Wet Bulk Micromachining a STIMESI II tutorial 23

Wet Bulk Micromachining – a STIMESI II tutorial 23. 02. 2011 Per Ohlckers, Vestfold University College www. hive. no Per. Ohlckers@hive. no Daniel Lapadatu, Senso. Nor Technologies www. multimems. com daniel. lapadatu@sensonor. no

Outline 1. Background and Motivation 2. The Silicon Crystal 3. Isotropic Wet Etching 4. Anisotropic Wet Etching 5. Selective Etching 6. Convex Corners Slide 2

Outline 1. Background and Motivation 2. The Silicon Crystal 3. Isotropic Wet Etching 4. Anisotropic Wet Etching 5. Selective Etching 6. Convex Corners Slide 3

Manufacturing Processes Serial (e. g. Focused Ion Milling - FIB) vs. batch (e. g. bulk Si micromachining) vs. continuous (e. g. doctor’s blade). Additive (e. g. evaporation) or subtractive (e. g. dry etching). Projection (almost all lithography techniques) vs. truly 3 D. Mold vs. final product. There are many different techniques that are being used. However, in general, batch processing is the most powerful technique and used mostly. FIB Slide 4

Micromachining Bulk Micromachining is a process that produces structures inside the substrate by selective etching. Surface Micromachining is a process that creates structures on top of the substrate by film deposition and selective etching. Bulk Micromachining Surface Micromachining Slide 5

Classification of Bulk Silicon Etching Bulk Micromachining is a process that produces structures inside the substrate by selective etching. Wet Etching Dry Etching Crystal orientation dependent Process dependent Isotropic Anisotropic Acidic Etchants Alkaline Etchants Br. F 3 Xe. F 2 F-based plasmas Slide 6

Etching Features Etch rate: ► Rate of removal of material/film. ► Varies with concentration, agitation and temperature of etchant, porosity and density of etched film. Etch selectivity: ► Relative etch rate of mask, film and substrate. Etch geometry: ► Etching depth (R); ► Mask undercut (U); ► Slope of lateral walls (S); ► Bow of floor (B); ► Anisotropy. Slide 7

Bulk Silicon Etching: Examples Deep cavity by wet, anisotropic etching Release etch by RIE Recess etch by RIE Slide 8

Wet Silicon Etching: Examples Isotropic etching with HNA (HF : Nitric Acid : Acetic Acid) Anisotropic etching with KOH (110) (100) Slide 9

Outline 1. Background and Motivation 2. The Silicon Crystal 3. Isotropic Wet Etching 4. Anisotropic Wet Etching 5. Selective Etching 6. Convex Corners Slide 10

Structure of Single Crystal Silicon Face-centred cubic (fcc) structure (diamond structure) with two atoms associated with each lattice point of the unit cell. One atom is located in position with xyz coordinates (0, 0, 0), the other in position (a/4, a/4), a being the basic unit cell length. Lattice constant a = 5. 43 Å. The arrangement of the silicon atoms in a unit cell, with the numbers indicating the height of the atom above the base of the cube as a fraction of the cell dimension. Slide 11

Miller Indices Miller indices are a notation system in crystallography for planes and directions in crystal lattices. A lattice plane is determined by three integers h, k and l, the Miller indices, written (hkl). The indices are reduced to the smallest possible integers with the same ratio. Determining the Miller indices for planes by using the intercepts with the axes of the basic cell. Slide 12

Determining Miller Indices Example: ► Take the intercepts of the plane along the crystallographic axes, e. g. 2, 1 and 3. ► The reciprocal of the three integers are taken: 1/2, 1/1 and 1/3. ► Multiply by the smallest common denominator (in this case 6): 3, 6 and 2. ► The Miller indices of the plane are: (362). Slide 13

Crystallographic Planes and Directions (abc) denotes a plane. {abc} denotes a family of equivalent planes. [abc] denotes the direction perpendicular on (abc) plane. <abc> denotes a family of equivalent directions. {100}, {110} and {111} are the most important families of crystal planes for the silicon crystal. Slide 14

Single Crystal Silicon Wafers Primary and secondary flats indicate the dopant type and surface orientation. Wafer diameter in current fab standards: from 100 to 300 mm. Wafer thickness in current fab standards: from 250 to 600 µm. Surface orientation: ► (100) for MOS and MEMS; ► (110) for MEMS; ► (111) for bipolar. Secondary flat Primary flat Slide 15

Standard 100 mm Wafers The position of the flat(s) indicates the surface orientation and the type of doping. The primary flat on (100) and (110) wafers is along the [110] direction. Orientation of flats for 100 mm wafers p-(111) Primary flat n-(111) p-(100) n-(100) Secondary flat Slide 16

Wafers Used in Multi. MEMS P-type, 150 mm Si wafer. (100) ± 0. 5º Surface. [110] ± 0. 5º Primary Flat. Si Wafer Slide 17

Outline 1. Background and Motivation 2. The Silicon Crystal 3. Isotropic Wet Etching 4. Anisotropic Wet Etching 5. Selective Etching 6. Convex Corners Slide 18

Isotropic Wet Etching of Silicon All crystallographic directions are etched at the same rate. Features: with stirring ► Etchants are usually acids; ► Etch temperature: 20. . . 50 °C; ► Reaction is diffusion-limited; without stirring ► Very high etch rate (e. g. up to 50 µm/min); ► Significant mask undercutting. Masking is very difficult: ► Au/Cr or LPCVD Si 3 N 4 is good. ► Si. O 2 may also be used for shallow etching. Si Slide 19

Isotropic Etching of Silicon: Etchants Etchant (Diluent) Typical Composition Temp. Etch Rate [µm/min] HF HNO 3 CH 3 COOH + H 2 O 10 ml 30 ml 80 ml 22 °C 0. 7. . . 3. 0 HF HNO 3 CH 3 COOH + H 2 O 25 ml 50 ml 25 ml 22 °C 40 HF HNO 3 CH 3 COOH + H 2 O 9 ml 75 ml 30 ml 22 °C 7. 0 Mechanism hole injection: oxidation: oxide removal: – HNO 3 + H 2 O + HNO 2 → 2 HNO 2 + 2 OH + + 2 h 4+ – Si + 4 OH → Si. O 2 + H 2 Si. O 2 + 6 HF → H 2 Si. F 6 Slide 20

Silicon Etching with HNA: mixture of ► 49, 23% HF, ► 69, 51% HNO 3 and ► acetic acid (CH 3 COOH) or water (H 2 O) as diluent. (µm/h) HNO 3 oxidizes the silicon, HF removes the oxide: ► High HNO 3: HF ratio, - Etch limited by oxide removal. ► Low HNO 3: HF ratio - Etch limited by oxide formation. Dilute with water or acetic acid: ► CH 3 COOH is preferred because it prevents HNO 3 dissociation. Iso-Etch Curve (from Robbins et al. ) Slide 21

Isotropic Etching of Glass Single- or double-side etching of glass wafers is achieved either by using HF-water solution or HNA. Typical etch rate for borosilicate glass in HNA: 1. 9 µm/min. Applications: ► Etching cavities and through-holes; ► Etching gas/fluid channels. Through-hole Mask Cavity Undercut Slide 22

Outline 1. Background and Motivation 2. The Silicon Crystal 3. Isotropic Wet Etching 4. Anisotropic Wet Etching 5. Selective Etching 6. Convex Corners Slide 23

Anisotropic Wet Etching of Silicon Crystallographic directions are etched at different rates. Features: ► Etchants are usually alkaline; ► Etch temperature: 85. . . 115 °C; ► Reaction is rate-limited; ► Low etch rate (ca. 1 µm/min); ► Small mask undercutting. Masking is very difficult: ► LPCVD Si 3 N 4 is good; ► Si. O 2 may also be used with some etchants. Si Slide 24

Etching Setup Laboratory setup for wet chemical etching of silicon. The principles for industrial manufacturing equipment are the same. Slide 25

Anisotropic Etching of Silicon: Etchants Etchant (Diluent) Typical Etch Rate Etch Ratio Masking Temp. Composition [µm/min] (100)/(111) Film Etylenediamine Pyrocathecol Water 750 ml 120 gr 100 ml 35: 1 Si. O 2 Si 3 N 4 metals Etylenediamine Pyrocathecol Water 750 ml 120 gr 240 ml 1. 25 35: 1 Si. O 2 Si 3 N 4 metals 115 °C KOH Water + Isopropyl 44 gr 100 ml 85 °C 1. 4 400: 1 Si. O 2 Si 3 N 4 TMAH Water + Isopropyl 220 gr 780 ml 90 °C 1. 0 100: 1 Si. O 2 Si 3 N 4 115 °C 0. 75 Slide 26

Chemistry of Anisotropic Etching phases: ► Transport of reactants to the silicon surface; ► Surface reaction; ► Transport of reaction products away from the surface. Key etch ingredients: ► Oxidisers; ► Oxide etchants; ► Diluents and transport media. Mechanism – 2+ – Si + 2 OH → Si(OH)2 + 4 e – – 4 H 2 O + 4 e → 4 OH + 2 H 2 (gas) 2+ – 2– Si(OH)2 + 4 OH → Si. O 2(OH)2 + 2 H 2 O Overall: – Si + 2 OH + 2 H 2 O → 2– → Si. O 2(OH)2 + 2 H 2 (gas) Slide 27

Anisotropic Etching of (100)-Si Cavity defined by: ► {111} walls – slow-etching planes; ► {100} floor – fast-etching plane. Final shape of cavity depends on: ► Mask geometry; ► Etching time. Shape of cavity: ► Truncated pyramid; ► V-groove; ► Pyramid. Slide 28

Cavity Geometry for (100)-Si Anisotropically etched cavity in (100) silicon with a square masking film opening oriented parallel to the <110> directions. Wb = W 0 – 2·l·cot(54. 7°) Slide 29

Mask Undercutting (a) is a pyramidal pit bounded by the {111} planes. (b) is a type of pit expected from slow undercutting of convex corners. (c) is a type of pit expected from fast undercutting of convex corners. In (d), further etching of (c) produces a cantilever beam suspended over the pit. (e) illustratates the general rule for undercutting assuming a sufficiently long etching time. Slide 30

Anisotropic Etching of (110)-Si Cavity defined by: ► {111} walls – slow-etching planes; ► {110} floor – fastest-etching plane; ► {100} bottom side walls – fast-etching planes; Final shape of cavity depends on: ► Mask geometry; ► Etching time. Cavity shape: ► Rhombic prisms; ► Hexahedric prisms. Slide 31

Cavity Geometry for (110)-Si Slide 32

Outline 1. Background and Motivation 2. The Silicon Crystal 3. Isotropic Wet Etching 4. Anisotropic Wet Etching 5. Selective Etching 6. Convex Corners Slide 33

Methods for Selective Etching Time etching methods: ► Calculate the needed etching time on the basis of the etching rate. – Easy, but inaccurate method, as etching rate varies with the chemical condition of the etchant and geometrical factors limiting the agitation of the etch. Typical accuracy: ± 20 µm. ► Inspect the depth of the etched cavity in appropriate time intervals until desired depth is reached. – Time consuming, but improved accuracy. Uneven etching depth from cavity to cavity due to chemical and geometrical factors is still a problem. Typical accuracy: ± 10 µm. Chemical selective techniques: ► The etching stops when a chemically resistive layer is reached. – Typical accuracy: ± 3 µm. Electrochemical selective techniques: ► The etching stops on reverse biased pn junctions. – Typical accuracy: ± 1 µm. Slide 34

Time-Stopped Etching Example of etching stopped at an arbitrary depth, exhibiting a flat floor. Slide 35

Etching Stopped by {111} Walls Example of etching stopped by the intersecting {111} walls, exhibiting a pyramidal groove. Slide 36

Boron Etch-Stop Technique Chemical selective etching: ► Etch rate depends on boron concentration. ► Etching stops if boron concentration exceeds 5· 1018 cm– 3. Boron stop layer is manufactured: ► By diffusion deposition, implantation or both; ► On the opposite surface of the wafer with respect to the etch cavity. Boron-dependent etch rate of silicon (from Seidel et al. ) Slide 37

Boron Etch-Stop Mechanism Interstitial bonds require more energy to be broken. The electrons supplied by the etchant recombine with the holes in the bulk, rather than participating in the chemical reaction. Interstitial boron atom Silicon atom Substitutional boron atom Slide 38

Boron Etch-Stop Shortcomings Electronics cannot be integrated in the boron stop layer. ► Solution: depositing an epitaxial layer atop the stop layer, with appropriate doping as substrate material for integrated devices. – Controlling the autodoping of the epi-layer is challenging. n-type epitaxial layer p+ type boron stop layer n-type substrate Slide 39

Electrochemical Etch-Stop (ECES) Electrochemical selective etching: ► Etch rate depends on the applied potential. ► Etching stops if the applied potential exceeds a threshold value, called passivation potential. ► Low-doped material, both p- and n-type, can be passivated: – To be used as substrate for integrated components such as piezoresistors. High accuracy, typically ± 1 µm: ► Achieved by using well-controlled implantation and diffusion techniques. KOH and TMAH can be used: – Both avoid the health dangers of EDP. Slide 40

Wafer Holder for ECES A practical way to make a wafer holder to be used for electrochemical selective etching. The principles for industrial manufacturing equipment are the same. Slide 41

Electrochemical Etch-Stop Mechanism Etch-stop achieved by reverse biasing the pn junctions. More in the Bulk Silicon Etching tutorial. . . Slide 42

ECES for Multi. MEMS Electrochemical etch-stop allows 3 different thicknesses: ► Full-wafer thickness (400 µm) For heavy seismic masses; ► Epi-layer thickness (3 µm) For thin membrane, springs; ► N-well thickness (23 µm) For thick membranes, masses, bosses… Slide 43

Etch-Stop on Multi-Level Junctions Very Thin Membrane Thick Membrane (Mass/Well) Thin Membrane Slide 44

Outline 1. Background and Motivation 2. The Silicon Crystal 3. Isotropic Wet Etching 4. Anisotropic Wet Etching 5. Selective Etching 6. Convex Corners Slide 45

Undercutting of Convex Corners High etch-rate of high-index planes: ► Severe undercutting of convex corners; ► Truncated pyramids or V-grooves as final cavities. Mask will be undercut until {111} planes are exposed. Mask {111} Slide 46

Compensation for Convex Corners Etching without corner compensation structure Etching with corner compensation structure Corner compensation of mask is difficult to establish as a repeatable process; highly dependant on etching parameters. Corner Compensation in Silicon (from Gupta et al. ) Slide 47

Corner Compensation Structures Compensation Structure Desired Result A simple approach to convex corner compensation (from Wei Fan et al. ) Slide 48

Designing with Undercut Corners Fabrication of MEMS - MEMS Technology Seminar (from Burhanuddin Yeop Majlis) Slide 49

Thank you for your attention ! Slide 50