Sidewall anti-fuse structure and method for making

Abstract: A described embodiment of the present invention includes an anti-fuse (22) comprising: a first conductive layer (14) having a horizontal major surface and having a substantially vertical sidewall; a thick insulating layer (16) formed on the horizontal major surface of the first conductive layer; a dielectric layer (18) formed on the sidewall; and a second conductive layer (20) formed on the dielectric layer. In an additional embodiment, the first and/or second conductive layers comprise polycrystalline silicon and a conductive material selected from the group of titanium, tungsten, molybdenum, platinum, titanium silicide, tungsten silicide, molybdenum silicide, platinum silicide, titanium nitride and combinations thereof.

Description: Field of the Invention The present invention relates to the field of integrated circuit technology. More specifically, the present invention relates to a semiconductor device including an anti-fuse. Background of the Invention In integrated circuit fabrication, it is often desirable to allow a system whereby a user may customize an integrated circuit to their particular needs. Because of the great expense involved in designing specific integrated circuits for many specific tasks, programmable integrated circuits have been developed which allow the user to program the integrated circuit to his specific needs. An emerging type of programmable device are field programmable gate arrays (FPGAs). These devices provide large arrays of fusible type structures which allow the user to program the functional operation of the devices by altering the conductive state of these fusible devices. One such fusible device is called an anti-fuse. An anti-fuse operates in the opposite of the traditional meaning of the term "fuse". An anti-fuse is programmed by providing a voltage above a threshold determined by the characteristics of the device which causes a large current to pass through a dielectric layer between two conductive layers. After this threshold voltage has been reached, a conductive connection between the two conductive layers is permanently established. This is opposite the traditional meaning of a fuse in that when a high current is passed through a traditional fuse, the fuse is burned open and thus a conductive connection is broken. An example of the anti-fuse technology can be found in Mohsen, et. al., "Programmable Low Impedance Anti-fuse Element", U.S. Patent 4,823,181 issued April 18, 1989. A field programmable gate array structure which utilises anti-fuse elements is described in Gamal et. al., "An Architecture For Electrically Configurable Gate Arrays", IEEE Journal of Solid State Circuits, Vol. 24, No. 2, Pgs. 394-398, (April 1989). Anti-fuse structures have been disclosed in published European Patent Publications EP-A-0,250,078 A2 and EP-A-0,323,078 A2. Each of these anti-fuse structures is formed on a horizontal area of the top surface of the integrated circuit device which it is to interconnect. Therefore the density of devices on an integrated circuit structure is limited by photolithographic capabilities. Additionally, very thin dielectrics are used which results in a high capacitance between conductive leads in a gate array. As in all integrated circuits, it is desirable to providc a circuit which operates as rapidly as possible. Prior art anti-fuse structures provide horizontal areas which arc limited by the lithography capabilities used to fabricate the integrated circuit. These devices arc in arrays with a very thin dielectric (6-20nm/60-200Å as disclosed in the Mohsen, et. Al. Patent). Because these dielcctrics must be very thin, a very high capacitance is provided between the conductive leads forming the gate array. In addition, because these are many of these devices along a particular lead, the resistive/capacitive (RC) time constant for a particular lead is very high. This creates a very large time lag from when a voltage is applied to a certain lead until the lead is charged up to the desired voltage. Thus it is desirable to minimise the capacitive coupling provided by an anti-fuse element. In addition, it is desirable to minimise the lateral area covered by an anti-fuse structure to allow for greater packing density of anti-fuse elements. This allows for shorter conductive leads for the same amount of anti-fuse elements as compared to the prior art structures. Because the anti-fuse leads are shorter, the resistance along the length of the lead is minimised and the RC constant is further reduced. SUMMARY OF THE INVENTION An aspect of the present invention provides a semiconductor device including an anti-fuse as defined in claim 1. A further aspect of the invention is set forth in claim 7. In some embodiments, said first and/or second conductive layers comprise polycrystalline silicon and a conductive material selected from the group of titanium, tungsten, molybdenum, platinum, titanium silicide, tungsten silicide, molybdenum silicide, platinum silicide, titanium nitride and combinations thereof. Another aspect of the present invention provides a method for forming an anti-fuse on a semiconductor device, the method comprising the steps defined in claim 8. Yet another aspect of the present invention provides a method of forming an array of anti-fuses comprising the steps defined in claim 14. Description of the Drawing Figures 1 through 3 are side view schematic diagrams showing the processing steps, which comprise one embodiment of the present invention, used to fabricate another embodiment of the present invention; Figures 4a through 6a and 4b through 6b are plan view diagrams with the associated side view schematic diagrams showing the processing steps, which comprise another embodiment of the present invention, used to fabricate an additional embodiment of the present invention. Detailed Description Figures 1-3 are a side view schematic diagrams showing the fabrication steps, which comprise one embodiment of the present invention, used to fabricate another embodiment of the present invention. Figures 4a-6a are plan view diagrams depicting the processing steps, which comprise another embodiment of the present invention, for fabricating yet another embodiment of the present invention. Figures 4b-6b are side view diagrams showing cutaway AA of plan view diagrams 4a-6a. As shown in Figure 1, a substrate 10 is initially provided. Substrate 10 may be any of a number of materials but in this preferred embodiment compriscs crystalline silicon, thus allowing the fabrication of other devices such as transistors and diodes. Silicon dioxide layer 12 is formed on the surface of substrate 10 using thermal oxidation to a thickness of approximately 500nm (5000Å). Polycrystalline silicon layer 14 is formed on the surface of silicon dioxide layer 12 using chemical vapor deposition (CVD). Polycrystalline silicon layer 14 has a thickness of between 200-400nm (2000-4000Å) in this preferred embodiment. To provide additional conductivity and thus reduce the resistance of polycrystalline silicon, layer 14, additional materials such as titanium, tungsten, molybdenum, platinum, titanium silicide, tungsten silicide, molybdenum silicide, platinum silicide or titanium nitride may be incorporated into polycrystalline silicon layer 14 as additional layers or incorporated into the material itself. In addition, combinations of these materials or other highly conductive materials may be incorporated into polycrystalline silicon layer 14. The use of polycrystalline silicon in layer 14 is exemplary and many other materials will become apparent to those skilled in the art in light of this specification and arc considered within the scope of the present invention. A thick silicon dioxide layer 16 is then formed on the surface of polycrystalline silicon 14. Silicon dioxide layer 16 may be formed by chemical vapor deposition or thermal oxidation of polycrystalline silicon layer 14. If thermal oxidation is used, then an additional thickness of polycrystalline silicon layer 14 must be provided to allow for consumption of this additional area by the thermal oxidation process. In the present embodiment, silicon dioxide layer 16 comprises approximately 200nm (2000Å) of silicon dioxide. In additional embodiments, silicon dioxide layer 16 may be other insulators or compound insulators to provide the function of silicon dioxide layer 16. The thickness of silicon dioxide layer 16 is provided to minimise the capacative coupling between conductive layers to be formed on the surface of silicon dioxide layer 16 and underlying polycrystalline silicon layer 14. Polycrystalline silicon layer 14 and silicon dioxide layer 16 are then patterned and etched to provide the structure shown in Figure 2. Preferably, this etching is accomplished using an anisotropic etching process using for example, a plasma of hydrofluoric acid to etch silicon dioxide layer 16 and a plasma of carbon tetrachloridc to etch polycrystalline silicon layer 14. This carbon tetrachloride etch is carcfully controlled by controlling the flow rates, temperature and plasma energy to provide a high rate of selectivity of polycrystalline silicon over silicon dioxide. The precise settings of the etching process are highly dependent upon the equipment used. This selectivity allows the etch to stop when the etch passes through polycrystalline silicon layer 14 to silicon dioxide layer 12. Although it is preferable to use a highly selective etch, this step is not critical in that silicon dioxide layer 12 is very thick and allows for some etching of silicon dioxide layer 12. A dielectric layer 18 is then deposited on the surface of the structure of the Figure 2 as shown in Figure 3. A layer of silicon nitride is deposited on the surface of the structure of Figure 2. This is accomplished using chemical vapour deposition in an atmosphere such as silane and ammonia. The silicon nitride layer is then subjected to thermal oxidation in a steam ambient to provide an oxynitride layer on the surface of the silicon nitride layer. The combined effective thickness (i.e. relative to silicon dioxide) of dielectric layer 18 is approximately 6.5nm (65Å). A polycrystalline silicon layer 20 is then deposited on the surface of dielectric layer 18. As with polycrystalline silicon layer 14, the materials and structures of polycrystalline silicon layer 20 may be modified to provide higher conductivity using the materials described with regard to polycrystalline silicon layer 14 or with other materials which will become obvious to those skilled in the art in light of this specification. The use of the nitride-oxynitride (NO) dielectric provides a two-way characteristic for the breakdown of the anti-fuse provided on the vertical sidewall of polycrystalline silicon layer 14, dielectric layer 18 and polycrystalline silicon layer 20. This anti-fuse structure is indicated by the number 22 in Figure 3. When a positive potential is provided between polycrystalline silicon layer 20 and polycrystalline silicon layer 14 (i.e. a positive voltage of 0 volts is applied to polycrystalline silicon layer 14), dielectric layer 18 provides a breakdown voltage of approximately 13 1/2 volts. When the positive potential is applied to polycrystalline silicon layer 14, the breakdown voltage is approximately 10 1/2 volts. Because anti-fuse 22 is formed on the sidewall of polycrystalline silicon layer 14, the interface area of anti-fuse 22 is the width of the anti-fuse in the thickness of the page times the thickness of polycrystalline layer 14. (The thickness is diminished slightly by the thickness of dielectri layer 18, but dielectric layer 18 is much thinner than polycrystalline silicon layer 14). Because one dimension of the anti-fuse interface area is defined by the thickness of polycrystalline silicon layer 14 rather than the minimum feature size allowed by the lithography used to fabricate the integrated circuit, the area of anti-fuse 22 is minimised. Because the area of anti-fuse 22 is minimised, the capacitive coupling between polycrystalline silicon layer 20 and polycrystalline silicon layer 14 is also minimized. Another important advantage is the small surface area required for this structure. Because the anti-fuse area is determined by the vertical edge of polycrystalline silicon layer 14, the area necessary for fabricating anti-fuse layer 22 is limited only by the alignment tolerance area required for a single edge. In prior art anti-fuse structures using a horizonal structure, the area which needed to be provided was the area of the anti-fuse itself plus alignment tolerances around the periphery of the anti-fuse. Thus, the anti-fuse of the present embodiment occupies a greatly reduced portion of the surface area of an integrated circuit incorporated the anti-fuse 22 as compared to that of the prior art. Figures 4a-6a and 4b-6b are plan view diagrams with the associated schematic side view diagrams showing the processing steps of another embodiment of the present invention. Substrate 110 and silicon dioxide layer 112 are fabricated using similar processing steps as described with regard to substrate 10 and silicon dioxide layer 12, respectively, as described earlier. Polycrystalline silicon layer 114 is deposited using chemical vapor deposition. Polycrystalline layer 114 is then patterned using masking and anisotropic etching to provide the structure shown in Figure 4a. As can be seen, polycrystalline silicon layer 114 is patterned in a ladder type structure. This ladder structure allows for the fabrication of a large number of anti-fuse structures using relatively simplified processing steps. Thick silicon dioxide layer 116 is formed by chemical vapor deposition on the surface of silicon dioxide layer 112 and polycrystalline silicon layer 114. Silicon dioxide layer 116 is deposited after polycrystalline silicon layer 114 has been patterned. Silicon dioxide layer 116 is shown in Figure 4b but is omitted from the plan view of Figure 4a for clarity. An etch mask (not shown) is then formed having a thin horizonal opening perpendicular to the "rungs" of the ladder structure of polycrystalline silicon layer 114. This etch mask is used to etch opening 117 as shown in Figure 5a. A plasma of hydrofluoric acid is used to etch silicon dioxide layer 116 and a plasma of carbon tetrachloride is used to etch polycrystalline silicon layer 114 down to silicon dioxide layer 112. As a part of this process the portions of silicon dioxide layer 116 between the rungs of the ladder will be etched away. The width of the rungs of the ladder are selected so as to allow alignment tolerances for opening 117 so that opening 117 will only intersect the rungs of the ladder and will not contact the edges, although some overlap may be tolerated. The resulting structure is shown in side view in Figure 5b. A dielectric layer 118 is then formed using the techniques described with regard to dielectric layer 18 on the surface of the structure of Figures 5a and 5b. A layer of polycrystalline silicon 120 is then deposited and patterned in a series of parallel strips overlapping the rungs of the ladder as shown in Figure 6a. Although polycrystalline silicon is used for layer 120, a number of conductive materials, particularly refractory metals and refractory metal silicides, may be employed. A schematic side view of this structure along cutaway AA is shown in Figure 6b. Perfect overlap between the rungs of the ladder of polycrystalline silicon layer 114 and the strips of polycrystalline silicon layer 120 is not necessary. Therefore, no additional alignment tolerance other than the minimum lithography dimension provided as the width of the rungs of the ladder is necessary. Thus, a plurality of anti-fuse elements 122 are fabricated using a highly compact array structure and providing a minimum of capacitive coupling between the conductive leads provided by polycrystalline silicon layer 120 and polycrystalline silicon layer 114. Although specific embodiments of the present invention are described herein, they are not to be considered to be limiting to the scope of the present invention. The scope of the present invention is limited only by the claims appended hereto.

Claims: A semiconductor device including an anti-fuse, said anti-fuse comprising: a first conductive layer (14, 114) formed on a substrate and having a substantially planar major surface; an insulating layer (16, 116) formed on the major surface, said insulating layer (16, 116) and said first conductive layer being arranged such that the first conductive layer has a substantially vertical sidewall; a dielectric layer (18, 118) formed over at least said vertical sidewall, the thickness of the dielectric layer being less than that of the first conductive layer (14, 114) and the insulating layer (16, 116); a second conductive layer (20, 120) formed overlying the dielectric layer (18, 118) such that the anti-fuse is substantially defined by a region of said dielectric layer formed on said vertical sidewall, said first conductive layer (14, 114) and said second conductive layer (20, 120) adjacent said dielectric layer (18, 118); and wherein, the insulating layer (16, 116) has a thickness such that capacitive coupling between the major surface and the second conductive layer (20, 120) is substantially prevented. A device as claimed in claim 1, wherein the dielectric layer (18, 118) is formed from a material selected from the group of materials containing silicon dioxide, silicon nitride or a combination thereof. A device as claimed in any preceding claim, wherein the insulating layer (16, 116) is formed from silicon dioxide and has a thickness of approximately 200nm (2000Å). A device as claimed in any preceding claim, wherein the dielectric layer (18, 118) has a thickness of 6.5nm (65Å) or less. A device as claimed in any preceding claim, wherein the first conductive layer (14, 114) is formed on an insulator layer (12, 112) of said semiconductor device. A device as claimed in any preceding claim, wherein the first and/or second conductive layers (14, 114, 20, 120) comprise polycrystalline silicon or polycrystalline silicon and a material selected from the group containing titanium, tungsten, molybdenum, platinum, titanium silicide, molybdenum silicide, platinum silicide, titanium nitride, and combinations thereof. A device as claimed in any preceding claim comprising an array of anti-fuses, wherein the anti-fuses are arranged in rows and columns on the semiconductor device. A method for forming an anti-fuse on a semiconductor device, the method comprising: forming a first conductive layer (14, 114) on a substrate having a substantially planar major surface; forming an insulating layer (16, 116) on the major surface, said insulating layer (16, 116) and said first conductive layer (14, 114) being arranged such that the first conductive layer (14, 114) has a substantially vertical sidewall; forming a dielectric layer (18, 118) over at least the vertical sidewall, the thickness of the dielectric layer (18, 118) being less than that of the first conductive layer (14, 114) and the insulating layer (16, 116); forming a second conductive layer (20, 120) overlying the dielectric layer (18, 118) such that the anti-fuse is substantially defined by a region formed on at least said vertical sidewall, said first conductive layer (14, 114) and said second conductive layer (20, 120) adjacent said dielectric layer (18, 118); and forming the insulating layer (16, 116) to a thickness such that capacitive coupling between the major surface and the second conductive layer (20, 120) is substantially prevented. A method as claimed in claim 8, wherein the method further comprises: forming the dielectric layer (18, 118) from a material selected from the group of materials containing silicon dioxide, silicon nitride or combinations thereof. A method as claimed in claim 8 or claim 9, further comprising: forming the insulating layer (16, 116) from silicon dioxide to a thickness of approximately 200nm (2000Å). A method as claimed in any of claims 8-10, further comprising: forming the dielectric layer (18, 118) to a thickness of 6,5nm (65Å) or less. A method as claimed in any of claims 8-11, further comprising: forming the first conductive layer (14, 114) on an insulator layer (12, 112) of said semiconductor device. A method as claimed in any of claims 8-12, further comprising: forming the first and/or second conductive layers (14, 114, 20, 120) from polycrystalline silicon or polycrystalline silicon and a material selected from the group of materials containing titanium, tungsten, molybdenum, platinum, titanium silicide, molybdenum silicide, platinum silicide, titanium nitride and combinations thereof. A method for forming an array of anti-fuses, the method comprising: forming a first conductive layer (114) on a substrate such that it comprises a plurality of first conductive elements extending substantially parallel to one another in a first direction, each conductive element having a substantially planar major surface and a plurality of conductive extensions extending substantially perpendicular to said major surface; forming an insulating layer (116) over the major surface of each first conductive element, said insulating layer (116) and each first conductive element being arranged such that each first conductive element has a substantially vertical sidewall; forming a dielectric layer (118) over at least the vertical sidewall of each first conductive element, the thickness of the dielectric layer being less than that of the first conductive layer (114) and the insulating layer (116); forming a second conductive layer (120) comprising a plurality of second conductive elements extending substantially parallel to one another and substantially perpendicular to the first conductive elements, the second conductive elements extending onto the dielectric layer (118) on each vertical sidewall in a region in which each second conductive element intersects each first conductive element thereby defining said anti-fuses; and forming the insulating layer (116) to a thickness such that capacitive coupling between the major surfaces and the second conductive elements is substantially prevented.

Inventors: CHEN KUEING-LONG, LIU DAVID KUAN-YU, RIEMENSCHNEIDER BERT R, CHEN, KUEING-LONG, LIU, DAVID KUAN-YU, RIEMENSCHNEIDER, BERT R.
Applicants: TEXAS INSTRUMENTS INC, TEXAS INSTRUMENTS INCORPORATED
Application year: 1992
Main classification: H01L23
IPC codes: H01L23