WO2004019130A1

WO2004019130A1 - Process for producing a patterned oxide film on a substrate

Info

Publication number: WO2004019130A1
Application number: PCT/GB2003/003760
Authority: WO
Inventors: Robert Andrew Dorey; Qi Zhang; Silvia Marson; Roger William Whatmore
Original assignee: Cranfield University
Priority date: 2002-08-24
Filing date: 2003-08-22
Publication date: 2004-03-04
Also published as: GB0219829D0; AU2003271838A1

Abstract

A process for producing a patterned oxide film on a substrate by dissolving an oxide forming powder in a solvent that is photosensitive and/or to which a photosensitiser is added to form a photosensitive solution of the oxide forming powder, depositing the solution on a substrate and drying it to form a film, masking an area of the film and exposing an unmasked area to a light source such as ultraviolet light to reduce the solubility of the unmasked area in a suitable solvent whereby the masked area can be selectively dissolved to leave the unmasked area having a desired pattern, and heating the unmasked area to form an oxide film having the desired pattern.

Description

PROCESS FOR PRODUCING A PATTERNED OXIDE FILM ON A SUBSTRATE

This invention relates to a process for producing an oxide-forming film. The invention has particular application to a process for producing an oxide-forming film that can be patterned by exposure to ultraviolet light.

Chemical Solution Deposition (CSD) has several advantages over other fabrication methods. It permits coating of complex geometries and offers a very good control of the material composition. Ceramic films have been deposited in a single layer with sol-gel techniques up to about 1 μm thick. Thicker films are normally produced by successive coatings. With the composite route the thickness achievable by a single deposition step is usually greater than 1 μm but the resulting film is very porous and needs to be infiltrated to increase the density and to improve the electrical properties. In both cases a long time is required to finish the whole process; therefore the manufacture of a dense thick layer in one stage would offer great time savings.

A significant issue related to thick film manufacture is the cracking that occurs during the thermal treatments. The shrinkage taking place when the organics are removed from the film creates a stress that is released by the formation of cracks. Moreover, in order to exploit the desired properties of the ceramic, it is essential that the ceramic film should be crack-free.

The present invention has been made from a consideration of the foregoing problems and disadvantages of the known processes. According to one aspect the present invention provides a process for producing an oxide-forming film that can be patterned by exposure to light that consists of dissolving an oxide-forming powder in a solvent that is photosensitive and/or to which is added a photosensitiser, the film being formed by depositing said solution onto a substrate, drying it and then exposing it to light through a mask, dissolving the unexposed areas in a suitable solvent and then heating the remaining material to form a patterned layer of oxide on the substrate.

The use of a photosensitive solvent allows the films to be patterned prior to firing thereby reducing the probability of cracking. Preferably, the solvent and/or photosensitiser is sensitive to UN light. This is different to existing processes in which films are patterned after firing by photolithography .

Preferably, the oxide-forming powder is amorphous allowing high concentrations of oxide to be used in the solutions. This reduces the organic loss during the thermal treatments and the subsequent shrinkage thereby further reducing the probability of cracking.

In this way, a patterned layer up to 1.9 μm thick can be obtained using a single photosensitive solution deposition step. Lead zirconate titanate (PZT) or niobium doped PZT may be used as the oxide material, but it will readily be appreciated that the process could be applied to a wide range of other oxides, such as zirconia, alumina, titania, other titanates and zirconates such as barium strontium titanate etc.

Preferably, the amorphous oxide powder is made by the addition of water to a solution of metal containing precursors in an alcohol and then drying the resulting precipitate. Suitable precursors include metal alkoxides, acetates or acetyl acetonates. The alcohol may be ethanol or methanol.

Preferably, the solvent and/or where employed photosensitiser has molecules containing carbon-carbon double bonds. For example, the solvent may be acrylic acid and the photosensitiser may be furylacrylic acid.

According to a preferred embodiment of the invention, a highly concentrated photo-patternable oxide-producing solution was prepared by dissolving a powder, preferably obtained by a sol-gel process, in acrylic acid. The solution was suitable for spin-coating and photosensitive to UN radiation. After deposition and subsequent UN exposure, the exposed portions of the film showed a decreased solubility in several organic compounds. These properties were used to create features on platinised Si substrates. A thermogravimetric analysis was performed on the solution to determine the best thermal profile for the burn out of the organics in order to avoid crack formation. Lead zirconate titanate (PZT) may be employed as the oxide material and PZT features up to 2 μm thick may be obtained after the firing process. The process may allow features of this thickness to be produced on the substrate by a single deposition step. The firing process may reduce the thickness by approximately 50%.

According to another aspect of the invention there is provided a process for producing an oxide-forming film that can be patterned by exposure to ultraviolet light that consists of dissolving an amorphous oxide-forming powder in a solvent for which the molecules contain carbon-carbon double bonds and/or to which is added a photosensitiser, the film being formed by depositing said solution onto a substrate, drying it and then exposing it to ultra-violet light through a mask, dissolving the unexposed areas in a suitable solvent and then heating the remaining material to form a patterned layer of oxide on the substrate.

According to yet another aspect of the invention there is provided a process for producing an oxide-forming film by dissolving an amorphous oxide-forming powder in a solvent, depositing said solution onto a substrate, and heat treating it to form a layer of oxide on the substrate.

The invention will now be described in more detail by way of example with reference to the accompanying drawings wherein:

Figure 1 shows a thermal program applied to films produced by the process according to the invention;

Figure 2 shows the results of thermogravimetric analysis (TGA) on PZT solution employed in the process according to the invention;

Figure 3 shows schematically the processing of films produced by the process according to the invention;

Figure 4 shows the appearance of the developed film after 15 minutes exposure; and

Figure 5 shows the appearance of the developed film after 20 minutes exposure.

An amorphous metalorganic powder was prepared by an ethanol based sol-gel process. The composition was such that

[Pbι.ιNbo.o2(Zr₀.5₂Ti₀.4₈)₀.9₈Oj] was obtained. The starting materials were lead acetate trihydrate, titanium iso-propoxide, zirconium n-propoxide and niobium ethoxide. It will be understood that other precursors could be used, such as other alkoxides, acetates or acetyl acetonates.

The lead solution was prepared with 10 mol% excess lead. Lead acetate was dehydrated and dissolved in ethanol. Mono-ethanolamine was then added and the mixture was gently heated in a nitrogen atmosphere to help the complete dissolution of the lead acetate.

The weighing and the mixing of the precursors of titanium, zirconium and niobium were carried out under nitrogen to prevent the moisture sensitive stock solutions from being hydrolysed. The precursors were dissolved in ethanol. Acetic acid was then added to the mixture, which was then refluxed under nitrogen for 1 hour.

The lead solution and the mixture of titanium, zirconium and niobium precursors were then mixed together and refluxed under nitrogen for 2 hours in order to obtain the PZT sol. When cool the sol was filtered and stabilised by the addition of ethylene glycol. The concentration of the resulting sol was 0.58M.

0.5% w/w of water was added to the sol to start the hydrolysis reaction and after 20 hrs of ageing a light yellow gel was obtained. The gel was then dried in a reduced atmospheric pressure environment to remove the solvents and to obtain the organic powder. The powder was found to be shelf stable and highly soluble in certain organic solvents.

The powder produced with the procedure described above was mixed with acrylic acid (Aldrich; 99% purity stabilised with 0.02% hydro- quinonmonomethylether) and stirred for 3-4 hrs to produce a clear (35% w/w) solution. Acrylic acid was used because of its photosensitivity caused by the double carbon-carbon bonds in its chemical structure. It will be understood, however, that other solvents containing such double bonds would also be expected to work in this role and provide a photosensitivity.

3 wt% of furylacrylic acid (Aldrich; 99% purity) was then added to the mixture to shift the light absorption into the visible region and enhance the photosensitivity of the solution. It will be understood that other photosensitisers containing conjugated double carbon-carbon bonds could be expected to work in this role.

TGA-MS was performed on the solution to obtain information on the releasing of the organics during the thermal treatments. In order to identify the evolution of the volatile compounds a few m/z values were plotted as a function of time. The carrier gas used was dry air at a flow rate of 50 ml/min. A heating rate of 2°C/min was applied.

Figure 2 shows the result of the thermogravimetric analysis performed on the solution. The results are expressed in terms of weight loss (% of mass) versus temperature of the PZT solution (curve A). The derivative of the TG curve is also shown to highlight the temperatures at which the major weight losses occur (curve B) . The coupling of TGA with mass spectroscopy allowed the volatile compounds to be identified in the four main steps.

The decomposition of the PZT solution is initially dominated by the release of acrylic acid (mass losses from room temperature up to circa 200 °C). This is supported by the observation on the mass spectrum of two specific ions for the acrylic acid (m/q = 55 and 72) . The acrylic acid is released in two different steps (around 50 and 170°C) indicating that it might be present in two different forms: as a pure solvent and as a chelating agent. A plateau is then found at circa 50% weight loss. The decomposition increases again above approximately 315°C. Two more steps (at around 380 and 465 °C) take the sample at the 26.7% of the initial weight. In these two final steps the organics contained in the powder, more acrylic acid and furylacrylic acid are released. In the final decomposition step the oxygen level decreases below the background level of dry air, which indicates that combustion is taking place.

The prepared solutions were deposited on platinised silicon substrates using a spin coater. The clear solution was deposited on the Pt/Si substrates leading to transparent films with a "wet" appearance. All the samples were prepared in the same conditions: 3000 revolution per minute for 30 seconds. The "wet" samples were than prebaked at 70°C for 25 minutes to permit contact between the films and the patterning mask. Following the prebaking and the exposure steps the films appeared transparent and glassy. A schematic of the film processing is shown in Figure 3.

After prebaking the films were patterned using UV irradiation from a mercury lamp source (radiance power: 13.4 mW/cm²). The effect of different exposure times (5, 10, 15, 20 and 25 minutes) was studied. A profilometer (Dektak-3-ST) was used to determine the thickness and width of the obtained features.

After exposure to UV light the film showed a decreased solubility in several organic compounds such as methanol, ethanol, methyl-isobutyl ketone, acrylic acid and acetic acid. The best results were obtained by dipping the films in acetic acid (Aldrich; 99,7% purity) for 10 seconds and washing the developed film with copious water free iso-propyl alcohol (IPA) to stop the development process. IPA was used because it is capable of dissolving the acetic acid remaining on the substrate and because it is easily removed by drying. The washed films were then dried with a nitrogen flow.

The film exposed for 5 minutes was completely removed once developed with acetic acid. This indicates that the exposure time was not long enough to permit the polymerisation of the film and the subsequent change in solubility.

The films exposed for 10 and 15 minutes appeared inhomogeneous and porous (see Figure 4) . This is probably due to the inflowing of the acid into the pores of the film not yet fully polymerised. Their thicknesses before firing were respectively 0.2 and 1.5 μm. The width of the features was found to be 10% larger than the original dimensions on the mask. This effect might be due to a polymerisation of the acrylic solution over the borders of the mask.

When exposed for 20 and 25 minutes the films were transparent and uniform in appearance (see Figure 5). Their thickness before firing was in both cases 1.9 μm and the dimensions of the features exceeded those on the mask by 5-6%.

The results show that developed film thickness increases with exposure time from 0.2 μm (10 minutes exposure) up to 1.9 μm (20 min exposure). This suggests that the development process might be responsible not only for the removal of the unexposed area but also of a superficial layer of the features. Following developing the samples were fired in a furnace to obtain a ceramic PZT film. The results obtained from the TG analysis were taken into account when selecting the thermal treatments of the films. The first temperature profile (prebaking at 70 °C on a hot plate) was necessary in order to prevent the "wet" film from sticking to the mask.

The subsequent thermal treatments were carried-out on a programmable furnace using a ramp of 2°C/min. As shown in Figure 1 , dwelling times of 20 minutes were used at 170, 382 and 465 °C. These are the temperatures at which the weight loss is maximum. This precaution was taken to permit a slow release of the organics and to prevent the formation of cracks. Parameters such as the quality of the film and the thickness before and after the firing process were taken into account to evaluate the best exposure time.

The fired films showed white features adherent to the substrate. The thickness of the developed film was reduced in all cases by approximately 50% leading to features up to 0.9 μm thick.

The above process was repeated with a more concentrated PZT solution (40% w/w powder, 3% w/w furylacrylic acid) in order to increase the thickness of the film. The film obtained spinning this solution on a Pt/Si substrate was prebaked at 70 °C for 25 minutes and exposed for 20 minutes under UV light.

The viscosity of the solution led a film that after the exposure step was 3.5 μm thick. During the firing process the thickness of the film decreased by approximately 50% leading to features around 2 μm thick. It will be understood that the invention is not limited to the embodiment above-described and various modifications and improvements will be apparent to those skilled in the art. For example photosensitivity of the oxide forming solution for deposition on the substrate may be achieved by the choice of solvent and/or by the addition of photosensitiser. The solution may be photosensitive to UV light or any other suitable light source.

Claims

1. A process for producing an oxide-forming film that can be patterned by exposure to ultraviolet light that consists of dissolving an oxide-forming powder in a solvent that is photosensitive and/or to which is added a photosensitiser, the film being formed by depositing said solution onto a substrate, drying it and then exposing it to ultra-violet light through a mask, dissolving the unexposed areas in a suitable solvent and then heating the remaining material to form a patterned layer of oxide on the substrate.

2. A process as described in claim 1 , wherein the oxide powder is amorphous.

3. A process as described in claim 2, wherein the amorphous oxide powder is made by the addition of water to a solution of metal containing precursors in an alcohol and then drying the resulting precipitate.

4. A process as described in claim 3, wherein the precursors are metal alkoxides, acetates or acetyl acetonates

5. A process as described in claim 3 or claim 4, wherein the alcohol is ethanol or methanol

6. A process as described in any preceding claim, wherein the solvent has molecules containing unsaturated carbon-carbon bonds, for example double bonds.

7. A process as described in any preceding claim, wherein the solvent is acrylic acid.

8. A process as described in any preceding claim, wherein the photosensitiser is furylacrylic acid.

9. A process as described in any preceding claim, wherein the oxide is substantially lead zirconate titanate

10. A process as described in claim 9, wherein the oxide is niobium- doped lead zirconate titanate

11. A patterned oxide film made by the process according to any preceding claim.