A key requirement for future integrated circuit fabrication is lower processing temperatures at all stages of fabrication. Typical rapid thermal annealing processes, utilizing infrared radiation, heat the film as well as the substrate and create the potential for device reliability problems. Microwave annealing may enable a novel solution to the problem of selective annealing of metallic or other conductive thin films, which is possible in the limit where the metal film thickness is less than the skin depth of the material at microwave frequencies. This technique is advantageous because the relatively low absorption of silicon deters substrate heating and the metallic thin film shields the rest of wafer.

We have investigated the quality factor, Q, and temperature of silicon as a function of its doping concentration and power input into the microwave cavity. As a particular application of microwave selective annealing, we have microwave annealed sub-micron Cu thin films sputtered on patterned SiO2/Si substrates and demonstrated an improvement in trench filling for use in interconnect metallization applications.

The phase transformation and densification behavior of gas condensation synthesized γA12O3 sintered with microwave radiation has been studied. Nucleation and growth phase transformations, which produce α-A12O3 occurred as the material was heated through the temperature range of 800–1300°C. These phase transformations resulted in anomalous grain growth with a distinct change in particle morphology, crystallite size and surface area. A12O3 derived from a chemically synthesized boehmite precursor has been shown to exhibit the same nucleation and growth phase transformation behavior when conventionally heated. It is concluded that nanocrystalline γ or β alumina will not be a viable starting material for the production of dense bodies with grain sizes of less than 100 nm.

Polyphase ceramic and metal ceramic composites with at least one high dielectric loss component are potential candidates for application of microwave sintering, because of the high effectiveness of power dissipation and short sintering times. In order to identify material parameters essential for effective microwave sintering, different mixtures of conductive, semiconductive and insulating phases were investigated. When only a limited amount of a highly dissipative component is present, a modification of the grain size and morphology of the dissipative phase has been identified as major factor leading to optimized microwave heating behavior. Surprisingly, microwave sintering is also possible in the presence of a metallic liquid phase.

Microwave heating of alumina/silica fiber tows in a single-mode microwave cavity at 2.45 GHz has produced a surprising thermal spike behavior on the fiber bundles. During a thermal spike, a “hot spot” on the tow brightens rapidly, persists for a few seconds, and then rapidly extinguishes. A hot spot can encompass the entire tow in the cavity or just a localized portion of the tow. Some local hot spots propagate along the fiber. Thermal spikes are triggered by relatively small (<15%) increases in power, thus having obvious implications for the development of practical microwave fiber processing systems. A tow can be heated through several successive thermal spikes, after which the tow is left substantially cooler than it was originally, although the applied microwave electric field is much larger. X-ray diffraction studies show that after each temperature spike there is a partial phase transformation of the tow material into mullite. After several excursions the tow has been largely transformed to the new, less lossy phase and is more difficult to heat. Heating experiments with Nextel 550 tows are examined for a plausible explanation of this microwave heating behavior.

Microwave sintering of zirconia toughened mullite (ZTM) has been performed in a single mode applicator. In comparison with conventional sintering, microwave processing of ZTM leads to a higher density and finer grain size. Microstructure of microwave sintered ZTM was characterized by TEM and HRTEM techniques. The pinning of intergranular ZrO2 dispersoids retarded the grain growth of mullite matrix. The observation of a considerable number of trans-granular microcracks indicates that microcracking toughening is the main toughening mechanism for ZTM.

Continuous yttria-stabilized zirconia ceramic fibers approximately 10–15 μm in diameter have been rapidly sintered by pulling them through a tuned, 2.45 GHz single-mode TE103 microwave cavity in ambient air. The resulting fibers were analyzed by X-ray diffraction, scanning electron microscopy, and single-filament tensile tests. They were found to be unsplit, to have a submicron grain structure and a tetragonal crystal structure, and to exhibit considerable strength and flexibility.

The microwave sintering technology is still at the developmental stage due to the limitation of the volume (or size) and batch of the ceramic products that can be fabricated by microwave processing. The present paper introduces a new microwave processing technique, i.e. a continuous microwave sintering, which makes it possible to sinter ceramic products with an infinite length in a microwave field. By this technique, alumina-mullite ceramic rollers of 40 mm in diameter and 2400 mm in length have been successfully microwave sintered. The exploratory results show this new technique could offer some new opportunities to scale up microwave sintering for industrial applications.

The spatial distribution and sintering activity of ZrO2 governs the microstructure evolution upon microwave sintering of alumina-zirconia ceramics. Using the same microwave sintering profile thermal runaway occurs, when ZrO2-powders with high specific surface area are employed, yielding ceramics with eutectic Al2O3-ZrO2 inclusions, whereas ZTA-type ceramics are obtained from low specific surface area powders. Microwave sintering yields ceramics with a higher density at a particular sintering temperature as compared to conventional sintering.

Application of paraffin as a MW-transparent matrix for microwave pyrolysis of metalorganic compounds enables synthesis of large quantities of nanosized powders with excellent storability and packing behavior. Fully crystalline, nanosized t-ZrO2 powders with specific surface area > 200 m 2 g -1 are obtained from Zr-alcoholate, and alumina powders with > 500 m2g-1 from Al-alcoholates. Some differences exist between microwave pyrolysis and conventional pyrolysis of such mixtures. Powders from microwave pyrolysis transform at lower calcination temperature to α-A12O3 and exhibit after calcination a more homogeneous morphology as compared to powders from conventional pyrolysis. Furthermore, a different composition of the volatile pyrolysis products is found. t-ZrO2 powder from the microwave processing route exhibits an increased stability towards transformation to monoclinic ZrO2 as well as smaller initial grain sizes than a conventionally synthesized powder.

The microwave heating of a thin carbon coated ceramic fiber is modeled and analyzed in the small Biot number regime. The electric field is assumed known and uniform throughout the cross-section, and does not vary along the axis of the cylinder. We present a model which gives a functional relationship between the coating thickness and the final temperature of the fiber. It gives a plausible explanation of the experimentally observed time evolution of temperature for a tow of fibers [1].

Comparative study of microwave sintering at frequencies 30 and 83 GHz, and the comparison between microwave and fast conventional sintering were undertaken with the aim to ascertain the possibility of specific non-thermal effects at microwave heating. It is shown that an enhancement of sintering can not be explained by the action of thermal stresses caused by peculiar distibution of the temperature field within the microwave-heated samples.

Nanociystalline Z1O2 powders, prepared by plasma-chemical method, were sintered by microwave heating. The experimental results indicated that the thermal runaway and hot spot are two important obstacles to successful microwave sintering of nanocrystalline ZrO2. By controlling ratio of dielectric loss between sintered ZrO2 compact and adjacent thermal insulator, the ZrO2 compact with diameter larger than 30 mm could be microwave-heated rapidly and uniformly from room temperature to 1600 °C in 80 minutes, the mean grain size of ZrCh ( 96% T.D. sintered at 1500°C for 5 minutes) is lower than 20 nm determined by means of quantitative XRD and TEM. The change of ZrO2 grain size may be related to phase transformation (t→m).

Conventional sintering of baryum hexaferrite Co2Z leads to a mixture of phases with a low density. In this paper we try to obtain a pure densified CO2Z hexaferrite by microwave heating. The design of the applicator is computed from both the electromagnetic physical characteristics, the dimensions and the shape of the hexaferrite sample. The travelling microwave mode is preferred to the resonant one. Microwave process consists in a debinding step followed by the high temperature sintering cycle. Sintering temperature evaluated by infrared sensors seems to be over-valued. A skin effect is observed which can be attributed to electronic conductivity due to high temperature formation of Fe2+ ions.

Direct microwave sintering and joining of low temperature ceramics such as mullite have been extensively studied. However, it is recognised that microwave sintering and joining of high temperature oxide and non-oxide ceramics poses serious problems, primarily because of the difficulty of coupling microwave energy into these non-absorbing materials which make up the bulk of advanced ceramics. Attempts to overcome this problem by using indirect auxiliary heating or utilisation of intermediary or impure materials have been, by and large, unsatisfactory. Indeed, this fundamental physical barrier has seriously impaired the penetration of microwave heating in ceramic processing.

In order to overcome this seemingly insurmountable fundamental barrier, a radical approach is required. This has been recently achieved by using a Microwave Induced Plasma (MIP) whereby microwave energy is utilised in a hybrid fashion. In this method, a microwave plasma is used to rapidly heat the ceramic to temperatures at which it becomes absorbing. At this point, microwave radiation is directly coupled to the ceramic and the plasma is extinguished since energy is no longer available to ionise the gas and sustain a plasma. This hybrid approach circumvents the limitations of conventional microwave heating for sintering and joining of advanced materials. In this review paper the fundamentals of microwave/materials interaction and conventional approach to microwave heating of ceramics will be thoroughly discussed. In addition, the new hybrid heating developed by the authors will be described and the possibility of MIP-assisted sintering/joining of industrially important materials will be demonstrated.

This paper reports on the use of frequency as a control parameter in the generation of microwave-excited plasmas for applications such as chemical vapor deposition, chemical vapor infiltration, or plasma etching. By changing the excitation frequency of a plasma in a multi-mode chamber, the locations of the maxima and minima of the power density within the chamber are also changed. This allows localization of the plasma discharge in the precise processing area desired in order to increase the process' efficiency. In a similar fashion, sweeping of the processing frequency during the process cycle can theoretically “scan” the plasma across an arbitrarily-shaped target surface. A series of experiments was conducted to evaluate the flexibility of variable frequency microwave energy as a plasma process tool. The results of these tests are presented.

Fundamental characteristics such as the oxide breakdown fields, oxide charges and interface state density of various ultra-thin silicon oxides (≤ 8 nm) grown by microwave plasma afterglow oxidation at low temperatures (400 °C and 600 °C) were investigated. The effective Oxide charge density of 600 °C as-grown oxide was as low as 6×1010 cm-2. The breakdown fields of the oxides were further enhanced and the interface state densities were reduced by employing fluorination (HF soaked) and low temperature N2O plasma annealing. The breakdown field of the thin oxide grown at 600 °C with 15 min N2O plasma annealing was 12 MV/cm. The reduction of interface state density was about 35% for 600 °C fluorinated oxide. When integrated with poly-gate process, the interface state density was as low as 5×1010 cm-2eV-1.

Based on the experience of the synthesis of nanophased ceramic powders with particle size in the range of a few nanometers, a new device for the synthesis of ceramic nanocomposite powders was designed. The new equipment is based on a 915 MHz microwave system built of IEC 153 R9 waveguide elements. The microwaves coming from the magnetron are split up in two branches using an E-plane power splitter. The reaction tube, made of quartz, passes consecutively both branches of the wave-guide system. Behind the applicators with the reaction tube, the waveguides are terminated with sliding shorts. These sliding shorts are adjusted in a way to ignite and operate a plasma in both branches of the waveguide system simultaneously. The system allows the production of homogeneous powders for nanocomposites. “Nanocoated particles” can be obtained in some special cases;for example ZrO2 particles synthesized with sizes in the range between 4 and 5 nm can be coated with a 1 to 2 nm layer of Al2O3 or vice versa. Metal chlorides are selected as precursors for the reactions. A mixture of, argon with 20 vol% oxygen is used as a plasma gas.

Titanium alloys possess several attractive properties such as light weight, high strength and excellent corrosion resistance. However, wider application of these alloys, especially for load bearing components operating under sliding conditions, are restricted due to poor tribological (friction and wear) behaviour. It has been shown previously [1] that nitriding of titanium by using DC plasma is an effective means of increasing the hardness and reducing friction and wear. In this work nitriding response of titanium alloy (Ti-6A1–4V) was investigated using a Microwave Induced Plasma (MIP). The plasma was generated in a TM012 stainless steel cylindrical cavity using 1.5 kW power supply operating at 2.45 GHz. The cavity was water cooled and tuned by two sliding shorts. Nitriding experiments in nitrogen-hydrogen mixture at 70 to 100 torr pressure established that MIP provides an excellent mass transfer medium for nitriding titanium alloy. Furthermore, significant advantages over DC plasma have been discerned. For instance, reduced sputtering, uniformity of treatment and temperature stability can be cited. In this work, preliminary results of characterisation of MIP nitrided surfaces by x-ray diffraction and optical microscopy will be presented to demonstrate the effectiveness of MIP for nitriding titanium.

The goal of this research is to develop an automatic feedback control system for ceramic samples being heated by microwave energy for the purpose of joining the samples. This paper describes two major topics connected with the research. First, the temperature control algorithm is described, and experimental results from testing that control algorithm with actual ceramic samples are presented. Secondly, the microwave cavity is modeled using a transmission line approach to relate iris and plunger positions to absorbed power. Equations are presented which give the proper positions for maximizing the absorbed power, and a control strategy for the iris and plunger is discussed. This research has been funded under NSF Grant No. DDM - 9115417.

A new single mode applicator has been designed and fabricated to provide effective and nearly uniform heating of long ceramic tubes or rods at 2.45 GHz. The applicator is made of two double mitered waveguide corners. A comparison between the new cavity and a rectangular cavity has been carried out to heat various ceramic samples. Results show a significant improvement in the heating rate as well as the reduction of the power needed to attain a given temperature. This paper describes the preliminary test results of the new cavity and the potential to achieve joining of long tubes or rods.