BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a band diagram of the p-i-n hetero-junction structure having an α-Si:H(i)/α-SiC:H(n) interface.
FIG. 2 is a schematic diagram showing the tensile and tunneling problems of the edge corner of the pixel electrode.
FIG. 3(
a) is a cross-sectional schematic diagram of a stacked p-i-n layer structure in pixels of an image sensor according to the prior art.
FIG. 3(
b) is a proposed equivalent circuitry of the image sensor of FIG. 3(a)
FIG. 4 is a schematic band diagram of the pixel electrode and electrode gap shown in FIG. 3.
FIG. 5 is an energy band diagram of the device structure show in FIG. 3 for both the pixel electrode and the electrode gap region vertically.
FIG. 6 is a sectional schematic diagram of an image sensor according to the present invention.
FIG. 7 is a top view layout of a portion of the image sensor shown in FIG. 6.
FIG. 8 is an energy band diagram of the image sensor shown in FIG. 6.
FIG. 9 is a potential diagram of the prior art image sensor of FIG. 3(a).
FIG. 10 is a potential diagram of the invention image sensor of FIG. 6.
FIGS. 11-15 are fabrication process diagrams of the image sensor according to the present invention.
DETAILED DESCRIPTION
With reference to FIGS. 6-7, FIG. 6 is a sectional schematic diagram of an image sensor 100 according to the present invention, and FIG. 7 is a top view layout of a portion of the image sensor 100 shown in FIG. 6. The image sensor 100 is a photoconductor-on-active-pixel (POAP) image sensor, and is formed on a semiconductor chip 102 comprising a silicon substrate 104. The image sensor 100 comprises a dielectric layer 106 positioned on the substrate 104 and a plurality of pixels 108 defined on the substrate 104, wherein the pixels 108 are arranged as a pixel array 110, as shown in FIG. 7. Each pixel 108 comprises a pixel circuit 112 positioned in the dielectric layer 106, which may comprise at least a metal-oxide-semiconductor filed effect transistor (MOSFET), and a pixel electrode 114 of the pixel 108. The pixel electrodes 114 comprise metal materials, such as titanium nitride (TiN) and are electrically connected to the corresponding pixel circuits 112 vertically through the via plugs 136. In various embodiments, the pixel electrodes 114 may comprise other conductive materials, such as tungsten (W), aluminum (Al), or copper (Cu). An electrode gap region G represents the spacing between the fringes of the adjacent pixel electrodes 114.
In addition, a shield electrode 116 is positioned in the electrode gap region G and between any two adjacent pixel electrodes 114. Therefore, the shield electrode 116 is arranged as a mesh around each pixel electrode 114, as shown in FIG. 7. In a preferable embodiment, the shield electrode 116 is positioned in the middle of the electrode gap region G so that the adjacent pixel electrodes 114 have the same distances d from the shield electrode 116, and the shield electrode 116 is served as the boundary between the pixels 108. Furthermore, the shield electrode 116 may have the same material as that of the pixel electrodes 114, such as TiN, and may be formed together with the pixel electrodes 114 though the same processes. A ground voltage level can be supplied to the shield electrode 116, where the shield electrode 116 has no current path within the pixel area. Here, the shield electrode 116 plays an important role to separate the pixels 108 electrically by lowering the potential level near the surface at the pixel electrode gap region G, while the structure of a conventional image sensor has a large fringing effect across the pixels, which brings the cross-talk current.
According to the present invention, an insulating layer 118 is positioned on the dielectric layer 106, the shield electrode 116, and on the edge portions of the pixel electrodes 114. The insulating layer 118 may be a thin oxide layer (such as silicon oxide, SiO2), which exposes most the central areas of the pixel electrodes 114 so that the pixel electrodes 114 are electrically connected to the photo conductive layer 120 thereon.
The image sensor 100 further comprises a photo conductive layer 120 and a transparent conductive layer 122 covering the insulating layer 118 and the pixel electrodes 114. The photo conductive layer 120 comprises an n-type layer (n-layer) 130, an intrinsic layer (i-layer) 132, and a p-type layer (p-layer) 134 from bottom to top. Wherein, the the i-layer 132 is formed with α-Si:H materials, and the p-layer 134 and the n-layer 130 comprise α-SiC:H materials. For providing good sensitivity and the color balance of the image sensor 100, the i-layer 132 should be thick enough, and may have a thickness H of about 5000 angstroms or more than 5000 angstroms. The transparent conductive layer 122 serves as a top electrode plate that may be formed with indium tin oxide (ITO). In addition, the image sensor 100 may comprise a first planarization layer 124, a color filter layer 126, and a second planarization layer 128 positioned on the photo conductive layer 120 in order, wherein the color filter layer 126 may comprise color filters with different colors, such as red, green, and blue, in different pixels 108.
The effect provided by the present invention can be explained with an equivalent circuitry, which is also shown in FIG. 6. Oriented from the node at the i-layer 132/n-layer 130 interface in the middle of the electrode gap region G, the Csub indicates the shield electrode 116 capacitance, the Cpd represents the capacitance component for the transparent electrode (ITO) 122, and the C1 and C2 are coupled with the adjacent metal pixel electrodes 114, respectively. Accordingly, the Csub becomes much larger than that of the conventional image sensor structure without the shield electrode 116. Therefore, the shield electrode 116 can strongly force the potential near the surface to a low level and there is 1-dimentioanl barrier height in the pixel electrode gap region G, thereby preventing the carrier cross-talk over the adjacent pixels occurred in the conventional image sensor as shown in FIG. 4.
FIG. 8 is an energy band diagram of the image sensor 100 shown in FIG. 6 for the adjacent pixel electrodes 114 and the shield electrode 116. Since the potential under the shield electrode 116 is forced to a lower level through the thin insulating layer 118 and the α-SiC based n-layer 130, it is obvious that the cross-talk effect occurred through the i-layer 132/n-layer 130 interface will be cut-off by the large potential barrier height (i.e. 1-dimentional barrier height) in the electrode gap region G. This effect enables to use the thicker i-layer 132 for enhancing the quantum efficiency with the existing bias conditions.
On the other hand, the thickness of the thin insulating layer 118 on the shield electrode 116 may be determined for the potential level at the interface between the α-Si:H based i-layer 132 and the α-SiC:H based n-layer 130, wherein it is preferable to adjusted the thickness of the insulating layer 118 to maximize the gate capacitance (Csub). Besides, another role for determining the thickness of the insulating layer 118 is to protect the pointed edge portions of the pixel electrodes 114 to avoid anomaly hole tunneling due to the concentrated electric field, which will cause dark leakages. In addition, it will also contribute to prevent the deep traps in α-SiC:H based n-layer 130 created by the tensile stress that will bring the image lag problem. With reference to FIG. 7, the shield electrode 116 and the edge portions of the pixel electrodes 114 are covered by the insulating layer 118, thus the present invention image sensor 100 structure eliminates the causal root of leakage problem occurred at the edge corners of the pixel electrodes 114 for that there is no longer strong electric fields and tensile stress on the contact region of the α-SiC:H based n-layer 130 and the pixel electrodes 114.
FIG. 9 is a potential diagram of the prior art image sensor 10 of FIG. 3(a) with the i-layer 20 thickness of 5000 Å, 7000 Å, and 10000 Å. As shown in FIG. 9, the two adjacent pixel electrodes 12 respectively have a potential of 1.2V and 2.6V, and the gap region between the two pixel electrodes 12 has no or small potential barrier height. Therefore, the electrons produced in the i-layer 20 easily move from the right side pixel electrode 12 with high potential to the left side pixel electrode 12 with low potential, which results in the cross-talk problem. Comparatively, FIG. 10 is a potential diagram of the invention image sensor 100 of FIG. 6 with the i-layer 132 thickness of 5000 Å, 7000 Å, and 10000 Å. As shown in FIG. 10, the two adjacent pixel electrodes 114 respectively have a potential of 1.2V and 2.6V, and the gap region G between the two pixel electrodes 114 has a great barrier height. Consequently, the lateral electric field due to the two adjacent pixel electrodes 114 with a potential difference is not so strong, and the shield electrode 116 of the invention image sensor 100 generates a great potential barrier height between the two adjacent pixel electrodes 114 so as to prevent the cross-talk problem. The i-layer having a thickness greater than 5000 Å is applicable in the invention image sensor 100.
Please refer to FIGS. 11-15, which are fabrication process diagrams of the image sensor 100 according to the present invention. First, as shown in FIG. 11, a semiconductor chip 102 is provided, and the semiconductor chip 102 comprises a silicon substrate 104. Then, a plurality of electric elements is formed on the substrate 104, which forms the pixel circuits 112 in the dielectric layer 106. Thereafter, a conductive layer 138 is formed on the dielectric layer 106, above the pixel circuits 112. The conductive layer 138 may comprise metal materials, preferable TiN and have a thickness about 300 angstroms. With reference to FIG. 12, a photolithography-etching process is performed to remove portions of the conductive layer 138 so as to form a plurality of pixel electrodes 114 in each of the pixels 108 and a shield electrode 116 positioned between the pixel electrodes 114. Therefore, the shield electrode 116 is positioned at the same plane as that of the pixel electrodes 114. In addition, the shield electrode 116 has the same distances to the adjacent pixel electrodes 114. In this embodiment, the shield electrode 116 has a width about 0.2 μm and respectively has a distance about 0.2 μm to the adjacent pixel electrodes 114.
Thereafter, a thin insulating layer 118 is deposited on the substrate 104 to cover the pixel electrodes 114 and the shield electrode 116, as shown in FIG. 13. The insulating layer 118 may comprise insulating materials, such as silicon oxide, and have a thickness about 200 angstroms. Referring to FIG. 14, another photolithography-etching process is performed to remove portions of the insulating layer 118 to expose most portions of the pixel electrodes 114, while the edge portions of the pixel electrodes 114 and the shield electrode 116 between the pixel electrodes 114 are still covered by the insulating layer 118. Sequentially, an α-SiC:H based n-layer 130, an α-Si:H based i-layer 132, and an α-SiC:H based p-layer 134 that form the photo conductive layer 120 are successively formed on the substrate 104, wherein the n-layer 130 are electrically connected to the pixel electrodes 114, and each pixel electrodes 114 are coupled to the corresponding pixel circuits 112 though the via plugs 136. In an embodiment of the present invention, the p-type layer 134 has a thickness about 50 angstroms, the i-layer 132 has a thickness about 5000 angstroms, and the n-layer 130 has a thickness about 100 angstroms. Then, a transparent conductive layer 122 is formed on the photo conductive layer 120. Sequentially, a first planarization layer 124, a color filter layer 126, and a second planarization layer 128 are formed on the transparent conductive layer 122 to finish the fabrication of the image sensor 100, as shown in FIG. 15.
In contrast to the prior, the present invention provides an image sensor structure have a high potential barrier between the pixel electrodes to prevent cross-talk. Furthermore, the insulating layer covering the shield electrode and the edge portions of the pixel electrodes prevents tunneling effect to improve the image lag and dark leakage current problems. Therefore, according to the present invention, an image sensor with good performance may be provided.
Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.
Patent Application
20080042230
