Soft X-ray Projection Lithography Research Articles

Extreme ultraviolet lithography: Status and prospects

Extreme ultraviolet lithography (EUVL) using 13.5nm wavelength light is the leading candidate to succeed 193nm immersion lithography, enabling semiconductor chips with features smaller than 22nm. Several major programs worldwide have developed this technology in recent years [D. A. Tichenor et al., OSA Proceedings on Soft X-Ray Projection Lithography, edited by A. M. Hawryluk and R. H. Stuten (1993), Vol. 18, p. 79; H. Kinachita, OSA Proceedings on Soft X-Ray Projection Lithography, edited by A. M. Hawryluk and R. H Stulen (1993), Vol. 18, p. 74; J. P. H. Benschop, W. M. Kaiser, and D. C. Ockwell, Proc. SPIE 3676, 246 (1999)] and in 2006, ASML shipped the first EUV Alpha Demo tools (NA=0.25 full-field scanners) to IMEC in Belgium [A. M. Goethals et al., Proc. SPIE 6517, 651709 (2007)] and CNSE in Albany [O. Wood et al., Proc. SPIE 6517, 6517–041 (2007)], USA. Currently the development of preproduction tools with targeted shipment of 2009 is well under way. This paper discusses the most critical items for EUVL development, namely, EUV imaging and EUV sources. Furthermore, it elaborates on the necessary development of masks and resists and, for example, quantifies how resist diffusion length can impact imaging capabilities. Results obtained and lessons learned with the Alpha Demo tools are discussed, as well as potential solutions to some of the remaining challenges. Additionally, this paper explains how EUV can realize high productivity (>100wafers∕h) and high resolutions (<22nm) to continue the cost-effective shrink of semiconductors for several generations.

Fabrication and evaluation of Ni/C multilayer soft X-ray mirrors

To develop multilayer mirrors for use at wavelengths shorter than 12.4 nm in soft X-ray projection lithography, Ni/C multilayers were fabricated using r.f. sputtering. Evaluation of their layered structure and reflectivity showed that multilayer deposited at about 80 °C had substantial interface roughness, while those deposited at about −20 °C had well-formed layers with a periodic length of about 5.5 nm and an interface roughness of 0.25 nm. A fabricated Ni/C multilayer with 35 pairs of 2.3 nm Ni and 3.2 nm C layers showed a high measured reflectivity of 18% at a wavelength of about 7 nm.

Interferometry using undulator sources (invited, abstract)

Optical systems for extreme ultraviolet (EUV) lithography need to use optical components with subnanometer surface figure error tolerances to achieve diffraction-limited performance [M.D. Himel, in Soft X-Ray Projection Lithography, A.M. Hawryluk and R.H. Stulen, eds. (OSA, Washington, D.C., 1993), 18, 1089, and D. Attwood et al., Appl. Opt. 32, 7022 (1993)]. Also, multilayer-coated optics require at-wavelength wavefront measurement to characterize phase effects that cannot be measured by conventional optical interferometry. Furthermore, EUV optical systems will additionally require final testing and alignment at the operational wavelength for adjustment and reduction of the cumulative optical surface errors. Therefore, at-wavelength interferometric measurement of EUV optics will be the necessary metrology tool for the successful development of optics for EUV lithography. An EUV point diffraction interferometer (PDI) has been developed at the Center for X-Ray Optics (CXRO) and has been already in operation for a year [K. Goldberg et al., in Extreme Ultra Lithography, D.T. Attwood and F. Zernike, eds. (OSA, Washington, D.C., 1994), K. Goldberg et al., Proc. SPIE 2437, to be published, and K. Goldberg et al., J. Vac. Sci. Technol. B 13, 2923 (1995)] using an undulator radiation source and coherent optics beamline at the Advanced Light Source (ALS) at Lawrence Berkeley National Laboratory. An overview of the PDI interferometer and some EUV wavefront measurements obtained with this instrument will be presented. In addition, future developments planned for EUV interferometry at CXRO towards the measurement of actual EUV lithography optics will be shown.

13 nm advanced SASE free electron laser with a quasi-stable optical klystron configuration

Abstract For soft X-ray projection lithography, we propose a 13 nm advanced SASE FEL with a quasi-stable optical klystron configuration. To shorten the wiggler length and decrease the overall size of the FEL system, we put forward the concept of quasi-stable operation of an asymmetric optical klystron. With a similar tolerance, our advanced SASE FEL can cut down the magnetic length of a single-pass SASE FEL from 37.12 to 20.3 m at the saturation power 42.8 MW (3D corrected), at the cost of an energy-modulated e-beam bunch and an adjustable dispersion magnet system. The detail analyses are based on the Pellegrini's scaling law and the one-dimensional FEL simulation code ILT1D.

Electroplated Reflection Masks for Soft X-Ray Projection Lithography

A new process for fabricating reflection masks for soft X-ray projection lithography (SXPL) using an electroplating method was developed. Resist patterns were formed on molybdenum/silicon (Mo/Si) multilayers with the Mo layer on top, and nickel (Ni) absorber patterns were then electroplated on the multilayers using the top Mo layer as the electrode in nickel sulfamate solution. Subsequently, the resist patterns were removed. Using the reflection masks fabricated by the above process, SXPL was performed using 1/32 reduction Schwarzschild optics tuned to the wavelength of 13 nm and the synchrotron radiation source at SORTEC. Line-and-space patterns down to 0.1 µ m were printed in polymethylmethacrylate (PMMA) resist.

Evaluation of Large-Area Mo/Si Multilayer Soft X-Ray Mirrors Fabricated by RF Magnetron Sputtering

It was determined by calculation that the soft X-ray reflectivities of Tc/Si, Mo/Si, Nb/Si, and Ru/Si multilayers are highest at 13 nm wavelength. Six-inch-diameter Mo/Si multilayers, selected from among these multilayers, were fabricated by an rf magnetron sputtering system. Their periodic length uniformity was good (better than +/-1% deviation over the entire surface area) and the layer interfaces were fairly well defined. Reflectivity of 63% at around 13.7 nm wavelength was achieved at 3-degrees from normal incidence. Large-area high-quality multilayer mirrors for soft X-ray projection lithography can thus be fabricated using this rf magnetron sputtering system.

New optics design methodology using diffraction grating on spherical mirrors for soft x-ray projection lithography

Diffraction gratings on spherical mirrors can be used instead of aspheric mirrors in soft x‐ray projection lithography. A method for converting aspheric mirrors to gratings on spherical mirrors is introduced, and used to design optics for soft x‐ray projection lithography. Ray tracing through the new grating optics shows resolution capability better than 0.1 μm which is comparable to that of the conventional aspheric mirror optics. To reduce chromatic aberration, however, the spectral width of the system should be less than 0.05 nm for 13 nm wavelength. The gratings typically have spacing of 50–1000 μm, and can be fabricated using a binary process. This approach reduces stringent accuracy requirements of aspheric mirror optics for soft x‐ray projection lithography.

Design and analysis of diffraction mirror optics for EUV projection lithography

Reflective gratings on a sphere are used instead of aspheric mirrors in soft X-ray projection lithography. In doing so, typical three-mirror optics are converted into reflective grating optics. Ray tracing through the new grating optics shows a resolution of better than 0.1 μm, which is comparable to that of aspheric mirror optics. Although there are problems with chromatic aberration and diffraction efficiency, this approach relaxes the stringent accuracy requirements of aspheric mirror optics for soft X-ray projection lithography.

Resist performance in 5nm and 13nm soft X-ray projection lithography

Imaging experiments in 5nm and 13nm soft X-ray projection lithography (SXPL) were performed using two separate 32:1 reduction Schwarzschild optics with a Ni/CrC multilayer and a Mo/Si multilayer, respectively. The optics were illuminated with synchrotron radiation (SR) light source from the SORTEC ring. Sensitivities of 0.7μm-thick PMMA and ZEP are 285 and 34mJ/cm 2 at 5nm and those of 0.2μm-thick PMMA and ZEP are 134 and 38mJ/cm 2 at 13nm. Resist contrasts (γ-values) of 0.7μm-thick PMMA and ZEP are 2.5 and 1.6 at 5nm, and those of 0.2μm-thick PMMA and ZEP are 1.8 and 1.4 at 13nm. A 0.05μm-wide pattern can be replicated by 13nm exposure, while only just a 0.1μm-wide pattern can be replicated by 5nm exposure. The high resolution of 0.05μm at 13nm is limited by diffraction, while that of 0.1μm at 5nm is limited by the wavefront error of the multilayer-coated optics.

Resist Performance in 5 nm Soft X-Ray Projection Lithography

Imaging experiments in 5 nm soft X-ray projection lithography (SXPL) were performed using 32:1 reduction Schwarzschild optics with NiCr/C multilayer, which was illuminated with synchrotron radiation (SR) from the SORTEC ring. Sensitivities of 0.7 µ m-thick polymethylmethacrylate (PMMA), ZEP and AZ-PN100 are 285, 34, and 15 mJ/cm2, respectively. Resist contrasts (γ-values) of 0.7 µ m-thick PMMA, ZEP and AZ-PN100 are 2.5, 1.6 and 3.1, respectively. A 0.15 µ m line-and-space pattern can be replicated by 5 nm exposure. Resist performance in 5 nm SXPL was investigated with a special emphasis on the effect of resist film thickness. Resist contrasts of PMMA did not change markedly with increasing resist thickness. By 5 nm exposure, a 0.3 µ m line-and-space pattern was clearly delineated in 0.9 µ m-thick PMMA. This result confirms that a single resist scheme is applicable in 5 nm SXPL.

Power loading limitations in soft x-ray projection lithography

Soft x-ray projection lithography (SXPL) is an attractive technique for the fabrication of high-speed, high-density integrated circuits. In an SXPL stepper, the x-ray imaging mirrors consist of multilayer coatings deposited onto high precision substrates. The stepper is intended to fabricate ultra-high spatial-resolution structures with a minimum feature size of <0.1 μm. To achieve this resolution, the imaging mirrors must maintain a very precise surface figure while being exposed to x radiation. Failure to achieve and maintain the mirror surface figure will distort the wavefront propagating through the imaging system and will degrade system resolution. The required surface figure accuracy for each imaging mirror depends upon the required resolution, the wavelength, and the optical design. For conventional SXPL stepper designs, the total (peak-to-valley) surface figure error budget per mirror is approximately ±1 nm. Due to material properties at soft x-ray wavelengths and practical fabrication considerations, x-ray multilayer mirrors have limited reflectivities. A fraction of the incident x radiation is absorbed in the multilayer coating. This absorbed radiation constitutes a thermal load on the mirror, thereby distorting its shape and compromising the accuracy of its surface figure. In this paper, we analyze the thermally induced distortion on the imaging optics and conclude that the maximum allowable thermal distortion limits the maximum allowable x-ray power transported to the wafer and limits the minimum acceptable multilayer mirror reflectivity. The penalty for either insensitive x-ray resists or inefficient mirror reflectivity is a decrease in system throughput which cannot be compensated with increased source power either collected by condenser optics or generated by the source.

Laser plasma sources for soft X-ray projection lithography

Results are reported concerning high-repetition-rate excimer lasers with average powers up to 415 W and their usage for generating laser-plasma soft X-ray sources. A conversion efficiency of laser light into monochromatized soft X-ray radiation of 0.7% at 13.5 nm (2% bandwidth) was achieved using an excimer laser of which the beam quality was adapted for this application. Two methods to mitigate the production of plasma debris have been analyzed: tape targets and the use of Kr as a buffer gas. The optimum coating thickness of tape targets coated with Ta has determined to be 1 mum. Ta tape targets and the Kr buffer were used in a debris contamination test of 10(5) pulses. After this exposure, the reflectivity of a normal incidence Mo-Si multilayer mirror that faced the plasma, was found to be 18% lower. The contamination could be removed by cleaning, which restored the reflectivity to 97% of the initial value.

High-performance Mo/Si and W/B 4C multilayer mirrors for soft X-ray imaging optics

Soft x-ray projection lithography and microscopy require a high throughput and diffraction-limited performance of a multielement imaging system. To meet these requirements for a specific design it is necessary to (1) achieve high normal incidence reflectivity on each optical element while optimizing the d -spacing variation across the surface of the optical element and (2) match the d -spacing on each optical element to that of the others according to the ray-tracing design. A technique used to achieve normal incidence reflectivity greater than 60% at 13 nm for Mo/Si and greater than 2.7% at the “water window” region for W/B 4 C coatings is discussed. In addition, methods to obtain a d -spacing uniformity of ±0.4% and to match the d -spacing between imaging mirrors with an accuracy of ±0.5% are considered. A method for characterizing multilayer coatings on curved surfaces, using cylindrical witness optics with precalculated shape and radius of curvature to simulate final optics, and a manufacturing method for witness optics are also presented.

PHASE CONJUGATED LASERS APPLIED TO X-RAY GENERATION

Three laser systems that are being developed for use in X-ray generation which incorporate SBS phase conjugate mirrors are described. A 25 J/pulse Nd:glass laser is being developed for commercial proximity print X-ray lithography; a 0.5 J/pulse, 1.3 kHz pulse repetition frequency laser is being built for soft X-ray projection lithography; and a 1 kJ/pulse laser driver for a table top X-ray laser has been designed. The results of prototypical experimental investigations are presented and the basic design principles for high average power phase conjugated laser systems shared by each of these lasers are discussed.

Power Loading Limitations in Soft X-Ray Projection Lithography

Soft x-ray projection lithography (SXPL) is an attractive technique for the fabrication of high-speed, high-density integrated circuits. In an SXPL stepper, the x-ray imaging mirrors consist of multilayer coatings deposited onto high precision substrates. The stepper is intended to fabricate ultra-high spatial-resolution structures with a minimum feature size of <0.1 μm. To achieve this resolution, the imaging mirrors must maintain a very precise surface figure while being exposed to x radiation. Failure to achieve and maintain the mirror surface figure will distort the wavefront propagating through the imaging system and will degrade system resolution. The required surface figure accuracy for each imaging mirror depends upon the required resolution, the wavelength, and the optical design. For conventional SXPL stepper designs, the total (peak-to-valley) surface figure error budget per mirror is approximately ±1 nm. Due to material properties at soft x-ray wavelengths and practical fabrication considerations, x-ray multilayer mirrors have limited reflectivities. A fraction of the incident x radiation is absorbed in the multilayer coating. This absorbed radiation constitutes a thermal load on the mirror, thereby distorting its shape and compromising the accuracy of its surface figure. In this paper, we analyze the thermally induced distortion on the imaging optics and conclude that the maximum allowable thermal distortion limits the maximum allowable x-ray power transported to the wafer and limits the minimum acceptable multilayer mirror reflectivity. The penalty for either insensitive x-ray resists or inefficient mirror reflectivity is a decrease in system throughput which cannot be compensated with increased source power either collected by condenser optics or generated by the source.

Fabrication of 0.1 µm Line-and-Space Patterns using Soft X-Ray Reduction Lithography

Soft X-ray projection lithography with a reduction rate of 32 was examined using Mo/Si-multilayer-coated Schwarzschild optics. The optics were designed to function at 13 nm, and were aligned with the synchrotron radiation light source. The patterns on a transmission mask were imaged in a 0.18-µm-thick polymethyl methacrylate resist. Line-and-space patterns down to 0.1 µ m were fabricated.

Enhancement of reflectivity of multilayer mirrors for soft x-ray projection lithography by temperature optimization and ion bombardment

Abstract In this paper we discuss two techniques to optimize the quality of multilayer x-ray mirrors, namely optimization of the temperature of the substrates during deposition and ion-bombardment of the layers. We produced Mo/Si multilayers applying both methods and present the effect on the near normal incidence reflectivity for λ = 13–14 nm radiation. Furthermore an analysis of the homogeneity of the deposited layers is given.

High-Performance Mo/Si and W/B4C Multilayer Mirrors for Soft X-Ray Imaging Optics

Soft x-ray projection lithography and microscopy require a high throughput and diffraction-limited performance of a multielement imaging system. To meet these requirements for a specific design it is necessary to (1) achieve high normal incidence reflectivity on each optical element while optimizing the d-spacing variation across the surface of the optical element and (2) match the rf-spacing on each optical element to that of the others according to the ray-tracing design. A technique used to achieve normal incidence reflectivity greater than 60% at 13 nm for Mo/Si and greater than 2.7% at the "water window" region for W/B4C coatings is discussed. In addition, methods to obtain a rf-spacing uniformity of ±0.4% and to match the d-spacing between imaging mirrors with an accuracy of ±0.5% are considered. A method for characterizing multilayer coatings on curved surfaces, using cylindrical witness optics with precalculated shape and radius of curvature to simulate final optics, and a manufacturing method for witness optics are also presented.

Issues of laser plasma sources for soft x-ray projection lithography

Results of optimization of an excimer laser-induced plasma x-ray source for projection lithography in the range λ = 13–15 nm are reported. A conversion efficiency of >0.7% in 2% BW has been achieved with high-Z target materials. Two methods of reducing contamination of optical elements by target debris have been tested: usage of a thin target layer (for Ta 1 μm was found to be optimal) and of a heavy buffer gas. The effect of the use of Ta-tape target and Kr buffer gas has been measured by determining the reflectivity of a Mo-Si multilayer sample after 10 5 shots.

Sub-0.1 µm Resist Patterning in Soft X-Ray (13 nm) Projection Lithography

Poly (methylmethacrylate) (PMMA) resist performance for soft X-ray projection lithography (SXPL) at an exposure wavelength of 13 nm was investigated with an special emphasis on development condition dependence. The resist contrast value γ for 13 nm SXPL showed strong dependence on the developer, in contrast with proximity X-ray lithography (XRL) at the peak-wavelength of 0.7 nm. With an optimized developer, a resolution as high as 0.05 µm was achieved. A wide focus range of 1.5 µm with a 0.1 µm line-and-space pattern was also confirmed.