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Gettering Effects of Silicon Nitride Films FromVarious Plasma-Enhanced Chemical Vapor

Deposition ConditionsAnYao Liu , Ziv Hameiri, Yimao Wan, Chang Sun , and Daniel Macdonald

Abstract—This paper investigates and compares the impuritygettering effects of silicon nitride (SiNx ) films that are synthesizedby plasma-enhanced chemical vapor deposition (PECVD) undervarious conditions. Both industrial- and laboratory-scale PECVDsystems are employed to deposit SiNx films with a wide range ofproperties (with refractive indices from 1.93 to 2.45 at 632 nm),which covers the entire range of SiNx used for silicon solar cells.The gettering effects are quantified by monitoring the reductionkinetics of the interstitial iron concentration in the silicon waferbulk as iron becomes gettered to the surface SiNx layers duringcumulative annealing at 400 °C. The results show that the verydifferent SiNx films generate similar gettering kinetics, indicatingthat the impurity gettering effect is likely present in most PECVDSiNx films for silicon solar cells. The gettering kinetics and theSiNx film properties of refractive index, Si–N, Si–H, N–H bonddensities, and H content, are found to have no clear correlations.

Index Terms—Gettering, iron, plasma-enhanced chemical vapordeposition (PECVD), silicon nitride.

I. INTRODUCTION

AMORPHOUS silicon nitride (SiNx ) films synthesized byplasma-enhanced chemical vapor deposition (PECVD)

are an integral part of industrial silicon solar cells. They arewidely used as an excellent antireflection and surface passiva-tion coating [1]–[4]. In addition, PECVD SiNx films are wellknown to improve the quality of the silicon wafer bulk duringcontact firing and moderate-temperature anneals by reducingthe recombination activity of certain defects in the silicon bulk,a phenomenon that is commonly referred to as bulk hydrogena-tion [5]–[8].

It was recently reported that the PECVD SiNx films are alsocapable of gettering metallic impurities from the silicon wafer

Manuscript received July 19, 2018; revised September 26, 2018; acceptedOctober 9, 2018. This work was supported by the Australian Renewable EnergyAgency through projects RND017 and 2017/RND001. The work of A. Liu and Y.Wan was supported by the ARENA ACAP postdoctoral fellowship scheme. Thework of Z. Hameiri was supported by the Australian Research Council throughthe Discovery Early Career Researcher Award under Project DE150100268.(Corresponding author: AnYao Liu.)

A. Liu, Y. Wan, C. Sun, and D. Macdonald are with the Research Schoolof Engineering, College of Engineering and Computer Science, The AustralianNational University, Canberra, ACT 2601, Australia (e-mail:, anyao.liu@anu.edu.au; yimao.wan@anu.edu.au; chang.sun@anu.edu.au; daniel.macdonald@anu.edu.au).

Z. Hameiri is with the School of Photovoltaic and Renewable Energy Engi-neering, University of New South Wales, Sydney, NSW 2052, Australia (e-mail:,z.hameiri@unsw.edu.au).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JPHOTOV.2018.2875871

bulk during annealing at elevated temperatures, where the metalsare sufficiently mobile to reach the surface SiNx layers [9]. Thisdemonstrates an additional property of the SiNx films that couldpotentially benefit the silicon wafer bulk for improved deviceefficiencies. However, at this stage, little is known about theunderlying gettering reactions, or whether the reported getteringphenomenon is only associated with certain types of SiNx filmsor certain deposition conditions.

To examine the robustness of the SiNx gettering effects andto identify the dominant contributor to the gettering reaction,this paper investigates the gettering effects of a wide rangeof PECVD SiNx films of different properties. The SiNx filmswere deposited by two different PECVD systems: One industrialinline tool, and one laboratory reactor. We focus on correlatingthe gettering kinetics with the relevant SiNx film properties,namely the refractive index (optical property), and chemicalbond densities and hydrogen content (chemical properties thatreflect the composition of the films). Iron (Fe) is again used as amarker impurity in silicon to monitor the kinetics of the getteringreaction. Fe is intentionally introduced into high-quality cleansilicon wafers via ion implantation and annealing. The loss of Fefrom the silicon wafer bulk to the surface SiNx films during thegettering process is monitored by carrier lifetime measurements[10], which enable determination of the dissolved interstitial Feconcentration ([Fei]) in the silicon wafer bulk via an Fe–B pairdissociation technique [11], [12].

II. EXPERIMENTAL DETAILS

Float-zone (FZ) p-type boron-doped silicon wafers were usedin this paper. The wafers had a resistivity of 0.9 Ω-cm, andwere 180 ± 10 μm thick after surface chemical etching. Somewafers were subjected to ion implantation with 56Fe to a doseof 1.8 × 1011 cm−2, using a relatively low implantation energyof 70 keV. The Fe implanted wafers were then cleaned and an-nealed at 1000 °C for 2 h (1.5 h in dry oxygen and 0.5 h innitrogen), which served to uniformly distribute the Fe through-out the wafer thickness, to annihilate any possible implantationdamages [13], and to grow thin silicon dioxide (SiO2) layers onthe wafer surfaces as a contamination barrier and surface passi-vation coating. The resulting volumetric Fe concentration in thesilicon wafer bulk was (1.0 ± 0.1) × 1013 cm−3, as confirmedby carrier lifetime-based [Fei] measurements after the distribu-tion anneal. The thermally grown SiO2 layers were then chem-ically removed in dilute HF solution (1 wt%) within minutes.

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TABLE IPECVD SINx DEPOSITION CONDITIONS AND FILM PROPERTIES

The Fe-implanted samples, together with the no-Fe controls,were cleaned and coated with PECVD SiNx films on both sides.We examined a total of nine deposition conditions: five from anindustrial inline MAiA XS PECVD system (Meyer Burger), lo-cated at the University of New South Wales [14]; and four froma laboratory-scale static Roth&Rau AK-400 microwave/radio-frequency PECVD reactor, at the Australian National University[15], [16]. The process conditions, along with the film proper-ties, are listed in Table I. The temperatures (T) in Table I arethe actual temperatures of the wafers during PECVD deposi-tions, rather than the reactor temperature set-points, which aregenerally higher than the sample temperatures. All films have athickness in the range of 80–120 nm. The refractive index (n)of the SiNx films at a wavelength of 632 nm was determined byspectrophotometry reflectance measurements and modeling, asdetailed in [14] and [15].

The Si–N, Si–H, and N–H bond densities in the as-depositedSiNx films were measured by Fourier transform infrared(FTIR) transmission spectra, using FTIR spectrometers witha wavenumber resolution of 6 cm−1 (Thermo Nicolet 5700 forthe MAiA deposited films and Bruker Vertex 80 V for the AK-400 films). Three distinctive absorption peaks at around 880,2220, and 3340 cm−1 were identified, which are associated withSi–N, Si–H, and N–H vibrational modes, respectively [17], [18].A small shift in peak position with changing film propertieswas observed. The bond density [A-B] was determined by thefollowing [19]:

[A − B] = kA−B

∫α(ω)

ωdω (1)

where α(ω) is the absorption coefficient at wavenumber ω andkA−B is the proportionality constant as taken from [17]. Base-line correction was carried out using an identical bare siliconwafer before each set of measurements. The fraction of hydro-gen (H) in the films were estimated from the Si–N, Si–H, andN–H bond densities.

The PECVD deposition process is known to cause some get-tering effects [9], [20], as the wafers were held at elevated tem-peratures where a small fraction of Fe could reach the surfaceSiNx layers. However, due to the moderate diffusivities of Fein silicon [21] and the short deposition times (on the orderof minutes), its impact on the remaining bulk [Fei] is small

Fig. 1. Interstitial Fe (Fei ) concentration in the silicon wafer bulk with re-spect to the cumulative annealing time. The Fe-contaminated FZ silicon waferswere annealed at 400 °C with different SiNx films coated on both sides. Thecorresponding SiNx deposition conditions and film properties can be found inTable I.

compared to the subsequent cumulative anneals. Moreover, therate of the [Fei] reduction is compared among the different SiNxfilms, rather than the absolute Fei concentrations.

Both the Fe-contaminated and the no-Fe control samples,with the nine different SiNx films, were cumulatively annealedat 400 °C on a hotplate in the air. After each annealing step,photoconductance-based lifetime measurements [10] were usedto determine the effective carrier lifetime of the samples andthe associated Fei concentrations. The Fei concentrations in thesilicon wafer bulk were estimated by the technique of compar-ing the effective lifetimes before and after dissociating Fe–Bpairs via strong illumination [11], [12]. Prior to measuring thelifetimes in the Fe–B paired state, the samples were left in thedark at room temperature for at least 4 h, which is sufficientto allow an almost complete pairing of the Fe–B pairs in these0.9 Ω-cm boron-doped silicon wafers [22]. A non-uniform Feidistribution across the wafer thickness is shown to have littleimpact on the average Fei concentration that is determined fromthis lifetime-based Fe–B dissociation technique [23]. The re-ported error bars in the Fei concentrations arose from assuming

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LIU et al.: GETTERING EFFECTS OF SILICON NITRIDE FILMS FROM VARIOUS PLASMA-ENHANCED CHEMICAL VAPOR 3

Fig. 2. [Fei ] reduction time constants, extracted from fitting an exponential decay curve to the [Fei ] kinetics shown in Fig. 1, are plotted against the various SiNxfilm properties—refractive index, Si–N, Si–H, N–H bond densities, and hydrogen content.

Fig. 3. [Fei ] reduction time constants are plotted against PECVD depositiontemperatures.

a 5% uncertainty in the measured lifetimes [24] before and afterFe–B dissociation.

The same p-type 0.9 Ω-cm FZ silicon wafers as those used forFe implantation were co-processed in order to monitor the SiNxsurface passivation quality and process-induced contaminations.All deposition conditions were found to give reasonable surfacepassivation, with the controls having an as-deposited effectivelifetime of 0.2–1 ms at an excess carrier density of 1015 cm−3.The passivation quality of some of the films, mainly the onesdeposited by the laboratory AK-400, which had high initial as-deposited lifetimes near 1 ms, was found to degrade graduallywith the cumulative anneals at 400 °C, but was still sufficient toallow accurate bulk [Fei]measurements. Changes in the surfacepassivation quality are accounted for in the [Fei] calculations.

III. RESULTS AND DISCUSSION

Fig. 1 presents the gettering kinetics of the different SiNxfilms by plotting the remaining bulk Fei concentrations with re-spect to the cumulative annealing times. All films are shownto result in effective gettering of bulk Fei . The Fei reduc-tion kinetics can be fitted by an exponential decay curve,as shown in Fig. 1, with different reduction time constants.Figs. 2 and 3 plot the extracted reduction time constants asa function of various SiNx film properties and depositiontemperatures.

The reduction time constants for the nine different SiNx filmsare found to only differ by a factor of two, indicating that thevarious SiNx deposition conditions result in similar getteringeffects. The differences in the film properties may not have

been large enough to significantly vary the gettering kinetics.However, the range of the film properties examined in this paperdoes cover the range that is normally used for silicon solar cellapplications. This suggests that industrial SiNx films, typicallywith a refractive index in the range 2.05–2.15 [25], [26], mostlikely possess similar impurity gettering effects.

The Fei reduction kinetics shown in Fig. 1 can also be fittedby a diffusion-limited surface-gettering model [27], as demon-strated in [9], which assumes an infinite gettering rate at thewafer surfaces and gives a fitted “apparent Fe diffusivity.” In[9], we showed that the fitted apparent diffusivities in the tem-perature range of 250–700 °C agree in general with the reportedFe diffusivities, with the differences being only a factor of 1–3.The SiNx films used in [9] were deposited by the same AK-400 PECVD tool, and had a refractive index of 1.93. A similarfinding was observed for the gettering of Fei by atomic layerdeposited aluminum oxide (Al2O3) films at 425 °C, where thefitted apparent diffusivities are only smaller than the reportedFe diffusivity by a factor of 2–3 [28]. The fitted apparent Fediffusivities for the data shown in Fig. 1 are in the range of(3.3 − 7.4) × 10−9 cm2/s, while the reported Fe diffusivity at400 °C is 9.6 × 10−9 cm2/s [21]. This is consistent with theprevious findings [9], [28]. These results suggest that the getter-ing of Fei by these dielectric films, including the various SiNxfilms examined in this study, is largely diffusion limited at suchtemperatures, if the discrepancy with the reported Fe diffusiv-ities is related to uncertainties in the diffusivity values and themeasured kinetics. However, the results could also suggest thatthe gettering reaction at the dielectric films is not of an unlimitedrate, and/or the presence of the dielectric films during anneal-ing may have somehow affected the diffusivity of Fe in silicon.Further work is required to examine these conjectures. In anycase, the differences in gettering kinetics between the variousfilms are small, and would not significantly affect their getteringeffectiveness in a device.

It is interesting to note from Fig. 2 that the Fei reductiontime constants, which reflect the SiNx gettering kinetics, haveno clear correlation with the film properties of refractive index,bond densities, or H content. The H content is considered hereas it has been speculated in the literature that the injection ofH into silicon may somehow affect the Fe diffusivity in silicon[29]. The correlation with the H fraction in Fig. 2 is not strongenough to support this speculation, although neither could it beruled out.

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4 IEEE JOURNAL OF PHOTOVOLTAICS

All four of the AK-400 deposited SiNx films demonstrate ef-fectively the same gettering kinetics, regardless of the differentdeposition conditions and film properties. The MAiA depositedfilms exhibit slightly slower and more varied gettering kinet-ics. This variation in the gettering kinetics might relate to thenon-uniform films generated by the inline MAiA tool [30]. Thedifferent gettering kinetics could also be caused by the differentdeposition temperatures of the films, as suggested by Fig. 3.Although the deposition temperature is only one of the param-eters that determine the final film properties, the film densitygenerally increases with the increasing temperature (e.g., [31]).The results in Fig. 3 could therefore signal a possible correlationof the gettering kinetics with the mass density of the films. Forinstance, the film density may have an impact on the availablegettering sites (e.g., defects and voids in the films), or affectthe impurity mobility in the films, similar to H in SiNx [32].The bond densities of the films (Si–N, Si–H, and N–H), on theother hand, are not strongly correlated with the gettering ki-netics, as shown in Fig. 2. Future investigations, possibly on amicroscopic scale, may offer some insights into the underlyinggettering reactions.

IV. CONCLUSION

In summary, a wide range of silicon nitride films have beeninvestigated in this study for their impurity gettering effects. Thefilms were deposited by both the industrial- and laboratory-scalePECVD systems under various deposition conditions. Interest-ingly all of them demonstrate a similar gettering effect at 400 °C.As the tested SiNx films span the range that is normally usedin industrial silicon solar cells, as well as in most laboratorycell fabrication, the impurity gettering effect is likely a commoncharacteristic of PECVD SiNx films. The SiNx gettering kinet-ics is found to have no obvious correlation with the refractiveindex, chemical bond densities, and hydrogen content of thefilms. The small variations in the gettering kinetics may relateto the PECVD deposition temperatures.

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