Lamda Rules Layout Tutorial.docx

CMOS lambda Design Rules CMOS 'λ' Design Rules: The MOSIS stands for MOS Implementation Service is the IC fabrication service available to universities for layout, simulation, and test the completed designs. The MOSIS rules are scalable λ rules. The MOSIS design rules are as follows: (1) Rules for N-well as shown in Figure below. 1. Minimum width = 10λ 2. Wells at same potential with spacing = 6λ 3. Wells at same potential = 0λ 4. Wells of different type, spacing = 8λ

(2) Rules for Active area shown in Figure below. 1. Minimum width = 3λ 2. Minimum spacing = 3λ 3. Source/Drain active to well edge = 5λ 4. Substrate/well contact active to well edge = 3λ

3) Rules for poly 1 as shown in Figure below. 1. Minimum width = 2λ 2. Minimum spacing = 2λ 3. Minimum gate extension of active = 2λ 4. Minimum field poly to active = 1λ

(4) Rules for contact to poly 1 as shown in Figure below. 1. Exact contact size = 2 λ  2 λ 2. Minimum poly 1 overlap = 1 λ 3. Minimum contact spacing = 2 λ

(5) Rules for contact to active as shown in Figure below. 1. Exact contact size = 2λ  2λ

2. Minimum active overlap = 1λ 3. Minimum contact spacing = 2λ 4. Minimum spacing to gate of transistor = 2λ

(6) Rules for metal 1 as shown in Figure below. 1. Minimum width = 3λ 2. Minimum spacing = 3λ 3. Minimum overlap of poly contact = 1λ 4. Minimum overlap of active contact = 1λ

(7) Rules for via 1 as shown in Figure below. 1. Minimum size = 2λ  λ 2. Minimum spacing = 3λ 3. Minimum overlap by metal 1 = 1λ

(8) Rules for metal 2 as shown in Figure below. 1. Minimum size = 3λ 2. Minimum spacing = 4λ

(9) Rules for metal 3 as shown in Figure below. 1. Minimum width = 6λ 2. Minimum spacing = 4λ

Design Rule Check : In order to ensure that none of the design rules are violated CAD tools named Design Rule Checking (DRC) is used. If DRC is not verified then it leads to the non functional design. The layout rules are grouped in three categories that are transistor rules, contact and via rules and well and substrate contact rules. Transistor rules : The transistor can be created by overlapping the active and poly-silicon layers. The minimum length of transistor equals 0.24 m which is minimum width of polysilicon, whereas the width of the transistor is at-least 0.3 m which is the minimum width of active layer. Figure below shows the layout of PMOS transistor.

Fig1-Design-Rule-Check Contact and Via rules :

A contact forms an interconnection between metal and active or polysilicon layer whereas via forms an interconnection between two metal lines. A contact or via is formed by overlapping the two interconnecting layers and provides a contact hole filled with metal between the two. Figure below shows the contacts and via used in layout.

Fig1-Design-Rule-Check Well and substrate contact rules : For digital circuit design it is important for the well and substrate regions to be connected to the supply voltages. If this is not done then a resistive path is created between the substrate contact of the transistors and the supply rails which leads to parasitic effects such as latch up. Inverter Layout : The schematic diagram of the inverter is as shown in Figure.

Fig1-Inverter-Layout The stick diagram of the schematic shown in Figure.

Fig2-Inverter-Layout Here, the most important point to note is that as we change the placing of the components in the schematic the stick diagram and hence, the layout of the circuit will change accordingly. For example, if we place the components vertically the stick diagram will be vertical and if we place the components horizontally the stick diagram will be horizontal. Figure below shows the physical layout of inverter which is drawn in tanner tool.

Fig2-Inverter-Layout Lambda-based-design-rules Lambda based design rules : The Mead-conway approach is to characterize the process with a single scalable parameter called lambda, that is process-dependent and is defined as the maximum distance by which a geometrical feature on any one layer can stray from another feature, due to overetching, misalignment, distortion, over or under exposure etc. with a suitable safety factor included.

The purpose of defining lambda properly is to make the design itself independent of both process and fabrication and to allow the design to be rescaled at a future date when the fabrication tolerances are shrunk. Scaling Theory : The Scaling theory deals with the shrinking transistor and directs the behaviour of a device when its dimensions are reduced. The most commonly used scaling models are the constant field scaling and constant voltage scaling. And another model for scaling the combination of constant field and constant voltage scaling. The scaling parameter s is the prefactor by which dimensions are reduced. It is s < 1. (1) The scaling factors used are, 1/s and 1/ . (2) 1/ is used for supply voltage VDD and gate oxide thickness . (3) 1/s is used for linear dimensions of chip surface. (4) For the constant field model and the constant voltage model,  = s and  = 1 are used. Layout Design Rules : The layout design rules provide a set of guidelines for constructing the various masks needed in the fabrication of integrated circuits. Design rules are consisting of the minimum width and minimum spacing requirements between objects on the different layers. The most important parameter used in design rules is the minimum line width. This parameter indicates the mask dimensions of the semiconductor material layers. Layout design rules are used to translate a circuit concept into an actual geometry in silicon. The design rules is the media between circuit engineer and the IC fabrication engineer. The Circuit designers requires smaller designs with high performance and high circuit density whereas the IC fabrication engineer requires high yield process. Minimum line width (MLW) is the minimum MASK dimension that can be safely transferred to the semiconductor material. For the minimum dimension design rules differ from company to company and from process to process. To address this issue scalable design rule approach is used. In this approach rules are defined as a function of single parameter called ''. For an IC process '' is set to a value and the design dimensions are converted in the form of numbers.

Typically a minimum line width of a process is set to 2 e.g. for a 0.25 m process technology '' equals 0.125 m. Layered Representation of Layout : The layer representation of layout converts the masks used in CMOS into a simple layout levels that are easier to visualise by the designers. The CMOS design layouts are based on following components : (1) Substrates or Wells : These wells are p type for NMOS devices and n type for PMOS devices. (2) Diffusion regions : At these regions the transistors are formed and also called as active layer. These are defined by n+ for NMOS and p+ for PMOS transistors. (3) Polysilicon layers : These are used to form the gate electrodes of the transistors. (4) Metal interconnects layers : These are used to form the power supply and ground rails as well as input and output rails. (5) Contact and Via layers : These are used to form the inter layer connections. CMOS-Layout-Design Layout of Logic gates: Three Input NAND Gate : Figure below shows, the schematic, stick diagram and layout of three input NAND gate.

Two Input NAND Gate : Figure below shows the schematic, stick diagram and layout of two input NAND gate implemented using complementary CMOS logic.

Two Input NOR Gate : Figure below shows the schematic, stick diagram and layout of two input NOR gate implemented using complementary CMOS logic.

Transmission Gate : Figure below shows the schematic, stick diagram and layout of the transmission gate.

Micron-Design-Rules Micron () Design Rules : Industry uses the micron design rules and code designs in terms of these micron dimensions. The micron design rules are as follows : (1) Rules for N-well as shown in Figure below. 1. Width = 3 2. Space = 9

2) Rules for active area as shown in Figure below. 1. Minimum size = 3 2. Minimum spacing = 3 2. N+ active to N-well = 7

(3) Rules for poly 1 as shown in Figure below. 1. Width = 2 2. Spacing = 3 3. Gate overlap of active = 2 4. Field poly 1 to active = 1

(4) Rules for contact to poly 1 as shown in Figure below. 1. Exact contact size = 2  2 2. Minimum poly overlap = 1 3. Minimum contact spacing = 2

(5) Rules for contact to active as shown in Figure below. 1. Exact contact size = 2  2 2. Minimum active overlap = 1 3. Minimum contact spacing = 2 4. Minimum spacing to gate = 2

(6) Rules for metal 1 as shown in Figure below. 1. Width = 3 2. Spacing = 3 3. Overlap of contact = 1 4. Overlap of via = 2

(7) Rules for metal 2 as shown in Figure below. 1. Width = 3 2. Space = 3 3. Metal 2 overlap of via = 2

Stick-Diagrams Stick Diagrams : A stick diagram is a kind of diagram which is used to plan the layout of a transistor cell. The stick diagrams uses "sticks" or lines to represent the devices and conductors. Figure below shows the schematic of an inverter. In order to draw the layout of this circuit it is necessary to define the direction and metalization of the power supply, ground, input and output. The rules for drawing stick diagrams are : Fig_Interconnect Routing Techniques 1) Power and ground lines run horizontally in metal 1. 2) The input and output are accessible from the top or bottom of the cell and

will be in Metal 2 running vertically. 3) To draw the stick diagrams the conventions used in this book are shown in Figure. These conventions are: For Metal-1 use thick solid line, for Metal-2 use thin solid line, for poly use thick dashed line, for active ( n+ or p+ ) use thin dashed line, for contact use "X" and for via use "O".

Fig-Stick-Diagrams