Abstract:
A non-volatile memory cell comprising a metal oxide semiconductor field effect transistor (MOSFET) fabricated to read back a logic level “one” state and programmable by a gate to drain fusing to read back to a logic level “zero” state. The drain is patterned to enhance the formation of a localized hot spot during device lateral NPN transistor snapback for a controlled meltdown of gate oxide and the creation of an ohmic gate to drain path. A metal oxide semiconductor (MOS) integrated circuit typically includes a plurality of memory cells composing a programmable array. The drains of each memory cell are tied together in parallel and connected to a high-level programming voltage, and the sources are tied to ground. Each memory cell further comprises a series pair of switches for connecting a low-level bias voltage to the gate for cell programming and for connecting a bias current to the gate for cell read-back, and a parallel pair of switches for shunting the gate to the source to inhibit cell programming.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. §119(e) of U.S. Provisional Application No. 60/126,106, filed Mar. 24, 1999; the disclosure of which is hereby incorporated by reference. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     N/A 
     BACKGROUND OF THE INVENTION 
     This invention relates generally to non-volatile memory cells and, more particularly, to a non-volatile memory cell for a linear metal oxide semiconductor (MOS) integrated circuit. 
     Precision performance by a linear MOS integrated circuit is typically accomplished by internally trimming the circuit currents and voltages. Conventional trimming techniques are performed as part of the manufacturing process and must be permanent so that the trimming does not change when power is removed from the circuit. Several techniques use a fuse-based technology, such as the metal spike shorting of the emitter-base junction of a NPN transistor, or the opening of metal or thin film links, that require high current pulses to sever the subject material. Another method requires specialized equipment for the laser trimming of thin film links or the vernier trim of thin film resistor tabs, which may drift over time. All of these techniques are intended for low-density memory applications. High-density storage techniques use either “anti-fuse” technology or place a long-term charge storage on the MOSFET gates. These techniques, however, add complex processing steps to wafer fabrication and extra cost to the device. 
     What is needed, therefore, is an inexpensive and user-programmable technique for performing a parametric trim of a linear MOS integrated circuit. 
     BRIEF SUMMARY OF THE INVENTION 
     In accordance with the invention, a non-volatile memory cell comprises a metal oxide semiconductor field effect transistor (MOSFET) fabricated to read back a logic level “one” state and programmable by a gate to drain fusing to read back to a logic level “zero” state. The MOSFET drain has a blunted and notched “V” shape to enhance the formation of a localized hot spot during lateral NPN transistor snapback for a controlled meltdown of gate oxide and the creation of an ohmic gate to drain path. The MOSFET gate oxide is also made thick enough to achieve snapback upon application of a gate bias voltage without gate oxide rupture. 
     A metal oxide semiconductor (MOS) integrated circuit typically includes a plurality of memory cells composing a programmable array. The MOSFET drains of each memory cell are tied together in parallel and connected to a high-level programming voltage, and the MOSFET sources are tied to ground. Each memory cell may further comprise a series pair of switches connecting a low-level bias voltage to the MOSFET gate for cell programming based on “snapback selectivity” and connecting a bias current to the MOSFET gate for cell read-back, and a parallel pair of switches shunting the gate to the source to inhibit cell programming. The array may be user-programmed to selectively engage parametric elements distributed throughout a linear MOS integrated circuit and thereby accomplish a parametric trim of the circuit for precision performance in an intended application. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     The invention will be more fully understood by reference to the following Detailed Description of the Invention in conjunction with the drawings, of which: 
     FIG. is a block diagram of a linear BiCMOS integrated circuit incorporating a programmable array of non-volatile memory cells in accordance with the invention; 
     FIG. 2A is a cross sectional view of a MOSFET of a memory cell of the array of the circuit of FIG. 1 having fused gate oxide and its lateral NPN parasitic; 
     FIG. 2B is a topological view of the MOSFET of FIG. 2A having fused gate oxide; 
     FIG. 3 is a plot of drain current and drain voltage for the MOSFET of FIG. 2 during cell programming; 
     FIG. 4 is an electrical diagram of a non-volatile memory cell of the array of the circuit of FIG. 1 comprising a MOSFET, a row FET and a column FET; 
     FIG. 5 is an electrical diagram of an eight by eight array of non-volatile memory cells; 
     FIG. 6 is a flow diagram of a procedure for programming the array of FIG. 5; and 
     FIG. 7 is a flow diagram of a procedure for read back of the array of FIG.  5 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A bipolar complementary oxide semiconductor (BiCMOS) linear integrated circuit  10  incorporating the invention is illustrated in FIG.  1 . The circuit  10  comprises a plurality of blocks  12   a - 12   f , or modules, each performing a part of a linear function, such as a digital-to-analog conversion. To achieve the most accurate output, each block typically requires a permanent and precise trimming of internal voltages and currents. This trimming is accomplished by a one-time programming of the appropriate non-volatile (NV) memory cells  40  of a cell array  50  and a power-up serial distribution of the array contents to distribution or holding registers  14   a - 14   f  associated with the blocks  12   a - 12   f  of the circuit  10 . 
     Each memory cell  40  includes a metal oxide semiconductor field effect transistor (MOSFET) device. A cross-sectional view of an n-type metal oxide semiconductor field effect transistor (MOSFET)  20  and its parasitic lateral NPN transistor  29  is illustrated in FIG. 2A and a topological view of the MOSFET  20  is illustrated in FIG.  2 B. The MOSFET  20  is comprised of a substrate  21 , a drain region  22  and source region  24 . The drain  22  and source  24  are diffused into the substrate  21  and are separated by a channel  21   a  in the substrate  21 . The drain  22  preferably has a ballast resistance  22   a  of approximately 130-150 ohms to limit the drain current during cell programming. The drain ballast resistance  22   a  serves to equalize the drain currents of MOSFET devices that are connected in parallel. A polysilicon gate  26  is deposited partially into an insulating oxide layer  28  and directly above the channel  21   a . That portion of the insulating oxide layer  28  directly beneath the gate  26  is referred to as gate oxide while the remainder of the oxide layer  28  is referred to as field oxide. The insulating oxide layer  28  has openings  28   a ,  28   b ,  28   c ,  28   d  above the drain  22 , gate  26 , source  24 , and substrate  21 , respectively. Metal traces  23 ,  25  contact the drain  22  and source  24  through the openings  28   a ,  28   c , respectively, and provide connectivity to other devices or external circuitry (not shown). 
     Gate-to-channel oxide breakdown, or rupture, is well known in the art and considerable attention has been given to avoiding a breakdown, or minimizing the damage caused by a breakdown, in a conventional MOSFET by maximizing its hardness to electrical overstress (EOS), such as electro-static discharge (ESD). In particular, a breakdown may occur when a sufficiently high energy/voltage pulse is applied to the gate so as to cause the parasitic lateral transistor to go into an “avalanche” condition. The current continues to increase until a voltage fold-back phenomena called lateral NPN transistor “snapback” occurs. Within a few micro-seconds hot spots form at the gate-drain edge causing degradation of the gate oxide and essentially destroying the device by shorting the gate to the drain, source or substrate. 
     The present invention uses this phenomenon advantageously for efficient and permanent electrical programming on a linear MOS integrated circuit. The insulating oxide layer  28  of the present MOSFET is made thick enough, typically 400-600 angstroms, to support a drain-to-gate voltage high enough to achieve lateral NPN transistor snapback without rupturing the insulating oxide layer  28 . In addition, the drain  22  has a blunted “V” shape to facilitate the creation of at least one pinpoint micro furnace when a programming voltage is applied to the drain  22 . The micro-furnace produces a hot spot  27  reaching a temperature over 1000 degrees Celsius, preferably 1500 degrees Celsius. If a programming voltage is maintained for a sufficient duration, approximately 100 milli-seconds, the insulating oxide layer  28  in the vicinity of the hot spot  27  chemically changes from an insulator to a glassy conductor to thereby create an ohmic path between the gate  26  and the drain  22 . Depending on the duration of the applied programming voltage, several hot spots may be created on the channel edge of the drain so that the resulting gate-to-drain resistance is relatively low, typically less than 500 k ohms. 
     The ohmic path is a permanent breakdown of the insulating oxide layer  28 . When the drain  22  is grounded and a bias current is injected into the gate  26 , a voltage below the threshold for a logic “one” level, typically 2.4 volts for 5 volt logic, is developed at the gate  26 . For example, a 10 micro-ampere bias current flowing through an approximately 4K ohmic path produces a 40 millivolt gate voltage. 
     The channel pattern should be kept to the process minimum feature size to, inter alia, maximize the lateral NPN transistor beta and the hot electron effects, and minimize the required voltage applied for cell programming. Typical dimensions for the channel are a 2 micron width and a 3 micron length. The source channel edge is preferably narrow, typically 2 microns. The bottom end of the “V” shaped drain is preferably blunted to produce a wide drain channel edge, typically 3-4 microns. The wide drain channel edge facilitates multiple parallel hot spots that help to increase the programming yield. The “V” shaped drain is also notched through the upper portion to produce the ballast resistance region  22   a . The ballast resistance regulates the drain current, typically to about 40 milli-amperes, and ensures a uniform distribution of drain current from cell to cell. The drain current, in turn, controls the rate of temperature rise at the channel edge of the drain and the radius of the hot spot. 
     FIG. 3 is a plot  30  of a programming current  32 , a programming voltage (V P )  34 , the drain voltage  36  and the gate voltage  38  during cell programming. A bias voltage (V B ), typically 5 volts, is applied to the gate  26  to enable the cell for programming. The programming voltage (V P ), typically 15 volts, is then applied to the drain to bring the parasitic lateral NPN transistor  29  into avalanche at point ‘a’. When the current density reaches point ‘b’, voltage snapback occurs. During snapback there is a significant power dissipation in the channel  21   a  and the temperature of the device continues to rise in the vicinity of the hot spot  27  until the oxide melts down at point ‘c’ to create the ohmic path between the gate  26  and the drain  22 . 
     It is the application of the bias voltage to the gate  26  of a particular cell that results in a “snapback selectivity” wherein only a biased cell is fused despite the fact that the programming voltage is applied to the drain  22  of every cell in the array during programming. The bias voltage reduces the level of the voltage that must be applied to the drain  22  to cause parasitic lateral NPN transistor snapback. Thus, while the biased cell achieves snapback, the other array cells are unaffected because the level of the programming voltage is inadequate to produce the drain current density necessary for snapback. 
     FIG. 4 is an electrical diagram of the non-volatile memory cell. In addition to the MOSFET  20 , each cell  40  also includes a serial pair of signal switching devices, field effect transistors (FET)  42   a ,  42   b , and a parallel pair of shunt switching devices, FETs  44   a ,  44   b . As described below, column and row control signals (COL, COL/, ROW, ROW/) are applied to the gates  43   a ,  45   a ,  43   b ,  45   b  of the signal FETs  42   a ,  42   b  and shunt FETs  44   a ,  44   b , respectively, to select a particular cell for programming. The signal switching devices are used either to apply the bias voltage to the MOSFET gate  26  during programming, or to connect the cell to a holding register  14  (FIG. 1) during power-up read back. The shunt switching devices are used to inhibit cell programming by shunting the MOSFET gate  26  to the ground potential of the MOSFET source  24 . 
     FIG. 5 is an electrical diagram of a sixty-four cell array  50 . The cells  40   C,R  are arranged in eight columns  52   a - 52   h  by eight rows  54   a - 54   h . It should be appreciated that smaller or larger sized arrays may be implemented. In an array programming mode, the drain  22  of each cell  40  is connected to the common programming voltage, V P , while in an array read back mode the drains are connected to ground. The source  24  of each cell  40  is connected to a common ground. As is described below, the gate  26  of all inactive cells are shunted to the grounded source  24 , while the gate  26  of the active cell is connected to an I/O line. In the program mode, the I/O line is an input signal, i.e., the applied bias voltage, V B , whereas in the read back mode the I/O line is an output signal, i.e., the voltage developed by a bias current, typically 10 microamperes, applied to the gate  26  of the active cell. 
     The array  50  also includes a column shift register  56  generating for each column a pair of column control signals, COL and COL/ (not labeled in FIG.  5 ), connected to the switch gates  43   a ,  45   a  of each cell  40   c,a - 40   c,h  in the corresponding column  52   a - 52   h . The array also includes a row shift register  58  generating for each row a pair of row control signals, ROW and ROW/ (not labeled in FIG.  5 ), connected to the switch gates  43   b ,  45   b  of each cell  40   a,R - 40   h,R  in the corresponding row  54   a - 54   h . A reset signal, RESET, is connected to reset pins of the column and row shift registers. A column clock signal (CLKC) and a row clock signal (CLKR) are used to shift a column bit and a row bit, respectively, across the shift register banks  56 ,  58 . A DONE signal changes from a logic level “zero” state to a logic level “one” state when the row bit is shifted out of the last register indicating that each cell in the array has been programmed or read back. 
     FIG. 6 is a flow diagram of an array programming procedure for the illustrated embodiment. This programming procedure is performed only one time, either as a final step in the manufacturing process, or by a user tailoring the device for a specific application. The programming may be accomplished by a variety of different techniques. For example, custom software may be written to download data and control signals through the parallel port of a personal computer. Alternatively, a commercially available data programmer used to program memory devices, such as the 27256 EPROM family, may be utilized. 
     At step  601 , column shift register outputs  56   b - 56   h  are reset so that the COLb-COLh signals (not labeled in FIG. 5) are at a logic level “zero” state, while the first column shift register output  56   a  is preset so that the COLa signal (not labeled in FIG. 5) is at a logic level “one” state. Also at step  601 , row shift register outputs  58   b - 58   h  are reset so that the ROWb-ROWh signals (not labeled in FIG. 5) are at a logic level “zero” state, while the first row shift register output  58   a  is preset so that the ROWa signal (not labeled in FIG. 5) is at a logic level “one” state. Thus, the cell at column a, row a is the first cell to be selected for programming. The MOSFET gate  26  of the cell at column a, row a is connected to the IO signal, while the MOSFET gates of the remaining cells are shunted to the MOSFET source  24 . 
     At decision step  603 , it is determined whether the selected cell, i.e., the active cell, is to be programmed to a logic level “zero” state, or is to remain at the fabricated logic level “one” state. If it is determined at decision step  603  that the active cell is not to be programmed to a logic level “zero” state, the procedure moves directly to decision step  611 . On the other hand, if the active cell is to be programmed to a “zero” logic level, a logic level “one” state is placed on the I/O line causing the 5 volt bias voltage, V B , to be placed onto the gate  26  at step  605 . The programming voltage, V P , on the drain  22  is ramped up from zero to 15 volts, at step  607 . At step  609 , the programming voltage, V P , is held at 15 volts for a predetermined time, typically 200-500 milliseconds. 
     As illustrated in FIG. 5, the programming voltage is placed on the drain  22  of each cell. Device selectivity is accomplished by placement of the bias voltage on the gate  26  of the target cell. As described above, the bias voltage produces hot electrons at the drain and lowers the breakdown voltage for the selected device. The heat in the channel  21   a  causes the gate oxide proximate to the drain openings  28   a  to meltdown and thereby create an ohmic path, typically less than 500K ohms, between the gate  26  and the drain  22 . 
     At decision step  611 , a DONE signal is checked to determine whether all of the array cells have been programmed. If the DONE signal is set, the array programming has finished. If the DONE signal is not set, the column shift register  56  and/or the row shift register  58  are shifted to select a new active cell at step  613 . In the present example, the active cell is shifted to the next column so that the cell at column b, row a is the active cell. When the last column, column h, is reached, the active cell is shifted from the cell at column h, row a to the cell at column a, row b. It should be appreciated that the sequence of cell programming is not critical and a variety of different cell shifting procedures may be used to program the array  50 . After the new active cell has been selected, the procedure returns back to decision step  603 . 
     Referring back to FIG. 1, the sixty-four bits stored in the cell array are distributed through the circuit  10  each time power is applied. The bits are read serially from the array  50  via the I/O line and shifted through holding shift registers  14   a - 14   f  that are assigned to a particular block  12 . Serial distribution reduces the silicon area required for interconnections and simplifies the structure of the holding shift registers. 
     FIG. 7 is a flow diagram of an array distribution procedure for the illustrated embodiment. This procedure is executed by the integrated circuit each time it is powered up. The contents of the array may also be read back out of the integrated circuit using a personal computer, commercially available data programmer, etc. 
     At step  701 , column shift register outputs  56   b - 56   h  are reset to a logic level “zero” state, while the first column shift register output  56   a  is preset to a logic level “one” state, and row shift register outputs  58   b - 58   h  are reset to a logic level “zero” state, while the first row shift register output  58   a  is preset to a logic level “one” state. Thus, the cell at column a, row a is the first cell to be selected for read back. At step  703 , a bias current, typically 10 microamperes, is injected into the gate  26  of the selected cell through the pair of signal switching devices  43   a ,  43   b . The voltage developed at the gate  26  indicates the logic level of the stored bit. A fused MOSFET offers low resistance so that only a very low voltage is developed and the gate  26  reads as a logic level “zero” state. The logic level is clocked into the first location of the serially connected holding shift registers  14   a - 14   f  (FIG. 1) at step  705 . 
     At decision step  707 , the DONE signal is checked to determine whether all of the array cells have been read. If the DONE signal is set, the read back has finished. If the DONE signal is not set, the column shift register  56  and/or the row shift register  58  are shifted to select a new active cell at step  709 . It should be appreciated that the sequence of cell read back is not critical and a variety of different cell shifting procedures may be used to program the array  50 . After the new active cell has been selected, the procedure returns back to step  703 . 
     It should be appreciated that a memory cell may alternatively comprise a p-type MOSFET. The use of a p-type MOSFET simply requires voltage potentials that are the inverse of the voltage potentials shown in the illustrated embodiment. The p-type MOSFET consumes more area as each p-type MOSFET requires a well diffusion for substrate isolation. 
     It should also be appreciated that the memory cells  40  need not be arranged as an array and that an individual cell may be addressed by a variety of methods. In addition, the memory cells may be fully integrated into the circuit blocks  12  so that read back of the cell data for distribution is not necessary. Furthermore, those skilled in the art will recognize that more efficient techniques for programming the array may be employed to support the simultaneous programming of two or more cells. For example, data specifying two or more cells in a particular column or row may be entered into the column and row shift registers  56 ,  58  of FIG. 5 via the DATA input line. Although the programmable array of the illustrated embodiment is used to store parametric trim data, those skilled in the art should further appreciate that the array may store data for a variety of different applications. 
     It should further be appreciated that the memory cell can be implemented using other MOS processes, such as CMOS, in addition to the BiCMOS process of the illustrated embodiment. 
     Having described a preferred embodiment of the invention, it will be apparent to one of skill in the art that other embodiments incorporating its concepts may be used. Accordingly, the invention should be limited only by the spirit and scope of the appended claims.