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Vacuum 84 (2010) 958–961

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Vacuum

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Mid-frequency deposition of a-C:H films using five different precursors

S. Peter a,*, M. Gunther a, D. Hauschild a, D. Grambole b, F. Richter a

a Chemnitz University of Technology, Institute of Physics, D-09107 Chemnitz, Germanyb Forschungszentrum Dresden-Rossendorf, Institute of Ion Beam Physics and Materials Research, D-01314 Dresden, Germany

Keywords:Diamond-like carbona-C:HPECVDPulsed dischargePrecursor

* Corresponding author. Fax: þ49 371531838258.E-mail address: s.peter@physik.tu-chemnitz.de (S.

0042-207X/$ – see front matter � 2010 Elsevier Ltd.doi:10.1016/j.vacuum.2010.01.023

a b s t r a c t

The plasma-enhanced chemical vapour deposition (PECVD) of amorphous hydrogenated carbon filmsfrom pulsed discharges with frequencies in the range from 50 kHz to 250 kHz was investigated. Fivedifferent hydrocarbons (acetylene C2H2, isobutene C4H8, cyclopentene C5H8, toluene C7H8 and cyclo-heptatriene C7H8) were probed as film growth precursors. In addition, two types of pulse-generatorswith somewhat different waveforms were used to power the discharges in the so called mid-frequencyrange. The a-C:H films deposited in a parallel-plate reactor were characterised for their thickness/deposition rate, hardness and hydrogen content. The hydrogen concentration in the films varied between19 at.-% and 37 at.-%. With the substrate temperature held constant, it is roughly in inverse proportion tothe hardness. The film with the highest hardness of 25 GPa was formed at a deposition rate of 0.8 mm/h inthe C2H2 discharge at the lowest investigated pressure of 2 Pa. With increasing molecular mass of theprecursor mostly weaker films were deposited. Relatively high values of both deposition rate andhardness were achieved using the precursor isobutene: a hardness of 21 GPa combined with a depositionrate of 4.1 mm/h. From the probed precursors, isobutene is also most advantageous for a-C:H depositionat higher pressures (up to 50 Pa investigated). But, as an over-all trend, the a-C:H hardness decreaseswith increasing deposition rate.

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1. Introduction

Films of hard hydrogenated amorphous carbon, a kind of dia-mond-like carbon (DLC), combine several attractive properties suchas high hardness, low friction coefficient, chemical inertness,dielectric strength and transparency for infrared light. For largearea deposition of a-C:H the plasma decomposition of hydrocar-bons is commonly used. Typical growth rates of DLC by PECVD are0.3–3 mm/h [1]. Both radicals and ions contribute to the filmformation process. The sub-surface modification by energetic ionsis important for cross-linking of the amorphous carbon networkand hydrogen abstraction. About 50 eV ion energy per depositedcarbon atom is necessary to create hard films [2].

Radio frequency discharges are applicable to small reactors(<0.1 m3). The use of asymmetric bipolar pulsed discharges oper-ating between 50 and 350 kHz is advantageous for the effectivePECVD of DLC films on an industrial scale ([3,4]).

The precursor has a large influence on the plasma and filmproperties. Several presumptions about favourable precursorproperties have been published:

Peter).

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- The deposition rate increases approximately exponentiallywith decreasing ionisation energy [5].

- The H/C ratio of the precursor influences the hydrogen contentin the plasma. Hydrogen reduces the deposition rate by satu-ration of dangling bonds and by chemical sputtering [6].

- The size of the precursor molecule plays a role as at the sameflow rate larger molecules deliver more carbon [7].

- Finally, the hybridisation of the precursor plays a role as well,since unsaturated sp1- and sp2-groups can polymerise [8].

Basic requirements concerning the precursor are: sufficientvapour pressure, low toxicity, easy to store and handle, and lowprice. Table 1 summarises some properties of the selected fiveprecursors.

The main object of the investigations presented was the gain ofbasic knowledge of the required properties of suitable precursorsfor high-rate deposition of hard a-C:H films.

2. Experimental

The a-C:H films were deposited onto silicon substrates (n-Si(100); r< 0.004 Ucm) in a computer controlled deposition systemMicroSys 400 (Roth&Rau) equipped with a load-lock (see [2]). Thedischarge in the parallel-plate reactor was powered by two types of

Table 1Precursors probed for a-C:H deposition and some of their properties (Mmol.: molarmass, Tboil.: boiling temperature; H/C: hydrogen to carbon ratio, IE: ionisationenergy).

Name Acetylene Isobutene Cyclopentene Toluene Cycloheptatriene

CAS nr. 74-86-2 115-11-7 142-29-0 108-88-3 544-25-2Formula C2H2 C4H8 C5H8 C7H8 C7H8

Structure

Mmol. [g] 26 56 68 92 92Tboil. [�C] �84

(gas)�6

(gas)44 111 117

H/C 1 2 1.6 1.14 1.14IE [eV] 11.4 9.2 9.0 8.8 8.2

S. Peter et al. / Vacuum 84 (2010) 958–961 959

PinnaclePlusþ power supplies (Advanced Energy) with differentmaximum output DC-voltages (Generator A: 650 V; Generator B:800 V) and different voltage–time characteristics (see Fig. 1a). Thepulse frequency f was varied between 50 kHz and 250 kHz. Voltageand current probes were placed very close to the vacuum feed-through. The power supplied to the plasma chamber was measuredcontinuously by means of a digital oscilloscope DPO 7054 (Tek-tronix) by integrating the product of voltage and current over oneperiod.

0

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0 5 10Time [µs]

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Fig. 1. Time-resolved characteristics of the two mid-frequency generators: (a) voltage,(b) current and (c) integrated power (120 sccm C2H2þ 40 sccm Ar; p¼ 10 Pa;f¼ 100 kHz; Tþ¼ 2.5 ms).

Fig. 1 a–c shows typical waveforms of voltage, current and power.The voltage waveforms consist – after an initial overshoot – ofa positive pulse of around 100 V, followed by a negative pulse. Thenegative peak amplitudes between �700 V and �1400 V are clearlyhigher than the nominally DC-voltages of the generators. Beside thepulse period T (reciprocal of the frequency f) also the duration of thepositive voltage Tþ (reverse time) was varied. The negative pulsesformed by generator B are characterised by shorter rise times andnearly constant voltage values within the first four microseconds.The capacities of the feed-through and the substrate electrodeinduce some reactive power (see high displacement current inFig. 1b and power oscillations in Fig. 1c). The values of the plasmapower (typically a few ten watts) were much lower than the powervalues indicated by the 5 kW/10 kW-generators. Therefore, thegenerators were operated in voltage regulation mode. Under theinvestigated experimental conditions the voltage waveforms werenearly unaffected by the process parameters, e.g. the pressure andthe precursor gas.

The flow rates of the gaseous precursors acetylene (99.6%) andisobutene (99%) were set by mass flow controllers (Bronkhorst). Theflows of the liquid precursors cyclopentene C5H8 (97%), tolueneC7H8 (99%) and cycloheptatriene C7H8 (95%) were regulated bya Liqui-Flow mass flow controller system for liquids (range 0.10 g/hfor water; Bronkhorst) with argon as carrier gas for the vaporisedhydrocarbons. Argon (99.999%) was partially also added to acety-lene and isobutene discharges. The operation chamber pressure wasmeasured using a capacitance manometer (MKS) and controlled byan automatic throttle valve (MKS). The base pressure of the stain-less-steel vacuum chamber was <3�10�5 Pa. The temperature ofthe powered electrode was controlled to 100 �C by radiation heatingfrom the backside. Oxygen plasma etching was used to clean theelectrodes after each deposition experiment. The area of the pow-ered electrode was 133 cm2, the area of the grounded electrodeamounted 200 cm2 and the interelectrode distance was fixed at5 cm.

The thickness of the deposited films was measured with a surfaceprofilometer Form Talysurf 50 (Rank Taylor Hobson). A nano-indentation system UMIS 2000 (CSIRO) equipped with a Berkovich-type diamond indenter tip was used to measure the hardness of thefilms. The thickness of films used for hardness measurements wasabove 1 mm to prevent any influence of the substrate.

The total hydrogen content in the a-C:H films was measured bynuclear reaction analysis (NRA; [9]).

3. Results and discussion

Several series of deposition experiments including all fiveprecursors were realised with generator A operated at its maximumoutput voltage of 650 V. The parameters varied were the pressure, thegas flow ratios (precursor to argon), the frequency and the reversetime. Fig. 2 shows the correlation of deposition rate and hardness forfilms deposited at a constant frequency of 100 kHz and a fixed reversetime of 2.5 ms, but different pressures in the range from 2 Pa to 30 Pa.Stable operation of the mid-frequency discharges at pressures below2 Pa was not possible. For all precursors deposition at 2 Pa resulted inmaximum film hardness and the highest value of 25 GPa wasobtained with the precursor acetylene at a rate of 0.8 mm/h. The samehardness value was measured for a-C:H deposited from thisprecursor and at this pressure in a capacitively coupled 13.56 MHzdischarge in the same equipment [2]. Using generator A, filmsdeposited from the other four precursors at maximum reacheda hardness in the range from 17 GPa to 19 GPa. Worth mentioning isthe observed slow decrease in hardness with increasing depositionrate for the precursor isobutene. Looking at films with hardnessabove 12 GPa, cycloheptatriene yields the lowest a-C:H growth rate in

0

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0 5 10 15Deposition rate [µm/h]

Har

dnes

s [G

Pa]

Acetylene

Isobutene

Cyclopentene

Toluene

CycloheptatrieneGen. A / 650 V

Fig. 2. Reduction of hardness with increasing growth rate for films deposited from thefive precursors and varying the pressure in the range from 2 Pa to 30 Pa (generator A;U¼ 650 V; f¼ 100 kHz; Tþ¼ 2.5 ms).

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[at.%

]

Fig. 3. Influence of the pressure on (a) the discharge power and deposition rate, and(b) hardness and hydrogen concentration of films deposited from acetylene (120 sccmC2H2þ 40 sccm Ar; Generator B, U¼ 800 V; f¼ 100 kHz; Tþ¼ 2.5 ms).

0

5

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25

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15 20 25 30 35 40Hydrogen concentration in a-C:H [at.%]

Har

dnes

s [G

Pa]

Fig. 4. Correlation of film hardness and total hydrogen concentration in a-C:Hdeposited from the five precursors in mid-frequency discharges (substrate tempera-ture: 100 �C).

S. Peter et al. / Vacuum 84 (2010) 958–961960

spite of the fact that this precursor molecule is characterised by thelowest ionisation energy (cf. Table 1).

There are only a few publications about the relation hardness –deposition rate for a-C:H films deposited by mid-frequency PECVD.With methane as precursor gas and 22 kHz bipolar pulseddischarges growth rates between 1.5 mm/h and 4 mm/h wereaccompanied by hardness of 12 GPa to 22 GPa [3]. Using theprecursor acetylene, a pressure of 2.5 Pa, a frequency of about200 kHz and voltage waveforms very similar to generator A,a deposition rate of 1.3 mm/h combined with a film hardness of19 GPa was achieved [10]. Here, using generator A this hardnessvalue was measured for a film deposited from C2H2 at 8 Pa ata growth rate of 3 mm/h (see Fig. 2). One can deduce therefore, thatalso other parameters like gas flow rate, used generator or reactordesign should have large influence on the deposition process.

At pressures of a few Pa the a-C:H deposition rate was limited bythe low power absorption in the mid-frequency discharges. Hence,to achieve maximum discharge power, generator A was operatedwith the output voltage fixed to its maximum value of 650 V.A second generator B offers higher output voltage and thereforehigher discharge power. Fig. 3 shows the influence of the pressureof a C2H2–Ar gas mixture on a-C:H deposition with generator Boperated at 800 V. From Fig. 3a it becomes evident, that thedeposition rate is directly correlated to the discharge power.Compared to generator A (650 V), the deposition rates withgenerator B for any pressure in this range are higher by one thirdwhen working with maximum output voltage (800 V) and lower byabout ten percent with its DC-voltage set also to 650 V. Thebehaviour of the deposition rates again matches with the respectivedischarge powers. By contrast, for any pressure from 4 Pa to 10 Pa,the hardness of films deposited with generator B is equal for bothvoltage settings and up to 3 GPa higher compared to depositionswith generator A. Using the precursor C2H2 at a pressure of 2 Pa themaximum film hardness of 25 GPa could be realised with bothgenerators.

With isobutene, stable discharges were possible also at highpulse-frequencies. Thus an a-C:H film with a hardness of 21 GPacould be deposited at a rate of 4.1 mm/h (2 Pa C4H8; f¼ 250 kHz;Tþ¼ 1 ms)

As visible from Fig. 3b, the hardness behaves contrary to thetotal hydrogen content of the films. In Fig. 4 this correlation isshown for in total 69 a-C:H films deposited from the five precursorsunder very different process conditions. From this, at constantdeposition temperature the total hydrogen content controls thehardness to a large extent. But, the data points from some experi-mental series – especially with the intensively investigatedprecursor isobutene – trend across the main correlation. Further

film- and plasma-diagnostic measurements are required to gain aninsight into the responsible mechanisms.

As above mentioned, argon was used as carrier gas for the liquidprecursors. For experimental purposes it was also added to the twogaseous hydrocarbons. Since the ionisation energies of all fiveprecursors (cf. Table 1) are below the energy of the metastable stateof the argon atom (11.55 eV) in principle Penning-ionisation ispossible. Another imaginable effect of metastable argon states isthe enhancement of molecular dissociation.

As a positive effect, the experiments showed, that argon additionstabilises the mid-frequency discharges. But in view of the a-C:Hdeposition characteristics it seems to act above all as a diluent – justas it reduces the pressure. With increasing argon admixture, thea-C:H deposition rate is reduced more and more. But above all, foreach precursor the behaviour of the hardness was found to be similarto the effect of a pressure reduction. Argon addition especially tocycloheptatriene or toluene discharges led to distinctly harder a-C:H

S. Peter et al. / Vacuum 84 (2010) 958–961 961

films, whereas with isobutene the hardness practically was noteffected. Thus, the energy transfer from excited argon to theprecursor and the growing film seems to be less important.

From published results about the interaction of metastableexcited argon with hydrocarbon molecules one can conclude, thatthe quenching of this states is very strong. During measurementsperformed at pressures above 67 Pa [11] a marked Penning effectwas observed for acetylene, but only for very low acetylene admix-tures (about 0.1% C2H2 in Ar). Excitation transfer from metastablestates of argon to benzene was analysed in the afterglow of micro-second-pulsed discharges [12]. Again, at somewhat higher pressures(>266 Pa), the density of metastable excited argon was drasticallyreduced when adding as little as 400 ppm benzene to argon.

Another aspect is the strong reduction of electron temperatureswhen changing from atomic to molecular gas discharges. This mayexplain the observations in [13], originally attributed to thePenning effect: a much higher degree of methane dissociation in anAr-rich (95% Ar) argon–methane glow discharge compared to puremethane and a hydrogen–methane mixture with 95% H2. Mole-cules, and in particular large hydrocarbon species, show higherelastic collision cross sections for electrons and they can take upelectron energy by rotational and vibrational excitation. In thehydrocarbon–argon discharges hence the electron energies may belower than the relatively high threshold energies necessary toexcite and ionise argon.

Experimental results about the electron energies have beenpublished for asymmetric 100 kHz bipolar pulsed discharges inmethane at 10 Pa [14]. The discharges were analysed at the centreof the interelectrode gap by time-resolved Langmuir probemeasurements. From these measurements, the electron tempera-ture and energy are relatively low during the off phase (reversetime Tþ). During the on phase, when ions bombard the growingfilm, a bi-Maxwellian population of electrons was found. Averagedover one pulse, the cold electrons had (nearly independent of thedischarge power) an electron temperature of only 0.6 eV anda density of 2�1010 cm�3. For the hot electron populationa constant averaged energy of nearly 6 eV and an increasing withthe discharge power (from 18 W to 52 W) electron density from1�109 cm�3 to 1.9�109 cm�3 were obtained. The energy gain ofthe hot electrons was attributed to stochastic heating from therapidly advancing sheath edge during the polarity reversal [14].

Another possible opportunity is, that the hot electrons primarywere secondary electrons emitted from the powered electrode byion bombardment and accelerated in the sheath. Due to the verysmall energies of the majority of electrons, ionising collisions ofthe hot electrons should again be responsible for the formation ofthe ions bombarding the growing a-C:H. Argon has a high ion-isation energy of 15.8 eV. Compared with the hydrocarbons, itselectron impact ionisation cross section at low electron energysion.(Ee�) is very small: for argon sion.(17 eV)¼ 0.017�10�16 cm2

[15]; acetylene sion.(15 eV)¼ 0.68� 10�16 cm2 [16]; toluene: sio-

n.(15 eV)¼ 2.5�10�16 cm2 [17]. Thus, the share of argon ions inthe ion flux to the growing film is expected to be marginal.

From the comparison of the five hydrocarbons, no specificprecursor property (e.g. ionisation energy, C/H-ratio) is the crucialone for the high-rate deposition of hard a-C:H films. The key factorseems to be the appropriate ion bombardment of the growing film. Itis specific for a precursor since at least (a) the initial cross-linking atthe surface depends on the radicals formed from the precursor

molecules and (b) the relative contributions of radicals and ions tofilm growth differ. Additionally, heavier hydrocarbon ions requireappropriate more energy to have the same energy per carbon atom.Finally, one has to take into account also the energy losses by colli-sions, relevant in the investigated pressure range. Thus the lowhardness reached with cyclopentene, toluene and cycolheptatrieneis not a specific of this hydrocarbons but a consequence of theinsufficient ion bombardment. When depositing a-C:H at lowerpressure in a capacitively coupled, strongly asymmetricRF-discharge, film hardness increased with increasing precursormass [18].

The precursor isobutene showed similar deposition character-istics to methane, investigated in RF-discharges [2], but offers muchhigher deposition rate. It is an attractive candidate for the deposi-tion of medium-hard a-C:H at relatively high pressures.

4. Conclusions

An experimental comparison of the a-C:H deposition processesin bipolar pulsed mid-frequency discharges using five differentprecursors has been carried out. When keeping the substratetemperature fixed, the hardness of the films is determined to a highdegree by their total hydrogen content. Using discharges pulsed atfrequencies in the order of 100 kHz, an acceptable power inputtakes place at pressures above a few Pa. Under these conditions,large hydrocarbon molecules are not applicable as DLC precursors.The critical parameter is the ion flux to the growing film. From theprobed precursors, isobutene was found out to be best suitable forthe mid-frequency PECVD of DLC films.

Acknowledgment

The authors would like to thank Petr Belsky for his contributionin film deposition and hardness measurement.

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