J. Cent. South Univ. (2012) 19: 168−173 DOI: 10.1007/s11771−012−0987−4

Design of an 8 bit differential paired eFuse OTP memory IP reducing sensing resistance

JANG Ji­Hye, JIN Li­yan(金丽妍), JEON Hwang­Gon, KIM Kwang­Il, HA Pan­Bong, KIMYoung­Hee

Department of Electronic Engineering, Changwon National University, 9 Sarim­Dong, Changwon 641­773, Korea

© Central South University Press and Springer­Verlag Berlin Heidelberg 2012

Abstract: For the conventional single­ended eFuse cell, sensing failures can occur due to a variation of a post­program eFuse resistance during the data retention time and a relatively high program resistance of several kilo ohms. A differential paired eFuse cell is designed which is about half the size smaller in sensing resistance of a programmed eFuse link than the conventional single­ended eFuse cell. Also, a sensing circuit of sense amplifier is proposed, based on D flip­flop structure to implement a simple sensing circuit. Furthermore, a sensing margin test circuit is proposed with variable pull­up loads out of consideration for resistance variation of a programmed eFuse. When an 8 bit eFuse OTP IP is designed with 0.18 µm standard CMOS logic of TSMC, the layout dimensions are 229.04 µm × 100.15 µm. All the chips function successfully when 20 test chips are tested with a program voltage of 4.2 V.

Key words: eFuse; differential paired efuse cell; one time programmable memory; sensing resistance; D flip­flop based sense amplifier

1 Introduction

In general, program memories used for PMICs (Power management ICs), LCD display ICs, and CMOS image sensors are OTP (One­time programmable) memories since they can be designed based on a logic process of eFuse or antifuse type which does not require any additional processes [1−2]. The OTP memory of antifuse type is programmed by shortening electrically from the breakdown mechanism when a high voltage is applied to a MOS transistor with a thin gate oxide [3−4]. The OTP cell of antifuse type requires a comparatively elaborate sensing circuit [5−6] since its post­program resistance becomes several mega ohms. In contrast, the OTP memory of eFuse type is programmed by flowing an over­current through the eFuse [7−8]. The pre­ program resistance of the eFuse cell is 50−100 Ω and the post­program resistance is more than several kilo ohms. Thus, the eFuse is programmed into either a conductive state or highly resistive state [9−10]. Generally, the eFuse­type OTP memory is applied to performing analog trimming as a small­density memory [11−12].

An eFuse OTP cell is classified into a single­port eFuse cell [13−14] and a dual­port eFuse cell [15]. The single­port eFuse cell shares its read and write ports, has the program resistance of several kilo ohms, and adopts an analog sensing scheme. In contrast, the dual­port eFuse cell has separated read and write ports and has the program resistance of several ten kilo ohms since it uses

a NMOS transistor with large channel width to flow a large programming current. The peripheral circuit is simple since a digital sensing scheme can be adopted. A dual­port eFuse cell consists of an eFuse link, a program NMOS transistor with large channel width that can flow a large programming current, and a read NMOS transistor with small channel width that can reduce a read current in the read mode [2].

For the eFuse cell, sensing failures occur since the resistance of a programmed eFuse link can vary during the data retention time and hence the post­program eFuse resistance decreases. Also, sensing failures can occur and yield can drop for some eFuse OTP memory of which the program resistance is about several kilo ohms.

In this work, a proposed differential paired eFuse cell is described, which can reduce sensing resistance of a programmed eFuse link. In addition, a simpler sensing circuit of D flip­flop structure is described. Finally, a sensing margin test circuit with a variable pull­up load is also described out of consideration for resistance variations of programmed eFuse. An 8 bit eFuse OTP memory IP is designed with 0.18 µm generic CMOS process of TSMC.

2 Circuit design

The trends in the development of an eFuse OTP cell are summarized in Table 1. The capacities of recently published OTP memory IPs are in the range of

Received date: 2011−04−26; Accepted date: 2011−10−10 Corresponding author: KIM Young­Hee, Professor, PhD; Tel: +82−55−285−1023; E­mail: youngkim@changwon.ac.kr

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Table 1 Trends in development of eFuse OTP cell

Ref. Process/ nm Cell circuit Cell size/ μm 2

Memory density/ bit

Sensing scheme

[13] 32 1.37 4 096 Analog

[14] 45 3.6 4 096 Analog

[15] 90 74.23 512 Digital

512−4 kbit [13−15]. For the conventional single­ended eFuse cell, sensing failures can occur due to a variation of a post­program eFuse resistance during the data retention time and a relatively high program resistance of several kilo ohms.

Figure 1 shows a simplified circuit of an eFuse OTP memory using a proposed differential paired eFuse OTP cell, which consists of a differential paired eFuse cell circuit, high­impedance pull­up loads, and a differential amplifier. The layout size of the eFuse OTP cell is 19.41 µm × 24.11 µm. The width and length of the eFuse cell are 0.35 µm and 2.1 µm, respectively. The proposed differential paired eFuse cell is made by connecting conventional dual­port eFuse cells in pair. The left circuit (eFuse1, MN1 and MN2) of the proposed eFuse cell stores its program datum and the right one (eFuse2, MN3 and MN4) does its complementary program datum. MN1 and MN3 are program transistors. MN2 and MN4 are read transistors. FSOURCE receives an external supply voltage of 4.2 V directly and flows an over­current in the program mode. The eFuse1 is blown by an over­current through the eFuse1 and MN1 since PD (Program data) and PDb (Program data bar) are applied with 4.2 V and 0 V in the case that the program datum of a differential paired eFuse cell is ‘1’. The eFuse2 is not blown since MN2 is in the off state. In addition, the eFuse2 is blown since MN1 is in the off state and MN3 is in the on state in the case that the program datum is ‘0’. In the read mode, BL and BLb are pulled up to VDD by high­impedance pull­up loads and then the BL of the blown eFuse link keeps VDD and the BL of the non­blown eFuse drops to 0 V if NMOS transistors (MN2 and MN4) are selected by RWL (Read word line) signal. If the voltages are developed on BL and BLb like

Fig. 1 Simplified circuit of eFuse OTP memory using differential paired eFuse OTP cell

this, the differential amplifier senses the differential voltage ∆VBL(=VBL−VBLb) between the two voltages. DOUT is VDD in the case that the eFuse1 is blown and DOUT is 0 V in the case that the eFuse2 is blown.

Table 2 gives major specifications of the designed 8 bit eFuse OTP memory IP using differential paired eFuse cells. The 8 bit OTP memory has a cell array of 1 row × 8 columns and has logic supply voltage of VDD (=1.8 V) and an external program voltage of FSOURCE (=4.2 V). There are program and read modes as operation modes. Since the programming current per bit for the eFuse OTP memory IP is about 30 mA, it is impossible to program byte by byte. Thus, programming is done bitwise as shown in Table 2. Reading is done byte by byte. The programming voltage and time are 4.2 V and 200 µs, respectively.

Table 2 Major specifications of 8 bit eFuse OTP memory IP using differential paired eFuse

Item Main feature

Process TSMC 0.18 μm generic process

OTP cell array size 1 row×8 columns

Fuse type Poly­fuse (Co silicide)

Supply (VDD)/V (1.8±10)%

FSOURCE voltage/V 4.2

Temperature range/°C −40−125 °C

Operating mode Program//read

Program bit/read bit 1/8 (Separate I/O)

Program time/μs 20

As shown in Fig. 2, an 8 bit eFuse OTP IP consists of an eFuse OTP cell array of 1 row × 8 columns, a column decoder selecting the corresponding column bits by decoding A[2:0], a PD (Program data) driver circuit

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driving the selected program data to the OTP cells, a BL S/A (Bit­line sense amplifier) sensing and outputting the BL datum to the DOUT port, and a control logic that supplies internal control signals which are suitable for both the program and read modes with the control signals (RD, PGM, TM_EN). The BL S/A senses and outputs a datum coming out from BL[7:0] according to the programmed states to DOUT[7:0] digitally in the read mode. The functional test read mode and the normal read mode are classified by TM_EN (Test mode enable) signal.

Fig. 2 Block diagram of 8 bit eFuse OTP IP

Figure 3 shows a cell array circuit of 1 row × 8 columns using differential paired eFuse cells. There is RWL signal in the row direction, and there are BL[7:0], BLb[7:0], PD[7:0] and PDb[7:0] signals in the column direction. Also, there are FSOURCE and VSS as powers.

Fig. 3 Cell array of 1 row × 8 columns

Table 3 gives bias voltage conditions for various operation modes of each OTP cell node. Since RWL keeps at 0 V in the program mode, a read NMOS transistor turns off. PD and PDb of the non­selected cell by A[2:0] keep at 0 V. On the other hand, PD and PDb of the selected cell drive 0 V and 4.2 V in the case that DIN is ‘0’, and 4.2 V and 0 V in the case that DIN is ‘1’. In the program mode, the eFuse connected to BLb of the selected cell is blown in the case that DIN is ‘0’ and the eFuse connected to BL is blown in the case that DIN is ‘1’. FSOURCE voltage is forced to be 4.2 V in the program mode. On the other hand, the program transistor turns off since PD and PDb keep at 0 V. FSOURCE stays in the floating state but is biased to 0 V circuitally. BL and BLb are pulled up to VDD by the high­impedance pull­up loads, and BL and BLb of the programmed cell with ‘0’ are 0 V and VDD and those of the programmed cell with ‘1’ are VDD and 0 V by RWL signal.

Figure 4 shows a layout image of a differential paired eFuse OTP memory cell. The layout dimensions of the OTP cell are 19.41 µm × 24.11 µm. The width and length of an eFuse link are 0.35 µm and 2.1 µm, respectively, as shown in Fig. 4.

Since COL_SEL is 0 V for non­selected cells by decoding A[2:0] in the program mode, the output signals of a PD driver, PD and PDb, keep at 0 V. On the other hand, PD and PDb drive 0 V and VPP in the case that DIN is ‘0’, and VPP and 0 V in the case that DIN is ‘1’ since COL_SEL is VDD for selected cells. An internal program voltage, VPP, is 4.2 V in the program mode. PD[7:0] and PDb[7:0] signals drive 0 V independently of DIN and COL_SEL signals since IPGMb (Internal program bar) keeps VDD in the read mode. VDD_PD signal protects the VDD­to­VPP level translator circuit in Fig. 5 from flowing a short­circuit current from the level translator before VDD is generated from the voltage regulator [2]. VPP switches to a higher supply between VDD and FSOURCE.

Figure 6 shows is a BL pull­up load circuit which precharges BL and BLb to VSS by PRECHARGE signal

Table 3 Bias voltage conditions for various operation modes of each OTP cell node

Program mode Signal

Unselected cell Selected cell Read mode

DIN 0 1 0 1 0 1

RWL 0 0 0 0 VDD VDD

PD/V 0 0 0 4.2 0 0

PDb/V 0 0 4.2 0 0 0

FSOURCE/V 4.2 4.2 4.2 4.2 Floating Floating

BL Floating Floating Floating Floating 0 VDD

BLb Floating Floating Floating Floating VDD 0

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Fig. 4 Layout image of designed OTP cell

Fig. 5 PD driver circuit

Fig. 6 BL pull­up load circuit

in the normal read mode. When RWL signal of an OTP cell is activated to VDD, BL and BLb are pulled up to VDD by pull­up load transistors with large channel width (MP1 and MP2) while BL_LOADb signal is 0 V. At this moment, pull­up transistors with small channel width (MP3 and MP4) for the test read mode are in the off state. All the pull­up load transistors in Fig. 6 are designed to stay in the high­impedance state. Since the impedances of the pull­up transistors are large, the bit line connected to a programmed eFuse is pulled up to VDD while the bit line connected to a non­programmed

eFuse keeps at VSS. In the case of reading a non­ programmed eFuse cell, a BL precharging voltage, VDD, discharges to 0 V through the read NMOS transistor and the eFuse link. At this moment, the eFuse can be blown by the EM (Electro­migration) phenomenon since a high­density current flows the eFuse link with narrow width. But we use a scheme of precharging a BL pair to VSS to reduce a read current through a non­programmed cell in this work. Thus, we can reduce a read current through a non­programmed eFuse.

In general, sensing failures can occur when the resistance of an eFuse link decreases during the data retention time for an eFuse OTP memory cell. Thus, a sensing margin test circuit is proposed with a variable pull­up load out of consideration for resistance variation of a programmed eFuse. The variable pull­up load can vary the impedance of the pull­up load in the BL charging circuit used in the functional test read and normal read modes. Among pull­up transistors in Fig. 6, MP3 and MP4 are used only for a functional test of the chip, and MP1 and MP2 are in the off state. In the test read mode, the differential input voltage of the differential amplifier decreases and a sensible eFuse resistance is larger than that in the normal read mode. Thus, the difference of the sensible eFuse resistances between in the functional test read mode and in the normal read mode is a marginal resistance during the data retention time.

Figure 7 shows a BL S/A circuit based on D flip­flop which senses and latches the differential input voltage. The BL S/A is a negative edge­triggered D flip­flop. While SAENb is high, N1 and N2 nodes keep high, and the SR latch circuit latches the previous­state datum. When SAENb is activated from high to low, BL and BLb are sensed and then the read data are transferred to the DOUT port.

Fig. 7 BL S/A circuit based on D flip­flop circuit

Figure 8 shows a layout image of an 8 bit eFuse OTP IP. The layout dimensions of designed 8 bit eFuse OTP IP with 0.18 µm generic process of TSMC are 229.04 µm×100.15 µm (= 0.022 9 mm 2 ).

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Fig. 8 Layout image of 8 bit eFuse OTP IP

3 Simulation and measurement results

Figure 9 shows a simulation result of BL sensing voltage with respect to post­program eFuse resistance according to the two read modes. Sensing resistances are 5.05 kΩ in the test mode and 1.58 kΩ in the normal mode when the sensing voltage of the BL S/A circuit is about 100 mV. Thus, we secure a margin of the resistance variation of programmed eFuse during the data retention time. Also, program yield increases since the eFuse sensing resistance of the differential paired eFuse OTP memory is several kilo ohms compared with several ten kilo ohms of the dual­port eFuse OTP memory. Furthermore, the circuit of the differential paired eFuse OTP memory is simplified since the BL S/A circuit does not require the reference voltage generator compared with the single­port eFuse OTP memory.

Fig. 9 Simulation results of BL sensed voltages with respect to post­program eFuse resistances according to read modes

Figure 10 shows simulation results in the normal read mode and in the program mode. At first, BL and BLb are precharged by PRECHARGE signal in the normal read mode. If RWL signal is activated to VDD, the read transistor of the eFuse OTP cell turns on. Then, the read data are transferred to BL and BLb while BL_LOADb signal is 0 V. If small signal voltages are developed on BL and BLb sufficiently, SAENb is

activated to be low, and the read data of BL and BLb are latched into the D flip­flop and output to the DOUT port. The access time is 139 ns in the read mode. As shown in Fig. 10(b) in the program mode, non­selected PD and PDb are all 0 V, and selected PD and PDb are 4.2 V and 0 V when DIN is ‘1’.

Fig. 10 Simulation results: (a) In normal read mode; (b) In program mode

Figure 11 shows a DOUT waveform of an eFuse OTP IP according to function modes of a test chip. The DOUT functions normally for both a programmed cell with ‘0’ and a programmed cell with ‘1’. Also, all the chips function successfully when 20 test chips are tested with a program voltage of 4.2 V.

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Fig. 11Measured DOUT waveform of eFuse OTP IP according to function modes

4 Conclusions

1) A differential paired eFuse cell is proposed with about half the size smaller in sensing resistance of a programmed eFuse link than the conventional single­ ended eFuse cell.

2) A sensing circuit of sense amplifier based on D flip­flop structure is proposed to implement a simple sensing circuit.

3) A sensing margin test circuit is proposed with variable pull­up loads out of consideration for resistance variations of programmed eFuse. Sensing can be done normally although the sensed resistance of a programmed eFuse in the normal mode is lower by about 3.5 kΩ than that in the test mode since the sensed voltage in the normal mode is higher than that in the test mode for the programmed eFuse.

4) An 8 bit eFuse OTP memory IP is designed with 0.18 µm standard CMOS process of TSMC. All the chips function successfully when 20 test chips are tested with a program voltage of 4.2 V.

Acknowledgement This work is sponsored by ETRI System

Semiconductor Industry Promotion Center, Human Resource Development Project for SoC Convergence, Korea.

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(Edited by YANG Bing)