Sensing Electric Currents in an a-IGZO TFT-Based Circuit Using a Quantum Diamond Microscope

Abstract

The Quantum Diamond Microscope (QDM) is an emerging magnetic imaging tool enabling noninvasive characterization of electronic circuits through spatially mapping current densities. In this work, we demonstrate wafer-level current sensing of a current mirror circuit composed of 16 amorphous-indium-gallium-zinc oxide (a-IGZO) thin-film transistors (TFTs). a-IGZO TFTs are promising for flexible electronics due to their high performance. Using QDM, we obtain two-dimensional (2D) magnetic field images produced by DC currents, from which accurate current density maps are extracted. Notably, QDM measurements agree well with conventional electrical probing measurements, and enable current sensing in internal circuit paths inaccessible via conventional methods. Our results highlight QDM's capability as a noninvasive diagnostic tool for the characterization of emerging semiconductor technologies, especially oxide-based TFTs. This approach provides essential insights to fabrication engineers, with potential to improve yield and reliability in flexible electronics manufacturing.

Figures (6)

• Figure 1: (a) Cross-sectional schematic of an a-IGZO TFT. (b) Transfer characteristics for 6 TFTs over a 4” x 4” substrate. The dimensions of the transistor channel are width ($W$) = 40 $\mu$m and length ($L$) = 20 $\mu$m. (c) A zoomed-in micrograph of an individual a-IGZO transistor with channel dimensions $W$ = 320 $\mu$m and $L$ = 20 $\mu$m. (d) Schematic of the current mirror circuit. (e) Micrograph of the current mirror circuit with one transistor at the input, and 15 ($M_a$, $M_b$, - - - - -, $M_o$) transistors at the output. (f) Measured response of the current mirror circuit, showing a mirroring ratio of 14.92.
• Figure 2: (a) Schematic diagram of the QDM setup used for sensing DC electric currents in the a-IGZO TFT-based CM circuit. (b) The diagram presents a cross-sectional side view of the setup. The diamond is placed on top of the DUT (CM circuit) such that the surface containing the NV layer is in contact with the circuit. Laser beam, delivered through the objective lens, illuminates the NV layer at the region of interest. The card edge connector facilitates current stimulus to the circuit by establishing a reliable connection to the conducting pads of the circuit. (c) The micrograph shows the diamond placed on top of the CM circuit, which consists of 16 TFTs. The current stimulus is applied through the $\mathcal{I}_{\text{in}}$ conducting pad, and the application of the voltage $V_{\text{DD}}$ to the $\mathcal{I}_{\text{out}}$ pad scales the output current (see \ref{['fig:fig1circuit']}d). (d) A zoomed-in micrograph of the circuit displaying a close-up view of the region $\mathcal{A}$ near the $\mathcal{I}_{\text{out}}$ terminal, and a close-up view of the region $\mathcal{B}$ near the ground terminal. We obtained magnetic field images of the current paths in these two regions. The width of the current paths in the regions $\mathcal{A}$ and $\mathcal{B}$ are $\sim$9.5 $\mu$m and $\sim$15 $\mu$m, respectively. (e) Defect structure of an NV center in diamond. The gray spheres represent the carbon atoms, while the blue and white spheres together represents the NV center. An external magnetic field $\vb*{B}$ acting along the NV axis is also shown with a blue arrow. (f) The energy level diagram of the ground-state spin of the NV center. Here, $D$ is the zero-field splitting (ZFS) between $\ket{m_s=0}$ and $\ket{m_s=\pm 1}$ spin states. With the application of a magnetic field, the degeneracy between $\ket{m_s=\pm 1}$ is lifted due to the Zeeman effect, resulting in spin transition frequencies $f_-$ for the transition between $\ket{m_s=0}$ and $\ket{m_s= - 1}$, and $f_+$ for the transition between $\ket{m_s=0}$ and $\ket{m_s= - 1}$ spin states.
• Figure 3: Imaging of the Oersted magnetic field produced by the current-carrying conducting strips. (a) The photoluminescence (PL) image of the region $\mathcal{A}$. (b) Image of the magnetic field ($B_{\text{NV}}$) due to the current $\mathcal{I_A}$ flowing through the conducting path in the region $\mathcal{A}$. (c) The magnetic field profile is measured along the black dashed line in (b). Red crosses represent experimental data, and the blue curve is a fit (see "\ref{['sec:methods']}") providing a standoff distance of 2.3 $\mu$m. (d) The PL image of the region $\mathcal{B}$. (e) Image of the magnetic field ($B_{\text{NV}}$) due to the current $\mathcal{I_B}$ flowing through the conducting path in the region $\mathcal{B}$. (f) The magnetic field profile is measured along the black dashed line in (e). The blue curve is a fit to the experimental data (red crosses) giving a stand-off distance of 2.8 $\mu$m. For the images shown in (a), (b), (d) and (e), the edges of the conducting strips are marked by orange dashed lines.
• Figure 4: Reconstructed current density images. (a, d) Images of the $K_x$ component of the current density of the regions $\mathcal{A}$ and $\mathcal{B}$. (b, e) Images of the $K_y$ component of the current density. (c, f) Images of the resultant current density magnitude $\abs{\vb*{K}}$. The edges of the conducting strips are marked by orange dashed lines in all the images.
• Figure 5: Reconstructed current density images near the corner regions of the circuit. (a) The micrograph of the a-IGZO TFT CM circuit. (b, e) The zoomed-in images at the regions $\mathcal{A}$ and $\mathcal{B}$ focusing on the corner regions chosen for this study. The red and green solid arrows show the direction of the current flow. (c, f) Magnetic field maps obtained at the respective corner regions. Black dashed lines outline the conductor edges (d, g) Reconstructed current density $\abs{\vb*{K}}$ maps at the corner regions near $\mathcal{A}$ and $\mathcal{B}$, obtained from the respective magnetic field maps. White dashed lines outline the conductor edges. The black arrows represent the current density vector $\vb*{J}$ indicating the direction of the current flow. The length of each arrow is proportional to $\abs{\vb*{K}}$. For plotting the arrows, a threshold of $\abs{\vb*{K}} > 12\,\mathrm{A/m}$ in (d) and $5\,\mathrm{A/m}$ in (g) is set.