RC24545 (W0805-007) May 1, 2008 Electrical Engineering

IBM Research Report

SOI Series MOSFET for Embedded High Voltage Applications and Soft-Error Immunity

Jin Cai, Tak Ning, Philip Oldiges*, Anthony Chou*, Arvind Kumar*, Werner Rausch*, Wilfried Haensch, Ghavam Shahidi IBM Research Division Thomas J. Watson Research Center P.O. Box 218 Yorktown Heights, NY 10598

*IBM Research & Development Center Hopewell Junction, NY 12533

Research Division Almaden - Austin - Beijing - Cambridge - Haifa - India - T. J. Watson - Tokyo - Zurich

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SOI Series MOSFET for Embedded High Voltage Applications and Soft-Error Immunity

Jin Cai, Tak Ning, Philip Oldiges*, Anthony Chou*, Arvind Kumar*, Werner Rausch*, Wilfried Haensch and Ghavam Shahidi

IBM Research Division, T. J. Watson Research Center, Yorktown Heights, NY 10598 * IBM Semiconductor Research & Development Center, Hopewell Junction, NY 12533 Phone: 914-945-2310; Fax: 914-945-2141; E-mail: [email protected].

ABSTRACT floating-body design and the body-tied design. The breakdown We demonstrate a simple and novel scheme to achieve high drain voltage increases with gate length but is limited to about 4V in a long breakdown voltage (BV) in a high-speed -on-insulator (SOI) channel (2%LNOM) body-tied design. The floating body devices show logic technology. In an SOI device with two FETs in series, the an abrupt increase in ID near VDS l1.5V, which is the well-known kink common floating node provides a negative feedback that limits the effect due to the onset of impact ionization near the drain. increase of avalanche current. This unique property of SOI provides a The output characteristics of two-FET in series is shown in Fig. 6. way to enhance the breakdown voltage by simply adding more devices The breakdown voltage is increased by 2% to >5V for the sub-nominal in series. Data from 45nm SOI technology show BV>6.5V for two- device and >6.5V for the nominal device. There are two ‘kinks’ in NFET in series, and BV>10V for three and four-FET in series. Other each ID-VDS curve, the first one at VDS<2V comes from impact benefits of SOI series device include >300% reduction in stand-by ionization in FET 2, and the second one at higher VDS is from impact leakage current and soft-error immunity as compared to a single SOI ionization in FET 1. The second kink can be eliminated by using a device. Our result suggests the prospect of integrating high-voltage body-tied SOI design for FET 1 only, as shown by the line in Fig. 6. functions on SOI-based technologies with little or no additional cost. The increase of breakdown voltage in the series device is due to a negative feedback mechanism, which is confirmed by the measured INTRODUCTION common floating node potential (V1) in Fig. 7. The calculated voltage In advanced CMOS technologies optimized for circuit speed, the drop across FET 2 (VDS2= VDS - V1) is plotted along with measured V1. drain breakdown voltage in short channel MOSFET is usually quite The applied drain voltage is initially dropped across FET 2 until VDS2 low, due to heavily doped abrupt junction profiles. In particular, SOI reaches about 3.5V where avalanche multiplication starts. The technologies give even lower breakdown voltage than similar bulk avalanche multiplication in FET 2 is self-limited by the floating node technologies due to the parasitic bipolar effect [1]. A body-tied SOI which charges up and keeps VDS2 from rising further, thus avoiding a can restore the breakdown voltage to the bulk value at the expense of runaway breakdown. The additional drain terminal voltage is then high parasitic capacitance. For some ASIC applications, voltage dropped across FET 1. When V1 increases, VGS2=VG - V1 reduces and handling capability of ~5V is desirable, which is becoming difficult to the current conduction mechanism in FET 2 changes from surface- meet. Series devices on (semi-)insulating substrate have been used as channel FET current to lateral parasitic bipolar current. The family of high-voltage RF switches, but can not be used to handle high voltages ID-VD curves in Fig. 8 clearly shows the breakdown voltage advantage at lower frequencies or in general logic [2]. For the first time, we of the SOI series device over the best-case single SOI device. As show that SOI series device from a high-speed CMOS process can be more devices are added in the series, the drain voltage is dropped on used for high-voltage logic applications. In comparison, bulk devices each device sequentially from the one closest to the drain to the one in series can not offer the same breakdown voltage benefit due to closest to the source, enabling higher breakdown voltages. In Fig. 9, drain-to-substrate breakdown. Measured results based on 528 gate- output characteristics of series devices with 3 FETs and 4 FETs are oxide FETs from a 45nm SOI technology [3] are presented. Physical shown. No signs of runaway breakdown is seen up to VDS=10V mechanism responsible for the high breakdown voltage is delineated. (limited by gate dielectric breakdown). Fig. 10 shows measured Since breakdown voltage is usually limited by NFETs rather than floating node potentials in a 4-FET series device. The successive rise PFETs due to the higher impact ionization rate of electrons, only of V3 and V2 with increasing VDS indicates the saturation of voltage NFET series device results are shown. Other unique SOI series device drops across FET 4 and FET 3 respectively. The breakdown voltage properties such as low stand-by leakage current and soft-error of the SOI series device is only limited by the number of devices in immunity are also presented. series, provided that the gate dielectric is thick enough. This is a BREAKDOWN AND LEAKAGE CHARACTERISTICS unique property for SOI. Bulk devices in series are not suitable for high-voltage application due to drain-to-substrate breakdown concern. Fig. 1 shows schematics of SOI series devices. The diffusion regions from the two neighboring devices can be merged (Fig. 1a) or SOFT-ERROR IMMUNITY electrically-connected (Fig. 1b), and the number of devices in series The SOI series device has built-in redundancy which makes it can be more than two (Fig. 1c). The merged diffusion design offers immune to single-event upsets. Fig. 11 and 12 compare the effect of higher density but requires heavy source/drain doping down to the 1MeV -particle strike for a single SOI device and a merged-diffusion buried oxide to be effective. All the measurement data reported here (Fig. 1a) series SOI device based on simulations. In the off state, are from connected-diffusion structures. Fig. 2 compares subthreshold when one FET is hit, the other FET keeps the series device off, thus characteristics of a series SOI device and a single SOI device, at the avoiding upset. The total collected charge is reduced by 40% if the - nominal channel length (LNOM) of ~0.22m. The off current (IOFF) in particle strikes FET 2 and by 400% if it strikes FET 1. the series device is reduced by 350% to 10pA/m at a VDS of 2.5V, and DIBL is reduced by 230mV. A similar trend is observed in Fig. 3 CONCLUSIONS where the gate length is scaled to LNOM / 2=0.11m. It should be We showed that high-speed SOI CMOS logic devices can be noted that even two sub-nominal devices in series results in 35% lower simply connected in series to achieve arbitrarily large drain IOFF and lower DIBL than a single nominal device. The mechanism breakdown voltage. The only requirement is to add a thick gate oxide that is responsible for the series device property can be understood that is commensurate with the desired voltage. This unique property from the measured common floating node potential in Fig. 4. In the of SOI CMOS opens up possibilities for low-cost SoC applications. off-state, the floating node is charged up to ~1.5V under a drain REFERENCES voltage of 2.5V, by the hole current generated from the high field 1. K. Young and J. Burns, IEEE Trans. Electron Devices, p. 426, 1988. region near the drain. This effectively reduces VDS across FET 2 and 2. M. Shifrin, et.al., IEEE Trans. Microwave Theory and Technology, v37, hence reduces IOFF and DIBL. The output ID-VDS characteristic under p. 2134, 1989. 3. S. Narasimha, et.al., IEDM Tech. Dig., session 27.3, 2006. VGS=1.5V is shown in Fig. 5 for single SOI devices in both the regular

-3 -3 (a) (b) 10 10 Gate Gate

Source Drain Source Drain m) m) 12 12 µ µ 10-6 10-6 n+ n+ n+ n+ n+ n+ n+

Lg=Lnom Lg=Lnom/2 (c) Gate 10-9 Single NFET 10-9 Single NFET Source12 N-1 N Drain 2-NFET in series 2-NFET in series V =0.05, 2.5V V =0.05, 2.5V DS DS n+ n+ n+ n+ n+ n+ (A/ CURRENT DRAIN (A/ CURRENT DRAIN £ 10-12 10-12 0123 0123 GATE VOLTAGE (V) GATE VOLTAGE (V) Fig. 1. Schematics of SOI series devices: (a) two NFETs in series with merged common diffusion, (b) Fig. 2. ID-VG characteristics of a single device vs. a Fig. 3. ID-VG characteristics of a single device vs. a two NFETs in series with an electrically-connected series SOI device, at nominal gate length. Series series SOI device, at half of the nominal gate length. floating node, and (c) N devices in series. device shows significant IOFF and DIBL reduction. Series device shows IOFF and DIBL reduction.

2.0 600 500 Single NFET V =1.5V V =1.5V Regular (solid) GS GS V =2.5V m) m) DS 500

µ µ 400 2-FET in series 1.5 Body-tied (open) Lnom/2 2-NFET in series A/ A/ Lnom/2 µ 400 µ Lnom Lg=Lnom Lnom 300 1.0 Lg=Lnom/2 300 2⋅Lnom 200 200 0.5 100 100 Body-tied SOI for FET1

0.0 ( CURRENT DRAIN 0 ( CURRENT DRAIN 0 0123 012345 01234567 FLOATING NODE VOLTAGE (V) VOLTAGE NODE FLOATING GATE VOLTAGE (V) DRAIN VOLTAGE (V) DRAIN VOLTAGE (V)

Fig. 4. The common floating node potential in SOI Fig. 5. Output characteristics of regular and body-tied Fig. 6. Output characteristics of two NFETs in series. series devices measured by a voltage probe. The SOI devices at various gate length (Lg). Breakdown Breakdown voltage is improved to over 6.5V for two common node floats up in the off-state (near VG~0V). voltage increases with Lg but is limited to about 4V. nominal gate length devices in series. 4 1000 300 V =1.5V 2-NFET in series, Lnom 3-FET, Lnom/2 V =1.5V GS Single NFET, 2⋅Lnom GS m) m) 3-FET, Lnom µ µ 800 Lg=Lnom V =0 to 4V in 0.5V steps 3 GS

A/ 4-FET, Lnom/2 A/ µ µ 200 4-FET, Lnom V = 600 DS2 2 V −V D 1 V = VG 400 DS1 100 1 V 12 VOLTAGE (V) 1 VD VS 200 V1 Body-tied SOI for FET1 (3-FET, Lnom) DRAIN CURRENT( 0 ( CURRENT DRAIN 0 0 02468 01234567 0246810 DRAIN VOLTAGE (V) DRAIN VOLTAGE (V) DRAIN VOLTAGE (V)

Fig. 7. Measured common floating-node potential Fig. 8. Family of ID-VD curves from two nominal Fig. 9. Output Characteristics of three NFETs in nd (solid line) and extracted VDS across the 2 FET NFETs in series (solid lines, Lg=LNOM) and a single series and four NFETs in series. The drain (dash line) as a function of drain voltage. long channel NFET (dash lines, Lg=2LNOM). breakdown voltage can be increased to over 10V. 800 100

m) Single FET

V =1.5V Lg=Lnom µ Single FET 6 GS 2-FET in series G2 strike A/ 2-FET in series 10

µ 2-FET in series G1 strike VG 600 G2 strike 2-FET in series 123 4VD 1 G1 strike 4 400 0.1 V1 V2 V3 200 2 V3 0.01 V2 1E-3 DRAIN CURRENT ( V1 0 COLLECTED CHARGE (fC) 0.01 0.1 1 10 100 0.01 0.1 1 10 100 0 0246810 TIME (psec) TIME (psec) FLOATING NODE VOLTAGE VOLTAGE FLOATING NODE (V) DRAIN VOLTAGE (V) Fig. 11. Comparisons of simulated drain transient Fig. 12. Comparison of simulated collected charge Fig. 10. Measured floating node potential in a 4-FET current after 1MeV -particle strikes. SOI series after 1MeV -particle strikes. SOI series devices show 40% to 400% reduction in collected charge than series device vs. drain voltage. Rise of V3 and V2 devices show lower transient current over a much shorter time span than that of a single device. a single device. shows saturation of VDS in FET 4 and 3 respectively.