EUV lithography requirements continue to present new challenges and opportunities for multi-beam mask writer. Driven by sub-10nm node mask requirements for higher resolution, CD uniformity, pattern placement accuracy, lower line edge roughness (LER), and zero writer-induced defects, the multi-beam mask patterning technology must keep the pace, continue to innovate, and work hand-in-hand with mask makers to overcome these challenges to meet the mask and wafer manufacturing metrics and requirements.
In this paper we will review some of these challenges from the mask maker point of view. Also, we will shed light on a bigger challenge of transitioning to curvilinear mask ILT (Inverse Lithography Technology) data structure and the support needed from multi-beam writer to handle large data volume. Processing and managing this large growth of data helps the mask industry speed up the process of adapting to this technology and enabling EUV mask and wafer manufacturing reach its ultimate goals.
Multi-beam mask writers (MBMW) manufactured by IMS Nanofabrication have been increasingly been accepted into mainstream mask making. Over the past decade, this new class of tools has successfully transitioned from the concept, to development and finally to the production phase. Significant technical challenges specific to the architecture were encountered and overcome. Many of these challenges are related to the large image size used by this writer. In this paper, we will review the motivation to develop this new class of writers and the key technical challenges which had to be overcome to realize lithographic promise. Current status and future opportunities to improve the architecture will be discussed.
A raster multibeam lithography tool is in Etec’s roadmap to meet the stringent requirements of sub 100 nm mask fabrication. The tool leverages the long experience obtained with the ALTA laser pattern generators and the high resolution capabilities of e-beam lithography. A photocathode controlled by acousto-optic modulated 257nm laser beams is utilized to generate 32 electron beams. The beams are accelerated at 50 KV in an electron column, demagnified and focussed on the mask or wafer substrate. The performance of the photocathode and other system components will be presented together with preliminary lithographic patterning.
In response to next-generation mask requirements, Etec Systems, Inc has developed a complete raster-based patterning solution to meet the production needs of the 130 nm IC device generation as well as those for early 100 nm production. In developing this new MEBES system, we have aimed at versatility, extendability, and compatibility with conventional high-contrast resists and redesigned it form the ground up. This MEBES system incorporates many technological innovations, such as anew 50 kV electron-beam (e-beam) column, a new raster graybeam writing strategy, a new stage, an integrated automated material handling system, on-board diagnostics, and environmental/thermal control. A discussion of architectural details of the new MEBES system designed to meet the tight requirements of 130-100 nm technology nodes is presented. This comprehensive patterning solution offers the best combination of benefits to the user in terms of versatility, overall system throughput, and extendability. Initial throughput and lithographic performance benchmarks are also presented and are very promising in predicting the ability to meet critical dimension uniformity requirements of 10nm or better, as predicted by the ITRS requirements.
Finite element (FE) numerical models were proposed to simulate and predict substrate thermal expansion in photomask substrates and were found to be computationally expensive and dependent on the mask-writing strategy. The present work describes a newly developed model that predicts and corrects for the substrate heating effects in the photomask. This prosed model provides a practical way of predicting in-plane distortions during real-time patterning that is not limited to nay writing strategy or pattern density distribution. The main advantage of this model is that it significantly reduces the computational time by using the linear superposition theory. By adopting the concept of linear superposition, pattern placement errors of mask substrate scan be determined at any time during writing using lookup tables from precomputed FE models. IF the thermal distortion of the substrate at the time during writing using lookup tables from, precomputed FE models. IF the thermal distortion of the substrate at the time of writing is known, beam deflection can be introduced to correct for the distorted substrate. The result predicted by the linear superposition FE model showed a difference of less than 10 percent compared with those predicted using a real-time calculated Fe mode, in a worst case scenario. The accuracy of the linear superposition FE model was found to be partially dependent on the size of the simulated patterning field. The results presented in this paper illustrate the effect of other parameters on the performance of the newly developed model, such as the shape of the patterning fields and pattern coverage uniformity. The overview of this work focuses on fused silica mask substrate materials.
System architecture choices for an advanced mask writer (100 - 130 nm) have been evaluated. To compare and contrast variably shaped beam vector architecture with raster-based architecture, factors such as beam accelerating voltage and its effects on lithographic performance and system throughput for complex patterns have been studied. The results indicate that while both architectures have strengths and weaknesses, in the final analysis, raster-based systems offer the best combination of benefits to the user in terms of versatility and overall system throughput. Furthermore, other system requirements needed to support the challenges of the next generation mask writers are discussed. An architecture that includes a 50 kV raster graybeam (RGB), based architecture, a new writing strategy, a new stage system, an advanced environmental/thermal control management system, an automated material handling system, and a new resist and process is proposed.
Bulk (or global) heating of photomasks due to e-beam energy deposition during patterning causes thermal expansion of the mask substrate and leads to pattern placement errors. Finite element calculations were performed to simulate the in-plane distortions (IPD) due to the single pass writing of a 6 in. X 6 in. optical reticle. Comparison studies were performed to identify the effects of material properties (such as thermal conductivity and the coefficient of thermal expansion) when pattering SiO2 and CaF2 substrates. Final IPD maps illustrate that thermal distortions of the CaF2 will need to be controlled in order to satisfy increasingly stringent error budgets.
As feature sizes decrease and the demand for throughput increases, the semiconductor industry must concentrate on pattern positioning accuracy and process efficiency. Thermomechanical distortions induced in the photomask during fabrication may act to constrain the desired range of operating conditions to meet the manufacturing requirements for pattern placement accuracy and throughput. 3D finite element heat transfer and structural models have been developed to determine the global in-plane distortions induced in the photomask during e-beam patterning. Results obtained from these models show that the thermal-induced distortions, caused by global heating, are significant. Whereas, distortions due to the mechanical loading, caused by resist in situ stress relief, are minimal and can be neglected.
We present a technique for the measurement of the 2D distribution of the line-average void fraction in a transparent, gas-liquid flow, based on the attenuation of visible laser light by the gas bubbles in the flow. The technique is demonstrated in a test chamber of rectangular cross section. Bubbles are generated by flowing air through twenty holes in the side of a tube at the bottom of the chamber. The collimated beam of an Ar-ion laser traverses the test chamber through front and back Lexan walls, is refocused onto a pinhole and imaged with a CCD camera. Mie scattering by the air bubbles causes a spatial modulation of the laser beam, with the distribution of the logarithm of the light intensity linearly proportional to the distribution of the line average of the interfacial area density. The images are normalized against background non- uniformities and the interfacial area density calculated from the 2D transmittance distribution. The bubble diameter is estimated from the contours of the interfacial area density field and the line-average void fraction is calculated is estimated from the contours of the interfacial area density field and the line-average void fraction is calculated from the product of the interfacial area and bubble diameter fields. The results compare very favorably with measurements of volume-average void fraction based on the swell of the water level consequent to the air injection.