INTRODUCTION TO CMOS CIRCUITS

Transcription

1 UNIT II INTRODUCTION TO CMOS CIRCUITS CONTENTS 1. INTRODUCTION 2. CMOS FABRICATION 2.1 n-well CMOS process 2.2 p-well CMOS process 2.3 Twin -Tub Process 3. LOGIC GATES CMOS Inverter NAND Gate NOR Gate 4. STICK DIAGRAM AND LAYOUT REPRESENTATION Components in CMOS technology CMOS Joining Rules Inverter stick diagrams NAND gate stick diagrams NOR gate stick diagram 5. LAYOUT DESIGN RULES 6. SCALING OF CMOS CIRCUITS 7. OTHER CMOS LOGIC Pseudo nmos Logic Clocked Cmos Logic( C2 MOS Logic) Dynamic CMOS Logic CMOS Domino Logic n-p CMOS LOGIC

2 1. INTRODUCTION One of the most popular MOSFET technologies available today is the Complementary MOS or CMOS technology. CMOS technology is the dominant semiconductor technology for microprocessors, memories and application specific integrated circuits (ASICs). The main advantage of CMOS over NMOS and BIPOLAR technology is the much smaller power dissipation. Unlike NMOS or BIPOLAR circuits, a CMOS circuit has almost no static power dissipation. Power is only dissipated in case the circuit actually switches. This allows to integrate many more CMOS gates on an IC than in NMOS or bipolar technology, resulting in much better performance. In CMOS technology, both N-type and P-type transistors are used to realize logic functions. The same signal which turns on a transistor of one type is used to turn off a transistor of the other type. This allows the design of logic devices using only simple switches, without the need for a pull-up resistor. In CMOS logic gates a collection of n-type MOSFETs is arranged in a pulldown network between the output and the lower voltage power supply rail (Vss or quite often ground). Instead of the load resistor of NMOS logic gates, CMOS logic gates have a collection of p-type MOSFETs in a pull-up network between the output and the highervoltage rail (often named Vdd). Thus, if both a p-type and n-type transistor have their gates connected to the same input, the p-type MOSFET will be on when the n-type MOSFET is off, and vice-versa. 2. CMOS FABRICATION There are a number of approaches to CMOS fabrication, including the p-well, the n-well, the twin tub and the silicon - on insulator processors. 2.1 n-well CMOS process Step 1: Si Substrate The n-well CMOS process starts with a moderately doped (with impurity concentration around impurities/m 3 ) p-type silicon substrate. Step 2: Oxidation This step deposits a thin layer of SiO 2 over the complete wafer by exposing it to high-purity oxygen and hydrogen at approx o C

3 SiO 2 Step 3: Photoresist Coating Photoresist is a light sensitive polymer evenly applied to a thickness of 1µm by spinning the wafer. This photoresist cross link when exposed to light, making the affected regions insoluble Photoresist SiO 2 Step 4: Masking The first lithographic mask defining the n-well region is brought in close proximity to the wafer. The mask is opaque in the regions we want to process and transparent in the other regions The combination of mask and wafer is now exposed to Ultraviolet rays. Where the mass is transparent, the photoresist becomes insoluble. Uv rays n-type mask Photoresist SiO 2 Step 5: Removal of Photoresist The soluble photoresist are removed by treating the wafer with acidic or basic solution. Then the wafer is soft baked at low temperature to harden the the remaining photoresist

4 Photoresist SiO 2 Step 6: Acid Etching SiO 2 is selectively removed from areas of wafer that are not covered by photoresist by using hydrofluoric acid. Photoresist SiO 2 Step 7: Removal of Photoresist Strip off the remaining photoresist SiO 2 Step 8: Formation of n-well DIFFUSION n-well is formed by diffusion or ion implantation. In diffusion, the wafers are placed in a quartz tube embedded in a heated furnace. A gas containing the dopant is introduced in the tube. The high temperatures of the furnace (900 to 1000 o C) cause the dopants to diffuse into the exposed surface both vertically and horizontally ION IMPLANTATION Dopants are introduced as ions into the material. The ion implantation system directs and sweeps a beam of purified ions over the semiconductor surface. The acceleration of ions determines how deep they will penetrate the material, while the beam current and the exposure time determine the dosage.

5 SiO 2 n well Steps 2 to 8 constitutes the photolithographic process. Step 9: Removal of SiO 2 Strip off the remaining oxide using HF and we have bare wafer with n-well n well Step 10: Polysilicon deposition Deposit very thin layer of gate oxide and then using chemical vapor deposition process a layer of polysilicon is deposited. In this process silane gas flows over the heated wafer coated with SiO 2 at a temperature of approx. 650 o C.The resulting reaction produces a noncrystaline or amorphous material called polysilicon. Polysilicon Thin gate oxide n well Use same lithography process to pattern polysilicon Polysilicon Thin gate oxide n well Step 11: N- diffusion Carry on photolithographic process to diffuse n- type material. N-diffusion forms nmos source, drain, and n-well contact. Oxidation

6 n well Uv rays n+ Diffusion n well Masking n well Etching n+ n+ n+ n well Diffusion or Ion implantation n+ n+ n+ n well Removal of SiO 2 Step 12: P- diffusion

7 Step 11 repeated with p diffusion mask produces pmos source,drain and substrate contact. p+ n+ n+ p+ p+ n+ n well Step 13: Contact cuts Now we need to wire the devices. Cover chip with thick field oxide. Etch oxide where contact cuts are needed p+ n+ n+ p+ p+ n+ Thick field oxide n well Step 14: Metallization Aluminum interconnect layers are deployed using a process known as Sputtering. Aluminum is evaporated in vacuum with heat for evaporation delivered by electron-beam or ion-beam bombarding. Other metal interconnects such as copper require different deposition techniques. p+ n+ n+ p+ p+ n+ Metal Thick field oxide n well 2.2 p-well CMOS process The fabrication of p-well cmos process is similar to n-well process except that p-wells acts as substrate for the n-devices within the parent n-substrate Advantages of n-well process n-well CMOS are superior to p-well because of lower substrate bias effects on transistor threshold voltage lower parasitic capacitances associated with source and drain region Latch-up problems can be considerably reduced by using a low resistivity epitaxial p-type substrate

8 However n-well process degrades the performance of poorly performing p-type transistor 2.3 Twin -Tub Process A combination of p-well and n-well process is the Twin-Tub process. Here we start with a substrate of high resistivity n-type material and then create both n-well and p-well regions. It is possible to preserve the performance of n-type transistors without compromising the p-type transistors In general, the Twin-tub process allows separate optimization of the n and p transistors 3. LOGIC GATES 3.1 CMOS INVERTER The circuit below is the simplest CMOS logic gate. Cmos inverter When a low voltage (0 V) is applied at the input, the top transitor (P-type) is conducting (switch closed) while the bottom transitor behaves like an open circuit.

9 Therefore, the supply voltage (5 V) appears at the output. Conversely, when a high voltage (5 V) is applied at the input, the bottom transitor (N-type) is conducting (switch closed) while the top transitor behaves like an open circuit. Hence, the ouput voltage is low (0 V). The function of this gate can be summarized by the following table: Input Output High(1) Low(0) Low(0) High(1) The output is the opposite of the input this gate inverts the input. Notice that always one of the transistor will be an open circuit and no current flows from the supply voltage to ground. 3.2 NAND Gate NAND Circuit and Standard Symbol The circuit has two inputs and one output. Whenever at least one of the inputs is low, the corresponding P-type transistor will be conducting while the N-type transistor will be closed. Consequently, the ouput voltage will be high. Conversely, if both inputs are high, then both P-type transistors at the top will be open circuits and both N-type transistors will be conducting. Hence, the output voltage is low.

10 The function of this gate can be summarized by the following table: V1 V2 Output Low Low High Low High High Low High High High High Low If logical 1 s are associated with high voltages then the function of this gate is called NAND for negated AND. Again, there is never a conducting path from the supply voltage to ground. 3.3 NOR Gate NOR Circuit and Standard Symbol The circuit has two inputs and one output. Whenever at least one of the inputs is high, the corresponding N-type transistor will be closed while the P-type transistor will be open. Consequently, the ouput voltage will be low. Conversely, if both inputs are low, then both P-type transistors at the top will be closed circuits and the N-type transistors will be open.

11 Hence, the output voltage is high. The function of this gate can be summarized by the following table: V1 V2 Output Low Low High Low High High Low High High Low Low Low If logical 1's are associated with high voltages then the function of this gate is called NOR for negated OR. Again, there is never a conducting path from the supply voltage to ground. 4. STICK DIAGRAM AND LAYOUT REPRESENTATION Stick diagrams and layout representation are used to convey layer information through the use of a color code. The Stick diagrams and layout representation for CMOS are a logical extension of the nmos style. The exception is yellow is used to identify p-type transistors and wires as depletion mode devices are not used. 4.1 Components in CMOS technology Component Colour Use power and metal 1 signal wires metal 2 polysilicon n-type diffusion p-type diffusion contact via power wires signal wires and transistor gates signal wires, source and drain of transistors signal wires, source and drain of transistors signal connection connection between metal layers insulator none -

12 Metal Two types of metal are used to represent the supply rails metal1 ( ) or metal2 ( ) Polysilicon Polysilicon ( ) is used to represent the gate. n-type diffusion n-type diffusion ( ) is used to represent the source and drain n-transistors. p-type diffusion p-type diffusion ( ) is used to represent the source and drain p-transistors. Contacts Contacts ( ) are used to represent electrical connections. Via Cuts called vias are used to make contact between two metal layers 4.2 CMOS Joining Rules n-type diffusion p-type diffusion poly metal1 metal2 n-type diffusion p-type diffusion poly metal1 metal2 symbol key allowed - connection formed

13 prohibited transistor formed contact needed if connection required, otherwise no connection is made via needed if connection required, otherwise no connection is made wherever red cross green an n-transistors is formed and p-transistor is formed wherever red cross yellow When wires of the same type are crossed connection is made even if you don't use contact or via. Diffusions of the opposite types cannot cross and contact poly (red) cannot contact n-diff (green) and p-diff (yellow). when you want to connect poly (red) to n-diff (green) or p-diff (yellow), you must first connect poly (red) to metal1 (blue) and then metal1 (blue) to n-diff (green) or p- diff (yellow).

14 Metal1 (blue) can contact ndiff (green), pdiff (yellow) and poly (red), but remember contact is needed, otherwise no connection is made, even if the layers are crossed Metal1 (blue) can contact metal2 (purple), but remember, via is needed, otherwise no connection is made even if the layers are crossed. 4.3 Inverter Stick Diagram Starting from the circuit diagram of inverter, we can design the stick layout step by step. 1. Two horizontal wires are used for connection with VSS and VDD. This is done in metal2, but you can decide to use metal1 instead.

15 2. Two vertical wires (p-diff and n-diff) are used to represent the p-transistor (yellow) and n- transistor (green). 3. Then the gates of the transistors are joined with a polysilicon wire, and connected to the input. 4. The drains of two transistors are then connected with metal1 and joined to the output. There cannot be direct connection from n-transistor to p-transistors.

16 5. The sources of the transistors are next connected to VSS and VDD with metal1. Notice that vias are used, not contacts. 4.4 NAND Gate Stick Diagram

17 4.5 NOR Gate Stick Diagram 5. LAYOUT DESIGN RULES The physical mask layout of any circuit to be manufactured using a particular process must conform to a set of geometric constraints or rules, which are generally called layout design rules. These rules usually specify the minimum allowable line widths for physical objects on-chip such as metal and polysilicon interconnects or diffusion areas, minimum feature dimensions, and minimum allowable separations between two such features. If a metal line width is made too small, for example, it is possible for the line to break during the fabrication process or afterwards, resulting in an open circuit. If two lines are placed too close to each other in the layout, they may form an unwanted short circuit by merging during or after the fabrication process. The main objective of design rules is to achieve a high overall yield and reliability while using the smallest possible silicon area, for any circuit to be manufactured with a particular process. Note that there is usually a trade-off between higher yield which is obtained through conservative geometries, and better area efficiency, which is obtained through aggressive, high- density placement of various features on the chip. The layout design rules which are specified for a particular fabrication process normally represent a reasonable optimum point in terms of yield and density. It must be emphasized, however, that the design rules do not represent strict boundaries which separate "correct" designs from "incorrect" ones. A layout which violates some of the specified design rules may still result in an operational circuit with reasonable yield, whereas another layout observing all specified design rules may result in a circuit which is not functional and/or has very low yield. To summarize, we can say, in general, that observing the layout design rules significantly increases the probability of fabricating a successful product with high yield. The design rules are usually described in two ways : Micron rules, in which the layout constraints such as minimum feature sizes and minimum allowable feature separations, are stated in terms of absolute dimensions in micrometers. Lambda rules, which specify the layout constraints in terms of a single parameter (λ) and, thus, allow linear, proportional scaling of all geometrical constraints.

18 Lambda-based layout design rules were originally devised to simplify the industry- standard micron-based design rules and to allow scaling capability for various processes. It must be emphasized, however, that most of the submicron CMOS process design rules do not lend themselves to straightforward linear scaling. The use of lambdabased design rules must therefore be handled with caution in sub-micron geometries. In the following, we present a sample set of the lambda-based layout design rules devised for the MOSIS CMOS process and illustrate the implications of these rules on a section a simple layout which includes two transistors MOSIS portable cmos design rules Rule number Description L-Rule R1 Minimum active area width 3 L R2 Minimum active area spacing 3 L R3 Minimum poly width 2 L R4 Minimum poly spacing 2 L R5 Minimum gate extension of poly over active 2 L R6 Minimum poly-active edge spacing 1 L (poly outside active area) R7 Minimum poly-active edge spacing 3 L (poly inside active area) R8 Minimum metal width 3 L R9 Minimum metal spacing 3 L R10 Poly contact size 2 L R11 Minimum poly contact spacing 2 L R12 Minimum poly contact to poly edge spacing 1 L R13 Minimum poly contact to metal edge spacing 1 L R14 Minimum poly contact to active edge spacing 3 L R15 Active contact size 2 L R16 Minimum active contact spacing 2 L (on the same active region) R17 Minimum active contact to active edge spacing 1 L R18 Minimum active contact to metal edge spacing 1 L R19 Minimum active contact to poly edge spacing 3 L R20 Minimum active contact spacing 6 L (on different active regions) SCALING The size of the circuits in IC s continues to increase. Proper scaling allows to shrink a design. Technology scaling rate is approximately 13% / year, halving every 5 years. Besides increasing the number of devices, scaling has had a profound impact on both speed and power. Full Scaling (Constant Electrical Field Scaling)

19 In the ideal model, all the dimensions of the MOS devices, e.g., the voltage supply level and depletion widths are scaled by the same factor S. Keeping the electric field patterns constant avoids breakdown and other secondary effects. This leads to greater device density, higher speed and reduced power consumption. Ron remains constant -- performance is improved because of the reduced capacitance. Circuit speed increases linearly while the power scales down quadratically! Both clearly indicate the benefits of scaling. Dimensions are scaled by S while voltages are scaled by U. Fixed Voltage Scaling Full scaling is not a feasible option. For example, to keep new chips compatible with existing chips, voltages cannot be scaled arbitrarily. Providing multiple voltage supplies is expensive. 5V was used up through the early 90s. Voltages of 3.3 and 2.5 used since the introduction of 0.5 µm. The change from a fixed-voltage scaling model to the general scaling model used today can be justified by reviewing the rightmost column. In velocity saturated device, keeping the voltage constant while scaling the device dimensions: Does not provide a performance advantage over full scaling model (1/S vs. 1/S) But has a major power penalty associated with it (1/S2 vs 1). Note the gain of increased current level is offset by the higher voltage swing, which only hurts power dissipation. Also note that this is very different from the situation when transistors were operating in the long-channel mode.

20 Here, current was a quadratic function of the voltage. In this scenario, keeping voltage constant gave a performance advantage (net reduction in "on resistance" Other reasons for scaling the supply voltage include hot-carrier effect and oxide breakdown. These latter reasons played a significant role in the trend we see today. Bear in mind that this is a first order analysis -- in reality, there is a (small) performance benefit with fixed voltage due to, e.g., channel length modulation. Supply voltage is now being scaled, but at a slower rate than feature size. For example, from 0.5 μm to 0.1 μm, supply voltage reduced from 5 V to 1.5V. Then why not stick with full scaling model if there is no benefit to keeping the supply voltage higher. Some device voltages, e.g., silicon bandgap and built-in junction potential, are material parameters and cannot be scaled. VT scaling is limited since making it too low makes it difficult to turn off the devices completely. This is aggravated by large process variations. A more general scaling model is needed, where dimensions and voltages are scaled independently using S and U respectively. Under fixed voltage scaling, U = 1 as shown in the last column of the table. General Scaling Under general scaling model, performance scenario is identical (1/S) to other models but power dissipation lies between the two models, S > U > 1. Recent CMOS technologies and projections of the future. OTHER CMOS LOGIC Pseudo nmos Logic Using a PMOS transistor simply as a pull-up device for an n-block is called pseudo-nmos logic. In this type of logic the transistor widths must be chosen properly, i.e., The pull-up transistor must be chosen wide enough to conduct a multiple of the n-block's leakage and narrow enough so that the n-block can still pull down the output safely.

21 The advantage of pseudo-nmos logic are its high speed (especially, in largefan-in NOR gates) and low transistor count. On the negative side is the static power consumption of the pull-up transistor as well as the reduced output voltage swing and gain, which makes the gate more susceptible to noise. Furthermore, when the gate of the pull-up transistor is connected to a appropriate control signal it can be turned off, i.e., pseudo-nmos supports a power-down mode at no extra cost. Clocked Cmos Logic( C2 MOS Logic) The logic is implemented in both n- and p- transistors in the form of a pull up p- block and a complementary n-block pull down structure. However the logic in this case is evaluated only during the on period of the clock Dynamic CMOS Logic

22 The basic structure of dynamic CMOS logic is shown below When the clock is low, the NMOS device is cutoff while the PMOS is turned ON. This has the effect of disconnecting the output node from ground while simultaneously connecting the node to VDD. Since the input to the next stage is charged up through the PMOS transistor when the clock is low, this phase of the clock is known as the precharge phase. When the clock is high however, the PMOS is cutoff and the bottom NMOS is turned ON, thereby disconnecting the output node from VDD and providing a possible pulldown path to ground through the bottom NMOS transistor. This part of the clock cycle is known as the evaluation phase, and so the bottom NMOS is called the evaluation NMOS. When the clock is in the evaluation phase, the output node will either be maintained at its previous logic level or discharged to GND. In other words, the output node may be selectively discharged through the NMOS logic structure depending upon whether or not a path to GND is formed due to inputs of the NMOS logic block. If a path to ground is not formed during the evaluation phase, the output node will maintain its previous voltage level since no path exists from the output to VDD or GND for the charge to flow away. There are many advantages to using dynamic CMOS logic over static CMOS logic or Pseudo NMOS logic. The elimination of the complimentary PMOS transistors significantly reduces the surface area needed to implement the various logic functions not only because the physical number of transistors is nearly cut in half, but because the physical size of the PMOS transistors tend to be much larger than the size of an NMOS transistor. The switching speeds are also increased using the dynamic logic configuration since the speed bottleneck caused by the lengthier time the PMOS requires to pull-up the output node is eliminated. Since this node is already precharged high through the PMOS during the precharge phase, the output node needs only to be selectively discharged during the evaluation phase. Discharging the output node

23 through the NMOS devices is significantly faster than the time needed to charge up the output node through the PMOS device. There are several potential problems with the implimentation of this design that need to be considered. Since the basic dynamic CMOS logic configuration causes the output node to be disconnected from VDD during the evaluation phase, even if the output is also disconnected from GND, the charge of the output node will begin to diminish due to the non-ideal effects of the system. Parasitic capacitances, for example, may leak the charge away from the output node and eventually cause a logic error. Since there is, however, a finite time needed for the charge to erroneously escape, the use of faster the clock speeds will eliminate this kind of error. This implies however, that there is a minimum clock speed at which dynamic CMOS logic structures may be operated. It also eliminates the possibility to idle the basic dynamic CMOS logic circuit. CMOS Domino Logic CMOS domino gates are formed by cascading dynamic gates. This kind of design is referred to as Domino Logic since the pull-down of one stage can conditionally cause the pull-down of succeeding stages and so on like falling dominoes. The number of Domino Logic stages that may be cascaded is limited only by the sum of the total pull-down times in all cascaded logic blocks which must be contained within the evaluation clock phase. The dynamic gate works or operates in two phases: Precharge phase and evaluation. During the precharge phase, the clock input is low, turning off the NMOS device and switching on the PMOS device, this pulls the output high, as there is no path from the output to the ground.

24 During evaluation the clock input is high, turning the PMOS transistor off and switching on the NMOS transistor. In this case the output of the device depends on the inputs. According to the precharge rule there should be no active path from the output to the ground during the precharge cycle, otherwise there will be a dissension between the PMOS precharge transistor and the NMOS transistors pulling to ground, consuming excess power and leaving the output at indeterminate value. Drawbacks to this design are The addition of two additional components to each dynamic block. Extra design consideration must also be observed when using dynamic CMOS logic blocks in conjunction with static CMOS logic blocks. Only non inverting logic functions can be realized n-p CMOS LOGIC An alternative to Domino Logic is n-p cmos logic or Domino-Zipper Logic. n- p logic is characterized by alternating the MOSFETs in the logic block from PMOS to NMOS logic gates and so on. The function of the clocked n- and p- fets in the PMOS logic stage are reversed compared to the NMOS logic stage. Although this structure eliminates the cascading problem, the excess use of PMOS in forming the logic gates reduces the maximum clocking speed and increases the surface area of the system. Another significant drawback to this configuration is the use of the two-phase clock The signals of both clock phases must be delivered at nearly the same instant for the circuit to operate correctly.

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