**4. Polyimide films' characterization**

**Figure 6** shows the main intrinsic electrical properties of spin-coated polyimide films measured at wafer level. On the one hand, the DC bulk conductivity of polyimide shows very low values around 10−16 S/m over an applied electric field range up to 40 V/μm (i.e., ohmic range), but remaining quite low at least up to 150 V/μm. On

### **Figure 6.**

*Main intrinsic electrical properties of spin-coated polyimide films measured at wafer level: (a) DC conductivity versus electric field and (b) AC breakdown field distribution.*

**Figure 7.** *Surge performance for isolators with polyimide films 30 μm thick.*

the other hand, the AC breakdown field of the polyimide films exhibits a minimum value of 450 Vrms/μm at 60 Hz. All these make spin-coated polyimide films very good insulating materials for reliable digital isolator application.

**Figure 7** shows the surge performance for isolators with 30-μm thick polyimide films. As it can be seen, these isolators will pass surge tests up to 18 kV, and the first failure voltage is 19 kV for negative pulse and the first failure voltage is 20 kV for positive pulse.
