Comprehensive ESD protection for RF inputs

https://doi.org/10.1016/j.microrel.2004.05.012Get rights and content

Abstract

We demonstrate that narrow-band tuned circuits may be used for ESD protection of RF inputs, and a figure of merit for optimization of these circuits is presented. The performance of the ESD-protected RF circuit is dependent on the quality factor of the ESD device, and various protection devices are evaluated in this work. Record-breaking human body model (HBM) protection levels, exceeding 5 kV, have been achieved without significantly degrading the RF performance at 5 GHz. Broadband circuit protection is also addressed.

Introduction

ESD protection of RF circuits is challenging because of the performance-degrading shunt admittance that the protection device introduces at the RF input. This shunt path is primarily due to the parasitic capacitance of the protection device. The problem worsens with increasing RF operation frequency. A protection circuit may be characterized by a figure of merit,FOM=ZShunt·VHBM=VHBMωCESDwhere VHBM is the HBM-ESD failure level, and ZShunt is the impedance of the parasitic shunt path to AC ground, at the RF operation frequency.

Recent works [1], [2] have focused on improving the FOM by minimizing the capacitance of the protection device. But as frequency increases, one eventually reaches a point at which the FOM is too low––and this can happen at frequencies as low as 2 GHz, depending on the technology [1].

To provide ESD protection without compromising RF performance, one may instead use a resonator to provide ESD protection [3], [4]. Here, for the first time, we present tuned circuits that protect against the full range of ESD transients––from the relatively slow HBM pulse to the rapid oscillations of a CDM discharge. Furthermore, we derive an appropriate FOM for this new class of protection circuits.

Section snippets

LC resonator and cancellation

Leroux and Steyaert [3] proposed using an inductor as an ESD protection device. The inductance is selected such that it is resonant with the parasitic capacitances (e.g. the bond-pad capacitance) at the RF operation frequency (Fig. 1a). For a narrow band around the resonant frequency of this parallel LC circuit, it has minimal effect on the RF performance, because its shunt impedance is very large compared to the 50-Ω input impedance of the RF circuit.

Furthermore, the HBM waveform contains

Quality factor and FOM

Assume that a shunt resistor of 1000 Ω may be added to the input of the LNA without significantly affecting the RF performance, i.e., noise figure, gain, and S11. At the RF operation frequency, which is also the resonant frequency of the cancellation circuit, the shunt resistance may be calculated asRShunt=QRESωCESD,where CESD represents the capacitance of the ESD protection circuit, and the resonator quality factor QRES is defined asQRES=11QIND+1QESD.In (3), QIND represents the inductor

MOS-based protection

First, we compare two floating-body, grounded-gate NMOS transistors (FB–GGNMOS). This protection device is described in [7]. The devices lie within a floating p-well. Simplified cross-sections of the test devices, built in Motorola's 0.35-μm BiCMOS process [8], are shown in Fig. 4a and b. In Fig. 4a, the p-well is surrounded with a floating n-type region, consisting of n-wells and an n+ buried layer. In Fig. 4b, deep trenches are used in place of n-wells alongside the p-well sidewalls. This

Isolation techniques

If any of the tuned protection circuits presented thus far has a small quality factor, it may unacceptably degrade the RF circuit performance. Therefore, alternative protection circuit topologies are needed for technologies in which high-Q inductors and/or high-Q protection devices are not available. One such alternative is to isolate the ESD protection device from the input terminal.

Consider the circuit shown in Fig. 6a, in which an LC tank is placed in series with the ESD protection device.

Experimental results

To test the effectiveness of the tuned protection circuits described above, three 5.25-GHz, 5-mW LNAs were designed. The first LNA was protected with a slightly modified cancellation circuit (identical to the one shown in Fig. 1b, but with a DC-blocking capacitor in series with the inductor). The second LNA was protected by the isolation circuit shown in Fig. 6a. The third LNA was left unprotected for comparison.

The simulated RF performance of each LNA is listed in Table 2. Apparently, the ESD

Broadband ESD protection

The protection circuits presented above can only be used in narrow-band applications, as the tuned protection circuit's off-state impedance is large only in a small band (width inversely proportional to Q) around the resonant frequency. For broadband circuits, options are somewhat limited.

A patent application by Atheros Communications, Inc. [19] describes a circuit in which a single inductor is used to isolate the ESD protection device from the RF input, as shown in Fig. 9. Essentially, the

Summary and conclusions

We have shown that, for radio-frequency, narrow-band applications, high levels of ESD protection can be achieved through the use of tuned circuits. We have demonstrated that new criteria should be considered when choosing and optimizing the protection, one being the quality factor of the ESD protection device. We have presented a new figure of merit to give a tool for this optimization. A comparison of quality factors for different ESD protection devices was made, to elucidate the best devices

Acknowledgements

The work was funded by Semiconductor Research Corporation. The authors would like to thank Motorola Inc. for processing some of the LNAs, Gary Kaatz of Motorola for numerous valuable technical discussions on RF design, and Timothy Maloney of Intel and Pascal Salome of ST Microelectronics for discussions on CDM stress.

References (19)

  • Richier C, Salome P, Mabboux G, Zaza I, Juge A, Mortini P. Investigation on different ESD protection strategies devoted...
  • Worley ER, Bakulin A. Optimization of input protection diode for high speed applications. In: Proceedings of the 24th...
  • P. Leroux et al.

    High-performance 5.2 GHz LNA with on-chip inductor to provide ESD protection

    Electron. Lett.

    (2001)
  • S. Hyvonen et al.

    Cancellation technique to provide ESD protection for multi-GHz RF inputs

    Electron. Lett.

    (2003)
  • A. Amerasekera et al.

    ESD in silicon integrated circuits

    (1995)
  • Joshi S, Rosenbaum E. Compact modeling of vertical ESD protection NPN transistors for RF circuits. In: Proceedings of...
  • Joshi S, Juliano P, Rosenbaum E, Kaatz G, Kang SM. ESD protection for BiCMOS circuits. In: Proceedings of the IEEE...
  • Chai FK, Kyono C, Ilderem V, Kaneshiro M, Zupac D, Bigelow S, et al. A cost-effective 0.25 mm Leff BiCMOS technology...
  • Voldman S, Juliano P, Schmidt J, Johnson R, Lanzerotti L, Joseph A, et al. Electrostatic discharge characterization of...
There are more references available in the full text version of this article.

Cited by (0)

View full text