J. Cent. South Univ. (2012) 19: 3484–3491 DOI: 10.1007/s11771-012-1433-3 Design of 32-bit differential paired eFuse OTP memory in a form of two-dimensional array KIM Yoon-kyu, JANG Ji-hye, YOON Geon-soo, LEE Dong-hoon, HA Man-yeong, HA Pan-bong, KIM Young-hee Department of Electronic Engineering, Changwon National University, 9 Sarim-Dong, Changwon 641-773, Korea © Central South University Press and Springer-Verlag Berlin Heidelberg 2012 Abstract: A differential paired eFuse OTP (one-time programmable) memory cell which can be configured into a 2D (two-dimensional) eFuse cell array was proposed. The sensible resistance of a programmed eFuse link is a half smaller than that of the single-ended counterpart and BL datum can be sensed without a reference voltage. With this 2D array of differential paired eFuse OTP memory cells, we design a 32-bit eFuse OTP memory IP. We use a sense amplifier based D F/F circuit as the BL (bit-line) SA (sense amplifier) and design a sensing margin test circuit with a variable pull-up load. It is confirmed by the function test that the designed 32-bit OTP memory IP functions normally on 30 sample dies. Key words: eFuse; one-time programmable memory; 2-dimensional array NMOS transistor with big channel width that can flow a 1 Introduction big programming current, and a read NMOS transistor with small channel width that can reduce a read current In general, small-density program memories used in the read mode [2]. In addition, the differential paired for the analog trimming of PMICs (power management eFuse cell has a form that dual-port eFuse cells are ICs) are OTP (one-time programmable) memories of connected in pair, and can make the peripheral circuit eFuse type rather than EEPROMs or flash memories simpler without any reference voltage generator and the since they can be designed based on a logic process sensing resistance of the programmed eFuse link a half which does not require any additional processes [12]. smaller. The OTP memory of eFuse type is programmed by There is, however, a problem that the previously flowing an over-current through poly-silicon fuses, or proposed differential paired eFuse cells can have just eFuses [3]. The pre-program resistance of the eFuse cell one-dimensional cell array since they have BL[7:0] and is about 50–200 Ω and the post-program resistance is BLb[7:0], PD[7:0] and PDb[7:0] in the column direction. more than several kilos ohms. Thus, the eFuse is Memory failures can also occur since there is a programmed into either a conductive state or highly possibility that the eFuse link can be shortened to the resistive state [3]. VSS-biased p-substrate by the thermally ruptured eFuse. An eFuse OTP cell is classified into a single-port In addition, an additional layout area is required since an eFuse cell [34], a dual-port eFuse cell [5], and a external pad is required to supply a program voltage in differential paired eFuse cell [6]. The single-port eFuse the program mode. cell shares its read and write port, has the program In this work, we propose a differential paired eFuse resistance of several kilo ohms, and adopts an analog OTP (one-time programmable) memory cell which can sensing scheme. In contrast, the dual-port eFuse cell has be configured into a 2D (two-dimensional) eFuse cell its separate read and write port and has the program array. The sensible resistance of a programmed eFuse resistance of several tens kilos ohms since it uses a link is a half smaller than that of the single-ended NMOS transistor with big channel width to flow a big counterpart and BL datum can be sensed without a programming current. The peripheral circuit is simple reference voltage. Also, we remove an external pad to since a digital sensing scheme can be adopted. A supply a program voltage since we use VIO (I/O voltage) dual-port eFuse cell consists of an eFuse link, a program for programming in the PMIC chip. Foundation item: Project supported by the Second Stage of Brain Korea 21 Projects Received date: 2012–02–09; Accepted date: 2012–04–20 Corresponding author: KIM Young-hee, Professor, PhD; Tel: +82−55−285−1023; E-mail: [email protected] J. Cent. South Univ. (2012) 19: 3484–3491 3485 Furthermore, we use a sense amplifier based D F/F is ‘0’ and the eFuse1 connected to BL is blown in case circuit as the BL (bit-line) SA (sense amplifier) and that DIN is ‘1’. The anodes of eFuse1 and eFuse2 are design a sensing margin test circuit with a variable connected to FSOURCE commonly and they are selected pull-up load in consideration of the variation of the column decoded PD[7:0] and PDb[7:0] in the differential programmed eFuse resistance. Also, we solve a problem paire eFuse cell of Fig. 1. Thus, the differential paired of an electrical shortage between an eFuse link and the eFuse cell do not have the row decoding function and VSS-biased p-substrate by placing a floated n-well under can be used only in a one-dimensional eFuse cell array. the eFuse link. We design the 32-bit eFuse OTP IP with Also, it requires an additional layout area since an MagnaChip’s 0.18 µm CMOS process. external pad is required to supply the programming voltage in the program mode. 2 Circuit design As shown in Fig. 2, differential paired eFuse OTP memory cells require the row decoded and column As shown in the simplified circuit of the eFuse OTP decoded signal to be configured in the two-dimensional memory using differential paired eFuse cells which can array. The proposed differential paired eFuse OTP be configured in one-dimensional array, it consists of memory cell consists of two program transistors (MN1 differential paired eFuse memory cell circuit, a and MN3), two read transistors (MN2 and MN4), and high-impedance pull-up loads, and differential amplifier two eFuses (eFuse1 and eFuse2). The used devices in Fig. [6]. The proposed differential paired eFuse cell is made 2 and the functions of RWL and BL/BLb are the same as by connecting conventional dual-port eFuse cells in pair. those in Fig. 1 while WWL and PD/PDb are used instead The left circuit (eFuse1, MN1 and MN2) of the proposed of FSOURCE. eFuse cell stores its program datum and the right one (eFuse2, MN3 and MN4) stores its complementary program datum. MN1 and MN3 are program transistors. MN2 and MN4 are read transistors. FSOURCE receives an external supply voltage directly and flows an over-current in the program mode. In the program mode, program voltage of FSOURCE should be applied with 5.5 V instead of 4.2 V in designing with MV transistors of 5 V rather than 3.3 V. Table 1 shows bias voltage conditions for various operation modes of each conventional one-dimensional configurable differential paired OTP memory cell node. The read NMOS transistors turn off since RWL keeps at 0 V in the program mode. Also, PD (program data) and PDb (program data bar) of the non-selected cell by A[2:0] keep at 0 V while PD (program data) and PDb (program data bar) of the selected cell keep at 0 V and 5.5 V in the case of DIN= ‘0’; 5.5 V and 0 V in the case of DIN= ‘1’. In the program mode, the eFuse2 connected Fig. 1 Simplified circuit of conventional differential paired to BLb of the selected cell is blown in the case that DIN eFuse OTP memory cell Table 1 Bias voltage conditions for various operation modes of each conventional differential paired OTP cell node Program mode Cell Read mode Unselected Cell Selected Cell DIN 0 1 0 1 X X RWL 0 0 0 0 VDD VDD PD 0 V 0 V 0 V 5.5 V 0 V 0 V PDb 0 V 0 V 5.5 V 0 0 V 0 V FSOURSE 5.5 V 5.5 V 5.5 V 5.5 V Floating Floating BL Floating Floating Floating Floating 0 V VDD BLb Floating Floating Floating Floating VDD 0 V eFuse1 Unblown Unblown Unblown Blowing Unblown Blown eFuse2 Unblown Unblown Blowing Unblown Blown Unblown 3486 J. Cent. South Univ. (2012) 19: 3484–3491 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. Figure 3 shows the layout image of the proposed differential paired eFuse OTP memory cell. The proposed eFuse cell size is 34.46 µm×6.94 µm (=239.15 µm 2). We use a p+ doped poly-silicon fuse as an eFuse, and the width and length of the eFuse link are 0.35 µm and 2.1 µm, respectively. There is, however, a possibility that the eFuse link can be shortened to the VSS-biased p-substrate by a thermal rupture and memory failures can occur. Thus, we solve a problem of an electrical shortage Fig. 2 Newly proposed differential paired eFuse OTP memory cell configured in a form of two-dimensional array between an eFuse link and the VSS-biased p-substrate by placing a floated n-well under the eFuse link, as shown Table 2 gives bias voltage conditions for various in Fig. 3 [7]. operation modes of each proposed two-dimensional Table 3 gives the major specifications of the 32-bit configurable differential paired OTP memory cell node.