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How do I measure the thickness of a film made of
– Copper
– Silicon oxide?
– Copper
Metal Films: Measuring Sheet Resistance
– Silicon oxide?
Dielectric Films: Interferometers
Name three distortions of the sensor output caused by slow response.
1. Phase shift: The electric signal will be a little later than the measured physical effect.
2. Reduced sensitivity: The electric signal will not reach the full amplitude.
3. Signal distortion: A rectangular signal will be distorted in the form of a “shark fin”.
What is the difference between accuracy and resolution?
Accuracy is the maximum difference between the sensor output and the physically true value, which is guaranteed by the supplier under all specified circumstances (e.g., working temperature: -40 ◦C to +120 ◦C).
The resolution can be measured. The accuracy is specified by the supplier.
The resolution is the smallest signal that can be distinguished from a neighboring value. Limits of resolution may be:
– Noise
– Number of bits in A/D conversion
– Number of decimal places on the display
What do we use as thermocouple material in the thermopile? Why?
polysilicon versus tungsten–titanium oder Aluminium
- polysilicon- Hoher Thermoelektrischer Koeffizient, Standartprozess (keine Verunreinigung)
- Aluminium Niedriger Thermoelektrischer Koeffizient
- tungsten–titanium - Nidriger Thermoelektrischer Koeffizient (+Hohe temperaturen)
What material do we use for the thermopile membrane? Why?
membrane from very thin (300 nm) silicon nitride
Nichtleitend sonst Kurzschluss, Standartprozess, Geringe Termische leitfähigkeit, hohe intrinsische
Zugspannung, Chemisch stabiel => widerstandsfähig gegen chemisches ätzen
What can we learn for noise reduction from this formula?
U_N = sqrt( 4 k_B R T ∆f )
- möglichst kleiner Widerstand
- Sensor kühlen
- keine unnötig hohe Bandbreite
T: The noise increases with temperature, since the movement of the electrons increases with temperature. Thus, the noise can be reduced by cooling the sensor. This is done in laboratory equipment. When thermopiles are used for IR spectroscopy or astronomy, they are cooled with liquid nitrogen to 77 K.
R: Due to Ohms law, the noise increases with the resistivity of the sensor. Unfortunately, the thermopile has a high internal resistivity due to the interconnects in thin-film technology and due to the many contacts between the two metals. A typical value is 50 kΩ. Therefore, we would like to make sensors with less resistance to reduce Nyquist noise. We could reduce the number of thermopairs in a thermopile, let us say, in half (1/2). The noise would reduce to 1/√ 2 = 71%, but the sensitivity would reduce to 50%. Obviously, it does not pay out.
∆f: The noise increases with the bandwidth of measurement. To reduce noise, the bandwidth can be reduced using a filter (like the “treble” filter in an audio amplifier). There is a trade-off between noise and response time. Small bandwidth reduces noise, but increases response time
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