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)
Starting with some of the most common microfabrication techniques (lithography, deposition, and etching), we present an array of micromachining and MEMS technologies that can be used to fabricate microstructures down to ∼ 1 µm. These techniques have attained an adequate level of maturity to allow for a variety of MEMS-based commercial products (pressure sensors, accelerometers, gyroscopes, etc.). More recently, nanometer-sized structures have attracted an enormous amount of interest. This is mainly due to their unique electrical, magnetic, optical, thermal, and mechanical properties. These could lead to a variety of electronic, photonic, and sensing devices with a superior performance compared to their macro counterparts. Subsequent to our discussion on MEMS and micromachining, we present several important nanofabrication

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)
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 to a UV source.(why?)
Fig. 5.2 Schematic drawing of the photolithographic steps
with a positive photoresist (PR)

7. LIGA Fig. 5 SEM of assembled LIGA-fabricated nickel structures
(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. LIGA - acceleration sensor onto
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:
High pressure mercury lamp (436 nm g-line and 365 nm i-line).
Above 0.25μm
Deep UV sources such as excimer lasers (248 nm KrF and 193 nm ArF)
Between 0.25 and 0.13μm
e-beam and X-ray, extreme UV (EUV) with a wavelength of 10–14 nm
Below 0.13μm

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 diffusion of 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 a gas 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).
Chemical vapor deposition 化学汽相淀 积, 蒸镀

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.
The PECVD process is performed in plasma systems such as the one represented in Fig The use of RF energy to create highly reactive species in the plasma allows for the use of lower temperatures at the substrates (150 to 350 ◦C). Parallel-plate plasma reactors normally used in microfabrication can only process a limited number of wafers per batch. 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. RF energy to create highly reactive species in the plasma
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.
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. As an example, copper electro deposition is frequently done in copper sulfide-based solutions. The reaction taking place at the surface is Cu2++2e-→Cu(s). Recommended current densities for electro deposition processes are on the order of 5 to 100mA/cm2.

20. MEMS Fabrication -Etching and Substrate Removal
In micro/nanofabrication, in addition to thin film etching, very often the substrate (silicon, glass, GaAs, etc.) also needs to be removed in order to create various mechanical micro-/nanostructures (beams, plates, etc.). Two important figures of merit for any etching process are selectivity and directionality. Selectivity is the degree to which the etchant can differentiate between the masking layer and the layer to be etched. Directionality has to do with the etch profile under the mask. In an isotropic etch, the etchant attacks the material in all directions at the same rate, creating a semicircular profile under the mask,
Fig. 5.10a–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.
As will be explained in subsequent sections, some microfabrication techniques utilize these effects to pattern the deposited layer. 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
Reactive ion 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
The three most important silicon etchants in this category are potassium hydroxide (KOH), ethylene diamine pyrochatechol (EDP), and tetramethyl ammonium hydroxide (TMAH).
Fig Colloidal(胶质的) particle self-assembly onto solid substrates upon drying in vertical position

31. MEMS Fabrication -Assembly and Template Manufacturing
Fig 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 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
Fig. 7.6
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.