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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.

Polyimides (PI) are well-known materials used as passivation and insulating layers in microelectronics or power electronics. Though the electric field and temperature withstanding of polyimides have been investigated for long, little information is available on the space charge behavior in relatively thin polyimide films. In this work, the space charge behavior was investigated with the Laser Intensity Modulation Method (LIMM) which is suited for films of several µm in thickness. It is complemented by DC conductivity measurements. The analysis is made on 12 and 18 µm thick PIlayers deposited on Si-substrates with using Al or Au top electrodes. A build-up of negative charges can be observed, irrespective of the polarity of the applied voltage, as the external field is increasing in the range 25-125 kV/mm. With decreasing film thickness, the DC conductivity increases and a diminution of the internal electric field distortion occurs. The native alumina formed between the aluminium electrode and the PI could act as a barrier to electrons injection from the top electrode.

Polyimide is widely used in film form as a passivation material for power semiconductor devices such as Si, SiC, and GaN. The magnitude of the electric field at the edge termination area of these semiconductor devices is becoming 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 the accumulation of space charge in the encapsulation and passivation material may affect the insulation performance of these devices, for example, the degradation of withstand voltage due to distortion of the internal electric field caused by space charge accumulation. To design space charge resistance of semiconductor devices, it is important to understand the space charge behavior in polyimide films with a thickness of several to several tens of micrometers. This chapter addresses practical implementation, specifications, and issues on space charge in polyimide insulation on power semiconductor devices focusing on the space charge measurements in thin polyimide films using the latest developed LIMM method and DC conductivity measurements.

Precise measurements of the optical properties of electrodes add value to the interpretation of the recorded current spectra when applying Laser Intensity Modulation Method (LIMM) for space charge density measurements, including also its version with focused laser ((F)LIMM). They allow to correctly account for the energy deposition in the metal electrode and in the dielectric material under study. In this work the refractive index (n) and the extinction coefficient (k) have been extracted from the recorded by spectroscopic ellipsometry spectra and then integrated in the model to account for the nature and real conditions of the electrode. Such approach improves modeling of the temperature required for extraction of the space charge. It is found that higher refractive indexes of the gold electrodes lead to an important increase of the signal/noise ratio, thus improving the sensibility of the method for space charge measurements. It also allows to probe much lower levels of space charge densities.

This work presents a method for megavoltage X-ray Bremsstrahlung assessment by means of Monte Carlo (MC) (PENELOPE) simulations. The proposed method provides suitable spectrum characterisation by means of a few parameters, namely the acceleration potential and incident angle of electron beam on the anode. The obtained spectra have been used as incident beam for in-depth dose distribution determination with the aim of performing further comparisons with experimental data. This comparison procedure has been employed for Bremsstrahlung assessment optimisation. Once the Linac Bremsstrahlung is well established, it may be used as incident beam for further dosimetric purposes, such as spatial dose distribution calculations.The whole process has been applied for the characterisation of the megavoltage Bremsstrahlung from a typical Linac, namely the 10 MV Varian Clinac 18. The obtained results for in-depth dose distributions, when compared with experimental measurements, show an overall good agreement, which may suggest the feasibility of the method. Results and DiscussionBremsstrahlung assessment: spectral and angular distributionsThe first approach consisted on the comparison of the different theoretical and semi-empirical Bremsstrahlung models described earlier. Actually, it is expected that when β tends to unity, the scattering angular distribution would concentrate at forward directions.