Electronic Design

New Design Tools Can Make Your Analog Layouts "Shape Up"

Shape-based routing has significant advantages for time-to-yield and design-for-manufacturing for analog, mixed-signal, and embedded memory designs.

There is no doubt that the challenge of designing analog integrated circuits gets more intense every day. Market demand and price pressures for end products reduce the design time . Add to that demanding performance and frequency requirements and burgeoning manufacturing issues and you have a potentially volatile mix. Although there' s plenty of ?time-to-market? talk, an increasingly important metric is time-to-yield. As a result, the analog portion of a design becomes critical to meet these target s.

Many analog devices are still routed by hand, relying on the experience of analog designers to produce the most cost- effective layout. Engineers will spend months tweaking a design to optimize the layout for maximum yield. And they will trade off many revisions of a mask set, costing millions of dollars, just to get a one percentage point increase in yield.

Now, there's a big push toward automated design tools, particularly in routing, that can produce similar levels of performance in a fraction of the time. Using an automated tool can reduce the time to route a typical analog chip from three months for designs done by hand to three hours, and shrink die size by 30%.

Minimizing the time-to- yield means getting the design right as quickly as possible. T hat doesn't simply involve creating ultra-fast design tools, though. It' s more important to cut down the amount of iterations, as well as reduce each iteration's number of data-transfer cycles between tools.

Since many capabilities are accessed from one tool, it's vital to check all the rules that pervade it. That way, designs become more accurate as they progress through the flow, reducing and/or eliminating iterations.

These issues aren't just relevant for pure analog chips. Many, if not most, large chip designs have some element of mixed-signal design, whether through analog interfaces, or analog parts of digital subsystems ( such as high- speed buses, power management, internal voltage pumps, phase-locked loops ( PLLs), and non volatile RAMs) .

Grid-Based Routing
Most routing tools today use grid-based algorithms, which take a completed design and break it up into a grid of small squares in a database. Areas of the grid containing components are blocked out. T he wires are then routed along the grid lines to link up the design's components, using the number of squares in the grid to determine the minimum path for routing. Because t his technique is so memory intensive, the majority of the design data (types of components, potential wire interaction) is dropped from the routing database.

To accelerate these grid-based routers and lower the memory requirements, it's best to use global routing. Instead of breaking the design into a very large number of wire-grid squares, it' s broken into a smaller number of larger squares, each with sides equal to many wire pitches. Due to the size of these squares, the router really can't consider the detailed positioning of the interconnects. However, the squares can solve global problems such as congestion. Once the global routing is complete, the wire-grid approach is used to join up the pins and edges of the global routing grid, one at a time.

This technique allows for very rapid routing of multimillion-gate designs and benefits from the falling cost of PC memory and greater PC-processor performance. But it suffers from two main problems, particularly for analog designs.

The main issue is that there' s no data on which other nets are around the net being routed. Therefore, controlling the interactions between different classes of nets (e. g., analog and digital) or managing signal-integrity issues is difficult. The grid may also force the wires to be at a non-optimal pitch (due to the requirement to via between layers where they have common grid points) . Leaving space for vias, even in areas that don't need them, can lead to an inefficient and costly layout. And, the rigid grid makes it costly to increase the spacing between nets to solve signal-integrity issues.

Shape-Based Routing
Utilizing alternative approaches to traditional grid-based routing algorithms can be very beneficial, improving a design's time-to-market and time-to-yield. Shape-based routing has been around for many years in board design. Now its strengths in area efficiency, signal integrity, and yield improvement?for analog and mixed- signal designs in particular?are driving its use in chip design.

The basic algorithms were originally developed at pc-board tool vendor Racal-Redac (now Zuken) in the 198 0s. These don't use an abstract grid, but create a ? flood? in one direction until the flood reaches an obstruction (Fig. 1). An unobstructed ? edge? in the direction of the target is found. Then t he algorithm floods in that direction until it reaches another obstruction. T he process repeats until it reaches the tar get.

Each edge is assessed not only for the distance it takes, but also for factors such as parasitics and signal integrity. This adds much more flexibility to the technique. All assessment s are tracked throughout the run, since one direction of exploration may end up in a dead end or in an undesirable route. But a previously unfavored edge may result in a more optimal overall route. (The technique is also coupled with ?rip up and re-try?). In this scenario, the calculation takes into account a path that causes an error.

Because the real shapes of objects are used, tracks can be placed as close as possible to obstructions. This results in a more compact routing pattern. It also factors in yield and process information, particularly for optical proximity correction (OPC). And because the net associated with each object is known, more powerful rules can easily be applied to control the spacing from those objects. So, for instance, the spacing between an analog and a digital nets can be larger than that used between two digital nets. This can dramatically improve the end design's yield.

To get the most efficient routing the first time around, a wide range of data can be used in the routing process, comparing one specific path against design rules and constraints. This eliminates going through the many iterative cycles of routing and post-processing necessary with the grid-based approach.

Using the detailed connectivity information with the true shapes means ?push- aside? techniques can be used, rather than going a long way around or creating an error. T he routing algorithm identifies areas where it could pass simply by moving an existing wire out of the way. This produces shorter wires with fewer vias, as well as an overall higher routing efficiency. The result is smaller, more cost-effective designs. This approach to routing also allows the tool to spread tracks apart to reduce capacitance and make tracks wider to lower resistance (Fig. 2).

Both techniques help reduce RC delays and enhance timing closure. T he key is that these steps happen during the routing process, rather than afterward in a separate tool. As a result, designs are routed correctly without iteration. This can' t be done in grid-based algorithms because the change of width or space will often require tracks to move over a whole extra grid pitch, leaving a much larger than necessary space.

Designer-Directed Routing
With the shape-based routing system, the design process can be interactive and iterative. If something is obviously wrong, the designer can stop the automatic routing and adjust the floorplan, the routing costs, or other rules?or even route a particular section by hand?and continue with the auto routing. This simply isn't possible with grid-based routing.

Designers can also use push- aside routing on an individual net to add their own experience. On line design rule checking (DRC) and persistent connectivity can help with this type of manual routing .

All of these advantages compensate for the fact that each net is routed individually . It's slower than the grid-based approach, but doesn't involve multiple iterations to converge on a solution. Individual routing of the nets also allows the designer to interact with the process, which isn't possible with the grid-based approach.

The shape-based approach works at the manufacturing resolution of the reticles used in masks, rather than at a coarse wire pitch. Having a tighter resolution makes it possible to use more of the chip area effectively, producing a higher-efficiency, cost-effective die that' s easier to manufacture (Fig. 3).

Integrating Analysis Tools
Because all of the design data is available during the routing process using shape-based algorithms, the real-time parasitics and signal- integrity analysis becomes part of the assessment analysis and is tied directly into the DRC tools. As a result, the routed path meets the design requirements from the first attempt.

Even when one track is pushed aside to make room for new tracks, the change is incorporated into the routing process. New decisions in the auto router take this into account because it uses the underlying design database. That' s not to say post-layout analysis tools and iterations aren't needed. Instead, they're used primarily to check the results, rather than to cause new iterations.

Applications
Analog and mixed-signal designs aren't the only ones benefiting from this approach. It's become popular with embedded- memory companies like Hynix and Elpida.

Elpida identified peripheral-logic routing as the most time-consuming activity. In this case, rows of standard or custom cells are positioned beside the design's memory cores, and it uses the tool in its 0.11-µm flow for DRAM designs.

This approach is particularly suited to designs that involve extreme- aspect- ratio cell areas (often 30 times as wide as they are high), limited numbers of routing layers (normally two or three), and a large fraction of signals traveling a significant distance. Thus, it's good for DRAM, SRAM, flash, and imaging- sensor designs (Fig. 4).

Design For Manufacturing (DFM)
Once an automated tool is adopted, other tools can be integrated into the router to help with DFM. T he adopted tool is then able to easily add in new rules for etching and photolithography, such as width and length-based spacing rules, and maximum spacing rules.

One of these tools automatically handles layout ? tidying? and ? smoothing.? This involves removing ugly and redundant wiring patterns, which reduces the number and length of routing segments, cuts down the number of vias, and increases the spacing between the tracks. The tool can also identify and cure process ? antennas.?

Another tool analyzes yield risks in the design, such as tightly spaced tracks and single-cut vias. Other tools then automatically push those tracks apart and try to change single-cut vias to double vias for better-yielding versions.

Because the tool doesn't work to a grid, it can make very good use of the available area on each routing layer. It's possible, then, for some designs to use fewer routing layers than that required by other tools . This significantly shrink s the manufacturing cost of the chips involved.

Another DFM concern is metal density. Both etching and planarization processes used in manufacturing require an even density of metal to be left across the chip, even if there' s no wiring in some areas of the design. The tool can balance density by adding patches of metal in sparse areas of the chip, and by making ?slots? in existing wide tracks such as power and ground lines.

Engineering Change Orders (ECOs)
The incremental push- aside capability of shape-based routing is a key advantage in reducing the time taken to incorporate ECOs engineering Change Orders . If devices must be added or removed, it can be done locally by pushing tracks aside and re routing them without having to reroute large areas of the design. This means that changes have only local consequences, so it' s likely that the process will converge rapidly. Many grid-based routers can only handle ECOs by performing large-scale rerouting of the design.

The shape-based approach has also been extended into other tools. For example, it's included in the full automatic placement of cells into placement regions to generate suitable floorplans for driving the placement engine, as well as in automatic power ring and mesh generation.

Analog design has struggled to find tools that can effectively automate the design process. Shape-based routing offers significant advantages for time-to-yield and DFM for analog, mixed-signal, and embedded memory designs.

Routing individual nets with full, real-time analysis significantly reduces the time to produce a design and delivers a cost-effective, high-yielding die. Even better, designers can still use their expertise by interacting with the tool, bringing the best of both the automated and the handcrafted worlds. That's the way to hit the tighter time-to-market and cost requirements facing designers today.
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