We have proposed a new resolution enhancement technology using attenuated mask with phase shifting aperture, named "Mask Enhancer", for random-logic contact hole pattern printing. In this study, we apply a new mask blank on Mask Enhancer in order to prevent the light intensity loss caused by the mask topography effect. We also perform to expose the new Mask Enhancer on the first ArF immersion scanner, ASML AT1150i. We demonstrate that the new Mask Enhancer can achieve 45nm-node contact hole printing with sufficient lithographic performance with combination of immersion lithography.
At the panel discussion of Photomask Japan 2004, we discussed about "Issues on mask technology for 65nm lithography with ArF". The summary of the PMJ2004 panel discussion is as follows: (1) 65nm node will be achieved with ArF immersion/dry lithography, (2) Attenuated PSM, Alternative PSM and Gate-Shrink will be used for 65nm device production., (3) there are no red brick walls for 65nm mask making, though there are many issues to be solved for 65nm mask fabrication; CD control, inspection, writer, repair, metrology and mask cost.
The message from the panel discussion of PMJ2004 is "The mask technology will be ready for 65nm device development and production at 2007" For the business success, chip makers, mask manufacturers, EDA tool and equipment suppliers should work together in order to reduce the mask cost and cycle time.
At the panel discussion of Photomask Japan 2003, we discussed about Mask cost and specification. The topics are (1) Mask price trend and its impact, (2) How to reduce the mask costs; solutions from a mask shop, mask writing tool and mask inspection tool 3) Partnering mask suppliers with mask users; reasonable mask specification and OPC strategies. The choice of DUV laser writer instead of e-beam writer is one solution for reduction of mask cost. The continuous improvement of e-beam writer and resist sensitivity for high throughput is another solution. The partnership between designer, EDA vender, mask maker and wafer lithographer becomes more important.
This paper describes the CD control of using bottom anti- reflective coating (BARC) processes on topographic substrates for DUV lithography. The CD variation using organic anti-reflective coating (ARC) and inorganic anti- reflective layer (ARL) is different because of their coverage characteristics on topography. We have compared the CD variation of different BARC processes on various step widths and heights. The CD range using highly planarizing ARC was smaller than that using another less planarizing local step is advantageous to CD control on topographic substrates. We analyzed the cause of large CD variation using ARL and found that the impact of the oblique reflection from a photoresist/ARL interface at steps.
In this paper, evaluation of lens coma aberration in projection lens of a stepper by various measurement methods is described. The measurement methods were based on asymmetry of the printed images caused by coma aberration. We used three measurement methods. They are conventionally used method and two new methods. The three methods are: (a) measuring the CD difference between both ends of line-and-space, (b) observing the side lobe patterns using an attenuated phase shifting mask (a-PSM), and (c) measuring the registration error using overlay patterns that have assist patterns. These measurement patterns are printed on Si wafers by a KrF stepper (NA equals 0.55) with various (sigma) . The dependence of the feature size and (sigma) on the influence of coma aberration was easily measured by these methods. As each method has merits and demerits, proper use of each method is necessary. Then the influence of coma aberration was evaluated by method (a). The influence of coma aberration had two components and they were due to primary coma aberration and de-centering coma aberration respectively. We estimated the influence of coma aberration by simulation and got good agrement with the experimental results. The measurement methods we demonstrated here are applicable for lens evaluation of steppers by the users.
Overlay errors of fine patterns using modified illumination are experimentally measured and analyzed. Overlaid `vernier' patterns which fine lines and holes are overlaid on standard line patterns are used to measure the overlay errors. The experimental results using a KrF excimer laser stepper with various illumination conditions show that the overlay errors depend on the illumination coherence factor (sigma) and the pattern feature. These overlay errors consist of magnification and offset and these errors depend on the pattern feature. The offset error between line pattern and hole pattern are considerably large for device fabrication. The magnification component originates from coma aberration of the projection lens. We will confirm and estimate the effect of coma aberration on the overlay error by optical simulation. The origin of the offset error is considered to be coma aberration due to lens element decentering. This `decentering' coma aberration causes the offset error and the effect of `decentering' coma aberration has the feature dependence.
While modified illumination with various illumination apertures is the promising method to get finer patterns, there are many problems in putting it into practical use. In particular, the different apertures give different effects on the lens distortion and they cause the pattern displacement. In this paper, we studied the image placement error of fine patterns under 1 micrometer and its pattern pitch dependence using various illumination apertures. The first layer of 0.8 micrometer lines and spaces is patterned using conventional aperture (NA equals 0.57, sigma equals 0.6) and the second layer is exposed on the etched patterns with various apertures such as conventional, annular and small sigma. The patterns of the second layer are 0.4 micrometer lines with various spaces. Pattern displacements of the second layer are measured within the lens field along the X axis. The results show that the displacements are affected by the illumination status. In addition, the displacement varies with the pattern pitch and the placements of the fine patterns are different from those of large overlay patterns such as box-in-box. These results are confirmed by the optical simulator. The simulated results correspond to the experimental results and they point out the third order coma aberration most effects the pattern displacement. The pattern size and pitch dependence of the image displacement reveals that the measurement result of the conventional overlay patterns such as box-in-box with the sizes of 10 - 20 micrometer doesn't represent the overlay of the fine patterns of actual devices.
The optimized design and fabrication of optical proximity correction (OPC) masks with serifs have been described for the application of mask ROM programming layer, which has 1 micrometers square patterns with 0.6 micrometers separation on a g-line stepper with 0.45 numerical aperture (NA). We have optimized the trade-off among the optical correction effects, practical mask fabrication problems, and inspection problems. Firstly, to obtain the sufficient correction effect on the topographic substrate, we have executed not only the simulation and experiments on the flat substrate, but also the experiments on the topographic substrate. Secondly, from the practical mask fabrication considerations we fixed the rules that the size of serifs must be larger than 1.0 micrometers square and the minimum separation width of mask patterns must be larger than 1.5 micrometers on 5X reticle. Thirdly, to maximize the detection capability of mask defects and to minimize the detection of false defects, we have fitted the fabricated mask patterns to the designed data by optimizing the electron dose.
An application of a mask with serif patterns to a 0.8 micrometers rule mask ROM programming layer is discussed. A serif pattern is the unprinting size pattern added to the corner of original pattern. It suppresses the corner rounding caused by the lack of resolution performance of a lithography exposure system. We aimed to make 1.0 micrometers square pattern with g-line (436 nm). There are some difficulties in application of optical proximity correction (OPC) mask. One of the problems is the optimization of the mask design to be applied to get the appropriate correction effect. We evaluated it by simulation and experiment. The second is the increase in EB data volume. We split mask data into periodic common data with serifs for cell pattern and random ROM code data for programming to compact the data volume. The other is the printability and inspection of mask defects. The OPC mask shows the high printability of defects because of the violation of the mask design. To detect all the printable defects, mask inspection needs high sensitivity. In the inspection with high sensitivity, the extrusion of pattern caused by EB proximity effect becomes to be detected as false defect. To reduce the false defects, we optimized the EB exposure process. In order to use the OPC masks in actual production, mask design for application should be optimized not only in the viewpoint of pattern correction effect but also in the viewpoint of mask fabrication and inspection.
We have investigated the application of phase-shifting mask (PSM) or DRAM cell capacitor fabrication. Narrow spaces formed by PSM enables us to attain a high DRAM cell capacitance. Four types of phase-shifting masks are tested: a rim-type mask, transparent type mask (I) (or `shifter-shutter' type), Levenson type mask and transparent type mask (II) (or `shifter-edge' type). The improvement in resolution and depth-of-focus (DOF) by the former two types of PSMs is small, although we can continue to use conventional positive resist with these PSMs. The latter two PSMs provide a significant increase in resolution and DOF. Since the transparent type (II) mask has the difficulty in mask fabrication, we select a Levenson type PSM for the fabrication of the DRAM cell capacitor. Though 0.28 micrometers spaces with the DOF of 1.5 micrometers can be formed on a bare silicon substrate by the Levenson type PSM, the printed patterns on an actual device substrate are deformed by the reflection from the substrate. Dyed negative resist is used to reduce the effects of the reflection, the patterns of 0.28 micrometers spaces with 0.6 micrometers DOF on the actual substrate can be successfully printed. We confirm the effectiveness of the Levenson type phase-shifting mask combined with a dyed negative resist for the DRAM cell capacitor fabrication.
This paper introduces the novel concepts of 'multistage phase shifter' and 'comb-shaped shifter' for resolving the problems of a transparent type phase shifting mask. The use of a multistage shifter decreases the light intensity dip at the shifter edges. The use of the comb- shaped shifter enables control of the pattern width. The effectiveness of a multistage shifter and a comb-shaped shifter were demonstrated by experiments and simulations. These technologies make it possible to fabricate a wide range of patterns for VLSI using the transparent phase shifting mask.
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