Backside illuminated image sensor and method of formation

Abstract

The application provides a back-illuminated image sensor and a forming method. The application forms an epitaxial layer with a plurality of recesses on the front surface of a first substrate. The bottom and at least part of the sidewall of each recess are covered with a dielectric layer, which is used as an etching stop layer. When the first substrate is removed and the epitaxial layer is thinned, the etching stops at the dielectric layer, thereby accurately controlling the etching endpoint. The recesses extend from the surface of the first epitaxial layer to the first substrate, releasing the stress of the first epitaxial layer, thereby avoiding warping or deformation caused by stress. The interface between the dielectric layer and the first epitaxial layer can adsorb impurity metal ions, which are used to isolate impurity metal ions from the first substrate.

Description

Technical Field

[0001] This invention relates to the field of semiconductor technology, and more specifically to a back-illuminated image sensor and its fabrication method. Background Technology

[0002] With the development of image sensing technology, CMOS image sensors are increasingly replacing CCD image sensors in various electronic products. CMOS image sensors include front-illuminated (FSI) image sensors and back-illuminated (BSI) image sensors.

[0003] In existing back-illuminated image sensors, after the photosensitive device layer is formed on the first substrate, the first substrate is flipped and bonded to the second substrate. Then, during the process of removing the first substrate by grinding and / or wet etching, the device stack is damaged and the performance is degraded due to the low etching selectivity or grinding selectivity.

[0004] Therefore, it is necessary to provide a method for fabricating an image sensor to solve the above-mentioned problems in the prior art. Summary of the Invention

[0005] The purpose of this invention is to provide a back-illuminated image sensor and a method for forming it, wherein an epitaxial stack is covered on the front side of a first substrate, and a groove with a dielectric layer at the bottom is formed in the epitaxial stack so that the removal of the first substrate stops at the dielectric layer.

[0006] Based on the above considerations, the present invention provides a method for forming a back-illuminated image sensor, characterized in that it includes: forming an epitaxial stack with a plurality of grooves on the front side of a first substrate, wherein the bottom and at least part of the sidewalls of each groove are covered with a dielectric layer, and removing the first substrate and thinning the epitaxial stack stops at the dielectric layer.

[0007] Preferably, the groove is used to release stress in the epitaxial stack and further includes removing the dielectric layer.

[0008] Preferably, the epitaxial stack includes a first epitaxial layer and a second epitaxial layer stacked together, each of the grooves extending from the surface of the first epitaxial layer toward the first substrate, and the second epitaxial layer closing the top of each of the grooves.

[0009] Preferably, the second epitaxial layer also covers the remaining sidewalls of each of the grooves.

[0010] Preferably, the second epitaxial layer further extends to the bottom of each of the grooves and covers a portion of the dielectric layer.

[0011] Preferably, forming the dielectric layer covering the bottom and at least part of the sidewalls of each of the grooves includes: forming a dielectric layer covering the bottom and sidewalls of each of the grooves on the first epitaxial layer, and etching back to remove the dielectric layer at least on the first epitaxial layer.

[0012] Preferably, the dielectric layer is also recessed and closes the top of each of the grooves.

[0013] Preferably, the back etching also removes at least the dielectric layer at the top of each of the grooves.

[0014] Preferably, the dielectric layer includes a first sub-dielectric layer and a second sub-dielectric layer stacked together, wherein the second sub-dielectric layer is recessed and closes the top of each of the grooves.

[0015] Preferably, the material of the first sub-dielectric layer includes one of silicon nitride, silicon oxide, silicon carbonitride, silicon carbide, and silicon oxynitride, and the material of the second sub-dielectric layer includes another of silicon nitride, silicon oxide, silicon carbonitride, silicon carbide, and silicon oxynitride.

[0016] Preferably, the first substrate is removed and the process stops at the first epitaxial layer before the epitaxial stack is thinned.

[0017] Preferably, thinning the epitaxial stack includes: thinning the first epitaxial layer and stopping at the dielectric layer.

[0018] Preferably, the dielectric layer is removed after stopping at the dielectric layer.

[0019] Preferably, after removing the dielectric layer, the first epitaxial layer is removed and the second epitaxial layer is planarized.

[0020] Preferably, the thickness of the first epitaxial layer is 0.2 μm to 2 μm, and the thickness of the second epitaxial layer is 3 μm to 4 μm.

[0021] Preferably, before removing the first substrate, a bonding layer is formed on the epitaxial stack and the first substrate is flipped to bond to the second substrate.

[0022] Another aspect of the present invention provides a front-illuminated image sensor formed using the above-described forming method.

[0023] The present invention forms an epitaxial stack with a plurality of grooves on the front side of a first substrate. The bottom and at least part of the sidewalls of each groove are covered with a dielectric layer. The dielectric layer serves as an etching stop layer, stopping the etching at the dielectric layer when the first substrate is removed and the epitaxial stack is thinned, thereby precisely controlling the etching endpoint.

[0024] Furthermore, the groove extends from the surface of the first epitaxial layer toward the first substrate, relieving the stress of the first epitaxial layer and thus preventing warping or deformation due to stress.

[0025] Furthermore, the interface between the dielectric layer and the first epitaxial layer can adsorb impurity metal ions, which can be used to isolate impurity metal ions from the first substrate, thus appropriately reducing the requirements for the first substrate. Attached Figure Description

[0026] Other features, objects, and advantages of the invention will become more apparent from the following detailed description of non-limiting embodiments, taken in conjunction with the accompanying drawings.

[0027] Figures 1 to 8 The diagram shows a cross-sectional structure of each step in a method for forming a back-illuminated image sensor according to an embodiment of the present invention.

[0028] Throughout the figures, the same or similar reference numerals denote the same or similar devices (modules) or steps. Detailed Implementation

[0029] To make the content of this invention clearer and easier to understand, the following description, in conjunction with the accompanying drawings, further illustrates the invention. Of course, this invention is not limited to this specific embodiment, and common substitutions well-known to those skilled in the art are also covered within the scope of protection of this invention.

[0030] It should be noted that in the following specific embodiments, in order to clearly illustrate the structure of the present invention and facilitate explanation, the structure in the accompanying drawings is not drawn to scale and has been partially enlarged, deformed and simplified. Therefore, it should be avoided to interpret this as a limitation of the present invention.

[0031] To address the problems in the prior art, the present invention forms an epitaxial stack with a plurality of grooves on the front side of a first substrate. The bottom and at least part of the sidewalls of each groove are covered with a dielectric layer. The dielectric layer serves as an etching stop layer, stopping the etching at the dielectric layer when the first substrate is removed and the epitaxial stack is thinned, thereby precisely controlling the etching endpoint.

[0032] The following detailed description of a method for forming an image sensor according to the present invention, with reference to specific embodiments and accompanying drawings, will illustrate the invention in detail.

[0033] like Figure 1 As shown, a first substrate 100 is provided, the first substrate 100 having a front side (top as shown) and a back side (bottom as shown).

[0034] In this embodiment, the material of the first substrate 100 is silicon. In other embodiments, the material of the first substrate 100 may also be germanium, silicon germanide, silicon carbide, gallium arsenide, or indium gallium ionide.

[0035] A first epitaxial layer 101 is formed on the front side of the first substrate 100, and the first epitaxial layer 101 is patterned to form a plurality of grooves 102.

[0036] like Figure 1 As shown, the groove 102 extends from the surface of the first epitaxial layer 101 toward the first substrate 100.

[0037] Next, as Figure 2 As shown, a dielectric layer covering the bottom and sidewalls of each of the grooves 102 is formed on the first epitaxial layer 101.

[0038] In this embodiment, the first sub-dielectric layer 103 is formed on the first epitaxial layer 101 and covers the bottom and sidewalls of the groove 102, and its thickness is less than half of the lateral dimension of the groove 102. The material of the first sub-dielectric layer 103 includes one of silicon nitride, silicon oxide, silicon carbonitride, silicon carbide, and silicon oxynitride. The interface between the first sub-dielectric layer and the first epitaxial layer can adsorb impurity metal ions to isolate impurity metal ions from the first substrate 100.

[0039] The first sub-dielectric layer 103 can be formed using deposition processes such as thermal oxidation, chemical vapor deposition, or atomic layer deposition. For example, thermal oxidation can be used to form the first sub-dielectric layer 103 made of silicon oxide. Because thermal oxidation can form a thick oxide layer in a short time, it can achieve high production efficiency and low production cost. At the same time, it can also form a first sub-dielectric layer 103 with complete morphology and uniform thickness on the sidewalls and bottom of the groove 102 with a large depth-to-width ratio.

[0040] The second sub-dielectric layer 104 is formed on the first sub-dielectric layer 103, and its material includes another one of silicon nitride, silicon oxide, silicon carbonitride, silicon carbide, and silicon oxynitride.

[0041] The second sub-dielectric layer 104 can be formed on the first sub-dielectric layer 103 by a deposition process. For example... Figure 2 As shown, the second sub-dielectric layer located on both sides of the groove 102 is fused to the top of the groove 102, thereby closing the top of each groove 102.

[0042] In this embodiment, the first sub-dielectric layer 103 is made of silicon oxide, which has good adhesion to the first epitaxial side 101, while the second sub-dielectric layer 104 is made of silicon nitride, which has good isolation properties.

[0043] Due to the large depth-to-width ratio of the groove 102, the second sub-dielectric layer 104 is insufficient to completely fill the groove 102. After a long period of deposition, the top surface of the second sub-dielectric layer 104 at the top of the groove 102 closes and forms a hollow cavity 105 inside the groove 102. The hollow cavity is beneficial for stress release.

[0044] In another embodiment, the dielectric layer is a single-layer first sub-dielectric layer 103 or a single-layer second sub-dielectric layer 104. By extending the thermal oxidation time or deposition time, the dielectric layer does not completely fill the groove 102 and is recessed and closes the top of each groove 102, so as to form a hollow cavity 105 in the recess 102.

[0045] Next, the dielectric layer on at least the first epitaxial layer 101 is removed by etchback. For example, while removing the dielectric layer on the first epitaxial layer 101 using the etchback process, the etchback process also removes at least the dielectric layer closing the top of each of the grooves 102.

[0046] like Figure 3 As shown, in this embodiment, the second sub-dielectric layer 104 and the first epitaxial layer 101 are etched back to a predetermined depth to expose the top and part of the sidewalls of the first epitaxial layer 101 and the groove 102.

[0047] Then, a second epitaxial layer 106 is formed on the first epitaxial layer 101.

[0048] like Figure 4 As shown, the second epitaxial layer 106 also fills the top of the groove 102 and covers the remaining sidewalls of each groove 102. Through the epitaxial growth time of the second epitaxial layer 106, the second epitaxial layer 106 can also extend to the bottom of each groove 102, that is, fill into the hollow inner wall 105 and cover part of the dielectric layer.

[0049] The first epitaxial layer 101, the second epitaxial layer 106 and the first substrate 100 have the same conductivity type, which can all be of the first conductivity type or all be of the second conductivity type.

[0050] The doping concentration of the first epitaxial layer 101 and the second epitaxial layer 106 can be up to 5E15 atom / cm. 3 The second epitaxial layer 106 is used to form several photosensitive areas (not shown) arranged in an array.

[0051] In this embodiment, both the first epitaxial layer 101 and the second epitaxial layer 106 have a first conductivity type, and the photosensitive region has a second conductivity type. The thickness of the first epitaxial layer 101 is 0.2 μm to 2 μm, and the thickness of the second epitaxial layer 106 is 3 μm to 4 μm.

[0052] Then, a device stack is formed on the second epitaxial layer 106, the device stack including a metal interconnect layer (not shown) and a first bonding layer (not shown) located on the metal interconnect layer. The metal interconnect layer has a plurality of interconnect structures and may also include other devices (not shown), such as a passive device stack (not shown) and radio frequency devices (not shown), etc.

[0053] The method and structure for forming the device stack can be selected from conventional manufacturing methods. Those skilled in the art can choose according to actual needs and refer to existing technologies, which will not be elaborated here.

[0054] like Figure 5 As shown, a second substrate 200 is then provided, and the first substrate 100 is flipped so that the back side of the first substrate 100 faces away from the second substrate 200, and the first substrate 100 and the second substrate 200 are bonded together.

[0055] The second substrate 200 is a carrier wafer used to support and protect the photosensitive device layer during subsequent processes such as removing the first substrate 100, thinning the first epitaxial layer 101, and other subsequent processes. The second substrate 200 can be a silicon substrate or other suitable substrate, and is not limited thereto.

[0056] The second substrate 200 is bonded to the surface of the photosensitive device layer on the front side of the first substrate 100 by a bonding process, which can be performed by any method known to those skilled in the art.

[0057] like Figure 6 As shown, the first substrate 100 is removed from the back side.

[0058] The first substrate 100 is covered with the first epitaxial layer 101 on its front side. A chemical mechanical polishing process is used to remove the first substrate 100 and expose the first epitaxial layer 101, and the dielectric layer stops at the bottom of the groove.

[0059] like Figure 7 As shown, the dielectric layer is removed.

[0060] In this embodiment, the first sub-dielectric layer 103 and the second sub-dielectric layer 104 are removed by etching process to expose the groove 102.

[0061] In this embodiment, as Figure 8 As shown, after removing the dielectric layer, the first epitaxial layer 101 is removed by chemical mechanical polishing, and the second epitaxial layer 106 is thinned until it is planarized.

[0062] The present invention forms an epitaxial stack with a plurality of grooves on the front side of a first substrate. The bottom and at least part of the sidewalls of each groove are covered with a dielectric layer. The dielectric layer serves as an etching stop layer, stopping the etching at the dielectric layer when the first substrate is removed and the epitaxial stack is thinned, thereby precisely controlling the etching endpoint.

[0063] Furthermore, the groove extends from the surface of the first epitaxial layer toward the first substrate, relieving the stress of the first epitaxial layer and thus preventing warping or deformation due to stress.

[0064] Furthermore, the interface between the dielectric layer and the first epitaxial layer can adsorb impurity metal ions to isolate impurity metal ions from the first substrate.

[0065] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from the spirit or essential characteristics of the invention. Therefore, the embodiments should be considered exemplary and not restrictive in any way. Furthermore, it is clear that the word "comprising" does not exclude other elements and steps, and the word "a" does not exclude a plurality. Multiple elements recited in the apparatus claims may also be implemented by a single element. The terms "first," "second," etc., are used to denote names and do not indicate any particular order.

Claims

1. A method for forming a back-illuminated image sensor, characterized in that, include: A first epitaxial layer is formed on the front side of a first substrate, and the first epitaxial layer is patterned to form a plurality of grooves, each groove extending from the surface of the first epitaxial layer toward the first substrate; a dielectric layer is formed on the first epitaxial layer to cover the bottom and sidewalls of each groove, the dielectric layer not completely filling each groove and closing the top of each groove, so as to form a hollow cavity in each groove. The dielectric layer is etched back to remove at least the dielectric layer located on the first epitaxial layer and the dielectric layer closing the top of each of the grooves, so as to expose the top and part of the sidewalls of each of the grooves; A second epitaxial layer is formed on the first epitaxial layer, the second epitaxial layer closing the top of each of the grooves, the first epitaxial layer and the second epitaxial layer constituting an epitaxial stack; the first substrate is removed, and the epitaxial stack is thinned, the thinning of the epitaxial stack stopping at the dielectric layer remaining at the bottom and at least part of the sidewalls of each of the grooves.

2. The forming method as described in claim 1, characterized in that, The groove and the hollow cavity are used to release the stress of the epitaxial stack; after stopping at the dielectric layer, the process also includes removing the dielectric layer.

3. The forming method as described in claim 1, characterized in that, The second epitaxial layer also covers the remaining sidewalls of each of the grooves.

4. The forming method as described in claim 3, characterized in that, The second epitaxial layer also extends to the bottom of each of the grooves and covers a portion of the dielectric layer.

5. The forming method as described in claim 1, characterized in that, The dielectric layer includes a first sub-dielectric layer and a second sub-dielectric layer stacked together, wherein, prior to the back etching, the second sub-dielectric layer is recessed and closes the top of each of the grooves.

6. The forming method as described in claim 5, characterized in that, The material of the first sub-dielectric layer includes one of silicon nitride, silicon oxide, silicon carbonitride, silicon carbide, and silicon oxynitride, and the material of the second sub-dielectric layer includes another of silicon nitride, silicon oxide, silicon carbonitride, silicon carbide, and silicon oxynitride.

7. The forming method as described in claim 1, characterized in that, Before thinning the epitaxial stack, the first substrate is removed and the process stops at the first epitaxial layer; thinning the epitaxial stack includes thinning the first epitaxial layer and stopping at the dielectric layer.

8. The forming method as described in claim 7, characterized in that, After stopping at the dielectric layer, the dielectric layer is removed.

9. The forming method as described in claim 8, characterized in that, After removing the dielectric layer, the first epitaxial layer is removed and the second epitaxial layer is planarized.

10. The forming method according to claim 1, characterized in that, The thickness of the first epitaxial layer is 0.2 μm to 2 μm, and the thickness of the second epitaxial layer is 3 μm to 4 μm.

11. The forming method as described in claim 1, characterized in that, Before removing the first substrate, a bonding layer is formed on the epitaxial stack and the first substrate is flipped to bond to the second substrate.

12. A back-illuminated image sensor, characterized in that, The method includes an epitaxial stack, which includes a first epitaxial layer and a second epitaxial layer; the first epitaxial layer has a plurality of grooves, the bottom and at least part of the sidewalls of each groove are covered with a dielectric layer, the dielectric layer does not completely fill each groove and forms a hollow cavity in each groove, and the second epitaxial layer closes the top of each groove; wherein, the dielectric layer constitutes a stop layer when performing back-side thinning on the epitaxial stack.

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