A printhead comprising a number of etched layers includes a thermal ink ejection actuator assembly formed in a first layer. The actuator assembly has a number of heater arms that operate an ink ejection member. A drive transistor to provide current to the heater arms is located in a position corresponding to the heater arms but in a second layer displaced from the first layer. Planarization may be applied to a surface of the second layer to prevent the actuator being formed with corrugations. However, the introduction of a planarization step is undesirable from a manufacturing perspective. Consequently, in a preferred embodiment the heater arms are formed with corrugations extending in a direction transverse to a bending axis of the actuator assembly. This results in an actuator that is only 2% less efficient than would be the case for a flat actuator formed with the planarization step.