Sensor

1. A sensor comprising:

a first semiconductor chip; and

a second semiconductor chip, wherein

the first semiconductor chip includes:

a substrate;

a reference electrode provided above the substrate;

a lower electrode provided above the reference electrode via an insulating film; and

an upper electrode provided above the lower electrode via a physical quantity detecting film, and

the second semiconductor chip includes:

a driver configured to apply a first alternate current drive signal to the upper electrode and apply a second alternate current drive signal to the reference electrode, the first alternate current drive signal and the second alternate current drive signal being in opposite phases; and

a charge-to-voltage converter configured to convert an electric charge into a voltage, the electric charge being output from the lower electrode.

2. The sensor according to claim 1 , wherein the second semiconductor chip includes:

a sample and hold circuit configured to selectively sample and hold the voltage output from the charge-to-voltage converter, and output the held voltage from each of a first output terminal and a second output terminal; and

an analog-to-digital converter configured to convert a differential value between an output voltage of the first output terminal and an output voltage of the second output terminal into a digital signal, and output the digital signal.

3. The sensor according to claim 2 , wherein

the charge-to-voltage converter includes:

an operational amplifier;

a switch; and

a capacitor, and wherein

the sample and hold circuit samples and holds a voltage output from the first output terminal in a period of time in which the switch is turned off, the first alternate current drive signal is at a low level, and the second alternate current drive signal is at a high level, and outputs the held voltage, and

the sample and hold circuit samples and holds a voltage output from the second output terminal in a period of time in which the switch is turned off, the first alternate current drive signal is at a high level, and the second alternate current drive signal is at a low level, and outputs the held voltage.

4. The sensor according to claim 3 , wherein

the upper electrode and the lower electrode form a parallel-plate-type detection capacitor,

the lower electrode and the reference electrode form a parallel-plate-type reference capacitor, and

wherein a differential value ΔV between the output voltage of the first output terminal and the output voltage of the second output terminal is expressed by

Δ V= 2 ×VDD ×( Cs−Cr )/ C 1

wherein Cs represents the parallel-plate-type detection capacitor, Cr represents the parallel-plate-type reference capacitor, C 1 represents the capacitor in the charge-to-voltage converter, and VDD represents a voltage when the first alternate current drive signal or the second alternate current drive signal is at a high level.

5. The sensor according to claim 1 , wherein

the first semiconductor chip is stacked above the second semiconductor chip,

the first semiconductor chip includes a first driving terminal connected to the upper electrode, a second driving terminal connected to the reference electrode, and a signal terminal connected to the lower electrode, the first alternate current drive signal being applied to the first driving terminal, and the second alternate current drive signal being applied to the second driving terminal, and wherein

each of the first driving terminal and the second driving terminal are electrically connected to the driver of the second semiconductor chip via a bonding wire, and

the signal terminal is electrically connected to the charge-to-voltage converter of the second semiconductor chip via a bonding wire.

6. The sensor according to claim 1 , wherein a shielding layer is disposed to surround the lower electrode.

7. The sensor according to claim 6 , further comprising two or more shielding layers.

8. The sensor according to claim 7 , wherein the two or more shielding layers include a first shielding layer disposed to surround a part of the lower electrode and a second shielding layer disposed to surround another part of the lower electrode, and respective drive signals are applied to the first shielding layer and the second shielding layer, the drive signals being in opposite phases.

9. The sensor according to claim 6 , wherein the shielding layer covers signal lines, the signal lines being respectively connected to the reference electrode, the lower electrode, and the upper electrode.

10. The sensor according to claim 1 , wherein an area of the lower electrode is smaller than an area of the reference electrode.

11. The sensor according to claim 10 , wherein an opening is formed in the upper electrode, and an area of the upper electrode includes an area of the opening.

12. The sensor according to claim 1 , wherein the physical quantity detecting film includes a polymeric material whose dielectric constant varies in accordance with an amount of adsorbed moisture.

13. The sensor according to claim 12 , wherein the polymeric material is polyimide.