Fabrication and properties of ITO films treatedby excited atomic oxygen
Valeriy A. Sterligov,1,* Evgeny V. Shun’ko,2 Iaroslav O. Grytsaienko,1,3 andLeonid V. Poperenko3
1Institute of Semiconductor Physics, NAS of Ukraine, 41 Nauki Prospect, 03028 Kiev, Ukraine2Wintek Electro-Optics Corporation, 1665 Highland Drive, Ann Arbor, Michigan 48108, USA
3National University of Kiev, 2 Acad. Glushkov Avenue, 03680 Kiev, Ukraine
Dielectric barrier discharge powered with radio fre-quency (DBD-RF) or DBD-pulse discharges in Ar orN2 admixed with certain percentage of O2 at atmo-spheric pressure [1,2] can generate a significantquantity of atomic oxygen, excited to the metastablestate 2S22P4�1S0�. This oxygen can transfer furtherthe energy of transition from 2S22P4�1S0� to2S22P4�1D2� state to materials, thus cleaning theirsurfaces and making them hydrophilic [1]. In viewof these results, utilization of excited atomic oxygenfor crystallization of indium-tin-oxide (ITO) films isof practical interest for various applications in dis-play technologies. This paper comprises four sec-tions: 1. Introduction, 2. Experimental Setup andTreatment Procedure, 3. Results of Measurements,and 4. Summary. In Section 2, we (i) describe anexperimental setup that provides controllable tem-perature of the treated substrate (with ITO film
deposited) and produces excited atomic oxygen and(ii) consider briefly a procedure of film treatment.In Section 3, the physical parameters of the ITO filmmeasured before and after treatment are compared.It is shown, in particular, that the film impedancedecreased significantly and transmittance of visiblelight increased after treatment under certain condi-tions. A comparison of x-ray diffraction (XRD) on thefilm before and after treatment wasmade. The initial(as-deposited) film structure was almost amorphous.After treatment, clearly pronounced peaks corre-sponding to several crystalline configurations ofIn2O3 were found. Specially developed modificationof elastic light scattering (ELS) diagnostics was ap-plied to investigate the film surface characteristicsbefore and after treatment. The same diagnostics en-abled us to find that the RMS surface roughness in-creased from 2.65 nm up to 4.07 nm due to treatment.
2. Treatment Procedure and Experimental Setup
To obtain ITO films, the round magnetron ONYX-2produced by Angstrom Sciences, Inc. (www.angstromsciences.com), and equipped with ITO
(93 wt:% of In2O3 and 7% of SnO2) target were in-stalled against a glass substrate in a vacuum cham-ber provided with a controllable Ar-gas leak. Thesubstrate temperature could be controlled and re-mained invariable in a range from room temperatureup to 300 °C (for considerable part of experimentsdescribed in our work, this temperature was set to200 °C). A pressure of Ar-gas in the vacuum chamberand a value of RF power applied to magnetron wereselected empirically to obtain the ITO film 300 nmthick with about 36 Ω∕□ surface resistivity, andthese parameters depended on the type and produc-tivity of operating vacuum pumps.
In Fig. 1, a sketch of the setup used in our experi-ments is shown, combined with a partial inset of itsphotograph. A glowing beam first was a source of ex-cited atomic oxygen [1], secondly was a controllableelectric heater of substrate (a glass plate coated withITO film), and lastly was a thermocouple used formeasuring the heater temperature. To ensure relia-bility of the spectral measurements of excited gasflow, the gas system, including a mass-flow control-ler, inlet tubing, quartz tube of the device, and con-trolled atmosphere box (connected to the yield end ofthe quartz tube), was checked with a leak detector ata residual pressure of 10−4 Pa to exclude possibleleakage of the ambient air containing nitrogen. Formost experiments with ITO film, the controlled at-mosphere box was dismantled. A flow of Ar gas (rateof 2.7 1∕min) admixed with a small percentage of O2(rate of 0.01 1∕min) went along a quartz tube atatmospheric pressure. It passed through two collar-type electrodes (not shown in Fig. 1) powered withRF (13.56 MHz, 150 W) and yielded a beam (Fig. 1)comprising, in particular, the excited atomic oxygenas a partial component [1].
A mixture of high purity (99.99999%) Ar admixedwith ∼0.4% of high purity (99.99998%) O2 at atmo-spheric pressure was used in our experiments. Thearrangement of the experimental device enabled usto measure O2 and O concentrations with a mass-spectrometer PSM 003, www.hidenanalytical.net,in excited gas.
To reveal and measure crystallization effects onthe friable ITO film (thickness δ � 300 nm) depos-ited onto a glass plate, the following diagnosticsand techniques were applied in the experiments:
1) Measurements of electrical impedance of theITO layer performed by a 4-point ohmmeter, 2) Mea-surements of light spectral transmittance, 3) XRD,and 4) ELS diagnostics.
At first, all parameters of the ITO film were mea-sured before treatment (as-deposited). Then, a glassplate (substrate) coated with amorphous ITO wasplaced on the heater (2 in Fig. 1) with the ITO layerface up and heated up to a steady initial temperatureTst. After that, the gas flow comprising excited atom-ic oxygen (1 in Fig. 1) was turned on for exposuretime Texp, and the corresponding parameters of thetreated film were measured again and comparedwith those of the initial (untreated) film. The treatedzone of the sample had circular shape with diameterof ∼5 mm.
In ELS measurements, the sample was illumi-nated with linearly polarized light from a He–Ne la-ser (λ � 632.8 nm, 2 mW, spot size in the samplesurface was ∼0.2 mm) that was incident on the sub-strate surface along a normal to it after passing atuning fork chopper and additional focusing lensL1; see Fig. 2. The half-sphere of the scattered lightwas collected with an elliptical mirror M that redir-ected it to a CCD camera and registered as I�θ;φ�
Fig. 1. Sketch of the experimental setup with partial inset ofits photograph: 1—jet of Ar gas mixed with excited oxygen; 2—controllable electric heater; 3—thermocouple.
Fig. 2. Schematic of elastic light scattering measurements: He–Ne, He–Ne laser; Ch, fork chopper; L1, focusing lens; M, elliptic mirror;S, substrate with ITO film; BS, beam splitter; PhD, photodetector; L2, conjunction lens; CCD, CCD camera.
distribution, where θ, φ are polar- and azimuth-scattering angles, respectively. Such registration waspossible because of a conjunction between the outputaperture of the mirror M and surface of CCD sensorby a lens L2 that was placed at the second focal pointof this mirror.
The results of direct measurements were normal-ized to the intensity of the incident light I0�θ;φ� andthe solid angle dΩ of a photodetector to obtain AngleResolved Scatter, ARS
ARS�θ;φ� � I�θ;φ�I0dΩ
(1)
A beam splitter BS redirected part of the hemispheri-cally scattered light to a photodiode PhD that waslocated in the plane of the image of the illuminatedsample area. The PhD signal was proportional to thetotal scattered light intensity (TSLI) that is incorpo-rated in a total integrated scatter (TIS) expression
TIS � TSLII0R
�2�
for illuminated area. Here R is the substrate specu-lar reflectivity coefficient. It is known that for thesurface of optical quality, TIS is directly related tothe rms surface roughness σ by the relation
σ � λ4π
���������
TISp
(3)
Any rough and nonplanar surface can be decomposedinto Fourier series of its components. An ensemble ofsuch components is a two-dimensional set of reliefdiffraction gratings, and sum of light scattering fromthese gratings gives the same spatial scattering dis-tribution as could be measured from a real surface.Each of these diffraction gratings can be character-ized by its period d or the corresponding spatialfrequency f � 1∕d. Conversion from polar scatteringangle θ to spatial frequency of corresponding diffrac-tion grating f can be done according to the relation
f � sin θk · λ �4�
where k is diffraction order, for our case k � 1. Equa-tion (4) is valid for normal incidence of light on thesample surface. Based on Eq. (4), one can calculatespatial frequency range that can be analyzed by thepresented ELS setup. The range of measured θ va-lues is �∼6° 90°�. The minimal θ value is determinedby the size of the sample shadow. The correspondingf range is �0.15 1.49� μm−1. The range of periods ofdiffraction gratings that can by analyzed by thismethod is �0.67 6.66� μm. The angular resolution ofthe ELS method is about 0.4°.
For the data obtained with atomic force microscopy(AFM) that also was used to analyze the surface ofthe sample, the range of the corresponding spatial
frequencies is �0.2 44� μm−1, so it partly overlapswith the frequency range of ELS data.
Note that the I0�θ;φ� distribution was measuredfor the substrate areas with minimal local TIS valueonly, thus reducing influence of surface defects andcontaminations on the final results.
3. Results of Measurements
Numerous measurements of physical properties ofvarious ITO film samples before and after treatmenthave shown that the as-deposited film had surfaceresistivity ρ□ � 36.11� 3.24 Ω∕□ that changed toρ□ � 4.63� 0.372 Ω∕□ after treatment at Tst �127� 3 °C for texp � 1 min. The charge carrier mobi-lity in the corresponding samples remained practi-cally invariable, 15.1� 1.2 cm2∕Vs; however, thecharge carrier concentration increased from �7.7�0.8� × 1019 cm−3 up to �1.1� 0.17� × 1021 cm−3. Notethat all the data measured for the treated films wereobtained at a surface area around the beam axis (1 inFig. 1) limited by diameter 5 mm; see inset in Fig. 9.Each sample surface impedance value was obtainedas a result of averaging of 4 measurements at differ-ent angular positions of a 4-point sensor of ohmmeterand then the data obtained were averaged over 16samples. Uniformity of the ITO film impedance in-side the mentioned spot of 5 mm diameter remainedapproximately invariable within ∼� 6% its spreadfor all the samples measured.
Note that ρ□ � 36.11� 3.24 Ω∕□ was found as aninvariable value for the film heated up to Tst ≈ 260 °Conly during reasonably long time (∼1 hour). The tem-perature Tst � 127 °C was found empirically as thelowest initial temperature at which all the effectsdescribed in this paper were observed in the wholerange of their magnitude.
The second kind of measurable changes of the filmwere observed in its transmittance T; see Fig. 3. Herecurve 1 (2) corresponds to the untreated (treated)film. Figure 3 shows that the transmittance in thewhole visible spectral range increased for the ITO
1
0.8
0.6
0.4
0300 400 500 600 700 800
T
λ, nm
0.2
12 2
Fig. 3. Transmittance T of ITO film (thickness δ � 300 nm)versus wavelength λ: 1 (2), curve corresponding to untreated(treated) film.
film after its treatment at Tst � 127 °C for texp �1 min mentioned above, while it decreased slightlyin the near IR range. Explanation of such trans-mittance behavior needs some additional studies.Perhaps a change of film refractive index can beresponsible for such a result. Some informationabout origin of the observed changes of the ITO filmsproperties can be found from comparison of theemission spectra of neutral excited atomic oxygen[1] with those of glowing plasma generated in N2 �O2 mixture under RF excitation, including NO2yellow-green continuum [3], as seen in Fig. 4. A lin-ear structure in near UV range is not only completelyabsent for spectra of the excited oxygen, but shapeand position of the wide peak in the red spectralrange is also sufficiently and unavoidable differentrelative to the NO2 yellow-green continuum.
The elimination of oxygen from the operating mix-ture of the device shown in Fig. 1 leading to the dis-appearance of the yellow glow of beam (1 in Fig. 1)and complete absence of any changes in the physicalparameters of the film treated should be noted.
Doubts in possibilities to generate and transportatomic oxygen can be rejected by several considera-tions [4]. An analysis of spectra of emitted dischargelight presented in Fig. 4 clearly shows significant dif-ferences between the spectra, measured in this studyand those of NO2 yellow-green continuum. In addi-tion, in our experiments we used very clean chemicalcomponents and the construction of producing unitexcludes noticeable impurities of other chemicalcomponents (including nitrogen). Therefore, nitrogenresponsible for yellow-green glow is absent in thedischarge area. More details can be found in Shun’koet al. [2].
Other kinds of sufficient change of the film proper-ties were found with the XRD technique (Fig. 5). InFig. 5, a gray trace corresponds to the untreated filmand a black trace indicates the treated one (Tst �127 °C and texp � 1 min). Several crystalline config-urations can be recognized in the ITO film after its
treatment. It should be noted that direct heating (an-nealing) of the film up to a temperature Tst � 320 °Cin the ambient air (without exposure to excited oxy-gen flow) resulted in ITO film changes similar tothose described above, except for observation of crys-talline forms In2O3�400� and In2O3�622�, presentedin Fig. 5 [4]. The spread of experimental data mag-nitudes (from sample to sample) presented in Fig. 5did not exceed �6%.
Spatial distribution of normalized scattered lightintensity provides valuable information on surfacestructure and its properties [6–8]. In present commu-nication, ELS was used to reveal changes of surfacestructure and their characteristics after treatment.
Polar presentation of averaged results of ARS�θ;φ�measurements is shown in Fig. 6. These data aremapped on a plane as a function of θ and φ. On thismapping, the intensity scattered in the normal direc-tion is at the center, the grazing one is on the border-line of the circle. The gray levels represent theARS�θ;φ� intensity. θ scale is at the bottom of eachfigure, the origin of φ scale is placed along horizontaldirection, started from the center of the circle; apositive direction ofφ is counterclockwise andmarkedbya circular arrow that is placednear origin ofφ scale.A dark area near the center of the circle is a shadow ofthe sample and its holder. The log ARS�θ;φ� datawere calculated, averaged over many points of thetreated zone of the sample and presented inFig. 6(a); similar data for untreated zone are shownin Fig. 6(b), and their difference is presented inFig. 6(c) at the linear scale.
An analysis of Figs. 6(a) and 6(b) implies that scat-tering properties of both treated and untreated zonesare azimuthally isotropic, i.e., there were no azimuthdirections in which significant deviation of scatteredintensity from its average value was observed. Basedon this observation, one can conclude that the surfaceroughness structure was mainly isotropic, i.e., itsgeometrical properties were almost invariable alongdifferent azimuth directions. Nevertheless, if oneaverages the ARS�θ;φ� dependence over the 35°–40°range of θ angle (ARS�35°…40°;φ�), then an azimuth
Fig. 4. Comparison of experimental spectra emitted by excitedatomic oxygen in transition O�1S0� → O�1D2�[2], dashed gray line,with NO2 yellow-green continuum [3], black line.
400
300
200
015 45 60 75 90
X r
ay in
tens
ity, p
.c.s
100
In O2 3(221)
In O2 3(440)
In O2 3(400)
In O2 3(222)
In O2 3(622)
30
Fig. 5. Angular dependence of intensity of XRD on ITO filmbefore (gray trace) and after (black trace) treatment.
dependence of scattered intensity shown in Fig. 7 canbe revealed. It follows from Fig. 7 that there was asmall difference between the scattering intensitiesalong the 0°–180° and 90°–270° planes. However,the effects of a similar anisotropy were absent
practically for the scattered intensities differenceARSTR �35°…40°;φ�‒ARSUNTR�35°…40°;φ� (the de-viation of obtained data from their average levelwas approximately comparable to the errors of mea-surements). Perhaps azimuth anisotropy of theARSTR �35°…40°;φ� and ARSUNTR�35°…40°;φ� dis-tributions could be explained in terms of orientationof polarization plane of the incident light. Note thatsome additional investigations should be performedto find the origin of this anisotropy precisely that arebeyond the scope of this work.
A spatial distribution of the differenceARSTR�θ;φ�‒ARSUNTR�θ;φ� was calculated and plot-ted in Fig. 6(c) to recognize changes of scatteringcharacteristics caused by treatment. It follows fromFig. 6(c) that the treatment resulted in a scatteredintensity increase for small polar scattering angles.
Let us analyze the measured ARS�θ;φ� distribu-tions in more detail. Scattering data in the rangeof φ�−10° 10°� are azimuthally isotropic, and the sha-dows of the sample and its holder are relatively smallin this range; see Figs. 6(a) and 6(b). Therefore, azi-muth averaging of these data gives correct polar an-gular dependence of averaged data. A scale of polarangles was then recalculated to obtain the scale ofspatial frequencies. From these data, the range ofspatial frequencies that are shadowed by the sampleis excluded. The results of such averaging are pre-sented in Fig. 8. When analyzing these data, theconclusion can be drawn that high-frequency (f �0.3…1.1 μm−1 or short-periodic, 0.6–3 μm) diffractiongratings of surface decomposition were not changedsignificantly after treatment. However, the compo-nents with low spatial frequencies (f � 0.2…0.3 μm−1 or periods 3–5.5 μm) increased significantly.Therefore, roughness of the sample surface increasedin such a way that, for the treated areas, the
(a)-3
-4
-5
0 30 60 90
θ°
θ°
θ°
-3
-4
-5
log(ARS)
(b)
0 30 60 90
0 30 60 90
ARS
14
10
6
2
18x10-6
0
ϕ°
ϕ°
ϕ°
0
0
log(ARS)
(b)
(c)
Fig. 6. Logarithm of normalized spatial distribution of scatteredlight (log ARS�θ;φ�) for (a) treated zone of substrate (averaged over9 points); (b) untreated zone of substrate (averaged over 10 points);(c) difference between normalized average spatial distributions ofscattered light of treated and untreated zones (linear scale).
050 100 150 200 250 300 350
ϕ°
-1
-5
1
2
3
AR
S, s
tr
x10
1
2
3
4
5
6
7
Fig. 7. Azimuthal distributions of scattered light intensities(ARS) averaged over the 35° ≤ θ ≤ 40° range: 1, treated zone; 2, un-treated zone; 3, difference between the treated and untreatedzones. A dip in the vicinity of ∼300° is owed to the shadow ofthe substrate holder.
contribution from the biggest surface componentsizes was more pronounced than that from the smal-lest ones.
Apart from the statistical properties of surfacestructure, the surface relief presented above can becharacterized by a value of rms roughness deter-mined according to Eq. (3). To obtain average infor-mation on changes of the surface structure aftertreatment, a spatial distribution of the ELS in sev-eral measurement points in the treated zone and out-side of it was measured and surface roughness σ wascalculated. The positions of these measurementpoints on the substrate were chosen in such a wayas to minimize the influence of surface defects of dif-ferent nature (scratches, dust particles, etc.) on themeasurement results. A map of measurement pointson the substrate surface is presented in the inset ofFig. 9. TIS value as well as the calculated rms surfaceroughness value σ has been determined for each area
of the sample surface where ARS data were mea-sured. In Fig. 9, histograms of σ values obtainedfor the treated and untreated zones are presented.From these data, the conclusion can be drawn thatthe average σ value increased due to treatment, andthe σ distribution function became wider. The aver-age rms surface roughness for a treated zone wasσTR � 4.069 nm, with a standard deviation of0.667 nm, while for an untreated zone these valueswere σUNTR � 2.654 nm and 0.324 nm, respectively.The standard deviation of the data obtained charac-terizes the data spread over the points of measure-ments rather than the error of measurements. Thatthe results of measurements for the ITO film surfaceafter treatment obtained by AFM (see Fig. 10) are ingood agreement with the ELS data described in thissection should be noted. For additional informationon deposition and modification of ITO, see [9–11].
4. Summary
The friable amorphous ITO films can be crystallizedby flow of gas (Ar) containing atomic oxygen ex-cited to the metastable state 2S22P4�1S0�. As a resultof crystallization, the film impedance decreasedfrom the value ρ□ � 36.11� 3.24 Ω∕□ to ρ□ �4.63� 0.372 Ω∕□, and the film transmittance in-creased approximately by 20% in almost the wholevisible range. The results of the film crystallizationby the excited atomic oxygen differ from those ob-tained using conventional heating (annealing) in am-bient air on account of two crystalline configurations,In2O3�400� and In2O3�622� (see Fig. 5) that were pro-nounced feebly after the film annealing [5]. The low-est initial temperature of substrate, Tst � 127 °C at
Fig. 8. Normalized intensity of scattered light, ARS, averagedover the 35° ≤ φ ≤ 40° range versus spatial frequency f : 1, treatedzone; 2, untreated zone; 3, difference between the intensities oftreated and untreated zones.
Fig. 9. Histograms of rms surface roughness for treated zone(black bars) and untreated zone (gray bars). Inset is a map of mea-surement points on the substrate. Treated area was inside of theinner circle and untreated area was outside of the outer circle. Thearea between the inner and outer circles was treated partially.
2
3
1
0 1 2 3 4 5
4
H, nm
0
2.5
5
7.5
10
2
1
0 2.5 5 7.5 10 12.5σ, nm
H,%
Fig. 10. Surface profile of treated zone measured by AFM. Insetcorresponds to histogram of AFM heights distribution.
which the effects of the film crystallization condi-tioned by excited atomic oxygen were observed onthe saturation scale, is significantly lower than thesubstrate temperature Tst � 320 °C required forcrystallization at conventional heating (annealing)of the film. Application of ELS diagnostics for analy-sis of the film surface enabled us to reveal sufficientdifference in surface geometry before and after treat-ment. In particular, from ELS measurements wefound that rms surface roughness increased due totreatment from 2.65� 0.32 nm up to 4.07�0.67 nm, mainly because of increase of amplitudeof surface Fourier components with 3–5.5 μmperiods.These results were in good agreement with the AFMdata. The changes of surface structure due to treat-ment were approximately azimuth isotropic.
The results of elastic light scattering intensitydecomposition only considered. However, the ELSdiagnostics possibilities can be sufficiently extendedif data processing by calculating amplitudes andheights of corresponding imaginary diffraction grat-ings components would be performed.
The authors would like to thank H.H. Huang andA.E. Belyaev for continuous support of our researchwork, D. Stevenson for important discussions andpermanent attention to experimental works, QiHua Fang for useful consulting and practical assis-tance, and R. Richmond for indispensable assistancein experiments.
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