1. Layout design method
The layout design method can be classified from different angles. If the degree of automation is divided according to the degree of design, the layout design method can be divided into 2 categories: manual design and automatic design. The design method can be divided into 2 categories, full custom and semi customizing, if they are divided according to the limitation of layout and wiring location and the limitation of layout modules. For the full custom design pattern, there are 3 kinds of CAD tools that serve him: interactive graphic editing of geometric figures, symbol method and automatic layout of block blocks. For the example of the layout design of the bipolar operational amplifier, the Tanner company's L-Edit software is used. This is an interactive graphical editor widely used on microcomputers. The designer uses an interactive graphic editor to enter the computer and edit the sketches that are manually designed. Therefore, this method is also classified as a manual design method. Because the manual design method inevitably produces misunderstandings, it is necessary to verify the layout after the layout is edited. Layout verification includes design rules check DRC, electrical rules check ERC, layout parameter extraction LPE, layout and schematic contrast check LVS. Of course, these validations can be done by L-Edit.
2. design rule
Circuit designers generally want the circuit to be designed as compact as possible. But the technology engineer wants to be a high yield process. Design rules are the tradeoffs that satisfy both of them. Design rules are good normative documents. He lists the minimum widths of components (conductors, active regions, resistors, etc.), the allowable minimum spacing between adjacent components, the necessary overlaps and other sizes matched with a given process. For a process when determining the design rules, to consider the factors of mask alignment, mask film, nonlinear bending and external diffusion (lateral diffusion), oxide growth profile, lateral drilling, optical resolution and their relationship with the circuit performance and yield. The design rules specify the horizontal alignment of each geometric figure on the mask plate on the other mask, which is related to each other. In addition to the distinct points, all rules refer to the minimum interval between the corresponding geometric figures. A design rule is to represent the minimum size directly by the number of micrometers. But even if the smallest size is the same, different companies have different design rules for different processes, which makes it very time-consuming to design import and export among different processes.
One way to solve the problem is to use advanced CAD tools, which can facilitate the conversion of compatible processes easily. In addition, we can use second design rules, the proportionate design rules which are popularized by Mead and Conway, also called design rules. He sets a parameter to the entire layout as the smallest of all design rules, and the number of other design rules is an integer multiple of this parameter. This parameter has different micron values for different processes. So that other rules can be changed linearly. Of course, they have shortcomings, too.
(1) linearity is only applicable to a certain range (for example, it is linear and effective between 1~2 m). When the range is far away, the relationship between rule and lambda is not linear.
As the rule of lambda represents different process technologies, the design rules must be considered comprehensively for the complete set of requirements of each process, which will inevitably bring about the reduction of super size and tightness. But it can get better safety factors or improve reliability.
3. design examples
This example is designed for the layout of a two pole operational amplifier as shown in Figure 1.