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He atoms were implanted in crystalline and pre-amorphized silicon wafers at doses in the 2×10 16 1×10 17 cm −2 range. Using transmission electron microscopy (TEM) we monitored the evolution of He bubbles into voids upon thermal annealing. Bubbles are formed in both crystalline and amorphous silicon. However, in amorphous material bubble interaction with the moving crystalline–amorphous interface during the epitaxial regrowth prevents their evolution into voids. By implanting He at different target temperatures in crystalline Si, thus by changing the structure of radiation damage, we found that the interaction between point defects and He atoms is essential for the generation of He bubbles and for their subsequent evolution into voids.

Formation and properties of voids in silicon obtained by high dose helium implants are described and discussed in view of their applications in localised lifetime engineering. Voids are stable even after huge thermal budget as observed by transmission electron microscopy analyses. Leakage measurements on p + n diodes as a function of temperature allowed us to determine the hole trap level ( ΔE =0.16 from the Fermi level above the valence band) and the generation and recombination lifetime values. To demonstrate the advantage of the method in power device applications, high-speed IGBTs were fabricated both with voids in the buffer layer or with unlocalised recombination centres. The devices with voids show a lower on-resistance and a fast turn-off behaviour. Map measurements on 150 mm silicon wafers demonstrate the good uniformity that can be reached by the method in an industrial environment.

Boron ions were implanted at 80 keV for a dose of 2 × 1015 cm−2 in crystalline silicon. The silicon crystal was recovered using different annealing processes to produce layers with different secondary defect densities. The quality and the characteristics of the silicon oxide grown on the different surfaces were found to depend on the defects present in the layer immediately under it. The oxide quality was improved by using implanted and annealed silicon with a defect free surface. Iron contaminations, introduced in the wafers at different concentrations (up to 1 × 1015 cm−2), do not affect the oxide layer grown on unimplanted silicon wafers, while it degrades the oxide characteristics of layers grown on implanted and annealed samples. This degradation decreases if the residual damage is reduced by annealing processes.

The diffusion and the electrical behavior of Al implanted in the dose of 1 x 10[sup 13] to 5 x 10[sup 15] cm[sup [minus]2] at 300 keV in capped and uncapped Si is investigated. The Al-based precipitates which are formed when Al concentration exceeds its solid solubility in Si are electrically inactive. The out-diffusion phenomenon that is always present in uncapped samples reduces the Al dose diffused into Si substrate. A study on the electrical activity of Al implanted in Si through SiO[sub 2], Si[sub 3]N[sub 4], and Si[sub 3]N[sub 4]/SiO[sub 2] capping films also is presented. In these capped samples Al segregation in SiO[sub 2] layer occurs. The electrically active doses are small and comparable to that of uncapped samples. The authors studied the diffusivity of Al in bulk SiO[sub 2] and Si[sub 3]N[sub 4] at 1,200 C. The fast Al diffusion through SiO[sub 2] thin layers is driven by a chemical reaction between Al and SiO[sub 2] starting from the SiO[sub 2]/Si interface.

Diffusion mechanisms in crystalline silicon are reviewed emphasizing the role played by the structural defects like vacancies and self-interstitials. These defects control the diffusion process of some transition metals, such as Au and Pt, which undergo fast long-range diffusion as interstitials and become substitutional by replacing a Si atom in a kick-out reaction. The influence of boundary conditions and sample surfaces on the concentration profiles of these metals are analysed in detail. These profiles can be precisely tailored using ion-implantation to achieve a low fluence diffusion source. Fine tuning of the metal profiles is shown to improve greatly the trade-off between dynamic and static characteristics of some silicon power devices like metal-oxide-semiconductor field effect transistors. Moreover, the possibility to obtain a preferential reduction of lifetime by metal doping in a selected area of a semiconductor device is demonstrated.

The electrical behaviour of ion implanted aluminum into silicon was investigated by varying the beam energy in the 80 keV-6 MeV range, the dose in the 1 × 1013–1 × 1014/cm2 range and the annealing procedure. Aluminum atoms precipitate into exten defects at the end of range damage and where the concentration exceeds the solid solubility value (about 2 × 1019/cm3 at 1200°C Escape of Al atoms occurs very easily as soon as they reach the external surface during the thermal diffusion. Using high energy implants, 6 MeV, it was possible to follow in detail the broadening of the diffused profiles. The measured trends between the retained dose and the junction depth and between the outdiffused dose and the annealing time are quite well predicted by the solution of the diffusion equation with the surface acting as a perfect sink for the dopant.

An innovative method to control carrier lifetime locally and efficiently in Insulated Gate Bipolar Transistors (IGBTs) is presented. It is based on the formation of void layers by low-energy and high-dose He implants and annealing. Voids introduce two well-defined midgap trap levels in silicon. HFIELDS simulations demonstrate the increase of surface hole concentration when a well localized recombination region is introduced in the buffer layer. High-speed IGBTs were fabricated both with voids in the buffer layer or with unlocalized recombination centres. Devices with localized bandgap centres show a lower on-resistance with a fast turn-off behavior.

Aluminum-based metallizations are extensively used as electrical interconnections of integrated power microelectronic devices. It is well known that this metal exposed to moisture is highly sensitive to corrosion due to the chemical interaction of aluminum with water. In this paper we report a study of some Al passivation treatments against moisture corrosion, The proposed treatment can be done on the finished wafer without any damage of other device materials and satisfies the die attach and wire-bonding requirements. It consists of a simple chemical dipping into an organic bath containing a phosphating agent. The passivation behavior of the resulting surfaces has been checked by means of pressure cooker test both on the wafer and power metal oxide semiconductor devices assembled in plastic packages. The surfaces resulting in each process step have been analyzed by means of electron spectroscopy for chemical analysis, The passivated surface is formed of one to two monolayers of ortho- and polyphosphate phases directly grafted onto the alumina surface, The same growth process allows fluorine surface contamination reduction.

Localized lifetime control in silicon bipolar devices is presented and discussed. It was achieved by formation of a void layer by He ion implantation. The void formation is reviewed and the void properties are described and carefully considered. Simulations demonstrate the advantages of using localized lifetime control, while the innovative method is applied to fabrication of high speed Insulated Gate Bipolar Transistors.

Double He implants at 40 and 100 keV at several combinations of doses were performed in (100) silicon to form two layers of voids after annealing. Two layers of voids were observed only for thermal treatments lower than 1100/spl deg/C, At higher temperatures a strong interaction between the two void bands is observed inducing their collapse into a single layer.