Influence of ion energy on the surface morphology of tetrahedral amorphous carbon films

Abstract

Tetrahedral amorphous carbon (ta-C) is a hydrogenfree carbon and typically contains up to 80±90% sp-bonded carbon, giving rise to remarkable properties close to those of diamond [1, 2]. One of the most exciting applications of ta-C could lie in ®eld emission displays. Our investigations [3, 4] found that ®eld emission from the ta-C ®lms was observed at an electric ®eld intensity of less than 20 V imy1, and an emission current as high as 20±40 iA was achieved. Such a low threshold ®eld provides the designer of at panel displays with an opportunity to develop a planar emitter, rather than the conventional Spindt-tip ®eld emitter with lithographic and microstructural disadvantages. For ®eld emission application, uniform emission is very important. The emission is sensitive to the surface morphology and uniformity of emitter materials. However, there has been less work published on the surface morphology of ta-C ®lms. In this letter, we report the in uence of ion energy on the surface morphologies of ta-C ®lms. The variation in the mechanism of surface morphology with ion energy is discussed. ta-C ®lms were prepared by ®ltered arc deposition (FAD). The detailed FAD system used in this work is shown in Fig. 1. The cathode used in this work was a high-purity (99.99%) graphite. For a graphite cathode, the emitted material is primarily C‡ ions with kinetic energies broadly peaked at around 22 eV [5]. The ta-C ®lms of 80 nm thickness were deposited on a p-type Si(1 0 0) wafer at different ion energies. The energy of the carbon atoms reaching the substrate can be adjusted by application of a negative bias to the substrate holder. The surface morphologies of ta-C ®lms were investigated by atomic force microscopy (AFM). Fig. 2 shows three-dimensional AFM images of FAD ta-C ®lms grown at different ion energies. The topography shown in Fig. 2a suggests that the surface of sample 1 grown at an ion energy of 22 eV is composed of columns with regular domed tops separated by a larger number of boundaries, while the surface morphology of sample 2 prepared at an ion energy of 220 eV illustrated in Fig. 2b is composed of relatively large regular domed tops. Sample 2 still exhibits a columnar structure to some extent; however, the boundaries between the columnar grain look shallower than in Fig. 2a. From Fig. 2c, it can been observed that the columnar growth in sample 3 deposited at an ion energy of 320 eV is weakened and the size of the domed top increases, resulting in a more compact surface structure of ®lm. Despite the same thickness, sample 3 has a much smoother and atter surface than samples 1 and 2 have. Here, it should be pointed out that the dominant morphology size (e.g., domed top surface of a columnar grain) is not the crystalline size of the ®lm. In the case of an amorphous ®lm there is even no crystallite. The nominal overall root-mean-square (RMS) roughness of all these ta-C ®lms as determined from the AFM image was found to be less than 2 AE , and the RMS roughness of ta-C ®lms decreases with increasing ion energy. AFM images revealed that the surface of FAD ta-C ®lms are uniform and smooth on a subnanometre scale. Furthermore, the surface morphologies of FAD taC ®lms are controlled by the ion energy, i.e., the smoothness and compactness of the surface of ®lms increase with increasing ion energy. As a plasma deposition technique, cathode arc evaporation provides an ideal method for assisted ®lm deposition in that the cathodic spot is an intense source of ionized materials with energies suf®cient for self-densi®cation when condensed onto a substrate surface. However, the principal disadvantage of this technique is the presence of micron-sized