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

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.). Remove solvent ,improve adhesion Positive negative Fig. 5.1 Lithography process flow( following generation of photomask)

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

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)

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

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

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

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

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)

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). Chemical vapor deposition 化学汽相淀 积, 蒸镀

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. 5.6. 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

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. 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.

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

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

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

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

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

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).

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

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 fabricated using the dissolved wafer process

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. 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: 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

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

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

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

MEMS/NEMS Devices 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 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.