The Use of Spark Ablation to Produce Calcium Phosphate Films on Silicon

Abstract

A method for fabrication of calcium phosphate films on crystalline silicon substrates which employs a high energy dc spark on silicon surfaces in the presence of Ca 10 ͑PO 4 ͒ 6 ͑OH͒ 2 is described. Such a process kinetically traps the desired calcium phosphate on the substrate with concomitant formation of a porous layer. Scanning electron micrographs of the silicon substrate show a nanoporous surface morphology for those areas exposed to spark processing. Energy dispersive X-ray analysis and infrared vibrational spectroscopy of the spark-processed regions of the Si substrate are clearly consistent with the presence of the desired biologically relevant material. In conjunction with an automated stage, facile formation of deliberate micrometer-size patterns is demonstrated.While long considered as the paradigm of semiconductor microelectronics, crystalline silicon ͑Si͒ has, with few exceptions, received minimal attention for its potential as a biomaterial. 1-4 Recent efforts by Canham and co-workers have clearly demonstrated that one approach to induce bioactivity in Si is to electrochemically form nanoporous Si films followed by the cathodic growth of calcium phosphate ͑the common inorganic mineral phase of bone͒. 5-7 Such materials are attracting attention as possible drug delivery vehicles 8,9 and a host of other applications. 10 As an alternative to the anodic etch process, previous studies have shown that the high-frequency arc from a Tesla coil can ablate the surface of crystalline silicon, producing nanoporous surfaces. [11][12][13] It has also been subsequently demonstrated that if a rare earth salt is deposited on the silicon surface and then spark processed, the resulting Si porous layer is doped with ''clusters'' of rare earth ions coordinated to oxygen. 14,15 These latter studies suggest that it should be possible to kinetically trap a variety of useful, thermally stable materials as the ablated Si coalesces onto the surface. Thus, this paper describes the use of a spark ablation process to produce calcium phosphate films anchored to Si. These materials are characterized via scanning electron microscopy ͑SEM͒, energy dispersive X-ray analysis ͑XEDS͒, and infrared vibrational spectroscopy ͑FTIR͒. These results confirm that the porous spark-processed regions of the substrate are clearly consistent with the presence of the desired biologically relevant material. The impact of electrode gap and spark duration, parameters that most sensitively determine film thickness and composition, are also discussed.Square pieces (15 ϫ 15 mm) of p-type, ͗100͘, boron-doped Czochralski ͑CZ͒ Si ͑6-8 ⍀ cm͒ were employed in these experiments. Electrical contact was achieved using Ni wire and silver epoxy. An excess of Ca 10 ͑PO 4 ͒ 6 ͑OH͒ 2 ͑Aldrich͒ is deposited on the wafer surface by either a slurry in acetone ͑or water͒ or alternatively by direct application of the dry calcium phosphate powder onto the surface with compression by a glass rod. The sample was allowed to dry, producing a layer of calcium phosphate covering...

Silicon Porosification: State of the Art

“…At that the surface is very rough and the surface morphology consists of "cauliflower" shaped clusters with large holes regularly penetrating the Si surface. 287 Pulse frequency and applied voltage are only two of the many processing parameters that influence the spark-processing time and the PL intensity. Other processing parameters include either the environment in which the sparking is conducted or the wafer temperature.…”

Routes to calcified porous silicon: implications for drug delivery and biosensing

“…Previous investigations from our laboratories established that the major factors controlling film growth in this process are the spark electrode gap distance and duration of the ablation event [10]. Films ranging in 10 2 nm to ~10 µm were initially reported [10]. By the repetitive ablation and re-application of calcium phosphate slurry to the silicon surface, we find that it is now possible to form films as thick as 20 µm by this technique.…”

“…Previous investigations from our laboratories established that the major factors controlling film growth in this process are the spark electrode gap distance and duration of the ablation event [10]. Films ranging in 10 2 nm to ~10 µm were initially reported [10].…”

“…As an alternative to the anodic etch process, we have recently reported that the high-frequency arc from a Tesla coil can ablate the surface of crystalline silicon in the presence of Ca 10 (PO 4 ) 6 (OH) 2 , producing nanoporous surfaces containing calcium phosphate [10]. Here we describe the diversity of this process in fabricating deliberate patterns of this material on crystalline Si.…”

Luminescent Properties of Porous Silicon

“…The resulting product is a grayish layer (see Figure 10.24). At that, the surface is very rough and the surface morphology consists of "cauliflower" shaped clusters with large holes regularly penetrating the Si surface (Weis et al 2002).…”