ME 141B: The MEMS Class Introduction to MEMS and MEMS Design Sumita Pennathur UCSBFundamentals of Wafer Level Bonding • Two separate and distinct steps  The wafers are aligned to each other ina bond aligner with a possible alignment accuracy of one micron or less  The bond fixture is loaded into a vacuum bond chamber where the wafers are contacted together • There most prevalent types  Direct or fusion wafter bonding (high temperature, ~1000C)  Anodic or field-assisted bonding (~500C)  Bonding with an intermediate “glue” layer • Gold (thermocompression), ~300C • Polymer or epoxy layer 10/28/10 2/45Wafer Bonding Motivation • For pressure sensors – allows creation of cavities • For fluidic channels, allows for easy fabrication • For MEMS device, allows for formation of 3D structures, cavities by combining etched wafers  Lithography and etching intrinsically allows limited range of shapes in 3D (prisms, cylinders, “extrusions”) • For MEMS and microelectronics allows for wafer level packaging  Minimize connections from chip to macroscale  Hermetic sealing prior to die sawing  Parallel manufacturing  3D interconnects 10/28/10 3/45MEMS applications of bonding 10/28/10 4/45Wafer Bonding Technologies • Direct Fusion bonding  Si to Si (also Si/SiO2, Si/Al2O3) • Anodic bonding  Si to glasses containing conductors • Glass frit bonding  Glass powder/paste softened/sintered to form bond • Solder/braze bonding (not generally used at wafer level) • Thermo-compression bonding • Polymeric adhesives (not generally used for permanent bonding but sometimes used for temporary attachment for handling) 10/28/10 5/45Silicon Direct Bonding • Grew our of efforts to replace deposition processes for creating device layers or microelectronics (c 1985)  Layer transfer by bonding and thinning doped wafer  Silicon on insulator (SOI) • Very rapidly adopted or MEMS  Cavity formation • Commerical application for SOI has driven transition to volume production – mainstream VLSI  Process very reproducible, well-controlled • Evolution to other applications, dissimilar materials 10/28/10 6/45Basics of direct bonding • Silicon fusion bonding is the process of bonding two mirror-polished silicon wafers with no intermediate adhesive layer • The baseline process consists of: • The primary advantages include:  Bonds with strengths that approach that of bulk silicon  No CTE mismatch  Compatible with CMOS processing  Ability to inspect and re-bond after contacting stage 10/28/10 7/45Cavity formation using Bonding 10/28/10 8/45Direct Wafer Bonding 10/28/10 9/45Alignment fixture for bonding 10/28/10 10/45EV Wafer Bonder 10/28/10 11/45Imaging of Bonds • Silicon is translucent in infra red at wavelengths of 1 um • Unbonded regions in close proximity generate interference fringes • Very useful tool for inspecting bonds after contacting  Quick, simple • Bonds can be separated at this stage, re-cleaned, and re-bonded • Also can use ultrasound inspection methods  Unbonded interfaced generate echos  Commonly use in packaging inspections  Use for non IR transparent materials 10/28/10 12/45IR Imaging 10/28/10 13/45Acoustic Microscopy 10/28/10 14/45Comparison of inspection techniques 10/28/10 15/45Acoustic image of poorly bonded pair 10/28/10 16/45IR Image contact bonding • Contact initiated at center • Intimate contact of surfaces required for subsequent bonding  Short range surface forces responsible for adhesion  Surfaces deform to achieve contact 10/28/10 17/45“Hydrophilicity” of surface clean/preparation solutions 10/28/10 18/45Hydrogen bonded surface chemistry - hydrophilic 10/28/10 19/4510/28/10 20/45 Hydrogen bonded surface chemistry - hydrophobicTypical Surface Energies 10/28/10 21/45Surface Preparation: RCA Clean • Developed in 1965 at Radio Corporation of America • The industry standard for removing contaminants from wafers • Required in most fabs prior to high temperature oxidation, difusion and deposition • Three primary steps:  Organic Clean – Removal or insoluble organic contaminants with 5:1:1 H2O:H2O2:NH4OH (or H2SO4)  Oxide Strip – Removal a thin silicon dioxide layer where metallic contaminants may have accumulated as a result of (1), using diluted 50:1 H2O:HF solution  Ionic Clean – Removal of ionic and heavy metal atomic contaminants using a solution of 6:1:1 H2O: H2O2:HCl 10/28/10 22/45Can go Wrong! 10/28/10 23/45Wafer geometry impacts bonding • Spontaneous wafer bonding reduces surface energy  Two smooth, clean, perfectly flat wafers will bond spontaneously • When wafers are not perfectly flat, bonding requires them to bend  Strain energy increases • How far will two wafers bond?  Wafers bond until the surface energy reduction equals the strain energy costs • Important factors  Wafer thickness  Radius of curvature • Wafer bow – innate or from stressed films • Waviness – locally greater curvature 10/28/10 24/45Wafer Geometry Impacts Bonding • Bonding order and strain energy  For given total stack thickness, the strain energy accumulates fastest for wafers of equal thickness (goes as thickness cubed)  To bond n wafers, add them one at a time • Etched features  Shallow etch hinders bonding (less interaction area)  Deep etch aids bonding (less stiffness) 10/28/10 25/45Bonding Defects 10/28/10 26/45Key Attributes of Direct Bonding • Ability to inspect in IR • Ability to debond and rebond in IR • Bond toughness/strength approaches that of Si after annealing • For Si-Si wafer bonds no thermal mismatch – important for sensitive instruments • High temperature technique, allows high temperature operation  But must come early in temperature hierarchy of processing • Relatively defect sensitive, particularly in intitial contacting stage  Particles, roughness, bow 10/28/10 27/45Wafer bonding and yield • Yield in MEMS can require a whole-wafer outlook, unlike IC processing • A micon-scale defect can create a mm- to cm-scale defect  Amplification by wafer stiffness • Can have a die