Abstract:
Methods, systems, and apparatus, including computer program products for programming memory. In one aspect, a program circuit includes a first transistive element; a second transistive element coupled to a first end of the first transistive element; a burn subcircuit, the burn subcircuit including a third transistive element coupled to a fourth transistive element, where the drain of the third transistive element is coupled to a second end of the first transistive element, and the source of the third transistive element is coupled to the drain of the fourth transistive element; and a fifth transistive element coupled in parallel to the fourth transistive element. Control logic coupled to the first transistive element, the burn subcircuit, and the fourth transistive element selectively enables the second transistive element, selectively enables the fourth transistive element, and selectively enables the fifth transistive element to enable a read mode or a program mode.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation application of and claims priority to U.S. application Ser. No. 12/751,764, filed on Mar. 31, 2010, the entire disclosure of which is incorporated by reference. 
    
    
     BACKGROUND 
     This specification relates to programming memory. 
     A value of a bit can be stored in a memory cell (a bit-cell) of a memory. The memory can be volatile or non-volatile. One type of non-volatile memory is a one time programmable (OTP) memory. OTP memory is a memory that is typically programmed only once. In practice, some OTP memories rely on breakdown of a device, e.g., a fuse, in a program circuit to change the value of the bit. As a result, programming of an OTP memory is not typically changed by an end user. 
     In practice, some OTP memories have memory cells and program circuits fabricated on a single integrated circuit (IC). OTP memory can be used in integrated circuits that require high operating voltages, e.g., voltages greater than about 12 V. In such applications, high-voltage devices, e.g., high-voltage transistors, are typically required to construct the program circuits which must be able to withstand the high operating voltages. The use of high-voltage devices increases the cost of fabricating the OTP memory. 
     SUMMARY 
     This specification describes technologies relating to a memory program circuit, particularly to a one-time programmable (OTP) bit-cell program circuit. 
     In general, one aspect of the subject matter described in this specification can be embodied in a semiconductor device that includes a first low-voltage transistive element; a second high-voltage transistive element coupled to a first end of the first low-voltage transistive element; a burn subcircuit coupled to the first low-voltage transistive element and a latch, the burn subcircuit including a third high-voltage transistive element coupled to a fourth low-voltage transistive element, where the drain of the third high-voltage transistive element is coupled to the a second end of the first low-voltage transistive element, and the drain of the fourth low-voltage transistive element is coupled to the latch; and a fifth low-voltage transistive element coupled in parallel to the fourth low-voltage transistive element. Control logic coupled to the burn subcircuit enables a voltage divider comprising the first low-voltage transistive element, the third high-voltage transistive element, and the fifth low-voltage transistive element, in a read mode; or enables the third high-voltage transistive element and the fourth low-voltage transistive element, and burns the first low-voltage transistive element, in a program mode. Other embodiments of this aspect include corresponding systems, methods, and computer program products. 
     These and other embodiments can optionally include one or more of the following features. One or more of the transistive elements can be stepped gate laterally diffused metal oxide semiconductor (LDMOS) transistors. 
     In general, another aspect of the subject matter described in this specification can be embodied in a program circuit that includes a first transistive element; a second transistive element coupled to a first end of the first transistive element; a burn subcircuit, the burn subcircuit including a third transistive element coupled to a fourth transistive element, where the drain of the third transistive element is coupled to a second end of the first transistive element, and the source of the third transistive element is coupled to the drain of the fourth transistive element; and a fifth transistive element coupled in parallel to the fourth transistive element. Control logic coupled to the first transistive element, the burn subcircuit, and the fourth transistive element selectively enables the second transistive element, selectively enables the fourth transistive element, and selectively enables the fifth transistive element to enable a read mode or a program mode. Other embodiments of this aspect include corresponding systems, methods, and computer program products. 
     These and other embodiments can optionally include one or more of the following features. The circuit further may include a leakage device coupled in parallel with the first transistive element and configured to enhance time dependent dielectric breakdown reliability (TDDB) reliability of the first transistive element. 
     The first transistive element may be a low-voltage stepped gate capacitor, the second transistive element may be a high-voltage diode, the third transistive element may be a high-voltage stepped gate laterally diffused metal oxide semiconductor (LDMOS) transistor, the fourth transistive element may be a low-voltage transistor, and the fifth transistive element may be a low-voltage transistor. The breakdown voltage of the gate oxide in a stepped gate LDMOS transistor may be less than the breakdown voltage of the junction in the stepped gate LDMOS transistor. The gate oxide in a stepped gate LDMOS may include a first portion and a second portion, where a thickness of the second portion is less than a thickness of the first portion. The thickness of the second portion may be less than about 40 Å. The thickness of the first portion may be greater than about 200 Å. An impedance of the first transistive element may be between 1 kΩ and 10 kΩ. An impedance of the fourth transistive element may be between 10 kΩ and 100 kΩ. 
     The fourth transistive element and the fifth transistive element may each include a channel of a same type of semiconductor material. The type of semiconductor material may be n-type. The fifth transistive element may be a weak transistor that is configured to create a voltage divider in the read mode. The circuit further may include a latch coupled to the source of the second transistive element. 
     In general, another aspect of the subject matter described in this specification can be embodied in methods that include the actions of applying a first voltage signal to a first low-voltage transistive element, where the first voltage signal is greater than a breakdown voltage of the first low-voltage transistive element, and where a first end of the first low-voltage transistive element is coupled to a second high-voltage transistive element; applying a second voltage signal to enable a third high-voltage transistive element, where the drain of the third high-voltage transistive element is coupled to a second end of the first low-voltage transistive element; and applying a third voltage signal to enable a fourth low-voltage transistive element, where the fourth low-voltage transistive element is coupled to the third low-voltage transistive element, the drain of the fourth low-voltage transistive element is coupled to a latch, and the fourth low-voltage transistive element is coupled in parallel to a fifth low-voltage transistive element. Other embodiments of this aspect include corresponding systems, apparatus, and computer program products. 
     In general, another aspect of the subject matter described in this specification can be embodied in methods that include the actions of applying a first voltage signal to a first low-voltage transistive element, where a first end of the first low-voltage transistive element is coupled to a second high-voltage transistive element; applying a second voltage signal to enable a third high-voltage transistive element, where the drain of the third high-voltage transistive element is coupled to a second end of the first low-voltage transistive element, the third high-voltage transistive element is coupled to a fourth low-voltage transistive element, and the drain of the fourth low-voltage transistive element is coupled to a latch; and applying a third voltage signal to enable a fifth low-voltage transistive element and to enable a voltage divider comprising the first low-voltage transistive element, the third high-voltage transistive element, and the fifth low-voltage transistive element; where the fifth low-voltage transistive element is coupled in parallel to the fourth low-voltage transistive element. Other embodiments of this aspect include corresponding systems, apparatus, and computer program products. 
     Particular embodiments of the subject matter described in this specification can be implemented to realize one or more of the following advantages. The program circuit and related systems and techniques described herein allow high-voltage devices to be used with low-voltage devices in high-voltage applications. In some situations, fabrication using low-voltage devices costs less than fabrication using high-voltage devices. Therefore, replacing high-voltage devices with low-voltage devices decreases a cost of OTP memory that can be used in the high-voltage applications. Use of stepped gate laterally diffused metal oxide semiconductor (LDMOS) in transistors in the program circuit increases the breakdown voltage of the transistors in the off state and minimizes on-resistance (resistance drain-to-source) during transistor conduction, thereby decreasing a response time, i.e., a decreased RC time constant. Increasing the breakdown voltage of the transistors in the program circuit allows the IC to operate at high voltages, reducing or eliminating a need for a separate charge pump or pin, thereby increasing an ease of IC fabrication and further reducing costs. In addition, implementation of OTP memory in a process that uses low-voltage and high-voltage devices leverages available device inventory and reduces a need for dedicated layers, process enhancements, and devices to implement memory functionality. 
     The details of one or more embodiments of the subject matter described in this specification are set forth in the accompanying drawings and the description below. Other features, aspects, and advantages of the subject matter will become apparent from the description, the drawings, and the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram including an example program circuit. 
         FIG. 2  is a schematic diagram of an example one time programmable (OTP) bit-cell. 
     
    
    
     Like reference numbers and designations in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
       FIG. 1  is a block diagram including an example program circuit  100 . The program circuit  100  includes a first switching element  110 , a second switching element  114 , a burn subcircuit  120  including a third switching element  122  and a fourth switching element  124 , and a fifth switching element  130 . The first switching element  110  can be a low-voltage switching element, the second switching element  114  can be a high-voltage switching element, the third switching element  122  can be a high-voltage switching element, the fourth switching element  124  can be a low-voltage switching element, and the fifth switching element  130  can be a low-voltage switching element. A high-voltage switching element is designed to have a greater breakdown voltage than a low-voltage switching element. For example, a high-voltage switching element can be designed with a breakdown voltage greater than 10.0 V, and a lower-voltage switching element can be designed with a breakdown voltage between 3.0 V and 10.0 V. 
     The program circuit  100  can be coupled to a latch  150 , e.g., a latch memory circuit used to store a bit value (0 or 1). 
     In program circuit  100 , the first switching element  110  includes a switch that is coupled between nodes x 1  and x 3 . The second switching element  114  includes a switch that is coupled between nodes x 1  and x 2 , and controlled by an input signal received at node x 0 . The third switching element  122  includes a switch coupled between nodes x 3  and x 4 , and is controlled by an input signal received at node x 5 . The fourth switching element  124  includes a switch coupled between nodes x 4  and x 6 , and is controlled by an input signal received at node x 7 . The fifth switching element  130  includes a switch coupled between nodes x 4  and x 6  in parallel with the switch of the second switching element  124 , and is controlled by an input signal received at node x 8 . The output of the program circuit  100  is provided to the latch  150  at node x 4 . 
     Each of the first switching element  110 , second switching element  114 , third switching element  122 , fourth switching element  124 , and fifth switching element  130  can be a semiconductor switching element, e.g., a transistor. In addition, the fourth switching element  124  and the fifth switching element  130  can each have a channel of a same type, i.e., doping, of semiconductor material. Furthermore, the fifth switching element  130  can be a weak switching element and the fourth switching element  124  can be a strong switching element in relation to the fifth switching element  130 . The strength of a switching element can be measured by pulldown strength of a device. For example, the strength of a transistor can be determined by the length of a channel underneath a gate of the transistor. As a particular example, the channel length of a weak switching element can be greater than, e.g., six times greater than, the channel length of a strong switching device. 
     Control logic  160  can be coupled to the program circuit  100  to select a mode of operation of the program circuit  100 . By selectively opening and closing the switching elements, the program circuit can be configured to be in a read mode for reading a value from a bit cell, e.g., by transferring the value of the bit cell into the latch  150 , or in a program mode (write mode) for writing a value into the bit cell. In the program circuit  100 , the first switching element  110  effectively serves as the bit cell to store the value of the bit. In particular, if the first switching element  110  is operating normally, with node x 1  (an input node) generally electrically isolated from node x 3  (a control node), then the program circuit  100  can be considered to have a first value, e.g., 0. In contrast, if the first switching element  110  has been shorted out, e.g., so that node x 1  is permanently electrically coupled to node x 3 , then the program circuit  100  can be considered to have a different second value, e.g., 1. Since shorting is a non-reversible process, once the first switching element  110  has been shorted out, the value of the bit stored by the program circuit  100  is permanently changed. 
     When the program circuit  100  is in a program mode, a write operation can be performed, e.g., the value of the bit stored by the program circuit  100  can be changed. In particular, the value of the bit stored by the program circuit  100  can be modified, e.g., changed from 0 to 1. Since the program circuit  100  is a one-time programmable memory, once the value of the bit is changed, it cannot be reset. 
     Enabling the program mode of the program circuit  100  can include using the control logic  160  to apply a sufficiently high voltage to the first switching element  110  to cause shorting of the first switching element, e.g., a voltage greater than a breakdown voltage of the first switching element  110 , such that node x 1  becomes electrically coupled to node x 3 . Shorting the first switching element  110  includes coupling node x 1  to a high voltage source, and enabling the third and fourth switching elements  122  and  124  while the fifth switching element  130  remains disabled. As a result, the high voltage applied to the first switching element  110  “blows”, and thus, programs the first switching element  110 . 
     When the program circuit is in a read mode, a read operation can be performed. Thus, in the read mode, the value of the bit stored by the program circuit  100  can be read, e.g., by transferring the value to the latch  150  and then reading the output of the latch  150 . The program circuit  100  can set the latch  150  in the read mode in the manner described in further detail below. Enabling the read mode of the program circuit  100  can include applying voltage signals at node x 2  and x 0 , while using the control logic  160  to enable, i.e., close, the third switching element  122  and the fifth switching element  130 , e.g., by applying signals to the nodes x 5  and x 8 . The fourth switching element  124  remains disabled throughout the duration of the read operation. As an example, a signal of 1.8 V can be applied to the third switching element  122  at node x 5 , and a signal of 1.8 V can be applied to the fifth switching element  130  at node x 8 ; the program circuit  100  can be operable in the range of 1.4 V to 2.2 V. 
     Because the fifth switching element  130  is a weak switching element with relatively high impedance, when enabled, the fifth switching element  130  does not pull node x 4  all the way to ground. Rather, the fifth switching element  130  creates a voltage divider. As a result, a substantial portion of the voltage applied at node x 2  will appear at node x 4  and serve as input for the latch circuit  150  during the read operation. The voltage at node x 4  is less than the voltage at node x 2 , but much greater than zero. As examples, a weak switching element can have an impedance greater than 100 kΩ, a strong switching element can have an impedance between 10 kΩ and 100 kΩ, and a blown switching element can have an impedance between 1 kΩ and 10 kΩ. 
     As a particular example, if the first switching element  110  has been blown, and signals of 1.8 V are applied at each of the nodes x 0 , x 1 , x 2 , x 5 , and x 8 , the fifth switching element  130  serves as a weak pulldown to ground, resulting in a voltage of approximately 1.0 V at node x 4 . The voltage at node x 4  is greater than a threshold voltage, e.g., 0.45 V, of a series inverter that can be coupled to the latch  150 , resulting in a bit value of 1 at the output of the latch  150 . 
       FIG. 2  is a schematic diagram of an example one time programmable (OTP) bit-cell. The OTP bit-cell includes a program circuit, e.g., an implementation of the program circuit  100  of  FIG. 1 . In general, the circuit of  FIG. 2  operates in the same fashion as the circuit of  FIG. 1 , with differences explained below. The program circuit includes a first transistive element  210  (e.g., an example of the first switching element  110  of  FIG. 1 ), a second transistive element  214  (e.g., an example of the second switching element  114  of  FIG. 1 ), a third transistive element  222  (e.g., an example of the third switching element  122  of  FIG. 1 ), a fourth transistive element  224  (e.g., an example of the fourth switching element  124  of  FIG. 1 ), and a fifth transistive element  230  (e.g., an example of the fifth switching element  130  of  FIG. 1 ). 
     In the example of  FIG. 2 , the first transistive element  210  is an OTP capacitive element; the second transistive element  214  is a p-channel diode; the third transistive element  222  is a high-voltage n-channel transistor; the fourth transistive element  224  is a strong, low-voltage n-channel transistor; and the fifth transistive element  230  is a weak, low-voltage n-channel transistor. In addition, a high voltage LDMOS device  211  is coupled in parallel with the first transistive element  210  to provide additional leakage path that improves the time dependent dielectric breakdown reliability (TDDB) of the unprogrammed capacitive cells in the first transistive element  210 . 
     Control logic can be coupled to the program circuit of  FIG. 2  to select a mode of operation of the program circuit of  FIG. 2  so that it operates in a manner as described with respect to  FIG. 1 . In particular, by selectively enabling and disabling the transistive elements, the program circuit of  FIG. 2  can be configured to be in a read mode for reading a value from a first transistive element  210 , e.g., by transferring the value of the first transistive element  210  into a latch  250  (e.g., an example of the latch  150  of  FIG. 1 ), or in a program mode (write mode) for writing a value into the first transistive element  210 . 
     In the example of  FIG. 2 , each of the transistive elements  211  and  222  in the program circuit are high voltage (HV) devices and can be LDMOS transistors. In addition, a uniform gate or a stepped gate structure can be used in the implementation of the HV devices. In the stepped gate LDMOS transistor, a first gate and a second gate are disposed on gate oxide including a first portion and a second portion. The first gate is disposed on the first portion and the second gate is disposed on the second portion. The thickness of the gate oxide in the first portion can be different from the thickness of the gate oxide in the second portion. For example, the thickness of the second portion can be less than the thickness of the first portion. 
     In some implementations, the thickness of the second portion is less than about 40 Å, e.g., 35 Å, and the thickness of the first portion is greater than about 200 Å. By varying the thickness of the gate oxide, the thinner portion of the gate oxide, e.g., the second portion, can have a lower turn-on voltage, e.g., a voltage that enables the transistor. In addition, the thinner portion of the gate oxide can also have a breakdown voltage that is less than the breakdown voltage of the junctions in the transistor, so that the gate oxide breaks down before the junctions. The stepped gate structure also provides an ability to maximize the breakdown voltage in a transistor&#39;s “off” state (disabled) and low Rds(on), i.e., on resistance (resistance drain-to-source). In some implementations, the first transistive element  210  includes both thin oxide and thick oxide portions. 
     Returning to the program circuit in  FIG. 2 , in some implementations, a first end of the first transistive element  210 , defined by a poly gate, is coupled to node x 1 , i.e., node VddTrim. A second end of the first transistive element  210  is coupled to the drain of the third transistive element  222 . The source of the third transistive element  222  is coupled to the drain of the fourth transistive element  224 . The source of the fourth transistive element  224  is coupled to node x 6 , i.e., node vn, and the gate of the fourth transistive element  224  is coupled to node x 7 , i.e., node Trim. 
     The fifth transistive element  230  is coupled in parallel to the fourth transistive element  224 . In particular, the drain of the fifth transistive element  230  is coupled to the drain of the fourth transistive element  224 , and the source of the fifth transistive element  230  is coupled to the source of the fourth transistive element  224 . The gate of the fifth transistive element  230  is coupled to node x 8 , i.e., node rd, and the source of the fifth transistive element  230  is coupled to node vn. 
     In some implementations, the OTP-first switching also includes the latch  250  and a series inverter circuit  270 . The latch  250  is coupled to the source of the third transistive element  222  and stores a bit value. The series inverter circuit  270  improves the quality and voltage gain of signals received at its input. An input node of the series inverter  270  is coupled to the latch  250 , as illustrated in  FIG. 2 , and an output node of the series inverter  270  is coupled to an output of the latch  250 . 
     As generally described above with reference to  FIG. 1 , the program circuit can operate in a read mode for reading the bit value in the latch  250  or in a program mode for writing a bit value to the latch  250 . 
     In the read mode, the second transistive element  214  is enabled by setting node vp to high, and node vddTrim can be set to the same value as node vp. The read mode can be in a low voltage domain (e.g., less than about 3.0 V). For example, vp can be about 1.8 V. When vddTrim is set to vp, a voltage at vddTrim is also 1.8 V. A sel/data signal and a read signal are also enabled on node sel/data and node rd, respectively, enabling the fifth transistive element  230  and the third transistive element  222 . Note that the sel/data signal and read signal can be low voltage signals because they are electrically coupled to thin gate transistive elements. 
     Because the fifth transistive element  230  is a weak transistive element, the fifth transistive element  230  provides a weak pulldown and creates as a voltage divider. As a result, the voltage at node x 4  will be less than the voltage at node x 2 , i.e., node vp, but greater than zero. 
     In the program mode, node vddtrim is connected to a high voltage source, e.g., greater than about 7.0 V), the third transistive element  222  is enabled by applying a voltage signal to sel/data, e.g., 1.8V. A pulsed trim signal is applied to the trim node of the fourth transistive element  224 . Because the fourth transistive element  224  is a strong device, the fourth transistive element  224  provides a strong pulldown and pulls the negative node of the first transistive element  210  to a voltage level close to zero. The voltage at node vddTrim will blow the first transistive element  210 , resulting in a resistive short between nodes x 1  and x 3 . 
     Other implementations are possible. In some implementations, the bit-cell, i.e., the first transistive element  210 , is coupled to another bit-cell in a multiple-bit OTP block through a high voltage switch. 
     A leakage device can be coupled in parallel with the first transistive element and configured to enhance time dependent dielectric breakdown reliability (TDDB) reliability of the first transistive element. 
     While this specification contains many specific implementation details, these should not be construed as limitations on the scope of any implementation or of what may be claimed, but rather as descriptions of features that may be specific to particular embodiments of particular implementations. Certain features that are described in this specification in the context of separate embodiments can also be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment can also be implemented in multiple embodiments separately or in any suitable subcombination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a subcombination or variation of a subcombination. 
     Particular embodiments of the subject matter described in this specification have been described. Other embodiments are within the scope of the following claims.