High Aspect Ratio Reactive Ion Etching Technology of 4H-SiCOI +
4H-SiC has shown great potential in high-temperature, high-frequency, high-power electronic devices and integrated photonics due to its wide bandgap, high thermal conductivity, high mechanical strength, and excellent optical nonlinear properties. Especially in microelectromechanical systems and integrated photonics, the 4H-SiC on Insulator (4H-SiCOI) platform provides an ideal platform for achieving high-performance bulk acoustic resonators, nonlinear optical devices, and quantum light sources. However, the chemical inertness and high hardness of SiC materials make their micro nano processing, especially high aspect ratio reactive ion etching, a key technological bottleneck that restricts their development. PAM-XIAMEN supplies 4H-SiCOI wafers. For more details, please contact: [email protected]
1. Preparation and Etching Challenges of 4H–SiCOI Platform
The 4H-SiC platform is typically prepared through wafer bonding and thinning techniques, bonding single crystal 4H-SiC onto a silicon oxide (SiO2) substrate to form a structure with high optical confinement and good thermal management capabilities. However, the difference in thermal expansion coefficient between SiC and silicon can lead to stress issues during the bonding process, affecting device performance.
In terms of etching, the high chemical stability of SiC makes traditional silicon dioxide or photoresist masks less selective, and metal hard masks such as nickel electroplating must be used to achieve high aspect ratio structures. In addition, a NiCxFy passivation layer is easily formed during the etching process, which not only limits the etching depth but also affects the sidewall roughness and opening size.
Fig. 1 (a) SEM image of electroplated nickel mask on 4H-SiCOI, showing non perpendicularity of sidewalls; (b) SEM images of high aspect ratio grooves, displaying the morphology of the passivation layer and sidewalls
2. Hard Mask Selection and Etching Process Optimization for 4H–SiCOI
The most commonly used hard mask currently is electroplated nickel, which has high selectivity and adjustable thickness, but faces problems such as uneven thickness and lateral scallop effect in wafer level etching. To improve surface roughness, Hardin explored electroless nickel phosphorus and nickel boron alloys, which have significantly better surface smoothness than electroplated nickel and no transverse scallop phenomenon. Another potential mask material is aluminum nitride(AlN), which has a selectivity for silicon of up to 5800:1 and does not form a fluorinated passivation layer. However, the patterning of AlN is difficult, the sidewall angle is difficult to control, and vertical opening has not yet been achieved.
In terms of etching process, parameters such as coil power, platform power, SF6 flow rate, argon and oxygen ratio jointly determine the etching rate, sidewall angle, and passivation layer formation. Researches have shown that reducing pressure and increasing bias power can help improve etching uniformity, but at the expense of mask selectivity and sidewall quality.
3. 4H–SiCOI Wafer Level Etching Results and Device Performance
By optimizing the nickel plating mask and etching parameters, Lotfi et al. achieved high aspect ratio etching from 15:1 to 20:1 on a 100mm 4H-SiCOI wafer, with an etching depth of 57µm, sidewall cone angle of 88.5°, and roughness less than 120nm. The etching uniformity was controlled within 2%, successfully penetrating to the buried oxygen layer, and the footing effect was less than 100nm.
The bulk acoustic resonator prepared based on the processed 4H-SiCOI achieved Q>2 × 10 ⁶ at a frequency of 6.255MHz, demonstrating the enormous potential of SiC in ultra-high Q value resonators. In addition, this process also provides a reliable platform for the integration of nonlinear optical devices and quantum light sources.
Fig. 2 Frequency response and Q value of 4H-SiCOI based resonator
The high aspect ratio reactive ion etching technology on the 4H-SiCOI platform has made significant progress, and its success lies in the continuous optimization of the core process steps. The researches focus on improving the process of electroplating nickel masks, effectively controlling the nickel carbon fluorine passivation layer during the etching process, and achieving wafer level etching uniformity. These breakthroughs lay the foundation for manufacturing high-performance devices. There is a clear path for improvement in this technology: firstly, it is necessary to develop new mask materials (such as ScAlN) in order to achieve higher selectivity and better sidewall quality; Secondly, efforts should be made to promote the development of all SiC-on-SiC platforms, fundamentally solving the problem of thermal mismatch caused by mismatched thermal expansion coefficients of substrate materials. With the further improvement of etching technology, 4H-SiC will play a more important role in high-tech fields such as integrated photonics, quantum optics, and high-temperature MEMS sensors due to its excellent material properties.
Whether you need 4H-SiCOI wafer for research or for industrial applications, please contact us email at [email protected] and [email protected].
References：
- Zhang, Q., Wang, J., & Poon, A. W. (2024, July). Silicon carbide microring resonators for integrated nonlinear and quantum photonics based on optical nonlinearities. In Photonics (Vol. 11, No. 8, p. 701). MDPI.
- Hardin, M. P. (2023). Hard mask exploration for high aspect ratio deep-reactive ion etching of silicon carbide.
- Lotfi, A., Hardin, M. P., Liu, Z., Wood, A., Bolton, C., Riddell, K., … & Ayazi, F. (2022). Wafer-level high-aspect-ratio deep reactive ion etching of 4H-silicon carbide on insulator substrates. In Proc. Solid-State Sensors, Actuators Microsyst. Hilton Head (pp. 5-8).