silicon oxide films from the plasmodul®

*Corresponding author: Tel.: #49-0-711-685-2300; 2302; fax: #49-711-685-3102.E-mail address: [email protected] (M. Walker).

Vacuum 57 (2000) 387}397

Silicon oxide "lms from the Plasmodult

M. Walker!,*, K.-M. BaumgaK rtner!, J. Feichtinger!, M. Kaiser", A. Schulz!,E. RaK uchle!

!Institut fu( r Plasmaforschung, der Universita( t Stuttgart, Pfawenwaldring 31, D-70569 Stuttgart, Germany"Fraunhofer Institut fu( r Chemische Technologie, Joseph-von-Fraunhofer-Strasse 7, 76327 Pxnztal, Germany

Received 14 February 2000; accepted 8 March 2000

Abstract

Gas mixtures of hexamethyldisiloxane (HMDSO) and oxygen (O2) are used for plasma-enhanced chemical

vapour deposition of silicon oxide "lms. These "lms were deposited in a newly developed plasma reactor,called Plasmodult. The plasma is excited by 2.45 GHz microwaves in the pressure range of 0.05}200 mbar.The Plasmodul is a modular device consisting of a plasma source, a gas inlet system, a reaction and substratechamber and a diagnostic port. It is a #exible equipment, comfortable in handling and simple in construction.The special arrangement allows a "lm deposition remote from the plasma source. The chemical compositionof the "lms was investigated in dependence on the HMDSO : O

2monomer mixture ratio by Fourier-

transform infrared spectroscopy. ( 2000 Elsevier Science Ltd. All rights reserved.

Keywords: Microwave plasma source; Plasma deposition; Silicon oxide "lms; Hexamethyldisiloxane

1. Introduction

Thin "lms of silicon oxide are widely used in microelectronics, optoelectronics, optics and theyhave outstanding properties for the protection of metals and polymers [1}5]. These coatings o!erseveral advantages : they are transparent and chemically inert, they absorb UV radiation (which isdesirable for UV-sensitive polymers), and they have su$cient hardness. Silicon oxide is used,e.g. for protective layers in re#ectors of automobiles or halogen lamps and can be used for scratchresistant layers on transparent polymers like polycarbonate (PC) or polymethlymetacrylate(PMMA) [6]. Another important application pro"ts from the barrier performance of packaging

0042-207X/00/$ - see front matter ( 2000 Elsevier Science Ltd. All rights reserved.PII: S 0 0 4 2 - 2 0 7 X ( 0 0 ) 0 0 1 5 2 - 4

Fig. 1. A photography of a plasma array consisting of two plasmalines.

polymers like polyethyleneterephtalate (PET) or polypropylene (PP), which can be stronglyimproved by a thin silicon oxide coating [7].

Besides the evaporation and sputter techniques, the plasma-enhanced chemical vapour depos-ition (PECVD) of silicon containing gases is well suited for the deposition of silicon oxide layers.Silane (SiH

4) and organosilicon monomers like hexamethyldisiloxane (HMDSO), tetraethyl-

orthosilicate (TEOS) and tetramethylsilane (TMS) are the most common precursors described inliterature. Organosilicon monomers as HMDSO have the advantages of lower cost relative toSiH

4and TEOS, and compared to SiH

4the handling is uncomplicated.

In order to establish PECVD processes economically, it is desirable to develop simple, inexpen-sive plasma sources for large scale deposition. In addition, such plasma sources should allow highdeposition rates with good uniformity in thickness and composition. Microwave discharges are oneattractive way of plasma production [8]. Especially the highly developed 2.45 GHz microwavetechnology for magnetrons and power supplies has reached a low cost standard that makes theplasma excitation by microwaves pro"table. Homogeneous linearly extended plasmas can beproduced by a speci"c arrangement of the emitting antenna where the microwave power is coupledfrom a linearly extended waveguide into the plasma [9], or by a horn antenna coupling themicrowave along an extended magnetic "eld structure into a low-pressure plasma using theelectron}cyclotron resonance (ECR) mechanism [10].

Recent investigations on a simple and inexpensive plasma source started with the Gigatront[11], where a linearly extended glass tube with a concentric metallic inner conductor was mountedin a low-pressure chamber. One end was closed, the other one was connected to the microwavegenerator. The plasma is produced in the low-pressure regime outside the tube. The microwavespropagate in the plasma as outer surface waves [12]. The axial homogeneity of the plasma wasconsiderably improved by feeding the microwaves symmetrically from both ends of the tube. ThisDuo-Plasmalinet forms a linearly extended and homogeneous plasma up to a length of severalmeters [13]. A combination of such plasmalines can be used to obtain a two-dimensional plasmaarray. A side view of such a plasma array with two plasmalines, each with a length of 80 cm anda distance between the lines of 9 cm is shown in Fig. 1. Measurements show that at a distance ofapproximately 6 cm perpendicular to the plasmalines the electron density and the deposition rate

388 M. Walker et al. / Vacuum 57 (2000) 387}397

Fig. 2. Schematic view of the Plasmodult.

are homogeneous over an area of 10 cm]50 cm. The e!ective plasma area can be further increasedby using additional plasmalines.

In research laboratories often a smaller, #exible plasma equipment is desirable. In this work,such a plasma device, called Plasmodult [14], is presented. Here, the plasmaline principle is usedfor medium and small devices as test devices in plasma technology.

2. Experimental

A schematic view of the modular concept of the Plasmodul is shown in Fig. 2. It consists ofa plasma source, two gas inlet systems, a reaction chamber and a diagnostic port. The whole device

M. Walker et al. / Vacuum 57 (2000) 387}397 389

Fig. 3. Side view of the plasma source for di!erent operating conditions. For details see text.

is a cylinder of aluminium with a diameter of 35 cm and a height of 40 cm. The pumping systemconsists of a rotary pump (Alcatel, Typ 2012) with a pumping speed of 2 m3/h. The achievableend pressure is 2]10~2 mbar. The pressure is controlled by a leak valve and a Baratron gauge(MKS Instruments).

Each of the 4 plasmalines consists of a copper rod as inner conductor centered in a glass tubewith a diameter of 1.5 cm. The microwaves generated by two magnetrons with a total power of2]600 W are fed in at both ends. The microwave power is splitted via a coaxial arrangement intothe 4 plasmalines. The inside of the glass tubes are at atmospheric pressure whereas the outside is atlow-pressure. The microwaves propagate along the copper rods. When the electric "eld strengthexceeds the breakdown "eld strength the discharge ignites outside the tubes. The length of theformed plasma column mainly depends on the microwave power and the process pressure. At lowmicrowave power, e.g. 50 W the plasma is concentrated at the ends of the tubes only (see Fig. 3a).With increasing microwave power the plasma grows from both ends along the tubes, and an axiallyhomogeneous plasma is formed (see Fig. 3b and c). Details of the measured electron densities andelectron temperatures are described in Refs. [12,15]. Here only a short summary will be given. Theinvestigations of the electron density distribution at di!erent positions parallel and perpendicularto the plasmalines show an excellent axial homogeneity. In the arrangement with 4 parallelplasmalines the electron density perpendicular to the plasmalines re#ects the geometric structureof the plasma source. At a distance near to one tube, the electron densities show the expectedmaximum under each plasmaline, with a local minimum between the lines. With an increasingdistance from the plasma source, the electron densities of the single plasmalines are broadened. Theelectron densities decreases, but the shape is more homogeneous. At a distance of 6 cm the electrondensity is homogeneous over an area of 10 cm]15 cm. The excellent homogeneity of the electrondensity re#ects the good homogeneity of the deposition rate. For the above-mentioned conditions,an inhomogeneity of the deposition rate of (5% can be obtained.

The plasma "lms were deposited on aluminium foils which were placed on a heatable substrateholder. The pressure during the plasma process was 0.6 mbar, the microwave power was 2]300 W.

The experiments were performed in gas mixtures of HMDSO and O2, supplied to the plasma


Fig. 4. IR spectra of HMDSO vapour (a) and of a plasma-polymerized HMDSO (PP-HMDSO) "lm (b).

chamber by means of two gas inlet systems, consisting of metal rods with holes acting as a gasshower (see Fig. 2). The O

2gas was fed via an electronic #ow controller to the "rst gas inlet, the

HMDSO monomer via an electronic #ow meter to the second gas inlet. This special arrangementallows a "lm deposition remote from the plasma source.

IR spectra of the plasma-polymerized "lms were recorded on a Bruker Vector 22 spectrometer.An attenuated total re#ectance (ATR) unit of Graseby Specac with a KRS-5 crystal (25 re#ections)was used. The angle of incidence was set at 453. The IR spectrum of the HMDSO vapour wasmeasured in the KBr transmission mode. For each spectrum, 32 scans with 8 cm~1 resolution wererecorded.

3. Results and discussion

The chemical composition of the plasma-polymerized "lms were investigated depending on theHMDSO : O

2monomer mixture ratio, and in some cases depending on the temperature.

3.1. IR spectra of HMDSO vapour and of a plasma-polymerized xlm from pure HMDSO

The hexamethyldisiloxane (HMDSO) molecule consists of a disiloxane (Si}O}Si) backbonewith three methyl groups (}CH

3) bound to each silicon atom. Fig. 4 shows the IR spectra of

(a) HMDSO vapour and (b) of a plasma-polymerized HMDSO (PP-HMDSO) "lm. The thicknessof the PP-HMDSO "lm was approximately 1 lm. The "lm was deposited with an HMDSO #owrate of 10 sccm (standard cm3 min~1) at a substrate temperature of 503C.

The very strong absorption band at 1070 cm~1 in the spectrum of HMDSO vapour is due to theasymmetric (Si}O}Si) stretching vibration. The corresponding symmetric stretching vibrationresults in a weak absorption band at 520 cm~1 [16}18]. In comparison, the IR spectrum of thePP-HMDSO "lm shows a broad (Si}O}Si) absorption band at 1020 cm~1 and a weak absorption


band at 450 cm~1. The symmetric }CH3

deformation vibration of methylsilyl (Si}CH3) groups

gives a strong infrared band at 1260 cm~1 in the IR spectrum of the HMDSO monomer as well asin the spectrum of the PP-HMDSO "lm. The absorption band at 850 cm~1 in the spectrum of theHMDSO monomer and the absorption band at 840 cm~1 in the spectrum of the PP-HMDSO "lmare due to Si}CH

3rocking vibrations. The absorption bands at 690 and 620 cm~1 in the HMDSO

monomer and the weak absorption in the PP-HMDSO "lm correspond to Si}C stretchingvibrations. The stretching vibrations of CH in Si-CH

3groups results in the absorption bands at

2965 and 2905 cm~1.The di!erences in the absorption intensities of the two IR spectra can be attributed to the

di!erent measuring procedures. Spectrum (a) is measured with the KBr method, whereas spectrum(b) is measured with the ATR technique. Especially in the range of large wavenumbers, the ATRtechnique is less sensitive than the KBr method.

Comparing the two spectra, the spectrum of the PP-HMDSO "lm shows bands which aredi!erent from the vapour spectrum: the strong absorption band at 800 cm~1 and the weakabsorption band at 2125 cm~1. The band at 800 cm~1 corresponds to Si}C stretching vibrationsand CH

3rocking vibrations of Si(CH

3)2

groups. Si}H groups in the PP-HMDSO "lm lead to theabsorption band at 2125 cm~1 [19}21].

Plasma polymerization of HMDSO is initiated by the collision of electrons with monomermolecules, resulting in the formation of hydrogen atoms, }CH

3, }CH

2and }CH groups, electrons,

ions, excited atoms and radicals [22]. The fragments di!use to the surface of the substrate and areabsorbed. Monomer molecules react with the absorbed radicals and produce radicals of highermolecular weight. Polymerization by further addition of monomer results in the growth of polymerchains. Additionally, the growing polymer surface is bombarded by particles formed in the plasma* especially electrons in low-pressure plasmas* and by photons emitted from the recombinationprocess of excited atoms. The energy of the photons and electrons are high enough to crack bondsin the polymer, thus forming new radical sites, which can act as centres for further polymerization.Based on this simpli"ed sequence of events a plasma polymer "lm is formed, which shows many sitechains and crosslinks. The monomer fragmentation via the abstraction of }CH

3groups from

silicon atoms and hydrogen atoms from }CH3

groups leads to the above mentioned Si(CH3)2

andSi}H groups in the plasma-polymerized "lm. Additionally, the di!erent (Si}O}Si) chain lengthscause di!erent (Si}O}Si) stretching frequencies in the IR spectrum. The superposition of thesevibration modes leads to the broad absorption band observed at 1020 cm~1.

It should be mentioned that the plasma polymerization process described here is only a qualitat-ive interpretation. A `completea understanding of the mechanism of polymer formation requiresthe reaction kinetics of all species involved.

3.2. IR spectra of diwerent HMDSO : O2 mixture ratios

The dilution of HMDSO by oxygen in the discharge leads to changes in the chemical composi-tion of the "lm as shown through infrared spectroscopy. Spectra of plasma "lms deposited fordi!erent HMDSO : O

2#ow rate ratios are shown in Fig. 5. The O

2#ow rate was varied between

4 and 40 sccm, the HMDSO #ow rate was 4 sccm. In the case of the "lm with a mixture ratio of1 : 20, the O

2#ow rate was 40 sccm and the HMDSO #ow rate was 2 sccm. The "lms were

deposited at a substrate temperature of 503C with a thickness of about 1 lm.


Fig. 5. IR spectra of plasma polymerized "lms grown at di!erent O2#ow rates.

Fig. 5 shows that the intensity of the symmetric }CH3

deformation band decreases withincreasing O

2concentration in the gas mixture. In the case of an HMDSO : O

2mixture ratio of

1 : 20 the }CH3

absorption band disappears. Additionally, the peak position changes as theO

2#ow rate changes, and shifts from 1257 cm~1 towards higher wavenumbers by as much as

20 cm~1 (Fig. 6). The peak position of the Si}CH3

absorption band is known to be sensitive to thenumber of methyl groups which are bonded on a silicon atom [23]. Therefore, a decrease of thenumber of methyl groups leads to an increase in the wavenumber and as shown in Fig. 5 toa related decrease in intensity. The peak position of the (Si}O}Si) stretching vibration also shifts tohigher wavenumbers for increasing O

2: HMDSO mixture ratio (Fig. 7). It is known that the

(Si}O}Si) peak position and the shape are sensitive to the SiOx

stoichiometry [24]. The frequencyof the peak scales monotonically with the oxygen atom concentration from 1075 cm~1 instoichiometric SiO

2to 1000 cm~1 in SiO. Films prepared with an HMDSO : O

2mixture ratio of

1 : 20 have a peak position at around 1040 cm~1. This results in a stoichiometry coe$cientof x+1.6.

For silicon oxide with x(2, the silicon atoms have one or more silicon atoms or, for example,}OH groups as neighbours and this shifts the (Si}O}Si) stretching frequency. Another possibility,which is discussed in the literature, is the shift of the peak frequency due to variations in the(Si}O}Si) bond angle [1].

As shown in Fig. 5, the absorption bands in the range between 950 and 450 cm~1 and in therange of 2970 and 2900 cm~1 change with increasing O

2concentration in the gas mixture. The

Si}CH3

rocking vibration at 840 cm~1 and the asymmetric and symmetric stretching vibrations of


Fig. 6. Variation of the peak position of the symmetric CH3

deformation vibration as a function of the O2

: HMDSOmixture ratio.

Fig. 7. Variation of the peak position of the (Si}O}Si) stretching vibration as a function of the O2

: HMDSO mixtureratio.

CH in Si}CH3

groups at 2960 and 2905 cm~1 disappear, whereas the intensity of the absorptionbands at 800 and 440 cm~1 increases. The IR spectrum of a "lm from an HMDSO : O

2mixture

ratio of 1 : 20 shows three characteristic (Si}O}Si) IR bands [25]. The strong absorption band at440 cm~1 corresponds to SiO rocking vibrations, the weak absorption band at 800 cm~1 is due tobending vibrations of SiO, and "nally the previous discussed strong absorption at 1040 cm~1corresponds to SiO stretching vibrations. The absorption bands at about 3300 and 930 cm~1 areassociated with stretching and bending motions, respectively, of hydrogen atoms in Si}OH groups,formed as oxidation products during the deposition process.

In conclusion, the measurements show that the dilution of HMDSO by oxygen in the dischargeresults in changes of the composition of the "lm. The chemical composition changes from anorganic polymer-like "lm to an inorganic, quartz-like "lm. The addition of oxygen causes anoxidation of the }CH

3groups to CO, CO

2, }OH and H

2O. This leads, as shown in Fig. 5 to


Fig. 8. IR spectra of SiOx"lms grown at different substrate temperatures.

a decrease in intensity of the CH absorption bands and to an increase in the (Si}O}Si) absorptionbands.

3.3. IR spectra of moderately heated substrates

Fig. 8 shows the IR absorption spectra for two "lms grown at di!erent substrate temperatures(¹"503C and 1003C), but with the same HMDSO : O

2mixture ratio of 1 : 3. The thickness of the

"lms was 1 lm.The intensity of the symmetric }CH

3deformation band at 1276 cm~1 and the Si}CH

3rocking

vibration at 840 cm~1 decreases with increasing substrate temperature. The "lm grown at thehigher temperature shows a smaller CH content and has accordingly a higher inorganic character.Additionally, the peak position of the (Si}O}Si) stretching vibration shifts towards higherwavenumbers. As mentioned above, the peak position is sensitive to the SiO

xstoichiometry. The

"lm prepared at a temperature of 1003C shows a peak position of the (Si}O}Si) stretching vibrationat 1042 cm~1. This means that the stoichiometry is x+1.6. This value corresponds to thestoichiometry of a "lm grown at a substrate temperature of 503C and an HMDSO : O

2mixture

ratio of 1 : 20.The two IR spectra show that the composition of the polymer "lms depends strongly on the

substrate temperature [25]. The balance of organic/inorganic content in such "lms can be shiftedtowards the inorganic character by increasing the substrate temperature. Therefore, the depositiontemperature is a powerful tool for the modi"cation and control of the chemical composition of theplasma polymer "lms.

4. Conclusions

The investigations contribute to the development of a microwave plasma source for industrialapplications and to the characterization of plasma-polymerized "lms from the monomerhexamethyldisiloxane (HMDSO).


The used plasma device, called Plasmodult, is a #exible equipment which is comfortable inhandling and simple in construction. The "lms deposited from di!erent HMDSO : O

2monomer

mixture ratios were characterized by FT-IR spectroscopy. It is shown that the chemical com-position of the "lms grown at di!erent O

2concentrations vary widely. Films grown at low

O2

concentrations contain many CH-groups, and a relatively soft, organic "lm is polymerized. Anincrease of the O

2concentration causes an oxidation of the CH-groups and the chemical

composition changes to an inorganic, quartz-like "lm. In the case of an SiOx"lm, plasma-

polymerized from an HMDSO : O2

mixture ratio of 1 : 20, the stoichiometry is x+1.6. Addition-ally, it is shown that at a "xed HMDSO : O

2monomer mixture ratio the balance of

organic/inorganic content in the "lms can be shifted to the inorganic character by increasing thesubstrate temperature.

Acknowledgements

This work was performed using microwave equipment from MUEGGE ELECTRONICGmbH, Mergbachstr. 56, 64385 Reichelsheim, Germany. The authors are very grateful toK. Muegge and H. Muegge. The authors thank Prof. Dr. U. Schumacher from the Institut fuK rPlasmaforschung der UniversitaK t Stuttgart for useful discussions. The authors would like to thankH. Petto and K.-H. Duhme for their technical assistance.

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