The present disclosure provides methods and apparatus for sensor element isolation in a backside illuminated image sensor. In one embodiment, a method of fabricating a semiconductor device includes providing a sensor layer having a frontside surface and a backside surface, forming a plurality of frontside trenches in the frontside surface of the sensor layer, and implanting oxygen into the sensor layer through the plurality of frontside trenches. The method further includes annealing the implanted oxygen to form a plurality of first silicon oxide blocks in the sensor layer, wherein each first silicon oxide block is disposed substantially adjacent a respective frontside trench to form an isolation feature. A semiconductor device fabricated by such a method is also disclosed.