Microengineering MEMs and Interfacing - Danny Banks Part 6 pdf

Fabrication of polyimide structures is normally performed on a polished silicon wafer to provide a convenient flat rigid substrate to which the material can be applied and which holds it

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80 Microengineering, MEMS, and Interfacing: A Practical Guide widespread form is photolithographic processing in the form of photoresists, poly-imides, and photoformable epoxies (SU-8; the latter also being a class of photore-sist) At the time of writing, these were closely followed by PDMS casting, but hot embossing appears to be making its mark as a mass production technique This section also addresses stereolithography and microcontact printing The latter is a lithographic technique that is not restricted to polymers but can also pattern bio-molecules, for instance However it usually requires a polymer (PDMS) original

3.6.1 P OLYIMIDES

Polyimides are UV photoformable polymers that are common in the electronics industry These have several different trade names and different properties Fabrication of polyimide structures is normally performed on a polished silicon wafer to provide a convenient flat rigid substrate to which the material can be applied and which holds it flat during subsequent machining steps Polyimides are usually spun on and patterned using conventional UV lithography techniques, usually to several microns thickness Metal films can also be deposited, patterned, and sandwiched between layers to provide a variety of different electrode or inter-connection structures Polyimide structures are often used as part of the packaging

of silicon microsystems — they are flexible and more robust than individually bonded wires The silicon die must be bonded to the ribbon cable either by con-ventional wire-bonding processes or flip-chip techniques (see Chapter 9) Unfortu-nately many polyimides are not very resistant to ingress of water

3.6.2 P HOTOFORMABLE E POXIES (SU-8)

SU-8 (from Microchem Corp., U.S.) is a photoformable epoxy (negative photo-resist) that has gained a considerable following among the MEMS community

It is easy to see why — it is available in several different formulations and can

be applied in films of 1 µm to 200 µm thickness in a single spin process, can be exposed using standard UV exposure equipment, and produces high-aspect-ratio structures (10:1 or better) with relatively straight sidewalls It is also highly resilient to chemical attack As a result, microstructures can be produced in SU-8 with a relatively low initial capital investment

Owing to its popularity, there are several data sheets and application notes available directly from manufacturers and distributors and other data available

on the Internet Once the processes required to apply, expose, and develop SU-8 have been mastered, the main problem encountered appears to be its removal As

an epoxy, it is exceedingly stubborn to remove and, at the time of writing, it seems most appropriate to advise the users that if they need to remove the hard-baked (cured) SU-8, they are probably not going to be able to do it very well Nonetheless, there are three possible options that have been suggested: plasma ashing, laser ablation, and use of release layer

A release layer is particularly useful if the SU-8 is to be used with electro-plating to create metal microstructures (as in the LIGA process) The release layer is basically a thin coat of photoresist that is applied beneath the SU-8 film DK3182_C003.fm Page 80 Monday, January 16, 2006 12:44 PM

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Nonsilicon Processes 81

When the SU-8 is to be removed, the release layer is simply stripped away, taking the SU-8 with it (see Figure 3.5)

One additional caveat: although the cured SU-8 may be particularly stubborn, this does not mean that it can sustain prolonged attack in KOH or EDP; it may suffer adhesion problems

3.6.3 P ARYLENE AND PTFE

Two other polymers that can often be found in MEMS laboratories are parylene and polytetrafluoroethylene (PTFE) Parylene is usually deposited by CVD It is

a particularly stubborn material and difficult to pattern It is also difficult to achieve a good conformal coating without pinholes or defects

PTFE is normally available in spin-on form Again, it is difficult to pattern and usually only used if absolutely necessary (such as encapsulating devices for implantation in the body) It is very difficult to get anything to adhere to PTFE, and it usually requires some sort of surface treatment if additional films are to

be deposited and patterned on it The most common of these is treatment with oxygen plasma to roughen and chemically activate the surface to some degree

3.6.4 D RY F ILM R ESISTS

Developed for printed circuit board (PCB) processing, dry film resists are not commonly used for micromachining They can, however, be used to create various microstructures

The resists are normally available as films of different thicknesses ranging from 50 to 100 µm They are laminated onto the substrate (or a proceeding patterned and developed resist layer) by a roller laminator at an elevated temperature The material can then be patterned and developed to create various microstructures,

FIGURE 3.5 LIGA using SU-8: (a) a substrate (coated with a nickel seed layer for electroplating) is coated first with a thin layer of photoresist and then SU-8, (b) the SU-8 and resist are patterned, (c) electroplating is used to form a nickel structure, (d) the photoresist layer is then stripped off, taking the SU-8 with it and leaving the metal structure (A detailed description can be found on the OmniCoat data sheet from MicroChem Corp, Newton, MA, U.S www.microchem.com )

SU-8 Resist DK3182_C003.fm Page 81 Monday, January 16, 2006 12:44 PM

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82 Microengineering, MEMS, and Interfacing: A Practical Guide

although there are limitations — aspect ratios and the angle of channel walls are somewhat limited, as is the resolution achievable By applying the resist to conventional PCB substrate material (FR4), it is possible to make use of the expertise and relatively low costs available for PCB production Figure 3.6 illus-trates construction of a simple chamber using dry film resist Closed channels for microfluidic applications can also be produced in this manner

3.6.5 E MBOSSING

One of the most promising methods for mass production of microstructures is the hot-embossing process In theory, the process is relatively simple A mold insert is created by one of a number of micromachining processes, usually bulk silicon micromachining or nickel electroplating in a LIGA-related process The mold and target material are heated until the chosen polymer becomes plastic, and the mold

is then pressed into the plastic so that it takes up the impression of the structure The mold is removed and the plastic sets in the desired shape (Figure 3.7)

In practice, the process is not quite so simple The mold insert will probably have a different coefficient of thermal expansion than the polymer, so in the best of circumstances the final dimensions of the plastic structure will not be the same as those of the mold Furthermore, the process has to be controlled to ensure a clean release, and parameters will have to be adjusted for each different material to be used Fortunately, however, there are two commercially available processes The first (Figure 3.8) is based on macroscale mass production techniques and can be run continuously The insert has to be flexible enough to be fitted around a roller,

FIGURE 3.6 Creating a chamber using dry film resist: (a) the resist is applied to the substrate using a roller, (b) it is exposed through a mask to UV light, (c) the first layer is developed and a second layer is applied, (d) the second layer can then be patterned and developed as required.

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FIGURE 3.9 Jenoptik hot-embossing machine with microstructured polymer wafer Reproduced courtesy of Application Center for Microtechnology (AMT), Jena, Germany ( www.amt-jena.de ).

FIGURE 3.10 Nickel mold insert for hot embossing, 100-mm diameter (Courtesy of Application Center for Microtechnology [AMT], Jena, Germany www.amt-jena.de.)

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FIGURE 3.11 Polymer microstructures formed by hot embossing: (a) channels in a cyclic olefin copolymer substrate, (b) PMMA microtitre plate (Courtesy of Application Center for Microtechnology [AMT], Jena, Germany www.amt-jena.de )

Acc.V 16.0 kV Magn400x 50 µm

Topas

(a)

Acc.V

Titerplatten

(b) DK3182_C003.fm Page 85 Monday, January 16, 2006 12:44 PM

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PDMS can be cast or spun over a master structure created in a silicon wafer (or other material, including SU-8) (Figure 3.12) One of the key problems to be overcome in this case is to effectively release the cast PDMS from the master Effenhauser and colleagues [1] silanized the silicon with a dimethyloctadecyl-chlorosilane solution, but others have tried alternative approaches The next problem is that of ensuring good adhesion between the PDMS and the silicon, PDMS, or glass substrate that has been used to close the channel system As with all micromachining processes, both clean flat surfaces and ensuring that the structure has not been exposed to the environment for a long period after casing, appear to help Effenhauser and colleagues found that they could place the PDMS down and peel it off again should the channels become clogged After cleaning the PDMS they could replace it and continue to use the device

Others are seeking more permanent bonds for their devices or attempting to make more complicated three-dimensional structures Various approaches have been tried with various degrees of success These include treating both surfaces with PDMS prior to bonding and use of HDMS primer Ensuring that the substrate is very dry (drive off water on a hot plate) also seems to be a key to achieving a good bond

FIGURE 3.12 Process for creating a microchannel in PDMS: (a) original structure, (b) PDMS is cast over the mold, the PDMS is then peeled off, and holes are punched

in it if necessary, (c) it is then applied to a flat substrate, such as a glass slide, creating

a microchannel with two reservoirs.

(c)

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3.6.7 M ICROCONTACT P RINTING

This is an interesting approach that makes use of PDMS structures to create quite

complex structures, usually in metal A variety of related techniques have been

developed by Whitesides’ group [2,3] The basic approach involves the use of

PDMS masters to print long-chain molecules onto an appropriate substrate

This has been used to print biomolecules onto various substrates, but the approach

favored by Whitesides’ group involves chemistry related to self-assembled

mono-layers; long-chain molecules that spontaneously self-organize when printed onto

the appropriate substrate — in particular, alkanethiols on silver and gold

sub-strates (again, this chemistry has also be used in the creation of biosensors)

The process is shown in outline in Figure 3.13 One of the advantages of this

approach is that it is not limited to flat substrates; by applying it imaginatively

to capillaries, Jackman and colleagues [2] have produced a variety of interesting

structures

Stereolithography is a well-developed process that is employed to produce

three-dimensional prototype structures for macroscale engineering The overall process

is illustrated in Figure 3.14

A stage is immersed just under the surface in a bath of UV-curable polymer

UV light is then focused onto the surface of the liquid, causing the polymer to

solidify where the illumination is most intense One layer of the structure is

formed by scanning the spot over the surface of the polymer, turning the beam

on and off as required The stage is then lowered deeper into the liquid and

FIGURE 3.13 Outline of microcontact printing process: (a) a silicon pattern is used to create

a PDMS stamp, (b) a PDMS stamp, (c) this is inked, (d) this is applied to an appropriately

prepared (e.g., gold coated) substrate, (e) the chemical ink remains on the substrate at points

of contact; the PDMS stamp can be wrapped around a roller and used in a continuous process.

(d) (c)

(e)

PDMS Silicon pattern

Ink DK3182_C003.fm Page 87 Monday, January 16, 2006 12:44 PM

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by the “Super IH” process appear a little more organic and less precise than those

created by more conventional processes, but this can probably be improved upon

3.7 ELECTRICAL DISCHARGE MACHINING

Electrical discharge milling (EDM), also commonly known as “spark erosion,” is

another precision macroscale machining process, capable of working to micrometer

tolerances, that is being used or adapted for micromachining of metals

EDM can only be used with conducting materials and is usually employed

in the precision machining of very hard metal alloys As one may anticipate, the

process involves creating a series of sparks between the workpiece (substrate)

and an electrode (mandrel), which is maintained at a positive voltage with respect

to the workpiece Each spark takes with it some small quantity of the material

being machined A dielectric liquid is employed to control the spark discharge

process and cool the workpiece

EDM is normally deployed in one of three modes (Figure 3.16): hole boring,

shaped working electrode, and wire EDM The first two rely on the fact that EDM

is a noncontact process (so no mechanical forces are applied to the working

electrode) in order to use a soft and easily shaped material to machine a much

harder material EDM hole boring is capable of creating holes with micrometer

dimensions Note, however, that the working electrode will degrade with use and

may even have to be reshaped (by reversing the EDM) or replaced during the

process For this reason, wire EDM was developed (Figure 3.16c) Here, the

working electrode is a wire that is continually drawn past the workpiece Thus,

there is always a new part of the electrode available for machining

Micro-EDM systems have been developed for microengineering applications

These typically employ three-axis positioning systems with micrometer XY

accu-racy and a smaller working electrode This, by itself, poses a problem because

FIGURE 3.16 EDM modes: (a) hole boring, (b) shaped working electrode, (c) wire EDM.

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the smaller electrode degrades very rapidly during machining For this reason,

different machining procedures and electrode shapes are under development to

extend the electrode life

3.8 PHOTOSTRUCTURABLE GLASSES

Micromachined glass is a popular alternative to silicon in many applications,

especially biological, where glass is relatively inert and transparent,

allow-ing biological processes to be viewed directly under an optical microscope

The normal process for patterning glass is to apply a photoresist, expose, and

pattern it using either BHF or RIE, the latter being used where anisotropic

etching is required To be able to pattern and anisotropically etch glass plates

without the use of a photoresist or RIE system would considerably simplify

the process of producing glass components and result in cheaper components

To this end, various photosensitive glasses have been developed These are

generally based on silver compounds introduced into the glass UV exposure

results in free silver atoms being released in the exposed areas The glass is

then heat treated so that it crystallizes around the free atoms The result is that

the exposed glass etches at up to 20 times faster than unexposed glass in 10:1

HF The process is outlined in Figure 3.17 Note that the depth of the structure

produced in the glass will be dependent on the etching time and etching rate

of the exposed glass, and the sidewall quality will depend both on the quality

of the illumination (divergence, for instance) and the selectivity ratio of exposed

to unexposed areas

There are two problems that presently limit the wider uptake of this process

The first is that the etch rate and final results depend strongly on process

param-eters, and it can take some time to set up a reliable system The second is that

heat treatment of the glass may take several hours Although it only needs to be

held at an elevated temperature of between 500°C and 600°C for 1 to 2 h to effect

crystallization, considerable time may be required to ramp up and down to these

temperatures to ensure goodresults

FIGURE 3.17 Process for patterning photosensitive glass (FOTURAN): (a) the glass is

exposed to UV light through a mask, (b) following heat treatment, the exposed glass

crystallizes, (c) the crystallized glass is then etched in 10:1 HF.

UV DK3182_C003.fm Page 90 Monday, January 16, 2006 12:44 PM

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Nonetheless, the process can achieve quite remarkable results given the

rel-atively simple equipment requirements Figure 3.18 shows the cross section of a

high-aspect-ratio channel etched in photostructurable glass

3.9 PRECISION ENGINEERING

Various techniques developed under the banner of precision engineering have

either been adapted directly as microengineering processes (CMP and EDM, for

example, or wafer-dicing techniques from Chapter 2) or fall into the categories

of microengineering and nanotechnology by dint of the results that they are

capable of achieving Many of the tools involved in precision engineering have

been discussed elsewhere; they include:

• Solid cutting or abrasive tools (e.g., diamond saw blades used in wafer

dicing Chapter 2, section 2.8.1)

• Free abrasives (in fixed abrasive processes; e.g., CMP Chapter 3,

section 3.2)

• Scanning tip tools (e.g., STM and AFM Chapter 10, section 10.3)

FIGURE 3.18 Cross section of channel etched in photostructurable glass (FOTURAN),

1-mm-thick substrate, 1 degree slope on wall, 100-µm-wide channel (approximate) (Image

courtesy of mgt mikroglas technik AG, Mainz, Germany www.mikroglas.com )

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