it is to be understood that the following disclosure provides many different embodiments , or examples , for implementing different features of various embodiments . specific examples of components and arrangements are described below to simplify the present disclosure . these are , of course , merely examples and are not intended to be limiting . in addition , the present disclosure may repeat reference numerals and / or letters in the various examples . this repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and / or configurations discussed . moreover , the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact , and may also include embodiments in which additional features may be formed interposing the first and second features , such that the first and second features may not be in direct contact . referring to fig1 , an image sensor 50 provides a grid of backside illuminated ( or back - illuminated ) pixels 100 . in the present embodiment , the pixels 100 are photosensitive diodes or photodiodes , for recording an intensity or brightness of light on the diode . alternatively , the pixels 100 may also include reset transistors , source follower transistors , pinned layer photodiodes , and transfer transistors . the image sensor 50 can be of various different types , including a charge - coupled device ( ccd ), a complimentary metal oxide semiconductor ( cmos ) image sensor ( cis ), an active - pixel sensor ( acp ), or a passive - pixel sensor . additional circuitry and input / outputs are typically provided adjacent to the grid of pixels 100 for providing an operation environment for the pixels and for supporting external communications with the pixels . referring now to fig2 , the sensor 50 includes a silicon - on - insulator ( soi ) substrate 110 including silicon and carbon dioxide . alternatively , the substrate 110 may comprise an epitaxial layer or other combination of layers . in other embodiments , the substrate 110 may comprise an elementary semiconductor such as silicon , germanium , and diamond . the substrate 110 may also comprise a compound semiconductor such as silicon carbide , gallium arsenic , indium arsenide , and indium phosphide . the substrate 110 may comprise an alloy semiconductor such as silicon germanium , silicon germanium carbide , gallium arsenic phosphide , and gallium indium phosphide . in the present embodiment , the substrate 110 comprises p - type silicon formed over a silicon dioxide base . silicon doping may be implemented using a process such as ion implantation or diffusion in various steps . the substrate 110 may comprise lateral isolation features to separate different devices formed on the substrate . the thickness of the substrate 110 has been thinned to allow for etching of the backside of the substrate . this reduction in thickness may be accomplished by back grinding , diamond scrubbing , chemical mechanical planarization ( cmp ), or other similar techniques . the sensor 50 includes a plurality of pixels 100 formed on the front surface of the semiconductor substrate 110 . for the sake of example , the pixels are further labeled 100 r , 100 g , and 100 b to correspond with example light wavelengths of red , green , and blue , respectively . as noted above , the pixels 100 ( also referred to as radiation - sensing elements ) sense different wavelengths of radiation ( light ) and record an intensity or brightness of the radiation ( light ). the pixels 100 each comprise a light - sensing region ( or photo - sensing region ) which in the present embodiment is an n - type doped region having dopants formed in the semiconductor substrate 110 by a method such as diffusion or ion implantation . in continuance of the present example , the doped regions are further labeled 112 r , 112 g , and 112 b to correspond with the pixels 100 r , 100 g , and 100 b , respectively . in some embodiments , the doped regions 112 can be varied one from another , such as by having different material types , thicknesses , and so forth . the sensor 50 further includes additional layers , including first and second metal layers 120 , 122 and inter - level dielectric 124 . the dielectric layer 124 comprises a low - k material , as compared to a dielectric constant of silicon dioxide . alternatively , the dielectric layer 124 may comprise carbon - doped silicon oxide , fluorine - doped silicon oxide , silicon oxide , silicon nitride , and / or organic low - k material . the metal layers 120 , 122 may include aluminum , copper , tungsten , titanium , titanium nitride , tantalum , tantalum nitride , metal silicide , or any combinations thereof . additional circuitry also exists to provide an appropriate functionality to handle the type of pixels 100 being used and the type of light being sensed . it is understood that the wavelengths red , green , and blue are provided for the sake of example , and that the pixels 100 are generally illustrated as being photodiodes for the sake of example . referring now to fig3 , the substrate 110 includes a plurality of absorption depths 114 r , 114 g , and 114 b located beneath the corresponding pixels 100 r , 100 g , and 100 b , respectively . each wavelength ( e . g ., red , green , and blue light ) has a different effective absorption depth when it passes through the substrate 110 . for example , blue light experiences a more shallow effective absorption depth , as compared to red light . thus , the absorption depth 114 r , 114 g , and 114 b for each color pixel 100 r , 100 g , and 100 b varies accordingly . as an example , the absorption depth 114 r beneath the pixel 100 r for red light is between 0 . 35 μm to 8 . 0 μm . the absorption depth 114 g beneath the pixel 100 g for green light is between 0 . 15 μm to 3 . 5 μm . the absorption depth 114 b beneath the pixel 100 b for blue light is between 0 . 10 μm to 2 . 5 μm . the absorption depths 114 may be formed by a variety of different techniques . one technique is to apply a photosensitive layer to the backside of the substrate 110 , pattern the photosensitive layer , and etch the substrate according to the pattern . for example , a wet etch process may be used to remove the unwanted silicon substrate . this process can be repeated to create different absorption depths . referring now to fig4 , the sensor 50 includes a planarization layer 130 located between the pixels 100 r , 100 g , and 100 b and the color filters 160 r , 160 g , and 160 b ( shown in fig5 ). the planarization layer 130 is made up of an organic or polymeric material that has a high transmittance rate for visible light . this allows light to pass through the planarization layer 130 with very little distortion so that it can be detected at the light - sensing regions in the substrate 110 . the planarization layer 130 may be formed by a spin coating method which provides for a uniform and even layer . referring now to fig5 , the sensor 50 is designed to receive light 150 directed towards the back surface of the semiconductor substrate 110 during applications , eliminating any obstructions to the optical paths by other objects such as gate features and metal lines , and maximizing the exposure of the light - sensing region to the illuminated light . the illuminated light 150 may not be limited to visual light beam , but can be infrared ( ir ), ultraviolet ( uv ), and other radiation . the sensor 50 further comprises a color filter layer 160 . the color filter layer 160 can support several different color filters ( e . g ., red , green , and blue ), and may be positioned such that the incident light is directed thereon and there through . in one embodiment , such color - transparent layers may comprise a polymeric material ( e . g ., negative photoresist based on an acrylic polymer ) or resin . the color filter layer 160 may comprise negative photoresist based on an acrylic polymer including color pigments . in continuance of the present example , color filters 160 r , 160 g , and 160 b correspond to pixels 100 r , 100 g , and 100 b , respectively . the sensor 50 may comprise a plurality of lenses 170 , such as microlenses , in various positional arrangements with the pixels 100 and the color filters 160 , such that the backside - illuminated light 150 can be focused on the light - sensing regions . referring to fig6 , a graph 200 shows a comparison of the sensitivities for the various pixels when responding to red , green , or blue light . the vertical axis of the graph 200 shows light or radiation sensitivity , and the horizontal axis shows light or radiation wavelength . as can be seen from the graph 200 , if the absorption depths are uniform , the light sensitivity 205 between the different pixels in response to red , green , and blue radiation wavelengths would be different . the blue light has a shorter wavelength than the green and red light and thus , the blue light has a shorter effective absorption depth in the substrate . in the present example , the pixel for receiving blue light would have a reduced level of light sensitivity , as compared to the pixels for receiving green and red light . referring now to fig7 , a graph 210 shows a comparison of the sensitivities for the pixels 100 r , 100 g , and 100 b , when responding to red , green , or blue light , respectively . since the sensor 50 has absorption depths 114 r , 114 g , and 114 b with varying thicknesses , then a more even distribution of light sensitivity 215 can be obtained between the different pixels 100 r , 100 g , and 100 b in response to different wavelengths of radiation . in the present example , the wavelengths are red , green , and blue , and the pixels 100 r , 100 g , and 100 b have corresponding color filters 160 r , 160 g , and 160 b . it is understood that variations in junction depths and dopant concentrations may be combined with aspects of the present disclosure to achieve a more uniform spectral response and to improve performance of the sensor 50 . thus , provided is an improved sensor device and method for manufacturing same . in one embodiment , a backside illuminated sensor includes a semiconductor substrate having a front surface and a back surface and a plurality of pixels formed on the front surface of the semiconductor substrate . the sensor further includes a plurality of absorption depths formed within the back surface of the semiconductor substrate . each of the plurality of absorption depths is arranged according to each of the plurality of pixels . in some embodiments , the plurality of pixels are of a type to form a cmos image sensor . in other embodiments , the plurality of pixels are of a type to form a charge - coupled device . in other embodiments , the plurality of pixels are of a type to form an active - pixel sensor . in still other embodiments , the plurality of pixels are of a type to form a passive - pixel sensor . in some other embodiments , the sensor includes red , green , and blue color filters aligned with corresponding red , green , and blue pixels and a planarization layer that lies between the color filters and the pixels . the sensor further includes microlenses over the color filters , a dielectric layer disposed above the front surface of the semiconductor substrate , and a plurality of metal layers over the semiconductor substrate . in another embodiment , a method is provided for forming a backside illuminated sensor . the method includes providing a semiconductor substrate having a front surface and a back surface and forming a first , second , and third pixel on the front surface of the semiconductor substrate . the method further includes forming a first , second , and third thickness within the back surface of the semiconductor substrate , wherein the first , second , and third thickness lies beneath the first , second , and third pixel , respectively . in some embodiments , the method includes forming color filters aligned with the plurality of pixels and forming a planarization layer between the color filters and pixels . the method further includes providing a dielectric layer and a plurality of metal layers above the front surface of the semiconductor substrate . the foregoing has outlined features of several embodiments so that those skilled in the art may better understand the detailed description that follows . those skilled in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and / or achieving the same advantages of the embodiments introduced herein . those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure , and that they may make various changes , substitutions and alterations herein without departing from the spirit and scope of the present disclosure .