What is the Standard Cell in chip design?

In the field of digital chip design, standard cells are the building blocks that make up complex chip functions. It refers to standardized circuit modules that have been pre-designed, optimized, and validated to have specific logic functions and can be reused. From basic logic gates such as andgates, or gates, and non-gates, to operation units such as triggers and adders, standard units cover the core requirements of digital circuits. The essence of this is "standardization and reusability": once a unit has been verified, it can be reused an unlimited number of times throughout the chip design, greatly reducing design complexity and error rates.

Physical characteristics and layout specifications of standard units

The height of all standard units is the same, and the width can be flexibly varied according to the complexity of the function. The height of the cell is usually measured by the number of tracks in its internal metal layer, such as 6T, 9T, 12T, etc., and the track height of different process nodes is gradually compressed from the early 7.5T to 3T. The width unit is usually CPP (Contact Polycrystalline Spacing), which is the minimum distance between two parallel gates. This "contour width" design allows standard units to be neatly arranged in cell rows, simplifying the layout process.

In terms of power supply and interface specifications, all standard units are uniformly arranged with VDD power rails on the top and VSS ground rails on the bottom, and they are connected to the underlying metal layer. The input and output pin positions and metal layer specifications of the unit are also completely unified, and these interface information can be directly called by the front-end logic synthesis and the back-end layout and wiring without additional adaptation.

The design process and optimization method of the standard unit

The design process of the standard unit covers multiple aspects from logic to physics. The logic circuit design phase identifies the functional requirements of the unit and then maps it to a transistor-level netlist. The transistor positions are arranged under the premise that the design rules are met, with PMOS located in the N-well region of the upper half of the cell and NMOS in the P-well region of the lower half. After completing the transistor layout, the wiring inside the unit is carried out, and then the layout optimization is carried out, covering the area, delay, power consumption and other indicators, and through the design rule check and the layout and circuit diagram consistency verification, the LIB file (timing and power consumption parameter library) and LIF file (physical information library) are finally generated for chip design calling.

Under the advanced process node, the design space is huge and the optimization goals are complex. To improve performance, a variety of optimization techniques can be adopted: through logic transformation, the same logic function can be implemented with different Boolean expressions, resulting in a variety of circuit structures to choose from; Optimize the Euler path by adjusting the transistor stacking order to reduce wiring complexity; The netlist splitting technology is used to split large driver units into multiple small units to reduce latency. Multiple standard cells can also be fused into a single composite unit, reducing the number of pins and compressing the area.

PPA trade-offs and applications of standard units of different specificationsThe standard unit has completed multi-dimensional optimization of power consumption, performance, and area in the design stage. Units of different specifications are complementary: small cells (such as 6T cells) have the smallest area and power consumption, making them suitable for mobile applications, ultra-low-power applications, and embedded microcontrollers; Medium-sized units (such as 9T units) strike a balance between area and performance for general-purpose computing and graphics processors; Large units (such as 12T units) have strong driving capacity and optimal performance for high-speed computing and critical modules. Engineers can choose the appropriate size of the unit according to the needs of different areas of the chip.

Challenges and collaborative optimization under advanced processes

With the evolution of new device structures from planar transistors to FinFETs, to ring-gate transistors and complementary FETs, the design rules are becoming more complex, the orbital height of standard cells is constantly compressed, and the transistor density continues to increase. The layout quality of the standard cell directly affects the final PPA performance of the entire chip, and the placement of each transistor, the direction of the metal wire, and the position of the vias will affect the performance and yield.

The future standard unit design is no longer a simple layout drawing, but a system-level optimization project across logical, topological and physical layers. Combining the concepts of design-process co-optimization and system-technology co-optimization, the design paradigm of standard units is expected to be redefined with the introduction of new devices (such as MESO, TFT).