The IC Ultra-Thin Back Surface - A Field of Real Nanoscale Fault Isolation Opportunities Requiring a Skillful Sample Preparation

The backside approach of contactless fault isolation (CFI) was comfortable as long as it could be carried out with Near Infra-Red (NIR) optical techniques. But even with a solid immersion lens (SIL), the resolution was limited to ~180nm, corresponding to ca. 40nm node integrated circuit (IC) technologies. However, with failure analysis (FA) experience and circuit simulation, it was still successful down to 14 nm FinFET technology. There are several attempts to keep optical CFI competitive because the FA community has enormous experience to read and interpret the obtained signals. Two major strategies are out to save optical CFI for smaller nanoscale IC technologies: (1) shorter wavelength increases resolution by practically max. 2X, but then optical absorption is increasing by orders of magnitude so bulk silicon has to get very thin, and (2) sticking to NIR resolution and work with the signal mix coming from ca. 10 FETs inside the optical spot, requiring an increasing level of circuit and device knowledge involving big data and Artificial Intelligence/Machine Learning (AI/ML).Here, another way out will be presented: (3) fault isolation techniques with real nanoscale resolution like e-beam probing, backside nanoprobing and even near-field optical microscopy are possible if only the back surface of the IC is very close to the active device. This Ultra-Thin Silicon Back Surface (UTSBS) has already been explored to a certain extent. This work shows an overview about the results that are available and the still open field of opportunities. These techniques also support CFI in 3D systems. The sample preparation is very challenging as it has to get down very close to the device of interest but gives more degrees of freedom as only local planarity in a trench is required. No space for a SIL has to be created and the imaging or probing techniques have a long working distance.So, the ultra-thinning may be only necessary in local area, offering a number of preparation solutions consisting of mainly FIB trenching and laser etching. They can as well be composed of these techniques. It will also be presented how beneficial chemical mechanical polishing (CMP) can be.