JP2008021692A - Charge transfer device, solid photographing apparatus, and their manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a CCD solid photographing apparatus in which the handling amount of charges can be increased or the transfer efficiency can be improved, and the lifetime can be prolonged or the readout voltage can be reduced for the same lifetime by reducing increase in readout voltage. <P>SOLUTION: The solid photographing apparatus comprises a semiconductor substrate, a first oxide film formed on the semiconductor substrate, and a first nitride film and a second oxide film between a plurality of first gate electrodes arranged on the first oxide film. The apparatus is further provided with the first oxide film and the second nitride film or a third oxide film formed between adjacent first gate electrodes, and a plurality of second gate electrodes separated from the semiconductor substrate by a composite film of the second nitride film and the third oxide film. The thickness of the first nitride film and the second nitride film is different from each other and different from those of the second oxide film and the third oxide film. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a charge transfer device, a solid-state imaging device, and manufacturing methods thereof, and more particularly to a gate insulating film.

In recent years, the demand for a solid-state imaging device as an imaging device for a camera module of a digital still camera, a digital video camera, or a mobile terminal device typified by a mobile phone is increasing.
In the solid-state imaging device, there is a strong demand for further improvement in image quality and further reduction in power consumption. In order to meet the demand, it is necessary to suppress noise and reduce drive voltage.
In recent years, solid-state imaging devices can be broadly classified into MOS type and CCD type, but due to the history of development, CCD type solid-state imaging devices dominate in the market.

  In a vertical CCD in a first conventional CCD type (hereinafter referred to as “IT-CCD type”) solid-state imaging device adopting an interline transfer system, an area corresponding to one pixel of the vertical CCD as shown in FIG. 1, a gate insulating film in which a silicon oxide film 102, a silicon nitride film 103, and silicon oxide films 104 and 106 are laminated in this order is disposed on the main surface of the semiconductor substrate 101, and the first surface is disposed on the main surface of the silicon oxide film 104. The gate electrode 105 and the second gate electrode 108 are disposed on the main surface of the silicon oxide film 106, respectively. The first gate electrode 105 is dedicated to charge transfer, and the second gate electrode 108 has a function of reading out charges from the light receiving portion and transferring the read charges. Although not shown, the semiconductor substrate 101 includes a signal transfer path.

In the gate insulating film, a so-called ONO structure has been a trend in the manufacturing process in order to suppress noise generation caused by a difference in film thickness for each region immediately below each gate electrode (patents). Reference 1).
In the gate insulating film having a so-called ONO structure in which the silicon oxide films 102, 104, and 106 are laminated so as to sandwich the silicon nitride film 103, every time signal charges are read from the light receiving portion to the vertical CCD, electrons are generated by the hot electron effect. Is trapped in the silicon nitride film 103.

As can be seen from the characteristic diagram showing the relationship between the read voltage increase amount and the usage time shown in FIG. 9, this problem is caused by the manufacturing lot, but in the conventional solid-state imaging device, the usage time elapses. The voltage necessary for reading the signal charge is increased. Therefore, with a gradually specified read voltage, it becomes difficult to read signal charges from the light receiving unit to the vertical CCD, causing degradation of image quality and causing a problem that goes against the demand for higher image quality for the CCD type solid-state imaging device.
JP-A-4-335572

  As a means for solving this problem, it is conceivable to increase the driving voltage of the solid-state imaging device at the time of reading, but if this means is adopted, not only does it go against the demand for low power consumption, but in the CCD type solid-state imaging device, There are adverse effects such as an increase in white flaws caused by an increase in the electric field, which is against the demand for image quality improvement. On the other hand, the inventor reduces the signal charge by reducing the thickness of the silicon nitride film 103, as can be seen from the characteristic diagram showing the relationship between the read voltage increase and the thickness of the silicon nitride film shown in FIG. It was found that an increase in read voltage can be suppressed.

  However, if the silicon nitride film 103 is made too thin, the potential of the semiconductor substrate 101 constituting the vertical CCD becomes difficult to be affected by the fringe electric field due to the adjacent electrodes, and the drift is dominantly defined when transferring charges in the vertical CCD. It becomes difficult to transfer charges in time, which is contrary to the improvement of image quality. On the contrary, it is necessary to apply a high voltage to transfer within a specified time, which is contrary to the demand for low power consumption.

  Therefore, as shown in FIG. 11B, silicon oxide films 204 and 206 covering the silicon nitride film 203 in order to suppress an excessive change in the potential as compared with the first conventional IT-CCD type solid-state imaging device. Was thickly arranged. FIG. 11A is a schematic configuration diagram showing one pixel 306 of the second conventional IT-CCD type solid-state imaging device, and FIG. 11B is a cross-sectional view taken along A in FIG. 11A. It is the schematic sectional drawing which looked at.

Then, as shown in FIG. 12B, in the oxidation process after forming the first gate electrode 205, the end of the first gate electrode 205 is warped in the direction opposite to the gate insulating film. Appeared prominently.
As a result, the uniform region of the thickness of the silicon oxide film 204 is remarkably reduced as compared with the vertical CCD of the first conventional IT-CCD type solid-state imaging device, that is, the effective required for the electrode for charge transfer. Area significantly decreases from the C region to the D region, and the amount of charge that can be handled in the second conventional vertical CCD is significantly reduced as compared with the first conventional vertical CCD, thereby inducing deterioration in image quality. A new problem has occurred. FIG. 12A is a schematic configuration diagram showing one pixel 306 of the second conventional CCD type solid-state imaging device, and FIG. 12B is a view of the B cross section shown in FIG. It is a schematic sectional drawing.

  The present invention has been made in view of the above problems, and when applied to a CCD type solid-state imaging device, the increase in signal charge read voltage can be reduced, and the reduction in the amount of charge handled can be suppressed to reduce the image quality. It is an object of the present invention to provide a charge transfer device that can be suppressed and a manufacturing method thereof.

  In order to achieve the above object, in the charge transfer device according to the present invention, a first insulating film is stacked on the upper surface of a semiconductor substrate, and a first gate electrode and a second gate electrode are formed on the first insulating film in the charge transfer direction. For the charge transfer device having a structure in which the second insulating film is interposed between the first gate electrode and the second gate electrode, the first insulating film is stacked in an alternately arranged state. Then, an oxide film is laminated on the nitride film, and the oxide film includes a first thickness of a portion interposed between the nitride film and the first gate electrode, and the nitride film. And the second thickness of the portion interposed between the second gate electrode and the second gate electrode, and in the nitride film, the third thickness of the portion immediately below the first gate electrode and the second gate electrode The fourth thickness of the portion immediately below was made different.

  In the method of manufacturing a charge transfer device according to the present invention, the step of forming a nitride film on one main surface of the semiconductor substrate, and the first so as to be scattered in one axial direction along the main surface on the nitride film. Forming the first oxide film and the first gate electrode in a stacked state in this order, and oxidizing the exposed surface using oxygen radicals from the exposed surface of the first gate electrode to the exposed surface of the nitride film. The step of forming the second oxide film so that the thickness of at least the portion on the nitride film is larger than the thickness of the first oxide film, and so as to be adjacent to the first gate electrode. Forming a second gate electrode on the exposed surface of the second oxide film in the uniaxial direction.

  Further, in the method for manufacturing the charge transfer device according to the present invention, the step of forming a nitride film on one main surface of the semiconductor substrate, and the step of forming a scatter in the uniaxial direction along the main surface on the nitride film. Forming a first oxide film and a first gate electrode in a stacked state in this order, and oxidizing using an oxidation method using oxygen radicals from the exposed surface of the nitride film to the exposed surface of the first gate electrode. Then, the exposed surface is oxidized by using a pyro-oxidation method, or the exposed surface is oxidized in the reverse order, so that the second oxide film is at least nitridized compared to the thickness of the first oxide film. Forming a thickness of a portion on the film, and forming a second gate electrode in the uniaxial direction on the exposed surface of the second oxide film so as to be adjacent to the first gate electrode; Included.

  In the present invention, the thickness of each of the oxide film and the nitride film constituting the first insulating film is not uniform in the surface, and is covered by the portion covered with the first gate electrode and the second gate electrode. Therefore, the thickness of the nitride film can be adjusted to a thickness that can suppress the adverse effects caused by the hot electron effect in each of the portions, and the thickness of the oxide film can be warped in each portion. Therefore, both problems can be solved at the same time.

  When each conventional charge transfer device is applied to a solid-state imaging device, the thickness of each of the oxide film and nitride film constituting the gate insulating film is adjusted to be uniform in the surface, thereby suppressing an increase in signal charge read voltage. If the thickness of the nitride film is reduced as much as possible, the thickness of the oxide film must be increased from the standpoint of maintaining the capacity, resulting in the disadvantage of reducing the amount of charge already described. Conversely, the thickness of the oxide film can be reduced in order to suppress the decrease in the amount of charge handled. However, the thickness of the nitride film must be increased from the standpoint of maintaining the capacity, leading to an increase in the signal charge read voltage. .

On the other hand, in the present invention, focusing on the fact that the appropriate values of the thicknesses of the oxide film and the nitride film are different between immediately below the first gate electrode and immediately below the second gate electrode, the oxidation that constitutes the first insulating film is performed. Since the thickness of each of the film and the nitride film can be individually adjusted to an appropriate value, the present invention can simultaneously suppress each of the above disadvantages.
Specifically, the charge transfer device according to the present invention is incorporated in a solid-state imaging device, the second gate electrode is assigned to the gate electrode that reads out the charge accumulated in the photoelectric conversion element to the charge transfer path, and the first When driving by assigning the gate electrode to the gate electrode for charge transfer, the thickness of the portion of the nitride film immediately below the second gate electrode is made smaller than the thickness of the portion immediately below the first gate electrode. Since the thickness of only the nitride film portion corresponding to the portion where the hot electron effect occurs can be reduced, the charge generated by the hot electron effect is higher than that of the first conventional IT-CCD solid-state imaging device. Trapping by the nitride film can be suppressed.

  In this case, the thickness of the portion of the oxide film interposed between the first gate electrode and the nitride film is the thickness of the portion of the oxide film interposed between the second gate electrode and the nitride film. Since the thickness of the oxide film portion that causes a reduction in the amount of charge handled as described above can be reduced, the second gate electrode interposed between the first gate electrode and the second gate electrode can be reduced. When the insulating film, specifically, the second insulating film is formed of an oxide film, the amount of oxygen atoms transmitted through the oxide film portion interposed between the first gate electrode and the nitride film can be suppressed. In comparison with the second conventional IT-CCD type solid-state imaging device, the warpage of the electrodes can be suppressed.

  Therefore, when the charge transfer device according to the present invention is incorporated in a solid-state imaging device, the voltage applied when reading the signal charge from the photoelectric conversion element is compared with the first conventional IT-CCD solid-state imaging device. A rise with time can be suppressed, and a reduction in the amount of charge handled can be suppressed and deterioration of image quality can be suppressed as compared with the second conventional IT-CCD solid-state imaging device.

  Of the oxide film, the thickness of the portion interposed between the first gate electrode and the nitride film is smaller than the thickness of the portion interposed between the second gate electrode and the nitride film, When the thickness of the portion immediately below the first gate electrode is greater than the thickness of the portion immediately below the second gate electrode, the capacitance between the first gate electrode and the semiconductor substrate, the second gate electrode, and the It is preferable because the capacitance between the semiconductor substrate and the semiconductor substrate can be made substantially equal, and a decrease in transfer efficiency can be suppressed.

Even if the first gate electrode has a function of transferring charges along the charge transfer path, the same effect as described above can be obtained.
In the manufacturing method according to the present invention, the oxide film is formed only by oxidizing the exposed surface using oxygen radicals from the exposed surface of the first gate electrode to the exposed surface of the nitride film. Even if the materials are different, they can be oxidized at the same rate.

In general, each of the gate electrodes contains polysilicon as a main component, and it is known that there is a great difference in oxidation rate between a nitride and a member containing polysilicon as a main component.
From the manufacturing process described above, the first gate electrode containing the polysilicon as a main component and the nitride film are oxidized at the same time. However, if oxidation is performed without using oxygen radicals in the oxidation process, the polysilicon is mainly used. While the oxidation of the first gate electrode as a component proceeds, the oxidation of the nitride film does not proceed, and the first gate electrode is excessively oxidized until the oxide film having a desired thickness is obtained by oxidizing the nitride film, It becomes difficult to secure an effective area necessary for functioning as a gate electrode.

  On the other hand, in the manufacturing method according to the present invention, the oxide film is formed only by oxidation using oxygen radicals. Therefore, even if the first gate electrode and the nitride film are made of different materials, they are equivalent. It is possible to oxidize at a rate and to suppress excessive oxidation of the first gate electrode while forming an oxide film that electrically insulates the first gate electrode and the second gate electrode. Thus, an effective area necessary for the first gate electrode to perform its function can be ensured.

  Moreover, in the manufacturing method according to the present invention, the second oxide film is oxidized using oxygen radicals so that the thickness of at least a portion on the nitride film is larger than the thickness of the first oxide film. Therefore, the second oxide film can be formed so that the thickness of the portion of the second oxide film on the nitride film is larger than the thickness of the first oxide film. The thickness of the nitride film directly under the electrode can be made smaller than the thickness of the nitride film directly under the first gate electrode.

Therefore, in the manufacturing method according to the present invention, the thickness of the oxide film portion immediately below the first gate electrode can be formed larger than the thickness of the oxide film portion directly below the second gate electrode, and at the same time, the thickness of the nitride film portion directly below the second gate electrode Since the thickness can be made smaller than the thickness of the nitride film portion directly below the first gate electrode, the charge transfer device (solid-state imaging device) according to the present invention can be suitably manufactured.
Further, in the manufacturing method according to the present invention, when an oxide film is formed by oxidizing these from the exposed surface of the first gate electrode to the exposed surface of the nitride film, oxidation using oxygen radicals and pyro-oxidation are performed. Oxidizes in combination with.

A rapid thermal oxidation method is known as an oxidation method for forming a thin oxide film, and a dry oxidation method is known as an oxidation method for forming a high-quality oxide film. An oxide film is formed by a pyro oxidation method. Then, compared with the oxide film formed by those oxidation methods, an oxide film having a low defect density in the film and an equivalent thickness can be obtained.
Therefore, the dielectric breakdown lifetime of the oxide film can be extended by using the pyrooxidation.

  If the oxidation treatment is performed using oxygen radicals, the first gate electrode and the second gate electrode can be oxidized at the same rate regardless of whether the first gate electrode and the nitride film are made of different materials. The first gate electrode can be prevented from being excessively oxidized while forming an oxide film that electrically insulates the first gate electrode, and the first gate electrode is effective for exhibiting its function. A large area can be secured.

  In the manufacturing method according to the present invention, the second oxide film is oxidized using an oxygen radical so that the thickness of at least a portion on the nitride film is larger than the thickness of the first oxide film. And the pyro-oxidation method can be used so that the thickness of the portion of the second oxide film on the nitride film is larger than the thickness of the first oxide film, Accordingly, the thickness of the nitride film immediately below the second gate electrode can be made smaller than the thickness of the nitride film immediately below the first gate electrode.

  Therefore, in the manufacturing method according to the present invention, the thickness of the oxide film portion immediately below the first gate electrode can be formed larger than the thickness of the oxide film portion directly below the second gate electrode, and at the same time, the thickness of the nitride film portion directly below the second gate electrode Since the thickness can be made smaller than the thickness of the nitride film portion directly below the first gate electrode, the charge transfer device (solid-state imaging device) according to the present invention can be suitably manufactured.

(Embodiment 1)
The IT-CCD type solid-state imaging device according to the first embodiment of the present invention will be described below with reference to the drawings.
FIG. 1 is a schematic configuration diagram of an IT-CCD type solid-state imaging device according to the present embodiment. As shown in FIG. 1, in the IT-CCD type solid-state imaging device according to the present embodiment, a plurality of light receiving portions 2 are two-dimensionally arranged on a semiconductor substrate 1 and are arranged in the Y direction. A vertical transfer unit (vertical CCD) 3 is arranged for each column of the light receiving unit 2 along the horizontal line 2, and a horizontal transfer unit (horizontal CCD) 4 is arranged in the X direction so as to be adjacent to the last row of the light receiving unit 2. An amplifier 5 is provided at the end of the horizontal CCD 4 column. The light receiving unit 2 is a photodiode and has a function as a photoelectric conversion element that accumulates electric charges according to the intensity of received light, and includes one light receiving unit 2 and a part of the vertical CCD 3 adjacent thereto. Thus, one pixel 6 is configured.

As indicated by the arrows in FIG. 1, the charges accumulated in the light receiving units 2 are read out to the vertical CCDs 3 adjacent to the respective light receiving units 2, transferred in the Y direction by the columns of the vertical CCDs 3, and by the columns of the horizontal CCDs 4. It is transferred in the X direction, amplified by the amplifier 5 and output to the outside.
FIG. 2 is an enlarged cross-sectional view of a main part of a vertical CCD in the CCD solid-state imaging device according to the present embodiment. As shown in FIG. 2, in the vertical CCD according to the present embodiment, a silicon oxide film 12, a silicon nitride film 13, and silicon oxide films 14 and 16 are laminated in this order on one main surface of a semiconductor substrate 1. A gate insulating film is provided, and a first gate electrode 15 and a second gate electrode 18 are provided on the silicon oxide films 14 and 16 for transferring signal charges. The first gate electrode 15 and the second gate electrode 18 are separated by an interlayer insulating film 17 and are electrically insulated. Although not shown, the semiconductor substrate 1 is provided with a well-known signal transfer path.

In the present embodiment, the second gate electrode 18 is used not only as an electrode for charge transfer, but also reads out the signal charge accumulated therein from the photodiode functioning as the light receiving unit 2 (see FIG. 1). Is used as a signal charge readout electrode.
In the portion of the gate insulating film covered with the second gate electrode 18, the film thickness t13b of the silicon nitride film 13 is larger than the thickness of the silicon nitride film covered with the signal charge readout electrode in the first conventional vertical CCD. Since it is small, it is possible to suppress trapping of electrons in the silicon nitride film 13 by the hot electron effect when reading out the signal charges.

  On the other hand, in the portion of the gate insulating film covered with the first gate electrode 15, the thickness t14 of the silicon oxide film 14 is smaller than the thickness of the silicon oxide film in contact with the charge transfer dedicated electrode in the second conventional CCD. Compared to the second conventional CCD, the edge of the first gate electrode 15 is warped when an interlayer insulating film for electrically separating the first gate electrode 15 and the second gate electrode 18 is formed. Of the first gate electrode 15 that contributes to the potential of the portion of the semiconductor substrate 1 covered by the first gate electrode 15 in the charge transfer unit (vertical CCD 3) of the CCD solid-state imaging device. A reduction in effective area can be suppressed.

  In the present embodiment, when the portion covered with the second gate electrode 18 and the portion covered with the first gate electrode 15 in the gate insulating film are compared, the silicon oxide film 16 in the covered portion of the second gate electrode 18 is compared. The film thickness t16 is thicker than the film thickness t14 of the silicon oxide film 14 covering the first gate electrode 15, whereas the film thickness t13b of the silicon nitride film 13 covering the second gate electrode 18 is the first gate. A gate insulating film is disposed so as to be thinner than the film thickness t13a of the silicon nitride film 13 covering the electrode 15.

When the gate insulating film has the above structure, a difference between the capacitance between the first gate electrode 15 and the semiconductor substrate 1 and the capacitance between the second gate electrode 18 and the semiconductor substrate 1 occurs. This can be suppressed.
<Example of capacitance adjustment for each gate electrode>
An example for making the electrostatic capacitance between the first gate electrode 15 and the semiconductor substrate 1 substantially equal to the electrostatic capacitance between the second gate electrode and the semiconductor substrate 1 is shown. For example, the film thickness t13a of the silicon nitride film 13 covered with the first gate electrode 15 is set to 40 [nm], and the film thickness t14 of the silicon oxide film 14 covering the silicon nitride film 13 is set to 5 [nm]. In this case, the film thickness t13b of the silicon nitride film 13 covered with the second gate electrode 18 is set to 20 [nm], and the film thickness t16 of the silicon oxide film 16 covering the silicon nitride film 13 is 15 [nm]. ], The capacitance between the first gate electrode 15 and the semiconductor substrate 1 and the capacitance between the second gate electrode and the semiconductor substrate 1 can be made substantially equal.

  Because the dielectric constant of the silicon nitride film is about twice that of the silicon oxide film, the ratio ε1 (n / o) of the dielectric constant of the silicon nitride film to the dielectric constant of the silicon oxide film is about twice, When calculating the film thickness to make the capacitances substantially equal, if the silicon oxide film is used as a reference, the film thickness of the silicon nitride film is multiplied by the reciprocal of the ratio ε1 (n / o), that is, the film of the silicon nitride film This is because the calculation was performed with the thickness approximately halved, and each film thickness was determined based on the calculation result.

  Specifically, the silicon oxide film 12 is uniformly distributed with a thickness of 20 [nm] from the portion covered with the first gate electrode 15 to the portion covered with the second gate electrode 18, and the silicon nitride film 13 is formed. Silicon oxide is disposed so that the film thickness of the portion covered by the first gate electrode 15 is 40 [nm] and the film thickness of the portion covered by the second gate electrode 18 is 20 [nm]. When the film 14 was disposed with a thickness of 5.0 [nm] and the silicon oxide film 16 was disposed with a thickness of 15 [nm], the gate insulating film was covered with the first gate electrode 15. When the film thickness is converted to equalize the capacitance with respect to the silicon oxide film in the portion, the converted film thickness is 20+ (40/2) +5, which is 45 [nm]. Of these, in the portion covered by the second gate electrode 18 When the film thickness conversion for equalizing the capacitance with respect to the silicon oxide film is performed, the converted film thickness is 20+ (20/2) +15, which is 45 [nm], and the first gate in the gate insulating film. When comparing the portion covered with the electrode and the portion covered with the second gate electrode, the film thicknesses converted from the viewpoint of equalizing the capacitance are equal, and the gap between the first gate electrode 15 and the semiconductor substrate 1 is the same. The capacitance and the capacitance between the second gate electrode 18 and the semiconductor substrate 1 can be made substantially equal.

  Even if the dielectric constant of the silicon nitride film may deviate from twice that of the silicon oxide film, the ratio εx (n / o) of the dielectric constant of the silicon nitride film to the dielectric constant of the silicon oxide film is changed as appropriate. When calculating and calculating the film thickness to make the capacitances substantially equal, with the silicon oxide film as a reference, the film thickness of the silicon nitride film is multiplied by the reciprocal of the newly calculated ratio εx (n / o). It is only necessary to calculate and determine each film thickness.

<< Effects of IT-CCD Solid-State Imaging Device According to Embodiment 1 >>
In the present embodiment, the second gate electrode 18 is not only used as an electrode for charge transfer, but also as a signal charge readout electrode for reading out signal charges accumulated therein from a photodiode as a light receiving unit. It is used. In the portion of the gate insulating film covered with the second gate electrode 18, the thickness t13b of the silicon nitride film 13 is equal to the thickness of the silicon nitride film covered with the signal charge readout electrode in the first conventional vertical CCD. Since it is smaller than that, it is possible to suppress trapping of electrons in the silicon nitride film 13 due to the hot electron effect when reading out the signal charges.

  Therefore, in the IT-CCD type solid-state imaging device according to the present embodiment, it is possible to suppress the adverse effect that it becomes difficult to read with the specified read voltage every time the signal charge is read, and the image quality can be maintained even if the read voltage is maintained. Can be prevented. Further, in the IT-CCD type solid-state imaging device according to the present embodiment, since the above-described adverse effects can be suppressed, there is no need to increase the readout voltage in order to suppress the deterioration of the image quality, and an increase in white scratches can be achieved. Can be suppressed.

  On the other hand, in the present embodiment, the first gate electrode 15 is used as an electrode dedicated to charge transfer. In the portion of the gate insulating film covered with the first gate electrode 15, the film thickness t14 of the silicon oxide film 14 is that of the silicon oxide film in contact with the charge transfer dedicated electrode in the second conventional IT-CCD solid-state imaging device. Since the thickness is smaller than the thickness, the edge of the first gate electrode 15 is warped when an interlayer insulating film for electrically separating the first gate electrode 15 and the second gate electrode 18 is formed. As a result, the area of the uniform region of the thickness t14 of the silicon oxide film 14 in the finished product is smaller than the area expected in the design, and the effective charge transfer portion is reduced. It can suppress that an area decreases.

Therefore, in the IT-CCD type solid-state imaging device according to the present embodiment, compared with the second conventional IT-CCD type solid-state imaging device, a reduction in the amount of charge handled when transferring charges in the vertical CCD is suppressed. And deterioration of image quality can be suppressed.
That is, as compared with the conventional IT-CCD type solid-state imaging device, in the IT-CCD type solid-state imaging device according to the present embodiment, the gate is interposed between portions of the gate insulating film covered with gate electrodes having different functions. By making the thicknesses of the silicon oxide film and silicon nitride film that make up the insulating film different, it is possible to suppress adverse effects when reading signal charges from the light receiving unit, and transfer the read signal charges It is possible to suppress adverse effects when doing so.

  In the present embodiment, a portion of the gate insulating film disposed so as to be covered from the first gate electrode 15 to the second gate electrode 18 is covered with the portion covered with the first gate electrode 15 and the second gate electrode 18. In contrast, the thickness t14 of the silicon oxide film 14 in the portion covered with the first gate electrode 15 is smaller than the thickness t16 of the silicon oxide film 16 in the portion covered with the second gate electrode 18. On the other hand, the thickness t13a of the silicon nitride film 13 in the portion covered by the first gate electrode 15 is larger than the thickness t13b of the silicon nitride film 13 in the portion covered by the second gate electrode 18. Since the arranged configuration is adopted, the capacitance between the first gate electrode 15 and the semiconductor substrate 1 and the capacitance between the second gate electrode 18 and the semiconductor substrate 1 are adopted. It can be made substantially equal to.

Therefore, in the present embodiment, the potential of the portion covered with the first gate electrode and the potential of the portion covered with the second gate electrode in the gate insulating film can be made substantially equal, and the transfer efficiency is reduced. Can be suppressed.
(Embodiment 2)
Hereinafter, a manufacturing method of the IT-CCD type solid-state imaging device according to the second embodiment of the present invention will be described with reference to the drawings.

3 and 4 are schematic process diagrams illustrating a method for manufacturing the IT-CCD type solid-state imaging device according to the second embodiment.
In the manufacturing method of the IT-CCD type solid-state imaging device according to the present embodiment, there is a feature in the method of depositing the gate insulating film, and other configurations are manufactured in the same process as the conventional one. The description of other configurations is omitted.

  First, a silicon oxide film 12 is deposited by, for example, thermal oxidation on the main surface of the semiconductor substrate 1 on which the signal transfer path is formed, and a silicon nitride film 13 is deposited on the main surface of the silicon oxide film 12 by, for example, a CVD method. A silicon oxide film 14 is deposited on the main surface of the nitride film 13 by, for example, a CVD method or formed by directly oxidizing the silicon nitride film 13 by a pyro-oxidation method, and the main surface of the silicon oxide film 14 is formed by, for example, a CVD method. Thus, a first polysilicon film 505 is deposited (see FIG. 3A).

Then, a predetermined portion of the first polysilicon film 505 is selectively removed by dry etching to form the first gate electrode 15. At this time, the silicon oxide film 14 and a part of the silicon nitride film 13 are also removed (see FIG. 3B).
Through the deposition / removal process, the silicon oxide film 14 and the first gate electrode 15 can be stacked in this order on the silicon nitride film 13.

In the present embodiment, the first gate electrode 15 is arranged as an electrode dedicated for signal charge transfer.
Next, by burning hydrogen and oxygen to produce of H _{2 O} in the known diffusion furnace kept at normal pressure in the same diffusion furnace by a so-called pyro-oxidation method to perform oxidation processing using the H _{2 O} The surface exposed from the first gate electrode 15 to the silicon nitride film 13 and the silicon oxide film 14 is subjected to oxidation treatment, and the surface of the first gate electrode 15 made of polysilicon has a silicon of about 20 [nm], for example. An oxide film 17 is deposited, and a silicon oxide film 16 of about several nm is deposited on the surface of the silicon nitride film 13 (see FIG. 3C). When the deposited silicon oxide film 16 and the silicon oxide film 17 are compared, the difference in film thickness occurs because the silicon nitride film 13 is less likely to be oxidized than the first gate electrode 15 made of polysilicon. is there.

  Subsequent to the pyro-oxidation process, the silicon oxide film 17 is formed by performing an oxidation process using an oxidation method including radical oxygen (for example, plasma oxidation, ISSG (In-Situ Steam Generator) or the like). While the silicon oxide film 16 is grown until the thickness reaches about 40 [nm], the silicon oxide film 16 provided so as to cover the silicon nitride film 13 is approximately ten times corresponding to about 80% of the increase in the thickness of the silicon oxide film 17. [Nm] is grown (see FIG. 4A).

  Since an oxidation method including radical oxygen is used in the oxidation step, the oxidation rate of the first gate electrode 15 and the oxidation rate of the silicon nitride film 13 can be brought close to each other, as described in the first embodiment. In addition, the film thickness may be adjusted so that the capacitance between the first gate electrode 15 and the semiconductor substrate 1 and the capacitance between the second gate electrode 18 and the semiconductor substrate 1 are substantially equal. it can.

  In the oxidation step, a pyro-oxidation method is combined with the oxidation method containing radical oxygen. When an oxide film is formed by the pyro-oxidation method, an oxide film having a low defect density can be formed with the same thickness as an oxide film formed by a rapid thermal oxidation (RTO) method or a dry oxidation method. The dielectric breakdown of the insulating film over time can be suppressed.

Next, a second polysilicon film 508 is deposited on the surface of the silicon oxide film 16 to the silicon oxide film 17 by, eg, CVD (see FIG. 4B), and the second polysilicon film 508 is formed. A predetermined portion is selectively removed by dry etching to form a second gate electrode 18 having a function of reading and transferring signal charges (see FIG. 4C).
When the gate insulating film is formed by laminating the silicon nitride film 13 between the silicon oxide film 12 and the silicon oxide films 14 and 16 through the above manufacturing process, the first gate electrode of the gate insulating film is formed. When the portion covered with 15 and the portion covered with the second gate electrode 18 are compared, the film thickness t14 of the silicon oxide film 14 in the portion covered with the first gate electrode 15 covers the second gate electrode 18. The silicon oxide films 14 and 16 can be deposited so as to be smaller than the film thickness t16 of the silicon oxide film 16 in the broken portion, and the film thickness of the silicon nitride film 13 in the portion covered with the first gate electrode 15 The silicon nitride film 13 is deposited so that t13a is larger than the film thickness t13b of the silicon nitride film 13 in the portion covered with the second gate electrode 18. It is possible.

  In the present embodiment, the deposited silicon nitride film 13 is oxidized after removing a part thereof by, for example, dry etching. However, the present invention is not limited to this, and a radical can be formed without removing a part of the nitride film 13. An oxidation process may be performed on the surface exposed from the first gate electrode 15 to the silicon nitride film 13 and the silicon oxide film 14 using an oxidation method including oxygen. By adopting an oxidation method including radical oxygen, the oxidation rate of the first gate electrode 15 mainly composed of polysilicon and the oxidation rate of the nitride film 13 can be made substantially equal. The first gate electrode 15 mainly composed of polysilicon is nitrided when the surface exposed from the first gate electrode 15 to the silicon nitride film 13 and the silicon oxide film 14 is oxidized without removing the first gate electrode 15. Excessive oxidation as compared with the film 13 can be suppressed, and the oxide films 16 and 17 are deposited on the exposed surface of the first gate electrode 15 and the exposed surface of the nitride film 13 so as to have a desired thickness. Can be made.

<< Effect of Manufacturing Method of IT-CCD Solid-State Imaging Device According to Embodiment 2 >>
In the manufacturing method of the IT-CCD type solid-state imaging device according to the present embodiment, the second gate electrode compared to the film thickness t13a of the silicon nitride film 13 in the portion covered with the first gate electrode 15 in the gate insulating film. Since the gate insulating film can be deposited so that the film thickness t13b of the silicon nitride film 13 in the portion covered with 18 is reduced, the silicon nitride film in the portion covered with the second gate electrode 18 in the gate insulating film 13 can be deposited in a thinning tendency. Therefore, in the portion covered with the second gate electrode 18 having the charge transfer function and the charge reading function from the light receiving portion, the film thickness t13b of the silicon nitride film 13 is The conventional vertical CCD may be deposited so as to be smaller than the thickness of the portion of the silicon nitride film covered with the signal charge readout electrode. Come, it is possible to manufacture the IT-CCD type solid-state imaging device having a vertical CCD capable of suppressing the electrons are trapped in the silicon nitride film 13 by hot electron effects in reading out the signal charge from the light receiving unit.

  Therefore, the manufacturing method of the IT-CCD type solid-state imaging device according to the present embodiment includes a vertical CCD that can suppress the adverse effect of being difficult to read at a specified read voltage every time signal charges are read from the light receiving unit. An IT-CCD solid-state image pickup device can be manufactured, and a CCD solid-state image pickup device including a vertical CCD capable of suppressing deterioration of image quality over time even when a conventional readout voltage is maintained can be manufactured. Further, in the manufacturing method of the CCD solid-state imaging device according to the present embodiment, a CCD type solid-state imaging device capable of suppressing the above-described adverse effects can be manufactured. Therefore, in order to suppress deterioration of image quality over time, a readout voltage is set. A CCD type solid-state imaging device having a vertical CCD capable of suppressing an increase in white scratches can be manufactured without the need to raise it.

  On the other hand, in the manufacturing method of the CCD type solid-state imaging device according to the present embodiment, the first gate electrode is compared with the film thickness t16 of the silicon oxide film 16 in the portion covered with the second gate electrode 18 in the gate insulating film. Since the gate insulating film can be deposited so that the film thickness t14 of the silicon oxide film 14 in the portion covered with 15 is reduced, the silicon oxide film 14 in the portion covered with the first gate electrode in the gate insulating film. Therefore, in the portion covered with the first gate electrode 15 having only the transfer function, the film thickness t14 of the silicon oxide film 14 is the transfer rate of the second conventional vertical CCD. The first gate electrode 15 and the second gate electrode 18 are electrically connected to each other so as to be smaller than the thickness of the silicon oxide film covered by the electrode. When the interlayer insulating film for isolation is formed, the first gate electrode 15 can be prevented from being warped, so that the silicon oxide film 14 can be formed as compared with the second conventional vertical CCD. It can suppress that the area | region with a uniform film thickness t14 reduces and the area of an effective charge transfer part reduces.

Therefore, in the manufacturing method of the IT-CCD type solid-state imaging device according to the present embodiment, the amount of charge handled when transferring charges in the vertical CCD is reduced as compared with the second conventional IT-CCD type solid-state imaging device. It is possible to manufacture an IT-CCD type solid-state imaging device including a vertical CCD that can suppress image quality deterioration and image quality deterioration.
That is, the manufacturing method of the IT-CCD type solid-state imaging device according to the present embodiment is covered with the gate electrode having a different function in the gate insulating film as compared with the manufacturing method of the conventional IT-CCD type solid-state imaging device. Since the gate insulating film can be formed so that the thicknesses of the silicon oxide film and the silicon nitride film constituting the gate insulating film are different between the portions, the adverse effect of reading the signal charge from the light receiving portion It is possible to manufacture an IT-CCD type solid-state imaging device including a vertical CCD that can suppress the adverse effect of transferring the read signal charges.

  In the present embodiment, when the gate insulating film is stacked, when the portion of the gate insulating film that is to be covered by the first gate electrode 15 is compared with the portion that is to be covered by the second gate electrode 18, While the film thickness t14 of the silicon oxide film 14 in the portion to be covered by the first gate electrode 15 is deposited so as to be smaller than the film thickness t16 of the silicon oxide film 16 in the portion to be covered by the second gate electrode 18, Since the film thickness t13a of the silicon nitride film 13 in the portion to be covered with the first gate electrode 15 is deposited so as to be larger than the film thickness t13b of the silicon nitride film 13 in the portion to be covered with the second gate electrode 18, The capacitance between the first gate electrode 15 and the semiconductor substrate 1 and the capacitance between the second gate electrode 18 and the semiconductor substrate 1 are substantially equal. It is possible to manufacture the IT-CCD type solid-state imaging device including a possible vertical CCD.

Therefore, in the present embodiment, the potential of the portion covered by the first gate electrode 15 and the potential of the portion covered by the second gate electrode 18 in the gate insulating film can be made substantially equal, and the transfer efficiency can be improved. An IT-CCD type solid-state imaging device including a vertical CCD capable of suppressing the decrease can be manufactured.
<Others>
FIG. 5 is a cross-sectional view of the main part showing another variation of part of the vertical CCD in the IT-CCD solid-state imaging device according to the first embodiment.

In this variation, the thickness t37 of the interlayer insulating film 37 is larger than the thickness t36 of the silicon oxide film 36, as shown in FIG.
In general, the difference between the drive voltage applied to the first gate electrode 35 and the drive voltage applied to the second gate electrode 38 is 10 [V] or more, and by adopting the above configuration to ensure a withstand voltage. In the interlayer insulating film 37, the withstand voltage capability can be improved, and the function of suppressing a short circuit between the first gate electrode 35 and the second gate electrode 38 during driving can be improved.

FIG. 6 is a cross-sectional view of the main part showing another variation of part of the vertical CCD in the IT-CCD solid-state imaging device according to the first embodiment. As shown in FIG. 6, the second gate electrode 48 may be disposed so as to cover the first gate electrode 45.
FIG. 7 is a cross-sectional view of the main part showing another variation of part of the vertical CCD in the IT-CCD solid-state imaging device according to the first embodiment. In this variation, as shown in FIG. 7, no silicon oxide film is disposed between the first gate electrode 55 and the silicon nitride film 53 in the gate insulating film.

  When this configuration is employed, the first gate electrode 55 is excessively oxidized when the interlayer insulating film 57 that insulates the first gate electrode 55 and the second gate electrode 58 is deposited. 55 can be further suppressed from being warped, so that the thickness of the gate insulating film in the portion covered with the first gate electrode 55 becomes more uniform, and the effective area of the charge transfer portion is reduced. The reduction can be further suppressed, and the decrease in the amount of charge handled when transferring charges in the vertical CCD can be further suppressed. As a result, the deterioration of image quality can be further suppressed, which is preferable.

In the IT-CCD solid-state imaging device according to Embodiment 1, the gate electrode has a single-layer structure, but the structure of the gate electrode is not limited to this. For example, the gate electrode is arranged so as to cover the gate insulating film. The formed gate electrode may have a multi-layer structure in which a silicon oxide film is sandwiched between polysilicon.
In the manufacturing method of the IT-CCD type solid-state imaging device according to the second embodiment, the silicon oxide film 16 and the silicon oxide film 17 are formed using a pyro-oxidation method and an oxidation method containing radical oxygen. Only an oxidation method containing radical oxygen may be used.

  In the manufacturing method of the IT-CCD type solid-state imaging device according to the second embodiment, when the silicon oxide film 16 and the silicon oxide film 17 are formed, an oxidation method including radical oxygen is performed after the pyrooxidation method is performed. However, these oxidation methods may be performed by changing the order, that is, the pyro-oxidation method may be performed after the oxidation method including radical oxygen is performed first.

  Further, the solid-state imaging device and the manufacturing method thereof according to each of the above embodiments have been described as an IT-CCD solid-state imaging device, but the solid-state imaging device is a CCD solid-state imaging device of another type. May be.

  The solid-state imaging device according to the present invention can suppress a decrease in the amount of charge handled, improve transfer efficiency, and reduce the increase in the read voltage, thereby extending the lifetime or having the same lifetime, It can also be applied to uses such as camera devices that require reduction.

1 is a schematic configuration diagram of a solid-state imaging device according to a first embodiment of the present invention. 1 is a cross-sectional view of a main part showing a part of a vertical CCD in a solid-state imaging device according to a first embodiment of the present invention. It is a schematic process drawing which shows the manufacturing method of the solid-state imaging device which concerns on the 2nd Embodiment of this invention. It is a schematic process drawing which shows the manufacturing method of the solid-state imaging device which concerns on the 2nd Embodiment of this invention. It is principal part sectional drawing which showed the other variation of a part of vertical CCD in the solid-state imaging device which concerns on the 1st Embodiment of this invention. It is principal part sectional drawing which showed the other variation of a part of vertical CCD in the solid-state imaging device which concerns on the 1st Embodiment of this invention. It is principal part sectional drawing which showed the other variation of a part of vertical CCD in the solid-state imaging device which concerns on the 1st Embodiment of this invention. It is principal part sectional drawing which showed a part of vertical CCD in the 1st conventional solid-state imaging device. It is the characteristic view which showed the relationship between the increase amount of an electric charge read-out voltage, and use time in the 1st conventional solid-state imaging device. FIG. 6 is a characteristic diagram showing a relationship between an increase in charge read voltage and a film thickness of a silicon nitride film in the first conventional solid-state imaging device. (A) is the schematic block diagram which showed one pixel in the 2nd conventional solid-state imaging device, (b) is principal part sectional drawing which looked at the A cross section shown by (a). (A) is the schematic block diagram which showed one pixel in the 2nd conventional solid-state imaging device, (b) is principal part sectional drawing which looked at the B cross section shown by (a).

Explanation of symbols

1, 31, 41, 51 Semiconductor substrate 2 Light receiving part 3 Vertical CCD
4 Horizontal CCD
5 Amplifier 12, 32, 42, 52 Silicon oxide film 13, 33, 43, 53 Silicon nitride film 15, 35, 45, 55 First gate electrode 17, 37, 47, 57 Interlayer insulating film 18, 38, 48, 58 Second gate electrode

Claims (8)

A first insulating film is stacked on one main surface of the semiconductor substrate, and a first gate electrode and a second gate electrode are provided in parallel on the first insulating film, and the first gate electrode is disposed between the first gate electrode and the second gate electrode. A charge transfer device having a structure in which a second insulating film is interposed between
The first insulating film has a configuration in which an oxide film is stacked on a nitride film,
In the oxide film, a first thickness of a portion interposed between the nitride film and the first gate electrode, and a second thickness of a portion interposed between the nitride film and the second gate electrode. And in the nitride film, the third thickness of the portion immediately below the first gate electrode is different from the fourth thickness of the portion immediately below the second gate electrode. .   2. The charge transfer device according to claim 1, wherein the first thickness is smaller than the second thickness, and the third thickness is larger than the fourth thickness.   The first thickness to the first gate electrode so that a capacitance between the first gate electrode and the semiconductor substrate is substantially equal to a capacitance between the second gate electrode and the semiconductor substrate. The charge transfer device according to claim 2, wherein a fourth thickness is set. A charge transfer path is formed on the substrate corresponding to the position of the first gate electrode and the second gate electrode, and a photoelectric conversion element is provided so as to be adjacent to the charge transfer path. ,
The second gate electrode has a function of reading charges from the element to the charge transfer path, and the first gate electrode does not have a function of reading charges from the element to the charge transfer path. A solid-state imaging device comprising the charge transfer device according to Item 3. The second gate electrode has a function of reading the charge and a function of transferring the charge along the charge transfer path,
The solid-state imaging device according to claim 4, wherein the first gate electrode has a function of transferring the charge along the charge transfer path.   Forming a nitride film on one main surface of the semiconductor substrate; and arranging the first oxide film and the first gate electrode in this order on the nitride film so as to be scattered in one axial direction along the main surface. Forming the second oxide film by only oxidizing the exposed surface using oxygen radicals from the exposed surface of the first gate electrode to the exposed surface of the nitride film; Forming at least a portion on the nitride film to be thicker than a thickness of the first oxide film; and a second surface on the exposed surface of the second oxide film so as to be adjacent to the first gate electrode. Forming a gate electrode in the one-axis direction.   Forming a nitride film on one main surface of the semiconductor substrate; and stacking a first oxide film and a first gate electrode in this order on the nitride film so as to be scattered in a uniaxial direction along the main surface A step of forming the exposed surface of the nitride film from the exposed surface of the nitride film to the exposed surface of the first gate electrode using an oxidation method using oxygen radicals, and then oxidizing the exposed surface using a pyro-oxidation method. Or forming the second oxide film by oxidizing in the reverse order so that the thickness of at least the portion on the nitride film is larger than the thickness of the first oxide film; Forming a second gate electrode on the exposed surface of the second oxide film in the one-axis direction so as to be adjacent to the one gate electrode.   A method for manufacturing a solid-state imaging device, comprising the method for manufacturing a charge transfer device according to claim 6.

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