Horizontal MOS transistor

This application is based upon and claims the benefit of priority from each of the prior Japanese Patent Application No. 2004-085937 filed on Mar. 24, 2004, the entire contents of which are incorporated herein by reference.

1. Field of the Invention

The present invention relates to layout structure of a MOS transistor, and more particularly to layout structure of a horizontal MOS transistor.

2. Description of Related Art

A semiconductor device disclosed in JP Laid-open Patent publication No. 9-129867 intends to reduce ON resistance by shortening a distance between source contacts with respect to a horizontal DSA (Double-diffused Self Alignment system) power MOSFET. As shown in FIG. 9 , unit cells of the semiconductor device is structured such that a drain contact 27 is arranged in a rectangular region surrounded by adjoining four source contacts 25 each of which is surrounded by gate layer 23 and periphery of the rectangular region is surrounded by gate layer 23 . The drain contact 27 has four sizes parallel to diagonal lines of the rectangular region and is arranged facing source contacts 25 or arranged on the diagonal lines so that ON resistance can be reduced due to shortened distance between two contacts.

In JP Laid-open Patent publication No. 9-129867, a distance between a source contact and a drain contact in diagonal-line direction of a rectangular region surrounded by adjoining source contacts is shortened and ON resistance is reduced, actually.

However, as a matter of fact, drain contacts are arranged to face one another sandwiching gate layer which constitutes each side of the rectangular region. Furthermore, source contacts are arranged to face one another along with gate layer which constitutes the rectangular region. It never works as MOS transistor in a region of MOS transistor structure where drain contacts face one another and source contacts face one another. Unit cells including MOS transistor structure inoperable as MOS transistor restricts its current drivability and ON resistance, which is problematic.

The present invention has been made to resolve the above- described problems. Accordingly, it is an object of the present invention to provide a horizontal MOS transistor capable of enhancing current drivability and reducing ON resistance by optimizing wiring structure of gate layer and layout structure of source-drain regions.

According to one aspect of the present invention, there is provided a horizontal MOS transistor structured such that first gate wirings wired in a first direction and second gate wirings wired in a second direction are arranged on same wiring layer and the fist gate wirings arranged by first predetermined pitch and pairs of the second gate wirings arranged by second predetermined pitch constitute mesh structure. Furthermore, the second gate wirings has bend sections between adjoining two of the first gate wirings and meander by the first predetermined pitch.

Thereby, in the mesh structure constituted by the first gate wirings and the second gate wirings crossing one another, MOS transistors are constituted in four directions sandwiching the mesh structured gate wirings. Therefore, nevertheless a gate width per unit area can be made wider, regions of same electric field such as source layer or drain layer exist only at diagonal position and regions of same electric field never face one another sandwiching gate wirings. Therefore, MOS transistor structure of not-for transistor operation can be minimized. Furthermore, since second gate wirings are wired in second direction meanderingly, gate width per unit length can be widened.

According to the present invention, while MOS transistor structure of not-for transistor operation can be minimized, gate width per unit width and per unit area can be widened. Thereby, there can be provided a horizontal MOS transistor such that an effective transistor region per unit region can be enlarged, current drivability per unit region is excellent, and ON resistance is very little.

The above novel features of the invention will more fully appear from the following detailed description when the same is read in connection with the accompanying drawings. It is to be expressly understood, however, that the drawings are for the purpose of illustration only and are not intended as a definition of the limits of the invention.

FIG. 1 is partial plan view of a horizontal MOS transistor directed to an embodiment;

FIG. 2 is a sectional view of section A—A in FIG. 1 ;

FIG. 3 is a sectional view of section B—B in FIG. 1 ;

FIG. 4 is a sectional view of section C—C in FIG. 1 ;

FIG. 5 is a partial plan view of comparison pattern ( 1 );

FIG. 6 is a partial plan view of comparison pattern ( 2 );

FIG. 7 is a partial plan view of comparison pattern ( 3 );

FIG. 8 is a partial plan view of comparison pattern directed to the embodiment; and

FIG. 9 is a partial plan view of a conventional horizontal MOS transistor.

Hereinafter, the preferred embodiments of the horizontal MOS transistor of the present invention will be described in detail with reference to FIGS. 1–8 .

FIG. 1 shows the layout of the horizontal MOS transistor of this embodiment and FIGS. 2–4 show sectional views taken along the lines A—A to C—C shown in FIG. 1 . The horizontal MOS transistor of this embodiment is a transistor of output stage incorporated in a control IC for controlling the power of a switching power supply and the like.

As shown in FIGS. 2–4 , P-type diffusion layers 24 A– 24 E are formed as source/drain layers in the surface portion of an N-type substrate 22 which is a base of a chip constituting a control IC or in the surface portion of the N-type well layer 22 constructed in the form of a semiconductor substrate so as to form a PMOS transistor.

Although FIGS. 2–4 exemplify the PMOS transistor, the NMOS transistor can be constructed in the same manner. The reason is that a difference between the P conductive type and N conductive type is determined depending on the sectional longitudinal structures shown in FIGS. 2–4 , so that the layout structure shown in FIG. 1 can be applied regardless of the difference between the P conductive type and N conductive type.

The layout structure will be described with reference to FIG. 1 . Along an X direction in this Figure, a first gate wiring 11 is disposed with a pitch Yl in the Y direction. Further along the Y direction, two second gate wirings 12 are disposed as a pair with a pitch X 1 in the X direction. The first and second gate wirings 11 , 12 form a gate electrode of the MOS transistor having a mesh structure formed of the same wiring layers. The wiring layer mentioned here is a poly-crystalline silicone layer for example. Because the first gate wiring 11 is disposed linearly in the X direction without being bent, the first gate wiring 11 is disposed with the shortest route in the X direction. Gate resistance is reduced by the wiring route on the first gate wiring 11 and the wiring route up to the second gate wiring 12 branched from the first gate wiring 11 . A region surrounded by the first and second gate wirings 11 , 12 is a diffusion layer region, which constitutes electrode regions such as source layer and drain layer.

The second gate wiring 12 has a bent section 14 which is bent obliquely substantially in the center of a pitch Y 1 of the first gate wiring 11 . In the diffusion layer region sandwiched by the first gate wiring 11 and the second gate wiring 12 , the bent sections 14 of the second gate wirings 12 are bent in opposite directions having a relation of mirror symmetry, so that the region surrounded by the second gate wirings 12 is formed in the shape of a bottle consisted of a wide-width region in which the width is large and a narrow-width region in which the width is narrowed.

Because the adjacent second gate wirings 12 are in the relation of mirror symmetry, the bottle-like shaped diffusion layer region constituted of the wide-width region and narrow-width region is disposed in the X direction with the disposition direction in the Y direction inverted alternately. Because the second gate wiring 12 intersects the first gate wiring 11 at right angle, the diffusion layer regions located adjacent oppose each other in each direction of the X and Y directions.

A contact 16 for connecting the diffusion layer region to a wiring layer 18 composed of a metal layer and the like is disposed in the wide-width region in the bottle-like shaped diffusion layer region. In this case, because the bottle-shapes of the diffusion layer regions invert alternately along the X direction, two wide-width regions are arranged alternately along the X direction within a Y-direction pitch Y 1 . Consequently, the contacts 16 in the wide-width region are arranged in two rows and the wiring layers 18 are arranged in the X direction along the two rows of the contacts 16 . The two wiring layers 18 disposed within the pitch Y 1 are connected to the diffusion layer region disposed in the X direction alternately thereby each constructing a source layer and a drain layer.

A wire picked up by the wiring layer 18 as the source layer and drain layer of each Y-direction pitch Y 1 is connected to an upper wiring layer 28 (see FIGS. 2–4 ) through a upper contact 20 . The upper wiring layer 28 is composed of for example, a metal layer and in this case, the upper contact 20 is a so-called via contact. When the MOS transistor shown in FIG. 1 controls the switching of a large current, preferably, wiring resistance in a current route from the source layer and the drain layer is reduced as much as possible. Thus, the upper wiring layer 28 is constructed having a wide wiring width on the upper layer of the MOS transistor structure. Further, it can be considered that this is constructed in the form of a plane shape having two-dimensional expanding.

Because the upper contact 20 for connecting the wiring layer 18 to the upper wiring layer 28 is located above the gate wirings 11 , 12 , it can be disposed just above the gate wirings 11 , 12 . Referring to FIG. 1 , the upper contact 20 is disposed in the narrow-width region in the diffusion layer region. The frequency of the disposition can be determined considering the connecting resistance between the wiring layer 18 and the upper wiring layer 28 .

A diffusion layer region and two diffusion layer regions adjacent in each of the X and Y directions on four sides constitute a different electrode region, so that the source and drain layers are formed across the gate wirings 11 , 12 on the four sides which section a single diffusion layer region so as to construct a MOS transistor.

Because a mesh structured gate wiring configuration is possessed by the first and second gate wirings 11 , 12 and MOS transistors are constructed on four sides of the mesh structure, the gate width of each unit area in the MOS transistor layout can be increased. Further, because the second gate wiring 12 meanders in the Y direction, the gate width per unit length in the Y direction can be expanded.

Because the diffusion layer regions of different electrodes of the source layer and drain layer oppose each other in the X and Y directions and the diffusion layer regions oppose each other in each of the X and Y directions, the diffusion layer regions of the same pole oppose only in the diagonal direction like source layer to source layer or drain layer to drain layer. A region in which the gate wirings 11 , 12 intersect each other surrounded by the diffusion layer regions in the diagonal direction turns to a non-active region which does not contributes to the operation of a transistor. This non-active region can be reduced to its minimum area because the first gate wiring 11 and the second gate wiring 12 intersect each other at right angle.

Consequently, expansion of an effective gate width can be achieved with the minimum area of the non-active region so that the structure of the MOS transistor which performs a transistor operation effectively in each unit region can be increased. As a result, the horizontal MOS transistor excellent in a current drivability in each unit area and having a low ON resistance can be constructed.

FIGS. 2–4 are sectional views taken the lines A—A to C—C shown in FIG. 1 . The sectional view taken the line A—A ( FIG. 2 ) is a sectional view of the narrow-width region. With a P-type diffusion layer region 24 B in the narrow-width region as a common diffusion layer region, the gate wirings 12 are provided at both end portions in the X direction and constructs the MOS transistor together with the adjacent P-type diffusion layer regions 24 A, 24 C. The P-type diffusion layer region 24 B sandwiched by the gate wirings 12 is a narrow-width region, which has no sufficient width for disposing the contact 16 for connecting the wiring layer 18 and thus, the upper contact 20 is disposed instead of the contact 16 and the wiring layer 18 is connected to the upper wiring layer 28 . In the meantime, reference numerals 26 , 27 denote interlayer insulating film.

The sectional view taken along the line B—B is a sectional view of the wide-width region. With the P-type diffusion layer region 24 B located in the wide-width region as a common diffusion layer region, the gate wiring 12 is provided on both end portions in the X direction and constructs a MOS transistor together with the adjacent P-type diffusion layer regions 24 A, 24 C. The P-type diffusion layer region 24 B surrounded by the gate wirings 12 has a sufficient width and the contact 16 is disposed therein so that the P-type diffusion layer 24 B and the wiring layer 18 are connected. In the meantime, the reference numerals 26 , 27 in FIG. 3 denote an interlayer insulating film.

The sectional view taken the line C—C ( FIG. 4 ) is a sectional view of the bottle-like shaped diffusion layer region. The P type diffusion layer region 24 B is divided to two sections across the center thereof, the narrow-width region (FIG. 4 ( 1 )) and the wide-width region (FIG. 4 ( 2 )). The upper contact 20 is disposed in the narrow-width region and the contact 16 in the wide-width region. In the meantime, the reference numerals 26 , 27 in FIG. 4 denote an interlayer insulating film.

As for the layout pattern of the embodiments, the gate width n and the ON resistance RON per unit area will be compared between comparison patterns ( 1 )–( 3 ) shown in FIGS. 5–7 . Upon this comparison, the comparison pattern ( 1 ) in FIG. 5 is used as a reference pattern, and the gate width n and ON resistance RON per each unit area on the comparison pattern ( 1 ) standardize the layout pattern of the embodiments.

The comparison pattern ( 1 ) of FIG. 5 which is the reference pattern is a stripe pattern in which the gate wirings 11 A placed linearly in the X direction are disposed at a constant pitch in the Y direction. This is the most basic layout for constructing the MOS transistor. The basic unit U 1 contains a gate wiring 11 A, which has a 1-pitch width in the Y direction while a single contact 16 and a single upper contact 20 are contained in the X direction. The contacts 16 , 20 are disposed at a constant pitch in the X direction. A gate width UG 1 in the basic unit U 1 has a length of two pitches for disposition of the contacts in the X direction.

In the comparison pattern ( 2 ) shown in FIG. 6 , by meandering the gate wiring 11 a of the comparison pattern ( 1 ) ( FIG. 5 ), the gate wiring 11 B is produced. This structure intends to expand the gate width by meandering the gate wiring. The basic unit U 2 includes a single pitch of meandering with a starting point of the meandering of the gate wiring 11 B as a boundary and a pitch of the disposition of the gate wiring 11 B in the Y direction. The gate width UG 2 of the basic unit U 2 is as long as a 1-pitch of the gate wiring 11 B.

The gate wiring in the comparison pattern ( 3 ) of FIG. 7 is constructed in a grid structure with gate wirings 11 C placed linearly in the X direction with a constant pitch in the Y direction and a gate wiring 12 C placed linearly in the Y direction with a constant pitch in the X direction. This is a so-called mesh pattern. This structure intends to expand the gate width by forming gate wirings in the grid structure. The basic unit U 3 contains the gate wirings 11 C, 12 C in its central portion, which are sectioned in the center of the gate wiring pitch of each in the X direction and Y direction. The gate width UG 3 of the basic unit U 3 is a sum of a length UG 31 which is a 1-pitch length in the X direction minus the length of the intersecting portion of the gate wirings 11 C and 12 C and a length U 32 which is a 1-pitch length in the Y direction minus the length of the intersecting region, while UG 3 =UG 31 +UG 32 .

FIG. 8 shows the basic unit U 0 in the layout pattern of this embodiment. The basic unit U 0 is a region which contains the gate wirings 11 , 12 and sectioned by the center between the adjacent gate wirings in the X direction and the center of a bent portion 14 in the Y direction. The gate length UG 0 of the basic unit U 0 is a sum of a length UG 01 which is a 1-pitch length in the X direction minus the length of the intersecting portion and a length UG 02 which is a 1-pitch length in the Y direction minus the length of the intersecting region, while UG 0 =UG 01 +UG 02 .

Table 1 shows a result of comparison between the gate width η×(x=0–3) and ON resistance RON×(x=0–3) per each unit area of the layout pattern ( FIG. 1 ) of the embodiment and the comparison patterns ( 1 )–( 3 ). Here, the gate width η×per unit area is a value calculated according to η×=UGx/Sx with an area Sx (x=0–3) of the basic unit Ux (x=0–3) and a gate width UGx (x=0–3) of each basic unit. Table 1 shows a ratio of the gate width η1 per each unit area and ON resistance RON 1 with the comparison pattern ( 1 ) adopted as a reference pattern.

According to this embodiment, the η ratio is 1.5 times or more larger than the stripe type of the comparison pattern ( 1 ) and larger than the mesh type of the comparison pattern ( 3 ). Consequently, the gate width per unit area can be larger than other patterns. Further, the RON ratio is reduced by 20% as compared to the stripe type of the comparison pattern ( 1 ), so that it is smaller than the mesh type of the comparison pattern ( 3 ). Consequently, the ON resistance can be reduced with respect to the other patterns.

Here, the X direction and Y direction are an example about the first direction and the second direction, which corresponds to a case where the first and second directions intersects at right angle each other. In this case, the first predetermined pitch is pitch Y 1 and the second predetermined pitch is pitch X 1 .

As described in detail above, according to the horizontal MOS transistor of this embodiment, when the second gate wiring 12 is wired in the Y direction at a pitch X 1 , the diffusion layer regions between the wirings are disposed with an excellent area efficiency such that the narrow-width region and the wide-width region repeats alternately. As for the Y direction, the diffusion layer regions opposing across the first gate wiring 11 are never deflected from each other with the same opposing side in width, so that they are disposed to oppose each other properly throughout that entire opposing side. Although the diffusion layer regions adjacent to the opposing diffusion layer region is constructed as a different electrode, there never occurs a case where the opposing side deflects so that the diffusion layer regions of the same pole oppose each other. Thus, there never occurs a case where the MOS transistor structure which does not operate as a transistor is formed, thereby making it possible to perform the transistor operation efficiently.

The wide-width regions disposed with two rows in the pitch Y 1 are diffusion layer regions each having a different electrode and composed of the source layer and drain layer. The wiring layer which forms a bias route between the source layer and the drain layer is disposed linearly along the X direction, thereby forming an efficient layout.

Because the first gate wiring 11 is disposed linearly, the first gate wiring 11 can be wired with the shortest route in the X direction. In the wiring route of the first gate wiring 11 and the wiring route from the first gate wiring 11 up to the second gate wiring 12 branched in the Y direction, the gate resistance can be reduced.

Because the first gate wiring 11 and the second gate wiring 12 intersect each other at right angle, the intersecting area of the gate wirings can be reduced to a minimum area necessary. The structure portion which is composed of a diffusion layer region of the same pole located at the diagonal position, not contributing to the transistor operation, can be reduced to a minimum area necessary.

The MOS transistors are formed on four sides of the mesh structure constituted of the first gate wiring 11 and the second gate wiring 12 , thereby increasing the gate width per each unit area. Because the region of the same electrode exists only in the diagonal direction and the gate wiring sandwiched therein is a minimum area necessary, the MOS transistor structure which is not related to the transistor operation can be reduced extremely. Further, because the second gate wiring 12 is disposed in the meandering condition in the Y direction, the gate width per each unit length in the Y direction can be expanded.

While the transistor structure not related to the transistor operation is minimized, the gate width of each unit area can be expanded. Consequently, the MOS transistor structure which executes an effective transistor operation in each unit region can be increased, and therefore, the horizontal MOS transistor having an excellent current drivability for each unit region and a small ON resistance can be constructed.

In the meanwhile, the present invention is not restricted to the above-described embodiment, and needless to say, the invention may be improved or modified in various ways within the scope not departing from the gist of the present invention.

Although this embodiment has been described by taking an example of the PMOS transistor in FIGS. 2–4 , the present invention is not restricted to this example, but applicable to the NMOS transistor also.

Although for this embodiment, an example that the upper contact 20 which connects the wiring layer 18 to the upper wiring layer 28 is disposed in the narrow width region of the diffusion layer region at every pitch Y 1 , for some case, the upper contacts 20 may be disposed more dispersedly depending on the contact resistance and the like and for other case, it may be disposed more densely.

The present invention makes it possible to provide a horizontal MOS transistor capable of enhancing current drivability and reducing ON resistance by optimizing wiring structure of gate layer and layout structure of source-drain regions.