Semiconductor Device Failure Analysis and Temperature Mapping
As electronic devices continue to decrease in size, heat generation and thermal dissipation become increasingly important. Most failures in electronic packages, components, and assemblies are either caused by excessive heat or they generate excessive heat. Localized heating induces temperature stresses that can affect device reliability and performance. For example, short circuits can grow over time and cause catastrophic failures.
The Micro thermal microscope measures and displays the temperature distribution over the surface of semiconductor devices, enabling quick detection of hot spots and thermal gradients which can indicate a defect site and frequently lead to decreased efficiency and early failure. Temperature mapping can be used to verify thermal design rules, verify die and solder attach, and optimize thermal packages. Micro can even be used to detect logic circuit failures by displaying the heat generated in active areas of an operating device.
- Semiconductor Device Failure Analysis
- Junction Temperature Measurement
- Thermal Resistance Measurement
- Thermal Design
- Die Bond Defect Identification
- IC Logic Circuit Failure Detection
- Circuit Board Failure Analysis
- MEMS Thermal Analysis
- Fiber Optic Thermal Analysis
Device Failure Analysis
Micro is often used to detect localized heating on an integrated circuit (IC) that is indicative of many different failure modes, including:
- Shorts in mold compound of packaged devices
- Latch-up sites
- Resistive shorts between gate and drain
- Shorts in metallization
- Gate oxide breakdown
- Reverse leakage current
And the root cause of these failure sites is often one of the following:
- Device fabrication problems
- Package integrity failures
- Wire bond defects
- Die attach problems
- Thermal stress
Hot Spot Detection Procedure
Detecting and locating hot spots on an IC requires first placing the device on the thermal stage so that its temperature can be precisely controlled. The device is biased by routing one of the leads through the relay module so that the software can control the timing of applied power. Next, sophisticated software tools, such as Hot Spot Detection or Lock-in Thermography, are used to examine the temperature response of the device while being automatically and repeatedly powered. After a hot spot has been detected and located, optical or X-ray inspection can then be used to examine the defect.
Temperature Mapping Procedure
When imaging semiconductor materials, much of the contrast on the image is due to emissivity variations on the device, not to temperature variations. Measuring the true temperature on a semiconductor device requires compensating for these emissivity variations on the device surface. First, the unpowered device is placed on the thermal stage so that its temperature can be precisely controlled. After it has reached a stable temperature, the Emissivity Tables software tool is used to automatically create an emissivity map of the device. By applying the emissivity map to thermal images, accurate temperatures can be obtained at any point on the device .
Steady-state thermography is limited to detecting hot spots that heat up at least 100 mK (0.100°C) and dissipate at least 20 mW of power. This may be useful for locating shorts on high-power devices, but is inadequate for detecting lower power defects such as reverse leakage current or hot spots in packaged devices. Steady-state thermography also suffers from poor spatial resolution as the heat from localized hot spots diffuses rapidly, blurring the location of the heat source.
Lock-in Thermography is a process of automatically and repeatedly powering a device at regular intervals while the temperature response of the device is monitored. By integrating temperature data over many power cycles, dramatic improvements in signal/noise and detection sensitivity can be obtained. Using this technique, hot spots that heat up below 1 mK (0.001°C) and dissipate under 100 µW can be detected.
Reverse Leakage Current
Undamaged ICs typically have reverse-biased leakage current less than 10 µA that can be very difficult to detect using thermal microscopy due to the very low amount of power dissipated. Damage from electrostatic discharge (ESD) can damage a dialectric layer or p-n junction, resulting in an increase in leakage current to levels that can often be detected. A good rule of thumb is that reverse leakage current doubles for every 10°C increase in temperature. When leakage current levels are below 100µA, we can use this characteristic to increase leakage current to detectable levels by placing the IC on the Micro thermal stage and heating it to a high (50 to 120°C) steady-state temperature. And to increase leakage current even higher, reverse-bias testing voltage can be increased.
Comparison of Microthermography Systems
Liquid Crystal Thermal Analysis
Liquid crystal hot spot detection involves coating the semiconductor surface with a uniform thickness of liquid crystal material and then analyzing the resulting surface color changes in response to temperature variations using a visual microscope. Hot spots exhibiting a temperature rise of 100 mK (0.1°C) or more can be detected using this technique with a spatial resolution of 1 micron. Disadvantages of this technique include low temperature sensitivity and slow thermal response (especially from the backside), the requirement that devices must be coated, ability to detect a smaller range of defects than mid and long wavelength infrared methods (below), and handling toxic liquid crystal materials.
Mid Wavelength Infrared Imaging
Mid wavelength thermal imaging cameras operate between 3 to 5 microns in wavelength. They are typically photon detectors that require cryogenic cooling to achieve thermal sensitivity below 20 mK (0.02°C) and frame rates of up to 200 Hz. Due to the wavelength in which they operate, mid wavelength IR cameras are limited to a spatial resolution of approximately 5 microns. Disadvantages include high cost, low thermal emittance in the 3-5 micron range at device measurement temperatures, low emissivity of semiconductor materials in the mid wavelength range, and decreased reliability and increased maintenance due to the cryogenic cooling plant.
Micro Long Wavelength Imaging
The Micro thermal imaging camera operates in the long infrared (IR) wavelength band from 7 to 14 microns. The camera's microbolometer detector does not require cooling and achieves a thermal sensitivity of 50 mK (0.05°C). With a response time of 7 ms, it can capture images at a frame rate of 30 Hz. Due to the long wavelength IR band of operation, microbolometers are limited to a spatial resolution of approximately 10 microns. The thermal image on the right display an area on an IC that is 800 microns (0.8mm) in width.
Advantages are low cost, quick start-up time, high reliability and low maintenance, hot spot detection sensitivity below 1 mK (0.001°C) with Lock-in Thermography, backside "through the die" analysis, true temperature mapping via Emissivity Tables, and no device surface coating is required. Additionally, thermal emittance and emissivity in the long IR range is higher than in the mid IR range. In other words, in the long IR range, there is more energy emitted from the die to work with.
Another advantage of the Micro infrared microscope is the ability to image the entire die at once using the microscopic lens with 6.4 x 4.8mm field-of-view. The precise location of the hot spot can then be determined within a resolution of 2.5 µm using the Image Enhancement and Picture Transparency software tools.
Junction Temperature Measurement
During semiconductor device operation, internal junction self-heating leads to a large concentration of heat at the junction. The peak temperature in a device is at the junction itself and heat conducts outward from the junction into the package. For this reason, accurate junction temperature measurement during device operation is an integral part of thermal characterization. The Emissivity Tables software tool enable you to perform accurate temperature measurements of the junction by automatically compensating for emissivity variations across the die surface.
In the development of micro-electro-mechanical systems (MEMS), such as microreactors, micro heat exchange systems, microactuators, and microsensors, spatial temperature distribution and thermal response time are among the most important parameters. Because a non-contact approach is required to measure the temperature of MEMS components, the Micro infrared microscope is a powerful tool for microthermographic characterization, providing detailed thermographic images with 20μm spatial resolution. The thermal image on the left displays an area on a micro heater that is 6400 microns (6.4mm) in width.
Circuit Board Failure Analysis
The Micro infrared microscope can also be used to perform failure analysis on circuit boards. The included wide-angle lens enables analysis of circuit boards both small and large. The Hot Spot Detection software tool helps detect and locate difficult-to-find short circuits.