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.
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.
Rigorous computer simulations of propagating electromagnetic fields have become an important tool for optical
metrology and optics design of nanostructured components. As has been shown in previous benchmarks some of
the presently used methods suffer from low convergence rates and/or low accuracy of the results and exhibit very
long computation times1, 2 which makes application to extended 2D layout patterns impractical. We address 3D
simulation tasks by using a finite-element solver which has been shown to be superior to competing methods by
several orders of magnitude in accuracy and computational time for typical microlithography simulations.2 We
report on the current status of the solver, incorporating higher order edge elements, adaptive refinement methods,
and fast solution algorithms. Further, we investigate the performance of the solver in the 3D simulation project
of light diffraction off an alternating phase-shift contact-hole mask.
Experiments and full resist simulations of contact patterns using both infinitely thin masks (2D) and 3-dimensional mask topography (3D) were performed to examine the quality of prediction by simulation. Experimental data were acquired by CD-SEM measurements of contact patterns in resist which were generated using a 193 nm scanner with a numerical aperture of 0.75, circular illumination (σ=0.5), and an attenuated phase shifting mask with 6% transmission. Analysis of the data is performed in terms of dose to size, process window, mask error enhancement factor (MEEF), and printed critical dimension (CD) in resist. Furthermore, an error analysis is performed with respect to mask CD, illumination source, dose and focus error. For the same contact size in resist a parabola like dependence of the mask contact length on contact width was found by experiment and simulation. Fair agreement between 2D and 3D simulation was obtained above 180 nm mask CD whereas a strong difference was observed below this region. Especially the location of the minimum at around 140 nm mask CD can be reasonably described only by 3D simulation. Thus, the prediction of accurate mask biases and process windows in the lower mask CD region is only possible by 3D simulation. Simple corrections of the 3D effect like the consideration of a mask CD offset or dose offset fail. Apart from that, 2D simulation in conjunction with a well calibrated resist model is sufficient for delivering reliable predictions for process window, MEEF, and CD.
In times of continuing aggressive shrinking of chip layouts a thorough understanding of the pattern transfer process from layout to silicon is indispensable. We analyzed the most prominent effects limiting the control of this process for a contact layer like process, printing 140nm features of variable length and different proximity using 248nm lithography. Deviations of the photo mask from the ideal layout, in particular mask off-target and corner rounding have been identified as clearly contributing to the printing behavior. In the next step, these deviations from ideal behavior have been incorporated into the optical proximity correction (OPC) modeling process. The degree of accuracy for describing experimental data by simulation, using an OPC model modified in that manner could be increased significantly. Further improvement in modeling the optical imaging process could be accomplished by taking into account lens aberrations of the exposure tool. This suggests a high potential to improve OPC by considering the effects mentioned, delivering a significant contribution to extending the application of OPC techniques beyond current limits.