The decreasing feature dimensions required in the semiconductor manufacturing industry are placing ever increasing demands upon metrology instruments. Atomic force microscopes (AFMs), which can have ~1 nm lateral resolution and sub-angstrom vertical resolution, are being used increasingly as metrology tools in the semiconductor industry. Nanometer level accuracy with AFM metrology, however, remains challenging. The scales of an instrument must be calibrated to perform accurate pitch or height measurements, while surface roughness or feature width measurements are further complicated by the 'convolution' of the probe geometry with that of the sample. Recent round-robin measurements of micro-roughness on silicon wafers revealed a considerable , variation in the results as measured by different instruments. Although many commercial AFMs employ some form of position sensor (capacitance or split diode) to linearize scan motions and improve repeatability, the scales of these instruments must still be calibrated using AFM because no instrument with the required accuracy has been available. Therefore. the National Institute of Standards and Technology (NIST) is developing a metrology AFM that will be used to calibrate traceable AFM standards. These, in turn, can be used to calibrate commercial AFMs. The NIST calibrated atomic force microscope (C-AFM) uses a flexure stage driven by piezoelectric actuators to scan the sample and commercial heterodyne interferometers to measure the lateral position of the sample. The control systems use two feedback loops that read from the X and Y interferometers, respectively, and adjust the piezoelectric voltages to keep the lateral position accurate. A commercial stage containing a piezoelectric actuator and capacitance transducer is used to generate and measure Z displacements (heights). A specially designed Z-interferometer head was used to calibrate the capacitance gauge in a process-intermittent manner. Presently, there is no commercial height artifact available in the sub-nanometer range. A promising candidate for such a standard is a specially prepared silicon sample, which has single or multiple atomic steps on the surface. In this work, we discuss our analysis algorithms and the status of our measurements of these surfaces using the C-AFM.
Proceedings Title: Proceedings of American Society for Precision Engineering
Conference Dates: January 1, 1996
Conference Location: Unknown, USA
Conference Title: American Society for Precision Engineering
Pub Type: Conferences
atomic force microscope, interferometer, metrology, step height