Abstract:
An improved pull-up transistor is provided for use as an open drain input/output structure. The transistor includes a source and a drain that define a channel between them. An impurity implantation region in the channel does not reach both the source and the drain. The impurity can reach only the source, only the drain, or none of them. As such, it presents a discontinuity, which serves as a p-type channel. The transistor therefore can act as an enhancement transistor used for pull-up. The invention can be implemented with a mask ROM embedded MCU, or an EPROM embedded MCU.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a semiconductor device and manufacturing method thereof, and more particularly to an open drain input/output structure and manufacturing method.  
           [0003]    2. Description of the Prior Art  
           [0004]    In general, when an input/output (I/O) of a MASKROM embedded MCU is implemented, it is necessary to establish a single layout for an open drain option and for a pull-up option. Accordingly, when devices in the MASKROM embedded MCU are manufactured, two I/Os (open drain I/O and pull-up I/O) are typically implemented as transistors, and more particularly as follows.  
           [0005]    A pull-up I/O is first formed in such a manner that a contrary type of impurity to a substrate is ion-implanted into the channel region so that a gate is formed. Thereafter an open drain I/O is formed by starting with what would be a depletion transistor, and converting it into an enhancement transistor by further ion-implanting an impurity of the same type as the substrate into only a specific region. The specific region is the channel region of a cell which would be used as an open drain option during after gate programming (AGP) process.  
           [0006]    The selective change of the depletion transistor into the enhancement transistor is for cutting off the depletion transistor for a pull-up resistance by the impurity ion implantation process (because a current flow occurs through the pull-up resistance, thereby causing an external component not to be controlled, when the both terminals of the pull-up resistance of the pull-up resistance type I/O are applied with an electric voltage source of a chip and an external high voltage). Here, the open drain I/O controls components by using an external high voltage.  
           [0007]    That is, the depletion transistor is used as the pull-up resistance on the condition that the depletion transistor is changed into the enhancement transistor by the impurity ion implantation process for a channel region after being patterned a gate when the depletion transistor is intended to use as the open drain I/O.  
           [0008]    [0008]FIG. 1 shows a circuit corresponding to a conventional open drain I/O structure. Reference numeral C represents a cutting-off node of the open drain circuit, D an open drain I/O input terminal, E an external component and Vdd an internal voltage terminal.  
           [0009]    Two transistors A, B are respectively connected to a second and first internal logic circuits  10   b  and  10   a . Transistor A is a n-channel open drain transistor. Transistor B is a pull-up or enhancement transistor, which has been so made by changing an n-channel depletion transistor by ion implanting impurities, after the gate is formed. The two transistors are connected in series each other through cutoff node C, which in turn is connected to an input/output pad  20 . The pad  20  is connected with an external analog IC for applying an external high voltage unlike MOS-type LSI.  
           [0010]    Pull-up transistor B is connected between a Vdd terminal and a pad  20  by the source and the drain. Because it should be maintained in the cut-off state, the first internal logic circuit  10   a  should be established to output a low level signal. When the second internal logic circuit  10   b  keeps a high level, an external signal is applied through the pad  20 , and then a current flows through the open drain transistor A, so as to operate the external component.  
           [0011]    [0011]FIG. 2 shows a conventional structure of a n-channel open drain transistor A. A gate insulating layer  34  is formed on an active region of a p-type semiconductor substrate  30  which is formed with a field oxide film  32 .  
           [0012]    A gate  26  is formed on a predetermined portion of the gate insulating layer  34 . The gate is preferably formed with a layer of a W-silicide  26   b  accumulated on a polysilicon layer  26   a.    
           [0013]    An insulative spacer  28  is formed on each side wall of the gate  26 . N-type source and drain regions  42   a  and  42   b  are formed in substrate  30 , provided with LDD (lightly doped drain) regions  40 . A channel region is defined between them.  
           [0014]    Referring to FIGS. 3 and 4, an enhancement transistor B is shown. It has a very similar structure to transistor A of FIG. 2, except for the following. The gate is numbered  36 , is comprised of respective layers  36   a ,  36   b , and its line width is W 2 . Spacers  38  are formed around it.  
           [0015]    Further, at the channel region under gate  36 , a n-type impurity implantation region  44  is formed. It is over a region that is adjacent to or even overlaps both the source and drain, and thus forms a continuous path of a n-type impurity between them.  
           [0016]    Additionally, a p-type impurity implantation region  46  is further formed at a middle section of region  44 . The p-type impurity implantation region serves as the pull-up resistance after the gate is formed. This ensures that a constant off state is kept, except when the gate is provided with a high level signal.  
           [0017]    The conventional method for forming the open drain I/O has the following drawbacks: First, in transistor B, the additional impurity ion implantation process must be performed one more time for forming the p-type impurity implantation region  46 , after the gate is formed. This causes not only the process to be complicated, but also cost to be increased.  
           [0018]    Second, when a system maker intends to achieve a EPROM embedded MCU by using a non-volatile memory (for example EPROM for the purpose of developing a program and for applying to the market), there is no problem to achieve the open drain I/O by the process and the layout different from a conventional mask ROM embedded MCU. However, there is a problem for the open drain I/O to achieve by using the same layout as the conventional layout. That is, since an AGP (after gate programming) coding is not used for the EPROM embedded MCU, it is not necessary the impurity ion implantation process after the gate is formed. Therefore, it is not possible to achieve selectively between the I/O for the pull-up resistance of the EPROM embedded MCU and the open drain I/O. That is, it is possible for the mask ROM embedded MCU to achieve the open drain I/O and the I/O for pull-up resistance, but it is possible for the EPROM embedded MCU to achieve only the I/O for pull-up resistance.  
           [0019]    Therefore, it is required for the open drain I/O having the same layout to apply in the mask ROM embedded MCU and the EPROM embedded MCU.  
         SUMMARY OF THE INVENTION  
         [0020]    The present invention overcomes these problems of the prior art. The invention provides an improved pull-up transistor that is to be used in place of the enhancement transistor B of the prior art. The transistor includes a source and a drain that can be connected to a Vdd terminal and to an I/O pad. In the channel of the transistor there is an impurity implantation region that does not reach both the source and the drain. In other words, it presents a discontinuity, which serves as a p-type channel. This avoids the need of the prior art for the additional p-type ion-implantation process, after forming the gate.  
           [0021]    According to the first embodiment, the impurity reaches the source, but not the drain. In the second embodiment, the impurity reaches the drain, but not the source. In the third embodiment the impurity reaches neither the source nor the drain, thus presenting at least two discontinuities. The invention can be implemented with a mask ROM embedded MCU, an EPROM embedded MCU.  
           [0022]    Another object of the present invention is to provide an open drain input/output transistor manufacturing method, which enables to effectively achieve an open drain structure of the input/output.  
           [0023]    The invention also provides a method of manufacture of the device. A gate insulating layer is formed in an active region on a p-type semiconductor substrate; then an impurity implantation region is formed at a predetermined first sector within the substrate by ion-implanting a n-type impurity. Then a gate is formed on the gate insulating layer over at least a portion of the first sector and over a region adjacent to the first sector. Source and drain regions are then formed within the substrate at opposite sides of the gate, by ion-implanting an impurity of the second conductive type. The first sector having been predetermined such that it does not reach both the source region and the drain region.  
           [0024]    As a result, using the open drain I/O of the invention enables implementing a MASKROM embedded MCU, I/O for pull-up resistance of EPROM embedded MCU, and open drain I/O with a single lay out structure, which makes it compatible for manufacture with MCU, and more economical.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0025]    The above object, and other features and advantages of the present invention will become apparent after a reading of the following detailed description when taken in conjunction with the drawings, in which:  
         [0026]    [0026]FIG. 1 is a schematic circuit diagram illustrating an open drain input/output stage structure of a conventional semiconductor device;  
         [0027]    [0027]FIG. 2 is a sectional view of open drain transistor A in FIG. 1;  
         [0028]    [0028]FIG. 3 is a sectional view of pull-up transistor B in FIG. 1;  
         [0029]    [0029]FIG. 4 is a plan view illustrating the layout structure after the gate is formed in the transistor of FIG. 3;  
         [0030]    [0030]FIGS. 5 a  to FIG. 5 c  are views illustrating an input/output stage structure of a semiconductor device made according to the first embodiment of the present invention, and more specifically:  
         [0031]    [0031]FIG. 5 a  is a sectional view illustrating the enhancement transistor structure of the open drain input/output stage,  
         [0032]    [0032]FIG. 5 b  is a plan view illustrating the layout structure after the gate is formed, and  
         [0033]    [0033]FIG. 5 c  is an equivalent circuit of FIG. 5 a;    
         [0034]    [0034]FIG. 6 a  to FIG. 6 c  are views illustrating an input/output stage structure of a semiconductor device made according to the second embodiment of the present invention, and more specifically:  
         [0035]    [0035]FIG. 6 a  is a sectional view illustrating the enhancement transistor structure of the open drain input/output stage,  
         [0036]    [0036]FIG. 6 b  is a plan view illustrating the layout structure after the gate is formed, and  
         [0037]    [0037]FIG. 6 c  is an equivalent circuit of FIG. 6 a;    
         [0038]    [0038]FIG. 7 a  to FIG. 7 c  are views illustrating an input/output stage structure of a semiconductor device made according to the third embodiment of the present invention, and more specifically:  
         [0039]    [0039]FIG. 7 a  is a sectional view illustrating the enhancement transistor structure of the open drain input/output stage,  
         [0040]    [0040]FIG. 7 b  is a plan view illustrating the layout structure after the gate is formed, and  
         [0041]    [0041]FIG. 7 c  is an equivalent circuit of FIG. 7 a.    
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0042]    As has been mentioned, the present invention is directed to an improved pull-up transistor that is to be used in place of the enhancement transistor B of FIG. 1 of the prior art. The transistor includes a source and a drain that can be connected to a Vdd terminal and to an I/O pad. In the channel of the transistor there is an impurity implantation region that does not reach both the source and the drain.  
         [0043]    In the following description, the line width F of the active region becomes important, along with the line widths of the impurity implantation region and of the gate. But for the special impurity implantation region, the transistor would be a n-channel open drain transistor, whose separate description is thus omitted.  
       Embodiment 1  
       [0044]    The first embodiment H 1  is now described with reference to FIGS. 5 a ,  5   b  and  5   c . As shown in FIG. 5 a , a gate insulating layer  34  with a field oxide layer  32  is formed at an active region of a p-type semiconductor substrate  30 . N-type source and drain regions  42   a  and  42   b  are formed, defining between them a channel region. They are optionally provided with LDD regions  40 .  
         [0045]    A n-type impurity implantation region  54  is formed at a first sector of the channel region. While it is coupled to source region  42   a , but maintains a predetermined distance -o- from drain region  42   b , which is different than in the prior art.  
         [0046]    A gate  56  is formed on the gate insulating layer  34 . What is further different is that gate  56  is formed on a predetermined portion of the gate insulating layer. Specifically, W 3  indicates the line width of gate  56 , and F indicates the line width of the active region. The gate is formed over a first portion of the first sector and over a portion of the area adjacent the first sector.  
         [0047]    The gate is formed by accumulating a polysilicon layer  56   a  and a W-silicide (W-silicide/polysilicon) layer  56   b . Other embodiments are possible, such as a one step polysilicon layer. Then both side walls are formed with an insulating spacer  58 .  
         [0048]    The enhancement transistor H 1  having the above-structure is manufactured through the following four steps.  
         [0049]    As a first step, the gate insulating layer  34  is formed at the active region on the p-type semiconductor substrate  30 , which is formed with the field oxide layer  32 . Then an n-type impurity is selectively implanted on a selected portion of the gate insulating layer  34 . Accordingly, an n-type impurity implantation region  54  is formed at the portions in the substrate  30 , which are collectively known as the first sector of the substrate.  
         [0050]    As a second step, the gate  56  is formed on the gate insulating layer  34 . The gate is over a first portion of the first sector, i.e. over at least some of the impurity implantation region  54 . The gate is also over a second portion of the surface of the substrate  30  that is adjacent region  54 . The gate can be manufactured by depositing a conductive layer and then selectively etching it to conform to the desired shape.  
         [0051]    Because the n-type impurity implantation region  54  should be formed only at the certain portions of the channel region in order to achieve the open drain structure without the p-type impurity ion implantation process for opening the channel, the gate  56  should be formed to have a little longer length W 3  than the conventional length W 2  of FIG. 2. This is best seen with reference to FIG. 5 b.    
         [0052]    As a third step, the n-type impurity in low concentration is ion-implanted to the substrate  30  through the gate  56 , as a mask so as to form LDD regions  40  in the substrate  30  at both sides of the gate.  
         [0053]    As a fourth step, insulating spacers  58  are formed at both side walls of the gate  56 . Then the n-type impurity in high concentration is ion-implanted to the substrate  30 , using the gate and the spacer  38  as a mask, so as to form the source/drain regions  42   a  and  42   b  in the substrate  30 .  
         [0054]    [0054]FIG. 5 c  illustrates the operation of transistor H 1 . Transistor H 1  operates as a depletion transistor HD in the first sector, which is where the n-channel region includes the n-type impurity implantation region  54 . However, it operates as an enhancement transistor HE at the region adjacent the first sector, i.e. the p-channel region (portion “O” in drawing) where the channel lacks the impurity implantation region  54 . Accordingly, the enhancement transistor can be cut-off only when a voltage Vdd is applied to the source region, and a low level signal is applied to the gate.  
       Embodiment 2  
       [0055]    The second embodiment H 2  is now described with reference to FIG. 6 a ,  6   b  and  6   c . It is the same as the first embodiment, except that the first sector reaches the drain region instead of the source region. In other words, a n-type impurity implantation region  64  is formed contiguously to, or overlapping with the drain region, but maintains a predetermined distance -o- from a source region.  
         [0056]    [0056]FIG. 6 c  shows an equivalent circuit for transistor H 2  of FIG. 6 a . Transistor H 2  operates similarly to transistor H 1  of FIG. 5 c.    
       Embodiment 3  
       [0057]    The third embodiment H 3  is now described with reference to FIG. 7 a ,  7   b  and  7   c . It is the same as the first two embodiments, except that the first sector does not reach either one of the source and drain regions. More specifically, a n-type impurity implantation region  74  is formed at predetermined distances from source/drain regions  42   a ,  42   b . Preferably the predetermined distances are equal.  
         [0058]    The enhancement transistor having the above-structure is manufactured through substantially the same steps, taking care that the n-type impurity implantation region  74  is formed at portions of the substrate that do not overlap or even contact the source and drain. Further, the gate  56  is formed on the gate insulating layer  34  so as to be over a first portion of the first sector and over a second portion of the surface of the substrate  30  adjacent the first sector. Preferably the first portion is in a predetermined ratio with the second portion.  
         [0059]    [0059]FIG. 7 c  shows an equivalent circuit for the transistor in FIG. 7 a . P-channels (parts “o” in the drawing) are formed at both sides of the n-channel region, adjacent to the impurity implantation region  44 . Transistor H 3  having the above structure operates at the p-channel regions as enhancement transistors HE, HE′, and operates at the n-channel regions as depletion transistor HD. Therefore, transistors HE, HE′ can be cut off only when a low level signal is applied to the gate, with the voltage Vdd being applied to the source region.  
         [0060]    A person skilled in the art will be able to practice the present invention in view of the present description, where numerous details have been set forth in order to provide a more thorough understanding of the invention. In other instances, well-known features have not been described in detail in order not to obscure unnecessarily the invention.  
         [0061]    Having illustrated and described the principles of the invention in its preferred embodiments, it should be readily apparent to those skilled in the art that the invention can be modified in arrangement and detail without departing from such principles. For example, the conductivity types (p-type and n-type, also collectively known as conductive types) can be interchanged. All modifications coming within the spirit and scope of the accompanying claims are claimed as follows.