Christoph Noelscher

Assessment and application of focus drilling for DRAM contact hole fabrication

By assessment of options for the fabrication of small contact holes in DRAM devices the method of focus drilling was identified and investigated to overcome the depth of focus limitations. By use of ArF-dry lithography a practical shrink of the target CD by 15nm can be achieved both with a focus offset double exposure (FODEX) and with a tilted stage approach. This was optimized in simulation and demonstrated by CD measurement on wafer, as well as by electrical measurement on integrated lots. Application of dual lambda focus drilling is limited by the chromatic magnification error of the lens. The increase of hole-to-hole CD variations due to a lower dose latitude and to increased MEEF was characterized. As improvement option the use of a high transmission attPSM was identified.

AltPSM contact hole application at DRAM 4xnm nodes with dry 193nm lithography

To avoid expensive immersion lithography and to further use existing dry tools for critical contact layer lithography at 4Xnm DRAM nodes the application of altPSM is investigated and compared to attPSM. Simulations and experiments with several test masks showed that by use of altPSM with suitable 0°/180° coloring and assist placement 30nm smaller contacts can be resolved through pitch with sufficient process windows (PW). This holds for arrays of contacts with variable lengths through short and long side pitches. A further benefit is the lower mask error enhancement factor (MEEF). Nevertheless 3D mask errors (ME) consume benefits in the PW and the assist placement and coloring of the main features (MF) put some constraints on the chip design. An altPSM compatible 4Xnm full-chip layout was realized without loss of chip area. Mask making showed very convincing results with respect to CDU, etch depth uniformity and defectiveness. The printed intra-field CD uniformity was comparable to attPSM despite the smaller target CDs. Room for improvement is identified in OPC accuracy and in automatic assist placement and sizing.