A method for manufacturing a magnetic sensor that allows the sensor to be constructed with a very narrow track width and with smooth, well defined side walls. A tri-layer mask structure is deposited over a series of sensor layers. The tri-layer mask structure includes an under-layer, a Si containing hard mask deposited over the under-layer and a photoresist layer deposited over the Si containing hard mask. The photoresist layer is photolithographically patterned to define a photoresist mask. A first reactive ion etching is performed to transfer the image of the photoresist mask onto the Si containing hard mask. The first reactive ion etching is performed in a chemistry that includes CF4, CHF3, O2, and He. A second reactive ion etching is then performed in an oxygen chemistry to transfer the image of the Si containing hard mask onto the under-layer, and an ion milling is performed to define the sensor.