Measurement of accurate optical parameters is now possible using Litel's technology, and importantly can improve Optical Proximity Correction models.

Optical Proximity Correction (OPC) is the technique of pre-distorting mask layouts so that the printed patterns are as close to the desired shapes as possible. For (model-based) OPC, a Lithographic Model to predict the edge position (contour) of patterns on the wafer after lithographic processing is needed.

Deep sub wavelength design for manufacturing (DFM) requirements have forced the design and manufacturing communities into very tight collaboration. With the help of new EDA tools and flows, designers are starting to embed design information in manufacturing data, permitting downstream tools to optimize analysis on structures that are critical to the design. However, new design tools are emerging that allow designers to apply more proactive methods to ensure compliance with downstream lithographic requirements. Sophisticated design rule sets that encode lithographic constraints are allowing these tools to identify and correct problematic structures well before tape-out, also known as a Lithography-aware design.

All are based on the assumption that a predictive or calibrated lithography model is accurate enough to cover all the design variations. Months are sometimes required to perform fine-tuning and validation of that one model. In many cases, the calibrated model is only accurate enough for one layer and focus condition.

The end effect is that all design data must eventually pass through some sort of RET treatment and lithography simulation using one or more models.

The Lithography Process and Model Calibration

The limitation of what can be printed is a function of not just the scanner wavelength, but also many physical effects that come into play at the sub-resolution nodes. With advanced simulation tools, a scanner and resist process can be optimized. For example, a variety of measured illuminator profiles, intensity and transmission profiles, NA, Partial Coherence and Lens Aberration, in addition to one or more calibrated models, can be used in a variety of simulation tools downstream, such as Litel’s MPACE. This gives other engineers in the design chain the opportunity to test other conditions that may fall outside the space of the calibrated models provided.

The lithography-aware design methodology is an integrated approach of having lithography simulation capability during the early design stages. With the current trust in the expanded models used in commercial OPC software, the assumptions behind the model now warrant an investigation.

For example in this case, Quadrupole Illumination has been used in the exposing scanner, and the nominal ideal map of a quadrupole illumination source is as shown in figure 1a below. However, this clean illumination pattern (figure 1a) is not what emerges from any real illumination system. Replacing the illumination pattern in the simulator with the actual pattern (figure 1b) of the real illumination system produces simulation results that agree very well with the measured values — with no change in the underlying optical simulation models themselves. In this case, the simulation engine itself was accurate, but the performance was only as good as the input to the model.

1. These maps show an idealized illumination source map for Quadrupole illumination (a); and actual measurement of the illumination source (b).

Measurement of accurate optical parameters is now possible using Litel's technology, and importantly can improve RET / OPC and Lithographic Modeling minimizing the number of iterations necessary to perform fine-tuning and validation of the RET and OPC.