JPH0760895B2 - Charge coupled device and driving method thereof - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a charge-coupled device and a driving method thereof, and in particular,
The present invention relates to a charge-coupled device capable of increasing the density of an output section and a driving method thereof.

(Prior Art) Charge-coupled devices (hereinafter referred to as CCDs) are based on the conventional high-level integrated circuit technology, and are rapidly developed along with their development. In recent years, various applications such as solid-state imaging and memory have been made. It became so. In particular, a solid-state image sensor using a CCD has many features such as low power consumption, small size, and light weight, and its development has been brisk in recent years. At present, the solid-state image sensor has a large number of pixels and a high density, but it is a general tendency. Along with this, there are problems such as a decrease in dynamic range and deterioration of S / N.

FIG. 2 shows a schematic cross-sectional view of the output part of a conventional charge coupled device. In the following description, an N-channel element using an N-type semiconductor substrate will be described for convenience. In FIG. 2, reference numeral 1 is an N-type semiconductor substrate 2, 3 is first and second semiconductor regions of opposite conductivity type, that is, P-type, formed on the semiconductor substrate. Call it well. Reference numeral 4 denotes an N type semiconductor layer (N well) formed in these P wells, which constitutes a buried channel CCD. 5 and 6 are regions in which high-concentration N-type impurities are diffused, and particularly 5 is a floating diffusion layer which constitutes a part of the output structure of the CCD.
Used to detect transfer charge. 7 is a P-type high-concentration diffusion layer forming a channel stopper. Reference numeral 8 is an insulating film, 9 is a reset electrode, 10 is an output gate electrode, and 11 and 12 are transfer gate electrodes. In the operation of this element, a reverse bias voltage V _SUB is normally applied between the first and second P wells 2 and 3 and the N-type semiconductor substrate 1. The second P well in this device is formed immediately below the N well 4, and CC
It effectively acts to increase the transfer charge amount of D or suppress smear in solid-state imaging. Also, the CCD of this device is 4 for convenience.
Was assumed to be driven by a phase drive pulses phi ₁ to [phi] _4, the drawings show only the transfer gate electrodes 11 and 12 corresponding to phi _3, phi ₄ near the output. A normal voltage V _OG is applied to the output gate electrode 10. Further, a periodic reset pulse φ _R is applied to the reset electrode 9, the channel immediately below the reset electrode is made conductive, and the potential of the floating diffusion layer 5 is periodically set to the reference potential V _RD . The operation of this element will be described below. The signal charge transferred through the CCD channel flows into the floating diffusion layer 5 directly below the output gate electrode 10. At this time, the potential of the floating diffusion layer 5 changes in proportion to the signal charge amount and in inverse proportion to the capacitance of the floating diffusion layer. This potential change is taken out to the outside through an output amplifier (not shown) coupled to the floating diffusion layer. After that, the reset pulse φ _R is applied to the reset electrode 9 and the floating diffusion layer 5
The potential of is set to the reference potential V _RD , and the signal charge can be detected again.

(Problems of Prior Art) From the series of operations described above, it is understood that the detection sensitivity of the signal charge of this element is larger as the floating diffusion layer capacitance is smaller. The floating diffusion layer 5 has a capacitance for the first and second P wells, a capacitance for the reset electrode 9 and an output gate electrode.
It is the combined capacitance of the capacitance for 10 and the input capacitance of the output amplifier (not shown).

Usually, the second P well 3 has a relatively high concentration. For this reason, in the conventional device, the depletion layer does not extend so much from the floating diffusion layer toward the P well, the capacitance of the P well is relatively large, and as a result, the capacitance of the floating diffusion layer is large, and the signal detection sensitivity is deteriorated. There was a problem such as a decrease in S / N ratio.

(Object of the Invention) It is an object of the present invention to provide a charge-coupled device having a high signal detection sensitivity and a higher S / N ratio and a method for driving the same by solving the problems of the related art.

(Structure of the Invention) A charge-coupled device according to a first invention of the present invention includes a semiconductor substrate of one conductivity type, and a first semiconductor region of a conductivity type opposite to the semiconductor substrate formed on a surface portion of the semiconductor substrate. A second semiconductor region which has the same conductivity type as that of the first semiconductor region and has a higher concentration than that of the first semiconductor region, which is formed on the surface portion of the first semiconductor region except for the predetermined region, and the predetermined region and the second semiconductor region. A semiconductor layer of a buried channel of one conductivity type formed on the surface portion of the semiconductor region, and a semiconductor diffusion layer of one conductivity type formed as a floating diffusion layer in the portion corresponding to the predetermined portion of the semiconductor layer and in the vicinity thereof. It has a built-in output structure.

The charge-coupled device driving method of the second invention of the present invention is
A semiconductor substrate of one conductivity type, a first semiconductor region of an opposite conductivity type to the semiconductor substrate formed on a surface portion of the semiconductor substrate, and a surface portion of the first semiconductor region except a predetermined portion thereof. A second semiconductor region having the same conductivity type as that of the first semiconductor region and having a higher concentration than that of the first semiconductor region, and a semiconductor layer of a buried channel of one conductivity type formed in the predetermined portion and the surface portion of the second semiconductor region. And a driving method of a charge-coupled device having an output structure including a portion corresponding to the predetermined portion of the semiconductor layer and a semiconductor diffusion layer of one conductivity type formed as a floating diffusion layer in the vicinity thereof. A reverse bias voltage is applied between the substrate and the first semiconductor region.

(Detailed Description of Configuration) In the charge-coupled device of the first invention of the present invention, by removing one of the two P wells which are located immediately below the floating diffusion layer, the impurity concentration of the P well immediately below the floating diffusion layer is removed. It is characterized by lowering the depletion layer so that it can spread more easily.

Then, the object of the present invention is achieved by performing the following operations by the method for driving a charge coupled device according to the second aspect of the present invention. That is, the reverse bias voltage applied between the P-well and the N-type substrate is sufficiently increased so that the P-well directly under the floating diffusion layer is completely depleted, and the depletion layer immediately under the floating diffusion layer is N-type. Try to reach the substrate.

Thus, according to the present invention, the depletion layer immediately below the floating diffusion layer can be enlarged, and the capacitance of the floating diffusion layer can be reduced, and the signal detection sensitivity and the S / N ratio can be improved.

(Example) Hereinafter, the Example of this invention is described with reference to drawings.

FIG. 1 is a schematic cross-sectional view of one embodiment according to the first aspect of the present invention, showing a main part of an element output section.
In FIG. 1, the same numbers as in FIG. 2 indicate the same objects. In FIG. 1, 13 and 14 are second P wells,
The portion directly below the floating diffusion layer 5 is omitted. Reference numeral 15 indicates a first P well region directly below the floating diffusion layer 5. In the structure of this device, the region 15 has a lower impurity concentration as a whole than the P well region of the main part of the device.
A reverse bias voltage V _SUB is applied between the P well 15 and the N type substrate 1. This reverse bias voltage is characterized by being sufficiently biased so that the region 15 immediately below the floating diffusion layer 5 is completely depleted. This area 15
In the device of the present invention, since the concentration is lower than that of the main part of the device as described above, it can be depleted relatively easily. The depletion layer has a configuration in which a depletion layer extending from the floating diffusion layer 5 toward the P well and a depletion layer extending from the N-type substrate 1 toward the P well are connected in series.
Therefore, the width of the depletion layer immediately below the floating diffusion layer 5 becomes extremely large as compared with the conventional element. That is, in the conventional device, the depletion layer is the floating diffusion layer 5 and the first and second P wells.
It was formed between 2 and 3 and its width was at most about 2 to 5 μm. On the other hand, according to the element and the driving method thereof according to the present invention, the depletion layer is formed between the floating diffusion layer 5 and the N-type substrate 1, so that the width thereof can be easily set to 15 μm or 20 μm or more. Therefore, the capacitance of the depletion layer just below the floating diffusion layer of the device according to the present invention is extremely small, and the signal detection sensitivity and the S / N ratio are improved.

(Effects of the Invention) As described in detail above, according to the present invention, a charge coupled device having a high signal detection sensitivity and a good S / N ratio and a driving method thereof can be realized by the above configuration and driving method. .

[Brief description of drawings]

FIG. 1 is a schematic sectional view showing an essential part of an embodiment of the first invention of the present invention, and FIG. 2 is a schematic sectional view of an essential part of a conventional charge coupled device. 1 ... N-type semiconductor substrate, 2 ... first P well, 3,13,1
4 …… Second P well, 4 …… Embedded channel, 5 ……
Floating diffusion layer, 6 ... High-concentration N-type impurity diffusion layer, 7 ... Channel stopper, 8 ... Insulating film, 9 ... Reset gate electrode, 10 ... Output gate electrode, 11, 12 ... Transfer gate electrode.

Claims (2)

[Claims] 1. A semiconductor substrate of one conductivity type, a first semiconductor region of a conductivity type opposite to that of the semiconductor substrate formed on a surface portion of the semiconductor substrate, and a predetermined portion on a surface portion of the first semiconductor region. A second semiconductor region of the same conductivity type as that of the first semiconductor region formed except for a portion and having a higher concentration than that of the first semiconductor region; and one conductivity type formed on the predetermined portion and the surface portion of the second semiconductor region. A charge coupling having a semiconductor layer of a buried channel, an output structure including a semiconductor diffusion layer of one conductivity type formed as a floating diffusion layer in a portion corresponding to the predetermined portion of the semiconductor layer and in the vicinity thereof. element. 2. A semiconductor substrate of one conductivity type, a first semiconductor region of a conductivity type opposite to that of the semiconductor substrate formed on a surface portion of the semiconductor substrate, and a predetermined portion on the surface portion of the first semiconductor region. A second semiconductor region of the same conductivity type as that of the first semiconductor region formed except for a portion and having a higher concentration than that of the first semiconductor region; and one conductivity type formed on the predetermined portion and the surface portion of the second semiconductor region. A method for driving a charge-coupled device having a semiconductor layer of a buried channel, an output structure including a portion corresponding to the predetermined portion of the semiconductor layer and a semiconductor diffusion layer of one conductivity type formed as a floating diffusion layer in the vicinity thereof is provided. A charge-coupled device driving method, characterized in that a reverse bias voltage is applied between the semiconductor substrate and the first semiconductor region.