Detailed circuit simulations have demonstrated that a classical two-pattern robust test for a path delay fault may not excite the worst case delay of the target path. We have developed a new definition of robust test that maintains the desirable properties of classical robust tests while incorporating two additional considerations, namely side-fan-in transitions and pre-initialization, which are shown to have a significant impact on the delay of the target path. The associated test generation problem was formulated as a constrained optimization problem, and an ATPG system developed to generate three-pattern robust tests that excite the worst case delay of the target path. The ATPG works on a gate level model that is augmented to capture the necessary switch level details. Experimental results show that the quality of robust delay tests varies dramatically and that the proposed high quality robust delay tests are needed for improving test quality.