BACKSIDE ILLUMINATED IMAGE SENSOR WITH SELF-ALIGNED METAL PAD STRUCTURES
An image sensor comprises a semiconductor material having a front side and a back side opposite the front side; a dielectric layer disposed on the front side of the semiconductor material; a poly layer disposed on the dielectric layer; an interlayer dielectric material covering both the poly layer and the dielectric layer; an inter-metal layer disposed on the interlayer dielectric material, wherein a metal interconnect is disposed in the inter-metal layer; and a contact pad trench extending from the back side of the semiconductor material into the semiconductor material, wherein the contact pad trench comprises a contact pad disposed in the contact pad trench, wherein the contact pad and the metal interconnect are coupled with a plurality of contact plugs; and at least an air gap isolates the contact pad and side walls of the contact pad trench. The poly layer and the semiconductor material between adjacent two STI structures of a plurality of first and second STI structures are used as hard masks to form the plurality of contact plugs by selectively removing the dielectric materials between a first side of the plurality of first STI structures and the metal interconnect, wherein each of the plurality of contact plugs extends from each of the first side of the plurality of first STI structures through each of the plurality of first STI structures into the interlayer dielectric material and vertically abuts the metal interconnect.
This disclosure relates generally to semiconductor image sensors, and in particular but not exclusively, relates to backside illuminated semiconductor image sensors.
BACKGROUND INFORMATIONImage sensors have become ubiquitous. They are widely used in digital still cameras, cellular phones, security cameras, as well as medical, automobile, and other applications. The device architecture of image sensors has continued to advance at a great pace due to increasing demands for higher resolution, lower power consumption, increased dynamic range, etc. These demands have also encouraged the further miniaturization and integration of image sensors into these devices.
The typical image sensor operates as follows. Image light from an external scene is incident on the image sensor. The image sensor includes a plurality of photosensitive elements such that each photosensitive element absorbs a portion of incident image light. Photosensitive elements included in the image sensor, such as photodiodes, each generate image charge upon absorption of the image light. The amount of image charge generated is proportional to the intensity of the image light. The generated image charge may be used to produce an image representing the external scene.
While there are a variety of ways to make image sensors, reducing the number of steps with fewer photo masks in semiconductor processing applications is always important. Since every fabrication step adds cost and time on the assembly line, new techniques to enhance image senor throughput are needed.
Non-limiting and non-exhaustive examples of the invention are described with reference to the following figures, wherein like reference numerals refer to like parts throughout the various views unless otherwise specified.
Corresponding reference characters indicate corresponding components throughout the several views of the drawings. Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of various embodiments of the present invention. Also, common but well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention.
DETAILED DESCRIPTIONExamples of an apparatus and method for an image sensor with simplified process are described herein. In the following description, numerous specific details are set forth to provide a thorough understanding of the examples. One skilled in the relevant art will recognize; however, that the techniques described herein can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring certain aspects.
Reference throughout this specification to “one example” or “one embodiment” means that a particular feature, structure, or characteristic described in connection with the example is included in at least one example of the present invention. Thus, the appearances of the phrases “in one example” or “in one embodiment” in various places throughout this specification are not necessarily all referring to the same example. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more examples.
Throughout this specification, several terms of art are used. These terms are to take on their ordinary meaning in the art from which they come, unless specifically defined herein or the context of their use would clearly suggest otherwise. It should be noted that element names and symbols may be used interchangeably through this document (e.g., Si vs. silicon); however, both have identical meaning.
Referring back to
In one example, after the image sensor photodiode/pixel in pixel array 312 has acquired its image data or image charge, the image data is readout by readout circuitry 311 and then transferred to function logic 315. In various examples, readout circuitry 311 may include amplification circuitry, analog-to-digital conversion (ADC) circuitry, or otherwise. Function logic 315 may simply store the image data or even manipulate the image data by applying post image effects (e.g., crop, rotate, remove red eye, adjust brightness, adjust contrast, or otherwise). In one example, readout circuitry 311 may readout a row of image data at a time along readout column lines (illustrated) or may readout the image data using a variety of other techniques (not illustrated), such as a serial readout or a full parallel readout of all pixels simultaneously.
In one example, control circuitry 321 is coupled to pixel array 305 to control operation of the plurality of photodiodes in pixel array 312. For example, control circuitry 321 may generate a shutter signal for controlling image acquisition. In one example, the shutter signal is a global shutter signal for simultaneously enabling all pixels within pixel array 312 to simultaneously capture their respective image data during a single acquisition window. In another example, the shutter signal is a rolling shutter signal such that each row, column, or group of pixels is sequentially enabled during consecutive acquisition windows. In another example, image acquisition is synchronized with lighting effects such as a flash.
In one example, imaging system 300 may be included in a digital camera, cell phone, laptop computer, automobile or the like. Additionally, imaging system 300 may be coupled to other pieces of hardware such as a processor (general purpose or otherwise), memory elements, output (USB port, wireless transmitter, HDMI port, etc.), lighting/flash, electrical input (keyboard, touch display, track pad, mouse, microphone, etc.), and/or display. Other pieces of hardware may deliver instructions to imaging system 300, extract image data from imaging system 300, or manipulate image data supplied by imaging system 300.
As illustrated in
In one example, each of the plurality of contact plugs 205 is formed by a self-aligned patterning process, wherein the portions 206 of the semiconductor material 102 are used as a hard mask to define the plurality of the contact plugs 205 and the air gaps 208a. Therefore, the self-aligned patterning process comprises only a fifth selective etch process without a fifth photolithography process, which may help to reduce the fabrication cost and simplify the fabrication process by minimizing the number of photolithography steps.
During the fifth selective etch process, the dielectric materials defined by the portions 206 of the semiconductor material 102 are selectively removed by at least one of selective anisotropic plasma dry etch and a selective wet etch processes. The removed dielectric materials include the dielectric materials in each of the plurality of the first STI structure 201a and a partial second STI structure 201b, the dielectric materials in each of the plurality of open slots 203 in the poly layer 202, a portion of the dielectric layer 104, and a portion of the interlayer dielectric material 105. The portion of the dielectric layer 104 and the portion of the interlayer dielectric material 105 are self-aligned with the STI structures, wherein the portions 206 of the semiconductor material 102 are used as a hard mask during the selective etch processes. The selective plasma dry etch and the selective wet etch processes have significantly higher etch rates for the dielectric materials in the STI structures 201a and 201b, the dielectric layer 104 and the interlayer dielectric material 105, than the etch rates for the conductive materials in the metal interconnect 107 and the semiconductor materials in the portions 206 of the semiconductor material 102 and the poly layer 202. As a result, the selective etch processes automatically slow down greatly at the interface 114 between the interlayer dielectric material 105 and the metal interconnect 107 so as to form the plurality of contact plugs 205, wherein each of the plurality of contact plugs 205 lands on the metal interconnect 107. Moreover, the selective etch processes also automatically slow down greatly at the interface between the poly layer 202 and the dielectric layer 104, so as to form at least one air gap 208a, wherein each of the air gaps lands on the poly layer 202. In one example, after the selective anisotropic etch processes are finished, there are a portion of the dielectric materials remained in each of the plurality of the second STI structures 201b between the air gaps and the semiconductor material 102.
In one example, the contact pad 207 and the air gap 208a and 208b are formed by a sixth patterning process which comprises at least a sixth photolithography process followed by at least one of a sixth anisotropic plasma dry etch and a sixth selective wet etch process. In the sixth etch processes, the sixth etch rates for the conductive materials in the contact pad 207 and the contact plugs 205 are significantly higher than the sixth etch rates for the dielectric materials in the STI structures (201a and 201b) and the semiconductor material 102 and 206. Therefore, the sixth etch processes stop on the first side 218 of the STI structures (201a and 201b) and there is no undercut on sidewalls of the contact pad trench 204.
The above description of illustrated examples of the invention, including what is described in the Abstract, is not intended to be exhaustive or to limit the invention to the precise forms disclosed. While specific examples of the invention are described herein for illustrative purposes, various modifications are possible within the scope of the invention, as those skilled in the relevant art will recognize.
These modifications can be made to the invention in light of the above detailed description. The terms used in the following claims should not be construed to limit the invention to the specific examples disclosed in the specification. Rather, the scope of the invention is to be determined entirely by the following claims, which are to be construed in accordance with established doctrines of claim interpretation.
Claims
1. An image sensor, comprising:
- a semiconductor material having a front side and a back side opposite the front side;
- a dielectric layer disposed on the front side of the semiconductor material;
- a poly layer disposed on the dielectric layer;
- an interlayer dielectric material covering both the poly layer and the dielectric layer;
- an inter-metal layer disposed on the interlayer dielectric material, wherein a metal interconnect is disposed in the inter-metal layer; and
- a contact pad trench extending from the back side of the semiconductor material into the semiconductor material, wherein the contact pad trench comprises:
- a contact pad disposed in the contact pad trench, wherein the contact pad and the metal interconnect are coupled with a plurality of contact plugs; and
- at least an air gap between the contact pad and side walls of the contact pad trench.
2. The image sensor of claim 1, wherein the air gap extends from side walls of the contact pad trench through the semiconductor material and the dielectric layer, and vertically abuts the poly layer.
3. The image sensor of claim 1, wherein each of the plurality of contact plugs extends through the semiconductor material, the dielectric layer, the poly layer and the interlayer dielectric material, and vertically abuts the metal interconnect.
4. The image sensor of claim 1, wherein a buffer layer is disposed on the back side of the semiconductor material, wherein the buffer layer comprises at least one of dielectric materials including silicon oxide and silicon nitride.
5. The image sensor of claim 1, wherein the interlayer dielectric material, the dielectric layer and the inter-metal layer are made of at least one of dielectric materials comprising silicon oxide and silicon nitride.
6. The image sensor of claim 5, wherein the interlayer dielectric material, the dielectric layer and the inter-metal layer are made of same dielectric materials.
7. The image sensor of claim 1, wherein the contact pad and the plurality of contact plugs are made of at least one of conductive materials comprising Al.
8. The image sensor of claim 7, wherein the contact pad and the plurality of contact plugs are made of same conductive materials.
9. The image sensor of claim 1, wherein the metal interconnect is made of at least one of metals comprising Cu and TiN.
10. A method of image sensor fabrication, comprising:
- providing a semiconductor material having a front side and a back side opposite the front side;
- forming a plurality of first shallow trench isolation (STI) structures and a plurality of second STI structures extending from the front side of the semiconductor material into the semiconductor material, wherein each of the plurality of first STI structures is fully surrounded by adjacent first and second STI structures and each of the plurality of second STI structures is partially surrounded by adjacent first and second STI structures;
- disposing a dielectric layer and a poly layer on the front side of the semiconductor material, wherein the poly layer is disposed on the dielectric layer, wherein a plurality of open slots are formed in the poly layer extending from a first side of the poly layer to the interface of the poly layer and the dielectric layer, wherein each of the plurality of open slots aligns up with each of the plurality of first STI structures and has same two dimensional lateral dimensions as the aligned first STI structure;
- disposing an interlayer dielectric material at the front side of the semiconductor material, wherein the dielectric layer and the poly layer are covered by the interlayer dielectric material, wherein the plurality of open slots are filled with the interlayer dielectric material;
- forming a metal interconnect in an inter-metal dielectric layer, wherein the inter-metal dielectric material is disposed on the interlayer dielectric material;
- forming a contact pad trench by partially removing the semiconductor material between the backside of the semiconductor and a first side of the plurality of first and second STI structures, wherein the edge of the contact pad trench locates on the plurality of second STI structures;
- forming a plurality of contact plugs by selectively removing the dielectric material between the first side of each of the plurality of first STI structures and the metal interconnect; wherein each of the plurality of contact plugs extends from each of the first side of the plurality of first STI structures through each of the plurality of first STI structures into the interlayer dielectric material and vertically abuts the metal interconnect; and
- forming a contact pad by disposing a conductive material in the contact pad trench, wherein the contact pad is isolated from the sidewalls of the contact pad trench, and are coupled with the metal interconnect by the plurality of contact plugs wherein the plurality of contact plugs are filled with the conductive material.
11. The method of image sensor fabrication in claim 10, wherein the poly layer and the semiconductor material between adjacent two STI structures are used as hard masks to form the plurality of contact plugs by selectively removing the dielectric materials between the first side of the plurality of first STI structures and the metal interconnect.
12. The method of image sensor fabrication in claim 10, further comprising a buffer layer disposed on the back side of the semiconductor material, wherein the buffer layer comprises at least one of dielectric materials including silicon oxide and silicon nitride.
13. The method of image sensor fabrication in claim 10, wherein the interlayer dielectric material, the dielectric layer and the inter-metal layer are made of at least one of dielectric materials comprising silicon oxide and silicon nitride.
14. The method of image sensor fabrication in claim 13, wherein the interlayer dielectric material, the dielectric layer and the inter-metal layer are made of same dielectric materials.
15. The method of image sensor fabrication in claim 10, wherein the contact pad and the plurality of contact plugs are made of at least one of conductive materials comprising Al.
16. The method of image sensor fabrication in claim 15, wherein the contact pad and the plurality of contact plugs are made of same conductive materials.
17. The method of image sensor fabrication in claim 10, wherein the metal interconnect is made of at least one of metals comprising Cu and TiN.
18. The method of image sensor fabrication in claim 10, wherein the contact pad is isolated from the sidewalls of the contact pad trench by at least one air gap.
19. The method of image sensor fabrication in claim 18, wherein the air gap extends from the side wall of the contact pad trench through the semiconductor material and the dielectric layer, and vertically abuts the poly layer.