PROCESS FOR FORMING A FUSE PROGRAMMABLE READ-ONLY MEMORY DEVICE

Abstract: In a process for forming an electrically programmable fuse structure and at least one MOSFET in a semiconductor substrate, a diffusion region (17), an oxide layer (26) and a polysilicon layer (22) for the fuse structure are formed simultaneously with source/drain regions (18, 19), a gate oxide layer (24) and a polysilicon gate electrode (23) for the MOSFET. A silicon nitride layer (27) is then deposited and etched away except for sidewall regions (28, 29) adjacent the polysilicon layer (22) and gate electrode (23). The MOSFET structure is then masked, the exposed nitride sidewall region (28) is removed from the fuse structure, and the oxide layer (26) is partially removed to form a thin oxide layer (39). The mask (32) is then removed and tungsten (33, 34) is selectively deposited on the polysilicon gate electrode (23) and on the polysilicon layer (22) of the fuse structure, including the sidewall thereof so as to contact the thin oxide layer (39) over a narrow stripe having a width corresponding to the tungsten thickness.

Description: PROCESS FOR FORMING A FUSE PROGRAMMABLE READ-ONLY MEMORY DEVICE Technical Field This invention relates to a process for forming an integrated circuit structure including at least one MOSFET transistor formed on the substrate and an electrically programmable fuse structure, programmable by the application of a voltage between a conductive layer and a diffusion region in the substrate to form a permanent conductive path between said conductive layer and said diffusion region. Background Art U.S. Patent No. 3,576,549 discloses a process for forming a contact on an insulating layer over a semiconductor substrate. The substrate contains a device such as a diode or bipolar transistor. The device is programmed by applying to the conductor a voltage of sufficient magnitude and duration to break down the insulator between the contact and the device so that the conductor makes oh ic contact with the device. Disclosure of the Invention It is an object of the present invention to provide a process of the kind specified which is simple and which provides programming current limitation. Therefore, according to the present invention, there is provided a process of the kind specified characterized by the steps of: forming a gate oxide layer, a polysilicon gate electrode, and source/drain diffusion regions for said MOSFET simultaneously with the respective formation of an oxide layer, a polysilicon component and said diffusion region for said fuse structure, whereby said polysilicon component overlies said oxide layer adjacent said diffusion region; forming a conformal silicon nitride layer over the resulting structure; anisotropically etching said silicon nitride layer to selectively remove silicon nitride except for sidewall nitride regions on the sidewalls of said polysilicon gate electrode and said polysilicon component; forming a mask on the resulting structure having a window exposing said sidewall nitride region on said polysilicon component; etching silicon nitride in the presence of said mask to remove said sidewall silicon nitride region on said polysilicon component; and selectively depositing tungsten to form a tungsten shunt layer on said polysilicon gate electrode and a tungsten layer over said polysilicon component, including the sidewalls thereof and contacting the oxide layer along a region having a width corresponding to the thickness of said tungsten layer. It will be appreciated that in a process according to the invention a fuse structure can be provided in a MOSFET type integrated circuit structure using only a single additional masking step. Furthermore, since the tungsten layer contacts the oxide layer over a region having a width corresponding to the thickness of the tungsten layer, programming current limitation is effected structurally. Brief Description of the Drawings One embodiment of the present invention will now be described by way of example, with reference to the accompanying drawings, in which: Figs. 1 through 5 are cross-sectional representations of a MOSFET integrated circuit sequentially illustrating the process of forming the sidewall tungsten fuse structure of the present invention; Fig. 6 is a layout of the tungsten sidewall fuse structure in a memory array; Fig. 7 is a simplified schematic of a programmable memory organization which utilizes the tungsten sidewall fuse structure of the present invention; and Figs. 8 and 9 are plots of current versus voltage for a p-channel transistor structure employing a tungsten sidewall fuse of the present invention, taken before and after programming, respectively. Best Mode for Carrying Out the Invention Figs. 1 through 5 are cross-sectional representations of the fabrication of an integrated circuit, taken sequentially during the process of forming the tungsten sidewall fuse structure of the present invention. (The proportions used in the drawings are selected for clarity and are not to scale.) This fuse structure is formed simultaneously with and is integrated with the process of forming peripheral circuit field effect transistors having tungsten shunted polysilicon gate electrode structures. A representative tungsten sidewall fuse structure 50 and the peripheral transistor structure 51 appear in Fig. 5. Fig. 6 is a layout for and Fig. 7 is a circuit schematic of a programmable read-only memory array 12 which uses the sidewall fuse structure of the present invention. In this configuration, the fuse polysilicon/tungsten line 60 is shared by fuse cells 61 and 62 of array 12, which cells are individually addressable via FETs 63 and 64 to form between fuse polysilicon/tungsten 60 and the respective source/drain electrodes a conductive element 65 or a dielectrically coupled capacitive element 66. The former, resistive element 65, represents a "blown" fuse while the latter, capacitive element 66, represents an intact, "unblown" fuse structure exhibiting MIS characteristics. Referring now to the fabrication process as first depicted in Fig. 1, that figure shows an NMOS or PMOS integrated circuit structure at an intermediate step during a standard integrated circuit processing sequence, which is the beginning of the processing sequence of the present invention. The intermediate form structure 10 evolves into the tungsten sidewall fuse structure 50 in Fig. 5. Initially fuse diffusion 17 is formed into substrate 13 within the region between field oxides 14. Similarly, structure 11 corresponds to the shunted polysilicon gate transistor having the concluding structure 51 as appears in Fig. 5, comprised of source 18, drain 19, gate oxide 24 and polysilicon gate electrode 23, all formed in an active region bounded by field oxides 14. Note that at the stage of fabrication depicted in Fig. 1 structure 10 includes a polysilicon electrode layer 22, a gate type oxide layer 26, and diffused region 17 preferably self-aligned with polysilicon electrode layer 22. Oxide layer 26 can be formed as part of the same oxidation which forms the typically 37.5 nanometer thick peripheral transistor 11 gate oxide 24 and can be subsequently thinned to form a program oxide, or can in the alternative be selectively formed to a 10- 20 nanometer thickness. Preferably, however, polysilicon conductor layer 22, oxide 26 and diffusion 17 of fuse structure 10 are formed by the same process sequence which is used to form the poly gate electrode 23, gate oxide 24 and source/drain regions 18/19 of transistor structure 11. Thereafter, a conformal layer of silicon nitride 27 is deposited using conventional techniques. Preferably, the layer is about 500 nanometers thick. This completes the structures 10 and 11 as shown in Fig. 1. At this point, the conventional MNOS or PMOS process has been used to form the diffusion 17, oxide 26, and polysilicon layer 22, and has been modified by the formation of the silicon nitride layer 27. Referring to Fig. 2, an anisotropic etch process such as ion-assisted reactive ion etching is used to remove the nitride 27 everywhere except for the sidewall masking sections 29-29 which are left at the sides of the gate electrode 23, and the section 28 on the sidewall of the polysilicon electrode 22. One suitable anisotropic dry etch process is reactive ion etching using a S g+02 etching gas. In reactive ion etching, the mechanical component or ion bombardment component dominates the chemical reaction component and provides the very well defined vertical etch characteristics which are responsible for removing the nitride everywhere except at the vertical sidewalls. It is important to note that oxide layers 24 and 26, as well as polysilicon layers 22 and 23 are not materially removed by this nitride selective etch. This selectivity ensures the retention of an oxide layer over diffusion 17. Next, an etching mask is formed on the structures 20 and 21 of Fig. 2. Referring to Fig. 3, the mask 32 is preferably a photoresist deposited and delineated using conventional photolithographic techniques. The resulting etching mask 32 is created to have a window 35 which exposes the polysilicon electrode 22 and sidewall nitride 28 while covering FET structure 21. Note that mask 32 exposes the fuse cell regions, as further depicted in Figs. 6 and 7, of any composite FET/fuse array. The exposed sidewall nitride is then removed preferably using an isotropic etch, for example, hot phosphoric acid, while photoresist mask 32 ensures the retention of sidewall nitride 29-29 for the FETs 31. The goal of this etch, whether it be wet or dry, is to remove the sidewall nitride 28 while retaining an acceptable thickness of programming oxide 39 at the time of selective tungsten deposition. Preferably, the thickness of oxide layer 26 is next, or possibly in concurrence with the isotropic nitride etch, reduced to form a programming oxide layer 39 overlying diffusion 17 between polysilicon electrode 22 and field oxide 14 to a thickness of about 10-20 nanometers by, for example, using hydrofluoric acid etchant and the mask 32 a second time. The resulting intermediate sidewall fuse structure 30 shown in Fig. 3 has the programming oxide 39 tailored to the desired thickness and has the sidewall of polysilicon conductor 22 devoid of nitride. However, the intermediate form FET structure 31 retains the nitride sidewall mask 29-29. Referring now to Fig. 4, next, the mask 32 is removed and tungsten layers 33 and 34 are simultaneously and selectively deposited on any exposed silicon, here on the polysilicon conductors 22 and 23, respectively. Preferably, the tungsten is selectively deposited to a thickness of about 80 to 120 nanometers using a WF5+H2 process. This process deposits the tungsten over all the exposed areas of the polysilicon electrodes 22 and 23, that is, on the top and sidewalls of electrode 22, but only on the top of the nitride sidewall masked transistor structure 41. That is, the tungsten layer 33 is deposited only on the top surface of the polysilicon gate electrode 23, due to the masking sidewall nitride 29-29 of transistor structure 41. However, tungsten layer 34 is deposited both on the top and sidewall of conductor 22, to thus form the tungsten-34 - program oxide-39 - diffusion-17 fuse structure 40. During the selective deposition step, no tungsten is formed over the oxide regions such as the field oxide 14 gate oxide 24 or the program oxide 26, to the extent not initiated by the growth progressing from polysilicon layer 22. The tungsten conducting members 33 and 34 formed in this manner grow radially from and retain excellent adhesion to the underlying polysilicon conductors, such as 22 and 23. As the tungsten deposits selectively and radially from the polysilicon, the tungsten layer forms an automatically patterned, low resistance shunt layer 33 over the polysilicon gate 23 and other polysilicon conductors/interconnects (not shown) as well as the tungsten 34-program oxide 26-diffusion 17 fuse structure. Note that in the context of the fuse structure 40 in Fig. 4 the region in which the "fuse" action occurs, namely the breakdown of oxide layer 39 and the formation of conductive regions therethrough, is substantially defined by the thickness to which tungsten layer 34 is deposited. As such, not only is the region of the reaction highly localized to accentuate and accelerate the conversion from dielectric to conductor, but the dimensional constraints on the conductive path from polysilicon 22 to diffusion 17 tends to self-limit the programming currents. For the particular example defined above, conduction during programming is focused through a 0.08 to 0.12 micrometer wide (for tungsten 0.08 to 0.12 micrometers thick) stripe of electric field degraded silicon dioxide which is 0.1 to 0.2 micrometers thick. As shown in Fig. 5, the structures 40 and 41 of Fig. 4 may then be completed using conventional chemical vapor deposition and metallization techniques to form the interlevel oxide layer 36 and aluminum conductors 37, 38, 39 and 42. Fig. 7 depicts a representative circuit arrangement for a programmable read-only memory array 12 that incorporates the tungsten sidewall fuse structure 50 (tungsten 34-program oxide 39-diffusion 17) depicted in Fig. 5 in its individual memory cells 61 and 62. A layout for such an array is shown in Fig. 6. The cells 61 and 62 are addressed by a specific combination of row and column line FETs. For example, cell 61 is addressed by enabling row line FET 64 and column line FET 67 to provide cell state information via line 68 to a conventional sense amplifier. Cell 62 is similarly addressed using FETs 63 and 67. Consider now the programming of a memory cell 61 (Fig. 7) using the tungsten sidewall fuse structure 50 (Fig. 5). In this configuration, unprogram ed (open) cells are arbitrarily defined to be logic 1 whereas programmed (shorted) cells are defined as logic 0. To program a cell, the row select decoder selects the specified row and turns on transistor 64 by raising voltage on the row line 69 to V " DΓ Also, the column select decoder turns on transistor 67 for the selected column. The Vgi^g is applied to transistor 71 while the programming voltage (typically -20 volts) is applied to Vpp. Vpp and the thickness of programming oxide are selected so that application of Vpp degrades the programming oxide in the fuse structure to establish an ohmic contact between the tungsten and the underlying diffusion. In the context of Fig. 7, the ohmic contact is represented by resistor 65 in cell 61, while the absence of such contact is represented by capacitor structure 66 in cell 62. One of skill in the art will also appreciate that capacitor 66 will exhibit characteristics of a MIS diode. To read the state of cell 61, the unique row and column select lines 69 and 71 are energized. If the cell is programmed, the potential on the column line 68 will be subject to the current drawn through resistive element 65. The same is true whether column line is statically biased or merely precharged using the column line 68 distributed capacitance. In contrast, if cell 62 were addressed the potential on column line 68 would remain substantially intact. The voltage on line 68 is then read by a conventional sense amplifier to develop binary logic 0 and 1, for respective cells 61 and 62. Field effect transistors with polysilicon gate electrodes and selectively formed sidewall tungsten-to-source/drain silicon fuse structures have been built and tested by programming on selected sides of a gate electrode structure. Fig. 8 shows a representative set of IQQ versus V^g curves 72 for an unprogrammed p-channel MOSFET incorporating a tungsten gate electrode shunt structure, such as 51 in Fig. 5, yet modified during fabrication to omit the sidewall masking nitride regions. The gate oxide 24, and consequently the structure programming oxide, was about 40 nanometers in thickness. Plot 73, Fig. 9, illustrates the IQD (gate- to-drain) versus VQD behavior after the same MOSFET was programmed using the circuit configuration 74. In effect, programming degenerated the oxide between the polysilicon gate sidewall tungsten and drain diffusion to create a permanent conductive path having a nominal resistance of 400 ohms. Similar results were obtained for gate-to- source sidewall tungsten fuse structures. Thus, there has been described a novel sidewall tungsten-to-diffusion overlap fuse structure that is implemented using only one additional masking step; relies on the selective tungsten deposition which is already used for shunting polysilicon structures used in interconnects and in peripheral circuitry; uses a simple programming technique and a low programming voltage; allows for the selective thickness adjustment of the dielectric subject to programming by degeneration; provides for the formation of a programming electric field in concentrated and dimensionally constrained regions to control programming current and time characteristics; provides fast operating speeds due to the tungsten; and can be implemented as a very dense structure.

Claims: CLAIMS 1. A process for forming an integrated circuit structure including at least one MOSFET transistor formed on the substrate and an electrically programmable fuse structure, programmable by the application of a voltage between a conductive layer (34) and a diffusion region (17) in the substrate to form a permanent conductive path between said conductive layer (34) and said diffusion region (17), characterized by the steps of: forming a gate oxide layer (24), a polysilicon gate electrode (23), and source/drain diffusion regions (18, 19) for said MOSFET simultaneously with the respective formation of an oxide layer (26), a polysilicon component (22) and said diffusion region (17) for said fuse structure, | whereby said polysilicon component (22) overlies said oxide layer (26) adjacent said diffusion region (17); forming a confor al silicon nitride layer (27) over the resulting structure; anisotropically etching said silicon nitride layer (27) to selectively remove silicon nitride (27) except for sidewall nitride regions (28, 29) on the sidewalls of said polysilicon gate electrode (23) and said polysilicon component (22); forming a mask (32) on the resulting structure having a window (35) exposing said sidewall nitride region (28) on said polysilicon component (22); etching silicon nitride in the presence of said mask (32) to remove said sidewall silicon nitride region (28) on said polysilicon component (22); and selectively depositing tungsten to form a tungsten shunt layer (33) on said polysilicon gate electrode (23) and a tungsten layer (34) over said polysilicon component (22), including the sidewalls thereof and contacting the oxide layer (26) along a region having a width corresponding to the thickness of said tungsten layer (34). 2. A process according to claim 1, characterized in that, after said sidewall nitride region (28) on said polysilicon component (22) has been removed and prior to said step of selectively depositing tungsten, there is effected the step of etching said oxide layer (26) of said fuse structure to a predetermined thickness. 3. A process according to claim 1, characterized in that said step of anisotropically etching is selective, such that substantial amounts of exposed polysilicon or oxide are not removed. 4. A process according to claim 1, characterized in that said step of etching silicon nitride in the presence of said mask (32) is effected by an isotropic etch suitable to thin said oxide layer (26). 5. A process according to claim 1, characterized in that the oxide thickness between said tungsten layer (34) and said diffusion region (17) is about 10-20 nanometers and is thereby degradable by a program voltage of about 10-20 volts. 6. A process according to claim 1, characterized in that said polysilicon component (22) is adjacent to two separate diffusions electrically interconnected to the source/drain regions of two separate MOSFET transistors (63, 64).

Inventors: METZ WERNER ADAM JR, MILLER GAYLE WILBURN, MOLL MAURICE MICHAEL, SZLUK NICHOLAS JOHN, METZ, WERNER, ADAM, JR., MILLER, GAYLE, WILBURN, MOLL, MAURICE, MICHAEL, SZLUK, NICHOLAS, JOHN
Applicants: NCR CO, NCR CORPORATION
Application year: 1987
Main classification: H01L21
IPC codes: G11C17, H01L21, H01L23, H01L27