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Solar Cell and NMOS Transistor Process EE290G Joey Greenspun
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An SOI Process For Fabrication of Solar Cells, Transistors and Electrostatic Actuators Power, computation, actuation Process geared towards creation of microrobots Two processes Metal gate NMOS + Solar 25 V beak down voltages 11% efficient Poly gate CMOS + Solar 50 V break down voltages 14% efficient Bellew, Hollar, Pister Transducers ‘03 400x400 um 2 Solar Cell 2 Stage NMOS Buffer Inchworm Clutch
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Considerations for Solar + Transistor Process Power microrobot actuators Inchworm needs ~50V V oc for Si is ~.5V Controlled by SCµM GPIOS currently 3.3V Transistors must have appropriate V t 0.5V
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Simulated Solar Cell 1.ISO - Etch isolation 2.Grow oxide liner 3.Deposit Nitride 4.Deposit Poly 5.CMP 6.NPLUS Implant 7.NMINUS Implant 8.PPLUS Implant 9.Dopant activation 10.CONT etch 11.METAL, Al liftoff Process Steps
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Isolation 40 um SOI wafer Boron doped to 10 -15 Grow wet oxide (protect Si) Pattern with ISO, STS-OX (Oxide RIE), STS2 (DRIE) 1.ISO - Etch isolation 2.Grow oxide liner 3.Deposit Nitride 4.Deposit Poly 5.CMP 6.NPLUS Implant 7.NMINUS Implant 8.PPLUS Implant 9.Dopant activation 10.CONT etch 11.METAL, Al liftoff Process Steps 2um Line/Space Design Rule
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Isolation Oxide for clean interface Nitride for dielectric strength, and robustness to HF Poly deposits quickly, and much lower stress than Si x N y 1.ISO - Etch isolation 2.Grow oxide liner 3.Deposit Nitride 4.Deposit Poly 5.CMP 6.NPLUS Implant 7.NMINUS Implant 8.PPLUS Implant 9.Dopant activation 10.CONT etch 11.METAL, Al liftoff Process Steps
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Chemical Mechanical Polishing Alternative to CMP: Blanket plasma etch, stopping on the nitride Hot phosphoric acid to remove nitride Hydrofluoric acid to remove protective wet oxide 1.ISO - Etch isolation 2.Grow oxide liner 3.Deposit Nitride 4.Deposit Poly 5.CMP 6.NPLUS Implant 7.NMINUS Implant 8.PPLUS Implant 9.Dopant activation 10.CONT etch 11.METAL, Al liftoff Process Steps
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Inplants Solar Cell on the left NMOS on the right 1.ISO - Etch isolation 2.Grow oxide liner 3.Deposit Nitride 4.Deposit Poly 5.CMP 6.NPLUS Implant 7.NMINUS Implant 8.PPLUS Implant 9.Dopant activation 10.CONT etch 11.METAL, Al liftoff Process Steps 2um Line/Space Design Rule
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Dopant Drive in and Activation Drive in and activate dopants Choose oxide thickness for AR coating and gate dielectric Solar Cell on the left NMOS on the right 1.ISO - Etch isolation 2.Grow oxide liner 3.Deposit Nitride 4.Deposit Poly 5.CMP 6.NPLUS Implant 7.NMINUS Implant 8.PPLUS Implant 9.Dopant activation 10.CONT etch 11.METAL, Al liftoff Process Steps
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Contacts Etch contact holes in oxide Solar Cell on the left NMOS on the right 1.ISO - Etch isolation 2.Grow oxide liner 3.Deposit Nitride 4.Deposit Poly 5.CMP 6.NPLUS Implant 7.NMINUS Implant 8.PPLUS Implant 9.Dopant activation 10.CONT etch 11.METAL, Al liftoff Process Steps 2um Line/Space Design Rule
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Metalization Aluminum liftoff (gates might be hard) Alternative, sputter and etch metal Solar Cell on the left NMOS on the right 1.ISO - Etch isolation 2.Grow oxide liner 3.Deposit Nitride 4.Deposit Poly 5.CMP 6.NPLUS Implant 7.NMINUS Implant 8.PPLUS Implant 9.Dopant activation 10.CONT etch 11.METAL, Al liftoff Process Steps 2um Line/Space Design Rule
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Metalization Aluminum liftoff (gates might be hard) Alternative, sputter and etch metal Solar Cell on the left NMOS on the right 1.ISO - Etch isolation 2.Grow oxide liner 3.Deposit Nitride 4.Deposit Poly 5.CMP 6.NPLUS Implant 7.NMINUS Implant 8.PPLUS Implant 9.Dopant activation 10.CONT etch 11.METAL, Al liftoff Process Steps 2um Line/Space Design Rule
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MEMS Optional DRIE to pattern MEMS structures AR coating will be etched during MEMS release Switch to Nitride? Germanium protection possible, but process intensive 1.ISO - Etch isolation 2.Grow oxide liner 3.Deposit Nitride 4.Deposit Poly 5.CMP 6.NPLUS Implant 7.NMINUS Implant 8.PPLUS Implant 9.Dopant activation 10.CONT etch 11.METAL, Al liftoff 12. MEMS Process Steps 2um Line/Space Design Rule
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Preliminary Simulation Results 200x200 um 2 Solar Cell Simulation V oc =.63 V FF = 82.7% Efficiency = 10.0% Colby’s 200x200 um 2 Solar Cell V oc =.64 V FF = 64.4% Efficiency = ~8%
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