W.M. Arnoldbik

Electronic sputtering of thin SiO2 films by MeV heavy ions

Abstract The rate of removal of material from SiO2 as a result of heavy ion irradiation, with energies in which energy loss via excitation and ionization of the solid predominates, depends strongly on the stopping power and angle of incidence of the incoming ions. There appears to be a threshold stopping power for SiO2 of 500 eV/(1015 at/cm2) (or 3.5 keV/nm). This electronic sputter yield has been found to reach values as large as 104 atoms/incoming ion for 66 MeV Ag ions at an angle of incidence of 7° with the plane of the surface. Strikingly, the electronic sputter yield is very small for thin SiO2 layers of a thickness ⩽1 nm when grown on c-Si, but it is appreciable for such layers deposited on the insulator silicon nitride. The data are discussed in the light of existing models for electronic sputtering invoking also models for potential sputtering of SiO2 by low-energy, highly charged ions.

On the ion and neutral atom bombardment of the growth surface in magnetron plasma sputter deposition

The energy distribution of positive argon ions bombarding the substrate during radiofrequency magnetron sputter deposition has been measured as a function of the argon pressure. The results are related to measurements of the plasma potential distribution and understood invoking the occurrence of resonant charge transfer reactions. This effectively lowers the ion bombardment energy and causes the bombardment of the growth surface with neutrals of a few eV kinetic energy in the pressure range of 0.1–1Pa.

Structure of sputtered silicon suboxide single- and multi-layers

The microscopic structure of silicon-rich and oxygen-rich SiOx (0<x<2) layers are very different. Generally, the Random Mixing Model (RMM) is used to describe the oxygen-rich SiOx layer structure in terms of microdomains of high- and low-oxygen content, respectively. We have studied the dimensions of spatial inhomogeneities in a-SiO2/a-Si multilayer stacks obtained by sputter deposition of Si in an Ar–O2 mixture. By using stacks of very thin layers, we have fabricated spatially inhomogenous structures as a model for the RMM. All stacks have the same total thickness (256 nm) and the thickness/layer is from 128 nm down to 2 nm. The composition and spatial inhomogeneities in the stacks were investigated by ion beam analyses techniques (Rutherford backscattering spectrometry (RBS) and high resolution RBS) and electron paramagnetic resonance (EPR). Infrared spectroscopy (IR) was used to study the local atomic structure of the samples. The EPR measurements, using different values of the microwave power, revealed two types of uncharged dangling bond defects. Their density amounts to approximately 1020 cm−3. We are able to detect spatial inhomogeneities down to 2 nm. This value is a firm upper limit for the spatial extension of domains in an RMM material.

Deuterium diffusion into plasma-deposited silicon oxynitride films

Abstract Plasma-deposited silicon oxynitride films have been annealed in the temperature range 800–1000°C in a D2/N2 gas mixture in order to study the H-D exchange reaction in the surface region of these materials and the diffusion rate of hydrogen in these materials. The hydrogen and deuterium depth profiles and the H and D bonding configurations were examined using Elastic Recoil Detection and Fourier Transform Infrared Absorption Spectroscopy. In the as-deposited materials the diffusion of hydrogen appeared to be very fast. This allowed a separate study of the hydrogen-deuterium exchange reaction. We deduced the activation energy for the reaction of nitrogen-bonded hydrogen in the material with D2-bonded deuterium from the gas phase to be 1.5 eV. In the pre-annealed oxynitrides the diffusion rate appeared to be decreased down to values of 10-12 to 10-13 cm2/s in the range 900–1000°C. Both the exchange rate and the diffusion coefficient increase with the O/(O + N) concentration ratio for O/(O + N)>0.3.

Distinct processes in radio-frequency reactive magnetron plasma sputter deposition of silicon suboxide films

A detailed investigation of the distinct processes in radio-frequency reactive magnetron plasma sputter deposition of SiOx films in a O2∕Ar atmosphere has been carried out, using the experimental evaluation of the individual growth rates of silicon and oxygen and of the ion impingement on the growth surface. Experimental variables are the total pressure, the oxygen partial pressure necessary to grow layers with 0⩽x⩽2, the RF power, the substrate temperature during deposition and the height of the cathode with respect to the growth surface. The various possible contributions to the silicon and oxygen incorporation on the growth surface have been distinguished and the magnitude of their contribution estimated, including that of sputtered SiO molecules. A model concerning the oxygen coverage on the cathode erosion area during sputtering is discussed, including the transition from the metallic cathode to the poisoned, nonmetallic, cathode.

THE ROLE OF HYDROGEN IN THE FORMATION, REACTIVITY AND STABILITY OF SILICON (OXY)NITRIDE FILMS

In the last decennium it has become clear how hydrogen and hydrogenated gases are involved in the formation of thin dielectric nitride and oxynitride films As a consequence hydrogen is incorporated in the deposited or grown films, where it plays a role in their physical, chemical and electrical reactivity and stability. Hydrogen is able to migrate in and desorb from the films via several mechanisms. These mechanisms are concisely reviewed. We consider the processes of wet oxidation and nitridation in the Si-O-N system as two manifestations of a single chemical reaction system. In this system hydrogen stabilizes intermediate reaction products, allowing multi-step reactions to proceed. Interruption of the process or, more specifically, isolation of the intermediate species from the reactants results in incorporation of these hydrogenated intermediates in the material. It appears that the reactivity of oxynitrides strongly increases for increasing 0/N concentration ratio of the material. The use of isotope-sensitive high-energy ion beam methods is emphasized.

Hot-wire chemical vapor deposition of silicon nitride for multicrystalline silicon solar cells

A new regime of high rate deposition of silicon nitride by hot wire chemical vapor deposition was investigated. The present design of the filament arrangement and the showerhead gas supply system allows for virtually unlimited scale-up. The deposition rates obtained were in excess of 5 nm/s. The refractive index (n at 2 eV; wavelength /spl sim/630 nm) could be controlled from 1.90 /spl plusmn/ 0.05 to 2.5 /spl plusmn/ 0.05, by varying the SiH/sub 4/ flow and the extinction coefficient (k) at 3.1 eV (wavelength 400 nm) was < 0.007 for all films of interest. The layers were tested on multicrystalline silicon solar cells in order to assess their passivation and antireflection properties. The cells had state-of-the-art values for all photovoltaic parameters, similar to cells with a microwave plasma deposited SiN/sub x/ anti-reflection coating. An efficiency of 14.3% was reached using HWCVD-SiN/sub x/ for multi-crystalline Si solar cells with an industrial process using screen printing.

Modeling of the Band Gap Profile in the Intrinsic Layer of the a-SiGe:H Material: Application in Solar Cells

A new band gap profile (exponential profile) for the active layer of the a-SiGe:H single junction solar cell deposited by plasma enhanced CVD (PECVD) has been designed and experimentally demonstrated. In this work we study the difference between the experimental results and the computer simulation. Using Rutherford Backscattering Spectrometry (RBS) significant differences were found between the composition profile as expected from variations in the gas phase ratio GeH 4 /SiH 4 during the deposition and the actual Ge/Si depth profile as determined by RBS. This has important implications for the design of bandgap-graded a-SiGe:H cells. Among others, a delayed response of the plasma conditions due to the changes in the GeH 4 flow is studied.