Semiconductor design refers to the process of creating integrated circuits (ICs) or chips that are used in electronic devices such as computers, smartphones, and other consumer electronics. It involves the design of the individual transistors, circuits, and systems that make up the chip, as well as the layout and fabrication of the chip itself.
Semiconductor design begins with a set of specifications that define the desired functionality of the chip. The designer then creates a high-level design that defines the overall architecture and function of the chip. This is followed by a detailed design phase where the individual transistors, circuits, and systems are designed and optimized for performance, power consumption, and area.
Once the design is complete, the chip is fabricated using a process called photolithography. This involves creating a pattern on a silicon wafer that defines the location of each transistor and circuit on the chip. The wafer is then processed to create the individual layers of the chip, including the transistors, interconnects, and packaging.
Semiconductor design is a complex process that requires a deep understanding of semiconductor physics, circuit design, and fabrication technology. It is constantly evolving as new technologies and manufacturing processes are developed, and is a critical component of the electronics industry.
The semiconductor design process can be divided into several key steps, including:
Specification: This involves defining the functionality, performance, and power consumption requirements for the chip. The specifications are typically developed in collaboration with the customer or end-user.
Architecture design: This involves creating a high-level design that defines the overall structure and function of the chip. The designer selects the appropriate building blocks, such as digital logic gates, analog circuits, memory cells, and interfaces, and determines how they will be interconnected to achieve the desired functionality.
Circuit design: This involves designing the individual circuits and systems that make up the chip. The designer selects the appropriate transistors, capacitors, resistors, and other components to implement the desired functions. Circuit simulations are used to verify that the circuits meet the performance, power consumption, and reliability requirements.
Layout design: This involves creating a physical layout of the chip that defines the location and interconnects of each circuit element. The designer uses specialized software to optimize the layout for performance, power consumption, and area. This step involves detailed considerations like placement of contacts, alignment, and orientation.
Verification: This involves testing the design to ensure that it meets the specifications and performs correctly under different operating conditions. Verification is done at various stages of the design process, including functional verification, timing verification, power verification, and design rule checking.
Mask preparation: This involves creating the masks used in the photolithography process to pattern the chip on the silicon wafer. The mask design is created from the layout design and typically involves multiple layers of masks that are aligned and exposed in sequence.
Fabrication: This involves the actual manufacturing of the chip. The mask patterns are transferred onto the silicon wafer using a process called photolithography. The wafer is then processed to create the individual layers of the chip, including the transistors, interconnects, and packaging.
Testing: This involves testing the completed chips to ensure that they meet the specifications and perform correctly under different operating conditions. Testing includes both functional and parametric tests to verify the chip’s functionality and performance.
Overall, semiconductor design is a highly iterative process that involves multiple rounds of refinement and verification to ensure that the final chip meets the desired specifications and performance targets.
System Specifications (Design Specifications) #
The first step of any design process is to lay down the specifications of the system. System specification is a high-level representation of the system. The factors to be considered in this process include performance, functionality, and physical dimensions. The fabrication technology and design techniques are also considered. The specification of a system is a compromise between market requirements, technology, and economic viability. The end results are specifications for the size, speed, power, and functionality of VLSI system.
To clear this concept, let’s take an example of Ripple Carry Adder (4-bit wide), which means that it takes two inputs namely A, B each of four-bit wide adds them and gives resultant sum as output. In this example, system specifications are inputs A, B, and output sum, functionality as the name indicates it adds two numbers, how much delay it should take, how much power it should consume, with what frequency it should work, with what technology (45nm, 90nm, 180nm, etc.) it should fabricate and many more comes into picture. The design specifications of Ripple Carry Adder is shown below:
Figure – 2: Design Specifications of Ripple Carry Adder
Note: Design Specifications are based on customer requirement.
Architectural Design #
The basic architecture of the system is designed in this step. This includes such decisions as RISC (Reduced Instruction Set Computer) versus CISC (Complex Instruction Set Computer), number of ALUs, Floating point units, number, and structure of pipelines, and size of caches among others. The outcome of architectural design is a Micro-Architectural (MAS) Specifications. While MAS is a textual description, architects can accurately predicts the performance, power and die size of the design based on such a description.
To clear this concept, by considering Ripple Carry Adder (4-bit wide) as an example, inside the architecture of Ripple Carry Adder we have four Full Adder modules namely FA0, FA1, FA2, FA3, and their interconnections as shown in the figure below:
Figure – 3: Architectural Design of Ripple
Functional Design (Behavioral) #
As shown in figure – 3, In this step the main functional units of the system are identified. This also identifies the interconnect requirements between the units. The area, power, and other parameters of each unit are estimated. The key idea is to specify behavior, in terms of inputs, outputs, and timing of each unit, without specifying its internal structure.
The outcome of functional design is usually a timing diagram or other relationships
between units. This information leads to improvement of the overall design process and reduction of the complexity of subsequent phases. The functional or behavioral design provides quick emulation of the system and allows fast debugging of the full system. Behavioral design is largely a manual step with little or no automation help available.
Logic Design #
In this step, the control flow, word widths, register allocation, arithmetic operations, and logic operations of the design that represent the functional design are derived and tested. This description is called Register Transfer Level (RTL) description. RTL is expressed in a Hardware Description Language (HDL), such as VHDL or Verilog. This description can be used in simulation and verification. This description consists of boolean expressions and timing information. The Boolean expressions are minimized to achieve the smallest logic design which confirms to the functional design. This logic design of the system is simulated and tested to verify its correctness. In some special cases, logic design can be automated using high-level synthesis tools. These tools produce an RTL description from a behavioral description of the design.
To clear this concept, let’s see the logic design by taking an example Ripple Carry Adder (4-bit wide) using Verilog HDL as shown below:
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//Code for FULLADDER
module FullAdder(cout,sum,a,b,cin); input a,b,cin; output cout,sum; xor v2(sum,a,b,cin); and b1(t1,a,b); and b2(t2,cin,b); and b3(t3,a,cin); or j1(cout,t1,t2,t3); endmodule //Code for RippleCarryAdder module Ripple_Carry_Adder(cout,S,A,B); input [3:0]A,B; output [3:0]S; output cout; wire [3:0]S; wire c0,c1,c2; FullAdder FA0 (c0,S[0],A[0],B[0],1’b0); FullAdder FA1 (c1,S[1],A[1],B[1],c0); FullAdder FA2 (c2,S[2],A[2],B[2],c1); FullAdder FA3 (cout,S[3],A[3],B[3],c2); endmodule //Testbench for RippleCarryAdder //Note: This file is not included as design but this testbench code is used to verify whether the RippleCarryAdder is properly working or not. module tb; reg [3:0]A,B; wire [3:0]S; wire cout; Ripple_Carry_Adder inst(cout,S,A,B); initial begin repeat(10) begin A=$random; B=$random; #1; $display(A,B,”::”,{cout,S}); end end endmodule |
Circuit Design #
The purpose of circuit design is to develop a circuit representation based on
the logic design. The Boolean expressions are converted into a circuit representation by taking into consideration the speed and power requirements of the original design. Circuit simulation is used to verify the correctness and timing of each component. The circuit design is usually expressed in a detailed circuit diagram. This diagram shows the circuit elements (cells, macros, gates, transistors) and interconnection between these elements. This representation is also called a netlist. Tools used to manually enter such description are called schematic capture tools. In many cases, a netlist can be created automatically from logic (RTL) description by using synthesis tools.
To clear this concept, let’s see the Circuit Design of our example Ripple Carry Adder (4-bit wide) as shown in the figure below:
Figure – 4: Circuit Design of Ripple Carry Adder
Physical Design:
In this step, the circuit representation or net list is converted into a geometric representation. This geometric representation of a circuit is called a layout. The layout is created by converting each logic component like cells, macros, gates, transistors into a geometric representation of specific shapes in multiple layers, which perform the intended logic function of the corresponding component. Connections between different components are also expressed as geometric patterns typically line in multiple layers. The exact details of the layout also depend on design rules, which are guidelines based on the limitations of the fabrication process and the electrical properties of the fabrication materials. Physical design is a very complex process and therefore it is usually broken down into various sub-steps. Various verification and validation checks are performed on the layout during physical design. In many cases, physical design can he completely or partially automated and layout can be generated directly from net list by Layout Synthesis tools. Layout synthesis tools, while fast do have an area and performance penalty. Which limit their use to some designs. Manual layout, while slow and manually intensive, does have a better area and performance as compared to the synthesized layout. However, this advantage may dissipate as larger and larger designs may undermine human capability to comprehend and obtain globally optimized solutions.
The flow of physical design is shown in the below figure:
Figure – 5: Flow of Physical design
Below are some schematics of physical design for our example Ripple Carry Adder (4-bit wide) designed using Cadence tool:
Figure – 6: Schematic of Optimized Full Adder
Figure – 7: Schematic of Optimized Ripple Carry Adder
Figure – 8: Layout of Ripple Carry Adder
Physical Verification #
After the layout, verification is done which includes the steps shown in the flow above. Then the design is ready for fabrication. Since layout data is typically sent to fabrication on tape, the event of the release of data is called Tape Out.
Wafer Sorting
Wafer testing is a step performed during semiconductor device fabrication. Wafer Testing is performed before a wafer is sent to die preparation. All individual integrated circuits that are present on the wafer are tested for functional defects by applying special test patterns.
Fabrication :
VLSI chips are made up of a series of fundamental stages, including crystal growth and wafer preparation, epitaxy, dielectric and polysilicon layer deposition, oxidation, lithography, and dry etching. The devices are formed on the chip during the fabrication process in VLSI. Devices are generated when a fixed-size substance crosses another material. To guarantee that the circuit functions properly, a set of design criteria must be followed while creating the devices. A fabrication facility is where VLSI chips are made. The circuit designer must understand the functions of various masks used in the manufacturing process, as well as how the masks are employed to specify various aspects of the devices on-chip. An Integrated Circuit (IC) is a semiconductor-based electronic network constructed in a single component. Impurities and other materials are introduced to the semiconductor surface in precise geometrical patterns throughout various manufacturing processes. The manufacturing operations are organised in three dimensions to create transistors and interconnects that make up the network.
Packaging :
This article is about the final stage in the manufacturing process of integrated circuits. For an article about the physical enclosure that surrounds integrated circuits, see Semiconductor package.
In electronics manufacturing, integrated circuit packaging is the final stage of semiconductor device fabrication, in which the block of semiconductor material is encapsulated in a supporting case that prevents physical damage and corrosion. The case, known as a “package”, supports the electrical contacts which connect the device to a circuit board.
In the integrated circuit industry, the process is often referred to as packaging. Other names include semiconductor device assembly, assembly, encapsulation or sealing.
The packaging stage is followed by testing of the integrated circuit.
The term is sometimes confused with electronic packaging, which is the mounting and interconnecting of integrated circuits (and other components) onto printed-circuit boards.