In wire bonding, the thickness of the aluminum pad under the bonds needs to be measured for bonding process optimization purposes. In Cu wire bonding, the deformation of the Al bond pad is larger than in Au wire bonding. Because of this, the uniformity and thickness of Al that remains under the ball is often used as a metric in the process optimization. The thickness of the Al pads can be inspected with manual cross sectioning or FIB (Focused Ion Beam). However, these two methods are time consuming and expensive. This paper presents a method that helps to quickly measure the aluminum pad, which involves creating a graph of penetration of electron beam at different EDS (Energy Dispersive Spectroscopy) voltages versus Al thickness.
The method basically consists of: a) crafting Al layers of different thicknesses, b) obtaining EDS for each layer at different EDS voltages, c) performing cross sectioning of those layers and measuring the thickness of the layers, d) generating the metric based on the acquired data, and e) testing the acquired metric.
Thermo-mechanical description of material diffusion during an ultrasonic wire bonding process
