Diamond-like carbon: state of the art

High power pulsed magnetron sputtering (HPPMS) is an emerging technology that has gained substantial interest among academics and industrials alike. HPPMS, also known as HIPIMS (high power impulse ...

High power pulsed magnetron sputtering: A review on scientific and engineering state of the art

Modern computer microprocessor chips are marvels of engineering complexity. For the current 45 nm technology node, there may be nearly a billion transistors on a chip barely 1 cm2 and more than 10 000 m of wiring connecting and powering these devices distributed over 9-10 wiring levels. This represents quite an advance from the first INTEL 4004B microprocessor chip introduced in 1971 with 10 μm minimum dimensions and 2 300 transistors on the chip! It has been disclosed that advanced microprocessor chips at the 32 nm node will have more than 2 billion transistors.1 For instance, Figure 1 shows a sectional 3D image of a 90 nm IBM microprocessor, containing several hundred million integrated devices and 10 levels of interconnect wiring, designated as the back-end-of-the-line (BEOL). Since the invention of microprocessors, the number of active devices on a chip has been exponentially increasing, approximately doubling every two years. This trend was first described in 1965 by Gordon Moore,2 although the original discussion suggested doubling the number of devices every year, and the phenomenon became popularly known as Moore’s Law. This progress has proven remarkably resilient and has persisted for more than 50 years. The enabler that has permitted these advances is known as scaling, that is, the reduction of minimum device dimensions by lithographic advances (photoresists, tooling, and process integration optimization) by ∼30% for each device generation.3 It allowed more active devices to be incorporated in a given area and improved the operating characteristics of the individual transistors. It should be emphasized that the earlier improvements in chip performance were achieved with very few changes in the materials used in the construction of the chips themselves. The increase of performance with scaling * Corresponding author. E-mail: gdubois@us.ibm.com. † IBM Almaden Research Center. ‡ Stanford University. Willi Volksen received his B.S. in Chemistry (magna cum laude) from New Mexico Institute of Mining and Technology in 1972 and his Ph.D. in Chemistry/Polymer Science from the University of Massachusetts, Lowell, in 1975. He then joined the research group of Prof. Harry Gray/Dr. Alan Rembaum at the California Institute of Technology as a postdoctoral fellow and upon completion of the one-year appointment joined Dr. Rembaum at the Jet Propulsion Laboratory as a Senior Chemist in 1976. In 1977 Dr. Volksen joined the IBM Research Division at the IBM Almaden Research Center in San Jose, CA, where he is an active research staff member in the Advanced Materials Group of the Science and Technology function.

Low dielectric constant materials.

Diamond-like carbon (DLC) films combine several excellent properties like high hardness, low friction coefficients and chemical inertness. The DLC coating material can be further classified in two main groups, the hydrogenated amorphous carbon (a-C:H, ta-C:H) and the hydrogen free amorphous carbon (a-C, ta-C). By adding other elements like metals (a-C:H:Me) or non-metal elements like silicon, oxygen, fluorine or others (a-C:H:X), several modifications of the properties can be adjusted according to application requirements. First reports on hard amorphous carbon films were published in the 1950s and about 20 years later there began worldwide intensive research activities on DLC. In the following years the number of publications increased continuously and the importance for industrial applications became more and more evident. Several deposition techniques were applied to prepare a-C:H, ta-C, metal containing a-C:H:Me and non-metal containing a-C:H:X coatings. In parallel the structure and deposition mechanisms of DLC coatings were extensively studied. An essential obstacle for a broad industrial application was the high compressive stress level in a-C:H films causing delamination and limiting the film thicknesses. With metal based intermediate layer systems most adhesion problems could be solved satisfactorily and thus from the mid-1990s the pre-conditions for a broad application especially in the automotive industry were given. With modified a-C:H:X and a-C:X coatings a considerable friction reduction or surface energy adjustments could be achieved.

History of diamond-like carbon films — From first experiments to worldwide applications

A comprehensive study of the stress release and structural changes caused by postdeposition thermal annealing of tetrahedral amorphous carbon (ta-C) on Si has been carried out. Complete stress relief occurs at 600–700 °C and is accompanied by minimal structural modifications, as indicated by electron energy loss spectroscopy, Raman spectroscopy, and optical gap measurements. Further annealing in vacuum converts sp3 sites to sp2 with a drastic change occurring after 1100 °C. The field emitting behavior is substantially retained up to the complete stress relief, confirming that ta-C is a robust emitting material.

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Stress reduction and bond stability during thermal annealing of tetrahedral amorphous carbon

Determination of elastic modulus and thickness of surface layers by ultrasonic surface waves

The deposition of superhard amorphous carbon films demands high energies of all impinging particles for film formation by subplantation instead of condensation. Such conditions may be realized by vacuum arc discharge. By using pulsed arc currents and laser controlled ignition (Laser-Arc) the usual problems in arc ablation of carbon targets have been overcome. In this way, smooth and very hard films with hardness in the superhard range between 40 and 80 GPa have been prepared with high deposition rates comparable to conventional industrial arc deposition. Due to the high ion energies, low deposition temperatures are possible without reducing the adherence. They are even necessary for the formation of the diamond bonds by avoiding relaxation towards a graphitic structure. Hence, materials besides metals such as steels or aluminum, temperature-sensitive materials such as polymers have been successfully coated with these hard layers.

Deposition of superhard amorphous carbon films by pulsed vacuum arc deposition

Evaluating ultra-thin films of a few nanometers thickness is still a challenge for mechanical testing. The capability of the laser-acoustic technique was investigated to measure the Young’s modulus of diamond-like carbon films with thickness down to 5 nm. The elastic modulus has proved to characterize an important mechanical property of hard films which are used for wear protection. It can non-destructively be determined by the laser-acoustic technique that is based on measuring the dispersion of surface acoustic waves. This acoustic wave mode is very sensitive to thin films which was previously shown for diamond-like carbon films with thickness down to 60 nm. Diamond-like carbon films with thickness from 5 to 30 nm were deposited by the pulsed arc technique on pure silicon and on silicon coated with a metal layer of 80 nm thickness. The demand for the accuracy of the measuring instruments and for the measuring conditions, such bandwidth and sample dimension, is discussed for testing such ultra-thin films.

Testing ultra-thin films by laser-acoustics

Carbon films are distinguished by their broad structural variability and a correspondingly wide range of properties varying from diamond-like to graphite-like. This variability is determined by the relation of diamond bonds (sp 3 ) to graphitic bonds (sp 2 ) and can be influenced by addition of other elements. The highest degree of diamond-likeness, e.g. expressed by high hardness, is achieved in pure carbon films. Such super hard films, so-called tetrahedral bonded amorphous carbon films (ta-C), reach a hardness from 40 GPa up to 80 GPa and a Young's modulus from 400 GPa up to 800 GPa. By incorporation of additional elements, not only the mechanical properties but also the surface energy can be varied. PTFE-like properties can be obtained by addition of fluorine or other elements. At Fraunhofer IWS Dresden such doped amorphous carbon films (a-C:X) have been deposited by a special pulsed vacuum-arc technique, the laser-induced pulsed arc (Laser-Arc). The deposition process is characterized by producing a high activated carbon plasma from a pure or doped graphite cathode and its deposition on substrates under vacuum or reactive conditions. The mechanical properties of the doped a-C:X-films were investigated in dependence on the content of the doped elements. Furthermore, the surface energy was determined by means of contact angle measurements. To get ultra hydrophobic properties of a surface additionally to a low surface energy a defined structuring of the surface topography is necessary. Different kinds of such structuring methods and their effects in conjunction with a doped a-C:X-film on the super hydrophobic behaviour are shown.

Ultra hydrophobic wetting behaviour of amorphous carbon films

http://materials.web.psi.ch/Publications/Publ_MatDev_files/2007/DLC_Stefan_2007.pdf

Bernd Schultrich

Papers

Determination of elastic modulus and thickness of surface layers by ultrasonic surface waves

Deposition of superhard amorphous carbon films by pulsed vacuum arc deposition

Testing ultra-thin films by laser-acoustics

Ultra hydrophobic wetting behaviour of amorphous carbon films

Structural characterization of diamond-like carbon films for ultracold neutron applications