edfas.org ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 25 NO. 2 10 If one is new to both, start with FIB because it’s the key to pinpointing, imaging, and manipulating the target area to be edited. Next, develop proficiency with the positioning of gas nozzles and the application of the necessary chemistry. GETTING STARTED When preparing to sit down and start a circuit-edit session, it is critical to be familiar with device technology, process, and validation. This includes the structure of each device, including: the number of metal layers, the technology nodes, and the cross section. It is also useful to know the electrical equivalent circuits and the preferred test methods for the target device. Below are three keys for increasing the odds of a successful circuit edit: 1. Get familiar with the circuit-edit solution: Precision, efficiency, and success are more likely when familiar with the capabilities of the circuit-edit solution, including basic capabilities, advanced features, and everything in between. Mastery of the basics serves an essential foundation for precise surgery on advanced devices. Next-level mastery adds the ability to identify which situations are best addressed by one or more of the special features of the instrument. For example, the availability of lower beam energy helps minimize the chances of destroying sensitive circuits (or even the sample itself) during the edit process. 2. Understand the circuit layout: Circuit layout is specific to each device, and it is important to prepare as much as possible. For example, it is imperative to know which circuit elements are sacrificial: silicon capacitors, buffers, input/output (I/O), ring oscillators, etc. These elements can also provide reference points as the editing process progresses; sacrificial devices (i.e., disposable samples) are often exposed (via milling or trenching) to establish reference points within the computer-aided design (CAD) layout to ensure precise alignment. It is therefore critical to know what can be exposed without altering or destroying device functionality. It is also important to know which layers the FIB can access and connect to, such as metal 0, metal 1, and contacts. This is easier if the device has been designed with FIB in mind, but this is unfortunately not a universally accepted practice. In many cases, it is best to acquire the correct CAD data and develop an edit scheme in collaboration with the circuit designer. They will be able to identify the signals of interest and point out which, if any, obstructing metal lines can be cut. 3. Remember a few basics: Before embarking on the editing process, it is useful to remember four key fundamentals: • Create a circuit-edit plan and clearly state the goal of the editing process. If the realities of the target location make it difficult to perform circuit editing, meet with the designer or a test engineer and discuss the electrical equivalent circuit. Then, before starting the actual work, clarify and refine the edit plan and the goal. • Always wear gloves when handling the device and any of the editing system’s vacuum parts. • Review and understand the best-known method (BKM) for setting up the target device. • Know how to insert unique packaging, and then mount the device to ensure it is well grounded. Avoid electrostatic build up when handling and operating on the device. THE GENERAL WORKFLOW Circuit editing begins with sample preparation. Naturally, if the device is no longer intact after this step, no amount of circuit editing can revive it into a functional device. Whether it’s backside or frontside editing, begin by decapping the package. For backside editing, the next step is grinding and polishing of the device. For frontside editing, the next step is removal of the polyamide layer. Next, global alignment is performed via CAD layout. For backside editing, the initial alignment is accomplished by using the IR camera to identify features and an optical image is then calibrated to the CAD layout. In the case of frontside editing, the FIB is used to image fiducials on the surface. The FIB is then aligned to the CAD layout. Both of these can be achieved using specialized CAD tools from a variety of vendors. The next steps apply to every location that requires circuit editing, with some variation between backside editing (e.g., bulk silicon trenching) and frontside editing (e.g., delayering). BULK SILICON TRENCHING, DELAYERING, REVISING LINES, AND REFILLING PLUS INSULATING With backside editing, trenching is used to access the layer or line to be edited. The key parameters are box size, application file, chemistry, beam current, and contrast setting. When performing backside editing, it is also important to review the workflow for backside silicon (Si) trenching:
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