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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)
Micro/Nanofabrication
E-beam, high resolution lithography, high cost Self-assembly, nano-imprint
lithography
Micro/Nanofabrication
• Basic microfabrication techniques lithograhpy Depositon and Doping Electroplating Etching and substrate removal
• MEMS Fabrication Techniques• Nanofabrication Techniques
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.).
Fig. 5.1 Lithography process flow( following generation of photomask)
Positive negative
Remove solvent ,improve adhesion
Micro/Nanofabrication- Lithography
Lithography machine structure ( high resolution)Wafer fabrication
Fig. 5.2 Schematic drawing of the photolithographic stepswith a positive photoresist (PR)
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?)
LIGA
Fig. 5 SEM of assembled LIGA-fabricated nickel structures
(in German: LIthographie GlvanoformungAbformung)
a high-aspect-ratio micromachining processthat relies on X-ray lithography and electroplating
with lateral dimensions down to 0.2μm (aspectratio > 100 : 1).
LIGA - acceleration sensor onto electronic circuit
Ni height 165 µm
Combination of integrated circuits
and variety of LIGA materials
0,0 0,4 0,8 1,2 1,6 2,0
1 g
- 1,42 g
+ 1,42 g
5 Hz
ampl
itude
[g]
time [s]
Micro/Nanofabrication- Lithography
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. ).
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
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
NAKCD
1
22 NAKDF
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
Fig. 5.4 Schematic representation of a typical oxidation furnace(controlling the conditions to get the desired thickness and achieve the high accuracy)
Thin Film Deposition and Doping
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.
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).
Plasma enhanced CVD (chemical vapor deposition )
Fig. 5.6 Schematic representation of a typical PECVD system
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.
Physical Vapor Deposition
Fig. 5.7 Schematic representation of an electron-beam deposition system
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.
MEMS Fabrication -Etching and Substrate Removal
Fig. 5.10a–d Formation of isolated metal structures byelectroplating through a mask: (a) seed layer deposition,(b) photoresist spinning and patterning, (c) electroplating,(d) photoresist and seed layer stripping
Fig. 5.9 Typical cross section evolution of a trench while being filled with sputter deposition
MEMS Fabrication Techniques-Electroplating
MEMS Fabrication Techniques
Fig. 5.8 Shadow effects observed in evaporated films. Arrows show the trajectory of the material atoms being deposited
One way to improve the step coverage is by rotating and/or heating the wafers during the deposition.
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.
Etching and Substrate Removal
Fig. 5.12a,b Anisotropic etch profiles for: (a) (100) and (b) (110) silicon wafers
Silicon wafers etched with an anisotropic wet etching.
Wet Etching
Top view and cross section of a dielectric cantileverbeam fabricated using convex corner undercut
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).
(a) ion milling, (b) high-pressure plasma
etching, and(c) RIE(Reactive ion etching)
Dry Etching
Simplified representation of etching mechanisms for
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.
SEM photograph of a micro-accelerometer fabricatedusing the dissolved wafer process
MEMS Fabrication -Assembly and Template Manufacturing
Fig. 5.54 Colloidal(胶质的 ) particle self-assembly onto solid substrates upon drying in vertical position
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
HEXSIL (HEXagonal honeycomb poly SILicon)
HEXSIL process flow: (a)DRIE(deep reactive ion etching
of silicon), (b)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
MEMS Fabrication Techniques
Fig. 5.41 SEM micrograph of an angular microactuatorfabricated using HEXSIL
HEXSIL (HEXagonal honeycombpoly SILicon)
HARPSS
HARPSS process flow. (a)Nitride deposition and patterning, DRIE etching and oxide
deposition, (b) poly 1 deposition and etch back, oxide patterning and poly 2
deposition and patterning,(c) DRIE etching, (d) silicon isotropic etching
HARPSS (The high aspect ratio combined with poly and single-crystal silicon)
MEMS Fabrication Techniques
SEM photograph of a micro-gyroscope fabricatedusing HARPSS process
The high aspect ratio combined with poly and single-crystal silicon (HARPSS)
SEM micrograph of a 3C-SiC nanomechanical beam resonator fabricated by electron-beam lithography and dry etching processes
MEMS/NEMS Devices
SEM micrograph of a surface-micromachined polysilicon micromotor fabricated using a SiO2 sacrificial layer
MEMS/NEMS Devices
SEM micrograph of a poly-SiC lateral resonant structure fabricated using a multilayer, micromolding-based micromachining process
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.
MEMS/NEMS Devices
