Optical fault localization methods rely on the combination of a microscope with an imaging system and an electrical connection to send current through damaged circuits in a semiconductor. Imaging systems span the spectrum; from sensitive visible-light cameras which count photons coming from recombination of holes and electrons exposed in failure sites, to IR light cameras which look for the heat signature of a short circuit often called the “Hot Spot.” There are advanced laser voltage imaging and probing and dynamic laser stimulation that can produce current using light which allows precise mapping of activity in the semiconductor.
It is up to the failure analysis engineer to determine the appropriate test conditions for each of these methods, optimizing the signal to the optical system and obtaining the best image. This article focuses on how RTI’s test solutions can test and electrically stimulate devices during these types of procedures.
Manual Test Solutions for Fault Localization
Manual Switch Box with Test Fixture
RTI’s manual switch box has been a staple of the Failure Analysis workbench since the beginning of IC failure analysis. The switch box offers a straightforward way to interface with a curve tracer and a DUT board.
Automated Test Solutions for Fault Localization
MultiTrace with OctoPogo Interface
The MultiTrace curve tracer is one of the most advanced ways to bias a device while observing it for hot spots. The primary purpose of the curve tracer is to find electrical signatures of failed devices. This often includes current “leakage” at a single pin or high supply current in one or more of the device’s power pins. You can use these same signatures during the characterization phase of analysis to stimulate failure during the fault localization process.
Unpowered Curve Tracing to Isolate Electrical Damage
Screenshot of Unpowered Curve Tracing in MultiTrace Software
Damage directly to an IO pin is often found with unpowered curve tracing. This type of electrical damage is easier to isolate and easier to stimulate again by the MultiTrace. Or you can move the device to the manual switch box with a simple monitored power supply because the test conditions are easily replicated.
Powered Curve Tracing to Isolate Electrical Damage
Screenshot of Powered Curve Tracing in MultiTrace Software
Subtle pin-related damage or failures in the device’s core or power domains can be difficult to isolate or measure in the unpowered curve trace. So, powered curve tracing is used instead. Often the device’s damage is only in the core and can only be seen with a high supply current. Here, powered curve tracing capability and the ability to toggle input pins to switch the operational states of the device are very useful. Powered curve tracing can manipulate biasing voltages to obtain the best contrast between the failure and the background of the rest of the circuitry.
Lock-in Thermography & Curve Tracers
One of the more sophisticated optical tools is called lock-in thermography. This technique seeks to synchronize the stimulus to the hot spot with the acquisition of the image. In curve tracing, the operator wants to look at the image while the curve trace is running and ignore the background when it is not. This helps filter out all random activity that may occur as the device operates. To address this on the MultiTrace, we make use of one unused SMU (Source Measure Unit) to produce a voltage pulse immediately before the curve trace begins and immediately again after it ends. This provides timing markers for the lock-in system to position its observation window. The imaging tool also has its own controls for optimization of the settings, including when the image is obtained regarding delays from these timing markers.
To configure this, we typically use the HV drive and its banana jack interface. The HV drive is configured for 1 sample and 0V to 5V sweep. When the test runs, the drive momentarily sweeps to 5V and back to 0V, and then allows the rest of the curve trace to proceed normally. There are several variations of settings that are useful. Most of these settings are in the ‘drive settings’ window but also in the control sequence and measurement windows in the StdTrace software. For more details on how to configure this, look for the application note titled “Creating Timing Pulses for Lock-in Thermography” included with the MultiTrace or request a copy from RTI.
More Test Solutions
RTI’s DUT boards and test sockets offer a wide range of design options useful in optical fault localization such as open-top lids, open bottom test sockets, and in-socket device heaters. RTI’s fixtures for failure analysis are typically low-profile and interface with flexible ribbon cables. Fixtures are also flexible, allowing you to configure them with a wide range of probe stations, light-tight boxes, and bench setups.
If you are interested in learning more about RTI’s curve tracers and how they can help with your specific needs, please contact us to schedule a free live demo of our products and software.