Patent Publication Number: US-7713766-B2

Title: Light sensor located above an integrated circuit

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is a division of prior application Ser. No. 11/323,074, filed on Dec. 30, 2005, now U.S. Pat. No. 7,492,026 entitled “Light Sensor Located Above An Integrated Circuit” which application claims the priority benefit of French patent application number 04/532624, filed on Dec. 30, 2004, entitled “Light Sensor Located Above An Integrated Circuit” which applications are hereby incorporated by reference to the maximum extent allowable by law. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to the field of micro-electronics and more specifically to light sensors associated with an integrated circuit. 
   2. Discussion of the Related Art 
   Light sensors are devices which enable turning light signal into an electric signal. Several millions of these sensors can be distributed on a surface to create an electric image comprising millions of points. Usually, a sensor is formed of a light-emitting diode and of a MOS circuit enabling collecting and processing the electric signal issued by the light-emitting diode. 
   The light-emitting diode may be formed of single-crystal silicon. In this case, the MOS circuit and the light-emitting diode coexist in a single-crystal silicon substrate and are located substantially in the same plane. The MOS transistors which enable collection of the electric current of the light-emitting diodes are located between the light-emitting diodes. Such an architecture is expensive since the surface area taken up in the silicon is the sum of the surface areas of the light-emitting diodes and of the MOS transistors. It has a low performance since not all the light reaches the light-emitting diodes and part of it is lost between the light-emitting diodes. The light hitting the light-emitting diodes is likely to diffuse and to disturb the operation of the MOS transistors adjacent to the light-emitting diodes. 
   Another solution consists of forming the light-emitting diodes above the integrated circuit comprising the MOS transistors. Such a structure is described in U.S. patent application 2004/0135209.  FIG. 1  shows a structure corresponding to this US application. An integrated circuit is formed in a substrate  1 . An insulating layer  2  comprising metal interconnects is formed above the integrated circuit. The light-emitting diodes are formed above the insulating layer. The light-emitting diodes comprise a lower titanium nitride (TiN) electrode  3  and an upper indium and tin oxide (ITO) electrode  4  transparent to light. The lower electrode is in electric contact with a P-type doped amorphous silicon layer  5  located above. The upper electrode is in electric contact with an underlying N-type doped amorphous silicon layer  6 . These two P- and N-type doped amorphous silicon layers are separated by a lightly-doped N-type amorphous silicon layer. Thus, the structure of  FIG. 1  is formed of a vertical PN-type amorphous silicon light-emitting diode having its lower electrode connected to a region  8  of the integrated circuit by a via  9  made of a conductive material. 
   In this type of architecture, the space lost between the light-emitting diodes is minimum, which enables collecting a maximum light signal for a minimum occupied surface area. It is further possible to use the entire surface of the underlying silicon to achieve complex electronic functions such as, for example, the image storage and processing. The light-emitting diode, made of amorphous silicon, exhibits a chromatic response substantially equivalent to that of the human eye. Such is not the case for the light-emitting diodes formed of single-crystal silicon which are particularly sensitive to infrared wavelengths. This chromatic response enables simplifying the arrangement of the color filters in the case where a light-emitting diode matrix capable of rendering a color image is desired to be formed. 
   However, up to now, such architectures result in low performance for the light sensor, in particular in low light. To collect the current generated by the light-emitting diode, the light-emitting diode junction must be reverse-biased. The light signal creates carriers close to the PN junction of the light-emitting diode. Such carriers are collected in the space charge area of the reverse-biased junction and then form a photocurrent which is processed by the underlying integrated circuit. The reverse biasing of the junction also generates a leakage current, called a dark current, which is a parasitic current. In the case of the light-emitting diode of  FIG. 1 , this dark current is greater by several decades than that obtained in the case of a light-emitting diode formed in single-crystal silicon. Under such conditions, the current generated by the light for low lightings is on the same order of magnitude as the dark current. The sensitivity of the sensor for low lighting levels is low. This requires, in the case of patent application No. US2004/0135209, implementing specific means to compensate for this dark current (“dark reference average circuit”, FIG. 8 of the US application). 
   SUMMARY OF THE INVENTION 
   An object of the present invention is to provide a light sensor integrable above an integrated circuit and exhibiting optimal electric qualities, that is, both a chromatic response substantially similar to that of the human eye and a minimum dark current. 
   Another object of the present invention is to provide such a light sensor that can be formed with as few technological steps as possible. 
   Another object of the present invention is to provide such a light sensor exhibiting a high density of light-emitting diodes of minimum size and comprising color filters which are easy to form. 
   To achieve these and other objects, the present invention provides a light sensor located above an integrated circuit comprising a lower electrode, a heavily-doped amorphous silicon layer of a first conductivity type, and a lightly-doped amorphous silicon layer of a second conductivity type. The lightly-doped amorphous silicon layer rests on a planar surface at least above and in the vicinity of the lower electrode. 
   According to an embodiment of the present invention, the light sensor comprises a lower chromium electrode in electric contact with the lightly-doped amorphous silicon. 
   According to an embodiment of the present invention, the light sensor comprises an upper ITO electrode in electric contact with heavily-doped amorphous silicon. 
   According to an embodiment of the present invention, the interfaces between the electrodes and the amorphous silicon layers are planar. 
   According to an embodiment of the present invention, the lower electrode is in contact with heavily-doped P-type amorphous silicon. 
   According to an embodiment of the present invention, the upper electrode is in electric contact with a heavily-doped N-type amorphous silicon layer. 
   The present invention also aims at an image sensor formed of light-emitting diodes such as mentioned hereabove. 
   According to an embodiment of the present invention, the repetition step of the light-emitting diodes is smaller than 1.5 μm. 
   The present invention also aims at a communicating object comprising an image sensor such as mentioned hereabove. 
   The present invention also aims at a method for forming a light sensor located above an integrated circuit comprising the steps of forming an integrated circuit comprising at its upper surface a first insulating component; depositing a second insulating layer; creating cavities across the thickness of the second insulating layer; depositing a metal, making the surface planar to leave the metal only in the cavity and to obtain a planar surface above and close to the cavity; depositing an amorphous silicon layer; depositing a doped silicon layer; and depositing an electrode layer. 
   The foregoing objects, features, and advantages of the present invention will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows an integrated light sensor above an integrated circuit of prior art; 
       FIG. 2  shows an integrated light sensor above an integrated circuit; 
       FIG. 3  shows an integrated light sensor above an integrated circuit according to an embodiment of the present invention; 
       FIG. 4  shows an image sensor formed above an integrated circuit according to the present invention; and 
       FIG. 5  shows an integrated light sensor above an integrated circuit according to another embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
   As usual in the representation of integrated circuits, the scales are not respected in the different cross-section views to better show the different layers and elements of the devices. 
   In an attempt to simplify the prior art structure illustrated in  FIG. 1 , the applicant has formed the structures shown in  FIG. 2 .  FIG. 2  describes an integrated circuit formed in a substrate  1 . An insulating layer  2 , comprising metal interconnects, is formed above the circuit. Above insulating layer  2  are formed the light-emitting diodes. The light-emitting diodes comprise a lower electrode  10  formed of a titanium nitride layer (TiN) entirely covered with a chromium layer  11  and an upper indium and tin oxide (ITO) electrode  12 . The upper electrode is in electric contact with an underlying P-type doped amorphous silicon  13 . Between electrode  11  and the P-doped amorphous silicon layer, a lightly-doped N-type amorphous silicon layer  14  has been formed. Thus, the structure of  FIG. 2  likely to sense a light signal is formed of a vertical NP-type amorphous silicon light-emitting diode. The contact on the N area is a Schottky contact ensured by the chromium. The first lower electrode is connected to a region  8  of the integrated circuit by a via  9  made of a conductive material. 
   The operation of this light-emitting diode is similar to that of the light-emitting diode of prior art. It can be seen that, despite changes in the light-emitting diode structure, the dark current is still significant and even greater than that measured in prior art. 
     FIG. 3  provides, according to an embodiment of the present invention, a novel light-emitting diode structure above an integrated circuit formed in a substrate  1 . An insulating layer  2 , comprising the metal interconnects, is formed above the integrated circuit. Above insulating layer  2  is formed a light-emitting diode  20  which forms the light sensor. This light-emitting diode successively comprises, from bottom to top, a lower electrode  21 , a first lightly-doped N-type amorphous silicon area  22 , a second P-type doped amorphous silicon  23 , and an upper electrode  24 . Lower electrode  21  is surrounded with an insulator layer  25  of same thickness. The width of lower electrode  21  is smaller than 3 μm, and preferably on the order of 1.5 μm. As seen from above, lower electrode  21  has a square shape but any other shape is possible. 
   Lower electrode  21  is preferably made of chromium which exhibits a barrier height with respect to amorphous silicon  22  capable of ensuring an electric contact with the lightly-doped N-type amorphous silicon. Those skilled in the art know that an electric contact can also be formed, for example, with a metal and an intermediary heavily-doped N-type silicon layer. In such a case, the forming method is more complex, but the use of chromium, which can exhibit a low adherence on oxide, is avoided. In the illustrated embodiment, this chromium adherence problem is solved by the decrease in the dimensions of the lower electrode and by the fact that the lower electrode is embedded in an insulator layer. Lower electrode  21  is electrically connected to a region  8  of the integrated circuit by a via  9  made of a conductive material. 
   First amorphous silicon layer  22  is not doped during the deposition. There however exists a natural N-type doping. This amorphous silicon deposited at low temperature (under 200° C.) does not exhibit clusters of measurable size. 
   Second amorphous silicon layer  23  is P-type doped. 
   ITO upper electrode  24  is connected to the integrated circuit by a metal connection not shown. 
   Light-emitting diode  20  of  FIG. 3  comprises an NP junction between first amorphous silicon  22  and second amorphous silicon  23 . This junction is reverse-biased by the underlying integrated circuit. When the light-emitting diode is lit, carriers are generated in lightly-doped amorphous silicon  22  and are then collected by the space charge area of the NP junction. This results in a photocurrent directed towards the integrated circuit by conductive via  9 . 
   The structures of  FIGS. 2 and 3  are similar. The dark current measured on the diode of  FIG. 2  is on the order of 100 pA/cm 2  for a 2-volt reverse voltage. However, the dark current measured on the diode of  FIG. 3  is on the order of 10 pA/cm 2  for a 2-V reverse voltage. This value is 10 times smaller than the former; the light-emitting diode performances are thus significantly improved. 
   There is no simple explanation for this significant decrease in the dark current. The present inventors however consider that the high dark current of prior art is linked to the presence of drops in the amorphous silicon layer. The device of  FIG. 3  is planar and the amorphous silicon in region  22 , at the level of the edge of electrode  21  is deposited and rests freely on a planar surface. Such is not the case for the devices of  FIGS. 1 and 2  in which the amorphous silicon at the level of the edges of electrodes  3  or  10  and  11  is deposited on a surface exhibiting steps due to the electrode etchings. The amorphous silicon is not very stable and its crystal structure varies according to the relief on which it rests. Silicon clusters having a size ranging between 8 and 20 nm may form locally either during the depositing of amorphous silicon or after this deposition. The electric results indicate that such clusters induce an additional dark current. In the case of  FIGS. 1 and 2 , the dark current would then have its source at the periphery of the light-emitting diodes. 
   The present invention provides for the amorphous silicon to rest on a planar surface across its entire useful surface and at least at the immediate periphery of this useful surface. A surface obtained by a chem.-mech polishing is considered as planar in the context of the present invention. A layer deposited on a surface exhibiting steps greater than 20 nm is considered as non-planar in the context of the present invention. 
     FIG. 4  shows, according to the present invention, an image sensor formed above an integrated circuit and obtained from the light-emitting diodes shown in  FIG. 3 . This sensor is formed of several millions of light-emitting diodes which comprise a plurality of lower electrodes  21  connected to the integrated circuit by conductive vias  9 . An amorphous silicon layer  22  rests on a planar surface extending over the entire area taken up by the lower electrodes. The image sensor structure is completed by a layer  23  formed of P-doped amorphous silicon and by an ITO layer  24 . To obtain a color image, the filter portions colored in blue B, green G, red R, are deposited and etched above each lower electrode. 
   It has been seen hereabove that, in the opinion of the present inventors, the dark current is generated by the periphery of the light-emitting diodes. According to the present invention, this contribution to the dark current has been suppressed. It is then possible to significantly decrease the surface of each light-emitting diode while keeping a high ratio between the photocurrent and the dark current. The lower electrode being embedded in insulator, it is no longer useful to use a bonding layer such a titanium nitride shown in  FIG. 2 . The distance to be respected for the covering of the titanium nitride with chromium becomes aimless. The structure provided by  FIG. 4  is particularly dense. The repetition threshold of the lower electrodes may reach 1.5 μm. A significant density of sensors can thus be obtained with a very good quality. 
   The B, G, R color filters must however also be formed according to the sensor step. These filters are formed from negative photosensitive resists which, once insolated, remain on the image sensor. Such resins are particularly sensitive to parasitic reflections of the light on the sides exhibited by the underlying layers. In other words, to obtain a good definition for these negative resins, it is necessary to have perfectly planar underlying surfaces. This is performed according to the present invention without using an intermediary layer capable of making the surface planar. 
   The integrated circuit underlying the image sensor may be an integral part of a communication object such as a digital photographic camera, a fax, a portable phone, or a laptop computer. The integrated circuit may also actively take part in the operation of the communication object in which it is inserted. 
   An embodiment of light-emitting diodes according to  FIG. 3  or  4  is the following, the numerical values being indicated as an example only:
         forming an integrated circuit comprising a TiN-type insulating upper layer, for example;   depositing a TEOS oxide layer with a thickness from 200 to 500 nm;   digging a cavity intended to receive the lower electrode;   substantially at the center of this cavity, digging a via down to an internal conductive layer of the integrated circuit;   depositing a chromium layer of a thickness ranging between 300 and 800 nm to fill the cavity and the via;   performing a CMP step to disengage the upper surface of the TEOS oxide and to leave the chromium only in the cavities and the vias;   depositing an undoped amorphous silicon layer with a thickness ranging between 0.35 and 1.3 μm, and preferably with a thickness equal to 0.45 μm, the natural doping of this layer being of type N and preferably smaller than 5.10 14  at/cm 3 ;   depositing a P-type doped amorphous silicon layer with a thickness ranging between 10 and 30 nm with a boron concentration greater than 10 18  at/cm 3 , where the dopant can be brought during the deposition by trimethylborane (TMB);   depositing by PVD an ITO layer with a thickness of approximately 80 nm;   depositing a green negative-type resist of a thickness of approximately 1 μm, and etching it; and   depositing red and blue resists in the same conditions as hereabove, and etching them.       

   The present invention is likely to have various alterations, modifications, and improvements which will readily occur to those skilled in the art. In particular, the metal of the lower electrode may be different, intermediary amorphous silicon layers may be added, the dopings of the amorphous silicon layers may be modified. 
     FIG. 5  illustrates another embodiment of the present invention. An integrated circuit is formed in a substrate  1  and comprises diffused conductive areas  8 . The interconnects of this integrated circuit are formed in various insulator layers  2  deposited above the integrated circuit. An insulating layer  25  rests above insulating layer  2 . Cavities  27  in this layer  25  successively contain a very conductive material  28  and heavily-doped P-type amorphous silicon  29 . Conductive material  28  may be a metal, titanium nitride, or any other conductive material. It is preferably directly in electric contact with conductive area  8  of the integrated circuit via a via  30 . The surface above layer  25  is planar. The cavities are filled so that the upper surface of layer  29  is substantially at the same level as the surface of insulator  25 . A lightly-doped N-type silicon layer  22  rests on this planar surface. The structure of the light-emitting diode is then completed with an N-type heavily-doped amorphous silicon layer  31  and with an upper ITO electrode. 
   The light-emitting diode is formed by the junction of the P-type and N-type amorphous silicon materials  29  and  22 . Heavily-doped N-type amorphous silicon layer  31  is used to obtain an ohmic contact of upper electrode  24  with lightly-doped amorphous silicon  22 . 
   This structure can be made out from those of prior art shown in  FIG. 1  by the planeness of the different amorphous silicon layers. The dark current of the structure of  FIG. 5  is minimum since according to the present invention, amorphous silicon  22  is deposited on a planar surface. Another advantage as compared to prior art is a better density and a better insulation of the P-type amorphous silicon  29  embedded in an insulator. 
   Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and the scope of the present invention. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. The present invention is limited only as defined in the following claims and the equivalents thereto.