Imagine a world without the electronic gadgets that you posses now. I am certain it’s quite impossible to even hold that thought. Very Large Scale Integration or VLSI is the entire process of designing a compact semiconductor chip that contains different elements together in an integrated fashion rather than individual elements connected together. 

Today, we will discuss what is VLSI and discuss in brief the different stages in designing an integrated chip. A VLSI course will directly help you understand the intricate details that goes behind building an integrated chip from scratch and how it powers our world.

Table of Content:

  • What is VLSI? 
  • VLSI Design Flow
    Gathering the system specification
    Architectural Design
    Behavioral or Functional Design
    Logic Design (LD)
    Designing the circuit
    Physical Design
    Fabrication
    Testing and Packaging
  • Conclusion

VLSI Design Flow

In this section we will be understanding the VLSI Design Flow in detail to help you learn the back end process better: 

1. Gathering the system specification

Like any other design process, defining the system’s specifications comes first. The system specification provides a high-level representation of the system. Performance, usability, and chip dimensions are a few of the factors that must be taken into account. Design techniques and fabrication technology are also considered.

2. Architectural Design

The system’s basic architecture is designed at this stage. This includes, among other things, Floating Point units, the number of Arithmetic Logic Units (ALUs), the quantity and configuration of pipelines, the size of caches, and the differences between Reduced Instruction Set Computers (RISC) and Complex Instruction Set Computers (CISC).

The outcome of architectural design is a Micro-Architectural Specification (MAS). Architects can use MAS, a textual description, to precisely predict a design’s performance, die size, and power.

3. Behavioral Or Functional Design

This step identifies the main structural components of the system. It also points out the necessity of connecting the units. The power, area, and other characteristics of all units are finally estimated.

Without being aware of the implementation’s specifics, the behavioural aspects of the system are taken into account. 

4. Logic Planning (LD)

The control flow, word widths, logic operations, register allocation, and arithmetic operations—which serve to represent the functional design—are derived from and tested in this step.

The Register Transfer Level (RTL) description is another name for this description. RTL is expressed using a Hardware Description Language (HDL), such as Verilog/VHDL. This description can be used to simulate and verify. Timing information and Boolean expressions are included in this description. To achieve the smallest LD that adheres to the functional design, boolean expressions are minimised. To ensure its accuracy, the system’s logic design is tested and simulated. In a few special circumstances, high-level synthesis tools can automate logic design. These programmes convert the behavioural description of a design into an RTL description.

5. Designing the Circuit

The objective is to create a circuit representation based on logic design. The circuit representation of the Boolean expressions takes into account the power and speed requirements of the design. To verify the precision and timing of each component, a circuit simulation is performed.

Usually, the circuit design is expressed using a thorough circuit diagram. It contains the parts of the circuit and the connections between them. Schematic capture tools are used for the manual entry of such descriptions, which are known as “netlist” arrangements. A netlist can frequently be automatically generated from a logic description using logic synthesis tools (RTL).

6. Physical Design

The netlist or circuit representation is converted into a geometric representation by physical design flow in VLSI. This geometric representation of a circuit is known as a layout. The layout is created by transforming each logic element (be it transistors, gates, macros, or even cells) into a geometric representation (certain shapes in multiple layers). The intended logic function of the corresponding component is carried out by these geometric representations. Another way to express connections between different components is through geometric patterns, which are typically lines in multiple layers.

The layout of a microprocessor is extremely precise, and its design principles are based on rigid rules. These recommendations are based on the limitations of the fabrication process and the electrical characteristics of the fabrication materials. Due to its complexity, this stage is frequently broken down into several more manageable steps. Numerous validations and verification checks are performed on the precise layout during physical design.

7. Fabrication

Layouts are created, verification checks are finished, and the design is then ready for fabrication. Typically, tape is used to deliver layout data to fabrication. Tape Out is the term used to describe data release. The layout data for each layer is used to produce one photo-lithographic mask.

Masks are used to mark locations on the wafer where particular materials need to be diffused, deposited, or removed. Silicon crystals are grown and then cut into wafers. To achieve extremely small dimensions for VLSI devices, the wafers must be polished to nearly perfect radii. Different materials are deposited and diffused on the wafer during various fabrication steps. Each step makes use of a single mask. It’s possible that several dozen masks will be used in the fabrication process.

8. Testing and packaging

A fabrication facility creates the wafer and divides it into individual chips in the final step. To make sure that all chips meet all design specifications and operate as intended, they are all tested and packaged. Chips are packaged for use in printed circuit boards in a Ball Grid Array (BGA), Pin Grid Array (PGA), Dual In-line Package (DIP), and Quad Flat Package (QFP). The chips used in Multi-Chip Modules (MCMs) are not packaged because MCMs use bare or naked chips.

Conclusion

As discussed in the intro, a world without electronic revolution and all the modern technologies is simply unthinkable. VLSI is the backbone of this revolution.

So, I hope this blog has helped you in understanding VLSI in brief and the detailed stages of designing that goes behind building an integrated chip from scratch.