Summer03 taking aim

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Taking Aim at the Overlay Metrology Budget for 70 nm John Allgair, Dave Benoit, and Gary Seligman, Motorola, Inc. Mike Adel, Mark Ghinovker, Elyakin Kassel, Chris Nelson, and John Robinson, KLA-Tencor Corporation

The current box-in-box (BiB) overlay mark has been the mainstay of optical overlay metrology for the past 15 years. It has served the industry well since its inception, when design rules were substantially greater than one micron and overlay metrology budgets were measured in hundreds of nanometers. Today, three key challenges are faced by metrology engineers: (1) reduction of total metrology uncertainty (TMU) in order to meet advanced design rule process requirements, (2) improved process robustness in the face of advanced processes, and (3) correlation between scribeline metrology and real overlay in the chip at design rule feature size, type, and pitch. A new grating overlay mark and measurement algorithm has been developed to address these concerns, called AIM (Advanced Imaging Metrology).

Introduction

As design rules shrink and wafer processing becomes more complex, traditional metrics of overlay metrology performance such as precision, tool induced shift (TIS), and tool induced shift variation (TIS-3Ďƒ) are becoming less and less adequate to describe all sources of overlay metrology uncertainty. Contributions to the overlay metrology error budget from wafer processing such as chemical-mechanical planarization (CMP), 1 or the impact of stepper aberrations on metrology mark2 placement are playing a more dominant role and must be understood as we move towards the 70 nm node. In the current study, overlay metrology performance on short loop wafers processed with 193 nm lithography for front-end (poly to STI active) as well as back-end (via to copper single damascene metal) was evaluated. In particular, we explore the comparative performance of periodic structure overlay marks with conventional BiB marks, with and without design rule segmentation. We have demonstrated that both non-segmented and design rule segmented grating marks shows significantly improved mark fidelity

and reduced overall measurement uncertainty when compared with non-segmented and design rule segmented BiB respectively. Measurement methodology and overlay mark design

Conceptually, overlay metrology using periodic structure overlay marks is similar to the BiB method.3 In both cases, two independent structures, one on each layer are printed concentrically and the displacement between their centers of symmetry is measured. However there are a number of significant differences. In BiB, this displacement is measured in units of pixels on the CCD. For periodic structure overlay marks, however, the pitch of the overlay mark is the unit of measure. This may be user-defined or measured in real-time. The measurement algorithm is also significantly different. Periodic signals enable many new methodologies compared with edge-based symmetry detection. In the current study, the majority of data was collected on 40 Âľm and 20 Âľm square overlay marks containing both X and Y overlay capability, examples of which are shown in Figure 1. Several parameters are available to the overlay mark designer and are subject to optimization. This optimization can be based on any combination of standard performance metrics, process robustness, and Summer 2003

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