Patent Publication Number: US-7211867-B2

Title: Thin film memory, array, and operation method and manufacture method therefor

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a semiconductor memory and an integrated circuit built therefrom, and more specifically, to a technique which uses for a channel formation region a semiconductor thin film such as SOI (Semiconductor On Insulator) or SON (Semiconductor On Nothing). The semiconductor thin film is formed on an insulating substrate (SOI) in some cases, is suspended and held at both ends by substrates in a hollow state (SON) in some other cases, and has a projecting portion which is connected at one end to a substrate in still other cases. 
     2. Description of the Related Art 
     H. J. Wann et al. have proposed in 1993 to obtain a dynamic memory that does not use a capacitor by incorporating two complementary transistors in a partially depleted SOIMOS transistor structure (See Non-patent Document 1, for example). 
     Recently, a memory has been proposed in which carriers are generated utilizing a carrier multiplication phenomenon such as avalanche breakdown in a drain high electric field region of a partially depleted SOIMOS transistor and the obtained carriers are used to charge a neutral body, resulting in a change in current flowing between the drain and the source of the transistor (See Non-patent Document 2, for example). 
     The term partially depleted SOI, abbreviated as PD SOI, refers to SOI in which a depletion layer spreads only partially in the depth direction of its semiconductor thin film to give it a neutral region. ‘Body’ is a simplified term for the above semiconductor thin film in which a channel is formed. 
     [Non-Patent Document 1] 
     H. J. Wann, C. Hu, “A capacitor-less DRAM cell on SOI substrate”, 1993, IEDM (International Electron Device Meeting) Technical Digest, pp. 635–638 
     [Non-Patent Document 2] 
     S. Okhonin et al., “A Capacitor-less 1T-DRAM Cell”, IEEE Electron Device letters, Volume 23, Number 2, pp. 85–87, February 2002 
     On the other hand, fully depleted (FD) SOI is used for low power consumption uses or for advanced miniaturization of SOIMOS transistor, thereby creating the need for SOI memory cells that can be applied to FDSOI. The term FD (fully depleted) SOI refers to SOI having such thickness and impurity concentration that makes the depletion layer cover the entire depth of the semiconductor thin film under some gate bias of a transistor fabricated there. 
     The method of utilizing carrier multiplication in a drain high electric field portion also causes carrier multiplication of a small degree in a not-selected cell which is connected to a bit line for driving at high voltage a drain of a cell to which a signal is to be written. This leads to erroneous, albeit mild, writing called ‘write disturb’ and therefore makes it difficult to assemble a large array in which a large number of cells are connected to each bit line. Furthermore, the method also needs a relatively large current, which prevents parallel programming of a large number of the cells. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in view of the above, and an object of the present invention is therefore to provide a capacitor-less SOI or other semiconductor thin film memory cell and memory cell array which are applicable to FDSOI. Another object of the present invention is to provide an SOI or other semiconductor thin film memory cell and memory cell array in which data is written or erased without using carrier multiplication in a drain high electric field portion, as well as an operation method and manufacture method for the memory cell and array. 
     To attain the above objects, the present invention employs a method of supplying carriers from a third semiconductor region, which is not a drain nor a source, (1) to a body (2) without using carrier multiplication in a drain high electric field portion. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings: 
         FIG. 1  is a sectional view showing a principle of the present invention; 
         FIGS. 2A and 2B  are a plan view showing an embodiment of the present invention and a sectional view thereof, respectively; 
         FIG. 3  is a plan view showing another embodiment of the present invention in which a first conductive gate and a second conductive gate are continuous; 
         FIGS. 4A and 4B  are a plan view and a sectional view, respectively, showing another embodiment of the present invention in which a third conductive gate is placed on a second principal surface of a semiconductor thin film; 
         FIGS. 5A and 5B  are a plan view of an embodiment in which memory cells of the present invention are arranged and connected to form an array structure and a sectional view of a cell portion, respectively; 
         FIGS. 6A to 6G  are sectional views showing an example of a process of manufacturing the memory cells and array of the embodiment shown in  FIGS. 5A and 5B ; 
         FIG. 7  is an equivalent circuit diagram of the memory cell array shown in  FIGS. 5A and 5B ; 
         FIG. 8  is a plan view of memory cells and array in which a writing bit line and a reading bit line are shared; and 
         FIG. 9  is an equivalent circuit diagram of the memory cell array of  FIG. 8 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiment Mode 
     A memory cell of the present invention is shown in  FIG. 1  and  FIGS. 2 and 2B .  FIG. 1  is an example of a sectional view thereof.  FIG. 2A  is an example of a plan view thereof and  FIG. 2B  is a sectional view taken along the line X–X′ of  FIG. 2A . As shown in these drawings, the memory cell includes: 
     a semiconductor thin film  100  having a first principal surface  101  and a second principal surface  102  that faces the first principal surface; 
     a first gate insulating film  210  formed on the first principal surface of the semiconductor thin film; 
     a first conductive gate  310  formed on the first gate insulating film; 
     a first semiconductor region  110  and a second semiconductor region  120  which are spaced apart from each other across the first conductive gate, which are insulated from the first conductive gate, which are in contact with the semiconductor thin film  100 , and which have a first conductivity type; and 
     a third semiconductor region  130  which has the opposite conductivity type and which is in contact with the semiconductor thin film. 
     The semiconductor thin film  100  has such a combination of thickness and impurity concentration relation that there exists an electric potential of the first conductive gate which causes depletion of carriers between the first principal surface  101  and the second principal surface  102  between the first and second semiconductor regions below the first conductive gate. 
     In the memory cell provided by the present invention, the semiconductor thin film is extended to the third semiconductor region  130  from a semiconductor thin film portion which is sandwiched between the first semiconductor region  110  and the second semiconductor region  120  and which is denoted by  103 , and on the extended portion of the semiconductor thin film which is denoted by  104 , a second gate insulating film  220  is formed and a second conductive gate  320  is formed on the second gate insulating film  220 . 
     In  FIGS. 2A and 2B , reference symbol  421  represents an inter-gate insulating film which is provided, if necessary, to insulate the first conductive gate and the second conductive gate which overlap each other. Denoted by  400  is a so-called field insulating film.  413  and  431  represent an insulating film formed on the third semiconductor region and an insulating film on the first conductive gate, respectively.  113 ,  123 ,  133 ,  313 , and  323  are provided, if necessary, to serve as contacts leading to the first second, and third semiconductor regions and contacts leading to the first and the second conductive gates.  FIG. 1  corresponds to a sectional view taken along the line Y–Y′ in  FIG. 2A . It is not always necessary for each cell to have the above contacts. In particular, a contact leading to a conductive gate can be shared among a large number of cells since a conductive gate often constitutes a part of a word line. 
     A first conductivity type channel is induced in the semiconductor thin film portion  103  that is sandwiched between the first semiconductor region  110  and the second semiconductor region  120  by an electric potential exceeding the gate threshold voltage of the first conductive gate. In the present invention, the semiconductor thin film portion  103  is called a first channel formation semiconductor thin film portion. 
     In the semiconductor thin film extended portion  104 , carriers of the opposite conductivity type are induced, or a channel for carriers of the opposite conductivity type is formed by an electric potential difference between the second conductive gate and the third semiconductor region. The extended portion  104  is referred to in the present invention as a second channel formation semiconductor thin film portion and is disposed in direct contact with the first channel formation semiconductor thin film portion. A portion  105  which is different in conductivity type or impurity concentration from the other extended portion  104  may be formed in the extended portion  104  in order to adjust the gate threshold voltage of the opposite conductivity type carrier channel which is viewed from the second conductive gate. In the present invention, the above expression ‘electric potential exceeding the gate threshold voltage’ means an electric potential whose absolute value is larger than the gate threshold voltage in the positive direction if the transistor is an n-channel transistor and an electric potential whose absolute value is larger than the gate threshold voltage in the negative direction if the transistor is a p-channel transistor. 
     The distance between the first principal surface and the second principal surface is called in the present invention as the thickness of the semiconductor thin film. 
     By a first combination of an electric potential of the second conductive gate and an electric potential of the third semiconductor region, carriers  2  of the opposite conductivity type are injected from the third semiconductor region through the second channel formation semiconductor thin film portion into the first channel formation semiconductor thin film portion to change into a first value Vth 11  the gate threshold voltage of the first conductivity type channel in the first channel formation semiconductor thin film portion which is viewed from the first conductive gate into a first value Vth 11 . This operation is called ‘writing’ in the present invention. 
     According to the first combination of the electric potentials, a value obtained by subtracting the electric potential of the third semiconductor region from the electric potential of the second conductive gate exceeds a gate threshold voltage Vth 2r  of the channel that is induced in the second channel formation semiconductor thin film portion to deliver the opposite conductivity type carriers from the third semiconductor region. Vth 2r  is the gate threshold voltage viewed from the second conductive gate. 
     With carriers of the opposite conductivity type injected to the first channel formation semiconductor thin film portion, a gate voltage necessary for the first conductive gate to induce the first conductivity channel is reduced by a level corresponding to the number of or the electric charges of the injected carriers of the opposite conductivity type. This means that the gate threshold voltage has shifted toward the depletion side equivalently. If the gate threshold voltage changes in an enhancement type range, it means that the absolute value of the gate threshold voltage is reduced. 
     The first electric potential combination allows multilevel setting. For example, on the premise that a value obtained by subtracting the electric potential of the third semiconductor region from the electric potential of the second conductive gate sufficiently exceeds a gate threshold voltage Vth 2r , which is viewed from the second conductive gate, of the channel that is induced in the second channel formation semiconductor thin film portion to deliver the opposite conductivity type carriers from the third semiconductor region, the electric potential of the third semiconductor region with respect to the gate electric potential is set to multilevel. This makes it possible to change the gate threshold voltage of the first conductive channel which is viewed from the first conductive gate into multilevel values Vth 11 , Vth 12 , Vth 13  . . . for writing. In short, this makes it possible to store plural bit information in one cell. 
     The opposite conductivity type carriers  2  injected into the first channel formation semiconductor thin film portion gradually diminish because of recombination with carriers of the first conductivity type or efflux from the first channel formation semiconductor thin film portion due to self-field. Accordingly, it is necessary to read the amount of opposite conductivity carriers accumulated in the first channel formation semiconductor thin film portion and re-write based on the readout. This is called ‘refreshing’. 
     By a second combination of an electric potential of the second conductive gate and an electric potential of the third semiconductor region, the carriers  2  of the opposite conductivity type are drawn into the third semiconductor region from the first channel formation semiconductor thin film portion to change the gate threshold voltage of the first conductivity type channel in the first channel formation semiconductor thin film portion which is viewed from the first conductive gate into a second value Vth 10 . This operation is called ‘erasing’ in the present invention. 
     According to the second combination of the electric potential relation, a value obtained by subtracting the electric potential of the opposite conductivity type carriers injected into the first channel formation semiconductor thin film portion from the electric potential of the second conductive gate exceeds the gate threshold voltage Vth 2r  of the opposite conductivity type channel in the second channel formation semiconductor thin film portion which is viewed from the second conductive gate. 
     Alternatively, the erasing operation is achieved by giving an electric potential of a direction that attracts carriers of the opposite conductivity type to the first or second semiconductor region (or instance, 0.6 V or higher in the negative direction for holes). In this case, carries of the first conductivity type are also supplied to the first channel formation semiconductor thin film portion to accelerate a decrease of carriers of the opposite conductivity type through recombination. In this erasing operation, data is erased from every cell whose second semiconductor region or first semiconductor region is connected to a common line or bit line. 
     Information stored in a memory cell of the present invention is judged by whether or not carriers of the opposite conductivity type are stored in the first channel formation semiconductor thin film portion of the memory cell or from the stored amount. To judge stored information in this way, the voltage of the first conductive gate with respect to the second semiconductor region is set to a prescribed value that exceeds one or both of the first gate threshold voltage and the second gate threshold voltage and whether a current flowing between the first semiconductor region and the second semiconductor region is large or small is detected (‘small’ including zero). For instance, the voltage of the first conductive gate with respect to the second semiconductor region is set to a level between the first gate threshold voltage and the second gate threshold voltage and whether or not a current flows between the first semiconductor region and the second semiconductor region is detected to judge the stored information. 
     In the case where multilevel of first gate threshold voltages are written, the voltage of the first conductive gate is set to a level between any two out of those levels to identify the stored data. Alternatively, the voltage of the first conductive gate with respect to the second semiconductor region is set to a voltage that exceeds both the first gate threshold voltage and the second gate threshold voltage and the stored information is judged from the amount of current flowing between the first semiconductor region and the second semiconductor region. 
     For detection of the current, a known method such as comparative detection using a reference current and a comparator circuit, or detection by time constant of charging or discharging a bit line or a bit line with additional capacitance can be employed. This operation is called ‘reading’. 
     Through the reading operation, the electric potential of the valence band or conduction band in the energy band of the first channel formation semiconductor thin film portion is moved in a direction that eliminates carriers of the opposite conductivity type. In addition, a large amount of first conductivity type carriers are supplied to the first channel formation semiconductor thin film portion to accelerate recombination of the opposite conductivity type carriers stored in the first channel formation semiconductor thin film portion and to cause information loss in some cases. In this case, the refreshing operation has to be conducted immediately after the reading. 
     The semiconductor thin film  100  in  FIG. 1  is supported by a substrate  10  having an insulating layer  20  formed on its surface. In most cases, the substrate  10  is formed of silicon and the insulating layer  20  is a silicon oxide film. The supporting substrate that has an insulating layer on its surface is called an insulating substrate. An insulating substrate that is entirely formed of an insulating material, such as a quartz substrate, can also serve as the supporting substrate. An alternative structure is that at least one end of the semiconductor thin film, or an end of the first semiconductor region, the second semiconductor region, or the third semiconductor region, is supported by a substrate. 
     In the present invention, if voltages of the first conductive gate and the second conductive gate during the writing, erasing, and reading operations are chosen carefully, a common voltage for both the first and second conductive gate can be used for each operation mode. Then the first and second conductive gates can be made continuous or shared as  FIG. 3  shows its example. Furthermore, the same material and thickness can be employed for the gate insulating films. As a result, the number of manufacture steps and the area the cell occupies are reduced. In this case, by replacing the ‘second conductive gate’ in the description on the writing and erasing operations of the present invention with the ‘first conductive gate’, it becomes possible to realize the writing and erasing operations. 
     It is also possible in the present invention to write the first gate threshold voltage value under a certain condition and the second gate threshold voltage value under another condition. For example, the first gate threshold voltage is written when a value obtained by subtracting the electric potential of the third semiconductor region from the electric potential of the second conductive gate sufficiently exceeds a gate threshold voltage Vth 2r  of the channel that is induced in the second channel formation semiconductor thin film portion to deliver the opposite conductivity type carriers from the third semiconductor region which is viewed from the second conductive gate while the electric potential of the third semiconductor region is biased forward with respect to the electric potential of the second semiconductor region On the other hand, the second gate threshold voltage is written (equals to erasing) when the electric potential of the third semiconductor region is zero-biased or biased backward with respect to the second gate voltage. 
     Another mode of a preferred memory cell for effectively carrying out the present invention is a memory cell shown in  FIGS. 4A and 4B . The memory cell is characterized by including at least: 
     a semiconductor thin film (broken into portions  103  and  104 ) having a first principal surface  101  and a second principal surface  102  that faces the first principal surface; 
     a first gate insulating film  210  formed on the first principal surface of the semiconductor thin film; 
     a first conductive gate  310  formed on the first gate insulating film; 
     a first semiconductor region  110  and a second semiconductor region  120  which are spaced apart each other across the first conductive gate, which are insulated from the first conductive gate, which are in contact with the semiconductor thin film, and which have a first conductivity type; 
     a third semiconductor region  130  which has the opposite conductivity type and which is in contact with the semiconductor thin film; 
     a third gate insulating film  230  formed on the second principal surface of the semiconductor thin film portion (first channel formation semiconductor thin film portion)  103  that is sandwiched between the first semiconductor region and the second semiconductor region; and 
     a third conductive gate  330  which is in contact with the third gate insulating film  230 . The semiconductor thin film portion  104  is also called as a second channel formation semiconductor thin film portion in the present invention. 
     Carriers of the opposite conductivity type are stably stored in the first channel formation semiconductor thin film portion if the third conductive gate is given an electric potential exceeding a gate threshold voltage Vth 2r , which is the gate threshold voltage viewed from the third conductive gate with respect to opposite conductivity type carriers induced in the first channel formation semiconductor thin film portion. However, the refreshing operation is necessary in this case too, for carriers of the opposite conductivity type are gradually generated and stored in the first channel formation semiconductor thin film portion by thermal excitation, slight carrier multiplication in normal electric field, and the like after the erasing operation. 
       FIG. 4A  is a plan view of the thin film memory cell of the above embodiment and  FIG. 4B  is a sectional view taken along the dot-dash line X–X′ of the plan view of  FIG. 4A . In  FIGS. 4A and 4B , reference symbol  10  denotes a supporting substrate and  20 , an insulating film on a surface of the supporting substrate  10 . Denoted by  103  and  104  are the first and second channel formation semiconductor thin film portions, respectively, which are a part of the semiconductor thin film  100 .  210  and  220  represent gate insulating films formed on the semiconductor thin film portions  103  and  104 . The gate insulating films  210  and  220  in the drawing are continuous. Denoted by  310  is the first conductive gate, which is also continuous from the second conductive gate.  110  and  120  are the first and second semiconductor regions, respectively.  130  denotes the third semiconductor region. 
       113  and  123  represent wiring contacts leading to the first and second semiconductor regions, respectively.  133  represents a wiring contact leading to the third semiconductor region. Denoted by  400  is a so-called field insulating film which is placed under an interconnection film or the like.  431  denotes an insulating film placed on the first conductive gate, and  410 , an insulating film placed between the semiconductor thin film  100  and the insulating film  20 .  313  denotes a wiring contact leading to the first conductive gate.  333  denotes a wiring contact provided, if necessary, to lead to the third conductive gate. 
     It is not always necessary for each cell to have the above contacts. In particular, a contact leading to a conductive gate can be shared among a large number of cells since a conductive gate often constitutes a part of a word line. An impurity region  105  is not always necessary if the electric field of the third conductive gate influences the portion  104  less than the portion  103  (in other words, if the third conductive gate does not overlap the portion  130  as shown in  FIG. 4B  or if the third conductive gate overlaps the portion  130  while an insulating film thicker than the film  230  is sandwiched between the two). 
     In the embodiment modes described above, the first and second conductive gates can have different gate threshold voltages if the conductivity type of an impurity or impurity concentration of the second channel formation semiconductor thin film portion, or the second conductive gate material is different from the conductivity type of an impurity or impurity concentration of the first channel formation semiconductor the film portion, or the first conductive gate material. Opposite conductivity type carriers injected to the second channel formation semiconductor thin film portion is prevented from flowing back to the third semiconductor region if the gate threshold voltage of the second conductive gate with respect to the channel for opposite conductivity type carriers from the third semiconductor region is set to a level further in the enhancement type direction than the gate threshold voltage of the first conductive gate. 
     Embodiments 
     Described below is the memory cell operation of when the first conductivity type is the n type and the opposite conductivity type is the p type. The principles and effects given in the following description also apply to the case where the first conductivity type is the p type, although polarity and the direction of changes is reversed.  FIG. 5A  is a plan view of an embodiment of memory cells of the present invention and an array of the memory cells.  FIG. 5B  is a sectional view taken along the dot-dash line X–X′ of the plan view of  FIG. 5A . 
     Reference symbol  10  denotes a supporting substrate, which, in this example, is an n type silicon &lt;100&gt; plane wafer of high resistance. Denoted by 20 is a silicon oxide film with a thickness of about 100 nm.  103  represents a semiconductor thin film about 30 nm in thickness which serves as a first channel formation semiconductor thin film portion of a thin film memory cell  1000  of this embodiment.  104  represents a second channel formation semiconductor thin film portion.  105  represents a high impurity concentration portion in the second channel formation semiconductor thin film portion.  110  represents a drain (first semiconductor region).  114  is a drain extension.  120  is a source (second semiconductor region).  124  represents a source extension.  130  is a third semiconductor region of the opposite conductivity type.  210  is a first gate nitrided oxide film with a thickness of 2.7 nm.  220  is a second gate nitrided oxide film.  310  and  320  are a first conductive gate and a second conductive gate that is continuous from the first conductive gate. ( 300  is a symbol as a conductive gate thin film.  1001  is a functional symbol as a local word line.) 
       210  and  220  are continuous gates. The first conductive gate is about 100 nm in length and, in this embodiment, is formed from a silicon thin film doped with boron. The first, second, and third semiconductor regions include a semiconductor film that is formed by epitaxial growth on the semiconductor thin film. The first and second channel formation semiconductor thin film portions  103  and  104  in one cell are separated from the first and second channel formation semiconductor thin film portions  103  and  104  in an adjacent cell by a separation insulating film  401 . 
     Denoted by  113  is a contact leading to the first semiconductor region, and the contact is connected to a reading bit line  1005 .  113  represents a contact leading to the third semiconductor region and the contact is connected to a writing bit line  1004 . The continuous first and second conductive electrodes  310  ( 320 ) are continuously extended between cells in the word direction, thereby forming a local word line  1001 . The second semiconductor region is extended between cells in the word direction, thereby forming a local common line  1003 . The local word line and the local common line are extended as long as the direct resistance does not affect the array operation, and are respectively connected to a global word line and a global common line through a selection transistor or directly. In a large capacity array, the above two types of bit lines are also connected to their respective global bit lines through a selection transistor. 
     In the array arrangement of  FIG. 5A , cells  1000  are repeatedly arranged with a mirror image relation in the bit direction. As a result, the contacts  113  and  133  are shared between cells adjacent to each other in the bit direction. The first and third semiconductor regions are continued from a cell to its adjacent cell in one way of the bit direction whereas the second semiconductor region is continued from a cell to its adjacent cell in the other way in the bit direction. The array area is thus reduced.  FIG. 5A  shows two cells in the word direction and four cells in the bit direction, eight cells in total (a cell  1000  (j, k), . . . , a cell  1000  (j+1, k+3)). The mirror image arrangement of cells is also employed in an embodiment of  FIG. 8  which will be described later. 
     A manufacture process of this embodiment is described below with reference to sectional views of  FIGS. 6A to 6G  and  FIG. 5B . 
     (a) A high resistivity silicon wafer is used as a supporting substrate  10  and a silicon oxide film  20  with a thickness of about 100 nm and a silicon thin film  100  which has an n type impurity concentration of about 2×10 17  atoms/cc and which has a thickness of about 35 nm are layered on the substrate to prepare an SOI substrate. 
     (b) On the thus obtained SOI, an oxide film  41  is let grow until it reaches a thickness of about 7 nm by thermal oxidation and a silicon nitride film  42  with a thickness of about 50 nm is formed thereon by CVD. Thereafter, a photoresist pattern  51  is formed by known photolithography to protect necessary portions of the silicon thin film such as regions where memory cells are connected in the word direction and the bit direction, selection transistor regions, and peripheral circuit regions. 
     (c) Using the photoresist pattern  51  as a mask, the silicon nitride film is etched under etching conditions that provide a selective ratio with respect to the silicon oxide film to leave the silicon oxide film. The photoresist pattern is then removed and the substrate surface is cleaned. The exposed surface of the silicon oxide film which is exposed by the removal of the silicon nitride film is oxidized by pyrogenic oxidation until a silicon oxide film  401  grows to a thickness of about 60 nm. Through this step, the silicon thin film  100  is divided into pieces leaving the necessary portions given in the above, 
     Alternatively, a known STI (shallow trench isolation) technique may be used to divide the silicon thin film. An insulating film for dividing the silicon thin film in the planar direction is called an insulating separation film  401 . 
     The silicon nitride film  42  is removed by a hot phosphoric acid-based etchant and the silicon oxide film  41  is removed by a buffer hydrofluoric acid-based etchant to expose the surface of the silicon thin film  100 . 
     A silicon oxide film  200  is formed on the surface of the silicon win film  100  by thermal oxidation to a thickness of 2.7 nm. Thereafter, ECR (Electron Cyclotron Resonance), ICP (Inductively Coupled Plasma), or like other high density plasma apparatus is used for surface nitridation at a nitridation ratio of 5 to 7% by introducing nitrogen radical from plasma of nitrogen gas, hydrogen gas, or xenon gas to the substrate surface and setting the substrate temperature to 400° C. Then the substrate is transferred in a highly pure nitrogen gas atmosphere and subjected to heat treatment at 800° C. in nitrogen to anneal surface defects. The silicon oxide film thus nitrided is used as the first and second gate oxide films. 
     (d) Next, a conductive gate thin film  300  is formed by deposition. For the first 10 nm or so, pure silicon is deposited to form a pure silicon thin film  301 . Then a boron-doped silicon thin film  302  is formed by deposition to a thickness of 200 nm. The material gases used are mono-silane (SiH 4 ) and di-borane (B 2 H 6 ). A silicon nitride film  43  is formed thereon by deposition to a thickness of about 100 nm. Ion implantation may be employed as an alternative method for the above boron doping. 
     Using a known technique such as ArF lithography or electron beam lithography, a gate-shaped photoresist pattern for a conductive gate/local word line having a gate length of about 100 nm is formed on the above silicon nitride film/conductive gate thin film. The photoresist pattern is used as a mask to etch the silicon nitride film and the conductive gate thin film in order by a RIE technique. 
     A photoresist film shaped by photolithography and the silicon nitride film/conductive gate thin film are used as selection masks to selectively form, by ion implantation at low acceleration voltage (about 15 KeV for arsenic), an extension region ( 114 ) of the n type drain (the first semiconductor region) and an extension region ( 124 ) of the source (the second semiconductor region). In the implantation, the dose is set so as to obtain an impurity concentration of about 1×10 19  atoms/cc (which is about 3×10 13  atoms/cm 2 ). 
     Similarly, a photoresist film shaped by photolithography and the silicon nitride film/conductive gate thin film are used as selection masks to selectively implant arsenic in the portion that forms the third semiconductor region at a dose of about 8.5×10 12  atoms/cc. A high impurity concentration region  105  is thus formed in the second channel formation semiconductor thin film portion so that it is in contact with the third semiconductor region formed in the subsequent step. This shift the gate threshold voltage Vth 2r  of the channel for holes from, the third semiconductor region which is viewed from the second conductive gate to the enhancement side. 
     (e) Using a known gate side wall insulating film process, insulating film side walls  403  each having a thickness of about 30 nm are formed on the side faces of the first and second conductive gates. The side walls are a two-layer laminate consisting of a silicon nitride film  404  with a thickness of about 7 nm and a silicon oxide film  405  with a thickness of about 23 nm. At this stage, the silicon nitride film  404  is left on the semiconductor thin film. 
     Lithography is used to form a photoresist pattern having an opening in the portion where the third semiconductor region is to be formed. A portion of the silicon nitride film  404  under the opening is etched by RIE. Then the photoresist is removed and a residual silicon oxide film in the opening is subjected to wet etching, followed by hydrogen termination. 
     A boron-doped silicon crystal film  135  is selectively grown in the opening until it reaches a thickness of about 100 nm. The boron concentration is about 4×10 19  atoms/cc. Through thermal oxidation at 850° C., an oxide film  406  with a thickness of about 30 nm is let grow on the top and side faces of the p type silicon crystal film. In actuality, boron is diffused at this point from the silicon crystal film  135  into the semiconductor thin film  100  to give a portion of the semiconductor thin film that is under  135  the p type conductivity. In  FIG. 6E , the portion is distinguished from the silicon crystal film selectively grown. 
     (f) The silicon nitride film  404  left on the memory cell portions of the semiconductor thin film  100  is etched by RIE. The silicon oxide film remaining on the etching surface is subjected to wet etching and then hydrogen termination is conducted. In etching the silicon nitride film, a portion of the silicon nitride film is excluded which is under the oxide film  406  on the side face of the crystal thin film that has been selectively grown to form the semiconductor region  130 . 
     Arsenic-doped silicon crystal films  115  and  125  are selectively grown in the opening to a thickness of about 100 nm each. The arsenic concentration is about 5×10 20  atoms/cc. The oxide film  406  on the side face separates the p type high impurity concentration silicon crystal film  135  from the n type high impurity concentration silicon crystal films  115  and  125 . 
     Instead of the above selective crystal growth, selective ion implantation using as masks a photoresist pattern and the conductive gate thin film and the silicon nitride film thereon may be employed to form the first, second, and third semiconductor regions. 
     During the crystal growth and subsequent heating step, the impurities of these silicon crystal films obtained through the selective crystal growth are diffused into the semiconductor thin film  100  starting from points where the crystal films and the semiconductor thin film meet. As a result, the third semiconductor region  130 , the first semiconductor region  110 , and the second semiconductor region  120  are formed at the same time the silicon crystal thin films are formed by selective crystal growth. 
     (h) The silicon nitride film  43  on the conductive gate thin film ( 300 ) is subjected to wet etching by hot phosphoric acid or the like. The surface is then rinsed, nickel is deposited by evaporation to a thickness of about 20 nm, and next follows sintering. The nickel on the insulating film that has not reacted is etched by an acid to leave a nickel silicide layer. Through high temperature sintering, a silicide layer  110   s  is formed on the first semiconductor region (drain), a silicide layer  120   s  is formed on the second semiconductor region (source), and a silicide layer  300   s  is formed on the gate thin film. 
     A silicon oxide film is formed as a interlayer insulating film  440  for interconnection by CVD on the surface. Contact holes are opened in the film as needed and contact plugs  133  and  113  are formed from titanium nitride, tungsten, or the like. Then a TiN thin film and a tungsten thin film are formed by evaporation. A wiring pattern is formed by photolithography and RIE (reactive ion etching) to obtain a local writing bit line  1004  and a local reading bit line  1005  (at this point, the state of  FIG. 5B  is reached). Thereafter, an additional interlayer insulating film and a multilayer interconnection composed of an Al film, a copper film or the like are formed as needed and, lastly, a passivation film is formed. 
     The features of this embodiment are (1) that the third semiconductor region and the first semiconductor region are insulated by the insulating film  406  formed on the side face of the crystal thin film that is obtained by selective epitaxial growth and (2) that the gate threshold voltage for inducing carriers of the opposite type differs between in the first channel formation semiconductor thin film portion [to] and in the opposite conductivity type carrier channel which leads to the first channel formation semiconductor thin film portion from the third semiconductor region. 
     Since the opposite conductivity type carrier channel which leads to the first channel formation semiconductor thin film portion from the third semiconductor region crosses the high impurity concentration region  105  in contact with the third semiconductor region, the impurity concentration of the second channel formation thin film portion differs from that of the first channel formation thin film portion. Thus, the gate threshold voltage which induces the carriers of the opposite conductivity type differs between in the first channel formation thin film portion and in the opposite conductivity type carrier channel which leads to the first channel formation semiconductor thin film portion from the third semiconductor region. 
     If a barrier against carriers of the opposite conductivity type is formed between the first channel formation semiconductor thin film portion and the third semiconductor region as described above, it reduces the amount of opposite conductivity type carriers pushed back to the third semiconductor region upon reading where a voltage in the direction that induces carriers of the first conductivity type is applied to the first conductive gate. Therefore reading can be carried out without fear of erasing the stored information. 
     In the embodiment illustrated in  FIGS. 5A and 5B , the first semiconductor regions of cells arranged in the column direction are connected to the reading bit line  1005  and the third semiconductor regions of these cells are connected to the writing bit line  1004 . The first and second common conductive gates of cells arranged in the row direction are connected to the word line  1001 . The second semiconductor regions of the cells arranged in the row direction are connected to the common line  1003 . The reading bit line and the writing bit line extend in the column direction whereas the word line and the common line extends in the row direction. Arrangement of cells and the vertical and horizontal relation of the bit line and the word line can be interchanged without causing any problem. 
     A description is given below on the operation of a single cell that is manufactured in accordance with the above embodiment. Data is written in this cell by setting the electric potential of the second conductive gate with respect to the electric potential of the third semiconductor region to a level that exceeds the threshold voltage Vth r2  of the opposite conductivity type carrier channel below the second conductive gate. 
     In the cell manufactured by the above manufacture process, Vth r2  is about −0.5 V when the electric potential of the second semiconductor region is 0 V and therefore it is desirable to set the electric potential of the third semiconductor region to 0.2 to 0.3 V while setting the second conductive gate to −0.3 to −0.4 V. To keep data, setting the first conductive gate to 0 to 0.2 V and giving the first semiconductor region the same electric potential as the second semiconductor region are desirable. 
     To erase data, the second semiconductor region is set to −0.6 V or lower (when the electric potential of the first semiconductor region is 0 V and the first conductive gate is 0 V), or the electric potential of the third semiconductor region is set to 0 to −0.4 V and the electric potential of the second conductive gate is set to −0.55 V or lower. In this way, opposite conductivity type carriers holes) stored in the first channel formation semiconductor thin film portion are drawn into the second semiconductor region or into the third semiconductor region. 
     To read data, whether the current flowing between the first and second semiconductor regions is large or small is detected by applying, to the first conductive gate, a voltage about the first conductivity type carrier gate threshold voltage Vth 10  (0.2 V larger at most) of the first conductive gate in the cell whose data has been erased. In the case of multi-valued storing, a voltage between Vth 10  and Vth 11 , a voltage between Vth 11  and Vth 12 , a voltage between Vth 12  and Vth 13  . . . are applied to the first conductive gate to detect stored information. A voltage applied between the first and second semiconductor regions is from 0.2 V to 0.9 V. The intermediate current value between the current of a cell to which data is written and the current of a cell from which data is erased is taken to serve as a reference value. In the case where a voltage between one stored threshold voltage and another stored threshold voltage is applied to the first conductive gate, information is judged from the presence or absence of the cell current. 
     In order to prevent writing error (erasing and reading error also) caused by opposite polarity carriers generated in the high electric field region of the first channel formation semiconductor thin film portion, it is safer to avoid applying a voltage equal to or larger than the energy gap value of the semiconductor thin film which is converted into voltage in the unit (1.1 V if the film is silicon) between the first and second semiconductor regions. 
     In Embodiment 1, cells of the present invention in  FIGS. 5A and 5B  are connected as shown in an equivalent circuit diagram of  FIG. 7  to obtain a memory array. The memory array is operated by combination of voltages shown in Table 1 below. This array is suitable as a memory for a specific use because data can be written in cells of a word while data is read from cells of another word. The array is also suited for high speed refreshing operation. Table 1 shows the voltage relation among the word line, the writing bit line, the reading bit line, and the common line when the array is operated by a single polarity power supply of 1.2 V. Operation on a single polarity power supply is made possible by biasing the common line at a positive electric potential, usually, 0.5 V. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Operation Voltage Example of a Memory Array according to Embodiment 1 of the Present Invention 
               
            
           
           
               
               
            
               
                   
                 Non-selected cell voltage (V) 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 Selected cell voltage (V) 
                 Writing 
                 Erasing 
                 Reading 
                 [Keeping] 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 Writing 
                 Erasing 
                 Reading 
                 Retention 
                 CW 
                 CB 
                 CW 
                 CB 
                 CW 
                 CB 
                 Retention 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Word line 
                 0.1 
                 0 
                 1.2 
                 0.5 
                 0.1 
                 0.5 
                 0 
                 0.5 
                 1.2 
                 0.5 
                 0.5 
               
               
                 Writing bit line 
                 0.7 
                 0 
                 0.5 
                 0.5 
                 0.5 
                 0.7 
                 0 
                 0 
                 0.5 
                 0.5 
                 0.5 
               
               
                 Reading bit line 
                 0.5 
                 0.5 
                 0.8 
                 0.5 
                 0.5 
                 0.5 
                 0.5 
                 0.5 
                 0.5 
                 0.8 
                 0.5 
               
               
                 Common line 
                 0.5 
                 0.5 
                 0.5 
                 0.5 
                 0.5 
                 0.5 
                 0.5 
                 0.5 
                 0.5 
                 0.5 
                 0.5 
               
               
                   
               
               
                 CW (common word): cells sharing a word line 
               
               
                 CB (common bit): cells sharing a bit line 
               
            
           
         
       
     
     In the operation shown in Table 1, data is erased from all cells that are connected to the same word line upon erasing. An acceptable change in voltage of one line is within ±0.1 V when the voltage of another line has the standard value. If the voltage of every line is changed in the same direction, the acceptable electric potential change is larger. 
     It is sufficient if the difference between the electric potential of each line and the electric potential of the common line satisfies the relation shown in Table 1. Accordingly, it may also be expressed as Table 2. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Relative Expression of Operation Voltage Example of a Memory 
               
               
                 Array according to Embodiment 1 of the Present Invention 
               
            
           
           
               
               
            
               
                   
                 Selected cell voltage (V) 
               
            
           
           
               
               
               
               
               
            
               
                   
                 Writing 
                 Erasing 
                 Reading 
                 Retention 
               
               
                   
               
               
                 Word 
                 Common 
                 Common 
                 Common 
                 Common 
               
               
                 line 
                 line electric 
                 line electric 
                 line electric 
                 line electric 
               
               
                   
                 potential −0.4 
                 potential −0.5 
                 potential +0.7 
                 potential 
               
               
                 Writing 
                 Common 
                 Common 
                 Common 
                 Common 
               
               
                 bit 
                 line electric 
                 line electric 
                 line electric 
                 line electric 
               
               
                 line 
                 potential +0.2 
                 potential 
                 potential 
                 potential 
               
               
                 Reading 
                 Common 
                 Common 
                 Common 
                 Common 
               
               
                 bit 
                 line electric 
                 line electric 
                 line electric 
                 line electric 
               
               
                 line 
                 potential 
                 potential 
                 potential +0.3 
                 potential 
               
               
                   
               
            
           
         
       
     
     According to the array structure for memory cells of the present invention, the third semiconductor region and the first semiconductor region are connected to the same bit line (one bit line doubles as a writing bit line and a reading bit line different from the case in the memory array of  FIGS. 5A and 5B ). 
     This array structure makes it possible to reduce the array area.  FIG. 8  is a plan view of memory cells used in this array connection and the cell area thereof is 6F 2  to 4F 2 . To achieve a cell area of 4F 2 , a self-alignment contact technique is needed. 
     The cell arrangement in the array of  FIG. 8  is similar to the one in  FIGS. 5A and 5B ; the conductive gates in one cell are above the first, second, and third semiconductor regions and this positional relation is reversed in every other cell in the longitudinal direction while the first, second, and third semiconductor regions in one cell are continuous with those in its adjacent cells in the column direction. For instance, the first and third semiconductor regions of the k-th cell in the column direction are continuous with those in the (K+1)-th cell. The second semiconductor region of the j-th cell is continuous with that of the (j+1)-th cell. The first semiconductor region in one cell and the first semiconductor region in a cell adjacent to the one cell in the word direction are electrically insulated from each other by the third semiconductor region of their adjacent cell when biased with a backward bias and a slight forward voltage. 
     The semiconductor thin film  100  extending in the word direction which includes the first semiconductor region and the third semiconductor region is physically continuous. On the other hand, in the example shown in  FIGS. 5A and 5B , the first semiconductor regions in adjacent cells in the word direction are separated by an insulating film. However, if the distance between the writing bit line and the reading bit line is narrowed by, for example, forming the two bit lines from different interconnection layers.  FIGS. 5A and 5B  can also take a structure in which the semiconductor thin film is continuous on the first semiconductor region side too and the adjacent first semiconductor regions are electrically insolated by the third semiconductor region. 
     In any case, the first channel formation semiconductor thin film portion or the second channel formation semiconductor thin film portion of one cell is separated from that of a cell adjacent to the one cell in the word direction. 
     In the plan view of  FIG. 8  also, the first conductive gate and the second conductive gate are continuous and are further continued to the first or second conductive gate of a cell adjacent in the row direction. The gate has a series resistance component and therefore limits the operation speed. To improve this, a metal wire is used as a main word line and is connected to the conductive gates of a group of cells for example, 32 to 512 cells form one group) before the series resistance reaches the limit value. 
     In Embodiment 2, cells of the present invention in  FIG. 8  are connected as shown in an equivalent circuit diagram of  FIG. 9  to obtain a memory array. The memory array of Embodiment 2 is operated by combination of voltages shown in Table 3 below. 
     Table 3 shows an example of the voltage relation among the word line, the bit line, and the common line when the array is operated by a single polarity power supply of 1 V. Operation on a single polarity power supply is made possible by biasing the common line at a positive electric potential, usually, 0.3 V. 
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 Operation Voltage Example of a Memory Array 
               
               
                 according to Embodiment 2 of the Present Invention 
               
            
           
           
               
               
            
               
                   
                 Non-selected cell voltage (V) 
               
            
           
           
               
               
               
               
               
            
               
                   
                 Selected cell voltage (V) 
                 Writing 
                 Writing 
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                 Writing 
                   
                   
                 (CW) 
                 (CB) 
                 Reading 
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 “1” 
                 “0” 
                 Reading 
                 Retention 
                 “1” 
                 “0” 
                 “1” 
                 “0” 
                 CW 
                 CB 
                 Retention 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Word line 
                 0 
                 0 
                 1.0 
                 0.3 
                 0 
                 0 
                 0.3 
                 0.3 
                 1.0 
                 0.3 
                 0.3 
               
               
                   
                   
                   
                 (in advance) 
               
               
                 Bit line 
                 0.6 
                 0 
                 0.5 
                 0.3 
                 0.3 
                 0.3 
                 0.6 
                 0 
                 0.3 
                 0.5 
                 0.3 
               
               
                 Common line 
                 0.3 
                 0.3 
                 0.3 
                 0.3 
                 0.3 
                 0.3 
                 0.3 
                 0.3 
                 0.3 
                 0.3 
                 0.3 
               
               
                   
               
               
                 CW (common word): cells sharing a word line 
               
               
                 CB (common bit): cells sharing a bit line 
               
            
           
         
       
     
     The word line voltage upon reading is supplied before the bit line voltage. 
     An acceptable change in voltage of one line is within ±0.1 V when the voltage of another line has the standard value. If the voltage of every line is changed in the same direction, the acceptable electric potential change is larger. 
     It is sufficient if the difference between the electric potential of each line and the electric potential of the common line satisfies the relation shown in Table 3. Accordingly, it may also be expressed as Table 4. 
     
       
         
           
               
             
               
                 TABLE 4 
               
             
            
               
                   
               
               
                 Relative Expression of Operation Voltage Example of a Memory 
               
               
                 Array according to Embodiment 2 of the Present Invention 
               
            
           
           
               
               
            
               
                   
                 Selected cell voltage (V) 
               
            
           
           
               
               
               
            
               
                   
                 Writing 
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 “1” 
                 “0” 
                 Reading 
                 Retention 
               
               
                   
               
               
                 Word 
                 Common 
                 Common 
                 Common 
                 Common 
               
               
                 line 
                 line electric 
                 line electric 
                 line electric 
                 line electric 
               
               
                   
                 potential −0.3 
                 potential −0.3 
                 potential +0.7 
                 potential 
               
               
                   
                   
                   
                 (in advance) 
               
               
                 Bit line 
                 Common 
                 Common 
                 Common 
                 Common 
               
               
                   
                 line electric 
                 line electric 
                 line electric 
                 line electric 
               
               
                   
                 potential +0.3 
                 potential −0.3 
                 potential +0.2 
                 potential 
               
               
                   
               
            
           
         
       
     
     In the present invention, the semiconductor thin film may be a silicon germanium single crystal thin film or a strained silicon/silicon germanium laminate other than a silicon single crystal thin film. The gate insulating film may be a silicon oxynitride film, a silicon nitride film, an alumina film, a hafnium oxide film, a film of a silicon—hafnium oxide mixture, a zirconium oxide film, or a film of a silicon—zirconium oxide mixture other than a silicon oxide film. The conductive gates may be a tungsten film, a titanium nitride film, or a titanium/titanium nitride laminate other than a polysilicon film or a silicon germanium film. The first, second, and third semiconductor regions may be formed not only in the semiconductor thin film but also on top of the semiconductor thin film, and a metal silicide film or a metal thin film may be added thereon to form a laminate. Thus the present invention can be carried out within a modification range which comes easy to the skilled in the art. 
     The first, second, and third semiconductor regions are described in this specification as “being in contact with” the semiconductor thin film. This contact state is obtained either by introducing impurity atoms into the semiconductor thin film and forming the first, second, and third semiconductor regions in the film or by forming the first, second, and third semiconductor regions on the semiconductor thin film by deposition. 
     The present invention may employ a structure for capacitance coupling of the second principal surface or the side face to the first channel formation semiconductor thin film portion, so that data stored is kept longer and the amount of opposite conductivity type carriers stored is increased. 
     The present invention is applicable to both PDSOI and FDSOI and, when applied to FDSOI, the present invention can provide effects that have been difficult to attain in prior art. 
     The present invention can provide a memory cell having the FDSOIMOS structure and the FDSONMIS structure as well as the PDSOIMIS structure, and a memory array using the memory cell. The memory cell does not need a large capacitor. 
     The memory can be mounted together with FDSOI logic that is capable of low power operation, and the operation voltage of the memory is in a range that matches the range of the low power logic. 
     If the cell takes a so-called double gate MIS structure, carriers of the opposite conductivity type are securely stored in the memory cell by the electric potential given to the third conductive gate of the cell.