Horst Neumann

Sputter process diagnostics by negative ions

We measured the energy distributions of negative ions during reactive sputtering of silicon in oxygen. Various oxygen containing negative ions are formed in the cathode sheath or directly at the sputter target, respectively. These negative ions are accelerated away from the cathode by the electrical field, and can be detected using a mass spectrometer facing the sputter magnetron. The origin of each ion can be determined from peak structures in the energy distribution. Additionally the flux of different negative ions provides information on poisoning of the target by oxide films.

DESIGN OF ION ENERGY DISTRIBUTIONS BY A BROAD BEAM ION SOURCE

We characterize the performance of a built‐in hot filament broad beam ion source by mass spectrometry, energy analysis, and beam profile measurements. In the ion energy distribution we detect various peak structures which can be explained by the potential across the ion source and different charge transfer processes. Depending on the typical cross sections for these processes, differences between the energy distributions of the ion species are observed. The total ion current obtained with the source is determined by the ionization rate in the discharge and the current share directed toward the extraction grid system. The performance of the source is strongly dependent on the process gas used. We observe much broader energy distributions in oxygen and nitrogen than in argon. This broadening is explained by spatial inhomogeneities in the discharge region and can be reduced by a suitable setting of the source parameters. The main contribution to the ion flux is caused by species generated directly from the p...

Interstitial nitrogen induced by low-energy ion beam nitridation of AIII–BV semiconductor surfaces

The nitridation of GaAs, InAs, and InSb by low-energy N2+ ion bombardment at room temperature was studied by near-edge x-ray absorption fine structure (NEXAFS) and x-ray photoelectron spectroscopy measurements. The formation of thin surface nitride layers, consisting mostly of GaN or InN but also containing minor amounts of mixed nitrides, was observed. Besides the nitride-related features, sharp peaks in the NEXAFS due to π* resonance at 401.0 eV and correlated peaks at 403.8 eV in N 1s core level spectra were detected. Both spectral features could be assigned to the presence of interstitial nitrogen, most likely molecular nitrogen. It was found that the amount of interstitial nitrogen in the surface layer strongly depends on the AIII–BV semiconductor system and may be affected by modification of the conditions during low energy ion bombardment.

Ion energy distributions in a dc biased rf discharge

We measured quasisimultaneously the energy distributions of positive ions at the powered rf and grounded electrode of a parallel plate 13.56 MHz discharge using an energy selective mass spectrometer. The resulting ion energy distributions reflect the discharge potential conditions expected from a capacitive plasma sheath model. By means of an externally supplied dc bias of the powered electrode we are able to influence the potential structure and to control ion energy and ion flux independently. The ratio between mean ion energy and mean sheath thickness reflects the effect of collisions on the ion energy distributions and enables estimates of sheath thickness and bulk plasma parameters to be made which are compared with values obtained by Langmuir probe measurements. We are able to demonstrate that changes in sheath potential also affect, via secondary electrons, the ionization regime in the discharge and this can be utilized to control the species composition in the discharge.

Optimisation and characterisation of a TCP type RF broad beam ion source

A very compact type of broad beam ion source based on transformer coupled plasma excitation (TCP) is described. Our ion source consists of a cylindrical RF coil surrounding the discharge vessel and a very compact, patented matching unit from special ceramics capacitors attached directly to the discharge arrangement. However, the TCP excitation as well as the sophisticated layout of the matching unit require an optimum source arrangement due to the beam parameters, the source lifetime and the performance of the RF elements. For that reason, a global discharge model was applied together with an RF replacement circuit diagram to calculate for the optimum source layout. An advanced plasma and beam diagnostics is used for controlling the source performance due to beam composition, beam profile and ion energy distribution. In this way, our RF source is adapted to different beam requirements in inert and reactive beam processes for etching, modification and sputter deposition.

A unique ECR broad beam source for thin film processing

We present the special microwave excited ECR (electron cyclotron resonance) type broad beam ion source EC/A 125 together with first results in ion beam deposition. Our source concept overcomes different disadvantages of common broad beam ion sources. By means of a modular source design and an autotuning microwave power supply, an adaptation at different process requirements is possible. The efficiency of the source is demonstrated analysing the performance in inert and reactive environment. We analyse and discuss the resulting beam composition and draw important conclusions on the plasma-chemical processes occurring in the source from the measurement of the ion energy distributions. The source is operated at the CYBERITE ion beam equipment and results in deposition of magneto-resistive films and multilayer films are illustrated.

Examples for application and diagnostics in plasma?powder interaction

Low-pressure plasmas offer a unique possibility of confinement, control and fine tailoring of particle properties. Hence, dusty plasmas have grown into a vast field and new applications of plasma-processed dust particles are emerging. There is demand for particles with special properties and for particle-seeded composite materials. For example, the stability of luminophore particles could be improved by coating with protective Al2O3 films which are deposited by a PECVD process using a metal-organic precursor gas. Alternatively, the interaction between plasma and injected micro-disperse powder particles can also be used as a diagnostic tool for the study of plasma surface processes. Two examples will be provided: the interaction of micro-sized (SiO2) grains confined in a radiofrequency plasma with an external ion beam as well as the effect of a dc-magnetron discharge on confined particles during deposition have been investigated.

Characterization of a modular broad beam ion source

We characterize the performance of a modular broad beam ion source by energy resolved mass spectrometry and beam profile measurements. Using the same source housing and grid system, we performed our experiments powering the source with a hot filament or an ECR excitation, respectively. In the ion energy distribution we detect various peak structures reflecting the potential profile across the ion source as well as different charge exchange and dissociation processes occurring in the beam. The position of these peaks on the energy scale allows conclusions to be formed about the original charge state of the ion, when produced by charge exchange, or on the original molecule, when produced by dissociation. The most important contribution to the ion beam is caused by process gas ions with important impurities from source materials appearing with reactive process gases. Using electronegative gases, a large amount of negative ions generated in the beam is observed. The performance of both excitation types due to the beam parameters is very similar. A comparable power supplied to the source plasma delivers similar beam currents and profiles. The resulting beam current is determined by the density and the potential structure in the source plasma. The axial potential gradient in the plasma, and with it the resulting beam current, is strongly affected by the total voltage applied between the grids and can be derived from the shape of the main peak in the ion energy distribution. The detected radial beam profile is determined simultaneously by the ion optical parameters of the grid system and the space charge potential in the beam. For a fixed grid geometry the shape of the plasma sheath at the screen grid is an important factor controlling the source performance. The shape of the plasma sheath is simultaneously controlled by the voltage difference between the grids and the Debye length. Changes in the shape can be derived from the ion energy distribution with the experimental results being in good agreement with numerical beam simulations. The only important difference between both excitation types is that using the Kaufman type excitation the potential of the source plasma is exactly fixed by the beam potential, whereas with the ECR source it deviates by several tens of volts depending on microwave power and process gas.

Inductive coupled radio frequency plasma bridge neutralizer

A 13.56 MHz radio frequency plasma bridge neutralizer (rf-PBN) for ion thruster applications as well as ion beam surface processing of insulating materials is presented. The energy for the plasma excitation is inductively coupled into the plasma chamber. Because no components are located inside the plasma, the lifetime of the rf-PBN is expected to be very long. A compact tuning system adapts the input power to the plasma impedance. The electron current may be controlled over a wide range by the rf input power. An electron current of up to 1.6 A has been extracted.

Influence of Microstructure on Nitriding Properties of Stainless Steel

Very hard and wear-resistant layers are formed after energetic nitrogen insertion into stainless steel. Here, a systematic investigation of the influence of the microstructure is presented. Nitrogen implantation was performed in austenitic, martensitic, and ferritic steels with the samples investigated with respect to formation of expanded phase, nitrogen depth distribution, hardness, and wear. Microstructure strongly affects the diffusion in austenite and has negligible effect in the case of martensitic/ferritic transformations