H. Hilgers

Raman spectroscopy measurements of DC-magnetron sputtered carbon nitride (a-C:N) thin films for magnetic hard disk coatings

Abstract As a protective coating for hard disks in magnetic storage applications, amorphous carbon nitride (a-C:N) thin films have proved superior to DLC (diamond-like carbon) a-C:H films in terms of durability, wear-resistance and adhesion properties. In this study, we present Raman spectroscopy investigations of a-C:N films which were produced by DC-magnetron sputtering systems. The layers were deposited with a variable nitrogen content, thickness and substrate temperature. Raman measurements were carried out with two different excitation lasers at wavelengths of 488 and 532 nm. The spectra show that besides the typical carbon D- and G-bands, two other characteristic bands are present at approximately 690 and 1090 cm−1. The meaning and identification of these bands is not clear in the literature. In order to obtain more information, the films were also characterized by various analytical techniques, e.g. time-of-flight secondary ion mass spectrometry (ToF-SIMS), Auger electron spectroscopy (AES), ellipsometry, and n+k optical measurements. The Raman G-band position shows a systematic shift with the varying nitrogen content of the films. A comparison of layer thickness and the total area of D-, G- and 1090 cm−1 bands also shows a significant correlation. The results offer Raman spectroscopy as a possible monitoring tool for carbon nitride coatings in the production of magnetic hard disk drives.

Analytical and functional characterization of ultrathin carbon coatings for future magnetic storage devices

Abstract Ultrathin protective carbon coatings (5 nm) on top of magnetic hard disks deposited by filtered high current pulsed arc (HCA) were analyzed by various techniques. The films were compared to currently employed magnetron sputtered CN x overcoats in terms of functionality-determining film properties. X-Ray reflectivity (XRR) was used to determine film thicknesses and has been proved to be accurately applicable to carbon coatings down to ∼1 nm thickness. Time of flight-secondary ion mass spectrometry (ToF-SIMS) sputter depth profiling has proved similar carbon penetration depths into the unerlying magnetic media, both for HCA deposition and magnetron sputtering (MS), in spite of considerably higher deposition energies occurring during the HCA process. This fact suggests no negative impact on the macroscopic magnetic properties due to HCA deposition. The minimum thicknesses for full coverage of the media were evaluated by applying X-ray photoelectron spectroscopy (XPS) to the different film types. The results show a coverage limit of ∼1.0 nm for HCA coatings in contrast to ∼2.0–3.0 nm for CN x coatings indicating a significantly improved corrosion performance for overcoat thicknesses below 3 nm. Characterization with an AFM-based Nanoscratching technique suggests a more than two times increased resistivity against plastic deformation for HCA films compared to magnetron sputtered CN x overcoats.

Characterisation of amorphous carbon coatings for magnetic storage devices via AFM‐Nanoscratching devices. AFM‐Nanoscratching an amorphen Kohlenstoffschichten für die Speichertechnologie

The mechanical properties of ultra-thin amorphous carbon films used as protective coatings for magnetic storage devices were investigated by means of atomic force microscopy (AFM). Diamond-tipped cantilevers were used in order to generate scratches with residual scratch depths of only a few Angstroms and even below. The presented method simulates mechanical strains at the head-disk interface. A driftcompensating image subtracting technique allows the visualisation of these ultra-shallow scratches and enables the mechanical characterisation of only few Nanometer films widely independent from the hardness of the substrate. The scratch resistance as it is defined here correlates well with the mass density and the sp3 content of the investigated films.

Hochstrom-Filterbogen: eine neue Technik zur Abscheidung von Kohlenstoffschutzschichten in der Magnetspeichertechnologie. High Current Filtered Arc: a new Deposition Technique for Protecting Carbon Overcoats in Magnetic Storage Devices

Eine Zunahme der Speicherkapazitat auf Magnetspeicherplatten setzt eine Verkleinerung der speichernden, magnetischen Domanen voraus. Damit die dadurch schwacheren Signale dennoch gelesen werden konnen, muss gleichzeitig der Abstand der Schreib-Lese-Kopfe zur Magnetschicht verringert werden. Da zur Funktionalitat eine ausreichende Flughohe der Kopfe eingehalten werden muss, kann eine Abstandsverringerung nur durch eine zusatzliche Verringerung der Schichtdicke der schutzenden Kohlenstoffschicht erreicht werden. n n n nZur Abscheidung ultradunnen Kohlenstoffschichten auf Magnetspeicherplatten (Disk) wurde ein gepulster Hochstrom-Filterbogen (HCA High Current Arc) fur den Einsatz in der industriellen Festplattenproduktion untersucht. Die mit der HCA-Quelle erzeugten Schichten zeigen eine weitaus hohere Kratzharte und eine vielfach geringere RMS-Rauhigkeit als konventionell gesputterte Kohlenstoffschichten. Es konnte zudem gezeigt werden, dass die mit dem Hochstrom-Filterbogen abgeschiedenen, nur 2 nm dicken Kohlenstoffschichten frei von sog. pin-holes sind und einen ausreichenden Korrosionsschutz aufweisen. Die Abscheiderate der HCA-Quelle liegt zwischen 0,07 nm und 0,3 nm pro Puls, d. h. fur die Beschichtung einer Disk werden weniger als 2 Sekunden benotigt. Die empfindliche Magnetschicht wird durch dieses Verfahren nicht beschadigt. Zur Vermeidung von Makropartikeln wurde ein magnetisches Filtersystem eingesetzt und iterativ weiterentwickelt. Durch wiederholte Versuchsserien mit einem anschliesenden Funktionstest (glide yield) der Disks hat sich der magnetische Filter als sehr wirkungsvoll erwiesen. n n n nInsgesamt konnte gezeigt werden, dass die HCA-Quelle eine vielversprechende Technik zur Beschichtung von Kohlenstoffschutzschichten auf Magnetspeicherplatten darstellt. In Testreihen, die den industriellen Produktionskriterien entsprechen, wurde eine Ausbeute an funktionierenden Disks von bis uber 90 % erreicht. n n n nDue to the lateral size reduction of stored bits on a hard disk, the head-to-media spacing has to be reduced as well as the thickness of the protecting carbon overcoats. In order to obtain a thickness in the 2 nm region a new process technology is needed. n n n nIn the present paper, a high current pulsed arc (HCA) technique is presented as an innovative source for ultra thin carbon coating for future industrial disk production. The hardness and scratching resistance of these films are remarkable higher than conventional magnetron sputtered films. n n n nWith the HCA-Source we are able to produce pinhole-free carbon films with thicknesses down to 2 nm. The deposition rate is 0.07 – 0.3 nm per pulse, therefore the coating time is below 2 sec per disk. The magnetic layer is left undamaged during the HCA deposition process. These aspects are very important for industrial disk production efficiency. n n n nStrong particle reduction due to a magnetic filter tube is confirmed by repeatable glide tests. Particles are confined in the magnetic filter tube and do not reach the substrate. n n n nIn several important tests we showed that the HCA source is capable of producing carbon layers within a realistic disk production environment with a yield over 90 % which proved to be comparable to current magnetron sputtered overcoats.

Ellipsometrische Charakterisierung von C-Schutzschichten für die SpeichertechnologieEllipsometric characterization of protective carbon coatings for storage technology

Die spektroskopische Ellipsometrie ist eine schnelle, zerstorungsfreie und zuverlassige Methode zur Charakterisierung von Dunnschichten, die auf der Wechselwirkung von Licht mit einem Schichtsystem beruht. Bei den hier durchgefuhrten Untersuchungen wurde dieses Verfahren im sichtbaren Spektralbereich zur Charakterisierung von Kohlenstoff-Schutzschichten (kurz: C-Schutzschichten) von 2-7nm Dicke auf Magnetspeicherplatten eingesetzt. Der spezifische Schichtaufbau einer Magnetspeicherplatte konnte mit einem angepassten optischen Einschicht-Modell beschrieben werden. Im Hinblick auf eine genaue Analyse der bedeckenden C-Schutzschicht wurde ein zuverlassig reproduzierbares Verfahren zur Bestimmung des unterliegenden metallischen Materials entwickelt. Das verwendete Modell zeigt sich bis zu Kohlenstoff-Dicken von 2 nm geeignet, was sich in einer linearen Abhangigkeit der ellipsometrischen Parameter (ψ, Δ) von der Schichtdicke ausert. Uber Simulationsberechnungen konnten Kriterien zur Selektion von optimierten Wellenlangen zur Schichtcharakterisierung gefunden werden. Weiterhin wurde ein Anstieg des Extinktionskoeffizienten κ fur einen zunehmenden Stickstoff-Gehalt in der Schicht festgestellt. Auftretende zeitliche Instabilitaten bei der Bestimmung der Schichtdicke d und des Extinktionskoeffizienten κ der C-Schutzschicht liesen sich durch Adsorptionsprozesse an der Oberflache erklaren. n n n nSpectroscopic Ellipsometry is a fast, non-destructive and reliable method for characterizing thin films, based on interaction between incident light and a multilayer system. For our investigations, light in the visible spectral range has been used to characterize protective carbon coatings with thicknesses of 2-7nm on magnetic hard disks. The specific disk layer stack has been described with an adequate one-layer model. With regard to an accurate analysis of the covering carbon coating a reproducible procedure for determining the underlying metallic material has been developed. The measured ellipsometric parameters (ψ, Δ) display a linear dependence on carbon film thickness which shows an appropriate application of the used model down to a thickness of 2 nm. By means of simulation calculations, criteria for the selection of optimized wavelengths with respect to film characterization has been established. Furthermore, an increasing extinction coefficient κ with rising nitrogen content in the carbon coating could be stated. Apparent time instabilities in the determination of layer thickness d and extinction coefficient κ of the carbon film could be explained as due to adsorption processes on the surface.

Optical noninvasive and nondestructive sensing of the glass transition temperature of 1-um-thick polymer films

Conditions for the glass transition temperature evaluation of a 1pm thick polymer film on an aluminum substrate using laser induced surface acoustic waves propagating along the aluminum surface have been investigated.