FEP-171 resist is commonly used both together with 50 keV VSB and DUV laser mask writers. To improve resolution and other lithographic parameters, the industry has strived towards thinner resist and absorber films on the mask blank. The chrome thickness and etch resistance limit how thin the resist can be. The NTAR7 (730Å) chrome was optimized for binary masks for 193 nm lithography, while NTAR5 (590Å) chrome is used for attenuated PSM blanks with a MoSi absorber beneath the chrome film.
Resolution and lithographic performance can be improved further by integrating improved processes, including PEB, development and dry-etch. Micronic has in a series of papers described improvements to the FEP-171 process in combination with different chrome films and the SLM-based DUV (248 nm) Sigma7300 mask writer. The thickness of FEP-171 for Sigma7300 has been optimized for NTAR7 chrome and improvements have been described for the PEB and dry-etch process of the FEP-171/NTAR7 blanks.
In this paper we describe the FEP-171 process development further. We have investigated improvements to the develop process for FEP-171/NTAR7 blanks using Design of Experiments (DOE) and a Steag Hamatech ASP-5000. Improved performance on mask, especially for CD linearity and clear-field/dark-field deviation, was achieved using the resulting development recipe together with the Sigma7300. Better than 5 nm (range) CD linearity in chrome was demonstrated for isolated spaces in the range 200-1400 nm.
This work also covers a process study of FEP-171 on NTAR5 chrome. The resist thickness was optimized to 3200Å for the Sigma7300 and the performance was tested in terms of resolution, resist profile, CD linearity and CD uniformity. Resolution of 120 nm isolated lines and 140 nm isolated spaces was demonstrated, as well as 4 nm (range/2) global CD uniformity.
With each new technology generation, photomask manufacturing faces increasing complexity due to shrinking designs and accelerating use of reticle enhancement techniques. Denser and more complex patterns on the mask result in lower yields and long write and turn-around times, important factors for the rapidly increasing mask related costs in IC manufacturing. Laser pattern generators operating at DUV wavelengths were recently introduced to provide cost effective alternatives to electron-beam systems for printing of high-end photomasks. DUV wavelengths provide the required resolution and pattern fidelity. Optical tools that use raster writing principles and massively parallel printing ensure short and predictable write times for photomasks almost independent of pattern complexity.
One such high-volume production system, the Sigma7300, uses spatial light modulator (SLM) technology and a 248 nm excimer laser for printing. Partially coherent imaging and multi-pass printing as in a lithography scanner further increases resolution and pattern accuracy. With four-pass printing the system provides resolution and pattern accuracy meeting mask requirements for critical layers at the 90-nm node and sub-critical layers at the 65-nm node and beyond.
The paper discusses how mask layout can be optimized to take full advantage of the speed potential provided by the SLM-based writer. It shows how flexible use of the writing principle can provide cost effective writing solutions for many layers in high-end mask sets. Resolution and pattern accuracy results from the Sigma7300 will be presented together with write times for different types of designs.
For many years, laser pattern generation has been printing on i-line resists. As features sizes continue to shrink, laser pattern generation is moving to DUV laser wavelengths, and a production worthy resist process is needed. Characteristics such as standing waves, resist foot and CD drift under and after exposure have previously challenged efforts to migrate 248nm stepper chemically amplified resists (CARs) to mask making applications. In this study the performance of a commercially available 248nm laser/e-beam resist solution is examined in the Sigma7000 series laser pattern generators. To achieve virtually no resist foot as well as tight CD control the optimum process conditions for DUV laser applications were determined. Cross-sectional and top-down scanning electron microscopy analysis was performed to evaluate the resist and dry etch processes. A comparison is made with the resist DX1100P, used in initial stages for DUV pattern generators development. The new resist also benefits from being well established in mask making e-beam mask writers.
Mask cost is a key challenge for the semiconductor industry and a major issue is the write times of e-beam pattern generators. DUV pattern generators can provide high throughput, but there is a cost and time involved in qualifying these tools for IC production. To minimize this time and cost, the masks from the DUV tool should have pattern fidelity similar to that of e-beam tools. This can be done with corner enhancements on an imaging DUV mask writer. Here, we describe such a corner enhancement scheme and present results for the 65-nm-node requirements. We demonstrate how the corner radius can be tuned in a range of radii with a negligible effect on the process latitude.
The Micronic Omega6500, a new high performance scanned laser mask lithography system, has been installed in a production mask facility of DuPont Photomasks, Inc. (DPI) The Omega6500 is a 5-beam system with an exposure wavelength of 413 nm, acousto-optic modulation beam intensity control and acousto-optic deflection. The use of a fast expandable datapath architecture along with a hierarchical data format allows extremely dense files to be printed at the full area coverage rate. Due to the differences between this tool and existing tools within DPI, and since this tool is completely new to the photomask industry, a cooperative project was initiated between DPI and Micronic to characterize the performance of the Omega6500 in a mask production environment. In specific we examined the optimization of the resist process, evaluated data handling capability and procedures, and determined changes required to job planning and manufacturing flow. Performance test procedures were created and used to evaluate the precision performance of the system. A production emulation plate suite was used to characterize pattern integrity and precision on real masks. Keywords: scanned laser lithography, photomask
180 nm photomasks require resolution and CD control that is normally not available from laser based systems. This paper describes the new Micronic Omega6000 laser pattern generator targeted at 180 nm as well as results from the system. The Omega6000 uses an architecture based on acousto-optics that improves the CD control. A 0.86 NA lens results in a high resolution. A dose modulation method provides a 5 nm address grid using single pass writing. The system incorporates a multi-processor data path designed to handle complex 180 nm patterns without reducing the writing speed. The data path is scalable which allows expansion with increasing data complexity.
With the shrinking design rules for semiconductors mask data complexity increases continuously. The increasing use of OPC, which has become common for advanced masks, reinforces this trend. The requirements on data processing increase and make it a possible bottleneck. Increasing write times directly impact the cost of the photomask. These facts raise the question of how to design a data path that will not limit the writing speed and throughput of a pattern generator.
CD uniformity is one of the most critical parameters for mask making today. The mask error factor (MEF) in lithography for features that are smaller than the stepper wavelength means that any CD error is transferred to the wafer to a greater extent than the stepper reduction factor would indicate. CD results form a new laser pattern generator, the Omega6000 product line, will be presented. The system features an acousto-optic deflection architecture specifically designed to meet the CD requirements of 180 nm photomasks. A 0.86 NA final lens provides the high resolution of the system. The CD control and the high resolution makes the system well suited for today's advanced photomasks.
The prevailing technique for assessing surface roughness and related parameters from engineering surfaces in industry is mechanical stylus profilometry. Though it is a conceptually direct method it suffers from several drawbacks; the measurement is not area-covering while many important features of engineering surfaces are related to 3D topography; the technique is slow; and, the technique involves contact with the surface, potentially damaging it. For surfaces of optical quality, elastic light scattering provides an efficient way of characterizing surfaces, and two ASTM standards exist pertaining to such measurements. However, the theoretical models based on first order perturbation theory break down when the rms roughness approaches one tenth of the incident wavelength. We have used infrared scattering at 10.6 micrometers wavelength to provide area-covering, non-contact surface characterization of engineering surfaces. The surface power spectral density is calculated from the scattering distribution by Rayleigh-Rice vector perturbation theory. Measurement of bandwidth limited surface roughness is demonstrated for ground and polished surfaces in the root mean square roughness range of 0.03 micrometers to 1.7 micrometers . Good correlation with contact stylus measurements is achieved for nearly 1D surfaces, anisotropic surfaces, and isotropic surfaces. Measurement of scratch and lay characteristics are also demonstrated.
A rigorous one-dimensional integral solution has been utilized to bring up some ideas about possible ways for detecting isolated surface defects by means of angle resolved scattering. Our preliminary results indicate that a suitable combination of polarization and different incidence angles can be useful. Numerical examples are given both for modelled defects and real defects measured with a Talystep profilometer.