Allan L. Sagle

Design considerations for an electron-beam pattern generator for the 130-nm generation of masks

The critical dimension (CD) requirements of the SIA roadmap require continued improvements in pattern generation (PG) tool technology. This includes electron-beam (e-beam) delivery, resist and process, writing strategy, and overall system throughput. In this paper, we discuss these interrelated topics and evaluate their impacts on the CD control, linearity, and uniformity performance of PG tools. By means of Monte Carlo simulations and experimental comparisons, we evaluate various parameters of e-beam delivery systems, including beam energy, spot size, writing strategy, and throughput. We also perform a thorough evaluation of mask heating effects due to e-beam exposure. Finally, we perform comparative studies of various resist and process combinations. The totality of our investigations allows us to conclude that a 50 kV raster scan e-beam system, using a high- contrast, high-sensitivity resist, such as SPR 700, with GHOST proximity effect correction (PEC), can meet the CD control, linearity, and uniformity requirements of the 130 nm technology node.

Exploring the fundamental limit of CD control: shot noise and CD uniformity improvement through resist thickness

We have observed that increasing the thickness of the resist can improve the critical dimension uniformity (CDU) in electron beam lithography. This is our first experimental demonstration that increasing the acid generation in the resist by incident electrons is a pathway to reduce the effect of shot noise on CDU. The measurements were made with our Quadra raster shaped beam lithography system. The resist was REAP 200, a chemically amplified resist. The thicknesses were 200, 300 and 600 nm. The phenomenon is consistent with our model prediction that there would be a reduction of the shot-noise-induced CDU as the number of acid molecules generated in the chemically amplified resist increased with the resist thickness. We used the model to estimate the acid generation efficiency and the resist blur. We have also observed deviations from this trend in the thick resist (600 nm) suggesting complexity that may not be explained by the model. We are continuing our investigation to confirm these preliminary results.

Exploring the fundamental limit of CD control: a model for shot noise in lithography

We have made measurements with our Quadra raster shaped beam lithography system to evaluate the shot-noise-induced critical dimension uniformity (CDU). We found that at the isofocal dose, the shot-noise-induced CDU is directly proportional to the edge blur, and is linear with the rate of CD change with dose. Here we propose a phenomenological model which permits an experimentalist to relate the CDU to controllable lithographic parameters. The model considers both the counting statistics of the incident electrons and the noise from the electron induced chemistry. The model suggested that the shot-noise induced CDU may be minimized by reducing the beam blur, forward scattering, through the optimization of the resist smoothing distance and maximizing the number of acid molecules created in the resist by an incident electron. With the proper parameters, this model matches the experimental observations well. It also predicts the limit of the improvements and suggests guides for future resist development. Shot-noise induced line edge roughness is also discussed.

System architecture choices for an advanced mask writer (100 to 130 nm)

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.

Exploring the fundamental limit of CD control: a measurement of shot noise induced CDU in e-beam lithography

We have used our Quadra lithography system to evaluate the shot-noise-induced critical dimension uniformity (CDU). We found that at the isofocal dose, the shot-noise-induced CDU is directly proportional to the edge blur, and hence the rate of CD changes with dose. This emphasizes the importance of minimizing beam blur of the system. We used a phenomenological model to analyze our experimental data. The model included the counting statistics of the incident electrons and that of the electron induced chemistry. With the proper parameters, this model matches the experimental observations well. It also predicts the limit of the improvements and suggests guides for the optimization of the lithographic process.

Raster Shaped Beam Pattern Generation for 70 nm Photomask Production

Photomask complexity is rapidly increasing as feature sizes are scaled down and as optical proximity correction (OPC) methods become widespread. The growing data content of critical mask levels requires that pattern generator solutions be adapted to maintain productivity. Raster shaped beam (RSB) technology has been developed to enable the production of 70 nm photomasks and the development of 50 nm masks. RSB is built on and extends the capability of the 50 kV MEBES platform. The beam is shaped as it is scanned, printing the mask pattern on a calibrated flash grid. Complex OPC patterns are efficiently tiled by combining a relatively small maximum shape size with a high flash rate of 100 MHz. The maximum shape size and the current density can be adjusted to match a wide set of mask applications. Proximity effects are corrected with dose modulation using a real-time computation.

0.25-μm lithography using a 50-kV shaped electron-beam vector scan system

Performance data from a prototype 50 kV shaped electron-beam (e-beam) pattern generator is presented. This technology development is targeted towards 180-130 nm device design rules. It will be able to handle 1X NIST X-ray membranes, glass reduction reticles, and 4- to 8-inch wafers. The prototype system uses a planar stage adapted from the IBM EL-4 design. The electron optics is an 50 kV extension of the AEBLE%+TM) design. Lines and spaces of 0.12 micrometers with < 40 nm corner radius are resolved in 0.4 micrometers thick resist at 50 kV. This evolutionary platform will evolve further to include a new 100 kV column with telecentric deflection and a 21-bit (0.5 mm) major field for improved placement accuracy. A unique immersion shaper, faster data path electronics, and 15-bit (32 micrometers ) minor field deflection electronics will substantially increase the flash rate. To match its much finer address structure, the pattern generator figure word size will increase from 80 to 96 bits. The data path electronics uses field programmable gate array (FPGA) logic allowing writing strategy optimization via software reconfiguration. An advanced stage position control (ASPC) includes three-axis, (lambda) /1024 interferometry and a high bandwidth dynamic corrections processor (DCP). Along with its normal role of coordinate transformation and dynamic correction of deflection distortion, astigmatism, and defocus; the DCP improves accuracy by modifying deflection conditions and focus according to measured substrate height variations. It also enables yaw calibration and correction for Write-on-the FlyTM motion. The electronics incorporates JTAG components for built-in self- test (BIST), as well as syndrome checking to ensure data integrity. The design includes diagnostic capabilities from offsite as well as from the operator console. A combination of third-party software and an internal job preparation software system is used to fracture patterns. It handles tone reversal, overlap removal, sizing, and proximity correction. Processing of large files in a commercial mask shop environment is made more efficient by retaining hierarchy and using parallel processing and data compression techniques. Large GDSIITM and MEBES data files can be processed. Data includes timing benchmarks for a 1 Gbit DRAM on both proximity and reduction reticles. The paper presents 50 kV results on silicon and quartz substrates along with examples of overlay to an external grid, field butting, and critical dimension (CD) control data. Selective experiments testing system stability, calibration accuracy, and local correction software implementation on a VAX control computer are also given.