High-throughput, high-spatial-frequency measurement of critical dimension variations using memory circuits as electrical test structures

Abstract

Articles you may be interested inIntegrated microcircuit on a diamond anvil for high-pressure electrical resistivity measurement Appl. Phys. Lett. 86, 064104 (2005); 10.1063/1.1863444Measurement of semi-isolated polysilicon gate structure with the optical critical dimension technique Practical approach to separating the pattern generator-induced mask CD errors from the blank/process-induced mask CD errors using conventional market measurements Critical dimension ͑CD͒ errors are traditionally specified and characterized without reference to their spatial frequency spectra. However, a given amplitude of CD variation can have very different consequences depending on its spectrum. CD errors whose variation is over a few micrometers can be much more serious than those of the same magnitude that extend over several chips. Existing CD metrology tools, such as scanning electron microscopy or electrical resistance measurements, are seldom used to characterize these short-range CD variations, particularly those with spatial wavelengths below 100 m, because of the large amount of data required and the difficulty of collecting data in such a dense grid. We report a new method of measuring CD variations using static random-access memory ͑SRAM͒ circuits in which direct measurements of bit-line currents reveal the individual transistor gate length variations within each memory cell. With the compactness and regularity of the SRAM layout we can measure CD variations with spatial periodicities down to 6 m. By repeatedly measuring each cell in a memory chip and recording the corresponding currents we can achieve sufficient data to minimize noise, and through two-dimensional bandpass filtering 0.2 nm CD variations can be detected. Two designs of 4 Mbit SRAMs fabricated using 250 nm design rules were studied. The resulting CD variations yielded spectra that were dominated by peaks whose origins included uncorrected electron beam and optical proximity effects. Pattern-independent variations ascribable to the reticle generator itself appeared to contribute only a small fraction of the total error observed.

High-throughput mapping of short-range spatial variations using active electrical metrology

“…8(b)], we can see that the poly layer has a corresponding two-cell periodicity in direction, while the diffusion layer has a corresponding two-cell periodicity in direction. Similar to previous study of SRAMs [7], [10], these layout differences may have caused the observed current periodicities. We should notice that transistor gate width (determined by the diffusion width) may have affected the transistor saturation current here, thus causing the two-cell periodicity in direction.…”

“…Furthermore, the intertransistor variations, which are defined as those short-range variations with spatial wavelengths from 1 to 100 m, are particularly serious for many types of integrated circuits such as most analog circuits that are sensitive to device mismatch problems, or some digital circuits with sense amplifiers or other balanced circuit designs [7]. However, such short-range parametric variations are not systematically measured and characterized mainly due to their metrology difficulties.…”

“…7(a)]. In this plot, the spatial variations are shown in spatial-frequency domain by Fourier transform [7], [10]- [14]. By checking the layout of the SRAM [ Fig.…”

“…We have previously used a modified SRAM to study spatial distribution of CD variations and discussed its measurement noise and speed [7], [10]. However, SRAM as a metrology tool has certain disadvantages: the measured gate length variations may have the influence from other error contributors such as variations of doping and gate oxide thickness, and the size of the SRAM cell is not minimized for metrology purpose.…”

“…Finally we will present the results of intertransistor CD and overlay variations measured with the test chips. Fourier analysis will be used and the results will be shown in spatial-frequency domain [7], [10]- [14]. Fig.…”