Khanh B. Nguyen

Impact of optical enhancement techniques on the mask error enhancement function (MEEF)

Resolution, R, in optical lithography is often described by the Rayleigh equation: R equals k1(lambda) /NA. Since the 0.25 um generation there has been a trend of aggressive gate length reduction for high performance devices. Leading edge logic technologies require gate CDs equal to ½to 2/3 the wavelength of the exposure system. Even with high NA steppers and scanners low k1 patterning is a requirement. Development of processes utilizing OPC and PSM technology is critical to achieving adequate process latitude and CD control. As k1 factor falls below 0.5 the image quality and contrast degrades substantially. One result of low contrast images is that the CD variation in the photomask gives rise to larger than expected printed CD changes: the so-called MEEF. The MEEF can be simply defined as the ratio of the change of the resist feature width to the change in the mask feature width, assuming constant process and illumination conditions. For a 4x mask the MEEF can be calculated.

Exploiting structure of wafer distortion in global alignment

One of the most critical emerging challenges in lithography is achieving rapid and accurate alignment. The problem is exacerbated by wafer and stage distortions. Thus, an effective learning process is needed to rapidly acquire the best possible positional information from an array of marks across the wafer. An algorithm based on a neural network model of global alignment was presented [A. A. Ghazanfarian et al., Proc. SPIE 3051, 629 (1997); J. Vac. Sci. Technol. B 15, 2146 (1997)], which incorporated both wafer and stage distortion. Yet in almost all cases, the stepper machines are precalibrated and the stage distortion is well controlled. Therefore, the aforementioned methods need to be revisited to specifically address the wafer distortion problem. In this article, we propose a new global alignment algorithm based on array processing techniques [J. Rissanen et al., Automatica 14, 465 (1978); M. Wax et al., IEEE Trans. Acoust., Speech, Signal Process], which exploits the structure of the overlay data fro...

Characterization of the manufacturability of ultrathin resist

A study was conducted to explore the manufacturability of ultrathin resist by focusing on two key issues, defects and etch resistance. Defects in ultrathin resist were characterized by optical inspection and scanning electron microscopy reviews. A number of representative defect types in the ultrathin resist/hardmask process were identified. With process optimization, defect density in ultrathin resist was reduced to levels that are comparable to that of a baseline 0.5 μm thick resist process on nontopographic wafers. Etch resistance sufficient for patterning metal–oxide–semiconductor transistor gate film stacks was demonstrated for a 100–150 nm thick resist layer.

Mask technology for EUV lithography

Extreme UV Lithography (EUVL) is one of the leading candidates for the next generation lithography, which will decrease critical feature size to below 100 nm within 5 years. EUVL uses 10-14 nm light as envisioned by the EUV Limited Liability Company, a consortium formed by Intel and supported by Motorola and AMD to perform R and D work at three national laboratories. Much work has already taken place, with the first prototypical cameras operational at 13.4 nm using low energy laser plasma EUV light sources to investigate issues including the source, camera, electro- mechanical and system issues, photoresists, and of course the masks. EUV lithograph masks are fundamentally different than conventional photolithographic masks as they are reflective instead of transmissive. EUV light at 13.4 nm is rapidly absorbed by most materials, thus all light transmission within the EUVL system from source to silicon wafer, including EUV reflected from the mask, is performed by multilayer mirrors in vacuum.

CD uniformity consideration for DUV step and scan tools

In this paper, we evaluate the CD-uniformity performance of an advanced DUV step and scan exposure tool. Using high quality reticles CD-uniformity at semi-dense lines is shown to be less than 10nm at best focus and less than 20nm at 0.6 micrometers focus range. Isolated lines show 10nm CD-uniformity at best focus and 20nm at 0.4 micrometers focus range. Removing reticle contribution leads to a decreases of intra-field CD- uniformity up to 6nm. At 0.18 micrometers lines, 0.45 micrometers pitch we found that the Mask Error Factor (MEF) is around 1. Going to smaller feature sizes and/or defocus MEF increases rapidly. We show that proper focus control is crucial for isolated line intra-field CD-control. Horizontal and vertical lines behave very similarly at semi-dense pitch. The isolated horizontal lines show a considerably higher DOF and tighter intra-field CD-control than isolated vertical lines. We are able to show both reticle and scanner contributions are not a limiting factor for fulfilling the requirements for CD-control of state of the art microprocessors. In case of 0.15micrometers linewidth and/or smaller pitches mask eros become more critical due to a considerable increase of the MEF.