lect4 stick diagram
TRANSCRIPT
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Lecture 4:
MOS and CMOS circuitdesign processes
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Outline
Layer representations
Stick Diagrams
nMOS design style
CMOS design style
Design rules
Need for design rules
Simple layout examples
Sheet resistance Resistance estimation
Capacitance estimation
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Layer Representations
MOS circuits are produced by turning a specification
into masks for processing silicon to meet the
specification
Four basic layers to form a MOS circuit
n diffusion
p diffusion
polysilicon
metal The layers are separated from one another by thick
or thin (thinox) silicon dioxide
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Stick diagram
VLSI design aims to translate circuit concepts onto
silicon.
stick diagrams are a means of capturing topography
and layer information using simple diagrams.
Stick diagrams convey layer information through
colour codes (or monochrome encoding).
Acts as an interface between symbolic circuit and
the actual layout.
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Stick diagram
Does show all components/vias.
It shows relative placement of components.
Goes one step closer to the layout Helps plan the layout and routing
A stick diagram is a cartoon of a layout.
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Stick diagram
Does no t show
Exact placement of components
Transistor sizes
Wire lengths, wire widths, tub boundaries.
Any other low level details such as
parasitics..
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Stick diagram notations
Metal 1
poly
ndiff
pdiffCan also draw
in shades of
gray/line style.
or
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Stick Diagram Colour Code
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P diffusion :Yellow/
BrownMetal1 : Blue
N diffusion : Green Metal2 : Magenta/Purple
Polysilicon : Red Metal3 : Cyan/L.Blue
Contacts & Taps : Black
If you deviate from these colours you will need to include
a key with your stick diagram.
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Stick diagram some rules
Rule 1.
When two or more sticks of the same type cross or
touch each other that represents electrical contact.
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Stick diagram some rules
Rule 2.
When two or more sticks of different type cross or
touch each other there is no electrical contact.
(If electrical contact is needed we have to show theconnection explicitly).
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Stick diagram some rules
Rule 3.
When a poly crosses diffusion it represents a transistor.
Note: If a contact is shown then it is nota transistor.
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Stick diagram some rules
Rule 4.
In CMOS a demarcation line is drawn to avoid touchingof p-diff with n-diff. All pMOS must lie on one side of theline and all nMOS will have to be on the other side.
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How to draw Stick Diagrams
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How to draw Stick Diagrams
Gnd
VDD
xx
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Method to draw stick diagram
Stick diagrams are constructed in two steps.
1) The first step is to construct a logic graph of
the schematic (Figure 1).
A) Identify each transistor by a unique name
of its gate signal (A, B, C, D, E in the example of
Figure 1).B) Identify each connection to the transistor
by a unique name (1,2,3,4 in the example of
Figure 1).
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Method to draw stick diagram
2) The second step is to construct one Euler path for
both the Pull up and Pull down network (Figure 2).
A) Euler paths are defined by a path the traverses
each node in the path, such that each edge is visited
only once.
B) The path is defined by the order of each
transistor name.i) If the path traverses transistor A then B then
C. Then the path name is {A, B, C}
C) The Euler path of the Pull up network must be
the same as the path of the Pull down network.
D) Euler paths are not necessarily unique.
F) It may be necessary to redefine the function tofind a Euler path.
i) F = E + (CD) + (AB) = (AB) +E + (CD)
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Method to draw stick diagram
3) Once the Euler path is found it is time to lay out the stick
diagram (Figure 3).
A) Trace two green lines horizontally to represent the
NMOS and PMOS devices.
B) Trace the number of inputs (5 in this example)
vertically across each green strip. These represent the gate
contacts to the devices that are made of Poly.C) Surround the NMOS device in a yellow box to
represent the surrounding Pwell material.
D) Surround the PMOS device in a green box to
represent the surrounding Nwell material.
E) Trace a blue line horizontally, above and below the
PMOS and NMOS lines to represent the Metal 1 of VDD and
VSS.
F) Label each Poly line with the Euler path label, inorder from left to right.
G) Place the connection labels upon the NMOS and
PMOS devices.
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Method to draw stick diagram
i) In the example of Figure 2 the connection labels are1, 2, 3, 4. Connection 1 is the node that lies between the
PMOS transistors A, B and E. The Euler path defines
the transistor ordering of {A, B, E, D, C} therefore,
transistor B is physically located beside transistor E.
Place the connection label 1 between the transistors B
and E. Later, we will route a Metal 1 connection from
the drain of transistor A to the connection label of 1.
ii) Connection 2 is the node that connects the
PMOS transistors of E, D, and C. Since the Euler path
places transistors E and D next to each other, place the
connection label between these two. Later, we will
route a Metal 1 strip from the source of C to connection
label 2.
iii) Connection label 3 lies between the
NMOS transistors of A and B.iiii) Connection label 4 lies between the
NMOS transistors of D and C.
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Method to draw stick diagram
H) Place the VDD, VSS and all output names upon theNMOS and PMOS devices (Figure 4).
i) For the example of Figure 2 there is only one
output signal, called output.
a) This signal is connected to the PUP device
through a node located between transistors D and C.
b) The signal is connected to the PDN at the
node that the three transistors of A, E and D share. The
Euler graph connects transistors E and D together so an
output connection will be located there. Transistor A has
one remaining contact that is unused, so the output label is
placed at that position.
ii) VDD is located upon the PMOS device at the
node shared between transistors A and B.
iii) VSS is located upon the NMOS device at a
node that is shared between transistors B, E and C.
a) The Euler path places transistors B and E
together so place a VSS label between the transistorsthere.
b) Transistor C has one remaining contact
that is unused. Place a VSS label there.
I) Place a blue line on the diagram to represent the
output metal one material (Figure 4). Note: this line may
have to be moved around depending on how the diagram
connections will lay out.
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Method to draw stick diagram
4) Now its time to interconnect the device. You will
probably have to experiment to find the best routing.
A) Notice that Poly and Metal 1 can overlap.
B) Avoid routing signals that are side by side for
long lengths. This adds capacitance to the device.
C) Avoid all interconnect overlap if possible. This
adds capacitance to the device.
D) Strive for simplicity. This will later provide the
smallest and fastest devices.E) You can use Poly, Metal 2, and even Active to
interconnect your device.
i) Poly and especially Active adds resistance to
you device.
ii) Avoid using Metal 2 if possible. Metal 2 is
another layer to your device that you will probably need
in the next heiarchy up.
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