Apple is hardly the first to bring a fingerprint sensor to a smartphone — Motorola Mobility baked one such sensor into the back of its 2011 flagship the Motorola Atrix, which allowed users to swipe their fingers across it to unlock the device. The crucial issue though was that the sensor itself seemed awfully finicky and wouldn’t always correctly accept a user’s finger inputs. That little stumbling block ultimately meant that the fingerprint sensor wound up being more hassle than it was worth in most cases, and I’m sure that’s a problem that Apple had to tackle head-on when developing Touch ID.
So how exactly has Touch ID been implemented? The home button is now made of sapphire glass to reduce the potential for damaging one of the 5S’ most notable features, and it’s bounded by a steel “detection ring†that determines when your finger is on the home button and fires up the Touch ID sensor. Touch ID is also capable of keeping tabs of multiple different fingerprints, so you (hopefully) won’t have to worry about your friends buying embarrassing songs with your iTunes account with a single touch.
http://techcrunch.com/2013/09/10/ap...rint-sensor-built-into-iphone-5s-home-button/