Sho Communica ion
Silicon dioxide a omic laye deposi ion a low empe a u e o PDMS
mic olenses coa ing
Flo i al M. Cunha
a
, Jo˜
ao R. F ei as
a
, Sa a Pimen a
a,b,*
, Manuel F. Sil a
a,b
, Jos´
e H. Co eia
a,b
a
CMEMS-UMinho, Uni e si y o Minho, 4800-058, Guima ˜
aes, Po ugal
b
LABBELS-Associa e Labo a o y, B aga/Guima ˜
aes, Po ugal
ARTICLE INFO
Keywo ds:
Polyme ic mic olenses
Su ace oughness
Op ical e iciency
ABSTRACT
The op ical pe o mance o a mic olens is s ongly a ec ed by i s su ace oughness, which depends on i s
ab ica ion p ocess. High-su ace oughness leads o sca e ing issues, dec easing he op ical e iciency o a
mic olens. In his wo k, a polydime hylsiloxane mic olens was coa ed wi h a e y hin- ilm o silicon dioxide (30
nm), deposi ed by plasma-enhanced a omic laye deposi ion a a low empe a u e (50 ◦C). The main goal was o
passi a e he polyme ic mic olens and educe i s su ace oughness. A omic o ce mic oscopy was pe o med
be o e and a e he silicon dioxide coa ing, con i ming he su ace oughness educ ion by a ac o o 3.2. The
mic olens was obse ed by scanning elec on mic oscopy a e silicon dioxide coa ing. Su ace elemen al
composi ion analysis o he silicon dioxide hin- ilm was also pe o med h ough X- ay pho oelec on spec os-
copy, con i ming he o ma ion o silicon dioxide and a s oichiome ic a io o silicon o oxygen (Si:O) close o
27:58. The e ac i e index o he deposi ed ilm was measu ed by ellipsome y, ob aining a alue o 1.4617 a
470 nm.
1. In oduc ion
The main challenge o using dielec ic coa ings in op ical mic o-
de ices is he achie emen o low-abso p ion and low-sca e ing losses.
Addi ionally, he coa ing ilms mus be deposi ed wi h p ecise hickness
and uni o mi y [1].
A omic laye deposi ion (ALD) is a high-quali y hin- ilm deposi ion
echnique wi h unique ea u es, including excellen ilm hickness con-
ol, high uni o mi y, and con o mali y, e en on complex o s uc u ed
su aces [2–7].
ALD is a special ype o chemical apo deposi ion (CVD), whe e
gaseous p ecu so s a e sequen ially and independen ly exposed o he
subs a e. The p ecu so pulses a e sepa a ed by pu ging cycles. Due o
his cycle-based p ocess, he eac ion does no happen in he gas phase
bu is only su ace-limi ed. Using ALD, he esul ing ilm hickness is
p ecisely con olled (a a sub-nanome e scale) by he numbe o ALD
cycles [1,8,9].
Due o he high con o mi y o he ALD, he oughness o he subs a e
can dec ease because he deposi ion ills all he holes in he subs a e
and u ns he su ace less ough [10].
ALD deposi ion o dielec ic hin- ilms a low- empe a u e subs a e
(<250 ◦C) ypically esul s in amo phous ilms wi h low-su ace
oughness, and hus low-sca e ing losses can be expec ed [1,11].
Silicon dioxide (SiO
2
) is he mos widely used dielec ic hin- ilm in
he semiconduc o indus y o pho onics and elec onic applica ions
due o i s excellen insula ing p ope ies and can be easily deposi ed by
ALD a low- empe a u e deposi ions. This is especially in e es ing
conside ing polyme s as subs a e ma e ials, making i possible o coa a
subs a e wi h an ul a hin oxide ilm wi h good elec ical p ope ies
[12–14].
Ul a- hin SiO
2
ilms ha e also been epo ed as good componen s o
he nano echnology ield, such as dielec ic ma e ials in mic oelec onic
de ices, an ico osion/passi a ion ilms, e c. [15–17].
O he oxides (e.g., i anium dioxide (TiO
2
) and alumina (Al
2
O
3
)) a e
epo ed in he li e a u e as good coa ings o inc ease he o ganic sol en
esis ance o a polyme (polydime hylsiloxane (PDMS)) o inc ease i s
hyd ophilici y [18,19].
The su ace oughness o a mic olens is an impo an pa ame e o
e alua ing i s op ical pe o mance and i is mainly a ec ed by he
ab ica ion p ocess. High-su ace oughness leads o sca e ing issues,
which dec eases he op ical e iciency o he mic olens [20].
In his wo k, a PDMS mic olens, p e iously mic o ab ica ed wi h he
* Co esponding au ho . CMEMS-UMinho, Uni e si y o Minho, 4800-058, Guima ˜
aes, Po ugal.
E-mail add ess: [email p o ec ed] (S. Pimen a).
Con en s lis s a ailable a ScienceDi ec
Op ical Ma e ials
jou nal homepage: www.else ie .com/loca e/op ma
h ps://doi.o g/10.1016/j.op ma .2024.116167
Recei ed 29 Ap il 2024; Recei ed in e ised o m 26 Augus 2024; Accep ed 24 Sep embe 2024
Op ical Ma e ials 157 (2024) 116167
A ailable online 24 Sep embe 2024
0925-3467/© 2024 The Au ho s. Published by Else ie B.V. This is an open access a icle unde he CC BY-NC-ND license ( h p://c ea i ecommons.o g/licenses/by-
nc-nd/4.0/ ).
p ocess epo ed a [21], was coa ed wi h a SiO
2
hin- ilm (30 nm hick),
deposi ed a a low empe a u e (50 ◦C) by plasma-enhanced a omic
laye deposi ion (PEALD), using a SENTECH Ins umen s GmbH sys em.
The e o e, he main goal o his wo k was o pe o m he passi a ion o a
he mally sensi i e PDMS mic olens by PEALD echnique and o educe
i s su ace oughness, achie ing he educ ion o op ical sca e ing los-
ses. Fu he mo e, he cha ac e iza ion echniques used du ing his
s udy, speci ically a omic o ce mic oscopy (AFM), scanning elec on
mic oscopy (SEM), X- ay pho oelec on spec oscopy (XPS), and ellips-
ome y p o e he PDMS mic olens op imiza ion.
2. Ma e ials and me hods
As p e iously e e ed, he SiO
2
PEALD was pe o med using a
SENTECH Ins umen s GmbH sys em. This ALD sys em in eg a es an
ellipsome e o eal- ime ilm hickness moni o ing. The Bis(die h-
ylamino)silane (BDEAS) p ecu so was selec ed wi h he O
2
-plasma as a
co- eac an . BDEAS is a usual p ecu so o SiO
2
ALD due o i s chemical
s abili y [15]. The subs a e (PDMS mic olens) and he eac o we e
hea ed up o 50 ◦C, he p ecu so BDEAS up o 60 ◦C, and he p ecu so ’s
lines up o 125 ◦C. The O
2
-plasma was exci ed a 13.6 MHz by adio
equency (RF) powe o 200 W and a low a e o 200 sccm. The eac o
acuum p essu e was h o le-con olled a 20 Pa. Table 1 shows he
ecipe condi ions used.
AFM was used o measu e he su ace oughness o he PDMS
mic olens be o e and a e he SiO
2
PEALD coa ing. The AFM mea-
su emen s we e pe o med wi h a BRUKER sys em (Dimension Icon) wi h
a ip model RTESPA-150 in con ac mode.
SEM measu emen s we e also pe o med a e he SiO
2
PEALD
coa ing, using a NanoSEM sys em (FEI No a 200) by no mal seconda y
elec on (SE) mode, wi h an accele a ion ol age o 10 kV, and wi h a
Gold/Palladium (Au/Pd) an is a ic coa ing o high conduc i i y.
XPS measu emen s we e pe o med o ob ain a chemical composi-
ion analysis o he SiO
2
hin- ilm, using a The moFishe Scien i ic sys em
(SCALAB250Xi) wi h a monoch oma ed X- ay sou ce Al K
α
(h =
1486.68 eV) adia ion, ope a ed a 220 W, 14.6 kV, spo size 650
μ
m.
The XPS spec a we e collec ed a pass ene gies 100 eV and 40 eV o
su ey spec a and indi idual elemen s espec i ely (N 1s, Al 2p, Si 2p, O
1s, and C 1s scan). The ene gy s ep o indi idual elemen s was 0.1 eV.
The XPS spec a we e peak- i ed using A an age da a p ocessing so -
wa e. Fo peak i ing he Shi ley- ype backg ound sub ac ion was used.
All he XPS peaks a e o be e e enced o ad en i ious ca bon C 1s, C–C
peak a 284.8 eV. Quan i ica ion has been done using sensi i i y ac o s
p o ided by he A an age lib a y. Cha ge neu aliza ion was achie ed
wi h bo h low-ene gy elec on and a gon ion lood guns (<0.1 eV, 120
μ
A, and 70
μ
A cu en , espec i ely) du ing XPS measu emen s.
Finally, ellipsome y o he SiO
2
hin- ilm was also pe o med o
access he e ac i e index o he hin- ilm, using a J.A. Woollam Co.
sys em (alphaSE Ellipsome e ) wi h wa eleng hs om 381 nm o 893 nm,
a di e en measu emen angles (65◦, 70◦, and 75◦) and applying he
Sellmeie model. The gene al o m o he model is gi en by (1) [22]:
n(λ)2=
ε
∞+AUV λ2
λ2−λ2
UV
+AIR λ2
λ2−λ2
IR
(1)
whe e nis he e ac i e index o he ma e ial a a gi en alue o λ,A
UV
and A
IR
a e he ampli ude o he ul a iole (UV) and in a ed (IR) poles,
espec i ely, λ
UV
and λ
IR
a e he posi ions o he UV and IR poles, and
ε
∞
is he low- equency o se . In he so wa e o he ellipsome e , he
Sellmeie model was used o i he pole pa ame e s and he low-
equency o se , ex ac ing he SiO
2
e ac i e index and hickness.
The mean squa e e o (MSE) o he i ing p ocess was 0.947.
3. Resul s and discussion
The SiO
2
PEALD hin- ilm was moni o ed using he ALD sys em in si u
ellipsome e . Fig. 1 shows he g ow h pa ame e s o he SiO
2
PEALD
hin- ilm a 50 ◦C. The ecipe s a s wi h a s abiliza ion pe iod o
app oxima ely 10 min be o e he i s cycle o ensu e he main enance o
all he deposi ion pa ame e s (p essu e, empe a u e, gas low a e). A
g ow h pe cycle (GPC) o 1.32 Å/cycle was measu ed by he ellips-
ome e . A mo e de ailed iew o he i s h ee cycles is shown in Fig. 1
(b). The exposi ion ime o he BDEAS and he O
2
-plasma is indica ed in
he shaded egions. A hickness gain was obse ed du ing he BDEAS
exposu e, while a dec ease in hickness occu ed du ing he O
2
-plasma
exposu e. This dec ease is a ibu ed o he eplacemen o die hylamine
g oups ((CH
3
CH
2
)
2
N) on he su ace wi h oxygen a oms du ing he O
2
-
Table 1
PEALD ecipe o he SiO
2
hin- ilm deposi ion.
Numbe o ALD
cycles
BDEAS O
2-
plasma
Pulse ime
(ms)
Pu ge ime
(s)
Pulse ime
(ms)
Pu ge ime
(s)
227 180 5 5000 1
Fig. 1. Thickness measu ed by he ALD sys em in si u ellipsome e o SiO
2
PEALD hin- ilm a 50 ◦C: (a) g ow h o e many cycles; (b) de ailed g ow h o e he i s
h ee cycles.
F.M. Cunha e al. Op ical Ma e ials 157 (2024) 116167
2
plasma exposu e, he eby educing he amoun o ma e ial a ached o
he su ace.
AFM measu emen s we e pe o med in he PDMS mic olens be o e
and a e he SiO
2
coa ing. Fig. 2 shows he AFM su ace opog aphy o
he PDMS mic olens be o e and a e he SiO
2
coa ing. Conside ing an
a ea o 1 ×1
μ
m
2
on he op o he mic olens, i was measu ed an
a e age su ace oughness (R
a
) o 1.6 nm be o e he SiO
2
coa ing, and
0.5 nm a e he SiO
2
coa ing. Thus, i was con i med ha he su ace
oughness was educed by a ac o o 3.2, which b ings less sca e ing
issues and imp o es he op ical e iciency o he PDMS mic olens. As an
example, an a e age su ace oughness o 3.7 nm is epo ed in he
li e a u e o a PDMS mic olens and conside ing he same su ace a ea
on he op o he mic olens o 1 ×1
μ
m
2
[23]. The e o e, he PEALD
coa ing could be an al e na i e me hod o educing he su ace
oughness and imp o ing polyme ic-mic olens op ical e iciency.
Fig. 3 (a) shows a op iew SEM image o he a ay o PDMS
mic olenses coa ed wi h he SiO
2
PEALD hin- ilm, highligh ing he
analyzed PDMS mic olens in his wo k. Fig. 3 (b) shows a c oss-sec ion
SEM image o a PDMS mic olens coa ed wi h a SiO
2
PEALD hin- ilm.
XPS is a highly sensi i e echnique o su ace elemen al composi ion
analysis o e y hin- ilms (below 40 nm hick). Fig. 4 shows he XPS o
he SiO
2
PEALD hin- ilm. The s oichiome ic a io o silicon o oxygen
(Si:O) was ound o be close o 27:58, by calcula ing he XPS peak a eas
o he Si 2p and O 1s. This alue is e y close o he expec ed alue o 1:2.
The appea ance o Si 2p a ≈103 eV con i med he o ma ion o SiO
2
[24].
Fig. 5 shows he e ac i e index as a unc ion o wa eleng h o he
SiO
2
PEALD hin- ilm, ob ained by ellipsome y (applying he Sellmeie
model). The e ac i e index o he SiO
2
a 470 nm is 1.4617, which is
e y close o he PDMS e ac i e index, 1.4014 also a 470 nm [25].
This small e ac i e index di e ence doesn’ ha e a signi ican impac
on he SiO
2
coa ed PDMS mic olens e ac i e p ope ies, which is
p e ended in his wo k. To ob ain he SiO
2
e ac i e index, 30 nm o
SiO
2
was g own on a small ba e Si wa e nex o he mic olens sample.
Ellipsome y measu emen s we e pe o med in si u du ing i s g ow h
p ocess and ex si u on a di e en ellipsome e o con i m i s hickness.
Fig. 2. AFM su ace opog aphy a he op o he PDMS mic olens (a ea o 1 ×1
μ
m
2
): (a) wi hou coa ing; (b) wi h he PEALD hin- ilm SiO
2
coa ing. R
a
: a e age
su ace oughness.
Fig. 3. SEM image o PDMS mic olenses wi h he SiO
2
PEALD coa ing: (a) op iew; (b) c oss-sec ion iew.
F.M. Cunha e al. Op ical Ma e ials 157 (2024) 116167
3
4. Conclusions
In his wo k, a PDMS mic olens was coa ed wi h a e y hin- ilm o
SiO
2
deposi ed by PEALD a a low empe a u e (50 ◦C). The main goal
was o passi a e a he mally sensi i e PDMS mic olenses and o educe
i s su ace oughness. AFM measu emen s pe o med be o e and a e
he coa ing con i med he su ace oughness educ ion by a ac o o 3.2.
Also, he XPS esul s con i med he o ma ion o a SiO
2
hin- ilm by
PEALD. Finally, ellipsome y cha ac e iza ion con i med ha he SiO
2
PEALD hin- ilm e ac i e index is e y close o he PDMS e ac i e
index a he same wa eleng h, as equi ed. PEALD coa ing is an in e -
es ing app oach o passi a e op ical s uc u es (e en polyme ic) o in-
c ease hei e iciency and educe sca e ing issues.
CRediT au ho ship con ibu ion s a emen
Flo i al M. Cunha: W i ing –o iginal d a , Visualiza ion, Valida-
ion, Me hodology, In es iga ion, Fo mal analysis, Concep ualiza ion.
Jo˜
ao R. F ei as: W i ing – e iew &edi ing, Valida ion, Me hodology,
In es iga ion, Fo mal analysis, Concep ualiza ion. Sa a Pimen a:
W i ing –o iginal d a , Valida ion, Me hodology, Fo mal analysis,
Concep ualiza ion. Manuel F. Sil a: W i ing – e iew &edi ing, Vali-
da ion, Me hodology, Concep ualiza ion. Jos´
e H. Co eia: W i ing –
e iew &edi ing, Supe ision, Resou ces, Concep ualiza ion.
Decla a ion o compe ing in e es
The au ho s decla e ha hey ha e no known compe ing inancial
in e es s o pe sonal ela ionships ha could ha e appea ed o in luence
he wo k epo ed in his pape .
Da a a ailabili y
Da a will be made a ailable on eques .
Acknowledgmen s
This wo k was suppo ed by he CMEMS-UMinho S a egic P ojec
(UIDB/04436/2020 and UIDP/04436/2020) and he MPho onBiopsy
(PTDC/FIS-OTI/1259/2020, h p://doi.o g/10.54499/PTDC/FIS-OTI/
1259/2020). Flo i al M. Cunha and Jo˜
ao R. F ei as hank FCT
(Fundaç˜
aopa aaCiˆ
enciaeaTecnologia) o hePh.D.g an s(2023.00594.
BD, h ps://doi.o g/10.54499/2023.00594.BD and 2020.07708.BD,
h ps://doi.o g/10.54499/2020.07708.BD, espec i ely). Sa a Pimen a
hanksFCT o heg an 2022.00101.CEECIND/CP1718/CT0008,h ps://
doi.o g/10.54499/2022.00101.CEECIND/CP1718/CT0008. Manuel F.
Sil a hanks FCT o he g an CEECINST/00156/2018/CP1642/CT0004,
h ps://doi.o g/10.54499/CEECINST/00156/2018/CP1642/CT0004.
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