Best depth of focus on 22-nm logic wafers with less shot count

Abstract

The contact layer for the 22 nm logic node faces many technological hurdles. Even using techniques such as multipleexposure patterning and 193 nm immersion, it will be difficult to achieve the depth of focus and CD uniformity required for 22 nm production. Such difficulties can be mitigated by recent advances in Inverse Lithography Technology (ILT). For example, circular main features combined with complex curvilinear assist features can provide superior CD uniformity with the required depth of focus, particularly for isolated contacts. However, such a solution can lead to long mask write times, because the curvilinear shapes necessitate a higher shot count induced by inefficient data fracturing, even without considering the circular main features. The current approach is to Manhattanize the curvilinear features resulting in a nearly equivalent image quality on the wafer; but a further reduction in mask write times could help lower costs. This paper describes a novel mask-writing method that uses a production e-beam mask writer to write main features as circles, with curvilinear assist features, while reducing shot count compared to traditional Manhattanized masks. As a result the new method makes manufacturing of ideal ILT-type masks feasible from a technical as well as from an economic standpoint. Resist-exposed SEM images are presented that validate the new method.

Optimization of mask shot count using MB-MDP and lithography simulation

“…MB-MDP as introduced in previous papers [10][11][12][13][14] combines shot count reduction by using overlapping shots and Mask Process Correction (MPC) in one step. Since MB-MDP simulates the resulting mask contour in a similar way that OPC does on the wafer level, MB-MDP enables the use of overlapping shots, dose modulation of individual e-beam shots and character projection.…”

Generalization of shot definition for variable shaped e-beam machines for write time reduction

“…Contact wafer targets with "curvy" assist features [3] are potentially well suited to the technique: exact matching of the assist feature shapes is not required and contact "main feature" shapes are faithfully represented by circles.…”

Inverse lithography technology: 30 years from concept to practical, full-chip reality

“…In 2010, D2S introduced the concept of using model-based mask data preparation (MB-MDP) that employs overlapping VSB shots to significantly reduce the curvilinear ILT mask shot count while improving dose margin. Fujimura et al 70,71 showed an ideal ILT mask pattern for 22-nm contact holes, with curvilinear patterns written with about the same number of shots as a simplified Manhattanized mask (Fig. 34).…”

“…Figure 38(a) shows an example contact array with curvilinear ILT for 193i producing desired curvilinear mask target shapes, then the VSB shots generated to produce it using overlapping shots. 70,71,119,120 In the figure, green lines show the wafer target, red lines show the wafer image simulated from mask images simulated from the VSB shots in a double simulation process. The VSB shots are shown as hatched blue rectangles.…”