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 (${\mathrm{SnO}}_2$), 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 ${\mathrm{SiO}}_2$ as an insulating layer (a common dielectric for microelectronics), whereas some others have used ${\mathrm{SiN}}_{\mathrm{x}}$, ${\mathrm{Al}}_2{\mathrm{O}}_3$, and polymers. The reliability of the microshutters depends strongly on the performances (for example, breakdown voltage and leakage current) of the dielectric layer(s).^43–46 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.

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 groups^17,26,31 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 groups^22,27,32 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 groups^28–31 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 (${\mathrm{cm}}^2$). 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 publications^43,44 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.

2.1.

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 $5\text{-}\mu \mathrm{m}$ 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 $10\text{-}\mu \mathrm{m}$ 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 glass^14,15 and market reports.^19–21 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).

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 $60\mu \mathrm{m}$ [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 ${\mathrm{SiN}}_{\mathrm{x}}$, 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 ${\mathrm{SiO}}_2$ 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 $200\text{-}\mu \mathrm{m}$ long, $30\text{-}\mu \mathrm{m}$ wide, and $0.3\text{-}\mu \mathrm{m}$ 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 ${\mathrm{SiN}}_{\mathrm{x}}/\mathrm{Al}$ bilayer top electrode. Their second report pointed out a detrimental temperature sensitivity when using ${\mathrm{SiN}}_{\mathrm{x}}/\mathrm{Al}$; 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).

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 ${\mathrm{SiN}}_{\mathrm{x}}$. The sacrificial layer is $0.5\text{-}\mu \mathrm{m}$ 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/${\mathrm{SiO}}_2$/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 $20\mu \mathrm{s}$. 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 $18\mu \mathrm{s}$. 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).

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 ${\mathrm{SiN}}_{\mathrm{x}}$ 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 $50\mu \mathrm{s}$.