IMAGE PICKUP UNIT, ENDOSCOPE, AND METHOD FOR MANUFACTURING IMAGE PICKUP UNIT

An image pickup unit includes: a bonded device provided with a light receiving surface, an undersurface, and four side faces, the bonded device including a light receiving element provided with a light receiving circuit, and a circuit element provided with a peripheral circuit and direct-bonded to the light receiving element; a first protective layer covering the four side faces, the first protective layer being made of an inorganic material; a second protective layer covering the first protective layer, the second protective layer being made of metal; and a third protective layer covering the second protective layer, the third protective layer being made of an organic material.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image pickup unit that includes a bonded device made up of direct-bonded two semiconductor devices, an endoscope containing the image pickup unit that includes the bonded device made up of the direct-bonded two semiconductor devices, and a method for manufacturing the image pickup unit that includes the bonded device made up of the direct-bonded two semiconductor devices.

2. Description of the Related Art

Japanese Patent Application Laid-Open Publication No. 2012-164870 discloses a high-sensitive back-illuminated image pickup device with a large light-receiving area. The back-illuminated image pickup device is produced by cutting a bonded wafer made up of direct-bonded image pickup device wafer and peripheral circuit wafer. A direct-bonded interface may contain a gap, in principle. Therefore, an image pickup unit that includes an image pickup device, the direct-bonded interface of which is exposed to a cut surface, might be liable to lower reliability.

Japanese Patent No. 6315859 discloses an image pickup unit in which an image pickup device and cover glass are adhered together using an adhesive layer and a cut surface of an interface of the adhesive layer is covered with sealing resin to prevent penetration of moisture through the cut surface.

SUMMARY OF THE INVENTION

An image pickup unit according to an embodiment includes: a bonded device provided with a light receiving surface, an undersurface on a side opposite the light receiving surface, and four side faces, the bonded device including a light receiving element provided with a light receiving circuit, and a circuit element provided with a peripheral circuit and direct-bonded to the light receiving element; a first protective layer covering the four side faces, the first protective layer being made of an inorganic material; a second protective layer covering the first protective layer, the second protective layer being made of metal; and a third protective layer covering the second protective layer, the third protective layer being made of an organic material.

An endoscope according to an embodiment includes an image pickup unit, wherein the image pickup unit includes a bonded device provided with a light receiving surface, an undersurface on a side opposite the light receiving surface, and four side faces, the bonded device including a light receiving element provided with a light receiving circuit, and a circuit element provided with a peripheral circuit and direct-bonded to the light receiving element, a first protective layer covering the four side faces, the first protective layer being made of an inorganic material, a second protective layer covering the first protective layer, the second protective layer being made of metal, and a third protective layer covering the second protective layer, the third protective layer being made of an organic material.

A method for manufacturing an image pickup unit according to an embodiment includes: direct-bonding a first wafer including a light receiving circuit and a second wafer including a peripheral circuit and thereby producing a bonded wafer provided with a light receiving surface and an undersurface on a side opposite the light receiving surface; forming a frame-shaped grooves in the undersurface of the bonded wafer, the grooves surrounding the light receiving circuit and the peripheral circuit, the grooves being deeper than a bonding interface between the first wafer and the second wafer; placing a first protective layer made of an inorganic material in the grooves; and cutting the bonded wafer along the grooves.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

First Embodiment

In the following description, the drawings based on each embodiment are schematic. A relationship between thickness and width of each component as well as thickness ratios and relative angles among individual components are different from actual ones. Some of dimensional relationships or ratios may differ among the drawings. Illustrations of some components are omitted. The direction from which light enters is designated as an upper direction.

As shown inFIGS.1to4, in an image pickup unit1according to the present embodiment, a bonded device15and cover glass40are adhered together using an adhesive layer30made of transparent resin.

The bonded device15is a back-illuminated image pickup device in which a light receiving element10provided with a light receiving circuit11and a circuit element20provided with a peripheral circuit22are direct-bonded. In other words, a first rewiring layer of the light receiving element10connected to the light receiving circuit11and a second rewiring layer of the circuit element20connected to the peripheral circuit22are direct-bonded. The bonded device15is a substantially rectangular parallelepiped shape, including a light receiving surface15SA, an undersurface15SB on a side opposite the light receiving surface15SA, and four side faces15SS. The light receiving element10includes the light receiving surface15SA and the circuit element20includes the undersurface15SB.

The cover glass40is placed on the light receiving surface15SA of the light receiving element10. With an upper surface of the circuit element20being bonded to the light receiving element10, the undersurface15SB on the side opposite the upper surface is covered with a solder resist film70. Solder80is placed in a hole H70in the solder resist film70. The solder80is connected to the peripheral circuit22through an interconnect layer65(conductor layer60) placed in a via H20.

The bonded device15includes a frame-shaped notch C15provided with a step and provided on outer edges. Notched side faces15SS are covered with an insulating layer50, which is a first protective layer made of an inorganic material, the conductor layer60, which is a second protective layer made of metal, and the solder resist film70, which is a third protective layer made of an organic material. In other words, a bonding interface BI direct-bonded and exposed to the side faces15SS of the bonded device15is covered with an insulating layer50. The insulating layer50is covered with the conductor layer60. The conductor layer60is covered with the solder resist film70. The third protective layer may be an organic material, mesoporous organosilica, or an inorganic material such as silicon nitride or silicon oxide, a film of which is formed by plasma CVD.

The side faces15SS of the bonded device15are not exposed to four side faces1SS of the image pickup unit1shaped as a substantially rectangular parallelepiped. In other words, the bonding interface BI is covered with three types of protective layer made of different materials. This increases reliability of the image pickup unit1.

As described later, the first protective layer combines the insulating layer50for use to insulate the interconnect layer65from a base body made of silicon. The second protective layer, which is the conductor layer60, is placed simultaneously with the interconnect layer65. The solder resist film70is essential in order to place the solder80. The interconnect layer65on a wall surface of the via H20is sandwiched between the insulating layer50and the solder resist film70.

Because the three types of protective layer covering the bonding interface BI is placed to interconnect the solder80and the peripheral circuit22, it is easy to manufacture the image pickup unit1.

<Method for Manufacturing Image Pickup Unit>

A method for manufacturing the image pickup unit will be described with reference to a flowchart ofFIG.5.

Although not illustrated, using a publicly known semiconductor manufacturing technique, a plurality of the light receiving circuits11are placed on a silicon wafer, respective first rewiring layers are placed on the plurality of light receiving circuits11, and consequently, a first wafer10W, which is an image pickup device wafer, is produced. The light receiving circuits11are CMOS (complementary metal oxide semiconductor) light receiving circuits or CCDs (charge coupled devices). Although not illustrated, color filters, microlenses, and the like are placed on the light receiving circuits11.

On the other hand, using a publicly known semiconductor manufacturing technique, a plurality of the peripheral circuits22are placed on a silicon wafer, respective second rewiring layers are placed on the plurality of peripheral circuits22, and consequently, a second wafer20W, which is a peripheral circuit wafer, is produced. The peripheral circuits22primarily process output signals of the light receiving circuits11and process a drive control signal. The second rewiring layers of the second wafer20W include internal electrodes21connected to the interconnect layer65.

The first rewiring layer of the first wafer10W and the second rewiring layer of the second wafer20W are direct-bonded. The direct-bonding is a bonding form in which atoms exposed to bonding surfaces are bonded together without the use of a bonding member in a bonding interface between two layers to be bonded together.

For example, surface-activated bonding is used for direct-bonding. First, for example, ion milling is done to irradiate the respective bonding surfaces of the first wafer10W and second wafer20W with an argon atom beam for 3 minutes, thereby activating the surfaces.

Then, in a high vacuum with an ultimate vacuum of 10−4Pa or below, the first wafer10W and the second wafer20W are stacked together and pressure-bonded (1 N/mm2) at room temperature for 10 minutes, and then heat-treated at 120° C. for 1 hour.

Preferably, the bonding surfaces are polished flat at the atomic level by CMP (chemical mechanical polishing) or the like. For example, the bonding surfaces are processed to 10 nm or below in terms of surface roughness in maximum height (Rmax) or 1 nm or below in terms of center line average roughness (Ra), where Rmax and Ra are defined by JIS-B0601:2001.

Direct-bonding conditions are selected as appropriate. For example, a plasma irradiation process may be used for activation in surface-activated bonding. Pressure bonding conditions are selected, for example, from the following ranges: a pressure of 0.1 N/mm2to 10 N/mm2, a duration of 1 minute to 1 hour, a temperature of room temperature to 200° C.

The first wafer10W of a bonded wafer15A is processed to be thin. For example, a back grinding step and a CMP (chemical mechanical polishing) step are performed from an upper surface (surface on the side opposite the bonding interface) side of the first wafer10W.

In the back grinding step, a diamond wheel called a back grinding wheel is used. The CMP step is a polishing process that involves a chemical action and a mechanical action to reduce surface roughness of the surface ground in the back grinding step.

The first wafer10W is reduced in thickness down to a thickness of 5 μm to 50 μm. Then, a glass wafer40W is adhered to the light receiving surface15SA, which is a polished surface, using an adhesive layer30W. It is sufficient that the glass wafer40W is transparent in a wavelength band of light used for image pickup, and the glass wafer40W is made, for example, of borosilicate glass, quartz glass, single-crystal sapphire, or other glass.

The adhesive layer30W is made of a BCB (benzocyclobutene) resin, an epoxy-based resin, a silicone-based resin, or the like, which has properties such as high transparency (e.g., transmittance at visible wavelengths is 90% or above), high adhesive strength, and high resistance to heat or the like in downstream operations. Regarding a curing method of the adhesive layer30W, as long as predetermined characteristics are satisfied, any of a heat curing method, a UV curing method, a UV curing method+a heat curing method, a UV curing method+a moisture curing method, and a cold setting method may be used depending on the resin.

FIG.6Ashows the bonded wafer15A, to an upper surface of which the glass wafer40W is adhered via the adhesive layer30W. The steps described below are carried out with respect to the undersurface15SB of the bonded wafer.FIG.6Bis an enlarged cross-sectional view of the part enclosed by a dotted line in the lower left ofFIG.6A.

As shown inFIGS.7A and7B, first grooves T20are formed in a lattice pattern in the undersurface15SB of the bonded wafer15W. A plurality of vias (closed-end holes) H20are formed simultaneously with the formation of the first grooves T20.

Although not illustrated, an etching process is performed after an etching mask is placed on the undersurface15SB. The etching mask is an inorganic film such as a silicon oxide film or a silicon nitride film, or an organic film of photoresist, polyimide, BCB, or the like.

In the etching process, the first grooves T20and the vias H20are formed, for example, by wet etching using an alkaline solution such as KOH or TMAH or by dry etching using ICP-RIE.

The internal electrodes21of the second wafer20W serve as etch-stop layers for the vias H20. The first grooves T20, the etching of which is finished simultaneously with the vias H20, is substantially equal in depth to the vias H20, and thus, the first grooves T20do not reach the bonding interface BI.

As shown inFIGS.8A and8B, second grooves T10, deeper than the bonding interface BI, are formed in a lattice pattern along bottom faces of the first grooves T20. Therefore, grooves T15have a plane parallel to the light receiving surface15SA, which was the bottom faces of the first grooves T20.

Regarding a method for forming the second grooves T10, for example, an ion milling process or a blade dicing process is used. The second grooves T10penetrate the first wafer10W, and have bottom faces that are the adhesive layer30W.

The first grooves T20in which the second grooves T10are formed are referred to as the grooves T15. It is sufficient if the grooves T15are at least deeper than the bonding interface BI.

The second grooves T10are deeper than the bonding interface BI, but can be formed in a short period of time because the second grooves T10are formed in the bottom faces of the first grooves T20, which are formed simultaneously with the vias H20.

As described later, the bottom faces of the grooves may be located either on the first wafer10W or on the glass wafer40W. If the step of first-groove formation is carried out separately from the formation of the vias H20, to a depth at least greater than the bonding interface BI using a dicing blade, it is unnecessary to form the second grooves.

As shown inFIG.9, an insulating layer50L, which is a first protective layer made of an inorganic material, is placed on the entire undersurface15SB of the bonded wafer15W. The side faces15SS of the grooves T15are also covered with the insulating layer50L used to place through wirings on the wall surfaces and bottom faces of the vias H20. The insulating layer50L is 0.1 μm to 3 μm thick.

The insulating layer50L is, for example, a silicon oxide film or a silicon nitride film formed using plasma CVD, photo-CVD, or the like. The film formation processes are low-temperature processes, and thus tetraethoxysilane (TEOS), octamethylcyclotetrasiloxane (OMCTS), or the like is used as a source gas in forming a silicon oxide film. In forming a silicon nitride film, a mixed gas such as SiH4+NH3, SiH2CL2+NH3, SiH4+N2, or SiH4+NH3+N2is used as a source gas.

Openings are formed in the insulating layer50L on the bottom faces of the vias H20using, for example, the ion milling process.

As shown inFIG.10, a conductor layer60L, which is a second protective layer made of metal, is placed, covering the insulating layer50L. The conductor layer60L, which is made of aluminum or copper, is placed using, for example, a sputtering process or a vapor deposition process. The conductor layer60L may be placed using a plating process after a seed layer is placed by the sputtering process or the vapor deposition process. The conductor layer60L is 1 μm to 10 μm thick.

The conductor layers60L placed in the vias H20are electrically connected to the internal electrodes21. Although not illustrated, the conductor layers60L extended from the plurality of vias H20each undergoes patterning to become the interconnect layers65insulated from one another. The side faces15SS of the grooves T15are also covered with the conductor layers60L used to place through wirings in the vias H20.

As shown inFIGS.11A and11B, a solder resist film70L, which is a third protective layer made of an organic material, is placed. The solder resist film70L is placed by spin coating, spray coating, screen printing, or the like, covering the patterned conductor layers60L (interconnect layers65). The solder resist film70L made of a resin such as polyimide, is a solder mask used to prevent solder from spreading in a solder placement step S70described later.

Holes H70are provided in the solder resist film70L at locations corresponding to the conductor layers60L extended from respective ones of the plurality of vias H20. The side faces15SS of the grooves T15are also covered by the solder resist film70L for use to place the solder80.

The solder resist film70L is 1 μm to 30 μm thick. Note that it is not necessary that the vias H20and the grooves T15are filled with the solder resist film70L. For example, there may be elongated recesses along the grooves T15on a surface of the solder resist film70L.

As shown inFIG.12, the solder80is placed in the holes H70in order to electrically connect to the outside world. Solder balls or the like are used as the solder80.

As shown inFIGS.12A and12B, the bonded wafer15W is cut along the lattice-patterned grooves T15, i.e., along cutting lines CL.

Note that by adhering a stacked lens wafer including a plurality of lens unit to the glass wafer40W before the cutting step, the bonded wafer15W may be cut together with the stacked lens wafer.

The bonded wafer15W is diced into a plurality of the image pickup units1. The image pickup unit1does not have the bonding interface BI exposed to the side faces which are diced surfaces. The bonding interface BI is covered with the insulating layer50L, which is a first protective layer made of an inorganic material, the conductor layer60L, which is a second protective layer made of metal, and the solder resist film70L, which is a third protective layer made of an organic material.

With the method for manufacturing the image pickup unit1according to the present embodiment, since the bonding interface BI is covered with three different types of material (inorganic material, metal material, and organic material), the image pickup unit1can be made highly reliable. Because an etching step, an insulating layer placement step, a conductor layer placement step, and a solder resist film placement step used to place electrical interconnects in the image pickup unit1allow the bonding interface BI to be protected, it is easy to implement the method for manufacturing the image pickup unit1according to the present embodiment.

Modifications of First Embodiment

Image pickup units1A to1C according to modifications of the first embodiment have effects similar to the effects of the image pickup unit1. Therefore, components having the same functions as the image pickup unit1are denoted by the same reference numerals as the corresponding components of the image pickup unit1, and description thereof will be omitted.

Modification 1 of First Embodiment

In the image pickup unit1A according to the present modification shown inFIG.13, the side faces of the bonding interface BI are covered only with the insulating layer50, which is a first protective layer made of an inorganic material.

In the image pickup unit1A, since the first grooves T20and the vias H20are formed by a wet etching process, wall surfaces are tilted. Bottom faces of the second grooves T10formed by a dicing blade with a curved tip is located in the light receiving element10.

The image pickup unit1A is higher in reliability than the image pickup unit1, which has the side faces of the bonding interface BI exposed. The image pickup unit1A, which is free of the need to cut the conductor layer60and the solder resist film70, requires a shorter time for the cutting step than the image pickup unit1.

Modification 2 of First Embodiment

In the image pickup unit1B according to the present modification shown inFIG.14, the side faces of the bonding interface BI are covered with the insulating layer50, which is a first protective layer made of an inorganic material. The insulating layer50is covered with the conductor layer60, which is a second protective layer made of metal.

Apart from the formation of the vias H20, the image pickup unit1B has grooves T15A formed by a dicing blade with a V-shaped tip, where the grooves T15A are to become notches C15. Bottom faces of the grooves T15A are located in the cover glass40. The notches C15do not have a plane parallel to the light receiving surface15SA.

The image pickup unit1B is higher in reliability than the image pickup unit1A. The image pickup unit1B, which is free of the need to cut the solder resist film70, requires a shorter time for the cutting step than the image pickup unit1.

Modification 3 of First Embodiment

In the image pickup unit1C according to the present modification shown inFIG.15, the side faces of the bonding interface BI are covered with the insulating layer50, which is a first protective layer made of an inorganic material. The insulating layer50is covered with the solder resist film70, which is a third protective layer made of an organic material.

The second grooves T10, which are to become the notches C15of the image pickup unit1C, is formed by a dicing blade with a curved tip. The bottom faces of the grooves T15are located in the cover glass40.

The image pickup unit1C is higher in reliability than the image pickup unit1A. The image pickup unit1C, which is free of the need to cut the insulating layer50, requires a shorter time for the cutting step than the image pickup unit1.

In the image pickup unit1, the shape of the notches C15, i.e., the shapes of the first grooves T20and second grooves T10, may be the same as any of the image pickup units1A to1C. On the other hand, the shape of the notches C15, in the image pickup units1A to1C may be the same as the image pickup unit1.

Second Embodiment

As shown inFIG.16, an endoscope9according to the present embodiment includes a distal end portion9A, an insertion portion extended from the distal end portion9A, an operation portion9C disposed on a proximal end side of the insertion portion9B, and a universal cord9D extending form the operation portion9C.

The image pickup unit1(1A to1C) is disposed on the distal end portion9A. An image pickup signal outputted from the image pickup unit1is transmitted to a processor (not shown) through a cable passing through the universal cord9D. A drive signal from the processor to the image pickup unit1is also transmitted through a cable passing through the universal cord9D.

As already described, the image pickup unit1(1A to1C) has high reliability. Consequently, the endoscope9has high reliability as well.

The endoscope9may be either a flexible endoscope, the insertion portion9B of which is flexible or a rigid endoscope, the insertion portion9B of which is rigid. The use of the endoscope9may be either medical or industrial.

The present invention is not limited to the embodiments and the like described above, and various alterations, combinations, and applications are possible without departing from the gist of the invention.