Investigation of charge trapping in thin Kapton® polyimide films by means of photo-stimulated discharge spectroscopy
Photo-stimulated discharge-current spectroscopy was performed on 50 μm Kapton® 200HN polyimide thermo- and photoelectrets formed in the temperature range between −40ºC and +130ºC. Films were either charged in the DC electric field of 20 MV/m or photo-charged by illumination with light at the wavelength of 420 nm under the electric field of the same strength. Photocurrent spectra scanned in the wavelength interval 600–350 nm demonstrate a pronounced peak at approximately 470 nm revealing the presence of one charge-trapping level. The long-wavelength cut-off of the peak is at 550 nm corresponding to 2.3 eV trap depth.
Space charge formation in polyimide films and polyimide/SiO2 double-layer measured by LIMM
For Polyimides (PI) used as insulating inter-layers or passivation layers in microelectronics and power electronics, reliability and failure mechanisms are intimately linked to phenomena such as non-linear conduction, distortion of field distribution and electrical ageing that have all some relation to space charge processes. Knowledge about these space charge distributions is therefore of importance to optimize structures and designs involving PIs. However, in the thickness scale of interest for such applications, of the order of 10 μm, no results were reported for PI, and only scarce studies for other materials. Space charge measurements carried out on PI using the laser intensity modulation method (LIMM) are reported in this work. PI-containing metal-insulator-semiconductor structures were pre-stressed under DC fields up to 2 MV/cm. The impact of the doping type of the substrate (either n-type or, p-type silicon) has been evaluated to address the mechanisms of space charge formation. The nature of electrode substrate substantially impacts the measured charge amount and charge nature, pointing towards charge injection as the origin of space charge build-up. The interpretation is substantiated by results obtained using a SiO2 layer as a barrier against electronic injection from the substrate into the PI layer.
Recalculation of Mass Defects
THE well-known mass defect curve of the old nuclear scheme calculated with regard to α-particles and protons presented a difficulty with its minimum of binding energy for tin and an increasing portion between tin and lead. On the other hand, the mass defect values against protons give a rather smoothly decreasing curve.1 As has already been pointed out,2 the number of α-particles must be considerably reduced from the point of view of the new scheme, which does not admit any electrons in nuclei, but only neutrons and protons (presumably joined as α-particles). The curve of mass defect against protons and neutrons (perhaps with a single ‘central’ α-particle) is very similar to the old curve against protons, but decreases less rapidly. Clearly the new mass defect values relatively to α-particles, neutrons and protons must lie somewhere between the old values computed relative to protons and α-particles respectively (because the number of α-particles is decreased). We may emphasise that this new mass defect curve shows no increasing portion between tin and lead. For illustration we give two typical values: Old: 50Sn124 = 31α + 12&#38;epsiv;; mass defect = 0.158 (in mass units); 82Pb208 = 52α + 22&#38;epsiv;; mass defect = 0.035. New: 50Sn124= 25α + 24ω (ω = neutron); mass defect = 0.304; 82Pb208 = 41α + 44ω; mass defect = 0.366.