Characterization of silicon films deposited in presence of nitrogen plasma

Vacuum 65 (2002) 91–100

Characterization of silicon films deposited in presence of nitrogen plasma Sheetal J. Patila, Dhananjay S. Bodasa, A.S. Ethirajb, R.C. Purandareb, G.J. Phatakc, S.K. Kulkarnib, S.A. Gangala,* a

Department of Electronic Science, University of Pune, Ganeshkhind Road, Pune 411 007, India b Department of Physics, University of Pune, Ganeshkhind Road, Pune 411 007, India c Center for Materials for Electronics Technology (C-MET), Off. Pashan Road, Pune 411 008, India Received 22 June 2001; accepted 7 August 2001

Abstract Silicon films are deposited in presence of nitrogen plasma with the technique known as ‘activated reactive evaporation (ARE)’ with a view to deposit silicon nitride films on silicon substrate at room temperature. The in-house fabricated ARE system consists of a high vacuum chamber (e-beam gun housing) and a low vacuum chamber (reaction chamber) separated by a plate with an opening at the center. Silicon was evaporated by an electron beam (e-beam) gun in the presence of nitrogen plasma and films were deposited on a silicon substrate at room temperature. A number of experiments were carried out for testing the system and for checking the repeatability of the deposition of the films. The characterization of the films deposited on silicon substrates was done using X-Ray Diffraction, X-ray Photoelectron Spectroscopy and Energy Dispersive Analysis of X-ray (EDAX). Refractive index of 1.97 obtained from ellipsometric measurements is in good agreement with that of the standard value of silicon nitride. Observation under SEM showed particulates of silicon on the surface of the film. The same was confirmed from spot EDAX analysis. These results indicate that the deposited films are non-stoichiometric and contain both silicon nitride and a phase of silicon oxynitride. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: Silicon nitride; Activated reactive evaporation; CVD; XPS; XRD; SEM; EDAX

1. Introduction Silicon dioxide ðSiO2 Þ formed by thermal oxidation of silicon has been the material of choice for various uses in silicon technology such as, gate or capacitor dielectric, insulator, *Corresponding author. Tel.: +91-20-569-9841; fax: +9120-565-3899. E-mail address: [email protected] (S.A. Gangal).

primary passivation, mask against diffusion or etching process [1–3], etc. The drive towards miniaturization of devices and increasing circuit complexity has already shown that silicon dioxide cannot satisfy all the needs in the future [4,5]. The practical limit of obtaining pinhole free 50 nm films of silicon dioxide has led to a search for compatible, higher dielectric constant material, which can be of greater thickness. One of the prime contenders for this application is silicon nitride ðSi3 N4 Þ: Presently, silicon nitride

0042-207X/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 0 1 ) 0 0 4 1 2 - 2

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is already being used as storage dielectric in memory devices, as final passivating layer and as mask for oxidation and dopant diffusion [6–8]. However, before silicon nitride films can be used for specific application, many issues such as, interface state density, thermo-electrical stability etc. need to be solved. The chosen material must also satisfy the other general requirements, such as low density of pinholes, good chemical stability, etc. Passivating films of silicon nitride are generally deposited by plasma enhanced chemical vapor deposition (PECVD), whereas, films used as storage dielectric are deposited by low pressure chemical vapor deposition (LPCVD). In both the techniques, films are typically deposited using a mixture of silane ðSiH4 Þ and ammonia ðNH3 Þ: The major advantage of PECVD is its low deposition temperature ð3001CÞ as compared to LPCVD ð7001CÞ: However, silicon nitride films deposited by both CVD methods have been reported to contain 5–30 atomic% hydrogen [9], which may be the cause of their high bulk trap densities [10]. Such a bulk trap in the gate dielectric assists the conduction of carriers, causing an increase in the gate leakage current and making them unusable as gate dielectrics. These films also show poor thermal stability, as they are prone to the release of hydrogen at higher temperatures [11,12]. Thus, it is evident that the incorporation of hydrogen renders the device unstable. Activated reactive evaporation (ARE) has been developed mainly for the deposition of tribological [13], transparent and conducting [14] coatings. It involves evaporation of metals in the presence of the plasma of reacting gas. The ‘‘activation’’ of the evaporated species as well as the gaseous reactant by plasma, helps in depositing high quality compound films at very low temperatures. Coatings of SnO2 ; In2 O3 and CdO have been reported using this technique [15–17]. In the present ARE system, the evaporated silicon atoms react with excited=ionized nitrogen species on the substrate surface to form silicon nitride or a non-stoichiometric compound of silicon and nitrogen. Compared to reactive sputtering, this method offers advantages of low power

levels of plasma and higher deposition rates. Moreover, it is expected to offer all the advantages of PECVD and it eliminates the problem of hydrogen in the films. Present work uses the electron beam (e-beam) evaporation method for the evaporation of pure elemental silicon (99.999%), because of its precise evaporation control, relative cleanliness and a high temperature achieved as required for silicon. This work refers to a low temperature plasma evaporation process, which has already been used successfully for the deposition of silicon dioxide as well as silicon nitride films [18,19]. In the present paper we report, to the best of our knowledge for the first time, the results of the room temperature (substrate temperature) deposition of silicon films in the presence of nitrogen plasma. The detailed physical and chemical characterization of the ARE deposited silicon films was done using techniques, such as Ellipsometry, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), energy dispersive analysis of X-ray (EDAX) and scanning electron microscopy (SEM). The results are compared with those available in the literature and discussed here.

2. Experimental The ARE system used for the deposition of silicon nitride films is shown schematically in Fig. 1. The system is divided into two parts, viz. high vacuum chamber (e-beam gun housing) and low vacuum chamber (reaction chamber). The pumping system comprises a rotary-backed diffusion pump having a liquid nitrogen trap between the diffusion pump and the chamber. The high vacuum part houses an e-beam gun (1801; 3 kW), which evaporates silicon through a crucible kept 5 cm below the opening of 2 cm diameter connecting the two chambers. Placed above the separating plate is a 12 cm diameter glass chamber, which defines the low vacuum part (reaction chamber) of the system. An externally placed inductive coil for plasma excitation is wound around the reaction chamber to avoid the submersion of coils in the plasma and the consequent contamination.

S.J. Patil et al. / Vacuum 65 (2002) 91–100 Pressure Measurement

93

Nitrogen Inlet

Low Vacuum Chamber (Reaction Chamber)

Inductive Coil Substrate

Matching Network

~

Conductance Plate

RF Source 13.56MHz

Evaporated Silicon E-Beam Gun (Angle =180o) and Crucible Base Plate

High Vacuum Chamber Vacuum Pump

Fig. 1. RF Exited ARE system.

2.1. Substrate and materials used

2.3. Deposition conditions

During the present experiments, deposition of films is done on n-type, ð1 0 0Þ oriented silicon wafers having 3 O cm (ohm centimeter) bulk resistivity. The native oxide was removed using HF=HNO3 and rinsed using 18 MO resistance de-ionized (DI) water. Pure elemental silicon (99.999%) powder was used as silicon source and UHP grade nitrogen gas (99.999%) as plasma source. The substrates were treated by nitrogen plasma as a final cleaning step.

A series of experiments were carried out for checking the repeatability of the deposition of the films. Deposition parameters are e-beam current 200 mA; e-beam voltage 5 kV; nitrogen flow rate 4:5 sccm; time of evaporation was 25 min and RF plasma power of 80 W: Silicon substrates were kept at room temperature. The distance between the source and substrate was 12 cm; which is the minimum possible distance in the existing system. 2.4. Characterization techniques

2.2. Deposition process Before the deposition, the system was evacuated to 106 Torr: Ultra pure nitrogen gas was then introduced from the top into the reaction chamber. The system was flushed with nitrogen gas 2–3 times, so as to reduce residual gas content in the chamber. One order of magnitude difference in the pressure is maintained between the reaction chamber ðB103 TorrÞ and in e-beam gun chamber ðB104 TorrÞ: The nitrogen gas was excited by an inductively coupled RF source ð13:56 MHzÞ and then silicon was evaporated by e-beam gun.

The refractive index and the thickness of the films were measured and calculated using Ellipsometry (Gaester L118 null, Mathcad-7 software). The structural characterization of these films was done using XRD technique (Philips, PW 1729, CuKa source). The topography of the films was observed using SEM (Cambridge, SEM S120). The chemical and compositional analysis of the films has been done using XPS (ESCA MK-II System of VG SCIENTIFIC Ltd. UK, MgKa source 1253:6 eV). Base vacuum was B109 Torr: Concentric hemispherical analyzer pass energy B50 eV was used. Au4f 7=2 at 85:0 eV was used

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as an external reference and C1s at 285:0 eV as an internal reference. This system uses argon ion ðArþ Þ gun for in situ sputtering and cleaning. The compositional analysis of the films was also done using EDAX (Phillips, XL-30, with EDAX detector).

3. Results and discussion The X-ray diffraction patterns were taken in order to evaluate the crystallinity of the deposited films. The films deposited at 80 W plasma power show maximum XRD peak intensity as compared with the films deposited at other powers of 60, 100 and 120 W: Therefore, for further investigations the films deposited at plasma power of 80 W were used. Deposition at 80 W power was tested for repeatability. A typical XRD pattern of the above deposited film is shown in Fig. 2. The pattern shows a strong peak corresponding to silicon substrate ½2yðthetaÞ ¼ 69:111; which was common in all the patterns. The films are deposited on single crystalline silicon (1 0 0 orientation) substrate and hence, this expected peak is neglected from the rest of the analysis. The other peaks at ‘d’ ( 1:501 A; ( 1:340 A ( correspond to values of 2:698 A;

either silicon nitride or silicon oxynitride [20]. These peaks have low intensity as compared to that of silicon peak and may be attributed to the smaller thickness of the film. The percentage difference ð½dðstdÞ  dðobsÞ=dðstdÞ100Þ of observed peaks from the standard peaks is calculated for silicon nitride (b-phase) and silicon oxynitride, and is given in Table 1. It is evident from Table 1 that the difference observed in the ‘d’ values for silicon nitride (b-phase) is less as compared to that for silicon oxynitride. However, still the presence of silicon oxynitride in the film cannot be neglected. The refractive index (at the He–Ne laser wavelength 632:8 nm) and thickness of the films were obtained from ellipsometric analysis. Refractive index was found to be 1.97, which was closer to the standard value of refractive index for silicon nitride ðZ ¼ 2:0Þ [21]. Thickness obtained by ( ellipsometric measurement was B800 A: Composition of the film was further investigated with the help of XPS technique. Fig. 3 shows the survey scans for the ‘as received’ sample and after Arþ bombardment for 40 min: It can be noticed that silicon and nitrogen are present in the sample and there are no other impurities except oxygen and a small amount of carbon which may be

Fig. 2. XRD plot of the film deposited at 80 W RF plasma power.

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S.J. Patil et al. / Vacuum 65 (2002) 91–100 Table 1 % difference of the observed XRD peaks from standard (film deposited on silicon /1 0 0S at 80 W plasma power) ( Std ‘d’ values A and /h k lS silicon nitride

( Std ‘d’ values A and /h k lS silicon oxynitride

Observed ‘d’ ( values A

% Difference silicon nitride (b-Phase)

% Difference silicon-oxynitride

2.66 ð1 0 1Þ 1.51 ð3 2 0Þ 1.34 ð3 2 1Þ

2.60 ð1 3 0Þ 1.52 ð1 1 3Þ 1.32 ð4 0 1Þ

2.680 1.501 1.344

0.75 0.9 0.29

3.07 1.25 1.81

Fig. 3. XPS survey scans of the film deposited at 80 W plasma power (i) As received (ii) 40 min.

incorporated during deposition or arise due to ex situ measurements. Nitrogen appears to have increased after Arþ bombardment i.e. sputtering of the ‘as received’ film. The concentration profile of the film is plotted in Fig. 4. It can be observed, that ‘oxygen’ reduces with Arþ bombardment. However, contamination level due to carbon is more or less uniform. Nitrogen level increases from less than a percent on the as received sample to B16% on 40 min sputtered film. Silicon increases by B10% from the as received sample to 40 min sputtered sample. It is well known that ion bombardment can decompose the compound and preferentially sputter some elements over the

others. However, decomposition is negligible in the case of compounds of light elements like SiO2 ; Al2 O3 ; etc. [22]. Also the sputtering coefficients of all these light elements are not expected to vary substantially and can be used to understand the composition and chemical nature of the film with depth. Further information about the bonding can be obtained by considering Si2p, N1s and O1s peaks as shown in Figs. 5(a)–(c), respectively. All these spectra are corrected for any charging effects that may arise, by referencing them to C1s ð285:0 eVÞ present in the sample. On the as received sample Si2p (Fig. 5a) peak appears at B104:6 eV which

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Si 2p

Conc. at %

50 40 30

O 1s

20

N 1s 10

C 1s 0 0

10

20

30

40

Ar + Bombardment Time (min) Fig. 4. Concentration Depth profile.

shifts to B102:6 eV after 40 min bombardment. Correspondingly, N1s (Fig. 5b) initially at 403:1 eV also shifts to 398:5 eV: There is, however, a broad but weak N1s component visible around 403:0 eV: N1s peaks in silicon nitride and silicon oxynitride are given by Shallenberger et al. [23]. They have attributed the peak at 402:8 eV to Si– N–ðOÞ2 ; and peak between 399.7 and 401:0 eV to ðSiÞ2 –N–O. Peak due to (–SiÞ3 N is between 397.4 and 398:4 eV [24]. Lu et al. [25] have assigned N1s at B397:0 eV to ðSiÞ3 N and higher binding energy N1s peaks, to silicon oxynitride in which oxygen being more electronegative than nitrogen causes N–O bond with charge transfer from nitrogen to oxygen. In our analysis, the ‘as received’ sample has N1s at a higher binding energy due to the large amount of oxygen incorporation, but with bombardment, nitride component in the film is more predominant. O1s peak (Fig. 5c) is broad and appears between 531.2 and 533:5 eV: However exact nature of oxide is difficult to judge from O1s. It is concluded from this data that the formation of non-stoichiometric silicon rich silicon nitride is more in depth of the film than at surface, which indicates that the nitride formation is more in the initial phases of deposition. The above observation was confirmed by subjecting these films to EDAX. It is possible to

obtain the elemental composition at different sampling depths and it depends upon the accelerating voltage of the incident electrons. The depth of penetration or electron range ðRe Þ of these electrons can be roughly calculated from the following equation [26]. Re ¼ ð4:28  106 =rÞE 1:75 ðcmÞ; where r (rho) is the density of the material in gm=cc and E is incident electron energy in keV. Table 2 represents the elemental concentrations found from EDAX at different accelerating voltages and the calculated penetration depth considering the total thickness of silicon or of silicon nitride. The penetration depths at high electron energy are very large in comparison with ( the deposited silicon nitride thickness ðB800 AÞ and are comparable to low electron energies (e.g. 3 keV). It can be observed from Table 2 that, as the penetration depth reduces, the concentration of nitrogen and oxygen increases and at the same time the concentration of silicon reduces. Since the thickness of the film is constant, the volume of the film scanned increases in relation to the volume of substrate scanned as the energy of incident electron decreases or penetration depth reduces. Therefore, the increase observed in the concentration of nitrogen and oxygen can be considered to be because of the film. The observed 10%

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Si 2p

(v)

(iv) (iii)

Intensity (arb. Unit)

Intensity (arb. Unit)

N 1s

(v)

(ii)

(iv) (iii) (ii) (i)

(i) 98

(a)

100

106

104 102 Binding Energy ( eV)

396

108

398

(b)

400 402 Binding Energy ( eV)

404

406

O 1s

(v)

Intensity (arb. Unit)

(iv)

(iii)

(ii)

(i) 529

(c)

531

533 535 Binding Energy ( eV)

537

539

Fig. 5a. (a) Si (2p) shift in binding energy in the depth with time variation (i) As received (ii) 1 min (iii) 5 min (iv) 20 min and (v) 40 min. (b) N (1s) shift in binding energy in the depth with time variation (i) As received (ii) 1 min (iii) 5 min (iv) 20 min and (v) 40 min. (c) O (1s) shift in binding energy in the depth with time variation (i) As received (ii) 1 min (iii) 5 min (iv) 20 min and (v) 40 min.

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Table 2 Relative concentration of silicon, nitrogen and oxygen at different depths found from EDAX analysis Acc. voltage (keV) 15 10 7 5 3

Calculated penetration depth ðmmÞ

EDAX results (standard less) at%

Silicon

Silicon nitride

Silicon

Oxygen

Nitrogen

2.10 1.03 0.55 0.30 0.12

1.42 0.69 0.37 0.20 0.08

95.98 83.78 78.40 71.99 58.53

4.02 13.85 16.27 20.67 31.88

0.00 2.36 5.33 7.34 9.59

Fig. 6. SEM pictures of the deposited silicon nitride film. (a) Texture near the film edge. (b) Surface texture and (c) Film texture after ultrasonic treatment.

concentration of nitrogen indicates that the film comprises the mixture of silicon nitride and a metastable phase of silicon oxynitride.

The surface topography of the deposited films is observed using SEM. SEM pictures of the films are shown in Fig. 6(a)–(c). Fig. 6a shows the film near

S.J. Patil et al. / Vacuum 65 (2002) 91–100

99

Fig. 7. XRD plot of the film deposited at 80 W plasma power after ultrasonic treatment.

its edge. This film was observed without any conducting film coating and therefore, large difference in the contrast of the film and the silicon substrate and also the charging effects observed during the experiments indicate the insulating nature of the film. The variation of the film thickness near the edge of the film was observed. Fig. 6b shows the surface texture of same film at 912X magnification. Particulate deposition of size between 0:5 mm and 10 mm diameter was observed over the entire film. The large size particles on the film make the film texture rough. These large size particles on the surface of the film were observed under the spot EDAX. It is confirmed that the particles are that of silicon. In order to check the adhesion of these silicon particles, the films were subjected to ultrasonic agitation. It was found that, these particles had an extremely poor adhesion and could be easily removed [27]. Micrograph of the film after ultrasonic removal of loosely bound particles is shown in Fig. 6c. The film surface is observed to be smooth and some black pinholes appear on the film at the places

where probably the above-mentioned silicon particles were present before. This supports the observation by spot EDAX of silicon particles on the top surface. In order to cross check the chemical composition, the ultrasonically treated films were analyzed by XRD again. Fig. 7 presents the XRD, which shows no appreciable change from the diffraction pattern before ultrasonic treatment. This confirms that the film indeed is composed of silicon nitride (b-phase) and metastable phase of silicon oxynitride.

4. Conclusion Silicon has been evaporated in the presence of nitrogen plasma on the silicon substrates maintained at room temperature in an in-house designed and fabricated ARE system. The present system has high and low vacuum chambers, pumped by a single pumping system. Results show that, the films comprise the mixture of silicon nitride and metastable phase of silicon oxynitride. Crystallized phase was observed for the films

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deposited even at room temperature. Further, work is in progress to deposit stoichiometric films of silicon nitride through optimization of process parameters.

Acknowledgements The authors gratefully acknowledge the continuous financial support of the Department of Atomic Energy, BRNS, India. S.K. Kulkarni thanks UGC, India, for a continuous support and A.S. Ethiraj thanks IUC-DAEF, India for fellowship. They would also like to express their sincere thanks to Dr. Murli Sastry (Scientist, National Chemical Laboratory (NCL), Pune, India) for providing Ellipsometry facility.

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