3.155J/6.152 J Fall2003 MOSCAP MOSCAP LAB SESSION 2 Back-side Strip Location: ICL (2 floor, Building 39) Overview of lab session The first step of this lab session will be to characterize the oxide and polysilicon films deposited in lab session 1. The main processing taking place in this lab session will be the removal of the oxide and polysilicon layers from the back side of the wafers. This will allow electrical contact to the back side of the wafer. The wafers will be coated with a photoresist solution using an automatic coating system which has several modules operating in a pipelined fashion. The first module treats each wafer with Hexamethyldisilazane(HMDs)vapor, an adhesion promoter. Next, a photoresist solution is dispensed onto the wafer and evenly distributed by a spin-on technique Finally, the wafer is baked to expel solvents from the photoresist. Since the wafers will not be patterned photolithographically, the wafers will be post-baked directly(rather than pre-baked exposed, and then post-baked) to harden the resist into a protective masking layer. The back sides of the wafers will be etched dry-etched in He/Cl2 plasma. Next, the oxide will be wet-etched in buffered oxide etch(BOE), a solution of hydrofluoric acid. Finally, the photoresist will be stripped by oxygen plasma etch Lab Objectives: Complete characterization of Oxide and Polysilicon films grown in Lab Session 1 Remove oxide and polysilicon films from back side of wafers Instruction on the following pieces of lab equipment KLA Tencor UV1280 Film Thickness Measurement System SSI Coater Track LAM 490B Plasma Etcher Oxide etch sink Matrix 106 System One Stripper Before attending this lab session, make sure to read the Standard Operating Procedures for the above equipment. SOPs can be accessed at: http://www-mtl.mitedu/mtlhome/3mfab/sop.html Lab procedure 1. Characterize Oxide and Polysilicon deposited in Lab session 1 Measure the oxide and polysilicon thickness using the UV1280 Note: An oxide color chart is enclosed in the Appendix. The color chart assumes perpendicular illumination and viewing of wafer surface under white fluorescent lighting 2. Etch oxide in boe (Oxide etch sink Using the Oxide Etch Sink, etch the native oxide on the polisilicon surface of the wafer in Buffered-Oxide-Etch(BOE). The etch rate for bOE is about 1000 A/min the thickness of the native oxide is around 200 a so estimate the approximate etch time required. When the BOE etch is complete, the BOE should'de-wet'or bead up off the wafer. After completing etch, rinse thoroughly with DI water and spin dry Overall reaction for etching Sio2 with BOE SiO2+ 6HF == H2+ SiF6+ 2H2O where a buffering agent, ammonium fluoride(NH4 F), is added to maintain HF concentration and to control pH to minimize photoresist attack) NH4F <== NH3 HF3.155J / 6.152J Fall 2003 MOSCAP 4 MOSCAP LAB SESSION 2 Back-side Strip Location: ICL (2nd floor, Building 39) Overview of Lab Session: The first step of this lab session will be to characterize the oxide and polysilicon films deposited in lab session 1. The main processing taking place in this lab session will be the removal of the oxide and polysilicon layers from the back side of the wafers. This will allow electrical contact to the back side of the wafer. The wafers will be coated with a photoresist solution using an automatic coating system which has several modules operating in a pipelined fashion. The first module treats each wafer with Hexamethyldisilazane (HMDS) vapor, an adhesion promoter. Next, a photoresist solution is dispensed onto the wafer and evenly distributed by a spin-on technique. Finally, the wafer is baked to expel solvents from the photoresist. Since the wafers will not be patterned photolithographically, the wafers will be post-baked directly (rather than pre-baked, exposed, and then post-baked) to harden the resist into a protective masking layer. The back sides of the wafers will be etched dry-etched in He/Cl2 plasma. Next, the oxide will be wet-etched in buffered oxide etch (BOE), a solution of hydrofluoric acid. Finally, the photoresist will be stripped by oxygen plasma etch. Lab Objectives: • Complete characterization of Oxide and Polysilicon films grown in Lab Session 1. • Remove oxide and polysilicon films from back side of wafers. • Instruction on the following pieces of lab equipment • KLA Tencor UV1280 Film Thickness Measurement System • SSI Coater Track • LAM 490B Plasma Etcher • Oxide Etch Sink • Matrix 106 System One Stripper Before attending this lab session, make sure to read the Standard Operating Procedures for the above equipment. SOPs can be accessed at: http://www-mtl.mit.edu/mtlhome/3Mfab/sop.html Lab Procedures: 1. Characterize Oxide and Polysilicon deposited in Lab session 1. Measure the oxide and polysilicon thickness using the UV1280. Note: An oxide color chart is enclosed in the Appendix. The color chart assumes perpendicular illumination and viewing of wafer surface under white fluorescent lighting. 2. Etch oxide in BOE. (Oxide etch sink) Using the Oxide Etch Sink, etch the native oxide on the polisilicon surface of the wafer in Buffered-Oxide-Etch (BOE). The etch rate for BOE is about 1000 Å/min. The thickness of the native oxide is around 200 Å so estimate the approximate etch time required. When the BOE etch is complete, the BOE should ‘de-wet’ or bead up off the wafer. After completing etch, rinse thoroughly with DI water and spin dry. Overall reaction for etching SiO2 with BOE: SiO2 + 6HF ==> H2 + SiF6 + 2H2O where a buffering agent, ammonium fluoride (NH4F), is added to maintain HF concentration and to control pH (to minimize photoresist attack): NH4F <==> NH3 + HF