At the University of Arizona, my research focused on high quality factor microwave circuits constructed using micromachining technology.  One of these circuits is a microwave frequency diplexer.  A diplexer is a specialized filter used on transmit / receive systems - they are used in cellular phones, wireless network cards, radar systems and other similar radio devices.  The diplexer I am building operates at 19GHz and 21 GHz with channel bandwidths of around 2%.  The circuit is based two micromachined resonant cavity filters. 

Initially, the cavity dimensions were calculated assuming  TE 101 cavity modes.  This hand design was then simulated and optimized using  Ansofts High Frequency Structure Simulator .  This software simulates the microwave properties of 3D structures.  The simulations were run on a 500MHz PC with 768Megs of RAM, and took 12-14 hours to run each simulation.

The completed design was fabricated first on  Rogers Duroid circuit board material (er=10.8).  This circuit was tested to verify the design. 

The structure was altered for silicon construction (er=11.9), and the circuit was re-simulated to verify proper operation.  Currently I am building this diplexer using micromachining technology.  The fabrication is taking place in the clean room at the Electrical and Computer Engineering building.
 

  One of the first steps in this process is growing oxide on a high resistivity silicon wafer.  In this picture I am loading the wafers in to an oven used to grow the oxide.
 
 
 
 
 
 
 

After 14+ hours in the 1000 degree Celsius oven, the wafers are carefully removed. 
 
 
 
 
 
 
 
 

After the wafers cool they are ready for more processing.
 
 
 
 
 
 
 
 
 

To align the various masks used during fabrication, we use an MJB-3 mask aligner.  In this picture I am aligning a mask to a wafer which has been partially processed.  The lights in this room are yellow to prevent exposure of the photoresist placed on the wafers.
 
 
 
 
 
 

Finally, the circuits are measured using an HP8510 Network Analyzer.  To see the results of my experiments see the  papers section of my webpage.
 

Electromagnetic band gap (EBG) structures are periodic structures that, because of their periodic nature, cause interesting effects when illuminated with an electromagnetic wave.  Much research has been done in the area of Photonic Bandgap Structures - EBG's that operate at the frequencies of light waves.  More recently this concept has been applied to waves in the microwave frequency range. 

By creating a defect in the EBG structure (by breaking the structure symmetry slightly), EBG's can operate like high - Q filters.  Depending on the geometry of the defect, the filter response can be changed. 

My research also focused on making electronically controllable defects.  By having electronic control of the defects in a EBG, reconfigureable filters can be created.  These could be useful in wireless systems that need to operate in more than one frequency band.

Here is a CAD drawing of an EBG crystal I designed.

This crystal was simulated using the Finite Difference Time Domain Technique (FDTD) - a technique commonly used to simulate microwave structures.  The FDTD software used was developed by Dave Wittwer here at the University of Arizona.

After the simulations were complete, I constructed the crystal using brass rods.  After machining the crystal support structure, the rods were assembled on the supporting structure and the crystal was measured using the network analyzer. 

To create the controlled defects, I designed a special rod that contains one or more PIN diodes.  These diodes are electrically controlled switches, and allow the defect to be electronically removed from the structure, thereby altering the filter's properties.  Details of this experiment can be found in the  papers section of my web page.
 

By combining the controlled defect EBG technology with the micromachined diplexer technology it will be possible to create a reconfigureable diplexer.  Unlike the EBG crystal above, this diplexer will be compatible with standard IC processing technology.

Keywords: Microwave Band Gap, PBG, MBG, Controlled Degect, High Q Microwave Filters, Micromachining, Cavity Diplexer.