High Reliability of  POSIFA’s Thermal Water Flow Sensors

Xiang Zheeng Tu

 

A POSIFA thermal water flow sensor is fabricated in a silicon substrate. A combination of a heater and two thermopiles is used as sensing element of the sensor. A porous silicon layer is formed in the substrate for thermal insulation between the sensing element and the substrate, while the top layer is made of a SiO₂/Si₃N₄ stack thin film. The mechanism of water flow detection mainly depends on measuring the change in the electrical voltage of the thermopiles, associated with the heat convection transfer caused by the water flow.  In operation the sensor is heated by applying an electric voltage pulse to the heater. The pulse can be rectangular with pulse width 20ms, repeat frequency 1Hz resulting in 1.8mw power consumption. It has been measured that the Instantaneous peak temperature of the sensor is lower than 50⁰C.

It has been proved that Arrhenius' equation can be used for calculation of the failure rate of a semiconductor. The equation is expressed as

L = A exp (Ea / k T)                                                  (1)

Where

L is the lifetime of a semiconductor device

T is the absolute temperature (in kelvin)

A is the pre-exponential factor, a constant for each semiconductor device

Ea is the activation energy for each failure mechanism (in Joules mol-1)

R is the universal gas constant

Activation energy refers to the minimum amount of energy required to trigger a temperature-accelerated failure mechanism. The following table shows some activation energy values obtained for various failure mechanisms commonly encountered in the semiconductor devices.

Failure Mechanism	Accelerating Factors	Activation Energy
Oxide Film Defect	Electric Field, Temperature	0.3- 1.1 eV
Al Wire Corrosion	Humidity, Temperature, Voltage	0.7 - 0.9 eV
Al Wire Electromigration	Temperature, Current Density	0.5 – 0.7 eV

Since the sensors normally driven with electric voltage pulses the mean current density is very low. So Al wire electromigration for the sensor failure can be ignored and the main failure mechanism is Al wire correction. If voltage is applied, the leakage current between Al conductors will be added as a factor for Al corrosion. Al corrosion reaction proceeds as follows:

(a) Reaction on anode side

Under the normal ambient conditions, since the surface of “Al” is covered with oxide film, “Al” is in the passive state and exists stably. At the bias voltage application status, if the surface of the anode side adsorbs the Cl- ions diffused from the inside of the sealed resin, the Al wire protected by the passive state gibbsite may react and finally melt as:

At first, the hydroxide on the surface reacts with the Cl- ions to generate fusible salt.

Al(OH)3 + Cl- → Al(OH)2Cl + OH-                          (2)

The substrate Al exposed by this reaction reacts with the Cl- ions.

Al + 4Cl- → AlCl4 - + 3e-                                          (3)

In addition, when the sealed resin absorbs moisture, reaction with the moisture may start.

AlCl4 - + 3H2O → Al(OH)3 + 3H+ + 4Cl-              (4)

Finally Al(OH)₃ will be generated. Different from the protective oxide film, the generated Al(OH)₃ is not soluble, but has a high enough cubic expansion rate to cause cracking on the protective oxide film. So the generated Al(OH)₃ promotes corrosion.

(b) Reaction on cathode side

As the sealed resin absorbs moisture, the hydroxide ion concentration will be increased near the electrode due to oxygen reduction by application of bias and reaction generates hydrogen as

O2 + 2H2O + 4e- → 4(OH)-                                   (5)

H2O + e- → (OH)- + (1/2)H2                                 (6)

The OH- ions generated by the above reaction are diffused from the defect such as pinhole, void, crack, etc. on the Al protective oxide film to the substrate Al, and then react as:

Al + 3(OH)- → Al(OH)3 + 3e-                                (7)

The reaction on the cathode side also generates aluminum hydroxide.

The graph in above figure shows the relationship between the lifetime and the operation temperature of semiconductor devices. The red slash line is the activity energy of 0.7 eV, which represents Al wire corrosion mechanism and the red vertical line represents the typical operation temperature of the water thermal flow sensors. This means that the lifetime of the sensors is expected to be very high when compare with other semiconductor devices which need to be operated at least at 125⁰C.