Substrate Coupling in RF Analog/Mixed Signal IC Design: A Review

Substrate Coupling in RF Analog/Mixed Signal IC Design: A Review Ashish C Vora, Graduate Student, Rochester Institute of Technology, Rochester, NY, USA. Abstract : Digital switching noise coupled into the sensitive analog circuits is a critical problem in the large scale integration of the mixed analog and digital circuits. Switching transients in the digital MOS circuits can perturb the analog circuits integrated on the same die by means of the coupling through the substrate. Parasitic substrate coupling can also severely degrade the performance of high speed ICs. This paper describes the noise coupling of this kind, especially through the substrate in the CMOS integrated circuits. Various models to model mixed signal coupling have been reviewed. Also various techniques used to reduce this coupling are suggested in the latter part of the paper. The paper also describes experimental technique for observing the effect of substrate noise on RF analog circuit. Index Terms : Vdd bounce, Gnd bounce, guard band, substrate noise, noise coupling INTRODUCTION Monolithic mixed signal and RF integrated circuits are becoming ubiquitous in the semiconductor industry. Substrate coupling is the major problem in the field of mixed signal and RF circuit design due to the trend to integrate as many circuits as possible on the same die. Substrate coupling is the phenomenon by which the transistors in the chip on the same substrate interact with each other through the common substrate. In mixed signal systems the coupling of noise between the on chip analog and digital can corrupt the low level analog circuit and impair the performance of such mixed signal circuits. In the digital portion there are a large number of gates undergoing transitions periodically at a high frequency. When such a transition occurs, a spike of current is absorbed from the power bus. Usually a great portion of this current is passes through the ground bus through direct feed through or it is injected into the substrate. The chip complexity and the higher level of integration do not allow the analog and the digital grounds tie together. The cumulative contribution of currents injected by switching gates in the substrate is felt in the sensitive circuits in the form of spurious signal, which is known as substrate noise. This high-speed substrate noise changes the desired analog output and degrades the overall IC performance. One of the most important issues in mixed signal design is to model the substrate and to predict the signal coupling between the digital circuit and the analog circuit on the same substrate correctly. The design methodology to reduce this substrate noise is also very important. The author is a Graduate Student in Electrical Engineering at Rochester Institute of Technology. This paper is written as a part of the Graduate Course Advanced Analog IC Design, Spring 2003. 1

Various lumped elements have been proposed for heavily and lightly doped substrate. Numerical techniques for efficient calculation of the substrate resistance have also been investigated. In this paper is presented a review of the sources of substrate noise, its effect in the mixed signal environment and various means to reduce these effects. Also simulation results are demonstrated showing the effect of the substrate noise due to an inverter operating at high frequencies on the performance of the Low Noise Amplifier at 2.4 GHz. SOURCES OF SUBSTRATE NOISE All the current that is injected into the substrate will cause fluctuations of the substrate voltage. This is called substrate noise and is caused by the coupling of the switching or noisy signals to the substrate. This noise can be caused due to different reasons: Coupling from the digital power supply, coupling from the switching source-drain nodes and impact ionization in the MOSFET channel. Noise on the digital power supply is caused by the voltage drops due to the resistances and the inductances of the power bus on the chip. This inductor and the on chip capacitor between the power and ground cause a ringing in the power supply. Fig. 1. Substrate as a parasitic return path Normally the substrate is used as a digital ground in every CMOS gate and this results in a low resistance path between the digital ground and substrate and hence all the digital noise and ringing will be present in the substrate. Also the resistance and the capacitance of the substrate causes the phenomenon of cross talk in the circuit as substrate acts as a parasitic return path for the signals. The second source of substrate noise is the capacitive coupling from the switching source and drain nodes of the transistors. SUBSTRATE NOISE COUPLING EFFECTS IN MIXED-SIGNAL INTEGRATED CIRCUITS The lesser power consumption and lower cost of single chip solutions motivates technology improvements in mixed signal (analog and digital) designs. But as there is a common substrate for the digital and analog circuit, which causes coupling between the analog and the digital part of the chip. Fig 2 shows that the digital switching node causes fluctuations in the underlying voltage as it is capacitively coupled to the substrate through junction capacitances and 2

interconnected bonding pad capacitances. Thus, a substrate current pulse flows between the surrounding substrate contacts and the switching node. When the digital circuit is operating at extremely high frequencies and many transistors are switching at a same time it causes a large instantaneous current to flow through. This current spike flows through the parasitic resistances and the inductances of the substrate causing power supply noise voltage spikes known as Vdd bounce or Gnd bounce. A part of this noise inevitably propagates to the sensitive analog circuitry through the substrate, power supply lines, bonding wires, package pins etc., as shown in Fig. 3. This noise current ranges from 0.1 ma to several ma depending on the circuit, number of transistors and their sizes. The signal coupling through the substrate causes variations in the gain and the bandwidth of the LNAs and other circuits at the front end of the receiver. Also the signal loss in the substrate is a significant concern both in the design of receiver front end circuitry and in the realization of on chip high Q inductors. Fig. 2. Substrate noise coupling in mixed signal ICs Fig. 3. The mixed signal problem of noise coupling from the switching portion of the IC to the RF/analog portion via the substrate, package and supply lines. 3

MODELING MIXED SIGNAL COUPLING Various Models have been described to explain the coupling through the substrate. i. Impact Ionization Model As the speeds of the operation of the circuits are increasing and the feature size is increasing, impact ionization is becoming the main cause of substrate currents in the mixed signal environment. When the electric field in the depleted drain end of the transistor increases beyond a certain limit it causes impact ionization and as a result the electron hole pairs are generated which causes the current to flow in the substrate. ii. Device Interconnect Capacitance Model Each and every transistor on the IC die is coupled capacitively to the substrate through its p-n junction depletion region capacitances. Also every interconnect routed on the chip has some capacitance to the substrate. This capacitive coupling to the substrate causes the coupling of the signal to the substrate which becomes very important in the mixed signal IC because of the large number of transistors which are switching and injecting current into the substrate at any moment of time. This affects the high impedance analog circuits. Moreover with the decreasing technology feature size this interconnects capacitances to the substrate are becoming very important contributors of the injected current. iii. Package Model The effect of the non-ideal power supplies has a significant impact on the amount of substrate coupled switching noise in any IC design. Since the bond wires and the package pins have a significant amount of inductance and capacitance associated with them, any substrate current picked up by them can cause glitches in the value of the substrate supply voltage. iv. Substrate Model The substrate can be modeled as layers of uniformly doped semiconductor material of varying doping densities. Neglecting the effects of the magnetic fields on the chip, a simplified form of the Maxwell s equations can be applied to the substrate giving 1 2 2 V ( r, t) + ε [ V ( r, t) ] = q. ( r, t) ρ t Solving this equation using the complex mathematics one can obtain the model for the substrate. This thing has been done in many previous papers and is not described here. SUBSTRATE NOISE COUPLING REDUCTION TECHNIQUES Some bad side effects of substrate noise are mentioned in Section II, the following section provides brief descriptions of some design techniques and guidelines for noise-aware physical design, in order to reduce substrate noise coupling effects. In order to minimize the coupling of substrate noise, three different aspects should be taken into account. 4

I) The amount of noise generated in the digital circuitry II) The sensitivity of the analog circuitry to noise III) The transfer of the noise from the digital portion of the chip into the analog section. By minimizing these three areas, the substrate noise can be reduced. The strategies for decoupling or reducing digital noise in the mixed signal integrated circuits can be classified as follows: I) Avoiding or suppressing noise generated by the digital circuit (the noise source) II) Preventing or limiting noise transmission through the substrate. III) Eliminating or compensating for the noise influence on the analog circuit. These strategies can be applied through the device and process technologies, through the circuit and layout techniques. Some of the methods to reduce the substrate coupling are shown here. GUARD RINGS: The guard ring is commonly utilized in the prevention of the substrate noise in the IC design. The ring is a surface region heavily doped with the majority-carrier dopant and is intended to form a Faraday shield around any sensitive devices, which need to be protected from the substrate noise. A typical layout of guard bands is shown in Fig. The structures of the guard ring are around the noisy and sensitive circuitry, and usually separate the digital circuits from the analog circuits. Digital Noise Bands Analog Circuit Guard Ring Fig. 4. Guard Rings to reduce substrate coupling NWELL Trench NWELL trenches can be used in between the noisy and sensitive circuitry to block the substrate current flowing near the surface of the substrate. SUPPLY BOUNCE REDUCTION A cross section of a package cavity with bond wires connecting the chip to package traces is depicted in Fig. 5. This inductance of the package and bond wires can lead to supply bounce. The supply bounce can cause the voltage drop between the board supply and the chip so that the digital power and ground can be very noisy. There are two methods to minimize this bad effect: 5

i) a separate power and ground are used in the analog portion of the chip to isolate the more sensitive analog circuitry from the digital supply noise. ii) lower package parasitic inductance can be accomplished through multiple or shorter bond wire connections. This is a very effective solution, but it is expensive due to the extra package costs. Fig. 5. On-chip supply bounce due to the voltage drop across bond-wire package inductance FLOORPLANNING When the space in the circuit is available, careful floorplanning can be used to reduce the effect of the substrate noise coupling. During floorplanning, specific well-isolated areas can be allocated to noisy circuits as illustrated in Fig. 6. It means that the further the sensitive and noisy circuits are apart, the less substrate noise coupling can affect the performance of the circuit. Minimum distance requirements can be computed based on the overall noise spectral energy produced by such circuits and the maximum levels of spurious energy tolerated by sensitive circuits. As with design trade-off, modifying a design to obtain better noise rejection may have an associated cost due to extra die areas, more package pins, and more expensive packages. Fig. 6. Good Floor planning to reduce the effect of the substrate noise coupling 6

SIMULATION AND RESULTS A sample Low Noise Amplifier operating at a frequency of 2.4GHz is simulated to study the effect of the substrate noise on the performance of the analog circuits. An inverter operating at a frequency of 500 MHz is considered to act as a digital circuit on the same substrate as the inverter. The substrate is modeled as a resistor of a suitable value. Effects of change in the resistance value due to the change in the resistivity of the substrate or the change in the spacing between the digital and the analog circuit of the chip are studied. The substrate current is modeled as a dc current of suitable low value. The different responses of the LNA like the S parameters, the Gain, the Noise figure etc. are measured for both the case viz. without the inverter connected i.e. isolated analog circuit and also with inverter connected and the substrate current provided. Responses in both cases are compared and suitable conclusions are made. Output Matching Fig 7. Output Matching of a LNA without substrate noise and with substrate noise The figure here shows us that how the output matching get affected due to substrate noise. Also the peak frequency of resonance is reduced due to noise. This parameters affect the front end performance of the receiver. The Gain Plot 7

Fig 8. Gain of a LNA without substrate noise and with substrate noise The gain value changes dramatically due to the substrate noise coupling effect. Also the frequency at which the gain get maximum also changes. Noise Figure Fig 9. Noise Figure of a LNA without substrate noise and with substrate noise The figure here shows how the Noise Figure of the LNA gets worse due to the substrate noise of Inverter 8

PowerGain Fig 10. Power Gain of a LNA without substrate noise and with substrate noise The figure here shows how the power gain is reduced due the substrate noise of inverter on LNA. CONCLUSION Nowadays, the trend for IC design is high system complexities, short delay time and small die areas. These systems may consist of analog, digital and even RF circuitry on the same substrate, so modeling the substrate noise is very difficult because of the heterogeneity in functionality and spurious signals in the mixed signal devices. However, substrate noise can reduce the performance and impair the functionality of circuits, so substrate noise coupling becomes a significant consideration in the high-speed design. Based on several techniques introduced in this paper, there are three common trends for the development of substrate noise coupling techniques: i) simple modeling, ii) modeling methods for high frequency and iii) prelayout. Simple substrate coupling techniques are good for IC designers due to extremely short design cycle. Only a few experiments need to be done for the parameters of modeling techniques. In addition, the demand for high frequency methods is increasing because the operating frequency of IC communication system is increasing. Consequently, modeling techniques as well as modeling equivalent circuits for high frequency should be explored and developed. Finally, designers would prefer that substrate coupling analysis is performed before circuit layout extraction in order to eliminate the time consumption for redesigning the circuits if substrate noise problems appear in the layout extraction. 9

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