Abstract:
A manufacturing method of a semiconductor device is provided to manufacture an increased number of semiconductor devices per single substrate such as, e.g., a wafer while obviating damages like those caused by conventional dicing method. The manufacturing method comprises steps of performing a first etching process to etch a separation area on a front surface of a substrate, arranging a supporter on a back surface of the first substrate to prevent semiconductor devices from coming apart, coating with a thin film a non-etching area including a sidewall of the etched separation area and excluding a bottom of the etched separation area on the front surface of the first substrate, and performing a second etching process to etch the first substrate from the front surface through an area not coated by the thin film to divide the substrate into multiple semiconductor devices.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention generally relates to devices and methods such as a semiconductor device formed by dividing a semiconductor element area formed on a substrate and a manufacturing method of the semiconductor device. This invention more particularly relates to a semiconductor device separated from another semiconductor device under the use of etching technology, a manufacturing method of the semiconductor device, a manufacturing equipment of the semiconductor device, a light emitting diode head, and an image forming apparatus. 
     2. Description of Related Art 
     There has been conventionally known a dicing method for separating multiple semiconductor devices formed on a semiconductor wafer into individual semiconductor devices (chips). In the dicing method, the wafer is cut by the dicing blade which adversely impacts the wafer, and such an impact is likely to chip end portions of the semiconductor device, namely, to cause chipping. In order to prevent such chipping, an area wider than a width actually being cut is reserved as a cutting area. Japanese Patent Application Publication No. H08-32110 discloses such method for dicing LED array chips used for an exposure device (LED head) of an image forming apparatus making use of an electrophotographic technology such as, e.g., a printer. 
     There exists a problem that the above-mentioned dicing method requires a relatively large area between semiconductor devices to adversely limit the number of semiconductor devices made from a single semiconductor wafer. 
     This invention is made to solve such problems, and it is an object to provide a semiconductor device and a manufacturing method of the semiconductor device that can produce more semiconductor devices from a single wafer such as, e.g., a semiconductor wafer while obviating damages like those caused by the dicing process. 
     BRIEF SUMMARY OF THE INVENTION 
     To solve such problems, the present invention provides a semiconductor device comprising a semiconductor element area arranged on a first substrate, wherein an etching process etches at least a portion of the first substrate to separate the semiconductor device from another neighboring semiconductor device. The present invention surely reduces occurrence of chipping by employing the required etching process as a separation method between devices instead of the conventional dicing process. 
     In preferred embodiments of this invention, an anisotropic dry etching process, an isotropic dry etching process, a wet etching process, and the like can be employed as an etching process, and the semiconductor device can be separated from another neighboring semiconductor device by the etching process penetrating the first substrate from the front surface to the back surface thereof. The substrate can employ various kinds of material such as, e.g., a semiconductor substrate, such as, e.g., silicon substrate, GaAs substrate, and InP substrate, and a Silicon On Insulator (SOI) substrate. 
     To solve such problems, the present invention provides a manufacturing method of a semiconductor device in which a first substrate comprising a semiconductor element area is divided into a plurality of semiconductor devices, the manufacturing method comprising steps of performing a first etching process to etch a separation area on a front surface of the first substrate, arranging a supporter on a back surface of the first substrate to prevent the plurality of semiconductor devices from coming apart after the first substrate is divided, coating with a thin film a non-etching area including a sidewall of the etched separation area and excluding an aperture area on a bottom of the etched separation area on the front surface of the first substrate, and performing a second etching process to etch the front surface of the first substrate through the aperture area as an etching window to penetrate the first substrate. 
     The present invention further provides a manufacturing method of a semiconductor device in which a first substrate comprising a semiconductor element area is divided into a plurality of semiconductor devices, the manufacturing method comprising steps of performing a first etching process to etch a separation area on a front surface of the first substrate, coating with a thin film a non-etching area including a sidewall of the etched separation area and excluding an aperture area on a bottom of the etched separation area on the front surface of the first substrate, performing a second etching process to etch the front surface of the first substrate through the aperture area as an etching window, arranging a supporter on the front surface of the first substrate to prevent the plurality of semiconductor devices from coming apart after the first substrate is divided, and grinding a back surface of the first substrate to cause a hollow portion etched by the first and second etching process to penetrate through the front surface to the back surface of the first substrate. 
     The present invention further provides a manufacturing method of a semiconductor device in which a first substrate comprising a semiconductor element area is divided into a plurality of semiconductor devices, the manufacturing method comprising steps of performing a first etching process to etch a separation area on a front surface of the first substrate, coating with a thin film a first non-etching area on a back surface of the first substrate excluding a first aperture area corresponding to the etched separation area, performing a second etching process to etch the back surface of first substrate through the first aperture area as an etching window, arranging a supporter on the back surface of the first substrate to prevent the plurality of semiconductor devices from coming apart after the first substrate is divided, coating with a thin film a second non-etching area on the front surface of the first substrate including a sidewall of the etched separation area and excluding a second aperture area on a bottom of the etched separation area, and performing a third etching process to etch the front surface of the first substrate through the second aperture area as an etching window to penetrate the first substrate. 
     The semiconductor device according to the present invention surely reduces occurrence of chipping by employing the etching process as a separation method between devices instead of the conventional dicing method. Therefore, the present invention is capable of making smaller a margin area for chipping around the semiconductor device, and making higher a ratio of an area in which the semiconductor devices can be formed to an area for manufacturing step, and thereby, the present invention increases the number of devices that can be manufactured. 
     With the manufacturing method of the semiconductor device according to the present invention, a combination of etching processes can surely divide the semiconductor element area for each of the semiconductor devices, a cutting width can be narrowed and controlled more precisely compared with the dicing method, and the etching process gives less mechanical damages to the substrate. Therefore, the manufacturing method according to the present invention is especially highly effective in a case of attempting to make finer elements. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The foregoing summary, as well as the following detailed description of the invention, will be better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, there are shown in the drawings embodiments which are presently preferred. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown. 
       In the drawings: 
         FIG. 1  is a diagram showing a process of separating semiconductor devices through an etching process according to the manufacturing method of the semiconductor device of the first embodiment of this invention; 
         FIG. 2  is a top view showing an arrangement of the semiconductor device according to the manufacturing method of the semiconductor device of the first embodiment of this invention; 
         FIG. 3  is a top view showing an adhesion step of a semiconductor wafer according to the manufacturing method of the semiconductor device of the first embodiment of this invention; 
         FIG. 4  is a cross section of the wafer before the wafer is divided into the semiconductor devices according to the manufacturing method of the semiconductor device of the first embodiment of this invention; 
         FIG. 5  is a cross section of the wafer before the wafer is divided into the semiconductor devices according to the manufacturing method of the semiconductor device of the second embodiment of this invention; 
         FIG. 6  is a cross section of the wafer after the isotropic etching process has been performed according to the manufacturing method of the semiconductor device of the second embodiment of this invention; 
         FIG. 7  is a cross section of the wafer after the adhesion sheet has been pasted according to the manufacturing method of the semiconductor device of the second embodiment of this invention; 
         FIG. 8  is a cross section of the wafer after the back surface of the wafer has been grinded according to the manufacturing method of the semiconductor device of the second embodiment of this invention; 
         FIG. 9  is a cross section of the wafer after the first etching process has been performed according to the manufacturing method of the semiconductor device of the third embodiment of this invention; 
         FIG. 10  is a cross section of the wafer after the second etching process has been performed according to the manufacturing method of the semiconductor device of the third embodiment of this invention; 
         FIG. 11  is a cross section of the wafer after the isotropic etching process has been performed according to the manufacturing method of the semiconductor device of the fourth embodiment of this invention; 
         FIG. 12  is a cross section of the wafer after the anisotropic etching process has been performed according to the manufacturing method of the semiconductor device of the fourth embodiment of this invention; 
         FIG. 13  is a cross section of the wafer according to the manufacturing method of the semiconductor device of the fifth embodiment of this invention; 
         FIG. 14  is a cross section of the wafer after the first etching process has been performed according to the manufacturing method of the semiconductor device of the fifth embodiment of this invention; 
         FIG. 15  is a cross section of the wafer after the second etching process has been performed according to the manufacturing method of the semiconductor device of the fifth embodiment of this invention; 
         FIG. 16  is a cross section of the wafer after the third etching process has been performed according to the manufacturing method of the semiconductor device of the fifth embodiment of this invention; 
         FIG. 17  is a diagram showing an embodiment of an LED print head according to this invention; 
         FIG. 18  is a top view showing an exemplary structure of the LED print head according to this invention; and 
         FIG. 19  is a diagram showing an essential portion of an embodiment of an image forming apparatus according to this invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Particular embodiments of a semiconductor device and a manufacturing method of the semiconductor device according to this invention is hereinafter described in details with reference to the figures. 
     First Embodiment 
     The first embodiment discloses a manufacturing method of a semiconductor device with the use of a silicon wafer. The first embodiment is especially advantageous for forming many semiconductor light emitting elements, such as, e.g., LED array chips, at a time. 
     In the first embodiment, semiconductor element areas  20  each in a rectangular shape are formed to line up in a lateral and longitudinal direction as shown in  FIG. 2 . Each of the semiconductor element areas  20  in a rectangular shape are formed on a silicon substrate  11 , namely, a semiconductor wafer. An area between the semiconductor element areas  20  in a rectangular shape is a separation area  16  that is to be etched for separating the semiconductor element areas  20  into individual devices. Particularly in the first embodiment, the separation area  16  is a belt-like area extending in a grid. 
     After a required semiconductor device has been formed, namely, for example, an electrode layer, an active layer, and the like have been formed on the semiconductor element areas  20  formed on the silicon substrate  11 , the silicon wafer is divided into the semiconductor element areas  20  as shown in  FIG. 1 . First, a masking material layer is applied to cover an entire surface of the silicon substrate  11  including the separation area  16  that is to be etched. Then, the masking material is removed in the separation area  16  to reveal the surface of the semiconductor wafer that is to be etched to form a window portion. The masking martial can employ, for example, one of organic material, silicon oxide, silicon nitride, or metal material, or a combination thereof. It is preferable that the masking material layer  12  remain as a layer of a ultimate device. 
     Subsequently, a first dry etching process etches the separation area  16  of the silicon substrate  11  from a surface  24  to a certain etched depth  23 . At this moment, the etched depth  23  should be, for example, 5 micrometers or more. The etched depth can be shallower as needed on condition that the etched depth does not affect the device. A sidewall of an etched area is preferred to be perpendicular to the surface of the wafer in a cross section thereof, and therefore, an anisotropic etching process is preferred that does not etch in a direction parallel to the surface of the wafer. Radio frequency dry etching process using SF 6  and O 2  gas as etching gas can be employed in the dry etching process. 
     The masking material layer  12  is applied to cover a surface  12   a  and an etched sidewall  12   b . However, an area that is to be the separation area  16 , i.e., a bottom surface of an etched area  15  in the certain etched depth  23 , is left uncovered. 
     Subsequently, as shown in  FIG. 3 , a wafer  17  (namely, the silicon substrate  11 ) is adhesively fixed to a fixation frame  19  with the use of an adhesive sheet  18  as a supporter so as to prevent each of the semiconductor devices from being separated after a following second dry etching process. 
     After the wafer  17  is fixed to the fixation frame  19 , the second dry etching process is performed. The second dry etching process employs an isotropic dry etching process in order to increase an etching rate in a depth direction. Where a silicon substrate  11  is thin, namely, for example, the thickness of the silicon substrate  11  is 50 micrometers or less, an etching process that is the same as the first etching process (the anisotropic etching process) can be performed again instead. The second dry etching process may employ radio frequency dry etching process using SF 6  and O 2  gas as etching gas. The second dry etching process is performed to render the cross section of an etched sidewall  14 , for example, substantially in a dome shape as shown in  FIG. 1 . The adhesive sheet  18  previously adhered is exposed at the bottom of the separation area  16 . 
     The second dry etching process extends an etched area edge  22  on the bottom of the wafer toward the devices from an etched area edge  21  of the first dry etching process. Therefore, the semiconductor element areas are separated by only a width etched by the anisotropic etching process capable of performing precise etching, and thus the semiconductor device is adequately separated from a neighboring semiconductor device even where the semiconductor devices are arranged in high density. 
     After completing the second dry etching process, each of device chips  26 , namely, semiconductor devices, is held by the adhesive sheet  18 . In a subsequent step, each of the device chips  26  is removed from the adhesive sheet  18  to become an individual microscopic chip respectively. 
     The manufacturing method according the first embodiment employs a process in which the isotropic dry etching process and the anisotropic dry etching process are performed in combination after an element formation portion of the semiconductor devices are made, thus providing high spatial precision defined in a wafer process, freeing finished separated edges from defects such as, e.g., chipping, and obviating mechanical damages affecting the devices. Furthermore, the first embodiment can also be applied to thick substrates since the first embodiment forms the separation area fast with the use of the isotropic etching process having a high etching rate. The first embodiment prevents the semiconductor devices from being contaminated since the first embodiment does not use cutting method such as, e.g., dicing. 
     Second Embodiment 
       FIG. 5  is a diagram showing a cross section of the wafer after the separation area has been etched through the manufacturing method according to the second embodiment and before each of the devices is separated. The masking material layer  32  is formed on the surface of a silicon substrate  31 . Then, with the help of the masking material layer  32 , an aperture area  35  of a narrow width is formed from a surface  44  to a depth  43  in a same manner as the first embodiment. On a sidewall of the aperture area  35 , a sidewall protection layer  32   a  is formed to incorporate with the surface  32   b.    
     Below the narrow aperture area  35 , a hollow space  36  (separation area) in a dome shape is formed through the isotropic etching process. A larger amount of the wafer is removed in a hollow space  36  than in the aperture area  35 , and thus a sidewall  36   w  in the hollow space  36  is in a shape further etching the wafer than the width of the aperture area  35 . 
     In order to obtain the above-mentioned structure, following steps shown in  FIG. 6  through  FIG. 8  are performed. First, as shown in  FIG. 6 , the anisotropic etching process etches an area that is to be the aperture area  35  from the surface  44  to the depth  43 . The anisotropic etching process described in the first embodiment can be employed. 
     Subsequently, the isotropic etching process etches to the depth  47 . The depth  47  should be shallower than the thickness (a back surface  48 ) of the silicon substrate  31 , that is, the isotropic etching process should be stopped at an extent in which the substrate is not penetrated from the front surface to the back surface. As the isotropic etching process, a method described in the first embodiment can be employed. The isotropic etching process forms the hollow portion  36  whose bottom is deeper than the depth  46  at the point straightly below the edge for the aperture area  35 , and the horizontal width of the hollow portion  36  is wider than the aperture area  35 . 
     Subsequently, as shown in  FIG. 7 , a chip fixation sheet  37  is pasted on the front surface, and the back surface  48  of the silicon substrate  31  is grinded until the hollow portion  36  is revealed. The grinding results in exposing the hollow portion  36  extended horizontally by the previous etching process, and an aperture area  49  of the exposed hollow portion  36  is made to further advance toward the semiconductor elements than the edge  41  (namely, the width of the aperture area  35 ). 
     Subsequently, as shown in  FIG. 8 , an adhesive sheet  39  for fixation is pasted on the back surface, and the sheet  37  on the front surface is removed. Alternatively, the chips can also be directly removed from the sheet  37  without the adhesive sheet  39  pasted instead of the sheet  37 . 
     In the second embodiment, the back surface  48  of the wafer is grinded after the second dry etching process is performed so as to reveal the hollow portion  36 , namely, an etched portion, from the back side. Thus, even where the silicon substrate  31  is thick, the semiconductor devices can be separated into each of individual devices without requiring as much etching amount as in the etching process for penetrating the entire thickness of the wafer. Therefore, the manufacturing method according to the second embodiment shortens an etching time and reduces a horizontal extension of the hollow portion more than necessary, thus improving the size reproducibility. 
     Third Embodiment 
       FIG. 9  and  FIG. 10  are cross sections of the wafer showing steps of the manufacturing method of the semiconductor device according to the third embodiment of this invention. First, as shown in  FIG. 9 , a masking material layer  52  is formed on a silicon substrate  51  except a separation area  56  that is to be left uncovered. Then, the anisotropic etching process forms an aperture area  55  at a location corresponding to the location of the separation area  56  from a surface  54  to a depth  53 . The anisotropic etching process forms a substantially vertical sidewall on an edge position  57  of the aperture area  55 . 
     Subsequently, as shown in  FIG. 10 , a second masking material layer  60  is formed on the back surface of the substrate except the separation area  56 , namely, an area that is to be etched to form an etching aperture area  60   a  on the back surface of the wafer. After forming the etching aperture area  60   a  as mentioned above on the second masking material layer  60 , a chip fixation sheet  59  is pasted on the front of the wafer. After the chip fixation sheet  59  is pasted to adhere each of the semiconductor devices to prevent the devices from scattering, the isotropic etching process etches the wafer from the back surface through the etching aperture area  60   a  to form the hollow portion  56 . The isotropic etching process may etch the wafer to a depth reaching a bottom  53  made by the first dry etching process, and the isotropic etching process can etch the wafer to expand the hollow portion  56  to a position  58  further extended horizontally from an edge position  57  of the aperture area  55 . 
     In the manufacturing method according to the third embodiment as hereinabove described, the second dry etching process is performed from the back surface of the substrate to manufacture the devices with highly precise size reproducibility while reducing influence of the horizontal etching amount of the separation area on the surface of the wafer. 
     Fourth Embodiment 
       FIG. 11  and  FIG. 12  are figures showing the manufacturing method of the semiconductor device according to the fourth embodiment of this invention. In a same manner as the previous embodiments, a masking material layer  62  is formed on the silicon substrate  61  except an area corresponding to a hollow portion  66  (a separation area ) that is to be left uncovered, and then the anisotropic dry etching process, namely, the first dry etching process, etches the separation area  66  from a surface  64  to a depth  63  to form an aperture area  65  corresponding to the separation area  66 . After completing the first dry etching process, a masking material layer  69  for etching is formed on the back surface of the substrate, and a supporter  68  is pasted on the front surface of the substrate. Then, the second dry etching process is performed from the back surface of the substrate. The second dry etching process can be the isotropic dry etching process. The second dry etching process can etch the substrate  61  to a certain depth but does not penetrate the substrate  61 . If the second dry etching process penetrates the substrate to reach the aperture area  65 , the devices will come apart, or the relative position of the semiconductor device (the chip) is shifted to cause deficiencies in the following wafer level processes. The second dry etching process widens the hollow portion  66  to a position  67   b  which is further horizontally extended from an edge position  67   a  of the aperture area  65 . The position  67   b  is an edge, facing the aperture area, of the silicon substrate  61  in contact with the masking material layer  69 . 
     Subsequently, a sidewall, made by the first dry etching process, of the aperture area  65  on the front surface of the substrate is coated with an etching mask, and a chip fixation sheet  70  is pasted on the back surface of the substrate. Then, the third dry etching process is performed from the front surface of the wafer. The third dry etching process can be the anisotropic dry etching process, and the third dry etching process can be self-aligned with the etching mask coating the sidewall of the aperture area  65 . 
     In the manufacturing method according to the fourth embodiment, the dry etching process is performed from both the front surface and the back surface of the substrate to better reduces influence of the horizontal extension of etching processes on the area of the devices. 
     Fifth Embodiment 
       FIG. 13  and  FIG. 16  are cross sections showing steps of the manufacturing method of the semiconductor device according to the fifth embodiment of this invention. As shown in  FIG. 13 , an adhesive sheet  76  as a supporter is pasted to the back surface of a silicon substrate  71 , and a masking material layer  72  is formed on the front surface thereof. The anisotropic etching process (the first etching process), the isotropic etching process (the second etching process), and another isotropic etching process (the third etching process), each of which is of a different condition, are performed through an aperture area  75  of the masking material layer  72  to form a hollow portion  77  in a shape extending in two steps. Therefore, it can be configured to have the latter isotropic etching process (the third etching process) etch the substrate faster than the former isotropic etching process (the second etching process) to improve the overall etching rate while preventing the device area from being adversely affected by the horizontal etching. 
     The fifth embodiment will be hereinafter described with reference to  FIG. 14  through  FIG. 16 . First, as shown in  FIG. 14 , a masking material layer  82  is formed on the front surface of a substrate  81 . Then, the anisotropic dry etching process etches the substrate to form a first aperture area  85  with the use of the masking material layer  82 . Subsequently, a sidewall of the first aperture area  85  is masked with the masking material. 
     Subsequently, as shown in  FIG. 15 , a second dry etching process is performed from the front surface of the substrate after a chip fixation sheet  86  is formed on the back surface of the substrate  81 . The second dry etching process employs a condition that widens a hollow portion  88  in a horizontal direction more than the first dry etching process but that has a lower etching rate than a general isotropic etching process to restrict etching in a horizontal direction. The second dry etching process uses, for example, an etching gas including SF 6  and O 2  and including more O 2  than SF 6 . The second dry etching process etches the substrate  81  to a certain depth but does not penetrate the substrate  81 . 
     Subsequently, a third dry etching process is performed from the front surface of the substrate  81 . The third dry etching process employs the isotropic etching process. A horizontal expansion of the hollow portion  87  near the bottom of the substrate is tolerated in the third dry etching process. As shown in  FIG. 16 , a space etched by the third dry etching process is made to be wider than a space etched by the second dry etching process in a horizontal direction. The third dry etching process uses, for example, an etching gas including SF 6  and O 2  and including more SF 6  than O 2 . 
     In the manufacturing method according to the fifth embodiment, the second dry etching process is performed under a condition that a horizontal etching amount thereof is greater than the first dry etching process but less than the isotropic etching process in general, and the third dry etching process is the isotropic etching process. Therefore, the fifth embodiment is advantageous in capable of not only preventing the device areas from being adversely affected by the etching in a horizontal (lateral) direction but also improving the etching ratio. The third dry etching may also be performed from the back surface of the wafer. A method described in the third embodiment can be employed to perform the etching from the back surface of the wafer. 
     As an example of the semiconductor apparatus manufactured according to the manufacturing method as described above, a light emitting diode (LED) print head will be hereinafter described with reference to  FIG. 17 . 
       FIG. 17  is a figure showing an LED print head  200  according to an embodiment of LED array chips of the present invention. As shown in  FIG. 17 , an LED array unit  202  is arranged on a base material  201 . The LED array unit  202  is manufactured with the semiconductor devices formed according to any one of the first through fifth embodiments and arranged on a mounting substrate. Thin film LED&#39;s are bonded and disposed in a row on the Si substrate to form LED array chips.  FIG. 18  is a top view showing a configuration example of the LED array unit  202 . On a mounting substrate  202   e , the semiconductor device chips described in the above embodiments are arranged as an LED array  202   a  in a longitudinal direction. Furthermore, on the mounting substrate  202   e , arranged are units such as, e.g., an electric component mounting area  202   b  and  202   c  on which electric components and wirings are formed and a connector  202   d  for providing control signal and power source received from outside. 
     A rod lens array  203  as an optical element for focusing light emitted from the LED array is arranged above the LED array  202   a . The rod lens array  203  is formed by arranging many columnar optical lenses arranged linearly along the light emitting unit, and is held at a prescribed position by a lens holder  204  corresponding to an optical element holder. 
     The lens holder  204  is formed to cover the base material  201  and the LED unit  202  as shown in  FIG. 17 . The base material  201 , the LED unit  202 , and the lens holder  204  are collectively held by a damper  205  arranged through an aperture area  201   a ,  204   a  formed on the base material  201  and the lens holder  204 . Thus, a light emitted from the LED unit  202  passes through the rod lens array  203  and irradiates a prescribed external photosensitive drum. The LED print head  200  is used as, for example, an exposure device of an electrophotographic printer, electrophotographic copying machine, and the like. 
     As hereinabove described, the LED print head according to the present embodiment employs any one of the semiconductor devices described in the aforementioned embodiments, and thus, a high quality and reliable LED print head is provided. 
       FIG. 19  is a figure showing a structure of an essential portion of an image forming apparatus  300  according to an embodiment of an image forming apparatus of the present invention. As shown in  FIG. 19 , four process units, namely, a process unit  301 ,  302 ,  303 , and  304 , respectively forming images of yellow, magenta, cyan, and black, are respectively arranged from upstream along a conveyance route  320  for a recording medium  305  in the image forming apparatus  300 . Since the process unit  301 ,  302 ,  303 , and  304  has the same interior structure, only the process unit  303  will be hereinbelow described as an example. 
     The process unit has a photosensitive drum  303   a  as an image carrier arranged rotatably in a direction indicated by an arrow. A charging unit  303   b  for charging the surface of the photosensitive drum  303   a  by providing the electricity thereto and an exposure unit  303   c  for forming electrostatic latent images by selectively irradiating light to the surface of the charged photosensitive drum  303   a  are arranged from upstream in the direction of rotation around the photosensitive drum  303   a . Furthermore, a developing unit  303   d  for generating a toner image by applying a toner in a prescribed color, namely, cyan in the present unit, and a cleaning unit  303   e  for removing a residual toner on the surface of the photosensitive drum  303   a  are arranged around the photosensitive drum  303   a . The drum and rollers used in each unit are driven in synchronization by a driving source and gears, not shown. 
     On the bottom of the image forming apparatus  300 , there is a paper cassette  306  for containing a recording medium  305  such as, e.g., paper in a stacked state. Above the paper cassette  306 , arranged is a hopping roller  307  for separating, sheet by sheet, and conveying the recording medium  305 . On downstream of the hopping roller  307  in a conveyance direction of the recording medium  305 , a resist roller  316  and  311  are arranged to correct skew and convey the recording medium to the process unit  301 ,  302 ,  303 , and  304  by sandwiching the recording medium  305  with a pinch roller  308  and  309 . The hopping roller  307 , the resist roller  310 , and  311  are driven in synchronization by a driving source and gears, not shown. 
     On the opposite of the respective photosensitive drum of the process unit  301 ,  302 ,  303 , and  304 , arranged are transfer rollers  312  made of a semiconductor rubber and the like. In order to cause the toners on the photosensitive drum  301   a ,  302   a ,  303   a , and  304   a  to adhere to the recording medium  305 , a prescribed potential difference is generated between the surface of the photosensitive drum  301   a ,  302   a ,  303   a , and  304   a  and the surface of each of the transfer rollers  312 . 
     A fusing unit  313  has a heating roller and a backup roller, and pressurizes and heats the toners on the recording medium  305  to fix the toners. A discharging roller  314  and  315  deliver the recording medium  305  discharged by the fusing unit  313  to a recording medium stacker unit  318  by sandwiching the recording medium with a pinch roller  316  and  317 . The discharging roller  314  and  315  are driven in synchronization by a driving source and gears, not shown. The LED print head  200  previously described is used as the exposure unit  303   c.    
     Subsequently, operation of the image forming apparatus is hereinafter described. First, the hopping roller  307  separates, from the top, sheet by sheet, the recording medium  305  stacked and contained in the paper container  306  and convey the separated recording medium  305 . Subsequently, the resist roller  310  and  311  and the pinch roller  308  and  309  sandwich and convey the recording medium  305  to the photosensitive drum  301   a  and the transfer roller  312  of the process unit  301 . Subsequently, the photosensitive drum  301   a  and the transfer roller  312  sandwich the recording medium  305  to transfer a toner image to the surface of the recording medium and, at the same time, convey the recording medium by the rotation of the photosensitive drum  301   a.    
     In a similar manner, a recording medium  305  passes through the process unit  302 ,  303 , and  304  in sequence so that toner images of each color developed by the developing unit  301   d ,  302   d ,  303   d  and  304   d  based on electrostatic latent images formed by the exposure unit  301   c ,  302   c ,  303   c , and  304   c  are transferred in sequence onto the surface of the recording medium  305  as overlapped. After the toner images of each color are overlapped on the surface of the recording medium  305 , the fusing unit  313  fuses the recording medium  305  having the toner images on the surface, and the discharging roller  314  and  315  and the pinch roller  316  and  317  sandwich and delivers the recording medium  305  to the recording medium stacker unit  318  on the outside of the image forming apparatus  300 . A color image is formed on the recording medium  305  through the above mentioned processes. 
     As hereinabove described, the image forming apparatus according to the present embodiment employs the LED print head described in the aforementioned embodiment, and thus, a high quality and reliable image forming apparatus is provided. 
     Although a light emitting device (LED) is described as an example of semiconductor device formed in the semiconductor element area of the semiconductor device in the above embodiments, this invention is not limited thereto. Obvious modifications will occur to those skilled in the art. For example, a photoreceptive element may be formed in place of the light emitting element, or various other semiconductor elements may be formed other than such light related elements. 
     It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but it is intended to cover modifications within the spirit and scope of the present invention as defined by the appended claims.