Micro/Nanofabrication

1. Micro/Nanofabrication • Micro/nanofabrication techniques are used to manufacture structures in a wide range of dimensions (mm–nm). (what?) • The most common microfabrication techniques: lithography, deposition, and etching… (how?) • Micromachining and MEMS technologies that can be used to fabricate microstructures down to ∼ 1 µm, have attained an adequate level of maturity to allow for a variety of MEMS-based commercial products (pressure sensors, accelerometers, gyroscopes, etc)

2. Micro/Nanofabrication E-beam, high resolution lithography, high cost Self-assembly, nano-imprint lithography

3. Micro/Nanofabrication • Basic microfabrication techniques • lithograhpy • Depositon and Doping • Electroplating • Etching and substrate removal • MEMS Fabrication Techniques • Nanofabrication Techniques

4. Micro/Nanofabrication- Lithography Lithography is the technique used to transfer a computer generated pattern onto a substrate (silicon, glass, GaAs, etc.). This pattern is subsequently used to etch an underlying thin film (oxide, nitride, etc.) for various purposes (doping, etching, etc.). Remove solvent ,improve adhesion Positive negative Fig. 5.1 Lithography process flow( following generation of photomask)

5. Micro/Nanofabrication- Lithography Wafer fabrication Lithography machine structure ( high resolution)

6. Photoresist 0.5–2.5μm ( positive or negative). Soft baked (5–30 min at 60–100 oC) Subsequently, the mask is aligned to the wafer and the photoresist is exposed toa UV source.(why?) Fig. 5.2 Schematic drawing of the photolithographic steps with a positive photoresist (PR)

7. LIGA (in German: LIthographie Glvanoformung Abformung) a high-aspect-ratio micromachining process that relies on X-ray lithography and electroplating with lateral dimensions down to 0.2μm (aspect ratio > 100 : 1). Fig. 5 SEM of assembled LIGA-fabricated nickel structures

8. Micro/Nanofabrication- Lithography LIGA - acceleration sensor onto electronic circuit Ni height 165 µm  Combination of integrated circuits and variety of LIGA materials

9. Micro/Nanofabrication- Lithography • Depending on the separation between the mask and the wafer, three different exposure systems are available: • 1) contact, • 2) proximity, and • 3) projection (most widely used system in microfabrication and can yield superior resolutions compared to contact and proximity methods. ).

10. Micro/Nanofabrication- Lithography Light source and line width:

11. Resolution in projection systems • deep ultraviolet (DUV) light with wavelengths of 248 and 193 nm, which allow minimum feature sizes down to 50 nm. • CD is the minimum feature size • Df is the depth of focus, which restricts the thickness of photoresist and depth of the topography on the wafer

13. Thin Film Deposition and Doping Mechanical structure • Electrical isolation • Electrical connection • Sensing or actuating • Mask for etching and doping • Support or mold during deposition of other materials (sacrificial materials) • Passivation

14. Thin Film Deposition and Doping Fig. 5.4 Schematic representation of a typical oxidation furnace(controlling the conditions to get the desired thickness and achieve the high accuracy)

15. Doping The process of creating an n-type region by diffusionof phosphor from the surface into a p-type substrate. A masking material is previously deposited and patterned on the surface to define the areas to be doped.

16. Chemical Vapor Deposition and Epitaxy • As its name suggests, chemical vapor deposition (CVD) includes all the deposition techniques using the reaction of chemicals in agas phase to form the deposited thin film. • The energy needed for the chemical reaction to occur is usually supplied by maintaining the substrate at elevated temperatures. Other alternative energy sources such as plasma or optical excitation are also used, with the advantage of requiring a lower temperature at the substrate. • The most common CVD processes in microfabrication are LPCVD (low pressure CVD) and PECVD (plasma enhanced CVD).

17. Plasma enhanced CVD (chemical vapor deposition ) RF energy to create highly reactive species in the Parallel-plate plasma reactors. Use of lower temperatures at the substrates (150 to 350 ◦C). The wafers are positioned horizontally on top of the lower electrode, so only one side gets deposited. Typical materials deposited with PECVD include silicon oxide, nitride, and amorphous silicon. Fig. 5.6 Schematic representation of a typical PECVD system

18. Physical Vapor Deposition Fig. 5.7 Schematic representation of an electron-beam deposition system

19. MEMS Fabrication Techniques-Electroplating • Electro plating (or electro deposition) is a process typically used to obtain thick (tens of micrometers) metal structures. • The sample to be electroplated is introduced in a solution containing a reducible form of the ion of the desired metal and is maintained at a negative potential (cathode) relative to a counter electrode (anode). The ions are reduced at the sample surface and the insoluble metal atoms are incorporated into the surface.

20. MEMS Fabrication -Etching and Substrate Removal Fig. 5.10a–d Formation of isolated metal structures by electroplating through a mask: (a) seed layer deposition, (b) photoresist spinning and patterning, (c) electroplating, (d) photoresist and seed layer stripping

21. MEMS Fabrication Techniques-Electroplating Fig. 5.9 Typical cross section evolution of a trench while being filled with sputter deposition

22. MEMS Fabrication Techniques One way to improve the step coverage is by rotating and/or heating the wafers during the deposition. Fig. 5.8 Shadow effects observed in evaporated films. Arrows show the trajectory of the material atoms being deposited

23. Etching and Substrate Removal The anisotropic etchants attack silicon along preferred crystallographic directions. In an isotropic etch, the etchant attacks the material in all directions at the same rate, creating a semicircular profile under the mask, Fig. 5.11a. In ananisotropic etch, the dissolution rate depends on specific directions, and one can obtain straight sidewalls or othernoncircular profiles, Fig. 5.11b.

24. Etching and Substrate Removal Silicon wafers etched with an anisotropic wet etching. Fig. 5.12a,b Anisotropic etch profiles for: (a) (100) and (b) (110) silicon wafers

25. Wet Etching Top view and cross section of a dielectric cantilever beam fabricated using convex corner undercut

26. Dry Etching • Most dry etching techniques are plasma-based. They have several advantages compared with wet etching: • These include smaller undercut (allowing smaller lines to be patterned) and • higher anisotropicity (allowing high-aspect-ratio vertical structures).

27. Dry Etching Simplified representation of etching mechanisms for • ion milling, • (b) high-pressure plasma etching, and • (c) RIE(Reactive ion etching)

28. Drying Etching SEM photograph of a structure fabricated using Drying Etching process: (a) comb-drive actuator, (b) suspended spring, (c) spring support, (d) moving suspended capacitor plate, and (e) fixed capacitor plate.

29. SEM photograph of a micro-accelerometer fabricated using the dissolved wafer process

30. MEMS Fabrication -Assembly and Template Manufacturing Fig. 5.54 Colloidal(胶质的) particle self-assembly onto solid substrates upon drying in vertical position

31. MEMS Fabrication -Assembly and Template Manufacturing Fig. 5.55 Cross-sectional SEM image of a thin planar opal silica template (spheres 855 nm in diameter) assembled directly on a Si wafer

32. HEXSIL (HEXagonal honeycomb poly SILicon) • HEXSIL process flow: • DRIE(deep reactive ion etching of silicon), • sacrificial layer deposition, • (c) structural material deposition and trench filling, • (d) etch structural layer from the surface, • (e) etch sacrificial layer • and pulling out of the structure, • (f) example of a HEXSIL fabricated structure

33. MEMS Fabrication Techniques HEXSIL(HEXagonal honeycomb poly SILicon) Fig. 5.41 SEM micrograph of an angular microactuator fabricated using HEXSIL

34. HARPSS HARPSS (The high aspect ratio combined with poly and single-crystal silicon) • HARPSS process flow. • Nitride deposition and patterning, DRIE etching and oxide deposition, • poly 1 deposition and etch back, oxide patterning and poly 2 deposition and patterning, • (c) DRIE etching, • (d) silicon isotropic etching

35. MEMS Fabrication Techniques The high aspect ratio combined with poly and single-crystal silicon (HARPSS) SEM photograph of a micro-gyroscope fabricated using HARPSS process

36. MEMS/NEMS Devices SEM micrograph of a 3C-SiC nanomechanical beam resonator fabricated by electron-beam lithography and dry etching processes

37. MEMS/NEMS Devices SEM micrograph of a surface-micromachined polysilicon micromotor fabricated using a SiO2 sacrificial layer

38. MEMS/NEMS Devices SEM micrograph of a poly-SiC lateral resonant structure fabricated using a multilayer, micromolding-based micromachining process

39. MEMS/NEMS Devices SEM micrograph of the folded beam truss of a diamond lateral resonator. The diamond film was deposited using a seeding based hot filament CVD process. The micrograph illustrates the challenges currently facing diamond MEMS.