SEMILAB Optimizes Microwave Transfer: How Virtual Testing Refined a Mode-Transforming Mirror

SEMILAB provides optical and electrical metrology solutions for semiconductor wafer and device manufacturers. As recounted in a previous user story,  SEMILAB designed a system to measure the microscopic contact area between a needle and glass. Similar to that situation, this case requires precise control, and designing components for microwave systems also demands careful consideration.  

This user story explores how COMSOL Multiphysics helped us optimize a crucial component called a mode-transforming mirror for a microwave system, with the goal of achieving optimal impedance matching between a rectangular and a cylindrical waveguide, minimizing reflected power. 

Challenge 

Understanding how each component works is key to product development. In the process of achieving that knowledge, it is important to define the nominal parameters and to compare them with alternatives. Here, the challenge was to engineer a mode-transforming element. This component links a rectangular microwave waveguide operating in the TE10 mode with a cylindrical waveguide operating in the TE11 mode. The ultimate goal was to achieve this connection with the least amount of reflected microwave power.

Solution 

Understanding the mode coupling between the two waveguides, two distinct designs were put to the test: a tilted mirror- and a doorknob design. Each design functions as a mode-transforming mirror. The mirror pairs a rectangular microwave waveguide to a cylindrical waveguide. Schematic diagrams depicting each design can be seen below. On the left, is a tilted mirror functioning as a mode-transforming element, while on the right is the “doorknob” arrangement in which a spherical metallic indentation resembling a spherical doorknob is placed at the junction of the two waveguides.

These mirrors, if correctly adjusted, will transform the TE10 mode, propagating in the single mode rectangular waveguide to the TE11 mode. If the conversion is perfect, the mirror will match the wave impedance between the two waveguides. Consequently, in a matched state, the reflection term returning to the source is 0%. During our simulations, we derived the S₁₁ parameter, which indirectly measures the reflection. 

The diagrams below depict the field strength distribution of these mode-matching elements, with the microwave propagation illustrated from left to right. Since COMSOL Multiphysics computes the superposed field, no interference is generated, i.e. the microwave power returning to the source is negligible. 

Result 

Following an evaluation of coupling performance, the tilted mirror model was selected for further development. Consequently, we incorporated additional microwave elements into it. An in-depth analysis of the mirror’s parameters, such as inclination and position, in relation to these new elements, was conducted. The results are depicted in the figure below. The relative position and inclination of the mirror in it have been adjusted to find the position where system wave impedance matching is achieved, i.e. the microwave power returned to the source is negligible. The RF module helped us visualize and understand microwave propagation, and also enabled us to establish the system setup procedures and setpoints. Moreover, COMSOL Multiphysics simulations allowed us to create virtual prototypes. These prototypes enabled us to select a system for live testing on the basis of various metrics. 

Conclusion 

COMSOL Multiphysics simulations enabled us to visualize and understand microwave propagation within the system, determine the optimal placement and inclination of the mirror for impedance matching, and virtually prototype the system to select the best design for real-world testing. 

Summary 

Challenge

Designing microwave components often involves trial and error testing, which can be time-consuming and expensive. This user story highlights the difficulty of finding the optimal design for a mode-transforming mirror that efficiently transfers microwaves between waveguides. 

Solution

COMSOL Multiphysics offered a virtual environment to test various mirror designs.  

Results

By simulating microwave propagation and analyzing reflection, we were able to identify the optimal mirror design. Compared to traditional methods, this protocol streamlined the development process and saved resources. 

Featured products

All products mentioned are developed by COMSOL. 

COMSOL Multiphysics 

RF Module 

Learn more 

• User story: SEMILAB’s Metrology Breakthrough: Optimizing Metal Needle-Glass Interactions with COMSOL Multiphysics
  SEMILAB tackles the complexities of inspecting a metal probe’s condition through interactions with glass. The results are a successful correlation between simulated and measured results, improving measurement arrangements by offering a streamlined and cost-effective solution for semiconductor manufacturers.
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