**6. Polyimide structural improvements**

To improve high voltage endurance for the polyimide, a charge injection barrier can be used as shown in **Figure 13** [6, 7]. The charge injection barrier is preferred to be oxide or nitride with a large bandgap and high dielectric constant. High dielectric constant will help to reduce the electric field close to the electrode, while the large bandgap raises the energy barrier for charge injection.

### **Figure 13.**

*Isolation transformer without (a) and with SiN charge injection barrier (b).*

**Figure 14.** *Band diagram for charge injection.*

To analyze the charge injection for a given isolation system, a band diagram can be drawn as shown in **Figure 14**. Four key materials in the isolation system shown in **Figure 12** are Au, the top coil material; polyimide, the isolation material between the top coil and bottom coil; oxide, the charge injection barrier; and TiW, the seed layer under the Au. Charge injection from Au or TiW into polyimide or oxide for electrons or holes can be calculated from the band diagram.

**Figure 15** presents the charging currents over time for polyimide and polyimide with SiN injection barriers measured under 1000 V. The steady-state current when the SiN barrier is introduced is reduced by more than five times compared to that of polyimide only. This highlights a significant reduction of the charge injection processes that are well known to be responsible of the electrical aging at high electric field.

**Figure 16** presents the time-to-failure (HVE tests) versus AC applied voltage from 1 kVrms up to 3.5 kVrms at 60 Hz for isolators with polyimide and polyimide/SiN barriers single die configurations. The lifetime at 50% and the extrapolation at 1 ppm of the data set are presented. Moreover, for both cases, the extrapolated working voltages at 30-year lifetime are reported. Digital isolator devices with polyimide insulation exhibit a 400 Vrms working voltage, while the improved design involving SiN injection barriers shows >900 Vrms working volatge at 1 ppm (750 Vrms after

### **Figure 15.**

*Charging currents comparison for polyimide and polyimide with SiN injection barrier under 1 kV.*

**Figure 16.** *Time-to-failure comparison for polyimide isolators with and without SiN charge injection barrier.*
