Mechanical stability of DLC films on metallic substrates: Part I—Film structure and residual stress levels

ii ii

ELSEVIER

i

t h i n Solid Films 312 (1998) 2 0 7 - 2 1 8

Mechanical stability of DLC films on metallic substrates: Part I Film structure and residual stress levels X.L. Peng, T.W. Clyne * lhyuo'lment ~1~Maleri.l.~ Sc'it,m.es omt Ah'talho'gy, C~¢mhrid.t,,e Unit'er.~'ity. Pembroke Street, C'~mthrid,t,,e ('B2 3QZ. UK Received 25 April 1997; accepted 16 July 19t)7

Abstract

Hydrogenated diamo,ad-like ca='bon (DLC) films have been deposited onto aluminium, titanium and silicon substrates using a 13.56 MHz capacitively-coupled RF gl¢~w discharge in a methane atmosphere. All of the DLC films, which were prepared with a negative bias voltage in the range 40 V to 345 V and a ga.,, pressure in the range I Pa to 40 Pa. showed strong sp3-CH, FTIR ab,~orbance peaks at 2900 tin ~ i and a broad Raman peak at ! 540 cm ~. Deposition temperatures, which were measured using irreversible sell-stick thermax, varied from 25°C to 182°C. Stress levels in the fihns were measured by monitoring the curvature changes of thin 1125 #m) Ti foil alter depositing thick (several btm) DLC films, using an AI bonding layer. This intrinsic (deposition) stress was compressive and varied l ~ m - 0.3 GPa to - 2.0 GPa. It increased initially with the energy of the bombarding ions, as implantation became more pronounced, reached a Inaxirnum and then decreased, as a consequence of thermally-activated structural relaxation. The experimental stress data agree well with predictions from a model prt~posed by Davis [C.A. Davis, A simple model lor the formation of compressive stress in thin Olin by ion bombardment. Thin Solid Films. 226 11993) 30-34]. © 1998 Elsevier Science S.A. Keywords: i)1.(" films: Iq'lP.: AI himding layer

!. I n t r o d u c t i o n Diamond-like carbon films (hydrogenated or non-hydrogenated) have been the subject of intensive research over the past 20 years or so. Various deposition techniques [ I - 6 ] have been employ"d. While there is no universally accepted definition, L,LC films are generally lbrmed by condensation of I,;gh energy (up to sever;,l hundred eV) carbon species [5.7] and exhibit high ~,ensity, hardness, electrical resistivity, chemical inertness, IR transparency, nanosmoothness ( R a = I nm) and no long-range structural order. Large area, homogeneous DLC films can be prepared by RF glow discharge [8] or pulsed laser deposition [9]. Substrate temperatures arc relatively low: values as low as 77 K have been rcptwted [10]. However, bombardment of DLC films by energetic species (which is necessary for obtaining DLC structures) tends to cause high intrinsic stresses, which are normally compressive. This is primarily a result of the implantation

('~rt'C~l~tmding .'lkllhi)r, Tel.: 4- 44.1223-334332; 33451W, e-mail: two 10(.n'Ci|ll|,at'.ltk.

t'a~,: +44-1223-

(X)41)-('~01~[1/98/$1t).1~) ,*'~ I~Jt)8 i-lsevicr Science S.A. All riuht,,, rc.,,cr~cd, PI! St)l)40- b r i g ( ) ( 9 7 )(b(D5 8 ~- t)

of carbon atoms into the growing film during bombardment. This intrinsic stress is expected to be sensitive to the impact energies [I I. 12]. Although Zelez [13] reported relalively low intrinsic stress levels ( + 7 0 MPa to - 7 5 0 MPa). most studies [I 1.14-18] have indicated higll compressive stress levels in the range - 0 , 5 GPa to - 1 2 . 5 GPa. Differential thermal contraction during cooling also tends to make the stress in the film more compressive, pallicularly with metallic substrates. Table I lists some reported stress levels in DLC films prepared by a variety of methods, it should be noted here that all these data are based on application of the Stoney equation to curvature measurements and did not take the thermal stress into account. The errors introduced via the approximation involved with the use of Stoney's equation and via neglect of thermal stresses are considered below. Accurate evaluation of the stress level from curvalure measuren'hents requires a value lbr the Young's naodulus of the DLC film, Reported values [21-23] range t~ma I(X) GPa to 500 GPa. largely depending on the s p 3 / s p -" ratio in the films, These two papers describe work on the mechanical slabilily of DLC fihns deposited onto metallic subslrates

X.I.. Pen.t.. 7~ W. ('!v,<'/ Thin Solid i"ih,.~ 312 1199S1 207 21.~'

2O8 Table 1

Reported stress levels in I)I+(" fihus Deposilitm method

Sub,~trate material and Itlickncss

I)I.C Iilll+ thicknes,, (

C2H, ~ RF discharge C sputtering + but:me RF discharge C.: I-taRF discha,'ge lon ["g'iltll: grapltite sputtering Ar + c H +RF discharge C: H 2 RF discharge C arc: ion 110;.1111:PECVD ('FI =: C¢,Hf, + He discharge Methyl-pr-pan¢ RF discharge

(;I,'tss. 1.5 mm (;lass, '? ( ] l a s " , , O. 17 m m St. ? St, '.' St, I).22 m m St. (I.3-tl.8 mm (;lass. 7.*/am St. 1L38 MIll

<1 '?

#IIi}

().5

? < 0.35 < I ().4 O. 12 ?

Slrcss (GPa)

t,~,'ference

-(1.6 |o - 7 , 2 + 0,07 to -0.75 - I to - 5 - 0.5 t,tt, - 3 (] to - 7.5 - 11.5 to - 1.3 +-0.6 to-- 12.5 - 1.22 to - 4.67 "- 1).5 to - 1.5

[15] [ t 3] [I-1] [ I I] [ 18] [ 19] [16] [201 [ 17]

All of Ihese data were obtained by applyin,g the Nlt~n¢} etttlalion tt~ cttr,,alurc nle;InllretlrlClllx obtained al room temperalurc, with rm, accottnI taken o f stresses I'rolll dil'l~'rentiat thernlal t'C)lllrtlClioll.

using an RF glow discharge technique. In this first paper, the structure and stress state of the films are investigated.

the stress levels in the fihns. Sitmples deposited on Si were subjected to various heat treatn~ents. The substrates were polished to a mirror finish using i gin diamond paste, ultrasonically cleaned for 20 rain in an acetone bath and blown dry. Prior to deposition of DLC, or any interlayer, substrates were cleaned for 10 rnin in sittt, using an Ar or tAr + I(Y,:/ CH 4) RF plasma at I Pa and -290 V. The chamber was then pumped dc,wn to below 10 r, Pa. Deposition of thin ( -- 81) nm) AI bonding layers wits carried out using a DC magnetron sputter source at 30-40 W and 0.5-0.8 Pa. Thc DI,C deposition parameters are listed in Table 2. The DLC t'ilms were characterised using FFIR, Laser Rarnall spectroscopy, micro-hardness indentation and curvature rneast~rements for the determination of stress levels.

2. Experimental procedures DLC films were deposited using the ,3.56 MHz RF plasma-assisted CVD system shown in Fig. I. The substrates employed were (a) aluminium of thickness 2 ram, (b) commercial purity titanium (CP-Ti) of thickness 125 # m and (c) silicon single crystals ((100) surface) of thickness 0.38 mm. Deposition on AI substrates was carried out to investigate the temperatt, re during the process and on silicon to study the structure and thickne,,s of the film,;. Work with the CP-Ti substrates was aimed :tt establishing

Baratron vacuum gauge

--

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I

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sputter target

-----¥/~1r . / / . / / / / Z / z f / / ~ / / / / - ~

~.

~

Cathode shield

Substrate

I

~ C h t ) k e

I Sputter power source ,

;" l

Impedance matching unit

I

shutter

Glow 1.5 M.C2/~Q - -

Gas inlet Turb~F_ pump

Fig. I. Schentalic illustration o t

I

DC voltmeter ~ . _

Rota~vump the RF ~l(ll~k,r db, ch;.|i'gu" ( ' v l )

syntcm u,,t.'d f~t' I ) ] . ( ' dclm,,ilion.

RF source

X.L. I'eng, 7~ W, Clym" / T h i , Solid Film.~" 312 (19~Av,) 2 0 7 - 218 Table 2 Expcriinenlal condiliolm used for all DI.C dcfmsititm

RF power (W) Cathode area (cm 2 ) Base presst, re (Pa) Pro-cleaning gas Working gas Gas flow rate (seen|) Working pressure (Pa) Deposition time (mill)

5-1511 21) < Ill ~' Ar or t a r 4 - 11)';~ ('It 4 ) ('|'14, 3--5 I --41) 5-2Iit)

2(~')

into a powder, ,nixing it with KBr, pressing the mixture into :l 10 mm diameter disk and using direct transmittance, Raman spectroscopy was performed using a SPEX 1401 double spectrometer at 488 nm wavelength and 100 mW output power.

3. Microstructure of deposited films

3. I. Negatit'e se!f-hias The bias voltage makes measurement of the substrate temper:tture impossible using a conventional thermocouple [17], Recording the temperature after switching off the plasma [24] is likely to be inaccurate, since the substratc cools down quickly as a rest, It o1" heat loss to the cathode cooling system. Furthermore, the presence of the glow discharge makes the use of a pyrometer unreliable. In view of this, an irreversible colour change sensor system was used. A micro-strip of thermax (supplied by RS) was attached to the rear face of an AI substrate. In view of the high thermal diffusivity of AI. the temperatu,'e at this point can be taken as representative of the fihn temperature during growth. This allowed the peak temperature during DLC deposition to be deduced from the colour change of the thermax strip. Temperatures taken by this method were accurate to about +5°C. Film thickness meast, rernents were carried out using SEM, by viewing cross sections. Vacuum heat treatments were performed on D L C / S i samples under a pressure of 5 Pa at temperatures of 300°C. 400°C or 500°C. lbr periods of 0.5 h, I h or 2 h. After each heat treatment, samples ,a.':'e examined by FTIR. Raman and SEM. All the FI"IR specu'a were obtained in reflection at 60 ° incident angle, using; Perkin-Elmer 1751} machine. Fo,' comparative purposes. VI'IR spectra of diamond films were obtained by grinding free-standing diamond films

In the system used. the high asymmetry of the powered cathode (small area) and earthed anode (large area) generates a high negative sell-bias voltage on the cathode (specimen) relative to the anode [4]. This bias arises [25] from the large difference between 'he mobilities of electrons and ions. Because the pow..'red electrode is capacitively coupled, the steady-state dr current must be zero, A surplus of electrons accumulates on the electrode, until a sufficient negative potential develops to ensure that the net current flow ove," a tull cycle is zero. Therefore, ion bombardment occurs on a snmll cathode area. This s e l f bias will depend on both the system geometry and the deposition parameters (power and pressure). Data on the measured self-bias as a function of power density and gas pressure are presented in Table 3 and Fig. 2. Clearly. the bias wdtage is proportional to the square root of the power density. This is consistent with previous observations [6]. It can also be :"en from Fig. 2 that increasing the methane pressure leads to a small but progressive decrease in the bias voltage, particularly at higher power densities. This is caused by the expansion of the plasma at lower pressure. which results [261 in an increase of the contact area between plasma and chamber wall (anode). Since the nominal RF input powc," is partially dissipated within the impedance matching circuit, it is more reliable to use the

Ti,bl¢ 3 Experimcnlal data from I)I.C I]lms uept~sited onto AI xubstrates undet dil'fcrcn! conditions Run code

Power density (roW into 2)

Methane pressure /' (i'u)

Negative bias Vt, (kV)

l.,i, p (kV Pa)

(i,k,)2( p )l (kV 2 P;.I~ ": )

(im,a Ih rate, /{1 #.till h I)

Subs~rat¢ temp. ("CI

W I W2 W3 W4 W5 W6 W7 PI 1'2 P3 IN I15 P6 1'7 I18

2.5 I I) 25 411 511 65 75

IO 1(I I0 I11 111 10 I l) I 5 II) 15 211 25 30 411

(L04 O. 105 O. Iq5 11.245 0.285 O, 3115 0,345 11.2t) 1) 29 0.285 11.27 11.205 0.245 11,225 11,215

0.4 1.115 1,95 2,45 2.85 3.05 3.45 O. 29 1.45 2.85 4.115 5,3 6,125 h.75 8.(~

lull o 0.035 O. 12 O. I q I),257 0.294 11.371"~

-

25 41) 71 118 182 93 I t0 I t,"I 12 I 127 157

511 50 5(i 50 50 511 50 50

0.68 II.92 1.112 1.83 I ,"14 1.93 l, 2 1.8 1,83

O.084 O, 188 0,257 0282 11.314 O,325 0.277 11.292

2.f~4 4. I (~ 5_39

];or ¢~l~|| I'ulI. 1t1¢ p0'¢,'¢1" I|110 ~LIn prcsxl|l't2 were .,,t'lu,uqCd alld It1~ bia.~ vollagu,, gro~,lh talC a||tl nt|b.,,Irate k'1111~r~d~.,~ ~,~t2rC the11 mca.~un.'d.

2 I0

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t I¢,19,'¢)

207-218

500

>

,--, 400 ""

- -A

.,='~

t

25 Pa

--o--p ( C H ) =

- --:. -P ICI-14~= I5 Pa ---¢- --P (CH4}= I0 Pa

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~- - e ~cH)

. . . . . .

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I

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0

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x-- . . . .

0.2

3,._. . . . . . . . . . . . 0.3

L ......... 0.4

I 0,5

Vb.~ ptn (kV: PatnD

Fig. 2, Variation of the negative self-bias as a funclion of the square root of the power density.

Fig. 4. Variation of suhstrate lenlpcr;tlure ;is ;t function of IllC square of negative self-bias times the square root of pressure.

measured self-bias voltage and gas pressure to determine the ion energy (see Section 3.3).

tional to the gas pressure [27]. During DLC deposition, the self-bias will accelerate positive ions towards the substrate, raising the net flux. Bubenzer et al. [28] obtained the same dependence using benzene as the hydrocarbon gas.

3.2, Deposition rates Theoretically. the growth rate, R. can be predicted from the flux of the growth species. Ji' since

R~C,J

i

(!)

where C, i:; the sticking coefficient for the growth species (ions and/or neutrals). Assuming C, has no significant variation with temperature in the range of interest here ( < 180°C). the growth rate is proportional to the growth species flux Rct J~

(2)

Measured growth rates for different pressure and power densities (bias voltages) are listed ir~ l'able 3 and presented in Fig. 3. The data in Fig. 3 indicate th.~t the growth rate is approximately proportional to the product of bias voltage and gas pressure, implying that Ji ~ Vh P

(3)

For an ideal gas, the flux of gas molecules is propor6 L

_""

..........

~ ' ~ ~ :

........................

i

5

/

=

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=

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3.3. Energetics o.f boud~archnent The temperature of the substrate is raised as a result of bombardment by energetic ion or neutrals and the impingement of UV radiation fronl the glow discharge. It is, however, mounted oll a water-cooled cathode, so this temperature increase is expected to be relatively small. Temperature data are given in Table 3 and are also presented in Fig. 4. This plot shows that the temperature increase during deposition is approximately proportional to the product ol" the square or the bias voltage and the square root of the gas pressure. In addition to the heating effect from ions accelerated to the substrate, high energy neutrals, tbrmed by charge exchange in the cathode sheath region [25]. rnay also contribute. Assuming the heat flux carried away by the cooling water to be constant, the ternperature increase will be proportional to the energy tlux injected by the bombarding particles. This can be expressed [28] as T - 7~ ct J,E (4) where Ji is the llux of energetic ions (or neutrals) and E is their average energy. Since this temperature rise has been shown to be approximately proportional to V~P~/~ '1

::'.'/"

JiT~ ¢g V h - P i'/2

(5)

Substituting for J, from Eq, (3), the average ion energy can be expressed in the form T'gct VhP ,/2 (6)

,

0~O

~

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2

,--

,

,

__L~x..-~----.~=+ 4

6

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8

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Vb P (kV Pa) Fig. 3. Measured growth rate,', plotted ag.'finst the prt,ducl of him, vollag¢ and gas pressure,

Since the plasma voltage, ~,, is usually much smaller [29] than the setfbias voltage, andjleglecting the effect of any collisi',ms in the ion sheath. E is approximately proportional to v,. r':a ,,( V,, 4. V,,) = ,,V,, (7) As the gas pressure is increased, however, the mean free path of ions is greatly reduced and the ion energy is likely

X.L. Pc.g, 7. W, Clym" / 7"hi. Solid Films 312 11998) 21)7-2 I/¢

[8] to be significantly less than eVt, ;is a result o1" collisions in the ion sheath. At a particular pressure, therefore, a relationship of the t'orm

E cx neV,

I

v =345v

r" [

" vt~ =285V

i!

(8)

is expected, where n is a dimensionless constant with a value ranging from I at very low pressure to O at very higr~ pressure. For example, Koidl et al. [8] fi)und the value of n to be about 0.4. Theretbre. by controlling the self-bias and the pressure. the energy of individual bombarding ions, which is one of the most important parameters for DLC l'ilm deposition, can be adjusted. Most DLC films are prepared with bonabardment energies [7] in the range I-100 eV. it should be noted here that Vh is sensitive to the geometry of the deposition chamber [4].

211

. v .195v l vt,=1(15v I

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3000

I

,

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2950 2900 2850 2800 Wavenumber (cm 4)

2750

2700

t

i F

3.4. Alomic bomting cltarttcteristi('.~" 3.4. I. FTIR spe¢'troscol~y ttydrogenated amorphous carbon displays a wide FTIR absorption band centred at 2900 c m ~, which can be deconvoluted inlo different individual peaks corresponding to various types of C - H stretching vibrations. Those at 2850 and 2920 cm ~ correspond to spLCH,, there is a peak at 2920 cm ~ corresponding to sp-LCH, those at 30{)0 and 3060 cm- ~ correspond to sp-'-CH and that at 3300 cm-~ corresponds to spLCH stretching vibrations [30,311. Fig. 5a shows FTIR spectra of DLC films deposited with different negative self bias voltages. For comparative purposes, a spectrum is also shown (Fig. 5b) from a diamond fiiln deposited by HFCVD using a H , - i .5t)~ CH.~ gas mixture at 8,5 C. Basically. the 2900 cm i absorbance band is composed of peaks centred at 3000 cm 1 (sp-'-CH), 2970 cm 1 (spLCH~). 2920 cm i {spLCH, spLCIta), 2875 cm -I (spLCH.~) and 2850 c m - i (spL CH.~). Palshin et al. [32] obtained similar FTIR spectra lbr ion-beam-deposited DLC. The C t l , absorbance peaks are sharper and better resolved in diamond films than in DLC films. Fig. 5a shows that negative self-bias has little inlluence on the shape of the F'TIR spectrum, but trte relative intensity of CH, (t .>_a > 3) absorbance peaks does show some variation as a result of changes ill the bonding preference of hydrogen lbr sp ~ and sp a carbon networks. (Quantitative analysis of the s p / s p - ratio was not carried out, in view of various difficulties associated with accurately deconvoluling the F'I'IR spectra.)

i '-

}, 3 /

',

c

" \

r

2950 2990 2850 2800 Wavenumber (cm t)

3tC,~ 3050 3000

2750

2700

Fig, 5. l:'l'lR speclra from (a} DLC films deposited at IO Pa, with different self-bian voltages and tb} a diamond film deposited by H F C V D at $25"C. with a gas ¢olnposilioll of H , - I.St;~ CH 4,

makes the D peak more prominent. The ratio of the integrated areas under the D and G peaks (ID/i~;) increases significantly as V~, increases. This effect, which is illustrated by the data in Fig. 6, is caused by an increase in the size and incidence of graphite microcrystais [33,34] on raising the negative bias and hence the botnbarding ion energy. This effect is probably associated with the higher

y

1.5

.3

3.4.2. Rama, ,wectros('opy DLC l'ihns deposited at 10 Plt pressure and different negative bias voltages from 40 V to 345 V were cxanlincd by Raman spectroscopy. All the films displaycd a Broad Raman peak ccntrcd al around 1540 cm ~ and a shoulder at lower frequency of about 1350 cm t , which correspond to G and D peaks respectively. Raising the bias voltage shifts the G peak position to higher frequency and also

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i

. . . .

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t

0.35

l:i~. (~. Ratio o f ttl,,: integrawd area, raider tile D a , d (; r,eak,, in Raman spectra, ph~lled ;is a functioll o1' tlt¢ ilcgatixc bias ,,ldta~e. Fur DI.C fiillts deposited a! tO Pa,

212

X.L Peng. 7: W. ('lyre,/77fio Solid Fih,s 312 ¢199fi; 2117-218

substrate temperature, which may encourage the growth of such crystals. This certainly occurs durir~g annealing ;.It relatively high temperatures (see Section 3.4.4).

i " '" . . . .

' (a) , ~= :>,

'= 3.4.3. M i c r o h a r d n e s s

3.4.4. Hydrogen release a m l gr+q~hiti:ati+m on annealing

The sp 3 network in DLC films is expected to transtbrm to the more stable sp 2 network (graphitization) on annealing at high temperature. For hydrogcn:lted DLC fiirns, it has been shown [30.31] that, on almealing above 400°C, the hydrogen concentration decreases and the sp ~ carbon network is transformed to an sp 2 network. FTIR analysis can be used to monitor the dehydrogenation process and the associated changes of carbon bonding state [30.36.37]. In the current study, only the high frequency FTIR spectrt, m range (2700 to 3100 c l n ) ) and low frequency range (700 to 1300 c m 1). which respectively contain the major C - H and C - C absorbance peaks, were investigated. Fig. 7a shows FTIR spectra from DLC films prepared at a bias voltage of 105 V and a methane pressure of I0 Pa. alter annealing for half an hour or 2 h at three difl'erent temperatures, while Fig. 7b refers to films produced with different bias voltages, alter annealing at 400°C. it is clear from Fig. 7a that. on raising the annealing temperature, a progressive transformation occurs from a predorninantly sp 3 structure in the as-deposited state to one which largely exhibits sp 2 bonding. It would appear that the transformation accelerates sharply above about 400°C. it was also noted that the heat treatments at 400°C and 500°C produced a marked change in the colour of the very thin films (from yellow-brown to deep blue). The data in Fig. 7b show that the temperature sensitivity of this transfornmtion is not strongly dependent on the bias voltage during deposition. Spectra are presented in Fig. 8 from the low frequency regime, tor films deposited at 285 V negative bias. These confirm the progressive disappearance of sp ~ bonding ;ind the Iormation of sp 2 bonds as the annealing temperature is raised. These spectra confirm that bonded hydrogen (only hydrol,.en which is bonded to carbon c:m be detected by !.-v IR) will be released on anncaling above about 40()°C. Hydrogen liberation is associated with the transformation from an sp3-C-C network to an sp2-C=C network and the associated rernoval of dangling spLC bonds. It should be pointed out that tiny unbondcd hydrogen present would bc

......

1'~"'++

.....

~ .................. A>, depo.',llcd

i

t+.

Micmhardncss tests on 2-/.tmlthick DLC films deposited on Si with 285 V bias and a pressure of 10 Plt showed that the hardness was typically around 30-40 GPa ( H , ) at 25 g load. (The hardness of Si is about 12 GPa.) it should be noted that pronot, nced elastic recovery of the DLC films can lead to overestimates of H, measured in this way and dynamic measurement has been recommended [35]. Nevertheless. these results confirrn that the DLC fihns being studied contain a rehltively high proportion o f s p ~ bonds.

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=

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=-

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2950 2900 2850 28110 2750 2700 Wavenumber{cmi)

3{XX)

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j .,

lr

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3100 3050 3{RD 2950 2till 28511 2800 2750 2700 W a v e n u m b e r ( c m ~) Fig. 7. Iq'lR spectra of I)LC tillns deposited (a) al IO5 V '.rod t(} Pa. altcr annci,ling tit differcr~l t¢lll[~craiurtrs alld Lime alld (b) at dilIL'rcn[ bias ?olt;.tges and I{) I},:l.after a)mcttling for 2 h ;.It 40{i"C.

undetectable by FTIR. Liberation of such unbonded hydrogen could occur at lower ternperatures than 400°C, Raman spectroscopy also conlirmed these conclusions. The transformation to a graphilic structure can be clearly seen in the Raman spectra shown in Fig. 9. As-deposited DLC films show a broad peak centred tit around 1540 cm +, which corresponds to an anmrphous carbon strut-

1 P, e-

{

-

2 h at 3 1 X F C

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.,

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Wavenulnber{cm t) Fig, 8. VI'IR spedra in the low hcquency rwlgc, frum I)I+C I]hus I:.reparcd vdth a sdl'-bJa:, of 285 V and a ~.:r', Irrc.",Sttle of IO P~.,. after amu.'uli,g for 3() mm ()r 2 h at dillcrctll |cmpcruture,...

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which relate curvatures and stress distributions to the thicknesses and physical properties of substrate and coating.

l/ ~'

~ As-dept~siled - - - :mnealcd 2 tt at 400C . . . . . . annealetl 0.5 h at 5('~)C

1600

213

= E[t'-h" + 41"~,l:~h'H + 61i~,E:h'H"

l.."tte: hE~, + HI:.~

'

17(X) 180{/

,r~l,.

~, =

¢rdl

. = --3`/:2

'

- ~,,

)

( ,."m~' ).,, .

+ 4i-~,l-:ld/'

+ l:,,a:(

(10)

+ E',~-H "~

h -

(lla)

~ )

(lIb)

--E, IK'~

Id-", + HI':

W a v e n u m b e r (cm' ~) Fig, 9. rmnan'~ speclra o f I)I,C films d e p o s i t e d at lO5 V and 10 Pa, hetorc alld

;Iflcr

~.llll'lC;.lliltg.

It = 3, e

¢rdJ ....

(l':'hE:)-l'~'~(tt+6) lsE,n+ HE', "

( , )

(12a)

E a di.

{r,~],~. = 3,~," hlf~. + HE~ lure [38-40]. This broad peak can be fitted using two Gaussian peaks centred at 1336 c m ~ (D peak) and 1541 cm ~ (G peak). After heat treatment at 400°C, the G peak shifts to higher i'rcquency and two well-resolved peaks appear at 1600 crn " I and 1360 crn '. The ratio (/l~/l~;) typically increased from about 0.64 Ior as-deposited DLC to about 2.8 after annealing at 4110°C and 3. I at 500°C, O1| raising the heat treatment temperature, graphite crystal lisation is accelerated and these crystallites contribute strongly to the D peak. After reaching a certain size, momentum conservation begins tn donainate and (1~>/!~;) starts to decline. At this stage [33], the crystallitc size is inversely pmpcwtiotlal to ( ID/I~; ).

(5=

- E'.Kgi

h" G - H "E~

'

2(~ tl:.,~

(13)

+ HI.:',)

where ~$ ix the distance of the netitral axis from the interface.

I|Ht : •. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

o, . . . . . . . . . . . . . . .

I E = lOOGPa lEa = 100GPa

"~

I

",;

--, o,

o1',£11"('s.~'e.£ .[l'olH ('llt'l'Llllll'e Itt[,a.£1tl'¢,mettL~'

/

6 trd h

/( - - --'----'W

t-', H-

Od)l L_ c. . . . . . . . . . . . . . . . . . . . . . IO

H

~:----

E~.a,,:l,

~

Exact, A¢ = 0.003

= 0,3 mm

, 10 "

f

i(}'~

I0

T h i c k n e s s ratio, I,JH

(9)

where H and h are the thicknesses of substrate and deposit and 1:'~ ( = E , / ( I - t,)) is the biaxial modulus of the substrate, is comnlonly used to calculate a film stress. ¢r~ from a nncasurcd curw.lttlrc. K, Tile equation is accurate only in the limit where tile film thickness. It, lends to zero. Unfortunately, in this limit the curvature. ~c. must also tend to zero. It Ilas been pointed out by Brenner ;,rid Senderoff [41] that the experimental error arising from the curvature being too small to measure accurately tends to exceed tile error introduced via the approximation incorporated into the Stoney equation when the ratio of the thickness of the coating to that of the stlbstrate, It/tl. is less than about 5~,4. in practice, the details are dependent on tile Young's nlodtdi and or1 tile absolute II'iicklless of the substratc. This is illustrated by the plots in Fig. 10. These were produced using the cxacl expressions [42]

Exa~:t

-Stot~ey Exact. Ea = tOOl) GPa

l

~,' ' i ' ~

Tile Stoney equation

7~- - - - - - ~ ' -

.,i"

4. Residual stresses 4, l. l ) e d m ' l i o n

(12b)

t. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

;I ...... E.,~,',,,,~ 0.011

I

I0"

| It} ~

..

"~, 10"

Thickiless ralio, iffH | ' i t , IlL P r e d M c d del~emleuce n f ~al ,,l',ccimcn cur~alur.." aud (b) .',lre,,~, k'veln oll the ratio t+l the thickllu',,n t+l Ihe k'O.~.tlil|~, h. to that o1" the stlb'..lt-,~|lu', !t, The I'qop, v, ere oblaitied tl,,,il|~ the e\;ict relatiolt~,hiPn I'¢P|'k",.,~.'l|led b,~ I'kl,,. (11i), ( I la), ( I I b ) , { 12a), ( 12b} and ( 131 and |lnill~ the Nhme.~ equation (liq. (q)),

214

X. L, Peng, 7: IV. ~'lym, / Thin S~lid !'71oIs.i' / 2 ~'199b;~ 20 7 . 2 1 8

These equations refer to an elastic systeni exhibiting an equal hiaxial stress slate, re.~uliiilg from the inti'oductiol~ of a tinifornl in-rlliln¢ irlisfil slrain, ~,,:. between the two conslituents. Note lllal, for relalively tllick coatings, the adoption of curvature lie! drily changes lhe coaling sti'ess, hut also inll'oduce.~ differences between the level al the free stlrlace and ihlil ill lhe inlerface. This is one reason why il is illore rigorou.~ 1o define il nlisfit Sll'ain !111.1il ;i coating stress, although it should be mentioned thai through-thickness variations within the coaling would not lle significant for most DLC films. The plots in Fig. I0 confirnl ihal llle Sloney equation is expected to be quite accurate for thickness ratios below a few percent or so, depending on the stiffnesses. The experimental studies included in Table t all salisfy Ihi:; condition, since a lypical vllluc for h is I /.zm and H ranges trom about 100 # m to over I mill, ltowever, the plots in Fig. IOa highlight the problcnl identified by Brenner and Senderoff [41] for ca.,,cs where the h / H ratio is so low: tot a misfit strain of 10 ~, many of the combinations Ifi" H and !1 in Table I would correspond to curvatures below 0.1 m ~, or even below 0.01 m ~ (101)m radius of curvattlre), Such t'urvatures are exlrenle!y diffictlll If<) nleasure with any precision. Of course, tile presence of much higher misfit strains will piish the curvalitre Lip IO nlore readily measurable levels. Nevertheless, it should he rec~lgnised that. in these regimes of small h / H and relalively large H, minor errors in measured curvature lead to major changes in deduced stress level. This may account for some of thc very high reported values zind also for the wide scatter in the data in Table I. The approach adopted in the present work was to generate relatively largc curvatures by using high h / H ratios and to apply Eqs. (10), ( I la), ( Iib). (12a). (12b) and (13) to deduce the misfit strains and hence the ~tl'es.,, levels. Curvatures were typically in the range i - I l l m ~, and were hence readily measured with high precision. This can bring problems of adhesiorl t'ailure as the coaling thickness is increased and the corresponding strain energy retcase rate becomes high (see Part !1), However, hy using thin ( - - 8 0 nm) AI interlaycrs, adequate adhesion was maintained between DLC films of several ,am thickness and the Ti substrates. The effect of the intertayers on the stress distribution was assurned to he ncgligible, The use of thin ( < 200 /.tin) substrates presents dangers of fracture foi brittle materials such as glass and Si, :a) a metal suhstr:lte was emFIoyed. The use of metal substrates ix also of interest in its own right, hut they introduce the likelihood of differential thermal contraction stresses being signil'icani. It ix also important thai plastic deformation be avoided. Expcrimcnlal data arc presented in Table 4. These include measured curvatures, corrcsf~ondii~g misfit strains and, after subtraction of tile Ihcrmal conlponcl~t, the intrinsic Sil'CSses which would bt: produced hy derlosiiJon on an infinilcly ill)ok sub, trait. It .,it~ould bl: )1died thai, in obtain-

ing these wilues, the Ihermorlhysical properties have been lakcn as conslanl. Neglect of tiny dependence ell lelltllei','llure is probahly reliahle in view of the limited ienlperalure rallgC. However, it is nlol'¢ difficuh lo justify the assunlplion thai all the DI,(" films had a YoLiilg'S inodulus equal tO thai reported by Blech and Wood [221. There is a severe shorlage of exl~erimenlal data in thi~ area. bul il seems likely that the value is sensitive to the sp ~ ¢Olllent and hence to the growth conditions. In lhct, !he deduced stress level is not ver b' sensitive Io E,'i (and ix indepcndenl of I:'~i when lhe Stoney equation is employed,) However, the strain enei'gy release rate. and hence the likelihood of doi~onding, is sensitive to ihis p:u'ameter (see Parl II). Tllc quoted or, values should he suitable for comparison with data from other work in which thermal stresses were noel)gillie (gluss or Si silbslrales) and the substl'ales close to being int'initely thick, h Call he ~een thai the valllcs obtained in the present work riln~e Lip tO abotil 2 GPa (compressive). This is in broad agrcemenl with prey)otis work, although the very high values ( > 3 GPa) ol~rained in sonic studies have llOl been recorded. The trends observed are considered in detail below (Section 4.3). 4.2. 77u>rmal slre,vse.~

Although the post-deposition drop in lenlperalure is relatively snlall for most DLC coatings, differential thermal c'onlraclioil SilCS.~¢s call he sJgnifJeanl, parlicularly wilh metallic ~ubsli'ates. For ll~e pl'esenl work, this C~lll be seen h)' COlllp;.irJil~ die rallies of the lh0rinal nlisfil strain in Table 4, distained ushiL.z It ,~;ll I ---- ,-~7"( (t', -- {l'lll/.

)

(14)

where A7" is the tClllpel';.lltlr¢ change, with the corr¢.~l~ondin 7 net mi~t'ii sli'ahis obtained from CllrValitre nleasuremenls. Ill general, sublraclion of the ihcrlllal nlisl'ii sli'aill imikes a relatively small hul significant difference to the deduced value f(ir the intrinsic stress. This would ilol, however, bc the case for suhstt'ales of glass tll" silicon, which have expansivily values nluch closer to thai of I)I,C and IlaVe corlespondiligly Sllialier misl'ii StlaillS.

4.3. hm'in,~ic ,~tru,~'se,~ The intrinsic stress rallies recorded in Table 4 ~ll'C plotted in Fi~. II a.~ a function of (a) nceative hia.~ vollage and (h) nlelhane l)ressui12. The trends ohserved in this figtire ).ire readily e×plainetl. Initially, the sire.~s rapidly becollleS slron~ly conll~ressive as Vl> rises. Thi.~ is II~e result of pronoullced iitlrJlanlalion i)l" I~olnbartlill~Z carbon ioils, once they have ellOt.lgh energy Io penetrate the slrLiclure. l:LIllher illCl'ea.~os in bOlllhardnlcnl energies lead to intensive local healing (ihernial spike), all iilcl'el.is¢ 111 Ihc .~tlb.Mral¢ Iclllperalul'e and COll.~¢tltlenl rcdtl~:liOll in the Colllpro~sjve Stl'e~s as the strtl¢ltn'¢ tlildl2rgoes Ih¢l'lnal re-

-.I.

W I

WI

W I

W2

W2

W2

W3

W3

W5

W5

~.~7

~,V7

%t-7

P I

P ]

P I

P2

P2

|'6

P6

PN

P,~

PN

T24

T25

T2~

T7

-[f~

T3 !

TI3

T I4

T5

T6

T~,

TI I

T I2

T32

T33

T34

T4rl

T~, I

T42

T J,3

T36

T37

r3~

fihn

2. t4

2.17

2.17

4.113

5.55

325

3.0,";

2.4

1.9

I.7

5.2

5.8

6

4.03

4. I 6

5.6

5.8

i.7

1.03

I r()5

2.4

2.4

2.4

h i /,~m)

thicknc~,,,.

DLC

-

-

- ~. 13

- 3.46

- 3. t g

- 5.5 f

- 8.75

- 6.35

- 7. | 3

- 4.61

- 3.33

- 3.28

t 0.6

- 5.82

t0

- 9.2

-- 7 . 3

-- I 0 . 6 7

-- 1 7 . 3 5

-- 6.1";2

-- 5 . ( ) !

--4.7

- 2.21

- 3.19

- 2.36

~." ( m - i )

mid, tit .,,train.

3.50

- 3.83

- 4 . t 14

- 3.84

- 3.75

- 4.49

- 5.2f~

- 6.20

- 5.(16

- 4.50,

- 6.49

~ 5.75

- 2.87

- 4.811

- 6.26

-- 4.83

-- 5.43

--8.57

-- 1 0 . 4 f )

-- 12.40

-- l 1.4()

- 2.43

-

- 2+59

.3, r. ( m i I t ( s t r a i n )

Net

f o r ~ ari~m,+ D I + C f i l m , , d e p o . ~ i t e d

curvalure.

curvature,

Mea~,ured

retain ternpcralurc

onto

0

0

(|

~:,h

~,uh,,trate-.

mi~,lit .,,tram.

CP+Ti

( milli~,lrain J

- I).82

- i).82

- 11.82

- 11.63

- 11.63

- 11.53

- 0.53

- 0.42

- o.42

- 11.42

- 0.97

- 11.97

- 0.97

- 1t.58

-- 0.58

-- 0 . 2 8

--O.28

--().09

--11.t19

--(}.()O

3,

Thermal

125 /.tm thick Intrin.sic mi.,,lit strain,

1.g0

- 3.111

- 3.36

- 3.02

- 3.12

- 3.86

- 4.73

- 5.67

-4.64

- 4.14

- 6.117

- 4.71',;

-

- 3.82

- 5.68

-- 4.25

-- 5. I 5

--8.29

- - 10+31

-- f 2 . 3 1

--11.31

- 2.,13

-3.511

- 2.50

3+,=:, ( r e ( I l i a , t r a i n }

lmrin~i¢

1.05

- 0.52

- i).58

- 11.53

- 11.54

- 0.67

- 0.82

- 0.99

- 0.81

- 11.72

-

- 0.83

- 11.33

- ().6h

- 11.99

-- 0.74

-- 0.90

-- 1.4--1"

-- 1.79

- - 2 . I--!"

-- 1.97

- 11.42

-0.61

- 11.45

,rr, ( G P a )

~,tre~s

('{,rre,,p~mdm~ mi.,|il qrairl'-. ;ire Ihcn sht~v,n. ¢alcula'¢d using F+qs. (H)). ( l l a ) . (I l hL (12a). (12hi and (131. The thermal re(slit smfin, deduced from Ihe deposition temperature (~,¢e Table 3) and the Ihcrmo[,h}qc;=l d:,la in Table 5+ is ,,ub:ractud Imrn this t=+ ~i+.c Ihc intrin,,ic (dcpt+sili~+n] mi',l]l strain. Finally. this b, presented as an inlrim.ic +tre~,,, b) ;=.,su,ning il i., entirel) ac,.-on+mt+daled in the lihn (infinite ,.gbqrat: c t,¢).

Run

code

f o r trilnl t h i c k n e , , ~ , a n d

Specimen

dala

~:odc

|-xperimental

Table

r~

]

e.a,

',c

e~

X.L. t'e,g, 7~ tt % ( ' h m ' / Thin Salid I.'ihn.s .¢12 ¢ 199,~¢) 207- 2 h~'

216 •2 5

.......

-2 .~ " .......... ' . . . . . . . . . . ~-~ " ~

: ............................................. lal

!

•

~2

............ '"~' ....... ]

:"

' ....

Experinlentat

*.........

d,,ta I

.

-2

~" rO

it

!

o " 1.5,

.,

ca

-I.5

.'¢. .~~a

-1 !*

=

e.-0.5

'"

""

~

!~.

i

-

-0.5 '

r:

_

i

! (| .............. ~. . . . . . . . . . .

0

005

..L_. . . . . .

0.1

" .....

(/.2

025

N e g a t i v e bias v o h a g c , V - 1.2 ...........

r ............

r ----~--~

:

~. . . . . . . . . . . . . . . . . . . . . . . . . . .

I).15

0 3

..

0 ; ......................................... .so

0.35

I.~.

Ion energy, E = 0.5 e Vt, (eV)

(kV)

...................

..,

.............

,. . . . .

Fig. 12. ( ' o l n p a r i s o n

(b)!

•

b e l w c e n Ihe i n l r i n s i c ,,tress l e v e l s rne;tsured e x p e r i -

i l l c n l a l l y , for f i l m s d c l m s i t c d al i11 Pa i"q'cSstlre w i l h v u r i o u s ne~2",livc hia.,, p r e s s u r e s , a n d file p r c d k l c d

c u r v e o b t a i n e d w, ing file I ) a v i s m o d e l in lh¢

f o r m iff F.t 1, ( 151. T h e stres.,, l e v e l is p l o l l e d again.sl io~l e n e r g y , l a k e n :l'. .O.gl

~5

.o 6 i .

,.

h a v i n g .'l v a l u e o f 0.5 c V , . T h e '¢ah,cs cn phLvcd for the p a r a m c l c r s in Ett.

,

(15) were:('~ t,V.

....

2(iPacVl

_' R / . I = 1. I-~;'- 1 . 7 5 e V a n d

I': = 1 9 . 8

.P-

based on the ,:onlpeting effc¢ls of implantaliorl and relax;.xtic,n. q he form of his cqtlati,on used here is

_o.4i..

'~, --~

i' . 0 . 2 i-

C.( E-

L

0 i ................... 0 5 iO

.L . . . . . . ~. . . . . . . . . . . . . . . . . . . . . . . . . 15 20 25 30

35

(15)

R

411

Fig. I I , l n l r i n s i c slre,,.,, level,, a,, a futlClion o r ( a ) n e g a | i v c scll'-bia~, ;.tl 111 P:I Ill~?lha.ll~2 !prt",.,qll't_'. iUld (h) lllt'thitllt.' prc,,,,urc, at 51t I11~¢ Illlll 2 pt~,0.t.'r dcnsil,,.

laxation. The data in Fig. lib show that the stress level progressively reduces as the methane pressure in increased. lncrcasing this prcssure leads to a sharp decrease in the [lte;.in free path of ions bombarding the specimen. This tends to inhibit implantation by i'educing the energy of ions reaching the specimen and hence tO reduce the stress levels. These results arc similar to those reported by Zou el al. [14] (sec also Table 5). Davis [12] has developed a simple model to describe the variation of compressive stress in bombarded ttlin films,

5

Thcrmoph.~sical

pr~pcrl~ d a t a

I__.431 tp, c d m

slrcss ~md cur~ :durc ¢ a l c u -

lulio,s t'r~pcrl?

I)1.("

T h e r m a l c x p a n , , i ~ i l ~ . ~ (..: I[) " K YOtltl~'s mod|lhl.',. /: ( ( ; P a ) IMisson's

'rhc

ratio

Yotln~',,

I)

2.3 t3t.l.2 0,2

Illothlhl,, 4~1 DI.(" u, as oblaJlWd frotll B i t c h

x a l u c [22] I'<~r the l q a x i a l Y o t l n f , , Ihfi,,,,on rali~ o f ft.2.

( t:" )~ '

-- + 0.016

M e t h a n e pressure, P ( P a l

"l';thle

I-, ) ~ '

~r=

, , ,

('l'-Ti ~.5

which in the same as Ih;It proposed originally, except that a critical tluvshold energy, E , has been introduced to accotint for the need to implant into the DLC structure in order to generate any compressive stress. In this equution, ('1 in a fitting coefficient, R is the growth rate, ,/ is the bombarding ion flux. I;. in tile :tclivation energy for relaxation and E in the incident ion energy. Exl~erimental dala for a gas pressure of I0 Pa are presented in Fig. 12. together with a best fit plot of Eq. (15). This plot corresponds to the following values: C, = - 2 GPa eV t/2, E = 0 . 5 eV b, R / J = I, E = 19.8 eV and / ' . = 1.75 eV. These values are plausible, since the deposition rate and bombarding species flux tend to very similar ( R/.I ~- I ) in the RF plasma process, the threshold energy for carbon ion penetration into DLC has been reported [44] as about 20 eV and tile average ion etlergy has been reported [8] to be around 0.4 eV,. It can be seen that excellent agreement in observed between the predictions of this model and the measured stress levels. Silnilar levels ol agreement are observed for dat:, eblained at other gas pressures. It I l l ; . l y be concluded dial the fll;.lin I';.ICIOI'S ¢olllrollhlg tile stress levels in DI.C have been identified and their effects have

been successfully quantified, al least to a first appmximalion.

1(16 0,36

a m l W
illl+duhls t~l 174 (il>a. bx t:,,~ttlllill~ a

5.

Conclusions Tile following co ncll.zSiolr,, can he d,'~wn I'r,,~ln this

work.

X, L, Pc,J,', 7" W. C/vm' / Th/n Solid bVh+ts .¢12 f 1998) 2 0 7 - 2 1 8

1) DLC films with a high proportion of sp ~ bonding have been prepared using the RF glow discharge method. with methane as the precursor gas. 2) The self-bias voltage is proportional to the square root el" the imposed powcr density, bt, t is only very weakly dependent on gas pressure, at leasl in the range of i - 4 0 Pa. 3) The average energy of individual bombarding ions is approximaiely proportional to the self-bias w)ltage divided by the square root of the gas pressure. 4) On annealing above abot, t 400°C, hydrogen is released from the film and the p,'cdominantty sp j carbon network transforms to an sp -~ carbon nclwork. The incidence of graphitic rnicrocrystallites in the DLC films increases with annealing lemperaturc or with bombarding ion energy flux during deposition. 5) The inlrinsic stress associated with film deposition varies from about - 0 , 3 GPa to - 2 . 0 GPa. depending on the growth conditions. The intrinsic stress increases inilially with average ion bombardnaent energy, as implantation becomes pronounced, and li~en decreases as a resuh of local h e a l i n g and tJ~tel'lllai relaxalion of the Stl'Ueture. Excel-

lent agreement has been obtained bcLween cxpcriment:d slress data and predictions fron~ the model of Davis.

Acknowledgements We are gratetill to Georges Adomopoulos and Dr. Kai Gilkes at University of Easl Angelia. Norwich for considerable assistance with the Ranlan analysis. One of the authors (XLP) would like to thank Cambridge Overseas Trust (COT) and ORS for finaric!,:,l support. ,.'

References

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