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Technology
 
- -- What are the features of the MEMS Flow Sensor chip?
 - Sasaki:
The main feature of the MEMS Flow Sensor is its ability to measure flow speed from 1 mm to 40 m/s, which covers the range from the fluttering of a butterfly's wings to the roar of a typhoon. At the heart of the MEMS Flow Sensor, there is a tiny sensor element. That is the MEMS Flow Sensor chip which is only 1.5 mm square.
Conventional flow sensors use a resistance measurement method based on a natural characteristic that causes the electrical resistance of a material to change due to changes in temperature. This method has a number of disadvantages, though, such as the high cost required for the extremely time-consuming adjustment of the resistance balance. (See Figure 1.)
- In contrast, the MEMS Flow Sensor was the industry's first to apply the thermopile method, which uses an element called a thermopile that converts thermal energy into electrical energy. This new method offered a variety of previously nonexistent advantages, including low-cost operation because there are few adjustments required, low power consumption, and high sensitivity.
| Resistance method |
Thermopile method |
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High costs are involved because of the difficulty in adjusting the resistance balance between the upstream side and the downstream side. |
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Electric power is required for measurement. |
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Sensitivity depends entirely on the materials used, and cannot be raised. |
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Costs are low because there are few adjustments, since the upstream and downstream sides are formed from the same battery. |
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Electric power is generated from thermal energy, so no other power is necessary. |
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The output can be increased as desired by simply connecting batteries in series. |
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- Fig.1 Resistance and thermopile methods
- -- With all of its advantages, it seems strange that it wasn't available sooner.
- Sasaki:
Well, even though the advantages were clear, it was very difficult to use the thermopile method because there was a serious obstacle in the way.
As you can see in Figure 2, when a wind is blowing, the thermopile in the MEMS Flow Sensor chip detects cooling on the upstream side and warming on the downstream side. In order to mount a large number of thermopiles and raise the sensitivity, it was necessary to form the thermopile into a wide, thin film.
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| Fig.2 Operating principle for MEMS Flow Sensor chip measurement |
- It may sound like this would be easy to do, but because the film would only have a thickness of 2 microns, its strength would be reduced as it was widened and it would inevitably break because it would not be able to withstand the wind pressure.
Faced with the challenge of widening the film, but knowing that this would reduce its strength, the key to developing the thermopile was in finding a way to form a wide but strong thin film.
- -- So how were you able to form a wide but strong thin film?
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Fig.3
Unique H-shaped etching hole | - Sasaki:
The trick was to use a unique H-shaped etching hole. (See Figure 3.)
The structure of the MEMS Flow Sensor chip consists of a silicon substrate, a space, and a thin film, as shown in Figure 2. The space is formed by pouring solvent into an etching hole that is made in the thin film, and dissolving part of the substrate.
- The substrate material, single-crystal silicon, has two crystal orientations, one that is easy to dissolve and one that is difficult to dissolve. When the etching hole is made correctly, it is quite easy to form the necessary space, but the problem is that making the hole greatly changes the strength of the thin film.
| Cross-shape |
H-shape |
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Fig.4 Stress differences according
to etching hole shape- Sasaki:
We ran into problems with the first etching hole that we used, which was cross-shaped, because we couldn't achieve the necessary strength. After simulating a variety of different shapes, we discovered that an H-shape is able to effectively disperse the stress and create an extremely durable film. (See Figure 4.)
Also, because the H-shape covers a large area, the number of thermopiles can be increased to further raise sensitivity.
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