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8 October 2019 Review of microshutters for switchable glass
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1.

Introduction

Most species on Earth require access to sunlight for their physical and mental health. In ancient Greece and China, dwellings were oriented to take advantage of the sun.1 Even after much technological advancements, houses are still not constructed to save energy using the sun, even in view of climate changes. Aeschylus, a playwright of ancient Greece, would surely consider us to be barbarians based on our houses' orientations.1 New buildings incorporate more and more windows but too often with curtains or unaesthetic blinds. “Switchable glass,” also referred to as smart glass or smart windows, utilizes light-blocking elements directly embedded into or onto the glass. This technology could transform the way we interact with exterior and daylighting. Windows could be more energy-efficient than regular walls, by adjusting the solar heat gain when needed. Moreover advanced facades could provide better access to daylighting, a significant well-being improvement for workplaces and homes. Moreover, public health could benefit from these advancements; for example, the positive effect of daylighting through glass on microbial control inside of buildings was recently demonstrated.2

Smart windows and switchable glass technologies (dynamic glazing) allow control of light transmission. Switchable glass gives access not only to a better human interaction with exterior life but also to an optimized use of sun energy according to the seasons, leading to significant building as well as vehicle energy savings.310

Switchable glass technologies can be classified as either passive or active. Microshutters are considered active since their state (open or closed) can be remotely controlled based on users’ requests. Currently, the most popular types of active switchable glasses are based on electrochromism (EC), suspended particle devices, or liquid crystal devices (LCDs).1016 Electrochromic switchable glass was considered very promising from 2000 to 2010, but after several decades of research-development (R-D), thousands of patents, dozens of startups, and one decade of commercial availability, one has to acknowledge that there are important issues (such as speed, memory, tint, cost, blockage, and stability) that still prevent wide customer acceptance. Each switchable technology has the potential to find a niche application depending on its performance; moreover, it may be time to turn our efforts toward other switchable technologies, for example, a technology quite different from the rest, namely microshutters. Microshutters can take different shapes, but this review will concentrate on microshutters made of curling electrodes actuated by electrostatic forces. They could almost be considered in the same family as cantilevers. “Microblinds” are microshutters developed by the National Research Council (NRC) Canada. They have been described in a limited number of publications [in Ref. 17 (republished three times under requests) and slideshare18]. Microblinds were also compared with other switchable glass technologies in review papers14,15 and market reports.1921 Several groups have worked on various versions of microshutters, namely NRC,17 New Visual Media Group (NVMG),22,23 University of Kassel,24,25 Institut National d’Optique (INO),26 University of Tokyo,27 Samsung,28 U.S. Air Force (USAF),29 Korea Advanced Institute of Science and Technology (KAIST),30,31 Microelectronic Center of North Carolina (MCNC),32 Fiat,33,34 and University of Stuttgart.35 These groups used various versions of curling electrodes actuated by electrostatic forces for switchable glass applications (window, display, imaging, eyewear, etc.). Other groups have studied the use of curling electrodes for microfluidics,36 interconnects,37 and energy storage.38,39

This review will describe the various approaches used in controlling the light transmission using microshutters based on curling electrodes and microelectromechanical system (MEMS) principles. It will compare performances and comment on challenges. Figure 1 presents an example of trapezoidal microblinds developed and fabricated at NRC.

Fig. 1

Scanning electron microscopic (SEM) image of an array of trapezoidal microblinds (microshutters) fabricated at the NRC.

JM3_18_4_040901_f001.png

1.1.

Microelectromechanical System Microshutters

MEMS devices are now everywhere, including in pressure sensors, accelerometers, inkjet printers, microphones, telecommunications switches, projectors, etc. They are also often identified as “micromachines” or “micro systems technology.” MEMSs usually have a mobile part that is actuated by either external forces (pressure, movement, etc.) or internal forces, such as electrostatic–magnetic forces. Their small sizes make them very sensitive to external or internal forces, as well as very robust mechanically. Their fabrication processes are derived from microelectronic manufacturing. Most MEMS devices are made or built on silicon (Si) substrates with some exception on glass substrates. Electrostatic MEMSs are particularly attractive because of their efficiency (high-energy density and large forces), design simplicity, fast response, and low power consumption.40

This review focuses on microshutters fabricated on glass substrates to control the light transmission. Other groups such as the one at NASA41 have worked on microshutter arrays based on Si substrates The devices described in this review are similar to cantilevers (basic MEMS structure). Nevertheless, microshutters have the top mobile electrode curling up based on stress gradient. The last layer(s) deposited get freestanding or mobile after etching away the sacrificial or release layer. When applying a voltage (electric field) between the bottom and the top electrodes, the top electrodes roll down onto the surface, thus blocking the light transmission.

1.2.

Materials

The choice of materials being deposited for the various layers is critical for the reliability of MEMS devices.42 It also determines their operating conditions as well as their manufacturing cost over large areas. The bottom electrode is usually a transparent conductive oxide (TCO), such as indium tin oxide (ITO), tin oxide (SnO2), zinc oxide (ZnO), or even silver (Ag)-based conductive layers. These layers will reduce the light transmission by a certain amount; some of them may also significantly reduce the transmission of near-infrared light critical for maximum solar heat gain in cold climates. The insulating or dielectric layer(s) are critical as well since a high electric field will exist between both electrodes. Most research groups have used SiO2 as an insulating layer (a common dielectric for microelectronics), whereas some others have used SiNx, Al2O3, and polymers. The reliability of the microshutters depends strongly on the performances (for example, breakdown voltage and leakage current) of the dielectric layer(s).4346 A sacrificial (or release) layer is then deposited on the dielectric. The top electrode is usually made of metal (Cr, Al, Au, sometimes combined with a dielectric layer). Once the sacrificial layer is etched away, the top electrode becomes freestanding and mobile.

The choice of materials is also critical for the general reliability of the device, for example, their stabilities in chemical environment (humidity, oxygen), temperature cycling, or UV exposure are keys for long-term reliability. In this context, hard, high melting point, inorganic materials are often preferred over less stable materials. Some switchable glass technologies that are sensitive to UV have added UV-blocking layers or stabilizers at the expense of reducing the UV (long wavelengths) benefits of daylight.

1.3.

Fabrication Steps

Most microshutters are based on standard microelectronic fabrication processes, such as e-beam evaporation, magnetron sputtering, optical lithography, and plasma etch and deposition. Figure 2 presents the main fabrication steps for the microshutters presented in this work.

Fig. 2

Typical fabrication scheme for microshutters. (a) Glass substrate coated with TCO, insulator, and sacrificial layer; (b) patterning and etching of the sacrificial layer to define the anchor; (c) deposition of the top electrode; (d) patterning the geometry of the microshutters by etching the top electrode; and (e) releasing the microshutters by removing the sacrificial layer.

JM3_18_4_040901_f002.png

The lithography (layer patterning) steps are particularly critical for defining the various planar geometries. More lithography steps result in an increased cost of manufacturing. These steps could be performed using conventional optical lithography (contact or projection) or printing techniques or could be laser-based.

The gradient of stress in the top electrode determines the curling behavior. The control of the stresses is crucial in order to get reproducible and uniform results (radius of curvature, actuation voltage, etc.) Many groups17,26,31 have used magnetron sputtering to deposit the top electrode to tune the intrinsic stresses. University of Tokyo27 has used evaporation techniques, relying on material dependent stresses. Other groups22,27,32 have taken advantages of the different coefficients of thermal expansion (CTEs) of bilayers. Universities of Kassel24 and Stuttgart35 have used plasma-enhanced vapor deposition (PECVD) layers with stress gradient. Controlling and characterizing the stress are the keys to reproducible and reliable devices.

The release of the mobile top electrode is performed by removing (etching) the sacrificial layer. This could be done by dry etching or wet etching, depending on the nature of the layers. The choice of the sacrificial layer is often made in such a way that it can be selectively removed without attacking the remaining layers in the device such as the dielectric layer and the top electrode. For example, NRC’s microshutters (microblinds) use a very thin sputtered amorphous Si layer as a sacrificial layer, which is etched away using fluorine-based plasmas or wet chemistries. This release step is delicate and can lead to fabrication issues such as improper curling up.29 Some groups2831 have included special and extra patterning steps (such as corrugation) to promote the proper release or curling behavior.

1.4.

Design

Depending on the required performances, individual microshutter area may vary from a few square microns to a more macroscopic order (cm2). They can be individually connected together or grouped by areas. The patterning steps required for manufacturing the microshutters may be viewed as a weakness, compared to other switchable glasses technologies, but it can also be considered a strength for applications that demand selectively actuated areas or even individually addressed microshutters using passive or active matrix thin film transistor (TFT) technologies. For this purpose, and for facilitating the dimming or improving the reliability of the devices, various groups have chosen to also pattern the bottom electrode.

Most research groups did not report any analytical or mathematical approaches to design their devices. However the U.S. Air Force29 used mathematical expressions developed for cantilevers. Other groups performed modeling using simulations software such as Fiat (Ansys), NRC (Comsol), and INO (Intellisuite).

1.5.

Actuation

Microshutters are based on MEMS devices, more specifically on capacitive MEMSs. There are many publications43,44 describing how to reliably actuate capacitive MEMS devices. The most popular methodology to actuate electrostatically driven microshutters is by using bipolar square waves at frequency around 100 to 1000 Hz. It is also possible to actuate them using DC voltages. The operation voltage varies from 10 V to a few hundred volts depending on the dielectric used, its size, the release layer thickness, and the top electrode (stress and thickness). There is a lot of know-how behind the actuation especially when millions of microshutters are simultaneously actuated on the same device.

2.

Various Versions of Microshutters

As mentioned in Sec. 1, many versions of microshutters have been reported in the literature and on the web. This section will briefly describe each of the many versions reported in the published literature and compare them.

2.1.

New Visual Media Group, United States

NVMG has developed electropolymeric shutters based on shrinkable polymer-laminated using rolls.22 They may be considered milli- or mini-shutters instead of microshutters because of their size of the order of millimeters and larger. They follow the work done by Charles G. Kalt some 30 years ago (resulting in more than 20 patents, for example, Ref. 47). These devices are included in this review since their working principle (electrostatic forces unrolling a mobile electrode) is the same as the usual microshutters. Moreover, they are the only ones commercially available on large area. The stressed curling top electrode is based on shrinkable polymer (1- to 5-μm thick). Once rolled up into multiple turn coils, their diameter is around 1 to 5 mm or greater. The dielectric is based on polymer (4- to 10-μm thick). Their fabrication involves roll laminator and adhesives. The operation voltage usually ranges between 100 and 500 V DC.

Most of the details are available from their patents, but they also have a website23 where videos demonstrating the technology are available.

2.2.

National Research Council Canada

NRC’s microshutters have been called microblinds.17

Details about microblinds are available through few conference proceeding paper,17 patent, and web presence.18,48 Their video has been quite popular, and they have been cited in few reviews on switchable glass14,15 and market reports.1921 They successfully tried various TCOs. The desired stress gradient in the top electrode is obtained using magnetron sputtering (varying deposition conditions) [Fig. 3(a)]. They optimized the dielectric to improve the reliability. The thin sacrificial layer (a:Si, 40-nm thick) and the thin top electrode allow the microblinds to be operated at relatively low voltages (around 20 V). Their fabrication scheme is relatively simple. Nevertheless, it leads to an impressive blockage in closed state (around 99.9%). The microblinds can be fabricated by regular microelectronic or flat panel display (FPD) technologies but also possibly using laser patterning for yielding a much lower fabrication cost.

Fig. 3

SEM images of microshutters developed at (a) NRC, (b) University of Kassel (reproduced from Ref. 25 with permission of Springer), (c) INO (reproduced from Ref. 26 with permission of SPIE), and (d) University of Tokyo (reproduced from Ref. 27 with permission of IEEE).

JM3_18_4_040901_f003.png

2.3.

University of Kassel, Germany

University of Kassel has been developing optical MEMS micromirrors for daylight steering.24,25 They are a kind of microshutters with curling hinges and a flat micromirror [Fig. 3(b)]. Their micromirrors are also actuated by electrostatic voltages (40 to 100 V). Their sacrificial layer is usually based on photoresist, and their usual stress gradient is obtained using PECVD. Many papers and dissertation theses in which they described the various fabrication schemes are available.

2.4.

Institut National d’Optique, Canada

INO developed microshutters for spectrometry applications.26 They are not aimed for large areas but mainly to cover slits in front of spectrometers. Each individual microshutter has a length of 1 mm and a width of 60  μm [Fig. 3(c)]. Contrary to most microshutters, they do not use TCO as a bottom electrode, but rather side electrodes made of Al layer. The dielectric is SiNx, the sacrificial layer is based on photoresist, and the top electrode is based on MoCr. The stress gradient is obtained by varying deposition conditions during the magnetron sputter deposition.

2.5.

University of Tokyo, Japan

The University of Tokyo developed microshutters using stress gradient based on evaporated metal on SiO2 at 180°C.27 The different CTEs create the stress gradient at room temperature but may make them sensitive to the operating temperature. Each individual shutter is 200-μm long, 30-μm wide, and 0.3-μm thick [Fig. 3(d)]. The sacrificial layer is photoresist. The closing speed is around 3 ms and the operating voltage is 55 V.

2.6.

Samsung, Korea

Samsung developed a 2-mm diameter iris shutter for camera using MEMS microshutters.28,49 The iris is formed out of 36 individual 1.4-mm-long triangular shutters [Fig. 4(a)]. On their first reported attempt, they used SiNx/Al bilayer top electrode. Their second report pointed out a detrimental temperature sensitivity when using SiNx/Al; they switched to Mo/Mo for better results. Mo/Mo describes two layers of Mo with different intrinsic stresses and ideal insensitivity to temperature. Their operating voltage is 30 V. They used corrugation to improve the curling and dimples to reduce stiction.

Fig. 4

Images of microshutters developed at (a) Samsung (reproduced from Ref. 28 with permission of IEEE), (b) USAF (reproduced from Ref. 29 with permission of SPIE), (c) KAIST (reproduced from Ref. 31 with permission from SID), and (d) MCNC (reproduced from Ref. 32 with permission from SPIE).

JM3_18_4_040901_f004.png

2.7.

United States Air Force

USAF Research Laboratory developed microshutters for adaptive coded aperture imaging and nonimaging applications.29 They used AlZnO as the TCO on fused silica, and the dielectric is SiNx. The sacrificial layer is 0.5-μm photoresist and the top electrode is made of Ti and Au [Fig. 4(b)]. Their research compared models of the DC actuation to their fabricated devices. These shutters used corrugations to improve the curling up but also revealed the difficulty of releasing depending on the geometry of the microshutters.

2.8.

Korea Advanced Institute of Science and Technology

KAIST developed cascaded microshutters for variable transmission in next-generation displays.30,31 Their microshutters are composed of ITO/SiO2/photoresist (sacrificial)/Ti-Au and require four patterning steps, including one to make corrugations [Fig. 4(c)]. The gradient of stress in the top electrode is based on different intrinsic stresses of sputtered Ti and Au layers. They measured a pull-in voltage of 20 V and a closing speed of 20  μs. Reliability testing of the first-level microshutters revealed some dielectric charging; nevertheless, they estimated their lifetime to 500 billion cycles.

2.9.

Microelectronic Center of North Carolina, United States

MCNC and the University of Florida developed microshutters for protecting optical sensors.32 Their microshutters are based on polyimide as the dielectric and Al as a sacrificial layer [Fig. 4(d)]. The top electrode is based on a sandwich of polyimide/Cr/Au/polyimide. The stress gradient is induced during the curing of the polyimide. They also experienced curling issues. Their operating voltage is around 100 to 300 V; the closing speed is measured as 18  μs. They reported that low operating voltages yield longer lifetime, and 450 million cycles were tested.

2.10.

Fiat, Italy

They developed microshutters for display and spectroscopy applications [Fig. 5(a)].33,34 They did not reveal much about the fabrication; nevertheless, their microshutters are relatively big (0.5 to 2 mm) and required around 80 V actuation voltage. Modeling results were obtained in collaboration with the Université de Bordeaux.

Fig. 5

SEM images of microshutters developed at (a) FIAT (reproduced from Ref. 34 with permission of Springer), (b) University of Stuttgart (reproduced from Ref. 35 with permission of SID), (c) Philipps (reproduced from Ref. 36 with permission from The Royal Society of Chemistry), and (d) PARC (reproduced from Ref. 37 with permission of SPIE).

JM3_18_4_040901_f005.png

2.11.

University of Stuttgart, Germany

Following a collaboration with NRC, University of Stuttgart decided to implement microshutters with their TFT expertise.35 Their microshutters are mainly aimed toward display application with low aperture ratio (15%) [Fig. 5(b)]. They are fabricated on TFT-based active matrices with a wet-etched Si sacrificial layer (150 to 200 nm thick). The top electrode is SiNx with gradient of stress obtained during PECVD, and a molybdenum-tantalum (MoTa) conductive layer (total thickness around 500 nm). The operating voltage is around 30 V, and they estimated a response time below 50  μs.

3.

Comparison of the Various Approaches

Table 1 presents the various versions of microshutters developed as a function of year (first publication known to us), material choice (bottom electrode, dielectric, sacrificial, top electrode), sizes, and fabrication processes. The references shown usually correspond to the latest publications. The first dates correspond to the date of first publication (paper or patent), although we recognize that work started before publications were made. Many of the research groups developed microshutters several years ago and stopped the R-D project for unknown reasons. Most of the groups used ITO for the bottom electrode. There is a diversity of materials (polymer, Si, and Al) used for the sacrificial layer. This choice is critical for the fabrication (ability to etch quickly, manufacturing cost) as well as for the operating voltage. The ability to coat a very thin sacrificial layer without pinholes reduces the actuation voltage and thus increases the microshutter lifetime. Most of the microshutters have sizes around 200  μm; those dimensions should make them reliable (MEMS) as well as practically invisible to the eye.

Table 1

Comparison of the various microshutters depending on materials and fabrication.

MaterialsFabrication
Research groupRef.TimeBottom electrodeDielectricSacrificialTop electrodeSizeProcesses, comments
NVMG222007 to presentTCOPolymer (4- to 10-μm thick)Shrinkable polymer layer (1- to 5-μm thick) with Al, NiCr, SS (100- to 500-A thick)Centimeter (r: 0.5 to 2 mm or greater)Temperature-induced stress, electropolymeric, roll laminator
NRC172005 to presentSnO2, ITO, Ag low-eSiO2, SiNx, Al2O3Si and others, around 40 nmCr and others, 1000 A50 to 300  μm (r: 10 to 100  μm)Optical or laser patterning, magnetron sputtering
University of Kassel242003 to presentITO, ZnO or Ag low-eSiO2, polymerPhotoresist, 0.5  μmSiO2SiNxAlCr150×400  μmPECVD stress and Al thickness, optical litho or imprinting
INO262008 to 2009AlSiNxPolyimideMoCr60×1000  μm (r:  370  μm)Stress controlled during sputter dep
University of Tokyo272015 to 2016ITO, 200 nmSiO2, 200 nmPhotoresist, 0.5 to 2  μmSiO2, 200  nm/Al, 100 nm200×100  μm (r:  70  μm)Al evaporated at 180°C on SiO2
Samsung492009 to 2011ITOPECVD SiO20.5  μmPeryleneSiNxAl (3500/3000 A), Mo/MoIris, 2-mm diameter, 36 1.4-mm-long triangular shutters (r: 230 to 500  μm)Similar CTE is much better. Corrugations and dimples are patterned.
Air Force292008AlZnOSiNxPhotoresist, 0.5  μmTi–Au200×50Corrugation, difficulty to roll them up depending on geometry
KAIST312010 to 2016ITO, 0.2  μmSiO2, 0.5  μmPhotoresist, 0.2  μmTi–Au 100 nm200×160  μmFour patterning steps, different intrinsic stress of sputtered Ti and Au
MCNC322000 to 2002ITOPolyimide, 0.5  μmAlPolyimide–Cr–Au–polyimideVarious sizes 50  μm to 1 mm (r: 50 to 100  μm)PI cured at 400°C
Fiat331999 to 2005ITO458  μm to 2.4 mm
University of Stuttgart352016 to presentMoTaSiNxSi, 150 to 200 nmMoTa on stressed SiNx (400 to 600 nm)200  μm (r: 64 to 300  μm)Structural SiNx with stress gradient, release by wet etching the Si

There are different strategies used to make the top electrode. This is a very crucial step: the gradient of stress in the top electrode needs to be well controlled and uniform over relatively large areas depending on the applications.

It seems that more reproducible and uniform stress gradients are expected with magnetron sputtered layers (given the fact that targets can be over 4-m long and that it is a low-temperature process) than with evaporated, PECVD, or thermally cured layers for large areas.

Moreover, as Kim and Hong (Samsung)49 pointed out, with experimental results, that stress gradients in the same material is much more temperature-stable than in bilayers such as Al/SiNx. The different CTEs may have detrimental effects on switchable glass, unless used on thermally activated switchable glass.

Contrary to most switchable glass, microshutters require patterning steps to define the anchors and the microshutters. These steps are usually based on optical lithography and represent a major manufacturing cost. Most microshutters require two patterning steps, but many research groups reported a required extra one for corrugating the top electrode. This extra step makes it easier to release and curl up successfully the top electrode but increases the manufacturing costs. Two groups have investigated other patterning approaches, such as imprinting (University of Kassel) and laser patterning (NRC). These approaches could lead to much lower manufacturing costs.

As for the reliability, many researchers have proven experimentally that the microshutters (at least on small areas) are very reliable, cycling them millions of time.24,30,32

Table 2 presents the various microshutters based on their actuation voltages, speeds, demonstrated sizes, visible transmissions, and applications. The required performances depend on the application; however, there are obvious general advantages in low operating voltage, high contrast, temperature stability (no CTE gradient in top electrode, high-melting-point materials), UV durability (inorganic), and low manufacturing cost (thin layers and simple patterning).

Table 2

Comparison of the various microshutters depending on performances.

Research groupRef.ActuationSpeedDemo size (active area)Max/min transmission contrastApplications comments
NVMG22100 to 500 Vsecond5000  cm2High contrastMacro-curling shutters, commercialized, eight U.S. patents issued, impressive demos
NRC1715 to 25 VAround 40  μs to close20  cm260/0.1 to 600High contrast, low voltage
University of Kassel2440 to 100 V100  cm280/5 to 16Mirror steering sunlight; lifetime >50 years, startup
INO26110 V2 ms to close, 7 ms to open<1  cm2Low contrastSpace instrumentation (slit for spectrometer), substrate: sapphire
University of Tokyo2755 V3 ms0.25  cm253/36Implemented on TFT
Samsung4930 V1 to 2 msIris of 0.04  cm2High contrastShutter for camera
Air Force29Low contrastAdaptive coded aperture imaging
KAIST3120 V20  μs to closeSmallLow contrastActive transparent display
MCNC32100 to 300 V18  μs5  cm2Low contrastEyelid for protection
Fiat3380 V0.1 msLow contrastDisplay
University of Stuttgart3525 to 45 V<50  μs (model)2  cm2Low contrastDisplay, implemented on TFT, low transmission

The operating voltage is a critical parameter for many applications and the electric field is critical for the reliability of the devices. They depend on the dielectric, the mechanical properties of the top mobile electrode, the radius of curvature, the thicknesses of dielectric, space, and top electrode. The operating speed is inversely proportional to the size and thickness of the microshutters.

Scaling-up. Most research reports on microshutters are for devices covering only small areas. Although a few groups have made devices on opaque wafers, most work is designed to control light transmission (switchable glass) on small areas, such as spectrometer slits and eyeglasses, or large areas, such as car sunroof, aircraft windows, or even building windows. The electrostatic principle of operation scales to large areas as demonstrated by NVMG.22,23 FPDs based on LCDs are functionally similar to switchable glass and integrate additional complex electronics. These devices are made on glass panels as large as 3.4 m and require many high resolution patterning steps. Large area microshutter panels are less complex than LCD panels but can be fabricated with similar manufacturing processes lines. Most research groups already used glass as a substrate and fabrication processes already employed in large scale manufacturing.

The operation voltage does not depend on the size of the area covered by the microshutters (voltage driven). Nevertheless, the total current will not only depend on area but also mostly depend on the operation frequency. The NRC modeled the effect of size, electrodes’ resistivities, dielectric and operation frequency on actuation. They found that scaling-up is not limited by the voltage distribution if the right materials and geometries are used. Realistically, it makes sense to target small- and mid-sized applications, before addressing the building window market. As pointed out earlier, laser processing could allow the microshutters to become a very competitive switchable glass technology for large areas such as building windows.

4.

Weaknesses and Challenges

Microshutters are very different from the current most common switchable glass technologies, such as EC. For example, microshutters are particularly fast switching, without unintentional tint, low power consumption, superior for solar use-control, and area-selective. Moreover, the microshutters with single material top electrode and without organic materials might be temperature-stable and UV-durable. One of the requirements for many applications, such as car sunroof and augmented reality, is the possibility to implement the switchable glass on curved glass substrates. EC did not succeed yet to reach that goal. Some microshutters based on high-melting-temperature materials (e.g., Ref. 17), can be implemented on glass substrates to be bent. Nevertheless, microshutters still have weaknesses, such as visual disturbance, required enclosure/encapsulation, manufacturability on large areas at low cost, reliability, and stress control. For example, the microshutters once curled up become tiny opaque lines that induce Fresnel diffraction, resulting in haze or visual disturbance. This diffraction is inherent to the technology, but it is also possible to reduce the resulting haze to an indiscernible level. There is a real market pull to develop such a switchable glass. The microshutters might not replace completely the competitive technologies. However, for applications where speed, contrast, durability, and temperature stability are critical, they might be the best candidate.

5.

Conclusion

Microshutters have very interesting performance levels, making them good candidates for switchable glasses or light modulators. Many research groups have investigated various approaches to fabricate microshutters. They proved that microshutters can be reliable on small scale, fast, low power, high contrast, and relatively easy to manufacture. There is a need to prove that they can also be scaled-up and cost-competitive.

Acknowledgments

We wish to thank Heping Ding, Kelly Laliberté, Richard Dudek, Mark Malloy from NRC, Rob Vandusen from Carleton University, and Patrick Schalberger from University of Stuttgart.

References

1. 

K. Butti and J. Perlin, A Golden Thread: 2500 Years of Solar Architecture, Cheshire Books, Palo Alto, California (1980). Google Scholar

2. 

A. K. Fahimipour et al., “Daylight exposure modulates bacterial communities associated with household dust,” Microbiome, 6 (1), 175 –187 (2018). https://doi.org/10.1186/s40168-018-0559-4 Google Scholar

3. 

D. R. Roberts, “Preliminary assessment of the energy-saving potential of electrochromic windows in residential buildings,” (2009). Google Scholar

4. 

A. H. Sadek and R. Mahrous, “Adaptive glazing technologies: balancing the benefits of outdoor views in healthcare environments,” Sol. Energy, 174 719 –727 (2018). https://doi.org/10.1016/j.solener.2018.09.032 Google Scholar

5. 

S. Attia et al., “Current trends and future challenges in the performance assessment of adaptive façade systems,” Energy Build., 179 165 –182 (2018). https://doi.org/10.1016/j.enbuild.2018.09.017 Google Scholar

6. 

A. Piccolo et al., “Energy performance of an electrochromic switchable glazing: experimental and computational assessments,” Energy Build., 165 390 –398 (2018). https://doi.org/10.1016/j.enbuild.2017.12.049 Google Scholar

7. 

A. Cannavale, U. Ayr and F. Martellotta, “Innovative electrochromic devices: energy savings and visual comfort effects,” Energy Procedia, 148 900 –907 (2018). https://doi.org/10.1016/j.egypro.2018.08.096 1876-6102 Google Scholar

8. 

J.-M. Dussault, L. Gosselin and T. Galstian, “Integration of smart windows into building design for reduction of yearly overall energy consumption and peak loads,” Sol. Energy, 86 (11), 3405 –3416 (2012). https://doi.org/10.1016/j.solener.2012.07.016 Google Scholar

9. 

E. Syrrakou, S. Papaefthimiou and P. Yianoulis, “Eco-efficiency evaluation of a smart window prototype,” Sci. Total Environ., 359 (1–3), 267 –282 (2006). https://doi.org/10.1016/j.scitotenv.2005.10.023 Google Scholar

10. 

J. Al Dakheel and K. T. Aoul, “Building applications, opportunities and challenges of active shading systems: a state-of-the-art review,” Energies, 10 (10), 1672 –1703 (2017). https://doi.org/10.3390/en10101672 NRGSDB 0165-2117 Google Scholar

11. 

Y. Wang, E. L. Runnerstrom and D. J. Milliron, “Switchable materials for smart windows,” Annu. Rev. Chem. Biomol. Eng., 7 283 –304 (2016). https://doi.org/10.1146/annurev-chembioeng-080615-034647 ARCBCY 1947-5438 Google Scholar

12. 

R. Baetens, B. P. Jelle and A. Gustavsen, “Properties, requirements and possibilities of smart windows for dynamic daylight and solar energy control in buildings: a state-of-the-art review,” Sol. Energy Mater. Sol. Cells, 94 (2), 87 –105 (2010). https://doi.org/10.1016/j.solmat.2009.08.021 SEMCEQ 0927-0248 Google Scholar

13. 

M. Casini, “Active dynamic windows for buildings: a review,” Renew. Energy, 119 923 –934 (2018). https://doi.org/10.1016/j.renene.2017.12.049 RNENE3 0960-1481 Google Scholar

14. 

J. Marchwiński, “Architectural evaluation of switchable glazing technologies as sun protection measure,” Energy Procedia, 57 1677 –1686 (2014). https://doi.org/10.1016/j.egypro.2014.10.158 1876-6102 Google Scholar

15. 

Y. P. Singh and M. S. Gupta, “Future of intelligent windows,” Int. J. Res. Eng. Technol., 4 (3), 17 –26 (2016). Google Scholar

16. 

S. D. Rezaei, S. Shannigrahi and S. Ramakrishna, “A review of conventional, advanced, and smart glazing technologies and materials for improving indoor environment,” Sol. Energy Mater. Sol. Cells, 159 26 –51 (2017). https://doi.org/10.1016/j.solmat.2016.08.026 SEMCEQ 0927-0248 Google Scholar

17. 

B. Lamontagne et al., “The next generation of switchable glass: the micro-blinds,” in Proc. Conf. Glass Perform. Days, (2009). Google Scholar

18. 

B. Lamontagne, “Smart automotive headlamp based on arrays of optical shutters,” (2019) https://www.slideshare.net/BorisLamontagne/smart-automotive-headlamp-based-on-arrays-of-optical-shutters September ). 2019). Google Scholar

19. 

MarketsAndMarkets, “Smart glass market by technology (Suspended particle display, electrochromic, liquid crystal, photochromic, thermochromic), application (architecture, transportation, consumer electronics), and geography—Global Forecast to 2023 2017,” (2017) https://www.marketsandmarkets.com/Market-Reports/smart-glass-market-907.html September 2019). Google Scholar

20. 

Research and Markets, “The global market for smart glass and windows to 2030,” (2018) https://www.researchandmarkets.com/research/7p62fr/global_smart?w=5 September 2019). Google Scholar

21. 

BCC Research, “Global markets and technologies for smart glass,” (2018) https://www.bccresearch.com/report/download/report/AVM065D September 2019). Google Scholar

22. 

E. Schlam, J. Finch and J. Koskulics, “Highly reflective electrostatic shutter display,” in Proc. Int. Disp. Workshops, (2017). Google Scholar

23. 

New Visual Media Group, (2019) https://newvisualmediagroup.com/index.php/technology September ). 2019). Google Scholar

24. 

H. Hillmer et al., “Optical MEMS-based micromirror arrays for active light steering in smart windows,” Jpn. J. Appl. Phys., 57 (8S2), 08PA07 (2018). https://doi.org/10.7567/JJAP.57.08PA07 Google Scholar

25. 

Q. Li et al., “Design of self-assembling micromirror arrays for light guiding applications,” Microsyst. Technol., 16 895 –899 (2010). https://doi.org/10.1007/s00542-009-1001-4 0946-7076 Google Scholar

26. 

S. Ilias et al., “Programmable optical microshutter arrays for large aspect ratio microslits,” Proc. SPIE, 7099 70992D (2008). https://doi.org/10.1117/12.807187 PSISDG 0277-786X Google Scholar

27. 

K. Mori et al., “A MEMS electrostatic roll-up window shade array for house energy management system,” IEEE Photonics Technol. Lett., 28 (5), 593 –596 (2016). https://doi.org/10.1109/LPT.2016.2514299 IPTLEL 1041-1135 Google Scholar

28. 

C. Kim, K. Jung and W. Kim, “A wafer-level micro mechanical global shutter for a micro camera,” in Proc. IEEE 22nd Int. Conf. Micro Electro Mech. Syst., 156 –159 (2009). Google Scholar

29. 

D. Langley, S. Rogers and L. Starman, “Fabrication studies for scaling photonic MEMS micro-shutter designs,” Proc. SPIE, 7096 709601 (2008). https://doi.org/10.1117/12.800475 PSISDG 0277-786X Google Scholar

30. 

K.-H. Lee, J. Chang and J.-B. Yoon, “High performance microshutter device with space-division modulation,” J. Micromech. Microeng., 20 (7), 075030 (2010). https://doi.org/10.1088/0960-1317/20/7/075030 JMMIEZ 0960-1317 Google Scholar

31. 

K.-S. Lim et al., “Wide bandwidth reflective microshutter blind panel for transparent organic light-emitting diode display,” in SID 2016 Int. Symp., 1389 –1391 (2016). https://doi.org/10.1002/sdtp.10936 Google Scholar

32. 

S. Goodwin-Johansson et al., “Reduced voltage artificial eyelid for protection of optical sensors,” Proc. SPIE, 4695 451 –458 (2002). https://doi.org/10.1117/12.475193 PSISDG 0277-786X Google Scholar

33. 

P. Roux et al., “Fem modelling of an electro-optical micro-shutter,” Sens. Actuators A: Phys., 119 (1), 1 –7 (2005). https://doi.org/10.1016/j.sna.2004.06.022 Google Scholar

34. 

M. Pizzi et al., “Electrostatically driven film light modulators for display applications,” Microsyst. Technol., 10 (1), 17 –21 (2003). https://doi.org/10.1007/s00542-002-0244-0 0946-7076 Google Scholar

35. 

P. Schalberger, S. A. A. Nusayer and C. Raichle, “Parallel fabrication for integration of electronic and microelectromechanical systems,” in SID 2016 Int. Symp., 731 –734 (2016). Google Scholar

36. 

J. Toonder et al., “Artificial cilia for active micro-fluidic mixing,” Lab Chip, 8 (4), 533 –541 (2008). https://doi.org/10.1039/b717681c LCAHAM 1473-0197 Google Scholar

37. 

M. A. Rosa et al., “A novel external electrode configuration for the electrostatic actuation of MEMS based devices,” J. Micromech. Microeng., 14 (4), 446 –451 (2004). https://doi.org/10.1088/0960-1317/14/4/003 JMMIEZ 0960-1317 Google Scholar

38. 

P. Deb Shuvra and S. McNamara, “The strain capacitor: a novel energy storage device,” AIP Adv, 4 127158 (2014). https://doi.org/10.1063/1.4905460 AAIDBI 2158-3226 Google Scholar

39. 

C. C. B. Bufon et al., “Self-assembled ultra-compact energy storage elements based on hybrid nanomembranes,” Nano Lett, 10 2506 –2510 (2010). https://doi.org/10.1021/nl1010367 NALEFD 1530-6984 Google Scholar

40. 

Y. Zhang and Y.-P. Zhao, “Flexural contact in MEMS stiction,” Int. J. Solids Struct., 49 (17), 2203 –2214 (2012). https://doi.org/10.1016/j.ijsolstr.2012.03.032 IJSOAD 0020-7683 Google Scholar

41. 

D. E. Burns et al., “2-D electrostatic actuation of microshutter arrays,” J. Microelectromech. Syst., 25 (1), 101 –107 (2016). https://doi.org/10.1109/JMEMS.2015.2498411 JMIYET 1057-7157 Google Scholar

42. 

V. T. Srikar and S. M. Spearing, “Materials selection for microfabricated electrostatic actuators,” Sens. Actuators A, 102 279 –285 (2003). https://doi.org/10.1016/S0924-4247(02)00393-X Google Scholar

43. 

W. M. van Spengen, “Capacitive RF MEMS switch dielectric charging and reliability: a critical review with recommendations,” J. Micromech. Microeng., 22 074001 (2012). https://doi.org/10.1088/0960-1317/22/7/074001 JMMIEZ 0960-1317 Google Scholar

44. 

H. Camon and C. Ganibal, “Advantages of alternative actuating signal for MEMS actuators,” Sens. Actuators A: Phys., 136 (1), 299 –303 (2007). https://doi.org/10.1016/j.sna.2006.11.025 Google Scholar

45. 

R. Saha et al., “Analysis of pull-in-voltage and figure-of-merit of capacitive MEMS switch,” Trans. Electr. Electron. Mater., 17 (3), 129 –133 (2016). https://doi.org/10.4313/TEEM.2016.17.3.129 Google Scholar

46. 

Y. Huang et al., “MEMS reliability review,” IEEE Trans. Device Mater. Reliab., 12 (2), 482 –493 (2012). https://doi.org/10.1109/TDMR.2012.2191291 Google Scholar

47. 

C. G. Kalt, “Electro-static device with rolling electrode,” patent USPTO US 4094590A (1976).

48. 

B. Lamontagne, “Micro-blinds for smart glass,” (2011) https://www.youtube.com/watch?v=RqwL2egaqYY Google Scholar

49. 

C.-H. Kim and S. Hong, “Study on the reliability of the mechanical shutter utilizing roll actuators,” in Proc. IEEE 24th Int. Conf. Micro Electro Mech. Syst., 501 –504 (2011). https://doi.org/10.1109/MEMSYS.2011.5734471 Google Scholar

Biography

Boris Lamontagne received his PhD in engineering physics from École Polytechnique, Montréal, in 1992. Postdocs in Paris and Quebec city were followed by a researcher employment at the National Research Council (NRC) in 1995. His research interests are fabrication of photonic and MOEMS devices.

Norman R. Fong is a research associate at the NRC, Ottawa, Ontario, Canada. He received his PhD in electrical engineering from Carleton University (2015) with research focus on micro- and nano-fabrication of integrated plasmonics and microelectromechanical system (MEMS) sensors.

In-Hyouk Song received his PhD in electrical engineering from Louisiana State University, Baton Rouge, Louisiana, in 2005. He served as a research associate at Ecole Polytechnique de Montreal, Montreal, and NRC, Ottawa, Canada. Since 2010, he has been at the Department of Engineering Technology, Texas State University, Texas, where he is currently an associate professor. His research efforts have focused on developing the physical and biosensing device combining MEMS techniques.

Penghui Ma received his PhD from University of Western Ontario in 1997. He is a senior research officer at NRC, Canada. His main research interests are in thin-film depositions, multilayer optical coatings, and their applications.

Pedro Barrios received his BSc degree in electronic engineering from the IUPFAN, Venezuela, in 1989, and his MS degree and PhD in electrical engineering from the University of Pittsburgh in 1993 and 1997, respectively. He worked as a postdoc at the NanoFAB Center of Texas A&M University from 1998 to 1999 and at the EE Department of the University of Notre Dame from 1999 to 2000. Currently, he pursues research in fabrication of optoelectronic devices on III–V semiconductors at the NRC of Canada in Ottawa.

Daniel Poitras received his PhD in physics engineering from Ecole Polytechnique Montreal with a thesis on plasma-deposited optical films and coatings in 2000. Currently, he is working as a senior researcher at the NRC of Canada. His main fields of interest are the application, design, fabrication, and characterization of optical coatings, with a recent focus on waveguide facet optical coatings.

© 2019 Society of Photo-Optical Instrumentation Engineers (SPIE)
Boris Lamontagne, Norman R. Fong, In-Hyouk Song, Penghui Ma, Pedro J. Barrios, and Daniel Poitras "Review of microshutters for switchable glass," Journal of Micro/Nanolithography, MEMS, and MOEMS 18(4), 040901 (8 October 2019). https://doi.org/10.1117/1.JMM.18.4.040901
Received: 11 May 2019; Accepted: 6 September 2019; Published: 8 October 2019
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