January 200236

by George Marsh

The image above shows a SEM micrograph of a poroussilicon structure. (Reproduced with permission from Journal of PorousMaterials (2000) 7 201-204. Courtesy of F. Maier, Max Planck Institut für Mikrostrukturphysik.)

When semiconductor developers noticed in the

1950s that electropolishing of bulk silicon left certain

areas rougher than the rest and somewhat porous,

they regarded these simply as imperfect areas. It was

not until Leigh Canham, a scientist with DERA (the

UK’s Defence Evaluation and Research Agency),

discovered in 1990 that porous silicon (PSi) emits

visible light when activated by external ultraviolet

sources1 that this morphological state of the material

came to attract significant research interest.

Elementary photonic sensors were soon proposed.

In 1992 researchers discovered that PSi also emits

light when an electric current is applied, a finding

that raised prospects for new optronic sensors and

other devices coupling light to electronics, including

future high-speed computers. Technology extensions

have since been found that make the material chemi-

and bio-luminescent as well as photo- and electro-

luminescent.

In sight by the mid- to late-1990s, therefore, was a

whole new class of solid-state sensors offering

significant advantages over solid-state gas and other

sensors. Many of these were based on bulk silicon and

semiconducting oxides such as tin or indium oxide

and alumina. Compared with them, PSi offers a high

surface area-to-volume ratio and hence high

reactivity and, as researchers have established, a

porous structure whose morphology could be

engineered for high selectivity to particular

molecules. Along with high sensitivity and selectivity,

comes a rapid response time.

Further boosts to PSi’s sensor prospects followed with the

discovery that luminous intensity is not the only parameter

influenced by the environment. Other properties such as

dielectric capacity, conductivity (PSi is normally less

conductive than bulk silicon), and resonant frequency, are

also mediated. Porosity can be graded to provide zonal

responses to different stimuli, raising the prospect of multi-

sensor arrays including, for instance, full olfactory ‘electronic

noses’. The many possibilities that now exist for innovative

new sensors that can be integrated onto silicon chips are

proving powerful inducement to further development.

Sensor possibilities for PSi have been indicated in fields as

diverse as environmental monitoring, laboratory testing,

process control, chemical warfare, biochemistry, and

medicine. Although PSi-based technologies have not yet

made the grade for full commercial exploitation, and have to

contend with significant competitors, the challenge is strong

and growing.

a useful imperfectionPorous silicon

APPLICATIONS FEATURE

Menu of properties

The photoluminescence (PL) of PSi, arising from the quantum

confinement effect in the presence of an enlarged band gap,

was initially observed at the red end of the visible spectrum,

but has since been demonstrated over the entire spectrum

from red to blue. It is not unique – it occurs weakly in bulk

silicon too – but is notable for its relative intensity, with

conversion efficiency of up to10%. This effect can be used

directly in simple photodetectors that emit visible light when

stimulated by non-visible radiation. Similarly,

electrodetectors that light up in the presence of electrical

activity can also be realized.

Early enthusiasm for more advanced photonic and electro-

optical applications has largely evaporated in the face of

practical difficulties. In particular the fact that PSi, in

unmodified form, is fragile, easily oxidized, and open to

chemical attack. However, ways are being found to stabilize it

sufficiently for sensor use and, thanks to a remarkable menu

of properties of which PL is just one, new and novel

applications are constantly being reported.

Of the many sensors now in the advanced stages of

development, paradoxically most do not rely on direct light

emission. Humidity sensors, for example, are based on the

change of either conductivity or dielectric constant that

occurs when moisture is adsorbed. These PSi devices are

proving more sensitive than conventional ceramic elements

that are established in this application, and offer other

favorable characteristics including reproducibility, low

hysteresis, short response time, good thermal tolerance, and

low power requirement. Response is proportional to moisture

content over a wide relative humidity range.

Gas detectors that are super-sensitive, but at the same

time simple and cheap, could be based on resistive PSi

elements. Researchers at the University of Brescia, for

example, have patented a technique using a PSi membrane on

an alumina substrate, which can sense concentrations of

nitrogen dioxide (NO2) of 100 parts per billion (ppb) – and

potentially as low as 20 ppb – with minimal interference

from contaminant organic vapors2. The work shows promise

for inexpensive, selective sensors for NO2, capable of

operating at low power and room temperature.

Such sensitivities can be attributed to PSi’s ability to

adsorb numerous molecules of the target gas or vapor onto

its surface, thanks to its high specific area – often better than

500 m2/cm3. Sensitivity can also be enhanced by surface

derivitization. It has been found, for instance, that electro-

depositing a coat of copper sulfide film onto microporous

silicon provides a sensor that is responsive to ammonia, with

much higher sensitivity than the semiconductor oxide and

cuprous sulfide-based films that are used conventionally.

Introducing dyes into the cavities is another technique that

enhances sensitivity to specific molecules.

The high surface-to-volume ratio of PSi is a crucial

advantage, believes Philippe Fauchet, co-founder of the

January 2002 37

Fig. 1 Standard methods for producing porous silicon: a) etching in a PTFE etching cell; and b) the stain etch process. (Reproduced with permission from7. Courtesy of A. Splinter.)

APPLICATIONS FEATURE

January 200238

Center for Future Health at the University of Rochester and

one of a number of researchers working to develop novel

biochemical and medical sensors. As he so graphically puts it,

“It’s an extremely useful property. The available surface is

enormous. A gram of PSi can have an area the size of a

football pitch.”

It is possible to coat the internal surfaces of PSi with

biomaterials that are attractive to particular target

molecules. In Fauchet’s work, for example, biomolecules that

attract their complements in a ‘lock and key’ targeting

approach have been applied in innovative biodetectors that

are highly specific as well as sensitive. His research has used

particular DNA sequences to attract complementary DNA

forms, and has recently produced sensors that can

differentiate between the main two classes of bacteria, gram-

positive and negative.

Crucial to selectivity is another PSi attribute, the ability to

tailor the material’s morphology to the desired application.

During manufacture, porous layers of a few microns to

100 µm thick are etched into bulk silicon, doped or undoped,

by standard anodise or stain (dry) etch processes (Fig. 1).

Cross-sectional scanning electron micrographs of PSi (Fig. 2)

show that pore formation occurs unidirectionally from the

surface into the bulk, leaving aligned pores and columnar

silicon structures. So fine is the control that can be achieved

by specifying particular etching parameters that micropores

(1-4 nm diameter), mesopores (4-50 nm) or macropores

(50 nm to 1µm range) can be created. Different pore sizes

and morphologies suit different molecules, so these can be

tailored to the sensing capability required. (Pores of 1-30 nm

in diameter, for instance, offer the best sensitivity to

humidity.) Moreover, it has proven practical to produce pores

of graded size on the same substrate to achieve multiple

target sensing capability on the same chip (Fig. 3).

Graded morphology is the key to multiple gas sensing

capability within the same array, and the possibility of an

‘electronic nose’. Current multi-sensors relying on metal

oxide semiconductors, gas sensitive field effect transistors or

conducting polymers are generally inferior at detecting low

concentrations in the presence of background odors. They are

also bulky and rely on pattern recognition algorithms. A

compact, sensitive PSi-based electronic nose could bring

greater reliability and repeatability into the professional

sensing of food and wine odors, or other applications

currently served by the human counterpart. Reproducing the

olfactory capabilities of animals could enhance safety in

counter-terrorism and defense applications.

Solid-state olfaction, though in its infancy, is available.

Researchers at the University of Oxford have developed

portable real-time, PSi-based sensor arrays that operate at

room temperature. Although commercial exploitation efforts

have since focused on carbon materials, which Allen Hill

claims can be micropatterned in fewer steps, the multi-sensor

capability of PSi was not in doubt. A Caltech microelectronicsFig. 2 Cross-sectional electron micrograph of luminescent porous silicon. (Photo courtesyof M. Sailor, University of California at San Diego.)

Fig. 3 This image shows four different types of porous silicon, where the pore size is varied.Thus, since pore size determines the luminescence spectrum the chips are different colors.(Courtesy Rochester University.)

research group brings an interdisciplinary approach spanning

neurobiology, chemistry, and electronic engineering to the

development of olfaction techniques, the goal a single silicon

chip neuromorphic electronic nose capable of advanced odor

detection and classification.

Another attraction of PSi is that fabrication techniques can

be easily adapted from those used by the microelectronics

industry. High reactivity means that the material can easily

be micromachined by etching and oxidization, and readily

patterned by lithographic processes. Fauchet believes that

this advantage is vital to the future development of

inexpensive sensors for healthcare applications. “Silicon is a

plentiful material and we can piggy-back onto mass-produced

silicon microelectronics to realize compact solid-state sensors

that are cheap and reliable. Functionalizing porous silicon is

little different to other silicon. You can fabricate it easily,

pattern it in the same way, and integrate sensors with

electronics in tiny, disposable devices,” says Fauchet.

Some applications require that the physical and chemical

vulnerability of PSi is overcome. Fauchet favors mild

oxidation in a furnace or by exposure to peroxide to leave “a

couple of monolayers” of silicon dioxide on the surface. For

many healthcare applications, he says, this provides all the

stabilization needed. Using this to replace the hydrogen-

silicon bonds left after hydrofluoric (HF) acid etching of the

material provides a good protective cover. “Silicon may be a

very ordinary semiconductor,” says Fauchet, but it has this

useful oxide interface. This is a significant advantage over

germanium or gallium arsenide, which do not.

In applications where longer life is required, alternative

methods are needed. Jillian Buriak, a chemist at the

University of Purdue has developed a passivation technique

relying on silicon-carbon bonds (Fig. 4)3. The use of a Lewis

acid results in a coating that protects the surface without

compromising PSi’s properties. The coating has, she reports,

minimal effect on sensitivity to target chemicals and is

robust – treated surfaces stand up well to aging tests in

which samples are boiled in potassium hydroxide and other

aqueous solutions. Fauchet comments that the method is of

great interest, with the reservation that, unlike the surface

oxidation method, it is foreign to the semiconductor industry.

Various other passivation techniques, mainly thermal

methods such as post-fabrication furnace and laser annealing,

have been tried with mixed results to date.

Complex moleculesAmong the more fascinating sensor trends at present is the

development of viable, compact detectors for traces of

complex chemical and biological entities. Currently topical,

for example, is the work of Michael Sailor, professor of

chemistry at the University of California, San Diego, who has

been developing sensors for, inter alia, explosives and nerve

agents. His work is particularly interesting as it has explored

the potential of a range of nanophase materials with unusual

APPLICATIONS FEATURE

January 2002 39

Fig. 4 Purdue chemist Jillian Buriak (left) has found a way to stabilize the surface of PSi sothat its light emitting properties can be harnessed in durable sensors and optronic devices.Here, Purdue student Matthew Allen shines ultraviolet light on a dish containing smallgrey PSi structures, causing them to emit bright orange light.(Courtesy of Purdue University.)

APPLICATIONS FEATURE

January 200240

optical or electronic properties, including luminescent

silicates, polysilanes, and silicone polymers as well as PSi. Of

the latter, Sailor comments, “As well as its high surface area,

controllable porosity, and compatibility with conventional

silicon microfabrication technologies, porous silicon has

convenient surface modification chemistry and can easily be

carved into elaborate optical structures such as Bragg stacks.

On the other hand it is more expensive than glass or plastic

substrates – though only slightly when the cost of

modification with DNA or other high value-added species is

taken into account – and it is not as stable in aqueous

environments as many plastics.”

Luminescence energy transfer studies have led Sailor to

develop detectors for explosives like TNT and DNT, with

sensitivities reaching parts per billion, while easy-use rapid-

throughput sensors are under development for

fluorophosphonate ester-based nerve agents such as Sarin

and Soman. Simple, disposable sensors for explosive vapors

are based on the measured quenching of PSi luminescence,

and a general purpose vapor detector has been developed in

conjunction with AlphaMOS America.

Fluorophosphonate detection, however, relies on optical

interferometry in which regular (Fabry-Perot) fringe patterns

formed when incoming white light fed through a PSi filter

fabricated on bulk silicon is reflected back from both the PSi

interfaces – that with bulk silicon and with air. The filter

consists of a PSi layer with graded porosity and hence graded

refractive index. A shift in the fringe pattern occurs when the

PSi’s native silicon hydride-terminated surfaces come into

contact with a target vapor, the resulting surface

modification altering the optical thickness of the PSi layer.

The degree of shift is a measure of this optical change and

hence of the vapor concentration. Advantages of the method,

apart from its relative simplicity, are that it is highly sensitive

and can take place with the light source and detector located

slightly away from the PSi sensor. It therefore lends itself to

non-contact on-line monitoring.

Sailor, in a fruitful collaboration with his colleague Ghadiri,

showed that this transduction technique can also be used to

sense biomolecular interactions4. Using various coupling

chemistries to attach biological receptors to PSi, they note

that when target molecules come into contact with the

receptors, the binding that occurs produces a Fabry-Perot

fringe shift. Very high sensitivities, pico- and femto-molar

concentrations, are achieved when a DNA strand ‘installed’

on the PSi binds to its complementary strand, for example.

With Létan, Sailor developed a sensor for hydrofluoric acid,

which is widely used in the semiconductor industry. Unlike

conventional electrochemical sensors that are sensitive but

do not sufficiently distinguish HF from hydrochloric acids, the

Létan-Sailor sensor does, and can detect HF in concentrations

as low as 30 ppm on an exposure time of 10 minutes.

Other researchers have developed silicon-based

spectroscopy techniques, promising an affordable and

compact alternative to conventional spectroscopy in process

control applications. A team from the University of Aachen,

for example, uses optical filters made of multiple PSi layers to

carry out spectral decomposition5. Such filters, which can be

tailored to particular sensing requirements, could contribute

to future low-cost optical on-line quality monitoring. A Swiss

team, from the Federal Institute of Technology’s Institute of

Microsystems in Lausanne, has taken this further and is

developing a scanning semiconductor microspectrometer

(Fig. 5) in which the PSi optical filter is presented over a

range of angles to the incident light as a means of tuning the

emergent wavelengths6. Such low-cost scanning devices can

be used, the researchers hope, for fast, visible, and infrared

light analysis of gases and liquids. Feasibility has been

Fig. 5 Working principle of a scanning microspectrometer comprising a PSi optical filterand a detector. The transmission of a tunable bandpass filter, or alternatively thereflection of a tunable band reflector, is measured. The wavelength can be scanned byvarying the incident angle of the multilayer filter. (Reproduced with permission5. Imagecourtesy of Gerhard Lammel, EPFL, now with Robert Bosch GmbH.)

demonstrated at device level and tests are now taking place

at the system level.

BiocompatibilityCanham is now among workers who finds PSi’s biological

properties the most interesting. The fact that, for example,

leading tissue engineers now use it as base material for

biological ‘scaffolds’ to support new tissue growth inside the

human body shows just how biocompatible PSi is. In fact, PSi

compares favorably with titanium, ceramics, composites,

polymers, and other materials commonly used for biological

implants. Moreover, the known tendency of PSi to break

down in aggressive chemical environments, far from being a

drawback, suggests Canham and DERA co-worker Roger

Aston, is a positive advantage. “The discovery that

nanostructured silicon was fully degradable in vivo was a

stunning result. It meant that the human body might be able

to dissolve and excrete silicon,” explains Canham. Further, the

material is bioactive and can stimulate the growth of certain

living tissues, bone in particular.

PSi’s biological properties make it possible to consider

novel drug delivery systems as well that could offer more

than a progressively degrading capsule. By incorporating its

own sensor, a system could release its drug dose in a

controlled manner at specific sites in the body, once the

required biological trigger had been sensed in blood or other

body fluid. Porosity control and surface passivation can be

used to determine the material’s durability in body

environments ranging from blood to gastric juices.

In his role heading up PsiMedica, Canham is working to

exploit these biological properties. An immediate focus is

simple drug delivery systems but, looking ahead, the

prospects are exciting. Affordable biosensors could be taken

into the body and signal their results to the outside world by

telemetry. Derivitivized PSi mirrors placed under the skin

could be used in minimally invasive optical monitoring of

biochemical markers for cancer. PSi cages could be used to

protect insulin-secreting cells from the immune system of

diabetic patients. The pores would be large enough to let

nutrients in and insulin out, but small enough to stop cells

getting in or out.

Possibilities for porous silicon chip biosensors are legion.

For instance, it has been shown that a PSi-based capacitive

field effect microsensor can detect the enzyme penicillinase.

More recently, light addressable potentiometric sensors

(LAPS) able to detect penicillin in low concentrations have

been realized. Hill achieved some early successes in the

biosensor field and has developed sensors for glucose and

other blood constituents. Although Hill has now shifted his

focus to carbon, a number of other researchers are producing

solid-state sensors small enough to be incorporated in

catheters and other biological probes.

Biosensors can also use the photoluminescence

transduction mechanism of PSi. Fauchet, along with physician

Alice Pentland in collaboration with the Massachusetts

Institute of Technology, has developed ‘smart’ socks, in which

PSi pressure sensors monitor blood pressure in the feet of

diabetic patients. Diabetics are at risk from poor blood supply

and the monitor can warn them to put their feet up when

pressure drops too low! Light emitted by PSi elements when

pressure is high is registered on a small sensor worn by the

patient. According to Fauchet, unlike much of the medical

technology that confronts patients, this preventative

technology is “very inexpensive, consumer friendly and

doesn’t require a complex change in lifestyle”.

For Fauchet, low cost is the future. “We are aiming for the

50 cent throw-away sensor,” he says. “Disposability is the

preferred trend in healthcare and tiny chips, manufactured in

thousands by established microelectronics processes, would

fit well with this.”

The Center for Future Health is also working on other

worn devices, including a bandage that alerts wearers to the

beginnings of infection by detecting the bacterium

responsible. PSi derivatives designed to attract specific DNA

sequences emit light when they are detected.

The potential for biological monitoring and diagnostic

technology is huge. Strategists at Intel believe there

is a $5 billion market for chips integrating healthcare

devices. Could this be where the promise of porous silicon is

greatest? MT

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January 2001 41

REFERENCES

1. Canham, L.T. Appl Phys. Lett. (1990) 57 p. 1046

2. Baratto, C. et al. Sensors and Actuators B (2001) 77 p. 62-66

3. Stewart, P. and Buriak, J.M. Advanced Materials (2000) 12 p. 859-869

4. Lin, V.S.Y. et al. Science (1997) 278 p. 840

5. Hilbrich, S. et al. Thin Solid Films (1997) 297 p. 250-253

6. Lammel, G. et al. Sensors and Actuators A (2001) 92 p. 52-59