passes

Software:
Maya 2018 | Arnold 5

1. In the Render Settings > Common tab, choose EXR as the output file format.
2. In the Render Settings > AOVs tab, open the drop-down menu on the right side of one of the AOVs in the list, and choose Select Driver.
   This will bring up the defaultArnoldDriver node Attributes.
3. In the defaultArnoldDriver node Attributes, under Advanced Output, check Merge AOVs.