In this paper the application of solid immersion lenses (SIL) in combination with Lock-in Thermography will be demonstrated for backside defect localization. The paper will give an introduction into Lock-in Thermography technique and presents a new developed easy-to-use holding system to adapt SIL for high resolution thermal imaging. It will be shown that defect localization can be applied from the backside of the chip up to a silicon thickness of 250µm using the same SIL. The relationship between the bulk silicon thickness and the resulting optical parameters was investigated.