JP4406558B2 - Solid-state image sensor - Google Patents

Description

The present invention relates to a solid-state imaging device , and more particularly to improvement in reliability of a solid-state imaging device having a fine single-layer electrode CCD (charge coupled device) structure.

  A solid-state imaging device using a CCD, which is an imaging device such as an area sensor, has, as a basic structure, a photoelectric conversion unit such as a photodiode, a charge readout unit from the photoelectric conversion unit, and a charge transfer for transferring the readout charge. And a charge transfer portion including an electrode. A plurality of these charge transfer electrodes are arranged adjacent to each other on a charge transfer channel formed on the surface of the semiconductor substrate, and are sequentially driven by a clock signal.

  In recent years, in a solid-state imaging device, pixel miniaturization has progressed due to an increase in the number of imaging pixels. Along with this, miniaturization of the photoelectric conversion part has progressed and it has become difficult to maintain high sensitivity.

  Therefore, various methods for efficiently condensing the light that has reached the periphery of the opening on the photoelectric conversion unit have been proposed.

For example, Patent Document 1 proposes a method of improving the light collection efficiency to the photoelectric conversion unit by mounting an on-chip lens on a semiconductor chip constituting a solid-state imaging device. However, the light collection efficiency by the on-chip lens is almost approaching the limit, and an in-layer lens has been proposed as a new technology different from the on-chip lens.
For example, as shown in Patent Document 1, a structure in which a recess surrounded by a planarizing film is formed and a high refractive index film is formed therein is proposed.

JP-A-11-121715

However, in recent years, the miniaturization of cells has been progressing, and it has become difficult to form an ideal in-layer lens. In addition, it is difficult to fabricate metal wiring with an uneven surface structure, resulting in short circuit of the wiring, Deterioration of optical characteristics due to coating unevenness due to the level difference of the part has been a problem. In particular, a fine one having a cell pitch of 2 μm or less has been proposed. In this case, the opening diameter of the photodiode part constituting the photoelectric conversion part has become extremely small, being half or less of the cell pitch, and has a wavelength of 600 There is a problem that light in a long wavelength region of ˜800 nm is difficult to reach the photoelectric conversion unit from this opening.
The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a solid-state imaging device that is small and highly reliable by further improving the light collection efficiency even when miniaturized .

Therefore, in the present invention, a light-shielding film that includes a photoelectric conversion unit and a charge transfer unit including a charge transfer electrode that transfers charges generated in the photoelectric conversion unit, and has an opening in a light receiving region of the photoelectric conversion unit A solid-state imaging device coated with
The opening is covered with a high refractive index film having a columnar single layer structure, and the periphery of the high refractive index film is an insulating film having a refractive index lower than that of the high refractive index film and having a flat surface. Covered ,
The high refractive index film is shaped so that the upper surface forms a convex lens shape .
With this configuration, the opening is covered with a high refractive index film having a single layer structure, so that light in a long wavelength region can also be efficiently collected in the light receiving region, and there is little decrease in sensitivity at low illuminance. Even in the case of oblique incident light, total reflection occurs at the interface between the insulating film and the high refractive index film, and most of the incident light is collected on the photodiode. Further, the shape is simple, and the size can be reduced. Furthermore, since the surface of the insulating film is flattened, even when a wiring is formed in this upper layer, it can be formed without causing a decrease in pattern accuracy.
In addition, a chip-on lens is not required, and further miniaturization can be achieved.

In the solid-state imaging device of the present invention, the high refractive index film is a silicon nitride film formed by a plasma CVD method.
  With this configuration, a high refractive index film having a refractive index of 1.9 or more can be obtained.

In the solid-state imaging device of the present invention, the light shielding film includes a tungsten film.
  With this configuration, the wiring pattern of the peripheral circuit portion can also be used, and a wiring pattern having a high light shielding property and a low resistance can be formed in the same process.

  As described above, in the present invention, since the opening is covered with the high refractive index film having a single layer structure, the light in the long wavelength region can be efficiently condensed on the light receiving region, and it can be used at low illuminance or obliquely. Even in the case of incident light, most of the incident light is collected on the photodiode. In addition, the shape is simple and downsizing is possible.

  Next, an embodiment of the present invention will be described with reference to FIGS. 1 and 2 are a cross-sectional view and a plan view of the solid-state imaging device of the present embodiment, and FIGS. 3 to 9 are manufacturing process diagrams thereof. In the following embodiments, a method for forming an optical system on a semiconductor substrate including a photodiode of a photoelectric conversion unit and a charge transfer unit, which is a feature of the present invention, will be described in the order of steps.

Embodiments of the present invention will be described below with reference to the drawings.
(First embodiment)
As shown in FIGS. 1 and 2, the solid-state imaging device has a gate oxide film formed on the surface of a silicon substrate 1 on which a p-well and an n-type semiconductor layer are formed. A plurality of charge transfer electrodes 40 (4a, 4b) arrayed via 2 are separated into a plurality of charge transfer electrodes by an interelectrode insulating film 3 formed on the gate oxide film 2 at a predetermined interval. Thus, the solid-state imaging device is covered with the light-shielding film 7 having an opening in the light receiving region of the photoelectric conversion unit PD, and the opening is covered with a lens made of a high refractive index film 10 having a columnar single layer structure. The periphery of the high refractive index film 10 is covered with insulating films 8 and 9 made of a silicon oxide film having a lower refractive index than that of the high refractive index film and having a planarized surface. It is characterized by that. Other areas are the same as those of a conventional solid-state imaging device.

  The high refractive index film 10 is made of a silicon nitride film formed by a plasma CVD method, and constitutes a lens having a convex surface, and its periphery is covered with a BPSG film 9.

  As shown in FIG. 1, a plurality of photodiodes 30 are formed on the silicon substrate 1, and the charge transfer electrodes 40 for transferring signal charges detected by the photodiodes have a meandering shape between the photodiodes 30. It is formed to exhibit. Although not shown, the charge transfer channel through which the signal charge transferred by the charge transfer electrode 40 moves is formed to have a meandering shape in a direction intersecting with the direction in which the charge transfer electrode 40 extends. In FIG. 1, the description of the interelectrode insulating film 3 formed near the boundary between the photodiode region and the charge transfer electrode 40 is omitted.

A photodiode 30 having a pn junction, a charge transfer channel, a channel stop region, and a charge readout region are formed in the silicon substrate 1 in which the p-well is formed. A gate oxide film 2 is formed on the surface of the silicon substrate 1. Is formed. An interelectrode insulating film 3 made of a silicon oxide film and a charge transfer electrode 40 are formed on the surface of the gate oxide film 2.
Although not shown, a thin p-type region is formed on the surface of the photodiode portion 30.

  Although the charge transfer electrode 40 is as described above, the silicon oxide film 5 as an interlayer insulating film is formed on the upper surface of the charge transfer electrode 40.

Above the solid-state imaging device, a light shielding film 7 having an opening o formed in the photodiode 30 is provided, and a color filter (not shown) is further provided as necessary. Except for the charge transfer electrode 40 and the interelectrode insulating film 3, the description is omitted because it is the same as the usual one. Further, FIG. 1 shows a so-called honeycomb-structured solid-state imaging device, but it goes without saying that the present invention can also be applied to an interline-type solid-state imaging device.
The charge transfer electrode is filled in between the first layer conductive film pattern 4a made of highly doped amorphous silicon and the interelectrode insulating film 3 and is made of highly doped amorphous silicon. The pattern 4b of the two-layer conductive film is composed of a pattern 4S that is a polycrystalline silicon film by annealing.

Next, the manufacturing process of this solid-state image sensor will be described.
First, as shown in FIG. 3A, for example, an n ⁻ type CCD buried channel serving as a charge transfer channel is formed on the surface of the silicon substrate 1 on which a p type well having an impurity concentration of about 10 ¹⁶ atoms / cm ³ is formed. At the same time, a silicon substrate is prepared in which ap ⁺ type channel stopper serving as an element isolation portion is formed in a predetermined region of the substrate surface. A silicon oxide film 2a having a film thickness of 25 to 50 nm (preferably 35 nm), a silicon nitride film 2b having a film thickness of 50 nm, and a silicon oxide film having a film thickness of 10 nm are formed on the surface of the n-type silicon substrate 1 on which the p-well is formed. A film 2c is formed, and a gate oxide film 2 having a three-layer structure is formed. Subsequently, a film thickness of 0.3 μm is formed on the gate oxide film 2 by a low pressure CVD method using a mixed gas of SiH ₄ (100%) 1000 SCCM and PH ₃ (1%: N ₂ diluted) 90 SCCM as a reactive gas. A highly doped amorphous silicon film is formed. At this time, the substrate temperature is 530 ° C., and the deposition pressure is 0.60 Torr. Subsequently, the upper insulating film 5 made of, for example, a silicon oxide film 5a having a thickness of 10 nm and a silicon nitride film 5b having a thickness of 150 nm is formed by a low pressure CVD method.

  Then, as shown in FIG. 3B, a resist pattern (GKR (registered trademark)) R1 is applied to the upper layer so as to have a thickness of 0.6 to 1.2 μm, and a desired mask is formed by photolithography. The resist pattern R1 having a pattern width of 0.35 μm is formed as shown in FIG. 3B.

Thereafter, as shown in FIG. 3C, the resist is formed by reactive ion etching using a mixed gas of CHF ₃ , CF ₄ and Ar (or a mixed gas of CHF ₃ , C ₂ F ₆ , O ₂ ). Using the pattern R1 as a mask, the upper insulating film 5 is patterned.
Further, as shown in FIG. 3D, the resist pattern R1 is removed. Thereafter, as shown in FIG. 3E, the first conductive film 4a (amorphous silicon film) is patterned using the upper insulating film 5 as a mask and the silicon nitride film 2b of the gate oxide film 2 as an etching stopper. . Here, it is desirable to use an etching apparatus such as ECR or ICP.

Thereafter, an interelectrode insulating film 3 made of a silicon oxide film (HTO) having a film thickness of 30 nm is formed by a low pressure CVD method using a mixed gas of SiH ₄ and N ₂ O. Here, an APCVD (atmospheric pressure CVD) method, a low pressure CVD method or the like using O ₃ and TEOS (tetraethoxysilane) as reaction gases may be used. Then, as shown in FIG. 4A, the film thickness of the remaining film is changed from 25 so that the etching proceeds only in the vertical direction by anisotropic etching and the silicon oxide film is left only on the side wall of the region to be the electrode. Etching is performed until the thickness is about 50 nm. The silicon oxide film remaining on the surface is removed by wet etching to form an interelectrode insulating film 3 made of a sidewall insulating film. (Here, as the interelectrode insulating film, a silicon oxide film formed by oxidation of the first conductive film may be used, or a combination of a film formed by oxidation and a film formed by deposition may be used.)

  According to this method, a fine, dense and high-quality interelectrode insulating film can be easily formed by leaving a sidewall using anisotropic etching when forming an insulating film pattern as an interelectrode insulating film. . Accordingly, it is possible to form a solid-state imaging device having a fine interelectrode insulating film smaller than the resolution limit.

Then, as shown in FIG. 4B, the film is again formed by a low pressure CVD method using a mixed gas of SiH ₄ (100%) 1000 SCCM and PH ₃ (1%: N ₂ diluted) 90 SCCM as a reactive gas. A second conductive film 4b made of a highly doped amorphous silicon film having a thickness of 0.8 μm is formed. Also at this time, the substrate temperature is 530 ° C. and the film forming pressure is 0.60 Torr.

  Thereafter, an RTA heat treatment is performed at 600 ° C. for 30 minutes or 700 ° C. for 30 seconds, and as shown in FIG. 4C, the amorphous silicon layer is annealed to form a doped polycrystalline silicon layer 4S. Here, the film thickness of the first layer was 400 nm or more (first layer conductive film (polycrystalline silicon film) + upper insulating film (silicon nitride film) = 400 nm + 150 nm = 550 nm).

  Further, as shown in FIG. 4D, the surface is flattened by CMP. As an etchant, organic amine-based colloidal silica particles were used, and planarized at a temperature.

  Thereafter, as shown in FIG. 5A, a second resist mask R2 having a PD opening pattern is formed by using a well-known photolithography technique. In the second resist mask R2, a large number of photodiode portion forming openings (PD openings) that become the photodiode regions of the photoelectric conversion portion are two-dimensionally arranged at regular intervals.

Subsequently, as shown in FIG. 5B, the second resist mask R2 is used as an etching mask, and the second layer conductive film 4S and the silicon oxide film 2c covering the PD formation region are subjected to reactive ion etching (RIE). Are sequentially dry-etched. Here, for example, C ₄ F ₈ is used as the etching gas for the silicon oxide film 2c, and the above-described mixed gas of HBr and O ₂ is used as the etching gas for polysilicon constituting the second layer conductive film. The first layer conductive film 4S and the second layer conductive film, which become the charge transfer electrode having a single layer structure, are only removed by etching the second layer conductive film 4S under the PD opening using the second resist mask R2 as an etching mask. The conductive film 4S is formed at a predetermined pitch.

  Subsequently, as shown in FIG. 5C, the second resist mask R2 is removed by a well-known ashing method, and an annealing process is performed at 900 ° C. in a nitrogen gas atmosphere. This annealing treatment activates phosphorus impurities in the silicon substrate 1 and recovers implantation damage.

  In this manner, a photodiode is formed by the p-well of the silicon substrate 1 and the n-type diffusion layer (not shown).

  In this way, a solid-state image sensor body having a charge transfer electrode is formed.

  Thereafter, a silicon oxide film 6a and a silicon nitride film 6b as an antireflection film are sequentially formed on the silicon substrate 1 in which the PD opening is formed, and after patterning, a silicon oxide film 6c as an insulating film is further formed. To do. Thereafter, as shown in FIG. 6A, a tungsten film 7 having a thickness of 200 nm is formed as a light-shielding film through titanium nitride (not shown) as an adhesive film.

Further, as shown in FIG. 6B, a resist pattern R4 for forming an opening in the light shielding film is formed in the upper layer.
Thereafter, as shown in FIG. 6C, the tungsten film and the silicon oxide film 6c are etched by the RIE method using CF ₄ or SF ₆ to form a light receiving region o.

  Then, as shown in FIG. 6D, the resist R4 is removed by ashing.

  Thereafter, after a silicon oxide film 8 is formed on the surface, a silicon nitride film having a refractive index of about 1.9 is formed to a thickness of about 1.0 to 1.2 μm by plasma CVD as shown in FIG. A high refractive index film 10 is formed.

  Further, as shown in FIG. 7B, a resist pattern R5 for forming a high refractive index film into a columnar body is formed in this upper layer.

  Further, as shown in FIG. 7C, a high refractive index film is formed into a columnar body using the resist pattern R5 as a mask.

Thereafter, as shown in FIG. 8A, the resist pattern R5 is removed by ashing.
Then, as shown in FIG. 8B, after forming an insulating film 11 made of a silicon oxide film having a thickness of about 100 nm, a BPSG film 9 having a thickness of about 1.0 μm is formed.

  Thereafter, as shown in FIG. 8C, the surface is flattened by CMP.

Further, as shown in FIG. 9A, a silicon nitride film 10 is formed again by plasma CVD.
Then, as shown in FIG. 9B, a resist pattern is formed in a circular shape by photolithography, and then heated to about 170 ° C., and the resist pattern R6 is shaped into a hemisphere and cured.
Finally, etch back is performed under etching conditions such that the resist pattern R6 and the silicon nitride film have the same etching rate, thereby forming a high refractive index film having hemispherical convex portions.
In this way, a columnar high refractive index film in which lenses are integrated is formed.

  In subsequent steps, a color filter, a condensing microlens, or the like may be further formed.

  In the present embodiment, the surface of the high refractive index film is formed in a convex shape so as to serve also as a chip-on lens. However, as shown in FIG. An insulating film, a color filter, and a microlens may be formed.

In this way, the high refractive index film is formed on the light receiving portion, and it is possible to provide a solid-state imaging device with high light collection efficiency.
Further, the surface is flat, the wiring can be easily formed, and the manufacturing is easy and the reliability is high.
In addition, a series of manufacturing processes is made efficient, and manufacturing costs can be easily reduced.

(Second embodiment)
Next, a second embodiment of the present invention will be described.
After the etching of the tungsten film 7 as the light-shielding film shown in FIG. 6C in the first embodiment, the resist pattern R4 used as a mask in this step is contracted by ashing with oxygen plasma or oxygen radicals. A resist pattern R5 ′ having a large opening O is formed (FIG. 10A).
Then, the silicon nitride film 10 is similarly formed by spin coating while leaving the resist pattern R5 ′ (FIG. 10B).
Thereafter, the resist pattern R5 ′ is removed to form a pattern of a high refractive index film only in the opening (FIG. 11A).

Thereafter, as shown in FIG. 11B, the SOG film 12 is formed around the pattern of the high refractive index film by a coating method, and the surface is flattened. Further, it may be planarized by etch back.
In this method, the pattern of the high refractive index film is formed in a self-aligning manner without changing the resist pattern used for forming the opening in the light shielding film in order to define the light receiving region. In addition to being able to be formed without deviation, the number of steps is simplified, and a highly accurate and reliable mounting structure can be obtained. Further, by using the BPSG film, the heat treatment temperature for planarization can be further lowered.

In the above embodiment, the case where the charge generated by photoelectric conversion is an electron has been described, but the present invention can be similarly applied even when the charge is a hole. However, in this case, all the conductivity types of the impurities described above may be reversed. In the above embodiment, the gate oxide film is described as having an ONO structure. However, the gate oxide film may be formed of only a silicon oxide film.
In the above embodiment, a doped polysilicon film formed by annealing a doped amorphous silicon layer is used as a conductive film for forming an electrode. However, after forming a non-doped amorphous silicon layer, Doping may be performed.
Further, not only SOG by spin coating but also a silicon oxide film may be formed by high-density plasma CVD and planarized by CMP.

  Note that the present invention is not limited to the above-described embodiment, and can be appropriately made within the scope of the technical idea of the present invention.

  As described above, the solid-state imaging device of the present invention can increase the light collection efficiency even when miniaturized, and can be miniaturized and easily manufactured. It is extremely effective as a small-sized image sensor.

1 is a plan view of a solid-state imaging device for explaining a first embodiment of the present invention. It is sectional drawing of the solid-state image sensor explaining the 1st Embodiment of this invention. It is sectional drawing of the manufacturing process of the solid-state image sensor explaining the 1st Embodiment of this invention. It is sectional drawing of the manufacturing process of the solid-state image sensor explaining the 1st Embodiment of this invention. It is sectional drawing of the manufacturing process of the solid-state image sensor explaining the 1st Embodiment of this invention. It is sectional drawing of the manufacturing process of the solid-state image sensor explaining the 1st Embodiment of this invention. It is sectional drawing of the manufacturing process of the solid-state image sensor explaining the 1st Embodiment of this invention. It is sectional drawing of the manufacturing process of the solid-state image sensor explaining the 1st Embodiment of this invention. It is sectional drawing of the manufacturing process of the solid-state image sensor explaining the 1st Embodiment of this invention. It is sectional drawing of the manufacturing process of the solid-state image sensor explaining the 2nd Embodiment of this invention. It is sectional drawing of the manufacturing process of the solid-state image sensor explaining the 2nd Embodiment of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Silicon substrate 2 Gate oxide film 3 Interelectrode insulating film 4a 1st layer conductive film 4b 2nd layer conductive film 5 Insulating film 6 Light-shielding film R1 Resist pattern R2 Resist pattern R3 Resist pattern R4 Resist pattern R5 Resist pattern R6 Resist pattern 10 High refractive index film

Claims (3)

Solid-state imaging comprising: a photoelectric conversion unit; and a charge transfer unit including a charge transfer electrode that transfers charges generated in the photoelectric conversion unit, and covering the light receiving region of the photoelectric conversion unit with a light shielding film having an opening An element,
The opening is covered with a high refractive index film having a columnar single layer structure, and the periphery of the high refractive index film is an insulating film having a refractive index lower than that of the high refractive index film and having a flat surface. Covered ,
The high-refractive index film is processed so that the upper surface forms a convex lens shape . The solid-state imaging device according to claim 1, wherein the high refractive index film is a silicon nitride film formed by a plasma CVD method. The solid-state imaging device according to claim 1, wherein the light shielding film includes a tungsten film.

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