Characterization and Optimization of a Lithography Stepper

Summary: Over the last decade the importance of the litography has pushed the study of the physical and chemical mechanismn of the litography process. Nevertheless, litographic modelin played a dramatic role in gving a comprehension of the experimental outcomes rapidyl and straightforwrdly. Therefore, in order to characterize and optimize the application of a new Ultratech "Sapphire 100E" wafer stepper at Fraunhofer IISB, we combined litographic simulation and exposure experiments.

To achieve our aim several aspects were tackled on the simulation and experiment level. This work showed the elements of an optical lithography tool to get a better understanding of each element task and utility in the i-line wafer stepper. Furthermore, the chapter introduced the DNQ-based positive photoresit QZ ECI3007, and explained the optical lithography process steps: wafer preparation, spin coting, soft-bake, exposure, post exposure and development.

the aerial image in optical projection litography was familiarized and simulated aerial images of the mask pattern with different features size of the dark and bright mask field were predented. The images showed that the stepper can print with good resolution, though the optical proximity effects, down to 0.6 μm.

The process windows of the features for different sizes of lines and spaces were plotted and the focus and threshold latitudes were extracted. The Focus latitudes are relatively huge even for the smaller features of 0.6 μm. The process linearity curve was presented as well and it showed that the stepper can print resolvable features szie only down to 0.8  μm.

Next, the litographic modeling was introduced for the bulk image, exposrue, post-exposure bake, development model and the full resist model simulation. Additionally the genetic calibration algorithm was announced along with the optimization problem setting.

The choice of experimental exposure dose and development time in practices was diccussed. Furthermore the influence of PEB on the resit profile was shown for the three cases: no PEB applied, PEB applied at a temperature of 90°C for 60sec, and PEB applied at a temperature of 110°C for 60sec. The profiles showed that the PEB at very higher temperature is able to suppress standing waves effects and results in vertical sidewalls.

The experimental sets of feature CDs data were introduced with different conditions:

  • Exposure dose of 115 mJ/cm2, no PEB applied.
  • Dose matrix from 60 mJ/cm2 220 mJ/cm2. PEB applied.
  • Focus matrix from -7.0 μm to 7.0  μm, PEB applied.

 

The differences that appeard between the data sets were discussed.

The last part of this work showed the two calibrated models one for the case of no PEB application and another for a PEB application. The simulated resist models results were compared with the experimental data and the aerial image extracted data. The first model showed fitting data with the experimental data. As for the second model it exhibits a slight difference due to SEM measurements erros.

The aerial image comparison data with the resist model simulated data shows typical agreement at best dose and focus; yet a difference start appearing when looking at CDs away from these best conditions.

The process windows based on rsist model as well and we have registered a drop in the values of the focus latitudes.

The lithographic modeling provided the opportunity to investiage and optimize the Ultratech "Sapphire 100E" wafer stepper under a variety of lithographic conditions with accurate results.