Paper published at the 2008 RCJ Symposium in Japan
Yasuhiro Fukuda (1), Toshikazu Kuroda (1), Koen Verhaege (2), Bart Keppens (2)
1) Oki Electric Industry Co., Ltd., now LAPIS Semiconductor
(2) Sarnoff Europe BVBA, now Sofics BV, 32 Sint-Godelievestraat, B-9880 Aalter, Belgium
This paper is co-copyrighted by LAPIS Semiconductor, Sofics and the RCJ symposium
Abstract
The Silicon Controlled Rectifier (‘SCR’) is widely used for ESD protection due to its superior performance and clamping capabilities. However, many believe that SCR based ESD protection is prone to latch-up, competitive triggering, long development cycles and slow trigger speed. This paper provides an overview of the problems and corresponding design solutions available.
- Introduction
- I. SCR for ESD protection
- II. Common SCR fears
- A. Latch-up concerns
- B. Process tuning and fab portability
- C. SCR trigger speed for CDM protection
- D. Competitive triggering
- E. Advanced technologies
- Conclusions
- Acknowledgements
- References
Introduction
Despite the efforts in prevention of charge build-up, Electro Static Discharge (‘ESD’) is still a very common cause for Integrated Circuit (‘IC’) failure. To protect the sensitive semiconductor chips against the harsh conditions during manufacturing, handling, transportation, assembly and use by the consumers, IC manufacturers include on-chip ESD protection circuits at the IO’s and power cells. Many different topologies, circuits and protection devices exist to achieve robust and effective ESD protection
Early-on, the Thyristor or Silicon Controlled Rectifier (‘SCR’) was considered a perfect solution due to its superior ESD performance and clamping capabilities [1]. However, the use of the basic SCR brought with it a large number of issues such as latch-up, increased development and tuning cost, fab portability problems, ESD failures due to competitive triggering, CDM failures and a limited portability to advanced processes including SOI
Although recent advances in SCR devices can solve most -if not all- of these problems, there is still a negative image associated with the SCR. This paper provides an overview of the problems and corresponding design solutions available
After the introduction the typical problems related to SCR based ESD protection are summarized. For each of the topics, design solutions are demonstrated. Finally the benefits for the optimized SCR technology are highlighted in the conclusions
I. SCR for ESD protection
The Basic SCR is constructed as a four layer (NPNP) element. The schematic, cross section in a P-substrate process and the typical IV curve are depicted in Figure 1. The N+/Nwell tie (G2) is connected to the anode while the P+/P-substrate tie (G1) is connected to the Cathode
This basic implementation has two inter coupled parasitic devices (lateral NPN and vertical PNP). The configuration creates a positive feedback once either one of the bipolar elements is turned on. The regenerative action is the key behind the snapback and low holding voltage (~1.2V) seen in the IV characteristic. At high currents, the structure behaves as an intrinsic PiN diode. To start the positive feedback, the Anode-Cathode voltage must exceed the Nwell/Pwell avalanche breakdown voltage
A measurement from a mature process technology (0.25um) is shown in Figure 2. Two problems are visible. Besides the low holding voltage, below the Vdd potential, a high Vt1 trigger voltage is visible. New SCR approaches have been introduced to cope with this trigger problem such as the MLSCR and the LVTSCR (Figure 3) [2]
Certainly the LVTSCR has been widely used in industry in the mature process nodes. IC designers quickly realized the many benefits. The SCR is nearly an ideal switch device with low leakage in off-state combined with low impedance once triggered.
Further, due to the very high ESD performance, area compact and low capacitance protection clamps can be created. However, the basic and LVTSCR did not solve all the problems. People reported latch-up problems due to the low clamping voltage. Moreover, it was difficult to correctly fit the SCR IV curve within the ESD design window. ESD engineers required a number of TEG runs to tune the trigger and holding voltage. Large semiconductor corporations noted serious IV curve fluctuations between their different fabrication plants. Further problems included difficulties to protect output buffers due to competitive trigger issues, lower than expected CDM results due to slow trigger speed and integration problems into advanced technologies (STI isolation, SOI)
In the next sections, more information is provided for each of these reported issues. Examples from different technologies and mass produced products solutions are presented based on Sarnoff Europe’s SCR solutions. When the optimized SCR devices are combined with the right process and product expertise the SCR device brings a great deal of benefits
II. Common SCR fears
When ESD designers discuss about Silicon Controlled Rectifiers it seems that all of them have suffered from some of the problems in the past. Most of these problems can be solved through improved SCR design as shown in the next 5 sections
A. Latch-up concerns
Latch-up problems are probably feared the most when Silicon Controlled Rectifiers are used. Because the holding voltage is only about 1.2V which is below the Vdd potential in most applications, a latch-up situation can easily occur. The main condition is that the ESD device is triggered during biased condition of the IC and that it opens a continuous current path that draws an over-current from the supply
Such unexpected triggering can occur through different scenarios where noise is applied at the supply lines or IO pads
- Over voltage applied at the IC terminals. If the applied voltage is higher than the trigger voltage the SCR is triggered. This can occur through wrong user interaction or through inductive effects.
- ESD stress applied during biased conditions. Such events mainly occur in automotive and industrial applications.
- Other noise events such as photo current are mainly important in space applications.
Based on these examples it is possible to define different constraints for the application of SCR based ESD protection:
Holding voltage engineering.
For certain CMOS applications the holding voltage can easily be increased above Vdd by adding diodes in series with the SCR. This is demonstrated in Figure 5 below. Another possibility is to stack multiple SCR’s in series.
Trigger point engineering.
The trigger voltage and trigger current of the protection clamp must and can be increased above absolute maximum ratings. By applying a separate trigger element the latch-up conditions can be shifted to this trigger element [3,4]
Holding current engineering.
Besides a sustaining voltage (Vhold) every SCR has a certain holding current Ihold. If the current through the SCR is made smaller than Ihold, the positive feedback mechanism is interrupted. The holding current can be increased with special layout technique called Anode/Cathode segmentation to reduce the effective Nwell/Pwell resistance as shown on Figure 7. [3]
These SCR improvements were applied at a number of IC products in mass production available today, including over 50 IC’s in high voltage applications where the latch-up problem is even more threatening. The example below (Figure 8) is an OKI LCD driver IC where the SCR protection was included at the high voltage power cell. High latch-up immunity of >300mA was achieved.
B. Process tuning and fab portability
Due to the many benefits of SCR based ESD clamps several ESD designers have included SCR’s in the protection approach of their ASIC’s. Al kinds of trigger and clamping options were conceived similar to the LVTSCR [2] in the late 1980-ties whereby the trigger circuit is integrated within the SCR body. Triggering mostly relied on avalanche breakdown.
However, people quickly realized that these SCR types required extensive silicon experiments to tune the trigger and clamping behavior to match the ESD design window. Typically, layout improvements to control the trigger behavior also influenced the clamping behavior. Many companies reported issues when these SCR based protection schemes were ported to different fabs: even minimal process variations induced a strong influence on the ESD behavior because non standard elements were used. Many people lost faith in the Silicon Controlled Rectifier.
A solution to this problem was presented in 2001 [5]. In the new approach the trigger and clamping behavior are separated from each other such that both aspects can be controlled independently. The SCR clamping behavior (ESD robustness, resistivity, trigger speed) is optimized by minimal AnodeCathode spacing (LAC) and close to the parasitic base triggering. The trigger behavior is controlled separately through the use of external trigger elements based on controlled devices such as resistors, diodes, MOS transistors. Several variations have since been used [6-7] (Figure 9).
This new approach is currently used in several hundred mass produced IC’s in at least the eight most recent CMOS generations. Consistent ESD behavior is achieved. In Figure 10 Diode Triggered SCR (DTSCR) devices on different process corners are compared in a 65nm process technology. Almost no variation is visible.
C. SCR trigger speed for CDM protection
Despite some of the problems with LVTSCR’s as detailed above different corporations are still using the device as ESD protection clamp. In 2000, Wu and Rosenbaum discovered yet another problem when the LVTSCR device (Figure 3) is used in advanced CMOS technology [8]. They provided proof that the LVTSCR device did not respond fast enough to some ESD transients similar to CDM. The device was unable to protect a thin gate oxide monitor connected in parallel for pulses with a rise time of 1ns.
The new solution introduced in 2001 by Russ et al. [5] did overcome this trigger speed issue. An example is shown in Figure 11 where a thin oxide (1.4nm in 65nm CMOS) gate monitor device is protected using a Diode Triggered SCR (DT-SCR) clamp even for an ultra-fast applied rise time of 200ps. Recently, various independent sources have provided proof of CDM protection capabilities of the Sarnoff DT-SCR [9-12].
Russ concluded that the main problem with the LVTSCR was the large Anode-Cathode distance (LAC) due to the integrated trigger junction. When the SCR device is optimized (shortest LAC – Figure 12) and external trigger element is correctly defined the protection device is fast enough.
D. Competitive triggering
For the ESD protection of output buffers people have long relied on the intrinsic ESD properties of the parasitic bipolar device within the MOS transistors. Certainly in mature technologies this strategy is both simple and adequate. Through the application of silicide and LDD blocked drains the same approach was extended to more advanced CMOS technologies. Because of the severe silicon area consumption for this NPN based protection, many ESD designers have looked at smaller alternatives such as the LVTSCR.
However, the application of the LVTSCR as a parallel protection element for the NMOS output transistor resulted in ESD failures due to competitive triggering. Both the sensitive buffer element and the clamp device have a very similar trigger mechanism: avalanche breakdown of an N+/Pwell junction. Furthermore, the trigger voltage of the sensitive output buffer NMOS device is reduced due to the undefined potential at its gate. This resulted in a broad statistical spread of the protection performance when the NMOS driver is triggered into NPN conduction before the LVTSCR.
In 2005, Van Camp et al. introduced a straight forward solution to the problem of competitive triggering [13] (Figure 13). The sensitive device was promoted to become the trigger device for the SCR clamp. Instead of detecting ESD as an overvoltage stress, this RT-SCR approach relies on the detection of an excess current flowing through the sensitive elements in an output buffer. A small resistance of about 2 ohm is added in series with the output buffer.
E. Advanced technologies
When Shallow Trench (STI) isolation replaced Field Oxide (FOX) schemes, many process engineers believed that the SCR benefits would disappear due to the much deeper isolation. Further it was believed that SCR based protection was not possible in SOI technology. Both statements proved to be wrong.
Marichal [14] provided information about the construction and application of SCR devices in SOI technologies. A special layout technique is needed to create the Nwell/Pwell junction and to connect the NPN/PNP bases.
Further, based on recent measurements on 40nm process technology, there is no reason to write-off the SCR approach for ESD protection in the most advanced CMOS nodes (Figure 14).
Conclusions
Many ESD engineers do not like to use SCR based ESD protection related to bad experiences in the past such as latch-up, competitive or slow triggering and long development cycles with early LVTSCR-like device types. This paper reviewed and explained these issues and provided design solutions to cope with each problem.
Latch-up immunity problems can be solved, even for High Voltage process technologies, by careful SCR design. The paper provided demonstration of mass produced ICs where the SCR power clamp is made latch-up immune to high current injection, even at high temperatures.
The common experience of long development cycles and extensive process tuning required to fine-tune basic SCR triggering behavior can be solved easily through the use of external trigger circuits coupled to the G1/G2 well ties. The trigger circuits can be based on standard and well characterized elements (diode, MOS, resistor).
Many publications in the past have pointed to the slow trigger speed of the SCR as a main problem. The paper showed examples and references where recent innovations in SCR design can lead to effective ESD protection even for fast transients including CDM and IEC stress.
Finally, the optimized CMOS SCR structure has been ported to various technologies including SOI, BCD and HVCMOS where it is used as an effective ESD protection structure without influencing the normal operation conditions and bringing great benefits.
It is clear that when the optimized solutions are combined with strong expertise, the Silicon Controlled Rectifier device can be safely used for ESD protection bringing great benefits like reduced ESD area, low parasitic capacitance, low leakage current and excellent clamping behavior.
As is the case with many published ESD design solutions, most of the techniques and protection solutions described in this paper are covered under patents and cannot be copied freely.
Acknowledgements
The Authors want to thank Shiono-san from RCJ for his valued support for the publication of this work.
References
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- [6] Mergens M. et al, “Advanced SCR ESD Protection Circuits for CMOS / SOI Nanotechnologies”, CICC proceedings 2005
- [7] Mergens, M. et al., “Speed Optimized Diode-Triggered SCR (DTSCR) for RF-ESD Protection of UltraSensitive IC Nodes in Advanced Technologies. IEEE TDMR 2005.
- [8] Wu, J. et al., “Breakdown and Latent Damage of UltraThin Gate Oxides under ESD Stress Conditions”, EOS/ESD 2000.
- [9] Brennan, C.J.,“Implementation of Diode and Bipolar Triggered SCRs for CDM Robust ESD protection in 90nm CMOS ASICs”, EOS/ESD 2005
- [10]Watanabe, K., “New Protection Techniques and Test Chip Design for Achieving High CDM Robustness”, EOS/ESD 2008
- [11]Suzuki, T., “CDM Analysis on 65nm CMOS: Pitfalls when Correlating Results between Test Chips and Product Level”, EOS/ESD 2008
- [12]Di Sarro, J., “A Dual-Base Triggered SCR with Very Low Leakage Current and Adjustable Trigger Voltage”, EOS/ESD 2008
- [13]Van Camp, B. et al., “Current detection trigger scheme for SCR based ESD protection for Output drivers in CMOS technologies avoiding competitive triggering”, EOS/ESD 2005.
- [14]Marichal, O. et al., “SCR based ESD protection in nanometer SOI technologies”, EOS/ESD 2005
SOFICS - Solutions for ICs 