',$021'0,&526758&785(5(3/,&$6)520 6,/,&210$67(56 H. Björkman, P. Rangsten, U. Simu, J. Karlsson, P. Hollman, and K. Hjort Department of Materials Science, Uppsala University Box 534, S-751 21 Uppsala, Sweden.
[email protected] $%675$&7 We are developing a microstructure technology for thick film diamond replicas, using deposition by hot filament CVD on micro structured silicon. This technology is primarily intended to make micromechanical structures for building-sets, fluidic cooling systems, systems for biochemical analyses and processes, and moulds for thermoplastic microstructures. In the thick film deposition on trenches in silicon, complete filling was possible with an aspect ratio up to 1.6. At higher aspect ratios voids or channels are formed within the diamond replica. Ridges, trenches and capillary channels with high resolution coverage and low roughness, rms < 2 nm, were created. Demonstrator structures for microfluidic, building-sets and polymer moulding applications are presented ,1752'8&7,21 'LDPRQG0LFUR6WUXFWXUHV Diamond is one of the most interesting materials under consideration for micro-electromechanical systems (MEMS) applications [1, 2]. As it consists of group IV atoms and with its wide band gap it is possible to make it both semiconducting or insulating. It is the hardest and stiffest material we know. Single crystalline diamond is thought to be the strongest of all materials, with measured tensile strengths above 25 GPa. It has the widest electromagnetic radiation transparency range of all materials known ranging from UV to far IR, and it is therefore frequently used as windows for, e.g., X-ray detectors. Also, diamond shows the highest thermal conductivity of all known materials, about 5 times higher than copper. Combined with the possible insulating property this makes it an ideal heat sink for high effect semiconductor devices. Additionally, diamond has a very low friction coefficient, and is extremely high wear resistant and has a fracture strength of 1.8 GPa for CVD polycrystalline miniature beams [3, 4]. The chemical inertness gives substantial benefits in microfluidic applications and when exposed to reactive gases. Today much research is made on thin film synthetic
polycrystalline diamond. The applications are tribological, heat conducting and optical as well as in the areas of MEMS and microstructure technology. Examples of microstructures are high efficiency electron emission microtips [5], diamond cantilevers with tips for SPM applications [6], a diamond motor structure [7], diamond gears [8], and diamond burrs [2]. Diamond is very difficult to micromachine due to its hardness and chemical inertness. Therefore it is preferable to make use of diamond replication. The well-developed area of silicon micromachining makes it possible to fabricate complicated structures that can be used as moulds for diamond. Kang HW DO have made microtips using microwave plasma CVD deposition of polycrystalline diamond film in arrays of pyramidal silicon cavity moulds, and diamond diaphragms were produced by subsequent etching of the silicon [5]. Integrated diamond cantilevers with tips for SPM applications have been fabricated in a similar way [6]. Other substrate materials such as polycrystalline Mo and Cu have also been tested with good results [9, 10]. +RWILODPHQW&9'GLDPRQGV\QWKHVLV The first documented growth of diamond at low pressures was reported in the mid-twentieth century [1]. The development of the chemical vapor deposition (CVD) synthesis technique for diamond makes it possible to transfer the properties of diamond to large areas and complicated shapes and forms which are not available from nature or from the high pressure high temperature diamond synthesis method. Frequently used CVD synthesis techniques for diamond film deposition are hot (combustion) flame, hot filament CVD (HFCVD), and microwave plasma CVD. HFCVD is considered as a suitable process for industrial scale-up [2]. In HFCVD diamond films are deposited on a substrate from a mixture of hydrocarbon and hydrogen activated by a filament placed close to the substrate. The carbon source is often methane, CH4, but other hydrocarbons can be used, such as C2H2 or C3H8. With methane as
precursor the diamond layers grow through interaction with the reagent CH3, which is formed as a result from the reaction of atomic hydrogen with CH4 molecules in the vicinity of the substrate surface. Atomic hydrogen is formed on the surface of the heated filament. Hydrogen further activates the growth surface and suppresses the graphite modification of carbon [11]. The deposition rate is normally about 1 to 2 µm/h. Higher deposition rates up to 15 µm/h have been presented using higher filament temperatures and higher methane concentrations [12]. To create good surface coverage, ultrasonic agitation in ethanol containing nanocrystalline diamond is often used to give uniform distribution of nucleation sites. Another method to enhance the nucleation density is the application of an electric bias to the substrate during the initial growth period. CVD diamond film growth typically results in rough faceted top surfaces, which is not desirable in many applications. In the case of free-standing diamond structures by CVD it is important to be able to release these structures from each other and to do desirable posttreatment. As diamond is extremely hard, mechanical polishing, thinning or cutting of diamond is time consuming and costly. Common diamond polishing techniques are laser ablation, ion beam irradiation, electric discharge and diffusional reaction with metals. (;3(5,0(17$/ A microstructure technology for diamond replicas following the process flow scheme below, see Fig. 1, is suggested. Each step is described in detail in the text below.
6LOLFRQ0LFURPDFKLQLQJ Different silicon micro structures have been manufactured by standard bulk micromachining processes i.e. lithography, wet etching, and dry etching, to evaluate micro replication of diamond (as a moulding technique). For a general background of silicon micromachining, see, e.g. [13, 14]. Both single sided and double sided anisotropic wet etching have been performed. Thermal silicondioxide has been used as masking material for the wet etching in potassium hydroxide (KOH) solution. When etching through the silicon wafers from the backside tetramethyl ammonium hydroxide (TMAH) was used due to its better selectivity of silicon to oxide. The only etch stop technique used was time stop. To fabricate some of the silicon microstructures, e.g. trenches and ridges, etching was done on (100) oriented wafers. The masks were aligned both parallel to the directions (i.e. 45° from the flat of the wafers) and along the directions. Deep trenches with different aspect ratios, i.e. depth to width relations, were fabricated in (110) oriented wafers to evaluate the filling properties of diamond. The trenches were etched by anisotropic KOH-etching (70g/100ml) at 90°C. This combination can be used to etch trenches with very high aspect ratios [15]. Trenches of aspect ratios differing between 0.67 and 3.8 were fabricated. When fabricating clamped beams, used as mould for diamond channels, a second etch step from the backside was necessary. First, U-grooves were etched in KOH (70g/100ml) at 80°C, on (100) oriented wafers, followed by a reoxidation step. The backside was patterned and the oxide was opened by reactive ion etching (RIE) equipment, in CHF3, to keep the mask layer intact on the topside. TMAH was used to etch through the entire wafer and eventually all oxide was stripped off in a HF-solution (1:10) prior to the diamond deposition. For the building-set demo-structures, where the crystallographic anisotropy in wet etching limits the design, standard RIE processes with SF6/O2 have been used. +)&9''HSRVLWLRQRI'LDPRQG
)LJ A process flow scheme for the manufacturing of diamond replicas.
Polycrystalline diamond films were deposited on the surfaces of microstructrured silicon wafers by HFCVD using a mixture of 15-sccm hydrogen and 0.15-sccm methane gas at substrate temperatures 850-900°C and chamber pressure 50 mbar. The substrate was pretreated in a HF-dip (1:50) to remove oxides and with ultrasonic agitation with diamond seeds in ethanol to promote diamond nucleation. The temperature was measured by a thermocouple on the substrate surface. A 1-mm thick, straight tungsten wire, precarbonized in a methane
atmosphere for two hours, held at 4-8 mm distance from the silicon wafer, was used as filament. The wire was put under a small tensile stress to remain in the same shape during the whole deposition. The experimental set-up is shown in Fig. 2. Tungsten filament
V
Gas inlet
plate thermo-mechanical thinning was used [16]. The diamond was thinned using hot metal diffusion, i.e. by just pressing an iron plate to the diamond. The diamond, still on the silicon substrate, was put in a vacuum chamber at 50 mbar and with reducing hydrogen atmosphere. The thinning rate is approximately 2 µm/hours [16]. 6DFULILFLDO6LOLFRQ(WFKLQJ
Prepared substrate
Thermocouple
Gas outlet
Substrate holder
)LJ Principle sketch of HFCVD set-up 7KHUPR0HFKDQLFDO7KLQQLQJRI'LDPRQG After the thick film deposition the top diamond surface is rough, with rms>1 µm. To thin the diamond film, hot iron
Following the deposition of diamond, the silicon mould had to be sacrificially etched away. Due to the good chemical resistance of diamond the most aggressive and fast etching solutions can be used. An isotropic etch was used consisting of 7:3 of HNO3:HF, which etched through a 380 µm silicon wafer at 100°C in less than 10 minutes without stirring. For all critical sacrificial etching, e.g. inside the diamond channels, etching was performed at room temperature. &KDUDFWHULVDWLRQRIWKH'LDPRQG5HSOLFDV Raman spectroscopy were used to characterise the quality of the deposited coatings. The thickness and the filling properties of the coatings were investigated by analysing cross-sections in a SEM. Roughness was measured in an atomic force microscope (AFM).
)LJ Examples of different geometrical structures which have been manufactured and investigated.
Fig. 3 illustrates different kinds of geometrical structures which have been manufactured and investigated. All structured samples show low roughness and very good coverage. Diamond coverage at the convex corners of the of the silicon mould gives a radius of curvature not visible in our SEM. However, for the pyramidal top formed in a silicon etch pit, the radius of curvature was large, approximately 100 nm. The close-ups in Table 1 show resolution and coverage. Still, there are problems with the nucleation density on larger flat surfaces on our samples. The roughness measured in an AFM on a flat diamond surface after sacrificial etching is rms 2 nm and 4 nm for 1*1 and 50*50 µm scanning areas, respectively. On the structured surfaces the nucleation density is substantially higher and the roughness consequently lower. In order to investigate the filling properties of the deposited diamond, cross-sections of trenches were investigated in an SEM. Trenches with aspect ratios
