Our target
Chapter 5. Acquisition of sensor response for detecting gas molecules in the air
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Chapter 5. Acquisition of sensor response for detecting gas molecules in the air
5 . 1 P r e f a c e
I n t he exper i m ent descr i bed i n t hi s chapt er, PMMA act i ng as a gas - sensi t i ve f i l m was spi n- coat ed on t he f abr i cated cavi t y i nt erf er om et er and t he r esponse of PMMA t o t he absor pti on of vol at i l e et hanol was acqui r ed. Fi r st , we obt ai n ed t he r esponse of et hanol exposur e t o i nt er f er om et er s f orm ed wi t h di ff er ent ai r - gap l engt hs and conf i r m ed t he i m pr ovem ent of t he spect r al response wi t h nar r ower gaps. Next , usi ng an i nt er f er om et er t hat exhi bi t ed t he hi ghest spect r al r esponse, we eval uat e d t he co ncent r at i on dependence and L OD of vol a t i l e et hanol gas.
Fi nal l y, we com par e d our f abr i cat ed se nsor wi t h ot her sensor s t hat m easure vol at i l e et hanol i n a r oom - t em per at ur e envi r onm ent and dem onst r at ed t he super i or i t y of t he f orm er.
5 . 2 I m p r o v e m e n t o f s p e c t r a l r e s p o n s e b y n a r r o w i n g t h e a i r -g a p l e n -g t h o f t h e i n t e r f e r o m e t e r
To eval uat e t he per f or m ance i m pr ovem ent s r esul t i ng f r om nar r owi ng t he gap l engt h, PMMA was sp i n - coat ed ont o i nt er f er om et er s wi t h gap l engt hs of 0.4, 0 .8, and 2.6 µm . The def l ect i on of t he def orm abl e m em br ane upon exposur e t o Et OH gas was m easur ed as a shi f t i n t he r ef lect i on spect r um . Fi g . 5.1 shows t he det ect i on system used i n t he experi m ent . Thr ee i nt erf er om et er chips wi t h di ff er ent gap l engt hs wer e ar r anged on a m ovabl e stage. The r ef l ect i on spect r um , at 10 µm f r om t he cent r e of t he def or m abl e m em br ane, was acqui r ed at 20 - s i nt er val s wi t h whi t e l i ght i r r adi at i on. I n addi t i on, a sm al l pet r i di sh wi t h 0.4 m L of Et OH sol ut i on ( dil ut ed wi t h 50% DI W) was pl aced near t he chi ps and seal ed wi t h a l arge pet r i di sh t o pr event t he l eakage of t he vol at i l i zed Et OH gas i nt o t he sur r oundi ng . The Et OH sol ut i on w as pl aced i n t he vi ci ni t y of t he chi ps f or 9 t o 20 m i n f r om t he begi nni ng of m easur em ent and t hen r em oved. Fi g . 5.2 show s t he r esponse of t he vol at i l e Et OH gas t o t he sensor. As shown i n Fi g . 5.2a, i n t h e sub - m i cr o scal e nar r ow- gap i nt er f er om et er, t he m em brane def l ect i on was obt ai ned as an i nt er f er om et ri c col our change. Fi g . 5.2b shows t he t i m e cour se of t he r ef l ect i on spect r a i n t he i nt er f er om et er wi t h an ai r g ap of 0.8 µm . Af t er e xposur e t o Et O H gas, t he r ef l ect i on spect r um bl ue - shi ft ed f r om ( 1) t o ( 2) . I n ot her wor ds,
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shor t eni ng t he opt i cal pat h l engt h cause d a down war d m em br ane def or m at i on, suggest i ng t hat t he absor pt i on of Et OH cause d t he PMMA f i l m t o shr i nk. Lat er, i n t he absence of expo sur e t o t he Et O H gas, t he i nt er f er ence wavef or m r ed- shi ft ed t o ( 3) , r esul t i ng i n over l aps wi t h t he i nt er f er ence wavef or m bef or e t he gas exposur e. Fi g . 5.2c s hows t he t i m e cour se of t he peak shi f t s as soci at ed wi t h gas exposur e i n al l i nt er fer om et er s. When t he exposur e t o Et OH gas was st opped, t he peak shi f t r et ur n ed t o t he i ni t i al st at e, which m ean t a r ever si bl e r esponse due t o t he change i n gas concent r at i on was obt ai ned i n al l i nt er f er om et er s. Fi g . 5.2d shows t he m axi m um peak shi f t dur i ng gas e xposur e i n al l t he i nt er f er om et er s. The change i n t he peak shi f t of t he 0.4 - µm - gap i nt er f er om et er wi t h r educed i nt er f er ence or der was 2.0 an d 11.1 t i m es hi gher t han t hat of t h e 0.8 an d 2.6 µm gap i nt er f er om et er s, r espect i vel y. The val ue of t he 0.8 - µm i nt er f er om et er was appr oxi m at el y cl ose to t he opt i cal anal ysi s val ue of 2.1, whi l e the val ue of t he 2.6- µm i nt erf er om et er was 1.8 t i m es hi gher t han t he anal ysi s val ue. Thi s m ean t t hat t he 0.4 - and 0.8 - µm sensor s had t he s am e def l ect i on am ount dur i ng t he gas r esponse, whi l e t he 2.6 - µm - gap sensor s had a l ower def l ect ion. Thi s m ay be because of t he t hi ckness of t he PMMA fi lm used as t he gas - r eact i ve f il m , whi ch was t hi cker t han t hat of t he 0.4 - µm - gap i nter f er om et er. Because t he sur f ace - str ess sensi t i vit y i s i nver s el y pr opor t i onal t o t he squar e of t he fi l m t hi ckness, t he sur f ace - str ess sensi tivi t y decr eased by 1.8 t i m es wi t h a 35% t hicker PMMA f i lm i n an i nt er f er om et er wi t h an ai r - gap l engt h of 2.6 µm . The PMM A l ayer deposi t ed by spi n coat i ng m ay have def or m ed t he par yl ene C m em br ane downwar d because of t he i m m edi at e pr essur e f r om addi ng t he l i qui d. Because t he PMMA on t he def or m abl e m em br ane wi t h a r el at i vel y deep cavi t y was l ocal ly f or m ed t hi cker t han ot her ar eas, i t w as assum ed t o have d ecr eased t he sur f ace -st r ess sensi t i vi t y of t he 2.6 - µm i nt er f er om et er, t her eby decreasi ng t he m em br ane def l ect i on. The nar r ow- gap i nt er f er om et er dem onst r at ed gas det ect i on by t he change i n i nt er f er ence col our wi t h m em br ane def l ecti on and i m pr ovem ents i n t he spect ral r esponse.
To eval uat e char act eri st i c s of t he sensor response dependi ng on gas speci es, we m easur ed r efl ecti on spect r a duri ng exposur e t o 90% dil ut i on of Et OH, am m oni a, m et hanol (MeOH) , and wat er va pour ( 92% r el at i ve hum i di t y) . Fi g . 5.3 shows t he r esul t s of acqui r i ng t he peak shi f t s i n t he r ef l ecti on spect r um of t he sensor af t er exposur e t o 90% di l ut i on of Et OH, am m oni a, MeOH, and wat er vapour.
The am ount of peak shi f t on t he ver t i cal axi s was posi t i ve f or t he di r ect i on i n
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whi ch t he spect r um shi f t ed t o t he shor t er wavel engt h si de. The l argest peak shi ft was obt ai ned f or Et OH am ong t he exposed gases, whi l e exposur e t o gases ot her t han
Fi gur e 5.1 : Schem at i c di agr am of t he experi m ent al set up
(a) (b)
(c) (d)
Fi gur e 5.2: Com par i son of i nt er f er om et er s wi t h di ff er ent ai r g aps under Et O H exposur e. ( a) Opt i ca l m i cr oscope i m ages wi t h col our change associ at ed wi t h
Spectrometer (USB4000)
Movable stage Objective lens
(20x)
Sensor chips
Ethanol
diluted by DIW
Petri dish Light source
(LAX-C100) CCD camera
(DP-22)
0.4 µm gap
0 0.2 0.4 0.6 0.8 1 1.2
0 10 20 30
Normalized peak shift (a.u.)
Time (min)
Air Ethanol Air
0.8 µm gap 2.6 µm gap
times11.1
2.6 0.8 0 0.4
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
Air gap length (µm)
Change amount under EtOH (a.u.)
times2.0 Initial
(Air) 20 min
(Ethanol) 30 min (Air) 0.4 µm
gap 0.8 µm gap 2.6 µm gap
Initial
12 min. later
17 min. later
33 min. later (1)
400 500 600 700 800
Reflectance (a.u.)
Wavelength (nm) Initial position
(2)
(3)
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m em br ane def l ect i on, ( b) t ypi cal spect r a shi f t i n t he i nt er f er om eter of 0.8 µm gap , ( c) change i n peak shi f t t o ti m e, and ( d) com par i son of change am ount i n peak shi f t i n t he Et OH exposur e.
Et OH r esul t ed i n a negat i ve peak shi f t . I n ot her wor ds, si nce t he def l ect i on of t he m em br ane occur r ed in t he di r ect i on of cavi t y expansi on, t he PMMA l ayer of t he gas - r eact i ve m em br ane m ay have absor b ed t he gas and expand ed , r esul t i ng i n com pr essi ve str ess to t he def or m abl e m em br ane. Thus, t he expansi on r at e of PMMA di ff er ed dependi ng on t he gas speci es ; we consi der ed t hat t her e was a di ff er ence i n t he am ount of peak shi f t . The r esul t i ndi cat ed t hat di scr im i nat i ng gas speci es wi t h a singl e devi ce i s di ff i cul t , gas speci es can be di st i ngui s hed t hr ough m achi ne l earni ng of t he di ff er ences i n t he r esponse pat t er ns of m ul t i pl e gas - sensi t i ve m em branes [ 82] . Not e t hat we conf i r m ed t hat t he PMMA l ayer cont r act ed and expanded dur i ng t he absor pt i on of Et OH and MeOH, r espect i vel y, whi ch can be usef ul i n di scr i m i nat i on gas speci es usi ng m achi ne l ear ni ng.
Fi gur e 5.3 : Com par i son of gases r esponse i n i nt er f er om et er s wi t h a 0.4 µm ai r gap and a 100 µm di am et er. ( Err or bar m eans st andar d devi at i on of 3 i nt er f er om et er s on t he sam e chi p.)
-10 -5 0 5 10 15 20
90% diluted
Ethanol 90% diluted
Ammonia 90% diluted
Methanol Water vapor (RH 92 %)
Peak shift (nm)
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5 . 3 E f f e c t o f c h a n g e s i n r e l a t i v e h u m i d i t y a n d t e m p e r a t u r e We m easur ed t he effect s of t em per at ur e and hum i di t y changes on t he sensor.
Under t he condi t i on of no et hanol i n a pet r i di sh, Fi g . 5.4a sho ws t he t i m e cour se of t he peak shi f t when t he t em per at ur e was changed by a hot pl at e. I n t hi s exper i m ent , t he sensor chi p was heat ed at 20 °C f or t he f i r st 5 m in , 27.5 °C f or 5 t o 22 m i n, and 35 °C f or 22 t o 45 m i n. Imm edi at el y af t er heat ing, t he peak shi f t exhi bi t ed a negat i ve val ue by t he expansi on of t he f i lm owi ng t o t her m al expansi on, whi l e t h e peak shi f t appr oa che d zer o wi t h t i m e . Ther ef or e, t he def l ect i on of t he m em br ane because of t em per at ur e changes coul d be sol ved by agei ng. Fi g . 5.4b sho ws t he t i m e cour se of t he peak shi f t of 90% di l ut ed et hanol exposur e at 20 °C a nd r el at i ve hum i di t ie s of 55% and 92% ( m easur ed by a hum i di t y sensor ( HS1101LF, TE Connect i vi t y) ) , r espect i vel y. The am ount of change i n t he peak shi f t decr eased under hi gh hum i di t y condi t i ons whi l e t he nanom echani cal r esponse was obt ai ned ev en i n t he hi gh hum i di t y envi r onm ent . Accor di ng t o t hi s r esul t , when t he r el at i ve hum i di t y change d f rom 55% t o 92% , t he peak shi f t am ount decr ease d by appr oxi m at el y 63% . The out put r esponse has been r epor t ed t o decr ease by 50% or m or e when t he r el at i ve hum i di t y changes f r om 50% t o 90% i n a sem i conduct or - based gas sensor usi ng a heat er [ 84] . Thus, t he decr ease i n r espo nse i n hi gh - hum i di t y envi r onm ent s i s t he sam e as t hat of ot her gas sensor s.
( a) ( b)
Fi gur e 5.4 : I m pact of changes i n t em per at ure and hum i di t y i n i nt er f er om et er s wi t h a 0.4 µm ai r gap and a 100 µm di am et er. Ti m e cour se of peak shi f t wi t h ( a) t em per at ur e change and ( b) hum i di t y change.
-2 2 6 10 14 18
0 5 10 15 20
Peak shift (nm)
Time (min)
RH 55%
RH 92%
Air 90% diluted
ethanol Air
-14 -10 -6 -2 2 6
0 15 30 45
Peak shift (nm)
Time (min)
27.5℃ 35℃
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5 . 4 C o n c e n t r a t i o n d e p e n d e n c e a n d L O D
To eval uat e t he concent r at i on dep endence of t he 0.4 µm i nt er f erom et er, whi ch showed t he hi ghest response t o et hanol i n t he above - ment i oned exper i m ent s, we per f or m ed experi m ents t o obt ai n t he peak shi f t s wi t h changes i n t he di l ut i on r at e of et hanol f r om 60 t o 100% ( Fi g . 5.5) . T he spect r al r esponse t o changes i n t he et hanol di l ut i on r at e, on t hr ee i nt er f er om et er s wi t h a 0.4 - µm gap, was m easur ed ni ne t i m es and expr essed as t he st andar d devi at i on usi ng er r or bar s. At a noi se l evel of 1.5 nm , det erm i ned by t he spect r om et er and m ovi ng aver age pr ocessi ng, t he shi f t am ount of 4.4 nm at a di l ut i on rate of 97.5% was t he m i ni m um LOD. I n addi t i on, t he MEMS i nt er f er om et er had an appr oxi m at el y li near r es ponse when t he di l ut i on r at e was bet ween 60 % and 97. 5% . To obt ai n t he cor r el at i on bet ween t he et hanol concent r at i on and sensit i vit y, the concent r at i on was i dent i f i ed wi t h a com m er ci al sem i conduct or gas sensor ( TGS2620, F i gar o ) . As Fi g . 5.6a shows , t he sem i conduct or gas sensor had a li near r esponse i n t he range bel ow 80% di l uti on r at e, whi ch i s t he guarant eed oper at i ng r ange r epr esent ed by t he rat i o of r esi st ance change of 0.18 –3.4 [ 2 6] . I n cont r ast , t he st abi l i t y was ext r em el y degr aded i n t he l ow- concent r at i on r ange, wher e t he di l ut i on r at e exceed ed 80% , al t hough t he r esponse changes accor di ng t o t he concent r at i on. Fi g . 5.6b shows t he r esul t of est i m at i ng t he concent r at i on of Et OH f r om the obt ai ned r esi st i vi ty f or t he di l ut i on r at e bel ow 80% , i ndi cat i ng a l i near r esponse i n t he sem i conduct or sensor.
Assum i ng t he Et OH concent r at i on of 0 ppm at 100% dil ut i on, and ext r ap ol at i ng t he l i near r esponse r egi on of t he semi conduct or gas sensor, t he concent r at i on of Et OH at 97.5% di l ut ion was obt ai ned as 5 ppm , whi ch was t he lower LOD of t he MEMS i nt er f er om et er. Fur t her m or e, i n t he l ow concent r at i on r ange ( di l ut i on r at e of 90 –97.5% ) exceedi ng t he guar ant eed oper at i on r ange of t he com m er ci al gas sensor, a st abl e r esponse coul d be obt ai ned wi t h a st andar d devi at i on of 0.62 – 1.24 com par ed wi t h 1.72 –3.03 f or t he sem i conduct or sensor.
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Fi gur e 5. 5: Concent r at i on de pendence under Et OH exposur e i n i nt er f er omet ers wi t h a 0.4 µm ai r gap and a 100 µm di am et er. ( Er r or bar m eans st andar d devi at i on of 9 i nt er f er om et er s.)
(a) (b)
Fi gur e 5. 6: Est i m at ion of t he LOD of Et OH i n MEMS I nt e r f er om et er usi ng sem i conduct or gas sensor. ( a) Out put r esponse of semi conduct or gas sensor and ( b) cor r el ati on bet ween di l ut i on r at e and concent r at i on of Et OH. ( Er r or bar m eans st andar d devi at i on of 3 sem i conduct or gas sensor s.)
-20 0 20 40 60 80 100 120 140 160 180
50 60 70 80 90 95 97.5 98.75 99.375 100
Peak shift (nm)
Dilution rate of Ethanol (%)
Noise level in spectrometer (USB4000, Ocean optics) 0
40 80 120 160
60 70 80 90 100
Peak shift (nm)
Dilution rate of EtOH (%)
40 50 60 70 80 90 100 110 0
5 10 15 20
Dilution rate of EtOH (%)
Ratio of resistance change (a.u.)
3.4
Non-linear region
Linear region
40 50 60 70 80 90 100
0 20 40 60 80 100 120
Dilution rate of EtOH (%)
Ratio of resistance change (a.u.)
45 ppm 70 ppm 80 ppm
110 ppm
Extrapolation region Limit of detection：5 ppm
Coefficients Intercept 216.67 X Variable -2.17
Concentration of EtOH (ppm)
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5 . 5 I m p r o v e m e n t o f L O D b y o p t i m i z i n g g e o m e t r y p a r a m e t e r s For f ur t her i m pr ovem ent s i n m i ni m um LOD, t he sensi t i vi t y of t he MEMS sur f ace - str ess sensor was adj ust ed by changi ng t he m em br ane di am et er and t hi ckness. Fi g . 5.7a shows changes i n t he sur f ace - st r ess sensi t i vi t y when t he di am et er of t he def orm abl e m em br ane expanded f r om 100 µm t o 200 and 300 µm . The sur f ace - st r ess sensi t i vi t y i ncr ease d by ni nef ol d when t he di am et er was ext ended by a f act or of t hr ee.
The def or m abl e m em br ane was f ur t her t hi nned wi t hout expandi ng t he di am et er.
Subsequent l y, t he sur f ace st r ess sensi t i vity of t he i nt er f er om eter s wi t h 50 nm -t hi ck par yl ene C and 100 - nm - -t hi ck PMMA, whi ch ar e -t he m i ni m um -t hi ckness f or m ed by dr y t r ansfer and spi n coat i ng, r espect i vel y, wer e m easur ed. Her e , t he sur f ace - str ess sensi t ivi t y i ncr eased by appr oxi m at el y f our f ol d (Fi g . 5.7b) , whi ch was equi val ent t o 125 µN/ m , whi ch was t he anal ysi s val ue of sen si t i vi t y when t he di am et er was doubl ed. Ther ef or e, t he det ec t i on l i m it was expect ed t o i ncr ease by m or e t han one or der of m agni t ude by si mul t aneousl y i ncr easi ng t he ar ea of t he def or m abl e m em br ane and r educi ng t he t hi ckness . The sensi ng area si ze and LOD of t he et hanol sensor s oper at ed under r oom t em per at ur e ar e sum m ari zed i n Tabl e 5.1. The f abr i cat ed i nt er f er omet er ar ea was m or e t han t wo or der s of m agni t ude sm al l er t han t hat of t he convent i onal pi ezor esi st i ve sur f ace - st ress sensor, and i t had super i or det ect i on l i m it s. The sub - m i cr on gap i nt er f er om etri c sur f ace -st r ess sensor pr esent ed here exhi bi t ed ppm -l evel gas det ect i on, whi ch was al m ost equi val ent t o t he per f or m ance of t he l at est sem i conduct or - based gas sensor at r oom t em per at ur e. By opt i m i zi ng t he geom et r y par am et er s, a sensor t hat can det ect sub- ppm et hanol concent r at i ons can be devel oped, w hi ch exceeds t he det ect i on per f orm ance of convent i onal r oom - t em per at ur e gas sensor s.
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(a) (b)
Fi gur e 5. 7: Imp ro v e me n t o f d e te c tio n li mit s b y o p t im iz in g t h e su rfa c e st re ss se n si tiv ity o f in te rfe r o me te r s. In th e c a se o f (a ) e x p a n d in g th e d ia me te r a n d (b ) th in n in g o f th e d e fo rm a b le me mb ra n e .
Tabl e 5.1 Com par i son of Et OH sensi ng r ep or t s at r oom t em per atur e.
D e v i c e t y p e S e n s i n g a r e a ( µ m2)
L O D f o r E t O H ( p p m )
L O D f o r s u r f a c e
s t r e s s ( µ N / m )
R e f .
S n O2– r G O
h y b r i d f i l m N / A 1 N / A [ 8 6 ]
P i e z o r e s i s t i v e c a n t i l e v e r
1 . 2 × 1 06 S i b r i d g e s t r u c t u r e
( 1 . 1 m m × 1 . 1 m m )
2 0 0 N / A [ 8 8 ]
( a ) T h i s s t u d y ( E x p e r i m e n t )
7 . 9 × 1 03 1 0 0 µ m i n d i a m e t e r 3 0 0 n m i n t h i c k n e s s
( 1 5 0 x s m a l l e r t h a n t h e p i e z o r e s i s t i v e t y p e )
5 5 0 0 N / A
( b ) T h i s s t u d y ( A n a l y s i s )
2 . 4 × 1 04 3 0 0 µ m i n d i a m e t e r 3 0 0 n m i n t h i c k n e s s
( 5 0 x s m a l l e r t h a n t h e p i e z o r e s i s t i v e t y p e )
N / A 1 2 5 N / A
( c ) T h i s s t u d y ( A n a l y s i s )
2 . 4 × 1 04 3 0 0 µ m i n d i a m e t e r 1 5 0 n m i n t h i c k n e s s
( 5 0 x s m a l l e r t h a n t h e p i e z o r e s i s t i v e t y p e )
N / A 5 5 N / A
10-6 10-5 10-4 10-3
10-4 10-3 10-2 10-1
1.5 nm
PMMA / Parylene, φ100 µm (thickness : 300 nm) PMMA / Parylene, φ100 µm (thickness : 150 nm)
Deflection (µm)
Surface stress (N/m)
µN/m125 500 LOD for the deflection of µN/m
deformable membrane
Surface stress (N/m)
Deflection (µm)
10-6
µN/m55 125
µN/m 500
µN/m
10-5 10-4 10-3
10-4 10-3 10-2 10-1
100 µm in diameter 200 µm in diameter 300 µm in diameter
1.5 nm
LOD for the deflection of deformable membrane
90 5 . 6 C o n c l u s i o n
Thi s chapt er descri bes t he deposi t i on of PMMA, whi ch act s as a gas - r eacti ve l ayer, ont o t he cavi t y - seal ed MEMS i nter f er om et er wi t h hi gh sur f ace - st r ess sensi t i vit y, and t he c oncent r at i on dependence and L OD of t he s ensor t o vol at i l e et hanol exposur e i s eval uat ed by obt ai ni ng t he spect r al r esponse. We dem onst r at e t hat t he spect r al r esponse ca n be i m pr oved by nar r owi ng t he ai r - gap l engt h of t he i nt er f er om et er t o 0.4 µm ; we successf ul l y det ect ed vol at il e et hanol at a concent r at i on of 5 ppm i n a r oom - t em per atur e envi r onm ent . The r esul t s i ndi cat ed t hat t he sensit i vi t y of t he sensor i s com parabl e t o t hat of a sem iconduct or - based sensor, whi ch has t he hi ghest sensi t i vi t y f or m easur i ng et hanol at r oom t em per at ur e and suggest ed t he f easi bi l it y of a sensor t hat can det ect sub - ppm et hanol concent r ati ons at r oom t em per at ure by o pt i m i zi ng t he shape par am et ers of t he i nt er f er om et er. The key poi nt s of t hi s chapt er ar e as f ol l ows:
【Exper i m ent al r esult s】
1. The spect r al r esponse of 50% di l ut ed vol ati l e et hanol t o i nt er f erom et er s wi t h di ff er ent ai r- gap l engt hs of 0.4, 0 .8, an d 2.6 µm was acqui r ed and t he r ever si bl e r esponse because of t he change in gas concent r at i on was conf i rm ed f or al l i nt er f er omet ers.
2. I n t he i nt er fer om et er wi t h an ai r - gap l ength of 0.4 µm , t he change i n t he peak shi f t i ncr eased by 2.0 and 11.1 t i m es com par ed wi t h t he 0.8 - and 2.6 - µm i nt er f er om et er s, r espect i vel y.
3. I n an i nt er fer om et er wi t h an ai r - gap lengt h of 0.4 µm , a li near r esponse bet ween 5–110 ppm of vol at i l e et hanol concent r at i on was o btai ned , and t he LOD of t he sensor wa s 5 ppm .
4. The opt i m i zat i on of the di am et er and t hi ckness of t he def or m able m em br ane i n an i nt er f er om et er suggest ed t he f easibi l i t y of a sensor t hat can det ect et hanol at a concent r at i on of sub - ppm or der, sur passi ng con vent i onal gas sensor s t hat can operat e i n r oom - t em per atur e envi r onm ent s.
Chapter 6. Detection of neurotransmitter by MEMS interferometer with molecularly imprinted polymer
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