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Sentris

Thermal Imaging Microscope for Semiconductor Device Failure Analysis

Introduction

Due to the continued decrease in integrated circuit feature size and supply voltages, detecting and locating the miniscule amount of heat generated by failure sites has become increasingly difficult. The Sentris thermal imaging microscope pinpoints the low-level infrared thermal emissions by IC faults such as short circuits and leakage current.

Using a non-destructive testing (NDT) process called lock-in thermography, failures can be isolated on both bare (front or backside) and packaged devices and no surface coating is required. Sentris can also locate low-power fail sites on SMD components, such as capacitor leakage. The x, y position of defect sites can be located, as well as defect depth. Depth analysis can be very helpful when isolating faults in stacked die packages.

In addition to fault isolation, Sentris also includes thermal analysis tools for true temperature mapping, junction temperature measurement, die attach evaluation, and thermal resistance evaluation.

Overview

Faults Detected

    Semiconductor ESD related faults
    Leakage current and hot spots
    Resistive shorts between gate and drain
    Shorts in mold compound of packaged devices
    Latch-up sites
    Shorts in metallization
    Defective transistors and diodes
    Oxide layer breakdown
    SMD component leakage


Functional and Uncomplicated

The innovative Sentris fault isolation process was developed by Optotherm to simplify and reduce the cost of the lock-in thermography process. Traditional lock-in thermography systems include complex function generators and power supplies in order to synchronize device power with the capture of thermal images. By eliminating the need for these complicated and expensive instruments, Sentris provides a straightforward and affordable technique to semiconductor defect isolation.

Advantages Over Liquid Crystal Thermography

Liquid crystal thermography has low sensitivity, slow response, and requires the surface of devices to be coated. These systems are limited to detecting hot spots greater than 0.1°C and because they involve steady-state device powering, the heat generated by faults diffuses through semiconductor material, blurring the location of the defect source. Sentris overcomes the limitations of liquid crystal thermography. No surface coating is required, and hot spots below 0.001°C can be detected. Furthermore, high-frequency lock-in thermography minimizes heat diffusion, enabling the precise location of faults.

Compliments Other FA Tools

Semiconductor devices are becoming smaller, faster, highly integrated, and multi-functional with the result that failures often cannot be found using only one analysis technique. It is often necessary to use multiple tools to pinpoint different types of faults. Sentris thermal emission analysis is often used in conjunction with photon emission and OBIRCH analysis. Photon emission is used primarily to analyze leakage current resulting from gate oxide defects, latch-ups and ESD failures. OBIRCH is most often used to detect leakage current, short circuits and areas of high resistance.

Affordable

The price for a Sentris system (included Sentris components) is less than half the cost of competing cooled MWIR lock-in thermography systems. Optional components are available to extend Sentris features and capabilities (optional Sentris components).

Features and Capabilities

    High sensitivity Lock-in Thermography fault isolation
    Defect depth analysis of stacked die
    True temperature mapping using Emissivity Tables
    Visual camera probing
    Junction temperature measurement
    Bare and packaged device analysis
    Front and backside analysis
    Detection of die attach problems
    Thermal resistance evaluation

Sentris

Thermal Imaging Microscope for Semiconductor Device Failure Analysis

Capabilities


Lock-in Thermography Fault Isolation

Lock-in thermography is a process of automatically and repeatedly powering a device at regular intervals using a laboratory power supply and reed relays while the temperature response of the device is integrated and analyzed over time. Using this technique, hot spots that heat up less than 1mK (0.001°C) and dissipate below 100 µW can be detected.

In addition to identifying the x, y location of a defect, fault depth within a stacked-die can also be determined by analyzing the phase angle between device power and subsequent surface heating.

True Temperature Mapping

When imaging semiconductor devices, much of the contrast on the image is usually due to emissivity variations, not to temperature variations. Measuring true temperature requires compensating for these emissivity variations according to the following procedure.

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 a device.

Junction Temperature Measurement

During integrated circuit 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 allows accurate temperature measurement of the junction by automatically compensating for emissivity variations across the die surface.

Die Attach Evaluation

Die attach defects can be due to a number of causes such as inadequate or contaminated die attach material, delamination, or voids. Sentris thermal analysis tools like Image Sequence analysis can be used to assess the flow of heat away from a device in order to determine the integrity of the die bond.

Thermal Resistance Evaluation

A common method of characterizing the thermal performance of packaged devices is using the concept of thermal resistance. Thermal resistance is the steady-state temperature rise of a device junction above the temperature of a reference point (device package or heat sink) for each watt of power dissipated in the junction. Once a device's thermal resistance is known, the junction temperature can be readily calculated using temperature measurements on the reference point. Sentris enables accurate measurement of device packaging and heat sinks for reliable thermal resistance evaluation.

Competitive Advantages

Defects Detected

Sentris can perform the same temperature analysis and detect identical types of semiconductor device defects as competing thermal emission systems.

    Leakage current
    Resistive shorts
    Oxide layer breakdown
    Device temperature mapping
    Thermal resistance evaluation

Infrared Camera

The primary difference between Sentris and competing Lock-in Thermography systems for semiconductor failure analysis is the infrared camera technology used. Sentris includes an uncooled LWIR (7-14 microns) microbolometer camera with <40mK NETD* and 60Hz image capture rate. Competing systems utilizes cooled MWIR (3-5 microns) cameras with ~20mK NETD* and ~100Hz image capture rate.

* Noise Equivalent Temperature Difference (NETD) specifies the smallest temperature difference that can be detected.

Test Sensitivity

Lock-in thermography (LIT) test sensitivity is primarily dependent on NETD and the number of images captured during a test. LIT Sensitivity (theoretical) = 0.5 * NETD / (square root(# test images captured)). Therefore, for a specific test time length, tests can be more than twice as sensitive using cooled cameras. Sentris test sensitivity can be increased however, by simply extending the test time.

Spatial Resolution

When lenses are properly designed, spatial resolution is limited by diffraction to approximately ½ the working wavelength. The working wavelength of the Sentris LWIR camera is approximately 10 microns, and therefore the minimum possible spatial resolution is 5 microns. The working wavelength of cooled MWIR cameras is approximately 4 microns, and therefore the minimum possible spatial resolution is 2 microns. It should be noted however, that in practice, an increase in lens magnification will generally lead to a reduction in lens numerical aperture (and working distance), and an increase in diffraction, limiting the effective spatial resolution to <2 microns.

Reliability

The cameras in competing systems include detector assemblies that are typically cooled by mechanical Stirling engines. Stirling engines are expensive, complex devices whose reliability should be considered when purchasing a system intended for many years of use.

Lock-in Hardware

The Sentris Lock-in Thermography software-driven process was developed by Optotherm to simplify and reduce the cost of semiconductor failure analysis. Competing lock-in thermography systems include complex and expensive lock-in amplifiers in order to synchronize captured thermal images with device power.

System Cost

By eliminating the expense involved with cooled infrared cameras and lock-in hardware, Sentris can be offered for approximately 1/3 the cost of competing systems.