Elsevier

Microelectronic Engineering

Volume 88, Issue 2, February 2011, Pages 145-149
Microelectronic Engineering

Accelerated Publication
Laser transfer of sol–gel ferroelectric thin films using an ITO release layer

https://doi.org/10.1016/j.mee.2010.09.021Get rights and content

Abstract

A new laser transfer process is reported which allows damage-free transfer of ferroelectric thin films from a growth substrate directly to a target substrate. The thin film ferroelectric material is deposited on a fused silica growth substrate with a sacrificial release layer of ITO (indium tin oxide). Regions of the film that are to be transferred are then selectively metallised, and bonded to the target substrate. Separation from the growth substrate is achieved by laser ablation of the ITO release layer by a single pulse from a KrF excimer laser, with the laser light being incident through the growth substrate. The residual ITO on the transferred ferroelectric layer is electrically conducting, and may be suitable for incorporation into the final device, depending on the application. The new process has been demonstrated for 500 nm-thick layers of sol–gel PZT which were thermosonically bonded to a silicon target substrate prior to laser release. The transferred films show ferroelectric behaviour and have a slightly reduced permittivity compared to the as-deposited material.

Introduction

Ferroelectric materials such as lead zirconate titanate (PZT) have potential applications in a wide range of miniaturised devices, including RF components (e.g. fixed capacitors, varactors and resonators), ferroelectric memories, and piezoelectric sensors and actuators [1], [2], [3]. However, the high processing temperatures required to produce dense and fully crystallised ferroelectric layers can make integration of these materials into such devices challenging. For example, thin film processing methods typically involve annealing temperatures in the range 500–700 °C. This precludes fully monolithic fabrication on low-temperature (e.g. polymer) substrates which are of increasing interest in consumer electronics. Even with traditional substrate materials such as silicon the design possibilities are reduced because the ferroelectric film has to be deposited at an early stage before other materials are introduced. These compatibility issues are even more severe for thick films produced by tape-casting or screen printing, where sintering temperatures in the range 800–1000 °C are typical.

The difficulties associated with fully monolithic integration can be avoided by forming the ferroelectric film on a high-temperature growth substrate and then transferring it to a second ‘target’ substrate where the rest of the device fabrication will take place. This kind of transfer can be achieved by bonding the film to the target substrate and then removing the growth substrate by some combination of mechanical grinding and chemical etching [4]. However, this approach is laborious and wasteful of growth substrate material. Another possibility is to use a growth substrate with a thin sacrificial layer, for example a metal oxide, which can be selectively etched away to release the film. A third option, which avoids the use of any wet chemicals during release, is to use laser transfer processing (LTP), also referred to as laser lift-off (LLO). Here the film is released from the growth substrate by a pulse of laser radiation incident through the substrate. The laser wavelength is typically in the ultra-violet (UV), and is chosen such that the substrate is highly transmissive while the film to be transferred is strongly absorbing. The incident laser energy is absorbed in a thin layer of the film adjacent to the interface with the growth substrate, causing delamination to occur. The delamination process is generally attributed at least partly to ablative decomposition of the film, although in principle it could result purely from thermally induced shear stresses at the interface.

Laser transfer for ferroelectric films was first demonstrated by Tsakalakos et al. who applied it to lanthanum-modified PZT thin films produced by pulsed laser deposition on MgO (magnesium oxide) substrates [5], [6]. The 1.4 μm-thick films were first bonded to a stainless steel foil target using palladium–indium bonding. The same group have also demonstrated laser transfer for thin films of unmodified PZT. Other groups have applied laser transfer to thin and thick film PZT [7], [8], and to various other ferroelectric materials in thin or thick film form, including barium strontium titanate, bismuth titanate, lanthanum-modified bismuth titanate and bismuth ferrite–lead titanate [8], [9]. In most of the work to date, silver-loaded epoxy resin has been used to bond the film to the target substrate prior to laser release.

Laser transfer by direct absorption in the ferroelectric film provides an elegant, simple and fast method for release from the growth substrate. However, this approach does have one significant drawback: it leaves a laser-damaged layer at the surface of the released film in which the ferroelectric properties are severely degraded. This layer results from the transient heating induced by the laser pulse, and is typically of the order of 100 nm thick for films released by excimer laser. The damaged layer can occupy a significant fraction of the total film thickness, in which case the ferroelectric and dielectric behaviour of the film as a whole will be compromised. Previous work has shown that the damage layer can be removed, and the electrical properties recovered, by ion milling [5], [6] or by polishing [8]. However, the first of these approaches is too slow to apply over large areas, while the second may not be sufficiently controllable for thin films.

This paper reports on a modified laser transfer process in which the laser energy is absorbed in a sacrificial release layer of indium tin oxide (ITO) that is deposited on the growth substrate prior to deposition of the ferroelectric film. In this way, any laser damage during release occurs in the ITO layer which can subsequently be removed by wet or dry etching. Alternatively, since ITO is electrically conducting, it may be appropriate to retain the ITO as a top electrode in low-frequency applications where highly conductive electrodes are not essential. Fig. 1 shows an overview of the process, including both the film preparation and laser transfer. The process has been demonstrated for PZT films deposited on fused silica substrates by sol–gel processing and released using a KrF excimer laser (248 nm wavelength). Thermosonic bonding was used to attach the films to silicon substrates prior to laser release.

Laser transfer using release layers has been applied previously to micromachined components fabricated directly on silica wafers [10], and to components fabricated on silicon wafers and transferred to glass carriers by bonding and substrate removal by grinding and etching [11], [12]. It has also been used to release electronic components from glass carriers [13]. Polymer release layers have been used in all such processes to date. Polymers are particularly attractive because they combine strong UV optical absorption with relatively poor thermal conductivity and low decomposition temperature, resulting in a low ablation threshold and hence a low fluence (energy per unit area) threshold for release. Unfortunately, however, polymer materials cannot be used as release layers on growth substrates for ferroelectric films, because of the high processing temperatures involved. Certain refractory metals could be used, but the much higher thermal conductivity, coupled with the high ablation threshold, would increase the likelihood of thermally-induced damage in the adjacent ferroelectric film. The high ablation threshold would also probably make the release process relatively violent and likely to produce mechanical damage. ITO was chosen for the present work because it combines a high melting point (ca. 2000 K) with strong optical absorption and a low ablation threshold in the UV. The ablation properties of ITO and other transparent conductive oxides have been studied extensively because of their potential applications in solar panels and displays. The ablation threshold in an air ambient is around 100 mJ/cm2 at 248 nm wavelength [14] which is comparable to that of many polymer materials.

Section snippets

Film preparation

Fig. 1a outlines the process used to produce thin PZT films for bonding and laser transfer. The growth substrates were prepared by depositing a 0.5 μm-thick layer of ITO onto a 4″-diameter, 500 μm-thick fused silica wafer. The ITO was deposited by DC sputtering from a powdered ITO target (90/10 composition) in an argon plasma containing 0.5% oxygen. The resulting films were ∼80% transmissive at 500 nm wavelength and had a DC sheet resistance of ∼100 Ω/□.

The sol–gel PZT films were deposited using

Results and discussion

A key requirement for any process of the type reported here is that it must be possible to form properly crystallised ferroelectric films on the release layer. Fig. 2 shows a typical XRD measurement for a PZT film deposited on Pt/Ti/ITO/silica. The peaks attributed to PZT are all for the perovskite phase, confirming that crystallisation has been achieved. However, many different perovskite orientations are represented, indicating that the film is not predominantly (1 1 1)-oriented as can be

Conclusion

A new laser transfer process for ferroelectric thin films has been demonstrated in which the laser energy is absorbed in an ITO release layer, thereby avoiding laser damage to the transferred film. To our knowledge this is the first report of thin film laser transfer directly from a high-temperature growth substrate using a release layer. The residual ITO on the transferred film can be removed by selective etching, although for some applications it may be appropriate to retain the ITO as a top

Acknowledgement

The authors gratefully acknowledge the financial support of the UK Engineering and Physical Sciences Research Council under the Flagship Project EP/D064805/1, “Integrated functional materials for system-in-package applications”.

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