Deep-UV Lithographic Approaches For 1 Gb DRAM

Abstract

Introduction Phase-shifting masks (PSMs) [1][2] and modified illumination techniques [3] have shown promise in improving the resolution and process latitude of lithography. For 1 Gb-DRAM application, these techniques are essential because printing 175 nm features is pushing the limits of even high numerical aperture (NA) deep-UV exposure systems, and wavelength reduction is not an alternative until the availability of 193 nm exposure systems around the turn of the century. This paper explores the application of attenuated PSM [2] and alternating PSM [l], as well as annular illumination [3] in the optimization of lithographic performance at the 175 nm groundrule. Lithographic simulation including the effects of photoresist processing [4] was used to identify optimal conditions for critical levels of a 1 Gb-DRAM cell design [5], and improvement in performance was quantified experimentally. Process Optimization To ensure reliability of the 1 Gb cell requires the control of feature edge placement to within 17.5nm (&lo%). Based on this criterion, exposure-defocus analyses [6] were performed on the viable resolution enhancement techniques, and their relative merits are quantified in terms of depth-of-focus (DOF) with 10% exposure dose variation. Simulation is performed using SPLAT [7] and an internal IBM program LEOPOLD which models the important effects of photoresist processing. Fig. 1 shows the mask and illumination techniques examined. For each critical level, the three approaches which give the largest DOF (at 10% exposure latitude) are listed in Table 1 together with the design layout. The latitude obtained with conventional chromium (COG) mask and standard illumination is also included for reference. In general, annular illumination and alternating PSM are strong candidates for grating-like levels, while attenuated PSM provides the most benefit for contact levels. It is of interest to note the use of negative resist for the active area (AA) level. Fig. 2 illustrates the improvement in image integrity with resolution enhancement techniques for the AA level. The contours represent aerial image intensity in steps of 0.1 normalized to the clear field intensity. For a COG mask with standard illumination, the intensity contours are sparse in both the width (horizontal) and length (vertical) directions, indicating poor image quality and resulting in a DOF of only 0.4 pm. For an attenuated PSM with annular illumination, the intensity contours are denser, resulting in an improved DOF of 1.2 pm. With an alternating PSM at a reduced partial coherence factor of 0.3, the intensity gradient is especially steep at the ends of the feature due to the effects of phase-shifting. This manifests as a better DOF of 1.4 pm and improved line-end shortening behavior: the length is only biased at lOOnm as opposed to 150nm on the COG and attenuated PSM. Mask imperfection limits and in some cases obliterates the benefits of PSMs. Fig. 3 shows the DOF for a 225nm bitline contact (CB) with different types of mask imperfection. With an ideal attenuated PSM, the best DOF exceeds 2.0 pm. This DOF decreases to 1.4 pm with f5 nm of mask critical dimension (CD) error. With the addition of 410.5% transmission and f 5 " phase variation, the DOF is further degraded to 0.4 pm, virtually indistinguishable from that of a COG mask with f 5 nm of mask CD error. Thus, transmission and phase control of the attenuated PSM must be better than f0.5% and ~ t 5 ' respectively for it to have any advantage over COG masks on the CB level. Experimental Results Biased COG and PSM reticles were fabricated for the deep trench (DT), AA, gate conductor (GC), bitline (MO), and CB levels. The reticles were exposed on a Nikon deep-UV step-andscan system (A = 248nm, N A = 0.6, CT = 0.6). For annular illumination, the inner and outer radii correspond respectively to U = 0.5 and (r = 0.75. The positive [8] and negative [9] resists are 0.6 pm and 0.5 pm thick, respectively. Table 2 summarizes the DOF (at 10% EL) for various levels determined from top-down scanning electron micrograph (SEM) measurement. The use of resolution enhancement techniques improve the process latitude of all levels. Of particular interest is the AA level, which calls for the use of negative photoresist. Fig. 4 shows the top-down SEMs of AA patterns exposed with annular illumination and attenuated PSM in negative resist over a 1 .O pm focus range. The DOF is approximately 0.8 pm between the focus at which the pattern loses integrity (-1.2 pm) and the focus at which stringers are observed (-0.2 pm). In a DRAM chip, it is important to print both the array and peripheral patterns. In some cases, the use of resolution enhancement techniques improves the process latitude of array features but degrades the integrity of peripheral patterns. For example, while the use of annular illumination in the MO level improves the grating-like array patterns, the end line of the peripheral feature shows signs of necking and bridging as shown by the SEM in Fig. 5. This problem can be remedied by biasing of the outer line and modifying the etch process. Conclusion Level-specific lithography optimization for a 1 Gb DRAM cell has been demonstrated based on simulation and experimental studies. Results indicate that the optimal lithographic approach is strongly dependent on feature pattern. The use of resolution enhancement techniques for process improvement within the array may lead to modifications of design rule for peripheral features. PSM imperfection due to fabrication may also limit the benefits of such masks. References [I] M. 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