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Preparation and properties of sub-micron thick and free-standing diamond membranes

Preparation and properties of sub-micron thick and free-standing diamond membranes

Diamond and Related Materials 11 (2002) 721–725 Preparation and properties of sub-micron thick and free-standing diamond membranes Sh. Michaelson, R...

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Diamond and Related Materials 11 (2002) 721–725

Preparation and properties of sub-micron thick and free-standing diamond membranes Sh. Michaelson, R. Akhvlediani, A. Hoffman* Chemistry Department, Technion, Haifa 32000, Israel

Abstract Free-standing and continuous diamond films of sub-micron thickness were obtained by hot filament chemical vapor deposition (HF-CVD) on silicon substrates following wet etching of the underlying substrate. By an ultrasonic surface abrasion pre-treatment of the silicon substrates in a slurry consisting of diamond and titanium particles prior to CVD deposition, it is possible to induce nucleation densities in excess of 1010 particleycm2 . As a result, continuous free-standing films of sub-micron thickness attached to a silicon frame of ;1 cm in diameter were obtained. As determined by atomic force microscopy (AFM), the membranes display nanometer roughness of the back side and sub-micron roughness of the as-grown surface. To ensure the removal of impurities, the free-standing membranes were exposed to a hydrogen plasma. Following this treatment, no impurities such as O and Si, were detected by X-ray photoelectron spectroscopy (XPS) and RBS. Furthermore, the electron energy loss spectroscopy (EELS) of both sides of the membrane display the characteristic bulk and surface plasmon of diamond showing that the asprepared surfaces are free of non-diamond carbon components. The surface electronic properties of the films were monitored by ultra-violet photoelectron spectroscopy (UPS). These measurements show that the as-hydrogenated surfaces after heating to 500 8C in situ display negative electron affinity. The Raman spectrum of the films displays a peak at 1328 cmy1 and a lower intensity broad band centered at 1546 cmy1. The optical transparency of the membranes in the 200–1100-nm range is also reported. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Free standing; Diamond; CVD; Etching; Sub-micron

1. Introduction Diamond polycrystalline thin films possess a number of unique chemical and physical properties that make them attractive for a broad range of applications in research and industry. Some of these applications require the use of free-standing film. To the best of our knowledge, free-standing diamond films of sub-micron thickness and their properties have not been reported. In this work, we report on the preparation and properties of free-standing diamond films up to 500-nm thickness deposited on silicon (100) substrates by the hot filament chemical vapor deposition (HF-CVD) method. To obtain films of such fine dimension it is necessary to deposit continuous films of sub-micron thickness. This was achieved by a pre-treatment consisting of ultrasonication in a slurry composed of diamond and titanium particles w1,2x. Such pretreatment *Corresponding author. E-mail address: [email protected] (A. Hoffman).

may lead to diamond particle densities (DPD) in excess of 1010 particlesycm2. The minimal continuous films thickness in this case is-100 nm w3x. Free-standing films were obtained following chemical etching of the silicon substrate resulting in a free-standing film or membrane supported by a silicon frame. The as-prepared membranes were characterized in terms of their structural properties by atomic force microscopy (AFM), and phase composition by Raman and electron energy loss spectroscopy (EELS). The chemical purity of the membranes was recorded by X-ray photoelectron spectroscopy (XPS) and Rutherford backscattering (RBS). Ultra-violet photoelectron spectroscopy (UPS) was applied to monitor the surface electronic properties of the back- and front sides of the film. Finally, the optical transmission properties of the films in the 200–1100nm range were measured. 2. Preparation of the diamond membranes Polycrystalline diamond was deposited on silicon (100) substrate by conventional vertical HF-CVD sys-

0925-9635/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 9 6 3 5 Ž 0 1 . 0 0 6 5 8 - 6

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tem. Before deposition, the silicon substrate was pretreated by ultrasonification using a mixed DiqTi slurry procedure developed in our laboratory as previously described. The CVD process was carried out using a gas mixture of 1% CH4 and 99% H2 at a pressure of 50 torr. Activation was achieved using rhenium filaments heated to 2000 8C positioned 8 mm above the sample. The temperature of the sample was maintained at 800 8C during deposition. The growth rate of the films deposited after this special pre-treatment and after pretreatment with the pure diamond slurry was measured by cross-section scanning electron microscopy (SEM). High resolution SEM was used to determined the continuity of the films. The growth rate of the films was determined to be 750 nmyh. In this work, films of 400nm-thickness were used to prepare the free-standing membranes. Thinner films were not sufficiently robust. Free-standing membranes were prepared by chemically etching the silicon substrate leaving a frame of f1 cm in diameter using a wax frame. The etching of the silicon substrate was carried out using a standard procedure using a mixture of concentrated hydrofluoric, nitric and acetic acids at room temperature w4x. Acetic acid was added to slow down the etching process to prevent fracture of the diamond membrane. The ratio of the acids was found to be optimal at 1:1:1. Diamond films of thickness less than 400 nm were found to tear under these etching conditions. After etching the silicon substrate the wax was removed by rinsing in CH3CCl3, followed by a short clean in acetone and ethanol. To remove impurities from the membranes surfaces (back and front) and to ensure a true diamond surface termination the as-prepared films were exposed to a hydrogen microwave (MW) plasma. The plasma exposure was carried out using a MW power of 600 watt, hydrogen pressure of 40 torr, and substrate temperature of 500 8C for 20 min. Higher substrate temperatures resulted in film fracture due to thermal stresses. 3. Properties of the membranes The topography of the as-prepared diamond membranes was examined by non-contact AFM. Both sides of the membranes were measured using a Topometrix 2010 system scanning areas of 2=2 mm and 5=5 mm with a 8=8-mm2 scanner. This scanner was calibrated for absolute dimension by a 0.2=0.2-mm2 standard grid. The radius of the tips used for measurements (NTMDT) was less than 10 nm (typical resonant frequencys360 kHz, angle -208, and a force constant of 48 Nym). Using these measurement mode and conditions, artifacts associated with the tip–film surface interactions were minimized. It should be stressed that due to the complex morphology of the films, the contact mode AFM introduced artifacts associated with tip degradation

and frictional forces resulting in a pronounced distortion of the pictures. In Fig. 1a,b, non-contact AFM pictures of the back and front sides, respectively, of the diamond membrane are shown. From these pictures, the area root mean square (RMS) and the average height of the film’s topography were calculated. The area RMS for the back and front sides were calculated to be 15.8 and 155.2 nm, respectively. The smoother surface topography of the backside is a reflection of the silicon topography. The topographical features measured on the back side are associated with nucleation sites. The density of these features is ;1010 ycm2. This value is similar to the diamond particle density measured after a deposition time of 5–10 min, in support of our argument. The front side (Fig. 1b), on the other hand, displays a rougher surface topography with an area RMS of 155.2 nm. The increase in surface roughness is due to the growth kinetics of the diamond film that favors the growth of some crystallites and inhibits that of others w5x. These effects result in a higher density of grain boundaries at the back side of the membrane. The chemical composition of the back and front surfaces of the membrane can be affected by the etching processes. Following the preparation procedure described above, the membrane surface was examined by XPS and RBS. XPS measurements were carried out in a ‘Specs’ RQ 20y38 system with a base pressure of 1=10y9 torr and using AlKa (1486.5 eV) X-ray radiation for photoelectron excitation. The only peak measured in the XP spectra is the C(1s) at ;284 eV (not shown). From these spectra it is concluded that the as prepared surfaces are relatively free of impurities ("0.1%). However, it is expected that both surfaces are hydrogen-terminated because of the hydrogen plasma treatment. According to RBS, no other element than carbon (and hydrogen) is present in the films. These results show that the silicon etching procedure and the subsequent hydrogen plasma treatment result in films free of any impurities. The phase purity of the film was monitored by Raman and electron energy loss spectroscopy (EELS) using a DILOR XY and SPECS electron spectrometer system. The Raman measurements were carried out using the Ar line at 514.5 cmy1 and laser spot size of f1 mm. The laser power was f5 mW. The EELS spectra were recorded by measuring the low energy losses to the C(1s) X-ray photoelectron line at f286 eV using AlKa incident photon radiation. In Fig. 2, Raman spectrum of the membrane is shown. Two peaks are clearly seen in the spectrum: a peak at 1328.7 cmy1 characteristic of diamond, and another broad one centered at ;1540 cmy1 associated with disordered carbon. The shift of the diamond peak to lower wave numbers evidences the presence of stresses within the films. The 1540-cmy1 peak is most likely

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Fig. 1. AFM images of (a) back- and (b) front side of the film. The roughness of the back side is approximately one order of magnitude smaller than that of the front side. DPD of the back side was observed at 1010 cmy2, whereas that of the front side was 108 cmy2.

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Fig. 4. Ultraviolet photoelectron spectrum of the back side of the film before and after annealing to 500 8C with an applied bias of 20 eV. Fig. 2. Raman spectrum of a free-standing diamond membrane.

associated to disordered carbon presence at grain boundaries. EELS spectra of the films recorded by measuring the energy losses to the C(1s) photoelectron line for the front and back sides of the films are shown in Fig. 3. The EELS spectra from both sides display the characteristic bulk and surface plasmons of diamond at 33 and 23 eV, respectively. No evidence for features associated with graphitic andyor amorphous carbon at ;6 eV could be observed in the spectra. These results show that the silicon etching and subsequent hydrogen plasma treatment result in well-defined diamond surfaces. This is necessary for applications involving the electronic properties of the diamond membranes. The electronic properties of the membrane’s surfaces were monitored by UPS using He(I) incident radiation. In Fig. 4 the UPS spectra of the back side of the film (previously in contact with the silicon substrate) are shown for the as hydrogenated surface and after annealing in vacuum to 500 8C. During the UPS measurements a bias voltage of 15 eV was applied to the sample. As observed from this figure, following heating to 500 8C in vacuum a 0.6 eV shift in the low-energy electron emission threshold was measured. This shift in electron

Fig. 3. Electron energy loss spectrum associated with the XPS C(1s) photoelectron of both sides of the membrane.

emission is interpreted to be due to the production of negative electron affinity (NEA) of the back side surface following the annealing process. This effect is most likely associated with desorption of oxygen species adsorbed on the diamond surface after hydrogenation and exposure to ambient conditions w6x. Similar results were obtained for the front surface. The NEA of the back surface obtained after annealing is not obvious as this side of the surface has a very high density of grain boundaries as established by AFM (Fig. 1), which may had have an adverse effect on the electron emission properties of the surface. Finally, the optical properties of the membranes are reported. The optical transmittance spectrum was recorded using a Shimadzu double-beam UV-1601 visible spectrophotometer in the 200–1100-nm range and it is shown in Fig. 5. From this result, the optical transmittance of the membrane suggests its use for optical applications. In summary, in this work we report on the production and properties of diamond membranes of sub-micron thickness and some of the structural, morphological, surface and optical properties. It was determined that free-standing membranes of ;1-cm diameter may be produced by increasing the diamond nucleation density

Fig. 5. Optical transmittance spectrum of the membrane in the 200– 1100-nm range.

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above 1010 particlesycm2. We found that the as-prepared surfaces display electronic properties characteristic to high quality diamond and are free of impurities. References w1x Y. Chakk, R. Brener, A. Hoffman, Appl. Phys. Lett. 66 (1995) 2819–2821.

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w2x Y. Chakk, R. Brener, A. Hoffman, Diamond Rel. Mater. 5 (1996) 286–291. w3x R. Akhvlediani, I. Lior, Sh. Michaelson, A. Hoffman, Diamond Rel. Mater. (2001) (submitted to this conference). w4x M.C. Salvadory, M. Caltani, V. Mammana, O.R. Monteiro, J.W. Ager III, I.G. Brown, Thin Solid Films 290–291 (1996) 157. w5x L. Huimin, D.S. Dandy, Diamond Rel. Mater. 4 (1995) 1173. w6x G. Piantanida, A. Breskin, R. Chechik, O. Katz, A. Laikhtman, A. Hoffman, C. Coluzza, J. Appl. Phys. 89 (2001) 8259–8264.