Passive Distortion Compensation for Package Level Interconnect

1
Passive Distortion Compensation for Package Level
Interconnect

• Chung-Kuan Cheng
• UC San Diego

Dongsheng Ma Janet Wang Univ. of Arizona
2
Outline

1. Motivation
2. Review of High-Speed Serial Links
3. Passive Distortion Compensation
4. Theory
5. Implementation
6. Simulation Results
7. Power Management and System Integration
8. Research Direction

3
1. Motivation ITRS Bandwidth Projection
Courtesy of Hamid Hatamkhani et al., DAC 06

• Abundant on-chip bandwidth
• Off-chip bandwidth is the bottleneck
• Many chip are I/O limited

4
2. Review of High-Speed Serial Links
Techniques On-Chip Off-Chip
Pre-emphasis and equalization v v
Clocked Discharging (M. Horowitz, ISVLSI03) v
Frequency Modulation (S. Wong, JSSC 03 Jose ISVLSI 05) v v
CDMA on wireline (Jongsun Kim et al.) v v
Non-linear Transmission Line (E. Hajimiri JSSC05, E.C. Kan CICC 05) v
Resistive Termination (Tsuchiya et al., EPEP M. Flynn ICCAD 05) v v
5
3. Passive Distortion Compensation
Distortionless Transmission Line
Typical RLC Transmission Line

• Intentionally make leakage conductance satisfy
  R/GL/C
• Frequency response becomes flat from DC mode to
  Giga Hz

• Frequency dependent phase velocity (speed) and
  attenuation

6
3.1 Theory Telegraphers Equations

• Telegraphers equations

• Wave Propagation

• Propagation Constant

• Characteristic Impedance

• and correspond to attenuation
  and phase velocity. Both are frequency dependent
  in general.

7
3.1 Theory Distortionless Lines

• Distortionless transmission line

If
Both attenuation and phase velocity become
frequency independent
8
3.1 Theory Differential Case
Common Mode Current flowing in the same
direction
Differential Mode Current flowing in the
opposite direction
Shunt between each line to ground
Shunt between the two lines
9
3.2 Implementation

• Evenly add shunt resistors between the signal
  line and the ground
• Non-ideality

Ideal Assumption In Practice Implication
Homogeneous and distributive line Discrete shunts Whats the optimal spacing? Are the shunt resistors realizable?
Frequency independent RLGC Frequency dependent RLGC Whats the optimal frequency for the matching?
10
3.2 Implementation MCM trace

• MCM trace vs. On-chip interconnect

MCM
On-chip
10 cm 10 mm
Length
Series Resistance
1 O/mm 1 O/µm
Frequency dependency of line parameters
Large Small
Operation region
RLC RC
11
3.2 Implementation A MCM Stripline Case

• Control the signal line thickness to minimize
  skin effect (cost vs. distortion)
• Assume LCP dielectric

Geometry based on IBM high-end AS/400 system
12
3.3 Simulation Methodology

• Transient simulation in Hspice
• Each transmission line segment is modeled by
  W-element using frequency-dependent tabular model
• Discrete resistors
• Used CZ2D tool from IBM for RLGC extraction
• Part of IBM EIP (Electrical Interconnect
  Packaging) suite.
• Fast and accurate
• Ensures causality of transient simulation

13
3.3 Simulation RLGC vs. Frequency
R
C
L
G
Z0 78 O, delay 57.78 ps/cm

• Match at DC
• Boost up low frequency traveling speed
• Balance low frequency attenuation and high
  frequency attenuation

R1MHz11.07 O/cm, L1MHz5.52e-3 µH/cm, C1MHz
0.74 pF/cm
Rshunt L1MHz/R1MHzC1MHz 669.5 O/cm
14
3.3 Simulation Shunt Resistor Spacing

• Number of shunt resistors N
• Resistors are implemented with embedded carbon
  paste film
• Spacing depends on the target data rate

15
3.3 Attenuation

• W8µm/t2µm/b20µm

16
3.3 Phase Velocity

• W8µm/t2µm/b20µm

17
3.3 Simulation Pulse Response
DC saturation voltage determined by the resistor
ladder
less severe ISI effect
18
3.3 Jitter and Eye opening for 2um case

• W8µm/t2µm/b20µm

10 cm 10 cm 20 cm 20 cm
Jitter (ps) Eye opening (volt) Jitter (ns) Eye opening (volt)
1 shunt/1 cm 1 5.565 0.42563 9.369 0.095785
Terminated with Z0 2 5.0228 0.37449 13.87 0.14595
Terminated with Rdc 3 5.8183 0.33906 12.117 0.090432
Open end 22.5 0.51 gt 70 lt 0.14

1. Each shunt resistor is 669.5 ohm
2. Z078 ohm
3. For 10cm line, Rdc 66.9 ohm for 20 cm line,
   Rdc33.5 ohm

19
3.3 Jitter and Eye opening for 4.5um case

• W8µm/t4.5µm/b20µm

10 cm 10 cm 20 cm 20 cm
Jitter (ps) Eye opening (volt) Jitter (ns) Eye opening (volt)
1 shunt/1 cm 1 22.83 0.525 23.34 0.238
Terminated with Z0 2 7.3764 0.48916 37.327 0.21423
Terminated with Rdc 3 12.026 0.57114 37.443 0.20064
Open end Unrecognizable Unrecognizable Unrecognizable Unrecognizable

1. Each shunt resistor is 1232 ohm
2. Z071.1 ohm
3. For 10cm line, Rdc 123.2 ohm for 20 cm line,
   Rdc61.6 ohm

20
3.3 Simulation Eye Diagrams

• W8µm/t2µm/b20µm/L10cm
• 1000 bit PRBS at 10Gbps
• W-element tabular RLGC model in HSpice

Without shunt resistors
With 10 shunts (each 669.5)
Reduced amplitude
Clear eye opening
Jitter 5.57 ps Eye opening 0.426 V
Jitter 22.5 ps Eye opening 0.51 V
21
3.3 Best Eye Diagram for 2um thick case

• W8µm/t2µm/b20µm/L10cm, 10 distributed resistors

Eye opening
Jitter
Best case when each shunt is 500 ohm Jitter
4.63 ps Eye opening 0.35645 V
Jitter eye opening v.s. shunt value
22
Best eye diagram when only terminator is used,
2um thick case

• W8µm/t2µm/b20µm/L10cm, terminator only

Eye opening
Jitter
Jitter eye opening v.s. R_term
Best case when terminator 90 ohm Jitter 4.97
ps Eye opening 0.40647 V
23

• W8µm/t2µm/b20µm/L20cm, 20 distributed resistors

Eye opening
Jitter
Best case when each shunt is 600 ohm Jitter
9.816 ps Eye opening 0.08379 V
Jitter eye opening v.s. shunt value
24

• W8µm/t2µm/b20µm/L20cm, terminator only

Eye opening
Jitter
Jitter eye opening v.s. R_term
Best case when terminator 40 ohm Jitter 11.95
ps Eye opening 0.10111 V
25
3.3 Eye Diagram for 4.5um thick case when matched
at DC

• W8µm/t4.5µm/b20µm/L10cm

Open ended
10 shunts matched at DC
Jitter 22.8 ps eye opening 0.525 V
Sleepy Eye
26
3.3 Best Eye Diagram for the 4.5um thick case

• W8µm/t4.5µm/b20µm/L10cm, 10 distributed resistors

Eye opening
Jitter
Best case when each shunt is 500 ohm Jitter
11.97 ps Eye opening 0.44036 V
Jitter eye opening v.s. shunt value
27

• W8µm/t4.5µm/b20µm/L10cm, terminator only

Eye opening
Jitter
Best case when the terminator is 80 ohm Jitter
7.18 ps Eye opening 0.51672 V
Jitter eye opening v.s. R_term
28

• W8µm/t4.5µm/b20µm/L20cm, 20 distributed resistors

Eye opening
Jitter
Best case when each shunt is 800 ohm Jitter
21.762 ps Eye opening 0.18288 V
Jitter eye opening v.s. shunt value
29

• W8µm/t4.5µm/b20µm/L20cm, terminator only

Eye opening
Jitter
Best case when the terminator is 110 ohm Jitter
37.595 ps Eye opening 0.24859 V
Jitter eye opening v.s. R_term
30
3.3 Eye Diagram for the MCM trace

• W8µm/t4.5µm/b20µm/L20cm

Terminated with Z0
20 shunts matched at DC
Jitter 38.834 ps Eye opening 0.21418
Jitter 23.24 ps eye opening 0.238 V
31
4. Adaptive Power Management (APM)

• The distortionless signaling simplifies the
  interface circuitry. However, the twice heavier
  attenuation due to passive compensation calls for
  adaptive power management
• With adaptive power management, we adaptively
  regulate the power supply of the transmitter
  according to attenuation
• The regulated supply voltage guarantees the speed
  of transmission while keeping the minimal power
  overhead and well-controlled bit-error rate.

32
4. APM Preliminary Results
33
4. APM Controller
34
4. System Integration

• The reduction of the jitter leaves larger design
  margin for interface circuit design
• To enable an effective and accurate
  communication, the operation of transmitter and
  receiver must be well synchronized. This requires
  accurate clock positioning and phase locking
• Synergic method will be taken to achieve mutual
  compensation and joint leverage on signal
  accuracy, attenuation and system power.

35
5. Research Direction

• Develop analysis models for the technology
• Eye diagram analysis via step responses
• Power consumption
• Optimize technologies
• Chip carrier and board technologies
• Redistribution
• Physical dimensions
• Shunts, terminators
• Prototype fabrication measurement
• More applications clock trees, buses
• Incorporate transmitter/receiver design

36
Remark

• Distortion Compensation
• Source termination Impedance
• Receiver termination Voltage Clamp, Matched Z,
  Optimized Z.
• Distributed shunts
• Combination of above techniques
• Packaging
• Current Products Improve signal quality based on
  current fabrication technologies.
• Future Products Devise the optimal combination.

37
The End
Thank you!