Aluminum Nitride Replacing Beryllium Oxide in Substrates

Safety concerns have led to aluminum nitride being used as a substitute for beryllium oxide in high-power electronics.

High-power microelectronic devices and applications have always needed materials with a high thermal conductivity. Beryllium oxide fitted this need for the last few decades and became the electronic sector’s most desirable material for use in substrates. Its main applications have been in RF or high frequency resistors as well as terminators.

However, it has since been discovered that beryllium oxide particles are toxic when inhaled. They can inflame the lungs, causing berylliosis. Conscious of similar experiences with other toxic materials such as asbestos, health regulators worldwide have been issuing stricter regulations. As a result, manufacturers have been limiting the use of beryllium oxide in ceramic substrates and replacing it with aluminum nitride.

It’s a bit of a return to the future for the microelectronics sector. Aluminum nitride has been used as a substrate since the 1960s. This compound has a thermal conductivity far higher than alumina or aluminum oxide and a dielectric constant that was much the same. But manufacturers lumped it together with alumina without realising its full potential.

The main advantage for Alumina is that it is a safe material to handle and is non-toxic. Its thermal conductivity is just short of that of beryllium nitride, while its dielectric constant at 8.9 is higher and can’t replace the latter in 100% of applications. This has caused problems for some circuit designers, as it will cause a higher shunt capacitance. So designers have had to devise new rules to overcome the problems of additional capacitance.

Another problem has been in thick film paste systems – materials used to bind electronic components to substrates. The thick film paste systems that have been used for beryllium oxide and alumina are unsuitable for aluminum nitride, as when this compound comes into contact with water at high temperatures, it will decompose into aluminum hydroxide. It also tends to react with any cleaning solvents that contain ammonia and may become unreliable when exposed to any moisture at temperatures above 100 degrees Celsius.

So circuit designers have had to develop new highly controlled and compatible processes that involved thin as well as thick films. This has included protecting the film elements from any moisture or damage when they are being handled during the processing procedure.

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