Design and Fabrication of MEMS Microchannels for Particle Detection

Annual Research Report

2003 – 2004

Prof. Richard Nelson, Prof. John LaRue and Allen Kine

Mark Villamor

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Table of Contents

Background . . . . . . . . . . 3

Objectives . . . . . . . . . . 4

Methods and Approaches

Fabrication Method I . . . . . . . . 5

Testing and Difficulties . . . . . . . 13

Fabrication Method II . . . . . . . . 14

Components Used . . . . . . . . 16

Next Steps . . . . . . . . . . 17

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Background

MEMS microfluidic channels are traditionally used for particle detection, particle size estimation

and particle counting are constructed using silicon and wafer processing techniques. A lower cost

approach with a quicker fabrication time uses standard laboratory microscope slides and a

photosensitive polymeric material for the channel walls. The range of available photosensitive

materials include the fabrication of microfluidic channels from tens of microns high to tenths of

microns high using similar electrode materials and fabrication methods. The application of

microfluidic channels encompasses individual cells, cell fragments, and DNA. The Coulter

counter approach for the particle detection, particle size estimation and particle counting will be

the basis for the proposed device. The Coulter counter is based on the change in the channel

resistance due to the size of a particle.

The electrode geometry has to be scaled in relation to the particle size and the associated

electronics optimized. A common problem in the design of a microprocessor or a microfluidic

channel is in the connection of the Macro world to the Micro world and the efficient transfer of

data and materials. For microchannels, which use continuous rather than batch measurements, a

fundamental problem can be that the inner diameter of the tube transferring the fluid to the

microchannel can easily be larger the microchannel.

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Objectives

Microfluidics is the world of the small (dimensions on the scale of micrometers) while the

macroworld in which we live has typical dimensions of centimeters and meters. Microfluidics

handles fluid samples in the nanoliter to picoliter range while the macroworld deals with cubic

centimeters and liters. The size of electrical structures in the microworld are determined by the

size of the sample to be analyzed or processed while the interconnections in the macroworld are

driven by the size of human hands. Interfacing these two realms is important to capitalize on the

advantages of MEMS processing of fluids. This research will develop methods for interfacing

these two worlds and then will demonstrate the selected interfaces with a useful

microfluidic/MEMS device.

The pulse shape resulting from particle detection in Coulter Counters is highly variable. This

limits the information that can be extracted from the waveform, such as the particles size and

shape. Incorporating a new electrode configuration and hydrodynamic focusing of the particle

flow can reduce this variability. The purpose of this project is to design and fabricate a channel

that can successfully count and estimate particles with a diameter ranging from five to ten

microns. This is accomplished with the design of a symmetric electrode structure, hydrodynamic

focusing, and input coupling. The symmetric electrode structure reduces the variability of the

generated pulse and hydrodynamic focusing centers the particles down the length of the channel.

Both increase the symmetry of particle placement with respect to the electrodes, resulting in a

more spatially uniform electric field distribution.

A feature to incorporate in the design is a hydrodynamic coupler at the input to the device. This

will allow a fluid input structure that is larger then the cross section of the flow channel. Fluid

dynamic analysis will be performed to minimize the perturbance and dead volume in the flow

field at the fluid input.

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Fabrication Method I The following fabrication method was applied to the microchannel that is coupled to the fluidic input and output ports on its sides. Also, all the sensing electrodes are fabricated on one substrate.

I. RCA Cleaning

All the glass slides that will be used in this fabrication method are required to be clean

and free of dirt and residue. To ensure that the chrome and photoresist has an adequate surface to

be deposited on, the glass is cleaned using RCA cleaning.

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II. Deposition of Chrome Layer

Before Deposition Process -Come into room and find E-Beam on HiVac. Should be around 1 E-6 torr. -HiVac off -hiss to click noise -Turn on Vent -wait for 7 E+1 torr -seal will release -With Vent on, proceed to chamber -As Chamber raises, turn off vent -Check shutter -Load in crucible and turn to Au to prevent contamination from shutter -Please note the position of the sample on the circle diagram

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-Once crucible is loaded, close shutter -Make sure to see reflection of crucible in the mirror -Load samples onto platform -tape 2-3 edges, do not cover center hole. -lift upside down to check that no samples fall -place platform into E-Beam machine -Close chamber -when closed, put to stop -Rough Pump on -air noises occur -target 5 E-2 torr -about 7-10 minutes -takes longer, may have leak -Turn off Rough Pump, turn on HiVac -target 5 E-6 -about 40-60 minutes -longer time needed means leak -Set display to IG1 -Once target pressure is met, power up in the following sequence: -Breaker On -Monitor On -Key turned to On -Please make sure that E-Beam gun is off (twist nob all the way CCW) -High Voltage On -pre-set to 7.5V -Gun On -Check Blue Book for metal density and z-ratio -In monitor, go to Film # -check the display for the correct densities and z-ratio

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-Go to "Data-8" (monitor deposition rate) -Press ZEROTHK -Set Film # -Again, make sure that Gun Control is set all the way to zero -Position chrome crucible in correct location During Deposition Process -Proceed to turning on electron beam to about 0.06 A -slowly turn up current to heat -check for glowing -allow about 10 minutes for heating -Once heated, turn down gun control to zero -Open Shutter -Adjust gun control to 1-2 A/s for beginning deposit (better contact to surface) -Do this until .075 kAngstroms -Proceed to increasing gun control to 3-4 A/s -would like a 3-4 A/s for middle process -If target thickness is 5kAngstroms -at around 4.7kAngstroms, bring gun control down to 1-2 A/s -Before target is met, turn off gun control After Deposition Process -Shutter Close -Wait about 10 mins for cooling -can check heat from chamber and from breaker -Proceed to shutting down in reverse -Can use monitor as a timer -When cool down process completed, turn of monitor and breaker -turn off HiVac -listen for hiss and click -Vent On -wait for 7 E+1 torr -seal will also release -Raise Chamber -Turn off venter when raising begins -Check for warmness and remove samples -Remove Crucible -Close shutter -Lower Chamber -Rough On -Wait for 5E-2 torr -Rough Off, HiVac On NOTE: The E-Beam is one of the most popular and widely used pieces of equipment in the INRF. Please be aware of all steps and be extremely careful when attempting an evaporation. A malfunction of any sort jeopardizes the majority of everyone’s research and causes a lot of delays. If at any point you are unsure of a step, ask any INRF staff person for help.

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NOTE: It is important to schedule about a week ahead of planned evaporation. Also, remember to keep detailed comments of any problems encountered or observations. Staff and other users are interested in how the E-Beam is “behaving” or if maintenance may be required (I.E. switching XTAL). III. Fabricating Chrome Electrodes Photoresist is spread over the top of the chrome layer to protect the areas of the chrome

that will not be removed.

The chrome electrode gets its pattern from the photoresist pattern. To do this, a mask and

photolithography is used.

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Areas of the PR exposed to the UV light source will be dissolved and removed when placed in a

developer solution, leaving behind the areas of PR unaffected by the UV light exposure.

The slide is placed in a chrome etchant to remove the layer of chrome not protected by the

patterned PR layer.

The PR is then removed from the slide using Acetone, leaving behind a patterned layer of

chrome, which makes up the sensing electrodes.

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IV. Constructing the Microchannel

An edge bead forms at the corners of a single glass slide when photoresist is spun on. This edge

bead can have a height of around 40 – 50 um above the uniformed PR surface and becomes a

problem when trying to seal the microchannel. This problem is solved by creating a continuous

surface of glass by placing halved glass slides to the left and right of the glass slide where the

channel is to be fabricated. By creating this continuous surface, it pushes the edge bead away

from the critical areas of the center slide to the outer slide where the development of an edge

bead is not a problem.

The center slide is removed from the wafer and the microchannel is made by photolithography.

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V. Sealing the Microchannel

Using a thin layer of about 2 – 3 um of Shipley 1827 seals the microchannel. Once the PR is

spun on a 1in x 1in piece of glass slide, the slide is placed on top of the channel to seal it.

The channel is now complete and can be placed in its holder. The following pictures shows how

the channel is lined up to the input and output fluidic ports.

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VI. Testing and Difficulties

There was a lot of difficulty when trying to create an adequate seal between the cover plate and

the microchannel. The following pictures show channels being checked for an adequate seal.

The test did not require the use of electrodes, which is why they were not fabricated onto the

channel for this test. An epoxy was used to help with the seal at the top edge, which helped a lot

in the securing of the top plate to the channel. This made the handling of the channels easier

since it lessened the likelihood of the top cover plate coming of when the channel was being

secured in its holder. The discoloration shows that the solution is not being contained within the

channel as hoped.

Additional Problems with solutions:

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Fabrication Method II

The following fabrication method is for the microchannel with the sensing electrodes

fabricated vertically, on two separate glass slides. The method of connecting the microchannel

to the macroworld is through an input and output ports attached to the top glass cover plate.

The channel will be constructed out of a photosensitive polymer, SU-8. To aid in the

coupling of the micro-channel to the outside world, the input and output of the channel well be

made larger for easy alignment to the top cover plate. A new approach will be used which will

require coupling the channel through the glass cover plate. The previous design that used four

electrodes on the bottom of the channel will be changed to a two electrodes positioned vertically

from each other. The smaller microchannel dimensions require photo reduction of the CAD

artwork.

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I. Fabrication of Bottom Electrodes

-Clean 3 in x 3 in flat glass plate.

-Deposit chrome metal layer using E-Beam deposition.

-Spread Shipley 1827 over metal layer.

-Use bottom electrode mask to define areas for chemical etching.

-Develop exposed photoresist.

-Etch exposed areas of chrome using Ceric Sulfate solution.

II. Fabrication of Top Electrodes

-Verify the location of the two 1 mm diameter drilled holes top glass cover plate.

-Clean top cover.

-Deposit chrome layer using E-Bean deposition.

-Spread Shipley 1827 over metal layer.

-Use top electrode mask to define areas for chemical etching.

-Develop exposed photoresist.

-Etch exposed areas of chrome using Ceric Sulfate solution.

III. Fabrication of Hydrodynamically Shaped Channel

-Clean and dehydrate glass plate with etched bottom electrodes.

-Spin a 15 um layer of SU-8.

-Soft bake.

-Important to align the channel registration marks with bottom

Electrode registration marks.

-Expose SU-8 using Channel Mask set.

-Develop exposed SU-8.

-Check channel features with microscope.

-Hard bake the SU-8.

Bottom Electrode Mask

Top Electrode Mask

Channel Mask

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IV. Mating Cover plate and top electrodes to channel and bottom electrodes

-The cover plate and its top electrodes may is attached by spreading a very thin layer of SU-8 (1

– 2 um)

-Align the top electrode registration marks to the channel’s registration marks.

-Press gently and allow to set.

-Verify alignment and bake SU-8.

V. Coupling Electrodes to Sending Electronics

-Using silver epoxy, adhere the wires from the sensing electronics to the channel’s electrodes.

-Includes the driving current source and voltage pick-off wires.

VI. Components Used

-The following components are used in the construction of the fluid coupling and supplying the

channel with a filtered down, micro particle filled solution. The Nanoport assemblies are aligned

to predrilled holes in the top glass plate. The holes have a diameter of 1 mm. When aligned, the

Nanoport assemblies are adhered to the glass plate, creating a tight seal. The inline filter will

filter down the solution to microparticles with diameters less than 10 um. This will also lower

the risk of having the channel become blocked with particles larger than the cross-sectional area

of the channel.

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VII. Next Steps

-Mask Set Current mask set is designed using FreeHand. 20in x 20in printed film mask to be reduced to 1in x 1in using photo-reduction. -Have mask set printed All masks can be printed as film masks by: Page One Digital

2372 Morse Avenue Irvine, CA 92614 949-851-1530 http://www.pageonedigital.com/

Price for printing: $0.18/in2

Largest film print possible: 21.5in x 12.5 in = 462.25 in2

-Photo-reduction See 20:1 photo-reduction notes.

Phoro reduction plates are available in the clean room. The box of high precision mask plates belonging to the Nelson group can be found in Photolithography room, on the shelf next to the KarlSuss Mask aligner. The box is black and taped up with the name of the group written on the label. Slightly exposed edges, but still useable since the centers can still be exposed and developed during mask reduction. IMPORTANT: only open box in dark room, where photoreduction takes place (red light source, not yellow) -Cleaning See RCA cleaning. Note: Try to use exact measurements. Miscalculations usually result in a residue being deposited on the glass surface, which can greatly affect the quality of the metal being evaporated onto it. -Photolithography Future channel will be realized using SU-8. See SU-8 datasheet. It is recommended to use a thinner SU-8 to achieve thicknesses of around 15 um. It is also suggested to use a thicker SU-8 but to mix in a thinner to bring down the thickness. Glass plate replacements can be bought at any store like “Home Depot” Dimensions: FeO masks can be bought from the INRF. Ask Vu for help in all INRF purchases. vutp@uci.edu

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-E-Beam See notes on E-Beam evaporation process. Cr has only been used as electrode material. Look into using other metals like Gold which can stand up to a NaCl solution and not prone to oxidation. -Drilling 1 mm diameter holes can be drilled into 1in x 3in glass slides by: UC Irvine Glass Shop 123 Rowland Hall 949-824-6643 -Order tubing, and connectors from Upchurch: Upchurch Scientific, a division of Scivex P.O. Box 1529 619 Oak Street Oak Harbor, WA 98277 http://www.upchurch.com/ National and International Phone (800) 426-0191 or (360) 679-2528 National and International Fax (800) 359-3460 or (360) 679-3830 Customer Service & Sales customerservice@scivex.com General Information upchurch@scivex.com Marketing info@scivex.com Technical Service customerservice@scivex.com