ANTIFUSE PROGRAMMED PROM CELL

Abstract: An antifuse based PROM cell design allows large currents to be sinked during cell programming to ensure low programmed resistance of the cell while using minimum-geometry select devices. This is achieved by utilizing a pseudo SCR latchup effect during programming. The regions in the semiconductor substrate (28a) forming lower antifuse electrodes for the antifuses in the PROM cells are doped at low levels with phosphorus. An antifuse layer (32a) formed from an oxide, oxide-nitride, or oxide-nitride-oxide antifuse layer, is formed over the lower antifuse electrode, and an upper antifuse electrode is formed from polysilicon. A minimum-geometry N-Channel select transistor is formed in series with the antifuse to complete the PROM cell. The drain (26a) and source (22a) diffusions of the select transistor are arsenic doped and the drain diffusion is contiguous with the lower antifuse electrode. A bit line (18a) is contacted to the upper antifuse electrode (34x) and the select transistor gate is part of a polysilicon word line. The source diffusion of the select transistor is grounded.

Description: &EE£IFI£A!IQN ANTIFUSE PROGRAMMED PROM CELL BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to antifuse technology. More particularly, the present invention relates to a novel programmable read only memory cell fabricated using antifuse technology. 2. The Prior Art In applications employing normal ONO antifuses having their lower electrodes disposed in the silicon substrate, substrate regions comprising lower antifuse electrodes have typically been doped to very high levels (i.e., Ie20) with arsenic, usually in the same masking step used to form source/drain diffusions for active devices in the substrate. Programming antifuses often takes a large amount of current, in excess of 4mA and as much as 15mA if an output impedance of 250 ohms or less is desired in the programmed antifuse. This means that a rather large MOS transistor is required, as much as 20 μ. This large transistor size is rather prohibitive for programmable read only memory (PROM) cells. The tradeoff between on resistances of programmed antifuse PROM cells higher than 250 ohms and the need for such large select transistors has been unacceptable in the prior art. BRIEF DESCRIPTION OF THE INVENTION A novel antifuse based PROM cell design allows large currents to be sinked during cell programming to ensure low programmed resistance of the cell antifuse while using minimum-geometry select devices. This is achieved by utilizing a pseudo SCR latchup effect during programming. In this invention the regions in the semiconductor substrate forming lower antifuse electrodes for the antifuses in the PROM cells are doped at low levels, i.e., - 1 - around lel8, with phosphorus. An antifuse layer formed from an oxide, oxide- nitride, or oxide-nitride-oxide antifuse layer, is formed over the lower antifuse electrode, and an upper antifuse electrode is formed from polysilicon. A minimum-geometry N-Channel select transistor is formed in series with the antifuse to complete the PROM cell. The drain and source diffusions of the select transistor are arsenic doped and the drain diffusion is contiguous with the lower antifuse electrode. A bit line is contacted to the upper antifuse electrode and the select transistor gate is part of a polysilicon word line. The source diffusion of the select transistor is grounded. An array of antifuse PROM cells according to the present invention may be formed in closely spaced orientation having common polysilicon word lines and metal interconnect layer bit lines. BRIEF DESCRIPTION OF THE DRAWING FIGURES FIG. 1 is a schematic diagram of a PROM cell according to the present invention. FIG. 2 is a top view of the layout of a group of four PROM cells fabricated on a p-type substrate with an N-Channel select transistor and an antifuse as the programming element. FIG. 3 is a cross sectional view of a single one of the PROM cells of the view of FIG. 2, taken through the center of one of the metal lines. FIG. 4 is a schematic diagram of an illustrative PROM array according to the present invention. DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT Those of ordinary skill in the art will realize that the following description of the present invention is illustrative only and not in any way limiting. Other embodiments of the invention will readily suggest themselves to such skilled persons. - 2 Referring first to FIG. 1, a schematic diagram of a PROM cell 10 according to the present invention is presented. PROM cell 10 comprises an antifuse 12 connected between a bit line 14 and the drain of an N-Channel MOS select transistor 16. The gate of the N-Channel MOS select transistor 16 is connected to a word line 18 and the source of the N-Channel MOS select transistor 16 is connected to ground. The N-Channel select transistor may be formed at minimum feature size for the fabrication technology employed. The antifuse is of the type disclosed in United States Patent No. to Mohsen et al., and in United States Patent No. to Hamdy et al. The lower electrode of the antifuse 12 is formed from a diffused region in a p-type semiconductor substrate, preferably a P+ epi substrate as is known in the art. However, instead of following the teachings of the prior art and forming the lower antifuse electrode from a heavily doped arsenic region in the substrate, the lower antifuse electrode is formed from a lightly doped phosphorus region. According to a presently preferred embodiment of the invention, the lower electrode of the antifuse is formed from a phosphorus diffusion doped to a concentration of between about lel7 and about lel9. An antifuse layer preferably comprising an oxide, oxide-nitride, or oxide- nitride-oxide antifuse layer, as known in the prior art, is formed over the lower antifuse electrode, and an upper antifuse electrode is formed from heavily doped polysilicon preferably as taught by Hamdy et al. in United States Patent No. Referring now to FIG. 2, a top view is presented of the layout of a group of four PROM cells 10a, 10b, 10c, and lOd fabricated on a grounded p-type substrate comprising a P+ region 20a and an epitaxial P- region 20b. Each cell employs an antifuse as the programming element.with an N-Channel select transistor. FIG. 3 is a cross sectional view through the substrate at the center of a pair of the PROM cells of the present invention. Viewing FIGS. 2 and 3 together will facilitate an understanding of the present invention. Reference is made to cell 10a in the upper left-hand corner of FIG. 2 for a disclosure of the PROM cell of the present invention. Cells 10b, 10c, and lOd are virtually identical mirror images of cell 10a. Elements in each of the PROM cells in - 3 FIG. 2 will be given the alphabetical suffix corresponding to the alphabetical suffix of their cell's reference numeral. For the purposes of this disclosure, FIG. 3 will be assumed to depict all of PROM cell 10a and a portion of PROM cell 10b, as well as a portion of an unidentified cell at the right hand side of the figure. In both FIGS. 2 and 3, elements of FIG. 1 which appear therein will be given the same reference numerals as in FIG. 1, followed of course by the appropriate alphabetical suffix. Bit line 14 of FIG. 1 is a metal line running vertically in FIG. 2 on both the left and right portions of the figure. The metal line at the left portion of FIG. 2 is bit line 14a in the top half of the figure and bit line 14b in the bottom half of the figure. The metal line at the right portion of FIG. 2 is bit line 14c in the top half of the figure and bit line 14d in the bottom half of the figure. Word line 18 of FIG. 1 is a polysilicon line running horizontally in FIG. 2 on both the top and bottom portions of the figure. The polysilicon line at the top portion of FIG. 2 is bit line 18a in the left half of the figure and bit line 18c in the right half of the figure. The polysilicon line at the bottom portion of FIG. 2 is bit line 18b in the left half of the figure and bit line 18d in the right half of the figure. PROM cell 10a in FIG. 3 starts on the left at diffused region 22a and 22b which forms the source diffusion for the N-Channel select transistor of PROM cell 10a as well as the source o the N-Channel select transistor of PROM cell 10b mirrored to the left. PROM cell 10a ends on the right at the center of field oxide region 24. Everything to the right of the center of field oxide region 24 is a portion of another PROM cell mirrored to the right. Referring still to FIG. 3, immediately to the left of field oxide region 24 is an N+ diffused region 26a including an N- diffused region 28a in its center. N- region 26a comprises a phosphorus doped region at a concentration of between about lei 7 and about lei 9, and forms the lower antifuse electrode. The surrounding N+ region 26a comprises an arsenic doped region at a concentration of between about lel9 and about le21, preferably about le20, and forms the drain of N-Channel MOS select transistor 16 of FIG. 1. The fact that regions 26a and 28a are contiguous assures connection between the lower antifuse electrode and the drain of the select transistor. - 4 - As best seen in FIG. 3, an oxide layer 30a is disposed over diffused regions 26a and 28a. Antifuse layer 32a is disposed in an aperture formed in the oxide layer 30a in alignment with diffused region 28a. As presently preferred, antifuse layer 32a comprises about 35 angstroms of thermal silicon dioxide, about 65 angstroms of deposited silicon nitride, and about 25 angstroms of thermal silicon dioxide converterd from the layer of silicon nitride. The left half of a polysilicon layer identified by reference numeral 34a forms the upper electrode of antifuse 12a. The right half of that same polysilicon layer 34x forms the upper electrode of an antifuse 12x in a neighboring cell. Metal line 18a connects to polysilicon layer 34a at contact region 36. To the left of diffused region 26a a channel 38a for the select transistor is bounded on its left by diffused region 22a, the source of the N-Channel MOS select transistor 16a. Diffused region 22b, the left half of the same diffusion, begins PROM cell 10b as the source of the N-Channel MOS select transistor 16b. This diffusion is connected to ground. A gate oxide region 40a overlying channel region 38a, separates channel region 38a from the polysilicon word line 18a,c of FIG. 2 and a gate oxide region 40b overlying channel region 38b, separates channel region 38b from the polysilicon word line 18b,d of FIG. 2. In FIG. 3, the rightmost polysilicon strip is the portion 18a of polysilicon word line 18a,c of FIG. 2 which forms the gate of N- Channel MOS select transistor 16a, and the leftmost polysilicon strip is the portion 18b of the polysilicon word line 18b,d which forms the gate of N-Channel MOS select transistor 16a. Another portion of oxide layer 30a is shown in FIG. 3 disposed above polysilicon gates 18a and 18b . As will be appreciated by those of ordinary skill in the art, the structures of PROM cells 10b, 10c, and lOd would be apparent from the above disclosure of PROM cell 10a. The PROM cell of the present invention may be fabricated as a part of a conventional CMOS semiconductor fabrication process. The process preferably starts with a P+ substrate including a P- epitaxial layer formed thereon. Conventional front-end steps are performed to create field oxide channel stop - 5 - regions for all active devices to be formed on the substrate. Field oxide region 24 of FIG. 3 is created at this time. Assuming the transistor devices are formed using self-aligned gate techniques, a gate oxide layer is formed and a polysilicon layer is deposited thereover, doped, and defined to form the word lines and select transistor gates. Next, the N+ and N- regions for the antifuse and select transistors are formed using conventional CMOS implant techniques. Persons of ordinary skill in the art will recognize that the step for formation of the N- region for the lower antifuse electrode may already be present in a CMOS process for formation of lightly-doped drain (LDD) regions for other transistors. In such an event, the lower antifuse electrode may be formed simply by including the additional apertures in the LDD mask already used in the process. Otherwise a second implant step will be inserted into the process to form the N- lower antifuse electrode diffusion. Next, an oxide layer is formed to cover the word line polysilicon and form the interlayer dielectric for the antifuses. Masking and etching steps are then performed to form the antifuse apertures and the antifuse dielectrics are formed in the apertures. Conventional semiconductor processing techniques are then used to form another insulating layer through which contact apertures (including the contact for the metal to cell polysilicon connection) are etched as is known in the art. A metal interconnect layer is deposited and defined over that layer to form the metal bit line. Conventional rear-end steps are then performed to complete the semiconductor structure. In the case when programming of the PROM cell 10a is performed with a positive voltage in the range of from about 10 to about 18 volts, depending on the overall dielectric thickness and composition, applied to the upper electrode 34a of the antifuse via metal line 18a, the antifuse layer 32a will break down and the local temperature of the N-diffusion region 28a will exceed the melting point of the silicon. A large quantity of holes and electrons will be generated in this process. The electrons will flow to the top plate and the holes will flow into the N- diffused region 28a. Since N- region 28a is doped lightly, the probability of the holes recombining is very small. Thus, the holes flow straight to the grounded P+ substrate 20 as - 6 - shown in FIG. 3 by arrows 42. To maintain this current flow a voltage of at least 2.5 volts must be maintained across the antifuse dielectric. The N- doped region 28a must therefore receive a supply of electrons from ground. These electrons are initially supplied by the N-Channel select transistor 16a. If the demand for this current is exceeded, the localized substrate potential will be raised by the injected holes thus forward biasing the source of the N-Channel select transistor 16a. This condition will supply any excess electrons required. Those of ordinary skill in the art will appreciate that, under these conditions, any amount of programming current can be achieved. The SCR action of the antifuse structure depicted in FIGS. 2 and 3 relies on a "virtual" P+ region at the top of the N- region 28a caused by the melting silicon material during the programming process. The hot spot in the silicon generated by the breakdown of the dielectric during programming is a source of holesjust as a P+ region would be in normal semiconductor operation. Since the programming process generates temperatures in excess of the melting point of the silicon in region 28a, holes and electrons are produced in large numbers. The electrons are attracted to the positive programming potential at the metal bit line 18a and the holes will be attracted to the grounded P+ substrate 20a just as holes from a P+ region would be so attracted. This virtual P+ region is forward biased with respect to the N- region 28a, which thus injects a large number of holes into the P- epi region 22b of the substrate, thus forward biasing that region with respect to N+ region 22a, the source of the select transistor 16, thus injecting many electrons back into P- region 22b and sustaining the forward biasing of the "virtual" P+ region in N- region 28a. This is the common four-layer structure for SCR action, even though one of the layers is a virtual layer. The sustaining voltage for the hot spot will be a bias of at least about 2.5 to 3 volts across the dielectric to keep the temperature high. This phenomenon allows substrate hole injection only during programming. The injected substrate current will discharge the adjacent antifuse substrate nodes to ground and will result in applying the voltage on the bit line 18a across whatever other unprogrammed antifuses in the array containing PROM cell 10a are connected to that bit line 18a. This not a problem because the impedance of the devices supplying the bit lines with programming current will clamp the voltage on the bit line to a value below that of the programming voltage of the antifuse dielectric at the instant the selected antifuse is blown. In the read mode the current through the antifuse connection is low enough to avoid heating up the silicon enough to cause it to create the "virtual" P+ region and go into SCR latchup. The only remaining issue is the fact that the lightly doped N-region 28a does have a significant spreading resistance. This can be tailored somewhat by increasing the doping as much as possible without stopping the SCR action. This limit is presently believed to be about lei 9. In many applications this is not very important though as electrons injected into the N- region 28a by the surrounding N+ region 26a will lower the resistance of the N- region 28a. This will create a slight nonlinearity in the resistance of the device of only about 0.3 V. This can easily be accounted for in the design of the circuit by setting the trip point of the sense amplifier somewhat higher, i.e., about 0.5 volt. The PROM cell of the present invention may be advantageously employed as an element in a practical PROM array. FIG. 4 is a schematic diagram of the core of an illustrative PROM array 50 according to this aspect of the present invention. While PROM array 50 is shown as a 3 by 3 array, persons of ordinary skill in the art will observe that it is merely illustrative and that PROM arrays of arbitrary size are possible using the technology of the present invention. Bit line 52 is associated with a first column of array 50. In addition, bit lines 54 and 56 are associated with second and third columns of array 50. Similarly, word line 58 is associated with a first row of array 50. In addition, word lines 60 and 62 are associated with second and third rows of array 50. Array 50 includes nine PROM cells. PROM cells 64, 66, and 68 comprise the first row of the array; PROM cells 70, 72, and 74 comprise the second row of the array; PROM cells 76, 78, and 80 comprise the third row of the array. PROM cells 64, 70, and 76 comprise the first column of the array; PROM cells 66, 72, and 78 comprise the second column of the array; PROM cells 68, 74, and 80 comprise the third column of the array. - 8

Claims: Each bit line is connected to the lower antifuse electrode of the antifuse in all of the cells in its column. Bit line 52 is connected to the lower electrodes of antifuses 64-1, 70-1, and 72-1, bit line 54 is connected to the lower electrodes of antifuses 66-1, 72-1, and 78-1, bit line 56 is connected to the lower electrodes of antifuses 68- 1 , 74- 1 , and 80- 1. Similarly, each word line is connected to the upper antifuse electrode of the antifuse in all of the cells in its row. Bit line 52 is connected to the upper electrodes of antifuses 64-1 , 66-1, and 68-1, bit line 54 is connected to the upper electrodes of antifuses 6706-1, 72-1, and 74-1, bit line 56 is connected to the upper electrodes of antifuses 76-1, 78-1, and 80-1. The peripheral circuits necessary for implementation of array 50 are well known to those of ordinary skill in the art and will not be disclosed in detail herein. As will be appreciated by skilled persons, bit lines 52, 54, and 56 are driven by column decoder/driver 82 and word lines 58, 60, and 62 are driven by row decoder/driver 84. Address decoding is well understood in the art. To program any PROM cell in array 50, the bit line and word line associated with that cell are asserted in programming mode with programming mode voltages, causing the appropriate antifuse to program as taught herein. For example, to program PROM cell 72, programming mode voltages are asserted on bit line 54 and word line 60. To read any cell in the array 50, the bit line and word line associated with that cell are asserted in read mode with read mode voltages, causing the appropriate antifuse to program as taught herein. For example, to read PROM cell 72, read mode voltages are asserted on bit line 54 and word line 60. While embodiments and applications of this invention have been shown and described, it would be apparent to those skilled in the art that many more modifications than mentioned above are possible without departing from the inventive concepts herein. The invention, therefore, is not to be restricted except in the spirit of the appended claims. What is claimed is: 1. A PROM cell having an antifuse programming element, formed in a semiconductor substrate comprising: an antifuse PROM cell including a first region in the semiconductor substrate forming a lower antifuse electrode, said first region doped at low levels of about lelδ with phosphorus, an antifuse layer formed over said lower antifuse electrode, and an upper antifuse electrode formed from a selected portion of an antifuse polysilicon layer; a bit line coupled to said upper antifuse electrode; an N-Channel select transistor having a drain formed from a first N+ region formed in said substrate, said first N+ region connected to said lower antifuse electrode, a source formed from a second N+ region formed in said substrate and spaced apart from said first N+ region to form a channel therebetween, said source coupled to ground, and a gate disposed over and insulated from said channel, said gate coupled to a word line. 2. The PROM cell of claim 1 wherein said first and second N+ regions are doped with arsenic in a concentration from between about lei 9 to between about le21. 3 The PROM cell of claim 1 wherein said gate comprises a portion of a polysilicon layer forming a word line. 4. The PROM cell of claim 1 wherein said bit line is a metal interconnect line and is coupled to said upper antifuse electrode via a contact. 5. The PROM cell of claim 1 wherein said N-Channel select transistor has a width smaller than 8 microns. 6. A semiconductor structure for programming an antifuse comprising: an antifuse including a first region in a semiconductor substrate forming a lower antifuse electrode, said first region doped at low levels around lelδ with phosphorus, an antifuse layer formed over said lower antifuse electrode, and an upper antifuse electrode formed from a selected portion of an antifuse polysilicon layer; an interconnect line coupled to said upper antifuse electrode; - 10 -

Inventors: MCCOLLUM JOHN, MCCOLLUM, JOHN
Applicants: ACTEL CORP, ACTEL CORPORATION
Application year: 1998
Main classification: H01L29
IPC codes: H01L27