Anadigics gets in trim with laser patent
US-based RFIC manufacturer Anadigics has been awarded 14 new US patents, including patent 6,664,500 describing laser trimming or severing of the resistive links fabricated on a GaAs substrate.
The technique can cleanly process a thin-film resistor fabricated on an undoped GaAs substrate without damaging adjacent circuits. It can also be used to trim thin-film transistors, enabling, for example, bias current adjustments to a power amplifier to tune its quiescent current (the current that flows when no signal is applied to the circuit).
Lasing trimming of resistors on silicon substrates has already been successfully employed, but Anadigics believes no-one has transferred the technique to GaAs.
Anadigics method ideally recommends using a laser with emission at 1047nm, although satisfactory results may be obtained with wavelengths between 900nm and 1500nm. Laser power is also critical - if the power is too high, GaAs can be damaged - and energies of 0.25-0.45µJ are advised.
Details of the thin-film resistor layers are also described: on a 500µm undoped GaAs substrate should be deposited 84nm of a protective dielectric, then a 43nm layer of NiCR, and then preferably a second dielectric layer, 94nm thick.
According to Anadigics, only one other patent, number 5,569,398, mentioned laser trimming of resistors on GaAs substrates. This work suggested that a wider window of laser wavelengths from 1.0 µm - 3.0µm was suitable, but workers at Anadigics have revised these figures.
Other patents awarded to Anadigics include:
• 6,759,922, which describes an RF power amplifier coupler with a length considerably less than that of a quarter wavelength coupler, which achieves directivity requirements for both the GSM and DCS/PCS bands.
• 6,645,790 granted for a new methodology that reduces the time required to bring new RF and microwave devices to market by enabling designers to selectively couple and uncouple device blocks late in the design process.
• 6,559,722 granted for a new bias circuit that improves cellular handset battery life by operating near Class B mode while the driver signal level is low.
