We make a new model for pattern failure, which is the pattern collapse and bridging of resist patterns of 43-nm 1:1 lines and spaces (L/S) exposed as a focus-exposure matrix, to explain and predict the process window of the pattern failure. It is found that the conventional Imax-Imin model cannot be fitted to the experimental pass/fail data. Instead of Imax and Imin, we select the critical dimension (CD) and normalized image log slope (NILS) as the model input. The new CD-NILS model corresponds well to the experimental pass/fail data. Good correspondence is assumed to be due to the properly selected model input. Pattern collapse, which occurs during the drying of the water at the rinse of the resist patterns, is expected to be accelerated by the smaller line CD and the larger line width roughness (LWR) due to smaller NILS. Pattern bridging, which occurs during resist development, is expected to be accelerated by the larger line CD and the larger LWR. The CD-NILS model predicts the process window precisely when a new process condition (a new illumination in this case) is adopted. It suggests that the CD-NILS model is a powerful methodology for predicting the process window to optimize the process condition and optimize the lithography design.
We have studied both the mask CD specification and the mask defect specification for spacer patterning
technology (SPT). SPT has the possibility of extending optical lithography to below 40nm half-pitch devices. Since
SPT necessitates somewhat more complicated wafer process flow, the CD error and mask defect printability on wafers
involve more process factors compared with conventional single-exposure process (SEP). This feature of SPT implies
that it is very important to determine mask-related specifications for SPT in order to select high-end mask fabrication
strategies; those are for mask writing tools, mask process development, materials, inspection tools, and so on. Our
experimental studies reveal that both mask CD specification and mask defect specification are somehow relaxed from
those in ITRS2007. This is most likely because SPT reduces mask CD error enhanced factor (MEF) and the reduction
of line-width roughness (LWR).
We made a new model for the pattern failure, which was the pattern collapse and the pattern bridging, of the resist patterns of 43nm 1:1 lines and spaces (L/S) exposed as a focus-exposure matrix, in order to explain and predict the process window of the pattern failure. It was found that the conventional 'Imax-Imin' model was unable to be fitted to the experimental pass/fail data. Instead of the Imax and Imin, we selected the critical dimension (CD) and the normalized image log slope (NILS) as the model input. The new 'CD-NILS' model corresponded well to the experimental pass/fail data. The good correspondence was assumed to be due to the properly selected model input. The pattern collapse, which occurs during the drying of the water at the rinse of the resist patterns, is expected to be accelerated by the smaller line CD and the larger line width roughness (LWR) due to smaller NILS. The pattern bridging, which occurs at the resist development, is expected to be accelerated by the larger line CD and the larger LWR. The CD-NILS model predicted the process window precisely when a new process condition (= a new illumination in this case) was adopted. It suggests that the CD-NILS model is a powerful methodology for predicting the process window in order to optimize the process condition and to optimize the lithography design.
A spacer patterning technology (SP) has the possibility of extending optical lithography to below 40nm half-pitch devices. Since the spacer patterning process necessitates somewhat more complicated wafer process flow, the CD variation on wafers involves more process error sources compared with conventional exposure patterning process. This implies that, for the spacer patterning process innovation in determining specifications for each unit process is requried. In particular, it is important to determine mask-related specifications in order to select high-end mask fabrication strategies for mask writing tools, mask process development, materials, inspection tools, and so on. The purpose of this paper is to discuss how to consider mask specification in spacer patterning process for 40nm half-pitch and beyond.
We have successfully achieved accurate alignment to remove stacked TiN/Ti/Al/TiN/Ti films on damascene W marks by using laser ablation technology. Because, the Al films deposited on the damascene W marks lead to poor quality of alignment accuracy due to the asymmetric topography of the deposited Al surfaces. In the case that the TiN/Ti film is not formed on the Al surface, high-density plasma is formed above the Al surface during laser irradiation. This plasma screens off the surface from laser irradiation. Therefore, it is difficult that the naked Al films are removed completely by the laser irradiation. From the results of thermal analysis of the TiN/Ti/Al films during laser irradiation, it is concluded that the irradiation fluence should be controlled as abrupt evaporation occurs at the Al surfaces without evaporation of the top TiN/Ti films. In this condition, the plasma cannot be formed above the TiN/Ti surfaces during laser irradiation. Therefore, the irradiation energy is absorbed efficiently in the TiN/Ti films and the TiN/Ti/Al/TiN/Ti films could be removed completely by laser irradiation.
The global alignment random of wafer alignment after the TiN/Ti/Al/TiN/Ti film ablation on the W marks is equal to that of the ideal W marks before the Al film deposition. These results mean that the laser ablation is the most effective technology for locally removing thin metal films on alignment marks to achieve accurate alignment.
We propose a methodology that separates critical dimension (CD) variation depending on local pattern density into error sources; CD error on exposure mask, mid-range flare of exposure tool, and acid evaporation during post exposure baking (PEB). This methodology consists of two particular processes. One is over-coating process onto resist film before exposure, and the other is double exposure process using test reticles. The test reticles have line-and-space (L/S) array region and peripheral region that contains opaque area to avoid overlapping with L/S array region in double exposure process. Over-coating process allows separation of acid evaporation from other error sources. On the other hand, double exposure process enables elimination of CD error on exposure mask depending on local pattern density since double exposure process can make various pattern densities around fixed L/S mask pattern. In fact, the experimental CD results could provide the good agreement with estimated CD results using mid-range flare model. It has become clear that the influence of local pattern density increases with design rule shrinkage. Furthermore, the methodology revealed that the influence of each error source is greatly dependent on local pattern density. Consequently, the methodology is effective to separate CD variation on local pattern density into error sources.
Mid-range flare (MRF) of an ArF exposure tool induces resist critical dimension (CD) variation depending on local mask (or resist) pattern density. CD correction techniques such as mask CD modification are needed to compensate MRF-induced CD variation and obtain sufficient resist CD uniformity. For this purpose, MRF should be accurately characterized and distinguished from other factors of similar influence caused by photoresist, mask, and so on. We have investigated a method of measuring magnitude and affected range of MRF in an exposure tool easily. In this method, double exposure of L/S with large area and a small square "window" in a widely shielded area is executed on a positive photoresist layer. After bake and development, resist CDs of the L/S are measured from near the window to far from the window using scanning electron microscope. An overcoat layer is used to exclude the influence of acid evaporation from photoresist and re-sticking. L/S mask shows small CD variation because local pattern density is uniform, and so influence of mask error on MRF measurement is small. Influence of MRF is shown in the graph of distance from window edge versus CD. From the distance-ΔCD curve, point spread function (PSF) which represents the characteristics of MRF can be obtained. Comparison of experimentally obtained MRF-induced CD variation, which appears in the periphery of a memory cell area, and calculated CD variation using the PSF obtained in the exposure tool showed good agreement.
In the tri-level resist process, it is sometimes difficult to detect the alignment mark because of the anti-reflection performance of the organic thick anti-reflective (ARL). Laser ablation in running water was one of the most effective techniques for removing the organic thick ARL on the alignment mark. Generally, the ablation process produces many particles. The results of our experiment indicate that the particle distribution area greatly depends on the dome-shape bubble on the ablation area. The particle distribution area could be minimized by optimizing some ablation conditions according to the estimated size of the dome-shape bubble. By optimizing a shift of the narrow slit-laser-beam and its energy so as to keep the ablation/initial thickness ratio to less than 20%, fine ablation area could be obtained. This novel ablation technique is very useful for particle-free selective removal of the organic thick ARL film.
A development monitor system capable of highly accurate control of pattern width has been established. This system is composed of a unique monitor pattern on the process wafer, the 0th order diffraction light measuring unit, and the image analysis and process control unit. In the conventional development process in which no monitor system is employed, the CD variation in 200nm line width was about 15nm when +/- 5 percent dose error exist. However, using the new system, 1nm of CD variation was obtained. In this article, a high-sensitivity monitor pattern is proposed and its performance in controlling 200nm line and space patterns in the development process is reported.
Lithographic characteristics of dual-trench type alternating phase-shifting mask (PSM), whose shifters are made of perpendicular trenches with different depth alternately, are evaluated numerically and experimentally. The structure of dual-trench type PSM could reduce the difference of adjacent peak intensities created by topography on the mask. Exposure characteristics of the mask varied with depth of deep and shallow trenches, and depth of both trenches should be controlled so as to have the optimum value. Mainly, the difference in depth of deep and shallow trenches caused varying "effective phase" and depth of shallow trench caused varying "effective transmission". The depth of focus using the mask was sensitive to the effective phase difference controlled by adjusting etched depth difference between both trenches, and insensitive to depth of shallow portion. From analysis of mask process margin, respecting acceptable error of depth of both trenches, it was found that the effective transmission error caused reduction of acceptable depth error.
Topographical structures for a dual-trench type alternating phase-shifting mask whose shifters were made of perpendicular trenches with different depth alternately, have been successfully designed using direct Maxwell's equation solver. The structures could reduce the difference of the adjacent peak intensities of the grouped line image on the wafer due to light scattering effects at sidewalls of the trenches. Detailed design of the structures was performed in accordance with the concept of 'effective transmission' and 'effective phase error'. It was clear that the former could be controlled by shallow trench depth, and the latter, which was defined as the phase difference between 'effective phase difference' and 180 degrees, could be reduced by controlling the difference in depth between deep and shallow trenches. For 0.175micrometers lines and spaces, the optimum shallow and deep trench depths corresponded to approximately 270 degrees and 447 degrees in phase, respectively. After the optimization, the depth of focus obtained by exposure-defocus tree was about 0.9 times as large as that obtained for an ideal alternating PSM having rectangle-shaped distribution of complex transmission (Kirchhoff's assumption).
An aging process that makes SiNx single-layer halftone film stable for DUV (248 nm) exposure has been established. The light irradiation with a low pressure mercury lamp was used to age the SiNx halftone film from the tendency of the transmittance change caused by the DUV exposure. Taking account of the optical constants shift during aging process, a SiNx halftone film with transmittance T equals 9.3%, phase shift angle (theta) equals 178 degrees was obtained. At the SiNx film, no transmittance change was observed after 2800 J/cm2 DUV exposure. Using the mask, 0.2 micrometers hole patterns were obtained with above 1.0 micrometers depth of focus (DOF).