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

Critical dimension (CD) errors are traditionally specified and characterize d without reference to their spatial frequency spectra. However, a given am plitude of CD variation can have very different consequences depending on i ts spectrum. CD errors whose variation is over a few micrometers can be muc h more serious than those of the same magnitude that extend over several ch ips. Existing CD metrology tools, such as scanning electron microscopy or e lectrical resistance measurements, are seldom used to characterize these sh ort-range CD variations, particularly those with spatial wavelengths below 100 mu m, because of the large amount of data required and the difficulty o f collecting data in such a dense grid. We report a new method of measuring CD variations using static random-access memory (SRAM) circuits in which d irect measurements of bit-line currents reveal the individual transistor ga te length variations within each memory cell. With the compactness and regu larity of the SRAM layout we can measure CD variations with spatial periodi cities down to 6 mu m. By repeatedly measuring each cell in a memory chip a nd recording the corresponding currents we can achieve sufficient data to m inimize noise, and through two-dimensional bandpass filtering 0.2 nm CD var iations can be detected. Two designs of 4 Mbit SRAMs fabricated using 250 n m design rules were studied. The resulting CD variations yielded spectra th at were dominated by peaks whose origins included uncorrected electron beam and optical proximity effects. Pattern-independent variations ascribable t o the reticle generator itself appeared to contribute only a small fraction of the total error observed. (C) 1999 American Vacuum Society. [SO734-211X (99)08606-0].