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Inverse Lithography Technology (ILT)

Finding ideal shapes to use on a photomask.


Inverse lithography technology (ILT) is a method of using non-Manhattan shapes on the photomask to produce a wafer more resilient to manufacturing variation. It relies on multi-bream mask writing, which writes masks at the same speed regardless of the complexity of shape. Another challenge is the compute power required to calculate the shapes.

Curvilinear ILT masks are one of several types of photomasks, which are critical components in the IC manufacturing supply chain. The photomask is a master template for a given IC design. After a mask is developed, it is shipped to the fab. The mask is placed in a lithography scanner. The scanner projects light through the mask, which patterns the images on a wafer.

Typical masks, including those based on optical and extreme ultraviolet (EUV) lithography, consist of tiny features that resemble rectangles, or so-called Manhattan shapes. In some cases, photomasks have simple curve or rectilinear shapes. At advanced nodes, though, chipmakers want more advanced masks.

That’s where curvilinear ILT masks fit in. ILT uses a mathematical formula to calculate the desired shapes on a photomask, which are free-form curvilinear types. ILT not only increases the process windows in chip manufacturing, but it also enables the best performance for a shape on the wafer. That’s important for small and complex features that are difficult to print.

ILT differs from another resolution enhancement technique, optical proximity correction (OPC). In OPC, you start with a simple pattern, and grate through pitch and feature sizes – a calibration structure of sorts, according to one expert. Then, you assess the printability on the wafer and create a model to generate artifacts that are placed in specific locations on the design pattern. This gives you the intended results on the wafer.

ILT can be applied to an entire mask, but is also used for hotspot repair, where a portion of the mask will require curvilinear shapes using ILT. The remainder of the mask would use traditional and simple shapes.


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Tech Talk: Inverse Lithography