3D打印毫米波太赫茲無源器件（三）

4. Challenges and Solutions for 3D Printed MmWave and THz Devices

The two dominantly influential factors on the performance of 3D printed mmWave and THz devices are and dimensional tolerance and surface roughness. The former is decided by the material particle size, thermal shrinkage in sintering and postsintering, laser/electron beam size, and movement control of the laser/electron beam and the nozzle size. The latter is of more concern in 3D printing. It is related to material particle size, laser/electron beam size, density of the material powder, the Gaussianity of the laser beam, and the movement control of the laser/electron beam. Existing techniques to improve the surface quality of 3D printed devices include mechanical polishing, chemical polishing, MMP, and electroplating. However, limited by the existing process, effective improvement of the dimensional tolerance and surface roughness is difficult. Methodology to circumvent the dimensional tolerance would lead the 3D printed mmWave and THz device to a great leap forward to widespread applications. Accurate model to predict and evaluate the surface roughness related loss in 3D printed devices is valuable for quality control in industry.

4.1. Dimensional Tolerance

The influence of dimensional tolerance is reflected by deteriorated reflection coefficient, increased insertion loss, and severe passive intermodulation. It is more meaningful to avoid the dimensional tolerance under the premise that the improvement of dimensional tolerance of 3D printing technologies has come to a limit. The high order mode structure expands the volume of fundamental mode structure, thereby weakening the influence of the processing volume tolerance on the device performance. However, by doing this high order modes are excited. For example, as shown in Figure 8, after transforming a fundamental mode V-band (60–90?GHz) waveguide twist into a high order mode waveguide twist, its immunity to the dimensional tolerance is significantly improved at the cost of high order mode excitation. The flatness of the insertion loss of high order mode waveguides appears to deteriorate. The performance of the high order mode waveguides is still comparable to that of the standard fundamental mode waveguides in a narrow band (e.g., 66–74?GHz). In practice, most RF passive components are not required to function in a full bandwidth. The high order design methodology can effectively avoid the effect of dimensional tolerance on the device performance while guaranteeing the required functionality in a specific bandwidth.

Figure 8: Fundamental mode and high order mode waveguide twists: (a) V-band fundamental mode waveguide twist, (b) frequency response of the V-band fundamental mode waveguide twist, (c) V-band high order mode waveguide twist, and (d) frequency response of the V-band high order mode waveguide twist.

4.2. Surface Roughness

Not-well-controlled surface roughness may give rise to increased insertion loss and deteriorated passive intermodulation (PIM). Hammerstad and Jensen model, Huray model, and Hall models are representative classic roughness empirical models. They model the rough surface as a two-dimensional structure with periodic properties. These models have reference value in the analysis of conductor loss in planar transmission line structure. Depending on the material and process, different correction factors are introduced for specific applications, such as the correction factor 

 introduced in the Hammerstad and Jensen model:

However, the above models are only applicable to the analysis of roughness related losses in planar microstrip structures; there is no effective model for the roughness analysis of 3D printed THz devices. The 3D printed device is different from the traditional device in the roughness characterization due to the specific process and material. For example, the 3D printed surface roughness is limited by the material particle diameter; the same printer using different materials may result in obviously different surface finishing. Due to the nonuniform particle diameter of the material, the root mean square of surface roughness is usually larger than that of the conventional device in roughness. Since the device is printed layerwise and the laser used usually features a Gaussian beam, the roughness of the metal 3D printing device may be subject to periodic distribution in large scale and Gaussian distribution in small scale. Based on the classic model, a correction factor of the 3D printed THz device should be introduced, and the empirical formula of the device loss and roughness should be established. The establishment of the statistic model of the 3D printed surface roughness will benefit the academia in device performance prediction and the industry in quality control.

5. Conclusions

This paper reviews the state-of-the-art 3D printed mmWave and THz devices. They largely fall into the dielectric and the metallic categories. The dielectric 3D printed devices stand out with low body mass, while the thermal stability, physical stability, and process complexity are of concern. The metallic 3D printed devices outperform in the thermal stability and physical stability at the cost of increased body loading. The widespread applications of using 3D printing technology for mmWave and THz device fabrication had come to a bottleneck because of the limited dimensional tolerance and surface roughness. By adopting the methodology of high order device, the dimensional tolerance could be circumvented. A precise model of 3D printed surface roughness could be helpful to predict and evaluate the roughness related loss. Besides, a hybrid printing technology that merges with the traditional CNC process is recently available. The printing and machining processes are carried out within a single machine at intervals. This effectively improves the dimensional tolerance and surface roughness by the machining process, while remaining eco-friendly and cost-effective by the 3D printing technology. A quasi 3D printing process, or 2.5D printing process, is under development. It merges with mask printing for dimensional tolerance control. It is very promising that, with the development of 3D printing technology and disciplines in material science and mechanical engineering, 3D printed mmWave and THz device will become the mainstream solution in both academia and industry.

Conflicts of Interest

The authors declare there are no conflicts of interest regarding the publication of this paper.

Acknowledgments

This work was supported by the Startup Foundation for Young Talent from Sichuan University, Research Project of Guangdong Province (2013B090500035), Youth Foundation of Guangdong University of Technology (15ZK0038), and Science and Technology Program of Guangzhou (2014J4100202). This work was also supported by the Sichuan Provincial Science and Technology Project of China (2015FZ0112), the Foundation of Chengdu University of Information Technology (J201602), and the Scholarship from China Scholarship Council (201508515023).

References

1. X. Shang, M. Ke, Y. Wang, and M. J. Lancaster, “WR-3 band waveguides and filters fabricated using SU8 photoresist micromachining technology,” IEEE Transactions on Terahertz Science and Technology, vol. 2, no. 6, pp. 477–448, 2012. View at Google Scholar

2. F. Caspers and E. Neumann, “Optical-fiber end preparation by spark erosion,” Electronics Letters, vol. 12, no. 17, 1976. View at Google Scholar

3. C. Hull, Apparatus for production of three-dimensional object by stereolithography, U.S. Patent 4 575 330, 1986.

4. C. Deckard, J. Beaman, and J. Darrah, Method for selective laser sintering with layerwise cross-scanning, U.S. Patent 5 155 324, 1992.

5. S. Crump, Apparatus and method for creating three-dimensional objects, U.S. Patent 5 121 329, 1992.

6. ARCAM, Electron Beam Melting, 2016, http://www.arcam.com/technology/electron-beam-melting/.

7. SLM Solutions, SLM, 2016, http://slm-solutions.com/about-slm.

8. L. Schulwitz and A. Mortazawi, “A compact dual-polarized multibeam phased-array architecture for millimeter-wave radar,” IEEE Transactions on Microwave Theory and Techniques, vol. 53, no. 11, pp. 3588–3594, 2005. View at Publisher · View at Google Scholar · View at Scopus

9. K. F. Brakora, J. Halloran, and K. Sarabandi, “Design of 3-D monolithic MMW antennas using ceramic stereolithography,” IEEE Transactions on Antennas and Propagation, vol. 55, no. 3, pp. 790–797, 2007.View at Publisher · View at Google Scholar · View at Scopus

10. N. Delhote, D. Baillargeat, S. Verdeyme, C. Delage, and C. Chaput, “Ceramic layer-by-layer stereolithography for the manufacturing of 3-D millimeter-wave filters,” IEEE Transactions on Microwave Theory and Techniques, vol. 55, no. 3, pp. 548–554, 2007. View at Publisher · View at Google Scholar ·View at Scopus

11. T. Chartier, C. Duterte, N. Delhote et al., “Fabrication of millimeter wave components via ceramic stereo- and microstereolithography processes,” Journal of the American Ceramic Society, vol. 91, no. 8, pp. 2469–2474, 2008. View at Publisher · View at Google Scholar · View at Scopus

12. Y. Lee, X. Lu, Y. Hao et al., “Rapid prototyping of ceramic millimeterwave metamaterials: simulations and experiments,” Microwave and Optical Technology Letters, vol. 49, no. 9, pp. 2090–2093, 2007. View at Publisher · View at Google Scholar · View at Scopus

13. Y. Lee, X. Lu, Y. Hao, S. Yang, J. R. G. Evans, and C. G. Parini, “Directive millimetrewave antennas using freeformed ceramic metamaterials in planar and cylindrical forms,” in Proceedings of the 2008 IEEE International Symposium on Antennas and Propagation and USNC/URSI National Radio Science Meeting, APSURSI, 4, p. 1, San Diego, Calf, USA, July 2008. View at Publisher · View at Google Scholar · View at Scopus