Despite being crucial in an optical lithography process, “dose” has remained a relative concept in the computational lithography regime. It usually takes the form of a percentage deviation from a pre-identified “nominal condition” under the same illumination shape. Dose comparison between different illumination shapes has never been rigorously defined and modeled in numerical simulation to date. On the other hand, the exposure-limited nature of EUV lithography throughput demands the * illumination shape being optimized with the physical dose impact consciously taken into consideration. When the projection pupil is significantly obscured (as in the ASML EXE high NA scanner series), the lack of a proper physical dose constraint may lead to suboptimal energy utilization during exposure. In this paper, we demonstrate a method to accurately model the physical dose in an optical lithography process. The resultant dose concept remains meaningful in the context of a changing illumination pupil, which enables co-optimization of imaging quality and a throughput metric during the Source-Mask Optimization (SMO) phase, known as the Dose-Aware SMO. With a few realistic test cases we demonstrate the capability of Dose-Aware SMO in terms of improving EUV throughput via reducing the effective exposure time, in both regular and obscured projection systems. The physical dose modeling capability in computational lithography not only addresses those immediate challenges emergent from EUV throughput, but also opens the gate towards a broad class of exciting topics that are built upon physical dose, such as optical stochastic phenomena and so on.
With the adoption of extreme ultraviolet (EUV) lithography for high-volume production of advanced nodes, stochastic variability and resulting failures, both post litho and post etch, have drawn increasing attention. There is a strong need for accurate models for stochastic edge placement error (SEPE) with a direct link to the induced stochastic failure probability (FP). Additionally, to prevent stochastic failure from occurring on wafers, a holistic stochastic-aware computational lithography suite of products is needed, such as stochastic-aware mask source optimization (SMO), stochastic-aware optical proximity correction (OPC), stochastic-aware lithography manufacturability check (LMC), and stochastic-aware process optimization and characterization. In this paper, we will present a framework to model both SEPE and FP. This approach allows us to study the correlation between SEPE and FP systematically and paves the way to directly correlate SEPE and FP. Additionally, this paper will demonstrate that such a stochastic model can be used to optimize source and mask to significantly reduce SEPE, minimize FP, and improve stochastic-aware process window. The paper will also propose a flow to integrate the stochastic model in OPC to enhance the stochastic-aware process window and EUV manufacturability.
Source-mask optimization and EUV mask structure optimization are studied to maximize DOF and NILS for honeycomb hole arrays at 0.55NA. Optimization flow on mask structure conditions for good and stable NILS performance, such as peak-NILS, NILS-DOF, and NILS-MEEF, is proposed to minimize the local CD variation as a result of EUV mask properties. Using NILS metrics, the absorber thickness and the mask CD with optimized illumination conditions are determined for the maximum performance, together with the tolerance of absorber thickness and mask CD. EUV binary absorbers using high-n/mid-k and low-n/high-k materials, and EUV PSM absorbers using low-n/low-k and highreflectance materials, are compared for 28-, 26-, and 24-nm honeycomb hole arrays.
Advancing technology nodes in CMOS Image Sensors (CIS) continues to drive a shrinking process to acquire higher resolution and low power consumption as well as more cost-effective production. With the sensor pixel size scaling down, a thicker photoresist (with aspect ratios greater than 10:1) is introduced to block high-energy implants with extremely localized implant profiles. Then double exposures/double focus (DE/DF) is applied to make sure the resist profile and process window is comparable or better. However, this process is a big challenge at high volume manufacturing (HVM) phase because of throughput loss. To recover it due to DE/DF, we invented SE MFI which uses two wavelengths (“colors”) generated by the KrF excimer laser to solve the problem. Due to the chromatic aberrations in the lens, the focal plane shift of different wavelength produces nearly the same result as DE/DF. However, the use of two-wavelengths brings some challenges. The first is the loss of image contrast and the second is the impact of chromatic aberrations across the slit which results in image shift and image asymmetry. In this work, we demonstrated that the use of ASML’s Tachyon KrF MFI source mask optimization (SMO) that can match the MFI SE process to DE/DF process of record (POR). We first used Tachyon Focus-Exposure Modeling plus (FEM+) to calibrate a DE resist model by using DE POR wafer data. Then we converted the DE model to a SE MFI model. At the end, we use the Tachyon MFI-SMO to optimize the SE MFI to match the DE/DF and MFI sidewall profiles through process window conditions at the center slit. We achieved making the MFI and DE/DF sidewall difference significantly smaller than other noises which can be measured on wafer at the center slit. We evaluated the chromatic aberration impact on through slit sidewall profiles also meet the specification. The through slit matching between MFI and DE/DF was further improved by through-slit mask optimization. This is done by inserting asymmetry sub resolution assist features (SRAFs). Tachyon Optical Proximity Correction plus (OPC+) can support full chip mask corrections for full-chip HVM. The above MFI technology including Tachyon optimization capability will be verified by wafer exposure via comparison between MFI and DE wafer results.
Over the years, lithography engineers have continued to focus on CD control, overlay and process capability to meet node requirements for yield and device performance. Previous work by Fukuda1 developed a multi-exposure technique at multi-focus positions to image contact holes with adequate DOF. Lalovic2 demonstrated a fixed 2-wavelength technique to improve DOF called RELAX. The concept of multi-focal imaging (MFI) was introduced3 demonstrating two focal positions are created that are averaged over the exposure field, this wavelength “dithering” approach which can be turned on and off, thus eliminating any potential scanner calibration issues.
In this work, the application of this imaging method (1 exposure-2 focus positions) can be used in thick photoresist and high aspect ratio applications. An example of thick photoresist imaging is shown in figure 1. We demonstrate 5um line and space features in 10um of photoresist at 3 different imaging conditions. On the left, single focus imaging (SFI) at best dose and focus, the center image which is also SFI but at a defocus of +3.2um. On the right is MFI with 2 focus positions of 0 and 2.8um. Here we can see a significant improvement in the SWA linearity and image profile quality. A second example of high aspect ratio imaging using MFI is shown in figure 2. The aspect ratio of 13:1 is shown for this. The use of Tachyon KrF MFI source – mask optimization flow will be reviewed to demonstrate optimum conditions to achieve Customer required imaging to meet specific layer requirements.
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