Hybrid lithography is developed to combine high lithography resolution with high throughput of optical lithography in order to decrease wafer exposure time. For that, we used Chemically Amplified Resists (CAR) which have the advantage of being fully compatible with e-beam and optical lithography with high sensitivity and resolution.
For this technique, the main difficulty is to develop one process compatible with two types of exposure. Process should be optimized to get the best e-beam resolution and prevent standing waves generated by optical exposures. Then, we have used a new Sumitomo negative CAR, called NEB35, which allows us to get resolution down to 30-nm. E-beam exposures were achieved on a gaussian electron beam lithography equipment (Leica VB6UHR from Vistec) operating at 100 KV and DUV exposures on an ASML stepper with a wavelength of 248 nm.
For this study, amorphous carbon hard mask has been used on our structure. Carbon layer offers many options for high resolution lithography because of the fact that very thin resist films (below 100nm) may be exposed. In addition, it can also be removed very easily by O2 plasma. Therefore, carbon hard mask offers new possibilities for improving e-beam resolution. Thanks to a lower resist thickness budget to transfer into the underneath layer, ultimate resolution as well as the pattern collapse for dense lines should be improved. Finally, after plasma etching, we have succeeded in achieving 30-nm MOS transistor gates and improving gate resolution down to 20-nm by hard mask trimming.
At the nanometer scale, small variations on transistors dimensions can strongly influence their electrical properties. Therefore, full 3D characterizations have also been carried out by using 3D Atomic Force Microscope (CD-AFM from VEECO) to determine the best process window. In contrast with SEM experiments, this non destructive technique allows us to have access to various parameters such as height, side-wall angles, Critical Dimension (CD), line edge roughness and line width roughness. |