Werner Schustereder

Tailoring of field-stop layers in power devices by hydrogen-related donor formation

Hydrogen-related donors can be formed by using only a moderate thermal budget, so that this process can be used to create field-stop layers in thin power devices. The electrical characteristics of 1200 V IGBTs and diodes provided with such field-stop layers are presented and compared with the characteristics of conventionally processed devices. Moreover, tailoring the field-stop distribution by multi-energy proton implantations offers new opportunities for optimizing the performance of power devices.

Use of 300 mm magnetic Czochralski wafers for the fabrication of IGBTs

As in other semiconductor industries, there is a strong trend to use larger wafer diameters for the fabrication of power devices. However, for wafer diameters above 200 mm float-zone (FZ) silicon which is traditionally used for IGBTs is not available. Therefore, there is a need to use silicon material which has been fabricated by the magnetic Czochralski (Cz) method to make use of 300 mm wafers for IGBT-production. As this material contains a relatively high concentration of oxygen, the influence of carbon/oxygen-complexes has to be taken into account. CIOI-complexes can be decorated with hydrogen atoms resulting in donor-like complexes. Particularly, the application of proton-irradiation for the doping of the field-stop zone results in a relatively high concentration of interstitial carbon which is continuatively associated with the generation of undesired donors.

An electrical and physical study of crystal damage in high-dose Al- and N-implanted 4H-SiC

In this paper, an investigation into the crystal structure of Al-and N-implanted 4H-SiC is presented, encompassing a range of physical and electrical analysis techniques, with the aim of better understanding the material properties after high-dose implantation and activation annealing. Scanning spreading resistance microscopy showed that the use of high temperature implantation yields more uniform resistivity profiles in the implanted layer; this correlates with KOH defect decoration and TEM observations, which show that the crystal damage is much more severe in room temperature implanted samples, regardless of anneal temperature. Finally, stress determination by means of μRaman spectroscopy showed that the high temperature implantation results in lower tensile stress in the implanted layers with respect to the room temperature implantation samples.

Conversion Efficiency of Radiation Damage Profiles into Hydrogen-Related Donor Profiles

By introducing radiation damage and hydrogen and successively annealing with low thermal budgets, hydrogen-related donors are created in oxygen-lean silicon. Hydrogen-related donor profiles are induced in float-zone silicon by implanting hydrogen and/or helium and successive annealing with or without additional hydrogen introduction by a hydrogen plasma. The efficiency of the conversion of the radiation-induced damage into the hydrogen-related donors differs in dependence of the method of damage and hydrogen introduction. In proton implanted samples, the ultimate introduction rate of the donors is significantly lower than it is in helium and hydrogen co-implanted samples. Furthermore, the depth distribution of the hydrogen-related donors shows a deviance from the simulated distribution of the radiation damage induced by proton implantation not seen in case of helium-induced damage. The change in doping efficiency is discussed in respect to the hydrogen content in the different experiments.

Hydrogen decoration of radiation damage induced defect structures

The defect complexes that are formed when protons with energies in the MeV-range were implanted into high-purity silicon were investigated. After implantation, the samples were annealed at 400 °C or 450 °C for times ranging between 15 minutes and 30 hours. The resistivity of the samples was then analyzed by Spreading Resistance Profiling (SRP). The resistivity shows minima where there is a high carrier concentration and it is possible to extract the carrier concentration from the resistivity data. Initially, there is a large peak in the carrier concentration at the implantation depth where most of the hydrogen is concentrated. For longer anneals, this peak widens as the hydrogen diffuses away from the implantation depth. Following the changes in resistivity as a function of annealing time allows us to characterize the diffusion of hydrogen through these samples. Differences in the diffusion were observed depending on whether the silicon was grown by the magnetic Czochralski (m:Cz) method or the Float zone...

Ion implantation challenges for power devices

Ion Implantation processes contribute significantly to the development of power devices. In this case not smallest scale technologies are addressed, but accurate treatment of the frontside, backside and bulk material play a crucial role to guarantee for key parameters such as highest power densities and required switching behavior. Some representative examples of the development needs of high, medium and low power switching devices based on silicon technology are described and challenges for ion implantation processing are concluded.

Radiation protection for high energy implantation of light ions in a production environment

In recent years the irradiation of semiconductor devices by light ions with high energies has increasingly become a well-established method to adjust charge carrier lifetimes in semiconductor devices. To introduce this process into the end of frontend production environment several measures in terms of radiation safety will have to be obeyed. An overview on thresholds for particles induced gamma emission will be given, starting with the 11B(p,γ)12C reaction at 163keV. The environment has to be shielded from gamma rays, therefore those implanters have to be situated in adequate enclosures. Calculations for specific implant energies are done to define the thicknesses of tool enclosures. A set of radiation detectors monitors the values at well-defined positions online and radiation values are fed into a superordinate system that is directly coupled to the main power supply of the implanter.

Alternative Highly Homogenous Drift Layer Doping for 650 V SiC Devices

A new method for homogenous drift layer doping is introduced. Instead of in-situ doping during epitaxial growth a subsequent high energy ion implant step is used to dope the drift layer of 650V MPS (Merged-PN-Schottky) diodes. In order to avoid multiple implant steps with various energies for emulating a box-like doping profile, a novel “energy filter” membrane is used to transform the monochromatic ion beam to a beam with a continuous energy spectrum suited for box-like doping. The electrical characteristics of the diodes manufactured by this means show a very homogenous blocking behavior on wafer level, however the expected improved homogeneity in differential resistance of the wafers could not be confirmed by wafer level measurements. More work is needed to understand this discrepancy between experiment and theory.

A Novel Method for Simultaneous on Wafer Level Monitoring of Ion Implantation Energy and Dose

Semiconductor technology needs the control of processes on wafer level. For ion implanation control there are two well established methods, namely the photothermal response techniques on as-implanted product or monitor wafers and the determination of resistivity via sheet resistance measurement on as-implanted and annealed wafers. In fact, both methods are sensitive to implant dose but rather insensitive to implant energy. In particular the established methods are not capable to distinguish between effects of dose and energy. Especially in a high energy regime above 1 MeV it is of high interest to verify the implant energy on wafer level. This is in principle achievable by spreading resistance profiling (SRP) or secondary ion mass spectroscopy (SIMS) with the disadvantage of long feedback times, expensive processing and limited ability of resolution. It was shown that the wide frequency sweep used in the photothermal heterodyne equipment TWIN can be sensitive to the depth of damage profiles. In this study we present a developed energy and dose sensitive method based on a specially designed tool TWIN-SC4 produced by PVA Metrology&Plasma Solutions GmbH. Making use of the unique ability to vary the modulation frequency of the excitation laser enables inspection of different regions of the respective implant damage profiles. Consequently, we propose a method, combining measurements with two modulation frequencies and being sensitive to energy and dose which requires only a single as-implanted wafer.

The Efficiency of Hydrogen-Doping as a Function of Implantation Temperature

– For a conventional proton implantation doping process applied to crystalline silicon comprising proton implantation and subsequent furnace annealing the effect of the substrate temperature set during implantation is examined for temperatures between 50 °C and 200 °C. The formation efficiency of the hydrogen related donors in the maximum of the related doping profiles is shown to linearly increase with the implantation temperature. Regarding the dose rate, a reverted effect is found. The appearing effects are explained by considering the evolution of the initial implantation damage towards a vacancy related precursor species of the hydrogen related donor. Additional information about the implantation temperature dependent defect distribution is gained from Fourier-DLTS results.