DESIGN FOR SEMICONDUCTOR RELIABILITY George Denes, Dipl.Eng. Senior Semiconductor Reliability Consultant

1. DESIGN FORSEMICONDUCTOR RELIABILITYGeorge Denes, Dipl.Eng.Senior Semiconductor Reliability Consultant

2. SEMICONDUCTOR DEVICE LIFE STAGES • To evaluate the reliability of an electronic system, reliability information on the components used in that system is important. • Failure rates are often used as an index for reliability. A failure rate indicates how often a failure occurs per unit time, and failure-rate values generally change over time as shown in the graph: • Early failure stage Random failure stage Wear-out failure stage • • Early failure stage: During this stage, failures occur at a high rate following the initial operation of • semiconductor devices. They occur very soon and thus the failure rate declines rapidly • over time. This is because the potential failures that could not be removed through a • selective process are included and surface in a short time if a stress such as • temperature or voltage is applied after use of the device is started. In the case of • semiconductors, these failures are usually due to defects that could not be removed • during production, such as micro dust collecting on the wafer, or to material defects. • • Random failure stage: When early failures are eliminated, the failure rate drops to an extremely low value. • However, there is always the possibility of a potential failure accidentally occurring after a • long time. Consequently, the failure rate never decreases to zero. It is almost constant • because failures occur sporadically. • • Wear-out failure stage: During this stage, failures occur with increasing frequency over time and are caused by • age-related wear and fatigue. In the case of a semiconductor device, electronic • migration or oxide film destruction (TDDB) may occur (see CHAPTER 3 FAILURE • MODES AND MECHANISMS). • The failure rate is generally expressed in (fit = 10−9/h). This indicates the number of failures per unit time. • In general, the mean failure rate of a semiconductor device per unit time is calculated with the following expression. • Total number of failures in given period • Mean failure rate = • Number of base devices × Operation time • According to this expression, 1 (fit) is expressed as follows. • 1 × 109 • 1 (fit) = • Number of bases: 100000 × Operation time: 10000 hours • In other words, 1 (fit) is equivalent to a failure of one device per 100,000 devices operated for 10,000 hours.

3. SEMICONDUCTOR DEVICE LIFE STAGES • Early failure stage: - failures occur at a high rate following the initial operation - failure rate declines rapidly over time: potential failures that could not be screened in fabrication fail in a short time if stress such as temperature or voltage is applied, as: - micro particles collecting on the wafer - material defects - photolithography defects - oxide damage during the fabrication process, etc. • Expectations: memory devices: 1-5 PPM, ASICS, microprocessors: less than 20 PPM consumer devices: below 2-300 PPM • Random failure stage: • After early failures are eliminated, the failure rate drops to an extremely low value. • Some failures randomly occurring after a long time, the failure rate never decreases to zero. • FR is almost constant because the failures occur sporadically (f. mechanisms on further slides). • Expectations: memory devices: 1-5 FITs, microprocessors: 5-20 FITs, ASICs: 20-30 FITs, consumer: below 100 FITs. • Wear-out failure stage: - failures occur with increasing frequency over time and are caused by age-related wear and fatigue, as: - electronic metal migration - oxide film destruction (TDDB) - transistor wear out due to hot carrier damage. • Expectations: consumer: 10 yrs or less, special applications: up to 50-100 yrs.

4. MAIN ELEMENTS OF SEMICONDUCTOR RELIABILITY • BIR (Built In Reliability), DfR (Design for Reliability), DfT (Design forTestability) • (Packaging related DfR is not in this discussion). 1. BIR (Built In Reliability) methods designed and used during wafer fabrication technology process development and on-going reliability monitoring. • Reliability metrics: for each process module (can be 14-34) vertically integrated, as: - ion implantation - oxide growth - photo lithography, etching - metal processes, etc. • Special wafer level rel. stress-test structures being used to test module reliability, as: - gate oxide capacitors (large area) - poly silicon resistors - metal traces - interlayer dielectric capacitors (large area) - contact-and via chains - stand-alone minimum size transistors, etc. • WLR (Wafer Level Reliability) testing: utilizes the special test structures (on product wafers or on special test wafers within the production wafer lot). • WLR aids process design/development and on-going process reliability monitoring of critical process modules, as: - oxide reliability (TDDB), - hot carrier injection damage - metal electro migration: SWEAT (Standard Wafer-level Electro migration Acceleration Test) - ionic contamination, etc. • Maintaining tight distribution of reliability metrics is crucial for IC reliability.

5. DESIGN FOR SEMICONDUCTOR RELIABILITY 2.DfR (Design For Reliability) • Design rules: developed by the wafer processing facility for optimized maximum lifetime set by each physical failure mechanism for each process module, as: - max. allowed voltages - transistor channel length - max. current per unit metal line width - max. current per contact and via - interconnect layout rules - active area spacing - transistor layout rules, etc. • Failure to comply with the reliability design rules may lead to unpredictably shorter IC lifetime. • IC Design Engineering jointly with Reliability Engineering develops special reliability test chips if needed to emulate and evaluate the reliability of critical circuit design features and/or design concepts, examples: - ESD protection circuits - latch up immunity of the I/O-s and internal circuits - special circuits to emulate the most reliability-critical circuit modules of the planned IC with easy electrical access to stress and test, examples: - memory cell transistors (DRAM, SRAM, EPROM, Flash, etc.) - highest speed circuit modules - high power circuit modules - high voltage modules - voltage reference circuits - A/D and D/A conversion circuits - phase locked loops

6. DESIGN FOR SEMICONDUCTOR RELIABILITY • Reliability simulator software programs facilitate prediction of IC lifetimes. • Examples: • MULSIC on-the-chip metal interconnect processing simulator (like damascene metal process, etc.), it is also an interconnect behavioral and reliability simulator, can be integrated with overall IC circuit design simulation software (TCAD, SPICE, etc.) • APET (Georgia Tech.) interconnect reliability and circuit hot spot evaluation software. • TSMC contract wafer foundry (Taiwan) “eReliability Estimator” program for all their wafer process technologies, enabling customers to do lifetime estimation for the major IC reliability failure mechanisms in their design environment, as: • electromigration (metal interconnect, contact, via), • Time Dependent Dielectric Breakdown (TDDB) of MOS gate oxide and interlayer oxides, • Hot Carrier Damage of MOS transistors due to high conducting channel electrical fields, • negative gate bias induced device degradation (NBTI) effecting mostly PMOS transistors, etc. • Reliability Engineering should perform due diligence auditing of all previously listed activities if done by a contract IC manufacturing facility.

7. SEMICONDUCTOR DESIGN FOR RELIABILITY TESTABILITY • DfT (Design For Testability) • We are addressing IC chip reliability testability, closely related to volume production functionality screen testing. • IC life testing (HTOL) requires dynamic close to “lifelike” functioning of the device under stress during ALT. • To achieve dynamically stimulated functional realistic stimulation and loading of the IC devices during ALT, we do the followings: • For digital ASIC circuits min. 85 % gate toggle coverage is desired during ALT for a reasonable degree of confidence. The following methods are utilized to “exercise” the chip during ALT:

8. SEMICONDUCTOR DESIGN FOR RELIABILITY TESTABILITY • asserting a large number of functional test vectors (0-s and 1-s patterns) in parallel on all digital inputs (with the IC outputs loaded); these test vectors are a subset of the product functional test vectors (generated by circuit simulations) used for production pass/fail testing, • using JTAG serial boundary input scan vectors (IEEE Std. 1149.1) if the ASIC chip is designed with test circuits (shift registers) to use serial test input vectors to toggle all logic gates of the IC with a serial input vector during Dynamic Life Testing (ALT), • invoking special stress modes of reliability critical circuits during ALT, if such special stress modes are designed-in on the IC, example: for flash memories stressing all word lines and all bit lines simultaneously. • utilize the on-the-chip designed-in BIST (Built In Self Test) feature for embedded memories in ASICs (if available) during the Dynamic Life Test (ALT) to stimulate/exercise all transistors of the embedded memories.

9. DESIGN FOR SEMICONDUCTOR RELIABILITY 3. IC Package Reliability Modeling * Computer-aided engineering (CAE) is the modern tool of designing for IC package reliability. These tools are utilizing finite element modeling (FEM). * Validated computer models can be used for design of experiments (DOE) studies of IC package geometry, material properties, thermo-mechanical properties, etc. under application-and test conditions. * Modeling capabilities exist for both first-and second level package reliability prediction (package-die interaction and package-circuit board interaction). * Model types and potential reliability issues covered: - Thermo-MechanicalStress Modeling for: structural reliability, identifies high stress areas due to mismach of coefficients of thermal expansion. Correlated with warpage measurements from moire interferometry and cross section analysis. - Viscoelastic Warpage Modeling: more sophisticated than linear-elastic (stress) modeling. Good molding compound properties analyzer (requires time dependent viscoelastic material properties). * Moisture Diffusion Modeling: moisture induced IC package failures, as: popcorning and delamination (both can happen during solder reflow due to sudden vaporization).

10. DESIGN FOR SEMICONDUCTOR RELIABILITY IC Package Reliability Modeling (continued): * Hygroswelling Modeling: simulates hygroswelling or hygro-mechanical stress due to a mismatch of coefficient of moisture expansion of package materials when moisture is absorbed. * Vapor Pressure Modeling: simulates the distribution of vapor pressure during solder reflow process to asses the “popcorning” failure mechanism. A moisture diffusion model is applied to predict the local moisture concentration at the critical interfaces. The vapor pressure induces additional mismatch to the package, which is of the same order as the thermo-mechanical and viscoelastic mismatch stress. * Integrated Stress Modeling: combines all stress and associated failure mechanisms to enhance good IC package design for reliability. * Board Level Solder Joint Reliability Modeling (second level reliability): package-circuit board interaction. Critical issue for QFP, TFBG, QFN packages. The model is correlated to temperature cycling results . The solder joint faigue life calculated using this model.