Page 377 - A Practical Guide from Design Planning to Manufacturing

Silicon Debug and Test 347

infrared is used to monitor individual transistors as well as taking photos
of the whole die. An infrared laser focused on one transistor will be partially
reflected and the amount it is reflected depends upon the voltage at the tran-
sistor. This allows the monitoring of the voltage within each cycle as the
transistor switches. This process is called laser voltage probing (LVP). 10
Shmoo plots, IREM images, and LVP waveforms all provide additional
information along with the output of DFT circuits, but in the end it is the
ingenuity and reasoning of the debug engineers, which create a theory as
to the cause of a bug. More measurements may be taken to test this theory,
and ultimately a fix is proposed. Possible solutions to a silicon bug are listed
as follows:

Full-layer stepping
Metal-only stepping
Change fuse settings
Microcode patch
BIOS update
OS update
Document as errata

Many bugs can be fixed only by changing the processor design. Each
revision of the design is called a stepping, and very few processors are
ever sold on their first stepping. Typically about half of the mask layers
required for fabrication are used to create the transistors and about half
are used to create the interconnect. If the transistors and interconnect
must be changed, this is a full-layer stepping. Money and time are saved
if a design change affects only the interconnects and allows the same
transistor level masks to be used. This is called a metal-only stepping.
In either case, updated circuit schematics must be created and new
layout drawn. The new layout is converted to masks, and the new design
is fabricated. This whole process can take weeks or months, making it
extremely important that the change actually fixes the bug and does
not create new ones. One way to make sure of this is through focused
ion beam (FIB) edits. 11
A proposed design change may be implemented in hardware by phys-
ically altering the die. AFIB machine can cut through wires, disable tran-
sistors, or even trigger the deposition of new wires. FIB edits cannot
create transistors, but in anticipation of this problem some designs will

10
Yee, “Laser Voltage Probe (LVP).”
11
Livengood and Donna, “Design For (Physical) Debug.”