Proceedings Article | 18 December 1998
KEYWORDS: Image processing, Photomasks, Optical proximity correction, Image quality, Scattering, Printing, Fiber optic illuminators, Binary data, Optical lithography, Photoresist processing
This work discusses routes to extend optical lithography to the 70 nm technology node using proper selection of masks, mask design including choice of optical proximity correction (OPC), exposure tool, illuminator design, and resist design to do imaging process integration. The goal of this integration is to make each component of the imaging system work to the best benefit of the other imaging components so as to produce focus-exposure process windows large enough to use in a manufacturing environment. In order to maximize return on investment, the design of the photoresist and the exposure tool is used to simplify reticle design as much as possible. For masks, the choices of binary, alternating or attenuated phase-shift masks (PSM) are discussed. Alternating PSM produces the best image quality but the effective phase angle depends on NA, wavelength, sigma, magnification, pitch and duty cycle. Attenuated PSM has maximum image quality when using transmissions of 18% for contact holes and 30% to 40% for lines and spaces. Using high transmission masks increases working resolution of a wide range of feature sizes and shapes, but requires suppression of unwanted light. This suppression requires using ternary attenuated PSM and in many instances necessitates critical formation of a second layer on the mask that has both the proper size and placement of the second level features. For OPC, the use of scattering bar, sub-resolution assist features to make isolated lines mimic dense exposure-focus response is discussed. For illuminators, properly tuned weak off-axis illumination is used with binary and attenuated PSM to flatten image CD while maintaining image quality at an acceptable level for the resist. For resists, the need to balance resist bias and side-lobe printing is discussed. A 'work-in-progress' integration experiment is reviewed for 525 nm and 1050 nm pitches with 175 nm targeted line features imaged with a 0.53 NA, 248 nm stepper that has been modified with weak and strong off-axis illuminators and a binary reticle. Results show weak illumination produces a common process corridor for the two pitches that will need enhancement using OPC, but that the individual windows have acceptable imaging capability. Predictions of production resolution that are inferred by our simulation and experimental results are made and recommendations are given to make these predictions a reality. Based on our work we believe that, expect for dense contact holes, 248 nm has the potential to be used through the 130 nm technology node and 193 nm can be used through the 100 nm node and the beginning of the 70 nm technology node. Dense contact holes will require a next generation lithography technology.
