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).

March 2, 2012 — In Part 2, 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.

Furnace considerations

An AlN formulation that sinters below 1700°C enables new furnace options versus higher-temp materials. At 1700°C or below, a continuous tunnel kiln can be utilized. This furnace runs in a N₂ atmosphere with a small amount of H₂ present to protect the heating elements from oxidation. The heat shields are constructed of alumina and the heaters of molybdenum. The substrates are stacked on alumina plates, which are continuously pushed through the furnace at a rate of travel determined by the length of the hot zone and the required time at sintering temperature (about 3-5 hours). The longer the hot zone, the higher the sintering throughput. Since a continuous furnace runs in steady state, no heat up/cool down times are needed, key limitations in batch processing.

Table 4. A comparison of a typical batch furnace for sintering high-temperature AlN with the continuous furnace used to sinter low-temperature AlN.

Furnace throughput comparison

The goal of this analysis is to compare the throughput of a batch furnace and continuous furnace with approximately the same capital equipment cost.

Key assumptions:

• Both furnaces have a capital cost of approximately $500,000
• Batch furnace hot zone dimensions: 8” x 8” x 20”
• For batch firing, assume that 80% of the hot zone is usable for the high temperature firing process. This would be typical. The very top and bottom of the hot zone are too hot/cold to obtain the optimum microstructure/density.
• Continuous furnace opening dimensions of 8” x 8”
• Continuous furnace hot zone length of 36” with adjacent zones heated to achieve a uniform hot zone temperature
• Fired substrates 4.5” x 4.5” x 20 mils
• Kiln furniture the same for both furnaces
• Stack of 5 substrates separated by coarse powder on top of a setter forms the basic stacking unit
• Batch furnace has a loader arrangement so that stacking time is not included in the total  cycle time.

Figure 6. A commercial HTCC furnace (Model 4612-3Z Automated).

Using these assumptions, the throughput in fired substrates per hour is:

• 225 substrates per hour for the continuous furnace
• 22 substrates per hour for the batch furnace (432 substrates per batch, 20 hours per run)
• Continuous furnace throughput for the same capital expense is 10x higher. The same type of relative throughput enhancement will be achieved for flat firing.

Conclusion

In Part 1, the 5 major cost factors for AlN substrates (compared to Al₂O₃) were discussed: (1) higher cost powder; (2) separate BBO cycle; (3) batch sintering cycle; (4) batch flat fire cycle and (5) non-aqueous processing. By adopting a low-temperature sintering configuration, cost factors 4 and 5 are addressed, bringing the sintering and flat-firing operations in line with the process for alumina.

This process will only be appropriate for applications where a thermal conductivity of 130W/m-K is acceptable, which includes most HBLED, RF, and power semiconductor devices. The same advantages of AlN as a substrate material in HB-LED applications are also key in discrete power semiconductor packaging and in packaging for highly concentrated photovoltaics (HCPV) applications.  For laser diode telecommunications applications, 130W/m-K will most likely be too low and conventional higher-cost AlN will continue to be utilized.

The availability of a low temperature, continuous sintering process also provides strong motivation for the next phase of cost reduction for AlN, utilization of lower-cost/lower-performance AlN powder. Again, with a focus on HB-LED and power semiconductor applications, sensitivity to impurities such as iron (Fe) and silicon (Si), which drive up AlN powder costs, may not be anywhere as stringent as applications such as RF and microwave (where dielectric properties at high frequencies are important). The combination of lower cost powder and a continuous sintering process would move AlN substrate pricing much more in line with alumina.

The major limiting factor for widespread utilization of AlN ceramics in these applications — the cost barrier compared to alumina — is addressed by this new sintering technology. It takes into account 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. With the exception of a lower thermal conductivity, the properties of traditional high-cost materials and the HB-LED-grade AlN are very similar.

Read the series from the start with Part 1 on HB-LED-grade AlN vs other materials here.

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.