Patent Publication Number: US-8114719-B2

Title: Memory device and manufacturing method of the same

Description:
TECHNICAL FIELD 
     The present invention relates to a memory device, and more particularly to a nonvolatile memory device. 
     BACKGROUND ART 
     In modern society, many electronic appliances are used and various data is generated and employed, requiring a memory device to store the data. Various memory devices produced and used today each has different advantages and disadvantages, and is selected depending on the data to be stored and used. 
     For example, a volatile memory that loses its memory content when the power is turned off includes a DRAM and an SRAM. The volatile memory has limited applications because of the volatility; however, it is used as a main memory device or a cash memory of a computer taking advantage of a short access time. As each memory cell has a small size, a large-capacity DRAM can be produced easily, though it is controlled in a complex manner and consumes much power. Meanwhile, an SRAM includes a memory cell constituted by a CMOS and is easily manufactured and controlled, though a large-capacity SRAM is produced with difficulty since one memory cell needs six transistors. 
     A nonvolatile memory that holds its memory content even after the power is turned off includes: a rewritable memory where data can be rewritten many times; a write-once memory where data can be written by a user only once; and a mask ROM where data content is determined in the manufacture of the memory and cannot be rewritten thereafter. As the rewritable memory, there are an EPROM, a flash memory, a ferroelectric memory, and the like. The EPROM allows an easy writing of data and unit cost per bit is relatively low, though dedicated program device and eraser for writing and erasing are required. The flash memory and the ferroelectric memory allow rewriting on a substrate used, have a short access time, and consume low power, though manufacturing steps of a floating gate and a ferroelectric layer are required to increase unit cost per bit. 
     Each memory cell of a write-once memory is constituted by a fuse, an antifuse, a cross pointer diode, an OLED (Organic Light Emitting Diode), a bistable liquid crystal element, and other devices whose state is changed by heat or light. In general, a memory device stores data by selecting one of the two states of each memory cell. The write-once memory device is manufactured with all memory cells having a first state, and only memory cells specified by a writing operation are changed to a second state. The change from the first state to the second state is irreversible and the changed memory cell cannot be restored. 
     Manufacturing steps of a write-once memory have limited temperature and materials, thus it is not formed on a silicon substrate in many cases. That is, a write-once memory is manufactured in completely different steps than a central processing unit (hereinafter referred to as a CPU), an arithmetic circuit, a rectification circuit, a control circuit and the like (hereinafter collectively referred to as other functional circuits to be distinguished from a write-once memory), which are generally formed on a silicon substrate. For example, an antifuse write-once memory has a wiring, an antifuse layer, and a control element, which are formed on a plastic or metal substrate (see Patent Document 1). The memory device manufactured in this manner achieves low cost, large capacity, low power consumption, and short access time. In the case of forming a semiconductor device having a certain function, however, a memory does not operate by itself and other functional circuits are necessarily required. Therefore, it is necessary to form a memory such as a write-once memory and other functional circuits separately. 
     In recent years, an IC tag has been known as an example of a semiconductor device where a memory and other functional circuits are integrated on the same silicon substrate. An IC tag includes memories such as an SRAM, a mask ROM, a flash memory, and a ferroelectric memory. A mask ROM is a memory where data content is determined in the manufacture of the memory and cannot be rewritten by an IC tag user. In addition, a piece of data is determined by one photomask; therefore, the mask ROM requires as many photomasks as kinds of data. Thus, the mask ROM is not practical for cost reasons. 
     A flash memory and a ferroelectric memory require additional steps for forming a floating gate and a ferroelectric layer in a gate insulating film. Meanwhile, all the circuits other than the memory in an IC tag can be obtained by CMOS manufacturing steps. 
     In recent years, technologies for forming a thin film transistor (hereinafter referred to as a TFT) on an insulating substrate have been actively developed to manufacture a display device such as a liquid crystal display and an EL display. For example, a driver circuit for displaying images and a pixel portion are formed on the same substrate using TFTs. Since an insulating substrate is not capacitively coupled to a wiring, high speed operation of a circuit can be achieved. Accordingly, various functional circuits such as an arithmetic circuit and a memory circuit are proposed to be formed using TFTs. Another advantage of forming functional circuits on an insulating substrate is cost saving. A glass substrate and a plastic substrate are quite inexpensive as compared to a silicon substrate. Further, it is possible to use an insulating substrate with a larger area than a silicon substrate that is limited to a small area. Thus, the number of products manufactured on an insulating substrate increases than that manufactured on a silicon substrate, leading to a very inexpensive semiconductor device. 
     A memory device formed using TFT manufacturing technologies includes a mask ROM, an SRAM, and a flash memory. An SRAM including TFTs can be easily formed on the same substrate as other functional circuits, though it has limited applications because of the volatility. A mask ROM is not practical since different photomasks are required for different data. A flash memory requires additional steps for forming a floating gate, though other functional circuits such as an arithmetic circuit on an insulating substrate can be formed by TFT manufacturing steps. 
     As set forth above, the invention is made in view of the two technologies: a technology for forming a memory device and a technology for forming a circuit on an insulating substrate such as a glass substrate or an insulating surface. 
     Patent Document 1 
     
         
         Japanese Patent Laid-Open No. 2003-36684 
       
    
     DISCLOSURE OF INVENTION 
     It is difficult to form a nonvolatile memory and other functional circuits on the same substrate with conventional technologies, regardless of whether a semiconductor integrated circuit is formed on a silicon substrate or an insulating substrate. However, when a memory and other functional circuits are formed separately to obtain one device, they are required to be connected externally, which results in increased size of a completed device. In addition, at least two circuits that are a memory and other functional circuits are expensive to manufacture. Even when a memory and other functional circuits can be formed on the same substrate as a flash memory and a ferroelectric memory, additional steps are required to form the memory. In the manufacture of a semiconductor device, increase in manufacturing steps leads to high cost, limits product specifications, and decreases productivity. 
     In other words, almost all memory devices manufactured with conventional technologies require specific steps. Accordingly, they cannot be formed on the same substrate as other functional circuits or they require additional steps other than TFT manufacturing steps even when formed on the same substrate as other functional circuits. Such a problem leads to extra cost for a memory in the manufacture of a semiconductor device having a function, for example such as an IC tag. 
     In view of the foregoing, the invention provides a manufacturing method of a write-once memory that can be formed by TFT manufacturing steps similarly to other functional circuits formed on an insulating substrate. Further, the invention provides an easy-to-use and inexpensive memory device while maintaining product specifications and productivity even when a memory is formed on the same substrate as other functional circuit. 
     In view of the foregoing problem, the invention provides a memory device having a write-once memory function by altering a semiconductor film formed over an insulating substrate such as a glass substrate and a plastic substrate or a substrate having an insulating surface (hereinafter collectively referred to as an insulating substrate). 
     According to one mode of the invention, a memory device includes a memory cell formed over an insulating surface. The memory cell includes a semiconductor film having two impurity regions, a gate electrode, and two wirings connected to the respective impurity regions. The semiconductor film is altered by applying a voltage between the gate electrode and at least one of the two wirings, thereby the two wirings are insulated from each other. 
     According to another mode of the invention, a memory device includes a first memory cell and a second memory cell that are formed over an insulating surface. Each of the first memory cell and the second memory cell includes a semiconductor film having two impurity regions, a gate electrode, and two wirings connected to the respective impurity regions. The first memory cell has an insulating state between the two wirings by applying a voltage between the gate electrode and at least one of the two wirings to alter the semiconductor film. The second memory cell has an initial state. The memory device can have either the insulating state or the initial state. 
     According to another mode of the invention, a memory device includes a memory cell formed over an insulating surface. The memory cell includes a semiconductor film having one or two impurity regions, an electrode, and two wirings connected to the respective impurity regions. The semiconductor film is altered by applying a voltage between the electrode and at least one of the two wirings, thereby the two wirings are insulated from each other. 
     According to another mode of the invention, a memory device includes a first memory cell and a second memory cell that are formed over an insulating surface. Each of the first memory cell and the second memory cell includes a semiconductor film having one or two impurity regions, an electrode, and two wirings connected to the respective impurity regions. The first memory cell has an insulating state between the two wirings by applying a voltage between the electrode and at least one of the two wirings to alter the semiconductor film. The second memory cell has an initial state. When seen from the top of the insulating substrate, the electrode is interposed between the two wirings. 
     According to the invention, the memory device may include one or more gate electrodes or electrodes. 
     According to the aforementioned structures, a write-once memory can be formed on an insulating substrate by TFT manufacturing steps. That is, the memory device of the invention can be formed by TFT manufacturing steps similarly to other functional circuits formed on an insulating substrate, which can suppress an increase in extra cost due to additional manufacturing steps of a memory. In addition, since a memory and other functional circuits can be formed by the same steps, the memory does not limit product specifications and not decrease productivity. 
     A glass substrate and a plastic substrate are quite inexpensive as compared to a silicon substrate. Further, it is possible to use an insulating substrate with a larger area than a silicon substrate that is limited to a small area. Thus, the number of products manufactured on an insulating substrate increases than that manufactured on a silicon substrate, leading to a very inexpensive semiconductor device. 
     According to the invention, a write-once memory is formed by TFT manufacturing steps, thereby an easy-to-use and inexpensive memory device can be provided while maintaining product specifications and productivity even when a memory is formed on the same substrate as other functional circuit. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIGS. 1A and 1B  are schematic views showing operations of a memory cell in a memory device of the invention. 
         FIG. 2  is a diagram showing an example of a memory cell array. 
         FIGS. 3A to 3E  are top plan views and cross sectional views of a resistor. 
         FIG. 4  is a diagram showing an example of a memory cell array. 
         FIGS. 5A to 5E  are diagrams showing manufacturing steps of a TFT. 
         FIGS. 6A to 6D  are diagrams showing manufacturing steps of a TFT. 
         FIGS. 7A and 7B  are diagrams showing manufacturing steps of a TFT. 
         FIGS. 8A and 8B  are diagrams each showing an application of a memory device of the invention. 
         FIGS. 9A to 9E  are diagrams each showing an example of a memory cell in a memory device of the invention. 
         FIGS. 10A and 10B  are photographs showing TFTs of the invention before and after being applied a voltage, respectively. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Although the invention will be described by way of embodiment modes and embodiments with reference to the accompanying drawings, it is to be understood that various changes and modifications will be apparent to those skilled in the art. Therefore, unless such changes and modifications depart from the scope of the invention, they should be construed as being included therein. Note that in all the drawings, the same components or components having the same function are denoted by the same reference numerals, and description thereof is omitted. 
     Embodiment Mode 1 
     When a voltage higher than required for normal operation of a TFT is applied between a gate electrode and at least one of two impurity regions (including a high concentration impurity region) of the TFT formed on an insulating substrate, a channel region of the TFT is insulated. This operation is shown in  FIGS. 1A and 1B  that are cross sectional views of a TFT before and after being applied a voltage respectively. For example, a TFT shown in  FIG. 1A  has a semiconductor film  102  formed over an insulating substrate  101 , a gate insulating film  105  over the semiconductor film  102 , and a gate electrode  106  over the gate insulating film  105 . The semiconductor film  102  includes two high concentration impurity regions  103  and a channel region  104 .  FIG. 1B  shows the TFT after being applied a voltage. In the TFT, at least the channel region  104  of the semiconductor film  102  is altered and an insulating region  108  is formed under the gate electrode  106 . Then, the gate electrode  106  and the two high concentration impurity regions  103  are insulated from each other. The insulating region  108  shown in  FIG. 1B  is a typical example, and it may have various shapes practically. 
     For example, in a TFT formed on a glass substrate, which has a channel length (hereinafter abbreviated to L) of 4 μm, a channel width (hereinafter abbreviated to W) of 4 μm, and a gate insulating film with a thickness (hereinafter abbreviated to GI) of 20 nm, a voltage of 25 V is applied between the gate electrode and at least one of the two high concentration impurity regions for 500 μseconds. Then, the channel region of the TFT is insulated and the gate electrode and the two high concentration impurity regions are insulated from each other. Actual photographs of the TFT before and after being applied a voltage are shown in  FIGS. 10A and 10B .  FIG. 10A  is a photograph of the TFT before being applied a voltage and  FIG. 10B  is a photograph of the TFT after being applied a voltage, which is seen from the back side of the glass substrate. 
     Alteration in this specification means specifically a change of at least the channel region into an insulating state, as from  FIG. 10A  to  FIG. 10B , due to a voltage applied to the TFT. It is needless to say that by changing the voltage application conditions, at least a channel region of a TFT can be insulated even when the TFT has a different size than that shown in  FIGS. 10A and 10B . Note that the insulating state means a state where electricity and heat are not conducted. 
     In this manner, when a voltage higher than required for normal operation of a TFT is applied between a gate electrode and at least one of two impurity regions (high concentration impurity regions in this embodiment mode), a current flows through a gate insulating film. The insulating film is made of a highly resistant substance in many cases, and heat is generated when a current flows therethrough. When a large amount of heat is generated in a TFT formed on an insulating substrate, the heat cannot escape because of the low thermal conductivity of the insulating substrate, thereby the gate insulating film or a semiconductor film are burned. As a result, the gate electrode and the two high concentration impurity regions can be insulated from each other. On the other hand, in a transistor formed on a silicon substrate with high thermal conductivity, heat generated when a current flows through a gate insulating film does not burn the insulating film and the silicon substrate. 
     According to the experiment in the invention, when a voltage is applied between a gate electrode and at least one of two high concentration impurity regions, a channel region is altered to an insulating state no less than 97% of the time, thereby the gate electrode and the two high concentration impurity regions are insulated from each other, namely they are brought into a non-conductive state. A defective element is detected the rest 3% of the time, where a channel region functions as a resistor after being applied a voltage and three terminals are conducted to each other. Defective elements may be caused by dust in a semiconductor film or an insulating film. Therefore, improved accuracy of manufactured TFTs allows defective elements to be further reduced. The generation of defective elements can also be suppressed when a TFT has a double gate structure or a redundant circuit is provided additionally. 
     As another defect, two of three terminals that are a gate electrode and two wirings connected to an impurity region may be conducted to each other. The defects such as conduction between three terminals or two terminals may be caused by excess voltage applied in writing as well as dust. Accordingly, the number of defective elements can be reduced by optimizing a writing voltage and voltage application time. 
     A memory device stores data when a memory cell selects one of two states. The memory device of the invention can store data when a TFT as a memory cell selects one of two states: whether a channel region of the TFT is in an initial state or an insulating state. In a write-once memory manufactured in the invention, a TFT in the initial state before being applied a voltage has a state ‘1’ while the TFT including a channel region altered to an insulating state by applying a voltage has a state ‘0’. This relation between states ‘0’ and ‘1’ and the states of the TFT is not limited to this, though it is used in this specification for convenience. 
       FIG. 2  is a circuit diagram of a 4-bit memory cell array showing an example of the memory device of the invention. The memory cell array includes two word lines  201 , two bit lines  202 , two source lines  204 , and four TFTs  206  to  209 . The word lines  201 , the bit lines  202 , and the source lines  204  are denoted as W 0 , W 1 , B 0 , B 1 , S 0 , and S 1  respectively. When a voltage of 25 V or higher is applied between a gate electrode and at least one of two impurity regions for 500 μseconds, as set forth above, each channel region of the TFTs  206  to  209  is altered to an insulating state. 
     Described first is an example of a circuit operation for writing ‘0’ to the TFT  206 . The writing operation can be performed by applying a voltage between the gate electrode and at least one of the two impurity regions of the TFT  206 . For example, a voltage of 25 V is applied to W 0  and a voltage of 0 V is applied to B 0  and S 0  for 500 μseconds. At this time, it is necessary to determine the voltages of W 1 , B 1  and S 1  so that ‘0’ is not written to the other TFTs. For example, a voltage of 0 V is applied to W 1  and a voltage of 10 V is applied to B 1  and S 1 . By applying these voltages, a voltage of 25 V is applied between the gate electrode and at least one of the two impurity regions of the TFT  206 , thereby the channel region of the TFT  206  can be insulated. 
     Brief description is made on the operation of the other TFTs in writing ‘0’ to the TFT  206 . Since a voltage of 25 V is applied to W 0  and a voltage of 10 V is applied to B 1  and S 1 , a voltage of 15 V is applied between the gate electrode and at least one of the two impurity regions of the TFT  207 . However, ‘0’ is not written to the TFT  207  because a voltage of 25 V or higher is not applied thereto. Similarly, ‘0’ is not written to the TFT  208  since a voltage of 0 V is applied to W 1 , B 0  and S 0 . A voltage of 0 V is applied to W 1  while a voltage of 10 V is applied to B 0  and S 0 , therefore, a voltage of 10 V is applied between the gate electrode and the semiconductor film of the TFT  209 , though ‘0’ is not written thereto. Note that the voltages applied here are just examples, and a signal can be written only to a selected TFT by arbitrarily determining the voltages of each word line  201 , bit line  202  and source line  204 . 
     Described next is an example of a circuit operation for writing ‘1’ to the TFT  206 . When ‘1’ is written to the TFT  206 , no voltage is applied between the gate electrode and at least one of the two impurity regions while maintaining the TFT  206  in an initial state. Accordingly, for example, all the word lines  201 , bit lines  202  and source lines  204  may have the same voltage so that ‘0’ is not written to the TFT  206 . This is just an example and the potential of each word line  201 , bit line  202  and source line  204  may be determined by a controlling method of a circuit. 
     Reading operation of the TFT  206  is described next. The reading operation determines whether the TFT  206  is in the state ‘1’, namely in the initial state without being applied a voltage or the TFT  206  is in the state ‘0’, namely altered to an insulating state. Thus, a voltage of a threshold or higher is applied to the gate electrode of the TFT  206  to determine whether a current flows between the two high concentration impurity regions. First, before reading, B 0  is precharged to 5 V. Then, voltages of 5 V and 0 V are applied to W 0  and S 0  respectively to read the potential of B 0 . At this time, it is necessary to determine the voltages of W 1 , B 1  and S 1  so that the other TFTs are not selected. For example, a voltage of 0 V is applied to W 1  and S 1  so as not to read the potential of B 1 . When the TFT  206  is in the state ‘1’ without being applied a voltage, the two impurity regions are conducted and a voltage of 0 V is applied to B 0  since a voltage of 5 V is applied to W 0 . On the other hand, when the TFT  206  is applied a voltage and brought into the state ‘0’, B 0  is maintained at 5 V since B 0  and S 0  are insulated from each other. The reading operation of the TFT  206  can thus be performed by applying a voltage equal to or higher than a threshold to W 0  and reading a change in the potential of B 0 . 
     Brief description is made on the operation of the other TFTs in the reading operation of the TFT  206 . Since B 1  is not selected to read, the reading operation does not involve the TFT  207  and the TFT  209 . The TFT  208  does not change the potential of the bit line since W 0  has a voltage of 0 V. Therefore, the other TFTs do not influence the reading operation of the TFT  206 . 
     As set forth above, in this embodiment mode, a memory cell has two states: switching state and insulating state. Thus, a memory cell can be constituted by only one TFT, leading to reduced area of a memory cell array and increased memory capacity. 
     Embodiment Mode 2 
     In a memory device of the invention, a high concentration impurity may be added over the entire surface of a semiconductor film of TFT as a memory cell. Instead, an impurity may be added to any portion of a semiconductor film and two wirings may be connected thereto; however, when forming an impurity region arbitrarily, the element does not function as a transistor. Meanwhile, when a high concentration impurity is added over the entire surface of a semiconductor film, all the three terminals can be insulated from each other by applying a voltage between an electrode and at least one of two wirings. 
     Described in this embodiment mode is an example where one impurity region (high concentration impurity region in this embodiment mode) is formed over a semiconductor film on an insulating substrate and two wirings are connected to the semiconductor film with one electrode interposed therebetween.  FIGS. 3A and 3C  are top plan views of an element having such a structure.  FIGS. 3B ,  3 D, and  3 E are cross sectional views of an element having such a structure.  FIGS. 3A and 3B  show a normal TFT where a high concentration impurity is added to a semiconductor film before forming a gate electrode on an insulating film.  FIGS. 3C ,  3 D, and  3 E show a TFT including a void  307  with an arbitrary width in a gate electrode, where a high concentration impurity is added to a semiconductor film after forming the electrode. The width of the void  307  should be determined so that the two wirings are insulated when applying a voltage between the electrode and the semiconductor film. Both the elements shown in  FIGS. 3A and 3C  have the two wirings conducted through the high concentration impurity region of the semiconductor film, thus these elements are referred to as resistor elements in this specification to be distinguished from TFTs. 
     In each of the resistor elements shown in  FIGS. 3A to 3E , a semiconductor film  301  is formed over an insulating substrate  303 , an insulating film  305  is formed over the semiconductor film  301 , and an electrode  302  is formed over the insulating film  305 . Two wirings  306  are connected to a high concentration impurity region  304  in the semiconductor film  301 . The high concentration impurity region  304  in the semiconductor film  301  and the wirings  306  connected to the semiconductor film  301  may be formed anywhere as long as the electrode  302  is formed between the two wirings  306 . The form of the electrode  302  can also be determined arbitrarily as shown in  FIG. 3C  where the void  307  with an arbitrary width is formed in the gate electrode. Further, the form of the resistor element can be arbitrarily determined as well, and  FIGS. 3A to 3E  show just examples. 
     For example, the resistor element shown in  FIG. 3A  has an L of 4 μm, a W of 4 μm, and a GI of 20 nm, and a voltage of 25 V is applied between the electrode and at least one of the two wirings for 500 μseconds similarly to Embodiment Mode 1. Then, the electrode and the two wirings are insulated from each other. Needless to say, even when the element has a different size than that shown here, the three terminals can be insulated from each other by changing the voltage application conditions. In this embodiment mode, a write-once memory is manufactured utilizing such a structure. 
       FIG. 4  is a circuit diagram of a 4-bit memory cell array showing an example of this embodiment mode. The memory cell array includes two word lines  31 , two bit lines  32 , two selection control lines  33 , four resistor elements  34 , and four selection transistors  35 . The word lines  31 , the bit lines  32  and the selection control lines  33  are denoted as W 0 , W 1 , B 0 , B 1 , W′ 0 , and W′ 1  respectively. A memory cell  22  is selected by W 0  and B 0 , a memory cell  24  is selected by W 0  and B 1 , a memory cell  42  is selected by W 1  and B 0 , and a memory cell  44  is selected by W 1  and B 1 . Similarly to the example shown in  FIGS. 3A and 3B , an electrode and two impurity regions of the resistor element  34  are insulated from each other when applying a voltage of 25 V or higher between the electrode and at least one of the two impurity regions. Note that in this embodiment mode, the impurity region may be formed over the entire surface of the semiconductor film, thus a voltage is applied between the electrode and the semiconductor film. 
     Described first is an example of a circuit operation for writing ‘0’ to the memory cell  22 . The writing operation can be performed by applying a voltage between the electrode and the semiconductor film, namely at least one of the two wirings connected to the semiconductor film of the resistor element  34  in the memory cell  22 . For example, a voltage of 25 V is applied to W 0  and a voltage of 0 V is applied to B 0  and W′ 0  for 500 μseconds. At this time, it is necessary to determine the voltages of W 1 , B 1  and W′ 1  so that ‘0’ is not written to the other resistor elements. For example, a voltage of 0 V is applied to W 1  and W′ 1  and a voltage of 10 V is applied to B 1 . By applying these voltages, a voltage of 25 V can be applied between the electrode and the semiconductor film of the resistor element  34  in the memory cell  22 , thereby the electrode and the two wirings can be insulated from each other. The applied voltages shown here are just examples and the writing operation can be performed by other conditions as well. 
     Brief description is made on the operation of the other memory cells in writing ‘0’ to the memory cell  22 . Since a voltage of 25 V is applied to W 0 , a voltage of 10 V is applied to B 1 , and a voltage of 0 V is applied to W′ 0 , a voltage of 15 V is applied between the electrode and the semiconductor film of the memory cell  24 . However, ‘0’ is not written to the memory cell  24  because a voltage of 25 V or higher is not applied thereto. Similarly, ‘0’ is not written to the memory cell  42  since a voltage of 0 V is applied to W 1 , W′ 1  and B 0 . A voltage of 0 V is applied to W 1  and W′ 1  while a voltage of 10 V is applied to B 0 , therefore, a voltage of 10 V is applied between the electrode and the semiconductor film of the memory cell  44 , though ‘0’ is not written thereto. In this manner, ‘0’ can be written only to a selected memory cell by arbitrarily determining the voltages of the word lines  31 , the bit lines  32  and the selection control lines  33 . 
     Described next is an example of a circuit operation for writing ‘1’ to the memory cell  22 . When ‘1’ is written to the memory cell  22 , no voltage is applied between the electrode and the semiconductor film of the resistor element  34  while maintaining the memory cell  22  in an initial state. Accordingly, for example, all the word lines  31 , bit lines  32  and selection control lines  33  may have the same voltage such as 0 V so that ‘0’ is not written to the memory cell  22 . This is just an example and the potential of each word line  31 , bit line  32  and selection control line  33  may be determined arbitrarily by a controlling method of a circuit. 
     Reading operation of the memory cell  22  is described next. The reading operation determines whether the resistor element  34  in the memory cell  22  is in the state ‘1’, namely in the initial state without being applied a voltage or the resistor element  34  is applied a voltage and in the state ‘0’, namely altered to an insulating state. Thus, a voltage of a threshold or higher is applied to the gate electrode of the selection transistor  35  in the memory cell  22  to determine whether B 0  is conducted to a ground of the two high concentration impurity regions of the selection transistor  35 . First, before reading, B 0  is precharged to 5 V. Then, a voltage of 5 V is applied to W′ 0 . At this time, it is necessary to determine the voltages of W′ 1  and B 1  so that the other transistors are not selected. W 0  and W 1  are used only in the writing operation and are not required in the reading operation. For example, a voltage of 0 V is applied to W′ 1  so as not to read the potential of B 1 . When the resistor element  34  in the memory cell  22  is in the state ‘1’, namely in the initial state without being applied a voltage, B 0  is conducted to the ground and a voltage of 0 V is applied to B 0 . On the other hand, when the resistor element  34  in the memory cell  22  is applied a voltage and brought into the state ‘0’, namely in the insulating state, B 0  is maintained at 5 V even when a voltage of 5 V is applied to W 0  since B 0  is insulated from the ground. The reading operation of the memory cell  22  can thus be performed by applying a voltage to W′0 and reading a change in the potential of B 0 . 
     Brief description is made on the operation of the other memory cells in the reading operation of the memory cell  22 . Since B 1  is not selected to read, the reading operation does not involve the memory cell  24  and the memory cell  44 . The memory cell  42  does not change the potential of the bit line since W′ 0  has a voltage of 0 V. Therefore, the other memory cells do not influence the reading operation of the memory cell  22 . 
     In this embodiment mode, each memory cell includes the two elements, which increases the area of the memory cell array. In such a case, however, an element using a high voltage in writing (e.g., a decoder connected to W 0  or W 1 ) and an element using a low voltage in reading (e.g., a decoder connected to W′ 0  or W′ 1 ) can be formed separately. When using a high voltage, a TFT is required to have a large L to withstand a high voltage. However, the TFT having a large L is not suitable for high speed operation, thus a TFT using a low voltage generally has a small L. Accordingly, it is very advantageous that the elements are formed separately for high speed operation and easy operation control. In this embodiment mode, a TFT can be used instead of the resistor element  34  and a memory TFT and a selection TFT can be formed separately in a memory cell. 
     EMBODIMENT 
     Embodiment 1 
     In this embodiment, a manufacturing method of a TFT on a glass substrate is specifically described with reference to  FIGS. 5A to 5E ,  FIGS. 6A to 6D  and  FIGS. 7A and 7B . Description is made with reference to cross sectional views of an N-channel TFT and a P-channel TFT. 
     First, a peeling layer  501  is formed over a substrate  500  ( FIG. 5A ). In this embodiment, an a-Si film (amorphous silicon film) with a thickness of 50 nm is formed over a glass substrate (e.g., a 1737 substrate, product of Corning Incorporated) by low pressure CVD. As for the substrate  500 , a quartz substrate, a substrate made of an insulating material such as alumina, a silicon wafer substrate, a plastic substrate having enough heat resistance to the treatment temperature in the subsequent step, and the like may be employed as well as the glass substrate. The peeling layer  501  is preferably formed of a film mainly containing silicon such as polycrystalline silicon, single crystalline silicon and SAS (semi-amorphous silicon that is also referred to as microcrystalline silicon) as well as amorphous silicon, though the invention is not limited to these. The peeling layer  501  may be formed by plasma CVD or sputtering as well as low pressure CVD. A film doped with an impurity such as phosphorous may be employed as well. The thickness of the peeling layer  501  is desirably 50 to 60 nm, though it may be 30 to 50 nm in the case of employing an SAS. 
     Next, a protective film  502  (also referred to as a base film or a base insulating film) is formed over the peeling layer  501  ( FIG. 5A ). In this embodiment, the protective film  502  has a three-layer structure where a SiON film with a thickness of 100 nm, a SiNO film with a thickness of 50 nm, and a SiON film with a thickness of 100 nm are stacked in this order from the substrate side, though the material, the thickness, and the number of layers are not limited to these. For example, instead of the SiON film on the bottom layer, a heat resistant resin such as siloxane with a thickness of 0.5 to 3 μm may be formed by spin coating, slit coating, droplet discharging, or the like. Alternatively, a silicon nitride film (SiN, Si 3 N 4  or the like) may be employed. The thickness of each layer is preferably in the range of 0.05 to 3 μm and can be selected within this range as required. A silicon oxide film can be formed by thermal CVD, plasma CVD, atmospheric pressure CVD, bias ECRCVD, or the like using a mixed gas of SiH 4  and O 2 , TEOS (tetraethoxy silane) and O 2 , or the like. A silicon nitride film can be typically formed by plasma CVD using a mixed gas of SiH 4  and NH 3 . A SiON film or a SiNO film can be typically formed by plasma CVD using a mixed gas of SiH 4  and N 2 O. 
     Subsequently, TFTs are formed over the protective film  502 . Note that other thin film active elements such as organic TFTs and thin film diodes may be formed as well as the TFTs. In order to form a TFT, first, an island shape semiconductor film  503  is formed over the protective film  502  ( FIG. 5B ). The island shape semiconductor film  503  is formed using an amorphous semiconductor, a crystalline semiconductor or a semi-amorphous semiconductor, which mainly contains silicon, silicon germanium (SiGe), or the like. Note that if a material mainly containing silicon such as a-Si is employed for the peeling layer  501  and the island shape semiconductor film  503 , the protective film  502  that is in contact with them may be formed using SiO x N y  in view of the adhesiveness. In this embodiment, an amorphous silicon film with a thickness of 70 nm is formed and the surface thereof is treated with a solution containing nickel. Thermal crystallization is performed at a temperature of 500 to 750° C. so that a crystalline silicon semiconductor film is obtained. Then, the crystallinity thereof may be improved by laser crystallization. Note that the film may be formed by plasma CVD, sputtering, LPCVD, or the like. As a crystallizing method, laser crystallization, thermal crystallization, or thermal crystallization using a catalyst other than nickel (Fe, Ru, Rh, Pd, Os, Ir, Pt, Cu, Au, or the like) may be adopted, or such methods may be performed alternately a plurality of times. 
     Alternatively, the semiconductor film having an amorphous structure may be crystallized by a continuous wave laser. In order to obtain a crystal with a large grain size during crystallization, a solid state laser capable of continuous wave may be used and it is preferable to apply second to fourth harmonics of a fundamental wave (the crystallization in this case is referred to as CWLC). Typically, a second harmonic (532 nm) or a third harmonic (355 nm) of a Nd:YVO 4  laser (a fundamental wave: 1064 nm) is applied. When a continuous wave laser is used, laser light emitted from a continuous wave YVO 4  laser having an output of 10 W is converted into a harmonic by a non-linear optical element. There is also a method for emitting a harmonic by putting a YVO 4  crystal or a GdVO 4  crystal and a non-linear optical element in a resonator. Then, the laser light is preferably formed in a rectangular shape or an ellipse shape at an irradiated surface with an optical system to irradiate a subject. In that case, an energy density of about 0.01 to 100 MW/cm 2  (preferably 0.1 to 10 MW/cm 2 ) is required. Then, the semiconductor film is preferably irradiated with laser light while being moved relatively to the laser light at a speed of about 10 to 2000 cm/sec. 
     When a pulsed laser is used, a pulsed laser having a frequency band of several tens to several hundreds Hz is generally used, though a pulsed laser having an extremely higher oscillation frequency of 10 MHz or higher may be used as well (the crystallization in this case is referred to as MHzLC). It is said that it takes several tens to several hundreds nsec to solidify a semiconductor film completely after the semiconductor film is irradiated with the pulsed laser light. When the pulsed laser light has an oscillation frequency of 10 MHz or higher, it is possible to irradiate the next pulsed laser light before the semiconductor film is solidified after it is melted by the previous laser light. Therefore, unlike the case of the conventional pulsed laser, the interface between the solid phase and the liquid phase can be moved continuously in the semiconductor film, and thus the semiconductor film having a crystal grain grown continuously along the scanning direction can be formed. More specifically, it is possible to form an aggregation of crystal grains each of which has a width of 10 to 30 μm in the scanning direction and a width of about 1 to 5 μm in the direction perpendicular to the scanning direction. By forming such single crystal grains extending long in the scanning direction, a semiconductor film having few crystal grain boundaries at least in the channel direction of the TFT can be formed. Note that when the protective film  502  is partially formed using siloxane that is a heat resistant organic resin, heat leak from the semiconductor film can be prevented in the aforementioned crystallization, leading to effective crystallization. 
     The crystalline silicon semiconductor film is obtained through the aforementioned steps. The crystals thereof are preferably aligned in the same direction as the source, channel and drain direction. The thickness of the crystalline layer thereof is preferably 20 to 200 nm (typically 40 to 170 nm, and more preferably 50 to 150 nm). Subsequently, an amorphous silicon film for gettering of a metal catalyst is formed over the semiconductor film with an oxide film interposed therebetween, and heat treatment is performed at a temperature of 500 to 750° C. for gettering. Furthermore, in order to control a threshold value as a TFT element, boron ions are injected into the crystalline silicon semiconductor film at a dosage of 10 13 /cm 2 . Then, etching is performed with a resist used as a mask to form the island shape semiconductor film  503 . Alternatively, the crystalline semiconductor film may be obtained by forming a polycrystalline semiconductor film directly by LPCVD (Low Pressure CVD) using a source gas of disilane (Si 2 H 6 ) and germanium fluoride (GeF 4 ). The flow rate of the gas is such that Si 2 H 6 /GeF 4 =20/0.9, the temperature for forming the film is 400 to 500° C., and He or Ar is used as a carrier gas, though the invention is not limited to these conditions. 
     A TFT, particularly the channel region thereof is preferably added with hydrogen or halogen of 1×10 19  to 1×10 22  cm 3 , and more preferably 1×10 19  to 5×10 20  cm −3 . In the case of using an SAS, it is preferably added with hydrogen or halogen of 1×10 19  to 2×10 21  cm −3 . In either case, it is desirable that the amount of hydrogen or halogen be larger than that contained in silicon single crystals used for an IC chip and the like. According to this, local cracks that may be generated at the TFT portion can be terminated by hydrogen or halogen. 
     Then, a gate insulating film  504  is formed over the island shape semiconductor film  503  ( FIG. 5B ). The gate insulating film  504  is preferably formed of a single layer or stacked layers of a film containing silicon nitride, silicon oxide, silicon nitride oxide, or silicon oxynitride by a thin film forming method such as plasma CVD and sputtering. In the case of the stacked layers, a three-layer structure may be adopted for example, where a silicon oxide film, a silicon nitride film and a silicon oxide film are stacked in this order from the substrate side. 
     Subsequently, a gate electrode  505  is formed ( FIG. 5C ). In this embodiment, Si and W (tungsten) are stacked by sputtering, and etched with a resist  506  used as a mask to form the gate electrode  505 . Needless to say, the material, the structure and the forming method of the gate electrode  505  are not limited to these and can be selected appropriately. For example, a stacked structure of Si and NiSi (Nickel Silicide) doped with an N-type impurity, or a stacked structure of TaN (tantalum nitride) and W (tungsten) may be employed. Alternatively, the gate electrode  505  may be formed of a single layer employing any conductive material. A mask of SiO x  or the like may be used instead of the resist mask. In this case, a patterning step of the mask such as SiO x  and SiON (such a mask made of an inorganic material is referred to as a hard mask) is additionally required, while the mask film is less decreased in etching as compared with the resist, thereby a gate electrode layer with a desired width can be formed. Alternatively, the gate electrode  505  may be selectively formed by droplet discharging without using the resist  506 . As for the conductive material, various kinds of materials can be selected depending on the function of the conductive film. When the gate electrode and an antenna are simultaneously formed, the material may be selected in consideration of their functions. As an etching gas for etching the gate electrode, a mixed gas of CF 4 , Cl 2  and O 2 , or a Cl 2  gas is employed here, though the invention is not limited to this. 
     Subsequently, a resist  509  is formed so as to cover portions to be P-channel TFTs  507 . An N-type impurity element  510  (typically, P (phosphorous) or As (arsenic)) is doped to the island shape semiconductor films of N-channel TFTs  508  at a low concentration with the gate electrode used as a mask (a first doping step,  FIG. 5D ). The first doping step is performed under such conditions as a dosage of 1×10 13  to 6×10 13 /cm 2  and an accelerated voltage of 50 to 70 keV, though the invention is not limited to these conditions. In the first doping step, through doping is performed through the gate insulating film  504  to form a couple of low concentration impurity regions  511 . Note that the first doping step may be performed to the entire surface without covering the P-channel TFT regions with the resist  509 . 
     After the resist  509  is removed by ashing or the like, another resist  512  is formed so as to cover the N-channel TFT regions. A P-type impurity element  513  (typically, B (boron)) is doped to the island shape semiconductor films of the P-channel TFTs  507  at a high concentration with the gate electrode used as a mask (a second doping step,  FIG. 5E ). The second doping step is performed under such conditions as a dosage of 1×10 16  to 3×10 16 /cm 2  and an accelerated voltage of 20 to 40 keV. In the second doping step, through doping is performed through the gate insulating film  504  to form a couple of P-type high concentration impurity regions  514 . 
     After the resist  512  is removed by ashing or the like, an insulating film  601  is formed over the entire surface of the substrate ( FIG. 6A ). In this embodiment, a SiO 2  film with a thickness of 100 nm is formed by plasma CVD. Then the entire surface of the substrate is covered with a resist  602 , and resist  602 , the insulating film  601  and the gate insulating film  504  are removed by etch back to form a sidewall  603  in a self-aligned manner ( FIG. 6B ). As an etching gas, a mixed gas of CHF 3  and He is employed. Note that if the insulating film  601  is formed on the back side of the substrate as well, the insulating film on the back side is etched and removed with the resist  602  used as a mask (this step is called a back side treatment). 
     The forming method of the sidewall  603  is not limited to the aforementioned one. For example, methods shown in  FIGS. 7A and 7B  may be employed as well.  FIG. 7A  shows an insulating film  701  having a two or more layer stacked structure. The insulating film  701  has, for example, a two-layer structure of a SiON (silicon oxynitride) film with a thickness of 100 nm and an LTO (Low Temperature Oxide) film with a thickness of 200 nm. In this embodiment, the SiON film is formed by plasma CVD, and the LTO film is formed by low pressure CVD using a SiO 2  film. Then, etch back is performed with the resist  602  used as a mask, thereby forming the sidewall  603  having an L shape and an arc shape.  FIG. 7B  shows the case where etching is performed so that the insulating film  702  is not removed by the etch back. The insulating film  702  in this case may be formed of a single layer or stacked layers. The sidewall  603  serves as a mask when an N-type impurity is doped at a high concentration in the subsequent step to form a low concentration impurity region or a non-doped offset region under the sidewall  603 . In any of the aforementioned forming methods of the sidewall, the conditions of the etch back may be changed depending on the width of the low concentration impurity region or the offset region to be formed. 
     In the semiconductor device of the invention, the memory cell operates normally without sidewalls. Therefore, in  FIG. 6B  and thereafter, the two TFTs on the left side have side walls and the two TFTs on the right side does not have side walls. 
     Subsequently, another resist  604  is formed so as to cover the P-channel TFT regions. An N-type impurity element  605  (typically, P or As) is doped at a high concentration with the gate electrode  505  and the sidewall  603  used as masks (a third doping step,  FIG. 6C ). The third doping step is performed under such conditions as a dosage of 1×10 13  to 5×10 15 /cm 2  and an accelerated voltage of 60 to 100 keV. In the third doping step, a couple of N-type high concentration impurity regions  606  are formed. After the resist  604  is removed by ashing or the like, thermal activation of the impurity regions may be performed. For example, a SiON film with a thickness of 50 nm is formed, and then heat treatment is performed at a temperature of 550° C. for four hours in a nitrogen atmosphere. Alternatively, it is also possible that a SiN x  film containing hydrogen is formed to have a thickness of 100 nm and heat treatment is performed at a temperature of 410° C. for one hour in a nitrogen atmosphere. According to this, defects in the crystalline semiconductor film can be improved. This step enables to, for example, terminate a dangling bond in the crystalline silicon and is called a hydrotreatment step. Then, a SiON film with a thickness of 600 nm is formed as a cap insulating film for protecting the TFT. Note that the aforementioned hydrotreatment step may be performed after the formation of this SiON film. In that case, a SiN x  film and a SiON film thereon may be continuously formed. In this manner, the insulating film includes three layers of SiON, SiN x  and SiON that are stacked in this order from the substrate side over the TFT, though the structure and the material are not limited to these. Note that such an insulating film is preferably formed, since it also has a function to protect the TFT. 
     Subsequently, an interlayer film  607  is formed over the TFT ( FIG. 6D ). For the interlayer film  607 , a heat resistant organic resin such as polyimide, acrylic, polyamide, and siloxane may be employed. The interlayer film  607  may be formed by spin coating, dipping, spray application, droplet discharging (inkjet printing, screen printing, offset printing or the like), a doctor knife, a roll coater, a curtain coater, a knife coater, or the like depending on the material thereof. Alternatively, an inorganic material may be employed such as a film of silicon oxide, silicon nitride, silicon oxynitride, PSG (phosphosilicate glass), BPSG (boron phosphosilicate glass), and alumina. These insulating films may also be stacked to form the interlayer film  604 . A protective film  608  may be formed over the interlayer film  607 . As the protective film  608 , a film containing carbon such as DLC (Diamond Like Carbon) and carbon nitride (CN), a silicon oxide film, a silicon nitride film, a silicon nitride oxide film, or the like may be employed. The protective film  608  may be formed by plasma CVD, atmospheric pressure plasma, or the like. Alternatively, a photosensitive or nonphotosensitive organic material such as polyimide, acrylic, polyamide, resist, and benzocyclobutene, or a heat resistant organic resin such as siloxane may be employed. A filler may be mixed into the interlayer film  607  or the protective film  608  in order to prevent these films from being detached or cracked due to stress generated by a difference of thermal expansion coefficients between the interlayer film  607  or the protective film  608  and a conductive material or the like of a wiring that is formed later. 
     After forming a resist, etching is performed to form contact holes, so that a wiring  609  is formed ( FIG. 6D ). As an etching gas for forming the contact holes, a mixed gas of CHF 3  and He is employed, though the invention is not limited to this. The wiring  609  is formed by sputtering and patterning so as to have a five-layer structure where Ti, TiN, Al—Si, Ti, and TiN are stacked in this order from the substrate side. By mixing Si into the Al layer, hillock can be prevented from generating in the resist baking when the wiring is patterned. Instead of Si, Cu of about 0.5% may be mixed. When the Al—Si layer is sandwiched between Ti and TiN, hillock resistance can be further improved. In the patterning, the aforementioned hard mask of SiON or the like is preferably employed. Note that the material and the forming method of the wirings are not limited to these, and the aforementioned material for forming the gate electrode may be employed as well. 
     Through the aforementioned steps, a semiconductor device having TFTs is completed. The semiconductor device includes an IC tag, an IC chip, a wireless chip, and the like. Although a top gate structure is employed in this embodiment, a bottom gate structure (an inversely staggered structure) may be employed as well. A region where a thin film active element such as a TFT is not formed mainly includes a base insulating film material, an interlayer insulating film material and a wiring material. This region preferably occupies 50% or more, and more preferably 70 to 95% of the whole semiconductor device. Meanwhile, it is preferable that an island shape semiconductor region (island) of the active element including the TFT portion occupy 1 to 30%, and more preferably 5 to 15% of the whole semiconductor device. As shown in  FIG. 6D , the thickness of the protective film or the interlayer film in the semiconductor device is desirably controlled so that the distance (t over ) between the an semiconductor layer of the TFT and the lower protective film may be the same or substantially the same as the distance (t over ) between the semiconductor layer and the upper interlayer film (or protective film if formed). By disposing the semiconductor layer in the middle of the semiconductor device in this manner, stress applied to the semiconductor layer can be alleviated, thereby generation of cracks can be prevented. 
     Embodiment 2 
     Described in this embodiment is an example of a semiconductor device incorporating the memory device of the invention on the same substrate. An IC tag can be taken as an example of the semiconductor device where a memory and other functional circuits are integrated on the same substrate.  FIG. 8A  is a block diagram of an IC tag. An IC tag  801  includes an RF circuit  802 , a power supply circuit  803 , a command control circuit  804 , a clock  805 , a congestion control circuit  806 , a memory control circuit  807 , a memory  808 , and an antenna  809 . These functional circuits are formed on the same insulating substrate. Note that the antenna  809  may be formed on the same substrate or provided externally and connected to a terminal formed on the same substrate, thus the antenna  809  is surrounded by a dashed line in the drawing. 
     In the IC tag  801 , all the circuits except for the memory  808  can be formed by the TFT manufacturing steps. When the memory device of the invention is incorporated in the memory  808 , all the circuits can be formed by the same manufacturing steps. In the case where the semiconductor device is formed on one substrate as shown in this embodiment, improved productivity and cost saving can be achieved by forming all the circuits by the same manufacturing steps. 
     A write-once memory can be suitably applied to the IC tag because, similarly to a bar code, it is not necessary to change the data content of a memory once it is determined. The security of an IC tag that is used for individual recognition and goods management can be improved by preventing the data content from being rewritten. Further, the IC tag should hold data for a long period; therefore, a write-once memory performing an irreversible operation for writing data is suitably incorporated in the IC tag. In addition, if data should be written during the use of the IC tag, available memory can be reserved as needed. In this manner, by incorporating the memory device of the invention in the IC tag, high-security and easy-to-use product can be provided. 
     An IC tag operates with a semiconductor device formed on an insulating substrate. The memory device of the invention can be used as a part of a device. An example of this case is shown in  FIG. 8B . A home electronic appliance  810 , for example such as a rice cooker and an air conditioner, includes a CPU  812 , a memory  811 , an I/O controller  813 , and an external device  814 . The memory  811  incorporated in this electronic appliance is a program ROM where operating data of the appliance is written before shipment thereof. 
     The CPU  812 , the memory  811  and the I/O controller  813  are generally formed as individual ICs; however, they can be formed on the same insulating substrate using TFTs. It is very advantageous that the circuits are formed on the same substrate even for a part of the appliance as shown in this embodiment. For example, when the CPU  812 , the memory  811  and the I/O controller  813  are formed as individual ICs, they are connected to each other with external wirings. Meanwhile, when they are formed on the same substrate, external connecting wirings are no longer required, leading to significantly reduced size of the product. Further, steps and cost for connection are reduced, which results in cost saving of the product. 
     Since the operating data written to the memory is not required to be rewritten after shipment of the product, a write-once memory is suitably applied to the electronic appliance. In addition, the data can be written easily, therefore, the data content can be determined and written at the last stage of the manufacture of the product taking into consideration a change or update of the written data. 
     Embodiment 3 
     Described in this embodiment are measures for reducing writing defects of a memory cell. Description is made with reference to  FIGS. 9A and 9C  showing top plan views of a TFT. Description is made with reference to  FIGS. 9B ,  9 D, and  9 E showing cross sectional views of a TFT. A TFT includes a semiconductor film  901  formed over an insulating substrate  903 , a gate insulating film  905  over the semiconductor film  901 , and a gate electrode  902  over the gate insulating film  905 . The semiconductor film  901  has high concentration impurity regions  904  to which two wirings  906  are connected respectively. The memory cell in the memory device of the invention may have a writing defect  907  as shown in  FIGS. 9A and 9B . When a voltage is applied between a gate electrode and a semiconductor film, three terminals of a TFT are generally insulated from each other; however, in a defective element, the semiconductor film  901  and the gate insulating film  905  function as a resistor and the three terminals are conducted to each other. 
     As a measure for preventing such a defective element, it is suggested that a TFT adopt a double gate structure having two gate electrodes as shown in  FIGS. 9C and 9D . A defective element is generated at random since it is supposedly caused by dust in a semiconductor film or an insulating film. Even when the defect  907  occurs in one channel region of the two gate electrodes as shown in  FIG. 9E , for example, the two wirings  906  are insulated if the other channel region becomes an insulating region  908 , therefore, the TFT can be used as a normal memory cell. Specifically, according to the present data, a defective element is detected about 3% of the time. Since the defective element is generated at random, if a double gate structure is adopted, the generation can be probabilistically reduced to 0.1% or less. 
     By applying the aforementioned measure, a TFT may have a multi-gate structure to reduce a defective element. The multi-gate here means two or more gate electrodes in a TFT. The generation of defective elements can be reduced with the increase in the number of gate electrodes in a TFT. The number of gate electrodes is desirably optimized taking into consideration current consumption or applied voltage during the use of a memory, the area of a memory cell array, and the like. 
     A defective element is still generated with the increase in memory capacity. In such a case, a redundant circuit can be additionally provided similarly to a memory device manufactured today. Alternatively, by controlling an external circuit, access to the defective element can be prohibited as a flash memory. 
     EXPLANATION OF REFERENCE 
       22 : memory cell  24 : memory cell  31 : word line  32 : bit line  33 : selection control line  34 : resistor element  35 : selection transistor  42 : memory cell  44 : memory cell  100 : TFT  101 : insulating substrate  102 : semiconductor film  103 : high concentration impurity region  104 : channel region  105 : gate insulating film  106 : gate electrode  107 : TFT  108 : insulating region  201 : word line  202 : bit line  203 : TFT  204 : source line  206 : TFT  207 : TFT  208 : TFT  209 : TFT  301 : semiconductor film  302 : electrode  303 : insulating substrate  304 : high concentration impurity region  305 : insulating film  306 : wiring  307 : void  500 : substrate  501 : peeling layer  502 : protective film  503 : island shape semiconductor film  504 : gate insulating film  505 : gate electrode  506 : resist  507 : P-channel TFT  508 : N-channel TFT  509 : resist  510 : impurity element  511 : low concentration impurity region  512 : resist  513 : impurity element  514 : high concentration impurity region  601 : insulating film  602 : resist  603 : side wall  604 : resist  605 : impurity element  606 : high concentration impurity region  607 : interlayer film  608 : protective film  609 : wiring  701 : insulating film  702 : insulating film  801 : IC tag  802 : RF circuit  803 : power supply circuit  804 : command control circuit  805 : clock  806 : congestion control circuit  807 : memory control circuit  808 : memory  809 : antenna  810 : electronic appliance  811 : memory  812 : CPU  813 : I/O controller  814 : external device  901 : semiconductor film  902 : gate electrode  903 : insulating substrate  904 : high concentration impurity region  905 : gate insulating film  906 : wiring  907 : defect  908 : insulating region