Plasma-assisted CVD of hydrogenated diamond-like carbon films by low-pressure dielectric

barrier discharges

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2001 J. Phys. D: Appl. Phys. 34 1651

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INSTITUTE OF PHYSICS PUBLISHING JOURNAL OF PHYSICS D: APPLIED PHYSICS

J. Phys. D: Appl. Phys. 34 (2001) 1651–1656 www.iop.org/Journals/jd PII: S0022-3727(01)21330-8

Plasma-assisted CVD of hydrogenateddiamond-like carbon films bylow-pressure dielectric barrier dischargesDongping Liu1, Tengcai Ma2, Shiji Yu2, Yong Xu1 andXuefeng Yang1,3

1 Laboratory of Plasma Physical Chemistry, Box 288, Dalian University of Technology,Dalian 116024, People’s Republic of China2 State Key Laboratory for Material Modification by Laser, Ion and Electron Beams,Dalian University of Technology, Dalian 116024, People’s Republic of China

E-mail: labplpc@dlut.edu.cn

Received 25 January 2001

AbstractHydrogenated diamond-like carbon (DLC) films have been deposited onsilicon substrates from dielectric barrier discharge (DBD) plasmas of CH4 atroom temperature under a pressure of 0.4–4.0 Torr. The effects of dischargegas pressure (P ), the applied peak voltage and the distance of the dischargegas spacing (d) on the film quality have been systematically investigated.The film hardness is mainly dependent on the Pd value and applied peakvoltage. The best films of φ40–70 mm with Knoop hardness up to 20 GPacan be deposited at 30 kV peak voltage with a Pd value of about 2 Torr mm.The deposited films were characterized by scanning electron microscopy,Raman and FTIR spectroscopy. These analyses show that the depositedfilms are homogeneous hydrogenated amorphous carbon films with verysmooth surfaces containing significant amounts of sp3 C–C bonding.The high-voltage and current waveform measurements of the dischargeindicate that the low-pressure DBD consists of uniform (along the wholeelectrode) glow-like single breakdowns with half-widths of severalmicroseconds. The DBD-induced deposition technique used in this workhas many advantages, including the simplicity of the experimental set-up,large area deposition of DLC films and a lower consumption of feed gas andelectric power.

1. Introduction

Hydrogenated (a-C:H) or hydrogen-free (a-C) diamond-likecarbon (DLC) films exhibit many unique properties similarto those of diamond that make DLC films ideal for a widevariety of applications. Their superior hardness, high wear-and-corrosion resistance and low-friction coefficient enablethem to be used in magnetic recording media [1], whereastheir chemical inertness and infrared transparency favour theirfurther use in magneto-optic and opto-electronic devices [2, 3].The necessity to grow DLC films drove the development ofvarious deposition techniques in the last decade. A rather richvariety of low-pressure (usually 10−2–10−4 Torr) depositiontechniques, such as radio-frequency [4] or microwave

3 Author to whom correspondence should be addressed.

discharge [5] plasma enhanced chemical vapour deposition,ion beam assisted deposition [6] and filtered cathodic vacuumarc deposition [7], have been successfully used to prepare DLCfilms. In the above techniques, DLC films can be grown onvarious substrates from pure hydrocarbons, such as methaneand acetylene, at room temperature and negative substrate biasis usually needed to maximize the positive ion flux with suitableion energy at the substrate [8].

Dielectric barrier discharges (DBDs), also referred to asbarrier discharges or silent discharges, are characterized by thepresence of one or two layers of insulator (dielectric barrier)between the electrodes. Due to their ability to produce non-equilibrium plasmas around atmospheric pressure, numerousapplications of DBDs have been found in ozone production,surface treatment, high-power lasers, excimer ultraviolet

0022-3727/01/111651+06$30.00 © 2001 IOP Publishing Ltd Printed in the UK 1651

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Figure 1. The apparatus for a-C:H film deposition system assisted in low-pressure barrier discharge plasmas.

lamps, pollution control and large-area flat plasma displaypanels [9]. Very recently, this laboratory [10] has reported theattempt to deposit DLC films in atmospheric H2/CH4 DBDplasmas on heated substrates. The energy of hydrocarbon ionsformed in the atmospheric non-thermal plasmas hardly reaches100 eV level, which is believed to be the optimum value invarious low-pressure DLC film deposition techniques. Therelatively hard films (about 10 GPa) deposited in atmosphericDBD plasmas may be formed via interactions between thesolid surface and H/CH3 radicals, similar to those in diamondCVD. Besides the continuing effort to improve the DLC filmquality by atmospheric DBDs, in this article we report anothernovel approach to prepare DLC films at low-pressure (around1 Torr) CH4-DBD plasmas, which keeps the majority of theadvantages of DBD plasma-assisted CVD, for example simpleexperimental set-up, large-area deposition, low consumptionof feed gas and electric power.

2. Experimental details

The DLC film deposition system by low-pressure barrierdischarges is shown in figure 1. The ac power supply source iscapable of supplying bipolar sine wave output with 0–30 kVpeak voltage at an ac frequency of 1.4 kHz. The vacuum-tightdischarge chamber with gas volume of φ100 × 10 mm mainlyconsists of a stainless-steel ground electrode and a parallel5 mm thick glass dielectric barrier plate. The stainless-steel,high-voltage electrode is attached to the top of the glass plate inair at atmospheric pressure in order to prohibit the high-voltagebreakdown around the glass. High-purity methane (>99.99%)discharge gas flows at a rate of 1–3.5 standard cm3 min−1

(sccm) through the 1–10 mm thick adjustable gas spacingbetween the glass dielectric and the ground electrode, on topof which the silicon (Si) substrate is placed. The depositionconditions used in this work are summarized in table 1. a-C:Hfilms have been prepared at room temperature under variousdischarge conditions by varying peak voltage (Up), dischargegas spacing (d) and deposition pressure (P ).

FTIR absorption spectroscopy is mainly used to analysethe relative contents of various C–H bonds in the a-C:H films[7, 8]. These quantities depend on the state of the carbon atomin the bond, namely, the type of its hybridization (sp3, sp2 or sp)and hydrocarbon groups (CH3, CH2 or CH). The concentrationof any sort of C–H bonds reported here is proportional to the

Table 1. Deposition conditions.

Substrates Silicon (111) (φ40–70,550 nm thick)

Dielectric Glass (5 mm thick)CH4 gas flow rate 1–3.5 sccmTotal gas pressure(P ) 0.4–4 TorrFrequency of the power supply (f ) 1.4 kHzAc peak-to-peak voltage (Up) 18–30 kVDischarge gas spacing (d) 1–10 mmDeposition time ∼2 h

absorption integral:

Ni = Ai

∫(αi(ω)/ω) dω

where Ni is the concentration of a certain type of C–H bonds,αi(ω) is the absorption coefficient at wavenumber ω and Ai isan experimental coefficient.

Hardness measurements were performed using a micro-hardness tester under a load of 10 g. Scanning electronmicroscopy (SEM) and Raman spectroscopy were also utilizedto characterize the deposited a-C:H films. The SEM (PHILIPSLX-30) was carried out mainly on specimen cross sections thatwere produced by flexing to fracture a-C:H-coated Si crystalspecimens. Raman spectra, usually scanned from 1100 to1700 cm−1, were obtained by a double spectrometer of SPEXvia near back-scattering geometry using an Ar+ laser operatedat 514.5 nm. Simultaneous measurements of high-voltage andcurrent waveforms at different experimental conditions weremade using a high-voltage probe (EP-100K, Pulse ElectronicEngineering Co, Japan), a current probe (Tektronix A6303)and a digital oscilloscope (LeCroy, LT 322).

3. Results and discussion

3.1. Discharge parameters and their effects on film hardness

The dependence of DLC film Knoop hardness on applied peakvoltage (Up) at a discharge gas spacing (d) of 5 mm and CH4

pressure (P ) of 0.5 Torr is shown in figure 2. The film hardnessrises from 13 to 20 GPa when UP is increased from 18 to 30 kV.The dependence of film hardness on the deposition pressure atdifferent gas spacings is shown in figure 3, which indicatesthat the film hardness increases with the decreasing depositionpressure at a given gas spacing.

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Hydrogenated diamond-like carbon films

16 18 20 22 24 26 28 30 3210

12

14

16

18

20

UP (kV)

Kn

oo

p h

ard

nes

s (G

Pa)

Figure 2. The evolution of the Knoop hardness of a-C:H filmsdeposited at P = 0.5 Torr and d = 5 mm as a function of appliedpeak voltage (Up).

0.0 0.5 1.0 1.5 2.0 2.5 3.00

5

10

15

20 1mm 2mm 5mm 10mm

Har

dn

ess

(GP

a)

Pressure (Torr)

Figure 3. Dependence of a-C:H film Knoop hardness on depositionpressure (P ). d = 1, 2, 5 and 10 mm, UP = 30 kV.

A careful observation drawn from these experiments is thatthe film hardness mainly depends on the Pd value (product ofP and d) of the discharge, as shown in figure 4, where the filmshave been deposited at a pressure of 0.4–3.0 Torr and a gasspacing of 1–10 mm. The film hardness varies from 2 to about20 GPa when Pd value decreases from 14 to 2 Torr mm. Inorder to understand such a relationship, the breakdown voltage(Ub) of CH4 as a function of Pd value was measured by usinga plane-to-plane (brass electrode) dc glow discharge cell with30 mm discharge gap in this laboratory (figure 5). Consistentwith the well-known Paschen law of gas discharges, while thePd value decreases, the CH4 breakdown voltage is reducedat large Pd but turns to rise steeply when Pd is less than5 Torr mm. The deposition of DLC films arises from positiveions with 75–150 eV optimum energy penetrating surfacelayers and generating more sp3 C–C bonding [4, 5, 8]. Asshown in figure 5, the increased breakdown voltage at smallerPd causes a larger potential decline across the alternative andimpulsive DBD cathode sheath and the lower pressure causesan increase of the mean free path of collisions, so that thepositive hydrocarbon ions can become more energetic throughsuch a sheath, leading to the increase of film hardness.

The frequency of ac power supply has dramatic effectson the film hardness and its deposition rate. At an appliedvoltage of 30 kV and Pd value of about 2 Torr mm, when

0 2 4 6 8 10 12 14 160

2

4

6

8

10

12

14

16

18

20

22

Har

dn

ess

(GP

a)

Pd (Torr mm)

Figure 4. Dependence of a-C:H film Knoop hardness on the Pdvalue of the discharge. UP = 30 kV, P = 0.4–3.0 Torr,d = 1–10 mm.

1 10 100 1000200

400

600

800

1000

1200

1400

1600

1800

2000U

b (

V)

Pd (Torr mm)

Figure 5. Variation of breakdown voltage (Ub) for CH4 versus Pdvalue, obtained using a plane-to-plane dc glow discharge cell with30 mm discharge gap.

the ac frequency is varied from 1.4 kHz to 50 Hz, the filmhardness decreases from 20 to 6 GPa, while the deposition ratedecreases from ∼0.9 to less than 0.2 Å s−1. The density of thebest DLC films with Knoop hardness of ∼20 GPa was foundto be ∼1.8 g cm−3.

3.2. Film characterization by SEM, Raman and FTIRspectroscopy

φ40–70 mm DLC films with very smooth surfaces (meansurface roughness less than 0.02 µm) were produced under theconditions described previously. Figure 6 is a SEM micrographof a cross section of a fractured DLC film, which was depositedon a Si substrate at Up = 30 kV, d = 5 mm and a CH4 pressureof 0.5 Torr. The film thickness is approximately 640 nm. In themicrograph, the continuity is evident across the Si/DLC filminterface, indicating the good adherence between Si and thefilm. All the tested DLC films have uniform microstructures.

Raman spectra are sensitive to changes in translationalsymmetry and are thus useful for the study of disorder orcrystalline in the films. The Raman spectra of crystallinediamond and graphite consist of a single line at 1332 cm−1 (D)and 1580 cm−1 (G), respectively. In the Raman spectra of DLCfilms prepared using different techniques (e.g. [11, 12]), two

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Figure 6. The cross section SEM of a typical a-C:H film depositedon a Si substrate at P = 0.5 Torr, d = 5 mm and Up = 30 kV.

1100 1200 1300 1400 1500 1600 1700200

400

600

800

1000

1200

1400 1529cm-1

1329cm-1

Ram

an s

ign

al (

c/s)

Wavenumber (cm-1)

Figure 7. Raman spectrum of a typical a-C:H film deposited atP = 0.5 Torr, d = 5 mm and Up = 30 kV.

Table 2. Absorption peaks in the C–H stretching vibration region.

Wavenumber (cm−1) Bond type

2850 sp3 CH2 (sym)2870 CH3 (sym)2905 sp3 CH2925 sp3 CH2 (asym)2970 CH3 (asym)3000 sp2 CH

broad and continuous peaks around 1345 ± 30 cm−1 (D-band)and 1550 ± 30 cm−1 (G-band) have been observed. This isattributed to the lack of long distance translational symmetryin these DLC films. In this work, the Raman spectrum ofthe same DLC film used for the above SEM analysis wasobtained, as shown in figure 7. Two broad peaks, centredat 1329 and 1529 cm−1, respectively, were deconvoluted fromthe spectrum. According to Wagner et al’s model [13], theG-band in DLC Raman spectra is assigned to a graphite-likesp2-bonded phase and the D-band is assigned to the sp3-bondedphase. Consistent with this model, when UP increases from18 to 30 kV (P = 0.5 Torr, d = 5 mm), the intensity ratioof the D-band over the G-band for the DLC films obtained inthis work increases from 0.43 to 0.57, and the correspondingKnoop hardness rises from 13 to 20 GPa.

All the typical a-C:H samples deposited under variousexperimental conditions were studied by FTIR absorptionspectroscopy. Each spectrum clearly shows an absorption

2750 2800 2850 2900 2950 3000 3050

(a)

sp2CH

CH3

sp3CH2

sp3CH

CH3

sp3CH

2

Ab

sorp

tio

n

Wavenumber (cm-1)

2750 2800 2850 2900 2950 3000 3050

(b)

Ab

orp

tio

n

Wavenumber (cm-1)

2750 2800 2850 2900 2950 3000 3050

(c)

Ab

sorp

tio

n

Wavenumber (cm-1

)

Figure 8. Infrared absorption spectra of a-C:H films deposited atd = 2 mm and Up = 30 kV. The deposition pressure and filmKnoop hardness are: (a) 1 Torr and 17.5 GPa; (b) 1.4 Torr and12.4 GPa; (c) 4 Torr and 0.5 GPa.

section in the range 2700–3100 cm−1 due to C–H stretchvibrations. The CHx (x = 1–3) groups and their known stretchvibration frequencies are summarized in table 2 [14, 15].Figures 8(a)–(c) show the curve-fitted absorption spectra of thesamples deposited on Si substrates at d = 2 mm, UP = 30 kVand deposition pressures of 1, 1.4 and 4 Torr, respectively.The film deposited at a pressure of 1 Torr with hardness of18 GPa has a weak sp2 CH absorption peak at 3000 cm−1,while the one deposited at a pressure of 4 Torr with hardnessof 0.5 GPa has a large CH3 absorption peak at 2970 cm−1.The ratios of CH3/(CH + CH2) and sp3 (CH + CH2)/sp2

(CH + CH2) as functions of Pd value are shown in figures 9(a)and 9(b), respectively. The CH3/(CH + CH2) ratio increasesalmost linearly with increasing Pd value. It is evident that

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2 4 6 8 10 12

0.2

0.4

0.6

0.8

1.0 (a)

CH

3/(C

H+

CH

2)

Pd (Torr mm)

2 4 6 8 10 121

10

100(b)

sp3 (C

H+

CH

2)/s

p2 (C

H+

CH

2)

Pd (Torr mm)

Figure 9. The ratios of CH3/(CH + CH2) (a) and sp3

(CH + CH2)/sp2 (CH + CH2) (b) as functions of Pd value.UP = 30 kV, P = 0.4–4.0 Torr, d = 1–10 mm.

the higher content of CH3 group in the films causes theirhardness to be decreased greatly. This is because the CH3

group is a terminated one and does not contribute to theformation of the cross-linked three-dimensional network [16].The higher CH3 group content also leads to the formation ofsoft polymeric films with high H content [17]. In contrast, thesp3 (CH + CH2)/sp2 (CH + CH2) ratio rises with decreasingsmall Pd value (figure 9(b)) indicating the contribution of sp3

C–C bonding to the film hardness, which is very likely formedfrom energetic ion bombardment at very small Pd. In the largePd range (4–10 Torr mm), the sp3 (CH+CH2)/sp2 (CH+CH2)ratio turns so as to be slowly increased with the increasing Pd

value (similar to the variation tendency of breakdown voltagewith Pd), but this will not effect the film hardness much dueto the high CH3 content formed in this range.

3.3. Low-pressure dielectric barrier discharge physics

Figure 10 shows the applied voltage and current waveformobtained at d = 5 mm and P = 0.75 Torr and the plasma isproduced between the glass dielectric (G) and the Si substrate(S). It can be seen that the amplitude and the number of currentpulses in a given time interval are nearly proportional to theslope of the voltage sine wave. The current directions (positiveor negative) and the averaged amplitude of the current pulsesare different in the voltage-rising and voltage-falling half-cycles. Similar experimental results were acquired in an air-DBD plasma where the ozone generator having a glass-metaldischarge configuration was operated by a rectangular wavesource supply with 1–2 kHz ac frequency [18]. The above-observed differences in two half-cycles are due to the different

Figure 10. Voltage and current waveforms of the barrier dischargeproduced at d = 5 mm and P = 0.75 Torr. (a) in 200 µs/div; (b) in10 µs/div.

mobility of electrons and ions on the glass surfaces. In thecase of G(−)S(+) polarity, the glass becomes a cathode and thepositive ions accumulate on the glass dielectric. The positiveions have low mobility and the ‘net’ electric field applied tothe discharge is decreased very soon due to the rapid formationof the reversed electric field in the discharge gas space. In thecase of G(+)S(−) polarity, the glass becomes an anode and, bycontrast, the electrons accumulated on the glass surface withhigh mobility cause the ‘net’ electrical field to be decreasednot so rapidly.

The averaged power, W , of the discharge is calculated bythe following equation from the measured current waveform,I (t), and the voltage waveform, V (t), over one cycle:

W = f

∫ t+T

t

I (t)V (t) dt

where f and T are the ac frequency and the duration of thepower supply. About 5–8 W was consumed when ∼φ40 mmDLC films were deposited at Up = 30 kV, f = 1.4 kHz andPd = 2–3 Torr mm.

For clarification, the extended voltage and currentwaveforms measured at d = 5 mm and 3 mm (with the sameUp of 30 kV and P of 0.75 Torr) are shown in figure 10(b)and figure 11, respectively. When d is decreased from 5 to3 mm, due to the breakdown voltage Ub increasing with thedecreasing small Pd value (figure 5), the full-width-at-half-maximum of the current pulses varies from ∼1 to ∼2.7 µs and

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Figure 11. The extended voltage and current waveforms obtained atUP = 30 kV, d = 3 mm and P = 0.75 Torr.

the corresponding time interval between two adjacent pulsesvaries from ∼5.8 to ∼26 µs.

The peak current density, 0.3–1 mA cm−2, which can becalculated from the peak current value and the electrode area, iswithin the range of the glow discharge. Due to this fact and thenearly uniformly distributed current pulses, it can be concludedthat the low-pressure DBDs consist of glow-like and large-area uniform (along the whole electrode) single breakdownswith a half-width of several microseconds. This characteristiccontributes to the formation of smooth surfaces of the DLCfilms. The impulsive breakdowns in low-pressure DBDsproduce hydrocarbon ions, such as CHx (x = 1–4) and C2Hx

(x = 1–3), which have been detected by using a molecularbeam mass spectrometer with three-stage differential pumpingin this laboratory; the results will be reported later. Thehydrocarbon ions are accelerated in the cathode sheath regionand form bombardments on the Si substrate surface before thenext breakdown is initiated.

4. Conclusion

In conclusion, a simple vacuum-tight discharge chamber usedto produce low-pressure DBD plasma has been designed andDLC films have been successfully deposited on Si substrates inthe thus-produced DBD plasmas. The influences of dischargegas pressure (0.4–4.0 Torr), the applied peak voltage (up to30 kV) and the discharge gas spacing (1–10 mm) on thefilm quality have been systematically investigated. The bestfilms of φ40–70 mm with Knoop hardness up to 20 GPa canbe deposited at 30 kV peak voltage with Pd value around2 Torr mm. The Pd value in the deposition chamber hasgreat effect on film hardness and the film hardness obviouslyincreases with decreasing small Pd value. The SEM andRaman spectroscopy characterization of the deposited filmsshows that they are homogeneous amorphous carbon filmswith very smooth surfaces. From the FTIR absorptionspectra analysis of the films deposited in the small Pd

range, the CH3/(CH + CH2) ratio decreases and the sp3

(CH + CH2)/sp2 (CH + CH2) ratio increases with decreasingPd value, indicating the importance of sp3 C–C bonding in theDLC film formation. Simultaneous measurements of high-voltage and current waveforms of the DBD plasmas suggestthat the low-pressure barrier discharge consists of spatially

(along the electrode) uniform glow-like single breakdownswith pulse widths of several microseconds, and the breakdownnumber per voltage period mainly depends on the Pd valueand the applied peak voltage. It was also found that when thePd value is smaller than 5 Torr mm the breakdown voltage Ub

of CH4 is dramatically increased with decreasing Pd value.Hydrocarbon ions with higher energy in the cathode sheathregion near the substrates, which is mainly determined by Ub

and the mean free path of collisions, contribute to the increaseof DLC film hardness.

Although the DBD deposition of DLC films reported hereis possessed of some attractive merits, so far the DLC filmsprepared in this work are still not as hard as the optimumfilms deposited by rf discharge or ion beam techniques. Toincrease the applied discharge voltage (e.g. to 50 kV or higher)and ac frequency (e.g. to 50 kHz) and to reduce the dischargegas spacing (e.g. to a few tens of micrometres) may raise thefilm quality and the deposition pressure further (e.g. to a fewhundreds of Torr or even atmospheric pressure). It is obviousthat the most attractive character of DBD deposition would bethe atmospheric operation.

Acknowledgments

A grant from the National Natural Science Foundation of China(Key Project No 19835030) is greatly appreciated. We alsowish to express our gratitude to Professor Y Wu and Dr Z Y Liufor providing the high-voltage and pulse current probes andtheir helpful discussions.

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