Fault localisation of ICs by lock-in fluorescent microthermal imaging (Lock-in FMI)

Abstract

An improved method for high resolution surface temperature aquisition using fluorescent microthermal imaging (FMI) is introduced. In combination with lock-inthermographie it can be applied for localisation of leakage currents in microelectronic devices with a precision accuracy of about 1µm. As an example hot spot detection of an IC failed with a raised current consumption is demonstrated. The physical failure at the detected hot spot position is verified by TEM cross-section analysis. For quality management and process optimisation the localisation of faulty structures in high-density integrated circuits is an essential assignment. Due to the highly complex circuit structure, defects can not be detected electrically. Defects which cause leakage currents, e.g. gate oxide breakdowns at transistor structures or metallisation shorts, can be detected due to their local heat generation. Up to now this was done essentially by using liquid crystal thermography [1]. A method that is orders of magnitude more sensitive is the IR thermography in lock-in mode (LIT) [2]. An essential drawback of both methods is the limited spatial resolution of about 5 µm, which is often not enough for precise defect localisation in highest density ICs. An additional thermally detection method is the standard (steady-state) FMI [3] which can be reach high spatial resolutions of less than 1 µm. However the method has two major disadvantages. At first the sensitivity limit for emitted thermal power is only about 10 mW, which limits the usage to less integrated ICs and power components. Furthermore the images show a strong topographical contrast, which is caused by surface structure effects of the fluorochrome coated sample and mostly dominates over the contrast of the emitted fluorescence light (thermal contrast). Here a modified FMI technique will be introduced, which is based on lock-in method using a specially developed hardware. It will be shown that this technique essentially lowers the minimum of the detectable heat generation and eliminates the topographical contrast from the taken images. In combination with the lock-in thermography the detection sensitivity and spatial resolution is significantly improved.