In this 2-part series, Part 1 describes aluminum nitride (AlN) and what it accomplishes as a ceramic substrate for high-brightness light emitting diodes (HB-LEDs). Part 2 provides analysis of the impact of this new technology on sintering throughput.

February 24, 2012 — HB-LED packaging requirements push the materials envelope for low cost and high thermal performance. As manufacturers look to shrink LED size, the substrate is required to dissipate more heat. The commercial imperative to decrease the $/Watt figure of merit for light output is also increasing interest in low-cost substrates.

HB-LED devices are bonded to a ceramic tile, comprising a ceramic substrate metallized with thick-plated copper (Cu), with Cu-filled via interconnections to the printed circuit board (PCB). Heat conduction from the active device occurs through both the Cu vias and the ceramic. The ceramic material provides electrical isolation between the different polarity inputs that drive the LED.

Traditionally, 96% Al₂O₃ is used as the ceramic substrate in HB-LED applications because of its low cost and good mechanical stability. However, with a thermal conductivity of only 20W/m-K, alumina does not contribute significantly to heat transport in the tiles. This brings in the opportunity for using other ceramic materials with higher thermal performance such as AlN or Si₃N₄. Both of these alternatives cost more than alumina.

Aluminum Nitride

Aluminum nitride (AlN) is a polycrystalline, high melting temperature (refractory), ceramic material with an advantageous set of properties for die-level packaging of HB-LEDs and power semiconductors: good electrical insulation, high thermal conductivity, high flexural strength, stable in high temperatures, and ease of fabrication (laser drilled, metallized, plated and brazed).

Table 1. AlN properties.

As power densities of semiconductor devices increase, the need for thermal dissipation from packaging, particularly for temperature-sensitive devices such as LEDs. AlN has a thermal conductivity that is 8-9x higher than competitive materials such as Al₂O₃. It offers an excellent answer to increasing thermal demands on first-level packaging materials.

Also read: LED packaging report reveals costs, reliability impact of package

Applications with high and increasing thermal demand include: radio frequency (RF) power components for cellular infrastructure, HB-LED, power semiconductors for motor control, packaging for concentrated photovoltaic (CPV) installations, and packaging for semiconductor lasers used in telecommunications.

AlN ceramic substrates are typically made 15 to 60 mils thick, and up to 4.5” square (larger for some specialized applications). These substrates are fabricated using conventional ceramic processing technology.

Table 2. A typical fabrication sequence.

AlN has a range of beneficial properties for high-thermal-demand applications. However, the cost of AlN has limited its utilization. Typically, AlN costs 5-7x more than lower-performance alumina on a cost/square inch basis.

Key contributors to this higher cost structure:

• Currently available AlN powder is approximately 20x more expensive than alumina powder of comparable quality (purity, particle size).
• AlN tape must be fired in a non-oxidizing atmosphere. This means that binder removal, which is typically done through oxidation, must be done in a separate furnacing step (at a temperature well below the sintering temperature). A thick film continuous furnace can be used. For alumina, binder removal can be accomplished in the sintering furnace in one furnace step.
• AlN is sintered in a batch furnace with much lower throughput than continuous furnaces used for alumina. In addition, these batch furnaces are constructed using Mo and W metal heat shields and heating elements  because of the extremely high sintering temperatures (>1800C), so the overall furnace cost is very high.
• AlN can also be sintered in graphite batch furnaces. Though lower capital cost than W furnaces, the sintering fixtures for this type of furnace are very high cost and the throughput is still low due to batch processing. Also, the interaction of AlN with the carbon containing atmosphere is a graphite furnace must be limited to produce high quality product.
• The considerations of furnace cost and low throughput for sintering are also a factor for flat fire, so there is essentially a “double hit” for using batch processing.
• Alumina can be processed in an aqueous environment. This makes the tape fabrication less expensive than the AlN process which must utilize non-aqueous solvents. This is a significant factor for tape casting.

HB-LED-grade AlN

CMC Laboratories Inc. developed a new material that addresses the lower-throughput batch sintering of AlN, higher-cost graphite batch sintering fixtures, and “double” firing costs. This new technology allows AlN to be sintered at lower temperatures in a continuous furnace very similar to furnaces used for alumina.

Table 2. Key properties for the low-temperature-sintered, lower-cost HB-LED Grade AlN compared to the standard, high temperature sintered, higher cost AlN material that is currently commercially available.

All of the properties are very similar, except that the thermal conductivity of the HB-LED grade material is about 24% lower than the high-cost AlN, but still 6+ times higher than alumina. This makes the HBLED grade material suitable for all but the highest thermal demand applications for AlN.

HBLED grade AlN is made with the same basic processing steps outlined in Table 2 that are used for the high-temperature material. The key difference is the sintering additives that allow the material to densify at 1675°-1690°C as compared to the conventional 1820°-1835°C. Tape binder formulations, tape casting conditions, and the binder burn out process are also the same as, or very similar, to conventional AlN material.

Figure 1 shows a 4.5” x 4.5” x 20 mils substrate made from HBLED grade material that was fired at 1690°C in a nitrogen gas atmosphere with a hold time at sintering temperature of 3 hours.

Figure 1. Low-temperature sintered AlN substrate.

Sintering aids for AlN ceramics form a liquid phase at the sintering temperature that increases the rate of densification and they getter oxygen from the AlN grains during sintering. Since the oxygen content of the AlN grains controls AlN’s thermal conductivity, effective oxygen gettering is key to achieving the highest possible thermal performance. A plot of thermal resistivity vs. oxygen content is shown in Figure 2 [1].

Typical sintering aids for AlN are rare earth oxides with a large chemical driving force for reaction with oxygen in the AlN grains. For the conventional high temperature system, Y₂O₃ is added to the AlN. At high temperatures during sintering, the added Y₂O₃ reacts with oxygen from the AlN grains in the form of Al₂O₃ to form various Y-Al-O phases. The Y₂O₃– Al₂O₃ pseudo-binary phase diagram, which is key to understanding the conventional high temperature sintering process for AlN, is shown in Figure 3 [2]. As is evident from the phase diagram, there is a eutectic in the Y₂O₃– Al₂O₃ system at 20% Al₂O₃ which melts at 1780°C. This is the initial liquid phase that promotes sintering. This liquid phase reacts with the added Y₂O₃ to form more yttria-rich Y-Al-O compounds, which in turn reacts with oxygen from the AlN grains. This reaction shifts the composition toward more Al₂O₃ rich compositions as oxygen is gettered from the grains. For conventional high temperature sintered AlN, the final second phase composition after the sintering process is complete is composed of YAP (Al₂O₃:Y₂O₃), YAM (2Y₂O₃:Al₂O₃) or a combination of YAP and YAM.

Figure 2. Oxygen vs. thermal resistivity of AlN.

To summarize, there are two reasons for the high sintering temperature. First, the temperature must be high enough to melt the additive phase to form a liquid which enhances the rate of sintering by orders or magnitude. Second, the temperature must be high enough so that oxygen can diffuse out of the AlN grains during sintering to enhance the thermal conductivity of the AlN ceramic.

There is a third critical requirement for the additive phase during AlN sintering. While a liquid, the Y-Al-O phase will completely surround each AlN grain. If we define a wetting angle between the AlN and Y-Al-O measured at the 3 grain junctions, the microstructure has a very low wetting angle that is less than 60°C. This type of microstructure is shown in the SEM micrograph in Figure 4A. The dark grains in this figure, which are about 10µm large, are the AlN. The bright phase is the Y-Al-O.

Figure 3. Y2O3– Al2O3 pseudo-binary phase diagram.

There are two critical performance issues with a wetted microstructure. First, because AlN fracture is inter-granular, the presence of a Y-Al-O phase between the grains lowers the tensile strength of the ceramic by a large factor. The second problem is that a wetted microstructure results in Y-Al-O covering large portions of the surface of the substrate. This reduces the consistency of AlN metallization processes.

Figure 4A. Wetted microstructure- High-temperature AlN.

Figure 4B. De-wetted microstructure- High-temperature AlN.

A key requirement for the oxide second phase during AlN sintering is that the oxide phase de-wet the ceramic grains during the later stages of the sintering process so that the final microstructure will have a de-wetted Y-Al-O phase as shown in the micrograph in Figure 4B.

Figure 5. Microstructure sintered at 1675C (and zoom on a particular spot).

These same basic considerations for sintering of high temperature, conventional AlN are relevant to designing a low temperature sintering process:

• The sintering additive must melt at the sintering temperature to facilitate liquid phase sintering kinetics.
• The temperature must be high enough for oxygen to diffuse out of the AlN grains during sintering. This consideration puts somewhat of a lower limit on how low AlN can be sintered to produce high thermal conductivity.
• The liquid phase must de-wet from the AlN grains after densification to form a de-wetted microstructure and thus high flexural strength.
• This de-wetting is also required to produce ceramic with high electrical resistivity

Figure 5 shows the microstructure of a low temperature formulation that was fired at 1675°C. This has a modified sintering additive package that will melt at much lower temperature than the conventional Y-Al-O additives, but still has a strong chemical driving force to getter oxygen from the AlN grains.

As in the previous micrographs, the dark grey areas are the AlN ceramic grains, about 3-5µm in size, and the bright areas are the oxide sintering additive phase. The difference in color between the micrographs in Figure 4 and Figure 5 are due to imaging conditions, not material differences.

In Part 2 here, the furnace considerations are discussed, as well as furnace throughput. It covers the role of the oxide sintering phase in AlN in defining the materials microstructure and thus determining key properties such as thermal conductivity and mechanical strength.

Jonathan Harris, PhD is president of CMC Laboratories Inc., www.cmclaboratories.com.

References:

[1] J.H. Harris, R.A. Youngman and R.G. Teller, J. Mater. Res. 5, 1763 (1990)

[2] J. McCauley, and N. Corbin, High Temperature Reactions and Microstructures in the Al2O3-AlN System, Progress in Nitrogen Ceramics, ed. F.L. Rley, Martinus Nijhoff Pub., The Netherlands, 111- 118 © 1983.