Computer disks (large and small) have internal drives that store data. Key components in the drives are the “heads” that actually read/write the data. Heads are manufactured in clean rooms by a series of steps that deposit layers of materials on a wafer, and pattern those layers using photolithography and etching. During the processing the wafers are inspected and, when processing is finished, the devices are tested. As a result of the manufacturing process, a head could have 2 active elements that are:
- both good (1,1) and can be used anywhere as a read-write head
- both bad (0,0) and thus can not be used
- mixed (0,1) or (1,0) and can only be used in certain positions.
Wafers are typically manufactured to have multiple rows and columns of heads. For this project we will assume a wafer has 12 rows and 10 columns. Because each head has 2 active elements, a tracking matrix could look like this (12 rows, 20 columns):
At each manufacturing station in the production process, after a new layer of material has been deposited or after an etching or assembly step, automatic or human testers inspect the heads to see if the operation was successful. Some stations do 100% inspection (all heads / all elements), while some stations do sample or boundary testing.* The results are stored in a Parts Tracking computer that is networked to all of the testing stations. At the end of the wafer manufacturing process, the wafer is “diced” – cut into rectangles, essentially in one operation – so each head is separated. A final machine sorts the heads into 4 containers for use in the disk assembly process. The disposition of the heads is either: good (1,1), bad (0,0), ID (inside defect: 0,1) or OD (outside defect: 1,0).
* Usually, in a real process, the process parameters such as thickness of the layer, the size of the pattern, the alignment with the previous layer, the reduction in size on etching… are monitored. The 100% testing of completed elements is usually done at the end.