04-05-2012, 01:01 PM
Silicon technology for optical MEMS
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Introduction:
Components fabricated with the emerging technologies of micro-electro-mechanical systems (MEMS) are being incorporated in an increasing number of sensor and actuator applications. There is rapid progress in optical systems for telecommunications to meet the needs for increased bandwidth, for optical networks with terabit capacities per fiber link, and for local area networks. Some applications require precise features for optical alignment while others involve the precise movement of small optical parts to achieve advanced functionality. In the MEMS approach accurate, low-loss, optical connections are made between different guided wave optical components including fibers, waveguides and lasers.
Mechanisms to allow motion and structures to provide electrical actuation are combined in micro-opto- electro-mechanical systems (MOEMS). Many MOEMS components process light in miniature free-space optical beams, and the performance scaling laws are often different from those for guided wave optical systems. This paper surveys MEMS with an emphasis on the outstanding challenges for Physicists and Engineers. The silicon and other materials and technologies used to create MEMS devices are discussed in detail, as are example applications in packaging such as fixed and demountable connectors and methods for hybrid integration.
Fabrication technologies for MEMS
The sets cross-sectional diagrams shown in Figure 1 illustrate the two main processes used for micromachining. The first established process is silicon bulk micromachining using alkaline solutions, which have a much smaller, etch rate of the crystallographic plane in Si compared with other planes. In this process, a mask is first patterned in an etch-resistant surface layer such as thermally grown SiO2 or deposited Si3N4, as shown in Row 1 of Fig.1.
The silicon is then etched. Since the planes etch the slowest, V-shaped grooves are produced by etching standard oriented substrates to termination. V-grooves can be used for the precise positioning of optic fibres as shown in Figure 2. The features may be hundreds of micrometers deep and precisely defined by the initial planar lithographic process. Even grooves with diamond shape cross sections are made, although the range of possible shapes is restricted by the characteristics of the etch process. Encapsulated structures are made by fusion bonding of glass to bulk micromachined wafers, and multilayer structures are built up by bonding several silicon wafers together. Suspended structures can also be made by undercutting of etch-resistant features.
Polysilicon surface micromachining exploits differences between deposited polysilicon and silica layers to form three-dimensional features as seen in Fig.1 row 2. The process is adapted from conventional silicon integrated circuit technology, and the chemical vapour deposited polysilicon mechanical layer is typically 2 µm thick. The underlying silica sacrificial layer is later wet etched away to produce a free-standing polysilicon MEMS layer. The polysilicon layer can be incorporated into a wide variety of sensors and actuators such as electrostatic comb drives.
By careful control of the CVD process conditions, the stress in the polysilicon can be made reproducibly low. However, the thickness of the deposited layers is limited to a few tens of micrometers by cost considerations, and by the mechanical and electrical properties of deposited polysilicon which are inferior to those of single crystal Si. The cycle of deposition, patterning and etching of each material can be repeated several times to build up multilayer structures, and feature shapes can be arbitrary. Foundries operate semi-standard processes with several levels of polysilicon.
more advanced micromachining processes which are under development is the fabricated structures must be thicker than is achievable using polysilicon, an alternative surface micromachining process is to use lithographic exposure of thick photoresist, followed by electroplating, to form the mechanical parts The original Lithographie, Galvanoformung, Abformung (LIGA) process uses synchrotron radiation to expose the resist. The short X-ray wavelength allows deep (up to 1 mm) resist layers to be exposed without significant diffraction effects, and high aspect ratio structures are made. The released metal layer can be used in a variety of MEMS applications including packaging.
Cheaper alternatives under development use ultraviolet mask aligners to exposure special resist and achieve features several hundred micrometers thick with aspect ratios of order 15. Parts are usually electroplated in nickel and, after removal of the resist, they can be replicated in other materials. Only low temperatures are needed, so LIGA can be used as post processes to add microstructure integrated optical components. Silica-on-silicon has several major advantages over the earlier technologies such as ion-exchanged glass, Ti : LiNbO3, GaAlAs and InGaAsP. Because of this, silica-on-silicon components are likely to find widespread application in low- cost systems such as fibre-to-the-home.
A typical device comprises a single-crystal Si substrate with fibre alignment grooves, a thick buffer layer of silica to isolate the guided mode from the substrate, channel guide cores formed from doped silica, and a thick over-cladding to bury the cores. When combined with MEMS fabrication, devices may be constructed with a large number of ports to CMOS. A recently developed process for forming suspended single crystal silicon structures is based upon the use of bonded silicon-on-insulator (SOI) starting material, which is available as a by-product of the high-performance silicon integrated circuit industry.
The starting material is a silicon wafer that is thermally bonded to an oxidised silicon substrate. The bonded wafer is polished back to the desired thickness, usually in the range 5 µm - 200 µm. The bonded layer is then structured by deep reactive ion etching (DRIE) using an inductively coupled plasma etcher and specific gas chemistry to obtain high etch anisotropy. Movable structures may be made by removal of the buried oxide. There are more integrated optical components.
Silica-on-silicon has several major advantages over the earlier technologies such as ion-exchanged glass, Ti : LiNbO3, GaAlAs and InGaAsP. Because of this, silica-on-silicon components are likely to find widespread application in low- cost systems such as fibre-to-the-home. A typical device comprises a single-crystal Si substrate with fibre alignment grooves, a thick buffer layer of silica to isolate the guided mode from the substrate, channel guide cores formed from doped silica, and a thick over-cladding to bury the cores. When combined with MEMS fabrication, devices may be constructed with a large number of ports.
All of the above processes involve surface patterning and the resulting structures are quasi three-dimensional. Fully 3-D microstructures are made in polysilicon and in SOI by rotating surface micromachined parts out of the wafer plane, and latching them into position. The parts are held by micromachined staple hinges. Assembly is usually manual, but mass-parallel powered assembly has been demonstrated by differential shrinkage of a polymer due to surface tension forces. Surface micromachined engines are also used to push parts out of plane.
MEMS packaging in optoelectronics
It is well known that passive alignment features for connecting single-mode optical fibres can be made by anisotropically etching single crystal Si. As shown in Fig.1, etching of (100) oriented Si through an appropriate mask is used to make V-shaped grooves which act as kinematic mounts for optic fibres. When two fibres are aligned in a groove and then butt- coupled together, all degrees of freedom except uniaxial motion are fixed. The assembly may then be epoxied. Alternatively, as shown in Fig.2, flexible Si3N4 cantilevers may hold a fibre. Single crystal silicon cantilevers have also been demonstrated [5]. Demountable connectors for ribbon optical fibres requiring the simultaneous connection of many cores are made using sets of etched V-grooves in Si substrates. On the male half of the connector, two large grooves are used to locate a pair of precision steel pins, which mate with corresponding grooves on the female half. The joint is made by aligning the pins, and sliding the connector halves together. V-grooves can also be used to construct simple subsystems known as opto-hybrids such as the connection between an optical fibre and a photodiode. Systems with detectors, lasers and high speed interconnects are used as transceivers.
[attachment=21259]
Introduction:
Components fabricated with the emerging technologies of micro-electro-mechanical systems (MEMS) are being incorporated in an increasing number of sensor and actuator applications. There is rapid progress in optical systems for telecommunications to meet the needs for increased bandwidth, for optical networks with terabit capacities per fiber link, and for local area networks. Some applications require precise features for optical alignment while others involve the precise movement of small optical parts to achieve advanced functionality. In the MEMS approach accurate, low-loss, optical connections are made between different guided wave optical components including fibers, waveguides and lasers.
Mechanisms to allow motion and structures to provide electrical actuation are combined in micro-opto- electro-mechanical systems (MOEMS). Many MOEMS components process light in miniature free-space optical beams, and the performance scaling laws are often different from those for guided wave optical systems. This paper surveys MEMS with an emphasis on the outstanding challenges for Physicists and Engineers. The silicon and other materials and technologies used to create MEMS devices are discussed in detail, as are example applications in packaging such as fixed and demountable connectors and methods for hybrid integration.
Fabrication technologies for MEMS
The sets cross-sectional diagrams shown in Figure 1 illustrate the two main processes used for micromachining. The first established process is silicon bulk micromachining using alkaline solutions, which have a much smaller, etch rate of the crystallographic plane in Si compared with other planes. In this process, a mask is first patterned in an etch-resistant surface layer such as thermally grown SiO2 or deposited Si3N4, as shown in Row 1 of Fig.1.
The silicon is then etched. Since the planes etch the slowest, V-shaped grooves are produced by etching standard oriented substrates to termination. V-grooves can be used for the precise positioning of optic fibres as shown in Figure 2. The features may be hundreds of micrometers deep and precisely defined by the initial planar lithographic process. Even grooves with diamond shape cross sections are made, although the range of possible shapes is restricted by the characteristics of the etch process. Encapsulated structures are made by fusion bonding of glass to bulk micromachined wafers, and multilayer structures are built up by bonding several silicon wafers together. Suspended structures can also be made by undercutting of etch-resistant features.
Polysilicon surface micromachining exploits differences between deposited polysilicon and silica layers to form three-dimensional features as seen in Fig.1 row 2. The process is adapted from conventional silicon integrated circuit technology, and the chemical vapour deposited polysilicon mechanical layer is typically 2 µm thick. The underlying silica sacrificial layer is later wet etched away to produce a free-standing polysilicon MEMS layer. The polysilicon layer can be incorporated into a wide variety of sensors and actuators such as electrostatic comb drives.
By careful control of the CVD process conditions, the stress in the polysilicon can be made reproducibly low. However, the thickness of the deposited layers is limited to a few tens of micrometers by cost considerations, and by the mechanical and electrical properties of deposited polysilicon which are inferior to those of single crystal Si. The cycle of deposition, patterning and etching of each material can be repeated several times to build up multilayer structures, and feature shapes can be arbitrary. Foundries operate semi-standard processes with several levels of polysilicon.
more advanced micromachining processes which are under development is the fabricated structures must be thicker than is achievable using polysilicon, an alternative surface micromachining process is to use lithographic exposure of thick photoresist, followed by electroplating, to form the mechanical parts The original Lithographie, Galvanoformung, Abformung (LIGA) process uses synchrotron radiation to expose the resist. The short X-ray wavelength allows deep (up to 1 mm) resist layers to be exposed without significant diffraction effects, and high aspect ratio structures are made. The released metal layer can be used in a variety of MEMS applications including packaging.
Cheaper alternatives under development use ultraviolet mask aligners to exposure special resist and achieve features several hundred micrometers thick with aspect ratios of order 15. Parts are usually electroplated in nickel and, after removal of the resist, they can be replicated in other materials. Only low temperatures are needed, so LIGA can be used as post processes to add microstructure integrated optical components. Silica-on-silicon has several major advantages over the earlier technologies such as ion-exchanged glass, Ti : LiNbO3, GaAlAs and InGaAsP. Because of this, silica-on-silicon components are likely to find widespread application in low- cost systems such as fibre-to-the-home.
A typical device comprises a single-crystal Si substrate with fibre alignment grooves, a thick buffer layer of silica to isolate the guided mode from the substrate, channel guide cores formed from doped silica, and a thick over-cladding to bury the cores. When combined with MEMS fabrication, devices may be constructed with a large number of ports to CMOS. A recently developed process for forming suspended single crystal silicon structures is based upon the use of bonded silicon-on-insulator (SOI) starting material, which is available as a by-product of the high-performance silicon integrated circuit industry.
The starting material is a silicon wafer that is thermally bonded to an oxidised silicon substrate. The bonded wafer is polished back to the desired thickness, usually in the range 5 µm - 200 µm. The bonded layer is then structured by deep reactive ion etching (DRIE) using an inductively coupled plasma etcher and specific gas chemistry to obtain high etch anisotropy. Movable structures may be made by removal of the buried oxide. There are more integrated optical components.
Silica-on-silicon has several major advantages over the earlier technologies such as ion-exchanged glass, Ti : LiNbO3, GaAlAs and InGaAsP. Because of this, silica-on-silicon components are likely to find widespread application in low- cost systems such as fibre-to-the-home. A typical device comprises a single-crystal Si substrate with fibre alignment grooves, a thick buffer layer of silica to isolate the guided mode from the substrate, channel guide cores formed from doped silica, and a thick over-cladding to bury the cores. When combined with MEMS fabrication, devices may be constructed with a large number of ports.
All of the above processes involve surface patterning and the resulting structures are quasi three-dimensional. Fully 3-D microstructures are made in polysilicon and in SOI by rotating surface micromachined parts out of the wafer plane, and latching them into position. The parts are held by micromachined staple hinges. Assembly is usually manual, but mass-parallel powered assembly has been demonstrated by differential shrinkage of a polymer due to surface tension forces. Surface micromachined engines are also used to push parts out of plane.
MEMS packaging in optoelectronics
It is well known that passive alignment features for connecting single-mode optical fibres can be made by anisotropically etching single crystal Si. As shown in Fig.1, etching of (100) oriented Si through an appropriate mask is used to make V-shaped grooves which act as kinematic mounts for optic fibres. When two fibres are aligned in a groove and then butt- coupled together, all degrees of freedom except uniaxial motion are fixed. The assembly may then be epoxied. Alternatively, as shown in Fig.2, flexible Si3N4 cantilevers may hold a fibre. Single crystal silicon cantilevers have also been demonstrated [5]. Demountable connectors for ribbon optical fibres requiring the simultaneous connection of many cores are made using sets of etched V-grooves in Si substrates. On the male half of the connector, two large grooves are used to locate a pair of precision steel pins, which mate with corresponding grooves on the female half. The joint is made by aligning the pins, and sliding the connector halves together. V-grooves can also be used to construct simple subsystems known as opto-hybrids such as the connection between an optical fibre and a photodiode. Systems with detectors, lasers and high speed interconnects are used as transceivers.