As 90 nm devices enter into the pre-production phase, the quality assurance strategy of photomasks for those devices must be well established with the proper cost and turn-around-time in mind. Such devices will be manufactured with a state-of-the-art photolithography systems equipped with 193nm actinic light sources. Photomasks for these devices are being produced with the most advanced equipment, material and processing technologies and yet, quality assurance still remains an issue for volume production. These issues include defect classification and disposition due to the insufficient resolution of the defect inspection system, uncertainty of the impact the defects have on the printed feature as well as inconsistencies of classical defect specifications as applied in the sub-wavelength era. To overcome these issues, the authors propose a new strategy to assess photomask quality by checking the CD variation on wafer (defect printability) using aerial image simulation. This method of simulation-based mask qualification uses aerial image defect simulation in combination with a high resolution optical review system with shorter review wavelength (248nm) and smaller pixel size (22.5nm)- combining the defect inspection system with a longer inspection wavelength (365nm) and larger pixel size (150nm). This paper discusses a new strategy on mask quality assurance with several experimental results that proves the applicability for enabling 90nm technology nodes. Combining high-resolution optical images captured by DUV measurement tool with Virtual Stepper System has achieved better accuracy for 0.72um contact holes on ArF Att.PSM. However, we need further investigation for precise prediction of CD variation caused by defects on 0.4um line/space patterns on ArF Att.PSM. This paper also discusses future work to make the strategy production-worthy.
The influence of edge taper angle and edge roughness on transferred wafer image was investigated by computer simulation for sub-half-micron space and hole patterns on the wafer. The exposure latitude for a 0.3- micrometers space pattern on the wafer is almost unchanged for angles exceeding 60 degrees. The exposure latitude for 0.3-, 0.35-, and 0.4- micrometers hole patterns remains almost unchanged unless the edge roughness exceeds 0.04 micrometers . However, when shifter thickness on the pattern edge decreases 50%, a particularly bad case, and edge roughness of 0.01 micrometers results in 25% degradation of exposure latitude. Taking simulation results into consideration, we optimized the mask manufacturing process using wet etching for CrO-based phase shifters and obtained an edge roughness of approximately 0.01 micrometers and an edge taper angle greater than 60 degrees. Experiment showed that wet-etched mask performance is equivalent to that of the dry-etched mask.
We studied phase shifter defect repair techniques which use conventional mask repair tools. This paper describes two repair techniques applicable to phase shifter residue defects on the etch-stop layer of a quartz substrate. These new techniques, which use a conventional Nd:YAG laser and a Focused Ion Beam (FIB) are beam-on-edge and absorber implantation. Beam-on-edge repair removes phase shifter pin-dots by optimally applying an Nd:YAG laser under conventional mask repair conditions. Absorber implantation repair optimizes phase shifter form combining FIB and Nd:YAG laser tools.
An unexpected resist pattern due to the phase transition at the shifter edge is one of the problems for applying phase shifting masks. We have developed a novel subtractive process with a wet-etching technique. By using a controlled conventional resist process with some modifications, a gradually sloped shifter edge over 2 micrometers wide was produced when the quartz substrate was etched to 380 nm for a 180 degree(s) phase shift. By this etching process, the quartz covered with the chrome becomes only a 380 nm undercut and this chrome edge was durable during the physical cleaning process. This fabricated mask could easily provide 0.3 micrometers L/S patterns on the wafer with 160 mJ/cm2 on i-line stepper and the unexpected resist patterns totally disappeared at 110 mJ/cm2 and higher doses.
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