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A story of successful developments: Optimized dry etching process

The importance of obtaining clean surfaces of semiconductor materials and devices at different stages during the manufacturing process is well known for having an important impact to the quality of the subsequent layers overgrown or deposited on top of those.

This is not different for our Kontrox™ technology, that aims to improve the quality of the surfaces/ interfaces of III-V materials, by creating a protective and passivating layer on the cleaned surface of the compound semiconductor material, having as one of its main characteristics, the lowest defect states density achievable in the market.

One important focus area for our team is the optimization of the cleaning and surface preparation steps so that the resulting surface offers the optimal condition for the implementation of our Kontrox™ passivation process. An interesting and challenging area of application where the cleaning step is particularly important is in the passivation of facets of side edge emitting lasers.

Edge-emitting laser diodes emit very high optical powers from small emission areas, therefore, the optical intensity at the front facet is extremely high. A lot of heat is created in the front facet, where in addition to normal operation, a lot of photons are absorbed by different defects including dangling bonds, threading dislocations, amorphous native oxides, or in short high level of surface states defects. This absorption leads to heating of the semiconductor which shrinks the LD bandgap, which results in an ever increased absorption and a further shrinkage of the bandgap. Such a thermal runaway process ultimately melts the semiconductor in what is known as Catastrophic Optical Damage, or COD. Increasing the power threshold at which the COD happens is an ongoing target for almost all laser manufacturers.

Some processes consist on performing the cleaving of the laser bars in UHV conditions followed by a passivating protection, i.e. ZnSe layer, prior the deposition of the mirror layers. One of the disadvantages of this process is the difficulty and very costly implementation of the cleaving step in UHV. Another approach has been cleaving the laser bars under ambient conditions and then perform dry cleaning methods. The optimization and tuning of those methods is also difficult as the facets of the chips are easily damaged during the application of the dry plasma etching.

The team composed by PhD Arjun Mandal and PhD Jaakko Mäkelä, has developed and optimized a dry etching process with focus on AlGaAs/GaAs heterostructures using new approaches to tune the etching environments as well as in-situ characterization techniques such as RHEED or Auger spectrometry. They have implemented novel techniques to monitor the actual plasma characteristics making use of spectrometry techniques. Additionally, HW modifications were designed and implemented to the reactor to add a new level of precision and control on the plasma generation and composition.

The tuning of the etching processes has been carried out using different materials with different crystal orientations, for example GaAs (001) and GaAs (110). The optimized cleaning process results in perfect and pristine crystal surfaces without induced damages that can be easily resolved from the RHEED images.

These good quality crystal structures are very important to ensure a successful implementation of the Kontrox passivation process, and to ensure that no defects are induced to the surfaces that would lead to a device failure later on.

Further analysis on actual cleaned facets of laser bars that were cleaved under air exposure show a very good quality and smooth surfaces under AFM.

This optimized cleaning process opens new approaches and opportunities for laser facet passivation techniques, like Kontrox(tm), that can be implemented easier and in more economical way than other used techniques , such as the above mentioned UHV cleaving followed by deposition of ZnSe.