edfas.org 11 ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 25 NO. 2 • Measure the silicon thickness with IR • Validate the etching rate • Initiate backside trenching at the validated etching rate During trenching, uniform gas distribution is essential to ensuring even and predictable detection of the N-well. The moment that the N-well appears, it is critical to stop etching any further. Stand-alone trenching systems using laser assisted chemical etching are also available and can significantly speed up the backside editing process. In frontside editing, delayering via the top layer is used to access the underlying layer or line to be edited. The key parameters to consider are box size, application file, chemistry, and beam current. Next, specific lines are cut (if needed), and/or new lines are deposited. Certain lines are also extended to create test points or probing leads. This stage may also require the milling of vias within a layer or between layers. Key steps for revising lines include: • Performing local alignment and navigation to the region of interest • Using delayering to access the target layers • Initiating the final alignment process before cutting, positioning, and editing the line This work is made easier with a circuit-edit solution that enables milling of vias with high aspect ratios and provides precise visualization and detection of endpoints. Finally, material is deposited to refill the N-well trench and to insulate, or isolate, the edited area. An example of this process is the filling of vias. It also should be noted, insulator deposition is critical to passivate any exposed silicon and metal lines. For instance, XeF2 can unintentionally etch exposed silicon if applied. Iodine chemistry, typically reserved for silicon, SiO2, or aluminum etching, causes copper to corrode. A good practice is to ensure that anything that has been exposed gets covered to prevent unintended corrosion. It is also suggested that new practitioners write down their workflow on a whiteboard within eyeshot of their circuit-edit system. STARTING POINTS FOR SPECIFIC PARAMETERS AND BEST CHEMISTRIES After getting comfortable with the relevant workflows, one can get more specific. A few key parameters at the top of the list are: box size; minimum, maximum, and optimum current; and minimum, maximum, and optimum beam energy. Start by reviewing the particulars of the process node, the relevant cross-sectional information, and the constituent materials. From there, one can more easily define the best box size for the circuit edit before getting down to work. It is critical to understand the material composition of a device, as the right choice of beam chemistry can enhance the speed, efficiency, and precision of etching/ milling operations. Key questions to ask are: • Which chemistries work best with the materials in the device (Table 1)? • Which chemistries are incompatible with those materials? To help avoid potential problems, the following combinations require all due caution and attention: 1. Insulator enhanced etch with xenon difluoride (IEE XeF2) spontaneously etches silicon and also accelerates removal of SiO2 layers. Consequently, it should be used carefully when performing backside editing through silicon structures. Another IEE process benefit is that it Table 1 Common combinations of chemistries and materials Etching beam chemistry Action and target material Insulator enhanced etch with xenon difluoride (IEE XeF2) Backside etch of SiO2, Si Enhanced etch with diiodine (EE I2) Backside trench floor of Al, shallow Si Selective carbon mill with water (SCM H20) Mill thick Cu on SiO2, polyamide Delineation etch (DE) Expose tungsten and remove low-K dielectric DX chemistry Etch Cu on low-K dielectric THE MOMENT THAT THE N-WELL APPEARS, IT IS CRITICAL TO STOP ETCHING ANY FURTHER.
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