Filament-assisted Chemical Vapor Deposition Research Articles

SiOCH Hits the Gas

There is an increasing need for on-site and real-time analyses of complex gas mixtures in many application fields such as industry, air quality, security and medicine. One challenge consists in developing complete and portable multi-gas analysis systems allowing in situ and real-time quantitative analysis of complex gas mixtures. A promising approach is based on the integration of resonating devices on silicon chips by using “standard” microelectronic technologies. The fabrication of these gravimetric gas sensors based on Nano Electro Mechanical System (NEMS) requires the use of a chemical sensitive layer to ensure a better collection and concentration of the target molecules. This enhances the sensitivity in conjunction with the limit of detection. The integration of a chemical sensitive layer on to NEMS fabricated on Si wafers leads to specific material requirements. The use of solvent is prohibited in order to avoid the degradation of the Si nanocantilevers due to capillarity forces,Very thin film thickness (less than few hundred of nanometers) should be deposited on the Si beam in order to avoid filling the gap between the resonant cantilever and the electrodes,The deposition technique should provide good uniformity over a large surface (typically on 200 mm Si wafers) for a collective sensor fabrication.And the material should be optimized to absorb the highest level of targeted gas for a given thickness, reversibly and quickly. In this work, the objective was to develop materials deposited using microelectronic compatible techniques in order to detect light alkanes and aromatic volatile organic compounds such as BTEX (for Benzene, Toluene Ethylbenzene and Xylene). Organosilicate materials are potentially good candidates for this application especially because they are nonpolar or weakly polar materials. In order to better understand the impact of the material chemistry on the detection of hydrocarbon gas and optimize the sensitivity of the chemical layer, a large panel of organosilicate thin films were deposited by different chemical vapor deposition (CVD) techniques. First, SiOCH were deposited by Plasma Enhanced Chemical Vapor Deposition (PECVD), this technique being the most used in the microelectronic industry for dielectric thin-film deposition. Several precursors (diethoxymethylsilane, trimethylsilane or octamethyltrisiloxane) and deposition conditions were used in order to vary their chemical composition and their physical properties. In these SiOCH, the carbon content is mainly under the form of methyl groups bonded to silicon. At the same time, to allow a better flexibility in term of chemical functions, filament assisted chemical vapor deposition (FACVD) was used for the deposition of organosilicate thin films. Indeed, FACVD allow a better control over the precursor fragmentation pathway in comparison to plasma-based techniques. Then, it is possible to add polar groups (such as ethoxy to in case of FACVD of methyltriethoxysilane) or to introduce siloxane rings in the materials (in case of initiated CVD of cyclosiloxane). Finally, the introduction of additional porosity was investigated as another way to potentially improve the sensitivity of the chemical sensitive layers. By using a porogen approach, it is possible to introduce up to 40 % porosity in a PECVD SiOCH. Using a foaming approach, we can go even further (> 50 %). Film properties after deposition and after potential annealing were investigated using multiple characterization techniques (ellipsometry, FTIR and Tof-SIMS). Porosity was characterized by ellipsometric porosimetry and grazing incidence small angle X-ray Scattering. In these material, pores (if any) are nanometrics and porosity is mainly open and interconnected. Thin film response under toluene or pentane exposures was studied using Quartz Crystal Microbalance functionalized with the different SiOCH.Through the synthesis and characterization of these various SiOCH thin films, the role of hydrophobicity, carbon content and specific chemical bonding can be highlighted. It is shown that the hydrophobic nature of SiOCH materials composed of a Si-O-Si backbone with methyl groups bonded to silicon, combined with the presence nanoporosity, lead to very high sensibility. The presence of isolated polar bonds such as ethoxy groups seems beneficial especially for BTEX detection. High partition coefficients toward toluene and pentane can be obtained, at least ten time higher than those measured on more classical polymers such as PDMS. Both deposition techniques (PECVD or FACVD) are able to produce good chemical sensitive layers for the detection of light alkanes and aromatics VOCs and these organosilicates constitute promising solution for the functionalization of NEMS-based gas sensors.Acknowledgment: Developments on FACVD were performed in the frame of a collaboration with TEL, US-Technology Development Center, America.

Reusable Boron-Doped Diamond Electrodes for the Semi-Continuous Detection of Tetrabromobisphenol A

It is of great importance to measure tetrabromobisphenol A (TBBPA) owing to its high toxicity and wide existence in the environment. In this paper, an electrochemical sensor based on boron-doped diamond (BDD) thin film was developed for the detection of TBBPA. The diamond thin film was deposited on Si substrate via filament-assisted chemical vapor deposition. Due to the great electrochemical properties, the BDD electrode could be reactivated after a pretreatment process after each measurement, which realized a continuous detection of TBBPA and solved the problem of renewal. The electrochemical pretreatment procedure was studied in order to achieve better performance. The peak current of the BDD electrode displayed a good linearity toward TBBPA concentrations ranging from 50 nM to $10~\mu \text{M}$ , and the detection limit was 27 nM (S/N = 3).

Filament Assisted Chemical Vapor Deposited organosilicate as chemical layer for nanometric hydrocarbon gas sensors

The fabrication of gravimetric gas sensors based on Nano Electro Mechanical System (NEMS) requires the use of a chemical sensitive layer that is uniformly deposited by a solvent-free deposition technique. In this work, we show than an organosilicate layer deposited by Filament-Assisted Chemical Vapor Deposition using methyltriethoxysilane presents very high affinity toward hydrocarbon gas. High partition coefficients toward toluene and pentane were obtained on Quartz Crystal Microbalance sensors after being annealed at 400 °C. It is shown that the hydrophobic nature of this material composed of a SiOSi backbone with methyl groups bonded to silicon, combined with the presence of isolated ethoxy groups and microporosity, allow a very high sensibility in comparison to others SiOCH. Furthermore, the functionalization of NEMS-based gas sensors was demonstrated. This material, deposited using a CMOS compatible technique, constitutes a promising solution for the development of a complete and portable multi-gas analysis system.

Conformal isolation of high-aspect-ratio TSVs using a low-κ dielectric deposited by filament-assisted CVD

In this study, SiOCH thin films were deposited by filament assisted chemical vapor deposition using methyltriethoxysilane. It is shown that annealing the films at 400°C (with or without UV-assist) is required to meet specifications in terms of dielectric properties (low-κ values and good insulating properties). This work highlights FACVD as a promising technique to allow the deposition of conformal dielectric thin films within High-Aspect-Ratio TSVs. Step coverage higher than 70% were obtained within 10∗80μm TSVs. Then, FACVD deposited SiOCH films can be considered as a promising candidate for use within TSV technologies.

Characterization of Nitrogen Doped Diamond Electrodes Produced by Hot Filament Chemical Vapor Deposition

The purpose of this work is to study the structural and morphological modification of the surface of the n-type diamond electrodes as a function of nitrogen doping. The characterizations of these electrodes were made using Raman Spectroscopy, Contact Angle, X-ray diffraction and Scanning Electron Microscopy (SEM). The nitrogen-doped diamond (NDD) electrodes were produced using Hot Filament-assisted Chemical Vapor Deposition method (HFCVD) from methane, hydrogen and nitrogen in the gas mixture. The results from Raman spectroscopy show that the diamond films obtained with nitrogen addition presented one large band at 1100-1700 cm-1. The SEM images showed that the variation in the nitrogen doping influenced the growth rate of films by promoting changes in the sizes of grains from microcrystalline to nanocrystalline texture. This behavior supported the results obtained from X-ray diffraction analyses. It was possible to verify a decrease in the crystallite size as a function of the nitrogen increase.

Copper Electroless Process Optimization to Modify Boron Doped Diamond at Different Boron Levels

Composite of Cu/BDD (boron doped diamond) have the potential to use as solid electrode for application in environmental approaches. Among many Cu coating processes, the electroless plating has emerged in recent years due to its low cost, fast deposition rate, and good filling capability. In addition, its major advantage is its simplicity, where the processes can take place at room temperatures and pressures [1]. In the Cu electroless process, a hard pre-treatment is needed for cleaning the surface and for creating functional groups that can act as anchor points on the BDD samples. The pre-treatments can be obtained using different oxidation techniques. In this study the anodic electrochemical pre-treatment was carried out using 0.5 mol L-1 H2SO4 and a fixed potential of 3 V for 30 min. The present study was undertaken in order to investigate the influence of the BDD doping level in the electroless synthesis as well as the effect of some deposition parameters such as the solution pH and the deposition time, to obtain Cu/BDD composites. This material is very attractive for nitrate electroreduction related to environmental interest. BDD films were grown by hot filament-assisted chemical vapor deposition technique on the titanium substrate, activated by methane and hydrogen gas mixture with a pressure of 40 Torr, at 650ºC, for 24 h. Boron source was obtained by an additional hydrogen line passing through a bubbler containing B2O3 dissolved in methanol with a controlled B/C ratio that led to films with different doping level (5000 and 15000 ppm). Prior to the Cu electroless deposition, the sensitization on the BDD was achieved using a solution of 40 mL L-1 HCl containing 0.04 mol L-1 SnCl2 for 5 min and the activation was made using a solution containing 7x10-4 mol L-1 PdCl2 with 2.5 mL L-1 HCl for 5 min. Ultrasonic vibration was used for both steps. The electroless Cu deposition was carried out for different times (30, 60, 180 and 2400 s) at room temperature. The bath composition was 0.1 mol L-1 CuSO4 + 0.2 mol L-1 KNaC4H4O6 + 17.5 mL L-1 HCHO. The pH level was adjusted to 8, 10 and 12 by the addition of NaOH. The Raman scattering spectra showed the peak close to 1332 cm-1 corresponds to the vibration of a diamond first-order phonon, confirmed the quality of these BDD films. This peak decreased in intensity due to the boron incorporation in diamond films. The effect of boron doping is reflected in the spectral features. There is the appearance of the two bands located at 500 cm-1 and 1220 cm-1 attributed to B-B vibrations and B-C vibrations, respectively. According to Scanning Electron Microscopy images the Cu deposits showed an uniform particle distribution on the BDD surface for solution of pH 12. On the other hand, no Cu deposit was verified at pH 8 and at pH 10 in the range of time deposition from 30 to 180 s. However, for deposition time of 2400 s, the deposit presented low particle density. Similar experimental conditions for electroless Cu deposition on lightly doped diamond showed the same behavior as a function of pH. The Cu deposit rate increased with increasing the deposition time, for both BDD films. Comparing the highly and the lightly BDD electrode, higher deposition rate was observed in the highly doped electrode. This result can be attributed to the better conductivity of this electrode due to its higher boron content. It is noteworthy that the sensitization and activation processes are a crucial step for high quality coatings independently of the BDD doping level. Acknowledgments: Brazilian National Institute of Science and Technology (INCT) for Climate Change funded by CNPq Grant Number 573797/2008-0 e FAPESP Grant Number 2008/57719-9.

Single-chamber filament-assisted chemical vapor deposition of polymer and organosilicate films for air gap interconnect formation

Air gaps introduced at the trench level of advanced interconnects provide a means of lowering effective dielectric constant, k eff, without the use of mechanically weak ultra low-k films. Filament-assisted chemical vapor deposition (FACVD) is a promising technology for depositing polymers, dielectrics and metals. Initiated chemical vapor deposition (iCVD) is a novel one-step method of depositing polymers in the vapor phase while retaining properties found in solution chemistry. In this paper, we present a 300 mm FACVD tool employing iCVD and FACVD processes to deposit polymer adhesion promoter (AP), decomposable polymer (DP), and permeable SiCOH cap films for air gap integration. By decomposing the iCVD polymer to form voids, we show decreased capacitance for a 160 nm line-space single damascene structure.

Development and Characterization of a Diamond-Based Localized Surface Plasmon Resonance Interface

The paper reports on the fabrication of a localized surface plasmon resonance (LSPR) interface with nanocrystalline diamond (NCD) as overcoating. The LSPR interface is formed through thermal evaporation of a 4 nm gold film onto quartz substrates and postannealed at 800 °C. The resulting gold nanostructures (Au NSs) are further coated with nanocrystalline diamond (NCD) films produced by hot filament-assisted chemical vapor deposition (HFCVD) at 800 °C. The influence of the NCD film thickness on the wavelength at maximum absorption (λmax) was investigated by recording UV−vis transmission spectra in air. The experimentally obtained data were compared to the theoretically calculated one. The possibility to introduce chemical functions on the quartz/Au NSs/NCD interfaces was demonstrated by (i) photochemical oxidation of the NCD overcoating and subsequent reaction of the oxidized surface with perfluorodecyltrichlorosilane or (ii) chemical linking of the 4-nitrobenzene diazonium salt directly onto as-grown hydrogen-terminated NCD films.

Evaluation of Silicon Nitride Ceramic Cutting Tools with Diamond Coatings

There is a substantial increase on carbon-carbon composites use for engineering applications, considering its high temperature properties and low specific mass. However the machining costs are relatively high, and new cutting tools, mainly ceramics, must be developed to overcome such difficulty, aiming cost reductions. In this work, silicon nitride based ceramics has been prepared , by pressureless sintering of silicon nitride powders and appropriate amounts of Al2O3,Ce2O3, Y2O3 and AlN. Cutting tools were prepared from the sintered materials, with geometry according to ISO1832. Selected cutting tools were also diamond coated by a hot filament-assisted Chemical Vapor Deposition (HFCVD) diamond coating process. Carbon Fiber Reinforced Carbon (CFRP) composites machining was performed, to evaluate the diamond coating influence on machining performance. After the tests, the uncoated tools presented severe flank wear and shorter life than the diamond coated ceramic tools. This flank wear is caused by the abrasive carbon powder generated during the facing operation. On CVD diamond coated α-SiAlON ceramic tools, no flank wear was observed, and the cutting edge remained unmodified, even for severe test conditions, such as high cutting length and speed. Carbon particles, originated from the machined composite, do not promotes diamond film rupture, but instead, acts as lubricant film and reduces composite surface initial roughness.

Electronic structure of sulfur-modified nanocrystalline carbon films

Thin films of nanocrystalline diamond were grown by filament-assisted chemical-vapor deposition using methane as carbon precursor with high hydrogen dilution and hydrogen sulfide concentration ranging from 0to500ppm in the gas phase. The surface topography and electronic structure of these films (n-C:S) were investigated using ultrahigh-vacuum scanning tunneling microscopy and scanning tunneling spectroscopy (STS), respectively. Topographic image analyses depict that the root-mean-square roughness of the film surface and average grain size decreases with increasing sulfur incorporation either in gas phase or solid films. High-resolution scanning tunneling microscopy images reveal the localized regions of high conductivity (white) surrounded by less conductive regions (black) pointing at the existence of inhomogeneous mixture of sp2- and sp3-bonded carbon in aggregate or clustered and dispersed state. The surface density of states was determined using scanning tunneling spectroscopy where normalized differential conductivity, i.e., (dI∕dV)∕(I∕V) mimics local density of states (DOS). These methods were employed to understand the role of sulfur in the modification of both the surface microstructure and electronic structure near the Fermi level. The band edges were derived by taking tangents to the differential conductivity (dI∕dV) within a certain potential window of ±2eV of the Fermi level. The resulting band gap is found to be similar to that measured optically (Tauc gap). The Fermi level for undoped nanocrystalline carbon (n-C) was found just below the midgap indicating that n-C is a weakly p-type semiconductor. The STS DOS shows oscillatory behavior or peaks which we ascribe to states of the surface layer having relatively more graphitic or sp2-bonded carbon bonds. With higher sulfur addition, the Fermi level is found to move above the midgap. These results seem to agree quite well with our early work on electrical conductivity exhibiting n-type doping taking place and declining band gap at higher sulfur contents measured with spectroscopic ellipsometry technique on the same samples.

Diamond deposition on modified silicon substrates: Making diamond atomic force microscopy tips for nanofriction experiments

Fine-crystalline diamond particles are grown on standard Si atomic force microscopy tips, using hot filament-assisted chemical vapor deposition. To optimize the conditions for diamond deposition, first a series of experiments is carried out using silicon substrates covered by point-topped pyramids as obtained by wet chemical etching. The apexes and the edges of the silicon pyramids provide favorable sites for diamond nucleation and growth. The investigation of the deposited polycrystallites is done by means of optical microscopy, scanning electron microscopy and micro-Raman spectroscopy. The resulting diamond-terminated tips are tested in ultra high vacuum using contact-mode atomic force microscope on a stepped surface of sapphire showing high stability, sharpness, and hardness.

Dependence of the gas composition in a microwave plasma-assisted diamond chemical vapor deposition reactor on the inlet carbon source: CH4 versus C2H2

Molecular beam mass spectrometry was used to measure the gas-phase composition near a growing diamond surface in a microwave plasma-assisted chemical vapor deposition reactor. The dependencies of the gas composition on changes in: (1) the carbon mole fraction in the reactor feed, Xc; (2) the identity of the inlet carbon source (CH, versus C,H,); and (3) the surface temperature, T,, were studied. Unlike the hot-filament case, the gas composition was independent of the identity of the inlet hydrocarbon source over the entire range of Xc. As expected from gas-composition measurements, films grown using either hydrocarbon exhibited similar growth rates, morphology and Raman spectra. The gas composition was also generally insensitive to changes in surface temperature suggesting that reported temperature sensitivities of film growth properties are primarily due to changes in the kinetics of surface processes rather than changes in the gas composition near the surface. For Xc GO.01, the gas compositions measured in this study and in our previous studies conducted in a hot filament-assisted chemical vapor deposition *~-et,,\* Q-P n,,nnt;tst;.rel.r .im;l~v imnlwinn mlc~r mo._nh QQP rhm4rtr.r in hnth ,.esr+nm ..,hnn a,,ffi,.innt atnmir h.rrlrnnr=n ;o IIc&“~“I CAL” yuYYc’,u” .V’, U111111WI ““y”J’u6 UllllllUl 6YY-y“uY’ .AL”““Y”J 111 “V&U lVUVC”lY VTUIll UUYI~IUII~ UL”llllV UJU’“~‘” 10 present. However, attempts to simulate the gas composition in the plasma environment using a numerical model based only on neutral-neutral gas-phase chemistry were generally unsuccessful over a wide range of Xc.

Temperature dependence of growth rate for diamonds grown using a hot filament assisted chemical vapor deposition method at low substrate temperatures

Diamond particles were deposited using a hot filament-assisted chemical vapor deposition (HFCVD) method in which the substrate temperature ranged from 210 to 700 °C. The size of the diamond particles measured as a function of time showed that a diamond grows via two periods of incubation and growth. Compared with an activation energy of 10–25 kcal/mol for substrate temperatures higher than 600 °C as reported in literature, the growth rate for a diamond grown using a HFCVD method was much less dependent on the substrate temperature for that temperature range investigated in our study. The apparent activation energy, determined from the Arrhenius plot of the substrate temperature versus diamond growth rate, decreased from 5 to 1 kcal/mol with decreasing temperature.

Kinetics of Diamond-Like Film Growth Using Filament-Assisted Chemical Vapor Deposition

AbstractA detailed kinetic model of diamond-like film growth from methane diluted in hydrogen using low-pressure, filament-assisted chemical vapor deposition (FACVD) has been developed. The model includes both gas-phase and surface reactions. The surface kinetics include adsorption of CH3· and H·, abstraction reactions by gas-phase radicals, desorption, and two pathways for diamond (sp3) and graphitic carbon (sp2) growth. It is postulated that adsorbed CH2· species are the major film precursors. The proposed kinetic model was incorporated into a transport model describing flow, heat and mass transfer in stagnation flow FACVD reactors. Diamond-like films were deposited on preseeded Si substrates in such a reactor at a pressure of 26 Torr, inlet gas composition ranging from 0.5% to 1.5% methane in hydrogen and substrate temperatures ranging from 600 to 950°C. The best films were obtained at low methane concentrations and substrate temperature of 700°C. The films were characterized using Scanning Electron Microscopy (SEM) and Raman spectroscopy. Observations from our experiments and growth rate data from similar experiments reported in the literature [1] were used to estimate unknown kinetic parameters of surface reactions. The proposed model predicts observed film growth rates, compositions and stable species distributions in the gas phase. It is the first complete model of FACVD that includes gas-phase and surface kinetics coupled with transport phenomena.

High Rate, High Quality, Diamond Synthesis by the Combustion Flame Process

ABSTRACTAmong the various low pressure techniques being developed for the synthesis of diamond films and bulk diamond slabs the combustion flame synthesis process has some distinct advantages. In this approach the combustion reaction between acetylene and oxygen is utilized to generate the requisite energy to activate excess acetylene in the gas mix leading to the deposition of diamond films on a temperature controlled substrate brought into contact with the flame. Other diamond synthesis approaches, such as microwave enhanced and the filament assisted chemical vapor deposition processes, and the various arc jet techniques utilize mixtures of hydrogen and methane as the process gases. Oxygen and oxidizing specie ( such as OH radicals) in the flame ambient may be much more effective than atomic hydrogen in promoting the growth of diamond over the growth of graphite and other non- diamond forms of carbon. In addition this technique enables the growth of diamond at high rates and is relatively easily scaled for large area synthesis. In this paper a discussion of this technique is presented drawing upon recent research by the authors as well as published work to present a general discussion of the issues involved in the development of this technique of low pressure diamond synthesis.

Growth of Diamond Films Using C3F8AND H2in A New System

ABSTRACTIn the present study, the new system used C3F8and H2 as source gases. Filament-assisted chemical vapor deposition was utilized. Continuous diamond films were grown on the Si and Mo substrates without any surface pretreatment. The results of scanning electron microscopy, Raman spectroscopy, X-ray diffraction, and atomic force microscopy measurements indicate that the films deposited on the Mo substrates are of high quality. Homoepitaxial diamond films were grown on the high pressure synthetic single crystal diamond substrates. The results show that in our experimental conditions epitaxial films were easily grown on the (111) synthetic diamond substrates and sometimes epitaxial filmse also grown on the (100) synthetic diamond substrates.

Polycrystalline Diamond Film on Si(100) by XPS

Filament-assisted chemical vapor deposition (CVD) diamond film growth on Si(100) was studied using x-ray photoelectron spectroscopy (XPS) in a system that couples a growth chamber to an ultrahigh vacuum analytical chamber. Diamond nucleates and grows on a SiC layer formed on the Si(100) substrate [D. N. Belton and S. J. Schmieg, Appl. Phys. Lett. 54, 416 (1989)]. After about 17 h growth XPS showed no Si signal from the substrate, no detectable contaminants, and only carbon present in survey scans. Electron energy loss spectroscopy (EELS) spectra obtained by x-ray excitation of the C 1s level can be used as a fingerprint for distinguishing diamond from graphite or carbides [D. N. Belton and S. J. Schmieg, J. Vac. Sci. Technol. A 8, 2353 (1990)]. The identification of a continuous diamond film was confirmed with Raman spectroscopy and scanning electron microscopy (SEM).

Surface sensitive electron spectroscopy study of diamond films prepared by hot filament-assisted CVD

Diamond films prepared by the use of a hot filament-assisted chemical vapor deposition have been investigated by Auger electron, electron energy loss and UV photoelectron spectroscopies. Results are discussed in comparison with previous measurements done principally on natural diamond and with graphite data. It is shown that film surfaces have original electron structure induced properties which can be explained in a simple model where perturbed diamond surfaces must be implicated.

Chemical Kinetic Calculations of the Gas Phase in Filament‐Assisted Chemical Vapor Deposition of Diamond

A chemical kinetic model was developed for the gas mixture in filament‐assisted diamond chemical vapor deposition (CVD) systems. CH4/H2 mixture was studied as the starting gas mixture. The predictions of gas evolution calculated from this model were used to interpret certain published experimental observations. From the initial analysis, it was concluded that the thermodynamic state of the gas composition determined the effects of transport parameters on diamond growth. In many filament‐assisted diamond CVD systems the gas composition was found to be kinetically controlled. The analysis also suggested that CH3 is a major precursor of diamond films. When CH4/H2/O2 was used as the starting gas mixture, the results indicated that the oxygen additive catalyzed the process to increase the concentrations of CH3 and H, thereby increasing the growth rate and improving the quality of diamond films.

Atomic force microscope imaging for process characterization in diamond film deposition

Atomic force microscope (AFM) images are presented for diamond films deposited using two processes. Films were deposited on polished Si from a CH4–H2 mixture by microwave plasma chemical vapor deposition (CVD) for CH4 concentrations in the range 0.4%–2.0%. At the lowest CH4 concentration, the film surface was rough (Rq = 80 nm) and contained many grains wider than 1 μ and taller than 0.4 μ. At the highest CH4 concentration, the growth of large grains was suppressed, producing much smoother films (Rq = 25 nm), which may be suitable as mask substrates for x-ray lithography. Filament-assisted CVD was also used; deposition was halted before a continuous film formed. The tallest grains (100–200-nm high) were clustered in intersecting linear arrays corresponding to marks made by diamond-polishing (seeding). Features a few nm high found in flat regions may be diamond nuclei on bare Si formed much later.