Effects of Dy 2 O 3 on the Dielectric Property , Phase Composition , and Microstructure of the Low Temperature Sintered Aln Ceramics

The effects of Dy2O3 on the phase composition, microstructure, and the dielectric property, especially, dielectric constant, of AlN ceramics sintered at low-temperature (1650C), were investigated systematically. X-ray diffraction (XRD) was employed to indentify the phase compositions of the ceramics during the sintering. Scanning electronic microscopy (SEM) was used to observe the microstructures of the AlN ceramics. The results show that the dielectric constant changes with the amount of Dy2O3. When the amount of Dy2O3 is less than 0.93 wt.%, the dielectric constant increases with the increased amount of Dy2O3. The dielectric constant of the AlN ceramics is higher than 200 when the amount of Dy2O3 is 0.93 wt.%. However, when the amount of Dy2O3 is more than 0.93 wt.%, the dielectric constant decreases with the increased amount of Dy2O3. The dielectric constant of the AlN ceramics with 1.87 wt.% Dy2O3 is only about 15, which closed to the theoretical value of pure AlN ceramics. The analysis illuminated that the grain phases changed from Al2O3-rich aluminate to AlDyO3 which existed at triple pockets with the increased amount of Dy2O3, the former can improve the dielectric constant greatly due to ion relaxation polarization.


Introduction
AlN has received growing interests in the electronics industry because of the need for smaller and more reliable integrated circuits and the need for higher voltage devices for power applications (Luo & Zhang, 2005).AlN ceramics are used as electronics materials because of its excellent properties, such as high resistivity, high thermal conductivity, low dielectric constant and thermal expansion coefficient close to that of silicon.
However, it is difficult to sinter AlN ceramics due to its high covalent bonding (Sheppard, 1990).High temperature (e.g.1900 o C) and/or high pressures are required in most of cases (Kume & Yasuoka, 2005).A common solution to lower down sintering temperature is to add sintering additives to improve liquid-phase sintering (Qiao & Zhou, 2003;Qiu & Hotta, 2006).Especially, using multiple component additives (Qiao & Zhou, 2003;Watari & Hwang, 1999;Boey & Tok, 2002) was the effective route to lower down sintering temperature.Apart from enhancing densification, the additives could also affect the dielectric properties since these additives mainly existed in grain boundaries as a second phase (Hagen & Yu, 2002;Kume & Yasuoka, 2005;Kume & Yasuoka, 2005).Using nanopowders as raw materials is another novel resolution.The sinterability is found to be improved greatly by lowing down the size (Panchula & Ying, 2003;Qiu & Hotta, 2006).However, there are some opposite opinions on using nanopowders due to their high agglomeration (Suehiro & Hirosaki, 2003).
When AlN ceramics were used as substrate and package materials for integrated circuits (ICs), not only dielectric loss but also dielectric constant should be necessary low for the sake of shortening signal delay time.As far as high pure AlN ceramics with few or only one additive are concerned, their dielectric constants are almost same to one another.However, when multiple component additives were used for low-temperature sintering, the complicated microstructure of AlN ceramics was always unavoided.Under this condition, the dielectric constant and dielectric loss varied significantly.To date, many literatures paid much more attention on the dielectric loss, especially for high pure AlN ceramics (Kume & Yasuoka, 2005;Hagen & Yu, 2002), but less on dielectric constant.Since the dielectric constant is equally important to substrate and package materials, it is significant to explore the effects of those sintering aids on the dielectric constant.
Dy 2 O 3 is one of rare earth oxides that are typically used as sintering aids for AlN.It has been reported that Dy 2 O 3 is effective for improving the densification and thermal conductivity (Du & Qin, 2007).In this study, AlN with 0-1.87 wt.% Dy 2 O 3 sintered at 1650 o C were prepared, and the effects of Dy 2 O 3 addition on the phase composition, microstructure and dielectric properties of AlN ceramics are investigated.

Experimental
The starting materials were commercially available CaO, Y 2 O 3 , Dy 2 O 3 (analytical purity), AlN with micrometer size powder (Tokuyama soda, Japan, grade H, D 50 =2.55μm, oxygen content 0.85 wt.%) and high purity Al 2 O 3 (purity 99.99%, Xinmeiyu, China, D 50 =0.6μm).Samples were prepared through conventional ceramic fabrication process.With polyvinylbutyral (PVB) solution (5%-10%, ethanol as solvent) as binder, AlN powder and additives were planetary milled using ethanol as mixing medium (mass ratio of raw powders: ball: ethanol = 1:2.2:1.2) for 6 hours.After drying, the powder mixture was uniaxially die-pressed into 20 mm diameter and 3 mm thick pills at 10 MPa for 1 min.The compositions of the samples are shown in Table 1.The green bodies were placed into Al 2 O 3 crucibles which is full of carbon powders.Then the crucibles were covered with lids to form a 'sealed' environment.These green bodies were embedded within carbon powders and sintered under a weak reducing atmosphere formed by carbon powder.Sintering process was as follows: firstly sintering at 1650 o C for 3h; then cooled to annealing temperature (1600 o C) for 5 minutes, and soaking for 6 h at 1600 o C; finally, the samples are cooled to room temperature, naturally.
In order to understand the formation of the liquid during sintering, for samples DO1, DO2 and DO3, Al 2 O 3 were used to replace AlN in D1, D3 and D5, which added as one of the reactants to examine the phases in the sintering.The densities and water absorbability of the sintered pellets were measured using Archimedes's method and the data was obtained by averaging 3-5 readings.The fracture surfaces of the pellets were observed by scanning electron microscopy (JSM-6700F, Japan).The XRD patterns were recorded with a X'Pert-Pro MPD diffractometer (PANalytical, Holland) using Cu Kα radiation (λ=1.5406Å).To measure dielectric property of the AlN samples, machining and polishing and then coating Ag were performed.Dielectric properties were measured from 1 Hz-10 MHz at room temperature by using Novocontrol Concept, Broad Band Dielectric Spectrometer.

Physical Properties
Figure 1 shows the water absorbability and density of the samples with different amount of Dy 2 O 3 .It can be observed that the water absorbability of all of the samples is less than 0.1% regardless of the amount of Dy 2 O 3 , which means that all the samples were almost fully densified.The water absorbability depends on the amount of Dy 2 O 3 .When the amount of the Dy 2 O 3 is less than 0.93%, the water absorbability decreased with increasing amount of Dy 2 O 3 .When the amount of the Dy 2 O 3 is more than 0.93%, the water absorbability then increased with increasing amount of Dy 2 O 3 .In contrast to water absorbability, the density shows different trend, it increased with the increase of the amount of Dy 2 O 3 .It can be explained from the microstructure of the ceramics above.The dielectric constant could be deduced according to Lichtenecker's mixture formulae which related with the dielectric constant and volume of each phase, as well as microstructure in ceramics.Since the structure of Al 2 O 3 -rich aluminate was much more relaxed than that of AlN grain and AlDyO 3 , the dielectric constant would be much higher than the latters due to ion relaxation polarization.When the content of Dy 2 O 3 was low, only limited Al 2 O 3 -rich aluminate was formed as discrete and relative thick two-grain boundary phase, under which condition both electric conductivity and dielectric constant were low.When the Dy 2 O 3 was added, an increased amount of Al 2 O 3 -rich aluminate existed among AlN grains to form thin grain boundary layer to improve the dielectric constant.As a result of increased amount of grain boundary layers, electric conductivity also increased greatly.This kind of ion relaxation polarization was further proved by dielectric loss: the tan δ rapidly increased as the frequency increased in high frequency band.When Dy 2 O 3 was added to a certain value, AlDyO 3 formed as the second phase.The Al 2 O 3 -rich aluminate network among grain boundary disappeared and isolated AlDyO 3 emerged at the triple pockets.As a result, the dielectric constant was lowered down again.
In order to explore the phase change explicitly, several additional experiments (Table 1) were also conducted by using ).This observation was consistent with literature (Yoshikawa & Katsuda, 2005) in which the ratio of AlSmO 3 /Sm-β-alumina (a kind of Al 2 O 3 -rich phase) as grain boundary phases raised as the increased amount of Sm 2 O 3 .Some researchers (Yoshikawa & Katsuda, 2005;Bondar & Toropov, 1966;Nakano & Watari, 2003) suggested that Al 2 O 3 -rich phase was easily formed at the grain boundary.

Conclusion
AlN ceramics with dielectric constant only of 15, close to the theoretical value of pure AlN ceramics, was prepared at low-temperature of 1650 o C with the additive of Dy 2 O 3 .It was found that the amount of Dy 2 O 3 (0 -1.87 wt.%) has significant effects on the composition, microstructure and dielectric constant of low temperature sintered AlN ceramics.The composition of the second phases varied from Al 2 O 3 -rich aluminate phase to AlDyO 3 phase with increasing the amount of Dy 2 O 3 .The different wettability of the second phases resulted in dissimilarity of both the microstructure and the dielectric properties of these samples.The AlN ceramics with 0.93 wt.% Dy 2 O 3 additive contained both Al 2 O 3 -rich aluminate phase and AlDyO 3 phase, whose wetting behavior to the AlN grain were good and poor, respectively.The large amount of wetting Al 2 O 3 -rich aluminate phase contributed to a high dielectric constant value (higher than 200).Sample D5 with 1.87 wt.% of Dy 2 O 3 containes only AlDyO 3 as a second phase at the triple pockets, yielding the lowest dielectric constant among five samples.

Figure 1 .
Figure 1.Densification and porosity of AlN ceramics with various amount of Dy 2 O 3

Figure 2 .
Figure 2. SEM photographs of fracture surfaces of sintered samples

Figure 3 .
Figure 3. XRD patterns of D1, D3 and D5 Al 2 O 3 reacted with Dy 2 O 3 -CaO-Y 2 O 3 in the same sintering procedure.The XRD patterns of the sintered samples (DO 1 -DO 3 ) are shown in Figure 6.In the Al 2 O 3 -Dy 2 O 3 system, depending on the amount of Dy 2 O 3 , there are two kinds of compounds: Al 2 O 3 -rich (Dy, Y)-aluminate and Al(Dy, Y)O 3 .The chemical formula of (Dy, Y)-aluminate are mainly Al 2 O 3 -rich phase: (Dy, Y) 3 Al 2 [AlO 4 ] 3 and DyCaAl 3 O 7 .The present results demonstrated that as the content of Dy 2 O 3 increased, the grain phases changed from Al 2 O 3 -rich phase, i. e., (Dy, Y)-aluminate (sample DO 1 ) to multi-phases, (Dy, Y)-aluminate and Al (Dy, Y)O 3 (sample DO 2 ) and finally changed to Al (Dy, Y) O 3 (sample DO 3

Figure 6 .
Figure 6.XRD patterns of DO1, DO2 and DO3 During sintering, these Al 2 O 3 -rich liquid phase would gradually migrated from interior to the surfaces of ceramic which resulted that the liquid phase gradually formed continuous network through the ceramic body.When the amount of Dy 2 O 3 increased to 0.93%, there was a large amount of liquid Al 2 O 3 -rich aluminate formed which rapidly migrated through the ceramic.There were also some Al(Dy, Y)O 3 formed at the same time.At the highest amount of Dy 2 O 3 , AlDyO 3 would instead formed primarily.

Table 1 .
Chemical compositions of the starting materials