Design and Use of Memory-Specific Test Structures to Ensure SRAM Yield and Manufacturability F. Duan, R. Castagnetti, R. Venkatraman, O. Kobozeva and S. Ramesh LSI Logic Corporation

ISQED Outline n Products need very high-density and high-performance memories without sacrificing yield and manufacturability n LSI Logics Industry-leading 1.87um 2 embedded SRAM bitcell in 130 nm CMOS SoC technology n Need for SRAM-specific test structures to ensure robustness and manufacturability –Accurately shows process-design interaction –Correlate to SRAM yield –Direct feedback for rapid process-improvement –Accurate SRAM device and cell characterization –RAMPCM for design rule robustness validation n Summary

ISQED LSI Logic SRAM Technology for SoC n Smallest production high-density SRAM in 180nm and 130nm technology n Fabricated by standard CMOS SoC process High-Density and High-Performance 6T-SRAM for System-on-Chip in 130 nm CMOS Technology, 2001 VLSI Technology Symposium, pp , W. Kong, R. Venkatraman, R. Castagnetti, F. Duan and S. Ramesh.

ISQED Why Do We Need for SRAM-specific Test Structure? An Example (b)(c) (a) Structures to test metal bridging

ISQED Electrical Test Data Only structure (a) detects the early metal /contact bridging

ISQED Features of Our Structures n Compared to the conventional structures, our structures: –More product-driven than process-development-driven –Accurately show process-design interaction –Correlate to functionality and yield directly –Identify the yield limiting factors quickly for fast process or design improvements. –Sensitive enough for ongoing monitoring and process transfers.

ISQED Test Structure Design and Results n Yield correlation and improvement n SRAM manufacturability n SRAM device and cell characterization n RAMPCM chip –Functional SRAM test chip –Used for SRAM design rule validation

ISQED Test Structures of Front-end Critical Layer Monitor n Critical layers: island, poly and contact n Monitor bridging current from intra- and inter- layers n Monitor shared-contact connection

ISQED Test Structures of Back-end Monitor n Monitor bridging current from: –Contact and metal 1 (blue arrows) –Metal 2 (red arrows) –Metal 3 (black arrow)

ISQED Example of a Test Structure For Poly Bridging Test cell Test array

ISQED Poly Bridging Data 100,000 cells No poly bridging found in SRAM

ISQED Metal Bridging and Correlation to SRAM Yield 100,000 cells Detected early metal /contact bridging

ISQED Metal Bridging and Correlation to SRAM Yield (cont.) Metal bridging identified as yield limiting factor

ISQED Metal 1 Bridging Data after Process Improvement 100,000 cells No metal bridging seen after improvement

ISQED Test Structures for Manufacturability n An illustration of test structure n List of test structures n Electrical data

ISQED An Illustration of the Test Structure n Sizing poly by 5% per side n Build test cell to test its effects on: –Poly to poly bridging –Poly to contact bridging –Pull down transistor leakage

ISQED List of Test Structures for SRAM Manufacturability n Sizing island –Island to island bridging –Transistor leakage n Sizing poly –Poly to poly bridging –Poly to contact bridging –Transistor leakage n Sizing contact –Contact to poly bridging –Contact / metal 1 bridging –Contact resistance n Sizing metal 1 –Metal 1 / contact bridging

ISQED Effect of Poly Sizing – SRAM Poly to Poly Bridging 100,000 cells Poly to poly spacing in SRAM is robust

ISQED Effect of Poly Sizing – SRAM Poly to Contact Bridging 100,000 cells Poly to contact spacing in SRAM is robust

ISQED Effect of Poly Sizing – SRAM Pull Down Transistor Leakage 100,000 cells All leakage currents are within device model spec

ISQED Test Structures for Characterization n Transistors in SRAM –Due to the environmental difference (OPC, etc), transistors in SRAM may behave differently compared to isolated devices –Need to measure transistor in the real SRAM array for accurate characterization n An illustration of test structures n Electrical data for transistor measurement n Cell characterization

ISQED Illustration of Measuring SRAM Transistor

ISQED Transistors in SRAM Cell n Measure all the 6 transistors in 6T SRAM –To compare cell symmetry between left and right –To compare with isolated devices n Measure 4 orientations

ISQED Saturation Current of Pass Gate Transistors (Left, Right, Isolated Counterpart) Good symmetry and little difference between iso. / dense

ISQED Saturation Current of Pull Down Transistors (Left, Right, Isolated Counterpart) Good symmetry and little difference between iso. / dense

ISQED Threshold Voltage of Pull Down Transistor in 4 Directions (0, 90, 180, 270) Little difference of 4 different orientations

ISQED SNM and Cell Current of 1.87 um 2 Bitcell (130nm Generation) Cell Current (A) Butterfly curve Cell current (L&R)

ISQED RAMPCM Test Chip n RAMPCM is a functional SRAM test chip that is used to prove robustness of the most critical SRAM design rules. n Size of the chip is 1M with 1024 row by 1024 columns n Every 64 columns evaluate one critical SRAM design rule

ISQED RAMPCM Test Chip (continued) n 8 most critical design rules are evaluated n Each design rule has 4 variations, distributed across array n Data logs from failing dies are used to extract numbers of failing bits per design rule variation

ISQED RAMPCM Test Data -- Normalized Failure Rate Design rules used in 1.87 um 2 cell Within process window Note: More than 50Mb data for each variation. This data proves the robustness and manufacturability of the 1.87 um 2 bitcell

ISQED Use of Test Structures in 90nm and Beyond n Increased design-process interaction in 90nm and beyond (Ex. OPC variation) n The test structure methodology we have presented today becomes even more necessary for 90 nm and beyond n Gate leakage impact for SRAM must be accurately evaluated n We have designed such necessary test structures for the 90nm node

ISQED Summary n We have designed and used SRAM-specific test structures as effective tool for SRAM technology development. n The data from these SRAM test structures provides us direct feedback on process-design interactions and helps to identify yield-limiting factors early and quickly. n The data (including from RAMPCM) is analyzed to prove the robustness and manufacturability of our industry- leading 1.87 um 2 SRAM cell.