**3.1 Space charge distribution in polyimide film**

In this subsection, the LIMM measurement results of PI film in a typical case using cumulative DC voltage application. **Figure 5** shows the sample structure and the example of LIMM measurement protocol. The thickness of PI film is 20 μm. The polarization steps consist in applying a DC electric field from 25 to 125 kV/mm with positive or negative polarity. The corresponding positive or negative DC voltages are applied on the top electrode of the sample in air at room temperature, whereas the Si substrate is grounded. After each polarization step, a depolarization process is performed by short-circuiting the sample. A cumulative protocol is applied for successive voltage steps on the sample as shown in the right side of **Figure 5**. Volt-on and volt-off steps of each 60 min are applied on the sample from 25 to 125 kV/mm, with the steps of 25 kV/mm. LIMM measurements are performed throughout the whole protocol, with every 10 min scanning of 63 frequency points from 10 Hz to 10 kHz.

**Figure 6** shows LIMM current waveform under the field of 25, 50, 75, 100, and 125 kV/mm, positive polarity. The space charge and internal electric field

### **Figure 5.**

*Typical sample structure and LIMM measurement protocol (cumulative voltage application of 25–125 kV/ mm, positive polarity).*

**Figure 6.** *Example of LIMM current waveform (25–125 kV/mm, positive polarity).*
