A Cost and Yield Analysis of Wafer-to-wafer Bonding

Abstract

When a product requires the bonding of two die or wafers, there are a number of methods that may be used. Not only does the type of bonding process itself have to be selected, but it must also be determined whether the items being bonded will be in wafer or die form. This paper will focus on wafer-to-wafer bonding, which has the highest throughput compared to die-to-wafer and die-to-die bonding; it also has the potential to be the lowest cost option if proper yields are achieved. This paper will introduce the background and general pros and cons of wafer-to-wafer, die-to-wafer, and die-to-die bonding. Activity based cost modeling will be used to construct a generic flow of a wafer-to-wafer bonding process. The process flow will be divided into a series of activities, and the total cost of each activity will be identified. The cost of each activity will be determined by analyzing the following attributes: time required, amount of labor required, cost of material required (consumable and permanent), tooling cost, depreciation cost of the equipment, and yield loss associated with the activity. The model will be used to explore multiple variables that affect the total cost of the wafer-to-wafer bonding process, including: incoming wafer cost, incoming wafer defect density, time required for the dicing process, time required for the bonding process, cost of the equipment for the bonding process, and the yield of the bonding process. First, a sensitivity analysis will be conducted to determine the impact each variable has on the total cost. Then scenarios will be created to conduct trade-offs between multiple variables. Only one, generic wafer-to-wafer bonding model will be created, but there will be enough variables to accurately reflect different bonding methods in use by the industry today. Methods for bonding two wafers together will also be discussed in the paper, as well as the cost and yield issues associated with each. An example of these methods are thermo compression bonding and direct bonding. The goal of this analysis will be to understand the cost and yield drivers associated with wafer-to-wafer bonding, and to determine scenarios in which wafer-to-wafer bonding is a suitable, cost effective technology selection.