Patent Application
20110042552
This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2009-190309, filed Aug. 19, 2009; the entire contents of which are incorporated herein by reference.
Embodiments described herein relate generally to a MOS solid-state imaging device with an improved pixel separation structure and a method of manufacturing the same.
Solid-state imaging devices, such as CMOS sensors, have been used in a wide variety of applications, including digital still cameras, video cameras, and surveillance cameras. Recently, a back-illuminated solid-state imaging device has been proposed to suppress a decrease in the signal-to-noise ratio due to a decrease in the pixel size. In the device, light is caused to enter the back side of the silicon substrate opposite the front side where a signal scanning circuit and its interconnection layer have been formed. Therefore, light entering the pixels can reach a light-receiving region formed in the silicon layer without being impeded by the interconnection layer. Accordingly, this offers the advantage of realizing a high quantum efficiency even when pixels are very small.
However, a back-illuminated solid-state imaging device has the following problem. Incident light leaks into adjacent pixels because the incident light is not impeded by the interconnection layer. When pixels are miniaturized, the aperture pitches of microlenses and color filters decrease, which permits diffraction to take place particularly when light entering R pixels corresponding to the long-wavelength passes through the color filter. In this case, light obliquely entering the silicon light-receiving region goes toward adjacent pixels. When the light crosses the boundary between pixels and enters the adjacent pixels, it generates photoelectrons in the adjacent pixels, causing crosstalk, which permits color mixture to take place. Therefore, color reproducibility is deteriorated on the replay screen, which causes the problem of degrading the image quality.
In the field of MOS solid-state imaging devices, to prevent color mixture caused by light that enters obliquely, a method of forming a multilayer film so as to enclose photoelectric conversion parts and separating the adjacent photoelectric conversion parts from one another electrically has been proposed. However, it is difficult to directly apply this configuration to a back-illuminated solid-state imaging device which has a signal scanning circuit and others provided in a semiconductor layer different from the photoelectric conversion parts.
In general, according to one embodiment, there is provided a solid-state imaging device with an array arrangement of unit pixels including photoelectric conversion parts for generating signal charges by photoelectric conversion and a signal scanning circuit part, the signal scanning circuit part being provided on a second semiconductor layer different from a first semiconductor layer including the photoelectric conversion parts, the second semiconductor layer being stacked above the front side of the first semiconductor layer via an insulating film, and the first semiconductor layer being so configured that a pixel separation insulating film is buried in pixel boundary parts and read transistors for reading signal charges generated by the photoelectric conversion parts are formed at the front side of the first semiconductor layer.
Hereinafter, embodiments will be explained in detail with reference to the accompanying drawings.
An example of the configuration of a MOS solid-state imaging device according to a first embodiment will be explained with reference to
The imaging region 110 is such that unit pixels are arrayed in a row and a column direction two-dimensionally on a semiconductor substrate, including photoelectric conversion parts and a signal scanning circuit part. The photoelectric conversion parts include unit pixels 130 including photodiodes that convert light into electricity (signal charges) and accumulate signal charges. The photoelectric conversion parts function as imaging parts. The signal scanning circuit part includes amplification transistors 133 and others as described later. The signal scanning circuit part reads a signal from the photoelectric conversion parts, amplifies the signal, and then transmits the amplified signal to an analog-to-digital conversion circuit 150. In the first embodiment, the light-receiving surface (photoelectric conversion parts) is provided on the back side of the semiconductor substrate opposite the front side of the substrate where the signal scanning circuit part is formed.
In the driving circuit region 120, there are provided element driving circuits, including a vertical shift register 140 for driving the signal scanning circuit part and an analog-to-digital conversion circuit 150.
While the analog-to-digital conversion circuit has been a part of the overall configuration of the CMOS sensor, the first embodiment is not limited to this. For instance, the analog-to-digital conversion circuit may not be arranged in the column position of the pixel array and may be arranged in the chip level. Alternatively, the analog-to-digital conversion circuit may not be arranged on the sensor chip.
The vertical shift register 140 outputs signals LS1 to SLk to the pixel array 110 and functions as a selection part for selecting unit pixels 130 on a row basis. Each of the unit pixels 130 in the selected row outputs an analog signal Vsig corresponding to the amount of incident light via a vertical signal line VSL. The analog-to-digital conversion circuit 150 is configured to convert an analog signal Vsig input via the vertical signal line VSL into a digital signal and output the digital signal. Although not shown in
As shown in
Each of the unit pixels 130 includes a photodiode 131, a read transistor 132, an amplification transistor 133, an address transistor 134, and a reset transistor 135.
The photodiode 131 constitutes a photoelectric conversion part. The amplification transistor 133, reset transistor 135, and address transistor 134 constitute a signal scanning circuit part. The cathode of the photodiode 131 is grounded.
The amplification transistor 133 is configured to amplify a signal from a floating diffusion layer 136 and output the amplified signal. The amplification transistor 133 has its gate connected to the floating diffusion layer 136, its source connected to the vertical signal line VSL, and its drain connected to the source of the address transistor 134. The CDS noise limiter circuit 122 eliminates noise from the output signal of a unit pixel 130 transmitted via the vertical signal line VSL and outputs the resulting signal at an output terminal 123.
The read transistor 132 is configured to control the accumulation of signal charges at the photodiode 131. The read transistor 132 has its gate connected to a read signal line TRF, its source connected to the anode of the photodiode 131, and its drain connected to the floating diffusion layer 136.
The reset transistor 135 is configured to reset the gate potential of the amplification transistor 133. The reset transistor 135 has its gate connected to a reset signal line RST, its source connected to the floating diffusion layer 136, and its drain connected to a power supply terminal 124.
The gate of the address transistor (transfer gate) 134 is connected to an address signal line ADR. The load transistor 121 has it gate connected to a selection signal line SF, its drain connected to the source of the amplification transistor 133, and its source connected to a control signal line DC.
The pixel array structure carries out a read driving operation as follows. First, the address transistor 134 in a row to be read is turned on by a row selection pulse sent from the vertical shift register 140.
Then, the reset transistor 135 is turned on by a reset pulse sent from the vertical shift register 140, resetting the potential of the floating diffusion layer 136. Thereafter, the reset transistor 135 goes off.
Then, the read transistor 132 goes on, causing the signal charges accumulated in the photodiode 131 to be read into the floating diffusion layer 136. The potential of the floating diffusion layer 136 is modulated according to the number of signal charges read.
Then, the modulated signal is amplified by the amplification transistor 133 constituting a source follower and read onto the vertical signal line VSL, which completes the read operation.
Next, a plane configuration of color filters the solid-state imaging device of the first embodiment has will be explained with reference to
In the layout of
In the first embodiment, the Bayer arrangement, the most widely used color filter arrangement, is used. As shown in
Next, a plane configuration of the pixel array 110 included in the solid-state imaging device of the first embodiment will be explained with reference to
As shown in
The pixel separation insulating film 15 is composed of an insulating film whose refractive index is lower than that of Si. For example, it is desirable that the pixel separation insulating film 15 should be made of an insulating material which has a refractive index of about 3.9 or less with respect to incident light whose wavelength is about 400 nm to 700 nm. More specifically, for example, the pixel separation insulating film 15 is made of an insulating material, such as a silicon dioxide film (SiO2 film), a silicon nitride film (Si3N4 film), or a titanium oxide (TiO) film.
As shown in
In a plane configuration shown in
While in the first embodiment, the pixel separation insulating films 15 have been provided discontinuously in the through-holes in the plane configuration, some parts of the pixel separation insulating films 15 may be formed continuously.
A cross-section configuration of the pixel array 110 included in the solid-state imaging device will be explained with reference to
In
More specifically, in the first Si layer 13, a pixel separation insulating film 15 marks off adjacent unit pixels is provided and read transistors are formed at the front side (lower surface) of the Si layer 13. At the front side (lower surface) of the Si layer 13, the second Si layer 33 is formed via the interlayer insulating film 16. In the Si layer 33, the amplification transistor, address transistor, reset transistor, and others are formed to constitute a signal scanning circuit.
On the surface of the Si layer 33, an interlayer insulating film 36 is formed. On the interlayer insulating film 36, an interconnection layer 50 composed of insulating films 51 and metal interconnections 52 is provided. On the back side (upper surface) of the Si layer 13, RGB filters 62 are provided via an Si nitride film 61. Microlenses 63 are formed on the individual filters 62. Incident light L1 enters the back side of the Si layer 13.
To connect the transistors in the Si layer 13 to the transistors in the Si layer 33, via holes 37, 38 are made so as to pass through the Si layer 33 and insulating films 16, 36.
As shown in
Next, the optical effect of the solid-state imaging device of the first embodiment will be explained with reference to
With a configuration without the pixel separation insulating film 15, light L2 obliquely entering the light-receiving region of the Si layer goes toward adjacent unit pixels, cross the boundary between pixels, and enters the adjacent unit pixels. As a result, this causes photoelectrons to be generated in the adjacent unit pixels, permitting crosstalk and color mixture to take place. Therefore, the color reproducibility is deteriorated on the replay screen.
In contrast, as shown in
When pixels are miniaturized, the aperture pitches of the microlenses 63 and color filters 62 decrease, which permits diffraction to take place particularly when light entering R pixels corresponding to the long-wavelength passes through the color filter 62. In this case, light L2 obliquely entering the light-receiving region in the Si layer 13 goes toward adjacent pixels, crosses the boundary between adjacent pixels, and enters the adjacent pixels. Light entering the adjacent pixels generates photoelectrons in the adjacent pixels, causing crosstalk, which permits color mixture to take place. Therefore, the color reproducibility is deteriorated on the replay screen, which degrades the image quality. In contrast, with the first embodiment, even when light enters the long-wavelength R pixels, crosstalk and the generation of color mixture can be prevented.
With the first embodiment, as shown in
In addition, since the solid-state imaging device is of the back-illuminated type, light can enter the back side of the Si substrate opposite the front side of the substrate where the signal scanning circuit and its interconnection layer have been formed. Therefore, light entering the pixels can reach a light-receiving region formed in the Si layer without being impeded by the interconnection layer, making it possible to realize high quantum efficiency even when pixels are very small. As a result, this offers the advantage of suppressing the deterioration of the quality of reproduced images even when pixels are miniaturized further.
Furthermore, since not only are the light-receiving region and signal scanning circuit provided in the separate Si layers, but also the read transistor 32 is provided in the Si layer 13 acting as a light-receiving layer, signal electrons are read from the photodiode 31 in the crystalline Si layer. Therefore, no signal charge will be left over in a read operation. Accordingly, neither afterimages nor kTC noise is generated, enabling images to be reproduced with low noise.
Next, a method of manufacturing the MOS back-solid-state imaging device of
First, as shown in
Next, as shown in
Then, as shown in
Next, as shown in
Next, as shown in
Next, as shown in
Then, as shown in
Next, as shown in
Next, as shown in
Then, as shown in
Next, as shown in
Next, as shown in
Then, as shown in
As described above, with the second embodiment, after the grooves for pixel separation are made and the insulating film is buried in the grooves in the SOI substrate 10, the read transistors are formed in the Si layer 13 and the substrate 11 of the SOI substrate is finally removed as shown in
This invention is not limited to the above embodiments. While in the embodiments, an SOI substrate has been used to form a first Si layer, the SOI substrate is not necessarily used, provided that any suitable auxiliary substrate is used as a substrate for the Si layer. For instance, after an Si substrate is caused to adhere to an auxiliary substrate, the Si substrate may be made thinner, thereby forming a first Si layer. In this case, the first Si layer is formed on the auxiliary substrate. Then, various processes are carried out as in the above embodiments to finally remove the auxiliary substrate.
Furthermore, a semiconductor substrate for forming photoelectric conversion parts is not necessarily limited to Si and other semiconductor materials may be used instead. In addition, the insulating film materials for various parts, the interconnection materials, and others may be changed suitably according to the specification. In the embodiments, a so-called type of four transistors including the address transistor was described as the signal scanning circuit part, it can apply to the type of three transistors where the address transistor is not comprised either.
While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2009-190309 | Aug 2009 | JP | national |
