Fullerton D.: ITO Sputtering 5” Annual Microelectronic Engineering Conference, 1997

DC Reactive Sputtering Of TransparentConducting Indium Tin Oxide

Dan FullertonSenior Microelectronic Engineering Student

Rochester Institute of TechnologyRochester, NY 14623

Abstract -- A DC reactive sputtering processfor thin transparent conducting films of indium tinoxide was developed at RIT for a variety of uses. Thinfilms were sputtered in a CVC-601 batch sputtererfrom a 200mm indium-tin target in an argon/oxygenambient onto 3-inch silicon wafers and 2x3-inch glassslides. An experiment utilizing a central compositedesign with variables of oxygen flow, argon flow, andDC power was performed.

As-sputtered films exhibited characteristics ofeither high resistivity and high transmittance due toan abundance of oxygen in the film, or low resistivityand low transmittance due to an oxygen deficiency inthe film. Process repeatability was poor. Films with avariety of characteristics were annealed on a hotplatein an air ambient. Low resistivity, low transmittancefilms tended to increase transmittance during theanneal, coupled with a further decrease in resistivity.The sputtering process with anneal yielded films ofresistivity 1.75*10-3 C2-cm, average transmission of 76.0percent over the visible spectrum (400-700 nm),background corrected for a glass substrate, for athickness of 2000 A, index of refraction near 2.13, anda deposition rate greater than 150 A/mm.

1. INTRODUCTION

Thin films of indium tin oxide (ITO), also known astin-doped indium, exhibit a high degree ofconductivity while also providing high

transmission for electromagnetic radiation in the visiblespectrum. The combination of these two properties makeITO a valuable material for optoelectronic devicefabrication such as transparent electrodes for displaytechnologies, electrostatic discharge layers, solar cells,heat mirrors, de-icing elements, transparent gates forelectronic imagers, and electrochromic devices.’

ITO is gaining popularity over other conductingtransparent materials such as tin oxide, indium oxide, zincoxide, cadmium oxide and cadmium tin oxide due to itslower resistivity and better etchability in both dry and wet

etch processes.2 The film conducts due to its nonstoichiometric nature. Tin in the indium oxide acts as acationic dopant, substituting for the cation indium andreleasing free carriers, which are also donated by oxygenvacancies in the film. These carriers provide the film’sconductive properties.3 ITO can be readily etched in avariety of wet chemicals, including dilute HF or HI, aswell as dry plasma processes using HI or CR4.

Deposition techniques for ITO include DC andRF sputtering, evaporation, chemical vapor deposition,and a number of high temperature deposition techniquesinvolving spray or dip processing. Sputtering was chosenfor this investigation due to the ability to carefully controlthe stoichiometry of the film by controlling filmdeposition rate combined with the amount of oxygen inthe sputter chamber.

II. EXPERIMENTAL

Films were deposited using a CVC-601 batchsputterer with a 200mm indium-tin (90 1000) magnetrontarget onto 2x3-inch glass slides and 3-inch siliconwafers. Base pressure was less than 2*1 06 Torr. Targetto-substrate distance was 6 cm. Figures 1 and 2 illustratethe experimental setup.

Figure 1: CVC-601 Schematic, Side View

Fullerton D.: ITO Sputtering 15th Annual Microelectronic Engineering Conference, 1997

Figure 2: CVC-601 Schematic, Top View

Holder

Films were sputtered in an oxygen argonambient. Samples were not rotated throughout thechamber, but were held stationary over the target.Chamber pressure could not be controlled directly, so gasflows were varied, keeping the throttle valve in the fullyclosed position during sputtering. Prior to eachexperimental run, the target was cleaned by pre-sputteringin a pure argon ambient at a power of 300W for aminimum of five minutes, continuing until the resultingpotential and current stabilized. Target cleaning wasfollowed by a pre-sputter using the gas flows and powerthe current experiment called for in order to condition thetarget. This step continued a minimum of five minutes,also continuing until the resulting potential and currentstabilized.

An initial screening experiment was run todetermine a process window for further optimization.Gas flow settings for argon and oxygen were varied toprovide a window encompassing both transparent,insulating, oxygen-rich films and opaque, conducting,oxygen-deficient films. Oxygen flows ranged from 11 to17 sccms and argon flows ranged from 240 to 280 sccms,providing a chamber pressure between I * 1 O~ and 5 * 1 O~Torr. Chamber pressure, measured with a Piranhi gauge,was monitored as an extraneous variable. DC Powerranged from 200 to 300 watts using a state-of-the-art ENIRPG-50 DC Plasma Generator. From these settings acentral composite experimental design with star pointswas created which required 17 experimental runs, ofwhich three were center points to test processrepeatability.

Measured responses included film resistivity,measured with a four-point probe on films deposited overglass slides. Refractive index was obtained fromellipsometric measurements of ITO films on bare silicon.

Film thickness was measured by ellipsometry along withfringe interference counting. Transmission was measuredat g-line by irradiating a photoradiometer from a GCA gline (436 nm) stepper with a plain glass slide and theITO-coated glass slide and comparing the detectedirradiance. Spectral plots were obtained from a PerkinElmer UVIVIS Lambda 11 Spectrometer, backgroundcorrected for air.

Following characterization of as-sputtered filmsselected films were annealed on a hot plate in air at250 C for 10 minutes. The films were then re-measuredto provide post-anneal characteristics.

III. RESULTS AND DISCUSSION

The original designed experiment showed poorrepeatability from run to run as well as low statisticalcorrelation. This is attributed largely to variations inchamber conditions given the same experimentalparameters. For a constant gas flow rate of 240 sccmsargon and 11 sccms oxygen, pressure readings variedfrom 3.6 mTorr to 7 mTorr, a 51 percent variation in aparameter critical to the process. Possible explanationsfor this variation include using the mass flow controlsnear their maximum and minimum flow specificationsleading to inaccurate gas flows, vacuum leaks, mass flowcontroller failure, Piranhi gauge failure, or throttle valvevariations.

Several general trends in film performance wereevident with adequate statistical significance, however.Figures 3 and 4 show how films of low oxygen contenttended to display metallic properties of low resistivity andlow optical transmittance, much like the alloyed targetfrom which they were sputtered, while films of highoxygen content tended to display glass-like properties,exhibiting high optical transmittance and high electricalresistivity.

Figure 3: Transmission as a Function of Gas Flows

5d~ Annual Microelectronic Engineering Conference, 1997Fullerton D.: ITO Sputtering

Figure 4: Resistivity as a Function of Gas Flows

The measured responses index of refraction anddeposition rate each showed a slight correlation with gasflow rates at a DC power setting of 300W, depicted infigures 5 and 6. Statistical significance of these responsesat other power settings for these variables was minimal.

.,~ of Refraction ~s. Ga~ FIo~~ Rates [sccm] 1 8_8C - _.l

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As expected. index of refraction tends to varyover the selected process window between 1.9 and 2.1,typical values for index of refraction measurements forITO films. Deposition rate tends to increase as oxygenflow decreases, indicating that at higher oxygen flows adielectric layer of oxygen-rich indium tin oxide isforming a capacitive layer over the indium tin target,reducing the sputtering rate. Lower oxygen flows tend toproduce more conductive films and therefore a higherdeposition rate is realized. A trend toward higherdeposition rates with a lower argon flow is also visibleand may be attributed to a higher sputter pressure causingincreased scattering of the deposited material due to anincrease in collisions.4

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Figure 6: Deposition Rate as a Function of Gas Flows at300W DC Power

Annealing of the oxygen-rich glass films in airshowed no significant effects on either transmission orresistivity. Annealing of the oxygen-deficient metallicfilms showed a profound shift upward in the transmissioncharacteristics of the films over the visible wavelengths asshown in Figures 7 and 8 combined with a furtherdecrease in resistivity. The increase in transmission ispartially attributed to further incorporation of oxygen intothe film combined with an increase in film crystallinity.The increase in conductivity is attributed to an increase incarrier mobility, also a result of improved crystallization.5

Figure 7: Transmission vs. Wavelength for Selected Films

By sputter depositing films in the oxygen-deficient state and annealing them in air for 10 minutes at250 C, thin ITO films of high visible transmittance andlow resistivity with an index of refraction near 2.1 anddeposition rates of greater than 150 Aimin wererepeatably obtained. Typical characteristics for a filmdeposited as described are presented in table 1.

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Fullerton D.: ITO Sputtering 15th Annual Microelectronic Engineering Conference, 1997

Figure 8: Transmission vs. Wavelength for Annealed ITOFilm, Corrected for Glass Substrate

Parameter Value

Argon flow (sccm) 240Oxygen flow (sccm) 1 1DC Power (W) 300Anneal temperature (C) 250Anneal time (mm.) 10Film thickness (Angstroms) 2011Sheet resistance (ohms sq) 87Resistivity (ohm-cm) 0.00 175Index of refraction 2.128Avg. Trans. [400-700 nm], corrected for 76.0glass substrate (° o)

Table 1: Typical Annealed ITO Film Process Parametersand Characteristics

bipolar pulsed DC process, insulators on the target aresputtered faster than the base material in a process calledpreferential sputtering, eliminating target poisoning andincreasing film quality and deposition rate.6

V. ACKNOWLEDGEMENTS

I would like to express my gratitude to Dr.Michael Jackson and Dr. Santosh Kurinec for theiradvisement and encouragement, and also acknowledgethe assistance of John Guida, Karl Hirschman, BudLangley, Clay Reynolds, Dr. Bruce Smith, and LenaZavyalova.

VI. REFERENCES

R.B. Goldner eta!., “Electrochromic Behavior in ITOand Related Oxides,” Applied Optics, 24, 2283(1985).

2 K.L. Chopra, S. Major and D.K. Pandya, Thin Solid

Films, 102, (1983).

T. Coutts, Transparent Conducting Oxides: TheirScience, Fabrication, Properties and Applications,Short Course Executive Committee of the AmericanVacuum Society, p.1-7, (1996).

P. Infante, Characterization ofDC Reactive SputteredIndium Tin Oxide Thin Films, RIT Master’s Thesis,unpublished.

Ibid.IV. SUMMARY

A process for sputter deposition of transparentconducting thin films of ITO was developed. Filmsdemonstrated resistivity in the 2*10.3 Q-cm range at athickness of approximately 2000 A with an index ofrefraction near 2.1 and deposition rates over 150 A/mm.Average transmission over the visible spectrum, whendeposited on a glass slide, approached 7O0o. Due toequipment variability, an anneal step is required forsatisfactory film performance.

Further investigation of CVC-601 gas flow andpressure control is recommended, as is optimization ofthe anneal process. Future work may also includedevelopment of an asymmetric bipolar pulsed DCreactive sputtering process utilizing the capabilities of thestate-of-the-art ENI RPG-50 DC Plasma Generatorcurrently installed on the CVC-601 equipment. Thistechnology allows for high quality, high rate reactivedeposition from metallic targets. Using an asymmetric

6 J~ Sellers, Asymmetric Bipolar Pulsed DC - The

Enabling Technologyfor Reactive P VD, ENI TechNotes, (1996).

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