Computer Numerically Controlled (CNC) machining

of splitblock waveguide circuits has become the primary method

of constructing terahertz waveguide circuits. The majority of

these circuits have been made on traditional CNC machining

centers or on custom-made laboratory machining systems. At

both the University of Arizona and Arizona State University, we

have developed techniques for machining splitblock waveguide

circuits using purpose-built ultra high precision CNC

machining centers designed for micromachining. These systems

combine the automation of a traditional CNC machining center,

including a high capacity toolchanger, workpiece and tool

metrology systems and a large work volume, with the precision

of custom laboratory systems. The systems at UofA and ASU

are built by Kern Micro and deliver typical measured

dimensional accuracies of 2-3 microns. Waveguide surface

finish has been measured with a Veeco white light

interferometric microscope to be Ra~75 nm. Tools of sizes

between 25 microns and 10mm are available, with toolchanger

capacities of 24-32 tools

Groppi, Christopher E.

Love, Brian

Underhill, Matthew

Walker, Christopher

Caltech Authors - Main

21ST INTERNATIONAL SYMPOSIUM ON SPACE TERAHERTZ TECHNOLOGY, OXFORD, 23-25 MARCH, 2010 
Automated CNC Micromachining for Integrated 
THz Waveguide Circuits 
Christopher E. Groppi1*, Brian Love2, Matthew Underhill1, Christopher Walker2 
1Arizona State University School of Earth and Space Exploration, Tempe, AZ 85287, USA 
2 Steward Observatory, University of Arizona, Tucson, AZ 85721, USA 
*Contact: cgroppi@asu, phone +31-50-363 4074 
 
 
 
Abstract— Computer Numerically Controlled (CNC) machining 
of splitblock waveguide circuits has become the primary method 
of constructing terahertz waveguide circuits. The majority of 
these circuits have been made on traditional CNC machining 
centers or on custom-made laboratory machining systems. At 
both the University of Arizona and Arizona State University, we 
have developed techniques for machining splitblock waveguide 
circuits using purpose-built ultra high precision CNC 
machining centers designed for micromachining. These systems 
combine the automation of a traditional CNC machining center, 
including a high capacity toolchanger, workpiece and tool 
metrology systems and a large work volume, with the precision 
of custom laboratory systems. The systems at UofA and ASU 
are built by Kern Micro and deliver typical measured 
dimensional accuracies of 2-3 microns. Waveguide surface 
finish has been measured with a Veeco white light 
interferometric microscope to be Ra~75 nm. Tools of sizes 
between 25 microns and 10mm are available, with toolchanger 
capacities of 24-32 tools. 
 
The automated toolchanger and metrology systems allow a 
metal blank to be machined into the final part in one machining 
cycle, including both micromachining operations and traditional 
machining operations. This allows for perfect registration 
between all block features, in addition to very short cycle times. 
Even the most complicated blocks have machining cycle times of 
no more than a few hours. Workpiece and tool metrology 
systems also allow for fast setup times and straightforward part 
re-work. In addition, other high-throughput techniques such as 
palletization are enabled for the simultaneous manufacture of 
large numbers of blocks. 
 
Using these machines, we have successfully produced waveguide 
circuits at frequencies ranging from W-band to 2.7 THz, 
including highly integrated blocks. The Supercam project relied 
on these machines to produce integrated 8-pixel SIS mixer array 
units with integrated low noise amplifiers, bias tees and blind 
mate connectors. In addition, the 64-way corporate power 
divider used for LO multiplexing was machined using these 
techniques. This system consists of 17 split-block circuits 
containing E-plane power dividers, waveguide twists, diagonal 
horns and all associated flanges. The final system consists of a 
single WR-3 UG-387 input and an 8x8 array of 11mm aperture 
diagonal feedhorn outputs. This is one of the largest 
submillimeter waveguide circuits ever constructed. Future large 
focal plane arrays and other applications requiring highly 
integrated waveguide circuits will critically depend on this type 
of highly automated micromachining technology. 
 
We present the capabilities and machining process used with 
these machining centers, along with several waveguide circuits 
that were manufactured with this process including measured 
results from these circuits. Future directions for improving 
manufacturing quality and automation for large focal plane 
arrays will be discussed, including the use of palletization, in-
situ metrology, and automatic workpiece changers. Using these 
techniques, construction of the necessary waveguide blocks for 
even kilopixel class heterodyne array receivers should be 
realizable in a manageable time with high part yield and 
relatively low incremental cost. 
 
Figure 1: The Kern Model 44 (left) and Kern MMP (right) micromilling 
systems used at Arizona State University and the University of Arizona. 
MICROMILLING SYSTEMS 
The state of the art Kern micromilling systems at ASU 
(Model 44) and the University of Arizona (MMP) allow for 
the automated production of terahertz waveguide and 
quasioptical components with micron level accuracy and 
nanometer scale surface roughness (See figure 1). Both 
machines are equipped with laser tool measurement systems 
and strain-gauge touchprobes which allow the measurement 
and control of both tool sizes (both diameter and length) and 
workpiece location to the micron level. These machines are 
capable of maintaining their rated accuracies over their entire 
work volume, which enables very large integrated terahertz 
circuits to be produced, while maintaining dimensional 
accuracy and splitblock alignment. Both machines are 3-axis 
fully CNC controlled systems and are equipped with large 
capacity toolchangers. This allows all machining operations 
(both standard and micromachining) to be completed in one 
clamping, eliminating the need to align micromachined 
features to traditionally machined components. This 
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21ST INTERNATIONAL SYMPOSIUM ON SPACE TERAHERTZ TECHNOLOGY, OXFORD, 23-25 MARCH, 2010 
 
dramatically increases productivity while maintaining the 
best possible quality. Future upgrades to 5-axis machining 
are possible with both machines. 
These machines are capable of cutting traditional 
waveguide block materials (brass, copper, and aluminum) as 
well as more exotic materials including stainless steel, 
hardened steel, ceramics and silicon. Their high precision and 
large work volumes also make them particularly well suited 
to the production of terahertz optical components, optical 
systems and precision mechanical components. 
TABLE I 
MICROMILLING SYSTEM SPECIFICATIONS 
 Kern Model 44 Kern MMP 
Positioning 
accuracy 
+/- 0.5 micron  +/- 1.0 micron  
Surface 
Finish 
Ra<0.1 microns Ra<0.2  microns 
Work 
Volume 
300x280x250mm  250x220x200mm  
Max 
workpiece 
mass 
50kg 30 kg 
Spindle Vector controlled 50 
krpm  
Vector controlled 40 
krpm  
Toolchanger 32 positions 24 positions 
Tool 
measurement 
Laser diameter and 
length 
Laser diameter and 
length 
Workpiece 
setting 
Touchprobe Touchprobe 
Cutting 
lubricant 
Temperature 
controlled cutting 
fluid 
Oil mist lubrication 
system 
 
MICROMACHINING PROCESS 
The micromachining process at the UofA and ASU 
combines electromagnetic design, mechanical design, CAM 
programming, machining, metrology, assembly and RF 
testing in one location. This allows for rapid progress from 
design to finished, working components, and also allows for 
multiple design cycles for the refinement and optimization of 
complex THz circuits. The CAM system used with both 
processes (Openmind Hypermill) is integrated into the CAD 
package used for design. This system automatically updates 
CAM programming when the solid model is changed in the 
CAD package. Toolpaths are calculated to an accuracy of 0.1 
microns. 
 MICROMACHINED TERAHERTZ COMPONENTS 
The Kern MMP at the University of Arizona was 
originally purchased to fabricate the waveguide structures for 
the Supercam 64 beam array receiver [1]. The Supercam 8-
pixel mixer module is shown in figure 2. The block consists 
of eight single ended SIS waveguide mixers, with integrated 
diagonal horns. LNA modules, IF and bias distribution and 
electromagents are all contained in this single, large 
(160x50x11mm) waveguide splitblock. All waveguide, feed 
and classical machining is done in a single clamping per side 
in the Kern MMP. 20 tools, and approximately 4 hours of 
machine run time are required to complete a single splitblock 
half. 
 
 
Figure 2: The Supercam 8-pixel mixer module. The single splitblock 
contains 8 single ended SIS mixers, 8 low noise amplifier modules, 8 
electromagnets, and all DC and IF distribution circuitry. 
LO power multiplexing for Supercam is also accomplished 
using a micromachined waveguide structure. The 64-way LO 
power divider shown above-left consists of 17 separate 
waveguide splitblocks with custom interface flanges. The 
first waveguide splitblock consists of 7 WR3 E-plane Y-
splitters in a binary tree. This tree of splitters divides the 
output of a single Virginia Diodes LO chain by 8, with equal 
waveguide pathlength for each output port (see figure 3). The 
output ports end with splitblock waveguide twists. This 
block’s 8 output ports then meet 8 additional waveguide 
splitblocks. These blocks have an idential tree of 7 E-plane 
Y-splitters, but the output ports end with diagonal feedhorns 
(see figure above-right). The diagonal feedhorns are then 
extended with another set of splitblocks to reach 11mm 
aperture size. The final block achieves even pixel-to-pixel 
splitting of LO power to within 10%, with a total loss of 2 dB 
compared to a lossless divider [2]. 
 
 
Figure 3: The completed Supercam LO power divider (left) and a close up 
photo of one of the E-plane Y-junctions inside the splitter (right). 
 
In addition to Supercam, many other waveguide 
components have been produced. The spatial filter shown in 
figure 4 was designed to improve the beam quality of a 2.7 
THz QCL [3]. This is currently the highest frequency 
waveguide structure to be designed, fabricated and tested 
using our micromachining process. 
339
21ST INTERNATIONAL SYMPOSIUM ON SPACE TERAHERTZ TECHNOLOGY, OXFORD, 23-25 MARCH, 2010 
 
Figure 4: A solid model and photo of a 2.7 THz spatial filter made at the 
University of Arizona using the micromachining process described in this 
paper. 
Highly integrated THz waveguide circuits are possible to 
fabricate using our micromaching process. The 660 GHz 
sideband separating mixer shown in figure 5 is fabricated 
from a splitblock containing two branch line directional LO 
couplers, 1 branch line hybrid coupler, two SIS mixers, an 
integrated diagonal LO feedhorn and associated IF circuitry 
and magnets [4]. The 1.45 THz mixer shown in figure 6 was 
fabricated using a 25 micron diameter cutter [5]. The b-
dimension of the waveguide is 42 microns. Feature depths 
are 18 microns for the waveguide backshort and 8 microns 
for the device channel. The coupler slot shown below right 
was cut with a 100 micron tool with a 5.5:1 aspect ratio. 
 
 
Figure 5: A 660 GHz sideband separating mixer block (left), and the 
splitblock alignment of the two halves of this block (right). The size of the 
full-height waveguide is 145x310 microns, with +/- 2 micron splitblock 
alignment. 
 
Figure 6: A 1.4 THz HEB mixer block for the Stratospheric Terahertz 
Observatory experiment. The dimensions of the waveguide backshort are 
160x42 microns, with a 18 micron depth. Features were machined with a 25 
micron diameter endmill. 
METROLOGY 
Metrology is a critical component to any micromachining 
process. High performance waveguide circuits are impossible 
 to successfully fabricate without the ability to measure 
micron-level dimensions and nanometer scale surface  
roughnesses. Both micromilling systems are equipped with 
laser tool measurement systems and touchprobe systems 
which allow the measurement of tool length and diameter to 
the micron level, and the location and rotation of the 
workpiece to the same level of precision. Combined with the 
exceptional positioning accuracy of the machine axes, these 
systems allow on-the-part accuracies of ~2um. Structure 
dimensions can then be measured to micron accuracies using 
a 3-axis measurement microscope with photographic 
capability. At the University of Arizona, a Veeco white light 
interferometric microscope is available to measure surface 
roughnesses. Typical surface roughness is Ra~75 nm. An 
example of such a waveguide is shown in figure 7. 
 
 
Figure 7: A white light interferometric microscope image of the floor of a 
350 GHz waveguide. The measured surface roughness is Ra~75 nm.  
FUTURE DIRECTIONS 
Automation techniques used for decades in classical 
machining systems can now be applied to high precision 
micromachining. Rather than producing one or a few 
waveguide blocks at a time, automation systems can allow 
the production of hundreds of blocks at a time without user 
intervention. The Kern Evo’s toolchanger can be expanded to 
up to 96 tools. The System 3R workpiece chuck allows 
straightforward palletization of workpieces, and allows the 
removal and reinstallation of the chuck with a precision of 
2um or better. The chuck can be combined with an automatic 
workpiece changer to allow the automated loading and 
unloading of pallets. Combined with the laser tool metrology 
system and workpiece touchprobe, very large numbers of 
THz waveguide blocks could be produced economically and 
quickly. Future instruments with large (~1000) pixels are 
feasible using these production techniques, in addition to the 
expansion of THz technology to the private sector. 
 
REFERENCES 
[1] Groppi, C.E., Walker, C.K., Kulesa, C., Golish, D., Kloosterman, 
J, Puetz, P., Weinreb, S., Kuiper, T., Kooi, J., Jones, G., Bardin, 
J., Mani, H., Lichtenberger, A., Cecil, T., Hedden, A., 
Narayanan, G. SuperCam: a 64 pixel heterodyne imaging 
spectrometer, Millimeter and Submillimeter Detectors and 
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21ST INTERNATIONAL SYMPOSIUM ON SPACE TERAHERTZ TECHNOLOGY, OXFORD, 23-25 MARCH, 2010 
 
Instrumentation for Astronomy IV, Edited by Duncan, William, 
Holland, Wayne, Withingtonm Stafford, Zmuidzinas, Jonas, 
Proc. SPIE 7020, 26, 2008. 
[2] Christopher Groppi, Christopher Walker, Craig Kulesa, Dathon 
Golish, Jenna Kloosterman, Sander Weinreb, Glenn Jones, 
Joseph Barden, Hamdi Mani, Tom Kuiper, Jacob Kooi, Art 
Lichtenberger, Thomas Cecil, Patrick Puetz, Gopal Narayanan, 
Abigail Hedden, Testing and Integration of Supercam, a 64-Pixel 
Array Receive for the 350 GHz Atmospheric Window, 21st 
International Symposium on Space Terahertz Technology, in 
press, 2010. 
[3] Abigail Hedden, Patrick Pütz, C. d'Aubigny, Dathon Golish, 
Christopher Groppi, Christopher Walker, Benjamin Williams, 
Qing Hu, John Reno, Micromachined Spatial Filters for Quantum 
Cascade Lasers, 17th International Symposium on Space 
Terahertz Technology, P2-11, 2006.  
[4] F.P. Mena, J. Kooi, A.M. Baryshev, C.F.J. Lodewijk, T.M. 
Klapwijk, R. Hesper, W. Wild, RF Performance of a 600 - 720 
GHz Sideband- Separating Mixer with All-Copper 
Micromachined Waveguide Mixer Block, 19th International 
Symposium on Space Terahertz Technology, pp. 90-92, 2008. 
[5] J. Kawamura, A. Skalare, J. Stern, E. Tong, C. Groppi, THz 
waveguide HEB mixer using Silicon-on-Insulator substrates for 
STO, 21st International Symposium on Space Terahertz 
Technology, in press, 2010. 
 
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Automated CNC Micromachining for Integrated

THz Waveguide Circuits

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Automated CNC Micromachining for Integrated THz Waveguide Circuits

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