Process window is the range of depth of focus (DoF) and exposure latitude within which the intended design is correctly transferred to the wafer within tolerance. The largest possible process window – which contributes to robust wafer yields – for every shape of the design is the goal for the shapes printed on a photomask. variation. D2S solutions provide better process windows by increasing depth-of-focus and exposure latitude, reducing MEEF/eMEEF, and improving mask uniformity.
It has been demonstrated that curvilinear masks produced by inverse lithography technology (ILT) create wafers with 2x larger process windows. It’s also important to remember that while the mask shop will do its best to make a physical mask that is as close to the ILT output as possible, with both local and global uniformity across the mask, what the mask shop can do is limited to the shape it is asked to produce.
It is the critical role of a good ILT solution to output shapes that meet the wafer edge-placement error (EPE) and critical dimension (CD) specifications, with good process window, and with mask shapes that are manufacturable on mask and resilient to mask process variation. Every variation on mask is a systematic variation on every exposure on every wafer. Improving mask uniformity is one of the most important ways to improve wafer uniformity and thus process window.
This is why mask error enhancement factor (MEEF), and for more modern masks, extended MEEF (eMEEF), is also an important metric. MEEF measures how mask errors are magnified when transferred to the wafer. eMEEF is an advanced measure that is used for the complex features (whether Manhattan or curvilinear) produced by ILT. The ILT technology must carefully choose shapes that are less susceptible to a given nanometer change as indicated by MEEF, or a given % dose change as indicated by eMEEF.
The D2S ILT solution, TrueMask® ILT, creates Entirely Manufacturable™, therefore entirely curvilinear, masks with proven, better process window, with good EPE, CDU, and MEEF/eMEEF.
