A semiconductor chip
undergoes synthesis, placement, clock tree synthesis and routing processes
before going for fabrication. All these processes require some time, hence, it
requires time (9 months to 1 year for a normal sized chip) for a new chip to be
sent for fabrication. As a result of cut-throat competition, all the
semiconductor companies stress on Cycle-time reduction to be ahead of others in
the race. New ways are being found out to achieve the same. New techniques are
being developed and more advanced tools are being used. Sometimes, the new chip
to be produced may be an incremental change over an existing product. In such a
case, there may not be the need to go over the same cycle of complete
synthesis, placement and routing. However, everything may be carried out in
incremental manner so as to reduce engineering costs, time and manufacture
costs.
It is a known fact that
the fabrication process of a VLSI chip involves manufacture of a number of
masks, each mask corresponding to one layer. There are two kinds of layers –
base and metal. Base layers contain the
information regarding the geometry and kind of transistors, resistors,
capacitors and other devices. Metal layers contain information regarding metal
interconnects used for connection of devices. For a sub-micron technology,
the mask costs may be greater than a
million dollars. Hence, to minimize the cost, the tendency is to reuse as
many masks as possible. So, it is tried to implement the ECO with minimal
number of layers change. Also, due to cycle time crunch, it is a tradition to
send the base layers for the manufacture of masks while the metals are still
modified to eliminate any kind of DRC’s. This saves around two weeks in cycle
time. The base layer masks are developed while metal layers are still being
modified.
What conditions cause an Engineering Change Order: As
mentioned above, ECO are needed when the process steps are needed to be
executed in an incremental manner. This may be due to-
- Some functionality enhancement of the existing device. This functionality enhancement change may be too small to undergo all the process steps again
- There may be some design bug that needs to be fixed and was caught very late in the design cycle. It is very costly to re-run all the process cycle steps for each bug in terms of time and cost. Hence, these changes need to be taken incrementally.
Normally, there is a case
that design enhancements/functional bug fixes are being implemented after the
design has already been sent for fabrication. For instance, the functional bug
may be caught in silicon itself. To fix
the bug, it is not practical to restart the cycle.
The ECO process starts
with the changes in the definition to be implemented into the RTL. The
resulting netlist synthesized from the modified netlist is, then, compared with
the golden netlist being implemented. The logic causing the difference is then
implemented into the main netlist. The netlist, then, undergoes placement of
the incremental logic, clock tree modifications and routing optimizations based
upon the requirements.
Kinds
of ECO: The engineering change orders can be classified into two
categories:
- All layers ECO: In this, the design change is implemented using all layers. This kind of ECO provides advantage in terms of cycle time and engineering costs. It is implemented whenever the change is not possible to be carried out without all layer change e.g. there is an updation in a hard macro cell or the change may require updation of 100’s of cells. It is almost impossible to contain such a large change to a few layers only.
- Metal-only ECO: As discussed above, due to incurring costs, sometimes, it may not be practical to use all the layers (base + metal) to do the ECO. In that case, to minimize the cost, it is required to be completed with changes only in minimal number of metal layers. These days, it is expected that every design will be re-opened for the ECOs. So, an adequate number of spare cells are sprinkled during the implementation all over the design to be used later on. These cells are spread uniformly over the design. The inputs of these cells are tied. Whenever the need for an ECO arises, the cells to be implemented can be mapped into the existing spare cells. Hence, there is no need to change the base layers in such a case. Only the connections need to be updated which can be done by changing the metal layers only. Hence, the base layer cost is saved.
Steps
to carry out an ECO: The ECOs are best implemented manually. There
exist some automated ways to carry out the functional ECOs, but the most
efficient and effective method is to implement manually. Generally, following
steps are employed to carry out Engineering Change Orders:
- The RTL with ECO implemented is synthesized and compared with the golden netlist.
- The delta is implemented into the golden netlist. The modified netlist is then again compared with the synthesized netlist to ensure the logic has been implemented correctly.
- The logic is the placed incrementally. However, if it is metal-only ECO, spare cells in the proximity of the changed logic are found out.
- The connections are, then, modified in metal layers.
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Such a comprehensive article! Great job!!!
ReplyDeletethank you for this article
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