Biomedical Engineering Reference
In-Depth Information
Many foundries still produce three inch wafers as standard. The hope is
that eventually the growth of high purity GaAs in larger diameter slugs will
help raise processing yields and lower the cost of MMIC devices. One way
in which MMIC manufacturers are increasing their yield percentages is by
the use of process control monitors (PCMs) on the wafers. By using PCMs in
conjunction with specialized test equipment that has been developed spe-
cifically for MMIC production purposes, manufacturers are able to pull bad
wafers from the production line before too much time and effort has been
spent on processing a wafer that will not yield a profitable amount of use-
able MMICs. Electrostatic discharge of the GaAs devices is accomplished
by integrating shunt diodes onto the die at the device inputs and outputs.
In designing an optical circuit employing monolithic microwave devices,
the devices used in the design must be within the standards set by the
foundry chosen to manufacture the chips. The use of computer-aided design
(CAD) and computer-aided engineering (CAE) tools for the MMIC portion
of the system usually produces significant cost efficiency. Fraser and Rode of
Triquint Semiconductor have published a list of design strategies they feel
will provide a firm basis for efficient and reliable MMIC designs. A few of
the major points from the list follow [95]:
1. MMIC costs are not based on the number of FETs used in the design,
so they should be used freely.
2. MMIC is best suited for medium performance functions; circuit
complexity is inversely proportional to the yield.
3. Start with simple designs to prove concepts and then move on to
more complex topologies.
4. The use of lumped elements below 18 GHz will reduce circuit size
without sacrificing performance.
5. Make use of the standard cell libraries provided by the foundry to
avoid reinventing the wheel.
6. Design for testability by providing ample test points at easy to access
places on the wafer.
7. During computer optimizations of the design it is imperative that
the effects of process variations and material spreads are included.
A typical MMIC design sequence from performance definition through the
foundry layout review is as follows [96]:
1. Define MMIC performance requirements.
2. Select the basic circuit and layout topology.
3. Simulate circuit performance using CAE techniques.
4. Analyze the design over temperature and process variations.
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