OPTICAL METROLOGY: Overlay system measures wafer-alignment errors

Nov. 1, 1996
Accurate mask-to-wafer alignment during lithography is critical to semiconductor fabrication. As circuit feature sizes decrease and manufacturers turn to more sophisticated exposure techniques, calibration of overlay metrology instruments becomes ever more critical.

Accurate mask-to-wafer alignment during lithography is critical to semiconductor fabrication. As circuit feature sizes decrease and manufacturers turn to more sophisticated exposure techniques, calibration of overlay metrology instruments becomes ever more critical.

Researchers at the National Institute of Standards and Technology (NIST, Gaithersburg, MD) have built an optical overlay metrology system that can resolve nanometer-scale three-dimensional mask-to-wafer alignment errors. Led by Richard Silver, the group will use the highly accurate instrument to characterize a standard reference artifact for industry overlay metrology systems calibration. "A previous instrument has demonstrated 10-nm uncertainty," says Silver. "The new instrument has a number of improvements, so we expect to do better than this. We`ll be competitive with the precision of commercial instruments but we`ll have the advantage of NIST-traceable accuracy."

The instrument consists of a confocal optical microscope, fiber-delivered sample illumination, a metrology frame of reference, and a photometer and CCD camera, all mounted on a rigid platform on an optical bench to minimize vibration (see photo).

The instrument can operate in conventional bright-field imaging mode, fixed-aperture confocal mode, or Nipkow spinning-disk confocal imaging mode. In bright-field mode, users can locate sample features and navigate about; the off-the-shelf metrology CCD camera captures the image.

Fixed-aperture confocal mode yields the most accurate measurements. The instrument acquires data on-axis to minimize measurement degradation by optical aberrations. Mounted on a crossed-roller-bearing translation stage, the sample under test passes under the optical head, where a high-sensitivity photometer detects the reflected light. Differential interferometers provide synchronous sample position data, tracking the linear motion of the stage along the orthogonal axes with a resolution of 0.6 nm; an autocollimator monitors angular motion of the stage about the optical axis.

Spinning images

The Nipkow confocal technique involves a spinning disk placed between the object and the imaging optics. Several thousand 20- to 40-µm-diameter apertures are arrayed on the disk in a spiral pattern. As the disk spins, the microscope produces multiple confocal images across the object field, essentially providing a real-time, full-field image with higher resolution than the conventional bright-field image. To provide spatial resolution, the CCD camera captures the image and sends it to a computer for processing.

In addition to characterizing calibration artifacts, the instrument will help scientists understand overlay metrology instrument design and performance. "The main reason to have both [fixed-aperture and Nipkow spinning disk] confocal capabilities," Silver notes, "is to compare instrument performance using similar imaging capabilitiesfor example, CCD array detectors versus photometersor to investigate the effects of working off-axis during a measurement process."

The 100X optical system has a numerical aperture of 0.95. A trinocular tube permits simultaneous eyepiece observation, CCD data acquisition, and photometer measurements. A fiber-coupled mercury source provides system illumination; the lamp itself is located 1 m away from the instrument to avoid thermal contamination.

Thermo-mechanical stability

To minimize vibration effects, the microscope and illumination optics are set on a Stewart platform, a three-point kinematic mount. NIST-patented strut joints allow the dimensions of the mount to be easily upgraded to 300-mm-wafer operation. The team built the Stewart platform of aluminum, with a coefficient of thermal expansion (CTE) of 24 × 10-6/°C, while the reference frame and mirror mounting hardware are formed of Invar, with a CTE of 1.2 × 10-6/°C. Web flexures compensate for the 20-µm/(°C m) mismatch in material expansion.

"We expect the instrument to meet or exceed state-of-the-art industry capabilities," says Silver. "Resolution of the optical system is sub-quarter micron, which demonstrates the capability of confocal microscopy today." Scientists at NIST have designed an overlay metrology tool calibration artifact, now in the prototype phase, based on a stepped micro-cone design with z-axis, as well as x-y plane relief. The new metrology instrument will allow them to fully characterize the artifact, then make it available to semiconductor manufacturers for calibration of their own overlay metrology systems.

About the Author

Kristin Lewotsky | Associate Editor (1994-1997)

Kristin Lewotsky was an associate editor for Laser Focus World from December 1994 through November 1997.

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