In-situ Surface Texturing of Conductive Polymer Composite Substrates for Field-emission Applications

Abstract

Amorphous carbon films have been identified as apotential large-area cold-cathode material for appli-cations requiring intense electron field emission [1].These not only include relatively complex flat-paneldisplays [2] but also traffic signals, dot-matrixinformation signs, liquid crystal display (LCD)back-lighting and vehicle lights.For these applications to become reality, field-emission systems will need to be inexpensive toproduce, mechanically robust and reliable to operate.Therefore, although silicon wafers are presently usedas substrates for testing and developing cathodes inthe laboratory, they are unlikely to be the best choiceof material for use as inexpensive substrates in thefield.The use of very smooth substrates, such ascrystalline silicon, can also give rise to unexpectedresults, due to small perturbations focusing theelectric field and creating localized ‘‘hot-spots’’ ofemission. This is often detrimental to the perform-ance of the electron field-emission system as awhole. Recent work has indicated that someamorphous nitrogenated carbon films self-textureduring deposition and that this may be beneficialfor their field-emitting properties [3].In this letter, we report the use of a conductivepolymer composite (CPC) [4] substrate for amor-phous carbon films designed for use in field-emis-sion systems. CPCs, in which a polymer matrix isloaded with a sufficient quantity of carbon black [5]so that the percolation threshold is exceeded, alreadyhave widespread applications in the electronicsindustry. These include moldable component hous-ings to provide electrostatic shielding [6] and activedevices such as resettable fuses [7] and self-regulating heaters [8]. Therefore, they are alreadyestablished as an electronic material and have theadded advantage of mechanical stability, goodprocessability and being relatively inexpensive tomanufacture. In addition, the use of a carbon-basedsystem for the substrate (polymer), the conductivemedium (carbon black) and the cathode (amorphouscarbon film) helps to maintain a degree of materialcompatibility between the different components.More importantly, we also show that CPCs can betextured by plasma etching and that this processingstep can be easily integrated with the depositionstage of the cathode.Hydrogenated amorphous carbon films containingnitrogen (a–C:H:N) have been deposited using aPlasma Technology DP800 radio frequency plasmaenhanced chemical vapor deposition (rf-PECVD)system. The DP800 was configured with an upper-driven electrode and a similar-sized lower-earthedsubstrate table ,400 mm in diameter. Therefore, it isideally suited for scaling-up to deposit films over alarge area. We have considered substrates attached toboth electrodes, enabling comparisons to be madewith research where denser films have been depos-ited under the higher self-bias of the drivenelectrode. During this study, films were depositedon both CPC and 1–2 Ucm (100) phosphorous-doped n-type silicon for comparison, and on Corning7059 glass for optical characterization.During all of the processing stages, the plasmapower was 200 W and the chamber and substratetable were water-cooled so that their temperatureremained ,30 8C. Substrates were pre-cleaned for210 s with a helium gas flow rate of 75 sccm and aprocess pressure of 175 mTorr. Alternatively, someof the CPC sampels were plasma etched for 300, 600or 900 s, using an oxygen gas flow rate of ,10 sccmand a process pressure of ,200 mTorr. The deposi-tion of a–C:H:N onto the substrates was performedin-situ directly after the cleaning or etching stagewithout the substrate being exposed to the atmo-sphere. The deposition was for 300 s using gas flowratios of 30:10:75 sccm of methane, nitrogen andhelium, respectively. The process pressure wasmaintained at 175 mTorr.A Hitachi 3200 N scanning electron microscope(SEM) was used to investigate the surface morph-ology of a gold-coated sample of each of the plasma-etched CPC substrates after film deposition. Fig. 1shows a series of these SEM images. The oxygenplasma preferentially etched the polymer matrix, sothat the initially flat and featureless surface (a) beganto reveal the carbon black primary particles (b) after300 s treatment on the lower-earthed electrode. Thisled to regular surface texturing by the exposedprimary particles after a treatment for a total of600 s (c) and very pronounced exposure of theaggregates after 900 s (d). On the driven electrode,the plasma etching was more severe so that after just300 s most of the primary carbon black particleswere exposed (e). Parts (f) and (g) indicate thedegree of etching achieved after 600 and 900 s,respectively. Close analysis of the morphology of thecarbon black particles in (d) and (g) reveal that theyhad also been attacked by the plasma. A schematic