DPT and Color conflict in Physical design

Multiple patterning lithography (MPL) techniques have been used to extend the 193nm lithography to 22nm/14nm nodes.
Possibly further due to the delay of extreme ultra violet lithography and electric beam lithography (EBL) Generally speaking, the MPL consists of double patterning lithography (DPL) and triple patterning lithography (TPL).
There are two main types of DPL with different manufacturing processes:
                  litho-etch-litho-etch (LELE) and
                 self-align double patterning (SADP).
Both of them can be extended for triple patterning. The most important issue for multiple patterning technique is how to successfully decompose the layout into several masks that can be manufactured under current 193nm optical lithography.
 When the pitch between two patterns is less than the lithography threshold, the patterns have to be separated into different masks. This process is called layout decomposition, or coloring. There are many studies on MPL layout decompositions at the mask synthesis stage to resolve the coloring conflicts, minimize the stitches, balance the mask density, or even mitigate the undesirable overlay effects
 

Meanwhile, there are studies showing that it is very important to consider the multiple patterning implications at earlier physical design stages so that the overall design and manufacturing closure can be reached.
 Both multiple patterning aware standard cell library design and multiple patterning aware physical design flow are necessary to avoid undecomposable patterns in final layout.

For example, if placement/routing techniques are not multiple patterning friendly, the final layout may introduce some conflicts.
Since redesigning indecomposable patterns in the final layout requires high ECO efforts.



we propose a unified physical design flow for standard cell compliance, pin access, routing, and placement to bridge the gap from mask/layout decomposition to physical design. With further scaling of feature size into very deep sub-wavelength, the mask-level manipulation will be limited if the final layout after physical design is not lithography friendly. Therefore, it is necessary to integrate lithography awareness into nanometer physical design flow. Our preliminary results from such a unified flow are very promising. As multiple patterning lithography is being actively adopted by industry and it can also be used together with EUV, e-beam lithography, DSA, etc., we expect to see more research activies to bridge such gap. 






The double-patterning technology (DPT) that means we can continue to use 193nm lithography to produce features at a 64nm pitch in 20nm processes is a breakthrough for manufacturing but an added complication for design. In double patterning, features that are too close together to be resolved with conventional lithography are separated onto two masks, which are exposed sequentially, with an etch step in between, to form the necessary densely packed features.

Designers must create layouts that can be split (‘decomposed’) into two layers, a process known as coloring, after map-coloring theory. Achieving such layouts means following new design rules that encapsulate the limitations of the lithographic process and prohibit two polygons of the same color from having particular geometric relationships

Physical designers have to work with these new rules during placement and routing, considering the costs of decomposability alongside other costs such as timing, power and area. The physical verification process also has to change, so that it can show that an entire layout can be successfully split onto two masks in a way that meets the foundry’s specifications.
Achieving decomposability

Although foundries will often decompose a layout themselves so that they can optimise the resultant masks for manufacturability, producing decomposable designs means being able to show that applying the foundry rules to a layout can result in successful mask assignments.

The essence of the coloring problem is to create layouts made up of closed loops of polygons (so-called ‘cycles’) that can be given alternating colors along the whole length of the loop. Physical verification tools such as Synopsys’s IC Validator include analysis engines that enable users to define complex geometric relationships, such as the minimum spacing between polygon edges and the corners of facing sides or line ends assigned to the same mask, and then apply them to a candidate design. More elaborate rules allow single polygons to be subdivided on to two masks (a process known as ‘stitching’), which enables greater design freedom by offering a way to avoid some coloring violations.

Meeting DPT requirements in library development

Double-patterning requirements have to be taken into account in both the design and placement of 20nm cell libraries.

Cell designers must ensure that their cell layouts can be safely decomposed into two colors, using physical verification tools to check that their work will meet this requirement and to suggest possible adjustments if it doesn’t. Cell designers also need to think about possible interactions between the cells they design when they are placed adjacent to each other. One way to overcome this issue is to build enough space into every cell that they can’t create coloring violations. The other is to design cells without this margin and rely on the placer to avoid any potential violations caused by cell adjacencies. Both approaches are supported by In-Design technology, which seamlessly couples signoff-quality analysis in IC Validator to the placement engine in IC Compiler.

Here’s an example of a 20nm cell library in which the designer has created an XOR gate and wants to know whether it can be successfully decomposed and its properties in placement.

The cell is first decomposed according to some basic conditions: both power rails must be assigned to the same color and polygons must not be stitched. This approach results in DPT conflicts