A method for controlling a variable geometry turbine of a turbocharger to increase the temperature of the exhaust gas delivered to an after-treatment system. The invention provides a compound of the formula for use in such methods. The cutting area defines a datum plane, and a feed table with a shuttle vise is used to move the workpiece toward the cutting area. The etchant may be applied to chemically non-homogeneous layers such as shallow trench isolation fill oxide layers, or to layers having a non-homogeneous composition or density at different depths within the layers, such as spin-on-glass or spin-on-dielectric films.