Abstract:
The present invention provides a multi-cavity Fabry-Perot ambient light filter apparatus. The multi-cavity Fabry-Perot ambient light filter apparatus comprises a plurality of Fabry-Perot cavities, each of the plurality of Fabry-Perot cavities covering one of a plurality of photodiodes; wherein each of the plurality of Fabry-Perot cavities has two partially reflective layers and one interferometric layer sandwiching between the two partially reflective layers, and shares one of the two partially reflective layers with a neighboring Fabry-Perot cavity and thereby stair stacking with the neighboring Fabry-Perot cavity. The plurality of Fabry-Perot cavities are capable of blocking the ambient light except for a wavelength spectrum that is recognizable for human eyes, thereby effectively accomplishes excellent IR blocking from non-visible light spectra.

Description:
FIELD OF THE INVENTION 
   The present invention generally relates to a multi-cavity Fabry-Perot ambient light filter structure together with an array of photodiode elements to detect and convert the ambient light into electrical signal. More particularly, this invention relates to a multi-cavity Fabry-Perot ambient light filter structure capable of sensing color information of ambient light and provides blocking of infrared (IR) light within the wavelength ranging from 700 nm to 1100 nm. The multi-cavity Fabry-Perot structure senses not just the ambient light brightness but also the fundamental red, green and blue color components of the ambient light. 
   BACKGROUND OF THE INVENTION 
   Ambient light sensors are now in widespread use, including cameras, camcorders, scanners, electrical microscopes, and so forth. The function of the ambient light sensors is to detect and convert ambient light brightness into electrical signal. For instance, knowing the brightness information of the ambient light, the display system brightness could be adjusted accordingly to reduce the power consumption of the backlight illumination. For most of the conventional ambient light sensor solutions, the sensor spectral response is not matched with the ideal human eye photometric response. The non-ideal ambient light sensor has a much wider spectral response range and also there are multiple peaks exhibited within the entire photodiode detection range of 400 nm to 1100 nm. Please refer to  FIG. 1 , which shows a chart of spectral response regarding the wavelength spectrum of a conventional ambient light sensor. Generally, the human eyes are capable of sensing visible light within wavelength ranging between 400 nm and 700 nm  11 . The response of the conventional ambient light sensor not only detect visible light in the range of wavelength spectrum like human eyes, but also captures infrared light with wavelength above 700 nm that human eye is unable to respond. Therefore, within the range between 700 nm and 1200 nm, two peaks  12  are produced without IR blocking according to the conventional ambient light sensor. Consequently, the inconsistency would be developed such that the human eye feels the ambient light is insufficient while, on the other hand, the conventional ambient light sensor senses sufficient ambient light. In other words, the ambient light sensor senses non-visible light that human eye is unable to response and the process for sensing non-visible light causes unnecessary backlight power consumption. For this reason, this invention provides a multi-cavity Fabry-Perot filter structure utilizing the Fabry-Perot optical interference theory in order to effectively block the range from 700 nm to 1100 nm and reduce power consumption, thereby both brightness and color image processing adjustments are provided. 
   SUMMARY OF THE INVENTION 
   Therefore, it is one objective of the present invention to provide a multi-cavity Fabry-Perot ambient light filter apparatus. The multi-cavity Fabry-Perot ambient light filter apparatus comprises a plurality of Fabry-Perot cavities. Each of the plurality of Fabry-Perot cavities covers one of a plurality of photodiodes. Each of the plurality of Fabry-Perot cavities has two partially reflective layers and one interferometric layer sandwiching between the two partially reflective layers, and shares one of the two partially reflective layers with a neighboring Fabry-Perot cavity, and thereby stair stacks with the neighboring Fabry-Perot cavity. The plurality of Fabry-Perot cavities is capable of blocking infrared portion of the ambient light except for a wavelength spectrum that is recognizable for human eyes. 
   It is another objective of the present invention to provide a multi-cavity Fabry-Perot ambient light filter apparatus, which comprises a first Fabry-Perot cavity, having a first partially reflective layer, a second partially reflective layer and a first interferometric layer deposited between the first partially reflective layer and the second partially reflective layer; a second Fabry-Perot cavity, having a third partially reflective layer, a fourth partially reflective layer and a second interferometric layer deposited between the third partially reflective layer and the fourth partially reflective layer; and a third Fabry-Perot cavity, having a fifth partially reflective layer, a sixth partially reflective layer and a third interferometric layer deposited between the fifth partially reflective layer and the sixth partially reflective layer; wherein the second Fabry-Perot cavity is stair stacked on the first Fabry-Perot cavity such that the second partially reflective layer and the third partially reflective layer share a first common partially reflective layer, and the first partially reflective layer and the fourth partially reflective layer are different partially reflective layers; and wherein the third Fabry-Perot cavity is stair stacked on the second Fabry-Perot cavity such that the fourth partially reflective layer and the fifth partially reflective layer share a second common partially reflective layer, and the third partially reflective layer and the sixth partially reflective layer are different partially reflective layers. 
   Thus, the multi-cavity Fabry-Perot ambient light filter structure can effectively accomplish excellent IR blocking from non-visible light spectra and the typical transmittance of less than 2% for the entire IR range of 700 nm to 1100 nm. Furthermore, the green channel spectral response of the ambient light filter structure could well match with the spectral response of human eyes by utilizing the Fabry-Perot optical cavity. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a chart of spectral response of a conventional ambient light sensor; 
       FIG. 2  is a cross-sectional diagram explaining an example of the composition of a single Fabry-Perot ambient light filter structure according to an embodiment of the present invention; 
       FIG. 3  is a chart of spectral responses explaining an example of the wavelength spectrum of the ambient light filter structure with IR blocking characteristics according to an embodiment of the present invention and human eyes; 
       FIG. 4  is a cross-sectional diagram explaining an example of the composition of multi-cavity Fabry-Perot ambient light sensing system explaining an example of the composition of a Fabry-Perot filter stack for ambient color sensing according to an embodiment of the present invention; 
       FIG. 5  is a top view explaining an example of three Fabry-Perot filter regions sensing the fundamental RGB color components according to an embodiment of the present invention; and 
       FIG. 6  is a top view explaining an example of three complementary RGB color sensing regions of a multi-cavity Fabry-Perot filter according to an embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The preferred embodiments of the present invention will be explained below with reference to the drawing. 
     FIG. 2  shows an example of a green channel ambient light filter structure according to an embodiment of the present invention. The ambient light filter structure comprises a silicon substrate  21 , a first silicon nitride (Si 3 N 4 : 3200 Å±200) thin film layer  22 , a first silver (Ag: 285 Å±35) partially reflective layer  23 , a second silicon nitride (Si 3 N 4 : 920 Å±50) thin film layer  24 , a second silver (Ag: 285 Å±35) partially reflective layer  25 , and a third silicon nitride (Si 3 N 4 : 3500 Å±200) thin film layer  26 . The preferred embodiment of the present invention has a P-type silicon substrate  21  which includes an array of N+ junction a photodiode element (not shown). On top of the N+/P-type photodiode, the first silicon nitride (Si 3 N 4 : 3200 Å±200) thin film layer  22  is deposited on the silicon substrate  21 , the first silver (Ag: 285 Å±35) partially reflective layer  23  is deposited on the first silicon nitride thin film layer  22 , the second silicon nitride (Si 3 N 4 : 920 Å±50) thin film layer  24  is deposited on the first silver partially reflective layer  23 , the second silver (Ag: 285 Å±35) partially reflective layer  25  is deposited on the second silicon nitride thin film layer  24 , and the third silicon nitride (Si 3 N 4 : 3500 Å±200) thin film layer  26  is deposited on the second silver partially reflective layer  25 . By way of the manufacturing process mentioned above, the single Fabry-Perot ambient light filter structure can be made and constitutes a simple five layers process plus the photodetector silicon substrate  21 . The conventional all dielectric thin film photometric filters require forty-two layers of thin-film coating. The first silicon nitride thin film layer  22  is a bottom spacer layer, the first silver partially reflective layer  23  is a bottom partial reflector layer, the second silicon nitride thin film layer  24  is a center interferometric dielectric layer, the second silver partially reflective layer  25  is a top partial reflector layer, and the third silicon nitride thin film layer  26  is a top moisture protective layer. The second silicon nitride thin film layer  24  is a Fabry-Perot interferometric nitride layer, for filtering a certain spectral band of light, and a dielectric material such as silicon dioxide (SiO 2 ) or oxy-nitride may be further applied thereon. The second silicon nitride thin film layer  24  can be shaped by Plasma Enhanced Chemical Vapor Deposition (PECVD). The first silver partially reflective layer  23 , the second silicon nitride thin film layer  24 , and the second silver partially reflective layer  25  are formed the core of the Fabry-Perot optical cavity. The first silicon nitride thin film layer  22  and the third silicon nitride thin film layer  26  are to protect the first silver partially reflective layer  23  and the second silver partially reflective layer  25  from moisture. The ambient light filter structure can be made by the Complementary Metal Oxide Semiconductor (CMOS) technology, the bipolar technology, and the Bi-Complementary Metal Oxide Semiconductor (BiCMOS) technology. Furthermore, combining the single Fabry-Perot ambient light filter structure with a metal three light shield layer is to provide an effective stray light rejection structure for integrated electrical circuits (the metal three light shield layer is deposited between the silicon substrate). The design of the multi-cavity Fabry-Perot ambient light filter structure is based on the 1 st  order optical interference theory to provide an excellent IR blocking characteristic for wavelength of 700 nm to 1100 nm. 
   Next, the responses of the ambient light filter structure according to the present invention and the human eye will be explained with  FIG. 3 . 
   As shown in  FIG. 3 , the chart introduces two responses, the first response  31  is the response of the ambient light filter structure according to the present invention and the second response  32  is that of the ideal human eye. Obviously, regarding the first response  31 , the wavelength spectrum ranging from 700 nm to 1100 nm is effectively blocked by the ambient light filter structure and the response of the ambient light filter structure is proximate to the response of the ideal human eye at the range of 400 nm to 700 nm. The peak wavelength of the ambient light filter structure locates at around 555 nm  312 . The spectral response of the ambient light filter structure substantially matches the response of the human eye. 
   Next, a multi-cavity Fabry-Perot ambient light color filter stack structure will be explained.  FIG. 4  shows a cross-sectional diagram of a multi-cavity Fabry-Perot ambient light color filter stack structure according to an embodiment of the present invention. The multi-cavity Fabry-Perot ambient light color filter stack structure is deposited on a photodiode array element ( 44 ) which comprises three photodiodes ( 41 ,  42 ,  43 ), such as the N+/P-substrate photodiodes shown in  FIG. 4 . The multi-cavity Fabry-Perot ambient light color filter stack structure comprises seven layers, they are: a first silver (Ag) partially reflective layer  411  deposited to cover the region of the first photodiode  41 ; a first silicon nitride (Si 3 N 4 ) interferometric layer  412  deposited on the first silver partially reflective layer  411 ; a second silver (Ag) partially reflective layer  413  deposited the first silicon nitride interferometric layer  412  and the region of the second photodiode  42 ; a second silicon nitride (Si 3 N 4 ) interferometric layer  421  deposited on the second silver partially reflective layer  413  to cover the region of the second photodiode  42 ; a third silver (Ag) partially reflective layer  422  deposited to cover both the second silicon nitride interferometric layer  421  and the region of the third photodiode  43 ; a third silicon nitride (Si 3 N 4 ) interferometric layer  431  deposited on the third silver partially reflective layer  422  to cover the region of the third photodiode  43 ; and a fourth silver (Ag) partially reflective layer  432  deposited on the third silicon nitride interferometric layer  431 . The first silver partially reflective layer  411 , the first silicon nitride interferometric layer  412 , and the second silver partially reflective layer  413  constitute a first Fabry-Perot optical cavity. The second silver partially reflective layer  413 , the second silicon nitride interferometric layer  421 , and the third silver partially reflective layer  422  constitute a second Fabry-Perot optical cavity. The third silver partially reflective layer  422 , the third silicon nitride interferometric layer  431 , and the fourth silver partially reflective layer  432  constitute a third Fabry-Perot optical cavity. It should be noted that the second silver partially reflective layer  413  extends from the region of the first photodiode  41  to the region of the second photodiode  42 ; and the third silver partially reflective layer  422  extends from the region of the second photodiode  42  to the region of the third photodiode  43 . In other words, the second silver partially reflective layer  413  is a common Fabry-Perot reflector shared by the first photodiode  41  and the second photodiode  42 ; and the third silver partially reflective layer  422  is a common Fabry-Perot reflector shared by the second photodiode  42  and the third photodiode  43 . The multi-cavity Fabry-Perot ambient light color filter stack structure can be made as a stair stack according to the present invention. The first silicon nitride interferometric layer  412 , the second silicon nitride interferometric layer  421 , and the third silicon nitride interferometric layer  431  are the interferometric center dielectric layer of the ambient light color filter structure. The deposition thickness of each silicon nitride interferometric layer may be implemented using the modern thin film deposition equipment, such as the Plasma Enhanced Chemical Vapor Deposition, which is a well controlled thickness deposition process. The seven layers of the ambient light color filter stack structure are usually used for a three-color system. The three-color system is a three fundamental color separation that human eye can recognize such as red, green, and blue. The aforementioned region of the first photodiode  41  may be implemented for capturing blue light, with a peak value near 450 nm in the wavelength spectrum. The aforementioned region of the second photodiode  42  may be implemented for capturing green light with a peak value near 550 nm in the wavelength spectrum. The aforementioned region of the third photodiode  43  may be implemented for capturing red light with a peak value near 650 nm in the wavelength spectrum. Furthermore, this type of seven layers of the ambient light color filter structure offers a modular flexible filter stack solution (the modular Fabry-Perot filter cell is formed by two silver partially reflective layers plus a silicon nitride interferometric layer and the silicon nitride interferometric layer is placed between two silver component layers) for any additional color filtering and detection. Each additional color filter cell requires only an extra modular Fabry-Perot filter stack masking layer and silicon nitride interferometric thickness layer deposition defines a specific optical passing spectrum. The partially reflective layer such as silver deposition and mask photo patterning process is based on either lift-off or dry etching process to define the modular filter regions. 
     FIG. 5  is a top view explaining an example of three fundamental color sensing regions of an ambient light color filter structure according to an embodiment of the present invention. The first rectangle  51  of the ambient light color filter, corresponding to the aforementioned region of the first photodiode  41 , may be designed for capturing blue light. The second rectangle  52  of the ambient light color filter, corresponding to the aforementioned region of the second photodiode  42 , may be designed for capturing green light. The third rectangle  53  of the ambient light color filter, corresponding to the aforementioned region of the third photodiode  43 , may be designed for capturing red light. 
   Besides the three rectangles for capturing each fundamental color by the ambient light color filter structure, they may also be implemented to capture complementary color. Please refer to  FIG. 6 , another bird&#39;s eye view of complementary color rectangles along with the ambient light color filter structure  54  according to an embodiment of the present invention is shown. The complementary color includes cyan, magenta, and yellow. Therefore, the first rectangle  61  of the ambient light color filter, corresponding to the aforementioned region of the first photodiode  41 , may be designed to capture yellow light. The second rectangle  62  of the ambient light color filter, corresponding to the aforementioned region of the second photodiode  42 , may be designed to capture magenta light. The third rectangle  63 , corresponding to the aforementioned region of the third photodiode  43 , may be designed to capture cyan light. 
   In summation of the description above, the present invention of multi-cavity Fabry-Perot filter stack filter structure is novel and useful and definite enhances the performance over the conventional CMOS polymer based RGB filter and further complies with the patent application requirements and is submitted to the Patent and Trademark Office for review and granting of the commensurate patent rights.