A novel design of grating projecting system for 3D reconstruction of wafer bumps

Abstract

A challenge in the semiconductor industry is the 3D inspection of solder bumps grown on wafers for direct die-to-die bonding. In an earlier work we proposed a mechanism for reconstructing wafer bump surface in 3D, which is based upon projecting a binary grating to the surface with an inclined angle. For the purpose of 3D reconstruction with high speed and accuracy, the requirements for the projection lens system are the followings: (1) having a tilted angle between the projection plane and the optical axis; (2) having high bandwidth to let high-spatial-frequency harmonics contained in the binary grating pass through the lens and be projected onto the inspected surface properly; (3) having high Modulation Transfer Function (MTF); (4) having large Field of View (FOV); and (5) having a large Depth of Field (DOF) that corresponds to the depth range or height of the inspected surface. The above requirements lead to great challenges in the design of the projection lens system. In this paper, we describe a design consisting of a grating and several pieces of spherical lens, that addresses the requirements. To reduce the lens aberrations, the grating is laid out with a tilting angle specifically to make the grating, the lens, and the image plane intersect at the same line. Such a system can project a high spatial-frequency binary grating onto the inspected surface properly. Simulation results, including performance analysis and tolerance analysis, are shown to demonstrate the feasibility of the design.