ABSTRACT Heat and mass transfer devices are being fabricated by International Mezzo Technologies that utilize micro features with increasingly aggressive combinations of both feature height and feature aspect ratio. Improvements in x-ray lithography using SU-8 now make it possible to lithographically define densely packed arrays of features with heights of 3 mm and with aspect ratios of around 25. These arrays potentially serve as the starting point for increasingly aggressive LIGA-based micro machining of heat exchangers, regenerators, recuperators, etc. that have superior performance due to length scale advantage

Kevin W Kelly

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VERY DEEP X-RAY LITHOGRAPHY FOR  HEAT AND MASS TRANSFER  
 APPLICATIONS 
 
Kevin W. Kelly/Louisiana State University/ 
International Mezzo Technologies 
 
Charles Becnel/Louisiana State University/ 
Mezzo International Technologies 
 Yohannes Desta/CAMD 
 
 
Proceedings of IMECE04 
2004 ASME International Mechanical Engineering Congress and Exposition 
November 13-20, 2004, Anaheim, California USA 
 
 
IMECE2004-62438  
 
 
 
ABSTRACT 
Heat and mass transfer devices are being fabricated by 
International Mezzo Technologies that utilize micro features 
with increasingly aggressive combinations of both feature 
height and feature aspect ratio. Improvements in x-ray 
lithography using SU-8 now make it possible to 
lithographically define densely packed arrays of features with 
heights of 3 mm and with aspect ratios of around 25. These 
arrays potentially serve as the starting point for increasingly 
aggressive LIGA-based micro machining of heat  exchangers, 
regenerators, recuperators, etc. that have superior  performance 
due to length scale advantage. 
 
INTRODUCTION 
 
International Mezzo Technologies, a company that 
was spawned from LIGA-based research activities at Louisiana 
State University, is currently manufacturing a number of heat 
and mass transfer products that offer performance advantages 
resulting from the reduced scale of the flow passages.  Products 
include a cross flow heat exchanger with extremely high values 
of heat transfer/unit volume and heat transfer/unit weight [1], 
[2], a mechanical seal with integral heat exchanger [3], a micro 
jet cooling array that relies on a field of impinging jets to 
achieve heat flux removal rates on the order of 1 kW/cm2, 
regenerators and recuperators for Stirling heat engines and cryo 
coolers, and chemical reactors.  An example of one product 
being developed is the cross flow heat exchanger shown in 
Figure 1a.  The cross flow fluid (usually a gas) passes through 
the slotted channels shown in Figure 1b.  The characteristic 
diameter of the slot is controlled by the slot width, which is 
typically on the order of 200-500 µm.  The heat exchanger is 
about 2-4 mm thick and the in-plane fluid (often a high 
pressure gas or a liquid) passes through channels shown in 
Figure 1b.  Mezzo is investigating a variety of possible d From: https://proceedings.asmedigitalcollection.asme.org on 07/02/2019 Terms of Useconfigurations of such panels to build heat exchangers that can 
transfer tens of kW.  Experiments currently being performed by 
Mezzo validate the excellent heat transfer that has previously 
been reported for this heat exchanger [1].   
 
This heat exchanger shown in Figure 1 provides an 
excellent example to motivate the manufacture of these smaller 
scale devices. For the case of a gas-liquid heat exchanger (such 
as a car radiator), the fluid with the higher volumetric flow rate 
(the gas) passes in crossflow through the heat exchanger while 
the liquid flows within the plane of the heat exchanger.  For a 
given mass flow rate/unit area, and given difference in 
temperature between the inlet temperature of the two fluids, the 
heat transfer/frontal areas of two heat exchangers will be very 
similar if the quantity L/Dh2 are equal, where L is the thickness 
of the heat exchanger (the length of the gas passage through the 
heat exchanger) and Dh is the hydraulic diameter of the through 
plane gas passage [1], [2].   Therefore, if the mass flow 
rate/unit area is maintained constant and Dh is decreased by a 
factor of 5 (i. e. from 2 mm to 400 µm), then the required 
thickness of the heat exchanger for equal heat transfer 
decreases by a factor of 25!  Interestingly, the pressure drop of 
the gas across the heat exchanger is predominantly, to first 
order, a function of the same L/Dh2 ratio.  Therefore, the same 
heat transfer occurs in a smaller volume (with less mass) and 
there is virtually no penalty in pressure drop.  This scaling 
advantage is at the core of the design philosophy of most of 
Mezzo’s products.   
 
 
1 Copyright © 2004 by ASME 
: http://www.asme.org/about-asme/terms-of-use
Do 
Figure 1a:  Plan view of nickel heat exchanger panel 
showing cross flow channels of width 350 µm 
 
Figure 1b: Cross section of nickel heat exchanger showing 
in-plane channels of width 300 µm and height 2.5 mm 
 
Another product that is being developed is a micro jet 
cooling array (MJCA) that is designed to remove extremely 
high heat fluxes (1000 W/cm2) from a surface via an array of 
impinging micro jets.  The MJCA is a derivative of the 
previously described cross flow heat exchanger where an extra 
matrix of holes is machined (either by EDM or laser drilling) 
into one of the faces of the heat exchanger.  A schematic of the  
MJCA is shown in Figure 2.  The MJCA  is positioned  a short 
distance above the surface to be cooled.  Coolant passes from a 
reservoir above the MJCA through the plane of the MJCA via 
an array of micro jets and impinges upon the surface of interest.  
Then, the extra set of holes machined in the bottom face of the 
MJCA allows coolant to flow into the gap between the upper 
and lower faces of the MJCA before escaping radially outward.   
If designed properly, the MJCA design provides an array of 
impinging micro jets whose flow does not interfere 
significantly with that of neighboring micro jets.  This is the 
important concept of this design.   
 
In-plane channels   
wnloaded From: https://proceedings.asmedigitalcollection.asme.org on 07/02/2019 Terms of Us  
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 2: Schematic of Micro Jet Cooling Array 
 
 
An actual MJCA is shown in Figure 3a and the MJCA 
assembly is shown in Figure 3b.  Heat transfer experiments 
have been performed that demonstrate heat transfer removal 
rates exceeding 1000 W/cm2 can be achieved.  These results 
will be published soon. 
 
 
 
Figure 3a:  Microjet cooling array 
 
 
 
 
2 Copyright © 2004 by ASME 
e: http://www.asme.org/about-asme/terms-of-use
Dow 
Figure 3b:  Microjet connected to reservoir 
 
Various derivatives of the LIGA micromachining 
process are used to fabricate the products being fabricated by 
Mezzo.  The LIGA micro machining process [4], [5] is a three 
step process sequence that begins by lithographically patterning 
micro features in a polymer sheet using a source of collimated 
x-rays.  Next, an electroforming process is used create a mold 
tool with features that are the inverse of those patterned during 
the lithography step.  Finally, the mold tool is used to 
inexpensively mass produce via embossing or injection 
molding parts with the same geometry as originally defined 
during the lithography process.  Over the last few years, Mezzo 
has steadily sought to increase both the absolute height of the 
micro features of their mold inserts as well as their aspect ratio. 
Presently, heights 2-5 mm are needed with aspect ratios 
(height/width ratios between 10 and 30).  Usually, the feature 
pattern is quite dense.  Finally, if possible, the features should 
be tapered to facilitate demolding [ 6].   Recent research at LSU 
has focused on developing the capability to lithographically 
define features with increasingly aggressive combinations of 
feature density, aspect ratio, and absolute height that are 
required for these and other applications.   
 
Deep X-ray lithography using SU-8 
  
X-ray lithography using SU-8 as the resist offers great 
potential to produce resist micro features having the desired 
heights between 2-5 mm and aspect ratios of 10-30 that Mezzo 
requires.  SU-8 has absorption characteristics very similar to 
the most common resist used in x-ray lithography, PMMA, but 
is much more sensitive.  For the case of the available radiation 
spectra available at CAMD, the time required to expose an SU-
8 sample of given thickness is over 100-fold less than that 
required for PMMA [ ].   For the last few years, SU-8 features 
with heights on the order of 1-1.5 mm have been routinely 
lithographically defined [7].    
 
 
nloaded From: https://proceedings.asmedigitalcollection.asme.org on 07/02/2019 Terms of UsUnfortunately, lithographically defining SU-8 features 
with aggressive combinations of heights/aspect ratio/feature 
density has been problematic.  However, recent experiments 
have isolated a number of parameters that seem to affect the 
ability to lithographically define very tall, densely packed, high 
aspect ratio SU-8 micro features [8], [9].  In general, process 
parameter changes that reduce the diffusion of the cross link 
initiating photo active generator (PAG) through the SU-8 are 
believed to improve the quality of the features. These 
parameters include solvent content and solvent content 
uniformity in the SU-8 after the pre-bake is finished,   absolute 
values of dose and top-to-bottom dose ratio, post bake 
temperature, and pattern geometry.  The most important of the 
above parameters seems to be solvent content, which should be 
uniform and no more than 7 weight percent.  
 
The result of recent experiments is a greatly improved 
capability to define very tall, very high aspect ratio micro 
features in SU-8.   The array of SU-8 hexagonal posts shown in 
Figure 4 demonstrates some of the current potential of X-ray 
lithography using SU-8.  The distance between opposite faces 
of the post is 480 µm, the height of the posts is 3 mm, and the 
gap between adjacent posts is 150 µm.  To our knowledge, 
combinations of aspect ratio and absolute height cannot be 
duplicated using competing micro machining methods (EDM, 
micro milling, etc.).  It should also be noted that the time 
required to expose a relatively large area (10 cm x 10 cm) using 
the high energy “wiggler” beamline using the synchrotron 
radiation source at the Center for Advanced Microstructures 
and Devices (CAMD) in Baton Rouge is only a few minutes.  
So, extremely high precision machining is effectively being 
accomplished in a relatively short period of time.   
 
 
Figure 4:  SU-8 posts of height 3 mm and gap 
between posts of 150 µm 
 
Finally, it has been shown in a  previous efforts that 
for certain geometries, a multiple-tilt, multiple-exposure 
process can produce SU-8 features with taper that is desired to 
facilitate part release during molding processes.  The tapered 
hexagonal posts shown in Figure 5 [6] demonstrate the ability 
to produce tapered SU-8 features.  These tapered SU-8 features 3 Copyright © 2004 by ASME 
e: http://www.asme.org/about-asme/terms-of-use
Dowwere then used to fabricate the nickel mold insert shown in 
Figure 6 that was subsequently used to injection mold tapered 
posts identical to the lithographically defined posts shown in 
Figure 5.   
 
 
Figure 5:  SU-8 features 1 mm in height with taper 
 
 
Figure 6:  Nickel mold tool electroformed over the tapered 
SU-8 features shown in Figure 5 
CONCLUSION 
 
Mezzo is now using a combination of micro 
machining technologies Significant recent improvements with 
respect to x-ray lithography using SU-8 make it possible to 
lithographically define features with extremely aggressive 
combinations of height and aspect ratio, possibly with taper.  
Mezzo is using these new capabilities, and is combining them 
with other micro machining technologies (micro milling, LIGA 
micromachining, micro welding, EDM) to fabricate numerous 
heat and mass transfer devices having flow passages on the 
order of a few hundred micrometers in diameter that offer 
excellent performance.   
ACKNOWLEDGMENTS 
 
The authors are grateful for support from the State of 
Louisiana for providing funding for the Center for Advanced  
nloaded From: https://proceedings.asmedigitalcollection.asme.org on 07/02/2019 Terms of UsMicrostructures and Devices and for providing a fellowship 
that has supported Mr. Becnel.   
REFERENCES 
 
1. C. Harris, K. Kelly, T. Wang, A. McCandless, Shariar 
Motakef, "Fabrication, Modeling and Testing of Micro 
Cross Flow Heat Exchangers",  Journal of 
Microelectromechanical Systems, Dec.  2002, Volume 
11, Number 6, pages 726-735 
 
2. C. Harris, K. W. Kelly, M. Despa, "Design and 
Fabrication of a Cross Flow Micro Heat Exchanger", 
Journal of Microelectromechanical Systems, Vol. 9, 
No. 4, December 2000, pages 502-508. 
 
3. L. S. Stephens, K. W. Kelly, D. Kountouris, J. 
McClean, "A micro heat sink for cooling macro-scale 
conformal surfaces under the influence of thrust and 
frictional forces", Journal of MEMS, Vol. 10, No. 2, 
June 2001. 
 
4. E. W. Becker, W. Ehrfeld, P. Hagmann, A. Maner and 
D. Munchmeyer,  “Fabrication of microstructures with 
extreme structural heights by synchrotron radiation 
lithography, galvanoforming and plastic moulding 
(LIGA process)”,  Microelectronic Engineering,  4,  
35-56,  (1986). 
 
5. A. Maner, S. Harsh and W. Ehrfeld, “Mass Production 
of Microdevices with Extreme Aspect Ratios by 
Electroforming”, Plating and Surface Finishing, 
March, 60-65, (1988). 
 
 
6. R. Turner, Y. Desta, K. Kelly, J. Zhang, E. Geiger, S. 
Cortez, D. Mancini, "Tapered LIGA HARMs", 
Journal of Micromechanics and Microengineering, 
Volume 13, Issue 3 (May 2003), pp 367-372 
 
7. L. Jian, Y. Desta, J. Goerttert, M. Bednarzik, B. 
Loechel, J. Yoonyoung, G. Aigeldinger, V. Singh, G. 
Ahrens, G. Gruetzner, R. Ruhman, R. Degen, “SU-8 
based deep x-ray lithography/LIGA”,  
 
8. C. Becnel, Y. Desta, K. Kelly, “Ultra deep X-ray 
lithography of densely packed SU-8 features, Part II: 
Process performance as a function of dose, feature 
height, and post bake temperature”, in progress. 
 
9. C. Becnel, Y. Desta, K. Kelly, “Ultra deep X-ray 
lithography of densely packed SU-8 features, Part I: 
An SU-8 casting procedure to obtain uniform solvent 
content”, in progress.  4 Copyright © 2004 by ASME 
e: http://www.asme.org/about-asme/terms-of-use


VERY DEEP X-RAY LITHOGRAPHY FOR HEAT AND MASS TRANSFER APPLICATIONS

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VERY DEEP X-RAY LITHOGRAPHY FOR HEAT AND MASS TRANSFER APPLICATIONS

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