Low Crosstalk Differential Transmission Line Interconnect on Si ULSI
1. Background ・Si ULSI: Global Interconnect Freq. > GHz, Length = cm-order1, Signal wavelengths ≒ Global interconnect length2, The inductance ( L): Cannot be neglected
Interconnect must be designed as transmission line. (= RLC distribution constant circuit model)
Using the inductance positively→ High-speed signal transmission can be achieved.
3. Differential Transmission Line Structure for Long Interconnects
2. PurposeEmployment of Differential Transmission Line to Interconnect Structures on Si ULSI
Differential Transmission Line vs. Single-ended Transmission Line・Ground planes or ground lines are not required・Superior common-mode noise robustness
1. Differential Transmission Line Structure for Long Interconnects2. Robustness to Crosstalk Noise
Required characteristic of differential transmission lines1. Low losses (Large bandwidth)2. Zdiff=100Ω3. Low interconnect-resource4. No GND-plane
Si
M3
Frequency [GHz]Imag
inar
y pa
rt o
f Zdi
ff [Ω
]
-300
-200
-100
0
1 10 1000
100
200
300
1 10 100Rea
l par
t of Z
diff
[Ω]
Frequency [GHz]
Frequency [GHz]A
ttenu
atio
n [d
B/c
m]
-40
-30
-20
-10
0
1 10 100
W=1 µmW=4 µmW=8 µmW=12 µmW=16 µmW=20 µm
w 1 4 8 12 16 20
d 2.2 4.2 6.8 9.8 13.3 16.5 [µm]
Co-planar Line
FreeParameter: W, d
W Wd
→
+ -
Si
M3 ILDM2
Frequency [GHz]
Atte
nuat
ion
[dB
/cm
]
-40
-30
-20
-10
0
1 10 100
Rea
l par
t of Z
diff
[Ω]
Frequency [GHz]0
100
200
300
1 10 100Frequency [GHz]Im
agin
ary
part
of Z
diff
[Ω]
-300
-200
-100
0
1 10 100
w 1 4 8 12 16 20
d 1.3 3.9 7.2 10.5 14.5 18.0 [µm]
Diagonal-pair LineW Wd Free
Parameter: W, d
W=1 µmW=4 µmW=8 µmW=12 µmW=16 µmW=20 µm
→
+-
Stacked-pair Line
Cross-section
W = 0.29 µm
RDC = 2.4 kΩ/cm
Use: M3 & M2
Si
M3 ILDM2
+-
Film Thickness: Fixed
Free Parameter: W
→ Large DC-resistance→ It is not feasible to use such lossy line for long interconnect.
Si
M3ILD
M1
W = 1 µm
RDC =670 Ω /cm
+
-
AlSiO2
Use: M3 & M1
4. Robustness to Crosstalk Noise
5. Conclusion & Future Work
+ - + -
4 µm 4 µm4.2 µm
Line Pitch
+
-
+
-
4 µm 4 µm3.9 µm
Line Pitch
+
-
+
-
ILD : 3 µm
4 µm 4 µm2.6 µm
Line Pitch
+
-
+
-
4 µm 4 µm1.2 µm
Line Pitch
Co-planar Line
Diagonal-pair Line
Diagonal-pair Linewith Thick ILD
Diagonal-pair Lineof Cu/Low-k (εr=2)
Zdiff=100Ω (for Lossless Metal)
Consideration oftransmission line structures
Line Parameter: AMS 0.35 µm CMOSM3: 0.9µm, M2&M1: 0.6µm, ILD: 1µm
EM SimulationAnsoft 2D Extractor
Forward Crosstalk Coefficient@ Line Length l: 1cm Rise Time tr: 10psec
Backward Crosstalk Coefficient
Crosstalk Margin :| Crosstalk Coefficient | ≦ 0.05
×: Individual Inductance, Capacitance: Mutual Inductance, Capacitance
Line Pitch [µm]
Cro
ssta
lk C
oeffi
cien
t
-0.4
-0.2
0
0.2
0.4
4 6 8 10 12 14 16Line Pitch [µm]
Cro
ssta
lk C
oeffi
cien
t
0
0.1
0.2
0.3
0.4
12 14 16 18 20 22 24
Line Pitch [µm]
Cro
ssta
lk C
oeffi
cien
t
-0.4
-0.2
0
0.2
0.4
4 6 8 10 12 14 16Line Pitch [µm]
Cro
ssta
lk C
oeffi
cien
t
-0.4
-0.2
0
0.2
0.4
4 6 8 10 12 14 16
Kf12Kb12Kf13Kb13
Co-planar Line Diagonal-pair Line
Diagonal-pair Linewith Thick ILD
Diagonal-pair Linewith Cu/Low-k
0
0.1
0.2
0.3
4 6 8 10 12 14 16Line Pitch [µm]
Diagonal-pair Line
Capacitive = InductiveCoupling Coefficient:
→ Kf12 = 0
Coupling Coefficient (pair1 - pair2)
8.1µm ≦ Line Pitch≦ 8.6µm12.6 µm ≦ Line Pitch
5.0µm ≦ Line Pitch≦ 5.2µm11.2 µm ≦ Line Pitch
15.7 µm ≦ Line Pitch
18.2 µm ≦ Line Pitch
Line Pitch18.2µm
pair 1 pair 2 pair 3
8.1µm
pair 1 pair 2 pair 3
5.0µm
pair 1 pair 2 pair 3
15.7µm
pair 1 pair 2 pair 3
Co-planar Line
Diagonal-pair Line
Thick ILD
Cu/Low-k
-14%
-56%
-73%
-62% vs. Co-planar Line of Cu/Low-k
1. Co-planar and diagonal-pair lines have superior attenuation-characteristics as a long interconnect.2. Diagonal-pair lines reduce the line pitch and crosstalk noise concurrently. → Better design-flexibility for Si ULSI→ Comparison between the electromagnetic simulation and the mesurement
Line Pitch vs.Crosstalk Noise(Differential-mode Noise)
Common-modeNoise→ Cancel
Differential-modeNoise
+
-
+
-
+
-
H. Ito, K. Okada, K. Masu
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