Steven E. Gianoulakis

Thermal management of EUV lithography masks using low-expansion glass substrates

Lithographic masks must maintain dimensional stability during exposure in a wafer stepper. In extreme UV lithography, multilayer coatings are deposited on a flat mask, substrate to make the mask surface reflective at EUV wavelengths. About 40 percent of the incident EUV radiation is absorbed by the multilayer coatings causing a temperature rise. The choice of mask substrate material affects dimensional stability due to thermal expansion and/or deformation. Finite element modeling has ben used to investigate the proper choice of mask substrate material and to explore the efficacy of various thermal management strategies. This modeling indicates that significant machine design and engineering challenges are necessary in order to employ Si as a mask substrate. Even if these challenges can be met, the thermal expansion of Si is likely to be too large to meet overlay error budgets for lithography at ground rules beyond the 100 nm technology node. ULE - a single phase, fused silica glass doped with titania - has near zero thermal expansion at the temperatures where EUV lithography is performed. Due to its small coefficient of thermal expansion, ULE does not undergo appreciable instantaneous or transient thermal expansion that results in image placement error.

Impact of thermal and structural effects on EUV lithographic performance

Thermal and structural effects will be an important consideration for all advanced lithography approaches targeting the 100nm technology generation and beyond. Such effects can contribute to loss of CD control, decrease in process latitude and reticle-wafer overlay error. This necessitates a system approach to account for thermo- mechanical effects in a complete system performance analysis of an EUV lithography tool. Multilayer-coated mirrors will typically absorb 35-40 percent of the in-band radiation causing thermal deformation of the mirror figure. In addition, Mo-Si multilayer films are deposited with compressive stress of approximately 350 MPa, which will also serve to deform the mirror substrate. To study these effects, we have inter-connected the capabilities of several software packages which include thermal and structural finite element, optical, and lithographic analysis. This enables us to determine the impact of mechanical effects on lithographic metrics such as the exposure-defocus process window, pattern placement and throughput. This paper includes result from a theoretical study of an EUV alpha tool with a wafer throughput of 20 200 mm wafers per hour for the 100nm technology generation.

Thermal–mechanical performance of extreme ultraviolet lithographic reticles

Thermal deformation of reticles will likely become an important consideration for all advanced lithography techniques targeting 130 nm features and below. Such effects can contribute to image placement errors and blur. These issues necessitate the need to quantify the reticle distortion, induced by the absorption of illumination power, for candidate substrate and coating materials. To study the impact of various substrate and coating materials on reticle performance, detailed three-dimensional transient thermal and solid mechanical models have been developed and extensively applied to predict total placement errors, residual placement errors, and blur on an extreme ultraviolet lithography (EUVL) reticle during scanning. The thermal model includes a bidirectional scanning heat source representative of the illumination incident on the reticle. The heat loads on the reticle are characteristic of an EUVL engineering test stand with a wafer throughput of twenty 200 mm wafers per hour (assuming 80% die coverage...

Low-power electrothermal actuation for microelectromechanical systems

Abstract. Electrothermal actuation has been used in microelectrome-chanical systems where low actuation voltage and high contact force arerequired. Power consumption to operate electrothermal actuators hastypically been higher than with electrostatic actuation. A method of de-signing and processing electrothermal actuators is presented that leadsto an order of magnitude reduction in required power while maintainingthe low voltage, high force advantages. The substrate was removed be-neath the actuator beams, thereby discarding the predominant powerloss mechanism and reducing the required actuation power by an orderof magnitude. Measured data and theoretical results from electrother-mally actuated switches are presented to confirm the method.

Development of an electric capillary discharge source

We report on the development of an electric capillary discharge source that radiates with comparable efficiency at both 13.5 nm and 11.4 nm, two wavelengths of interest for EUV lithography. The discharge source is comprised of a low- pressure, xenon-filled, small diameter capillary tube with electrodes attached to both ends. A high-voltage electric pulse applied across the capillary tube generates an intense plasma that radiates in the EUV. This source is capable of producing 7 mJ/steradian per pulse in a 0.3 nm bandwidth centered at 13.4 nm. In this paper we will address three significant issues related to the successful development of this source: minimization of debris generation, thermal management, and imaging quality.

System performance modeling of extreme ultraviolet lithographic thermal issues

Numerical simulation is used in the development of an extreme ultraviolet lithography Engineering Test Stand. Extensive modeling was applied to predict the impact of thermal loads on key lithographic parameters such as image placement error, focal shift, and loss of CD control. We show that thermal issues can be effectively managed to ensure that their impact on lithographic performance is maintained within design error budgets.

Experimental validation of a thermal model used to predict the image placement error of a scanned EUVL reticle

Lithographic masks must maintain dimensional stability during exposure in a lithographic tool to minimize subsequent overlay errors. In extreme ultraviolet lithography (EUVL), multilayer coatings are deposited on a mask substrate to make the mask surface reflective at EUV wavelengths. About 40% of the incident EUV light is absorbed by the multilayer coating which leads to a temperature rise. The choice of mask substrate material and absorber affects the magnitude of thermal distortion. Finite element modeling has been used to investigate potential mask materials and to explore the efficiency of various thermal management strategies. An experimental program was conducted to validate the thermal models used to predict the performance of EUV reticles. The experiments closely resembled actual conditions expected within the EUV tool. A reticle instrumented with temperature sensors was mounted on a scanning stage with an electrostatic chuck. An actively cooled isolation plate was mounted in front of the reticle for thermal management. Experimental power levels at the reticle corresponding to production throughput levels were utilized in the experiments. Both silicon and low expansion glass reticles were tested. Temperatures were measured a several locations on the reticle and tracked over time as the illuminated reticle was scanned. The experimental results coupled with the predictive modeling capability validates that the assertion that the use of a low expansion glass will satisfy image placement error requirements down to the 30 nm lithographic node.