Gijsbertus A. C. M. Spierings

Stresses in Pt/Pb(Zr,Ti)O3/Pt thin‐film stacks for integrated ferroelectric capacitors

A study of the stresses in a ferroelectric capacitor stack deposited on an oxidized silicon substrate is presented. The capacitor stack was prepared with sputtered Pt bottom and top electrodes and a ferroelectric film of composition PbZrxTi1−xO3 (PZT) with x≊0.5 which was deposited using a modified sol‐gel technique. The stresses were determined by the changes in the radius of curvature of the wafer following the deposition steps, during and after annealing treatments, and after etching steps in which the top electrode, the PZT film, and the bottom electrode were successively removed. The largest stress effects are found in the Pt electrodes which are deposited under conditions giving an intrinsic compressive stress. An annealing treatment exceeding 500 °C changed the stress of the bottom electrode from ≊−750 MPa (compressive) to a large tensile stress (≊1 GPa). This stress is largely thermal and is caused by the differences in thermal‐expansion coefficients of the Pt film and the Si substrate. The stress...

Nanosecond switching of thin ferroelectric films

The switching time (ts) for polarization reversal in ferroelectric films of PbZr0.53Ti0.47O3 and La‐substituted PbTiO3 has been investigated. The films were prepared by spin‐on and metalorganic decomposition followed by processing into ferroelectric capacitors having electrode areas ranging from 9 to 50 000 μm2. Pulse measurements show that under all our experimental conditions ts is instrumentally limited and that therefore the true switching time is smaller than the experimental resolution of 1.8 ns. This very fast polarization reversal can be explained by a nucleation‐rate‐controlled switching mechanism.

Contact bonding, including direct-bonding in a historical and recent context of materials science and technology, physics and chemistry historical review in a broader scope and comparative outlook

Abstract Bonding is a subject matter, which on the one hand is at least as old as written history, and on the other hand is as modern as ultrahigh-vacuum (UHV) technology. In this paper, we present the main threads of its historical evolution and modern evaluation. Bonding has always been a high-tech technology, which used to be governed by an ‘ object in view ,’ and nowadays is governed by the ‘ state-of-the-art .’ Direct-bonding, i.e. the glueless joining of two solid bodies, is more or less embodied in what we have called ‘ contact bonding ,’ i.e. a large variety of bonding and annealing techniques. Reasonably weak van der Waals attractions are transferred into strong chemical bonds by annealing. Sir Isaac Newton was the first to see direct-bonding, as testified by his famous central black spot surrounded by ‘ Newton rings ,’ established between an optical contact of a flat and a convex optical surface. Before World War II, direct-bonding was mainly applied in classical optical instruments (such as interferometers); after World War II it was primarily applied in semiconductor technology, optoelectronics, micromechanics and microelectromechanics. This leads to the need for the thinning of one of the wafers for appropriate applications, such as silicon-on-insulator (SOI). More recently, direct-bonding has been investigated for a large variety of materials, thus leading to significant upgrades in terms of flatness, smoothness and cleanliness. A polishing strategy is one consequence of this, which we will deal with in some detail. During the last decade of the 20th century, great progress was made in UHV-bonding, a technology comparable to lateral solid-phase epitaxial growth (SPEG). Bonding and crystal growing have, therefore, become united disciplines. Wafer thinning now has a new impact, for example, by dedicated ion implantation and low-temperature annealing, called ‘ smart-cut .’ A great deal of effort has been exerted to master lattice mismatch in the form of dislocations, i.e. compliant layers. The outlook of these technologies is promising, to say the least, and might one day surpass the physical limits of those of bulk monocrystalline materials such as silicon. All these subject matters are treated step-by-step in this paper. We take a phenomenological approach, sometimes alone or in combination with other disciplines, but not specifically application-directed. The paper covers pragmatic issues and also treats know-how.

SURFACE PREPARATION AND PHENOMENOLOGICAL ASPECTS OF DIRECT BONDING

Abstract Various intrinsic and extrinsic parameters that play a role in the preparation of materials for direct bonding are discussed in this paper. The constitution of a material or a wafer can be described on the basis of its shape and its mechanical, chemical and physical surface finish. Subsurface damage is also of importance with respect to direct bonding applications. Different polishing strategies have been evaluated for polishing the surfaces of different materials to a finish suitable for direct bonding. Optical elements can be polished by means of mechanical polishing; refractory metals by means of dedicated mechanical polishing; III–V compounds by means of chemical polishing; semiconductors by means of tribochemical, i.e. chemomechanical polishing; hard materials by means of enhanced tribochemical polishing; noble metals by means of organo-liquid-supported tribochemical polishing; non-noble metals by means of oxidation-stimulated polishing. After such preparative treatments the material or wafer has to be cleaned, using a suitable method. Certain aspects of the bonding phenomenon itself will also be discussed in this paper.

Ferroelectrics for non-volatile memories

In the late 1980s the interest in ferroelectric materials for memory applications has been renewed on the basis of concepts where ferroelectric thin film capacitors are embedded in Integrated Circuit processes. This paper discusses the application of ferroelectric thin films in memories. First ferroelectric thin film capacitors are reviewed followed by a discussion on the deposition techniques for ferroelectric thin films and on the integration with an IC technology. Next measured data of some important electrical characteristics for non-volatile memories are treated. Finally different ferroelectric memory cell configurations and their consequences on memory characteristics are discussed.

Surface-related phenomena in the direct bonding of silicon and fused-silica wafer pairs

Abstract Direct bonding is the result of a complex interaction between chemical, physical and mechanical properties of the surfaces to be bonded and is therefore strongly correlated with the surface state of the materials. Phenomena characteristic of the actual bonding process are (a) the formation of an initial bond area, (b) bond energy, and (c) bond-front velocity. The effects of variations in surface state on these process characteristics have been investigated for silicon, oxidized silicon and fused-silica wafer pairs. The surface bond energy of hydrophilic wafers is in the range of 0.05–0.2 J/m 2 and is largely determined by the hydrogen bonds formed. The bond energy of hydrophobic wafers is a factor of 10 smaller and is determined by Van der Waals attractive forces. The bond-front velocity is determined by the surface state and the stiffness of the wafer. Both bond energy and bond-front velocity show ageing effects.

Direct bonding of organic polymeric materials

Abstract Direct bonding of organic polymeric materials can be realized when their surfaces are prepared in such a way that they are clean, smooth and susceptible to direct-bonding. In the surface-preparation process, tribo-chemical polishing is an essential step. Polymeric materials such as polymethylmethacrylate (PMMA), polyarylate, polyimide and polycarbonate were bonded either to themselves, to another polymer or to an inorganic material such as silicon or fused silica. The surfacial bond energy of the room temperature bond is surprisingly high: 0.1–0.2 J/m 2 . Heating strengthens the direct bond; for example, for a bonded PMMA/PMMA wafer pair annealed at the glass-transition temperature of PMMA (105°C), the surfacial bond strength increases to 7.8 J/m 2 . This indicates that the bonded surfaces are fused and are interlinked by chemical bonds. When polymers are bonded to low-thermal-expansion materials such as Si and fused silica, during annealing treatments, thermal stresses can induce fracturing of the inorganic part of the bonded wafer pair. By limiting the maximum annealing temperature or the size of the bonded area, fracturing can be avoided.

Processing and performance of integrated ferroelectric and CMOS test structures for memory applications

Abstract The feasibility of integrating ferroelectric thin films with silicon CMOS technology was investigated by processing a ferroelectric process evaluation module which contains ferroelectric and CMOS test structures and some memory cells. The smallest cells have a ferroelectric capacitor (FECAP) of 25 μm2. The FECAPs were made with Pt/Ti electrodes and with Pb(Zr,Ti)O3 deposited by a modified sol-gel technique or by organometallic chemical vapour deposition. The back-end processing includes the insulation and interconnection of the FECAPs and the MOS transistors. The ferroelectric processing has only a slight influence on the CMOS properties. The properties of the FECAPs improve significantly by an additional anneal in oxygen. Both CMOS and FECAP properties allow a proper functioning of the memory cells. These can be reliably operated at supply voltages as low as 3 V and pulse widths down to 20 ns. The endurance of the memory cells exceeds 1013 read/write cycles.

Ferroelectrics and high permittivity dielectrics for memory applications

Abstract In this paper an overview will be given of the field of non-volatile ferroelectric memories with the emphasis on material and processing aspects for high density applications. The emerging field of high permittivity dielectric materials for dynamic random access memories, applying materials closely related to those for ferroelectric memories, will be treated as well.

Direct bonding of organic materials

Direct bonding of polymethylmethacrylate (PMMA) is described as an example of the applicability of direct bonding to organic materials such as polymers. Direct bonding of a PMMA wafer to itself and to silicon and fused silica wafers is realized. At room temperature, the value of the bond energy indicates the presence of weak chemical interactions at the bonded interface. Heating a bonded PMMA/PMMA wafer pair to the glass transition temperature of PMMA (105 °C) and higher temperatures causes the two surfaces to fuse and the interface to become bridged by polymer chains.