JP2013181983A - Dust sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a compact dust sensor.SOLUTION: A dust sensor 1 includes a detection unit 2 and a circuit unit electrically connected to the detection unit 2. The detection unit 2 has an elastically deformable cantilever unit 7 which has a piezoresistive layer 10 formed thereon. The circuit unit measures changes in resistance in a resonance frequency range of the cantilever unit 7.

Description

  The present invention relates to a dust sensor.

  The dust sensor is used in a clean room, for example. The clean room has a controlled suspended particle concentration and is provided to minimize the inflow, generation and stagnation of suspended particles in the room. In a clean room, it is necessary to measure the amount of fine particles with a dust sensor in order to maintain the cleanliness class.

  Conventionally, a dust sensor including an optical system and a flow path through which gas passes is disclosed (for example, Non-Patent Document 1). The optical system includes a light emitting unit that emits laser light, a focusing lens that focuses the laser light, and a photodiode that receives the laser light. Gas is drawn into the flow path by a vacuum pump, thereby forming an air flow in the flow path. When the suspended particles pass through the flow path together with the air current, the suspended particles block or scatter the laser light. Thus, the amount of suspended particles is measured by detecting the state in which the laser beam is blocked or scattered by the suspended particles with a photodiode. The concentration of suspended particles can be calculated from the flow rate of the gas sucked by the vacuum pump and the number of suspended particles detected by the photodiode.

Makynen et al 1982, Allen 1996, Binnig, et al, 2007

  However, although the non-patent document 1 can accurately measure the amount of suspended particles, an optical system is required, and thus there is a problem that the dust sensor becomes large.

  Accordingly, an object of the present invention is to provide a dust sensor that can be miniaturized.

  A dust sensor according to the present invention includes a detection unit and a circuit unit electrically connected to the detection unit, the detection unit includes an elastically deformable cantilever unit, and the cantilever unit includes a piezoresistive layer. The circuit part measures the resistance change in the resonance frequency region of the cantilever part.

  According to the present invention, since the amount of suspended particles contained in the airflow can be measured by the detection unit including the cantilever unit, the size can be reduced as compared with the conventional case.

It is a perspective view showing the whole dust sensor composition concerning this embodiment. It is a perspective view which shows the structure of a detection part. FIG. 3A is a longitudinal cross-sectional view showing a manufacturing method of a detection unit step by step, FIG. 3A is a state in which piezoresistive layers are stacked, FIG. 3B is a state in which a gap is formed, FIG. It is a figure which shows the state which formed and formed the cantilever part. FIG. 4A is a longitudinal sectional view showing a usage state of the dust sensor according to the present embodiment, FIG. 4A is a state where there is no air flow, FIG. 4B is a state where the cantilever portion is deformed by the air flow, and FIG. FIG. It is an example of the graph which shows the resistance change rate measured by a dust sensor. It is a schematic diagram which shows the apparatus used when measuring displacement sensitivity. It is a graph which shows the measurement result of displacement sensitivity. It is a schematic diagram which shows the apparatus used when measuring a resonant frequency. It is a graph which shows the measurement result of resonance frequency. It is a schematic diagram which shows the apparatus used when measuring an airflow speed. It is a graph which shows the measurement result of an airflow velocity. It is a schematic diagram which shows the apparatus used when measuring the number of suspended particles. It is a graph which shows the measurement result of the number of suspended particles. It is the photograph which image | photographed from before the collision before the collision of a floating particle to a pressure receiving part at the time of a collision. FIG. 15A is a graph showing the measurement result of the resistance change rate measured in synchronization with the photograph shown in FIG. 14, and FIG. 15B is a graph showing the result of examining the relationship between the air velocity including the suspended particles and the resistance change rate.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

(overall structure)

A dust sensor 1 shown in FIG. 1 includes a main body 4 in which a flow path 3 is formed, and a detection unit 2 provided in the flow path 3. The flow path 3 is formed so that gas passes from the inlet 4 _IN toward the outlet 4 _OUT . In the case of this embodiment, the main body 4 consists of a cylindrical member, and the column-shaped flow path 3 is formed.

Preferably, the main body 4 is provided with a blower 5 at the outlet 4 _OUT . The blower 5 draws gas from the inlet 4 _IN of the flow path 3 into the flow path 3 and forms an air flow in the flow path 3.

  As shown in FIG. 2, the detection unit 2 includes a cantilever unit 7. The cantilever portion 7 has a piezoresistive layer 10 and an electrode 12 formed on the surface.

  The cantilever part 7 has a flat plate-like pressure receiving part 8 and a pair of hinge parts 11 integrally formed at the base end of the pressure receiving part 8, and is fixed to the lumen of the flow path 3 in the hinge part 11. ing. Thereby, the detection part 2 is fixed so that the pressure receiving part 8 protrudes into the flow path 3.

The surface of the pressure receiving portion 8 is arranged in a direction orthogonal to the air flow (in the direction of the arrow in the figure) from the inlet 4 _IN to the outlet 4 _OUT of the flow path 3. As a result, the air pressure contacts the surface of the pressure receiving portion 8 and the suspended particles 15 included in the air current collide.

  The cantilever part 7 is formed so that it can be elastically deformed around the hinge part 11 when the air current comes into contact with it or the suspended particles 15 included in the air current collide with the surface of the pressure receiving part 8. The hinge part 11 is electrically connected to the electrode 12.

  In practice, a circuit portion (not shown) is electrically connected to the electrode 12. The circuit unit supplies current to the piezoresistive layer 10 through the electrode 12 and detects an output signal output from the detecting unit 2, that is, a change in the resistance value of the piezoresistive layer 10. Preferably, the circuit unit includes an amplifier circuit that amplifies the output signal and a filter circuit that separates the output signal by frequency.

(Production method)
Next, the manufacturing method of the detection part 2 is demonstrated with reference to FIG. First, an insulating layer 21 made of SiO ₂ is formed on a substrate 20 made of Si, and a silicon layer 22 made of Si is formed on the insulating layer 21 to thereby form the substrate 20, the insulating layer 21, and the silicon layer. A SOI structure 23 composed of 22 is formed. The thickness (Si / SiO ₂ / Si) of each layer of the SOI 23 can be set to 0.3 / 0.4 / 300 μm, for example, in order from the top. Next, an impurity is doped on the silicon layer 22 to form a piezoresistive layer 10 in which a part of the silicon layer 22 is an N-type or P-type semiconductor (FIG. 3A).

  Next, the metal layer 25 is patterned on the piezoresistive layer 10 on the SOI 23, and then the silicon layer 22 and a part of the piezoresistive layer 10 are etched (FIG. 3B). At this time, a resist (not shown) is further formed on a part of the upper surface of the metal layer 25.

  Thereafter, the metal layer 25 is further patterned to remove a portion where a resist (not shown) is not formed, and then the resist is also removed to form the electrode 12 (FIG. 3C). And the cantilever part 7 is formed by etching the board | substrate 20 and the insulating layer 21 from the bottom face side (FIG. 3D).

(Function and effect)
Next, the operation and effect of the dust sensor 1 according to the present embodiment configured as described above will be described. The dust sensor 1 supplies current to the piezoresistive layer 10 through the electrode 12 and outputs an output signal. The output signal changes as the resistance value of the piezoresistive layer 10 changes. The resistance value of the piezoresistive layer 10 changes in proportion to the deformation amount of the cantilever portion 7. That is, when the deformation amount of the cantilever portion 7 is large, the absolute value of the resistance change rate becomes large.

  The output signal has a low frequency when the deformation of the cantilever section 7 is steady, that is, when the deformation is caused by an air current. On the other hand, when the deformation of the cantilever part 7 is caused by the collision of the suspended particles 15, the output signal has a pulse response and the frequency becomes high. The frequency in this case is a resonance frequency.

  Then, the dust sensor 1 can measure the air velocity (flow rate) by measuring an output signal having a low frequency, and measure the number of suspended particles 15 by measuring the number of output signals in the resonance frequency region. be able to.

  This will be specifically described below. In a state where there is no airflow in the flow path 3, the cantilever part 7 of the dust sensor 1 is not deformed (FIG. 4A). At this time, the output signal output from the detector 2, that is, the resistance change rate is zero (FIG. 5 (i)).

When an air flow from the inlet 4 _IN to the outlet 4 _{OUT of the} flow path 3 is formed by starting the blower 5 or the like, the cantilever part 7 and the piezoresistive layer 10 of the detection part 2 are pushed by the air current in the hinge part 11. Deforms (FIG. 4B). If it does so, the output signal output from the detection part 2 will change with the change of the resistance value of the piezoresistive layer 10 (FIG. 5 (ii)). The output signal in this case has a steady response and a low frequency because the cantilever part 7 is deformed by the contact of the airflow. The air velocity can be measured by measuring the absolute value of the output signal having a low frequency, that is, the resistance change rate.

  Further, when the suspended particles 15 included in the airflow collide with the pressure receiving portion 8, the cantilever portion 7 and the piezoresistive layer 10 are deformed in the hinge portion 11 by the kinetic energy of the suspended particles 15 (FIG. 4C). If it does so, the output signal output from the detection part 2 will change with the change of the resistance value of the piezoresistive layer 10 (FIG.5 (iii)).

  The kinetic energy of the suspended particles 15 is larger by the mass of the suspended particles 15 than the kinetic energy of the airflow. Therefore, the rate of change of the output signal caused by the collision of the suspended particles 15 contained in the air stream is larger than the rate of change of the output signal caused by the air stream.

  The output signal generated by the collision of the suspended particles 15 has a pulse response, and the frequency becomes the resonance frequency. Therefore, the dust sensor 1 can measure the number of suspended particles 15 by measuring the number of pulse responses in the resonance frequency region in the output signal.

  As described above, the dust sensor 1 can measure the flow rate and the number of suspended particles 15 included in the airflow with the detection unit 2 including the cantilever unit 7, and thus can be reduced in size compared to the conventional one.

  Further, the dust sensor 1 can measure the change in resistance value caused by the air flow and the change in resistance value caused by the collision of the suspended particles 15 by separating the output signal by frequency using a filter circuit. Therefore, the dust sensor 1 can measure the flow rate and the number of suspended particles 15 included in the airflow at the same time with the single detection unit 2, and thus can be further downsized.

(Example)
Actually, a cantilever portion 7 having a length of 250 μm, a width of 200 μm, and a thickness of 0.3 μm was manufactured and evaluated. The hinge portion 11 has a length of 50 μm and a width of 50 μm.

(Displacement sensitivity measurement)
Displacement sensitivity was measured using the apparatus 28 shown in FIG. The device 28 includes an actuator 29 formed of a piezo element. The actuator 29 is arranged so as to give a displacement to the tip of the cantilever part 7. The actuator 29 was reciprocated at an amplitude of 18 μm and a frequency of 300 mHz. A circuit was formed by using the piezoresistive layer 10 of the detector 2 as one of four resistors of a Wheatstone bridge circuit (not shown). Although not shown, the Wheatstone bridge circuit is connected to the oscilloscope via an amplifier circuit. The measurement result of the displacement sensitivity is shown in FIG. In FIG. 7, the vertical axis represents the resistance change rate ΔR / R × 10 ⁻³ calculated from the output voltage, and the horizontal axis represents time (s). From the results shown in this figure, the displacement sensitivity is 1.44 × 10 ⁻⁴ μm ⁻¹ as a result of dividing the amplitude 2.6 × 10 ⁻³ of the resistance change rate by the amplitude 18 μm applied to the cantilever portion 7 by the actuator 29. And calculated.

(Resonance frequency measurement)
The resonance frequency of the cantilever part 7 was measured using the apparatus 32 shown in FIG. The device 32 includes an actuator 30, an amplifier 33, a network analyzer 34, and a heterodyne interferometer 35. The heterodyne interferometer 35 is an interferometer that converts a phase of an interference fringe into a phase of an electric signal by measuring a frequency shift between two interfering lights and detecting the optical heterodyne. The detection unit 2 was installed on the actuator 30. The actuator 30 was vibrated from 10 Hz to 10 kHz by the network analyzer 34, and the vibration of the cantilever unit 7 of the detection unit 2 was measured by the heterodyne interferometer 35. The measurement results are shown in FIG. In FIG. 9A, the vertical axis indicates the output magnitude (dB), and the horizontal axis indicates the frequency (kHz). In FIG. 9B, the vertical axis represents phase (degree) and the horizontal axis represents frequency (kHz). From this figure, the resonance frequency of the cantilever part 7 was measured as 3.1 kHz.

(Airflow velocity measurement)
The air velocity was measured using the apparatus 37 shown in FIG. The apparatus 37 includes a linear stage 39 as a moving unit and a fixing unit 38 that fixes the detection unit 2 to the linear stage 39. Although not shown, the linear stage 39 is movably provided on a guide rail that is linearly provided on the base. Although not shown, the fixing unit 38 is provided with a bridge circuit and an amplifier circuit. The detection unit 2 is fixed so that the surface of the pressure receiving unit 8 is orthogonal to the moving direction of the linear stage 39. The length of the guide rail was 1800 mm.

The linear stage 39 was moved at a constant speed from one end of the guide rail to the other end. The speed was increased from 0 to 2.5 m / s in steps of 0.25 m / s. The measurement results are shown in FIG. In FIG. 11, the vertical axis represents the resistance change rate ΔR / R × 10 ⁻³ , and the horizontal axis represents the air flow velocity v (m / s). From this figure, it was confirmed that the resistance change rate was proportional to the air velocity. The straight line shown in this figure is represented by ΔR / R = 1.2 × 10 ⁻³ v.

(Measurement of the number of suspended particles)
The number of suspended particles 15 included in the airflow was measured using the device 42 shown in FIG. The device 42 includes a main body 4 made of an acrylic pipe having a diameter of 50 mm and a length of 150 mm, and a blower 5 attached to the outlet 4 _OUT side of the main body 4. A honeycomb structure 43 was provided at the inlet 4 _IN and the outlet 4 _OUT of the main body 4 in order to rectify the airflow passing through the flow path 3. The detection unit 2 is arranged in the center in the length direction of the flow path 3 and in a direction orthogonal to the air flow (in the direction of the arrow in the figure) where the surface of the pressure receiving unit 8 is directed from the inlet 4 _IN to the outlet 4 _OUT of the flow path 3. did.

Suspended particles 15 having a diameter of 35 μm were generated around the device 42 using a particle generator (not shown). As a result, the suspended particles 15 were stably introduced into the airflow passing through the flow path 3. The measurement results are shown in FIG. In FIG. 13, the vertical axis represents the resistance change rate ΔR / R × 10 ⁻³ , and the horizontal axis represents time (s). As is clear from this figure, it was confirmed that the output signal includes a steady response and a pulse response. The steady response is a result of the cantilever portion 7 being deformed by the airflow. Using the result of the airflow velocity measurement (FIG. 11), the airflow velocity can be calculated as 1.0 m / s from the resistance change rate in the steady response.

  In order to confirm that the pulse-like response was generated by the collision of the suspended particles 15, the cantilever portion 7 at the moment when the pulse-like response occurred was observed. FIG. 14 is a photograph taken from before the suspended particles 15 collide with the pressure receiving unit 8 until after the collision. The image was taken with a high-speed camera (VW-9000, Keyence) placed on the entrance side of the main unit 4. The frame rate was 10,000 fps (Frames Per Second). 14A is a photograph taken at a timing of 0.8 ms, (B) is 0.9 ms, (C) is 1.0 ms, (D) is 1.1 ms, and (E) is 1.2 ms. In FIG. 14, it can be confirmed that one suspended particle 15 collides with the upper right corner of the pressure receiving portion 8 at 1.0 ms.

FIG. 15A shows the resistance change rate measured in synchronization with the above photograph. In FIG. 15A, the vertical axis represents the resistance change rate ΔR / R × 10 ⁻³ , and the horizontal axis represents time (ms). The resistance change rate was a value obtained by subtracting the resistance change rate (steady value) due to the airflow.

From FIG. 15A, it was confirmed that the amplitude of the resistance change rate reached 1.2 × 10 ⁻³ . The frequency of the output signal was about 3 kHz, which is the same as the resonance frequency of the cantilever part 7. From these results, it was confirmed that the collision of the suspended particles 15 generates a pulse-like response. Therefore, the number of suspended particles 15 can be measured by measuring the number of pulse responses in the resonance frequency region.

Subsequently, the air velocity was increased from 0.5 to 2.5 m / s in 0.5 m / s increments, and 12 to 26 suspended particles 15 were measured by the detector 2 for each air velocity. The measurement results are shown in FIG. 15B. In FIG. 15B, the vertical axis represents the rate of change ΔR / R × 10 ⁻³ due to the suspended particles 15 and the air flow, and the horizontal axis represents the air flow velocity v (m / s). Each point represents the average resistance change rate in the measured pulse response, and the error bar represents the standard deviation. From this figure, it was confirmed that the resistance change rate increased in proportion to the air flow velocity. The approximate expression obtained using the least square method was ΔR / R = 2.0 × 10 ⁻³ . From this result, it was confirmed that the kinetic energy of the suspended particles 15 was converted into the elastic energy of the cantilever part 7 according to the law of energy conservation. Further, from the above result, the mass of the suspended particles 15 can be calculated.

(Modification)
The present invention is not limited to the above-described embodiment, and can be appropriately changed within the scope of the gist of the present invention.

DESCRIPTION OF SYMBOLS 1 Dust sensor 2 Detection part 3 Flow path 5 Blower 7 Cantilever part 10 Piezoresistive layer 15 Airborne particle

Claims (5)

A detection unit;
A circuit unit electrically connected to the detection unit,
The detection unit has an elastically deformable cantilever unit,
The cantilever part is formed with a piezoresistive layer,
The dust sensor according to claim 1, wherein the circuit unit measures a resistance change in a resonance frequency region of the cantilever unit. The circuit section is
Having a filter circuit;
A flow rate of gas passing around the detection unit;
The dust sensor according to claim 1, wherein the amount of suspended particles contained in the gas is measured. A flow path through which the gas passes;
The dust sensor according to claim 2, wherein the detection unit is provided in the flow path. The dust sensor according to claim 3, wherein a blower is provided at an outlet of the flow path, and draws the air from an inlet of the flow path to form an air flow in the flow path.   The dust sensor according to claim 1, wherein the detection unit is provided in a moving unit that is movable on a base.

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