handbook of modern sensors

398 13 Humidity and Moisture Sensors
Fig. 13.3. Capacitive moisture sensing system.
(A)
(B)
Fig. 13.4. Capacitive thin-film humidity sensor: (A) interdigitized electrodes form capacitor
plates; (B) cross section of the sensor.
of moisture measurement is quite useful in the process control of pharmaceutical
products. The dielectric constants of most medical tablets is quite low (between 2.0
and 5.0) as compared with that of water (Fig. 3.7 of Chapter 3). The sampled material
is placed between two test plates which form a capacitor connected into an LC
oscillating circuit. The frequency is measured and related to the moisture. The best
way to reduce variations attributed to environmental conditions, such as temperature
and room humidity, is the use of a differential technique; that is, the frequency shift
f = f 0 − f 1 is calculated, where f 0 and f 1 are frequencies produced by the empty
container and that filled with the sampled material, respectively. The method has
some limitations; for instance, its accuracy is poor when measuring moistures below
0.5%, the sample must be clean of foreign particles having relatively high dielectric
constants (e.g., metal and plastic objects, a packing density), and a fixed sample
geometry must be maintained.
A thin-film capacitive humidity sensor can be fabricated on a silicon substrate [8].
A layer of SiO 2 3000 Å thick is grown on an n-Si substrate (Fig. 13.4B). Two metal
electrodes are deposited on the SiO 2 layer. They are made of aluminum, chromium,
or phosphorus-doped polysilicon (LPCVD). 2 The electrode thickness is in the range
2000–5000 Å. The electrodes are shaped in an interdigitized pattern as shown in Fig.
13.4A. To provide additional temperature compensation, two temperature-sensitive
resistors are formed on the same substrate. The top of the sensor is coated with a
dielectric layer. For this layer, several materials can be used, such as chemically
2 LPCVD - low pressure chemical vapor deposition.