TWO-TRANSISTOR ZERO-POWER ELECTRICALLY-ALTERABLE NON-VOLATILE LATCH

Abstract: A two-transistor, zero-power, electrically-alterable non-volatile latch element comprises an input node, an output node, and an erase node. A P-Channel MOS transistor has a source connected to a source of first electrical potential, a drain connected to the output node, a control gate connected to the input node, and a floating gate capacitively coupled to the control gate. An N-Channel MOS transistor has a source connected to a source of second electrical potential lower than the first electrical potential, a drain connected to the output node, a control gate connected to the input node, and a floating gate capacitively coupled to the control gate and electrically connected to the floating gate of the P-Channel MOS transistor. The floating gates of the P-Channel MOS transistor and the N-Channel MOS transistor are capacitively coupled to the erase node via a tunnel dielectric.

Description: Title Of The Invention Two-Transistor Zero-Power Electrically- Alterable Non-Volatile Latch Background Of The Invention 1. Field of the Invention The present invention relates to solid state switching devices. More particularly, the present invention relates to a two-transistor, zero-power, electrically-alterable non-volatile latch element. Typical environments for the present invention include using the switch to control a pass transistor for user-programmable circuit applications. 2. The Prior Art The problem of providing non-volatile latch elements has been addressed in the prior art. Several solutions have been proposed. United States Patent No. 4,132,904 to Harrari, discloses a static-RAM-like structure employing a cross-coupled CMOS latch in which the N-Channel transistors are provided with floating gates which may be programmed to impart a desired state to the latch. While th Harrari circuit does provide a non-volatile latch which provides complementary data which can b reprogrammed, it requires high-voltage n-well structures and provides indeterminate data on firs powerup. In addition, the floating gate structures are prone to the well known read-distur phenomenon by which repeated read operations can degrade the stored data United States Patent No. 4,300,212 to Simko discloses a non-volatile static RA circuit. It employs a very large cell size including at least eight transistor devices, some of whic are complicated semiconductor structures. United States Patent No. 4,858, 185 to Kowshik et al. discloses a structur employing a CMOS non-volatile latch. While the Kowshik et al. latch does provide a non-volatil latch which provides complementary data which can be reprogrammed, can assume a known stat on powerup and does not require high-voltage n-well technology, it requires a very large cell siz employing ten transistors as well as other structures. It is therefore an object of the present invention to provide a non-volatile latc element which overcomes some of the shortcomings of the prior art. Another object of the present invention is to provide a non-volatile latch elemen which employs a small cell size. Another object of the present invention is to provide a non-volatile latch elemen which employs a minimum number of transistor devices. A further object of the present invention is to provide a non-volatile latch element which does not require high-voltage n-well technology. Yet another object of the present invention is to provide a non-volatile latch element which has improved read disturb immunity. It is a further object of the present invention to provide a non-volatile latch element which assumes a known state on powerup. Brief Description Of The Invention According to a first aspect of the present invention, a two-transistor, zero-power, electrically-alterable non-volatile latch element includes an input node, an output node, and an erase gate. A P-Channel MOS transistor has a source connected to a source of first electrical potential, a drain connected to the output node, a control gate connected to the input node, and a floating gate capacitively coupled to the control gate. An N-Channel MOS transistor has a source connected to a source of second electrical potential lower than the first electrical; potential, a drain connected to the output node, a control gate connected to the input node, and a floating gate capacitively coupled to its control gate and electrically connected to the floating gate of the P- Channel MOS transistor. The floating gates of the P-Channel MOS transistor and the N-Channel MOS transistor are capacitively coupled to the erase node via a tunnel dielectric. The control gate and the erase gate may comprise regions in the substrate capacitively coupled to the floating gate or may comprise polysilicon lines capacitively coupled to the floating gate. According to another aspect of the present invention, a MOS transistor switch may be connected to the output node of the two-transistor, zero-power, electrically-alterable non¬ volatile latch element of the present invention. The state of the latch controls whether the transistor is turned on or off. The drain and source of the MOS ttansistor switch may be connected to circuit nodes which may be selectively connected together by programming the two-transistor, zero- power, electrically-alterable non-volatile latch element. The two-transistor, zero-power, electrically-alterable non-volatile latch element of the present invention may be fabricated using a single polysilicon gate process in which the control gate and erase nodes are formed from diffused regions in a semiconductor substrate. Alternately, the two-transistor, zero-power, electrically-alterable non-volatile latch element of the present invention may be fabricated using a double polysilicon gate process in which the control gate and erase nodes are formed from regions of the second level polysilicon layer. Brief Description Of The Drawings FIG. 1 is a schematic diagram of a two-transistor, zero-power, electrically-alterable non-volatile latch element according to the present invention. FIG. 2 is a graph showing the transfer characteristics of the two-transistor, zero- power, electrically-alterable non-volatile latch element of FIG. 1 in both its programmed and erased states. FIG. 3 is a top view of an illustrative semiconductor layout of the two-transistor zero-power, electrically-alterable non- volatile latch element of FIG. 1. FIG. 4 is a cross-sectional view of the illustrative semiconductor layout of the two transistor, zero-power, electrically-alterable non-volatile latch element of FIG. 3, taken throug lines 4-4. FIG. 5 is a schematic diagram of a two-by-two array of two-transistor, zero power, electrically-alterable non-volatile latch elements according to another aspect of the presen 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. Referring first to FIG. 1 , a two-transistor, zero-power, electrically-alterable non- volatile latch element 10 of the present invention is seen to comprise a P-Channel MOS transistor 12 and an N-Channel MOS transistor 14 configured in the manner of a well-known inverter. The source 16 of P-Channel MOS transistor 12 is a diffused region in a semiconductor substrate and is connected to Vcc potential, its drain 18 is a diffused region in a semiconductor substrate and is also the drain of N-Channel MOS transistor 14. The source 20 of N-Channel MOS transistor 14 is a diffused region in a semiconductor substrate and is connected to ground potential. The common drains 18 of P-Channel MOS transistor 12 and N-Channel MOS transistor 14 form an output node 22 for the latch element. A common floating gate 24, which may be formed from a layer of doped polysilicon as is known in the art, is associated with both P-Channel MOS transistor 12 and N- Channel MOS transistor 14. Floating gale 24 is capacitively coupled to an erase "gate" node 26 via a tunneling dielectric as is known in the art. P-Channcl MOS transistor 12 and N-Channel MOS transistor 14 share a common control "gate" node 28. Control gate 28 is capacitively coupled to floating gate 24 and may comprise, for example, a separate diffused region in the semiconductor substrate. The two-transistor, zero-power, electrically-alterable non-volatile latch element 10 of the present invention is stable in two states, a programmed, or "on" state, and an erased, or "off state. The two- transistor, zero-power, electrically-alterable non-volatile latch element of the present invention 10 may be programmed by raising the common control gates 28 of the P- Channel and N-Channel MOS transistors 12 and 14 to a programming voltage Vpp, while the erase node 26 is held at zero volts. The floating gate 22 is coupled high because of the capacitive coupling to the control gates 28. If the electric field across the tunnel dielectric exceeds the minimum field required for Fowlcr-Nordheim tunneling, electrons will tunnel across the tunnel dielectric to the floating gate 24. The trapped electrons increase (i.e., make more positive) the threshold of the N-Channel MOS transistor 14 and decrease (i.e., make more negative) the threshold of the P-Channel MOS transistor 12. After the programming voltage is removed, the control gates 28 are held at ground potential. The output of the latch at node 22 will be at approximately Vcc since the N-Channcl MOS transistor 14 is turned off and the P-Channel MOS transistor 12 is turned on. To erase the two-transistor, zero-power, electrically-alterable non-volatile latch element 10 of the present invention, the control gates 28 are held at ground potential and the erase node 26 is raised to Vpp. The electrical field established across the tunnel dielectric will cause electrons to tunnel out of the floating gate 24, thus making it more positive. The positive charge on the floating gate decreases (i.e., makes less positive) the threshold of the N-Channel MOS transistor 14 and increases (i.e., makes more positive) the threshold of the P-Channel MOS transistor 12. After the programming voltage is removed, the control gates 28 are held at ground potential. The output of the latch at node 22 will be at approximately ground since the N-Channel MOS transistor 14 is turned on and the P-Channel MOS u * ansistor 12 is turned off. FIG. 2 is a graph showing the transfer characteristics of the two-transistor, zero- power, electrically-alterable non-volatile latch element of FIG. 1 in both its programmed and erased stales. FIG. 2 illus ues the operation of the latch as described above As may be seen from an examination of FIG. 2, the solid line 30 represents the normal voltage transfer characteristic of a CMOS inverter. The dashed line 32 represents the current flow through the inverter as a function of the output voltage. The negative peak of the current flow curve represents the point at which both the P-Channel MOS transistor and the N-Channel MOS transistor are conducting. The straight dashed line 34 in FIG. 2 represents the operating point of the inverter when electrons have been tunneled onto the floating gate by the programming operation. The output voltage is at a maximum and the current flow is zero. The straight dashed line 3 represents the operating point of the inverter when electrons have been tunneled off of the floatin gate by the erase operation. The output voltage is at a minimum and the current flow is zer Thus it may be seen that the latch of the present invention exhibits no current flow in either it programmed or erased slate. Those of ordinary skill in the art will recognize that the latch structure of the prese invention is a stable structure. Experience with floating gate transistor devices has shown th such devices reliably retain their floating gate charge for periods in excess of ten years. The latch structure of FIG. 1 may be used to drive a number of different device As will be disclosed herein, the latch of the present invention may be advantageously employed t drive a pass transistor whose state may be used to control the selective formation o interconnection nets in an integrated circuit. Referring now to FIGS. 3 and 4, an illustrative embodiment of the latch of th present invention as embodied in semiconductor material is shown. FIG. 3 is a top view of a illustrative semiconductor layout of the two-transistor, zero-power, electrically-alterable non volatile latch element of FIG. 1. FIG. 4 is a cross-sectional view of the illustrative semiconducto layout of the two-transistor, zero-power, electrically-alterable non-volatile latch element of FIG. 3 taken through lines 4-4. The P-Channcl MOS transistor 12 of FIG. 1 is disposed in n-well 40 and its sourc and drain comprise p+ diffused regions 42 and 44, respectively. The N-Channel MOS transisto 14 of FIG. 1 is disposed in the p-type semiconductor substrate 46 (more easily seen in FIG. 4 and its drain and source regions comprise n+ diffused regions 48 and 50, respectively. Metal stri 52 is used to connect p+ diffused 44 to n+ diffused region 48 via contacts 54 and 56. Vcc meta line 58 is connected to p+ diffusion 42 via contacts 60 and ground metal line 62 is connected t n-t- diffusion 50 via contacts 64. Those of ordinary skill in the art will observe that doubl contacts are shown in the layout of FIG. 3, but will realize that single contacts may be employed instead. Floating gate 24 of FIG. 1 is formed from a single layer of polysilicon 66 having extending finger 68 disposed above the channel region between diffused regions 42 and 44 forming the floating gate portion of P-Channel MOS transistor 12. A second extending finger 70 is disposed above the channel region between diffused regions 48 and 50 forming the floating gate portion of N-Channcl MOS transistor 14. As will be appreciated by those of ordinary skill in the art, the source and drain diffusions for both the P-Channel and N-Channel MOS transistors 12 and 14 may be formed using self-aligned gate techniques as are known in the art. The common control gates of P-Channel MOS transistor 12 and N-Channel MOS transistor 14 may be formed as a high-voltage n-type diffused region 72 in the substrate. High- voltage n-type diffused region 72 should be formed so as to withstand the contemplated Vpp voltage with an appropriate margin as is well understood in the art. The high-voltage diffused region 72 is capacitively coupled to an enlarged portion of the overlying polysilicon floating gate strip 66 to enhance capacitive coupling. A dielectric layer 74, typically formed from high quality oxide as is known in the art, to a thickness of from about 50 to about 130 angstroms, preferably about 80 angstroms, separates the diffused region 72 from the polysilicon floating gate strip 66 and forms coupling capacitor 76. The area of a typical coupling capacitor 76 is about 7 to 10 times the area of the tunneling capacitor. The erase node of laich 10 of FIG. 1 is formed as a high-voltage n-type diffused region 78 in the substrate. As with high-voltage n-type diffused region 72, high-voltage n-type diffused region 78 should also be formed so as to withstand the contemplated Vpp voltage with an appropriate margin as is well understood in the art. A tunneling capacitor 80 is formed by the intersection of the diffused region 78 with the overlying polysilicon strip 66. The polysilicon layer 66 is separated from the diffused region 78 by a tunneling dielectric layer 82 typically formed from high quality oxide as is known in the art, to a thickness of about 50 to about 130 angstroms, preferably about 80 angstroms. The area of a typical tunneling capacitor 82 is about the smallest feature size which can be printed using phototlithography techniques, or may be formed by an overlap of the source/drain diffusion with the polysilicon floating gate. Those of ordinary skill in the art will recognize that, while the embodiment illustrated in FIGS. 3 and 4 employs "gate" elements which are actually disposed in the substrate, diffused regions 72 and 78 could easily be replaced by polysilicon lines formed in a second layer of polysilicon as is well-known in the EPROM and EEPROM art. Such structures are equivalent to the structures depicted in the figures herein and are thus intended to fall within the scope of the present invention. In a typical embodiment of the present invention, the latch elements may be programmed and erased by a Vpp voltage in the range of about 10 to about 15 volts. As may be seen from an examination of FIGS. 3 and 4, because the control and erase "gates" are formed from doped regions in the substrate, the two-transistor, zero-power, electrically-alterable non-volatile latch element of the present invention may be fabricated as a par of a conventional single-level polysilicon gate CMOS integrated circuit fabrication process. Suc a process begins with the conventional steps for forming the n-wells, the field implant and fiel oxidation steps. The high-voltage n-type regions comprising the erase gate and control gate lines a then formed in the substrate using buried n+ mask and implant. Alternatively, the erase gate ca also be formed in the n-type source/drain diffusion using known high-voltage techniques. Next, a tunnel window is etched to expose the high-voltage n diffusion comprisin the global erase line and the tunnel dielectric is formed, preferably from high quality oxide, and i then defined in the tunnel window. The polysilicon layer which will comprise the floating gate i then formed, doped and defined, using conventional semiconductor processing technology. The gate oxide step for the formation of other P-Channel and N-Channel transisto devices on the integrated circuit is then performed, followed by deposition, doping, and definition of the polysilicon gates for those other transistor devices. If the global erase and control gates are to be formed from polysilicon layers, an inter-poly oxide is formed, windows are etched to expose regions of the floating gate, and the tunnel dielectric and control gate capacitor dielectrics are formed prior to performing the gate poly deposition step. An implant oxide is formed, followed by the masking and implant steps necessary to form both the P-Channel and N-Channel self- aligned source and drain regions. Next, BPSG deposition, contact window formation, and first level metal steps are performed using conventional semiconductor processing techniques. Subsequent metal layers and other back-end processing may then be performed as is well known in the art. The details of the individual process steps used in the fabrication of the two- transistor, zero-power, electrically-alterable non-volatile latch element, such as times, temperatures, doses, thicknesses, etc., arc well known in the art for formation of CMOS devices and have therefore not been recited herein. As will be recognized by persons of ordinary skill in the art, the process described herein for the fabrication of the latch of the present invention may be practiced using standard CMOS processing steps, and employing conventional layer thicknesses, implant dosages, etc. The additional processing steps used to form the floating gate, tunnel dielectric, and capacitors for the control and erase gates are well known in the EEPROM and EPROM art and arc not repeated here in order to avoid overcomplicating the disclosure. As may also be seen from an examination of FIGS. 3 and 4, the latch 10 of the present invention may be formed as a compact structure. The layout shown in FIGS. 3 and 4 readily lends itself to the layout of an anay of latches. Such an array could be usefully employed in devices such as user-programmable circuits (e.g., FPGA integrated circuits). The outputs of the latches would be used to drive transistor switches to selectively make interconnections in the user-programmable integrated circuits. Such an array 90 of two-transistor, zero-power, electrically-alterable non-volatile latch elements is shown in FIG. 5 in schematic form according to another aspect of the present invention. The array 90 includes a plurality of two-transistor, zero-power, electrically-alterable non-volatile latch elements arranged in rows and columns.. In FIG. 5, the latch elements are shown as squares and designated with reference numerals 10-1, 10-2 10-3, and 10-4. Although the illustrative array 90 of FIG. 5 is shown comprising two rows and two columns, those of ordinary skill in the art will readily recognize that the array of FIG. 5 is for purposes of illustration and that much larger arrays are practical according to the present invention. Latch elements 10-1 and 10-2 form the first row of array 90 and latch elements 10- 3 and 10-4 form the second row of anay 90. Similarly, latch elements 10-1 and 10-3 form the first column of array 90 and latch elements 10-2 and 10-4 form the second column of array 90. Each latch element has associated with it a pass transistor. These transistors are labelled with reference numerals 92-1 through 92-4, the numeric suffixes corresponding to the numeric suffixes of the latch element with which each pass transistor is associated. The gate of each pass transistor is connected to the output node of its associated latch element. As illustrated in FIG. 5, pass transistors 92-2 through 92-4 arc shown as N-Channel MOS transistors, but those of ordinary skill in the art will recognize that other types of devices may be used for different applications without departing from the principles of the present invention. The individual latch elements 10- 1 through 10-4 in the array 90 of the present invention may be addressed and programmed or may be erased. In the configuration of the array depicted in FIG. 5, a global erase mode is provided, although those of ordinary skill in the art will easily understand how to configure an array in which the latch elements could be individually erased. In the array 90 of the present invention, select transistors 94-1 through 94-4 are used to select the individual latch elements for programming. As with the pass transistors.the numeric suffixes of the select transistors in FIG. 5 correspond to the numeric suffixes of the latch element with which each pass transistor is associated. The select transistors 94-1 through 94- are used to deliver the programming voltage to selected ones of the latch elements to progra them. The gates of select transistors 94-1 and 94-2 are connected to a first row line 9 The gates of select transistors 94-3 and 94-4 are connected to a second row line 98. The drains o select transistors 94- 1 and 94-3 are connected to a first column line 100. The drains of selec transistors 94-2 and 94-2 are connected to a second column line 102. The source of each of th select transistors is connected to the input node (the common control gates) of its correspondin latch element. Global erase line 104 is shown connected to each of the latches 10-1 through 10-4 The global erase line is connected to the erase node of each of the latches and is used to erase o reset all of the latches simultaneously. During the operating mode of the array 90 of the present invention, the row line 96 and 98 are kept at zero volts so that select transistors 94-1 through 94-4 are turned off. When i is desired to program one of the latch elements 10-1 through 10-4 of the array 90, the row line and the column lines are used to select the latch to be programmed and to deliver them appropriat voltages to the selected latch. To program one or more desired latches, it is first advisable, but not necessary t erase all of the latches in order to set them all to a known state. This is done by connecting al column lines 102 and 104 to ground, turning on all select transistors 94-1 through 94-4 to plac ground on the common control gates of the N-Channel and P-Channel MOS transistors, and by applying the programming voltage Vpp to the global erase line 104. As described previously for single latch example, this action will cause electrons to tunnel from the floating gates of all of the latches, thus rendering them more positive. After erasing, all of the N-Channel MOS transistors 14 (FIG. 1) in the individual latches will be turned on and all of the P-Channel MOS transistors 12 (FIG. 1) will be turned off. The outputs of all of the latches will go to ground. Once all of the latches are placed in a known state (erased), any one or more of the latches 10-1 through 10-4 may be programmed. Using the example of programming latch 10-3, second row line 98 is brought to Vpp while first row line 96 is kept grounded. Vpp is also placed on first column line 100 while second column line 102 is kept grounded. The global erase line 104 is also kept grounded. This action turns on select transistor 94-3, placing Vpp on the common control sates of the N-Channel and P-Channcl MOS transistors in latch 10-3. Under these conditions, electrons are caused to tunnel onto the floating gate inside of latch 10-3, shifting the thresholds of the N-Channel and P-Channel MOS transistors in latch 10-3 such that the P- Channel MOS transistor is turned on and the N-Channel MOS transistor is turned off, causing the output of latch 10-3 to rise to a voltage of approximately Vcc. Because first row line 96 is held at ground, both select transistors 94-1 and 94-2 are turned off. Because second column line 102 is held at ground, Vpp does not appear at the control gates of the transisiors in latch 1 -4 even though the gate of select transistor 94-4 is at a voltage which would turn it on if there was any voltage at its drain. Thus, no tunneling of electrons will occur onto the floating gates of latches 10-1, 10-2, and 10-4. The driving of the various voltages onto lines 96, 98, 100, 102, and 104 is provided by programming circuitry 106. Circuitry for generating and switching the necessary voltages for programming the devices of the present invention is well known in the art and is a matter of simple design choice. The voltages may be generated on-chip by charge pumps as is known in the art or may be supplied from off chip via selected I/O pins as is well known in the art. The two-transistor, zero-power, electrically-alterable non-volatile latch element of the present invention will always power up to a desired state once it has been pre-programmed. Unlike some of the prior art latch elements, it employs independent read and write paths, thus minimizing the read disturb problem. It does not require high-voltage n-well technology. It employs a small cell size, making it an excellent candidate for array applications, and enjoys excellent lon 4 C<.-ieιτn data retention. 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.

Claims: Claims We claim: 1 . A two-transistor, zero-power, electrically-alterable non-volatile latc element comprising: an input node; an output node; an erase node; a P-Channel MOS transistor having a source connected to a source of first electrica potential, a drain connected to said output node, a control gate connected to said input node, and floating gate capacitively coupled to said control gate; an N-Channcl MOS transistor having a source connected to a source of secon electrical potential lower than said first electrical; potential, a drain connected to said output node, control gate connected to said input node, and a floating gate capacitively coupled to said contro gate and common to said floating gate of said P-Channel MOS transistor; said floating gates of said P-Channel MOS transistor and said N-Channel MO transistor capacitively coupled to said erase node via a tunnel dielectric. 2. The two- transistor, zero-power, electrically-alterable non-volatile latc element of claim 1, further including a MOS transistor having a drain, a source, and a gate, sai gate connected to said output node. 3. An anay of two-transistor, zero-power, electrically-alterable non-volatil latch elements comprising: a row line for each row in said array; a column line for each column in said array, each column line forming an intersection with each row line in said array; a plurality of two-transistor, zero-power, electrically-alterable non-volatile latch elements arranged in array of rows and columns, each of said two-transistor, zero-power, electrically-alterable non-volatile laich elements disposed at one of said intersections and comprising an input node, an output node, an erase node, a P-Channel MOS transistor having a source connected to a source of first electrical potential, a drain connected to said output node, a control gate connected to aid input node, and a floating gate capacitively coupled to said control gate, an N-Channel MOS transistor having a source connected to a source of second electrical potential lower than said fust electrical; potential, a drain connected to said output node, a control gate connected lo said input node, and a floating gate capacitively coupled to said control gate and common to said floating gate of aid P-Channel MOS transistor, said floating gates of said P- Channcl MOS transistor and said N-Channcl MOS transistor capacitively coupled to said erase node via a tunnel dielectric, and a select transistor, each of said select transistors having, a source, source connected to aid input node, a drain connected to the one of said column lines associated with its intersection, and a gate connected to the one of said row lines associated with its intersection; a common erase line connected to the erase node of each of said two-transistor, zero-power, electrically-alterable non-volatile latch element in said array; circuitry for selectively connecting each of said row lines to a voltage selected from ground and Vpp; and circuitry for selectively connecting each of said column lines to a voltage selected from ground and Vpp.

Inventors: KOWSHIK VIKRAM, KOWSHIK, VIKRAM
Applicants: ACTEL CORP, ACTEL CORPORATION
Application year: 1996
Main classification: H03K3
IPC codes: H01L27, H03K3, H03K17, H03K19