Bookham Claims New Fab Technique

KANATA, Ontario -- Bookham Technology plc (LSE: BHM, NASDAQ: BKHM), a leading supplier of communications lasers, will present a paper at ECOC 2003 describing a novel method of fabricating ridge-waveguide DFB lasers with only a single growth stage. Decreasing the number of growth stages and associated processes promises significant cost reductions and higher yields, especially for chips integrating multiple lasing wavelengths, as in DFB laser arrays, or several optical functions, as in integrated laser modulators.

The fabricated corrugated-ridge DFB lasers show very good single-mode behaviour, with a side-mode suppression ratio (SMSR) of over 55dB and linewidths of less than 1MHz up to injection currents of 700mA. They offer high-power operation of more than 100mW of optical power at 500mA injection current. This is the highest power demonstrated to date for a single-growth DFB laser.

“Most single-frequency lasers, like distributed feedback lasers (DFB) or multisection distributed Bragg reflector (DBR) lasers, used in transmitters require complicated multiple growths and processing steps for fabrication. This usually drives down yields and lengthens cycle times, which means higher costs,” says Benoit Reid and the paper’s lead author. “There is now a lot of pressure to decrease the cost of optical components installed into optical communication systems. So, ideally, you want to minimise the number of growth and processing steps. For chips with multiple single-frequency laser sources, such as laser arrays, or with more than one optical function, such as a laser integrated with a modulator, you really do want simpler fabrication technology.”

Single-growth DFB lasers have already been demonstrated, but most of the designs are based on e-beam lithography, which can be a slow and expensive technique. Also, many proposals require very different approaches, using metallic gratings or very deep grating etches, which are not supported by extensive historic data and therefore raise issues of performance and reliability.

The paper presents Bookham’s initial fabrication of DFB lasers with no overgrowth. The design goal was to develop a process flow close to that of a simple ridge-waveguide process using stepper photolithography.

“To achieve this, we have designed DFB lasers with third-order gratings written on the ridge sidewalls, forming a corrugated-ridge DFB laser,” says Reid. “Defining the gratings on the sidewalls gives a lot of flexibility in the laser design; one can easily vary the grating pitch and strength along the cavity to achieve a given performance.”

The devices use pre-existing epiwafers for 14xy pump lasers and so their structure was not optimised for the design. The structure is based on a nominally undoped active region of four 55Å-thick and 1.3% compressively strained InGaAsP quantum wells separated by 100Å-thick InGaAsP barriers. The grating strength was targeted to minimise spatial hole burning.

For corrugated ridge fabrication, an I-line stepper was used for photolithography and patterns were transferred to an SiO2 hard-mask layer. The corrugated ridge waveguide was formed by etching the semiconductor in an induction-coupled plasma. Both HBr and Cl2/N2 etch chemistries were explored. Conformal p-contact metallization and full wafer back end processing completed device fabrication.

To demonstrate the potential of the approach, the paper also describes devices fabricated with two different grating pitches on the ridge sidewalls. The lasers emit light at one of two wavelengths, depending on the drive current and the temperature. This behaviour could be used to achieve wider wavelength tunability than that of a conventional temperature-tuned DFB laser.

Says Reid: “With conventional approaches it would be challenging — and perhaps impossible — to produce such effects. Having the gratings on the sidewalls, and also the associated flexibility, can open up the way to new types of devices or new functionality on a single chip.”

Bookham Technology plc