TW201351623A - Back-illuminated image sensor and fabricating method thereof - Google Patents

Abstract

A fabricating method of a back-illuminated image sensor includes the following steps. First, a silicon wafer having a first surface and a second surface is provided, wherein a number of trench isolations are formed in the first surface, and at least one image sensing member is formed between the trench isolations. Then, a first chemical mechanical polishing (CMP) process is performed to the second surface using the trench isolations as a polishing stop layer to thin the silicon wafer. Because the polishing rate of the silicon material in the silicon wafer is different with that of the isolation material of the trench isolations in the first CMP process, at least one dishing depression is formed in the second surface of the silicon wafer. Finally, a microlens is formed above the dishing depression, and a surface of the microlens facing the dishing depression is a curved surface.

Description

Back-illuminated image sensor and manufacturing method thereof

The present invention relates to a method of fabricating an image sensor, and more particularly to a method of fabricating a back-illuminated image sensor.

A method of fabricating a back-illuminated image sensor is to first complete a sensor element, a signal processing element, a dielectric layer, a metal wiring layer, etc. on the front side of the germanium wafer. Thereafter, a thinning process is performed on the back side of the germanium wafer to reduce the thickness of the germanium wafer from hundreds of micrometers to several micrometers (μm), for example, from 750 micrometers to about 3 micrometers. Next, optical elements such as color filters and microlenses are formed on the back surface of the thinned germanium wafer to introduce light from the back surface to avoid shielding of the light by the metal wiring layer. However, it is possible to accurately control the thinning process so that the thickness of the germanium wafer after thinning for hundreds of micrometers can achieve good uniformity in both the cross wafer and the wafer to wafer (uniformity). ), it is not easy to work. If a single germanium wafer is excessively thinned or thinned, it will damage the sensor components in front of the germanium wafer or affect the performance of the optical components behind the wafer. If the wafer is thin and uneven, It also affects the quality of the image sensor.

Therefore, how to solve the above problems to improve the yield and performance of the back-illuminated image sensor is precisely the purpose of developing the present invention.

It is an object of the present invention to provide a method of fabricating a back-illuminated image sensor, the method comprising the following steps. First, a wafer is provided. The germanium wafer includes a first surface and a second surface. The first surface is formed with a plurality of isolation trenches, and at least one image sensing element is completed between the plurality of isolation trenches. Secondly, a plurality of isolation trenches are used as a polishing stop layer to perform a first chemical mechanical polishing process on the second surface to thin the germanium wafer, and the first chemical mechanical polishing process is used to polish the germanium material between the wafers at a rate and isolation. The spacer material in the trench has a different polishing rate and at least one dishing is formed on the second surface of the wafer between the plurality of isolation trenches. And, a microlens is formed above the dish-shaped recess, and a surface of the microlens facing the dish-shaped recess is a curved surface.

It is still another object of the present invention to provide a back-illuminated image sensor comprising a germanium wafer and at least one microlens. The germanium wafer includes a first surface and a second surface, wherein the first surface has completed at least one image sensing element and at least one peripheral circuit. The microlens is disposed above the second surface of the tantalum wafer, and the surface of the microlens adjacent to the second surface of the tantalum wafer is curved.

The method for manufacturing the back-illuminated image sensor of the present invention uses a plurality of isolation trenches as a polishing stop layer to perform a CMP process with a high polishing selectivity ratio to thin the germanium wafer, thereby thinning the germanium wafer The round thickness achieves good uniformity in both the cross wafer and the wafer to wafer. On the other hand, the back-illuminated image sensor of the present invention has a curved surface due to the surface of the microlens adjacent to the second surface of the silicon wafer, thereby enabling the external incident light to be accurately focused on the image sensing element, thereby increasing the backlight. The performance of the image sensor.

The above and other objects, features and advantages of the present invention will become more <RTIgt;

1A to FIG. 1E are partial schematic diagrams showing an embodiment of a method for manufacturing a back-illuminated image sensor according to the present invention.

Referring to the cross-sectional view shown in FIG. 1A, first, a germanium wafer 100 is provided. The germanium wafer 100 includes a first surface 110 and a second surface 120. The first surface 110 is formed with a plurality of deep trench isolations 111. At least one image sensing element 112 is completed between the two deep isolation trenches 111, and at least the periphery of the image sensing component 112 has been completed. A peripheral circuit 113. Next, a plurality of dielectric layers 114a, 114b, and 114c are sequentially formed on the first surface 110, and metal wiring layers M1, M2, and M3 are formed in each of the dielectric layers. Then, an electrode layer 115 is formed on the outermost dielectric layer 114c, and an insulating layer 116 is formed on the electrode layer 115. And, a carrier wafer 199 is bonded to the insulating layer 116. In this embodiment, the vertical depth d1 of the deep isolation trench 111 is not more than 3 micrometers, and the image sensor element 112 between the deep isolation trenches 111 is, for example, a photodiode for receiving the outside world. The light is converted to a charge. The peripheral circuit 113 is, for example, a signal processing circuit composed of a metal oxide semi-transistor (MOS), which can read the charge of the photodiode and convert it into a digital image signal output. It is worth mentioning that the implementation of the present invention can increase or decrease the number of dielectric layers on the first surface of the wafer and the number of metal wiring layers according to actual needs. FIG. 1 is merely illustrative and not intended to limit the present invention. A method, pattern, or number of dielectric layers, metal wire layers, and peripheral circuits.

Referring to the cross-sectional view of FIG. 1B, after the carrier wafer 199 is bonded to the wafer 100, the wafer 100 is supported by the carrier wafer 199. Next, a thinning process is performed from the second surface 120 to reduce the thickness of the wafer 100 to about 3 microns without exposing the surface of the deep isolation trench 111. The thinning process includes a second chemical mechanical polishing process or an etching process or a combination of the two, wherein the second chemical mechanical polishing process may be performed to reduce the thickness of the germanium wafer 100 to about 40 micrometers and then to the etching process. Continue to reduce the thickness of the germanium wafer 100 to approximately 3 microns, or etch first The process further performs the second CMP process, and the two processes described above may be alternately performed to reduce the thickness of the ruthenium wafer 100, which is not limited by the present invention. The main purpose of performing the thinning process described above is to be able to quickly reduce the thickness of the germanium wafer 100. Therefore, the slurry in the second chemical mechanical polishing process and the etching solution in the etching process may not be considered. Whether other materials have a selection ratio, and only need to consider the polishing speed and etching speed of the germanium material, and control the progress time of the thinning process to reduce the thickness of the wafer 100 to a value (for example, about 3 micrometers) and then stop. The principle is not to contact the deep isolation trench 111.

Referring to the cross-sectional view of FIG. 1C, the deep isolation trench 111 is used as the polishing stop layer, and the second surface 120 is further subjected to a first chemical mechanical polishing process having a high polishing selectivity ratio to continue thinning. Silicon wafer 100. Since the slurry formulation selected in the first CMP process has a different polishing rate for the tantalum wafer 100 than the spacer material for the deep isolation trench 111 (for example, cerium oxide), it is ground to deep isolation. After the trench 111, the deep isolation trench 111 is not easily removed, so that the thinned germanium wafer can be controlled in combination with the change of the light reflection signal and the change of the electrical resistance generated by the increase of the polishing resistance, thereby avoiding excessive thinning of the wafer or thin Insufficient wafers are used to achieve good uniformity in thinned wafers and wafer to wafer thickness. In detail, because of the germanium material of the germanium wafer 100 and the isolation material (such as ceria) filled in the deep isolation trench 111, the two materials have different refractive indices for the light and are ground in the above slurry formulation. The rate is different, so in the present invention, the deep isolation trench 111 can be reused as a polishing stop layer to perform a first chemical mechanical polishing process having a high polishing selectivity.

In addition, the high abrasive selection ratio characteristics allow the first CMP process to create a dish-shaped recess 120a as shown in the second wafer 120 between the deep isolation trenches 111.

Referring to the cross-sectional view of FIG. 1D, the amount of charge after photoelectric conversion is reduced in order to prevent the hole-electron pair recombination generated after the image sensing element 112 receives external light, and the second surface 120 can be The ion implantation process is performed to improve. Then, because the thinning process and the ion implantation process cause defects in the germanium wafer 100 (not shown), and such germanium structure defects cause leakage current, an annealing process can be performed to repair the germanium structure. defect. Thereafter, the anti-reflection layer 121 may be further formed to reduce improper reflection of incident light. Then, a microlens 122 is formed over the dishing recess. Since the microlens 122 can be formed in accordance with the shape of the dish-shaped recess 120a, the surface of the microlens 122 facing the dish-shaped recess 120a is a curved surface. The shape of the microlens in the image sensor structure is a convex lens having a condensing effect. According to the path of the incident light, the surface of the convex lens is divided into a light receiving surface for receiving light and a light transmitting surface for allowing light to pass through, wherein the light receiving surface is convex and the light transmitting surface is defined as a convex flat lens; the light receiving surface is flat and transparent. The convex surface of the smooth surface is defined as a plano-convex lens; the convex surface of both the light receiving surface and the light transmitting surface is defined as a convex convex lens. Under the condition that the surface radii of the above three types of convex lenses are the same, when the external light enters the above three forms of lenses in parallel directions, the focal length formed by the convex convex lens is the shortest and the focal length formed by the convex flat lens is the longest. Further, if the above three forms of lenses are to form the same focal length, the radius of the curved surface of the convex flat lens is the smallest and the radius of the curved surface of the convex convex lens is the largest, that is, the curved surface surface forming the convex flat lens is curved and the curved surface of the convex convex lens The surface shape can be relatively flat.

Compared with the front-illuminated image sensor, the back-illuminated image sensor has a shorter distance from the microlens to the image sensor element in order to increase the received light. It can be known from the above optical principle that the radius of the curved surface of the convex flat microlens is small to form a small focal length. Therefore, manufacturing a back-illuminated image sensor in which a process for forming a microlens must be precisely controlled to form a small radius of curvature, in order to produce an accurate lenticular focal length to focus external light on the image sensing element. In the present invention, the dish-shaped recess 120a The upper microlens 122 may be formed as a plano-convex lens or a convex-convex lens with respect to the traveling direction of the incident light, and its focal length is shorter than that of the convex flat lens under the same curved surface radius. Therefore, the microlens 122 is relatively easy to form the focal length of the microlens within the distance range of the microlens 122 to the image sensing element 112 in the process condition, so that the incident light of the outside is accurately focused on the image sensing element 112 to improve its photoelectric conversion. Rate, increase the performance of back-illuminated image sensors. On the other hand, the first chemical mechanical polishing process having a high polishing selectivity has a different ratio of the polishing rate of the tantalum material to the polishing rate of the insulating material, and a dishing is formed between the deep isolation trenches 111 on the second surface 120 of the tantalum wafer 120. The shape of the surface of 120a will also be different. For example, the larger the polishing rate of the crucible material, the smaller the radius of the curved surface of the dish-shaped recess, and the smaller the radius of the curved surface of the dish-shaped recess 120a, the smaller the radius of the curved surface of the microlens 122 formed by the dish-shaped recess 120a. Therefore, by adjusting the slurry formulation of the first chemical mechanical polishing process having a high polishing selection ratio to produce the dish-shaped recesses 120a of different curved shapes, the light-transmissive surface of the microlenses 122 can be formed with respect to the curved shape of the dish-shaped recess 120a. Different surface radii. In the present invention, the first CMP process having a high polishing selectivity has a ratio of the rate of the abrasive raft material to the rate of the abrasive spacer material as large as possible, preferably greater than 200.

It is worth mentioning that, as shown in the cross-sectional view of the back-illuminated image sensor shown in FIG. 1D, other methods may be used to make the surface of the microlens 122 facing the second surface 120 curved. For example, after thinning the germanium wafer, etching the second surface to form at least one recess; then, forming an anti-reflection layer on the second surface; and then performing another chemical mechanical polishing process with a high polishing selectivity ratio, grinding The rate of the antireflection layer material is greater than the rate at which the crucible material is ground, so that the surface of the antireflection layer filled in the groove produces dishing depressions; and the microlenses are formed on the dishing depressions on the surface of the antireflection layer. Thus, the surface of the microlens facing the second surface also forms a curved surface.

Referring to the cross-sectional view shown in FIG. 1E, finally, the back-illuminated image sensor manufacturing method of the present invention may further comprise the steps of: forming a first flat layer on the microlens 122. 123. Then, the metal shielding layer 124 is formed on the first flat layer 123 to prevent the external light from directly illuminating the peripheral circuit 113. The material forming the metal shielding layer 124 may be aluminum, copper or titanium, which is not limited in the present invention. Thereafter, above the microlens 122, a color filter 125 is formed on the first flat layer 123. And, the second flat layer 126 and the second microlens 127 are selectively formed on the color filter 125 to complete a back-illuminated color image sensor. It is worth mentioning that, according to the direction of travel of the external light, the second microlens 127 is defined as a convex flat lens, and the microlens 122 and the second microlens 127 form a lenticular lens combination, and the lenticular lens combination is easier to adjust the second microlens in the process. 127 The radius of the curved surface of the light-receiving surface forms an accurate focal length to improve the photoelectric conversion rate of the image sensing element and improve the performance of the back-illuminated image sensor.

In summary, the technical solution of the present invention can not only accurately control the thinned germanium wafer process, but also achieve a good uniformity of the thinned germanium wafer thickness on a single wafer and wafer to wafer. The focal length of the microlens can be more accurately formed to improve the photoelectric conversion efficiency of the image sensing element. Therefore, the back-illuminated image sensor provided by the invention has good performance and quality.

While the present invention has been described in its preferred embodiments, the present invention is not intended to limit the invention, and the present invention may be modified and modified without departing from the spirit and scope of the invention. The scope of protection is subject to the definition of the scope of the patent application.

100‧‧‧矽 wafer

110‧‧‧ first surface

111‧‧‧Deep isolation trench

112‧‧‧Image sensor components

113‧‧‧ peripheral circuits

114a, 114b: 114c‧‧‧ dielectric layer

115‧‧‧electrode layer

116‧‧‧Insulation

120‧‧‧second surface

120a‧‧ dish-shaped depression

121‧‧‧Anti-reflective layer

122‧‧‧Microlens

123‧‧‧First flat layer

124‧‧‧Metal shielding

125‧‧‧Color filters

126‧‧‧Second flat layer

127‧‧‧second microlens

199‧‧‧ carrying wafer

M1, M2, M3‧‧‧ metal wire layer

d1‧‧‧Deep isolation trench 111 vertical depth

1A to 1E are partial schematic diagrams showing the steps of manufacturing a back-illuminated image sensor of the present invention.

100‧‧‧矽 wafer

111‧‧‧Deep isolation trench

112‧‧‧Image sensing components

113‧‧‧ peripheral circuits

120‧‧‧second surface

120a‧‧ dish-shaped depression

121‧‧‧Anti-reflective layer

122‧‧‧Microlens

199‧‧‧ carrying wafer

Claims (15)

A method of manufacturing a back-illuminated image sensor, the method comprising the steps of: providing a germanium wafer, the germanium wafer comprising a first surface and a second surface, wherein the first surface is formed with a plurality of isolation trenches Having at least one image sensing element between the isolation trenches; using the isolation trenches as a polishing stop layer and performing a first chemical mechanical polishing process on the second surface to thin the germanium wafer, The first CMP process produces a polishing rate of one of the germanium wafers different from a polishing rate of one of the isolation trenches and at least a second surface of the germanium wafer between the isolation trenches a dish-shaped recess; and a microlens formed above the dish-shaped recess, a surface of the microlens facing the dish-shaped recess being a curved surface. The method for manufacturing a back-illuminated image sensor according to claim 1, wherein the method further comprises: performing a thinning process to reduce the thickness of the germanium wafer by a value, and then performing the chemical mechanical polishing process to thin the矽 Wafer. The method of manufacturing a back-illuminated image sensor according to claim 2, wherein the thinning process comprises a second chemical mechanical polishing process and an etching process. The method of manufacturing a back-illuminated image sensor according to claim 2, wherein the value is not less than 3 μm. The method of manufacturing a back-illuminated image sensor according to claim 1, wherein the first chemical mechanical polishing process has a polishing rate for the tantalum material greater than a polishing rate of the insulating material. The method of manufacturing a back-illuminated image sensor according to claim 5, wherein the ratio of the polishing rate of the tantalum material to the polishing rate of the insulating material is greater than 200. The method of manufacturing the back-illuminated image sensor according to claim 1, wherein before the microlens is formed on the dish-shaped recess, the method further comprises: performing an ion implantation process on the second surface of the germanium wafer and An annealing process; and forming an anti-reflection layer on the second surface. The method of manufacturing a back-illuminated image sensor according to claim 1, wherein the method further comprises the steps of: forming a first flat layer on the microlens; and forming a color filter on the first flat layer sheet. The method of manufacturing a back-illuminated image sensor according to claim 8, wherein the method further comprises the steps of: forming a second planar layer on the color filter; and forming a first layer on the second flat layer Two microlenses. A back-illuminated image sensor comprising: a germanium wafer comprising a first surface and a second surface, wherein the first surface has completed at least one image sensing element and at least one peripheral circuit And at least one microlens disposed on the second surface of the germanium wafer, and a surface of the microlens adjacent to the second surface of the germanium wafer is a curved surface. The back-illuminated image sensor of claim 10, wherein the A surface further includes a plurality of isolation trenches, and the image sensing element is completed between the isolation trenches. The back-illuminated image sensor of claim 11, wherein the second surface of the silicon wafer further comprises at least one dish-shaped recess located between the isolation trenches, the microlens being disposed on the dish Above the depression. The back-illuminated image sensor of claim 11, wherein one of the isolation trenches has a depth dimension that is perpendicular to the first surface and is no greater than 3 microns. The back-illuminated image sensor of claim 10, further comprising: an anti-reflection layer disposed on the second surface; a first planar layer disposed on the microlens and the anti-reflection a metal shielding layer disposed on the first planar layer; and a color filter disposed above the microlens and disposed on the first planar layer. The back-illuminated image sensor of claim 14, further comprising: a second planar layer disposed on the color filter; and a second microlens disposed on the second flat On the floor.