Michael Cangemi

Improved prediction of across chip linewidth variation (ACLV) with photomask aerial image CD metrology

Critical dimension (CD) metrology is an important process step within the wafer fab. Knowledge of the CD values at resist level provides a reliable mechanism for the prediction of device performance. Ultimately tolerances of device electrical performance drive the wafer linewidth specifications of the lithography group. Staying within this budget is influenced mainly by the scanner settings, resist process and photomask quality. At the 65nm node the ITRS roadmap calls for sub-3nm photomask CD uniformity to support a sub-3nm wafer level CD uniformity. Meeting these targets has proven to be a challenge. What can be inferred from these specifications is that photomask level CD performance is the direct contributor to wafer level CD performance. With respect to phase shift masks, criteria such as phase and transmission control are also tightened with each technology node. A comprehensive study is presented supporting the use of photomask aerial image emulation CD metrology to predict wafer level Across Chip Linewidth Variation (ACLV). Using the aerial image can provide more accurate wafer level prediction because it inherently includes all contributors to image formation such as the physical CD, phase, transmission, sidewall angle, and other material properties. Aerial images from different photomask types were captured to provide across chip CD values. Aerial image measurements were completed using an AIMSTMfab193i with its through-pellicle data acquisition capability including the Global CDU MapTM software option for AIMSTM tools. The through-pellicle data acquisition capability is an essential prerequisite for capturing final CD data (after final clean and pellicle mounting) before the photomask ships or for re-qualification at the wafer fab. Data was also collected on these photomasks using a conventional CD-SEM metrology system with the pellicles removed. A comparison was then made to wafer prints demonstrating the benefit of using aerial image CD metrology.

Tunable transmission phase mask options for 65/45nm node gate and contact processing

Today the industry is filled with intensity-balanced c:PSM and much more focus is being placed on innovative approaches such as CPL (and in conjunction with IML for Contacts) and tunable transmission embedded attenuating phase shift mask (TT-EAPSM). Each approach has its own merits and demerits depending on the manufacturing strategy and lithography performance required. Currently the only commercially available photomask blanks are different chrome thickness binary and 6% attenuating blanks using molybdenum-silicide, making the accessibility to alternate transmissions much more challenging. This paper investigates the mask manufacturability of a tunable transmission embedded attenuating phase shift mask. New film materials that are used in the mask blank manufacture are modeled, deposited and characterized to determine its ability to meet performance requirements. Sputtering models, by rate and gas component, determines film stacks with tunable transmissions and thicknesses. Chemical durability, etch selectivity and thickness are a few parameters of the films that have been characterized to enhance the manufacturability and process reliability of the masks. Lithography simulation models using measured optical properties were developed and test masks that include actual device designs were fabricated. Analysis of CD variation, pattern fidelity and process margin was performed using 3D mask simulation to understand the impact on 65nm design rules. Feasibility and performance of tunable transmission photomasks for use in design and lithography are verified. Moreover, the mask manufacturability and lithography performance is compared to other enhancement techniques and their merits presented.

Implementation of a transparent etch stop layer for an improved alternating PSM

Alternating phase shift masks (alt. PSM) are emerging as an attractive resolution enhancement technique. Although alt. PSM is a technique that clearly improves resolution, there are some inherent disadvantages that are induced by the manufacturing process. Intensity imbalance, phase non-uniformity and quartz defects diminish the performance of an alternating PSM. Many of these disadvantages can be a result of imprecise quartz etching. By implementing a transparent etch stop layer, these deficiencies can be minimized. The etch stop layer ensures that all of the quartz is etched and that over-etching will not induce a phase-shift error. This produces improved phase uniformity and eliminates quartz defects. The etch stop layer also has the ability to improve the image intensity balancing by reducing the intensity through the zero degree region. This paper discusses the advantages and manufacturability of alt. PSM using a transparent etch stop layer.

Feasibility study of embedded binary masks

Towards hyper-NA lithography, the mask blank and mask topography have the opportunity to be optimized for imaging performance. At the resolution limit of hyper-NA imaging, depth of focus and MEEF become critical for conventional mask stacks. Although conventional binary masks (BIM) are the simplest and the most cost-effective to manufacture, other mask types can provide better imaging performance. This study explores the feasibility and imaging performance of an embedded binary mask (EBM). The EBM emphasizes the simple binary manufacturing process with the application of an additional transparent layer. Two types of EBMs, topographic and planar, were evaluated. The mask diffraction properties are studied by both measurements using an ellipsometer (Woollam VUV-VASE) and simulations using Solid-E 3.2.0.2 (Sigma-C). In this first phase, the imaging performance is assessed by rigorous simulations for three different illumination conditions (cross-quad, quasar and annular). By comparing metrics such as contrast, NILS, MEEF, and process windows, simulations determined that an optimized topographic EBM has a better overall through-pitch imaging performance than a conventional binary mask. This preliminary investigation suggests that an embedded binary mask may be considered as an RET option for hyper-NA imaging improvement.

Improved phase uniformity control using a new AAPSM etch stop layer technique

One of the major challenges in alternating aperture phase shift mask (AAPSM) production is the variability of the glass etch rate as a function of exposed area (pattern loading) on the mask. The lack of an endpoint system means that the etch is entirely based on time, and the result is increased variability in the mean etch depth as well as decreased yields against ever tightening phase specifications. If a transmissive etch stop layer were placed underneath an appropriate thickness of glass to obtain a 180-degree phase shift, the result is a forced endpoint at exactly 180 degrees every time. Such a film system also leads to many process advantages over conventional AAPSM processes. This paper discusses the film stack deposition and maskmaking at Photronics, Inc. and details the process advantages of using AAPSM blanks with etch stop layers.

Polarization effects: EAPSM vs. TT EAPSM

Today novel RET solutions are gaining more and more attention from the lithography community that is facing new challenges in attempting to meet the new requirement of the SIA roadmap. Immersion, high NA, polarization, and mask topography, are becoming common place terminology as lithographers continue to explore these areas. Here with, we compare a traditional 6% MoSi based EAPSM reticle and a high transmission solution made of a SiON/Cr film stack. Insights into the manufacturability of high transmission material are provided. Test patterns have been analyzed to determine the overall impact of imaging performance when used with immersion scanners and polarized light. Some wafer results provide reliability of simulations, which are used to make further investigation on polarization and immersion effects.

Manufacturability and printability of AAPSM with transparent etch stop layer

Phase-shift mask (PSM) technology in combination with 193nm illumination remains a viable option for high contrast imaging towards 45nm half-pitch applications. The advent of hyper NA (immersion) lithography increases the imaging sensitivity towards the photomask properties, such as mask-induced polarization. In addition, the use of PSM technology implies taking into account the inherent photomask topography effects for a balanced through pitch imaging. A good quartz etch depth control of +/-1o through pitch is required for optimized wafer imaging [1]. Therefore, a new PSM material stack was proposed based on a transparent etch stop layer (TESL) in order to meet the stringent phase depth requirements beyond 65nm half-pitch [2]. This extra layer allows over-etching of the quartz, resulting in a good etch depth linearity and uniformity. This study examines the manufacturability and printability of TESL-based masks. We examine the effect of an improved quartz etch depth linearity on the through-pitch process windows for a TESL-based alternating aperture (AA)PSM. Moreover, due to the different stack of photomask material compared to a classical photomask blank, the impact on printability is investigated by simulations, AIMS and wafer imaging. The image imbalance compensation by trench biasing needs to be optimized for through-pitch process windows. The actual depth and line width of the structures is systematically probed within the photomask field. Based on photomask metrology data, rigorous electro-magnetic field simulations are compared to wafer prints, obtained on an ASML XT1250Di ArF immersion scanner working with a 0.85NA projection lens and to AIMS results from Zeiss AIMS fab 193i. Furthermore, feature sizes on the order of the lithography wavelength induce photomask polarization effects in the imaging path [3]. The degree of polarization is compared to the polarization behavior of a conventional PSM. In summary, this study assesses the capability of TESL PSM towards the 65nm node through-pitch imaging.

EMF simulation with DDM to enable EAPSM masks in 45-nm manufacturing

One of the enabling RET candidates for 45 nm robust imaging is high transmission (20-30%) EAPSM masks. However, the effectiveness of these masks is strongly affected by the electromagnetic field (EMF) that is ignored in most commercial full-chip OPC applications that rely on the Kirchhoff approximation. This paper utilizes new commercial software to identify and characterize points in a design that are especially sensitive to these EMF effects. Characterization of conventional 6% and 30% High Transmission photomasks were simulated and compared with experimental results. We also explored, via simulation-driven design of experiment, the impact of mask variations in transmission, phase, and SRAF placement and size to the imaging capability. The simulations are confirmed by producing a photomask including the experimental variations and printing the mask to silicon. Final analysis of the data will include exact mask measurements to confirm match to simulation assumptions of mask stack, and phase.

Through-pitch characterization and printability for 65nm half-pitch alternating aperture phase shift applications

Alternating aperture phase shift masks (AAPSM) continue to offer high contrast imaging for 65nm half-pitch using conventional 193nm illumination. The transition to high NA lithography systems including immersion lithography, and the ever-decreasing feature sizes have made the topography of the photomask a significant issue in the final resist image. Therefore, the influence of the alternating phase shift depth, the trench profile, and the critical dimension control through variable feature width must be considered and understood for optimized wafer imaging. This paper will examine the impact on imaging based on three photomasks, each employing different quartz etch chemistries. The three methods used to define the well structures include two all dry and a partial wet etch approach. As the photomask features continue to decrease, slight changes in the quartz etched trench profile and depth can severely affect the wafer prints, as the effective 180 degree phase shift for imaging is not achieved. In this work we correlate the imaging performance through pitch to a systematic evaluation of the photomask topography using complementary photomask metrology techniques. The actual depth and profile of the structures is obtained on a FEI Stylus nano-profilometer (SNP-XT) and from destructive cross sections. The CD linearity is measured on a top-down reticle CD SEM (KLA 8100XR). Based on photomask metrology data, rigorous electro-magnetic field (EMF) simulations of the various topographic profiles are performed. As a first printing performance estimate the photomasks are evaluated on a Zeiss AIMSfab193. Comparisons between the different evaluations will be made against wafer prints, obtained on an ASML PAS5500/1100 ArF scanner working with a 0.75NA projection lens. This study will lead to an understanding of the impact of possible limitations of the current quartz etching processes on the imaging performance of alternating phase-shift masks for 65nm half-pitch.

Comparisons of 9% versus 6% transmission attenuated phase-shift mask for the 65-nm device mode

The minimum gate pitch for the 65nm device node will push 193nm lithography toward k1 ~ 0.35 with NA = 0.85. Previous work has analyzed the challenges expected for this generation. However, in the simplest terms, optical lithography for the 65nm node will be difficult. Lithographers are, therefore, looking into high-transmission attenuated phase shift mask (high-T attPSM), where T > 14%, to improve process margins. The benefits of a high-t attPSM are substantial, but drawbacks like inspection difficulty, defect free blanks manufacture, and sidelobe printing may make the use of such masks impractical. One possible solution to this problem is to employ medium transmission (med-T) attPSM, such as T = 9%, to image critical levels of the 65nm node with 193nm lithography. Earlier work shows that the problems High-T attPSMs face are manageable for med-T attPSM. Sidelobe printing in particular will be treated in this work with simulation and experiment. A primary goal of this effort is to determine if the lithographic benefit of moving from industry-standard 6% attPSM to 9% attPSM is worth the risks associated with such a transition. This goal will be met through a direct comparison of experimental 0.75NA 193nm λ results for 6% versus 9% attPSM on gate, contact/via, and metal layers at 65nm generation target dimensions with leading edge resists. Additional information on the inspectability and reticle blank manufacture of % AttPSM will also be given to provide a cohesive analysis of the transition tradeoffs.