Sidewall Metrology Expects Clear Sailing to 32 nm.

Semiconductor International, March, 2007

Alexander E. Braun, Senior Editor

As technology progresses from node to node, metrology standbys such as AFM, CD-SEM and scatterometry continue to meet measurement requirements.

As semiconductor technology evolves toward the 32 nm node, there is some concern whether traditional metrology solutions such as CD-SEM, scatterometry, atomic force microscopy (AFM) and others will continue providing those measurements that are crucial to process development and control. While there is every indication that CD-SEM and scatterometry will be capable of measuring CDs at the 32 nm node -- and AFM has measured CD lines <25 nm -- continuous improvement will be necessary to meet the requirements imposed by each succeeding node.

Profile and sidewall angle

Bob Monteverde, director of marketing at Timbre Technologies (Santa Clara, Calif.), thinks a significant benefit of optical digital profilometry (ODP) is that it is a true profile measurement, not just a top-down image or CD measurement. "ODP gives a complete profile: CD at the structure's bottom, at the top, and the sidewall shape," he said. For simplicity's sake, fabs have traditionally distilled the profile shape metric into a single CD value for use in statistical process control (SPC) and advanced process control (APC). However, in advanced processes, the control of profile shape is increasingly critical, and improved sidewall information is necessary. Serguei Komarov, senior applications engineer at Timbre, works on complex 3-D aspect-ratio applications. He views sidewall angle (SWA) metrology as important because a small angle change results in a huge alteration in dimensions with high aspect ratio. "Destructive cross-section measurements aren't production-worthy, and top-down CD-SEM cannot provide true sidewall information. ODP gives accurate sidewall shape measurement. As to extendibility, at 32 nm we can show performance with 3-D applications on very high aspect ratios." (Fig. 1)

Scatterometry performs dense line space measurement best because it uses a periodic grating. As lithography is driven to make dense features with increasingly smaller linewidths, ODP advances provide detailed metrology for emerging lithography techniques, such as double patterning. For litho process control, SWA is a focus indicator. Previously, when using CD metrology for process control in lithography, CDs were measured, and the dose on the scanner was adjusted to tune the CD to the right size. With ODP providing both the CD and SWA measurement, there is the capability to monitor both dose and focus.

The progression to 32 nm is challenging. With 3-D transistors, structures with dimensions that were lateral are now vertical, making profile metrology critical. A gate is no longer represented by the lateral distance between the structure's right and left edges; it is the distance along the vertical wall.

At 45 and 32 nm, with traditional scaling unable to keep up and with technologies like finFETs and double patterning (which metamorphose overlay into a CD problem), profile measurements and SWA are the link between lateral and vertical measurements. If they vary, they affect one another through the SWA. These two measurements are now inseparable, requiring a complete profile measurement and understanding of how these metrics interact.

The roughness problem

Major challenges for sidewall metrology lie in deriving repeatable and accurate methods to assess line edge roughness (LER) and sidewall roughness (SWR), and separately characterizing them to reduce process development times. "As gate lengths drop to 30 nm, LER will become a significant part of the CD budget, and must be characterized and controlled," said Dean Dawson, senior director of marketing for automated AFMs at Veeco Instruments (Santa Barbara, Calif.). Excessive linewidth roughness (LWR) can cause device failure and CD measurement uncertainties. Performance of new device structures will be affected by excessive SWR. Additionally, SWR of low-k materials requires characterization to overcome porosity and copper electromigration issues.

Profile accuracy of critical features becomes necessary at 45 nm to overcome CD-SEM and scatterometry bias issues. Accurate profile metrology also provides calibration and regular monitoring of scatterometry models, enabling their use as a stable production metrology tool. Sidewall metrics, such as LER/SWR, are essential for 45 nm processing, and may be adopted as process control monitors.

Recent 3-D AFM scanning technology improvements and a new CD "finger" probe design now enable high-resolution, accurate measurement of SWR and LWR over the entire profile. LER has been assessed by SEM technology, which is limited in its resolution and measures from the top down only. Prior to 3-D AFM, the only methods for assessing SWR were to take a horizontal TEM along a feature, or cross-section trench features and measure with an AFM. Both are costly, time-consuming and lack a statistical basis for process development. Additionally, 3-D AFM enables accurate profile and angle metrology, including undercut features. Reference metrology for profile is significant because of bias issues on current CD metrology systems.