Curvilinear Masks: Motivations and Metrology (Invited Paper)

The photomask industry is experiencing a fundamental shift from Manhattan masks to curvilinear masks. Recent lithography and mask technology conferences have featured many papers and talks on curvilinear masks, curvilinear OPC, curvilinear ILT, curvilinear mask process correction (MPC), and curvilinear mask formats. Step by step, the photomask industry has started a transition from Manhattan to curvilinear, enabled by the adoption of the new multi-beam mask writers and the advent of practical full-chip curvilinear inverse lithography technology (ILT). The benefits of curvilinear masks go much deeper than is immediately obvious. In this paper we will share our insight on the motivations for the mask world to move toward curvilinear mask shapes. We will evaluate the benefits of curvilinear mask shapes in terms of process window, mask rules, mask error enhancement factor (MEEF), and mask variation. In addition, we will propose a new specification for curvilinear mask metrology, as an equivalent to critical dimension (CD), and we will also demonstrate that curvilinear masks have a tighter mask process variation (PV) band than Manhattan masks using this proposed specification.