as higher than 2 keV.

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.

Completion of the β tool and the recent progress of low energy e-beam proximity projection lithography

We have proposed low energy electron-beam proximity projection lithography [T. Utsumi, Jpn. J. Appl. Phys., Part 1 38, 7046 (1999); J. Vac. Sci. Technol. B 17, 2897 (1999)] (LEEPL) for LSI production lithographic processes below 100-nm-feature size. One and one half years ago, the proof of concept (POC) of LEEPL was successfully completed using an α tool. Now, a β tool of LEEPL, which is similar to a mass production tool in the 100- and 70-nm-technology node, has been developed. We have already completed the performance evaluations of the β tool and confirmed proof of lithography. We obtained patterning resolution of 45-nm-L/S patterns and 48-nmφ-hole patterns in resist images and overlay accuracy of 23 nm (3σ) in the x direction and 31 nm (3σ) in the y direction over an effective area of 8 in. wafer. Furthermore, functionality of complementary mask alignment was demonstrated and logic type device-like patterns in 100-nm-technology node were fabricated.

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.

Low energy electron-beam proximity projection lithography: Discovery of a missing link

Low energy electron(e)-beam proximity projection lithography is proposed for integrated circuit lithography for minimum feature sizes ⩽0.1 μm. This new e-beam lithography is similar to optical projection lithography except that photons are replaced by low energy electrons of 2 kV. The low e-beam energy permits the use of single crystal 0.5 μm thick silicon membrane masks without an absorbing metal layer of high atomic number. This membrane mask is thick enough for good heat conduction and thin enough for feature sizes ⩽0.1 μm. The mask distortion caused by the fabrication can be corrected by a fine-tuning deflector. Therefore the mask is allowed to have a residual distortion of more than 100 nm. The proposed system does not suffer from the space charge effect in the electron optics column nor the proximity effect with respect to both the wafer and mask writings, and it is fundamentally a low power lithography which needs no special cooling system. The analysis shows that the e-beam column can be made entirely of electrostatic components to achieve sufficient resolution. For an appropriate resist process for this low energy e-beam we propose a bilayer process which consists of a chemically amplified thin deep ultraviolet photoresist and a thick antireflective layer. We estimated a throughput of 30 12 in. wafers per hour and a resolution of less than 50 nm.