edfas.org 17 ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 28 NO. 1 LASER SCAN ALGORITHM A standard SDL laser scanning algorithm employs a fixed laser dwell time either set by the user or using a synchronization signal driven by an external source like the tester. SAILS uses a variable synchronization pulse signal driven by a microcontroller. An inexpensive microcontroller-based data acquisition system is employed to establish signal communication between the tool and a host computer running a LabVIEW application. Figure 2 shows how these three systems communicate with each other. The reason for using an external microcontroller with custom LabVIEW script is to make SAILS tool agnostic. SAILS can easily be adapted to any laser scanning tool as long as it supports synchronization capability. Under normal operation, the laser will move its position after each test loop iteration on the DUT. The raster path begins at the top left of the frame, and ends at the bottom right, moving in a typical raster fashion. In this implementation, specific regions in the frame are traversed at different laser trigger frequencies than others based on real time image processing. The steps in the SAILS technique are executed in a non-linear sequence, so a state machine architecture shown in Fig. 3 was chosen. After the scan parameters are set, the user can start the program, which runs autonomously until the scan is complete. The state machine begins in an idle state where the user can input scan parameters such as the slow and fast dwell times, site detection criteria, and the total number of frames to scan. After these are set, the user can initiate the scan sequence. To obtain a rough scan for site detection, the scan will proceed with normal operation for a user defined number of frames to acquire an initial image. Once these frames are collected, the state machine will pass this rough scan data to the site detection state in real time, where a thresholding/image processing algorithm is executed and a scan mask is generated. For the remaining frames, SAILS modulates the dwell time of the laser across the frame according to the scan mask. TOOL SCAN RECONSTRUCTION For SAILS to identify sites correctly, laser scanning data must be replicated correctly external to the laser tool. Each time a quantity of trigger pulses is sent, the tool reads the same number of pass/fail states. To ensure that the site detection algorithm makes a conservative estimate of the sites, a Gaussian blur (σ = 1.5) is applied to the rough scan, which helps by radially diffusing intensity of each individual peak. With this approach, Fig. 4 shows that SAILS can reproduce images that preserve the location and size of the sites. The next step after replication of the image is to detect sites in real time. SITE DETECTION ALGORITHM TRADITIONAL SITE DETECTION The site detection algorithm aims to reduce scan time of an image by identifying potential sites early in the imaging process. It does so by using averaged rough scan data gathered from a user defined number of initial frames. Fig. 2 Diagram showing system configuration of SAILS. Fig. 3 State machine diagram for the LabVIEW program.
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