In this paper, two negative-tone chemically amplified resists (CAR) are evaluated. The methodology and results are compared and discussed. The resists include EN-024M from TOK, and NEB 31 from Sumitomo. Both resists show high contrast, good dry etch selectivity, and high environmental stability. EN-024M showed good coating uniformity while NEB31 showed a coating uniformity problem. This was a round “dimple” approximately one centimeter in diameter of different thickness and density at the center of the plate. We addressed the “dimple” coating problem as described in the paper. Optimum PAB and PEB temperatures and nominal to maximum doses for isolated features were determined by running a matrix of PAB and PEB temperatures along with a dose series. We evaluated the process and compared the lithographic performance in terms of dose sensitivity, dose and bake latitude, resolution, resist profile, OPC (Optical Proximity Correction) pattern fidelity, CD uniformity, environmental stability, Line Edge Roughness (LER) and etching bias and resistance.
Improvements in the final uniformity of masks can be shrouded by error contributions from many sources. The final Global CD Uniformity (GCDU) of a mask is degraded by individual contributions of the writing tool, the Post Applied Bake (PAB), the Post Exposure Bake (PEB), the Develop sequence and the Etch step. Final global uniformity will improve by isolating and minimizing the variability of the PEB and Develop. We achieved this de-coupling of the PEB and Develop process from the whole process stream by using “dark loss” which is the loss of unexposed resist during the develop process. We confirmed a correspondence between Angstroms of dark loss and nanometer sized deviations in the chrome CD. A plate with a distinctive dark loss pattern was related to a nearly identical pattern in the chrome CD. This pattern was verified to have originated during the PEB process and displayed a [Δ(Final CD)/Δ(Dark Loss)] ratio of 6 for TOK REAP200 resist. Previous papers have reported a sensitive linkage between Angstroms of dark loss and nanometers in the final uniformity of the written plate. These initial studies reported using this method to improve the PAB of resists for greater uniformity of sensitivity and contrast. Similarly, this paper demonstrates an outstanding optimization of PEB and Develop processes.
KEYWORDS: Etching, Quartz, Photomasks, Reactive ion etching, Chemistry, Critical dimension metrology, Electron beam lithography, Scanning electron microscopy, Electron beams, Helium
High resolution etching of MoSi for photomask processing places new requirements on etching processes. As resist features are sized to 100 nm and below, it is first necessary to duplicate these features first into a chrome over-layer. After resist is stripped, this chrome over-layer is used for etching MoSi. Both chrome and MoSi etched profiles require near-vertical sidewalls, good CD (critical dimension) uniformity, good linearity, and CD mean-to-target (MTT). Additional requirements of etched MoSi include minimal roughness on exposed quartz, selectivity to chrome and quartz, phase angle target and phase angle uniformity, etch depth global uniformity, and etch depth uniformity as a function of feature size. An ETEC integrated process is used for the application of resist, patterning, and all subsequent processing. Chemically amplified resist is patterned with the 50 kV MEBES Quadra or MEBES eXara raster scan electron beam writer, allowing for patterning of small features with vertical resist profiles. Plates are etched in a Tetra photomask etch system for projecting resist images into chrome and MoSi. Etch processes have been developed specifically for etching small features in order to meet the requirements of 65 nm node lithography. An optimized etch process window is capable of patterning MoSi features below 100 nm sizes with near-vertical sidewall, < 20 nm etch bias, and with similar profile and etch bias for lines and spaces between 100 nm and > 1 um. Excellent CD uniformity and CD etch loading performance are demonstrated. Micro-profilometry is employed to measure the MoSi etch depths of features of varying sizes, and to quantify the effect of loading on MoSi etch depth. SEM micrographs illustrate sidewall profiles resulting from small feature etching.
In this paper, a process established with a positive-tone chemically amplified resist (CAR) from TOK REAP200 and Fujifilm Arch FEP171 and 50kV MEBES system is discussed. This TOK resist is developed for raster scan 50 kV e-beam systems. It has high contrast, good coating characteristics, good dry etch selectivity, and high environmental stability. In the mask industries, the most popular positive tone CAR is FEP171, which is a high activation energy type CAR. REAP (Raster E-beam Advanced Process) 200 is low activation energy type and new acetal protecting polymer. In this study, we compared to these different type resists in terms of contrast, PAB and PEB latitude, resist profile, footing, T-topping, PED stability, LER, Global CDU (Critical Dimension Uniformity) and resolution. The REAP200 Resist obtained 75nm isolated lines and spaces, 90nm dense patterns with vertical profile, and a good stability of delay time.
Photomask complexity threatens to outpace mask pattern generator productivity, as semiconductor devices are scaled down and optical proximity correction (OPC) becomes commonplace. Raster scan architectures are well suited to the challenge of maintaining mask throughput and mask quality despite these trends. The MEBES eXara mask pattern generator combines the resolution of a finely focused 50 keV electron beam with the productivity and accuracy of Raster Graybeam writing. Features below 100 nm can be imaged, and OPC designs are produced with consistent fidelity. Write time is independent of resist sensitivity, allowing high-dose processes to be extended, and relaxing sensitivity constraints on chemically amplified resists. Data handling capability is enhanced by a new hierarchical front end and hiearchical data format, building on an underlying writing strategy that is efficient for OPC patterns. A large operating range enables the MEBES eXara system to support the production of 100 nm photomasks, and the development of 70 nm masks.
Photo mask etching for the 100nm technology node places new requirements on dry etching processes. As the minimum-size features on the mask, such as assist bars and optical proximity correction (OPC) patterns, shrink down to 100nm, it is necessary to produce etch CD biases of below 20nm in order to reproduce minimum resist features into chrome with good pattern fidelity. In addition, vertical profiles are necessary. In previous generations of photomask technology, footing and sidewall profile slope were tolerated, since this dry etch profile was an improvement from wet etching. However, as feature sizes shrink, it is extremely important to select etch processes which do not generate a foot, because this will affect etch linearity and also limit the smallest etched feature size. Chemically amplified resist (CAR) from TOK is patterned with a 50keV MEBES eXara e-beam writer, allowing for patterning of small features with vertical resist profiles. This resist is developed for raster scan 50 kV e-beam systems. It has high contrast, good coating characteristics, good dry etch selectivity, and high environmental stability. Chrome etch process development has been performed using Design of Experiments to optimize parameters such as sidewall profile, etch CD bias, etch CD linearity for varying sizes of line/space patterns, etch CD linearity for varying sizes of isolated lines and spaces, loading effects, and application to contact etching.
A chemically amplified resist (CAR) process has been recognized as an approach to meet the demanding critical dimension (CD) specifications of 100nm node technology and beyond. Recently, significant effort has been devoted to optimizing CAR materials, which offer the characteristics required for next generation photomask fabrication. In this paper, a process established with a positive-tone CAR from TOK and 50kV MEBES eXara system is discussed. This resist is developed for raster scan 50 kV e-beam systems. It has high contrast, good coating characteristics, good dry etch selectivity, and high environmental stability. The coating process is conducted in an environment with amine concentration less than 2 ppb. A nitrogen environment is provided during plate transfer steps. Resolution using a 60nm writing grid is 90nm line and space patterns. CD linearity is maintained down to 240nm for isolated lines or spaces by applying embedded proximity effect correction (emPEC). Optimizations of post-apply bake (PAB) and post-expose bake (PEB) time, temperature, and uniformity are completed to improve adhesion, coating uniformity, and resolution. A puddle develop process is optimized to improve line edge roughness, edge slope, and resolution. Dry etch process is optimized on a TetraT system to transfer the resist image into the chrome layer with minimum etch bias.
This paper reports on the results of the optimization of a high throughput sputter etch process prior to interconnect deposition with no device damage, performed on the Endura HP PVD system. The second level metallization for a 2-level metal interconnect scheme requires a pre- metallization sputter etch of the vias in order to remove the native oxide and the fluoro-carbon residues left after the ILD oxide etch. For high throughput processing, high etch rates are required along with the essential prerequisites of plasma processing, namely, good process uniformity and no electrical device damage. A high density dual-frequency system with independent control of plasma and bias powers was used to optimize the pre-metallization etch using the second level of a 256 K SRAM device test structure fabricated in a 150 mm wafer production line. Response surface methodology (RSM) was used to explore the parameter space of the sputter etch process. The rf plasma power, rf bias power and the SiO2 thickness etched were chosen as the 3 independent variables. The SiO2 etch rate, SiO2 etch uniformity, via resistance & uniformity and threshold voltage & uniformity were measured and modeled as the response variables. SiO2 etch rates from 214 A/min to 926 A/min, SiO2 etch uniformities from 1.88 to 7.27% 1-sigma, via resistances from 0.32 to 0.42 Ohms, and threshold voltages from 0.74 V were obtained. A suitably wide process window was established with the excellent process/device results and a 40% reduction in process time.
We have examined several metal films currently used in IC manufacturing and reported the results of reflectivity as a function of different processing parameters. The objective when characterizing a film to be used as an anti-reflective coating (ARC) is to locate and optimize the process window to achieve the minimum reflectance at the operating wavelength of a stepper. Reflectivity measurements as a function of thickness and process conditions, across broadband wavelengths, are presented. These results show the variation in minimum reflectance as a function of these variables. Various thicknesses of coherent titanium nitride (TiN) and titanium tungsten (TiW) films have been studied to understand the application of these films to interconnect metallization schemes as anti-reflective coatings. Additionally, a series of aluminum (Al) wafers are created to show the variation in measured absolute reflectivity as a function of process parameters. The aluminum deposition temperature was varied from 100 degree(s)C to 500 degree(s)C.
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