**1. Introduction**

In recent years, high voltage electronics and power electronics applications have emerged needing the use of power semiconductor devices with widely used Si and wide bandgap materials such as SiC and GaN. In these devices, thin polymer material has been widely used as passivation coating to protect device surfaces. Particularly, polyimide (PI) is of great interest due to its excellent thermal and electrical properties and its easy processing. Some of the most important applications of these material films are as inter-level dielectric insulators and as electronic device surface passivation [1]. The typical image of PI layer at the edge surface of semiconductor chip is shown in **Figure 1**.

**Figure 1.**

*Power module and edge termination structure of semiconductor chip. (a) HV IGBT power module. (b) Edge termination structure of semiconductor chip*

**Figure 2.** *Avalanche voltage simulation results of HVIGBT chip for different Qss.*

Space charge is generally reported as a triggering mechanism for the degradation of insulators [2] and considered as a cause of the decrease of electrical breakdown voltage of those polymer films. Furthermore, the space charge could degrade semiconductor leakage current characteristics by strengthening the electric field in semiconductor substrate region. The influence of space charge transportation and accumulation on surface of semiconductor chip has been discussed in [3–6] using TCAD simulation. Reliability tests of IGBT devices under accelerated conditions were reported in [7, 8], showing that the degradation (increase) of leakage current of the device is caused possibly by the formation of space charge on the surface at semiconductor edge termination area. The decrease of withstand voltage of HV IGBT due to accumulated space charge (*Qss*) on the surface of device edge termination area has been demonstrated by TCAD simulation as shown in **Figure 2**. Attempts have also been made to evaluate the space charge resistance of a real chip by evaluating the withstand voltage of a semiconductor chip using a guard probe electrode to simulate an external electric field due to space charge as shown in **Figure 3** [9].

To evaluate the actual influence by the space charge accumulation, it is necessary to clarify the space charge distribution around the edge termination area of

*Behavior of Space Charge in Polyimide and the Influence on Power Semiconductor Device… DOI: http://dx.doi.org/10.5772/intechopen.92165*

**Figure 3.**

*(a) Schematic view of guard probe (GP) method and (b) Leakage current dependent on the guard prove voltage.*

semiconductor chip. Many trials have been performed to measure the space charge characteristics in polyimide and encapsulation material such as silicone gel as described below.

Earlier reports featuring space charge in PI [10–12] reported the processes indirectly, for example, through charging current measurements or directly as with pulsed electroacoustic (PEA) method with relatively thick films (125 μm) [10]. 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 [14–17]. 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 [18–20], respectively.

As described, many attempts have been made to evaluate the influence of space charge, and quantitative evaluation has been under development, especially in thin PI. However, the space charge distribution in the PI film, which is important for providing space charge tolerance of semiconductor chip, is gradually becoming apparent along with the influence of the operating environment. In this chapter, the latest developments with space charge measurements in thin polyimide films using the LIMM method, with the focus on local field strengthening and correlation with conductivity measurements, are discussed.
