Sombel Diaham

Space-charge-limited currents in polyimide films

Space-charge-limited currents have been identified in thin polyimide film capacitor structures as the main conduction process in the very high temperature range from 320 °C to 400 °C before the breakdown. The transition field between the trap-filled-limit conduction and the trap-free conduction is reported versus temperature. Assuming an exponential distribution of the traps in the forbidden gap, both the characteristic temperature and trap energy are obtained at 446 °C and 62 meV, respectively. The total trap density is accurately estimated at 1.5 × 1017 cm−3 using the Kumar approximation [Kumar et al., J. Appl. Phys. 94, 1283 (2003)]. Finally, the mobility temperature dependence of free charges is reported between 1.6 × 10−6 and 2.3 × 10−6 cm2 V−1 s−1 in the range from 340 °C to 400 °C.

Evaluation of Encapsulation Materials for High-Temperature Power Device Packaging

High-temperature power electronics represent an increasing demand. Higher power density or severe ambient temperature applications become the trend, while silicon carbide components with 250-300 °C Tjmax are emerging. Among materials used in high-voltage power module, soft encapsulants play a significant role in improving both semiconductor die and module package voltage ratings, especially under enhanced electrical and thermal constraints. In operation close to their upper temperature limit, two silicone materials were selected among the most thermally stable soft insulators available today. Up to 300 °C, dielectric properties and their stability under isothermal aging in air ambient tests were characterized. The gel, tested using sandwich structures, exhibits cracking of its exposed-to-air face, at an aging temperature as low as 250 °C after less than 100 h. The elastomer, tested as free films, presents no cracking, no degraded electrical characteristics, and a 6 % relative mass loss, after 500 h aging. Moreover, the elastomer insulating properties, at low and high electric field, remains stable up to 300 °C (short-term tests), contrary to the gel which shows a strong increase in dc electrical conductivity. So the elastomer shows promising properties for improved encapsulation performance at 250 °C, to be further investigated in package configurations.

Concentration and mobility of charge carriers in thin polymers at high temperature determined by electrode polarization modeling

Charge carrier concentration (n0) and effective mobility (μeff) are reported in two polymer films (<10 μm) and in a very high temperature range (from 200 to 400 °C). This was possible thanks to an electrode polarization modeling of broadband dielectric spectroscopy data. It is shown that the glass transition temperature (Tg) occurrence has a strong influence on the temperature dependence of n0 and μeff. We carry out that n0 presents two distinct Arrhenius-like behaviors below and above Tg, while μeff exhibits a Vogel-Fulcher-Tamman behavior only above Tg whatever the polymer under study. For polyimide films, n0 varies from 1 × 1014 to 4 × 1016 cm−3 and μeff from 1 × 10−8 to 2 × 10−6 cm2 V−1 s−1 between 200 °C to 400 °C. Poly(amide-imide) films show n0 values between 6 × 1016 and 4 × 1018 cm−3 from 270 °C to 400 °C, while μeff varies between 1 × 10−10 and 2 × 10−7 cm2 V−1 s−1. Considering the activation energies of these physical parameters in the temperature range of investigation, n0 and μeff values appe...

Time and frequency domains dc conductivity analysis in thin dielectric films at high temperature

Electrical conductivity of a thin dielectric film has been analysed at high temperature in both time and frequency domains (TD/FD). Two disturbing ionic space-charge phenomena have been highlighted in two different temperature ranges and a correlation of their electrical signature between TD and FD is carried out. These two phenomena were related to the thermal activation of ions coming from two different trap levels (shallow and deep traps). We validate here also the fact that the FD method is a powerful way to estimate the dc conductivity in dielectric solids at high temperature thanks to a better discrimination of ionic contributions and injection phenomena.

Broadband dielectric spectroscopy of BPDA/ODA polyimide films

Dielectric spectroscopy of a high-temperature photosensitive polyimide was investigated in wide temperature and frequency ranges during heating and cooling cycles (from −150 to 370 ◦ C and from 0.1 to 1 MHz). During the heating phase measurements two sub-glass relaxation processes were observed, noted as γ and β relaxations. The γ relaxation appears at a low temperature (around −60 ◦ C at 1 kHz) with an activation energy of 0.44 eV during the heating phase and disappears during the cooling one, indicating that the peak is initially related to the presence of water in the polyimide films. The β relaxation appears at higher temperatures (around 180 ◦ C at 1 kHz) with a higher activation energy of about 1.5 eV. The β peak location and intensity for low temperatures (between 100 ◦ C and 120 ◦ C) change slightly on comparing the heating and cooling spectra, indicating also the effect of water molecules, which may act as a plasticizer. However, for higher temperatures, the β peak does not show any significant effect of the thermal cycle, and the relaxation is mainly attributed to the non-cooperative relaxation of the polyimide molecules. The ac conductivity (σ � ) values show that the electronic hopping process is influenced by the dynamics of the segmental and macromolecular chains of the polyimide in the γ and β relaxation regions. At high temperatures (>250 ◦ C) a plateau region appears in the ac conductivity allowing the extraction of the dc conductivity values, which are not affected between the heating and cooling measurements. This leads us to conclude that there are no significant morphological or chemical changes in the polyimide even for temperatures higher than its glass transition one under N2 for short periods. For temperatures above 300 ◦ C an increase in the values of relative permittivity is observed and referred to the Maxwell–Wagner–Sillars or to the electrode polarization phenomena. In this range the activation energy of the polarization peak frequency, conductivity relaxation peak frequency and the dc conductivity is the same and equal to 2.4 eV, indicating that those three parameters are governed by the same underlying mechanism. (Some figures may appear in colour only in the online journal)

Dielectric strength of parylene HT

The dielectric strength of parylene HT (PA-HT) films was studied at room temperature in a wide thickness range from 500 nm to 50 μm and was correlated with nano- and microstructure analyses. X-ray diffraction and polarized optical microscopy have revealed an enhancement of crystallization and spherulites development, respectively, with increasing the material thickness (d). Moreover, a critical thickness dC (between 5 and 10 μm) is identified corresponding to the beginning of spherulite developments in the films. Two distinct behaviors of the dielectric strength (FB ) appear in the thickness range. For d ≥ dC , PA-HT films exhibit a decrease in the breakdown field following a negative slope (FB  ∼ d −0.4), while for d < dC , it increases with increasing the thickness (FB  ∼ d 0.3). An optimal thickness doptim  ∼ 5 μm corresponding to a maximum dielectric strength (FB  ∼ 10 MV/cm) is obtained. A model of spherulite development in PA-HT films with increasing the thickness is proposed. The decrease in FB above dC is explained by the spherulites development, whereas its increase below dC is induced by the crystallites growth. An annealing of the material shows both an enhancement of FB and an increase of the crystallites and spherulites dimensions, whatever the thickness. The breakdown field becomes thickness-independent below dC showing a strong influence of the nano-scale structural parameters. On the contrary, both nano- and micro-scale structural parameters appear as influent on FB for d ≥ dC.

Dielectric properties of polyamide-imide

The dielectric properties of poly(amide imide) (PAI) films are investigated in a large temperature range. A moisture-dependent relaxation (γ-relaxation between −100 °C and 20 °C) and a non-cooperative local dipole relaxation (β-relaxation between 40 °C and 200 °C) display an Arrhenius-type behaviour with an activation energy of 0.50 eV and 1.22 eV, respectively. In the near glass transition (Tg) region at 277 °C, a relaxation process (α-relaxation) occurs due to cooperative segmental motions of the chains and follows a non-linear Vogel–Fulcher–Tamman (VFT) temperature dependence with a strong fragility. Simultaneously in the same temperature range, both a conduction process (σ-conduction) and an electrode polarization phenomenon (ρ-relaxation) are also present. The σ-conduction also follows a VFT behaviour but it possesses a lower fragility in the above-Tg region. This discrepancy is assigned to a partial decoupling between ionic transport and segmental chain motions. The dielectric strength of PAI films exhibits a negative temperature dependence. The near-Tg region corresponds to a change from a thermal breakdown mechanism to an electromechanical breakdown due to the glass–liquid phase transition. Above Tg, the formation of a space-charge probably also involves an electro-thermal breakdown mechanism.

Weibull Statistical Dielectric Breakdown in Polyimide up to 400°C

In spite of the growing maturity of silicon carbide (SiC) technology, several difficulties still remain and limit its use for high temperature applications up to 200degC. Due to its excellent physical properties, SiC material offers the ability to design electronic devices working at junction (or ambient) temperature and at power level much higher than those of the present silicon based semiconductors. Thus, the environment of the SiC die will endure severe electrical and thermal stresses. Particularly, the passivation layer must present good thermal stability of its dielectric strength, which must remain high enough to ensure a proper electrical insulation on top of the SiC surface. In this study, polyimide material has been chosen as a candidate for SiC power device passivation. This paper presents the changes of the dielectric strength of a BPDA/PPD polyimide for temperatures ranging up to 400degC.

Dielectrics for High Temperature SiC Device Insulation: Review of New Polymeric and Ceramic Materials

The keys to successful high power electronic systems are located as much in the ability to build high temperature power devices and to package them with the appropriate materials, as in the aptitude to reduce and control switching and conduction power losses. Particularly, high temperature low loss operation allows an increase in the power rating of these devices. The recent development of wide band gap semiconductor devices should allow improving power electronic systems. Wide band gap semiconductor materials, especially the most mature silicon carbide (SiC), should allow the electronics operation at high junction temperatures (>200°C), high voltages (>10 kV) or in harsh thermal environment, with faster switching and lower power losses active devices than the silicon (Si) counterparts. Such SiC devices impose more severe electrical and thermal stresses to the surrounding insulating materials (polymeric passivation and encapsulation materials and ceramic substrates). Lots of improvements have already been built-up at the die level; however, superior device performance degrees could be reached using higher performance insulation materials. Among the power device packaging materials for a high temperature operation, typical organic passivation and encapsulation appear nowadays as the most sensitive to the thermal constraints (Tmax=250 °C). Moreover, even if ceramic materials present a high isothermal stability (up to 600°C) they are very sensitive to the large passive or active thermal cycling induced by the power devices or by severe environmental constraints during operation. Therefore, research on high temperature dielectric materials tries to identify new polymeric and ceramic materials electrically, thermally and mechanically suited for the packaging of SiC power devices and to determine their effective limits (properties and durability). In this chapter after a section on the high temperature applicative needs and the new thermal and electrical constraints imposed by SiC devices on the surrounding insulating materials, a complete review of the polymers and ceramics insulating materials which are reported to potentially answer to the packaging issues is carried out through a presentation of their different main physical properties and the sensitive aging parameters in link with microstructure. Among the polymeric materials, BPDA/PDA polyimide (PI), fluorinated parylene (PA-F), polyamide-imide (PAI), and silicone (PDMS) will be studied. On the other hand, mainly aluminium nitride (AlN) and silicon nitride (Si3N4) ceramics will be presented.

Huge nanodielectric effects in polyimide/boron nitride nanocomposites revealed by the nanofiller size

The dielectric properties of polyimide/boron nitride (PI/BN) nanocomposite films are investigated as a function of the BN nanofiller size from 20 to 350 °C and at low filler content (1–2 vol.%). The role of the BN nanofiller size on the large reduction of the electrode polarization relaxation phenomenon due to ionic movements is reported. For the two smallest BN nanoparticles (95 nm and 35 nm), the permittivity, dielectric losses and dc conductivity are strongly attenuated above 200 °C by a factor of 10 to 1000 compared to neat PI. Thus, the dc conductivity at 350 °C is reduced from 4 × 10−8 Ω−1 cm−1 for neat PI to 3 × 10−11 Ω−1 cm−1 for PI/BN (35 nm). Moreover, a further decrease is obtained by functionalizing the nanofiller surface with a silane coupling agent which improves the grafting of PI chains on those latter nanoparticles. These results highlight the trapping efficiency in the interphase region introduced by the small BN nanofillers (<100 nm) and provides evidence as to the huge nanodielectric effects on the charge carrier transport controlled by the nanoparticle diameter. This finding should be of great importance for advanced high temperature electrical insulation in the future.