RELIABLE FABRICATION OF SUB-40 NM PERIOD GRATINGS USING A NANOLITHOGRAPHY SYSTEM WITH INTERFEROMETRIC DYNAMIC FOCUS CONTROL

Electron beam nanolithography tools readily permit the fabrication of ultra small structures. However, the disadvantage of such a system is that the depth of field is very small, making it difficult to maintain the focus of the system as the sample is moved due to sample topograp hy, tilt, and stage run out. This presents a serious Limitation to nan ofabrication. To overcome this problem, we have built and installed a two color, heterodyne common path interferometer on a converted JEOL 1 00 CXII scanning transmission electron microscope. The interferometer compensates for the very small depth of field of the system by detecti ng variations in the sample working distance with respect to the elect ron optical objective lens, and dynamically correcting the lens curren t. The advantage of this system is that optical access to the sample c an be made in a microscope with a very small working distance or sampl e space. To demonstrate this system, we have fabricated 38 nm period g ratings on a silicon substrate. The process was a conventional one usi ng a single 30 nm layer of 350 k molecular weight poly methyl methacry late resist. The grating was exposed with a linear dose of 1.6 nC/cm a nd developed in 3:1 IPA:MIBK for 30 s at 23 degrees C. Patterns were t ransferred by lift off of an evaporated Ni/Au film. The gratings have a 1:1 mark:space ratio and a Line-width variation of 5 nn at 4 sigma. The stage can be moved over large areas and from chip to chip, and the lithography repeated successfully. This system therefore permits reli able state-of-the-art nanolithography. (C) 1996 American Vacuum Societ y.