Morihisa Hoga

Evaluating next-generation reticle demands on lithography equipment

Industry trends indicate that the next generation of exposure tool will be scanning steppers. Scanning steppers have a 25 mm by 33 mm field using 6 inch reticles. For device manufacturing, first generation 256M DRAM chips are roughly 12.5 mm by 25 mm, while 1G DRAM are expected to be 30 mm by 15 mm. As a result, a 25 mm by 33 mm field does not allow the exposure of two or more chips per shot. Therefore introduction of a larger mask is expected. To determine next generation reticle size standard, many factors have been investigated. First, an assumption was made that DRAM chip size trends will remain constant. Then, the throughput of scanning stepper was calculated with 6, 7, 8 and 9 inch reticles. The advantages were dependent on chip generation and device (memory or logic). Finally, a 9 inch reticle standard was chosen. Making the leap to a 9 inch reticle standard avoids the development time and costs of incremental changes in the standard. The thickness of a reticle is dependent upon reticle distortions, projection lens focus error. The deformation of the reticle has been simulated for the 0.25 to 0.5 inch thickness range. The final outcome of these simulations was that 9 mm (approximately 0.35 inch) was selected as the best thickness.

Performance improvement in e-beam reticle writer HL-800M

An advanced e-beam mask-writing system HL-800M has been developed for the 0.25-micrometer rule-devices. To meet the design-rule, the targets of this system specifications are critical dimension (CD) control of 30 nm, positioning accuracy of 40 nm, and throughput over 0.5 plate per hour. To achieve CD control, we judged that it was inevitable to increase the acceleration voltage up to 50 kV for patterns smaller than 2 micrometer. However, for patterns larger than 5 micrometer, the e-beam proximity-effect causes the pattern-width linearity to be worse. To achieve the sufficient linearity, proximity correction on the hardware module of the systems was performed. This hardware module executes proximity effect correction for each patterns over the area on the plate, so that total throughput was improved compared with that of the correction by software. Besides, a noise cancellation module was introduced to reduce the errors in the e-beam shot positions. This module detects the vibration noise caused by with the power-supply frequency and feeds the correction signal back to the e-beam deflectors. For positioning accuracy, in addition to the mirror correction using hardware for the stage interferometer, a new positioning-correction function depending on the coordinates of the system was developed. In the results of the exposure evaluations, CD uniformity on a 6025 plate showed width-deviations of 3 sigma were 31 nm (X) and 18 nm (Y). Pattern-width linearities for various kinds of patterns were within plus or minus 50 nm. Furthermore, the noise cancellation module was made the amplitude of the e-beam vibration reduced from 33 nm to less than 8 nm. For positioning accuracy, evaluation patterns measured by the LMS2020 (Leica) showed sufficient results for our target. For throughput, the average of the writing time per 6-inch plate for ten patterns is shorter than our targeted throughput with a dosage of 4 (mu) C/cm2. The HL-800M system is capable of producing reticles for 0.25-micrometer design-rule.

Error Analysis in Electron Beam Lithography System -Thermal Effects on Positioning Accuracy-

Thermal effects on positioning accuracy in an electron beam lithography system have been evaluated. In order to make 5X reticles for 0.3-µm ULSIs, positioning accuracy as small as 0.06 µm is required. Thermal effects are significant problems in achieving highly accurate reticle judging from positioning error analysis. In this study, internal thermal effects were investigated; these were (1) heating by electron beam illumination and (2) stage friction with step-and-repeat movement. As a result, the positioning error due to electron beam illumination is negligibly small compared with 0.06 µm and the stage temperature fluctuation must be controlled within 0.07°C to maintain a positioning error within 0.06 µm. Furthermore, internal thermal effects of the system could be fatal in next-generation electron beam lithography systems without improvement of materials and systems.

Chemically amplified positive resist for next-generation photomask fabrication

We have been developing novolak-based chemically amplified positive resists for the next generation photomask fabrication. In this paper, we report two different types of EB resists: RE-5150P and RE-5160P. Our resist materials consist of four components: a novolak matrix resin, a polyphenol compound, an acid generator and a dissolution inhibitor. We applied two different types of dissolution inhibitors to our resist materials. RE-5150P and RE-5160P employed respective a high and a low activation energy type of a dissolution inhibitor. RE-5150P has high contrast and RE- 5160P has wide process window. As a result, we confirmed RE- 5150P could achieve 0.24 micrometer line-and-space vertical resist pattern profiles at 8 (mu) C/cm2 using the 50 kV EB- writer HL-800M, and RE-5160P has wide process window: post exposure delay stability is over 24 hrs. and post coating delay stability is over 30 days.

Stitching Error Analysis in an Electron Beam Lithography System: Column Vibration Effect

The column vibration effects on field stitching accuracy in an electron beam lithography system are investigated. Field stitching error analysis shows that beam placement error caused by column vibration is about 0.04 µm. This corresponds to 80% of the required accuracy for 0.3-µm ULSIs. In this study, the relation between column vibration and field stitching accuracy is clarified using modal analyses by measurement of column acceleration and by computer simulation. Based on these analyses, a new anti-vibration system was designed. As a result, stitching error due to column vibration is reduced to less than 0.01 µm, and field stitching accuracy of 0.05 µm (|Mean|+3σ) is achieved. In the next-generation, quantitative understanding of disturbance effects such as vibration will be the most critical issue in designing more accurate electron beam lithography systems.

Advantage in using the combination of HL-800M and CAR process

The advanced 50 kV e-beam mask writing system HL-800M (Hitachi Co. Ltd.) was developed for 0.25 - 0.18 micrometer design-rule mask fabrication and widely applied. The combination of 50 kV e-beam writing system (EB) and Chemically Amplified Resist (CAR) is one of the solutions to improve accuracy for the fabrication of further high-end masks. The purpose of this study is to show the advantages of Critical Dimension (CD) accuracy in using the combination of 50 kV EB;HL-800M and positive-CAR; RE-5120P (Hitachi Chemical Co. Ltd.). In order to control CD, Proximity Effect Correction (PEC) is indispensable for the high acceleration voltage EB. Therefore, HL-800M has a high-speed-PEC system with hardware circuits. In this study, the PEC condition of HL-800M was optimized to improve CD accuracy. As a result, CD linearity of 18 nm was obtained in the pattern width from 0.7 micrometer to 3 micrometer. Besides, we evaluated the CD variation due to resist heating in using this combination. And, in the experiment of the resist heating effect, the CD variation was less than plus or minus 7 nm in the range of dosage ratio from 100% (11 (mu) C/cm2) to 500%. In other words, the CD variation due to resist heating is not so much serious problem for practical use in using the combination of the 50 kV EB and CAR.

Improvement of CD accuracy for next-generation reticles using HL-800M and CA resists

The 50 kV electron-beam (EB) writing system HL-800M (Hitachi Co. Ltd.) was developed for 0.25 - 0.18 micrometer design-rule mask fabrication and widely applied. Chemically Amplified Resist (CAR) has merits of high sensitivity, high resolution and dry-etching durability. The combination of 50 kV EB and CAR is one of the best solutions to improve accuracy and throughput of next generation reticles such as 0.18 micrometer design-rule mask and beyond. The purpose of this study is to optimize the exposure and process conditions of the combination of 50 kV EB and CAR for improving Critical Dimension (CD) accuracy. At first, new positive-type CAR; RE515OP (Hitachi Chemical Co. Ltd.) has been evaluated. This resist shows the high resolution of 0.25 micrometer. Because of the vector-scanning EB such as HL-800M, the use of negative-type resist improves throughput of exposure. Negative-type CAR; NEB-22A (Sumitomo Chemical Co. Ltd.) has been also evaluated. This resist shows also the high resolution of 0.14 micrometer. It is clarified that both resists have the characteristic to meet the 0.18 - 0.15 micrometer design-rule mask fabrication. Besides, in order to improve CD accuracy with HL-800M, Proximity Effect Correction (PEC) condition has been optimized. Especially, as parameters of mesh-size and times of smoothing area-density, CD errors are investigated. As a result, CD linearity of 18 nm is obtained in the pattern-widths from 0.7 micrometer to 3 micrometer.

High-performance and stability reticle writing system HL-800M

HL-800M has been developed as electron beam reticle writing system (EB) for advanced reticle production. It is very important for EB to keep high performance constantly in the actual advanced reticle production. To meet such a requirement, this system adopts accelerated voltage of 50kV, variable shaped beam, continuous moving stage and 3-stage deflector. Especially, to improve the positioning accuracy, this system has temperature control system, active vibration-isolation system and the new software for position error correction. The proximity effect correction which changes exposure shot time depending on the pattern density and the multi-exposure function are also installed. As a result, the positing accuracy of 32nm and the long term placement of 28 nm are obtained. The line-width linearity from 1 micrometers to 10 micrometers is within the range of 70 nm, and 40 nm form 1 micrometers to 3 micrometers . The stitching accuracy at the stripe boundary is 26nm, and 20nm in case of the 3-path exposure.

Practical method of phase-shifting mask fabrication

Phase-shifting mask (PSM) fabrication techniques have been investigated in order to use PSMs in the manufacturing of 0.3 micrometers memory devices. Comparison of various PSM methods resulted in selection of the SOG (spin on glass)-on-chrome structure. The alternating-type method is used for the wiring layer and the outrigger-type method is used for the hole layer. New techniques are needed for fabricating the PSMs, and the following technologies have been developed or are under development: (1) e-beam writing and process, (2) SOG process, and (3) inspection and repair. Defect free SOG-on-chrome PSMs are available because SOG process is a low defect density process. PSMs and negative i-line resist have been used to experimentally manufacture 0.35 micrometers memory devices, thus demonstrating that SOG-on- chrome PSMs are currently the most practical for memory device manufacturing.

Improvement of the resolution and accuracy of chemical-amplification positive resist for 0.13-μm reticle fabrication

We have developed a novolak-based chemical-amplification resist for 0.13-micrometers or later reticle fabrication. For the 0.13-micrometers or later design-rule reticle-fabrication with OPC patterns, the resist resolution is required under 0.2-micrometers on the mask substrate. To improve the chemical-amplification resist resolution, it is necessary to control the acid- diffusion in the resist film. We have developed the technique of the acid-diffusion control with neutral-salt additives. By use of the resist with this technique, we could fabricate 0.14-micrometers 1/s patterns on a CrOx substrate at a dose of 9.3-(mu) C/cm2. The resist has a good margin of doses.