In this paper, an Energy Intensity Distribution (EID) model considering dose latitude for Variable Shaped Beam (VSB) has been developed. η values (i.e. back-scattering ratio) versus dose and process threshold have been investigated by using the EID model. Additionally, a new procedure to find optimum PEC values (η) taking into account of the process threshold is proposed through simulation. For fogging effect correction, we have adopted a Gauss model and created a new simulation algorithm to find the most suitable parameters regarding η value, process threshold, dose and the EID model.
In this paper, an Energy Intensity Distribution (EID) model considering dose latitude for Variable Shaped Beam (VSB) has been developed. η values (i.e. back-scattering ratio) versus dose and process threshold have been investigated by using the EID model. Additionally, a new procedure to find optimum PEC values (η) taking into account of the process threshold is proposed through simulation. For fogging effect correction, we have adopted a Gauss model and created a new simulation algorithm to find the most suitable parameters regarding η value, process threshold, dose and the EID model.
Cleaning is one of the most important processes in photomask manufacturing, because the smallest particles may be printable on wafers. Moreover, mask cleaning requirements are stricter than that for wafers because masks are the master image from which all wafers prints will be made. We now face difficult challenges as we enter the 90nm era with 193nm DUV lithography and more prominent use of phase shifting applications. As defect sizes to be controlled in the cleaning process decrease, cleaning performance depends not only on conventional chemical treatments and megasonic hardware, but also on new cleaning methods such as UV/Ozone treatment. We investigated and compared the cleaning performance of UV/Ozone treatment + traditional chemical cleaning methods with standalone conventional wet chemical cleaning methods on glass, chrome, and MoSiON blank surfaces over pattern densities at 70% and 30% clear in the pattern area. Contact angle measurements and wettability tests were performed as well to evaluate cleaning performance results. The cleaning effectiveness with different drying methods on EAPSMs has been also investigated by controlling phase and transmission of KrF EAPSMs to within ±3'and ±0.3% respectively. Overall, it was found that the UV/Ozone pre-treatment combined with the traditional chemical cleaning process results in a better particle removal rate compared to conventional cleaning methods when it comes to removing the smallest mask particles., and it did not adversely affect EAPSM optical properties.
For the latest photomask fabrication, better critical dimension (CD) control and pattern fidelity to design size are required. According to the latest ITRS roadmap, masks for the 90 nm technology node should have CD uniformity of 6~8nm (3σ). Moreover, CD control is particularly critical for isolated opaque lines, such as those found in gate layers, whose loading is primarily clear field. The high acceleration voltage electron beam (EB) systems that employ variable shaped beams (VSBs) are used for mask writing due to their high throughput. To minimize write time and fogging effects, and to control mean CD and improve CD uniformity for mask production, it is well known that negative tone resists enable better VSB mask writing system performance. In these circumstances, positive and negative tone chemically amplified resists (CARs), FEP171 (Fuji Films) and FEN270 (Fuji-Films), were evaluated empirically for mask making. We investigated and compared resolution, sensitivity, resist profiles, CD variation vs. exposure dose, proximity effect correction (PEC), fogging effect, pattern fidelity, and so on. Furthermore, write tool data volume and throughput, defect trends, and other process parameters on the positive and negative tone resists were evaluated and compared by applying test patterns.
As the requirement of specification on photomask continues to be tightening with advanced logic and memory devices, the combined process of chemically amplified resist (CAR) and high acceleration voltage e-beam writing tool is widely used to meet the resolution and throughput for advance photomask fabrication. It is well known that the post exposure baking (PEB) condition makes serious effect on the characteristic of CAR due to its de-protection reaction with thermal acid catalyzation. In this paper, we present the PEB temperature effect on pattern resolution such as line edge roughness (LER) and proximity effect correction (PEC) latitude that is practical limitation in the combined process of 50 kV writng tool and CAR resist. Our results show that LER and PEC lattitude are strongly dependent on PEB temperature due to resist contrast variation. At higher PEB temperature, increasing the contrast value can reduce the LER and it can increase the optimum PEC latitude.
As minimum feature size of device shrink down below 100 nm, the process margin for the mask fabrication reduced dramatically. Mask makers are enlisting equipment and material suppliers in their efforts to achieve wide process margin from existing processes. One of the most promising methods is thinning Cr thickness as low as possible. However, briging the thin Cr blank into mass production line could cuase some problem for advance photomask fabrication using 50 kV electron beam writing tools. In this paper, we verified the feasibility of Cr film ranged from 400 Å to 1000 Å. The results categorized into two sections. At first, we verified the writing property change with thinning Cr thickness and then investigated the etching characteristics. As a result, we found that Cr thickness don't affect writing properties regardless of Cr thickness. However, the thinner Cr blank represented superior etching characteristics to a conventional one. It showed low etching bias and loading effects. From these results, we concluded the thinner Cr blank could not only make the process wider but also improve the mask quality.
As critical dimensions (CDs) continue to approach the 90 nm node, it is inevitable that the industry has employed the use of chemically amplified resist (CAR) with 50 kV e-beam writing tool. However, the fogging effect by re-scattered incident electron at a high acceleration e-beam writer and the loading effect at dry etching step due to pattern density are critical issues since these effects make the variation of CD mean to target (MTT) and the degradation of CD uniformity. Tracking the CD error sources in CD uniformity and minimizing the error are very important task for high technology node mask production. In this paper, we focus on finding the source of the radial error in CD uniformity for each process step since the radial error occupy the main part of total CD uniformity. Also we present the radial error modeling using convolution equation between Gaussian CD error distributions with pattern densities. Finally, we describe the radial error correction method by phantom exposure with rectangle representing local pattern density. Fogging effect at writing process is one of the main sources of the radial error in global CD uniformity. The error by fogging effect is linearly proportional to mask pattern density, whereas loading effect at dry etch process increases the radial error in the case of the higher pattern density. The correction method using defocused beam based on our CD uniformity model effectively reduces the radial error and total error to 50% of their original value.
Writing fogging effect in chemically amplified resist process makes critical effect on global CD distribution in the advanced 90nm node photomask with higher pattern density and smaller geometries. High contrast feature of chemically amplified resist makes difficult to correct the global CD uniformity in resist develop process compared with conventional ZEP resist. In this paper we examine the fogging effect in the combination chemically amplified resist with 50KeV writing tool and the consequential problem for production mask with higher pattern density. We will present the feasibility of the global CD uniformity correction technique in post exposure baking process using gradient temperature hotplate.
Chemically amplified resist (CAR) provides superior lithographic performance compared to traditional e-beam resists in production maskmaking. Parameters benefiting the most are contrast, resolution, and sensitivity. In spite of CAR's advantages, defect control and tighter 50KeV e-beam CAR process restrictions are significantly more critical thanks to smaller geometries, tighter CD specifications, and optical proximity correction (OPC) for 90nm node mask technology. Among defect root causes, resist development is considered to be the one of the most important steps because post-development residue can generate printable defects on finished masks.
We investigated the CAR development process across different resist development methods, such as binary and fan-type nozzle spin spray, and puddle development. Several high density binary and embedded-attenuated phase shift masks (EAPSMs) with 70% clear area in the main pattern field were evaluated in an effort to identify and contain post-develop defects in a typical mask production flow. Development step process residue was examined at the after-develop inspection (ADI) step and scanning electron microscopy (SEM) was used for individual defect review. The KLA-Tencor SLF77 TeraStar inspection tool was used to inspect patterns after the development, Cr/MoSiON layer dry etch, and clean steps. The effectiveness of the various CAR development methods has been also studied following development, dry etch, and cleaning inspection by using identical binary and EAPSM masks from production. The mechanism and defect source during the stepwise process and inspections were scrutinized and discussed.
Experimental results showed that stepwise process inspection was effective in identifying defects and their sources to prevent defects, and in optimizing each process step. It was found that CAR development and dry etch processes are the most important process steps to control defects in CAR-based mask production. Suggested optimized develop process parameters for 90nm-node mask
For high-voltage vector e-beam writing systems, solving the resist heating effect problem is one of the highest priorities because it is a major factor affecting localized critical dimension (CD) uniformity. In order to write patterns for 90nm node devices, the utilization of proximity effect correction (PEC) is essential for e-beam mask writers to achieve high CD performance.
In this study, the dependence of CD variation on e-beam write conditions was investigated under optimum PEC parameter conditions. Writing conditions such as current density, shot size, number of writing passes, and settling time were tested to see their affects on resist heating. Industry-standard Nippon Zeon ZEP 7000 resist was written by a Toshiba EBM-3500B 50KeV vector e-beam writer using patterns found in sub-130nm node devices. Results indicated that the main factor affecting resist heating CD variation for ZEP 7000 was in fact the e-beam writer shot size selected. Multi-pass writing was effective in reducing the CD variation, and the settling time of each shot in the EBM-3500B had very little influence.
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