Avago has introduced the world's smallest packaged RF amplifiers - 1x0.5x0.25mm - using a wafer-scale technique.
"The process is the critical thing," applications manager Alan Rixon told Electronics Weekly. "It is being able to cap a wafer with a wafer to produce a hermetic package."
For the RF amplifiers, the firm is using a GaAs wafer to cap a GaAs wafer in what is essentially an extension of its FBAR process for silicon filters.
"We extended it to GaAs which is a somewhat more brittle material so we had to develop special techniques," said Rixon.
The wafer scale hermetic package allows air-bridges and other low loss structures to be incorporated
The packaged ICs are made by forming RF circuits on one wafer, and etching corresponding rectangular hollows it in the other.
When aligned and stuck - using a proprietary bonding technique - the hollows line-up with the circuits forming air-filled hermetic cavities.
At 1x0.5x0.25, these 0402 RF amplifiers are claimed to be the world's smallest.
The 0.25µm RF process makes use of air bridges and other air-spaced elements including couplers.
"With air circulating above the circuit area of the die, losses and noise are lower than with plastic encapsulation," said Rixon.
The capping layer strengthens the structure, allowing the bottom wafer to be thinned more than is possible with self-supporting GaAs wafers.
All the family have three contacts. Alongside ground there is a single output/power pad, and an input/control pad.
Connections to the outside world are made through full-depth vias in the bottom wafer, filled with plating to retain the hermetic seal. These connect to gold contact areas plated on the bottom of the package. Over-plating with another metal makes the package compatible with Pb-free processing, said Rixon.
Avago estimates the package is good up to 100GHz, and actually quietly introduced it a month or two ago housing the VMMK-12xx family of single FETs which are now shipping.
Future work includes the search for alternative capping wafers. The GaAs cap is a low risk thermal match to the GaAs substrate, but hardly a low-cost material.
Packages larger than the current 0402 size are also planed, with more than the three contacts possible in the limited 0.125mm2 bottom area. "You could put a lot of functionality into a larger package," said Rixon.
The thin bottom layer also conducts heat well, said Rixon, and power devices are also under development.
With three pins available, the amplifiers are powered through their outputs - through an inductor or resistor to Vcc - Avago's once unique enhancement-mode GaAs allows for single rail operation.
Where necessary, for control of bypass or shutdown, a DC connection is made to the input pin in addition to the AC coupled signal.
Dubbed the VMMK-2x03 family, the devices are sampling now and due to be available properly in the last quarter of this year.
The 1x0.5x0.12mm GaAs amplifier family at a glance
- VMMK-2103 0.5-6GHz LNA with bypass and shutdown mode
- VMMK-2203 1-10GHz LNA with 15dB gain at 5GHz
- VMMK-2303 0.5-6GHz 1.8-3.3V LNA
- VMMK-2403 2-4GHz high linearity with 2.5dB NF
- VMMK-2503 1-12GHz high linearity flat to 8GHz
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