|
|
|
|
Back
to Research Interests Page 
Back to Filler-property’
Contribution to Rheology
“Research Outline (Enumeration of
Current Projects)”(2) Flame
Synthesis of Aluminum Nitride Filler-powder
“Why are
‘Spherical’ & ‘Filler-size (0.5-30micron)’ necessary?”
[ Contents ]
1.
INTRODUCTION (Semiconductor Packaging Technology)
2.
Recent Trend of AlN Powder Production
3. Recent Trend of Silica
“Filler-powder” Production
4. Aims of My Work & Recent Results
[ Research Examples ]
Flame Synthesis of Aluminum
Nitride Filler-powder
Functions of New Aerosol Multilayered Gas Sensor
Semiconductor packaging material encapsulates IC chips etc., and necessitates a high thermal conductivity, a low thermal expansion and a good moldability. Higher the packing content of filler, higher the thermal conductivity (lower the thermal expansion); but then, the moldability degrades.
“New filler-powder“ having a high thermal conductive coefficient and a very spherical morphology is one of highly promising approach to improve the semiconductor-packaging properties.
Presently, a spherical SiO2 particle flame-fused natural quartz ingredient has provided for the most part of practical filler-powder, having a bimodal size distribution and a roughly 10μm in average size (Ogawa et al., 1990; Otsuka, 1993; Unger, 1994; Abe et al., 1996; Kitano, 1998), e.g., as shown in Fig. 1 ( Abe et al., 1996). AlN, initially, has received much interest in the print-circuit board as a substitute of conventional ceramic substrate. It has following advantages; (1) theoretical thermal conductive coefficients of AlN, metal Si, Al2O3 and SiO2 are roughly 300, 90, 20 and 2 Wm-1K-1 at 400K; (2) the thermal expansion coefficient of AlN is close to that of Si substrate, 5×10-6 K-1 (Sheppard, 1990; Nakajima, 1998). The common methods for commercial AlN powder are; (1) the direct-nitridation of metallic Al (Weimer et al., 1994; Nagai et al., 1997) and (2) the reduction-nitridation of Al2O3 with carbon reductant in the presence of nitrogen (Kuramoto et al.,, 1989; Komeya et al., 1993). Whereas, the direct-nitridation necessitates a crushing/milling procedure, and the resultant powder has an angular shape mostly, such as shown in Fig. 2 ( Nagai et al., 1997). The main target of commercially-circulated powder via reduction-nitridation is for the raw powder of ceramic sintered body, which has a monomodal size distribution and far often smaller in mean diameter than the practical SiO2 filler-powder (e.g., submicron). The process, furthermore, is an endoergic reaction and needs a pretty high heating temperature in powder preparation, around 1700 ℃. It is not certain to be possible to nitride the large-sized raw Al2O3 necessitated to get the large-sized filler-powder. Several attempts in new aerosol processes such as CVD, plasma and fluidized-bed, were suggested too as a new production route of AlN, but all in all, they were likely to be insufficient for filler-powder needs or economical powder supply (Hotta et al., 1994; Wakimura et al., 1995; Pratsinis et al., 1995).
Furthermore, the SiO2 filler-powder, practically, is produced by flame fusion thechnology, and the flame aerosol synthesis is an established industrial process bringing sizable profits for large-scale manufacture of spherical-shaped particle, also at a pigmentary titania, a fumed silica for optical fiber and a super-paramagnetic particle (McMillin et al., 1996; Pratsinis et al., 1996). AlN filler-powder via flame synthesis route is reasonable because of its simplification, spherical-shaped particle, large-scale manufacture and cost-effectiveness. It could be also put the hopes to use the pre-existing manufacturing apparatus and scientific foundation built at the SiO2 filler-powder. However, there were very little concerns on it except SiO2 thus far, although only a few patents/articles were reported such as the flame-fused δ-Al2O3 particle (Hiragushi et al., 1982; Ogawa et al., 1986), or the diamond synthesis via the imperfect-combustion flame, which meant an insufficient O2 ratio to LPG (liquefied petroleum gas) (Hirose, 1996). Conceivable reason for the little concern in flame synthesis AlN might be its obscurity in melting point or its apt tendency to make angular shape. Al2O3 also has an angular shape based on their crystalline phases, but very spherical α-Al2O3 was developed by one of gas-phase synthesis (Tanaka et al., 1997). Ease-to-make tendency of spherical shape, which was an essentiality of aerosol synthesis, was utilized in the preparation; AlN is just as anxious to do likewise.
My work concerned with a flame AlN aerosol synthesis and its effect of flame temperature, which offers a new filler-powder supply for resin polymer composite system filled with ceramic particles. It appears to be available the AlN powder having suitable properties as filler-powder, i.e., a spherical morphology and a fairly large aerosol particle size, roughly 10 μm in average size.
