handbook of modern sensors

146 4 Optical Components of Sensors
metal in atmosphere of low-pressure nitrogen [8]. The most effective way of creating
a highly absorptive (emissive) material is to form it with a porous surface [9]. Particles
with sizes much smaller than the wavelength generally absorb and diffract light. The
high emissivity of a porous surface covers a broad spectral range; however, it decreases
with the increased wavelength. A film of gold black with a thickness corresponding to
500 µg/cm 2 has an emissivity of over 0.99 in the near-, mid- and far-infrared spectral
ranges.
To form porous platinum black, the following electroplating recipe can be used
[10]:
Platinum chloride H 2 PTCl 6 aq: 2 g
Lead acetate Pb(OOCCH 3 ) 2 · 3H 2 O: 16 mg
Water:
58 g
Out of this galvanic bath, the films were grown at room temperature on silicon wafers
with a gold underlayer film. A current density was 30 mA/cm 2 . To achieve an absorption
better than 0.95, a 1.5 g/cm 2 film is needed.
To form a gold black by evaporation, the process is conducted in a thermal evaporation
reactor at a nitrogen atmosphere of 100 Pa pressure. The gas is injected via a
microvalve, and the gold source is evaporated from the electrically heated tungsten
wire from a distance of about 6 cm. Due to collisions of evaporated gold with nitrogen,
the gold atoms lose their kinetic energy and are slowed down to thermal speed.
When they reach the surface, their energy is too low to allow surface mobility and
they stick to the surface on the first touch event. Gold atoms form a surface structure
in the form of needles with linear dimensions of about 25 nm. The structure resembles
surgical cotton wool. For the best results, gold black should have a thickness in the
range from 250 to 500 µg/cm 2 .
Another popular method to enhance emissivity is to oxidize a surface metal film
to form metal oxide, which, generally, is highly emissive. This can be done by metal
deposition in a partial vacuum.
Another method of improving the surface emissivity is to coat a surface with
an organic paint (visible color of the paint is not important). These paints have farinfrared
emissivity from 0.92 to 0.97; however, the organic materials have low thermal
conductivity and cannot be effectively deposited with thicknesses less than 10 µm.
This may significantly slow the sensor’s speed response. In micromachined sensors,
the top surface may be given a passivation glass layer, which not only provides
an environmental protection but has an emissivity of about 0.95 in the far-infrared
spectral range.
4.10 Electro-optic and Acousto-optic Modulators
One of the essential steps of a stimulus conversion in optical sensors is their ability to
modify light in some way (e.g., to alter its intensity by a control signal). This is called
modulation of light. The control signal can have different origins: temperature, chemical
compounds with different refractive indices, electric filed, mechanical stress, and
so forth. Here, we examine light modulation by electric signals and acoustic waves.