136
Sensors and Actuators A, 4142 (1994) 136140
Applications of fluorocarbon polymers in micromechanics and
micromachining
H.V
Jansen,
J.G.E.
Gardenlers,
J Elders,
H.A.C.
Ttlmans
and M. Elwenspoek
MESA Research insbtrrte, Umverstty of i’knte, PO Box 217, 7500 A.5 Enschede (Netherlands)
Abstract
Several thm-tilm deposttlon and etchmg techmques of the polymer fluorocarbon are mvestigated and the resultmg
thm-fihn propemes wdl be compared wth those of commercially available bulk polytetrailuoroethylene The most
promrsmg deposltlon tccbmque IS performed m a conventional reactwe ion etcher using a carbonhydrotnfluonde
(CHF3) plasma By changmg the deposmon parameters, control of the properttes and step coverage of the
deponted thm films within a certain range IS possible, eg, um-dnectlonal and couformal step coverage of
deposlted thm films can be obtained Etchmg IS performed wth the help of an evaporated alumrrnum oxide
mask using an oxygen, mtrogen, or sulfurhexaflumde plasma for lsotrop~c etching, or a CHF, plasma gwmg a
duecuonal etch profile The combmatlon of the unique properties, deposItion and etchmg techniques make
fluorocarbon thm tibna a promlsmg tool for muxomachmmg, a number of apphcations wll be dlscnssed and
demonstrated
IUtt.OdllCtiOll
The hnear fluorocarbon (FC) polymer polytetrafluoroethylene (FTFE) LSbetter known under several trademarks such as Teflon”, Fluon”, or Host&on@ Due to
the many exceptlonal thermal, electncal, and chenucal
properties, it IS an mterestmg matenal m all kmds of
dlsclplmes, such as mlcroelectromcs, rmcromechamcs
and (bio)chenustry Table 1 grves a summary of hterature
data [l-3] on the properties of PTFE These properties
are considered mdlcatlve for a whole class of FC films
Applications m nucromechamcs generally reqmre
PTFE m the form of thm films and therefore we have
mvestlgated several deposition methods of PTFE-hke
films, 1 e , FC polymers wtth dtierent chemical structures These &ns can be plasma-etched and their surface
IS moddied durmg this etchmg This behavlour IS lmportant because many properties of the FC films are
directly related to the surface and not to the bulk of
the film The combmatron of the unique properties,
deposition and etchmg techmques make FC films promlsmg for nucromechamcal purposes A few apphcations
will be demonstrated
Deposition techniques
Thm films of FC polymers can be made by several
methods [l, 4-111 In thus sectlon spm-coating, plasma
deposItIon, and e-beam evaporation wdl be dlscussed
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The adhesion of plasma-deposlted films 1s much better
than the spm-coated and evaporated iilms This IS
probably caused by the creation of danghng bonds at
the surface of the underIymg film when the film IS
deponted and the subsequent attachment of free radical
copolymers (CF;) ongmatmg from the glow region
(I) Spu-counng FC thm Films (down to 20 nm) can
be spm-coated from a nuxture of FC722* and FC!40@
[9] gwmg umform and pmhole-free layers Figure 1
shows an example of an FC film lymg on top of a
s&con (Sl) wafer This iihn 1s obtamed by spmmng
an FC722 FC40 = 1 1 mrxture on the SI substrate for
60 s at 4000 rpm and baked for 5 mm at 90 “C, resultmg
m a layer of 70 nm f 1% The low surface free energy
of the FC f&n reduces the adhesion to the substrate,
adhesion IS improved with the deposItion of an mtermediate layer, such as a polymude film [4]
(II) Evapomtron FC films have also been deposlted
by e-beam evaporation of bulk PTFE Because of the
drrectlonal nature of this techmque, structures are only
top-coated Figure 2 shows a SEM photograph of an
alummmm structure covered with an evaporated FC
fihn
(m) Plasma deposttwn FC polymer fihns have been
deposIted yla plasma polymenxation of carbonhydrotnfiuonde (CHF,) m a commercially avadable reactwe
Ion etcher (RIE) In the plasma mode (25 seem CHF,,
130 mTorr, 25 “C, 5 X 10M3W/cm’, 0 V,, ) conformal
coverage of three-dimensional structures IS observed
at a deposition rate of 5 nm/mm Figure 3 shows an
TABLE 1 Propemes
of bulk PTFB from hterature data [l-3]
Electncal
Volume reslstmty lOI Ohm m
Surface reststance lot6 Ohm
Bulk breakdown voltage -4x 10’ V/m
Thm-film breakdown voltage * 4 X 10’ V/m
Dlelectnc constant 2 1 (50 Hz-10 GHz)
Dmupatlon factor 0 0003 (50 Hz-10 GHz)
Chermcal
Stable and mert (-190
Ondatwe stable
up to +250 “C)
Young’s modulus 4 x 10s N/m’
Bemlmg strength 18x16 N/m*
Tcnsde strength 25 x lb N/m2
Fracture stram 30@-500%
Fe 2 An evaporated FC thm tilm on top of an alunumum mask
layer
Optical
Refracuve mdex 14
Optical transmission
> 95%
Thermal
Stable, unflammable and shape memory
Mnnmum temperature -190 “C
Maxnnum temperature 225 “C
Melhng pomt 327 “C
Decomposition temperature -400 “C
Melt vlscoslty 10’” Pa s
Thermal conductloo 024 W/mK
Thermal capacity 105 J/gK
Expansion cocfficlent 8 X 1O-5 l/K
PhysIcal
C&Cal surface tenslon 18 X lo-” N/m
Contact angle wth water 10s’
Frlctlon cocffictent v&h resp to steel 004
Specific gravity 2 20 g/cm3
Non-permeable for fluIds (except He)
Fig 3 A plasma-deposited FC thm film (0 V,,,) gwmg a conformal
step coverage over an ahummum beam
Fig 1 A spm-coated PC thm film on a &con
substrate
alummlum beam that 1s coated Hnth an
the same tickness on both sides of the
In the RIE mode (25 seem CHF3,
25 “C, 5 X 10e2 W/cm2, target potential -
FC film mth
beam
130 mTorr,
100 V, c) the
film grows faster (26 nm/mm) at the places which are
directed to the plasma glow In Fig 4 an alummmm
beam is completely surrounded by an FC film In this
Figure three ddferent phenomena are seen Firstly, the
film is thicker at those places that are Qrected to the
plasma glow This 1s probably caused by the formation
of active sites due to Impmgmg Ions and/or photons
from the plasma glow Secondly, the bad adhesion
between the alurmmum beam and its surroundmg film
1s clearly seen Thirdly, at the edges of the beam a
thicker layer has been deposited (arrow), this higher
deposition rate rmght be caused by a higher flux of
radicals that strikes the edges
Unidxectional coverage of structures appears, Just
as was the case for the evaporated films, if an external
bias (- 400 V, =) 1s applied m the RIE mode The
depositIon rate IS higher than 50 nm/mm m this mode
In FQJ 5 a free polyslhcon beam 1s top-coated with an
FC film
138
Fig 4 A plasmadepoeted
FC thm Nm (- 100 V, E) gwmg an
enhanced deposmon rate on the top of an ahunmrum beam
The arrow mdvzates the thicker layer at the edges of the beam
Fig 5 A plasma-depoated
FC tlun film (-400
to a top-coated polysrhcon beam
V,,) gwmg nse
The FC films have been charactermed by energy
drspersrve X-ray analysrs (EDX), X-ray photoelectron
spectroscopy (XPS), and Founer-transform Infrared
spectroscopy (IT-IR) The analyses revealed that carbon 1s mamly bonded to two fluor atoms, although
carbon bonded to one or three fluor atoms 1s also
observed The contact angle, refractive mdex and the
chemical properties were comparable wtth those of
PTFE
Etching techniques
Fluorocarbon lihns can readily be etched tsotroprcally
m an RIE usmg an oxygen (O,), nitrogen (NJ, or
sulfurbexafluorrde (SF,) plasma and also m a carbonhydrotnfluonde (CHF,) plasma m the lower pressure
reme resultmg in a umdrrectional etch profile Thus,
Rg 6 A surface mxomachmed
alummmm oxide beam An
oxygen plasma IS used to sacntice the underlymg FC thm film
m the lower pressure regune the CHF, plasma etches
an FC film whereas m the higher regtme rt w111deposit
a film FC films have an etch rate of =SOO &mm m
0, plasmas and of y 100 mn/mm m SF, plasmas, which
IS 60 tmes lower than that of srhcon under the same
condrtlons Inorganic matenals, e.g , alummmm, are
commonly used as a maskmg layer Thrs versatrhty with
respect to patternmg leads to increased processmg
flexibihty
(1) S@-coafed flms Direct patterning of FC film
with photoresrst IS not possrble because, due to the
low surface free energy and high wettmg angle of the
FC film, the spin-coated resist cannot adhere at the
FC surface However, evaporated alummmm oxide
(Alox) does adhere and can be patterned In Fig 6
an FC film 1s coated first with an alumnuum oxide film
and after patternmg the alummmm oxlde mask, the
FC film IS rsotroprcally etched wrth an 0, plasma The
~sotroprc underetch 1s clearly observed
(11) Evaporated and plasma-depm~tedFC jibns Ebeam evaporated and plasma-deposited FC films can
be patterned duectly wtth photoresrst This difference
with respect to the spin-coated films nnght be caused
by the rougher surface of the plasmadeposrted films
Applkations
A number of possible apphcatrons of FC films will
be drscussed and demonstrated below
(1) Possrvutron layer Micromechamcal structures and
electronic circuitry can be coated wrth FC polymer to
prevent corrosron In this way, high-quality mirrors,
mrcrognppers, and electrostatic actuators can be fabncated Because of then chemical inertness and low
surface free energy, FC films are bmcompatrble and
could be used as a passlvauon layer m the medical
and chenucal analysts systems For example, an ionsenstttve field-effect transistor (ISFET) [12] was coated
wrth a spin-coated and a plasma-deposited fihn m order
to decrease the pH sensrtrvrty It was found 113) that
the pH sensitmty of such a so-called reference FET
(REFET) was decreased by a factor six, which makes
it a possible standard REFET Moreover, no CO,
response of this REFET was measurable mdrcatmg
that the fihn is non-porous and pmhole free
(n) Insulator layer FC flhns have a htgh resistwuy,
therefore only very thm layers are required for electrrcal
insulation Moreover, because of thetr high resrstlvlty
and hydrophobic nature, FC films are suitable as electrets m, e g , mrcromachmed sdicon microphones [14]
It was found [13] that the spin-coated FC electret 1s
as good as the sihcon dioxrde/hexamethyldrsdrxane
(StO,/HMDS) electret [15] When chargmg the FC
electret beyond the breakdown voltage (4 X lo7 V/m),
the electret voltage dropped quickly to this voltage
where tt stabdlzed It was also found that StOz electrets
treated with an FC passlvation layer do not discharge
when wetted, while those with HMDS do
(111)Sucnficzallayer Because very thm layers ( <20
nm) can be deposited with conformal step coverage
and because selective etchants are known, the FC
polymers are excellent sacnlicial layers to be used m
surface mmromachmmg This IS demonstrated 111Rg 6
where a 70 nm thick, spin-coated FC flhn beneath an
evaporated alummmm oxide beam 1sisotropically etched
under with the help of a dry 0, plasma etch The etch
rate is approximately 0.5 pm/nun
(iv) Stmcturullayer FC films are practically unbreakable and chemrcally mert For this reason they are
useful as mechamcal construction material m nncromechanss, e g , beams or membranes The low Young’s
modulus IS required for devmes generatmg large deformation under moderate forces Because the tihns
do not stick to almost any substrate, specral construction
methods must be developed (eg, mechamcal anchormg) In Fig 7 an FC beam rs shown that 1s made
of a plasma-deposited FC film which is patterned with
the help of an 0, plasma and an alummmm oxtde
mask
(v) Ann-stroking layer FC surfaces have extremely
low free energy and, as a result, they tend to have non
adhesrve character and low coefficrents of frrcnon
Therefore, FC films can be used as a coatmg over a
solid surface to prevent or reduce adhesion when a
material is brought into contact with the sohd, e g,
self-lubncatmg bearings, pumps, pipelmes and valves
in the chemmal industry In Fig. 8 an tsotropically
etched silicon flow channel IS coated with a very thm
plasma-deposited FC layer Other opportumties are the
release of mmromechamcal structures wrthout the use
of a sacnficial layer Figure 9 shows the bending of an
Fg 7 An FCbeam, lymg on top of a al~con substrate, constructed
with the help of an alummmm oxide mask layer and an oxygen
plasma
Fig 8 A sd~con flow channel coated wth a plasmadeposlted
FC thm fdm
alurmmum beam which IS situated duectly on top of
a spm-coated FC film This structure can be used as
the electrostatically driven element m the actrve lomt
[16] It is fabncated by applymg an FC flhn on a s&on
substrate and an alummmm layer on top of this Now,
when the alunnnmm 1s patterned the beams wdl bend
directly because of then, built-m, mtnnstc stress wrthout
etchmg the FC layer
Although the opportumtres of the followmg examples
are not demonstrated yet, they are also of interest m
rmcromechanms and mmroelectromcs
(1) FC films are transparent and are found to make
excellent optical fihns for, e g , fibres and planar waveguides Teflon AF2400m [lo] can be used as a low
refractive-index (1291) coatmg for optical devices
(u) Fluonnated polymers are available for nnpartmg
or1 and water resistance to mmromechamcal structures
because of then nonwettabdrty We can unagme a
Acknowkdgements
The authors are grateful to Geert Langerers, Wouter
Olthms, and Poet Bergveld for putting then measurement results on the REFET and electret to theu
disposal Specral thanks to Bert Otter for makmg the
SEM photos and Erwm Berenschot for hts work on
the actrve Joint These mvestrgatrons m the programme
of the Foundanon for Fundamental Research on Matter
(FOM) have been supported by the Netherlands Technology Foundanon (STW)
References
Fig 9 The bendmg of alummmm beams released directly from
the substrate, wlthout the use of a sacrdicml layer, due to the
underlymg FC thm film
number of umque uses for matenals of thrs type, e g,
adhesives wrth selccttve bonding proper&s and barrrer
mater& to prevent the spreadmg of hqurds from pomts
of contact on ball beanngs, motors, and other devrces
(III) The low Young’s modulus makes it possrble to
use FC tilms as a ‘rubbery’ coatmg for mrcrovalves m
preventmg leakage of flurds
(IV)FC tihns can be compounded wrth other plastrcs
or metals by smmltaneously sputtermg metals m the
reactor chamber dunng PECVD deposrtron of FC films
WI
Conclusions
The usefulness of FC polymers m mrcromechamcs
has been demonstrated Three drtferent kmds of deposrtron techmques are treated spur coahng, evaporation, and plasma deposrtron Especrally the plasmadeposited films are very attractrve because rt ISpossible
to cover structures m dtierent ways Etchmg 1s performed m the same plasma reactor that 1s used for
depositron and rt ts possible to etch thm FC fihns
umdrrectronal as well as rsotroprc Many matenals,
mostly morgamc, are known to act as a maskmg layer
m order to copy the geometry of the developed photoresrst mto the FC films The unique propertres and
the drversrty m deposrtron, etching, and masking matenals make the use of FC frhns m mrcromechamcs
and mrcroelectromcs very simple and strarghtfonvard
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