ALUMINUM NITRIDE VERSUS BERYLLIUM OXIDE FOR MICROWAVE APPLICATIONS

Component General, Inc. manufactures a product line of aluminum nitride thick film terminations and resistors. These devices are currently illustrated on our website. The European communities currently do not wish to use beryllium oxide in their products and this has provided the need for aluminum nitride devices.

For many decades, beryllium oxide has been the ceramic of choice when high temperature heat sinking of microwave devices was needed. In addition, beryllium oxide has a very favorable dielectric constant ideal for microwave resistive products. Many countries believe that BeO (beryllium oxide) is dangerous. Generally speaking, BeO is not dangerous unless it is abraded or in some way turned into powder that can be inhaled. As a result of this BeO shortcoming, the electronics industry has been searching for a viable alternative. Aluminum nitride boasts a thermal conductivity far superior to alumina with a dielectric constant nearly matching alumina (aluminum oxide). Subsequently, it has been named as a viable replacement in many applications. Aluminum nitride does not, however, serve as a complete and direct replacement for BeO since its thermal conductivity is only about 63% of BeO. Aluminum nitride is non-toxic, however, and fully suitable for resistors, terminations, and attenuators. Generally, the design rules for BeO do not apply to AlN (aluminum nitride). Due to the difference in dielectric constant, the AlN designs will often require a more extensive matching network. This is due to the increased capacitance as a result of the dielectric constant. It was almost 1996 before the full properties of AlN were realized and utilized.

Standard thick film pastes used for BeO (oxide chemistries) do not work well with aluminum nitride ceramics. It was some time before paste manufacturers could formulate the proper pastes for AlN that would provide consistent adhesion and long MTBF’s. The key to a successful thick film paste system centers on the long-term adhesion reliability of the fired thick film onto the ceramic surface. Surface preparation of the AlN ceramic is very important to achieve good adhesion. Cleaning after lapping is essential in order to remove any residue from the lapping that might interfere with the adhesion. AlN is typically a very reactive surface which reacts with water and acids/bases in a negative way.

These agents typically etch the surface making it more porous. Consequently, adhesion suffers as a result. To date, only IPA alcohol has proven to be an adequate cleaning agent.

It is interesting to note that thin films can readily be used with AlN. The surface must be polished and clean in order to accept the thin film but adhesion can be greatly enhanced by the use of sputtered MolyMag as an adherence layer. Subsequent layers are then “plated up” to achieve the final solderable metallization. AlN will accept both nickel/chromium and tantalum nitride resistive thin films. Component General, Inc. will be producing nickel/chromium thin film products on AlN.

A property chart comparison of BeO, AlN and Alumina is shown below. Component General, Inc. utilizes all of these materials.

PROPERTY                                                                         AlN                       BeO                 Al2O3          

Dielectric Constant   (1 MHz.)                                  8.9                         6.7                       9.8

Dielectric Loss  (1 MHz.)                                             0.0001                 0.0003             0.0002

Electrical Resistivity (Ohm-cm)                              <10E14                <10E14              <10E14

Thermal Conductivity    (W/m.K)                            170-190                260                      36

Coefficient of Thermal Expansion (ppm/C)       4.6                          8.5                       8.2

Density (g/cmE3)                                                                3.30                       3.85                    2.89

Bending Strength (m/Pa)                                               290                         230                      380

Hardness (knoop) (GPa)                                                11.8                         9.8                      14.1

Young’s modulus (GPa)                                                   331                          345                      372                                                                            

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