MEMS Class 5 Micromachining Technologies Mohammad Kilani

MEMS

Class 5

Micromachining Technologies

Mohammad Kilani

Photolithography Steps, Home Work #1

Surface Cleaning

Adhesion Promoter

Resist Application

Prebake (Soft Bake)

Exposure

Developing

Post Bake

Etching/deposition/doping

Removal of particulates, organic films, adsorbed metal ions

Sometimes used to achieve better adhesion of the resist

Thickness varies with rotational speed of the spinner andviscosity of the resist

70 °C – 90 ºC, necessary to drive solvent out of the resist

Contact/proximity printing, projection printing

Negative resist: solvent; positive resist: alkaline developer

90 ºC – 140 ºC, necessary to increase both adherence and etch resistance

Resist Removal Stripping solutions, plasma etching in oxygene atmosphere

Surface Cleaning

• Why is it important?

• How is it done?

• Physics/chemistry?

• Parameters affecting it?

Report + 10 min. PowerPoint presentation

Micromachining Technologies

• Lithographic Technologies:• Bulk Micromachining• Surface Micromachining• LIGA

• Nonlithographic Technologies:• Ultraprecision Mechanical Machining• Laser Machining• Electrodischarge Machining• Screen Printing• Microcontact Printing• Nanoimprint Lithography• Hot Embossing• Ultrasonic Machining

Bulk Micromachining

• Realize micromechanical structures within the bulk of a single-crystal silicon wafer by selectively removing (‘etching’) wafer material.

• Significant amounts of silicon are removed from a substrate to form membranes and a variety of trenches, holes, or other structures.

• It emerged in the early 1960s and has been used since then in the fabrication of different microstructures.

• It is utilized in the manufacturing of the majority of commercial devices – almost all pressure sensors and silicon valves and 90% of silicon accelerometers.

• The microstructures fabricated may cover the thickness range from submicron to full wafer thickness (200 to 500 µm) and the lateral size range from submicron to the lateral dimensions of a full wafer.

• Can be divided into wet etching and dry etching of silicon according to the phase of etchants. Wet etching relies on aqueous chemicals, while dry etching relies on vapor and plasma etchants.

• Wafer-bonding is often necessary for the assembled MEMS devices.

Bulk Micromachining – Wet Etching

• Wet etching occurs by dipping substrate into an etching bath or spraying it with etchants which may be acid or alkaline.

• Can either be isotropic or anisotropic depending on the structure of the materials or the etchants used. If the material is amorphous or polycrystalline, wet etching is always isotropic etching. Single-crystal silicon can be anisotropically etched.

• During isotropic etching (etchants used are acid solution), resist is always undercut, meaning the deep etching is not practical for MEMS.

• Anisotropic wet etchants such as solutions of potassium hydroxide (KOH), ethylenediamine pyrocatechol (EDP), tetramethylammonium hydroxide (TMAH) and hydrazine-water are used. These etchants have different etch rates in different crystal orientations of the silicon.

• The etch process can be made selective by the use of dopants (heavily doped regions etch slowly), or may even be halted electrochemically (e.g. etching stops upon encountering a region of different polarity in a biased p–n junction).

• By combining anisotropic etching with boron implantation (P+ etch-stop), and electrochemical etch-stop technique, varied silicon microstructures can be bulk machined

Bulk Micromachining – Dry Etching

• Dry etching occurs through chemical or physical interaction between the ions in the gas and the atoms of the substrate.\

• Nonplasma, isotropic dry etching can be possible using xenon difluoride or a mixture of interhalogen gases and provides very high selectivity for aluminum, silicon dioxide, silicon nitride, photoresist, etc.

• The most common dry etching of bulk silicon are plasma etching and reactive ion etching (RIE) etching, where the external energy in the form of RF power drives chemical reactions in low-pressure reaction chambers. A wide variety of chlorofluorocarbon gases, sulfur hexafluoride, bromine compounds and oxygen are commonly used as reactants.

• The anisotropic dry etching processes are widely used in MEMS because of the geometry flexibility and less chemical contamination than in wet etching.

• Arbitrarily oriented features can be etched deep into silicon using anisotropic dry etching. Very deep silicon microstructures can be obtained by the deep RIE (DRIE) dry etching

Bulk Micromachining Example – Vibration Sensor

Vibration Sensor: Masks Layout

Vibration Sensor: Masks Layout

Diffusion InsulationSi Over

Si Under Metal


Cleaning in Acetone Cleaning in H2O2/H2SO4

Deposition of PECVD SiO2

Patterning of SiO2 Photolithography through Diffusion Mask

Diffusion Mask


Etching of SiO2

Stripping of Resist

Deposition of Si3N4

Deposition of SiO2

Patterning of SiO2 and Si3N4

Photolithography through Si Under mask

Si Under mask


Etching of Si bulk in KOHPatterning of SiO2 and Si3N4

SiO2 / Si3N4 mask 001 Silicon 111 Silicon Silicon membrane


SiO2 deposition, etching of Si3N4

Diffusion doping of phosphorous, etching of SiO2 mask.

Deposition and patterning of SiO2 insulation layer.

Insulation mask


Deposition and structuring of conduction layer

Metal mask


After diffusion doping of piezoresistor

After sputtter desposition of Au (metalization)


Deposition and structuring of SiO2 using Silicon Over mask

Silicon Over Mask

Through etching of silicon to release tongue


Silicon Oxide mask

Free Silicon Paddle

Before etching of silicon

After through etching of silicon to release paddle

Surface Micromachining

• Surface micromachining does not shape the bulk silicon but builds structures on the surface of the silicon by depositing thin films of ‘sacrificial layers’ and ‘structural layers’ and by removing eventually the sacrificial layers to release the mechanical structures.

• The dimensions of surface micromachined structures can be several orders of magnitude smaller than bulk micromachined structures.

• The prime advantages of surface-micromachined structures is their self assembly and easy integration with IC components.

• As miniaturization in immensely increased by surface micromachining, the small mass structure involved may be insufficient for a number of mechanical sensing and actuation applications.


• Silicon microstructures fabricated by surface micromachining are usually planar structures (or are two dimensional). Other techniques involving the use of thin-film structural materials released by the removal of an underlying sacrificial layer have helped to extend conventional surface micromachining into the third dimension.

• By connecting polysilicon plates to the substrate and to each other with hinges, 3D micromechanical structures can with rotational freedom can be realized.


• Surface micromachining requires a compatible set of structural materials, sacrificial materials and chemical etchants.

• The structural materials must possess satisfactory mechanical properties; e.g. high yield and fracture stresses, minimal creep and fatigue and good wear resistance.

• The sacrificial materials must have good mechanical properties to avoid device failure during fabrication. These properties include good adhesion and low residual stresses in order to eliminate device failure by delamination and/or cracking.

• The etchants to remove the sacrificial materials must have excellent etch selectivity and they must be able to etch off the sacrificial materials without affecting the structural ones. In addition the etchants must have proper viscosity and surface tension characteristics.

Surface Micromachining – Compatible Sets

Polysilicon-Silicon dioxide-HF:LPCVD polysilicon as the structural material and LPCVD oxide as the sacrificial material. The oxide is readily dissolved in HF solution without the polysilicon being affected. Together with this material system, silicon nitride is often used for electrical insulation. This is the common IC compatible materials used in surface micromachining e,g. MUMPS and SUMMiT.

Polyimide-aluminum- Acid-based etchants; Polyimide is the structural material and aluminum is the sacrificial material. Acid-based etchants are used to dissolve the aluminum sacrificial layer.

Silicon nitride-polysilicon- KOH and EDP; Silicon nitride is used as the structural material, whereas polysilicon is the sacrificial material. For this material system, silicon anisotropic etchants such as KOH and EDP are used to dissolve polysilicon.

Tungsten-silicon dioxide- HF:CVD deposited tungsten is used as the structural material with oxide as the sacrificial material. HF solution is used to remove the sacrificial oxide.


Single crystal wafer LPCVD sacrificial oxide Spin on Photoresist Expose to UV Develop PR

Etch unprotected oxide Strip off PR PECVD conformal poly Spin on PR

Expose to UV

Develop PR Etch unprotected Poly

Strip off PR Etch Sacrificial oxide

Multilevel Surface Micromachining

Thermal SiO2, 0.63 micronsSilicon Nitride, 0.8 microns

P0, 0.3 microns

S1, 2.0 microns

P1, 1.0 micronsS2, 0.3 microns

Substrate6-inch wafer, <100> n-type

P2, 1.5 microns

S3, 2.0 microns

P3, 2.25 microns

S4, 2.0 microns

P4, 2.25 microns

LPCVD

LPCVD

PECVD

PECVD

Dimple 1 gap0.5 microns



LPCVD

LIGA

• LIGA is a German acronym for Lithographie, Galvanoformung, Abformung (lithography, galvanoforming, moulding). It was developed in Germany in the early 1980s using X-ray lithography for mask exposure to form the metallic parts and moulding to produce microparts with plastic, metal, ceramics, or their combinations

• Used to produce complex microstructures that are thick and three-dimensional achieve high-aspect-ratio (height-to-width) and 3D devices.

• Microstructures’ height can be up to hundreds of microns to millimeter scale, while the lateral resolution is kept at the submicron scale.

LIGA

• Electroplated MEMS structures can take the shape of the underlying substrate and a photoresist mold.

• First, a conducting seed layer (e.g., of gold or nickel) is deposited on the substrate.

• 5- to 100-µm thick resist is then deposited and patterned using optical or x-ray lithography.

• X-ray lithography is used to define very high aspect ratio features (>100) in very thick (up to 1,000 µm) poly(methylmethacrylate) (PMMA), the material on which Plexiglas® is based.

• The desired metal is then plated. Finally, the resist and possibly the seed layer outside the plated areas are stripped off.

LIGA

• Various materials can be incorporated into the LIGA process, allowing electric, magnetic, piezoelectric, optic and insulating properties in sensors and actuators with a high-aspect ratio, which are not possible to make with the silicon-based processes.

• By combining the sacrificial layer technique and LIGA process, advanced MEMS with moveable microstructures can be built.

• Disadvantage is high production cost due to the fact that it is not easy to access X-ray sources limits the application of LIGA.

• Another disadvantage is that structures are not truly three-dimensional, because the third dimension is always in a straight feature.

Multilevel Surface Micromachining – Gear Example

Poly 0

Sac Ox 1 Cut Pin Joint Cut

Sac Ox 2 Cut Poly 2

Poly 2 Cut (All layers)

Dimple 1 Cut

Thermal SiO2, 0.63 micronsSilicon Nitride, 0.8 microns

P0, 0.3 microns

S1, 2.0 microns

P1, 1.0 micronsS2, 0.3 microns

Substrate6-inch wafer, <100> n-type

P2, 1.5 microns

S3, 2.0 microns

P3, 2.25 microns

S4, 2.0 microns

P4, 2.25 microns

LPCVD

LPCVD

PECVD

PECVD




LPCVD


Section through Gear

SI Substrate UNIFORM DEPOSITION



Grow Thermal Oxide CONFORMAL DEPOSITION



Nitride Deposition LPCVD



Nitride Etch DRY ETCH



Thermal Oxide Etch DRY ETCH



Poly0 Deposition LPCVD



Poly0 Etch Dry Etch

Poly 0



Sacrificial Oxide 1 Deposition LPCVD



Dimple 1 etch DRY ETCH (Timed)

Dimple 1 Cut



Sac Ox 1 etch DRY ETCH

Sac Ox 1 Cut



Poly 1 Deposition LPCVD



Pinjoint cut etch DRY ETCH

Pinjoint Cut



Pinjoint undercut etch part 1 DRY ETCH

Pinjoint Cut



Pinjoint undercut etch part 2 WET ETCH (ISOTROPIC)

Pinjoint Cut



Sac Ox2 Deposition LPCVD



Sac Ox2 Etch Dry Etch

SacOx2 Mask



Poly 2 Deposition LPCVD



Poly 2 Etch DRY Etch

SacOx2 Mask



Release RELEASE ETCH

Homework # 2

Consider Photochemical Fabrication Processes available in Jordan, and which can be used to demonstrate MEMS fabrication processes. Examples:

1. B&W Photography2. Zincograph3. PCB fabrication4. Glass Etching

Describe the following1. The process, photos etc.2. The physics and chemistry3. Equipment cost4. Possibility of implementing in a MEMS

lab

LIGA

• LIGA is a German acronym for Lithographie, Galvanoformung, Abformung (lithography, galvanoforming, moulding). It was developed in Germany in the early 1980s using X-ray lithography for mask exposure to form the metallic parts and moulding to produce microparts with plastic, metal, ceramics, or their combinations.

• Used to produce complex microstructures that are thick and three-dimensional achieve high-aspect-ratio (height-to-width) and 3D devices.

• Microstructures’ height can be up to hundreds of microns to millimeter scale, while the lateral resolution is kept at the submicron scale.

Electroplating

• Electroplating (galvanoforming) is the process of using electrical current to coat an electrically conductive object with a relatively thin layer of metal.

• The process used in electroplating is called electrodeposition. The part to be plated is the cathode of the circuit. In one technique, the anode is made of the metal to be plated on the part.

• Both components are immersed in a solution called an "Electrolyte" containing one or more dissolved metal salts as well as other ions that permit the flow of electricity. A rectifier supplies a direct current to the cathode causing the metal ions in the electrolyte solution to lose their charge and plate out on the cathode. As the electrical current flows through the circuit, the anode slowly dissolves and replenishes the ions in the bath.

http://electrochem.cwru.edu/ed/encycl/fig/e01/e01-f01b.gif

LIGA

• Electroplated MEMS structures can take the shape of the underlying substrate and a photoresist mold.

• First, a conducting seed layer (e.g., of gold or nickel) is deposited on the substrate.

• 5- to 100-µm thick resist is then deposited and patterned using optical or x-ray lithography.

• X-ray lithography is used to define very high aspect ratio features (>100) in very thick (up to 1,000 µm) polymethylmethacrylate (PMMA), the material on which Plexiglas® is based

LIGA

• The desired metal is then plated. Finally, the resist and possibly the seed layer outside the plated areas are stripped off.

Electroform Planarize

Remove PMMA Release

LIGA

• Various materials can be incorporated into the LIGA process, allowing electric, magnetic, piezoelectric, optic and insulating properties in sensors and actuators with a high-aspect ratio, which are not possible to make with the silicon-based processes.

• By combining the sacrificial layer technique and LIGA process, advanced MEMS with moveable microstructures can be built.

• Disadvantage is high production cost due to the fact that it is not easy to access X-ray sources limits the application of LIGA.

• Another disadvantage is that structures need to be manually assembled, and they are not truly three-dimensional, because the third dimension is always in a straight feature.

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MEMS Class 5 Micromachining Technologies Mohammad Kilani