The TSV Revolution and Fermilab’s MPW Run Experiences R. Yarema Fermilab TIPP 2011, Chicago June 8-13

Introduction A revolution is taking place in the semiconductor world that is due to acceptance of through silicon vias (TSVs) as an option to improve circuit performance and as a complementary approach to transistor scaling. With TSVs, an IC can now be considered a device where connections can be made to either the top or bottom side. The ability to have TSVs leads to several different applications such as WLP (Wafer Level Packaging), SiIP (Silicon Interposers), and 3D integrated circuits. 2TIPP 2011, Chicago

3 3D Integration Platforms with TSVs 3D wafer level packaging – Relatively large vias on a coarse pitch added at wafer level – Usually for peripheral connections – Backside contact allows stacking of chips or adding a sensor on top – Low cost – Small package 3D Silicon Interposers (2.5D) – Built on blank silicon wafers – Provides pitch bridge between IC and substrate – Can integrate passives 3D Integrated circuits – Small vias added at wafer level permit internal chip connections between tiers – Opens door to multilevel high density vertical integration – Reduces interconnect paths between circuit elements Vertex D Wafer level package 3D Silicon interposer MIT LL 3D integrated APD Pixel Circuit 8 22um

TIPP 2011, Chicago4 Through Silicon Via History Not a new concept. In 1975, a GaAs IC used a TSV for backside grounding. More than 10 years ago backside illuminated CCDs were fabricated by thinning the CCD and opening a long trench behind the normal bond pads to allow wire bonding from the back side of the die. In 2005 the technology was applied in HEP to a MAPS device wherein separate openings were made behind each bond pad to allow for wire bonding from the backside and thus allow backside illumination (BSI). 7 The above examples are similar to WLP. Will now examine more recent effort in HEP to use TSVs to develop true 3D integrated circuits. Add Remove

MIT LL 3D MPW Run Experience 5TIPP 2011, Chicago Step 1: Fabricate individual SOI tiers (FEOL + BEOL) Note: Wafer 1 can be SOI or bulk Step 2: invert, align, and bond wafer 2 to wafer 1 using an oxide bond Step 3: remove handle silicon from wafer 2, etch vias, deposit and CMP tungsten. (The BOX acts as an etch stop when removing the handle silicon.) Note: additional tiers can be stacked by using a face to back configuration on top of wafer 2 Fermilab made two submissions to MIT LL of a 3 tier device for an ILC pixel detector. The first in 2006 had processing and some design issues resulting in poor yield and took 13 months to fabricate. The second iteration in 2008 used a conservative design approach which yielded much better results but still took 20 months to fabricate MIT LL uses a via last process on their SOI wafers Space must be left on all metal layers for via insertion An oxide bond is used to mate wafers as shown here. Oxide bond 3D via

Tezzaron 3D MPW Run Experience TIPP 2011, Chicago6Vertex Assume identical wafers Flip 2 nd wafer on top of second wafer Bond 2 nd wafer to 1 st wafer using Cu-Cu thermocompression bond Thin 2 nd wafer to about 12um to expose super via Add metallization to back of 2 nd wafer for bump or wire bond After FEOL fabricate 6 um super contact (via) Complete BEOL processing 12 um Additional wafers can be stacked face to back on top of 2 nd wafer TSV 6um Cu-Cu bond In late 2008, consortium of 15 institutions formed to fabricated 3D integrated circuits using the Tezzaron/Chartered process. – Chartered uses a via middle process to add vias to 130nm CMOS process – Tezzaron performs 3D stacking using Cu-Cu thermo compression bonding

Tezzaron MPW Run Frame TIPP 2011, Chicago7 Design approach – Two tiers with a single mask set – Top tiers on left side and bottom tiers on right side of frame More than 25 two tier designs (circuits and test devices) – ATLAS pixels – CMS strip ROIC for track trigger – X-ray imaging – B-factory and Linear Collider pixels – Test circuits Vertex Frame shows symmetry about center line HH* II*JJ* Fermilab designs: H: VICTR – pixel readout chip mating to two sensors for track trigger in CMS I: VIP2b – ILC pixel chip with time stamping and sparcification J: VIPIC – fast frame readout chip for X-ray Photon Correlation Spectroscopy at a light source TX and TY – test chips TXTX*TYTY* MPW run frame showing top tiers on the left and bottom tiers on the right Note symmetrical placement of frames on wafer

Design and Submission Problems Some Design Problems – All designers did not use the same design kit leading to several problems such as layer map in consistencies. – Some TSV design rules were interpreted incorrectly. – Manual fill on designs had to be redone with fill program. – Bugs found in MicroMagic software used to assemble the fame for 3D submission created errors. Some Submission problems – Chartered requested extra frame space after the frame was completed requiring multiple frame revisions. – Design labels outside the design area had to be moved into design area or be removed. – Masks for frame sections were incorrectly mirrored at mask house. – Most error waivers were unacceptable by Chartered – Some designs were submitted with incorrect mirroring TIPP 2011, Chicago8

9 Fabrication Problems 3D wafer fabrication done in Chartered prototype line Chartered was bought by Global Foundries which slowed our wafer fabrication process – Personnel knowledgeable in 3D fab issues were moved – Some equipment use for 3D fab moved to higher profit production line Global/Chartered did not properly place frames on wafers for 4 different lots of wafers being processed for Tezzaron. The wafers could not be aligned properly for 3D bonding. – Never happened before These wafers however could be used for some 2D IC testing as discussed later. Frames are not placed symmetrically about the wafer center lines 1.2 mm misalignment

TIPP 2011, Chicago10 Fabrication Problems A new lot of 31 wafers was fabricated at no cost to Tezzaron or us except for time (3 months) Due to delays in fabrication, the 3D wafer bonding facilities were not available when the second batch of wafers were ready. The wafers have 400 nm of protective nitride which must be removed from the surface before bonding at about 240 lb/in2 and 400 degrees C. Newly fabricated wafer with proper frame placement on the wafer

Assembly Problems After the nitride removal, 3 wafer pairs were bonded but all 3 had large unbonded areas in the center of the wafer making thinning impossible. One bonded pair was broken and a SEM image taken showing a residue 3-7 nm on the wafer surface. A Auger electron microscope showed the residue to be carbon The remaining unbonded wafers used CMP to remove the carbon. The unbonded wafers were then sent for bonding At this point 3 bonded wafers pairs are being thinned in Singapore Back side metalization follows the thinning process to complete the 3D assembly. TIPP 2011, Chicago11 carbon

Tests Performed on Global/Chartered Parts 12TIPP 2011, Chicago Unfortunately 3D circuits and test structures are still not available. Fortunately some circuits have been fabricated in 2D for testing and some 3D wafers had pads added so testing could be done of individual tiers on our misaligned 2D wafers. All circuits tested by various collaborators in 2D have been found to performed as expected with no deleterious effects due to the TSVs.

TIPP 2011, Chicago13 VIP2a design for ILC pixels is separated into separate digital and analog tiers. Circuits on analog tier could be tested independently. Functionality of each block of analog circuit was verified. Good linearity and range Process findings – NMOS thresholds ~ 100 mv lower than simulations – NMOS gm a few % lower than simulations – PMOS gm 10-15% lower than simulations – MiM caps ~4% lower than expected – 75 ns time constant is equal to 8e + 0.5e/fF* Cin Tests on Fermilab Circuits

TIPP 2011, Chicago14 Idea for track trigger is to discriminate on tracks with high pt Compare hits locally on two closely spaced strip sensors. Very aggressive use of 3D technologies. One tier processes signals from top tier Other tier processes hits from bottom tier, accepts hit information from top tier, performs comparison and transmits data off detector. Functionality of short strip tier has been proven on 2D chip – Downloading of registers – Control of front end bias – Front end response – Backend readout – DAQ system – Strip Vth sigma = 197e – Noise mean= 75e, sigma= 13e Tests on VIPIC for CMS Track Trigger Long strips Short strips Short strip readout tier Two rings of strip sensors shown with bent track

Device Testing Radiation tests – Tests performed by CPPM – ELT devices and core linear devices with TSVs tested at CERN’s X-ray test lab Linear PMOS and ELT NMOS and PMOS show insignificant rad effects. NMOS leakage current shows peak around 1 Mrad which is similar to other 130 nm processes tested by CERN. The main difference observed is that the Chartered NMOS Vt shift is positive rather than negative as seen in other processes tested by CERN NMOS and PMOS Vt shifts are acceptable Cryogenic tests – Fermilab is interested in designing CMOS cryo electronics – The main problem reported in the literature is that Hot Carrier Effects cause lifetime degradation and operating voltage derating is one means of correcting the problem. – Stress tests were performed on minimum size Chartered 130 nm devices. – Preliminary results indicate that the Chartered transistors will not require any voltage derating whatsoever to achieve a lifetime greater than 20 years. TIPP 2011, Chicago15

Commercial Silicon Brokers Move Toward 3D Circuits using Chartered/Tezzaron Partnership announced for 3D circuit fabrication – CMP will provide and maintain 3D/Chartered design kit – CMC and CMP will accept designs and send them to MOSIS for interfacing with Tezzaron – Tezzaron will handle NDAs and submission of designs to Global/Chartered – 3D assembly will be done by Tezzaron – Parts will be distributed by MOSIS 16TIPP 2011, Chicago

17 A comprehensive design kit has been assembled by CMP. Tools included for – Cadence Cadence data base Open access Encounter for 3D – Calibre – Hercules – Mentor (Eldo, HSPICE) – Micromagic – ARM libraries including physicals Numerous programs and libraries provided by HEP Consortium Monte Carlo models Automatic fill program User set up files Two packages are available – Design kit with ARM libraries – Design kit without ARM libraries Design Kit Features

Future 18TIPP 2011, Chicago Tezzaron working to improve process flow by moving all steps except TSVs to NC Tezzaron moving toward using wafers from other foundries and inserting TSVs at SVTC Tezzaron TSV process has been installed at Honeywell on SOI process Tezzaron and IBM are having discussions about running 65 nm with TSVs at IBM MOSIS is planning to have two 3D runs this year. – A run in Sept 2011 is scheduled for non-HEP customers – Another run is scheduled for HEP designs a few months after the first run chips have been tested.

Summary Through silicon Vias have begun to receive significant attention not only in industry but now in HEP. Wafer level packaging using TSVs is being investigated for current HEP projects and silicon interposers are being investigated for future projects. Perhaps the most dramatic change is the use of TSVs to design multi-tier 3D integrated circuits for HEP. The efforts of the 3D design consortium when successful will open the door for a new wave of detector circuits which will surely revolutionize some approaches to detector design. 19TIPP 2011, Chicago