Solid-state imaging device and manufacturing method therefor

A solid-state imaging device includes a plurality of two-dimensionally arranged photo diodes and a plurality of microlenses having substantially hemispherical shapes which cover the respective photo diodes. The microlens has a multilayer structure including at least a transparent resin upper layer which forms at least a portion of the substantially hemispherical shape, and a colored lower layer provided on a portion of the transparent resin upper layer which is located above the photo diode, with an interface between the colored lower layer and the transparent resin upper layer having a shape conforming to a surface of the photo diode.

This application is based upon and claims the benefit of priority from prior Japanese Patent Applications No. 2002-199558, filed Jul. 9, 2002; and No. 2003-023297, filed Jan. 31, 2003, the entire contents of both of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a solid-state imaging element typified by a light-receiving device such as a C-MOS or CCD.

2. Description of the Related Art

An area (opening portion) on a solid-state imaging device such as a CCD in which photo diodes contribute to photoelectric conversion is limited to about 20 to 40% of the total area of the solid-state imaging device, although it depends on the size and the number of pixels of the solid-state imaging device. A small opening portion directly leads to low sensitivity. In order to compensate for it, a microlens for condensing light is generally formed on a photo diode.

Recently, however, strong demands have arisen for a solid-state imaging device having a high resolution of over 3,000,000 pixels. Serious problems have been posed in terms of a reduction in the open area ratio (i.e., a reduction in the sensitivity) of a microlens attached to this high-resolution solid-state imaging device and image quality deterioration due to an increase in noise such as flare and smear. Imaging devices such as C-MOSs and CCDs have almost reached a sufficient number of pixels. Competition for the number of pixels among device makers is now changing to competition for image quality.

A known technique associated with a technique of forming microlenses is disclosed relatively in detail in, for example, Jpn. Pat. Appln. KOKAI Publication No. 60-53073. This reference discloses, in detail, a technique using the heat flow properties (heat flow) of a resin due to heat as a technique of forming a lens into a hemispherical shape and a technique of processing a lens by several etching methods. The reference also discloses, as measures against the loss of the light condensing performance of a lens surface due to light scattering, a technique of forming, on the lens surface, an organic film such as a poly(glycidylmethacrylate) (PGMA) film or an inorganic film made of OCD (an SiO2-based film formation coating solution available from Tokyo Ohka Kogyo Co., Ltd.) and the like.

A technique of forming a single-layer or multilayer antireflection film on a microlens to prevent reflection by the microlens is also disclosed in, for example, Jpn. Pat. Appln. KOKAI Publication No. 4-223371. In addition, a technique of dry-etching a microlens other than the above techniques is disclosed in detail in Jpn. Pat. Appln. KOKAI Publication No. 1-10666. Furthermore, a technique for chromatic microlenses (colored microlenses) is disclosed in, for example, Jpn. Pat. Appln. KOKAI Publication Nos. 64-7562 and 3-230101.

FIG. 1Ais a sectional view of a typical conventional solid-state imaging device. As shown inFIG. 1A, for example, planarized layers81and82, a color filter83, and if circumstances require, an inner-layer lens are formed on a photo diode80. As consequence, in general, an under-lens distance D1is about 5 to 6 μm, which is relatively large (relatively thick).

FIG. 1Bis a sectional view of another conventional solid-state imaging device (having chromatic lenses90). The arrangement of the solid-state imaging device can be simplified by each chromatic lens90having a color filter function.

The conventional solid-state imaging devices, however, have, for example, the following problems.

First, the arrangements of the conventional solid-state imaging devices have difficulty in reducing under-lens distances. More specifically, referring toFIG. 1A, reducing (thinning) the under-lens distance D1is a promising means for improving the condensing performance with respect to incident light from microlenses85and also increasing the S/N (signal-to-noise) ratio in the photo diodes80. If, however, the thickness of each microlens85(lens height D2) is simply reduced, it is difficult to form a microlens into a substantially hemispherical shape by using the method of manufacturing microlenses by heat flow. Therefore, a suitable microlens cannot be manufactured.

This problem is especially obvious in a C-MOS imaging device, which has recently attracted a great deal of attention because it consumes low power and is integrated with a driving circuit to realize space saving. This is because in a C-MOS imaging device, the distance from a microlens to a photo diode tends to be large owing to its structure, and hence this arrangement is disadvantageous in reducing the

Second, with the conventional arrangement, color purity degrades to cause a deterioration in image quality depending on the incident position of light. More specifically, referring toFIG. 1B, light L1incident near the center of the chromatic lens90is transmitted through a portion of the chromatic lens which has a sufficient thickness, and hence an almost intended color filter effect can be expected for transmitted light L3. In contrast to this, light L2incident from an end portion of the chromatic lens90is transmitted through a thin portion of the chromatic lens serving as a color filter, and hence transmitted light L4becomes considerably whitish. As a result, the color purity greatly degrades.

BRIEF SUMMARY OF THE INVENTION

The present invention has been made in consideration of the above situation, and has as its object to provide a solid-state imaging device which can improve the light condensing performance and S/N ratio by reducing the under-lens distance, set the substantial thickness of each microlens to 0.5 μm or more, and improve the open area ratio by suppressing degradation in the color purity of each chromatic lens, and a manufacturing method for the device.

According to a first aspect of the present invention, there is provided a solid-state imaging device comprising a plurality of two-dimensionally arranged photo diodes and a plurality of microlenses having substantially hemispherical shapes which cover the respective photo diodes, each microlens comprising a multilayer structure lens including at least a transparent resin upper layer which forms at least a portion of the substantially hemispherical shape, and a colored lower layer provided on a portion of the transparent resin upper layer, which is located above the photo diode, with an interface between the colored lower layer and the transparent resin upper layer having a shape conforming to a surface of the photo diode.

According to a second aspect of the present invention, there is provided a solid-state imaging device manufacturing method for a solid-state imaging device comprising a plurality of two-dimensionally arranged photo diodes and a plurality of microlenses having substantially hemispherical shapes which cover the respective photo diodes, comprising forming a planarized layer on a plurality of photo diodes two-dimensionally arranged on a semiconductor substrate, forming colored lower layers in a plurality of colors on the planarized layer by photolithography using photosensitive colored resists containing coloring matters as coloring materials, forming transparent resin upper layers on the plurality of colored lower layers by coating a first resin coating solution, forming a lens matrix on the transparent resin upper layer by photolithography and annealing using a lens material having alkali solubility, photosensitivity, and heat flow properties, and transferring a pattern of the lens matrix onto at least the transparent resin upper layer by performing dry etching on the lens matrix, and forming the microlens having at least the transparent resin upper layer and the colored lower layer.

DETAILED DESCRIPTION OF THE INVENTION

Each embodiment of the present invention will be described below with reference to the views of the accompanying drawing. Note that the same reference numerals denote constituent elements having substantially the same functions and arrangements throughout the following description, and repetitive descriptions will be made only when required.

FIG. 2Ais a top view of a solid-state imaging device1according to the first embodiment.FIG. 2Bis a sectional view taken along a line A—A inFIG. 1. The arrangement of the solid-state imaging device1will be described first with reference toFIGS. 2A and 2B.

The semiconductor substrate11is a substrate for mounting the photo diodes13and the like. The photo diode13converts light incident through the microlens10into an electric charge. The planarized layer15planarizes the mount surface for the microlenses10.

The microlens10is hemispherical and has a hemispherical transparent resin upper layer10awhich forms the upper portion of the microlens10and a colored lower layer10bwhich forms the bottom portion of the microlens10. The boundary between the transparent resin upper layer10aand the colored lower layer10bhas a shape conforming to the surface of the photo diode13, i.e., a flat shape. The area of this flat surface corresponds to part of the effective area (the surface having a condensing function) of the colored lower layer10b. In the case shown inFIG. 2B, part of the colored lower layer10bforms part of the hemispherical shape of the microlens10, as shown inFIG. 2B. As described above, the colored lower layer10bpreferably forms part of the hemispherical shape of the microlens10.

The thickness T1of the transparent resin upper layer10ais not specifically defined, but is preferably 0.4 μm or more, which is the lower limit of thickness in heat flow. The upper limit of the thickness T1of the transparent resin upper layer10ais preferably about 1 μm because this embodiment is directed to a fine pixel pitch.

The thickness T2of the colored lower layer10bsuffices if it corresponds to a color filter film thickness necessary for intended color separation, and is not specifically limited. In general, it suffices if this thickness falls within the range of 0.5 μm to 1.5 μm. The flat interface between the colored lower layer10band the transparent resin upper layer10ais preferably as large as possible within the range permitted in terms of pixel size in consideration of color separation.

Although the colored lower layer10bmay be colored by using an organic pigment as a coloring material, the layer is preferably colored with a dye (the coloring material means materials including coloring agents). For example, the following are the reasons for this. If organic pigments are used, etching rates in dry etching vary depending on the types of pigments used, and hence lens shapes tend to vary for the respective colors. The surfaces become rough. In an imaging device with a fine pixel pitch to which this embodiment is directed, the particle size (particle) of a pigment itself is likely to affect the S/N ratio, and it is difficult to perform filtration (foreign substance removal) of the coloring resist material.

When a colored layer containing an organic pigment as a coloring material is etched deeply, its surface becomes considerably rough. When part of the colored lower layer which has become the rough surface is formed into a microlens, it is difficult to hold the microlens shape. If colored lower layers in the respective colors before etching vary in thickness, final thickness adjustment is done in a dry etching step. Inevitably, the thickness of a colored lower layer to be dry-etched is increased. In order to make the roughness of etched surfaces fall within an allowable range, the differences in thickness between colored lower layers in the respective colors may be made to fall within 0.3 μm. As the differences in thickness between these color lower layers increase, microlenses with better lens shapes can be obtained.

The reflective index difference between the transparent resin upper layer10aand the colored lower layer10bis preferably as small as possible to minimize a reduction in the amount of light incident on the photo diode. In addition, the refractive index of the transparent resin upper layer10ais preferably as low as possible to reduce its surface reflection. In consideration of these points, a thin optical film for the reduction of reflection may be inserted in the interface between the transparent resin upper layer10aand the colored lower layer10b. In this case, although the interface between the transparent resin upper layer10aand the thin optical film need not be flat, the interface between the thin optical film and the colored lower layer10bneeds to be flat. Alternatively, an antireflection film may be stacked on the entire surface of the microlens10. Considering as well that the transparent resin upper layer10ahaving a low refractive index can be formed thicker than that having a high refractive index, an arrangement in which an antireflection film is stacked is preferable for the solid-state imaging device1having fine pixels.

The microlenses10are formed by dry etching using a lens matrix. In this dry etching using the lens matrix, etching tends to speed up relatively in the recess portions between the lenses, resulting in a deterioration in the finished shape of each microlens. In order to reduce this deterioration, the entire lens matrix is preferably covered with a thin transparent resin layer having a thickness of about 0.05 μm to 0.3 μm before dry etching. Inserting this step can execute lens matrix transfer more smoothly.

In manufacturing the solid-state imaging device1, in order to reduce the under-lens distance, the dry etching depth is set to be as large as possible. In this case, if etching proceeds to the underlying layer (i.e., the planarized layer15) of the colored lower layer, the planarized surface (effective area) of the colored lower layer decreases. Consequently, the amount of light which has reduced color purity and is incident from a bottom portion of the microlens increases, resulting in a deterioration in image quality. For this reason, the dry etching depth is preferably made to correspond to a midway position in the colored lower layer in the direction of thickness. If a portion of the colored lower layer is left unetched by a thickness of about 0.4 μm, more preferably 0.7 μm, deterioration of color purity can be suppressed.

In general, O2gas is used for dry etching. If a reducing flon-based gas is used as an etching gas, since lens matrices can be transferred with narrow gaps, a lens shape can be easily ensured. Flon-based gases that can be used include CF4, C2F6, C3F8, C4F8, CHF3, C2HF5, and the like. These gases can be used singly or in combination. Increasing the ratio of C or H with respect to F is effective in holding narrow gaps. More specifically, a gas mixture containing CF4as a basic gas with a small amount of C3F8, C4F8, or the like added is preferably used. Note, however, that the composition of an etching gas greatly depends on the dry etching apparatus used in order to obtain an optimal lens shape or inter-lens gaps. Therefore, the gas composition is not limited to any specific one.

If the transparent resin upper layer10ais made of an acrylic-based resin, a photosensitive colored resist resin is preferably an acrylic-based photosensitive resin in consideration of adhesive force, refractive index, and the like. A dye may be used in a dissolved form into the prime solvent of a photosensitive colored resist, in a dispersed form, or an embedded form in a resin skeleton, i.e., a so-called pendant form.

Note that a general dyeing method using a dye bath is not preferable in terms of cost because of an increase in the number of steps. A color filter using a dye as a coloring material can perform high filtration (foreign substance removal) of 0.2 μm to 0.1 μm in the stage of a colored resist, and hence an imaging device having high image quality and greatly increased S/N ratio can be obtained as compared with the case wherein a colored resist dispersed with an organic pigment whose filtration is limited to 1 μm to 0.5 μm is used.

Dyes that can be used include azo-based dyes, xanthenium-based dyes, phthalocyanine-based dyes, anthraquinone-based dyes, coumarin-based dyes, styryl-based dyes, and the like. Primary color dyes, i.e., red, green, and blue dyes, complementary color dyes, i.e., cyan , magenta, and yellow dyes, and dyes obtained by adding a green dye to them can be used.

EXAMPLE 1 OF MANUFACTURING METHOD

A method of manufacturing the solid-state imaging device1will be described in detail next.

FIGS. 3A to 3Care views for explaining a method of manufacturing the solid-state imaging device1.

As shown inFIG. 3A, a planarized layer15is formed on a semiconductor substrate11, on which photo diodes13, light-shielding films, and passivations (both of which are not shown) are formed, by spin coating using a thermosetting acrylic resin coating solution. In addition, colored lower layers10bare formed using R (red), G (Green), and B (Blue) photosensitive colored resists by performing photolithography three times. The respective photosensitive colored resists in R (Red), G (Green), and B (Blue) are coated by spin coating, and exposure is performed by using a stepper exposure apparatus.

As shown inFIG. 3B, a transparent resin upper layer10ais formed on the R (red), G (Green), and B (Blue) colored lower layer10bby spin coating using a thermosetting acrylic resin coating solution.

The transparent resin upper layer10ais coated with a photosensitive acrylic-based resin having heat flow properties by spin coating, and hemispherical lens matrices19are formed by exposure, development, and heat flow. The temperature in a heat flow process is set to, for example, 190° C. Thereafter, the entire upper surface of the lens matrix19is coated with the same acrylic-based resin coating solution as that used for the formation of the transparent resin upper layer10asuch that the resultant layer has a thickness of about 0.1 μm after drying, thus forming a thin transparent resin layer (not shown).

The semiconductor substrate11on which the lens matrix19is formed is etched by a dry etching apparatus using O2gas. This etching process is executed at a substrate temperature of room temperature, a pressure of 5 Pa, an RF power of 500 W, and a bias of 100 W to obtain the solid-state imaging device1having the shape shown inFIG. 3C. Note that an antireflection film may be stacked on the formed microlens10.

In the above manufacturing, as resin materials for the transparent resin upper layer10a, colored lower layer10b, and planarized layer15, acrylic resins which have almost the same refractive index in the refractive index range of 1.51 to 1.55 at a light wavelength of 550 nm are used. It is relatively difficult to accurately measure the refractive index of the colored lower layer10bdue to the coloring materials contained in the layer. However, the refractive index of an R (red) portion is 1.61 at 700 nm (R (red) exhibits large absorption with respect to 500-nm green light, and hence it is difficult to accurately measure a refractive index at 550 nm).

The R (red), G (Green), and B (Blue) colored lower layers10bare formed by using acrylic-based photosensitive colored resists obtained by preparing coloring materials mainly including dyes represented by color indices, C.I. Acid Red 114, C.I. Acid Green 16, and C.I. Acid Blue 896, together with acrylic-based resins, and a cyclohexane solvent. The amount of coloring material added is about 20% in terms of solid content ratio in each resist.

As shown inFIG. 3C, the solid-state imaging device1obtained by such a manufacturing method is designed such that the microlens10constituted by the photo diode13, colored lower layer10b, and transparent resin upper layer10ais formed on the semiconductor substrate11. According to the experiment conducted by the present inventors, the peak thickness (the thickness of the central portion) T1of the transparent resin upper layer10aof the obtained solid-state imaging device1was 0.6 μm, and a thickness T5of the microlens10, which is the sum of the peak thickness and the depth of the notched portion of the colored lower layer10bin the form of a lens, was about 1.1 μm. The thickness T2of the colored lower layer10balone was 0.9 μm. The under-lens distance (the distance from the colored lower layer10bto the photo diode13) was about 3.4 μm. The under-lens distance in the prior art is 5.5 μm. This distance in the solid-state imaging device1could be greatly reduced to about 60% of that in the prior art. In this embodiment, the microlens pitch was 3.5 μm, and the inter-lens gap was 0.3 μm.

EXAMPLE 2 OF MANUFACTURING METHOD

Another method of manufacturing the solid-state imaging device1will be described in detail next.

The solid-state imaging device1was manufactured by the same method as that in Example 1 described above except that dry etching was performed by using a dry etching gas constituted by 80% CF4gas and 20% C3F8gas instead of O2gas.

As shown inFIG. 3C, the solid-state imaging device1obtained by such a manufacturing method is designed such that a microlens10constituted by a photo diode13, colored lower layer10b, and transparent resin upper layer10ais formed on a semiconductor substrate11. According to the experiment conducted by the present inventors, the peak thickness (the thickness of the central portion) T1of the transparent resin upper layer10aof the obtained solid-state imaging device1was 0.4 μm, and the thickness T5of the microlens10, which is the sum of the peak thickness and the depth of the notched portion of the colored lower layer10bin the form of a lens, was about 0.6 μm. The thickness T2of the colored lower layer10balone was 0.8 μm. The under-lens distance (the distance from the colored lower layer10bto the photo diode13) was about 2.5 μm. The under-lens distance in the prior art is 5.5 μm. This distance in the solid-state imaging device1could be greatly reduced to about 45% of that in the prior art. In this example, the microlens pitch was 2.7 μm, and the inter-lens gap was 0.05 μm.

As described above, the solid-state imaging device1and its manufacturing method according to this example can obtain at least any one of the following effects.

First, the under-lens distance can be reduced, and hence the light condensing performance and S/N ratio can be improved for the following reason. The solid-state imaging device1has at least a two-layer structure constituted by the transparent resin upper layer10aand colored lower layer10b, and includes the microlens10in which the interface between the transparent resin upper layer10aand the colored lower layer10bconforms to the surface shape of the photo diode13(conforms to the horizontal shape in this embodiment). Therefore, the layer formed under the lens can be minimized by incorporating the colored lower layer10bin the microlens10.

Second, even a solid-state imaging device with a small pixel can be easily processed. This is because the solid-state imaging device1allows the microlens10to have a substantial thickness of 0.5 μm or more by reducing the under-lens distance, and this thickness allows easy formation of a hemispherical lens shape by heat flow.

According to the experiment conducted by the present inventors, it was very difficult to form a lens shape with a thickness of 0.4 μm or less. In the case of a pixel pitch of 3 μm, the limit thickness of a microlens was 0.4 μm in consideration of mass productivity. When the thickness was 0.3 μm, a microlens was formed into a trapezoidal shape instead of a substantially hemispherical shape. In contrast to this, when the substantial thickness of the microlens10was set to 0.5 μm or more as in this solid-state imaging device, a substantially hemispherical shape could be easily formed.

Third, degradation in the color purity of a chromatic lens can be suppressed to increase the open area ratio. This is because the interface between the colored lower layer10band the transparent resin upper layer10aof the solid-state imaging device1conforms to the surface shape of the photo diode13(conforms to the horizontal shape in this embodiment), and hence the effective area of the colored lower layer10bis larger than that in the prior art.

Fourth, a solid-state imaging device in which the under-lens distance is small to have improved light condensing performance and S/N ratio, and degradation in the color purity of a chromatic lens is suppressed to increase the open area ratio can be easily manufactured even with a small pixel pitch. This is because this solid-state imaging device manufacturing method includes a step of forming colored lower layers in a plurality of colors on photo diodes, a step of forming a transparent resin upper layer on the colored lower layers in the plurality of colors, a step of forming lens matrices on the transparent resin upper layer, and a step of performing dry etching on the lens matrices to transfer a lens matrix pattern onto the transparent resin upper layers and colored lower layers.

FIG. 4is a top view of a solid-state imaging device20according to the second embodiment.FIG. 5Ais a sectional view taken along a line A—A of the solid-state imaging device20inFIG. 4. As shown inFIG. 5A, the solid-state imaging device20includes a semiconductor substrate11, photo diodes13, microlenses10, light-shielding layers16, and a planarized layer15.

As shown inFIG. 5B, an ultraviolet absorbing layer14can be placed between the planarized layer15and the colored lower layer10b.

With recent advances in miniaturization of solid-state imaging devices, pixels (or microlenses) tend to become extremely small regions with a 3 or 2 μm pitch or less. With these minute pixels, a pattern shape fluctuation affects image quality in the form of image quality unevenness or the like.

In order to prevent the reflection of light from an underlayer which causes a pattern shape fluctuation (re-reflected light in a stepper exposure apparatus (ultraviolet light having an exposure wavelength of 365 nm), a layer having an ultraviolet absorbing function is preferably formed in advance as an underlayer of a colored lower layer. The ultraviolet absorbing layer14may be formed on the planarized layer15or the planarized layer15may have an ultraviolet absorbing function. It suffices if a layer having an ultraviolet absorbing function can be formed under the colored lower layer10band the ultraviolet absorbing layer14may also have an ultraviolet absorbing function.

The ultraviolet absorbing layer14is manufactured by inserting a step of coating an ultraviolet light absorbing layer between a step of forming an infrared absorbing layer and a step of forming a lens matrix. Forming an ultraviolet absorbing layer in the manufacturing process in this manner can form a high-precision microlens pattern while preventing halation in the stepper exposure apparatus. In addition, a function of protecting an infrared absorbing layer with relatively low light resistance against ultraviolet light can be added.

For the ultraviolet absorbing layer14, a transparent resin can be used such as acrylic resin, epoxy resin, polyester resin, urethane resin, melamine resin, urea resin, styrene resin, phenol resin, or a copolymer thereof.

The ultraviolet absorbing layer14is directed to aniline (365 nm) used in the manufacturing process for a solid-state imaging device and ultraviolet light contained in external light when a camera in which the solid-state imaging device is mounted is used. In the former case, the ultraviolet absorbing layer14ensures the lens matrix pattern shape by preventing halation of aniline (365 nm). In the latter case, the ultraviolet absorbing layer14absorbs ultraviolet light to prevent a deterioration in the function of the infrared absorbing layer.

In addition, an ultraviolet absorbing function can be implemented by adding an ultraviolet absorbing compound or ultraviolet absorbing agent to the above transparent resin or planarized layer formation resin or by the pendant method (the agent in the form of a reactive ultraviolet absorbing agent or the like is bonded to resin molecular chains). Ultraviolet absorbing agents that can be used include a benzotriazole-based compound, benzophenone-based compound, salicylic-acid-based compound, and coumarin-based compound. For example, a light stabilizer such as hindered-amine-based compound or a quencher (e.g., a singlet oxygen quencher) may be added to such an ultraviolet absorbing agent. Alternatively, an ultraviolet absorbing agent made of metallic oxide particles such as cerium oxide or titanium oxide may be used.

The microlens10includes the transparent resin upper layer10aand colored lower layer10b. The transparent resin upper layer10aand planarized layer15have an infrared absorbing function. For this reason, the solid-state imaging device20requires no infrared cut filter. Therefore, the under-lens distance is smaller than that in the prior art without any degradation in color reproducibility.

In addition, an infrared absorbing function can be implemented by adding an infrared absorbing compound or infrared absorbing agent to the above transparent resin or by the pendant method (the agent in the form of a reactive dye or reactive infrared absorbing agent is bonded to resin molecular chains).

The absorption wavelength ranges of many infrared absorbing agents are limited, so that it is difficult to cover the entire near-infrared region and infrared region (e.g., 650 nm to 1,100 nm) required in a photo diode of a C-MOS, CCD, or the like with one type of infrared absorbing agent. For this reason, a plurality of infrared absorbing agents, e.g., two to six types of agents, are preferably used in combination, or one constituent element is preferably formed into multiple layers.

In order to add a sufficient infrared absorbing function while ensuring high transmittance in the visible range (400 nm to 700 nm), the infrared absorbing function is preferably distributed to a plurality of constituent elements arrayed on photo diodes such as C-MOSs, CCDs, or the like. For example, identical infrared absorbing agents are preferably contained in different constituent elements to enhance the infrared absorbing function. Alternatively, infrared absorbing agents with different absorption wavelength ranges are preferably contained in different constituent elements to distribute an infrared absorbing function. Furthermore, in consideration of, for example, the heat resistance of an infrared absorbing agent, a specific constituent element may be selected as one in which the agent should be contained.

When the colored lower layer10bis made to have an infrared absorbing function, the types and contents of infrared absorbing agents with different absorption ranges are preferably adjusted before they are contained. This is because in a primary color (RGB) or complementary color (YMC) filter layer, the spectral characteristics (absorption) in the infrared region vary for the respective colors.

The depth of dry etching, conditions for gases to be used, and conditions for colored resins to be used for the colored lower layer10bin forming the microlens10are the same as those in the above embodiment. In order to increase the open area ratio of the microlens10by reducing a non-opening portion25or improve the infrared absorbing function, a thin infrared absorbing layer may be stacked on the microlens10by coating.

To reduce re-reflection of incident light from the surface or non-opening portion of the microlens10, a thin, low refractive index resin film is preferably formed on the microlens10or the above thin infrared absorbing layer. In addition, a thin film made of a low refractive index material may be stacked on the non-opening portion25(FIG. 4) exposed between the microlenses10to absorb stray light reflected by the surface of the microlenses so as to reduce noise (re-incidence of reflected light in this case) produced in the solid-state imaging device.

EXAMPLE 1 OF MANUFACTURING METHOD

A method of manufacturing the solid-state imaging device20will be described in detail next.

FIGS. 6A to 6Care views for explaining the method of manufacturing the solid-state imaging device20.

As shown inFIG. 6A, after a planarized layer15is formed on a semiconductor substrate11, on which a photo diodes13and light-shielding layers16are formed, colored lower layers10bin three colors are sequentially formed by known photolithography using colored resists in three colors, i.e., R (Red), G (Green), and B (Blue), and a stepper exposure apparatus. Each colored lower layer10bhas a thickness of, for example, 0.9 μm to 0.8 μm.

Note that colored resists available from Toyo Ink Mfg. Co., Ltd. which use organic pigments as coloring materials can be used for the R (Red), G (Green), and B (Blue) colored lower layers10b. In Example 1, as a color arrangement, a so-called Beyer arrangement is used, in which one pixel is constituted by two G (Green) elements, one R (Red) element, and one B (Blue) element, i.e., a total of four elements.FIG. 6is a plan view of a solid-state imaging device20viewed from the microlens side, and is also a view showing a two-dimensional (planar) arrangement of color filter layers and microlenses in the Beyer arrangement.

As shown inFIG. 6B, a 1-μm thick infrared absorbing layer26is formed on the colored lower layers10bby using a resin coating solution containing three types of infrared absorbing agents. In addition, the resultant structure is coated with a photosensitive phenol resin having heat flow properties by spin coating, and a hemispherical lens matrix19is formed by exposure, development, and heat flow. The heat flow temperature at this time is, for example, 200□, and the thickness (lens height) of the lens matrix19is 0.7 μm.

In this example, as a resin coating solution having an infrared absorbing function, a resin coating solution is used, which is obtained by dissolving 100 parts by weight of thermosetting acrylic resin and 20 parts by weight of a combination of three types of agents, i.e., infrared absorbing agents YKR-3080, YKR-3030, and YKR-200 available from Yamamoto Kasei K.K., in an organic solvent such as cyclohexanone.

As shown inFIG. 6C, the semiconductor substrate11on which the lens matrices19are formed is subjected to an etching process (white arrows) using O2gas by a dry etching apparatus. This process is performed at a substrate temperature of room temperature, a pressure of 1 Pa, an RF power of 500 W, and a bias of 50 W to completely transfer the lens matrices19to the underlying infrared absorbing layer, thereby forming the transparent resin upper layer10ahaving an infrared absorbing function.

Note that the shape of a microlens can be matched with optimal optical characteristics by using resin materials having different etching rates, e.g., a resin with a low etching rate, such as phenol resin, for the lens matrix19(or using a material with a high etching rate as a resin for an underlying infrared absorbing layer).

EXAMPLE 2 OF MANUFACTURING METHOD

Another method of manufacturing the solid-state imaging device20will be described in detail next.

FIG. 7is a sectional view taken along a line B—B of the solid-state imaging device20inFIG. 6, which is manufactured by the method according to Example 2. As shown inFIG. 7, in the solid-state imaging device20, a planarized layer15having an average thickness of 0.6 μm is formed on a semiconductor substrate11on which photo diodes13are formed, and a 0.5-μm thick ultraviolet absorbing layer14is stacked on the planarized layer15by coating. In addition, 0.9-μm thick colored lower layers10bin three colors are formed by using reactive dyes as coloring materials. Note thatFIG. 7shows only G (Green) pixels in a Beyer arrangement becauseFIG. 7is a sectional view taken along a line B—B inFIG. 6.

A thin film26as an infrared absorbing layer and a thin film of a low refractive index resin27, each having a thickness of about 0.1 μm, are formed on the colored lower layers10bby spin coating. A portion of a thin film as an infrared absorbing layer which is located in the recess portion between microlenses10has a relatively large thickness of about 0.5 μm. This is because a recess having a depth of about 0.4 μm is formed in advance between colors of the color filter by dry etching, as will be described later.

FIGS. 8A to 8Care views for explaining the method of manufacturing the solid-state imaging device20. First of all, as shown inFIG. 8A, each planarized layer15having an infrared absorbing function and the ultraviolet absorbing layer14is formed on the semiconductor substrate11by coating using a spin coating technique. These layers are hardened by using, for example, a hot plate at 230° C. In addition, the colored lower layers10bin three colors are sequentially formed by photolithography, as in Example 1, using colored resists (photosensitive acrylic resin base) containing dyes as coloring materials.

After the infrared absorbing layer26and lens matrices19are formed, the lens matrices are transferred by dry etching to form the microlenses10, as in Example 1. In this case, the colored lower layers10bare partly etched. A recess28having a depth of 0.4 μm is formed between the pixels of the colored lower layers10b.

As shown inFIG. 8B, the thin film26of the infrared absorbing layer having a thickness of about 0.1 μm (having a larger thickness in a recess between microlenses) is formed. As shown inFIG. 8C, the low refractive index resin27(fluorine-based acrylic resin: refractive index of 1.45) having a thickness of about 0.1 μm is formed by coating. Stacking the low refractive index resin27can decrease the reflectance by about 2% (i.e., a 2% increase in transmittance) as compared with an arrangement without the low refractive index resin27(e.g., the arrangement shown inFIG. 8B).

As described above, the solid-state imaging device20and its manufacturing method according to this embodiment can obtain at least any one of the following effects.

First, since the transparent resin upper layer10aand planarized layer15of the solid-state imaging device20have the infrared absorbing function, there is no need to use any conventional infrared cut filters. This makes it possible to easily reduce the size of a camera.

Second, since a plurality of types of infrared absorbing agents having different infrared absorption wavelength ranges are distributed to the respective constituent elements to give them absorption abilities, a wide-range infrared absorbing function can be arbitrarily set in the solid-state imaging device20without any difficulty. In addition, this function can be provided in an optimal place in consideration of the heat resistance or light resistance of each infrared absorbing agent.

Third, since the lens matrices19are transferred to the infrared absorbing layer26by dry etching, a solid-state imaging device having a thin-film arrangement with high utilization efficiency of light can be provided. In addition, since part of each colored lower layer10bis also etched, a further reduction in thickness can be achieved. This makes it possible to provide a solid-state imaging device with higher image quality.

Fourth, providing an ultraviolet absorbing function for the surface of each microlens10or an underlayer of each colored lower layer10bcan protect an infrared absorbing agent with relatively poor light resistance.

Fifth, by forming thin, low refractive index resin films on the surfaces and non-opening portions of the microlenses10, reflected light can be reduced. This can improve the image quality of the solid-state imaging device. In general, reflected light from a microlens or the surface of a thin infrared absorbing layer becomes re-reflected light from the cover glass of a solid-state imaging device to be re-incident on the solid-state imaging device. This light becomes noise to cause a deterioration in image quality. However, the solid-state imaging device20can reduce such noise, and hence can obtain high image quality.

Sixth, the solid-state imaging device20can be manufactured, which can eliminate the necessity of a conventional infrared cut filter by making the transparent resin upper layer10aand planarized layer15have an infrared absorbing function. This is because the above manufacturing method includes a step of forming the planarized layer15having the infrared absorbing function on each photo diode13on the semiconductor substrate11, a step of forming the colored lower layers10b, a step of forming the infrared absorbing layer26, a step of forming the lens matrices19by photolithography and annealing, and a step of transferring a lens matrix pattern to the infrared absorbing layer26by dry etching to form the infrared absorbing layer26into the transparent-resin upper layers10a.

FIG. 9is a sectional view taken along a line A—A of a solid-state imaging device30according to the third embodiment inFIG. 4. The arrangement of the solid-state imaging device30will be described first with reference toFIG. 9.

As shown inFIG. 9, the thickness T5of the microlens10is the sum of the thickness T1of a transparent resin upper layer10aand the thickness T4of a lower colored layer forming part of the microlens (the depth of a notched portion of a colored lower layer10bin the form of a lens) (T5=T1+T4).

The preferable thickness T1of the transparent resin upper layer10a, the preferable thickness T2of the colored lower layer10b, and the area of the interface between the transparent resin upper layer10aand the colored lower layer10bare the same as those in the above embodiments.

This embodiment is also the same as the first embodiment in that the surface of a portion of the colored lower layer10bwhich corresponds to a bottom portion of the microlens10is a curved surface formed by extending the curved surface of the transparent resin upper layer10a.

The outer resin layer31is a thin film formed on the S portion of each colored lower layer10bwhich corresponds to a bottom portion of the microlens10. The outer resin layer31is preferably made of a transparent resin material (low refractive index resin) having a lower refractive index than the colored lower layer10b. In addition, the outer resin layer31is preferably formed by coating to a thickness that can easily obtain an antireflection effect by light interference at the colored lower layer and low refractive index resin. This is because the colored lower layer10bcontains a color material (pigment or dye), and hence tends to optically have a high refractive index. Owing to this antireflection effect, the influence of reflected light from a non-opening portion25can be reduced to prevent a deterioration in image quality due to re-incident light.

The refractive index of the transparent resin upper layer10aas part of the microlens10is preferably decreased to reduce surface reflection. In order to increase the amount of light transmitted, a thin optical film for the reduction of reflection may be inserted between the transparent resin upper layer and the colored lower layer. Alternatively, an antireflection film may be stacked on the entire surface of the microlens10. The transparent resin upper layer10awith a low refractive index is preferable for the present invention directed to minute pixels because a thicker film can be formed as compared with a case wherein the transparent resin upper layer has a high refractive index.

The transparent resin upper layer10ais formed from a fluorine-based acrylic resin which is a low refractive index resin. This makes it possible to reduce reflected light at the microlens10.

In general, the focal lengthfof a lens having a radiusris given by
f=n1/(n1−n0)·r(1)
whereris the radium of the spherical surface, n0is the refractive index of air, and n1is the refractive index of the lens. For example, a lens with refractive index n1=1.61 has a focal length of 2.64 r in an air medium (refractive index n0=1).

As described above, it is generally difficult to form a hemispherical microlens with a thickness of 0.4 μm or less. If, however, the transparent resin upper layer10ais formed by using a transparent resin with a low refractive index of 1.5 or less, and preferably a refractive index in the range of 1.45 to 1.40, a relatively thick, hemispherical microlens10can be stably formed. For example, using a transparent fluorine-based acrylic resin with a refractive index of 1.43 makes it possible to increase the thickness of the microlens from 0.4 μm by 1.25 times to 0.5 μm.

The depth of dry etching, conditions for gases to be used, and conditions for colored resins to be used for the colored lower layer10band resins and dyes for photosensitive colored resists to be used for the formation of the colored lower layer10bin forming the microlens10are the same as those in the above embodiments.

EXAMPLE OF MANUFACTURING METHOD

A method of manufacturing the solid-state imaging device30will be described in detail next.

In the solid-state imaging device30according to this example, the peak thickness T1of a transparent resin upper layer10a(the height from the bottom surface to the vertex of the central portion) is set to 0.3 μm, and the thickness T5, i.e., the sum of the peak thickness and the depth of a notched portion of a colored lower layer10bin the form of a lens, is set to about 0.8 μm. In addition, the thickness T2of the colored lower layer10balone is set to 0.9 μm. With such settings, the under-lens distance becomes as low as about 3.1 μm, which is 56% of 5.5 μm in the prior art.

The R (red), G (Green), and B (Blue) colored lower layers10bare formed by using acrylic-based photosensitive colored resists obtained by preparing coloring materials mainly including dyes represented by color indices, C.I. Acid Red 114, C.I. Acid Green 16, and C.I. Acid Blue 86, together with acrylic-based resins, and a cyclohexane solvent. The amount of coloring material added is about 20% in terms of solid content ratio in each resist.

FIGS. 10A to 10Care views for explaining the method of manufacturing the solid-state imaging device30. First of all, as shown inFIG. 10A, photo diodes13, light-shielding films16, and passivations are formed on a semiconductor substrate11. A planarized layer15is formed on the semiconductor substrate11by spin coating using a thermosetting acrylic resin coating solution. In addition, colored lower layers (33) are formed using R (red), G (Green), and B (Blue) photosensitive colored resists by performing photolithography three times. The respective photosensitive colored resists in R (Red), G (Green), and B (Blue) are coated by spin coating, and exposure is performed by using a stepper exposure apparatus.

As shown inFIG. 10B, a transparent resin upper layer10ais formed on the R (red), G (Green), and B (Blue) colored lower layers10bby spin coating using a thermosetting acrylic resin coating solution (first resin coating solution).

The transparent resin upper layer10ais coated with a photosensitive acrylic-based resin by spin coating, and a hemispherical lens matrix19is formed by exposure, development, and heat flow. Note that the temperature in a heat flow process is set to, for example, 190° C.

The semiconductor substrate11on which the lens matrices19are formed is etched by a dry etching apparatus using O2gas. This etching process is executed at, for example, a substrate temperature of room temperature, a pressure of 1 Pa, an RF power of 500 W, and a bias of 50 W.

An outer resin layer31which is a thin transparent resin film is formed, as shown inFIG. 10C, by spin-coating a thermosetting fluorine-based acrylic resin having a refractive index of 1.45 (second resin coating solution) (obtained by diluting the first resin coating solution in an organic solvent) to a thickness of about 0.09 μm.

In this example, as resin materials for the colored lower layer10band planarized layer15, acrylic resins which have almost the same refractive index in the refractive index range of 1.51 to 1.55 at a light wavelength of 550 nm are used. The transparent resin upper layer10ais formed by using a fluorine-based acrylic resin with a refractive index of 1.45 which is available from Nippon Kayaku Co., Ltd. It is relatively difficult to accurately measure the refractive indices of the colored lower layers10bdue to the coloring materials contained in the layers. However, the refractive index of the R (red) layer which is 1.61 at 700 nm (the R (red) layer exhibits large absorption with respect to 550-nm green light, and hence it is difficult to accurately measure a refractive index at 550 nm).

FIG. 11is a sectional view taken along a line B—B inFIG. 4.FIG. 4shows non-opening portions25of the solid-state imaging device30. In the stage shown inFIG. 10B, the colored lower layers with a high refractive index are exposed on the non-opening portions25and portions S corresponding to bottom portions of microlenses. In the final stage, however, this surface is coated with the outer resin layer31having a thickness of about 0.09 μm. The light interference effect by the outer resin layer31, together with light absorption by the colored lower layer, can greatly reduce re-reflected light from the non-opening portion25. The bottom portion S of the microlens, which is the surface of the colored lower layer, is slightly roughened by dry etching or the like. This also provides the effect of reducing reflected light.

In this example, a description of a step of exposing pad portions (electrical connection portions) of an imaging device is omitted. If the outer resin layer31is used in the form of an alkali soluble photosensitive resin, the step of exposing pad portions can be replaced with the exposure and development steps. In addition, in this example, the thin outer resin layer31is stacked. However, the outer resin layer31may be omitted. In this arrangement, although the amount of reflected light from the non-opening portion25inFIG. 11slightly increases, since the etching process described in this example as well can also be used as the step of exposing pad portions, the omission of the step will achieve a reduction in cost.

The solid-state imaging device30and its manufacturing method according to this embodiment described above can obtain at least any one of the following effects.

First, the under-lens distance is reduced to improve the light condensing performance, and a device can be easily processed even with a small pixel pitch for the following reasons. In this solid-state imaging device, each microlens has at least a two-layer structure constituted by a transparent resin upper layer and colored lower layer, and the interface between the transparent resin upper layer and the colored lower layer is flat. In addition, the surface of a portion of the colored lower layer which corresponds to a bottom portion has a curved surface formed by extending the curved surface of the transparent resin upper layer, and the refractive index of the transparent resin upper layer is lower than that of the colored lower layer. Therefore, the under-lens distance can be made smaller than that in the prior art, and each microlens10having a predetermined thickness or more can be formed.

Second, degradation in the color purity of a chromatic lens is suppressed to contribute to high image quality, and the S/N ratio can be increased by reducing reflected light from each non-opening portion for the following reasons. In this solid-state imaging device, the surface of each colored lower layer is covered with a thin transparent resin film having a lower refractive index than the colored lower layer, and the transparent resin upper layer is made of a fluorine-based acrylic resin.

FIG. 12Ais a top view of a solid-state imaging device40according to the fourth embodiment viewed from the microlens side, and is also a view showing a two-dimensional (planar) arrangement of colored lower layers and microlenses in the Beyer arrangement.FIG. 12Bis a sectional view taken along a line A—A of the solid-state imaging device40inFIG. 12A. The arrangement of the solid-state imaging device40will be described first-with reference toFIG. 12B.

As shown inFIG. 12B, the solid-state imaging device40includes substantially hemispherical microlenses41, a semiconductor substrate11, photo diodes13, a planarized layer15, light-shielding layers (also serving as electrodes)16, and an outer resin layer31.

Each microlens41has a lens matrix41aformed by dry etching or the like, a transparent resin intermediate layer41b, and a colored lower layer10b. At least part of the transparent resin intermediate layer41band colored lower layer10bforms part of a substantially hemispherical shape.

The transparent resin intermediate layer41bis formed as an underlayer of the lens matrix41a, and is made of the same material as that for the transparent resin upper layer10ain the first to third embodiments described above. The colored lower layer10bis formed as an underlayer of the transparent resin intermediate layer41b. The interface between a transparent resin upper layer41aband the colored lower layer10bhas a shape conforming to the surface of the photo diode13, i.e., a flat shape. The area of this flat surface corresponds to the effective area of the colored lower layer10b.

The above arrangement of the microlens41makes it possible to decrease an under-lens distance D1. This allows the substantial lens thickness to be 0.5 μm or more so as to facilitate microlens processing with a pixel pitch of 3 μm or less.

FIG. 13is an enlarged view of the microlenses41, and is also a view for explaining the thickness of each colored lower layer10b. As shown inFIG. 13, the solid-state imaging device40according to this embodiment has an arrangement which satisfies the condition T4≦0.52T2where T4is the thickness of a portion of the colored lower layer10bwhich forms a curved portion of the microlens41, and T2is the thickness of the colored lower layer10b. In this arrangement, an interface portion of the colored lower layer10bis used as a lens to minimize the under-lens distance D1, and at the same time, degradation in the color impurity of the colored pixel layer can be prevented.

Basically, in order to decrease the under-lens distance, dry etching is performed as deeply as possible. If, however, etching is done to the underlayer surface of the colored pixel layer, the flat surface (effective surface) of the colored pixel layer decreases. As a consequence, incident light with degraded color impurity from the periphery of each microlens increases in amount, leading to a deterioration in image quality. Excessive etching (T4>0.5T) will produce gaps between color filters, reducing the open area ratio. In addition, with T4>0.5T, as shown inFIG. 14, a wavelength l3of light crossing each colored pixel becomes excessively small, adversely affecting the color impurity (image quality). For this reason, it is necessary for the thickness of part of a colored pixel layer to satisfy the condition T4≦0.5T2.

Note that the lower limit of T4preferably satisfies the condition 0.02T2≦T4for the following reason. A resin is dry-etched with a resolution of about 0.02. This resolution corresponds to about 0.02T2in scale on a colored lower layer. It is therefore believed that when the colored lower layer is etched, the depth of the etched portion becomes equal to or more than the resolution of dry etching, i.e., equal to or more than 0.02T2.

In addition, the planarized layer15is formed by using a resin with a transmittance of 40% or less at the exposure wavelength (365 nm) and a transmittance of 90% in the visible range. This arrangement is employed because the transmittance of the colored lower layer10bat the exposure wavelength (365 nm) and the transmittance of the planarized layer15as an underlayer of the colored lower layer10bat the exposure wavelength (365 nm) greatly influence the pixel shape reproducibility of the colored lower layer10b, as described with reference the layer having the ultraviolet absorbing function which is part of the solid-state imaging device according to the second embodiment. That is, the reflectance at the wavelength of ultraviolet light (356 nm) which is the exposure wavelength used when the colored lower layer10bis formed can be suppressed low, and the pixel shape reproducibility of the colored lower layer10bat a pixel size of 3.5 μm or less can be improved. In the solid-state imaging device40with a pixel size of 3.5 μm or less, or a pixel size of 2.5 μm or less, or a pixel size of 2 μm or less, in order to ensure high optical characteristics or high image quality, the pixel size of the colored lower layer10bmust be controlled on the submicron order. Adding an ultraviolet absorbing function to an underlayer of a colored lower layer can provide a noticeable pixel shape improving effect in the range of 2.5 μm to 2 μm.

A method of manufacturing the solid-state imaging device40will be described next.FIGS. 15A to 15Gare views sequentially showing steps in an example of the method of manufacturing the solid-state imaging device40.

As shown inFIG. 15A, first of all, a planarized layer15having a predetermined thickness is formed on a semiconductor substrate11, which has photo diodes13, light-shielding layers16, and the like, by coating a resin solution obtained by adding an ultraviolet absorbing agent to a transparent resin such as acrylic resin by spin coating or the like, and heating/hardening the solution. For example, as a transparent resin for the formation of the planarized layer15, one of the following, other than the above acrylic resin, can be used: epoxy, polyester, urethane, melamine, urea resin such as area, styrene resin, phenol resin, and copolymers thereof.

A method of reducing the transmittance at the exposure wavelength (365 nm) to 40% or less can be implemented by adding an ultraviolet absorbing compound or ultraviolet absorbing agent to the above transparent resin or by the pendant method (the agent in the form of a reactive ultraviolet absorbing agent or the like is bonded to resin molecular chains).

Ultraviolet absorbing agents that can be used include a benzotriazole-based compound, benzophenone-based compound, triazine-based compound, salicylate-based compound, coumarin-based compound, xanthene-based compound, methoxy-cinnamate-based compound, and the like. Alternatively, an ultraviolet absorbing agent made of particles of a metal oxide such as cerium oxide or titanium oxide may be used.

Table 1 below shows the results of colored lower layer shape evaluation with the reflectances of the colored lower layers10bin the respective colors at the exposure wavelength (365 nm) and a pixel size of 3.5 μm or less upon formation of the planarized layers15respectively having transmittances of 10%, 20%, 30%, 40%, and 50% at the exposure wavelength (365 nm).

TABLE 1Colored lower layer shape evaluation onreflectance (%) with respect to each transmittanceof planarized layer at 365 nm50%Shape40%Shape30%Shape20%Shape10%ShapeC2.2%Δ1.4%◯0.8%◯0.3%◯0.1%◯M11.3X7.2X4.1X1.8◯0.5◯Y5.0X3.2X1.8◯0.8◯0.2◯R2.0%Δ1.2%◯0.7%◯0.3%◯0.1%◯G0.8◯0.5◯0.3◯0.1◯0.0◯B0.3◯0.2◯0.1◯0.0◯0.0◯NoteC: CyanM: MagentaY: YellowR: RedG: GreenB: BlueNoteSee Table 2 for transmittance of each colored lower layer alone (containing no planarized layer ultraviolet absorption) at 365 nm. Numeral (%) on right side of each colored lower layer is product of squares of transmittance of planarized layer at 365 nm and transmittance of planarized layer at 365 nm, and indicates reflectance at 365 nm.NoteColored lower layer shape is evaluated with fine pixel size of 3.5 μm or less.

As shown in Table 1, with regard to complementary color pixels (C, M, Y), when the transmittance of the planarized layer15is 20% or less at the exposure wavelength (365 nm), colored lower layer shapes in all three colors are reproduced with high precision. With regard to primary color pixels (R, G, B), when the transmittance is 40% or less, colored lower layer shapes are reproduced with high precision.

Table 2 below shows the transmittances of colored lower layers alone (thickness: 1 μm) at the exposure wavelength (365 nm).

It is obvious from the results in Tables 1 and 2 that the transmittances of these colored lower layers10bat the exposure wavelength (365 nm) and the transmittances of the planarized layers15as underlayers of the colored lower layers10bat the exposure wavelength (365 nm) greatly influence the pixel shape reproducibility of the colored lower layers10b. This tendency becomes apparent when the pixel size is 3.5 μm or less, and especially apparent when the pixel size is 3.0 μm or less.

As shown inFIG. 15B, the planarized layer15is spin-coated with colored resists in which dyes are contained in advance, thereby forming colored photosensitive layers. A series of patterning processes including pattern exposure, development, and the like are performed for the layers to form the colored lower layers10bin the respective colors on the planarized layer15.

It suffices if each colored lower layer10bhas a thickness sufficient for intended color separation, and the thickness is not specifically limited. In general, it suffices if this thickness falls within the range of 0.4 μm to 1.5 μm. A colored resist and a resin material for the transparent resin intermediate layer41bformed on the colored lower layer10bare preferably acrylic-based photosensitive resins in consideration of adhesive force, refractive index, and the like.

A dye may be used in a dissolved form into the prime solvent of a colored resist, in a dispersed form, or a form in which a dye is contained in a resin skeleton, i.e., a so-called pendant form. A general dyeing method using a dye bath is not preferable in terms of cost because of an increase in the number of steps. A color filter using a dye as a coloring material can perform high filtration (foreign substance removal) of 0.2 μm to 0.1 μm in the stage of a colored resist, and hence an imaging device having high image quality and greatly increased S/N ratio can be obtained as compared with the case wherein a colored resist dispersed with an organic pigment whose filtration is limited to 1 μm to 0.5 μm is used.

Dyes that can be used include azo-based dyes, xanthenium-based dyes, phthalocyanine-based dyes, anthraquinone-based dyes, coumarin-based dyes, styryl-based dyes, and the like. Primary color dyes, i.e., red, green, and blue dyes, complementary color dyes, i.e., cyan , magenta, and yellow dyes, and dyes obtained by adding a green dye to them can be used.

Although colored resists in which dyes are contained in advance are used as materials for the above colored lower layers10b, colored resins obtained by using organic pigments as coloring materials may be used. If organic pigments are used, etching rates in dry etching vary depending on the types of pigments used, and hence lens shapes tend to vary for the respective colors. The surfaces become rough. In an imaging device with a fine pixel pitch, the particle size (particle) of a pigment itself is likely to adversely affect the S/N ratio, and it is difficult to perform filtration (foreign substance removal) of the coloring resist material. For these reasons, the colored resins containing dyes as coloring materials are preferably used.

As shown inFIG. 15C, a photosensitive resin layer is formed by spin-coating a phenol-based photosensitive resin solution having heat reflow properties and drying/hardening it. A series of patterning processes including pattern exposure, development, and the like are performed to form a transparent resin intermediate layer41bhaving a predetermined thickness and a patterned resin layer43having an opening portion42above the light-shielding layer16are formed on the colored lower layers10b.

In this case, the thickness of the transparent resin intermediate layer41b(the peak thickness of the layer in the form of a lens) is not specifically limited. However, the lower thickness limit that can absorb unevenness of a color filter as an underlayer is preferably 0.2 μm or more. The upper limit thickness of the transparent resin intermediate layer41bis preferably 1 μm because this solid-state imaging device is directed to a fine pixel pitch.

As shown inFIG. 15D, a photosensitive resin layer44having a predetermined thickness is formed by spin-coating an acrylic-based photosensitive resin solution having heat reflow properties and drying/hardening it.

As shown inFIG. 15E, a series of patterning processes including pattern exposure, development, and the like are performed for the photosensitive resin layer44to form lens patterns44aon the colored lower layers10b.

As shown inFIG. 15F, heating reflow is performed for the lens patterns44aat a predetermined temperature to form lens matrices44beach having a predetermined curvature. In this case, the radius of curvature of each lens matrix44bis about 0.7 μm.

As shown inFIG. 15G, the semiconductor substrate11on which the lens matrices44bare formed is processed by a dry etching apparatus to etch the lens matrices44b, transparent resin intermediate layers41b, colored lower layers10b, and planarized layer15to form the microlenses41and electric connection pads45.

Through the respective steps described above, the solid-state imaging device40can be obtained, in which the microlenses41constituted by the lens matrices41a, transparent resin intermediate layers41b, and colored lower layers10band the electric connection pads45are formed on the semiconductor substrate11on which the photo diodes13and light-shielding layers16are formed.

Note that the etching end point of dry etching is set such that the thickness T4of part of the colored lower layer10bbecomes ½ or less of the thickness T of the colored lower layer10b. In dry-etching the lens matrices41a, etching tends to relatively speed up in the recess portions between the lens matrices41a, resulting in a deterioration in the finished shape of each microlens. In order to reduce this deterioration, the entire lens matrix is preferably covered with a thin transparent resin layer having a thickness of about 0.05 μm to 0.3 μm before dry etching. Inserting this step can execute lens matrix transfer more smoothly.

In addition, an antireflection film may be formed on the entire surface of each microlens41. The depth of dry etching, conditions, gases that can be used for dry etching, and the like are the same as those in the first embodiment.

EXAMPLE OF MANUFACTURING METHOD

An example of a method of manufacturing the solid-state imaging device40will be described in detail next with reference toFIGS. 15A to 15G.

First of all, as shown inFIG. 15A, a 0.6-μm thick planarized layer15is formed on a semiconductor substrate11, on which photo diodes13, light-shielding films16, and passivations, and the like are formed, by coating a resin solution obtained by adding an ultraviolet absorbing agent to a thermosetting acrylic resin or the like, and heating/hardening it. In this case, the transmittance of the 0.6-μm thick planarized layer15at the exposure wavelength (365 nm) is 40%.

Coloring materials mainly including dyes represented by color indices, C.I. Acid Red 114, C.I. Acid Green 16, and C.I. Acid Blue 86, are mixed in acrylic-based resins to be formed into photoresists, together with a cyclohexane solvent, to form R, G, and B acrylic-based colored resists. The amount of coloring material added is about 20 wt % in terms of solid content ratio (the sum of polymer, monomer, coloring material, and the like) in each resist.

As shown inFIG. 15B, a patterning process including the formation of a colored photosensitive layer, pattern exposure, development, and the like is performed three times by using R, G, and B acrylic-based colored resists to form 1.2-μm thick R, G, and B colored lower layers10b. In this case, the respective colored photosensitive layers are formed by spin coating, and pattern exposure is performed by using a stepper exposure apparatus using the exposure wavelength (365 nm).

As shown inFIG. 15C, a photosensitive resin layer is formed by spin-coating a photosensitive, thermosetting phenol-based resin solution having sensitivity with respect to ultraviolet light of 365 nm, and drying/hardening it. Thereafter, a series of patterning processes including pattern exposure, development, and the like are performed to form a 0.4-μm thick transparent resin intermediate layer41bon the colored lower layers10band a patterned resin layer43having opening portions42on the light-shielding layers16.

As shown inFIG. 15D, a photosensitive resin layer44having a predetermined thickness is formed by spin-coating an acrylic-based photosensitive resin solution having heat reflow properties and drying/hardening it. As shown inFIG. 15E, a series of patterning processes including pattern exposure, development, and the like are performed for the photosensitive resin layer44to form lens patterns44aon the colored lower layers10b.

As shown inFIG. 15F, a heating reflow process is performed for the lens patterns44aat a temperature of 190° C. to form lens matrices44beach having a radius of curvature of about 0.7 μm.

An etching process is performed for the semiconductor substrate11, on which the lens matrices44bare formed, by using a dry etching apparatus using O2gas. This etching process is executed at, for example, a substrate temperature of room temperature, a pressure of 5 Pa, an RF power of 500 W, and a bias of 100 W.

Through the respective steps described above, as shown inFIG. 15G, the solid-state imaging device40can be obtained, in which the microlenses41constituted by lens matrices41a, the transparent resin intermediate layers41b, and the colored lower layers10band the light-shielding layers16are formed on the semiconductor substrate on which the photo diodes13and light-shielding layers16are formed.

According to the experiment conducted by the present inventors, the thickness T2of the colored lower layer10bwas 0.7 μm, whereas the thickness T4of a portion of the colored lower layer10b(a portion which forms the curved surface of the microlens41) was 0.3 μm. The under-lens distance in the solid-state imaging device40was about 2.1 μm. That is, an under-lens distance ½ or less of the under-lens distance in the conventional solid-state imaging device, which is 5.5 μm, could be realized.

According to the solid-state imaging device40and its manufacturing method according to this embodiment described above, at least any one of the following effects can be obtained.

First, the under-lens distance can be greatly reduced, and hence the incident light condensing performance greatly improves. In addition, since oblique incidence of noise light can be greatly reduced, the image quality of the solid-state imaging device can be improved.

Second, the reduction in lens thickness (or a reduction in lens matrix thickness during the manufacturing process) accompanying a reduction in pixel size can be reduced. This makes it possible to provide a solid-state imaging device with a microlens thickness of 0.5 μm or more which causes no problems in manufacture.

Third, a portion of the colored lower layer10bis etched to form a lens shape, and hence when a chromatic microlens is used, the difference in color between incident light at the central portion and that at the peripheral portion is eliminated, thereby providing a solid-state imaging device with high image quality. At the same time, since etching is stopped midway in the direction of thickness of the colored lower layer10b, even if slight variations in etching behavior occur in the direction of thickness, the influences on colors and light condensing performance can be reduced.

Fourth, since the recess portions between the microlenses41are colored, reflected light components from the recess portions can be reduced. This can lead to a further improvement in image quality.

Fifth, according to the solid-state imaging device manufacturing method, the conventional complicated step in exposing electrical connection pads can be omitted, and a solid-state imaging device having electrical connection pads obtained by only a simple step of dry etching can be manufactured.

FIG. 16is a top view of a solid-state imaging device50according to the fifth embodiment viewed from the microlens side, and is also a view showing a two-dimensional (planar) arrangement of colored lower layers and microlenses in the Beyer arrangement.FIG. 17is a sectional view taken along a line B—B inFIG. 16.FIG. 18is a sectional view taken along a line A—A inFIG. 16. The arrangement of the solid-state imaging device50will be described first with reference toFIGS. 16 to 18.

As shown in each drawing, the solid-state imaging device50includes substantially hemispherical microlenses51, a semiconductor substrate11, photo diodes13, a planarized layer15, light-shielding layers16, and non-opening portion layers52. Each microlens51includes a transparent resin upper layer10amade of a fluorine-based acrylic resin and a colored lower layer10b.

The non-opening portion layer52is a thin film made of a transparent resin material (low refractive index resin) having a low refractive index, such as a fluorine-based acrylic resin, and formed in a non-opening portion25between the microlenses51on the upper surface of the colored lower layer10b. The solid-state imaging device50is designed to reduce reflected light from each microlens by forming the microlens51and non-opening portion layer52using a fluorine-based acrylic resin which is a low refractive index resin. In addition, since the colored lower layer10bcontains a coloring material (pigment or dye), its refractive index tends to be optically high. For this reason, a thin transparent resin film for forming the non-opening portion layer52is preferably formed by coating to a thickness that allows easy acquisition of an antireflection effect based on light interference between the colored lower layer10band the low refractive index resin. This makes it possible to reduce the influence of reflected light from the non-opening portion25and prevent a deterioration in image quality due to re-incident light.

In addition, the solid-state imaging device50is aimed at improving heat resistance by forming each microlens51and non-opening portion layer52using a fluorine-based acrylic resin which is a heat-resistant resin. The use of a fluorine-based acrylic resin prevents discoloration of the microlenses even after annealing at about 250° C. for about 1 hr.

Preferable conditions associated with the thickness T1of the transparent resin upper layer10aand the thickness T2of the colored lower layer10bare the same as those for the transparent resin upper layer10aand colored lower layer10bdescribed in the first embodiment.

In general, the focal lengthfof a lens having a radiusris represented by equation (1) described above. For example, a lens with refractive index n1=1.61 has a focal length of 2.64 r in an air medium (refractive index n0=1). As described above, it is difficult to form a hemispherical microlens with a thickness of 0.4 μm or less. If, however, a microlens is formed by using a transparent resin with a low refractive index of 1.5 or less, and preferably a refractive index in the range of 1.45 to 1.40, a hemispherical microlens having a thickness of 0.5 μm or more can be stably formed. For example, using a transparent fluorine-based acrylic resin with a refractive index of 1.43 makes it possible to increase the thickness of the microlens from 0.4 μm by 1.25 times to 0.5 μm.

A fluorine-based acrylic resin is a resin having a low refractive index and a high transmittance (reflectance is lower about 2%). This transmittance is higher than that of a high refractive index resin containing the above phenol resin skeleton and having a refractive index of 1.6 to 1.7. That a fluorine-based acrylic resin has a high transmittance is effective in improving the sensitivity and image quality of a solid-state imaging device such as a CCD or C-MOS.

EXAMPLE OF MANUFACTURING METHOD

An example of a method of manufacturing the solid-state imaging device50will be described in detail next with reference toFIGS. 18 and 19Ato19C.FIGS. 19A to 19Care sectional views taken along a line B—B inFIG. 16which explain a manufacturing process for the solid-state imaging device5. As shown inFIG. 18, in the solid-state imaging device50according to this example, microlenses51constituted by photo diodes13, colored lower layers10b, and transparent resin upper layers10amade of a fluorine-based acrylic resin are formed on a semiconductor substrate11.

In this example, as resin materials for the colored lower layer10band a planarized layer15, acrylic resins which have almost the same refractive index in the refractive index range of 1.51 to 1.55 at a light wavelength of 550 nm. The transparent resin upper layer10ais formed by using a fluorine-based acrylic resin with a refractive index of 1.45 which is available from Nippon Kayaku Co., Ltd. It is relatively difficult to accurately measure the refractive indices of the colored lower layers10bdue to the coloring materials contained in the layers. However, the refractive index of the R (red) layer is 1.61 at 700 nm (the R (red) layer exhibits large absorption with respect to 500-nm green light, and hence it is difficult to accurately measure a refractive index at 550 nm).

In addition, the colored lower layer10bhas a refractive index different from that of the matrix resin (shifts to the higher refractive index side) due to the influence of a coloring material dispersed in the resin. As shown inFIG. 16, as a color arrangement in this example, a so-called Beyer arrangement is used, in which one pixel is constituted by two G (Green) elements and one each of R and B (Blue) elements, i.e., a total of four elements. Note that photosensitive colored resists available from Toyo Ink Mfg. Co., Ltd. which use organic pigments as coloring materials can be used for the R (Red), G (Green), and B (Blue) colored lower layers10b.

FIGS. 19A to 19Care views for explaining the method of manufacturing the solid-state imaging device50. As shown inFIG. 19A, the planarized layer15is formed on the semiconductor substrate11, on which the photo diodes13, light-shielding layers16, and passivations are formed, by spin-coating a thermosetting acrylic resin coating solution. In addition, the colored lower layers10bare formed by performing photolithography three times using R (Red), G (Green), and B (Blue) photosensitive colored resists. The R (Red), G (Green), and B (Blue) photosensitive colored resists are coated by spin coating, and exposure is done by a stepper exposure apparatus.

As shown inFIG. 19B, the transparent resin upper layer10ais formed on the R (Red), G (Green), and B (Blue) colored lower layers10bby spin coating using a thermosetting fluorine-based acrylic resin coating solution (available from Nippon Kayaku Co., Ltd.).

A photosensitive acrylic-based resin having heat flow properties is coated on the transparent resin upper layer10aby spin coating, and is subjected to exposure, development, and heat flow to form hemispherical lens matrices19. The heat flow temperature at this time is, for example, 200° C.

An etching process is then performed for the semiconductor substrate11, on which the lens matrices19are formed, by using a dry etching apparatus using O2gas. This etching process is executed at a substrate temperature of room temperature, a pressure of 1.2 Pa, an RF power of 500 W, and a bias of 200 W.

Finally, the solid-state imaging device50shown inFIG. 19Ccan be obtained by executing an etching process so as to leave a 0.1-μm thick transparent resin (fluorine-based acrylic resin) on each non-opening portion25between the microlenses51.

FIGS. 16 and 17show the non-opening portions25of the solid-state imaging device50. On the non-opening portion25, a color filter having a relatively high refractive index is formed as an underlayer, and a fluorine-based acrylic resin which is a low refractive index resin is deposited on this surface to a thickness of about 0.1 μm. The light interference effect by this thin low refractive index resin film and light absorption by the color filter can greatly reduce re-reflected light from the non-opening portion25.

According to an experiment conducted by the present inventors, the peak thickness T1of the transparent resin upper layer10a(the height from the interface with the colored lower layer10bto the central portion of the lens) of the solid-state imaging device50obtained by this example was 0.9 μm, and the thickness T6of a non-opening portion layer52was 0.1 μm. The thickness T5of the microlens was 0.8 μm, which was obtained by subtracting the thickness T6of the non-opening portion layer52from the thickness T1of the transparent resin upper layer10a. The thickness T7of the colored lower layer10balone was 0.8 μm. In addition, the under-lens distance was about 3.3 μm, which was much smaller than 5.5 μm in the prior art, i.e., 60% thereof. In this example, the microlens pitch was set to 3.5 μm, and the inter-lens gap was set to 0.3 μm.

The states of reflected light in the solid-state imaging device50according to this example and in conventional solid-state imaging device using a lens material with a high refractive index (refractive index of 1.6) for comparison were measured/compared by using an integrating sphere and variable angle goniometer (both available from Murakami Shikisai K.K.). In this case, the integrating sphere is used to check the total amount of reflected light on the entire device surface. The variable angle goniometer is used to check the state of reflected light at variable angles (locally) by changing the angle of the light-receiving portion with respect to incident light (parallel light).

The solid-state imaging device50according to this example decreased in reflectance by 2 to 3% as compared with the prior art throughout the entire visible range when measured with the integrating sphere. In measurement using the variable angle goniometer, light was incident at −5° in almost the regular reflection direction, and the angle of the light-receiving element was changed from +5° to +20°. It was found that the intensity value of reflected light in the solid-state imaging device50was as low as half or less of that in the prior art.

In this example, a description of a step of exposing the pad portions (electrical connection portions) of the solid-state imaging device50is omitted. If a low refractive index resin is used in the form of an alkali soluble photosensitive resin, the step of exposing pad portions can be replaced with the exposure and development steps.

The solid-state imaging device50and its manufacturing method according to this embodiment described above can obtain at least any one of the following effects.

First, the S/N ratio and image quality can be improved by minimizing reflected light from the non-opening portions between the microlenses and the surfaces of the microlenses. This is because in this solid-state imaging device, a transparent resin upper layer made of a fluorine-based acrylic resin is formed on the surface of each colored lower layer, and a non-opening portion layer made of a fluorine-based acrylic resin is formed on each non-opening portion between microlenses so as to prevent reflection from the microlenses.

Second, the substantial lens thickness can be increased from 0.5 μm to 0.3 μm as described above to 0.5 μm or more while the under-lens distance is reduced. This makes it possible to easily process microlenses on an imaging device with a small pixel pitch of 3 μm or less.

Third, since transparent resin upper layers and non-opening portion layers are made of a fluorine-based acrylic resin, heat resistance adaptable to severer processing conditions can be realized as compared with the prior art.

According to the solid-state imaging device and its manufacturing method described above, light condensing performance and S/N ratio can be improved by decreasing the under-lens distance. The substantial thickness of each microlens can be set to 0.5 μm or more. In addition, the open area ratio can be increased by suppressing degradation in the color purity of a chromatic lens.