Radiant energy – Infrared-to-visible imaging – Including detector array
Reexamination Certificate
2002-03-27
2004-07-06
Hannaher, Constantine (Department: 2878)
Radiant energy
Infrared-to-visible imaging
Including detector array
C348S296000
Reexamination Certificate
active
06759657
ABSTRACT:
CROSS REFERENCE TO RELATED APPLICATIONS
This application is based upon and claims the benefits of priority from the prior Japanese Patent Applications No. 2000-91173 filed on Mar. 27, 2001 and No. 2001-95301 filed on Mar. 29, 2001, the entire contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
This invention relates to an infrared sensor and, more particularly, it relates to a signal read circuit of a thermal type infrared sensor.
The technology of infrared imaging can find a broad scope of application because images can be picked up regardless of day or night even through smoke and fog to provide a great advantage over visible light imaging. Additionally, infrared imaging can obtain thermal information on the object of imaging. The broad scope of application of infrared imaging covers military defense devices, monitor cameras and fire detection cameras.
In recent years, massive efforts have been paid for developing no cooling thermal type infrared solid state imaging devices that do not require the use of a cooling mechanism for low temperature operations because the need of using a cooling mechanism is the largest problem of quantum type infrared solid state imaging devices that have been in the main stream. Thermal type infrared solid state imaging devices are so designed that the incoming infrared rays with a wavelength of about 10 ìm are transformed into heat by means of an IR rays absorption system and the temperature change of the heat sensing section produced by the weak heat is converted into an electric signal by means of a thermoelectric converter. Then, infrared image information can be obtained by reading the electric signal.
Thermal type infrared solid state imaging devices realized by forming silicon pn junctions for converting a temperature change into a voltage change by means of a constant forward electric current in an SOI (silicon on insulator) region have been reported (Tomohiro Ishikawa, et al., Proc. SPIE Vol. 3698, p 556, 1999).
Silicon pn junction type devices using an SOI substrate provide an advantage that they can be manufactured by using only a silicon LSI manufacturing process and hence are highly adapted to mass production.
Another advantage of silicon pn junction type devices is that the pn junctions that operate as thermoelectric conversion means have a pixel selecting function of utilizing the current rectifying ability of pn junctions and therefore it is possible to simplify the internal structure of pixels.
Meanwhile, the temperature change in the pixel section of a thermal type infrared solid state imaging device is generally about 5×10
−3
times of the temperature change of the object of imaging although it may vary depending on the absorption coefficient of the infrared rays absorption layer and the performance of the optical system. In other words, when the temperature of the object of imaging changes by 1[K], the pixel temperature changes by 5[mK].
When eight silicon pn junctions are connected in series to a single pixel, the thermoelectric conversion efficiency is in the order of about 10 [mV/K]. Therefore, when the temperature of the object of imaging changes by 1[K], a signal voltage of 50 [ìV] is generated in the pixel section.
In reality, the thermal type infrared solid state imaging device is required to detect a temperature change of about 0.1 [K]. Then, it has to read a generated voltage signal of about 5 [ìV].
As means for reading such a very weak signal voltage, a circuit adapted to amplify the generated signal voltage as the gate voltage of a MOS amplifier transistor for amplifying the electric current and integrate the amplified current with time by means of a storage capacitor is known.
Such a circuit is referred to as gate modulation integration circuit. An effect of limiting the signal bandwidth and reducing the random noise can be achieved by arranging such a circuit as column amplifying circuit in each column of a matrix for the purpose of parallel amplification of the electric current of a row.
The voltage gain: G of a gate modulation integration circuit is determined by the mutual conductance of the amplifier transistor:gm=äId/äVg, the integration time:ti and the storage capacity Ci and expressed by G=(ti×gm)/Ci. When the integration time:ti and the storage capacity:Ci are given, the gain is dominated by the mutual conductance:gm of the amplifier transistor. The value of gm is approximately expressed by formula (1) below when an n-type MOS transistor operates in a saturation region;
gm
=(
W/L
)·(
åox/Tox
)·
ìn
·(
Vgs−Vth
) (1),
where W: channel width,
L: channel length,
åox: dielectric constant of gate oxide film,
Tox: film thickness of gate oxide film,
ìn: electron mobility,
Vgs: gate/source voltage and
Vth: threshold voltage of transistor.
As pointed out above, a thermal type infrared solid state imaging device is required to detect a temperature change of about 0.1[K] in the temperature of the object of imaging. Therefore, it is necessary to read a signal of about 5 [ìV] that is generated in the pixel section. This voltage level is very low if compared with a CMOS sensor that is used to obtain an image by means of visible light. According to Nakamura and Matsunaga, “High Sensitivity Image Sensor”, the Journal of the Institute of Image Information and Television Engineers, Vol. 54, No. 2, p. 216, 2000, the noise voltage is about 0.4 [mV]=400 [ìV]. In view of this noise level, the noise level of the above infrared solid state imaging device is as low as about {fraction (1/80)} of that of a CMOS sensor and hence the signal voltage the former deals is as low as about {fraction (1/80)} of that of the latter.
Therefore, if the sensor output is processed by means of a circuit similar to a circuit to be for processing the output of a CMOS sensor that is a typical imaging device, a column amplifier comprising gate modulation integration circuits and showing a gain of about 80 times will be required.
However, a variance greater than the pixel output of about 5 [ìV] is found at the gate of the amplifier transistor of a gate modulation integration circuit and hence such a circuit needs to be designed with a relatively low gain. The variance is attributable to the variance of the threshold voltage of the MOS amplifier transistors and that of the threshold of the load MOS transistors to be used as constant current source and it is known that both show an amplitude of about 30 [mV].
The amplitude of the fluctuations of the threshold voltage means that the storage capacity can show fluctuations of about 2.4[V] when an about 80 times greater gain is used as design value because the fluctuations are amplified by the amplifier/read circuit like the pixel output signal applied as the gate voltage of the amplifier transistor. Of course, the fluctuations of the thresholds are specific to the individual MOS amplifier transistors and the individual load MOS transistors and a fixed pattern appears in the picked up image so that the obtained image can be corrected by means of an external circuit. However, such corrections use most of the voltage swing of the storage capacitor and expand the dynamic range that the external circuit is required to show.
Therefore, until now, the load applied to the external circuit has to be inevitably reduced at the cost of the gain of the amplifier/read circuit. Furthermore, it has not been possible to sufficiently suppress random noises such as current shot noises and 1/f noises of the amplifier/read circuit in order to secure a large gain.
Additionally, in many cases, an electric current has to be made to flow to the thermoelectric converter of the thermal type infrared sensor in order to read the thermal information of the thermoelectric converter as electric signal. Then, a so-called self heating problem arises because Joule's heat is generated due to the bias current
Iida Yoshinori
Shigenaka Keitaro
Gabor Otilia
Hannaher Constantine
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