Electric field enhancements due to space charge in thin polyimide film

Abstract

Polyimide (PI) is widely used in film form as a passivation material for power semiconductor devices of Si, SiC, GaN, etc. The magnitude of the electric field at the edge termination area of these semiconductor devices is becoming higher and higher due to the increase of operational voltage and /or demand for shrinking the edge termination area to increase device active area. Hence it is concerned that space charge accumulation in the encapsulation and passivation material affects the insulation performance of those devices. In this report, space charge distribution and electric field enhancement associated with the charge accumulation have been investigated for thin PI film with thicknesses of about 20 μm, using Laser Intensity Modulation Method (LIMM). It is shown that the enhancement factor of the internal electric field (the maximum internal electric field divided by the applied field) in PI film is negligibly small (less than 1.1) for applied fields less than 10 kV/mm. However, it can reach much higher like 1.4 at higher applied field like 25 kV/mm. This result implies that electric field at edge termination area should be carefully designed under such an operational electric field as is the case of SiC devices.

Behavior of Space Charge in Polyimide and the Influence on Power Semiconductor Device Reliability

“…In [10], the effect of humidity in air and the difference due to electrode material (Al and Au) on space charge formation with time evolution are discussed. Recently, the Laser Intensity Modulation Method (LIMM) [13] has been performed to investigate space charge characteristics in thin PI films with few micrometers in thickness under the DC field up to 125 kV/mm, close to breakdown voltage [14][15][16][17]. In [17], the space charge characteristics were reported under DC voltage of from low field of 2.5 to 125 kV/mm with the correlation with DC conductivity trends.…”

“…Recently, the Laser Intensity Modulation Method (LIMM) [13] has been performed to investigate space charge characteristics in thin PI films with few micrometers in thickness under the DC field up to 125 kV/mm, close to breakdown voltage [14][15][16][17]. In [17], the space charge characteristics were reported under DC voltage of from low field of 2.5 to 125 kV/mm with the correlation with DC conductivity trends. Considering the encapsulation material, some studies have been performed through charge accumulation measurement in silicone gel by using PEA [18,19] and LIPP (Laser-Induced Pressure Pulse) [20] methods, and space charge characteristics of only surface information and total amount of charge have been measured in [18][19][20], respectively.…”