AlN laser diode carriers
Thin film metallised Aluminium Nitride (AlN) submounts/carriers for mounting of laser diodes and associated components and devices for optical sub-assemblies.
Ceramic material
The ceramic substrate material is Aluminium Nitride (AlN). Standard grade is 170W/m·K. 200W/m·K is also available. Typical properties are:
Thickness, flatness and surface finish
The AlN substrate material is available with as-fired, lapped or polished surface finishes. Standard substrate thicknesses are 0.63mm and 1.0mm with as-fired finish. Non-standard thickness material can also be fabricated. Substrates can be lapped and polished from 0.25mm thick up to 1.5mm thick.
The flatness and surface finish of the substrate material affects conductor definition and adhesion and also the quality of the laser die attachment. Typical substrate properties are:
| Surface | Thickness | Flatness | Surface Finish | Notes |
|---|---|---|---|---|
| As-fired | ±15% | 50 µm/25 mm | ~1.6 µm Ra | Lowest cost |
| Lapped | ±0.02 mm | 5 µm/25 mm | ~1.0 µm Ra | Better thickness and flatness control but poor surface integrity |
| Near polish | “ | “ | ~0.15 µm Ra | Best for conductor and AuSn adhesion |
| Polished | “ | “ | <0.05 µm Ra | Best for conductor definition and minimising epoxy bleedout |
Mechanical dimensions
Small mounts are commonly used for individual chip-on-carrier assemblies, and large substrates with complex patterning for optical sub-assemblies. General size limits are:
Parts can be machined using diamond saws for the highest edge quality and dimensional control. Laser profiling/scribing are used for complex shapes, holes, etc. Typical tolerances available:
| Diamond sawing: | Typical | High |
|---|---|---|
| Length and width | ±0.05 mm | ±0.02 mm |
| Edge chipping | <0.05 mm | <0.02 mm |
| Pattern to edge | ±0.05 mm | ±0.01 mm |
| Laser Profiling: | ||
| Holes | ±0.05 mm | ±0.025 mm |
| Pattern to edge/feature | ±0.05 mm | ±0.025 mm |
Metallisations
Various metallisation schemes can be applied to form conductors, integral resistors, solder barriers, solder dams etc. Typical schemes include:
| Conductors | Notes |
|---|---|
| Ti/Pt/Au | Best overall conductor scheme but highest cost |
| TiW/Pd/Au (+ Au) | Pd barrier layer (plus electroplated Au) for standard low cost scheme |
| TiW/Au (+ Ni/Au) | Ni barrier layer for extra protection when soldering with Sn and Pb based solders |
| TiW/Au (+ Au) | For high temperature soldering. Suitable for Au bearing solders only |
| Resistors | |
| Ta2N or NiCr | Typical sheet resistivity of 50 ohms per sq, with a standard tolerance of 20% or 0.2% if laser trimmed |
| Dielectrics | |
| Polyimide | For conductor cross overs, solder dams and passivation protection |
The Au and barrier layer thicknesses depend on the application. Generally for Au wire bonding 0.5µm minimum of Au is required. For Pt or Pd barrier layers a minimum of 0.2µm is usually specified.
Plated thru-holes (substrate vias)
Front and back faces of the component can be electrically interconnected using pre-drilled metallised thru-holes. Typical hole diameter = substrate thickness.
Wrap-around metallisation
Plain submounts can also be fabricated with metallisation that wraps around one or multiple sides walls to connect the top and bottom faces. Wrap around connections can also be accomplished on patterned mounts, with due consideration to process limitations and cost.
Patterning
Circuit features are realised using wet or dry film resists and chemical or plasma etching. Typical capabilities:
Alignment guides/fiducials can be incorporated into the design for subsequent automated assembly.
Pre-deposited AuSn
For laser mounting a thin layer of vacuum-deposited AuSn can be selectively applied over conductor areas. The standard alloy ratio is nominally 76Au/24Sn, normally at ~4-5 microns thick. On reflowing, the Au in the underlying conductor and on the underside of the laser chip, combine with the pre-deposited solder to form an alloy approximating to the eutectic ratio of 80Au/20Sn. The resulting alloy flows and wets very well to both the laser and the carrier, resulting in excellent low-void joints minimising thermal resistance and hotspots. The alloy ratio can also be tailored for specific applications.
Typical AuSn features:
For further details please contact us.