EE 143: Microfabrication TechnologyLecture 21m: Interconnects & ContactsCTN 4/8/10Copyright @ 2010 Regents of the University of California at BerkeleyEE 143: Microfabrication Technology LecM 7 C. Nguyen 3/14/10 8Sintering• Usually need a forming gas anneal @ 400-450oC (called sintering) at the end of an NMOS or CMOS processª Forming gas = mixture of H2and N2ª Improves the metal to silicon contactª Reduces the oxide interface charge, allowing the threshold voltage to be the right valueª The H2ties up dangling bonds that would otherwise contribute fixed interface charge • If you finish an MOS process and your thresholds are off, don’t despair – just do a sintering step & this will probably fix your problems (almost like magic)EE 143: Microfabrication Technology LecM 7 C. Nguyen 3/14/10 9Al Spiking & Junction Penetration• Problem: taking the temperature up too high causes silicon to diffuse into Alª Supply of Si tends to come from a few points, that then leave caves that Al can then go into to form spikesª This happens because the silicon wants to diffuse into the Al till it reaches its solid solubility limit of 0.25-1.5%• To prevent this, can:ª Incorporate Si into the Al so that it’s closer to its solid solubility limit – usually add 1% Si to the Al target in sputteringª Use a barrier metal (e.g., TiW)ª Use a barrier silicideEE 143: Microfabrication Technology LecM 7 C. Nguyen 3/14/10 10Contact Resistance• Definition: Resistance associated with the contact between two materials• Inversely proportional to the area of the contact• Strong function of the sintering temperatureARccρ=EE 143: Microfabrication Technology LecM 7 C. Nguyen 3/14/10 11Measuring Contact Resistance• Your lab layout includes contact chains:• Each chain: 14 series contacts and 7 series resistive pads• Strategy: if one uses enough contacts, the contact resistance becomes large enough to measure• Problem: segment resistance also rises; how can one delineate the contact resistance?Metal-Diffusion ContactsMetal-PolySiContactsEE 143: Microfabrication TechnologyLecture 21m: Interconnects & ContactsCTN 4/8/10Copyright @ 2010 Regents of the University of California at BerkeleyEE 143: Microfabrication Technology LecM 7 C. Nguyen 3/14/10 12Measuring Contact Resistance• Solution: use a 4-pt. probe strategy (also on the layout)Metal-PolySiContact Resistance Test StructureEE 143: Microfabrication Technology LecM 7 C. Nguyen 3/14/10 13Electromigration• Definition: The movement of atoms in a metal film due to momentum transfer from the electrons carrying the currentª Happens under high current density• Mean time to failure (MTF) formula:⎥⎦⎤⎢⎣⎡∝−kTEJMTFAexp2whereJ = current densityEA= activation energy (0.4-0.5eV for Al)EE 143: Microfabrication Technology LecM 7 C. Nguyen 3/14/10 14Electromigration• To reduce electromigration can add a small percentage of a heavier metal, like Cu (to Al)ª Cu has higher mass Ö more resistant to electromigrationª Targets composed of 95% Al, 4% Cu, and 1% Si often used in sputtering systemsEE 143: Microfabrication Technology LecM 7 C. Nguyen 3/14/10 15Issue: Polysilicon-to-Metal Contact• Polysilicon-to-metal contact takes up space• Need to eliminate waste of space for the smallest circuits, like memory arrays• Below: conventional layoutWasted SpaceEE 143: Microfabrication TechnologyLecture 21m: Interconnects & ContactsCTN 4/8/10Copyright @ 2010 Regents of the University of California at BerkeleyEE 143: Microfabrication Technology LecM 7 C. Nguyen 3/14/10 16Buried Contact• Need to add a mask to allow a buried contact, but it can save space and make the most compact layout• Diffusion from the n+ polySi merges with the S/D diffusion around the gate (the diffusion cannot be initially below the gate, since the gate serves as a mask against the source/drain implant)EE 143: Microfabrication Technology LecM 7 C. Nguyen 3/14/10 17Butted Contact• Saves area, since it only needs one contact to connect polysilicon and metalEE 143: Microfabrication Technology LecM 7 C. Nguyen 3/14/10 18Silicide• Sheet resistance of polysilicon and shallow diffusions used in CMOS are generally on the order of 10-20Ω/• Can reduce this resistance to 15-50 μΩ/ by reacting silicon with a noble or refractory metal to form a silicideEE 143: Microfabrication Technology LecM 7 C. Nguyen 3/14/10 19The Silicidation Process• Expose silicon areas where silicidation is desired• Blanket deposit metal• Heat to needed temperature; can be done via rapid thermal anneal (RTA)• Remove unreacted metalEE 143: Microfabrication TechnologyLecture 21m: Interconnects & ContactsCTN 4/8/10Copyright @ 2010 Regents of the University of California at BerkeleyEE 143: Microfabrication Technology LecM 7 C. Nguyen 3/14/10 20The Silicidation ProcessRemarks:1. Often can be self aligned to the region to be silicided, in which case it’s called a salicide2. Polycide: a silicide over polysiliconª Also pretty much self-alignedª Just put the metal down everywhere, heat, and reaction will only occur over polysilicon areas3. Achieve resistivities from 15-50 μΩ-cm4. Can oxidize the surface of a silicide, since silicon diffuses through the silicide to combine with the oxidant5. Unlike silicon-metals that are unreacted, many silicides can take temperatures much higher than the eutectic temperature, over 1000oC ª not true for all silicides, e.g., nickel silicide (900oC), platinum silicide (800oC), and palladium silicide (700oC)EE 143: Microfabrication Technology LecM 7 C. Nguyen 3/14/10 21Lift-Off• Many metals (e.g., nickel, copper) do not have a recipe for dry etching, so cannot be etched anisotropicallywith good resolution• Lift-off provides a method for patterning a metal without the need for etching• Right: comparison of conventional and lift-off based metallization processesConventional Lift-OffEE 143: Microfabrication Technology LecM 7 C. Nguyen 3/14/10 22Multilevel Metallization• Big Problem (for multi-levels of metal): topographyª Interferes with lithography, degrading resolutionª Creates stringers that then force overetching• Solution: planarization and contact plugsEE 143: Microfabrication Technology LecM 7 C. Nguyen 3/14/10 23Damescene Process• Employs plating through molds to achieve contact plugs in a fully-planarized (via CMP) cross-sectionEE 143:
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