The improvement of DOF for sub-100nm process by focus scan

Abstract

As the design rule of device shrinks down, it is difficult to enlarge the process window, especially DOF (Depth of Focus). It has shown good results in resolution issues with short wavelength, high NA aperture and several RET (Resolution Enhancement Technique) like special illuminator and mask techniques and so on. But it needs to be challenged for DOF process window in contact / via process having various pitch and pattern location. It is a key point in sub 100nm process development and product. It is demonstrated that focus scan method is effective for DOF improvement in contact and via layers. Focus Scan method is one of the focus drilling techniques; it is realized to tilt wafer stage so that the same point on the wafer field can be exposed in limited continual focus range using multiple focal planes through the slit of scanner tool. In this study, confirmation was inspected for simulation and wafer evaluation for focus scan effects in view of process feasibility. DOF increased over 50% with focus scan in contact mask process even though there are several issues to be solved and considered. Energy Latitude (EL) decreased a little by image contrast drop, but if we consider the process window for evolution of device, it is relatively enough for process. OPC or Bias tuning is needed for application in contact layer having various pitch and location, and overlay issues are needed to confirm for each illuminator. From these experiments, it is found that DOF margin can easily be enhanced using focus scan method. Also some fine tuning is required to adequately use this method on production devices.

90nm Contact Hole Study with KrF Process

“…Focus scan [11] and CDP [12] technology are well known and cost effective solutions. The DOF can be improved obviously but image contrast will be decreased.…”

The Focus Margin Gain and Limitation Realized by Focus Drilling in 28 nm Hole-Like Design Rules

“…When semiconductor technology node achieves to 28 nm, some function of scanner comes out contribution on process window increase. Focus drilling is the scanner implementation to increase the depth of focus (DoF) for contact-hole or via fabrication processes [1][2][3][4][5][6][7]. The basic principle is to smear out the maximum image contrast at best focus (BF) over a specific defocus range.…”