s 16 (2007) 220–224www.elsevier.com/locate/diamond

Diamond & Related Material

Diamond-like carbon films with End-Hall ion source enhancedchemical vapour deposition

Y.Q. Pan a, Y. Yin b,⁎

a Shaanxi Province Thin Film Technology and Optical Test Open Key Laboratory, Xi'an institute of Technology, Xi'an, Chinab School of Physics, University of Sydney, NSW 2006, Australia

Received 29 August 2005; received in revised form 3 April 2006; accepted 9 May 2006Available online 10 July 2006

Abstract

A custom-designed End-Hall ion source was used to deposit diamond-like carbon (DLC) films in a plasma enhanced chemical vapourdeposition (PECVD) mode. The deposition system was characterised and optimised for infrared transmission enhancement applications and largearea deposition onto silicon or germanium substrates. Ion bombardment energy (in eV) on substrate was found to scale about 60% of the dischargevoltage. Uniformity was about 2.5% and 5% for substrate diameters of 20 cm and 40 cm respectively. For the infrared enhancement applicationsthe optimised ion bombardment energy was about 54 eV with a high deposition rate approximate 30 nm/min. Coating the DLC onto a single sideof double-sided polished silicon wafers resulted in a transmission of 69.5% in the wavelength of about 4 μm, very close to the ideal value.Mechanical and reliability properties of the DLC films on silicon wafers were analysed at different environmental conditions. It was found that theDLC films produced in the ion source PECVD deposition system were satisfied with the requirements for the infrared transmission enhancementapplications.© 2006 Elsevier B.V. All rights reserved.

Keywords: End-Hall ion source; PECVD; Diamond-like carbon films; Infrared transmission enhancement

1. Introduction

Diamond-like carbon (DLC) thin films are unique because ofa number of excellent properties, including infrared transpar-ency, high mechanical hardness, very low friction coefficient,and chemical inertia. DLC films have been used in microelec-tronics, mechanical surface protection, optical devices, andbiomedical applications. Extensive study has been done on itsdeposition method and material properties. Deposition methodsinclude magnetron sputtering, reactive sputtering, plasmaenhanced chemical vapour deposition (PECVD), vacuum arc,and vacuum arc assisted chemical vapour deposition [1–5]. Inthese methods, the ion energy during deposition is generallyrecognized as one of the most important parameters influencingthe properties of DLC films. Unless a RF (or high frequency)source is applied to the substrate, however, the ion energy

⁎ Corresponding author.E-mail address: yyin@physics.usyd.edu.au (Y. Yin).

0925-9635/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.diamond.2006.05.003

control is not convenient in many deposition methods due to itselectrically insulating property of DLC materials.

Ion source can overcome this problem to provide the desiredion bombardment energy. Cold cathode ion source was used forPECVD deposition [6]. However, it is know that cold cathodeion source can only provide ion energy in the range betweenapproximately 300 and 1000 eV. It is desirable in DLCdeposition to extend the energy range to the region between 300and a few tens of eV. It was reported that very hard and high sp3

content DLC films were reported to be deposited in this energyregion [7]. It is also true that cold cathode ion source can onlygenerate small ion current so that its plasma enhancement effectand thus the deposition rate is limited significantly. In this work,we proposed to use an End-Hall ion source for DLC PECVDdeposition. End-Hall ion source can provide ion energy fromapproximately 30 eV to 200 eV, a useful extension of ion energyof the cold cathode ion sources. The ion current of an End-Hallion source is approximately one order of magnitude higher thanthat of a typical cold cathode ion source. The technical design ofthe End-Hall ion source has been well developed, and its

Fig. 2. Discharge current as a function of discharge voltage.

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operating mechanism is well understood. The End-Hall isnamed because the ion beam exists the acceleration region at thecircular end of the magnetic field as described by Kaufman et al.[8]. Recently, the circular End-Hall ion sources were arranged toform a linear version for large area plasma treatment [9].

DLC films deposited in this work were aimed for applica-tions of infrared transmittance enhancement in the wavelengthregion between 3 and 5 μm. Infrared sensors or transducers forthis window are usually made of materials such as Si and Ge.Those materials generally have high refractive index in thewavelength region, resulting in high reflection losses. It is thusnecessary to deposit antireflection thin films to enhance thetransmission. Unfortunately, conventional infrared antireflec-tion thin films have some drawbacks such as poor mechanicalproperty, low density, and moisture sensitive. DLC thin film isideal for infrared transmission enhancement application, due toits infrared transparency, high mechanical hardness, moistureinsensitive, chemical inertia, and ideal refractive index forsilicon or germanium substrates [10–12]. For the application inthe wavelength region above 4 μm, DLC films should beapproximately thicker than 550 nm. This means a lowmechanical stress DLC film is necessary.

2. Experimental setup and procedure

Fig. 1 shows the schematic diagram of the custom-designedEnd-Hall ion source. End-Hall ion source designed in this workcan form a glow discharge in the region as shown in the figure.The magnetic field lines cross the electric field lines so that theelectrons can move in circular trajectories to generate the socalled Hall current. Reactive gases were injected from bottom ofthe ion source as shown in the figure. The cathode was placed infront of the ion source. The electrons emitted from the cathodetravel into the discharge region along the magnetic field lines toionize gas molecules. Ions are extracted by the potentialdifference between the cathode and the anode and acceleratedonto substrate.

Base pressure of the system was 5×10−3 Pa. A grided ionenergy analyser was used to measure the ion energy. It is knownthat the ion energy analysis method is problematic in plasmaswith significant fraction of carbon species. In order to simplifythe ion energy analysis, ion energy analysis was performed inargon atmosphere to avoid poisoning the grided ion energy

Fig. 1. Schematic diagram of the custom-designed End-Hall ion source.

analyser. Methane (CH4) was used as reactive gas with purity of4 N. All DLC experimental results presented in this paper wereobtained at flow rate of 20 SCCM and operating pressure of2.0×10−2 Pa. DLC films were deposited on to both sides of thepolished p-type (100) silicon substrates. During deposition, thesubstrate temperature was about 50 °C. All deposition andanalysis in this work were conducted at distance of 20 cm fromthe ion source, unless specified in the text. Thin film thicknessand optical constants were measured using J.A. Woollam M-2000UI ellipsometer in the wavelength region from 400 to1600 nm. Three incident angles, 65, 70, 75°, were used in theellipsometry analysis. Hardness was analysed using HXD-1000microhardness instrument. Infrared transmission spectrum wasobtained using P.E. Fourier transform infrared (FTIR) spectros-copy in the wave number range from 400 to 4000 cm−1.

3. Results

Discharge current is a function of discharge voltage in theion sources. Fig. 2 shows the dependence of the dischargecurrent on the discharge voltage at argon pressure of2×10−2 Pa. At the distance of 20 cm from the ion source, ionenergy was measured using a grided ion energy analyser. Theions were generated in the discharge region as shown in Fig. 1.The ions coming out of the discharge region were thenaccelerated at the region between the cathode and the dischargeregion. The analysis showed that the ion energy was correlatedto the discharge current and the ion energy in eV, which wasapproximately scaled as 60% of the discharge voltage with anenergy window ±5 eV.

Fig. 3. Discharge current vs. cathode current.

Fig. 5. Hardness of DLC films vs. discharge current.

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Cathode current is an important parameter influencing thedischarge current. Fig. 3 shows the dependence of the dischargecurrent on the cathode current. The threshold cathode currentwas about 10 A. An approximately linear dependence of thedischarge current was found for cathode current between 10 and15 A, followed by a decrease of the discharge current. Thelinear range was resulted from that electron emission currentfrom the cathode which enhanced the discharge. Furtherincreasing the cathode current, the emitted electron currentcompensated the discharge current, resulting in decrease of thedischarge current.

The deposition rate was measured as a function of thedischarge current and the distance from the substrate to the ionsource as shown in Fig. 4. An approximately linear relationshipof deposition rate was found for discharge current up to about3 A as shown in Fig. 4(a). Beyond 3 A, it is expected thatenhanced ion bombardment on substrate would result in etchingdeposited films. Increasing the substrate distance, the depositionrate decreased as shown in Fig. 4(b). The deposition rate was30 nm/min for substrate distance about 20 cm, which is areasonable rate for practical applications.

Fig. 5 shows the hardness of DLC films as a function ofdischarge current. The hardest DLC film was correspondent tothe discharge current about 2 A. As shown in Fig. 2, thedischarge voltage corresponding to this discharge current wasabout 90 V, and thus the ion energy was about 54 eV (scaledabout 60% of the discharge voltage). Energetic ion bombard-ment of DLC films during deposition is regarded as one of themost important factors affecting the hardness and intrinsicstress. Thornton schematically illustrated that thin films changefrom a porous or columnar to a dense structure as sputtering

Fig. 4. (a) Deposition rate as a function of discharge current, (b) deposition rateas a function of distance from substrate to ion source.

voltage increased [13]. The transition from a columnar to adense material is often associated with changes in microstruc-ture and preferred orientation.

The hardness data in Fig. 5 was replotted in Fig. 6 as afunction of ion energy. Experimentally, a universal behaviour isfound in thin film materials showing intrinsic stress, reaching amaximum and then decreasing with increasing ion energy.Models were proposed by Davis [14] and other researchers [15]to explain the behaviour of compressive stress in thin filmsdeposited with simultaneous ion bombardment. It was proposedthat increasing compressive stress in DLC films resulted inincreasing sp3 content and thus the hardness of the films [16].The maximum position of the hardness in Fig. 6 is consistentwith previous observations.

The hardness dependence on ion energy suggested that theEnd-Hall ion source is very useful for ion source enhancedchemical vapour deposition of DLC films. Cold cathode ionsource used in previous study [7] would be expected lesseffective to produce hard DLC films as its ion energy windowwas beyond 300 eV.

In the infrared transmission enhancement application, opticalrefractive index is an important parameter. Ideally, one expectedto have a relationship for the optical refractive index of amaterial as nf

2 =ns, where nf is the optical index of the thin film,and ns is the optical index of the substrate. In the infrared region,the optical index of silicon is equal to 3.5, thus the ideal opticalindex of the DLC films is 1.87 for this application when siliconis used as a substrate. Fig. 7 shows the refractive index of DLCfilms as a function of discharge current. A reasonable widerange of discharge current was found between 1 and 3 A.

Fig. 6. Hardness of DLC films vs. ion energy.

Fig. 9. Infrared spectrum of a DLC film coated on one side of silicon substrate.

Fig. 7. Effect of discharge current on refractive index of DLC films.

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Considering the hardness effect in Fig. 6, we determined that theoptimised condition of discharge current was 2 A.

One of the advantages of the End-Hall ion source is its wideangle ion emission feature. We found that a large area uniformdeposition can be obtained in the ion source enhanced PECVDmode. Fig. 8 shows the thickness profile for substrate distanceof 20 cm. The uniformity was about 2.5% for a 20 cm diametersubstrate. Increasing substrate diameter to 40 cm, the uniformitywas still about 5%.

Five hundred nanometer thick DLC film was deposited ontoone side of a double-sided polished silicon substrate. A typicalinfrared transmission spectrum was shown in Fig. 9. Atwavelength equal to 4 μm, transmittance is about 69.4%, veryclose to the ideal value. Hydrogen was found to be incorporatedinto the DLC, evidenced by its associated absorption peaks inthe region between 2800 and 3000 cm−1. The insert in Fig. 9shows the details of these absorption peaks. The presence ofthese peaks was suggested as an indication of the carbonbonding states to be of predominantly sp3 type [17].

The bonding characteristics of the DLC films were studiedfurther using Raman technique. Fig. 10 shows a Ramanspectrum of a 500 nm thick as-deposited sample. The DLCwas deposited with the optimised conditions at dischargecurrent of 2 A. The Raman intensity spectrum showed a typicalpeak at 1350 cm−1 assigned as a D band overlapping with apeak at 1570 cm−1 assigned as a G band. The intensity ratio Id/Ig is commonly used as a measure of the sp3 content in the filmsby deconvoluting and fitting to two Lorentzians [17]. For thissample, the fit resulted in a ratio Id/Ig of 2.06.

The hardness and the Raman intensity of D band relative tothat of G band appeared to be higher than expected. However,the observations in this work were not entirely contrary toresults reported by other researchers. For example, Druz et al.

Fig. 8. Relative thickness vs. distance from ion source axis.

[18] reported hardness higher than 30 GPa (or the hardness inthis work). Their DLC films were obtained by using a Veeco RFICP ion source with reactive gas methane or acetylene.However, Raman analysis on their films was not reported.Fang et al. reported both hardness and Raman absorption peaksof DLC films recently [19]. In their microwave plasmadeposition, the substrate was biased to 30 V with mixture gasof methane and hydrogen. The relative absorption intensity of Dband in their films appeared even higher than that in our result.Their hardness was about 27 GPa, very close to our result.Taking plasma potential as 25 V (which is very common for RFplasma), their ion energy should be approximately 55 eV, whichis very close to our ion energy. Nevertheless, furtherinvestigation on this issue is required.

The mechanical properties of the DLC coated samples werealso analysed. The adhesion of the DLC film was very good.Adhesion of the DLC films on silicon substrate was analysedusing a standard tape test. At the required thickness forapplications in the 3–5 μm region, all the DLC films survivedthe adhesion tests and their stresses were between 1 and 3 GPa.The DLC films on silicon substrates were also found thermallyand chemically stable. At 200 °C and −40 °C in air for 8 hrespectively, both hardness and infrared transmittance were notchanged after 4 times repeating the experiment. We placed theDLC films into 5% NaCl, 5%HCl, and 5%NaOH aquaticsolution respectively for one week. We found no evidence ofdegradation on mechanical and optical properties of the DLCfilms.

Fig. 10. Raman spectrum of an as-deposited DLC sample.

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4. Conclusion

A custom-designed End-Hall ion source was used to depositdiamond-like carbon (DLC) films in an ion source plasmaenhanced chemical vapour deposition mode. The system wascharacterised and optimised for infrared transmission enhance-ment applications and for deposition of large area DLC ontosilicon or germanium substrates. The ion energy range of the ionsource was approximately between 30 and 200 eV. A large dis-charge current can be produced in this ion source. Ion bomb-ardment energy (in eV) on substrate was found to scale about 60%of the discharge voltage, determined by using a grided ion energyanalyser. Large area deposition was achieved with uniformityabout 2.5% and 5% for substrate diameters of 20 cm and 40 cmrespectively. Mechanical hardness and optical index of the DLCfilms were also analysed. For the infrared enhancement ap-plications using silicon substrates the optimised ion bombardmentenergywas about 54 eVwith a high deposition rate of 30 nm/min.Depositing DLC onto single side of double-sided polished siliconwafers resulted in a transmission of 69.5% in the wavelength ofabout 4 μm, which was very close to the ideal value. Ramananalysiswas also conducted.Mechanical properties and reliabilityof DLC films on silicon wafers were analysed at differentenvironmental conditions.We found that the DLC films producedin the ion source PECVD deposition system satisfied the require-ments for the infrared transmission enhancement applications.

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