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  • Blob Defect Prevention in 193nm Topcoat-free Immersion Lithography

    Deyan Wang, Jinrong Liu, Doris Kang, Cong Liu, Tom Estelle, Cheng-Bai Xu, George Barclay and

    Peter Trefonas

    The Dow Chemical Company Dow Electronic Materials

    455 Forest Street, Marlborough, Massachusetts, USA, 01752

    1. Introduction

    In 193nm immersion lithography, immersion top coat was the first proposed technique for preventing the leaching of photoresist (resist) components, such as photoacid generator (PAG) and quencher base, into the immersion fluid (DI water). In this approach, the top coat is coated onto a resist film in a separate step including coating the top coat film and baking the film. This approach certainly adds extra cost to the device manufacturing and incurs reduced throughput as compared to the dry lithography process. The embedded barrier layer (EBL) technology1-5 developed at Dow Electronic Materials has been demonstrated to be a revolutionary approach, in which a suitable EBL material is formulated into an existing resist, and in a spin coating process the EBL material comes to the resist surface to forms a leaching barrier in situ. This approach has now been widely accepted and implemented in the integrated circuit manufacturing industry for replacing the conventional immersion top coat process. In addition to being an excellent leaching barrier, EBL materials, in general, result in a resist surface with a high receding angle for water. This property makes the EBL approach more desirable in topcoat free immersion lithography, since it allow for the latest scanners to perform at their maximum scan speed without generating watermark defects. For immersion lithography, the most important issue for mass production is defectivity control. This is true for both top coat and topcoat free approaches. In the top coat approach, the formulation optimization for both top coat and resists was extensively involved for this technique finally to reach an acceptable defectivity level for mass production of semiconductor devices. As a later developed technology, the EBL approach has gone through a series of research and development stages particularly in material innovation to reach the same low defectivity level as that of an immersion top coat process. After achieving the target of low defectivity in lithography, the challenges left to the EBL approach were to solve high defectivity in bulk exposed and bulk unexposed regions, which became prominent in both bright field and dark field lithographic applications. To solve the high defectivity issues, a thorough understanding of the blob defect formation mechanism was imperative. In this paper, the defect formation mechanism in both bulk exposed and unexposed regions is proposed, and this proposed mechanism is applicable not only to the EBL approach but also to the immersion top coat approach in general. 2. High Blob Defectivity Issues with Earlier Generation EBL Materials In earlier stages of EBL development, EBL materials were designed primarily for reaching a high receding angle while functioning as an effective leaching barrier to DI water for PAG and other resist components. These properties were achieved without much effort because of the inherent hydrophobic nature of the EBL materials. Efforts were then focused on achieving the same low defectivity levels as the top coat process in lithography patterning. This was eventually achieved by ensuring developer solubility of the EBL materials in the unexposed regions. 2 The blob defects in bulk exposed and unexposed regions on a processed wafer, however, remained a prominent issue. Shown in Figure 1 is a comparison of wafer maps showing defectivity between top coat and EBL with EPIC2096 Photoresist (Dow Electronic Materials).

    Advances in Resist Materials and Processing Technology XXIX, edited by Mark H. Somervell, Thomas I. Wallow, Proc. of SPIE Vol. 8325,83252G 2012 SPIE CCC code: 0277-786X/12/$18 doi: 10.1117/12.916818

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  • Fig. 1. Defect wafer map comparison for a checkerboard defectivity test. Exposure dose is the dose for printing 50nm, 1:1 line space patterns. Film thickness is 1200. The EBL material used is an EBL that does not change its hydrophobicity during a development/rinsing step. Its loading is 3% relative to the total solid content of the resist. The top coat is OCTM2000 (Dow Electronic Materials) with its POR process condition (coating at 1500 rpm, and bake at 90C for 60 seconds) Each dot in the wafer map represents an actual defect regardless of its nature. Although some of the defects are attributed to particles or defects from the BARC (bottom antireflective coating) onto which the resist was coated, a majority of the defects are classified as satellite defects. As suggested by their name, satellite defects are a type of defect in which a larger mass is normally at the center and a few smaller pieces randomly reside around the center mass, as shown in Figure 2 6,7

    Figure 2. SEM Images of typical satellite (blob) defects. The scale bar is 1m. When contact angles were determined on the EBL resist surface for a processed wafer, we found that the EBL containing resist remains more hydrophobic on its surface than its counterpart processed with a top coat. To be specific, the static contact angle of the EBL resist was 79 and its receding angle was 49.5, whereas the static and receding angles of the same resist but processed with the top coat were 58 and 20, respectively. We rationalize the higher hydrophobicity of the processed EBL resist surface as follows: Although the EBL material tested was designed to have a certain developer solubility, the developer (an aqueous solution of 0.26N TMAH) removes only the outermost surface of the resist film where the EBL concentration is essentially 100%. As illustrated in Figure 3, which shows the EBL concentration profile across the depth of a resist film, immediately under the EBL enriched surface, the EBL concentration drops rapidly to zero in the range of a few nanometers. It is not difficult to imagine that after the EBL enriched surface is removed in the development process, a new resist surface is created from the sharp gradient region where the EBL polymer and the resist polymer coexist. Because of the hydrophobic nature of the EBL, the new surface is colored with the hydrophobicity of the EBL.

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    Figure 3. SIMS spectra of an EBL containing resist created by monitoring signal intensities corresponding to the characteristic mass ratios of the EBL polymer. Regardless of the defect formation mechanism, the high level of blob defects seems closely related to the new, hydrophobic EBL resist surface. The question then becomes, how does the high hydrophobicity correspond to the high level of blob defects? 3. Exploring Blob Defect Formation Mechanism of EBL Resists There is a popular theory in this industry for the formation of blob defects on a processed wafer. Based on this theory, the blob defects are due to the aggregation of the resist components during alkaline developer development and DI water rinse steps, and the aggregated resist components then fall back onto the wafer surface forming the blobs. This mechanism is also referred to as pH shock or reduced zeta potential of the resist components in the developing/rinsing transition stage.7 This theory provides a good guideline for the selection or design of resist materials including resist polymers, PAGs, quencher bases and other additives. Experimentally, people do see that a resist formulation with poor developer soluble components often results in high defectivity, particularly blobs. Is this the mechanism that governs blob defect formation for hydrophobic EBL resists? Our meticulous studies lead us to the belief that these satellite defects are, in fact, water stains. On a hydrophobic resist surface, water droplets bead up, and their shapes or contact angles depend on the hydrophobicity of the surface. This can be measured not only by droplet static contact angle (s) but also by its receding angle (r). The latter is more sensitive to the interaction strength between a liquid and a solid surface, and therefore is a very important parameter in characterizing the behaviour of water droplets on a surface, particularly in a dynamic condition such as spinning of a wafer. Under a dynamic condition, i.e., wafer spinning, water droplets on a resist surface start to deform, skewing towards the wafer edge, and then followed by one of two actions as follows depending on the different situations: Situation 1. Resist surface is hydrophobic (high s and high r). In this situation, water droplets slide outwards when centrifugal force exceeds the interaction strength between the droplet and the wafer surface, otherwise staying on the wafer surface and possibly becoming dried during spinning. This is often called poor dynamic wetting. Situation 2. Resist surface is hydrophilic (low s and low r). This corresponds to a strong water-resist interaction. In this situation, instead of sliding, the droplets spread outwards to become coated on the resist surface and are coated evenly and dried during the wafer spinning. To elaborate the first situation, lets look at the force analysis diagram of a droplet on a spinning wafer surface in Figure 4.

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