Technical PaperGaAs manufacturing processes conditions for micro- and nanoscale devices
Introduction
Micro- and nanotechnologies using ICP-RIE technology are widely used in both manufacturing and research laboratories [1], [2], [3], [4]. III–V semiconductors suit high performance applications beyond optoelectronics due to the higher electron mobilities and peak velocities compared to silicon based devices [5], [6], [7]. However, the different processing techniques on III–V materials are still challenging and needs further developments. For instance, there is a growing interest for the advancement of reliable plasma etching technics on III–V materials for electronic and photonic applications. ICP-RIE etching technology enables plasma etching to operate at lower pressures (1–30 mTorr) with anistropic features and etch rates comparable to or higher than conventional reactive-ion etching (RIE) [8], [9], [10], [11], [12]. There have been numerous reports on the etching of silicon [12], [13], [14] and semiconductor devices based on GaAs/AlGaAs, which are widely used for the fabrication of microsystems [15], [16], electronic and photonic devices [17], [18], [19]. These various applications require high etching rates, anisotropic etching profiles, smooth sidewalls and good surface morphology in the microscale as well as in the nanoscale. However, the critical manufacturing step to obtain nearly perfect anisotropic profiles, with respect to the various devices, is sensitive to the choice of both plasma etching chemistry and masking material. Therefore, the latter needs to be finely selected and tuned.
Conventional III–V compounds etch masks consist of hard masks based on metals [20], dielectric layers (silicon oxide or silicon nitride) [21] and thick photoresists [22], [23]. Some mask materials adapted to chlorine chemistry processes already exist but little is known about the reliability of these materials used for pattern transfer on III–V compounds, particularly concerning the defects encountered during the micro- and nano-fabrication. Since the mask interacts with the plasma, it is crucial to understand each mask behavior to obtain a high fidelity pattern transfer on the III–V compounds over the entire etching period. Moreover, information on the etching rate and selectivity for each mask enables a better and reliable manufacturing process. Several investigations on etchants targeting specific materials have been made but would only report on the etching rates on III-V compounds, and would rarely discuss the selectivity of the mask transfer from microscale towards nanoscale (viz. GaAs vs mask transfer layer). At the micrometer scale, the degree of anisotropy can be improved by adding inert gases such as argon (Ar) with a mixture of , BCl gases on GaAs/AlGaAs substrate [24], [25]. In contrast, at the nanometer scale, the choice of an ideally balanced BCl/Cl/Ar chemistry is difficult to obtain due to under etching [26]. To protect the sidewalls, N gas is added into BCl/Cl/Ar plasma. The presence of N also improves the selectivity, cleans the surfaces, and maintains a high anisotropy for different scale ranges [27], [28], [29], [30], [31].We have previously demonstrated the feasibility of the chlorine-based GaAs/AlGaAs etch plasma chemistries at the microscale [32] and we have improved the etch rate, the selectivity, the quality of sidewall surfaces and the verticality of the ridge structure commonly used for quantum cascade lasers [18]. Nevertheless, the lack of knowledge in how to apply and integrate engineering processes from microscale towards nanoscale technologies is an obstacle to the selection of specific plasmas in dry etching systems. Hence, we develop in this article manufacturing engineering methodologies that integrate the chlorinated plasma chemistry to obtain with high reliable microdevices and nanodevices of predetermined shapes.
We demonstrate that the desired etching characteristics on GaAs, meaning vertical sidewalls, very low or insignificant roughness on the surface sidewalls, can be obtained in ICP-RIE with a BCl/Cl/Ar/N plasma chemistry and using a photoresist mask or a selected hard mask. We present here the different mask transfer methods and etching recipes which allow to obtain straight and smooth sidewalls at the nanoscale supported by GaAs etching profiles analysis.
Section snippets
Experimental section
The experiments were performed in a Surface Technology Systems (STS) reactor, equipped with a maximum available power of 900 W, 13.56 MHz RF coil generator. The gases employed in this study were BCl, Cl, Ar, N and O. During all the experiments, the temperature of the electrode was fixed at C. To avoid contamination of the ICP reactor using exclusively chlorine chemistries, two chambers are dedicated separately for metallic masks and photoresist mask. A 5-min-long oxygen cleaning
Microscale
There are numerous important parameters which affect the etching process such as etching rate, masking materials, selectivity and surface roughness. Kovacs et al. [33] have provided with a general comparison between different etchants properties regarding the aforementioned important etching characteristics required. They also discussed about the suitable masking materials for such etching processes. In our work, micro- and nanometer-sized patterns were fabricated using a top-down approach with
Discussion
In this ICP- DRIE study, we have investigated both photoresist masks and metal masks during the etching processes: microfabrication was optimized to achieve straight, smooth sidewalls and a deeper depth, while the nanofabrication etching was improved to obtain the desired nanopillar shape. There are many parameters in the processing such as pressure, coil power/platen power and gas mixture, and we find that:
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Various demands on mask selectivity, etching rate and profile can be met in the
Conclusion
This work reports on an optimized ICP etching process at the micrometer and nanometer scale of GaAs substrate through our optimized BCl/Cl/Ar/N plasma chemistries conditions. For microdevices manufacturing where straight and smooth sidewalls are requested, the photoresist masks or Cr-masks have better results than Ni-mask. We highlighted the selection of the hard mask properties towards the nanoscale, and some interesting effects may arise. In particular, for the manufacturing of nano-scale
Conflict of interest
None declared.
Declaration of Competing Interest
The authors report no declarations of interest.
Acknowledgment
The authors acknowledge the clean room and characterization laboratory staff. This work was partly supported by the French RENATECH network with technological facilities. They also would like to thank David Bouville and Samson Edmond for fruitful discussions.
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