For future technology nodes, highly accurate dimensional metrology will become more and more important. At this stage,
measuring layer thickness in planar test structures or geometrical dimensions in simplified proxy structures may be not
sufficient for accurate control of highly sophisticated process steps. Model-based dimensional metrology has the
potential to provide critical parameters of interest for process control in high volume manufacturing, while during
process and technology development the constrained flexibility of models and the required model-building efforts may
be a serious limitation. On the other hand, model-free dimensional metrology may provide sufficient flexibility for
process development, while in some cases it may not be production-worthy in high volume manufacturing. This article
details advantages and disadvantages of the different methods during the lifetime of a product starting from early
development to high-volume production.
Infrared spectroscopic ellipsometry (IRSE) metrology is an emerging technology in semiconductor production environment. Infineon Technologies SC300 implemented the first worldwide automated IRSE in a class 1 clean room in 2002. Combining properties of IR light -- large wavelength, low absorption in silicon -- with a short focus optics -- no backside reflection -- which allow model-based analysis, a large number of production applications were developed. Part of Infineon IRSE development roadmap is now focused on depth monitoring for arrays of 3D dry-etched structures. In trench DRAM manufacturing, the areal density is high, and critical dimensions are much lower than mid-IR wavelength. Therefore, extensive use of effective medium theory is made to model 3D structures. IR-SE metrology is not limited by shrinking critical dimensions, as long as the areal density is above a specific cut-off value determined by trenches dimensions, trench-filling and surrounding materials. Two applications for depth monitoring are presented. 1D models were developed and successfully applied to the DRAM trench capacitor structures. Modeling and correlation to reference methods are shown as well as dynamic repeatability and gauge capability results. Limitations of the current tool configuration are reviewed for shallow structures.
As aspect ratios become higher, features become smaller, and requirements for planarity tighten, Atomic Force Microscopy (AFM) has begun to replace profilometry for topographic measurements such as trench and via depths, step height, and micro-planarity measurements, both in development and in production. In this paper, we describe the application of a new, high throughput AFM for line monitoring in the STI and trench capacitor modules. We focus on
two key applications: the post-CMP height difference between the active area and the isolation area in the STI module, and the post-etch depth of a DRAM trench capacitor. We begin by describing the two initial AFM applications. Next, we introduce a statistical approach for determining optimal lot sampling for these applications. From the gap between throughput of our current AFMs, and statistically determined sampling requirements, we validate the need for a high throughput AFM. Next, we describe the design of
such an AFM, recently developed by KLA-Tencor, and its expected benefits. Finally, we discuss the economic benefit to Infineon of detecting metrology problems in-line, without the delay and cost of cross-sectional SEM analysis.
Polysilicon recess etch process control in deep trench arrays of a DRAM requires reliable measurements of the recess depth directly in the trench array. Until now Atomic Force Microscopy (AFM) has been used for post etch depth measurements. However, with decreasing lateral trench dimensions, AFM may approach its limits especially with respect to the available bottom travel length. Consequently, alternative metrology methods are of interest. Scatterometry is an optical, model based measurement technique which potentially allows a full reconstruction of the measured structure. The measurement of the polysilicon recess presents a number of challenges: (1) the recess depth (150nm to 300nm) is much smaller than the total height of the complete structure (several microns), (2) spacer-like sidewall layers are present, while (3) unpredictable effects may be present (e.g. voids in the polysilicon fill) and would be difficult to include into a grating model. In addition, for measurements within the trench array 3D capability is required. In this work we analyze the capability of 2D and 3D scatterometry for polysilicon recess depth process control. We evaluate parameter sensitivities, parameter correlations, measurement robustness, depth correlation to the trench array, precision and accuracy for a wide range of process variations by comparing results obtained by scatterometry to those obtained by AFM and SEM cross sections. We show that a simplified grating model provides accurate measurements in lines/spaces structures (2D). However, in trench arrays (3D) the trench depth sensitivity is critical.