Image sensor and method for fabricating the same

An image sensor comprises a substrate, a plurality of photoelectric transducer devices, an interconnect structure, at least one dielectric isolator and a back-side alignment mark. The substrate has a front-side surface and a back-side surface opposite to the front-side surface. The interconnect structure is disposed on the front-side surface. The photoelectric transducer devices are formed on the front-side surface. The dielectric isolator extends downwards into the substrate from the back-side surface in order to isolate the photoelectric transducer devices. The back-side alignment mark extends downwards into the substrate from the back-side surface and references to a front-side alignment mark previously formed on the front-side surface.

FIELD OF THE INVENTION

The present invention relates to a semiconductor device and the method for fabricating the same, more particularly to an image sensor and the method for fabricating the same.

BACKGROUND OF THE INVENTION

An image sensor, such as a metal-oxide-semiconductor (MOS) image sensor is a photoelectric device which can convert optical images into electrical signals and has been widely applied in various consumer products such as, digital cameras, camcorders, personal communication systems (PCSs), game equipment, requiring for improved image resolution.

In order to satisfy the demands for finer image resolution capabilities, it is necessary to increase the pixel integration of MOS image sensors, and the size and pixel pitch of the constituent photoelectric transducer device, (e.g., a photodiode) shall be shrank. However, reduction in the physical size and pixel pitch of the photoelectric transducer devices may result in electrical and optical crosstalk between two adjacent photoelectric transducer devices, the sensitivity of the MOS image sensor is thus correspondingly reduced and, in the worst case, image distortion may occur.

Shallow trench isolations (STI) which are typically formed between two adjacent photoelectric transducer device in a conventional image sensor have been adopted by the prior art to reduce the possibility of electrical crosstalk. However, the STI which has limited depth may not possible to satisfactorily provide an electrical crosstalk barrier. Furthermore, the STI does not provide an effective optical crosstalk barrier when a backside image sensor is fabricated.

Therefore, it is necessary to provide an advanced image sensor and the fabricating method thereof to obviate the drawbacks and problems encountered from the prior art.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, an image sensor comprises a substrate, a plurality of photoelectric transducer devices, an interconnect structure, at least one dielectric isolator and a back-side alignment mark. The substrate has a front-side surface and a back-side surface opposite to the front-side surface. The interconnect structure is disposed on the front-side surface. The photoelectric transducer devices are formed on the front-side surface. The dielectric isolator extends downwards into the substrate from the back-side surface in order to isolate the photoelectric transducer devices. The back-side alignment mark extends downwards into the substrate from the back-side surface and references to a front-side alignment mark previously formed on the front-side surface.

In one embodiment of the present invention, the dielectric isolator comprises a plurality of anti-reflective coating (ARC) layers. In some embodiments of the present invention, the image sensor further comprises an ion doping layer disposed in the substrate and surrounding the dielectric isolator.

In some embodiments of the present invention, the image sensor further comprises a STI extending downwards into the substrate from the front-side surface and connecting to the dielectric isolator. In some embodiments of the present invention, the back-side alignment mark directly aligns to the front-side alignment mark or makes a reference to the front-side alignment mark by a predetermined spatial relation.

In some embodiments of the present invention, the image sensor further comprises a color filter and a plurality of lenses disposed on the back-side surface. In some embodiments of the present invention, the image sensor further comprises a metal shielding layer disposed between the color filter and the dielectric isolator. In some embodiments of the present invention, the back-side alignment mark comprises a recess extending downwards into the substrate from the back-side surface. In some embodiments of the present invention, the back-side alignment mark further comprises a dielectric layer and a metal shielding layer disposed on the bottom and the sidewalls of the recess.

In accordance with another aspect of the present invention, a method for fabricating an image sensor is provided, wherein the method comprises steps as follows: A plurality of photoelectric transducer devices and an interconnect structure are formed on a front-side surface of a substrate. At least one dielectric isolator extending downwards into the substrate from a back-side surface of the substrate opposite to the back-side surface is then formed in order to isolate the photoelectric transducer devices. And a back-side alignment mark extending downwards into the substrate from the back-side and referencing to a front-side alignment mark previously formed on the front-side surface is formed.

In some embodiments of the present invention, the dielectric isolator and the back-side alignment mark are formed simultaneously. In some embodiments of the present invention, the formation of the dielectric isolator and the back-side alignment mark comprises following steps: An etching process is firstly performed on the back-side surface to form at least one deep trench and a recess in the substrate. A dielectric layer is then formed to fulfill the deep trench and partially fill the recess. Subsequently, a planarization process is performed to remove a portion of the dielectric layer in order to expose a portion of the substrate. In some embodiments of the present invention, the deep trench exposes a STI previously formed on the front-side surface.

In some embodiments of the present invention, the method further comprises performing an ion implantation process and a laser annealing process after the formation of the deep trench and the recess. In some embodiments of the present invention, the method further comprises forming a hard mask on the back-side surface prior the etching process is carried out.

In some embodiments of the present invention, after the planarization process is performed the method further comprises steps of performing a surface treatment on the substrate and the back-side alignment mark; forming a metal layer on the substrate, the dielectric isolator and the back-side alignment mark; and patterning the metal layer to form a metal shielding layer overlaying on the dielectric isolator and the back-side alignment mark. In some embodiments of the present invention, the surface treatment comprises steps of performing an ion plantation process and a laser annealing process on the substrate, the dielectric isolator and the back-side alignment mark; and forming an ARC layer on the substrate, the dielectric isolator and the back-side alignment mark.

In some embodiments of the present invention, the back-side alignment mark aligns to the front-side alignment mark or makes a reference to the front-side alignment mark by a predetermined spatial relation. In some embodiments of the present invention, the method further comprises steps of forming a color filter and a plurality of lenses on the back-side surface and the dielectric isolator, after the dielectric isolator and the back-side alignment mark are formed.

In some embodiments of the present invention the method further comprises steps of attaching a working wafer on the interconnect structure and performing a thinning process on the back-side surface to thin the substrate down, prior to the dielectric isolator is formed.

In accordance with another aspect of the present invention, a method for fabricating an image sensor is provided, wherein the method comprises steps as follows: A plurality of photoelectric transducer devices and an interconnect structure are formed on a front-side surface of a substrate. An etching process is then performed on a back-side surface of the substrate to form at least one deep trench in the substrate. Subsequently, a dielectric layer is then formed to fulfill the deep trench, whereby at least one dielectric isolator extending downwards into the substrate from a back-side surface of the substrate is then formed in order to isolate the photoelectric transducer devices, and the dielectric layer is remained as-deposited.

In accordance with the aforementioned embodiments of the present invention, an image sensor and the fabricating method thereof are provided, wherein a plurality of photoelectric transducer devices and an interconnect structure are firstly formed on a front-side surface of a substrate. At least one dielectric isolator extending downwards into the substrate from a back-side surface of the substrate opposite to the back-side surface is then formed in order to isolate the photoelectric transducer devices, whereby both of the incident light passing into the substrate and the photo-carriers generated in the substrate can be effectively isolated. Therefore, the electrical and optical crosstalk between two adjacent photoelectric transducer devices can be avoided.

In addition, because the formation of the dielectric isolator and the front-side process for forming the photoelectric transducer devices and the interconnect structure are respectively performed on opposite surface of the substrate, the high temperature generated by the liner oxidation process and the gap-fill material densification process for fabricating the dialectic isolator may not interfere the performance of two adjacent photoelectric transducer devices, and the quality of the interconnect structure which is formed on the front-side surface shall not be adversely affect by a u-scratch resulted from the planariztion process (e.g. a chemical mechanical polishing process) performed on the back-side surface.

Furthermore, since a back-side alignment mark referencing to a front-side alignment mark previously formed on the front-side surface of the substrate for the front-side process can be simultaneous with the formation of the dielectric isolator on the back-side surface by the same steps, thus the subsequent process can be performed more precisely without performing any additional process. In other words, the processing accuracy of the image sensor can be significantly increased without increasing any additional cost.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention is to provide an image sensor and the method for fabricating the same to avoid the problems of electrical and optical crosstalk, meanwhile, the processing accuracy of the image sensor can be significantly increased. The present invention will now be described more specifically with reference to the following embodiment for fabricating a MOS image sensor100. It is to be noted that the following descriptions of preferred embodiments of this invention are presented herein for purpose of illustration and description only. It is not intended to be exhaustive or to be limited to the precise form disclosed.

FIGS. 1A to 1Jare cross sectional views illustrating a method for fabricating a MOS image sensor100in accordance with one embodiment of the present invention. As shown inFIG. 1A, a substrate101having a front-side surface101aand a back-side surface101bopposite to the front-side surface101ais firstly provided. A front-side process is then performed on the front-side surface101ato form a plurality of photoelectric transducer devices102on the front-side surface101aof the substrate101. An interconnect structure103is subsequently formed on the front-side surface101aand electrically connects to the photoelectric transducer devices102.

In some preferred embodiments of the present invention, the substrate101may be a silicon substrate. In some other embodiments of the present invention, the substrate101may be, otherwise, a silicon-on-insulator (SOI). The plurality of the photoelectric transducer devices102are divided by a plurality of STIs111which are formed on the front-side surface101aand extending downwards in to the substrate101. Each of the photoelectric transducer devices102comprises a photodiode102a, a drain102band a gate structure102c, wherein the photodiode102aand the drain102bare both embedded in the substrate101and the gate structure102cis formed on the front-side surface101a. The interconnect structure103is a stacked structure constituted by a plurality of metal layers103astacked in sequence, a dielectric layer103bused to isolate the metal layers103aand at least one conductive via103cused to electrically connect with two of the metal layers103a.

It should be appreciated that the interconnect structure103consisting of the metal layers103a, the dielectric layer103band the conductive via103cas shown inFIG. 1Ais merely illustrative. Various interconnect structures having circuit integrity and line width the same with or different from that of the interconnect structure103may be formed by the front-side process.

Next, a working wafer is bonded on the interconnect structure103, the substrate101is then flipped, and a thinning process is performed on the back-side surface101bto thin the substrate101down to a thickness less than 3 μm. The preferred thickness of the thinned substrate101may range from 3 μm to 2 μm (seeFIG. 1B).

A patterned hard mask107is then formed on the back-side surface101band an etching process is performed by using the patterned hard mask107as a mask to form a plurality of deep trenches105and a recess106in the substrate101. The patterned hard mask107consists of silicon oxide, silicon nitride or the combination thereof. In some embodiments of the present invention, each of the deep trenches105extends downward into the substrate101from the back-side surface101band exposes a corresponding STI111. However, in some other embodiments of the present invention, the deep trenches do not align to the STIs111but stagger with them, such that the STIs111can not be exposed from the deep trenches. The recess106also extends downward into the substrate101from the back-side surface101band aligns to a front-side alignment mark113previously formed on the front-side surface101a(seeFIG. 1C) for the purposes of performing reticle alignment steps of the front-side process.

Although, the front-side alignment mark113illustrated inFIG. 1Cis a single mark formed in the substrate101, in some other embodiments, the front-side alignment mark113may comprise a plurality of elements either formed on the substrate101or formed in the substrate101; and the front-side alignment mark113may be formed during, prior to or after the front-side process. In other words, the front-side alignment mark113comprises any structure which can serve as a mark being aligned by the recess106subsequently formed on the back-side surface101b.

Thereafter, an ion implantation process is performed by using the patterned hard mask107as a mask to implant a plurality of p type dopants into the sidewalls of the deep trenches105and the recess106, whereby a plurality of p+ doping layers108each surrounding the corresponding deep trenches105are formed in the substrate101(seeFIG. 1D).

Subsequently, a dielectric material layer109is formed on the patterned hard mask107to fulfill the deep trenches105and partially fill the recess106, so as to define another recess114in the recess106(seeFIG. 1E). In the present embodiment, because the recess106has a size largely greater than that of the deep trenches105, as well as, the deep trenches105and the recess106are formed by the same etching process, thus the recess may have a thickness less than the deep trench105. Therefore, when the dielectric material layer109is formed to fulfill the deep trenches105, the recess106may not be fulfilled, and the non-fulfilled portion of the recess106can be referred as the recess114.

In some embodiments of the present invention, the dielectric material layer109is made of a plurality of anti-reflective materials. In the present embodiment, the dielectric material layer109is an ARC-multilayer structure constituted by a silicon oxide layer, a silicon nitride layer a silicon oxide layer stacked in sequence. But in some other embodiments, the dielectric material layer109is an ARC-multilayer structure constituted by a silicon oxide layer and a silicon nitride layer.

An optional planarization process, such as a chemical mechanical polishing (CMP) process, is then performed by using the back-side surface101bof the substrate101as the stop layer to remove a portion of the dielectric material layer109in order to expose a portion of the substrate101, meanwhile a plurality of dielectric isolators110used to isolate the photoelectric transducer devices102are formed, wherein each of the dielectric isolators110extends downwards in to the substrate101from the back-side surface101band connects with the corresponding STI111. Simultaneously, a back-side alignment mark112is formed in the recess106, wherein the back-side alignment mark112comprises a portion of the dielectric material layer109remaining in the recess106extending downwards in to the substrate101from the back-side surface101band refers to the front-side alignment mark113previously formed on the front-side surface101a. In the present embodiment, the back-side alignment mark112aligns with the front-side alignment mark113(seeFIG. 1F). However, in some other embodiment, the back-side alignment mark112dose not aligns to the front-side alignment mark113directly but makes a reference to the front-side alignment mark113by a predetermined spatial relation, e.g. a coordinate diagram.

Next, a surface treatment is optionally performed on the on the substrate101, the dielectric isolator110and the back-side alignment mark112, and an optional metal shielding layer115ais then formed on the dielectric isolator110and the back-side alignment mark112. In some embodiments of the present invention, the surface treatment comprises the following steps: An ion implantation process and a laser annealing process are performing on the back-side surface101bto form a p+ doping region (not shown) in the substrate101. An ARC layer119is then formed on the back-side surface101b, the dielectric isolator110and the sidewalls and the bottom of the recess114(seeFIG. 1G). The formation of the optional metal shielding layer115acomprises steps of forming a metal layer115on the back-surface101b, the dielectric isolator110and the sidewalls and the bottom of the recess114(seeFIG. 1H), and patterning the metal layer115to remaining a portion of the metal layer115overlaying on the dielectric isolator110and partially filling the recess114(seeFIG. 1I).

Afterward, a plurality of color filters116and a plurality of lenses117are formed on the back-side surface101band the dielectric isolator110and meanwhile the MOS image sensor100shown asFIG. 1Jis formed.

Referring toFIG. 1Jagain, the MOS image sensor100fabricated by the aforementioned method may comprise a substrate101, a plurality of photoelectric transducer devices102, an interconnect structure103, a plurality of dielectric isolator110, a back-side alignment mark112, an optional metal shielding layer115a, a color filter116and a plurality of lenses117. Wherein, the substrate101has a front-side surface101aand a back-side surface101bopposite to the front-side surface101a. The photoelectric transducer devices102are formed on the front-side surface101a. The interconnect structure103is disposed on the substrate101adjacent to the front-side surface101a. The dielectric isolator110extends downwards into the substrate101from the back-side surface101bin order to isolate the photoelectric transducer devices102. The back-side alignment mark112extends downwards into the substrate101from the back-side surface101band references to a front-side alignment mark113previously formed on the front-side surface101a. The color filter116and the lenses117are formed on the back-side surface101band the dielectric isolator110. The optional metal shielding layer115ais disposed between the dielectric isolator110and the color filter116.

Since the gap-fill procedural of the embodiments of the present invention is arranged after the front-side process which requires precise alignment, thus, in some preferred embodiments, the optional the planarization process may be omitted. Therefore, the dielectric material layer109can be remained as-deposited; and after the subsequent process as described above are carried out, the MOS image sensor200shown asFIG. 2can be formed.

In accordance with the aforementioned embodiments of the present invention, an image sensor and the fabricating method thereof are provided, wherein a plurality of photoelectric transducer devices and an interconnect structure are firstly formed on a front-side surface of a substrate. At least one dielectric isolator extending downwards into the substrate from a back-side surface of the substrate opposite to the back-side surface is then formed in order to isolate the photoelectric transducer devices, whereby both of the incident light passing into the substrate and the photo-carriers generated in the substrate can be effectively isolated. Therefore, the electrical and optical crosstalk between two adjacent photoelectric transducer devices can be avoided.

In addition, because the formation of the dielectric isolator and the front-side process for forming the photoelectric transducer devices and the interconnect structure are respectively performed on opposite surface of the substrate, the high temperature generated by the liner oxidation process and the gap-fill material densification process for fabricating the dialectic isolator may not interfere the performance of two adjacent photoelectric transducer devices, and the quality of the interconnect structure which is formed on the front-side surface shall not be adversely affect by a u-scratch resulted from the planariztion process (e.g. a chemical mechanical polishing process) performed on the back-side surface.

Furthermore, since a back-side alignment mark referencing to a front-side alignment mark previously formed on the front-side surface of the substrate for the front-side process can be simultaneous with the formation of the dielectric isolator on the back-side surface by the same steps, thus the subsequent process can be performed more precisely without performing any additional process. In other words, the processing accuracy of the image sensor can be significantly increased without increasing any additional cost.