Semiconductor memory device and fabrication process thereof

This is a divisional of U.S. application Ser. No. 13/726,940, filed Dec. 12, 2012, which is a divisional of U.S. application Ser. No. 12/068,692, filed Feb. 11, 2008, which is based on Japanese priority application No. 2007-042498 filed on Feb. 22, 2007, the entire contents of which are hereby incorporated by reference.

The present invention generally relates to semiconductor devices and more particularly to a semiconductor memory device that includes a static random access memory.

A static random access memory (referred to hereinafter as SRAM) is a high-speed semiconductor memory device including a transfer transistor selected by a word line and two CMOS inverters forming together a flip-flop connection, wherein each of the CMOS inverters is connected to a corresponding bit line via a corresponding transfer transistor. SRAMs are used extensively in high-speed logic circuit devices together with high-speed logic elements such as CMOS circuit.

FIG. 1 is an equivalent circuit diagram of a typical SRAM 10 .

Referring to FIG. 1 , the SRAM 10 includes a first CMOS inverter I 1 in which a first load transistor LT 1 and a first driver transistor Dt 1 are connected in series, a second CMOS inverter I 2 in which a second load transistor LT 2 and a second driver transistor LD 2 are connected in series. The first CMOS inverter I 1 and the second CMOS inverter I 2 form together a flip-flop circuit FF, wherein a node N 1

N 1 connecting the first load transistor LT 1 , and the first driver transistor DT 1 , with each other, is connected to a first bit line BL via a first transfer transistor TF 1 , while the first transfer transistor TF 1 is controlled by a word line WL. Similarly, a node N 2 connecting the second load transistor LT 2 and the second driver transistor DT 2 with each other, is connected to a first bit line /BL via a second transfer transistor TF 2 controlled by the word line WL.

In SRAMs of such a construction, current drivability of the load transistors LT 1 and LT 2 that drive the driver transistors DT 1 and DT 2 is extremely important for attaining high-speed operation of the SRAM.

Patent Reference 1 Japanese Laid-Open Patent Application 2006-41035 Patent Reference 2 Japanese Laid-Open Patent Application 7-131003 Patent Reference 3 Japanese Laid-Open Patent Application 7-169858 Patent Reference 4 Japanese Laid-Open Patent Application 2002-329798 Patent Reference 5 Japanese Laid-Open Patent Application 2002-190534

According to an aspect of an embodiment, a semiconductor memory device includes: a semiconductor substrate; a first CMOS inverter comprising first and second MOS transistors having respective, mutually different channel conductivity types and connected in series at a first node on said semiconductor substrate; a second CMOS inverter comprising third and fourth MOS transistors having respective, mutually different channel conductivity types and connected in series at a second node on said semiconductor substrate, said second CMOS inverter forming, together with said first CMOS inverter, a flip-flop circuit; a first transfer transistor provided on said semiconductor substrate between a first bit line and said first node, said first transfer transistor having a first gate electrode connected to a word line and driven by a selection signal on said word line; a second transfer transistor provided on said semiconductor substrate between a second bit line and said second node, said second transfer transistor having a second gate electrode connected to said word line and driven by a selection signal on said word line; a polysilicon resistance element formed on a device isolation region on said semiconductor substrate, each of said first and third MOS transistors being formed in a device region of a first conductivity type defined in said semiconductor substrate by said device isolation region, each of said first and third MOS transistors including: a polysilicon gate electrode formed on said semiconductor substrate via a gate insulation film and having gate sidewall insulation films at respective sidewall surfaces thereof; a second conductivity type source region formed in said semiconductor substrate at a first side of said polysilicon gate electrode such that an end part of said second conductivity type source region invades into a part of said semiconductor substrate right underneath said polysilicon gate electrode; a second conductivity type drain extension region formed in a surface part of said semiconductor substrate at a second side opposite to said first side of said polysilicon gate electrode such that an end part of said second conductivity type drain extension region invades into a part of said semiconductor substrate right underneath said polysilicon gate electrode; and a second conductivity type drain region formed in said semiconductor substrate at an outer side of said gate sidewall insulation film of said second side in overlapping with said drain extension region with a depth larger than a depth of said second conductivity type drain extension region, said source region being formed deeper than said drain extension region, said polysilicon gate electrode having a film thickness identical to a film thickness of said polysilicon resistance element, said source region and said polysilicon resistance element being doped with the same dopant element.

According to another aspect of an embodiment, a method for fabricating a semiconductor memory device, said semiconductor memory device including: a first CMOS inverter comprising first and second MOS transistors having respective, mutually opposite channel conductivity types and connected in series at a first node on a semiconductor substrate; a second CMOS inverter comprising third and fourth MOS transistors having respective, mutually opposite channel conductivity types and connected in series at a second node on said semiconductor substrate, said second CMOS inverter forming, together with said first CMOS inverter, a flip-flop circuit; a first transfer transistor provided on said substrate between a first bit line and said first node, said first transfer transistor having a first gate electrode connected to a word line and driven by a selection signal on said word line; a second transfer transistor provided on said substrate between a second bit line and said second node, said second transfer transistor having a second gate electrode connected to said word line and driven by a selection signal on said word line; and a polysilicon resistance element formed on a device isolation region on said semiconductor substrate, said method includes the steps of: forming a first polysilicon pattern constituting a gate electrode of said first MOS transistor on a device region of a first conductivity type defined on said semiconductor substrate by said device isolation region via a gate insulation film, simultaneously with a second polysilicon pattern constituting said polysilicon resistance element on said device isolation region, as a result of patterning of a polysilicon film; introducing an impurity element of said second conductivity type into said device region at a first side of said first polysilicon pattern and further into said second polysilicon pattern such that there is formed a source region of said second conductivity type at said first side of said first polysilicon pattern in said device region and doping said second polysilicon pattern with said impurity element; introducing an impurity element of said second conductivity type into said device region at said first side and at a second side opposite to said first side of said first polysilicon pattern and further into said second polysilicon pattern such that there is formed a drain extension region in a surface part of said device region at said second side of said first polysilicon pattern with an impurity concentration lower than an impurity concentration of said source region and increasing an impurity concentration of said second polysilicon pattern; forming sidewall insulation films on respective sidewall surfaces of said first and second polysilicon patterns; and introducing an impurity element of said second conductivity type into said device region and further into said second polysilicon pattern while using said first polysilicon pattern and said sidewall insulation films at said first side and said second side of said first polysilicon pattern as a mask such that there are formed drain regions of said second conductivity type at respective outer parts of said sidewall insulation films of said first side and said second side of said first polysilicon pattern and such that an impurity concentration of said second polysilicon pattern is increased.

According to further another aspect of an embodiment, a semiconductor memory device includes: a semiconductor substrate; a first CMOS inverter comprising first and second MOS transistors having respective, mutually opposite channel conductivity types and connected in series at a first node on said semiconductor substrate; a second CMOS inverter comprising third and fourth MOS transistors having respective, mutually opposite channel conductivity types and connected in series at a second node on said semiconductor substrate, said second CMOS inverter forming, together with said first CMOS inverter, a flip-clop circuit; a first transfer transistor provided on said semiconductor substrate between a first bit line and said first node, said first transfer transistor having a first gate electrode connected to a word line and driven by a selection signal on said word line; a second transfer transistor provided on said semiconductor substrate between a second bit line and said second node, said second transfer transistor having a second gate electrode connected to said word line and driven by a selection signal on said word line, said first MOS transistor being formed in a first device region of a band shape formed on said semiconductor substrate by a device isolation region, said first MOS transistor having a gate electrode of a first polysilicon pattern traversing said first device region, said third MOS transistor being formed in a second device region of a band shape formed on said semiconductor substrate by said device isolation region, said third MOS transistor having a gate electrode of a second polysilicon pattern traversing said second device region, said first polysilicon pattern being connected to a first end part of said second device region by a first via-plug, said second polysilicon pattern being connected to a first end of said first device region by a second via-plug, a third via-plug being in contact with a part of said first device region at a side opposite to a side of said first via-plug with regard to said first polysilicon pattern as a power contact, a fourth via-plug being in contact to a part of said second device region at a side opposite to said second via-plug with regard to said second polysilicon pattern as a power contact, said third via-plug having a diameter larger than a width of said first device region, said fourth via-plug having a diameter larger than a width of said second device region, said third via-plug being offset from a central line of said first device region, said fourth via-plug being offset from a central line of said second device region.

FIG. 1 is a diagram showing an equivalent circuit diagram of SRAM;

FIGS. 2A and 2B are diagrams explaining the construction of an SRAM according to a related art;

FIG. 3 is a diagram explaining the problems to be solved;

FIGS. 4A and 4B are further diagrams explaining the problems to be solved;

FIGS. 5A-5C are further diagrams explaining the problems to be solved;

FIG. 6 is a further diagram explaining the problems to be solved;

FIGS. 7A and 7B are diagrams explaining the construction of an SRAM according to a first embodiment;

FIG. 8 is a cross-sectional diagram showing the construction of the SRAM of FIGS. 7A and 7B in detail;

FIGS. 9A-9H are diagrams explaining the fabrication process of an SRAM of FIG. 8 ;

FIG. 10 is a diagram explaining the problems to be solved;

FIG. 11 is a diagram showing the construction of an SRAM according to a second embodiment;

FIG. 12 is a diagram showing the construction of an SRAM according to a second embodiment;

FIGS. 13A-13D are diagrams showing the construction of the SRAM according to the second embodiment;

FIG. 14 is a diagram showing the construction of an SRAM according to a third embodiment;

FIG. 2A shows an SRAM layout corresponding to the equivalent circuit diagram of FIG. 1 according to a related art of the present invention. Further, FIG. 2B shows a cross-sectional diagram taken along a line A-A′ of FIG. 2A .

Referring to FIGS. 2A and 2B , there are formed an n-type well 11 N and a p-type well 11 P on the surface of the silicon substrate 11 by an ion implantation process, wherein it can be seen that there are formed n-type device regions 11 A 1 and 11 A 2 in the n-type well 11 N by a device isolation region 11 I in a symmetric relationship. Further, in the p-type well 11 P, there are formed p-type device regions 11 B and 11 C by the device isolation region 11 I.

In the device regions 11 A 1 and 11 A 2 , there are formed load transistors LT 1 and LT 2 of FIG. 1 in the form of a p-channel MOS transistor respectively, wherein the load transistor LT 1 has a p-type polysilicon gate electrode G 1 crossing the device region 11 A 1 . Likewise, the load transistor LT 2 has a p-type polysilicon gate electrode G 2 crossing the device region 11 A 2 .

In a part of the n-type device region 11 A 1 located at one side of the gate electrode G 1 , there is formed a power contact V DD . Further, there is formed a via-plug V 2 in the n-type device region 11 A 1 at the other side of the gate electrode G 1 for connecting the polysilicon gate G 2 to the device region 11 A 1 .

Similarly, in a part of the n-type device region 11 A 2 located at a one side of the gate electrode G 2 , there is formed a power contact V DD , while it can be seen that there is formed a via-plug V 4 in the n-type device region 11 A 2 at the other side of the gate electrode G 2 for connecting the polysilicon gate G 1 to the device region 11 A 2 .

Further, in the p-type well 11 P located at the left side of the n-type well 11 N, it can be seen that the device isolation region 111 defines the p-type device region 11 B and that there is formed an re-channel MOS transistor having an n-type polysilicon gate electrode G 3 in a region of increased width of the p-type device region 11 B as the driver transistor DT 1 . Further, there is formed an n-channel MOS transistor having an n-type polysilicon gate electrode G 4 in a part of the p-type device region 11 B of narrow width as the transfer transistor TF 1 .

Similarly, in the p-type well 11 P located at the right side of the n-type well 11 N, it can be seen that the device isolation region 11 I defines the p-type device region 11 C and that there is formed an re-channel MOS transistor having an n-type polysilicon gate electrode G 5 in a region of increased width of the p-type device region 11 C as the driver transistor DT 2 . Further, there is formed an n-channel MOS transistor having an n-type polysilicon gate electrode G 6 in a part of the p-type device region 11 C of narrow width as the transfer transistor TF 2 .

Further, there is formed a via-contact V 1 in a part of the device region 11 B between the gate electrodes G 3 and G 4 , wherein the via-contact V 1 is connected to the via-contact V 2 by way of a local interconnection pattern (not illustrated). Similarly, there is formed a via-contact V 3 in a part of the device region 11 C between the gate electrodes G 5 and G 6 , wherein the via-contact V 3 is connected to the via-contact V 4 by way of a local interconnection pattern (not illustrated). Here, the via-contacts V 1 and V 2 constitute the node N 1 of FIG. 1 , while the via-contacts V 3 and V 4 constitute the node N 2 .

Further, there is formed a ground contact in a part of the p-type device region 11 B at an opposite side of the via-contact V 1 with regard to the gate electrode G 2 , and there is formed a via-contact V 5 connected to the bit line BL in a part of the p-type device region 11 B at the opposite side of the via-contact V 1 with regard to the gate electrode G 4 . Similarly, there is formed a ground contact V SS in a part of the p-type device region 11 C at an opposite side of the via-contact V 3 with regard to the gate electrode G 5 , and there is formed a via-contact V 6 connected to the bit line /BL in a part of the p-type device region 11 C at the opposite side of the via-contact V 3 with regard to the gate electrode G 6 .

Further, with the layout of FIG. 2 , there is formed a gate electrode G 7 of a different memory cell across the n-type device region 11 A 1 .

Meanwhile, the inventor of the present invention has obtained interesting results shown in FIG. 3 as a result of analysis of the characteristics of the load transistor in a single bit defective part of the SRAM of such a construction.

Referring to FIG. 3 , the horizontal axis represents the gate voltage and the vertical axis represents the drain current, wherein it can be seen that a large drain current is attained with the experiment indicated by the numeral 2 , while there is caused a decrease of drain current with the experiment indicated by the numeral 1 for the same gate voltage.

Here, the experiment designated with the numeral 1 represents the case shown in FIG. 4A in which a supply voltage V DD is applied to the source side of the p-channel MOS transistor constituting the load transistor while grounding the drain side. On the other hand, the experiment designated with the numeral 2 represents the case shown in FIG. 4B in which a supply voltage V DD is applied to the drain side of the same p-channel MOS transistor while grounding the source side. The fact that a small drain current is obtained in the experiment of FIG. 3 designated by the numeral 1 and that a large drain current is obtained in the experiment designated by the numeral 2 indicates that there exists a resistance R at the source side of the load transistor as shown in FIGS. 4A and 4B . In the case of FIG. 4A , the existence of the resistance R at the source side, to which the supply voltage V DD is applied, induces a decrease of the voltage V GS applied effectively across the source and drain, while this tends to lead to incomplete conduction of the transistor. In the case of FIG. 4B , on the other hand, there exists no such a resistance at the source side and there is caused no voltage drop in the source-gate voltage as a result of the resistance R.

In the equivalent circuit diagram, such a resistance R is thought to be formed in the part circled in the drawing by a broken line.

FIG. 5A shows a plan view of the SRAM of FIG. 2 for the part surrounded in FIG. 2 by a broken line, while FIG. 5B shows a cross-sectional diagram taken along a line A-A′ of FIG. 5A . Further, FIG. 5C shows a cross-sectional diagram taken along a line B-B′ of FIG. 5A . Further, FIG. 6 is a diagram showing the part surrounded by the broken line in FIG. 5B with enlarged scale.

Referring to FIG. 5A , it can be seen that the n-type diffusion region 11 A 1 is defined on the silicon substrate 11 by the device isolation region 11 I, and the gate electrodes G 1 and G 7 extend across the n-type device region 11 A 1 . Further, a power contact V DD is formed between the gate electrodes G 1 and G 7 . Further, the via-contact V 2 is formed in a manner to connect the gate electrode G 2 and the re-channel region 11 A 1 .

As shown in the cross-sectional diagram of FIG. 5B , the gate electrodes G 1 , G 2 and G 7 are formed respectively in the form of polysilicon patterns 13 A, 13 B and 13 C each doped to n-type, wherein each of the gate electrodes G 1 , G 2 and G 7 carries, on respective sidewall surfaces, sidewall insulation films SW of typically a silicon oxide film.

In the cross-sectional diagram of FIG. 5B , the polysilicon patterns 13 A and 13 C are formed on the n-type device region 11 A 1 via gate insulation films 12 A and 12 C, while the polysilicon pattern 13 B is formed on the device isolation insulation film 11 I.

In the device region 11 A 1 , there is formed a p-type source extension diffusion region 11 a at one side of the gate electrode pattern 13 A and a p-type drain extension diffusion region 11 b is formed in the device region 11 A 1 at an opposite side of the gate electrode pattern 13 A. Similarly, there is formed a p-type source extension diffusion region 11 c at one side of the gate electrode pattern 13 C and a p-type drain extension diffusion region 11 d is formed in the device region 11 A 1 at an opposite side of the gate electrode pattern 13 C.

Further, in the n-type device region 11 A 1 , there is formed a p + -type diffusion region 11 e in a part between the gate electrode patterns 13 A and 13 C so as to be located at the respective outer sides of the sidewall insulation films SW as the source regions of the respective load transistors having the gate electrodes G 1 and G 7 .

Further, in the n-type device region 11 A 1 , there is formed a drain region 11 f of similar p + -type diffusion region at the opposite side of the source region 11 e with regard to the gate electrode pattern 13 A so as to be located between the sidewall insulation film SW and the device isolation insulation film 11 I.

Further, there are formed silicide layers 14 A- 14 C on the respective surfaces of the polysilicon gate electrode patterns 13 A- 13 C, and silicide layers 14 e and 14 f are formed on the respective surfaces of the source region 11 e and the drain region 11 f.

Further, the polysilicon gate electrode patterns 13 A- 13 C, including the silicide layers 14 A- 14 C and the sidewall insulation films SW, are covered with an interlayer insulation film 15 formed on the silicon substrate 11 , and a via-plug 15 A constituting the via-contact V DD is contacted to the source region lie via the silicide layer 14 e . Further, a via-plug 15 B constituting the via-contact V 2 makes a contact with the drain region 11 f via the silicide layer 14 f . Thereby, it should be noted that the sidewall insulation film SW of the polysilicon gate electrode 13 B at the side of the polysilicon gate electrode pattern 13 A is removed, and with this, the via-plug 15 B performs the function of connecting the polysilicon gate electrode pattern 14 B to the drain region 11 f electrically.

Further, with reference to FIG. 5C , it will be noted that, because of the extreme miniaturization of the SRAM of the present invention, the via-plug 15 A has a diameter larger than the width of the device region 11 A 1 when viewed in the cross-sectional view taken along the line B-B′ of FIG. 5A .

Now, in the case the via-plugs 15 A and 15 B are formed at the respective, predetermined or nominal positions, the via-plugs 15 A and 15 B cause electrical contact with the source regions lie and 11 f via the silicide layers 14 e and 14 f , respectively. As a result, the source current is injected efficiently from the via-plug 15 A into the source region 11 e via the silicide layer 11 e . In the case there is caused a positional offset in the via-plugs 15 A and 15 B as shown in FIG. 15B by the continuous lines, on the other hand, there is a concern that the via-plug 15 A may contact directly with the p-type diffusion region 11 a as represented in FIG. 5B by a small circle.

FIG. 6 shows the part of FIG. 5B in the vicinity of the via-plug 15 A with enlarged scale. Referring to FIG. 6 , it can be seen that the via-plug 15 A has the tip end thereof invading into the p-type diffusion region 11 a , and as a result, there is caused a restriction of carrier path from the p + -type source region 11 e to the p-type source extension region 11 a and further to the channel region right underneath the polysilicon gate electrode pattern 13 A. The via-plug 15 A is generally formed of a metal such as W (tungsten), wherein the surface thereof is covered by a metal such as Ta or Ti or a conductive nitride such as TiN. Thus, the depletion layer extending from the surface of the via-plug 15 A into the interior of the p − -type source region 11 a restricts the carrier path further. It is believed that this is the cause of the source resistance explained with reference to FIG. 3 and FIGS. 4A and 4B .

When such a source resistance appears in one of the load transistors constituting an SRAM at the location shown by the broken line in FIG. 1 , for example, there is caused instability in the operation of the SRAM, while this leads to formation of defective memory cell.

Such a problem of misalignment of via-plug may be avoided by using the technology of so-called self-aligned contact that forms the sidewall insulation films SW with a material having resistance against the etching used for formation of the via-hole corresponding to the via-plug 15 A. In the case of the SRAM having the layout of FIG. 2 , however, there is a need of removing one of the sidewall insulation films of the polysilicon gate electrode G 2 as shown in FIG. 5B , and thus, it is difficult to construct the sidewall insulation films with a material having etching resistance.

In a first aspect, the present invention provides a semiconductor memory device comprising: a semiconductor substrate; a first CMOS inverter comprising first and second MOS transistors having respective, mutually different channel conductivity types and connected in series at a first node on said semiconductor substrate; a second CMOS inverter comprising third and fourth MOS transistors having respective, mutually different channel conductivity types and connected in series at a second node on said semiconductor substrate, said second CMOS inverter forming, together with said first CMOS inverter, a flip-flop circuit; a first transfer transistor provided on said semiconductor substrate between a first bit line and said first node, said first transfer transistor having a first gate electrode connected to a word line and driven by a selection signal on said word line; a second transfer transistor provided on said semiconductor substrate between a second bit line and said second node, said second transfer transistor having a second gate electrode connected to said word line and driven by a selection signal on said word line; a polysilicon resistance element formed on a device isolation region on said semiconductor substrate, each of said first and third MOS transistors being formed in a device region of a first conductivity type defined in said semiconductor substrate by said device isolation region, each of said first and third MOS transistors comprising: a polysilicon gate electrode formed on said semiconductor substrate via a gate insulation film and carrying gate sidewall insulation films at respective sidewall surfaces thereof; a second conductivity type source region formed in said semiconductor substrate at a first side of said polysilicon gate electrode such that an end part of said second conductivity type source region invades into a part of said semiconductor substrate right underneath said polysilicon gate electrode; a second conductivity type drain extension region formed in a surface part of said semiconductor substrate at a second side opposite to said first side of said polysilicon gate electrode such that an end part of said second conductivity type drain extension region invades into a part of said semiconductor substrate right underneath said polysilicon gate electrode; and a second conductivity type drain region formed in said semiconductor substrate at an outer side of said gate sidewall insulation film of said second side in overlapping with said drain extension region with a depth larger than a depth of said second conductivity type drain extension region, wherein said source region is formed deeper than said drain extension region, said polysilicon gate electrode having a film thickness identical to a film thickness of said polysilicon resistance element, said source region and said polysilicon resistance element being doped with the same dopant element.

In another aspect, the present invention provides a method for fabricating a semiconductor memory device, said semiconductor memory device comprising: a first CMOS inverter comprising first and second MOS transistors having respective, mutually opposite channel conductivity types and connected in series at a first node on a semiconductor substrate; a second CMOS inverter comprising third and fourth MOS transistors having respective, mutually opposite channel conductivity types and connected in series at a second node on said semiconductor substrate, said second CMOS inverter forming, together with said first CMOS inverter, a flip-flop circuit; a first transfer transistor provided on said substrate between a first bit line and said first node, said first transfer transistor having a first gate electrode connected to a word line and driven by a selection signal on said word line; a second transfer transistor provided on said substrate between a second bit line and said second node, said second transfer transistor having a second gate electrode connected to said word line and driven by a selection signal on said word line; and a polysilicon resistance element formed on a device isolation region on said semiconductor substrate, said method comprising the steps of: forming a first polysilicon pattern constituting a gate electrode of said first MOS transistor on a device region of a first conductivity type defined on said semiconductor substrate by said device isolation region via a gate insulation film, simultaneously with a second polysilicon pattern constituting said polysilicon resistance element on said device isolation region, as a result of patterning of a polysilicon film; introducing an impurity element of said second conductivity type into said device region at a first side of said first polysilicon pattern and further into said second polysilicon pattern such that there is formed a source region of said second conductivity type at said first side of said first polysilicon pattern in said device region and doping said second polysilicon pattern with said impurity element; introducing an impurity element of said second conductivity type into said device region at said first side and at a second side opposite to said first side of said first polysilicon pattern and further into said second polysilicon pattern such that there is formed a drain extension region in a surface part of said device region at said second side of said first polysilicon pattern with an impurity concentration lower than an impurity concentration of said source region and increasing an impurity concentration of said second polysilicon pattern; forming sidewall insulation films on respective sidewall surfaces of said first and second polysilicon patterns; and introducing an impurity element of said second conductivity type into said device region and further into said second polysilicon pattern while using said first polysilicon pattern and said sidewall insulation films at said first side and said second side of said first polysilicon pattern as a mask such that there are formed drain regions of said second conductivity type at respective outer parts of said sidewall insulation films of said first side and said second side of said first polysilicon pattern and such that an impurity concentration of said second polysilicon pattern is increased.

In a further aspect, the present invention provides a semiconductor memory device comprising: a semiconductor substrate; a first CMOS inverter comprising first and second MOS transistors having respective, mutually opposite channel conductivity types and connected in series at a first node on said semiconductor substrate; a second CMOS inverter comprising third and fourth MOS transistors having respective, mutually opposite channel conductivity types and connected in series at a second node on said semiconductor substrate, said second CMOS inverter forming, together with said first CMOS inverter, a flip-clop circuit; a first transfer transistor provided on said semiconductor substrate between a first bit line and said first node, said first transfer transistor having a first gate electrode connected to a word line and driven by a selection signal on said word line; a second transfer transistor provided on said semiconductor substrate between a second bit line and said second node, said second transfer transistor having a second gate electrode connected to said word line and driven by a selection signal on said word line, said first MOS transistor being formed in a first device region of a band shape formed on said semiconductor substrate by a device isolation region, said first MOS transistor having a gate electrode of a first polysilicon pattern traversing said first device region, said third MOS transistor being formed in a second device region of a band shape formed on said semiconductor substrate by said device isolation region, said third MOS transistor having a gate electrode of a second polysilicon pattern traversing said second device region, said first polysilicon pattern being connected to a first end part of said second device region by a first via-plug, said second polysilicon pattern being connected to a first end of said first device region by a second via-plug, a third via-plug being in contact with a part of said first device region at a side opposite to a side of said first via-plug with regard to said first polysilicon pattern as a power contact, a fourth via-plug being in contact to a part of said second device region at a side opposite to said second via-plug with regard to said second polysilicon pattern as a power contact, said third via-plug having a diameter larger than a width of said first device region, said fourth via-plug having a diameter larger than a width of said second device region, said third via-plug being offset from a central line of said first device region, said fourth via-plug being offset from a central line of said second device region.

According to the present invention, the problem of increase of source resistance is avoided even in the case there is caused positional offset of power contact in the load transistor that constitutes the SRAM, and it becomes possible to avoid occurrence of defects.

First Embodiment

FIG. 7A shows the layout of an SRAM 20 according to a first embodiment. Further, FIG. 7B shows a cross-sectional diagram taken along a line A-A′ of FIG. 7A .

Referring to FIGS. 7A and 7B , the SRAM 20 is formed on a silicon substrate 21 and has a construction to form an equivalent circuit diagram identical to that shown in FIG. 1 .

More specifically, there are formed an n-type well 21 N and a p-type well 21 P on the surface of the silicon substrate 21 by an ion implantation process, wherein there are formed n-type device regions 21 A 1 and 21 A 2 in the n-type well 21 N by a device isolation region 21 I in point symmetry. Further, in the p-type well 21 P, there are formed p-type device regions 21 B and 21 C by the device isolation region 21 I.

In the device regions 21 A 1 and 21 A 2 , there are formed load transistors LT 1 and LT 2 of FIG. 1 in the form of a p-channel MOS transistor, respectively, wherein the load transistor LT 1 has a p-type polysilicon gate electrode G 1 crossing the device region 21 A 1 , while the load transistor LT 2 has a p-type polysilicon gate electrode G 2 traversing the device region 21 A 2 .

In a part of the n-type device region 11 A 1 located at one side of the gate electrode G 1 , there is formed a power contact V DD , while it can be seen that there is formed a via-plug V 2 in the n-type device region 21 A 1 at the other side of the gate electrode G 1 for connecting the polysilicon gate G 2 to the device region 21 A 1 .

Similarly, in a part of the n-type device region 21 A 2 located at a one side of the gate electrode G 2 , there is formed a power contact V DD , while it can be seen that there is formed a via-plug V 4 in the n-type device region 11 A 2 at the other side of the gate electrode G 2 for connecting the polysilicon gate G 1 to the device region 21 A 2 .

Further, in the p-type well 21 P located at the left side of the n-type well 21 N, it can be seen that the device isolation region 21 I defines the p-type device region 21 B and that there is formed an re-channel MOS transistor having an n-type polysilicon gate electrode G 3 in a region of increased width of the p-type device region 11 B as the driver transistor DT 1 . Further, there is formed an n-channel MOS transistor having an n-type polysilicon gate electrode G 4 in a part of the p-type device region 21 B of narrow width as the transfer transistor TF 1 .

Similarly, in the p-type well 21 P located at the right side of the n-type well 21 N, it can be seen that the device isolation region 21 I defines the p-type device region 21 C and that there is formed an re-channel MOS transistor having an n-type polysilicon gate electrode G 5 in a region of increased width of the p-type device region 21 C as the driver transistor DT 2 . Further, there is formed an n-channel MOS transistor having an n-type polysilicon gate electrode G 6 in a part of the p-type device region 21 C of narrow width as the transfer transistor TF 2 .

Further, there is formed a via-contact V 1 in a part of the device region 21 B between the gate electrodes G 3 and G 4 , wherein the via-contact V 1 is connected to the via-contact V 2 by way of a local interconnection pattern (not illustrated). Similarly, there is formed a via-contact V 3 in a part of the device region 21 C between the gate electrodes G 5 and G 6 , wherein the via-contact V 3 is connected to the via-contact V 4 by way of a local interconnection pattern (not illustrated). Here, the via-contacts V 1 and V 2 constitute the node N 1 of FIG. 1 , while the via-contacts V 3 and V 4 constitute the node N 2 .

Further, there is formed a ground contact in a part of the p-type device region 21 B at an opposite side of the via-contact V 2 with regard to the gate electrode G 2 , and there is formed a via-contact v 5 connected to the bit line BL in a part of the p-type device region 11 B at the opposite side of the via-contact V 1 with regard to the gate electrode G 4 . Similarly, there is formed a ground contact V SS in a part of the p-type device region 21 C at an opposite side of the via-contact V 3 with regard to the gate electrode G 5 , and there is formed a via-contact v 6 connected to the bit line /BL in a part of the p-type device region 21 C at the opposite side of the via-contact V 3 with regard to the gate electrode G 6 .

Further, with the layout of FIG. 7A , there is formed a gate electrode G 7 of another memory cell across the n-type device region 21 A 1 , and there is further formed a resistance element R of polysilicon pattern on the device isolation structure 21 I in a different part of the same silicon substrate 21 . It can be seen that the resistance element R is formed with via-contacts V R1 and V R2 .

FIG. 8 shows a cross-sectional diagram taken along the line B-B′.

Referring to FIG. 8 , the gate electrodes G 1 , G 2 and G 7 are formed respectively in the form of polysilicon patterns 23 A, 23 B and 23 C each doped to n-type, wherein each of the gate electrodes G 1 , G 2 and G 7 carries, on respective sidewall surfaces, sidewall insulation films SW of typically a silicon oxide film.

In the cross-sectional diagram of FIG. 8 , the polysilicon patterns 23 A and 23 C are formed on the n-type device region 21 A 1 via gate insulation films 22 A and 22 C, while the polysilicon pattern 23 B is formed on the device isolation insulation film 21 I.

With the SRAM 20 of the present embodiment, a p + -type diffusion region 21 a is formed in the device region 21 A 1 at one side of the gate electrode pattern 23 A as the source region, and there is formed a p-type diffusion region 21 b at the other side as the drain extension region. Similarly, there is formed a p-type diffusion region 21 c in the device region 21 A 1 at one side of the gate electrode pattern 23 C as the drain extension region, the p + -type diffusion region 21 a is formed as a common source region at the other side.

Further, in the n-type device region 21 A 1 , there is formed a drain region 21 d of a p + -type diffusion region at the opposite side of the source region 21 a with regard to the gate electrode pattern 23 A at a location between the sidewall insulation film SW and the device isolation insulation film 21 I.

Further, there are formed silicide layers 24 A- 24 C on the respective surfaces of the polysilicon gate electrode patterns 23 A- 23 C, and silicide layers 24 a and 24 d are formed on the respective surfaces of the source region 21 a and the drain region 21 d.

On the silicon substrate 21 , there is formed an interlayer insulation film 25 so as to cover the polysilicon gate electrode patterns 23 A- 23 C including the silicide layers 24 A- 24 C and the sidewall insulation films SW, wherein the interlayer insulation film 25 is formed with a via-plug 25 A constituting the via-contact V DD in correspondence to the source region 21 a such that the via-plug 25 A makes a contact thereto via the silicide layer 24 a . Similarly, the interlayer insulation film 25 is formed with a via-plug 25 B constituting the via-contact V 2 in correspondence to the drain region 21 d such that the via-plug 25 B makes a contact thereto via the silicide layer 24 d . Thereby, it should be noted that the sidewall insulation film SW of the polysilicon gate electrode 23 B at the side of the polysilicon gate electrode pattern 23 A is removed, and with this, the via-plug 25 B performs the function of connecting the polysilicon gate electrode pattern 23 B to the drain region 21 d electrically.

Further, with the SRAM 20 of FIG. 8 , there is formed a polysilicon pattern 23 D on another part of the same device isolation region 21 I on the same silicon substrate 21 as a polysilicon resistance element R, and the interlayer insulation film 25 is formed with via-plugs 25 E and 25 F so as to make a contact with the polysilicon pattern 23 D via respective silicide layers 24 e and 24 f . It should be noted that the polysilicon pattern 23 D is formed simultaneously to the polysilicon gate patterns 23 A- 23 C and is formed with the sidewall insulation films SW similarly. Further, with the illustrated example, it can be seen that there is left a dielectric pattern 25 d , which is used for the mask at the time of formation of the silicide layers 24 e and 24 f , on the polysilicon pattern 23 D.

With the present embodiment, the polysilicon pattern 23 D constituting the resistance element R is formed simultaneously to the polysilicon gate electrode patterns 23 A- 23 C as a result of patterning of the same polysilicon film, and thus, the polysilicon pattern 23 D has the same film thickness, and hence the same height, to each of the polysilicon gate electrode patterns 23 A- 23 D. Further, the polysilicon pattern 23 D is doped by the same impurity element to the same concentration level as in the case the polysilicon gate electrode patterns 23 A- 23 D.

Next, the fabrication process of the SRAM 20 of FIG. 8 will be explained with reference to FIGS. 9A-9H .

Referring to FIG. 9A , the polysilicon patterns 23 A- 23 D are formed on the silicon substrate 21 as a result of patterning of a polysilicon film (not shown), such that the polysilicon patterns 23 A and 23 C are formed on the device region 21 A 1 respectively via gate insulation films 22 A and 22 C, and such that the polysilicon pattern 23 B and 23 D are formed on the device isolation insulation film 21 I that defines the device region 21 A 1 . In the description hereinafter, the polysilicon pattern 23 D used to form the resistance element R is carried upon the device isolation insulation film 21 I that also carries thereon the polysilicon pattern 23 B.

Next, in the step of FIG. 9B , the device region 21 A 1 is covered with a resist pattern R except for the part located between the polysilicon pattern 23 A and the polysilicon pattern 23 B, and B+ ions are injected into the part of the device region 21 A 1 not covered with the resist pattern R by an ion implantation process conducted under an acceleration voltage of 8 keV with a dose of 4.0×10 15 cm −2 . As a result, the p + -type diffusion region 21 a is formed in the device region 21 A 1 between the polysilicon patterns 23 A and 23 C. At the same time, with the step of FIG. 9B , the polysilicon patterns 21 A- 21 C and the polysilicon pattern 23 D on the device isolation insulation film 21 I are doped with B to pt-type.

Next, in the step of FIG. 9C , the resist pattern R is removed, and ion implantation of B+ is conducted into the device region 21 A 1 while using the polysilicon patterns 23 A and 23 C as a mask. With this, there are formed p-type drain extension regions 21 b and 21 c on the device region 21 A 1 at respective sides of the polysilicon patterns 23 A and 23 C away from the source region 21 a . At the same time, a similar injected region 21 b ′ is formed in a surface part of the source region 21 a in overlapping therewith.

Further, in the ion implantation process of FIG. 9C , the same ion implantation of B is conducted to the polysilicon patterns 23 A- 23 C under the same condition.

Next, in the step of FIG. 9D , an insulation film (not shown) is deposited on the structure of FIG. 9C by a CVD process, followed by an etchback process conducted by an anisotropic etching process that works generally perpendicularly to the substrate surface. With this, sidewall insulation films SW are formed on the respective sidewall surfaces of the polysilicon patterns 23 A- 23 D. Further, in the next step of FIG. 9E , the device region 21 A 1 is introduced with B+ by an ion implantation process while using the polysilicon patterns 21 A and 21 C as a mask. With this, the p + -type drain region 21 d is formed between the polysilicon pattern 23 A and the device isolation insulation film 21 I carrying the polysilicon pattern 23 B at the part outside of the sidewall insulation film SW. Further, similar ion implantation of B is conducted also into the interior of the p + -type source region 21 a , and there is formed a p+-type ion implantation region 21 d ′ in overlapping with the p + -type source region 21 a.

Further, in the step of FIG. 9E , the ion implantation of B is conducted to the polysilicon patterns 23 A- 23 D under the same condition.

Next, in the step of FIG. 9F , a silicide layer is formed on the exposed silicon surface of the structure of FIG. 9E by a salicide process, and as a result, there are formed silicide layers 24 A- 24 C on the polysilicon patterns 23 A- 23 C respectively. Further, silicide layers 21 a and 24 d are formed on the source region 21 a and the drain region 21 d . Further, in the polysilicon pattern 23 D constituting the resistance element R, there are formed two silicide formation regions in separation from each other by a dielectric pattern 25 d , and the silicide layers 24 e and 24 f are formed on such silicide formation regions.

Further, with the step of FIG. 9G , there are formed via-holes in the interlayer insulation film 25 so as to expose the silicide layers 24 a and 24 d , and the SRAM 20 of the equivalent circuit explained with reference to FIG. 1 is obtained by filling the via-holes with the via-plugs 25 A and 25 B.

Further, with the step of FIG. 9G , there are formed via-holes exposing the silicide layers 24 e and 24 f of the polysilicon pattern 23 D in the interlayer insulation film 25 , and the resistance element R of the polysilicon pattern 23 D is formed simultaneously by filling the via-holes with the via-plugs 25 D and 25 E.

While FIG. 9G shows the state in which there is no positional offset in the via-plugs 25 A and 25 B, while even in the case the via-plugs 25 A and 25 B are displaced in the direction to the polysilicon patterns 23 A and 23 B as shown in FIG. 9H , the via-plug 25 A maintains the contact to the high-concentration diffusion region 21 a , and there arises no increase of source resistance in the load transistor that uses the polysilicon pattern 23 A as the gate electrode. Thus, there arises no problem of change of transistor characteristics explained previously with reference to FIG. 3 .

Further, with the present embodiment, in which the formation of the high-concentration diffusion region 21 a is conducted simultaneously to the formation of the polysilicon resistance element, there arises no problem of increased fabrication process steps or increase of fabrication cost.

Second Embodiment

Meanwhile, the problem of change of characteristics of the load transistor caused by the positional offset of the via-plug 15 A explained with reference to FIG. 5B is caused by the lack of margin of positional alignment, which in turn is caused as a result of excessive miniaturization of the SRAM such that the width of the device region 11 A 1 is now smaller than the diameter of the via-plug as explained with reference to FIG. 5C . Thus, one may be tempted to the solution as shown in FIG. 10 to expand the width of the device region 11 A 1 or 11 A 2 in the vicinity of the via-contact V DD . Here, FIG. 10 is a diagram extracting the n-well part including the device regions 11 A 1 and 11 A 2 from the layout of FIG. 2 .

However, such a construction cannot be used in practice. As shown in FIG. 10 , the n-type device region 11 A 1 comes too close to the adjacent p-type well with such a construction. In view of the fact that the contact V DD is applied with the supply voltage, there is a substantial risk that the SRAM causes malfunctioning.

On the other hand, FIG. 11 is a diagram showing the layout of an SRAM according to a second embodiment. In FIG. 11 , those parts explained before are designated by the same reference numerals and the description thereof will be omitted.

Referring to FIG. 11 , it should be noted that the present embodiment does not increase the width of the device regions 11 A 1 and 11 A 2 in correspondence to he via-contact V DD as in the case of the layout of FIG. 10 but displaces the via-contact V DD in one direction.

With such a construction, it is possible to avoid the problem of degradation of the transistor characteristics caused by the increase of source resistance explained before, even in the case the via-contact V DD is displaced and comes closer to the gate electrode G 1 or G 2 and the carrier flowing from the via-contact V DD to the channel of the load transistor is blocked as represented in FIG. 11 by x, in view of the fact that the carrier can flow along the path not blocked even in such a situation. Further, because sufficient distance is secured between the n-type device region 11 A 1 and the adjacent p-type well, there arises no problem of degradation of breakdown voltage. In FIG. 11 , it should be noted that the exposed parts of the device regions 11 A 1 and 11 A 2 shown in blank are actually covered with a silicide layer.

FIG. 12 shows the device region 11 A 1 of FIG. 11 in a plan view.

Referring to FIG. 12 , each of the polysilicon pattern 13 C that corresponds to the gate electrode G 7 , the polysilicon pattern 13 A that corresponds to the gate electrode G 1 and the polysilicon pattern 13 B that corresponds to the gate electrode G 2 carries the sidewall insulation films SW thereon and extends across the device region 11 A 1 , wherein it can be seen that the via-plug 15 A constituting the via-contact V DD is formed between the polysilicon pattern 13 A and 13 C such that the via-plug 15 A is located between a pair of the sidewall insulation films SW. Further, the sidewall insulation film SW is removed locally at the side of the polysilicon pattern 13 B facing the polysilicon pattern 14 A, and the via-plug 15 B constituting the via-contact V 2 connects the polysilicon pattern 13 B to the device region 11 A 1 .

In FIG. 12 , too, the exposed part of the device region 11 A 1 shown by blank is covered with a silicide film such as CoSi 2 or NiSi.

FIG. 13A is a plan view extracting a part of FIG. 12 in the vicinity of the via-contact V DD , and hence the contact plug 15 A, FIG. 13B is a cross-sectional diagram taken along a line bx 1 -bx 1 ′ shown in FIG. 13A , FIG. 13C is a cross-sectional diagram taken along a line bx 2 -bx 2 ′ in FIG. 13A , and FIG. 13D is a cross-sectional diagram taken along a line by 1 -by 1 ′ in FIG. 13A .

Referring to FIGS. 13A-13C , it can be seen in the cross-sectional diagram of FIG. 13C taken along the line bx 2 -bx 2 ′ that there is formed a structure similar to that of FIG. 5B , and thus, there is caused an increase of source resistance for the load transistor that has the polysilicon gate electrode 13 A as a result of restriction of the path of the carriers injected into the channel region as represented in FIG. 13C by x. On the other hand, with the cross-section taken along the line bx 1 -bx 1 ′, it can be seen that the via-plug 15 A is offset from the plane of the cross-section, and the carriers are injected efficiently into the channel region from the source region 11 e into the channel region right underneath the gate electrode pattern 14 C.

In the present embodiment, too, the via-plug 15 A has a diameter W 1 larger than the width W 2 of the device region 11 A 1 (W 1 >W 2 ) as shown in the cross-section of FIG. 13D . For example, the via-plug 15 A may have the diameter of about 100 nm at the bottom end part thereof while the device region 11 A 1 only has the width of about 70 nm. Even in such a case, it is possible to secure a width of about 10 nm for the path of the carriers circumventing the via-plug 15 A by displacing the via-plug 15 A at a central line c 2 thereof by 25 nm with respect to a central line c 1 of the device region 11 A 1 by 25 nm (5=25 nm in FIG. 12 ).

In the present embodiment, too, the part of the device regions 11 A 1 and 11 A 2 in FIGS. 11 and 12 is covered with a silicide film except for the parts covered by the via-plugs 15 A and 15 B that the via-contacts V DD and V 2 or by the polysilicon patterns 13 A- 13 C constituting the gate electrodes G 1 , G 2 and G 7 including the sidewall insulation films SW thereof.

While the present embodiment has displaced the via-contact V DD to the side of a well boundary WB between the n-type well 11 N and the p-type well 11 P as shown in FIG. 11 , it is also possible to displace the via-contact V DD in the direction away from the well boundary WB.

Third Embodiment

FIG. 14 is a diagram showing the layout of an SRAM according to a third embodiment. In FIG. 14 , those parts explained before are designated by the same reference numerals and the description thereof will be omitted.

Referring to FIG. 14 , the present embodiment displaces one of the via-contacts V DD in one direction similarly to the layout of FIG. 11 and further expands the width of the device region 11 A 1 at the part where the via-contact V DD is formed in a direction away from the adjacent p-type well.

With such a construction, it is possible to avoid the problem of degradation of the transistor characteristics caused by the increase of source resistance explained before, even in the case the via-contact V DD is displaced and comes closer to the gate electrode G 1 or G 2 and the carrier flowing from the via-contact V DD to the channel of the load transistor is blocked as represented in FIG. 11 by x, in view of the fact that the carrier can flow along the path not blocked even in such a situation. Further, because sufficient distance is secured between the n-type device region 11 A 1 and the adjacent p-type well, there arises no problem of degradation of breakdown voltage. Thereby, there is caused no problem of degradation of breakdown voltage in view of the fact that the width of the device region 11 A 1 is expanded in the direction away from the well boundary WB shown in FIG. 14 by a one-dotted line.

In the present embodiment, too, the diameter W 1 of the via-plug 15 A is larger than the width W 2 of the device region 11 A 1 (W 1 >W 2 ), while it is possible to secure a width of about 30 nm for the carrier path circumventing the via-plug 15 A by expanding the width of the device region 11 A 1 by about 40 nm and displacing the central line c 2 of the via-plug 15 A by 25 nm with regard to the central line c 1 of the device region 11 A 1 .

In the present embodiment, too, the part of the device regions 11 A 1 and 11 A 2 in FIG. 14 is covered with a silicide film except for the parts covered by the via-plugs 15 A and 15 B that the via-contacts V DD and V 2 or by the polysilicon patterns 13 A- 13 C constituting the gate electrodes G 1 , G 2 and G 7 including the sidewall insulation films SW thereof.

Thus, with the second and third embodiments, the problem of increase of the source resistance associated with displacement of the contact is successfully avoided by securing the circumventing path of carriers around the via-contact V DD . Thereby, it should be noted that sufficient effect is attained when such a circumventing path is provided with the width of 10-30 nm.

The conductivity type of p-type and n-type can be reversed in each of the foregoing embodiments.

While not specified, CoSi 2 or NiSi can used for the silicide layer.