Total dose effects and hardening-by-design methodologies for implantable medical devices

Implantable medical device technology is commonly used by doctors for disease management, aiding to improve patient quality of life. However, it is possible for these devices to be exposed to ionizing radiation during various medical therapeutic and diagnostic activities while implanted. This commands that these devices remain fully operational during, and long after, radiation exposure. Many implantable medical devices employ standard commercial complementary metal-oxide-semiconductor (CMOS) processes for integrated circuit (IC) development, which have been shown to degrade with radiation exposure. This necessitates that device manufacturers study the effects of ionizing radiation on their products, and work to mitigate those effects to maintain a high standard of reliability. Mitigation can be completed through targeted radiation hardening by design (RHBD) techniques as not to infringe on the device operational specifications. This thesis details a complete radiation analysis methodology that can be implemented to examine the effects of ionizing radiation on an IC as part of RHBD efforts. The methodology is put into practice to determine the failure mechanism in a charge pump circuit, common in many of today's implantable pacemaker designs, as a case study. Charge pump irradiation data shows a reduction of circuit output voltage with applied dose. Through testing of individual test devices, the response is identified as parasitic inter-device leakage caused by trapped oxide charge buildup in the isolation oxides. A library of compact models is generated to represent isolation oxide parasitics based on test structure data along with 2-Dimensional structure simulation results. The original charge pump schematic is then back-annotated with transistors representative of the parasitic. Inclusion of the parasitic devices in schematic allows for simulation of the entire circuit, accounting for possible parasitic devices activated by radiation exposure. By selecting a compact model for the parasitics generated at a specific dose, the compete circuit response is then simulated at the defined dose. The reduction of circuit output voltage with dose is then re-created in a radiation-enabled simulation validating the analysis methodology.