VLSI Design Cycle

Very Large Scale Integration(VLSI) is the process of making Integrated Circuits (ICs) by combining a number of components like resistors, transistors, and capacitors on a single chip.

VLSI Design is an iterative cycle. Designing a VLSI Chip includes a few problems such as functional design, logic design, circuit design, and physical design. The design is verified for accuracy by the process of simulation. If any design errors are found at any stage of verification, at least one of the previous design steps must be repeated to correct the error during the process of designing.

What is VLSI Design Flow?

VLSI stands for Very Large Scale Integration where by Integration Circuits (ICs) are formed by interconnecting a large number of components including resistors, transistors, capacitors and other components on a single chip. VLSI design is a process of designing these advanced ICs which are miniaturized and involves one or more intrinsic loops to generate the final product owning to certain specifications.

The VLSI design process includes several steps: software requirements, architectural design, behavioral or functional design, logical design, circuit design, physical design, exemplary design, fabrication and packaging, and testing and packaging. All of them are essential for the creation of the new IC starting from the idea and up to its turnkey functioning.

Description of VLSI Design Flow Chart

1. System specification: The objective of the desired final product is written in this step. During system specification, the designated cost of the system, its performance, architecture, and how the system will communicate with the external world are to be determined. During this step, the design specification should be provided by the users or clients.

2. Architectural design: The basic architecture of the desired design must meet the system specifications of the desired design. The architecture of the desired design is decided and the layout for the same is designed by design engineers. Architectural design includes the integration of analog and mixed-signal blocks, memory management, internal and external communication, power requirements, and choice of process technology and layer stacks.

3. Functional design or Behavioural design: It consists of refining the design specification of the desired design in order to design the functional behavior of the desired system. The main objective of this is to generate design a high-performance architectural design within the cost requirements posed by the specifications.

4. Logic Design: In this step, the structure of the desired design is added to the behavioral representation of the desired design. The main specifications to be considered for logic design are logic minimization, performance enhancement, and testability. Logic design must also consider the problems associated with test vector generation, error detection, and error correction. Many logic synthesis tools have been developed for the automation of the process of logic design.

5. Circuit Design: In this step, the logic blocks of the desired design are replaced by the electronic circuits, which are consists of electronic devices such as resistors, capacitors, and transistors. Circuit simulation of the desired design is done at this stage, in order to verify the timing behavior of the desired system. Kirchhoff's laws are used to know the behavior of the electronic circuit in terms of node voltages and branch circuits. The result of integrodifferential equations is then solved in discrete- time. SPICE is a well-known program for circuit simulation.

6. Physical Design: In this step, the actual layout of the desired system is done, where all the components will be placed in the circuit and all these components are interconnected. The actual layout of the desired system can affect the area, correctness, and performance of the final desired product. The correctness of the chip is also controlled by the physical design. A circuit design that passes the test of a circuit simulator may be faulty after it has been packaged. This is because of geometric design rule errors. These design rules must be followed to ensure the correctness of the chip fabrication. Errors such as short circuits, open circuits, open channels, etc may result if the design rules are not respected.

7. Fabrication: After the actual layout and verification of the desired design, the design is sent for manufacturing. The handoff of the desired design to the manufacturing process is called tapeout. Generation of the data for manufacturing is referred to as streaming out. The desired design is onto the different layers of the design using the photolithographic process. ICs are manufactured on round silicon wafers with a diameter from 200mm to 300mm, these ICs are then tested and are marked as either functional or defective ICs.

8. Packaging and Testing: After fabrication of desired design, functional chips are then packed. Packaging is configured early in the desired design process and the application along with the cost and form factor requirements. Packaged types may include Dual In-Line Packaged (DIPs), Pin Grid Array (PGAs), and Ball Grid Arrays (BGAs). After a die is positioned in the package cavity, its pins are connected to the pins of the package, e.g., with wire bonding or solider bumps (flip-chip). The package of the desired design is then sealed and then sent to the end-users or clients.

Importance of Design Flow in VLSI

The VLSI design flow is essential for several reasons:

• Systematic Approach: It gives a defined approach to addressing the intricacies of designing ICs, to the extent that each phase is detail-oriented.
• Error Detection and Correction: When the designing process is divided into stages, the mistakes committed in the designing process are easily detected and corrected before the manufacturing and development of the product is complete.
• Performance Optimization: The nature of the design flow means that designs can be refined over and over for greatest performance, and then further optimized to meet all the requirements for the product.
• Cost Efficiency: One way cost management is achieved through efficient design flow because during its implementation one is able to notice that there are some problems that could have been foreseen at an early stage hence it means that one will not incur a lot of money in trying to correct such mistakes in the later stages.

Types of VLSI Design Flow

VLSI design flows can be broadly classified into different types based on the design methodology and tools used:

• Top-Down Design Flow: Initiates from high-level system specification and advances to venturing the design at more and more detailed phases namely: architectural design/functional design and logic design.
• Bottom-Up Design Flow: Starting the highest level of the system and breaking it down to the lower levels of the system.
• Iterative Design Flow: Stresses the progressive elaboration and repeated cycle of activities in each phase and sub phase in the design of a system with the view to enhancing the design and correcting any shortcoming realized through the simulation exercise and testing.
• Design for Testability (DFT) Flow: It contains provisions that would make it possible to test the IC to check for faults with the view of ascertaining its reliability.

Challenges in VLSI Design Flow

Designing VLSI circuits comes with several challenges:

• Complexity: The more the components on the on-chip, the higher the density of the design and hence the interactions between the different components also tend to be high.
• Performance Constraints: It can be quite challenging to meet the performance specifications, factor in power usage and thermal management.
• Verification and Validation: That there is a way in which the design has to be configured to achieve the best of it in terms of performance can be a herculean task as well as costly.
• Design Rule Constraints: Sticking to design rules mainly geometry and manufacturing to avoid designs that would lead to formation of short circuits and open circuits.

Future Trends in VLSI Design Flow

The future of VLSI design is likely to be influenced by several emerging trends

• Advanced Process Technologies: For instance, the current generation and the next generation process nodes of 5nm as well as below it will require new advancement in design techniques and tools.
• Machine Learning and AI: These technologies are believed to have a great effect on the enhancement of efficiency in the designing activity, on automation of certain tasks, and on increasing accuracy of the effects of the design.
• Integration of Heterogeneous Systems: The call for having as many technologies as possible including Analog, digital and RF in a single chip will push the design methodologies to new ground.
• Quantum Computing: With the emergence of the possibility of quantum computing on the horizon, new problems and solutions in VLSI design are on the horizon and a new technique in PCB construction will be needed.

Conclusion

VLSI design flow is actually a flow which is the essential part of the modern electronics that informs and guides the circuit designers. This way, engineers are able to handle vast number of design criteria and complexities that arise in the development of integrated circuits, while at the same time delivering the best optimized and well performing product that will meet the required performance specifications. That is why faced with the further development of technology, the future of methodology and tool for VLSI design will also be far from solid, and it will only deepen.