Search

EP-4135038-B1 - SEMICONDUCTOR IMAGE SENSOR

EP4135038B1EP 4135038 B1EP4135038 B1EP 4135038B1EP-4135038-B1

Inventors

  • KURACHI, IKUO
  • TAKANO, HIROSHI
  • KASHIMA, YASUMASA

Dates

Publication Date
20260506
Application Date
20200410

Claims (4)

  1. A semiconductor image sensor (1000) comprising: a light receiving element (100a) formed in a silicon substrate (101) under an insulation film (102) of an SOI substrate comprising the silicon substrate (101) having a main surface and a backside surface opposed to the main surface, the insulation film (102) formed on the main surface of the silicon substrate (101) and having a backside surface contacting with the main surface of the silicon substrate (101) and a frontside surface opposed to the backside surface, and a semiconductor layer (107) formed on the frontside surface of the insulation film (102), and composed of a pn junction diode formed in a vertical direction to the main surface of the silicon substrate (101) and having sensitivity to near-infrared light; a high voltage generating circuit (200) configured to generate an applied voltage for applying a reverse bias voltage to the pn junction diode; and a buried oxide film (BOX) capacitor (112); wherein the semiconductor image sensor is characterized in that the BOX capacitor (112) has a first electrode as a portion of the semiconductor layer (107) formed on the frontside surface of the insulation film (102) and a second electrode as a first P + diffusion layer (105) formed in a P-well layer (106) formed in the silicon substrate (101) so as to be located near the main surface of the silicon substrate (101), the first P + diffusion layer (105) is contacting with the backside surface of the insulation film (102), the first electrode is connected to an output of the high voltage generating circuit (200), the pn junction diode contains a second P + diffusion layer (105) formed in the silicon substrate (101) so as to be located near the main surface of the silicon substrate (101) and an N + diffusion layer (103) formed on the backside surface of the silicon substrate (101), an impurity concentration of a portion of the silicon substrate (101) excluding the first P + diffusion layer (105), the second P + diffusion layer (105) and the P-well layer (106) is in a range of 1×10 12 /cm 3 to 1×10 14 /cm 3 , a film thickness of the silicon substrate (101) is in a range of 300 µm to 700 µm, and the applied voltage is in a range of 10 V to 60 V.
  2. The semiconductor image sensor (1000) according to claim 1, wherein a film thickness of the insulation film (102) of the SOI substrate is in a range of 100 nm to 300 nm.
  3. The semiconductor image sensor (1000) according to claim 1 or 2, comprising a first region and a second region formed in the semiconductor layer (107) on the insulation film (102) and in contact with a channel region with the channel region therebetween, and a gate electrode formed on the channel region, the first region and the channel region having the same conductivity type, the second region and the channel region having different conductivity types from each other, wherein the gate electrode and the second region are connected to use the first region and the second region via the channel region as a diode, and a charge pump circuit configured to output a high voltage by connecting a plurality of the diodes in series and giving a signal to each diode is the high voltage generating circuit.
  4. The semiconductor image sensor (1000) according to claim 1, 2 or 3, wherein an impurity concentration of the semiconductor layer (107) is in a range of 1×10 15 /cm 3 to 3×10 18 /cm 3 , and a film thickness of the semiconductor layer (107) is in a range of 10 nm to 100 nm.

Description

Technical Field The present invention relates to a semiconductor image sensor, in particular, to a semiconductor image sensor that has high sensitivity to near-infrared light and can be integrated in a small area. Background Art As a well-known semiconductor image sensor (hereinafter referred to as an optical sensor), one that uses a pn junction diode formed in a silicon substrate as a light receiving element is well-known. In order to operate this optical sensor, it is necessary to first apply reverse bias voltages to the pn junction diode, i.e., apply a negative bias voltage to a p-type semiconductor layer and a positive bias voltage to an n-type semiconductor layer. This forms a depletion layer without carriers in the pn junction. When this depletion layer is irradiated with light, electron-hole pairs (carriers) are generated by the light energy (referred to as a photoelectric effect), electrons are attracted into the n-type semiconductor layer to which a positive voltage is applied while holes are attracted to the p-type semiconductor layer to which a negative voltage is applied, by an electric field in the depletion layer. With this, since the electric charge amount between the terminals of the pn junction diode varies in accordance with an optical signal, the optical signal can be converted into an electrical signal (referred to as a photoelectric conversion) . The document US10312275B is a relevant prior art. In the photoelectric conversion by the pn junction diode using the silicon substrate, a limit of detectable light on the long wavelength side (low light energy side) is determined by a bandgap width in the silicon. Since the bandgap width of silicon is about 1.1 eV, the optical sensor using the pn junction diode made of silicon can only detect light having a wavelength of about 1,100 nm or less. This wavelength (about 1,100 nm) is in a near-infrared region. FIG. 1 shows absorption coefficient of light using silicon as a medium, and the light absorption coefficient of near-infrared light having a wavelength near 1,100 nm is small and detection sensitivity is low. Therefore, the sensitivity to near-infrared light has been conventionally enhanced by making some improvements of a silicon optical sensor. As one of the improvements, Patent Document 1 and Non-Patent Document 1 describe that near-infrared light incident on the optical sensor is dispersed in the optical sensor, thereby extending an optical path length along which the near-infrared light passes through the optical sensor to enhance the sensitivity. Specifically, this has been achieved by forming pyramidal unevenness on a surface of the silicon in which a light receiving element is formed. Further, the sensitivity has been enhanced by forming a specific layer that is referred to as a diffuser on a surface of the light receiving element to disperse near-infrared light. However, these conventional methods cause an increase in a manufacturing process and are accompanied by an increase in cost. In addition, the dispersion of the near-infrared light alone has not led to enough sensitivity improvement and has limitations. On the other hand, a method of thickening a depletion layer serving as a photoelectric conversion region is effective to enhance the sensitivity. FIG. 2 shows a relationship between a depletion layer width and light absorptivity for each light wavelength. This drawing shows that if a depletion layer width can be controlled at 300 µm or greater, sufficient light absorption to light in a near-infrared region can be obtained. FIG. 3 shows a relationship between a reverse bias voltage and a depletion layer width for an impurity concentration of the silicon substrate in which a pn junction is formed. Although a concentration in a silicon substrate in general use is about 1×1015/cm3, it is shown that when a low-concentration substrate (to 1×1012/cm3) is used, the depletion layer width is increased by about one order of magnitude at the same bias voltage. FIG. 4 shows a relationship between a reverse bias voltage and a substrate impurity concentration to have a similar degree of sensitivity to visible light, using a light wavelength as a parameter. It shows that it is necessary to apply a bias voltage of about 50V, in order to obtain an optical sensor that has a similar degree of sensitivity to visible light, for near-infrared light having a wavelength of 940 nm, using an FZ substrate having the substrate concentration of about 2×1012/cm3. Therefore, the optical sensor includes a high voltage generating circuit that generates a high voltage to apply a reverse bias voltage to a pn junction diode. The high voltage generating circuit is generally a circuit for boosting a power-supply voltage (VCC) to obtain a predetermined high voltage, and a charge pump circuit is known. The charge pump circuit is a circuit that is realized by turning an input signal (power-supply voltage: VCC) on and off using capacitors (C1 to C9) and diodes (D1 to D9