Thermal Emission Microscope for Semiconductor Device Fault Isolation
How It Works
Sentris employs sophisticated Lock-in Thermography software and hardware to dramatically improve detection sensitivity and fail site spatial resolution. Lock-in thermography is a process of automatically and repeatedly powering a device at regular intervals using a laboratory power supply and solid-state relays while the temperature response of the device is integrated and analyzed over time. Over many power cycles, the sum of thermal images captured while the device is unpowered are subtracted from the sum of thermal images captured while the device is powered.
Increasing the number of test cycles results in improved test sensitivity. Using this technique, hot spots that heat up less than 1mK (0.001°C) and dissipate below 100 µW can be detected. Weak sources of heat arising during normal operation of the device may even be detected.
Cycle Frequency can be set between 0.125 to 7.5 Hz. Performing lock-in tests at lower frequencies improves test signal/noise due to higher device heatup. Higher frequency tests improve hot spot spatial resolution by reducing thermal diffusion into adjacent areas of the device.
Test sensitivity is directly dependent upon lock-in test time. As a test continues to run, lower and lower power dissipation levels can be detected. There is no limitation on how long a Sentris lock-in test can be performed. Consequently, finding an extremely small heat signature is only a matter of time. The image on the left was produced while testing a short on a semiconductor die with 100µW dissipation over a 10 minute period.
Lock-in Test Results
The amplitude image, which displays total temperature increase on a device during power cycling and is commonly used to determine fault location in the x, y direction, as shown in the image below. Performing lock-in tests at lower frequencies improves test signal/noise due to higher device heatup. Higher frequency tests improve hot spot spatial resolution by reducing thermal diffusion into adjacent areas of the device.
Lock-in Test Procedure
First, the semiconductor device is placed on the Thermal Stage so that its temperature can be raised slightly for ease of focusing. In cases of very low leakage current, the temperature of the device can be raised further in order to increase leakage current to improve the probability of detection.
Device biasing is accomplished via exposed leads, package pins, or by probing the device using the Probe Positioners. One or more of the biasing leads are routed through the Relay Module so that the Sentris software can control the timing of applied power. The applied voltage level is generally set so that approximately 1-2 mW is dissipated within the defect. This level of power is generally sufficient to localize the fault within seconds or minutes.
Next, a Lock-in Thermography software test captures continuous thermal images while device power is cycled until the fail site becomes apparent and can be located precisely. At this point, X-ray inspection may be used to examine the defect in more detail.