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194 E. Meng et al.

Fig. 4.1 Chemical structure

of SU-8 which is a glycidyl

ether of bisphenol-A novolac

(Adapted from [5])

Chemicals), gamma-butyrolactone (GBL) solvent, and triaryl sulfonium salts as

a photosensitizer (5–10% by weight) [1–3]. The high functionality of the epoxy

(eight reactive epoxy groups per monomer unit) gives rise to t he polymer name and

also affords improved sensitivity over other formulations (Fig. 4.1)[4]. Subsequent

SU-8 chemistries utilized other solvent formulation including methyl iso-butyl

ketone (MIBK) and propylene glycol methyl ether acetate (PGMEA) [4]. In 1996,

MicroChem Corporation (Newton, MA) commercialized SU-8 offering the product

line NANO

XP SU-8 which was then followed by the SU-8 2000 and 3000 lines.

These new formulations address some coating and adhesion issues encountered in

the original formulation. For example, SU-8 2000 was formulated with cyclopen-

tanone (C

O) solvent, which according to MicroChem, offers improved coating

and adhesion properties.

SU-8 is a negative, near-UV resist, and a thermoset polymer. Although ﬁrst

used in 1997 as a replacement for X-ray lithography in the LIGA process (a

German acronym for Lithographie Galvanoformung und Abformung)[6], SU-8 is

an extensively employed MEMS material and was reviewed in [7].

A typical SU-8 lithography process includes the following step sequence.

• Spin coating

• Softbaking (extraction of solvent)

• UV exposure

• Post/hard baking (cross-link polymer)

• Development (may be enhanced with ultrasonic/megasonic bath or by spray)

Softbaking conveniently enables SU-8 ﬁlms to self-planarize and smooth any

edge bead build-up [3]. SU-8 possesses a high solids content (72–85% by weight)

and low absorbance in the near-UV spectrum (∼46% at 365 nm) which enables thick

structures of a few hundred microns (over 500 μm with a single spin) [2, 4]. Thicker

structures can be created with multiple layers; just two layers of SU-8 can produce

1.2 mm thick structures with an aspect ratio of 18 [3]. By repeating lithography

steps, it is even possible to build open multilevel SU-8 structures [8–11]. When

individual layer thicknesses greater than 1 mm are desired, it is advantageous to use

a single constant volume injection step over multiple spin coating steps. SU-8 is

4 Additive Processes for Polymeric Materials 195

dispensed over a deﬁned area and spread manually (at 80

◦

C) to produce ﬁlms up to

1.5 mm thick [12]. Thicker layers are limited by resist overﬂow at substrate edges.

Six-layer SU-8 tissue engineering scaffolds have been reported [11].

UV exposure triggers generation of a strong acid that initiates the cross-linking

process [13, 14]. Inclined or inclined/rotated lithography enable the production of

three-dimensional structures using just a single layer of SU-8 and one mask [15].

Reﬂected UV energy can also be recovered to produce exotic structures. Three-

dimensional structures can also be produced by exposing (with or without rotation)

in multiple directions through a UV-transparent substrate (e.g., glass, sapphire, or

quartz; Fig. 4.2)[16]. In addition to standard near-UV lithography, it is also pos-

sible to employ SU-8 as an X-ray resist. In fact, SU-8 possesses greater sensitivity

and shorter exposure times compared to standard polymethylmethacrylate (PMMA)

resists. Aspects ratios of up to 100:1 have been reported [17–19].

Fig. 4.2 (a) Backside exposure through a UV-transparent substrate. (b) Three-dimensional SU-

8 structures produced by multidirectional and rotational back-side inclined lithography (Original

The postbaking step completes cross-linking of the UV-exposed polymer. SU-

8 exhibits a high degree of cross-linking that enhances its physical properties but

also induces stress on ﬁlms and structures. Stress in SU-8 and methods to reduce its

effects are discussed in detail in the “Lessons Learned” section below. Finally, SU-8

is usually developed in PGMEA.

4.1.1 Material Properties

SU-8 is well known to possess excellent thermal and chemical stability as a result of

the aromatic nature of the resin and high degree of cross-linking [6, 20]. In particu-

lar, its chemical stability facilitates its application as an electroplating mold and as

an etch mask, especially for prolonged plasma etching [4]. Its mechanical strength

allows it to be a planarization layer in chemical mechanical polishing applications

196 E. Meng et al.

[21]. However, it is well documented that the properties of SU-8 can vary widely

with processing conditions [22, 23]. Further control of SU-8 properties is achieved

by the addition of functional materials to the resist; tuning of electrical [24, 25],

magnetic [26], optical [27, 28], and mechanical [29–31] properties was reported by

several groups.

Bare SU-8 surfaces are hydrophobic and possess low surface energy (∼73–80

◦

and 45 mJ/m

, respectively [32]) which limits their usefulness i n microﬂuidics and

as substrates for cell culture. Several surface modiﬁcation techniques have been

suggested to obtain hydrophilic SU-8 surfaces required for various applications.

To facilitate ﬁlling of microchannels, an ethanolamine treatment was used [33].

Protein immobilization or cell attachment was promoted by grafting various poly-

mers (polyethylene glycol (PEG), polyacrylic acid, and polyacylamide) to SU-8

via cerium(IV) ammonium nitrate [34]. Hydrogels grafted to SU-8 improved sur-

face wettability and integration of bioanalytical functional layers such as enzymes

and antibodies [35]. PEG has also been attached covalently to SU-8 surfaces [36].

By modifying PEG concentration or molecular weight, protein adsorption and cell

attachment can be tuned. PEG as a MEMS material is discussed later in this chapter.

In addition to the aforementioned wet surface modiﬁcation methods, hydrophilic

SU-8 surfaces were also demonstrated using dry techniques. Hot wire chemical

vapor deposition of amine groups facilitated immobilization of antibodies onto

SU-8 cantilevers [37]. Oxygen plasma reduced the contact angle to 21

◦

to facili-

tate ﬁlling of microchannels [38, 39]; argon plasma oxidized and roughened SU-8

surfaces (contact angle reduced to 29

◦

)[32]. In contrast, efforts to render SU-

8 superhydrophobic with low hysteresis using an excimer laser technique were

reported (165

◦

)[40]. It is also possible to increase the hydrophobicity of SU-

8 simply by increasing the density of patterns on a surface (contact angle up

to 147

◦

)[41].

The versatility of SU-8 as a MEMS material has led to its extensive applica-

tion in microﬂuidics and biomedical devices. However, SU-8 is not a USP Class

VI material and the manufacturer (MicroChem) prohibits its use in implantable

devices. Even so, initial studies indicate acceptable chemical compatibility and bio-

compatibility [42]. SU-8 coupons implanted in rats resulted in reduced biofouling

in comparison to other common MEMS materials [43].

4.1.2 Processing Variations

4.1.2.1 Partial Exposure

By using multiple exposure wavelengths (313 and 365 nm), multiple exposure

depths are possible in SU-8. Thus, multilevel structures are achievable with only

a single spin coat [44]. This method was improved by adding antireﬂection coatings

to the substrate to reduce unwanted reﬂections and achieve more accurate con-

trol of feature dimensions [45]. Layers that partially absorb UV exposure (SU-8

mixed with positive resist) have been successfully employed to produce multilevel

4 Additive Processes for Polymeric Materials 197

SU-8 structures [27]. Embedded metal masks have also been used in a similar

manner [46]. Absorbing layers were particularly useful in the creation of embedded

microchannels.

4.1.2.2 Direct Writing

Intricate structures having ﬁne nanometer-scale dimensions have been produced by

direct writing with e-beam writing [47]. Nd:YAG lasers and proton beams have

also been explored for creating embedded microchannels and cantilever structures

[48–50]. By combining UV lithography with stereolithography, three-dimensional

SU-8 structures were fabricated [51, 52].

4.1.2.3 Removal of SU-8

Chemical removal of SU-8 occurs through crazing, peeling, delamination, or crack-

ing but not through dissolution. Dissolution remains difﬁcult due to the high

cross-link density of the polymer. Chemistries suitable for removing SU-8 include:

• Hot N-methyl-2-pyrrolidone (NMP) [6, 53]

• H

[54]

• H

[53]

• Fuming HNO

[53]

• NANO

RemoverPG and Omnicoat

(MicroChem Corporation, Newton,

MA) [55]

• ACT-1(Air Products and Chemicals, Inc., Allentown, PA) (dimethylacetamide

and monoethanolamine) [55]

• QZ3322 (Arch Chemicals, Norwalk, CT) (ethanolamine and tetrahydro-2-

furanmethanol) [55]

• MS-111 (Miller-Stephenson, Danbury, CT) (methylene chloride, phenol, and

organic acids) [56]

• Magnastrip (Inland Technologies, Tacoma, WA) (NMP-based) (70

◦

C) [56]

• RS-120 (Cyantek, Fremont, CA) (sulfolane-based) (100–120

◦

C) [56]

• Molten salt bath (Kolene

 No. 5 and Kolene

 No. 10, Kolene Corporation,

Detroit, MI) (350

◦

C) [56]

Note that although MS-111 is more effective than NMP i n removing SU-8, it is

rather toxic. Instead, Magnastrip and RS-120, which are less toxic, can be used [56].

Other SU-8 removal methods include pyrolysis, plasma etching, laser ablation,

and water jet machining:

• Pyrolysis in O

or air (300–1000

◦

C) [54, 56–58]

• O

and O

/CF

plasma etching (0.9 μm/min for O

and 1.27 μm/min for

/CF

)[6, 53, 59, 60]

• O

/CF

downstream chemical etching (2.1 and 10 μm/min at 185 and 275

◦

respectively) [56]

198 E. Meng et al.

• O

/SF

plasma etching (1.2–2 μm/min and improved sidewall angle but

decreased etch rate with the addition of Ar) [61]

• Excimer laser ablation (248 nm KrF laser) [62]

• Water jet machining [63]

4.1.2.4 Release of SU-8

SU-8 release allows liftoff of electroplating molds and the formation of free-

standing structures. The following techniques have been investigated for SU-8

release:

• NANO

RemoverPG (80

◦

C) [64]

• Thin metal sacriﬁcial layer and wet chemical etching (SiO

[26], Al [3, 65], Ti

[66], Cu [67, 68], Cr [69–71], Cr/Au/Cr [72]) (for release of 1–2 mm structures)

• Thick ﬁlms (electroplated Cu [73], polystyrene [74], positive photoresist [66, 75],

low temperature oxide [76], thermal oxide [13], whole Pyrex wafers [13], other

oxides [77–79], polysilicon [80], whole Si wafers [81]) (for release of structures

>2 mm)

• Peeling of SU-8 from release layer with poor adhesion to SU-8 (release layers

include: spin on Teﬂon

[82], plasma-deposited ﬂuorocarbon [83], poly-

imide [20, 23, 84, 85], polyethylene terephthalate (PET) [86], self-assembled

monolayers (SAMs) [77, 87, 88]) [70]

• Ultrasonic agitation of SU-8 on oxidized substrate [10]

4.1.2.5 Bonding

SU-8 can be joined to other polymers either through adhesive bonding or by

lamination processes. Adhesives compatible with SU-8 include epoxy resin [89],

UV curable epoxies [9, 12, 90], polymethylmethacrylate (PMMA) [91–93], and

polydimethylsiloxane (PDMS) [94]. Lamination is achieved by using Riston

(DuPont, Research Park Triangle, NC) (50

◦

C) [95], benzocyclobutene (BCB) (Dow

Chemical, Midland, MI) (pressure for 2 h and 200

◦

C) [95], and GHQ120 crystal

clear laminating ﬁlm (GMP, Germany) (120–125

◦

C) [94].

In addition, SU-8 itself is also widely used as an adhesive agent to join individual

coupons, dies, and wafers together. SU-8 can be used in a variety of different states

to achieve effective bonds including wet [96], softbaked [10], and semisolid [20].

Wafer-level bonding can be achieved with or without commercial wafer-bonding

tools by controlling the vacuum level, pressure, and temperature [97–101]. In addi-

tion, there are several reports of achieving wafer-level bonding with very thin SU-8

layers [94, 102–105]. Special structures such as moats around channels prevent the

reﬂow of uncross-linked SU-8 from ﬁlling channels and other gaps [101]. Also,

conversion of the SU-8 surface to a hydrophilic one can promote bonding [39, 94].

For example, hydrophilic SU-8 (obtained by adding surfactant (10–40% trisiloxane

alkoxylate, Silwet∗618, GE)) can be bonded to glass, Si, and PDMS without the

application of pressure [106].

4 Additive Processes for Polymeric Materials 199

4.1.2.6 Transfer

An alternative route to the fabrication of multilayered or even free-standing struc-

tures in SU-8 combines bonding with transfer of SU-8 ﬁlms. Flexible carrier

substrates support the SU-8 during transfer and are then peeled off. Multilayer

transfer has been demonstrated [ 84]. Carrier substrates used include Riston [8],

polyimide [20, 84, 85], and PET [86, 107].

4.1.2.7 SU-8 as an Etch Mask

SU-8’s excellent chemical resistance enables its use as an etch mask in both wet and

dry processes:

• HF etching of glass [54]

• Electrochemical etching of nanoporous and macroporous Si [108]

• Reactive ion etching of Si in SF

/CBrF

and SF

/CHF

[2, 5]

• Plasma etching processes involving SF

[5] and CHF

/Ar [109]etch

chemistries

4.1.3 Lessons Learned

The many processing challenges encountered when using SU-8 are the focus of

several process optimization articles [14, 22, 54, 110–113]. Dimensional accuracy

and sidewall proﬁle are linked to exposure. Thicker ﬁlms require longer wavelengths

for uniform exposure [113]. A “T-topping” phenomenon where the sidewall proﬁle

has a T-shape from the higher adsorption of shorter wavelengths is associated with

using broadband UV sources. This effect is eliminated by applying a ﬁlter in the

exposure path that removes wavelengths below 350 nm [44]. Aspect ratios can be

increased by using megasonic agitation during development (up to 100:1, standard

lithography) [114]. Megasonic development also decreases the overall development

time.

Large stress gradients in SU-8 structures are often accompanied by out-of-plane

curvature or fracture (Fig. 4.3)[115]. SU-8 ﬁlms undergo a high degree of cross-

linking during the postbake step. Although the cross-link density imparts many

desirable physical properties, it also leads to brittleness and signiﬁcant shrinkage

(∼7.5%) [8, 70]. If SU-8 is spun on an Si substrate, the fourfold difference in the

coefﬁcients of thermal expansion ( CTE) of the two materials also results in high

residual stress (16–19 MPa tensile stress [116]). At the very least, bowing of the

substrate results [2, 3, 53] and in the worst case, there is delamination of t he SU-8

[70].

Stress reduction in SU-8 ﬁlms is achieved through a variety of processing mod-

iﬁcations. A low-speed spin coating process followed by a polymer relaxation

200 E. Meng et al.

Fig. 4.3 Out-of-plane

bending in a free-standing

SU-8 spring element

(Reprinted from [115] with

permission from Elsevier,

period prior to softbaking was found to reduce stress [14]. The curvature of SU-

8 parts can be either positive, negative, or near-zero simply by optimizing exposure

and postbaking steps [110, 117]. Both of these parameters follow similar trends:

increasing the duration of exposure or postbake temperature increases the glass

transition temperature as well as the cross-link density [23, 110]. Controlled tem-

perature ramping at slow rates and multistep baking reduces thermal shock [14,

73]. It is noted that postbake steps carried out in vacuum environments may relieve

stress [70].

Stress is further minimized during the design process. Large patterns should be

divided into smaller areas to reduce thermal shrinkage-induced stress [9, 12, 73,

105]. For example, microchannels should be designed with thin walls and support

posts [105]. Structures such as expandable fences and circular geometries may guide

and distribute intrinsic stress [73].

Two additional stress-reduction strategies include modifying the SU-8 for-

mulation (the addition of silica nanoparticles [29] or direct modiﬁcation of

the resist chemistry [118]) or the use of CTE matching layers or stress bar-

rier layers. Multiple polymer matching layers have proved effective (Parylene

C[119], PMMA, polyetheretherketone (PEEK) [20], and adhesive PET [86])

[53, 77]. Metal layers serve dual roles as a stress barrier and compensator

[14, 78].

The use of intermediate layers may also promote adhesion of SU-8. SU-8 adhe-

sion is gained by using adhesion promoter on substrates such as glass, Al, Cu, and Cr

[55, 70]. SU-8 does, however, exhibit good adhesion on Au and Si without the use

of adhesion promoters [6]. Examples of adhesion promoters include metal, thin SU-

8 layers, and silanes (3-aminopropyltriethoxysilane (APTES), (for glass and Al),

and AP300 (Silicon Resources, Chandler, AZ) (for stainless steel)) [55, 70, 105].

Reducing exposure dose and softbake time have been shown to improve adhesion

but compromise aspect ratio [55].

4 Additive Processes for Polymeric Materials 201

4.1.4 Examples of SU-8 Application

SU-8 is a versatile material for the fabrication of microﬂuidic and lab-on-a-chip

devices using a variety of different processing methods [120]. A diverse range of

methods exist to produce simple or complex microﬂuidic channels and cavities

[13, 45, 46, 48, 54, 65, 84, 86, 95, 98, 102, 105, 106, 121–123]. Microreactors

are conveniently formed by deﬁning cavities bounded by SU-8 walls [89, 90, 124].

Soft Parylene microchannel access ports are strengthened by patterning SU-8 sup-

ports [125, 126] or by deﬁning anchors to secure PDMS septa for needle-based

access to channels [119]. Other microﬂuidic structures produced with SU-8 include

micromixers [ 10], microﬂuidic oscillators [93], nozzles [100], needles [127, 128],

and check valves [67, 73].

4.2 PDMS

Polydimethylsiloxane (PDMS) is a commercially available cleanroom compatible

silicon-based organic polymer. PDMS has a repeating unit of (CH

)

SiO [129]as

shown in Fig. 4.4. The properties of PDMS are attributed to and can be explained by

the molecular structure. For example, the relatively low density and high diffusivity

of gases are primarily the consequence of the larger Si–O and Si–C bond lengths

compared to the C–C bond length, and the very low glass transition temperature

and deformability are mostly related to the polymer chains packing and the amount

of so-called “free volume.” The solidiﬁcation (curing) of PDMS is rationalized by

an organometallic cross-linking reaction, as illustrated in Fig. 4.5. Notice that the

siloxane base oligomers contain vinyl groups. The cross-linking oligomers contain

at least three silicon hydride bonds each. The platinum-based curing agent catalyzes

the addition of the Si–H bond across the vinyl groups, forming Si–CH

–CH

–Si

linkages. The multiple reaction sites on both the base and cross-linking oligomers

allow for three-dimensional cross-linking. If the ratio of curing agent to base is

increased, a harder, more cross-linked elastomer results. Heating the prepolymer

also accelerates the cross-linking reaction.

Thanks to the unique properties and simple processing, PDMS is currently used,

for instance, as the mechanical interconnection layer between two silicon wafers, as

Fig. 4.4 The molecular structure of PDMS (R = CH

in the structure shown here for PDMS)

202 E. Meng et al.

Fig. 4.5 Three dimensional cross-linking of PDMS. Used with permission from [134], copyright

1999, Division of Chemical Education, Inc.)

ion-selective membranes on ISFETs [130], and as the spring material in accelerom-

eters [131]. It can be used as the top elastomer on a tactile sensor [132] without

inﬂuencing the sensitivity of the device and a ﬂexible encapsulation material in

order to mechanically and chemically decouple sensors f rom their environment

[133]. Furthermore, PDMS is used in sensors with integrated electronics due to its

low-curing temperature.

4.2.1 Material Properties

This polymer is optically transparent with a UV cutoff of 240 nm, allowing for

optical detection from 240 to 1000 nm. Some physical and chemical attributes of

PDMS are (compared to other polymers) a low glass transition temperature (T

∼ –125

◦

C) [135], a unique ﬂexibility (the shear modulus, G, may vary between

100 kPa and 3 MPa) [135], a very low loss tangent (tan δ<<0.001) [136], small

temperature variations of the physical constants (except for the thermal expansiv-

ity, α ∼ 20 × 10

−5

−1

[137]), a high dielectric strength (∼14 V μm

–1

)[137],

a high permeability to water vapor, gases, and nonpolar organic solvents [138],

an impermeability to liquid water, a high compressibility, usability over a wide

temperature range (at least from −100

◦

Cupto+100

◦

C) [139], a low chemical reac-

tivity, and a nontoxic nature. Some typical material properties of PDMS are listed

in Table 4.1.

PDMS is a viscoelastic material [146–148], and its mechanical properties

change with loading frequencies [149, 150], and elapsed time durations [151,

152]. Although traditionally the viscoelastic properties have been neglected, recent

advancements in material characterization have shown that ignoring the time- and

frequency-dependent characteristics of PDMS results in errors in cellular force

measurements, and erroneous interpretation of the measurement data of biologi-

cal mechanisms. Hence, a more accurate model was determined through a material

4 Additive Processes for Polymeric Materials 203

Table 4.1 Typical material properties of PDMS

Property Value

Glass transition temperature (T

)

[135]

≈−125

◦

Mass density [140] 0.97 kg/m

Young’s modulus 360–3000 kPa

Poisson ratio [140]0.5

Tensile or fracture strength [140]2.24MPa

Speciﬁc heat [140] 1.46 kJ/kg K

Thermal conductivity [140]0.15W/mK

Dielectric constant [140] 2.3–2.8

Index of refraction [ 140]1.4

Electrical conductivity [140]4×10

m

Magnetic permeability [140]0.6× 105 cm

Wet etching method [141] Tetrabutylammonium ﬂuoride (C

FN) +

n-methyl-2-pyrrolidinone (C

NO) 3:1

Plasma etching method [141]CF

Adhesion to silicon dioxide [142] Excellent

Biocompatibility [143–145] Nonirritating to skin, no a dverse effect on rabbits and mice,

only mild inﬂammatory reaction when implanted

characterization on PDMS materials for biological applications. Lin et al. deter-

mined the viscoelastic constitutive law of PDMS materials and developed a more

appropriate model for cellular traction force measurement using the punch test sys-

tem and ﬁnite element analysis (FEA), respectively [153]. Using the viscoelastic

FEA model in dimensionless form (i.e., a dimensionless deﬂection X

∗

= X/L and

a dimensionless loading P

∗

= P/P

, where L is the beam length and P

is the cel-

lular force calculated by the small deﬂection beam theory), the mechanical forces

generated by different loading rates could be determined (Fig. 4.6). The applica-

tion of viscoelastic material characterization is not limited to the cellular traction

force measurement and the results of their research are applicable to the analysis of

other soft polymer materials at micro- and nanoscales (e.g., PMMA, in the above

case, its glass transition temperature; and polyurethane, PU) [154] commonly used

in biomedical industries, as well as biodegradable polymers (e.g., polybutylene suc-

cinate/adipate, PBD/SA) [155], soft lithography materials, and ﬂexible electronic

device materials [156].

4.2.2 Processing Techniques

The most commonly used fabrication technique for PDMS specimens is soft lithog-

raphy which can duplicate inverted structures from a rigid mold [157]. PDMS can be

easily molded to form and replicate features sized less than 0.1 μm. Because of its

low surface energy, it can be lifted easily from molds and reversibly sealed to other

materials without requiring intensive expertise. This ease of handling ensures rapid