What Is an AlN Submount?

A definition of the aluminum nitride laser-diode submount: what it does, why 170–210 W/m·K and a 4.3–4.6 ppm/°C CTE fit InP and GaAs, and when to move to SiC.

What an AlN submount does

An AlN submount is the ceramic piece between a laser diode and the metal header or butterfly floor. Aluminum nitride is the usual material for InP and GaAs lasers because it removes heat and expands at nearly the same rate as the die.

Thermal spreading. Polycrystalline AlN conducts 170–210 W/m·K, about 6–8 times standard alumina, so a 200–500 mA CW laser stays inside its junction-temperature limit.
Mechanical match. CTE of 4.3–4.6 ppm/°C sits next to InP (~4.5 ppm/°C) and GaAs (~5.7 ppm/°C), which limits stress when the module cycles.
Electrical routing. The ceramic is an insulator. Ti/Pt/Au or AuSn pads carry the die bond and the wire bonds out to the package pins.

AlN compared with other submount materials

Material	Thermal conductivity	CTE	Typical use
AlN (FerraLink)	170–210 W/m·K	4.3–4.6 ppm/°C	InP/GaAs DFB, FP, and datacom lasers
Single-crystal SiC	350–400 W/m·K	3.7–4.3 ppm/°C	Pulsed lidar, laser bars, GaN RF
Alumina	26–30 W/m·K	Higher than AlN	Low-power, cost-sensitive builds

AlN is polycrystalline. Grain boundaries scatter heat, which is why it sits below single-crystal SiC. For most telecom lasers that margin is enough, and the CTE match is the reason teams stay on AlN. The broader definition is in what a laser diode submount is , and the side-by-side choice is in AlN vs SiC.

Catalog sizes

FerraLink AlN samples start at $25 and ship in 2–4 weeks. A common footprint is FL-ALN-035455-001 at 3.5 × 4.55 × 0.30 mm with Ti/Pt/Au and AuSn. Smaller squares run from FL-ALN-005 (0.5 × 0.5 mm) through FL-ALN-020 (2.0 × 2.0 mm). A 10-piece box is $250 and includes a material certificate and lot traceability.

Specs and the sample form are on the AlN submount page. If the die is a pulsed lidar emitter or a GaN device, read what a SiC submount is before locking the material.

The part that depends on your die

The rules above hold for most edge-emitter modules. What changes from program to program is geometry, duty cycle, and how hard you are pushing junction temperature — those inputs decide material, thickness, and whether catalog samples are enough.

Footprint, thickness, and solder pad art for your specific die.
Reliability vs your duty cycle and cycling profile.
A short internal-review memo your team can sign off before prototyping.

Go deeper — Pick material

These guides answer adjacent questions teams ask while choosing a submount. Each ends the same way: what you can decide in general, then what needs your die and power.

More topics coming — thermal path, attach yield, qualification, and packaging context.

Frequently asked questions

What is an AlN submount?

A polycrystalline aluminum nitride ceramic that sits between a laser diode and the package. It spreads heat, provides a CTE match to InP and GaAs, and gives metallized pads for die attach and wire bonds.

What is the thermal conductivity of an AlN submount?

FerraLink polycrystalline AlN submounts are 170–210 W/m·K, about 6–8 times standard alumina and below single-crystal SiC at 350–400 W/m·K.

When should I choose AlN instead of SiC?

Start with AlN for InP and GaAs telecom and datacom lasers, where CTE match matters as much as heat. Move to SiC for pulsed lidar, laser bars, and GaN RF when the AlN thermal margin is not enough.

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