We developed an optimized substrate removal process for GaAs-based opto-electronic devices by establishing a reliable two-step process. In this process, the substrate is first thinned by mechanical lapping and then followed by selective high density plasma etching. The adopted inductively coupled plasma etching, having an optimized boron trichloride (BCl3)/sulfur hexafluoride (SF6)/argon composition, shows a nearly infinite etching selectivity for the GaAs substrate over the aluminum gallium arsenide (AlxGa1-xAs) etch-stop layer. The surface of the die is perfectly smooth and mirror-like after processing. In addition to its simplicity, the process is also highly reproducible and shows no damage to the underlying detector material.