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Silica and polyimide coated quartz crystal resonators for analysis of liquids
B ELSEVIER
Sensors and Actuators B 35-36 (! 996) ! 46-153
CHL=MIP~IU"
Silica and polyimide coated quartz crystal resonators for analysis of liquids H o n g - T a o S u n a,*, Z h i - H o n g C h e n b, W o j c i e c h W l o d a r s k i b, M a l c o l m M c C o r m i c k c aMicroelec:ronics and Materials Technology Centre, Royal Melbourne bistitute of Technology, 124 Latrobe Street, Melbourne, Vic. 3000, Australia bDepartment of Communication and Electronic Engineering, Royal Melbourne Institute of Technology, 124 Latrobe Street, Melbourne. Vic. 3000, Australia CDq~artment of Applied Chemistry, Royal Melbourne hlstitute of Technology, 124 Latrobe Street. Melbourne, Vic. 3000, Australia
Abstract
Silica and polyimide coatings were successfully applied to both sides of 10 MHz AT-cut quartz crystal resonators with silver and gold electrodes. A similar oscillation was observed for the uncoated and coated resonators in air, deionised water, 0. i M NaC! and propylene glycol, in terms of the complex impedance analysis. The impedance module decreased with increasing conductance of the surrounding media while the resonant frequency appeared to depend on the liquid viscosity and density. However, differences in the oscillation were evident when these devices were immersed in waste water from a clean room for microelectronic fabrication. Relevant chemical analyses were also made on the waste water samples. Hourly testing results indicated that changes in the impedance were due to metal ions (Na+, K÷, Ca 2+, etc.) and the frequency shift comes from the complicated interaction between the solute molecules (propylene glycol) and the coatings or electrodes. This study highlights that the multi-sensors with various coatings and electrodes provide possibilities to monitor waste water in real-time.
KeywoMs: Quartz crystal; Resonant frequency; Impedance; Liquid; Waste water 1. Introduction Recent advances in the research and development of quartz crystal microbalance (QCM) or resonator offer new possibilities for chemical and biochemical analyses of liquid media like waste water [i,2]. However, the QCM contacting liquid exhibits more complicated resonance than the device in vacuum and gaseous atmospheres. Its frequency shift is not only due to mass loading but is also affected by energy coupling at the interface between the solid and liquid. The oscillation pattern depends greatly on the electrical and mechanical conditions of the crystal, which are changed by the adjacent liquid. Compared to simple oscillator circuits, impedance spectroscopy provides a better description and enables more liquid properties to be determined with an equivalent circuit analysis or pattern recognition. The QCM usually contacts with liquid on one side only in order to eliminate the electrical influence of the
* Corresponding author.
0925-4005/96/$15.00 © 1996 Elsevier Science S.A. All rights reserved Pll S 0 9 2 5 - 4 0 0 5 ( 9 6 ) 0 2 0 8 5 - 0
liquid. In this manner, the oscillation is stable but does not have any information on the electrical properties of the liquid [3]. It was reported that a totally immersed resonator could overcome this disadvantage [4,5]. With the aid of impedance analysis and circuit simulation, the resonator is sensitive to minor changes in viscoelastic and electrical properties of water caused by temperature variations. In order to monitor industrial waste water, relevant electronic design was made [6]. When the waste water conductivity was high enough, however, the oscillation disappeared because the surrounding media formed a 'short' circuit for the applied voltage. On other hand, device materials including quartz crystal, electrodes and wires may be contaminated or etched by some chemicals in the water, as a result of poor long-term stability. It seems useful to solve these problems by applying coatings to the quartz crystal. Wet chemistry is one of the best candidates for preparation of metal oxide and polymer coatings. Silica is considered as a unique material with excellent chemical and physical behaviour. Sol-gel derived silica coatings were used as gutters of alkali ions in soda-silica glass substrates
147
H. - T. Sun et al. / Sensors and Actuators B35-36 (1996) 146-153
and improved the weathering resistance of the glass [7]. The silica protective layer on AT-cut resonators was found to be very stable even in severe chemical environments. Besides the chemical inertness, chemical and physical similarities between the crystal and coating are invaluable. It is also beneficial that silica glass, with controllable pore distribution can be prepared via a sol-gel process at low temperature. Polyimides show excellent thermal and mechanical behaviour, are also chemically unreactive. Applications include electrical insulation of a variety of components, such as solder-resistant printed circuit boards and encapsulation for integrated circuits
[81. 2. Silica and polyimide coatings 2. !. Preparation Starting coating solutions were spin-on silica glass (P114A, Allied Signal Inc., USA) and polyimide (PIX1400, Hitachi Chemical Co. Ltd., Japan). Quartz crystal resonators (10Mhz) with gold and silver electrodes (Shizuoka University, Japan) were cleaned in acetone, isopropanol and deionised water, and dried at 120°C for 30 min. The cleaned crystals were fully immersed in coating solutions to cover the whole surface of the devices. After removal, the devices were immediately mounted on a spinner to achieve uniform coatings. These
coatings were then baked in a vacuum oven and cured in a diffusion furnace, according to the details illustrated in Fig. 1. The final coatings were about 0.2 ~m thick for the silica, and 1/,tm for the polyimide. The preparation was performed in a clean room.
2.2. Surface characterisation The silica and polyimide coatings were also spun on micro slide glass substrates (Superior, Germany) following the procedure described above. Their surface morphology was characterised with an Alpha-step 250 profilemeter (Tencor Instruments, USA). Fig. 2 shows the surface profiles of the silica and polyimide coatings in a 0.5 nm vertical resolution. The depth range is about 40 nm for the naked glass surface, and 20 nm and 100 nm alter applying the polyimide and silica coatings. The glass substrate had an average roughness of 6 nm, while the silica and polyimide coatings gave an average roughness of 12 nm and 6 nm, respectively. The quartz crystals and electrodes showed high surface roughness, 139 nm for Ag/quartz and 71 nm for Au/ quartz. Minor decreases in the roughness were observed for the silica surface coated onto the both electrodes. However major improvements were obtained with the polyimide coating, see Table 1. This may be due to the difference in the viscosity of the two solutions (0. ! P for silica, 11 P for polyimide).
LCleaning Au, Ag/Quartz I
Silica Solution (Spin-on silica glass)
Polyimide Solution (Polymeric coating)
3000 rpm, 60 sec
2000 rpm, 20 sec Hydrolysis:
Dipping 1
OEt
Evaporation: -- NH
I EtO--
~n
HO~C
Si~OEt
r Spinning ]
I
OH
N
II O
II
~
O
Ilz
z~
C m
~ C m O
II
O
NI
HJ
Imide Rings: 200 *C, 30 min
Condensation: 120°C, 30 min
It
Decomposition: 400°C, 60 min
I VacuumBaking I I Thermal Curing ]
(.
O
N ~ r
O
I 0
C
--
~R=~
c 0
m
NI|Z
It O
Polymerisation: 350°C, 60 rain
SiO2
O
~
II C --N II O
Fig. I. Flowchart of the silica and polyimide coating preparation.
O
II O
In
148
H.-T. Sun et al. / Sensors and Actuators B35-36 (1996) 146-153 Silica/Glass
a 1M
too
41o 5OO
~t~
3OO
Distance (~m)
b
Polyimldoh31an 190
Pohlmid~I(di~
J-
3.2. In waste water
o
40 0
mum impedance occurs, was 9.991 MHz for As/quartz. In other cases, the fr was relatively constant, ]Z] = 2040Q, 0 = 2 0 - 3 5 °, caused by the large conductance (106.7 mS cm -l) of 0.1 M NaCl solution [9]. However, the fr was decreased by 47 _+ 1 kHz, apparently by the higher viscosity of propylene glycol, and [ZI rose to around 1000 Q, as shown in Fig. 4. It can be seen that the oscillation behaviour observed was similar regardless of whatever coating or electrode type was used. Only the polyimide coating made the impedance module higher and the phase angle lower. The dense polyimide may eliminate the current leakage in the NaCI solution. However no significant difference could be found in the uncoated and porous silica-coated resonators. These results highlight that the coatings do not appear to interrupt the electromechanical coupling between the piezoelectric crystal and the liquid.
100
200
300
Distance (.m)
Fig. 2. Surface characteristics of the (a) silica and (b) polyimide coatings on the glass substrate.
2.3. Oscillation of coated quartz
The oscillation of the quartz crystals in air and liquid was tested with a network/spectrum analyser (HP 4195A, USA), with an applied signal 0.2 V and frequency step I kHz. The coating effect on the oscillation in air is shown in Fig. 3. The resonant frequency shift of the polyimide-coated device was 70 kHz, while the silica coating gave only 40 kHz, because of the thicker polyimide coating. Beyond the oscillation region, the phase angle (0) was about -50 ° for all the devices tested. The impedance module of the polyimide-coated resonator was almost the same as that of the uncoated one, however, that of the silica-coated device was 120 Q lower, probably because of the humidity and gases (CO:,, NOx) adsorbed in the porous silica coating.
3. Analysis in liquid media
A clean room for microelectronic fabrication was selected as a pilot source of industrial waste water. The investigated clean room is equipped with the wet processes of lithography for microelectronic and optoelectronic fabrication. Miniaturised complex devices and circuits are built into and on the surface of a silicon wafer in a succession of process steps. Each step begins with coating the silicon wafer with a light-sensitive material called a photoresist. A pattern is delineated in this material by shining ultraviolet light onto it through a mask containing the desired pattern. The exposed region is made soluble in positive resists. The pattern of the polymer stencil is repeated in the metal film and insulator film by etching away the undesired area. The metal can also be patterned by an additive lift-off process. After the resist has been stripped, the silicon is doped through the oxide mask to produce the desirable semiconductor dev4ces [10]. Some typical chemicals employed in the clean room are summarised in Table 2. The waste water was sampled at the inlet of the waste tank, and analysed with the quartz resonators, as illustrated in Fig. 5. A sample was taken every hour from 0900 until 1800 h. Although the oscillation was maintained, it was quite different for each device. Fig. 6 shows changes in the impedance parts (R = lZl cos 0, X = [Z] sin 0) and resonant frequencies of these devices in the waste water during a working period. These three
3. i. in pure solutions
These devices were also tested in deionised water and other commercial liquids, such as 0.1 M NaC! and propylene glycol. The oscillation of these devices in the deionised water was selected as the reference, it had an impedance module (~l) of 450-500 Q and phase angle (0) of 20-35 ° in NaCl, -50" in others at non-resonant frequencies. The resonant frequency (fr), at which mini-
Table i Average surface roughness (nm) of the uncoated and coated quartz crystals
Ag/quartz Au/quartz
Uncoated
Silica-coated
Polyimide-coated
139 71
127 67
90 59
! 49
H.- T. Sun et al. / Sensors and Actuators B35-36 (! 996) 146-153
El
AglQuartz in Air
b
100000
AglQuartz in Air 120
silica-coated 100
silica-coated
A
10000
80
°'Y'mide'c°a'e A A uncoated
A
polyimide-coated
R uncoated
6O
'10 v N
40 e-
"D o
E
20
~
0
o.
c m
--
u) (9 J~
1000
E
--
-20
-40
-60
t00 9.70
-80 9.80
0.90
t0.00
10.10
10.20
10.30
Frequency (MHz)
9.70
9.80
0.00
10.00
t0.10
:b.20
10.30
Frequency (MHz)
Fig. 3. (a) Impedance module and (b) phase angle spectra of the uncoated, silica-coated and polyimide-coated Ag/quartz resonator in air.
parameters of the resonators in deionised water are plotted as reference lines. Differences from the relevant reference lines reflect changes in chemical composition from time to time. From 1400 to 1500 h, therefore, contaminalion appears to be the highest in terms of the largest variances of R and X compared with the references. The R and X changes with time are similar for the six devices. At beginning of the testing period, both R and X values were close to the reference since deionised waster is used for re-cleaning glassware in the morning. Disposal of used chemical, which commonly happens after lunch, appears to have caused a simultaneous decrease in R and an increase in X values after which a period of recovery was observed. The conductivity and pH value of waste water were also measured. Conductivity and pH meters (Hanna, Germany) were calibrated with standard KC! solutions, and buffer solutions at pH 4, 7 and 9. Fig. 7 reports the conductivity and pH values of the same waste samples as shown in Fig. 6. Within the washing period, the investigated water appeared weakly basic (pH < 9.04) and the conductivity ranged from 5 to 7 mS cm -I. During the period of change, the pH value was above l O and the conductivity as high as 140-300 mS cm -I. This means that chemicals present were strongly basic. During the recovery period, the pH value (>9) and conductivity
(>34 mS cm -I) maintained high levels Chemical analyses were also conducted on these waste water samples. In the case of metal ions, the samples were digested in acidic solution and concentrated prior to being analysed by inductively coupled plasma with mass spectrometric detection. Organic molecules were detected by direct injection into a capillary gas chromatograph with mass spectrometric detection. The mass spectrometer was in full scan mode and separation was achieved using a low bleed capillary column. Peak ider,tification was confirmed using a standard mass spectral data base. Table 3 reports the analytical results during the three periods, i.e. washing, pouring and recovery for which individual samples were composited. Examination of the pH and conductivity data for these samples (see Fig. 6) clearly shows that the impedance is very much more sensitive to changes in conductivity than to changes in pH, but changes in ionic compesition can certainly be detected. On the other hand, detection of molecular solutes is far more complex. Fig. 4 shows that the presence of propylene glycol caused a measurable drop in the resonant frequency and the chemical analytical data indicate show that the concentration of the propylene glycol is much higher in the samples collected between 1400 and 1500 h. However, as Fig. 6 shows, the resonant frequency did not drop as expected
H.-T. Sun et al. / Sensors and Actuators B35-36 (1996) 146-153
150
a
AglQuartz
b
10000
,.,,
Silica/AglQuartz 10000
In prolylcneglycol
1000
g
N
1000
In d
r,,j
e
i
o
n
~
| 8C --
1o0
100
In O,IM NaCIsolution
In O.IM Na('l solution
10 9.70
9.so
0.9o lO.OO lO.lO Frequency (MHz)
1o,2o
10 9,70
10.30
I
I
I
'
:
9,80
9.90
10.00
10,10
10.20
10.30
Frequency (MHz) Polyimide/AglQuartz
10000
In prolyleneglycol 1000
zcd water
N Q)
C
8. S[
100
In O.IM NaCI solution 'N/
10 9.?0
I
I
o
I
I
9.80
9.90
10.00
10.10
10.20
-, 10.30
Frequency (MHz) Fig. 4. Oscillation of the Ag/quartz resonators in deionised water, O. 1 M NaCI and propylene glycol: (a) uncoated, (b) silica-coated, (c) polyimidc, coated.
H.-T. Sun et al. / Sensors and Actuators B35-36 (1996) 146-153
Table 2 Main chemicals currently used in the clean room. Category
Products
Chemical components
Photoresist
AZ ! 5 ! 2, A Z A 5 6 2
Developer
AZ450, AZ726, MIF319
Wafer Organic
Silicon, quartz, etc. Solvents, adhesive
Metal Etchant Cleaning
Mask, electrode Mask, electrode, wafer Acid, base, salt
Diazoquinone, Novolak, Cellosolve acetate KOH,NaOH, KMnO4, polyethylene glycol Si, SiO2, LiNbO 3, Glass Acetone, isopropanol, hexamethyldisilazan Cr, Ti, Ni, AI, Au (NH4)2Ce(NO3)6,HCIO4 HF, HNO3, HCI, NaOH, Na2CO3
for all resonator systems used. Only the uncoated Ag/quartz and coated Au/quartz had a drop in frequency and then to different extents. The silicaJAg/quartz in fact, appeared quite a large increase. It is apparent theretbre, that some components of the waste water samples can mask the effect of the propylene glycol on some resonator systems since all systems gave similar results with pure propylene glycol, see Fig. 4. The reason for the abnormal frequency shift may be that some organic molecules with low density, selectively adsorbed in the porous silica layer, interact with the silver electrode. Since the chemical composition of th,: waste water is very complicated, interactions of chemical molecules with the silver electrode or silica coating require further study. The concentration of propylene glycol and sodium increased dramatically at 1400-1500 h. Major increases in
Clean-Room
I A
SinP
151
calcium, copper, nickel and titanium and minor increases in potassium, silicon and aluminium were also observed. These results are in a good agreement with the work records. Main tasks performed on the day are patterning and etching mask and waveguides. Therefore, a large amount of developers and some etchants are discarded during the afternoon and are the probably cause of the contamination observed. No etching effect was found for the resonators in these basic solutions. However, the electrodes of the resonators without coatings were stripped off after immersing in 0.5 M HCI at room temperature for 24 h. With the coatings in place, the electrodes were still undamaged after 48 h immersion, which demonstrates the protective properties.
4. Conclusions In this study, the quartz crystal resonators with and without coatings were applied to liquid samples. Major conclusions might be summarised as follows. The silica and polyimide films can be conveniently applied on the quartz crystal by spin-coating. The crystal surface becomes tough and smooth with the silica and polyimide coatings, respectively. The oscillation of the coated resonators were maintained in air, deionised water and solutions tested. The effect of propylene glycol appears to .be masked with other combinations of coating and electrode which may be more sensitive to other minor components of the samples. Results from the waste water samples are complex
Network Anaiyser
t= ® ®
®l
OOe]
¢
Neutralization
Waste Water
Tank
FTo city sewage treatment
Fig. 5. Schematic diagram of the w~'-tewater analysis using the quartz resonators.
152
H.-T. Sun et al. / Sensors and Actuators B35-36 (1996) 146-153 Silica/AulQuartz in Waste Water
Ag/Qu~rtz in Waste Water
a
we
d
i ~°°°
5oo,. ]
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.
.
water
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.
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.
.
waste water
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11:00
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17:00
18:00
Time (hours)
Time (hours)
A ~ Q u a r t : in Waste Water
PolylmldslAglQuartz in Waste Water $oo ..................................................................................................................................................
.
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.
.
.
.
.
.
demonized'~,~to~ . . .
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.
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10:00
11:00
12:00
Time (hours)
demnlzedwater
13;00
........
15;00
16:00
11:00
PolylmldelAu/Quartz In Waste Water ...............................................................................................
dOiOnlZSdwster
18:00
~,J
",/
4000
Oe~onizedwater. . . . . . . . . . . . . . . . . . . .
East
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Time (hours)
Sillca/Ag/Quartz in Waste W a t e r
I
/
i
°
~oo__
x
/
\
wasle water 400 i
0:Q0
"--'
\ %
/ ,,t
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1O00
tt:O0
1;100
Time (t~rs)
13:00
1400
lS:00
16:00
I1P:00
18:00
Time (hours)
Fig. 6. Ho'.,rly waste water monitoring based on the ,~,artz resonators: (a) Ag/quartz, (b) silica/Ag/quartz, (c) polyimide/Ag/quartz, (d) Aulquartz, (e) silica/Au/quartz, (f) polyimide/Au/quartz.
pH value and Conductivity
"[
T ""
Table 3 Chemical analysis of the waste water (results are expressed as mg I-!) pl!
1 !
- -
$:W
't~.'W
11~
'l):eO
11,1~
14:N
15.1)0
~
,
rs:W
1T.'~
.I00
18:00
T~me (hours)
Fig. 7. Conductivity and pH values of the waste water at the same time as above.
Analytes
0900-1300 h
1400-1500 h
1600-1800 h
Propylene glycol Sodium Potassium Calcium Silicon Copper Aluminium Nickel Titanium
4.0 10 2.9 1.4 1.4 0.04 0.09 0.013 <0.01
2800 89 3.3 2.5 2.0 0.26 0. ! 3 0.026 0.017
! 00 54 2. ! ! .3 2.0 0. ! I 0.12 0.004 <0.01
H.-T. Sun et aL /Sensors and Actuators B35-36 (1996) 146-153
but the variability from one coating and electrode to another may be valuable in designing sensors which are more selective for specific components. The coated resonator can be used for on-line analysis to detect changes in the chemical composition of waste water from a clean room facility. The difference of oscillation due to various coatings and electrodes provides a promising way to realise a multi-sensor system to distinguish individual chemicals in liquids.
Acknowledgements One of the author (HTS) wishes to acknowledge the R M I T for the research fellowship, and ZHC to the Department of Communication and Electronic Engineering for the postgraduate research scholarship. Thanks are due to Ms. Chiping Wu for her technical support in the clean room, to Mr. Massood Zandi tbr assistance in the chart plotting, and to Mr. Carlos Jimenez for helpful suggestions.
References [I] J. Auge, P. Hauptmann,J. Hartmann, S. Rosier and R. Lucklum, New design for QCM sensors in liquids, Sensors and Actuators B, 24-25 (1995) 43--48.
153
[2] N.B. Tytler and S.J. Tully, Air, earth, fire and waste - the analysis of waste, Anal. Pro~'., 30 (! 993) 69-71. [3l D.W. Paul, S.T. Clark and T.L. Beeler, Instrumentation for simultaneous measurement of double-layer capacitance and solution resistance at a QCM electrode, Sensors and Actuators B, 17 (1994) 247-255. [4] H.T. Sun, L.Y. Zhang, X. Yao and W. Wlodarski, AT-cut quartz resonators for determination of viscoelastic and dielectric properties of water/glycerol, Sensors and Actuators A, 43 (i 994) 208212. [5] H.T. Sun, M. Faccio, C. Cantalini and M, Pelino, Impedance analysis and circuit simulation of quartz resonator in water at different temperature, Sensors and Actuators B, in press. [6] M. Faceio, H.T. Sun, C. Cantalini and M. Pelino, Waste water monitoring with quartz crystal sensor, in Z.G. Zhou (ed.), Proc. Int. Conf. on Electronic Components and Materials', Sensors and Actuators, International Academic Publishers, Beijing, 1995, pp.
319-322. [7] C.J. Brinker and G.W. Scherer, Sol-Gel Science, Academic Press, San Diego, CA, 1990. [8] J.M. Charrier, Polymeric Materials and Processing: Plastics, Elastomers and Composites, Hanser, Munich, 1990, pp. 130-137. [9] G.W.C. Kaye and T,H. Laby, Tables oJ' Physical and Chemical Constants, Longman, London, !973, pp. 215. [10] W.M. Moreau, Semiconducwr Lithography: Principles, Practices and Material.t, Plenum, New York, 1988, pp. 1-22.
Sensors and Actuators B 35-36 (! 996) ! 46-153
CHL=MIP~IU"
Silica and polyimide coated quartz crystal resonators for analysis of liquids H o n g - T a o S u n a,*, Z h i - H o n g C h e n b, W o j c i e c h W l o d a r s k i b, M a l c o l m M c C o r m i c k c aMicroelec:ronics and Materials Technology Centre, Royal Melbourne bistitute of Technology, 124 Latrobe Street, Melbourne, Vic. 3000, Australia bDepartment of Communication and Electronic Engineering, Royal Melbourne Institute of Technology, 124 Latrobe Street, Melbourne. Vic. 3000, Australia CDq~artment of Applied Chemistry, Royal Melbourne hlstitute of Technology, 124 Latrobe Street. Melbourne, Vic. 3000, Australia
Abstract
Silica and polyimide coatings were successfully applied to both sides of 10 MHz AT-cut quartz crystal resonators with silver and gold electrodes. A similar oscillation was observed for the uncoated and coated resonators in air, deionised water, 0. i M NaC! and propylene glycol, in terms of the complex impedance analysis. The impedance module decreased with increasing conductance of the surrounding media while the resonant frequency appeared to depend on the liquid viscosity and density. However, differences in the oscillation were evident when these devices were immersed in waste water from a clean room for microelectronic fabrication. Relevant chemical analyses were also made on the waste water samples. Hourly testing results indicated that changes in the impedance were due to metal ions (Na+, K÷, Ca 2+, etc.) and the frequency shift comes from the complicated interaction between the solute molecules (propylene glycol) and the coatings or electrodes. This study highlights that the multi-sensors with various coatings and electrodes provide possibilities to monitor waste water in real-time.
KeywoMs: Quartz crystal; Resonant frequency; Impedance; Liquid; Waste water 1. Introduction Recent advances in the research and development of quartz crystal microbalance (QCM) or resonator offer new possibilities for chemical and biochemical analyses of liquid media like waste water [i,2]. However, the QCM contacting liquid exhibits more complicated resonance than the device in vacuum and gaseous atmospheres. Its frequency shift is not only due to mass loading but is also affected by energy coupling at the interface between the solid and liquid. The oscillation pattern depends greatly on the electrical and mechanical conditions of the crystal, which are changed by the adjacent liquid. Compared to simple oscillator circuits, impedance spectroscopy provides a better description and enables more liquid properties to be determined with an equivalent circuit analysis or pattern recognition. The QCM usually contacts with liquid on one side only in order to eliminate the electrical influence of the
* Corresponding author.
0925-4005/96/$15.00 © 1996 Elsevier Science S.A. All rights reserved Pll S 0 9 2 5 - 4 0 0 5 ( 9 6 ) 0 2 0 8 5 - 0
liquid. In this manner, the oscillation is stable but does not have any information on the electrical properties of the liquid [3]. It was reported that a totally immersed resonator could overcome this disadvantage [4,5]. With the aid of impedance analysis and circuit simulation, the resonator is sensitive to minor changes in viscoelastic and electrical properties of water caused by temperature variations. In order to monitor industrial waste water, relevant electronic design was made [6]. When the waste water conductivity was high enough, however, the oscillation disappeared because the surrounding media formed a 'short' circuit for the applied voltage. On other hand, device materials including quartz crystal, electrodes and wires may be contaminated or etched by some chemicals in the water, as a result of poor long-term stability. It seems useful to solve these problems by applying coatings to the quartz crystal. Wet chemistry is one of the best candidates for preparation of metal oxide and polymer coatings. Silica is considered as a unique material with excellent chemical and physical behaviour. Sol-gel derived silica coatings were used as gutters of alkali ions in soda-silica glass substrates
147
H. - T. Sun et al. / Sensors and Actuators B35-36 (1996) 146-153
and improved the weathering resistance of the glass [7]. The silica protective layer on AT-cut resonators was found to be very stable even in severe chemical environments. Besides the chemical inertness, chemical and physical similarities between the crystal and coating are invaluable. It is also beneficial that silica glass, with controllable pore distribution can be prepared via a sol-gel process at low temperature. Polyimides show excellent thermal and mechanical behaviour, are also chemically unreactive. Applications include electrical insulation of a variety of components, such as solder-resistant printed circuit boards and encapsulation for integrated circuits
[81. 2. Silica and polyimide coatings 2. !. Preparation Starting coating solutions were spin-on silica glass (P114A, Allied Signal Inc., USA) and polyimide (PIX1400, Hitachi Chemical Co. Ltd., Japan). Quartz crystal resonators (10Mhz) with gold and silver electrodes (Shizuoka University, Japan) were cleaned in acetone, isopropanol and deionised water, and dried at 120°C for 30 min. The cleaned crystals were fully immersed in coating solutions to cover the whole surface of the devices. After removal, the devices were immediately mounted on a spinner to achieve uniform coatings. These
coatings were then baked in a vacuum oven and cured in a diffusion furnace, according to the details illustrated in Fig. 1. The final coatings were about 0.2 ~m thick for the silica, and 1/,tm for the polyimide. The preparation was performed in a clean room.
2.2. Surface characterisation The silica and polyimide coatings were also spun on micro slide glass substrates (Superior, Germany) following the procedure described above. Their surface morphology was characterised with an Alpha-step 250 profilemeter (Tencor Instruments, USA). Fig. 2 shows the surface profiles of the silica and polyimide coatings in a 0.5 nm vertical resolution. The depth range is about 40 nm for the naked glass surface, and 20 nm and 100 nm alter applying the polyimide and silica coatings. The glass substrate had an average roughness of 6 nm, while the silica and polyimide coatings gave an average roughness of 12 nm and 6 nm, respectively. The quartz crystals and electrodes showed high surface roughness, 139 nm for Ag/quartz and 71 nm for Au/ quartz. Minor decreases in the roughness were observed for the silica surface coated onto the both electrodes. However major improvements were obtained with the polyimide coating, see Table 1. This may be due to the difference in the viscosity of the two solutions (0. ! P for silica, 11 P for polyimide).
LCleaning Au, Ag/Quartz I
Silica Solution (Spin-on silica glass)
Polyimide Solution (Polymeric coating)
3000 rpm, 60 sec
2000 rpm, 20 sec Hydrolysis:
Dipping 1
OEt
Evaporation: -- NH
I EtO--
~n
HO~C
Si~OEt
r Spinning ]
I
OH
N
II O
II
~
O
Ilz
z~
C m
~ C m O
II
O
NI
HJ
Imide Rings: 200 *C, 30 min
Condensation: 120°C, 30 min
It
Decomposition: 400°C, 60 min
I VacuumBaking I I Thermal Curing ]
(.
O
N ~ r
O
I 0
C
--
~R=~
c 0
m
NI|Z
It O
Polymerisation: 350°C, 60 rain
SiO2
O
~
II C --N II O
Fig. I. Flowchart of the silica and polyimide coating preparation.
O
II O
In
148
H.-T. Sun et al. / Sensors and Actuators B35-36 (1996) 146-153 Silica/Glass
a 1M
too
41o 5OO
~t~
3OO
Distance (~m)
b
Polyimldoh31an 190
Pohlmid~I(di~
J-
3.2. In waste water
o
40 0
mum impedance occurs, was 9.991 MHz for As/quartz. In other cases, the fr was relatively constant, ]Z] = 2040Q, 0 = 2 0 - 3 5 °, caused by the large conductance (106.7 mS cm -l) of 0.1 M NaCl solution [9]. However, the fr was decreased by 47 _+ 1 kHz, apparently by the higher viscosity of propylene glycol, and [ZI rose to around 1000 Q, as shown in Fig. 4. It can be seen that the oscillation behaviour observed was similar regardless of whatever coating or electrode type was used. Only the polyimide coating made the impedance module higher and the phase angle lower. The dense polyimide may eliminate the current leakage in the NaCI solution. However no significant difference could be found in the uncoated and porous silica-coated resonators. These results highlight that the coatings do not appear to interrupt the electromechanical coupling between the piezoelectric crystal and the liquid.
100
200
300
Distance (.m)
Fig. 2. Surface characteristics of the (a) silica and (b) polyimide coatings on the glass substrate.
2.3. Oscillation of coated quartz
The oscillation of the quartz crystals in air and liquid was tested with a network/spectrum analyser (HP 4195A, USA), with an applied signal 0.2 V and frequency step I kHz. The coating effect on the oscillation in air is shown in Fig. 3. The resonant frequency shift of the polyimide-coated device was 70 kHz, while the silica coating gave only 40 kHz, because of the thicker polyimide coating. Beyond the oscillation region, the phase angle (0) was about -50 ° for all the devices tested. The impedance module of the polyimide-coated resonator was almost the same as that of the uncoated one, however, that of the silica-coated device was 120 Q lower, probably because of the humidity and gases (CO:,, NOx) adsorbed in the porous silica coating.
3. Analysis in liquid media
A clean room for microelectronic fabrication was selected as a pilot source of industrial waste water. The investigated clean room is equipped with the wet processes of lithography for microelectronic and optoelectronic fabrication. Miniaturised complex devices and circuits are built into and on the surface of a silicon wafer in a succession of process steps. Each step begins with coating the silicon wafer with a light-sensitive material called a photoresist. A pattern is delineated in this material by shining ultraviolet light onto it through a mask containing the desired pattern. The exposed region is made soluble in positive resists. The pattern of the polymer stencil is repeated in the metal film and insulator film by etching away the undesired area. The metal can also be patterned by an additive lift-off process. After the resist has been stripped, the silicon is doped through the oxide mask to produce the desirable semiconductor dev4ces [10]. Some typical chemicals employed in the clean room are summarised in Table 2. The waste water was sampled at the inlet of the waste tank, and analysed with the quartz resonators, as illustrated in Fig. 5. A sample was taken every hour from 0900 until 1800 h. Although the oscillation was maintained, it was quite different for each device. Fig. 6 shows changes in the impedance parts (R = lZl cos 0, X = [Z] sin 0) and resonant frequencies of these devices in the waste water during a working period. These three
3. i. in pure solutions
These devices were also tested in deionised water and other commercial liquids, such as 0.1 M NaC! and propylene glycol. The oscillation of these devices in the deionised water was selected as the reference, it had an impedance module (~l) of 450-500 Q and phase angle (0) of 20-35 ° in NaCl, -50" in others at non-resonant frequencies. The resonant frequency (fr), at which mini-
Table i Average surface roughness (nm) of the uncoated and coated quartz crystals
Ag/quartz Au/quartz
Uncoated
Silica-coated
Polyimide-coated
139 71
127 67
90 59
! 49
H.- T. Sun et al. / Sensors and Actuators B35-36 (! 996) 146-153
El
AglQuartz in Air
b
100000
AglQuartz in Air 120
silica-coated 100
silica-coated
A
10000
80
°'Y'mide'c°a'e A A uncoated
A
polyimide-coated
R uncoated
6O
'10 v N
40 e-
"D o
E
20
~
0
o.
c m
--
u) (9 J~
1000
E
--
-20
-40
-60
t00 9.70
-80 9.80
0.90
t0.00
10.10
10.20
10.30
Frequency (MHz)
9.70
9.80
0.00
10.00
t0.10
:b.20
10.30
Frequency (MHz)
Fig. 3. (a) Impedance module and (b) phase angle spectra of the uncoated, silica-coated and polyimide-coated Ag/quartz resonator in air.
parameters of the resonators in deionised water are plotted as reference lines. Differences from the relevant reference lines reflect changes in chemical composition from time to time. From 1400 to 1500 h, therefore, contaminalion appears to be the highest in terms of the largest variances of R and X compared with the references. The R and X changes with time are similar for the six devices. At beginning of the testing period, both R and X values were close to the reference since deionised waster is used for re-cleaning glassware in the morning. Disposal of used chemical, which commonly happens after lunch, appears to have caused a simultaneous decrease in R and an increase in X values after which a period of recovery was observed. The conductivity and pH value of waste water were also measured. Conductivity and pH meters (Hanna, Germany) were calibrated with standard KC! solutions, and buffer solutions at pH 4, 7 and 9. Fig. 7 reports the conductivity and pH values of the same waste samples as shown in Fig. 6. Within the washing period, the investigated water appeared weakly basic (pH < 9.04) and the conductivity ranged from 5 to 7 mS cm -I. During the period of change, the pH value was above l O and the conductivity as high as 140-300 mS cm -I. This means that chemicals present were strongly basic. During the recovery period, the pH value (>9) and conductivity
(>34 mS cm -I) maintained high levels Chemical analyses were also conducted on these waste water samples. In the case of metal ions, the samples were digested in acidic solution and concentrated prior to being analysed by inductively coupled plasma with mass spectrometric detection. Organic molecules were detected by direct injection into a capillary gas chromatograph with mass spectrometric detection. The mass spectrometer was in full scan mode and separation was achieved using a low bleed capillary column. Peak ider,tification was confirmed using a standard mass spectral data base. Table 3 reports the analytical results during the three periods, i.e. washing, pouring and recovery for which individual samples were composited. Examination of the pH and conductivity data for these samples (see Fig. 6) clearly shows that the impedance is very much more sensitive to changes in conductivity than to changes in pH, but changes in ionic compesition can certainly be detected. On the other hand, detection of molecular solutes is far more complex. Fig. 4 shows that the presence of propylene glycol caused a measurable drop in the resonant frequency and the chemical analytical data indicate show that the concentration of the propylene glycol is much higher in the samples collected between 1400 and 1500 h. However, as Fig. 6 shows, the resonant frequency did not drop as expected
H.-T. Sun et al. / Sensors and Actuators B35-36 (1996) 146-153
150
a
AglQuartz
b
10000
,.,,
Silica/AglQuartz 10000
In prolylcneglycol
1000
g
N
1000
In d
r,,j
e
i
o
n
~
| 8C --
1o0
100
In O,IM NaCIsolution
In O.IM Na('l solution
10 9.70
9.so
0.9o lO.OO lO.lO Frequency (MHz)
1o,2o
10 9,70
10.30
I
I
I
'
:
9,80
9.90
10.00
10,10
10.20
10.30
Frequency (MHz) Polyimide/AglQuartz
10000
In prolyleneglycol 1000
zcd water
N Q)
C
8. S[
100
In O.IM NaCI solution 'N/
10 9.?0
I
I
o
I
I
9.80
9.90
10.00
10.10
10.20
-, 10.30
Frequency (MHz) Fig. 4. Oscillation of the Ag/quartz resonators in deionised water, O. 1 M NaCI and propylene glycol: (a) uncoated, (b) silica-coated, (c) polyimidc, coated.
H.-T. Sun et al. / Sensors and Actuators B35-36 (1996) 146-153
Table 2 Main chemicals currently used in the clean room. Category
Products
Chemical components
Photoresist
AZ ! 5 ! 2, A Z A 5 6 2
Developer
AZ450, AZ726, MIF319
Wafer Organic
Silicon, quartz, etc. Solvents, adhesive
Metal Etchant Cleaning
Mask, electrode Mask, electrode, wafer Acid, base, salt
Diazoquinone, Novolak, Cellosolve acetate KOH,NaOH, KMnO4, polyethylene glycol Si, SiO2, LiNbO 3, Glass Acetone, isopropanol, hexamethyldisilazan Cr, Ti, Ni, AI, Au (NH4)2Ce(NO3)6,HCIO4 HF, HNO3, HCI, NaOH, Na2CO3
for all resonator systems used. Only the uncoated Ag/quartz and coated Au/quartz had a drop in frequency and then to different extents. The silicaJAg/quartz in fact, appeared quite a large increase. It is apparent theretbre, that some components of the waste water samples can mask the effect of the propylene glycol on some resonator systems since all systems gave similar results with pure propylene glycol, see Fig. 4. The reason for the abnormal frequency shift may be that some organic molecules with low density, selectively adsorbed in the porous silica layer, interact with the silver electrode. Since the chemical composition of th,: waste water is very complicated, interactions of chemical molecules with the silver electrode or silica coating require further study. The concentration of propylene glycol and sodium increased dramatically at 1400-1500 h. Major increases in
Clean-Room
I A
SinP
151
calcium, copper, nickel and titanium and minor increases in potassium, silicon and aluminium were also observed. These results are in a good agreement with the work records. Main tasks performed on the day are patterning and etching mask and waveguides. Therefore, a large amount of developers and some etchants are discarded during the afternoon and are the probably cause of the contamination observed. No etching effect was found for the resonators in these basic solutions. However, the electrodes of the resonators without coatings were stripped off after immersing in 0.5 M HCI at room temperature for 24 h. With the coatings in place, the electrodes were still undamaged after 48 h immersion, which demonstrates the protective properties.
4. Conclusions In this study, the quartz crystal resonators with and without coatings were applied to liquid samples. Major conclusions might be summarised as follows. The silica and polyimide films can be conveniently applied on the quartz crystal by spin-coating. The crystal surface becomes tough and smooth with the silica and polyimide coatings, respectively. The oscillation of the coated resonators were maintained in air, deionised water and solutions tested. The effect of propylene glycol appears to .be masked with other combinations of coating and electrode which may be more sensitive to other minor components of the samples. Results from the waste water samples are complex
Network Anaiyser
t= ® ®
®l
OOe]
¢
Neutralization
Waste Water
Tank
FTo city sewage treatment
Fig. 5. Schematic diagram of the w~'-tewater analysis using the quartz resonators.
152
H.-T. Sun et al. / Sensors and Actuators B35-36 (1996) 146-153 Silica/AulQuartz in Waste Water
Ag/Qu~rtz in Waste Water
a
we
d
i ~°°°
5oo,. ]
~4ooo .
.
.
water
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w~sle wa~r
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.
.
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waste water
/
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/
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10:00
11:00
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IS:O0
17:00
18:00
Time (hours)
Time (hours)
A ~ Q u a r t : in Waste Water
PolylmldslAglQuartz in Waste Water $oo ..................................................................................................................................................
.
400
.
.
.
.
.
.
demonized'~,~to~ . . .
4000
.
2000 CI
2O00
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11:00
13:00
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10:00
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10:00
11:00
12:00
Time (hours)
demnlzedwater
13;00
........
15;00
16:00
11:00
PolylmldelAu/Quartz In Waste Water ...............................................................................................
dOiOnlZSdwster
18:00
~,J
",/
4000
Oe~onizedwater. . . . . . . . . . . . . . . . . . . .
East
• "1
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4000
Time (hours)
Sillca/Ag/Quartz in Waste W a t e r
I
/
i
°
~oo__
x
/
\
wasle water 400 i
0:Q0
"--'
\ %
/ ,,t
.2000
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1O00
tt:O0
1;100
Time (t~rs)
13:00
1400
lS:00
16:00
I1P:00
18:00
Time (hours)
Fig. 6. Ho'.,rly waste water monitoring based on the ,~,artz resonators: (a) Ag/quartz, (b) silica/Ag/quartz, (c) polyimide/Ag/quartz, (d) Aulquartz, (e) silica/Au/quartz, (f) polyimide/Au/quartz.
pH value and Conductivity
"[
T ""
Table 3 Chemical analysis of the waste water (results are expressed as mg I-!) pl!
1 !
- -
$:W
't~.'W
11~
'l):eO
11,1~
14:N
15.1)0
~
,
rs:W
1T.'~
.I00
18:00
T~me (hours)
Fig. 7. Conductivity and pH values of the waste water at the same time as above.
Analytes
0900-1300 h
1400-1500 h
1600-1800 h
Propylene glycol Sodium Potassium Calcium Silicon Copper Aluminium Nickel Titanium
4.0 10 2.9 1.4 1.4 0.04 0.09 0.013 <0.01
2800 89 3.3 2.5 2.0 0.26 0. ! 3 0.026 0.017
! 00 54 2. ! ! .3 2.0 0. ! I 0.12 0.004 <0.01
H.-T. Sun et aL /Sensors and Actuators B35-36 (1996) 146-153
but the variability from one coating and electrode to another may be valuable in designing sensors which are more selective for specific components. The coated resonator can be used for on-line analysis to detect changes in the chemical composition of waste water from a clean room facility. The difference of oscillation due to various coatings and electrodes provides a promising way to realise a multi-sensor system to distinguish individual chemicals in liquids.
Acknowledgements One of the author (HTS) wishes to acknowledge the R M I T for the research fellowship, and ZHC to the Department of Communication and Electronic Engineering for the postgraduate research scholarship. Thanks are due to Ms. Chiping Wu for her technical support in the clean room, to Mr. Massood Zandi tbr assistance in the chart plotting, and to Mr. Carlos Jimenez for helpful suggestions.
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153
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