Three-Dimensional Microelectronics Integration: Design, Analysis and Characterization Zeynep Dilli Ph.D. Program Dissertation Proposal

Introduction & Motivation: 3-D Integration Current trend in electronics: Tighter integration at every integration level:  Device  Gate  Chip  …  Board  Main Board  System  Still Planar! Limitations: Speed, compactness, signal clarity, robustness… “Smart Dust” systems  Ideally self-contained, self-powered and small  May require mixed-signal integration  3-D stacking might be the ideal answer Stacks with cap chips……with intertier vias …with sidewall connections

3-D vs. 2-D Integration: Advantages & Disadvantages 1.Net system size reduction 2.Increased active Si area/chip footprint 3.Delay reduction: faster clocks or higher bandwidth Shorter interconnects Lower parasitic & load impedance 4.Potential intra-system noise reduction 5.Potential substrate noise reduction 6.Heterogeneous integration 7.More freedom in geometric design 8.Lower power consumption. 1.Increased heat-dissipation problems 2.Increased design complexity. Challenge to the Designer 3-D integration is a subject that ties together chip design, chip physics, device design, circuit design, electromagnetics, and geometrical layout problems.

Outline Proof of concept: A 3-D integrated self- powering system Circuit performance in 3-D integration Passive devices for self-contained 3-D systems

Self-Powered Electronics by 3-D Integration: Proof of Concept 3D system concept: Three tiers  Sensor (Energy harvesting: Photosensor)  Storage (Energy: Capacitor)  Electronics (Local Oscillator and Output Driver)

Process & Circuit 0.18 μm fully-depleted SOI process  3 metals  p-type substrate, ~10 14  Implants: Threshold adjustment, CBN and CBP (5x10 17 ), source-drain, PSD and NSD (0.5x10 19, 1x10 19 ) ipip

Process information Silicon islands 50 nm thick Three-metal process Three tiers stacked Through-vias Top two tiers turned upside- down Figure adapted from MIT_LL 3D01 Run Application Notes

Photodiodes: Design Issues Photocurrent=Responsivity [A/W] x Incident Power Responsivity= Quantum efficiency x λ [μm] /1.24  For red light, λ [μm] /1.24 = 0.51 Incident Power = Intensity [W/μm 2 ] x Area [μm 2 ]  Sunlight intensity ≈ 1x10 -9 W/ μm 2 Quantum Efficiency: η = [# electron-hole pairs]/ [# incident photons]  Depends on reflectance, how many carrier pairs make it to the outer circuit, and absorption  At 633 nm (red light), absorption coef. ≈3.5e-4 1/nm  amount of photons absorbed in 50 nm depth is (1-exp(-αd)) ≈  η = x reflectance x ratio of non-recombined pairs ≈ x 0.75=0.013  Photocurrent=0.013 x 0.51 x 1x10 -9 x Area [μm 2 ] = 6.63 pA/μm 2 Major problem: The material depth is very small

Photodiodes: Design Issues  Photocurrent=0.013 x 0.51 x 1x10 -9 x Area [μm 2 ] = 6.63 pA/μm 2 Photosensitive area: pn-junction depletion region width (W d ) times length Available implants: Body threshold adjustment implants (p-type CBN and n-type CBP, both 5x10 17 cm -3 ); higher-doped source-drain implants and capacitor implants; undoped material is p-type, ~10 14 cm -3. Two diode designs: CBN/CBP diode and pin diode (CBP/intrinsic junction)  CBN/CBP diode W d = μm; A= μm 2  Pin-diode W d ≈ 1.5 μm; A=15 μm 2; possibly problematic To increase: Higher-intensity light; optimal wavelength (higher wavelength increases λ/1.24, but decreases absorption) Layout Constraints: As many diodes as possible; diodes in regular arrays; need three bonding pads of a certain size; assigned area 250 μm by 250 μm only Layout: 2062 CBN/CBP diodes: 7.48 nA; 52 pin diodes: 5.17 nA  Expect about 12 nA

Photodiodes: CBN/CBP Diode Layout

Photodiodes: pin diode layout

Layout: Tier 3, Diodes and Pads “GND” “VDD” Oscillator output

Layout: Tier 2, Capacitor Top plate: Poly Bottom plate: N-type capacitor implant, CAPN Extracted value: 29 pF Expected value: 30 pF

Layout: Tier 1, Local Oscillator

Circuit Operation C storage is charged up to a stable level depending on i ph.

Circuit Operation V rail =270 mV, f osc =1.39 MHz for i ph =15 nA.

3-D System, Further Research, 1 Test the device once fabrication is completed:  Self-contained system, 250 μm x 250 μm x 700 μm Design a version to be fabricated at LPS  Greater active photosensor depth  Voltage regulator to prevent diode forward bias current overtaking the photocurrent Investigate rectifying antennas as alternate power source  Preliminary investigation done on circuit board level  See further work on passive structures

3-D System, Further Research, 2 Compare yield of 3-D system with planar system of the same footprint area Codify self-powering system design methodology  Photodiode-based: Photodiode power generation ability: Diode design and chip optical design (microlenses/AR coatings…) Charge storage system: Capacitor, high-k dielectric use Power regulation circuit requirement  Rectifying-antenna based: Antenna properties: Possible low-k dielectric use Need for a transformer Rectifier diode design  Tied to load circuit characteristics

Outline Proof of concept: A 3-D integrated self- powering system Circuit performance in 3-D integration Passive devices for self-contained 3-D systems

3-D Integration: Performance Study Speed  Intra-chip communication Heat  Generation and dissipation Noise  Substrate noise  External Interference Signal Integrity Compare performance with planar integrated circuits and connections

Performance: Speed Bonding pads, wires: Extra load + parasitics: Slow things down Left: “External” ring oscillator, 11 stages (two stages are shown) Internal Osc. External Osc. One-stage delay 112 MHz (31- stage) (equivalent to 1.16 GHz for 3 stages) 398 KHz (11- stage) (equivalent to 1.46 MHz for 3 stages) ~330 ps for internal, ~330 ns for external dev. Speed ratio: Load ratio: ~1000 Right: Internal ring oscillator, 31 stages, output to divide-by- 64 counter Both are comprised of minimum- size transistors, simulated speed for 31 stages: 132 MHz.

3-D Connections vs. Planar Off-chip Connections Chip-to-chip communication between different chips with vertical vias that require 12  m x 12  m metal pads Cadence-extracted capacitance for a pad 9.23 fF: Same order of magnitude as inverter load cap in2out2 out1in1

3-D Connections: “Symmetric” Chip Structures that can be connected in 3D and planar counterparts for comparison

3-D Connections: “Symmetric” Chip A 31-stage planar ring oscillator and A 31-stage 3-D ring oscillator (In the figure, groups of ). The proper pairs of pads have to be connected to each other through vertical through-chip vias post-fabrication for the circle to close. To counter input Simulation results: Planar: 142 MHz 3-D, six “layer”s : 122 MHz (six vertical pads as extra load) “symmetry” axis

3-D Integration: Performance Study Speed  Intra-chip communication Heat  Generation and dissipation Noise  Substrate noise  External Interference Signal Integrity Compare performance with planar integrated circuits and connections

Performance: Heat Coupled simulation at device and chip level to characterize chip heating: Generation, distribution and dissipation

Performance: Heat Modified heat flow equation: Integrated over a volume to obtain a “KCL” eqn.: Discretized: “C(dV/dt) + (V2-V1)/R = I” General Algorithm: Solve device equations for a range of temperatures; set up the chip thermal network including heat generation; assume initial temperatures and solve the thermal network; obtain heat generated by each transistor at that temperature; reevaluate heat generated by each transistor, repeat until convergence Also Possible: Use solver to suggest layout solutions for heat dissipation

Performance: Heat Device operation characteristics depend on temperature (e.g. I-V characteristic of a diode) Affects circuit operation (e.g. f osc of a ring oscillator)

3-D Integration: Performance Study Speed  Intra-chip communication Heat  Generation and dissipation Noise  Substrate noise  External Interference Signal Integrity Compare performance with planar integrated circuits and connections

Performance: Noise Substrate Noise  Modeled as substrate currents [1] or lumped- element networks [2]  Characterized with test circuits [3] or S- parameter measurements [4] [1] Samavedam et al., 2000 [2] Badaroglu et al., 2004 [3] Nagata et al., 2001, Xu et al., 2001[4] Bai, 2001

Performance: Signal Integrity On-chip interconnects on lossy substrate: capacitively and inductively coupled [1] Characterized with S-parameter measurements Equivalent circuit models found by parameter-fitting [1] Zheng et al, 2000, 2001; Tripathi et al, 1985, 1988…

Performance: Signal Integrity Substrate properties and return current paths affect interconnect characteristics  Three modes of operation, affecting loss and dispersion [1]  Mutual inductance from a return current with a complex depth to calculate interconnect p.u.l inductance [2] Effect of a second substrate stacked in proximity not investigated Effect of vertical interconnects not investigated [1] Hasegawa et al, 1971[2] Weisshaar et al, 2002

Performance: Further Research, 1 Speed: 3-D integrated chip in design revision Heating: Planar heating characterization chip in fabrication; 3-D heating characterization chip being designed Noise: Design a planar chip to model and characterize the substrate noise coupling within; design a 3-D integrated chip to compare noise performances

Performance: Further Research, 2 Signal Integrity: Adapting the heat-elements network to coupled interconnect models  Evaluate the sensitivity of different interconnect layouts to external pulses Set up a coupled network Use random current sources as external pulses to generate a coupling map Design interconnect chips for experimental verification  Interconnect equivalent circuit model/parameter alterations for 3-D integration Investigate capacitive and inductive coupling to a nearby conductive, electrically disconnected substrate in addition to the associated substrate Derive transmission line parameters Design chips for experimental verification

Outline Proof of concept: A 3-D integrated self- powering system Circuit performance in 3-D integration Passive devices for self-contained 3-D systems

On-Chip Inductors and Transformers Self-contained integrated systems may require passive structures:  Communication with other units in the network: on-chip antennas  Power harvesting: Rectifying antennas and transformers  Various analog circuits--- mixers, tuned amplifiers, VCOs, impedance matching networks: Inductors, transformers, self-resonant LC structures

On-Chip Inductors and Transformers Planar inductor design  Number of turns  Total length  First/last segment length  Trace width  Trace separation  Metal layer  Substrate doping  Substrate shields  Stacked or coiled structure Planar inductor modeling  Define an equivalent circuit, parameter- match to measurements  Separate physics- based approaches for serial or shunt parameters

De-embedding and Extraction  Measured reference frame for DUT_full   -----Ref. frame after Open is taken out   ----DUT----  An on-chip inductor has frequency ranges where it behaves inductively or capacitively:

A Qualitative Look at the Inductance Curve

On-Chip Inductors: Measurements

Different Metal Layers M3: Lowest capacitance, highest Q factor

Planar vs. Stacked Inductors micron 2 vs micron 2

Transformers

Tuning an Inductor with Light Bottom right: Shunt capacitance increases; f sr drops ~ MHz Top right: Substrate resistance decreases: peak impedance increases

Exploiting Self-Resonance Certain circuits need LC tanks  Mixers, tuned LNAs (as a tuned load)  LC filters There is some interest on intentionally integrating inductors with capacitors to obtain an LC tank Use the equivalent circuit model in a circuit design to exploit this effect and investigate optimization; verify experimentally

Passives: Further Research 3-D inductors in chip stacks  Investigate multiple-substrate inductor geometries  Electromagnetic modeling, experimental verification  Low-k dielectrics Tunable Self-Resonant Structures  Controlled tuning methodology Photoelectrical Electrical Electromagnetic  Circuit applications

Summary 3-D Self-contained System Design 3-D System Performance Analysis  Speed  Heat  Noise  Signal integrity Passive Structures for Self-Contained Systems  3-D passives  Tunable passives

…And on another track… Engineering education research Past work:  Helped direct ECE program for Maryland Governor’s Institute of Technology program, Summer 2001 Co-authored textbook Benjamin Dasher Best Paper Award in FIE 2002  Contributed to the design of ENEE498D, Advanced Capstone Design Presented paper in ITHET 2004  Participated in departmental outreach programs GE program for high school teachers, MERIT students, WIE summer programs Attending PHYS 708: Physics Education Research Seminar  Engineering education is an open research field as well  Considering several questions adapted from PER: Student knowledge body characterization; concept lists for EE…