ELSEVIER Microelectronie Engineering 41/42 (1998) 83-86 MICROELECTRONIC ENGINEERING Sub-Resolution Feature OPC as an Enabler for Manufacturing at 0.2gm and Below J. Randall@, A. Tritchkov*, K. Ronse*, and P. Jaenen* @Texas Instruments, Dallas, Texas - on Assignment at IMEC *IMEC, Kapeldreef 75, B-3001, Leuven, Belgium Optical proximity correction (OPC) with sub-resolution assisting features (outriggers) aided by adjustable numerical apertures and partial coherence is shown to provide improvement in process windows through tightening printed critical dimensions (CDs) and improving the depth of focus. Printed CD variations due to mask errors are not exacerbated by the use of outriggers and may even be reduced by this type of OPC. 1. INTRODUCTION As the critical dimensions (CDs) printed by optical lithography shrink below the physical wavelength of the exposing radiation, optical proximity effects (OPE) are increasingly hard to ignore. We address three important issues created by OPE: The shift in size even at best focus of linewidths as a function of the local pattern environment (the pitch of an array of lines), depth of focus (DOF) limitations, and the magnification of mask CD errors[l]. This paper will deal primarily with the printing of 0.2p.m lines at various pitches on a clear field mask with 248mn lithography. We assume that a manufacturable lithography process will require mutually overlapping process windows with at least a 0.6gm depth of field and 10% exposure latitude for all relevant pattern pitches, not just the extremes of the range of pitches. The variable numerical aperture (NA) and partial coherence (if) as well as off-axis illumination options on state-of-the-art 248nm lithography systems am important tools in dealing with OPE. Because different pattems contain significantly different spatial frequencies, the optimum optical configuration for each pattern can be quite different. However, these variables create a large parameter space that must be explored to find suitable conditions for specific applications. Feature size biasing is the most common type of OPC. However, feature biasing can only shift CDs and does little to improve the change in CD with defocus that is particularly acute with isolated lines. The use of outriggers has been shown to be effective in both correcting the CD and decreasing the change in CD as the image is defocussed. The selection of appropriate optical conditions and the use of outriggers is useful for establishing overlapping process windows for patterns with a wide variety of pitches. The use of outriggers does require relatively small features on photomasks, but commercial mask makers have already demonstrated the ability to make such masks. Commercial software exists that automatically assigns placement of outriggers to mask layouts[2]. 2. PROCESS WINDOW IMPROVEMENT A test reticle called RTP4 was loaned to IMEC by the MicroUnity Systems Engineering Inc. for evaluation of its perfommnce at 248nm. The reticle contains many test structures and had outriggers placed according to several different rule sets for different target CDs. The 4X mask was fabricated by Photronics. We investigated primarily 0.2gm line patterns with fine-to-space ratios of 1:1, 1:2, 1:3, and 1:4. The outriggers applied to these patterns were nominally 75nm (IX) and placed (edge-to- edge) 0.251ares (IX) from isolated features. There were no outriggers placed between the 1:1 and 1:2 line arrays. The 1:3 array had a single 75nm outrigger placed in the center of the space between the lines. The 1:4 array had two 75nm outriggers in the space between the lines, each placed 0.25gms from the nearest 0.2gm line. By selecting these particular line to space ratios, we avoided the transition regions at approximately 1:2.5 and 1:3.5 where first one then two outriggers are placed between the 0.2gm lines. MicroUnity is exploring techniques to produce smooth transitions[3]. Prior to making test exposures with this mask on the ASML PAS 5500 /300 deep-UV (248nm) stepper, we used aerial image simulation to select settings for the NA, o, and for annular illumination conditions an inner boundary for the partial coherence (o-inner). In the simulations, NA was varied from 0.4 to 0.63, o was varied over 0.3 to 0.8, c-innercovered the range 0 to ~-0.3. We used 0167-9317/98/$19.00 © Elsevier Science B.V. All rights reserved. PII: S0167-9317(98)00018-5