J. Plasma Fusion Res. SERIES, Vol.7 (2006) 145-149

Optical Emission Spectroscopy of Low Temperature CVD Diamond
PATACSIL Crismar1,2 , MALAPIT Giovanni1,2 and RAMOS Henry2
1 Department of Physical Sciences, College of Science, University of Philippines Baguio, Baguio City
2 Plasma Physics Laboratory, National Institute of Physics, University of the Philippines, Diliman, Quezon City

（Received: 4 October 2004 / Accepted: 4 October 2005）

Abstract
The gas phase chemistry in low temperature (< 300 ◦ C) diamond and DLC chemical vapor deposition
process was investigated using optical emission spectroscopy (OES) to determine the eﬀects of the diﬀerent
deposition parameters such as methane concentration, substrate temperature, plasma current and gas pressure
in relation to diamond growth on silicon substrates. The OES data were analyzed to determine the importantfactors favorable for diamond deposition such as the mean electron temperature, T e , and the relative concentration levels of methane radicals (CH and CH+ ). From the OES result, the optimum deposition parameters
were determined for the low temperature diamond CVD system. Using these optimized parameters, film samples were attempted to be deposited at diﬀerent substrate temperatures (175 ◦ C − 300 ◦ C). X-ray Diﬀraction(XRD), Raman Spectroscopy and Scanning Electron Microscopy (SEM) characterization of the deposited
samples confirmed that diamond films were deposited at temperatures from 200 ◦ C − 300 ◦ C.
Keywords:
optical emission spectroscopy, low temperature, diamond1．Introduction
Previous researches at the Plasma Physics Laboratory, National Institute of Physics have shown that diamond and DLC thin films can be grown on silicon substrates with the Plasma Enhanced Chemical Vapor Deposition (PECVD) system via DC Glow discharge [1,2].
However, since the processes and mechanisms responsible for the diamond growth are still unclear [3,4], it
was recommended that plasma diagnostics was needed
to determine the optimum parameters such as plasma
current, substrate temperature and chamber pressure
to ensure successful and consistent deposition of goodquality diamond and DLC films [5,6].
The Plasma Physics Laboratory acquired the Triax
320 Spectrometer to carry out such investigation byanalyzing the optical emissions of the plasma. Optical Emission Spectroscopy (OES) is a non-invasive
technique, which enables individually emitting plasma
species to be monitored. In addition to identifying thechemical species present in the plasma, changes in themean electron temperature, T e , and relative concentration levels of the species can be monitored from theOES data. The electron temperature is an important
plasma parameter since it is found to be directly proportional to the growth rate of diamond films [7]. This
work reports the results of the electron temperature andconcentration levels of CH and CH+ radicals based on

the relative optical emission intensities of the plasma at
diﬀerent plasma deposition conditions.
2．Methodology
2.1 The experimental set-up

The experimental set-up is shown schematically inFig. 1. It consists of the vacuum system, power source,
plasma electrodes, diagnostic ports for probes, theheater, thermocouple and the vacuum gauge. Includedin the diagram are the gases, mass flow controllers,
multi gas controller, vacuum gauge controller and tem-Fig. 1 Schematic Diagram of the PECVD system
C 2006 by The Japan Society of Plasma

Science and Nuclear Fusion Research

Corresponding author’s e-mail: cppatacsil@upb.edu.ph145

Optical Emission Spectroscopy of Low Temperature CVD Diamond

the transition i → j of the species x is given by:I x (i → j) = C x k xi ne N x g(λ)(1)

where Cx is a constant; k xi is the rate constant for excitation of the species x to the state i by electron collisions; ne is the concentration of electrons in the plasma;N x is the concentration of the species x; and g(λ) is theprobability that the photon of that wavelength will becollected and counted.
From equation (1), the emission intensity ratio of twolines, I1 /I2 is given by:
I1 /I2 ∼ (k1 /k2 )(N1 /N2 )Fig. 2 Photograph of the plasma produced as seen fromthe viewport.

(2)

For the same species, the intensity ratio of two lines, e.g.
IHβ /IHα is proportional to the rate constant of excitationof the species, which reflects an electron mean energy inthe plasma, i.e. the electron temperature [8,13]. For different species existing in an excited environment, the intensity ratio reflects their relative concentrations [5,14].

perature controller. The chamber has a view port forobserving the plasma inside the chamber.
Figure 2 shows the DC glow discharge produced,
cylindrical in shape (approx. 5.0 cm diameter). The
dispersed plasma emission was collimated and focusedon one end of a fiber optic and relayed into the spectrometer interfaced into a computer for data processing.

3．Results and discussion
3.1 The effect of methane concentration

Previous papers on diamond deposition showed thatthe key parameter in diamond film depositions, wherehydrogen and methane gases are used, is the methane
concentration [11,12]. Figure 3 shows that the eﬀect ofmethane concentration on the electron temperature as
seen in the Hβ /Hα plot is not significant. However, thesame figure shows that higher methane concentration
results in relatively high levels of CH and CH+ confirming the results of other researches [5,13,14]. This meansthat lower methane concentration favors diamond deposition.
The SEM profiles of the deposited films using varying methane concentration are shown on Fig. 4.

2.2 Theoretical background2.2.1 CVD diamond and DLC films

Chemical vapor deposition process involves agas phase chemical reaction that transforms gaseous
molecules, which are called precursors, into a solidmaterial in the form of thin film on the surface of asubstrate. For the chemical vapor deposition of diamond films, hydrocarbon gas (e.g. methane) is used inexcess of hydrogen. The molecules are excited, dissociated and ionized in the plasma principally byelectron-impact bringing about excited species (atoms,molecules, radicals, atomic and molecular ions). Moreover, these species react with each other and are thentransported to the surface of the substrate where nucleation and growth processes occur. The successful
deposition of diamond film on the surface requires
the following important conditions for the plasma: (1)high H concentration level to have a higher probabilityof hydrogen abstraction from the CH radicals leaving
only carbon on the surface [9,10], (2) low CH radical
concentration to prevent formation of graphitic carbon[11,12].
2.2.2 Optical emission spectroscopy

An electronically excited atom can undergo a spontaneous transition to a lower energy level and releasea photon of light. This photon will have energy E ji =hc/λ ji where i and j correspond to the lower and upper
energy levels respectively. The emission intensity for

Fig. 3 Methane concentration vs. intensity ratio

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Optical Emission Spectroscopy of Low Temperature CVD DiamondFig. 6 Pressure vs. intensity ratio
Fig. 4 SEM images of samples deposited at varying methane concentration (pressure = 10 Torr;
plasma current 50 mA): (a) Silicon substrate, (b)graphitic deposit on the silicon substrate (8 %methane used), (c) DLC deposits on the silicon
substrate (5 % methane used), (d) Diamond deposited on the silicon substrate (1 % methaneused)

3.3 The effect of pressure

The CVD chamber is evacuated to a base pressure of
10−6 Torr. Successively, hydrogen gas is introduced tothe desired pressure, after which a potential diﬀerence
between the electrodes ignited the plasma. For this research, stable plasma was attained only for pressure of
10 − 20 Torr because the power supply used has maximum output of about 600 V only. Figure 6 is seen toindicate that the electron temperature decreases as thepressure increases. Increasing levels of CH and CH+
concentrations were observed as the pressure increases.
Both cases indicate that the lower the pressure, the morefavorable the deposition of diamond.

3.2 The effect of plasma current

The plasma current is controlled by the plasma voltage applied between the two electrodes. Figure 5 shows
the relation of the relative intensity ratios as a function of plasma current. A very stable plasma is obtained at about 30 to 55 mA. The CVD facility wasoperated on these current ranges for the stable plasma(30 mA − 55 mA).
The graph indicates increasing electron temperatureas the current increases from 30 mA to 55 mA. Figure
5 also shows an inverse relation between that the concentration levels of CH and CH+ and the plasma current. That is, the concentration levels of CH and CH+
decreases as plasma current increases. This therefore
suggests that the higher the plasma current, the morefavorable the diamond deposition.

3.4 The effect of substrate temperature

Figure 7 shows the relation of the intensity ratio asa function of the substrate temperature. It can be seenthat the electron temperature does not vary much between temperatures 180 − 275 ◦ C. However, the trend
seems to indicate that the electron temperature increases
significantly for substrate temperature above 250 ◦ C.
Figure 7 also shows that as the substrate temperature
is increased, the CH and CH+ concentration levels are

Fig. 7 Substrate temperature vs. intensity ratio

Fig. 5 Plasma current vs. intensity ratio
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Optical Emission Spectroscopy of Low Temperature CVD DiamondFig. 9 Raman spectrograph of diamond samples deposited at different substrate temperatures showing diamond peaks (a) 200 ℃ and (b) 275 ℃.

Fig. 8 XRD proﬁles of samples deposited at different
substrate temperatures (1 % CH4 in H2 , pressure = 20 Torr; plasma current 50 mA): , (a) 225℃, showing diamond (220) peak at 2θ = 76.08
and (b) 275 ℃, showing diamond (220) peak at
2θ = 75.458 ℃.

rent and pressure, diamond films were successfully
deposited on silicon substrate at low temperature of200 ◦ C to 300 ◦ C.

decreased. Although the results shows that higher substrate temperature favors diamond deposition, the researchers were able to deposit diamond at 200 ◦ C using
the optimum values of the previously discussed parameters. Figs. 8 and 9 respectively shows the XRD profile and Raman spectrograph of the sample deposited at
diﬀerent substrate temperatures with the diamond peakindicated.

[1]
[2]

[3]
[4]

Conclusion
Using the OES technique, the electron temperatureand the concentration levels of methane radicals (CHand CH+) were inferred from the intensity ratios ofspectral lines. Their values were plotted as functionsof the diﬀerent deposition parameters (plasma current,
substrate temperature and pressure) and were analyzed
to determine the optimum values of parameters for low
temperature diamond deposition. Using these optimized values for methane concentration, plasma cur-[5]

[6]
[7]

148

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