PHYS 534 (Fall 2008) Lecture 2 Introduction to Microfabrication 1 Srikar Vengallatore, McGill University How are Microsystems Designed? Market need Creativity Evaluation & experience CtConcept of competition Manufacturing Embodiment Modeling and considerations Analysis In-house DtilDetail Management expertise decisions Product Specification 2 1 Structural Embodiment Phase of Design •Selection of materials, structures, and shapes to optimize performance and reliability. •Is electrostatic actuation optimal? •Is a torsional hinge optimal? •Is aluminum the best material? •Is sputtering the best method to deposit aluminum? 3 But,…. Structural Design is Severely Constrained by Process Limitations Micro Engine MACRO Engine 4 2 Metallic vs. Ceramic Materials Silicon Microengine 5 Kalpakjian and Schmid What is the Origin of the Process Limitations? Starting Material: Substrate (wafer) Photolithography Patterning E-beam lithoggpyraphy Ion beam lithography Subtractive Soft lithography Processes Processes Evaporation Wet etching Additive Sputtering Dry etching Processes CVD Plasma etching Electrodeposition DRIE Wafer bonding Polishing Package Microdevice 6 3 Microfabrication differs from macro-machining in several ways: •Material removal is by selective corrosion •Structural thin films are not available from a catalog. ItdInstead, we have to creatthfilte the film bfbefore s hap ing it •Defining a pattern follows a process that resembles Photography (of the old-fashioned type using photosensitive films) •Massively parallel manufacture •Simultaneous manufacture and assembly 7 Catalog of Manufacturing Processes Patterning Techniques: Photolithography, Microstamping, Electron/ion beam lithography, Soft lithography,… Additive processes: Thin-film deposition, wafer bonding, oxidation, epitaxy, ….. Subtractive processes: Wet etching, dry etching, ion milling, deep reactive ion etching,… 8 4 Starting Material: Substrate •Typical characteristics: •Shappp(e: Circular plates (also called wafers) •Size: ~1 mm thickness ~10 cm diameter 9 Common Substrate Materials •Single crystal silicon •Single crystal Quartz (silicon oxide) •Commercially Available •Amorphous silica glasses •Pyrex •Gallium arsenide •SiC •…… 10 5 How are Silicon Wafers Specified? Chemistry: Purity; Dopant concentration Electrical Properties: Resistivity Geometry: Diameter; Thickness; Total thickness variations; Surface finish and polish; BdBow and warpage; Crystallographic Orientation; Primary & secondary flats Virginia Semiconductor (http://www.virginiasemi.com/) and many others 11 Specification of Chemistry Impurities: Carbon, oxygen, heavy metals,… Typically, O2: 5 – 25 ppm C: 1 – 5 ppm Metals: < 1 ppb Some impurities are intentionally added in small, well-defined, quantiti es Such impurities are called DOPANTS 12 6 Crystallography of Substrates Based on atomic arrangement, materials can be Amorphous Single Crystals Grain boundary Polycrystalline 13 [Ohring] Unit Cells of Cubic Crystals Simple Cubic Body Center Cubic (BCC) Face Center Cubic (FCC) 14 7 Directions of a Cubic Crystals z [0,0,1] [1,0,1] Unit Distance y [0,1,0] [1,1,0] x [1,0,0] 15 Miller indices of Crystal Planes [0,0,1] [0,1,0] [1,0,0] Recipe: 1. Determine intercepts on each axis 1, 1, ∞ 2. Take reciprocals of these numbers 1, 1, 0 3. Reduce to smallest integers (110) 16 8 Examples of Low Index Planes 17 [Senturia] Two Important Results for Cubic Crystals Result 1. Plane (h k l) has unit normals [h k l] [0,0,1] (1 1 0) [0,1,0] [1,0,0] [1 1 0] Notation: {1 0 0} indicates a family of (1 0 0) planes <1 0 0> indicates a family of [1 0 0] directions 18 9 Two Important Results for Cubic Crystals Result 2. The angle (γ) between two planes with indices (h1 k1 l1) and (h2 k2 l2) is given by h h + k k + l l cosγ = 1 2 1 2 1 2 2 2 2 2 2 2 h1 + k1 + l1 h2 + k2 + l2 Example: (i) The angle between (100) and (111) is −1 ⎡1+ 0 + 0⎤ −1 1 0 γ = cos ⎢ ⎥ = cos = 54.7 ⎣ 1 3 ⎦ 3 19 Crystallography of Single-Crystal Silicon Wafers (111) Wafers Look for the primary flat (100) Wafers 20 [Maluf] 10 Representation of a Simple Process Flow 1 mm 10 cm Golden Rule of Process Representation: NOTHING is EVER drawn to scale! 21 Thin-Film Deposition 1 mm Thin Film 1 μm 22 11 Photolithographic Patterning •Apply thin layer of photosensitive polymer Photoresist (~ 1 μm thick) 23 Patterning Using Photolithography •Selectively expose to light using a RETICLE Opaque coating Transparent plate •Reticles are also called Photo-masks: Commercially available24 12 Exposure to Ultraviolet Radiation 25 Exposed photoresist is soluble Development of Exposed Photo-resist (Use solvent) 26 13 Etching: Selective Corrosion to Remove Material Chemical 1: Removes Blue only Chemical 2: Removes only red 27 28 14 Next: A Surface-Micromachining Process.. •Addition, Patterning & Selective Removal of Thin Films Silicon Oxide Silicon 29 [Maluf] Silicon oxide Deposit polysilicon thin film 30 15 Mask Photo resist Pattern and etch polysilicon polysilicon Silicon Oxide 31 [Maluf] Sacrificial Oxide Release the structure 32 [Maluf] 16 2-d Representation of Process Flows Again, Not to Scale! 33 2-d Representation of Process Flow (Photolithographic details not shown) 34 Substrate Silicon Oxide Polysilicon 17 Interpretation of Floating Structures Silicon Light Machines •Examine different cross-sections to find anchoring locations 35 BULK MICROMACHINING: Selective Removal Material from Substrate 36 (Maluf) 18 Clarifies angle of this surface 37 Overview of Microdevice Manufacture Starting Material: Substrate (wafer) Photolithography Pattern E-beam lithoggpyraphy Formation Ion beam lithography Subtractive Soft lithography Processes Processes Evaporation Wet etching Additive Sputtering Dry etching Processes CVD Plasma etching Electrodeposition DRIE Wafer bonding Polishing Package Microdevice 38 19 Lithos: Stone graphy: to write Photolithography: Pattern transfer using photons HdHardware: Source ofUVf UV ra ditidiation (a ligner ) Reticle (master pattern) Photoresist (polymer) Chemical solvents 39 Basic Steps in Photolithography Coat Photoresist Expose to Ultraviolet radiation Develop mask 40 20 Spin Coating and the Importance of Low Viscosity Centrifugal forces vs. Viscosity photoresist 1000 – 8000 rpm 10 – 30 s ω Partial evaporation of solvent during spin coating 41 How thin must the photoresist be? •Depends on details of process flow •To create small features (<1.0 μm), use thin resists (1.0 μm) •For bulk micromachining, use thick (~10 μm) photoresist layer Rule of Thumb Photoresist thickness scales with feature size 42 21 Two Types of Photoresists Exposed regions Exposed regions become soluble become insoluble POSITIVE resist NEGATIVE resist 43 Mechanisms linked to Bond Formation/Destruction Before Exposure After Exposure Positive Resist Negative Resist 44 22 Examples of Positive & Negative Resists Positive: PMMA (poly methyl methacrylate) DQN (diaquinone ester + phenolic novolak) Negative: bis(aryl)azide rubber resists •Photoresists are commercially available 45 [Madou] Photolithographic Aligners Source of Radiation Focusing optics Tooling for Alignment 46 [Senturia] 23 http://www.physics.mcgill.ca/nanotools/ ALIGNER 47 Overview of Microdevice Manufacture Starting Material: Substrate (wafer) Photolithography Pattern E-beam lithoggpyraphy Formation Ion beam lithography Subtractive Soft lithography Processes Processes Evaporation Wet etching Additive Sputtering Dry etching Processes CVD Plasma etching Electrodeposition DRIE Wafer bonding Polishing Package Microdevice 48 24 THIN FILM USUALLY IMPLIES…. •Deposited on substrate & subsequently processed •Lateral film dimensions much larger than thickness. •Thin Films vs. Thick Films: Thin films 0.1 μm < hfilm < 2 μm Thick films 5 μm<hm < hfilm <50< 50 μm 49 THIN FILM PROCESSING TECHNIQUES Wet (Solution) Dry (Vapor) •Spin casting •Evaporation •Electrodeposition •Sputter-deposition •Sol-gel & colloidal •Chemical vapor techniques deposition (CVD) •Pulsed-laser deposition •Oxidation 50 25 GENERIC VAPOR-DEPOSITION PROCESS Source of atoms (target) Vapor of atoms substrate vacuum Nucleation Growth Coalescence 51 QUALITY OF DEPOSITED FILMS Geometry: Thickness & Thickness uniformity Lateral dimensions & uniformity Conformality vs. Line-of-sight coatings Kinetics: Rate of film growth Chemistry: Fidelity of composition Compositional uniformity Mechanical stress: Intrinsic (growth-related) 52 26 GEOMETRIC PARAMETERS Lateral uniformity & Thickness uniformity No lateral uniformity lateral uniformity Thickness uniformity No thickness uniformity No lateral uniformity No thickness uniformity 53 CONFORMALITY OF COATING •Ability to coat topographic features i.e., ability to conform to surface features Sidewall Top surface CONFORMAL NON-CONFORMAL 54 27 Material Addition Using Wafer Bonding •Direct Wafer Bonding Wafer 1 Wafer 2 •Intermediate Wafer Bonding Metal; Glass; Oxide; Polymer 55 High Strength Bonding is Possible (Bonded regions as strong as lattice!) 56 28 IR Image of Bond Formation •Relatively simple process to implement 57 Overview of Microdevice Manufacture Starting Material: Substrate (wafer) Photolithography Pattern E-beam lithoggpyraphy Formation Ion beam lithography Subtractive Soft lithography Processes Processes Evaporation Wet etching Additive Sputtering Dry etching Processes CVD Plasma etching Electrodeposition DRIE Wafer bonding Polishing Package Microdevice 58 29 Key Concept: Material Removal by Chemical Corrosion Example 1: Development of Photoresist (Wet Etching) Exposed photoresist is soluble 59 How to Design a Etch Process •What etchants