Sensors and Materials, Vol. 11, No. 8 (1999) 493-505

MYUTokyo
S &M0391

Capacitive SiO Humidity Sensors with
Novel Microstructures
Andy T. Wu, Mary Seto and Michael J. BrettDepartment of Electrical and Computer Engineering, University of Alberta,
Edmonton,AB, Canada, T6G 2G7(Received June 7, 1999; accepted August 24, 1999)
sensor, new sensing material, new sensor fabrication technique, humidity sensor, SiO
film, glancing angle deposition

Key words:

It is demonstrated that porous SiO films fabricated by a newly developed glancing
angle deposition (GLAD) technique can be used for relative humidity (RH) sensing. The
sensing characteristics of capacitive SiO sensors depend critically on the details of theirmicrostructures. It is shown that a SiO sensor with a microst:rncture of cylindrical rods anda porosity of 25% can exhibit a response time of less tl).an three seconds for a stepwisechange in RH from 15% to 80%. The sensor does not degrade after water immersion andhas a dynamic response range over five orders of magnitude. The conventional equivalentcircuit model is shown to bie inadequate in explaining the observed results. It is suggested
that the detailed microstructure of the entire sensor and the interactions between watermoisture, the SiO film and the applied field must be taken in.to consideration in calculating,the total parallel capacitance of the SiO sensor when RH reaches a certain level. Our results
indicate that the GLAD technique has broad potential in various sensing applications, due
to the ease of fabrication of porous films with structure controllable in three dimensions on
the IO nm scale.

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Sensors and Materials, Vol. 11, No. 8 (1999)

Introduction

Relative humidity (RH) sensing is an important process required in many areas, forexample, precision instruments, semiconductor chip manufacturing, food and chemical
processing industries, automobile industry, and environmental chambers, as well as air
conditioners. Among different types of sensorsOl used for RH sensing, the capacitive typehas been playing an important role because it can be fabricated cheaply in different sizes
and can be incorporated relatively easily into integrated circuits. Nowadays, two types ofcapacitive RH sensors are commonly used and investigated: polymer and ceramic. Capaci­
tive polymer RH sensors usually have good linearity in their working range. Hysteresis as
small as 0.5% RH is achievable in the sensing range from 10 to 90% RH when an
appropriate polymer is chosen as the sensing material.C2J Typical response time is aboutfour minutes,C3l which can be improved considerably by using an obliquely deposited metalelectrode_C4l Due to the inherent water-reactive nature of most polymers, intensive expo­
sure to high RH or water immersion can degrade or even destroy polymer sensors. The
dynamic response range of polymer sensors is narrow, typicallyC3l from 110 to 144 pf for achange of RH from 8% to 90%. Ceramic capacitive RH sensors usually have a wider
dynamic response range and do not react with liquid water. A dynamic response range offour orders of magnitude has been recorded. C5l On the other hand, linearity is not as good
and they often exhibit a relatively large hysteresis loop.C6l Although RH sensing has beenused and investigated since the invention of the first hygroscope by Nicolas Cryfts in 1450,
humidity measurement remains a difficult task for the design engineer,Ol and research anddevelopment are being actively pursued.
Recently, a new film deposition technique for porous film fabrication, called glancing
angle deposition (GLAD), has been developedC8l at the University of Alberta. By combin­
ing oblique angle evaporation with controlled substrate motion, this technique allows us tofabricate films with three-dimensional control of the structure on the 10 nm scale. Possible
optical applications of this technique have been demonstrated,C9- l and potential applica­
tions in biological, mechanical, magnetic, and electrical fields0 tJ have been suggested. In
this paper, we demonstrate that this technique can be used for sensing applications.
Capacitive humidity sensors are fabricated using SiO films, deposited by the GLAD
technique, as the sensing material. It is shown that the sensors have fast response times, awide dynamic response range, and do not degrade after water immersion. Initial experi­ments show two advantages of these sensors: the sensing characteristics of the sensors canbe finely tuned to different sensing needs by tailoring their microstructures, and a wide
range of dielectric substances may be used as the sensing material. The latter point isfurther supported by the recent advance in the GLAD technique. It has been shown that inaddition to thermal and e-beam evaporation methods,csJ GLAD-fabricated films can also begrown by sputteringC 1 2J and laser ablationC13l methods, thus enabling the deposition of
metals, semiconductors, dielectric materials, organic materials, and refractory materials.
Here, we use RH sensing only as an example to illustrate the utility of the GLAD techniquein sensing applications. Based on this example, we suggest that the GLAD technique canbe used to produce films for many other sensing applications where a large surface/mass
ratio is desired or a particular microstructure is required.
10

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Sensors and Materials, Vol. 11, No. 8 (1999)

2.

Experimental

There are several reasons why we have chosen SiO as the RH sensing material. First, it
is well known that SiO is relatively chemically inert and does not react with liquid water.
Therefore, the sensors are less likely to degrade after water immersion or after long-term
use under conditions of high humidity. Also of importance is the fact that the dielectric
properties of SiO films are relatively well studied, and SiO films may be readily fabricatedusing thermal or e-beam evaporation, to which the GLAD technique is commonly applied.
The basic principle of the GLAD technique is schematically illustrated in Fig. 1. The
substrate is rotated about two axes. One is perpendicular to the substrate plane (<)>rotation),the other parallel (a tilt). A computer is used to control the<)>rotation anda tilt. Different
combinations of the speed and the direction of the <I> rotation and the tilting angle of thesubstrate allow us to fabricate films with various microstructures. Film porosity isdetermined primarily bya, with increasing porosity for increasinga. Various microstruc­tures, for instance, helices, "zigzags", "staircases", or posts, can be produced by the GLAD
process with porosity controllable from 10 to 80%. A detailed description of the GLAD
process and achievable microstructures is given in ref. 8.
The layout of our SiO RH sensor is shown in Fig. 2. Aluminum electrodes were first
deposited onto an insulating substrate. Then the GLAD technique was used to thermallyevaporate porous SiO films with different microstructures surmounted by denser capping
layers. The vapor source material we used was random pieces of SiO. Typical dimensionof the SiO pieces was from 3 to 6 mm and purity was 99.9%. Deposition conditions were

substrate.------­\tfvapoursource

c:::::::::>

Fig. 1. The basic principle of the GLAD technique.

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Sensors and Materials, Vol. 11, No. 8 (1999)
SiO Film

Gold Common Electrode

Insulating Substrate

Aluminum ElectrodeFig. 2.

Schematic layout of our SiO RH sensor.

kept fairly constant, with typical deposition rates of 20-30 Als and base pressures of
3.5xI0-6 Torr. Energy-dispersive X-ray diffraction (EDX) measurements indicated thatthe obtained films were mainly SiO. Finally, a gold layer was sputter deposited on top of
the capped SiO as a common electrode. In this report, we will focus our attention on two
representative examples of SiO sensors with post and helix microstructures.
Capacitance-RH (C-RH) curves were measured using the control system shown in Fig.
3. This system allowed RH to be varied between 1% and 97%. When a RH below room
humidity was desired, we put desiccants into the container inside the humidity chamber.
For high RH, water was placed into the container instead. A switch was used to change theelectronic circuit from one state to the opposite when RH was varied from below room
humidity to above room humidity or vice versa. During the change, we needed to put a dishof either water or desiccants behind the outer fan. The electronic circuit was essentially aproportional-integral-differential control loop. It compared the output of the standard RH
sensor with the value set by a potentiostat, and produced a pulse width modulation to the
fan driver according to the magnitude of the difference between the output of the standard
RH sensor and the set value. The internal fan was used to accelerate the evaporation ofliquid water or the absorption of moisture by desiccants inside the humidity chamber and to
make the environment in the humidity chamber more homogeneous. Dry or wet air was
blown into the humidity chamber employing the outer fan. The dry or wet air was thenmixed with the air inside the humidity chamber to produce a particular RH. The sample
was placed as close to the standard RH sensor as possible. A commercial capacitancemeter was used to measure the parallel capacitance of the sample at 120 Hz.
Measurements of the response to a step change of RH were carried out by putting thesample into a sealed dish at room humidity inside the humidity chamber. The RH of thechamber was increased to 80%, and the cover of the sealed dish was removed. The timerequired for the capacitance of the sample to increase to 90% of its capacitance at 80% RH
was measured as the response time. Since the seal might not be completely tight, we tooka small amount of leakage into account by viewing the step change of RH as being from°
15% to 80%. All measurements were carried out at 23 C.

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Sensors and Materials, Vol. 11, No. 8 (1999)

Capacitance Meter

Control Loop
Humidity
ChamberTube

Pulse Width
Modulator

Fan Drive

Fig. 3.

3.

Schematic block diagram of our measuring system.

Results and Discussion

A scanning electron microscope (SEM) cross-sectional image of one of our sensors isshown in Fig. 4. The SiO capacitor layer was grown by choosing an appropriate tilting. angle a with substrate rotation to produce a porous film with helical microstructure. A
more compact layer of SiO film (called a capping layer) was then deposited on top of thehelical SiO film by exponentially decreasing the tilting angle a while linearly increasing
the speed of the substrate rotation <jl. Finally, a gold layer was sputter deposited on top of
the capping layer with a thickness varying from sensor to sensor, depending on the detailed
microstructure of the capping layer and desired gold film porosity. For the sensor (SensorA) shown in Fig. 4, the thickness of the gold layer was estimated to be around 30 nm. The
thicker appearance of the gold layer in Fig. 4 is an artifact created by the high ductility of· Au during cleaving and sample preparation. This thin gold layer contained natural
perforations due to grain boundaries and islanding, which formed passages through which498

Sensors and Materials, Vol. 11, No. 8 (1999)

Fig. 4.

SEM cross-sectional image of Sensor A.

water molecules can move freely in or out of the interior of the sensor. The capacitance of
this sensor was measured as a function of RH at room temperature (see Fig. 5). The
observed capacitance first increased slowly and linearly with RH up to about 85% RH
where a sharp increase in sensitivity occurred. With decreasing RH, hysteresis developed
down to 60% RH.

3 .1 Effect of microstructure of SiO film

The importance of the microstructures of the sensing materials to the characteristics ofthe sensors was clearly demonstrated in the work by Shimizu and coworkers.c 4, J Theirresults indicated that pore size distribution in ceramic humidity sensors had a decisive
influence on the sensing characteristics of the sensors. To study this effect, we prepared
Sensor B with a different microstructure, as shown in Fig. 6. The fabrication procedure ofthis sensor is similar to that of Sensor A except that the tilting angle a is larger and the
speed of substrate rotation <I> is much faster. The relatively more compact layer near thesubstrate is caused by the fact that during nucleation and initial bulk growth stages, the self­
shadowing effect is less prominent. The corresponding C-RH curve is shown in Fig. 5. A
dramatic change in the shape of the curve, the hysteresis loop, and the dynamic response
range is observed. The hysteresis loop of Sensor B is present over a much wider RH range
than that of Sensor A. Most surprisingly, the dynamic response range of Sensor B covers
five orders of magnitude in capacitance. This wide dynamic response range was common
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100000499

L�._s��L-J--1---::...k��d�l---J

b10000 ..---t-----+---+r--::.ail�iie::...t----7'"!---t-----+-�--t-��

10 ..._...._.._.....___...,....___________..__..
90 100
30
40
50
60
70
80
10
20
Relative Humidity (%)
Fig. 5. Capacitance _versus relative humidity for Sensors A and B. Arrows indicate the direction of
RH cycling.

Fig. 6.

The microstructure of Sensor B.

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to a number of sensors fabricated with similar post microstructures. Other researchers haverecorded a wide dynamic response range for a thick-film ceramic humidity sensor fabri­cated by a sophisticated procedure, where a change of four orders of magnitude<5l incapacitance was detected. Theoretical explanation for the sensing characteristics of the
thick-film ceramic humidity sensor was found to be very complicated by Qu and Meyer(5land was not discussed in their paper.
The hysteresis loops in Fig. 5 are caused by the difference in RH during adsorption anddesorption for a given amount of water condensed in the pores due to the capillary effect.
The capillary effect may be described mathematically by the well-known Kelvin equa­tion.06l Based on this equation, we know that the wider RH range for the hysteresis loop of
Sensor B implies a larger range in pore size distribution in the SiO film. This is consistent
with the microscopic observations (see Figs. 4 and 6). One may simply attribute the wider
dynamic response range in Sensor B to its larger space which water moisture can fill, sincethrough a comparison between Fig. 4 and Fig. 6 we can easily see that the SiO film in
Sensor B is much more porous than that in Sensor A However, it will be shown later thatsuch a simple explanation cannot account for the observed results.

3.2 Microstructure of top layer

The microstructure of the top layer also plays an important role in determining the
response characteristics of the sensor to the change of RH, since the top layer is the "gate"
which controls the passage of moisture to the dielectric. One of the most difficult tasks inthe process of this sensor fabrication is to produce a proper capping layer on top of thesensing SiO film. The capping layer must be compact enough to serve as a common
electrode and yet still has enough perforations to allow water molecules to move freely inor out of the sensor. A series of deposition experiments showed that the capping layer
should be fabricated so that a gold layer with a thickness of a few hundred A deposited on
top of the capping layer can be used as a porous common electrode.
SEM images of the top layers of Sensors A and B are shown in Fig. 7. Analysis of thephotos has revealed that the top electrode of Sensor B is more porous than that of Sensor A,
and that the pores are more homogeneously distributed in Sensor B than those in Sensor A
The response time of Sensor A is about 75 s, whereas for Sensor B it is less than 3 s. Since
water molecules penetrate into the interior of the SiO film mainly through the holes on thesurface, the difference in response time between Sensors A and B may at least be partiallyrelated to the difference in the microstructures of the top layers. However, since thediameter of water molecules is about 3.87 A, the surface of Sensor A is presumably porous
enough to allow penetration of water molecules. It is plausible, in this particular case, thatthe slower response time of Sensor A is mainly a result of the helical and relatively compact
microstructure of the SiO film, which may create a less permeable barrier to watermoisture.

3.3 Equivalent circuit model

As shown in Fig. 5, the dynamic response range of Sensor B extends over a surprising
five orders of magnitude. To understand this phenomenon, we calculate the largest
capacitance that Sensor B can have if the change is caused entirely by the filling of the501

Sensors and Materials, Vol. 11, No. 8 (1999)

(b)

(a)
Fig. 7.

SEM top view of (a) Sensor A and (b) Sensor B.

pores by water moisture. Conventionally, the behaviors of RH sensors are understood
through the equivalent circuit model. The equivalent circuit of Sensor Bmay be expressedas shown in Fig. 8, where a detailed equivalent circuit is drawn only for the SiO film. The
top capping layer and the bottom more compact SiO film (see Fig. 5) have similarequivalent circuits. Z, and Zb in the figure represent the complex impedance of the top
capping layer and the bottom compact SiO film, respectively. C, and R, are the capacitanceand resistance for the SiO film, respectively. Similarly, Cw and Rw are for the pores inside
the SiO film, which are presumed to be filled completely by liquid water at high humidity.
It may be shown that the total parallel capacitance (Cp) of the system is expressed as(1)

where co is the angular frequency of the ac current. R and Care defined as:C=w(C, +Cw )R,, 2Rw 2(R,, +Rw ) 2 +w 2R/R/(C, +Cw ) 2(2)

Using a computer, the apparent porosity of the SiO film of Sensor Bis estimated to be 25%
from the top views of SEM images. The porosities of the top capping layer and the bottom

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Sensors and Materials, Vol. 11, No. 8 (1999)
Cs

Zb

Zt
Cw

Rw

Fig. 8. The equivalent circuit of Sensor B (see text for details).

compact SiO film are taken to be 1 5%. The dielectric constants of SiO film07l and liquidwaterC18l are 5.8 and 78.54 respectively. By taking the resistivities of SiO fihn°91 and liquidwaterC 15l to be l.l lxl 07 nm and 1.22xl04 nm, respectively, and using the geometry shownin Fig. 6, the total parallel capacitance is calculated to be 433 pf. This is significantly
smaller than the measured value at 97% RH, which is 425 nf (Fig. 5). Note here that in
calculating Cp, we take only the pore fraction of the SiO film that contributes to the chang�
in capacitance into account. Those possible pores, voids, or vacancies which are embedded
inside the SiO film are not filled with liquid water or water moisture at high RH and are
therefore neglected.
To check whether the equivalent circuit model is reasonable, we also calculate theparallel capacitance under dry conditions using the same formulae, assuming Rw is infinite
and the dielectric constant of the pores is 1. The result is 75.0 pf. This is remarkably close
to the 63 pf measured at 1.1% RH (Fig .. 5). The small discrepancy may be a result of thefact that in calculating C,, we chose the largest possible dielectric constantC17l of 5.8 for the
SiO film. Another source of the small discrepancy is a possible uncertainty in determining
the apparent porosity of the SiO film.
One may wonder whether the discrepancy in CP between measured and calculated
values at high humidity is a result of the possible small uncertainty in determining theapparent porosity of the SiO film. This is highly unlikely since the calculated CP under dry
conditions agrees with the measured value quite well, implying therefore, that the apparent
porosity of SiO estimated from SEM top-view images is reasonable. As an extreme
example, one might assume the apparent porosity of the SiO film to be 80% which is the
maximum porosity achievable with the GLAD technique. If it is assumed that the pores
inside the SiO f1lm are filled completely by liquid water at high humidity, the calculated
total parallel capacitance becomes 81 2 pf. Again this is significantly smaller than themeasured value of 425 nf.

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503

This simple model calculation, therefore, suggests that the capacitive response to RH isdetermined by phenomena other than the simple filling of pores by moisture or liquidwater. After RH reaches a certain level, one cannot treat the system as if it were a simple
mixture of independent water moisture (or liquid water) and SiO film. It is necessary totake the detailed microstructure of the entire sensor and the interactions between SiO film,
water moisture, and the applied field into consideration in calculating CP . On the other
hand, at low RH, the interactions are weak in the present case and can be neglected.

3.4 Possible explanations

Since the sensors fabricated here are new in terms of the sensing material, thefabrication technique, and the microstructures of the sensors, it is difficult to apply any
existing sensing theories directly to the present case without major modifications, in order
to explain the observed sensing characteristics. Development of a new sensing model may
be required, particularly for explaining the observed wide dynamic response range of oursensors and for finding a way to minimize or eliminate the undesirable large hysteresis
loops shown in Fig. 5. Any appropriate model should consider at least the following threeeffects on the sensing characteristics of the cap acitive SiO RH sensors: a) the effect of
capillary condensation of water moisture inside the pores of the sensor, b) the effect of thedetailed microstructure of the entire sensor, and c) the effect of the interactions between
SiO film, water moisture, and the applied field.
To confirm that the increase of the capacitance in our sensors is due to the admission of
the water moisture into the pores of the SiO films, we have carried out measurements on asample made of a compact SiO film. A change in capacitance from 104 to 156 pf wasobserved when RH was varied from 0.6% to 98.2%. This change is negligible whencompared with those of Sensors A and B (Fig. 5). It is also clear that little, if any, chemical
reaction takes place between water and SiO. We immersed a sensor into water for one min,then removed it. One and a half hours later, the measured capacitance was only about 5%
larger than that before the immersion.
To explain our observations, first we must investigate the nature of the hygroscopic
behavior of SiO films. The hygroscopic property of a material is normally described by theadsorption isotherm. Various adsorption isotherms, resulting from physical adsorption,
may be classified into five types according to the BET classification.<20-21 J For porous
materials, there exist only three types of adsorption isotherms, namely types one, four, andfive. These three types can describe the adsorption isotherms of pores of different sizes.
Type one is for micropores and types four and five are for intermediate or macroporesaccording to the classification of pores by Dubinin, <22) although there are no sharp border­-lines between the classes. From Figs. 4 and 6, we can see that the pore size in Sensor A ismuch different from that in_ Sensor B. Therefore, the adsorption isotherm of Sensor A is
expected to be significantly different from that of Sensor B. This is one of the major factors
contributing to the observed differences in C-RH curves between Sensors A and B.
When porous media are placed in contact with water moisture, several processes occur.
With the increase of RH, water molecules may adsorb on the surf aces of the pores, forminga monolayer, then multilayers, and eventually liquid water. The total number of watermolecules retained by a porous medium in the first three processes depends on the specific504

Sensors and Materials, Vol. 11, No. 8 (1999)

area of the medium. Different porosities and microstructures. in different porous media
give different specific areas.
Since both water and SiO are dipolar in nature, the dipole-dipole interaction and theinteraction with the applied field cannot be neglected when RH reaches a certain level.
Moreover, the strength of the interactions changes with a change in the stage of adsorption:
For instance, the interaction at the stage of monolayer coverage of the surfaces of the poresis much different from that when liquid water is formed. These and other factors may re�ult
in a remarkable effect on the physical properties at and close to the interface between waterand the SiO film, leading to an enhanced dielectric constant for the system. A recently
reported and unexplained large capacitance in porous electrochemical capacitorsC23l may bebased on similar effects. Since the described effects are all microstructure-sensitfve,
detailed microstructure-related parameters should be included in calculating CP. Further
study regarding the detailed mechanism is under way.

4.

Conclusions

We have used relative humidity (RH) sensing as an example to demonstrate thepotential of a newly developed film deposition technique called glancing angle deposition(GLAD) in sensing applications. Novel capacitive SiO RH sensors have been fabricatedusing the GLAD technique which allows the microstructures of deposited films to becontrolled in three dimensions on the IO nm scale. It is shown that the sensing character­istics of the SiO sensors depend critically on the microstructures of the SiO films and theircapping layers. By tailoring the microstructures of the SiO films and their capping layers,
it is possible to optimize the performance of the sensors if the sensing mechanism is wellunderstood.
At present, it is not completely clear whether humidity sensors based on porous
dielectrics fabricated by GLAD have significant advantages over other commonly usedcapacitive RH sensors. However, our results suggest that the versatility of the GLAD
process in engineering the film structure and porosity wanants evaluation of these films inother sensor structures and applications.

Acknowledgments
The authors thank the Natural Sciences and Engineering Research Council of Canada
and the Alberta Microelectronic Corporation for financial support, and G. Braybrook for
the SEM images.

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