Study on the Preparation of Diamond Film Substrates on AlN Ceramic and Their Performance in LED Packaging

In this paper, a structure was proposed to promote thermal management of deep ultraviolet light-emitting diodes (DUV-LEDs) by introducing the aluminum nitride (AlN) ceramic substrate fabricated with copper-filled thermal holes (CFTHs). Different numbers of CFTHs ( $0,2\times 2, 3\times 3$ , and $4\times 4$ ) were formed in AlN ceramic substrates using direct-plated-copper process. The thermal resistance of DUV-LEDs was determined by a thermal transient tester. Meanwhile, to validate the experiment results, thermal simulation using finite element analysis was developed by considering the various model of AlN ceramic substrates. Experimental results indicated that the thermal resistance and junction temperature decreased with the increasing number of CFTHs. Compared with a conventional structure, the thermal resistance of DUV-LED-based $4\times 4$ CFTHs was reduced by 34.6% and its junction temperature was decreased by 7.3 °C at 100 mA. This approach is believed to provide a simple and effective strategy for improving the heat dissipation and thermal reliability of DUV-LEDs.

Frequency agile antennas based on aluminum nitride ceramics

This paper will discuss the technological aspects of direct bonding copper (DBC) process conducted on a copper foil (M00б) with high thermal conductive alumina (BK-96, BK-100) and aluminum nitride ceramics (AIN) being utilized in bonding. There is a wide use of DBC structure in the construction of high power semiconductor devices. The paper will also provide the results of the research on compound interfaces that was carried out based on DBC technology. The results of scanning electron microscopy (SEM) and X-ray diffraction (XRD) show that the DBC interface contains interphases of spinel-type copper aluminates CuAl 2 O 4 and CuAlO 2 , which affect an adhesive strenght of DBC joining interface. The bonding strenght of DBC interfaces was measured, with the results of said measurements presented in this paper.

Understanding of chemical corrosion resistance of aluminum nitride (AlN) ceramics is strongly desired for their applications as structural parts in semi-conductor process. However, there is not enough information on the chemical corrosion. Two kinds of commercial AlN ceramics with high thermal conductivity of 170 and 200 W/mK were examined in acids and basic aqueous solutions such as KOH, NaOH, HNO{sub 3} and H{sub 2}SO{sub 4} with different concentration. Weight changes, phases present and microstructures on the surfaces before and after testing were evaluated. As a result, basic aqueous solutions exactly corroded AlN ceramics in comparison with acid aqueous solutions. Although secondary phases, such as yttrium aluminate, were mainly dissolved in the case of acid aqueous solutions, AlN itself was corroded into basic aqueous solutions. Furthermore, it was observed that weight loss of AlN ceramics with higher thermal conductivity was larger than that of lower thermal conductivity. It seems that oxygen content dissolved into AlN grain has a influence on corrosion of AlN ceramics. (orig.)

A newly developed rapid densify technique, spark plasma sintering (SPS), is used to prepare full-densified aluminum nitride (AlN) ceramics which are poorly sinterable. Some sintering aids are also used to promote the AlN ceramics' densification and improve its thermal conductivity. In this work, effects of sintering aid Li2O on densifica- tion, microstructure and thermal property of SPS sintered AlN ceramics were investigated. Results suggest that the ini- tial sintering temperature of AlN samples reduce from 1550℃ to lower than 1200℃ with 1.0wt% Li2O and 1.5wt% Sm2O3 (or Y2O3) adding as sintering aids . With Li2O addition, AlN compacts can be fully densified at 1650℃. The microstructure of AlN compacts indicates that Li2O is beneficial to generate aluminate liquid phase with better wet- tability and promote the densification of AlN ceramics, but it is unable to obtain higher relative density of AlN com- pacts because the escapement of gas phase though out liquid phase is very difficult in a rapid sintering process. Meanwhile, Li2O addition affects the growth of AlN grains, and the better wettability of aluminate with AlN grain in- duces the homogeneous distribution of grain boundary phase. The deterioration of thermal conductivity of AlN ceram- ics is caused by the fact that the scattering of phonon is enhanced by small grain size and the secondary phase spreading adequately along the AlN grain boundaries. The thermal conductivity of AlN samples with 1.0wt% Li2O and 1.5wt%Sm2O3 as sintering aids is lower than that of sample only with Sm2O3 as sintering aids.

Although there is a strong need to understand the chemical corrosion resistance of aluminum nitride (AlN) ceramics for their applications as structural parts in semiconductor processes, there is insufficient information on chemical corrosion. We examined two kinds of commercial AlN ceramics with high thermal conductivity (170 and 200 W m -1 K -1 ) in both acid and basic aqueous solutions, such as KOH, NaOH, HNO 3 and H 2 SO 4 , using different concentrations. Weight changes, phases present, and microstructures on the surfaces before and after testing were evaluated. Basic aqueous solutions corroded AlN ceramics in exactly the same way as acid aqueous solutions. Although secondary phases, such as yttrium aluminate, were mainly dissolved in acid aqueous solutions, AlN itself was corroded in basic aqueous solutions. Furthermore, we observed that the weight loss of AlN ceramics with higher thermal conductivity exceeded that of lower thermal conductivity. It seems that oxygen dissolved in AlN grain influences on corrosion of AlN ceramics.

We propose the use of optical films to enhance the light extraction efficiency (LEE) and wide-angle emission of traditional packaged deep-ultraviolet light-emitting diodes (DUV-LEDs). Total internal reflection occurs easily in DUV-LEDs because they contain sapphire, which has a high refractive index. DUV-LEDs also contain an aluminum nitride (AlN) ceramic substrate, which has high light absorption in the ultraviolet band. Photons are absorbed by the sapphire and AlN ceramic substrate, which reduces the LEE of DUV-LEDs. By adding a brightness enhancement film (BEF) on the sapphire surface and a high-reflection film (HRF) on the surface of the AlN ceramic substrate, the LEE of DUV-LEDs can be increased. Moreover, we designed a single-layer metal reflective film (SMRF) on the upper surface of the quartz glass in order to achieve wide-angle emission. Experimental results indicated that compared with traditional packaged DUV-LEDs, the light output power and external quantum efficiency of DUV-LEDs with a plated BEF, HRF, and SMRF increased by 18.3% and 18.2%, respectively. Moreover, an emission angle of 160° was achieved. In a reliability test, DUV-LEDs maintained more than 95% of the initial forward voltage and light output power after 1000 h of operation at 25°C, which indicated that the addition of an optical film can improve the light efficiency and long-term reliability of DUV-LEDs.

This study uses the T3Ster transient thermal resistance measuring device to investigate the effects to heat transfer performances from different LED crystal grains, packaging methods and heat-sink substrates through the experimental method. The experimental parameters are six different types of LED modules that are made alternatively with the crystal grain structure, the die attach method and the carrying substrate. The crystal grain structure includes the lateral type, flip chip type and vertical type. The die attach method includes silver paste and the eutectic structure. The carrying substrates are aluminum oxide (Alumina) and aluminum nitride (AIN) ceramic substrates and metal core PCB (MCPCB). The experimental results show that, under the conditions of the same crystal grain and die attach method, the thermal resistance values for the AIN substrate and the Alumina substrate are 2.1K/W and 5.1K/W, respectively and the total thermal resistance values are 7.3K/W and 10.8K/W. Compared to the Alumina substrate, the AIN substrate can effectively lower the total thermal resistance value by 32.4%. This is because the heat transfer coefficient of the AIN substrate is higher than that of the Alumina substrate, thus effectively increasing its thermal conductivity. In addition, under the conditions of the same crystal grain and the same substrate, the packaging methods are using silver paste and the eutectic structure for die attach. Their thermal resistance values are 5.7K/W and 2.7K/W, respectively, with a variance of 3K/W. Comparisons of the crystal grain structure show that the thermal resistance for the flip chip type is lower than that of the traditional lateral type by 0.9K/W. This is because the light emitting layer of the flip chip crystal grain is closer to the heat-sink substrate, shortening the heat dissipation route, and thus lowering the thermal resistance value. For the total thermal resistance, the crystal grain structure has a lesser effect to the heat dissipation performance whereas the packaging method and the choice of the substrate have a relatively higher effect to the LED heat dissipation performance. Combining the above results, we see that selecting the flip chip crystal grain with a closer light emitting layer to the substrate, the eutectic structure that has better thermal conductivity and the AIN substrate can effectively increase the heat dissipation performance of the LED module.

Balancing thermal conductivity and strength of hot-pressed AlN ceramics via pre-sintering and annealing process

Aluminum nitride (AlN) ceramics exhibit exceptional properties that render them highly valuable for diverse industrial applications. However, conventional manufacturing techniques encounter significant challenges in fabricating complex AlN components with precise geometries. To address these limitations, digital light processing (DLP) has emerged as a promising additive manufacturing approach for AlN ceramics. This study presents a systematic investigation of the monomer composition in the photopolymer resin system through a comprehensive experimental evaluation. The results demonstrate that an optimized mixture of monomers ACMO (56.7 wt%), DEGDA (2.7 wt%), and TMPTA (40.6 wt%) yields photopolymer resin with superior comprehensive performance. Utilizing this optimized formulation, a 50 vol% solid loading AlN ceramic slurry was successfully prepared, and subsequently, dense AlN ceramic components were fabricated through DLP. This provides an important basis for optimizing the slurry preparation of AlN ceramic fabrication based on DLP 3D printing.

Effects of adding yttrium nitrate on the mechanical properties of hot-pressed AlN ceramics

Aluminum nitride shaping by non-aqueous gelcasting of low-viscosity and high solid-loading slurry

The electrical conductivity of CaF2‐doped aluminum nitride (AlN) ceramics was characterized at high temperatures, up to 500°C, by AC impedance spectroscopy. High thermal conductive CaF2‐doped AlN ceramics were sintered with a second additive, Al2O3, added to control the electrical conductivity. The effects of calcium fluoride (CaF2) on microstructure and related electrical conductivity of AlN ceramics were examined. Investigation into the microstructure of specimens by TEM analysis showed that AlN ceramics sintered with only CaF2 additive have no secondary phases at grain boundaries. Addition of Al2O3 caused the formation of amorphous phases at grain boundaries. Addition of Al2O3 to CaF2‐doped AlN ceramics at temperatures 200°C–500°C revealed a variation in electrical resistivity that was four orders of magnitude larger than for the specimen without Al2O3. The amorphous phase at the grain boundary greatly increases the electrical resistivity of AlN ceramics without causing a significant deterioration of thermal conductivity.

In order to overcome the purification difficulty of aluminum nitride (AlN) ceramics, the sintering of AlN ceramics with ammonium fluoride (NH4F) as an additive had been studied. The results demonstrate that the addition of NH4F evidently affects the phase compositions, the microstructure of grains and the contents of oxygen and nitrogen in the AlN sintered samples. NH4F not only removes oxygen out of AlN grains but also reduces the total oxygen content in AlN ceramics. It is found that relatively high purity of AlN can be acquired when the molar ratio of NH4F/O (oxygen element in raw AlN powder) increases to 0.8. With adequate amount of NH4F, the Al–O–N phases are removed. SEM and TEM results show the hexagonal structures of AlN grains with clean triple-grain junctions. The oxygen content decreases to 0.55 wt% and nitrogen content increases to 33.7 wt%. Thermodynamic analysis illustrates the oxygen removing effects of NH4F by the reaction of NH3 and Al2O3, which inhibits the formation of Al–O–N. NH4F should be at least 2/3 of the oxygen content.

Metal–organic precursor for aluminum nitride (AlN) ceramics was synthesized by reacting aluminum tri-chloride (AlCl3) with bis(trimethylsilyl)carbodiimide (BTSC). Fourier transform infra-red (FT-IR) spectrum of the synthesized precursor exhibited characteristic absorption bands assigned to the carbodiimide (N=C=N) group at 2150–2250, and 851 cm−1, while the solid state 27Al magic angle spinning nuclear magnetic resonance (MAS NMR) spectrum of the precursor exhibited single signal at 103 ppm which was thought to correspond to Al(N=C=N)4 unit. To examine the potential as a precursor for AlN ceramics, the intrinsic thermal conversion behavior up to 1800°C of the synthesized precursor was investigated under argon atmosphere. X-ray diffraction analysis revealed that the crystallization of AlN was found to start above 800°C, and fully crystallized AlN ceramics was synthesized by the additional heat treatment at 1800°C. In addition to the FT-IR and NMR spectroscopic analyses for studying the synthetic parameters such as reaction temperature and use of catalyst for the formation of polymeric precursors derived from AlCl3 and BTSC, the effects of heat treatment condition on the polymer/ceramics conversion yield, impurity and crystallinity of the AlN ceramics have been studied by using a thermogravimetric analyzer coupled with a quadrupole mass spectrometer (TG-MS). The results were discussed from a viewpoint to develop a novel synthesis method for AlN ceramics through the polymer precursor route.