Patent Document

TECHNICAL FIELD 
       [0001]    The present disclosure relates to a nonvolatile programmable semiconductor memory, and more particularly to a one-time programmable (OTP) anti-fuse memory array. 
       BACKGROUND 
       [0002]    Today&#39;s consumer electronic and wireless markets are under heavy pressure to reduce costs, increase performance, minimize power consumption and increase security. Configurability and programmability, which are accomplished with OTP memories, are the key enablers to achieving these goals. 
         [0003]    Electrical fuse (eFuse) technology is a one-time programmable memory that is programmed by forcing a high current density through a conductor link to completely rupture it or make its resistance significantly higher.  FIG. 1  shows a conventional eFuse  101  after programming. The eFuse is typically formed from a polysilicon layer  103 , over an oxide layer  105 , on substrate  107 , with a passivation layer  109  over the polysilicon layer. The polysilicon layer may include cobalt or nickel silicide on top, and the fuse is programmed by an electromigration mechanism, in which electron momentum pushes the silicide atoms out of the conductor link along the direction of electron flow. Debris and shards  111  remain after programming. However, CMOS technology has advanced to metal gates at  32  nanometer technology nodes and beyond. Therefore, eFuses based on polysilicon (and silicide) are no longer available. Thin metal fuses can be utilized instead, but they require a much larger programming current as a draw-back. Further, all e-fuses require an extra mask and extra process steps (such as thin-film deposition and etch) and are large in size. 
         [0004]    An antifuse is the opposite of an eFuse. The circuit is open (i.e., high resistance) at the start and is programmed to close (or short) the circuit by applying electrical stress that creates a low resistance conductive path. In the prior art, a hard gate oxide breakdown is used as the one-time programmable non-volatile memory mechanism.  FIG. 2  illustrates a conventional 2-transistor (2-T) antifuse bitcell. As shown, one transistor servers as the anti eFuse with logic poly program gate (WLP)  201  and another transistor serves as the selector transistor  203  are formed over gate oxide  205  and oxide  207  (typically thicker than  205 ), respectively, over substrate  209 , with spacers  211  on opposite sides of each gate. N+ region  213  is formed in N-type lightly doped drain (NLDD) region  215  in the substrate between the two gates, and a bit line contact  217  is formed over another N+ region on the opposite side of selector transistor  203 . The breakdown is intended by applying a high voltage on WLP  201 , so that the gate oxide breakdown can occur between gate  201  and NLDD region  215 . Before the breakdown, the gate oxide isolates the gate and the source like a capacitor, and after breakdown, it behaves like a resistor between the gate and the source. 
         [0005]    However, programming a 2-T antifuse can result in the oxide breaking down in one of three areas, the transistor channel  219  (below the gate), from the gate to the NLDD region (shown at  221 ), and from the gate to a halo (leakage reduction) implant region (shown at  223 ). Oxide breakdown to the channel is the desirable mechanism, since it results in a more uniform and tighter current distribution for the programmed bit cells. Breakdown to the NLDD region produces a resistive path from the gate to the substrate and a low-threshold voltage (Vt) tail, resulting in a current spread across bit cells in the array. Breakdown to the halo implant produces a high-Vt tail and results in a second peak of cell current for programmed cells. Thus, the three regions in a 2T structure produce a multi-modal current spread for bit cells. 
         [0006]      FIG. 3  illustrates another known antifuse, a metal gate antifuse including 1T1C bitcell. Adverting to  FIG. 3 , the capacitor based antifuse includes an N+ metal gate  301  formed with a thin high-k dielectric layer  303  and N+ source/drain regions  305  in Nwell  307  of substrate  309 , with access transistor  311  connected to a row. When a programming voltage is applied to the gate, breakdown can occur between the gate and either the N+ region or the Nwell, and a high reading current is sensed. The main drawback of the 1T1C bitcell is that the access transistor includes a thick oxide, such that the transistor is bulky and occupies significant silicon area. 
         [0007]    As illustrated in  FIG. 4 , selected wordline  401  is biased with Vread, and selected bitline  403  is biased as ground, for reading bit  405  (at the cross point of the selected wordline and bitline), and other unselected wordlines and bitlines  407  are floating. For a selected bit (at an intersection of a wordline and a bitline) at high resistance, little current will flow from the wordline to the bitline, and for the selected bit at low resistance, a large current will flow from the wordline to the bitline. Therefore, if selected bit  405  is at high resistance and unselected bits  409  have a low resistance status, a current (called a sneak current) will flow along the arrows in the picture, bypassing the selected bit. This current does not tell us information about the selected bit and would lead to misread of the bit  405 . 
         [0008]    To prevent sneak currents in a simple cross-point memory array, a select transistor is required for each bit. Since two transistors are still needed for one bit, this antifuse is area inefficient. Attempts have been made to utilize diodes for such cross-point memory arrays to prevent sneak current, as illustrated in  FIG. 5 . Specifically, memory element  501 , including top electrode  503 , bottom electrode  505 , and memory layer  507  sandwiched between  503  and  505 , is formed with diode  509  at each intersection of a wordline  511  and a bitline  513 . However, diodes incur process complexity and/or area penalty. 
         [0009]    A need therefore exists for methodology enabling manufacture of an area-efficient, low power, high-performance, reliable OTP memory which is compatible with CMOS system-on-chip (SOC) technology, and the resulting device. 
       SUMMARY 
       [0010]    An aspect of the present disclosure is a method of manufacturing a 2-D cross-point antifuse memory array with no separate selector for each bit. 
         [0011]    Another aspect of the present disclosure is 2-D cross-point antifuse memory array with no separate selector for each bit. 
         [0012]    Additional aspects and other features of the present disclosure will be set forth in the description which follows and in part will be apparent to those having ordinary skill in the art upon examination of the following or may be learned from the practice of the present disclosure. The advantages of the present disclosure may be realized and obtained as particularly pointed out in the appended claims. 
         [0013]    According to the present disclosure, some technical effects may be achieved in part by a method including: forming wells of a first polarity in a substrate; forming a bitline in each well, the bitline being of the first polarity; and forming plural metal gates across each bitline, wherein no source/drain regions are formed between the metal gates. 
         [0014]    Aspects of the present disclosure include forming shallow trench isolation (STI) regions in the substrate; forming the wells of the first polarity extending below the STI regions; and forming top portions of the bitlines separated by the STI regions. Further aspects include forming the metal gates by: forming polysilicon gates on the substrate across the bitlines; covering the polysilicon gates with an interlayer dielectric (ILD); removing the polysilicon, forming cavities; and filing the cavities with metal. Other aspects include forming a high-k dielectric in the cavities prior to filling with metal. Another aspect includes forming wells of a second polarity between the wells of the first polarity below the STI regions prior to forming the metal gates. An additional aspect includes the substrate being a substrate of the second polarity. Further aspects include forming well contact regions of the first polarity in each bitline. Other aspects include programming a selected bit at an intersection of one metal gate and one bitline by applying a positive voltage to the one metal gate, the positive voltage being sufficient to cause dielectric breakdown of the gate oxide under the metal gate at the selected bit. An additional aspect includes the dielectric breakdown forming an element above the well of the first type at the selected bit showing non-linear current-voltage (I-V) characteristics, wherein a forward current is significantly greater than a reverse current. 
         [0015]    Another aspect of the present disclosure is a device including: a substrate; a well of a first polarity in the substrate; a bitline of the first polarity in the well; and plural metal gates across each bitline, wherein no source/drain contacts are formed between the metal gates. 
         [0016]    Aspects include shallow trench isolation (STI) regions in the substrate, wherein: the wells of the first polarity extend below the STI regions; and top portions of the bitlines are separated by the STI regions. Further aspects include the metal gates being formed by: forming polysilicon gates on the substrate across the bitlines; covering the polysilicon gates with an interlayer dielectric (ILD); removing the polysilicon, forming cavities; and filing the cavities with metal. Another aspect includes a high-k dielectric layer lining the cavities, under the metal gates. An additional aspect includes wells of a second polarity between the wells of the first polarity below the STI regions. A further aspect includes the substrate being a substrate of the second polarity. Other aspects include well contact regions of the first polarity in each bitline. 
         [0017]    Another aspect of the present disclosure is a method including providing a 2-D cross-point memory array having wells of one polarity, bitlines of the one polarity in the Nwells, metal gates across the bitlines, each with a gate oxide thereunder, and no source/drain regions between the metal gates; selecting a bit at an intersection of one metal gate and one bitline; and programming the selected bit by applying a positive voltage to the one metal gate, the positive voltage being sufficient to cause dielectric breakdown of the gate oxide under the metal gate at the selected bit. 
         [0018]    Aspects include applying a voltage equal to one half of the positive voltage to metal gates and bitlines other than the one metal gate and the one bitline. Further aspects include reading the selected bit after programming by applying a positive reading voltage of 0.2 to 1V to the one metal gate. An additional aspect includes the dielectric breakdown forming an element above the well at the selected bit showing non-linear current-voltage (I-V) characteristics, wherein a forward current is significantly greater than a reverse current. 
         [0019]    Additional aspects and technical effects of the present disclosure will become readily apparent to those skilled in the art from the following detailed description wherein embodiments of the present disclosure are described simply by way of illustration of the best mode contemplated to carry out the present disclosure. As will be realized, the present disclosure is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the present disclosure. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as restrictive. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    The present disclosure is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawing and in which like reference numerals refer to similar elements and in which: 
           [0021]      FIG. 1  schematically illustrates a conventional eFuse; 
           [0022]      FIG. 2  schematically illustrates a conventional 2-T antifuse; 
           [0023]      FIG. 3  schematically illustrates a conventional metal gate antifuse; 
           [0024]      FIG. 4  schematically illustrates a sneak current in the metal gate of  FIG. 3 ; 
           [0025]      FIG. 5  schematically illustrates a diode selector with each antifuse of a cross-point antifuse memory array; 
           [0026]      FIG. 6A  schematically illustrates a top view of a 2-D cross-bar antifuse memory array, in accordance with an exemplary embodiment, and  FIGS. 6B and 6C  schematically illustrate cross-sectional views along lines  6 B- 6 B′ and  6 C- 6 C′, respectively, of the memory array illustrated in  FIG. 6A ; 
           [0027]      FIGS. 7A through 13A  and  7 B through  13 B schematically illustrate cross-sectional views along lines  6 B- 6 B′ and  6 C- 6 C′, respectively, of method steps for forming the 2-D cross-bar antifuse memory array of  FIGS. 6A through 6C ; and 
           [0028]      FIG. 14  illustrates programming a bit of the 2-D cross-bar antifuse memory array of  FIGS. 6A through 6C . 
       
    
    
     DETAILED DESCRIPTION 
       [0029]    In the following description, for the purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of exemplary embodiments. It should be apparent, however, that exemplary embodiments may be practiced without these specific details or with an equivalent arrangement. In other instances, well-known structures and devices are shown in block diagram form in order to avoid unnecessarily obscuring exemplary embodiments. In addition, unless otherwise indicated, all numbers expressing quantities, ratios, and numerical properties of ingredients, reaction conditions, and so forth used in the specification and claims are to be understood as being modified in all instances by the term “about.” 
         [0030]    The present disclosure addresses and solves the current problems of process complexity, area penalty, high power requirements, unreliability, and incompatibility with CMOS technologies attendant upon forming OTP non-volatile memories with eFuses or antifuses. In accordance with embodiments of the present disclosure, bitlines are formed as part of Nwells in a substrate, and plural metal gates are formed across each bitline with no contacts between the metal gates. When a selected bit is programmed by applying a high voltage to the selected gate, an element with non-linear I-V characteristics is formed over the Nwell at the selected bit, which acts as a selector, thereby eliminating the need for a separate selector for each bit. 
         [0031]    Methodology in accordance with embodiments of the present disclosure includes forming Nwells and Pwells in a substrate, forming an N-type bitline in each Nwell, and forming plural metal gates across each bitline, wherein no source/drain regions are formed between the metal gates. 
         [0032]    Still other aspects, features, and technical effects will be readily apparent to those skilled in this art from the following detailed description, wherein preferred embodiments are shown and described, simply by way of illustration of the best mode contemplated. The disclosure is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as restrictive. 
         [0033]    Adverting to  FIGS. 6A through 6C , a 2-D cross-point antifuse memory array is illustrated in accordance with an exemplary embodiment.  FIG. 6A  shows a top view, and  FIGS. 6B and 6C  show cross-sectional views along lines  6 B- 6 B′ and  6 C- 6 C′, respectively. As illustrated, N-type active regions  601  are formed in Nwells  603 . The N-type active regions  601  define shallow trench isolation (STI) regions  605 . 
         [0034]    The Nwells, in turn, are formed within a Pwell  607  in a P-type substrate  609 . The Nwells  603  act as bitlines. Multiple bits share the same Nwell bitline. Metal gates  611  are formed across active regions  601 , each with a high-k dielectric layer  613  (an oxide having a dielectric constant greater than that of silicon dioxide, such as hafnium oxide) thereunder. An N+ well tap region  615  is formed in the active region  601 , and bitline wire out lines  617  are connected to the well tap regions. 
         [0035]    In the resulting structure a parasitic element with non-linear I-V characteristics is formed, after the metal gate-to-well breakdown, which serves as the selector. Consequently, there is no sneak current in the cross-point array, and, therefore, no misreads. Further, no source/drain regions for each bit, no source/drain contacts between the gates (or wordlines), and multiple bits sharing the same Nwell bitline enable an aggressive design rule and small cell size. In addition, wordlines can be strapped by multiple metal layers in back-end-of-line processes. 
         [0036]      FIGS. 7A through 12A  and  7 B through  12 B shown cross-sectional views (along lines  6 B- 6 B′ and  6 C- 6 C′, respectively) of the method steps for forming the 2-D cross-point antifuse memory array illustrated in  FIGS. 6A through 6C . Adverting to  FIGS. 7A and 7B , STI trenches are etched in a P-type substrate  701  to a depth of 100 to 200 nm and a width of 100 to 1000 nm. The trenches are filled with an oxide and planarized, for example by chemical mechanical polishing (CMP), to form STI regions  703 . 
         [0037]    Next, Nwell  801  and Pwell  803  implants are performed, as illustrated in  FIGS. 8A and 8B , by conventional methods. The wells may be formed to a depth of 100 to 500 nm. Bitlines are formed in the entire Nwell. The STI regions  703  are defined by an active region in upper portion of the Nwell/bitline 
         [0038]    Adverting to  FIGS. 9A and 9B , an oxide layer  901  is deposited over the substrate to a thickness of 1 to 3 nm, followed by a polysilicon layer  903  to a thickness of 30 to 200 nm. The two layers are then patterned and etched to form gate oxide  1001  and dummy gates  1003 , as illustrated in  FIGS. 10A and 10B . Gate oxide  1001  and dummy gates  1003  are etched to a width of 14 to 500 nm. Gate spacers, not shown for illustrative convenience, are formed on opposite sides of each dummy gate. 
         [0039]    As illustrated in  FIGS. 11A and 11B , N+ well tap regions  1101  are formed in active region  901  by conventional source/drain implantation. Although shown at opposite ends of the active region, an N+ well tap region  1101  will be formed every 16 or 32 wordlines. Then, an oxide interlayer dielectric (ILD)  1201 , such as silicon dioxide, is formed over the entire substrate, filling all spaces, as illustrated in  FIGS. 12A and 12B . The oxide is planarized, for example by CMP, down to the upper surface of the polysilicon dummy gates  1003 . 
         [0040]    Next a replacement gate process is performed. Specifically, the polysilicon dummy gates  1003  and underlying gate oxide  1001  are removed forming cavities between the gate spacers. A thin high-k dielectric  1301  (an oxide having a dielectric constant greater than that of silicon dioxide, such as hafnium oxide) is deposited in the cavities to a thickness of 1 to 3 nm, thereby lining the cavitites. Then, a metal gate  1303  is formed on the high-k dielectric  1301  in each cavity. The metal gate may, for example, be formed of titanium nitride (TiN) or aluminum. Further processing may include etching a contact for each active region  1101  or gate region and conventional middle-of-line (MOL) and back-end-of-line (BEOL) processes. 
         [0041]    Adverting to  FIG. 14 , the memory of  FIGS. 6A through 6C  is programmed by selecting a bit (at the intersection of a metal gate  1401  and a bitline  1403  in Nwell  1405 ), supplying a positive voltage (Vpp), for example 2 to 5 V, to the metal gate  1401 , which drives the N-type bitline  1403  (grounded) to accumulation. This creates a high field across the thin oxide  1407 , which causes dielectric breakdown (at breakdown location  1409 ). At the same time, Vpp/2 is supplied to un-selected wordlines and bitlines to prevent disturbances. The gate dielectric breakdown forms an element with non-linear I-V characteristics at the breakdown location, and the high resistance at a reverse-biased condition blocks sneak current during reading. The programmed bit is read by applying a low positive voltage, e.g., 0.2 to 1 V, to the metal gate to check the resistance between the wordline and the bitline. 
         [0042]    It should be noted that although the substrate has been described as a P-type substrate, and the bitlines formed in Nwells, all references to N-type and P-type are merely exemplary and could be reversed. Specifically, the substrate could be an N-type substrate, with the bitlines formed in Pwells, which are separated by STI regions and Nwells, and P+ well tap regions could be formed in the Pwells. 
         [0043]    If the disclosed OTP memory is implemented in Finfet technology, the enhanced field at the fin tip could further reduce the programming voltage. More specifically, at the top corner of the fin, the bottom oxide E field would be enhanced, thereby helping oxide breakdown during programming. 
         [0044]    The embodiments of the present disclosure can achieve several technical effects, such as a small OTP cell (about 4F 2  in the cross-point array) with no sneak current in the cross-bar array, multiple bits sharing the same Nwell bitline, low power operation, and compatibility with 20 nm and 14 nm replacement metal gate CMOS process flow. This OTP array is also compatible with Finfet technology, to further reduce programming voltage due to field enhancement at the fin tip. The present disclosure enjoys industrial applicability in any of various types of highly integrated semiconductor devices used in microprocessors, smart phones, mobile phones, cellular handsets, set-top boxes, DVD recorders and players, automotive navigation, printers and peripherals, networking and telecom equipment, gaming systems, and digital cameras, particularly in any of various types of semiconductor devices, particularly in the 20 nm technology node and beyond. 
         [0045]    In the preceding description, the present disclosure is described with reference to specifically exemplary embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the present disclosure, as set forth in the claims. The specification and drawings are, accordingly, to be regarded as illustrative and not as restrictive. It is understood that the present disclosure is capable of using various other combinations and embodiments and is capable of any changes or modifications within the scope of the inventive concept as expressed herein.

Technology Category: 3