BİLKENT UNIVERSITY NANOTECHNOLOGY RESEARCH CENTER

HIGH PERFORMANCE NEAR-INFRARED SEMICONDUCTOR

PHOTODETECTORS and LASERS

FABRICATION AND CHARACTERIZATION OF HIGH POWER LASER DIODES

In Bilkent University Nanotechnology Research Center, microfabrication and characterization of AlGaAs high power laser diodes is another branch of research. Our research center is capable of doing all the fabrication and characterization of AlGaAs lasers in the form of a single diode to a stack of arrays.

1.Fabrication

For the microfabrication we use a resist spinner, mask aligner, reactive ion etching system, box coater, surface profilometer, plasma enhanced chemical vapor deposition system and a rapid thermal annealing system.

The fabrication starts with a regular sample cleaning operation. Following that we coat the samples with 1.4 µm thick resist. The patterns for the active areas that will lase is printed using Karl Süss MJB3 mask aligner system. The first step of the fabrication depends on the structure of the wafer . If there is a layer that must be removed we first etch the surface of the sample using reactive ion etching system. After that we print different size of diodes on _our sample. The laser diode masks are also designed in our facility. On these masks we have 50 µm, 75 µm, 100 µm, 150 µm and 200 µm devices with different spacing. After the exposure a developer solution is used to form the shapes on the sample. The first step of fabrication goes on with depositing contact metals of lasers using Leybold high vacuum box coater system. The metal deposited on unwanted areas is removed with a lift off process. The contacts are formed after annealing the sample. In the second step of fabrication image reversal of the same mask is printed on the sample protecting the deposited metals. Clear areas are etched using dry etch technique. In fact removing only the cap layer of the structure is enough during etching but we also try different regions during the fabrication. The third step is the most critical step of a laser diode fabrication. In this step we thin our 350 µm _samples down to 100 µm thickness to cleave them easily. We use two different wet etching solutions to make the rough and fine polishing of the _backside of our wafer. The thickness of the resulting sample and the roughness of the backside are observed. If this is fine enough we can move to the _next step. In the fourth step first backside metallization and contact formation by annealing is done. The fifth step is coating the surface of the sample by _a dielectric for isolation except the active areas. We use PECVD system to coat Si3N4 on the surface. After that we deposit thick films to both back _and front side of the sample. Now our sample is ready to be cleaved, packaged and measured. Below you can see SEM images taken at different _steps of fabrication of laser diodes.

Figure 1 SEM image of laser diode taken after first metallization and etching

Figure 2 SEM image of laser diode taken after dielectric coating of sampl

Figure 3 SEM image of laser diode taken after thick metallization on the surface

After the process, the device fabrication goes on with cleavage of the samples for a suitable package. For single diodes we cut our samples in 1mmx1mm size which will cover a single diode with the suitable process mask. We also cut samples for arrays and stacks. For example in a 1 cmx1mm dimension sample we have 40 of 50 µm laser diodes. To form a stack we put the same size, same type bars on top of each other and make the necessary bondings

Figure 4 Packaged single laser diode and array

2.Characterization

After preparing a full package of diodes we perform all kinds of measurements from the diode spectrum to a lifetime test in our facility. For these measurements we use optical spectrum analyzer, advanced laser diode drivers and temperature controllers, integrating sphere for light collection, related detectors, scopes and some software to collect and analyze the data.

First we couple the light into a fiber connected to the optical spectrum analyzer to define the spectrum of our diodes. After that we try to define the change in the spectrum of diode with changing current level and temperature. We can drive the laser diodes in CW and QCW mode. We get optical power versus biased current graphs. Then we calculate the efficiency, quantum efficiency, FWHM, threshold current, threshold current density. Now we are capable of reaching several watts from a single array.

Figure 5 Laser light from a single diode and a laser diode array

Figure 6 Laser light from a 5 diode array

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