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Optics Express

Optics Express

  • Editor: Michael Duncan
  • Vol. 11, Iss. 12 — Jun. 16, 2003
  • pp: 1406–1410
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Optical properties of spin-on deposited low temperature titanium oxide thin films

J. T. Rantala and A. H. O. Kärkkäinen  »View Author Affiliations


Optics Express, Vol. 11, Issue 12, pp. 1406-1410 (2003)
http://dx.doi.org/10.1364/OE.11.001406


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Abstract

This letter presents a method to fabricate high quality, high refractive index titanium oxide thin films by applying liquid phase spin-on deposition combined with low temperature annealing. The synthesis of the liquid form titanium oxide material is carried out using a sol-gel synthesis technique. The material can be annealed at low temperature (150 C°) to achieve relatively high refractive index of 1.94 at 632.8 nm wavelength, whereas annealing at 350 C° results in index of 2.03 at 632.8 nm. Film depositions are demonstrated on silicon substrates with 0.5% uniformity in thickness. Refractive indices and extinction coefficients are characterized over a broad wavelength range to demonstrate the optical performance of this novel aqueous phase spin-on deposited hybrid titanium oxide material.

© 2003 Optical Society of America

1. Introduction

Liquid phase deposition of glass materials synthesized by a sol-gel technique has been actively used for several years in research laboratories for fabrication of optical materials and components. Recently, the technique has also demonstrated its potential towards commercial manufacturing in various fields of applications such as planar optical circuits and micro-optic devices [1

1. S. Toyoda, N. Ooba, M. Hikita, T. Kurihara, and S. Imamura, “Propagation loss and birefringence properties around 1.55 µm of polymeric optical waveguides fabricated with cross-linked silicone,” Thin Solid Films 370, 311 (2000). [CrossRef]

5

5. A. H. O. Kärkkäinen, J. T. Rantala, A. Maaninen, G. E. Jabbour, and M. R. Descour, “Siloxane based hybrid glass materials for binary and gray-scale mask photoimaging,” Adv. Mat. 14, 535 (2002). [CrossRef]

]. Conventionally, sol-gel based materials are produced from metal alkoxide precursors, e.g., silicon tetraethoxide and titanium isopropoxide, which however, require high annealing temperatures of up to 1000 C° to obtain densified forms and high optical refractive index values. Moreover, there has been growing interest to fabricate low temperature and spin-on processable materials for optical purposes based on sol-gel hybrid glasses. Devotion to low temperature processing arises from the fact that many optical materials do not necessarily withstand high temperature CVD (Chemical Vapor Deposition) type deposition steps for high index coatings, e.g., anti-reflection coatings and pass-band filters. In addition, vacuum processing steps are typically batch type methods, demand high capital investment, and especially a diversification from an integrated circuit type spin-on process line. That is why recent activities are targeting the development of fabrication procedures where vacuum-processing steps can be excluded.

In this letter we demonstrate a new titanium oxide material based on organically modified chlorinated titanium precursors. The synthesized material is low temperature processable in terms of liquid phase method in normal atmospheric pressure by applying a standard low cost and short duty-cycle spin-on process. The material results in high refractive index films at 150 C° annealing temperature. Spectral refractive index and extinction co-efficient properties are studied and they were found to depend highly on the annealing temperature.

2. Experimental

The liquid phase material for optical titanium oxide films was synthesized by using hydrolysis and condensation chemistry for titanium chloride precursors. 0.4 mol of titanium tetrachloride was stabilized and complexed with 0.1 mol of methacrylic acid. The solution hydrolyzed with 10 times extent of ultra pure water with dichloromethane as an organic solvent reservoir and reaction stabilizer. The material solution was allowed to react for 2 hours under vigorous stirring and in addition the solution was aged additional 12 hours without stirring. Finally, aqueous phase was extracted from the solvent by an extraction funnel resulting in a stable aqueous liquid form material that was ready for spin-on deposition. Non-reacted methacrylic acid and in-situ formed hydrochloric acid remained mainly in the organic solvent since the aqueous deposition solution was near to neutral.

Optical thin films were deposited on p-type 4” and 6” silicon substrates by applying spin-on processing method. The solution was poured on a static substrate after which the material was spun-on the wafer in two stages: first the solution was spread on the substrate with 300 rpm speed for 5 seconds and then the speed was accelerated in 2 seconds to 2000 rpm and allowed spin for 30 seconds. Edge bead removal (5 mm removal from the wafer edge) and backside rinse were accomplished manually using 2-propanol as a rinsing solvent. The film annealing was done with conventional open-air laboratory hot plate with ±2 °C temperature uniformity over the plate. Initial film pre-bake was done at 60 C° for 5 minutes. Then the film was taken through temperature sequenced annealing process (85 °C, 105 °C, 150 °C, 200 °C, 250 °C and 350 °C) each for 5 minutes. Between each step the film was cooled down and optical measurements were carried out using reflectometer. Moreover, two 6” silicon wafers were fabricated interchangeably at 150 °C and 350 °C annealing for spectrophotometry-based metrology characterization. Adhesion and wetting ability on a polypropylene plastic substrate was also tested. Finally, the films refractive index stability was tested with a “pressure cooking test” (120 °C, ~2 atm for 2 hours).

3. Characterization and results

The film thickness, refractive index and extinction co-efficient measurements were performed by using Filmetrics 20 reflectomer and SCI FilmTek 4000, which is a spectrophotometry-based metrology tool. The spectral optical data was acquired from 200 nm to 1700 nm. The surface-topography and rms surface roughness values of the deposited films were characterized with an optical non-contact surface profiler (WYKO NT-3300).

The synthesized material was very sensitive against elevated temperature treatments. The films annealed at 105 C° for 5 minutes, resulted already as stable films that were resistant against common organic solvents as well as acidic and basic aqueous solutions without any changes in optical properties. In addition, the scratch resistance of the films annealed at 105 C° was excellent, although actual hardness values were not acquired. Adhesion for both substrates, i.e. polypropylene and p-type silicon, materials was also found to be good for 105C° treated samples as they passed standard “scott tape test”. Stability of 150 C° and 350 C° annealed films were tested with “pressure cooking test” where the samples are treated in 2 atm supercritical water pressure at 120 C° for two hours. The test indicated that the films annealed at 150 C° or less are not fully densified to withstand very aggressive environments, since the film’s refractive index degreased more than 1% or 2.1×10-2 at He-Ne wavelength. However, the film annealed at 350 C° showed good stability against the “pressure cooking test” and index change was in the order of ±2×10-4 at 632.8 nm.

Extinction co-efficient (k) showed slight increment as a function of annealing temperature as the film got more densified. At UV region (<400 nm) the extinction co-efficient increased rapidly by reaching the maximum value at the end of the measurement range (250 nm). The k values at 250 nm range were 0.0125 nm-1 and 0.0215 nm-1 respectively for 150 C° and 350 C° annealed samples. Spectral extinction co-efficiencies are presented in Fig. 1. The k value saturated to “zero-level” in terms of measurement accuracy (approximately 1.0×10-4) at 390 nm for both samples and no changes were obtained at visible and Near Infra-Red (NIR) regions up to 1700 nm (see insert in Fig. 1).

Fig. 1. Extinction co-efficiency of titanium oxide films annealed at 150 °C and 350 °C. The extinction co-efficiency saturates to “zero-level” at 390 nm range (see insert) and no attenuation is detectable at visible and NIR regions.

Spectral refractive indices between 250–1700 nm are shown in Fig. 2. Annealing temperature has analogous effect to the refractive index of the films than to the extinction coefficient values. Therefore higher refractive index was obtained for the sample treated at 350 °C. Refractive index difference between the annealing temperatures was 0.0928 at 632.8 nm with corresponding values of 1.9407 and 2.0336 of 150 °C and 350 °C treated samples, respectively. The indices reached the maximum at 290 nm for the lower temperature sample and at 285 nm for the higher temperature sample, thereafter indices steadily degreased. The maximum values for 150 C° and 350 C° samples were respectively 2.5892 and 2.8464.

Fig. 2. Refractive index of titanium oxide films as a function of wavelength for 150 C° (solid) and 350 C° (dashed) annealed films.

The achieved film uniformity was better than 0.5% and 0.9% (five points measured over the wafer) on a 4” and 6” silicon wafers, respectively. The film uniformities are comparable to standard chemical vapor deposition or physical deposition based thin film processing techniques. The film treated at 150 °C resulted in a rms surface roughness of 1.43 nm within a 400 µm×400 µm rectangular region, after tilt removal from the surface (see Fig. 3(a)). The film treated at 350 °C resulted in a rms surface roughness of 0.97 nm within a 400 µm×400 µm rectangular region, after tilt removal from the surface (see Fig. 3(b)).

Fig. 3. Surface-topography measurement of the deposited films. Part (a) shows a 3D image of the film treated at 150 °C. The rms surface roughness was measured to be 1.43nm. Part (b) shows a 3D image of the film treated at 350 °C. The rms surface roughness was measured to be 0.97nm. See text for measurement details.

Significant film shrinkage was noticed between the films baked at 60 C° and films annealed at 350 C°. The shrinkage of almost 50% for 350 C° annealed films can be explained by two main reasons, hydroxyl and methacrylic acid group condensation reactions and residual water (a part which was not removed in the extraction) removal during the annealing at elevated temperature. The same reactions are also reasons for a surface re-flow and sintering, which resulted in better surface smoothness for the high temperature annealed film. The effect of the shrinkage was also seen as relatively high birefringence of the films that is formed through stresses in the films. The optical birefringence for 150 C° and 350 C° were 1×10-3 and 9×10-3, respectively. The film thicknesses as a function of temperature are presented in Table 1. Table 1 presents also refractive indices for corresponding processing temperatures.

Table 1. Refractive indices (at 633 nm) of the film at various processing temperatures and corresponding film thicknesses based on reflectomer measurements.

table-icon
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4. Conclusions and discussion

A new composition and a low temperature spin-on deposition process for high throughput titanium oxide thin films are demonstrated. High refractive indices, i.e., 1.94, are readily achieved after 5 minutes annealing at 150 C°. Optical properties of the films change clearly as a function of the annealing temperature. These changes are mainly due to further densification of the material at higher temperatures, which is caused due to condensation reaction of the titanium hydroxides towards pure titanium dioxide composition. The films processed at low (150 C°) annealing temperature yielded slightly unstable film material against aggressive “pressure cooking test”, which was seen as refractive index reduction during the test. Reason for this behavior is either adsorption of water into the metal-oxide matrix or bleaching out of the remaining organic moieties, i.e., methacrylic acid, from the film. A refractive index value of 2.0 was nearly achievable at 250 C° and at 350 C° the index was 2.03. Optical transparency was found to be good at visible and NIR regions and in the UV region the optical attenuation reduced remarkably. The obtained broad range of optical properties adjustable with processing temperature proposes that the introduced material composition has a wide variety of applications.

Due to fact that the synthesized material posses excellent adhesion for plastic and silicon surfaces and since the material can be densified at low temperatures to achieve desired optical properties, the material is a good candidate for mass-manufacturing optical thin film coatings on polymeric and semiconductor surface and on top of various optical components. The film quality presents good film uniformity for 4 and 6 inch silicon substrates as well as excellent smoothness in terms of surface roughness. Achieved film thicknesses with one spin-on step are good candidates for optical l/4 plates and 1/4-wave stacks. Current material performance also shows potential for anti-reflection coatings, such as BARC (bottom anti-reflection coatings) to enhance lithography quality and resolution in UV patterning processes and as AR-coatings on plastic and other low temperature stability optical film materials. The material exhibits as well a capability for photoresist top coatings due to its aqueous nature, i.e., most of the photoresists are not soluble into neutral water solutions. The process simplicity indicates that the material is adaptable for standard deposition tracks used in integrated circuit (IC) manufacturing and therefore gives an opportunity to develop “all spin-on deposition” type manufacturing of IC, micro-optical and photonic components.

Acknowledgements

AK acknowledges financial support from Emil Aaltonen Foundation, Tauno Tönning Foundation and Seppo Säynäjäkangas Science Foundation.

References and links

1.

S. Toyoda, N. Ooba, M. Hikita, T. Kurihara, and S. Imamura, “Propagation loss and birefringence properties around 1.55 µm of polymeric optical waveguides fabricated with cross-linked silicone,” Thin Solid Films 370, 311 (2000). [CrossRef]

2.

R. R. A. Syms and A. S. Holmes, “Deposition of thick silica-titania sol-gel films on Si substrates,” J. Non-Cryst. Solids 170, 223 (1994). [CrossRef]

3.

J. T. Rantala, P. Äyräs, R. Levy, S. Honkanen, M. R. Descour, and N. Peyghambarian, “Binary phase zone-plate arrays based on hybrid sol-gel glass,” Opt. Lett. 23, 1939 (1998). [CrossRef]

4.

M. R. Descour, A. H. O. Kärkkäinen, J. D. Rogers, C. Liang, B. Kilic, E. Madenci, J. T. Rantala, R. R. Richards-Kortum, E. V. Anslyn, and R. D. Dupuis, “Toward the development of miniaturized imaging systems for detection of pre-cancer,” IEEE J. Quantum Electron. 38, 122 (2002). [CrossRef]

5.

A. H. O. Kärkkäinen, J. T. Rantala, A. Maaninen, G. E. Jabbour, and M. R. Descour, “Siloxane based hybrid glass materials for binary and gray-scale mask photoimaging,” Adv. Mat. 14, 535 (2002). [CrossRef]

6.

M. Langlet, M. Burgos, C. Coutier, C. Jimenez, C. Morant, and M. Manzo, “Low temperature preparation of high refractive index and mechanically resistant sol-gel TiO2 films for multilayer antireflective coating applications,” J. Sol-gel Sci. Tech. 22, 139 (2001). [CrossRef]

7.

P. Belleville, P. Prené, and B. Lambert, “A UV-cured sol-gel broadband antireflective and scratch-resistant coating for CRT,” Proc. SPIE - Int. Soc. Opt. Eng. 3943, 67 (2000).

OCIS Codes
(160.4670) Materials : Optical materials
(160.6060) Materials : Solgel

ToC Category:
Research Papers

History
Original Manuscript: April 21, 2003
Revised Manuscript: May 29, 2003
Published: June 16, 2003

Citation
J. Rantala and A. H. O. Kärkkäinen, "Optical properties of spin-on deposited low temperature titanium oxide thin films," Opt. Express 11, 1406-1410 (2003)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-11-12-1406


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References

  1. S. Toyoda, N. Ooba, M. Hikita, T. Kurihara, S. Imamura, ???Propagation loss and birefringence properties around 1.55 µm of polymeric optical waveguides fabricated with cross-linked silicone,??? Thin Solid Films 370, 311 (2000). [CrossRef]
  2. R. R. A. Syms, A. S. Holmes, ???Deposition of thick silica-titania sol-gel films on Si substrates,??? J. Non-Cryst. Solids 170, 223 (1994). [CrossRef]
  3. J. T. Rantala, P. ??yräs, R. Levy, S. Honkanen, M. R. Descour and N. Peyghambarian, ???Binary phase zoneplate arrays based on hybrid sol-gel glass,??? Opt. Lett. 23, 1939 (1998). [CrossRef]
  4. M. R. Descour, A. H. O. Kärkkäinen, J. D. Rogers, C. Liang, B. Kilic, E. Madenci and J. T. Rantala; R. R. Richards-Kortum, E. V. Anslyn and R. D. Dupuis, ???Toward the development of miniaturized imaging systems for detection of pre-cancer,??? IEEE J. Quantum Electron. 38, 122 (2002). [CrossRef]
  5. A. H. O. Kärkkäinen, J. T. Rantala, A. Maaninen, G. E. Jabbour and M. R. Descour, ???Siloxane based hybrid glass materials for binary and gray-scale mask photoimaging,??? Adv. Mat. 14, 535 (2002). [CrossRef]
  6. M. Langlet, M. Burgos, C. Coutier, C. Jimenez, C. Morant, M. Manzo, ???Low temperature preparation of high refractive index and mechanically resistant sol-gel TiO2 films for multilayer antireflective coating applications,??? J. Sol-gel Sci. Tech. 22, 139 (2001). [CrossRef]
  7. P. Belleville, P. Prené, B. Lambert, ???A UV-cured sol-gel broadband antireflective and scratch-resistant coating for CRT,??? Proc. SPIE - Int. Soc. Opt. Eng. 3943, 67 (2000).

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