Polycrystalline silicon contact structure of an SRAM cell

Abstract: A contact structure provides electrical contact between two polycrystalline silicon interconnect layers. The lower layer has a silicide layer on its upper surface. The upper polycrystalline silicon layer can be doped with a different conductivity type, and makes an ohmic contact with the silicided region of the lower polycrystalline silicon layer.

Description: The present invention is related generally to integrated circuits, and more specifically to a contact between different layers of polycrystalline silicon interconnect. In semiconductor circuits, it is known that ohmic contacts are desirable between interconnect layers. An ohmic contact is one in which no P-N junction is formed. When polycrystalline silicon interconnect lines having different conductivity types make contact, a P-N junction is formed. A similar junction can be formed when polycrystalline silicon having the same conductivity type, but very different doping levels (such as N -- to N + ) make contact. For various reasons, it is often desirable to have interconnect having different conductivity types make contact, and it would be desirable to provide an ohmic contact for such structures. It is therefore an aim of the present invention to provide an ohmic contact between polycristalline silicon interconnect layers of different conductivity types or very different doping levels. JP-A-60068634 discloses a method of fabricating a semiconductor device in which a high melting point metal layer is formed on a first polycrystalline silicon layer, a heating step is employed to for a silicide layer, and a second polycrystalline silicon layer is formed over the silicide layer. JP-A-63081984 discloses a method of fabricating a polycrystalline semiconductor diode. The diode comprises a silicon island formed on a semiconductor substrate which is divided into regions of p- and n-type doping. EP-A-0032025 discloses a static random access memory device including a first polycrystal semiconductor layer which is relatively heavily doped and a second polycrystal semiconductor layer which is relatively lightly doped. Where the second polycrystal semiconductor layer contacts the first polycrystal semiconductor layer or a relatively heavily doped region of a substrate upward diffusion of impurities from the relatively heavily doped parts to the second polycrystal semiconductor layer is provided. Thereby, ohmic contacts with the second polycrystal semiconductor layer can be made, whilst the general light level of doping of the second polycrystal semiconductor layer can be selected so that that layer provides a resistor of a selected value. It is another aim of the present invention to provide such a contact which is easily formed with a process compatible with existing process technologies. It is a further aim of the present invention to provide such a contact which is suitable for use in an SRAM structure to provide a load. The present invention provides an SRAM cell comprising first and second cross-coupled transistors and first and second access transistors, said cross-coupled transistors each being field-effect transistors having respective gates, sources and drains, the gate of said first cross-coupled transistor being connected to the drain of said second cross-coupled transistor and the gate of said second cross-coupled transistor being connected to the drain of said first cross-coupled transistor and first and second load elements respectively connecting the drains of said first and second cross-coupled transistors to a positive voltage supply, characterised in that the first and second load elements each comprise a contact structure comprising a first polycrystalline silicon interconnect layer of a first conductivity type connected to the drain of the respective cross-coupled transistor, a silicide layer on said first polycrystalline silicon layer, an insulating layer overlying said first polycrystalline silicon layer and said silicide layer, said insulating layer having a contact opening therein which exposes a portion of an upper surface of said silicide layer, a second polycrystalline silicon interconnect layer of a second conductivity type overlying a portion of said insulating layer and contacting said silicide layer in the contact opening, wherein an ohmic contact is formed in the contact opening to provide a resistor connection to the drain of the respective cross-coupled transistor and a doped region of a third conductivity type in the second polycrystalline interconnect layer at a location spaced from the contact opening, the doped region of a third conductivity type being more heavily doped than the second polycrystalline interconnect layer and being connected to the positive voltage supply whereby a p-n junction is formed within the second polycrystalline interconnect layer to provide a diode connection between the resistor and the positive voltage supply. The present also provides a method of fabricating an SRAM cell comprising first and second cross-coupled transistors and first and second access transistors, said cross-coupled transistors each being field-effect transistors having respective gates, sources and drains, the gate of said first cross-coupled transistor being connected to the drain of said second cross-coupled transistor and the gate of said second cross-coupled transistor being connected to the drain of said first cross-coupled transistor and first and second load elements respectively connecting the drains of said first and second cross-coupled transistors to a positive voltage supply, characterised in that the first and second load elements each comprise a contact structure formed by the method steps of forming a first polycrystalline silicon interconnect layer of a first conductivity type, forming a silicide layer on the first polycrystalline silicon interconnect layer, forming an insulating layer over the entire device, forming a contact opening in the insulating layer wherein a contact region on an upper surface of the silicide layer is exposed, forming a second polycrystalline silicon interconnect layer of a second conductivity type over the insulating layer, wherein the second polycrystalline silicon interconnect layer makes ohmic contact with the silicide layer through the contact opening thereby providing a resistor connection to the drain of the respective cross-coupled transistor and forming a doped region of a third conductivity type within the second polycrystalline silicon interconnect layer at a location spaced from the contact opening, the doped region of a third conductivity type being more heavily doped than the second polycrystalline interconnect layer and being connected to the positive voltage supply whereby a p-n junction is formed within the second polycrystalline silicon interconnect layer thereby to provide a diode connection between the resistor and the positive voltage supply. The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself however, as well as a preferred mode of use, and further objects and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein: Figures 1-3 illustrate a preferred method for forming a contact ; and Figure 4 is a schematic diagram of an SRAM cell according to the present invention. The process steps and structures described below do not form a complete process flow for manufacturing integrated circuits. The present invnetion can be practiced in conjunction with integrated circuit fabrication techniques currently used in the art, and only so much of the commonly practiced process steps are included as are necessary for an understanding of the present invention. The figures representing cross-sections of portions of an integrated circuit during fabrication are not drawn to scale, but instead are drawn so as to illustrate the important features of the invention. Referring to Figure 1 , a semiconductor substrate 10 is partially covered with an oxide layer 12 . The oxide layer 12 is not complete over the entire surface of the substrate 10 , but that portion of interest to the present description has no openings to the substrate 10 . A polycrystalline silicon layer 14 lies on the oxide layer 12 . In the illustrative embodiment, layer 14 is doped N-type. The polycrystalline silicon layer 14 has been silicided to form a silicide layer 16 thereon. The polycrystalline silicon 14 and silicide layer 16 have been patterned in a previous processing step as known in the art to form a signal line. The polycrystalline silicon layer 14 may be a first polycrystalline silicon layer, such as commonly used to form gate electrodes of field effect devices. Alternatively, polycrystalline silicon layer 14 may be a second or later level used for interconnect between different portions of an integrated circuit device. At the processing stage shown in Figure 1 , the transistors of the device have already been formed. Once the polycrystalline silicon and silicide layers 14 , 16 have been formed and patterned, an oxide layer 18 is formed over the surface of the device. Oxide layer 18 is typically a thin oxide layer, having a thickness of between 500 and 1000 angstroms (10Å = 1 nm). The thickness of oxide layer 18 may be any thickness which is compatible with the fabrication process with which the invention described herein is being used. Referring to Figure 2 , oxide layer 18 is patterned and etched to define a contact opening 20 to the upper surface of the silicide layer 16 . A layer of polycrystalline silicon 22 is then deposited over the surface of the device. A light dosage of boron is implanted into the polycrystalline silicon layer 22 in order to convert it to a P-type conductor. A typical dosage would be approximately 10 13 atoms/cm 2 . Referring to Figure 3 , the polycrystalline silicon layer 22 is then masked, and a heavy arsenic implant made to define an N + region 24 . A typical dosage for such implant is 5 x 10 15 atoms/cm 2 . Such doping level is used to allow the N + region 24 to be used as a power supply line. A P-N junction 26 is formed at the interface between the N + region 24 and the lightly P-doped polycrystalline silicon layer 22 . The doping of polycrystalline silicon layer 22 is low enough to define a resistor, but is sufficiently high to cause degeneration in the contact opening 20 , providing an ohmic contact between the polycrystalline silicon layer 22 and the silicide layer 16 . Thus, although the polycrystalline silicon layer 14 is N-type, no P-N junction is formed at the contact between the two layers 14 , 22 . After formation of the highly doped N + regions 24 , the polycrystalline silicon layer 22 is etched to define interconnect, leaving the structure shown in Figure 3 . The device is then ready for formation of further oxide and interconnect levels as desired. Referring to Figure 4 , a 4-transistor SRAM cell is shown. The contact structure formed in Figure 1-3 is suitable for use as a load element in the cell of Figure 4 . Cross-coupled field effect devices 30 , 32 form the basis of the SRAM cell. Access transistors 34 , 36 connect the bit line BL and complemented bit line BL' to common nodes 38 , 40 , respectively. Access transistors 34 , 36 are driven by the word line 42 as known in the art. Node 38 is connected to the power supply line V cc through resistor 44 and diode 46 . Node 40 is connected to V cc through resistor 48 and diode 50 . Node 38 corresponds to contact opening 20 in Figure 3 . Resistor 44 corresponds to polycrystalline silicon region 22 of Figure 3 , with diode 46 being formed at the junction 26 . Node 40 , resistor 48 , and diode 50 correspond to Figure 3 in a similar manner. Since the contact at contact opening 20 , corresponding to nodes 38 and 40 , is an ohmic contact, the load for the SRAM cell is formed by a resistor and a diode rather than back-to-back polycrystalline silicon diodes. In some SRAM cell designs, this can provide improved performance over the use of a resistor alone, or back-to-back polycrystalline silicon diodes. A similar ohmic contact can be formed between a lower polycrystalline silicon layer which is doped P-type and an upper N-type layer. The silicide layer prevents formation of a P-N junction in the contact opening. While the invention has been particularly shown and described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the scope of the invention.

Claims: An SRAM cell comprising: first and second cross-coupled transistors (30,32) and first and second access transistors (34,36), said cross-coupled transistors (30,32) each being field-effect transistors having respective gates, sources and drains, the gate of said first cross-coupled transistor (30) being connected to the drain of said second cross-coupled transistor (32) and the gate of said second cross-coupled transistor (32) being connected to the drain of said first cross-coupled transistor (30) ; and first and second load elements respectively connecting the drains of said first and second cross-coupled transistors (30,32) to a positive voltage supply; characterised in that the first and second load elements each comprise a contact structure comprising: a first polycrystalline silicon interconnect layer (14) of a first conductivity type connected to the drain of the respective cross-coupled transistor; a silicide layer (16) on said first polycrystalline silicon layer (14); an insulating layer (18) overlying said first polycrystalline silicon layer (14) and said silicide layer (16), said insulating layer (18) having a contact opening (20) therein which exposes a portion of an upper surface of said silicide layer (16); a second polycrystalline silicon interconnect layer (22) of a second conductivity type overlying a portion of said insulating layer (18) and contacting said silicide layer (16) in the contact opening (20), wherein an ohmic contact is formed in the contact opening (20) to provide a resistor connection to the drain of the respective cross-coupled transistor; and a doped region (24) of a third conductivity type in the second polycrystalline interconnect layer (22) at a location spaced from the contact opening (20), the doped region (24) of a third conductivity type being more heavily doped than the second polycrystalline interconnect layer (2) and being connected to the positive voltage supply whereby a p-n junction is formed within the second polycrystalline interconnect layer (22) to provide a diode connection between the resistor and the positive voltage supply. The structure of claim 1, wherein the first conductivity type is n-type, and the second conductivity type is p-type. The structure of claim 1, wherein the first conductivity type is p-type, and the second conductivity type is n-type. The structure of claim 1, wherein the first and second conductivity types are of the same type, and wherein the second polycrystalline silicon layer (22) is lightly doped relative to the first polycrystalline silicon layer (14). The structure of claim 1, wherein said second polycrystalline silicon interconnect layer (22) is lightly doped in the contact opening (20). The structure of claim 5, wherein the first and second conductivity types are of opposite type and the first and third conductivity types are of the same type. A method of fabricating an SRAM cell comprising: first and second cross-coupled transistors (30,32) and first and second access transistors (34,36), said cross-coupled transistors (30,32) each being field-effect transistors having respective gates, sources and drains, the gate of said first cross-coupled transistor (30) being connected to the drain of said second cross-coupled transistor (32) and the gate of said second cross-coupled transistor (32) being connected to the drain of said first cross-coupled transistor (30); and first and second load elements respectively connecting the drains of said first and second cross-coupled transistors (30,32) to a positive voltage supply; characterised in that the first and second load elements each comprise a contact structure formed by the method steps of: forming a first polycrystalline silicon interconnect layer (14) of a first conductivity type; forming a silicide layer (16) on the first polycrystalline silicon interconnect layer (14); forming an insulating layer (18) over the entire device; forming a contact opening (20) in the insulating layer (18) wherein a contact region on an upper surface of the silicide layer (16) is exposed; forming a second polycrystalline silicon interconnect layer (22) of a second conductivity type over the insulating layer (18), wherein the second polycrystalline silicon interconnect layer (22) makes ohmic contact with the silicide layer (16) through the contact opening (20) thereby providing a resistor connection to the drain of the respective cross-coupled transistor; and forming a doped region (24) of a third conductivity type within the second polycrystalline silicon interconnect layer (22) at a location spaced from the contact opening (20), the doped region (24) of a third conductivity type being more heavily doped than the second polycrystalline interconnect layer and being connected to the positive voltage supply whereby a p-n junction (26) is formed within the second polycrystalline silicon interconnect layer (22) thereby to provide a diode connection between the resistor and the positive voltage supply. The method of claim 7, wherein the first and second conductivity types are of the same type, and the second polycrystalline silicon interconnect layer (22) is lightly doped relative to the first polycrystalline silicon interconnect layer (14). The method of claim 7, wherein the first and second conductivity types are of opposite type and the first and third conductivity types are of the same type.

Inventors: BRADY JAMES, CHAN TSIU CHIU, CULVER DAVID SCOTT, BRADY, JAMES, CHAN, TSIU CHIU, CULVER, DAVID SCOTT
Applicants: ST MICROELECTRONICS INC, STMICROELECTRONICS, INC.
Application year: 1991
Main classification: H01L23
IPC codes: H01L23, H01L21