SPIE Photomask Technology + EUV Lithography 2010
Conference paper

Understanding the trade-offs of thinner binary mask absorbers

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Mask topography is only one of the challenges for extending 193nm immersion lithography to 22nm and beyond. Migration to binary but thinner mask absorbers, from the previously employed attenuated phase-shift mask (attnPSM) technology, traded a tolerable loss in contrast for better mask making performance and reduced electromagnetic field (EMF) impact [1]. The relentless technological advances in 193nm lithography required to enable 22nm technology, however, continue to drive the dimensions of mask features deep into sub-wavelength scale. In this regime, residual mask EMF effects can still degrade the imaging performance of critical mask patterns, often in the form of featuredependent biasing and shifts of the plane of best focus that shrink the common process window and magnify the impact of mask errors. In this paper we investigate the potential benefits in EMF effects mitigation provided by further thinning the mask absorber to the minimum possible while retaining the required opacity. This study was motivated by the narrower process variability bands observed on a 22nm structure with high EMF sensitivity, when computed with rigorous EMF simulations using a thinner absorber. Resist measurements on wafers exposed with the same EMF sensitive structure built on either the standard binary mask or the thinner sample, confirmed the lower sensitivity to mask topography of the latter while also providing a significant process window improvement. We further observed that thinner topography allowed for a smaller topography induced bias, resulting in improved mask manufacturability with less risk for mask corrections to be limited by mask manufacturability rules such as small assist features and small corner to corner gaps. Thinning the absorber, however, is typically accompanied by an increase in reflectivity of the mask blank which may influence the nature of stray light in the imaging system. To understand the consequences of increasing the blank reflectivity, a double expose scheme was used to measure stray light and determine the relative contribution from the imaging system optics, the mask blank reflectivity, and pellicle thickness. Initial results show that the increased reflectivity of the thinner absorber film has minimal impact on stray light effects in the scanner and that the overall mask reflectivity at high angles is typically dominated by the pellicle thickness. The thin binary absorber should be used in conjunction with a thin pellicle. In this paper we will also explore other relevant characteristics of these novel mask blanks, such as diffraction efficiency, EMF-induced focus drift, changes in contrast, and the implications for increased reflectivity, smaller assist feature size and other lithographic considerations. These results will be demonstrated with rigorous electromagnetic simulations as well as AIMS and wafer measurements on a set of EMF sensitive structures for 22nm contact and metal layers, while simultaneously verifying that the imaging performance of the remaining patterns is unaffected. The use of thinner absorber film also improves the mask making process, contributing to better mask critical dimension (CD) uniformity and overall better lithographic performance as discussed in [2]. © 2010 SPIE.