real-time in situ ellipsometric monitoring of aluminum ...real-time in situ ellipsometric monitoring...

Real-time in situ ellipsometric monitoring of aluminum nitride film growth via hollow-

cathode plasma-assisted atomic layer deposition

Adnan Mohammad, Deepa Shukla, Saidjafarzoda Ilhom, Brian Willis, Blaine Johs, Ali Kemal Okyay, and NecmiBiyikli

Citation: Journal of Vacuum Science & Technology A 37, 020927 (2019); doi: 10.1116/1.5085341

View online: https://doi.org/10.1116/1.5085341

View Table of Contents: https://avs.scitation.org/toc/jva/37/2

Published by the American Vacuum Society

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Real-time in situ ellipsometric monitoring of aluminum nitride film growthvia hollow-cathode plasma-assisted atomic layer deposition

Adnan Mohammad,1 Deepa Shukla,1,2 Saidjafarzoda Ilhom,1 Brian Willis,3 Blaine Johs,4

Ali Kemal Okyay,5,6 and Necmi Biyikli1,a)1Department of Electrical and Computer Engineering, University of Connecticut, 371 Fairfield Way, Storrs,

Connecticut 062692Department of Materials Science and Engineering, University of Connecticut, 97 North Eagleville Road,

Storrs, Connecticut 062693Department of Chemical and Biomolecular Engineering, University of Connecticut, 191 Auditorium Road,Storrs, Connecticut 062694Film Sense LLC, 500 W South St, Suite 7, Lincoln, Nebraska 68522

5Department of Electrical Engineering, Stanford University, Stanford, California 943056Okyay Technologies Inc., Ankara 06374, Turkey

(Received 11 December 2018; accepted 11 February 2019; published 26 February 2019)

The authors report on the real-time monitoring of self-limiting aluminum nitride growth process byusing multiwavelength in situ ellipsometry. Aluminum nitride (AlN) thin films were grown onSi(100) substrates via hollow-cathode plasma-assisted atomic layer deposition (HCPA-ALD) usingtrimethylaluminum (TMA) and Ar/N2/H2 plasma as metal precursor and coreactant, respectively.Growth saturation experiments within 100–250 °C temperature range were carried out withoutinterruption as extended single runs featuring 10-cycle subruns for each parameter change. Thesensitivity of the multiwavelength ellipsometry provided sufficient resolution to observe not onlythe minuscule changes in the growth-per-cycle (GPC) parameter, but also the single chemicaladsorption (chemisorption) and plasma-assisted ligand removal events. GPC values showed a slightincreasing slope within 100–200 °C, followed by a stronger surge at 250 °C, signaling the onset ofthermal decomposition. The real-time dynamic in situ monitoring revealed mainly the followinginsights into the HCPA-ALD process of AlN: (i) film growth rate and TMA chemisorption amountexhibited plasma power dependent saturation behavior, which was also correlated with the substratetemperature; (ii) time-dependent refractive index evolution indicated a nonconstant relationship: afaster increase within the first ∼100 cycles followed by a slower increase as the AlN film getsthicker; and (iii) a considerable improvement in crystallinity was observed when the substratetemperature exceeded 200 °C. Besides in situ optical characterization, ex situ optical, structural, andchemical characterization studies were also carried out on 500-cycle grown AlN films as a functionof substrate temperature. All AlN samples displayed a single-phase wurtzite polycrystallinecharacter with no detectable carbon and relatively low (

ellipsometry (SE) has already been demonstrated for a numberof materials grown by PA-ALD, including oxides (Al2O3,HfO2, TiO2, ZnO, Er2O3, Ta2O5, PtO2),

23,31,32 metals (Pt, Pd,Ru),25 and nitrides (TiN, TaN, AlN).33–37 Here, instead of SE,we verified that a cost-effective multiwavelength ellipsometer(MWE) can be used effectively for real-time in situ analysisof the PA-ALD process. Despite its limitations with respect toSE, we demonstrated for the first time that real-time dynamicin situ MWE measurements not only convey film depositionrate, but also resolve single chemisorption and ligand removalevents with clarity. Moreover, forcing the limits for fitting theacquired in situ MWE data, we were able to track the evolu-tion of the AlN optical constants along the ALD cycles,which revealed thickness dependency.

Our main motivation behind this study was twofold: (i)real-time in situ MWE analysis of the III-nitride PA-ALDprocess with a focus on resolving individual surface reactions(chemisorption, ligand removal, and nitrogen incorporation)as a function of ALD growth parameters and (ii) perfor-mance evaluation of our custom designed hollow-cathodeplasma-assisted atomic layer deposition (HCPA-ALD)reactor featuring an improved hollow-cathode plasma (HCP)source by comparing our current results with previousPA-ALD grown AlN reports.

II. EXPERIMENT

A. Film growth

AlN thin film deposition is performed in anOKYAYTECHALD P100 reactor (Okyay Technologies Inc.,Turkey) equipped with a capacitively coupled HCP source(Meaglow Ltd., Canada). Figure 1 shows the schematic rep-resentation of the remote plasma-ALD reactor equipped withoptical ports integrated to FS-1 MWE (Film Sense LLC,NE). Prior to growth experiments, Si(100) substrates were

cleaned sequentially using isopropanol, acetone, and deion-ized water in an ultrasonic bath. Followed by nitrogendrying, samples were loaded immediately into the reactor.After reaching the reactor base pressure of ∼20 mTorr, N2carrier and Ar/N2 plasma gas flows were adjusted to 10 and50/50 sccm, respectively, resulting in a process pressure of∼700 mTorr. To minimize the negative impact of atmo-spheric exposure during the sample loading process and toremove the adsorbed water vapor from reactor walls, eachgrowth run started with a 10 min-long N2-only (50 sccm)plasma cleaning process at 100W. Trimethylaluminum(TMA) (Strem Inc., electronic grade, purity ⩾99.999%) wasused as the Al precursor with exposure times less than60 ms. As a nitrogen half-cycle, Ar/N2/H2 plasma exposurewas applied with 50/50/50 sccm flow rate at rf-plasma power25–200W, and plasma durations up to 40 s. 5–30 s Ar/N2purge intervals were utilized in between the TMA pulse andplasma exposures to evacuate the excess unreacted precursormolecules and reaction by-products.

Figure 2 shows the schematic layout of a unit PA-ALDcycle used for AlN deposition. For saturation experiments,10-cycle subsequent growth runs monitored via in situellipsometry were carried out for each controlled parameterchange including TMA exposure time, plasma duration,plasma power, and purge time for each substrate temperature.This way, the real-time in situ MWE monitoring capabilityenabled us to obtain the full saturation curves for each sub-strate temperature within a single run on a single substrate.Without in situ monitoring capability, such a saturation studywould take ∼20 separate >200-cycle ALD runs at each sub-strate temperature. The reduction in experiment time and pre-cursor consumption is significant, while the negative effectsof atmospheric exposure and contamination are also elimi-nated. After the completion of saturation curves, 500-cycleruns were conducted at 100, 150, 200, and 250 °C, with thesame TMA pulse time, plasma power, plasma duration, andpurge time to obtain linearity curves and thicker films for thecharacterization of ex situ materials.

B. Film characterization

1. In situ characterization

Real-time in situ monitoring of the film growth process isachieved by using the FS-1 MWE unit, which utilizes fourvisible light emitting diodes (LEDs) as light sources centeredat 464.44, 523.56, 599.12, and 637.29 nm, with spectralwidths of 28.78, 36.47, 15.17, and 20.54 nm, respectively.The relatively wide spectral bandwidths of LED sources aretaken into account by the FS-1 software in the model calcu-lation process. The ellipsometry angle of incidence was keptconstant at ∼70°. The internal optics of the source unit formsa uniform beam, which is fed into the detector unit afterreflecting from the sample surface. Since there are nomoving parts, the ellipsometric data are quickly acquired(10 ms minimum acquisition time) with long-term measure-ment stability.38 During our real-time in situ measurements,we recorded the dynamic data every 1 s, which resulted inlower data noise along with sufficient time resolution to

FIG. 1. Schematic of an HCPA-ALD reactor featuring a hollow-cathodeplasma source and an integrated in situ ellipsometer.

020927-2 Mohammad et al.: Real-time in situ ellipsometric monitoring of AlN film growth via HCPA-ALD 020927-2

J. Vac. Sci. Technol. A, Vol. 37, No. 2, Mar/Apr 2019

observe the surface reaction events. We should note that the1-s measurement time is only for raw data acquisition anddoes not include any layer modeling/fitting/analysis process.For saturation experiments, growth-per-cycle (GPC) andTMA chemisorption values are calculated by averaging thethickness gain and observed individual TMA chemisorptionevents over ten ALD cycles for each single growth parameterchange. To ensure that the film refractive index and growthrate were stabilized, our thickness change measurementswere all done after ∼150 ALD cycles of initial AlN growthwhere stable GPC values were observed in consecutive10-cycle runs.

For thicker 500-cycle AlN films, the optical model usedto analyze the in situ FS-1 ellipsometric data consisted of a“pseudo” substrate with a transparent film. In some cases, asurface roughness layer was added to the model. The pseudooptical constants of the substrate (“n” and “k” values at eachmeasurement wavelength) were directly inverted from theellipsometric data acquired before the deposition. The indexof refraction for the transparent film was parameterized by atwo-term Cauchy dispersion formula, while the optical con-stants of the surface roughness layer were calculated usingthe Bruggeman Effective Medium approximation, assuminga 50/50% mixture of the underlying film with “void.”

2. Ex situ characterization

Ex situ ellipsometric measurements were recorded in the370–1000 nm wavelength interval for three different anglesof incidence (65°–70°–75°) by a variable-angle spectroscopicellipsometer (M-2000V, J.A. Woollam Co. Inc, NE). Opticalconstants, film thicknesses, and surface roughness values ofthe samples were extracted from the measured ellipsometerdata by applying a fitting procedure (COMPLETEEASE software)with the Cauchy dispersion function.

To determine the crystallinity, density, thickness, andsurface roughness of the grown AlN films, grazing-incidencex-ray diffraction (GIXRD), x-ray reflectivity (XRR), and

gonio scan measurements were performed with the RigakuSmartLab multipurpose x-ray diffractometer (RigakuCorporation, Japan) operated at 45 kV and 40 mA by usingCu Kα radiation. GIXRD spectra were collected at 0.02° stepsize and 1.0 s counting time in the 2ϴ range of 25°–75°.Omega-2ϴ scan was performed from 0 to 2.0° in 0.01° stepsize for XRR measurements. GLOBALFIT software was utilizedfor the reflectivity data fitting process where a three-layermodel of AlN/SiO2/Si was used. The same diffractometersystem is used for XRD measurements with the BraggBrentano geometry. Gonio scans were performed with a stepsize of 0.02°in between 34° and 37° using a counting timeof 1.0 s. The values of the average crystal grain size wereestimated using the Scherrer equation,

D ¼ 0:9λ=d(cosθ),

where D is the crystallite size, λ is the x-ray wavelength, d isthe full-width-at-half-maximum (FWHM) of the XRD peakof interest, and θ is the angle in radian.

For elemental composition, chemical bonding states, andimpurity incorporation analysis, x-ray photoelectron spectro-scopy (XPS) measurements were performed using a mono-chromated Al Kα x-ray source (Kratos AXIS165, UK). Therecorded spectra were uncorrected for charging. Pass energiesof 80 and 20 eV were used for survey and high-resolutionspectra, respectively. Quantification is based on transmissioncorrections and relative sensitivity factors provided by themanufacturer. Sputtering was done using Ar ions at 4 keV;the sputter depth for film bulk measurements was estimatedat 20–25 nm into the film based on the observed time toremove AlN films of known thickness. Atomic force micros-copy (AFM) scans of AlN sample surface were carried outon an MFP3D microscope (Asylum Research-OxfordInstruments, UK) operating in the tapping mode with the fol-lowing imaging parameters: 256 × 256 pixel density, animaging speed of 1 Hz line rate, and a setpoint amplitudethat is 70% of the free resonant lever amplitude.

FIG. 2. Process layout for a typical AlN PA-ALD cycle and the corresponding reactor pressure variation over time.


JVST A - Vacuum, Surfaces, and Films

III. RESULTS

Figure 3 summarizes the AlN saturation experimentswhere average GPC values are extracted from the opticalthickness changes recorded via in situ ellipsometric measure-ments. Saturation curves as a function of plasma exposuretime, plasma power, TMA pulse time/purge time, and sub-strate temperature are shown in Figs. 3(a)–3(d), respectively.The GPC values exhibit a relatively monotonous characterwith a very slight increasing trend for longer TMA pulse andpurge times. We can safely conclude that the minimum avail-able TMA pulse time of 15 ms and 5 s purge is sufficient toachieve surface saturation for the entire temperature range.To keep at a safe side, a 10 s purge time was preferredthroughout the study. The GPC values obtained for thesamples grown at 100, 150, and 200 °C are relatively closeto each other, while the 250 °C sample shows a considerablyhigher growth rate, as shown in Fig. 3(d). We might attribute

this behavior to the onset of thermal decomposition of thechemisorbed surface groups (methyl ligands) that get partlyremoved before interacting with hydrogen radicals during theplasma half-cycle. This would possibly lead to more avail-able reactive sites on the surface, which will increase thesubsequent nitrogen incorporation. The average saturatedGPC values for the 100–200 °C temperature range are within1.05–1.2Å, while 250 °C shows values higher than 1.4Å.We reported a similar observation in our previous AlNexperiments, where a relatively constant GPC was observedup to 200 °C, and the subsequent increasing behavior wasattributed to the thermal decomposition of TMA.39,40

The plasma duration influence on GPC [Fig. 3(a)] con-firms that the plasma exposure should be at least 20 s for theentire temperature range. 200 and 250 °C results indicate thatfull saturation occurs at 30 s of plasma exposure, so wedecided to fix the plasma duration at 30 s. The most

FIG. 3. AlN saturation curves as a function of substrate temperature: (a) plasma exposure time (fixed parameters: plasma power 100W, TMA dose 15 ms, purgetime 10 s); (b) plasma power (fixed parameters: plasma duration 30 s, TMA dose 15 ms, purge time 10 s); (c) TMA dose (fixed parameters: plasma power 100W, plasma duration 30 s, and purge time 10 s) and purge time (fixed parameters: plasma power 100W, plasma duration 30 s, purge time 10 s); (d) substratetemperature dependence of GPC (fixed parameters: plasma power 100W, TMA dose 15 ms, plasma duration 30 s, purge time 10 s).



interesting saturation result was obtained for the plasma powerscan between 25 and 200W, which is shown in Fig. 3(b). Weobserved apparent saturation of GPC beyond certain plasmapower levels at all temperatures, but with different trajectories,signaling the existence of a plasma activation energy. We cangroup the three samples grown at 100, 150, and 200 °C asthey represent very similar increasing trends with lateral shiftsin the plasma power axis. All three curves show relativelysharp transitions toward saturation values occurring at 125,100, and 75W for 100, 150, and 200 °C, respectively. 100and 150 °C samples display similar limited GPC values for75W and lower plasma power exposures, whereas at 200 °C,this regime shifts toward 50W. What happens in this low-power regime can be explained by insufficient ligand removaldue to lower energy hydrogen radicals, resulting in partialsurface coverage of methyl groups, reduced reaction sites, andtherefore limited nitrogen incorporation. The GPC valuesrecorded within the 100–200 °C temperature range show anear-ideal saturation behavior up to 200W, where all GPCvalues are confined between 1.1 and 1.2Å.

On the other hand, when the substrate temperature reaches250 °C, the GPC shows an altered plasma power depen-dence. Instead of a similar sharp transition toward saturationvalues, it exhibits a more gradual and steady increase up to125W, where it saturates and stays relatively constant until200W at an average GPC value of ∼1.45Å. A comparisonof the 200 and 250 °C data shows that the lateral shift inplasma power dependence transforms into a rather verticalshift, confirming the elevated GPC due to partial thermaldecomposition taking place on the film surface. The verticalGPC shift at 250 °C is observed for the entire plasma powerrange (25–200W), verifying the strong possibility of thesurface thermal decomposition process which is independentof the exposed plasma power.

In order to gain a better insight of the individual surfacereactions, using the in situ ellipsometric data, we extractedthe measured average thickness changes due to TMA chemi-sorption as a function of TMA pulse time, plasma exposuretime, plasma power, and purge time (Fig. 4). At first sight,these plots look quite similar to the GPC saturation curves(Fig. 3) in terms of variation behavior: relatively insensitiveagainst TMA dose and purge time, and a clear saturationtrend with respect to plasma exposure time and power.However, several critical differences exist as well. The fourtemperature data can be classified under two subgroups:lower temperature (100 and 150 °C) and higher temperature(200 and 250 °C) samples. The thickness increase with TMAchemisorption is almost identical within these subgroups,except only two specific data points (5 s plasma duration and75W plasma power), where 100 and 150 °C samples exhibita notable difference. Particularly, AlN films grown at 200and 250 °C show almost identical TMA chemisorptionvalues over the entire parameter scans. Considering the dif-ferences observed in GPC behavior for these films and thepostulated thermal decomposition onset on the surface forthe 250 °C sample, this similarity is noteworthy.

Comparing these two subgroups, we noted that a lowersubstrate temperature leads to a consistently higher amount

of thickness change: while the lower temperature samplesshow an average thickness gain of ∼2.5Å, this decreases toless than ∼2.3Å for the higher temperature group [Fig. 4(d)].We believe that this difference stems from the more pro-nounced physisorption component at lower temperatures.These physisorbed surface groups possibly reduce the avail-able reaction sites for the subsequent plasma half-cycle, thusresulting in lower nitrogen incorporation and an overalldecreased GPC value. Despite the higher thickness gain, theactual chemisorbed TMA amount is less than the higher tem-perature counterparts. On the contrary, at higher substrate tem-peratures, the thickness gain is dominantly due to chemisorbedsurface groups, while minor physisorbed groups are quicklyremoved from the surface due to higher surface energy.

The plasma power dependence of TMA thickness gain[Fig. 4(b)] depicts that TMA half-cycle saturation is achievedat 75 and 100W for higher and lower temperature grownsamples, respectively. This result conforms well to the GPCsaturation behavior and indicates a minimum plasma activa-tion energy needed for effective ligand removal and nitrogenincorporation surface reactions. On the other hand, the rathersignificant difference in the observed GPC values between200 and 250 °C samples was attributed to the onset ofthermal decomposition of the chemisorbed surface groups.Taking into account the almost identical TMA chemisorptionvalues, one can explain the higher film growth rate by thethermally broken Al–CH3 bonds during the purge period thatcreate extra surface reaction sites for effective nitrogenbonding during the plasma half-cycle. A supportive observa-tion for this is the slightly increasing GPC value at longerpurge times for AlN deposition at 250 °C [Fig. 4(c), inset].As the purge time gets longer, more thermal decompositionand desorption events are possibly leading to slightly moreefficient nitrogen incorporation.

Figures 5(a) and 5(b) display the in situ measured singleALD cycles as a function of temperature at a fixed plasmapower (150W) and plasma power at a fixed substrate temper-ature (250 °C), respectively. Each individual cycle data repre-sents an average over at least five cycles during a 10-cyclesubrun. The fast and strong TMA chemisorption is evident atall temperature and plasma power values, while plasmaexposure shows an initial faster decay followed by a slowthickness decrease [Fig. 5(a)]. The upward kink at the end of30 s plasma exposure (between 45 and 50 s) is due to aplasma artifact that is also responsible for the faster-than-normal initial thickness reduction. The slower ligandremoval and N incorporation process can be visualized fromthese observations. Figure 5(b) confirms the ineffectiveplasma surface interaction for lower rf-power (

TMA pulse time, plasma exposure time, plasma power, andpurge time at 15 ms, 30 s, 100W, and 10 s, respectively.

The recorded in situ optical thickness data look reason-ably linear, and we noticed that after the early growth stage(first ∼100 cycles) the extracted GPC values (slopes of thethickness curves) increase with deposition temperatures.Averaged in situ single-cycle ellipsometer measurements as afunction of cycle numbers from various stages of the growthprocess are plotted in Fig. 7. These results demonstrateclearly that the optical thickness gain in an individual ALDcycle increases with cycle number and therefore with cumu-lative film thickness, particularly for the first 50 cycles offilm growth. While 150 and 200 °C data show similar tran-sient behavior with lower optical thickness change around 20cycles, cycle numbers higher than 60 depict almost similarthickness gain. Again, it is the 250 °C data that stand out interms of a continuously increasing growth rate. To better

understand the reason behind this observation, we carried outmore detailed fitting procedures on the measured dynamicin situ ellipsometer data for 150 and 250 °C samples.

The dynamic in situ data were analyzed with differentmodels to determine the best model for each deposition run,as evidenced by the lowest fit difference (FD) value (whichquantifies the “goodness” of the model fit to the data). Theplot in Fig. S1(a) of the supplementary material53 shows FDversus time for the 150 °C run, analyzed with refractiveindex fixed at the value determined at the run, whileFig. S1(b) in the supplementary material53 depicts how FDand refractive index fit evolve along the film growth. Fittingfor the refractive index significantly improves the FD (bymore than fourfold in the middle of the run), and thereforethe “thickness and index fit” model was selected as the bestmodel. For this run, adding surface roughness to the modeldid not significantly reduce the FD parameter. The film

FIG. 4. Average TMA chemisorption thickness variations extracted from dynamic in situ data as a function of substrate temperature: (a) plasma duration (fixedparameters: plasma power 100W, TMA dose 15 ms, purge time 10 s), (b) plasma power (fixed parameters: plasma duration 30 s, TMA dose 15 ms, purge time10 s), (c) TMA dose (fixed parameters: plasma power 100W, plasma duration 30 s, purge time 10 s) and purge time (fixed parameters: plasma power 100W,plasma duration 30 s, purge time 10 s), and (d) substrate temperature dependence of TMA chemisorption (fixed parameters: plasma power 100W, TMA dose15 ms, plasma duration 30 s, purge time 10 s).



thickness and refractive index versus deposition time for the150 °C run are shown in Fig. 8(a). Note that the indexbecomes noisier for thinner film thicknesses, mainly due toparameter correlation which is inherent when optically char-acterizing very thin films (

Table I summarizes the ellipsometric analysis resultsacquired from in situ and ex situ MWE and ex situ SE.Ex situ MWE results are obtained assuming a constant refrac-tive index throughout the entire thickness of the AlN film.The results show close agreement between ex situ and in situresults, confirming the data fitting via a variable refractiveindex for the in situ analysis. The difference is highest forthe 250 °C sample, where the increased surface roughnessmade the fitting analysis more difficult.

The crystalline structure of the HCPA-ALD grown AlNfilms was investigated by means of GIXRD. Figure 10shows the measured GIXRD patterns of the 500-cycle AlNfilms grown at 100, 150, 200, and 250 °C. (100), (002),(101), (102), (110), (103), and an ensemble of (200)/(112)/(201) reflections of the hexagonal wurtzite phase (h-AlN)were observed. For 100 and 150 °C samples, both spectradisplay very similar intensity values reminding similar crys-talline properties in this temperature range. (100) peak dis-plays the highest intensity value for both samples. However,when substrate temperature increases to 200 °C and beyond,the (002) peak becomes both dominant and stronger, signal-ing a change and improvement in crystalline quality in the200 and 250 °C samples. Particularly, at 250 °C, we might

comment that the sample exhibits a preferred orientation inthe (002) plane, which indicates that increasing substratetemperature induces crystalline AlN growth in (002) orienta-tion. To confirm the stronger crystal orientation, we alsocarried out θ–2θ gonio scans in the Bragg–Brentano configu-ration. The scan results are shown in Fig. S2 of the supple-mentary material53 and confirm the higher crystal orientationin the (002) plane for the high-temperature samples, while100 and 150 °C samples do not show any notable diffractionsignal.

Using the FWHM values from the GIXRD spectra,average crystal grain size values were calculated for the AlNsamples, which are shown in Table II. Here, we notedthat the grain size increases with temperature (from ∼12 to∼20 nm) and is very similar for lower (100 and 150 °C) andhigher temperature (200 and 250 °C) sample couples.Regarding the position of the (002) diffraction peak, weobserved a slight shift from 35.90° toward a smaller angle of35.76° when the substrate temperature reaches 200 °C.Comparing with the reference (002) peak position of 36.02°(Powder Diffraction File No. 25-1133, International Centerfor Diffraction Data, PDF-2), we might comment that ourpeaks shift further with increasing substrate temperature,

FIG. 7. Averaged in situ single-cycle ellipsometer measurements as a function of cycle numbers from various stages of growth at (a) 100 °C, (b) 150 °C, (c)200 °C, and (d) 250 °C.



reminding of a possible stress buildup within the AlN filmsgrown at 200 and 250 °C.

The x-ray reflectivity method was used for analyzing thefilm density, film thickness, and surface roughness of AlNthin film samples. Figure 11 shows the measured XRR curvesfor the AlN samples grown at 100, 150, 200, and 250 °C. Theanalysis results are summarized in Table II. Probably due tothe higher surface roughness of 250 °C sample, we were notable to achieve reliable film density and surface roughnessvalues. Density values extracted for 100 and 150 °C sampleswere almost identical, confirming similar material propertiessuch as crystallinity and average grain size. However, the 200°C sample exhibited a considerable increase in film density,reaching 3.20 g/cm3, which was quite close to the densityvalue of bulk AlN (3.26 g/cm3).41 This result was also con-firmed by the variation in the critical angle (θc), which wasestimated by analyzing the second derivative of the reflectivitycurve (Fig. S3 in the supplementary material).53 While 100and 150 °C samples exhibited θc∼ 0.23°, this value increasedto ∼0.25° for 200 and 250 °C, confirming the density increase

FIG. 8. Real-time measured dynamic in situ ellipsometric film thickness andfitted refractive index to obtain the lowest FD parameter. (a) 150 °C run and(b) 250 °C run.

FIG. 9. Ex situ spectroscopic ellipsometer measurements of the spectralrefractive index for the 500-cycle AlN films grown at 150 and 250 °C.(inset) Extracted spectral extinction coefficient data for the same samples.

TABLE I. Film thickness, GPC, and refractive index measurement results

obtained for the 500-cycle HCPA-ALD grown AlN samples at different

substrate temperatures via in situ/ex situ multiwavelength and ex situ

spectroscopic ellipsometry.

Tsub(°C)

Ex situ MWE In situ MWE Ex situ SE

tavg(nm)

GPC

(Å) navg

tavg(nm)

GPC

(Å) navg

tavg(nm)

GPC

(Å) navg

100 47.82 0.96 1.89 — — — — — —

150 50.40 1.01 1.93 48.75 0.98 1.93 47.76 0.96 1.95

200 55.68 1.11 1.97 — — — — — —

250 64.23 1.28 2.00 59.01 1.18 2.04 62.66 1.25 2.03

FIG. 10. GIXRD measurement spectra for 500-cycle AlN samples grown atdifferent substrate temperatures.



in parallel to crystallinity enhancement. The surface roughnesswas observed to be proportional to the substrate temperature,increasing from ∼1.5 nm at 100 °C to ∼2.8 nm at 200 °C.

To provide an additional insight into the surface rough-ness of the grown AlN samples, we carried out AFM mea-surements (Fig. S4 in the supplementary material).53 Both150 and 250 °C samples showed rather consistent subnanom-eter surface roughness values, which slightly deviates fromthe XRR results while being considerably different than theellipsometric modeling outcomes. We are currently planningfor additional experiments and analysis studies to solvethis discrepancy between the ellipsometric modeling andXRR/AFM results. We will put additional effort on the mod-eling aspect by employing different models other than theconventional Cauchy model that was being used throughoutthe study.

The chemical composition and bonding states of synthe-sized AlN films were characterized via XPS after Ar-sputtering∼20–25 nm deep into the film where surface carbon isremoved and oxygen concentration has already saturated to itsminimum value. The results are summarized in Table III for all

four AlN films grown at different substrate temperatures.Survey scans revealed the presence of aluminum, nitrogen, andoxygen, with Al 2p, N 1s, and O 1 peaks (Fig. S5 in the sup-plementary material).53 Carbon C 1s signal was detectableonly at the very surface, while it was below the XPS detectionlimit for all samples, confirming the effective ligand removalprocess across all temperature values. Oxygen impurity con-centration reduced from ∼6 to ∼3 at. %, with the substrate tem-perature increasing from 100 to 250 °C. Considering that asignificant fraction of O impurities possibly segregate at thecrystal grain boundaries, this result is in agreement with theincrease in grain size determined by GIXRD. Probable sourcesfor this oxygen impurity include residual oxygen and watervapor presence in organometallic precursors and plasma gasesand/or trapped oxygen/water vapor inside the multilayer coat-ings on the inner walls of the plasma-ALD vacuum reactor.On the other hand, the almost ideal film stoichiometry (Al/N≅ 1.0) throughout the entire temperature range is rather signifi-cant and indicates the highly efficient nitrogen incorporationvia the hollow-cathode Ar/N2/H2 plasma process.

Figure 12 shows the high-resolution XPS scans for Al 2p, N1s, O 1s, and C 1s, all recorded after 10min of Ar-sputtering.Please note that these plots represent the raw data obtaineddirectly from the measurements, without any corrections withrespect to Ar and without background subtraction. Al 2p and N1s peaks are almost symmetric, centered at 74.8 and 397.8 eV,respectively. Using handbook data for embedded Ar (fromsputtering) as a binding energy reference, we applied a rigidshift that moves Al 2p and N 1s peak centers to 73.2 and396.2 eV, respectively, which closely agree with nitridebonding states.42–44 The O 1s peak is relatively symmetric aswell, and the Ar-corrected peak energy of 531.3 eV can beattributed to Al–O.42 Finally, the C 1s data with no clear peakconfirm that carbon content within the AlN film is below theXPS detection limit.

IV. DISCUSSION

Above, real-time in situ ellipsometric monitoring resultsof AlN thin film growth via HCPA-ALD have been pre-sented in detail along with ex situ optical, structural, andchemical characterization. Here, we would like to discussand correlate the major experimental outcomes. Moreover,we would also like to compare our research findings withpreviously reported PA-ALD grown AlN results.

TABLE II. Structural material parameters extracted from GIXRD and XRR

measurements of the 500-cycle HCPA-ALD grown AlN samples.

Tsub(°C)

GIXRD XRR

(002) position

(deg)

FWHM

(deg)

Grain size

(nm)

Density

(g/cm3)

Thickness

(nm)

Roughness

(nm)

100 35.82 0.723 11.6 2.96 50.87 1.50

150 35.90 0.666 12.5 2.94 50.99 2.01

200 35.76 0.461 18.1 3.20 52.50 2.82

250 35.76 0.425 19.6 — 69.60 —

FIG. 11. XRR measurements of the four 500-cycle HCPA-ALD grown AlNthin film samples. Inset shows the measurement and software-fitted calcula-tion data for the 150 °C sample.

TABLE III. Chemical composition of the 500-cycle AlN samples in terms of

atomic concentration (measured from the bulk of the films after 10 min

Ar-sputtering).a

Tsub(°C)

Al

(at. %)

N

(at. %)

O

(at. %)

C

(at. %)

100 46 46 6

(1) The relatively fast metal precursor chemisorptionand slower plasma-assisted ligand exchange reactionswere in agreement with previous PA-ALD reportswhere in situ spectroscopic ellipsometer was utilized.However, the plasma power dependence of GPC andTMA chemisorption thickness gain showed a clear sat-uration behavior, which was also correlated with thesubstrate temperature. This relation was not reportedpreviously for AlN films grown by PA-ALD withinductively coupled plasma (ICP)37,39,40,45–49 or HCP(Ref. 50) sources. Either the plasma power was fixed ata certain value or it was already adjusted to itsmaximum available limit. In our previous AlN efforts,the GPC value for AlN did not saturate with appliedrf-power until its maximum value of 300W.39,40,50,51

The observation of this plasma power saturation regimeis attributed to the custom designed compactHCPA-ALD reactor equipped with a modified HCPsource, which provides a higher plasma density andtherefore enhanced radical flux at the substrate surface.Moreover, we observed that the power saturation to bearound 100W, which is considerably lower than our

previous AlN experiments (300W) with ICP and HCPsources. The real-time in situ probing capability alsorevealed the existence of a plasma activation powerthat shifts toward lower power values with increasingsubstrate temperature. Once this plasma activationpower level is surpassed, a relatively sharp increase indeposition rate is observed at all growth temperaturesexcept at 250 °C, which displays a more gradual GPCincrease. The thermal decomposition component at250 °C possibly leads to higher GPC values even atplasma power lower than the activation energy needed.

(2) Temperature-dependent GPC values indicated a ratherslow increase within the (100–200 °C) regime, while arelatively stronger surge at 250 °C. We postulated thatat this temperature we start to see thermal decomposi-tion of the chemisorbed surface groups during thepurge period after the TMA pulse. We do not thinkthat the TMA thermal decomposition occurs in the gasphase before interacting with the film surface. Such agas-phase thermal decomposition should have resultedin nonsaturating GPC increase with longer TMApulses, which is not observed during the saturation

FIG. 12. High-resolution XPS measurements of the 500-cycle AlN sample grown at 250 °C. (a) Al 2p, (b) N 1s, (c) O 1s, and (d) C 1s. Measurements werecarried out after 1 min of Ar-sputtering which corresponds to ∼20–25 nm film depth.



experiments. Moreover, the gas-phase thermal decom-position would possibly result in Al-rich stoichiometrydue to the ligand-free Al atoms arriving on the surface.The excellent near-ideal film stoichiometry preservedover the entire temperature range (100–250 °C) isanother strong indication that gas-phase reactions arenot taking place. Instead, we attribute the higher GPCat 250 °C to the onset of thermal decomposition ofsurface chemisorbed groups, partially releasing theirmethyl ligands before their interaction with hydrogenradicals. This conclusion is also supported by the obser-vation of near-identical TMA chemisorption behaviorand thickness gain with the 200 °C sample. If theamount of TMA chemisorption is equal, it has to be thenitrogen incorporation that becomes more efficient at250 °C, which can be explained via the additional reac-tion sites provided by thermally broken surface groups.

(3) Another major observation was the evolution of filmoptical constants as a function of ALD cycles and filmthickness, which is in agreement with the previousobservations by Motamedi and Cadien47 and Van Buiet al.37 Both studies utilized in situ SE measurementsand reported a rather fast increase in the refractiveindex until ∼30 nm of AlN film thickness. Our dataanalysis of the dynamic in situ MWE measurementsalso revealed a gradual enhancement of the refractiveindex. After the first ∼100 cycles, a fairly stable increasein refractive index with film growth is observed. Whilefilm densification and grain size increase might beresponsible for the gradual increase, the exact reason forthe initial considerable refractive index change is notclear yet. One possibility might be the negative influ-ence of the native oxide layer, on top of which the AlNfilms were grown. A further in-depth study on the initialgrowth stage of AlN films needs to be carried out. Theoutcomes of such a study are particularly important ifdevice-quality ultrathin III-nitride films and heterostruc-tures are targeted. We believe that real-time in situellipsometry will provide additional valuable insightinto such a follow-up study.

(4) We observed a correlation between substrate tempera-ture and film crystallinity and optical absorption. Asthe growth temperature exceeds 200 °C, the considerableenhancement of crystallinity is accompanied by the lackof optical absorption. As AlN is a wide bandgap mate-rial with an ideal optical band edge around 200 nm,higher quality AlN films should not exhibit opticalabsorption in the visible spectrum. The spectroscopicellipsometer measurement of the 150 °C sample revealedfilm absorption with a decaying nonzero extinction coef-ficient, while 250 °C sample ellipsometer data are fittedbest without any absorption component. This result con-firms the significantly higher (002) peak intensityobtained from the 250 °C sample which is a sign of apreferred orientation film growth. Correlating theseobservations with the higher surface roughness remindsthe onset of a possible higher degree of crystallizationactivity. Despite the similar grain size values estimated

from XRD peak FWHM data, a more detailed investiga-tion is needed to understand the possible reasons for thehigher surface roughness extracted from in situ andex situ ellipsometer measurements. A focused study inthis direction is also planned.

(5) When compared with our previous PA-ALD grownAlN results obtained with Fiji 200LL (Veeco/CNTInc., MA) reactor integrated with ICP (Refs. 39, 40,and 51) and HCP (Ref. 50) sources, we notice the fol-lowing major differences:(i) Less precursor consumption and shorter plasma

durations: We achieved AlN growth saturationalready at a 15 ms TMA pulse, which is the limitof the hi-speed ALD valve. However, saturationwas reported at longer than 60 ms TMA pulsetimes for both ICP and HCP sourced Fiji 200LLreactors. Similarly, 20 s of plasma exposureseemed sufficient for saturation, while it was atleast 40 s previously. We attribute the loweramount of precursor consumption to the smallervolume of the reactor as well as the shorter dis-tance from the precursor inlet line to the sub-strate surface. Shorter plasma exposure time isassociated with both the higher radical flux pro-duced by the modified HCP source and theshorter distance between the substrate holder andplasma source.

(ii) Higher deposition rate at lower plasma power:At a common substrate temperature of 200 °Cand identical plasma gas mixture of Ar/N2/H2,we achieved a saturated GPC of ∼1.1Å at 100W plasma power when compared with ∼0.6 and∼0.96Å obtained previously at 300W with ICPand HCP, respectively. The higher GPC valuesat lower plasma power confirm the effectivenessof plasma-assisted ligand removal and nitrogenincorporation reactions in our current reactorsettings.

(iii) Improved, near-ideal stoichiometry: We achievednear-ideal stoichiometry over the entire tempera-ture range studied (100–250 °C), while previouslywe had obtained aluminum-rich and nitrogen-deficient films in ICP (55 at. % Al, 41 at. %N)39,40,51 and HCP (51 at. % Al, 45 at. % N)50

configuration. Other PA-ALD grown AlN reportsindicated Al-rich47 or N-rich45,52 film stoichiome-try with the use of N2/H2 or NH3 plasma, respec-tively. We attribute this improvement to the moreefficient nitrogen incorporation performance andfully saturated plasma power impact establishedby both the reactor and optimized plasma sourcedesigns.

(iv) Increased film density: When compared withpreviously obtained AlN film density values(∼2.6 and ∼2.8 g/cm3) obtained at 200 °C usingAr/N2/H2 plasma, our present study reports asignificant improvement. Even the lower temper-ature (100 and 150 °C) AlN samples showed



higher than 2.9 g/cm3 density, while the samplegrown at 200 °C reached a remarkable value of∼3.2 g/cm3. Although not measurable with XRRdue to surface roughness, we think that the 250 °C sample has a similar density value, which isconsiderably close to the reported bulk AlNdensity of 3.26 g/cm3.

V. CONCLUSIONS

We have presented a systematic experimental study on thegrowth characterization of AlN thin films via real-time in situellipsometric process monitoring. To the best of our knowl-edge, this report represents the first real-time in situ ellipso-metric analysis of III-nitride films grown by PA-ALD byusing an MWE. Our process and materials characterizationresults suggest that the customized plasma-ALD reactor withan optimized HCP source provides a considerable improve-ment in AlN film properties. 10-cycle saturation and500-cycle growth runs probed by dynamic in situ ellipsometrymeasurements revealed critical insight into TMA chemisorp-tion/physisorption, Ar/N2/H2 plasma-assisted ligand removal,and nitrogen incorporation surface reactions. Saturation curvesconfirmed the self-limiting growth character within 100–250 °C.Plasma power dependence of the deposition rate indicatedclear saturation at relatively low plasma power levels (1.0Å GPC,which showed a relatively strong increase beyond 250 °C,signaling the onset of surface thermal decomposition.Dynamic in situ ellipsometry analyses of 500-cycle ALDruns resulted in thickness-dependent refractive index valuesfor AlN films, reaching 2.04 at 250 °C. All films showed asingle-phase hexagonal polycrystalline wurtzite structure,while a preferred orientation in (002) domain was observed at250 °C. XRR measurements showed AlN mass densityvalues above 2.9 g/cm3. The chemical composition of theAlN films was near ideal with ∼1:1 stoichiometry, with nodetectable carbon and less than 5 at. % oxygen impurity exist-ing within the bulk of the films for the entire temperaturestudy range. Our findings prove strong potential for character-ization of plasma-ALD processes using real-time in situMWE analysis as a powerful dynamic metrology technique.

ACKNOWLEDGMENTS

The authors thank Bryan Huey, Thomas Moran, and LuisOrtiz Flores for the AFM measurements. The authors wouldlike to thank Daniela Morales and the support staff of theInstitute of Materials Science (IMS) for their support in XRDand XRR measurements. This work was financially supportedby the University of Connecticut, School of EngineeringStartup Research Funding.

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Understanding the role of rf-power on AlN film properties in hollow-cathode plasma-

assisted atomic layer deposition

Saidjafarzoda Ilhom, Deepa Shukla, Adnan Mohammad, John Grasso, Brian Willis, and Necmi Biyikli

Citation: Journal of Vacuum Science & Technology A 38, 022405 (2020); doi: 10.1116/1.5128663

View online: https://doi.org/10.1116/1.5128663

View Table of Contents: https://avs.scitation.org/toc/jva/38/2

Published by the American Vacuum Society

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Understanding the role of rf-power on AlN film

properties in hollow-cathode plasma-assistedatomic layer deposition

Cite as: J. Vac. Sci. Technol. A 38, 022405 (2020); doi: 10.1116/1.5128663

View Online Export Citation CrossMarkSubmitted: 20 September 2019 · Accepted: 20 December 2019 ·

Published Online: 14 January 2020

Saidjafarzoda Ilhom,1 Deepa Shukla,1,2 Adnan Mohammad,1 John Grasso,3 Brian Willis,3 and Necmi Biyikli1,a)

AFFILIATIONS

1Department of Electrical and Computer Engineering, University of Connecticut, 371 Fairfield Way, Storrs, Connecticut 062692Department of Materials Science and Engineering, University of Connecticut, 97 North Eagleville Road, Storrs,

Connecticut 062693Department of Chemical and Biomolecular Engineering, University of Connecticut, 191 Auditorium Road, Storrs,

Connecticut 06269

Note: This paper is part of the 2020 Special Topic Collection on Atomic Layer Deposition (ALD).a)Electronic mail: [email protected]

ABSTRACT

In this study, the authors have carried out real-time process monitoring via in situ ellipsometry to understand the impact of rf-plasmapower and plasma exposure time on self-limiting aluminum nitride (AlN) growth character and the corresponding film properties. AlNthin films were grown on Si(100) substrates with plasma-enhanced atomic layer deposition using trimethyl-aluminum (TMA) as a metalprecursor and Ar/N2/H2 plasma as a coreactant. Saturation experiments have been employed in the range of 25–200W plasma powerand 30–120 s plasma exposure time. In situ multiwavelength ellipsometry identified single chemical adsorption (chemisorption) andplasma-assisted ligand removal events, as well as changes in growth per cycle (GPC) with respect to plasma power. The real-timedynamic in situ monitoring study revealed that GPC and TMA chemisorption thickness gain exhibited plasma power dependent satura-tion behavior. The amount of chemisorption saturated at ∼2.3 Å for higher rf-power levels, while for 25 and 50W it went below 1.0 Å,which is mainly attributed to incomplete ligand removal. Besides in situ characterization, ex situ measurements to identify optical, struc-tural, and chemical properties were also carried out on 500-cycle AlN films as a function of plasma power. AlN samples displayed asingle-phase hexagonal wurtzite crystal structure with (002) preferred orientation for 150 and 200W, while the dominant orientationshifted toward (100) at 100W. 50W and lower rf-power levels resulted in amorphous material with no apparent crystal signature.Furthermore, it was found that when the plasma exposure time was increased from 30 to 120 s for 25 and 50W, the amount ofchemisorption exceeded the thickness gain values recorded for 150–200W (∼2.4 Å). However, such a recovery in the chemisorptionthickness gain did not restore the crystallinity as the AlN films grown at sub-50W showed amorphous character independent of plasmaexposure time.

Published under license by AVS. https://doi.org/10.1116/1.5128663

I. INTRODUCTION

Aluminum nitride (AlN) is an attractive binary member ofthe III-nitride compound semiconductor family (AlN, GaN,and InN) due to its unique properties such as ultrawide anddirect bandgap (6.2 eV), high dielectric constant (κ � 8), pie-zoelectric property, high thermal conductivity, and good chemi-cal stability.1–4 Owing to these qualities, AlN is employed over a

wide range of applications, such as in high power and high fre-quency electronics, metal-insulator-semiconductor structures, deepultraviolet light emitting diodes and photodetectors, surface acousticwave devices, and gas sensors.5–10 AlN has conventionally beengrown using chemical and physical vapor deposition techniquesincluding metal-organic chemical vapor deposition (MOCVD),11

molecular beam epitaxy (MBE),12 sputtering,13 and pulsed laser

ARTICLE avs.scitation.org/journal/jva

J. Vac. Sci. Technol. A 38(2) Mar/Apr 2020; doi: 10.1116/1.5128663 38, 022405-1

Published under license by AVS.

https://doi.org/10.1116/1.5128663https://doi.org/10.1116/1.5128663https://www.scitation.org/action/showCitFormats?type=show&doi=10.1116/1.5128663http://crossmark.crossref.org/dialog/?doi=10.1116/1.5128663&domain=pdf&date_stamp=2020-01-14mailto:[email protected]://doi.org/10.1116/1.5128663https://avs.scitation.org/journal/jva

deposition (PLD).14 Although high-temperature epitaxial grownMOCVD/MBE films show the highest film quality, the elevateddeposition temperature (up to 1100 °C) limits its application inlower temperature-compatible device layers such as in CMOS tech-nology and flexible electronics.15 Lower-temperature sputtering andPLD, on the other hand, result in lower crystalline quality whilelacking from precision thickness control, large-area uniformity, andthree-dimensional (3D) conformality.16,17

Atomic layer deposition (ALD) is a unique low-temperaturechemical vapor deposition technique, which is based on sequentialself-limiting gas-solid surface reactions. The major advantages ofALD over other thin film growth methods are submonolayerprecision thickness control, large-area uniformity, and ultimate3D conformality.18 Unlike conventional thermal-ALD, energeticplasma-enhanced atomic layer deposition (PEALD) features addi-tional benefits, as the plasma-generated highly reactive radicals enablethe surface ligand exchange reactions to occur more efficiently atlower substrate temperatures. This in turn potentially enables filmprocessing and device fabrication at CMOS-compatible temperaturesas well as on temperature-sensitive polymeric/organic substrates usedfor flexible/wearable electronics.19–25

Although the ALD-based self-limiting growth process and filmproperties of AlN have been widely explored,20–28 studies on thereal-time in situ investigation of the plasma-assisted surface chemis-try for PEALD-grown AlN have been considerably lacking.29–31 Inthe previously reported in situ ellipsometry studies of ALD-grownAlN films, the focus has been primarily directed on monitoring theoverall film growth characteristics.29–31 More recently, we havestudied the substrate-temperature dependence of AlN film proper-ties while monitoring the film thickness variation during unit-ALDcycles via real-time in situ ellipsometry.32 In this work, we have sys-tematically investigated the role of Ar/N2/H2 plasma rf-power onthe growth and properties of AlN films. We present the real-timedynamic in situ ellipsometry analysis for the plasma rf-powerimpact on individual trimethyl-aluminum (TMA) chemisorptionand ligand exchange events. Additionally, structural, chemical, andoptical properties of the grown AlN films are studied and correlatedwith the rf-plasma parameters.

II. EXPERIMENT

AlN thin film growth experiments are carried out in anOkyayTechALD P100 reactor (Okyay Technologies Inc., Turkey)equipped with a capacitively coupled hollow-cathode plasma(HCP) source (Meaglow Ltd., Canada). Before thin film deposi-tion, Si(100) substrates were cleaned using acetone, isopropanol,and deionized water, followed by immediate nitrogen drying.After loading the substrates into the chamber, the reactor waspumped down to ∼20mTorr base pressure, and a 10min longN2-only plasma cleaning at 100W was performed. This predeposi-tion in situ plasma-cleaning process aims to reduce the amount ofresidual water vapor that adsorbs on the chamber walls duringsample loading. TMA (Strem Inc., electronic grade, purity≥99.999%) was utilized as the metal precursor at 15ms dosing timeand Ar/N2/H2 plasma as a coreactant at 50/50/50 sccm flow rates.Ar/N2 purges were employed for 10 s each after TMA pulse andplasma exposure to remove the unreacted excess precursor molecules

and surface reaction byproducts. At each cycle, H2 is introduced tothe reactor only during the plasma period, where an additional 5 sstabilization time is introduced before igniting the plasma.Saturation experiments were carried out in the range of 25–200Wrf-plasma power and 30–120 s plasma exposure time within100–250 °C substrate temperature. The saturation experiments arecarried out by monitoring the film growth process in real-time byin situ multiwavelength ellipsometry (MWE) (FS-1, Film Sense,LLC). It should be noted that the data acquisition time for theellipsometer is about 1.04 s. Initially, ∼150-cycle run is performeduntil stable growth per cycle (GPC) is observed, whereafter consec-utive 10-cycle runs are done for each plasma power, plasma expo-sure time, and substrate temperature to observe the changes inGPC and TMA chemisorption for each parameter set. After com-pleting the saturation curves, 500-cycle runs were carried out with50, 100, 150, and 200W plasma powers at a fixed substrate tem-perature of 150 °C, with 15 ms TMA pulse time, 30 s plasma dura-tion, and 10 s purge times to obtain the linearity curves andthicker films for ex situ materials characterization.

Ex situ ellipsometric measurements were recorded by MWEand a variable-angle spectroscopic ellipsometer (SE) (M-2000V, J.A.Woollam Co. Inc., NE). Grazing-incidence x-ray diffraction (GIXRD)and x-ray reflectivity (XRR) measurements were performed with aRigaku SmartLab multipurpose x-ray diffractometer (RigakuCorporation, Japan). The crystalline structure and interfacial layer ofthe AlN films were also studied using cross-sectional TEM imagingusing an FEI Talos F200X Scanning/Transmission ElectronMicroscope (Thermo Fisher Scientific, USA). For elemental compo-sition, chemical bonding states, and impurity incorporation analysis,x-ray photoelectron spectroscopy (XPS) measurements were per-formed using a monochromated Al Kα x-ray source (KratosAXIS165, UK). The full description of the deposition system andfurther details of the characterization methods can be found in ourprevious report.32

III. RESULTS AND DISCUSSION

Figure 1 shows individual unit-ALD cycles obtained from theaverage of 10-cycle consecutive subruns at fixed substrate tempera-tures (150 and 200 °C) with respect to varying rf-power levels (25–200W) for 30 s of plasma exposure time. The effect of substratetemperature and plasma power on TMA chemisorption and ligandexchange reactions can be observed from Figs. 1(a) and 1(b). At150 °C, the amount of chemisorption thickness gain exhibits a dis-tinct rf-power dependence, gradually increasing from 25 to 100Wand showing relatively constant behavior within 100–200W. Onthe other hand, at 200 °C we notice two distinct behaviors: (i) TheTMA chemisorption is less at lower plasma powers (

available surface reaction sites, resulting in reduced chemisorptionat both process temperatures. Furthermore, a similar linear trend for75W at 150 °C [Fig. 1(a)] but relatively sharp decay for the samerf-power at 200 °C [Fig. 1(b)] also conforms with the observedlower and higher adsorption rates at this plasma condition, respec-tively. Based on these observations, it is worth noting that, in addi-tion to plasma-assisted ligand exchange reactions, there is alsosurface thermal energy component to the process, which furtherassists in the effective removal of methyl (–CH3) groups, and thusresulting in more efficient nitrogen incorporation into the films.

We have also carried out subsequent 10-cycle runs at differentrf-powers (50, 100, and 150W) while monitoring the thickness var-iations via an in situ ellipsometer when switching from onerf-plasma power to another (the transition is in the direction of thearrow as shown in Fig. 2). This miniseries further enabled us to

understand the time it takes to observe the influence of newlyswitched rf-power on the TMA chemisorption and interaction ofcoreactant species with the surface groups previously exposed todifferent plasma conditions. This effect can be clearly seen by com-paring the transitions between 50 and 100W (Fig. 2). It should benoted that for all the transition combinations shown, the initialthicknesses start at 0Å and we have shown only the first threecycles for visual purpose. As such, considering the rf-power changefrom 50 to 100W (2), initially there is low chemisorption(∼0.75Å) which is immediately followed by a considerably fasterligand removal inferred from the sharp decay rate during the100W plasma exposure. Indeed, the negative thickness decreaseseen during the first cycle of this transition is another confirmationthat most of the ligands which have not been fully removed atlower plasma power (50W) are now being successfully removedand substituted with nitrogen radicals at higher rf-power values(100W). In contrast, changing the power from 100 to 50W (1)leads to initially enhanced adsorption, which is followed by a ratherreduced removal rate of surface unreacted –CH3 groups, indicatedby the linearity of the curve during the plasma period. Anotherobservation from these sets of transitions is that the recentlyswitched rf-power demonstrates its full effect right at the beginningof the initial second half-cycle and throughout the next cycles. It isreflected as either improved or deteriorated chemisorption andligand removal depending on whether rf-power is increased ordecreased, respectively, as seen in Fig. 2. However, the transitionsbetween 100 and 150W [(3) and (4)] did not exhibit such an effecton the growth process.

Based on the above observations, one reason behind theincomplete ligand removal might be insufficient energetic radicalscreated at lower plasma powers (≤50W), which are unable to breakthe Al–C bonds. Yet, another reason might be the shorter plasmaexposure time (30 s) not allowing the radical species to effectivelyinteract with the surface-adsorbed TMA groups. In order to clarify,we have further performed consecutive 10-cycle runs at 25 and 50Wwith longer plasma durations (30–120 s), for which the averagedthickness variations over 10 cycles at each exposure time are plottedin Fig. 3. The results show that at both lower rf-powers (25 and50W), the TMA chemisorption significantly increases with extendedplasma exposure times: from 0.9 to 2.4Å for 30 and 120 s, respec-tively. These results suggest that by keeping the plasma running for

FIG. 1. Real-time measured and aver-aged in situ ellipsometric film thicknessdata showing the TMA chemisorptionand plasma-assisted ligand removalreactions for different rf-power valuesat substrate temperatures of (a) 150 °Cand (b) 200 °C.

FIG. 2. Average thickness variations extracted from dynamic in situ data as afunction of ALD cycles showing initial chemisorption and ligand removal behav-ior due to transition between various rf-plasma powers. The substrate tempera-ture is kept constant at 200 °C.




https://avs.scitation.org/journal/jva

longer periods, we can generate sufficiently energetic radical speciesto be able to fully remove the –CH3 groups and thus increase thenumber of available reactive sites for the next TMA half-cycle. Thisalso reveals that a true plasma activation energy barrier for the effec-tive ligand removal at lower rf-plasma powers (25 and 50W) doesnot exist in our HCP-ALD reactor, which we had hypothesized oth-erwise in our earlier report on the temperature study of AlN growthvia the same ALD system.32 However, as will be discussed in theXRD analysis part of the article, such lower rf-power ALD runs leadto amorphous films which reminds of a possible activation energyneeded to trigger the crystallization process of AlN. The main take-away of this substudy is that seemingly complete ligand removal onits own does not imply crystalline film growth.

Figure 4 shows the real-time in situ recorded data of500-cycle grown films at four different rf-plasma power values and

at a common substrate temperature of 150 °C. AlN grown at 50Wshows a linear trend with a relatively low growth rate as seen fromthe cumulative film thickness. However, the film deposited at100W exhibits two modes of growth, rather slow at the beginning(∼50 cycles) and followed by a higher GPC afterward. On theother hand, AlN grown at 150W displays a higher growth rate in themiddle of the run with rather slower rates both at the beginning andthe end stages. Finally, the sharp thickness gain at the early stages(∼40–50 cycles) of the growth with 200W rf-power is possibly due tothe enhanced nucleation stage leading to the highest GPC to thepoint of film closure after which the GPC stabilizes and follows alinear growth mode. Similar behavior has also been reported forPEALD of AlN and TiN.31,33 Averaged in situ single-cycles fromvarious stages of the deposition process can be seen in Fig. S1.34

Table I summarizes the extracted film thickness, GPC, andrefractive index (at 633 nm) of the grown AlN films, which wereacquired separately by two different ellipsometry systems: ex situMWE and ex situ spectroscopic ellipsometry (SE). The resultsindicate close agreement between measurements performed byboth ellipsometer systems. Figure 5 displays the dispersion of therefractive index and extinction coefficient obtained via SE within370–1000 nm. Overall, the refractive index of the AlN filmsimproves as a function of increasing plasma power, indicating anenhancement in the film quality. However, AlN grown at 150Wexhibited a slightly reduced refractive index toward longer wave-lengths. Moreover, the near-zero extinction coefficient of the AlN

FIG. 3. Time-dependent film thicknesschange during the plasma exposurecycles at (a) 25 W and (b) 50 W.

FIG. 4. 500-cycle linearity curves for AlN films grown at 150 °C substrate tem-perature and under varying plasma power obtained via real-time in situ ellipsom-eter recording. (Inset) Zoomed-in portion of cycles for 150–200 W samplesshowing relatively sharp individual chemisorption and ligand removal processes.

TABLE I. Film thickness, GPC, and refractive index measurement resultsobtained for 500-cycle PEALD grown AlN samples at 150 °C substrate tempera-ture and different rf-powers via ex situ multiwavelength and ex situ spectroscopicellipsometry.

rf-power(W)

Ex situ MWE Ex situ SE

tavg(nm)

GPC(Å) navg

tavg(nm)

GPC(Å) navg

50 15.01 0.30 1.68 — — —100 50.40 1.01 1.93 47.76 0.96 1.95150 55.74 1.11 1.95 60.85 1.22 1.96200 52.70 1.05 1.94 53.16 1.06 1.96





films produced with 150 and 200W in the range of 450–1000 nmis also another indication of higher quality films, while the filmgrown at 100W demonstrated non-negligible optical absorptionover the measurement spectrum, signaling a degraded film com-pared to higher rf-plasma powers. On the other hand, 50Wsample showed poor optical properties with a reduced refractiveindex of 1.68 as measured by an ex situ ellipsometer, reminding ofan amorphous film structure as confirmed by the GIXRD spectra(Fig. 6).

The structural properties of AlN films grown with varyingrf-plasma powers were studied by grazing-incidence x-ray diffraction(GIXRD), as shown in Fig. 6. The GIXRD spectra clearly show thepolycrystalline structure of the films grown at 100–200W range withmainly (100), (002), and (101) reflections of the hexagonal wurtzitephase (h-AlN), whereas the 50W sample showed amorphous char-acteristic. While for the film grown with 100W the preferred crystalorientation is mainly (100), for 150 and 200W samples, the (002)diffraction signal becomes the most dominant one with the signifi-cantly higher peak intensity for 150W film. The reduced (002) peakintensity for 200W film indicates crystal quality degradation possiblydue to plasma surface damage, the more pronounced plasma redepo-sition of ligand groups, and possibly more effective plasma-assistedincorporation of the residual oxygen.

Using the full-width-at-half-maxima (FWHM) obtained fromthe GIXRD spectra, the average crystal grain size values were calcu-lated by the Scherrer equation

D ¼0:9λd cos θ

,

where D is the crystallite size, λ is the x-ray wavelength, d is theFWHM of the XRD peak of interest, and θ is the angle in radians.The estimated grain sizes for (002) crystal orientation are summarized

in Table II. The grain size is largest for the 150W sample calculatedat 14.8 nm, which is also reflected in the (002) plane having the nar-rowest FWHM values and the highest peak intensity compared to100 and 200W samples (Fig. 6). Considering the position of (002)peak, a slight shift is observed as a function of increasing rf-powerfrom 35.82° to 35.22° at 100 and 200W, respectively, which is possi-bly due to stress build-up within the AlN films at elevated rf-powerand thus higher surface energy and temperature. Furthermore, theinset in Fig. 6 shows the GIXRD spectrum for the 500-cycle filmdeposited at 25W for a longer plasma duration of 150 s. We havenoticed above during in situ saturation studies that this plasma condi-tion resulted in efficient ligand removal and thus improved TMAchemisorption similar to the case of higher rf-powers (Fig. 3).However, this enhancement in plasma interactions at such reducedpowers did not result in a crystalline AlN film, as evidenced by theamorphous signature in GIXRD spectrum (inset, Fig. 6). A plausibleexplanation could be that although at such low (25–50W) rf-powersthe plasma radical energy is enough to successfully remove (–CH3)ligands under prolonged plasma exposures leading to higher subse-quent TMA adsorption, possibly the energy of the plasma species isnot sufficient for effective nitrogen incorporation and crystallizationprocess. Instead, the surface groups formed after the longer ligandremoval process possibly interact with the residual water vapor mole-cules within the chamber, as well as other impurities existing in theplasma gas mixture, and thus resulting in an amorphous filmshowing elevated oxygen and carbon incorporation.

The density, thickness, and surface roughness of AlN filmsgrown under 50, 100, 150, and 200W plasma powers were investigatedby the XRR method. Figure 7 depicts the measured XRR curves

FIG. 5. Ex situ spectroscopic ellipsometer measurements of the spectral refrac-tive index for the 500-cycle AlN films grown at 150 °C. (Inset) Extracted spectralextinction coefficient for the same samples. FIG. 6. GIXRD measurement spectra for 500-cycle AlN sample grown with vari-

able rf-plasma powers for 30 s exposure time at 150 °C. (Inset) GIXRD mea-surement spectrum for the sample deposited with 25 W rf-plasma power for150 s exposure time at 200 °C.





with the inset graph showing the curve for 100W film fitted withthe model. The extracted results from the analysis are summarizedin Table II. The density of the films increases considerably whenrf-plasma power is switched from 50 to 100W, which confirms theonset of crystallization at 100W. However, the density did notchange significantly for the 100–200W power range. Besides, thefilm surface roughness increased from ∼0.68 nm at 50W to 2.29 nmat 100W possibly due to the onset of crystallization which led tocrystal grains resulting in higher surface roughness.

XPS was utilized to study the chemical composition of theAlN films. XPS survey scans for the samples grown at 50, 100, 150,and 200W are shown in Fig. S2.34 Table III summarizes the atomicconcentrations for Al, N, O, and C within the film bulk. Carbonand oxygen present at the surface of 100, 150, and 200W samples(Fig. S2)34 are originating from the postdeposition atmosphericexposure, which reduces below the detection limits for carbon and

levelled off to its minimum value of ∼4 at. % for oxygen afterAr-sputtering. Moreover, the near-ideal film stoichiometry over the100–200W plasma power range is an important outcome anddemonstrates the effective nitrogen incorporation of the hollow-cathode plasma-assisted deposition process carried out in acompact reactor design setting. Particularly, 150W sample exhibit-ing an Al to N ratio of 1:0 along with the strongest GIXRD (002)peak intensity corresponds to the optimal plasma power at thissubstrate temperature (Fig. 6). The 50W amorphous sample exhib-ited relatively high impurity levels which saturated at ∼4 and34 at. % for carbon and oxygen, respectively. This confirms theellipsometer and GIXRD results which collectively indicate thepoor nitrogen incorporation and insufficient surface energy forcrystallization at reduced rf-plasma power.

The crystalline structure and interfacial layer of the AlN filmswere studied using cross-sectional TEM imaging. Figures 8(a)and 8(b) show the HR-TEM image and the selected-area electrondiffraction (SAED) of the AlN sample grown with 100W at 250 °C,respectively. The polycrystalline nature of the films can be observedon the HR-TEM image seen as various crystallites oriented at dif-ferent orientations, which is also reflected on the scattered arclikediffraction pattern on the SAED micrograph corresponding to hex-agonal AlN (h-AlN) crystallographic planes [Fig. 8(b)]. Likewise,the samples grown at 150W, 150 °C exhibited a polycrystallinefilm structure as evidenced by HR-TEM imaging [Figs. S3(a)and S3(b)].34 High-resolution STEM images and energy dispersivex-ray spectroscopy (EDX) based elemental mapping through thecross section of the AlN films deposited at 250 °C and 100W canbe seen in Figs. 8(c) and 8(d). The EDX mapping reveals homoge-neous AlN films with discernible ∼4.3 nm native oxide at the inter-face between the film and the substrate [Fig. 8(d), zoomed-in].Interestingly, there seems to be an apparent void layer close to the

TABLE II. Structural material parameters extracted from GIXRD and XRR measurements of the 500-cycle AlN samples grown at 150 °C.

rf-power(W)

GIXRD XRR

(002) position(deg)

FWHM(deg)

Grain size(nm)

Density(g/cm3)

Thickness(nm)

Roughness(nm)

50 — — — 2.28 16.13 0.68100 35.82 0.613 13.7 2.97 51.05 2.29150 35.54 0.569 14.8 3.05 53.42 3.42200 35.22 0.670 12.5 2.94 46.73 2.70

FIG. 7. XRR measurements of the four 500-cycle PEALD-grown AlN thin filmsamples. (Inset) The measurement and software-fitted calculation data for100 W sample.

TABLE III. Chemical composition of the 500-cycle AlN samples in terms of atomicconcentration (measured from the bulk of the films after Ar-sputtering).a

rf-power(W)

Al(at. %)

N(at. %)

O(at. %)

C(at. %)

50 39 22 34 4100 46 47 5

surface of the Si substrate apparent on the STEM and EDX micro-graphs [Figs. 8(c) and 8(d)], which is possibly created during the man-ufacturing process of Si wafers. The gold and carbon layers on top ofAlN film were deposited during the sample preparation for HR-S/TEM analysis via sputtering and focused ion beam, respectively.

IV. CONCLUSIONS

In this study, the role of Ar/N2/H2 plasma power on AlN filmgrowth via hollow-cathode plasma-assisted ALD in a compactreactor setting featuring an optimized plasma source has been inves-tigated. PEALD growth of AlN reveals that rf-power is critical fornot only efficiently removing the ligands but also improving thecrystalline structure and optical properties of the film by providingthe necessary surface energy needed for the crystallization process.With the dynamic in situ ellipsometry incorporated into our system,we have shown that TMA chemisorption-based thickness gain isnot only dependent on the intensity of rf-power, but also on thelength of the plasma exposure cycle. Moreover, it was found that

when switching between different rf-powers, the effect of plasma onthe TMA adsorption and ligand removal is observable immediatelyfollowing the second ALD half-cycle. The 633 nm refractive indicesof the films ranged between 1.68 and 1.96 for 50–200W as obtainedby an ex situ multiwavelength ellipsometer and spectroscopic ellips-ometry. All samples grown within 100–200W rf-power rangeshowed the hexagonal polycrystalline wurtzite structure while 50Wresulted in an amorphous AlN film. Although extending the plasmaexposure time at lower plasma powers (25–50W) enhanced theTMA chemisorption and possibly improved the nitrogen incorpora-tion, it did not result in a crystalline AlN film. XRR measurementsshowed AlN mass density reaching above 3.0 g/cm3. The chemicalcomposition of the films grown at 100–200W was near ideal at∼1:1 Al to N ratio, with ∼5 at. % O, and no XPS-detectable carbonwithin the bulk of the films. 50W amorphous sample, on the otherhand, showed significant O and C contamination. Additionally,cross-sectional HR-S/TEM analysis revealed that the films formcrystallite domains oriented randomly confirming the polycrystal-line nature of the grown AlN films.

FIG. 8. (a) HR-TEM image, (b) SAEDmicrograph, (c) high-resolution STEMimage, and (d) EDX elementalmapping through the cross section ofAlN films deposited at 250 °C, 100 W.





ACKNOWLEDGMENTS

The authors thank Daniela Morales for helping with the XRDand XRR measurements. The authors would like to thank RogerRistau and Lichun Zhang for their support in FIB sample prepara-tion and HR-TEM measurements. This work was financially sup-ported by the University of Connecticut, School of EngineeringStartup Research Funding, and the Research Excellence Program(REP) funded by the Office of the Vice President for Research(OVPR). The authors also acknowledge the financial support ofNational Science Foundation grant (NSF Award No. 1511138).

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