Life-Cycle Assessment of Semiconductors

Abstract

This thesis is the first complete and transparent study of the life-cycle environmental impacts of semiconductor chips using process-level data, as well as the first analysis of the changes in these impacts over time. LCA of complementary metal oxide semiconductor (CMOS) logic, flash memory and dynamic random access memory (DRAM) are presented. CMOS logic is the most common form of digital logic used today. This thesis provides a life cycle assessment for CMOS chips over 7 technology generations with the purpose of comparing impacts by life cycle stage, examining their trends over time and evaluating their sensitivity to data uncertainty and changes in production metrics such as yield. A hybrid life cycle assessment (LCA) model is used; Wafer production, electricity generation, water supply and certain materials are represented by process LCA data, while the remaining materials are described using economic input-output (EIO) LCA methods. It is determined that, in the case of CMOS logic, life-cycle impacts in all but one category are dominated by the use phase, and impacts are most sensitive to those variables which define use phase energy demand (chip power demand, usage patterns, power supply efficiency and chip lifetime). Using the same methodology, LCA of flash memory over 5 technology generations is presented. The most recent generation of flash memory is compared with magnetic storage, using a laptop hard-drive as a functional unit, and it is determined that in most impact categories, a flash-based drive will result in fewer impacts. Life-cycle impacts of DRAM, over 6 technology generations, are presented using as the functional unit, the memory requirements a popular operating system in each year of production. The influence of the choice of functional unit on results in semiconductor LCA is examined, and it is argued that functional unit is best defined as a computational power or memory capacity required for a given function, within a set time period. The LCA impact and inventory results for these three types of semiconductor products allow more accurate assessment of the environmental benefit or costs of information technology relative to traditional products and services.