Abstract:
A semiconductor device, particularly, a photoelectric conversion element having a semiconductor layer is demonstrated. The photoelectric conversion element of the present invention comprises, over a substrate, a photoelectric conversion layer and first and second electrodes which are electrically connected to the photoelectric conversion layer. The photoelectric conversion element further comprises a wiring board over which a third and fourth electrodes are provided. The characteristic point of the present invention is that a bonding layer, which readily forms an alloy with a conductive material, is formed over the first and second electrodes. This bonding layer improves the bonding strength between the first and third electrodes and the second and fourth electrode, which contributes to the prevention of the connection defect between the substrate and the wiring board and consequentially to high reliability of the photoelectric conversion element.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a photoelectric conversion element formed using a semiconductor and a semiconductor device including the photoelectric conversion element. 
     2. Description of the Related Art 
     As one mode of a photoelectric conversion element, a photoelectric conversion element having sensitivity to light with a wavelength of 400 to 700 nm in the visible light region is referred to as a light sensor or a visible light sensor. A known application of a light sensor or a visible light sensor is to detect a light signal and read information and to detect brightness of the ambient environment and control operation of electronic appliances or the like. 
     For example, a light sensor is used in a cellular phone or a television device so that brightness of a display screen is adjusted according to brightness of the ambient environment where the cellular phone or the television device is placed (see Patent Document 1: Japanese Published Patent Application No. 2002-62856). 
       FIG. 2A  shows a structure of a light sensor disclosed in Patent Document 1. A photoelectric conversion layer  1603  is provided over a substrate  1601  so as to be interposed between a light-reflective electrode  1604   b  and a light-transmitting electrode  1602  provided with openings  1605  and  1606 . The photoelectric conversion layer  1603  has a PIN junction and forms a diode in combination with the light-transmitting electrode  1602  and the light-reflective electrode  1604   b . That is, the light sensor has a mode as a two-terminal element. One of external connection terminals is formed of a light-reflective electrode  1604   a  which is connected to the light-transmitting electrode  1602  through an opening  1607  provided in the photoelectric conversion layer  1603 . The other is formed of the light-reflective electrode  1604   b . A light-receiving surface is the light-transmitting substrate  1601  side, and light transmitted through the substrate  1601  is incident on the photoelectric conversion layer  1603 . 
       FIG. 2B  shows a light sensor in which a light-reflective electrode  1611 , a photoelectric conversion layer  1612 , and a light-transmitting electrode  1613  are provided in this order over a substrate  1610 . This light sensor has a structure in which light is incident on the photoelectric conversion layer  1612  from the light-transmitting electrode  1613  side. Through holes are formed in the light-reflective electrode  1611  and the photoelectric conversion layer  1612 , and insulating layers  1614  and  1615  are provided in the through holes. The insulating layers  1614  and  1615  prevents the short-circuit between the light-reflective electrode  1620  and the photoelectric conversion layer  1621 , which are located in the neighborhood of an edge of the substrate  1610 , and the light-reflective electrode  1611  and the photoelectric conversion layer  1612 . The light-transmitting electrode  1613  and the light-transmitting electrode  1619  are electrically insulated from each other by an insulating layer  1616  provided over the photoelectric conversion layer  1612 . An external connection terminal  1617  is provided so as to be in contact with the light-transmitting electrode  1619  and is electrically connected to the photoelectric conversion layer  1612  and the light-reflective electrode  1611 . The other external connection terminal  1618  is provided so as to be in contact with the light-transmitting electrode  1613 . 
       FIG. 2C  shows a mode in which the light sensor shown in  FIG. 2A  is mounted on a wiring board  1800 . The wiring board  1800  and the light sensor are fixed to each other by a photo-curing resin or thermosetting resin  1852  so that the light-reflective electrodes  1604   a  and  1604   b  face wirings  1850 . The light-reflective electrodes  1604   a  and  1604   b  are electrically connected to the wirings  1850  by conductive particles  1851 . In addition,  FIG. 2D  shows a mode in which the light sensor shown in  FIG. 2B  is mounted on the wiring board  1800 . The wiring board  1800  and the light sensor are bonded to each other by a conductive material  1853  such as cream solder or silver paste so that the external connection terminals  1617  and  1618  face the wirings  1850 . 
     SUMMARY OF THE INVENTION 
     In the mode of the light sensor shown in  FIG. 2C , the light sensor is bonded to the wiring board  1800  with only surfaces where the light-reflective electrodes  1604   a  and  1604   b  are formed. In addition, in the mode of the light sensor shown in  FIG. 2D , the external connection terminals  1617  and  1618  are bonded to the wiring board  1800  only by the conductive material  1853 . 
     If an external terminal of a light sensor and a conductive material are incompatible, the bonding strength is small and separation readily occurs. 
     It is an object of the present invention to improve the bonding strength between a photoelectric conversion element such as a light sensor and a wiring board or the like and to solve problems such as poor contact and separation when the photoelectric conversion element is mounted on the wiring board or the like. 
     In the present invention, when an element substrate over which a photoelectric conversion layer is formed is bonded to a wiring board by a conductive material such as solder, a layer containing a material which forms an alloy with the conductive material is formed as an uppermost layer of the element substrate. Accordingly, the bonding strength between the element substrate and the wiring board is increased and separation between the element substrate and the wiring board can be suppressed. 
     The present invention relates to a semiconductor device below. 
     Specifically, the present invention relates to a semiconductor device which has a feature that a photoelectric conversion layer; an amplifier circuit including at least two thin film transistors, which amplifies output current of the photoelectric conversion layer; a first electrode which supplies high-potential power supply voltage and a second electrode which supplies low-potential power supply voltage, which are electrically connected to the photoelectric conversion layer and the amplifier circuit; and a bonding layer which forms an alloy with a conductive material as an uppermost layer of a first substrate are formed over the first substrate, and the conductive material which bonds a third electrode to a fourth electrode, the first electrode to the third electrode, and the second electrode to the fourth electrode is provided over a second substrate. 
     In the present invention, the bonding layer contains one of nickel (Ni), copper (Cu), zinc (Zn), palladium (Pd), silver (Ag), tin (Sn), platinum (Pt), and gold (Au), and the conductive material is solder. 
     In the present invention, the first electrode and the second electrode are formed with use of nickel paste. 
     In the present invention, a fifth electrode and a sixth electrode are formed over the first electrode and the second electrode, respectively, and the fifth electrode and the sixth electrode are formed with use of copper paste. 
     In the present invention, the amplifier circuit is a current mirror circuit. 
     Note that, in this specification, a semiconductor device means elements and devices in general, which operate by utilization of a semiconductor, and a photoelectric conversion element utilizing a semiconductor, a photoelectric conversion device including a photoelectric conversion element, and an electronic appliance with an element which functions by utilization of a semiconductor are included in the category. 
     According to the present invention, the bonding strength between a substrate and a photoelectric conversion element can be increased and separation between the substrate and the photoelectric conversion element can be prevented. Accordingly, reliability of the photoelectric conversion element is improved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings: 
         FIG. 1  is a cross-sectional view showing a manufacturing process of a photoelectric conversion element of the present invention; 
         FIGS. 2A to 2D  are cross-sectional views each showing a manufacturing process of a conventional photoelectric conversion element; 
         FIGS. 3A to 3D  are cross-sectional views showing a manufacturing process of a photoelectric conversion element of the present invention; 
         FIGS. 4A to 4C  are cross-sectional views showing a manufacturing process of a photoelectric conversion element of the present invention; 
         FIGS. 5A to 5C  are cross-sectional views showing a manufacturing process of a photoelectric conversion element of the present invention; 
         FIGS. 6A and 6B  are cross-sectional views showing a manufacturing process of a photoelectric conversion element of the present invention; 
         FIGS. 7A and 7B  are cross-sectional views showing a manufacturing process of a photoelectric conversion element of the present invention; 
         FIG. 8  is a cross-sectional view showing a manufacturing process of a photoelectric conversion element of the present invention; 
         FIGS. 9A and 9B  are cross-sectional views showing a manufacturing process of a photoelectric conversion element of the present invention; 
         FIG. 10  is a circuit diagram of a photoelectric conversion element of the present invention; 
         FIG. 11  is a circuit diagram of a photoelectric conversion element of the present invention; 
         FIG. 12  is a circuit diagram of a photoelectric conversion element of the present invention; 
         FIG. 13  is a top view of a photoelectric conversion element of the present invention; 
         FIG. 14  is a top view of a photoelectric conversion element of the present invention; 
         FIG. 15  is a diagram showing a device mounted with a semiconductor device of the present invention; 
         FIGS. 16A and 16B  are diagrams each showing a device mounted with a semiconductor device of the present invention; 
         FIGS. 17A and 17B  are diagrams each showing a device mounted with a semiconductor device of the present invention; 
         FIG. 18  is a diagram showing a device mounted with a semiconductor device of the present invention; 
         FIGS. 19A and 19B  are diagrams showing a device mounted with a semiconductor device of the present invention; 
         FIG. 20  is a cross-sectional view showing a manufacturing process of a photoelectric conversion element of the present invention; 
         FIG. 21  is a cross-sectional view showing a manufacturing process of a photoelectric conversion element of the present invention; 
         FIGS. 22A and 22B  are cross-sectional views showing a manufacturing process of a photoelectric conversion element of the present invention; and 
         FIG. 23  is a cross-sectional view showing a manufacturing process of a photoelectric conversion element of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiment Modes of the present invention will be hereinafter described with reference to the accompanying drawings. However, the present invention can be carried out in many different modes and it is easily understood by those skilled in the art that modes and details of the present invention can be modified in various ways without departing from the purpose and scope of the present invention. Therefore, the present invention should not be interpreted as being limited to the following description of Embodiment Modes. Note that in all drawings for describing Embodiment Modes, the same portions or portions having similar functions are denoted by the same reference numerals, and repeated explanation thereof will be omitted. 
     Embodiment Mode 1 
     This embodiment mode will be described with reference to  FIG. 1 ,  FIGS. 3A to 3D ,  FIGS. 4A to 4C ,  FIGS. 5A to 5C ,  FIGS. 6A and 6B ,  FIGS. 7A and 7B ,  FIG. 8 ,  FIGS. 9A and 9B ,  FIG. 10 ,  FIG. 11 ,  FIG. 12 ,  FIG. 13 ,  FIG. 14 ,  FIG. 21 ,  FIGS. 22A and 22B , and  FIG. 23 . 
     A manufacturing method of a photoelectric conversion element of this embodiment mode is described below. 
     First, an insulating film  202  is formed over a substrate  201  (see  FIG. 3A ). As the substrate  201 , a light-transmitting substrate, for example, any of a glass substrate, a quartz substrate, a ceramic substrate, or the like can be used. In this embodiment mode, a glass substrate is used as the substrate  201 . 
     As the insulating film  202 , a film may be formed of silicon oxide, silicon oxide containing nitrogen, silicon nitride, silicon nitride containing oxygen, or a metal oxide material by a sputtering method or a plasma CVD method. 
     Alternatively, the insulating film  202  may be formed as a two-layer structure of a lower insulating film and an upper insulating film. As the lower insulating film, for example, a silicon nitride film containing oxygen (SiO x N y : y&gt;x) is preferably used. As the upper insulating film, for example, a silicon oxide film containing nitrogen (SiO x N y : x&gt;y) is preferably used. The insulating film  202  is formed as the two-layer structure, whereby impurities such as moisture entering the element from the substrate  201  side can be prevented. 
     Next, a crystalline semiconductor film is formed over the insulating film  202  and the crystalline semiconductor film is etched into an island shape, whereby an island-shaped semiconductor film  212  for serving as an active layer is formed. 
     In addition, a gate insulating film  205  is formed to cover the island-shaped semiconductor film  212  (see  FIG. 3B ). Next, a lower gate electrode  213   a  and an upper gate electrode  213   b  are provided over the gate insulating film  205  (see  FIG. 3C ). Although, in  FIG. 3C , a gate electrode  213  has a two-layer structure of the lower gate electrode  213   a  and the upper gate electrode  213   b , the gate electrode  213  may be formed as a single-layer structure. In addition, a source region, a drain region, and a channel formation region are formed in the island-shaped semiconductor film  212 . 
     An interlayer insulating film  206  is formed to cover the gate insulating film  205  and the gate electrode  213  including the lower gate electrode  213   a  and the upper gate electrode  213   b.    
     Note that the interlayer insulating film  206  may be formed of a single insulating film or stacked insulating layers formed of different materials. 
     A source electrode  215  and a drain electrode  216  are formed over the interlayer insulating film  206 , which is to be electrically connected, respectively, to the source region and the drain region in the island-shaped semiconductor film  212 . Moreover, a gate wiring  214  is formed to be electrically connected to the gate electrode  213 . 
     Furthermore, electrodes  221 ,  222 , and  223 , which are formed of the same material and in the same process as the gate wiring  214 , the source electrode  215 , and the drain electrode  216 , are formed over the interlayer insulating film  206  (see  FIG. 3D ). These electrodes  221  to  223  may be formed of a material and in a process that are different from those of the gate wiring  214 , the source electrode  215 , and the drain electrode  216 . 
     The gate wiring  214 , the source electrode  215 , the drain electrode  216 , and the electrodes  221  to  223  are formed using a metal film, for example, a low-resistant metal film. As such a low-resistant metal film, an aluminum alloy, pure aluminum, and the like can be represented. In this embodiment mode, a three-layer structure in which a titanium film (Ti film), an aluminum film (Al film), and a titanium film (Ti film) are sequentially stacked is employed as a stacked-layer structure of a refractory metal film and a low-resistant metal film. 
     Instead of the stacked-layer structure of the refractory metal film and the low-resistant metal film, each of the gate wiring  214 , the source electrode  215 , the drain electrode  216 , and the electrodes  221  to  223  can be formed of a single-layer conductive film. As such a single-layer conductive film, the following can be used: a single-layer film formed of an element selected from titanium (Ti), tungsten (W), tantalum (Ta), molybdenum (Mo), neodymium (Nd), cobalt (Co), zirconium (Zr), zinc (Zn), ruthenium (Ru), rhodium (Rh), palladium (Pd), osmium (Os), iridium (Ir), or platinum (Pt), or an alloy or compound containing the aforementioned element as its main component; or a single-layer film formed of nitride of the element, for example, titanium nitride, tungsten nitride, tantalum nitride, or molybdenum nitride. 
     Note that, in this embodiment mode, each of the gate wiring  214 , the source electrode  215 , the drain electrode  216 , and the electrodes  221  to  223  is formed using a titanium (Ti) film formed to a thickness of 400 nm. 
     Although a TFT  211  is a top gate TFT in this embodiment mode, the TFT  211  may be a bottom gate TFT. Alternatively, the TFT  211  may be a single gate TFT which includes one channel formation region, or a multigate TFT which includes a plurality of channel formation regions. 
     Note that only one TFT is shown in  FIG. 3D . In practice, however, at least two TFTs are formed as the TFT  211 , which constitutes a part of an amplifier circuit configured to amplify photoelectric current obtained by a photo diode  101  which is to be described later, for example, a current mirror circuit. 
     Next, etching is performed so that edge portions of the interlayer insulating film  206 , the gate insulating film  205 , and the insulating film  202  have a tapered shape (see  FIG. 4A ). 
     The edge portions of the interlayer insulating film  206 , the gate insulating film  205 , and the insulating film  202  have the tapered shape, whereby coverage by a protective film  227  which is to be formed over these films is improved, giving an effect to reduce the probability that moisture, impurities, or the like enters these films. 
     Next, a p-type semiconductor film, an i-type semiconductor film, and an n-type semiconductor film are formed over the interlayer insulating film  206  and the electrode  222 , and these films are etched, whereby a photoelectric conversion layer  225  including a p-type semiconductor layer  225   p , an i-type semiconductor layer  225   i , and an n-type semiconductor layer  225   n  is formed (see  FIG. 4B ). 
     The p-type semiconductor layer  225   p  may be formed of an amorphous semiconductor film containing an impurity element of Group 13, for example, boron (B) by a plasma CVD method. 
     In  FIG. 4B , the lowest layer of the photoelectric conversion layer  225 , which is the p-type semiconductor layer  225   p  in this embodiment mode, is in contact with the electrode  222 . 
     As the i-type semiconductor layer  225   i , an amorphous semiconductor film may be formed by a plasma CVD method, for example. As the n-type semiconductor layer  225   n , an amorphous semiconductor film containing an impurity element of Group 15, for example, phosphorus (P) may be formed, or the impurity element of Group 15 may be introduced after the amorphous semiconductor film is formed. 
     Note that an amorphous silicon film, an amorphous germanium film, or the like may be used as the amorphous semiconductor film. 
     Note that, in this specification, the i-type semiconductor film means a semiconductor film in which impurities imparting p-type or n-type conductivity are contained at a concentration of 1×10 20  cm −3  or less, each of oxygen and nitrogen is contained at a concentration of 5×10 19  cm −3  or less, and which has photoconductivity of 100 or more times as high as dark conductivity. In addition, boron (B) may be added to the i-type semiconductor film at 10 to 1000 ppm. 
     Alternatively, not only an amorphous semiconductor film but also a microcrystalline semiconductor film (also referred to as a semi-amorphous semiconductor film) may be used as each of the p-type semiconductor layer  225   p , the i-type semiconductor layer  225   i , and the n-type semiconductor layer  225   n.    
     Further alternatively, a microcrystalline semiconductor film may be used as each of the p-type semiconductor layer  225   p  and the n-type semiconductor layer  225   n , and an amorphous semiconductor film may be used as the i-type semiconductor layer  225   i.    
     Note that a semi-amorphous semiconductor (in this specification, also referred to as “SAS”) film is a film containing a semiconductor having an intermediate structure between an amorphous semiconductor and a crystalline (including single crystal and polycrystalline) semiconductor. The semi-amorphous semiconductor film is a semiconductor film having a third state which is stable in terms of free energy, and is a crystalline substance having short-range order and lattice distortion. The crystal grain with a size of 0.5 to 20 nm can exist in a dispersed state in a non-single crystalline semiconductor film. Note that a microcrystalline semiconductor film (microcrystal semiconductor film) is also included in the category of the semi-amorphous semiconductor film. 
     A semi-amorphous silicon film is given as an example of the semi-amorphous semiconductor film. The Raman spectrum of the semi-amorphous silicon film is shifted to a wavenumber side lower than 520 cm −1 , and the diffraction peaks of (111) and (220) that are thought to be derived from a Si crystal lattice are observed in X-ray diffraction. In addition, hydrogen or halogen of at least 1 at % or more to terminate dangling bonds is added to the semi-amorphous silicon film. In this specification, such a silicon film is referred to as a semi-amorphous silicon film for the sake of convenience. Moreover, a rare gas element such as helium, argon, krypton, or neon may be added to the silicon film to further promote lattice distortion, whereby stability is enhanced and a semi-amorphous semiconductor film with favorable characteristics can be obtained. 
     The semi-amorphous silicon film can be obtained by glow discharge decomposition of a gas containing silicon. SiH 4  is a typical gas containing silicon, and Si 2 H 6 , SiH 2 Cl 2 , SiHCl 3 , SiCl 4 , SiF 4 , or the like can be used as well as SiH 4 . The semi-amorphous silicon film can be easily formed with use of the gas containing silicon diluted with hydrogen or a gas in which one or more of rare gas elements selected from helium, argon, krypton, and neon is/are added to hydrogen. It is desirable that the gas containing silicon be diluted at a dilution ratio in the range of 2 to 1000 times. Alternatively, a gas of a carbon-containing compound, such as CH 4  or C 2 H 6 , a gas including germanium, such as GeH 4  or GeF 4 , F 2 , or the like may be mixed into the gas containing silicon so that the energy bandwidth is adjusted to be 1.5 to 2.4 eV or 0.9 to 1.1 eV. 
     Note that, in this specification, the photoelectric conversion layer  225 , a photo diode including the photoelectric conversion layer  225 , and an element including the photodiode are referred to as a photoelectric conversion element. 
     Next, the protective film  227  is formed so as to cover an exposed surface (see  FIG. 4C ). In this embodiment mode, a silicon nitride film is used as the protective film  227 . The protective film  227  can prevent moisture or impurities such as an organic substance from entering the TFT  211  and the photoelectric conversion layer  225 . 
     Next, an interlayer insulating film  228  is formed over the protective film  227  (see  FIG. 5A ). The interlayer insulating film  228  also functions as a planarizing film. In this embodiment mode, polyimide is formed to have a thickness of 2 μm as the interlayer insulating film  228 . 
     Next, the interlayer insulating film  228  is etched, whereby contact holes are formed. At the time of the etching, the gate wiring  214 , the source electrode  215 , and the drain electrode  216  of the TFT  211  are not etched due to the presence of the protective film  227 . Next, the protective film  227  in regions where an electrode  231  and an electrode  232  are to be formed is etched, whereby contact holes are formed. In addition, over the interlayer insulating film  228 , the electrode  231  is formed to be electrically connected to the electrode  221  through the contact hole formed in the interlayer insulating film  228  and the protective film  227 , and the electrode  232  is formed to be electrically connected to the electrode  223  and the upper layer (in this embodiment mode, the n-type semiconductor layer  225   n ) of the photoelectric conversion layer  225  through the contact hole formed in the interlayer insulating film  228  and the protective film  227  (see  FIG. 5B ). Tungsten (W), titanium (Ti), tantalum (Ta), silver (Ag), or the like can be used for the electrodes  231  and  232 . 
     In this embodiment mode, a conductive film formed of titanium (Ti) formed to a thickness of 30 to 50 nm is used as the electrodes  231  and  232 . 
     Next, an interlayer insulating film  235  is formed over the interlayer insulating film  228  by a screen printing method or an ink jet method (see  FIG. 5C ). In the formation of the interlayer insulating film  228 , the interlayer insulating film  235  is not formed over the electrodes  231  and  232 . In this embodiment mode, an epoxy resin is used for the interlayer insulating film  235 . 
     Next, over the interlayer insulating film  235 , an electrode  241  is formed to be electrically connected to the electrode  231 , and an electrode  242  is formed to be electrically connected to the electrode  232  with use of nickel (Ni) past by a printing method, for example (see  FIG. 6A ). 
     Next, in order to electrically separate adjacent elements from each other, parts of the substrate  201 , the protective film  227 , the interlayer insulating film  228 , and the interlayer insulating film  235  are removed in a dicing process, whereby slits  261  are formed (see  FIG. 6B ). 
     Alternatively, parts of the interlayer insulating films  228  and  235  and the substrate  201  may be removed by laser irradiation instead of the dicing process. 
     A laser includes a laser medium, an excitation source, and an oscillator. As for the laser, a gas laser, a liquid laser, and a solid-state laser are provided when classified according to the medium, and a free electron laser, a semiconductor laser, and an X-ray laser are represented when classified according to the oscillation characteristics. However, any of the lasers may be used for element separation. Note that, it is preferable to use a gas laser or a solid-state laser, and it is more preferable to use a solid-state laser. 
     As a gas laser, a helium-neon laser, a carbon dioxide gas laser, an excimer laser, an argon ion laser, and the like are exemplified. As an excimer laser, a rare gas excimer laser and a rare gas halide excimer laser are given. As the medium of the rare gas excimer laser, argon, krypton, and xenon are represented. As a gas laser, a metal vapor ion laser is provided. 
     As a liquid laser, an inorganic liquid laser, an organic chelate laser, and a dye laser are exemplified. An inorganic liquid laser and an organic chelate laser use, as a laser medium, a rare-earth ion such as neodymium utilized in a solid-state laser. 
     A laser medium used in a solid-state laser is a solid base doped with active species which are capable of lasing. The solid base refers to a crystal or glass. The crystal refers to YAG (yttrium aluminum garnet crystal), YLF, YVO 4 , YAlO 3 , sapphire, ruby, or alexandrite. The active species which are capable of lasing refer to, for example, trivalent ions (such as Cr 3+ , Nd 3+ , Yb 3+ , Tm 3+ , Ho 3+ , Er 3+ , and Ti 3+ ). 
     Note that a continuous-wave laser or a pulsed laser can be used as a laser for the separation of the adjacent elements. The conditions of laser beam irradiation are controlled as appropriate in consideration of the thicknesses, materials, and the like of the substrate  201 , the protective film  227 , and the interlayer insulating films  228  and  235 . 
     In the case where the substrate  201  is a glass substrate, a solid-state laser beam with a wavelength of greater than or equal to 1 nm and less than or equal to 380 nm in the ultraviolet region is preferably used as a laser beam. This is because an ablation process can be easily performed, which results from the fact that laser light with a wavelength in the ultraviolet region is more easily absorbed by a substrate (especially a glass substrate) than another laser beam with a longer wavelength. More preferably, an Nd:YVO 4  laser beam with a wavelength of greater than or equal to 1 nm and less than or equal to 380 nm in the ultraviolet region is used. This is because an ablation process is easily performed especially when an Nd:YVO 4  laser beam is used. 
     When a laser beam is emitted to a glass substrate to form a groove in the glass substrate, a cut surface of the groove is rounded. If the cut surface is rounded, a chip of a corner of the cut surface, or crack can be prevented from being generated, in comparison with a case where the cut surface has a corner. Such an advantage facilitates the handling of a glass substrate mainly when the glass substrate is carried by a robot or the like. In addition, when the photoelectric conversion element formed on a glass substrate is mounted on a product, generation of a chip or a crack can be suppressed, and accordingly generation of damage or destruction of the substrate can be suppressed. 
     In the ablation process, a phenomenon is used in which a molecule which absorbs a laser beam undergoes bond-cleavage and then photolysis to result in decomposition products which are evaporated. In other words, in a formation method of the groove of this embodiment mode, a molecular bond of a molecule irradiated with the laser beam is cleaved, the molecule is photolyzed to give decomposition products, and the decomposition products are evaporated, whereby the groove is formed in the substrate. 
     Note that a laser irradiation apparatus for emitting the above-described laser beam includes a moving table, a substrate, a head portion, and a control portion. The moving table is provided with an adsorption hole. The substrate is held by the adsorption hole over the moving table. The head portion applies a laser beam which is emitted from a laser oscillation apparatus, through a laser head. The control portion moves one or both of the moving table and the head portion, which allows the laser head to be positioned at an arbitrary place over a surface of the substrate and to apply a laser beam thereat. Note that the control portion recognizes and determines a portion to be processed from a relative position on the basis of a mark for positioning over a substrate which is taken by a CCD camera. 
     Next, a bonding layer  271  is formed so as to cover the interlayer insulating film  235 , the electrodes  241  and  242 , and the slits  261  (see  FIG. 1 ). 
     The bonding layer  271  contains a metal which forms an alloy with solder, for example, at least one of nickel (Ni), copper (Cu), zinc (Zn), palladium (Pd), silver (Ag), tin (Sn), platinum (Pt), or gold (Au). In particular, at least one of nickel (Ni), copper (Cu), silver (Ag), platinum (Pt), or gold (Au) is preferably used. 
     In this embodiment mode, the bonding layer  271  is formed by a sputtering method in such a manner that a gold (Au) layer with a thickness of 50 nm as a first layer  271   a , a nickel (Ni) layer with a thickness of 300 to 500 nm as a second layer  271   b , and a titanium (Ti) layer with a thickness of 100 nm as a third layer  271   c  are stacked (see  FIG. 9A ). 
     The titanium (Ti) layer has a good contact with nickel paste which is a material of the electrodes  241  and  242 . The nickel (Ni) layer contains nickel which is a material of the electrodes  241  and  242  and a material which easily forms an alloy with solder. In addition, gold (Au) easily forms an alloy with solder and has a function of protecting a surface of the nickel layer. Note that the third layer  271   c  is not required to be formed if not necessary (see  FIG. 9B ). 
     Among the stacked layers which form the bonding layer  271 , the nickel layer which is the second layer  271   b  mainly forms an alloy with solder, and bonds the electrodes  241  and  242  to electrodes  283  and  282 , respectively, which are to be formed later. 
     Although the nickel layer is used as the second layer  271   b  in this embodiment mode, instead of the nickel layer, the above-described metal such as copper (Cu), zinc (Zn), palladium (Pd), silver (Ag), tin (Sn), platinum (Pt), or gold (Au) may be subjected to the film formation using a sputtering method. 
     Alternatively, instead of the gold layer, a layer which is formed by a sputtering method using tin (Sn) or solder as a target may be used as the first layer  271   a.    
     Note that, for example, an electrode  243  and an electrode  244  may be formed of copper (Cu) paste over the electrode  241  and the electrode  242 , respectively (see,  FIG. 20 ). In this case, the bonding layer  271  is formed over the electrode  243  and the electrode  244 . The bonding layer  271  in this case also preferably contains at least one of nickel (Ni), copper (Cu), zinc (Zn), palladium (Pd), silver (Ag), tin (Sn), platinum (Pt), or gold (Au). It is more preferable that the bonding layer  271  contain copper (Cu) because copper forms an alloy with solder easily and is bonded to the copper paste more easily. 
     Next, an opening  265  for electrical insulation between the electrode  241  and the electrode  242  is formed in the interlayer insulating film  235  by dicing or laser beam irradiation (see  FIG. 7A ). The electrodes  241  and  242  can be short-circuited due to the bonding layer  271  unless the opening  265  is formed. The above-described laser may be used for the laser beam. 
     Next, the slits  261  is further subjected to dicing, and thus individual elements are separated, whereby one photoelectric conversion element is formed (see  FIG. 7B ). 
     This separation process may also be performed by laser irradiation, instead of dicing. 
     The element is bonded to a printed circuit board  281  over which the electrodes  282  and  283  are formed (see  FIG. 8 ). The element and the printed circuit board  281  are bonded to each other with use of solder as conductive materials  285  and  286  so that the electrode  241  and the electrode  283  face each other and the electrode  242  and the electrode  282  face each other. Since the bonding layer  271  is formed, the bonding strength of the electrodes  241  and  242  and the conductive materials  285  and  286  using the solder is increased. Thus, separation between the printed circuit board  281  and the photoelectric conversion element can be suppressed. 
     In the case where solder is used as the conductive materials  285  and  286 , a reflow method, that is, a method in which solder in paste form is printed on the printed circuit board, parts are mounted thereon, and then the solder is melted by application of heat. As a heating method, an infrared ray method, a hot air method, and the like can be employed. 
     Alternatively, after forming the structure shown in  FIG. 6A , the bonding layer  271  may be formed over the electrodes  241  and  242 , and the interlayer insulating film  235  without forming the slit  261  (see  FIG. 21 ). After that, the opening  265  for electrical insulation is formed in the interlayer insulating film  235  by dicing or laser beam irradiation (see  FIG. 22A ). Individual elements are separated by dicing or laser irradiation, whereby one photoelectric conversion element is formed (see  FIG. 22B ). 
     Furthermore, the element is bonded to the printed circuit board  281  over which the electrodes  282  and  283  are formed (see  FIG. 23 ). The element and the printed circuit board  281  are bonded to each other with use of solder as the conductive materials  285  and  286  so that the electrode  241  and the electrode  283  face each other and the electrode  242  and the electrode  282  face each other. 
     Note that even in the case where the slit  261  is not formed, similarly to  FIG. 20 , the bonding layer  271  may be formed after the electrodes are formed of, for example, copper (Cu) paste over the electrodes  241  and  242 . 
     As for the semiconductor device including the photoelectric conversion element manufactured in this embodiment mode, the photoelectric conversion element and the printed circuit board which are included in the semiconductor device are bonded to each other more strongly; thus, the photoelectric conversion element and the printed circuit board are more difficult to separate than those in a conventional semiconductor device. Thus, the semiconductor device with high reliability is realized. 
     Hereinafter, a circuit configuration of the photoelectric conversion element of this embodiment mode is described with reference to  FIG. 10 ,  FIG. 11 , and  FIG. 12 . 
       FIG. 10  shows a circuit configuration of a photoelectric conversion element in the case of using the photodiode  101  including the photoelectric conversion layer  225  and, for example, a current mirror circuit  111  as an amplifier circuit configured to amplify the output of the photodiode  101 . The current mirror circuit  111  includes a reference-side TFT  104  and an output-side TFT  105 . Note that the above-described TFT  211  is either the TFT  104  or the TFT  105 . 
     In  FIG. 10 , a gate electrode of the TFT  104  which constitutes a part of the current mirror circuit  111  is electrically connected to a gate electrode of the TFT  105  which also constitutes another part of the current mirror circuit  111 , and furthermore, the gate electrode of the TFT  104  is electrically connected to a drain electrode (also referred to as a “drain terminal”) which is one of a source electrode and the drain electrode of the TFT  104 . 
     The drain terminal of the TFT  104  is electrically connected to the photodiode  101 , the drain terminal of the TFT  105 , and a high-potential power supply V DD . 
     The source electrode (also referred to as “source terminal”) which is the other of the source electrode and the drain electrode of the TFT  104  is electrically connected to a low-potential power supply V SS  and a source terminal of the TFT  105 . 
     In addition, the gate electrode of the TFT  105  which constitutes a part of the current mirror circuit  111  is electrically connected to the gate electrode and drain terminal of the TFT  104 . 
     Since the gate electrodes of the TFT  104  and the TFT  105  are connected to each other, common potential is applied to the gate electrodes. 
     In  FIG. 10 , the example of the current mirror circuit constituted by two TFTs is shown. In this case, when the reference-side TFT  104  and the output-side TFT  105  have the same characteristics, the ratio between reference current and output current is 1:1. 
       FIG. 11  shows a circuit configuration for increasing the output value by n times. The circuit configuration of  FIG. 11  corresponds to the configuration in which n pieces of TFT  105  shown in  FIG. 10  are provided. The number ratio between the TFT  104  and the TFT  105  is set to 1:n as shown in  FIG. 11 , whereby the output value can be increased by n times. This is a similar principle to that which increases the channel width W of the TFT and increases the allowable amount of the current which can be passed through the TFT by n times. 
     For example, in the case where the output value is set to be increased by 100 times, one n-channel TFT  104  and 100 n-channel TFTs  105  are connected in parallel, whereby the target current can be obtained. 
     In  FIG. 11 , the current mirror circuit  111  includes the TFT  104 , a circuit  118   i  (any one of n pieces of circuit including a circuit  118 α, a circuit  118 β, and the like), a terminal  119   i  (any one of n pieces of terminal including a terminal  119 α, a terminal  119 β, and the like), a terminal  120   i  (any one of n pieces of terminal including a terminal  120 α, a terminal  120 β, and the like), and a terminal  121   i  (any one of n pieces of terminal including a terminal  121 α, a terminal  121 β, and the like). 
     Note that as for the reference numerals in  FIG. 11 , the reference numerals with “i” are the same as those without “i” in  FIG. 10 . That is, for example, the TFT  105  in  FIG. 10  is the same as the TFT  105   i  in  FIG. 11 . 
     The circuit configuration of  FIG. 11  is based on the circuit configurations of  FIG. 10  and  FIG. 11 , and the same elements are denoted by the same reference numerals. That is, a gate electrode of the TFT  105   i  is electrically connected to the terminal  119   i  and also electrically connected to the terminal  120   i . In addition, a source terminal of the TFT  105   i  is electrically connected to the electrode  121   i  which is the terminal. 
     Thus, in  FIG. 11 , the TFT  105  includes n pieces of TFT  105   i . Accordingly, current flowing through the TFT  104  is amplified by n times and outputted. 
     That is, as described above, in the case where the ratio between the reference current and the output current is desired to be set to 1:1, one reference-side TFT and one output-side TFT are preferably formed, and  FIG. 10  shows the circuit diagram of such a case. In the case where the ratio between the reference current and the output current is desired to be set to 1:n, one reference-side TFT and n pieces of output-side TFT are preferably formed.  FIG. 11  shows the circuit diagram of this case. 
       FIG. 10  shows the current mirror circuit  111  as an equivalent circuit in which an n-channel TFT is used; however, a p-channel TFT may be used instead of the n-channel TFT. 
     In the case where a current mirror circuit  131  includes a p-channel TFT, an equivalent circuit shown in  FIG. 12  is realized. As shown in  FIG. 12 , the current mirror circuit  131  includes p-channel TFTs  134  and  135 . Note that the same components are denoted by the same reference numerals as those in  FIG. 10 ,  FIG. 11 , and  FIG. 12 . 
     In the case where the ratio between the reference current and the output current of the current mirror circuit  131  is desired to be set to 1:n, similarly to  FIG. 11 , n pieces of the output-side TFT  135  are preferably provided. 
     Each of  FIG. 13  and  FIG. 14  is a top view of the photoelectric conversion element shown in each of  FIG. 10  and  FIG. 7B . 
     The current mirror circuit  111  is electrically connected to the electrode  223  which is electrically connected to the high-potential power supply V DD , through a wiring  144 . In addition, the current mirror circuit  111  is electrically connected to the electrode  221  which is electrically connected to the low-potential power supply V SS , through wirings  145 . 
     The electrode  232  covers the whole photoelectric conversion layer  225 , and accordingly, electrostatic breakdown can be prevented. Note that the electrode  232  may be formed so as to overlap with part of the photoelectric conversion layer  225 , according to need. 
     Although the edge of the electrode  222  is formed into a semicircular shape in  FIG. 13 , a region of the electrode  222 , which overlaps with the photoelectric conversion layer  225 , may be formed into a rectangular shape as shown in  FIG. 14 . This makes it possible to suppress electric field concentration, and accordingly, electrostatic breakdown can be prevented. 
     As described above, the present invention makes it possible to provide a semiconductor device including a photoelectric conversion element with high reliability. 
     Embodiment Mode 2 
     In this embodiment mode, examples in which the photoelectric conversion element obtained in Embodiment Mode 1 is incorporated in various electronic appliances will be described. As examples of the electronic appliances described in this embodiment mode, computers, displays, cellular phones, TV sets are given. Specific examples of the electronic appliances are shown in  FIG. 15 ,  FIGS. 16A  and  16 B,  FIGS. 17A and 17B ,  FIG. 18 , and  FIGS. 19A and 19B . 
       FIG. 15  shows a cellular phone, which includes a main body A  701 , a main body B  702 , a chassis  703 , operation keys  704 , an audio input portion  705 , an audio output portion  706 , a circuit board  707 , a display panel A  708 , a display panel B  709 , a hinge  710 , a light-transmitting material portion  711 , and a photoelectric conversion element  712  obtained in Embodiment Mode 1. 
     The photoelectric conversion element  712  detects light which is passed through the light-transmitting material portion  711  and controls luminance of the display panel A  708  and the display panel B  709  depending on the illuminance of the detected extraneous light or controls illumination of the operation keys  704  depending on the illuminance obtained by the photoelectric conversion element  712 . In this manner, current consumption of the cellular phone can be suppressed. 
       FIGS. 16A and 16B  show other examples of cellular phones. In  FIGS. 16A and 16B , the cellular phone includes a main body  721 , a chassis  722 , a display panel  723 , operation keys  724 , an audio output portion  725 , an audio input portion  726 , and photoelectric conversion elements  727  and  728  obtained in Embodiment Mode 1. 
     In the cellular phone shown in  FIG. 16A , extraneous light is detected by the photoelectric conversion element  727  provided in the main body  721 , whereby luminance of the display panel  723  and the operation keys  724  can be controlled. 
     In the cellular phone shown in  FIG. 16B , the photoelectric conversion element  728  is provided inside the main body  721  in addition to the structure shown in  FIG. 16A . By the photoelectric conversion element  728 , luminance of a backlight provided in the display panel  723  can also be detected. 
       FIG. 17A  shows a computer, which includes a main body  731 , a chassis  732 , a display portion  733 , a keyboard  734 , an external connection port  735 , a pointing device  736 , and the like. 
       FIG. 17B  shows a display device, and a television receiver corresponds to the display device. The display device includes a chassis  741 , a support  742 , a display portion  743 , and the like. 
       FIG. 18  shows a detailed structure of a case where a liquid crystal panel is used for the display portion  733  of the computer shown in  FIG. 17A  and the display portion  743  of the display device shown in  FIG. 17B . 
     A liquid crystal panel  762  shown in  FIG. 18  is incorporated in a chassis  761  and includes substrates  751   a  and  751   b , a liquid crystal layer  752  interposed between the substrates  751   a  and  751   b , polarizing filters  755   a  and  755   b , a backlight  753 , and the like. In addition, a formation region  754  of the photoelectric conversion element including the photoelectric conversion element obtained in Embodiment Mode 1 is formed in the chassis  761 . 
     In the formation region  754  of the photoelectric conversion element, the amount of light from the backlight  753  is detected, and the information regarding the amount of light and the like is fed back, whereby the luminance of the liquid crystal panel  762  is adjusted. 
       FIGS. 19A and 19B  are views showing an example in which a photoelectric conversion element is incorporated in a camera, for example, a digital camera.  FIG. 19A  is a perspective view seen from the front side of the digital camera, and  FIG. 19B  is a perspective view seen from the back side of the digital camera. 
     In  FIG. 19A , the digital camera is provided with a release button  801 , a main switch  802 , a viewfinder  803 , a flash portion  804 , a lens  805 , a barrel  806 , and a chassis  807 . 
     In  FIG. 19B , the digital camera is provided with an eyepiece finder  811 , a monitor  812 , and operation buttons  813 . 
     When the release button  801  is pushed down to the half point, a focus adjustment mechanism and an exposure adjustment mechanism are operated. When the release button  801  is pushed down to the lowest point, a shutter is opened. 
     The main switch  802  is pushed down or rotated, whereby a power supply of the digital camera is switched on or off. 
     The viewfinder  803  is located above the lens  805 , which is on the front side of the digital camera, for checking a shooting range and the focus point from the eyepiece finder  811  shown in  FIG. 19B . 
     The flash portion  804  is located in the upper position on the front side of the digital camera. When the luminance of a subject is not sufficient, auxiliary light is emitted from the flash portion  804 , at the same time as the release button is pushed down and the shutter is opened. 
     The lens  805  is located at the front side of the digital camera and includes a focusing lens, a zoom lens, and the like. The lens forms a photographic optical system with a shutter button and a diaphragm which are not shown. In addition, behind the lens, an imaging device such as a CCD (charge coupled device) is provided. 
     The barrel  806  moves a lens position to adjust the focus of the focusing lens, the zoom lens, and the like. In shooting, the barrel is slid out so that the lens  805  moves forward. Further, when carrying the digital camera, the lens  805  is moved backward to be compact. Note that a structure is employed in this embodiment, in which the barrel is slid out, whereby the subject can be photographed by zoom; however, the present invention is not limited to this structure. A structure may also be employed for the digital camera, in which shooting can be performed by zoom without sliding out the barrel with use of a structure of a photographic optical system inside the chassis  807 . 
     The eyepiece finder  811  is located in the upper position on the back side of the digital camera for looking therethrough in checking a shooting range and a focus point. 
     The operation buttons  813  are buttons for various functions provided on the back side of the digital camera, which includes a set up button, a menu button, a display button, a function button, a selection button, and the like. 
     When the photoelectric conversion element obtained in Embodiment Mode 1 is incorporated in the camera shown in  FIGS. 19A and 19B , the photoelectric conversion element can detect whether light exists or not and light intensity, and accordingly, exposure adjustment of a camera, or the like can be performed. 
     In addition, the photoelectric conversion element obtained in Embodiment Mode 1 can also be applied to other electronic appliances such as projection TV sets and navigation systems. In other words, the photoelectric conversion element can be applied to any object as long as the object needs to detect light. 
     This application is based on Japanese Patent Application no. 2007-108795 filed with Japan Patent Office on Apr. 18, 2007, the entire contents of which are hereby incorporated by reference.