edfas.org 21 ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 27 NO. 1 after the sample preparation, where molding has been taken off and the die is exposed and thinned down. This is where previously mentioned methods such as EMMI, OBIRCH, as well as x-ray methods would be commonly used. Here QDM excels at providing µm resolution images in a short amount of time, as well as providing direct information about electrical activity in the form of an image. Another benefit of QDM is that it does not require sample polishing, even for highly doped samples, which decreases the overall time spent on sample preparation. Due to its depth sensing capabilities, the QDM can also identify a defective layer much earlier than many optical methods, saving the FA engineer hours of layer-by-layer thinning. Figure 2 illustrates QDM’s place in the FA workflow. This article focuses on demonstrating QDM’s capabilities for lateral resolution on two different samples and familiarizing the reader with the magnetic and current density measurement data. QDM’s capabilities of depth reach, depth resolution, and sensitivity will be analyzed in a later work. SPATIAL RESOLUTION OF THE QUANTUM DIAMOND MICROSCOPE Several factors influence the spatial resolution of magnetic field images, defining the accuracy with which magnetic phenomena can be mapped in integrated circuits. The optical spatial resolution of the system described in this article sets the lower limit and depends on the numerical aperture (NA) of the optical system. To achieve maximum resolution, diamond solid immersion lenses are available achieving up to 200 nm optical resolution. The primary limiting factor, however, is the standoff distance—the gap between the NV centers and the electronic circuit under test. This distance influences resolution due to the decay of magnetic field strength with increasing distance. Notably, while the standoff distance affects spatial resolution, it also offers valuable insights into the depth of the current under investigation. To demonstrate the spatial resolution of this system, the team acquired vectorial magnetic field images of a current-carrying wire sample, provided by Hamamatsu’s PHEMOS group. The measurement results using QDM are shown in Fig. 3a. The experimental setup included an infinity-corrected objective with 50x magnification and a numerical aperture (NA) of 0.55, optimizing the system for a balance between field depth and resolution. The sample, a 500 nm thick wire, carried a current of 1 mA, generating magnetic fields on the order of 50 µT, which were mapped across a predefined area as the current flowed through the wire’s path. As discussed in reference 14, there are multiple ways to define the spatial resolution of the MCI. Here, spatial resolution is defined as the ability to resolve contribution from two parallel current paths as described by Sparrow’s criterium.[15] By taking a line cut along the x-axis of the signal generated by a single wire where current flows in the y-direction, the spatial resolution can be estimated as the 15-85 % raise of the signal. Bz signals have a different behavior, and the peak-to-peak distance is a definition generally adopted since it is easy to identify. Looking at the Bx profile gives a spatial resolution of 3.0 ± 0.5 µm. Analyzing the magnetic field profiles generated by a current-carrying wire shows that Bx and Bz components exhibit distinct spatial behaviors. The Bx field has a more confined profile with a sharper peak, whereas the Bz field tends to display broad lateral extensions, resulting in a more gradual decay at greater distances from the wire. These broad extensions lead to the long-range behavior Fig. 3 Magnetic field images of a 1 mA current-carrying wire, used to determine the magnetic resolution. (a) Amplitudes of Bx, By, and Bz magnetic field components, where a positive Bx value indicates an upward current and a positive By value indicates a right-to-left current. (b) Line cut of Bx along the x-axis, indicated in (a) by a dashed black line. The resolution is determined using Sparrow’s criterion, with 15% and 85% of the total magnetic magnitude shown by dashed orange lines. (c) Line cut of Bz along the x-axis, also marked in (a) by a dashed black line. Resolution is determined by the peak-to-peak distance, indicated by dashed orange lines. (a) (b) (c)
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