To avoid expensive immersion lithography and to further use existing dry tools for critical contact layer lithography at
4Xnm DRAM nodes the application of altPSM is investigated and compared to attPSM. Simulations and experiments
with several test masks showed that by use of altPSM with suitable 0°/180° coloring and assist placement 30nm smaller
contacts can be resolved through pitch with sufficient process windows (PW). This holds for arrays of contacts with
variable lengths through short and long side pitches. A further benefit is the lower mask error enhancement factor
(MEEF). Nevertheless 3D mask errors (ME) consume benefits in the PW and the assist placement and coloring of the
main features (MF) put some constraints on the chip design. An altPSM compatible 4Xnm full-chip layout was realized
without loss of chip area. Mask making showed very convincing results with respect to CDU, etch depth uniformity and
defectiveness. The printed intra-field CD uniformity was comparable to attPSM despite the smaller target CDs. Room for
improvement is identified in OPC accuracy and in automatic assist placement and sizing.
Chromeless Phase Lithography (CPL) is discussed as interesting option for the 65nm node and beyond offering high resolution and small Mask Error Enhancement Factor. However, it was shown recently that at high NA CPL masks can exhibit large polarization and also phase effects. A well known phase effect occurring for CPL semi dense lines are through focus Bossung tilts.
However, another manifestation of phase effects for dense lines and spaces is a reduced contrast for a symmetrical off-axis illumination due to phase errors between 0<sup>th</sup> and 1<sup>st</sup> diffraction order. In this paper it is shown that these phase effects can lead to a significant contrast loss for dense features smaller than 60nm half pitch. While also present for trench structures, the contrast reduction is more pronounced for mesa style structures. It is shown that for mesa structures an adjustment of etch depth can not recover an effective pi-phase shift. Furthermore, significant polarization effects are observed. As an example, the optimum mesa structure for TE polarization is shifted to small lines.
For an experimental validation, a CPL mask containing dense lines and spaces was fabricated. Their imaging performance was characterized with an AIMS 45i offering NA's greater than 1 and linearly polarized illumination as well as by wafer printing. Gratings with pitches down to 100 nm with varying duty cycles were measured with TE, TM and unpolarized dipole illumination. Very good agreement between measurement and simulation results confirmed the validity of theoretical predictions.
Alternating Phase-Shifting masks (altPSM) are known to provide high contrast imaging combined with a low Mask Error Enhancement Factor (MEEF) at low k1. At feature sizes close to 60nm half-pitch and less the impact of mask topography effects increases. This applies in particular for altPSM. This is due to the quartz etch which is required for every second mask aperture to obtain the 180 degrees phase shift. It enlarges the mask profile height significantly. The influence of the quartz trench profile on the transmission and phase balancing performance has already been studied extensively. Basically it has been shown, that tighter quartz trench profile control, specifically for etch depth and width, is required with decreasing mask feature half pitch. The desired mask pattern geometry optimization is currently based on an evaluation of the printed resist pattern over defocus. However, a mask process engineer can use instead only AIMS measurements of the mask features. Therefore there is a mature interest to check, how good such measurements can replace resist pattern measurements. In the paper therefore it is evaluated how accurate AIMS measurements can describe the real printing performance of an alternating PSM in resist. Impact of differences of the image formation is investigated by use of analytical expressions. Furthermore, the influence of tool imperfections and the presence of resist are discussed. The theoretical results are compared to experimental data taken from AIMS measurements and wafer prints.
Advances in photo mask etch technology are clearing the way for 65nm alternating phase shifting masks (alt-PSM) to be used as a principal component in a typical mask set. As wafer features shrink to ever smaller sizes, the specifications on the photo mask etch performance become more and more stringent. To meet the challenging demands of 65nm technology, alt-PSM’s are employed to help deliver a reliable and repeatable pattern transfer to the wafer. Hence, especially in the framework of quartz dry etch technology for the production of high-end alt-PSM’s ever tightening specifications generate various efforts of machine vendors and mask making industry to meet the demands <sup>1</sup>. This paper covers data from a ten experiment two level three factorial Design of Experiment. Therein, the effects of changing quartz process conditions (i.e., ICP power, RIE power, and gas chemistry) on the Applied Materials Tetra<sup>TM</sup> II Photomask Etch System were investigated. As for alt-PSM's the universally agreed upon number one priority is phase angle uniformity followed closely by RIE lag, sidewall angle (SWA), and micro-trenching this was also taken into account during the optimization process of the DoE findings. The results show phase angle uniformity of less than 2.0° relative to a 180° etch depth and acceptable performance for RIE lag, SWA, and micro-trenching. Trends and graphs of the DoE are presented and discussed in detail.