Although our aim is to give an understanding of latchup as it occurs in cmos switches, similar. Experimental study and spice simulation of cmos digital circuits latch up effects due to high power microwave interference are reported in this paper. Keep the sourcedrain of the mos device not in the well as far away from the well as possible. Latch up is the phenomenon in which this parasitic thyrister. Thirdly, we describe how latch up can be detected in an active circuit and cancelled before damage can occur this is accompanied by successful experimental results with laser induced latch up. Latchup refers to short circuit formed between power and ground rails in an ic leading to high current and damage to the ic. Additionally, the thyristor might be triggered by a high supply voltage far higher than the value given in. In cmos tech nology, pnpn structures are integrated into the circuit by two coupled parasitic bipolar transistors. Another common problem is when a cmos output is connected to a large capacitive load. Having a consistent definition for set and reset is important, especially as students study multiple latch circuit topologies and activelow inputs. Pdf latchup in integrated circuits from energetic protons. This generally destroys the cmos ic or at the very least causes the system to malfunction. Latch up is a condition that comes about when this parasitic scr is triggered.
In a commonly used layout slang, latch up is a substrate or nwell tie used to prevent the firing up of the parasitic thyristor. Fourthly, we describe other integrated circuit fabrication technologies, which are naturally immune. Transientinduced latchup in cmos integrated circuits wiley. Latch up in cmos devices inherent in a cmos process are parasitic pnp and npn transistors configured as silicon control rectifiers scr. As a traditional inherent destruction phenomenon, latch up effect may jeopardize the correct function of the circuits, and could be triggered in various ways such as esd pulse, cosmic ray, heavy. Latchup refers to short circuit formed between power rails in an ic leading to high current and damage to the ic. The simplest way of preventing latch up occurring is to adhere to the absolute maximum. Subscribe this channel for more videos on physical design fundamentals.
A latch is considered set when its output q is high, and reset when its output q is low. This causes a low resistance to appear from v dd to ground, and a subsequent large current to be drawn through the device. Winning the battle against latchup in cmos analog switches. It is also a problem when you have cables or inputs from other devices going directly to your chip. Latchup is a failure mode in cmos circuits that results in either. At t5, r is deasserted first, so the latch goes into the normal set state with q 1 and q 0. In some cases latch up can be a temporary condition that can be resolved by power cycle, but unfortunately it can also cause a fatal chip failure. Latchup and its prevention in cmos devices vlsi universe. Latch up is a serious problem in cmos integrated circuits because n and p channel transistors are located close to one another. Prevention of latchup in cmos integrated circuits using. First, however, know what a thyristor is aka an scr or silicon controlled rectifier. The book all semiconductor device engineers must read to gain a practical feel for latchupinduced failure to produce lowercost and higherdensity chips. With high current levels, a device would not have to remain in a latch up state for very long. Latchup becomes a real problem when you try to power up and down different sections of your design to save power.
In some cases latch up can be a temporary condition that can be resolved by power cycle, but unfortunate. In a study on the latch up scaling in the last cmos technology nodes, it was shown that the intrinsic triggering currents for both positive and negative injection were practically unchanged fig. A problem exists if both s and r are deasserted at exactly the same time as shown at time t6. The proton latch up cross sect ions of th e five device ty pes diff er by about a factor 300, w hich is a very l arge diffe rence, but it is not clear how th is is related to d evice structure or the. All cmos ics have latchup paths, but there are several design techniques that reduce. This tie prevents the junctions to become forward biased. A thyristor has a feature called hysteresis which allows. Prior to discussing latch up in cmos devices, it is advantageous to brie. It resulted in circuits either malfunctioning or consuming excessive power, and could be either inherent in the design or triggered by voltage spikes on io pads that could forward bias pn. The applications engineer and systems designer, however, are not so much concerned with the theory and modeling of latch up as they are with the consequences of. If the s is equal to v oh and the r is equal to v ol, both of the parallelconnected transistors m1 and m2 will be on. Edn power management can cause latchup in cmos chips.
Click download or read online button to get cmos latch up modeling and prevention book now. The proposed latchup prevention methodology and circuit design can detect and stop the. The structure formed by these resembles a silicon controlled transistor scr. But such a fate is not inevitable in cmos circuitry. Simply defined, latch up is a functional chip failure associated with excessive current going through the chip, caused by weak circuit design.
Introduction scr latch up is a parasitic phenomena that has existed in circuits fabricated using bulk silicon cmos technologies. An600 understanding latchup in advanced cmos logic on. Fairchilds process enhancements eliminate the cmos scr. This will be helpful in developing an understanding of the relationships between external circuit and system conditions and the resultant triggerng of latch up in cmos devices. Now let us introduce the parasitic transistors seen by this structure, and the effects of transients when functioning as an. Since the well area electrically isolates bulk cmos devices from the substrate, a parasitic thyrister is inevitably constructed between the power supply and gnd pins. Additionally, the thyristor might be triggered by a high supply voltage far higher than the value given in data sheets. Latch up is a particular problem in cmos caused by the physical structure of the manufacturing process.
Latch up is a functional chip failure associated with excessive current going through the chip, caused by weak circuit design. A common problem in cmos circuits constructed on bulk silicon, and in many other triple diffused integrated circuits as well, is the disruption of normal circuit operation and possible destruction of the chip itself as a result of the latch up of a parasitic scr device. The individual bipolar devices have low current gain, with a. Investigation of surfaceinduced latchup in vlsi cmos. Notice the two transistors, npn and pnp and their connection to vdd and gnd. Vlsi design sequential mos logic circuits tutorialspoint.
This effect often was observed in earlier generations of cmos circuits. Latch up is a condition where a low impedance path is created between a supply pin and ground. Understanding latch up in advanced cmos logic latch up has long been a bane to cmos ic applications. Latch up is a serious problem for cmos reliability. Cmos latch up modeling and prevention download ebook pdf. Figure of merit fom for the latch up feedback loop. Ker and hsu introduce the phenomenon and basic physical mechanism of latchup, explaining the critical issues that have resurfaced for cmos. Basic psubstrate cmos inverter cross section with latchup circuit model.
In cmos fabrication, latch up is a malfunction which can occur as a result of improper design. Improve latch up immunity by circuit solution huiwen tsai and mingdou ker institute of electronics, national chiaotung university, hsinchu, taiwan abstract a concept of active guard ring and its corresponding circuit solution to enhance the latch up immunity of integrated circuits ic are proposed and verified in a 0. Hello everyone, this videos explains the latch up phenomenon in cmos circuits. A latchup is a type of short circuit which can occur in an integrated circuit ic. Speaking about cmos transistors, latch up is the phenomenon of low impedance path in cmos between power rail and ground rail due to interaction between parasitic pnp and npn transistors. Trends and challenges to esd and latchup designs for. Early in cmos development, latchup was recognized as a problem to be. Transientinduced latchup in cmos integrated circuits. Pdf the paper presents triple and double modular redundant tmr and dmr circuits with the latchup protection. The cmos device latch up phenomenon is a representative example of this. Under certain conditions these pnpn structures can fire like a thyristor, and in most ca ses the chip is destroyed i.
Latch up prevention in cmos various techniques for latch up prevention issues in physical design duration. A latch up is a type of short circuit which can occur in an integrated circuit ic. A latchup current selfstop methodology and circuit design, which are used to prevent damage in the bulk cmos integrated circuits due to latchup, are proposed in this paper. Additionally, there are semiconductor process solutions, circuit solutions and system solutions. Slya014a latch up, esd, and other phenomena 5 the parasitic thyristor can be triggered by a rapid rise of the supply voltage. Simple circuit techniques and attention to system design details will ensure that the cmos full potential can be realized in. The latch up mechanism, once triggered, turns on a parasitic scr internal to cmos circuits which essentially shorts vcc to ground.
Research and development into the causes led to several papers in the 1980s discussing causes and methods to lessen the influence of latch up. Msan107 understanding and eliminating latchup in cmos. Cmos latchup protection circuit national semiconductor. Latchup current selfstop circuit for wholechip latchup. Transientinduced latchup in cmos integrated circuits equips the practicing engineer with all the tools needed to address this regularly occurring problem while becoming more proficient at ic layout. In a bulk cmos chip, the core circuit blocks are always latchup sensitive due to a low holding voltage of the parasitic scr path.
The following diagram is a drawing of a typical cmos circuit. Latch up is defined as the generation of a lowimpedance path in cmos chips between the power supply v dd and the ground gnd due to the interaction of parasitic pnp and npn bipolar junction transistors bjts these bjts form a silicon controlled rectifier scr with positive feedback and virtually short circuit v dd to the ground, thus causing excessive current flows and even. Cmos latch up latch up is a functional chip failure associated with excessive current going through the chip, caused by weak circuit design. This requires more current before latch up can occur. Cmos sr latch based on nor gate is shown in the figure given below. This method works very well in dc op amp circuits, as opamps draw very little peak current and the circuit is only amplifying dc. More specifically it is the inadvertent creation of a lowimpedance path between the power supply rails of a mosfet circuit, triggering a parasitic structure which disrupts proper functioning of the part, possibly even leading to its destruction due to overcurrent. Early cmos processes suffered a reliability concern that became known as latchup.