10.1.1.1.6467

STR/03/072/ST

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Process Optimisation of Thick UV-patternable Hybrid Sol-gel Coating Development

X. Zhang, H. Lu and A. M. Soutar

ABSTRACT- We report on the fabrication and characterization of thick UV-patternable hybrid sol-gel films by spin coating using a modified sol-gel process. Thick coating of 25 microns with crack-free surface can be produced by one spin coating step. Thick layers of more than 100 µm can be obtained with a multi-coating proc-ess. Characterization using different techniques including optical microscope and atomic force microscope shows both a smooth surface of optical quality and precise patterns. Parameters for hybrid sol-gel coating preparation, photo-lithography processing including prebaking, ex-posure, development and postbaking were stud-ied in detail. Keywords: Hybrid sol-gel, Thin films, Spin-coating, MEMO, UV-patternable 1 BACKGROUND Organic-Inorganic hybrid sol-gel (HSG) materi-als, also called “ORMOSIL's”- organically modi-fied silicates or “ORMOCER”- organically modi-fied ceramic, are new function materials and their properties are similar to glasses (usually transparent in visible and NIR ranges) [1-2]. Continuous interest has been focused on these organic-inorganic glasses for different applica-tions including planar waveguide devices, and protective coatings for metal, plastic and electri-cal circuits [3-5]. Recently, these coatings have been considered for use as bonding and pas-sivation materials for semiconductor processes as well as hosts for biochemical systems [6-7].

UV-patternable hybrid sol-gel (UVPHSG) glass formulations contain one or more photosensitive organic groups, usually with unsaturated C=C bonds, which can be polymerised upon UV light irradiation. In general, the photo-behaviour of the UVPHSG coating is similar to that of the ordinary negative photoresists employed in semiconductor processing. Complex patterns of high precision can be developed readily with a photomask and a conventional UV mask aligner. By combining the characteristics of sol-gel and polymer, the UVPHSG glass has many advan-tages over pure inorganic sol-gel or organic polymer coatings. A general problem in devel-

oping pure inorganic sol-gel films is the cracking for thicker layers. Thus a multicoating process of up to 20 times is required to obtain a layer of about 2 µm thick. By using hybrid sol-gel pre-cursors, thick layers of up to several tens of mi-crons can be coated without any cracks. Low temperature processing, usually lower than 200oC, is another important advantage for the hybrid materials. This allows the hybrid sol-gel glass to be compatible with most semiconductor processes. By contrast, a thermal treatment at temperatures more than 600oC is required for most of inorganic materials. Furthermore, the UV patternable feature makes the UVPHSG convenient for developing passive or active pla-nar waveguide devices on chip or other sub-strates.

Compared with organic polymer coatings, the hybrid sol-gel films also manifest some advan-tages such as low optical propagation loss, high chemical and mechanical stabilities as well as good compatibility with different substrates to be coated. As mentioned by M. P. Andrews and S. I. Najafi [4], “Materials proper-ties hybridization means that sol-gel has the flexibility and facility to offer a host of photonics solution where other materials cannot.” Among the photosensitive sol-gel materials, much attention has been paid to the system based on MEMO (3-metha-cryloxypropyltrimethoxysilane) and zirconium 1-propanol precursors [4,8-10]. To produce a thick layer for planar waveguide devices, usually dip coating was employed because spin coating usually led to thin layers of less than 3 µm. A surface roughness of 10 nm was reported [4] for films deposited by dip-coating. In practice, how-ever, spin coating is often used in semiconduc-tor processes on wafer, Printed Circuit Board (PCB) or other substrates. Therefore, to ensure the process is compatible with fabrication proc-esses for integrated optics and electrical circuit boards, it is important to develop a process for thick sol-gel coating deposition with good sur-face roughness by using a spin coating tech-nique. In this project, we have developed a modified sol-gel fabrication process. Thick layers of up to 25 µm were prepared with one spin-coating step. The coating surface is very smooth with an

Process Optimisation of Thick UV-Patternable Hybrid Sol-gel Coating Development

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average roughness smaller than 1 nm as meas-ured by an atomic force microscope (AFM). These specifications meet the requirements for wafer level and PCB level optical waveguide interconnection and other passive optical de-vices such as optical splitter and couplers, ar-rayed waveguide gratings (AWGs), as well as thermal-optical switches. 2 OBJECTIVE The objective of the present project was to de-velop a process for UV patternable sol-gel opti-cal waveguides on different substrate materials such as Si wafer, glass, ceramic as well as PCBs for interchip optical interconnection. The scope of this report is the fabrication and formu-lation optimization of thick hybrid sol-gel coating for optical applications. 3 METHODOLOGY 3.1 Fabrication of patternable sol-gel

coatings The flow chart depicted in Fig. 1 shows the es-sential processes for sol-gel coating solution preparation. The hybrid organic-inorganic coat-ing solution is mainly based on MEMO, which has a methacrylate group that enables it to be polymerized upon UV light irradiation. Other materials include metal-organic alkoxide-zirconium n-propoxide, methacrylic acid (MAA) and n-propanol. To get a thick coating, the sol-vent was evaporated underreduced pressure. Depending on the desired final coating thick-ness, the amount of solvent evaporated from the solution can be varied. The prepared coat-ing solution can be stored for several weeks without apparent degradation. 3.2 Sol-gel patterns development Coating solutions were applied on different sub-strates including silicon, silica-on-silicon and glass wafers as well as PC boards by using the spin coating technique. To form a waveguide structure, 500 nm SiO2 on Si wafer were used as substrates for the hybrid coating. Photolithography processes were conducted in a 10k-grade cleanroom environment to avoid air particles from the coating films. After spin coat-ing the films were immediately prebaked at 110 to 115oC on a hot plate in air for 30min to stabi-lize the coating surface and to avoid sticking to photomask during UV light exposure. An EV620

UV mask aligner was employed to polymerise the coating through openings of a photomask. Fig. 1. Flow-chart of the modified hybrid sol-gel coat-ing preparation.

Fig. 2. Flowchart for sol-gel patterns development. The exposure time is between 20 and 30 min depending on the thickness of the coating. After photo imprinting, samples were developed with 1-propanol or ethanol for 1-3 min. A final thermal treatment was done at 160-180oC for 2-4 hrs under N2 gas flow.

MEMO Precursor

Add DI water with a ratio to (MEMO+Zr) 2:1

Coating solution

Memo+Zirconium

Reflux for 1 hour in water bath

Zirconium precursor +1-propanol+MCA

Evaporation under reduced pressure

Add photoinitiator

Stir for 30min

Cool to room temperature

Stir for 1 hour, filtering

Pre-baking at 110 oC for 30 min

Develop in n-propanol for 2 min

Post-heating at 160-180 oC for 2-4 hrs.

Exposure under UV light for 20-30 min

Spin coating on SiO2/Si

Stir for 30 min


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The flowchart for the patterns development is shown in Fig 2. 3.3 Characterisation The coating surface roughness was studied by AFM. Sol-gel patterns were observed with both an optical microscope and scanning electron microscope (SEM). Stylus profilometer was used to measure the coating thickness. 4 RESULTS 4.1 Formulation optimisation The formulation previously reported [4] for dip-coating was first taken as the starting formula-tion in this project. This formulation was modi-fied to optimize the composition for spin coating. The reported formulation for dip-coating can be described as follow: MEMO:0.01M HCl = 1:0.75 (mole) MEMO:Zr(OPr)4 =1:0.25 (mole) Zr(OPr)4:1-Propanol = 1:1 (volume) Zr(OPr)4:MAA = 1:1 (mole)

MEMO+Zr(OPr)4:H2O = 1:2.25 (mole, total water)

Coatings prepared with this formulation exhibit non-uniformities when spin-coated on wafer or glass substrates. The reason is probably insuffi-cient hydrolysis caused by insufficient water being used. This problem was overcome by two modifications: the concentration of catalyst HCl was increased to 0.02 M; the total water amount was raised to 3:1 moles with respect to total MEMO and Zirconium precursors. The new for-mulation can be represented as follow:

MEMO:0.02M HCl = 1:1 (mole)

MEMO:Zr(OPr)4 =1:0.15-0.65 (mole)

Zr(OPr)4 (70%):1-Propanol = 1:1 (mole)

Zr(OPr)4:MAA = 1:1-1.5 (mole) MEMO+Zr(OPr)4:H2O = 1:3 (mole, total water)

Through an investigation of the ratio between MEMO and Zirconium precursor, we found that both low and high zirconium contents are not good for the coating film quality. If zirconium composition is lower than 15%, no uniform film can be formed and if the ratio is higher than 65%, cracks are most likely to be observed even during prebaking. Increasing of the amount of methacrylic acid will led to a better surface qual-ity, especially with high zirconium content.

4.2 Parameters for photolithography The photo-polymerization process of MEMO based coating solution can be depicted in Fig. 3. In order to incur and accelerate the photo-polymerization, a photoinitiator is added in the coating solution. In this project, we use a UV-sensitive photoinitiator IRG 184 (1-hydroxycyclohexyphenylketone). In a first step, the photoinitiator will decompose to radicals upon the absorption of a UV photon. In the sec-ond step, the radicals react with the MEMO molecules to form combined MEMO radicals. The combine radicals will transfer and be en-larged to form the polymer like structure in the last step. The content of photoinitiator in the final coating solution is about 3% by weight. Due to the multi-step chemical reaction, the po-lymerization process for the hybrid sol-gel mate-rial is usually slow, from several minutes to sev-eral ten's minutes. This exposure time is also depending on the density of UV lamp used in the photolithography process. By using the EV620 mask aligner (UV density about 25 mW/cm2), the exposure time for a thin coating of less than 3 µm is about 10-15 min. For a thick layer of larger than 10 µm thick, however, the exposure time as long as 30 min is necessary to ensure a complete photo-polymerization. 4.3 Developer-solvent selection One of the advantages of using hybrid sol-gel material for planar waveguide development is that the development can be realized with com-mon solvents. Different solvents including etha-nol, methanol, n-propanol, iso-propanol, acetone and their mixtures were studied for being used as developers for the sol-gel coatings. The de-veloping speed, interaction with the photo-polymerized patterns were monitored for differ-ent solvents and we found n-propanol, ethanol and their mixtures are suitable developers. They show a moderate developing speed with a de-velop time between 1 and 3 min depending on coating thickness. In addition, no apparent etch-ing of the patterns was found for the time used. 4.4 Pre- and postbaking parameters The main parameters for pre- and postbaking are the temperature used and the duration of the baking. For postbaking, the heating and cooling rates are also important factors to be consid-ered. In order that complex patterns are repro-duced precisely, the prebaking temperature and duration are critical parameters and must be


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carefully controlled. If the temperature is too low or the duration is too short, the mask is found to stick to the coating layer. In contrast, if too high a temperature is used or the duration of baking is too long, the coating become partially ther-mally polymerized prior to photo-irradiation. As a consequence, the coating cannot be accurately developed. For postbaking, the temperature and time for baking are not as critical as for prebaking. To get a completely cured coating, the baking tem-perature can be varied from 160 to 200oC and the duration can be 1-4 hours depending on the temperature used. In this case, the heating and cooling rates are critical factors to be defined. Usually, a low temperature gradient for heating and cooling is adapted to avoid cracks arising from an abrupt shrinkage of the sol-gel materi-als.

When these factors are considered, the baking parameters are optimised as follow: for prebak-ing, 110-115oC, 30-50 min; postbaking, 160-180oC, 1 to 4 hours with heating rate of 0.5-1 oC/min and cooling rate of 1 oC/min. 4.5 Surface morphology The hybrid sol-gel coating developed in this pro-ject show very smooth surface with optical qual-ity. Coating surface morphology taken using an AFM was shown in Fig. 4. The thickness of the coating is about 8 µm and the scan area is 10 µm x10 µm. As shown in Fig. 4, the average surface roughness RMS is only 0.367 nm and the maximum value about 3.6 nm. These values are comparable with the best coatings prepared using the commercial BCB or polyimide photo-sensitive coatings. The results shown in this picture reveal a surface of optical quality ob-tained by the modified sol-gel process.

Fig. 4. AFM surface morphology of an 8 µm-thick hybrid sol-gel coating.

CH 2C H 3

OO

S i ( O C H 3 ) 3O

.

O

OH

O

. OH .UV light

.C H 3

OO

S i ( O C H 3 ) 3

H

C H 3

OO

S i ( O C H 3 ) 3

CH 2

Step 2

Step 3

Step 1

Fig. 3. Photo-polymerization based on IRG 184 photoinitiator and MEMO.


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4.6 Micrographs of sol-gel patterns de-veloped

Fig. 5 shows the optical microscope pictures of well-defined patterns obtained on SiO2/Si wa-fers. The pictures are obtained from different coatings with thickness changing from 4 to 20 µm. The line widths are changing from 5 to 200 µm. These pictures reveal that complex patterns can be developed precisely by the hybrid sol-gel coating with a conventional mask aligner. In a separate report, the development and charac-terization of planar waveguides by the hybrid sol-gel coatings will be discussed in details. 5 CONCLUSION Thick hybrid sol-gel coating was formulated based on 3-methacryloxypropyl-trimethoxysilane (MEMO) and Zirconium propoxide precursors. A new process combining the chemical reflux and solvent evaporation was developed in fabricat-ing thick hybrid coatings for optical waveguide applications by the conventional spin-coating technique. Films with thickness more than 12 micron were obtained with one spin-coat. Photolithography process was developed for the photopatterning of these films. For a thick layer, an exposure time as long as 30 min is required to form a stable pattern. Thicker layer can be produced with a multi-coating process and a 36

µm thick film was prepared with four consecu-tive coating layers. Different solvent including ethanol, n-propanol, iso-propanol, acetone and their mixtures were studied for being used as developer for the sol-gel material. In considering their interaction with the hybrid coating and the developing speed, n-propanol, ethanol and their mixture were chosen as developers. The develop time is between 1 to 3 min depending on the thickness of the coating. Prebaking and postbaking parameters were de-fined. The prebaking temperature is critical for photolithographic process. If the temperature is too low, serious mask sticking will happen and if it is too high, partially thermal polymerization will affect the quality of the developed patterns. The duration of prebaking has the same effect as temperature. The temperature and duration of thermal treatment for postbaking are not as criti-cal at for prebaking, but the heating and cooling rate must be controlled precisely to avoid crack-ing due to abrupt shrinkage of sol-gel coating. High quality Photolithography-developed pat-terns were studied by optical microscope, atomic force microscope, and profilometer. AFM results reveal a very smooth coating surface of optical quality with an average surface rough-ness of 0.367 nm for an area of 10µmx10µm. Complex patterns with high precision were ob-served with optical microscope.

Fig. 5. Optical microscopic pictures of different patterns developed on SiO2/Si wafers with the sol-gel coating pre-pared in this project. a), vertical angles of a series of 25 µm lines of 12 µm thick; b), letters, figures and lines (5 µm for the smallest ones) on a 20 µm-thick coating; c), circles with radii ranging from 10 to 100 µm from a 10 µm-thick coating and d), different patterns on a 4 µm-thick coating.


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6 INDUSTRIAL SIGNFICANCE The technique developed in this project can be used to fabricate thick hybrid sol-gel films by spin-coating method. This process will attract interests from the optoelectronic industry for component integration and packaging. The thick films developed through this work will find pos-sible applications in the ONFIG-related opto-electric projects in Singapore. FINISAR Singa-pore has shown interest in developing hybrid sol-gel coatings with high aspect ratio. The excellent chemical and mechanical proper-ties of the hybrid sol-gel films offer a possibility to use them in applications other than optical waveguides. Possible applications can be found for circuit protective coating, corrosion resistive coatings. The good dielectric property together with the excellent direct photo-imprinting charac-ter of these materials provides a new approach on the fabrication of on-chip component such as embeded capacitor on PCB. Other applications can be found in passivation and bonding mate-rials for semiconductor processing. The high precision patterning process has at-tracted attentions from other industry as well. Hewlett-Packard Singapore has shown great interest to introduce the sol-gel coating in its new generation print head fabrication process. The feasibility study is still on going. In a word, the hybrid sol-gel coating developed in this project can be used in various applica-tions including planar waveguides, metal and materials protection, bonding and passivation in semiconductor fabrication as well as on-chip devices on PC board. REFERENCES [1] U. Schubert, N. Husing and A. Lorenz, "Hy-

brid inorganic-organic materials by sol-gel processing of organo-functinal metal alkox-ides", Chem. Mater., Vol. 7, pp. 2010-2027, (1995).

[2] P. Judenstein and C. Sanchez, "Hybrid in-organic-organic materials: a land of multi-disciplinarity", J. Mater. Chem., Vol. 6, pp. 511-525, (1996).

[3] R. Buestrich, F. Kahlenberg and M. Popall, "ORMOCERs for optical interconnection technology", Journal of Sol-Gel Sci. Tech., Vol. 20, pp. 181-186, (2001).

[4] M.P. Andrews and S.I. Najafi, "Passive and active sol gel material and devices", Sol-Gel and Polymer Photonic Devices, Critical Re-view CR68, pp. 253-285, (1997). M.A. Far-dad, S.I. Najifi and M.P. Andrews, "Solvent-assisted lithographic process using photo-sensitive sol-gel derived glass dor deposit-ing ridge waveguides on silicon", United States Patent No. 6,054,253, 25 April 2000.

[5] T.P. Chou, C. Chanderasekaran, S. Lim-mer, C. Nguyen and G.Z. Cao, "Organic-inorganic sol-gel coating for corrosion pro-tection of stainless steel", J. Material Sci. Lett., Vol. 21, pp. 251-256, (2002).

[6] B. Arkles, "Commercial application of sol-gel-derived hybrid Materials", MRS Bulletin, pp. 402-407, May, (2001).

[7] C. Ohtsuki, T. Miyazaki and M. Tanihara, "Synthesis of bioactive organic-inorganic hybrid from methacryloxypropyltri-methoxysilane and 2- Hydroxyethyl-methacrylate", Proceeding 13th Int. Symp. on Ceramics in Medicine, Bologna, Italy, 22-26 November 2000, pp. 39, (2001).

[8] T. Watanabe, N. Ooba, S. Hayashida, T. Kurihar and S. Imamura, "Polymeric optical waveguide circuits formed using silicone resin", J. Lightwave Technol., Vol. 16, pp. 1049-1055, (1998).

[9] O.H. Park, J.I. Jung and B.S. Bae, "Photoinduced condensation of sol-gel hy-brid glass films doped with benzildi-methylketal", J. Mater. Res., Vol. 16, pp. 2143-2148, (2001).

[10] M.A. Fardad, O.V. Mishechkin and M. Fal-lahi, "Hybrid sol-gel materials for integration of optoelectronic components", J. Light-wave Techno., Vol. 19, pp. 84-91, (2001).

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