Development of pseudo-random binary gratings and arrays for calibration of surface profile metrology tools
Optical Metrology tools, especially for short wavelength (EUV and X-Ray), must cover a wide range of spatial frequencies from the very low, which affects figure, to the important mid-spatial frequencies and the high spatial frequency range, which produces undesirable flair. A major difficulty in using surface profilometers arises due to the unknown Modulation Transfer Function (MTF) of the instruments. Therefore, accurate calibration of profilometers, the understanding of their MTF limitations, and cross calibration between tools represents a considerable challenge for quantitative optical metrology. In previous work the instrumental MTF of a surface profiler was precisely measured using reference test surfaces based on binary pseudo-random (BPR) gratings. Here, they present results of fabricating and using two-dimensional (2D) BPR arrays that allow for a direct 2D calibration of the instrumental MTF. BPR sequences are widely used in engineering and communication applications such as Global Position System, and wireless communication protocol. The ideal BPR pattern has a flat 'white noise' response over the entire range of spatial frequencies of interest. The BPR array used here is based on the Uniformly Redundant Array prescription initially used for x-ray and gamma ray astronomy applications. The URA's superior imaging capability originates from the fact that its cyclical autocorrelation function very closely approximates a delta function, which produces a flat Power Spectrum Density (PSD). Three different BPR array patterns were fabricated by electron beam lithography and ICP etching of silicon. The basic size unit was 200 nm, 400 nm, and 600 nm. Figure 1 shows the fabrication sequence. The 2D BPR arrays were used as standard test surfaces for MTF calibration of the MicroMap{trademark}-570 interferometric microscope with all available objectives. Figure 2 shows representative scanning probe height data for the 400 nm BPR sample. Figure 2 shows the raw Power Spectral Density for 5 different objectives. They demonstrate that the two dimensional BPR array is a very effective calibration standard. However, departures from ideal, such as square sidewall, and uniform etch depth ultimately can limit the accuracy of the calibration. The effects of fabrication imperfections on the efficiency of calibration will be discussed.
- Research Organization:
- Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
- Sponsoring Organization:
- Materials Sciences Division
- DOE Contract Number:
- DE-AC02-05CH11231
- OSTI ID:
- 948464
- Report Number(s):
- LBNL-1414E-Ext-Abs; TRN: US200907%%50
- Resource Relation:
- Conference: The 53rd international conference on electron, ion, photon beam technology and nanofabrication, Marco Island, FL, May 26-29, 2009
- Country of Publication:
- United States
- Language:
- English
Similar Records
Development of pseudo-random binary arrays for calibration of surface profile metrology tools
Binary Pseudo-Random Gratings and Arrays for Calibration of Modulation Transfer Functions of Surface Profilometers