Ghodssi R., Lin P., MEMS Materials and Processes Handbook

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234 E. Meng et al.

Fig. 4.33 Process ﬂow for bilayer structure using differential adhesion method and single-layer

structure using sacriﬁcial layer method. (a) Deposition and patterning adhesion layer Cr (10–

30 nm). (b) Deposition structure layer Au (100 nm). (c) Electrodeposition and patterning of PPy

(1 μm). (d) Etching of the ﬁnal microactuator structure by removal of the excess Au. (e) Deposition

and patterning the sacriﬁcial layer Au (100 nm). (f) Deposition and patterning the anchor layer

Ti (100 nm). (g) Electrodeposition and patterning of the PPy (1 μm). (h) Etching of the ﬁnal

microactuator structure and underetching Au (Reprinted from [402] with permission from ASME)

The bilayer cantilever, similar to Jager et al. [422], was fabricated using the dif-

ferential adhesion method. The anchor part consisted of Cr and Au, and the moving

part was Au on top of glass. In our experiment, we ﬁrst used the patterned electrode

method to fabricate the structure on a glass slide (1 × 3 in.), but the Au part without

the Cr underneath could not be released after deposition. This could be due to the

lateral growth of the polymer which extends and adheres to the substrate; also the

rougher glass surface could result in higher adhesion to the Au layer.

Then we switched to the photoresist template method and used polished silicon

wafer as the substrate. To ensure wafer cleanliness prior to depositing the Au layer,

we added a special cleaning process: piranha solution immersion (hydrogen perox-

ide and sulfuric acid with a volume ratio of 1:3) for 10 min. The ﬁnal structure could

be released immediately after ﬁnal etching of Au (Fig. 4.34).

The single-layer cantilever was fabricated using the sacriﬁcial layer method.

Because the Ti etchant (Transene), which is typically hydrochloric acid, will attack

Cr as well, the undercut etching of Ti will cause the whole structure to peel off from

the substrate. Therefore, we used Au as the sacriﬁcial layer and Ti as the anchor.

4 Additive Processes for Polymeric Materials 235

Fig. 4.34 Bilayer structure (0.5 × 0.1 cm): (a) top view; (b) side view (Reprinted from [402]with

permission from ASME)

During the synthesis, polypyrrole was ﬁrst deposited on the Au layer and extended

to the Ti layer after sufﬁcient time. When the deposition was complete, the underly-

ing Au was etched to release the cantilever, leaving only polypyrrole as the moving

part (Fig. 4.35). This structure may function as a linear actuator.

Fig. 4.35 Single-layer cantilever (1 × 0.2 cm): (a) before etching and (b) after etching and release

(Reprinted from [402] with permission from ASME)

4.7 Other Polymers

4.7.1 Benzocyclobutene

Benzocyclobutene, or BCB, is a cross-linked aromatic polymer available in two

spin-on formats: photosensitive and dry-etch (Cyclotene, Dow Chemical Company,

Midland, MI). This thermoset polymer is formed from 1, 3-divinyl-1, 1, 3, 3-

tetramethyldisiloxane-bisbenzocyclobutene (DVS-bis-BCB) monomer (Fig. 4.36)

236 E. Meng et al.

Fig. 4.36 Chemical structure

of Cyclotene monomer (After

[429])

[426, 427]. The dry-etch formulation consists of partially cross-linked monomer

dissolved in mesitylene solvent whereas the photosensitive formulation contains an

additional photosensitizer (for broadband UV sensitivity) [428].

BCB is well known as an microelectronics material (with application, e.g., as

an interlevel dielectric, passivation coating, and packaging material) [430–434]

and was later demonstrated as a suitable planarization material MEMS processes

and packaging [435, 436]. The mechanical [437, 438], optical [439], and elec-

trical [428] properties of BCB were characterized. Its combination of properties

has led to its use as a power MEMS material for the fabrication of electrostatic

micromotors [440–442]. The low moisture uptake and low dielectric constant have

attracted biomedical applications [443], however BCB’s biocompatibility is not

fully characterized (Fig. 4.37).

The standard process features for dry-etch BCB are summarized in Table 4.5.

BCB is removed by plasma processes (∼0.6 μm/min in oxygen plasma) [429].

Control of the sidewall angle ( vertical and sloped) was obtained by varying the

process parameters in a reactive ion etching system (O

plasma) (Fig. 4.38)[435].

Au protects BCB during anisotropic etching of Si (in KOH) and Cr is used as an

intermediate adhesion promotion layer (Fig. 4.39)[429].

Table 4.5 Standard process features for dry-etch BCB (cyclotene)

Process Method Details

Surface treatment Adhesion promoter AP3000

Deposition Spin-coat 1–5 krpm

Soft cure Furnace N

, 1 h @ 210

◦

Hard cure Furnace N

, 1 h @ 250

◦

Etching Plasma O

/CF

or O

/SF

After [428]

Although BCB has superior planarization performance compared to polyimide,

its CTE mismatch with silicon (∼60 ppm/

◦

C[432, 444] versus 2.3 ppm/

◦

C[445])

leads to undesirable cracking. Sandwiches of polyimide–BCB–polyimide harness

both the planarization capability of BCB and the mechanical/chemical strength of

polyimide [435]. The mechanical robustness of BCB enables its use in chemical

mechanical polishing processes as a replacement for silicon dioxide, unlike other

softer polymers [446].

4 Additive Processes for Polymeric Materials 237

Fig. 4.37 BCB-based neural probes (three recording sites) with integrated ﬂuidic delivery chan-

nels (40 μm × 10 μm) (Cyclotene 4026) (Reprinted from [443] with permission from Elsevier,

In general, BCB exhibits poor adhesion to inorganic materials [447]. AP3000

(Dow Chemical Company) adhesion promoter enhances metal adhesion to BCB;

the detailed process and a thorough review on strategies to promote BCB adhesion

are given in [429]. Wafer-level packaging utilizing BCB membrane transfer bonding

was demonstrated for RF devices (950 mbar pressure, 2 bar force, 250

◦

C; ∼70%

yield) [448].

238 E. Meng et al.

Fig. 4.38 Sloped sidewalls in

a polyimide–BCB–polyimide

sandwich obtained by

reactive ion etching

(Cyclotene 3022). Original

ﬁgure from [435] used with

permission of Institute of

Physics Publishing Ltd

Fig. 4.39 BCB protected by Au mask for KOH etching of an underlying Si trench. Reprinted with

4.7.2 Liquid Crystal Polymer

Liquid crystal polymers (LCP) possess a unique combination of properties exploited

heavily in optical devices, especially consumer displays [449]. For example, they are

birefringent and possess dielectric anisotropy. The latter property results in electric

ﬁeld-induced molecular alignment. Later, LCPs were investigated as a packaging

material and printed circuit board (PCB) substrate [450, 451]. In the liquid phase,

LCPs are isotropic. They may enter one or more liquid crystal phases that exhibit

4 Additive Processes for Polymeric Materials 239

order and symmetry. There are two mechanisms by which the material changes from

the liquid to the liquid crystal phase; LCPs are either (1) thermotropic with ther-

mally driven transitions or (2) lyotropic in which the polymer can self-assemble in

response to solvent concentration [449]. We focus on thermotropic LCPs here which

are available commercially in thin sheet/ﬁlm format (either single- or multilayer).

This thermoplastic polymer contains a network of interlinked rigid and ﬂexi-

ble monomers, the alignment of which can be tuned by application of shear ﬂow

when heated and maintained following cooling [452]. LCPs are excellent moisture

and vapor barriers and chemically resistant over a broad temperature range [451].

The chemical compatibility is investigated for various acids, bases, and solvents

in [452] and has enabled exploration of LCPs as substrates for supporting under-

water and marine sensors [453]. Flow, tactile, and pressure sensors have also been

demonstrated for use in dry environments [452, 454].

LCP ﬁlms may be processed using standard microfabrication techniques. Prior

to lithography, it is necessary to apply a rigid supporting substrate [452]. Removal

of LCP is achieved by laser machining [455–457] or oxygen plasma etching. An

Al mask was using in oxygen plasma reaction ion etching to remove LCP ﬁlm

(Vectra A-950) at a rate of ∼0.22–0.27 μm/min (350 W, 500 mT). Films can also

be patterned by mechanical punching [454]. LCP ﬁlms can be bonded by thermal

lamination (260–270

◦

C) [452] or thermocompression bonding (400 lb load on a

hydraulic press at 275

◦

C, 30 min) [454].

4.8 Polymers for Embossing and Molding

4.8.1 Technical Overview

Hot embossing is the process whereby a pattern is impressed in a material that has

been softened by the application of heat. The materials most commonly embossed

for MEMS applications are thermoplastics, which are plastics that soften when

heated, eventually melt, and solidify again when cooled. This approach is not lim-

ited to plastics; more exotic materials such as chalcogenide glasses are embossed to

make lenses and waveguides, and microscale features have been embossed in bulk

metallic glasses [458]. Hot embossing is typically used in conjunction with injec-

tion molding as a process to make mold inserts. Many of the polymers described in

this section are not only used in hot embossing but also in complementary polymer

fabrication techniques such as injection molding and compression molding. These

complementary molding processes are brieﬂy summarized here; for further infor-

mation on speciﬁc mold-making processes, the reader is directed to the references

[459–462].

Injection molding is appropriate for both thermoplastics and thermosets. Unlike

thermoplastic materials, thermosetting plastics do not soften when heated. Instead,

they are only workable prior to being cured. The curing process produces irre-

versible cross-links that impart strength to the ﬁnal material. Polymers are formed

240 E. Meng et al.

by injecting by force a liquid polymer into a structured mold cavity. The ﬁnal poly-

mer part takes on the shape of the mold cavity. Overall, this versatile process l ends

itself to the production of polymer parts having complex shapes and in high volume.

Specialized equipment is required to perform the polymer injection and subsequent

molding process.

Compression molding shares similarities with injection molding in that a poly-

mer is forced against a mold cavity in order to take on its shape. Here, polymer

granules are placed into a mold and forced against it, typically using a combination

of heat and pressure (usually with a hydraulic ram). This simple process is low cost

and does not require expensive tooling. However, compression molding is not suit-

able for forming plastics into complex shapes as in injection molding but instead is

used for predominantly ﬂat or moderately curved parts. A variety of polymers may

be used, however, compression molding is usually performed on thermosets.

The basic process parameters of hot embossing are temperature, time, and force.

The substrate is heated until it begins to soften (step A in Fig. 4.40). Ideally when

embossing is initiated the material is pliable, but not yet bordering on becoming

liquid. The softening point or glass transition temperature (T

) is the temperature

when the amorphous component of the material is transitioning between its glassy

(brittle) and rubbery (pliable) states. The magnitude of the effect is dependent on the

ratio of amorphous and crystalline components of the material. The ideal embossing

temperature is not only dependent on the material being embossed, but also mold

features. Different feature and substrate geometries will pattern more accurately

with different polymer viscosities. If the material is too viscous it may adhere to the

mold and make demolding difﬁcult. This is a primary mode of mold wear.

Fig. 4.40 Process proﬁle time, temperature, and force

Force is applied to force between the mold and plastic (Fig. 4.40,stepB).The

force is maintained for the “embossing hold time” to allow the plastic adequate time

to ﬂow into the mold (Fig. 4.40, step C). Once the mold has been impressed in the

plastic to the desired depth, both the mold and plastic are cooled below the glass

transition temperature (Fig. 4.40, s tep D) to increase the rigidity of the newly cre-

ated features such that they will maintain their shape during the demolding process

4 Additive Processes for Polymeric Materials 241

(Fig. 4.40, step F). Some shrinkage of the new features relative to the mold can

assist demolding. Careful selection of temperature ramp rates and good uniformity

control help minimize stress in the ﬁnal part.

The force ramp times (Fig. 4.40, steps B and E) are relatively short, so the time

to emboss is dominated by the temperature ramp times and the hold time. The r amp

times are determined by the thermal mass of the tooling, the heaters and chillers

available, and the controllers. Ideally the ramp times are short, the main constraints

being reproducible performance while maintaining temperature uniformity between

mold and material being molded. The ideal emboss hold time is usually considered

to be the minimum time necessary for the plastic to ﬂow and ﬁll the mold as desired.

Additional time may be necessary to buffer for variability, but excessive hold times

can result in excellent bonding of the substrate to the mold.

The force required for embossing is determined by the mold ﬂow resistance and

viscosity of the material being molded. The mold ﬂow resistance is a function of

the amount of area to be pressed into the substrate, as well as the size of features in

the mold that need to be ﬁlled. For instance, a mold used to fabricate narrow pillars

of plastic will require more force than one used to fabricate narrow holes. A good

indicator of high ﬂow resistance is if not only the area pressed into the substrate is

high, but also the length of the perimeter that encompasses that area is also high.

It is useful to evacuate the embossing chamber to reduce problems associ-

ated with air trapped between the mold and substrate; however, the vacuum levels

necessary are not very difﬁcult to achieve (on the order of 1 mT).

The primary components of embossing are: the substrate embossed, the emboss-

ing machine, and the mold insert or tool.

4.8.2 Substrate Material Selection

The selection of material to be embossed will be driven by the requirements of the

device to be fabricated. Material properties commonly of interest are: mechanical

strength and durability, optical transmission, ﬂuorescence, water uptake, chemical

resistance, electrical properties, and UV stability. If multiple candidate materials are

suitable for the device, ease of fabrication and material cost may also factor in.

Fabrication cost is driven primarily by tool time. Because the tool time required

per part is dominated by the heating and cooling times, the selection of a material

with lower T

will cost less to emboss. A material that is easy to emboss can improve

mold lifetime and thus reduce cost.

4.8.2.1 Polymethylmethacrylate

Polymethylmethacrylate (PMMA) is relatively easy to emboss. It has good ﬂow

properties, with a relatively low T

. The mechanical properties at room tempera-

ture are suitable for a wide variety of applications; however, the material is brittle

and may crack if subject to impact. It has good UV resistance, and as such is com-

monly used in outdoor applications. Chemical resistance is not especially good; it is

242 E. Meng et al.

attacked by most solvents and does not hold up well to strong bases, but it is widely

used in microﬂuidic devices due to its low ﬂuorescence over a broad spectrum.

4.8.2.2 Polycarbonate

Polycarbonate (PC) has excellent mechanical properties, as is demonstrated by

the fact that it is widely used as bulletproof glass. These properties translate to

devices fabricated on the microscale, with very high strains displayed before fail-

ure. It also has a relatively high T

and heat deﬂection temperature, and thus is

mechanically stable to higher temperatures than PMMA. It displays poor resis-

tance to most solvents, and absorbs water (0.25–0.35%). Polycarbonate ﬂuoresces

(excitation wavelength 250 nm) which limits its compatibility with many biological

assays.

4.8.2.3 Polytetraﬂuoroethylene

Polytetraﬂuoroethylene (PTFE) is of interest for applications where excellent chem-

ical resistance is required. It is a semicrystalline thermoplastic, so even though the

is quite low (in many cases below the operational temperature), the emboss-

ing temperature will need to be much higher, closer to the melting temperature.

PTFE has several potentially useful physical properties including low friction, low

adhesion, hydrophobicity, is an excellent dielectric, and displays no ﬂuorescence

(although it may ﬂuoresce as a result of radiation-induced breakdown). It displays

high creep and is expensive.

4.8.2.4 Cyclic Oleﬁn Copolymer

Cyclic oleﬁn copolymer (COC) materials are widely used in optical devices inas-

much as they have very stable optical properties and display low ﬂuorescence.

They also have low moisture uptake (<0.01%), and consequently are popular for

use in microﬂuidic devices, especially ones deploying optical interrogation. They

have decent chemical resistance, and are compatible with alcohols and some acids.

These materials display glass transition temperatures in the range of 110–150

◦

The mechanical properties are reasonable, but adjusting composition to increase

Young’s modulus also increases brittleness.

An important consideration in material selection is the source. Depending on the

process used to produce the embossed stock, the material may have internal stress,

which can manifest during embossing. Also, if the material properties drift from lot

to lot, so will the embossing results.

4.8.3 Tool Selection

There are several commercial tools on the market for hot embossing on the micro-

and nanoscale, and many researchers have produced good results on homemade

4 Additive Processes for Polymeric Materials 243

tools. Unsurprisingly, the ideal setup is very application-dependent. Here are some

basic considerations for tool selection.

1. Temperature range: determines materials the tool can emboss.

2. Force range: high force capability increases the area that can be embossed, as

well as the range of acceptable mold ﬂow resistances.

3. Fixture: for both mold and substrates; easy substitution will improve throughput.

4. Alignment capability: alignment of mold to a patterned substrate, or alignment

of a lower second mold for double-sided embossing; in a tool, or external with

jig transfer into a tool.

5. Heating and cooling rates, uniformity, and control stability: determines through-

put and repeatability.

6. Mechanical precision: micro- versus nanoscale optimized.

7. Demolding mechanism.

8. Software: ease of process development, tracking and debugging.

Several of the tools are modiﬁed bonders, thus it may be worth considering this

alternate functionality when selecting a tool. Typically this means that alignment is

performed on separate platform.

4.8.4 Mold Material Selection and Fabrication

A wide variety of materials and methods can be used to fabricate molds. Primary

considerations for material selection are cost, fabrication complexity (and hence

turnaround time), and desired lifetime. The fabrication approach is determined by

the features to be embossed, and the material preference. Here we cover some

of the more common approaches for fabricating molds for embossing micro- and

nanoscale features.

4.8.4.1 Silicon

Silicon is not an ideal mold material, inasmuch as for many applications it does not

display good wear properties. Because it is crystalline, it is prone to fracturing and

chipping. It also has a much lower thermal coefﬁcient of expansion than some of the

more ductile metals used in tool mounting hardware (i.e., steel, stainless steel, and

bronze), which can complicate the ﬁxture. However, if the embossing process itself

is not especially mechanically challenging for the mold, silicon can be an excellent

choice.

It is very widely used in prototyping applications due to the widely available

and mature set of processes for machining, especially on the micro- and nanoscales.

The maturity of silicon processing means that advanced mold pattern geometries

can often be achieved in a relatively straightforward manner, with reasonable cost.

Good turnaround times allow multiple design and fabrication iterations to take place

in a short period of time.