JP2008078490A - Ccd image sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a CCD image sensor capable of improving efficiency of transfer of charges by reducing generation of crystal defect. <P>SOLUTION: In the CCD image sensor in which a large number of electrodes 30, 40 for charge transfer are disposed in an n well via an insulating film 20, the electrode 30 has a structure in which a p-type polysilicon 31 and an n-type polysilicon 32 are sequentially disposed toward a charge transfer direction, and the electrode 40 has a structure in which a p-type polysilicon 41 and an n-type polysilicon 42 are sequentially disposed toward a charge transfer direction. In such a configuration, a potential gradient due to a difference in work function between the p- and n-type polysilicons can be generated on an n-well 12 immediately below the electrodes 30, 40 without implanting ions onto the surface of a semiconductor substrate and the potential gradient matches the charge transfer direction, the charge transfer efficiency can be improved. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a CCD image sensor.

As this type of conventional technique, there is one disclosed in Patent Document 1, for example. That is, this Patent Document 1 relates to a charge transfer device having a structure in which a large number of MOS structure electrodes are arranged on the surface of a semiconductor substrate, and two or more kinds of impurity concentrations differing from those of a semiconductor substrate directly below an electrode of a potential barrier portion. It has an impurity region. With such a configuration, a potential gradient due to a difference in impurity concentration is generated in the semiconductor substrate of the potential barrier portion, so that the charge transfer efficiency (that is, transfer rate) can be improved using the potential gradient. .
JP-A-3-283629

  By the way, when the impurity region as described above is formed in the semiconductor substrate, the impurity is usually implanted near the surface of the substrate by using an ion implantation technique. Here, the ion implantation technique has a problem that crystal defects are generated on the surface of the semiconductor substrate when the impurities are implanted. If a crystal defect remains on the surface of the semiconductor substrate, the charge generated through the crystal defect is added to the transferred charge, that is, a noise component is added to the signal component, which may deteriorate the charge transfer characteristics.

Even if crystal defects occur due to ion implantation, the crystal defects can be eliminated by heating the semiconductor substrate at a sufficiently high temperature for a long time after that. However, in a fine process that is currently under intense research and development, the heating temperature tends to be lowered and the heating time tends to be shortened. In other words, heating at a sufficiently high temperature for a long time deviates from the basic concept of “suppressing impurity diffusion” in a fine process, so that the degree of freedom in design tends to be low. Therefore, even if it is desired to heat at a sufficiently high temperature for a long time, it is not allowed to perform the treatment from the viewpoint of design, and sometimes it is necessary to give up complete crystallinity recovery.
Therefore, the present invention has been made in view of such circumstances, and an object of the present invention is to provide a CCD image sensor capable of reducing the generation of crystal defects and improving the charge transfer efficiency.

[Invention 1] In order to solve the above problems, a CCD image sensor according to Invention 1 is a CCD image sensor in which a large number of charge transfer electrodes are arranged on the surface of a semiconductor substrate via an insulating film, The portion in contact with the insulating film has a structure in which a plurality of types of conductive layers having different work functions are sequentially arranged in the charge transfer direction. Here, examples of the “plurality of conductive layers having different work functions” of the present invention include a semiconductor layer having a conductivity type of P or N, a metal layer, and the like.

According to the CCD image sensor of the first aspect, a potential gradient is generated on the surface of the semiconductor substrate immediately below the electrode due to the work function difference, and the transferred charge is attracted toward the charge transfer direction directly below the electrode by this potential gradient. Such a potential gradient is maintained regardless of whether or not voltage is applied to the electrodes. Therefore, the charge transfer efficiency can be increased.
Compared with the prior art, the potential gradient is formed without ion implantation of impurities into the semiconductor substrate, so that an ion implantation process for the purpose of forming the potential gradient is not necessarily required. For this reason, generation | occurrence | production of the crystal defect in a semiconductor substrate can be decreased.

[Invention 2] The CCD image sensor of Invention 2 is a CCD image sensor in which a large number of charge transfer electrodes are arranged on the surface of a first conductivity type semiconductor substrate through an insulating film, The contact portion has a structure in which a second conductive type first semiconductor layer and a first conductive type second semiconductor layer are sequentially arranged in the charge transfer direction. .

With such a configuration, a potential gradient is generated on the surface of the semiconductor substrate immediately below the electrode due to a work function difference between the first semiconductor layer and the second semiconductor layer, and the transferred charge is transferred to the first semiconductor layer by the potential gradient. It is attracted from directly under the second semiconductor layer. Further, the direction in which transferred charges are attracted by this potential gradient coincides with the charge transfer direction, and this potential gradient is maintained regardless of whether or not voltage is applied to the electrodes. Therefore, the charge transfer efficiency can be increased.
Compared with the prior art, the potential gradient is formed without ion implantation of impurities into the semiconductor substrate, so that an ion implantation process for the purpose of forming the potential gradient is not necessarily required. For this reason, generation | occurrence | production of the crystal defect in a semiconductor substrate can be decreased.

[Invention 3] The CCD image sensor of Invention 3 is a CCD image sensor in which a large number of charge transfer electrodes are arranged on the surface of an N-type semiconductor substrate via an insulating film, and a portion of the electrode in contact with the insulating film Has a structure in which P-type polysilicon and N-type polysilicon are sequentially arranged in the charge transfer direction.
Here, the work function (WF) of P-type polysilicon is about 5.25 eV, and the work function (WF) of N-type polysilicon is about 4.17 eV.

According to the CCD image sensor of the invention 3, a potential gradient is generated on the surface of the semiconductor substrate immediately below the electrode due to the work function difference between the P-type polysilicon and the N-type polysilicon, and the electron e− immediately below the P-type polysilicon is It is drawn directly under N-type polysilicon. The direction in which the electrons e− are attracted by this potential gradient coincides with the charge transfer direction, and this potential gradient is maintained both when the voltage is applied to the electrode and when it is not. Therefore, the charge transfer efficiency can be increased.
Compared with the prior art, the potential gradient is formed without ion implantation of impurities into the semiconductor substrate, so that an ion implantation process for the purpose of forming the potential gradient is not necessarily required. For this reason, generation | occurrence | production of the crystal defect in a semiconductor substrate can be decreased.

[Invention 4] The CCD image sensor according to Invention 4 is the CCD image sensor according to Invention 3, wherein a plurality of P-type impurity layers are provided on the surface of the semiconductor substrate immediately below the electrodes at regular intervals along the charge transfer direction. Is further provided.
With such a configuration, the P-type impurity layer functions as a potential barrier, and the potential of the channel region immediately below the P-type impurity layer is lowered, so that a potential gradient is formed between adjacent regions, or The potential gradient can be made steeper.

  In addition, when forming a P-type impurity layer using an ion implantation technique, there exists a possibility that a crystal defect may generate | occur | produce in a semiconductor substrate. However, in the CCD image sensor of the invention 4, P-type polysilicon forming a part of an electrode is disposed immediately above the P-type impurity layer, and this P-type polysilicon is also used to form a potential gradient in the same manner as the P-type impurity layer. Contribute. Therefore, if the same charge transfer efficiency is required as compared with the conventional technique (for example, when the electrodes on the P-type impurity layer are all made of N-type polysilicon), the concentration of the P-type impurity layer can be reduced. In addition, since the dose at the time of forming the P-type impurity layer can be reduced, the occurrence of crystal defects can be reduced.

[Invention 5] The CCD image sensor of Invention 5 is the CCD image sensor of Invention 3 or Invention 4, wherein the P-type polysilicon includes P + polysilicon having a high concentration of P-type impurities and P− having a low concentration of P-type impurities. The N-type polysilicon includes N + polysilicon having a high N-type impurity concentration and N-polysilicon having a low N-type impurity concentration, and is in contact with the insulating film of the electrode. The portion has a structure in which the P + polysilicon, the P-polysilicon, the N-polysilicon, and the N + polysilicon are sequentially arranged in the charge transfer direction. It is a feature.
With such a configuration, a potential gradient is formed in steps from directly under P + polysilicon to immediately under N + polysilicon. For example, electrons e− existing immediately under P + polysilicon are directly below N + polysilicon. Can be guided smoothly.

  According to the present invention, in the CCD image sensor, the occurrence of crystal defects can be reduced and the charge transfer efficiency can be increased.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
(1) First Embodiment FIGS. 1A to 1C are a cross-sectional view showing a configuration example of a charge transfer region (that is, a shift register) of a CCD image sensor 100 according to a first embodiment, and these cross-sectional views. Is a potential diagram corresponding to. FIG. 2 is a diagram illustrating an example of the timing of the voltage signals φ1 and φ2. 1B shows the potential at time t1 in FIG. 2, and FIG. 1C shows the potential at time t2 in FIG.

  As shown in FIG. 1A, the CCD image sensor 100 includes a P-type silicon substrate 11, an N-type well (that is, an N-well) 12 formed on the silicon substrate 11, and the N-well 12 And a plurality of electrodes 30 and 40 for charge transfer formed on the insulating film 20. As shown in FIG. 1A, in this CCD image sensor 100, two types of electrodes 30 and 40 having different structures in cross-sectional view (that is, cross-sectional structures) are alternately arranged along the charge transfer direction. For example, the voltage signal φ1 is input to the electrode 30, and the voltage signal φ2 is input to the other electrode 40.

  A type of CCD that transfers charges with two-phase signals φ1 and φ2 is also called a two-phase drive CCD. As shown in FIGS. 1A to 1C, in the CCD image sensor 100, by applying different voltage signals φ1 and φ2 to the electrodes 30 and 40, respectively, the N well 12 directly below the electrodes 30 and 40 has a depth. Are created, and are used to hold charges (in this example, electrons e-). Further, as shown in FIG. 2, the voltage signals φ1 and φ2 applied to the electrodes 30 and 40 are switched on (ON) and off (OFF) at regular intervals, thereby directly below the electrode 30 (or the electrode 40). The charges held in the N well 12 are transferred directly below the electrode 40 (or electrode 30) adjacent to the charge transfer direction side.

  In the CCD image sensor 100, the electrode 30 is composed of P-type polysilicon 31 and N-type polysilicon 32, and the P-type polysilicon 31 and N-type polysilicon 32 are sequentially directed toward the charge transfer direction. It has an arranged structure. That is, the P-type polysilicon 31 is arranged upstream of the charge transfer region, and the N-type polysilicon 32 is arranged downstream thereof. Similarly, the electrode 40 is also composed of P-type polysilicon 41 and N-type polysilicon 42, and the P-type polysilicon 41 and N-type polysilicon 42 are sequentially arranged in the charge transfer direction. It has a structure.

  Here, the work function (WF) of P-type polysilicon is about 5.25 eV, and the work function (WF) of N-type polysilicon is about 4.17 eV. As shown in FIG. 3, the potential of the surface of the semiconductor substrate immediately below the P-type polysilicon is lower than the potential of the surface of the semiconductor substrate immediately below the N-type polysilicon, and a potential gradient is formed centering directly below the PN junction. Then, due to this potential gradient, the electrons e− are attracted from directly under the P-type polysilicon to directly under the N-type polysilicon.

  Referring back to FIGS. 1A to 1C, in the CCD image sensor 100, the potential gradient due to the work function difference between the P-type polysilicon 31 and the N-type polysilicon 32 on the surface of the N well 12 immediately below the electrode 30. The electrons e− immediately below the P-type polysilicon 31 are attracted directly below the N-type polysilicon 32. Similarly, a potential gradient is also generated on the surface of the N well 12 immediately below the electrode 40 due to the work function difference between the P-type polysilicon 41 and the N-type polysilicon 42, and the electrons e− immediately below the P-type polysilicon 41 It is drawn directly below the N-type polysilicon 42. The direction in which electrons e− are attracted by such a potential gradient coincides with the charge transfer direction in both electrodes 30 and 40.

  Further, as shown in FIGS. 1B and 1C, such a potential gradient (that is, a state in which the potential immediately below the P-type polysilicon is lower than the potential immediately below the N-type polysilicon) is on / off of φ1 and φ2. Therefore, the transfer of electrons e− in the charge transfer direction can be boosted by the potential gradient.

Next, a manufacturing method of the CCD image sensor 100 shown in FIG.
4A to 5C are process diagrams showing a method for manufacturing the CCD image sensor 100. FIG.
As shown in FIG. 4A, first, an N-type impurity is partially introduced into a P-type silicon substrate 11 using a photolithography technique and an ion implantation technique to form an N-well 12. Next, an insulating film 20 is formed on the N well 12. The insulating film 20 is, for example, a SiO ₂ film, a Si ₃ N ₄ film, a SiON film, or a high-k film. In the case of forming the SiO ₂ film as the insulating film 20, for example, a silicon substrate 11 to form a SiO ₂ film by thermal oxidation.

  Next, as shown in FIG. 4B, for example, a non-doped polysilicon film 21 is formed on the insulating film 20. The polysilicon film 21 is formed by CVD, for example. Then, a resist pattern 22 is formed on the polysilicon film 21 so as to open above the region where the N-type polysilicon is to be formed and to cover the other regions. The resist pattern 22 is used as a mask on the polysilicon film 21. N-type impurities are ion-implanted. As a result, N-type polysilicon 42 is formed in the polysilicon film 21 as shown in FIG. After the N-type polysilicon 42 is formed, the resist pattern 22 is removed by ashing, for example.

  Next, as shown in FIG. 4C, a resist pattern 23 is formed on the polysilicon film 21 so as to open above the region where the P-type polysilicon is formed and cover the other region. P-type impurities are ion-implanted into the polysilicon film 21 using the resist pattern 23 as a mask. As a result, P-type polysilicon 41 is formed in the polysilicon film 21 as shown in FIG. After the P-type polysilicon 41 is formed, the resist pattern 23 is removed by ashing, for example.

  Next, as shown in FIG. 5A, a resist pattern 24 is formed on the polysilicon film 21 so as to cover the region where the electrode 40 is to be formed and to expose the region above the other region. For example, the polysilicon film 21 is dry-etched using the mask 24 as a mask. As a result, as shown in FIG. 5A, an electrode 40 made of P-type polysilicon 41 and N-type polysilicon 42 is formed on the insulating film 20. After the electrode 40 is formed, the resist pattern 24 is removed by ashing, for example.

Next, the insulating film 25 is formed on the surface of the silicon substrate 11 exposed from below the electrode 40 and the surface of the electrode 40. The insulating film 25 is an SiO ₂ film, for example, and is formed by, for example, thermally oxidizing the silicon substrate 11. Then, as shown in FIG. 5B, for example, a non-doped polysilicon film 26 is formed on the entire upper surface of the silicon substrate 11. The polysilicon film 26 is formed by CVD, for example.

  Next, N-type impurities are partially introduced into the polysilicon film 26 by using a photolithography technique and an ion implantation technique, and the N-type polysilicon 32 is added to the polysilicon film 26 as shown in FIG. Form. Subsequently, P-type impurities are partially introduced into the polysilicon film 26 by using a photolithography technique and an ion implantation technique, and as shown in FIG. 5C, P-type polysilicon 31 is added to the polysilicon film 26. Form. Thereafter, the polysilicon film 26 is patterned by using a photolithography technique and an etching technique. Thereby, the electrode 30 composed of the P-type polysilicon 31 and the N-type polysilicon 32 shown in FIG. 1A is formed.

  As described above, according to the first embodiment of the present invention, potential gradients are generated on the surface of the N-well 12 immediately below the electrodes 30 and 40 due to the work function difference between the P-type polysilicon and the N-type polysilicon. Then, due to this potential gradient, the electrons e− immediately below the P-type polysilicon are attracted directly below the N-type polysilicon, the direction in which the electrons e− are attracted coincides with the charge transfer direction, and the potential gradient is φ1 , Φ2 is maintained regardless of whether it is turned on or off (that is, regardless of whether or not voltage is applied to the electrodes 30 and 40), so that the charge transfer efficiency can be increased.

Compared with the prior art, a potential gradient is formed without ion implantation of impurities into the silicon substrate 11, so that an ion implantation process for the purpose of forming a potential gradient is not necessarily required. For this reason, the generation of crystal defects in the silicon substrate 11 can be reduced.
In the first embodiment, the N well 12 corresponds to the “surface of the semiconductor substrate” of the present invention, and the P-type polysilicons 31 and 41 and the N-type polysilicons 32 and 42 have the “work functions different from each other”. It corresponds to “multiple types of conductive layers”. The P-type polysilicons 31 and 41 correspond to the “second conductive type first semiconductor layer” of the present invention, and the N-type polysilicons 32 and 42 correspond to the “first conductive type second semiconductor layer” of the present invention. It corresponds to.

(2) Second Embodiment FIGS. 6A to 6C are a cross-sectional view showing a configuration example of a charge transfer region (that is, a shift register) of a CCD image sensor 200 according to a second embodiment, and these cross-sectional views. Is a potential diagram corresponding to. FIG. 6B shows the potential at time t1 in FIG. 2, and FIG. 6C shows the potential at time t2 in FIG. 6A, components having the same structure as in FIG. 1A are denoted by the same reference numerals, and detailed description thereof is omitted.

  As shown in FIG. 6A, the CCD image sensor 200 includes a P-type silicon substrate 11, an N well 12 formed on the silicon substrate 11, and an insulating film formed on the N well 12. 20 and a plurality of charge transfer electrodes 30 and 140 formed on the insulating film 20. As shown in FIG. 1A, in this CCD image sensor 200, two types of electrodes 30 and 140 having different cross-sectional structures are alternately arranged along the charge transfer direction.

  Here, the electrode 140 is composed only of N-type polysilicon. Further, for example, when an integer number is given to the electrodes 30 arranged along the charge transfer direction, the voltage signal φ1 is applied to an electrode with an odd number, and the electrode with an even number is an electrode with an even number. A voltage signal φ2 is applied thereto. That is, the voltage signal φ1 is applied to every other electrode 30 arranged along the charge transfer direction, and the voltage signal φ2 is applied to the remaining electrodes 30. ing. Further, the voltage signal φ1 is applied to the electrode 140 adjacent to the downstream side of the electrode 30 to which the voltage signal φ1 is applied, and the voltage signal φ2 is applied to the electrode 140 adjacent to the downstream side of the electrode 30 to which the voltage signal φ2 is applied. Is applied. Further, a P-type impurity layer 150 is formed in the N well 12 immediately below the electrode 30.

  Even with such a configuration, a potential gradient is generated in the N well 12 immediately below the electrode 30 due to the work function difference between the P-type polysilicon 31 and the N-type polysilicon 32, and the electrons immediately below the P-type polysilicon 31. e− is drawn directly below the N-type polysilicon 32. The direction in which the electrons e− are attracted coincides with the charge transfer direction, and the potential gradient is maintained regardless of whether or not a voltage is applied to the electrodes, so that the charge transfer efficiency can be increased.

  Further, the P-type impurity layer 150 functions as a potential barrier, and the potential of the N well 12 immediately below the P-type impurity layer 150 can be lowered. For this reason, a potential gradient is also formed immediately below the electrode 140 with the same conductivity type and the adjacent N-type polysilicon 32 to which the same signal φ1 (or φ2) is applied. The transfer of electrons e− from directly under the electrode 140 can be boosted by the potential gradient caused by the P-type impurity layer 150.

Next, a method for manufacturing the CCD image sensor 200 will be described.
FIG. 7A to FIG. 8C are process diagrams showing a method for manufacturing the CCD image sensor 200.
As shown in FIG. 7A, first, an N-type impurity is partially introduced into a P-type silicon substrate 11 using a photolithography technique and an ion implantation technique to form an N-well 12. Next, a P-type impurity layer 150 is formed by partially introducing a P-type impurity into the N well 12 using a photolithography technique and an ion implantation technique. Then, the insulating film 20 is formed on the N well 12 in which the P-type impurity layer 150 is formed. Subsequently, a polysilicon film having an N type conductivity, for example, is formed on the insulating film 20. The introduction of the N-type impurity into the polysilicon film may be performed in-situ at the time of film formation, or may be performed by ion implantation of the N-type impurity after forming the non-doped polysilicon film.

Next, as illustrated in FIG. 7B, the electrode 140 is formed by patterning the N-type polysilicon film using a photolithography technique and an etching technique. Then, an insulating film is formed on the surface of the silicon substrate 11 exposed from below the electrode 140 and the surface of the electrode 140. This insulating film is, for example, a SiO ₂ film, and is formed by, for example, thermally oxidizing the silicon substrate 11.

  Next, as shown in FIG. 7C, for example, a non-doped polysilicon film 126 is formed on the entire upper surface of the silicon substrate 11 after the insulating film 125 is formed. The polysilicon film 126 is formed by CVD, for example. Then, as shown in FIG. 8A, a resist pattern 127 is formed on the polysilicon film 126 so as to open above the region where the N-type polysilicon is formed and cover the other region. N-type impurities are ion-implanted into the polysilicon film 126 using the pattern 127 as a mask. Thereby, N-type polysilicon 32 is formed in the polysilicon film 126. After the N-type polysilicon 32 is formed, the resist pattern 127 is removed by ashing, for example.

  Next, as shown in FIG. 8B, a resist pattern 128 is formed on the polysilicon film 126 so as to open above the region where the P-type polysilicon is to be formed and cover the other regions. P-type impurities are ion-implanted into the polysilicon film 126 using the resist pattern 128 as a mask. Thereby, the P-type polysilicon 31 is formed in the polysilicon film 126. After the P-type polysilicon 31 is formed, the resist pattern 126 is removed by ashing, for example. Thereafter, the polysilicon film is patterned using a photolithography technique and an etching technique. Thereby, the electrode 30 composed of the P-type polysilicon 31 and the N-type polysilicon 32 shown in FIG. 6A is formed.

  As described above, according to the second embodiment of the present invention, as in the first embodiment, the potential gradient is generated immediately below the electrode 30, so that the charge transfer efficiency can be increased. Further, the P-type impurity layer 150 functions as a potential barrier, and the potential of the adjacent N well 12 downstream of the P-type impurity layer 150 in the charge transfer direction is increased, so that a potential gradient is formed between adjacent regions. Alternatively, the potential gradient can be made steeper (when the N-type impurity concentration of the electrode 140 is higher than the N-type impurity concentration of the N-type polysilicon 32).

  In the case where the P-type impurity layer 150 is formed using the ion implantation technique, there is a possibility that crystal defects may occur in the silicon substrate 11. However, in this CCD image sensor 200, the P-type polysilicon 31 is disposed immediately above the P-type impurity layer 150, and this P-type polysilicon 31 contributes to the formation of a potential gradient in the same manner as the P-type impurity layer 150.

Therefore, the P-type impurity layer 150 is reduced in concentration if the same charge transfer efficiency is required as compared with the conventional technique (for example, when all the electrodes on the P-type impurity layer 150 are made of N-type polysilicon). In addition, since the dose at the time of forming the P-type impurity layer 150 can be reduced, it is possible to reduce the occurrence of crystal defects.
In the second embodiment, the P-type polysilicon 31 and the N-type polysilicon 32 correspond to “a plurality of types of conductive layers having different work functions” according to the present invention. The P-type polysilicon 31 corresponds to the “second conductive type first semiconductor layer” of the present invention, and the N-type polysilicon 32 corresponds to the “first conductive type second semiconductor layer” of the present invention. Yes.

(3) Third Embodiment FIGS. 9A to 9D are diagrams illustrating a plurality of configuration examples of the electrode 30. The left side of each figure is a diagram showing the electrode 30 in three dimensions, and the right side of each figure is a schematic diagram when the electrode 30 is cut along AA ′ along the charge transfer direction.
In the first embodiment, as shown in FIG. 9A, the electrode 30 is composed of P-type polysilicon and N-type polysilicon, and the P-type polysilicon is located upstream of the charge transfer region. The case where the N-type polysilicon is disposed on the downstream side of the charge transfer region has been described. However, the configuration of the electrode 30 is not limited to this, and may be, for example, a configuration as shown in FIGS.

  That is, in FIG. 9B, the electrode 30 is composed of P + polysilicon, P-polysilicon, N-polysilicon, and N + polysilicon, and these layers work in the charge transfer direction. The function is arranged in order from large to small. Here, P + polysilicon has a higher P-type impurity concentration and a higher work function than P-polysilicon. N + polysilicon has a higher N-type impurity concentration and a lower work function than N-polysilicon.

  In such a configuration, between P + polysilicon and P-polysilicon, between P-polysilicon and N-polysilicon, N-polysilicon and N + poly. A potential gradient that draws electrons e− toward the charge transfer direction is formed between the silicon and the silicon. That is, since a potential gradient is formed stepwise from directly under P + polysilicon to immediately under N + polysilicon, for example, electrons e− existing immediately under P + polysilicon are smoothly guided directly under N + polysilicon. Can do. Thereby, the transfer efficiency of the CCD image sensor can be increased.

  In FIG. 9C, the electrode 30 is composed of P-type polysilicon and N-type polysilicon, the P-type polysilicon is arranged on the upstream side of the charge transfer region, and the N-type polysilicon is in the charge transfer region. It is arranged downstream. In this example, as shown in FIG. 9C, the upper portion of the P-type polysilicon is covered with N-type polysilicon. Even in such a configuration, a potential gradient is formed so that electrons e− are attracted in the charge transfer direction from directly below the P-type polysilicon to directly below the interface in contact with the N-type polysilicon insulating film. Therefore, the transfer efficiency of the CCD image sensor can be increased.

  In FIG. 9D, the electrode 30 is composed of P-type polysilicon and N-type polysilicon, the P-type polysilicon is disposed upstream of the charge transfer region, and the N-type polysilicon is disposed in the charge transfer region. It is arranged on the downstream side. Further, the N-type polysilicon has a U-shape opening in the upstream side of the charge transfer region in a plan view, and P-type polysilicon is fitted into the opening. Even with such a configuration, a potential gradient that draws electrons in the direction of charge transfer is formed directly below the P-type polysilicon and directly below the N-type polysilicon, thereby increasing the transfer efficiency of the CCD image sensor. Can do.

In the example shown in FIG. 9D, it is desirable that the P-type polysilicon completely traverses the charge transfer region. The reason is that, when the P-type polysilicon does not completely cross the charge transfer region, the potential gradient may not be partially formed in the non-crossing portion, and as a result, the transfer efficiency can be improved. This is because there is a risk of damage.
In the third embodiment, the configuration example of the electrode 30 has been described with reference to FIGS. 9A to 9D, but such a configuration example is applied not only to the electrode 30 but also to the other electrode 40. Is possible.

(4) Fourth Embodiment In the first to third embodiments described above, it has been described that the electrodes 30 and 40 are composed of P-type polysilicon and N-type polysilicon. This is because the work function (WF) of N-type polysilicon is about 4.17 eV, the work function (WF) of P-type polysilicon is about 5.25 eV, and the difference between the two work functions is silicon. This is because a potential gradient is generated on the substrate surface.

  However, in the present invention, a conductive material having a WF value larger than that of N-type polysilicon can be used as an alternative material for P-type polysilicon constituting electrodes 30 and 40. In this case, a potential gradient is generated on the surface of the semiconductor substrate due to the work function difference between the alternative material and the N-type polysilicon. Examples of alternative materials for P-type silicon include the following.

  Silver (WF = 4.31 eV), Copper (WF = 4.52 eV), Gold (WF = 4.70 eV), Nickel (WF = 4.74 eV), Palladium (WF = 5.00 eV), Platinum (WF = 5) .30 eV), molybdenum (WF = 4.53 eV), tungsten (WF = 4.63 eV), molybdenum silicide (WF = 4.80 eV), tungsten silicide (WF = 4.80 eV).

Next, a method for manufacturing an electrode using the above alternative material will be described.
10A to 10E are process diagrams showing a method for manufacturing the electrode 300 using an alternative material. As shown in FIG. 10A, first, an insulating layer 203 is formed over a semiconductor substrate 201, and N-type polysilicon 205 is formed thereon. Here, the semiconductor substrate 201 is, for example, a silicon substrate. Further, the N-type impurity is added to the polysilicon in, for example, in-situ. Next, as shown in FIG. 10B, by using a photolithography technique and an etching technique, only the N-type polysilicon 205 in a region where the alternative material is formed is removed, and an opening 207 having the insulating layer 203 as a bottom surface. Form.

Next, as shown in FIG. 10C, an alternative material 210 of P-type polysilicon is formed on the entire upper surface of the semiconductor substrate 201 to fill the opening 207. This alternative material 210 is formed by, for example, CVD or sputtering technology. Next, the substitute material 210 is etched back or subjected to CMP to scrape the substitute material 210 by its thickness, so that the substitute material 210 remains only in the opening 207 as shown in FIG. Thereafter, as shown in FIG. 10E, a resist pattern 211 is formed on the semiconductor substrate 201 so as to cover the upper part of the region where the electrode is formed and expose the upper part of the other region. Then, the N-type polysilicon 205 is etched using the resist pattern 211 as a mask. Thereby, the electrode 300 made of the alternative material 210 and the N-type polysilicon 205 is completed.
In the fourth embodiment, the alternative material 210 and the N-type polysilicon 205 correspond to “a plurality of types of layers having different work functions” according to the present invention.

FIG. 2 is a cross-sectional view and a potential diagram showing a configuration example of a CCD image sensor 100. The figure which shows an example of the timing of voltage signal (phi) 1 and (phi) 2. The figure which shows the potential gradient from right under P-type polysilicon to right under N-type polysilicon. Process drawing which shows the manufacturing method of the CCD image sensor 100 (the 1). Process drawing which shows the manufacturing method of CCD image sensor 100 (the 2). FIG. 3 is a cross-sectional view and a potential diagram showing a configuration example of a CCD image sensor 200. Process drawing which shows the manufacturing method of CCD image sensor 200 (the 1). Process drawing which shows the manufacturing method of CCD image sensor 200 (the 2). The figure which shows the some structural example of the electrode 30. FIG. Process drawing which shows the manufacturing method of the electrode 300 using an alternative material.

Explanation of symbols

11 Silicon substrate 12 N well 20, 25, 125 Insulating film 21, 26, 126 Polysilicon film 22, 23, 24, 126, 127, 128 Resist pattern 30, 40, 140, 300 Electrode 31, 41 P-type polysilicon 32 42 N-type polysilicon 100, 200 Image sensor 150 P-type impurity layer 201 Semiconductor substrate 203 Insulating layer 205 N-type polysilicon 207 Opening 210 Alternative material 211 Resist pattern φ1, φ2 Voltage signal

Claims (5)

A CCD image sensor in which a number of electrodes for charge transfer are arranged on the surface of a semiconductor substrate via an insulating film,
The CCD image sensor, wherein a portion of the electrode in contact with the insulating film has a structure in which a plurality of types of conductive layers having different work functions are sequentially arranged in a charge transfer direction. A CCD image sensor in which a large number of charge transfer electrodes are arranged on the surface of a first conductivity type semiconductor substrate via an insulating film,
The portion of the electrode in contact with the insulating film has a structure in which a second conductive type first semiconductor layer and a first conductive type second semiconductor layer are sequentially arranged in the charge transfer direction. CCD image sensor. A CCD image sensor in which a large number of charge transfer electrodes are arranged on the surface of an N-type semiconductor substrate via an insulating film,
The CCD image sensor, wherein a portion of the electrode in contact with the insulating film has a structure in which P-type polysilicon and N-type polysilicon are sequentially arranged in a charge transfer direction.   The CCD image sensor according to claim 3, further comprising a plurality of P-type impurity layers provided at regular intervals along the charge transfer direction on the surface of the semiconductor substrate immediately below the electrodes. The P-type polysilicon includes P + polysilicon having a high concentration of P-type impurities and P-polysilicon having a low concentration of P-type impurities,
The N-type polysilicon includes N + polysilicon having a high concentration of N-type impurities and N-polysilicon having a low concentration of N-type impurities,
The portion of the electrode in contact with the insulating film is arranged such that the P + polysilicon, the P-polysilicon, the N-polysilicon, and the N + polysilicon are sequentially arranged in the charge transfer direction. The CCD image sensor according to claim 3, wherein the CCD image sensor has a structured structure.

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