Abstract:
A combination of materials is used to form the photodiodes of a vertical color imager cell. The materials used to form the photodiodes have different band gaps that allow the photon absorption rates of the photodiodes to be adjusted. By adjusting the photon absorption rates, the sensitivities of the photodiodes and thereby the characteristics of the imager can be adjusted.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to imager cells and, more particularly, to a bandgap tuned vertical color imager cell. 
     2. Description of the Related Art 
     A standard imager cell is a device that utilizes a photodiode to convert light energy into photo-generated electrons, and a number of transistors to control the operation of the photodiode and sense the number of photo-generated electrons that were produced by the light energy. 
     A color imager cell is a cell that produces photo information for more than one color, typically red, green, and blue. Digital cameras utilize millions of color imager cells that capture images based on the amount of reflected red, green, and blue light energy that strikes each imager cell. 
     One approach to implementing a color imager cell is to utilize three standard imager cells, and vary the depth of the sense point. Blue photons are high-energy photons that are absorbed at the surface of the photodiode, while green photons have less energy and are absorbed at a first depth that lies below the surface of the photodiode. Red photons have even less energy and are absorbed at a second depth that lies below the first depth. 
     Thus, by sensing the number of photo-generated electrons at the surface of a first cell, at the first depth in a second cell, and at the second depth in a third cell, blue, green, and red photo information, respectively, can be obtained from the three standared imager cells. One drawback of this approach, however, is that a color imager cell is three times the size of a standard imager cell. 
     Another approach to implementing a color imager cell that addresses the size problems of the three-cell approach is a vertical color imager cell. A vertical color imager cell is an imager cell that collects photo-generated electrons that represent different colors, such as red, green, and blue, at different depths in the same photodiode. 
     In an imager cell, the transistors required to operate the photodiode and sense the number of photo-generated electrons utilize a relatively small amount of space with respect to the amount of space utilized by the photodiode. Thus, one of the advantages of a vertical color imager cell is that since the cell collects multiple colors with the same photodiode, a vertical color imager cell is only slightly larger than a standard imager cell. 
     U.S. Patent Application Publication US 2002/0058353 A 1 published on May 16, 2002 describes a vertical color imager cell. FIG. 1 shows a combined cross-sectional and schematic diagram that illustrates a prior art color imager cell  100 . Cell  100  is substantially the same as the cell shown in FIG. 2A of the &#39;353 published application. 
     As shown in FIG. 1, imager cell  100  includes a first p− region  110 , a first n+ region  112  that contacts p− region  110 , and a first depletion region  114  that is formed at the junction between regions  110  and  112 . Imager cell  100  also includes a second p− region  120  that contacts n+ region  112 , a second n+ region  122  that contacts p− region  120 , and a second depletion region  124  that is formed at the junction between regions  120  and  122 . In addition, imager cell  100  further includes a third p-type region  130  that contacts n+ region  122 , a third n+ region  132  that contacts p− region  130 , and a third depletion region  134  that is formed at the junction between regions  130  and  132 . 
     In operation, as shown in FIG. 1, p− regions  110 ,  120 , and  130  are connected to ground. In addition, n+ regions  112 ,  122 , and  132  are connected to first, second, and third reset transistors  150 ,  152 , and  154 , respectively. Prior to collecting photo information, reset transistors  150 ,  152 , and  154  are pulsed on which, in turn, places a positive potential on n+ regions  112 ,  122 , and  132 . 
     The positive potential reverse biases the pn junction of regions  110  and  112 , thereby forming a red collecting photodiode, and the pn junction of regions  120  and  122 , thereby forming a green collecting photodiode. The positive potential also reverse biases the pn junction of regions  130  and  132 , thereby forming a blue collecting photodiode. 
     Once the positive potentials have been placed on n+ regions  112 ,  122 , and  132 , light energy, in the form of photons, is collected by the red, green, and blue photodiodes. The red photons are absorbed by the red photodiode which, in turn, forms a number of red electron-hole pairs, while the green photons are absorbed by the green photodiode which, in turn, forms a number of green electron-hole pairs. Similarly, the blue photons are absorbed by the blue photodiode which, in turn, forms a number of blue electron-hole pairs. 
     The red electrons from the electron-hole pairs that are formed in depletion region  114  move under the influence of the electric field towards n+ region  112 , where each additional electron collected by n+ region  112  reduces the positive potential that was placed on n+ region  112  by reset transistor  150 . On the other hand, the holes formed in depletion region  114  move under the influence of the electric field towards p− region  110 . 
     In addition, the electrons, which are from the electron-hole pairs that are formed in p− region  110  within a diffusion length of depletion region  114 , diffuse to depletion region  114  and are swept to n+ region  112  under the influence of the electric field. Further, the electrons that are formed in n+ region  112  remain in n+ region  112 . 
     Similarly, the green electrons from the electron-hole pairs that are formed in depletion region  124  move under the influence of the electric field towards n+ region  122 , where each additional electron collected by n+ region  122  reduces the positive potential that was placed on n+ region  122  by reset transistor  152 . On the other hand, the holes formed in depletion region  124  move under the influence of the electric field towards p− region  120 . 
     In addition, the electrons, which are from the electron-hole pairs that are formed in p− region  120  within a diffusion length of depletion region  124 , diffuse to depletion region  124  and are swept to n+ region  122  under the influence of the electric field. Further, the electrons that are formed in n+ region  122  remain in n+ region  122 . 
     As with the red and green electrons, the blue electrons from the electron-hole pairs that are formed in depletion region  134  move under the influence of the electric field towards n+ region  132 , where each additional electron collected by n+ region  132  reduces the positive potential that was placed on n+ region  132  by reset transistor  154 . On the other hand, the holes formed in depletion region  134  move under the influence of the electric field towards p− region  130 . 
     In addition, the electrons, which are from the electron-hole pairs that are formed in p− region  130  within a diffusion length of depletion region  134 , diffuse to depletion region  134  and are swept to n+ region  132  under the influence of the electric field. Further, the electrons that are formed in n+ region  132  remain in n+ region  132 . 
     After the red, green, and blue photodiodes have collected light energy for a period of time, known as the integration period, sense circuitry associated with the photodiodes detects the change in potential on n+ regions  112 ,  122 , and  132 . Specifically, in addition to a reset transistor, each photodiode also has an associated source follower transistor SF and a row select transistor RS. 
     The change in potentials on an n+ region is present on the gate of the associated source follower transistor SF, while the source of the source follower transistor SF is one diode drop below the potential. Thus, when the gate of the row select transistor RS is pulsed, an output potential equal to the photodiode potential less a diode drop is output to a sense cell to determine the output potential. Once the change in positive potential has been determined, the photodiodes are reset and the process is repeated. 
     One problem with imager cell  100  is that the red, green, and blue photodiodes of cell  100  do not produce an equal number of photo-generated electrons when exposed to a white light source. The blue photodiode produces the largest number, with the green photodiode next and the red photodiode producing the smallest number of photo-generated electrons. The difference in the numbers of electrons must then be compensated for to produce an equal response. 
     SUMMARY OF THE INVENTION 
     The present invention provides a vertical imager cell that utilizes a combination of materials to adjust the band gaps and, in turn, adjust the photon absorption rate of the color photodiodes used in an imager cell. By adjusting the photon absorption rate, the numbers of electrons collected by the photodiodes, and thereby the characteristics of the imager, can be adjusted. 
     An imager cell in accordance with the present invention includes a first layer of material, and a second layer of material that is formed on the first layer of material. In addition, the imager cell also includes a third layer of material that is formed on the second layer of material, and a fourth layer of material that is formed on the third layer of material. Further, a top layer of material is formed over the fourth layer of material. The top layer of material is different from the fourth layer of material. 
     The imager cell additionally includes a first region of a first conductivity type that is formed in the first layer of material and a lower portion of the second layer of material. The imager cell further includes a first region of a second conductivity type that is formed in an upper portion of the second layer of material to contact the first region of the first conductivity type. In addition, a first depletion region is formed between the first regions of the first and second conductivity types. 
     Further, the imager cell includes a second region of the first conductivity type that is formed in the third layer of material and the lower portion of fourth layer of material to contact the first region of the second conductivity type. In addition, the imager cell includes a second region of the second conductivity type that is formed in an upper portion of the fourth layer of material to contact the second region of the first conductivity type. Further, a second depletion region is formed between the second regions of the first and second conductivity types. 
     The imager cell also includes a third region of the first conductivity type that is formed in the top layer of material. In addition, the imager cell includes a third region of the second conductivity type that is formed in a surface of the top layer of material to contact the third region of the first conductivity type. Further, a third depletion region is formed between the third regions of the first and second conductivity types. 
     The present invention also includes a method of forming an imager cell that includes the steps of forming a first layer of material, and forming a second layer of material, which has a first conductivity type, on the first layer of material. The method also includes the steps of doping a top portion of the second layer of material to have a second conductivity type, and forming a third layer of material on the second layer of material. 
     The method additionally includes the steps of forming a fourth layer of material of the first conductivity type on the third layer of material, and doping a top portion of the fourth layer of material to have the second conductivity type. Further, the method includes the steps of forming a top layer of material of the first conductivity type over the fourth layer of material, and doping a top portion of the top layer of material to have the second conductivity type. 
     A better understanding of the features and advantages of the present invention will be obtained by reference to the following detailed description and accompanying drawings that set forth an illustrative embodiment in which the principles of the invention are utilized. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a combined cross-sectional and schematic diagram illustrating a prior art vertical color imager cell  100 . 
     FIG. 2 is a combined cross-sectional and schematic diagram illustrating an example of a vertical color imager cell  200  in accordance with the present invention. 
     FIGS. 3A-3D are a series of cross-sectional views illustrating an example of a method of forming the photodiode structure of FIG. 2 in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 2 shows a combined cross-sectional and schematic diagram that illustrates an example of a vertical color imager cell  200  in accordance with the present invention. As described in greater detail below, cell  200  utilizes a combination of materials to vary the photon absorption rate of the imager. 
     As shown in FIG. 2, cell  200  includes a first layer of material  210 , and a second layer of material  212  that is formed on the first layer of material  210 . In addition, cell  200  includes a third layer of material  214  that is formed on the second layer of material  212 , and a fourth layer of material  216  that is formed on the third layer of material  214 . Further, cell  200  includes a fifth layer of material  218  that is formed on the fourth layer of material  216 . 
     In addition to the five layers of material  210 ,  212 ,  214 ,  216 , and  218 , cell  200  also includes three p− regions and three n+ regions that form three vertical photodiodes. As further shown in FIG. 2, imager cell  200  includes a first p− region  220  that is formed in layer  210  and the lower portion of layer  212 , and a first n+ region  222  that is formed in the upper portion of layer  212  to contact p− region  220 . In addition, cell  200  has a first depletion region  224  that is formed between regions  220  and  222 . 
     Imager cell  200  additionally includes a second p− region  230  that is formed in layer  214  and the lower portion of layer  216  to contact n+ region  222 . Cell  200  further includes a second n+ region  232  that is formed in the upper portion of layer  216  to contact p− region  230 , and a second depletion region  234  that is formed between regions  230  and  232 . 
     Imager cell  200  further includes a third p− region  240  that is formed in layer  218  to contact n+ region  232 , and a third n+ region  242  that is formed in the surface of layer  218  to contact p− region  240 . In addition, cell  200  includes a third depletion region  244  that is formed between regions  240  and  242 . Cell  200  also includes a p+ sinker  250  that is connected to p− regions  220 ,  230 , and  240 , an n+ sinker  252  that is connected to n+ region  222 , and an n+ sinker  254  that is connected to n+ region  232 . 
     As further shown in FIG. 2, imager cell  200  includes three sets of control transistors, with each set having a reset transistor RT, a source-follower transistor SF, and a select transistor RS. The select transistor from each set can be either a row or column select transistor and can, for example, share the same select line. Sharing the same select line allows the three colors to be measured at the same time. In addition, each set can optionally include a second select transistor CS (a column or row select transistor) if an individual pixel read out is desired. 
     In operation, as shown in FIG. 2, p− regions  220 ,  230 , and  240  are connected to ground via p+ sinker region  250 . In addition, n+ regions  222 ,  232 , and  242  are connected to the sources of the reset transistors RT. Prior to collecting photo information, the reset transistors RT are pulsed on which, in turn, places a positive potential on n+ regions  222 ,  232 , and  242 . 
     The positive potential reverse biases the pn junction of regions  220  and  222 , thereby forming a red collecting photodiode, and the pn junction of regions  230  and  232 , thereby forming a green collecting photodiode. The positive potential also reverse biases the pn junction of regions  240  and  242 , thereby forming a blue collecting photodiode. 
     Once the positive potentials have been placed on n+ regions  222 ,  232 , and  242 , light energy, in the form of photons, is collected by the red, green, and blue photodiodes. The red photons are absorbed by the red photodiode which, in turn, forms a number of red electron-hole pairs, while the green photons are absorbed by the green photodiode which, in turn, forms a number of green electron-hole pairs. Similarly, the blue photons are absorbed by the blue photodiode which, in turn, forms a number of blue electron-hole pairs. 
     The red, green, and blue electrons from the electron-hole pairs that are formed in depletion regions  224 ,  234 , and  244 , respectively, move under the influence of the electric field towards n+ regions  222 ,  232 , and  242 , respectively. In addition, each additional electron collected by n+ regions  222 ,  232 , and  242  reduces the positive potential that was placed on n+ regions  222 ,  232 , and  242  by the reset transistors RT. On the other hand, the holes formed in depletion regions  224 ,  234 , and  244  move under the influence of the electric field towards p− regions  220 ,  230 , and  240 , respectively. 
     In addition, the electrons, which are from the electron-hole pairs that are formed in p− regions  220 ,  230 , and  240  within a diffusion length of depletion regions  224 ,  234 , and  244 , respectively, diffuse to depletion regions  224 ,  234 , and  244 , respectively. The electrons are then swept through depletion regions  224 ,  234 , and  244  to n+ regions  222 ,  234 , and  244 , respectively, under the influence of the electric field. Further, the electrons that are formed in n+ regions  222 ,  234 , and  244  remain in n+ regions  222 ,  234 , and  244 , respectively. 
     After the red, green, and blue photodiodes have collected light energy for an integration period, the control transistors associated with the photodiodes detects the change in potential on n+ regions  222 ,  232 , and  242 . Specifically, the change in potential on an n+ region is present on the gate of the associated source follower transistor SF, while the source of the source follower transistor SF is one diode drop below the gate potential. 
     Thus, when the gate of the select transistor RS is pulsed, an output potential equal to the photodiode potential less a diode drop is output to a sense cell to determine the output potential. Once the change in positive potential has been determined, the photodiodes are reset and the process is repeated. 
     As shown in FIG. 2, cell  200  is formed so that depletion region  224  of the red photodiode and a thin adjoining layer of p− region  220  (equal to an electron diffusion length) are formed in the second layer of material  212 . In accordance with the present invention, the second layer of material  212  is formed from a material that has a lower band gap than the material used to form the fifth layer of material  218 . 
     For example, when the fifth layer of material  218  is formed from single crystal silicon, which has a band gap of 1.1 eV, the second layer of material can be formed from silicon germanium, which has a band gap of 0.7 eV. Since silicon germanium has a lower band gap than single crystal silicon, silicon germanium is more sensitive to red photons than single crystal silicon. Thus, when silicon germanium is utilized, a larger number of photo-generated electrons are formed in depletion region  224  than are formed in the depletion region of a silicon vertical color imager, such as depletion region  114  of imager  100 . 
     Cell  200  is also formed so that depletion region  234  of the green photodiode and a thin adjoining layer of p− region  230  (equal to an electron diffusion length) are formed in the fourth layer of material  216 . The fourth layer of material  216  can also formed from a material that has a lower band gap than the material used to form the fifth layer of material  218 . 
     For example, when the fifth layer of material  218  is formed from single crystal silicon, the fourth layer of material  216  can be formed from silicon germanium. As above, since silicon germanium has a lower band gap than single crystal silicon, silicon germanium is more sensitive to green photons than single crystal silicon. Thus, when silicon germanium is utilized, a larger number of photo-generated electrons are formed in depletion region  234  than are formed in the depletion region of a silicon vertical color imager, such as depletion region  124  of imager  100 . 
     In the present example, cell  200  is further formed so that depletion region  244  of the blue photodiode and a thin adjoining layer of p− region  240  (equal to an electron diffusion length) are both formed in the fifth layer of material  218 . When the fifth layer of material  218  is formed from single crystal silicon, depletion region  244  collects about the same number of photo-generated electrons as the depletion region of a conventional vertical color imager, such as depletion region  134  of imager  100 . 
     When the second and fourth layers of material  212  and  216  are the same, the relative concentrations of the elements of the material can be the same or different. For example, when the second and fourth layers of material  212  and  216  are silicon germanium, the concentration of germanium in layers  212  and  216  can be the same or different. 
     One of the advantages of the present invention is that the photon absorption characteristics of the vertical photodiodes can be tuned to a desired value. For example, when exposed to a white light source, the number of photo-generated electrons collected by a red photodiode can be increased to be approximately equal to the number of photo-generated electrons collected by a blue photodiode by using a material with a lower band gap and, therefore, a greater sensitivity. 
     Similarly, the number of photo-generated electrons collected by a green photodiode when exposed to white light can be increased to be approximately equal to the number of photo-generated electrons collected by a blue photodiode by using a material with a lower band gap and, therefore, a greater sensitivity. 
     Although the above description used single crystal silicon as an example of a material that can be used to implement the fifth layer of material  218 , and silicon germanium as an example of a material that has a band gap that is less than single crystal silicon, the present invention is not limited to these materials. 
     Instead, the present invention includes all materials that have a lower band gap than the material used to form the fifth layer of material, and can withstand the fabrication environment. When the fifth layer of material is formed from single crystal silicon, a few examples of the materials that can be used to form the fourth layer of material include gallium nitride with different stochastics, gallium arsenide, and indium phosphide. 
     FIGS. 3A-3D show a series of cross-sectional views that illustrate an example of a method of forming the photodiode structure of FIG. 2 in accordance with the present invention. As shown in FIG. 3A, the first layer of material  210  is formed as a p− single crystal silicon layer, while the second layer of material  212  is epitaxially or otherwise formed on layer  210  as a p− silicon germanium layer. 
     Following the formation of silicon germanium layer  212 , the surface of layer  212  is blanket implanted to form n+ region  222 . In addition, the bottom portion of p+ sinker region  250  is formed through the n+ region  222  of layer  212  to contact the p− region of layer  212  using conventional masking and implanting steps. (The bottom portion of p+ sinker region  250  can alternately be formed later as part of a multi implant step that utilizes different implant energies.) 
     Next, as shown in FIG. 3B, the third layer of material  214  is epitaxially or otherwise formed on layer  212 . Third layer  214  can be formed as a layer of p− single crystal silicon, or a layer of p− silicon germanium. Following the formation of layer  214 , the next portion of p+ sinker region  250  is formed through silicon layer  214  using conventional masking and implanting steps. (The next portion of p+ sinker region  250  can alternately be formed later as part of a multi implant step that utilizes different implant energies.) Further, the lower portion of n+ sinker region  252  is formed through p− layer  214  to contact n+ region  222  using conventional masking and implanting steps. 
     Following this, as shown in FIG. 3C, the fourth layer of material  216  is epitaxially or otherwise formed on layer  214  as a p− silicon germanium layer. (When the third layer of material  214  is formed of silicon germanium, the third layer of material  214  and the fourth layer of material  216  are formed at the same time as a single layer of material.) Following the formation of silicon germanium layer  216 , the surface of layer  216  is blanket implanted to form n+ region  232 . 
     In addition, the next portion of p+ sinker region  250  is formed through layer  216  using conventional masking and implanting steps. (The next portion of p+ sinker region  250  can alternately be formed later as part of a multi implant step that utilizes different implant energies.) Further, the next portion of n+ sinker region  252  is formed through layer  216  using conventional masking and implanting steps. (The next portion of n+ sinker region  252  can alternately be formed later as part of a multi implant step that utilizes different implant energies.) An isolation trench T 1  is also formed in layer  216  to isolate n+ region  232  of layer  216  from n+ sinker region  252  using conventional trench formation techniques. 
     Next, as shown in FIG. 3D, the fifth layer of material  218  is epitaxially or otherwise formed on layer  216  as a layer of p− single crystal silicon. Following the formation of silicon layer  218 , the surface of layer  218  is masked and implanted using conventional steps to form n+ region  242 . 
     In addition, the top portion of p+ sinker region  250  is formed through layer  218  using conventional masking and implanting steps. (The portions below the top portion of sinker region  250  can be formed at this point as part of a multi implant step that utilizes different implant energies.) Further, the top portion of n+ sinker region  252  is formed through layer  218  using conventional masking and implanting steps. (The portions below the top portion of sinker region  252  can alternately be formed at this point as part of a multi implant step that utilizes different implant energies.) 
     An isolation trench T 2  is also formed through layer  218  to contact trench T 1  and isolate n+ region  242  of layer  218  from n+ sinker region  252  using conventional trench formation techniques. Further, sinker region  254  is formed through layer  218  using conventional masking and implanting steps to contact n+ region  232 . In addition, an isolation trench T 3  is also formed through layer  218  to isolate n+ region  242  of layer  218  from n+ sinker regions  252  and  254  using conventional trench formation techniques. 
     It should be understood that the above descriptions are examples of the present invention, and that various alternatives of the invention described herein may be employed in practicing the invention. The present invention can be modified to include prior art elements and steps, such as those disclosed in U.S. Patent Application Publication US 2002/0058353 A1 published on May 16, 2002 to Richard Merrill for Vertical Color Filter Detector Group and Array, which is hereby incorporated by reference. For example, a light shield can be added to cover the sinkers  250 ,  252 , and  254  along with the control transistors. 
     In addition, although the present invention has been described in terms of three photodiodes, the present invention is not limited to three photodiodes, but can be extended to cover four or more vertical color photodiodes. For example, when four photodiodes are used, the second layer of material, which includes the depletion region of the bottom photodiode, has a band gap that is less than the band gap of the top layer of material, which includes the depletion region of the top photodiode. 
     Further, the fourth layer of material, which includes the depletion region of the second from the bottom photodiode, has a band gap that is less than the band gap of the top layer of material, which includes the depletion region of the top photodiode. Thus, it is intended that the following claims define the scope of the invention and that structures and methods within the scope of these claims and their equivalents be covered thereby.