Model based metal fill tool

Very deep sub-micron technologies require that process related issues are taken into account during design. Starting with 130nm processes, a Chemical Mechanical Polishing (CMP) step has been adopted in the manufacturing process to flatten wafer surface between each metal layer. Designers have to insert “dummy tiles” into empty areas of their design to help flatten the surface for each metal layer, since metal density variations badly impact the CMP process.

This step, called metal filling or planarization, is necessary for most advanced processes to increase the foundry yield. While non uniform feature density causes CMP to over-polish empty area, and under-polish dense areas, resulting in degraded performance, the insertion of dummy tiles ensures a uniform pattern repartition over the chips and wafers, thus an increased factory yield. These additional polygons, though electrically inactive, may add parasitic effects, which must be taken into account early in the design process to ensure correct behavior and timing of the chip.

GTsmooth – Model-Based Metal Fill Engine

GTsmooth is a model-based dummy fill solution, which uses topology simulation algorithms to determine the least amount of dummy fill necessary to achieve optimal wafer planarization while minimizing parasitic effects.

Traditional multi-purpose Design Rules Checkers (DRCs) often fill empty areas in metal layers with the maximum number of dummy tiles, leading to degraded performance induced by metal-to-metal and layer-to-layer effects.

GTsmooth performs metal filling in two steps: at first a rule-based method is used to insert as much metal fill as possible, while respecting design rules. Then a second step, based on a process-specific topology simulation, removes as many fill patterns as possible. This results in a reduction of up to 95% filling, leading in turn to dramatically reduced parasitics and timing issues.

GTsmooth is ideally suited for the most ambitious designs, which cannot afford any performance impact due to the metal filling step.

Key benefits

Maximize factory yield with highly accurate tile insertion based on local chip thickness variation,
Minimize parasitic effects by limiting to a minimum the number of inserted tiles,
High flexibility with process independent thickness estimation fully customizable by user,
Take into account impact of dummy tile insertion early in the design process, thanks to fast computation,
Compatible with existing DRC flows.

Main features

User defined thickness estimation function

As opposed to other model-based metal-fill tools, GTsmooth is highly flexible and can be used for any process. By default GTsmooth identifies the areas to fill with XYALIS' post-CMP thickness estimation function.

GTsmooth offers a plug-in mechanism, which allows design and process engineers to use their own estimation function, or a model-specific function provided by the foundry as part of their design kit to drive the filling algorithm. Engineers can choose the function that best describes the effects of CMP on their process.

Keep Away Function

Inserting dummy tile in a chip creates parasitics that adversely affect the circuit behavior and performance. GTsmooth enables the user to define a “keep away” distance between inserted tiles and active geometries. In areas, where density targets allow for metal fills to be placed far from active nets, a reduction of 90% of parasitic capacitance has been observed.

It is also possible to specifically increase the spacing around critical nets, provided by any 3rd party extractor, to meet hard to reach timing specifications.

Uniform Dummy Distribution

Traditional placement of dummies along a grid parallel to the device geometries may result in unbalanced parasitics along different nets, some nets being much closer than others to the inserted tiles. GTsmooth supports non orthogonal grids to avoid this problem.

Insert any Type of Dummies

GTsmooth metal fill engine is very flexible. It supports full layer filling, as well as the insertion of any user-defined dummy. Dummy tiles can be any shape, they can even be complex elements such as stacked vias, diodes to ground, etc...

In order to better achieve density target, GTsmooth can concatenate dummies to create bigger metal filling patterns. And to minimize the impact of floating capacitances, GTsmooth enables dummy grounding to build an efficient shielding: interconnect cells may be inserted between contiguous dummies, dummies can be stacked, and it is easy to insert dummy stripes, since GTsmooth manages the antenna effect.

High Performance and Low Memory Usage

GTsmooth has been optimized in terms of speed, memory, and disk usage. This allows designers to perform metal filling early in the design process and take into account the possible induced parasitic effects.

GTsmooth is able to manage the largest databases since it directly operates on zipped or compressed files. Special optimizations in the filling algorithm have been developed to minimize the size of the generated GDSII or OASIS databases.

User Interface

GTsmooth offers an intuitive Graphical User Interface to define the insertion rules specific to the project and to perform the insertion step. It is also avaiable through the command line making it fully scriptable and enabling automatic processing and easy inclusion in an existing flow.

Model based flow

Metal-Fill in the design flow

Patented filling algorithm

At the heart of GTsmooth is a patented filling algorithm which optimizes the tiles distribution in critical areas.

User defined estimation function

As opposed to other model-based metal-fill tools, GTsmooth is highly flexible and can be used for any process. By default GTsmooth identifies the areas to fill with XYALIS’ post-CMP thickness estimation function. But GTsmooth offers a plug-in mechanism, which allows design and process engineers to use their own estimation function or a function provided by their foundry as part of the design kit, to drive the filling algorithm. Engineers can use the function that best describes the effects of CMP on their process.

3D viewing of post-CMP thickness

Performance

GTsmooth has been optimized in terms of speed, memory and disk usage. This allows designers to perform the metal-fill early in their design process, and take into account the possible induced parasitic effects.

GTsmooth is able to manage the largest databases, since it directly operates on zipped or compressed files. Special optimizations in the filling algorithm have been developed in order to minimize the size of the generated GDSII/OASIS databases.

Supported platforms

SUN Sparc Stations, 32 and 64 bits, Solaris 2.5 and + HP HPUX 11.x x86 PC Linux RedHat 3 and + x86-64 PC Linux RedHat 3 and + ItaniumII Linux RedHat 3 and +